Myh4, encoding myosin heavy chain IIB (MyHC-IIB), is crucial for muscle function. MyHC-IIB expression is associated with muscle hypertrophic growth [2]. Muscle fibers in adult rodent skeletal muscles are classified into 4 subtypes based on myosin heavy chain proteins, with Myh4 corresponding to type IIB fibers [3].

In Myh4 null mice, postnatal muscle growth is affected. The gastrocnemius muscle loses up to 50% of its fibers between 2-4 weeks of age, and to compensate, type I, IIA, and IIX(D) fibers increase in prevalence and size. Stimulating the oxidative program in muscle only partially protects from fiber loss. Traditional means of stimulating muscle hypertrophy, like breeding with myostatin null mice or feeding clenbuterol, have little impact on Myh4 -/- mice, suggesting existing fibers have maximized their compensatory capacity [1].

In conclusion, Myh4 is essential for muscle fiber maintenance, transition, and hypertrophy during postnatal muscle growth. The study of Myh4 null mice provides insights into the role of Myh4 in muscle development and offers new avenues for attenuating fast-twitch fiber loss in aged or diseased muscles [1].