Cellular differences in the regeneration of murine skeletal muscle: a quantitative histological study in SJL/J and BALB/c mice

Exploring the ability of low-level laser irradiation to reduce myonecrosis and increase Myogenin transcription after Bothrops jararacussu envenomation

Envenoming caused by snakebites is a very important neglected tropical disease worldwide. The myotoxic phospholipases present in the bothropic venom disrupt the sarcolemma and compromise the mechanisms of energy production, leading to myonecrosis. Photobiomodulation therapy (PBMT) has been used as an effective tool to treat diverse cases of injuries, such as snake venom-induced myonecrosis. Based on that, the aim of this study was to analyze the effects of PBMT through low-level laser irradiation (904 nm) on the muscle regeneration after the myonecrosis induced by Bothrops jararacussu snake venom (Bjssu) injection, focusing on myogenic regulatory factors expression, such as Pax7, MyoD, and Myogenin (MyoG). Male Swiss mice (Mus musculus), 6-8-week-old, weighing 22 ± 3 g were used. Single sub-lethal Bjssu dose or saline was injected into the right mice gastrocnemius muscle. At 3, 24, 48, and 72 h after injections, mice were submitted to PBMT treatment. When finished the periods of 48 and 72 h, mice were euthanized and the right gastrocnemius were collected for analyses. We observed extensive inflammatory infiltrate in all the groups submitted to Bjssu injections. PBMT was able to reduce the myonecrotic area at 48 and 72 h after envenomation. There was a significant increase of MyoG mRNA expression at 72 h after venom injection. The data suggest that beyond the protective effect promoted by PBMT against Bjssu-induced myonecrosis, the low-level laser irradiation was able to stimulate the satellite cells, thus enhancing the muscle repair by improving myogenic differentiation.

The effects of concentric and eccentric training in murine models of dysferlin-associated muscular dystrophy

INTRODUCTION

Dysferlin-deficient murine muscle sustains severe damage after repeated eccentric contractions.

METHODS

With a robotic dynamometer, we studied the response of dysferlin-sufficient and dysferlin-deficient mice to 12 weeks of concentrically or eccentrically biased contractions. We also studied whether concentric contractions before or after eccentric contractions reduced muscle damage in dysferlin-deficient mice.

RESULTS

After 12 weeks of concentric training, there was no net gain in contractile force in dysferlin-sufficient or dysferlin-deficient mice, whereas eccentric training produced a net gain in force in both mouse strains. However, eccentric training induced more muscle damage in dysferlin-deficient vs dysferlin-sufficient mice. Although concentric training produced minimal muscle damage in dysferlin-deficient mice, it still led to a prominent increase in centrally nucleated fibers. Previous exposure to concentric contractions conferred slight protection on dysferlin-deficient muscle against damage from subsequent injurious eccentric contractions.

DISCUSSION

Concentric contractions may help dysferlin-deficient muscle derive the benefits of exercise without inducing damage.

Functions of Vertebrate Ferlins

Ferlins are multiple-C2-domain proteins involved in Ca²⁺-triggered membrane dynamics within the secretory, endocytic and lysosomal pathways. In bony vertebrates there are six ferlin genes encoding, in humans, dysferlin, otoferlin, myoferlin, Fer1L5 and 6 and the long noncoding RNA Fer1L4. Mutations in DYSF (dysferlin) can cause a range of muscle diseases with various clinical manifestations collectively known as dysferlinopathies, including limb-girdle muscular dystrophy type 2B (LGMD2B) and Miyoshi myopathy. A mutation in MYOF (myoferlin) was linked to a muscular dystrophy accompanied by cardiomyopathy. Mutations in OTOF (otoferlin) can be the cause of nonsyndromic deafness DFNB9. Dysregulated expression of any human ferlin may be associated with development of cancer. This review provides a detailed description of functions of the vertebrate ferlins with a focus on muscle ferlins and discusses the mechanisms leading to disease development.

Modulation of Klotho expression in injured muscle perturbs Wnt signalling and influences the rate of muscle growth

NEW FINDINGS

What is the central question of this study? Does modulating the expression of Klotho affect myogenesis following acute injury of healthy, non-senescent muscle? What is the main finding and its importance? Klotho can accelerate muscle growth following acute injury of healthy, adult mice, which supports the possibility that increased delivery of Klotho could have therapeutic value for improving repair of damaged muscle.

ABSTRACT

Skeletal muscle injuries activate a complex programme of myogenesis that can restore normal muscle structure. We tested whether modulating the expression of klotho influenced the response of mouse muscles to acute injury. Our findings show that klotho expression in muscle declines at 3 days post-injury. That reduction in klotho expression coincided with elevated expression of targets of Wnt signalling (Ccnd1; Myc) and increased MyoD+ muscle cell numbers, reflecting the onset of myogenic cell differentiation. klotho expression subsequently increased at 7 days post-injury with elevated expression occurring primarily in inflammatory lesions, which was accompanied by reduced expression of Wnt target genes (Ccnd1: 91%; Myc: 96%). Introduction of a klotho transgene maintained high levels of klotho expression over the course of muscle repair and attenuated the increases in Ccnd1 and Myc expression that occurred at 3 days post-injury. Correspondingly, transgene expression reduced Wnt signalling in Pax7+ cells, reflected by reductions in Pax7+ cells expressing active β-catenin, and reduced the numbers of MyoD+ cells at 3 days post-injury. At 21 days post-injury, muscles in klotho transgenic mice showed increased Pax7+ and decreased myogenin+ cell densities and large increases in myofibre size. Likewise, treating myogenic cells in vitro with Klotho reduced Myod expression but did not affect Pax7 expression. Muscle inflammation was only slightly modulated by increased klotho expression, initially reducing the expression of M2-biased macrophage markers Cd163 and Cd206 at 3 days post-injury and later increasing the expression of pan-macrophage marker F480 and Cd68 at 21 days post-injury. Collectively, our study shows that Klotho modulates myogenesis and that increased expression accelerates muscle growth after injury.

Regulation of muscle growth and regeneration by the immune system

Diseases of muscle that are caused by pathological interactions between muscle and the immune system are devastating, but rare. However, muscle injuries that involve trauma and regeneration are fairly common, and inflammation is a clear feature of the regenerative process. Investigations of the inflammatory response to muscle injury have now revealed that the apparently nonspecific inflammatory response to trauma is actually a complex and coordinated interaction between muscle and the immune system that determines the success or failure of tissue regeneration.

The Tubulointerstitial Pathophysiology of Progressive Kidney Disease

Accumulating evidence suggests that the central locus for the progression of CKD is the renal proximal tubule. As injured tubular epithelial cells dedifferentiate in attempted repair, they stimulate inflammation and recruit myofibroblasts. At the same time, tissue loss stimulates remnant nephron hypertrophy. Increased tubular transport workload eventually exceeds the energy-generating capacity of the hypertrophied nephrons, leading to anerobic metabolism, acidosis, hypoxia, endoplasmic reticulum stress, and the induction of additional inflammatory and fibrogenic responses. The result is a vicious cycle of injury, misdirected repair, maladaptive responses, and more nephron loss. Therapy that might be advantageous at one phase of this progression pathway could be deleterious during other phases. Thus, interrupting this downward spiral requires narrowly targeted approaches that promote healing and adequate function without generating further entry into the progression cycle.

Early Regeneration of Whole Skeletal Muscle Grafts Is Unaffected by Overexpression of IGF-1 in MLC/mIGF-1 Transgenic Mice

Early myogenic events in regenerating whole muscle grafts were compared between transgenic MLC/mIGF-1 mice with skeletal muscle-specific overexpression of the Exon-1 Ea isoform of insulin-like growth factor-1 (mIGF-1) and control FVB mice, from day 3 to day 21 after transplantation. Immunocytochemistry with antibodies against desmin showed that skeletal muscle-specific overexpression of IGF-1 did not affect the pattern of myoblast activation or proliferation or the onset and number of myotubes formed in regenerating whole muscle grafts. Hypertrophied myotubes were observed in MLC/mIGF grafts at day 7 after transplantation, although such hypertrophy was transient, and the transgenic and control grafts had a similar appearance at later time points (days 10, 14, and 21). Immunostaining with antibodies to platelet endothelial cell adhesion molecule-1, which identifies endothelial cells, demonstrated no difference in the formation of new vascular network in grafts of transgenic and control mice. Skeletal muscle-specific overexpression of mIGF-1 does not appear to stimulate the early events associated with myogenesis during regeneration of whole muscle grafts. (J Histochem Cytochem 52:873–883, 2004)

Lipid accumulation in dysferlin-deficient muscles

Dysferlin is a membrane associated protein involved in vesicle trafficking and fusion. Defects in dysferlin result in limb-girdle muscular dystrophy type 2B and Miyoshi myopathy in humans and myopathy in A/J(dys-/-) and BLAJ mice, but the pathomechanism of the myopathy is not understood. Oil Red O staining showed many lipid droplets within the psoas and quadriceps muscles of dysferlin-deficient A/J(dys-/-) mice aged 8 and 12 months, and lipid droplets were also conspicuous within human myofibers from patients with dysferlinopathy (but not other myopathies). Electron microscopy of 8-month-old A/J(dys-/-) psoas muscles confirmed lipid droplets within myofibers and showed disturbed architecture of myofibers. In addition, the presence of many adipocytes was confirmed, and a possible role for dysferlin in adipocytes is suggested. Increased expression of mRNA for a gene involved in early lipogenesis, CCAAT/enhancer binding protein-δ, in 3-month-old A/J(dys-/-) quadriceps (before marked histopathology is evident), indicates early induction of lipogenesis/adipogenesis within dysferlin-deficient muscles. Similar results were seen for dysferlin-deficient BLAJ mice. These novel observations of conspicuous intermyofibrillar lipid and progressive adipocyte replacement in dysferlin-deficient muscles present a new focus for investigating the mechanisms that result in the progressive decline of muscle function in dysferlinopathies.

Stac3 is a novel regulator of skeletal muscle development in mice

The goal of this study was to identify novel factors that mediate skeletal muscle development or function. We began the study by searching the gene expression databases for genes that have no known functions but are preferentially expressed in skeletal muscle. This search led to the identification of the Src homology three (SH3) domain and cysteine rich (C1) domain 3 (Stac3) gene. We experimentally confirmed that Stac3 mRNA was predominantly expressed in skeletal muscle. We determined if Stac3 plays a role in skeletal muscle development or function by generating Stac3 knockout mice. All Stac3 homozygous mutant mice were found dead at birth, were never seen move, and had a curved body and dropping forelimbs. These mice had marked abnormalities in skeletal muscles throughout the body, including central location of myonuclei, decreased number but increased cross-sectional area of myofibers, decreased number and size of myofibrils, disarrayed myofibrils, and streaming Z-lines. These phenotypes demonstrate that the Stac3 gene plays a critical role in skeletal muscle development and function in mice.

Transforming growth factor β-induced peritoneal fibrosis is mouse strain dependent

BACKGROUND

Encapsulating peritoneal sclerosis (EPS) is a rare but devastating complication of peritoneal dialysis. The etiology is unclear, but genetic predisposition may be a contributing factor. We used adenovirus-mediated gene transfer of transforming growth factor (TGF) β1 to the peritoneum in four genetically distinct laboratory mouse strains to assess differences in fibrogenic response.

METHODS

Mice from four genetic backgrounds (C57BL/6J, DBA/2J, C3H/HeJ and SJL/J) received an intraperitoneal injection of an adenovirus expressing TGFβ1 (AdTGFβ1) or control adenovirus (AdDL) and were assessed 4 and 10 days after infection. Submesothelial thickening, angiogenesis and gene expression were quantified from peritoneal tissue. Protein was extracted from omental tissue and assessed for collagen, E-cadherin and TGFβ signaling pathway proteins.

RESULTS

There was a graded response among the mouse strains to the peritoneal overexpression of TGFβ1. TGFβ1 induced a significant fibrogenic response in the C57BL/6J mice, whereas the SJL/J mice were resistant. The DBA/2J and the C3H/HeJ mice had intermediate responses. A similar graded response was seen in collagen protein levels in the omental tissue and in fibrosis-associated gene expression. TGFβ type 1 receptor and SMAD signaling pathways appeared to be intact in all the mouse strains.

CONCLUSIONS

There were significant differences in mouse strain susceptibility to peritoneal fibrosis, suggesting that genetic factors may play a role in the development of peritoneal fibrosis and possibly EPS. As early TGFβ1 signaling mechanisms appear to be intact, we hypothesize that fibrosis resistance in the SJL/J mice lies further down the wound-healing cascade or in an alternate, non-SMAD pathway.

Distinct effects of contraction-induced injury in vivo on four different murine models of dysferlinopathy

Mutations in the DYSF gene, encoding dysferlin, cause muscular dystrophies in man. We compared 4 dysferlinopathic mouse strains: SJL/J and B10.SJL-Dysf^im/AwaJ (B10.SJL), and A/J and B6.A-Dysf^prmd/GeneJ (B6.A/J). The former but not the latter two are overtly myopathic and weaker at 3 months of age. Following repetitive large-strain injury (LSI) caused by lengthening contractions, all except B6.A/J showed ~40% loss in contractile torque. Three days later, torque in SJL/J, B10.SJL and controls, but not A/J, recovered nearly completely. B6.A/J showed ~30% torque loss post-LSI and more variable recovery. Pre-injury, all dysferlinopathic strains had more centrally nucleated fibers (CNFs) and all but A/J showed more inflammation than controls. At D3, all dysferlinopathic strains showed increased necrosis and inflammation, but not more CNFs; controls were unchanged. Dystrophin-null DMD^mdx mice showed more necrosis and inflammation than all dysferlin-nulls. Torque loss and inflammation on D3 across all strains were linearly related to necrosis. Our results suggest that (1) dysferlin is not required for functional recovery 3 days after LSI; (2) B6.A/J mice recover from LSI erratically; (3) SJL/J and B10.SJL muscles recover rapidly, perhaps due to ongoing myopathy; (4) although they recover function to different levels, all 4 dysferlinopathic strains show increased inflammation and necrosis 3 days after LSI.

Myogenic capacity of muscle progenitor cells from head and limb muscles

The restoration of muscles in the soft palate of patients with cleft lip and/or palate is accompanied by fibrosis, which leads to speech and feeding problems. Treatment strategies that improve muscle regeneration have only been tested in limb muscles. Therefore, in the present study the myogenic potential of muscle progenitor cells (MPCs) isolated from head muscles was compared with that of limb muscles. Muscle progenitor cells were isolated from the head muscles and limb muscles of rats and cultured. The proliferation of MPCs was analysed by DNA quantification. The differentiation capacity was analysed by quantifying the numbers of fused cells, and by measuring the mRNA levels of differentiation markers. Muscle progenitor cells were stained to quantify the expression of paired box protein Pax 7 (Pax-7), myoblast determination protein 1 (MyoD), and myogenin. Proliferation was similar in the head MPCs and the limb MPCs. Differentiating head and limb MPCs showed a comparable number of fused cells and mRNA expression levels of myosin-1 (Myh1), myosin-3 (Myh3), and myosin-4 (Myh4). During proliferation and differentiation, the number of Pax-7(+), MyoD(+), and myogenin(+) cells in head and limb MPCs was equal. It was concluded that head and limb MPCs show similar myogenic capacities in vitro. Therefore, in vivo myogenic differences between those muscles might rely on the local microenvironment. Thus, regenerative strategies for limb muscles might also be used for head muscles.

Dynamics of angiogenesis during wound healing: a coupled in vivo and in silico study

OBJECTIVE

The most critical determinant of restoration of tissue structure during wound healing is the re-establishment of a functional vasculature, which largely occurs via angiogenesis, specifically endothelial sprouting from the pre-existing vasculature.

MATERIALS AND METHODS

We used confocal microscopy to capture sequential images of perfused vascular segments within the injured panniculus carnosus muscle in the mouse dorsal skin-fold window chamber to quantify a range of microcirculatory parameters during the first nine days of healing. This data was used to inform a mathematical model of sequential growth of the vascular plexus. The modeling framework mirrored the experimental circular wound domain and incorporated capillary sprouting and endothelial cell (EC) sensing of vascular endothelial growth factor gradients.

RESULTS

Wound areas, vessel densities and vessel junction densities obtained from the corresponding virtual wound were in excellent agreement both temporally and spatially with data measured during the in vivo healing process. Moreover, by perturbing the proliferative ability of ECs in the mathematical model, this leads to a severe reduction in vascular growth and poor healing. Quantitative measures from this second set of simulations were found to correlate extremely well with experimental data obtained from animals treated with an agent that targets endothelial proliferation (TNP-470).

CONCLUSION

Our direct combination and comparison of in vivo longitudinal analysis (over time in the same animal) and mathematical modeling employed in this study establishes a useful new paradigm. The virtual wound created in this study can be used to investigate a wide range of experimental hypotheses associated with wound healing, including disorders characterized by aberrant angiogenesis (e.g., diabetic models) and the effects of vascular enhancing/disrupting agents or therapeutic interventions such as hyperbaric oxygen.

A Rodent Model to Advance the Field Treatment of Crush Muscle Injury During Earthquakes and Other Natural Disasters

Approximately 170 earthquakes of 6.0 or higher magnitude occur annually worldwide. Victims often suffer crush muscle injuries involving impaired blood flow to the affected muscle and damage to the muscle fiber membrane. Current rescue efforts are directed toward preventing acute kidney injury (AKI), which develops upon extrication and muscle reperfusion. But field-usable, muscle-specific interventions may promote muscle regeneration and prevent or minimize the pathologic changes of reperfusion. Although current rodent crush injury models involve reperfusion upon removal of the crush stimulus, an analysis of their methodological aspects is needed to ensure adequate simulation of the earthquake-related crush injury. The objectives of this systematic review are to (a) describe rodent crush muscle injury models, (b) discuss the benefits and limitations of these models, and (c) offer a recommendation for animal models that would increase our understanding of muscle recovery processes after an earthquake-induced crush muscle injury. The most commonly used rodent model uses a clamping or pressing crush stimulus directly applied to murine hindlimb muscle. This model has increased our understanding of muscle regeneration but its open approach does not adequately represent the earthquake-related crush injury. The model we recommend for developing field-usable, muscle-specific interventions is a closed approach that involves a nonclamping crush stimulus. Findings from studies employing this recommended model may have greater relevance for developing interventions that lessen the earthquake’s devastating impact on individual and community health and quality of life, especially in developing countries.

Sca-1 influences the innate immune response during skeletal muscle regeneration

Efficient muscle regeneration requires the clearance of dead and dying tissue via phagocytosis before remodeling. We have previously shown that mice lacking stem cell antigen-1 (Sca-1) display a defect in skeletal muscle regeneration characterized by increased fibrosis and decreased turnover of the extracellular matrix. In the present study we demonstrate that Sca-1(-/-) mice have a defect in their capacity to recruit soluble IgM, and subsequently C3 complement, to damaged muscle. We hypothesize that this defect in recruitment delays or decreases phagocytosis by macrophages, contributing to the previously observed fibrotic phenotype of these mice. As the primary source of soluble IgM is peritoneal B-1a cells, which are a subset of self-renewing B cells, we analyzed this cell population and observed a significant reduction in B-1a cells in Sca-1(-/-) animals. Interestingly, these mice are protected from ischemia-reperfusion injury, an acute inflammatory reaction also mediated by IgM and C3 complement that has been linked to a deficit in B-1a cells in previous studies. Collectively, these data reveal a novel role for Sca-1 in innate immunity during muscle regeneration and indicate that further elucidation of immuno-myogenic processes will help to better understand and promote muscle regeneration.

Stem cells for the treatment of skeletal muscle injury

Skeletal muscle injuries are extremely common, accounting for up to 35%-55% of all sports injuries and quite possibly affecting all musculoskeletal traumas. These injuries result in the formation of fibrosis, which may lead to the development of painful contractures, increases patients' risk for repeat injuries, and limits their ability to return to a baseline or pre-injury level of function. The development of successful therapies for these injuries must consider the pathophysiology of these musculoskeletal conditions. We discuss the direct use of muscle-derived stem cells and some key cell population dynamics as well as the use of clinically applicable modalities that may enhance the local supply of stem cells to the zone of injury by promoting angiogenesis.

Sonic hedgehog regulates angiogenesis and myogenesis during post-natal skeletal muscle regeneration

Sonic hedgehog (Shh) is a morphogen-regulating crucial epithelial-mesenchymal interactions during embryonic development, but its signalling pathway is considered generally silent in post-natal life. In this study, we demonstrate that Shh is de novo expressed after injury and during regeneration of the adult skeletal muscle. Shh expression is followed by significant up-regulation of its receptor and target gene Ptc1 in injured and regenerating muscles. The reactivation of the Shh signalling pathway has an important regulatory role on injury-induced angiogenesis, as inhibition of Shh function results in impaired up-regulation of prototypical angiogenic agents, such as vascular endothelial growth factor (VEGF) and stromal-derived factor (SDF)-1alpha, decreased muscle blood flow and reduced capillary density after injury. In addition, Shh reactivation plays a regulatory role on myogenesis, as its inhibition impairs the activation of the myogenic regulatory factors Myf-5 and MyoD, decreases the up-regulation of insulin-like growth factor (IGF)-1 and reduces the number of myogenic satellite cells at injured site. Finally, Shh inhibition results in muscle fibrosis, increased inflammatory reaction and compromised motor functional recovery after injury. These data demonstrate that the Shh pathway is functionally important for adult skeletal muscle regeneration and displays pleiotropic angiogenic and myogenic potentials in post-natal life. These findings might constitute the foundation for new therapeutic approaches for muscular diseases in humans.

Age influences the early events of skeletal muscle regeneration: studies of whole muscle grafts transplanted between young (8 weeks) and old (13-21 months) mice

Injured skeletal muscle generally regenerates less efficiently with age, but little is understood about the effects of ageing on the very early inflammatory and neovascular events in the muscle repair process. This study used a total of 174 whole muscle grafts transplanted within and between young and old mice to analyse the effects of ageing on the early inflammatory response in two strains of mice (BALB/c and SJL/J). There was a very slight delay in the early inflammatory response, and in the appearance of myotubes at day 4 in BALB/c muscle grafted into an old host environment (implicating systemic events). In SJL/J mice, the initial speed of the inflammatory response was slightly delayed with old muscle grafts regardless of host age (implicating muscle-derived factors), while an old host environment transiently affected myogenesis (myotube formation). The slight delays in inflammatory and neovascular responses in old mice did not dramatically impact on the overall formation of new muscle. The neovascular response to injured young and old muscle tissue was further analysed using the corneal micropocket assay. This showed a very clear 1-2 day delay in angiogenesis induced by old versus young BALB/c muscle tissue implanted into the young rat cornea, indicating that new blood vessel formation is at least partly determined by muscle-derived factors. Taken together these results indicate that, while there are slight age-associated delays in inflammation and neovascularisation in response to injured muscle, there is no detrimental effect on myogenesis in the mouse model used in this study.

Laser microdissection-based expression analysis of key genes involved in muscle regeneration in mdx mice

We have used the mdx mice strain (C57BL/10ScSn-mdx) as an experimental subject for the study of reiterative skeletal muscle necrosis-regeneration with basement membrane preservation. In young mdx muscle, by means of Hematoxylin-Eosin staining, different types of degenerative-regenerative groups (DRG) can be recognized and assigned to a defined muscle regeneration phase. To evaluate the expression of known key-regulatory genes in muscle regeneration, we have applied Laser Capture Microdissection technique to obtain tissue from different DRGs encompassing the complete skeletal muscle regenerative process. The expression of MyoD, Myf-5 and Myogenin showed a rapid increase in the first two days post-necrosis, which were followed by MRF4 expression, when newly regenerating fibers started to appear (3-5days post-necrosis). MHCd mRNA levels, undetectable in mature non-injured fibers, increased progressively from the first day post-necrosis and reached its maximum level of expression in DRGs showing basophilic regenerating fibers. TGFbeta-1 mRNA expression showed a prompt and strong increase following fiber necrosis that persisted during the inflammatory phase, and progressively decreased when new regenerating fibers began to appear. In contrast, IGF-2 mRNA expression decreased during the first days post-necrosis but was followed by a progressive rise in its expression coinciding with the appearance of the newly formed myofibers, reaching the maximum expression levels in DRGs composed of medium caliber basophilic regenerating myofibers (5-7 days post-necrosis). mdx degenerative-regenerative group typing, in conjunction with laser microdissection-based gene expression analysis, opens up a new approach to the molecular study of skeletal muscle regeneration.

Recovery of Function in Skeletal Muscle Following 2 Different Contraction-Induced Injuries

OBJECTIVE

To determine if the proliferation of myogenic cells is equally important to recovery of contractile function after 2 different types of contraction-induced muscle injuries.

DESIGN

Randomized trial.

SETTING

Muscle biology laboratory.

ANIMALS

Adult male Sprague-Dawley rats.

INTERVENTIONS

Tibialis anterior muscles were injured by a single lengthening contraction with large strain (1R) or multiple lengthening contractions with small strain (MR). The hindlimbs of some animals in each group were irradiated before injury to prevent proliferation of myogenic cells during recovery.

MAIN OUTCOME MEASURES

Contractile tension was measured immediately after injury and 3, 7, 14, and 21 days after injury. Permeation to Evans blue dye was used to assay membrane damage. Centrally nucleated fibers and reverse transcriptase-polymerase chain reaction of myoD and myogenin were used as measures of myogenesis.

RESULTS

Inhibiting myogenesis prevented the recovery of contractile function after MR, but not after 1R. Both protocols caused Evans blue dye uptake immediately after injury, but Evans blue dye was only retained in fibers for several days after 1R. This suggests that membranes reseal after 1R, but not after MR.

CONCLUSIONS

The mechanisms that underlie recovery after injuries caused by repeated lengthening contractions and injuries caused by a single lengthening contraction are different. The differences may be important when planning targeted rehabilitation strategies for each type of injury.

Differential effects of post-natal development, animal strain and long term recovery on the restoration of neuromuscular function after neuromyotoxic injury in rat

We have analysed the effect of long term recovery, post-natal development and animal strain on the extent of restoration of neuromuscular function after neuromyotoxic injury in the rat (Rattus norvegicus). Muscle isometric contractile properties of soleus muscle in response to nerve stimulation were measured in situ in snake venom injured muscles and compared to contralateral uninjured muscles. We show here that neuromuscular function was not fully recovered until 24 weeks after injury in young adult (2-3 month old) Wistar rats. Moreover, the level of functional recovery 3 weeks after injury induced in juvenile rats (1 month old) was not globally different from that in younger adult, adult (10 month old) and older adult (24 month old) Wistar rats. Furthermore, the level of recovery of some contractile parameters differed between Wistar and Sprague-Dawley strains 3 weeks after injury. In conclusion, a very long time (>12 weeks) is required for full neuromuscular recovery following neuromyotoxic injury of young adult rats. Moreover, neuromuscular recovery during post-natal development is not markedly different from that during adult stage in the Wistar rat strain. Finally, some rat strain differences are observed in the recovery after injury of young adult rats.

Macrophages and skeletal muscle regeneration: a clodronate-containing liposome depletion study

The study evaluates the influence of monocytes/macrophages in the mechanisms of skeletal muscle injury using a mouse model and selective depletion of peripheral monocyte with systemic injections of liposomal clodronate (dichloromethylene bisphosphonate). This pharmacological treatment has been demonstrated to induce specific apoptotic death in monocytes and phagocytic macrophages. In the current studies, the liposomal clodronate injections resulted in a marked attenuation of the peak inflammatory response in the freeze-injured muscle in the first three days after injury. The effect was accompanied by a transient reduction (at day 1 or 3 postinjury) of the expression of several genes coding for inflammatory, as well as growth-related mediators, including TNF, monocyte chemoattractant protein (MCP)-1, thioredoxin, high-mobility group AT-hook 1, insulin-like growth factor-binding protein (IGFBP), and IGF-1. In contrast, the expression of major myogenic factors (i.e., MyoD and myogenin) directly involved in the activation/proliferation and differentiation of muscle precursor cells was not altered by the clodronate liposome treatment. The repair process in the injured muscle of clodronate liposome-treated mice was characterized by prolonged clearance of necrotic myofibers and a tendency for increased muscle fat accumulation at day 9 and 14 postinjury, respectively. In conclusion, a significant reduction of the initial monocyte/macrophage influx into the injured muscle is associated with not improved, but moderately impaired, repair processes after skeletal muscle injury.

Differential Necrosis Despite Similar Perfusion in Mouse Strains after Ischemia1

BACKGROUND

Numerous mouse models have been used to study the tissue response to ischemia, but multiple technical differences make comparisons difficult. We have comprehensively characterized the mouse hind limb ischemia model and determined how different genetic backgrounds of mice affect recovery.

MATERIALS AND METHODS

Severity of tissue necrosis and restoration of perfusion after femoral artery excision or femoral artery transection, using five different surgical procedures, were evaluated using laser Doppler imaging in a mouse model of hind limb ischemia. Severity of necrosis was concurrently measured using a five-point scale.

RESULTS

Significant differences were observed depending upon the surgical procedure used to initiate ischemia as well as the strain of mouse used. First, a progressively delayed and incomplete recovery of vascular perfusion occurred in relation to the anatomical position and extent of the arterial defect. Second, among mouse strains, the severity of tissue necrosis varied despite similar restoration of perfusion. Thus, DBA/1J mice had significantly increased severity and incidence of tissue loss as compared with either C57Bl/6J (P = 0.01) or BALB/c (P = 0.01) mice. Finally, contrary to previous reports, T-cell-mediated immune events did not modify ischemia-induced hind limb perfusion and necrosis as responses in nude mice were not different than controls on either BALB/c or C57Bl/6J backgrounds.

CONCLUSIONS

Surgical approach, mouse strain, and measures of hind limb perfusion and tissue injury are crucial considerations in the study of ischemia. Understanding how different genetic backgrounds in mice can affect necrosis may provide insights into the diverse healing responses observed in humans.

Mice deficient in plasminogen activator inhibitor-1 have improved skeletal muscle regeneration

Skeletal muscle possesses a remarkable capacity for regeneration. Although the regulation of this process at the molecular level remains largely undefined, the plasminogen system appears to play a critical role. Specifically, mice deficient in either urokinase-type plasminogen activator (uPA−/−mice) or plasminogen demonstrate markedly impaired muscle regeneration after injury. In the present study, we tested the hypothesis that loss of the primary inhibitor of uPA, plasminogen activator inhibitor-1 (PAI-1), would improve muscle regeneration. Repair of the extensor digitorum longus muscle was assessed after cardiotoxin injury in wild-type, uPA−/−, and PAI-1-deficient (PAI-1−/−) mice. As expected, there was no uPA activity in the injured muscles of uPA−/−mice, and muscles from these transgenic animals demonstrated impaired regeneration. On the other hand, uPA activity was increased in injured muscle from PAI-1−/−mice to a greater extent than in wild-type controls. Furthermore, PAI-1−/−mice demonstrated increased expression of MyoD and developmental myosin after injury as well as accelerated recovery of muscle morphology, protein levels, and muscle force compared with wild-type animals. The injured muscles of PAI-1-null mice also demonstrated increased macrophage accumulation, contrasting with impaired macrophage accumulation in uPA-deficient mice. The extent of macrophage accumulation correlated with both the clearance of protein after injury and the efficiency of regeneration. Taken together, these results indicate that PAI-1 deficiency promotes muscle regeneration, and this protease inhibitor represents a therapeutic target for enhancing muscle regeneration.

Determinants of skeletal muscle contributions from circulating cells, bone marrow cells, and hematopoietic stem cells

To investigate the factors that regulate incorporation into uninjured or damaged skeletal muscle of donor markers derived from unfractionated bone marrow (BM) cells or from highly purified c-kit+ Thy1.1lo Lin- Sca-1+ hematopoietic stem cells (HSCs), we evaluated myofiber chimerism of multiple muscle groups in irradiated and transplanted recipient mice and in unirradiated parabiotic animals. Uninjured panniculus carnosus, diaphragm, and abdominal muscles infrequently incorporated donor markers into myofibers in a subset of animals after either BM or HSC transplantation; however, acute muscle injury was essential to elicit contributions to triceps surae (TS) and tibialis anterior muscles. The low level of incorporation of donor marker-expressing myofibers could not be enhanced either by transplantation into newborn recipients or by induced migration of HSCs into the periphery. Analysis of muscle chimerism in unirradiated animals joined surgically by parabiosis revealed that contributions of circulating cells to myofibers in the TS were injury dependent and that at least some circulating cells with the potential to contribute to regenerating muscle derive from BM, suggesting that hematoablative preconditioning is not required for such contributions. In all cases tested, donor-derived myofibers expressed both donor-specific and host-specific markers, suggesting that they arise by low-level fusion into skeletal muscle of cells that can include the progeny of HSCs. It is not yet clear whether such events represent a normal myogenic pathway or a pathological response to muscle damage.

Perivascular cells expressing annexin A5 define a novel mesenchymal stem cell-like population with the capacity to differentiate into multiple mesenchymal lineages

The annexin A5 gene (Anxa5) was recently found to be expressed in the developing and adult vascular system as well as the skeletal system. In this paper, the expression of an Anxa5-lacZ fusion gene was used to define the onset of expression in the vasculature and to characterize these Anxa5-lacZ-expressing vasculature-associated cells. After blastocyst implantation, Anxa5-lacZ-positive cells were first detected in extra-embryonic tissues and in angioblast progenitors forming the primary vascular plexus. Later, expression is highly restricted to perivascular cells in most blood vessels resembling pericytes or vascular smooth muscle cells. Viable Anxa5-lacZ+ perivascular cells were isolated from embryos as well as adult brain meninges by specific staining with fluorescent X-gal substrates and cell-sorting. These purified lacZ+ cells specifically express known markers of pericytes, but also markers characteristic for stem cell populations. In vitro and in vivo differentiation experiments show that this cell pool expresses early markers of chondrogenesis, is capable of forming a calcified matrix and differentiates into adipocytes. Hence, Anxa5 expression in perivascular cells from mouse defines a novel population of cells with a distinct developmental potential.

A novel role for non-muscle gamma-actin in skeletal muscle sarcomere assembly

Existing models describing sarcomere assembly have arisen primarily from studies using cardiac muscle. In contrast to cardiac muscle, skeletal muscle differentiation is characterised by dramatic changes in protein expression, from non-muscle to muscle-specific isoforms before organisation of the sarcomeres. Consequently, little is understood of the potential influence of non-muscle cytoskeletal proteins on skeletal sarcomere assembly. To address this issue, transfectant (gamma33-B1) and control mouse C2 myoblasts were differentiated to form myotubes, and various stages of skeletal sarcomere assembly were studied. Organisation of non-muscle gamma-actin and co-localisation with sarcomeric alpha-actinin, an early marker of sarcomere assembly and a major component of Z lines, was noted. gamma-Actin was also identified in young myotubes with developing sarcomeric myofibrils in regenerating adult mouse muscle. Localisation of gamma-actin in a different area of the myotube to the muscle-specific sarcomeric alpha-actin also indicated a distinct role for gamma-actin. The effects of aberrant gamma-actin expression in other myoblast lines, further suggested a sequestering role for gamma-actin. These observations make the novel suggestion that non-muscle gamma-actin plays a role in skeletal sarcomere assembly both in vitro and in vivo. Consequently, a modified model is proposed which describes the role of gamma-actin in skeletal sarcomere assembly.

The COX-2 pathway is essential during early stages of skeletal muscle regeneration

Skeletal muscle regeneration comprises several overlapping cellular processes, including inflammation and myogenesis. Prostaglandins (PGs) may regulate muscle regeneration, because they modulate inflammation and are involved in various stages of myogenesis in vitro. PG synthesis is catalyzed by different isoforms of cyclooxygenase (COX), which are inhibited by nonsteroidal anti-inflammatory drugs. Although experiments employing nonsteroidal anti-inflammatory drugs have implicated PGs in tissue repair, how PGs regulate muscle regeneration remains unclear, and the potentially distinct roles of different COX isoforms have not been investigated. To address these questions, a localized freeze injury was induced in the tibialis anterior muscles of mice chronically treated with either a COX-1- or COX-2-selective inhibitor (SC-560 and SC-236, respectively), starting before injury. The size of regenerating myofibers was analyzed at time points up to 5 wk after injury and found to be decreased by SC-236 and in COX-2−/−muscles, but unaffected by SC-560. In contrast, SC-236 had no effect on myofiber growth when administered starting 7 days after injury. The attenuation of myofiber growth by SC-236 treatment and in COX-2−/−muscles is associated with decreases in the number of myoblasts and intramuscular inflammatory cells at early times after injury. Together, these data suggest that COX-2-dependent PG synthesis is required during early stages of muscle regeneration and thus raise caution about the use of COX-2-selective inhibitors in patients with muscle injury or disease.

Anti-TNFalpha (Remicade) therapy protects dystrophic skeletal muscle from necrosis

Necrosis of skeletal muscle fibers in the lethal childhood myopathy Duchenne muscular dystrophy (DMD) results from defects in the cell membrane-associated protein, dystrophin. This study tests the novel hypothesis that the initial sarcolemmal breakdown resulting from dystrophin deficiency is exacerbated by inflammatory cells and that cytokines, specifically tumor necrosis factor-alpha (TNFalpha), contribute to muscle necrosis. To block in vivo TNFalpha bioactivity, young dystrophic mdx mice (a model for DMD) were injected weekly from 7 days of age with the anti-TNFalpha antibody Remicade before the onset of muscle necrosis and dystropathology that normally occurs at 21 days postnatally. The extent of inflammation, muscle necrosis, and myotube formation was measured by histological analysis from 18 to 28 days and muscle damage was also visualized by penetration of Evans blue dye into myofibers. Data from Remicade-treated and control mdx mice were compared with mdx/TNFalpha-/- mice that lack TNFalpha. Pharmacological blockade of TNFalpha activity with Remicade clearly delayed and greatly reduced the breakdown of dystrophic muscle, in marked contrast to the situation in mdx and mdx/TNFalpha-/- mice. Remicade had no adverse effect on new muscle formation. Remicade is a highly specific anti-inflammatory intervention, and clinical application to muscular dystrophies is suggested by this marked protective effect against skeletal muscle breakdown.

Cellular and molecular regulation of muscle regeneration

Under normal circumstances, mammalian adult skeletal muscle is a stable tissue with very little turnover of nuclei. However, upon injury, skeletal muscle has the remarkable ability to initiate a rapid and extensive repair process preventing the loss of muscle mass. Skeletal muscle repair is a highly synchronized process involving the activation of various cellular responses. The initial phase of muscle repair is characterized by necrosis of the damaged tissue and activation of an inflammatory response. This phase is rapidly followed by activation of myogenic cells to proliferate, differentiate, and fuse leading to new myofiber formation and reconstitution of a functional contractile apparatus. Activation of adult muscle satellite cells is a key element in this process. Muscle satellite cell activation resembles embryonic myogenesis in several ways including the de novo induction of the myogenic regulatory factors. Signaling factors released during the regenerating process have been identified, but their functions remain to be fully defined. In addition, recent evidence supports the possible contribution of adult stem cells in the muscle regeneration process. In particular, bone marrow-derived and muscle-derived stem cells contribute to new myofiber formation and to the satellite cell pool after injury.

Downhill running in rats: influence on neutrophils, macrophages, and MyoD+ cells in skeletal muscle

The accumulation of neutrophils and macrophages, as well as the activation of satellite cells, are early events following skeletal muscle injury. We examined the temporal relationship between changes in neutrophils, macrophages, and MyoD protein, a marker of satellite cell activation, after injurious exercise. Male rats ( n=47) performed an intermittent downhill (-16% grade) running (17 m/min) protocol and the solei were obtained at 0, 2, 6, 24, 48, or 72 h post-exercise. Neutrophils, macrophages (ED1 and ED2), and MyoD+ cells were determined in muscle cross sections using immunohistochemistry. Downhill running increased ( P<or=0.05) the percentage of injured fibers and elevated blood creatine kinase activity. Neutrophils were elevated 18-fold relative to controls at 24 h post-exercise. ED1 macrophages were elevated four- and twofold at 24 and 48 h post-exercise, respectively. Neither ED2 macrophages nor MyoD+ cells were elevated post-exercise. These observations may indicate that elevations in neutrophils and ED1 macrophages after injurious exercise are not temporally associated with an increase in satellite cell activation.

Regeneration of transgenic skeletal muscles with altered timing of expression of the basic helix-loop-helix muscle regulatory factor MRF4

In regenerating muscle cells, muscle regulatory factor (MRF) 4 is normally the last of the four MRFs to be expressed. To analyze how the timing of MRF4 expression affects muscle regeneration, we compared regeneration after local freeze injury of muscles from wild-type mice with muscles from transgenic mice in which MRF4 expression was under control of an approximately 1.6-kb fragment of the myogenin promoter. Three days after injury, masseter and tibialis anterior (TA) muscles in wild-type mice expressed little or no MRF4 mRNA; whereas these muscles in transgenic mice expressed abundant MRF4 mRNA from both the transgene and the endogenous gene. Thus, MRF4 up-regulation was accelerated in transgenic compared to wild-type regenerating muscles, and expression of the transgene appeared to activate, perhaps indirectly, expression of the endogenous MRF4 gene. At 11 days after injury, regeneration, as measured by cross-sectional area and density of regenerated fibers, was significantly impaired in transgenic TA compared to wild-type TA, whereas at 19 days after injury both transgenic and TA muscle fibers had fully recovered to preinjury values. Regeneration of masseter muscles, which normally regenerate much less completely than TA muscles, was unaffected by the transgene. Thus, the timing of MRF4 up-regulation, as well as additional muscle-specific factors, can determine the progress of muscle regeneration.

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.

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.

Physiological role of tumor necrosis factor alpha in traumatic muscle injury

degenerative and regenerative roles of tumor necrosis factor alpha (TNF-alpha), a pro-inflammatory cytokine with pleiotropic functions, were investigated by using TNF receptor 1 and 2 double knockout (TNFR-DKO) and TNF-alpha antibody neutralized mice following traumatic freeze injury to the tibialis anterior muscle. In wild-type control mice, TNF-alpha mRNA transcripts and protein increased following injury and gradually returned to control (uninjured) levels by 13 days. A reduction in MyoD mRNA expression occurred in TNF-alpha-deficient mice, although there were no visible differences in MyoD immunostaining or histological characteristics in regenerating muscles. At 5 days post-injury, the reductions in isometric strength in TNFR-DKO and TNF-alpha-depleted mice did not differ from that of wild-type mice but by 13 days after injury, the TNFR-DKO and TNF-alpha-depleted mice exhibited strength deficits twice that of wild-type mice (i.e., 27-31% vs 13%). Muscle injury was also accompanied by increased expression of interleukin-6 (IL-6), but IL-6-deficient mice demonstrated MyoD expression and recovery of isometric strength similar to that of wild-type mice. These data indicate that TNF-alpha is involved in the recovery of muscle function after traumatic muscle injury, and this effect might be associated with modulation of muscle regulatory genes, including MyoD.

Muscle inflammatory cells after passive stretches, isometric contractions, and lengthening contractions

We tested the hypotheses that lengthening contractions, isometric contractions, and passive stretches increase muscle inflammatory cells (neutrophils and macrophages) and that prior conditioning with lengthening contractions, isometric contractions, or passive stretches reduces neutrophils and macrophages after subsequent lengthening contractions. Extensor digitorum longus muscles in anesthetized mice were subjected in situ to lengthening contractions, isometric contractions, or passive stretches. Six hours or 3 days after a protocol of contractions or passive stretches, neutrophils and macrophages were quantified in muscle cross sections. Three days after isometric contractions or passive stretches, neutrophils were elevated (P < 0.05) 3.7- and 5.5-fold, respectively, relative to controls. Both macrophages and neutrophils were increased 51.2- and 7.9-fold, respectively, after lengthening contractions. Prior lengthening contractions, isometric contractions, or passive stretches reduced inflammatory cells after lengthening contractions performed 2 wk later. The major finding of this study was that passive stretches and isometric contractions elevated neutrophils without causing overt signs of injury. Because both passive stretches and isometric contractions elevated neutrophils and afforded some protection from contraction-induced muscle injury, neutrophils and/or the related inflammatory events may contribute to the induction of a protective mechanism.

Lengthening contraction-induced inflammation is linked to secondary damage but devoid of neutrophil invasion

Inflammation triggered by exercise-induced muscle damage (EIMD) has been postulated to influence the extent of tissue destruction. We tested the hypotheses that 1) repressing inflammation decreases secondary damage production and 2) EIMD leads to a sequential appearance of inflammatory cells in which neutrophil accumulation precedes macrophage invasion. Rat ankle dorsiflexor muscles were submitted to in situ lengthening contractions. Measurement of in vitro contractile properties, inflammatory cell concentrations, and histological staining were performed postprotocol. Rats were treated with diclofenac, a nonsteroidal anti-inflammatory drug (NSAID group) to repress inflammation or with the vehicle solution (EIMD group). Muscles from the NSAID group had smaller force deficits on days 2 and 3 postexercise. This effect was associated with significantly smaller increases in the concentration of muscle macrophage ED1+ and ED2+. Surprisingly, neutrophils did not accumulate post-EIMD. These results suggest that inflammation-induced ED1+ macrophage accumulation is responsible for the secondary damage observed 2-3 days post-EIMD. We further conclude that an increase in ED1+ macrophage concentration can occur in absence of previous neutrophil invasion.

Dysferlin expression after normal myoblast transplantation in SCID and in SJL mice

Limb girdle muscular dystrophy type 2B form and Miyoshi myopathy are both caused by mutations in the recently cloned gene dysferlin. In the present study, we have investigated whether cell transplantation could permit dysferlin expression in vivo. Two transplantation models were used: SCID mice transplanted with normal human myoblasts, and SJL mice, the mouse model for limb girdle muscular dystrophy type 2B and Miyoshi myopathy, transplanted with allogeneic primary mouse muscle cell cultures expressing the beta-galactosidase gene under control of a muscle promoter of Troponin I. FK506 immunosuppression was used in the non-compatible allogeneic model. One month after transplantation, human and mouse dysferlin proteins were detected in all transplanted SCID and SJL muscles, respectively. Co-localization of dysferlin and human dystrophin or beta-galactosidase-positive fibers was observed following the transplantation of myoblasts. Dysferlin proteins were monitored by immunocytochemistry and Western blot. The number of dysferlin-positive fibers was 40-50% and 20-30% in SCID and SJL muscle sections, respectively. Detection of dysferlin in both SCID mice and dysferlin-deficient SJL mouse shows that myoblast transplantation permits the expression of the donor dysferlin protein.

The role of tumor necrosis factor-alpha (TNF-alpha) in skeletal muscle regeneration. Studies in TNF-alpha(-/-) and TNF-alpha(-/-)/LT-alpha(-/-) mice

The role of tumor necrosis factor-alpha (TNF-alpha), an important mediator of the inflammatory response after injury, was investigated in regenerating skeletal muscle. The pattern of expression of TNF-alpha during muscle regeneration was examined by immunohistochemistry in tissue sections of crush-injured or transplanted muscle autografts and in primary cultures of adult skeletal muscle. TNF-alpha was highly expressed in injured myofibers, inflammatory cells, endothelial cells, fibroblasts, and mast cells. Myoblasts and myotubes also expressed TNF-alpha in primary muscle cultures and tissue sections. The essential role of TNF-alpha and its homologue lymphotoxin-alpha (LT-alpha) during muscle regeneration was assessed by basic histology in TNF-alpha(-/-) and TNF-alpha(-/-)/LT-alpha(-/-) mice. No difference was apparent in the onset or pattern of muscle regeneration (i.e., inflammatory response, activation and fusion of myoblasts) between the two strains of null mice or between nulls and normal control mice. However, both strains of null mice appeared more prone to bystander damage of host muscle and regeneration distant from the site of injury/transplantation. Although expression of TNF-alpha may play an important role in muscle regeneration, the studies in the null mice show that redundancy within the cytokine system (or some other response) can effectively compensate for the absence of TNF-alpha in vivo.

Effect of atherogenic diet on reverse cholesterol transport in vivo in atherosclerosis susceptible (C57BL/6) and resistant (C3H) mice

Mice susceptible (C57BL/6) or resistant (C3H) to atherosclerosis induced by a high cholesterol-cholate containing diet (A-diet) were used to study reverse cholesterol transport (RCT) in vivo as measured by loss of cholesterol from a depot created by injection of cationized LDL into the rectus femoris muscle. Plasma total and HDL-cholesterol (HDL-C), total and HDL phospholipid (HDL-PL) levels in chow fed C3H male and female mice were higher than in C57BL/6 mice. After one month on A-diet, plasma cholesterol more than doubled in both strains and genders. The decrease in HDL-C and HDL-PL was twice as great in C57BL/6 as in C3H female mice, while in male C3H mice there was no decrease. The loss of exogenous cholesterol mass (ECM) after injection of cationized LDL was more rapid in C3H than in C57BL/6 mice. In chow fed mice, ECM retained in muscle on day 12 was 37% in C57BL/6 and 20% in C3H females; in males it was 39% and 18% in C57BL/6 and C3H, respectively. On A-diet, 76% were retained in C57BL/6 and 28% in C3H females; these values were 59% and 28% in C57BL/6 and C3H males. Thus, the slow clearance of ECM (which represents RCT) in C57BL/6 mice on A-diet, that could be related to a marked decrease of HDL-PL, might contribute towards their susceptibility to atherosclerosis.

Secondary reduction in calpain 3 expression in patients with limb girdle muscular dystrophy type 2B and Miyoshi myopathy (primary dysferlinopathies)

Dysferlin is the protein product of the gene (DYSF) that is defective in patients with limb girdle muscular dystrophy type 2B and Miyoshi myopathy. Calpain 3 is the muscle-specific member of the calcium activated neutral protease family and primary mutations in the CAPN3 gene cause limb girdle muscular dystrophy type 2A. The functions of both proteins remain speculative. Here we report a secondary reduction in calpain 3 expression in eight out of 16 patients with a primary dysferlinopathy and clinical features characteristic of limb girdle muscular dystrophy type 2B or Miyoshi myopathy. Previously CAPN3 analysis had been undertaken in three of these patients and two showed seemingly innocuous missense mutations, changing calpain 3 amino acids to those present in the sequences of calpains 1 and 2. These results suggest that there may be an association between dysferlin and calpain 3, and further analysis of both genes may elucidate a novel functional interaction. In addition, an association was found between prominent expression of smaller forms of the 80 kDa fragment of laminin alpha 2 chain (merosin) and dysferlin-deficiency.

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.

Skeletal muscle regeneration is not impaired in Fgf6 -/- mutant mice

FGF6 is a member of the fibroblast growth factor family. The Fgf6 gene is almost exclusively expressed in adult and developing skeletal muscle. We have obtained mice deficient in FGF6 by targeting the Fgf6 gene by homologous recombination. We studied regeneration of adult skeletal muscle in Fgf6 -/- mice derived on a standard inbred background. Muscle degeneration was induced by notexin drug or crush injury. The defect in FGF6 did not modify the kinetics of muscle regeneration. We bred Fgf6 -/- mice with mdx dystrophin deficient mice; Fgf6 -/-:mdx and mdx muscles were similar. Our study suggests that FGF6 does not play a role in muscle regeneration, i.e., in satellite cell proliferation and fusion, or that this role is strictly compensated by other factors, possibly other FGFs.

Exposure to tissue culture conditions can adversely affect myoblast behavior in vivo in whole muscle grafts: implications for myoblast transfer therapy

The effects of tissue culture conditions on the viability of myoblasts in whole muscles transplanted in vivo were investigated. Whole male (SJL/J) donor muscles were exposed to various tissue culture reagents and proteolytic enzymes, and allografted into female (SJL/J) host mice. Desmin immunohistochemistry was used to assess the numbers of myogenic cells (as an index of myoblast viability and the extent of regeneration) in tissue sections of whole-muscle grafts sampled on days 7 and 14. DNA quantitation with a Y-chromosome-specific probe was used to determine the total Y-1 sequence DNA (as an index of myoblast survival and proliferation) in whole-muscle grafts sampled on days 1, 3, and 7. In grafts exposed to serum-free medium, there was a delay in myoblast fusion at 7 days that was recovered by 14 days, but exposure to serum (10% or 20%) had a prolonged adverse effect on myotube formation at 14 days. DNA quantitation demonstrated that either serum-free culture medium or 10% serum enhanced the number of male cells within whole-muscle grafts at 7 days. Proteolytic digestion (even for 5 min) of whole muscles prior to grafting was extremely detrimental to myoblast survival and viability at 7 and 14 days. The unexpected finding of adverse effects of tissue culture conditions on the regeneration of whole-muscle grafts in vivo appears to parallel the major problem of the rapid death of isolated cultured donor myoblasts after injection in myoblast transfer therapy. The use of whole-muscle grafts provides an alternative and sensitive model to analyze the crucial effects of various tissue culture components on the subsequent survival and proliferation of myogenic cells in vivo.

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.