COMPARATIVE BIOCHEMISTRY OF MYOPATHIES*

Pathogenesis of progressive muscular dystrophy: studies on free radical metabolism in an animal model

Evidence to suggest the presence of abnormal metabolism of oxygen free radicals in progressive muscular dystrophy is presented using an animal model. In the superficial pectoral muscles of dystrophic chickens, enzyme activities regulating the metabolism of oxygen free radicals, i.e., catalase, superoxide dismutases and glutathione peroxidase, were significantly elevated within 1 week of hatching. Activities of related enzymes, i.e., glutathione reductase, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase were also elevated. In contrast, the specific activity of phosphofructokinase, the rate-limiting enzyme of the glycolytic pathway, was normal during the first 4-week period. These results suggest that there is an increased turnover of oxygen free radicals in the dystrophic muscle. This concept appears important in a further investigation of the pathogenesis and treatment of progressive muscular dystrophies.

Glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase and glyceraldehyde-3-phosphate dehydrogenase activities in early stages of development in dystrophic chickens

Glucose-6-phosphate dehydrogenase (G6PDH), 6-phosphogluconate dehydrogenase (6PGDH) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) activities were assayed in superficial pectoral muscles of hereditary dystrophic chickens, 1 week, 2 weeks, 4 weeks and 4 months after hatching. In control chickens, activities of G6PDH and 6PGDH were very low at 4 months of age; however, at 1 week of age, they were much higher than those at 4 months of age. Activities of G6PDH and 6PGDH were significantly higher in dystrophic chickens compared with those in the controls at all the stages of development studied. These findings suggest that considerable activities of G6PDH and 6PGDH are present within the pectoral muscle cells at early stages of development, at least in dystrophic chickens. GAPDH activity was significantly lower in dystrophic chickens at 2 weeks, 4 weeks and 4 months of age compared with those in control chicken. These findings together with our previous studies (Mizuno 1984a,b) in which increased activities of superoxide dismutases, catalase, glutathione peroxidase and glutathione reductase were reported in dystrophic chickens, indicate the presence of an increased capacity for the turnover of oxygen-free radicals within muscle cells in dystrophic chickens, and that oxygen-free radicals and the related activated oxygen species may be playing a role in inducing cellular damage.

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.

The incorporation of isotopes into proteins of normal and dystrophic mouse heart and skeletal muscle

Studies are described of L-[3H-(G)]leucine and [5-3H]uridine incorporation into proteins in dystrophic mouse skeletal and heart muscle of the 129/Rej/dy Bar Harbor strain. The results show a remarkable difference in isotope incorporation in the 2 muscle types. While skeletal muscle in the dystrophic mouse shows a consistently high cytoplasmic incorporation of leucine and uridine respectively, the heart muscle shows a marked reduction of uptake in nuclear, myofibrillar, and cytoplasmic protein. These findings appear to suggest that in terms of protein synthesis the mouse heart is not affected at the age of 8 weeks by a recognizably similar dystrophic process to that in the skeletal muscle. Whilst our material showed no gross evidence of cardiomyopathy, cytomorphometric analysis of the dystrophic heart muscle is in keeping with an atrophic process. The possibility that this difference in protein synthesis pattern may be related to an embryologically different organogenesis of the heart and skeletal muscle altering the predystrophic synthesis pattern cannot be excluded.

Morphological changes in skeletal muscles in vitamin E-deficient and refed rabbits

1. Nutritional muscular dystrophy was induced in young rabbits by giving them a vitamin E-deficient diet. Dystrophic animals were rehabilitated by the addition of vitamin E to the diet. Controls were fed on a supplemented diet containing 50 mg DL-α-tocopheryl acetate/kg.

2. Muscle weight and fibre diameter were determined in seven skeletal muscles, from the fore-limb (biceps brachii), the trunk (pectoralis descendens), the hind-limb (gastrocnemius, semitendinosus, soleus, plantaris and tibialis cranialis). Muscle weights, except for those of the soleus and semitendinosus, were significantly reduced in vitamin E-deficient animals when compared with the control animals. A monophasic fibre diameter distribution was obtained for all muscles from all the groups studied. Distribution curves for vitamin E-deficient muscles showed a consistent decrease in fibre diameter. The extent of change in distribution varied among different muscles. On refeeding, the redistribution curve returned to the normal pattern in the biceps brachii, semitendinosus, gastrocnemius, plantaris, tibialis cranialis and pectoralis descendens but not in the soleus.

3. No significant changes in bone length or weight were detected in the humerus, femur and tibia in vitamin E-deficient rabbits compared to the control animals.

4. Transverse and longitudinal sections of the seven muscles were studied by light microscopy. In the vitamin E-deficient animals, typical hyaline degeneration was observed with much infiltration of collagen and lymphocytes. The extent of degeneration varied among muscles, the gastrocnemius being most affected and the semitendinosus least affected. Muscles in the rabbits refed vitamin E showed that the infiltration with collagen and lymphocytes had almost disappeared.

Studies on new intracellular proteases in various organs of rat. 3. Control of group-specific protease under physiological conditions

1. To study the role of group-specific protease in enzyme degradation, alternation of its activity under various physiological conditions was examined. 2. Studies on the distribution of group-specific protease in various organs of rats showed high activity in skeletal muscle and the muscle layer of small intestine, and rather low activity in liver. The activity varied in different muscles, but red muscle tended to have higher activity than white muscle. Activity was much lower in the muscles of the stomach and colon than in those of the small intestine. 3. Group-specific protease in skeletal muscle increased under various dietary conditions (starvation, protein-free diet or high protein diet), but the activities in the muscle layer of the small intestine and liver were not greatly influenced by dietary conditions. None of the hormones tested (i.e. hydrocortisone, glucagon, insulin, growth hormone and estrogen) influenced the activity of group-specific protease in liver. 4. The level of group-specific protease in skeletal muscle was increased markedly fifteen days after denervation, with a reciprocal decrease in the level of muscle phosphorylase, which is a good substrate of the protease. 5. Liver protease activity appeared in the late suckling period. The activity in skeletal muscle was high at the time of birth and attained the adult level 3 weeks after birth. The activity in the muscle layer of the small intestine did not change after birth. Thus the mechanism for evoking these three specific proteases during development are apparently different. The activity of liver protease began to decrease approximately 12 h after partial hepatectomy and reached a minimum after about 72 h. Recovery of the protease activity was very slow and activity had not returned to the normal value 7 days after the operation. This observation seems to be consistent with the fact that there is little or no protease activity in liver in the neonatal period.