IGF-I treatment improves the functional properties of fast- and slow-twitch skeletal muscles from dystrophic mice
Introduction
Muscle wasting is one of the major symptoms of many neuromuscular disorders, including, but not limited to, Duchenne muscular dystrophy (DMD), the most severe of the muscle diseases. Although much effort is currently being directed at developing gene therapies for DMD and related disorders, these techniques are far from perfect [1]. In the interim, it is essential that alternative therapies for treating neuromuscular disorders also be developed, with efforts directed towards preserving existing muscle tissue, enhancing muscle regeneration, and promoting muscle growth. Slowing the loss of muscle tissue will preserve muscle function. Restoring or increasing muscle mass to previous or higher levels will optimize the potential for improving muscle function.
Insulin-like growth factor-I (IGF-I) is a potent anabolic agent in skeletal muscle responsible for increasing rates of protein and nucleic acid synthesis, as well as inhibiting protein degradation [2], [3], [4]. IGF-I has been proposed for clinical trials for patients suffering from muscle wasting because of its potential for increasing the rate of muscle regeneration following injury [5], [42] and its ability to prevent or offset the muscle atrophy concomitant with glucocorticoid treatment [6] and contraction-induced injury [7]. Recent studies have demonstrated that IGF-I is implicated in skeletal muscle growth, hypertrophy and regeneration [8], [9], [10]. Virally-delivered IGF-I genes induced local skeletal muscle hypertrophy and attenuated age-related skeletal muscle atrophy, restoring and improving muscle mass and strength in mice [8].
Despite the potential for IGF-I to offset muscle atrophy, few studies have investigated the functional properties of dystrophic skeletal muscles following IGF-I treatment. In the neurological mutant wobbler mouse, a murine model of human motoneuron diseases such as amyotrophic lateral sclerosis and spinal muscular dystrophies, IGF-I treatment (1 mg/kg per day, 6 weeks) was shown to maintain muscle fibre diameter and increase ‘grip strength’ by 40% as estimated with a dynamometer [11], [12]. Such a measurement is a gross estimation of muscle strength, at best. Truly accurate measurements of functional improvements in skeletal muscle function following such an intervention can only be made by measuring maximum force production of isolated muscles either in situ or in vitro.
The mdx mouse has a similar genetic defect to that in DMD, but only the diaphragm muscle exhibits progressively severe dystrophic symptoms like that in DMD [13], [14], [15], [16], [17], [18], [19]. The 129 ReJ dy/dy dystrophic mouse, although it has different genetics to that of DMD, suffers from a deficiency in laminin, a muscle structural protein, resulting in severe hind limb muscle wasting and weakness. For the purpose of testing the efficacy of IGF-I administration on the structure and function of dystrophic skeletal muscles, the 129 ReJ dy/dy dystrophic mouse is the model that best represents the degree of muscle wasting common to the different muscular dystrophies [20]. In this study we investigated the effect of 4 weeks of IGF-I treatment on the properties of skeletal muscles of 129 ReJ dy/dy dystrophic mice. We tested the hypothesis that IGF-I treatment would maintain or increase the mass and improve the functional properties of dystrophic skeletal muscles.
Section snippets
Experimental groups and IGF-I treatment
All procedures were approved by the Animal Experimentation Ethics Committee of The University of Melbourne and conformed to the guidelines for the care and use of experimental animals as described by the National Health and Medical Research Council of Australia. Male dystrophic 129P1 ReJ-Lama2dy (129 ReJ dy/dy) and control mice were obtained from The Jackson Laboratory (Bar Harbor, ME) at 4 weeks old and then housed in the Biological Research Facility at The University of Melbourne in standard
Selected morphometric parameters in control and dystrophic mice following IGF-I treatment
Comparisons of selected morphometric parameters between control and dystrophic mice are presented in Table 1. Body mass of the dystrophic mice was 64% that of control mice (P<0.05). The mass of the EDL and soleus muscles from dystrophic mice were 47 and 54% of those from control mice, respectively (P<0.01). The ratio of muscle mass to body mass was unchanged between dystrophic and control mice for both EDL and soleus muscles.
In control mice, IGF-I treatment did not affect body mass or the mass
Discussion
The most important finding of this study was that treatment with IGF-I for 4 weeks increased the size and force producing capacity of fast- and slow-twitch skeletal muscles of 129 ReJ dy/dy dystrophic mice. IGF-I treatment increased the maximum Po of EDL and soleus muscles of dystrophic mice by 40 and 32%, respectively. This improvement in function following IGF-I treatment was associated with an increase in the mass and the cross-sectional area of fibres in the EDL and soleus muscles.
Acknowledgements
Supported by grants from the Muscular Dystrophy Association (USA), the Rebecca L. Cooper Medical Research Foundation, and the National Health and Medical Research Council of Australia.
References (42)
- et al.
Morphometric analysis of mdx diaphragm muscle fibres: comparison with hindlimb muscles
Neuromusc Disord
(1993) - et al.
Examining potential drug therapies for muscular dystrophy utilising the dy/dy mouse. I. Clenbuterol
J Neurol Sci
(1998) - et al.
Increase in oxidative capacity of muscle fibres in dystrophic mice and correlation with overactivity in these fibres
Exp Neurol
(1980) - et al.
Quantitative measurement of muscle strength in the mouse
J Neurosci Methods
(1995) - et al.
Higher content of insulin-like growth factor-I in dystrophic mdx mouse: potential role in the spontaneous regeneration through an electrophysiological investigation of muscle function
Neuromusc Disord
(1999) - et al.
High protein diet has beneficial effects in murine muscular dystrophy
J Nutr
(1995) Is there a future for gene therapy?
Neuromusc Disord
(1999)- et al.
Endocrine regulation of protein breakdown in skeletal muscle
Diabetes Metab Rev
(1988) - et al.
Insulin-like growth factor-1 and high protein diet decrease calpain-mediated proteolysis in murine muscular dystrophy
Proc Soc Exp Biol Med
(1998) - et al.
Metabolic and structural effects of insulin-like growth factor-I and high-protein diet on dystrophic hamster skeletal muscle
Proc Soc Exp Biol Med
(1997)