Muscle fibre type composition, motoneuron firing properties, axonal conduction velocity and refractory period for foot extensor motor units in dystrophia myotonica

Correction of Clcn1 alternative splicing reverses muscle fiber type transition in mice with myotonic dystrophy

In myotonic dystrophy type 1 (DM1), deregulated alternative splicing of the muscle chloride channel Clcn1 causes myotonia, a delayed relaxation of muscles due to repetitive action potentials. The degree of weakness in adult DM1 is associated with increased frequency of oxidative muscle fibers. However, the mechanism for glycolytic-to-oxidative fiber type transition in DM1 and its relationship to myotonia are uncertain. Here we cross two mouse models of DM1 to create a double homozygous model that features progressive functional impairment, severe myotonia, and near absence of type 2B glycolytic fibers. Intramuscular injection of an antisense oligonucleotide for targeted skipping of Clcn1 exon 7a corrects Clcn1 alternative splicing, increases glycolytic 2B levels to ≥ 40% frequency, reduces muscle injury, and improves fiber hypertrophy relative to treatment with a control oligo. Our results demonstrate that fiber type transitions in DM1 result from myotonia and are reversible, and support the development of Clcn1-targeting therapies for DM1.

MBNL-dependent impaired development within the neuromuscular system in myotonic dystrophy type 1

AIMS

Myotonic dystrophy type I (DM1) is one of the most frequent muscular dystrophies in adults. Although DM1 has long been considered mainly a muscle disorder, growing evidence suggests the involvement of peripheral nerves in the pathogenicity of DM1 raising the question of whether motoneurons (MNs) actively contribute to neuromuscular defects in DM1.

METHODS

By using micropatterned 96-well plates as a coculture platform, we generated a functional neuromuscular model combining DM1 and muscleblind protein (MBNL) knock-out human-induced pluripotent stem cells-derived MNs and human healthy skeletal muscle cells.

RESULTS

This approach led to the identification of presynaptic defects which affect the formation or stability of the neuromuscular junction at an early developmental stage. These neuropathological defects could be reproduced by the loss of RNA-binding MBNL proteins, whose loss of function in vivo is associated with muscular defects associated with DM1. These experiments indicate that the functional defects associated with MNs can be directly attributed to MBNL family proteins. Comparative transcriptomic analyses also revealed specific neuronal-related processes regulated by these proteins that are commonly misregulated in DM1.

CONCLUSIONS

Beyond the application to DM1, our approach to generating a robust and reliable human neuromuscular system should facilitate disease modelling studies and drug screening assays.

Increased autophagy and apoptosis contribute to muscle atrophy in a myotonic dystrophy type 1 Drosophila model

Muscle mass wasting is one of the most debilitating symptoms of myotonic dystrophy type 1 (DM1) disease, ultimately leading to immobility, respiratory defects, dysarthria, dysphagia and death in advanced stages of the disease. In order to study the molecular mechanisms leading to the degenerative loss of adult muscle tissue in DM1, we generated an inducible Drosophila model of expanded CTG trinucleotide repeat toxicity that resembles an adult-onset form of the disease. Heat-shock induced expression of 480 CUG repeats in adult flies resulted in a reduction in the area of the indirect flight muscles. In these model flies, reduction of muscle area was concomitant with increased apoptosis and autophagy. Inhibition of apoptosis or autophagy mediated by the overexpression of DIAP1, mTOR (also known as Tor) or muscleblind, or by RNA interference (RNAi)-mediated silencing of autophagy regulatory genes, achieved a rescue of the muscle-loss phenotype. In fact, mTOR overexpression rescued muscle size to a size comparable to that in control flies. These results were validated in skeletal muscle biopsies from DM1 patients in which we found downregulated autophagy and apoptosis repressor genes, and also in DM1 myoblasts where we found increased autophagy. These findings provide new insights into the signaling pathways involved in DM1 disease pathogenesis.

Central and peripheral components of exercise-related fatigability in myotonic dystrophy type 1

BACKGROUND

Fatigue frequently occurs in myotonic dystrophy type 1 (DM1), but its pathophysiology remains unclear. This study assessed central and peripheral components of exercise-related fatigability in patients with DM1, compared to controls.

METHODS

Examinations were performed before and after a contraction of the abductor digiti minimi (ADM) muscle sustained for 45 s at 60% of maximal voluntary contraction (MVC). Myoelectric activity was recorded using high spatial resolution surface EMG during twitch stimulations and MVC and was characterized by root mean square, mean power frequency (MPF), and muscle fiber conduction velocity (MFCV). Peripheral nerve excitability was assessed by stimulating the ulnar nerve at the wrist with ADM recordings. Motor cortex excitability testing to transcranial magnetic stimulation included measures of intracortical facilitation and inhibition of motor evoked potentials (MEPs) in ADM muscle.

RESULTS

At baseline, patients with DM1 showed altered peripheral nerve and cortical excitability (reduced intracortical facilitation) associated with impaired myoelectric properties. During the fatiguing exercise, the force remained stable, while MPF and MFCV decreased in both DM1 and control groups. After exercise, only refractoriness was reduced in patients with DM1, whereas controls showed marked neuromuscular and cortical changes.

CONCLUSION

Patients with DM1 showed altered excitability of various cortical and neuromuscular components at baseline. However, most of excitability parameters did not vary after exercise in patients with DM1, in contrast to controls. This suggests that excitability properties, frankly altered at baseline, were not prone to be affected further after exercise in patients with DM1.

Large CTG repeats trigger p16-dependent premature senescence in myotonic dystrophy type 1 muscle precursor cells

A CTG repeat amplification is responsible for the dominantly inherited neuromuscular disorder, myotonic dystrophy type 1 (DM1), which is characterized by progressive muscle wasting and weakness. The expanded (CTG)n tract not only alters the myogenic differentiation of the DM1 muscle precursor cells but also reduces their proliferative capacity. In this report, we show that these muscle precursor cells containing large CTG expansion sequences have not exhausted their proliferative capacity, but have entered into premature senescence. We demonstrate that an abnormal accumulation of p16 is responsible for this defect because the abolition of p16 activity overcomes early growth arrest and restores an extended proliferative capacity. Our results suggest that the accelerated telomere shortening measured in DM1 cells does not contribute to the aberrant induction of p16. We propose that a cellular stress related to the amplified CTG repeat promotes premature senescence mediated by a p16-dependent pathway in DM1 muscle precursor cells. This mechanism is responsible for the reduced proliferative capacity of the DM1 muscle precursor cells and could participate in both the impaired regeneration and atrophy observed in the DM1 muscles containing large CTG expansions.

Satellite cell dysfunction contributes to the progressive muscle atrophy in myotonic dystrophy type 1

AIMS

Myotonic dystrophy type 1 (DM1), one of the most common forms of inherited neuromuscular disorders in the adult, is characterized by progressive muscle weakness and wasting leading to distal muscle atrophy whereas proximal muscles of the same patients are spared during the early phase of the disease. In this report, the role of satellite cell dysfunction in the progressive muscular atrophy has been investigated.

METHODS

Biopsies were obtained from distal and proximal muscles of the same DM1 patients. Histological and immunohistological analyses were carried out and the past regenerative history of the muscle was evaluated. Satellite cell number was quantified in vivo and proliferative capacity was determined in vitro.

RESULTS

The size of the CTG expansion was positively correlated with the severity of the symptoms and the degree of muscle histopathology. Marked atrophy associated with typical DM1 features was observed in distal muscles of severely affected patients whereas proximal muscles were relatively spared. The number of satellite cells was significantly increased (twofold) in the distal muscles whereas very little regeneration was observed as confirmed by telomere analyses and developmental MyHC staining (0.3-3%). The satellite cells isolated from the DM1 distal muscles had a reduced proliferative capacity (36%) and stopped growing prematurely with telomeres longer than control cells (8.4 vs. 7.1 kb), indicating that the behaviour of these precursor cells was modified.

CONCLUSIONS

Our results indicate that alterations in the basic functions of the satellite cells progressively impair the muscle mass maintenance and/or regeneration resulting in gradual muscular atrophy.

A study of repeated lateral pinch grip in myotonic dystrophy

BACKGROUND AND PURPOSE

Subjects with myotonic dystrophy present with progressive muscle weakness, myotonia and fatigue. The aim of this study was to determine whether there was a difference in response to fatiguing exercise in myotonic dystrophy individuals compared to normal subjects. If no difference was found, a similar response to a physiotherapy exercise programme as seen in normal subjects might be expected.

METHOD

Ten individuals with myotonic dystrophy were compared to eight normal subjects in their response to ten repetitions of maximal lateral pinch grip efforts each five seconds in duration and separated by a ten-second rest. The root mean square (RMS) values, initial median frequency (Fmed) and slope of the median frequencies were recorded by electromyography (EMG) for the first dorsal interosseus, flexor pollicis brevis, flexor digitorum profundus and extensor digitorum communis muscles in the forearm and hand. Simultaneously, the rate of grip development, rate of grip release and work done during each grip effort were recorded by dynamometer. The RMS values for all muscles from subjects with myotonic dystrophy increased over the ten repetitions.

RESULTS

The initial Fmed for all myotonic dystrophy muscles was lower than for the normal subjects. The Fmed slopes for the first and last repetition showed no significant difference to the normal subjects. Rate of grip development was no different between groups over ten repetitions. Rate of grip release was slower and work done less for the individuals with myotonic dystrophy.

CONCLUSION

Results suggest the main difference between muscles affected by myotonic dystrophy and normal ones was the smaller size of muscle fibres. The increase in rate of grip release that was found is supportive of the 'warm-up' phenomenon. This appears to indicate that muscles affected by myotonic dystrophy could benefit from standard physiotherapeutic exercise methods.

Electrically-elicited surface mechanomyogram in myotonic dystrophy

The surface oscillation of the muscle during electrical stimulation is detectable by an accelerometer as a surface mechanomyogram (MMG). The aim of this study was to evaluate whether MMG properties reflect the mechanical muscle changes induced by myotonic dystrophy (MyD). To this end, the tibialis anterior of seven MyD patients and seven age- and sex-matched controls (C) was supramaximally stimulated at 1 (single twitches), 5, 10, 15, 20 Hz for 3 s at the most proximal motor point.

RESULTS SINGLE TWITCHES

The MMG amplitude was 67% less, the duration 37% longer, the electromechanical delay 64% longer, and the spectrum mean frequency, 44% lower in the MyD patients than in controls.

REPETITIVE STIMULATION

At each stimulation frequency, the average MMG peak-to-peak was less in the MyD patients than in C.

CONCLUSION

The differences between the MMGs of MyD patients and C support the hypothesis that, together with the well-known changes in sarcolemmal excitability, an alteration in electromechanical coupling and a failure in contractile machinery may coexist in MyD.

Muscle histopathology in myotonic dystrophy in relation to age and muscular weakness

We studied histopathological changes in the biceps brachii muscle in relation to age and the degree of muscle weakness in 64 patients (aged 11-59 years) with myotonic dystrophy. The proportion of type 1 fibers was unaltered in the adolescent patients compared with control values, but increased with age. The average diameters of all the fiber types were smaller than control values in the adolescents, suggesting immature development; however, there was an increase in diameter with age that was associated with an increase of hypertrophic type 2 fibers. At all ages, type 1 fibers were smaller than type 2 ones. Small angular fibers and small group atrophy consisted mainly of type 1 fibers, their incidences decreasing with age. The severity of muscular weakness was related to the predominance of type 1 fibers, the reduction in the number of hypertrophic type 2 fibers, and the accumulation of adipose cells, but not to the presence of small angular fibers or to small group atrophy.

Ca2+ homeostasis in Brody's disease. A study in skeletal muscle and cultured muscle cells and the effects of dantrolene an verapamil

Brody's disease, i.e., sarcoplasmic reticulum (SR) Ca(2+)-dependent Mg(2+)-ATPase (Ca(2+)-ATPase) deficiency, is a rare inherited disorder of skeletal muscle function. Pseudo-myotonia is the most important clinical feature. SR Ca(2+)-ATPase and Ca2+ homeostasis are examined in m. quadriceps and/or cultured muscle cells of controls and 10 patients suffering from Brody's disease. In both m. quadriceps and cultured muscle cells of patients, the SR Ca(2+)-ATPase activity is decreased by approximately 50%. However, the concentration of SR Ca(2+)-ATPase and SERCA1 are normal. SERCA1 accounts for 83 and 100% of total SR Ca(2+)-ATPase in m. quadriceps and cultured muscle cells, respectively. This implies a reduction of the molecular activity of SERCA1 in Brody's disease. The cytosolic Ca2+ concentration ([Ca2+]i) at rest and the increase of [Ca2+]i after addition of acetylcholine are the same in cultured muscle cells of controls and patients. The half-life of the maximal response, however, is raised three times in the pathological muscle cells. Addition of dantrolene or verapamil after the maximal response accelerates the restoration of the [Ca2+]i in these muscle cells. The differences in Ca2+ handling disappear by administration of dantrolene or verapamil concomitantly with acetylcholine. The reduced Ca2+ re-uptake from the cytosol presumably due to structural modification(s) of SERCA1 may explain the pseudo-myotonia in Brody's disease. Single cell measurements suggest a beneficial effect of dantrolene or verapamil in treating patients suffering from Brody's disease.

Deficiency of Na+/K(+)-ATPase and sarcoplasmic reticulum Ca(2+)-ATPase in skeletal muscle and cultured muscle cells of myotonic dystrophy patients

Since defective regulation of ion transport could initiate or contribute to the abnormal cellular function in myotonic dystrophy (MyD), Na+/K(+)-ATPase and sarcoplasmic reticulum (SR) Ca(2+)-ATPase were examined in skeletal muscle and cultured skeletal muscle cells of controls and MyD patients. Na+/K(+)-ATPase was investigated by measuring ouabain binding and the activities of Na+/K(+)-ATPase and K(+)-dependent 3-O-methylfluorescein phosphate (3-O-MFPase). SR Ca(2+)-ATPase was analysed by e.l.i.s.a., Ca(2+)-dependent phosphorylation and its activities with ATP and 3-O-methylfluorescein phosphatase (3-O-MFP). In MyD muscle the K(+)-dependent 3-O-MFPase activity and the activity and concentration of SR Ca(2+)-ATPase were decreased by 40%. In cultured muscle cells from MyD patients the activities as well as the concentration of both Na+/K(+)-ATPase and SR Ca(2+)-ATPase were reduced by about 30-40%. The ouabain-binding constant and the molecular activities, i.e. catalytic-centre activities with ATP or 3-O-MFP, of Na+/K(+)-ATPase and SR Ca(2+)-ATPase were similar in muscle as well as in cultured cells from both controls and MyD patients. Thus the decreased activity of both ATPases in MyD muscle is caused by a reduction in the number of their molecules. To check whether the deficiency of ATP-dependent ion pumps is a general feature of the pathology of MyD, we examined erythrocytes from the same patients. In these cells the Ca2+ uptake rate and the Ca(2+)-ATPase activity were lower than in controls, but the Ca(2+)-ATPase concentration was normal. Thus the reduced Ca(2+)-ATPase activity is caused by a decrease in the molecular activity of the ion pump. The Na+/K(+)-ATPase activity is also lower in erythrocytes of MyD patients. It is concluded that the observed alterations in ion pumps may contribute to the pathological phenomena in the muscle and other tissues in patients with MyD.

Modification in the expression and localization of contractile and cytoskeletal proteins in Schwartz-Jampel syndrome

Muscle biopsies taken from 4 patients with clinical diagnosis of Schwartz-Jampel syndrome were analyzed by enzyme-histochemical immunocytochemical and biochemical techniques. In situ distribution of the different myosin heavy chain (MHC) isoforms together with that of the cytoskeletal proteins vimentin, desmin and titin was determined in type I, type IIA, type IIB and type IIC fibers. The same muscle biopsies were analyzed for their content in myosin light chains (MLC) by two-dimensional gel electrophoresis and native myosin isoforms by pyrophosphate gel electrophoresis. The opportunity to study 4 patients of different ages, all members of the same family, permitted us to reveal several interesting features in this rare and so far poorly understood muscle pathology. (i) We observed a predominance of slow (type I) fibers in the oldest patient. (ii) Two classes of small clusters of atrophic type IIC fibers were observed. The first class corresponded to fibers which coexpressed embryonic, fetal and fast, but not slow, MHC isoforms. The fibers also displayed an abnormal distribution of desmin, vimentin and titin. The second class was composed by fibers coexpressing embryonic, fetal, fast and slow, MHC isoforms. In contrast to that observed for the first class, fibers in the second class displayed a normal pattern of expression of desmin, vimentin and titin. (iii) A familial heterogeneity was observed between the 4 patients. The pathological processes involved in the evolution of this syndrome are discussed.

Fibre-type composition, structure and cytoskeletal protein location of fibres in anterior tibial muscle. Comparison between young adults and physically active aged humans

Muscle biopsies were obtained from the anterior tibial muscle (TA) of 15 healthy, sedentary young (23-37 years) and 13 healthy and physically active elderly (66-77 years) volunteers. The mean frequency of type I fibres was lower in the young subjects compared with the elderly, but the mean type I fibre cross-sectional area was equal in the two groups. The type IIA fibres were, however, smaller in the elderly than in young subjects. Capillary density, capillary per fibre ratio, capillaries in contact with type I fibres (CC) and CC in relation to type I and type II fibre area did not differ in the two groups. The number of capillaries in contact with type IIA fibres was higher in the younger group. Only occasional and minor pathological changes were observed in the young subjects. In the elderly, such changes were much more common, including central nuclei, ring fibres, fibre splitting, scattered highly atrophic fibres, moth-eaten fibres and vacuoles. Ring fibres were most easily identified with anti-desmin labelling and highly atrophic fibres exhibited a rough network of labelling. Increased content of actin and spectrin was also observed at the periphery of ring fibres. In the elderly group, a qualitative ultrastructural analysis was also obtained and obvious changes included some myofilament loss, collections of lipofuscin which were also observed in satellite cells, proliferation of the SR-T systems and increased wrinkling of nuclear membranes and sarcolemma.

Intracellular elemental composition of single muscle fibres in muscular dystrophy and dystrophia myotonica

Seven patients with myotonic muscle dystrophy (MD), 6 with muscle dystrophy without myotonia (limb-girdle and facio-scapulo-humeral dystrophy) and 6 healthy volunteers were subjected to biopsy from the anterior tibial muscle (TA). Light microscopic examination of cryostat sections revealed pathological changes of different kind and degree--the occurrence of ring fibres and multiple central nuclei was the most consistent in advanced MD. X-ray microanalysis (XRMA) of single muscle fibres demonstrated the intracellular content of such elements as Na, Cl, K, Mg, S and P. The most conspicuous finding was the increase in Na and Cl and decrease in K demonstrated in MD. The highest levels of Na and Cl were found in ring fibres. Decrease in K was as high in structurally normal fibres of MD biopsies as in ring fibres and was also found in the muscle dystrophies without myotonia, but to a lesser degree than in MD. Thus, the decrease in K was not correlated to increase in Na and Cl. These changes in muscle fibre elemental content are discussed in relation to plasma membrane changes of erythrocytes in MD and especially the indications of disturbed anion channels.

Effects of excessive use of remaining muscle fibers in prior polio and LV lesion

Twenty-three subjects with weakness of musculus tibialis anterior (TA) due to prior poliomyelitis (n = 19) or prior LV lesion (n = 4) were examined regarding the use of remaining TA motor units during walking and the consequences of long-term overuse for the TA muscle fibers. Subjects with an excessive overuse exhibited almost only type 1 muscle fibers and with marked hypertrophy. Subjects who did not use remaining TA fibers because of too-severe paralysis had a normal fiber type differentiation and fiber atrophy. The use of remaining TA motor units was correlated to the type 1 muscle fiber percentage (r = 0.69) and to the type 1 fiber size (r = 0.78). Identical changes were observed in subjects with prior poliomyelitis and in subjects with prior LV lesion and were, thus, not specific for poliomyelitis. The changes in fiber type composition may be explained by a transition of type 2 to type 1 muscle fibers in accordance with previously reported experimental transneuronal stimulation through implanted electrodes in animals.