Differential distribution of vesicle associated membrane proteinisoforms in the mouse retina

[摘要] Purpose: Many proteins associated with synaptic vesicle exocytosisare differentially distributed among synapses in the retina andelsewhere in the central nervous system. The synapse-specificdistribution of these proteins and their isoforms is thought tocontribute to synapse-specific functional differences.Vesicle-associated membrane protein (VAMP, also known as synaptobrevin)is an integral synaptic vesicle membrane protein that is part of thefusion core complex needed for docking and fusing of synaptic vesiclesat the synaptic active zone. Two VAMP isoforms have been identified thatare considered to be synaptic, VAMP-1 and VAMP-2, however theirdistributions among the various synapses in the mammalian retina havenot been characterized.Methods: Single- and double-labeling immunocytochemistry was used toinvestigate the distribution of the synaptic VAMP isoforms, VAMP-1 andVAMP-2, in the mouse retina.Results: VAMP-2 was the predominant isoform in both synaptic layers.Double-labeling studies using conventional and ribbon-synapse-specificmarkers showed that VAMP-2 was broadly distributed among conventionaland ribbon synapses. In contrast, the distribution of VAMP-1 was verylimited. In the outer retina, only weak labeling was present inphotoreceptor terminals. In the inner retina, labeling for VAMP-1 wasfound in the dendrites, cell bodies, and axons of some ganglion cells,as demonstrated by double labeling with the ganglion cell markers,microtubule-associated protein-1 and Brn-3a. VAMP-1 labeling did notcolocalize with amacrine or bipolar cell markers, nor did it colocalizewith other pre-synaptic markers, suggesting that VAMP-1 is notassociated directly with neurotransmitter release in the inner retina.Labeling for VAMP-1 identified a set of large ganglion cells thatramified in the mid-IPL (inner plexiform layer), suggesting that theymay show ON-OFF responses. Some of these cells had cell bodies displacedto the inner nuclear layer. The dendrites of the largeVAMP-1-immunoreactive ganglion cells did not co-stratify with thecholinergic plexuses of the starburst amacrine cells (labeled forcholine acetyltransferase) and therefore are unlikely to showdirectional selectivity. However, these cells are likely to receiveinput from bipolar cells and a population of putative glutamatergicamacrine cells.Conclusions: VAMP-1 and VAMP-2 are differentially distributed amongthe synapses of the mouse retina. VAMP-2 is the predominant isoform andis widely expressed at ribbon and conventional synapses in bothplexiform layers. VAMP-1 expression in the mouse retina is much morelimited and is not restricted to presynaptic terminals. In the OPL,VAMP-1 is co-expressed with VAMP-2 presynaptically in photoreceptorterminals. However, VAMP-1 expression in the IPL is associated withganglion cells and does not appear to be localized to presynapticterminals. VAMP-1 is a specific marker for a set of large ganglion cellsand displaced ganglion cells that ramify in the mid-IPL and are likelyto have ON-OFF physiology.

[学科分类] 生物化学/生物物理