Insights into the role of Fsh signaling in ovarian differentiation of chorionic gonadotropin α (cgα)-deficient zebrafish

Chorionic gonadotropin α (Cgα) is the common subunit of Thyroid-stimulating hormone subunit β (Tshβ), Luteinizing hormone subunit β (Lhβ), and Follicle-stimulating hormone subunit β (Fshβ). Recently, these β-subunits have been extensively studied using effective gene knockout strategies in zebrafish, but that of Cgα remains elusive. Here, we found that the cgα-deficient fish, which were generated using a transcription activator-like effector nucleases (TALENs) targeting procedure, were viable but exhibited retarded growth, darker pigmentation, reduced thyroxine (T4) levels, and abnormal anterior swimming bladder inflation at juvenile stages. The gonadal development, secondary sex characters (SSCs), and mating behaviors were severely impaired in both cgα-deficient female and male fish as examined at the adult stage. Intriguingly, both testicular and ovarian differentiation were observed in cgα-deficient fish and lhβ-/-;fshβ-/- fish. Gonadal sex differentiation of cgα-deficient fish was tipped towards testis after the additional fshβ-depletion (cgα-/-;fshβ-/- fish with amh upregulation) or fshr-depletion (cgα-/-;fshr-/- fish). Upregulated cAMP response element (CRE) promoter activity was observed in Chinese hamster ovary (CHO) cells transfected with expressing vectors of Fshr, Fshβ/Fshr and Cgα/Fshβ/Fshr in CHO cells. The phenotypes of cgα-deficient fish duplicate the observations in zebrafish via the thyrotropin and gonadotropin inactivation, respectively, highlighting the essential role of Cgα in thyroid and gonadal function. Most importantly, we uncovered the role of Fsh signaling in maintaining proper ovarian differentiation in cgα-deficient zebrafish, including the Cgα-independent function of Fshβ and the constitutive activity of Fshr.