Involvement of pax2 in ovarian development and recrudescence of catfish: a role in steroidogenesis.

PAX2, a member of paired box family, is an essential transcription factor for the organ development in vertebrates including teleosts, yet no evidence has been shown for its involvement in reproduction. To study this, partial- and/or full-length cDNA of pax2 was isolated from the ovary of catfish, Clarias batrachus, along with its other Pax family members, pax1 and pax9 Tissue distribution and ontogeny expression analysis indicated the prevalence of pax2 but not pax1 and pax9 in ovary. Varied phase-wise expression during ovarian cycle and elevation of pax2 after human chorionic gonadotropin induction showed probable regulation by gonadotropins. Pax2 could be localized in various stages of oocytes and in follicular layer of vitellogenic and post-vitellogenic oocytes. To assess the functional significance of pax2, transient RNA silencing was performed using primary catfish ovarian follicle culture, in vitro, and in catfish, in vivo, through ovary-targeted injection of PEI-esiRNA. Pax2 siRNA treatment reduced the expression of various transcripts related to ovarian development like signaling molecules such as wnt4 and wnt5, estrogen receptors, several steroidogenic enzymes and transcription factors. These transitions in transcript levels might have been mediated by Pax2 acting upstream of wnt4/5 that may play a role in steroidogenesis and/or ovarian development along with ad4bp/sf-1 or by direct or indirect interaction with steroidogenic enzyme genes, which is evident from the change in the levels of serum estradiol-17β but not 17α,20β-dihydroxy-4-pregnen-3-one. Taken together, it seems that pax2 has a plausible role during ovarian development and/or recrudescence of catfish either directly or indirectly through Wnt signaling pathway.