PROJECT SUMMARY/ABSTRACT Myelin—the electrical insulator around neuronal axons—is essential in vertebrates for rapid nerve signaling, and its loss in diseases like multiple sclerosis and following injury causes severe disability in patients. In the central nervous system, oligodendrocytes build myelin by first extending their membrane processes to ensheath axons, then wrapping spirally around the axon while compacting their membranes to become electrically insulating. In chronic multiple sclerosis lesions, oligodendrocytes ensheath axons but fail to wrap, suggesting that wrapping is a rate-limiting step for remyelination. To ultimately understand why remyelination fails in multiple sclerosis, we first aim to understand the mechanism by which myelin wraps normally. It was long hypothesized that the assembly of actin filaments provides the force required to drive wrapping, like the lamellipodium of a motile cell or a neuronal growth cone. However, we and others recently discovered that the dramatic disassembly of the oligodendrocyte actin cytoskeleton is required for wrapping. This finding was completely unexpected and suggests two models for wrapping. Cycles of actin disassembly and reassembly could be required to “ratchet” the oligodendrocyte membrane forward. In contrast, based on our preliminary data, we propose that actin disassembly acts as a “trigger” to initiate actin-independent wrapping and that the major role of actin disassembly is to allow myelin to compact. To test these models, we are using a suite of innovative approaches including first-in-class genetic tools we created to experimentally induce actin disassembly (DeActs) or block actin disassembly (StablActs) in oligodendrocytes during wrapping in vivo, advanced microscopy techniques to resolve myelin in vivo, and live cell imaging of oligodendrocytes in culture. Our preliminary data demonstrate: (1) actin filaments disassemble in oligodendrocytes prior to wrapping, (2) experimentally inducing actin disassembly specifically in oligodendrocytes in vivo increases myelin wrapping, and (3) experimentally blocking actin disassembly impairs myelin membrane compaction in a culture model of myelination. These data support the “trigger” model of myelin wrapping, laying the foundation for future translational studies to test whether this actin disassembly-based mechanism is recapitulated or perturbed during remyelination. By defining the role of actin disassembly in myelin wrapping and compaction, this project will open up new research directions towards understanding myelin formation, plasticity, and disease in the central nervous system.