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

In vivo assessment of muscle membrane properties in myotonic dystrophy

INTRODUCTION

Myotonia in myotonic dystrophy types 1 (DM1) and 2 (DM2) is generally attributed to reduced chloride-channel conductance. We used muscle velocity recovery cycles (MVRCs) to investigate muscle membrane properties in DM1 and DM2, using comparisons with myotonia congenita (MC).

METHODS

MVRCs and responses to repetitive stimulation were compared between patients with DM1 (n = 18), DM2 (n = 5), MC (n = 18), and normal controls (n = 20).

RESULTS

Both DM1 and DM2 showed enhanced late supernormality after multiple conditioning stimuli, indicating delayed repolarization as in MC. Contrary to MC, however, DM1 showed reduced early supernormality after multiple conditioning stimuli, and weak DM1 patients also showed abnormally slow latency recovery after repetitive stimulation.

CONCLUSIONS

These findings support the presence of impaired chloride conductance in both DM1 and DM2. The early supernormality changes indicate that sodium currents were reduced in DM1, whereas the weakness-associated slow recovery after repetitive stimulation may provide an indication of reduced Na(+) /K(+) -ATPase activation. Muscle Nerve 54: 249-257, 2016.

Fluoxetine blocks myotonic runs and reverts abnormal surface electromyogram pattern in patients with myotonic dystrophy type 1

OBJECTIVES

To verify the effects of a muscular injection of fluoxetine both on needle electromyogram (EMG) "myotonic runs" and on the surface EMG pattern in patients affected by myotonic dystrophy type 1.

METHODS

Needle EMG recording: We performed needle EMG recordings on the tibialis anterior or opponent thumb muscle in 3 patients. The resting electrical activity and the myotonic discharge were detected before and after the local injection of 100 microL of fluoxetine. Surface EMG recording: A motor point stimulation protocol was carried out on the tibialis anterior of 3 patients. Stimulation consisted of 10-second, 15-Hz pulse train, 0.1 ms in duration. A supramaximal stimulation was applied, and the surface myoelectric signal was recorded. The averaged rectified value (ARV) of the amplitude was evaluated before and after the intramuscular injection of 300 microL of fluoxetine.

RESULTS

Needle EMG: The injection of fluoxetine induced a clear-cut reduction of the basal electrical activity and made it impossible to evoke "myotonic runs" in all the patients tested. The reversibility of the effect of the drug was checked in 2 patients who exhibited a partial recovery of myotonic EMG activity 40 minutes after the administration. Surface EMG: The patients showed the typical decreasing ARV pattern before the drug administration; the fluoxetine injection consistently provoked a clear and complete recovery of the normal increasing ARV curve.

CONCLUSIONS

We showed, for the first time, that the local application of fluoxetine produces functional modifications in myotonic dystrophy type 1 muscle electrical properties. The relevance of this study consists in the introduction of fluoxetine, a well-known and largely used drug, as a tool for investigating further therapeutical approaches in this disease.

Gene expression analysis in myotonic dystrophy: indications for a common molecular pathogenic pathway in DM1 and DM2

An RNA gain-of-function of expanded transcripts is the most accredited molecular mechanism for myotonic dystrophy type 1 (DM1) and 2 (DM2). To disclose molecular parallels and divergences in pathogenesis of both disorders, we compared the expression profile of muscle biopsies from DM1 and DM2 patients to controls. DM muscle tissues showed a reduction in the major skeletal muscle chloride channel (CLCN1) and transcription factor Sp1 transcript levels and an abnormal processing of the CLCN1 and insulin receptor (IR) pre-mRNAs. No essential differences were observed in the muscle blind-like gene (MBNL1) and CUG binding protein 1 (CUGBP1) transcript levels as well as in the splicing pattern of the myotubularin-related 1 (MTMR1) gene. Macroarray analysis of 96 neuroscience-related genes revealed a considerable similar expression profile between the DM samples, reflective of a common muscle pathology origin. Using a twofold threshold, we found six misregulated genes important in calcium and potassium metabolism and in mitochondrial functions. Our results indicate that the DM1 and DM2 overlapping clinical phenotypes may derive from a common trans acting mechanism that traps and influences shared genes and proteins. An RNA gain-of-function of expanded transcripts is the most accredited molecular mechanism for myotonic dystrophy type 1 (DM1) and 2 (DM2). To disclose molecular parallels and divergences in pathogenesis of both disorders, we compared the expression profile of muscle biopsies from DM1 and DM2 patients to controls. DM muscle tissues showed a reduction in the major skeletal muscle chloride channel (CLCN1) and transcription factor Sp1 transcript levels and an abnormal processing of the CLCN1 and insulin receptor (IR) pre-mRNAs. No essential differences were observed in the muscle blind-like gene (MBNL1) and CUG binding protein 1 (CUGBP1) transcript levels as well as in the splicing pattern of the myotubularin-related 1 (MTMR1) gene. Macroarray analysis of 96 neuroscience-related genes revealed a considerable similar expression profile between the DM samples, reflective of a common muscle pathology origin. Using a twofold threshold, we found six misregulated genes important in calcium and potassium metabolism and in mitochondrial functions. Our results indicate that the DM1 and DM2 overlapping clinical phenotypes may derive from a common trans acting mechanism that traps and influences shared genes and proteins.

Multilevel somatosensory system disinhibition in children with migraine

Although migraine is characterised by an abnormal cortical excitability level, whether the central nervous system is hyper- or hypo-excitable in migraine still remains an unsolved problem. The aim of our study was to compare the somatosensory evoked potential (SEP) recovery cycle, a marker of the somatosensory system's excitability, in a group of 15 children suffering from migraine without aura (MO) (mean age 11.7+/-1.6 years, five males, 10 females) and 10 control age-matched subjects (CS) (mean age 10.9+/-2.1 years, six males, four females). We calculated the SEP's latency and amplitude modifications after paired electrical stimuli at 5, 20 and 40 ms interstimulus intervals (ISIs), comparing it with a single stimulus condition assumed as the baseline. In MO patients, the amplitudes of the cervical N13 and of the cortical N20, P24 and N30 responses at 20 and 40 ms ISIs showed a higher recovery than in CS (two-way ANOVA, P<0.05). Since, the SEP recovery cycle depends on the inhibitory interneuron function, our findings suggest that a somatosensory system disinhibition takes place in migraine. This is a generalized phenomenon, not limited to the cerebral cortex, but concerning also the cervical grey matter. The SEP recovery cycle reflects the intracellular concentration of Na(+), therefore, the shortened recovery cycle in our MO patients suggests a high level of intracellular Na(+) and a consequent depolarized resting membrane potential, possibly due to an impaired Na(+) -K(+) ATPase function in migraine.

Myotonic dystrophy protein kinase phosphorylates phospholamban and regulates calcium uptake in cardiomyocyte sarcoplasmic reticulum

Myotonic dystrophy (DM) is caused by a CTG expansion in the 3'-untranslated region of a protein kinase gene (DMPK). Cardiovascular disease is one of the most prevalent causes of death in DM patients. Electrophysiological studies in cardiac muscles from DM patients and from DMPK(-/-) mice suggested that DMPK is critical to the modulation of cardiac contractility and to the maintenance of proper cardiac conduction activity. However, there are no data regarding the molecular signaling pathways involved in DM heart failure. Here we show that DMPK expression in cardiac myocytes is highly enriched in the sarcoplasmic reticulum (SR) where it colocalizes with the ryanodine receptor and phospholamban (PLN), a muscle-specific SR Ca(2+)-ATPase (SERCA2a) inhibitor. Coimmunoprecipitation studies showed that DMPK and PLN can physically associate. Furthermore, purified wild-type DMPK, but not a kinase-deficient mutant (K110A DMPK), phosphorylates PLN in vitro. Subsequent studies using the DMPK(-/-) mice demonstrated that PLN is hypo-phosphorylated in SR vesicles from DMPK(-/-) mice compared with wild-type mice both in vitro and in vivo. Finally, we show that Ca(2+) uptake in SR is impaired in ventricular homogenates from DMPK(-/-) mice. Together, our data suggest the existence of a novel regulatory DMPK pathway for cardiac contractility and provide a molecular mechanism for DM heart pathology.

Na+-K+ Pump Regulation and Skeletal Muscle Contractility

Clausen, Torben. Na+-K+ Pump Regulation and Skeletal Muscle Contractility. Physiol Rev 83: 1269-1324, 2003; 10.1152/physrev.00011.2003.—In skeletal muscle, excitation may cause loss of K+, increased extracellular K+ ([K+]o), intracellular Na+ ([Na+]i), and depolarization. Since these events interfere with excitability, the processes of excitation can be self-limiting. During work, therefore, the impending loss of excitability has to be counterbalanced by prompt restoration of Na+-K+ gradients. Since this is the major function of the Na+-K+ pumps, it is crucial that their activity and capacity are adequate. This is achieved in two ways: 1) by acute activation of the Na+-K+ pumps and 2) by long-term regulation of Na+-K+ pump content or capacity. 1) Depending on frequency of stimulation, excitation may activate up to all of the Na+-K+ pumps available within 10 s, causing up to 22-fold increase in Na+ efflux. Activation of the Na+-K+ pumps by hormones is slower and less pronounced. When muscles are inhibited by high [K+]o or low [Na+]o, acute hormone- or excitation-induced activation of the Na+-K+ pumps can restore excitability and contractile force in 10-20 min. Conversely, inhibition of the Na+-K+ pumps by ouabain leads to progressive loss of contractility and endurance. 2) Na+-K+ pump content is upregulated by training, thyroid hormones, insulin, glucocorticoids, and K+ overload. Downregulation is seen during immobilization, K+ deficiency, hypoxia, heart failure, hypothyroidism, starvation, diabetes, alcoholism, myotonic dystrophy, and McArdle disease. Reduced Na+-K+ pump content leads to loss of contractility and endurance, possibly contributing to the fatigue associated with several of these conditions. Increasing excitation-induced Na+ influx by augmenting the open-time or the content of Na+ channels reduces contractile endurance. Excitability and contractility depend on the ratio between passive Na+-K+ leaks and Na+-K+ pump activity, the passive leaks often playing a dominant role. The Na+-K+ pump is a central target for regulation of Na+-K+ distribution and excitability, essential for second-to-second ongoing maintenance of excitability during work.

Somatosensory evoked potential recovery in myotonic dystrophy

OBJECTIVE

To evaluate recovery functions of the sensory cortex using somatosensory evoked potentials (SEPs) elicited by paired stimuli of the median nerve in patients with myotonic dystrophy (MD).

SUBJECTS/METHODS

Twelve MD patients were enrolled in the present investigation. Five patients with facioscapulohumeral muscular dystrophy (FSH) and 12 healthy volunteers were studied as control groups. SEP was recorded from the hand sensory area contralateral to the median nerve stimulated at the wrist. Single pulse or paired-pulse stimuli at various interstimulus intervals (ISIs) (10, 20, 40, 60, 80, 100, 150, 200 and 300 ms) were given. Recovery functions of N9, N20onset-N20peak, N20-P25 and P25-N33 components were studied.

RESULTS

Conventional SEPs to a single stimulus were normal in the latency and amplitude in all the patients. Recovery functions of both N9 and N20o-N20p components were normal in the patients. In contrast, in MD patients, disinhibited or hyperexcitable recovery pattern was observed in recovery curves of the N20-P25 or P25-N33 components, whereas those were normal in FSH patients.

CONCLUSIONS

Disinhibited cortical excitability (or hyperexcitability) is present in the sensory cortex in patients with myotonic dystrophy. This may reflect cortical pathology or functional alteration of the sensory cortex in MD.

Fragile-X syndrome and myotonic dystrophy: parallels and paradoxes

Fragile-X syndrome and myotonic dystrophy are caused by triplet repeat expansions embedded in CpG islands in the transcribed non-coding regions of the FMR1 and the DMPK genes, respectively. Although initial reports emphasized differences in the mechanisms by which the expanded triplet repeats caused these diseases, results published in the past year highlight remarkable parallels in the likely molecular etiologies. At both loci, expansion is associated with altered chromatin, aberrant methylation, and suppressed expression of the adjacent FMR1 and DMAHP genes, implicating epigenetic mediation of these genetic diseases.

Myotonic dystrophy protein kinase is involved in the modulation of the Ca2+ homeostasis in skeletal muscle cells

Myotonic dystrophy (DM), the most prevalent muscular disorder in adults, is caused by (CTG)n-repeat expansion in a gene encoding a protein kinase (DM protein kinase; DMPK) and involves changes in cytoarchitecture and ion homeostasis. To obtain clues to the normal biological role of DMPK in cellular ion homeostasis, we have compared the resting [Ca2+]i, the amplitude and shape of depolarization-induced Ca2+ transients, and the content of ATP-driven ion pumps in cultured skeletal muscle cells of wild-type and DMPK[-/-] knockout mice. In vitro-differentiated DMPK[-/-] myotubes exhibit a higher resting [Ca2+]i than do wild-type myotubes because of an altered open probability of voltage-dependent l-type Ca2+ and Na+ channels. The mutant myotubes exhibit smaller and slower Ca2+ responses upon triggering by acetylcholine or high external K+. In addition, we observed that these Ca2+ transients partially result from an influx of extracellular Ca2+ through the l-type Ca2+ channel. Neither the content nor the activity of Na+/K+ ATPase and sarcoplasmic reticulum Ca2+-ATPase are affected by DMPK absence. In conclusion, our data suggest that DMPK is involved in modulating the initial events of excitation-contraction coupling in skeletal muscle.

Use of post mortem and in vitro tissue specimens for X-ray microanalysis

Post mortem changes in the distribution of elements, as well as changes induced by dissection and incubation of tissue slices, were investigated by X-ray microanalysis of brain tissue, liver, pancreas and submandibular gland. Dissection itself causes minor changes in the intracellular ionic concentrations, but even a brief exposure of dissected tissue slices to a physiological buffer causes an increase in intracellular Na and Cl and a decrease in intracellular K concentration. The effect is most marked in brain tissue and least marked in submandibular gland slices. Incubation in fluid resembling the extracellular compartment in its ion composition results in a further increase of Na and Cl in brain tissue (cortex and hippocampus) and liver; in pancreas and submandibular gland, on the other hand, a stable situation throughout 2 h of incubation can be obtained. Incubation at lower temperature, and exchanging NaCl in the incubation medium for Na gluconate, has only relatively minor effects on the intracellular K/Na ratio. Exchanging the NaCl for K gluconate results in a high intracellular K/Na ratio throughout the incubation, but morphological evidence of tissue oedema was nevertheless observed. Physiological changes in intracellular ion content induced by cholinergic stimulation are similar in in vitro preparations as compared with stimulation in situ. The effect of dissection and brief incubation on the ionic composition of brain tissue is less pronounced 6 h after death than in a living, anaesthetized animal, but this is largely due to the post mortem changes that already have taken place.

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.

Skeletal muscle bioenergetics in myotonic dystrophy

Skeletal muscle function of 15 patients with myotonic dystrophy (dystrophia myotonica, DM) was investigated using 31P magnetic resonance spectroscopy to evaluate bioenergetics and intracellular pH at rest and during exercise and recovery. Results from DM patients, normal controls and mitochondrial myopathy patients were compared in order to assess the possible contribution of abnormal mitochondrial metabolism to muscle dysfunction in DM. In resting DM muscle, intracellular pH (pHi) was normal, but there were significant elevations in the concentration ratios of Pi/ATP, phosphomonoesters/ATP and phosphodiesters/ATP. In patients with the most severe exercise intolerance the phosphocreatine/ATP ratio was also reduced. Resting muscle of 11 mitochondrial myopathy patients showed similar changes to those of the most exercise-intolerant DM patients. In exercising DM muscle, energy stores were rapidly depleted as in mitochondrial myopathy. Muscle acidified in all subjects, but in DM the decrease in pHi was less than in normal muscle. Recovery half-times for phosphocreatine, Pi and ADP were normal in DM but slow in mitochondrial myopathy. The initial rate of phosphocreatine repletion after exercise was rapid in DM, consistent with high [ADP], but slow in mitochondrial myopathy in spite of elevated [ADP]. Because recovery is an oxidative process, we conclude that there was no decrease in the oxidative capacity of the muscles in this group of DM patients. In the subjects in whom it could be measured, the rate of recovery of intracellular pH was greater in the 3 DM patients (0.14, 0.15 and 0.16 U/min) than in the 7 normal controls (0.08-0.12 U/min, mean 0.10). The results do not rule out a minor abnormality in glycogenolysis, but they suggest that the failure to acidify normally during exercise is probably due to rapid proton efflux.