Nodal regulates ovarian functions in zebrafish

Nodal, a member of the transforming growth factor-β (TGF-β) superfamily, plays critical roles during embryo development. Several studies suggest that Nodal also regulates reproduction. The objective of this study was to investigate if Nodal is expressed in zebrafish ovary and if it is involved in the regulation of ovarian functions. Using real-time PCR, we detected two Nodal homologs, nodal-related (ndr)1, and ndr2 in zebrafish ovarian follicles. We further compared the mRNA levels of ndr1, ndr2, and their receptors between maturational incompetent early vitellogenic follicles (stage IIIa) and mid- to late-vitellogenic follicles (stage IIIb) which are capable of undergoing maturation when they are induced by hormones. We found that mRNAs for ndr1 and ndr2, as well as a type I receptor, acvr1ba, were significantly increased in follicular cells isolated from stage IIIb follicles. In primary cultures of ovarian follicular cells, treatment with recombinant human Nodal inhibited cell proliferation. On the other hand, Nodal increased the mRNA levels of two steroidogenic enzymes hsd3b2 and cyp17a1, as well as paqr8, which encodes the membrane progestin receptor-β (mPR-β). Conversely, knockdown of ndr1 and ndr2 using siRNAs decreased the mRNA levels of hsd3b2, cyp17a1, and paqr8. Finally, treatment of Nodal significantly induced oocyte maturation. Taken together, these findings suggest that Nodal exerts multiple effects on zebrafish ovary to regulate follicle growth, steroidogenesis, and oocyte maturation.