Transcriptome analysis reveals fundamental differences between liver of neonatal calves and transition dairy cows

Abstract Text:

Primary hepatocytes isolated from neonatal calf liver have been used to infer aspects of liver metabolism of dairy cows, with the assumption that physiological responses of neonatal hepatocytes mimic those of cows. In order to evaluate more directly the usefulness of calf hepatocytes as model to study cow responses, in particular during the early postpartal period, liver RNA from 7 Holstein cows (20 d in milk) and 7 Holstein calves (4-d old) was used for transcriptome analysis. Individual samples were used to determine expression of 7 key metabolic genes via quantitative RT-PCR. Data were log₂ normalized and subjected to ANOVA using the Proc MIXED procedure of SAS. Compared with calves, the expression of genes related with lipoprotein synthesis (APOB, P = 0.01; MTTP, P = 0.01), gluconeogenesis (FBP1, P = 0.07), fatty acid oxidation (CPT1A, P = 0.01), and methionine metabolism (MAT1A, P = 0.03; BHMT, P = 0.07) was lower in calves than cows. Samples were pooled by type (calves or cows) and co-hybrized onto the 44K-Agilent bovine (V2) gene expression microarray chips (Agilent Technologies Inc.). This allowed for a direct comparison of transcriptomes in calves and cows. Out of 14,772 unique annotated genes detected by the array, 354 (2.39%) differentially expressed genes with expression ratio ≥ (calf specific) or ≤ (cow specific) than mean ± 2 SD were considered highly-expressed in calves or cows and used for pathway analysis using the Dynamic Impact Approach (DIA) with the KEGG database. Within this set, 221 (1.49%) had at least 40-fold greater expression in cows (e.g., ACSL6, APOBEC3A, IGF2BP1, PFKFB3, SLC11A1, VLDLR) or calves (e.g., AOX1, CYP1A1, CYP7A1, SLC27A2). Instead, expression of 73 genes was equal among cows and calves. The 25 most-impacted pathways from the DIA analysis indicated that critical metabolic processes involving amino acid, lipid, carbohydrate and vitamin metabolism (e.g., branched-chain amino acid degradation; primary bile acid biosynthesis; fatty acid elongation in mitochondria; and glycolysis/gluconeogenesis) were biologically more important and highly-activated in calves than cows. In contrast, signaling pathways related to immunity (e.g., NOD-like receptor, Toll-like receptor, and chemokine), as well as cofactor metabolism (e.g., folate, pantothenate and CoA biosynthesis) were highly-activated in cows than calves. Overall, results indicate that liver of calves and cows have unique transcriptome profiles, hence, hepatocytes isolated from calves might not be a suitable model to study hepatic function/metabolic responses of cows.

Keywords: Calf, dairy cow, liver transcriptome