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

SIMPLE SUMMARY: A recent mouse study reported that R-spondin2 promotes follicular development by activating WNT/Catenin beta-1 (CTNNB1) signaling in granulated cells. Our results demonstrate that RSPO2, CTNNB1, G-protein coupled receptor 4 (LGR4), and G-protein coupled receptor 5 (LGR5) factors play a major role in porcine follicular development, and epidermal growth factor receptor (EGFR) signaling is essential for in vitro maturation. The R-spondin2 and WNT/CTNNB1 signaling pathways are involved in porcine follicle development, and it is expected that the EGFR-ERK signaling pathway is also involved. ABSTRACT: The secretion of oocyte-derived paracrine factors, such as R-spondin2, is an essential mechanism for follicle growth by promoting the proliferation and differentiation of cumulus cells around oocytes. In the present study, we aimed to identify the effect of R-spondin2 during follicular development. First, R-spondin2-related factors (R-spondin2, CTNNB1, LGR4, and LGR5) were identified through immunofluorescence in porcine ovarian tissue. CTNNB1 was expressed in ooplasm, and CTNNB1 and LGR4 were expressed in granulosa cells. In addition, R-spondin2, LGR4, and LGR5 were expressed in the theca interna. These results imply that these proteins play a major role in porcine follicular development. In addition, the effects of R-spondin2 on the in vitro maturation process of porcine cumulus oocyte complexes and subsequent embryonic development were confirmed. A treatment of 100 ng/mL R-spondin2 in the in vitro maturation (IVM) process increased nuclear maturation and increased the expression of EGFR mRNA in cumulus cells. The EGFR-ERK signal is essential for oocyte maturation, ovulation, and luteinization. R-spondin2 treatment also increased the expression of CTNNB1 and EGFR in primary cultured cumulus cells. In conclusion, RSPO2 and WNT/CTNNB1 signaling pathways are required for porcine follicle development and are predicted to be involved in the EGFR-ERK signaling pathway.