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Synapsin, N-terminal domain
Symbol Synapsin
Pfam PF02078
InterPro IPR001359
SCOP 1auv
Synapsin, ATP binding domain
Symbol Synapsin_C
Pfam PF02750
InterPro IPR001359
SCOP 1auv

The synapsins are a family of proteins that have long been implicated in the regulation of neurotransmitter release at synapses. Specifically, they are thought to be involved in regulating the number of synaptic vesicles available for release via exocytosis at any one time.[1]

Current studies suggest the following hypothesis for the role of synapsin: synapsins bind synaptic vesicles to components of the cytoskeleton which prevents them from migrating to the presynaptic membrane and releasing transmitter. During an action potential, synapsins are phosphorylated by Ca2+/calmodulin-dependent protein kinase II, releasing the synaptic vesicles and allowing them to move to the membrane and release their neurotransmitter.

Gene knockout studies in mice (where the mouse is unable to produce synapsin) have had some surprising results. Mice lacking all three synapsins are prone to seizures, and experience learning defects.[2] These results suggest that while synapsins are not essential for synaptic function, they do serve an important modulatory role. Conversely, studies using transgenic mice in which neuronal signaling is abolished in specific circuitries showed that synaptic activity regulates, but is not essential to maintain, the expression of these proteins.[3]

Family members

Humans and most other vertebrates possess three genes encoding three different synapsin proteins.[4] Each gene in turn is alternatively spliced to produce at least two different protein isoforms for a total of six isoforms:[5]

Gene Protein Isoforms
SYN1 Synapsin I Ia, Ib
SYN2 Synapsin II IIa, IIb
SYN3 Synapsin III IIIa, IIIb

Different neuron terminals will express varying amounts of each of these synapsin proteins and collectively these synapsins will comprise 1% of the total expressed protein at any one time.[6]


  1. ^ Evergren E, Benfenati F, Shupliakov O (September 2007). "The synapsin cycle: a view from the synaptic endocytic zone". J. Neurosci. Res. 85 (12): 2648–56. doi:10.1002/jnr.21176. PMID 17455288.  
  2. ^ Rosahl TW, Geppert M, Spillane D, Herz J, Hammer RE, Malenka RC, Sudhof TC (1993). "Short-term synaptic plasticity is altered in mice lacking synapsin I". Cell 75: 661–670. doi:10.1016/0092-8674(93)90487-B. PMID 7902212.  
  3. ^ Kihara AH, Santos TO, Paschon V, Matos RJ, Britto LR (2008). "Lack of photoreceptor signaling alters the expression of specific synaptic proteins in the retina". Neuroscience 151: 995–1005. doi:10.1016/j.neuroscience.2007.09.088. PMID 18248909.  
  4. ^ Kao HT, Porton B, Hilfiker S, Stefani G, Pieribone VA, DeSalle R, Greengard P (December 1999). "Molecular evolution of the synapsin gene family". J. Exp. Zool. 285 (4): 360–77. PMID 10578110.  
  5. ^ Gitler D, Xu Y, Kao HT, Lin D, Lim S, Feng J, Greengard P, Augustine GJ (April 2004). "Molecular determinants of synapsin targeting to presynaptic terminals". J. Neurosci. 24 (14): 3711–20. doi:10.1523/JNEUROSCI.5225-03.2004. PMID 15071120.  
  6. ^ Ferreira A, Rapoport M (April 2002). "The synapsins: beyond the regulation of neurotransmitter release". Cell. Mol. Life Sci. 59 (4): 589–95. PMID 12022468.  


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