Pathophysiology of duchenne muscular dystrophy: current hypotheses

Structural and functional alterations of muscle fibres in the novel mouse model of facioscapulohumeral muscular dystrophy

We recently generated a mouse model of facioscapulohumeral muscular dystrophy (FSHD) by selectively overexpressing FRG1, a candidate gene for FSHD, in skeletal muscle. The muscles of the FRG-1 mice did not show any plasmamembrane defect suggesting a novel pathogenetic mechanism for FSHD. Here, we study structure and function of muscle fibres from three lines of mice overexpressing FRG1 at different levels: FRG1-low, FRG1-med, FRG1-high. Cross-sectional area (CSA), specific force (Po/CSA) and maximum shortening velocity (V(o)) of identified types of muscle fibres from FRG1-low and FRG1-med mice were analysed and found to be lower than in WT mice. Fast fibres and especially type 2B fibres (the fastest type) were preferentially involved in the dystrophic process showing a much larger force deficit than type 1 (slow) fibres. Consistent with the latter observation, the MHC isoform distribution of several muscles of the three FRG1 lines showed a shift towards slower MHC isoforms in comparison to WT muscle. Moreover, fast muscles showed a more evident histological deterioration, a larger atrophy and a higher percentage of centrally nucleated fibres than the soleus, the slowest muscle in mice. Interestingly, loss in CSA, Po/CSA and V(o) of single muscle fibres and MHC isoform shift towards a slower phenotype can be considered early signs of muscular dystrophy (MD). They were, in fact, found also in FRG1-low mice which did not show any impairment of function in vivo and of muscle size in vitro and in soleus muscles, which had a completely preserved morphology. This study provides a detailed characterization of structure and function of muscle fibres in a novel murine model of one of the main human MDs and suggests that fundamental features of the dystrophic process, common to most MDs, such as the intrinsic loss of contractile strength of muscle fibres, the preferential involvement of fast fibres and the shift towards a slow muscle phenotype can occur independently from obvious alterations of the plasma membrane.

Bex1 knock out mice show altered skeletal muscle regeneration

Bex1 and Calmodulin (CaM) are upregulated during skeletal muscle regeneration. We confirm this finding and demonstrate the novel finding that they interact in a calcium-dependent manner. To study the role of Bex1 and its interaction with CaM in skeletal muscle regeneration, we generated Bex1 knock out (Bex1-KO) mice. These mice appeared to develop normally and are fertile, but displayed a functional deficit in exercise performance compared to wild type (WT) mice. After intramuscular injection of cardiotoxin, which causes extensive and reproducible myotrauma followed by recovery, regenerating muscles of Bex1-KO mice exhibited elevated and prolonged cell proliferation, as well as delayed cell differentiation, compared to WT mice. Thus, our results provide the first evidence that Bex1-KO mice show altered muscle regeneration, and allow us to propose that the interaction of Bex1 with Ca(2+)/CaM may be involved in skeletal muscle regeneration.

Branched fibers in dystrophic mdx muscle are associated with a loss of force following lengthening contractions

We demonstrated that the susceptibility of skeletal muscle to injury from lengthening contractions in the dystrophin-deficient mdx mouse is directly linked with the extent of fiber branching within the muscles and that both parameters increase as the mdx animal ages. We subjected isolated extensor digitorum longus muscles to a lengthening contraction protocol of 15% strain and measured the resulting drop in force production (force deficit). We also examined the morphology of individual muscle fibers. In mdx mice 1–2 mo of age, 17% of muscle fibers were branched, and the force deficit of 7% was not significantly different from that of age-matched littermate controls. In mdx mice 6–7 mo of age, 89% of muscle fibers were branched, and the force deficit of 58% was significantly higher than the 25% force deficit of age-matched littermate controls. These data demonstrated an association between the extent of branching and the greater vulnerability to contraction-induced injury in the older fast-twitch dystrophic muscle. Our findings demonstrate that fiber branching may play a role in the pathogenesis of muscular dystrophy in mdx mice, and this could affect the interpretation of previous studies involving lengthening contractions in this animal.

Perioperative care of patients with neuromuscular disease and dysfunction

A variety of different pathologies result in disease phenotypes that are summarized as neuromuscular diseases because they share commonalty in their clinical consequences for the patient: a progressive weakening of the skeletal muscles. Distinct caution and appropriate changes to the anesthetic plan are advised when care is provided during the perioperative period. The choice of anesthetic technique, anesthetic drugs, and neuromuscular blockade always depends on the type of neuromuscular disease and the surgical procedure planned. A clear diagnosis of the underlying disease and sufficient knowledge and understanding of the pathophysiology are of paramount importance to the practitioner and guide optimal perioperative management of affected patients.

Proteomic profiling of animal models mimicking skeletal muscle disorders

Over the last few decades of biomedical research, animal models of neuromuscular diseases have been widely used for determining pathological mechanisms and for testing new therapeutic strategies. With the emergence of high-throughput proteomics technology, the identification of novel protein factors involved in disease processes has been decisively improved. This review outlines the usefulness of the proteomic profiling of animal disease models for the discovery of new reliable biomarkers, for the optimization of diagnostic procedures and the development of new treatment options for skeletal muscle disorders. Since inbred animal strains show genetically much less interindividual differences as compared to human patients, considerably lower experimental repeats are capable of producing meaningful proteomic data. Thus, animal model proteomics can be conveniently employed for both studying basic mechanisms of molecular pathogenesis and the effects of drugs, genetic modifications or cell-based therapies on disease progression. Based on the results from comparative animal proteomics, a more informed decision on the design of clinical proteomics studies could be reached. Since no one animal model represents a perfect pathobiochemical replica of all of the symptoms seen in complex human disorders, the proteomic screening of novel animal models can also be employed for swift and enhanced protein biochemical phenotyping.