Synaptic reserve pool

One characteristic of regulated exocytosis is the ability to store secretory vesicles in a reserve pool for utilization upon stimulation. In the presynaptic terminal, this principle is expanded to define multiple pools of synaptic vesicles a ready releasable pool, a recycled synaptic vesicle pool and a larger reserve pool. This reserve pool assures that neurotransmitter is available for release in response to even the highest physiological demands. Neurons can fire so many times per minute because synaptic vesicles from the ready releasable pool at a given synapse undergo exocytosis in response to a single action potential. 

Those vesicles have been primed by docking at the active zone and are therefore ready for exocytosis upon arrival of an action potential. However, for the synapse to respond rapidly and repeatedly under heavy physiological demand, these exocytosed vesicles must be rapidly replaced. This is accomplished first from the recycled pool of vesicles and, as the demand increases, from the reserve pool. To be recycled, synaptic vesicles must be reloaded quickly after they release their contents. The sequence of events that is triggered by neurotransmitter exocytosis is known as the synaptic vesicle cycle [73,74] (Fig. 9-8). 

FIGURE 2 3-7 Schematic diagram of the synaptic vesicle cycle. Neurotransmitter-filled vesicles held in the reserve pool are trafficked to a readily releasable pool where they are docked, primed and fused with the plasmalemma at the synaptic cleft. Also depicted is the clathrin-mediated endocy-tosis of the fused vesicles, which is followed by their uncoating and recycling via early endosomal fusion and budding of vesicles. This returns the vesicles to the reserve pool. Some of the phosphoproteins which regulate these steps are shown. For a more detailed description of this process and the phosphoproteins involved the reader is directed to the excellent text by Cowen et al. [67]. 

Inactivation of the a-synuclein gene by homologous recombination results in mice that appear largely normal [3]. Analysis of mice lacking y-synuclein has similarly failed to reveal any gross abnormalities [4]. In hippocampal slices from mice without a-synuclein, the replenishment of docked vesicles by reserve pool vesicles was slower than in slices from control mice. It suggests a physiological role for a-synuclein in the mobilization of synaptic vesicles. 

In addition to the proteins discussed above, neuronal SNAREs were reported to interact with numerous other proteins in a specific manner, but in most cases both the structural basis and the biological function of these interactions need to be defined. For instance, synaptophysin, a membrane protein of synaptic vesicles, forms a complex with synaptobrevin in which synaptobrevin is not available for interactions with its partner SNAREs syntaxin 1A and SNAP-25, suggesting that this complex represents a reserve pool of recruitable synaptobrevin (Becher et al. 1999) or regulates interactions between the vesicle-associated synaptobrevin and the plasmalem-mal SNAREs. Alternatively, it has been suggested that this complex is involved in synaptobrevin sorting to synaptic vesicles. 

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.

Dysbindin-1 is enriched in presynaptic fields at diverse locations in the CNS as described earlier (see O Section 2.2.63.23). Where investigated with immunoEM, the protein in those fields is localized primarily to synaptic vesicles (e.g.,0 Figures 2.2-16-2.2-18, Talbot et al., 2006). That maybe explained by the finding discussed above that dysbindin-1 and at least three other BLOC-1 members are peripheral membrane components of synaptic vesicles and PC-12 cell SVLMs derived from endosomes by an AP-3 mediated process (cf. Salazar et al., 2005b, 2006). These probably constitute the reserve pool of synaptic vesicles (Voglmaier and Edwards, 2007), which is estimated to include 80-90% of all synaptic vesicles in an axon terminal (Rizzoli and Betz, 2005). Loss of dysbindin-1 in homozygous sdy mice leads to a significant loss in the size of the reserve pool (Chen et al., 2008). A preferential localization of BLOC-1 in the reserve pool may... 

The third and largest synaptic vesicle pool is termed the reserve pool and does not contribute to neurotransmitter release under normal physiological conditions. It is proposed that reserve pool vesicles are only recruited with extremely intense extended bouts of synaptic stimulation, conditions under which the recycling pool of vesicles is depleted (17). When vesicle pool sizes are expressed as percentages of the total synaptic vesicle cluster, these percentages hold well across many synapse... 

Two pools of neurotransmitter-filled synaptic vesicles are present in axon terminals those docked at the plasma membrane, which can be readily exocytosed, and those in reserve in the active zone near the plasma membrane. Each rise in Ca triggers exocytosls of about 10 percent of the docked vesicles. Membrane proteins unique to synaptic vesicles then are specifically internalized by endocytosis, usually via the same types of clathrln-coated vesicles used to recover other plasma-membrane proteins by other types of cells. After the endocytosed vesicles lose their clathrln coat, they are rapidly refilled with neurotransmitter. The ability of many neurons to fire 50 times a second is clear evidence that the recycling of vesicle membrane proteins occurs quite rapidly. 

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