Duchenne dystrophy viewed as a disturbance of nerve-muscle interactions

Electrical Properties of Adult Mammalian Motoneurons

Motoneurons are the 'final common path' between the central nervous system (that intends, selects, commands, and organises movement) and muscles (that produce the behaviour). Motoneurons are not passive relays, but rather integrate synaptic activity to appropriately tune output (spike trains) and therefore the production of muscle force. In this chapter, we focus on studies of mammalian motoneurons, describing their heterogeneity whilst providing a brief historical account of motoneuron recording techniques. Next, we describe adult motoneurons in terms of their passive, transition, and active (repetitive firing) properties. We then discuss modulation of these properties by somatic (C-boutons) and dendritic (persistent inward currents) mechanisms. Finally, we briefly describe select studies of human motor unit physiology and relate them to findings from animal preparations discussed earlier in the chapter. This interphyletic approach to the study of motoneuron physiology is crucial to progress understanding of how these diverse neurons translate intention into behaviour.

Influence of Botulinumtoxin A on the Expression of Adult MyHC Isoforms in the Masticatory Muscles in Dystrophin-Deficient Mice (Mdx-Mice)

The most widespread animal model to investigate Duchenne muscular dystrophy is the mdx-mouse. In contrast to humans, phases of muscle degeneration are replaced by regeneration processes; hence there is only a restricted time slot for research. The aim of the study was to investigate if an intramuscular injection of BTX-A is able to break down muscle regeneration and has direct implications on the gene expression of myosin heavy chains in the corresponding treated and untreated muscles. Therefore, paralysis of the right masseter muscle was induced in adult healthy and dystrophic mice by a specific intramuscular injection of BTX-A. After 21 days the mRNA expression and protein content of MyHC isoforms of the right and left masseter, temporal, and the tongue muscle were determined using quantitative RT-PCR and Western blot technique. MyHC-IIa and MyHC-I-mRNA expression significantly increased in the paralyzed masseter muscle of control-mice, whereas MyHC-IIb and MyHC-IIx/d-mRNA were decreased. In dystrophic muscles no effect of BTX-A could be detected at the level of MyHC. This study suggests that BTX-A injection is a suitable method to simulate DMD-pathogenesis in healthy mice but further investigations are necessary to fully analyse the BTX-A effect and to generate sustained muscular atrophy in mdx-mice.

Oxidative stress regulation of stem and progenitor cells

In recent years, it has become clear that balanced regulation of reactive oxygen species is of critical significance for cell-fate determination as well as for stem cell development, function, and survival. Although many questions regarding intracellular redox status regulation of stem cell fate remain, we review here what is known regarding the impact of cell-fate signaling as shown with a variety of human cancer cells and more recently on cancer-initiating cells and on the regenerative capacity of skeletal muscle and hematopoietic tissue and their stem cells. We also discuss the role of altered intracellular redox status as a potential primary pathogenic mechanism in muscular dystrophy and hematopoietic pathologies. Studies discussed here illustrate how understanding altered redox regulation of stem cell behavior may contribute to the development of novel stem cell therapies.

Oxidative stress and the pathogenesis of muscular dystrophies

The muscular dystrophies represent a diverse group of diseases differing in underlying genetic basis, age of onset, mode of inheritance, and severity of progression, but they share certain common pathologic features. Most prominent among these features is the necrotic degeneration of muscle fibers. Although the genetic basis of many of the dystrophies has been known for over a decade and new disease genes continue to be discovered, the pathogenetic mechanisms leading to muscle cell death in the dystrophies remain a mystery. This review focuses on the oxidative stress theory, which states that the final common pathway of muscle cell death in these diseases involves oxidative damage.

Expression of nitric oxide synthase in the spinal cord in C57BL/6J mice with congenital muscular dystrophy

Autoradiography with the nitric oxide synthase (NOS) inhibitor ((3)H)nitro-L-arginine ([(3)H]L-NNA) was used to quantify NOS in cervical and lumbar spinal cord in normal and dystrophic mice. A single homogeneous population of binding sites was seen in all subregions of the gray matter in normal mice and in the superficial dorsal horn in dystrophic mice. However, in dystrophic mice, two populations were revealed in the deeper dorsal, intermediate, and ventral subregions. Pronounced immunoreactivity for neuronal NOS (nNOS), and weak immunoreactivity for endothelial NOS (eNOS), were revealed in all subregions in normal and dystrophic mice. Inducible NOS (iNOS) immunoreactivity was negligible in normal mice but intense in the deeper dorsal, intermediate, and ventral subregions in dystrophic mice. The higher affinity ((3)H)L-NNA binding site colocalized with nNOS and the lower affinity site with iNOS. It is suggested that expression of iNOS is associated with the pathological changes occurring in congenital muscular dystrophy.

Are motoneurons involved in muscular dystrophy?

In the early seventies, a suggestion that even in muscular dystrophy a neurogenic factor may be involved, was formulated. The argument which followed this suggestion, resulted in eventual abandoning of this concept even by its author. This discussion however has never been supported by any systematic study of motoneuron activity in muscular dystrophy. We examined an activity of motoneurons supplying brachial biceps in eight controls and 26 patients affected by Duchenne muscular dystrophy by studying single motor unit (MU) potentials picked up by fine wire bipolar electrodes. In the majority of patients, MU firing rates were higher as compared to controls and increased more rapidly with increasing force level. The relationship between standard deviation of interspike intervals and their mean value, SD(x), was shifted towards the shorter intervals and lower SDs. The numerical values describing these changes were correlated with severity of the disease. There is evidence that the break-point of the function SD(x) is correlated with motoneuron properties, in particular with after-hyperpolarization duration. In muscular dystrophy, this break-point corresponds to the shorter interspike intervals. We suggest that the motoneurons in muscular dystrophy are altered either in response to the muscle degeneration, or as a result of the disease itself.

Motoneurons are altered in muscular dystrophy

The activity of motoneurons supplying the brachial biceps muscle was examined in eight control subjects and 26 patients affected by Duchenne muscular dystrophy. The patients were subdivided into two groups: one whose motor units (MU) fired with normal rates (N group) and the other whose MU firing rates were higher as compared to controls (I group). Firing rates of motoneurons of patients from group I increased more rapidly with increasing force level. The relationship between the standard deviation of interspike intervals and their mean value, sigma(Tm), was shifted towards the shorter intervals and lower standard deviations in both groups of patients. The numerical values describing these changes correlated with the severity of disease. The MU recruitment was comparable for control subjects and for patients. Experimental results as well as computer simulations indicate that the break-point of the function sigma(Tm) is correlated with motoneuronal properties, and in particular with the afterhyperpolarization (AHP) duration. In muscular dystrophy this break-point corresponds to the shorter interspike intervals. Therefore, we propose that the motoneurons in muscular dystrophy are altered either in response to the muscle degeneration or as a result of the disease itself.

Modification of the dystrophic phenotype after transient neonatal denervation: role of MHC isoforms

While it recently has been demonstrated that it is possible to modify the phenotypic expression of murine dystrophy (dy/dy) (i.e., prevent myofiber loss) by subjecting the extensor digitorum longus (EDL) muscle of 14-day-old dy/dy mice to transient neonatal denervation (Moschella and Ontell, 1987), the mechanism responsible for this phenomenon has not been determined. Since it has been suggested that the effects of dystrophy vary according to fiber type, the fiber type frequency in 100-day-old normal (+/+) and dy/dy EDL muscles subjected to transient neonatal denervation has been determined by immunohistochemical analysis of their myosin heavy chain (MHC) composition. This frequency has been compared with that found in the EDL muscles of 14- and 100-day-old unoperated +/+ and dy/dy mice, in order to determine whether the reinnervation of transiently denervated neonatal muscle results in a preponderance of fibers of the type that might be spared dystrophic deterioration. In unoperated dy/dy muscle there is a progressive decrease in the frequency and in the absolute number of fibers that express MHC2B, with 100-day-old dy/dy muscles having approximately 32% of the number of myofibers fibers containing MHC2B as is found in age-matched +/+ muscles. The number of fibers containing the other fast isoforms (MHC2A and MHC2X) is similar in +/+ and dy/dy muscles at this age, indicating that fibers with MHC2B are most affected by the dystrophic process. Reinnervation following transient neonatal denervation of both the +/+ and the dy/dy EDL muscles results in a similar decrease (approximately 62%) in the number of myofibers containing MHC2B and an increase in myofibers containing the other fast MHC isoforms (MHC2A and MHC2X). The selective effect of dy/dy on fibers containing MHC2B and the sparing of myofibers in transiently denervated dy/dy muscle (which contains a reduced frequency of fibers containing MHC2B) are consistent with, although not direct proof of, the hypothesis that alterations in the fiber type may play a role in the failure of myofibers in transiently denervated dy/dy muscles to undergo dystrophic deterioration. Evidence is presented suggesting that neurons that supply myofibers containing MHC2B may be at a selective disadvantage in their ability to reinnervate neonatally denervated muscles.

Expression of myosin heavy chain isoforms in Duchenne muscular dystrophy patients and carriers

The expression of MHC isoforms in the skeletal muscles of nine patients with Duchenne muscular dystrophy (DMD) (from 2.5 to 15 yr of age) and three DMD carriers was studied using different specific anti-MHC MAbs. We also analyzed muscle fiber size and fiber reactivity with acridine orange and/or with a surface antigen marker. One-quarter of all fibers of DMD patients, or less with age, were of normal size and contained only adult slow MHC. Half of the muscle fibers contained adult and developmental MHCs. Only half of these fibers were representative of an active regenerative process. MHC co-expression also altered the proportion of normal fast or slow fibers. Adult fast MHCs were expressed as unique MHC only in small and very small fibers in the oldest DMD patients. In DMD carrier muscles, the greatest alterations in MHC expression were observed in patients with the most reduced dystrophin expression. However, MHC changes in dystrophin-positive fibers were similar to those observed in dystrophin-free fibers. In conclusion, disruptions or delays in the switching of all genes coding for adult fast and slow MHC and developmental MHC coincided with dystrophin deletion and with perturbations in its expression.

Opioid receptors in skeletal muscle of normal and dystrophic mice

Autoradiography was used to investigate the presence of opioid receptors in soleus, extensor digitorum longus (EDL) and diaphragm muscles in two strains (C57BL/6J and C57BL/10) of mice which can inherit muscular dystrophy. Binding sites for two radiolabelled opioid ligands [125I]beta-endorphin and [3H]naloxone, were seen in a few muscle fibres in the normal mice. However, a significantly greater number of fibres exhibited the binding sites in the dystrophic individuals of both strains. The binding sites were not restricted to the endplate regions but were present over the entire surface in these fibres.

Therapeutic possibilities of chronic low frequency electrical stimulation in children with Duchenne muscular dystrophy

To evaluate the therapeutic possibilities of chronic electrical stimulation, muscle function studies and quantitative tests of physical assessment were used to monitor the response of quadriceps femoris to prolonged low frequency stimulation. Comparative studies of the maximum voluntary and electrically elicited responses of muscles of young ambulant children with Duchenne muscular dystrophy, when compared to those of normal children's muscles, revealed lower values of maximum voluntary contraction, significant slowing (P less than 0.001) of mean relaxation times and a higher resistance to fatigue testing. Intermittent chronic low frequency stimulation resulted in a significant (P less than 0.01) increase in mean maximum voluntary contraction of the stimulated muscles compared with the mean force exerted by the unstimulated control muscles. There are clear therapeutic possibilities for the use of chronic low frequency stimulation in these children.

Effects of tenotomy on muscle histology and energy metabolism in normal and dystrophic mice

Creatine, phosphocreatine, ATP and phosphate concentrations were measured in extracts of tibialis anterior and gastrocnemius muscles from 42 normal and 39 dystrophic mice from 1 to 16 weeks of age. No differences were observed at 1 week, prior to the onset of histological abnormalities in dystrophic animals. Creatine and phosphocreatine concentrations were significantly reduced in older dystrophic mice, and phosphate levels were higher, while ATP levels generally did not differ. Tenotomy of gastrocnemius at 1 week prevented the development of dystrophy in this muscle but this was not associated with an increase in phosphogen concentrations. Serum creatine kinase levels were significantly higher in dystrophic mice than normal mice but only during the first two weeks of life; levels in older mice were not significantly different. This study shows that the reported deficits in phosphogen concentrations in dystrophic muscles are likely to reflect the results, rather than the cause, of the dystrophic process.

Serum cholinesterase activity in infantile and juvenile spinal muscular atrophy

Serum acetylcholinesterase (AChE) and pseudocholinesterase (ChE) activity in infantile and juvenile spinal muscular atrophy (SMA) was determined. The total AChE activity was either normal or decreased in the childhood SMA (Type 1), the other SMA groups and disease controls (ALS, X-linked SMA). In the majority of SMA Type 1 cases (6/7 tested) an absence of the asymmetric A12 form was found. This was accompanied by changes in the other asymmetric and globular forms. The latter was, however, not specific for SMA Type 1 cases. The ChE activity was increased in the majority of SMA cases as well as disease controls. The asymmetric A12 ChE form was increased in all SMA Type 3 cases, the values of this form in SMA Type 1 was variable. A change in the ChE globular forms in SMA Type 1 and SMA Type 2 was a frequent finding. It is suggested that the absence of the asymmetric A12 AChE form in SMA Type 1 arises because of muscle cell immaturity and undeveloped muscle-nerve interactions. The reason of ChE changes is obscure.

Accumulation of collagen and altered fiber-type ratios as indicators of abnormal muscle gene expression in the mdx dystrophic mouse

The growth and development of the X-linked muscular dystrophy mutant mouse (mdx) was compared with a control group from 3 weeks to 1 year old. Quantitative cytological analysis of the soleus muscle revealed cycles of degeneration, regeneration, and hypertrophy, and at any one time it was difficult to assess the extent of the disease based on muscle fiber size. One noticeable difference even in the youngest muscles studied was the reduced numbers of slow oxidative fibers and the increased number of fast glycolytic fibers in the mdx soleus muscles. The collagen of the connective tissue components of selectively stained sections was determined by computerized image analysis. Marked accumulation of collagen was found in both the endomysium and perimysium of the dystrophic muscles as compared with age-matched controls. Since the mdx mouse is a result of the same type of genetic defect as in human Duchenne muscular dystrophy, this model could thus be used to assess the effectiveness of various therapeutic approaches, including gene therapy using muscle fibrosis and fiber type proportions as the indicators.

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.

Changes in center of gravity in boys with Duchenne muscular dystrophy

A study was undertaken, using methods of stabilometry to compare stability of stance in normal children (n = 37) and those with Duchenne muscular dystrophy (n = 61). The purpose of this study was to monitor changes in the locus of the center of gravity and the range and frequency of sway and to evaluate the effect of orthotic application in an attempt to obtain information that would assist further development of orthoses. In group 1, boys with Duchenne muscular dystrophy who were still walking without assistance (mean age 7.2 +/- 1.76 years), the analysis of sway showed that, between 5 and 6 years of age, the boys already had ranges of anteroposterior (A/P) and lateral (Lat) sway that were significantly greater than those found in normal children (A/P P less than 0.05, Lat P less than 0.01). In group 2, boys with Duchenne muscular dystrophy when orthoses had been introduced (n = 23, mean age 10.4 +/- 1.47 years), the center of gravity was returned to a more normal position. There was a reduction of the anteroposterior range of sway, but the lateral range of sway remained significantly greater (P less than 0.01) as did the frequency of sway in both the anteroposterior and lateral directions (A/P P less than 0.001, Lat P less than 0.001).

Effect of nerve section on beta-endorphin and alpha-melanotropin immunoreactivity in motor nerves of normal and dystrophic mice

beta-Endorphin and alpha-melanotropin immunoreactivity was demonstrated in some motor nerves in histological sections of soleus and extensor digitorum longus muscles of the mouse. In two strains (C57BL/6J and 129/ReJ) which can inherit muscular dystrophy, the proportion of immunoreactive nerves was greater in the dystrophic individuals than in their healthy littermates. At 24 h after section of the sciatic nerve in the normal mice the proportion of immunoreactive nerve profiles had increased significantly in the denervated muscles even though most of the nerve axons had degenerated. Similar increases were also seen in nerves in the unoperated contralateral muscles.

Comparison of force and stiffness in normal and dystrophic mouse muscles

Isometric force and stiffness of fast- and slow-twitch muscles of affected and normal mice of the 129/ReJ dy/dy strain were studied at rest and during active contraction at a variety of lengths. Dystrophic muscles developed less force in response to stimulation, but the resting stiffness was not reduced as much, particularly at long muscle lengths. This is consistent with the replacement of muscle fibers by connective tissue that is considerably less elastic. When second and third stimuli are superimposed on the rising phase of a twitch in a normal muscle, a less-than-linear summation of force and stiffness generation (early depression) is followed by a more-than-linear summation (later facilitation). Dystrophic muscles showed a smaller early depression and a greater later facilitation of force and active muscle stiffness. Many of these phenomena can be predicted from a simple model of Ca2+ release and binding to troponin.

Postnatal development of Ca2+-sequestration by the sarcoplasmic reticulum of fast and slow muscles in normal and dystrophic mice

Ca2+-uptake activities of the sarcoplasmic reticulum (SR) were determined with a Ca2+-sensitive electrode in homogenates from fast- and slow-twitch muscles from both normal and dystrophic mice (C57BL/6J strain) of different ages. Immunochemical quantification of tissue Ca2+-ATPase content allowed determination of the specific Ca2+-transport activity of the enzyme. In 3-week-old mice of the dystrophic strain specific Ca2+ transport was already significantly lower than in the normal strain. It progressively decreased with maturation and reached only 40-50% and 30-50% of the normal values in fast- and slow-twitch muscles of adult dystrophic animals, respectively. Tissue contents of calsequestrin were reduced in both types of muscle leading to an increased Ca2+-ATPase to calsequestrin protein ratio. Equal amounts of the Ca2+-ATPase protein (detected by Coomassie blue staining of polyacrylamide gels) were present in SR vesicles isolated by Ca2+-oxalate loading from adult normal and dystrophic fast-twitch muscles. However, the specific ATP-hydrolysing activity of the enzyme was approximately 50% lower in dystrophic than in normal SR. The reduced ATP-hydrolysing activity was correlated with decreased Ca2+-transport activity, phosphoprotein formation and fluorescein isothiocyanate labeling as determined in total microsomal and heavy SR fractions. Although the Ca2+ and ATP affinities of the enzyme were unaltered, its ATPase activity was reduced at all levels of ATP in the dystrophic SR. Taken together, these findings point to a markedly impaired function of the SR and an increase in the population of inactive SR Ca2+-ATPase molecules in murine muscular dystrophy.

Comparison of fiber size and phenotypic gene expression in muscles of dystrophic C57BL/6J DY2J/DY2J mice

Considerable evidence has shown a correlation between fiber size and degree of necrosis in dystrophic muscles of hamster, X-linked muscular dystrophy (MDX) mice and humans. It has been proposed that small-caliber fibers have an immunity to the phenotypic expression of the dystrophic gene(s). The results from the present study show a discordance between fiber size and necrosis in dystrophic muscles of C57BL/6J dy2J/dy2J mice. Extensor carpi radialis longus and brevis muscles (ECRL and ECRB respectively) were compared in normal and dystrophic 2-week, 4-week and 12-month animals by measuring the mean cross-sectional area of type II fibers, determination of relative proportions of types IIA and IIB fibers and calculation of percentage of fibers exhibiting centronucleation in an entire cross-section of muscle (stained for haematoxylin and eosin or ATPase). The ECRL and ECRB muscles were found to have identical sizes of fiber at each of the 3 ages studied and similar proportions of fiber types, yet the former muscle developed and retained significantly more necrosis (manifest as centronucleation) than the latter.

Fast muscle fibers are preferentially affected in Duchenne muscular dystrophy

We show that Duchenne muscular dystrophy (DMD) selectively affects a subset of skeletal muscle fibers specialized for fast contraction. Muscle fiber types were characterized immunohistochemically with monoclonal antibodies that distinguish isoforms of fetal and adult-fast or adult-slow myosin heavy chain present in the same fiber. Fetal myosin expression increased with patient age and was not due to arrested development but rather to de novo synthesis, which served as a sensitive indicator of muscle regeneration. A subset of fast fibers were the first to degenerate (type IIb). Extensive fast fiber regeneration occurred before slow fibers were affected. These results suggest that the DMD gene product has a specific function in a subpopulation of muscle fibers specialized to respond to the highest frequency of neuronal stimulation with maximal rates of contraction.

Immaturity of muscle fibers in the congenital form of myotonic dystrophy: its consequences and its origin

Skeletal muscle maturation is impaired in children with congenital myotonic dystrophy. This immaturity is characterized at the light microscopy level by an abnormal presence of myotubes, small fascicles of muscle fibers, thin myofibers, and delayed muscle fiber type differentiation with a peripheral halo lacking mitochondrial oxidative enzyme activity. At an ultrastructural level, the characteristics are a paucity of myofibrils with a peripheral rim devoid of mitochondria and myofibrils in the fibers. In time the muscle is able to gain a certain degree of maturity as shown in one of our cases who had two successive muscle biopsies. The muscle, however, never becomes normal but retains discrepancies in fiber size and fiber type distribution and shows some fiber necrosis. Maturation of the motoneurons is normal, which may explain necrosis of immature muscle fibers. In an experimental study carried out to look for evidence of a circulatory factor in mothers of children with congenital myotonic dystrophy, it was found that sera from these mothers administered intra-peritoneally to newborn rats does in fact impair muscle maturation, whereas rats injected similarly with sera from control women showed normal muscle maturation.

Effect of denervation on adenine nucleotides in skeletal muscle from normal and dystrophic mice

The effect of denervation on the adenine nucleotide content of fast- and slow-twitch skeletal muscle of the C57BL mouse was studied by high-performance liquid chromatography. From the adenine nucleotide content the energy charge, a measure of high-energy phosphate available to the cell, was calculated. The energy charge of the extensor digitorum longus muscle was significantly higher than that of the same muscle from dystrophic mice (C57BL/6J dy2j/dy2j) and on denervation decreased to the values found in the innervated muscle from dystrophic animals. Denervation of the muscle in dystrophic mice did not change the energy charge of that muscle. The energy charge of the soleus muscle from both normal and dystrophic mice was similar and did not change on denervation. It is proposed that in the dystrophic process a functional denervation of skeletal muscle occurs which preferentially affects fast-twitch muscle, leading to a reduction in the energy charge.

Sciatic nerve protein composition in normal and dystrophic C57BL/6J mice

Proteins were extracted from homogenized sciatic nerves of normal C57BL/6J and dystrophic C57BL/6J dy2J/dy2J mice and were separated on SDS-polyacrylamide slab gel electrophoresis. Proteins visualized on Coomassie blue-stained gels were resolved into 43 bands. These were quantitated by densitometric scanning and continuous plotting of OD 595, then heights and areas of individual peaks were measured. The relative proportions of 5 proteins of intermediate molecular weight (37-92 kDa) were greater in dystrophic than normal nerves. These augmented proteins were not related to myelin or nuclear histone proteins.

Developmental changes in succinate dehydrogenase activity in muscle fibers from normal and dystrophic mice

The question of whether or not the development of dystrophic muscles is similar to that of normal muscles, prior to the manifestations of the symptoms of the disease, is investigated here. The developmental change in the activity of succinate dehydrogenase was therefore measured in individual fibers of prospectively dystrophic muscles from 10- to 28-day-old mice (strain C57Bl/6J dy2j) and compared with that of muscles from normal mice of the same age. It was found that up to 10 days of age, muscle fibers from normal and prospective dystrophic animals had low succinate dehydrogenase activities, and were all more or less uniform. Thereafter in the normal muscle the overall activity of the enzyme increased and the fibers became more heterogeneous with age. By 21 days the extensor digitorum longus muscle resembled that of the adult. At that time, fibers from prospectively dystrophic muscles had lower succinate dehydrogenase activities and were more homogeneous. Thus fibers from prospectively dystrophic muscles fail to achieve their adult characteristics by 21 days. On the basis of these results, it is suggested that muscle maturation is retarded in dystrophic animals.

Practical analysis of variability of muscle function measurements in Duchenne muscular dystrophy

To determine the possible sources of variation in performance indicators used in therapeutic trials, electrical stimulation techniques were used to measure contractile properties of the adductor pollicis and quadriceps muscles in boys with Duchenne muscular dystrophy. As no therapeutic effects were observed, longitudinal data obtained are taken to indicate changes in disease progress. Variance in voluntary contractions was found to be similar to that with electrically stimulated contractions; thus, variation could not be attributed to motivational changes, but rather to physiologic changes. Dystrophic muscle was slower to relax and less fatiguable than normal. However, such changes are of less significance to the overall disability compared to the loss of muscle bulk (cross-sectional area). Important variations in the function of individual muscles essential to complex performance, such as walking or getting up from the floor, could be masked by combining results from several muscle groups.

Responses of muscles of patients with Duchenne muscular dystrophy to chronic electrical stimulation

The effect of chronic low frequency stimulation on the tibialis anterior muscle of children with Duchenne muscular dystrophy was investigated. Baseline data from 16 boys established low values of maximum voluntary contraction which did not improve with age. Studies of the contractile properties revealed significant slowing (p less than 0.001) of mean relaxation time compared to that of normal children's muscles. There was no loss of force during fatigue testing, as in normal children, but in contrast to normal children, there was no potentiation at lower frequencies of stimulation. Intermittent chronic low frequency stimulation of muscles in six young ambulant children with Duchenne muscular dystrophy resulted in a significant increase (p less than 0.05) in mean maximum voluntary contraction compared with the mean forces exerted by the unstimulated control muscles of the contralateral leg.

Duchenne dystrophic muscle develops lesions in long-term coculture with mouse spinal cord

When strips of human skeletal muscle from biopsies of normal children and donors with Duchenne muscular dystrophy (DMD) are explanted in organotypic coculture with fetal mouse spinal cord, many regenerating muscle fibers develop, become innervated, and maintain a remarkable degree of mature structure and function for more than 3-6 months in vitro. Sequential light microscopy in correlation with electron-microscopic and electrophysiologic analyses showed that despite cross-species innervation, these human muscle fibers develop stable cross-striations, peripherally positioned myonuclei, and mature, functional motor endplates. Of special interest is the onset of significant progressive abnormalities, e.g., unusual focal myofibrillar lesions, in substantial numbers of innervated mature DMD muscle fibers after 2-4 months in culture. The focal myofibrillar lesions were not detected in normal muscle fibers maintained as long as 6 months in coculture, nor are they comparable to the generalized loss of cross-striations observed in muscle atrophy following in vitro denervation of mature DMD fibers.

Muscular dystrophy and muscle regeneration

An animal model of muscular dystrophy, the dystrophic (129ReJ dy/dy) mutant mouse, was used to evaluate the regenerative phenomenon in dystrophic muscle. The effect of age on "spontaneous" regeneration (i.e., regeneration in the absence of secondary trauma) was assessed by quantitative morphometric analysis and evaluation of myosatellite cell dynamics (i.e., myosatellite cell frequency, proliferative activity, and fusion capability). Spontaneous regeneration ceased by the time the mice were 8 weeks old. The findings suggested that the small "regenerating" myofibers found in older dystrophic muscle had been formed earlier in the time course of the disease and were growth-inhibited. To determine the cause of the cessation of regeneration, dystrophic muscle was subjected to the severe trauma of whole-muscle transplantation, a trauma that results in total myofiber necrosis followed by de novo myotube formation. When young dystrophic muscle (from 4- to 6-week-old dystrophic mice) was orthotopically transplanted, the time course of degeneration-regeneration was similar to that seen in age-matched normal muscle. Moreover, the regenerated dystrophic myofibers were capable of long-term survival (200 days or longer after transplantation), and they failed to show evidence of histologic changes consistent with murine dystrophy. When older dystrophic muscle (from 17-week-old dystrophic mice), muscle that failed to display spontaneous regeneration, was transplanted, it displayed remarkable regenerative capacity. It was suggested that the cessation of spontaneous regeneration in older dystrophic murine muscle is due not to exhaustion of myosatellite cell proliferative capacity, but rather to age-related loss of the mitogenic effect of dystrophy on the myosatellite cells of dystrophic muscle.

Low frequency chronic electrical stimulation of normal and dystrophic chicken muscle

The fast-twitch posterior latissimus dorsi muscle of normal and genetically dystrophic chickens was subjected to continuous indirect electrical stimulation at 10 Hz for periods of 4-8 weeks. To sustain this in vivo nerve stimulation an internally implantable miniature stimulator device was designed. This regime of stimulation caused complete fatigue of the normal muscle within 5 min of its initiation. The dystrophic muscles maintained a very small degree of contractile activity during this initial phase. Tangible twitching of the muscle returned in 5 week birds between 3 and 5 days and in 10 week birds between 11 and 16 days after implantation. After 4 weeks of stimulation, no significant change was measured in the time-to-peak of the isometric twitch response, nor in the half-relaxation time. The resistance to fatigue was significantly increased in the stimulated muscles when tested with a series of tetani at 40 Hz. The mean fibre area was decreased, in all muscles stimulated for longer than 3 weeks, in comparison to their contralateral controls, except where fibre splitting in dystrophic birds abnormally reduced the control value. The majority fibre type of the muscle was changed from type IIB to IIA. The histochemical reactions for both NADH-linked oxidation and phosphorylase were distinctly increased in the stimulated muscles. In normal muscle, stimulation increased somewhat the number of nuclei per unit area and changed their intracellular distribution, so that a greater proportion was found adjacent to the sarcolemma. The normal posterior latissimus dorsi muscle responded to chronic stimulation with increases of 3-6-fold in its acetylcholinesterase (AChE) activity. The maximum change in AChE occurred after 2 weeks stimulation; a steady level, 3 times that of the control unstimulated muscle, persisted at later times. Chronic stimulation suppressed the over-production of AChE that is characteristic of dystrophic chicken fast-twitch muscle, to attain a level comparable to the AChE activity in a stimulated normal muscle. Stimulation exerted a strong normalizing influence on dystrophic muscle, as assessed morphologically. The characteristic fibre rounding, fibre hypertrophy and myonuclear proliferation were reduced. This influence was most marked where the stimulation was initiated before the major pathological changes had occurred, but was also significant when commenced in strongly affected birds of 10-11 weeks.

Altered distribution of protein kinase C in dystrophic muscle cells and its modulation by liposome-delivered phospholipids

The activity and subcellular distribution of the calcium-phospholipid dependent protein kinase (protein kinase C) were studied in normal and dystrophic muscle cells in vitro. Clonal strains of satellite cells, isolated from normal and dystrophic (C57BL/6J/dydy) mice, differentiate in vitro at a comparable level (over 80% of fusion). Differentiated myotubes were homogenized and separated into a soluble and a particulate fraction. The activity of protein kinase C was assayed in both fractions, and was found to be mainly in the cytosol of normal cells, whereas it was mainly associated to the membrane fraction of dystrophic cells. This altered distribution of the enzyme was likely consequent to alterations in the phospholipid composition of the dystrophic cell membrane, since it was possible to partially revert the situation by modifying the membranes with liposome-delivered phospholipids. Splenic lymphocytes from dystrophic mice showed an altered distribution of protein kinase C similar to that observed in muscle cells. The possible biochemical basis and the functional consequences of this altered distribution of the enzyme in the dystrophic cells are discussed.

Electromyographic findings in different forms of infantile and juvenile proximal spinal muscular atrophy

Quantitative electromyography (EMG) was performed in 223 infantile and juvenile cases of spinal muscular atrophy (SMA), which were classified into 3 groups: (A) form Ia, Werdnig-Hoffmann disease; (B) forms Ib and II, intermediate forms; and (C) form III, Kugelberg-Welander disease. The groups differed in the occurrence of spontaneous activity; only groups A and B showed spontaneous rhythmic firing of motor units, whereas in long-standing cases, pseudomyotonic volleys appeared. The parameters of individual motor unit potentials (MUPs) differed in the different forms of the disease. Group A showed, in addition to long potentials of high amplitude, some short and low amplitude potentials, and the histograms of amplitudes and durations were bimodal. In the long-standing cases, the values of these parameters were shifted to longer durations and higher amplitudes of motor unit potentials. However, in long-standing cases of the benign group C, the short, low potentials appeared as well as so-called linked potentials. In the very early stage of the disease, the children who were found to be suffering from chronic forms of SMA--both malignant (form Ib and II or benign group C)--had an EMG record that was slightly different from that of acute form Ia. Their EMG record shows more so-called "spinal" signs, particularly in the benign group C (Kugelberg-Welander disease). These increasing features of chronic anterior horn cell involvement followed a pattern of reinnervation and hypertrophy of muscle fibers. These phenomena were particularly seen in the benign group C. These findings indicate that in the early stage of SMA, the EMG not only has diagnostic, but also prognostic, value.

Fetal myosin immunoreactivity in human dystrophic muscle

We report immunofluorescence observations on normal and dystrophic human muscle using an antibody (anti-bF) raised against bovine fetal myosin and specific for fetal myosin heavy chains. In rat skeletal muscle, anti-bF was previously found to react selectively with myosin isoforms expressed during fetal and early postnatal development and in regenerating muscles. Anti-bF stained most fibers in human fetal and neonatal muscle, whereas only nuclear chain fibers of muscle spindles were labeled in normal adult muscle. In muscle biopsies from patients with Duchenne's muscular dystrophy, numerous extrafusal fibers were stained: some were small regenerating fibers, others were larger fibers presumably resulting from previous regenerative events. Fetal myosin immunoreactivity in Duchenne's dystrophy appears to reflect the reexpression of fetal-specific myosin isoforms and provides a new valuable tool for identifying regenerating fibers and following their destiny in dystrophic muscle.

Modification of the phenotypic expression of murine dystrophy: a morphological study

The extensor digitorum longus muscles of 4-6-week-old normal mice (129 ReJ) and dystrophic mice (129 ReJ dy/dy) were orthotopically transplanted. Grafted muscles were examined 1, 3, 7, 14, 20, 50, and 100 days post-transplantation. The myofibers of both types of grafts underwent a similar time course of necrosis and regeneration. Other than during the initial necrotic response, no evidence of necrotic myofibers was found in either type of grafted muscle. At 100 days post-transplantation, the grafted normal and dystrophic muscles were essentially similar, except that the dystrophic graft was of smaller size. Based on a comparison of the number of myofibers found at the 100-day grafts' widest girths [631 +/- 59 SEM, for normal grafts (Bourke and Ontell, 1984); 631 +/- 74 SEM, for dystrophic grafts], it is suggested that the regenerative capability of traumatized 4-6-week-old dystrophic muscle is similar to that of traumatized normal muscle. At 100 days post-transplantation, the grafted dystrophic muscle appeared "healthier" than untraumatized muscle from age-matched dystrophic mice, having less variation in myofiber diameter, better fascicular organization, and less connective tissue. The transplantation system demonstrates the possibility of modifying the expression of genetic programming of myopathic disorders using environmental manipulation.

Impaired biosynthesis of highly unsaturated phosphatidylcholines: a hypothesis on the molecular etiology of some muscular dystrophies

A brief review of the literature concerning the synthesis of phosphatidylcholine and phosphatidylethanolamine in muscle suggests that the cytidine pathways are replaced by the recently proposed acyl-specific de novo and salvage glycerolphosphodiester pathways (Infante, 1984) in fully differentiated muscle. An analysis of published data suggests an impaired synthesis of 4,7,10,13,16,19-docosahexaenoic phosphatidylcholine, at the level of de novo sn-3-glycerolphosphorylcholine synthesis, as the primary defect in Duchenne and (dy) murine muscular dystrophies. This phosphatidylcholine species is postulated to be required for optimum sarcoplasmic Ca2+ transport activity. It is proposed that this impairment initiates the secondary series of events which lead to the observed pathology of these diseases. Based on some predictions of the hypothesis, potential diagnosis and treatments are suggested.

Presence of immunoreactive alpha-melanotropin and beta-endorphin in spinal motoneurones of the dystrophic mouse

Two derivatives of the stem hormone opiomelanocortin, alpha-melanotropin (alpha-MSH) and beta-endorphin, were detected immunocytochemically in the cell bodies and in the peripheral axon terminals of spinal motoneurones in immature but not in adult healthy mice. Immunoreactivity was demonstrated in motoneurones in both immature and adult mice with inherited muscular dystrophy. These results provide evidence for a motoneurone abnormality in murine dystrophy. The observations are discussed in the light of trophic influences over neuromuscular transmission which have been attributed to this family of neuropeptides.

Increased muscle action potentials by 5 Hz prolonged nerve stimulation in neurological and neuromuscular disorders--clinical usefulness for detecting underlying pathophysiology

The time courses of changes in amplitudes of muscle action potentials (MAPs) obtained from gastrocnemius and soleus muscles by 5 Hz prolonged tibial nerve stimulation were studied. Subjects included muscular dystrophy (MD), spinal muscular atrophy, Issacs syndrome, idiopathic muscle spasms, psychiatric disorders such as autism and schizophrenia, and normal controls. In normal subjects, MAPs obtained at 5 minutes from gastrocnemius muscles was 87-102% of those at initiation of the stimulation. In soleus muscles, MAPs at 5 minutes was 95-105% of those at the beginning. In gastrocnemius muscles, MAPs increased in disorders such as Duchenne MD, Fukuyama type congenital MD, facioscapulohumeral MD, myotonic dystrophy, dermatomyositis, Kugelberg-Welander syndrome, viral myelitis, malignant hyperpyrexia, autism and schizophrenia. In soleus muscles, the increase of MAPs was demonstrated in Duchenne MD, Fukuyama type congenital MD, myotonic dystrophy and autism. MAPs remained within normal range in infants with Werdnig-Hoffman disease, Issacs syndrome and idiopathic muscle spasms. In two cases with Duchenne MD, MAPs obtained from gastrocnemius muscles reduced in amplitudes by the administration of dantrolen sodium. While the pathogenesis of the increased MAPs is not clear, several possible factors are discussed. It is considered that this 5 Hz examination may provide an important information for detecting the effect of dantrolen sodium on Duchenne MD, and it is also suggested that the examination will be a useful test for finding latent malignant hyperpyrexia.