Vesicle-associated membrane protein isoforms in the tiger salamander retina

Abstract

Vesicle associated membrane protein (VAMP; also known as synaptobrevin) is a key component of the core complex needed for docking and fusion of synaptic vesicles with the presynaptic plasma membrane. Recent work indicates that the precise complement of presynaptic proteins associated with transmitter release and their isoforms vary among synapses, presumably conferring specific functional release properties. The retina contains two types of vesicular synapses with distinct morphologic, functional, and biochemical characteristics: ribbon and conventional synapses. Although the precise complement of presynaptic proteins is known to differ between conventional and ribbon synapses and among conventional synapses, the distribution of VAMP isoforms among retinal synapses has not been determined. The expression and localization of VAMP isoforms in the salamander retina, a major model system for studies of retinal circuitry, was examined by using immunocytochemical and immunoblotting methods. Both methods indicated that at least two VAMP isoforms were expressed in salamander retina. One isoform, recognized by an immunoglobulin M antibody that recognizes both mammalian VAMP-1 and VAMP-2, was associated with photoreceptor and bipolar cell terminals as well as many conventional synapses, and probably corresponds to mammalian VAMP-2. A different VAMP isoform associated with a subset of amacrine cells, was recognized only by antibodies directed against the N-terminus of mammalian VAMP-2. An antiserum directed against the N-terminus of mammalian VAMP-1 did not specifically recognize any salamander VAMPs in either immunocytochemical or immunoblotting experiments. Heterogeneous distribution of VAMP isoforms among conventional retinal synapses was confirmed by double labeling for synapsin I, a marker for conventional synapses. These studies indicate that VAMP isoforms are expressed heterogeneously among retinal synapses but cannot account for the differences in transmitter release characteristics at ribbon and conventional synapses. These results also corroborate previous studies in Xenopus indicating that the N-terminus of nonmammalian VAMP isoforms differs from their mammalian counterparts.

Cellular distribution and subcellular localization of molecular components of vesicular transmitter release in horizontal cells of rabbit retina

“…Moreover, other synaptic proteins have been localized to the OPL and horizontal cells in the mammalian retina, including synaptoporin (synaptophysin II) and SNAP-25 (Catsicas et al, 1992;Ullrich and Sü dhof, 1994;Grabs et al, 1996;von Kriegstein et al, 1999;West Greenlee et al, 2001). At present, the synaptobrevin (VAMP) isoform, the third component of the SNARE complex, which is present in mammalian horizontal cells, has not yet been identified, although those typically found synaptically do not appear to be present (Sherry et al, 2003b).…”

Synaptic transmission at retinal ribbon synapses

“…Syntaxin 3 is also present in insulin-secreting β cells where it inhibits L-type Ca 2+ channels and insulin secretion (Kang et al, 2002). The other components of the mininal core fusion complex, SNAP-25 and synaptobrevin, are now believed to be present in retinal ribbon synapses, despite earlier controversies (Von Kriegstein et al, 1999;Sherry et al, 2001Sherry et al, , 2003b. In mouse retina, the synaptobrevin isoform VAMP2 is expressed at both conventional and ribbon synapses.…”

“…In mouse retina, the synaptobrevin isoform VAMP2 is expressed at both conventional and ribbon synapses. VAMP1, however, is limited in distribution and is present in terminals of photoreceptors and a subset of ganglion cells, but not in bipolar cells (Sherry et al, 2003b). Thus, all retinal neurons, including those that utilize synaptic ribbons, possess the minimal components necessary for SNARE-mediated fusion.…”

“…Many other synaptic vesicle-associated proteins including SV2B, rab3, SCAMP1, synaptogyrin, Munc18 and synaptophysin1 are present on synaptic vesicles of photoreceptors and bipolar cells (Von Kriegstein et al, 1999;Von Kriegstein and Schmitz, 2003;Sherry et al, 2001Sherry et al, , 2003bMorgans et al, 1996;Morgans, 2000;Brandstatter et al, 1996a, b;Wang et al, 1997;Ullrich and Sudhof, 1994). A small number of vesicle proteins appear to be absent from some or all ribbon synapses.…”

“…Synaptic vesicles in both photoreceptor and bipolar cell terminals contain the neutrotransmitter glutamate, which is packed into the vesicle by the vesicular glutamate transporter type I (Sherry et al, 2003b;Johnson et al, 2003a). The concentration of glutamate in the vesicle is estimated to be on the order of 100 mM (Burger et al, 1989).…”