Title: Control of oocyte reawakening by Kit
Room: FENS L030
Date & Time: December 21, Wednesday, 13:40 - 14:30
Presenter: Hatice Duygu Saatcioglu
Control of oocyte reawakening by Kit
Hatice Duygu Saatcioglu, Ileana Cuevas and Diego H Castrillon
UT Southwestern Department of Pathology
Females are born with a finite number of oocytes stockpiled as primordial follicles. Oocytes are“reawakened” via an irreversible, metered process whereby they are selected from this reserve pool to initiate follicle growth. Oocyte reawakening must be delicately balanced to ensure that
some growing oocytes are available during each estrus cycle but at the same time, limit the number of growing follicles to avoid premature depletion of primordial follicles. Because the mechanisms controlling reawakening in effect balance fertility with reproductive senescence, their
elucidation is of fundamental importance. The forkhead transcription factor Foxo3 is a key regulator and suppressor of this process (1). Foxo3 serves as a molecular switch whose activity (localization in cytoplasm versus nucleus) is under the control of PI3K-AKT signalling within the
oocyte (2).However, the identity of the presumptive upstream signalling factor controlling the PI3K-AKT-Foxo3 axis has been controversial (3).
Kit is highly expressed in oocytes, but firm genetic evidence implicating Kit in oocyte reawakening has been lacking. To define the role of Kit in this process, we created two novel alleles in mice. In the first, we engineered a conditional mutation (KitD818V), which results in constitutive Kit activity.
In the second (Kit-), the kinase domain essential for Kit activity was floxed, permitting cell-specific conditional inactivation. These two alleles are complementary, as one is gain-of-function, while the other is loss-of-function. We employed a germ-cell specific Cre driver (Vasa-Cre) to explore
the effects of Kit hyperactivity vs. inactivity within oocytes. KitD818V adult females were infertile and showed elevations in both FSH and LH levels (P=0.0021 and 0.0114; n=3) consistent with ovarian failure. Histomorphological analysis revealed that oocyte diameters of the mutant females
were larger on postnatal day 7, 14, and 28 (P<10-5; n>6 for all time points), and their ovaries became depleted of all oocytes by 6 weeks of age, a classic global reawakening phenotype. Additionally, Foxo3 protein was exported from the nucleus in KitD818V oocytes, demonstrating that
reawakening was dependent on the action of Kit via PI3K-Akt-Foxo3. Kit- females were alsoinfertile with ovarian failure, but exhibited an opposite oocyte phenotype: they had smaller ovaries and viable oocytes with no oocyte loss up to 6 weeks of age. Histomorphological analysis
revealed that oocytes failed to grow by 6 weeks (i.e. a complete failure of reawakening). Also, oocyte Foxo3 remained constitutively nuclear, demonstrating that loss of Kit activity caused the oocytes to remain in a state of “suspended animation” via constitutive Foxo3 localization. Taken
together, our findings show that Kit is the key upstream signalling molecule and cell surface receptor, acting through the PI3K-AKT-Foxo3 axis, to control the reawakening of oocytes.
(1) D. H. Castrillon, et al., Science 2003; 301:215-218. (2) G. B. John, et al., Dev Biol. 2008;321: 197-204 (2008).(3) S. D. Sullivan and D. H. Castrillon, 2001. Semin Repred Med. 2011; 29: 283-298