Dystrophin-glycoprotein complex purified from hamster cardiac muscle. Comparison of the complexes from cardiac and skeletal muscles of hamster and rabbit

Natural History of Histopathologic Changes in Cardiomyopathy of Golden Retriever Muscular Dystrophy

Background

Duchenne muscular dystrophy (DMD) is an X-linked inherited myopathy that causes progressive skeletal and cardiac muscle disease. Heart lesions were described in the earliest DMD reports, and cardiomyopathy is now the leading cause of death. However, diagnostics and treatment for cardiomyopathy have lagged behind those for appendicular and respiratory skeletal muscle disease. Most animal model studies have been done in the mdx mouse, which has a relatively mild form of cardiomyopathy. Dogs with the genetically homologous condition, Golden Retriever muscular dystrophy (GRMD), develop progressive cardiomyopathy analogous to that seen in DMD. Previous descriptive studies of GRMD cardiomyopathy have mostly been limited to selective sampling of the hearts from young dogs.

Methods and Results

We systematically assessed cardiac lesions in 31 GRMD and carrier dogs aged 3 to 76 months and a separate cohort of 2–10-year-old normal hounds. Both semi-quantitative lesion scoring and quantitation of the cross-sectional area of fibrosis distinguished dogs with GRMD disease from normal dogs. The carriers generally had intermediate involvement but had even greater fibrosis than GRMD dogs. Fatty infiltration was the most prominent feature in some older GRMD dogs. Vascular hypertrophy was increased in GRMD dogs and correlated positively with lesion severity. Purkinje fiber vacuolation was also increased but did not correlate with lesion severity. Histopathologic changes correlated with late gadolinium enhancement on cardiac MRI.

Conclusion

These features are generally compatible with those of DMD and further validate GRMD as a useful model to study cardiomyopathy pathogenesis and treatment. Additionally, the nature of some degenerative lesions suggests that functional hypoxia or non-thrombotic ischemia may contribute to disease progression.

Dystrophic Cardiomyopathy: Complex Pathobiological Processes to Generate Clinical Phenotype

Duchenne muscular dystrophy (DMD), Becker muscular dystrophy (BMD), and X-linked dilated cardiomyopathy (XL-DCM) consist of a unique clinical entity, the dystrophinopathies, which are due to variable mutations in the dystrophin gene. Dilated cardiomyopathy (DCM) is a common complication of dystrophinopathies, but the onset, progression, and severity of heart disease differ among these subgroups. Extensive molecular genetic studies have been conducted to assess genotype-phenotype correlation in DMD, BMD, and XL-DCM to understand the underlying mechanisms of these diseases, but the results are not always conclusive, suggesting the involvement of complex multi-layers of pathological processes that generate the final clinical phenotype. Dystrophin protein is a part of dystrophin-glycoprotein complex (DGC) that is localized in skeletal muscles, myocardium, smooth muscles, and neuronal tissues. Diversity of cardiac phenotype in dystrophinopathies suggests multiple layers of pathogenetic mechanisms in forming dystrophic cardiomyopathy. In this review article, we review the complex molecular interactions involving the pathogenesis of dystrophic cardiomyopathy, including primary gene mutations and loss of structural integrity, secondary cellular responses, and certain epigenetic and other factors that modulate gene expressions. Involvement of epigenetic gene regulation appears to lead to specific cardiac phenotypes in dystrophic hearts.

Dominant-negative inhibition of Ca2+ influx via TRPV2 ameliorates muscular dystrophy in animal models

Muscular dystrophy is a severe degenerative disorder of skeletal muscle characterized by progressive muscle weakness. One subgroup of this disease is caused by a defect in the gene encoding one of the components of the dystrophin-glycoprotein complex, resulting in a significant disruption of membrane integrity and/or stability and, consequently, a sustained increase in the cytosolic Ca(2+) concentration ([Ca(2+)](i)). In the present study, we demonstrate that muscular dystrophy is ameliorated in two animal models, dystrophin-deficient mdx mice and delta-sarcoglycan-deficient BIO14.6 hamsters by dominant-negative inhibition of the transient receptor potential cation channel, TRPV2, a principal candidate for Ca(2+)-entry pathways. When transgenic (Tg) mice expressing a TRPV2 mutant in muscle were crossed with mdx mice, the [Ca(2+)](i) increase in muscle fibers was reduced by dominant-negative inhibition of endogenous TRPV2. Furthermore, histological, biochemical and physiological indices characterizing dystrophic pathology, such as an increased number of central nuclei and fiber size variability/fibrosis/apoptosis, elevated serum creatine kinase levels, and reduced muscle performance, were all ameliorated in the mdx/Tg mice. Similar beneficial effects were also observed in the muscles of BIO14.6 hamsters infected with adenovirus carrying mutant TRPV2. We propose that TRPV2 is a principal Ca(2+)-entry route leading to a sustained [Ca(2+)](i) increase and muscle degeneration, and that it is a promising therapeutic target for the treatment of muscular dystrophy.

Protective effects of Ca2+ handling drugs against abnormal Ca2+ homeostasis and cell damage in myopathic skeletal muscle cells

Deficiency of delta-sarcoglycan (delta-SG), a component of the dystrophin-glycoprotein complex (DGC), causes skeletal muscular dystrophy and cardiomyopathy in BIO14.6 hamsters. Here, we studied the involvement of abnormal Ca2+ homeostasis in muscle degeneration and the protective effect of drugs against Ca2+ handling proteins in vivo as well as in vitro. First, we characterized the properties of cultured myotubes from muscles of normal and BIO14.6 hamsters (30-60 days old). While there were no apparent differences in the levels of expression of various Ca2+ handling proteins (L-type Ca2+ channel, ryanodine receptor, SR-Ca2+ ATPase, and Na+/Ca2+ exchanger), muscle-specific proteins (contractile actin and acetylcholine receptor), or DGC member proteins except SGs, BIO14.6 myotubes showed a high degree of susceptibility to mechanical stressors, such as cyclic stretching and hypo-osmotic stress as compared to normal myotubes, as evidenced by marked increases in creatine phosphokinase (CK) release and bleb formation. BIO14.6 myotubes showed abnormal Ca2+ homeostasis characterized by elevated cytosolic Ca2+ concentration, frequent Ca2+ oscillation, and increased 45Ca2+ uptake. These abnormal Ca2+ events and CK release were significantly prevented by Ca2+ handling drugs, tranilast, diltiazem, and FK506. The calpain inhibitor E64 prevented CK release, but not 45Ca2+ uptake. Some of these drugs (tranilast, diltiazem, and FK506) also exerted a significant protective effect for muscle degeneration in BIO14.6 hamsters and mdx mice in vivo. These observations suggest that elevated Ca2+ entry through sarcolemmal Ca2+ channels predominantly contributes to muscle degeneration and that the drugs tested here may have novel therapeutic potential against muscular dystrophy.

Cardiac syntrophin isoforms: Species-dependent expression, association with dystrophin complex and subcellular localization

Syntrophin is known to be a component of the dystrophin-glycoprotein complex (DGC), a membrane/cytoskeleton-anchoring structure that is essential for the maintenance of viability of sarcolemma. We purified DGC from hearts of human and several animal species, and compared their protein composition. While almost all components of DGC were present in various species, proteins with the apparent molecular mass of 50-65 kDa corresponding to syntrophin isoforms were very different among them. Three isoforms of syntrophin (alpha1, beta1, beta2) were expressed in hamster, rat and canine ventricles, whereas only alpha1-isoform was mainly expressed in human and rabbit ventricles. Immunohistochemical analysis revealed that alpha1-and beta2-syntrophins were co-localized in sarcolemma and in T-tubules of canine ventricles. However, despite membrane localization of most syntrophins, subcellular fractionation revealed that part of syntrophins were recovered in the cytosolic fraction devoid of other components of DGC, raising the possibility that syntrophins may play multiple roles in various intracellular sites of cardiac muscle cells. Species-dependent expression and unique subcellular localization of syntrophins in cardiac muscle may contribute to the variable severity of muscle dysgenesis caused by the same primary defect in components of DGC of human and other animal species.

A novel mechanism of myocyte degeneration involving the Ca2+-permeable growth factor-regulated channel

Disruption of the dystrophin-glycoprotein complex caused by genetic defects of dystrophin or sarcoglycans results in muscular dystrophy and/or cardiomyopathy in humans and animal models. However, the key early molecular events leading to myocyte degeneration remain elusive. Here, we observed that the growth factor-regulated channel (GRC), which belongs to the transient receptor potential channel family, is elevated in the sarcolemma of skeletal and/or cardiac muscle in dystrophic human patients and animal models deficient in dystrophin or delta-sarcoglycan. However, total cell GRC does not differ markedly between normal and dystrophic muscles. Analysis of the properties of myotubes prepared from delta-sarcoglycan-deficient BIO14.6 hamsters revealed that GRC is activated in response to myocyte stretch and is responsible for enhanced Ca2+ influx and resultant cell damage as measured by creatine phosphokinase efflux. We found that cell stretch increases GRC translocation to the sarcolemma, which requires entry of external Ca2+. Consistent with these findings, cardiac-specific expression of GRC in a transgenic mouse model produced cardiomyopathy due to Ca2+ overloading, with disease expression roughly parallel to sarcolemmal GRC levels. The results suggest that GRC is a key player in the pathogenesis of myocyte degeneration caused by dystrophin-glycoprotein complex disruption.

Biochemical characterization of the epithelial dystroglycan complex

Dystroglycan is a widely expressed extracellular matrix receptor that plays a critical role in basement membrane formation, epithelial development, and synaptogenesis. Dystroglycan was originally characterized in skeletal muscle as an integral component of the dystrophin glycoprotein complex, which is critical for muscle cell viability. Although the dystroglycan complex has been well characterized in skeletal muscle, there is little information on the structural composition of the dystroglycan complex outside skeletal muscle. Here we have biochemically characterized the dystroglycan complex in lung and kidney. We demonstrate that the presence of sarcoglycans and sarcospan in lung reflects association with dystroglycan in the smooth muscle. The smooth muscle dystroglycan complex in lung, composed of dystroglycan, dystrophin/utrophin, beta-, delta-, epsilon-sarcoglycan, and sarcospan, can be biochemically separated from epithelial dystroglycan, which is not associated with any of the known sarcoglycans or sarcospan. Similarly, dystroglycan in kidney epithelial cells is not associated with any of the sarcoglycans or sarcospan. Thus, our data demonstrate that there are distinct dystroglycan complexes in non-skeletal muscle organs as follows: one from smooth muscle, which is associated with sarcoglycans forming a similar complex as in skeletal muscle, and one from epithelial cells.

Making sense of the limb-girdle muscular dystrophies

The clinical heterogeneity which has long been recognized in the limb-girdle muscular dystrophies (LGMD) has been shown to relate to the involvement of a large number of different genes. At least eight forms of autosomal recessive LGMD and three forms of autosomal dominant disease are now recognized and can be defined by the primary gene or protein involved, or by a genetic localization. These advances have combined the approaches of positional cloning and candidate gene analysis to great effect, with the pivotal role of the dystrophin-associated complex confirmed through the involvement of at least four dystrophin-associated proteins in different subtypes of autosomal recessive LGMD (the sarcoglycanopathies). Two novel mechanisms may have to be postulated to explain the involvement of the calpain 3 and dysferlin genes in other forms of LGMD. Using the diagnostic tools which have become available as a result of this increased understanding, the clinical features of the various subtypes are also becoming clearer, with useful diagnostic and prognostic information at last available to the practising clinician.

Mechanisms of resistance to pathogenesis in muscular dystrophies

A mechanistic definition of the dystrophic process is proposed, and the effects of growth factors vs. down-regulation of growth are critically analyzed. A conceptual scheme is presented to illustrate the steps leading to pathology, and various compensatory systems which ameliorate the pathology are examined, particularly in regards to the mdv mouse which is resistant to the deficiency of dystrophin, the main protein product of the Duchenne and Becker muscular dystrophy (DMD/BMD) gene. These compensatory systems are analyzed in terms of the differential resistance of fiber types to pathogenesis. The generation of a stable population of maturationally arrested centronucleated fibers which express the mature adult myosin isoforms is proposed to be the main strategy of mdx muscle to minimize apoptosis. Physiological properties of these fibers, such as utrophin expression, and high mitochondrial and endoplasmic reticulum content, together with probable increased glycerophosphorylcholine concentrations and facile access to the vascular system, are hypothesized to be instrumental in their resistance to pathogenesis. It is proposed that the major element that determines the susceptibility of most human muscles to the dystrophic process is their inability to arrest the maturation of regenerated fibers at the centronucleated stage with a concomitant expression of the adult myosins.

Alpha1-syntrophin has distinct binding sites for actin and calmodulin

Overlay and co-sedimentation assays using recombinant alpha1-syntrophin proteins revealed that two regions of alpha1-syntrophin, i.e. aa 274-315 and 449-505, contain high-affinity binding sites for F-actin (Kd 0.16-0.45 microM), although only a single high-affinity site (Kd 0.35 microM) was detected in the recombinant full-length syntrophin. We also found that actomyosin fractions prepared from both cardiac and skeletal muscle contain proteins recognized by anti-syntrophin antibody. These data suggest a novel role for syntrophin as an actin binding protein, which may be important for the function of the dystrophin-glycoprotein complex or for other cell functions. We also found that alpha1-syntrophin binds calmodulin at two distinct sites with high (Kd 15 nM) and low (Kd 0.3 microM) affinity.

Bidirectional signaling between sarcoglycans and the integrin adhesion system in cultured L6 myocytes

The rat L6 skeletal muscle cell line was used to study expression of the dystrophin-containing glycoprotein complex and its interaction with the integrin system involved in the cell-matrix adhesion reaction. A complex of dystrophin and its associated proteins was fully expressed in L6 myotubes, from which anti-dystrophin or anti-alpha-sarcoglycan co-precipitated integrin alpha 5 beta 1 and other focal adhesion-associated proteins vinculin, talin, paxillin, and focal adhesion kinase. Immunostaining and confocal microscopy revealed that dystrophin, alpha-sarcoglycan, integrin alpha 5 beta 1, and vinculin exhibited overlapping distribution in the sarcolemma, especially at focal adhesion-like, spotty structures. Adhesion of cells to fibronectin- or collagen type I-coated dishes resulted in induction of tyrosine phosphorylation of alpha- and gamma-sarcoglycans but not beta-sarcoglycan. The same proteins were also tyrosine-phosphorylated when L6 cells in suspension were exposed to Arg-Gly-Asp-Ser peptide. All of these tyrosine phosphorylations were inhibited by herbimycin A. On the other hand, treatment of L6 myotubes with alpha- and gamma-sarcoglycan antisense oligodeoxynucleotides resulted in complete disappearance of alpha- and gamma-sarcoglycans and in significant reduction of levels of the associated focal adhesion proteins, which caused about 50% reduction of cell adhesion. These results indicate the existence of bidirectional communication between the dystrophin-containing complex and the integrin adhesion system in cultured L6 myocytes.