Conformational changes that effect oligomerization and initiate pore formation are triggered throughout perfringolysin O upon binding to cholesterol

Immunogenic and neutralization efficacy of recombinant perfringolysin O of Clostridium perfringens and its C-terminal receptor-binding domain in a murine model

Clostridium perfringens is a Gram-positive anaerobe ubiquitously present in different environments, including the gut of humans and animals. C. perfringens have been classified in the seven toxinotypes based on the secreted toxins that cause different diseases in humans and animals. Perfringolysin O (PFO), a cholesterol-dependent pore-forming cytolysin, is one of the potent toxins secreted by almost all C. perfringens isolates. The PFO acts in synergy with α-toxin in the progression of gas gangrene in humans and necrohemorrhagic enteritis in the calves.C. perfringens infections spread very fast, and the animals die within a few hours of the onset of infection. This necessitates the use of vaccines to control clostridial infections. Though the vaccine potential of other toxins has been reported, PFO has remained unexplored. The present study describes the immunogenic and protective potential of native recombinant PFO (WTrPFO). Since the PFO is toxic to the host cells, the non-toxic C-terminal domain of PFO (rPFOC-ter) was also assessed for its immunogenicity and protective efficacy. Immunization of mice with the purified soluble recombinant histidine-tagged WTrPFO and rPFOC-ter, expressed in E. coli, generated robust mixed immune response and T cell memory. Pre-incubation of the WTrPFO with anti-WTrPFO and rPFOC-ter antisera negated its hemolytic activity in mice RBCs, as well as its cytotoxic effect in mice peritoneal macrophages in vitro. Thus, immunization with the WTrPFO and its non-toxic C-terminal domain generated neutralizing antibodies, suggesting their vaccine potential against the PFO. Thus, the non-toxic C-terminal domain of PFO could serve as an alternative to PFO as a vaccine candidate.

Optimization of cell permeabilization in electron flow based mitochondrial function assays

Permeable cell models have contributed much to the progress in mitochondrial research. Optimization of permeabilization is required to make the cell's plasma membrane permeable to small molecules while keeping the intracellular organelles and their membranes intact and fully functional. Here we report our assessment and optimization of commonly used permeabilizing agents including different saponin preparations, digitonin, and recombinant perfringolysin O employing a new electron flow based mitochondrial assay technology that utilizes a colorimetric redox dye. The results of this study provide guidance in optimizing the conditions for mitochondrial function assays with permeabilized cells using the novel redox dye-based format.

Imaging Sphingomyelin- and Cholesterol-Enriched Domains in the Plasma Membrane Using a Novel Probe and Super-Resolution Microscopy

In this chapter, we show the visualization of lipid domains using a specific lipid-binding protein and super-resolution microscopy. Lipid rafts are plasma membrane domains enriched in both sphingolipids and sterols that play key roles in various physiological events. We identified a novel protein that specifically binds to a complex of sphingomyelin (SM) and cholesterol (Chol). The isolated protein, nakanori, labels the SM/Chol complex at the outer leaflet of the plasma membrane in mammalian cells. Structured illumination microscopic images suggested that the influenza virus buds from the edges of the SM/Chol domains in MDCK cells. Furthermore, a photoactivated localization microscopy analysis indicated that the SM/Chol complex forms domains in the outer leaflet, just above the phosphatidylinositol 4,5-bisphosphate domains in the inner leaflet. These observations provide significant insight into the structure and function of lipid rafts.

19F NMR studies provide insights into lipid membrane interactions of listeriolysin O, a pore forming toxin from Listeria monocytogenes

Listeria monocytogenes is a mammalian pathogen that causes gastroenteritis, miscarriages and infections of the central nervous system in immunocompromised individuals. Its main virulence factor is listeriolysin O (LLO), a pore-forming cholesterol-dependent cytolysin (CDC), which enables bacterial escape from the phagolysosome and contributes to bacterial pathogenicity. Details of cholesterol (Chol) recognition and membrane binding mechanisms by LLO are still not known. Here we used ¹⁹F-NMR spectroscopy in order to assess LLO-Chol interactions in solution and in a Chol-rich membrane environment. LLO has six tryptophan residues located in the region of the molecule that is first in contact with lipid membranes. ¹⁹F-LLO, which contained 5-fluoro-tryptophans, was prepared by using isotopic labelling in an E. coli expression system. Signals in the ¹⁹F-NMR spectrum of ¹⁹F-LLO were unambiguously assigned by using a series of single Trp → Phe point mutations. The results employing various cholesterol preparations in solution indicate that tryptophan residues are not directly involved in Chol binding in solution. However, significant chemical shift changes were observed upon LLO binding to Chol-rich membranes, highlighting the role of tryptophan residues in membrane interactions (W512) and oligomerisation (W189 and W489).

Listeriolysin O: from bazooka to Swiss army knife

Listeria monocytogenes (Lm) is a Gram-positive facultative intracellular pathogen. Infections in humans can lead to listeriosis, a systemic disease with a high mortality rate. One important mechanism of Lm dissemination involves cell-to-cell spread after bacteria have entered the cytosol of host cells. Listeriolysin O (LLO; encoded by the hly gene) is a virulence factor present in Lm that plays a central role in the cell-to-cell spread process. LLO is a member of the cholesterol-dependent cytolysin (CDC) family of toxins that were initially thought to promote disease largely by inducing cell death and tissue destruction-essentially acting like a 'bazooka'. This view was supported by structural studies showing CDCs can form large pores in membranes. However, it is now appreciated that LLO has many subtle activities during Lm infection of host cells, and many of these likely do not involve large pores, but rather small membrane perforations. It is also appreciated that membrane repair pathways of host cells play a major role in limiting membrane damage by LLO and other toxins. LLO is now thought to represent a 'Swiss army knife', a versatile tool that allows Lm to induce many membrane alterations and cellular responses that promote bacterial dissemination during infection.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

Interaction of Cholesterol with Perfringolysin O: What Have We Learned from Functional Analysis?

Cholesterol-dependent cytolysins (CDCs) constitute a family of pore-forming toxins secreted by Gram-positive bacteria. These toxins form transmembrane pores by inserting a large β-barrel into cholesterol-containing membranes. Cholesterol is absolutely required for pore-formation. For most CDCs, binding to cholesterol triggers conformational changes that lead to oligomerization and end in pore-formation. Perfringolysin O (PFO), secreted by Clostridium perfringens, is the prototype for the CDCs. The molecular mechanisms by which cholesterol regulates the cytolytic activity of the CDCs are not fully understood. In particular, the location of the binding site for cholesterol has remained elusive. We have summarized here the current body of knowledge on the CDCs-cholesterol interaction, with focus on PFO. We have employed sterols in aqueous solution to identify structural elements in the cholesterol molecule that are critical for its interaction with PFO. In the absence of high-resolution structural information, site-directed mutagenesis data combined with binding studies performed with different sterols, and molecular modeling are beginning to shed light on this interaction.

Mechanistic Insights into the Cholesterol-dependent Binding of Perfringolysin O-based Probes and Cell Membranes

Cholesterol distribution in the cell is maintained by both vesicular and non-vesicular sterol transport. Non-vesicular transport is mediated by the interaction of membrane-embedded cholesterol and water-soluble proteins. Small changes to the lipid composition of the membrane that do not change the total cholesterol content, can significantly affect how cholesterol interacts with other molecules at the surface of the membrane. The cholesterol-dependent cytolysin Perfringolysin O (PFO) constitutes a powerful tool to detect cholesterol in membranes, and the use of PFO-based probes has flourished in recent years. By using a non-lytic PFO derivative, we showed that the sensitivity of the probes for cholesterol can be tuned by modifications introduced directly in the membrane-interacting loops and/or by modifying residues away from the membrane-interacting domain. Through the use of these biosensors on live RAW 264.7 cells, we found that changes in the overall cholesterol content have a limited effect on the average cholesterol accessibility at the surface of the membrane. We showed that these exquisite biosensors report on changes in cholesterol reactivity at the membrane surface independently of the overall cholesterol content in the membrane.

Domain 4 (D4) of Perfringolysin O to Visualize Cholesterol in Cellular Membranes-The Update

The cellular membrane of eukaryotes consists of phospholipids, sphingolipids, cholesterol and membrane proteins. Among them, cholesterol is crucial for various cellular events (e.g., signaling, viral/bacterial infection, and membrane trafficking) in addition to its essential role as an ingredient of steroid hormones, vitamin D, and bile acids. From a micro-perspective, at the plasma membrane, recent emerging evidence strongly suggests the existence of lipid nanodomains formed with cholesterol and phospholipids (e.g., sphingomyelin, phosphatidylserine). Thus, it is important to elucidate how cholesterol behaves in membranes and how the behavior of cholesterol is regulated at the molecular level. To elucidate the complexed characteristics of cholesterol in cellular membranes, a couple of useful biosensors that enable us to visualize cholesterol in cellular membranes have been recently developed by utilizing domain 4 (D4) of Perfringolysin O (PFO, theta toxin), a cholesterol-binding toxin. This review highlights the current progress on development of novel cholesterol biosensors that uncover new insights of cholesterol in cellular membranes.

Dynamics of sphingomyelin- and cholesterol-enriched lipid domains during cytokinesis

Sphingomyelin (SM) and cholesterol (Chol) are the major lipids in the mammalian cells, which are mainly localized to the plasma membrane. Multiple lines of evidence suggest that these lipids form local lipid domains in the plasma membrane, playing functional roles in the cell. Several observations have suggested that these lipid domains are required for cytokinesis. In this chapter, we show the methods for visualizing SM-rich and/or Chol-rich membrane domains at cytokinesis by using specific lipid-binding proteins. Lysenin, equinatoxin II, perfringolysin O, and pleurotolysin A2 bind specifically to clustered SM-rich domain, dispersed SM-rich domain, Chol-rich domain, and SM/Chol mixtures, respectively. Nontoxic forms of these lipid-binding proteins fused to fluorescent proteins are used for imaging lipid domains in biological membranes at cytokinesis. The image analysis reveals the structures and functions of SM-rich and/or Chol-rich domains at the time of cytokinesis.

Detectors for evaluating the cellular landscape of sphingomyelin- and cholesterol-rich membrane domains

Although sphingomyelin and cholesterol are major lipids of mammalian cells, the detailed distribution of these lipids in cellular membranes remains still obscure. However, the recent development of protein probes that specifically bind sphingomyelin and/or cholesterol provides new information about the landscape of the lipid domains that are enriched with sphingomyelin or cholesterol or both. Here, we critically summarize the tools to study distribution and dynamics of sphingomyelin and cholesterol. This article is part of a Special Issue entitled: The cellular lipid landscape edited by Tim P. Levine and Anant K. Menon.

Perfringolysin O Theta Toxin as a Tool to Monitor the Distribution and Inhomogeneity of Cholesterol in Cellular Membranes

Cholesterol is an essential structural component of cellular membranes in eukaryotes. Cholesterol in the exofacial leaflet of the plasma membrane is thought to form membrane nanodomains with sphingolipids and specific proteins. Additionally, cholesterol is found in the intracellular membranes of endosomes and has crucial functions in membrane trafficking. Furthermore, cellular cholesterol homeostasis and regulation of de novo synthesis rely on transport via both vesicular and non-vesicular pathways. Thus, the ability to visualize and detect intracellular cholesterol, especially in the plasma membrane, is critical to understanding the complex biology associated with cholesterol and the nanodomains. Perfringolysin O (PFO) theta toxin is one of the toxins secreted by the anaerobic bacteria Clostridium perfringens and this toxin forms pores in the plasma membrane that causes cell lysis. It is well understood that PFO recognizes and binds to cholesterol in the exofacial leaflets of the plasma membrane, and domain 4 of PFO (D4) is sufficient for the binding of cholesterol. Recent studies have taken advantage of this high-affinity cholesterol-binding domain to create a variety of cholesterol biosensors by using a non-toxic PFO or the D4 in isolation. This review highlights the characteristics and usefulness of, and the principal findings related to, these PFO-derived cholesterol biosensors.

How Lipid Membranes Affect Pore Forming Toxin Activity

Pore forming toxins (PFTs) evolved to permeate the plasma membrane of target cells. This is achieved in a multistep mechanism that usually involves binding of soluble protein monomer to the lipid membrane, oligomerization at the plane of the membrane, and insertion of part of the polypeptide chain across the lipid membrane to form a conductive channel. Introduced pores allow uncontrolled transport of solutes across the membrane, inflicting damage to the target cell. PFTs are usually studied from the perspective of structure-function relationships, often neglecting the important role of the bulk membrane properties on the PFT mechanism of action. In this Account, we discuss how membrane lateral heterogeneity, thickness, and fluidity influence the pore forming process of PFTs. In general, lipid molecules are more accessible for binding in fluid membranes due to steric reasons. When PFT specifically binds ordered domains, it usually recognizes a specific lipid distribution pattern, like sphingomyelin (SM) clusters or SM/cholesterol complexes, and not individual lipid species. Lipid domains were also suggested to act as an additional concentration platform facilitating PFT oligomerization, but this is yet to be shown. The last stage in PFT action is the insertion of the transmembrane segment across the membranes to build the transmembrane pore walls. Conformational changes are a spontaneous process, and sufficient free energy has to be available for efficient membrane penetration. Therefore, fluid bilayers are permeabilized more readily in comparison to highly ordered and thicker liquid ordered lipid phase (Lo). Energetically more costly insertion into the Lo phase can be driven by the hydrophobic mismatch between the thinner liquid disordered phase (Ld) and large protein complexes, which are unable to tilt like single transmembrane segments. In the case of proteolipid pores, membrane properties can directly modulate pore size, stability, and even selectivity. Finally, events associated with pore formation can modulate properties of the lipid membrane and affect its organization. Model membranes do not necessarily reproduce the physicochemical properties of the native cellular membrane, and caution is needed when transferring results from model to native lipid membranes. In this context, the utilization of novel approaches that enable studying PFTs on living cells at a single molecule level should reveal complex protein-lipid membrane interactions in greater detail.

The Relationship between Glycan Binding and Direct Membrane Interactions in Vibrio cholerae Cytolysin, a Channel-forming Toxin

Bacterial pore-forming toxins (PFTs) are structurally diverse pathogen-secreted proteins that form cell-damaging channels in the membranes of host cells. Most PFTs are released as water-soluble monomers that first oligomerize on the membrane before inserting a transmembrane channel. To modulate specificity and increase potency, many PFTs recognize specific cell surface receptors that increase the local toxin concentration on cell membranes, thereby facilitating channel formation. Vibrio cholerae cytolysin (VCC) is a toxin secreted by the human pathogen responsible for pandemic cholera disease and acts as a defensive agent against the host immune system. Although it has been shown that VCC utilizes specific glycan receptors on the cell surface, additional direct contacts with the membrane must also play a role in toxin binding. To better understand the nature of these interactions, we conducted a systematic investigation of the membrane-binding surface of VCC to identify additional membrane interactions important in cell targeting. Through cell-based assays on several human-derived cell lines, we show that VCC is unlikely to utilize high affinity protein receptors as do structurally similar toxins from Staphylococcus aureus. Next, we identified a number of specific amino acid residues that greatly diminish the VCC potency against cells and investigated the interplay between glycan binding and these direct lipid contacts. Finally, we used model membranes to parse the importance of these key residues in lipid and cholesterol binding. Our study provides a complete functional map of the VCC membrane-binding surface and insights into the integration of sugar, lipid, and cholesterol binding interactions.

Pore-forming activity of clostridial binary toxins

Clostridial binary toxins (Clostridium perfringens Iota toxin, Clostridium difficile transferase, Clostridium spiroforme toxin, Clostridium botulinum C2 toxin) as Bacillus binary toxins, including Bacillus anthracis toxins consist of two independent proteins, one being the binding component which mediates the internalization into cell of the intracellularly active component. Clostridial binary toxins induce actin cytoskeleton disorganization through mono-ADP-ribosylation of globular actin and are responsible for enteric diseases. Clostridial and Bacillus binary toxins share structurally and functionally related binding components which recognize specific cell receptors, oligomerize, form pores in endocytic vesicle membrane, and mediate the transport of the enzymatic component into the cytosol. Binding components retain the global structure of pore-forming toxins (PFTs) from the cholesterol-dependent cytotoxin family such as perfringolysin. However, their pore-forming activity notably that of clostridial binding components is more related to that of heptameric PFT family including aerolysin and C. perfringens epsilon toxin. This review focuses upon pore-forming activity of clostridial binary toxins compared to other related PFTs. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Perfringolysin O: The Underrated Clostridium perfringens Toxin?

The anaerobic bacterium Clostridium perfringens expresses multiple toxins that promote disease development in both humans and animals. One such toxin is perfringolysin O (PFO, classically referred to as θ toxin), a pore-forming cholesterol-dependent cytolysin (CDC). PFO is secreted as a water-soluble monomer that recognizes and binds membranes via cholesterol. Membrane-bound monomers undergo structural changes that culminate in the formation of an oligomerized prepore complex on the membrane surface. The prepore then undergoes conversion into the bilayer-spanning pore measuring approximately 250–300 Å in diameter. PFO is expressed in nearly all identified C. perfringens strains and harbors interesting traits that suggest a potential undefined role for PFO in disease development. Research has demonstrated a role for PFO in gas gangrene progression and bovine necrohemorrhagic enteritis, but there is limited data available to determine if PFO also functions in additional disease presentations caused by C. perfringens. This review summarizes the known structural and functional characteristics of PFO, while highlighting recent insights into the potential contributions of PFO to disease pathogenesis.

A novel cholesterol-insensitive mode of membrane binding promotes cytolysin-mediated translocation by Streptolysin O

Cytolysin-mediated translocation (CMT), performed by Streptococcus pyogenes, utilizes the cholesterol-dependent cytolysin Streptolysin O (SLO) to translocate the NAD(+) -glycohydrolase (SPN) into the host cell during infection. SLO is required for CMT and can accomplish this activity without pore formation, but the details of SLO's interaction with the membrane preceding SPN translocation are unknown. Analysis of binding domain mutants of SLO and binding domain swaps between SLO and homologous cholesterol-dependent cytolysins revealed that membrane binding by SLO is necessary but not sufficient for CMT, demonstrating a specific requirement for SLO in this process. Despite being the only known receptor for SLO, this membrane interaction does not require cholesterol. Depletion of cholesterol from host membranes and mutation of SLO's cholesterol recognition motif abolished pore formation but did not inhibit membrane binding or CMT. Surprisingly, SLO requires the coexpression and membrane localization of SPN to achieve cholesterol-insensitive membrane binding; in the absence of SPN, SLO's binding is characteristically cholesterol-dependent. SPN's membrane localization also requires SLO, suggesting a co-dependent, cholesterol-insensitive mechanism of membrane binding occurs, resulting in SPN translocation.

Mouse, but not human, ApoB-100 lipoprotein cholesterol is a potent innate inhibitor of Streptococcus pneumoniae pneumolysin

Streptococcus pneumoniae produces the pore-forming toxin pneumolysin (PLY), which is a member of the cholesterol-dependent cytolysin (CDC) family of toxins. The CDCs recognize and bind the 3β-hydroxyl group of cholesterol at the cell surface, which initiates membrane pore formation. The cholesterol transport lipoproteins, which carry cholesterol in their outer monolayer, are potential off-pathway binding targets for the CDCs and are present at significant levels in the serum and the interstitial spaces of cells. Herein we show that cholesterol carried specifically by the ApoB-100-containing lipoprotein particles (CH-ApoB-100) in the mouse, but not that carried by human or guinea pig particles, is a potent inhibitor of the PLY pore-forming mechanism. Cholesterol present in the outer monolayer of mouse ApoB-100 particles is recognized and bound by PLY, which stimulates premature assembly of the PLY oligomeric complex thereby inactivating PLY. These studies further suggest that the vast difference in the inhibitory capacity of mouse CH-ApoB-100 and that of the human and the guinea pig is due to differences in the presentation of cholesterol in the outer monolayer of their ApoB-100 particles. Therefore mouse CH-ApoB-100 represents a significant innate CDC inhibitor that is absent in humans, which may underestimate the contribution of CDCs to human disease when utilizing mouse models of disease.

Crucial role of perfringolysin O D1 domain in orchestrating structural transitions leading to membrane-perforating pores: a hydrogen-deuterium exchange study

Perfringolysin O (PFO) is a toxic protein that binds to cholesterol-containing membranes, oligomerizes, and forms a β-barrel transmembrane pore, leading to cell lysis. Previous studies have uncovered the sequence of events in this multistage structural transition to a considerable detail, but the underlying molecular mechanisms are not yet fully understood. By measuring hydrogen-deuterium exchange patterns of peptide bond amide protons monitored by mass spectrometry (MS), we have mapped structural changes in PFO and its variant bearing a point mutation during incorporation to the lipid environment. We have defined all regions that undergo structural changes caused by the interaction with the lipid environment both in wild-type PFO, thus providing new experimental constraints for molecular modeling of the pore formation process, and in a point mutant, W165T, for which the pore formation process is known to be inefficient. We have demonstrated that point mutation W165T causes destabilization of protein solution structure, strongest for domain D1, which interrupts the pathway of structural transitions in other domains necessary for proper oligomerization in the membrane. In PFO, the strongest changes accompanying binding to the membrane focus in D1; the C-terminal part of D4; and strands β1, β4, and β5 of D3. These changes were much weaker for PFO(W165T) lipo where substantial stabilization was observed only in D4 domain. In this study, the application of hydrogen-deuterium exchange analysis monitored by MS provided new insight into conformational changes of PFO associated with the membrane binding, oligomerization, and lytic pore formation.

The influence of natural lipid asymmetry upon the conformation of a membrane-inserted protein (perfringolysin O)

Eukaryotic membrane proteins generally reside in membrane bilayers that have lipid asymmetry. However, in vitro studies of the impact of lipids upon membrane proteins are generally carried out in model membrane vesicles that lack lipid asymmetry. Our recently developed method to prepare lipid vesicles with asymmetry similar to that in plasma membranes and with controlled amounts of cholesterol was used to investigate the influence of lipid composition and lipid asymmetry upon the conformational behavior of the pore-forming, cholesterol-dependent cytolysin perfringolysin O (PFO). PFO conformational behavior in asymmetric vesicles was found to be distinct both from that in symmetric vesicles with the same lipid composition as the asymmetric vesicles and from that in vesicles containing either only the inner leaflet lipids from the asymmetric vesicles or only the outer leaflet lipids from the asymmetric vesicles. The presence of phosphatidylcholine in the outer leaflet increased the cholesterol concentration required to induce PFO binding, whereas phosphatidylethanolamine and phosphatidylserine in the inner leaflet of asymmetric vesicles stabilized the formation of a novel deeply inserted conformation that does not form pores, even though it contains transmembrane segments. This conformation may represent an important intermediate stage in PFO pore formation. These studies show that lipid asymmetry can strongly influence the behavior of membrane-inserted proteins.

Perfringolysin O structure and mechanism of pore formation as a paradigm for cholesterol-dependent cytolysins

Cholesterol-dependent cytolysins (CDCs) constitute a family of pore forming toxins secreted by Gram-positive bacteria. These toxins form transmembrane pores by inserting a large β-barrel into cholesterol-containing membrane bilayers. Binding of water-soluble CDCs to the membrane triggers the formation of oligomers containing 35-50 monomers. The coordinated insertion of more than seventy β-hairpins into the membrane requires multiple structural conformational changes. Perfringolysin O (PFO), secreted by Clostridium perfringens, has become the prototype for the CDCs. In this chapter, we will describe current knowledge on the mechanism of PFO cytolysis, with special focus on cholesterol recognition, oligomerization, and the conformational changes involved in pore formation.

Disulfide-bond scanning reveals assembly state and β-strand tilt angle of the PFO β-barrel

Perfringolysin O (PFO), a bacterial cholesterol-dependent cytolysin, binds a mammalian cell membrane, oligomerizes into a circular prepore complex (PPC) and forms a 250-Å transmembrane β-barrel pore in the cell membrane. Each PFO monomer has two sets of three short α-helices that unfold and ultimately refold into two transmembrane β-hairpin (TMH) components of the membrane-embedded β-barrel. Interstrand disulfide-bond scanning revealed that β-strands in a fully assembled PFO β-barrel were strictly aligned and tilted at 20° to the membrane perpendicular. In contrast, in a low temperature-trapped PPC intermediate, the TMHs were unfolded and had sufficient freedom of motion to interact transiently with each other, yet the TMHs were not aligned or stably hydrogen bonded. The PFO PPC-to-pore transition therefore converts TMHs in a dynamic folding intermediate far above the membrane into TMHs that are hydrogen bonded to those of adjacent subunits in the bilayer-embedded β-barrel.

Characterization of pneumolysin from Streptococcus pneumoniae, interacting with carbohydrate moiety and cholesterol as a component of cell membrane

The cytolytic mechanism of cholesterol-dependent cytolysins (CDCs) requires the presence of cholesterol in the target cell membrane. Membrane cholesterol was thought to serve as the common receptor for these toxins, but not all CDCs require cholesterol for binding. One member of this toxin family, pneumolysin (PLY) is a major virulence factor of Streptococcus pneumoniae, and the mechanism via which PLY binds to its putative receptor or cholesterol on the cell membrane is still poorly understood. Here, we demonstrated that PLY interacted with carbohydrate moiety and cholesterol as a component of the cell membrane, using the inhibitory effect of hemolytic activity. The hemolytic activity of PLY was inhibited by cholesterol-MβCD, which is in a 3β configuration at the C3-hydroxy group, but is not in a 3α-configuration. In the interaction between PLY and carbohydrate moiety, the mannose showed a dose-dependent increase in the inhibition of PLY hemolytic activity. The binding ability of mannose with truncated PLYs, as determined by the pull-down assay, showed that mannose might favor binding to domain 4 rather than domains 1-3. These studies provide a new model for the mechanism of cellular recognition by PLY, as well as a foundation for future investigations into whether non-sterol molecules can serve as receptors for other members of the CDC family of toxins.

Altering hydrophobic sequence lengths shows that hydrophobic mismatch controls affinity for ordered lipid domains (rafts) in the multitransmembrane strand protein perfringolysin O

The hypothesis that mismatch between transmembrane (TM) length and bilayer width controls TM protein affinity for ordered lipid domains (rafts) was tested using perfringolysin O (PFO), a pore-forming cholesterol-dependent cytolysin. PFO forms a multimeric barrel with many TM segments. The properties of PFO mutants with lengthened or shortened TM segments were compared with that of PFO with wild type TM sequences. Both mutant and wild type length PFO exhibited cholesterol-dependent membrane insertion. Maximal PFO-induced pore formation occurred in vesicles with wider bilayers for lengthened TM segments and in thinner bilayers for shortened TM segments. In diC(18:0) phosphatidylcholine (PC)/diC(14:1) PC/cholesterol vesicles, which form ordered domains with a relatively thick bilayer and disordered domains with a relatively thin bilayer, affinity for ordered domains was greatest with lengthened TM segments and least with shortened TM segments as judged by FRET. Similar results were observed by microscopy in giant vesicles containing sphingomyelin in place of diC(18:0) PC. In contrast, in diC(16:0) PC/diC(14:0) PC/diC(20:1) PC/cholesterol vesicles, which should form ordered domains with a relatively thin bilayer and disordered domains with a relatively thick bilayer, relative affinity for ordered domains was greatest with shortened TM segments and least with lengthened TM segments. The inability of multi-TM segment proteins (unlike single TM segment proteins) to adapt to mismatch by tilting may explain the sensitivity of raft affinity to mismatch. The difference in width sensitivity for single and multi-TM helix proteins may link raft affinity to multimeric state and thus control the assembly of multimeric TM complexes in rafts.

Cholesterol microcrystals and cochleate cylinders: attachment of pyolysin oligomers and domain 4

Using an established organic solvent injection procedure for the preparation of aqueous cholesterol microcrystal suspensions, it has now been shown that a new, hollow, cylindrical, tightly-coiled, multi-bilayer form of cholesterol can be generated, termed the cochleate cylinder. Cholesterol cochleate cylinders are formed in larger numbers at intermediate temperatures (40-75°C) but are not formed at 100°C. The structure of the cholesterol microcrystals and cochleate cylinders is shown in negatively stained electron micrographs. Oligomerization and attachment of pyolysin to cholesterol microcrystals and cochleate cylinders is shown, as is the attachment of the pyolysin "cholesterol-binding" domain 4 (D4) fragment. The bound D4 domain forms a linear array on the two planar surfaces and edges of the cholesterol microcrystals and a quasi helical array on the surface of the cochleate cylinders. Little evidence has been obtained to support the possibility that interaction or hetero-oligomerization can occur between intact pyolysin and the pyolysin D4 fragment on the surface of cholesterol microcrystals. Using immobilized cholesterol crystals attached to a carbon support film, single-sided linear labelling of the cholesterol surface with pyolysin D4 has been achieved, which correlates well with the images from the microcrystal suspensions and our earlier data using non-cytolytic streptolysin O mutants.

Bacterial toxins and the nervous system: neurotoxins and multipotential toxins interacting with neuronal cells

Toxins are potent molecules used by various bacteria to interact with a host organism. Some of them specifically act on neuronal cells (clostridial neurotoxins) leading to characteristics neurological affections. But many other toxins are multifunctional and recognize a wider range of cell types including neuronal cells. Various enterotoxins interact with the enteric nervous system, for example by stimulating afferent neurons or inducing neurotransmitter release from enterochromaffin cells which result either in vomiting, in amplification of the diarrhea, or in intestinal inflammation process. Other toxins can pass the blood brain barrier and directly act on specific neurons.

Clostridial toxins

Clostridia produce the highest number of toxins of any type of bacteria and are involved in severe diseases in humans and other animals. Most of the clostridial toxins are pore-forming toxins responsible for gangrenes and gastrointestinal diseases. Among them, perfringolysin has been extensively studied and it is the paradigm of the cholesterol-dependent cytolysins, whereas Clostridium perfringens epsilon-toxin and Clostridium septicum alpha-toxin, which are related to aerolysin, are the prototypes of clostridial toxins that form small pores. Other toxins active on the cell surface possess an enzymatic activity, such as phospholipase C and collagenase, and are involved in the degradation of specific cell-membrane or extracellular-matrix components. Three groups of clostridial toxins have the ability to enter cells: large clostridial glucosylating toxins, binary toxins and neurotoxins. The binary and large clostridial glucosylating toxins alter the actin cytoskeleton by enzymatically modifying the actin monomers and the regulatory proteins from the Rho family, respectively. Clostridial neurotoxins proteolyse key components of neuroexocytosis. Botulinum neurotoxins inhibit neurotransmission at neuromuscular junctions, whereas tetanus toxin targets the inhibitory interneurons of the CNS. The high potency of clostridial toxins results from their specific targets, which have an essential cellular function, and from the type of modification that they induce. In addition, clostridial toxins are useful pharmacological and biological tools.

The cholesterol-dependent cytolysin family of gram-positive bacterial toxins

The cholesterol-dependent cytolysins (CDCs) are a family of beta-barrel pore-forming toxins secreted by Gram-positive bacteria. These toxins are produced as water-soluble monomeric proteins that after binding to the target cell oligomerize on the membrane surface forming a ring-like pre-pore complex, and finally insert a large beta-barrel into the membrane (about 250 A in diameter). Formation of such a large transmembrane structure requires multiple and coordinated conformational changes. The presence of cholesterol in the target membrane is absolutely required for pore-formation, and therefore it was long thought that cholesterol was the cellular receptor for these toxins. However, not all the CDCs require cholesterol for binding. Intermedilysin, secreted by Streptoccocus intermedius only binds to membranes containing a protein receptor, but forms pores only if the membrane contains sufficient cholesterol. In contrast, perfringolysin O, secreted by Clostridium perfringens, only binds to membranes containing substantial amounts of cholesterol. The mechanisms by which cholesterol regulates the cytolytic activity of the CDCs are not understood at the molecular level. The C-terminus of perfringolysin O is involved in cholesterol recognition, and changes in the conformation of the loops located at the distal tip of this domain affect the toxin-membrane interactions. At the same time, the distribution of cholesterol in the membrane can modulate toxin binding. Recent studies support the concept that there is a dynamic interplay between the cholesterol-binding domain of the CDCs and the excess of cholesterol molecules in the target membrane.

Cholesterol exposure at the membrane surface is necessary and sufficient to trigger perfringolysin O binding

Perfringolysin O (PFO) is the prototype for the cholesterol-dependent cytolysins, a family of bacterial pore-forming toxins that act on eukaryotic membranes. The pore-forming mechanism of PFO exhibits an absolute requirement for membrane cholesterol, but the complex interplay between the structural arrangement of the PFO C-terminal domain and the distribution of cholesterol in the target membrane is poorly understood. Herein we show that PFO binding to the bilayer and the initiation of the sequence of events that culminate in the formation of a transmembrane pore depend on the availability of free cholesterol at the membrane surface, while changes in the acyl chain packing of the phospholipids and cholesterol in the membrane core, or the presence or absence of detergent-resistant domains do not correlate with PFO binding. Moreover, PFO association with the membrane was inhibited by the addition of sphingomyelin, a typical component of membrane rafts in cell membranes. Finally, addition of molecules that do not interact with PFO, but intercalate into the membrane and displace cholesterol from its association with phospholipids (e.g., epicholesterol), reduced the amount of cholesterol required to trigger PFO binding. Taken together, our studies reveal that PFO binding to membranes is triggered when the concentration of cholesterol exceeds the association capacity of the phospholipids, and this cholesterol excess is then free to associate with the toxin.

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.