Glycolytic, pentose-phosphate shunt and transaminase enzymes in gastrocnemius muscle, liver, heart, and brain of two mouse mutants, 129 J-dy and A2g-adr, with abnormal muscle function

Transcriptional and post-transcriptional impact of toxic RNA in myotonic dystrophy

Myotonic dystrophy type 1 (DM1) is an RNA dominant disease in which mutant transcripts containing an expanded CUG repeat (CUG(exp)) cause muscle dysfunction by interfering with biogenesis of other mRNAs. The toxic effects of mutant RNA are mediated partly through sequestration of splicing regulator Muscleblind-like 1 (Mbnl1), a protein that binds to CUG(exp) RNA. A gene that is prominently affected encodes chloride channel 1 (Clcn1), resulting in hyperexcitability of muscle (myotonia). To identify DM1-affected genes and study mechanisms for dysregulation, we performed global mRNA profiling in transgenic mice that express CUG(exp) RNA, when compared with Mbnl1 knockout and Clcn1 null mice. We found that the majority of changes induced by CUG(exp) RNA in skeletal muscle can be explained by reduced activity of Mbnl1, including many changes that are secondary to myotonia. The pathway most affected comprises genes involved in calcium signaling and homeostasis. Some effects of CUG(exp) RNA on gene expression are caused by abnormal alternative splicing or downregulation of Mbnl1-interacting mRNAs. However, several of the most highly dysregulated genes showed altered transcription, as indicated by parallel changes of the corresponding pre-mRNAs. These results support the idea that trans-dominant effects of CUG(exp) RNA on gene expression in this transgenic model may occur at the level of transcription, RNA processing and mRNA decay, and are mediated mainly but not entirely through sequestration of Mbnl1.

Alanine infusion during hypoglycaemia partly supports cognitive performance in healthy human subjects

AIM

To investigate the potential for the non-glucose metabolic substrate alanine to support brain function during glucose deprivation in man.

METHODS

Seven healthy men were studied on two occasions using a hyperinsulinaemic glucose clamp to lower arterialized plasma glucose to 2.5 mmol/l, in the presence of either 2 mmol/kg/h alanine infusion or saline, measuring counter-regulatory hormonal responses, symptoms generated and cognitive function with a mini-battery of tests sensitive to hypoglycaemia.

RESULTS

Alanine infusion elevated plasma alanine (peak value 1481 +/- 1260 vs. 138 +/- 32 micro mol/l, P = 0.02 alanine vs. saline) and lactate (peak value 3.09 +/- 0.14 vs. 2.05 +/- 0.12 mmol/l, P = 0.02). Cognitive function assessed by the Stroop word and colour subtests deteriorated less with alanine than saline (P < 0.01 for both). Other cognitive function tests deteriorated equally and counter-regulatory hormones rose equally during hypoglycaemia in both studies (P > 0.34) except for increased glucagon with alanine (peak 260 +/- 53 vs. 91 + 8 ng/l, P = 0.03). There was no significant effect of alanine on either autonomic or neuroglycopenic symptom scores.

CONCLUSIONS

Some, but not all, aspects of cognitive performance may be supported by an alanine infusion during hypoglycaemia. It is not clear whether alanine supports brain function directly or via increased availability of lactate. These data contribute to the growing evidence that regional metabolic differences exist in the brain's ability to use non-glucose fuels during hypoglycaemia.

Calcium ion in skeletal muscle: its crucial role for muscle function, plasticity, and disease

Mammalian skeletal muscle shows an enormous variability in its functional features such as rate of force production, resistance to fatigue, and energy metabolism, with a wide spectrum from slow aerobic to fast anaerobic physiology. In addition, skeletal muscle exhibits high plasticity that is based on the potential of the muscle fibers to undergo changes of their cytoarchitecture and composition of specific muscle protein isoforms. Adaptive changes of the muscle fibers occur in response to a variety of stimuli such as, e.g., growth and differentition factors, hormones, nerve signals, or exercise. Additionally, the muscle fibers are arranged in compartments that often function as largely independent muscular subunits. All muscle fibers use Ca(2+) as their main regulatory and signaling molecule. Therefore, contractile properties of muscle fibers are dependent on the variable expression of proteins involved in Ca(2+) signaling and handling. Molecular diversity of the main proteins in the Ca(2+) signaling apparatus (the calcium cycle) largely determines the contraction and relaxation properties of a muscle fiber. The Ca(2+) signaling apparatus includes 1) the ryanodine receptor that is the sarcoplasmic reticulum Ca(2+) release channel, 2) the troponin protein complex that mediates the Ca(2+) effect to the myofibrillar structures leading to contraction, 3) the Ca(2+) pump responsible for Ca(2+) reuptake into the sarcoplasmic reticulum, and 4) calsequestrin, the Ca(2+) storage protein in the sarcoplasmic reticulum. In addition, a multitude of Ca(2+)-binding proteins is present in muscle tissue including parvalbumin, calmodulin, S100 proteins, annexins, sorcin, myosin light chains, beta-actinin, calcineurin, and calpain. These Ca(2+)-binding proteins may either exert an important role in Ca(2+)-triggered muscle contraction under certain conditions or modulate other muscle activities such as protein metabolism, differentiation, and growth. Recently, several Ca(2+) signaling and handling molecules have been shown to be altered in muscle diseases. Functional alterations of Ca(2+) handling seem to be responsible for the pathophysiological conditions seen in dystrophinopathies, Brody's disease, and malignant hyperthermia. These also underline the importance of the affected molecules for correct muscle performance.

The pentose phosphate pathway in skeletal muscle under patho-physiological conditions. A combined histochemical and biochemical study

Over the last 30 years, research into the neuromuscular apparatus, has expanded greatly. Multidisciplinary investigations have rapidly advanced our understanding both of diseases and of the basic neuromuscular mechanisms. The mode of pathological reaction of the neuromuscular apparatus is now quite well understood. The most notable aspect of the reaction of the injured neuromuscular apparatus is the remarkably stereotyped character of the resulting pathological changes as demonstrated by a wide variety of harmful causes, producing surprisingly similar effects. The findings of our combined histochemical and biochemical investigations presented in this monograph, are in complete harmony with the stereotyped character of the pathological changes. For example, it is particularly striking that many affected muscle fibres of patients with muscular dystrophies, congenital myopathies, inflammatory myopathies, metabolic myopathies, endocrine myopathies, or with diseases of the lower motor neuron, display an enhanced activity of both oxidative enzymes of the pentose phosphate pathway. Likewise, we found that experimental animals with disordered skeletal muscles, provoked by different types of agents or treatments, reveal the same marked rise in activity of GPDH and PGDH in the muscle fibres, with a positive correlation between the activity of both enzymes. Other findings of our investigations point to a positive correlation between the activity of GPDH and PGDH on the one hand and that of the non-oxidative enzymes of the pentose phosphate pathway, the enzymes TA, TK, RPI and RPE on the other hand. The rise in activity of PGDH and, in particular, of GPDH is regulated by two different mechanisms. The first represents a rapid control mechanism based on the stimulation of both oxidative enzymes of the pentose phosphate pathway by NADP+ and on their inhibition by NADPH. The other mechanism represents a long-term effect directed at the synthesis of the enzymes. It is this type of mechanism which is responsible for the rise in activity of GPDH and PGDH we observed. The findings obtained with the applied enzyme histochemical techniques clearly demonstrated that the rise in activity of both enzymes is not homogeneously distributed in the disordered skeletal muscles of man and experimental animals. For that reason, in order to obtain reliable quantitative information about enzyme activities in the muscle fibres themselves, the application of biochemical assays on a micro-scale was indispensable. The biochemical assay of enzyme activities was performed on histologically and histochemically selected dissected muscle specimens.(ABSTRACT TRUNCATED AT 400 WORDS)

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 myotonic mouse mutant ADR: physiological and histochemical properties of muscle

The muscle physiology and histochemistry of a hereditary neuromuscular syndrome of the mouse, "arrested development of righting response" (ADR), was studied. The speed of single twitches of fast ADR limb muscles was normal up to an age of about 60 days but decreased at later ages. At any age between 10 and 120 days postnatal, fast and slow muscles of the mutant displayed after-contractions of 1-3 (5) seconds duration. These coincided with electrical after-activity of muscle, as demonstrated by electromyography. After-contractions and EMG signals were suppressed by the membrane-stabilizing drug tocainide. These physiological data suggest that ADR is a myotonia. With a few exceptions, limb and trunk muscles of ADR animals showed a uniform oxidative phenotype with a lack of large diameter glycolytic fibers. The histochemical muscle phenotype of the ADR mouse was partially reversed by a long-term treatment with tocainide.

Additional biochemical criteria in the differential diagnosis of myositis

Thirty-six biopsy specimens of human biceps and vastus lateralis muscles were examined by histometric analysis and determination of enzyme activities (phosphorylase, triosephosphate dehydrogenase, 3-hydroxacyl-CoA-dehydrogenase, lactate dehydrogenase, hexose isomerase, citrate synthetase, 6-phosphogluconate dehydrogenase). The series included 13 specimens from patients suffering from a benign form of muscular dystrophy (limb girdle and Becker type of muscular dystrophy) and 12 specimens from patients with an acute (n = 5) or chronic (n = 7) form of myositis. Muscle fibres were atrophic in myositis and hypertrophic (with an increased variation of fibre diameters) in muscular dystrophies, as has been shown previously. When myositis samples were compared with either normal or dystrophic muscles, a highly significant lowering of glycolytic enzyme activity was found in chronic myositis, while the activity of 6-phosphogluconate dehydrogenase was elevated to highly significant levels. Measurements of the latter enzyme's activity might be of additional value in differentiating chronic forms of myositis from benign muscular dystrophies.

Free radicals: a potential pathogenic mechanism in inherited muscular dystrophy

Despite years of intensive work, the biochemical defect responsible for the pathogenesis of inherited muscular dystrophy has not been identified either in humans or animal models. This review examines evidence in support of the hypothesis that free radicals may be responsible for muscle degeneration in this disorder. A variety of cellular abnormalities noted in dystrophic muscles can be accounted for by free radical mediated damage. In addition, chemical by-products associated with free radical damage are found in dystrophic muscle tissue from humans and animals with this disease. Various enzymatic antioxidant systems can be enhanced as a normal cellular response to oxidative stress, and such changes are seen both in dystrophic muscle cells and certain other tissues of dystrophic animals. An increased level of free radical damage would follow from either: enhanced production of free radical species, or a deficient component of the cellular antioxidant system, such as vitamin E. The free radical hypothesis of muscular dystrophy can account for data supporting several alternative theories of the pathogenesis of this disease, as well as other observations which have not previously been explained.

Glyceraldehyde-3-phosphate dehydrogenase mRNA. Activity and amount in dystrophic hamster muscle

The activity and amount of glyceraldehyde-3-phosphate dehydrogenase (GAPDH) in muscle of young dystrophic hamsters was reduced to approximately half the level found in control animals. No changes in brain or liver enzyme activity were found. Several other glycolytic enzyme activities and creatine kinase activity in muscle were unchanged, except for modest decreases in aldolase and pyruvate kinase. To assess the synthesis of glyceraldehyde-3-phosphate dehydrogenase, the poly(A)+ RNA was isolated from muscle polysomes of dystrophic and control animals and its activity was assessed in an mRNA-dependent translation system. The translatability of the mRNA for GAPDH found in the dystrophic muscle preparations also was half of that found in the control muscle preparations. Decreases were also found in the translatability of mRNA for tropomyosin.

Activities of antioxidant enzymes in muscle, liver and lung of chickens with inherited muscular dystrophy

An inherited form of muscular dystrophy in chickens has been used as a model of Duchenne muscular dystrophy. The pectoralis major muscle of chickens with this disease showed a significantly elevated activity of catalase (CAT) one day after hatching, and by 7 days showed elevated superoxide dismutase (SOD), glutathione peroxidase (GPX) and glutathione-S-transferase (GST) activities. Increases were also found in tissues of the dystrophic birds that, unlike the pectoralis muscle, are considered to be unaffected by the pathology of muscular dystrophy. The soleus muscle contained significantly increased levels of SOD and GPX in 1 and 7 day old chickens, and increased GST in 1, 14, and 28 day old birds. CAT was significantly increased in liver from 1 and 7 day old chickens, while GPX was increased in lung from 1, 7 and 14 day old birds. These results support the possibility that excess oxygen free-radicals or altered cellular antioxidant defenses play some role in the pathogenesis of muscular dystrophy.

Gastrocnemius muscle lipids in relation to diet in two mouse mutants, 129Re-dy and A2G-adr, with abnormal muscle function

The lipids of gastrocnemius muscle from normal and dystrophic (dy) mice of the Bar Harbor, 129Re strain were studied. Animals were fed diets containing either 3.1% or 1.1% of total calories as linoleic acid. Lipid analyses were also done on muscle from a new mouse mutant, A2G-adr, which has abnormal muscle function, characterised by an arrested development of the righting response. These animals were fed the "high" linoleic acid diet only. Total lipid, triacylglycerol, and cholesterol were elevated in the 129Re-dy irrespective of the diet, whereas A2G-adr possessed significantly higher levels of cholesterol. Total phosphorus (micrograms P/g muscle) and cholesterol/phospholipid ratios were elevated in the dy strains only. Cardiolipin was raised in the dy ("low" linoleic diet) and adr muscle, whereas phosphatidylcholine was lower in the adr strain only. Linoleic acid esterified to phosphatidylcholine, phosphatidylethanolamine, and phosphatidylserine was elevated whereas arachidonic acid in phosphatidylserine was decreased in both mutants. Docosahexanoic acid (22:6) in all three dy phospholipids was decreased, independent of dietary treatment. The adr strain possessed normal levels of this fatty acid. The results specifically point to an abnormality in long-chain polyunsaturated fatty acid metabolism in gastrocnemius muscle in the 129Re-dy mutant; in the adr mutant they could reflect an abnormal increase in the number of muscle mitochondria.

Phosphorylation of the sarcolemma of dystrophic and normal hamster muscle following denervation

The rapid phosphorylation at 0 degree C of sarcolemma preparations of hamster leg muscle was compared with (Na+,K+)-ATPase activity in sham-operated and 7-day denervated muscle. The phosphorylation appeared to be under the trophic influence of the sciatic nerve since the denervated preparations exhibited a markedly reduced phosphorylation. In similar studies using dystrophic hamsters the sarcolemma preparations from sham-operated and denervated leg muscle both exhibited the same degree of phosphorylation.

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.

Developmental changes in lactate dehydrogenase and aldolase activity of the A2G-adr mouse with abnormal muscle function: further comparison with the 129Re-dy mutant

Lactate dehydrogenase and aldolase activity were reduced in lateral gastrocnemius muscle from two mouse mutants, A2G-adr and 129Re-dy, with abnormal muscle function. The activities of both of these enzymes were significantly reduced in the lateral gastrocnemius muscle from the A2G-adr mice at ages varying from 2 weeks to 32 weeks, whereas the activities in the soleus, heart, liver, and brain were the same as in the control animals. The lactate dehydrogenase isoenzymes in the lateral gastrocnemius and soleus muscles from the A2G mice were quantified, and although those of the soleus were comparable in mutant and control muscle, the lateral gastrocnemius from the adr mutant had reduced activity of LDH 5 and increased activities of the other four isoenzymes. The findings suggest that the adr mutation is expressed in the white (Type II) muscle fibres and not in the red (Type I) fibres or in any of the organs studied. It is suggested that the initiation of differentiation into Type II fibres from the embryonic form is absent or delayed in the A2G mutant. The reduced activities of lactate dehydrogenase and aldolase in 129Re-dy muscle confirm the findings of other workers.

Enzyme activities and activity profiles in muscle fibers of dystrophic, immature-normal, and adult-normal BL6 mice

Enzyme activities of energy metabolism were determined in both homogenates and single fibers of immature-normal (IN), adult-normal (AN), and adult-dystrophic (AD) tibialis anterior (TA) muscle of BL6 mice. Mitochondrial enzyme activities were similar in AN and AD, whereas lower activities were found in IN. Creatine kinase, glycogenolytic, and glycolytic enzyme activities were reduced but glucose 6-phosphate dehydrogenase was elevated in AD and IN as compared to AN. IN and AD both showed an increase in the H-subunit of lactate dehydrogenase. Microphotometric measurements of succinate dehydrogenase (SDH) revealed large fiber differences in AN whereas smaller scattering was observed both in IN and AD. Although similarities exist between enzyme and isozyme patterns of anaerobic metabolism in AD and IN, this does not hold for mitochondrial enzymes.