Muscle fibre breakdown in venom-induced muscle degeneration

Alterations of the skeletal muscle contractile apparatus in necrosis induced by myotoxic snake venom phospholipases A2: a mini-review

Skeletal muscle necrosis is a common clinical manifestation of snakebite envenoming. The predominant myotoxic components in snake venoms are catalytically-active phospholipases A2 (PLA2) and PLA2 homologs devoid of enzymatic activity, which have been used as models to investigate various aspects of muscle degeneration. This review addresses the changes in the contractile apparatus of skeletal muscle induced by these toxins. Myotoxic components initially disrupt the integrity of sarcolemma, generating a calcium influx that causes various degenerative events, including hypercontraction of myofilaments. There is removal of specific sarcomeric proteins, owing to the hydrolytic action of muscle calpains and proteinases from invading inflammatory cells, causing an initial redistribution followed by widespread degradation of myofibrillar material. Experiments using skinned cardiomyocytes and skeletal muscle fibers show that these myotoxins do not directly affect the contractile apparatus, implying that hypercontraction is due to cytosolic calcium increase secondary to sarcolemmal damage. Such drastic hypercontraction may contribute to muscle damage by generating mechanical stress and further sarcolemmal damage.

Changes in basement membrane components in an experimental model of skeletal muscle degeneration and regeneration induced by snake venom and myotoxic phospholipase A2

Skeletal muscle regeneration is impaired after myonecrosis induced by viperid snake venoms, but the mechanisms behind such poor regenerative outcome are not fully understood. This study compared the changes in basement membrane (BM) components in mouse skeletal muscle in two different scenarios of muscle injury: (a) injection of Bothrops asper venom, as a model of poor regeneration, and (b) injection of a myotoxic fraction (Mtx) isolated from this venom, as a model of successful regeneration. The degradation and reposition of laminin, type IV collagen and fibronectin were assessed over time by a combination of immunohistochemistry, Western blot, and real time polymerase chain reaction. Both treatments induced degradation of laminin and type IV collagen in areas of muscle necrosis since day one, however, there were differences in the pattern of degradation and reposition of these proteins along time. Overall, Mtx induced a higher synthesis of fibronectin and higher degradation of laminin at intermediate time points, together with higher levels of transcripts for the chains of the three proteins. Instead, venom induced a higher degradation of laminin and type IV collagen at early time intervals, followed by a reduced recovery of type IV collagen by 15 days. These differences in extracellular matrix degradation and remodeling between the two models could be associated to the poor muscle regeneration after myonecrosis induced by B. asper venom.

Mechanisms Regulating Muscle Regeneration: Insights into the Interrelated and Time-Dependent Phases of Tissue Healing

Despite a massive body of knowledge which has been produced related to the mechanisms guiding muscle regeneration, great interest still moves the scientific community toward the study of different aspects of skeletal muscle homeostasis, plasticity, and regeneration. Indeed, the lack of effective therapies for several physiopathologic conditions suggests that a comprehensive knowledge of the different aspects of cellular behavior and molecular pathways, regulating each regenerative stage, has to be still devised. Hence, it is important to perform even more focused studies, taking the advantage of robust markers, reliable techniques, and reproducible protocols. Here, we provide an overview about the general aspects of muscle regeneration and discuss the different approaches to study the interrelated and time-dependent phases of muscle healing.

Effect of spaghetti meat abnormality on broiler chicken breast meat composition and technological quality

The effects of spaghetti meat (SM) myopathy and sampling location on chicken breast meat physical traits, composition, and protein functionality were investigated using 30 normal (N) and 30 SM boneless fillets. Weight, drip loss, pH, and color traits were determined on intact fillets. Proximate composition, water holding capacity, mineral profile, SDS-PAGE, myofibrillar, and sarcoplasmic protein solubility, and emulsifying properties were assessed on both the superficial (S) and deep (D) layers of the breasts. SM fillets were heavier (P < 0.0001) and exhibited greater drip loss (P = 0.0131) and higher b* index on the skin side of the muscle (P < 0.0001). Muscle condition by layer interaction effect revealed that the superficial portion of SM fillets (SM-S) exhibited the highest moisture (P = 0.0003) and fat contents (P = 0.0011) coupled with the lowest protein (P < 0.0001) and ash contents (P = 0.0458). Total and soluble collagen amounts were higher in N-S and SM-S groups compared with N-D and SM-D (P < 0.0001). SM-S group exhibited the highest calcium (P = 0.0035) and sodium (P < 0.0001) levels. Overall, the myopathy had only minor impacts on protein profiles, while the muscle layer exerted a more remarkable effect. SM fillets exhibited higher pH but a lower myofibrillar protein solubility (P < 0.0001). Salt-induced water uptake, cooking loss, and final yield values suggested a potential impairment of water-holding capacity in SM-affected meat. Sarcoplasmic and myofibrillar emulsion activity indexes were similar between the 2 muscle conditions, but the stability of the emulsions was lower in SM meat. Overall, significant layer and muscle condition by layer effects were not observed in the functional properties of the breast meat. SM exerted a profound and negative impact on breast meat composition that led to detrimental consequences on functionality traits. Given the fundamental role of protein quality for meat processing, these data suggest that a further step toward the understanding of this myopathy should be the investigation of intrinsic protein characteristics.

Biotoxins in muscle regeneration research

Skeletal muscles are characterized by their unique regenerative capacity following injury due to the presence of muscle precursor cells, satellite cells. This characteristic allows researchers to study muscle regeneration using experimental injury models. These injury models should be stable and reproducible. Variety of injury models have been used, among which the intramuscular injection of myotoxic biotoxins is considered the most common and widespread method in muscle regeneration research. By using isolated biotoxins, researchers could induce acute muscle damage and regeneration in a controlled and reproducible manner. Therefore, it is considered an easy method for inducing muscle injury in order to understand the different mechanisms involved in muscle injuries and tissue response following injury. However, different toxins and venoms have different compositions and subsequently the possible effects of these toxins on skeletal muscle vary according to their composition. Moreover, regeneration of injured muscle by venoms and toxins varies according to the target of toxin or venom. Therefore, it is essential for researcher to be aware of the mechanism and possible target of toxin-induced injury. The current paper provides an overview of the biotoxins used in skeletal muscle research.

Attenuation of autophagy impacts on muscle fibre development, starvation induced stress and fibre regeneration following acute injury

Autophagy has been implicated as a major factor in the development of a number of diseases of skeletal muscle. However, its role in skeletal muscle homeostasis is still evolving. We examined skeletal muscle architecture in a mouse model, Atg16L1, where autophagy is attenuated but importantly still present. We show that muscle fibres from Atg16L1 mice were smaller than wild-type counterparts, proving a role for this process in the growth of these cells. We show that mild attenuation of autophagy results in accelerated muscle loss during the initial phase of acute starvation. Furthermore, we show that regeneration of skeletal muscle following cardiotoxin (CTX) mediated injury is slower in the Atg16L1 mouse compared to wild-type. Lastly, we show that autophagy controls the integrity of the sarcolemma. Attenuated autophagy makes muscle fibres more susceptible to infiltration by circulating immunoglobulins following muscle injury with CTX. These fibres internalise dystrophin and nNOS. Importantly these fibres are able to restore dystrophin and nNOS localisation and do not die. In conclusion, these studies shed new light into the ability of skeletal muscle fibres to cope with injury and establish a link between the fine-tuning of autophagy and skeletal muscle regeneration.

Secreted phospholipases A2 of snake venoms: effects on the peripheral neuromuscular system with comments on the role of phospholipases A2 in disorders of the CNS and their uses in industry

Neuro- and myotoxicological signs and symptoms are significant clinical features of envenoming snakebites in many parts of the world. The toxins primarily responsible for the neuro and myotoxicity fall into one of two categories--those that bind to and block the post-synaptic acetylcholine receptors (AChR) at the neuromuscular junction and neurotoxic phospholipases A2 (PLAs) that bind to and hydrolyse membrane phospholipids of the motor nerve terminal (and, in most cases, the plasma membrane of skeletal muscle) to cause degeneration of the nerve terminal and skeletal muscle. This review provides an introduction to the biochemical properties of secreted sPLA2s in the venoms of many dangerous snakes and a detailed discussion of their role in the initiation of the neurologically important consequences of snakebite. The rationale behind the experimental studies on the pharmacology and toxicology of the venoms and isolated PLAs in the venoms is discussed, with particular reference to the way these studies allow one to understand the biological basis of the clinical syndrome. The review also introduces the involvement of PLAs in inflammatory and degenerative disorders of the central nervous system (CNS) and their commercial use in the food industry. It concludes with an introduction to the problems associated with the use of antivenoms in the treatment of neuro-myotoxic snakebite and the search for alternative treatments.

TLR4 signaling protects from excessive muscular damage induced by Bothrops jararacussu snake venom

Immune cells and skeletal muscle express Toll-like receptors (TLRs) that participate as sensors of tissue injury triggering signals for activation of innate and adaptive immune responses. This study aimed to investigate the involvement of TLR4 in the process of skeletal muscle repair. Muscular injury was induced by injection of 0.6 mg/kg of Bothrops jararacussu snake venom in the gastrocnemius muscle of C3H/HeJ mice that express a non-functional TLR-4 receptor and C3H/HeN mice with functional receptor. TLR4-deficient mice had persistent muscular inflammation with few F4/80 macrophages at onset but increased MMP9 activity and collagen deposition during resolution of injury. Since such effect was not observed in the mouse strain with functional receptor it is concluded that TLR4 signaling exerts a protective role preventing from excessive muscular damage induced by B. jararacussu venom.

Osteopontin deficiency delays inflammatory infiltration and the onset of muscle regeneration in a mouse model of muscle injury

Osteopontin is secreted by skeletal muscle myoblasts and stimulates their proliferation. Expression of osteopontin in skeletal muscle is upregulated in pathological conditions including Duchenne muscular dystrophy, and recent evidence suggests that osteopontin might influence the course of this disease. The current study was undertaken to determine whether osteopontin regulates skeletal muscle regeneration. A whole muscle autografting model of regeneration in osteopontin-null and wild-type mice was used. Osteopontin expression was found to be strongly upregulated in wild-type grafts during the initial degeneration and subsequent early regeneration phases that are observed in this model. Grafted muscle from osteopontin-null mice degenerated more slowly than that of wild-type mice, as determined by histological assessment, fibre diameter and fibre number. The delayed degeneration in osteopontin-null grafts was associated with a delay in neutrophil and macrophage infiltration. Centrally nucleated (regenerating) muscle fibres also appeared more slowly in osteopontin-null grafts than in wild-type grafts. These results demonstrate that osteopontin plays a non-redundant role in muscle remodelling following injury.

Pulsed ultrasound therapy accelerates the recovery of skeletal muscle damage induced by Bothrops jararacussu venom

We studied the effect of pulsed ultrasound therapy (UST) and antibothropic polyvalent antivenom (PAV) on the regeneration of mouse extensor digitorum longus muscle following damage by Bothrops jararacussu venom. Animals (Swiss male and female mice weighing 25.0 ± 5.0 g; 5 animals per group) received a perimuscular injection of venom (1 mg/kg) and treatment with UST was started 1 h later (1 min/day, 3 MHz, 0.3 W/cm², pulsed mode). Three and 28 days after injection, muscles were dissected and processed for light microscopy. The venom caused complete degeneration of muscle fibers. UST alone and combined with PAV (1.0 mL/kg) partially protected these fibers, whereas muscles receiving no treatment showed disorganized fascicules and fibers with reduced diameter. Treatment with UST and PAV decreased the effects of the venom on creatine kinase content and motor activity (approximately 75 and 48%, respectively). Sonication of the venom solution immediately before application decreased the in vivo and ex vivo myotoxic activities (approximately 60 and 50%, respectively). The present data show that UST counteracts some effects of B. jararacussu venom, causing structural and functional improvement of the regenerated muscle after venom injury.

CD34 promotes satellite cell motility and entry into proliferation to facilitate efficient skeletal muscle regeneration

Expression of the cell surface sialomucin CD34 is common to many adult stem cell types, including muscle satellite cells. However, no clear stem cell or regeneration-related phenotype has ever been reported in mice lacking CD34, and its function on these cells remains poorly understood. Here, we assess the functional role of CD34 on satellite cell-mediated muscle regeneration. We show that Cd34(-/-) mice, which have no obvious developmental phenotype, display a defect in muscle regeneration when challenged with either acute or chronic muscle injury. This regenerative defect is caused by impaired entry into proliferation and delayed myogenic progression. Consistent with the reported antiadhesive function of CD34, knockout satellite cells also show decreased motility along their host myofiber. Altogether, our results identify a role for CD34 in the poorly understood early steps of satellite cell activation and provide the first evidence that beyond being a stem cell marker, CD34 may play an important function in modulating stem cell activity.

Muscle injury activates resident fibro/adipogenic progenitors that facilitate myogenesis

Efficient tissue regeneration is dependent on the coordinated responses of multiple cell types. Here, we describe a new subpopulation of fibro/adipogenic progenitors (FAPs) resident in muscle tissue but arising from a distinct developmental lineage. Transplantation of purified FAPs results in the generation of ectopic white fat when delivered subcutaneously or intramuscularly in a model of fatty infiltration, but not in healthy muscle, suggesting that the environment controls their engraftment. These cells are quiescent in intact muscle but proliferate efficiently in response to damage. FAPs do not generate myofibres, but enhance the rate of differentiation of primary myogenic progenitors in co-cultivation experiments. In summary, FAPs expand upon damage to provide a transient source of pro-differentiation signals for proliferating myogenic progenitors.

Differential recovery of neuromuscular function after nerve/muscle injury induced by crude venom from Notechis scutatus, cardiotoxin from Naja atra and bupivacaine treatments in mice

Different neuromyotoxic agents are frequently used in rodent models of skeletal nerve/muscle injury and repair. However, their differential effects are not well known. Right Tibialis anterior muscles of mice were injured by one of three different neuromyotoxic agents: crude venom from Notechis scutatus, cardiotoxin from Naja atra or bupivacaine (local anesthetic). Mice were studied 5, 14 and 56 days after injury by analysing the recovery of in situ muscle isometric function in response to nerve stimulation, muscle weights and muscle histology. Our results show that at day 5 venom treatment had a more debilitating effect on muscle weights and maximal tetanic force than cardiotoxin and bupivacaine treatments (p<0.05). Moreover, the degree of recovery of muscle parameters 14 days after neuromyotoxic treatment varies as follow: venom<bupivacaine<cardiotoxin. By day 56, we found that injured muscles still exhibit deficits in maximal tetanic force (cardiotoxin and bupivacaine treatments) and fatigue resistance (venom and cardiotoxin treatments) as compared to control muscles (p<0.05). In conclusion, these results indicate that neuromyotoxic agents induce differential destructive effects and recovery in mice and confirm the fact that full nerve/muscle repair is slow and in some cases may never be attained.

Notexin causes greater myotoxic damage and slower functional repair in mouse skeletal muscles than bupivacaine

Although the myotoxins bupivacaine and notexin are employed for studying processes that regulate muscle regeneration after injury, no studies have compared their efficacy in causing muscle damage or assessing functional regeneration in mouse skeletal muscles. Bupivacaine causes extensive injury in rat muscles but its effects on mouse muscles are variable. We compared functional and morphological properties of regenerating mouse extensor digitorum longus (EDL) muscles after notexin or bupivacaine injection and tested the hypothesis that muscle damage would be more extensive and functional repair less complete after notexin injection. Bupivacaine caused degeneration of 45% of fibers and reduced maximum force (Po) to 42% of control after 3 days. In contrast, notexin caused complete fiber breakdown and loss of functional capacity after 3 days (P < 0.05). At 7 and 10 days after bupivacaine, Po was restored to 65% and 71% of control, respectively, whereas Po of notexin-injected muscles was only 10% and 39% of control at these time-points, respectively (P < 0.05). At 7 and 10 days after bupivacaine, approximately 30% of fibers were centrally nucleated (regenerating), whereas notexin-injected muscles were comprised entirely of regenerating fibers (P < 0.05). The results demonstrate that notexin causes a more extensive and complete injury than bupivacaine, and is a useful model for studying muscle regeneration in mice.

Functional characterization of a basic D49 phospholipase A2 (LmTX-I) from the venom of the snake Lachesis muta muta (bushmaster)

The whole venom of Lachesis muta muta is preponderantly neurotoxic but moderately myotoxic on the chick biventer cervicis preparation (BCp). We have now examined these toxic activities of a basic phospholipase A(2), LmTX-I, isolated from the whole venom. LmTX-I caused a significant concentration-dependent neuromuscular blockade in the BCp. The time to produce 50% neuromuscular blockade was 14.7+/-0.75 min (30 microg/ml), 23.6+/-0.9 min (10 microg/ml), 34+/-1.7 min (2.5 microg/ml) and 39.2+/-3.6 min (1 microg/ml), (n=5/concentration; p<0.05). Complete blockade with all tested concentrations was not accompanied by inhibition of the response to ACh. At the highest concentration, LmTX-I (30 microg/ml) significantly reduced contractures elicited by exogenous KCl (20mM), increased the release of creatine kinase (1542.5+/-183.9 IU/L vs 442.7+/-39.8 IU/L for controls after 120 min, p<0.05), and induced the appearance of degenerating muscle fibers ( approximately 15%). Quantification of myonecrosis indicated 14.8+/-0.8 and 2.0+/-0.4%, with 30 and 10 microg/mlvenom concentration, respectively, against 1.07+/-0.4% for control preparations. The findings indicate that the basic PLA(2) present on venom from L. m. muta (LmTX-I) possesses a dominant neurotoxic action on isolated chick nerve-muscle preparations, whereas myotoxicity was mainly observed at the highest concentration used (30 microg/ml). These effects of LmTX-I closely reproduce the effects of the whole venom of L. m. muta in chick neuromuscular preparations.

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

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

Effect of anti-inflammatory and antioxidant drugs on the long-term repair of severely injured mouse skeletal muscle

Non-steroidal anti-inflammatory drugs are frequently prescribed after skeletal muscle injury. It is not known whether this type of medication can interfere with muscle repair, although inflammatory response is thought to play an important role in this process. Tibialis anterior muscles of mice were injured by myotoxic agent (snake venom) or crushed. Then, animals were treated daily for 10-14 days with different types of non-steroidal anti-inflammatory and antioxidant drugs. The long-term repair was studied 10-42 days after injury by analysing the recovery of in situ muscle force production, size of regenerating muscle cells and expression of myosin heavy chain. Our results show that diclofenac, diferuloylmethane (curcumin), dimethylthiourea or pyrrolidine dithiocarbamate treatment did not significantly affect muscle recovery after myotoxic injury (P > 0.05). Similarly, diferuloylmethane, dimethyl sulphoxide or indomethacin administration did not markedly change muscle repair after crush injury. However, we noted that high doses (> 2 mg kg(-1)) of diferuloylmethane or indomethacin increased lethality and reduced muscle repair after crush injury. In conclusion, non-steroidal anti-inflammatory and antioxidant drugs did not exhibit long-term detrimental effects on muscle recovery after injury, except at lethal doses.

Functional, cellular and molecular aspects of skeletal muscle recovery after injury induced by snake venom from Notechis scutatus scutatus

We have analysed the rate and ultimate extent of muscle functional recovery after snake venom-induced myotoxicity, as well as the relationships between functional, biochemical and structural indices of recovery. We also compared the effects of various injuries leading to muscle necrosis, loss of innervation/vasculature and/or precursors of muscle cells (pmc). We found that several parameters of rat soleus muscle such as maximal isometric force, slow myosin heavy chain, and citrate synthase, were fully and rapidly restored within 6 weeks after treatment with snake Notechis scutatus venom (im, 2 microg/muscle). In contrast, some muscle contractile properties (degree of tetanic fusion, fatigue resistance...) were not fully recovered even by 12 weeks after venom treatment. However, when compared to other injuries, recovery 3 weeks after venom treatment, was better than that observed after severing the terminal nerve and accompanying vessels and after cryodamage known to kill pmc. In conclusion, our studies demonstrate that-contrary to what is commonly believed -- muscle treated by myotoxic agent does not recover rapidly and fully. However, the degree or rate of muscle recovery after snake venom treatment was much better when compared to other types of injury. In addition, histological and biochemical parameters cannot be used as such to easily predict functional recovery following injury.

Comparative teratogenicity of chlorpyrifos and malathion on Xenopus laevis development

The embryotoxic potential of chlorpyrifos (CPF) and malathion (MTN), two organophosphorus insecticides (OPs), was evaluated by modified Frog Embryo Teratogenesis Assay-Xenopus (FETAX). CPF and MTN were not embryolethal even at the highest concentration tested (6000 microg/l), but both exhibited a powerful teratogenicity. The probit analysis of malformed larva percentages showed a TC(50) of 161.54mug/l for CPF, and a TC(50) of 2394.01 microg/l for MTN. Therefore, CPF teratogenicity was about 15 times higher than MTN. Larvae of both exposed groups were mainly affected by ventral and/or lateral tail flexure coupled with abnormal gut coiling. Histopathological diagnosis displayed abnormal myotomes and myocytes with marked hypertrophies localized at the cell extremity, probably due to a break away of myofibril extremities at the intersomitic junction level. We speculate that this muscular damage was related to inhibition of acetylcholinesterase that showed a clear concentration-response in CPF and MTN exposed larvae. The teratogenic effects of these anti-cholinesterase compounds on Xenopus laevis myogenesis suggest a possible role played by OPs on induction of congenital muscular dystrophy.

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.