Characterization of a direct role of GnRHs in the control of spermiogenesis and steroidogenesis in the small-spotted catshark Scyliorhinus canicula.

Hypothalamic gonadotropin-releasing hormone (GnRH) regulates the production of gonadotropins, which control reproduction. In elasmobranchs, unlike other gnathostomes, GnRH is released into the systemic circulation to stimulate gonadotrope cells located in the ventral lobe of the pituitary. The aim of this study was to investigate the potential role of systemic GnRH in the regulation of the testis in Scyliorhinus canicula. Phylogeny and synteny analyses identified three GnRHs and four GnRH receptor (ScGnRHR-I1, -IIa1, -IIa2 and -IIb2). In vitro functional hormone-receptor interactions using synthetic ScGnRHs showed that all ScGnRHs were effective at receptors, except ScGnRHRIIa2, at femtomolar to nanomolar concentrations, with lower efficiency for ScGnRH1/ScGnRHRIIb2. Real-time PCR analyses in a wide range of tissues, including male and female reproductive tracts, showed that all three gnrh were expressed mainly in the brain and all four gnrhr were expressed in the testis, particularly during spermiogenesis. Testicular explants containing cysts with spermatids were treated with ScGnRHs and their protein content analyzed by NanoLC-ESI-MS/MS, highlighting 1677 significantly differentially expressed proteins. Among them, the growth hormone receptor (GHR) and proteins involved in cholesterol and steroid metabolism, including several HSD17bs, were upregulated. In situ hybridization showed that ghr, hsd17b3 and hsd17b12 transcripts were localized in Sertoli cells, which are the main testicular steroidogenic cells in S. canicula. Fifteen steroids were assayed in the culture media, using LC-ESI-HRMS/MS, and an increase in 17β-estradiol concentrations was observed, consistent with hsd17b expressions. Furthermore, proteins involved in transcription and DNA structure were downregulated in response to GnRHs. In conclusion, this study showed that ScGnRHs may play a direct role in the regulation of elasmobranch testes by promoting spermiogenesis and modulating steroidogenesis.