Subfertility and reduced progestin synthesis in Pgrmc2 knockout zebrafish

Progestin receptor membrane component (Pgrmc1 & 2) is a heme-binding protein. Studies on Pgrmc1 have suggested possible roles in heme binding, activation of steroid-synthesizing P450s, along with binding and transferring of membrane proteins. However, the studies of Pgrmc1s paralog, Pgrmc2 are still lacking. In order to determine the physiologic function(s) of Pgrmc2, we generated a zebrafish mutant line (pgrmc2(-/-)). We found a reduction in both spawning frequency and the number of embryos produced in female pgrmc2(-/-). This subfertility is caused by reduced oocyte maturation (germinal vesicle breakdown, GVBD) in pgrmc2(-/-) in vivo. Nonetheless, oocytes from pgrmc2(-/-) had similar sensitivity to 17 alpha,20 beta-dihydroxy-4-pregnen-3-one (DHP, a maturation induced progestin in zebrafish) compared with wildtype (wt) in vitro. Therefore, we hypothesized that oocyte maturation tardiness found in vivo, could be due to lack of progestin in pgrmc2(-/-). Interestingly, we found significant reduced expression of hormones, receptors, and steroid synthesizing enzymes including lhcgr, egfra, ar, and esr2, cyp11a1 and hsd3b1. In addition, DHP levels in pgrmc2(-/-) ovaries showed a significant decrease compared to those in wt. In summary, we have provided a plausible molecular mechanism for the physiological functions of Pgrmc2 in the regulation of female fertility, likely via regulation of receptors and steroids in the ovary, which in turn regulates oocyte maturation in zebrafish.