Depolarization-induced contraction and SR function in mechanically skinned muscle fibers from dystrophic mdx mice

Sarcoplasmic reticulum calcium handling in unbranched, immediately post-necrotic fast-twitch mdx fibres is similar to wild-type littermates

NEW FINDINGS

Central question: What are the early effects of dystrophin deficiency on SR Ca²⁺ handling in the mdx mouse?

MAIN FINDING

In the mdx mouse, Ca²⁺ handling by the SR is little affected by the absence of dystrophin when looking at fibres without branches that have just regenerated following massive myonecrosis. This has important implications for our understanding of Ca²⁺ pathology in the mdx mouse.

ABSTRACT

There is a variety of results in the literature regarding the effects of dystrophin deficiency on the Ca²⁺ -handling properties of the SR in mdx mice, an animal model of Duchenne muscular dystrophy. One possible source of variation is the presence of branched fibres. Fibre branching, a consequence of degenerative-regenerative processes such as muscular dystrophy, has in itself a significant influence on the function of the SR. In our present study we attempt to detect early effects of dystrophin deficiency on SR Ca²⁺ handling by using unbranched fibres from the immediate post-necrotic stage in mdx mice (just regenerated following massive necrosis). Using kinetically-corrected Fura-2 fluorescence signals measured during twitch and tetanus, we analysed the amplitude, rise time and decay time of Δ[Ca²⁺ ]_i in unfatigued and fatigued fibres. Decay was also resolved into SR pump and SR leak components. Fibres from mdx mice were similar in all respects to fibres from wt littermates apart from: (i) a smaller amplitude of the initial spike of Δ[Ca²⁺ ]_i during a tetanus; and (ii) a mitigation of the fall in Δ[Ca²⁺ ]_i amplitude during the course of fatigue. Our findings suggest that the early effects of a loss of dystrophin on SR Ca²⁺ handling in mdx mice are subtle, and emphasise the importance of distinguishing between Ca²⁺ pathology that is due to lack of dystrophin and Ca²⁺ pathology that is due to muscle degeneration. This article is protected by copyright. All rights reserved.

Abnormal Calcium Handling in Duchenne Muscular Dystrophy: Mechanisms and Potential Therapies

Duchenne muscular dystrophy (DMD) is an X-linked muscle-wasting disease caused by the loss of dystrophin. DMD is associated with muscle degeneration, necrosis, inflammation, fatty replacement, and fibrosis, resulting in muscle weakness, respiratory and cardiac failure, and premature death. There is no curative treatment. Investigations on disease-causing mechanisms offer an opportunity to identify new therapeutic targets to treat DMD. An abnormal elevation of the intracellular calcium (Cai2+) concentration in the dystrophin-deficient muscle is a major secondary event, which contributes to disease progression in DMD. Emerging studies have suggested that targeting Ca²⁺-handling proteins and/or mechanisms could be a promising therapeutic strategy for DMD. Here, we provide an updated overview of the mechanistic roles the sarcolemma, sarcoplasmic/endoplasmic reticulum, and mitochondria play in the abnormal and sustained elevation of Cai2+ levels and their involvement in DMD pathogenesis. We also discuss current approaches aimed at restoring Ca²⁺ homeostasis as potential therapies for DMD.

Absence of Dystrophin Disrupts Skeletal Muscle Signaling: Roles of Ca2+, Reactive Oxygen Species, and Nitric Oxide in the Development of Muscular Dystrophy

Dystrophin is a long rod-shaped protein that connects the subsarcolemmal cytoskeleton to a complex of proteins in the surface membrane (dystrophin protein complex, DPC), with further connections via laminin to other extracellular matrix proteins. Initially considered a structural complex that protected the sarcolemma from mechanical damage, the DPC is now known to serve as a scaffold for numerous signaling proteins. Absence or reduced expression of dystrophin or many of the DPC components cause the muscular dystrophies, a group of inherited diseases in which repeated bouts of muscle damage lead to atrophy and fibrosis, and eventually muscle degeneration. The normal function of dystrophin is poorly defined. In its absence a complex series of changes occur with multiple muscle proteins showing reduced or increased expression or being modified in various ways. In this review, we will consider the various proteins whose expression and function is changed in muscular dystrophies, focusing on Ca(2+)-permeable channels, nitric oxide synthase, NADPH oxidase, and caveolins. Excessive Ca(2+) entry, increased membrane permeability, disordered caveolar function, and increased levels of reactive oxygen species are early changes in the disease, and the hypotheses for these phenomena will be critically considered. The aim of the review is to define the early damage pathways in muscular dystrophy which might be appropriate targets for therapy designed to minimize the muscle degeneration and slow the progression of the disease.

Protective effect of female gender-related factors on muscle force-generating capacity and fragility in the dystrophic mdx mouse

INTRODUCTION

The dystrophic features in hindlimb skeletal muscles of female mdx mice are unclear.

METHODS

We analyzed force-generating capacity and force decline after lengthening contraction-induced damage (fragility).

RESULTS

Young (6-month-old) female mdx mice displayed reduced force-generating capacity (-18%) and higher fragility (23% force decline) compared with female age-matched wild-type mice. These 2 dystrophic features were less accentuated in young female than in young male mdx mice (-32% and 42% force drop). With advancing age, force-generating capacity decreased and fragility increased in old (20 month) female mdx mice (-21% and 57% force decline), but they were unchanged in old male mdx mice. Moreover, estradiol treatment had no effect in old female mdx mice.

CONCLUSIONS

Female gender-related factors mitigate dystrophic features in young but not old mdx mice. Further studies are warranted to identify the beneficial gender-related factor in dystrophic muscle.

Sarcoplasmic reticulum Ca2+ permeation explored from the lumen side in mdx muscle fibers under voltage control

Under resting conditions, external Ca²⁺ is known to enter skeletal muscle cells, whereas Ca²⁺ stored in the sarcoplasmic reticulum (SR) leaks into the cytosol. The nature of the pathways involved in the sarcolemmal Ca²⁺ entry and in the SR Ca²⁺ leak is still a matter of debate, but several lines of evidence suggest that these Ca²⁺ fluxes are up-regulated in Duchenne muscular dystrophy. We investigated here SR calcium permeation at resting potential and in response to depolarization in voltage-controlled skeletal muscle fibers from control and mdx mice, the mouse model of Duchenne muscular dystrophy. Using the cytosolic Ca²⁺ dye Fura2, we first demonstrated that the rate of Ca²⁺ increase in response to cyclopiazonic acid (CPA)–induced inhibition of SR Ca²⁺-ATPases at resting potential was significantly higher in mdx fibers, which suggests an elevated SR Ca²⁺ leak. However, removal of external Ca²⁺ reduced the rate of CPA-induced Ca²⁺ increase in mdx and increased it in control fibers, which indicates an up-regulation of sarcolemmal Ca²⁺ influx in mdx fibers. Fibers were then loaded with the low-affinity Ca²⁺ dye Fluo5N-AM to measure intraluminal SR Ca²⁺ changes. Trains of action potentials, chloro-m-cresol, and depolarization pulses evoked transient Fluo5N fluorescence decreases, and recovery of voltage-induced Fluo5N fluorescence changes were inhibited by CPA, demonstrating that Fluo5N actually reports intraluminal SR Ca²⁺ changes. Voltage dependence and magnitude of depolarization-induced SR Ca²⁺ depletion were found to be unchanged in mdx fibers, but the rate of the recovery phase that followed depletion was found to be faster, indicating a higher SR Ca²⁺ reuptake activity in mdx fibers. Overall, CPA-induced SR Ca²⁺ leak at −80 mV was found to be significantly higher in mdx fibers and was potentiated by removal of external Ca²⁺ in control fibers. The elevated passive SR Ca²⁺ leak may contribute to alteration of Ca²⁺ homeostasis in mdx muscle.

Hsp72 preserves muscle function and slows progression of severe muscular dystrophy

Duchenne muscular dystrophy (DMD) is a severe and progressive muscle wasting disorder caused by mutations in the dystrophin gene that result in the absence of the membrane-stabilizing protein dystrophin. Dystrophin-deficient muscle fibres are fragile and susceptible to an influx of Ca(2+), which activates inflammatory and muscle degenerative pathways. At present there is no cure for DMD, and existing therapies are ineffective. Here we show that increasing the expression of intramuscular heat shock protein 72 (Hsp72) preserves muscle strength and ameliorates the dystrophic pathology in two mouse models of muscular dystrophy. Treatment with BGP-15 (a pharmacological inducer of Hsp72 currently in clinical trials for diabetes) improved muscle architecture, strength and contractile function in severely affected diaphragm muscles in mdx dystrophic mice. In dko mice, a phenocopy of DMD that results in severe spinal curvature (kyphosis), muscle weakness and premature death, BGP-15 decreased kyphosis, improved the dystrophic pathophysiology in limb and diaphragm muscles and extended lifespan. We found that the sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase (SERCA, the main protein responsible for the removal of intracellular Ca(2+)) is dysfunctional in severely affected muscles of mdx and dko mice, and that Hsp72 interacts with SERCA to preserve its function under conditions of stress, ultimately contributing to the decreased muscle degeneration seen with Hsp72 upregulation. Treatment with BGP-15 similarly increased SERCA activity in dystrophic skeletal muscles. Our results provide evidence that increasing the expression of Hsp72 in muscle (through the administration of BGP-15) has significant therapeutic potential for DMD and related conditions, either as a self-contained therapy or as an adjuvant with other potential treatments, including gene, cell and pharmacological therapies.

Overexpression of SERCA1a in the mdx diaphragm reduces susceptibility to contraction-induced damage

Although the precise pathophysiological mechanism of muscle damage in dystrophin-deficient muscle remains disputed, calcium appears to be a critical mediator of the dystrophic process. Duchenne muscular dystrophy patients and mouse models of dystrophin deficiency exhibit extensive abnormalities of calcium homeostasis, which we hypothesized would be mitigated by increased expression of the sarcoplasmic reticulum calcium pump. Neonatal adeno-associated virus gene transfer of sarcoplasmic reticulum ATPase 1a to the mdx diaphragm decreased centrally located nuclei and resulted in reduced susceptibility to eccentric contraction-induced damage at 6 months of age. As the diaphragm is the mouse muscle most representative of human disease, these results provide impetus for further investigation of therapeutic strategies aimed at enhanced cytosolic calcium removal.

Upregulation of store-operated Ca2+ entry in dystrophic mdx mouse muscle

Store-operated Ca(2+) entry (SOCE) is an important mechanism in virtually all cells. In adult skeletal muscle, this mechanism is highly specialized for the rapid delivery of Ca(2+) from the transverse tubule into the junctional cleft during periods of depleting Ca(2+) release. In dystrophic muscle fibers, SOCE may be a source of Ca(2+) overload, leading to cell necrosis. However, this possibility is yet to be examined in an adult fiber during Ca(2+) release. To examine this, Ca(2+) in the tubular system and cytoplasm were simultaneously imaged during direct release of Ca(2+) from sarcoplasmic reticulum (SR) in skeletal muscle fibers from healthy (wild-type, WT) and dystrophic mdx mouse. The mdx fibers were found to have normal activation and deactivation properties of SOCE. However, a depression of the cytoplasmic Ca(2+) transient in mdx compared with WT fibers was observed, as was a shift in the SOCE activation and deactivation thresholds to higher SR Ca(2+) concentrations ([Ca(2+)](SR)). The shift in SOCE activation and deactivation thresholds was accompanied by an approximately threefold increase in STIM1 and Orai1 proteins in dystrophic muscle. While the mdx fibers can introduce more Ca(2+) into the fiber for an equivalent depletion of [Ca(2+)](SR) via SOCE, it remains unclear whether this is deleterious.

Inhibitory control over Ca(2+) sparks via mechanosensitive channels is disrupted in dystrophin deficient muscle but restored by mini-dystrophin expression

BACKGROUND

In dystrophic skeletal muscle, osmotic stimuli somehow relieve inhibitory control of dihydropyridine receptors (DHPR) on spontaneous sarcoplasmic reticulum elementary Ca(2+) release events (ECRE) in high Ca(2+) external environments. Such 'uncontrolled' Ca(2+) sparks were suggested to act as dystrophic signals. They may be related to mechanosensitive pathways but the mechanisms are elusive. Also, it is not known whether truncated dystrophins can correct the dystrophic disinhibition.

METHODOLOGY/PRINCIPAL FINDINGS

We recorded ECRE activity in single intact fibers from adult wt, mdx and mini-dystrophin expressing mice (MinD) under resting isotonic conditions and following hyper-/hypo-osmolar external shock using confocal microscopy and imaging techniques. Isotonic ECRE frequencies were small in wt and MinD fibers, but were markedly increased in mdx fibers. Osmotic challenge dramatically increased ECRE activity in mdx fibers. Sustained osmotic challenge induced marked exponential ECRE activity adaptation that was three times faster in mdx compared to wt and MinD fibers. Rising external Ca(2+) concentrations amplified osmotic ECRE responses. The eliminated ECRE suppression in intact osmotically stressed mdx fibers was completely and reversibly resuscitated by streptomycine (200 microM), spider peptide GsMTx-4 (5 microM) and Gd(3+) (20 microM) that block unspecific, specific cationic and Ca(2+) selective mechanosensitive channels (MsC), respectively. ECRE morphology was not substantially altered by membrane stress. During hyperosmotic challenge, membrane potentials were polarised and a putative depolarisation through aberrant MsC negligible excluding direct activation of ECRE through tubular depolarisation.

CONCLUSIONS/SIGNIFICANCE

Dystrophin suppresses spontaneous ECRE activity by control of mechanosensitive pathways which are suggested to interact with the inhibitory DHPR loop to the ryanodine receptor. MsC-related disinhibition prevails in dystrophic muscle and can be resuscitated by transgenic mini-dystrophin expression. Our results have important implications for the pathophysiology of DMD where abnormal MsC in dystrophic muscle confer disruption of microdomain Ca(2+) homeostasis. MsC blockers should have considerable therapeutic potential if more muscle specific compounds can be found.

Towards developing standard operating procedures for pre-clinical testing in the mdx mouse model of Duchenne muscular dystrophy

This review discusses various issues to consider when developing standard operating procedures for pre-clinical studies in the mdx mouse model of Duchenne muscular dystrophy (DMD). The review describes and evaluates a wide range of techniques used to measure parameters of muscle pathology in mdx mice and identifies some basic techniques that might comprise standardised approaches for evaluation. While the central aim is to provide a basis for the development of standardised procedures to evaluate efficacy of a drug or a therapeutic strategy, a further aim is to gain insight into pathophysiological mechanisms in order to identify other therapeutic targets. The desired outcome is to enable easier and more rigorous comparison of pre-clinical data from different laboratories around the world, in order to accelerate identification of the best pre-clinical therapies in the mdx mouse that will fast-track translation into effective clinical treatments for DMD.

Stressed out: the skeletal muscle ryanodine receptor as a target of stress

Over the past century, understanding the mechanisms underlying muscle fatigue and weakness has been the focus of much investigation. However, the dominant theory in the field, that lactic acidosis causes muscle fatigue, is unlikely to tell the whole story. Recently, dysregulation of sarcoplasmic reticulum (SR) Ca(2+) release has been associated with impaired muscle function induced by a wide range of stressors, from dystrophy to heart failure to muscle fatigue. Here, we address current understandings of the altered regulation of SR Ca(2+) release during chronic stress, focusing on the role of the SR Ca(2+) release channel known as the type 1 ryanodine receptor.

Muscle-specific overexpression of IGF-I improves E-C coupling in skeletal muscle fibers from dystrophic mdx mice

Duchenne muscular dystrophy (DMD) is a lethal X-linked disease caused by the absence of functional dystrophin. Abnormal excitation-contraction (E-C) coupling has been reported in dystrophic muscle fibers from mdx mice, and alterations in E-C coupling components may occur as a direct result of dystrophin deficiency. We hypothesized that muscle-specific overexpression of insulin-growth factor-1 (IGF-I) would reduce E-C coupling failure in mdx muscle. Mechanically skinned extensor digitorum longus muscle fibers from mdx mice displayed a faster decline in depolarization-induced force responses (DIFR); however, there were no differences in sarcoplasmic reticulum (SR)-mediated Ca(2+) resequestration or in the properties of the contractile apparatus when compared with nondystrophic controls. The rate of DIFR decline was restored to control levels in fibers from transgenic mdx mice that overexpressed IGF-I in skeletal muscle (mdx/IGF-I mice). Dystrophic muscles have a lower transcript level of a specific dihydropyridine receptor (DHPR) isoform, and IGF-I-mediated changes in E-C coupling were associated with increased transcript levels of specific DHPR isoforms involved in Ca(2+) regulation. Importantly, IGF-I overexpression also increased the sensitivity of the contractile apparatus to Ca(2+). The results demonstrate that IGF-I can ameliorate fundamental aspects of E-C coupling failure in dystrophic muscle fibers and that these effects are important for the improvements in cellular function induced by this growth factor.

Ca2+-independent phospholipase A2 enhances store-operated Ca2+ entry in dystrophic skeletal muscle fibers

Duchenne muscular dystrophy is caused by deficiency of dystrophin and leads to progressive weakness. It has been proposed that the muscle degeneration occurring in this disease is caused by increased Ca2+ influx due to enhanced activity of cationic channels that are activated either by stretch of the plasma membrane (stretch-activated channels) or by Ca2+-store depletion (store-operated channels). Using both cytosolic Ca2+ measurements with Fura-2 and the manganese quench method, we show here that store-operated Ca2+ entry is greatly enhanced in dystrophic skeletal flexor digitorum brevis fibers isolated from mdx5cv mice, a mouse model of Duchenne muscular dystrophy. Moreover, we show for the first time that store-operated Ca2+ entry in these fibers is under the control of the Ca2+-independent phospholipase A2 and that the exaggerated Ca2+ influx can be completely attenuated by inhibitors of this enzyme. Enhanced store-operated Ca2+ entry in dystrophic fibers is likely to be due to a near twofold overexpression of Ca2+-independent phospholipase A2. The Ca2+-independent phospholipase A2 pathway therefore appears as an attractive target to reduce excessive Ca2+ influx and subsequent degeneration occurring in dystrophic fibers.

Sparks, signals and shock absorbers: how dystrophin loss causes muscular dystrophy

The dystrophin-glycoprotein complex (DGC) can be considered as a specialized adhesion complex, linking the extracellular matrix to the actin cytoskeleton, primarily in muscle cells. Mutations in several components of the DGC lead to its partial or total loss, resulting in various forms of muscular dystrophy. These typically manifest as progressive wasting diseases with loss of muscle integrity. Debate is ongoing about the precise function of the DGC: initially a strictly mechanical role was proposed but it has been suggested that there is aberrant calcium handling in muscular dystrophy and, more recently, changes in MAP kinase and GTPase signalling have been implicated in the aetiology of the disease. Here, we discuss new and interesting developments in these aspects of DGC function and attempt to rationalize the mechanical, calcium and signalling hypotheses to provide a unifying hypothesis of the underlying process of muscular dystrophy.

Optimizing plasmid-based gene transfer for investigating skeletal muscle structure and function

Intramuscular injection of naked plasmid DNA is a less cytotoxic alternative to viral vectors for delivering genetic material to skeletal muscle in vivo. However, the low efficiency of plasmid-based gene transfer limits its potential therapeutic efficacy and/or its use for many experimental applications. Current strategies to enhance transfection efficiency (i.e., electroporation) can cause significant muscle damage, confounding physiological assessments such as muscle contractility. Optimizing protocols to limit damage is critical for accurate physiological, biochemical, and molecular measurements. Following extensive testing, we developed an electroporation protocol that enhances transfection efficiency in skeletal muscles without causing muscle damage. Pretreating mouse tibialis anterior muscles with hyaluronidase and electroporation at 75 V/cm (using 50% vol/vol saline as a vehicle for plasmid DNA) resulted in 22 +/- 5% of the muscle fibers expressing a reporter gene. This protocol did not compromise contractile function of skeletal muscles assessed at both the intact (whole) muscle and the cellular (single fiber) level. Furthermore, ectopic expression of insulin-like growth factor I to levels that induced muscle fiber hypertrophy without causing tissue damage or compromising muscle function highlights the therapeutic potential of these methods for myopathies, muscle wasting disorders, and other pathophysiologic conditions.

Role of contraction-induced injury in the mechanisms of muscle damage in muscular dystrophy

1. Duchenne muscular dystrophy (DMD) is a severe disease of skeletal muscle, characterized by an X-linked recessive inheritance and a lack of dystrophin in muscle fibres. It is associated with progressive and severe wasting and weakness of nearly all muscles and premature death by cardiorespiratory failure. 2. Studies investigating the susceptibility of dystrophic skeletal muscles to contraction-mediated damage, especially after lengthening actions where activated muscles are stretched forcibly, have concluded that dystrophin may confer protection to muscle fibres by providing a mechanical link between the contractile apparatus and the plasma membrane. In the absence of dystrophin, there is disruption to normal force transmission and greater stress placed upon myofibrillar and membrane proteins, leading to muscle damage. 3. Contraction protocols (involving activation and stretch of isolated muscles or muscle fibres) have been developed to assess the relative susceptibility of dystrophic (and otherwise healthy) muscles to contraction-induced injury. These protocols have been used successfully to determine the relative efficacy of different (gene, cell or pharmacological) interventions designed to ameliorate or cure the dystrophic pathology. More research is needed to develop specific 'contraction assays' that will assist in the evaluation of the clinical significance of different therapeutic strategies for muscular dystrophy.

Skinned single fibers from normal and dystrophin-deficient dogs incur comparable stretch-induced force deficits

Intact dystrophin-deficient canine muscles were previously shown to incur greater-than-normal stretch-induced force deficits. Here we tested the hypothesis that maximally activated detergent-treated (skinned) single fibers from normal and dystrophin-deficient dogs would incur comparable force deficits after stretch. Skinned cranial sartorius (CS) fibers from dystrophin-deficient and normal dogs were calcium-activated (pCa 4.5) and rapidly stretched. A single 30% stretch induced force deficits of 27.07 +/- 3.9% and 29.7 +/- 4.8% in dystrophin-deficient (n = 22) and normal (n = 18) fibers, respectively. Our data support the hypothesis that maximally activated skinned single fibers from normal and dystrophin-deficient dogs incur comparable force deficits after stretch. Our findings suggest that knowledge of the extent of stretch-induced force deficits following repeated stretch-activations in the GRMD dog may be useful to assess future therapeutic interventions aimed at replacing dystrophin in the sarcolemmal membrane.

Interferon-{beta} inhibits bleomycin-induced lung fibrosis by decreasing transforming growth factor-{beta} and thrombospondin

Pulmonary fibrosis is the result of abnormal processes of repair that occur after lung injury. Transforming growth factor (TGF)-beta is a key molecule in the progression of pulmonary fibrosis. Although clinical use of interferon (IFN)-beta did not improve survival in patients with idiopathic pulmonary fibrosis, because some preclinical studies have suggested that IFN-beta is a potent inhibitor of fibrogenesis, beneficial effects of IFN-beta have been expected. We therefore attempted to determine effects of IFN-beta and investigated the mechanism of action of IFN-beta in bleomycin-induced pulmonary fibrosis. Bleomycin at Day 0 and IFN-beta for 4 wk were administered intravenously to ICR mice. At 28 d after bleomycin injection, histologic and chemical analysis was performed for evaluation of effects of IFN-beta. Tissue distribution and amounts of TGF-beta1 and thrombospondin (TSP)-1/2 were analyzed. IFN-beta attenuated prolylhydroxylase activity, resulting in inhibition of pulmonary fibrosis. Bleomycin-induced increase in TGF-beta1 in epithelial cells and extracellular matrix was attenuated by IFN-beta. TSP-1/2 was limited in platelets of control mice, but was present in foamy cells in fibrotic regions induced by bleomycin. These findings suggest that the antifibrotic effect of IFN-beta is inhibition of TGF-beta and its activation via decrease in TSP-1/2 in lung tissue and change in location of TSP-1/2 from platelets to foamy cells.

Red blood cells inhibit proliferation and stimulate apoptosis in human lung fibroblasts in vitro

Cell proliferation and apoptosis are both important mechanisms for the regulation of tissue homeostasis. For instance, proliferation is crucial in wound repair, whereas apoptosis is important for removal of damaged cells and resolution of inflammation. Imbalance between cell proliferation and apoptosis can therefore lead to pathological conditions and disease. In inflammatory and fibrotic lung disorders, red blood cells (RBCs) can interact with fibroblasts and connective tissue. In the present study, we therefore hypothesized that the presence of RBCs can affect fibroblast proliferation and apoptosis. Human foetal lung fibroblasts (HFL-1) were cultured in the presence or absence of purified whole RBCs and RBC-conditioned media. RBC significantly decreased fibroblast proliferation as determined both by DNA content analysis (Hoechst 33258 staining, P < 0.01; WST-1, P < 0.001) and BrdU incorporation. After treatment with staurosporine (STS) for 48 h, apoptosis was determined by TUNEL and propidium iodide staining followed by flow cytometry analysis. RBCs augmented STS-induced apoptosis (median: 46.4%; range 12.0-90.4) compared to control cells (median 26.2%; range 7.1-45.5). Thus, our data indicate that the presence of RBCs affects both fibroblast proliferation and susceptibility to undergo apoptosis. Our findings therefore suggest a role for RBCs in regulating fibroblast homeostasis after tissue injury.

Changes in contractile activation characteristics of rat fast and slow skeletal muscle fibres during regeneration

Damaged skeletal muscle fibres are replaced with new contractile units via muscle regeneration. Regenerating muscle fibres synthesize functionally distinct isoforms of contractile and regulatory proteins but little is known of their functional properties during the regeneration process. An advantage of utilizing single muscle fibre preparations is that assessment of their function is based on the overall characteristics of the contractile apparatus and regulatory system and as such, these preparations are sensitive in revealing not only coarse, but also subtle functional differences between muscle fibres. We examined the Ca(2+)- and Sr(2+)-activated contractile characteristics of permeabilized fibres from rat fast-twitch (extensor digitorum longus) and slow-twitch (soleus) muscles at 7, 14 and 21 days following myotoxic injury, to test the hypothesis that fibres from regenerating fast and slow muscles have different functional characteristics to fibres from uninjured muscles. Regenerating muscle fibres had approximately 10% of the maximal force producing capacity (P(o)) of control (uninjured) fibres, and an altered sensitivity to Ca(2+) and Sr(2+) at 7 days post-injury. Increased force production and a shift in Ca(2+) sensitivity consistent with fibre maturation were observed during regeneration such that P(o) was restored to 36-45% of that in control fibres by 21 days, and sensitivity to Ca(2+) and Sr(2+) was similar to that of control (uninjured) fibres. The findings support the hypothesis that regenerating muscle fibres have different contractile activation characteristics compared with mature fibres, and that they adopt properties of mature fast- or slow-twitch muscle fibres in a progressive manner as the regeneration process is completed.

Decreased expression of ryanodine receptors alters calcium-induced calcium release mechanism in mdx duodenal myocytes

It is generally believed that alterations of calcium homeostasis play a key role in skeletal muscle atrophy and degeneration observed in Duchenne's muscular dystrophy and mdx mice. Mechanical activity is also impaired in gastrointestinal muscles, but the cellular and molecular mechanisms of this pathological state have not yet been investigated. We showed, in mdx duodenal myocytes, that both caffeine- and depolarization-induced calcium responses were inhibited, whereas acetylcholine- and thapsigargin-induced calcium responses were not significantly affected compared with control mice. Calcium-induced calcium release efficiency was impaired in mdx duodenal myocytes depending only on inhibition of ryanodine receptor expression. Duodenal myocytes expressed both type 2 and type 3 ryanodine receptors and were unable to produce calcium sparks. In control and mdx duodenal myocytes, both caffeine- and depolarization-induced calcium responses were dose-dependently and specifically inhibited with the anti-type 2 ryanodine receptor antibody. A strong inhibition of type 2 ryanodine receptor in mdx duodenal myocytes was observed on the mRNA as well as on the protein level. Taken together, our results suggest that inhibition of type 2 ryanodine receptor expression in mdx duodenal myocytes may account for the decreased calcium release from the sarcoplasmic reticulum and reduced mechanical activity.

Administration of insulin-like growth factor-I improves fatigue resistance of skeletal muscles from dystrophicmdx mice

Muscle fatigue occurs in many neuromuscular diseases, including the muscular dystrophies, and it contributes to a loss of functional capacity and reduced quality of life for affected patients. An improvement in fatigue resistance has been observed in diaphragm muscles of mdx mice following insulin-like growth factor-I (IGF-I) administration. Whether similar treatment can improve locomotor muscle function in mdx mice is not known. We examined the efficacy of IGF-I administration (1 mg/kg daily s.c. for 8 weeks) on structural, metabolic, and functional properties of extensor digitorum longus (EDL) and soleus muscles of mdx mice, and tested the hypothesis that IGF-I treatment would improve function in these muscles. After treatment, muscles were more resistant to fatigue during repeated maximal contractions than muscles from untreated mice, an improvement associated with increased muscle fiber succinate dehydrogenase activity in the absence of changes in cellular (single-fiber) contractile activation characteristics. The findings have important clinical implications, not just for the dystrophinopathies, but for all neuromuscular pathologies where fatigue of locomotor muscles limits functional capacity and decreases quality of life.

Conditional expression of transforming growth factor-alpha in adult mouse lung causes pulmonary fibrosis

To determine whether overexpression of transforming growth factor (TGF)-alpha in the adult lung causes remodeling independently of developmental influences, we generated conditional transgenic mice expressing TGF-alpha in the epithelium under control of the doxycycline (Dox)-regulatable Clara cell secretory protein promoter. Two transgenic lines were generated, and following 4 days of Dox-induction TGF-alpha levels in whole lung homogenate were increased 13- to 18-fold above nontransgenic levels. After TGF-alpha induction, transgenic mice developed progressive pulmonary fibrosis and body weight loss, with mice losing 15% of their weight after 6 wk of TGF-alpha induction. Fibrosis was detected within 4 days of TGF-alpha induction and developed initially in the perivascular, peribronchial, and pleural regions but later extended into the interstitium. Fibrotic regions were composed of increased collagen and cellular proliferation and were adjacent to airway and alveolar epithelial sites of TGF-alpha expression. Fibrosis progressed in the absence of inflammatory cell infiltrates as determined by histology, without changes in bronchiolar alveolar lavage total or differential cell counts and without changes in proinflammatory cytokines TNF-alpha or IL-6. Active TGF-beta in whole lung homogenate was not altered 1 and 4 days after TGF-alpha induction, and immunostaining was not increased in the peribronchial/perivascular areas at all time points. Chronic epithelial expression of TGF-alpha in adult mice caused progressive pulmonary fibrosis associated with increased collagen and extracellular matrix deposition and increased cellular proliferation. Induction of pulmonary fibrosis by TGF-alpha was independent of inflammation or early activation of TGF-beta.