PROJECT SUMMARY Synaptic vesicles (SVs) are highly specialized organelles that store and release neurotransmitters. The accumulation of old or damaged proteins on SVs compromises neurotransmission and can lead to dysfunctional neural circuits and networks. Indeed, recent studies have shown that mutations in genes that regulate SV protein degradation are associated with neurological and neurodegenerative disorders, demonstrating the critical importance of SV protein turnover for nervous system health. Yet the molecular mechanisms responsible for SV turnover and degradation remain poorly understood. The overall goal of this project is to elucidate these mechanisms, providing critical insights into the etiology of diseases that afflict millions of Americans. Our recent work has shown that the ESCRT pathway mediates the activity-dependent degradation of SV membrane proteins. The ESCRT pathway comprises a series of protein complexes that sequentially recruit ubiquitinated cargo and catalyze the formation of multivesicular bodies (MVBs) for delivery of these cargo to lysosomes. Intriguingly, we find that increased neuronal firing stimulates the activation of de/ubiquitinating enzymes at the synapse, as well as the motility of axonal transport vesicles carrying initial ESCRT protein Hrs, and their recruitment to SV pools. We hypothesize that these events are critical rate-limiting steps for activity-dependent turnover of SV membrane proteins. We will test this hypothesis with three aims. In Aim 1, we will evaluate the role of de/ubiquitination in the recycling of SV membrane proteins. Here, we will use biochemical and fluorescence imaging assays to evaluate how ubiquitination regulates SV protein recycling vs. degradation in hippocampal neurons. We will also investigate whether the deubiquitinating enzyme UCHL1 is necessary for maintaining SV proteins on recycling SVs, counteracting their degradative sorting. In Aim 2, we will characterize Hrs vesicles and the impact of Hrs on downstream ESCRT protein recruitment to SV pools. We will use super- resolution fluorescence/electron microscopy and proximity biotinylation to characterize the morphology and molecular composition of these vesicles, and Hrs gain- and loss-of-function combined with live imaging to determine whether the recruitment of downstream ESCRT proteins to SV pools requires Hrs. In Aim 3, we will investigate the mechanisms of activity-dependent Hrs recruitment to SV pools. We will test the roles of specific kinesins in the axonal transport of Hrs, and test whether its recruitment to SV pools requires the lipid PI(3)P, the presence of ubiquitinated proteins, and/or the small GTPase Rab35. Together, these studies will uncover fundamental mechanisms underlying SV proteostasis in neurons.