Simultaneous loss of skeletal muscle myosin heavy chain IIx and IIb causes severe skeletal muscle hypoplasia in postnatal mice.

The skeletal muscle myosin heavy chain (MyHC) is a fundamental component of the sarcomere structure and muscle contraction. Two of the three adult fast MyHCs, MyHC-IIx and MyHC-IIb, are encoded by Myh1 and Myh4, respectively. However, skeletal muscle disorders have not yet been linked to these genes in humans. MyHC-IIb is barely detectable in human skeletal muscles. Thus, to characterize the molecular function of skeletal muscle MyHCs in humans, investigation of the effect of simultaneous loss of MyHC-IIb and other MyHCs on skeletal muscle in mice is essential. Here, we generated double knockout (dKO) mice with simultaneous loss of adult fast MyHCs by introducing nonsense frameshift mutations into the Myh1 and Myh4 genes. The dKO mice appeared normal after birth and until 2 weeks of age but showed severe skeletal muscle hypoplasia after 2 weeks. In 3-week-old dKO mice, increased expression of other skeletal muscle MyHCs, such as MyHC-I, MyHC-IIa, MyHC-neo, and MyHC-emb, was observed. However, these expressions were not sufficient to compensate for the loss of MyHC-IIb and MyHC-IIx. Moreover, the aberrant sarcomere structure with altered expression of sarcomere components was observed in dKO mice. Our findings imply that the simultaneous loss of MyHC-IIb and MyHC-IIx is substantially detrimental to postnatal skeletal muscle function and will contribute to elucidating the molecular mechanisms of skeletal muscle wasting disorders caused by the loss of skeletal muscle MyHCs.

An Analysis of Differentially Expressed Coding and Long Non-Coding RNAs in Multiple Models of Skeletal Muscle Atrophy.

The loss of skeletal muscle mass (muscle atrophy or wasting) caused by aging, diseases, and injury decreases quality of life, survival rates, and healthy life expectancy in humans. Although long non-coding RNAs (lncRNAs) have been implicated in skeletal muscle formation and differentiation, their precise roles in muscle atrophy remain unclear. In this study, we used RNA-sequencing (RNA-Seq) to examine changes in the expression of lncRNAs in four muscle atrophy conditions (denervation, casting, fasting, and cancer cachexia) in mice. We successfully identified 33 annotated lncRNAs and 18 novel lncRNAs with common expression changes in all four muscle atrophy conditions. Furthermore, an analysis of lncRNA-mRNA correlations revealed that several lncRNAs affected small molecule biosynthetic processes during muscle atrophy. These results provide novel insights into the lncRNA-mediated regulatory mechanism underlying muscle atrophy and may be useful for the identification of promising therapeutic targets.

Myoparr-Associated and -Independent Multiple Roles of Heterogeneous Nuclear Ribonucleoprotein K during Skeletal Muscle Cell Differentiation.

RNA-binding proteins (RBPs) regulate cell physiology via the formation of ribonucleic-protein complexes with coding and non-coding RNAs. RBPs have multiple functions in the same cells; however, the precise mechanism through which their pleiotropic functions are determined remains unknown. In this study, we revealed the multiple inhibitory functions of heterogeneous nuclear ribonucleoprotein K (hnRNPK) for myogenic differentiation. We first identified hnRNPK as a lncRNA Myoparr binding protein. Gain- and loss-of-function experiments showed that hnRNPK repressed the expression of myogenin at the transcriptional level. The hnRNPK-binding region of Myoparr was required to repress myogenin expression. Moreover, hnRNPK repressed the expression of a set of genes coding for aminoacyl-tRNA synthetases in a Myoparr-independent manner. Mechanistically, hnRNPK regulated the eIF2α/Atf4 pathway, one branch of the intrinsic pathways of the endoplasmic reticulum sensors, in differentiating myoblasts. Thus, our findings demonstrate that hnRNPK plays lncRNA-associated and -independent multiple roles during myogenic differentiation, indicating that the analysis of lncRNA-binding proteins will be useful for elucidating both the physiological functions of lncRNAs and the multiple functions of RBPs.