Muscle gene expression patterns associated with differential intramuscular fat in cattle and markers for skeletal muscle growth rate and major cell types

Abstract Text: Growth rate, intramuscular fat content (IMF%), and IMF composition influence the value of individual animals. However, for IMF there are many different pathways to the final common process of Triacylglyceride (TAG) synthesis and storage in intramuscular adipocytes. Gene expression data from a number of cattle and sheep experiments was used to identify the pathways involved in the synthesis of IMF and the genes correlated with growth rate and as markers of cell populations. The datasets were from a time course of longissimus muscle (LM) development in Piedmontese (PxH) and Wagyu cross Hereford (WxH) cattle, from the LM of a group of 48 Brahman cattle of similar age and from the LM of a group of 20 sheep of similar age. The differential expression of genes between WxH (high marbling) and PxH (high muscling) cattle and the correlation of gene expression with measured IMF in the Brahman and sheep datasets was integrated with known biochemical pathways. Expression of genes encoding proteins involved in the synthesis and deposition of TAG was most correlated with IMF%. In well-fed immature animals TAG deposition rate (estimated by TAG gene expression) was proportional to current IMF%. By comparing TAG gene expression and IMF% we identified a small number of animals with unexpectedly low or high rates of IMF deposition for their IMF%. The genes in the fatty acid (FA) synthesis pathway were less correlated with IMF%, presumably as IMF TAG can contain preformed FAs from circulation as well as those synthesized de novo by intramuscular adipocytes. By comparing changes in expression of the TAG and FA genes we estimated the relative contributions of synthesised and preformed FAs to IMF deposition on different diets. The expression of two groups of genes in the LM of the Brahman steers, significantly enriched for “cell cycle” and “ECM (extracellular matrix) organization” GO-terms, was correlated with average daily gain/kg liveweight. However, expression of the same genes was only partly related to growth rate across the development time course in (PxH and WxH). K-means clustering of genes with similar expression profiles to the ECM genes was undertaken. Analysis of the clusters and published markers of different cell types in muscle suggested that the “cell cycle” and “ECM” signals were from the fibro/adipogenic lineage. The increase in ECM remodelling required for increased IMF deposition probably altered the relationship between the expression of these genes and animal growth rate.

Keywords: cattle, lipid