Retinoic acid receptors and retinoid X receptors in the mature retina: Subtype determination and cellular distribution

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Abstract
PURPOSE. In the mature retina retinoic acid (RA) biosynthesis was reported to be regulated by light. RA is of vital importance for proper function of the retina. RA activity is mediated by interaction with nuclear retinoic acid receptors (RARs) and retinoid X receptors (RXRs). The purpose of this study was to determine if and which RARs and RXRs are present in the mature retina, and to determine their location within the retina. METHODS. The reverse transcriptase polymerase chain reaction (RT-PCR) was used to amplify cDNA fragments encoding RARa, RARß, RAR?, RXRa, RXRß, and RXR? from human retinal RNA. RT-PCR products were cloned, sequenced, and used in Northern blot experiments. Antibodies directed against each receptor subtype were used for immunocytochemical analysis. RESULTS. RT-PCR and Northern blot analysis indicated that all RAR and RXR subtypes are present in the mature retina. Western blot analysis, using a cytoplasmic protein fraction isolated from the bovine and human neural retina, showed the presence of RXRa. Immunocytochemical analysis of the human, bovine, and rat retina showed that RARa is highly expressed in the outer segments of cone photoreceptor cells. RXRa expression was observed in the rod inner segment layer. RXR? was detected in the nuclei and outer segments of cone photoreceptor cells. CONCLUSIONS. The retinal expression pattern of RARs and RXRs is heterogeneous. The observation that RXRa is present in rods whereas RARa is present in cones, suggests a mechanism by which RA that is produced upon bleaching, could generate different responses in the two photoreceptor cell subtypes.