Dynamics of extracellular vesicle-coupled microRNAs in equine follicular fluid associated with follicle selection

The mammalian follicular development is a well-orchestrated and complex molecular process characterized by sequences of follicular waves that yield the ovulatory follicle. It has been reported that bi-directional communication between the growing oocyte and the surrounding somatic cells is a hallmark of mammalian follicular development, partially mediated by extracellular vesicles (EVs) present in the follicular fluid (FF). Bioactive molecules such as the miRNAs encapsulated in the EVs mediate this communication within the follicular environment. Here, we aimed to decipher the dynamics of the miRNAs in the EVs of equine FF aspirated in vivo during different stages of follicular development. Hence, all follicles ≥6 mm were ablated on Days 10–11 of the estrous cycle (ovulation = Day 0) and the new induced wave was ultrasound-monitored daily until the largest growing follicle for at least three consecutive days was randomly allocated to one of the five groups (n = 8 follicles/group) namely: predeviation (preDEV; 18–20 mm), deviation (DEV; 22–25 mm), postdeviation (postDEV; 26–29 mm), preovulatory (preOV; 30–35 mm), and impending ovulation (IMP; ~40 mm). The FF was collected from each target group by ultrasound-guided transvaginal follicle aspiration. After removal of cells, cellular debris, and larger particles, EVs were isolated from FF groups (4 biological replicates/group; 0.5 mL FF/replicate) using high-speed ultracentrifugation (120,000xg for 70 min). Isolated EVs were characterized using western blot for EV marker proteins, NanoSight Tracking Analysis, and electron microscopy. MiRNA enriched total RNA was isolated from EVs using an Exosomal RNA Isolation kit and small-RNA library preparation and RNAseq (NextSeq500; Illumina) was performed by Novogene Co, Ltd. Approximately 180 known miRNAs were detected in each group with 148 mutually found in all groups. Among the expressed miRNAs, miR-148a, miR-21, miR-192, and miR-99a were the top highly abundant miRNAs in all groups. Cluster analysis exhibited 15 different expression patterns during follicular development. Among these patterns, a group of 23 miRNAs (including miR-106b, miR-199a-5p, and miR-125a-5p) exhibited a stable expression at the preDev until the postDev stage, a significant increase at the preOV stage, and then a significant decrease at the IMP stage. This miRNA cluster has been found to target genes involved in TGF-signaling and endocytosis pathways. Another interesting pattern with 22 miRNAs (including miR-146b-5p, miR-140, and miR-143) exhibited a sharp reduction in their expression from the preDEV until the preOV stage and was predicted to target genes associated with fatty acid biosynthesis, cell cycle, and oocyte meiosis pathways. In contrast, a group of 16 miRNAs (including miR-483, miR-128, and miR-192) gradually increased in expression from preDEV to the postDEV and then downregulated at the preOV stage. This miRNA cluster targets several signaling pathways, including ErbB, thyroid, and estrogen signaling. In conclusion, this study provides greater insights into the stage-specific expression dynamics of the FF EV-miRNAs during equine follicular development, which may play a role in folliculogenesis in mares. Quarter horse mares (n = 19; 8 to 14 years old) with normal reproductive tract as determined by ultrasonographic examination were used during the ovulatory season (June and July). Ultrasound-guided ablations of all follicles ≥6 mm were performed on Days 10–11 of the estrous cycle (ovulation = Day 0) to induce a new follicular wave. Thereafter, daily ultrasonographic follicle examinations were performed. Ultrasonographically healthy and growing follicles for at least three consecutive days were assigned to five groups based on the targeted diameters: predeviation (PreDev; 18–20 mm), deviation (Dev; 22–25 mm), postdeviation (PostDev; 26–29 mm), preovulatory (PreOV; 30–35 mm), and impending ovulation (IMP; follicles with signs of imminent ovulation). Samples of FF were collected in vivo by ultrasound-guided transvaginal follicle aspiration