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C. L. Decatur, V. M. Lopez, W. D. Stamer, R. M. Lynch, B. S. McKay; l-DOPA Is the Endogenous Ligand of OA1. Invest. Ophthalmol. Vis. Sci. 2008;49(13):5554.
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© ARVO (1962-2015); The Authors (2016-present)
OA1 is an orphan G-protein coupled receptor likely to control ocular pigmentation. No ligand has previously been identified, thus, the function of OA1 in signaling pathways in RPE remains unknown. In this study we hypothesized that OA1 and tyrosinase function in an autocrine loop accounting for the similarity among the various forms of albinism.
The human OA1 coding sequence was cloned from pigmented RPE, then transfected into CHO cells. Radioligand binding studies were used to characterize the interaction between l-DOPA and OA1. The ability of l-DOPA to stimulate OA1 function and signaling as a GPCR was investigated through second messenger assays. The release of intracellular calcium was measured using fluorescence ratiometry, and cAMP was quantified through direct binding.
The OA1 receptor bound l-DOPA in a single-site, saturable manner, with a Kd of 9.35x10-6M. We observed no effect of OA1 signaling on cAMP levels in the cytosol. Second messenger activity in response to l-DOPA was observed by calcium imaging. In these assays, we observed a specific dose-dependent release of intracellular calcium stores into the cytoplasm upon l-DOPA treatment. Two related molecules, tyrosine and dopamine, elicited no second messenger response through OA1.
l-DOPA, a by-product of pigment synthesis, is the endogenous ligand of OA1. OA1 activation resulted in the release of intracellular calcium stores with no effect on cAMP, suggesting that OA1 signals through a Gq subunit. Two related molecules, tyrosine and dopamine, did not elicit OA1 signaling, indicating highly specific interaction between OA1 and l-DOPA. Our results suggest an autocrine loop between OA1 and tyrosinase, linked by l-DOPA.
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