Susan K. McConnell

All Publications

• Transcriptional repression by FEZF2 restricts alternative identities of cortical projection neurons. Cell reports Tsyporin, J., Tastad, D., Ma, X., Nehme, A., Finn, T., Huebner, L., Liu, G., Gallardo, D., Makhamreh, A., Roberts, J. M., Katzman, S., Sestan, N., McConnell, S. K., Yang, Z., Qiu, S., Chen, B. 2021; 35 (12): 109269

  Abstract

  Projection neuron subtype identities in the cerebral cortex are established by expressing pan-cortical and subtype-specific effector genes that execute terminal differentiation programs bestowing neurons with a glutamatergic neuron phenotype and subtype-specific morphology, physiology, and axonal projections. Whether pan-cortical glutamatergic and subtype-specific characteristics are regulated by the same genes or controlled by distinct programs remains largely unknown. Here, we show that FEZF2 functions as a transcriptional repressor, and it regulates subtype-specific identities of both corticothalamic and subcerebral neurons by selectively repressing expression of genes inappropriate for each neuronal subtype. We report that TLE4, specifically expressed in layer 6 corticothalamic neurons, is recruited by FEZF2 to inhibit layer 5 subcerebral neuronal genes. Together with previous studies, our results indicate that a cortical glutamatergic identity is specified by multiple parallel pathways active in progenitor cells, whereas projection neuron subtype-specific identity is achieved through selectively repressing genes associated with alternate identities in differentiating neurons.

  View details for DOI 10.1016/j.celrep.2021.109269

  View details for PubMedID 34161768

• Transcription factor expression defines subclasses of developing projection neurons highly similar to single-cell RNA-seq subtypes. Proceedings of the National Academy of Sciences of the United States of America Heavner, W. E., Ji, S. n., Notwell, J. H., Dyer, E. S., Tseng, A. M., Birgmeier, J. n., Yoo, B. n., Bejerano, G. n., McConnell, S. K. 2020

  Abstract

  We are only just beginning to catalog the vast diversity of cell types in the cerebral cortex. Such categorization is a first step toward understanding how diversification relates to function. All cortical projection neurons arise from a uniform pool of progenitor cells that lines the ventricles of the forebrain. It is still unclear how these progenitor cells generate the more than 50 unique types of mature cortical projection neurons defined by their distinct gene-expression profiles. Moreover, exactly how and when neurons diversify their function during development is unknown. Here we relate gene expression and chromatin accessibility of two subclasses of projection neurons with divergent morphological and functional features as they develop in the mouse brain between embryonic day 13 and postnatal day 5 in order to identify transcriptional networks that diversify neuron cell fate. We compare these gene-expression profiles with published profiles of single cells isolated from similar populations and establish that layer-defined cell classes encompass cell subtypes and developmental trajectories identified using single-cell sequencing. Given the depth of our sequencing, we identify groups of transcription factors with particularly dense subclass-specific regulation and subclass-enriched transcription factor binding motifs. We also describe transcription factor-adjacent long noncoding RNAs that define each subclass and validate the function of Myt1l in balancing the ratio of the two subclasses in vitro. Our multidimensional approach supports an evolving model of progressive restriction of cell fate competence through inherited transcriptional identities.

  View details for DOI 10.1073/pnas.2008013117

  View details for PubMedID 32948690

• Compensatory Actions of Ldb Adaptor Proteins During Corticospinal Motor Neuron Differentiation CEREBRAL CORTEX Leone, D. P., Panagiotakos, G., Heavner, W. E., Joshi, P., Zhao, Y., Westphal, H., McConnell, S. K. 2017; 27 (2): 1686-1699

  Abstract

  Although many genes that specify neocortical projection neuron subtypes have been identified, the downstream effectors that control differentiation of those subtypes remain largely unknown. Here, we demonstrate that the LIM domain-binding proteins Ldb1 and Ldb2 exhibit dynamic and inversely correlated expression patterns during cerebral cortical development. Ldb1-deficient brains display severe defects in proliferation and changes in regionalization, phenotypes resembling those of Lhx mutants. Ldb2-deficient brains, on the other hand, exhibit striking phenotypes affecting layer 5 pyramidal neurons: Immature neurons have an impaired capacity to segregate into mature callosal and subcerebral projection neurons. The analysis of Ldb2 single-mutant mice reveals a compensatory role of Ldb1 for Ldb2 during corticospinal motor neuron (CSMN) differentiation. Animals lacking both Ldb1 and Ldb2 uncover the requirement for Ldb2 during CSMN differentiation, manifested as incomplete CSMN differentiation, and ultimately leading to a failure of the corticospinal tract.

  View details for DOI 10.1093/cercor/bhw003

  View details for Web of Science ID 000397257600061

• TBR1 regulates autism risk genes in the developing neocortex. Genome research Notwell, J. H., Heavner, W. E., Darbandi, S. F., Katzman, S., McKenna, W. L., Ortiz-Londono, C. F., Tastad, D., Eckler, M. J., Rubenstein, J. L., McConnell, S. K., Chen, B., Bejerano, G. 2016; 26 (8): 1013-1022

  Abstract

  Exome sequencing studies have identified multiple genes harboring de novo loss-of-function (LoF) variants in individuals with autism spectrum disorders (ASD), including TBR1, a master regulator of cortical development. We performed ChIP-seq for TBR1 during mouse cortical neurogenesis and show that TBR1-bound regions are enriched adjacent to ASD genes. ASD genes were also enriched among genes that are differentially expressed in Tbr1 knockouts, which together with the ChIP-seq data, suggests direct transcriptional regulation. Of the nine ASD genes examined, seven were misexpressed in the cortices of Tbr1 knockout mice, including six with increased expression in the deep cortical layers. ASD genes with adjacent cortical TBR1 ChIP-seq peaks also showed unusually low levels of LoF mutations in a reference human population and among Icelanders. We then leveraged TBR1 binding to identify an appealing subset of candidate ASD genes. Our findings highlight a TBR1-regulated network of ASD genes in the developing neocortex that are relatively intolerant to LoF mutations, indicating that these genes may play critical roles in normal cortical development.

  View details for DOI 10.1101/gr.203612.115

  View details for PubMedID 27325115

• Satb2 Regulates the Differentiation of Both Callosal and Subcerebral Projection Neurons in the Developing Cerebral Cortex. Cerebral cortex Leone, D. P., Heavner, W. E., Ferenczi, E. A., Dobreva, G., Huguenard, J. R., Grosschedl, R., McConnell, S. K. 2015; 25 (10): 3406-3419

  Abstract

  The chromatin-remodeling protein Satb2 plays a role in the generation of distinct subtypes of neocortical pyramidal neurons. Previous studies have shown that Satb2 is required for normal development of callosal projection neurons (CPNs), which fail to extend axons callosally in the absence of Satb2 and instead project subcortically. Here we conditionally delete Satb2 from the developing neocortex and find that neurons in the upper layers adopt some electrophysiological properties characteristic of deep layer neurons, but projections from the superficial layers do not contribute to the aberrant subcortical projections seen in Satb2 mutants. Instead, axons from deep layer CPNs descend subcortically in the absence of Satb2. These data demonstrate distinct developmental roles of Satb2 in regulating the fates of upper and deep layer neurons. Unexpectedly, Satb2 mutant brains also display changes in gene expression by subcerebral projection neurons (SCPNs), accompanied by a failure of corticospinal tract (CST) formation. Altering the timing of Satb2 ablation reveals that SCPNs require an early expression of Satb2 for differentiation and extension of the CST, suggesting that early transient expression of Satb2 in these cells plays an essential role in development. Collectively these data show that Satb2 is required by both CPNs and SCPNs for proper differentiation and axon pathfinding.

  View details for DOI 10.1093/cercor/bhu156

  View details for PubMedID 25037921

• Evidence for topographic guidance of dopaminergic axons by differential Netrin-1 expression in the striatum MOLECULAR AND CELLULAR NEUROSCIENCE Li, J., Duarte, T., Kocabas, A., Works, M., McConnell, S. K., Hynes, M. A. 2014; 61: 85-96

  Abstract

  There are two main subgroups of midbrain dopaminergic (DA) neurons: the more medially located ventral tegmental area (VTA) DA neurons, which have axons that innervate the ventral-lateral (VL) striatum, and the more laterally located substantia nigra (SN) DA neurons, which preferentially degenerate in Parkinson's disease (PD) and have axons that project to the dorsal-medial (DM) striatum. DA axonal projections in the striatum are not discretely localized and they arborize widely, however they do not stray from one zone to the other so that VTA axons remain in the VL zone and SN axons in the DM zone. Here we provide evidence that Netrin-1 acts in a novel fashion to topographically pattern midbrain DA axons into these two striatal zones by means of a gradient of Netrin-1 in the striatum and by differential attraction of the axons to Netrin-1. Midbrain DA neurons are attracted to the striatum in culture and this attraction is blocked by an anti-DCC (Netrin receptor) antibody. Mechanistically, outgrowth of both VTA and SN DA axons is stimulated by Netrin-1, but the two populations of DA axons respond optimally to overlapping but distinct concentrations of Netrin-1, with SN axons preferring lower concentrations and VTA axons preferring higher concentrations. In vivo this differential preference is closely mirrored by differences in Netrin-1 expression in their respective striatal target fields. In vivo in mice lacking Netrin-1, DA axons that reach the striatum fail to segregate into two terminal zones and to fully innervate the striatum. Our results reveal novel actions for Netrin-1 and provide evidence for a mechanism through which DA axons can selectively innervate one of two terminal zones in the striatum but have free reign to arborize widely within a terminal zone.

  View details for DOI 10.1016/j.mcn.2014.05.003

  View details for Web of Science ID 000340690400009

  View details for PubMedID 24867253

• A network of genetic repression and derepression specifies projection fates in the developing neocortex PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Srinivasan, K., Leone, D. P., Bateson, R. K., Dobreva, G., Kohwi, Y., Kohwi-ShigematSU, T., Grosschedl, R., McConnell, S. K. 2012; 109 (47): 19071-19078

  Abstract

  Neurons within each layer in the mammalian cortex have stereotypic projections. Four genes-Fezf2, Ctip2, Tbr1, and Satb2-regulate these projection identities. These genes also interact with each other, and it is unclear how these interactions shape the final projection identity. Here we show, by generating double mutants of Fezf2, Ctip2, and Satb2, that cortical neurons deploy a complex genetic switch that uses mutual repression to produce subcortical or callosal projections. We discovered that Tbr1, EphA4, and Unc5H3 are critical downstream targets of Satb2 in callosal fate specification. This represents a unique role for Tbr1, implicated previously in specifying corticothalamic projections. We further show that Tbr1 expression is dually regulated by Satb2 and Ctip2 in layers 2-5. Finally, we show that Satb2 and Fezf2 regulate two disease-related genes, Auts2 (Autistic Susceptibility Gene2) and Bhlhb5 (mutated in Hereditary Spastic Paraplegia), providing a molecular handle to investigate circuit disorders in neurodevelopmental diseases.

  View details for DOI 10.1073/pnas.1216793109

  View details for Web of Science ID 000311997200020

  View details for PubMedID 23144223

  View details for PubMedCentralID PMC3511157

• Primary cilia and Gli3 activity regulate cerebral cortical size DEVELOPMENTAL NEUROBIOLOGY Wilson, S. L., Wilson, J. P., Wang, C., Wang, B., McConnell, S. K. 2012; 72 (9): 1196-1212

  Abstract

  During neural development patterning, neurogenesis, and overall growth are highly regulated and coordinated between different brain regions. Here, we show that primary cilia and the regulation of Gli activity are necessary for the normal expansion of the cerebral cortex. We show that loss of Kif3a, an important functional component of primary cilia, leads to the degeneration of primary cilia, marked overgrowth of the cortex, and altered cell cycle kinetics within cortical progenitors. The G1 phase of the cell cycle is shortened through a mechanism likely involving reduced Gli3 activity and a resulting increase in expression of cyclin D1 and Fgf15. The defects in Gli3 activity alone are sufficient to accelerate cell cycle kinetics and cause the molecular changes seen in brains that lack cilia. Finally, we show that levels of full-length and repressor Gli3 proteins are tightly regulated during normal development and correlate with changes in expression of two known Shh-target genes, CyclinD1 and Fgf15, and with the normal lengthening of the cell cycle during corticogenesis. These data suggest that Gli3 activity is regulated through the primary cilium to control cell cycle length in the cortex and thus determine cortical size.

  View details for DOI 10.1002/dneu.20985

  View details for Web of Science ID 000307166400002

  View details for PubMedID 21976438

  View details for PubMedCentralID PMC3350755

• Sonic Hedgehog Expression in Corticofugal Projection Neurons Directs Cortical Microcircuit Formation NEURON Harwell, C. C., Parker, P. R., Gee, S. M., Okada, A., McConnell, S. K., Kreitzer, A. C., Kriegstein, A. R. 2012; 73 (6): 1116-1126

  Abstract

  The precise connectivity of inputs and outputs is critical for cerebral cortex function; however, the cellular mechanisms that establish these connections are poorly understood. Here, we show that the secreted molecule Sonic Hedgehog (Shh) is involved in synapse formation of a specific cortical circuit. Shh is expressed in layer V corticofugal projection neurons and the Shh receptor, Brother of CDO (Boc), is expressed in local and callosal projection neurons of layer II/III that synapse onto the subcortical projection neurons. Layer V neurons of mice lacking functional Shh exhibit decreased synapses. Conversely, the loss of functional Boc leads to a reduction in the strength of synaptic connections onto layer Vb, but not layer II/III, pyramidal neurons. These results demonstrate that Shh is expressed in postsynaptic target cells while Boc is expressed in a complementary population of presynaptic input neurons, and they function to guide the formation of cortical microcircuitry. VIDEO ABSTRACT:

  View details for DOI 10.1016/j.neuron.2012.02.009

  View details for Web of Science ID 000301998700010

  View details for PubMedID 22445340

  View details for PubMedCentralID PMC3551478

• Fezf1 and Fezf2 Are Required for Olfactory Development and Sensory Neuron Identity JOURNAL OF COMPARATIVE NEUROLOGY Eckler, M. J., McKenna, W. L., Taghvaei, S., McConnell, S. K., Chen, B. 2011; 519 (10): 1829-1846

  Abstract

  The murine olfactory system consists of main and accessory systems that perform distinct and overlapping functions. The main olfactory epithelium (MOE) is primarily involved in the detection of volatile odorants, while neurons in the vomeronasal organ (VNO), part of the accessory olfactory system, are important for pheromone detection. During development, the MOE and VNO both originate from the olfactory pit; however, the mechanisms regulating development of these anatomically distinct organs from a common olfactory primordium are unknown. Here we report that two closely related zinc-finger transcription factors, FEZF1 and FEZF2, regulate the identity of MOE sensory neurons and are essential for the survival of VNO neurons respectively. Fezf1 is predominantly expressed in the MOE while Fezf2 expression is restricted to the VNO. In Fezf1-deficient mice, olfactory neurons fail to mature and also express markers of functional VNO neurons. In Fezf2-deficient mice, VNO neurons degenerate prior to birth. These results identify Fezf1 and Fezf2 as important regulators of olfactory system development and sensory neuron identity.

  View details for DOI 10.1002/cne.22596

  View details for Web of Science ID 000291111200001

  View details for PubMedID 21452247

  View details for PubMedCentralID PMC3268373

• Endocytosis Regulates Cell Soma Translocation and the Distribution of Adhesion Proteins in Migrating Neurons PLOS ONE Shieh, J. C., Schaar, B. T., Srinivasan, K., Brodsky, F. M., McConnell, S. K. 2011; 6 (3)

  Abstract

  Newborn neurons migrate from their birthplace to their final location to form a properly functioning nervous system. During these movements, young neurons must attach and subsequently detach from their substrate to facilitate migration, but little is known about the mechanisms cells use to release their attachments. We show that the machinery for clathrin-mediated endocytosis is positioned to regulate the distribution of adhesion proteins in a subcellular region just proximal to the neuronal cell body. Inhibiting clathrin or dynamin function impedes the movement of migrating neurons both in vitro and in vivo. Inhibiting dynamin function in vitro shifts the distribution of adhesion proteins to the rear of the cell. These results suggest that endocytosis may play a critical role in regulating substrate detachment to enable cell body translocation in migrating neurons.

  View details for DOI 10.1371/journal.pone.0017802

  View details for Web of Science ID 000288809100009

  View details for PubMedID 21445347

  View details for PubMedCentralID PMC3062553

• Characterization of axon guidance cue sensitivity of human embryonic stem cell-derived dopaminergic neurons MOLECULAR AND CELLULAR NEUROSCIENCE Cord, B. J., Li, J., Works, M., McConnell, S. K., Palmer, T., Hynes, M. A. 2010; 45 (4): 324-334

  Abstract

  Dopaminergic neurons derived from human embryonic stem cells will be useful in future transplantation studies of Parkinson's disease patients. As newly generated neurons must integrate and reconnect with host cells, the ability of hESC-derived neurons to respond to axon guidance cues will be critical. Both Netrin-1 and Slit-2 guide rodent embryonic dopaminergic (DA) neurons in vitro and in vivo, but very little is known about the response of hESC-derived DA neurons to any axonal guidance cues. Here we examined the ability of Netrin-1 and Slit-2 to affect human ESC DA axons in vitro. hESC DA neurons mature over time in culture with the developmental profile of DA neurons in vivo, including expression of the DA neuron markers FoxA2, En-1 and Nurr-1, and receptors for both Netrin and Slit. hESC DA neurons respond to exogenous Netrin-1 and Slit-2, showing an increased responsiveness to Netrin-1 as the neurons mature in culture. These responses were maintained in the presence of pro-inflammatory cytokines that might be encountered in the diseased brain. These studies are the first to evaluate and confirm that suitably matured human ES-derived DA neurons can respond appropriately to axon guidance cues.

  View details for DOI 10.1016/j.mcn.2010.07.004

  View details for Web of Science ID 000283970100002

  View details for PubMedID 20637284

• The Rho GTPase Rac1 is Required for Proliferation and Survival of Progenitors in the Developing Forebrain DEVELOPMENTAL NEUROBIOLOGY Leone, D. P., Srinivasan, K., Brakebusch, C., McConnell, S. K. 2010; 70 (9): 659-678

  Abstract

  Progenitor cells in the ventricular zone (VZ) and subventricular zone (SVZ) of the developing forebrain give rise to neurons and glial cells, and are characterized by distinct morphologies and proliferative behaviors. The mechanisms that distinguish VZ and SVZ progenitors are not well understood, although the homeodomain transcription factor Cux2 and Cyclin D2, a core component of the cell cycle machinery, are specifically involved in controlling SVZ cell proliferation. Rho GTPases have been implicated in regulating the proliferation, differentiation, and migration of many cell types, and one family member, Cdc42, affects the polarity and proliferation of radial glial cells in the VZ. Here, we show that another family member, Rac1, is required for the normal proliferation and differentiation of SVZ progenitors and for survival of both VZ and SVZ progenitors. A forebrain-specific loss of Rac1 leads to an SVZ-specific reduction in proliferation, a concomitant increase in cell cycle exit, and premature differentiation. In Rac1 mutants, the SVZ and VZ can no longer be delineated, but rather fuse to become a single compact zone of intermingled cells. Cyclin D2 expression, which is normally expressed by both VZ and SVZ progenitors, is reduced in Rac1 mutants, suggesting that the mutant cells differentiate precociously. Rac1-deficient mice can still generate SVZ-derived upper layer neurons, indicating that Rac1 is not required for the acquisition of upper layer neuronal fates, but instead is needed for the normal regulation of proliferation by progenitor cells in the SVZ.

  View details for DOI 10.1002/dneu.20804

  View details for Web of Science ID 000280018700004

  View details for PubMedID 20506362

  View details for PubMedCentralID PMC2929936

• A central role for the small GTPase Rac1 in hippocampal plasticity and spatial learning and memory MOLECULAR AND CELLULAR NEUROSCIENCE Haditsch, U., Leone, D. P., Farinelli, M., Chrostek-Grashoff, A., Brakebusch, C., Mansuy, I. M., McConnell, S. K., Palmer, T. D. 2009; 41 (4): 409-419

  Abstract

  Rac1 is a member of the Rho family of small GTPases that are important for structural aspects of the mature neuronal synapse including basal spine density and shape, activity-dependent spine enlargement, and AMPA receptor clustering in vitro. Here we demonstrate that selective elimination of Rac1 in excitatory neurons in the forebrain in vivo not only affects spine structure, but also impairs synaptic plasticity in the hippocampus with consequent defects in hippocampus-dependent spatial learning. Furthermore, Rac1 mutants display deficits in working/episodic-like memory in the delayed matching-to-place (DMP) task suggesting that Rac1 is a central regulator of rapid encoding of novel spatial information in vivo.

  View details for DOI 10.1016/j.mcn.2009.04.005

  View details for Web of Science ID 000267686500003

  View details for PubMedID 19394428

  View details for PubMedCentralID PMC2705331

• The Fezf2-Ctip2 genetic pathway regulates the fate choice of subcortical projection neurons in the developing cerebral cortex PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Chen, B., Wang, S. S., Hattox, A. M., Rayburn, H., Nelson, S. B., McConnell, S. K. 2008; 105 (32): 11382-11387

  Abstract

  Pyramidal neurons in the deep layers of the cerebral cortex can be classified into two major classes: callosal projection neurons and long-range subcortical neurons. We and others have shown that a gene expressed specifically by subcortical projection neurons, Fezf2, is required for the formation of axonal projections to the spinal cord, tectum, and pons. Here, we report that Fezf2 regulates a decision between subcortical vs. callosal projection neuron fates. Fezf2(-/-) neurons adopt the fate of callosal projection neurons as assessed by their axonal projections, electrophysiological properties, and acquisition of Satb2 expression. Ctip2 is a major downstream effector of Fezf2 in regulating the extension of axons toward subcortical targets and can rescue the axonal phenotype of Fezf2 mutants. When ectopically expressed, either Fezf2 or Ctip2 can alter the axonal targeting of corticocortical projection neurons and cause them to project to subcortical targets, although Fezf2 can promote a subcortical projection neuron fate in the absence of Ctip2 expression.

  View details for DOI 10.1073/pnas.0804918105

  View details for Web of Science ID 000258560700063

  View details for PubMedID 18678899

  View details for PubMedCentralID PMC2495013

• MALS-3 regulates polarity and early neurogenesis in the developing cerebral cortex DEVELOPMENT Srinivasan, K., Roosa, J., Olsen, O., Lee, S., Bredt, D. S., McConnell, S. K. 2008; 135 (10): 1781-1790

  Abstract

  Apicobasal polarity plays an important role in regulating asymmetric cell divisions by neural progenitor cells (NPCs) in invertebrates, but the role of polarity in mammalian NPCs is poorly understood. Here, we characterize the function of the PDZ domain protein MALS-3 in the developing cerebral cortex. We find that MALS-3 is localized to the apical domain of NPCs. Mice lacking all three MALS genes fail to localize the polarity proteins PATJ and PALS1 apically in NPCs, whereas the formation and maintenance of adherens junctions appears normal. In the absence of MALS proteins, early NPCs progressed more slowly through the cell cycle, and their daughter cells were more likely to exit the cell cycle and differentiate into neurons. Interestingly, these effects were transient; NPCs recovered normal cell cycle properties during late neurogenesis. Experiments in which MALS-3 was targeted to the entire membrane resulted in a breakdown of apicobasal polarity, loss of adherens junctions, and a slowing of the cell cycle. Our results suggest that MALS-3 plays a role in maintaining apicobasal polarity and is required for normal neurogenesis in the developing cortex.

  View details for DOI 10.1242/dev.013847

  View details for Web of Science ID 000255337900006

  View details for PubMedID 18403412

  View details for PubMedCentralID PMC3016226

• Satb2 regulates callosal projection neuron identity in the developing cerebral cortex NEURON Alcamo, E. A., Chirivella, L., Dautzenberg, M., Dobreva, G., Farinas, I., Grosschedl, R., McConnell, S. K. 2008; 57 (3): 364-377

  Abstract

  Satb2 is a DNA-binding protein that regulates chromatin organization and gene expression. In the developing brain, Satb2 is expressed in cortical neurons that extend axons across the corpus callosum. To assess the role of Satb2 in neurons, we analyzed mice in which the Satb2 locus was disrupted by insertion of a LacZ gene. In mutant mice, beta-galactosidase-labeled axons are absent from the corpus callosum and instead descend along the corticospinal tract. Satb2 mutant neurons acquire expression of Ctip2, a transcription factor that is necessary and sufficient for the extension of subcortical projections by cortical neurons. Conversely, ectopic expression of Satb2 in neural stem cells markedly decreases Ctip2 expression. Finally, we find that Satb2 binds directly to regulatory regions of Ctip2 and induces changes in chromatin structure. These data suggest that Satb2 functions as a repressor of Ctip2 and regulatory determinant of corticocortical connections in the developing cerebral cortex.

  View details for DOI 10.1016/j.neuron.2007.12.012

  View details for Web of Science ID 000253075300007

  View details for PubMedID 18255030

• The determination of projection neuron identity in the developing cerebral cortex CURRENT OPINION IN NEUROBIOLOGY Leone, D. P., Srinivasan, K., Chen, B., Alcamo, E., McConnell, S. K. 2008; 18 (1): 28-35

  Abstract

  Here we review the mechanisms that determine projection neuron identity during cortical development. Pyramidal neurons in the mammalian cerebral cortex can be classified into two major classes: corticocortical projection neurons, which are concentrated in the upper layers of the cortex, and subcortical projection neurons, which are found in the deep layers. Early progenitor cells in the ventricular zone produce deep layer neurons that express transcription factors including Sox5, Fezf2, and Ctip2, which play important roles in the specification of subcortically projecting axons. Upper layer neurons are produced from progenitors in the subventricular zone, and the expression of Satb2 in these differentiating neurons is required for the formation of axonal projections that connect the two cerebral hemispheres. The Fezf2/Ctip2 and Satb2 pathways appear to be mutually repressive, thus ensuring that individual neurons adopt either a subcortical or callosal projection neuron identity at early times during development. The molecular mechanisms by which Satb2 regulates gene expression involves long-term epigenetic changes in chromatin configuration, which may enable cell fate decisions to be maintained during development.

  View details for DOI 10.1016/j.conb.2008.05.006

  View details for Web of Science ID 000257621700005

  View details for PubMedID 18508260

  View details for PubMedCentralID PMC2483251

• Ongoing sonic hedgehog signaling is required for dorsal midline formation in the developing forebrain DEVELOPMENTAL NEUROBIOLOGY Hayhurst, M., Gore, B. B., Tessier-Lavigne, M., McConnell, S. K. 2008; 68 (1): 83-100

  Abstract

  The division of the mammalian forebrain into distinct left and right hemispheres represents a critical step in neural development. Several signaling molecules including sonic hedgehog (SHH), fibroblast growth factor 8 (FGF8), and bone morphogenetic proteins (BMPs) have been implicated in dorsal midline development, and prior work suggests that the organizing centers from which these proteins are secreted mutually regulate one another during development. To explore the role of the ventral organizing center in the formation of two hemispheres, we assessed dorsal midline development in Shh mutant embryos and in wildtype embryos treated with the SHH signaling inhibitor HhAntag. Collectively, our findings demonstrate that SHH signaling plays an important role in maintaining the normal expression patterns of Fgf8 and Bmp4 in the developing forebrain. We further show that FGF8 can induce the expression of Zic2, which is normally expressed at the midline and is required in vivo for hemispheric cleavage, suggesting that FGF signaling may stimulate dorsal midline development by inducing Zic2 expression.

  View details for DOI 10.1002/dneu.20576

  View details for Web of Science ID 000252349300007

  View details for PubMedID 17948241

• Mutations in the BMP pathway in mice support the existence of two molecular classes of holoprosencephaly DEVELOPMENT Fernandes, M., Gutin, G., Alcorn, H., McConnell, S. K., Hebert, J. M. 2007; 134 (21): 3789-3794

  Abstract

  Holoprosencephaly (HPE) is a devastating forebrain abnormality with a range of morphological defects characterized by loss of midline tissue. In the telencephalon, the embryonic precursor of the cerebral hemispheres, specialized cell types form a midline that separates the hemispheres. In the present study, deletion of the BMP receptor genes, Bmpr1b and Bmpr1a, in the mouse telencephalon results in a loss of all dorsal midline cell types without affecting the specification of cortical and ventral precursors. In the holoprosencephalic Shh(-/-) mutant, by contrast, ventral patterning is disrupted, whereas the dorsal midline initially forms. This suggests that two separate developmental mechanisms can underlie the ontogeny of HPE. The Bmpr1a;Bmpr1b mutant provides a model for a subclass of HPE in humans: midline inter-hemispheric HPE.

  View details for DOI 10.1242/dev.004325

  View details for Web of Science ID 000250097900003

  View details for PubMedID 17913790

• Boc is a receptor for sonic hedgehog in the guidance of commissural axons NATURE Okada, A., Charron, F., Morin, S., Shin, D. S., Wong, K., Fabre, P. J., Tessier-Lavigne, M., McConnell, S. K. 2006; 444 (7117): 369-373

  Abstract

  In the spinal cord, sonic hedgehog (Shh) is secreted by the floor plate to control the generation of distinct classes of ventral neurons along the dorsoventral axis. Genetic and in vitro studies have shown that Shh also later acts as a midline-derived chemoattractant for commissural axons. However, the receptor(s) responsible for Shh attraction remain unknown. Here we show that two Robo-related proteins, Boc and Cdon, bind specifically to Shh and are therefore candidate receptors for the action of Shh as an axon guidance ligand. Boc is expressed by commissural neurons, and targeted disruption of Boc in mouse results in the misguidance of commissural axons towards the floor plate. RNA-interference-mediated knockdown of Boc impairs the ability of rat commissural axons to turn towards an ectopic source of Shh in vitro. Taken together, these data suggest that Boc is essential as a receptor for Shh in commissural axon guidance.

  View details for DOI 10.1038/nature05246

  View details for Web of Science ID 000242018300049

  View details for PubMedID 17086203

• FGF signalling generates ventral telencephalic cells independently of SHH DEVELOPMENT Gutin, G., Fernandes, M., Palazzolo, L., Paek, H., Yu, K., Ornitz, D. M., McConnell, S. K., Hebert, J. M. 2006; 133 (15): 2937-2946

  Abstract

  Sonic hedgehog (SHH) is required to generate ventral cell types throughout the central nervous system. Its role in directly specifying ventral cells, however, has recently been questioned because loss of the Shh gene has little effect on ventral development if the Gli3 gene is also mutant. Consequently, another ventral determinant must exist. Here, genetic evidence establishes that FGFs are required for ventral telencephalon development. First, simultaneous deletion of Fgfr1 and Fgfr3 specifically in the telencephalon results in the loss of differentiated ventromedial cells; and second, in the Fgfr1;Fgfr2 double mutant, ventral precursor cells are lost, mimicking the phenotype obtained previously with a loss of SHH signalling. Yet, in the Fgfr1;Fgfr2 mutant, Shh remains expressed, as does Gli1, the transcription of which depends on SHH activity, suggesting that FGF signalling acts independently of SHH to generate ventral precursors. Moreover, the Fgfr1;Fgfr2 phenotype, unlike the Shh phenotype, is not rescued by loss of Gli3, further indicating that FGFs act downstream of Shh and Gli3 to generate ventral telencephalic cell types.

  View details for DOI 10.1242/dev.02465

  View details for Web of Science ID 000239758500016

  View details for PubMedID 16818446

• Dose-dependent functions of Fgf8 in regulating telencephalic patterning centers DEVELOPMENT Storm, E. E., Garel, S., Borello, U., Hebert, J. M., Martinez, S., McConnell, S. K., Martin, G. R., Rubenstein, J. L. 2006; 133 (9): 1831-1844

  Abstract

  Mouse embryos bearing hypomorphic and conditional null Fgf8 mutations have small and abnormally patterned telencephalons. We provide evidence that the hypoplasia results from decreased Foxg1 expression, reduced cell proliferation and increased cell death. In addition, alterations in the expression of Bmp4, Wnt8b, Nkx2.1 and Shh are associated with abnormal development of dorsal and ventral structures. Furthermore, nonlinear effects of Fgf8 gene dose on the expression of a subset of genes, including Bmp4 and Msx1, correlate with a holoprosencephaly phenotype and with the nonlinear expression of transcription factors that regulate neocortical patterning. These data suggest that Fgf8 functions to coordinate multiple patterning centers, and that modifications in the relative strength of FGF signaling can have profound effects on the relative size and nature of telencephalic subdivisions.

  View details for DOI 10.1242/dev.02324

  View details for Web of Science ID 000236807200021

  View details for PubMedID 16613831

• Visualization of embryonic neural stem cells using Hes promoters in transgenic mice MOLECULAR AND CELLULAR NEUROSCIENCE Ohtsuka, T., Imayoshi, I., Shimojo, H., Nishi, E., Kageyama, R., McConnell, S. K. 2006; 31 (1): 109-122

  Abstract

  In the central nervous system, neural stem cells proliferate in the ventricular zone (VZ) and sequentially give rise to both neurons and glial cells in a temporally and spatially regulated manner, suggesting that stem cells may differ from one another in different brain regions and at different developmental stages. For the purpose of marking and purifying neural stem cells to ascertain whether such differences exist, we generated transgenic mice using promoters from Hes genes (pHes1 or pHes5) to drive expression of destabilized enhanced green fluorescent protein. In the developing brains of these transgenic mice, GFP expression was restricted to undifferentiated cells in the VZ, which could asymmetrically produce a Numb-positive neuronal daughter and a GFP-positive progenitor cell in clonal culture, indicating that they retain the capacity to self-renew. Our results suggest that pHes-EGFP transgenic mice can be used to explore similarities and differences among neural stem cells during development.

  View details for DOI 10.1016/j.mcn.2005.09.006

  View details for Web of Science ID 000235045600009

  View details for PubMedID 16214363

• Fgf8 expression defines a morphogenetic center required for olfactory neurogenesis and nasal cavity development in the mouse DEVELOPMENT Kawauchi, S., Shou, J. Y., Santos, R., Hebert, J. M., McConnell, S. K., Mason, I., Calof, A. L. 2005; 132 (23): 5211-5223

  Abstract

  In vertebrate olfactory epithelium (OE), neurogenesis proceeds continuously, suggesting that endogenous signals support survival and proliferation of stem and progenitor cells. We used a genetic approach to test the hypothesis that Fgf8 plays such a role in developing OE. In young embryos, Fgf8 RNA is expressed in the rim of the invaginating nasal pit (NP), in a small domain of cells that overlaps partially with that of putative OE neural stem cells later in gestation. In mutant mice in which the Fgf8 gene is inactivated in anterior neural structures, FGF-mediated signaling is strongly downregulated in both OE proper and underlying mesenchyme by day 10 of gestation. Mutants survive gestation but die at birth, lacking OE, vomeronasal organ (VNO), nasal cavity, forebrain, lower jaw, eyelids and pinnae. Analysis of mutants indicates that although initial NP formation is grossly normal, cells in the Fgf8-expressing domain undergo high levels of apoptosis, resulting in cessation of nasal cavity invagination and loss of virtually all OE neuronal cell types. These findings demonstrate that Fgf8 is crucial for proper development of the OE, nasal cavity and VNO, as well as maintenance of OE neurogenesis during prenatal development. The data suggest a model in which Fgf8 expression defines an anterior morphogenetic center, which is required not only for the sustenance and continued production of primary olfactory (OE and VNO) neural stem and progenitor cells, but also for proper morphogenesis of the entire nasal cavity.

  View details for DOI 10.1242/dev.02143

  View details for Web of Science ID 000234409000007

  View details for PubMedID 16267092

• The genetics of cerebral cortex development in the mouse McConnell, S. K. NATURE PUBLISHING GROUP. 2005: S9

  View details for Web of Science ID 000233442100026

• Fezl regulates the differentiation and axon targeting of layer 5 subcortical projection neurons in cerebral cortex PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Chen, B., Schaevitz, L. R., McConnell, S. K. 2005; 102 (47): 17184-17189

  Abstract

  During the development of the cerebral cortex, progenitor cells produce neurons that migrate to laminar positions appropriate for their birth dates, adopt specific neuronal identities, and form appropriate local and long-distance axonal connections. Here, we report that forebrain embryonic zinc-finger-like protein (Fezl), a putative zinc-finger transcriptional repressor, is required for the differentiation of projection neurons in cortical layer 5. In Fezl-deficient mice, these neurons display molecular, morphological, and axonal targeting defects. The corticospinal tract was absent in Fezl(-/-) mice, corticotectal and pontine projections were severely reduced, and Fezl-expressing neurons formed aberrant axonal projections. The expression of many molecular markers for deep-layer neurons was reduced or absent in the Fezl(-/-) cerebral cortex. Most strikingly, Ctip2, a transcription factor required for the formation of the corticospinal tract, was not expressed in the Fezl-deficient cortex. These results suggest that Fezl regulates the differentiation of layer 5 subcortical projection neurons.

  View details for DOI 10.1073/pnas.0508732102

  View details for Web of Science ID 000233463200050

  View details for PubMedID 16284245

  View details for PubMedCentralID PMC1282569

• Cytoskeletal coordination during neuroinal migration PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Schaar, B. T., McConnell, S. K. 2005; 102 (38): 13652-13657

  Abstract

  Discoveries from human and mouse genetics have identified cytoskeletal and signaling proteins that are essential for neuronal migration in the developing brain. To provide a meaningful context for these studies, we took an unbiased approach of correlative electron microscopy of neurons migrating through a three-dimensional matrix, and characterized the cytoskeletal events that occur as migrating neurons initiate saltatory forward movements of the cell nucleus. The formation of a cytoplasmic dilation in the proximal leading process precedes nuclear translocation. Cell nuclei translocate into these dilations in saltatory movements. Time-lapse imaging and pharmacological perturbation suggest that nucleokinesis requires stepwise or hierarchical interactions between microtubules, myosin II, and cell adhesion. We hypothesize that these interactions couple leading process extension to nuclear translocation during neuronal migration.

  View details for DOI 10.1073/pnas.0506008102

  View details for Web of Science ID 000232115100054

  View details for PubMedID 16174753

  View details for PubMedCentralID PMC1199551

• Gene targeting using a promoterless gene trap vector ("targeted trapping") is an efficient method to mutate a large fraction of genes PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Friedel, R. H., Plump, A., Lu, X. W., Spilker, K., Jolicoeur, C., Wong, K., Venkatesh, T. R., Yaron, A., Hynes, M., Chen, B., Okada, A., McConnell, S. K., Rayburn, H., Tessier-Lavigne, M. 2005; 102 (37): 13188-13193

  Abstract

  A powerful tool for postgenomic analysis of mammalian gene function is gene targeting in mouse ES cells. We report that homologous recombination using a promoterless gene trap vector ("targeting trapping") yields targeting frequencies averaging above 50%, a significant increase compared with current approaches. These high frequencies appear to be due to the stringency of selection with promoterless constructs, because most random insertions are silent and eliminated by drug selection. The promoterless design requires that the targeted gene be expressed in ES cells at levels exceeding a certain threshold (which we estimate to be approximately 1% of the transferrin receptor gene expression level, for the secretory trap vector used here). Analysis of 127 genes that had been trapped by random (nontargeted) gene trapping with the same vector shows that virtually all are expressed in ES cells above this threshold, suggesting that targeted and random trapping share similar requirements for expression levels. In a random sampling of 130 genes encoding secretory proteins, about half were expressed above threshold, suggesting that about half of all secretory genes are accessible by either targeted or random gene trapping. The simplicity and high efficiency of the method facilitate systematic targeting of a large fraction of the genome by individual investigators and large-scale consortia alike.

  View details for DOI 10.1073/pnas.0505474102

  View details for Web of Science ID 000231916300034

  View details for PubMedID 16129827

  View details for PubMedCentralID PMC1193537

• Genomic characterisation of a Fgf-regulated gradient-based neocortical protomap DEVELOPMENT Sansom, S. N., Hebert, J. M., Thammongkol, U., Smith, J., Nisbet, G., Surani, M. A., McConnell, S. K., Livesey, F. J. 2005; 132 (17): 3947-3961

  Abstract

  Recent findings support a model for neocortical area formation in which neocortical progenitor cells become patterned by extracellular signals to generate a protomap of progenitor cell areas that in turn generate area-specific neurons. The protomap is thought to be underpinned by spatial differences in progenitor cell identity that are reflected at the transcriptional level. We systematically investigated the nature and composition of the protomap by genomic analyses of spatial and temporal neocortical progenitor cell gene expression. We did not find gene expression evidence for progenitor cell organisation into domains or compartments, instead finding rostrocaudal gradients of gene expression across the entire neocortex. Given the role of Fgf signalling in rostrocaudal neocortical patterning, we carried out an in vivo global analysis of cortical gene expression in Fgfr1 mutant mice, identifying consistent alterations in the expression of candidate protomap elements. One such gene, Mest, was predicted by those studies to be a direct target of Fgf8 signalling and to be involved in setting up, rather than implementing, the progenitor cell protomap. In support of this, we confirmed Mest as a direct transcriptional target of Fgf8-regulated signalling in vitro. Functional studies demonstrated that this gene has a role in establishing patterned gene expression in the developing neocortex, potentially by acting as a negative regulator of the Fgf8-controlled patterning system.

  View details for DOI 10.1242/dev.01968

  View details for Web of Science ID 000232430900014

  View details for PubMedID 16079153

• Doublecortin microtubule affinity is regulated by a balance of kinase and phosphatase activity at the leading edge of migrating neurons NEURON Schaar, B. T., Kinoshita, K., McConnell, S. K. 2004; 41 (2): 203-213

  Abstract

  Doublecortin (Dcx) is a microtubule-associated protein that is mutated in X-linked lissencephaly (X-LIS), a neuronal migration disorder associated with epilepsy and mental retardation. Although Dcx can bind ubiquitously to microtubules in nonneuronal cells, Dcx is highly enriched in the leading processes of migrating neurons and the growth cone region of differentiating neurons. We present evidence that Dcx/microtubule interactions are negatively controlled by Protein Kinase A (PKA) and the MARK/PAR-1 family of protein kinases. In addition to a consensus MARK site, we identified a serine within a novel sequence that is crucial for the PKA- and MARK-dependent regulation of Dcx's microtubule binding activity in vitro. This serine is mutated in two families affected by X-LIS. Immunostaining neurons with an antibody that recognizes phosphorylated substrates of MARK supports the conclusion that Dcx localization and function are regulated at the leading edge of migrating cells by a balance of kinase and phosphatase activity.

  View details for Web of Science ID 000221457800007

  View details for PubMedID 14741102

• BMP ligands act redundantly to pattern the dorsal telencephalic midline GENESIS Hebert, J. M., Hayhurst, M., Marks, M. E., Kulessa, H., HOGAN, B. L., McConnell, S. K. 2003; 35 (4): 214-219

  Abstract

  The embryonic telencephalon is patterned into several areas that give rise to functionally distinct structures in the adult forebrain. Previous studies have shown that BMP4 and BMP2 can induce features characteristic of the telencephalic midline in cultured explants, suggesting that the normal role of BMP4 in the forebrain is to pattern the medial lateral axis of the telencephalon by promoting midline cell fates. To test this hypothesis directly in vivo, the Bmp4 gene was efficiently disrupted in the telencephalon using a CRE/loxP approach. Analysis of Bmp4-deficient telencephalons fails to reveal a defect in patterning, cell proliferation, differentiation, or apoptosis. The absence of a phenotype in the Bmp4-deficient telencephalon along with recent genetic studies establishing a role for a BMP4 receptor, BMPRIA, in telencephalic midline development, demonstrate that loss of Bmp4 function in the telencephalon can be compensated for by at least one other Bmp gene, the identity of which has not yet been determined.

  View details for DOI 10.1002/gene.10183

  View details for Web of Science ID 000182589400003

  View details for PubMedID 12717732

• Mouse models of holoprosencephaly CURRENT OPINION IN NEUROLOGY Hayhurst, M., McConnell, S. K. 2003; 16 (2): 135-141

  Abstract

  Holoprosencephaly (HPE) is the most common anomaly of forebrain development in humans. The pathogenesis of HPE results in a failure of the brain hemispheres to separate during early development. Here we review experimental models of HPE in which some of the genes known to cause HPE in humans have been disrupted in the mouse.To date, mutations that cause HPE have been identified in seven genes. Three of these genes encode members of the Sonic hedgehog (SHH) signaling pathway, which regulates the development of ventral structures throughout the neuraxis. Two other HPE mutations affect signaling by Nodal ligands, which also play important roles in neural patterning. The roles of the two other known HPE genes are not yet clear. Analysis of genetically altered mice has revealed that mutations in other members of the SHH and Nodal signaling pathways also result in HPE phenotypes.Studies of HPE in the mouse have provided a framework for understanding key developmental events in human brain development and may provide new candidate genes for human HPE. Despite this progress, fundamental mysteries remain about how molecules that pattern ventral brain regions ultimately disrupt the formation of the cerebral hemispheres in dorsal regions.

  View details for DOI 10.1097/01.wco.0000063761.15877.40

  View details for Web of Science ID 000182542200003

  View details for PubMedID 12644739

• FGF signaling through FGFR1 is required for olfactory bulb morphogenesis DEVELOPMENT Hebert, J. M., Lin, M., Partanen, J., Rossant, J., McConnell, S. K. 2003; 130 (6): 1101-1111

  Abstract

  During development, the embryonic telencephalon is patterned into different areas that give rise to distinct adult brain structures. Several secreted signaling molecules are expressed at putative signaling centers in the early telencephalon. In particular, Fgf8 is expressed at the anterior end of the telencephalon and is hypothesized to pattern it along the anteroposterior (AP) axis. Using a CRE/loxP genetic approach to disrupt genes in the telencephalon, we address the role of FGF signaling directly in vivo by abolishing expression of the FGF receptor Fgfr1. In the Fgfr1-deficient telencephalon, AP patterning is largely normal. However, morphological defects are observed at the anterior end of the telencephalon. Most notably, the olfactory bulbs do not form normally. Examination of the proliferation state of anterior telencephalic cells supports a model for olfactory bulb formation in which an FGF-dependent decrease in proliferation is required for initial bulb evagination. Together the results demonstrate an essential role for Fgfr1 in patterning and morphogenesis of the telencephalon.

  View details for DOI 10.1242/dev.00334

  View details for Web of Science ID 000181751300007

  View details for PubMedID 12571102

• Neurotrophin-3 is required for appropriate establishment of thalamocortical connections NEURON Ma, L., Harada, T., Harada, C., Romero, M., Hebert, J. M., McConnell, S. K., Parada, L. F. 2002; 36 (4): 623-634

  Abstract

  In the vertebrate brain, the thalamus serves as a relay and integration station for diverse neuronal information en route from the periphery to the cortex. Formation of the thalamocortical tract occurs during pre- and postnatal development, with distinct thalamic nuclei projecting to specific cortical regions. The molecular forces that underlie the invasion by axons into specific cortical layers followed by activity-dependent maturation of synapses are poorly understood. We show that genetic ablation of neurotrophin-3 (NT-3) in the mouse neocortex results in reduction of a set of anatomically distinct axonal bundles projecting from thalamus through cortical white matter. These bundles include thalamocortical axons that normally establish connections with retrosplenial and visual cortex, sites of early postnatal NT-3 expression. These results implicate neurotrophins in the critical stage of precise thalamocortical connections.

  View details for Web of Science ID 000179313100010

  View details for PubMedID 12441052

• BMP signaling is required locally to pattern the dorsal telencephalic midline NEURON Hebert, J. M., Mishina, Y., McConnell, S. K. 2002; 35 (6): 1029-1041

  Abstract

  BMPs have been proposed to pattern the medial-lateral axis of the telencephalon in a concentration-dependent manner, thus helping to subdivide the embryonic telencephalon into distinct forebrain regions. Using a CRE/loxP genetic approach, we tested this hypothesis by disrupting the Bmpr1a gene in the telencephalon. In mutants, BMP signaling was compromised throughout the dorsal telencephalon, but only the most dorsalmedial derivative, the choroid plexus, failed to be specified or differentiate. Choroid plexus precursors remained proliferative and did not adopt the fate of their lateral telencephalic neighbors. These results demonstrate that BMP signaling is required for the formation of the most dorsal telencephalic derivative, the choroid plexus, and that BMP signaling plays an essential role in locally patterning the dorsal midline. Our data fail to support a more global, concentration-dependent role in specifying telencephalic cell fates.

  View details for Web of Science ID 000178040000005

  View details for PubMedID 12354394

• FGFR1 is required for the development of the auditory sensory epithelium NEURON Pirvola, U., Ylikoski, J., Trokovic, R., Hebert, J. M., McConnell, S. K., Partanen, J. 2002; 35 (4): 671-680

  Abstract

  The mammalian auditory sensory epithelium, the organ of Corti, comprises the hair cells and supporting cells that are pivotal for hearing function. The origin and development of their precursors are poorly understood. Here we show that loss-of-function mutations in mouse fibroblast growth factor receptor 1 (Fgfr1) cause a dose-dependent disruption of the organ of Corti. Full inactivation of Fgfr1 in the inner ear epithelium by Foxg1-Cre-mediated deletion leads to an 85% reduction in the number of auditory hair cells. The primary cause appears to be reduced precursor cell proliferation in the early cochlear duct. Thus, during development, FGFR1 is required for the generation of the precursor pool, which gives rise to the auditory sensory epithelium. Our data also suggest that FGFR1 might have a distinct later role in intercellular signaling within the differentiating auditory sensory epithelium.

  View details for Web of Science ID 000177521000008

  View details for PubMedID 12194867

• Distinct origins of neocortical projection neurons and interneurons in vivo CEREBRAL CORTEX Anderson, S. A., Kaznowski, C. E., Horn, C., Rubenstein, J. L., McConnell, S. K. 2002; 12 (7): 702-709

  Abstract

  Recent studies in rodents have suggested that some cortical GABAergic interneurons arise within the neuroepithelium of the subcortical telencephalon then migrate dorsally into the cerebral cortex. These studies have relied heavily on short-term organotypic culture methods and on the analysis of mutant mice that die during the neonatal period. The purpose of this study is to ascertain directly whether cells labeled in the subcortical telencephalon in vivo differentiate into mature cortical interneurons and whether any cortical interneurons arise from the dorsal, cortical neuroepithelium. Mitotic cells within the neonatal cortex or subcortical telencephalon were labeled by focal injections of [(3)H]thymidine into the brains of neonatal ferrets. The fates of labeled cells were assessed in mature animals 6 weeks later. Our results suggest that many cortical interneurons, but not cortical projection neurons, derive from the subcortical telencephalon. Conversely, cortical projection neurons, but few if any interneurons, are generated within the proliferative zones of the neocortex.

  View details for Web of Science ID 000176227100003

  View details for PubMedID 12050082

• Telencephalon-specific Rb knockouts reveal enhanced neurogenesis, survival and abnormal cortical development EMBO JOURNAL Ferguson, K. L., Vanderluit, J. L., Hebert, J. M., McIntosh, W. C., Tibbo, E., MacLaurin, J. G., Park, D. S., Wallace, V. A., Vooijs, M., McConnell, S. K., Slack, R. S. 2002; 21 (13): 3337-3346

  Abstract

  Correct cell cycle regulation and terminal mitosis are critical for nervous system development. The retinoblastoma (Rb) protein is a key regulator of these processes, as Rb-/- embryos die by E15.5, exhibiting gross hematopoietic and neurological defects. The extensive apoptosis in Rb-/- embryos has been attributed to aberrant S phase entry resulting in conflicting growth control signals in differentiating cells. To assess the role of Rb in cortical development in the absence of other embryonic defects, we examined mice with telencephalon-specific Rb deletions. Animals carrying a floxed Rb allele were interbred with mice in which cre was knocked into the Foxg1 locus. Unlike germline knockouts, mice specifically deleted for Rb in the developing telencephalon survived until birth. In these mutants, Rb-/- progenitor cells divided ectopically, but were able to survive and differentiate. Mutant brains exhibited enhanced cellularity due to increased proliferation of neuroblasts. These studies demonstrate that: (i) cell cycle deregulation during differentiation does not necessitate apoptosis; (ii) Rb-deficient mutants exhibit enhanced neuroblast proliferation; and (iii) terminal mitosis may not be required to initiate differentiation.

  View details for Web of Science ID 000176784100013

  View details for PubMedID 12093735

  View details for PubMedCentralID PMC126087

• Regulated nuclear trafficking of the homeodomain protein Otx1 in cortical neurons MOLECULAR AND CELLULAR NEUROSCIENCE Zhang, Y. A., Okada, A., Lew, C. H., McConnell, S. K. 2002; 19 (3): 430-446

  Abstract

  Otx1 is a homeodomain protein required for axon refinement by layer 5 neurons in developing cerebral cortex. Otx1 localizes to the cytoplasm of progenitor cells in the rat ventricular zone, and remains cytoplasmic as neurons migrate and begin to differentiate. Nuclear translocation occurs during the first week of postnatal life, when layer 5 neurons begin pruning their long-distance axonal projections. Deletion analysis reveals that Otx1 is imported actively into cell nuclei, that the N-terminus of Otx1 is necessary for nuclear import, and that a putative nuclear localization sequence within this domain is sufficient to direct nuclear import in a variety of cell lines. In contrast, GFP-Otx1 fusion proteins that contain the N-terminus are retained in the cytoplasm of cortical progenitor cells, mimicking the distribution of Otx1 in vivo. These results suggest that ventricular cells actively sequester Otx1 in the cytoplasm, either by preventing nuclear import or by promoting a balance of export over import signals.

  View details for DOI 10.1006/mcne.2001.1076

  View details for Web of Science ID 000174845800010

  View details for PubMedID 11906214

• NudC associates with Lis1 and the dynein motor at the leading pole of neurons JOURNAL OF NEUROSCIENCE Aumais, J. P., Tunstead, J. R., McNeil, R. S., Schaar, B. T., McConnell, S. K., Lin, S. H., Clark, G. D., Yu-Lee, L. Y. 2001; 21 (24)

  Abstract

  NUDC is a highly conserved protein important for nuclear migration and viability in Aspergillus nidulans. Mammalian NudC interacts with Lis1, a neuronal migration protein important during neocorticogenesis, suggesting a conserved mechanism of nuclear movement in A. nidulans and neuronal migration in the developing mammalian brain (S. M. Morris et al., 1998). To further investigate this possibility, we show for the first time that NudC, Lis1, and cytoplasmic dynein intermediate chain (CDIC) colocalize at the microtubule organizing center (MTOC) around the nucleus in a polarized manner facing the leading pole of cerebellar granule cells with a migratory morphology. In neurons with stationary morphology, NudC is distributed throughout the soma and colocalizes with CDIC and tubulin in neurites as well as at the MTOC. At the subcellular level, NudC, CDIC, and p150 dynactin colocalize to the interphase microtubule array and the MTOC in fibroblasts. The observed colocalization is confirmed biochemically by coimmunoprecipitation of NudC with CDIC and cytoplasmic dynein heavy chain (CDHC) from mouse brain extracts. Consistent with its expression in individual neurons, a high level of NudC is detected in regions of the embryonic neocortex undergoing extensive neurogenesis as well as neuronal migration. These data suggest a biochemical and functional interaction of NudC with Lis1 and the dynein motor complex during neuronal migration in vivo.

  View details for Web of Science ID 000172654800002

  View details for PubMedID 11734602

• Doublecortin interacts with mu subunits of clathrin adaptor complexes in the developing nervous system MOLECULAR AND CELLULAR NEUROSCIENCE Friocourt, G., Chafey, P., Billuart, P., Koulakoff, A., Vine, M. C., Schaar, B. T., McConnell, S. K., Francis, F., Chelly, J. 2001; 18 (3): 307-319

  Abstract

  Doublecortin is a microtubule-associated protein required for normal corticogenesis in the developing brain. We carried out a yeast two-hybrid screen to identify interacting proteins. One of the isolated clones encodes the mu1 subunit of the adaptor complex AP-1 involved in clathrin-dependent protein sorting. We found that Doublecortin also interacts in yeast with mu2 from the AP-2 complex. Mutagenesis and pull-down experiments showed that these interactions were mediated through a tyrosine-based sorting signal (YLPL) in the C-terminal part of Doublecortin. The functional relevance of these interactions was suggested by the coimmunoprecipitation of Doublecortin with AP-1 and AP-2 from mouse brain extracts. This interaction was further supported by RNA in situ hybridization and immunofluorescence studies. Taken together these data indicate that a certain proportion of Doublecortin interacts with AP-1 and/or AP-2 in vivo and are consistent with a potential involvement of Doublecortin in protein sorting or vesicular trafficking.

  View details for Web of Science ID 000171705700006

  View details for PubMedID 11591131

• Targeted mutagenesis of Lis1 disrupts cortical development and LIS1 homodimerization PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA Cahana, A., Escamez, T., Nowakowski, R. S., Hayes, N. L., Giacobini, M., von Holst, A., Shmueli, O., Sapir, T., McConnell, S. K., Wurst, W., Martinez, S., Reiner, O. 2001; 98 (11): 6429-6434

  Abstract

  Lissencephaly is a severe brain malformation in humans. To study the function of the gene mutated in lissencephaly (LIS1), we deleted the first coding exon from the mouse Lis1 gene. The deletion resulted in a shorter protein (sLIS1) that initiates from the second methionine, a unique situation because most LIS1 mutations result in a null allele. This mutation mimics a mutation described in one lissencephaly patient with a milder phenotype. Homozygotes are early lethal, although heterozygotes are viable and fertile. Most strikingly, the morphology of cortical neurons and radial glia is aberrant in the developing cortex, and the neurons migrate more slowly. This is the first demonstration, to our knowledge, of a cellular abnormality in the migrating neurons after Lis1 mutation. Moreover, cortical plate splitting and thalomocortical innervation are also abnormal. Biochemically, the mutant protein is not capable of dimerization, and enzymatic activity is elevated in the embryos, thus a demonstration of the in vivo role of LIS1 as a subunit of PAF-AH. This mutation allows us to determine a hierarchy of functions that are sensitive to LIS1 dosage, thus promoting our understanding of the role of LIS1 in the developing cortex.

  View details for Web of Science ID 000168883700088

  View details for PubMedID 11344260

  View details for PubMedCentralID PMC33485

• Temporally and spatially regulated expression of a candidate G-protein-coupled receptor during cerebral cortical development JOURNAL OF NEUROBIOLOGY Chenn, A., Levin, M. E., McConnell, S. K. 2001; 46 (3): 167-177

  Abstract

  Genes expressed in layer-specific patterns in the mammalian cerebral cortex may play a role in specifying the identity of different cortical layers. Using PCR-differential display, we identified a cDNA that encodes rCNL3, a gene cloned previously by sequence homology to G-protein-coupled receptors. rCNL3 is expressed predominantly in layers 2-4 of the young rat cortex and in the developing and adult striatum. Cortical expression of rCNL3 begins postnatally at P3 and continues at high levels until around P15, while striatal expression begins at E20 and continues through adulthood. rCNL3 expression is not detectable in the ventricular zone precursors that generate the neurons of layers 2-4. The expression pattern of rCNL3 in the developing cortex suggests that rCNL3 is not involved in the initial specification of laminar fate, but rather may be involved with later differentiation events within the superficial cortical layers.

  View details for Web of Science ID 000166617900001

  View details for PubMedID 11169503

• Developmental expression pattern of the cdo gene DEVELOPMENTAL DYNAMICS Mulieri, P. J., Okada, A., Sassoon, D. A., McConnell, S. K., Krauss, R. S. 2000; 219 (1): 40-49

  Abstract

  CDO is a cell-surface protein of the immunoglobulin/fibronectin type III repeat family that positively regulates myogenic differentiation in vitro. To gain a better understanding of the role of cdo during vertebrate development, we carried out an extensive in situ hybridization study to characterize its expression pattern from postimplantation to late stages of mouse embryogenesis and in rat brain from E13 to adult. Our results show a broad pattern of cdo expression that is spatially and temporally restricted during embryogenesis. In the central nervous system (CNS), cdo expression is detected as early as E7.5 and maintained in the dorsal ventricular zones of the brain and spinal cord, becoming increasingly restricted in the adult. High levels of cdo are detected in developing sensory organs, such as the eye and ear. Outside the CNS, cdo is expressed mainly in neural crest and mesodermal derivatives, including skeletal muscle precursors. Overall, the highest levels of cdo expression are seen from E9.0 to E15.5. The temporal onset and restricted expression of cdo suggest that cdo plays a role in the determination and/or differentiation of a number of cell types during embryogenesis.

  View details for Web of Science ID 000089062600006

  View details for PubMedID 10974670

• Progressive restriction in fate potential by neural progenitors during cerebral cortical development DEVELOPMENT Desai, A. R., McConnell, S. K. 2000; 127 (13): 2863-2872

  Abstract

  During early stages of cerebral cortical development, progenitor cells in the ventricular zone are multipotent, producing neurons of many layers over successive cell divisions. The laminar fate of their progeny depends on environmental cues to which the cells respond prior to mitosis. By the end of neurogenesis, however, progenitors are lineally committed to producing upper-layer neurons. Here we assess the laminar fate potential of progenitors at a middle stage of cortical development. The progenitors of layer 4 neurons were first transplanted into older brains in which layer 2/3 was being generated. The transplanted neurons adopted a laminar fate appropriate for the new environment (layer 2/3), revealing that layer 4 progenitors are multipotent. Mid-stage progenitors were then transplanted into a younger environment, in which layer 6 neurons were being generated. The transplanted neurons bypassed layer 6, revealing that layer 4 progenitors have a restricted fate potential and are incompetent to respond to environmental cues that trigger layer 6 production. Instead, the transplanted cells migrated to layer 4, the position typical of their origin, and also to layer 5, a position appropriate for neither the host nor the donor environment. Because layer 5 neurogenesis is complete by the stage that progenitors were removed for transplantation, restrictions in laminar fate potential must lag behind the final production of a cortical layer. These results suggest that a combination of intrinsic and environmental cues controls the competence of cortical progenitor cells to produce neurons of different layers.

  View details for Web of Science ID 000088310500008

  View details for PubMedID 10851131

• Targeting of cre to the Foxg1 (BF-1) locus mediates loxP recombination in the telencephalon and other developing head structures DEVELOPMENTAL BIOLOGY Hebert, J. M., McConnell, S. K. 2000; 222 (2): 296-306

  Abstract

  The use of genetics to study the development of the telencephalon and derivatives such as the cerebral cortex has been limited. The telencephalon begins to form midway through gestation, and targeted mutations in genes suspected of playing roles in its development often lead to early phenotypes that preclude analysis of their role at later stages. This problem can be circumvented using a Cre/loxP recombination system. A mouse line was produced in which cre was targeted to the Foxg1 (BF-1) locus, a gene expressed specifically in the telencephalon and discrete head structures. Crosses between Foxg1-Cre mice and three separate loxP reporter mice generated embryos with recombination patterns matching that expected from the normal pattern of Foxg1 expression. Recombination occurs invariably in the telencephalon, anterior optic vesicle, otic vesicle, facial and head ectoderm, olfactory epithelium, mid-hindbrain junction, and pharyngeal pouches. Recombination in some animals also occurs less efficiently in tissues not known to express Foxg1. We show that the genetic background of the parental mice and the loxP target allele can each contribute to differences in the exact pattern of recombination. Collectively, these data show that Foxg1-Cre mice should be useful in the deletion or ectopic expression of any floxed target gene in a Foxg1-like pattern.

  View details for Web of Science ID 000087795200002

  View details for PubMedID 10837119

• Cortical degeneration in the absence of neurotrophin signaling: Dendritic retraction and neuronal loss after removal of the receptor TrkB NEURON Xu, B. J., Zang, K. L., Ruff, N. L., Zhang, Y. A., McConnell, S. K., Stryker, M. P., Reichardt, L. F. 2000; 26 (1): 233-245

  Abstract

  To examine functions of TrkB in the adult CNS, TrkB has been removed from neurons expressing CaMKII, primarily pyramidal neurons, using Cre-mediated recombination. A floxed trkB allele was designed so that neurons lacking TrkB express tau-beta-galactosidase. Following trkB deletion in pyramidal cells, their dendritic arbors are altered, and cortical layers II/III and V are compressed, after which there is an apparent loss of mutant neurons expressing the transcription factor SCIP but not of those expressing Otx-1. Loss of neurons expressing SCIP requires deletion of trkB within affected neurons; reduction of neuronal ER81 expression does not, suggesting both direct and indirect effects of TrkB loss. Thus, TrkB is required for the maintenance of specific populations of cells in the adult neocortex.

  View details for Web of Science ID 000086770500023

  View details for PubMedID 10798407

• Cortical neurons require Otx1 for the refinement of exuberant axonal projections to subcortical targets NEURON Weimann, J. M., Zhang, Y. A., Levin, M. E., Devine, W. P., Brulet, P., McConnell, S. K. 1999; 24 (4): 819-831

  Abstract

  Information processing in the nervous system depends on the creation of specific synaptic connections between neurons and targets during development. The homeodomain transcription factor Otx1 is expressed in early-generated neurons of the developing cerebral cortex. Within layer 5, Otx1 is expressed by neurons with subcortical axonal projections to the midbrain and spinal cord. Otx1 is also expressed in the precursors of these neurons, but is localized to the cytoplasm. Nuclear translocation of Otx1 occurs when layer 5 neurons enter a period of axonal refinement and eliminate a subset of their long-distance projections. Otx1 mutant mice are defective in the refinement of these exuberant projections, suggesting that Otx1 is required for the development of normal axonal connectivity and the generation of coordinated motor behavior.

  View details for Web of Science ID 000084495300011

  View details for PubMedID 10624946

• Regional differences in the developing cerebral cortex revealed by ephrin-A5 expression CEREBRAL CORTEX Mackarehtschian, K., Lau, C. K., Caras, I., McConnell, S. K. 1999; 9 (6): 601-610

  Abstract

  The development of axonal connections between thalamic nuclei and their cortical target areas occurs in a highly specific manner. To explore the mechanisms of thalamocortical axon pathfinding, we investigated the expression of several members of the ephrin and Eph gene families in the forebrain. The Eph ligand ephrin-A5 was expressed in three distinct gradients during the development of the telencephalon. The first gradient occurred in the cortical ventricular zone and established ephrin-A5 as one of the earliest markers distinguishing cells residing in the anterior versus posterior cortical neuroepithelium. The second gradient was apparent in the subplate and occurred in spatial opposition to a distinct gradient for the low-affinity NGF receptor p75. This finding reveals that different regions of the early subplate are molecularly heterogeneous. Third, we confirmed that ephrin-A5 is expressed in a bi-directional gradient in the cortical plate, with highest levels in the somatomotor cortex. Three putative receptors for ephrin-A5 -- EphA3, EphA4 and EphA5 -- showed distinct expression patterns in the developing thalamus. The graded distributions of ephrin-A5 in the developing subplate and cortex and the expression of its receptors in the thalamus are consistent with the notion that the Eph ligands and their receptors may function in the topographic mapping of thalamic axons to specific cortical areas.

  View details for Web of Science ID 000082308800010

  View details for PubMedID 10498278

• Doublecortin is a developmentally regulated, microtubule-associated protein expressed in migrating and differentiating neurons NEURON Francis, F., Koulakoff, A., Boucher, D., Chafey, P., Schaar, B., Vinet, M. C., Friocourt, G., Mcdonnell, N., Reiner, O., Kahn, A., McConnell, S. K., Berwald-Netter, Y., Denoulet, P., Chelly, J. 1999; 23 (2): 247-256

  Abstract

  Recently, we and others reported that the doublecortin gene is responsible for X-linked lissencephaly and subcortical laminar heterotopia. Here, we show that Doublecortin is expressed in the brain throughout the period of corticogenesis in migrating and differentiating neurons. Immunohistochemical studies show its localization in the soma and leading processes of tangentially migrating neurons, and a strong axonal labeling is observed in differentiating neurons. In cultured neurons, Doublecortin expression is highest in the distal parts of developing processes. We demonstrate by sedimentation and microscopy studies that Doublecortin is associated with microtubules (MTs) and postulate that it is a novel MAP. Our data suggest that the cortical dysgeneses associated with the loss of Doublecortin function might result from abnormal cytoskeletal dynamics in neuronal cell development.

  View details for Web of Science ID 000081218600011

  View details for PubMedID 10399932

• Imaging cells in the developing nervous system with retrovirus expressing modified green fluorescent protein EXPERIMENTAL NEUROLOGY Okada, A., Lansford, R., Weimann, J. M., Fraser, S. E., McConnell, S. E. 1999; 156 (2): 394-406

  Abstract

  To visualize the movements of cells and their processes in developing vertebrates, we constructed replication-incompetent retroviral vectors encoding green fluorescent protein (GFP) that can be detected as a single integrated copy per cell. To optimize GFP expression, the CMV enhancer and avian beta-actin promoter were incorporated within a retrovirus construct to drive transcription of redshifted (F64L, S65T) and codon-modified GFP (EGFP), EGFP tagged with GAP-43 sequences targeting the GFP to the cell membrane, or EGFP with additional mutations that increase its ability to fold properly at 37 degrees C (S147P or V163A, S175G). We have used these viruses to efficiently mark and follow the developmental progression of a large population of cells in rat neocortex and whole avian embryos. In the chick embryo, the migration and development of GFP-marked neural crest cells were monitored using time-lapse videomicroscopy. In the neocortex, GFP clearly delineates the morphology of a variety of neuronal and glial phenotypes. Cells expressing GFP display normal dendritic morphologies, and infected cells persist into adulthood. Cortical neurons appear to form normal local axonal and long-distance projections, suggesting that the presence of cytoplasmic or GAP-43-tagged GFP does not significantly interfere with normal development.

  View details for Web of Science ID 000079906200014

  View details for PubMedID 10328944

• Intrinsic polarity of mammalian neuroepithelial cells MOLECULAR AND CELLULAR NEUROSCIENCE Chenn, A., Zhang, Y. A., Chang, B. T., McConnell, S. K. 1998; 11 (4): 183-193

  Abstract

  Progenitor cells in the mammalian forebrain can undergo either symmetric or asymmetric cell divisions by varying their cleavage orientation. In asymmetric divisions, cells distribute apically and basally localized proteins differentially to their daughters. Here we explore the intrinsic polarity of neuroepithelial cells in the developing telencephalon. Actin microfilaments are concentrated apically, forming beltlike structures that encircle spots of gamma-tubulin immunoreactivity. Staining for N-cadherin, beta-catenin, and the tyrosine kinase substrates pp120 and paxillin is also enriched at the lumenal surface, presumably due to the localization of these proteins at adherens junctions. Phosphotyrosine immunoreactivity is concentrated apically in rings, suggesting that adherens junctions are enriched for signaling molecules. In mitotic cells it appears that adherens junction proteins and phosphotyrosine immunoreactivity may be inherited either symmetrically or asymmetrically, depending on the cell's cleavage orientation during mitosis. The differential inheritance of junctional proteins may determine whether a daughter cell can respond to extrinsic signals after mitosis.

  View details for Web of Science ID 000075037700002

  View details for PubMedID 9675050

• Determination of the migratory capacity of embryonic cortical cells lacking the transcription factor Pax-6 DEVELOPMENT Caric, D., Gooday, D., HILL, R. E., McConnell, S. K., Price, D. J. 1997; 124 (24): 5087-5096

  Abstract

  The cerebral cortex forms by the orderly migration and subsequent differentiation of neuronal precursors generated in the proliferative ventricular zone. We studied the role of the transcription factor Pax-6, which is expressed in the ventricular zone, in cortical development. Embryos homozygous for a mutation of Pax-6 (Small eye; Sey) had abnormalities suggesting defective migration of late-born cortical precursors. When late-born Sey/Sey precursors were transplanted into wild-type embryonic rat cortex, they showed similar integrative, migrational and differentiative abilities to those of transplanted wild-type mouse precursors. These results suggest that postmitotic cortical cells do not need Pax-6 to acquire the capacity to migrate and differentiate, but that Pax-6 generates a cortical environment that permits later-born precursors to express their full developmental potential.

  View details for Web of Science ID 000071276500015

  View details for PubMedID 9362466

• Postmitotic neurons migrate tangentially in the cortical ventricular zone DEVELOPMENT OROURKE, N. A., Chenn, A., McConnell, S. K. 1997; 124 (5): 997-1005

  Abstract

  Patterns of cell movement play a key role in the establishment of the brain's functional architecture during development. The migration of neuronal progenitor cells has been hypothesized to disperse clonally related cells among different areas of the developing cerebral cortex. To test this model, we explored the migratory patterns of cells in the proliferative zone of the intact cortex of the ferret. After focal injections of DiI, labeled cells migrated in all directions and over long distances within the ventricular and subventricular zones. These cells expressed the neuron-specific marker TuJ1 and did not incorporate BrdU after cumulative labeling. Our results reveal an extensive tangential dispersion of cortical cells mediated predominantly or exclusively by the non-radial migration of postmitotic neurons.

  View details for Web of Science ID A1997WQ22900007

  View details for PubMedID 9056775

• Induction of deep layer cortical neurons in vitro DEVELOPMENT BOHNER, A. P., Akers, R. M., McConnell, S. K. 1997; 124 (4): 915-923

  Abstract

  Transplantation studies suggest that the laminar fates of cerebral cortical neurons are determined by environmental signals encountered just before mitosis. In ferret, E29 progenitor cells normally produce neurons of layers 5 and 6. When transplanted during S-phase into an older ventricular zone, E29 progenitors produce neurons that change their fates and migrate to layer 2/3; however, cells transplanted later in the cell cycle migrate to their normal deep-layer positions even in an older environment (McConnell and Kaznowski, 1991). Here we utilize three culture systems to investigate the nature of the environmental signals involved in laminar specification. E29 cells were first cultured at low density to ascertain whether cell contact and/or short-range cues are required for deep layer specification. Neurons transplanted after a short time in low-density culture failed to adopt their normal fates and migrated instead to the upper layers. When crude cell contacts were restored by pelleting E29 cells together, most transplanted neurons cells became specified to their normal deep layer fates. Finally, E29 cells were transplanted after being cultured in explants that maintained the architecture of the cerebral wall. Explants allowed normal deep layer specification to occur, as transplanted cells migrated to layers 5 and 6. These results suggest that short-range cues induce multipotent progenitors to produce deep layer neurons.

  View details for Web of Science ID A1997WL96400016

  View details for PubMedID 9043072

• Restriction of late cerebral cortical progenitors to an upper-layer fate NEURON Frantz, G. D., McConnell, S. K. 1996; 17 (1): 55-61

  Abstract

  Early in development, neural progenitors in cerebral cortex normally produce neurons of several layers during successive cell divisions. The laminar fate of their daughters depends on environmental cues encountered just before mitosis. At the close of neurogenesis, however, cortical progenitors normally produce neurons destined only for the upper layers. To assess the developmental potential of these cells, upper-layer progenitors were transplanted into the cerebral cortex of younger hosts, in which deep-layer neurons were being generated. These studies reveal that late cortical progenitors are not competent to generate deep-layer neurons and are instead restricted to producing the upper layers.

  View details for Web of Science ID A1996UY98100007

  View details for PubMedID 8755478

• STRATEGIES FOR THE GENERATION OF NEURONAL DIVERSITY IN THE DEVELOPING CENTRAL-NERVOUS-SYSTEM JOURNAL OF NEUROSCIENCE McConnell, S. K. 1995; 15 (11): 6987-6998

  Abstract

  During development, the neural tube produces a large diversity of neuronal phenotypes from a morphologically homogeneous pool of precursor cells. In recent years, the cellular and molecular mechanisms by which specific types of neurons are generated have been explored, in the hope of discovering features common to development throughout the nervous system. This article focuses on three strategies employed by the CNS to generate distinct classes of neuronal phenotypes during development: dorsal-ventral polarization in the spinal cord, segmentation in the hindbrain, and a lamination in the cerebral cortex. The mechanisms for neurogenesis exemplified by these three strategies range from a relatively rigid, cell lineage-dependent specification with a high degree of subservance to early patterns of gene expression, to inductions and cell-cell interactions that determine cell fates more flexibly.

  View details for Web of Science ID A1995TF26400001

  View details for PubMedID 7472455

• CONSTRUCTING THE CEREBRAL-CORTEX - NEUROGENESIS AND FATE DETERMINATION NEURON McConnell, S. K. 1995; 15 (4): 761-768

  View details for Web of Science ID A1995TC64000006

  View details for PubMedID 7576626

• CLEAVAGE ORIENTATION AND THE ASYMMETRIC INHERITANCE OF NOTCH1 IMMUNOREACTIVITY IN MAMMALIAN NEUROGENESIS CELL Chenn, A., McConnell, S. K. 1995; 82 (4): 631-641

  Abstract

  Neurons in the mammalian central nervous system are generated from progenitor cells near the lumen of the neural tube. Time-lapse microscopy of dividing cells in slices of developing cerebral cortex reveals that cleavage orientation predicts the fates of daughter cells. Vertical cleavages produce behaviorally and morphologically identical daughters that resemble precursor cells; these symmetric divisions may serve to expand or maintain the progenitor pool. In contrast, horizontally dividing cells produce basal daughters that behave like young migratory neurons and apical daughters that remain within the proliferative zone. Notch1 immunoreactivity is distributed asymmetrically in mitotic cells, with Notch1 inherited selectively by the basal (neuronal) daughter of horizontal divisions. These results provide cellular and molecular evidence that cortical neurons are generated from asymmetric divisions.

  View details for Web of Science ID A1995RR73400014

  View details for PubMedID 7664342

• TANGENTIAL MIGRATION OF NEURONS IN THE DEVELOPING CEREBRAL-CORTEX DEVELOPMENT OROURKE, N. A., Sullivan, D. P., Kaznowski, C. E., Jacobs, A. A., McConnell, S. K. 1995; 121 (7): 2165-2176

  Abstract

  The mammalian cerebral cortex is divided into functionally distinct areas. Although radial patterns of neuronal migration have been thought to be essential for patterning these areas, direct observation of migrating cells in cortical brain slices has revealed that cells follow both radial and nonradial pathways as they travel from their sites of origin in the ventricular zone out to their destinations in the cortical plate (O'Rourke, N.A., Dailey, M.E., Smith, S.J. and McConnell, S.K. (1992) Science 258, 299-302). These findings suggested that neurons may not be confined to radial migratory pathways in vivo. Here, we have examined the patterns of neuronal migration in the intact cortex. Analysis of the orientations of [3H]thymidine-labeled migrating cells suggests that nonradial migration is equally common in brain slices and the intact cortex and that it increases during neurogenesis. Additionally, cells appear to follow nonradial trajectories at all levels of the developing cerebral wall, suggesting that tangential migration may be more prevalent than previously suspected from the imaging studies. Immunostaining with neuron-specific antibodies revealed that many tangentially migrating cells are young neurons. These results suggest that tangential migration in the intact cortex plays a pivotal role in the tangential dispersion of clonally related cells revealed by retroviral lineage studies (Walsh, C. and Cepko, C. L. (1992) Science 255, 434-440). Finally, we examined possible substrata for nonradial migration in dorsal cortical regions where the majority of glia extend radially. Using confocal and electron microscopy, we found that nonradially oriented cells run perpendicular to glial processes and make glancing contacts with them along their leading processes. Thus, if nonradial cells utilize glia as a migratory substratum they must glide across one glial fiber to another. Examination of the relationships between migratory cells and axons revealed axonal contacts with both radial and nonradial cells. These results suggest that nonradial cells use strategies and substrata for migration that differ from those employed by radial cells.

  View details for Web of Science ID A1995RH14100020

  View details for PubMedID 7635060

• Plasticity and commitment in the developing cerebral cortex 2nd Stanford International Neuroscience Symposium McConnell, S. K. ELSEVIER SCIENCE BV. 1995: 129–143

  View details for Web of Science ID A1995BE39G00012

  View details for PubMedID 7568871

• OTX1 AND OTX2 DEFINE LAYERS AND REGIONS IN DEVELOPING CEREBRAL-CORTEX AND CEREBELLUM JOURNAL OF NEUROSCIENCE Frantz, G. D., Weimann, J. M., Levin, M. E., McConnell, S. K. 1994; 14 (10): 5725-5740

  Abstract

  Within the cerebral and cerebellar cortices, neurons are organized in layers that segregate neurons with distinctive morphologies and axonal connections, and areas or regions that correspond to distinct functional domains. To explore the molecular underpinnings of pattern formation in layered regions of the CNS, we have characterized the patterns of expression of two homeodomain genes, Otx1 and Otx2, by in situ hybridization during embryonic and postnatal development in the rat. Otx1 and Otx2 are vertebrate homologs of the Drosophila gap gene orthodenticle, and are expressed during the development of the murine CNS (Simeone et al., 1992). Here we report that Otx1 mRNA is expressed in a subpopulation of neurons within cortical layers 5 and 6 during postnatal and adult life. This gene is also expressed by the precursors of deep-layer neurons within the developing cerebral ventricular zone, but is apparently downregulated by the progenitors of upper-layer neurons; Otx1 is never expressed by the neurons of layers 1-4. The spatial and temporal patterns suggest that Otx1 may play a role in the specification or differentiation of neurons in the deep layers of the cerebral cortex. Within the cerebellum, mRNAs for Otx1 and Otx2 are found within the external granular layer (EGL), but in three spatially distinct domains. During postnatal development, Otx1 is expressed within anterior cerebellar lobules; Otx2 mRNA is localized posteriorly, and a region of overlap in mid-cerebellum defines a third domain in which both genes are expressed. The boundaries of Otx1 and Otx2 expression correspond to the major functional boundaries of the cerebellum, and define the vestibulocerebellum, spinocerebellum, and pontocerebellum, respectively. Spatially restricted patterns of hybridization are observed early in postnatal life, at times that correspond roughly to the invasion of spinal and pontine afferents into the cerebellum (Arsénio-Nunes and Sotelo, 1985; Mason, 1987). These results raise the possibility that Otx1 and Otx2 play a role in cerebellar regionalization during early development.

  View details for Web of Science ID A1994PL02800001

  View details for PubMedID 7931541

• DIFFERENTIAL EXPRESSION OF SYNAPTIC VESICLE PROTEIN-2 (SV2) ISOFORMS JOURNAL OF NEUROSCIENCE Bajjalieh, S. M., Frantz, G. D., Weimann, J. M., McConnell, S. K., Scheller, R. H. 1994; 14 (9): 5223-5235

  Abstract

  The synaptic vesicle proteins SV2A and SV2B (SV2 = synaptic vesicle protein 2) are two highly related proteins belonging to a family of transporters. As a first step toward identifying the function of the SV2 proteins, we examined the expression of SV2A and SV2B in the rat brain by in situ hybridization, immunohistochemistry, and immunoprecipitation with isoform-specific antibodies. These analyses revealed that one isoform, SV2A, is expressed ubiquitously throughout the brain at varying levels. The other isoform, SV2B, has a more limited distribution with varying degrees of coexpression with SV2A. Immunoprecipitation of brain synaptic vesicles with isoform-specific antibodies followed by Western analyses suggests that both isoforms can be present on the same synaptic vesicle. The expression of the SV2 proteins did not correlate either with neurotransmitter phenotype or with the expression of other synaptic vesicle protein isoforms. SV2B expression was observed to change during development; it is more widely expressed in the immature brain and is found in cells that have yet to establish synaptic contacts. The ubiquitous and overlapping expression of the SV2s suggests that they perform a function common to all synaptic vesicles. Variable and changing coexpression of the SV2 isoforms may indicate that SV2 function is regulated by the isoform composition of synaptic vesicles. The observation that the synaptic vesicle proteins, all occurring in multiple isoforms, are differentially expressed with respect to each other indicates that up to 90 different vesicle types are possible.

  View details for Web of Science ID A1994PJ12600006

  View details for PubMedID 8083732

• SUBPLATE PIONEERS AND THE FORMATION OF DESCENDING CONNECTIONS FROM CEREBRAL-CORTEX JOURNAL OF NEUROSCIENCE McConnell, S. K., Ghosh, A., Shatz, C. J. 1994; 14 (4): 1892-1907

  Abstract

  The adult cerebral cortex extends axons to a variety of subcortical targets, including the thalamus and superior colliculus. These descending projections are pioneered during development by the axons of a transient population of subplate neurons (McConnell et al., 1989). We show here that the descending axons of cortical plate neurons appear to be delayed significantly in their outgrowth, compared with those of subplate neurons. To assess the possible role of subplate neurons in the formation of these pathways, subplate neurons were ablated during the embryonic period. In all cases, an axon pathway formed from visual cortex through the internal capsule and into the thalamus. In half of all cases, however, cortical axons failed to invade their normal subcortical targets. In the other half, targets were innervated normally. Subplate neurons are therefore likely to provide important cues that aid the process by which cortical axons grow toward, select, and invade their subcortical targets.

  View details for Web of Science ID A1994NF02600002

  View details for PubMedID 7512631

• REGULATION OF THE POU DOMAIN GENE SCIP DURING CEREBRAL CORTICAL DEVELOPMENT JOURNAL OF NEUROSCIENCE Frantz, G. D., BOHNER, A. P., Akers, R. M., McConnell, S. K. 1994; 14 (2): 472-485

  Abstract

  The mammalian cerebral cortex is patterned into layers of neurons that share characteristic morphologies, physiological properties, and axonal connections. Neurons in the various layers are thought to acquire their lamina-specific identities shortly before the time of their final mitosis in the cortical ventricular zone. In order to investigate the molecular basis of laminar patterning in the CNS, we have performed in situ hybridization studies of the POU homeodomain gene SCIP (also known as Tst-1 or Oct-6), which is expressed in proliferating Schwann cells in the PNS and O2A progenitor cells in the developing CNS. In the CNS of adult rats, SCIP is expressed at high levels in the cerebral cortex, specifically in layer 5 pyramidal neurons that form subcortical axonal connections. SCIP is both temporally and spatially regulated during cortical development. Its initial expression in the intermediate zone and cortical plate is correlated with the early migration and differentiation of layer 5 neurons. SCIP hybridization was not, however, observed within the ventricular zone during the period of neurogenesis. SCIP is also expressed at high levels in the neurons of cortical layer 2/3, during their migration and differentiation within the cortical plate. This expression in the upper layers is apparently downregulated during postnatal periods, with the adult pattern apparent by postnatal day 30 (P30). POU domain genes are thought to play a role in cell lineage and cell fate decisions in several systems; thus, SCIP may serve a function in generating discrete laminar phenotypes in the developing cerebral cortex. In addition, since SCIP is a putative repressor of myelin gene expression, our results suggest that SCIP plays a role in regulating transcription in differentiated CNS neurons as well as in proliferating glial precursors.

  View details for Web of Science ID A1994MV31900002

  View details for PubMedID 7905511

• CELL-MIGRATION IN CULTURED CEREBRAL CORTICAL SLICES DEVELOPMENTAL BIOLOGY Roberts, J. S., OROURKE, N. A., McConnell, S. K. 1993; 155 (2): 396-408

  Abstract

  In order to investigate the cellular mechanisms of migration and lamination in the mammalian cerebral cortex, we have cultured living slices of the developing telencephalon and tracked the migration of newly generated cortical neurons. Slice cultures made from neonatal ferret cortex were maintained in roller tubes and survived well for several weeks in vitro. Cells generated on Postnatal Day (P) 0 or 1 were labeled with the thymidine analog 5-bromo-2-deoxy-uridine (BrdU), and their movements were tracked during the subsequent culture period. At the beginning of the culture period, labeled cells were found almost exclusively in the proliferative zones of the cerebral wall, the ventricular and subventricular zones. Over the first week in culture, there was a dramatic movement of labeled cells out of the proliferative zones and into the cortical plate, the final destination of neocortical neurons. Comparison of the patterns of cell movement in cultured slices with that in intact littermate controls revealed that the initial migration of labeled cells into the cortical plate is similar in cultured slices and normal animals. The primary difference between migration in slice cultures and in vivo is that after longer times in culture, cells that migrate into the cortical plate fail to form the tightly clustered laminae characteristic of cells in the intact brain. Labeled cells are instead distributed more widely throughout the cortical plate and other regions of the cerebral wall. Thus, at least during the initial period of migration, cultured slices provide an experimentally manipulable system in which cell migration can be directly observed in a histotypically normal environment.

  View details for Web of Science ID A1993KL19200011

  View details for PubMedID 8432395

• DIVERSE MIGRATORY PATHWAYS IN THE DEVELOPING CEREBRAL-CORTEX SCIENCE OROURKE, N. A., Dailey, M. E., Smith, S. J., McConnell, S. K. 1992; 258 (5080): 299-302

  Abstract

  During early development of the mammalian cerebral cortex, young neurons migrate outward from the site of their final mitosis in the ventricular zone into the cortical plate, where they form the adult cortex. Time-lapse confocal microscopy was used to observe directly the dynamic behaviors of migrating cells in living slices of developing cortex. The majority of cells migrated along a radial pathway, consistent with the view that cortical neurons migrate along radial glial fibers. A fraction of cells, however, turned within the intermediate zone and migrated orthogonal to the radial fibers. This orthogonal migration may contribute to the tangential dispersion of clonally related cortical neurons.

  View details for Web of Science ID A1992JR86000034

  View details for PubMedID 1411527

• The control of neuronal identity in the developing cerebral cortex. Current opinion in neurobiology McConnell, S. K. 1992; 2 (1): 23-27

  Abstract

  Recent studies of the lineages and developmental potential of cortical neurons show that cell fates are progressively restricted during cerebral cortical development. Cell lineage experiments suggest that individual cortical precursors are multipotent, as their progeny can end up in different cortical areas, and in different layers within an area. Transplantation studies have shown that young neurons are committed very early on to adopting a given laminar position, in a manner correlated with their birth date in the ventricular zone. Neurons in different neocortical areas, however, retain a functional and anatomical equipotentiality well into cortical development, suggesting that positional cues determine a cell's area-specific identity.

  View details for PubMedID 1638129

• PERSPECTIVES ON EARLY BRAIN-DEVELOPMENT AND THE EPILEPSIES WORKSHOP ON MOLECULAR NEUROBIOLOGY OF EPILEPSY McConnell, S. K., Macdonald, R. L., MOURITZENDAM, A., Mello, L., Tobin, A. J., Swann, J. W., Engel, J., Buzsaki, G., Tempel, B. L. ELSEVIER SCIENCE BV. 1992: 183–191

  View details for Web of Science ID A1992KH38100019

  View details for PubMedID 1285907

• CELL-CYCLE DEPENDENCE OF LAMINAR DETERMINATION IN DEVELOPING NEOCORTEX SCIENCE McConnell, S. K., Kaznowski, C. E. 1991; 254 (5029): 282-285

  Abstract

  The neocortex is patterned in layers of neurons that are generated in an orderly sequence during development. This correlation between cell birthday and laminar fate prompted an examination of how neuronal phenotypes are determined in the developing cortex. At various times after labeling with [3H]thymidine, embryonic progenitor cells were transplanted into older host brains. The laminar fate of transplanted neurons correlates with the position of their progenitors in the cell cycle at the time of transplantation. Daughters of cells transplanted in S-phase migrate to layer 2/3, as do host neurons. Progenitors transplanted later in the cell cycle, however, produce daughters that are committed to their normal, deep-layer fates. Thus, environmental factors are important determinants of laminar fate, but embryonic progenitors undergo cyclical changes in their ability to respond to such cues.

  View details for Web of Science ID A1991GJ64200038

  View details for PubMedID 1925583

• MORPHOLOGY OF PIONEER AND FOLLOWER GROWTH CONES IN THE DEVELOPING CEREBRAL-CORTEX JOURNAL OF NEUROBIOLOGY Kim, G. J., Shatz, C. J., McConnell, S. K. 1991; 22 (6): 629-642

  Abstract

  In the developing nervous systems of both invertebrates and vertebrates, neurons must develop precise sets of axonal connections. One strategy used by both orders of animals is to generate a special class of neurons whose axons "pioneer" the first pathways between these cells and their targets. In the developing mammalian telencephalon, the subplate neurons (which are among the first neurons to be generated in development) extend axons to long-distance subcortical targets before the neurons of the deep cortical layers 5 and 6 have been generated. The axons of layer 5 and 6 neurons later follow a similar pathway to form permanent subcortical projections to the thalamus and tectum, and thereafter the vast majority of subplate neurons die. These results have generated the hypothesis that subplate axons may actually be required for the axons of layer 5 and 6 neurons to innervate their appropriate subcortical targets. The complexity of growth cones has previously been correlated with axonal decision making: differences in growth cone morphologies have been noted in comparisons of leading versus following axons (LoPresti, Macagno, and Levinthal, 1973; Nordlander, 1987; Yaginuma, Homma, Kunzi, and Oppenheim, 1991), and at choice points along axon pathways (Raper, Bastiani, and Goodman, 1983; Tosney and Landmesser, 1985; Caudy and Bentley, 1986a,b; Bovolenta and Mason, 1987; Holt, 1989; Bovolenta and Dodd, 1990; Yaginuma et al., 1991). Thus, as a first step toward addressing the question of whether the axons of deep-layer neurons simply follow subplate axons to their targets, we have studied the morphology of cortical growth cones at various points along the corticothalamic pathway and at different stages of development. We examined the brains of fetal ferrets and cats at ages ranging from embryonic days (E) 24 to E50, using the fluorescent lipophilic tracer 1,1-dioctadecyl-3,3,3',3'-tetramethyl indocarbocyanine perchlorate (DiI) to reveal the axons and growth cones of cortical neurons. Growth cones were drawn, and quantitative measurements of their complexity were made by counting filopodia and calculating their surface area. No morphological differences were found among growth cones at different points along the corticothalamic pathway at a given age. However, growth cones belonging to early-generated cells (likely to be subplate neurons) are significantly larger and more complex than are the growth cones of later-generated cortical neurons. This evidence is consistent with the suggestion that subplate growth cones actively pioneer the corticothalamic pathway, and that the axons of layer 5 and 6 neurons follow it.

  View details for Web of Science ID A1991GB72100007

  View details for PubMedID 1919567

• SUBPLATE NEURONS AND THE DEVELOPMENT OF NEOCORTICAL CONNECTIONS 3RD ANNUAL SYMP OF THE RETINA RESEARCH FOUNDATION : DEVELOPMENT OF THE VISUAL SYSTEM Shatz, C. J., Ghosh, A., McConnell, S. K., Allendoerfer, K. L., FRIAUF, E., Antonini, A. M I T PRESS. 1991: 175–196

  View details for Web of Science ID A1991BU03N00011

• THE GENERATION OF NEURONAL DIVERSITY IN THE CENTRAL-NERVOUS-SYSTEM ANNUAL REVIEW OF NEUROSCIENCE McConnell, S. K. 1991; 14: 269-300

  View details for Web of Science ID A1991EZ46700012

  View details for PubMedID 2031572

• THE SPECIFICATION OF NEURONAL IDENTITY IN THE MAMMALIAN CEREBRAL-CORTEX EXPERIENTIA McConnell, S. K. 1990; 46 (9): 922-929

  Abstract

  The determination of neuronal fate in the developing cerebral cortex has been studied by tracking normal cell lineages in the cortex, and by testing the commitment of young cortical neurons to their normal fates. These studies together suggest that neuronal progenitors are multipotent during development and have the potential to produce neurons destined for many or all of the cortical layers. However, the laminar identity of an individual neuron appears to be specified through environmental interactions at the time of the cell's terminal mitotic division, prior to its migration into the cortical plate.

  View details for Web of Science ID A1990EC20800007

  View details for PubMedID 2209801

• REQUIREMENT FOR SUBPLATE NEURONS IN THE FORMATION OF THALAMOCORTICAL CONNECTIONS NATURE Ghosh, A., Antonini, A., McConnell, S. K., Shatz, C. J. 1990; 347 (6289): 179-181

  Abstract

  The neurons of layer 4 in the adult cerebral cortex receive their major ascending inputs from the thalamus. In development, however, thalamic axons arrive at the appropriate cortical area long before their target layer 4 neurons have migrated into the cortical plate. The axons accumulate and wait in the zone below the cortical plate, the subplate, for several weeks before invading the cortical plate. The subplate is a transient zone that contains the first postmitotic neurons of the telencephalon. These neurons mature well before other cortical neurons, and disappear by cell death after the thalamic axons have grown into the overlying cortical plate. The close proximity of growing thalamocortical axons and subplate neurons suggests that they might be involved in interactions important for normal thalamocortical development. Here we show that early in development the deletion of subplate neurons located beneath visual cortex prevents axons from the lateral geniculate nucleus of the thalamus from recognizing and innervating visual cortex, their normal target. In the absence of subplate neurons, lateral geniculate nucleus axons continue to grow in the white matter past visual cortex despite the presence of their target layer 4 neurons. Thus the transient subplate neurons are necessary for appropriate cortical target selection by thalamocortical axons.

  View details for Web of Science ID A1990DY35200061

  View details for PubMedID 2395469

• FUNCTIONAL SYNAPTIC CIRCUITS IN THE SUBPLATE DURING FETAL AND EARLY POSTNATAL-DEVELOPMENT OF CAT VISUAL-CORTEX JOURNAL OF NEUROSCIENCE FRIAUF, E., McConnell, S. K., Shatz, C. J. 1990; 10 (8): 2601-2613

  Abstract

  Among the first postmitotic cells of the cerebral cortex is a special population located below the cortical plate: the subplate neurons. These neurons reach a high degree of morphological maturity during fetal life, well before the neurons of the cortical layers have matured, yet nearly all of these cells die after birth in the cat. Subplate neurons are also known to receive synaptic contacts. Here we have investigated whether these contacts are functional by making intracellular recordings from subplate neurons in cortical slices maintained in vitro. Subplate neurons were identified based on their location and morphology by injecting them with biocytin following the intracellular recordings. At all ages studied between embryonic day 50 and postnatal day 9, electrical stimulation of the optic radiations elicited EPSPs and synaptic and antidromic spikes in subplate neurons, indicating that some of the synapses seen at the ultrastructural level are indeed capable of synaptic transmission. The spiking patterns of 39 morphologically identified subplate neurons were examined by injecting depolarizing current, which revealed that a large majority gave only a single spike or a brief train of spikes in response to maintained depolarization, in contrast to the regular spiking pattern found in many neurons of adult cortex. Biocytin injections into subplate neurons revealed that they are a morphologically heterogeneous population with respect to their dendritic branching patterns; roughly half were inverted pyramids, the classic subplate neuron morphology. The axonal processes of subplate neurons were remarkable in that many not only arborized within the subplate, but also entered the cortical plate and terminated in the marginal zone. At early postnatal ages, these axons also gave off collaterals within cortical layer 4. The results of this study indicate that subplate neurons participate in synaptic microcircuits during development. While the presynaptic identity of the input to subplate neurons is not known conclusively, it is likely that geniculocortical axons, which wait in close proximity to subplate neurons, contribute significantly. The pattern of axonal branching of subplate neurons also implies that information conferred to subplate neurons may be relayed, in turn, to the neurons of cortical layer 4. Finally with the death of subplate neurons, the geniculocortical axons leave the subplate and invade the cortical plate to innervate directly the neurons of layer 4. Thus, subplate neurons may function as a crucial, but transient synaptic link between waiting geniculocortical axons and their ultimate target cells in the cortex.

  View details for Web of Science ID A1990DW22100009

  View details for PubMedID 2388080

• PIONEER NEURONS AND TARGET SELECTION IN CEREBRAL CORTICAL DEVELOPMENT COLD SPRING HARBOR SYMPOSIA ON QUANTITATIVE BIOLOGY Shatz, C. J., Ghosh, A., McConnell, S. K., Allendoerfer, K. L., FRIAUF, E., Antonini, A. 1990; 55: 469-480

  View details for Web of Science ID A1990HB91800045

  View details for PubMedID 1983445

• SUBPLATE NEURONS PIONEER THE 1ST AXON PATHWAY FROM THE CEREBRAL-CORTEX SCIENCE McConnell, S. K., Ghosh, A., Shatz, C. J. 1989; 245 (4921): 978-982

  Abstract

  During the development of the nervous system, growing axons must traverse considerable distances to find their targets. In insects, this problem is solved in part by pioneer neurons, which lay down the first axonal pathways when distances are at a minimum. Here the existence of a similar kind of neuron in the developing mammalian telencephalon is described. These are the subplate cells, the first postmitotic neurons of the cerebral cortex. Axons from subplate neurons traverse the internal capsule and invade the thalamus early in fetal life, even before the neurons of cortical layers 5 and 6, which will form the adult subcortical projections, are generated. During postnatal life, after the adult pattern of axonal projections is firmly established, most subplate neurons disappear. These observations raise the possibility that the early axonal scaffold formed by subplate cells may prove essential for the establishment of permanent subcortical projections.

  View details for Web of Science ID A1989AM93300029

  View details for PubMedID 2475909

• THE DETERMINATION OF NEURONAL FATE IN THE CEREBRAL-CORTEX TRENDS IN NEUROSCIENCES McConnell, S. K. 1989; 12 (9): 342-349

  Abstract

  During the embryonic development of the cerebral cortex, young neurons migrate out into characteristic laminar positions and form specific axonal connections with other neurons. The birthdate of a neuron, and its tangential location in the ventricular zone, can serve as markers that predict its normal laminar fate and pattern of connectivity. In order to test whether cells of the developing cerebral cortex are committed to their normal fates, several types of transplantation experiment have challenged young cortical neurons to alter their identities or connections in novel environments. Other recent experiments have employed retroviral vectors to trace neuronal lineages in the cortex. Together, these studies suggest that phenotypic commitment involves a series of decisions. Certain developmental restrictions--for example, commitment to a laminar identity--may occur at or around the time of the cell's final mitotic division, whereas the refinement of area-specific axonal projections occurs as the neuron differentiates within the cortex.

  View details for Web of Science ID A1989AN98500007

  View details for PubMedID 2480675

• FATES OF VISUAL CORTICAL-NEURONS IN THE FERRET AFTER ISOCHRONIC AND HETEROCHRONIC TRANSPLANTATION JOURNAL OF NEUROSCIENCE McConnell, S. K. 1988; 8 (3): 945-974

  Abstract

  In the mammalian cerebral cortex, neurons in a given layer are generated at about the same time in development. These cells also tend to share similar sets of morphological and physiological properties and have projection patterns characteristic of that layer. This correspondence between the birthday and eventual fate of a cortical neuron suggests the possibility that the commitment of a cell to a particular laminar position and set of connections may occur very early on in cortical development. The experiments described here constitute an attempt to manipulate the fates of newly generated cortical neurons upon transplantation. The first set of experiments addressed the normal development of neurons in the primary visual cortex (area 17) of the ferret. Injections of 3H-thymidine into newborn ferrets showed that neurons generated after birth are destined to sit in layer 2/3 of the cortex, whereas neurons born on embryonic day (E) 32 populate primarily layers 5 and 6. Many layer 2/3 neurons in adult ferrets could be retrogradely labeled with HRP from visual cortical areas 18 and 19, while about half of the neurons in layer 6 were found to project to the lateral geniculate nucleus (LGN). In the second set of experiments, presumptive layer 2/3 cells were labeled in vivo by injecting ferrets with 3H-thymidine on P1 and P2. Before the cells had a chance to migrate, they were removed from the donor brain, incubated in a fluorescent dye (DAPI or fast blue), and dissociated into a single-cell suspension. The labeled cells were then transplanted into the proliferative zone of a littermate host ferret ("isochronic" transplants). Over the next few weeks, many of these dye-labeled cells underwent changes in their position and morphology that were consistent with a radially directed migration and subsequent differentiation into cortical neurons. The final positions of isochronically transplanted neurons in the host brain were mapped out by using the 3H-thymidine marker after long survival periods. About 97% of radioactively labeled cells had migrated out into the visual cortex, where they attained a compact laminar distribution: 99% were found in layer 2/3, their normal destination. The labeled cells had normal, mostly pyramidal neuronal morphologies and appeared to be well integrated with host neurons when viewed in Nissl-stained sections. Ten isochronically transplanted neurons were successfully labeled after HRP injection into 2 normal target regions, areas 18 and 19.(ABSTRACT TRUNCATED AT 400 WORDS)

  View details for Web of Science ID A1988M530900018

  View details for PubMedID 3346731

• Development and decision-making in the mammalian cerebral cortex. Brain research McConnell, S. K. 1988; 472 (1): 1-23

  Abstract

  One of the fundamental tasks of neurobiology is to understand how the precision and specificity of the adult nervous system is achieved during development. This paper reviews the progress that has been made toward this end in studies of the developing mammalian cerebral cortex. Particular attention is focused on the problem of how cortical neurons make decisions during development: the correlation between a neuron's 'birthday' and its final laminar destination and projection patterns has raised the possibility that young neurons may be committed to their adult fates very early on in development, perhaps prior to migration. Indeed, several lines of evidence reviewed here suggest that at least some of the decisions made by cortical neurons are intrinsic properties of the cell itself. These studies include experiments on the reeler mouse mutant, and more recent attempts to manipulate developmental fates by pharmacological interventions and transplantation techniques. It is concluded that early commitment events in the cerebral cortex may specify a neuron's laminar position and restrict the range of potential axonal projections that the cell may form, but that local positional cues direct neurons to select (or maintain) only certain of the possible projections.

  View details for PubMedID 3277690

• FUNCTIONAL-ORGANIZATION OF PRIMARY VISUAL-CORTEX IN THE MINK (MUSTELA-VISON), AND A COMPARISON WITH THE CAT JOURNAL OF COMPARATIVE NEUROLOGY LeVay, S., McConnell, S. K., Luskin, M. B. 1987; 257 (3): 422-441

  Abstract

  The functional organization of geniculocortical afferents and the visual responses of neurons in primary visual cortex (area 17) were studied in barbiturate-anesthetized, paralyzed minks and cats. Responses of the afferents were studied after silencing intrinsic cortical activity with injections of kainic acid. In both species, afferents were segregated into patches on the basis of eye of origin. In the mink, but not in the cat, there was a further segregation on the basis of center type, with on- and off-center afferents terminating in alternating, partially overlapping patches. The visual responses of cortical neurons in the mink showed many similarities to those in the cat. Nearly all units were orientation-selective, and there was a columnar organization for preferred orientation. Many units were selective for one direction of movement. Within the binocular segment of cortex, although many units could be driven from either eye, there was a marked bias toward the contralateral eye compared to the cat. There was a columnar system for ocular dominance, but contralateral eye columns were wider than ipsilateral. In both species, a quantitative study was made of the responses of cortical neurons to stationary, flashing slits as a function of position in the receptive field. In the mink, and less clearly in the cat, units could be identified as simple or complex on the basis of the spatial separation or overlap of "on" and "off" discharge zones. In both species, simple cells were found most commonly in layers IV and VI, while layer V contained the greatest proportion of complex cells. The relative strengths of the on and off discharges of single cells were also measured. In the mink, many units gave better overall responses to the on or off phase of the stimulus, and 15% showed a strong (greater than 9:1) preference for one or the other, compared to 4% in the cat. In the mink, units with a common preference for the on or off phase of stationary stimuli were arranged in columnar aggregates, a feature of cortical organization that was not found in the cat. These columns probably result from the partial segregation of on-center and off-center geniculate afferents within layers IV and VI of the mink's cortex. On-dominated columns were, however, wider or more numerous than off-dominated columns.

  View details for Web of Science ID A1987G388800009

  View details for PubMedID 3558898

• ANATOMICAL ORGANIZATION OF THE VISUAL-SYSTEM OF THE MINK, MUSTELA-VISON JOURNAL OF COMPARATIVE NEUROLOGY McConnell, S. K., LeVay, S. 1986; 250 (1): 109-132

  Abstract

  The organization of the retinogeniculocortical visual system of the mink was studied by anterograde and retrograde tracer techniques, by physiological mapping, and by direct recordings from axonal terminals after injection of kainic acid. In the lateral geniculate nucleus, retinogeniculate afferents are segregated according to eye of origin between the two principal layers, A and A1. Within each of these layers there is a further parcellation according to functional type: on-center afferents terminate in the anterior leaflets of A and A1, and off-center afferents in the posterior leaflets. This separation is preserved in area 17: geniculocortical afferents terminate in ocular dominance patches in layer IV, and these patches coexist with an alternating, partially overlapping set of patches for on-center and off-center inputs that we have demonstrated previously (McConnell and LeVay: Proc. Natl. Acad. Sci. USA 81:1590-1593, '84). In both the lateral geniculate nucleus and in area 17, the contralateral eye predominates to a much greater extent than in the cat. Visual cortical areas corresponding to the cat's areas 17, 18, and 19 can be identified in the mink, but they are shifted posterolaterally in the hemisphere, and they show less emphasis on the representation of central retina. Mapping studies also revealed the existence of a fourth visual area in the splenial sulcus (area SV) adjacent to the representation of the far periphery in area 17. This area differs from the corresponding region in the cat in that it receives direct projections from the lateral geniculate nucleus and from areas 17 and 18. The lateral geniculate nucleus projects to each of the four cortical areas that were mapped. The bulk of the projection to area 17 is derived from the principal layers, A and A1, while most cells projecting to areas 18 and SV are found in the C-layer complex. The recurrent projection from area 17 to the lateral geniculate nucleus arises from pyramidal neurons in layer VI, and terminates through all layers of the lateral geniculate nucleus, but most densely in the interlaminar zones. Areas 18 and SV project predominantly to the C layers. Areas 17, 18, and SV are reciprocally connected with the claustrum and the LP-pulvinar complex, and project to the superior colliculus. All four visual cortical areas are mutually interconnected; these associational projections arise from both the supragranular and infragranular layers.

  View details for Web of Science ID A1986D271100009

  View details for PubMedID 3016036

• MIGRATION AND DIFFERENTIATION OF CEREBRAL CORTICAL-NEURONS AFTER TRANSPLANTATION INTO THE BRAINS OF FERRETS SCIENCE McConnell, S. K. 1985; 229 (4719): 1268-1271

  Abstract

  Cells from the cerebral proliferative zones of newborn ferrets were labeled with tritiated thymidine and a fluorescent dye and were transplanted as a single-cell suspension into the occipital region of newborn ferrets. The transplanted cells became thoroughly integrated into the host environment: many cells migrated through the intermediate zone and into the cortical plate, where they developed as pyramidal neurons. Other transplanted cells came to resemble glial cells. After 1 to 2 months most transplanted neurons had taken up residence in layer 2 + 3, the normal destination of neurons generated on postnatal days 1 and 2. Thus the sequence of morphological differentiation and the eventual laminar position of the isochronically transplanted neurons closely paralleled that of their normal host counterparts.

  View details for Web of Science ID A1985AQK7300038

  View details for PubMedID 4035355

• SEGREGATION OF ON-CENTER AND OFF-CENTER AFFERENTS IN MINK VISUAL-CORTEX PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA-BIOLOGICAL SCIENCES McConnell, S. K., LeVay, S. 1984; 81 (5): 1590-1593

  Abstract

  In the lateral geniculate nucleus of the mink, on-center and off-center neurons occupy separate layers [LeVay, S. & McConnell, S.K. (1982) Nature (London) 300, 350-351]. To study the mode of termination of geniculate afferents in area 17, we recorded from their terminal arborizations in layer IV after the destruction of cortical neurons by injection of kainic acid. At the majority of recording sites, multifiber responses were entirely or predominantly of one type: on-center or off-center. Responses obtained during perpendicular penetrations showed the same preferred sign of contrast throughout the thickness of layer IV. During tangential penetrations through the layer, we encountered sequences of on- and off-center activity separated by stretches of mixed responses. We conclude that on- and off-center afferents terminate in separate, alternating patches that occupy the full thickness of layer IV. These coexist with another set of patches in which the same afferents are segregated by eye of origin.

  View details for Web of Science ID A1984SJ69300066

  View details for PubMedID 6584894

• ON AND OFF LAYERS IN THE LATERAL GENICULATE-NUCLEUS OF THE MINK NATURE LeVay, S. 1982; 300 (5890): 350-351

  View details for Web of Science ID A1982PR77400041

  View details for PubMedID 7144889