Notexin-induced muscle injury in the dog

Preclinical Research in McArdle Disease: A Review of Research Models and Therapeutic Strategies

McArdle disease is an autosomal recessive disorder of muscle glycogen metabolism caused by pathogenic mutations in the PYGM gene, which encodes the skeletal muscle-specific isoform of glycogen phosphorylase. Clinical symptoms are mainly characterized by transient acute “crises” of early fatigue, myalgia and contractures, which can be accompanied by rhabdomyolysis. Owing to the difficulty of performing mechanistic studies in patients that often rely on invasive techniques, preclinical models have been used for decades, thereby contributing to gain insight into the pathophysiology and pathobiology of human diseases. In the present work, we describe the existing in vitro and in vivo preclinical models for McArdle disease and review the insights these models have provided. In addition, despite presenting some differences with the typical patient’s phenotype, these models allow for a deep study of the different features of the disease while representing a necessary preclinical step to assess the efficacy and safety of possible treatments before they are tested in patients.

Tissue-Engineered Human Myobundle System as a Platform for Evaluation of Skeletal Muscle Injury Biomarkers

Traditional serum biomarkers used to assess skeletal muscle damage, such as activity of creatine kinase (CK), lack tissue specificity and sensitivity, hindering early detection of drug-induced myopathies. Recently, a novel four-factor skeletal muscle injury panel (MIP) of biomarkers consisting of skeletal troponin I (sTnI), CK mass (CKm), fatty-acid-binding protein 3 (Fabp3), and myosin light chain 3, has been shown to have increased tissue specificity and sensitivity in rodent models of skeletal muscle injury. Here, we evaluated if a previously established model of tissue-engineered functional human skeletal muscle (myobundle) can allow detection of the MIP biomarkers after injury or drug-induced myotoxicity in vitro. We found that concentrations of three MIP biomarkers (sTnI, CKm, and Fabp3) in myobundle culture media significantly increased in response to injury by a known snake venom (notexin). Cerivastatin, a known myotoxic statin, but not pravastatin, induced significant loss of myobundle contractile function, myotube atrophy, and increased release of both traditional and novel biomarkers. In contrast, dexamethasone induced significant loss of myobundle contractile function and myotube atrophy, but decreased the release of both traditional and novel biomarkers. Dexamethasone also increased levels of matrix metalloproteinase-2 and -3 in the culture media which correlated with increased remodeling of myobundle extracellular matrix. In conclusion, this proof-of-concept study demonstrates that tissue-engineered human myobundles can provide an in vitro platform to probe patient-specific drug-induced myotoxicity and performance assessment of novel injury biomarkers to guide preclinical and clinical drug development studies.

Phosphorylase re-expression, increase in the force of contraction and decreased fatigue following notexin-induced muscle damage and regeneration in the ovine model of McArdle disease

McArdle disease is caused by a deficiency of myophosphorylase and currently a satisfactory treatment is not available. The injection of notexin into, or the layering of notexin onto, the muscles of affected sheep resulted in necrosis followed by regeneration of muscle fibres with the expression of both non-muscle isoforms of phosphorylase within the fibres and a reduction of the amount of glycogen in the muscle with an increase in the strength of contraction and a decrease in fatiguability in the muscle fibres. The sustained re-expression of both the brain and liver isoforms of phosphorylase within the muscle fibres provides further emphasis that strategies to enhance the re-expression of these isoforms should be investigated as a possible treatment for McArdle disease.

Periurethral skeletal myofibre implantation in patients with urinary incontinence and intrinsic sphincter deficiency: a phase I clinical trial

WHAT'S KNOWN ON THE SUBJECT? AND WHAT DOES THE STUDY ADD?: Cell therapy using muscle precursor cell (MPC) injections has shown promise for urinary incontinence due to intrinsic sphincter deficiency (ISD), but the cell-preparation process is complex and costly. Implantation of freshly isolated myofibres carrying MPCs, mainly satellite cells, was very efficient in repairing muscle damage in recent animal experiments. In a phase I clinical trial, we investigated whether periurethral myofibre implantation generated local myogenesis and improved continence in 10 patients (five men and five women) with ISD. We found that myofibre implantation increased intraurethral pressure and periurethral electromyographic activity in patients with ISD. There were no serious side-effects.

OBJECTIVES

To assess the safety of periurethral myofibre implantation in patients with urinary incontinence due to intrinsic sphincter deficiency (ISD) To assess the resulting myogenic process and effects on urinary continence.

PATIENTS AND METHODS

An open-label non-randomised phase I clinical trial was conducted in five men and five women with ISD (mean age, 62.5 years). A free muscle strip from the patient's gracilis muscle was implanted around the urethra as a means to deliver locally myofibres and muscle precursor cells (MPCs). Patients were assessed for collection formation and incomplete bladder emptying. The maximum urethral closure pressure (MUCP) and concomitant periurethral electromyographic (EMG) activity were recorded before surgery and 1 and 3 months after surgery. Continence was assessed using the 24-h pad test and self-completed questionnaires, for 12 months.

RESULTS

There were no serious side-effects. Continence improved significantly during the 12-month follow-up in four of the five women, including two who recovered normal continence. In the women, MUCP increased two-fold and de novo EMG periurethral activity was recorded. In the men, MUCP and EMG recordings showed similar improvements but the effect on continence was moderate. The few patients enrolled could affect these results.

CONCLUSIONS

This is the first report of a one-step procedure for transferring autologous MPCs via myofibre implantation in patients with ISD. EMG and urodynamic assessments showed improvement of periurethral muscle activity. Further work is needed to confirm and improve the therapeutic efficiency of this procedure.

The MRI assessment of intraurethrally--delivered muscle precursor cells using anionic magnetic nanoparticles

Autografting of cultured myogenic precursor cells (MPC) is a therapeutic strategy for muscle disorders, including striated urethral sphincter insufficiency. Implantation of myofibers with their satellite cells into the urethra is a recently described method of MPC transfer aimed at generating a new sphincter in incontinent patients. In this study, we magnetically labeled muscle implants with dextran-free anionic iron oxide nanoparticles (AMNP). The aim was to evaluate the biocompatibility of the labeling procedure and its utility for non-invasive MRI follow-up of cell therapy in a female pig model. After adsorption of AMNP to the implant surface, various cell types, including MPC, were magnetically labeled within the implants. Magnetic labeling did not affect cell proliferation or differentiation. Autograft detection in vivo by 0.3-T MRI was possible for up to 1 month. Ex vivo, Perl's, anti-desmin and anti-myosin heavy chain staining confirmed the co-localization of AMNP and regenerated myofibers. AMNP labeling was thus useful for locating myofiber implant autografts in vivo and for ex vivo monitoring of the biology of this cell transfer method.

Targeting the Neuromuscular Junction in Skeletal Muscles

Botulinum neurotoxins selectively weaken skeletal muscle by presynaptic blockade of vesicles containing the neurotransmitter acetylcholine. Because the location of the neuromuscular junction (found in motor endplates) is most highly concentrated in a band within the midbelly of the muscle, injection of botulinum neurotoxins are hypothesized to be more potent when specifically targeted toward the motor endplate zones. Few studies have attempted to describe the distribution of motor endplate zones in skeletal muscles, and scant experimental data exist that directly tests this hypothesis. In this article, a rationale is presented for motor endplate targeting within specific limb muscles with respect to botulinum neurotoxin therapy.

Skeletal muscle degeneration induced by venom phospholipases A2: insights into the mechanisms of local and systemic myotoxicity

Local and systemic skeletal muscle degeneration is a common consequence of envenomations due to snakebites and mass bee attacks. Phospholipases A2 (PLA2) are important myotoxic components in these venoms, inducing a similar pattern of degenerative events in muscle cells. Myotoxic PLA2s bind to acceptors in the plasma membrane, which might be lipids or proteins and which may differ in their affinity for the PLA2s. Upon binding, myotoxic PLA2s disrupt the integrity of the plasma membrane by catalytically dependent or independent mechanisms, provoking a pronounced Ca2+ influx which, in turn, initiates a complex series of degenerative events associated with hypercontraction, activation of calpains and cytosolic Ca(2+)-dependent PLA2s, and mitochondrial Ca2+ overload. Cell culture models of cytotoxicity indicate that some myotoxic PLA2s affect differentiated myotubes in a rather selective fashion, whereas others display a broad cytolytic effect. A model is presented to explain the difference between PLA2s that induce predominantly local myonecrosis and those inducing both local and systemic myotoxicity. The former bind not only to muscle cells, but also to other cell types, thereby precluding a systemic distribution of these PLA2s and their action on distant muscles. In contrast, PLA2s that bind muscle cells in a more selective way are not sequestered by non-specific interactions with other cells and, consequently, are systemically distributed and reach muscle cells in other locations.

The importance of electromyographic guidance and electrical stimulation for injection of botulinum toxin

The importance of EMG or electrical stimulation for guiding injections of botulinum toxin in limb muscles to treat dystonia or spasticity is based more on theoretical and preclinical data than on controlled clinical trials. Questions remain about the preferred administration of botulinum toxin for these conditions. Future clinical research is necessary to show clearly functional benefit of any particular injection localization method.

The regeneration process of the striated urethral sphincter involves activation of intrinsic satellite cells

The regeneration of adult skeletal muscle is mediated by satellite cells. Classically, these are considered to be somitically derived cells that colonize the limbs during early embryogenesis. The striated urethral sphincter presents specific developmental characteristics that distinguish it from skeletal muscles, such as the non-somitic origin of its precursor cells and the late formation of its myofibers. This prompted us to determine whether the striated urethral sphincter can regenerate after injury by the same mechanism as skeletal muscles. By means of the single myofiber explant culture technique we investigated the presence of satellite cells in the striated urethral sphincter of male mice and evaluated their ability to recapitulate a myogenic program. In addition, a myotoxic substance (notexin) was injected into the sphincter in order to provoke rapid destruction of the myofibers; the regeneration process was studied by means of electrophysiological and histological techniques. Satellite cells expressing pax7 were found attached to the sphincteric myofibers. They proliferated and expressed MyoD, Myf5 and desmin after 2 days in culture. After 10 days, they formed multinucleated myotubes expressing alpha-actinin-2. In vivo, complete recovery of the striated urethral sphincter, as assessed by normalization of muscle strength and of myofiber number and diameter, was observed after 3 weeks, and resulted from the fusion of myogenic cells. These results demonstrate that the striated urethral sphincter can regenerate by means of a myogenic program involving intrinsic satellite cells. The therapeutic implications of this knowledge and the possible origin of the sphincteric satellite cells are discussed.

Eccentric contraction injury in dystrophic canine muscle

OBJECTIVE

To test the hypothesis that eccentric contractions induce greater injury in dystrophic compared with normal canine muscle.

DESIGN

Blinded cohort study.

SETTING

Animal laboratory.

ANIMALS

Ten dogs with a homologue to Duchenne muscular dystrophy (Golden retriever muscular dystrophy [GRMD]) and 10 normal littermates.

INTERVENTIONS

Contractions induced in tibiotarsal flexors and extensors by sciatic nerve stimulation. Because more powerful extensors overrode flexors, eccentric contractions occurred in flexors. Concentric contractions were induced in contralateral flexors by peroneal nerve stimulation.

MAIN OUTCOME MEASURE

Tibiotarsal flexion force 3 days after contractions. Muscle was examined for injury (esterase activity, Evans blue dye penetration) and regeneration (embryonic myosin isoform expression).

RESULTS

Mean force deficit after eccentric flexor contractions was 43.3%+/-25.7% in GRMD dogs compared with 25.0%+/-18.4% in controls (P=.04, Wilcoxon rank-sum test). Concentric contractions induced force deficits in GRMD but not normal dogs; however, the difference between the 2 groups was not significant (P=.08, Wilcoxon rank-sum test). After concentric contractions in controls, force decrements correlated with esterase activity measured by area (r=.794, P=.006) and intensity (r=.697, P=.025, Spearman rank correlation). No other significant correlation was detected between force and biopsy data.

CONCLUSIONS

Force data support the hypothesis that eccentric contractions induce greater injury in dystrophic compared with normal canine muscle. Phenotypic features of the dystrophic canine model used here are similar to those of humans with Duchenne's.

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

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

Electrotransfer of naked DNA in the skeletal muscles of animal models of muscular dystrophies

The electrotransfer of naked DNA has recently been adapted to the transduction of skeletal muscle fibers. We investigated the short- and long-term efficacy of this methodology in wild-type animals and in mouse models of congenital muscular dystrophy (dy/dy, dy(2J)/dy(2J)), or Duchenne muscular dystrophy (mdx/mdx). Using a reporter construct, the short-term efficacy of fiber transduction reached 40% and was similar in wild-type, dy/dy and dy(2J)/dy(2J) animals, indicating that ongoing muscle fibrosis was not a major obstacle to the electrotransfer-mediated gene transfer. Although the complete rejection of transduced fibers was observed within 3 weeks in the absence of immunosuppression, the persistency was prolonged over 10 weeks when transient or continuous immunosuppressive regimens were used. Using therapeutic plasmids, we demonstrated that electrotransfer also allowed the transduction of large constructs encoding the laminin alpha2 chain in dy/dy mouse, or a chimeric dystrophin-EGFP protein in mdx/mdx mouse. The correct sarcolemmal localization of these structural proteins demonstrated the functional relevance of their expression in vivo, with a diffusion domain estimated to be 300 to 500 microm. However, degeneration-regeneration events hampered the long-term stability of transduced fibers. Given its efficacy for naked DNA transfer in these models of muscular dystrophies, and despite some limitations, gene electrotransfer methodology should be further explored as a potential avenue for treatment of muscular dystrophies.

Myoblast transplantation in whole muscle of nonhuman primates

The goal of the present study was to determine the feasibility, success, and toxicity of myoblast transplantation (MT) in the whole muscle of primates. Allogenic myoblasts transduced with the beta-galactosidase (beta-Gal) gene were transplanted in the whole Biceps brachii of 5 monkeys immunosuppressed with FK506. Myoblast injections were spaced at every 1 to 1.5 mm in 7 muscles, as well as at every 5 mm in 2 muscles. Myoblasts were resuspended in HBSS, notexin 1 microg/ml or notexin 5 microg/ml. Depending on the number of beta-Gal labeled myoblasts and the injection protocol, biopsies of transplanted muscles exhibited 7% to 74% beta-Gal+ fibers 1 month after MT. Beta-Gal+ fibers were present in muscle biopsies made 3, 8, and 12 months after MT. Myoglobinuria and hyperkalemia, the risk factors after extensive muscle damage and notexin toxicity, were not observed. The withdrawal of immunosuppression led to histological evidences of cellular rejection of the graft. We concluded that MT can be successfully performed in large primate muscles without toxicity due to muscle damage. An effective immunosuppression allowed the maintenance of beta-Gal+ fibers up to 1 year after MT. These results suggest parameters that may allow effective MT in humans.

Successful myoblast transplantation in primates depends on appropriate cell delivery and induction of regeneration in the host muscle

Myoblast transplantation (MT) may be a potential treatment for severe recessive hereditary myopathies. The limited results of MT in clinical trials led us to improve this technique in monkeys, an animal model phylogenetically similar to humans. Three Macaca mulata monkeys were used as donors and six as receivers for MT. Myoblasts were grown in culture from muscle biopsies of adult monkeys and infected with a retroviral vector encoding the LacZ gene. Different numbers of cells (i.e., 4 x 10(6), 8 x 10(6), and 24 x 10(6) cells) were transplanted into different muscles and 8 x 10(6) cells (resuspended in a notexin solution) were injected in one muscle of four monkeys. For these transplantations, the cell suspension (in a volume of about 100 microl) was injected at 35 sites less than 1 mm apart. Two other monkeys received 100 x 10(6) myoblasts resuspended in 1 ml of HBSS or 1 ml of notexin. For these two monkeys, the myoblasts were injected at 200-250 sites within a small portion of the muscle. All monkeys were immunosuppressed with daily injections of FK506. Four weeks after MT, the transplanted muscle portions were biopsied and the presence of beta-galactosidase-positive (beta-Gal+) muscle fibers was investigated. The number of beta-Gal+ fibers was 822 +/- 150 (site grafted with 4 x 10(6) cells), 1253 +/- 515 (8 x 10(6) cells), 1084 +/- 278 (24 x 10(6)), and 2852 +/- 1211 (notexin). In the monkeys grafted with 100 x 10(6) myoblasts, the number of beta-Gal+ fibers was 4850 (site without notexin) and 9600 (site with notexin). We demonstrated that a precise mechanical distribution of myoblasts into the tissue improves substantially MT in primates. The presence of notexin with the transplanted cells further increased the success of their transplantation. These are the best results obtained either with MT or gene therapy in primates and they encourage the possibility to human MT trials.

Impaired mitochondrial oxidative phosphorylation in skeletal muscle of the dystrophin-deficient mdx mouse

The mdx mouse, an animal model of the Duchenne muscular dystrophy, was used for the investigation of changes in mitochondrial function associated with dystrophin deficiency. Enzymatic analysis of skeletal muscle showed an approximately 50% decrease in the activity of all respiratory chain-linked enzymes in musculus quadriceps of adult mdx mice as compared with controls, while in cardiac muscle no difference was observed. The activities of cytosolic and mitochondrial matrix enzymes were not significantly different from the control values in both cardiac and skeletal muscles. In saponin-permeabilized skeletal muscle fibers of mdx mice the maximal rates of mitochondrial respiration were about two times lower than those of controls. These changes were also demonstrated on the level of isolated mitochondria. Mdx muscle mitochondria had only 60% of maximal respiration activities of control mice skeletal muscle mitochondria and contained only about 60% of hemoproteins of mitochondrial inner membrane. Similar findings were observed in a skeletal muscle biopsy of a Duchenne muscular dystrophy patient. These data strongly suggest that a specific decrease in the amount of all mitochondrial inner membrane enzymes, most probably as result of Ca2+ overload of muscle fibers, is the reason for the bioenergetic deficits in dystrophin-deficient skeletal muscle.

Myoblast transplantation in non-dystrophic dog

Dog myoblasts obtained from muscle biopsies were infected in vitro with a defective retroviral vector containing a cytoplasmic beta-galactosidase (beta-Gal) gene. These myoblasts were initially transplanted in the irradiated muscles of SCID mice and beta-Gal positive muscle fibers were observed. beta-Gal myoblasts were also transplanted back either in the donor dogs (autotransplantation model) or in unrelated recipient dogs (allotransplantation model). Following these myoblast injections, a rapid inflammatory reaction developed within the muscle as indicated by an expression of P-selectin and of pro-inflammatory cytokine mRNAs (interleukin 6 (IL-6) and transforming growth factor beta (TGF-beta), and by a neutrophil infiltration. Following either auto- or allotransplantation in inadequately or non-immunosuppressed dogs, a specific immune reaction also developed within 2 weeks as indicated by the infiltration of CD4+ and of CD8+ lymphocytes, the increased expression of IL-10 and granzyme B mRNAs and the presence of antibodies reacting with the injected cells. Some dogs were immunosuppressed with several combinations of FK506, cyclosporine (CsA) and RS-61443. In dogs immunosuppressed with CsA combined with RS-61443, only a few myoblasts and myotubes expressing beta-Gal were observed 1-2 weeks after the transplantation, but no muscle fibers expressing beta-Gal were observed after 4 weeks, and antibodies against the injected cells were formed. In dogs immunosuppressed with FK506 alone, although no antibodies against the injected cells were produced, there were no small cells and no muscle fibers expressing beta-Gal 1 month after the transplantation. However, FK506 triggered diarrhea and vomiting in dogs. When the dogs were immunosuppressed with FK506 combined with CsA and RS-61443, muscle fibers expressing beta-Gal were present 4 weeks after the transplantation and no antibodies reacting with donor myoblasts were detected. These results indicate that the combination of three immunosuppressive agents (i.e., FK506, CsA and RS-61443) is effective in controlling the specific immune reactions following myoblast transplantation in dogs and they underline that the outcome of myoblast transplantation is dependent in part on an adequate immunosuppression. These results obtained here in normal dogs may justify myoblast transplantation in dystrophic dogs despite the side effects of FK506.

Transplantation of myoblasts from a transgenic mouse overexpressing dystrophin prduced only a relatively small increase of dystrophin-positive membrane

Myoblast cultures from normal and Tg-MDA (transgenic mouse overexpressing dystrophin 50-fold) mice were transplanted into dystrophin-deficient mdx mouse muscles. Four weeks after transplantation, dystrophin-positive fibers were observed four times more frequently in cross sections of muscles injected with Tg-MDA. Myoblasts from Tg-MDA mice also expressing the beta-gal transgene (Tg-MDA/beta-gal) and myoblasts from beta-gal transgenic mice containing one normal dystrophin gene (normal/beta-gal) were also transplanted into mdx mouse muscles. Four weeks after transplantation, the fiber length positive for dystrophin (nuclear domain) was shorter (439 +/- 326 microm) than the beta-gal nuclear domain (1466 +/- 713 microm) of the same fiber when normal/beta-gal myoblasts were transplanted, but increased (1302 +/- 487 microm) when Tg-MDA/beta-gal myoblasts were used. These experiments show that despite the presence in Tg-MDA myoblasts of constructions which lead in vivo in transgenic mice to an overexpression of dystrophin 50-fold, the membrane area over which dystrophin was expressed was increased only threefold. This observation is also expected for vector-mediated gene therapy.

Effect of ammodytin L from the venom of Vipera ammodytes on Xenopus laevis differentiated muscle fibres and regenerating limbs

Ammodytin L is a non-catalytic, phospholipase-like snake venom toxin from Vipera ammodytes, which shows a cytotoxic activity on differentiated myotubes when tested in vitro. In the range of concentrations in which ammodytin L induced necrosis of myogenic cells in culture, other cell types (erythrocytes, platelets, fibroblasts) did not appear to be affected. To test the in vivo toxicity and the effective cytolytic specificity of ammodytin L we have followed the morphological changes in muscle tissue of Xenopus laevis limbs after intramuscular toxin injection. Only muscular cells were affected by ammodytin L, and the toxin did not induce any morphological change in other cell types. Further evidence of the muscle-specific action of the toxin was obtained from experiments carried out using the Xenopus kidney cell line B3.2 in culture. Ammodytin L was unable to affect parameters of cell viability such as lactate dehydrogenase leakage, [3H]thymidine incorporation, growth curves and morphological changes. Moreover, direct ammodytin L application to cultured regenerative limbs did not provoke alterations in undifferentiated myoblasts. These data suggest that ammodytin L, like other phospholipase-like toxins, exerts its toxicity by selectively damaging differentiated muscle fibres.

Phospholipase A2 myotoxins from Bothrops snake venoms

Several myotoxins have been isolated from Bothrops snake venoms during the last 10 years. All of them are group II basic phospholipases A2, although some lack enzymatic activity (i.e. Lys-49 variants). These myotoxins appear as an antigenically related family of proteins occurring in many, but not all, Bothrops venoms, bearing a close structural and antigenic relationship to toxins found in other crotalid venoms of the genera Agkistrodon and Trimeresurus. Myotoxins are quantitatively important venom components in some Bothrops species. Intramuscular injection of Bothrops myotoxins leads to a rapid series of drastic degenerative events, probably initiated at the plasma membrane level, which culminate in a selective skeletal muscle necrosis. This in vivo specificity contrasts with the ability of myotoxins to lyse many types of cells in culture. Muscle damage, as well as cytolysis and liposome disruption, occur in conditions where phospholipase A2 activity is inhibited, although enzymatic activity might enhance myotoxin actions. A membrane receptor for Bothrops myotoxins has not been identified yet. A working hypothesis on the mechanism of action is proposed. Current evidence suggests that these toxins interact with biological membranes via a molecular region distinct from their known catalytic site. The active region is likely to be formed by a combination of basic and hydrophobic amino acid residues near the C-terminus of the protein, which allow electrostatic interaction and bilayer penetration. These events may lead to membrane destabilization and loss of selective permeability to ions such as calcium, both of which appear to be important mediators in the process of muscle necrosis.

The cellular events of injured muscle regeneration depend on the nature of the injury

The cellular events of muscle degeneration and regeneration and their time course were studied in two experimental models of muscle injury mice; (i) the denervation-devascularization (DD) of the extensor digitorum longus (EDL) muscle, which is an ischaemic lesion; (ii) the injection of notexin (NOT), a snake venom, in the tibialis anterior (TA) muscle, resulting in a toxic lesion. Compared to the ischaemic lesion, the toxic lesion was characterized by a more extensive inflammatory infiltrate and a shortened phase of phagocytosis of the damaged myofibres. This allowed the proliferation and differentiation of muscle precursor cells (mpc) to take place earlier and may be further promoted by growth factors released by inflammatory cells. Compared to DD-EDL, NOT-TA showed also a greater conservation of the basement membranes of the necrotic myofibres, that can support the fusion of mpc into myotubes, and a better microvascularization. The onset of muscle regeneration is tightly related to the events which occur during the phase of degeneration.

Phospholipase A2 activity in dystrophinopathies

Phospholipase A2 activity in human muscle with or without dystrophin abnormality was studied. The results showed an increased phospholipase A2 activity in Duchenne muscular dystrophy (DMD) patients (1160 +/- 160, P < 0.01) compared to controls (< 200 U mg-1). DMD fetal muscle showed normal levels, but levels then increased dramatically postnatally. Highest levels were found at 5 yr of age (10 times normal) and then declined to 1.5-2 times normal by age 10. Steroid treatment did not change the phospholipase A2 levels significantly. In patients with abnormal dystrophin, i.e. Becker muscular dystrophy, phospholipase A2 activity was increased in the age group 3-15 (920 +/- 230 U mg-1, P < 0.01), while older patients (17-49) showed a non-significant (220 +/- 60 U mg-1) increase. The lack of phospholipase A2 activation in fetuses with DMD, indicates that activation is not a direct consequence of dystrophin deficiency. Phospholipase A2 activity has been shown to be connected to the formation of several inflammatory mediators such as prostaglandins, leukotriens, platelet activating factor and lysophospholipids. Phospholipase A2 activation may therefore play an important role in the development of inflammation and necrosis, with subsequent fibrosis and massive loss of muscle function, which develops in Duchenne and Becker muscular dystrophy.