The aim of this study was to investigate the function of the Hippo pathway member Yes-associated protein (Yap, gene name Yap1) in skeletal muscle fibres in vivo. Specifically we bred an inducible, skeletal muscle fibre-specific knock-in mouse model (MCK-tTA-hYAP1 S127A) to test whether the over expression of constitutively active Yap (hYAP1 S127A) is sufficient to drive muscle hypertrophy or stimulate changes in fibre type composition. Unexpectedly, after 5–7 weeks of constitutive hYAP1 S127A over expression, mice suddenly and rapidly lost 20–25% body weight and suffered from gait impairments and kyphosis. Skeletal muscles atrophied by 34–40% and the muscle fibre cross sectional area decreased by ≈40% when compared to control mice. Histological analysis revealed evidence of skeletal muscle degeneration and regeneration, necrotic fibres and a NADH-TR staining resembling centronuclear myopathy. In agreement with the histology, mRNA expression of markers of regenerative myogenesis (embryonic myosin heavy chain, Myf5, myogenin, Pax7) and muscle protein degradation (atrogin-1, MuRF1) were significantly elevated in muscles from transgenic mice versus control. No significant changes in fibre type composition were detected using ATPase staining. The phenotype was largely reversible, as a cessation of hYAP1 S127A expression rescued body and muscle weight, restored muscle morphology and prevented further pathological progression. To conclude, high Yap activity in muscle fibres does not induce fibre hypertrophy nor fibre type changes but instead results in a reversible atrophy and deterioration.

We have recently identified a number of Quantitative Trait Loci (QTL) contributing to the 2-fold muscle weight difference between the LG/J and SM/J mouse strains and refined their confidence intervals. To facilitate nomination of the candidate genes responsible for these differences we examined the transcriptome of the tibialis anterior (TA) muscle of each strain by RNA-Seq.

Citrate synthase (CS) is an enzyme of the Krebs cycle that plays a key role in mitochondrial metabolism. The aim of this study was to investigate the mechanisms underlying low activity of citrate synthase (CS) in A/J mice compared with other inbred strains of mice. Enzyme activity, protein content, and mRNA levels of CS were studied in the quadriceps muscles of A/J, BALB/cByJ, C57BL/6J, C3H/HeJ, DBA/2J, and PWD/PhJ strains of mice. Cytochrome c protein content was also measured. The results of the study indicate that A/J mice have a 50–65% reduction in CS activity compared with other strains despite similar levels of Cs mRNA and lack of differences in CS and cytochrome c protein content. CS from A/J mice also showed lower Michaelis constant ( K_m) for both acetyl CoA and oxaloacetate compared with the other strains of mice. In silico analysis of the genomic sequence identified a nonsynonymous single nucleotide polymorphism (SNP) (rs29358506, H55N) in Cs gene occurring near the site of the protein interacting with acetyl CoA. Allelic variants of the polymorphism segregated with the catalytic properties of CS enzyme among the strains. In summary, H55N polymorphism in Cs could be the underlying cause of low CS activity and its high affinity for substrates in A/J mice compared with other strains. This SNP might also play a role in resistance to obesity of A/J mice.

Properties of muscle fibers, i.e., their type, number and size, are important determinants of functional characteristics of skeletal muscle, and of the quality of meat in livestock. Genetic factors play an important role in determining variation in fiber properties, however, specific genes remain largely elusive. We examined histological properties of soleus muscle fibers in two strains of mice exhibiting a twofold difference in muscle mass, LG/J and SM/J, and their F2 intercross. The total number of muscle fibers (555 ± 106; mean ± SD) did not differ between the strains or between males and females. A higher percentage of type I fibers was observed in the LG/J compared to the SM/J strain (P < 0.001) in both males (45 ± 3 vs. 37 ± 4%) and females (58 ± 4 vs. 41 ± 3%). Across strains, females had a higher percentage of type I fibers than males (P < 0.001), and the sex effect was greater in the LG/J strain (strain-by-sex interaction, P < 0.001). The cross-sectional area (CSA) did not differ between type I and type IIA fibers, but was greater in the LG/J than the SM/J strain (1365 ± 268 vs. 825 ± 229 μm², P < 0.001). Three significant quantitative trait locus (QTL) affecting CSA for type I and type IIA fibers mapped to chromosomes (Chr) 1, 6, and 11 and three suggestive QTL for percentage of type I fibers mapped to Chr 2, 3, and 4. Within each significant QTL, regions of conserved synteny were also implicated in variation of similar traits in an analogous study in pigs. Our results provide the evidence that the intercross between the SM/J and LG/J strains is a promising model to search for genes affecting muscle fiber properties.

Berlin high (BEH) and Berlin low (BEL) strains selected for divergent growth differ threefold in body weight. We aimed at examining muscle mass, which is a major contributor to body weight, by exploring morphological characteristics of the soleus muscle (fiber number and cross sectional area; CSA), by analyzing the transcriptome of the gastrocnemius and by initiating quantitative trait locus (QTL) mapping. BEH muscles were four to eight times larger than those of BEL. In substrain BEH+/+, mutant myostatin was replaced with a wild-type allele; however, BEH+/+muscles still were two to four times larger compared with BEL. BEH soleus muscle fibers were two times more numerous ( P < 0.0001) and CSA was two times larger ( P < 0.0001) compared with BEL. In addition, soleus femoral attachment anomaly (SFAA) was observed in all BEL mice. One significant (Chr 1) and four suggestive (Chr 3, 4, 6, and 9) muscle weight QTLs were mapped in a 21-day-old F2 intercross ( n = 296) between BEH and BEL strains. The frequency of SFAA incidence in the F2 and in the backcross to BEL strain (BC_L) suggested the presence of more than one causative gene. Two suggestive SFAA QTLs were mapped in BC_L; however, their peak markers were not associated with the phenotype in F2. RNA-Seq analysis revealed 2,148 differentially expressed ( P < 0.1) genes and 45,673 single nucleotide polymorphisms and >2,000 indels between BEH+/+ and BEL males. In conclusion, contrasting muscle traits and genomic and gene expression differences between BEH and BEL strains provide a promising model for the search for genes involved in muscle growth and musculoskeletal morphogenesis.