Proteomic tools help understanding the metabolic adaptation to negative energy balance in dairy cows

High-yielding dairy cows have enormous energy and nutrient requirements for milk production which are generally not met by a sufficient feed intake resulting in a negative energy balance (NEB) characterized by mobilisation of body reserves. It is still controversial whether during early lactation the mobilization of body reserves causes insufficient feed intake or insufficient feed intake causes mobilization of body reserves. In order to distinguish between cause and effect, we designed feed-restriction studies modelling NEB as well as follow-up studies on periparturient dairy cows and examined metabolic adaptation processes during NEB. To this end, 2D-gel based proteomic approaches coupled with MALDI-TOF-MS and MALDI-TOF-TOF analyses are often used for the investigation of changes in protein expression, posttranslational modifications (PTMs) and protein identification, while subsequent Western Blots are applied to confirm the existence and expression of individual proteins. Proteomic profiling in tissues obtained from frequent liver and muscle biopsies or from slaughter provides insight into regulatory mechanisms at the translational and posttranslational level. We were able to demonstrate that phosphorylation of the adenosine monophosphate-activated protein kinase (AMPK), a cellular energy key sensor, is increased in hypothalamus and liver but not in skeletal muscle during NEB. Muscle tissue in early lactation showed reduced abundance of muscular cytoskeletal proteins and enzymes involved in glycogen synthesis, fatty acid degradation, and TCA cycling, while the expression of enzymes involved in glycolysis, lactate and ATP production was increased. The functional characterisation of the hepatic oxidative metabolism is of particular interest because of its role to provide substrates for the mammary gland and its involvement in the control of feed intake. While feed restriction down-regulated hepatic proteins associated with fatty acid oxidation, early lactation expression of enzymes participating in fatty and amino acid degradation, TCA cycling, ATP production, and oxidative stress defence was increased. The integration of proteome data with corresponding plasma metabolite and hormone concentrations allowed us to propose an inter-organ crosstalk model in which hepatic and skeletal muscle metabolism in early lactating cows supports gluconeogenesis for milk production while hepatic oxidation of fatty acids interferes with the control of feed intake in the brain.