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

Y 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 an ybx1 mutant 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 of cdkn1a (p21), a cell cycle inhibitor, increased dramatically in ybx1-/- follicles. Disrupting cdkn1a gene with CRISPR/Cas9 resulted in embryonic lethality. In p21 heterozygote (cdkn1a+/-), however, follicle activation and maturation in the ovary were both advanced, contrasting with the ybx1-/- mutant. Interestingly, partial loss of p21 could alleviate the phenotype of ybx1-/-. Folliculogenesis resumed in ybx1-/-;p21+/- females with normal follicle activation (PG-PV transition) and vitellogenic growth (PV-EV transition). Interestingly, the follicle cells from the ybx1-/- mutant displayed a poor proliferative activity both in vivo and in vitro; however, the cells from the ybx1-/-;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 of cdkn1a, a cell cycle inhibitor.