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

Abstract

R-spondin2 (RSPO2) is a member of the R-spondin family, which are secreted activators of the WNT/β-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.

R-Spondin 2 and WNT/CTNNB1 Signaling Pathways Are Required for Porcine Follicle Development and In Vitro Maturation

“…The results indicated that the major genes involved in RSPO2 and WNT signaling in porcine cumulus cells during IVM are CTNNB1 , LGR4 , LGR5 , EGFR, and RSPO2 . In a recent mouse study, it has been reported that oocyte-secreted RSPO2 promotes follicular development by activating WNT/CTNNB1 signaling in granulosa cells [ 3 ]. Our results indicate that RSPO2 activates CTNNB1 signaling through LGR4 or LGR5 in porcine.…”

“…At least three oocyte-derived factors have been identified to date to promote the growth of granulosa cells: roof plate-specific spondin2 (RSPO2), growth differentiation factor-9, and bone morphogenic protein-15 [ 1 ]. Recently, both Growth differentiation factor-9 (GDF9) and RSPO2 have been found to be essential for granulosa cell proliferation: RSPO2 activates WNT/Catenin Beta 1 (CTNNB1) signaling, and GDF9 signaling has been reported to act through the Bone morphogenetic protein receptor type II (BMPR2)/Activin receptor type-1B (ALK4) heterodimer receptor and the Mothers against decapentaplegic homolog 2/3 (SMAD2/3) pathway in granulosa cells [ 2 , 3 ].…”

Self-activation of Wnt signaling in pre-granulosa cells is required for ovarian folliculogenesis

“…Another mechanism may also operate to regulate the timing of Wnt signaling activation, however, given that such activation occurs during a narrower time window compared with that during which these Wnt ligand genes are expressed. It was recently suggested that production of functional Rspo2 by oocytes is important for the activation of canonical Wnt signaling in GCs (De Cian et al, 2020). Given that Rspo2 mRNA was found to be abundant in oocytes of growing follicles, a mechanism likely exists to inhibit Wnt signaling after the primary follicle stage.…”

“…More recently, oocyte-derived R-spondin 2 (Rspo2) was shown to contribute to the activation of Wnt signaling in GCs (De Cian et al, 2020). Rspo2 is a Wnt agonist that is secreted extracellularly and enhances canonical Wnt signaling (Kazanskaya et al, 2004), and follicle growth was found to be impaired in ovaries with loss of Rspo2 function by transplant experiments (De Cian et al, 2020). Although Wnt signaling is implicated together with other important factors such as GDF9 (growth differentiation factor 9) and BMP15 (bone morphogenetic protein 15) in PFA (Dong et al, 1996;Dube et al, 1998), its mechanism of action has been unknown.…”

lncRNA GCAT1 is involved in premature ovarian insufficiency by regulating p27 translation in GCs via competitive binding to PTBP1

“… 4 , 5 The proliferation and differentiation of GCs are critical for follicle maturation and ovulation, and aberrant apoptosis and degeneration of GCs results in follicular atresia. 6 , 7 , 8 Given that dysfunctional GCs have been implicated in POI, 9 , 10 identifying the regulatory network of functional GCs will provide important insights into the understanding of POI pathogenesis.…”