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# '''세포내이입''': 이 과정은 [[#완전융합 모델|완전한 막 융합 모델]]에서 시냅스 소포가 재흡수되는 과정이다.
 
== 소포 재활용 ==
Two leading mechanisms of action are thought to be responsible for synaptic vesicle recycling: full collapse fusion and the "kiss-and-run" method. Both mechanisms begin with the formation of the synaptic pore that releases transmitter to the extracellular space. After release of the neurotransmitter, the pore can either dilate fully so that the vesicle collapses completely into the synaptic membrane, or it can close rapidly and pinch off the membrane to generate kiss-and-run fusion.<ref name="3synapse">{{cite journal |author=Breckenridge LJ, Almers W |title=Currents through the fusion pore that forms during exocytosis of a secretory vesicle |journal=Nature |volume=328 |issue=6133 |pages=814–7 |year=1987 |pmid=2442614 |doi=10.1038/328814a0 |url=}}</ref>
=== 재흡수 ===
 
=== 완전융합 모델 ===
It has been shown that periods of intense stimulation at neural synapses deplete vesicle count as well as increase cellular capacitance and surface area.<ref name="pmid4348786">{{cite journal |author=Heuser JE, Reese TS |title=Evidence for recycling of synaptic vesicle membrane during transmitter release at the frog neuromuscular junction |journal=J. Cell Biol. |volume=57 |issue=2 |pages=315–44 |year=1973 |pmid=4348786 |pmc=2108984 |doi= |url=}}</ref> This indicates that after synaptic vesicles release their neurotransmitter payload, they merge with and become part of, the cellular membrane. After tagging synaptic vesicles with HRP ([[horseradish peroxidase]]), Heuser and Reese found that portions of the cellular membrane at the frog [[neuromuscular junction]] were taken up by the cell and converted back into synaptic vesicles.<ref name="pmid6607255">{{cite journal |author=Miller TM, Heuser JE |title=Endocytosis of synaptic vesicle membrane at the frog neuromuscular junction |journal=J. Cell Biol. |volume=98 |issue=2 |pages=685–98 |year=1984 |pmid=6607255 |pmc=2113115 |doi= |url=}}</ref> Studies suggest that the entire cycle of exocytosis, retrieval, and reformation of the synaptic vesicles requires less than 1 minute.<ref name="pmid8643616">{{cite journal |author=Ryan TA, Smith SJ, Reuter H |title=The timing of synaptic vesicle endocytosis |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=93 |issue=11 |pages=5567–71 |year=1996 |pmid=8643616 |pmc=39287 |doi= |url=}}</ref>
 
In full collapse fusion, the synaptic vesicle merges and becomes incorporated into the cell membrane. The formation of the new membrane is a protein mediated process and can only occur under certain conditions. After an [[action potential]], Ca<sup>2+</sup> floods to the presynaptic membrane. Ca<sup>2+</sup> binds to specific proteins in the cytoplasm, one of which is [[synaptotagmin]], which in turn trigger the complete fusion of the synaptic vesicle with the cellular membrane. This complete fusion of the pore is assisted by [[SNARE (protein)|SNARE]] proteins. This large family of proteins mediate docking of synaptic vesicles in an ATP-dependent manner. With the help of [[synaptobrevin]] on the synaptic vesicle, the t-SNARE complex on the membrane, made up of [[syntaxin]] and [[SNAP-25]], can dock, prime, and fuse the synaptic vesicle into the membrane.<ref name="pmid21987819">{{cite journal |author=Xu H, Zick M, Wickner WT, Jun Y |title=A lipid-anchored SNARE supports membrane fusion |journal=Proc. Natl. Acad. Sci. U.S.A. |volume=108 |issue=42 |pages=17325–30 |year=2011 |pmid=21987819 |pmc=3198343 |doi=10.1073/pnas.1113888108 |url=}}</ref>
 
The mechanism behind full collapse fusion has been shown to be the target of the [[Botulinum toxin|botulinum]] and [[tetanus]] toxins. The botulinum toxin has [[protease]] activity which degrades the [[SNAP-25]] protein. The [[SNAP-25]] protein is required for vesicle fusion that releases neurotransmitters, in particular acetylcholine.<ref name="pmid12381720">{{cite journal |author=Foran PG, Mohammed N, Lisk GO, Nagwaney S, Lawrence GW, Johnson E, Smith L, Aoki KR, Dolly JO |title=Evaluation of the therapeutic usefulness of botulinum neurotoxin B, C1, E, and F compared with the long lasting type A. Basis for distinct durations of inhibition of exocytosis in central neurons |journal=J. Biol. Chem. |volume=278 |issue=2 |pages=1363–71 |year=2003 |pmid=12381720 |doi=10.1074/jbc.M209821200 |url=}}</ref> Botulinum toxin essentially cleaves these SNARE proteins, and in doing so, prevents synaptic vesicles from fusing with the cellular synaptic membrane and releasing their neurotransmitters. Tetanus toxin follows a similar pathway, but instead attacks the protein [[synaptobrevin]] on the synaptic vesicle. In turn, these [[neurotoxin]]s prevent synaptic vesicles from completing full collapse fusion. Without this mechanism in effect, muscle spasms, paralysis, and death can occur.
 
=== "kiss-and-run" ===
The second mechanism by which synaptic vesicles are recycled is known as [[kiss-and-run fusion]]. In this case, the synaptic vesicle "kisses" the cellular membrane, opening a small pore for its neurotransmitter payload to be released through, then closes the pore and is recycled back into the cell.<ref name="3synapse" /> The kiss-and-run mechanism has been a hotly debated topic. Its effects have been observed and recorded; however the reason behind its use as opposed to full collapse fusion is still being explored. It has been speculated that kiss-and-run is often employed to conserve scarce vesicular resources as well as being utilized to respond to high-frequency inputs.<ref name="4synapse">{{cite journal |author=Harata NC, Aravanis AM, Tsien RW |title=Kiss-and-run and full-collapse fusion as modes of exo-endocytosis in neurosecretion |journal=J. Neurochem. |volume=97 |issue=6 |pages=1546–70 |year=2006 |pmid=16805768 |doi=10.1111/j.1471-4159.2006.03987.x |url=}}</ref> Experiments have shown that kiss-and-run events do occur. First observed by [[Bernard Katz|Katz]] and del Castillo, it was later observed that the kiss-and-run mechanism was different from full collapse fusion in that cellular [[capacitance]] did not increase in kiss-and-run events.<ref name="4synapse" /> This reinforces the idea of a kiss-and-run fashion, the synaptic vesicle releases its payload and then separates from the membrane.
 
=== 조작 ===
세포는 최소한 두 가지 기작, 즉 재활용과 키스앤런을 통하여 막을 재활용하는 것으로 알려져 있다. 특정한 조건에서 세포는 두 기작을 서로 바꿀 수 있다. Ca<sup>2+</sup> 농도가 낮을 때에는 시냅스 막에서 느린 전체 막 융합이 우세하게 일어나고, Ca<sup>2+</sup> 농도가 높을 때에는 키스앤런 기작이 뒤따른다.

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