Abstract
BackgroundThe ovulatory surge of gonadotropins triggers oocyte maturation and rupture of the ovarian follicle. The resumption of nuclear maturation in the oocyte from the prophase stage is characterized by germinal vesicle breakdown (GVBD). It has previously been shown that specific inhibition of cAMP degradation by PDE3 prevents the resumption of oocyte meiosis. However, no report has characterized the activity of PDE3 in the porcine oocyte, or the implication of the cAMP-PDE3 pathway in the entire nuclear maturation process. In this study, PDE3 activity in the oocyte was assessed during in vitro maturation (IVM) and the possible roles of the cAMP-PDE3 pathway in the resumption and progression of meiosis were investigated in terms of different models of oocyte maturation.ResultsCyclic AMP-degrading PDE activity was detected in the cumulus-oocyte complex (COC) and was partially inhibited by a specific PDE3 inhibitor, cilostamide. When measured only in the denuded oocyte, PDE activity was almost completely inhibited by cilostamide, suggesting that cAMP-PDE3 activity is the major cAMP-PDE in porcine oocytes. PDE3A mRNA was detected by RT-PCR. PDE3 activity did not vary significantly during the early hours of IVM, but a maximum was observed at 13 hours. In cumulus-oocyte complexes, meiosis resumed after 20.81 hours of culture. PDE3 inhibition no longer maintained meiotic arrest if sustained beyond 17.65 hours of IVM, 3 hours prior to resumption of meiosis. Thereafter, PDE3 inhibition progressively lost its efficacy in GVBD. When the protein phosphatase 1 and 2A inhibitor okadaic acid was continuously or transiently (3 hours) present during IVM, meiosis resumed prematurely; PDE3 inhibition was unable to prevent GVBD. However, PDE3 inhibition in COC treated with OA for 3 hours significantly delayed meiosis at the intermediate stage.ConclusionThe present investigation has demonstrated that PDE3A is the major cAMP-degrading PDE in the oocyte. It regulates the resumption of meiosis until 3 hours prior to GVBD and transiently affects meiotic progression.
Highlights
The ovulatory surge of gonadotropins triggers oocyte maturation and rupture of the ovarian follicle
Four lines of evidence support this conclusion: (1) cilostamide-sensitive cyclic adenosine monophosphate (cAMP)-PDE activity in the oocyte represents the main cAMP-PDE activity; (2) PDE3A mRNA was detected in oocyte cDNA by RT-PCR; (3) PDE3 inhibition after 15 hours of in vitro maturation (IVM) still prevents the resumption of meiosis; (4) PDE3 inhibition delays nuclear maturation in okadaic acid (OA)-treated oocytes
It has demonstrated that PDE3A is the major cAMP-degrading PDE activity and regulates meiotic resumption until 3 hours prior to germinal vesicle breakdown (GVBD)
Summary
The ovulatory surge of gonadotropins triggers oocyte maturation and rupture of the ovarian follicle. The resumption of nuclear maturation in the oocyte from the prophase stage is characterized by germinal vesicle breakdown (GVBD). It has previously been shown that specific inhibition of cAMP degradation by PDE3 prevents the resumption of oocyte meiosis. Recent findings have shown that cAMP-producing capacity is important in maintaining the prophase I-arrested state in oocytes [2]. High concentrations of cAMP and correspondingly high PKA activity within the oocyte prevent the resumption of meiosis, as shown using cAMP analogues [3,4], adenylyl cyclase activators [5,6] and invasive adenylyl cyclase treatments [7]. The PKA-mediated mechanism that maintains meiotic arrest is not fully understood, interesting advances have recently been made on possible PKA targets in mouse oocytes, such as CDC25B phosphatase and Wee1B kinase, two cell cycle regulators [9]
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