title: Passive diffusion as a mechanism underlying ribbon synapse vesicle release and resupply.
abstract: Synaptic ribbons are presynaptic protein structures found at many synapses that convey graded, "analog" sensory signals in the visual, auditory, and vestibular pathways. Ribbons, typically anchored to the presynaptic membrane and surrounded by tethered synaptic vesicles, are thought to regulate or facilitate vesicle delivery to the presynaptic membrane. No direct evidence exists, however, to indicate how vesicles interact with the ribbon or, once attached, move along the ribbon's surface to reach the presynaptic release sites at its base. To address these questions, we have created, validated, and tested a passive vesicle diffusion model of retinal rod bipolar cell ribbon synapses. We used axial (bright-field) electron tomography in the scanning transmission electron microscopy to obtain 3D structures of rat rod bipolar cell terminals in 1-μm-thick sections of retinal tissue at an isotropic spatial resolution of ∼3 nm. The resulting structures were then incorporated with previously published estimates of vesicle diffusion dynamics into numerical simulations that accurately reproduced electrophysiologically measured vesicle release/replenishment rates and vesicle pool sizes. The simulations suggest that, under physiologically realistic conditions, diffusion of vesicles crowded on the ribbon surface gives rise to a flow field that enhances delivery of vesicles to the presynaptic membrane without requiring an active transport mechanism. Numerical simulations of ribbon-vesicle interactions predict that transient binding and unbinding of multiple tethers to each synaptic vesicle may achieve sufficiently tight association of vesicles to the ribbon while permitting the fast diffusion along the ribbon that is required to sustain high release rates.
authors.affiliation: Synaptic Physiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, and.
authors.forename: Cole W
authors.initials: CW
authors.lastname: Graydon
authors.affiliation: Synaptic Physiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, and.
authors.forename: Jun
authors.initials: J
authors.lastname: Zhang
authors.affiliation: Synaptic Physiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, and.
authors.forename: Nicholas W
authors.initials: NW
authors.lastname: Oesch
authors.affiliation: Laboratory of Cellular Imaging and Macromolecular Biophysics, National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: Alioscka A
authors.initials: AA
authors.lastname: Sousa
authors.affiliation: Laboratory of Cellular Imaging and Macromolecular Biophysics, National Institute of Biomedical Imaging and Bioengineering, National Institutes of Health, Bethesda, Maryland 20892.
authors.forename: Richard D
authors.initials: RD
authors.lastname: Leapman
authors.affiliation: Synaptic Physiology Section, National Institute of Neurological Disorders and Stroke, National Institutes of Health, Bethesda, Maryland 20892, and diamond@ninds.nih.gov.
authors.forename: Jeffrey S
authors.initials: JS
authors.lastname: Diamond
mesh_terms: D000818:Animals; D003198:Computer Simulation; D004058:Diffusion; D055032:Electron Microscope Tomography; D005260:Female; D008297:Male; D008959:Models, Neurological; D009010:Monte Carlo Method; D018377:Neurotransmitter Agents / Q000378:metabolism*; D017729:Presynaptic Terminals / Q000502:physiology* / Q000648:ultrastructure; D051381:Rats; D017207:Rats, Sprague-Dawley; D051245:Retinal Bipolar Cells / Q000502:physiology* / Q000648:ultrastructure; D013572:Synaptic Vesicles / Q000378:metabolism*
chemical_list: D018377:Neurotransmitter Agents
keywords: diffusion; tether; vesicle