ESCRT-0 regulates AMPA receptor currents and Ca2+- dependent signaling

Abstract

Membrane protein trafficking is essential for synaptic growth, maintenance, function, and plasticity, and involves the regulated exocytosis and endocytosis of proteins to and from the pre-and post-synaptic membranes. Defects in the clearance of membrane proteins can lead to the accumulation of ubiquitinated membrane proteins and contribute to neurodegenerative disease. The ESCRT (endosomal sorting complexes required for transport) machinery binds and sorts ubiquitinated membrane proteins into lysosomes for degradation, yet the presence and function of ESCRTs in sorting ubiquitinated AMPA and other receptors at the post-synapse remain unclear. Here we show that the ubiquitin-binding ESCRT-0 protein, Hrs, localizes to both pre- and post-synapses, and levels are modulated by neuronal activity, increasing and decreasing with higher and lower neuronal activity, respectively. Phosphoproteomic profiling of Hrs-depleted post-synaptic membranes revealed a role for Hrs in glutamatergic synaptic transmission, including long-term potentiation. In addition, Hrs-depleted neurons showed faster AMPAR current kinetics and reduced amplitude in whole-cell patch-clamp recordings. Genetic deletion of neuronal Hgs in mice led to reductions in phosphorylated CaMKII- and -{beta} (T286/T287) and structural proteins, PSD-95 and gephyrin, suggestive of LTD (long-term depression)-like synaptic depression. In contrast, Hrs overexpression led to increases in Ca2+-dependent signaling, including protein kinase C (PKC) and PKC substrate, AMPAR subunit GluA1-S831, a site which increases conductance. Together, these findings identify a dynamic, bidirectional role for Hrs at the post-synapse as it both senses and is modulated by neuronal activity, ultimately impacting excitatory synaptic strength. Significance StatementSynaptic plasticity relies on dynamic trafficking and turnover of membrane proteins, including AMPA-type glutamate receptors (AMPARs), yet how receptor trafficking intersects with ubiquitin-mediated sorting pathways at synapses remains unclear. We show that the ubiquitin-binding ESCRT-0 protein, Hrs, localizes to both pre- and post-synapses, and its abundance is bidirectionally regulated by neuronal activity. Genetic depletion of Hrs in mice reduces CaMKII phosphorylation and impacts AMPAR channel surface localization. In contrast, neuronal-specific Hrs overexpression led to enhanced GluA1 and protein kinase C substrate phosphorylation, suggesting altered AMPAR trafficking, subunit composition, and/or function. Thus, Hrs emerges as a modulator of glutamatergic signaling, coupling ubiquitin-mediated receptor sorting to the fine-tuning of synaptic transmission, with direct implications for learning and memory in health and disease.