Altered protein synthesis and creatine kinase in breast muscle cell cultures from dystrophic chick embryos

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 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.

Content and synthesis of glycolytic enzymes and creatine kinase in skeletal muscles and normal and dystrophic chickens

A number of workers have reported that avian muscular dystrophy causes alterations in the levels of certain enzyme activities in "fast-twitch" muscle fibers but has little effect on enzyme activities in "slow-twitch" muscle fibers. In the present work, the effects of this disease on the content and relative rates of synthesis of a number of glycolytic enzymes and the skeletal muscle-specific MM isoenzyme of creatine kinase in chicken muscles was investigated. It was shown that (i) the approximate 50% reductions in steady-state concentrations of three glycolytic enzymes (aldolase, enolase, and glyceraldehyde-3-P dehydrogenase) in dystrophic breast (fast-twitch) muscle result predominantly from decreases in relative rates of synthesis, rather than accelerations in relative rates of degradation, of these proteins in the diseased tissue; (ii) in contrast to the situation with the glycolytic enzymes, muscular dystrophy has only minor effects (25% or less) on the content and relative rate of synthesis of MM creatine kinase in breast muscle fibers; (iii) the muscular dystrophy-associated alterations in content and synthesis of the glycolytic enzymes in breast muscle fibers become apparent only during postembryonic maturation of this tissue; and (iv) as expected, muscular dystrophy has no significant effect on the content or relative rates of synthesis of glycolytic enzymes in slow-twitch lateral adductor muscles of the chicken. These results are discussed in terms of the apparent similarities between the effects of muscular dystrophy and surgical denervation on the protein synthetic programs expressed by mature fast-twitch muscle fibers.

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.

Therapeutic trial with protease inhibitor (leupeptin) in chicken muscular dystrophy. A histologic and histochemical study

For the purpose of observing the therapeutic benefit of protease inhibitors for progressive muscular dystrophy, a large quantity of doses of leupeptin of 10 mg/kg/day and 50 mg/kg/day were administered i.p. to male chickens afflicted with hereditary muscular dystrophy (line 413) for 4 months starting on the 7th day ex ovo. No clinical improvement was identified in physical ability as a result of the examination by flip test, and creatine kinase (CK) values. The number of necrotic fibers in the pectoralis superficialis (PS) muscle which is known to be preferentially damaged in dystrophic chicken, did not decrease significantly in the birds treated with 10 mg leupeptin/kg/day (number of necrotic fibers; 47.7/mm2) and 50 mg/kg/day (46.4/mm2) as compared to that of the untreated ones (43.2/mm2). A morphometric analysis of fiber diameter distribution also showed no statistical difference between the treated and untreated birds. In the second group, 10 mg leupeptin/kg and a combination of leupeptin and bestatin of 10 mg/kg each were injected directly into the left lower half of the PS muscle three times a week for 4 months. Necrotic fibers were still present in the injected site, remote area of the left upper PS muscle treated with leupeptin (52.7/mm2), leupeptin and bestatin (52.2/mm2), and contralateral right upper PS muscle (41.6 and 53.5/mm2, respectively). The number of necrotic fibers in treated muscles was again not significantly different from that in untreated dystrophic ones (39.6/mm2). In fiber diameter analysis, no statistical difference was recognized between the treated and untreated dystrophic muscles.

Successful treatment of murine muscular dystrophy with the proteinase inhibitor leupeptin

Mice with genetic muscular dystrophy were treated with intraperitoneal injections of the proteinase inhibitor leupeptin, beginning before the onset of weakness. A significant number of the treated animals failed to develop histological evidence of dystrophy, compared with controls. Leupeptin treatment prevented (or delayed) the onset of muscular dystrophy in this experiment.

In vivo effects of protease inhibitors on chickens with hereditary muscular dystrophy

Beginning on day 4 ex ovo, and every 3 d thereafter, genetically dystrophic Line 413 chickens were given intraperitoneal injections (4 mg/kg body wt) of a protease inhibitor, leupeptin, pepstatin, or antipain. Experimental chickens received protease inhibitors dissolved in a water:ethanol:dimethyl sulfoxide solution (50:40:10, vol:vol:vol). Control untreated animals received diluent injections. Untreated dystrophic chickens typically reach around day 30 ex ovo a maximum ability to right from the supine position in a standardized functional test for muscle weakness. After day 30 ex ovo, the dystrophic chickens are found to decline progressively in their ability to right, compared with normal, nondystrophic controls, which have an unimpaired ability to right. Concomitantly, dystrophic chickens exhibit characteristically high levels of plasma creatine phosphokinase enzyme activity. In addition, an increased frequency of degenerating, regenerating, and vacuolated myofibers, and inflammatory cells appear in the affected pectoralis major muscles from the dystrophic chicken. Throughout the duration of the trial, there was no major enhancement in the functional righting ability of dystrophic chickens receiving any one of the protease inhibitors tested. However, there was a significant reduction in the abnormally high levels of plasma creatine phosphokinase in the treated chickens. Also, there was an apparent reduction in the mean number of vacuolated fibers in the pectoralis muscle from the protease inhibitor-treated birds. No significant reductions were observed in the relative frequency of degenerating and regenerating myofibers or inflammatory cells. In addition to the plasma creatine phosphokinase decrease, however, therapeutic benefit was seen in 31.0, 30.5, and 14.8% increases in the wet weight (and total noncollagen protein) of pectoralis muscle from dystrophic chickens receiving leupeptin, pepstatin or antipain, respectively.

Alterations in creatine kinase in fresh muscle and cell cultures in Duchenne dystrophy

Creatine kinase (CK) (total and isoenzymes) was measured in cultures obtained by dissociation and subsequent plating of cells from biopsied quadriceps muscle of 10 patients with Duchenne muscular dystrophy (DMD) and 20 controls. The total cellular CK and CK-MM reached highest values around 21 days in both DMD and control cultures, suggesting that DMD cultures do not show delayed myoblast fusion. There was a significant decrease of total cellular CK in DMD cultures at all stages, but the maximum differences were noted for the peak values. The CK isoenzyme pattern in DMD cultures demonstrated a higher percentage of CK-BB and CK-MB and a lower percentage of CK-MM than was observed in cultures from the controls. Addition of cytosine arabinoside after myoblast fusion to muscle cell cultures did not induce significant changes of CK isoenzyme pattern. There was no difference in the CK levels in culture medium from controls and DMD patients. The alterations of cellular CK were similar in fresh muscle and cell cultures from DMD patients.