PROJECT SUMMARY Current models of learning and memory have focused on cell-autonomous regulation of synaptic strength such as long-term potentiation and depression (LTP/LTD). Learning and experience induces a specific set of genes that mediate long-lasting maintenance of synaptic strength, epigenetic alterations of chromatin, and neuronal homeostasis. One key activity-dependent gene, Arc, is a critical mediator of long-lasting synaptic plasticity and memory. Recently, we identified a new type of intercellular communication mediated by Arc that could be central to its function. We unexpectedly observed that purified Arc protein spontaneously forms virus-like capsids and determined that the Arc gene originated from an ancient family of retrotransposons. We posited that if Arc forms capsids, perhaps other functional aspects of retroviruses have also been retained. We found that purified Arc capsids encapsulate RNA, including Arc mRNA. Endogenous Arc from brain also associates with its own mRNA and is released in extracellular vesicles (EVs) from primary cultured neurons. Strikingly, Arc-containing EVs can transfer Arc mRNA into Arc knock-out neurons. Thus, Arc has retained biochemical functions of the ancestral retrovirus Gag protein. These findings raise the possibility that Arc-dependent intercellular communication might underlie its function in learning and memory. Uptake of Arc EVs in neighboring neurons could result in Arc- mediated AMPAR endocytosis and synapse weakening. Arc has been implicated in the removal of AMPARs from weak synapses on the same dendrite in a form of heterosynaptic depression. However, Arc mRNA or protein transfer between synapses could mediate heterosynaptic plasticity at the circuit level, facilitating the stabilization of memory circuits by weaking surrounding neurons that were not active during learning. The main aim of this proposal is to test whether Arc mediates intercellular synaptic plasticity and mechanisms of Arc intercellular signaling. We have identified the BAR domain protein IRSp53 as an Arc interacting protein that mediates the release of Arc from cells after LTP induction. In preliminary data, we found that Arc EVs induce a loss of surface AMPA receptors in recipient cells. Based on these data, we will test the following model using live-imaging and electrophysiology techniques: LTP increases Arc protein expression in dendrites, which assembles into oligomeric virus-like capsids. LTP also increases IRSp53 expression in dendrites, where it interacts with Arc to facilitate Arc release in EVs. Once released, Arc EVs are taken up by neighboring neurons, inducing AMPAR endocytosis that results in non-cell autonomous weakening of synapse strength. The proposed studies will help uncover a new form of intercellular synaptic plasticity that may be important for memory and cognition. Disruption of Arc-dependent plasticity is implicated in various neurodevelopmental and psychiatric disorders – thus our work will hel...