Synapsins
Introduction to Synapsins
The nervous system endows every organism with the ability to perceive and appropriately respond to external signals. When the body receives different stimuli, our brains integrate and process information through complex systems of synapses and neurons. Therefore, the molecular mechanism of regulating synaptic function has become a hot spot in the field of neuroscience. Studies have shown that synapses are made up of the presynaptic and postsynaptic compartments, which are connected by different kinds of neurons. In addition, the functions of many members of the synaptic protein family have aroused much attention due to their essential role in synaptic transmission and plasticity. The synapsin family consists of 10 homologous proteins: Syns Ia-b, IIa-b, as well as IIIa-f. In mammals, different subtypes of synapsins are formed by specific gene alternative splicing. For example, SYN1, SYN2, and SYN3 genes have been identified on human chromosome X(XP11), human chromosome 3(3p25), and mice chromosome 22(22q12.3), respectively.
Fig.1 Molecular interactors of synapsins. (Cesca, 2010)
The Structure of Synapsins
Synapsins consist of a highly homologous N-terminal region and multiple variable C-terminal regions. The N-terminal region is composed of three main domains, domain A, B, and C. The C-terminal regions contain various spliced domains, such as domain D, domain E, domain F, and domain I. Among them, domain A is shared by all synaptic proteins. It generally includes highly conserved PKA phosphorylation sites (site 1), CaMKI as well as IV. Domain B is considered to be a connecting region between domain A and domain C. It consists of phosphorylation sites 4 and 5, mitogen-activated protein kinase, and extracellular signal-regulated kinase (Erk). The domain C is a large amino acid region with a size of about 300aa. These sequences are usually highly charged and hydrophobic, and play a critical role in maintaining α-helix and β-slice conformations of synapsins. Domain D can bind to a battery of SH3-containing proteins and several types of protein kinase, such as CaMKII.
Synapsins in the Neurotransmitter Release
Identifying the synapse proteins is one of the key issues in revealing the mechanism of neurotransmitter release. Pilot studies have demonstrated that many synapsins located at the end of the synapse are thought to be involved in the management of synaptic vesicle cycling. In general, synapsins are one of the most abundant neuron-specific proteins in synaptic vesicles. Moreover, synapsins can interact with a series of synaptic vesicle-associated proteins to help stabilize the bilayer structure of phospholipids and maintain vesicle integrity and size uniformity. Furthermore, recent studies have indicated that synapsins can be treated as ATP-using enzymes that can bind ATP with high affinities. Besides, synapsins can mediate the process of vesicle exocytosis. Meanwhile, disruption of synapsin function may increase the release of neurotransmitters. As a result, a group of synapsin-deficient models has been generated for selectively reducing the number of vesicles that are far from the plasma membrane.
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Reference
- Cesca, F.; et al. The synapsins: key actors of synapse function and plasticity. Progress in neurobiology. 2010, 91(4): 313-348.
