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News
2009-09-23
NEW MECHANISMS CAUSING FATIGUE IN DMD IDENTIFIED BY DMDRC SCIENTISTS
Duchenne muscular dystrophy (DMD) involves a complex pathophysiology that is not easily explained by the loss of the protein dystrophin, the primary defect in DMD. Instead, many features of the pathology are attributable to the secondary loss of neuronal nitric oxide synthase (nNOS) from dystrophin-deficient muscle. Researchers at the UCLA DMDRC tested whether loss of nNOS contributes to the increased fatigability of mdx mice, a model of DMD. The investigators found that genetic manipulations to restore nNOS to dystrophic muscle increased the endurance of mdx mice and enhanced glycogen metabolism during treadmill-running, but did not affect vascular perfusion of muscles. They also learned that the specific activity of phosphofructokinase (PFK; the rate limiting enzyme in glycolysis) is positively affected by nNOS in muscle; PFK specific activity is significantly reduced in mdx muscles and the muscles of nNOS null mutants, while significantly increased in nNOS transgenic muscles and muscles from mdx mice that express the nNOS transgene. PFK activity measured under allosteric conditions was significantly increased by nNOS, but unaffected by endothelial NOS or inducible NOS. The specific domain of nNOS that positively regulates PFK activity was assayed by cloning and expressing different domains of nNOS and assaying their effects on PFK activity. This approach yielded a polypeptide that included the FAD-binding domain of nNOS as the region of the molecule that promotes PFK activity. Smaller peptides in this domain were then synthesized and used in activity assays that showed a 36-amino acid peptide in the FAD-binding domain in which most of the positive allosteric activity of nNOS for PFK resides. Mapping this peptide onto the structure of nNOS shows that the peptide is exposed on the surface, readily available for binding. Collectively, these findings indicate that defects in glycolytic metabolism and increased fatigability in dystrophic muscle may be caused in part by the loss of positive allosteric interactions between nNOS and PFK. Details of this study are available in the following publication. Wehling-Henricks, M., M. Oltmann, C. Rinaldi, K. H. Myung, and J. G. Tidball. 2009. Loss of positive allosteric interactions between neuronal nitric oxide synthase and phosphofructokinase contributes to defects in glycolysis and increased fatigability in muscular dystrophy. Human Molec. Genetics 18:3439-3451.
2009-04-28
NEW LINK DISCOVERED BETWEEN MUSCULAR DYSTROPHY AND DEFECTS IN THE BRAIN
Duchenne muscular dystrophy is associated with defects in learning and memory that are normally attributable to the actions of a region of the brain called the hippocampus. However, the relationship between muscular dystrophy and defects in learning and memory are a mystery. Now, researchers at the UCLA-DMDRC have identified a new link between muscular dystrophy and brain dysfunction. The investigators found that dystrophin mutation disrupts adult neurogenesis (the formation of new nerves) by promoting cell proliferation in the hippocampus and suppressing neuronal differentiation. Because loss of dystrophin from muscle results in the secondary loss of neuronal nitric oxide synthase (nNOS), and NO is able to modulate neurogenesis, the scientists tested whether the genetic restoration of nNOS to mdx muscles corrected defects in adult, hippocampal neurogenesis. Assays of NO in the sera of active dystrophic mice, showed significant reductions in NO caused by the dystrophin mutation. However, over-expression of nNOS in the muscles of dystrophic mice increased serum NO and normalized cell proliferation and neuronal differentiation in the hippocampus. These findings indicate that muscle-derived NO regulates adult neurogenesis in the brain and loss of muscle nNOS may underlie defects in the central nervous system in DMD.
Details of this study are reported in a recent issue of The Journal of Physiology:
Deng, B., D. Glanzman and J.G. Tidball. 2009. Nitric oxide generated by muscle corrects defects in hippocampal neurogenesis and neural differentiation caused by muscular dystrophy. J. Physiol. (Lond.) 587: 1769-1778.
2009-02-18
CAN DIETARY SUPPLEMENTS WITH ARGININE HAVE NEGATIVE CONSEQUENCES IN DMD?
Dietary supplementation with products that are enriched with the amino acid arginine is provided to some boys and men with DMD, although the consequences of supplemental arginine on the pathology of muscular dystrophy are not well-known. Scientists at the UCLA DMDRC have discovered that two populations of inflammatory macrophages called M1 macrophages and M2a macrophages can compete for the use of arginine in dystrophic muscle, and the results of that competition can affect the course of the pathology.
Using the mdx mouse model of DMD, UCLA DMDRC researchers have shown that M1 macrophages convert arginine to toxic free radicals that can kill muscle fibers. However, killing by muscle by M1 macrophages is kept in check by another population of macrophages, the M2a macrophages that are also present in dystrophic muscle. M2a macrophages compete with M1 macrophages for arginine, and convert the arginine to non-toxic molecules. However, increases in arginine concentration can increase its availability to M1 macrophages and increase their killing of muscle cells, even in the presence of M2a macrophages. These findings suggest that increased killing of muscle cells by inflammatory cells may be a short-term consequence of dietary supplementation with arginine in DMD.
Details of this study are available in the following publication.
Villalta, S.A., H.X. Nguyen, B. Deng, T. Gotoh and J.G. Tidball. 2009. Shifts in macrophage phenotypes and macrophage competition for arginine metabolism affect the severity of muscle pathology in muscular dystrophy. Human Molecular Genetics 18:482-496.
NEW MECHANISM THAT INCREASES FIBROSIS IN MUSCULAR DYSTROPHY.
Researchers at the UCLA DMDRC in collaboration with Dr. Jamie Lee at the Mayo Clinic have shown a surprising role for the immune cells called eosinophils in promoting the pathology of muscular dystrophy. Eosinophils are commonly involved in allergic reactions or in immune responses to parasites. However, DMDRC investigators find these cells increase muscle fibrosis in the mdx mouse model of muscular dystrophy. By treating mdx mice with antibodies that bound to a protein on the surface of eosinophils, the researchers were able to reduce injury to the cell membrane of muscle cells. In addition, mutating the gene for the eosinophil protein called major basic protein (MBP) reduced fibrosis of muscle and hearts, a major cause of mortality in DMD. Further experimentation showed that MBP can influence the nature of the immune response in muscular dystrophy, and contribute to a shift in inflammatory cells to a type that causes fibrosis of dystrophic muscles and hearts.
Further details of this investigation are reported in:
Wehling-Henricks, M., Sokolow, S., Lee, J.J., Myung, K.H., Villalta, S.A., and J.G. Tidball. 2008. Major basic protein-1 promotes fibrosis of dystrophic muscle and attenuates the cellular immune response in muscular dystrophy. Human Molecular Genetics 17:2280-2292.
RESEARCHERS FIND A NEW, IMMUNE-BASED STRATEGY TO TREAT MUSCULAR DYSTROPHY
Previous work by UCLA DMDRC researchers identified the immune system as an important factor that can increase the pathology of muscular dystrophy. But how can immune cell involvement in DMD be manipulated to reduce pathology? Collaborative research between scientists at Ohio State University and the UCLA DMDRC identified a specific mechanism through which the immune system affects the pathology of muscular dystrophy. The scientists found that molecular signaling through a system that involves the proteins IkappaB kinase and NF-kappaB is persistently elevated in immune cells and regenerative muscle fibers in muscular dystrophy. However, genetic modification of part of the NF-kappaB gene was sufficient to reduce pathology in mdx mice, a model of DMD. The researchers also showed that the specific pharmacological inhibition of the protein IKK resulted in improved pathology and muscle function in mdx mice. These results underscore the critical role of NF-kappaB in the progression of muscular dystrophy and suggest the IKK/NF-kappaB signaling pathway as a potential therapeutic target for DMD.
Details of this investigation are reported in:
Acharyya, S., S.A. Villalta, N. Bakkar, T. Bupha-Intr, P.M.L. Janssen, M. Carathers, M. Karin, Z. Li, A. Beg, S. Ghosh, Z. Sahenk, M. Weinstein, K.L. Gardner, J.A. Rafael-Fortney, J.G. Tidball, A.S. Baldwin and D.C. Guttridge. 2007. IKK/NF-kB signaling interplay in macrophages and myofibers promotes muscle wasting in Duchenne muscular dystrophy. Journal of Clinical Investigation 117:889-901.
DOES OXIDATIVE STRESS CONTRIBUTE TO THE PATHOLOGY OF MUSCULAR DYSTROPHY?
Before the identification of the deficient proteins that underlie muscular dystrophies, such as Duchenne muscular dystrophy (DMD), oxidative stress was proposed as a major cause of the disease. Now, current knowledge supports the likelihood that interactions between the primary genetic defect and disruptions in the normal production of free radicals contribute to the pathophysiology of muscular dystrophies. Current evidence indicates three general routes through which free radical production can be disrupted in dystrophin deficiency to contribute to the ensuing pathology. First, differences in free radical production can disrupt signaling processes in muscle and other tissues and thereby worsen pathology. Second, tissue responses to the presence of pathology can cause a shift in free radical production that can promote cellular injury and dysfunction. Finally, behavioral differences in the affected individual can cause further changes in the production and relative quantities of free radicals and thereby contribute to disease. Unfortunately, the complexity of the free radical-mediated processes that are perturbed in complex pathologies such as DMD make it difficult to develop therapeutic approaches founded on systemic administration of antioxidants. A summary of current knowledge concerning the relationship between free radicals and muscular dystrophy is provided in the following paper by DMDRC researchers:
Tidball, J.G. and M. Wehling-Henricks. 2007. The role of free radicals in the pathophysiology of muscular dystrophy. Journal of Applied Physiology 102:1677-1686.
WHY DOES MUSCULAR DYSTROPHY CAUSE HEART DISEASE?
A large proportion of DMD patient deaths are attributable to cardiac dysfunction associated with ventricular fibrosis, arrhythmias and conduction abnormalities, although the relationships between the dystrophin mutation and the cardiac defects are unknown. Researchers at the UCLA Duchenne Muscular Dystrophy Research Center tested whether cardiac pathology in dystrophin-deficient mdx mice can be corrected by the elevated production of nitric oxide (NO) by the myocardium. Dystrophin-deficient mdx mice were produced in which there was myocardial expression of a neuronal nitric oxide synthase (nNOS) transgene. Expression of the transgene prevented the progressive ventricular fibrosis of mdx mice and greatly reduced myocarditis. Electrocardiographs (ECG) attained by radiotelemetry in mdx mice showed that the mice displayed cardiac abnormalities that are characteristic of DMD patients. All of the ECG abnormalities in mdx mice were improved or corrected by nNOS transgene expression. In addition, defects in mdx cardiac autonomic function, which were reflected by decreased heart rate variability, were significantly reduced by nNOS transgene expression. These findings indicate that increasing NO production by dystrophic hearts may have therapeutic value.
For details concerning this investigation, please see the publication:
Wehling-Henricks, M., M.C. Jordan, K.P. Roos, B. Deng and J. G. Tidball. 2005. Cardiomyopathy in dystrophin-deficient hearts is prevented by expression of a neuronal nitric oxide synthase transgene in the myocardium. Human Molecular Genetics 14: 1921-1933.