Mouse soleus (slow) muscle shows greater intramyocellular lipid droplet accumulation than EDL (fast) muscle: fiber type-specific analysis.

Skeletal muscle is the main tissue of lipid metabolism and accordingly is critical for homeostasis and energy production; however, the determinants of lipid accumulation in skeletal muscle are unknown. Here, we examined whether the soleus muscle (predominantly slow-twitch fibers) has a higher lipid accumulation capacity than that of the extensor digitorum longus (EDL, predominantly fast-twitch fibers) muscle in mice. Soleus and EDL muscles were harvested from male C57BL/6J mice. The mRNA levels of genes involved in fatty acid import and triglyceride synthesis and accumulation were examined in soleus and EDL muscles. The intramyocellular lipid (IMCL) droplets of muscle cross sections and isolated single fibers were visualized by staining with BODIPY493/503, and fiber types were determined by immunofluorescent detection of myosin heavy chain (MyHC) isoforms. We detected higher mRNA expression of genes related to lipid accumulation in the soleus than the EDL. We also observed a marked increase of IMCL in single fibers from the soleus, but not the EDL, after treatment with a high-fat diet plus denervation. Interestingly, greater accumulation of IMCL droplets was observed in type 2A and 2X fibers (MyHC2A- and MyHC2X-positive fibers) than type 1 fibers (MyHC1-positive fibers) in soleus muscles. These results suggest that the soleus contains more IMCL owing to the higher population of type 2A fibers, and the difference in lipid accumulation between the soleus and EDL could depend on fiber type composition.

Perioperative Oral ß-Hydroxy-ß-Methylbutyrate Supplementation Ameliorates Sarcopenia in Rats Undergoing Major Hepatectomy.

ß-Hydroxy-ß-methylbutyrate (HMB), a metabolite of leucine, is known to increase muscle mass and strength. However, the effect of perioperative HMB supplementation in liver surgery is unclear. Moreover, the impact of HMB on the skeletal muscle fiber type also remains unclear. We investigated the impact of HMB on the body composition and skeletal muscle fiber type in sarcopenic rats undergoing major hepatectomy. Nine-week-old male F344/NSlc rats were maintained in hindlimb suspension (HLS) and were forcedly supplemented with HMB calcium salt (HMB-Ca, 0.58 g/kg×2 times) or distilled water in addition to free feeding. After 2 wk of HLS, the rats underwent 70% hepatectomy and were sacrificed 3 d after surgery. Body composition factors and the proportion of slow-twitch fibers in hindlimb muscles were evaluated. HMB maintained the body composition and hindlimb force and acted against their deterioration in sarcopenic rats, exerting a particular effect on lean mass weight, which was significant. In the histological study, HMB significantly increased the proportion of slow-twitch fibers in the soleus (p=0.044) and plantaris (p=0.001) of sarcopenic rats. HMB ameliorated deterioration of the body composition and increased the proportion of slow-twitch fibers in sarcopenic rats undergoing major hepatectomy.

Analysis of Masticatory Muscle Tendon-aponeurosis Hyperplasia by Using Next-generation Sequencing.

BACKGROUND/AIM: Masticatory muscle tendon-aponeurosis hyperplasia (MMTAH) is a disease associated with a mouth opening limitation. Here, we conducted a bioinformatics analysis to examine gene expression patterns in patients with MMTAH in comparison to those with facial deformity (FD). MATERIALS AND METHODS: Seven MMTAH patients and three FD patients were recruited. We conducted RNA sequencing analysis, quantitative reverse transcription polymerase chain reaction and immunoblot analysis. RESULTS: Of the identified 19,767 mapped read tags that showed clear differential expression, 2,471 genes were significantly up-regulated and 2,849 genes were significantly down-regulated in patients with MMTAH compared to those in patients with FD. Among the up-regulated genes, ten genes were significantly increased. The distribution of up-regulated and down-regulated genes at different ages tended to be similar. Moreover, the protein levels of Ankyrin Repeat Domain 2, Troponin T1 and myosin heavy chain 7, which are associated with slow twitch fibers and mechanical loading, were strongly expressed in patients with MMTAH compared to those in patients with FD. CONCLUSION: The gene expression pattern in MMTAH patients was similar regardless of age. As the transition of fast-to-slow twitch in the skeletal muscle is induced by mechanical loading, and up-regulation of slow twitch molecules was observed in MMTAH patients, mechanical loading is suggested to be implicated in MMTAH.

Protein Structure-Function Relationship at Work: Learning from Myopathy Mutations of the Slow Skeletal Muscle Isoform of Troponin T.

Troponin T (TnT) is the sarcomeric thin filament anchoring subunit of the troponin complex in striated muscles. A nonsense mutation in exon 11 of the slow skeletal muscle isoform of TnT (ssTnT) gene (TNNT1) was found in the Amish populations in Pennsylvania and Ohio. This single nucleotide substitution causes a truncation of the ssTnT protein at Glu180 and the loss of the C-terminal tropomyosin (Tm)-binding site 2. As a consequence, it abolishes the myofilament integration of ssTnT and the loss of function causes an autosomal recessive nemaline myopathy (NM). More TNNT1 mutations have recently been reported in non-Amish ethnic groups with similar recessive NM phenotypes. A nonsense mutation in exon 9 truncates ssTnT at Ser108, deleting Tm-binding site 2 and a part of the middle region Tm-binding site 1. Two splicing site mutations result in truncation of ssTnT at Leu203 or deletion of the exon 14-encoded C-terminal end segment. Another splicing mutation causes an internal deletion of the 39 amino acids encoded by exon 8, partially damaging Tm-binding site 1. The three splicing mutations of TNNT1 all preserve the high affinity Tm-binding site 2 but still present recessive NM phenotypes. The molecular mechanisms for these mutations to cause myopathy provide interesting models to study and understand the structure-function relationship of TnT. This focused review summarizes the current knowledge of TnT isoform regulation, structure-function relationship of TnT and how various ssTnT mutations cause recessive NM, in order to promote in depth studies for further understanding the pathogenesis and pathophysiology of TNNT1 myopathies toward the development of effective treatments.

COPD elicits remodeling of the diaphragm and vastus lateralis muscles in humans.

A profound remodeling of the diaphragm and vastus lateralis (VL) occurs in patients with moderate-to-severe chronic obstructive pulmonary disease (COPD). In this mini-review, we discuss the following costal diaphragm remodeling features noted in patients with moderate-to-severe COPD: 1) deletion of serial sarcomeres, 2) increased proportion of slow-twitch fibers, 3) fast-to-slow isoform shift in sarco(endo)plasmic reticulum Ca(2+)-ATPase, 4) increased capacity of oxidative metabolism, 5) oxidative stress, and 6) myofiber atrophy. We then present the sole feature of diaphragm remodeling noted in mild-to-moderate COPD under the heading "MyHC and contractile remodeling noted in mild-to-moderate COPD." The importance of VL remodeling in COPD patients as a prognostic indicator as well as a major determinant of the ability to carry out activities of daily living is well accepted. We present the remodeling of the VL noted in COPD patients under the following headings: 1) Decrease in proportion of slow-twitch fibers, 2) Decreased activity of oxidative pathways, 3) Oxidative and nitrosative stress, and 4) Myofiber atrophy. For each of the remodeling features noted in both the VL and costal diaphragm of COPD patients, we present mechanisms that are currently thought to mediate these changes as well as the pathophysiology of each remodeling feature. We hope that our mechanistic presentation stimulates research in this area that focuses on improving the ability of COPD patients to carry out increased activities of daily living.