Evaluation of circulating miRNAs in mares approaching parturition

Circulating microRNAs (miRNAs) are stable in body fluids and can serve as biomarkers for various diseases and physiological states. Although pregnancy˗related miRNAs have been identified in various mammals, studies on parturition˗related circulating miRNAs in mares are limited. Therefore, this study aimed to identify parturition˗related miRNAs and examine their potential applications in the prediction of parturition date. miRNAs were extracted from the plasma of Thoroughbred mares 30 days (295–326 days pregnant) and 5 (323–352 days pregnant) – 0 (328–357 days pregnant) days before parturition, followed by small RNA sequencing (small RNA˗seq) and reverse transcription quantitative PCR (RT˗qPCR). Additionally, we measured plasma progestin concentrations in mares using an enzyme-linked immunosorbent assay. Small RNA˗seq data indicated that 18 miRNAs were affected by parturition proximity. Among the 18 miRNAs, two novel miRNAs and three known miRNAs (miR˗361˗3p, miR˗483, and miR˗99a) showed significant changes at 5–0 days before parturition compared with that at 30 days to parturition. Plasma progestin concentrations were higher at 5–3 days to parturition than at 30 days to parturition, and then decreased on the day of parturition. Conclusively, this study provides basic knowledge of parturition˗related circulating miRNAs in mares, and identifies miRNAs that could potentially be used as biomarkers to predict parturition in mares.