The role of PKG in oocyte maturation of zebrafish

Recently the importance of cyclic guanosine monophosphate (cGMP) signaling pathway in oocyte maturation has been well demonstrated in several species. However, as the primary downstream effector of the cGMP signaling pathway, little is known on the role of cGMP-dependent protein kinase (PKG) in oocyte maturation. In the present study, the expression, regulation and function of PKG in oocyte maturation was investigated in zebrafish. We identified four distinct PKG coding genes (named Prkg1a, Prkg1b, Prkg2, and Prkg3) in zebrafish. All prkgs are expressed in the ovary, and both prkg1a and prkg1b could be regulated by human chronic gonadotropin in follicular cells during oocyte maturation. We found that a cGMP analogue, 8-Br-cGMP, could stimulate oocyte maturation in a dose- and time-dependent manner. Such stimulatory effects of cGMP could be totally blocked by a PKG specific inhibitor, KT-5823. Intriguingly, we further found KT5823 could significantly attenuate spontaneous oocyte maturation in intact follicles but not in the denuded oocytes, suggesting that the activity of PKG in follicular cells is important for oocyte maturation. All of these results clearly demonstrate that PKG is involved in oocyte maturation in zebrafish.