Factors influencing bovine intramuscular adipose tissue development and cellularity

Abstract Text: Appearance, distribution and amount of intramuscular fat (IMF) or marbling are highly variable depending on nutrition, gender, environmental and genetic factors. On the molecular level, the concerted action of several factors, including hormones, receptors, transcription factors etc., determines where clusters of adipocytes arise. Therefore, the aim remains to identify biological markers of IMF to increase the ability to identify animals that deposit IMF early in age in order to ensure the competitiveness of meat products and increase efficiency of high quality meat production. In an attempt to unravel the cellular development of marbling, we investigated on the one hand the abundance of markers for adipogenic differentiation during fattening of cattle and on the other hand the transcriptome of muscle and dissected IMF from different breeds. Markers of different stages of adipogenic differentiation are well known from cell culture experiments. However, early markers are transiently expressed and late markers may reflect the number of mature adipocytes in the sample rather than gene activity in a tissue. On the cellular level, the development of marbling requires recruitment, proliferation, and differentiation of adipogenic cells. Hypertrophy of adipocytes is limited and hyperplasia occurs to store excess energy in form of lipids in new cells. Within muscles, hyperplasia and hypertrophy of adipocytes can be observed throughout life. In a recent study, we investigated the localization and abundance of delta-like homolog 1 (DLK1) and CCAAT/enhancer-binding protein beta (CEBPB), early markers of adipogenic differentiation, in bovine muscle tissue. Cell culture models demonstrated high expression of DLK1 in preadipocytes and complete disappearance during differentiation to adipocytes. Accordingly, we could demonstrate inverse relationship between IMF content and number of DLK1 positive cells in bovine muscle. Considering the cellular environment of differentiating adipocytes in muscle and accepting mature adipocytes and myocytes as secretory cells, it becomes obvious that cross-talk between cells via adipokines and myokines may be important for IMF development. Secreted proteins can act on other cells inhibiting or stimulating their development via autocrine and paracrine actions.  Among them, agouti signaling protein and thrombospondin 4 were further investigated. Thromospondin 4 is a potential myokine with supposed impact on IMF development, which has been identified in a cross species comparison of muscle transcriptomes and secretomes. Furthermore, results from transcriptome analysis suggest involvement of genes which are not directly related to adipogenesis and lipid metabolism providing new candidates for future research.

Keywords:

Intramuscular Adipogenesis, Adipokines, Myokines