115 Exosome-mediated microRNA expression profile in follicular fluid of metabolically divergent postpartum cows

Most high-milking cows enter a state of negative energy balance during the early lactation period. This phenomenon disturbs the metabolic status of the follicular fluid microenvironment, resulting in delayed ovulation. Cell-to-cell communication between the oocyte and the surrounding cells is crucial during folliculogenesis. Exosomes, evolutionarily conserved cargo molecules (30-150 nm in diameter) carrying RNA and proteins, are known to be involved in cell-to-cell communication. Here, we aimed to investigate the association between postpartum metabolic status and the expression of exosomal microRNA (miRNA) in follicular fluid of Holstein-Friesian cows. For this, follicular fluid was collected from antral follicles (>8 mm in diameter) using ovum pickup procedure from cows (n = 30) on a weekly basis between weeks 5 and 10 postpartum. Follicular fluid collected from heifers (n = 8) was used as a control. The energy status of each cow was assessed based on the blood metabolite (nonesterified fatty acids and β-hydroxybutyrate) concentration, body weight curve, and overall energy balance determined by dry matter intake. Afterwards, cows were categorized as early negative and late positive (cows show negative energy balance at early weeks and recovered at late weeks postpartum), always negative (cows did not recover until 15 weeks postpartum), and always positive (cows did not enter in to a state of negative energy balance). Following this, exosomes were isolated from pooled samples from each animal category using ultracentrifugation, and their morphology and size was characterised using electron microscopy and nanosight, respectively. Exosomal total RNA enriched with miRNA was isolated using an exosomal RNA isolation kit. Next-generation sequencing of miRNA was performed using Illumina NextSEqn 500 (Illumina Inc., San Diego, CA, USA). MicroRNAs with a fold change ≥2, P-value <0.05, and a false discovery rate of <0.1 were considered differentially expressed. The results showed that a total of 356 known and 156 novel miRNA were identified across samples. Differential expression analysis of miRNA between always-negative cows versus always-positive cows revealed down-regulation of all 6 differentially expressed miRNA, including bta-miR-451, bta-miR-132, and bta-miR-2285. Similarly, down-regulation of 14 miRNA, including bta-miR-20b, bta-miR-363, bta-miR-132, and bta-miR-451, and up-regulation of 3 miRNA was observed in always-negative cows compared to heifers. Furthermore, the target prediction analysis of the down-regulated miRNA have been shown to be involved in regulating different pathways including transforming growth factor-β signalling, cell cycle, hippo signalling, forkhead box O signalling, and endometrial cancer, among others. In conclusion, the results revealed that although negative energy balance in postpartum dairy cows suppressed exosomal miRNA expression in follicular fluid, the opposite was observed in metabolically unstressed cows. This divergence of exosome-mediated miRNA expression in the follicular fluid of metabolically stressed cows could be associated with the reduced fertility of those cows.