Control of mouse cumulus cell-oocyte complex integrity before and after ovulation: Plasminogen activator synthesis and matrix degradation

During the preovulatory period, cumulus cells (CCs) form a hyaluronan-protein extracellular matrix (cumulus expansion) that positively influences oocyte fertilization. Degradation of this matrix and CC-oocyte complex (COC) dissociation occurs within a few hours of ovulation and parallels the aging of oocytes. Modulation of CC proteolytic activity by gonadotropins and oocyte soluble factors has been hypothesized to determine such cumulus matrix changes. In the present study, we investigated plasminogen activator (PA) synthesis by COCs during the expansion and disassembly processes. Our results show that the secretion of tissue type PA and urokinase type PA (uPA) by oocytes and CCs, respectively, does not change significantly during expansion but dramatically increases thereafter. Compact COCs were isolated from immature mice, primed 48 h earlier with 5 IU PMSGs, and were induced to expand in vitro with 100 ng/ml FSH in the presence of 1% FCS. Full expansion was achieved at 16 h, when hyaluronan synthesis ceased. Release of hyaluronan and CCs from the COC matrix began between 18 and 20 h of culture, which indicates that matrix degradation started at this time. PA activities in culture media were determined by SDS-PAGE, followed by a zymography at various time intervals between 4 and 32 h of culture. Secreted tissue type PA and uPA activity abruptly increased between 16 and 20 h after FSH stimulation. Slot blot hybridization of CC messenger RNA showed that uPA messenger RNA levels correlated with the increase in uPA activity. Similar temporal patterns of PA synthesis and matrix degradation were found in COCs induced to expand in vivo by injection of 5 IU human CG into PMSG-primed mice. Cultures of CCs, both in the presence and absence of oocytes, revealed that uPA synthesis is repressed in FSH-stimulated CCs by an oocyte-soluble factor for the first 16 h of culture, whereas CC responsiveness to this factor is lost thereafter. In conclusion, the data show that a sophisticated interplay between oocyte and CCs causes the two cell types to simultaneously secrete PA activity after ovulation. The fact that matrix degradation parallels PA production strongly supports the hypothesis that these enzymes may destabilize the expanded COC matrix.