- Granulosa cells in the follicle of the ovary support the growth, maturation and release of oocytes
- This study showed that YAP1, the major effector of the Hippo pathway, plays a critical role in granulosa cell proliferation, differentiation and survival
- Transgenic mice in which YAP1 was knocked out from granulosa cells exhibited disrupted ovarian follicle development that led to severe subfertility
- Further studies in mice demonstrated that YAP1 interacts with epidermal growth factor receptor, gonadotropin and transforming growth factor-ß signaling pathways to regulate granulosa cell proliferation and differentiation
- YAP1 is a promising target for treatment of subfertility associated with abnormal ovarian granulosa cell function
Granulosa cells in the follicle of the ovary support the growth, maturation and release of oocytes. The mechanisms underlying granulosa cell proliferation and differentiation are not fully understood, but several studies have demonstrated that YAP1, the major effector of the Hippo pathway, may be involved.
In previous research from University of Nebraska Medical Center published in Endocrine-Related Cancer, Cheng Wang, PhD, an investigator in the Vincent Center for Reproductive Biology (VCRB) at Massachusetts General Hospital, and colleagues demonstrated that YAP1, which regulates cell proliferation, is highly expressed in ovarian granulosa cells and promotes the progression of ovarian cancer. Now, in The FASEB Journal, Dr. Wang and VCRB Research Fellow Xiangmin Lv, PhD, report on YAP1 as a contributor to female fertility.
Localization of YAP1 in Ovarian Cells
Using immunohistochemistry, the researchers observed that in adult human ovarian follicles, from the primary stage to preovulatory follicles, YAP1 protein was mainly localized to the nuclei of granulosa cells. In the corpus luteum, however, YAP1 was mainly localized to the cytoplasm of the luteal cells.
Nuclear YAP1 activates several transcription factors that promote cell proliferation. Translocation from the nucleus to cytoplasm will prevent YAP1 from binding to its transcription factors, leading to its inactivation. Thus, the transition of YAP1 from the nucleus to the cytoplasm during granulosa cell differentiation indicates that inactivation of YAP1 may be a necessary step for granulosa cell differentiation.
Observations in the granulosa cells and luteal cells of mouse ovaries were consistent with those in human ovaries.
Effects of Inactivating YAP1
In cultures of mouse ovaries, the researchers observed that pharmacologic inactivation of YAP1 suppressed granulosa cell growth and disrupted follicle development. In living mice, the same YAP1-suppressing drug significantly induced apoptosis in granulosa cells and significantly reduced the number of corpora lutea in the ovaries.
Effects of YAP1 Knockout
The research team then bred transgenic mice in which Yap1 was deleted specifically from ovarian granulosa cells. These animals exhibited reduced ovarian size, decreased numbers of corpora lutea and increased numbers of underdeveloped follicles. When these transgenic mice mated with normal male mice, litter sizes were significantly smaller than normal. They found that phosphorylation of AKT1, which is critical for granulosa cell proliferation and survival, was significantly inhibited. Also, a biomarker for apoptosis was highly expressed in the granulosa cells of abnormal follicles.
The researchers also bred transgenic mice in which Yap1 was deleted in ovarian luteal cells. They were surprised to observe no effects on the morphology of ovaries, the number of corpora lutea or fertility. Apparently, cytoplasmic YAP1 in the luteinized granulosa cells had a minimal effect on mouse fertility.
Other Key Findings
Based on other experiments with mouse cells, the researchers report that:
- YAP1 and the epidermal growth factor receptor signaling pathway formed a feedforward loop in granulosa cells to drive cell proliferation and control cell differentiation
- Hyperactivation of YAP1 in granulosa cells suppressed production of estrogen and progesterone, which are markers of granulosa cell differentiation. Thus, inactivation of YAP1 appears to be important for proper differentiation of granulosa cells
- Knockdown of Yap1 interrupted signaling of transformational growth factor β (TGF-β) in granulosa cells. This suggests that there is crosstalk between the Hippo-YAP pathway and the TGF-β pathway in regulating granulosa cell function
Findings show that follicle development and successful reproduction rely on balanced expression and activation of YAP1. As such, YAP1 appears to be a promising target for treatment of subfertility associated with abnormal ovarian granulosa cell function.
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