Skeletal muscles of the mammalian trunk and limbs comprise myofibers that express four types of myosin heavy-chain (MyHC) isoforms, each with distinct contractile and metabolic properties. Despite histochemical and immunohistochemical staining to identify myofiber types, all myofiber types cannot be identified simultaneously in vivo. In this study, we generated a novel knock-in mouse model, termed "MusColor," that enables the simultaneous identification of individual MyHC isoforms through the expression of four fluorescent proteins. The identification of fibre types by fluorescent expression in MusColor mice was consistent with that achieved by immunostaining and had higher sensitivity. By studying the aging-associated changes in myofiber types using the MusColor mice, we were able to identify changes in hybrid myofibers that simultaneously express multiple MyHCs. Furthermore, by culturing satellite cells isolated from MusColor mice and treatment of thyroid hormone or rapamycin, changes in myofiber type and metabolic function could be analysed in living cells. The MusColor mouse proved useful for elucidating the mechanisms of muscle fibre changes caused by diseases such as sarcopenia, neuromuscular and metabolic diseases, as well as by exercise and nutritional environments.
© 2025. The Author(s).

In response to various stimuli, vascular smooth muscle cells (SMCs) can de-differentiate, proliferate and migrate in a process known as phenotypic modulation. However, the phenotype of modulated SMCs in vivo during atherosclerosis and the influence of this process on coronary artery disease (CAD) risk have not been clearly established. Using single-cell RNA sequencing, we comprehensively characterized the transcriptomic phenotype of modulated SMCs in vivo in atherosclerotic lesions of both mouse and human arteries and found that these cells transform into unique fibroblast-like cells, termed 'fibromyocytes', rather than into a classical macrophage phenotype. SMC-specific knockout of TCF21-a causal CAD gene-markedly inhibited SMC phenotypic modulation in mice, leading to the presence of fewer fibromyocytes within lesions as well as within the protective fibrous cap of the lesions. Moreover, TCF21 expression was strongly associated with SMC phenotypic modulation in diseased human coronary arteries, and higher levels of TCF21 expression were associated with decreased CAD risk in human CAD-relevant tissues. These results establish a protective role for both TCF21 and SMC phenotypic modulation in this disease.