FOXL2 drives the differentiation of supporting gonadal cells in early ovarian development

Forkhead box L2 (FOXL2) is a forkhead box transcription factor expressed in the pituitary, ovaries, and eyelids. Human mutations in FOXL2 associate with the blepharophimosis, ptosis epicanthus and inversus syndrome (BPES), which can be linked to primary ovarian insufficiency, and are indirectly linked with differences of sex development (DSD). Animal studies have shown the crucial role that FOXL2 plays in the development, function, and maintenance of the ovary as well as in sex determination. However, the role of FOXL2 in early human somatic cell ovarian development is largely unknown. In this study, using CRISPR/Cas9 genome activation and a previously published in-house 14-day gonadal differentiation protocol, we show that FOXL2 downregulates coelomic epithelial markers GATA4 and LHX9, female gonadal markers RSPO1 and WNT4, and male gonadal markers SOX9 and NR0B1. Differentially expressed genes were mostly associated with Kyoto encyclopaedia of genes and genomes (KEGG) pathways relating to cell adhesion molecules and gene ontology (GO) pathways relating to extracellular matrix and junction formation. Moreover, comparative analysis with existing single cell RNA sequencing data from human in vivo-derived data elucidated that FOXL2 initiates downregulation of coelomic epithelial genes GATA4, LHX9 and UPK3B at day 6. At day 8, the genes ARX and GATA2 are transiently upregulated by FOXL2 induction and then downregulated as the genes LGR5, TSPAN8, OSR1 and TAC1 become upregulated. Thus, FOXL2 seems to support the exit of the differentiating cells from coelomic epithelium and first drives the cells towards a transitional identity and then into early supporting gonadal-like cells. The findings of this study will help largely in understanding normal gonadal development which can be used as a basis to elucidate pathological gonadal development underlying BPES.