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

Abstract

Chorionic gonadotropin α (Cgα) functions as the shared subunit for thyroid-stimulating hormone subunit β (Tshβ), luteinizing hormone subunit β (Lhβ), and follicle-stimulating hormone subunit β (Fshβ). While these β-subunits have been extensively studied using effective gene knockout models in zebrafish, the biological role of Cgα remains elusive. In this study, cgα-deficient zebrafish generated via transcription activator-like effector nucleases (TALENs) exhibited viability but displayed pronounced developmental abnormalities, including growth retardation, hyperpigmentation, reduced thyroxine (T4) levels, and defective anterior swim bladder inflation during juvenile stages. In adults, cgα deficiency led to disrupted gonadal development, impaired secondary sex characteristics (SSCs), and severely impacted reproductive behavior in both female and male fish. Notably, both testicular and ovarian differentiation were observed in cgα-deficient fish and lhβ ^-/- ; fshβ ^-/- mutants. Gonadal sex differentiation in cgα-deficient zebrafish exhibited a pronounced shift toward testicular fate upon additional disruption of fshβ ( cgα ^-/-; fshβ ^-/-), marked by elevated anti-Müllerian hormone ( amh) expression, or following loss of follicle-stimulating hormone receptor ( fshr) ( cgα ^-/-; fshr ^-/-). In vitro assays in Chinese hamster ovary (CHO) cells revealed increased cAMP response element (CRE) promoter activity following transfection with constructs encoding Fshr, Fshβ/Fshr, or Cgα/Fshβ/Fshr. Collectively, the phenotypes observed in cgα-deficient fish recapitulate those of thyrotropin- and gonadotropin-disrupted models, highlighting the essential role of Cgα in thyroid and gonadal function. Importantly, these findings uncover the role of Fsh signaling in maintaining proper ovarian differentiation in zebrafish, including Cgα-independent Fshβ activity and the constitutive functionality of Fshr.