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D.R. Copenhagen, R.C. Renteria, E.E. Strehler, P.G. Gillespie, D. Krizaj; Plasma Membrane Calcium ATPases 1b and 4b Catalyze Calcium Extrusion from Photoreceptor Synaptic Terminals . Invest. Ophthalmol. Vis. Sci. 2003;44(13):4157.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose: Calcium extrusion is critical for controling retinal photoreceptor transmitter release. Photoreceptors exclusively use plasma membrane calcium ATPases (PMCAs) for extrusion of calcium from their synaptic terminals. All four PMCA isoforms are subject to alternative splicing that yields major variants: "a" (containing additional exons and an altered C-terminal tail) and "b" (lacking all extra exons). The alternative splicing causes these molecules to have different calmodulin binding properties and different regulatory phosphorylation sites and confers PDZ-protein binding to the "b" variants. Here we identify which PMCA splice variants are expressed in photoreceptor synaptic terminals. Methods: We used antibodies that recognize the PMCA isoforms, the "b" variants of several isoforms, and the "a" variants of specific isoforms, along with various other retinal markers, to localize proteins in both Long-Evans adult rat and larval tiger salamander retinas. Antibodies were visualized after immunohistochemistry using a confocal microscope. Results: PMCA1 is prominently expressed in photoreceptor terminals in the outer plexiform layer (OPL), with lower expression in cone bipolar cells. The PMCA1a antibody staining pattern did not have the prominent terminal band in the OPL but did show low expression in cone bipolar cells. Two antibodies, each recognizing the "b" variants of several PMCA isoforms (i.e., 1b, 2b, and 3b), showed prominent staining of photoreceptor terminals. This is a result primarily of PMCA1b expression because these terminals show only low PMCA2 and no PMCA3 expression. PMCA2a was not present in the OPL. A PMCA4 antibody labels photoreceptor terminals, many types of bipolar cell, and several bands in the inner plexiform layer; a PMCA4a antibody, however, does not label the OPL terminals. PSD95, containing PDZ domains and known to be able to interact with the PMCA "b" variants, is expressed presynaptically in the same photoreceptor terminals. Conclusions: These data suggest that PMCA1b and PMCA4b are responsible for the PMCA-catalyzed calcium extrusion that occurs at photoreceptor terminals. PSD95 in the photoreceptor terminals may serve to localize the PMCA "b" variant proteins to the terminals and link them to the presynaptic release machinery.
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