LOCALIZATION OF CELLULAR RETINOIC ACID-BINDING PROTEIN (CRABP)II AND CRABP IN DEVELOPING RAT TESTIS

Retinoic acid (RA) has been implicated as a signaling molecule for the morphogenesis of some tissues and organs. The morphogenesis of the ra t testis occurs relatively late in development, culminating in puberty . Two members of the superfamily of small intracellular carrier protei ns for lipophilic compounds are cellular RA-binding protein (CRABP) an d cellular RA-binding protein II (CRABP-II). Both CRABP and CRABP-II a re present at various sites in the developing mouse embryo. Here we re port the developmental expression and localization of CRABP and CRABP- II in rat testis. Northern blot analysis of CRABP-II demonstrated the highest messenger RNA expression on day 4 (the earliest time point ass ayed by this technique), decreasing thereafter until day 20, when it b ecame undetectable, Western blot analysis, begun on day 19 of fetal de velopment, indicated that high levels of protein expression in the tes tis already existed at that time. CRABP messenger RNA expression reach ed its highest levels between postnatal days 16-20 and decreased there after. Immunolocalization revealed that CRABP-II was confined to the f etal population of Leydig and Sertoli cells. We observed that CRABP-II was expressed in certain cells that synthesized retinoic acid in the uterus and ovary (unpublished). The expression of CRABP-II in Sertoli cells and fetal Leydig cells suggested that these cells may well be th e site of RA synthesis in the developing testis. CRABP was localized t o gonocytes in earlier stages and spermatogonia later, where it was cl early excluded from the nucleus, indicating that the role of CRABP may be to protect these cells from the effects of RA. The reported expres sion of CRABP-II in embryonal tissues, which are RA responsive and und ergoing morphogenesis, coupled with CRABP-II expression in the testis at a critical morphogenic stage suggest that RA may play a prominent r ole in the morphogenesis of the testis.