Synapsins

Synapsins comprise a family of phosphoproteins that are found only in association with SSVs. Although they account for only about 9% of the total vesicular membrane protein they probably cover a large proportion of their surface. So far, synapsins la, Ib, Ila, Ilb and III, which are the products of different genes, have been identified. 

Much evidence supports this scheme. For example, neuronal depolarisation increases the amount of free synapsin in the cytosol and microinjection of CAM kinase II into the terminals of the squid giant axon or brain synaptosomes increases depolarisation-evoked transmitter release. By contrast, injection of dephosphorylated synapsin I into either the squid giant axon or goldfish Mauthner neurons inhibits transmitter release. 

Figure 4.11 Dephosphorylated synapsin, associated with SSVs, is thought to form a heteromeric complex with CAM kinase II (also partially embedded in the vesicular membrane) and actin filaments. An increase in intracellular Ca + triggers phosphorylation of S3mapsin I which dissociates from the vesicular membrane. This frees the vesicles from the fibrin microfilaments and makes them available for transmitter release at the active zone of the nerve terminal...

Greengard, P, Benfenati, F and Valtorta, F (1994) Synapsin I, an active-binding protein regulating synaptic vesicle traffic in the nerve terminal. In Molecular and Cellular Mechanisms of Neurotransmitter Release (Eds Starjne, L, Greengard, P, Grillner, S, Hokfelt, T and Ottoson, D), Raven Press, New York, pp. 31 5. 

Synapsins la, lb, Ha and lib Monotopic membrane proteins with common N-terminal domains, with phosphorylation sites for CaMKI and... 

Synaptobrevins (VAMPs) Synaptogyrin Synaptophysins PKA but diverge C-terminally. Synapsins Ia/b contain C-terminal phosphorylation sites for CaMKII and CDK 5. Interact with microfilaments, neurofilaments, microtubules, SH3 domains, calmodulin and annexin VI in vitro. Small-membrane proteins that are cleaved by tetanus toxin and by botulinum toxins B, D, F and G. Polytopic membrane protein that is tyrosine-phosphorylated. Function unknown. Polytopic membrane proteins, including synaptoporin, that are tyrosine-phosphorylated and bind to synaptobrevins. May regulate SNARE function... 

Ca2+,calmodulin-dependent May transiently associate with synaptic vesicles to phosphorylate synapsins and rabphilin-3A. May regulate various protein kinases I and II steps in neurotransmitter release. 

One proposed mechanism for targeting of organelles to terminals might have general implications. The synapsin family of phosphoproteins [20], which is concentrated in the presynaptic terminal, may be involved in targeting synaptic vesicles. Dephosphorylated synapsin binds tightly to both synaptic vesicles and actin microfilaments (MFs), while phosphorylation releases both of them. 

Dephosphorylated synapsin inhibits axonal transport of MBOs in isolated axoplasm, while phosphorylated synapsin at similar concentrations has no effect [21]. When a synaptic vesicle passes through a region rich in dephosphorylated synapsin, it may be cross-linked to the available MF matrix by synapsin. Such cross-linked vesicles would be removed from fast axonal transport and are effectively targeted to a synapsin- and MF-rich domain, the presynaptic terminal. 

McGuinness, T. L., Brady, S. T., Gruner, J. et al. Phosphorylation-dependent inhibition by synapsin I of organelle movement in squid axoplasm. /. Neurosci. 9 4138-4149, 1989. 

Terada, S., Tsujimoto, T., Takei, Y., Takahashi, T. and Hirokawa, N. Impairment of inhibitory synaptic transmission in mice lacking synapsin I. /. Cell Biol. 145 1039-1048, 1999. 

Some intracellular signal transduction molecules are reduced in schizophrenia. The release of neurotransmitters is regulated by a family of proteins that coordinate vesicular trafficking (see Ch. 9). Of these, the expression of complexin I and II appears to be decreased in prefrontal cortex and subfields of the hippocampal formation, and the ratio of complexin I to complexin II is elevated in the hippocampus [35], SNAP-25 (Synaptosomal Associated Protein, kDa 25) has inconsistently been found to be down-regulated in both these regions. Synapsin expression is also reduced, but more robust decrements have been observed in bipolar disorder (Ch. 55). 

There is evidence that a number of closely related phosphoproteins associated with the synaptic vesicles, called synapsins, are involved in the short-term regulation of neurotransmitter release. These proteins also appear to be involved in the regulation of synapse formation, which allows the nerve network to adapt to long-term passage of nerve impulses. 

Lastly, studies on the different polymorphic forms of the synapsins, that organize the mobilization of neurotransmitter vesicles thereby regulating neurotransmitter release, could account for some of the subtle changes in neurotransmission that occur in schizophrenia. However, to date linkage analysis studies have failed to reveal any positive associations between the various polymorphisms of the synapsin gene and schizophrenia. 

This enzyme [EC 2.7.1.123], also referred to as calcium/ calmodulin-dependent protein kinase type II, and micro-tubule-associated protein MAP2 kinase, catalyzes the reaction of ATP with a protein to produce ADP and an 0-phosphoprotein. The enzyme requires calcium ions and calmodulin. Proteins that can serve as substrates include vimentin, synapsin, glycogen synthase, the myosin light-chains, and the microtubule-associated tau protein. This enzyme is distinct from myosin light-chain kinase [EC 2.7.1.117], caldesmon kinase [EC 2.7.1.120], and tau-protein kinase [EC 2.7.1.135]. 

ATP -I- synapsin <1, 2> (<1> brain synapsin best substrate of chicken gizzard caldesmon kinase [1] <1> brain synapsin best substrate, phosphorylated at 950% the rate of caldesmon [5]) (Reversibility <1,2>... 

Synapsins la, lb, Ha, lib Rab3, rabphilin Cysteine string proteins (CSP) Synaptotagmins... 

Benfenati F, Ferrari R, Onofri F, Arcuri C, Giambanco I, Donato R. 2004. S100A1 codistributes with synapsin I in discrete brain areas and inhibits the F-actin-bundling activity of synapsin I. J Neurochem 89 1260-1270. 

---