Effect of Once Daily Milking on Mammary Transcriptome and Cell Turnover in Dairy Goat

Once daily milking is known to modify cell number and activity in the bovine mammary gland. However in goat, 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 two groups. All Goats were twice daily milked during a pre-experimental period of 2 weeks. Then, the goats were once daily milked for the 3 following weeks. Mammary biopsy samples were collected at D -1 and D 7 after the start of once daily milking for one group of 5 goats and at D -1 and D 21 for a second group of 5 goats. Cell apoptosis and proliferation rates were analysed in mammary tissue by immunohistological analyses after TUNEL and PCNA staining, respectively. RNA 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 down regulated, respectively, by once-daily milking compared with twice-daily milking. More than 1,000 transcripts were commonly regulated between the two 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 signalling 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.