Retarded myogenic cell replication in regenerating skeletal muscles of old mice: an autoradiographic study in young and old BALBc and SJL/J mice

Adipose tissue in older individuals: a contributing factor to sarcopenia

Sarcopenia is a geriatric syndrome characterized by a functional decline in muscle. The prevalence of sarcopenia increases with natural aging, becoming a serious health problem among elderly individuals. Therefore, understanding the pathology of sarcopenia is critical for inhibiting age-related alterations and promoting health and longevity in elderly individuals. The development of sarcopenia may be influenced by interactions between visceral and subcutaneous adipose tissue and skeletal muscle, particularly under conditions of chronic low-grade inflammation and metabolic dysfunction. This hypothesis is supported by the following observations: (i) accumulation of senescent cells in both adipose tissue and skeletal muscle with age; (ii) gut dysbiosis, characterized by an imbalance in gut microbial communities as the main trigger for inflammation, sarcopenia, and aged adipose tissue; and (iii) microbial dysbiosis, which could impact the onset or progression of a senescent state. Moreover, adipose tissue acts as an endocrine organ, releasing molecules that participate in intricate communication networks between organs. Our discussion focuses on novel adipokines and their role in regulating adipose tissue and muscle, particularly those influenced by aging and obesity, emphasizing their contributions to disease development. On the basis of these findings, we propose that age-related adipose tissue and sarcopenia are disorders characterized by chronic inflammation and metabolic dysregulation. Finally, we explore new potential therapeutic strategies involving specialized proresolving mediator (SPM) G protein-coupled receptor (GPCR) agonists, non-SPM GPCR agonists, transient receptor potential (TRP) channels, antidiabetic drugs in conjunction with probiotics and prebiotics, and compounds designed to target senescent cells and mitigate their pro-inflammatory activity.

The effect of age on stem cell function and utility for therapy

During development, stem cells generate all of the differentiated cells that populate our tissues and organs. Stem cells are also responsible for tissue turnover and repair in adults, and as such, they hold tremendous promise for regenerative therapy. Aging, however, impairs the function of stem cells and is thus a significant roadblock to using stem cells for therapy. Paradoxically, the patients who would benefit the most from regenerative therapies are usually advanced in age. The use of stem cells from young donors or the rejuvenation of aged patient-derived stem cells may represent part of a solution. Nonetheless, the transplantation success of young or rejuvenated stem cells in aged patients is still problematic, since stem cell function is greatly influenced by extrinsic factors that become unsupportive with age. This article briefly reviews how aging impairs stem cell function, and how this has an impact on the use of stem cells for therapy.

Diminished force production and mitochondrial respiratory deficits are strain-dependent myopathies of subacute limb ischemia

OBJECTIVE

Reduced skeletal muscle mitochondrial function might be a contributing mechanism to the myopathy and activity based limitations that typically plague patients with peripheral arterial disease (PAD). We hypothesized that mitochondrial dysfunction, myofiber atrophy, and muscle contractile deficits are inherently determined by the genetic background of regenerating ischemic mouse skeletal muscle, similar to how patient genetics affect the distribution of disease severity with clinical PAD.

METHODS

Genetically ischemia protected (C57BL/6) and susceptible (BALB/c) mice underwent either unilateral subacute hind limb ischemia (SLI) or myotoxic injury (cardiotoxin) for 28 days. Limbs were monitored for blood flow and tissue oxygen saturation and tissue was collected for the assessment of histology, muscle contractile force, gene expression, mitochondrial content, and respiratory function.

RESULTS

Despite similar tissue O₂ saturation and mitochondrial content between strains, BALB/c mice suffered persistent ischemic myofiber atrophy (55.3% of C57BL/6) and muscle contractile deficits (approximately 25% of C57BL/6 across multiple stimulation frequencies). SLI also reduced BALB/c mitochondrial respiratory capacity, assessed in either isolated mitochondria (58.3% of C57BL/6 at SLI on day (d)7, 59.1% of C57BL/6 at SLI d28 across multiple conditions) or permeabilized myofibers (38.9% of C57BL/6 at SLI d7; 76.2% of C57BL/6 at SLI d28 across multiple conditions). SLI also resulted in decreased calcium retention capacity (56.0% of C57BL/6) in BALB/c mitochondria. Nonischemic cardiotoxin injury revealed similar recovery of myofiber area, contractile force, mitochondrial respiratory capacity, and calcium retention between strains.

CONCLUSIONS

Ischemia-susceptible BALB/c mice suffered persistent muscle atrophy, impaired muscle function, and mitochondrial respiratory deficits during SLI. Interestingly, parental strain susceptibility to myopathy appears specific to regenerative insults including an ischemic component. Our findings indicate that the functional deficits that plague PAD patients could include mitochondrial respiratory deficits genetically inherent to the regenerating muscle myofibers.

Mitochondrial Regulation of the Muscle Microenvironment in Critical Limb Ischemia

Critical limb ischemia (CLI) is the most severe clinical presentation of peripheral arterial disease and manifests as chronic limb pain at rest and/or tissue necrosis. Current clinical interventions are largely ineffective and therapeutic angiogenesis based trials have shown little efficacy, highlighting the dire need for new ideas and novel therapeutic approaches. Despite a decade of research related to skeletal muscle as a determinant of morbidity and mortality outcomes in CLI, very little progress has been made toward an effective therapy aimed directly at the muscle myopathies of this disease. Within the muscle cell, mitochondria are well positioned to modulate the ischemic cellular response, as they are the principal sites of cellular energy production and the major regulators of cellular redox charge and cell death. In this mini review, we update the crucial importance of skeletal muscle to CLI pathology and examine the evolving influence of muscle and endothelial cell mitochondria in the complex ischemic microenvironment. Finally, we discuss the novelty of muscle mitochondria as a therapeutic target for ischemic pathology in the context of the complex co-morbidities often associated with CLI.

Pericytes at the intersection between tissue regeneration and pathology

Perivascular multipotent cells, pericytes, contribute to the generation and repair of various tissues in response to injury. They are heterogeneous in their morphology, distribution, origin and markers, and elucidating their molecular and cellular differences may inform novel treatments for disorders in which tissue regeneration is either impaired or excessive. Moreover, these discoveries offer novel cellular targets for therapeutic approaches to many diseases. This review discusses recent studies that support the concept that pericyte subtypes play a distinctive role in myogenesis, neurogenesis, adipogenesis, fibrogenesis and angiogenesis.

Aging and regeneration in vertebrates

Aging is marked by changes that affect organs and resident stem cell function. Shorting of telomeres, DNA damage, oxidative stress, deregulation of genes and proteins, impaired cell-cell communication, and an altered systemic environment cause the eventual demise of cells. At the same time, reparative activities also decline. It is intriguing to correlate aging with the decline of regenerative abilities. Animal models with strong regenerative capabilities imply that aging processes might not be affecting regeneration. In this review, we selectively present age-dependent changes in stem/progenitor cells that are vital for tissue homeostasis and repair. In addition, the aging effect on regeneration following injury in organs such as lung, skeletal muscle, heart, nervous system, cochlear hair, lens, and liver are discussed. These tissues are also known for diseases such as heart attack, stroke, cognitive impairment, cataract, and hearing loss that occur mostly during aging in humans. Conclusively, vertebrate regeneration declines with age with the loss of stem/progenitor cell function. Future studies on improving the function of stem cells, along with studies in fish and amphibians where regeneration does not decline with age, will undoubtedly provide insights into both processes.

Efficient myoblast expansion for regenerative medicine use

Cellular therapy using expanded autologous myoblasts is a treatment modality for a variety of diseases. In the present study, we compared the commercial skeletal muscle cell growth medium-2 (SKGM-2) with a medium designed by our group for the expansion of skeletal myoblasts. The use of an in-house medium [DMEM/F12 medium supplemented with EGF, bFGF, HGF, insulin and dexamethasone (DFEFH)] resulted in a greater number of myoblast colonies (>50%) and a 3-, 4- and 9‑fold higher proliferation rate, eventually resulting in a 3-, 7- and 87-fold greater number of cells at the 1st, 2nd and 3rd passage, respectively, compared with the cells grown in SKGM-2 medium. The average CD56 expression level was higher in the myoblasts cultured in DFEFH than in those culturd in SKGM-2 medium. At the 3rd passage, lower expression levels of myostatin and considerably higher expression levels of myogenin were observed in the cells that were grown in DFEFH medium. The results of our study indicated that myoblasts cultured in both medium types displayed fusogenic potential at the 3rd passage. Furthermore, it was shown that cells cultured in DFEFH medium created myotubes with a considerably higher number of nuclei. Additionally, we observed that the fusion potential of the cells markedly decreased with the subsequent passages and that the morphology of the myoblasts differed between the 2 cultured media. Our data demonstrate that culture in the DFEFH medium leads to an approximately 90‑fold greater number of myoblasts, with improved morphology and greater fusion potential, compared with culture in the commercial SKGM-2 medium.

p38 MAPK signaling underlies a cell-autonomous loss of stem cell self-renewal in skeletal muscle of aged mice

Skeletal muscle aging results in a gradual loss of skeletal muscle mass, skeletal muscle function and regenerative capacity, which can lead to sarcopenia and increased mortality. Although the mechanisms underlying sarcopenia remain unclear, the skeletal muscle stem cell, or satellite cell, is required for muscle regeneration. Therefore, identification of signaling pathways affecting satellite cell function during aging may provide insights into therapeutic targets for combating sarcopenia. Here, we show that a cell-autonomous loss in self-renewal occurs via alterations in fibroblast growth factor receptor-1, p38α and p38β mitogen-activated protein kinase signaling in satellite cells from aged mice. We further demonstrate that pharmacological manipulation of these pathways can ameliorate age-associated self-renewal defects. Thus, our data highlight an age-associated deregulation of a satellite cell homeostatic network and reveal potential therapeutic opportunities for the treatment of progressive muscle wasting.

Age-dependent alteration in muscle regeneration: the critical role of tissue niche

Although adult skeletal muscle is composed of fully differentiated fibers, it retains the capacity to regenerate in response to injury and to modify its contractile and metabolic properties in response to changing demands. The major role in the growth, remodeling and regeneration is played by satellite cells, a quiescent population of myogenic precursor cells that reside between the basal lamina and plasmalemma and that are rapidly activated in response to appropriate stimuli. However, in pathologic conditions or during aging, the complete regenerative program can be precluded by fibrotic tissue formation and resulting in functional impairment of the skeletal muscle. Our study, along with other studies, demonstrated that although the regenerative program can also be impaired by the limited proliferative capacity of satellite cells, this limit is not reached during normal aging, and it is more likely that the restricted muscle repair program in aging is presumably due to missing signals that usually render the damaged muscle a permissive environment for regenerative activity.

Skeletal muscle-specific genetic determinants contribute to the differential strain-dependent effects of hindlimb ischemia in mice

Genetics plays an important role in determining peripheral arterial disease (PAD) pathology, which causes a spectrum of clinical disorders that range from clinically silent reductions in blood flow to limb-threatening ischemia. The cell-type specificity of PAD pathology, however, has received little attention. To determine whether strain-dependent differences in skeletal muscle cells might account for the differential responses to ischemia observed in C57BL/6 and BALB/c mice, endothelial and skeletal muscle cells were subjected to hypoxia and nutrient deprivation (HND) in vitro, to mimic ischemia. Muscle cells were more susceptible to HND than were endothelial cells. In vivo, C57BL/6 and BALB/c mice displayed strain-specific differences in myofiber responses after hindlimb ischemia, with significantly greater myofiber atrophy, greater apoptosis, and attenuated myogenic regulatory gene expression and stress-responsive signaling in BALB/c mice. Strain-specific deficits were recapitulated in vitro in primary muscle cells from both strains after HND. Muscle cells from BALB/c mice congenic for the C57BL/6 Lsq-1 quantitative trait locus were protected from HND-induced atrophy, and gene expression of vascular growth factors and their receptors was significantly greater in C57BL/6 primary muscle cells. Our results indicate that the previously identified specific genetic locus regulating strain-dependent collateral vessel density has a nonvascular or muscle cell-autonomous role involving both the myogenic program and traditional vascular growth factor receptor expression.

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.

Heterogeneity in the muscle satellite cell population

Satellite cells, the adult stem cells responsible for skeletal muscle regeneration, are defined by their location between the basal lamina and the fiber sarcolemma. Increasing evidence suggests that satellite cells represent a heterogeneous population of cells with distinct embryological origin and multiple levels of biochemical and functional diversity. This review focuses on the rich diversity of the satellite cell population based on studies across species. Ultimately, a more complete characterization of the heterogeneity of satellite cells will be essential to understand the functional significance in terms of muscle growth, homeostasis, tissue repair, and aging.

Aging-associated oxidative stress modulates the acute inflammatory response in skeletal muscle after contusion injury

Inflammation is an integral component of the response of skeletal muscle to a contusion injury that can be modulated by acute oxidative stress. Less is known regarding the effect of aging-associated oxidative stress on the inflammatory response in injured skeletal muscle. The purpose of this project was to assess the level of oxidative stress in skeletal muscles of young, adult, and old rats and determine its effect on the acute inflammatory response to a contusion injury. Inherent oxidative stress in the muscle was determined by measuring the glutathione:glutathione disulfide ratio, and levels of GP91(phox). Elevated oxidative stress was observed in uninjured muscles of adult and old rats and was accompanied by increased levels of lipid peroxidation and neutrophil chemoattractant CINC-1. After injury, the acute inflammatory response (8h, 3 d) was determined from markers of neutrophil (myeloperoxidase) and macrophage (CD68) content and by expression of NFkappaB, CINC-1 and TGF-beta1. Compared to injured muscles of young rats, NFkappaB, myeloperoxidase activity (8h), macrophage content (3 d), and TGF-beta1 (8h and 3 d) were significantly greater in injured muscles of old rats. We conclude that aging-associated oxidative stress in muscles of old rats exacerbated the inflammatory response to contusion injury and leads to increased TGF-beta1-induced collagen content.

Skeletal muscle as a paradigm for regenerative biology and medicine

Tissue development and regeneration share common features, since modules of regulatory pathways and transcription factors that are crucial for prenatal development are redeployed for tissue reconstruction after trauma. Regenerative medicine has therefore gained important insights through the study of developmental and regenerative biology. Moreover, diverse experimental models have been used to investigate the regeneration process in different tissues and organs. Paradoxically, little is known regarding the relative contribution of stem cells with respect to the supporting tissue during tissue regeneration. Particular attention will be given to mouse models using distinct injury paradigms to investigate the regenerative biology of skeletal muscle. An understanding of the response of stem and parenchymal cells is crucial for the development of clinical strategies to combat the normal decline in tissue performance during aging or its reconstitution after trauma and during disease. This review addresses these issues, focusing on muscle regeneration and how different factors, including genes, cells and the environment, impinge on this process.

Hepatocyte growth factor (HGF) and the satellite cell response following muscle lengthening contractions in humans

The time-courses of satellite cell (SC) activation and protein expression of hepatocyte growth factor (HGF), HGF activator (HGFA), HGFA inhibitor-1 (HAI-1), and HGFA inhibitor-2 (HAI-2) in human skeletal muscle, as well as serum HGF following a single bout of muscle lengthening contractions, were determined. Eight recreationally active participants were recruited for the study. Subjects performed 300 lengthening contractions involving the quadriceps femoris muscles of a single leg at a fixed velocity of 180 degrees/s. Percutaneous muscle biopsies were taken before (PRE) and at 4 h (T4), 24 h (T24), 72 h (T72), and 120 h (T120) following the exercise. The protocol resulted in an increase in the number of SCs [neural cell adhesion molecule (NCAM)-labeled cells] expressed relative to total myonuclei, at T24, compared with both PRE and T4 (P<0.05), and peaked at T72 (approximately 80% increase vs. PRE, P<0.05). HGF protein increased significantly in serum from baseline (PRE) to T4 (P<0.05). Active HGF protein was detected in skeletal muscle at rest [14.4+/-1.3 average integrated density value (IDV)/actin average IDV] and tended to increase at early time-points (P=0.12). HGFA protein increased significantly from PRE to T24 (P<0.05). HAI-1 protein increased significantly from PRE to T24 (P<0.05). HAI-2 (32 kDa) increased significantly from baseline (PRE) by T24 (P<0.05), and also by T72 and T120 (P<0.05). We conclude that a single bout of lengthening muscle contractions is sufficient to activate SCs, which may involve both a local and systemic HGF response to contraction-induced injury.

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.

Intrinsic changes and extrinsic influences of myogenic stem cell function during aging

The myogenic stem cell (satellite cell) is almost solely responsible for the remarkable regeneration of adult skeletal muscle fibers after injury. The availability and the functionality of satellite cells are the determinants of efficient muscle regeneration. During aging, the efficiency of muscle regeneration declines, suggesting that the functionality of satellite cells and their progeny may be altered. Satellite cells do not sit in isolation but rather are surrounded by, and influenced by, many extrinsic factors within the muscle tissue that can alter their functionality. These factors likely change during aging and impart both reversible and irreversible changes to the satellite cells and on their proliferating progeny. In this review, we discuss the possible mechanisms of impaired muscle regeneration with respect to the biology of satellite cells. Future studies that enhance our understanding of the interactions between stem cells and the environment in which they reside will offer promise for therapeutic applications in age-related diseases.

Ibuprofen Inhibits Skeletal Muscle Hypertrophy in Rats

PURPOSE

We sought to determine whether cyclooxygenase (COX) activity is necessary for overload-induced growth of adult rat skeletal muscle, and whether nitric oxide synthase (NOS) activity is involved in upregulation of COX messenger RNA (mRNA) expression in skeletal muscle.

METHODS

Unilateral surgical removal of the gastrocnemius and soleus was performed on the right hindlimb of 16 female Sprague-Dawley rats (approximately 230 g) to induce chronic overload (OL) of the plantaris for 14 d, with sham surgeries performed on the contralateral leg as a normally loaded (NL) control. Half of the rats were treated with the nonspecific COX inhibitor, ibuprofen (0.2 mg.mL(-1) in drinking water; approximately 20 mg.kg(-1).d(-1)). In a second experiment, the plantaris was unilaterally overloaded for 5 or 14 d in male rats (approximately 350 g; N = 16 rats per time point) and half of the animals were treated with the NOS inhibitor, L-NAME (0.75 mg.mL(-1) in drinking water; approximately 90 mg.kg(-1).d(-1)).

RESULTS

Ibuprofen treatment inhibited plantaris hypertrophy by approximately 50% (P < 0.05) following 14 d of OL, as did L-NAME treatment (P < 0.05). COX-1 and COX-2 mRNA did not differ between any groups at 5 d. At 14 d, however, L-NAME caused a 30-fold increase in plantaris COX-1 mRNA expression independent of loading condition. Additionally, OL induced a 20-fold increase in COX-2 mRNA expression compared with NL (P < 0.05) at 14 d, without affecting COX-1 mRNA level. L-NAME treatment significantly inhibited OL-induced expression of COX-2 mRNA.

CONCLUSION

COX activity is important for in vivo muscle hypertrophy, and plantaris overload is associated with NOS activity-dependent COX-2 expression.

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.

Pattern of Pax7 expression during myogenesis in the posthatch chicken establishes a model for satellite cell differentiation and renewal

The paired-box transcription factor Pax7 plays a critical role in the specification of satellite cells in mouse skeletal muscle. In the present study, the position and number of Pax7-expressing cells found in muscles of growing and adult chickens confirm the presence of this protein in avian satellite cells. The expression pattern of Pax7 protein, along with the muscle regulatory proteins MyoD and myogenin, was additionally elucidated in myogenic cultures and in whole muscle from posthatch chickens. In cultures progressing from proliferation to differentiation, the expression of Pax7 in MyoD+ cells declined as the cells began expressing myogenin, suggesting Pax7 as an early marker for proliferating myoblasts. At all time points, some Pax7+ cells were negative for MyoD, resembling the reserve cell phenotype. Clonal analysis of muscle cell preparations demonstrated that single progenitors can give rise to both differentiating and reserve cells. In muscle tissues, Pax7 protein expression was the strongest by 1 day posthatch, declining on days 3 and 6 to a similar level. In contrast, myogenin expression peaked on day 3 and then dramatically declined. This finding was accompanied by a robust growth in fiber diameter between day 3 and 6. The distinctions in Pax7 and myogenin expression patterns, both in culture and in vivo, indicate that while some of the myoblasts differentiate and fuse into myofibers during early stages of posthatch growth, others retain their reserve cell capacity.

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.

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.

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.

Kinetics of myoblast proliferation show that resident satellite cells are competent to fully regenerate skeletal muscle fibers

The satellite cell compartment provides skeletal muscle with a remarkable capacity for regeneration. Here, we have used isolated myofibers to investigate the activation and proliferative potential of satellite cells. We have previously shown that satellite cells are heterogeneous: the majority express Myf5 and M-cadherin protein, presumably reflecting commitment to myogenesis, while a minority is negative for both. Although MyoD is rarely detected in quiescent satellite cells, over 98% of satellite cells contain MyoD within 24 h of stimulation. Significantly, MyoD is only observed in cells that are already expressing Myf5. In contrast, a minority population does not activate by the criteria of Myf5 or MyoD expression. Following the synchronous activation of the myogenic regulatory factor+ve satellite cells, their daughter myoblasts proliferate with a doubling time of approximately 17 h, irrespective of the fiber type (type I, IIa, or IIb) from which they originate. Although fast myofibers have fewer associated satellite cells than slow, and accordingly produce fewer myoblasts, each myofiber phenotype is associated with a complement of satellite cells that has sufficient proliferative potential to fully regenerate the parent myofiber within 4 days. This time course is similar to that observed in vivo following acute injury and indicates that cells other than satellite cells are not required for complete myofiber regeneration.

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 potential of muscle stem cells

Skeletal muscle contains two types of stem cells: satellite cells, which function as myogenic precursors, and a population of multipotent adult stem cells. Satellite cells are believed to form a stable, self-renewing pool of stem cells in adult muscle where they function in tissue growth and repair. An additional stem cell population in adult muscle displays a remarkable capacity to differentiate into hematopoietic cells as well as muscle following transplantation. This article discusses the characteristics and properties of these cell populations, the relationship between them, and the potential for stem cell-based muscle therapeutics.

The timing between skeletal muscle myoblast replication and fusion into myotubes, and the stability of regenerated dystrophic myofibres: an autoradiographic study in mdx mice

In mdx mice, a model for Duchenne muscular dystrophy, the timing between the replication of myoblasts and their incorporation into myotubes was determined autoradiographically. Thirty-eight mdx mice aged 23 d were injected with tritiated thymidine to label myoblasts replicating early in the dystrophic process. At intervals from 8 h to 30 d after injection the tibialis anterior muscles were removed, processed for autoradiography and analysed for labelled central myonuclei (derived from the progeny of myoblasts which had been labelled at 23 d). At 8 h after injection there were no labelled central myonuclei, showing that the labelled myoblasts had not fused within this time. At 1 d, 2 % of central myonuclei were labelled, at 2 d, up to 32% were labelled, at 3 d approximately 60% were labelled, and at 4 d the labelling peaked at 74%. In the 27 mice sampled from 5-30 d after injection, the levels of central myonuclear labelling varied enormously: from 1-63%. However, there was a consistent decrease in the numbers of labelled central myonuclei with time. This may have been due to dilution of the relative numbers of labelled myonuclei due to other, nonlabelled, myoblasts replicating after the availability of tritiated thymidine, and fusing. It was also possible that labelled myofibres underwent subsequent necrosis and were eliminated from the muscle. The proposal that a regenerated myofibre can undergo a subsequent cycle of necrosis and regeneration was supported by evidence of some necrotic myofibres with labelled and unlabelled central nuclei. These results have implications for understanding the cellular biology and pathology of dystrophic muscle, particularly in relation to myoblast transfer therapy as a potential treatment of Duchenne muscular dystrophy.

Regeneration of skeletal and cardiac muscle in mammals: do nonprimate models resemble human pathology?

Most of the available information regarding the regenerative potential and compensatory remodeling of mammalian tissues has been obtained from nonprimate animals, mainly rodent experimental models. The increasing use of transgenic mice for studies of the mechanisms controlling organogenesis and regeneration also requires a clear understanding of their applicability as experimental models for studies of similar processes in humans and other mammals. Application of modern cell biology methods to studies of regenerative processes has provided new insights into similarity and differences in cellular responses to injury in the tissues of different mammalian species. During more than 200-million years of progressive divergent evolution of mammals, cellular mechanisms of tissue regeneration and compensatory remodeling evolved together with increasingly adaptive functional specialization and structural complexity of mammalian tissues and organs. Rodents represent a phylogenetically ancient order of mammals that has conservatively retained a number of morphofunctional characteristics of early representatives of this class, which include enhanced regenerative capacity of tissues. A comparative analysis of regenerative processes in skeletal and cardiac muscle, as well as in several other mammalian tissues, shows that time courses and intensities of regeneration in response to the same type of injury vary even within taxonomically related species (e.g., rat, mouse, and hamster). The warm bloodedness of mammals facilitated the development of more complex mechanisms of metabolic, immune, and neurohumoral regulation, which resulted in a stronger dependence of regenerative processes on vascularization and innervation. For this reason, interspecies modifications of regenerative responses are limited by the capacity of the animal to resorb rapidly the foci of necrosis and to revascularize and reinnervate the volume of the regenerating tissue. These differences, among other factors, result in significantly lower rates of reparative regeneration in mammals possessing larger body sizes than rodents. A review of these data strongly indicates that the phylogenetic age and biological differences between different species should be taken into account before extrapolation of regenerative properties of nonprimate tissues on the regenerative responses in the primates.

Fibroblast growth factor promotes recruitment of skeletal muscle satellite cells in young and old rats

Although the role of satellite cells in muscle growth and repair is well recognized, understanding of the molecular events that accompany their activation and proliferation is limited. In this study, we used the single myofiber culture model for comparing the proliferative dynamics of satellite cells from growing (3-week-old), young adult (8- to 10-week-old), and old (9- to 11-month-old) rats. In these fiber cultures, the satellite cells are maintained in their in situ position underneath the fiber basement membrane. We first demonstrate that the cytoplasm of fiber-associated satellite cells can be monitored with an antibody against the extracellular signal regulated kinases 1 and 2 (ERK1 and ERK2), which belong to the mitogen-activated protein kinase (MAPK) superfamily. With this immunocytological marker, we show that the satellite cells from all three age groups first proliferate and express PCNA and MyoD, and subsequently, about 24 hr later, exit the PCNA+/MyoD+ state and become positive for myogenin. For all three age groups, fibroblast growth factor 2 (FGF2) enhances by about twofold the number of satellite cells that are capable of proliferation, as determined by monitoring the number of cells that transit from the MAPK+ phenotype to the PCNA+/MAPK+ or MyoD+/MAPK+ phenotype. Furthermore, contrary to the commonly accepted convention, we show that in the fiber cultures FGF2 does not suppress the subsequent transition of the proliferating cells into the myogenin+ compartment. Although myogenesis of satellite cells from growing, young adult, and old rats follows a similar program, two distinctive features were identified for satellite cells in fiber cultures from the old rats. First, a large number of MAPK+ cells do not appear to enter the MyoD-myogenin expression program. Second, the maximal number of proliferating satellite cells is attained a day later than in cultures from the young adults. This apparent "lag" in proliferation was not affected by hepatocyte growth factor (HGF), which has been implicated in accelerating the first round of satellite cell proliferation. HGF and FGF2 were equally efficient in promoting proliferation of satellite cells in fibers from old rats. Collectively, the investigation suggests that FGF plays a critical role in the recruitment of satellite cells into proliferation.

Heterogeneity among muscle precursor cells in adult skeletal muscles with differing regenerative capacities

Skeletal muscle has a remarkable capacity to regenerate after injury, although studies of muscle regeneration have heretofore been limited almost exclusively to limb musculature. Muscle precursor cells in skeletal muscle are responsible for the repair of damaged muscle. Heterogeneity exists in the growth and differentiation properties of muscle precursor cell (myoblast) populations throughout limb development but whether the muscle precursor cells differ among adult skeletal muscles is unknown. Such heterogeneity among myoblasts in the adult may give rise to skeletal muscles with different regenerative capacities. Here we compare the regenerative response of a masticatory muscle, the masseter, to that of limb muscles. After exogenous trauma (freeze or crush injuries), masseter muscle regenerated much less effectively than limb muscle. In limb muscle, normal architecture was restored 12 days after injury, whereas in masseter muscle, minimal regeneration occurred during the same time period. Indeed, at late time points, masseter muscles exhibited increased fibrous connective tissue in the region of damage, evidence of ineffective muscle regeneration. Similarly, in response to endogenous muscle injury due to a muscular dystrophy, widespread evidence of impaired regeneration was present in masseter muscle but not in limb muscle. To explore the cellular basis of these different regenerative capacities, we analyzed the myoblast populations of limb and masseter muscles both in vivo and in vitro. From in vivo analyses, the number of myoblasts in regenerating muscle was less in masseter compared with limb muscle. Assessment of population growth in vitro indicated that masseter myoblasts grow more slowly than limb myoblasts under identical conditions. We conclude that the impaired regeneration in masseter muscles is due to differences in the intrinsic myoblast populations compared to limb muscles.

Correlation between biomechanical and structural changes during the regeneration of skeletal muscle after laceration injury

A standardized and reliable model for muscle laceration injuries was developed. The biomechanical and morphological changes during the process of muscle repair after injury were analysed, and the reproducibility of the methods was evaluated. The soleus muscles of Sprague-Dawley rats were completely transected and were allowed to heal for 5, 7, 10, 14, 21, 28, or 56 days, when the muscles either were pulled to failure to measure different parameters of tensile strength or were removed for morphological analysis. During the repair process, the regenerating myofibers penetrated into the connective-tissue scar and formed new myotendinous junctions, thus restoring the functional continuity across the muscle stumps. The muscle atrophied significantly during the recovery period. Mechanical failure occurred in the scar until day 10, and thereafter it occurred within myofibers. Until day 10, the failure load, strain, and specific energy increased to as much as 46, 59, and 36% of the control level, respectively; thereafter, there were only minor changes. Stress (tensile strength per cross-sectional area) reached 86% of the control level by day 21 and further increased to as much as 96% of the control level until day 56. These results indicate that the scar becomes stronger than muscle within 14 days; thereafter, the weakest point is the atrophic muscle. The fact that the stress value was most rapidly normalized suggests that, qualitatively, the regenerated muscle had virtually regained its pretrauma strength by day 56 and that the low values of failure load, strain, and specific energy were mainly due to atrophy of the muscle. Thus, further increase in the tensile strength of the regenerated muscle-tendon unit may require active exercise to reverse muscle atrophy.

The identification of myogenic cells in skeletal muscle, with emphasis on the use of tritiated thymidine autoradiography and desmin antibodies

The identification of myogenic precursor cells (mpc) is a key factor in determining the early events in the myogenesis and regeneration of skeletal muscle. Although satellite cells have long been established as the providers of myoblastic cells, very little is really known (apart from their anatomical location in relation to muscle fibres and their ability to migrate) about the precise role of satellite cells in myogenesis. Numerous techniques for labelling mpc have been devised, but none of these has proven to be completely reliable in firmly establishing the origin of myogenic cells. The use of tritiated thymidine to label DNA in proliferating mpc (which are not specifically distinguishable at the time) and the subsequent location of their labelled progeny in myotube nuclei has revealed a great deal of data on the timing of myogenesis, but not about the nature of mpc themselves. DNA synthesis can also be detected by antibodies to the thymidine analogue, bromodeoxyuridine, and also by antibody staining for proliferating nuclear cell antigen. Like tritiated thymidine, these other markers are not specific for muscle but are general markers for DNA synthesis. In situ hybridisation of various muscle-specific genetic markers and their products has been informative, as has immunolabelling of myogenin, MyoD1 and desmin. Desmin labelling has been particularly instructive in identifying mpc because it is one of the first muscle-specific proteins to be produced in mpc. This review covers some of the techniques mentioned above and their usefulness in determining the early events in myogenesis.

Desmin-lacZ transgene expression and regeneration within skeletal muscle transplants

The purpose of this study was to investigate the initiation and time course of the regeneration process in fragments of skeletal muscle transplants as a function of muscle tissue age at implantation. The appearance of desmin occurs at the very beginning of myogenesis. The transgenic desmin nls lacZ mice used in the study bear a transgene in which the 1 kb DNA 5' regulatory sequence of the desmin gene is linked to a reporter gene coding for Escherichia coli beta-galactosidase. The desmin lacZ transgene labels muscle cells in which the desmin synthesis programme has commenced. We implanted pectoralis muscle fragments from fetal transgenic embryos and mature and old transgenic mice into mature non-transgenic mice. Early events of myogenesis occurring during regeneration started sooner in transplants from 4-month-old (day 3 post-implantation) muscle than in those from 24-month-old (day 5-6 post-implantation) muscle, and they lasted longer in those from young (day 17 post-implantation) than in those from old (day 14 post-implantation) muscle fragments. In adult muscle, transgene activation proceeded from the periphery toward the centre of the transplant. In transplants from fetal 18-day-old pectoralis, myotubes with transgene activity were observed from day 1 to day 19. Desmin immunoreactivity, which appeared about one day after transgene activation, was followed by myosin expression. In adult transplants, the continuity of laminin labelling was disrupted around degenerative fibres, illustrating alteration of the extracellular matrix. Our data suggest that satellite cells from old muscle tissue have lower proliferative capacity and/or less access to trophic substances released by the host (damaged fibres, vascularization) than those from fetal or young adult muscle.

The host environment determines strain-specific differences in the timing of skeletal muscle regeneration: cross-transplantation studies between SJL/J and BALB/c mice

The difference in the timing of the regeneration process of skeletal muscle between SJL/J and BALB/c mice was investigated using grafts of whole skeletal muscle (both autografts and allografts). Histological, autoradiographic and immunohistochemical techniques were used in the investigation. Infiltration of leucocytes into autografts, numbers of desmin-positive myogenic cells and myotube formation were all more advanced in the SJL/J compared with BALB/c mice. Furthermore, autoradiographic evidence showed that myoblasts in the SJL/J autografts were synthesising DNA 12 h earlier than myoblasts in BALB/c autografts. In allografts, where SJL/J host mice received BALB/c grafts, and vice versa, leucocyte infiltration and myotube formation occurred earlier in the BALB/c muscles grafted into SJL/J hosts, than in the reverse situation with BALB/c hosts. The results show that, at least for whole muscle grafts, it is the host environment which determines the speed and outcome of the regenerative process.