Dual roles of PDE9a in meiotic maturation of zebrafish oocytes

Abstract

The essential role of cyclic guanosine monophosphate (cGMP) signaling in regulating the oocyte meiotic cell cycle has been established. However, control of the level of cGMP in ovarian follicles is unclear. The cGMP-hydrolyzing phosphodiesterases (PDEs) are important in regulating the cellular cGMP level. We used zebrafish as a model to study the role of a cGMP-hydrolyzing phosphodiesterase-9a (PDE9a) in meiotic maturation of oocytes. Three PDE9a coding genes (PDE9aa, PDE9ab, and PDE9ac) were identified in zebrafish. Both pde9aa and pde9ac are expressed in most adult tissues including the ovary, but pde9ab is only expressed in the ovary, kidney, pituitary, and brain. All three pde9as mRNA exhibited different expression profiles during folliculogenesis. All of them are highly expressed in the oocyte but not in the follicular cell. The expression of both pde9aa and pde9ab, but not pde9ac, in ovarian follicles increases during oocyte maturation either in natural ovulatory cycle or induced by administration of hCG in vivo. We overexpressed pde9aa by injection of capped pde9aa mRNA into the oocytes. The cGMP level was decreased, and oocyte maturation was stimulated. When the activity of PDE9a was blocked by a specific inhibitor, Bay736691, the oocyte maturation was also stimulated. The stimulatory effect could be blocked by a gap junction blocker. However, the spontaneous oocyte maturation of denuded oocytes was not largely affected after treatment with Bay736691. All of the mature oocytes obtained by either treatment of Bay736691 or injection of pde9aa mRNA, could be fertilized in vitro. These results demonstrate the dual roles of PDE9a in oocyte maturation. The basal level of PDE9a is responsible for maintaining the meiotic arrest, and the increased level of PDE9a induced by LH signaling is helpful for stimulating meiotic maturation by hydrolyzing cGMP in oocytes.