Unilateral once daily milking locally induces differential gene expression in both mammary tissue and milk epithelial cells revealing mammary remodeling

Once daily milking reduces milk yield and alters mammary transcriptome resulting in a decrease in milk protein synthesis as well as an induction of the apoptotic signaling networks. A local regulation due to milk stasis in the tissue could contribute to this effect but such mechanisms have not yet been described. To challenge this hypothesis, cows were milked unilaterally, once daily on one udder half and twice daily on the other one, and variations in gene expression were studied in biopsies as well as in mammary epithelial cells (MEC) shed into milk during the lactation process (milk MEC). This study therefore also contributes to decipher if transcript variations in milk purified MEC can reflect that of the mammary tissue. We compared the mammary transcript profiles in biopsies collected from unilaterally once versus twice-daily milked udder halves, 504 transcripts were differentially expressed: 193 and 232 transcripts were up- and down-regulated, respectively. A first category of transcripts, which accumulation levels are mostly up-regulated, relates to mechanisms involved in cell renewal, such as cell cycle, cellular growth and proliferation, cell death and cellular development. A second family, mostly down-regulated, is involved in small molecule biochemistry, amino acid, lipid and carbohydrate metabolisms as well as molecular transport. A third category, mostly up-regulated, includes transcripts expressed in non-epithelial mammary cells such as adipocytes, endothelial and immune cells and cells from the connective tissue. These results are consistent with previous data showing a decrease in mammary synthesis activity and an activation of cell death during once daily milking. They further suggest that during once daily milking the local milk accumulation has a major effect on mammary remodeling. Interestingly, some transcripts belonged to a third family described as « molecular transports ». The expression of the 21 mRNA was then analyzed by RT-par in MEC (Table 4). Seven transcripts (NUCB2, RNASE4, ABCG2, RNASE1, SLC34A2, Cap1, FABP3, LALBA, SCD) were significantly down-regulated in milk purified MEC. Six were also significantly down-regulated in mammary biopsies. These transcripts are mostly involved in milk synthesis. We therefore conclude that milk purified MEC cannot be used as general markers of variations occurring in the mammary tissue but that variations in some transcripts listed above can be useful indicators. In this experimental design, udder halves were unilaterally milked once daily and the contralateral udder halves twice daily. Each mammary biopsy RNA sample from one udder half was compared to the one of the contralateral udder half in 5 dye-swaps corresponding to 5 animal replications, minimizing the animal variability. One dye swap corresponded to one comparison meaning two slides.