Ovulation is the timely release of a functional oocyte from the ovary. LH surge activates the key cAMP/PKA pathway, where it activates Adenyl Cyclase (AC), which converts ATP to cAMP. Increased cAMP activates Protein Kinase A (PKA), which upon translocation into the nucleus phosphorylates CREB (pCREB), which triggers ovulatory gene expression. The cAMP levels are controlled by phosphodiesterases (PDE). Among the 11 PDE isoforms, PDE4 and its subfamily short isoform Pde4d1 are shown to be expressed in the granulosa cells1.
Gene expression studies indicate that the LH-surge induces both PDE4b and PDE4d genes and more specifically, the short spliced variant isoforms of these two genes, Pde4d1 and Pde4b2 were significantly higher at 2 hours post-hCG, while the long isoforms were elevated at 8 hours post-hCG, followed by a downward trend. Immunohistochemistry staining showed that Pde4d and Pde4b were localized in follicles of all stages from small pre-ovulatory to large antral follicles. Time-lapse live fluorescent imaging of granulosa cells transfected with the cAMP sensor ‘Pink Flamindo’ revealed that a Pde4d specific inhibitor (D159687) at 10 µM and 3 µM increased cAMP levels in a dose-dependent manner, but a Pde4b specific inhibitor (A33) was less effective. Immunofluorescent analysis of pCREB induction, by treatment of GCs with D159687 and A33 showed a dose-dependent induction of pCREB within a dose range of 0.01 µM to 10 µM.
Taken together, our results demonstrate that PDE4 isoforms have unique expression patterns within the ovary during the peri-ovulatory phase and they distinctly regulate the cAMP dynamics in granulosa cells. Further research is needed to understand the functional relevance of each isoform in regulating ovulation.