Identification of miR-187 as a regulator of early oogenesis and follicle growth in medaka ovary

Abstract MicroRNAs (miRNAs) are known regulators of ovarian function in vertebrates, yet their physiological roles in fish reproduction remain poorly understood. Here, we identified miR - 187 as one of the most ovarian-enriched miRNAs in medaka ( Oryzias latipes ) and we uncovered its function in vivo using CRISPR/Cas9-mediated gene inactivation. We showed that miR-187-3p is expressed in oocytes and granulosa cells in the ovary, and in discrete brain areas. Its loss-of-function led to a significant reduction in female fecundity. High-resolution 3D imaging of whole ovaries revealed that mir-187 mutants accumulate early stage I follicles and fewer advanced follicles, suggesting a defect in follicle growth. Transcriptomic profiling of mutant ovaries revealed extensive gene-expression changes consistent with the altered follicular-stage composition, including downregulation of key regulators of steroidogenesis, Wnt/β-catenin signaling, and TGF-β pathways, and upregulation of genes associated with early germ cells and immature ovarian states. Using an expression-based target-prediction pipeline, we selected several putative miR-187-3p targets, including nr6a1a ( gcnf ) and dpagt1 , two genes previously implicated in oocyte differentiation and female fertility in mammals. Together, our results demonstrate that miR-187 acts as a previously unrecognized regulator of early folliculogenesis and female reproductive capacity in medaka, expanding the repertoire of miRNAs with essential in vivo roles in teleost oogenesis and female fecundity.