Membrane-perturbing activity of Viperidae myotoxins: an electrostatic surface potential approach to a puzzling problem

Lys49 myotoxins, secreted phospholipase A2-like proteins of viperid venoms: A comprehensive review

Muscle necrosis is a potential clinical complication of snakebite envenomings, which in severe cases can lead to functional or physical sequelae such as disability or amputation. Snake venom proteins with the ability to directly damage skeletal muscle fibers are collectively referred to as myotoxins, and include three main types: cytolysins of the "three-finger toxin" protein family expressed in many elapid venoms, the so-called "small" myotoxins found in a number of rattlesnake venoms, and the widespread secreted phospholipase A₂ (sPLA₂) molecules. Among the latter, protein variants that conserve the sPLA₂ structure, but lack such enzymatic activity, have been increasingly found in the venoms of many viperid species. Intriguingly, these sPLA₂-like proteins are able to induce muscle necrosis by a mechanism independent of phospholipid hydrolysis. They are commonly referred to as "Lys49 myotoxins" since they most often present, among other substitutions, the replacement of the otherwise invariant residue Asp49 of sPLA₂s by Lys. This work comprehensively reviews the historical developments and current knowledge towards deciphering the mechanism of action of Lys49 sPLA₂-like myotoxins, and points out main gaps to be filled for a better understanding of these multifaceted snake venom proteins, to hopefully lead to improved treatments for snakebites.

Myotoxicity induced by Cerastes cerastes venom: Beneficial effect of heparin in skeletal muscle tissue regeneration

Myonecrosis is a relevant tissue damage induced by snakes of Viperidae family often leading to permanent tissue and function loss and even amputation. The aim of this study was to evaluate the effect of heparin on skeletal muscle tissue regeneration after Cerastes cerastes envenomation. Mice received either the venom (1 LD₅₀) by i.m. route, or the venom followed, by heparin administration by i.v. route at 15 min and 4 h. Obtained results showed that Cerastes cerastes venom induced deep tissue structure alterations, characterized mainly by edema, hemorrhage, myonecrosis and inflammation. Myotoxicity was correlated with increased CK levels in sera, concomitant with their decrease in muscle tissue homogenates. Muscle wet weight was restored within 2 weeks after heparin treatment and 28 days in the envenomed group. Heparin treatment significantly decreased MPO activity, suggesting an anti-inflammatory effect. NO, HGF, VEGF and G-CSF levels were increased after heparin administration. These mitogenic factors constitute potent stimuli for satellite and endothelial cells improving, thus, muscle regeneration. This study showed that muscle tissue recovery was significantly enhanced after heparin treatment. Heparin use seems to be a promising therapeutic approach after viper envenomation.

Effect of monospecific antibodies against baltergin in myotoxicity induced by Bothrops alternatus venom from northeast of Argentina. Role of metalloproteinases in muscle damage

Myotoxicity, one of the most relevant local manifestations in envenomation by Bothrops genus, may result from a direct action of myotoxins or be due to an indirect vascular degeneration and ischemia. Baltergin, a snake venom metalloproteinase (SVMP), isolated from Bothrops alternatus venom has been used to obtain monospecific IgG, in order to determine the relative role of toxin in myotoxicity induced by whole venom. Bothrops diporus venom, another medical relevant genus of the northeastern region of Argentina, was also studied. Anti-baltergin IgG was able to neutralize completely the hemorrhagic activity of B. alternatus venom at an antibodies:venom ratio of 30:1 (w:w). However, mice injected with B. diporus venom showed a small spot remaining even at the highest ratio of IgG:venom assayed (50:1; w:w). Specific antibodies were efficient to neutralize the myotoxicity of B. alternatus venom at ratio 30:1 (w:w) but did not neutralize the same effects in B. diporus venom. Anti-baltergin polyclonal antibodies were useful tools for revealing the central role of SVMPs in the development of myotoxicity of B. alternatus venom, as well as, helping to suggest indirectly presence of potent myotoxic phospholipases A2 (PLA2s) in B. diporus venom.

Membrane cholesterol modulates the cytolytic mechanism of myotoxin II, a Lys49 phospholipase A2 homologue from the venom of Bothrops asper

Lys49 phospholipase A2 (PLA2) homologues present in crotalid snake venoms lack enzymatic activity, yet they induce skeletal muscle necrosis by a membrane permeabilizing mechanism whose details are only partially understood. The present study evaluated the effect of altering the membrane cholesterol content on the cytolytic activity of myotoxin II, a Lys49 PLA2 isolated from the venom of Bothrops asper, using the myogenic cell line C2C12 as a model target. Cell membrane cholesterol depletion by methyl-β-cyclodextrin (MβCD) treatment enhanced the cytolytic action of myotoxin II, as well as of its bioactive C-terminal synthetic peptide p(115-129) . Conversely, cell membrane cholesterol enrichment by preformed cholesterol-MβCD complexes reduced the cytolytic effect of myotoxin II. The toxic actions of myotoxin I, a catalytically active PLA2 from the same venom, as well as of the cytolytic peptide melittin from bee venom, also increased in cholesterol-depleted cells. Although physical and functional changes resulting from variations in membrane cholesterol are complex, these findings suggest that membrane fluidity could be a relevant parameter to explain the observed modulation of the cytolytic mechanism of myotoxin II, possibly influencing bilayer penetration. In concordance, the cytolytic effect of myotoxin II decreased in direct proportion to lower temperature, a physical factor that affects membrane fluidity. In conclusion, physicochemical properties that depend on membrane cholesterol content significantly influence the cytolytic mechanism of myotoxin II, reinforcing the concept that the primary site of action of Lys49 PLA2 myotoxins is the plasma membrane.

Biochemical and biological characterization of a PLA2 from crotoxin complex of Crotalus durissus cumanensis

A new PLA2 (Cdcum6) from crotoxin complex of Colombian Crotalus durissus cumanensis rattlesnake was purified using molecular exclusion chromatography and RP-HPLC. The molecular mass of Cdcum6 was determined by SDS-PAGE approximately 14 KDa and confirmed by MALDI-TOF (14321.98 Da). The enzyme showed Km 6.0 mM, Vmax 3.44 nmol/min, optimum pH was 8.0 and temperature was between 30 and 45 degrees C, and it had a strict requirement of Ca2+ for its activity. The N-terminal sequence of PLA2 was SLVQF EKMIK EVAGK NGVPWY. Comparison of amino acid sequence data with other PLA2 from South American Crotalus durissus rattlesnakes showed that Cdcum6 shares the highest sequence identity with Cdr13 an isoform PLA2 from Crotalus durissus ruruima, nevertheless, Cdcum6 showed high content of basic and hydrophobic amino acids. In mice, Cdcum6 presented higher LD50 than crotoxin complex from C d. cumanensis. Additionally, Cdcum6 induced a conspicuous local myotoxic effect and moderate footpad edema; in vitro, it was antigoagulant in doses as low as 0.5 microg/l ml, and it was not cytotoxic on myoblast but Cdcum6 was able to lyse myotubes.

Systemic and local myotoxicity induced by snake venom group II phospholipases A2: Comparison between crotoxin, crotoxin B and a Lys49 PLA2 homologue

The patterns of myotoxicity induced in mice by crotoxin, crotoxin B and a Lys49 phospholipase A(2) (PLA(2)) homologue were compared. Lys49 PLA(2)-induced local myotoxicity is reflected by creatine kinase (CK) loss in injected gastrocnemius muscle, and by a profile of CK increase in plasma characterized by a rapid increment and drop after intramuscular injection, and by a lack of CK increase in plasma after intravenous injection. In contrast, crotoxin and crotoxin B, which induce local and systemic myotoxicity, provoked a more prolonged increment in plasma CK activity upon intramuscular injection, and induced increments in plasma CK after intravenous injection. The three toxins promoted a similar extent of local myotoxicity, assessed by the loss of CK in injected gastrocnemius. A method for the quantitative assessment of the ability of toxins to induce systemic myotoxicity is proposed, based on the estimation of the ratio between the area under the curve in the plasma CK activity (total myotoxicity) to the loss of CK in injected gastrocnemius (local myotoxicity). The highest ratio corresponded to crotoxin, and the lowest corresponded to Lys49 PLA(2), the former being a systemic myotoxin and the latter a local myotoxin. Neutralization by antivenoms also differed between the toxins: a drastic reduction in plasma CK, with very poor neutralization of local CK loss, was achieved in the case of crotoxin B when antivenom was injected intravenously, whereas no neutralization was achieved in the case of Lys49 PLA(2). When tested in undifferentiated myoblasts in culture, Lys49 PLA(2) induced cytotoxicity, whereas crotoxin and crotoxin B did not, evidencing that the latter are devoid of widespread cytolytic activity. Molecular modeling analysis showed that Lys49 PLA(2) has a conspicuous cationic face, which is likely to interact with diverse membranes. In contrast, crotoxin B, despite its overall basic pI, has a lower density of positively charged residues at this molecular region. It is suggested that Lys49 PLA(2)s homologues interact, through this cationic face, with many different cell types, thus lacking specificity for muscle cells. In contrast, crotoxin B has a more selective interaction with targets in the muscle cell membrane. This selectivity might be the basis for the ability of crotoxin and crotoxin B to induce systemic myotoxicity.

Effect of calcineurin inhibitors on myotoxic activity of crotoxin and Bothrops asper phospholipase A2 myotoxins in vivo and in vitro

Previous studies have shown that calcineurin activity plays a critical role in the myotoxic activity induced by crotoxin (CTX), a group II phospholipase A(2) (PLA(2)) with neurotoxic and myotoxic actions. In order to address whether calcineurin is also important for the activity of non-neurotoxic group II PLA(2) myotoxins we have compared the effects of calcineurin inhibition on the myotoxic capacity of CTX and the non-neurotoxic PLA(2)s, myotoxin II (Mt II) and myotoxin III (Mt III) from Bothrops asper venom. Rats were treated with cyclosporin A (CsA) or FK506, calcineurin inhibitors, and received an intramuscular injection of either CTX, Mt II or Mt III into the tibialis anterior. Animals were killed 24 h after injection of toxins. Tibialis anterior was removed and stored in liquid nitrogen. Myofibers in culture were also treated with CsA or FK506 and exposed to CTX, Mt II and Mt III. It was observed that, in contrast to CTX, CsA and FK506 do not attenuate myotoxic effects induced by both Mt II and Mt III in vivo and in vitro. The results of the present study suggest that calcineurin is not essential for the myotoxic activity of Mt II and Mt III, indicating that distinct intracellular pathways might be involved in myonecrosis induced by neurotoxic CTX and non-neurotoxic Bothrops sp. PLA(2) myotoxins. Alternatively, calcineurin dependent fast fiber type shift might render the muscle resistant to the action of CTX, without affecting its susceptibility to Bothrops sp. myotoxins.

Inhibition of Myotoxic Activity of Bothrops asper Myotoxin II by the Anti-trypanosomal Drug Suramin

Suramin, a synthetic polysulfonated compound, developed initially for the treatment of African trypanosomiasis and onchocerciasis, is currently used for the treatment of several medically relevant disorders. Suramin, heparin, and other polyanions inhibit the myotoxic activity of Lys49 phospholipase A2 analogues both in vitro and in vivo, and are thus of potential importance as therapeutic agents in the treatment of viperid snake bites. Due to its conformational flexibility around the single bonds that link the central phenyl rings to the secondary amide backbone, the symmetrical suramin molecule binds by an induced-fit mechanism complementing the hydrophobic surfaces of the dimer and adopts a novel conformation that lacks C2 symmetry in the dimeric crystal structure of the suramin-Bothrops asper myotoxin II complex. The simultaneous binding of suramin at the surfaces of the two monomers partially restricts access to the nominal active sites and significantly changes the overall charge of the interfacial recognition face of the protein, resulting in the inhibition of myotoxicity.

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.

An overview of lysine-49 phospholipase A2 myotoxins from crotalid snake venoms and their structural determinants of myotoxic action

In 1984, the first venom phospholipase A2 (PLA2) with a lysine substituting for the highly conserved aspartate 49 was discovered, in the North American crotalid snake Agkistrodon p. piscivorus [J. Biol. Chem. 259 (1984) 13839]. Ten years later, the first mapping of a 'toxic region' on a Lys49 PLA2 was reported, in Bothrops asper myotoxin II [J. Biol. Chem. 269 (1994) 29867]. After a further decade of research on the Lys49 PLA2s, a better understanding of their structural determinants of toxicity and mode of action is rapidly emerging, with myotoxic effector sites identified at the C-terminal region in at least four proteins: B. asper myotoxin II, A. p. piscivorus K49 PLA2, A. c. laticinctus ACL myotoxin, and B. jararacussu bothropstoxin I. Although important features still remain to be established, their toxic mode of action has now been understood in its more general concepts, and a consistent working hypothesis can be experimentally supported. It is proposed that all the toxic activities of Lys49 PLA2s are related to their ability to destabilize natural (eukaryotic and prokaryotic) and artificial membranes, using a cationic/hydrophobic effector site located at their C-terminal loop. This review summarizes the general properties of the Lys49 PLA2 myotoxins, emphasizing the development of current concepts and hypotheses concerning the molecular basis of their toxic activities.

A structure based model for liposome disruption and the role of catalytic activity in myotoxic phospholipase A2s

Venom phospholipase A2s (PLA2s) display a wide spectrum of pharmacological activities and, based on the wealth of biochemical and structural data currently available for PLA2s, mechanistic models can now be inferred to account for some of these activities. A structural model is presented for the role played by the distribution of surface electrostatic potential in the ability of myotoxic D49/K49 PLA2s to disrupt multilamellar vesicles containing negatively charged natural and non-hydrolyzable phospholipids. Structural evidence is provided for the ability of K49 PLA2s to bind phospholipid analogues and for the existence of catalytic activity in K49 PLA2s. The importance of the existence of catalytic activity of D49 and K49 PLA2s in myotoxicity is presented.

Expression of an active recombinant lysine 49 phospholipase A(2) myotoxin as a fusion protein in bacteria

ACL myotoxin (ACLMT) is a K49 phospholipase A(2)-like protein isolated from the venom of the snake Agkistrodon contortrix laticinctus (broad-banded copperhead) that induces necrosis of skeletal muscle. We have previously cloned and sequenced the cDNA coding for ACLMT from a venom gland cDNA library. In order to perform structure and function studies, we have developed an expression system for production of ACLMT as a fusion protein with maltose binding protein (MBP) from the periplasm of bacteria, using the pMAL-p2 expression vector. The cDNA coding for the mature toxin without the signal peptide was amplified by PCR and subcloned into the pMAL-p2 vector. The new plasmid (pMAL-MT) was used to transform BL21(DE3) E. coli cells. Culture of transformed cells induced with IPTG led to the expression of a 60 kDa fusion protein which strongly reacts with anti-native ACLMT antibodies. The fusion protein was purified from the bacterial periplasm by affinity chromatography in an amylose column and by gel filtration. The purified fusion protein (MBP-rACLMT) was able to induce necrosis of skeletal muscle of mice very similar to that caused by the native myotoxin.

Effects of chemical modifications of crotoxin B, the phospholipase A(2) subunit of crotoxin from Crotalus durissus terrificus snake venom, on its enzymatic and pharmacological activities

Crotoxin B, the basic Asp49-PLA(2) subunit from crotoxin, the main component of Crotalus durissus terrificus venom, displays myotoxic, edema-inducing, bactericidal (upon Escherichia coli), liposomal-disrupting and anticoagulant activities. Chemical modifications of His (with 4-bromophenacyl bromide, BPB), Tyr (with 2-nitrobenzenesulphonyl fluoride, NBSF), Trp (with o-nitrophenylsulphenyl chloride, NPSC) and Lys (with acetic anhydride) residues of this protein, in addition to cleavage with cyanogen bromide (CNBr) and inhibition with ethylenediaminetetraacetic acid (EDTA), were carried out in order to study their effects on enzymatic and pharmacological activities. Lethality was reduced after modification of His or Lys residues, as well as after cleavage with CNBr, while enzymatic activity was completely abolished after modification of His or incubation with EDTA. Modification of Lys or Tyr, or cleavage with CNBr, partially reduced enzymatic activity. Anticoagulant activity was modified similarly to enzymatic activity, evidencing the dependency of this pharmacological effect on catalytic activity. Myotoxicity was reduced after modification of His or Lys, as well as after cleavage with CNBr, whereas EDTA reduced this effect to a lesser extent. Bactericidal effect was significantly reduced only after modification of Lys and after cleavage with CNBr. Edema-inducing activity was partially inhibited after treatment with EDTA and strongly reduced after acetylation of Lys residues and cleavage with CNBr, being only partially reduced after His alkylation. On the other hand, liposome disrupting activity was only partially reduced after modification of His and Tyr or after cleavage with CNBr. Modification of Trp residue partially reduced lethality and myotoxicity but did not affect enzymatic or anticoagulant activities. These data indicate that enzymatic activity is relevant for some pharmacological effects induced by crotoxin B (mainly lethal, myotoxic and anticoagulant activities), and also evidence that this subunit of crotoxin displays regions different from the active catalytic site which are involved in some of the toxic and pharmacological effects induced by this phospholipase A(2).