Differential miRNA expression between equine ovulatory and anovulatory follicles

Relatively little is known about the physiological roles of microRNAs (miRNAs) during follicular development. Previous evidence from in vitro studies suggests specific roles for a subset of miRNAs, including miR-21, miR-23a, miR-145, miR-503, miR-224, miR-383, miR-378, miR-132, and miR-212, in regulating ovarian follicle development. The objective of this study was to gain insight on the involvement of these miRNAs during follicle maturation. Follicular fluid was aspirated from dominant follicles (>32 mm) during the ovulatory season (July to October) and the anovulatory season (January to March) in each of 5 mares, and the levels of steroids, IGF1, and miRNAs were analyzed by immunoassays and quantitative PCR. Levels of progesterone, testosterone, and IGF1 were lower (P ≤ 0.05) in anovulatory than in ovulatory follicles. Relative to ovulatory follicles, anovulatory follicles had higher (P < 0.05) mean levels of miR-21, miR-23b, miR-378, and miR-202 and tended to have higher (P = 0.06) levels of miR-145. Levels of miR-224 and miR-383 could not be detected in follicular fluid. These novel results indicate a physiological association between increases in follicular miRNA levels and seasonal anovulation in mares; further studies should elucidate the precise involvement of miR-21, miR-23b, miR-145, miR-378, and miR-202 in follicle maturation in the mare.