Cellular and subcellular localization of six retinoid receptors in rat testis during postnatal development: identification of potential heterodimeric receptors.

Vitamin A is required in the testis for germ cell development. It acts through two families of retinoid receptors, retinoic acid receptors (RAR) and retinoid X receptors (RXR), each with three subtypes alpha, beta, and gamma. These receptors are postulated to dimerize and regulate the transcription of retinoid-responsive genes that are crucial for germ cell development. In this study, we determined the cellular and subcellular localization of six retinoid receptors in the developing rat testis to identify the specific cellular sites and times of receptor expression. Immunohistochemical results revealed the expression of RARalpha, RARbeta, RXRalpha, and RXRgamma proteins in somatic and germ cells throughout postnatal development. In contrast, the expression of RARgamma and RXRbeta did not increase until 30-35 days of age in somatic cells from the testis. Interestingly, RARalpha and RXRalpha had a similar subcellular localization pattern in Sertoli cells throughout postnatal testis development, while RARalpha and RXRgamma were both present in the nucleus of spermatocytes and elongating spermatids. These results suggest that RARalpha may potentially dimerize with RXRalpha in Sertoli cells and with RXRgamma in germ cells. In addition, we demonstrate that the only RAR in the nucleus of early meiotic germ cells is RARalpha.