Creation of knockin mice for the fluorescence protein based in vivo identification of skeletal myofiber types

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.