Characterization of a novel proteinous toxin from sea anemone Actineria villosa

The sea anemone Actineria villosa expresses a lethal protein toxin. We isolated a novel 120-kDa protein, Avt120, from partially purified toxin and found it to possess extremely strong lethal activity. The 3,453-bp Avt120 gene translates to a 995-amino acid protein. The 50% lethal dose (LD(50)) of purified Avt120 in mice was 85.17 ng. Among several tested cell lines, Colo205 cells were most sensitive to Avt120: 50% of them were damaged by 38.4 ng/mL Avt120. Avt120 exerted ATP degradation activity (10 μmol ATP h(-1) mg(-1)), which was strongly inhibited by ganglioside GM1 to decrease the cytotoxicity of Avt120.

Structures of sea anemone toxins

Sea anemones produce a variety of toxic peptides and proteins, including many ion channel blockers and modulators, as well as potent cytolysins. This review describes the structures that have been determined to date for the major classes of peptide and protein toxins. In addition, established and emerging methods for structure determination are summarized and the prospects for modelling newly described toxins are evaluated. In common with most other classes of proteins, toxins display conformational flexibility which may play a role in receptor binding and function. The prospects for obtaining atomic resolution structures of toxins bound to their receptors are also discussed.

Molecular mechanism of pore formation by actinoporins

Actinoporins are effective pore-forming toxins produced by sea anemones. These extremely potent, basic 20 kDa proteins readily form pores in membranes that contain sphingomyelin. Much has been learned about the molecular basis of their pore-forming mechanism in recent years. Pore formation is a multi-step process that involves recognition of membrane sphingomyelin, firm binding to the membrane accompanied by the transfer of the N-terminal region to the lipid-water interface and finally pore formation after oligomerisation of three to four monomers. The final conductive pathway is formed by amphipathic alpha-helices, hence actinoporins are an important example of so-called alpha-helical pore-forming toxins. Actinoporins have become useful model proteins to study protein-membrane interactions, specific recognition of lipids in the membrane, and protein oligomerisation in the lipid milieu. Recent sequence and structural data of proteins similar to actinoporins indicate that they are not a unique family restricted to sea anemones as was long believed. An AF domain superfamily (abbreviated from actinoporin-like proteins and fungal fruit-body lectins) was defined and shown to contain members from three animal and two plant phyla. On the basis of functional properties of some members we hypothesise that AF domain proteins are peripheral membrane proteins. Finally, ability of actinoporins to form transmembrane pores has been exploited in some novel biomedical applications.

The mauve stinger Pelagia noctiluca (Forsskål, 1775). Distribution, ecology, toxicity and epidemiology of stings. A review

The toxicity of Cnidaria is a subject of concern due to its influence on humans. In particular, jellyfish blooms can highly affect human economical activities, such as bathing, fishery, tourism, etc., as well as the public health. Stinging structures of Cnidaria (nematocysts) produce remarkable effects on human skin, such as erythema, swelling, burning and vesicles, and at times further severe dermonecrotic, cardio- and neurotoxic effects, which are particularly dangerous in sensitive subjects. In several zones the toxicity of jellyfish is a very important health problem, thus it has stimulated the research on these organisms; to date toxicological research on Cnidarian venoms in the Mediterranean region is not well developed due to the weak poisonousness of venoms of jellyfish and anemones living in this area. In spite of this, during last decades several problems were also caused in the Mediterranean by stinging consequent to Cnidarian blooms mainly caused by Pelagia noctiluca (Forsskål, 1775) which is known to be the most venomous Mediterranean jellyfish. This paper reviews the knowledge on this jellyfish species, particularly considering its occurrence and toxicity.

Physiological effects of hyperchloraemia and acidosis

The advent of balanced solutions for i.v. fluid resuscitation and replacement is imminent and will affect any specialty involved in fluid management. Part of the background to their introduction has focused on the non-physiological nature of 'normal' saline solution and the developing science about the potential problems of hyperchloraemic acidosis. This review assesses the physiological significance of hyperchloraemic acidosis and of acidosis in general. It aims to differentiate the effects of the causes of acidosis from the physiological consequences of acidosis. It is intended to provide an assessment of the importance of hyperchloraemic acidosis and thereby the likely benefits of balanced solutions.

Acidosis and contractility of heart muscle

The contractility of heart muscle is sensitive to small and physiological changes of extracellular pH. The reduction of contractility associated with an acidosis is determined by the fall of pH in the intracellular fluid. The function of many organelles within the cardiac cell is affected by hydrogen ions. The tension generated by isolated myofibrils at a fixed calcium concentration is reduced at low pH. The dominant mechanism for the reduction of contractility in whole tissue is competitive inhibition of the slow calcium current by hydrogen ions. The reduction of the slow calcium current is similar when the same fall of developed tension is induced by acidosis or by a reduction of extracellular calcium concentration. Measurement of tissue pH with fast-responding extracellular electrodes show that, in myocardial ischaemia, tissue acidosis develops at the same time or only seconds before the onset of contractile failure. Much of the reduced contractility can be accounted for by the severity of the acidosis. Although a mild acidosis can delay or prevent damage to the myocardium from ischaemia or hypoxia, a severe acidosis is not beneficial and may even cause tissue necrosis.

The malonyl CoA axis as a potential target for treating ischaemic heart disease

Cardiovascular disease is the leading cause of death and disability for people living in western societies, with ischaemic heart disease accounting for the majority of this health burden. The primary treatment for ischaemic heart disease consists of either improving blood and oxygen supply to the heart or reducing the heart's oxygen demand. Unfortunately, despite recent advances with these approaches, ischaemic heart disease still remains a major health problem. Therefore, the development of new treatment strategies is still required. One exciting new approach is to optimize cardiac energy metabolism, particularly by decreasing the use of fatty acids as a fuel and by increasing the use of glucose as a fuel. This approach is beneficial in the setting of ischaemic heart disease, as it allows the heart to produce energy more efficiently and it reduces the degree of acidosis associated with ischaemia/reperfusion. Malonyl CoA is a potent endogenous inhibitor of cardiac fatty acid oxidation, secondary to inhibiting carnitine palmitoyl transferase-I, the rate-limiting enzyme in the mitochondrial uptake of fatty acids. Malonyl CoA is synthesized in the heart by acetyl CoA carboxylase, which in turn is phosphorylated and inhibited by 5'AMP-activated protein kinase. The degradation of myocardial malonyl CoA occurs via malonyl CoA decarboxylase (MCD). Previous studies have shown that inhibiting MCD will significantly increase cardiac malonyl CoA levels. This is associated with an increase in glucose oxidation, a decrease in acidosis, and an improvement in cardiac function and efficiency during and following ischaemia. Hence, the malonyl CoA axis represents an exciting new target for the treatment of ischaemic heart disease.

Stycholysin II, a cytolysin from the sea anemone Stichodactyla helianthus promotes higher hemolysis in aged red blood cells

We have investigated the relationship between the status of red blood cells (RBCs) and their susceptibility to toxin sticholysin II (StII) hemolytic activity; we have evaluated this effect in different RBC ensembles, comprising young and old cells, and in cells partially damaged by their pre-exposition to a free radical source. Upon action of StII, young cell populations are less prone to hemolysis than the whole population, while old cell populations and peroxyl-oxidized red cells are lysed faster than the whole population. Cell K(+) content was higher in young cells and lower in both senescent cells and in peroxyl-damaged cells relative to whole cell population. The relevance of cell K(+) content in St II-induced lysis was shown when external Na(+) was partially replaced by K(+); under this condition, RBC lysed faster in the presence of St II but no difference was observed among young cells, whole cells population and peroxyl-damaged cells; only old cells lysed faster that the whole population, response that can be due to an enhanced St II-induced pore formation as supported by evaluation of St II irreversible binding to RBC. It is concluded that this factor and the amount of intracellular K(+) are the dominant parameters that modulate the resistance of RBC to St II-induced lysis.

Pore formation: an ancient yet complex form of attack

Bacteria, as well as higher organisms such as sea anemones or earthworms, have developed sophisticated virulence factors such as the pore-forming toxins (PFTs) to mount their attack against the host. One of the most fascinating aspects of PFTs is that they can adopt a water-soluble form at the beginning of their lifetime and become an integral transmembrane protein in the membrane of the target cells. There is a growing understanding of the sequence of events and the various conformational changes undergone by these toxins in order to bind to the host cell surface, to penetrate the cell membranes and to achieve pore formation. These points will be addressed in this review.

Potentiation of histamine release by Microfungal (1-->3)- and (1-->6)-beta-D-glucans

(1-->3)-beta-D-Glucans, a cell wall component in most microfungi, are suggested to play a role in the development of respiratory and general symptoms in organic dust-related diseases. The mechanisms by which they induce these effects are, however, not clear. In the present study, mediator release and its potentiation by the (1-->3)-beta-D-glucan as well as by the (1-->6)-beta-D-glucan found in yeast and other fungi were therefore examined. Blood leucocytes from healthy volunteers and from patients allergic to house dust mite were incubated with (1-->3)-beta-D-glucans with increasing 1,6-branchings: curdlan [a linear (1-->3)-beta-D-glucan], laminarin and scleroglucan, and furthermore with pustulan, a linear (1-->6)-beta-D-glucan. Histamine release was not observed on exposure to the glucans only, but in the presence of anti-immunoglobulin E (IgE) antibody or specific antigens, all the glucans investigated led to an enhancement of the IgE-mediated histamine release. The glucans induced a significant potentiation of the mediator release when present at concentrations in the range of 2-5 x 10(-5) M. These results suggest that (1-->3)-beta-D-glucan as well as (1-->6)-beta-D-glucan aggravates IgE-mediated histamine release. Knowledge concerning the effects of glucans on immune responses may be of importance for understanding and treating inflammatory and allergic diseases.

A protein toxin from the sea anemone Phyllodiscus semoni targets the kidney and causes a severe renal injury with predominant glomerular endothelial damage

Envenomation by the sea anemone Phyllodiscus semoni causes fulminant dermatitis and, rarely, acute renal failure in humans. Here, we investigated whether the venom extracted from the nematocysts (PsTX-T) was nephrotoxic when administered intravenously in rats and whether PsTX-T induced activation of the complement system. Although small dose of PsTX-T induced acute tubular necrosis in rats resembling pathology seen in patients, kidneys displayed glomerular injury with glomerular endothelial damage, thrombus formation, mesangiolysis, and partial rupture of glomerular basement membrane, accompanied by severe tubular necrosis at 24 hours after administration of 0.03 mg of PsTX-T per animal, similar to the glomerular findings typical of severe hemolytic uremic syndrome. The early stage injury was accompanied by specific PsTX-T binding, massive complement C3b, and membrane attack complex deposition in glomeruli in the regions of injury and decreased glomerular expression of complement regulators. A pathogenic role for complement was confirmed by demonstrating that systemic complement inhibition reduced renal injury. The isolated nephrotoxic component, a 115-kd protein toxin (PsTX-115), was shown to cause identical renal pathology. The demonstration that PsTX-T and PsTX-115 were highly nephrotoxic acting via induction of complement activation suggests that inhibition of complement might be used to prevent acute renal damage following envenomation by P. semoni.

Infrared spectroscopy study on the conformational changes leading to pore formation of the toxin sticholysin II

The structure of the actinoporin sticholysin II (StnII) in the pore state was investigated by Fourier transform infrared spectroscopy in the attenuated total reflection configuration. 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine/cholesterol unilamellar vesicles were employed. The alpha-helix content increases in approximately 30% upon lipid binding, which agrees with an extension of eight or nine residues at the N-terminal helix. Furthermore, analyses of dichroic spectra show that the extended N-terminal helix would have a 31 degrees tilt with respect to the membrane normal. The orientation of the central beta-sandwich was also estimated. In addition, it was detected that StnII alters the orientation of the lipid acyl chains. (1)H/(2)H exchange experiments sustain a mainly superficial interaction between StnII and the membrane, with no protection of the beta-sandwich. The implications of the results in the mechanism of pore formation are discussed.

Effect of sphingomyelin and cholesterol on the interaction of St II with lipidic interfaces

Sticholysin II (St II) is a cytolysin produced by the sea anemone Stichodactyla helianthus, characterized by forming oligomeric pores in natural and artificial membranes. In the present work the influence of the membrane lipidic components sphingomyelin (SM) and cholesterol (Cho) on binding and functional activity of St II, was evaluated using ELISA, lipid monolayers and liposomes. The aim of this work was to establish the promoting role of Cho and SM, both in St II binding and pore formation efficiency. In general the association (evaluated by ELISA and incorporation to phospholipid monolayers) of St II to lipids mixtures was better than to any one of the single components. Regarding the unique role of SM, it was found that, albeit inefficiently, St II binds to phosphatidylcholine (PC):Cho monolayers and liposomes, and is able to form active pores in these bilayers. The results in monolayers and liposomes show that the presence of SM and large amounts of Cho leads to the highest values of critical pressure and rate of association to monolayers, the most favorable interaction with liposomes, and the fastest rate of pore formation, in spite of the rigidity of the layers as suggested by the high generalized polarization (GP) of Laurdan incorporated to liposomes and FTIR data. Taken together, the present results show that the joint presence of SM and Cho, both in binary and ternary (PC containing) mixtures provide conditions particularly suitable for St II binding and function. We suggest that microdomains present in the bilayers could be important for toxin-membrane association.

The electrogenic Na+/HCO3- cotransport modulates resting membrane potential and action potential duration in cat ventricular myocytes

Perforated whole-cell configuration of patch clamp was used to determine the contribution of the electrogenic Na+/HCO3- cotransport (NBC) on the shape of the action potential in cat ventricular myocytes. Switching from Hepes to HCO3- buffer at constant extracellular pH (pH(o)) hyperpolarized resting membrane potential (RMP) by 2.67 +/- 0.42 mV (n = 9, P < 0.05). The duration of action potential measured at 50% of repolarization time (APD50) was 35.8 +/- 6.8% shorter in the presence of HCO3- than in its absence (n = 9, P < 0.05). The anion blocker SITS prevented and reversed the HCO3- -induced hyperpolarization and shortening of APD. In addition, no HCO3- -induced hyperpolarization and APD shortening was observed in the absence of extracellular Na+. Quasi-steady-state currents were evoked by 8 s duration voltage-clamped ramps ranging from -130 to +30 mV. A novel component of SITS-sensitive current was observed in the presence of HCO3-. The HCO3- -sensitive current reversed at -87 +/- 5 mV (n = 7), a value close to the expected reversal potential of an electrogenic Na+/HCO3- cotransport with a HCO3-:Na+ stoichiometry ratio of 2: 1. The above results allow us to conclude that the cardiac electrogenic Na+/HCO3- cotransport has a relevant influence on RMP and APD of cat ventricular cells.

Caissarolysin I (Bcs I), a new hemolytic toxin from the Brazilian sea anemone Bunodosoma caissarum: Purification and biological characterization

Two cationic proteins, C1 and C3, were purified to homogeneity from the hemolytic fraction of the venom of Bunodosoma caissarum sea anemone. The purification processes employed gel filtration followed by ion exchange chromatography, being the purity and molecular mass confirmed by SDS-PAGE and mass spectrometry. Protein C1 represented the second major peak of the hemolytic fraction and was previously believed to be a cytolysin belonging to a new class of hemolysins. The C1 protein has a molecular mass of 15495 Da and was assayed for hemolysis, PLA2 activity and acute toxicity in crabs and mice, showing no activity in these assays. It has an amino terminal with no similarity to all known hemolysins and, therefore, should not be considered a toxin, being its function completely unknown. The protein C3 (19757 Da), that also lacks PLA2 activity, was recognized by antiserum against Eqt II and presented high hemolytic activity to human erythrocytes (ED50 of 0.270 microg/ml), being named Caissarolysin I (Bcs I). Its activity was inhibited by pre-incubation with sphingomyelin (SM) and also when in presence of erythrocytes pre-treated with the SMase P2, a phospholipase D from the brown spider Loxosceles intermedia, indicating that SM is the main target of Bcs I. Caissarolysin I is the first hemolysin purified from a sea anemone belonging to the genus Bunodosoma and belongs to the Actinoporin family of sea anemone hemolysins.

Detergent-resistant membranes are platforms for actinoporin pore-forming activity on intact cells

Sticholysin II is a pore-forming toxin produced by the sea anemone Stichodactyla helianthus. We studied its cytolytic activity on COS-7 cells. Fluorescence spectroscopy and flow cytometry revealed that the toxin permeabilizes cells to propidium cations in a dose-dependent and time-dependent manner. This permeabilization is impaired by preincubation of cells with cyclodextrin. Isolation of detergent-resistant cellular membranes showed that sticholysin II colocalizes with caveolin-1 in fractions corresponding to raft-like domains. The interaction of sticholysin II with such domains is only lipid dependent as it also occurs in the absence of any other membrane-associated protein. Toxin binding to raft-like lipid vesicles inhibited cell permeabilization. The results suggest that sticholysin II promotes pore formation in COS-7 cells through interaction with membrane domains which behave like cellular rafts.

Biochemical and physiological analyses of a hemolytic toxin isolated from a sea anemone Actineria villosa

A species of venomous sea anemone Actineria villosa was recently found inhabiting the coastal areas of Okinawa, Japan. This marine animal produces various proteinous toxins, so that a local health organization was called for medical treatment for those who had accidental contact with this animal. In this study we analyzed the biochemical and physiological properties of hemolytic protein from A. villosa. The toxin purified from the tentacles of the animals was found to be a protein with a molecular weight of approximately 19 kDa. We named this newly found hemolytic toxin of A. villosa, Avt-I. Incubation of the toxin with sphingomyelin inhibited hemolytic activity by up to 85%, showing that Avt-I may target sphingomyelin on the erythrocyte membrane. The hemolytic activity was stably maintained at temperatures below 45 degrees C, however, a sharp linear decrease in heat stability was observed within the range of 45-55 degrees C. Our results provide the first evidence that A. villosa produces a toxin with strong hemolytic activity similar in biochemical and physiological properties to other members of actinoporin family previously isolated from related species of sea anemones.

Neurotoxic activity induced by a haemolytic substance in the extract of the marine sponge Geodia corticostylifera

In our search for marine bioactive compounds we chose a Brazilian Coast sponge, Geodia corticostylifera (Demospongiae), whose extracts showed previously antibacterial and antifungal activities. In the present work we studied the following toxic properties of G. corticostylifera extract: neurotoxic (in mouse neuromuscular junction); mouse acute toxicity (IP) and haemolytic (against mouse and frog erythrocytes). Insertion of ionic channels in planar lipid bilayers in presence of a haemolytic purified fraction of the extract was observed. The toxic activities of G. corticostylifera crude extract are related to the formation of ionic pores in the cell membrane, which induce the release of haemoglobin from erythrocytes, and depolarization of nerve and muscle membranes. These last physiological effects cause the blockade of the diaphragm contractions, leading to death through respiratory arrest.

Interaction of the eukaryotic pore-forming cytolysin equinatoxin II with model membranes: 19F NMR studies

Sea anemones produce a family of 18-20 kDa proteins, the actinoporins, which lyse cells by forming pores in cell membranes. Sphingomyelin plays an important role in their lytic activity, with membranes lacking this lipid being largely refractory to these toxins. As a means of characterising membrane binding by the actinoporin equinatoxin II (EqTII), we have used 19F NMR to probe the environment of Trp residues in the presence of micelles and bicelles. Trp was chosen as previous data from mutational studies and truncated analogues had identified the N-terminal helix of EqTII and the surface aromatic cluster including tryptophan residues 112 and 116 as being important for membrane interactions. The five tryptophan residues were replaced with 5-fluorotryptophan and assigned by site-directed mutagenesis. The 19F resonance of W112 was most affected in the presence of phospholipid micelles or bicelles, followed by W116, with further change induced by the addition of sphingomyelin. Although binding to phosphatidylcholine is not sufficient to enable pore formation in bilayer membranes, this interaction had a greater effect on the tryptophan residues in our studies than the subsequent interaction with sphingomyelin. Furthermore, sphingomyelin had a direct effect on EqTII in both model membranes, so its role in EqTII pore formation involves more than simply an indirect effect mediated via bulk lipid properties. The lack of change in chemical shift for W149 even in the presence of sphingomyelin indicates that, at least in the model membranes studied here, interaction with sphingomyelin was not sufficient to trigger dissociation of the N-terminal helix from the beta-sandwich, which forms the bulk of the protein.

Lysenin: A new tool for investigating membrane lipid organization

Sphingomyelin is a major sphingolipid species in animal cells and is a major lipid constituent of plasma membranes. Recent reports have established important roles for sphingomyelin and its metabolites as second messengers in signal transduction events during development and differentiation. Sphingomyelin is also a major component of sphingolipid, cholesterol-rich plasma membrane microdomains, known as 'lipid rafts'. However, little is known about the organization of sphingomyelin in biological membranes. Lysenin is a recently discovered sphingomyelin-specific toxin. In the present review, we summarize the current characterization of this protein and describe our recent attempt to elucidate the organization of sphingomyelin in cellular membranes using lysenin as a unique tool.

Isolation, properties and partial amino acid sequence of a new actinoporin from the sea anemone Radianthus macrodactylus

A new cytolytic toxin, actinoporin RTX-S II, was isolated from the sea anemone Radianthus macrodactylus with a high degree of purity by a combination of gel filtration, ion-exchange and reverse-phase chromatography. RTX-S II has molecular mass of 19,280 Da and isoelectric point of 10.0. The hemolytic activity of RTX-S II is inhibited by sphingomyelin. RTX-S II had an LD(50) of 70 mg/kg, and is lacking in phospholipase activity. The amino acid composition of this protein contains a high amount of basic and non-polar amino acids and no cysteine. The N-terminal sequence of RTX-S II was determined. The partial amino acid sequence (141 aa) of RTX-S II was deduced based on the cDNA sequence obtained with two oligonucleotides encoding the N-terminal portion of RTX-S II and the internal conserved cytolysin peptide by PCR. A comparison of the RTX-S II cDNA sequence and the rtx-s II gene obtained with the same PCR primers indicates that they are 100% identical at the nucleotide level. It shows that no introns are present in the corresponding region of the rtx-s II gene. Multiple alignments of RTX-S II with known sequences of actinoporins show that RTX-S II is highly homologous to magnificalysin II from Heteractis magnifica. The predicted secondary structure of RTX-S II is predominantly anti-parallel beta-structure, which is in good agreement with experimental data obtained from other sea anemones-actinoporins.

Adjunctive therapies in sepsis: An evidence-based review

OBJECTIVE

In 2003, critical care and infectious disease experts representing 11 international organizations developed management guidelines for adjunctive therapies in sepsis that would be of practical use for the bedside clinician, under the auspices of the Surviving Sepsis Campaign, an international effort to increase awareness and to improve outcome in severe sepsis.

DESIGN

The process included a modified Delphi method, a consensus conference, several subsequent smaller meetings of subgroups and key individuals, teleconferences, and electronic-based discussion among subgroups and among the entire committee.

METHODS

The modified Delphi methodology used for grading recommendations built on a 2001 publication sponsored by the International Sepsis Forum. We undertook a systematic review of the literature graded along five levels to create recommendation grades from A to E, with A being the highest grade. Pediatric considerations to contrast adult and pediatric management are in the article by Parker et al. on p. S591.

CONCLUSION

Glycemic control (maintenance of glucose <150 mg/dL) is recommended. The beneficial effect of glycemic control appears to be related control of glucose and not the administration of insulin. Glycemic control should be combined with a nutritional protocol. The dialysis dose is important in sepsis-induced acute renal failure. Continuous hemofiltration offers easier management of fluid balance in hemodynamically unstable septic patients but in the absence of hemodynamic instability is equivalent to intermittent hemodialysis. It is uncertain whether high-volume hemofiltration improves prognosis in sepsis. Bicarbonate therapy is not recommended for the purpose of improving hemodynamics or reducing vasopressor requirements in the presence of lactic academia and pH >7.15.

Crystal and electron microscopy structures of sticholysin II actinoporin reveal insights into the mechanism of membrane pore formation

Sticholysin II (StnII) is a pore-forming protein (PFP) produced by the sea anemone Stichodactyla helianthus. We found out that StnII exists in a monomeric soluble state but forms tetramers in the presence of a lipidic interface. Both structures have been independently determined at 1.7 A and 18 A resolution, respectively, by using X-ray crystallography and electron microscopy of two-dimensional crystals. Besides, the structure of soluble StnII complexed with phosphocholine, determined at 2.4 A resolution, reveals a phospholipid headgroup binding site, which is located in a region with an unusually high abundance of aromatic residues. Fitting of the atomic model into the electron microscopy density envelope suggests that while the beta sandwich structure of the protein remains intact upon oligomerization, the N-terminal region and a flexible and highly basic loop undergo significant conformational changes. These results provide the structural basis for the membrane recognition step of actinoporins and unexpected insights into the oligomerization step.

Lipid Phase Coexistence Favors Membrane Insertion of Equinatoxin-II, a Pore-forming Toxin from Actinia equina

Equinatoxin-II is a eukaryotic pore-forming toxin belonging to the family of actinoporins. Its interaction with model membranes is largely modulated by the presence of sphingomyelin. We have used large unilamellar vesicles and lipid monolayers to gain further information about this interaction. The coexistence of gel and liquid-crystal lipid phases in sphingomyelin/phosphatidylcholine mixtures and the coexistence of liquid-ordered and liquid-disordered lipid phases in phosphatidylcholine/cholesterol or sphingomyelin/phosphatidylcholine/cholesterol mixtures favor membrane insertion of equinatoxin-II. Phosphatidylcholine vesicles are not permeabilized by equinatoxin-II. However, the localized accumulation of phospholipase C-generated diacylglycerol creates conditions for toxin activity. By using epifluorescence microscopy of transferred monolayers, it seems that lipid packing defects arising at the interfaces between coexisting lipid phases may function as preferential binding sites for the toxin. The possible implications of such a mechanism in the assembly of a toroidal pore are discussed.

Bench-to-bedside review: treating acid-base abnormalities in the intensive care unit - the role of buffers

The recognition and management of acid-base disorders is a commonplace activity for intensivists. Despite the frequency with which non-bicarbonate-losing forms of metabolic acidosis such as lactic acidosis occurs in critically ill patients, treatment is controversial. This article describes the properties of several buffering agents and reviews the evidence for their clinical efficacy. The evidence supporting and refuting attempts to correct arterial pH through the administration of currently available buffers is presented.

Cross-reactivity and inhibition of haemolysis by polyclonal antibodies raised against St II, a cytolysin from the sea anemone Stichodactyla helianthus

The immunogenicity of sticholysin II (St II), a pore-forming polypeptide from the sea anemone Stichodactyla helianthus, was studied in rabbits using two adjuvants, Freund's and aluminium hydroxide. High titres of antibodies were raised against St II with Freund's adjuvant (FA). The structural homology between sticholysins I and II was also revealed by cross-reactivity assays. Since the oil constituent of FA neutralized the St II haemolytic activity, immunizations with St II-Freund's emulsions were carried out with the inactivated cytolysin. Purified anti-St II IgG also neutralized the St II haemolytic activity.

Oligomerization and pore formation of a sphingomyelin-specific toxin, lysenin

Lysenin is a novel protein derived from coelomic fluid of the earthworm Eisenia foetida, which specifically recognizes sphingomyelin and induces cytolysis. The mechanism underlying lysenin-induced cell lysis has not been clarified. In this report we studied the interaction of lysenin with red blood cells as well as artificial liposomes. Our results showed that lysenin bound membranes and assembled to SDS-resistant oligomers in a sphingomyelin-dependent manner, leading to the formation of pores with a hydrodynamic diameter of approximately 3 nm. Antibody scanning analysis suggested that the C-terminal region of lysenin was exposed, whereas the N-terminal was hidden in the isolated oligomer complex. Differential scanning calorimetry revealed that lysenin interacted with both hydrophilic head group and hydrophobic hydrocarbon tails of sphingomyelin. Oligomerization but not binding was affected by the amide-linked fatty acid composition of sphingomyelin, suggesting the role of membrane fluidity in the oligomerization step.

Effects of the eukaryotic pore-forming cytolysin Equinatoxin II on lipid membranes and the role of sphingomyelin

Equinatoxin II (EqtII), a protein toxin from the sea anemone Actinia equina, readily creates pores in sphingomyelin-containing lipid membranes. The perturbation by EqtII of model lipid membranes composed of dimyristoylphosphatidycholine and sphingomyelin (10 mol %) was investigated using wideline phosphorus-31 and deuterium NMR. The preferential interaction between EqtII (0.1 and 0.4 mol %) and the individual bilayer lipids was studied by (31)P magic angle spinning NMR, and toxin-induced changes in bilayer morphology were examined by freeze-fracture electron microscopy. Both NMR and EM showed the formation of an additional lipid phase in sphingomyelin-containing mixed lipid multilamellar suspensions with 0.4 mol % EqtII. The new toxin-induced phase consisted of small unilamellar vesicles 20-40 nm in diameter. Deuterium NMR showed that the new lipid phase contains both dimyristoylphosphatidycholine and sphingomyelin. Solid-state (31)P NMR showed an increase in spin-lattice and a decrease in spin-spin relaxation times in mixed-lipid model membranes in the presence of EqtII, consistent with an increase in the intensity of low frequency motions. The (2)H and (31)P spectral intensity distributions confirmed a change in lipid mobility and showed the creation of an isotropic lipid phase, which was identified as the small vesicle structures visible by electron microscopy in the EqtII-lipid suspensions. The toxin appears to enhance slow motions in the membrane lipids and destabilize the membrane. This effect was greatly enhanced in sphingomyelin-containing mixed lipid membranes compared with pure phosphatidylcholine bilayers, suggesting a preferential interaction between the toxin and bilayer sphingomyelin.

Biologically active polypeptides from the tropical sea anemone Radianthus macrodactylus

Some biologically active polypeptides, three high and two low molecular weight cytolysins and four trypsin inhibitors were isolated from the sea anemone Radianthus macrodactylus and characterized. The purification steps involved acetone precipitation, gel filtration, ion-exchange, and affinity chromatography, and ion-exchange and reverse-phase HPLC. The relative molecular weight of high molecular weight Radianthus cytolysins named according to their N-terminal amino acids RTX-A (Ala), RTX-S (Ser) and RTX-G (Gly) was about 20,000. The isoelectric points were 9.8 for RTX-A and RTX-S, and 10.5 for RTX-G. The hemolytic activities of RTX-A, RTX-S and RTX-G were 3.5 x 10(4), 5.0 x10(4), and 1.0 x10(4)HU/mg, respectively, and were inhibited by sphingomyelin. The N-terminal amino acid sequence of RTX-A was determined as ALAGAIIAGAGLGLKILIEVLGEG-VKVKI-. Molecular weight of low molecular weight Radianthus cytolysins RmI, RmII, and of one trypsin inhibitor InI were 5100, 6100 and 7100, respectively. Isoelectric points for RmI and RmII were 9.2 and 9.3. Their hemolytic activity worked out 25 and 20 HU/mg, and was not inhibited by sphingomyelin. Toxicity of RmI and RmII was assessed by their histaminolytic activity. Amino acid composition of RmI and RmII was similar to that of tealiatoxin, histaminolytic cytolysin from the sea anemone Tealia felina.

Identity between cytolysins purified from two morphos of the Caribbean sea anemone Stichodactyla helianthus

Stichodactyla helianthus is a sea anemone relatively abundant along Cuban coasts appearing in two morphos with different colors in their tentacles: green or brownish, probably due to their association with algal symbionts. Traditionally, the brownish morpho has been used as a source of sticholysins I and II, the most characterized cytolysins from this anemone, but the green morpho is the most abundant along the western coasts of Havana. The present work is aimed to establish if the cytolysins purified from the green morpho (StIg and StIIg) are similar to those purified from brownish anemones (StI and StII). Following the same chromatographic procedure used to purify the toxins from morphos, the electrophoretic mobilities, amino acid compositions, amino terminal sequences and molecular masses were practically identical between analogal cytolysins. In conclusion, homologous sticholysins purified from the green and brownish variants of Stichodactyla helianthus are the same molecular entities.

Properties of proteinaceous toxins in the salivary gland of the marine gastropod (Monoplex echo)

Three proteinaceous toxins (named echotoxins 1, 2, and 3) with both lethal and hemolytic activities were purified from the salivary gland of the marine gastropod (Monoplex echo) by gel filtration on Sephadex G-75 and cation-exchange FPLC on Mono S, although marked reduction in toxicity was observed in the latter chromatography. When subjected to reverse-phase HPLC on TSKgel Phenyl-5PW RP, echotoxin 2 afforded a single peak, while both echotoxins 1 and 3 were further separated into two components, respectively. All echotoxins are monomeric simple proteins with a molecular mass of about 25kDa and their N-termini are blocked. Amino acid compositions of echotoxins 1a, 1b, and 2 are closely similar to one another, being characterized by the abundance of Gly and Ala and the absence of half-Cys, Met, and Trp. Of the sugars, lipids and glycolipids tested, gangliosides potently inhibited the hemolytic activity of echotoxins. Furthermore, ganglioside G(T1b) inhibited both in vivo hemolysis and lethality in mice displayed by echotoxins. It is thus concluded that echotoxins injected into mice induce rapid hemolysis by binding to receptors (possibly gangliosides) gangliosides in the erythrocyte membrane, eventually leading to death of mice.

Cytolytic peptide and protein toxins from sea anemones (Anthozoa: Actiniaria)

More than 32 species of sea anemones have been reported to produce lethal cytolytic peptides and proteins. Based on their primary structure and functional properties, cytolysins have been classified into four polypeptide groups. Group I consists of 5-8 kDa peptides, represented by those from the sea anemones Tealia felina and Radianthus macrodactylus. These peptides form pores in phosphatidylcholine containing membranes. The most numerous is group II comprising 20 kDa basic proteins, actinoporins, isolated from several genera of the fam. Actiniidae and Stichodactylidae. Equinatoxins, sticholysins, and magnificalysins from Actinia equina, Stichodactyla helianthus, and Heteractis magnifica, respectively, have been studied mostly. They associate typically with sphingomyelin containing membranes and create cation-selective pores. The crystal structure of equinatoxin II has been determined at 1.9A resolution. Lethal 30-40 kDa cytolytic phospholipases A(2) from Aiptasia pallida (fam. Aiptasiidae) and a similar cytolysin, which is devoid of enzymatic activity, from Urticina piscivora, form group III. A thiol-activated cytolysin, metridiolysin, with a mass of 80 kDa from Metridium senile (fam. Metridiidae) is a single representative of the fourth family. Its activity is inhibited by cholesterol or phosphatides. Biological, structure-function, and pharmacological characteristics of these cytolysins are reviewed.

Solution structure of the eukaryotic pore-forming cytolysin equinatoxin II: implications for pore formation

Sea anemones produce a family of 18-20 kDa proteins, the actinoporins, that lyse cells by forming pores in cell membranes. Sphingomyelin plays an important role in their lytic activity, with membranes lacking this lipid being largely refractory to these toxins. The structure of the actinoporin equinatoxin II in aqueous solution, determined from NMR data, consists of two short helices packed against opposite faces of a beta-sandwich structure formed by two five-stranded beta-sheets. The protein core has extensive hydrophobic interfaces formed by residues projecting from the internal faces of the two beta-sheets. 15N relaxation data show uniform backbone dynamics, implying that equinatoxin II in solution is relatively rigid, except at the N terminus; its inferred rotational correlation time is consistent with values for monomeric proteins of similar mass. Backbone amide exchange rate data also support the view of a stable structure, even though equinatoxin II lacks disulfide bonds. As monitored by NMR, it unfolds at around 70 degrees C at pH 5.5. At 25 degrees C the structure is stable over the pH range 2.5-7.3 but below pH 2.5 it undergoes a slow transition to an incompletely unfolded structure resembling a molten globule. Equinatoxin II has two significant patches of positive electrostatic potential formed by surface-exposed Lys and Arg residues, which may assist its interaction with charged regions of the lipid head groups. Tyr and Trp residues on the surface may also contribute by interacting with the carbonyl groups of the acyl chains of target membranes. Data from mutational studies and truncated analogues identify two regions of the protein involved in membrane interactions, the N-terminal helix and the Trp-rich region. Once the protein is anchored, the N-terminal helix may penetrate the membrane, with up to four helices lining the pore, although other mechanisms of pore formation cannot be ruled out.

Effects of regional hypoxia and acidosis on Rb(+) uptake and energetics in isolated pig hearts: (87)Rb MRI and (31)P MR spectroscopic study

The study compared the effects of regional hypoxia and acidosis on Rb(+) uptake and energetics in isolated pig hearts perfused by the Langendorff method. The left anterior descending artery (LAD) was cannulated and the LAD bed was perfused with the same specific flow as the whole heart. Following equilibration with normal Krebs-Henseleit buffer (KHB, pO(2) 568 mm Hg, pH 7.42) the perfusate was switched to one that contained Rb(+) (Rb-KHB). Simultaneously, perfusion through the LAD was carried out with hypoxic (pO(2)=31 mm Hg), an acidemic (pH 7.12) or normal (pO(2)=550 mm Hg) Rb-KHB for 120 min. (87)Rb images of the entire heart or localized (31)P spectra from the left ventricular anterior wall were acquired. Hypoxia decreased the maximal (87)Rb image intensity and Rb(+) flux in the anterior wall to 79+/-9% and 85+/-7%, respectively, of that in the posterior wall. Extracellular acidosis did not affect (87)Rb image intensity and reduced Rb(+) flux (83+/-10%). During hypoxia phosphocreatine and ATP decreased to 36+/-10 and 50+/-15% of baseline, respectively and intracellular pH (pHi) decreased to 6.90+/-0.05. Extracellular acidosis did not affect the phosphocreatine or ATP levels but reduced pHi (7.06+/-0.18 vs. 7.26+/-0.06 in control). We suggest that intracellular acidosis plays a role in the inhibition of Rb(+) uptake during hypoxia.