Towards understanding skeletal muscle regeneration

Production of consistent crush lesions in murine quadriceps muscle--a biomechanical, histomorphological and immunohistochemical study

Poor healing of high-energy fractures is often associated with severe muscle damage. This may be partly due to the production, by the injured muscle, of inflammatory cytokines that somehow misdirect bone healing. In order to investigate this question, an animal model was established which embodies a controlled degree of muscle injury with a dose response to the energy absorbed, that can be characterised histologically. Using a custom crush jig, 60 CFLP mice had either 100 or 200 g masses dropped from a fixed height onto the quadriceps muscle, with mechanical measurement of the impact. Energy of impact was reliably and significantly different between the small and large impact conditions, though there was more variability when the large mass was used. Animals were sacrificed at day 2, 4, 8, 16, and 24 post-injury. Muscle histomorphometry at all time points and immunohistochemistry for IL-1beta, IL-6, and TNF-alpha up to day 8 were used as measures of muscle damage, inflammation and repair. Histological sections were analysed into areas of normal muscle fibres, damaged/regenerating muscle fibres and fibrous/inflammatory infiltrate. Early histological response was similar between the two groups; the large crush group displayed significantly greater areas of inflammatory infiltrate and damaged muscle at the later time points after day 8. In the large crush group, IL-1beta and IL-6 expression were significantly higher at day 2 and TNF-alpha was higher at day 8 when compared to the small crush group. The experiment demonstrated that more severe injury to muscle was reliably followed by increased inflammatory cytokine production and a greater degree of inflammation and fibrosis. Increased production of inflammatory cytokines such as TNF-alpha and IL-1beta in the damaged muscles may activate macrophages and recruit fibroblasts, promote scar formation and lead to delayed union or non-union of the adjacent fracture(s).

Gender dimorphism influences extracellular matrix expression and regeneration of muscular tissue in mdx dystrophic mice

Mdx mouse, the animal model of Duchenne muscular dystrophy, lacks dystrophin and develops an X-linked recessive inflammatory myopathy characterized by degeneration of skeletal muscle fibers and connective tissue replacement. The present work aimed to assess whether gender dimorphism in mdx mice would influence skeletal muscle pathology at ages corresponding to main histological changes in the microenvironment of muscular tissue: myonecrosis, regeneration, and fibrosis. At the height of myonecrosis (6 weeks postnatal), skeletal muscles of male mdx mice showed increased sarcolemmal permeability, numerous inflammatory foci, and marked deposition of the extracellular matrix components (ECM) type I collagen and laminin. In contrast, age-matched mdx females showed mild ECM deposition, discrete myonecrosis, but increased numbers of regenerating fibers expressing the satellite cell marker NCAM. In contrast ovariectomized mdx females showed decreased numbers of regenerating fibers. Older (24 and 48 weeks postnatal) mdx females showed extensive fibrosis with increased sarcolemmal permeability and marked deposition of ECM components than corresponding males. These results suggest a role for female hormones in the control of myonecrosis probably by promoting regeneration of muscular tissue and mitigating inflammation especially at ages under the critical influence of sex hormones.

Purification and cell-surface marker characterization of quiescent satellite cells from murine skeletal muscle by a novel monoclonal antibody

A novel monoclonal antibody, SM/C-2.6, specific for mouse muscle satellite cells was established. SM/C-2.6 detects mononucleated cells beneath the basal lamina of skeletal muscle, and the cells co-express M-cadherin. Single fiber analyses revealed that M-cadherin+ mononucleated cells attaching to muscle fibers are stained with SM/C-2.6. SM/C-2.6+ cells, which were freshly purified by FACS from mouse skeletal muscle, became MyoD+ in vitro in proliferating medium, and the cells differentiated into desmin+ and nuclear-MyoD+ myofibers in vitro when placed under differentiation conditions. When the sorted cells were injected into mdx mouse muscles, donor cells differentiated into muscle fibers. Flow cytometric analyses of SM/C-2.6+ cells showed that the quiescent satellite cells were c-kit-, Sca-1-, CD34+, and CD45-. More, SM/C-2.6+ cells were barely included in the side population but in the main population of cells in Hoechst dye efflux assay. These results suggest that SM/C-2.6 identifies and enriches quiescent satellite cells from adult mouse muscle, and that the antibody will be useful as a powerful tool for the characterization of cellular and molecular mechanisms of satellite cell activation and proliferation.

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.

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.

Timing of Human Insulin-Like Growth Factor-1 Gene Transfer in Reinnervating Laryngeal Muscle

OBJECTIVES/HYPOTHESIS

The authors have designed a rat laryngeal paralysis model to study gene transfer strategies using a muscle-specific expression system to enhance local delivery of human insulin-like growth factor-1 (hIGF-1). In preliminary studies, a nonviral vector containing the alpha-actin promoter and human hIGF-1 sequence produced both neurotrophic and myotrophic effects 1 month after single injection of plasmid formulation into paralyzed rat thyroarytenoid muscle in vivo. Based on these findings, it is hypothesized that the effects of hIGF-1 will enhance the results of laryngeal muscle innervation procedures. The timing of gene delivery relative to nerve repair is likely to be important, to optimize the results.

STUDY DESIGN

Prospective analysis.

METHODS

The effects of nonviral gene transfer for the delivery of hIGF-1 were evaluated in rats treated immediately following recurrent laryngeal nerve transection and repair and in rats receiving a delayed treatment schedule, 30 days after nerve transection and repair. Gene transfer efficiency was determined using polymerase chain reaction and reverse transcriptase-polymerase chain reaction techniques. Muscle fiber diameter, motor endplate length, and percentage of motor endplates with nerve contact were examined to assess hIGF-1 trophic effects.

RESULTS

Compared with reinnervated untreated control samples, both early and delayed hIGF-1 transfer resulted in significant increase in muscle fiber diameter. Motor endplate length was significantly decreased and nerve/motor endplate contact was significantly increased following delayed gene transfer, but not after early treatment.

CONCLUSION

We infer from results of the study that delayed hIGF-1 gene transfer delivered by a single intramuscular injection will enhance the process of muscle reinnervation. The clinical relevance of these findings supports the future application of gene therapy using nonviral vectors for management of laryngeal paralysis and other peripheral nerve injuries.

In vivo and in vitro differentiation of myocytes from human bone marrow-derived multipotent progenitor cells

OBJECTIVE

Recent studies have shown that bone marrow (BM) contains cells capable of differentiating into myocytes in vivo. However, addition of demethylation drugs has been necessary to induce myocyte differentiation from BM cells in vitro, and precise mechanisms of BM cells' conversion to myocytes and the origin of those cells have not been established. We investigated the expression of myogenic markers during differentiation and maturation of myocytes from BM-derived multipotent adult progenitor cells (MAPC) under physiological culture condition.

MATERIALS AND METHODS

Frozen BM samples from 21 healthy donors were used as a source of MAPC. To induce myocyte differentiation MAPC was cultured in the presence of 5% FCS, VEGF, bFGF, and IGF-1, and the expressions of myocyte markers were examined at various time points. We also investigated engraftment and differentiation of MAPC-derived myocytes in vivo.

RESULTS

Frozen BM-derived MAPC, cultured under the physiological myogenic condition, demonstrated spatial expression patterns of several myocyte markers similar to that of authentic myocyte differentiation. When injected into murine muscles, MAPC treated with the myogenic condition engrafted and differentiated into myocyte marker-positive cells and myotubes in vivo.

CONCLUSION

For the first time, we were able to induce myocyte formation from BM cells under the physiological condition in vitro and demonstrated that treating cells with this condition prior to intramuscular injection increased efficiency of engraftment and differentiation in vivo.

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.

Antifibrotic effects of suramin in injured skeletal muscle after laceration

Muscle injuries are very common in traumatology and sports medicine. Although muscle tissue can regenerate postinjury, the healing process is slow and often incomplete; complete recovery after skeletal muscle injury is hindered by fibrosis. Our studies have shown that decreased fibrosis could improve muscle healing. Suramin has been found to inhibit transforming growth factor (TGF)-beta1 expression by competitively binding to the growth factor receptor. We conducted a series of tests to determine the antifibrotic effects of suramin on muscle laceration injuries. Our results demonstrate that suramin (50 microg/ml) can effectively decrease fibroblast proliferation and fibrotic-protein expression (alpha-smooth muscle actin) in vitro. In vivo, direct injection of suramin (2.5 mg) into injured murine muscle resulted in effective inhibition of muscle fibrosis and enhanced muscle regeneration, which led to efficient functional muscle recovery. These results support our hypothesis that prevention of fibrosis could enhance muscle regeneration, thereby facilitating more efficient muscle healing. This study could significantly contribute to the development of strategies to promote efficient muscle healing and functional recovery.

Neutrophils do not contribute to local tissue damage, but play a key role in skeletal muscle regeneration, in mice injected withBothrops aspersnake venom

Local tissue damage induced by crotaline snake venoms includes edema, myonecrosis, hemorrhage, and an inflammatory response associated with a prominent cellular infiltrate. The role of neutrophils in the local tissue damage induced by Bothrops asper snake venom and by myotoxin I, a phospholipase A2 isolated from this venom, was investigated. Male Swiss mice were pretreated with either an antimouse granulocyte rat monoclonal immunoglobulin G (IgG) antibody or with isotype-matched control antibody. No significant differences in these local effects were observed between mice pretreated with antigranulocyte antibodies and those receiving control IgG. Moreover, myotoxicity induced by B. asper myotoxin I was similar in neutrophil-depleted and control mice. The role of neutrophils in the process of skeletal muscle regeneration was also assessed. Muscle regeneration was assessed by quantifying the muscle levels of creatine kinase and by morphometric histological analysis of the area comprised by regenerating cells in damaged regions of skeletal muscle. Mice depleted of neutrophils and then injected with B. asper venom showed a more deficient regenerative response than mice pretreated with control IgG. Moreover, a drastic difference in the regenerative response was observed in mice injected with myotoxin I, because animals pretreated with control IgG showed a successful regeneration, whereas those depleted of neutrophils had abundant areas of necrotic tissue that had not been removed 7 days after injection, associated with reduced contents of creatine kinase. It is concluded that (1) neutrophils do not play a significant role in the acute local pathological alterations induced by the venom of B. asper, and (2) neutrophils play a prominent role in the process of skeletal muscle regeneration after injection of B. asper venom and myotoxin I, probably related to the phagocytosis of necrotic material and the recruitment of other inflammatory cells, two events directly associated with a successful muscle regenerative response.

Inflammatory mediators and skeletal muscle injury: a DNA microarray analysis

Traumatic skeletal muscle injury causes a specific sequence of cellular events consisting of degeneration, inflammation, regeneration, and fibrosis. The role of early posttraumatic mechanisms, including acute inflammatory response, in muscle repair is not well understood. In the present study, oligonucleotide microarray analyses were used to examine the candidate genes that are involved in these early events of the muscle injury/repair process. cDNA was prepared from the injured and control tibialis anterior (TA) muscle of mice 24 h postinjury and labeled with the fluorescent dye Cy5 or Cy3 prior to hybridization to a DNA microarray. The microarray analysis, including 732 genes, was conducted in triplicate, and we describe only genes modulated by the injury showing a differential expression (both increased and decreased) 1.7-fold or greater (p < 0.05) from control uninjured TA muscle. Selected expression patterns were confirmed by other gene expression detection methods, including real-time reverse transcription-polymerase chain reaction (RT-PCR) and RNase protection assay (RPA) or immunohistochemistry detection methods. The upregulated genes (2.8%) were mainly associated with inflammation, oxidative stress, and cell proliferation, whereas the downregulated genes (3.2%) were related to metabolic and cell signaling pathways. In addition, the study suggested that chemokines, such as monocyte chemoattractant protein-1 (MCP-1), associated with monocyte/macrophage influx and activation, are abundantly expressed in postinjured muscle, and they might play a role in traumatic muscle injury/recovery processes.

Calcineurin initiates skeletal muscle differentiation by activating MEF2 and MyoD

Skeletal muscle differentiation is characterized by withdrawal from the cell cycle, expression of muscle specific genes, fusion into multinucleated cells, and assembly of the contractile apparatus. Although many of the key regulatory elements have been identified, the factors that initiate the differentiation process are not well understood. The calcium-dependent phosphatase calcineurin plays an important regulatory role early in myogenesis, but the downstream effectors of calcineurin in differentiation are not known. Here, we show that calcium and calcineurin regulate expression of the myogenin gene at the level of transcription. The myogenin promoter contains two essential elements; an E-box and an A/T rich element that bind MRF and MEF2 transcription factors, respectively. Both of these elements are responsive to calcium and calcineurin. In differentiating myoblasts, MyoD is the major MRF protein that binds to the myogenin promoter E-box. Calcineurin activates MyoD indirectly by decreasing the expression of the Id inhibitory proteins, probably by down-regulating Egr-1 expression, an upstream activator of Id transcription. These results demonstrate that calcineurin regulates skeletal muscle differentiation by activating MEF2 and MyoD transcription factors leading to the induction of myogenin expression.

Muscle regeneration in amphibians and mammals: passing the torch

Skeletal muscle in both amphibians and mammals possesses a high regenerative capacity. In amphibians, a muscle can regenerate in two distinct ways: as a tissue component of an entire regenerating limb (epimorphic regeneration) or as an isolated entity (tissue regeneration). In the absence of epimorphic regenerative ability, mammals can regenerate muscles only by the tissue mode. This review focuses principally on the regeneration of entire muscles and covers what is known and what remains to be elucidated about fundamental mechanisms underlying muscle regeneration at this level.

Gene therapy and tissue engineering for sports medicine

Sports injuries usually involve tissues that display a limited capacity for healing. The treatment of sports injuries has improved over the past 10 to 20 years through sophisticated rehabilitation programs, novel operative techniques, and advances in the field of biomechanical research. Despite this considerable progress, no optimal solution has been found for treatment of various sports-related injuries, including muscle injuries, ligament and tendon ruptures, central meniscal tears, cartilage lesions, and delayed bone fracture healing. New biological approaches focus on the treatment of these injuries with growth factors to stimulate and hasten the healing process. Gene therapy using the transfer of defined genes encoding therapeutic proteins represents a promising way to efficiently deliver suitable growth factors into the injured tissue. Tissue engineering, which may eventually be combined with gene therapy, may potentially result in the creation of tissues or scaffolds for regeneration of tissue defects following trauma. In this article we will discuss why gene therapy and tissue engineering are becoming increasingly important in modern orthopaedic sports medicine practice. We then will review recent research achievements in the area of gene therapy and tissue engineering for sports-related injuries, and highlight the potential clinical applications of this technology in the treatment of patients with musculoskeletal problems following sports-related injuries.

Targeted expression of IGF-1 transgene to skeletal muscle accelerates muscle and motor neuron regeneration

Currently, there is no known medical treatment that hastens the repair of damaged nerve and muscle. Using IGF-1 transgenic mice that specifically express human recombinant IGF-1 in skeletal muscle, we test the hypotheses that targeted gene expression of IGF-1 in skeletal muscle enhances motor nerve regeneration after a nerve crush injury. The IGF-1 transgene affects the initiation of the muscle repair process after nerve injury as shown by increased activation of SCA-1positive myogenic stem cells. Increased satellite cell differentiation and proliferation are observed in IGF-1 transgenic mice, shown by increased expression of Cyclin D1, MyoD, and myogenin. Expression of myogenin and nicotinic acetylcholine receptor subunits, initially increased in both wild-type and IGF-1 transgenic mice, are restored to normal levels at a faster rate in IGF-1 transgenic mice, which indicates a rescue of nerve-evoked muscle activity. Expression of the IGF-1 transgene in skeletal muscle results in accelerated recovery of saltatory nerve conduction, increased innervation as detected by neurofilament expression, and faster recovery of muscle mass. These studies demonstrate that local expression of IGF-1 augments the repair of injured nerve and muscle.

Time course of skeletal muscle repair and gene expression following acute hind limb ischemia in mice

DNA microarrays were used to measure the time course of gene expression during skeletal muscle damage and regeneration in mice following femoral artery ligation (FAL). We found 1,289 known sequences were differentially expressed between the FAL and control groups. Gene expression peaked on day 3, and the functional cluster "inflammation" contained the greatest number of genes. Muscle function was depressed for 3 days postligation, but returned to normal by day 7. Decreased muscle function was accompanied by reduced expression of genes involved in mitochondrial energy production, muscle contraction, and calcium handling. The induction of MyoD on day 1 denoted the beginning of muscle regeneration and was followed by the reemergence of the embryonic forms of muscle contractile proteins, which peaked at day 7. Transcriptional analysis indicated that the ischemic skeletal muscle may transition through a functional adaptation stage with recovery of contractile force prior to full regeneration. Several members of the insulin-like growth factor axis were coordinately induced in a time frame consistent with their playing a role in the regenerative process.

An evaluation of leukaemia inhibitory factor as a potential therapeutic agent in the treatment of muscle disease

The exogenous delivery of growth factors and cytokines is a potential therapeutic strategy to alleviate the degenerative effects of primary inherited myopathies such as Duchenne muscular dystrophy. The mdx mouse diaphragm is a model for examining the progressive degeneration of dystrophic muscle. We have delivered leukaemia inhibitory factor to the mdx diaphragm using slow release alginate gels. Previous studies have reported an improvement in the histology of mdx diaphragms after delivery of leukaemia inhibitory factor in a similar manner, but little attention has been paid to the mechanism by which leukaemia inhibitory factor acts. We have used autoradiography to examine cell proliferation, Evans Blue Dye to examine myofibre damage, and morphometric analysis to examine histology in leukaemia-inhibitory-factor-treated diaphragms and compared them with untreated mdx and normal C57Bl10/ScSn diaphragms. Autoradiography showed that although myoblast proliferation was significantly increased in leukaemia inhibitory factor-treated mdx diaphragms, leukaemia inhibitory factor did not reduce myofibre damage and no histological improvement was observed. The data presented here, while demonstrating a role for leukaemia inhibitory factor in myoblast proliferation, do not support a strong and consistent benefit of leukaemia inhibitory factor on dystrophic muscle in vivo as a means of alleviating the effects of chronic dystrophic muscle degeneration.

Adeno-associated virus-mediated vascular endothelial growth factor gene therapy in skeletal muscle before transplantation promotes revascularization of regenerating muscle

Successful clinical transplantation of whole skeletal muscles can be limited by impaired muscle revascularization and regeneration. The aim of this study was to enhance the revascularization (and hence speed of regeneration) of transplanted whole muscles by transducing muscles with the vascular endothelial growth factor (VEGF) gene before transplantation, using a recombinant adeno-associated virus (rAAV). The rAAV encoding VEGF and green fluorescent protein (GFP) (rAAV.VEGF.GFP) was injected into the tibialis anterior muscles of adult BALB/c mice. One month after injection whole muscle autotransplantation was performed. Muscles were sampled 7 days after autografting. GFP expression was examined as an indicator of persistent transgene expression after grafting, and immunohistochemistry was used to identify VEGF, blood vessels, and newly formed myotubes. After grafting, GFP expression persisted only in a few surviving myofibers in the periphery of rAAV.VEGF.GFP-pretreated muscles, although abundant VEGF expression was seen in myogenic cells in all grafted muscles. Quantitative analysis demonstrated that, although only small numbers of rAAV.VEGF.GFP-transduced myofibers were present, whole muscle grafts preinjected with rAAV.VEGF.GFP were significantly more vascular than saline-injected and uninjected control muscle grafts. Furthermore, rAAV.VEGF.GFP-injected whole muscle transplants were further advanced in terms of regeneration (myotube formation) compared with the uninjected control muscle transplants. This study clearly shows that rAAV-mediated VEGF expression persists only in myofibers that survive the necrosis induced by muscle transplantation; however, this amount of VEGF results in significantly increased revascularization and regeneration of whole muscle transplants.

Expression of alpha7beta1 integrin splicing variants during skeletal muscle regeneration

Integrin alpha7beta1 is a laminin receptor, both subunits of which have alternatively spliced, developmentally regulated variants. In skeletal muscle beta1 has two major splice variants of the intracellular domain (beta1A and beta1D). alpha7X1 and alpha7X2 represent variants of the alpha7 ectodomain, whereas alpha7A and alpha7B are variants of the intracellular domain. Previously we showed that during early regeneration after transection injury of muscle alpha7 integrin mediates dynamic adhesion of myofibers along their lateral aspects to the extracellular matrix. Stable attachment of myofibers to the extracellular matrix occurs during the third week after injury, when new myotendinous junctions develop at the ends of the regenerating myofibers. Now we have analyzed the relative expression of beta1A/beta1D and alpha7A/alpha7B and alpha7X1/alpha7X2 isoforms during regeneration for 2 to 56 days after transection of rat soleus muscle using reverse transcriptase-polymerase chain reaction and immunohistochemistry. During early regeneration beta1A was the predominant isoform in both the muscle and scar tissue. Expression of muscle-specific beta1D was detected in regenerating myofibers from day 4 onwards, ie, when myogenic mitotic activity began to decrease, and it became more abundant with the progression of regeneration. alpha7B isoform predominated on day 2. Thereafter, the relative expression of alpha7A transcripts increased until day 7 with the concomitant appearance of alpha7A immunoreactivity on regenerating myofibers. Finally, alpha7B again became the predominant variant in highly regenerated myofibers. Similarly as in the controls, alpha7X1 and alpha7X2 isoforms were both expressed throughout the regeneration with a peak in alpha7X1 expression on day 4 coinciding with the dynamic adhesion stage. The results suggest that during regeneration of skeletal muscle the splicing of beta1 and alpha7 integrin subunits is regulated according to functional requirements. alpha7A and alpha7X1 appear to have a specific role during the dynamic phase of adhesion, whereas alpha7B, alpha7X2, and beta1D predominate during stable adhesion.

Transected myofibres may remain permanently divided in two parts

During regeneration of transected myofibres a scar is formed between their stumps. Myofibres restore their tendon-muscle-tendon continuity and contractile function by attaching to the scar with new myotendinous junctions. The scar contracts with time, and thereby the stumps are pulled close to each other. During early regeneration, myoblasts and myotubes can fuse with the surviving parts of the transected myofibres. However, it is not known whether it is possible that the opposite stumps could eventually fuse to reunite the divided parts of the transected fibres. In this study, we show in rat that even after 12 months the stumps remain attached to the separating scar by myotendinous junctions without showing definite fusion of the stumps. We conclude that transected myofibres probably remain permanently divided in two consecutive tendon-muscle-tendon units.

Recombinant adeno-associated virus type 2 mediates highly efficient gene transfer in regenerating rat skeletal muscle

The recent identification of genes responsible for several muscle diseases, particularly inherited myopathies, has made gene transfer to pathologic muscle tissue an attractive research field. As early pathologic changes in myopathic muscle involve repeated necrosis-regeneration cycles, leading to the coexistence of myofibers at different stages of maturity, a delivery system for efficient, durable gene therapy of inherited muscle diseases should allow gene transfer into myofibers at any stage of maturity. Experiments with rat skeletal muscles showed that recombinant adeno-associated virus (rAAV) type 2 can be highly efficient and even improve gene transfer in regenerating as compared with mature muscle, provided that vector injection is performed during the myotube growth period of the regenerative process. At this early period of muscle regeneration, young regenerating myotubes strongly express heparan sulfate proteoglycan AAV type 2 receptor. Improvement was associated with a greater number of transduced myofibers in muscle samples and an increase in viral genomic copies in transduced muscle. No significant deleterious effects on muscle phenotype or any evident alterations in the regenerative process were observed in transduced muscles. Unlike other available viral vectors, whose transduction efficiencies are highly maturation-dependent, rAAV type 2-based vectors provide efficient in vivo gene transfer in myofibers at various stages of maturity, making AAV a promising delivery system for pathological muscle tissue.

Hemostatic effects induced by Thalassophryne nattereri fish venom: a model of endothelium-mediated blood flow impairment

Accidents by Thalassophryne nattereri fish venom are characterised by severe local symptoms and signs including pain of fast onset, oedema and necrosis with impaired muscle regeneration. These effects have been related to alterations in hemostatic mechanisms and cytolytic effects rather than to conventional inflammatory pathways. In this work we evaluated the effects induced by the venom on microcirculatory vessels, platelets and blood coagulation. Effects evoked by topical application of venom on cremaster muscle were visualised through intravital microscopy. Stasis was observed, concomitantly with the presence of thrombi in venules and focal transient constrictions in arterioles, all of which impaired the blood flow. Significant alterations on vessel walls took place few minutes after venom application, characterised by increment in thickness, probably by deposition of fibrin. Increase in vascular permeability was also observed in venules. Additionally, the action of the venom was locally restricted since no alteration on systemic blood coagulation was observed. Venom lacked a direct pro-coagulant activity, but exerted a strong cytolytic effect on platelets and endothelial cells in vitro. These data suggest that venom action on endothelium may contribute to blood stasis and to the formation of platelet and fibrin thrombi, with the consequent ischemia, contributing to the local effects of the venom.

SFRP2 expression in rabbit myogenic progenitor cells and in adult skeletal muscles

Satellite cells derived from fast- and slow-twitch muscles have different properties in culture. We have used the differential display technique to retrieve genes differentially expressed in fast- and slow-twitch muscle satellite cell cultures. Amongst these genes we have identified, cloned, sequenced and studied the expression in muscle of rabbit secreted frizzled related protein 2 (SFRP2) mRNA, whose importance in cell fate determination has been well documented. It has been shown that SFRP2 is widely expressed in the developing embryo but its expression in the adult is much more restricted. We show that primary cultures of satellite cells from adult rabbit fast- and slow-twitch muscles strongly and differentially express SFRP2 mRNA. Embryonic rabbit muscle cell primary cultures also strongly express SFRP2 mRNA whereas the myoblast C2.7 cell line shows little expression. We also studied SFRP2 mRNA expression in growing, regenerating and denervated muscles. Embryonic rabbit muscles express SFRP2 mRNA but this rapidly falls off after birth. In adult rabbit muscles SFRP2 mRNA is detected within 1 day of either muscle damage or denervation. Thereafter the SFRP2 mRNA expression profiles are different for fast- and slow-twitch muscle. The function of SFRP2 in muscle is unknown but its putative activity as a Wnt antagonist and its precocious expression after muscle damage suggest a role in satellite cell activation.

Tristetraprolin and LPS-inducible CXC chemokine are rapidly induced in presumptive satellite cells in response to skeletal muscle injury

Myogenic precursor cells known as satellite cells persist in adult skeletal muscle and are responsible for its ability to regenerate after injury. Quiescent satellite cells are activated by signals emanating from damaged muscle. Here we describe the rapid activation of two genes in response to muscle injury; these transcripts encode LPS-inducible CXC chemokine (LIX), a neutrophil chemoattractant, and Tristetraprolin (TTP), an RNA-binding protein implicated in the regulation of cytokine expression. Using a synchronized cell culture model we show that C2C12 myoblasts arrested in G0 exhibit some molecular attributes of satellite cells in vivo: suppression of MyoD and Myf5 expression during G0 and their reactivation in G1. Synchronization also revealed cell cycle dependent expression of CD34, M-cadherin, HGF and PEA3, genes implicated in satellite cell biology. To identify other genes induced in synchronized C2C12 myoblasts we used differential display PCR and isolated LIX and TTP cDNAs. Both LIX and TTP mRNAs are short-lived, encode molecules implicated in inflammation and are transiently induced during growth activation in vitro. Further, LIX and TTP are rapidly induced in response to muscle damage in vivo. TTP expression precedes that of MyoD and is detected 30 minutes after injury. The spatial distribution of LIX and TTP transcripts in injured muscle suggests expression by satellite cells. Our studies suggest that in addition to generating new cells for repair, activated satellite cells may be a source of signaling molecules involved in tissue remodeling during regeneration.

Identification of myogenic-endothelial progenitor cells in the interstitial spaces of skeletal muscle

Putative myogenic and endothelial (myo-endothelial) cell progenitors were identified in the interstitial spaces of murine skeletal muscle by immunohistochemistry and immunoelectron microscopy using CD34 antigen. Enzymatically isolated cells were characterized by fluorescence-activated cell sorting on the basis of cell surface antigen expression, and were sorted as a CD34+ and CD45- fraction. Cells in this fraction were approximately 94% positive for Sca-1, and mostly negative (<3% positive) for CD14, 31, 49, 144, c-kit, and FLK-1. The CD34+/45- cells formed colonies in clonal cell cultures and colony-forming units displayed the potential to differentiate into adipocytes, endothelial, and myogenic cells. The CD34+/45- cells fully differentiated into vascular endothelial cells and skeletal muscle fibers in vivo after transplantation. Immediately after sorting, CD34+/45- cells expressed only c-met mRNA, and did not express any other myogenic cell-related markers such as MyoD, myf-5, myf-6, myogenin, M-cadherin, Pax-3, and Pax-7. However, after 3 d of culture, these cells expressed mRNA for all myogenic markers. CD34+/45- cells were distinct from satellite cells, as they expressed Bcrp1/ABCG2 gene mRNA (Zhou et al., 2001). These findings suggest that myo-endothelial progenitors reside in the interstitial spaces of mammalian skeletal muscles, and that they can potentially contribute to postnatal skeletal muscle growth.

Skeletal muscle injury induces hepatocyte growth factor expression in spleen

Hepatocyte growth factor (HGF) is present in skeletal muscle and facilitates skeletal muscle regeneration by activating quiescent satellite cells and stimulating their proliferation. However, possible involvement of HGF from non-muscle organs during muscle regeneration is still uncovered. Since liver injury induces HGF expression in distal HGF-producing organs such as lung, kidney and spleen, we examined if this is the case in muscle injury in analogy. In rat femoral muscle, HGF protein levels were elevated within 1 h after muscle injury, with a simultaneous proteolytic activation of HGF protein. Semiquantitative RT-PCR analysis revealed an elevation of HGF mRNA expression after muscle injury in the liver and spleen, and also an increase of HGF protein levels in the spleen, suggesting the presence of endocrine HGF-inducing factor(s) during muscle regeneration. Indeed, the sera from the rat with muscle regeneration were capable of inducing HGF mRNA expression when applied to primary cultured spleen cells from intact rats. These results indicated that skeletal muscle injury induces HGF expression in the non-muscle HGF-producing organs, especially in the spleen, and suggested the possible involvement of non-muscle organ-derived HGF in activation/proliferation of satellite cells during muscle regeneration.

Characteristics of myofibres in the masseter muscle of mice during postnatal growth period

Functional maturation of muscles is related to the constitutional proportion of muscle protein isoforms during development and growth. Although the mouse masseter muscle (MS) is classified as a fast limb muscle, its functions are different from those of a limb muscle. This study investigated the differentiation of myosin heavy chain (MHC) isoforms during the postnatal development periods in mouse MS and mouse tibialis anterior (TA), which is a fast limb muscle. Many anti-MHC slow-type-positive fibres were observed in neonatal MS and TA; these fibres decreased during development. Adult MS was composed of anti-MHC fast-type-positive fibres. MHC isoforms in MS were composed of MHC-2a and MHC-2d soon after birth. MHC-2b was expressed, but MHC-2a was not seen after 21 days. Expression of MHC-2b agreed with the weaning period, that is 2-3 weeks after birth. This fact suggested that the transformation from suckling to mastication changed the MHC isoforms during this period. In this study, the expressions of MHC-2b agree with the weaning period.

Acceleration by MS-818 of early muscle regeneration and enhanced muscle recovery after surgical transection

The synthesized pyrimidine compound MS-818 has neurotrophic effects in several kinds of neuronal cells, but its effect with respect to muscle cells remains unknown. We therefore examined the effects of MS-818 on regeneration for 12 weeks in a wounded area (damaged and gap areas) of cut muscle in adult rats. The right semitendinosus muscles of treated and control groups were severed and sutured at the belly and the left semitendinosus muscles were left intact. MS-818 was administered intraperitoneally to the treated group at a dose of 5 mg/kg once daily. Control rats received an equal volume of physiological saline. A reference group underwent no surgical procedure. MS-818 significantly increased the maximal isometric twitch tension (Tmax) compared to control and reference rats after week 4 (approximately 1.4-fold control value; 0.6-fold reference value). Northern blotting showed that MS-818 enhanced myogenin mRNA expression to about 1.5-fold above the control level at 2, 4, and 7 days after surgery. Immunohistochemical and histochemical studies showed significant enhancement in the treated group since myogenic cells expressed desmin and were positive for neonatal myosin, and the fiber diameters and numbers of premature myofibers and end plates were increased when compared with those in the control group. These results show that MS-818 accelerated the proliferation and differentiation of activated satellite cells and the fusion of myotubes to form immature myofibers. At week 12, Tmax, fiber diameter, and number of end plates in the treatment group recovered 60, 85, and more than 100%, respectively, compared to the reference group. The mechanism of MS-818 effects on the accelerated regeneration of cut muscle is discussed.

The absence of MyoD in regenerating skeletal muscle affects the expression pattern of basement membrane, interstitial matrix and integrin molecules that is consistent with delayed myotube formation

MyoD is a member of a skeletal muscle specific family of transcription factors which directs the events of myogenesis during development and regeneration. Muscle cells that lack MyoD show delayed fusion in vivo and in vitro and defects have been observed in vitro in the attachment of MyoD(-/-) myoblasts to complex substrates such as Matrigel. Since interactions with the extracellular matrix (ECM) are important during myoblast fusion (i. e. myotube formation), it was hypothesised that expression of ECM molecules or their receptors may be altered in MyoD(-/-) muscle. The production of basement membrane molecules such as collagen type IV and several laminins, the interstitial molecules fibronectin and tenascin-C, and the cell surface molecules integrin alpha5 and alpha6 were quantitated in vitro using ELISA on cultured cells from MyoD(-/-) and wild type mice. Differences were observed in the production of fibronectin, tenascin-C, collagen type IV, laminin-1 and integrin alpha5 between control and MyoD(-/-) myotubes in vitro. This corresponded with delayed fusion of myoblasts in MyoD(-/-) cultures. On the basis of these findings with respect to matrix expression in vitro, fluorescent immunohistochemistry was carried out on adult whole muscle autografts to examine whether the expression of these molecules, as well as integrin alpha7, was altered in the complex in vivo environment. Some minor differences in expression patterns were observed in MyoD(-/-) as compared to normal BALB/c autografts. The overall expression of matrix components was consistent with the delayed onset of myotube formation. These results suggest that the delay in myotube formation in MyoD(-/-) muscle is not a direct result of altered expression of the matrix molecules collagen type IV, laminins, fibronectin, tenascin-C, and integrins alpha5, alpha6 or alpha7.

Lower environmental temperature delays and prolongs myogenic regulatory factor expression and muscle differentiation in rainbow trout (Onchrhynchus mykiss) embryos

The effect of different temperatures (4 degrees C and 12 degrees C) on myogenic regulatory factors (MyoD and myogenin) and myosin heavy chain (MyHC) expression was investigated in rainbow trout (Onchrhynchus mykiss) during early development. MyoD is first switched on at stage 14 [about 5 somites are formed (1/2 epiboly)] while myogenin mRNA is expressed at stage 15 [around 15 somites are visible (2/3 epiboly)] at both temperatures. Subsequently (up to at least stage 20), the most caudal somites exhibit less myogenin mRNA at 4 degrees C compared to 12 degrees C. At the eyed stage (stage 23-24), both myogenin mRNA and protein are present in greater amounts throughout all myotomes at the lower temperature, with mRNA levels in warmer (12 degrees C) embryos at 83% for MyoD and 72% for myogenin of the levels seen in 4 degrees C embryos. Conversely, however, at this same stage, fast-MyHC mRNA and protein are more abundant in 12 degrees C than in 4 degrees C embryos. This indicates relatively advanced muscle differentiation at the warmer temperature. At hatching, myogenin-positive cells are concentrated within the myosepta at both temperatures and they are also sparsely distributed in the myotome at 4 degrees C, but not at 12 degrees C. MyoD, myogenin, and MyHC levels provide an indication of differentiation of muscle cells. These findings suggest that myogenic regulatory factor expression is delayed but prolonged by the lowering of temperature.

The extracellular matrix of muscle--implications for manipulation of the craniofacial musculature

Successful adaptation of craniofacial skeletal muscle is dependent upon the connective tissue component of the muscle. This is exemplified by procedures such as distraction histo/osteogenesis. The mechanisms underlying remodelling of intramuscular connective tissue are complex and multifactorial and involve extracellular matrix (ECM) molecules, receptors for the ECM (integrins) and enzymes that remodel the ECM (MMPs). This review discusses the current state of knowledge and clinical implications of connective tissue biology as applied to craniofacial skeletal muscle.

The use of an antifibrosis agent to improve muscle recovery after laceration

Muscle injuries are challenging problems in traumatology and the most frequent injuries in sports medicine. Muscle injuries are capable of healing, although slowly and occasionally with incomplete functional recovery. We observed that lacerated muscle undergoes a rapid process of regeneration, which is hindered by the development of fibrosis. Biologic approaches to enhance muscle regeneration and prevent fibrosis are being investigated to improve muscle healing after injuries. We observed that growth factors can improve muscle regeneration but cannot prevent muscle fibrosis. We investigated the use of an antifibrosis substance, decorin, as an approach to prevent fibrosis and thereby improve muscle healing after injury in murine muscle. We observed that direct injection of human recombinant decorin can efficiently prevent fibrosis and enhance muscle regeneration in the lacerated muscle. More importantly, decorin can improve the recovery of strength in the injured muscle to a level similar to that observed in normal noninjured muscle. These results suggest that injection of decorin improves both the muscle structure and the function of the lacerated muscle to near complete recovery. This study will contribute significantly to the development of strategies to promote efficient muscle healing and complete functional recovery after muscle injuries.

Skeletal muscle regeneration after insulin-like growth factor I gene transfer by recombinant Sendai virus vector

We scrutinized the applicability and efficacy of Sendai virus (SeV) vectors expressing either LacZ or human insulin-like growth factor-I (hIGF-I) in gene transfer into skeletal muscle. Seven days after the intramuscular injection of LacZ/SeV X-gal labeled myofibers were demonstrated in rat anterior tibialis muscle with/without bupivacaine treatment and the transgene expression persisted up to 1 month after injection. Recombinant hIGF-I was detected as a major protein species in culture supernatants of a neonatal rat myoblast cell line L6 and thus induced the cells to undergo myogenetic differentiation. The introduction of hIGF-I/SeV into the muscle showed a significant increase in regenerating and split myofibers which were indicative of hypertrophy, and also an increase in the total number of myofibers, in comparison to that seen in the LacZ/SeV-treated control muscle. These results demonstrate that SeV achieves high-level transgene expression in skeletal muscle, and that hIGF-I gene transfer using SeV vector may therefore have great potential in the treatment of neuromuscular disorders.

Anabolic steroids increase exercise tolerance

The influence of an anabolic androgenic steroid (AAS) on thymidine and amino acid uptake in rat hindlimb skeletal muscles during 14 days after a single exhaustive bout of weight lifting was determined. Adult male rats were divided randomly into Control or Steroid groups. Nandrolone decanoate was administered to the Steroid group 1 wk before the exercise bout. [3H]thymidine and [14C]leucine labeling were used to determine the serial changes in cellular mitotic activity, amino acid uptake, and myosin synthesis. Serum creatine kinase (CK) activity, used as a measure of muscle damage, increased 30 and 60 min after exercise in both groups. The total amount of weight lifted was higher, whereas CK levels were lower in Steroid than in Control rats. [3H]thymidine uptake peaked 2 days after exercise in both groups and was 90% higher in Control than in Steroid rats, reflecting a higher level of muscle damage. [14C]leucine uptake was approximately 80% higher at rest and recovered 33% faster postexercise in Steroid than in Control rats. In a separate group of rats, the in situ isometric mechanical properties of the plantaris muscle were determined. The only significant difference was a higher fatigue resistance in the Steroid compared with the Control group. Combined, these results indicate that AAS treatment 1) ameliorates CK efflux and the uptake of [3H]thymidine and enhances the rate of protein synthesis during recovery after a bout of weight lifting, all being consistent with there being less muscle damage, and 2) enhances in vivo work capacity and the in situ fatigue resistance of a primary plantarflexor muscle.

The expression of intermediate filament protein nestin as related to vimentin and desmin in regenerating skeletal muscle

Intermediate filament (IF) proteins show specific spatial and temporal expression during development of skeletal muscle. Nestin, the least known muscle IF, has an important role in neuronal regeneration. Therefore, we analyzed the expression pattern of nestin as related to that of vimentin and desmin during skeletal muscle regeneration. Nestin and vimentin appear at 6 h post-injury in myoblasts, with maximum expression around day 3-5 post-injury. Thereafter, vimentin expression ceases completely, whereas that of nestin is downregulated to remain only in the sarcoplasm next to neuromuscular and myotendinous junctions. Desmin appears at 6-12 h post-injury and becomes the predominant IF in myofibers simultaneously with the appearance of cross-striations. The expression pattern and colocalization of nestin and vimentin, known to form heteropolymers, suggests that they are essential during the early dynamic phase of the myofiber regeneration when migration, fusion, and structural modeling of myogenic cells occurs, whereas desmin is responsible for keeping myofibrils in register in mature myofibers. In conclusion, the expression of nestin is dynamically orchestrated with that of vimentin and desmin during skeletal muscle regeneration and recapitulates that seen during myogenesis, i.e. these IFs have key functional roles in the construction and restoration of skeletal myofibers.

Leukemia inhibitory factor enhances regeneration in skeletal muscles after myoblast transplantation

Cell-based therapies, such as myoblast transfer therapy, are likely to become an integral part of any approach to treat myopathies such as Duchenne muscular dystrophy. Previous studies have shown that an increased level of regeneration in the host muscle enhances incorporation of donor myoblasts. Leukemia inhibitory factor (LIF) increases the number of dystrophic fibers expressing dystrophin after myoblast transplantation and enhances regeneration in injured and diseased muscle. Morphometric analysis was used to investigate whether an increased level of regeneration is induced by LIF after myoblast transplantation. We found that, in muscles treated with LIF, the number of fibers undergoing regeneration was increased. The increased incorporation of donor myoblasts and thus dystrophin expression induced by LIF may be due, at least in part, to an increased level of regeneration of dystrophic muscle.

Exercise-enhanced satellite cell proliferation and new myonuclear accretion in rat skeletal muscle

The effects of increased functional loading on early cellular regenerative events after exercise-induced injury in adult skeletal muscle were examined with the use of in vivo labeling of replicating myofiber nuclei and immunocyto- and histochemical techniques. Satellite cell proliferation in the soleus (Sol) of nonexercised rats (0.4 +/- 0.2% of fibers) was unchanged after an initial bout of declined treadmill exercise but was elevated after two (1.0 +/- 0.2%, P < or = 0.01), but not four or seven, daily bouts of the same task. Myonuclei produced over the 7-day period comprised 0.9-1.9% of myonuclei in isolated fibers of Sol, tibialis anterior, and vastus intermedius of nonexercised rats. The accretion of new myonuclei was enhanced (P < or = 0.05) in Sol and vastus intermedius by the initial exercise followed by normal activity (to 3.1-3.4% of myonuclei) and more so by continued daily exercise (4.2-5.3%). Observed coincident with a lower incidence of histological fiber injury and unchanged fiber diameter and myonuclei per millimeter, the greater new myonuclear accretion induced by continued muscle loading may contribute to an enhanced fiber repair and regeneration after exercise-induced injury.

Effect of rhythm and level of distraction on muscle structure: an animal study

To determine the optimal rhythm of distraction and level of osteotomy for maximal muscle preservation during limb lengthening, histologic and ultrastructural analysis of the peroneus tertius muscle was performed in 46 goats. Thirty-nine animals with 20% or 30% tibial lengthening at the rate of 0.75 mm per day at each osteotomy level were divided in two groups. The first group included 15 animals undergoing tibial middiaphyseal lengthening with automated distractor producing one (n = 5), four (n = 5), or 720 (n = 5) increments per day. The second group included 24 animals in which lengthening was performed at the proximal (n = 4), middle (n = 6), distal (n = 6), or double (n = 8) level of the tibia with distraction rhythm of three times per day. The remaining seven animals served as controls. Histomorphologic features of the lengthened muscles were characterized by a polymorphous appearance with foci of necrosis, atrophy, and sclerosis scattered among apparently normal muscle fibers. The intensity and dispersion of degenerative changes were in direct proportion to the amount of lengthening but in reverse proportion to the frequency of distraction. Tibial lengthening at the diaphysis caused intense fibroblast proliferation and remarkable muscle sclerosis, whereas metaphyseal distraction was associated with a predominance of muscle tissue regeneration over atrophy and sclerosis. Bifocal lengthening and a more fractionated rhythm of distraction were associated with enhanced preservation of existing muscle fibers and greater regenerative activity of the muscle.

Skeletal muscle necrosis and regeneration after injection of Thalassophryne nattereri (niquim) fish venom in mice

Stings by Thalassophryne nattereri are responsible for envenomation of fishermen in north-eastern Brazil. Its venom induces prominent local tissue damage, characterized by pain, oedema and necrosis. The pathogenesis of acute muscle damage induced by T. nattereri venom was studied in mice. Intramuscular injection induced myonecrosis within the first hours. Some muscle cells presented a hypercontracted morphology, but most necrotic fibres were not hypercontracted, being instead characterized by a disorganization of myofibrils, with Z line loss, mitochondrial swelling and sarcolemmal disruption. In addition, thrombosis was observed histologically in venules and veins, together with vascular congestion and stasis, evidenced by intravital microscopy. Venom induced a rapid increment in serum creatine kinase (CK) levels, concomitant with a reduction in gastrocnemius muscle CK activity, whereas no increments in muscle lactic acid were detected. A rapid cytolytic effect was induced by the venom on C2C12 murine myoblasts in culture. The inflammatory reaction in affected muscle was characterized by oedema and scarce cellular infiltrate of polymorphonuclear leucocytes and macrophages, with a consequent delay in the removal of necrotic material. Skeletal muscle regeneration was partially impaired, as evidenced by the presence of regenerating fibres of variable size and by the increase of fibrotic tissue in endomysium and perimysium. It is suggested that T. nattereri venom affects muscle fibres by a direct cytotoxic effect, and that the vascular alterations described preclude a successful regenerative process.

Identification and Purification of an Intrinsic Human Muscle Myogenic Factor That Enhances Muscle Repair and Regeneration

The limited ability of damaged muscle to regenerate after gross injuries is a major clinical problem. To date, there is no effective therapeutic treatment for muscle injuries. In the present study, we have examined the ability of crude and fractionated human skeletal muscle extracts to promote myogenic cell proliferation and differentiation. It was found that the crude muscle extract could significantly stimulate BrdU incorporation in C2C12 myogenic cell line. In addition, the extract also promoted myogenic cell alignment and fusion. Using electrophoresis techniques, in conjunction with in vitro refolding technique, a protein with molecular weight of approximately 40 kDa was identified that could produce the same effects as the crude muscle exdtract. We also tested the ability of semipurified (30-50 kDa) muscle extract to promote muscle repair in adult rats. Surgical intervention was used to induce muscle damage in the tibialis anterior. The semipurified muscle extract (fraction H) was injected subcutaneously over the tibialis anterior for a period of 5 days. It was found that the damaged muscle fibers were replaced by newly regenerated muscle fibers. These newly regenerated fibers originated from the fusion of differentiated satellite cells as revealed by BrdU-labeling analysis. In contrast, the injury site of muscles treated with BSA control protein contained mainly fibroblasts.

Immunological changes in human skeletal muscle and blood after eccentric exercise and multiple biopsies

1. A role of the immune system in muscular adaptation to physical exercise has been suggested but data from controlled human studies are scarce. The present study investigated immunological events in human blood and skeletal muscle by immunohistochemistry and flow cytometry after eccentric cycling exercise and multiple biopsies. 2. Immunohistochemical detection of neutrophil- (CD11b, CD15), macrophage- (CD163), satellite cell- (CD56) and IL-1beta-specific antigens increased similarly in human skeletal muscle after eccentric cycling exercise together with multiple muscle biopsies, or multiple biopsies only. 3. Changes in immunological variables in blood and muscle were related, and monocytes and natural killer (NK) cells appeared to have governing functions over immunological events in human skeletal muscle. 4. Delayed onset muscle soreness, serum creatine kinase activity and C-reactive protein concentration were not related to leukocyte infiltration in human skeletal muscle. 5. Eccentric cycling and/or muscle biopsies did not result in T cell infiltration in human skeletal muscle. Modes of stress other than eccentric cycling should therefore be evaluated as a myositis model in human. 6. Based on results from the present study, and in the light of previously published data, it appears plausible that muscular adaptation to physical exercise occurs without preceding muscle inflammation. Nevertheless, leukocytes seem important for repair, regeneration and adaptation of human skeletal muscle.

Myotube formation is delayed but not prevented in MyoD-deficient skeletal muscle: studies in regenerating whole muscle grafts of adult mice

We compared the time course of myogenic events in vivo in regenerating whole muscle grafts in MyoD(-/-) and control BALB/c adult mice using immunohistochemistry and electron microscopy. Immunohistochemistry with antibodies to desmin and myosin revealed a striking delay by about 3 days in the formation of myotubes in MyoD(-/-) autografts compared with BALB/c mice. However, myotube formation was not prevented, and autografts in both strains appeared similar by 8 days. Electron microscopy confirmed myotube formation in 8- but not 5-day MyoD(-/-) grafts. This pattern was not influenced by cross-transplantation experiments between strains examined at 5 days. Antibodies to proliferating cell nuclear antigen demonstrated an elevated level of replication by MyoD(-/-) myoblasts in autografts, and replication was sustained for about 3 days compared with controls. These data indicate that the delay in the onset of differentiation and hence fusion is related to extended proliferation of the MyoD(-/-) myoblasts. Overall, although muscle regeneration was delayed it was not impaired in MyoD(-/-) mice in this model.

Myostatin regulation during skeletal muscle regeneration

Myostatin, a member of the TGF-beta superfamily, is a key negative regulator of skeletal muscle growth. The role of myostatin during skeletal muscle regeneration has not previously been reported. In the present studies, normal Sprague-Dawley and growth hormone (GH)-deficient (dw/dw) rats were administered the myotoxin, notexin, in the right M. biceps femoris on day 0. The dw/dw rats then received either saline or human-N-methionyl GH (200microg/100g body weight/day) during the ensuing regeneration. Normal and dw/dw M. biceps femoris were dissected on days 1, 2, 3, 5, 9 and 13, formalin-fixed, then immunostained for myostatin protein. Immunostaining for myostatin revealed high levels of protein within necrotic fibres and connective tissue of normal and dw/dw damaged muscles. Regenerating myotubes contained no myostatin at the time of fusion (peak fusion on day 5), and only low levels of myostatin were observed during subsequent myotube enlargement. Fibres which survived assault by notexin (survivor fibres) contained moderate to high myostatin immunostaining initially. The levels in both normal and dw/dw rat survivor fibres decreased on days 2-3, then increased on days 9-13. In dw/dw rats, there was no observed effect of GH administration on the levels of myostatin protein in damaged muscle. The low level of myostatin observed in regenerating myotubes in these studies suggests a negative regulatory role for myostatin in muscle regeneration.

Snake venom metalloproteinases: their role in the pathogenesis of local tissue damage

The biochemical characteristics of hemorrhagic metalloproteinases isolated from snake venoms are reviewed, together with their role in the pathogenesis of the local tissue damage characteristic of crotaline and viperine snake envenomations. Venom metalloproteinases differ in their domain structure. Some enzymes comprise only the metalloproteinase domain, others have disintegrin-like and high cysteine domains and others present, besides these domains, an additional lectin-like subunit. All of them are zinc-dependent enzymes with highly similar zinc binding environments. Some metalloproteinases induce hemorrhage by directly affecting mostly capillary blood vessels. It is suggested that hemorrhagic enzymes cleave, in a highly selective fashion, key peptide bonds of basement membrane components, thereby affecting the interaction between basement membrane and endothelial cells. As a consequence, these cells undergo a series of morphological and functional alterations in vivo, probably associated with biophysical hemodynamic factors such as tangential fluid shear stress. Eventually, gaps are formed in endothelial cells through which extravasation occurs. In addition to hemorrhage, venom metalloproteinases induce skeletal muscle damage, myonecrosis, which seems to be secondary to the ischemia that ensues in muscle tissue as a consequence of bleeding and reduced perfusion. Microvessel disruption by metalloproteinases also impairs skeletal muscle regeneration, being therefore responsible of fibrosis and permanent tissue loss after snakebites. Moreover, venom metalloproteinases participate in the degradation of extracellular matrix components and play a relevant role in the prominent local inflammatory response that characterizes snakebite envenomations, since they induce edema, activate endogenous matrix metalloproteinases (MMPs) and are capable of releasing TNF-alpha from its membrane-bound precursor. Owing to their protagonic role in the pathogenesis of local tissue damage, snake venom metalloproteinases constitute relevant targets for natural and synthetic inhibitors which may complement antivenoms in the neutralization of these effects.

Telomeric profiles and telomerase activity in turkey satellite cell clones with different in vitro growth characteristics

The satellite cell population in postnatal skeletal muscle is heterogeneous because individual satellite cells isolated from a single muscle have differing abilities to proliferate under the same in vitro conditions. Telomeres are structures found at the ends of all eukaryotic chromosomes that are characterized by repetitive DNA sequences, and they are important in determining cellular proliferation potential. The relationship between satellite cell proliferative heterogeneity and telomeric DNA was examined by digesting genomic DNA from large-colony-forming and small-colony-forming turkey satellite cell clones with HinfI, separating the restriction fragments on an agarose gel, and hybridizing the gels with an oligonucleotide probe specific for telomeric DNA. Turkey satellite cells generated telomeric restriction fragments up to approximately 180 kB. The large-colony-forming satellite cell clones had a larger proportion (P<0.05) of total telomeric restriction fragments below 33 kB than the small-colony-forming satellite cell clones. However, telomerase expression was detected in cultures from large-colony-forming and small-colony-forming turkey satellite cells suggesting that the differences in telomeric restriction fragments may not be related to the differences in in vitro proliferative behavior and that telomerase may contribute to the high in vitro growth capacity of turkey satellite cells.

Limited myogenic response to a single bout of weight-lifting exercise in old rats

The purpose of the present study was to compare the myogenic response of hindlimb muscles in young (14-20 wk of age) and old (>120 wk of age) rats with a single exhaustive bout of heavy resistance weight lifting. [(3)H]thymidine and [(14)C]leucine labeling were monitored for up to 2 wk after the exercise bout to estimate serial changes in mitotic activity and the level of amino acid uptake and myosin synthesis. Histological, histochemical, and immunohistochemical [anti-5-bromo-2'-deoxyuridine and myogenic determination genes (MyoD)] analyses of whole muscles and analysis of muscle-specific gene expression (MyoD) using Western blotting and RT-PCR were performed. Old rats showed significant muscle atrophy and a lower exercise capacity than young rats. Exercise-induced muscle damage, as assessed in histological sections, and increases in serum creatine kinase activity were evident in both young and old exercised groups. Mitotic activity was increased in young, but not old, rats 2 days after exercise. There was a biphasic increase in [(14)C]leucine uptake during the 14 days postexercise (peaks at 1-4 and 10 days) in young rats: only the first peak was observed in old rats. There was a lower uptake of [(14)C]leucine in the myosin fraction and an impaired expression of MyoD at the protein (immunohistochemistry and Western blotting) and mRNA (RT-PCR) levels in old rats throughout the postexercise period. These results demonstrate a reduced reparative capability of muscle in response to a single bout of exercise in old compared with young rats.

T-cell-dependent fibrosis in the mdx dystrophic mouse

In Duchenne muscular dystrophy patients, the pathological hallmark of the disease, namely, the chronic accumulation of sclerotic scar tissue in the interstitial space of skeletal muscle is attributed to manifestation of secondary pathological processes. Such anomalous generation of matrix protein is thought to be driven by the continuous degeneration and regeneration of muscle both in Duchenne Muscular Dystrophy and in the mdx mouse homolog. We examined mdx and the control strain C57bl/10 mice over a range of ages with respect to the amounts of collagen present in muscles and other organs, finding that the mdx have significantly higher collagen content at later time points in their kidney and lung as well as their muscles. Surprisingly, when we bred the mdx mice on the nu/nu background, the time course of fibrogenesis was modified depending on the tissue and the collagen content was significantly different in age-matched mice. Transplantation of normal thymic tissue into the mdx-nu/nu mice replenished their T-cells and concomitantly altered the collagen content in their tissues to levels comparable with those in immunocompetent mdx mice. This suggests that T-cells play a role in the onset of the fibrotic events that undermines the ability of dystrophic muscle to regenerate.

Laminin alpha4 and integrin alpha6 are upregulated in regenerating dy/dy skeletal muscle: comparative expression of laminin and integrin isoforms in muscles regenerating after crush injury

The expression of laminin isoforms and laminin-binding integrin receptors known to occur in muscle was investigated during myogenic regeneration after crush injury. Comparisons were made between dystrophic 129ReJ dy/dy mice, which have reduced laminin alpha2 expression, and their normal littermates. The overall histological pattern of regeneration after crush injury was similar in dy/dy and control muscle, but proceeded faster in dy/dy mice. In vitro studies revealed a greater yield of mononuclear cells extracted from dy/dy muscle and a reduced proportion of desmin-positive cells upon in vitro cultivation, reflecting the presence of inflammatory cells and "preactivated" myoblasts due to ongoing regenerative processes within the endogenous dystrophic lesions. Laminin alpha1 was not detectable in skeletal muscle. Laminin alpha2 was present in basement membranes of mature myofibers and newly formed myotubes in control and dy/dy muscles, albeit weaker in dy/dy. Laminin alpha2-negative myogenic cells were detected in dy/dy and control muscle, suggesting the involvement of other laminin alpha chains in early myogenic differentiation, such as laminin alpha4 and alpha5 which were both transiently expressed in basement membranes of newly formed myotubes of dy/dy and control mice. Integrin beta1 was expressed on endothelial cells, muscle fibers, and peripheral nerves in uninjured muscle and broadened after crush injury to the interstitium where it occurred on myogenic and nonmyogenic cells. Integrin alpha3 was not expressed in uninjured or regenerating muscle, while integrin alpha6 was expressed mainly on endothelial cells and peripheral nerves in uninjured muscle. Upon crush injury integrin alpha6 increased in the interstitium mainly on nonmyogenic cells, including infiltrating leukocytes, endothelial cells, and fibroblasts. In dy/dy muscle, integrin alpha6 occurred on some newly formed myotubes. Integrin alpha7 was expressed on muscle fibers at the myotendinous junction and showed weak and irregular expression on muscle fibers. After crush injury, integrin alpha7 expression extended to the newly formed myotubes and some myoblasts. However, many myoblasts and newly formed myotubes were integrin alpha7 negative. No marked difference was observed in integrin alpha7 expression between dy/dy and control muscle, either uninjured or after crush injury. Only laminin alpha4 and integrin alpha6 expression patterns were notably different between dy/dy and control muscle. Expression of both molecules was more extensive in dy/dy muscle, especially in the interstitium of regenerating areas and on newly formed myotubes. In view of the faster myogenic regeneration observed in dy/dy mice, the data suggest that laminin alpha4 and integrin alpha6 support myogenic regeneration. However, whether these accelerated myogenic effects are a direct consequence of the reduced laminin alpha2 expression in dy/dy mice, or an accentuation of the ongoing regenerative events in focal lesions in the muscle, requires further investigation.