Vesicle recycling Synaptic vesicle




1 vesicle recycling

1.1 full collapse fusion
1.2 kiss-and-run
1.3 modulation





vesicle recycling

two leading mechanisms of action thought responsible synaptic vesicle recycling: full collapse fusion , kiss-and-run method. both mechanisms begin formation of synaptic pore releases transmitter extracellular space. after release of neurotransmitter, pore can either dilate vesicle collapses synaptic membrane, or can close rapidly , pinch off membrane generate kiss-and-run fusion.


full collapse fusion

it has been shown periods of intense stimulation @ neural synapses deplete vesicle count increase cellular capacitance , surface area. indicates after synaptic vesicles release neurotransmitter payload, merge , become part of, cellular membrane. after tagging synaptic vesicles hrp (horseradish peroxidase), heuser , reese found portions of cellular membrane @ frog neuromuscular junction taken cell , converted synaptic vesicles. studies suggest entire cycle of exocytosis, retrieval, , reformation of synaptic vesicles requires less 1 minute.


in full collapse fusion, synaptic vesicle merges , becomes incorporated cell membrane. formation of new membrane protein mediated process , can occur under conditions. after action potential, ca floods presynaptic membrane. ca binds specific proteins in cytoplasm, 1 of synaptotagmin, in turn trigger complete fusion of synaptic vesicle cellular membrane. complete fusion of pore assisted snare proteins. large family of proteins mediate docking of synaptic vesicles in atp-dependent manner. of synaptobrevin on synaptic vesicle, t-snare complex on membrane, made of syntaxin , snap-25, can dock, prime, , fuse synaptic vesicle membrane.


the mechanism behind full collapse fusion has been shown target of botulinum , tetanus toxins. botulinum toxin has protease activity degrades snap-25 protein. snap-25 protein required vesicle fusion releases neurotransmitters, in particular acetylcholine. botulinum toxin cleaves these snare proteins, , in doing so, prevents synaptic vesicles fusing cellular synaptic membrane , releasing neurotransmitters. tetanus toxin follows similar pathway, instead attacks protein synaptobrevin on synaptic vesicle. in turn, these neurotoxins prevent synaptic vesicles completing full collapse fusion. without mechanism in effect, muscle spasms, paralysis, , death can occur.


kiss-and-run

the second mechanism synaptic vesicles recycled known kiss-and-run fusion. in case, synaptic vesicle kisses cellular membrane, opening small pore neurotransmitter payload released through, closes pore , recycled cell. kiss-and-run mechanism has been hotly debated topic. effects have been observed , recorded; reason behind use opposed full collapse fusion still being explored. has been speculated kiss-and-run employed conserve scarce vesicular resources being utilized respond high-frequency inputs. experiments have shown kiss-and-run events occur. first observed katz , del castillo, later observed kiss-and-run mechanism different full collapse fusion in cellular capacitance did not increase in kiss-and-run events. reinforces idea of kiss-and-run fashion, synaptic vesicle releases payload , separates membrane.


modulation

cells appear have @ least 2 mechanisms follow membrane recycling. under conditions, cells can switch 1 mechanism other. slow, conventional, full collapse fusion predominates synaptic membrane when ca levels low, , fast kiss-and-run mechanism followed when ca levels high.


ales et al. showed raised concentrations of extracellular calcium ions shift preferred mode of recycling , synaptic vesicle release kiss-and-run mechanism in calcium-concentration-dependent manner. has been proposed during secretion of neurotransmitters @ synapses, mode of exocytosis modulated calcium attain optimal conditions coupled exocytosis , endocytosis according synaptic activity.


experimental evidence suggests kiss-and-run dominate mode of synaptic release @ beginning of stimulus trains. in context, kiss-and-run reflects high vesicle release probability. incidence of kiss-and-run increased rapid firing , stimulation of neuron, suggesting kinetics of type of release faster other forms of vesicle release.








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