Generation of MLC-2v-tdTomato knock-in reporter mouse line.

MLC-2v is a myosin light chain regulatory protein which is specifically expressed in ventricular cardiomyocytes and slow twitch skeletal muscle cells. MLC-2v plays critical roles in ventricular maturation during heart development. Mice lacking MLC-2v are embryonic lethal due to heart failure associated with abnormal myofibrillar organization of ventricular cardiomyocytes. To study the development of ventricular cardiac muscle and slow twitch skeletal muscle, we generated a new MLC-2v reporter mouse line by knocking-in a tdTomato reporter cassette into 3' UTR of the MLC-2v gene without disrupting the endogenous gene. Our results demonstrated specific MLC-2v-tdTomato knock-in reporter expression in ventricular cardiomyocytes and slow twitch muscle during myogenesis, precisely recapitulating the spatiotemporal expression pattern of endogenous MLC-2v. No tdTomato expression was observed in the atria, fast twitch muscle or other organs throughout development into adulthood. Isolated neonatal and adult ventricular cardiomyocytes uniformly express tdTomato. Taken together, MLC-2v-tdTomato knock-in reporter mouse model described in this article will serve as a valuable tool to study cardiac chamber and skeletal muscle specification during development and regeneration by overcoming the pitfalls of transgenic strategies.