Endocrine Abstracts

Wnt signaling has well-established roles in gonadal development, notably being required for proper sex determination. More recently, a series of descriptive studies has shown the expression of various Wnts and Wnt signaling pathway components in adult ovaries during different stages of follicular development. To determine the potential role of WNT4 in follicle development, a mouse strain bearing a floxed Wnt4 allele was created using standard gene targeting techniques, and mated to Amhr2^cre/+ mice which express Cre in granulosa cells. A 6-month mating trial produced similar numbers of litters from Wnt4^flox/;Amhr2^Cre/+ and control groups, however the average litter size from Wnt4^flox/−;Amhr2^Cre/+ females was only 54% that of controls, and Wnt4^flox/−;Amhr2^Cre/+ ovaries were less than half normal weight. Follicles from ovaries of 5 day and 42 day-old Wnt4^flox/−;Amhr2^Cre/+ animals were counted and categorized by stage of development and viability. Results at both ages showed no effect of genotype on numbers of primordial or primary follicles (P>0.05). However, 42d Wnt4^flox/−;Amhr2^Cre/+ ovaries had far fewer healthy antral follicles, with only 25.2% the number found in controls (P<0.05). This was attributed to increased follicular atresia, as 46.2% of all antral follicles were atretic in Wnt4^flox/−; Amhr2^Cre/+ ovaries, versus 16.0% in Wnt4^flox/− controls. Furthermore, a small subset (~5%) of Wnt4^flox/;Amhr2^Cre/+ mice had very small ovaries devoid of antral follicles and corpora lutea at 8wks of age, and a complete depletion of follicles at 8 months. To determine the mechanism of WNT4 action, cultured granulosa cells were infected with an adenovirus to overexpress WNT4. Affymetrix microarray analyses revealed that the majority of genes induced by WNT4 overexpression were genes previously shown to mediate cellular hypoxia and/or stress responses. Our findings suggest that WNT4 mediates ovarian follicle development by inducing a stress response in granulosa cells that is required for their survival during the later phases of follicle development.