BMP15 in catfish testis: Cellular distribution, seasonal variation, and its role in steroidogenesis

Considering the absence of information on testicular growth factors in fishes, present study was aimed to elucidate the existence of BMP15, an important member of TGF-β superfamily, in the testis of a seasonally breeding freshwater catfish, Clarias batrachus and its role in regulation of testicular activities. The study demonstrated the expression of BMP15 in the somatic cells (Sertoli and interstitial cells) in fish testis. The expression varied with changing testicular activity; the expression was very high in the quiescent and early recrudescing testis coinciding with the renewal of spermatogonial cells. Expression then declined gradually with progression of spermatogenesis and steroidogenesis. Expression of BMP15 showed positive correlation with seasonally changing testicular 17β-estradiol but negatively with testicular testosterone and 11-ketotestosterone. In vitro treatment of testis with recombinant human BMP15 enhanced the production of estradiol-17β but concurrently suppressed the production of testosterone and 11-ketotestosterone in testis. Though BMP15 did not alter the expression of StAR protein in the testis, it promoted the expression of 3β-hydroxysteroid dehydrogenase and aromatase in fish testis. Thus the present study for the first time demonstrates that fish testis is capable of producing BMP15 and is expressed by the somatic cells unlike mammals wherein it is produced exclusively by germ cells. Study also suggests that BMP15 may modulate the testicular steroidogenesis by altering the expression of steroidogenic enzymes. BMP15 also appears to play crucial role in renewal of spermatogial cells by augmenting the testicular production of 17β-estradiol.