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The primordial follicle pool is the main source of developing follicles in the ovary. The length of reproductive life and the onset of menopause are governed by the amount of primordial follicles in the ovary. The genetic factors and molecular mechanisms that maintain the primordial follicles in a dormant and surviving state for the whole of reproductive life are not well understood. The phosphatidylinositol 3 kinase (PI3K) signaling pathways in the oocyte that control oocyte growth and early follicular development are largely unknown. The major aim of this thesis was to investigate the functional role of the intra-oocyte PI3K pathway in the regulation of primordial follicle activation and survival.

 Phosphatase and tensin homolog deleted on chromosome ten (PTEN) is a major negative regulator of PI3K. The conditional deletion of Pten in the oocytes of primordial follicles led to the overgrowth of oocytes and activation of the entire pool of primordial follicles. There were higher numbers of activated primordial follicles at postnatal day 8 (PD8) in ovaries lacking PTEN in oocytes; by PD35 all the primordial follicles were activated and all the follicles were depleted by 12 weeks, causing premature ovarian failure (POF). In addition, the rate of follicular death that occurs during sexual maturity is reduced in ovaries that lack PTEN in oocytes. Further mechanistic studies revealed that loss of Pten in oocytes resulted in elevated Akt signaling and upregulation of both expression and activation of ribosomal protein S6 (rpS6). The overactivation of primordial follicles in ovaries that lack PTEN in oocytes is believed to be due to elevated expression and activation of rpS6. PTEN in oocytes is indispensable for the maintenance of primordial follicles in dormancy.

 To study the role of the intra-oocyte PI3K signaling pathway in controlling the survival and maintenance of primordial follicles, 3-phosphoinositide-dependent protein kinase-1 (PDK1) was deleted in oocytes of primordial follicle. The loss of Pdk1 in oocytes led to the depletion of most primordial follicles around the onset of sexual maturity, causing POF during early adulthood. Furthermore, the activation of Akt, p70 S6 kinase 1 (S6K1), and rpS6 was impaired in oocytes that lacked PDK1. The suppressed PDK1–Akt–S6K1–rpS6 signaling in oocytes appears to be responsible for the loss of primordial follicles. The excessive activation of primordial follicles seen in the absence of Pten in oocytes could be reversed by concurrent deletion of Pdk1. In addition, the elevated activation of Akt and S6K1 in the absence of PTEN in oocytes was not observed in PTEN and PDK1 double mutant mice. Similarly, the hyperphosphorylation of rpS6 in oocytes that lack PTEN was prevented in double mutant mice, which was most likely due to downregulation of S6K1 activation. Thus, inactivation of rpS6 in double mutant mice might be the reason for the prevention of excessive primordial follicular activation and survival.

 PTEN and PDK1 in oocytes are essential for the maintenance of quiescence and survival of primordial follicles. The molecular network involving PI3K/PTEN–PDK1 signaling in oocyte controls the survival, loss, and activation of primordial follicles, which together govern reproductive aging and determine the length of reproductive life in females. The results of the above studies indicate that the mammalian oocyte serves as the seat of programming of follicular activation and survival.

Immature ovarian primordial follicles are essential for maintenance of the reproductive lifespan of female mammals. Recently, it was found that overactivation of the phosphatidylinositol 3-kinase (PI3K) signaling in oocytes of primordial follicles by an oocyte-specific deletion of Pten (phosphatase and tensin homolog deleted on chromosome ten), the gene encoding PI3K negative regulator PTEN, results in premature activation of the entire pool of primordial follicles, indicating that activation of the PI3K pathway in oocytes is important for control of follicular activation. To investigate whether PI3K signaling in oocytes of primary and further developed follicles also plays a role at later stages in follicular development and ovulation, we conditionally deleted the Pten gene from oocytes of primary and further developed follicles by using transgenic mice expressing zona pellucida 3 (Zp3) promoter-mediated Cre recombinase. Our results show that Pten was efficiently deleted from oocytes of primary and further developed follicles, as indicated by the elevated phosphorylation of the major PI3K downstream component Akt. However, follicular development was not altered and oocyte maturation was also normal, which led to normal fertility with unaltered litter size in the mutant mice. Our data indicate that properly controlled PTEN/PI3K-Akt signaling in oocytes is essential for control of the development of primordial follicles whereas overactivation of PI3K signaling in oocytes does not appear to affect the development of growing follicles. This suggests that there is a stage-specific function of PTEN/PI3K signaling in mouse oocytes that controls follicular activation.

The molecular mechanisms that control reproductive aging and menopausal age in females are poorly understood. Here, we provide genetic evidence that 3-phosphoinositide-dependent protein kinase-1 (PDK1) signaling in oocytes preserves reproductive lifespan by maintaining the survival of ovarian primordial follicles. In mice lacking the PDK1-encoding gene Pdk1 in oocytes, the majority of primordial follicles are depleted around the onset of sexual maturity, causing premature ovarian failure (POF) during early adulthood. We further showed that suppressed PDK1-Akt-p70 S6 kinase 1 (S6K1)-ribosomal protein S6 (rpS6) signaling in oocytes appears to be responsible for the loss of primordial follicles, and mice lacking the Rps6 gene in oocytes show POF similar to that in Pdk1-deficient mice. In combination with our earlier finding that phosphatase and tensin homolog deleted on chromosome 10 (PTEN) in oocytes suppresses follicular activation, we have now pinpointed the molecular network involving phosphatidylinositol 3 kinase (PI3K)/PTEN-PDK1 signaling in oocytes that controls the survival, loss and activation of primordial follicles, which together determine reproductive aging and the length of reproductive life in females. Underactivation or overactivation of this signaling pathway in oocytes is shown to cause pathological conditions in the ovary, including POF and infertility.