Mable Lam

All Publications

• CNS myelination requires VAMP2/3-mediated membrane expansion in oligodendrocytes. Nature communications Lam, M., Takeo, K., Almeida, R. G., Cooper, M. H., Wu, K., Iyer, M., Kantarci, H., Zuchero, J. B. 2022; 13 (1): 5583

  Abstract

  Myelin is required for rapid nerve signaling and is emerging as a key driver of CNS plasticity and disease. How myelin is built and remodeled remains a fundamental question of neurobiology. Central to myelination is the ability of oligodendrocytes to add vast amounts of new cell membrane, expanding their surface areas by many thousand-fold. However, how oligodendrocytes add new membrane to build or remodel myelin is not fully understood. Here, we show that CNS myelin membrane addition requires exocytosis mediated by the vesicular SNARE proteins VAMP2/3. Genetic inactivation of VAMP2/3 in myelinating oligodendrocytes caused severe hypomyelination and premature death without overt loss of oligodendrocytes. Through live imaging, we discovered that VAMP2/3-mediated exocytosis drives membrane expansion within myelin sheaths to initiate wrapping and power sheath elongation. In conjunction with membrane expansion, mass spectrometry of oligodendrocyte surface proteins revealed that VAMP2/3 incorporates axon-myelin adhesion proteins that are collectively required to form nodes of Ranvier. Together, our results demonstrate that VAMP2/3-mediated membrane expansion in oligodendrocytes is indispensable for myelin formation, uncovering a cellular pathway that could sculpt myelination patterns in response to activity-dependent signals or be therapeutically targeted to promote regeneration in disease.

  View details for DOI 10.1038/s41467-022-33200-4

  View details for PubMedID 36151203

• pHusion: A robust and versatile toolset for automated detection and analysis of exocytosis. Journal of cell science O'Shaughnessy, E. C., Lam, M., Ryken, S. E., Wiesner, T., Lukasik, K., Zuchero, J. B., Leterrier, C., Adalsteinsson, D., Gupton, S. L. 2024

  Abstract

  Exocytosis is a fundamental process used by eukaryotes to regulate the composition of the plasma membrane and facilitate cell-cell communication. To investigate exocytosis in neuronal morphogenesis, previously we developed computational tools with a graphical user interface to enable the automatic detection and analysis of exocytic events from fluorescence timelapse images. Though these tools were useful, we found the code was brittle and not easily adapted to different experimental conditions. Here we developed and validated a robust and versatile toolkit, named pHusion, for the analysis of exocytosis written in ImageTank, a graphical programming language that combines image visualization and numerical methods. We tested this method using a variety of imaging modalities and pH-sensitive fluorophores, diverse cell types, and various exocytic markers to generate a flexible and intuitive package. We show that VAMP3-mediated exocytosis occurs 30-times more frequently in melanoma cells compared with primary oligodendrocytes, that VAMP2-mediated fusion events in mature rat hippocampal neurons are longer lasting than those in immature murine cortical neurons, and that exocytic events are clustered in space yet random in time in developing cortical neurons.

  View details for DOI 10.1242/jcs.261828

  View details for PubMedID 38690758

• SRF transcriptionally regulates the oligodendrocyte cytoskeleton during CNS myelination. Proceedings of the National Academy of Sciences of the United States of America Iram, T., Garcia, M. A., Amand, J., Kaur, A., Atkins, M., Iyer, M., Lam, M., Ambiel, N., Jorgens, D. M., Keller, A., Wyss-Coray, T., Kern, F., Zuchero, J. B. 2024; 121 (12): e2307250121

  Abstract

  Myelination of neuronal axons is essential for nervous system development. Myelination requires dramatic cytoskeletal dynamics in oligodendrocytes, but how actin is regulated during myelination is poorly understood. We recently identified serum response factor (SRF)-a transcription factor known to regulate expression of actin and actin regulators in other cell types-as a critical driver of myelination in the aged brain. Yet, a major gap remains in understanding the mechanistic role of SRF in oligodendrocyte lineage cells. Here, we show that SRF is required cell autonomously in oligodendrocytes for myelination during development. Combining ChIP-seq with RNA-seq identifies SRF-target genes in oligodendrocyte precursor cells and oligodendrocytes that include actin and other key cytoskeletal genes. Accordingly, SRF knockout oligodendrocytes exhibit dramatically reduced actin filament levels early in differentiation, consistent with its role in actin-dependent myelin sheath initiation. Surprisingly, oligodendrocyte-restricted loss of SRF results in upregulation of gene signatures associated with aging and neurodegenerative diseases. Together, our findings identify SRF as a transcriptional regulator that controls the expression of cytoskeletal genes required in oligodendrocytes for myelination. This study identifies an essential pathway regulating oligodendrocyte biology with high relevance to brain development, aging, and disease.

  View details for DOI 10.1073/pnas.2307250121

  View details for PubMedID 38483990

• Oligodendrocyte calcium signaling promotes actin-dependent myelin sheath extension. Nature communications Iyer, M., Kantarci, H., Cooper, M. H., Ambiel, N., Novak, S. W., Andrade, L. R., Lam, M., Jones, G., Münch, A. E., Yu, X., Khakh, B. S., Manor, U., Zuchero, J. B. 2024; 15 (1): 265

  Abstract

  Myelin is essential for rapid nerve signaling and is increasingly found to play important roles in learning and in diverse diseases of the CNS. Morphological parameters of myelin such as sheath length are thought to precisely tune conduction velocity, but the mechanisms controlling sheath morphology are poorly understood. Local calcium signaling has been observed in nascent myelin sheaths and can be modulated by neuronal activity. However, the role of calcium signaling in sheath formation remains incompletely understood. Here, we use genetic tools to attenuate oligodendrocyte calcium signaling during myelination in the developing mouse CNS. Surprisingly, genetic calcium attenuation does not grossly affect the number of myelinated axons or myelin thickness. Instead, calcium attenuation causes myelination defects resulting in shorter, dysmorphic sheaths. Mechanistically, calcium attenuation reduces actin filaments in oligodendrocytes, and an intact actin cytoskeleton is necessary and sufficient to achieve accurate myelin morphology. Together, our work reveals a cellular mechanism required for accurate CNS myelin formation and may provide mechanistic insight into how oligodendrocytes respond to neuronal activity to sculpt and refine myelin sheaths.

  View details for DOI 10.1038/s41467-023-44238-3

  View details for PubMedID 38177161

  View details for PubMedCentralID 9651929

• BMAL1 loss in oligodendroglia contributes to abnormal myelination and sleep. Neuron Rojo, D., Dal Cengio, L., Badner, A., Kim, S., Sakai, N., Greene, J., Dierckx, T., Mehl, L. C., Eisinger, E., Ransom, J., Arellano-Garcia, C., Gumma, M. E., Soyk, R. L., Lewis, C. M., Lam, M., Weigel, M. K., Damonte, V. M., Yalçın, B., Jones, S. E., Ollila, H. M., Nishino, S., Gibson, E. M. 2023

  Abstract

  Myelination depends on the maintenance of oligodendrocytes that arise from oligodendrocyte precursor cells (OPCs). We show that OPC-specific proliferation, morphology, and BMAL1 are time-of-day dependent. Knockout of Bmal1 in mouse OPCs during development disrupts the expression of genes associated with circadian rhythms, proliferation, density, morphology, and migration, leading to changes in OPC dynamics in a spatiotemporal manner. Furthermore, these deficits translate into thinner myelin, dysregulated cognitive and motor functions, and sleep fragmentation. OPC-specific Bmal1 loss in adulthood does not alter OPC density at baseline but impairs the remyelination of a demyelinated lesion driven by changes in OPC morphology and migration. Lastly, we show that sleep fragmentation is associated with increased prevalence of the demyelinating disorder multiple sclerosis (MS), suggesting a link between MS and sleep that requires further investigation. These findings have broad mechanistic and therapeutic implications for brain disorders that include both myelin and sleep phenotypes.

  View details for DOI 10.1016/j.neuron.2023.08.002

  View details for PubMedID 37657440

• Misfolded proteins bind and activate death receptor 5 to trigger apoptosis during unresolved endoplasmic reticulum stress ELIFE Lam, M., Marsters, S. A., Ashkenazi, A., Walter, P. 2020; 9

  Abstract

  Disruption of protein folding in the endoplasmic reticulum (ER) activates the unfolded protein response (UPR)-a signaling network that ultimately determines cell fate. Initially, UPR signaling aims at cytoprotection and restoration of ER homeostasis; that failing, it drives apoptotic cell death. ER stress initiates apoptosis through intracellular activation of death receptor 5 (DR5) independent of its canonical extracellular ligand Apo2L/TRAIL; however, the mechanism underlying DR5 activation is unknown. In cultured human cells, we find that misfolded proteins can directly engage with DR5 in the ER-Golgi intermediate compartment, where DR5 assembles pro-apoptotic caspase 8-activating complexes. Moreover, peptides used as a proxy for exposed misfolded protein chains selectively bind to the purified DR5 ectodomain and induce its oligomerization. These findings indicate that misfolded proteins can act as ligands to activate DR5 intracellularly and promote apoptosis. We propose that cells can use DR5 as a late protein-folding checkpoint before committing to a terminal apoptotic fate.

  View details for DOI 10.7554/eLife.52291

  View details for Web of Science ID 000518125300001

  View details for PubMedID 31904339

  View details for PubMedCentralID PMC7041945

• The Mars1 kinase confers photoprotection through signaling in the chloroplast unfolded protein response ELIFE Perlaza, K., Toutkoushian, H., Boone, M., Lam, M., Iwai, M., Jonikas, M. C., Walter, P., Ramundo, S. 2019; 8

  Abstract

  In response to proteotoxic stress, chloroplasts communicate with the nuclear gene expression system through a chloroplast unfolded protein response (cpUPR). We isolated Chlamydomonas reinhardtii mutants that disrupt cpUPR signaling and identified a gene encoding a previously uncharacterized cytoplasmic protein kinase, termed Mars1-for mutant affected in chloroplast-to-nucleus retrograde signaling-as the first known component in cpUPR signal transmission. Lack of cpUPR induction in MARS1 mutant cells impaired their ability to cope with chloroplast stress, including exposure to excessive light. Conversely, transgenic activation of cpUPR signaling conferred an advantage to cells undergoing photooxidative stress. Our results indicate that the cpUPR mitigates chloroplast photodamage and that manipulation of this pathway is a potential avenue for engineering photosynthetic organisms with increased tolerance to chloroplast stress.

  View details for DOI 10.7554/eLife.49577

  View details for Web of Science ID 000490547100001

  View details for PubMedID 31612858

  View details for PubMedCentralID PMC6794094

• Confirming a critical role for death receptor 5 and caspase-8 in apoptosis induction by endoplasmic reticulum stress CELL DEATH AND DIFFERENTIATION Lam, M., Lawrence, D. A., Ashkenazi, A., Walter, P. 2018; 25 (8): 1530-1531

  Abstract

  Several studies implicate specific death receptors (DRs) and caspase-8 in mediating apoptosis in response to endoplasmic reticulum (ER) stress; however, a recent paper challenges this conclusion. Here we validate the importance of DR5 and caspase-8 as critical signal conduits for apoptosis activation upon ER stress.

  View details for DOI 10.1038/s41418-018-0155-y

  View details for Web of Science ID 000442884000013

  View details for PubMedID 29991746

  View details for PubMedCentralID PMC6113221