A progestin (17α,20β-dihydroxy-4-pregnen-3-one) stimulates early stages of spermatogenesis in zebrafish

Recently, evidence has been provided for multiple regulatory functions of progestins during the late mitotic and meiotic phases of spermatogenesis in teleost fish. For example, our previous studies suggested that 17α,20β-dihydroxy-4-pregnen-3-one (DHP), potentially via Sertoli cells that express the progesterone receptor (pgr) gene, can contribute to the regulation of zebrafish spermatogenesis. To further our understanding of the function of DHP at early spermatogenetic stages, we investigated in the present study the expression of genes reflecting Sertoli cell function and spermatogenic development in adult zebrafish testis after DHP treatment in tissue culture. Moreover, using an in vivo model of estrogen-mediated down-regulation of androgen production to interrupt adult spermatogenesis, we studied the effects of DHP on estrogen-interrupted spermatogenesis. In this model, DHP treatment doubled the testis weight, and all differentiating germ cell types, such as type B spermatogonia and primary spermatocytes, were abundantly present and incorporated the DNA-synthesis marker (BrdU). Accordingly, transcript levels of germ cell marker genes were up-regulated. Moreover, transcripts of two Sertoli cell-derived genes anti-müllerian hormone (amh) and gonadal soma-derived growth factor (gsdf) were up-regulated, as were three genes of the insulin-like growth factor signaling system, insulin-like growth factor 2b (igf2b), insulin-like growth factor 3 (igf3) and insulin-like growth factor 1b receptor (igf1rb). We further analyzed the relationship between these genes and DHP treatment using a primary zebrafish testis tissue culture system. In the presence of DHP, only igf1rb mRNA levels showed a significant increase among the somatic genes tested, and germ cell marker transcripts were again up-regulated. Taken together, our results show that DHP treatment induced the proliferation of early spermatogonia, their differentiation into late spermatogonia and spermatocytes as well as expression of marker genes for these germ cell stages. DHP-mediated stimulation of spermatogenesis and hence growth of spermatogenic cysts and the associated increase in Sertoli cell number may in part explain the elevated expression of Sertoli cell genes, but our data also suggest an up-regulation of the activity of the Igf signaling system.