RSPO2 Coordinates with GDF9:BMP15 Heterodimers to Promote Granulosa Cell and Oocyte Development in Mice

The generation of mature oocytes, a cornerstone of reproduction, relies on the coordinated interactions between oocytes and surrounding follicular somatic cells. Central to this process is the bidirectional communication between the oocyte and granulosa cells, mediated by oocyte-secreted factors (OSFs), including GDF9 and BMP15. While GDF9 and BMP15 are well-established regulators of oocyte and follicle development, the role of additional OSFs and their coordination with GDF9 and BMP15 remains largely unclear. Here, RSPO2 is identified as a key OSF that coordinates with the GDF9:BMP15 heterodimer to regulate granulosa cell development and enhance oocyte competence. RSPO2, primarily expressed in oocytes, interacts with GDF9:BMP15 to preserve transcriptomic integrity in preantral granulosa cells. This coordination is gene-specific, exhibiting either synergistic or antagonistic effects depending on the target genes, and involves crosstalk between CTNNB1- and SMAD2-dependent pathways. Conditional knockout of Rspo2 in oocytes causes severe defects in granulosa cell and oocyte development, leading to subfertility and earlier reproductive lifespan termination. Transcriptomic analysis shows that RSPO2 loss disrupts key granulosa cell genes (e.g., Amh, Ccnd2, Inhbb, Kitl) and compromises oocyte mitochondrial function, reducing developmental competence. These findings establish RSPO2 as an essential factor in the oocyte-granulosa cell regulatory loop, crucial for ovarian function and fertility.