Increased turnover of proteins from the sarcoplasmic reticulum of dystrophic chicken muscle cells in tissue culture

Drastic reduction of calsequestrin-like proteins and impaired calcium binding in dystrophic mdx muscle

Although the reduction in dystrophin-associated glycoproteins is the primary pathophysiological consequence of the deficiency in dystrophin, little is known about the secondary abnormalities leading to x-linked muscular dystrophy. As abnormal Ca(2+) handling may be involved in myonecrosis, we investigated the fate of key Ca(2+) regulatory membrane proteins in dystrophic mdx skeletal muscle membranes. Whereas the expression of the ryanodine receptor, the dihydropyridine receptor, the Ca(2+)-ATPase, and calsequestrin was not affected, a drastic decline in calsequestrin-like proteins of 150-220 kDa was observed in dystrophic microsomes using one-dimensional immunoblotting, two-dimensional immunoblotting with isoelectric focusing, diagonal two-dimensional blotting technique, and immunoprecipitation. In analogy, overall Ca(2+) binding was reduced in the sarcoplasmic reticulum of dystrophic muscle. The reduction in Ca(2+) binding proteins might be directly involved in triggering impaired Ca(2+) sequestration within the lumen of the sarcoplasmic reticulum. Thus disturbed sarcolemmal Ca(2+) fluxes seem to influence overall Ca(2+) homeostasis, resulting in distinct changes in the expression profile of a subset of Ca(2+) handling proteins, which might be an important factor in the progressive functional decline of dystrophic muscle fibers.

Effect of phorbol esters and liposome-delivered phospholipids on the differentiation program of normal and dystrophic satellite cells

Satellite cells, isolated from hind limb of normal C57BL/6J mice, differentiate in culture in the presence of concentrations of phorbol esters which inhibit differentiation of embryonic myoblasts. However, if phosphatidylserine containing liposomes were added to the culture medium together with TPA, differentiation of satellite cells was reversibly inhibited. Under these conditions, the withdrawal of these cells from the cell cycle still occurred as in untreated cells. Phosphatidylserine liposomes alone or liposomes containing phosphatidylcholine (either alone or in combination with TPA) had no effect on satellite cell differentiation. In the case of satellite cells from dystrophic C57BL/6J/dydy mice, TPA addition (0.1 microM) to the culture medium partially (about 70%) inhibited morphological and biochemical differentiation. This effect could be prevented by preincubating dystrophic satellite cells with liposomes containing phosphatidylcholine but not other phospholipids. These data indicate that it is possible to change the sensitivity to TPA of satellite cells by modifying the phospholipid composition of their plasma membrane. Possible relationships of these phenomena with activation of protein kinase C or phosphatidylinositol breakdown have been investigated. The results obtained are discussed with regard to possible modulation of the intracellular response to agonist binding.

Tissue culture studies of muscle disorders: Part 2. Biochemical studies, nerve-muscle culture, metabolic myopathies, and animal models

This review continues with studies of protein, lipid, and purine metabolism of Duchenne muscular dystrophy (DMD) cells in vitro and of muscle cells in combined culture with nerve cells. In vitro studies of human metabolic myopathies are tabulated. Results using the hamster, chicken, and mouse (dy25, dy, mdg, and mdx) myopathies are discussed. Interesting findings include suggestions of altered collagen synthesis by DMD cells. Analysis of cell proteins by two-dimensional gel electrophoresis and the use of combined nerve-muscle cultures remain important areas of development. It is disappointing that so few attempts have been made to repeat significant findings in this field, and when a number of laboratories have examined the same phenomenon, the results are often contradictory. It remains to be shown how these various abnormalities found in cells in vitro are related to each other and to those pathologic features of diseased muscle observed in vivo.

Protein profiles of sarcoplasmic reticulum from normal and dystrophic mouse muscle

Sarcoplasmic reticulum (SR) was isolated from skeletal muscle of dystrophic (C57BL/6J dy2J/dy2J) mice and the protein composition analysed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Densitometric analysis of dystrophic SR preparations indicated a decrease in the Ca2+-ATPase and calsequestrin, and the appearance of a protein with molecular weight 72 000. These differences in the protein profiles between normal and dystrophic SR became more apparent as the disease progressed. The observations are discussed in relation to secondary changes in the dystrophic process such as changes in fibre type and the presence of immature fibres.

Protein degradation in cultured skeletal muscle from Duchenne muscular dystrophy patients

The loss of contractile protein in Duchenne muscular dystrophy could result from low rates of synthesis, abnormally high rates of protein degradation, or a combination of both. We measured overall protein degradation rates in cultured human muscle cells obtained at biopsy from patients with Duchenne dystrophy or various muscle diseases and normal subjects. Measurements were performed on confluent cultures exhibiting no growth and containing a mixed cell population of myoblasts, fibroblasts, and multinucleated myotubes. Using a new double-isotope labeling protocol, we found protein degradation rates in all three groups to be similar (KD = 0.0171-0.0176 hr-1), suggesting no detectable abnormality of overall protein degradation in cells derived from Duchenne dystrophy patients.

Changes in superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase activities and thiobarbituric acid-reactive products levels in early stages of development in dystrophic chickens

Cu-Zn superoxide dismutase, Mn superoxide dismutase, catalase, glutathione peroxidase, and glutathione reductase activities and thiobarbituric acid-reactive products were assayed in the superficial pectoral muscles of genetically dystrophic chickens (line 413) and their controls (line 412) 1, 2, and 4 weeks, and 4 months after hatching. In control chickens, all these enzyme activities declined as they grew older. In dystrophic chickens, all these enzyme activities were significantly elevated at all stages of development studied, and their developmental time courses were quite different from those in the controls. Thiobarbituric acid-reactive products were also significantly elevated in dystrophic chickens after 2 weeks of age. Invasion of macrophages and lipid cells were not manifest until 4 weeks after hatching in the dystrophic chickens studied. Therefore, observed abnormalities were considered to represent biochemical pathologies within muscle cells. Increased activities of the enzymes which are responsible for the regulation of active oxygen species and the elevated thiobarbituric acid-reactive products would indicate the presence of increased turnover of those active oxygen species. These findings indicated that active oxygen species were playing a significant role in the pathogenesis of muscular dystrophies. The possible mechanisms of cellular damage by active oxygen species are discussed.

Turnover of myofibrillar proteins in cultured muscle cells from normal and dystrophic chick embryos

Rates of protein degradation and synthesis were determined in myofibrillar and non-myofibrillar fractions and in myofibrillar subunits of cultured muscle cells from normal and dystrophic chick embryos of genetically closely related lines. Growth characteristics of normal and dystrophic cells were identical as measured by DNA, RNA and protein accumulation. Degradation rates of myofibrillar and non-myofibrillar protein determined from label-chase experiments were the same in normal and dystrophic cultures. In similar experiments in which degradation rates of eight different components of the myofibrillar fraction were measured, a spectrum of degradation rates was obtained indicating that myofibril components turn over individually or in groups, rather than as an intact unit. Myosin heavy chain was the slowest turning over component of those measured, while actin was among the most rapid. No differences were found between normals and dystrophics in the turnover of any of the components. Fractional rates of protein synthesis were also determined for the myofibrillar and non-myofibrillar fractions of cultured cells. The rates obtained with normal and dystrophic cells were indistinguishable and were the same for the myofibrillar and non-myofibrillar fractions. Initial amino acid incorporation rates into 17 separate components of the myofibrillar fractions were compared in normal and dystrophic cells. No significant differences were found.

Intracellular protein degradation in cultures of dystrophic muscle cells and fibroblasts

We have analysed protein degradation in primary cultures of normal and dystrophic chick muscle, in fibroblasts derived from normal and dystrophic chicks, and in human skin fibroblasts from normal donors and from patients with Duchenne muscular dystrophy (DMD). Our results indicate that degradative rates of both short- and long-lived proteins are unaltered in dystrophic muscle cells and in dystrophic fibroblasts. Longer times in culture and co-culturing chick fibroblasts with the chick myotubes do not expose any dystrophy-related abnormalities in protein catabolism. Furthermore, normal and dystrophic muscle cells and fibroblasts are equally able to regulate proteolysis in response to serum and insulin. We conclude that cultures of chick myotubes, chick fibroblasts, and fibroblasts derived from humans afflicted with DMD are not appropriate models for studying the enhanced protein degradation observed in dystrophy.