Neurons contact each other mostly by synaptic transmission at synapses. The maintenance of synaptic transmission relies on vesicle endocytosis, which recycles fused vesicles for the second round of exocytosis. My goal is to improve our understanding on the cellular and molecular mechanisms underlying synaptic vesicle endocytosis, which are the building block for the maintenance of synaptic transmission and thus the signaling process of the nervous system. Our progress in the last year is described below. 1. Vesicle fusion with the plasma membrane generates an Ω-shaped membrane profile. Its pore is thought to dilate until flattening (full-collapse), followed by classical endocytosis to retrieve vesicles. Alternatively, the pore may close (kiss-and-run), but the triggering mechanisms and its endocytic roles remain poorly understood. Here, using confocal and STED imaging of dense-core vesicles, we find that closure of fusion-generated Ω-profiles, which produces various sizes of vesicles, is the dominant mechanism mediating rapid and slow endocytosis within 1-30 s. Strong calcium influx triggers dynamin-mediated closure. Weak calcium influx does not promote closure. These results establish that fusion pore closure is the main mechanism to retrieve vesicles, a finding that challenges the current thinking that classical endocytosis is the main mechanism to retrieve vesicles. 2. Calcium influx triggers and accelerates endocytosis in nerve terminals and non-neuronal secretory cells. Whether calcium/calmodulin-activated calcineurin, which dephosphorylates endocytic proteins, mediates this process is highly controversial for different cell types, developmental stages, and endocytic forms. At three preparations where controversies arose, including large calyx-type synapses, conventional cerebellar synapses and neuroendocrine chromaffin cells containing large dense-core vesicles, we reported that calcineurin gene knockout consistently slowed down endocytosis, regardless of cell types, developmental stages, or endocytic forms (rapid or slow). In contrast, calcineurin and calmodulin blockers slowed down endocytosis at relatively small calcium influx, but did not inhibit endocytosis at large calcium influx, resulting in false-negative results. These results suggest that calcineurin is universally involved in endocytosis. They may also help explain the controversies in pharmacological studies. We therefore suggest including calcineurin as a key player in mediating calcium-triggered and -accelerated vesicle endocytosis. 3. A large number of studies suggest that calcium triggers and accelerates vesicle endocytosis at many synapses and non-neuronal secretory cells. However, many studies show that prolonging the duration of the stimulation train, which induces more calcium influx, slows down endocytosis;and several studies suggest that instead of triggering endocytosis, calcium actually inhibits endocytosis. Here we addressed this apparent conflict at a large nerve terminal, the calyx of Held in rat brainstem, in which recent studies suggest that transient calcium increase up to tens of micromolar at the micro/nano domain triggers endocytosis. By dialyzing 0-1M calcium into the calyx via a whole-cell pipette, we found that slow endocytosis was inhibited by calcium dialysis at a concentration-dependent manner. Thus, prolonged, small, and global calcium increase inhibits endocytosis, whereas transient and large calcium increase at the micro/nano domain triggers endocytosis and facilitates endocytosis. This yin and yang effect of calcium may reconcile apparent conflicts regarding whether calcium accelerates or inhibits endocytosis. Whether endocytosis is fast or slow depends on the net outcome between the yin and yang effect of calcium. 4. we provide a comprehensive review for Annual Review of Physiology (2014) on the progress over the last four decades regarding the modes, functions and coupling mechanisms of exo- and endocytosis.
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