Long-Term Temporal Dynamics of Synaptic Vesicles

Neurotransmission requires the release of neurotransmitters from synaptic vesicles. This \noccurs via fusion of the vesicle to the pre-synaptic membrane upon stimulation. However, not \nall synaptic vesicles are equally releasable, and it has long been debated why the majority of \nsynaptic vesicles do not respond to physiological levels of stimulation. I demonstrate here, \nusing live-cell antibody-tagging in rat hippocampal cultures, that only young synaptic vesicles \nare releasing neurotransmitter, and that they become more reluctant to release as they age. \nThis inactivation of synaptic vesicles is not strictly an ageing-dependent process, but \nconditional upon vesicle usage. I report here that synaptic vesicles release ~260 times, on \naverage, before becoming inactive, and that increasing usage frequency speeds up \ninactivation. The inactivation is caused by contamination of synaptic vesicles with the cell \nmembrane protein SNAP25. SNAP25 can interact with the vesicle protein CSPα in cis- \ncomplexes on the vesicle itself. This sequesters CSPα and prevents the formation of trans- \ncomplexes with SNAP25 on the cell membrane. This trans-interaction, however, promotes \nvesicle fusion to the cell membrane. The more often a vesicle has fused to the cell \nmembrane, the higher its chance is to be contaminated with SNAP25, and the less \ncompetent it is for future rounds of release. The inactivation of ageing synaptic vesicles is \npresumably coupled to usage to remove potentially damaged synaptic vesicles from \nneurotransmission. This hypothesis is strengthened by the observation of endocytosis \ndefects and neurite degeneration when aged vesicles are forced to release. I further provide \nseveral timing parameters for key events in the life of synaptic vesicles, which can serve as a \nframework towards a quantitative model of the synaptic vesicle life cycle.