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-prote in extracellular matrix (cumulus expansion) that positively influences oocy te fertilization. Degradation of this matrix and CC-oocyte complex (COC) di ssociation occurs within a few hours of ovulation and parallels the aging o f oocytes. Modulation of CC proteolytic activity by gonadotropins and oocyt e soluble factors has been hypothesized to determine such cumulus matrix ch anges. In the present study, we investigated plasminogen activator (PA) syn thesis by COCs during the expansion and disassembly processes. Our results show that the secretion of tissue type PA and urokinase type PA (uPA) by oo cytes and CCs, respectively, does not change significantly during expansion but dramatically increases thereafter. Compact COCs were isolated from imm ature 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 a nd CCs from the COC matrix began between 18 and 20 h of culture, which indi cates that matrix degradation started at this time. PA activities in cultur e media were determined by SDS-PAGE, followed by a zymography at various ti me 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. Slo t blot hybridization of CC messenger RNA showed that uPA messenger RNA leve ls 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 C Cs, both in the presence and absence of oocytes, revealed that uPA synthesi s is repressed in FSH-stimulated CCs by an oocyte-soluble factor for the fi rst 16 h of culture, whereas CC responsiveness to this factor is lost there after. In conclusion, the data show that a sophisticated interplay between oocyte and CCs causes the two cell types to simultaneously secrete PA activ ity after ovulation. The fact that matrix degradation parallels PA producti on strongly supports the hypothesis that these enzymes may destabilize the expanded COC matrix.