Molecular and Functional Determinants of Synaptic Vesicle Recycling In CNS Synapses
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Chemical neurotransmission is the basis for information processing in the brain, and presynaptic terminals respond to a large range of stimulation patterns including baseline rhythms of activity that coordinate neuronal ensembles, to short bursts of activity that encode information. They also release neurotransmitter spontaneously in the absence of any activity. The question then is how can a single subcellular compartment with approximately 100 synaptic vesicles coordinate these complex functions? We used a multifaceted approach to address this question. We first studied the role of synaptotagmin 7 (syt7), a highly alternatively spliced synaptic plasma membrane protein, whose short splice forms inhibit clathrin-mediated endocytosis. We found that in hippocampal synapses, the splice variants formed a bi-directional molecular switch targeting vesicles to kinetically distinct recycling pathways. Additionally, syt7 knockout synapses had less fast endocytosis, while calcium binding site mutant synapses showed increased vesicle endocytosis. We further investigated the slower recycling pathways by exploring rab5 function. A dominant negative rab5 mutation did not alter synaptic function, but constitutively active rab5 or the inhibition of vesicle budding from endosomes by the PI3-kinase inhibitor wortmannin, decreased vesicle pool size and release kinetics. This suggests that central synapses are tuned towards faster modes of recycling. The model of spontaneous neurotransmitter release from this same evoked recycling pool places additional constraints on this system. We explored this hypothesis by directly visualizing presynaptic recycling of spontaneous vesicles using FM dyes, syt1 antibodies and HRP uptake. We found that there are actually two sets of vesicle pools, one for evoked release, and one for spontaneous release that have minimal interaction with one another. Can presynaptic function be a substrate for diseases of the CNS? To test this important question, we studied a mouse model for infantile Batten disease. We found that underlying synaptic deficits in vesicle pool size and mini frequency could produce the neurological phenotypes exhibited by patients well before the onset of neurodegeneration. Taken together, these results show that synaptic vesicle recycling is a very plastic entity and that synapses have the intrinsic ability to modulate their vesicle trafficking pathways in response to the varying demands placed on them.