Understanding and Manipulating Lipid Metabolism in Ruminants

Lipid metabolism in ruminants underpins major production traits. Intramuscular fat content (IMF%) influences the value of individual animals, especially for higher marbling markets. We have used gene expression data from a number of cattle and sheep data sets to build a model of proteins involved in the synthesis of IMF in ruminants. Not surprisingly, genes encoding proteins involved in the synthesis and deposition of triacylglyceride (TAG), the TAG gene set, are most correlated with IMF%. The genes in the fatty acid (FA) synthesis pathway, the FA gene set, were less correlated with IMF%, presumably as IMF TAG can contain preformed FAs from circulation as well as those synthesized de novo by adipocytes within muscle. Less well correlated with IMF% were genes encoding a number of the enzymes responsible for the generation of the building blocks for FA synthesis. This may reflect the different sources of precursors for acetyl-CoA; circulating acetate and ketone bodies, intracellular FA oxidation, glucose, and lactate from muscle cells. By comparing changes in expression of the TAG and FA gene sets we estimated the relative contributions of synthesised and preformed FAs to TAG deposition. In well-fed immature animals TAG deposition (as estimated by TAG gene set gene expression) was proportional to current IMF%. By comparing TAG gene set expression and IMF% we could identify animals with unexpectedly low or high rates of IMF deposition. Approaches to the modification of ruminant lipid metabolism will be discussed.