Role of Y box-binding protein 1 (Ybx1/ybx1) in zebrafish folliculogenesis: Promoting follicle cell proliferation via suppression of cell cycle inhibitor p21 (cdkn1a)

Abstract

AbstractY box-binding protein 1 (YB-1; Ybx1/ybx1) regulates transcription and translation of targeted genes through DNA/RNA-binding. Our research in zebrafish has revealed a high abundance of Ybx1 in the primary growth (PG) follicles in the ovary, which decreases precipitously as the follicles enter the secondary growth (SG) phase. To understand the function of Ybx1 in folliculogenesis, we created anybx1mutant using TALEN and observed a disruption in folliculogenesis in the mutant (ybx1-/-) during the transition from previtellogenic (PV) to early vitellogenic (EV) stage of the SG phase, resulting in underdeveloped ovaries and reduced female fertility. Transcriptome and Western blot analyses identified several differentially expressed genes between mutant (ybx1-/-) and control (ybx1+/-) ovaries. Notably, the expression ofcdkn1a(p21), a cell cycle inhibitor, increased dramatically inybx1-/- follicles. Disruptingcdkn1agene with CRISPR/Cas9 resulted in embryonic lethality. In p21 heterozygote (cdkn1a+/-), however, follicle activation and maturation in the ovary were both advanced, contrasting with theybx1-/-mutant. Interestingly, partial loss of p21 could alleviate the phenotype ofybx1-/-. Folliculogenesis resumed inybx1-/-;p21+/- females with normal follicle activation (PG-PV transition) and vitellogenic growth (PV-EV transition). Interestingly, the follicle cells from theybx1-/- mutant displayed a poor proliferative activity both in vivo and in vitro; however, the cells from theybx1-/-;p21+/- follicles resumed normal proliferation. In conclusion, our study suggests that Ybx1 serves a pivotal role in controlling early folliculogenesis in zebrafish, and its acts, at least partly, by repressing the expression ofcdkn1a,a cell cycle inhibitor.