Abstract

Compact myelin enables rapid and precise impulse conduction in the vertebrate nervous system. During CNS development, oligodendrocytes wrap spirally around axons while compacting membranes, yet how cytoskeletal remodeling is coupled to these events remains unclear. Actin disassembly is required for wrapping, but whether wrapping is driven by iterative actin-based protrusion or follows a transition to actin-independent mechanisms is unresolved. Here we integrate tract-resolved developmental profiling, live-cell compaction mapping, and in vivo genetic perturbation to define how actin disassembly is coupled to myelin wrapping. We find that actin filaments undergo pronounced, sustained disassembly before active wrapping begins, with little evidence for persistent actin within sheaths during wrapping. We develop a live-cell assay that maps membrane compaction in cultured oligodendrocytes and show that compaction zones are depleted of actin filaments and expand when actin disassembly is promoted. Consistent with this model, oligodendrocyte-specific actin disassembly in vivo accelerates the appearance of thicker myelin early in development. Together, our results support a model in which actin disassembly promotes myelin wrapping by enabling membrane compaction, and provide a platform to dissect how compaction is regulated in development and disease. ### Competing Interest Statement The authors have declared no competing interest. ARRA Award from National Center for Research Resources, 1S10RR026780-01 NSF, NN1 NSF 1707356, NN2 NSF 2014862 NIH ORIP grant, 1 S10 OD021514-01 Stanford University, https://ror.org/00f54p054, The Walter V. and Idun Berry Postdoctoral Fellowship Program, Wu Tsai Neurosciences Interdisciplinary Graduate Fellowship, Stanford Medical Scientist Training Program (T32 GM007365-45), Stanford Medicine Postbaccalaureate Experience in Research Program, Stanford Bio-X Interdisciplinary Graduate Fellowship, Wu Tsai Neurosciences Interdisciplinary Postdoctoral Scholarship NINDS Diversity Supplements, R01NS119823-01S1, R01NS119823-04S1 Stanford University, https://ror.org/00f54p054, Regina Casper Stanford Graduate Fellowship National Multiple Sclerosis Society McKnight Endowment Fund for Neuroscience Myra Reinhard Family Foundation Koret Family Foundation National Institutes of Health, R01NS119823