Transcriptome and cellular evidence of depot-specific function in beef cattle intramuscular, subcutaneous, and visceral adipose tissues
Deposition of intramuscular adipose tissue (IMAT; marbling) is one of the primary determinants for beef quality grade within the U.S. However, IMAT accumulation is often secondary to subcuta-neous (SCAT) and visceral (VIAT) adipose tissue deposition, which results in lower product yield. The mechanisms that underlie the differences in the accumulation of IMAT, SCAT, and VIAT are still not fully understood. The aim of this study was to define the depot-specific transcriptome and adipocyte function in IMAT, SCAT and VIAT in beef cattle. Functional transcriptome analysis in-dicated the activation of pathways for greater lipid accumulation and immune function in VIAT and SCAT compared with IMAT. Florescent activated cell sorting analysis identified a greater percentage of adipocyte stem and progenitor cells (ASPC) within IMAT compared to SCAT and VIAT, but lower ASPC's proliferation in vitro, suggesting potential functional defects on IMAT's adipogenic capacity. In vitro culture of adipocytes revealed greater lipid accumulation and insulin responses, and lower lipolysis of SCAT compared to IMAT adipocytes, with VIAT adipocytes having a characteristic of both SCAT, and IMAT adipocytes. Our findings revealed the de-pot-specific transcriptional profile of IMAT, SCAT and VIAT in beef cattle, which were corrobo-rated by differences on adipocyte metabolic function in vitro. Overall design: Four beef animals (3 steers, 1 heifer) were euthanized following humane slaughter practices using a captive bolt stinner at the Texas Tech University meat Lab and tissue samples were collected immediately after exsanguination and hide removal to prevent tissue contamination. Animal history, age and background were not known. All animals were appraised be-fore slaughter and there was no sign of distress or illness at the time of entering the fa-cility. A section of the Longissimus dorsi (LD) muscle (± 500 g) was excised from the 9-11th rib section, placed into a stainless-steel tray, and immediately placed on ice for performing the dissection of IMAT and SCAT (backfat) samples using tweezers and a scalpel. VIAT samples were collected at evisceration from the greater omentum. Tissue subsamples of approximately 1g each were flash frozen in liquid nitrogen for subsequent RNA sequencing analysis.