PSPC1 regulates CHK1 phosphorylation through phase separation and participates in mouse oocyte maturation.

Liquid-liquid phase separation (LLPS) underlies the formation of membraneless compartments in mammal cells. However, there are few reports that focus on the correlation of mouse oocyte maturation with LLPS. Previous studies have reported that paraspeckle component 1 (PSPC1) is related to the occurrence and development of tumors, but whether PSPC1 functions in mouse oocyte maturation is still unclear. Sequence analysis of PSPC1 protein showed that it contains a prion-like domain (PrLD) that is required for phase separation of proteins. In this study, we found that PSPC1 could undergo phase separation. Moreover, the loss of PrLD domain of PSPC1 could greatly weaken its phase separation ability. The immunofluorescence assays showed that PSPC1 is present in mouse oocytes in the germinal vesicle (GV) stage. Knockdown of PSPC1 significantly impeded the maturation of mouse oocytes in vitro. CHK1 has been reported to play important roles in the GV stage of mouse oocytes. Co-IP experiment revealed that PSPC1 could interact with phosphatase serine/threonine-protein phosphatase 5 (PPP5C), which regulates CHK1 phosphorylation. Western blot analysis revealed that PSPC1 could regulate the phosphorylation of CHK1 through PPP5C; however, PSPC1 without PrLD domain was inactive, suggesting that the lack of phase separation ability led to the abnormal function of PSPC1 in regulating CHK1 phosphorylation. Thus, we conclude that PSPC1 may undergo phase separation to regulate the phosphorylation level of CHK1 via PPP5C and participate in mouse oocyte maturation. Our study provides new insights into the mechanism of mouse oocyte maturation.