Ryan T. Ash MD, PhD

All Publications

• Poster Session I: Noninvasive neuromodulation of subcortical visual pathways with transcranial focused ultrasound. Journal of vision Ash, R., Mohammadjavadi, M., Butts Pauly, K., Norcia, A. 2023; 23 (15): 23

  Abstract

  Transcranial ultrasound stimulation (TUS) is an emerging tool to noninvasively modulate neural activity in deep brain areas. In preparation for our first in-human TUS studies, we targeted TUS to the lateral geniculate nucleus (visual thalamus) in a large mammal (sheep). Full-field light flash stimuli were presented with or without concomitant TUS in randomly interleaved trials. Similar to what has previously observed by Fry et al (Nature 1959) in cats, EEG visual-evoked potentials (VEPs) were reversibly suppressed by TUS to the LGN. No changes in VEPs were observed in sheep who received sham-TUS to a control site in the basal ganglia, ruling out potential transducer auditory-somatosensory confounds. Magnetic resonance acoustic radiation force imaging (MR-ARFI), a technique to measure the ultrasound focus in situ, showed a focal volume of microscopic displacement at the expected target. Excitingly, MR-ARFI predicted the suppressive effect on VEPs in individual subjects, suggesting that MR-ARFI can be used to confirm TUS targeting and estimate neurophysiological impact. We are now translating this paradigm into human, targeting TUS to the LGN while participants perform a contrast detection task with EEG recording of steady-state VEPs. MR-ARFI will be measured to evaluate targeting and estimate TUS dosage in each participant. This work provides the foundation for a dissection of the roles of different subcortical nuclei in different aspects of human vision.

  View details for DOI 10.1167/jov.23.15.23

  View details for PubMedID 38109625

• Stability of steady-state visual evoked potential contrast response functions. Psychophysiology Ash, R. T., Nix, K. C., Norcia, A. M. 2023: e14412

  Abstract

  Repetitive sensory stimulation has been shown to induce neuroplasticity in sensory cortical circuits, at least under certain conditions. We measured the plasticity-inducing effect of repetitive contrast-reversal-sweep steady-state visual-evoked potential (ssVEP) stimuli, hoping to employ the ssVEP's high signal-to-noise electrophysiological readout in the study of human visual cortical neuroplasticity. Steady-state VEP contrast-sweep responses were measured daily for 4 days (four 20-trial blocks per day, 20 participants). No significant neuroplastic changes in response amplitude were observed either across blocks or across days. Furthermore, response amplitudes were stable within-participant, with measured across-block and across-day coefficients of variation (CV = SD/mean) of 15-20 ± 2% and 22-25 ± 2%, respectively. Steady-state VEP response phase was also highly stable, suggesting that temporal processing delays in the visual system vary by at most 2-3 ms across blocks and days. While we fail to replicate visual stimulation-dependent cortical plasticity, we show that contrast-sweep steady-state VEPs provide a stable human neurophysiological measure well suited for repeated-measures longitudinal studies.

  View details for DOI 10.1111/psyp.14412

  View details for PubMedID 37614220

• MeCP2 deficiency impairs motor cortical circuit flexibility associated with motor learning. Molecular brain Yue, Y., Ash, R. T., Boyle, N., Kinter, A., Li, Y., Zeng, C., Lu, H. 2022; 15 (1): 76

  Abstract

  Loss of function mutations in the X-linked gene encoding methyl-CpG binding protein 2 (MECP2) cause Rett syndrome (RTT), a postnatal neurological disorder. The loss of motor function is an important clinical feature of RTT that manifests early during the course of the disease. RTT mouse models with mutations in the murine orthologous Mecp2 gene replicate many human phenotypes, including progressive motor impairments. However, relatively little is known about the changes in circuit function during the progression of motor deficit in this model. As the motor cortex is the key node in the motor system for the control of voluntary movement, we measured firing activity in populations of motor cortical neurons during locomotion on a motorized wheel-treadmill. Different populations of neurons intermingled in the motor cortex signal different aspects of the locomotor state of the animal. The proportion of running selective neurons whose activity positively correlates with locomotion speed gradually decreases with weekly training in wild-type mice, but not in Mecp2-null mice. The fraction of rest-selective neurons whose activity negatively correlates with locomotion speed does not change with training in wild-type mice, but is higher and increases with the progression of locomotion deficit in mutant mice. The synchronization of population activity that occurs in WT mice with training did not occur in Mecp2-null mice, a phenotype most clear during locomotion and observable across all functional cell types. Our results could represent circuit-level biomarkers for motor regression in Rett syndrome.

  View details for DOI 10.1186/s13041-022-00965-0

  View details for PubMedID 36064580

• Increased reliability of visually-evoked activity in area V1 of the MECP2-duplication mouse model of autism. The Journal of neuroscience : the official journal of the Society for Neuroscience Ash, R. T., Palagina, G., Fernandez-Leon, J. A., Park, J., Seilheimer, R., Lee, S., Sabharwal, J., Reyes, F., Wang, J., Lu, D., Sarfraz, M., Froudarakis, E., Tolias, A. S., Wu, S. M., Smirnakis, S. M. 2022

  Abstract

  Atypical sensory processing is now thought to be a core feature of the autism spectrum. Influential theories have proposed that both increased and decreased neural response reliability within sensory systems could underlie altered sensory processing in autism. Here, we report evidence for abnormally increased reliability of visual-evoked responses in layer 2/3 neurons of adult male and female primary visual cortex in the MECP2-duplication syndrome animal model of autism. Increased response reliability was due in part to decreased response amplitude, decreased fluctuations in endogenous activity, and an abnormal decoupling of visual-evoked activity from endogenous activity. Similar to what was observed neuronally, the optokinetic reflex occurred more reliably at low contrasts in mutant mice compared to controls. Retinal responses did not explain our observations. These data suggest that the circuit mechanisms for combining sensory-evoked and endogenous signal and noise processes may be altered in this form of syndromic autism.SIGNIFICANT STATEMENT:Atypical sensory processing is now thought to be a core feature of the autism spectrum. Influential theories have proposed that both increased and decreased neural response reliability within sensory systems could underlie altered sensory processing in autism. Here, we report evidence for abnormally increased reliability of visual-evoked responses in primary visual cortex of the animal model for MECP2-duplication syndrome, a high-penetrance single-gene cause of autism. Visual-evoked activity was abnormally decoupled from endogenous activity in mutant mice, suggesting in line with the influential "hypo-priors" theory of autism that sensory priors embedded in endogenous activity may have less influence on perception in autism.

  View details for DOI 10.1523/JNEUROSCI.0654-22.2022

  View details for PubMedID 35831173

• Transcranial ultrasound neuromodulation of the thalamic visual pathway in a large animal model and the dose‐response relationship with MR‐ARFI Scientific Reports Mohammadjavadi, M., Ash, R. T., Li, N., Gaur, P., Kubanek, J., Saenz, Y., Glover, G. H., Popelka, G. R., Norcia, A. M., Butts Pauly, K. 2022; 12: 19588

  View details for DOI 10.1038/s41598-022-20554-4

• Stability and Plasticity of Steady-State Visual-Evoked Potential Contrast-Response Functions Ash, R., Nix, K., Lee, M., Pauly, K., Williams, N., Norcia, A. SPRINGERNATURE. 2021: 507

  View details for Web of Science ID 000725511402120

• Motor training improves coordination and anxiety in symptomatic Mecp2-null mice despite impaired functional connectivity within the motor circuit. Science advances Yue, Y., Xu, P., Liu, Z., Sun, X., Su, J., Du, H., Chen, L., Ash, R. T., Smirnakis, S., Simha, R., Kusner, L., Zeng, C., Lu, H. 2021; 7 (43): eabf7467

  Abstract

  [Figure: see text].

  View details for DOI 10.1126/sciadv.abf7467

  View details for PubMedID 34678068

• Inhibition of Elevated Ras-MAPK Signaling Normalizes Enhanced Motor Learning and Excessive Clustered Dendritic Spine Stabilization in the MECP2-Duplication Syndrome Mouse Model of Autism. eNeuro Ash, R. T., Buffington, S. A., Park, J., Suter, B., Costa-Mattioli, M., Zoghbi, H. Y., Smirnakis, S. M. 2021

  Abstract

  The inflexible repetitive behaviors and "insistence on sameness" seen in autism imply a defect in neural processes controlling the balance between stability and plasticity of synaptic connections in the brain. It has been proposed that abnormalities in the Ras-ERK/MAPK pathway, a key plasticity-related cell signaling pathway known to drive consolidation of clustered synaptic connections, underlie altered learning phenotypes in autism. However, a link between altered Ras-ERK signaling and clustered dendritic spine plasticity has yet to be explored in an autism animal model in vivo The formation and stabilization of dendritic spine clusters is abnormally increased in the MECP2-duplication syndrome mouse model of syndromic autism, suggesting that ERK signaling may be increased. Here, we show that the Ras-ERK pathway is indeed hyperactive following motor training in MECP2-duplication mouse motor cortex. Pharmacological inhibition of ERK signaling normalizes the excessive clustered spine stabilization and enhanced motor learning behavior in MECP2-duplication mice. We conclude that hyperactive ERK signaling may contribute to abnormal clustered dendritic spine consolidation and motor learning in this model of syndromic autism.Significance StatementIt has been proposed that autism-associated genetic mutations lead to altered learning phenotypes by perturbing cell signaling pathways that regulate synaptic plasticity in the brain. The Ras-ERK/MAPK signaling pathway, which promotes stabilization of dendritic spine clusters, has been particularly implicated in autism spectrum disorder (ASD). Here, we show that Ras-ERK signaling is increased in motor cortex following rotarod training in the MECP2-duplication syndrome mouse model of autism, and that the abnormal motor learning and excessive stabilization of clustered dendritic spines previously observed in MECP2-duplication mice can be rescued by pharmacological inhibition of Ras-ERK signaling. This provides additional support to hypotheses that autistic phenotypes arise from disrupted Ras-ERK signaling and synaptic plasticity and suggest potential future paths for therapeutic intervention.

  View details for DOI 10.1523/ENEURO.0056-21.2021

  View details for PubMedID 34021030

• Excessive formation and stabilization of dendritic spine clusters in the MECP2 duplication syndrome mouse model of autism. eNeuro Ash, R. T., Park, J., Suter, B., Zoghbi, H. Y., Smirnakis, S. M. 2020

  Abstract

  Autism-associated genetic mutations may perturb the balance between stability and plasticity of synaptic connections in the brain. Here we report an increase in the formation and stabilization of dendritic spines in the cerebral cortex of the mouse model of MECP2-duplication syndrome, a high-penetrance form of syndromic autism. Increased stabilization is mediated entirely by spines that form cooperatively in 10-micron clusters and is observable across multiple cortical areas both spontaneously and following motor training. Excessive stability of dendritic spine clusters could contribute to behavioral rigidity and other phenotypes in syndromic autism.Significance Statement The inflexible repetitive behaviors, "insistence on sameness," and at times exceptional learning abilities seen in autism imply a defect in the neural processes underlying learning and memory, potentially affecting the balance between stability and plasticity of synaptic connections in the brain. Here we report a pathological bias toward stability of newly formed dendritic spines in the MECP2-duplication mouse model of autism. Enhanced spine stability is mediated entirely by spines aggregating within 10 m of each other, in clusters. Enhanced clustered spine stability is observable in multiple brain areas both at rest and during motor training. The results suggest that some phenotypes of autism could arise from abnormal consolidation of clustered synaptic connections.

  View details for DOI 10.1523/ENEURO.0282-20.2020

  View details for PubMedID 33168618

• Contribution of apical and basal dendrites to orientation encoding in mouse V1 L2/3 pyramidal neurons. Nature communications Park, J., Papoutsi, A., Ash, R. T., Marin, M. A., Poirazi, P., Smirnakis, S. M. 2019; 10 (1): 5372

  Abstract

  Pyramidal neurons integrate synaptic inputs from basal and apical dendrites to generate stimulus-specific responses. It has been proposed that feed-forward inputs to basal dendrites drive a neuron's stimulus preference, while feedback inputs to apical dendrites sharpen selectivity. However, how a neuron's dendritic domains relate to its functional selectivity has not been demonstrated experimentally. We performed 2-photon dendritic micro-dissection on layer-2/3 pyramidal neurons in mouse primary visual cortex. We found that removing the apical dendritic tuft did not alter orientation-tuning. Furthermore, orientation-tuning curves were remarkably robust to the removal of basal dendrites: ablation of 2 basal dendrites was needed to cause a small shift in orientation preference, without significantly altering tuning width. Computational modeling corroborated our results and put limits on how orientation preferences among basal dendrites differ in order to reproduce the post-ablation data. In conclusion, neuronal orientation-tuning appears remarkably robust to loss of dendritic input.

  View details for DOI 10.1038/s41467-019-13029-0

  View details for PubMedID 31772192

• Increased Axonal Bouton Stability during Learning in the Mouse Model of MECP2 Duplication Syndrome ENEURO Ash, R. T., Fahey, P. G., Park, J., Zoghbi, H. Y., Smirnakis, S. M. 2018; 5 (3)

  Abstract

  MECP2 duplication syndrome is an X-linked form of syndromic autism caused by genomic duplication of the region encoding methyl-CpG-binding protein 2 (MECP2). Mice overexpressing MECP2 demonstrate social impairment, behavioral inflexibility, and altered patterns of learning and memory. Previous work showed abnormally increased stability of dendritic spines formed during motor training in the apical tuft of primary motor cortex (area M1) corticospinal neurons in the MECP2 duplication mouse model. In the current study, we measure the structural plasticity of axonal boutons in layer 5 pyramidal neuron projections to layer 1 of area M1 during motor training. In wild-type littermate control mice, we find that during rotarod training the bouton formation rate changes minimally, if at all, while the bouton elimination rate more than doubles. Notably, the observed upregulation in bouton elimination with training is absent in MECP2 duplication mice. This result provides further evidence of an imbalance between structural stability and plasticity in this form of syndromic autism. Furthermore, the observation that axonal bouton elimination more than doubles with motor training in wild-type animals contrasts with the increase of dendritic spine consolidation observed in corticospinal neurons at the same layer. This dissociation suggests that different area M1 microcircuits may manifest different patterns of structural synaptic plasticity during motor training.

  View details for DOI 10.1523/ENEURO.0056-17.2018

  View details for Web of Science ID 000442165100006

  View details for PubMedID 30105297

  View details for PubMedCentralID PMC6086213

• Loss and Gain of MeCP2 Cause Similar Hippocampal Circuit Dysfunction that Is Rescued by Deep Brain Stimulation in a Rett Syndrome Mouse Model. Neuron Lu, H. n., Ash, R. T., He, L. n., Kee, S. E., Wang, W. n., Yu, D. n., Hao, S. n., Meng, X. n., Ure, K. n., Ito-Ishida, A. n., Tang, B. n., Sun, Y. n., Ji, D. n., Tang, J. n., Arenkiel, B. R., Smirnakis, S. M., Zoghbi, H. Y. 2016; 91 (4): 739–47

  Abstract

  Loss- and gain-of-function mutations in methyl-CpG-binding protein 2 (MECP2) underlie two distinct neurological syndromes with strikingly similar features, but the synaptic and circuit-level changes mediating these shared features are undefined. Here we report three novel signs of neural circuit dysfunction in three mouse models of MECP2 disorders (constitutive Mecp2 null, mosaic Mecp2(+/-), and MECP2 duplication): abnormally elevated synchrony in the firing activity of hippocampal CA1 pyramidal neurons, an impaired homeostatic response to perturbations of excitatory-inhibitory balance, and decreased excitatory synaptic response in inhibitory neurons. Conditional mutagenesis studies revealed that MeCP2 dysfunction in excitatory neurons mediated elevated synchrony at baseline, while MeCP2 dysfunction in inhibitory neurons increased susceptibility to hypersynchronization in response to perturbations. Chronic forniceal deep brain stimulation (DBS), recently shown to rescue hippocampus-dependent learning and memory in Mecp2(+/-) (Rett) mice, also rescued all three features of hippocampal circuit dysfunction in these mice.

  View details for DOI 10.1016/j.neuron.2016.07.018

  View details for PubMedID 27499081

  View details for PubMedCentralID PMC5019177

• Dynamic Control of Excitatory Synapse Development by a Rac1 GEF/GAP Regulatory Complex DEVELOPMENTAL CELL Um, K., Niu, S., Duman, J. G., Cheng, J. X., Tu, Y., Schwechter, B., Liu, F., Hiles, L., Narayanan, A. S., Ash, R. T., Mulherkar, S., Alpadi, K., Smirnakis, S. M., Tolias, K. F. 2014; 29 (6): 701–15

  Abstract

  The small GTPase Rac1 orchestrates actin-dependent remodeling essential for numerous cellular processes including synapse development. While precise spatiotemporal regulation of Rac1 is necessary for its function, little is known about the mechanisms that enable Rac1 activators (GEFs) and inhibitors (GAPs) to act in concert to regulate Rac1 signaling. Here, we identify a regulatory complex composed of a Rac-GEF (Tiam1) and a Rac-GAP (Bcr) that cooperate to control excitatory synapse development. Disruption of Bcr function within this complex increases Rac1 activity and dendritic spine remodeling, resulting in excessive synaptic growth that is rescued by Tiam1 inhibition. Notably, EphB receptors utilize the Tiam1-Bcr complex to control synaptogenesis. Following EphB activation, Tiam1 induces Rac1-dependent spine formation, whereas Bcr prevents Rac1-mediated receptor internalization, promoting spine growth over retraction. The finding that a Rac-specific GEF/GAP complex is required to maintain optimal levels of Rac1 signaling provides an important insight into the regulation of small GTPases.

  View details for DOI 10.1016/j.devcel.2014.05.011

  View details for Web of Science ID 000338174600008

  View details for PubMedID 24960694

  View details for PubMedCentralID PMC4111230

• Viral transduction of the neonatal brain delivers controllable genetic mosaicism for visualising and manipulating neuronal circuits in vivo EUROPEAN JOURNAL OF NEUROSCIENCE Kim, J., Ash, R. T., Ceballos-Diaz, C., Levites, Y., Golde, T. E., Smirnakis, S. M., Jankowsky, J. L. 2013; 37 (8): 1203–20

  Abstract

  The neonatal intraventricular injection of adeno-associated virus has been shown to transduce neurons widely throughout the brain, but its full potential for experimental neuroscience has not been adequately explored. We report a detailed analysis of the method's versatility with an emphasis on experimental applications where tools for genetic manipulation are currently lacking. Viral injection into the neonatal mouse brain is fast, easy, and accesses regions of the brain including the cerebellum and brainstem that have been difficult to target with other techniques such as electroporation. We show that viral transduction produces an inherently mosaic expression pattern that can be exploited by varying the titer to transduce isolated neurons or densely-packed populations. We demonstrate that the expression of virally-encoded proteins is active much sooner than previously believed, allowing genetic perturbation during critical periods of neuronal plasticity, but is also long-lasting and stable, allowing chronic studies of aging. We harness these features to visualise and manipulate neurons in the hindbrain that have been recalcitrant to approaches commonly applied in the cortex. We show that viral labeling aids the analysis of postnatal dendritic maturation in cerebellar Purkinje neurons by allowing individual cells to be readily distinguished, and then demonstrate that the same sparse labeling allows live in vivo imaging of mature Purkinje neurons at a resolution sufficient for complete analytical reconstruction. Given the rising availability of viral constructs, packaging services, and genetically modified animals, these techniques should facilitate a wide range of experiments into brain development, function, and degeneration.

  View details for DOI 10.1111/ejn.12126

  View details for Web of Science ID 000317850800001

  View details for PubMedID 23347239

  View details for PubMedCentralID PMC3628093

• Dendritic arborization and spine dynamics are abnormal in the mouse model of MECP2 duplication syndrome. The Journal of neuroscience : the official journal of the Society for Neuroscience Jiang, M. n., Ash, R. T., Baker, S. A., Suter, B. n., Ferguson, A. n., Park, J. n., Rudy, J. n., Torsky, S. P., Chao, H. T., Zoghbi, H. Y., Smirnakis, S. M. 2013; 33 (50): 19518–33

  Abstract

  MECP2 duplication syndrome is a childhood neurological disorder characterized by intellectual disability, autism, motor abnormalities, and epilepsy. The disorder is caused by duplications spanning the gene encoding methyl-CpG-binding protein-2 (MeCP2), a protein involved in the modulation of chromatin and gene expression. MeCP2 is thought to play a role in maintaining the structural integrity of neuronal circuits. Loss of MeCP2 function causes Rett syndrome and results in abnormal dendritic spine morphology and decreased pyramidal dendritic arbor complexity and spine density. The consequences of MeCP2 overexpression on dendritic pathophysiology remain unclear. We used in vivo two-photon microscopy to characterize layer 5 pyramidal neuron spine turnover and dendritic arborization as a function of age in transgenic mice expressing the human MECP2 gene at twice the normal levels of MeCP2 (Tg1; Collins et al., 2004). We found that spine density in terminal dendritic branches is initially higher in young Tg1 mice but falls below control levels after postnatal week 12, approximately correlating with the onset of behavioral symptoms. Spontaneous spine turnover rates remain high in older Tg1 animals compared with controls, reflecting the persistence of an immature state. Both spine gain and loss rates are higher, with a net bias in favor of spine elimination. Apical dendritic arbors in both simple- and complex-tufted layer 5 Tg1 pyramidal neurons have more branches of higher order, indicating that MeCP2 overexpression induces dendritic overgrowth. P70S6K was hyperphosphorylated in Tg1 somatosensory cortex, suggesting that elevated mTOR signaling may underlie the observed increase in spine turnover and dendritic growth.

  View details for DOI 10.1523/JNEUROSCI.1745-13.2013

  View details for PubMedID 24336718

  View details for PubMedCentralID PMC3858623