Synaptic Vesicle Endocytosis

National Institute Of Neurological Disorders And Stroke

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Abstract

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. Since their discovery decades ago, the primary physiological and pathological effects of potassium channels have been attributed to their ion conductance, which sets membrane potential and repolarizes action potentials. For example, Kv3 family channels regulate neurotransmitter release by repolarizing action potentials. Here we report a surprising but crucial function independent of potassium conductance: by organizing the F-actin cytoskeleton in mouse nerve terminals, the Kv3.3 protein facilitates slow endocytosis and rapid endocytosis. A channel mutation that causes spinocerebellar ataxia inhibits endocytosis by disrupting the ability of the channel to nucleate F-actin. These results unmask novel functions of potassium channels in endocytosis. Potassium channel mutations that impair these non-conducting functions may thus contribute to generation of diverse neurological disorders. 2. Transformation of flat membrane into round vesicles is generally thought to underlie endocytosis and produce speed-, amount- and vesicle-size-specific endocytic modes. Visualizing depolarization-induced endocytic membrane transformation in live neuroendocrine chromaffin cells, we found that flat membrane is transformed into lambda-shape, omega-shape and then O-shape vesicles via invagination, lambda-base constriction and omega-pore constriction, respectively. Surprisingly, endocytic vesicle formation is predominantly not from flat-membrane-to-round-vesicle transformation, but from calcium-triggered and dynamin-mediated closure of 1) omega-profiles formed before depolarization and 2) fusion pores (called kiss-and-run). Varying calcium influxes control these pore closures speed, number, and vesicle size, resulting in speed-specific slow (>6-s), fast (<6-s) or ultrafast (<0.6-s) endocytosis, amount-specific compensatory (endocytosis=exocytosis) or overshoot endocytosis (endocytosis>exocytosis), and size-specific bulk endocytosis. These findings reveal major membrane transformation mechanisms underlying endocytosis, diverse endocytic modes, and exo-endocytosis coupling, calling for correction of the half-a-century concept that the flat-to-round transformation predominantly mediates endocytosis after physiological stimulation.