hnRNPU Safeguards Oocyte Development and Female Fertility via Regulation of Alternative Splicing.

RNA-binding proteins (RBPs) are essential for oocyte development, folliculogenesis, and ovarian homeostasis by regulating RNA metabolism. Heterogeneous nuclear ribonucleoprotein U (hnRNPU) plays critical roles in the regulation of multiple physiological processes as a key RBP, yet its function in mammalian oocytes remains poorly understood. Here, we generated two oocyte-specific Hnrnpu knockout mouse models by using Zp3-Cre and Gdf9-Cre transgenic mouse lines to dissect its role in female reproduction. Both models exhibited severe follicular developmental arrest and complete female infertility. Zp3-Cre-mediated deletion caused oocyte arrest at the germinal vesicle (GV) stage, whereas Gdf9-Cre-mediated deletion nearly abolished GV oocyte retrieval, indicating a more severe developmental block and revealing stage-specific requirements for hnRNPU. Mechanistic investigations focused on the Zp3-Cre model, which provided sufficient GV oocytes for molecular analyses. Loss of Hnrnpu in growing oocytes led to mitochondrial dysfunction, impaired oocyte-granulosa cell communication, and increased follicular apoptosis. Transcriptomic profiling revealed widespread dysregulation of genes involved in mitochondrial function and cell adhesion, with numerous mitochondrial-associated transcripts exhibiting aberrant pre-mRNA splicing. Collectively, our findings identify hnRNPU as an indispensable regulator of oocyte development and female fertility, acting through alternative splicing regulation to preserve mitochondrial function, maintain oocyte quality, and support folliculogenesis.