Content and synthesis of glycolytic enzymes in normal, denervated, and dystrophic skeletal muscle fibers

Denervation-induced skeletal muscle atrophy is associated with increased mitochondrial ROS production

Reactive oxygen species (ROS), especially mitochondrial ROS, are postulated to play a significant role in muscle atrophy. We report a dramatic increase in mitochondrial ROS generation in three conditions associated with muscle atrophy: in aging, in mice lacking CuZn-SOD ( Sod1−/−), and in the neurodegenerative disease, amyotrophic lateral sclerosis (ALS). ROS generation in muscle mitochondria is nearly threefold higher in 28- to 32-mo-old than in 10-mo-old mice and is associated with a 30% loss in gastrocnemius mass. In Sod1−/− mice, muscle mitochondrial ROS production is increased >100% in 20-mo compared with 5-mo-old mice along with a >50% loss in muscle mass. ALS G93A mutant mice show a 75% loss of muscle mass during disease progression and up to 12-fold higher muscle mitochondrial ROS generation. In a second ALS mutant model, H46RH48Q mice, ROS production is approximately fourfold higher than in control mice and is associated with a less dramatic loss (30%) in muscle mass. Thus ROS production is strongly correlated with the extent of muscle atrophy in these models. Because each of the models of muscle atrophy studied are associated to some degree with a loss of innervation, we were interested in determining whether denervation plays a role in ROS generation in muscle mitochondria isolated from hindlimb muscle following surgical sciatic nerve transection. Seven days postdenervation, muscle mitochondrial ROS production increased nearly 30-fold. We conclude that enhanced generation of mitochondrial ROS may be a common factor in the mechanism underlying denervation-induced atrophy.

Putative effects of nerve extract on carbonic anhydrase III expression in rat muscles

Carbonic anhydrase III (CA III), the predominant CA isoform in skeletal muscle is very sensitive to neuronal influences. We aimed to determine whether CA III expression could be influenced by neurotrophic factor(s) present in sciatic nerve extract (SNE). Intact muscles were thus compared with denervated soleus (SOL), extensor digitorum longus (EDL), and tibialis anterior (TA) muscles injected daily for 7 days with saline solution (SS) or with SNE. CA III activity was significantly increased in SS-treated EDL and TA muscles compared to control (CTR), while SNE injections partially prevented this increase. There was no significant difference for CA III activity in the SOL between CTR, SS, and SNE groups. The CA III mRNA increase observed in response to denervation was reduced by 40% in SNE-treated EDL and TA muscles. While SOL CA III mRNA level was not affected by denervation, a 52% decrease was observed with SNE. We concluded that neuronal modulation of CA III expression in type II fibers may involve a neurotrophic component.

The expression of glycogen phosphorylase in normal and dystrophic muscle

Specific cofactor labelling was employed to determine the degradation rate of glycogen phosphorylase in normal adult C57BL/6J mice and their dystrophic counterparts (C57BL/6Jdy/dy). The rate constant for the decay of phosphorylase-bound label was 0.125 day-1 in normal muscle and 0.49 day-1 in dystrophic muscle, i.e. a lower rate of catabolism of phosphorylase in dystrophic muscle. Quantitative Northern-blot analyses of total RNA isolated from normal and dystrophic muscle indicated that the abundance of phosphorylase mRNA as a percentage of total RNA was approx. 40% lower in dystrophic muscle. The specific activity of phosphorylase in dystrophic muscle is approx. 60% lower than in normal muscle, and is elicited by a lower rate of turnover of the enzyme, i.e. both synthesis and degradation are decreased.

Effect of denervation on the expression of glycogen phosphorylase in mouse skeletal muscle

After sciatectomy of the left hind-limb of C57BL/J mice, a denervation-induced muscular atrophy ensued and was accompanied by a decrease in the specific activity of glycogen phosphorylase to approx. 25% of control values. The cofactor of phosphorylase, pyridoxal 5'-phosphate, was used as a specific label in the determination of the degradation rate of the enzyme following nerve section. After a delay of 3-4 days, phosphorylase was degraded approx, twice as rapidly in the denervated gastrocnemius (0.20 day-1) as in the control muscle (0.12 day-1). The effect of denervation on phosphorylase mRNA was measured by quantitative Northern-blot analysis using a rat skeletal-muscle phosphorylase cDNA probe. After an initial rapid decline, phosphorylase mRNA levels stabilized in denervated muscle at 50% of the value measured in the contralateral control muscle.

Distribution and developmental transition of phosphoglycerate mutase and creatine phosphokinase isozymes in rat muscles of different fiber-type composition

Phosphoglycerate mutase and creatine phosphokinase have in mammals three isozymes (types MM, MB and BB) with similar tissue distribution and developmental transition in muscle cells. To assess whether the phenotype and the developmental switch of these isozymes differ in the diverse types of muscle fibers, the enzymatic activities and the isozyme patterns, analyzed by cellulose acetate electrophoresis, have been determined in rat soleus, extensor digitorum longus and gastrocnemius muscles during postnatal development. Both phosphoglycerate mutase and creatine phosphokinase activity increased in the three muscles, the increase in extensor digitorum longus and gastrocnemius being higher than in soleus. For the two enzymes the increase in activity was due to the progressive increment of the muscle-specific forms. It is concluded that whereas phosphoglycerate mutase and creatine phosphokinase type-B subunits are present at similar levels in both type I and type II muscle fibers, phosphoglycerate mutase and creatine phosphokinase type-M subunits exhibit much higher levels in type II fibers.

Oxidative stress and muscular dystrophy

Oxidative stress may be the fundamental basis of many of the structural, functional and biochemical changes characteristic of the inherited muscular dystrophies in animals and humans. The presence of by-products of oxidative damage, and the compensatory increases in cellular antioxidants, both indicate oxidative stress may be occurring in dystrophic muscle. Changes in the proportions and metabolism of cellular lipids, abnormal functions of cellular membranes, altered activity of membrane-bound enzymes such as the SR Ca2+-ATPase, disturbances in cellular protein turnover and energy production and a variety of other changes all indicate that these inherited muscular dystrophies appear more like the results of oxidative stress to muscle than any other type of underlying muscle disturbance. Particular details of these altered characteristics of dystrophic muscle, in combination with current knowledge on the processes of oxidative damage to cells, may provide some insight into the underlying biochemical defect responsible for the disease, as well as direct research towards the ultimate goal of an effective treatment.

The turnover of skeletal muscle glycogen phosphorylase studied using the cofactor, pyridoxal phosphate, as a specific label

The turnover of glycogen phosphorylase has been measured using the cofactor, pyridoxal phosphate, as a label specific for this enzyme in skeletal muscle. Radiolabelled pyridoxine administered in vivo is incorporated into a protein-bound fraction in skeletal muscle, shown by several criteria to be equivalent to glycogen phosphorylase. This pool of radiolabel disappears slowly with a half-life of 11.9 days, taken to be a good estimate of the intracellular half-life of the enzyme. The use of the cofactor in this fashion minimises overestimation of half-life that results from reincorporation of the label. Further, premature dissociation of the cofactor from native enzyme, which would lead to underestimation of half-life, is unlikely. At the level of sensitivity given by this method there was little evidence for the appearance of pyridoxal phosphate-labelled degradation intermediates of the enzyme.

Accelerated degradation of glycogen phosphorylase in denervated and dystrophic mouse skeletal muscle

Pyridoxal phosphate, the cofactor of glycogen phosphorylase, fulfils the criteria needed of a turnover label for this enzyme. The decay of protein-bound label following administration of [3H]pyridoxine is a good index of the rate of degradation of the enzyme in vivo. This method has been applied to the study of catabolism of the enzyme in normal, denervated and dystrophic mouse skeletal muscle. In both of the pathological conditions the enzyme is degraded more rapidly than normal.