MicroRNAs are Master Regulators of the Bovine Lactation Curve

The production capacity of the bovine mammary gland during a lactation cycle is determined by the number and activities of secretory cells, nutrition, health, and animal’s genetics including regulatory factors. The lactation curve (LC) is characterized by rapid increase in milk yield followed by gradual decrease after peak milk (~40-70DIM)until animal is dried off. This study examined the regulatory roles of microRNAs (miRNAs) throughout the LC.  Nine high producing Canadian Holstein cows were followed throughout the LC. Milk samples were collected on days 1 (day of calving) (d1), 7, 30, 70, 130, 170, 230, 290 and at dry off when production had dropped to 5kg/day (d5kg) for analyses of milk components and miRNA expression.  Milk fat% was constant throughout the LC while protein% increased progressively (P<0.0000) to the end of lactation. After Next generation miRNA sequencing, 335 known and 520 novel miRNAs were identified. 23 miRNAs were highly expressed across the LC. Top molecular and cellular functions of top five highly expressed miRNAs (Bta-miR-148A, miR-21-5p, miR-26A, miR-30A-5p and Let-7A-5p) suggest housekeeping roles in cellular growth and development, lipid metabolism, tissue morphology and organ development.  Differential gene expression showed that about 400 miRNAs were differentially expressed (DE) (FDR<0.05) throughout the LC. The highest number of DE miRNAs (133 to 338) was between d1 and d30/d70/d130/d170/d230/d290/d5kg followed by D7 and d70/d130/d170/d230/d293/d5kg (162 to 205 miRNAs). The least number of DE miRNAs (0 to 87 miRNAs) was between d1 and d7, d30 and d70/d130/d170/d230 and d290 and d5kg. Hierarchical cluster analysis separated the LC into 3 stages: d290/d5kg/d1/d7; d7/d30 and d70/d130/d170/d230. Our data shows that miRNAs play pivotal roles in regulating milk production throughout the bovine LC and in mammary gland growth and productivity.