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

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 ac id receptors (RARs) and retinoid X receptors (RXRs). The purpose of this st udy was to determine if and which RARs and RXRs are present in the mature r etina, and to determine their location within the retina. Methods. The reverse transcriptase polymerase chain reaction (RT-PCR) was u sed to amplify cDNA fragments encoding RAR alpha, RAR beta, RAR gamma, RXR alpha, RXR beta,and RXR gamma from human retinal RNA. RT-PCR products were cloned, sequenced, and used in Northern blot experiments. Antibodies direct ed against each receptor subtype were used for immunocytochemical analysis. Results. RT-PCR and Northern blot analysis indicated that all RAR and RXR s ubtypes are present in the mature retina. Western blot analysis, using a cy toplasmic protein fraction isolated from the bovine and human neural retina , showed the presence of RXR alpha. Immunocytochemical analysis of the huma n, bovine, and rat retina showed that RAR alpha is highly expressed in the outer segments of cone photoreceptor cells. RXR alpha expression was observ ed in the rod inner segment layer. RXR gamma was detected in the nuclei and outer segments of cone photoreceptor cells. Conclusions. The retinal expression pattern of RARs and RXRs is heterogeneo us. The observation that RXR alpha is present in rods whereas RAR alpha is present in cones, suggests a mechanism by which RA that is produced upon bl eaching, could generate different responses in the two photoreceptor cell s ubtypes.