Synapsin

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.

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. 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.

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

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. Synapsin Ia has been implicated in bipolar disorder and schizophrenia.