SNARE Synaptotagmin

Upon activation, neurons begin trafficking TRPVl to the membrane where the receptors become activated, desensitized and then recycled to the intracellular compartments. Translocation of TRPVl to the cell membrane occurs via SNARE (snapin and synaptotagmin IX)-mediated exocytosis [37]. Broadly speaking, activation involves phosphorylation by protein kinases (most notably, protein kinase A [PKA] and C [PKC]) and desensitization involves de-phosphorylation by phosphatases (e.g. calcineurin) [38]. Among PKC isozymes, PKCp seems to be of particular importance [39]. 

The decisive element in exocytosis is the interaction between proteins known as SNAREs that are located on the vesicular membrane (v-SNAREs) and on the plasma membrane (t-SNAREs). In the resting state (1), the v-SNARE synaptobrevin is blocked by the vesicular protein synaptotagmin. When an action potential reaches the presynaptic membrane, voltage-gated Ca "" channels open (see p. 348). Ca "" flows in and triggers the machinery by conformational changes in proteins. Contact takes place between synaptobrevin and the t-SNARE synaptotaxin (2). Additional proteins known as SNAPs bind to the SNARE complex and allow fusion between the vesicle and the plasma membrane (3). The process is supported by the hydrolysis of GTP by the auxiliary protein Rab. 

Fig. 4 Stages in synaptic vesicle exocytosis. Putative intermediate steps on the molecular pathway to synaptic vesicle fusion. Vesicle delivery and tethering to the presynaptic membrane most likely involves Rab-proteins and their effectors. So far, the nature of a speculative docking complex (dc) is unclear, but docking appears to be independent from SNARE proteins. In the primed state, SNAREs have assembled into a complex probably stabilized by complexin (Cpx). The fusion reaction is arrested until the intracellular calcium concentration increases. The putative calcium sensor for fast neurotransmitter release, synaptotagmin 1 (Syt), binds to intracellular calcium and in turn triggers fusion by associating with the presynaptic membrane and interacting with the SNARE complex, thereby displacing complexin (Tang et al. 2006).

Rizo J, Chen X, Arac D (2006) Unraveling the mechanisms of synaptotagmin and SNARE function in neurotransmitter release. Trends Cell Biol 16 339-50 Rizo J, Sudhof TC (2002) Snares and Muncl8 in synaptic vesicle fusion. Nat Rev Neurosci 3 641-53... 

Fig. 5 GPCR regulation of exocytosis downstream of Ca2+-entry. (a) Sequence of steps leading from recruitment to maturation of synaptic vesicles from a reserve pool (RP) to a readily-releasable pool (RRP) displaying slow (asynchronous) and fast (synchronous highly Ca2+-sensitive pool, HCSP synaptotagmin 1 (SYT 1) supported) components, (b) Protein-protein interactions of SNARES (SYX, syntaxin SYB, synaptobrevin and SNAP-2s-7S complex) and major putative regulatory proteins. Phosphoproteins are shown in shaded boxes (phosphorylation sites for PKA and PKC are indicated where known) with phosphorylation-dependent interactions depicted by arrows (increase indicated by filled arrows decrease indicated by open arrows). Circle-end connectors indicate a phosphorylation-independent or as yet unspecified interaction. Potential effects of interactions at various points of the sequence in A are discussed in the text.

Although we have discussed specific synaptic vesicle-associated proteins in this chapter, it is important to recognize that these proteins do not function alone. Extensive protein-protein interactions are required for the orchestrated release of neurotransmitter at the synaptic cleft. SNARE proteins form a complex that must interact with synaptotagmins, synaptophysin, and other molecules for fusion and exocytosis to occur. Synapsins bind to multiple signaling molecules as well as the synaptic vesicle and actin, in order to... 

Figure 7.3. Mechanism of transmitter release, a Thepresynap-tic action potential opens voltage-gated Ca channels. Ca triggers exocytosis of neurotransmitters stored in piesynaptic vesicles. b Some proteins (out of mat r more) that are involved in exocytosis. Ca is involved at multiple stages. By binding to calmodulin (CaM), it promotes phosphorylation of synapsin, which primes the transmitter vesicle but does not immediately lead to exocytosis. Adhesion of primed vesicles to the presynap-tic membrane is mediated by synaptobrevin and other SNARE proteins. Synaptotagmin is activated directly by Ca and participates in the final step of secretiom...

Davis AF, Bai J, Fasshauer D, Wolowick Ml, Lewis JL, Chapman ER. Kinetics of synaptotagmin responses to Ca2 - - and assembly with the core SNARE complex onto membranes. Neuron 1999 24 363-376. 

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