Endosomal sorting of readily releasable synaptic vesicles

摘要

Neurotransmitter release is achieved through the fusion of synaptic vesicles with the neuronal plasma membrane (exocytosis). Vesicles are then retrieved from the plasma membrane (endocytosis). It was hypothesized more than 3 decades ago that endosomes participate in vesicle recycling, constituting a slow endocytosis pathway re quired especially after prolonged stimulation. This recycling model predicts that newly endocytosed vesicles fuse with an endosome, which sorts (organizes) the molecules and buds exocytosis-compe tent vesicles. We analyzed here the endosome function using hip pocampal neurons, isolated nerve terminals (synaptosomes), and PC12 cells by stimulated emission depletion microscopy, photooxi dation EM, and several conventional microscopy assays. Surpris ingly, we found that endosomal sorting is a rapid pathway, which appeared to be involved in the recycling of the initial vesicles to be released on stimulation, the readily releasable pool. In agreement with the endosomal model, the vesicle composition changed after endocytosis, with the newly formed vesicles being enriched in plasma membrane proteins. Vesicle proteins were organized in clus ters both in the plasma membrane (on exocytosis) and in the endo some. In the latter compartment, they segregated from plasma membrane components in a process that is likely important for sort ing/budding of newly developed vesicles from the endosome.