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N.J. Mangini, B.G. Kennedy; Expression of the Calcium–Sensing Receptor (CaSR) in Retinal Pigment Epithelium (RPE) . Invest. Ophthalmol. Vis. Sci. 2006;47(13):2873.
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The retinal pigment epithelium (RPE) is a transporting epithelial monolayer that controls hydration and composition of the subretinal space (SRS). The calcium–sensing receptor (CaSR) is a G–protein–coupled plasma membrane receptor whose ligand is extracellular calcium. Calcium concentration in SRS varies with light/dark transitions, and the CaSR, if expressed in the RPE, could modulate RPE transport in response to these changes. The present study investigated the expression of CaSR in RPE.
CaSR expression was assessed by performing RT–PCR on total RNA isolated from primary cultures of human RPE, the established RPE cell line, ARPE19, and isolates of native bovine RPE. cDNAs were amplified using CaSR–specific primers and the products confirmed by direct sequencing. CaSR–specific antibodies were used in Western blotting studies to assess CaSR protein expression in cultured human RPE, in isolates of native human, bovine, and pig RPE and in the RPE cell line ARPE19. The same antibodies were used to localize the protein in formaldehyde–fixed frozen sections of native RPE/choroid by immunohistochemistry.
RT–PCR analysis of human and bovine RPE yielded PCR DNAs of 466 bp that encoded a fragment of CaSR with sequences homologous to published sequences. Western blotting documented protein expression. Reactivity was detected on immunoblots of isolates from native human and bovine RPE, isolates from primary cultures of human RPE and in isolates from ARPE19 cells. Immunocytochemistry of sections of native human and bovine RPE/choroid tissue showed that CaSR immunoreactivity was predominantly –– though not exclusively –– apical.
RPE expresses the CaSR. Presence of this receptor on the apical RPE surface supports the hypothesis that CaSR functions to monitor changes in SRS calcium concentration possibly regulating RPE transport activity in a feedback mechanism to control calcium levels in SRS.
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