Decrease in urinary excretion of 3-methylhistidine by patients with Duchenne muscular dystrophy during glucocorticoid treatment

Beta-hydroxy-beta-methylbutyrate (HMB) improves daily activity and whole-body protein metabolism in Duchenne muscular dystrophy dogs: a pilot study

Duchenne muscular dystrophy (DMD) is a severe neuromuscular disease due to loss of dystrophin, leading to progressive muscle wasting and physical inactivity. In this pilot study, we studied the effect of daily supplementation of the anabolic substrate beta-hydroxy-beta-methylbutyrate (HMB) on whole body protein and amino acid kinetics using novel isotope methods and daily activity in a canine model of DMD. Six DMD dogs were administered 3 g daily of HMB or placebo for 28 days according to a randomized, placebo-controlled, double-blinded crossover design. We measured pre- and post-intervention daily activity, biochemistry markers, and whole-body amino acid kinetics. We tracked daily activity with an activity monitoring device and measured plasma creatine kinase and standard clinical biochemistry panels to monitor muscle and organ function. To calculate whole body and intracellular amino acid production, we administered in the postabsorptive state an IV stable isotope solution containing 20 amino acid tracers. We collected blood before and six times after until two hours post tracer pulse administration and measured amino acid enrichments and concentrations by LC-MS/MS, subsequently followed by (non) compartmental modeling of the decay enrichment curves. Results were expressed as mean with 95% CI. Whole body production, plasma concentrations, and intra-/extracellular compartmental analyses of various amino acids were attenuated in HMB-dosed DMD dogs. Specifically, the plasma concentration of hydroxyproline (marker of collagen breakdown) was significantly higher in the placebo group compared to the HMB group. The intra- and extracellular pool sizes and flux between the two compartments of hydroxyproline was reduced in HMB treated dogs. DMD dogs treated with HMB as compared to placebo had a respective 40% increase in exertional (play) (951 [827, 1075] versus 569 [491, 647]; p < 0.0001) and 10.5% increase in non-exertional (active) activity (15,366 [14742, 15990] versus 13,806 [13148,14466]; p = 0.0016). In addition, a 6% reduction was found in rest time after HMB supplementation compared to placebo (23,857 [23,130, 24,584], versus 25,363 [24500, 26225]; p = 0.0122). Creatine kinase was not statistically different between groups. We did not observe any adverse clinical or biochemical-related effects of HMB dosing. Daily HMB supplementation in DMD dogs can safely and positively influence protein and amino acid metabolism and improve overall daily activity.

Corticosteroid Treatment Impact on Spinal Deformity in Duchenne Muscular Dystrophy

Duchenne muscular dystrophy is a progressive disease with loss of ambulation at around 9-10 years of age, followed, if untreated, by development of scoliosis, respiratory insufficiency, and death in the second decade of life. This review highlights the natural history of the disease, in particular, with regard to the development of the spinal deformity and how this complication has been modified by surgical interventions and overall by corticosteroid treatment. The beneficial effect of corticosteroids may have also an impact on the clinical trial design of the new emerging causative therapies.

Effect of deflazacort on cardiac and sternocleidomastoid muscles in Duchenne muscular dystrophy: a magnetic resonance imaging study

OBJECTIVE

To evaluate the involvement of cardiac and sternocleidomastoid muscles by magnetic resonance imaging (MRI) measurement of T2 relaxation time and the left ventricular systolic function in patients with Duchenne muscular dystrophy (DMD) on treatment with deflazacort and compare them with DMD patients without treatment.

SUBJECTS

Seventeen patients with DMD (aged 17-22 years) on treatment with deflazacort for at least 7 years and 17 boys with DMD of younger age (12-15 years) without steroid treatment. All patients were free of cardiac or respiratory symptoms and had normal ECG and Holter monitor examination.

METHODS

T2 relaxation time of the myocardium (H), left (SCM-L) and right sternocleidomastoid (SCM-R) muscles and left ventricular systolic function were evaluated by magnetic resonance imaging (MRI) in two groups of DMD patients. Myocardial and sternocleidomastoid muscles T2 relaxation time was calculated using 16 TEs (10-85 msec) and TR at least 2000 ms and T2 maps were created.

RESULTS

DMD on deflazacort had higher T2 relaxation time values of the heart and of both sternocleidomastoid muscles (T2H median (range): 47 (41-48) vs. 33 (31-37)ms, p<0.001, T2 SCM-L median (range): 35 (30-37) vs. 23 (20-26)ms, p<0.001, T2 SCM-R median (range): 35 (32-37) vs. 23 (20-27)ms, p<0.001) and left ventricular systolic function (LVEDV median (range): 95 (75-120) vs. 90 (80-105)ml, p=0.03, LVESV median (range): 45 (38-55) vs. 47 (41-51)ml, p=0.81(NS), LVEF median (range): 53% (51-57) vs. 48% (42-51), p<0.001) compared to DMD without treatment.

CONCLUSIONS

DMD patients on deflazacort are characterized by better preservation of the T2 relaxation time of myocardium and sternocleidomastoid muscles and better LV systolic function. The duration of this beneficial effect needs to be studied prospectively.

Evolving therapeutic strategies for Duchenne muscular dystrophy: targeting downstream events

Duchenne muscular dystrophy (DMD) is a progressive, lethal, muscle wasting disease that affects 1 of 3500 boys born worldwide. The disease results from mutation of the dystrophin gene that encodes a cytoskeletal protein associated with the muscle cell membrane. Although gene therapy will likely provide the cure for DMD, it remains on the distant horizon, emphasizing the need for more rapid development of palliative treatments that build on improved understanding of the complex pathology of dystrophin deficiency. In this review, we have focused on therapeutic strategies that target downstream events in the pathologic progression of DMD. Much of this work has been developed initially using the dystrophin-deficient mdx mouse to explore basic features of the pathophysiology of dystrophin deficiency and to test potential therapeutic interventions to slow, reverse, or compensate for functional losses that occur in muscular dystrophy. In some cases, the initial findings in the mdx model have led to clinical treatments for DMD boys that have produced improvements in muscle function and quality of life. Many of these investigations have concerned interventions that can affect protein balance in muscle, by inhibiting specific proteases implicated in the DMD pathology, or by providing anabolic factors or depleting catabolic factors that can contribute to muscle wasting. Other investigations have exploited the use of anti-inflammatory agents that can reduce the contribution of leukocytes to promoting secondary damage to dystrophic muscle. A third general strategy is designed to increase the regenerative capacity of dystrophic muscle and thereby help retain functional muscle mass. Each of these general approaches to slowing the pathology of dystrophin deficiency has yielded encouragement and suggests that targeting downstream events in dystrophinopathy can yield worthwhile, functional improvements in DMD.

Adverse and beneficial functions of proteolytic enzymes in skeletal muscle. An overview

Proteolytic enzymes (proteases) comprise a family of enzymes which hydrolyse protein or peptide substrates in the generalised process of intracellular protein degradation, a process essential for the normal functioning of all cells. Proteases may also have a wide range of additional functions, including metabolic control of physiologically active oligopeptides or precursor protein forms, antigen presentation/recognition by the major histocompatibility complex in the cellular immune response, as well as in digestion, blood clotting, complement activation, etc. In this article, the nomenclature and classification of proteolytic enzymes in skeletal muscle, and their role in normal muscle physiological processes have been reviewed, including exercise, muscle development and ageing. Although proteases play an important role in normal muscle functioning, in pathological situations the enzymes may themselves be regarded as 'toxic agents' in terms of their damaging effects on muscle tissue. Muscle damage resulting from inappropriate activity of proteolytic enzymes in muscle wasting associated with muscular dystrophies, denervation atrophy, inflammatory myopathies, cancer, sepsis, diabetes and alcoholism have been reviewed. In addition, evidence that the adverse effects of drugs known to induce muscle wasting, such as corticosteroids, (or beneficial effects of growth promoting drugs) may be mediated via proteolytic enzymes is also reviewed.

Corticosteroids in Duchenne muscular dystrophy: a reappraisal

Duchenne muscular dystrophy is the most common and most severe form of childhood muscular dystrophies, resulting in early loss of ambulation between the ages of 7 and 13 years and death in the teens and twenties. Despite the phenomenal advances made in the understanding of the molecular genetics of the disease, no definitive cure has been found. Of all of the therapeutic drugs studied in Duchenne muscular dystrophy, only prednisone seems to have the potential for providing interim functional improvement for boys with Duchenne muscular dystrophy while they wait for a cure with gene or cell therapy. There is still no consensus regarding recommending corticosteroids as standard therapy for boys. This is an evidence-based review of all of the studies of corticosteroids (prednisone, deflazacort, and oxandrolone) in Duchenne muscular dystrophy. From this review, it is clear that until a definitive treatment for Duchenne muscular dystrophy is available, the use of deflazacort and prednisone with judicious dietary control and close clinical monitoring for side effects seems the best intervention for interim preservation of function in such a common devastating disorder of young growing boys.

Calcium influx through calcium leak channels is responsible for the elevated levels of calcium-dependent proteolysis in dystrophic myotubes

To estimate calpain proteolysis, we measured the hydrolysis rate of a fluorogenic calpain substrate in individual resting normal and dystrophic mdx mouse myotubes in culture. Hydrolysis rates were high during myoblast and myotube alignment and fusion. After alignment and fusion ceased, hydrolysis rates declined. For normal myotubes, hydrolysis remained low after the development of contractile activity. In contrast, after the development of contractile activity, dystrophic mdx myotubes had abnormally high levels of hydrolysis that were dependent on external calcium and that could be abolished by calpeptin, an inhibitor of calpain. We eliminated the direct effects of contraction during measurements of hydrolysis by the addition of tetrodotoxin. Substrate hydrolysis by lysosomes or proteosomes was controlled for using NH(4)Cl and clasto-lactacystin beta-lactone, respectively. Increased activity of the calcium-activated protease in mature mdx myotubes was linked to the abnormal activity of calcium-specific leak channels because an antagonist of these channels reduced the higher levels of hydrolysis in dystrophic myotubes to nearly normal levels. The abnormal activity of these channels is linked to an increased frequency of transient sarcolemmal disruptions in the more fragile mdx myotubes (, ). Treatment of mdx myotubes with a pro-drug of methylprednisolone also reduced calpain substrate hydrolysis to nearly normal levels. However, this inhibition only required 2.5 h of pretreatment, which was not long enough to act by the known effects of prednisolone on calcium homeostasis.

Corticosteroid therapy does not alter the threshold for contraction-induced injury in dystrophic (mdx) mouse diaphragm

The effects of methylprednisolone therapy on the susceptibility of dystrophin-deficient myofibers to contraction-induced injury were evaluated in the mdx mouse diaphragm model of Duchenne dystrophy. Mdx myofibers were abnormally vulnerable to injury induced by high-stress eccentric contractions. However, methylprednisolone therapy did not significantly alter the degree of contraction-induced injury. These data suggest that beneficial effects of corticosteroid therapy in Duchenne dystrophy are unlikely to be related to a change in the threshold for contraction-induced myofiber damage.

Effect of prednisone on protease activities and structural protein levels in rat muscles in vivo

To further elucidate the biochemical mechanism by which the corticosteroid prednisone induces differential changes in muscle mass (via altered protein synthesis/degradation rates) in normal or degenerating muscle tissues, we have determined the activity of a range of proteolytic enzyme types, together with levels of muscle structural proteins, in five innervated and denervated muscle types from control and drug treated rats. In both normal and wasting muscles, the activity of many protease types was substantially down-regulated following treatment with prednisone; however, accompanying net decreases in muscle mass were observed (although the structural protein composition of muscles was unaltered following drug treatment). We conclude that whilst overall rates of protein degradation in both normal and degenerating muscle may be reduced (via protease down-regulation) following prednisone treatment, the effect of the latter in reducing protein synthesis rates must be proportionately greater (even in actively degenerating tissue). Thus, the data do not support the hypothesis that the beneficial effect of prednisone in maintaining muscle mass in pathological tissues (e.g., Duchenne muscular dystrophy (DMD)) operates principally via down-regulation of protease action/protein catabolism.

Expression of myoglobin gene in skeletal muscle of patients with neuromuscular diseases

Expression of the myoglobin (Mb) gene in skeletal muscle was studied in patients with Duchenne muscular dystrophy (DMD), polymyositis (PM), or amyotrophic lateral sclerosis (ALS) by measuring Mb concentration by radioimmunoassay and Mb messenger ribonucleic acid (RNA) (MbmRNA) levels by Northern blot analysis. Mb concentrations in the muscle cells (Mb/noncollagenous protein) were decreased in patients with DMD, PM, or ALS. However, while Mb concentrations per MbmRNA content (Mb/MbmRNA) were decreased in DMD and PM patients, these values were normal in ALS patients. These results suggest that Mb synthesis is increased in muscles of DMD and PM patients, but is not sufficient to compensate for the excessive loss of Mb from the affected muscles, and that the synthesis is decreased in the muscles of ALS patients.