PROJECT SUMMARY/ABSTRACT Myelin sheaths accelerate conduction velocity along axons, and its loss in neurological diseases, such as multiple sclerosis, leads to devastating disability. The importance of oligodendrocyte function and myelin for neuron health is also emerging in many neurological disorders, such as Alzheimer’s and Parkinson’s disease. Therefore, understanding how myelin is formed, remodeled, and regenerated may reveal new strategies with broad relevance to prevent or rescue neurological disorders. The majority of myelin generates during neurodevelopment, but recent discoveries demonstrate that new myelin forms following learning and sensory stimulation in humans and rodent models. Experiments in rodent models show that neuronal activity can directly stimulate new myelin formation and that new myelin formation is necessary for cognition, learning, and memory. This emerging form of neuroplasticity, termed activity-dependent myelination, can tune action potential timing and neural circuitry by adjusting myelination patterns along axons, changing the number, length, and thickness of myelin sheaths. Myelin sheaths form from oligodendrocytes that extend multiple processes and dramatically expand their cell surface to wrap axons in spiraling layers of membrane. How does neuronal activity regulate membrane expansion in oligodendrocytes? I recently discovered that exocytosis through VAMP2 and VAMP3 drive membrane expansion in oligodendrocytes during neurodevelopment. In many cell types, VAMP-mediated exocytosis can be stimulated by extracellular stimuli, but the cues that regulate oligodendrocyte exocytosis are unknown. I hypothesize that neuronal activity stimulates oligodendrocyte exocytosis to drive activity-dependent myelination within activated circuits. My preliminary data reveals that neuronal activity increases VAMP3 exocytosis in cultured primary oligodendrocytes by more than 2-fold. In my proposal, I will determine which VAMP proteins in primary oligodendrocytes are stimulated by neuronal activity (Aim 1.1) and which neuron-derived factors stimulate oligodendrocyte exocytosis (Aim 1.2). Then, I will obtain new training to investigate how human-derived neuron subtypes affect oligodendrocyte exocytosis in co-cultures (Aim 1.3). To determine the role of oligodendrocyte exocytosis in vivo, I will inhibit exocytosis specifically in oligodendrocytes of adult mice via AAVs (Aim 2.1) or Cre-inducible botulinum toxin (Aim 2.2) and use optogenetic stimulation to induce activity-dependent myelination. I will determine if oligodendrocyte exocytosis is necessary for activity-dependent myelin changes. Finally, with additional training, I will test if oligodendrocyte exocytosis is necessary for motor learning, a functional task that requires activity- dependent myelination (Aim 2.3). Altogether, my aims will uncover key cellular mechanisms that drive activity- dependent myelination and expand my scientific training to launch an exciting independen...