Cross-talk between liver and mammary tissue after experimental Escherichia coli mastitis in Holstein dairy cows using RNAseq

Abstract Text: Using RNAseq combined with bioinformatics tools, our objective was to identify cross-talk between liver and mammary tissue and key pathways altered during intramammary (IMI) challenge with Escherichia coli (E. coli). Six cows were inoculated with ~20-40 CFU of live E. coli into one mammary quarter at ~4-6 weeks in lactation. Biopsies were performed at -144 and 24 hours relative to challenge in liver and at 24h in both rear quarters (i.e. infected and non-infected) of the mammary gland. Each sample was sequenced using a 100 bp paired-end approach. Sequence reads were aligned to the Bovine genome and the number of reads that mapped to each of the 24,616 Ensembl genes was determined. A generalized linear model was fitted for the read count of each gene and differential expression was assessed using a likelihood ratio test statistic after adjustment for multiple testing (FDR). Ingenuity Pathway Analysis coupled with the Dynamic Impact Approach analysis of differentially expressed genes (overall time effect FDR≤0.05, post-hoc P≤0.05) indicated that IMI induced a large biological response in the liver and mammary tissue with a strong inhibition of metabolism, especially related to lipid, glucose, and xenobiotic metabolism, in the former and induction of inflammatory response/immune cells activation in the latter.  Analysis of upstream regulators indicated a prominent role of several cytokines, growth factors, and transcription regulators in the two tissues’ transcriptomics adaptation to IMI, clearly lipid-related and inhibited in the liver and inflammatory-related and activated in mammary tissue. The analysis uncovered a substantial cross-talk between the two tissues during IMI with a communication almost unidirectional (i.e., from mammary to the liver) via the induction of the hepatic proliferation, regeneration, and inflammatory response due to a large number of cytokines with an increased expression in the mammary gland and able to interact with highly induced hepatic receptors. The analysis indicated that only 3 proteins (SPP1, EPO, and GRP) with an increased hepatic expression due to IMI could potentially interact with receptors involved in leukocytes differentiation/proliferation with an increased expression due to IMI in mammary tissue. The larger enrichment of immune cell-related functions in the data from the mammary tissue suggests increased recruitment of active immune-cells to the mammary tissue. The analysis uncovered a large communication from the mammary to the liver to coordinate the inflammatory response with very few factors potentially released by the liver to control mammary gland response.

Keywords: Dynamic Impact Approach, Liver, Mastitis, RNAseq