Coordinated gene expression between skeletal muscle and intramuscular adipose tissue in growing beef cattle

Previous research indicates that metabolism and fiber type of skeletal muscle is related to intramuscular lipid content. It is hypothesized that changes in skeletal muscle gene expression influence adipose tissue development. The objective of this study was to determine differences in the metabolism and intercellular signaling of skeletal muscle fibers within the same muscle group that could be responsible for the initiation of intramuscular adipose tissue development and differentiation. Longissimus dorsi muscle samples were collected from steers (n = 12; 385 d of age; 378 kg BW) grazing wheat pasture. Longissimus muscle samples were dissected under magnification and sorted into 3 categories based on visual stage of adipose tissue development: immature intramuscular adipose tissue (MM), intermediate intramuscular adipose tissue (ME), and mature intramuscular adipose tissue (MA). Additionally, muscle fibers lying adjacent to each intramuscular adipose tissue (IM) category and those not associated with IM tissue were collected and stored separately. Quantitative real-time PCR was used to determine relative fold change in genes involved in metabolism, angiogenesis, formation of extracellular matrix, and intercellular signaling pathways in both LM and IM samples. Gene expression data were analyzed using a GLM that included the fixed effect of tissue. Pearson correlation coefficients were also computed between gene expression in LM and IM tissue samples that were at the same stage of development. Fatty acid binding protein 4 and peroxisome proliferator-activated receptor. mRNA expression were 3.56- and 1.97-fold greater (P < 0.05) in ME and MA IM compared with MM IM whereas delta-like 1 homolog mRNA expression was 1.43-fold less (P < 0.01) in MA IM compared with MM IM, indicating successful separation into different development categories. Genes associated with metabolism and angiogenesis in LM tissue showed no differences among stages of development. Myostatin expression did not change in LM tissue; however, expression of myostatin receptor and follistatin mRNA decreased (P < 0.01) as IM matured. Collagen type I and collagen type VI mRNA expression were 2.5- and 1.32-fold greater in LM associated with MM IM than in LM associated with ME IM. Angiogenic growth factors in MM IM tissue had a strong positive correlation (r >= 0.69) with angiogenic growth factors in LM associated with MM IM; however, no correlation was observed in ME or MA IM. These data indicate a coordinated effort between LM and IM in early stages of IM development.