Gonadotropin surge-induced upregulation of plasminogen activator system components within bovine periovulatory follicular and luteal tissue

Ovulation and subsequent release of a mature oocyte is essential for establishment of pregnancy. Maintenance of pregnancy requires transformation of this ruptured follicle into a corpus luteum (CL). The preovulatory LH surge initiates both the ovulatory process and CL formation. Both the ovulatory process and CL formation feature degradation and reorganization of follicular wall and extracellular matrix (ECM) components. The plasminogen activator/plasmin system includes specific activators, inhibitors and receptors that directly and indirectly promote ECM degradation. In rodents, the gonadotropin surge induces expression of specific plasminogen activator/plasmin system components prior to ovulation. In plasminogen activator double knockout mice, ovulation rate is reduced 30%. Therefore, the plasminogen activator/plasmin system presumably plays an integral role in ovulation and CL formation in cattle. My working hypothesis is that in cattle, the gonadotropin surge up-regulates the de novo expression of plasminogen activator/plasmin system components that promote site directed degradation of the follicular wall and subsequent CL development. To test this hypothesis, I will determine the effect of a gonadotropin releasing hormone-induced gonodotropin surge on the localization, expression and activity of the plasminogen activator/plasmin system components. To date, plasminogen activator/plasmin system components have not been investigated in cattle. An increased understanding of the mechanisms that control ovulation and CL development may lead to improved methods to promote reproductive efficiency in cattle. This study examined the effect of the preovulatory gonadotropin surge on the plasminogen activator system components in bovine preovulatory follicles and new CL collected at approximately 0, 6, 12, 18, 24 and 48 h after a GnRH-induced gonadotropin surge. Messenger RNAs for tPA, uPA, uPAR, PAI-1 and PAI-2 were all increased in a temporally specific fashion within 24 h of the gonadotropin surge. Messenger RNAs for uPA uPAR, PAI-1 and PAI-2 remained elevated in the developing new CL. Localization of tPA and PAI-2 mRNAs were to the granulosal layer while PAI-1 mRNA was localized to the thecal layer. Both uPA and uPAR mRNAs were detected in both the granulosal and thecal layers. Activity for tPA was increased in follicular fluid and the preovulatory follicle apex and base within 12 h following the gonadotropin surge. The increase in tPA activity in the follicle base was transient, whereas increased activity in the apex was maintained through the 24 h timepoint. Activity for uPA increased in the follicle apex and base within 12 h of the gonadotropin surge and remained elevated through the time of follicular rupture. Plasmin activity in follicular fluid also increased within 12 h following the preovulatory gonadotropin surge and was greatest at 24 h. Significant plasminogen activator inhibitor activity was detected in follicle extracts, but temporal or spatial differences in plasminogen activator inhibitor activity were not detected in response to the gonadotropin surge. My results indicate that all the plasminogen activator components are upregulated in bovine preovulatory follicles following the gonadotropin surge in a cell-specific manner. Increased plasminogen activator and plasmin activity may be a contributing factor in the mechanisms of follicular rupture in cattle.