Novel therapies for muscular dystrophy and other muscle wasting conditions

Therapeutic potential of PEGylated insulin-like growth factor I for skeletal muscle disease evaluated in two murine models of muscular dystrophy

OBJECTIVE

Duchenne muscular dystrophy (DMD) is a fatal monogenetic disease with affected males displaying severe and progressive muscle wasting and weakness eventually leading to premature death. Possible therapeutic benefits of insulin-like growth factor I (IGF-I) have been studied extensively in various models of muscle disease and DMD with IGF-I as a mediator of improved skeletal muscle regeneration by enhancing myoblast proliferation and differentiation.

DESIGN

We tested the efficacy of a novel IGF-I analogue, a polyethylene glycol modified IGF-I (PEG-IGF-I), to ameliorate the pathophysiology of muscular dystrophy in two mouse models of DMD. We used mdx mice which lack dystrophin (as in DMD) but exhibit only a relatively mild phenotype, and the dko mouse which is a transgenic model lacking utrophin in addition to dystrophin, and which exhibits a more severe, lethal phenotype like that in DMD.

RESULTS

In young mdx mice, twice-weekly PEG-IGF-I s.c. injections for 6 weeks protected the diaphragm muscle against fatigue and the tibialis anterior (TA) muscle against contraction-induced injury. However, this beneficial effect of PEG-IGF-I was less pronounced in mdx mice when treatment was initiated later in adulthood. In severely affected dko mice PEG-IGF-I treatment did not affect pathophysiological parameters including animal survival.

CONCLUSIONS

These data suggest a therapeutic benefit with PEG-IGF-I treatment only in mild muscle pathologies, since its potential to ameliorate the pathophysiology in models of severe muscular dystrophies was limited. Treatment should be initiated only for mild muscle pathologies if functional benefits are to be realised and therefore may be relevant as a short-term therapy to hasten the functional repair of otherwise healthy muscles after injury.

Low dose formoterol administration improves muscle function in dystrophic mdx mice without increasing fatigue

The beta(2)-adrenoceptor agonist (beta(2)-agonist), formoterol, has been shown to cause muscle hypertrophy in rats even when administered at the micromolar dose of 25 micro g/kg/day. We investigated whether a similar low dose of formoterol could improve muscle function in the dystrophic mdx mouse. Ten-week-old male mdx and wild-type (C57BL/10) mice were administered formoterol (25 micro g/kg/day, i.p.) for 4 weeks. Formoterol treatment increased extensor digitorum longus (EDL) and soleus muscle mass, increased median muscle fibre size in diaphragm, EDL, and soleus muscles, and increased maximum force producing capacity in skeletal muscles of both wild-type and mdx mice. In contrast to other studies where beta(2)-agonists have been administered to mice and rats, generally at higher doses, low dose formoterol treatment did not increase the fatiguability of EDL, soleus or diaphragm muscles. Although others have found formoterol can decrease ubiquitin mRNA and proteasome activity when administered to tumour bearing rats at high doses (2mg/kg/day), in the present study low dose formoterol treatment did not alter ubiquitin or the E1 and E3 ubiquitin ligases in diaphragm muscles of wild-type or mdx mice, but it did reduce the level of ubiquitinated proteins in diaphragm of wild-type mice. The findings indicate that formoterol has considerably more powerful anabolic effects on skeletal muscle than older generation beta(2)-agonists (like clenbuterol and albuterol), and has considerable therapeutic potential for muscular dystrophies and other neuromuscular disorders where muscle wasting is indicated.

Emerging drugs for sarcopenia: age-related muscle wasting

Sarcopenia is the term widely used to describe the progressive loss of muscle mass with advancing age. Even before significant muscle wasting becomes apparent, ageing is associated with a slowing of movement and a gradual decline in muscle strength, factors that increase the risk of injury from sudden falls and the reliance of the frail elderly on assistance in accomplishing even basic tasks of independent living. Sarcopenia is recognised as one of the major public health problems now facing industrialised nations, and its effects are expected to place increasing demands on public healthcare systems worldwide. Although the effects of ageing on skeletal muscle are unlikely to be halted or reversed, the underlying mechanisms responsible for these deleterious changes present numerous targets for drug discovery with potential opportunities to attenuate muscle wasting, improve muscle function, and preserve functional independence. Very few drugs have been developed with sarcopenia specifically in mind. However, because many of the effects of ageing on skeletal muscle resemble those indicated in many neuromuscular disorders, drugs that target neurodegenerative diseases may also have important relevance for treating age-related muscle wasting and weakness. This review describes a selection of the emerging drugs that have been developed during the period 1997 - 2004, relevant to sarcopenia.

Administration of insulin-like growth factor-I improves fatigue resistance of skeletal muscles from dystrophicmdx mice

Muscle fatigue occurs in many neuromuscular diseases, including the muscular dystrophies, and it contributes to a loss of functional capacity and reduced quality of life for affected patients. An improvement in fatigue resistance has been observed in diaphragm muscles of mdx mice following insulin-like growth factor-I (IGF-I) administration. Whether similar treatment can improve locomotor muscle function in mdx mice is not known. We examined the efficacy of IGF-I administration (1 mg/kg daily s.c. for 8 weeks) on structural, metabolic, and functional properties of extensor digitorum longus (EDL) and soleus muscles of mdx mice, and tested the hypothesis that IGF-I treatment would improve function in these muscles. After treatment, muscles were more resistant to fatigue during repeated maximal contractions than muscles from untreated mice, an improvement associated with increased muscle fiber succinate dehydrogenase activity in the absence of changes in cellular (single-fiber) contractile activation characteristics. The findings have important clinical implications, not just for the dystrophinopathies, but for all neuromuscular pathologies where fatigue of locomotor muscles limits functional capacity and decreases quality of life.

Muscular dystrophies and other inherited myopathies

It is certain that more inherited neuromuscular disorders of dogs and cats will be identified as the ability of practicing veterinarians to recognize disorders of muscle, nerve, and neuromuscular junction improves and newer diagnostic tests become available. Two specific points are critical. Before DNA-based genetic tests and specific therapies can be developed, an accurate description of the problem, clinically and histopathologically, must be performed. This is particularly important for the accuracy of a pedigree analysis, because inclusion of dogs with unrelated problems would alter the interpretation. Second, animals with inherited breed-associated disease should not be bred for generation of companion animals.

Therapies for improving muscle function in neuromuscular disorders

Muscle atrophy or wasting is a loss of muscle tissue resulting from disease or lack of use. This review examines recent pharmacologic or nutrition interventions for ameliorating wasting and improving muscle function in neuromuscular disorders. The information has application for treating the muscular dystrophies, cancer cachexia, weightlessness, immobilization, denervation, and disuse atrophy.