Transcriptome analysis unveils multiple reasons behind delayed and slower deposition of intramuscular fat compared to subcutaneous fat in cattle
Abstract Background: Intramuscular fat refers to the white adipose tissue deposited between muscle fibers, and its quantity and distribution directly impact the quality and value of beef. Compared to subcutaneous fat, intramuscular fat develops later and accumulates more slowly in cattle. The reasons for the delayed development and slower growth of intramuscular fat in cattle remain unclear. Results: Histological analysis showed that adipocytes in intramuscular fat were smaller than those in subcutaneous fat from the same mature cattle, indicating a delayed development or slower growth of intramuscular fat compared to subcutaneous fat. Intramuscular fat had a lower capacity for retaining or incorporating long-chain fatty acids into triglycerides than subcutaneous fat. Comparing the transcriptomes of intramuscular and subcutaneous fat by RNA sequencing identified more than 1,000 genes differentially expressed (DEGs) between the two adipose depots. Genes upregulated in intramuscular fat included FOXO6, SLC27A1, HDAC9, WWTR1, and PIK3C2A, which are known to inhibit adipose tissue development and growth. Genes downregulated in intramuscular fat included FABP4, AGPAT2, ADIG, ADIRF, and PLIN2, which are known to promote adipose tissue development and growth. Functional enrichment analyses of these DEGs suggested that intramuscular fat may have a lower capacity for fatty acid binding and adipogenesis compared to subcutaneous fat. Furthermore, genes downregulated in intramuscular fat were enriched in signaling pathways such as the PPAR signaling pathway, whereas genes upregulated in intramuscular fat were enriched in pathways including the Wnt signaling pathway. Stromal vascular fraction (SVF) cells from intramuscular fat exhibited a lower adipogenic potential than those from subcutaneous fat. Conclusion: Multiple factors may contribute to the delayed and slower deposition of intramuscular fat compared to subcutaneous fat in cattle, including reduced fatty acid binding capacity, lower triglyceride synthesis, and decreased adipogenesis in intramuscular fat. These differences are possibly driven by lower expressions of genes such as AGPAT2, FABP4, and ADIG, higher expression of genes such as FOXO6, HDAC9, and SLC27A1, reduced activation of the PPAR signaling pathway, and increased activation of the Wnt signaling pathway in intramuscular fat. Overall design: To investigate the molecular basis of the delayed and slower deposition of intramuscular fat (IMF) compared to subcutaneous fat (SF) in cattle, we performed RNA-seq on adipose tissues isolated from the IMF and SF depots of five Angus-crossbred heifers. Total RNA was extracted from each tissue sample, and libraries were prepared and sequenced using paired-end sequencing on the Illumina NovaSeq platform. Differential gene expression analysis was conducted to identify depot-specific gene expression profiles. This transcriptomic dataset was used to explore biological processes and pathways that may explain differences in adipogenic capacity and lipid metabolism between IMF and SF. We then validated the differential expression of key genes and functional differences between these fat depots through cellular experiments.