Satellite cells and myonuclei in long-term denervated rat muscles

The effects of heavy resistance training and detraining on satellite cells in human skeletal muscles

The aim of this study was to investigate the modulation of satellite cell content and myonuclear number following 30 and 90 days of resistance training and 3, 10, 30, 60 and 90 days of detraining. Muscle biopsies were obtained from the vastus lateralis of 15 young men (mean age: 24 years; range: 20-32 years). Satellite cells and myonuclei were studied on muscle cross-sections stained with a monoclonal antibody against CD56 and counterstained with Mayer's haematoxylin. Cell cycle markers CyclinD1 and p21 mRNA levels were determined by Northern blotting. Satellite cell content increased by 19% (P= 0.02) at 30 days and by 31% (P= 0.0003) at 90 days of training. Compared to pre-training values, the number of satellite cells remained significantly elevated at 3, 10 and 60 days but not at 90 days of detraining. The two cell cycle markers CyclinD1 and p21 mRNA significantly increased at 30 days of training. At 90 days of training, p21 was still elevated whereas CyclinD1 returned to pre-training values. In the detraining period, p21 and CyclinD1 levels were similar to the pre-training values. There were no significant alterations in the number of myonuclei following the training and the detraining periods. The fibre area controlled by each myonucleus gradually increased throughout the training period and returned to pre-training values during detraining. In conclusion, these results demonstrate the high plasticity of satellite cells in response to training and detraining stimuli and clearly show that moderate changes in the size of skeletal muscle fibres can be achieved without the addition of new myonuclei.

Continuous myofiber remodeling in uninjured extraocular myofibers: myonuclear turnover and evidence for apoptosis

Unlike normal mature limb skeletal muscles, in which satellite cells are quiescent unless the muscle is injured, satellite cells in mammalian adult extraocular muscles (EOM) are chronically activated. This is evidenced by hepatocyte growth factor, the myogenic regulatory factor, Pax-7, and the cell-cycle marker, Ki-67, localized to the satellite cell position using serial sections and the positional markers laminin and dystrophin. Bromodeoxyuridine (brdU) labeling combined with dystrophin immunostaining showed brdU-positive myonuclei, presumably the result of fusion of activated satellite cells into existing myofibers. One new myonucleus was added to every 1000 myofibers in cross-section using a 12-hour brdU-labeling paradigm. The EOM thus appear to retain a stable nuclear population by an opposing process of apoptosis that results in myonuclear removal as visualized by terminal deoxynucleotidyltransferase-mediated nick end labeling (TUNEL). Activated caspase-3 was present in localized cytoplasmic domains extending from 10 to 210 microm within individual myofibers, suggesting segmental cytoplasmic reorganization. Understanding the cellular mechanisms that maintain this process of continuous myonuclear addition and removal in normal adult EOM may suggest new hypotheses to explain the preferential involvement or sparing of these muscles in skeletal muscle disease.

Long-term morphometric and immunohistochemical findings in human free microvascular muscle flaps

Reinnervation, muscle regeneration, density of microvessels, and muscle-type specific atrophy were studied 3-4 years after surgery in surgically nonreinnervated free microvascular muscle flaps to 13 patients transplanted to the upper or lower extremities. Routine histology and immunohistochemistry for PGP 9.5 and S-100 (neuronal markers), Ki-67 (cell proliferation), myosin (muscle fiber types), and CD-31 (endothelium) were carried out, and results were analyzed morphometrically. Three to 4 years after surgery, severe atrophy of predominantly slow-type fibers was seen in 9 cases. In 4 cases, muscle-fiber diameter and fiber-type distribution were close to normal. Long intraoperative muscle ischemia and postoperative immobilization were associated with poor muscle bulk in flaps. The density of microvessels in flaps did not differ from control muscles. PGP 9.5 and S-100 immunopositive nerve fibers were detected in 7 patients. Reinnervation was associated with good muscle bulk. In 4 patients, activation of satellite cells was evident. The results suggest that in some cases, spontaneous reinnervation may occur in free muscle flaps, and that several years after microvascular free flap transfer, the muscle still attempts to regenerate.

Ubiquitin expression is up-regulated in human and rat skeletal muscles during aging

In this study, we have used two-dimensional electrophoresis, protein sequencing, immunoblotting, and immunohistochemistry to identify proteins that were differentially expressed during aging in human and rat skeletal muscles. Ubiquitin was identified. It was expressed at high levels in old fast-twitch muscles but at low levels in young fast-twitch muscles. It was also discovered that exogenous ubiquitin could suppress the growth of C2C12 cells, in vitro. The reduction in C2C12 cell growth was not attributed to an increase in apoptosis but to an inhibition in cell cycle entry. Furthermore, it was possible to induce muscles to degenerate in vivo by injecting a high dose of exogenous ubiquitin into young healthy skeletal muscles. These results suggest that hyperactivity of the ubiquitin-proteasome pathway is involved in the aging process of fast-twitch muscles. In addition, ubiquitin-dependent growth suppression in satellite cells may be associated with the poor healing potential of old skeletal muscles.

mRNA expression of fibroblast growth factors and hepatocyte growth factor in rat plantaris muscle following denervation and compensatory overload

We addressed the question of whether hypertrophy induced by compensatory overload differs according to innervation status, and how fibroblast growth factors (FGF) and hepatocyte growth factor (HGF) mRNAs are expressed in the rat plantaris muscle during overload (OL) and/or denervation. Male Wistar rats were divided into four groups (Normal-Cont, Normal-OL, Denervated-Cont, and Denervated-OL). according to the plantaris denervation and/or overload. Three weeks later, plantaris weight in Denervated-Cont and Denervated-OL was significantly lower than in the Normal-Cont. The muscle weights in the Normal-OL were higher than in the Normal-Cont. The muscle weights in the Denervated-OL were higher than in the Denervated-Cont. Three days after the treatment, FGF-2, FGF-6, FGF-7 and HGF mRNAs in the Normal-OL were significantly higher than those in the Normal-Cont. FGF-2, FGF-6, FGF-7 and HGF mRNAs in the Denervated-OL were also significantly higher after 3 days than those in the Denervated-Cont. After 7 days, FGF-2, FGF-5, FGF-6, FGF-7 and HGF mRNAs were significantly higher in the Normal-OL than those in the Normal-Cont. At 21 days, FGF-1, FGF-6 and HGF mRNA levels were significantly increased. In the Denervated-OL, FGF-2, FGF-7 and HGF mRNAs at 7 days, and FGF-2 mRNA at 21 days were significantly higher than those in the Denervated-Cont. FGF-2 and FGF-6 mRNA levels decreased significantly following denervation; however, FGF-1, FGF-5, FGF-7 and HGF mRNA levels increased and maintained this increase for the 21-days treatment period. Muscle hypertrophy was thus induced by compensatory overload irrespective of innervation status, possibly in association with certain FGFs and HGF. The differential mRNA expression patterns of FGFs and HGF observed following compensatory overload and/or denervation suggest distinct roles for individual FGFs and HGF in muscle hypertrophy and/or atrophy.

Morphometric ultrastructural analysis of satellite cells in denervated rat soleus muscle

Morphometric analysis of the ultrastructural changes of satellite cells (SCs) of rat soleus muscle 7, 14, and 36 days post denervation was performed. Denervation caused decrease of surface area cross-section of SCs and their nuclei, volume fractions of nucleus and Golgi complex elements and ribosomes number. In contrast, the increase of surface area/volume ratio of SCs and nucleus, volume fraction and number of endosome/lysosome-like structures, and number of caveolae-like structures was noticed. Ultrastructural changes of SCs in denervated muscles strongly suggest decline of cell activity accompanied by increased processes of degradation of material of endo-, and/or egzogenous origin.

M-cadherin transcription in satellite cells from normal and denervated muscle

Satellite cells (SC) in adult muscle are quiescent in the G0 phase of the cell cycle. In the present study we determined whether SC after denervation upregulate M-cadherin, an adhesion molecule that is upregulated with differentiation and fusion. We also monitored primary cultures of SC from denervated muscle for expression of the transcription factors of the MyoD family to determine whether SC from denervated muscle can be activated in vitro. Hindlimb muscles of rats were denervated under anesthesia, and rats were killed after 2-28 days. The SC of the denervated limbs were pooled and either assessed for M-cadherin mRNA by using real-time RT-PCR or cultured in vitro. The cultures were processed for RT-PCR or immunofluorescence for expression of the transcription factors of the MyoD family. Hindlimb muscles of M-cadherin knockout mice were denervated under anesthesia, mice were killed after 2-28 days, and cells were stained for beta-galactosidase activity by X-gal histochemistry. In vitro, primary SC cultures from rat muscle denervated for 2-28 days expressed transcripts of myf5, MyoD, myogenin, and MRF4 as SC from normal innervated muscle. In vivo, M-cadherin transcription was not upregulated in SC from denervated rat muscle when compared with normal muscle. Moreover, beta-galactosidase activity was not detected in denervated mouse muscle. The finding that SC do not upregulate M-cadherin after denervation supports the notion that they remain in the G(0) phase of the cell cycle in vivo. However, the cells retain the capacity to pass through the proliferative and differentiative program when robustly stimulated to do so in vitro.

Satellite cells and myonuclei in young and elderly women and men

The overall aim of this study was to assess the effects of aging on the satellite cell population. Muscle biopsies were taken from the tibialis anterior muscle of healthy, moderately active young (age range, 20-32 years; n = 31) and elderly (age range, 70-83 years; n = 27) women and men with comparable physical activity pattern. Satellite cells and myonuclei were visualized using a monoclonal antibody against neural cell adhesion molecule and counterstained with Mayer's hematoxylin. An average of 211 (range, 192-241) muscle fibers were examined for each individual. Compared with the young women and men, the elderly subjects had a significantly lower (P < 0.011) number of satellite cells per muscle fiber but a significantly higher (P < 0.004) number of myonuclei per muscle fiber. The number of satellite cells relative to the total number of nuclei [satellite cells/(myonuclei + satellite cells)] was significantly lower in the elderly than in the young women and men. These results imply that a reduction in the satellite cell population occurs as a result of increasing age in healthy men and women.

Satellite cell proliferation, reinnervation, and revascularization in human free microvascular muscle flaps

BACKGROUND

Satellite cell proliferation, reinnervation, and revascularization were studied in human nonreinnervated free microvascular muscle flaps to characterize mechanisms of muscle regeneration after flap surgery.

MATERIALS AND METHODS

Patient biopsies (n = 19) were taken at operation and five timepoints up to 9 months after operation, and corresponding clinical data were obtained. Immunohistochemistry for Ki-67 was used to detect proliferating satellite cells, CD-31 to identify endothelial cells, and S-100 and PGP 9.5 proteins to detect reinnervation.

RESULTS

Two weeks after operation, the expression of PGP 9.5 and S-100 had virtually disappeared in all larger nerve fibers and half of smaller nerve fibers. By 6 months, however, a strong expression of PGP 9.5 and S-100 had reappeared in larger nerve fibers in three of four flaps, suggesting that reinnervation had taken place. The number of mitotic satellite cells already peaked at 2 weeks, indicating onset of muscle regeneration. The number of intramuscular capillaries first increased but later decreased to lower than original level. Flaps with more muscle volume showed more reinnervation and satellite cell mitotic activity. In cases of a delay occurring in reconstructive surgery, a low level of reinnervation was seen.

CONCLUSION

Three patients of four showed spontaneous muscle reinnervation in microvascular free flaps with satellite cell activation followed by restored morphology. Late reconstruction and obesity lead to poor reinnervation, placing emphasis on timing of surgery and patient selection.

Effects of long-term denervation on skeletal muscle in old rats

We compared the reactions to denervation of limb muscles between young adult and old rats. After denervation for up to 4 months in 24-month-old rats, limb muscles were removed and analyzed for contractile properties, morphology, and levels of several key molecules, including the peptide elongation factors eEF1A-1 and eEF1A-2/S1, myogenin, gamma-subunit of the acetylcholine receptor, and cyclin D3. The principal difference between denervated old and young muscle is a somewhat slower rate of atrophy in denervated older muscle, especially among the type II fibers. Expression levels of certain molecules were higher in old than in young control muscle, but after denervation, levels of these molecules increased to the same absolute values in both young and old rats. Although many aspects of postdenervation reactions do not differ greatly between young and old animals, the lesser degree of atrophy in the old rats may reflect significant age-based mechanisms.

Skeletal muscle denervation increases satellite cell susceptibility to apoptosis

Peripheral motor nerve trauma severely compromises skeletal muscle contractile function. Satellite cells respond to denervation by dividing multiple times, ultimately fusing with other satellite cells or myocytes to form new muscle fibers. After chronic denervation, satellite cell numbers decline dramatically, impairing the ability to regenerate and repair myofibers. This satellite cell depletion may contribute to the mechanical deficit observed in denervated or reinnervated muscle. Apoptosis, an evolutionarily conserved form of cell suicide, is a potential mechanism for satellite cell depletion in denervated skeletal muscle. This work tested the hypothesis that skeletal muscle denervation increases satellite cell susceptibility to apoptotic cell death. Adult rats underwent sciatic nerve transection to denervate the distal hindlimb musculature; rats of similar age without the operation served as controls. Two, 6, 10, or 20 weeks after denervation (n = 6 each group), the gastrocnemius and soleus were excised, enzymatically digested, and plated for satellite cell culture. After reaching 95 percent confluence, satellite cells were treated for 24 hours with tumor necrosis factor-alpha (20 ng/ml) and actinomycin D (250 ng/ml), known pro-apoptotic agents. Immunostaining for activated caspases, terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling (TUNEL), and hematoxylin and eosin staining were performed to identify apoptotic satellite cells. Percentages of apoptotic cells were quantified histomorphometrically. In addition, the presence or absence of bcl-2 and bax was determined by Western blot analysis of control, 6 weeks of denervation, and 10 weeks of denervation specimens. At 6 and 10 weeks after nerve transection, TUNEL and caspase activity were increased more than two-fold in satellite cells isolated from denervated muscle compared with those isolated from control muscle (p < 0.05). In all experimental groups, retention of adherence to the collagen-coated substrate was strongly associated with satellite cell survival. Western blot analysis revealed that adherent satellite cells from all groups expressed both bcl-2 and bax. These data support the authors' hypothesis that skeletal muscle denervation increases satellite cell susceptibility to apoptotic cell death. Apoptosis may play a causative role in the depletion of satellite cells in long-term denervated skeletal muscle.

Reparative myogenesis in long-term denervated skeletal muscles of adult rats results in a reduction of the satellite cell population

This study, conducted on 25-month denervated rat hindlimb muscles, was directed toward elucidating the basis for the poor regeneration that is observed in long-term denervated muscles. Despite a approximately 97.6% loss in mean cross-sectional area of muscle fibers, the muscles retained their fascicular arrangement, with the fascicles containing approximately 1.5 times more fibers than age-matched control muscles. At least three distinct types of muscle fibers were observed: degenerating, persisting (original), and newly formed (regenerated) fibers. A majority of newly formed fibers did not appear to undergo complete maturation, and morphologically they resembled myotubes. Sites of former motor end-plates remained identifiable in persisting muscle fibers. Nuclear death was seen in all types of muscle fibers, especially in degenerating fibers. Nevertheless, the severely atrophic skeletal muscles continued to express developmentally and functionally important proteins, such as MyoD, myogenin, adult and embryonic subunits of the nicotinic acetylcholine receptor, and neural-cell adhesion molecule. Despite the prolonged period of denervation, slow and fast types of myosin were found in surviving muscle fibers. The number of satellite cells was significantly reduced in long-term denervated muscles, as compared with age-matched control muscles. In 25-month denervated muscle, satellite cells were only attached to persisting muscle fibers, but were never seen on newly formed fibers. Our data suggest that the absence of satellite cells in a population of immature newly formed muscle fibers that has arisen as a result of continuous reparative myogenesis may be a crucial, although not necessarily the only, factor underlying the poor regenerative ability of long-term denervated muscle.

Myogenic satellite cells: physiology to molecular biology

Adult skeletal muscle has a remarkable ability to regenerate following myotrauma. Because adult myofibers are terminally differentiated, the regeneration of skeletal muscle is largely dependent on a small population of resident cells termed satellite cells. Although this population of cells was identified 40 years ago, little is known regarding the molecular phenotype or regulation of the satellite cell. The use of cell culture techniques and transgenic animal models has improved our understanding of this unique cell population; however, the capacity and potential of these cells remain ill-defined. This review will highlight the origin and unique markers of the satellite cell population, the regulation by growth factors, and the response to physiological and pathological stimuli. We conclude by highlighting the potential therapeutic uses of satellite cells and identifying future research goals for the study of satellite cell biology.

Analysis for transcript expression of meltrin alpha in normal, regenerating, and denervated rat muscle

Meltrin alpha (a disintegrin and metalloprotease (ADAM) 12) is a recently discovered molecule of the metalloprotease-disintegrin family which has been shown to participate in myotube formation in vitro and in myogenesis in vivo. In this study we investigated meltrin alpha in regenerating rat muscle, which is a condition where satellite cells (SC) contribute to myofiber growth by fusing with one another and with myotubes or muscle fibers. We studied meltrin alpha mRNA expression by RT-PCR and in situ-hybridization in normal adult muscle, in soleus muscle regenerating for 2, 5, or 10 days, and in muscle which had been denervated 1 week, 4 weeks, or 6 months previously. SC do not fuse after denervation. They detach from the principal muscle fiber. Immunohistochemistry using an antibody against M-cadherin was performed in parallel in order to identify SC. Messenger RNA as revealed by RT-PCR was absent in normal adult muscle, but present in regenerating and also in denervated muscle. Meltrin alpha transcript detected by in situ-hybridization was present in regenerating muscle only, not in normal or denervated muscle. It was localized to SC. Taken together, meltrin alpha is absent in normal muscle, and localized to SC in fusing conditions. After denervation, the transcript is upregulated. However, it is so lowly abundant that it fails to be detected by in situ-hybridization. This expression profile suggests a role for meltrin alpha in the fusion of SC with myotubes or muscle fibers, but not in SC adhesion to the adjacent myofiber in normal adult muscle.

Parvalbumin Expression Is Downregulated in Rat Fast-Twitch Skeletal Muscles during Aging

In this study, the protein expression profile of extensor digitorum longous (EDL) and Soleus (SOL) muscles, representing fast- and slow-twitch skeletal muscles, respectively, was established using high resolution two-dimensional electrophoresis (2-DE). One protein spot was found uniquely expressed in EDL muscle. N-terminal sequence analysis identified the protein as parvalbumin. Parvalbumin is a high affinity calcium binding protein that regulates muscle contraction and relaxation. Our experiments revealed that parvalbumin expression in EDL muscle was down-regulated during aging. In addition, high-intensity exercise could reverse this age-related change. Soleus muscles do not normally express parvalbumin, but high-intensity exercise could ectopically induce its expression in both young and old SOL muscles. We have also confirmed our 2-DE findings by immunohistochemistry on muscle sections. Our results suggest that high-intensity training could be used to improve muscle functions during aging because parvalbumin play an important role in regulating skeletal muscle contraction and relaxation.

Apoptosis in atrophic skeletal muscle induced by brachial plexus injury in rats

BACKGROUND

Skeletal muscle atrophy induced by denervation is associated with apoptosis. This study was undertaken to determine the role of apoptosis and the expression of apoptosis-associated genes in rat skeletal muscle made atrophic by brachial plexus injury, and to study the apoptotic signal transduction pathway.

METHODS

An animal model of skeletal muscle atrophy was established in rats by severing the brachial plexus of one forelimb. Apoptosis of muscle cells was investigated with terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate end labeling, flow cytometry, deoxyribonucleic acid (DNA) electrophoresis, and electron microscopy. The apoptosis-associated genes Fas, FADD, Caspase 8, c-myc, p53, and Bcl-2 were detected by immunohistochemistry and Northern blot.

RESULTS

By terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate end labeling and flow cytometry we found that the percentage of apoptotic muscle cells was higher in atrophic than in healthy skeletal muscle (p < 0.05). DNA laddering could be seen in gel electrophoresis of DNA from atrophic muscle. By electron microscopy, we observed morphologic change of early apoptosis, such as aggregation of chromosomes, expansion of nucleic cistern, and contraction of the nucleus. Using immunohistochemistry, we determined that in atrophic muscle Fas, FADD, and Caspase-8 genes were highly expressed, whereas Bcl-2 was poorly expressed (p < 0.01). However, we did not detect a change in the expression of p53 or c-myc genes. Northern blots indicated that Fas messenger ribonucleic acid was higher and Bcl-2 messenger ribonucleic acid was lower in atrophic than in healthy muscle (p < 0.01).

CONCLUSION

There are many more apoptotic cells in muscle atrophied as a result of brachial plexus injury than in healthy muscle, and apoptosis plays an important role in the pathogenesis of atrophy. The apoptotic signal may be transmitted from Fas to FADD to Caspase-8, with a decrease in Bcl-2 expression aggravating the process.

Nerve terminals form but fail to mature when postsynaptic differentiation is blocked: in vivo analysis using mammalian nerve-muscle chimeras

To better understand the role of the postsynaptic cell in the differentiation of presynaptic terminals, we transplanted muscles that lacked postsynaptic differentiation from mutant mice into normal adult immunocompatible hosts and attached the host nerve to the grafts. Host motor axons innervated wild-type grafted muscle fibers and established normal appearing chimeric neuromuscular junctions. By repeated in vivo imaging, we found that these synapses were stably maintained. Results were different when nerves entered transplanted muscles derived from mice lacking muscle-specific receptor tyrosine kinase (MuSK) or rapsyn, muscle-specific components required for postsynaptic differentiation. Initial steps in presynaptic differentiation (e.g., formation of rudimentary arbors and vesicle clustering at terminals) occurred when wild-type neurites contacted MuSK- or rapsyn deficient muscle fibers, either in vivo or in vitro. However, wild-type terminals contacting MuSK or rapsyn mutant muscle fibers were unable to mature, even when the chimeras were maintained for up to 7 months. Moreover, in contrast to the stability of wild-type synapses, wild-type nerve terminals in mutant muscles underwent continuous remodeling. These results suggest that postsynaptic cells supply two types of signals to motor axons: ones that initiate presynaptic differentiation and others that stabilize the immature contacts so that they can mature. Normal postsynaptic differentiation appears to be dispensable for initial stages of presynaptic differentiation but required for presynaptic maturation.

Regulation of myogenin protein expression in denervated muscles from young and old rats

Myogenin is a muscle-specific transcription factor participating in denervation-induced increases in nicotinic ACh receptor (nAChR) gene expression. Although myogenin RNA expression in denervated muscle is well documented, surprisingly little is known about myogenin protein expression. Therefore, we assayed myogenin protein and RNA in innervated and denervated muscles from young (4 mo) and old (24-32 mo) rats and compared this expression to that of the nAChR alpha-subunit RNA. These assays revealed increased myogenin protein expression within 1 day of denervation, preceding detectable increases in nAChR RNA. By 3 days of denervation, myogenin and nAChR alpha-subunit RNA were increased 500- and 130-fold, respectively, whereas myogenin protein increased 14-fold. Interestingly, old rats (32 mo) had 6-fold higher myogenin protein and approximately 80-fold higher mRNA levels than young rats. However, after denervation, expression levels were similar for young and old animals. The increased myogenin expression during aging, which tends to localize to small fibers, likely reflects spontaneous denervation and/or regeneration. Our results show that increased myogenin protein in denervated muscles correlates with the upregulation of its mRNA.

Dynamics of nuclei of muscle fibers and connective tissue cells in normal and denervated rat muscles

The mitotic activity in muscles of growing rats and the effect of denervation were studied by means of continuous infusion of 5-bromo-2-deoxyuridine (BRDU). Denervated muscles after 10 weeks contained 20 to 60% fewer muscle nuclei than normal; BRDU labeled about 25% of the nuclei of normal soleus and extensor digitorum longus (EDL) and of denervated EDL muscles but only 5% in the denervated soleus muscle. Labeled nuclei persisted in denervated but not in normal muscles. After the main growth period, the turnover of myonuclei was at most 1 to 2% per week. The behavior of connective tissue nuclei was similar to that in muscle fibers. Infusion of BRDU had no effect on contractile properties. It is suggested that the exceptionally rapid atrophy of the denervated rat soleus associated with loss of satellite cells was due to loss of myonuclei and differentiation and fusion of satellite cells. The cause may possibly be that the phase of postdenervation fibrillation is shorter than in other muscles.

Diagnostic protein expression in human muscle biopsies

Using immunohistochemistry in diagnosing neuromuscular diseases is meant to enhance the diagnostic yield in two ways. The first application aims at visualizing molecules which are developmentally, neurally, and/or immunologically regulated and not expressed by normal muscle. They are upregulated in pathological conditions and may help assign a given muscular biopsy to one of the main diagnostic entities (muscular dystrophies, inflammatory myopathy, neurogenic atrophy). In the past, muscle-specific molecules with a defined expression pattern during fetal myogenesis served as antigens, with the rationale that the developmental program was switched on in new fibers. Recently, myofibers in diseased muscle are thought of as targets of stimuli which are released by macrophages in muscular dystrophy, by lymphocytes in inflammatory myopathies, or by a lesioned peripheral nerve in neurogenic atrophies. This has somewhat blurred the borders between the diagnostic groups, for certain molecules, e.g. cytokines, may be upregulated after experimental necrotization, denervation, and also in inflammatory myopathies. In the second part of this review we summarise the experiences of a Centre in the North of England that specialises in the diagnosis and clinical support of patients with muscular dystrophy. Emphasis is placed on the use of protein expression to guide mutation analysis, particularly in the limb-girdle muscular dystrophies (a group of diseases that are very difficult to differentiate on clinical grounds alone). We confirm that genetic analysis is essential to corroborate the results of protein analysis in certain conditions (particularly in calpainopathy). However, we conclude that analysing biopsies for abnormal protein expression is very useful in aiding the decision between alternative diagnoses.

Cell death in denervated skeletal muscle is distinct from classical apoptosis

Denervation of skeletal muscle is followed by the progressive loss of tissue mass and impairment of its functional properties. The purpose of the present study was to investigate the occurrence of cell death and its mechanism in rat skeletal muscle undergoing post-denervation atrophy. We studied the expression of specific markers of apoptosis and necrosis in experimentally denervated tibialis anterior, extensor digitorum longus and soleus muscles of adult rats. Fluorescent staining of nuclear DNA with propidium iodide revealed the presence of nuclei with hypercondensed chromatin and fragmented nuclei typical of apoptotic cells in the muscle tissue 2, 4 and to a lesser extent 7 months after denervation. This finding was supported by electron microscopy of the denervated muscle. We found clear morphological manifestations of muscle cell death, with ultrastructural characteristics very similar if not identical to those considered as nuclear and cytoplasmic markers of apoptosis. With increasing time of denervation, progressive destabilization of the differentiated phenotype of muscle cells was observed. It included disalignment and spatial disorganization of myofibrils as well as their resorption and formation of myofibril-free zones. These changes initially appeared in subsarcolemmal areas around myonuclei, and by 4 months following nerve transection they were spread throughout the sarcoplasm. Despite an increased number of residual bodies and secondary lysosomes in denervated muscle, we did not find any evidence of involvement of autophagocytosis in the resorption of the contractile system. Dead muscle fibers were usually surrounded by a folded intact basal lamina; they had an intact sarcolemma and highly condensed chromatin and sarcoplasm. Folds of the basal lamina around the dead cells resulted from significant shrinkage of cell volume. Macrophages were occasionally found in close proximity to dead myocytes. We detected no manifestations of inflammation in the denervated tissue. Single myocytes expressing traits of the necrotic phenotype were very rare. A search for another marker of apoptosis, nuclear DNA fragmentation, using terminal deoxyribonucleotidyl transferase mediated dUTP nick end labeling (the TUNEL method) in situ, revealed the presence of multiple DNA fragments in cell nuclei in only a very small number of cell nuclei in 2 and 4 month denervated muscle and to less extent in 7 month denervated muscle. Virtually no TUNEL reactivity was found in normal muscle. Double labeling of tissue denervated for 2 and 4 months for genome fragmentation with the TUNEL method and for total nuclear DNA with propidium iodide demonstrated co-localization of the TUNEL-positive fragmented DNA in some of the nuclei containing condensed chromatin and in fragmented nuclei. However, the numbers of nuclei of abnormal morphology containing condensed and/or irregular patterns of chromatin distribution, as revealed by DNA staining and electron microscopy, exceeded by 33-38 times the numbers of nuclei positive for the TUNEL reaction. Thus, we found a discrepancy between the frequences of expression of morphological markers of apoptosis and DNA fragmentation in denervated muscle. This provides evidence that fragmentation of the genomic DNA is not an obligatory event during atrophy and death of muscle cells, or, alternatively, it may occur only for a short period of time during this process. Unlike classical apoptosis described in mammalian thymocytes and lymphoid cells, non-inflammatory death of muscle fibers in denervated muscle occurs a long time after the removal of myotrophic influence of the nerve and is preceded by the progressive imbalance of the state of terminal differentiation. Our results indicate that apoptosis appears to be represented by a number of distinct isotypes in animals belonging to different taxonomic groups and in different cell lineages of the same organism.

Age-related changes of aqueous protein profiles in rat fast and slow twitch skeletal muscles

Two-dimensional electrophoresis was used to generate the aqueous protein expression patterns of rat extensor digitorum longus muscle (EDL, fast twitch muscle) and solues muscle (SOL, slow twitch muscle) of different ages. Two specific protein spots, S1 and S3, were identified from EDL muscles at the ages of 12 and 18 months onward respectively. In the EDL muscles of aged rat (24 months) after intensive exercise training, S3 was still detected while S1 disappeared. In addition, diaphragm muscle (DIA, fast twitch muscle), which retains physically active throughout the life span, was used as nondisuse control. The results showed that the expressions of S1 and S3 in 24-month DIA muscle were identical with the trained aged EDL muscle. It is suggested that exercise might delay the onset of S1 expression. However, the expression of S3 over age seemed to be progressive and exercise independent. Another protein spot, S2 was identified to express only in young EDL and SOL muscles, but its expression decreased over age. Furthermore, exercise has no effect on S2 expression since S2 could not be detected in aged DIA as well as trained aged EDL and SOL muscles. These results indicated that aqueous protein expression patterns of skeletal muscle undergo changes during aging. Some of these changes such as S2 and S3 appear progressively, and some such as S1 could be delayed by exercise. S3 was identified as ubiquitin, which might play an important role in protein degradation during skeletal muscle aging process.

The effect of age on in vitro motility speed of slow myosin extracted from single rat soleus fibres

The effect of age on the motor protein myosin was examined in a novel in vitro motility assay. Myosin was extracted from soleus fibres of young (3-6 month) and old (20-24 month) rats. All fibres expressed the type I myosin heavy chain (MyHC) and the slow isoforms of the myosin light chains (MyLCs). In vitro motility speed was significantly (P < 0.001) faster in the young adult (1.43 +/- 0.23 microm s-1) than in the aged group (1.27 +/- 0.23 microm s-1). The result indicates that the age-related decrease in contractile speed observed in slow fibres may be the effect of a change in the properties of myosin with age.

Satellite cells on isolated myofibers from normal and denervated adult rat muscle

Satellite cells (SCs) in normal adult muscle are quiescent. They can enter the mitotic program when stimulated with growth factors such as basic FGF. Short-term denervation stimulates SC to enter the mitotic cycle in vivo, whereas long-term denervation depletes the SC pool. The molecular basis for the neural influence on SCs has not been established. We studied the phenotype and the proliferative capacity of SCs from muscle that had been denervated before being cultured in vitro. The expression of PCNA, myogenin, and muscle (M)-cadherin in SCs of normal and denervated muscle fibers was examined at the single-cell level by immunolabeling in a culture system of isolated rat muscle fibers with attached SCs. Immediately after plating (Day 0), neither PCNA nor myogenin was present on normal muscle fibers, but we detected an average of 0.5 M-cadherin(+) SCs per muscle fiber. The number of these M-cadherin(+) cells (which are negative for PCNA and myogenin) increased over the time course examined. A larger fraction of cells negative for M-cadherin underwent mitosis and expressed PCNA, followed by myogenin. The kinetics of SCs from muscle fibers denervated for 4 days before culturing were similar to those of normal controls. Denervation from 1 to 32 weeks before plating, however, suppressed PCNA and myogenin expression almost completely. The fraction of M-cadherin(+) (PCNA(-)/myogenin(-)) SCs was decreased after 1 week of denervation, increased above normal after denervation for 4 or 8 weeks, and decreased again after denervation for 16 or 32 weeks. We suggest that the M-cadherin(+) cells are nondividing SCs because they co-express neither PCNA or myogenin, whereas the cells positive for PCNA or myogenin (and negative for M-cadherin) have entered the mitotic cycle. SCs from denervated muscle were different from normal controls when denervated for 1 week or longer. The effect of denervation on the phenotypic modulation of SCs includes resistance to recruitment into the mitotic cycle under the conditions studied here and a robust extension of the nonproliferative compartment. These characteristics of SCs deprived of neural influence may account for the failure of denervated muscle to fully regenerate. (J Histochem Cytochem 47:1375-1383, 1999)

Myonuclear domains in muscle adaptation and disease

Adult skeletal muscle fibers are among the few cell types that are truly multinucleated. Recently, evidence has accumulated supporting a role for the modulation of myonuclear number during muscle remodeling in response to injury, adaptation, and disease. These studies have demonstrated that muscle hypertrophy is associated with, and is dependent on, the addition of newly formed myonuclei via the fusion of myogenic cells to the adult myofiber, whereas muscle atrophy and disease appear to be associated with the loss of myonuclei, possibly through apoptotic-like mechanisms. Moreover, these studies also have demonstrated that myonuclear domain size, i. e., the amount of cytoplasm per myonucleus, is unchanged following the acute phase of hypertrophy but is reduced following atrophy. Together these data demonstrate that modulation of myonuclear number or myonuclear domain size (or both) is a mechanism contributing to the remodeling of adult skeletal muscle in response to alterations in the level of normal neuromuscular activity.

Quantitative study of the effects of denervation and castration on the levator ani muscle of the rat

The levator ani muscle (LA) of the rat is highly androgen-sensitive and, like all skeletal muscles, deteriorates structurally and functionally when denervated. In order to elucidate the interplay of neural and endocrine influences, the separate and combined effects of denervation and castration on myofiber cross-sectional area and nuclear populations were quantitatively studied. In one group of 4-month-old male rats (A), the LA was denervated. Another group (B) was surgically castrated and a third group (C) was both denervated and castrated. The control rats (D) remained both gonad- and nerve-intact. After two months, the LA was obtained for myofiber and nuclear enumeration, cross-sectional area and satellite cell frequency determination. In the denervated muscle of gonad-intact rats (Group A), myofiber cross-sectional area was markedly diminished (265.84+/-11.38 microm2; compared with controls [Group D]: 1519.98+/-79.41 microm2; P < 0.05). Satellite cell nuclei, as a percentage of total sublaminar nuclei (i.e., satellite cell ratio), increased significantly (4.26%, from a control value of 1.91%). Castration alone (Group B) resulted in pronounced myofiber atrophy (mean cross-sectional area: 754.03+/-89.63 microm2) but had no significant effect on satellite cell ratio (2.36%). The combination of castration and denervation (Group C) elicited the same degree of myofiber atrophy as denervation alone (Group A) but had no significant impact on satellite cell ratio. Instead, the nuclear count per myofiber declined to about a third of the control level (300.5+/-38.49 compared with 861.7+/-24.8; P < 0.05). The results indicate that the atrophic effects of denervation and castration on the LA are non-synergistic and mechanistically similar. They also show that the inability of satellite cells to respond mitotically to the withdrawal of neural input under disandrogenized conditions is a factor in the myonuclear depletion of the denervated muscle of castrated rats.

Comparison of the muscle fiber diameter and satellite cell frequency in human muscle biopsies

Satellite cells are responsible for the formation of postnatal muscle fibers. The number, mitotic activity, and differentiation potential of satellite cells and the muscle fiber diameter are tightly regulated events in normal muscle. The signal that induces satellite cells to stop proliferation once the determined muscle fiber size has been reached in normal growth is not known. The aim of the present study was to determine whether a correlation exists between satellite cell frequency and muscle fiber diameter in human muscle disease. Muscle biopsies from 7 cases of Duchenne muscular dystrophy (DMD), 8 other muscular dystrophies, 23 cases of inflammatory myopathy, and 22 cases of neurogenic atrophy were examined. The satellite cell number was elevated in DMD and neurogenic atrophy but not in other muscular dystrophies or inflammatory myopathies. Nevertheless, in all the diseased muscles, but not in normal controls, there was a significantly higher relative frequency of satellite cells with increasing fiber diameter. It has been shown before that satellite cells show ultrastructural and autoradiographic signs of activation and proliferation in myopathic and neurogenic conditions. We assume that we are dealing with activated, not quiescent, satellite cells in diseased muscle and that under these conditions the fiber diameter does not represent a stop signal for satellite cells to proliferate. The data suggest that not only the number of satellite cells matters in diseased muscle, as has been shown before, but that it is their behavior that influences, at least in part, progress and severity of muscle diseases.

Long-term regeneration of fast and slow murine skeletal muscles after induced injury by ACL myotoxin isolated from Agkistrodon contortrix laticinctus (broad-banded copperhead) venom

The aim of the present work was to analyze the regenerated muscle types I and II fibers of the soleus and gastrocnemius muscles of mice, 8 months after damage induced by ACL myotoxin (ACLMT). Animals received 5 mg/kg of ACLMT into the subcutaneous lateral region of the right hind limb, near the Achilles tendon; contralateral muscles received saline. Longitudinal and cross sections (10 microm) of frozen muscle tissue were evaluated. Eight months after ACLMT injection, both muscle types I and II fibers of soleus and gastrocnemius muscles still showed centralized nuclei and small regenerated fibers. Compared with the left muscle, the incidence of type I fibers increased in the right muscle (21% +/- 03% versus 12% +/- 06%, P = 0.009), whereas type II fibers decreased (78% +/- 02% versus 88% +/- 06%, P = 0.01). The incidence of type IIC fibers was normal. These results confirm that ACLMT induced muscle type fiber transformation from type II to type I, through type IIC. The area analysis of types I and II fibers of the gastrocnemius revealed that injured right muscles have a higher percentage of small fibers in both types I and II fibers (0-1,500 microm2) than left muscles, which have larger normal type I and II fibers (1,500-3,500 microm2). These results indicate that ACLMT can be used as an excellent model to study the rearrangement of motor units and the transformation of muscle fiber types during regeneration.

Dihydropyridine receptor and ryanodine receptor gene expression in long-term denervated rat muscles

Following disruption of the nerve supply, extensor digitorum longus (EDL) and soleus (SOL) muscles in rats are known to exhibit alterations in excitation-contraction coupling. After total RNA isolation from the denervated and the contralateral control muscles performed at 25 and 50 days following denervation, RNase protection assays were carried out with four cDNA probes specific for the skeletal and cardiac isoforms of both the DHPR alpha 1-subunit and the RyR. Longterm denervation increased the expression of the mRNA for skeletal DHPR and skeletal RyR in SOL muscle, but it also significantly increased the expression of the mRNA for the cardiac isoform of the DHPR alpha 1 subunit in EDL muscle.

Effects of long-term phasic electrical stimulation on denervated soleus muscle: guinea-pig contrasted with rat

Guinea-pig soleus muscles were denervated and electrically stimulated for periods of 43 to 66 days. Stimuli were in 1 s bursts of 40 Hz pulses, repeated every 5 min. Other guinea-pigs were denervated for 82 days without stimulation and, in a third group, the soleus muscle was necrotized and allowed to regenerate without reinnervation for 13-15 days. Isometric and isotonic recordings were made in vivo. Denervated guinea-pig muscles were embedded in epoxy resin for light and electron microscopy. Chronic stimulation of denervated guinea-pig soleus had no effects on the prolonged twitch or on reduced maximal shortening velocity, maximal rate of rise of tension and tetanic force. This contrasts with the slow-to-fast conversion produced by denervation and denervation-stimulation of rat soleus. Loss of force was much greater in rat than guinea-pig after denervation, and chronic stimulation increased force in rat to the same level as in guinea-pig after denervation (with or without stimulation). Eighty-day denervated guinea-pig soleus did not reveal those morphological signs of fibre breakdown and regeneration which are prominent in denervated rat soleus muscles. Those changes in rat resembled aneurally regenerated muscles in several aspects, especially the increased incidence of fibres with internal myo-nuclei which did not appear in guinea-pig soleus after denervation. Aneurally regenerated guinea-pig soleus became fast like aneurally regenerated rat muscle. Our data are compatible with the hypothesis that slow-to-fast transformation of denervated rat soleus is not directly brought about by chronic stimulation but by de-novo formation of fast-contracting regenerated fibres. The persistence of fibrillation in guinea-pig but not rat after denervation may account for the species difference.

The effect of duration of muscle denervation on functional recovery in the rat model

The effect of long-term denervation on neuromuscular recovery was studied in a rat hind limb model. The posterior tibial nerve was transected and repaired immediately or after denervation periods of 2 weeks, or 1, 3, 6, 9, or 12 months. Six months following reconstruction excellent axonal regeneration was seen across all nerve repairs irrespective of periods of denervation. However, there was a precipitous and profound decrease in the recovery of both muscle mass and integrated motor function if the reconstruction was delayed for longer than 1 month. Rather than a progressive change proportional to the length of the denervation period, significant, more discrete changes occurred sometime after 1 month of denervation that precluded a full recovery of muscle mass. Integrated motor function quantified using walking track analysis was impaired even after immediate nerve repair.

Electron microscopic study of long-term denervated rat skeletal muscle

BACKGROUND

This study describes the ultrastructure of long-term denervated rat extensor digitorum longus and tibialis anterior muscles, with particular emphasis on understanding the cellular basis for the reduced restorative capacity of long-term denervated muscle.

METHODS

In 30 male WI/HicksCar rats, the right hindleg was denervated for periods of 1, 2, 4, 5.5, 6, 7, 12, 14, and 18 months before tissues were prepared for electron microscopy.

RESULTS

Atrophy of muscle fibers was prominent by the second month post-denervation. At this time, type II fibers showed greater atrophy than type I fibers. At further periods of denervation, atrophy of all fibers was seen; and with increasing times of denervation the muscle fibers became surrounded by dense mats of collagen fibers. Muscle spindles persisted for the duration of this study. At two and four months, satellite cells showed signs of activation, such as elongated cytoplasmic processes and an increased concentration of cytoplasmic organelles. As denervation progressed, activated satellite cells became more widely separated from their associated muscle fibers, and basal lamina material was deposited between the satellite cells and muscle fibers. Some satellite cells broke free from their muscle fibers, and others acted as bridges between two muscle fibers. Evidence was seen of both muscle fiber degeneration and the regeneration of new muscle fibers, often more than one regenerating fiber beneath a single basal lamina. Loose folds of basal lamina were often present around atrophic muscle fibers. As denervation progressed, the morphology of individual muscle fibers varied. Some contained well-ordered lattice arrays of myofilaments, whereas in others considerable sarcomeric disorganization was evident. Mitochondria became smaller and rounded; elements of the sarcoplasmic reticulum proliferated and became more disorganized; lipid droplets, glycogen deposits, and autophagic vesicles were all present in the cytoplasm of atrophic muscle fibers.

CONCLUSIONS

In addition to muscle fiber atrophy, long-term denervated muscles show evidence of myofiber and capillary death, as well as the deposition of massive amounts of interstitial collagen. These changes, all of which would appear to reduce the restorative capacity of the denervated muscle, take place concurrently with the morphological activation of satellite cells. The latter indicates that even in the denervated condition, restorative processes occur concurrently with regressive processes.

Quantitative study of the effects of long-term denervation on the extensor digitorum longus muscle of the rat

BACKGROUND

In order to understand the cellular basis underlying the progressively poorer restorative capacity of long-term denervated muscle, we determined the effects of long-term denervation on the muscle fibers and satellite cell population of the rat extensor digitorum longus (EDL) muscle.

METHODS

In 36 male rats, the right hind legs were denervated, and EDL muscles were removed 2, 4, 7, 12, and 18 months later. Muscles were either fixed for electron microscopic analysis or were dissociated into individual muscle fibers for direct fiber counting or for confocal microscopic analysis.

RESULTS

The percentage of satellite cells rose from the 2.8% control value to 9.1% at 2 months of denervation; thereafter the percentage decreased to 1.1% at 18 months of denervation. The number of myonuclei per muscle fiber steadily declined from 410 in 4 month control muscle to 158 in 7 month denervated muscle. Up to 7 months of denervation, the total number of muscle fibers per muscle remained relatively constant at somewhat over 5,000. The calculated total satellite cell population in 4 month denervated EDL muscle was the same as that of controls at 65,000, but by 7 months of denervation it had declined to 21,000. With increasing time of denervation, the number of cross-sectional profiles of muscle fibers not containing nuclei rose from 14% in control muscle to 49% in 12 month denervated muscle. This was correlated with a pronounced regular clumping of the nuclei, with pronounced nonnucleated segments between nuclear clumps.

CONCLUSIONS

Increasing times of denervation are accompanied by a pronounced decline in the number of myonuclei per muscle fiber and an initial rise and subsequent fall in satellite cell number. These changes are correlated with a decreasing restorative ability of these muscles over the same periods of denervation. Further work on the proliferative capacity of the remaining satellite cells is necessary before firm quantitative conclusions can be made.

Myonuclear loss in atrophied soleus muscle fibers

BACKGROUND

A skeletal muscle fiber consists of many successive "territories," each controlled by the nucleus residing in that territory. Because nuclei appear to control a specific amount of territory (nuclear domain), nuclei must be added to accommodate an increase in fiber size. Because growth and hypertrophy require the addition of nuclei to fibers, it is of interest to determine whether atrophy causes a decrease in myonuclear number. This study compared the myonuclear population in the soleus muscles of rats that had undergone atrophy due to 10 days of spaceflight in the space shuttle, Endeavour, with muscles of ground-based control animals (10 rats each).

METHODS

Myofibrillar ATPase activity was used to determine the major skeletal muscle fiber types in control rats and those having spent 10 days in space, and dystrophin antibodies were used to label the sarcolemma to identify underlying myonuclei.

RESULTS

Type I and II fibers were atrophied after the flight, but type I fibers were atrophied twice as much as type II. Myonuclei were counted in identified and measured fibers, and the distribution normalized to number per millimeter of fiber circumference; this was significantly greater in type II than in type I fibers in both groups of rats. However, although the muscle fibers from flight animals were significantly atrophied, the normalized number of nuclei were identical between control and flight animals, indicating that nuclei decreased in numbers as the fibers atrophied.

CONCLUSION

The nuclear domain is under strict control, and a decrease in the domain, as induced by atrophy, will cause nuclear degeneration and loss, which maintains a relatively constant size of the nuclear domain.