hnRNPL-orchestrated alternative splicing governs germ cell integrity and primordial follicle pool establishment

The primordial follicle pool established during the perinatal period, serves as the lifelong source of oocytes, with its depletion leading to premature ovarian failure (POF). Germ cell development from primordial germ cells (PGCs) to primordial follicle formation is tightly regulated by transcription and extensive post-transcriptional alternative splicing (AS). However, the dynamics and role of AS during PGC development and follicle formation remains poorly understood. Analysis of single-cell transcriptomes from E14.5 to P5 mouse ovaries revealed dynamic AS changes and key splicing factors involved in primordial follicle formation. Here, we employed a conditional knockout (cKO) model to demonstrate that hnRNPL loss in pre-meiotic germ cells disrupts homologous synapsis, induces extensive DNA damage, and results in abnormal diplotene-stage germ cells. Further analysis revealed that hnRNPL deficiency reduces transcript levels of genes essential for primordial follicle maintenance, causes aberrant splicing and protein expression of synapsis-related genes. These cumulative effects culminate in POF and infertility. Our findings explore AS dynamics and highlight hnRNPL as a key regulator of AS in primordial follicle pool formation and activation.