Skeletal muscle protein and amino acid metabolism in hereditary mouse muscular dystrophy. Accelerated protein turnover and increased alanine and glutamine formation and release

Cross-sectional serum metabolomic study of multiple forms of muscular dystrophy

Muscular dystrophies are characterized by a progressive loss of muscle tissue and/or muscle function. While metabolic alterations have been described in patients’‐derived muscle biopsies, non‐invasive readouts able to describe these alterations are needed in order to objectively monitor muscle condition and response to treatment targeting metabolic abnormalities. We used a metabolomic approach to study metabolites concentration in serum of patients affected by multiple forms of muscular dystrophy such as Duchenne and Becker muscular dystrophies, limb‐girdle muscular dystrophies type 2A and 2B, myotonic dystrophy type 1 and facioscapulohumeral muscular dystrophy. We show that 15 metabolites involved in energy production, amino acid metabolism, testosterone metabolism and response to treatment with glucocorticoids were differentially expressed between healthy controls and Duchenne patients. Five metabolites were also able to discriminate other forms of muscular dystrophy. In particular, creatinine and the creatine/creatinine ratio were significantly associated with Duchenne patients performance as assessed by the 6‐minute walk test and north star ambulatory assessment. The obtained results provide evidence that metabolomics analysis of serum samples can provide useful information regarding muscle condition and response to treatment, such as to glucocorticoids treatment.

Long-term voluntary exercise, representing habitual exercise, lowers visceral fat and alters plasma amino acid levels in mice

OBJECTIVES

To determine the impact of long-term voluntary exercise, representing habitual exercise for the prevention of lifestyle-related diseases, on glucose, lipid, and amino acid metabolism in mice.

METHODS

Twenty-four mice aged 6 weeks were divided into three groups. Two groups (16 mice) were housed individually in either cages equipped with a running wheel (8 mice, exercising, Ex-mice) or without (8 mice, sedentary, Se-mice) for 24 weeks. The remaining group (8 mice) was sacrificed at 6 weeks of age. Biomarkers related to glucose, lipid, and amino acid metabolism were examined.

RESULTS

Ex-mice ran voluntarily, predominantly in the dark. The distance per day peaked at 4 weeks and then decreased until 12 weeks to around the level seen at the beginning of the experimental period, and was maintained at 4.9 ± 0.2 km/day from 12 to 24 weeks. Ex-mice showed a similar adrenal weight and vitamin C content to Se-mice but had a significantly lower body weight and higher food intake. Ex-mice also showed a higher skeletal muscle weight, a lower white adipose tissue and liver weight, associated with lower plasma leptin and insulin-like growth factor-1 levels, and a lower hepatic triglyceride content. Analysis of plasma amino acids showed that Ex-mice had significantly higher phenylalanine, tyrosine, and glutamine levels, resulting in a significantly lower Fischer's ratio.

CONCLUSIONS

We present an animal model of long-term voluntary exercise under low stress. Findings related to the effects of long-term voluntary exercise on lipid, and amino acid metabolism in our mouse model indicate that such an exercise regimen may affect pathophysiological states related to appetite and behavior.

Age-related changes and tissue distribution of parvalbumin in normal and dystrophic mice of strain 129 ReJ

In murine muscular dystrophy, hindlimb muscle contains a functionally defective thiol protease inhibitor (TPI) which has been implicated in the onset and progression of the disease in mice. More recently, this protease inhibitor has been identified as parvalbumin, a calcium binding protein. In this study, a polyclonal antibody against mouse muscle parvalbumin was used to study the concentration and distribution of this protein in normal and dystrophic male mice at various ages. Immunodetection assays were used to screen extracts of hindlimb, forelimb, brain, heart, lung, liver, and kidney in 60-day-old normal and dystrophic male mice for parvalbumin content. Parvalbumin was detected in relatively high amounts in both hindlimb and forelimb muscle extracts, while much lower concentrations were detected in brains of normal and dystrophic animals. No parvalbumin was detected in the lung, liver, heart, or kidney extracts using the immunoassay. With aging, the parvalbumin concentration in hindlimb muscle of normal mice remained fairly constant for 90 days, whereupon the level increased at 120 days. In contrast, the parvalbumin concentration in hindlimb muscle of dystrophic mice decreased steadily with age to about 22%% of normal animals at 120 days. The parvalbumin content was also reduced in dystrophic brain.

Lipid peroxide and antioxidant enzymes in muscle and nonmuscle of dystrophic mouse

To determine whether abnormality in redox metabolism occurs specifically in certain individual dystrophic muscles, thiobarbituric acid reactivity, free radical scavengers, and oxidative marker enzymes were measured in the liver, kidney, erythrocytes, heart, and four different individual skeletal muscles from C57BL/6J dy/dy mice. Superoxide dismutases were assayed by specific radioimmunoassays, which enabled the study of a small individual murine muscle. Glutathione peroxidase and catalase were increased markedly in each individual dystrophic skeletal muscle studied and less markedly in the heart. Manganosuperoxide dismutase and thiobarbituric acid reactivity were decreased to a similar extent in each dystrophic skeletal muscle. Cuprozinc superoxide dismutase was decreased in the soleus muscle. Only a minimal biochemical change occurred in nonmuscles. Fumarase activity correlated closely with the level of manganosuperoxide dismutase. These results suggest that muscle protein breakdown occurs independently of lipid peroxidation despite the presence of tissue-specific abnormality of redox metabolism in dystrophic muscle.

Radiation-induced increase in the release of amino acids by isolated, perfused skeletal muscle

Local exposure of the hindquarter of the rat to 15 Gy of gamma-radiation resulted, 4-6 h after irradiation, in an increased release of amino acids by the isolated, perfused hindquarter preparation, 70 per cent of which is skeletal muscle. This increase in release involves not only alanine and glutamine which are synthesized to a large extent de novo in muscle, but also those amino acids which are not metabolized by muscle and, therefore, released in proportion to their occurrence in muscle proteins. Because metabolic parameters and content of energy-rich phosphate compounds in muscle remain unchanged, it is unlikely that general cellular damage is the underlying cause of the radiation-induced increase in amino acid release. The findings strongly favour the hypothesis that the increased availability of amino acids results from enhanced protein breakdown in skeletal muscle which has its onset shortly after irradiation. This radiation-induced disturbance in protein metabolism might be one of the pathogenetic factors in the aetiology of radiation myopathy.

Accelerated protein turnover in the skeletal muscle of dystrophic mice

The excretion of 3-methylhistidine increased in the urine of dystrophic mice C57BL/6J. The content of 3-methylhistidine residue decreased in the muscle proteins of dystrophic mice, but not in other organs. Methylated proteins in the skeletal muscle, actin and myosin, were partially purified from the dystrophic and control muscles. The amount of 3-methylhistidine residue in unit weight of the actin and myosin preparations was normal in dystrophic muscle. These three facts indicate that the turnover rates of actin and myosin are increased in the muscle of the dystrophic mice.

Ribosomal protein synthesis in cultured skin fibroblast cells obtained from patients with Duchenne muscular dystrophy

Ribosomes and ribosomal subunits were extracted from cultured skin fibroblasts from patients with Duchenne muscular dystrophy (DMD). A poly(U)-directed polyphenylalanine synthesis system was used to test 80S ribosomes from DMD cells and normal controls as well as hybrid 80S couples of subunits from DMD cells and control cells. The activity of ribosomes extracted from the patients was 38-62% lower than that of normal controls. Of the 80S hybrid ribosomes, only those consisting of 40S subunits from DMD cells and 60S subunits from the control cells, showed a similar decrease in activity. That means that the defect is exclusively based on an alteration in the small ribosomal subunit.

Protein degradation in cultured skeletal muscle from Duchenne muscular dystrophy patients

The loss of contractile protein in Duchenne muscular dystrophy could result from low rates of synthesis, abnormally high rates of protein degradation, or a combination of both. We measured overall protein degradation rates in cultured human muscle cells obtained at biopsy from patients with Duchenne dystrophy or various muscle diseases and normal subjects. Measurements were performed on confluent cultures exhibiting no growth and containing a mixed cell population of myoblasts, fibroblasts, and multinucleated myotubes. Using a new double-isotope labeling protocol, we found protein degradation rates in all three groups to be similar (KD = 0.0171-0.0176 hr-1), suggesting no detectable abnormality of overall protein degradation in cells derived from Duchenne dystrophy patients.

Content and synthesis of glycolytic enzymes and creatine kinase in skeletal muscles and normal and dystrophic chickens

A number of workers have reported that avian muscular dystrophy causes alterations in the levels of certain enzyme activities in "fast-twitch" muscle fibers but has little effect on enzyme activities in "slow-twitch" muscle fibers. In the present work, the effects of this disease on the content and relative rates of synthesis of a number of glycolytic enzymes and the skeletal muscle-specific MM isoenzyme of creatine kinase in chicken muscles was investigated. It was shown that (i) the approximate 50% reductions in steady-state concentrations of three glycolytic enzymes (aldolase, enolase, and glyceraldehyde-3-P dehydrogenase) in dystrophic breast (fast-twitch) muscle result predominantly from decreases in relative rates of synthesis, rather than accelerations in relative rates of degradation, of these proteins in the diseased tissue; (ii) in contrast to the situation with the glycolytic enzymes, muscular dystrophy has only minor effects (25% or less) on the content and relative rate of synthesis of MM creatine kinase in breast muscle fibers; (iii) the muscular dystrophy-associated alterations in content and synthesis of the glycolytic enzymes in breast muscle fibers become apparent only during postembryonic maturation of this tissue; and (iv) as expected, muscular dystrophy has no significant effect on the content or relative rates of synthesis of glycolytic enzymes in slow-twitch lateral adductor muscles of the chicken. These results are discussed in terms of the apparent similarities between the effects of muscular dystrophy and surgical denervation on the protein synthetic programs expressed by mature fast-twitch muscle fibers.

Excretion of total and muscular N tau-methylhistidine and creatinine in muscle diseases

The daily urinary excretions of N tau-methylhistidine and creatinine from 52 adult patients were measured under standardized conditions. The ratio of N tau-methylhistidine to creatinine excretion was calculated on the basis of the total and muscle-specific excretion rates and correlated to the clinical status of the patients. In patients with muscular diseases and in those with diseases of the central nervous system, the total daily excretion of both metabolites was about 30% lower than in controls. The muscle-specific ratio in patients with diseases of the central nervous system and patients with muscular diseases was not different from that observed in controls. Only in patients with neurogenic atrophies was the ratio elevated, so that it was more than twice the control value. The ratio of excreted N tau-methylhistidine/creatinine is only valid as an indicator of myofibrillar protein breakdown after correction for the contribution of nonskeletal muscle tissues to the urinary excretion.

Content and synthesis of several abundant glycolytic enzymes in skeletal muscles of normal and dystrophic mice

In both 2- and 3-month-old 129 ReJ mice, the catalytic activity levels of three enzymes involved in glycogen breakdown (phosphorylase, enolase, and aldolase) were found to be 35-50% lower in hind limb muscles of dystrophic mice as compared with normal mice. The reduced activities of these enzymes in the diseased tissue was directly due to corresponding reductions in the number of enzyme molecules rather than being due to inactivation of the enzymes in the dystrophic muscle. Results of short term double isotope incorporation experiments conducted with muscle explants in vitro suggested that the rates of synthesis of these enzymes, and of most other abundant cytosolic proteins, relative to each other, were similar in hind limb muscles of normal and dystrophic mice. The present work on murine muscular dystrophy is discussed in terms of our previous studies into the influence of avian muscular dystrophy on the content and synthesis of abundant glycolytic enzymes in chicken skeletal muscles.

Increased content of glucocorticoid receptors in mouse muscular dystrophy

An analysis of the glucocorticoid receptor population of skeletal muscle and liver from dystrophic and control mice was performed. These data demonstrate that the cytosol of muscles from dystrophic animals contain a significantly higher concentration of glucocorticoid receptors (approximately 100%) relative to the cytosols of muscles from control mice. No differences in the glucocorticoid receptor population was noted in the livers removed from these animals. These data are consistent with previous observations on the chicken dystrophic model, and may suggest a general phenomenon associated with dystrophy regardless of species or type.

Cytosolic androgen receptor in skeletal muscle from normal and dystrophic mice

The cytosolic androgen receptor was studied in skeletal muscle of normal mice and of mice with hereditary muscular dystrophy (129/ReJ-dy). Mouse muscle contains a cytosolic androgen receptor with high affinity (KD 0.6 nM) and low capacity (Bmax 3fmol/mg protein). Dystrophic muscle contains a receptor with similar binding properties. The receptors from muscle of dystrophic and normal mice bind to DNA-cellulose affinity columns and are eluted with similar patterns. The reported failure of androgens to alleviate the clinical signs of murine muscular dystrophy is probably not due to decreased androgen responsiveness of the affected muscles.