Effect of once daily milking on mammary transcriptome and cell turnover in dairy goat
Résumé
Once-daily milking is known to modify cell number and activity in the bovine mammary gland. However, in goats, the effect of once-daily milking on mammary cell death is controversial. To assess the effect of once-daily milking on mammary transcriptome and cell turnover and the effect of the duration of
once-daily milking, 10 goats producing 2.5 kg of milk per day at 100 DIM were divided into 2 groups. All goats were twice daily milked during a pre-experimental period of 2 wk. Then, the goats were once daily milked for the 3 following weeks. Mammary biopsy samples were collected at d −1 and 7 after the start of once-daily milking for one group of 5 goats and at d −1 and 21 for a second group of 5 goats. Cell apoptosis and proliferation rates were analyzed in mammary tissue by immunohistological analyses after TUNEL and PCNA staining, respectively. Ribonucleic acid was extracted from mammary tissues. A transcriptomic analysis using the Agilent Bovine 4x44k microarrays has been performed to compare the effect of once-daily milking on mammary transcript profiles. Data were normalized and statistical significant raw P-values were adjusted for multiple comparisons using the Benjamini–Hochberg procedure. A similar reduction in milk yield (−19%) was
observed in both groups of goats during once-daily milking compared with twice-daily milking. Cell apoptosis was higher during once-daily milking than during twice-daily milking for both groups of goats (P < 0.05) whereas cell proliferation did not vary (P = 0.27). The transcriptomic analysis showed a differential gene expression of 4,039 transcripts, 2,238 and 1,801 transcripts up- and downregulated, respectively, by once-daily milking compared with twice-daily milking. More than 1,000 transcripts were commonly regulated between the 2 groups of goats. IPA analysis showed that these transcripts were part of networks associated with DNA replication, cellular growth and proliferation, and cell-to-cell signaling and interaction in both groups of goats. RT-qPCR analysis of 11 genes confirmed the differential gene expression with a downregulation
of genes involved in milk synthesis and an upregulation of genes involved in cellular cycle and apoptosis. These results showed that once-daily milking induce cell turnover in goat mammary tissue, with a small impact of time.