MicroRNA in the ovine mammary gland during early pregnancy: spatial and temporal expression of miR-21, miR-205, and miR-200.

The mammary gland undergoes extensive remodeling between the beginning of pregnancy and lactation; this involves cellular processes including cell proliferation, differentiation, and apoptosis, all of which are under the control of numerous regulators. To unravel the role played by miRNA, we describe here 47 new ovine miRNA cloned from mammary gland in early pregnancy displaying strong similarities with those already identified in the cow, human, or mouse. A microarray study of miRNA variations in the adult ovine mammary gland during pregnancy and lactation showed that 100 miRNA are regulated according to three principal patterns of expression: a decrease in early pregnancy, a peak at midpregnancy, or an increase throughout late pregnancy and lactation. One miRNA displaying each pattern (miR-21, miR-205, and miR-200b) was analyzed by qRT-PCR. Variations in expression were confirmed for all three miRNA. Using in situ hybridization, we detected both miR-21 and miR-200 in luminal mammary epithelial cells when expressed, whereas miR-205 was expressed in basal cells during the first half of pregnancy and then in luminal cells during the second half. We therefore conclude that miR-21 is strongly expressed in the luminal cells of the normal mammary gland during early pregnancy when extensive cell proliferation occurs. In addition, we show that miR-205 and miR-200 are coexpressed in luminal cells, but only during the second half of pregnancy. These two miRNA may cooperate to maintain epithelial status by repressing an EMT-like program, to achieve and preserve the secretory phenotype of mammary epithelial cells.