Synaptic vesicle endocytosis

Perspectives on kiss-and-run: role in exocytosis, endocytosis, and neurotransmission

Regulated exocytosis and endocytosis are critical to the function of many intercellular networks, particularly the complex neural circuits underlying mammalian behavior. Kiss-and-run (KR) is an unconventional fusion between secretory vesicles and a target membrane that releases intravesicular content through a transient, nanometer-sized fusion pore. The fusing vesicle retains its gross shape, precluding full integration into the planar membrane, and enough molecular components for rapid retrieval, reacidification, and reuse. KR makes judicious use of finite presynaptic resources, and mounting evidence suggests that it influences synaptic information transfer. Here we detail emerging perspectives on KR and its role in neurotransmission. We additionally formulate a restraining force hypothesis as a plausible mechanistic basis for KR and its physiological modulation in small nerve terminals. Clarification of the mechanism and function of KR has bearing on understanding the kinetic transitions underlying SNARE-mediated fusion, interactions between vesicles and their local environment, and the influence of release dynamics on neural information processing.

Corelease of dopamine and GABA by a retinal dopaminergic neuron

Numerous neurons release two transmitters of low molecular mass, but it is controversial whether they are localized within the same synaptic vesicle, with the single exception of GABA and glycine because they are ferried into the vesicle by the same transporter. Retinal dopaminergic (DAergic) amacrine cells synthesize both dopamine (DA) and GABA. Both transmitters are released over the entire cell surface and act on neighboring and distant neurons by volume transmission, but, in addition, DAergic cells establish GABAergic synapses onto AII amacrine cells, the neurons that transfer rod signals to cone bipolars. By combining recordings of DA and GABA release from isolated, genetically identified perikarya of DAergic cells from the mouse retina, we observed that a proportion of the events of DA and GABA exocytosis were simultaneous, suggesting corelease. Furthermore, a proportion of the secretory organelles in the perikaryon and synaptic endings of DAergic cells contained both vesicular transporters for DA [vesicular monoamine transporter 2 (VMAT2)] and GABA [vesicular GABA transporter (VGAT)]. Because the majority of the DA release events concerned a single transmitter and organelles were present that contained a single transporter, either VMAT2 or VGAT, we conclude that the secretory organelles of DAergic cells contain variable concentrations of the two transmitters, which are in turn determined by a variable mixture of the two transporter molecules in their limiting membrane. This variability can be explained if the relative numbers of transporter molecules is determined stochastically during the budding of the somatic organelles from the trans-Golgi network or the retrieval of the vesicular membrane from the plasmalemma after exocytosis.

Role of dynamin, synaptojanin, and endophilin in podocyte foot processes

Podocytes are specialized cells that play an integral role in the renal glomerular filtration barrier via their foot processes. The foot processes form a highly organized structure, the disruption of which causes nephrotic syndrome. Interestingly, several similarities have been observed between mechanisms that govern podocyte organization and mechanisms that mediate neuronal synapse development. Dynamin, synaptojanin, and endophilin are functional partners in synaptic vesicle recycling via interconnected actions in clathrin-mediated endocytosis and actin dynamics in neurons. A role of dynamin in the maintenance of the kidney filtration barrier via an action on the actin cytoskeleton of podocytes was suggested. Here we used a conditional double-KO of dynamin 1 (Dnm1) and Dnm2 in mouse podocytes to confirm dynamin's role in podocyte foot process maintenance. In addition, we demonstrated that while synaptojanin 1 (Synj1) KO mice and endophilin 1 (Sh3gl2), endophilin 2 (Sh3gl1), and endophilin 3 (Sh3gl3) triple-KO mice had grossly normal embryonic development, these mutants failed to establish a normal filtration barrier and exhibited severe proteinuria due to abnormal podocyte foot process formation. These results strongly implicate a protein network that functions at the interface between endocytosis and actin at neuronal synapses in the formation and maintenance of the kidney glomerular filtration barrier.