Abstract
Japanese Black cattle (Japanese Wagyu) have a unique phenotype in which ectopic intramuscular fat accumulates in skeletal muscle, producing finely marbled beef. However, the mechanism of intramuscular fat formation in Japanese Black cattle remains unclear. To investigate the key genes involved in intramuscular fat accumulation, we comprehensively analyzed mRNA levels in subcutaneous and intramuscular fat tissues using RNA sequence (RNA-seq) analysis, which detected 27,606 genes. We identified eight key genes, namely carboxypeptidase E, tenascin C, transgelin, collagen type IV alpha 5 (COL4A5), cysteine and glycine-rich protein 2, PDZ, and LIM domain 3, phosphatase 1 regulatory inhibitor subunit 14A, and regulator of calcineurin 2. These genes were highly and specifically expressed in intramuscular fat tissue. Immunohistochemical analysis revealed a collagen network, including COL4A5, in the basement membrane around the intramuscular fat tissue. Moreover, pathway analysis revealed that, in intramuscular fat tissue, differentially expressed genes are related to cell adhesion, proliferation, and cancer pathways. Furthermore, pathway analysis showed that the transforming growth factor-β (TGF-β) and small GTPases regulators RASGRP3, ARHGEF26, ARHGAP10, ARHGAP24, and DLC were upregulated in intramuscular fat. Our study suggests that these genes are involved in intramuscular fat formation in Japanese Black cattle.
Highlights
Japanese Black cattle (i.e., Japanese Wagyu) are among the most expensive meats, characterized by excellent marbling, rich and sweet aroma [1,2]
During the analysis of a small amount of intramuscular fat samples collected from Japanese Black cattle, we collected subcutaneous fat from the same cattle to compare with intramuscular fat (Figure 1a)
We found no difference in the appearance of adipocytes between subcutaneous and intramuscular fat tissues, the size of adipocytes was significantly smaller (p < 0.05) in intramuscular fat (Figure 1b,c)
Summary
Japanese Black cattle (i.e., Japanese Wagyu) are among the most expensive meats, characterized by excellent marbling, rich and sweet aroma (the so-called Wagyu beef aroma) [1,2]. Marbling is a phenotype in which ectopic intramuscular fat accumulates in muscle tissue. Excessive dietary lipid intake and age-related senescence lead to ectopic intramuscular fat in skeletal muscle. Fatty acid accumulation correlates with lipotoxicity-related cell dysfunction and is associated with sarcopenic obesity [4]. Unlike cattle, humans accumulate intramuscular fat due to senescence and illness. The marbling traits of Japanese Black cattle are considered a valuable research model for elucidating the molecular mechanism of ectopic intramuscular fat formation in various livestock, as well as in humans [5]
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