The Notch system during pubertal development of the bovine mammary gland.

The development of the mammary gland of cows during pre-weaning and puberty will condition its future productive capacity and warrants special study. In this respect, Notch signaling regulates tissue development and fate by modifying cell proliferation and differentiation and has been involved in stem cell maintenance, but has not been extensively studied in the developing mammary glands in cows. We therefore investigated Notch receptor expression and localization, as well as the expression of Notch ligands and target genes in the mammary gland of Holstein heifers in pre- and post-pubertal stages. Notch receptors 1 to 4 were detected by immunohistochemistry in the parenchyma and stroma of the developing gland. The subcellular localization of the four receptors was predominantly cytoplasmic except for NOTCH4, which was mostly nuclear. The membrane and the active intracellular domains of NOTCH paralogues were identified by western blot. NOTCH1 and NOTCH2 active domains increased during pubertal stages while NOTCH3 and NOTCH4 active domains decreased, suggesting strikingly different involvement of NOTCH paralogues in bovine mammary gland development and differentiation. The mRNA expression levels of the target genes HEY1 and HEY2 increased during peri-puberty whereas no variation of HES1 mRNA levels was observed. The mRNA levels of the Notch ligands JAGGED1 and DELTA1 also increased gradually during development. In conclusion, Notch signaling system dynamically varies throughout the development of the mammary gland during puberty pointing to specific time involvement of each component.