About lipids and toxins

Effect of late endosomal DOBMP lipid and traditional model lipids of electrophysiology on the anthrax toxin channel activity

Anthrax toxin action requires triggering of natural endocytic transport mechanisms whereby the binding component of the toxin forms channels (PA₆₃) within endosomal limiting and intraluminal vesicle membranes to deliver the toxin's enzymatic components into the cytosol. Membrane lipid composition varies at different stages of anthrax toxin internalization, with intraluminal vesicle membranes containing ~70% of anionic bis(monoacylglycero)phosphate lipid. Using model bilayer measurements, we show that membrane lipids can have a strong effect on the anthrax toxin channel properties, including the channel-forming activity, voltage-gating, conductance, selectivity, and enzymatic factor binding. Interestingly, the highest PA₆₃ insertion rate was observed in bis(monoacylglycero)phosphate membranes. The molecular dynamics simulation data show that the conformational properties of the channel are different in bis(monoacylglycero)phosphate compared to PC, PE, and PS lipids. The anthrax toxin protein/lipid bilayer system can be advanced as a novel robust model to directly investigate lipid influence on membrane protein properties and protein/protein interactions.

Membrane-Active Properties of an Amphitropic Peptide from the CyaA Toxin Translocation Region

The adenylate cyclase toxin CyaA is involved in the early stages of infection by Bordetella pertussis, the causative agent of whooping cough. CyaA intoxicates target cells by a direct translocation of its catalytic domain (AC) across the plasma membrane and produces supraphysiological levels of cAMP, leading to cell death. The molecular process of AC translocation remains largely unknown, however. We have previously shown that deletion of residues 375–485 of CyaA selectively abrogates AC translocation into eukaryotic cells. We further identified within this “translocation region” (TR), P454 (residues 454–484), a peptide that exhibits membrane-active properties, i.e., is able to bind and permeabilize lipid vesicles. Here, we analyze various sequences from CyaA predicted to be amphipatic and show that although several of these peptides can bind membranes and adopt a helical conformation, only the P454 peptide is able to permeabilize membranes. We further characterize the contributions of the two arginine residues of P454 to membrane partitioning and permeabilization by analyzing the peptide variants in which these residues are substituted by different amino acids (e.g., A, K, Q, and E). Our data shows that both arginine residues significantly contribute, although diversely, to the membrane-active properties of P454, i.e., interactions with both neutral and anionic lipids, helix formation in membranes, and disruption of lipid bilayer integrity. These results are discussed in the context of the translocation process of the full-length CyaA toxin.

Effects of membrane properties on the binding activities of the HN and HC heavy-chain domains of botulinum neurotoxin A

Binding behaviors of the H_N and the H_C domains of BoNT/A were investigated individually to identify if there exist any differences in their interaction with the cell membrane. Recombinant fragments corresponding to both BoNT/A H_N and H_C regions were prepared (H_N519-845 and H_C967-1296) and their binding to synaptic proteins was verified. The binding behaviors of these heavy-chain domains were analyzed by treating the Neuro 2a, a murine neuroblastoma cell line, with compounds known to alter membrane properties. Cholesterol depletion and lipid raft inhibition increased the binding of H_N519-845 to Neuro 2a cells without affecting H_C967-1296-cell interaction. Sphingolipid depletion decreased the binding of cells to both H_C967-1296 and H_N519-845 whereas, loading exogenous GD1a, on to the Neuro 2a cells, increased the binding of both the peptides to cells. Microtubule disruption of the Neuro 2a cells by nocodazole decreased the binding of both H_C967-1296 and H_N519-845 to the treated cells. Inhibition of the clathrin-mediated endocytosis using dynasore, chlorpromazine or potassium (K⁺) depletion buffer lowered the binding of both H_C967-1296 and H_N519-845 to the cells, but seemed to exert a more pronounced effect on the binding of H_C967-1296 than on the binding of H_N519-845. Results indicate that while both the H_N and H_C domains are involved in the binding of the toxin to neuronal cells there are differences in their behavior which probably stem from their respective amino acid composition and structural location in the toxin three-dimensional structure along with their intended role in translocation and internalization into the cells.

Purinergic control of lysenin's transport and voltage-gating properties

Lysenin, a pore-forming protein extracted from the coelomic fluid of the earthworm Eisenia foetida, manifests cytolytic activity by inserting large conductance pores in host membranes containing sphingomyelin. In the present study, we found that adenosine phosphates control the biological activity of lysenin channels inserted into planar lipid membranes with respect to their macroscopic conductance and voltage-induced gating. Addition of ATP, ADP, or AMP decreased the macroscopic conductance of lysenin channels in a concentration-dependent manner, with ATP being the most potent inhibitor and AMP the least. ATP removal from the bulk solutions by buffer exchange quickly reinstated the macroscopic conductance and demonstrated reversibility. Single-channel experiments pointed to an inhibition mechanism that most probably relies on electrostatic binding and partial occlusion of the channel-conducting pathway, rather than ligand gating induced by the highly charged phosphates. The Hill analysis of the changes in macroscopic conduction as a function of the inhibitor concentration suggested cooperative binding as descriptive of the inhibition process. Ionic screening significantly reduced the ATP inhibitory efficacy, in support of the electrostatic binding hypothesis. In addition to conductance modulation, purinergic control over the biological activity of lysenin channels has also been observed to manifest as changes of the voltage-induced gating profile. Our analysis strongly suggests that not only the inhibitor's charge but also its ability to adopt a folded conformation may explain the differences in the observed influence of ATP, ADP, and AMP on lysenin's biological activity.

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.

Pore-forming toxins: Properties, diversity, and uses as tools to image sphingomyelin and ceramide phosphoethanolamine

Pore-forming toxins (PFTs) represent a unique class of highly specific lipid-binding proteins. The cytotoxicity of these compounds has been overcome through crystallographic structure and mutation studies, facilitating the development of non-toxic lipid probes. As a consequence, non-toxic PFTs have been utilized as highly specific probes to visualize the diversity and dynamics of lipid nanostructures in living and fixed cells. This review is focused on the application of PFTs and their non-toxic analogs as tools to visualize sphingomyelin and ceramide phosphoethanolamine, two major phosphosphingolipids in mammalian and insect cells, respectively. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Engineered nanoparticles mimicking cell membranes for toxin neutralization

Protein toxins secreted from pathogenic bacteria and venomous animals rely on multiple mechanisms to overcome the cell membrane barrier to inflict their virulence effect. A promising therapeutic concept toward developing a broadly applicable anti-toxin platform is to administer cell membrane mimics as decoys to sequester these virulence factors. As such, lipid membrane-based nanoparticulates are an ideal candidate given their structural similarity to cellular membranes. This article reviews the virulence mechanisms employed by toxins at the cell membrane interface and highlights the application of cell-membrane mimicking nanoparticles as toxin decoys for systemic detoxification. In addition, the implication of particle/toxin nanocomplexes in the development of toxoid vaccines is discussed.

Revisiting caspase-11 function in host defense

Proinflammatory caspases play important roles in innate immunity. Much attention has focused on caspase-1, which acts to eliminate pathogens by obliterating their replicative niches as well as alerting the host to their presence. Now, emerging data have shed light on the lesser-studied proinflammatory caspase-11 in the combat between host and pathogens. Using the new tools available, researchers are further elucidating the mechanisms by which caspase-11 contributes to host defense. Here, we review the emerging understanding of caspase-11 functions and the mechanisms of activation and discuss the implications for human disease.

Characterization of a membrane-active peptide from the Bordetella pertussis CyaA toxin

Bordetella pertussis, the pathogenic bacteria responsible for whooping cough, secretes several virulence factors, among which is the adenylate cyclase toxin (CyaA) that plays a crucial role in the early stages of human respiratory tract colonization. CyaA invades target cells by translocating its catalytic domain directly across the plasma membrane and overproduces cAMP, leading to cell death. The molecular process leading to the translocation of the catalytic domain remains largely unknown. We have previously shown that the catalytic domain per se, AC384, encompassing residues 1-384 of CyaA, did not interact with lipid bilayer, whereas a longer polypeptide, AC489, spanning residues 1-489, binds to membranes and permeabilizes vesicles. Moreover, deletion of residues 375-485 within CyaA abrogated the translocation of the catalytic domain into target cells. Here, we further identified within this region a peptidic segment that exhibits membrane interaction properties. A synthetic peptide, P454, corresponding to this sequence (residues 454-485 of CyaA) was characterized by various biophysical approaches. We found that P454 (i) binds to membranes containing anionic lipids, (ii) adopts an α-helical structure oriented in plane with respect to the lipid bilayer, and (iii) permeabilizes vesicles. We propose that the region encompassing the helix 454-485 of CyaA may insert into target cell membrane and induce a local destabilization of the lipid bilayer, thus favoring the translocation of the catalytic domain across the plasma membrane.

Pathogenic pore-forming proteins: function and host response

Organisms from all kingdoms produce pore-forming proteins, with the best-characterized being of bacterial origin. The last decade of research has revealed that the channels formed by these proteins can be very diverse, thus differentially affecting target cell-membrane permeability and consequent cellular outcome. The responses to these toxins are also extremely diverse due to multiple downstream effects of pore-induced changes in ion balance. Determining the secondary effects of pore-forming toxins is essential to understand their contribution to infection.

Identification of a region that assists membrane insertion and translocation of the catalytic domain of Bordetella pertussis CyaA toxin

The adenylate cyclase (CyaA) toxin, one of the virulence factors secreted by Bordetella pertussis, the pathogenic bacteria responsible for whooping cough, plays a critical role in the early stages of respiratory tract colonization by this bacterium. The CyaA toxin is able to invade eukaryotic cells by translocating its N-terminal catalytic domain directly across the plasma membrane of the target cells, where, activated by endogenous calmodulin, it produces supraphysiological levels of cAMP. How the catalytic domain is transferred from the hydrophilic extracellular medium into the hydrophobic environment of the membrane and then to the cell cytoplasm remains an unsolved question. In this report, we have characterized the membrane-interacting properties of the CyaA catalytic domain. We showed that a protein covering the catalytic domain (AC384, encompassing residues 1-384 of CyaA) displayed no membrane association propensity. However, a longer polypeptide (AC489), encompassing residues 1-489 of CyaA, exhibited the intrinsic property to bind to membranes and to induce lipid bilayer destabilization. We further showed that deletion of residues 375-485 within CyaA totally abrogated the toxin's ability to increase intracellular cAMP in target cells. These results indicate that, whereas the calmodulin dependent enzymatic domain is restricted to the amino-terminal residues 1-384 of CyaA, the membrane-interacting, translocation-competent domain extends up to residue 489. This thus suggests an important role of the region adjacent to the catalytic domain of CyaA in promoting its interaction with and its translocation across the plasma membrane of target cells.

Small molecule inhibitors of anthrax toxin-induced cytotoxicity targeted against protective antigen

Two molecular scaffolds were designed using the CAVEAT molecular design package to inhibit the oligomerization of protective antigen (PA(63) ), a key protein component of anthrax toxin. The inhibitors were designed to prevent heptamerization of PA(63) by mimicking key residues of PA(63) needed for the intermolecular interactions that stabilize the heptamer. Using the scaffolds identified by CAVEAT, seven candidate inhibitors were synthesized and tested for their ability to inhibit anthrax toxin-induced cytotoxicity, with three of the agents demonstrating modest inhibition in murine J774A.1 macrophage cells.

Amyloid oligomer neurotoxicity, calcium dysregulation, and lipid rafts

Amyloid proteins constitute a chemically heterogeneous group of proteins, which share some biophysical and biological characteristics, the principal of which are the high propensity to acquire an incorrect folding and the tendency to aggregate. A number of diseases are associated with misfolding and aggregation of proteins, although only in some of them—most notably Alzheimer's disease (AD) and transmissible spongiform encephalopathies (TSEs)—a pathogenetic link with misfolded proteins is now widely recognized. Lipid rafts (LRs) have been involved in the pathophysiology of diseases associated with protein misfolding at several levels, including aggregation of misfolded proteins, amyloidogenic processing, and neurotoxicity. Among the pathogenic misfolded proteins, the AD-related protein amyloid β (Aβ) is by far the most studied protein, and a large body of evidence has been gathered on the role played by LRs in Aβ pathogenicity. However, significant amount of data has also been collected for several other amyloid proteins, so that their ability to interact with LRs can be considered an additional, shared feature characterizing the amyloid protein family. In this paper, we will review the evidence on the role of LRs in the neurotoxicity of huntingtin, α-synuclein, prion protein, and calcitonin.

Reconstituted lipoprotein: a versatile class of biologically-inspired nanostructures

One of biology's most pervasive nanostructures, the phospholipid membrane, represents an ideal scaffold for a host of nanotechnology applications. Whether engineering biomimetic technologies or designing therapies to interface with the cell, this adaptable membrane can provide the necessary molecular-level control of membrane-anchored proteins, glycopeptides, and glycolipids. If appropriately prepared, these components can replicate in vitro or influence in vivo essential living processes such as signal transduction, mass transport, and chemical or energy conversion. To satisfy these requirements, a lipid-based, synthetic nanoscale architecture with molecular-level tunability is needed. In this regard, discrete lipid particles, including reconstituted high density lipoprotein (HDL), have emerged as a versatile and elegant solution. Structurally diverse, native biological HDLs exist as discoidal lipid bilayers of 5-8 nm diameter and lipid monolayer-coated spheres 10-15 nm in diameter, all belted by a robust scaffolding protein. These supramolecular assemblies can be reconstituted using simple self-assembly methods to incorporate a broad range of amphipathic molecular constituents, natural or artificial, and provide a generic platform for stabilization and transport of amphipathic and hydrophobic elements capable of docking with targets at biological or inorganic surfaces. In conjunction with top-down or bottom-up engineering approaches, synthetic HDL can be designed, arrayed, and manipulated for a host of applications including biochemical analyses and fundamental studies of molecular structure. Also highly biocompatible, these assemblies are suitable for medical diagnostics and therapeutics. The collection of efforts reviewed here focuses on laboratory methods by which synthetic HDLs are produced, the advantages conferred by their nanoscopic dimension, and current and emerging applications.

Multivalent ions control the transport through lysenin channels

We report the effect of different ions on the conducting properties of lysenin channels inserted into planar lipid bilayer membranes. Our observations indicated that multivalent ions inhibited the lysenin channels conductance in a concentration dependent manner. The analysis performed on single channels revealed that multivalent ions induced reversible sub-conducting or closed states depending on the ionic charge and size. Good agreement is reported between experimental results and a theoretical model that is proposed to describe the interaction between divalent ions and lysenin channels as a simple isothermal absorption process.

Hijacking the endocytic machinery by microbial pathogens

Understanding the mechanisms that microbes exploit to invade host cells and cause disease is crucial if we are to eliminate their threat. Although pathogens use a variety of microbial factors to trigger entry into non-phagocytic cells, their targeting of the host cell process of endocytosis has emerged as a common theme. To accomplish this, microbes often rewire the normal course of particle internalization, frequently usurping theoretical maximal sizes to permit entry and reconfiguring molecular components that were once thought to be required for vesicle formation. Here, we discuss recent advances in our understanding of how toxins, viruses, bacteria, and fungi manipulate the host cell endocytic machinery to generate diseases. Additionally, we will reveal the advantages of using these organisms to expand our general knowledge of endocytic mechanisms in eukaryotic cells.

Dynamic partitioning of a glycosyl-phosphatidylinositol-anchored protein in glycosphingolipid-rich microdomains imaged by single-quantum dot tracking

Recent experimental developments have led to a revision of the classical fluid mosaic model proposed by Singer and Nicholson more than 35 years ago. In particular, it is now well established that lipids and proteins diffuse heterogeneously in cell plasma membranes. Their complex motion patterns reflect the dynamic structure and composition of the membrane itself, as well as the presence of the underlying cytoskeleton scaffold and that of the extracellular matrix. How the structural organization of plasma membranes influences the diffusion of individual proteins remains a challenging, yet central, question for cell signaling and its regulation. Here we have developed a raft-associated glycosyl-phosphatidyl-inositol-anchored avidin test probe (Av-GPI), whose diffusion patterns indirectly report on the structure and dynamics of putative raft microdomains in the membrane of HeLa cells. Labeling with quantum dots (qdots) allowed high-resolution and long-term tracking of individual Av-GPI and the classification of their various diffusive behaviors. Using dual-color total internal reflection fluorescence (TIRF) microscopy, we studied the correlation between the diffusion of individual Av-GPI and the location of glycosphingolipid GM1-rich microdomains and caveolae. We show that Av-GPI exhibit a fast and a slow diffusion regime in different membrane regions, and that slowing down of their diffusion is correlated with entry in GM1-rich microdomains located in close proximity to, but distinct, from caveolae. We further show that Av-GPI dynamically partition in and out of these microdomains in a cholesterol-dependent manner. Our results provide direct evidence that cholesterol-/sphingolipid-rich microdomains can compartmentalize the diffusion of GPI-anchored proteins in living cells and that the dynamic partitioning raft model appropriately describes the diffusive behavior of some raft-associated proteins across the plasma membrane.

Molecular pathogenesis of Shigella spp.: controlling host cell signaling, invasion, and death by type III secretion

Shigella spp. are gram-negative pathogenic bacteria that evolved from harmless enterobacterial relatives and may cause devastating diarrhea upon ingestion. Research performed over the last 25 years revealed that a type III secretion system (T3SS) encoded on a large plasmid is a key virulence factor of Shigella flexneri. The T3SS determines the interactions of S. flexneri with intestinal cells by consecutively translocating two sets of effector proteins into the target cells. Thus, S. flexneri controls invasion into EC, intra- and intercellular spread, macrophage cell death, as well as host inflammatory responses. Some of the translocated effector proteins show novel biochemical activities by which they intercept host cell signal transduction pathways. An understanding of the molecular mechanisms underlying Shigella pathogenesis will foster the development of a safe and efficient vaccine, which, in parallel with improved hygiene, should curb infections by this widespread pathogen.

Role of lipid rafts in the processing of the pathogenic prion and Alzheimer's amyloid-β proteins

The conformational conversion of the cellular form of the prion protein (PrP C) into the infectious form (PrP Sc) and the proteolytic processing of the amyloid-beta (Abeta) peptide are central pathogenetic events in the prion diseases and Alzheimer's disease, respectively. Cholesterol- and sphingolipid-rich lipid rafts have emerged as important sites for the conversion of PrP C into PrP Sc, and for the proteolytic production, degradation and aggregation of Abeta. Here, we discuss these findings and their implications for our understanding of these disease processes. In addition, the potential for rafts as sites for therapeutic intervention in prion diseases and Alzheimer's disease is considered.

The role of the inflammasome in cellular responses to toxins and bacterial effectors

Invading pathogens are recognized by mammalian cells through dedicated receptors found either at the cell surface or in the cytoplasm. These receptors, like the trans-membrane Toll-like Receptors (TLR) or the cytosolic Nod-like Receptors (NLR), initiate innate immunity after recognition of molecular patterns found in bacteria or viruses, such as LPS, flagellin, or double-stranded RNA. Recognition of molecules produced only by a specific pathogen, such as a viral envelop protein or a bacterial adhesin does not appear to occur. Bacterial protein toxins, however, might compose an intermediate class. Considering the diversity of toxins in terms of structure, it is unlikely that cells respond to them via specific molecular recognition. It rather appears that different classes of toxins trigger cellular changes that are sensed by the cells as danger signals, such as changes in cellular ion composition after membrane perforation by pore-forming toxins or type III secretion systems. The signaling pathways triggered through toxin-induced cell alterations will likely play a role in modulating host responses to virulent bacteria. We will here describe the few studied cases in which detection of the toxin by the host cell was addressed. The review will include not only toxins but also bacteria effectors secreted by the bacterium in to the host cell cytoplasm.