Early Embryo Initiated Uterine Receptivity Regulated By Local Apoptotic Caspase-3 Mediated Prostaglandin Action.

We observed a twenty-fold increase in caspase-3 activation in pregnant the mouse uterus on day 1 post coitus (1dpc) which rapidly declined on 2dpc to barely detectable levels. The function of caspase-3 at this early gestational time-point in pregnancy has not been determined. We have previously determined that elevated levels of apoptotic caspase-3 at term regulate uterine prostaglandin synthesis through the cleavage and activation of phospholipase-A2 (iPLA2). Therefore, the primary objective of this current study was to determine if elevated caspase-3 on 1dpc also regulates prostaglandin synthesis very early in pregnancy, which others have demonstrated to play a leuteoprotective role, thereby enhancing uterine receptivity. Our secondary objective was to determine if the conceptus was the trigger for this very early pregnancy surge in uterine caspase-3. Prospective laboratory animal study using pregnant mouse models. Utilizing an inbred strain of non-pregnant mice, the following mouse models were created. Unilaterally ligated and bilaterally ligated mouse models were generated with non-pregnant mice that underwent surgical ligation of uterine horn/horns just proximal to the oviduct. Mice which did not undergo surgical uterine horn ligation, served as controls. All mice were time mated, sacrificed and uteri collected from pregnant control mice on day 1-19 dpc, unilateral and bilateral and control pregnant mouse models on day 1, 3 and 6 of gestation and non-pregnant mice at estrous and diestrous (n=3 for each). Uteri were examined for apoptotic indices, including caspase-3 activation and TUNEL staining. Immunohistochemical analysis was performed to identify the site of apoptotic caspase-3 activation. Cytoplasmic and nuclear extracts were prepared and examined for caspase-3 activation, PARP cleavage and the appearance of cleaved iPLA2 by western blot analysis, PDI and NCOA3 served as our loading controls. Apoptotic caspase-3 and iPLA2 cleavage were isolated to the uterine endometrial compartment in control mice and both uterine horns of unilaterally ligated model solely on 1dpc. Apoptotic caspase-3 and cleaved iPLA2 were absent in the bilaterally ligated uterine horn model on all days examined. Our data supports, a novel link for apoptotic endometrial casepase-3 in promoting prostaglandin mediated uterine receptivity in early pregnancy. The absence of apoptotic caspase-3 and iPLA2 cleavage in the bilaterally ligated uterine horn supports that the early conceptus and not coitus acts as the trigger for endometrial caspase-3 in very early pregnancy. We therefore propose that the conceptus releases systemic signals as evidenced by caspase-3 activation and cleaved iPLA2 in both horns of the control and unilateral pregnant model on 1dpc, acting to increase prostaglandin E2 levels thereby supporting corpus luteal progesterone production and endometrial receptivity in very early pregnancy.