Yes-associated protein (YAP1) is required for proliferation and function of bovine granulosa cells in vitro$.

Yes-associated protein (YAP1) is a major component of the Hippo signaling pathway. Although the exact extracellular signals that control the Hippo pathway are currently unknown, increasing evidence supports a critical role for the Hippo pathway in embryonic development, regulation of organ size, and carcinogenesis. Granulosa cells within the ovarian follicle proliferate and produce steroids and growth factors, which facilitate growth of follicle and maturation of the oocyte. We hypothesize that YAP1 plays a role in proliferation and estrogen secretion of granulosa cells. In the current study, we examined expression of the Hippo signaling pathway in bovine ovaries and determined whether it was important for granulosa cell proliferation. MST1 and LATS2 were identified as prominent upstream components of the Hippo pathway expressed in granulosa and theca cells of the follicle and large and small cells of the corpus luteum. Immunohistochemistry revealed that YAP1 was localized to the nucleus of growing follicles. In vitro, nuclear localization of the downstream Hippo signaling effector proteins YAP1 and TAZ (WWTR1) was inversely correlated with granulosa cell density, with greater nuclear localization under conditions of low cell density. Treatment with verteporfin and siRNA targeting YAP1 or TAZ revealed a critical role for these transcriptional co-activators in granulosa cell proliferation. Furthermore, knockdown of YAP1 in granulosa cells inhibited FSH-induced estradiol biosynthesis. The data indicate that Hippo pathway transcription co-activators YAP1/TAZ play an important role in granulosa cell proliferation and estradiol synthesis, two processes necessary for maintaining normal follicle development.