141 The effect of anatomical location and marbling degree on the extracellular matrix properties in muscle and adipose tissue of finished beef steers.

Abstract Beef quality grade in the U.S. is mainly dependent on the deposition of intramuscular adipose tissue (IMAT), also known as marbling. Adipose tissue also deposits in the subcutaneous (SCAT) and visceral (VIAT) depots, but in excess can lead to unnecessary fatness in beef carcasses. The growth of adipose tissue through adipogenesis depends on the extracellular matrix’s (ECM) support, which is the non-cellular component of tissues majorly formed by collagens and proteoglycans. Our objective was to define the adipose tissue and muscle ECM characteristics in a depot- and marbling-specific manner in finished beef steers. Half-sibling, black Angus beef steers kept under similar feeding and management conditions were processed in a local abattoir. Paired IMAT and adjacent SCAT were dissected along with muscle from the same longissimus dorsi section (12th-13th rib), and VIAT obtained from the omentum. Upon marbling scoring, four steers were assigned to high marbling (HM; 798.5 ± 31) and four to lower marbling group (LM; 629.0 ± 7), grading prime and choice, respectively (Figure 1A). As expected, HM had increased muscle fat (21±2.17%) vs. LM (7.0±0.08%), as evaluated by Folch method in HM (Fig.1B), while backfat thickness (BFT) and KPH% were similar between groups. Tissue samples were analyzed for total collagen content (picrosirius red), gene expression of targeted ECM genes via RT-qPCR, and viscoelastic properties of whole and decellularized (native ECM) tissues via shear rheology. Effects of group, depot, and their interaction were analyzed via GLM procedure in SAS. While there were no differences in COL1A1, COL6A1, MMP9, MMP10, TIMP1, TIMP2 and LOX gene expression, TGFB1 was increased in HM vs. LM (Fig.1C) independent of depot. TGFB1 upregulates ECM remodeling genes to enhance lipogenesis. FN1, a fibrotic marker associated with obesity and insulin resistance in humans, was the highest in VIAT compared with IMAT and whole muscle (Fig.1D), independent of marbling. In line with FN1 expression, rheology (Fig.1E) revealed VIAT is stiffer and more viscous than IMAT in HM, but not in LM. Notably, viscoelasticity of whole and decellularized adipose and muscle tissues (ECM) were similar, suggesting cells do not contribute to stiffness. Lastly, preliminary picrosirius red results showed increased collagen in IMAT vs. VIAT and SCAT, which were similar among them (Fig.1F). Our results suggest that 1) marbling scores vary in beef steers with similar genetic background and nutritional management, even though overall fatness (BFT and KPH%) were similar; 2) specific genes involved on ECM remodeling, such as TGFB1, may be a key modulator of marbling deposition in beef steers; and 3) adipose tissue viscoelastic properties and collagen content are not directly affected by marbling degree, but softer ECM and lower expression of pro-fibrotic markers (i.e. FN1) in muscle appear to favor IMAT deposition in finished beef steers.