Transcriptomic Signatures of WNT-Driven Pathways and Granulosa Cell-Oocyte Interaction during Primordial Follicle Activation

Primordial follicle activation (PFA) is a pivotal event in female reproductive biology, orchestrating the transition from quiescent to growing follicles. This study employed comprehensive single-cell RNA sequencing to investigate the transcriptomic landscape of ovarian cells, focusing on the process of PFA. The aim of our study was to gain insights into the detailed regulatory mechanisms governing synchronized dormancy and activation between granulosa cells (GCs) and oocytes with decomposition of the PFA process. The Wntless (Wls) conditional knockout (cKO) mouse served as a unique model, suppressing the transition from pre-GC to GC, and disrupting somatic cell-derived WNT signaling in the ovary. Our data revealed immediate transcriptomic changes in GCs post-PFA in Wls cKO mice, leading to a divergent trajectory, while oocytes exhibited modest transcriptomic alterations. Subpopulation analysis identified molecular pathways affected by WNT signaling on GC maturation, along with specific gene signatures linked to dormant and activated oocytes. Despite minimal evidence of continuous up-regulation of dormancy-related genes in oocytes, loss of WNT signaling in (pre-)GCs impacted gene expression on oocytes even before PFA, subsequently influencing them globally. The infertility observed in Wls cKO mice was attributed to compromised GC-oocyte molecular crosstalk and a microenvironment for oocytes. Our study highlights the pivotal role of the WNT-signaling pathway and its molecular signature, emphasizing the importance of intercellular crosstalk between (pre-)GCs and oocytes in orchestrating folliculogenesis. To investigate cell type-specific alteration in gene expression during PFA with and without WNT-signaling, scRNA-seq analysis was performed on the ovaries of control and Wls cKO PD7 mice using the RamDA-seq, a full-length total RNA-sequencing method for single cells