Lack Of Dystrophin In Muscle Research Articles

Dmdmdx/Largemyd: a new mouse model of neuromuscular diseases useful for studying physiopathological mechanisms and testing therapies

SUMMARYAlthough muscular dystrophies are among the most common human genetic disorders, there are few treatment options available. Animal models have become increasingly important for testing new therapies prior to entering human clinical trials. The Dmdmdx mouse is the most widely used animal model for Duchenne muscular dystrophy (DMD), presenting the same molecular and protein defect as seen in humans with the disease. However, this mouse is not useful for clinical trials because of its very mild phenotype. The mouse model for congenital myodystrophy type 1D, Largemyd, harbors a mutation in the glycosyltransferase Large gene and displays a severe phenotype. To help elucidate the role of the proteins dystrophin and LARGE in the organization of the dystrophin-glycoprotein complex in muscle sarcolemma, we generated double-mutant mice for the dystrophin and LARGE proteins. The new Dmdmdx/Largemyd mouse model is viable and shows a severe phenotype that is associated with the lack of dystrophin in muscle. We tested the usefulness of our new mouse model for cell therapy by systemically injecting them with normal murine mesenchymal adipose stem cells (mASCs). We verified that the mASCs were hosted in the dystrophic muscle. The new mouse model has proven to be very useful for the study of several other therapies, because injected cells can be screened both through DNA and protein analysis. Study of its substantial muscle weakness will also be very informative in the evaluation of functional benefits of these therapies.

T.P.4.10 Towards a better understanding of truncated dystrophin instability

Duchenne Muscular Dystrophy and Becker Muscular Dystrophy are both genetic neuromuscular disorders due to mutations in the dystrophin gene. DMD patients are characterized by a lack of dystrophin in muscles and present a severe phenotype, while BMD display a much milder phenotype and express either lower amounts of dystrophin or still semi-functional truncated dystrophins. In some cases, such as in patients with 45–47 and 45–48 deletions (about 50% of the BMD population), resulting truncated dystrophins are unstable.

Expansion of revertant fibers in dystrophic mdx muscles reflects activity of muscle precursor cells and serves as an index of muscle regeneration

Duchenne muscular dystrophy and the mdx mouse myopathies reflect a lack of dystrophin in muscles. However, both contain sporadic clusters of revertant fibers (RFs) that express dystrophin. RF clusters expand in size with age in mdx mice. To test the hypothesis that the expansion of clusters is achieved through the process of muscle degeneration and regeneration, we analyzed muscles of mdx mice in which degeneration and regeneration were inhibited by the expression of micro-dystrophins or utrophin transgenes. Postnatal RF expansion was diminished in direct correlation to the protective effect of the transgene expression. Similarly, expansion of RFs was inhibited when muscle regeneration was blocked by irradiation. However, in irradiated muscles, irradiation-tolerant quiescent muscle precursor cells reactivated by notexin effectively restored RF expansion. Our observations demonstrate that revertant events occur initially within a subset of muscle precursor cells. The proliferation of these cells, as part of the regeneration process, leads to the expansion of RF clusters within degenerating muscles. This expansion of revertant clusters depicts the cumulative history of regeneration, thus providing a useful index for functional evaluation of therapies that counteract muscle degeneration.

Dystrophin and utrophin influence fiber type composition and post-synaptic membrane structure

The X-linked muscle wasting disease Duchenne muscular dystrophy is caused by the lack of dystrophin in muscle. Protein structure predictions, patient mutations, in vitro binding studies and transgenic and knockout mice suggest that dystrophin plays a mechanical role in skeletal muscle, linking the subsarcolemmal cytoskeleton with the extracellular matrix through its direct interaction with the dystrophin-associated protein complex (DAPC). Although a signaling role for dystrophin has been postulated, definitive data have been lacking. To identify potential non-mechanical roles of dystrophin, we tested the ability of various truncated dystrophin transgenes to prevent any of the skeletal muscle abnormalities associated with the double knockout mouse deficient for both dystrophin and the dystrophin-related protein utrophin. We show that restoration of the DAPC with Dp71 does not prevent the structural abnormalities of the post-synaptic membrane or the abnormal oxidative properties of utrophin/dystrophin-deficient muscle. In marked contrast, a dystrophin protein lacking the cysteine-rich domain, which is unable to prevent dystrophy in the mdx mouse, is able to ameliorate these abnormalities in utrophin/dystrophin-deficient mice. These experiments provide the first direct evidence that in addition to a mechanical role and relocalization of the DAPC, dystrophin and utrophin are able to alter both structural and biochemical properties of skeletal muscle. In addition, these mice provide unique insights into skeletal muscle fiber type composition.

Enhanced lipid peroxidation in Duchenne dystrophy muscle may be secondary to muscle danage

Although it is now recognised that Duchenne muscular dystrophy (DMD) results from a lack of dystrophin in muscle the mechanism by which the deficiency of this protein leads to the characteristic muscle damage and degeneration is currently unknown. Enhanced free radical-mediated lipid peroxidation has previously been suggested as a possible mechanism leading to degradation of muscle tissue in DMD [l-31 and increased amounts of malonaldehyde (or correctly thiobarbituric acid-reactive substances) have been reported in muscle biopsy samples from patients with DMD 11-31, but not from patients with other forms of muscular dystrophy [4]. These findings suggested that the elevated lipid peroxidation products in DMD muscle were either a specific consequence of the biochemical defect in Duchenne or due to the greater amount of damage and degeneration generally seen in this disorder in comparison to other forms of muscular dystrophy. Increased lipid peroxidation has been suggested to occur as a consequence of non-specific damage to tissues rather than as a cause of it [5].