Abstract
Simple SummaryThe aim of this study was to explore the genes associated with milk protein content in dairy cows. Seven down-regulated and three up-regulated proteins were found in isolated milk epithelial cells (MECs) from dairy cows with high milk protein level (group High). l-leucine depletion not only decreased the proteins synthesis (p < 0.05), but also decreased l-lactate dehydrogenase B chain (LDH-B) mRNA expression in bovine mammary alveolar cells MAC-T cells (p < 0.05). This study suggested that LDH-B is negatively associated with the milk protein content of dairy cows and has a positive relationship with l-leucine. These findings may provide some theoretical basis to study individual differences in the milk protein synthesis ability of dairy cows.This study aimed to explore genes associated with milk protein content in dairy cows and their relationships with l-leucine. Ten primiparous Holstein cows (93.8 ± 11.56 milking days) fed the same diet were divided into two groups depending on their milk protein contents (group High, 3.34 ± 0.10%; and group Low, 2.86 ± 0.05%). Milk epithelial cells (MECs) were isolated from the collected morning milk and differentially expressed proteins in MECs were explored by two-dimensional gel electrophoresis (2-DE). Then, the mRNA expression of these proteins was detected by real time PCR in MAC-T cells incubated with three different media named positive control (PC), negative control (NC), and l-leucine depletion (NO-leu). Results showed that ten proteins were differentially expressed in MECs from cows in group High. They included seven down-regulated ones (heat shock protein beta-1 (HSPB1), 78 kDa glucose-regulated protein (GRP-78), l-lactate dehydrogenase B chain (LDH-B), malate dehydrogenase, cytoplasmic (MDH1), annexin I (ANXA1), cytokeratin-7 (CK-7), and glyceraldehyde-3-phosphate dehydrogenase (GAPDH)), and three up-regulated ones (prohibitin (PHB), beta casein (CSN2), and alpha S1 casein (CSN1S1)). When l-leucine was depleted from the medium, not only proteins content was lowered (p < 0.05), but also the LDH-B mRNA expression was decreased in MAC-T cells (p < 0.05). In conclusion, LDH-B is negatively associated with the milk protein content of dairy cows and has a positive association with l-leucine.
Highlights
In milk, about 25% of total milk solids are proteins that are of great importance to milk processors for the manufacturing of a range of products [1]
In order to improve the understanding of leucine-specific signaling in milk protein synthesis, proteins differentially expressed were explored in isolated bovine milk epithelial cells (MECs) from cows with high milk protein level, and their relationships with l-leucine were checked in bovine mammary alveolar cells (MAC-T cells) in this study
The present data indicated significant differences existed in the milk protein synthesis ability of individual cows, even when fed the same diet
Summary
About 25% of total milk solids are proteins that are of great importance to milk processors for the manufacturing of a range of products [1]. Studies have indicated that fatty acid composition in milk can be changed by feeding, while protein composition is not significantly influenced by the cow’s diet [3,5]. Some amino acids were found serving as substrate, and acting as signaling molecules for protein synthesis [5,6]. L-leucine, one of the branched-chain amino acids, is recognized as a major regulator in protein synthesis [7,8]. We hypothesized that some proteins related to l-leucine are associated with the milk protein content of dairy cows. In order to improve the understanding of leucine-specific signaling in milk protein synthesis, proteins differentially expressed were explored in isolated bovine milk epithelial cells (MECs) from cows with high milk protein level, and their relationships with l-leucine were checked in bovine mammary alveolar cells (MAC-T cells) in this study
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