Some mechanical and electrical properties of distal hind limb muscles of genetically dystrophic mice (C57BL/6Jdy2j/dy2j)

MyoD distal regulatory region contains an SRF binding CArG element required for MyoD expression in skeletal myoblasts and during muscle regeneration

We show here that the distal regulatory region (DRR) of the mouse and human MyoD gene contains a conserved SRF binding CArG-like element. In electrophoretic mobility shift assays with myoblast nuclear extracts, this CArG sequence, although slightly divergent, bound two complexes containing, respectively, the transcription factor YY1 and SRF associated with the acetyltransferase CBP and members of C/EBP family. A single nucleotide mutation in the MyoD-CArG element suppressed binding of both SRF and YY1 complexes and abolished DRR enhancer activity in stably transfected myoblasts. This MyoD-CArG sequence is active in modulating endogeneous MyoD gene expression because microinjection of oligonucleotides corresponding to the MyoD-CArG sequence specifically and rapidly suppressed MyoD expression in myoblasts. In vivo, the expression of a transgenic construct comprising a minimal MyoD promoter fused to the DRR and beta-galactosidase was induced with the same kinetics as MyoD during mouse muscle regeneration. In contrast induction of this reporter was no longer seen in regenerating muscle from transgenic mice carrying a mutated DRR-CArG. These results show that an SRF binding CArG element present in MyoD gene DRR is involved in the control of MyoD gene expression in skeletal myoblasts and in mature muscle satellite cell activation during muscle regeneration.

Electrophysiology of the neuromuscular junction of the laminin-2 (merosin) deficient C57 BL/6J dy2J/dy2J dystrophic mouse

The C57 BL/6J dy2J/dy2J dystrophic mouse expresses an abnormal truncated form of the alpha2 subunit of the protein laminin-2 (or merosin), which is unable to form a stable link between the extracellular matrix and the dystrophin-associated proteins, resulting in muscular dystrophy. Morphological abnormalities of the peripheral nervous system and neuromuscular junction have also been reported. The electrophysiological properties of the neuromuscular junctions of diaphragm, extensor digitorum longus (EDL), and soleus from C57 BL/6J dy2J/dy2J mice and controls are described. No evidence for the presence of denervated fibres were found. Mean MEPP amplitudes were significantly increased in EDL and soleus but reduced in the diaphragm from affected mice. Mean MEPP frequencies were raised in all the dy2J/dy2J muscles studied. dy2J/dy2J muscles were paralysed by low concentrations of mu-conotoxin suggesting that embryonic (tetrodotoxin and mu-conotoxin resistant) sodium channels are not widespread on dy2J/dy2J muscle as has previously been reported. EPP latencies were significantly prolonged in the diaphragm and EDL but not soleus from dy2J/dy2J mice. Quantal contents were higher in all dy2J/dy2J muscles. In the dy2J/dy2J diaphragm failures in neurotransmission occurred and a faster rate of rundown of EPPs were apparent. Some changes appear from a direct effect of dystrophy, whilst increased MEPP frequency and quantal content, and failures in neurotransmission indicate neuronal abnormalities.

Long term effect of low frequency chronic electrical stimulation on the fast hind limb muscles of dystrophic mice

Low frequency chronic electrical stimulation can have a beneficial effect on dystrophic muscles. The present study was undertaken to assess the long term effect of such stimulation on the fast hind limb muscles of dystrophic mice. The relationship between the changes induced by stimulation and the initial condition of the dystrophic muscles, as well as other factors which might contribute to this relationship, were examined. The stimulation induced an increase in the force output of weak dystrophic muscles and a speeding of their time course of contraction and relaxation, as well as an increase in their fatigue resistance. In relatively strong dystrophic muscles, the stimulation induced similar changes in contractile speed and fatigue characteristics, but it led to a slight decrease in force output. Our results suggest that the stimulation promotes the growth and differentiation of the small regenerating fibres known to be present in the diseased muscles and, in addition, induces an increase in the mitochondrial content of the muscle fibres. Our results indicate that these effects are not permanent.

Inherited neuromuscular diseases in the mouse. A review of the literature

There are several neuromuscular disorders affecting the human being. Most of these are poorly understood and lack and effective treatment. Due to the limitation of experimental manipulation in "anima nobili", inherited neuromuscular diseases in laboratory animals constitute a valuable source of scientific information. Amongst several animal species affected by neuromuscular disorders the house mouse is of particular interest because of its small size, short pregnancy and low costs of maintanence. In the present review 20 murine mutants with diseases affecting peripheral nerves, skeletal muscles and motor end-plates are tabulated. Genetic, clinical and pathological aspects are discussed aiming to provide information about these mutants which might be of great interest as animal models for human neuromuscular diseases.

Contractile and EMG studies of murine myotonia (mto) and muscular dystrophy (dy/dy)

This report focuses on the myotonic (mto) mouse, an autosomal recessive neuromuscular mutant first described in 1982. Studies in vivo confirmed the presence of hindlimb rigidity during walking and typical myotonic electromyographic (EMG) discharges that persisted after nerve transection and complete neuromuscular blockade. Studies of the contractility of mto muscles in vitro revealed reduced peak isometric tetanic tension and greatly prolonged relaxation times. Tubocurarine did not affect tension parameters, but did antagonize the delayed relaxation in vitro. On the basis of EMG studies alone this mutant can accurately be described as myotonic. Reduction of the contractile abnormalities by tubocurarine in vitro, however, poses further questions regarding the nature of the disorder. Although the more familiar dystrophic mouse (dy/dy) has been termed "myotonic" by some, the new mto mutant differs from it in all aspects examined.

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.

Long-term peripheral nerve and muscle recordings from normal and dystrophic mice

A method for long-term recording of electrical activity from small mammalian nerves and muscles is described. Electrodes for stimulating and recording activity were implanted on nerves and muscles subserving ankle flexion and extension in normal and dystrophic mice. Activity was monitored on a regular basis for up to 200 days following implantation. Neural compound action potentials, compound EMG potentials and twitch tension were recorded. Shortly after implantation, evoked EMG and twitch tension declined, but recovered progressively to values measured at the time of implantation and subsequently remained steady in normal mice. However, while dystrophic mice did recover, with EMG levels reaching 50-60% of the values recorded at implantation, tension eventually dropped to 10% in flexor muscles and 25% in extensors.

Temperature effects on the kinetics of force generation in normal and dystrophic mouse muscles

The kinetics of isolated extensor digitorum longus and soleus muscles from normal and genetically dystrophic (129/ReJ dy/dy) mice were studied at temperatures from 8 to 38 degrees C. The rate constants for the exponential rise of tetanic force and for the exponential decay of force during an isometric twitch or short tetanus were similar in normal and dystrophic soleus muscles, but the decay rates were significantly reduced in dystrophic extensor digitorum longus muscles. The temperature dependence for several rate constants for isometric twitches and tetani was similar in all muscles studied, suggesting that the same rate limiting processes apply to fast and slow, normal and dystrophic muscles. Thus, the contractile proteins and those in the sarcoplasmic reticulum of dystrophic muscle are probably normal. The slower relaxation phase in dystrophic extensor digitorum longus muscles is compatible with a reduction in Ca2+-pumping sites in the sarcoplasmic reticulum, perhaps secondary to a change in motor unit composition. Some changes in the temperature dependence for measured times, toward those of soleus muscles, is consistent with the increased proportion of slow twitch motor units in dystrophic extensor digitorum longus muscles.

Acetylcholine-sensitivity in fast and slow twitch muscle of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice

Small bundles of muscle fibres were isolated from diaphragm, extensor digitorum longus (EDL) and soleus (SOL) muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice, and their isometric tension developed in response to acetylcholine (ACh) was recorded. For each type of muscle the relationship between the maximum amplitude of the ACh-contracture and log [ACh] was similar in normal and dystrophic animals. However, this relationship was steeper for normal and dystrophic SOL than for EDL and diaphragm muscles. Dystrophy did not induce changes in the time course of the ACh-contractures, except a significant 'speeding' of dystrophic SOL that appeared in the time to peak of the contractile response. The amplitude of ACh-contractures of both normal and dystrophic diaphragm preparations increased by about 50% after perfusion for 80-90 min in physiological solution containing phospholipase C 5 mU/ml. ACh-sensitivity was measured in normal and dystrophic diaphragm preparations by iontophoretic application of ACh from high resistance pipettes. ACh-potentials were similar in time course in the two types of muscle fibres, and there was also no significant difference in the length of sensitive fibre segments and maximum sensitivity values. Extrajunctional ACh-sensitivity was absent in normal as well as in dystrophic fibres. It is concluded that the absence in dystrophic muscles of stronger ACh-contractures and of extrajunctional sensitivity can be considered as evidence against a primary neuronal involvement in murine dystrophy of the dy2J strain.

Potassium and caffeine contractures in limb muscles of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice

The strength of contractures, produced by 15 to 146 mM [K]0 (as L-glutamate), was measured in isolated small bundles of muscle fibers from the fast-twitch extensor digitorum longus and from the slow-twitch soleus of normal and dystrophic (C57 BL/6J dy2J/dy2J) mice. The analysis of the relation between the maximal amplitude of the contracture vs the membrane potential and the time constant of relaxation of the K-contractures has shown that dystrophy induced an attenuation of the differences between fast- and slow-twitch muscles. The repriming of K-contractures was more affected by changes in [Ca]0 in normal soleus than in normal extensor digitorum longus and this difference was unaffected by dystrophy. For both types of muscles, the ability of caffeine to produce contractures was reduced in dystrophic muscle and this modification was not related to a change in the fiber typing.

An insulin receptor defect in murine muscular dystrophy

A study has been made of the I125 insulin binding and postbinding effects on excised soleus muscles from the 129 ReJ strain of dystrophic mice. Results are compared with those in sex- and weight-matched controls. The data suggest that, in the range of physiological hormone concentrations, the affinity of insulin receptors on dystrophic muscles is less than normal and that the insulin-dependent uptake of both 2-deoxyglucose (2-DG) and aminoisobutyric acid (AIB) is impaired. These findings are taken to indicate that many of the biochemical and electrophysiological abnormalities observed in murine dystrophy could arise from some genetic defect in the receptor proteins controlling uptake of raw materials.

Effects of pH on membrane resistance in normal and dystrophic mouse skeletal muscle fibers

Membrane cable properties of skeletal muscle fibers of dystrophic mice (Rej-129) and their littermate controls were examined using a conventional two-microelectrode recording technique. Fibers from dystrophic mice had a decreased membrane resistivity (Rm) compared with those from normal mice (517 +/- 27 vs 642 +/- 34 omega - cm2), while the internal resistivities (Ri) did not differ significantly. The increase in membrane specific conductance was due to an increased Cl- conductance (gCl) (2304 vs 1346 microseconds/cm2 for normal fibers), although the K+ conductance (gK) was actually decreased (234 vs 369 microseconds/cm2 for normal fibers). With changes in pH, membrane conductances of normal and dystrophic skeletal muscle fibers varied differently, mainly due to differences in effects on the Cl- conductance. This contrast may be due to altered regulation of internal pH in dystrophic muscle.

Restoration of functional continuity in dystrophic murine muscle after crushing

The recovery of functional continuity after crush injury was measured by a simple electrophsiological technique in semitendinosus muscles of normal and dystrophic mice of the C57BL/6J(dy2Jdy2J) strain. In contrast to virtually complete restitution in normal muscles, only one-third of fibers regained continuity in dystrophic muscles. The study also confirmed the lower resting membrane potentials of dystrophic fibers and the presence of "functional" denervation in some of them.

Slowing of twitch of dystrophic mouse muscle in partially due to altered activity pattern

Fast-twitch muscles of the hindlimb of dystrophic (dy2J) mice show a prolongation of both the contraction and relaxation phases of the isometric twitch. Comparable muscles of the forelimb of these mice exhibit relatively little increase in time to peak tension but time to half-relaxation is as severely affected as in the hindlimb. When examined with an immunohistochemical technique to demonstrate the presence of "slow" myosin it was apparent that there were no fibers containing the "slow" isoenzyme in either hindlimb or forelimb muscles of 6-month control mice. In dy2J mice hindlimb muscles contained many fibers with "slow" myosin whereas forelimb muscles did not. It is suggested that the spontaneous twitching activity produced in the hindlimbs, as a result of amyelination of the spinal roots, induces synthesis of "slow" myosin, which in turn leads to prolongation of time to peak twitch tension.

Effect of chronic electrical stimulation at low frequency on the passive membrane properties of muscle fibers from dystrophic mice

It has been reported that chronic electrical stimulation at low frequency applied to dystrophic muscles has a beneficial effect. In this study, the effect of this treatment on the passive membrane properties of muscle fibers from dystrophic mice was followed. Cable properties were assessed by the two-microelectrodes DC method and spacial decay analysis. Earlier results showing a decrease in resting potential, an increase in input resistance and in specific membrane resistance in muscle fibers from dystrophic mice were confirmed. In addition, the specific membrane capacitance of these muscle fibers was found to be lower than normal. This suggests that the membrane properties of fibers from dystrophic muscles are similar to those of immature muscle fibers. Muscle fibers from dystrophic animals that were stimulated for 2 to 4 weeks had membrane properties similar to those from normal muscles. This indicates that electrical stimulation at low frequency for 2 to 4 weeks restores membrane properties of dystrophic muscle fibers to normal and we suggest that an appropriate pattern of stimulation induces the maturation of dystrophic muscle fibers.

Membrane alterations in skeletal muscle fibers of dystrophic mice

Skeletal muscle fibers from dystrophic mice and littermate controls (ReJ-129) were characterized electrically and then injected with an intracellular marker. In this way they could be identified for examination with an electron microscope to correlate the relative time course of electrical and ultrastructural alterations resulting from the dystrophic process. On the average, dystrophic muscle fibers displayed decreased membrane potentials (-59 +/- 1.2 vs -79 +/- 0.7 mV for normals), decreased specific membrane resistivity (517 +/- 27 vs 642 +/- 34 omega-cm2 for normals), and depressed action potential (AP) maximum rates of rise (+Vmax) (352 +/- 9 vs 417 +/- 9 V/s for normals) and amplitudes (92 +/- 1.2 vs 102 +/- 1.0 mV for normals) at an experimentally polarized membrane potential of -90 mV. Membrane resistivity and AP +Vmax were decreased even in those fibers from dystrophic muscles that displayed normal ultrastructure (classified visually and by ratio of sarcoplasmic reticulum to total cell volume). These findings support the membrane hypothesis of muscular dystrophy that membrane lesions are the primary lesion in the disease process.

A freeze-fracture study of normal and dystrophic C57BL mouse muscle

Freeze-fracture replicas of normal and dystrophic C57BL mouse muscle and kidney were examined to see whether here was a deficit in plasmalemmal particles which others suggest is a feature of dystrophies. When compared with normal membranes there was an increase in the particle density in dystrophic extensor digitorum longus muscle, a decrease in dystrophic soleus muscle, and no change in dystrophic kidney. Therefore there was not a general deficit in intramembrane particles in this dystrophic tissue. Indirect evidence supported the hypothesis that abnormalities in dystrophic mouse muscles are caused by abnormal motor input. The density of indentations, parallel to the T-tubule, on the flat surface of the terminal cisternae can be modulated by the motor nerve. Changes were found in indentation density in dystrophic muscle which were similar to changes seen after transection of the spinal cord in the mid-thoracic region. There were parallel changes in contractile properties and indentation density in dystrophic fibers.

Stereological analyses of capillaries in muscles of dystrophic mice

Stereological analyses of the distribution of capillaries in skeletal muscles of congenitally dystrophic mice are described and reported. Two methods are used, each based on a different mathematical model of the 3-dimensional distribution of lines in space. For these analyses specimens of extensor digitorum longus muscle from clinically affected C57BL/6Jdy2J/dy2J dystrophic mice, and from non-littermate controls, were used. The analyses were carried out on transverse and longitudinally orientated semithin sections of these muscles. Although the two methods employed are based on different mathematical models and yield results relevant to each particular model, it is clear that there is a more extensive capillary network present per unit volume of the dystrophic muscle than in control muscles. These findings are relevant to the theories which involve a vascular aetiology for muscular dystrophy. It is apparent that, in order to explore the structure of the capillary network more fully, there is a need for the development of more sophisticated stereological techniques for analysis of capillaries in skeletal muscle.

Observations on the effects of low frequency electrical stimulation on fast muscles of dystrophic mice

The deterioration of tibialis anterior (TA) and extensor digitorum longus (EDL) muscles in dystrophic mice (C 57 BL dy/dy) was compared. The effects of chronic electrical stimulation on various characteristic properties of these muscles were also studied. The results indicate that EDL muscles are less affected by the disease than TA. This "selectivity" is difficult to explain since both muscles have similar fibre type composition. TA and EDL muscles that were stimulated for 10-28 days developed greater tetanic tensions than the contralateral muscles, but this effect was apparent only when the muscles were severely affected by the disease, that is the contralateral TA or EDL muscles developed less than 50% of the tension produced by muscles from normal animals. In all EDL muscles, stimulation increased the fatigue resistance. The time course of contraction and relaxation of dystrophic muscles is usually slower than that of normal muscles. The stimulation reduced this slowing effect, so that the stimulated muscles became similar to homologous muscles from normal littermates.

Oral kinesthesia in patients with Duchenne muscular dystrophy

In trials on normal subjects and patients with Duchenne muscular dystrophy (DMD), interdental dimension discrimination (IDD) was tested by assessing the ability of subjects to discriminate between pairs of sticks of different dimensions held between the upper and lower teeth. The IDD ability of the DMD patients was significantly inferior to that of the normal subjects. Further, DMD patients tended to overestimate the dimension of the first stick of each pair even more than did normal subjects. Vibration applied to the mandible seemed to lessen such inaccuracies of oral kinesthesia in the DMD patients. These results are compatible with the idea that muscle receptors, especially muscle spindles in jaw closing muscles, are mainly responsible for IDD.

Mechanical activation in dystrophic C57BL mouse muscle

Stimulus strength-duration curves for contraction threshold have been obtained from normal and dystrophic C57BL mouse muscle fibres using two microelectrode voltage clamp techniques. The threshold membrane potential for activation in dystrophic soleus fibres was further from the resting membrane potential than in normal fibres and was close to the threshold for extensor digitorum longus fibres. When a redistribution if fibre types in dystrophic soleus muscles is taken into account the dystrophic data fell within the normal range of values. It is apparent that the inability of dystrophic C57BL mice to use their hind limbs in the normal way cannot be attributed to a failure of excitation-contraction coupling.

Some factors affecting spontaneous transmitter release in dystrophic mice

Neuromuscular junctions of slow- and fast-twitch skeletal muscles from dystrophic (dy2J/dy2J) and control mice of the C57BL/6J strain were used to investigate the effect of muscular dystrophy on nerve-terminal regulation of their intracellular concentration of free calcium ions. The frequency of spontaneous miniature endplate potentials (MEPPs) was taken as an indicator of the intraterminal free calcium ion concentration. Dicoumarol, 2,4-dinitrophenol, ruthenium red, and the calcium ionophore A-23187 all potentiated the MEPP frequency in dystrophic muscles at concentrations which had negligible effects on normal muscles. Dystrophic muscle preparations were also more sensitive to an increased extracellular calcium concentration. Usually, these manipulations had more effect on the nerve terminals of dystrophic slow muscle than on those of dystrophic fast muscle. We conclude that muscular dystrophy alters the nerve terminal's ability to regulate the concentration of intracellular free calcium ions.

Dystrophic mice show age related muscle fibre and myelinated axon losses

Although many differences between age matched normal and dystrophic animals have been found, there have been few demonstrations of a time related quantitative change from normal to a characteristically dystrophic situation within the dystrophic strain itself. Here we report such a change-normal numbers of muscle fibres present in young dystrophic mice were rapidly lost, such that older animals showed the reduced number of muscle fibres characteristic of murine dystrophy. Although these losses began after a demonstrable loss of myelinated axons had occurred, it is not possible to say if the loss of muscle fibres was a result of the loss of nerve fibres.

Parabiotic reinnervation in normal and myopathic (BIO 14.6) hamsters

Parabiotic reinnervation is a technique whereby the muscles of one animal can be reinnervated by peripheral nerves of another animal. This technique has been used to test the hypothesis that the myopathy in the BIO 14.6 strain of hamster may result from a derangement of the trophic function that a nerve exerts upon the muscle which it innervates. The percentage of internal nuclei in muscle fibres has been used as a criterion to define the myopathic state. The percentage of internal nuclei was high in myopathic hamster muscles but very low in normal or "self-reinnervated" normal muscles. There was no indication that the percentage of internal nuclei in myopathic muscles fell after parabiotic reinnervation with normal nerves. Similarly, there was no evidence that the percentage increased in normal muscles that had been parabiotically reinnervated with dystrophic nerves. On the basis of these results, it was concluded that there was no positive evidence in favour of a neural aetiology for the myopathy in the BIO 14.6 strain of hamsters.