Regulation of cumulus expansion and hyaluronan synthesis in porcine oocyte-cumulus complexes during in vitro maturation

Abstract

This review deals with molecular mechanisms controlling three important ovarian follicular processes: 1) expansion of the cumulus, 2) synthesis of the hyaluronan (HA), and 3) production of the progesterone in oocyte cumulus complexes (OCCs). The expansion of the mice cumuli induced by FSH or 8-bromo cAMP is dependent upon aspecific factor(s) secreted by the oocyte (called "cumulus expansion enabling factor", CEEF). The porcine oocytes produce at least two factors that have influence on the formation and stability of the preovulatory extracellular cumulus matrix (ECM), although oocytectomy does not alter the ability of the cumulus cells to respond to FSH and forskolin by increased cAMP content, HA synthesis, and subsequent cumulus expansion, The net synthesis of HA, during the FSH-stimulated expansion of the OCCs in the presence of serum, correlates directly with accumulation of glycosaminoglycans in the ECM. In pig, insulin growth factor 1 (IGF1) is acomponent of the serum that promotes the FSH-stimulated synthesis and retention of HA within the expanded ECM by phosphatidylinositol 3-kinase (PI3K)/v-akt murine thymoma viral oncogene homolog (AKT)- and mitogen-activated kinase 3 and 1 (MAPK3/1)-dependent mechanisms. Mouse, porcine, bovine, and rat oocytes produce CEEF(s). Possible candidate for the CEEF in the mouse is growth differentiation factor 9 (GDF9) secreted by oocytes. In pig, GDF9 mRNA is expressed not only in the oocytes but also in the cumulus and mural granulosa cells of the growing and preovulatory follicles, although the relative abundance of the GDF9 in the somatic cells is approximately 4 times lower than in the oocytes. Cross talk between FSH/ epidermal growth factor receptor (EGFR) and transforming growth factor β (TGFβ)/GDF9 signaling pathways is essential for functional activities of the porcine OCCs, since FSH enhances EGF-induced tyrosine phosphorylation of EGFR, indicating that FSH signaling pathway may stimulate specific EGFR-regulating proteins. Also, FSH-induced synthesis of both HA and progesterone is reduced but not abolished by AG1478 (EGFR tyrosine kinase inhibitor), indicating that other signaling pathways elicited by FSH are operating in parallel. Furthermore, SMAD2/3 signaling pathway is involved in the control of both cumulus expansion and steroidogenesis in porcine OCCs, since SMAD2/3 activation by GDF9/ TGFβ produced by oocyte and/or cumulus cells, significantly affects gonadotropin-induced HA and progesterone synthesis by porcine cumulus cells. oocyte-cumulus complex, hyaluronan, progesterone, cumulus expansion.