Milk fat is a key factor affecting milk quality and is also a major trait targeted in dairy cow breeding. To determine how the synthesis and the metabolism of lipids in bovine milk is regulated at the miRNA level, primary mammary epithelial cells (pMEC) derived from two Chinese Holstein dairy cows that produced extreme differences in milk fat percentage were cultured by the method of tissue nubbles culture. Small RNA libraries were constructed from each of the two pMEC groups, and Solexa sequencing and bioinformatics analysis were then used to determine the abundance of miRNAs and their differential expression pattern between pMECs. Target genes and functional prediction of differentially expressed miRNAs by Gene Ontology and the Kyoto Encyclopedia of Genes and Genomes analysis illustrated their roles in milk fat metabolism. Results show that a total of 292 known miRNAs and 116 novel miRNAs were detected in both pMECs. Identification of known and novel miRNA candidates demonstrated the feasibility and sensitivity of sequencing at the cellular level. Additionally, 97 miRNAs were significantly differentially expressed between the pMECs. Finally, three miRNAs including bta-miR-33a, bta-miR-152 and bta-miR-224 whose predicted target genes were annotated to the pathway of lipid metabolism were screened and verified by real-time qPCR and Western-blotting experiments. This study is the first comparative profiling of the miRNA transcriptome in pMECs that produce different milk fat content.

Bovine mastitis is an inflammatory condition of the mammary gland often caused by (Staphylococcus aureus) S. aureus infection. The aim of this study was to identify mastitis-related miRNAs and their downstream target genes, and therefore elucidate the regulatory mechanisms involved in disease progression and resistance. Three healthy and three mastitic cows were identified on the basis of the somatic cell count and bacterial culture of their milk, and the histological examination of udder tissues. High-throughput RNA sequencing and bioinformatic analyses revealed that 48 differentially expressed miRNAs (DEMs) in the mastitic udder tissues relative to the healthy tissues. Among 48 DEMs, the expression level of bta-miR-223 was the most up-regulated. Overexpression of the bta-miR-223 in Mac-T cells mitigated the inflammatory pathways induced by S. aureus-derived lipoteichoic acid (LTA). The Cbl proto-oncogene B (CBLB) was identified as the target gene of bta-miR-223, and the direct binding of the miRNA to the CBLB promoter was confirmed by dual luciferase reporter assay using wild-type and mutant 3'-UTR constructs. Furthermore, overexpression of CBLB in the LTA-stimulated Mac-T cells significantly upregulated PI3K, AKT, and phosphorylated NF-κB p65, whereas CBLB knockdown had the opposite effect. Consistent with the in vitro findings, the mammary glands of mice infected with 10⁸CFU/100 μL S. aureus showed high levels of CBLB, PI3K, AKT, and p-NF-κB p65 48 h after infection. Taken together, bta-miR-223 is a predominant miRNA involved in mastitis, and bta-miR-223 likely mitigates the inflammatory progression by targeting CBLB and inhibiting the downstream PI3K/AKT/NF-κB pathway.

According to our previous studies, bta-miR-124a was differentially expressed in breast tissue between high-fat and low-fat dairy cows. However, the function of bta-miR-124a in lipid metabolism of dairy cows and the identification of its target genes have not been reported. Therefore, this study will identify the target gene of bta-miR-124a and explore its role in the regulation of milk lipid metabolism. First, preliminary bioinformatics prediction of bta-miR-124a candidate target genes was performed, and quantitative real-time polymerase chain reaction (qRT-PCR) was used to analyze relative expression changes of bta-miR-124a and its candidate target genes and the expression level of the downstream gene of the target gene in the lipid metabolism signaling pathway in dairy mammary epithelial cell lines (Mac-T), using the dual luciferase reporter system for the identification of the targeting relationship between bta-miR-124a and the candidate target gene. Then, the effect of transfection of bta-miR-124a mimics and inhibitors on triglyceride (TG) and free fatty acid (FFA) levels was analyzed. The results indicate that bta-miR-124a directly interacts with the 3′-untranslated region of peroxisomal trans-2-enoyl-CoA reductase (PECR) to downregulate its expression in Mac-T cells. Further, bta-mir-124a regulates the expression of PECR and the downstream gene extension of very long chain fatty acid protein 2 (ELOVL2) through an unsaturated fatty acid biosynthesis signaling pathway. In conclusion, bta-miR-124a is involved in lipid metabolism by directly downregulating the PECR gene and affecting the expression of the downstream gene ELOVL2 and regulates the content of some key secretory elements such as TG and FFA. The function of bta-miR-124a has a certain effect on the synthesis and secretion of milk fat in the mammary epithelial cells of dairy cows.

Staphylococcus aureus is a key pathogen in bovine mastitis, and antibiotic therapy is challenged by resistance and residue concerns. Milk-derived extracellular vesicles emerge as promising natural antimicrobials. This study aimed to evaluate the antimicrobial activity and explore potential associated mechanisms of milk-derived extracellular vesicles against S. aureus. Milk-derived EV-enriched fractions (mEVs) from healthy (HmEVs) and mastitic (MmEVs) bovine milk suppressed S. aureus growth in vitro and were associated with oxidative imbalance, with MmEVs showing stronger inhibition. In addition, MmEVs significantly reduced biofilm biomass, extracellular matrix production, and the expression of key biofilm-associated genes (sarA, icaB, fnbA, clfB, cidA). Small RNA sequencing revealed distinct miRNA profiles between HmEVs and MmEVs; in particular, MmEVs were enriched in miRNAs predicted to target the S. aureus biofilm-associated gene clfB. Although we did not directly demonstrate uptake of mEV-derived miRNAs by bacteria or their regulation of bacterial gene expression in this study, our small RNA sequencing data together with subsequent bioinformatic predictions suggest that vesicular miRNAs should be regarded as candidate contributors, rather than demonstrated mediators, of the observed antibacterial and antibiofilm effects. Taken together, these findings indicate the potential of mEVs as residue-free adjuncts for controlling bovine mastitis, while recognizing that the present conclusions are mainly derived from in vitro experiments with S. aureus and bioinformatic analyses. Therefore, functional validation of candidate miRNAs, in vivo studies, and evaluation of activity against other mastitis-associated pathogens are still required to clarify the underlying mechanisms, therapeutic potential, and spectrum of activity of mEVs.