Regulation of ACVR1 and ID2 by cell-secreted exosomes during follicle maturation in the mare

Background: Ovarian follicle growth and maturation requires extensive communication between follicular somatic cells and oocytes. Recently, intercellular cell communication was described involving cell-secreted vesicles called exosomes (50-150 nm), which contain miRNAs and protein, and have been identified in ovarian follicular fluid. The goal of this study was to identify a possible role of exosomes in follicle maturation. Methods: Follicle contents were collected from mares at mid-estrous (similar to 35 mm, before induction of follicular maturation) and pre-ovulatory follicles (30-34 h after induction of follicular maturation). A real time PCR screen was conducted to reveal significant differences in the presence of exosomal miRNAs isolated from mid-estrous and pre-ovulatory follicles, and according to bioinformatics analysis these exosomal miRNAs are predicted to target members belonging to the TGFB superfamily, including ACVR1 and ID2. Granulosa cells from pre-ovulatory follicles were cultured and treated with exosomes isolated from follicular fluid. Changes in mRNA and protein were measured by real time PCR and Western blot. Results: ACVR1 mRNA and protein was detected in granulosa cells at mid-estrous and pre-ovulatory stages, and real time PCR analysis revealed significantly lower levels of ID2 (an ACVR1 target gene) in granulosa cells from pre-ovulatory follicles. Exposure to exosomes from follicular fluid of mid-estrous follicles decreased ID2 levels in granulosa cells. Moreover, exosomes isolated from mid-estrous and pre-ovulatory follicles contain ACVR1 and miR-27b, miR-372, and miR-382 (predicted regulators of ACVR1 and ID2) were capable of altering ID2 levels in pre-ovulatory granulosa cells. Conclusions: These data indicate that exosomes isolated from follicular fluid can regulate members of the TGFB/BMP signaling pathway in granulosa cells, and possibly play a role in regulating follicle maturation.