Transgenic mutant D567G but not wild-type human FSH receptor overexpression provides FSH-independent and promiscuous glycoprotein hormone Sertoli cell signaling.

We have characterized the in vivo actions of human wild-type FSH receptor (FSHR) overexpressed in Sertoli cells of transgenic (Tg) mice (TgFSHRwt) compared with transgenic overexpression of the human activated mutant FSHRD567G (TgFSHRD567G). Testicular TgFSHRwt expression significantly elevated specific FSH binding (>2-fold, P < 0.01) relative to nontransgenic testes, similar to increased FSH binding in TgFSHRD567G testes. Isolated TgFSHRwt Sertoli cells exhibited higher FSH-stimulated cAMP levels compared with non-Tg or TgFSHRD567G cells but did not display the elevated FSH-independent basal cAMP levels found in TgFSHRD567G Sertoli cells. Furthermore, Sertoli cell overexpression of TgFSHRD567G but not TgFSHRwt allowed promiscuous cAMP responses to human chorionic gonadotropin (300 IU/ml) and TSH (30 mIU/ml), demonstrating increased constitutive signaling and altered glycoprotein hormone specificity via the intracellular D567G substitution rather than FSHR overexpression. Despite elevating Sertoli cell FSH sensitivity, overexpression of TgFSHRwt had no detectable effect upon normal testis function and did not stimulate Sertoli and germ cell development in testes of gonadotropin-deficient hypogonadal (hpg) mice, in contrast to the increased meiotic and postmeiotic germ cell development in TgFSHRD567G hpg testes. Increased steroidogenic potential of TgFSHRD567G hpg testes was demonstrated by elevated Cyp11a1 and Star expression, which was not detected in TgFSHRwt hpg testes. Androgen-regulated and Sertoli cell-specific Rhox5 gene expression was increased in TgFSHRD567G but not TgFSHRwt hpg testes, providing evidence of elevated LH-independent androgen activity due to mutant FSHRD567G. Hence, transgenic FSHR overexpression in Sertoli cells revealed that the D567G mutation confers autonomous signaling and steroidogenic activity in vivo as well as promiscuous glycoprotein hormone receptor activation, independently of FSHR overexpression alone.