Fsh controls gene expression in fish both independently of and through steroid mediation.

The mechanisms and the mediators relaying Fsh action on testicular functions are poorly understood. Unlike in mammals, in fish both gonadotropins (Fsh and Lh) are able to efficiently stimulate steroidogenesis, likely through a direct interaction with their cognate receptors present on the Leydig cells. In this context, it is crucial to understand if Fsh effects are mediated through the production of steroids. To address this issue we performed transcriptome studies after in vitro incubations of rainbow trout testis explants in the presence of Fsh alone or in combination with trilostane, an inhibitor of Δ4- steroidogenesis. Trilostane significantly reduced or suppressed the response of many genes to Fsh (like wisp1, testis gapdhs, cldn11, inha, vt1 or dmrt1) showing that, in fish, important aspects of Fsh action follow indirect pathways and require the production of Δ4-steroids. What is more, most of the genes regulated by Fsh through steroid mediation were similarly regulated by Lh (and/or androgens). In contrast, the response to Fsh of other genes was not suppressed in the presence of trilostane. These latter included genes encoding for anti-mullerian hormone, midkine a (pleiotrophin related), angiopoietine-related protein, cyclins E1 and G1, hepatocyte growth factor activator, insulin-like growth factor 1b/3. A majority of those genes were preferentially regulated by Fsh, when compared to Lh, suggesting that specific regulatory effects of Fsh did not depend on steroid production. Finally, antagonistic effects between Fsh and steroids were found, in particular for genes encoding key factors of steroidogenesis (star, hsd3b1, cyp11b2-2) or for genes of the Igf system (igf1b/3). Our study provides the first clear evidence that, in fish, Fsh exerts Δ4-steroid-independent regulatory functions on many genes which are highly relevant for the onset of spermatogenesis.