In brief: This study shows that ageing affects miRNA profiles in follicular fluid, and an miRNA that is highly abundant in the follicular fluid of young cows supports the growth of oocytes derived from early antral follicles.
Abstract: We examined age-associated changes in miRNA profiles in the follicular fluid (FF) of cows. The role of miR-19b, which is abundant in the FF of young cows, in in vitro growth of early antral follicles (EAFs)-derived oocytes was assessed. FF was collected from the antral follicles of young (20-40 months) and aged (>120 months) cows. The miRNA profiles were similar between the FF of both age groups, whereas the abundance of some miRNAs differed between these samples. The miRNA profiles in granulosa cells (GCs) and the spent culture medium of oocyte-GC complexes (OGCs) derived from EAFs were distinct. Some miRNA groups overlapped among the GCs, culture media, and FFs. miR-19b was highly abundant in the FF of young cows, GCs, and culture medium. The supplementation of OGC culture medium with miR-19b increased the diameter, acetylation levels, and fertilisation ability of the oocytes. To assess whether miR-19b was functional in the GCs, a dual-luciferase assay, suppression of target protein, and RNA-sequencing of the GCs followed by functional annotation of the differentially expressed genes (DEGs) were conducted. Functional annotation of the DEGs suggested that miR-19b influences genes associated with FoxO signalling, endocytosis, and NR3C1 in GCs. These results suggest that in FFs, ageing affects the abundance of miRNAs that have important roles in oocyte development.