Role of cyclic nucleotide signaling in oocyte maturation

The development of the ovarian follicle, oocyte maturation, and ovulation require a complex set of endocrine, paracrine, and autocrine inputs that are translated into the regulation of cyclic nucleotide levels. Changes in intracellular cAMP mediate the gonadotropin regulation of granulosa and theca cell functions. Likewise, a decrease in cAMP concentration in the oocyte has been associated with the resumption of meiosis. Using pharmacological and molecular approaches, we determined that the expression of cyclic nucleotide phosphodiesterases (PDEs), the enzymes that degrade and inactivate cAMP, is compartmentalized in the ovarian follicle of all species studied, with PDE3 present in the oocytes and PDE4s in granulosa cells. The PDE3 expressed in the mouse oocyte was cloned, and the protein expressed in a heterologous system had properties similar to those of a PDE3A derived from somatic cells. Inhibition of the oocyte PDE3 completely blocked oocyte maturation in vitro and in vivo, demonstrating that the activity of this enzyme is essential for oocyte maturation. Heterologous expression of PDE3A in Xenopus oocyte causes morphological changes distinctive of resumption of meiosis (GVBD), as well as activation of mos translation and MAPK phosphorylation. Using mRNA and antibody microinjection in the Xenopus eggs, we have shown that PDE3 is downstream from the kinase PKB/Akt in the pathway that mediates IGF-1 but not progesterone-induced meiotic resumption. The presence of a similar regulatory module in mammalian oocytes is inferred by pharmacological studies with PDE3 inhibitors and measurement of PDE activity. Thus, PDE3 plays an essential role in the signaling pathway that controls resumption of meiosis in amphibians and mammals. Understanding the regulation of this enzyme may shed some light on the signals that trigger oocyte maturation.