microRNAs and the mammary gland: A new understanding of gene expression

Abstract

MicroRNAs (miRNAs) have been identified in cells as well as in exosomes in biological fluids such as milk. In mammary gland, most of the miRNAs studied have functions related to immunity and show alterations in their pattern of expression during lactation. In mastitis, the inflammatory response caused by Streptococcus uberis alters the expression of miRNAs that may regulate the innate immune system. These small RNAs are stable at room temperature and are resistant to repeated freeze/thaw cycles, acidic conditions and degradation by RNAse, making them resistant to industrial procedures. These properties mean that miRNAs could have multiple applications in veterinary medicine and biotechnology. Indeed, lactoglobulin-free milk has been produced in transgenic cows expressing specific miRNAs. Although plant and animal miRNAs have undergone independent evolutionary adaptation recent studies have demonstrated a cross-kingdom passage in which rice miRNA was isolated from human serum. This finding raises questions about the possible effect that miRNAs present in foods consumed by humans could have on human gene regulation. Further studies are needed before applying miRNA biotechnology to the milk industry. New discoveries and a greater knowledge of gene expression will lead to a better understanding of the role of miRNAs in physiology, nutrition and evolution.

Keywords: lactation; mastitis; miRNA; milk.

Figure 1

A schematic representation of the canonical biogenesis of miRNA. Initially, a long hairpin-shaped (pri-miRNA) is transcribed by RNA polymerase II and then cleaved by Drosha (to yield pre-miRNA) prior to leaving the nucleus; the molecule is subsequently cleaved by a Dicer enzyme to yield double-stranded mature miRNA. Finally, miRNA is incorporated into the RNA-induced silencing complex (RISC), thereby allowing separation of the functional strand that interferes with mRNA by repressing translation or cleaving mRNA.

Figure 2

Diagram of the mammary epithelial cell response to infection by Streptococcus uberis in vivo and in vitro. (A) Mammary alveolus showing (a) epithelial cells, (b) myoepithelial cells, (c) basement membrane, (d) extracellular matrix and (e) capillary. In the alveolar lumen: (f) bacterial infiltration and (g) neutrophil infiltration. The accompanying box shows that 12 h after inoculation with S. uberis there was a decrease in the expression of miRNAs 15b, 16a, 31, 145 and 181a and an increase in miRNA 223 to modulate the inflammatory response (Naeem et al., 2012). (B) Mammary epithelial cells inoculated with S. uberis. No changes were observed at 1 h but there was an increase in miRNAs 29e and 708 at 2 h, an increase in miRNAs 7b and 98 and a decrease in miRNAs 29b-2, 193 and 130a at 4 h and, finally, an increase in 12 miRNAs (7b, 7d, 7e, 200c, 210, 24-2, 128-2, 128-1, 185, 652, 494 and 2342) concomitantly with a decrease in miRNA 29b2, 29e, 29c, 100 and 130a at 6 h (Lawless et al., 2013).

Figure 3

Bos taurus lactoglobulin B (LGB) mRNA sequence (GenBank accession number BC108213.1) showing the positions of miRNA6 and miRNA4 that targeted LGB, as designed by Jabed et al. (2012). The numbers refer to the nucleotide positions.