Invited review: MicroRNAs in bovine colostrum-Focus on their origin and potential health benefits for the calf

Colostrum is the first milk produced by a cow after she gives birth. Compared with mature milk, it has a high concentration of immunoglobulin G. Calves are born without circulating antibodies, thus ingestion of colostrum is necessary to protect the calf against pathogens in the first challenging weeks of life. In addition to the life-saving supply of antibodies, colostrum contains minerals, vitamins, growth factors, and immune cells. Recently, microRNAs (rriiRNAs) were added to that list. MicroRNAs are short, non-coding RNA molecules that can regulate gene expression at the post-transcriptional level. They are thought to act as key regulators of diverse biological and developmental processes. Colostrum contains higher amounts of miRNAs than mature milk; immune- and development-related miRNAs are prominent. Their expression pattern in milk is likely to be influenced by maternal nutrition and environment. The fat content of the maternal diet appears to have a major effect on expression of miRNAs in milk and in the neonate. The immunological state of the mammary gland seems to affect miRNA expression as well. In cows diagnosed with subclinical mastitis, alterations in the expression of miRNAs in milk have been observed. It is believed that miRNAs in colostrum and milk are signaling molecules passed from mother to newborn. They are packaged in extracellular vesicles, which makes them resistant to the harsh conditions in the gastrointestinal tract. Therefore, they can reach the small intestine, where they are absorbed and transferred into the bloodstream. MicroRNAs are important for the development of the intestines. For example, miRNAs stimulate cell viability, proliferation, and stem cell activity of the intestinal epithelium. Furthermore, miRNAs seem to act as key players in the development of the complete immune system. They can, among other things, regulate B- and T-tell differentiation and affect interleukin production of macrophages. The abundance of miRNAs in colostrum and milk and the possibility for their absorption in the intestines of the neonate supports the hypothesis that these tiny molecules are important for the development of the newborn. The probable relation of diet to the expression of miRNAs by the mother creates a possible avenue to optimize expression of miRNAs and improve neonatal maturation.