Cellular localization and seasonal variation in BMP15 expression in ovary of the catfish Clarias batrachus and its role in ovarian steroidogenesis.

Despite important actions of BMP15 (a TGFβ superfamily member) in follicular development in vertebrates, studies are mostly limited to mammals. The folliculogenic processes in lower vertebrates, particularly, fishes are quite different from mammals. It was, therefore, decided to detect the presence of BMP15 in ovarian follicles and what are its role in folliculogenesis and steroidogenesis in catfish? BMP15 protein was detected in different cellular compartments of ovarian follicles of the catfish collected during different reproductive phases, using immunohistochemical and Western blotting methods, with concurrent measurement of ovarian steroids through ELISA. In vitro effects of rhBMP15 on ovarian steroids, expression and activities of steroidogenic enzymes and StAR were also analyzed using established immunoblotting and spectrophotometric methods. BMP15 was localized distinctly in the nest of oogonia, perinucleolar oocytes and in oocytes as well as follicular cells of the primary ovarian follicles, which started diminishing gradually with the progression of folliculogenesis in oocytes-II and finally it was greatly reduced in the oocytes-III (fully grown follicles). BMP15 expression in follicles showed negative correlation with ovarian steroid levels. Further BMP15 also inhibited steroids production by suppressing the expression and activities of ovarian 3β-HSD, 17β-HSD and aromatase, however, it did not influence the expression of StAR. The findings of study suggests that BMP15 help in maintaining the early-stage oocytes in catfish and inhibits follicular growth by reducing ovarian steroidogenesis through suppression of the expression and activities of steroidogenic enzymes.