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 a, b, and g. 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 deter-mined the cellular and subcellular localization of six retinoid receptors in the developing rat testis to identify the specific cel-lular sites and times of receptor expression. Immunohistochem-ical results revealed the expression of RARa, RARb, RXRa, and RXRg proteins in somatic and germ cells throughout postnatal development. In contrast, the expression of RARg and RXRb did not increase until 30–35 days of age in somatic cells from the testis. Interestingly, RARa and RXRa had a similar subcellular localization pattern in Sertoli cells throughout postnatal testis development, while RARa and RXRg were both present in the nucleus of spermatocytes and elongating spermatids. These re-sults suggest that RARa may potentially dimerize with RXRa in Sertoli cells and with RXRg in germ cells. In addition, we dem-onstrate that the only RAR in the nucleus of early meiotic germ cells is RARa