Arrest of WNT/ß-catenin signaling enables the transition from pluripotent to differentiated germ cells in mouse ovaries.

Germ cells form the basis for sexual reproduction by producing gametes. In ovaries, primordial germ cells exit the cell cycle and the pluripotency-associated state, differentiate into oogonia, and initiate meiosis. Despite the importance of germ cell differentiation for sexual reproduction, signaling pathways regulating their fate remain largely unknown. Here, we show in mouse embryonic ovaries that germ cell-intrinsic ß-catenin activity maintains pluripotency and that its repression is essential to allow differentiation and meiosis entry in a timely manner. Accordingly, in ß-catenin loss-of-function and gain-of-function mouse models, the germ cells precociously enter meiosis or remain in the pluripotent state, respectively. We further show that interaction of ß-catenin and the pluripotent-associated factor POU5F1 in the nucleus is associated with germ cell pluripotency. The exit of this complex from the nucleus correlates with germ cell differentiation, a process promoted by the up-regulation of Znrf3, a negative regulator of WNT/ß-catenin signaling. Together, these data identify the molecular basis of the transition from primordial germ cells to oogonia and demonstrate that ß-catenin is a central gatekeeper in ovarian differentiation and gametogenesis.

Constitutive WNT/CTNNB1 activation triggers spermatogonial stem cell proliferation and germ cell depletion.

The differentiation of germ cells into oogonia or spermatogonia is the first step that eventually gives rise to fully mature gametes. In the female fetal gonad, the RSPO1/WNT/CTNNB1 signalling pathway is involved in primordial germ cell proliferation and differentiation into female germ cells, which are able to enter meiosis. In the postnatal testis, the WNT/CTNNB1 pathway also mediates proliferation of spermatogonial stem cells and progenitor cells. Here we show that forced activation of the WNT/CTNNB1 pathway in fetal gonocytes using transgenic mice leads to deregulated spermatogonial proliferation, and exhaustion of the spermatocytes by apoptosis, resulting in a hypoplastic testis. These findings demonstrate that a finely tuned timing in WNT/CTNNB1 signalling activity is required for spermatogenesis.

R-spondin2 signaling is required for oocyte-driven intercellular communication and follicular growth.

R-spondin2 (RSPO2) is a member of the R-spondin family, which are secreted activators of the WNT/ß-catenin (CTNNB1) signaling pathway. In the mouse postnatal ovary, WNT/CTNNB1 signaling is active in the oocyte and in the neighboring supporting cells, the granulosa cells. Although the role of Rspo2 has been previously studied using in vitro experiments, the results are conflicting and the in vivo ovarian function of Rspo2 remains unclear. In the present study, we found that RSPO2/Rspo2 expression is restricted to the oocyte of developing follicles in both human and mouse ovaries from the beginning of the follicular growth. In mice, genetic deletion of Rspo2 does not impair oocyte growth, but instead prevents cell cycle progression of neighboring granulosa cells, thus resulting in an arrest of follicular growth. We further show this cell cycle arrest to be independent of growth promoting GDF9 signaling, but rather associated with a downregulation of WNT/CTNNB1 signaling in granulosa cells. To confirm the contribution of WNT/CTNNB1 signaling in granulosa cell proliferation, we induced cell type specific deletion of Ctnnb1 postnatally. Strikingly, follicles lacking Ctnnb1 failed to develop beyond the primary stage. These results show that RSPO2 acts in a paracrine manner to sustain granulosa cell proliferation in early developing follicles. Taken together, our data demonstrate that the activation of WNT/CTNNB1 signaling by RSPO2 is essential for oocyte-granulosa cell interactions that drive maturation of the ovarian follicles and eventually female fertility.

Retinoic acid synthesis by ALDH1A proteins is dispensable for meiosis initiation in the mouse fetal ovary.

In mammals, the timing of meiosis entry is regulated by signals from the gonadal environment. All-trans retinoic acid (ATRA) signaling is considered the key pathway that promotes Stra8 (stimulated by retinoic acid 8) expression and, in turn, meiosis entry. This model, however, is debated because it is based on analyzing the effects of exogenous ATRA on ex vivo gonadal cultures, which not accurately reflects the role of endogenous ATRA. Aldh1a1 and Aldh1a2, two retinaldehyde dehydrogenases synthesizing ATRA, are expressed in the mouse ovaries when meiosis initiates. Contrary to the present view, here, we demonstrate that ATRA-responsive cells are scarce in the ovary. Using three distinct gene deletion models for Aldh1a1;Aldh1a2;Aldh1a3, we show that Stra8 expression is independent of ATRA production by ALDH1A proteins and that germ cells progress through meiosis. Together, these data demonstrate that ATRA signaling is dispensable for instructing meiosis initiation in female germ cells.