Organization of the Reserve Pool of Synaptic Vesicles in Nerve Terminals Lacking Protein Liquid Phase Components

The protein endophilin A, which in the mammalian genome is encoded by three genes, endophilin A1, A2, and A3, regulates the synaptic vesicle cycle during exo- and endocytosis, and it is present in the reserve pool of synaptic vesicles (SVs), where its function is unknown. In vitro experiments suggest that endophilin, via its SH3 domain interactions, incorporates several components into the protein liquid phase that organizes SVs in the reserve pool. We investigated the effect of deletion of the genes encoding endophilin and one of its binding partners, dynamin, on the organization of SVs in living synapses formed by cortical neurons in culture. Our experiments showed that deletion of endophilin genes does not change the density of SVs in the reserve pool. At the same time, the deletion of dynamin 1 and dynamin 3 genes leads to a significant increase in the vesicle density. We suggest that other SH3-domain-containing proteins, which are components of the protein liquid phase, complement the function of endophilin in the SV reserve pool.