A maturational defect in passive membrane properties of dystrophic mouse muscle

Type-1 pericytes participate in fibrous tissue deposition in aged skeletal muscle

In older adults, changes in skeletal muscle composition are associated with increased fibrosis, loss of mass, and decreased force, which can lead to dependency, morbidity, and mortality. Understanding the biological mechanisms responsible is essential to sustaining and improving their quality of life. Compared with young mice, aged mice take longer to recover from muscle injury; their tissue fibrosis is more extensive, and regenerated myofibers are smaller. Strong evidence indicates that cells called pericytes, embedded in the basement membrane of capillaries, contribute to the satellite-cell pool and muscle growth. In addition to their role in skeletal muscle repair, after tissue damage, they detach from capillaries and migrate to the interstitial space to participate in fibrosis formation. Here we distinguish two bona fide pericyte subtypes in the skeletal muscle interstitium, type-1 (Nestin-GFP(-)/NG2-DsRed(+)) and type-2 (Nestin-GFP(+)/NG2-DsRed(+)), and characterize their heretofore unknown specific roles in the aging environment. Our in vitro results show that type-1 and type-2 pericytes are either fibrogenic or myogenic, respectively. Transplantation studies in young animals indicate that type-2 pericytes are myogenic, while type-1 pericytes remain in the interstitial space. In older mice, however, the muscular regenerative capacity of type-2 pericytes is limited, and type-1 pericytes produce collagen, contributing to fibrous tissue deposition. We conclude that in injured muscles from aging mice, the pericytes involved in skeletal muscle repair differ from those associated with scar formation.

Skeletal muscle fiber type conversion during the repair of mouse soleus: potential implications for muscle healing after injury

We used a mouse model of cardiotoxin injury to examine fiber type conversion during muscle repair. We evaluated the soleus muscles of 37 wild-type mice at 2, 4, 8, and 12 weeks after injury. We also used antibodies (fMHC and sMHC) against fast and slow myosin heavy chain to classify the myofibers into three categories: fast-, slow-, and mixed (hybrid)-type myofibers (myofibers expressing both fMHC and sMHC). Our results revealed an increase in the percentage of slow-type myofibers and a decrease in the percentage of fast-type myofibers during the repair process. The percentage of hybrid-type myofibers increased 2 weeks after injury, then gradually decreased over the following 6 weeks. Similarly, our analysis of centronucleated myofibers showed an increase in the percentage of slow-type myofibers and decreases in the percentages of fast- and hybrid-type myofibers. We also investigated the relationship between myofiber type conversion and peroxisome proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha). The expression of both PGC-1alpha protein, which is expressed in both the nucleus and the cytoplasm of regenerating myofibers, and sMHC protein increased with time after cardiotoxin injection, but we observed no significant differential expression of fMHC protein in regenerating muscle fibers during muscle repair. PGC-1alpha-positive myofibers underwent fast to slow myofiber type conversion during the repair process. These results suggest that PGC-1alpha contributes to myofiber type conversion after muscle injury and that this phenomenon could influence the recovery of the injured muscle.

Higher content of insulin-like growth factor-I in dystrophic mdx mouse: potential role in the spontaneous regeneration through an electrophysiological investigation of muscle function

Insulin-like growth factor-I (IGF-I) is known to promote proliferation and differentiation of muscle cells during growth and regeneration. Both these conditions are characterized by acquisition of specialized muscle functions, such as a large macroscopic chloride conductance (GCl), a parameter that is a target of growth hormone (GH)/IGF-I axis action on skeletal muscle. The present study has been aimed at evaluating the role of IGF-I in the spontaneous regeneration occurring in hind limb muscle of dystrophic mdx mouse. IGF-I levels have been measured in hind limb muscles, plasma and liver of mdx and control mice of 8-10 weeks and 5 months of age by radioimmunoassay. In parallel the biophysical and pharmacological properties of muscle chloride channels of extensor digitorum longus (EDL) muscle fibers of mice belonging to the same age-group have been measured electrophysiologically in vitro. At 8-10 weeks of age, significantly greater amounts of IGF-I were found in plasma and hind limb muscles of mdx mice with respect to controls. Such a difference was only just detectable and no longer statistically significant at 5 months of age. No differences were found in hepatic IGF-I levels at either age. The EDL muscle fibers of mdx mice at 8-10 weeks of age were characterized by higher GCl values and by a different pharmacological sensitivity to the enantiomers of 2-(p-chlorophenoxy)-propionic acid (CPP), specific chloride channel ligands, with respect to age-matched controls. However, these differences were no longer detected at 5 months of age. Our results suggest a role of IGF-I in the high regenerative potential of muscles from mdx mice and support the hypothesis that the biophysical and pharmacological properties of chloride channels of EDL muscle fibers are sensitive indices of the action of regeneration-promoting factors on muscle function.

Electrical properties of diaphragm and EDL muscles during the life of dystrophic mice

The membrane electrical properties of diaphragm and extensor digitorum longus (EDL) muscle fibers of dystrophic mdx and control mice from 4 wk to 14-19 mo of age were recorded with the intracellular microelectrode technique. Up to 8 wk of age, the diaphragm and EDL muscles did not differ between the two strains. From 8 up to 20 wk, the mdx diaphragm fibers showed a higher membrane resistance (Rm), which was due to significantly lower values of resting chloride conductance (GCl) and an overexcitability with respect to age-matched controls. Oppositely, the mdx EDL muscle fibers had significantly lower Rm and higher GCl values than age-related controls at 8, 10, and 13 wk, along with a decreased membrane excitability. These differences were no longer detectable at 20 wk. The diaphragm and EDL muscles from 14- to 19-mo-old controls showed a decrease of GCl and an increase of potassium conductance with respect to adult animals. In aged mdx animals, these changes were very dramatic in diaphragm fibers, whereas no differences, with respect to adults, were found in the EDL muscle. Thus GCl is an index of the dystrophic condition of mdx muscles. In the degenerating diaphragm, the impairment of GCl can account for some of the pathological features of the muscle. In the EDL muscle, the changes of GCl can follow the high regenerative potential of the hindlimb muscles of the mdx phenotype.

Effect of caffeine and high potassium on normal and dystrophic mouse EDL muscles at various developmental stages

EDL muscles from normal and dystrophic (dy2j) mice of various ages were examined. Muscles were divided into three groups according to age: 7 to 14 days postnatal, 16 to 21 days postnatal, and 6 months old, to assess age and/or phenotype related differences in the muscle response to caffeine or high K+. The response of normal muscles to caffeine decreased with age and reached adult characteristics between the second and third week of postnatal life. Their response to high K+ also changed during postnatal development; specifically, the time taken to recover to 50% pretest twitch tension decreased with age, probably reflecting developmental changes in Cl- conductance. Up to 21 days of age, the sensitivity of dystrophic muscles to both caffeine and high K+ was essentially similar to normal, while marked differences were observed in the adult. Taken altogether, our results suggest that while the maturation of a number of systems might be delayed in dystrophic muscles at preclinical stages of the disease, their e-c coupling and SR function (Ca2+ release and reuptake) appear to be quite normal. Our results further suggest that the "abnormal" responses of dystrophic muscles at more advanced stages of the disease, when challenged by drugs acting on either of these systems, may be explained in terms of changes in muscle fiber type proportions.

Membrane ionic conductances in normal and denervated skeletal muscle of the rat during development

The development of membrane ionic conductances of rat extensor digitorum longus (EDL) muscle fibers was studied in vitro using intracellular recordings. At 7-8 days after birth, the potassium conductance (GK) dominated the total membrane conductance while the chloride conductance (GCl) was very low. A rapid increase of GCl towards adult values was observed after few days (12-14 day old rats), whereas GK did not decrease up to day 23. Denervation at 7-8 days after birth suppressed the maturation of the electrical parameters measured, and 15 days after the nerve crush, GCl was just detectable. These results suggest that the maturation of the electrical properties, and in particular that of the resting chloride conductance in mammalian striated muscle fibers, occurs during the first weeks of postnatal life and is dependent on innervation.

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.

Protein profiles of sarcoplasmic reticulum from normal and dystrophic mouse muscle

Sarcoplasmic reticulum (SR) was isolated from skeletal muscle of dystrophic (C57BL/6J dy2J/dy2J) mice and the protein composition analysed by sodium dodecyl sulfate polyacrylamide gel electrophoresis. Densitometric analysis of dystrophic SR preparations indicated a decrease in the Ca2+-ATPase and calsequestrin, and the appearance of a protein with molecular weight 72 000. These differences in the protein profiles between normal and dystrophic SR became more apparent as the disease progressed. The observations are discussed in relation to secondary changes in the dystrophic process such as changes in fibre type and the presence of immature fibres.

Ontogenetic aspects of changes in muscular potentials at medial gastrocnemius muscles of dystrophic mice due to prolonged stimulation

Changes in muscular potentials at medial gastrocnemius (MG) muscles induced by prolonged stimulation at 5 Hz were compared in dystrophic mice and their normal littermates at various ages. A rapid and notable reduction in the amplitude of muscular potentials at MG muscles was observed in normal mice. This was in contrast with a slight decrease or even an increase in the amplitude in dystrophic mice. The magnitude of reduction in normal mice increased with age, but in dystrophic mice where the change (a decrease or even an increase) was slight, it was similar in extent regardless of age. The slight change in dystrophic mice under the present regimen would be called a fatigue, resistant-like property, and this was discussed in conjunction with analogous properties observed in electrophysiological, histological and biochemical fields.

Differences among dystrophic, dwarf, and their crossbred mice in the time course of changes in extracellular muscle action potentials induced by 5-Hz stimulation

With urethane anesthesia, extracellular action potentials were recorded in medial gastrocnemius muscles of dystrophic, dwarf, and their crossbred mice. When repetitive stimulation was delivered at 5 Hz for a relatively long period, characteristic features were revealed. (i) Dystrophic mice showed a slight decrease or even an increase in action potentials whereas in littermate normal mice the amplitudes were rapidly and notably reduced. (ii) In both dwarf and their littermate normal mice, a considerable reduction in amplitude was observed. Slightly more depression was produced than in nondystrophic mice of a comparable age. (iii) Crossbred mice were in two classes. A rapid and notable reduction in the amplitude of muscle action potentials was observed in one class, and slight changes in the potentials were produced in another class showing dystrophy-specific symptoms.

Chronic phenytoin administration alters the metabolic profile of superficial gastrocnemius muscle fibers in dystrophic mice

Phenytoin is known to reduce neural overactivity (pseudomyotonia) affecting the hind limb musculature in C57B1/6J dystrophic (dy2J/dy2J) mice. This study reports a change in the metabolic profile of superficial gastrocnemius muscle fibers from dy2J/dy2J animals after chronic phenytoin treatment. The superficial gastrocnemius muscle region from normal mice is composed of 98% fast-twitch glycolytic muscle fibers. In dystrophic mice these fibers (FG) show increased oxidative capacity without evidence of morphologic degeneration during the first few months ex utero. Many of these fibers also store abnormally large amounts of glycogen as determined by periodic acid-Schiff histochemistry. After 104 days of phenytoin treatment, the dy2J/dy2J FG muscle fibers showed a reduction in abnormally high oxidative capacity as monitored by succinic dehydrogenase activity; there was also a reduction of glycogen storage in a number of dy2J/dy2J fibers. One hypothesis suggests that the increase in oxidative capacity of the dy2J/dy2J superficial gastrocnemius muscle fibers is the expected result of overstimulation by the pseudomyotonia. Our experiments indicated that the abnormal metabolic profile observed in those fibers can be altered simply by a reduction in pseudomyotonia. These results mimic those seen after short-term denervation of the same dy2J/dy2J muscle. After phenytoin treatment the mean dy2J/dy2J superficial gastrocnemius muscle fiber cross-sectional area was significantly increased compared with untreated animals. Cursory examination of the degenerated deep region of this same muscle suggested that similar changes did not occur after drug treatment. This suggests that the pseudomyotonia was partially different from the factor(s) causing early degeneration of the oxidative muscle fibers in the dy2J/dy2J animals.

Membrane electrical properties of developing fast-twitch and slow-tonic muscle fibres of the chick

Isolated single fibres from the anterior (a.l.d.) and the posterior (p.l.d.) lattissimus dorsi muscles of embryonic and young chicks were used to study in vivo development of membrane electrical properties. Isolated fibres were obtained by an enzymatic dissociation procedure. Intracellular micro-electrode recordings from isolated fibres and from fibres in intact muscles showed that the dissociation procedure did not significantly alter resting membrane potentials, input resistances or membrane time constants (tau m). The 14 day embryonic fibres of a.l.d. and p.l.d. did not have a measurable resting conductance to Cl-. At hatching, about 70% of the resting conductance in p.l.d. fibres was due to Cl-. Membrane electrical properties were estimated from the analysis of voltage responses to intracellular injection of rectangular pulses of current. At 14 days in ovo, membrane resistance (Rm) was approximately 20 k omega cm2 and membrane capacitance (Cm) was 1-2 microF/cm2 for both a.l.d. and p.l.d. The mean membrane length constants (lambda) were 1.7 mm for a.l.d. and 1.5 mm for p.l.d. For p.l.d., the values of Rm, tau m and lambda decreased as development proceeded. For a.l.d., there was no change in these values by the time of hatching (21 days). The decreases in the electrical constants for p.l.d. fibres were partly explained by the appearance of a resting Cl- conductance during the last week of embryonic development.

The acetylcholinesterase abnormality in dystrophic mice is a reflection of a maturational defect

Juvenile mice of the ReJ/129 strain exhibit a distribution of acetylcholinesterase molecular forms that is similar to the pattern previously observed in dystrophic mice. The change to an adult distribution of the enzyme occurs at about 3 weeks of age which is also when dystrophic signs first become evident. It is suggested that dystrophic mouse muscle fails to mature with respect to the molecular forms of acetylcholinesterase.

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.

Elimination of polyneuronal innervation in a fast muscle of normal and dystrophic mice

The changes in the pattern of innervation of extensor digitorum longus (e.d.l.) during post-natal development was studied in normal and dystrophic mice. As in other mammals, individual muscle fibres of new-born mice are supplied by more than one axon. Up to 10 days after birth there was no difference in the extent of this polyneuronal innervation between normal and dystrophic muscle fibres. During post-natal development the polyneuronal innervation gradually disappeared. In normal e.d.l. muscles the rate of the elimination of polyneuronal innervation was faster during the first 10 post-natal days and then slowed down. By 16 days the final value of less than 10% of muscle fibres receiving more than one input was reached. In the dystrophic muscles the rate of elimination was similar to normal up to 10 days of age, but continued to decrease rapidly so that already by 11 days of age polyneuronal innervation was reduced to its final level of less than 10%. Thus the elimination of polyneuronal innervation was completed at least 3 days earlier in the dystrophic animals. It is suggested that the increased nerve activity said to be present in dystrophic mice could account for this finding.

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.

Increase of muscle mitochondrial content with age in murine muscular dystrophy

Comparison of morphological features of gastrocnemius muscle fibers in normal and dystrophic (dy2J) mice during development was undertaken to determine the time course of increased oxidative capacity in dystrophic fibers. Measurements of mitochondrial volume percent and of Z-line width were made in superficial fast-twitch fibers using electron microscopy and stereological techniques. Dystrophic fibers develop a progressively higher mitochondrial volume percent than normal fibers after 1 month of age. Z-line width is positively correlated with mitochondrial volume percent. The results support the hypothesis that progressive changes in muscle fiber properties result from abnormal neural activity (pseudomyotonia) in dystrophic animals.

Properties of muscles from chickens with inherited muscular dystrophy

Inherited muscular dystrophy of the chicken is an abnormality affecting the normal development and function of fast-twitch skeletal muscles. Several different strains of dystrophic chickens have been developed by selection for high lipid content in the pectoralis muscle and early onset of the disorder or by outcrossing the original New Hampshire stock into an inbred White Leghorn breed. The purpose of this study was to determine whether fast-twitch dystrophic muscles differ in expressed properties within the same bird and to examine the differences in gene expression between dystrophic New Hampshire and White Leghorn breeds. The biochemical and physiological properties examined were lactate dehydrogenase and acetylcholinesterase activities, total lipid content, muscle fiber diameter and electromyographic insertion activity. Results showed that fiber diameter and lipid levels were different in muscles within individual birds of two dystrophic lines and that the dystrophic gene causes rapid fiber atrophy and high lipid content in the White Leghorn breed. In addition, differences in lactate dehydrogenase activity and electromyographic patterns were found between two dystrophic lines. The results suggest that the expressed properties differ within each muscle of the dystrophic bird and that the expression of the dystrophic genes is dependent upon the nature of the genetic background of the breed.

Electric membrane properties of adult mouse DRG neurons and the effect of culture duration

The electrical membrane properties (EMP) of adult mouse dorsal root ganglion (DRG) neurons were characterized by an extensive electrophysiological investigation of 450 cells. The neurons were divided into two types: an M-type having an action potential with monophasic falling phase and a B-type with a more complex biphasic or triphasic falling phase. Compared to M-type, B-type were "slow" neurons with a higher specific membrane resistance (Rm), and a longer time constant (tau), duration of action potential (delta t), and absolute refractory period (ARP). B-type also had a larger amplitude action potential, afterhyperpolarization and positive overshoot. The action potential of the M-type neuron had only a Na+ component while that of the B-type had both a Na+ and a Ca2+ component. After two days in culture, M-type neurons exhibited phase bright cytoplasmic granules, which were seldom observed for B-type neurons. Although neuron survival remained constant during the first six days in culture (DIV), the relative frequency of occurrence of the M-type decreased from 82 to 50%. Thereafter, it decreased more gradually to a final value of approximately 20% after 40 DIV. It was concluded that at least during the first 6 DIV and possibly through to 40 DIV, M-type neurons transformed into B-type. Both M- and B-type neurons showed significant and similar changes in their EMP with increasing DIV (up to 40 DIV). For M- and B-types combined, Rm increased approximately 142%, tau by 204%, and no significant change in specific membrane capacitance was observed. Rheobasic threshold depolarization decreased 58%, while the resting membrane potential decreased by only 19%. These changes in the EMP of adult neurons are strikingly similar to changes in EMP observed in adult denervated muscle and in cultures of either embryonic nerve or muscle. This similarity suggested that the adult DRG neurons in cell culture undergo progressive dedifferentiation because of isolation from their usual trophic interactions. Determination of neuronal membrane electrical characteristics provides a new method for evaluating the effects of various possible trophic agents, e.g., hormones and tissue extracts, on the state of differentiation of neurons in cell culture.