MOLECULAR DETERMINANTS OF MAMMALIAN FERTILITY: ROLE OF PROGESTERONE RECEPTOR MEMBRANE COMPONENT 1 (PGRMC1)

Reproductive efficiency in dairy cows is decreasing worldwide. The root cause of the declining fertility is probably a combination of a variety of physiological and management factors that have an additive effect on reproductive efficiency. These factors include increasing in milk production and herd size, greater use of confinement housing, labor shortages and higher inbreeding percentages. In addition, the reproductive physiology of dairy cattle has also changed in response to genetic selection for milk production. Compared with traditional dairy cows, modern dairy cows have longer intervals to first ovulation, a higher incidence of anestrus and abnormal luteal phases, lower blood progesterone and IGF-I concentrations, higher incidence of multiple ovulations and twinning rates as well as greater embryonic loss. Declining fertility represents an obstacle in maintaining profitability of dairy farms. It has been estimated that a slight increase in pregnancy rates results in a significant increase in profitability that could make the difference in whether a family farm remains in operation. These findings suggest that defining the factors and mechanisms that contribute to oocyte and embryo quality is essential for improving female fertility. In particular, basic knowledge of which proteins within the oocyte regulate meiosis, oocyte fertilizability and developmental potential would be advantageous. Starting from these observations, our studies were conducted to test the hypothesis that PGRMC1 is one of the key factors that regulate mammalian oocyte quality and therefore female fertility. Initial indications that PGRMC1 participates in progesterone signaling in the reproductive system come from studies in which PGRMC1 expression was silenced using siRNA in ovarian cells. These experiments demonstrated that Progesterone’s ability to inhibit ovarian cells from undergoing apoptosis in vitro is dependent on PGRMC1, indicating that PGRMC1 plays an essential role in promoting the survival of ovarian cell in vitro. This led us to start our investigations on the function of PGRMC1 in bovine fertility. Initial experiments were conducted to determine the presence and localization of PGRMC1 in various compartments of the bovine female reproductive organs, during the follicular and luteal phases of the estrous cycle. Importantly, these studies revealed the presence of PGRMC1 in the nucleus of bovine oocytes. Further studies revealed that PGRMC1 is present in both GV- and MII-stage oocyte, is associated with male and female pronuclei in the zygote and is highly expressed in the blastocysts, with typical localization at each of these stages. Since fertilization and embryonic development are mainly dependent on the completion of oocyte maturation, we focused our attention on PGRMC1’s role during this important step. Our localization as well as our functional data suggest an important role of PGRMC1 in oocyte maturation that may be specifically related to the mechanism by which chromosomes segregate and the first polar body extruded. Moreover, we have started to elucidate the mechanism by which PGRMC1 could act as a regulator of oocyte meiosis. In fact we demonstrated not only that PGRMC1 co-localize with the active form of AURKB, but also that the activities of these two molecules may be somehow related. In fact, inhibition of AURKB causes changes in the localization of PGRMC1 and alterations in the MII chromosomal plate. Furthermore, our data suggest that alteration in the localization of PGRMC1 and AURKB could account in part for the increased aneuploidy and low development competence of oocytes of ovaries isolated from cows with reduced ovarian reserve. In conclusion, the present study raise important questions regarding the role of PGRMC1 in the regulation of oocyte maturation and the loss of fertility of dairy cows. Despite the mechanism through which PGRMC1 regulates these processes is still unknown and remains to be deeply investigated, our data indicate that functional alterations of PGRMC1 could impair meiosis and reduce development competence of oocytes. Finally, understanding these mechanisms in cattle could provide insight into how these processes are regulated in other species, including humans.