Animal Models of Neuromuscular Diseases: Pathophysiology and Implications for Rehabilitation

Molecular Diagnosis and Novel Therapies for Neuromuscular Diseases

With the development of novel targeted therapies, including exon skipping/inclusion and gene replacement therapy, the field of neuromuscular diseases has drastically changed in the last several years. Until 2016, there had been no FDA-approved drugs to treat Duchenne muscular dystrophy (DMD), the most common muscular dystrophy. However, several new personalized therapies, including antisense oligonucleotides eteplirsen for DMD exon 51 skipping and golodirsen and viltolarsen for DMD exon 53 skipping, have been approved in the last 4 years. We are witnessing the start of a therapeutic revolution in neuromuscular diseases. However, the studies also made clear that these therapies are still far from a cure. Personalized genetic medicine for neuromuscular diseases faces several key challenges, including the difficulty of obtaining appropriate cell and animal models and limited its applicability. This Special Issue “Molecular Diagnosis and Novel Therapies for Neuromuscular/Musculoskeletal Diseases” highlights key areas of research progress that improve our understanding and the therapeutic outcomes of neuromuscular diseases in the personalized medicine era.

Muscle fatigue in animal models of neuromuscular disease

This review summarizes the literature regarding the fatigue properties of skeletal muscles in animal models of neuromuscular disease and the effect of exercise training on these properties in diseased muscles. Results of these studies are not consistent. Reasons for the variability may include the use of differing experimental conditions, varying stimulation protocols, different methods of muscle preparation, and a range of temperatures and ages. Despite the variability and small sample sizes, these studies of muscle fatigue have revealed definite trends. Whereas the slow oxidative soleus muscle from the diseased animals fatigued more slowly or at the same rate as controls, the extensor digitorum longus (a fast, glycolytic muscle) from the diseased animals fatigued more rapidly than controls. The diaphragm-a fast, oxidative, glycolytic muscle that is generally fatigue resistant-did not reveal significant changes in fatigability. Exercise training had a minimal impact on the fatigue properties of skeletal muscle from diseased animals. As molecular and biochemical mechanisms are further identified and defined, additional therapeutic interventions may become available for these disorders. It is hoped that this review will serve as a guide for investigators who use an animal model to evaluate interventions for neuromuscular diseases.

Adaptations to exercise training and contraction-induced muscle injury in animal models of muscular dystrophy

This article reviews the current status of exercise training and contraction-induced muscle-injury investigations in animal models of muscular dystrophy. Most exercise-training studies have compared the adaptations of normal and dystrophic muscles with exercise. Adaptation of diseased muscle to exercise occurs at many levels, starting with the extracellular matrix, but also involves cytoskeletal architecture, muscle contractility, repair mechanisms, and gene regulation. The majority of exercise-injury investigations have attempted to determine the susceptibility of dystrophin-deficient muscles to contraction-induced injury. There is some evidence in animal models that diseased muscle can adapt and respond to mechanical stress. However, exercise-injury studies show that dystrophic muscles have an increased susceptibility to high mechanical forces. Most of the studies involving exercise training have shown that muscle adaptations in dystrophic animals were qualitatively similar to the adaptations observed in control muscle. Deleterious effects of the dystrophy usually occur only in older animals with advanced muscle fiber degeneration or after high-resistive eccentric training. The main limitations in applying these conclusions to humans are the differences in phenotypic expression between humans and genetically homologous animal models and in the significant biomechanical differences between humans and these animal models.