Proteomic analysis of zebrafish folliculogenesis identifies YB-1 (Ybx1/ybx1) as a potential gatekeeping molecule controlling early ovarian folliculogenesis.

As in mammals, ovarian folliculogenesis in teleosts also consists of two phases: the primary growth (PG) and secondary growth (SG) phases, which are analogous to the preantral and antral phases respectively in mammals. In this study, we performed a proteomic analysis on zebrafish follicles undergoing the PG-SG transition aiming to identify factors involved in the event. Numerous proteins showed significant changes, and the most prominent one was Y-box binding protein 1 (YB-1; Ybx1/ybx1), a transcription factor and mRNA-binding protein. YB-1 belongs to the Y-box binding protein family, which also includes the gonad-specific YB-2. Interestingly, phylogenetic analysis showed no YB-2 homolog in zebrafish. Although ybx1 mRNA was expressed in various tissues, its protein Ybx1 was primarily produced in the gonads, similar to YB-2 in other species. In the ovary, Ybx1 protein started to appear in early follicles newly emerged from the germ cell cysts, reached the highest level in late PG oocytes, but decreased precipitously when the follicles entered the SG phase. In PG follicles, Ybx1 might function as a key component of the messenger ribonucleoprotein particles (mRNPs) in association with other RNA-binding proteins. Similar to mammalian YB-1, zebrafish Ybx1 also contains functional signals that determine its intracellular localization. In conclusion, Ybx1 may play dual roles of YB-1 and YB-2 in zebrafish. In the ovary, Ybx1 binds mRNAs to stabilize them while preventing their translation. At PG-SG transition, Ybx1 is removed to release the masked mRNAs for translation into functional proteins, leading to follicle activation.
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