Pore-forming toxins: ancient, but never really out of fashion

Structural Features of Apicomplexan Pore-Forming Proteins and Their Roles in Parasite Cell Traversal and Egress

Apicomplexan parasites cause diseases, including malaria and toxoplasmosis, in a range of hosts, including humans. These intracellular parasites utilize pore-forming proteins that disrupt host cell membranes to either traverse host cells while migrating through tissues or egress from the parasite-containing vacuole after replication. This review highlights recent insight gained from the newly available three-dimensional structures of several known or putative apicomplexan pore-forming proteins that contribute to cell traversal or egress. These new structural advances suggest that parasite pore-forming proteins use distinct mechanisms to disrupt host cell membranes at multiple steps in parasite life cycles. How proteolytic processing, secretion, environment, and the accessibility of lipid receptors regulate the membranolytic activities of such proteins is also discussed.

Genomic, Physiologic, and Symbiotic Characterization of Serratia marcescens Strains Isolated from the Mosquito Anopheles stephensi

Strains of Serratia marcescens, originally isolated from the gut lumen of adult female Anopheles stephensi mosquitoes, established persistent infection at high rates in adult A. stephensi whether fed to larvae or in the sugar meal to adults. By contrast, the congener S. fonticola originating from Aedes triseriatus had lower infection in A. stephensi, suggesting co-adaptation of Serratia strains in different species of host mosquitoes. Coinfection at high infection rate in adult A. stephensi resulted after feeding S. marcescens and Elizabethkingia anophelis in the sugar meal, but when fed together to larvae, infection rates with E. anophelis were much higher than were S. marcescens in adult A. stephensi, suggesting a suppression effect of coinfection across life stages. A primary isolate of S. marcescens was resistant to all tested antibiotics, showed high survival in the mosquito gut, and produced alpha-hemolysins which contributed to lysis of erythrocytes ingested with the blood meal. Genomes of two primary isolates from A. stephensi, designated S. marcescens ano1 and ano2, were sequenced and compared to other Serratia symbionts associated with insects, nematodes and plants. Serratia marcescens ano1 and ano2 had predicted virulence factors possibly involved in attacking parasites and/or causing opportunistic infection in mosquito hosts. S. marcescens ano1 and ano2 possessed multiple mechanisms for antagonism against other microorganisms, including production of bacteriocins and multi-antibiotic resistance determinants. These genes contributing to potential anti-malaria activity including serralysins, hemolysins and chitinases are only found in some Serratia species. It is interesting that genome sequences in S. marcescens ano1 and ano2 are distinctly different from those in Serratia sp. Ag1 and Ag2 which were isolated from Anopheles gambiae. Compared to Serratia sp. Ag1 and Ag2, S. marcescens ano1 and ano2 have more rRNAs and many important genes involved in commensal and anti-parasite traits.

The mystery behind membrane insertion: a review of the complement membrane attack complex

The membrane attack complex (MAC) is an important innate immune effector of the complement terminal pathway that forms cytotoxic pores on the surface of microbes. Despite many years of research, MAC structure and mechanism of action have remained elusive, relying heavily on modelling and inference from biochemical experiments. Recent advances in structural biology, specifically cryo-electron microscopy, have provided new insights into the molecular mechanism of MAC assembly. Its unique 'split-washer' shape, coupled with an irregular giant β-barrel architecture, enable an atypical mechanism of hole punching and represent a novel system for which to study pore formation. This review will introduce the complement terminal pathway that leads to formation of the MAC. Moreover, it will discuss how structures of the pore and component proteins underpin a mechanism for MAC function, modulation and inhibition.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

The peroxisomal matrix protein translocon is a large cavity-forming protein assembly into which PEX5 protein enters to release its cargo

A remarkable property of the machinery for import of peroxisomal matrix proteins is that it can accept already folded proteins as substrates. This import involves binding of newly synthesized proteins by cytosolic peroxisomal biogenesis factor 5 (PEX5) followed by insertion of the PEX5-cargo complex into the peroxisomal membrane at the docking/translocation module (DTM). However, how these processes occur remains largely unknown. Here, we used truncated PEX5 molecules to probe the DTM architecture. We found that the DTM can accommodate a larger number of truncated PEX5 molecules comprising amino acid residues 1-197 than full-length PEX5 molecules. A shorter PEX5 version (PEX5(1-125)) still interacted correctly with the DTM; however, this species was largely accessible to exogenously added proteinase K, suggesting that this protease can access the DTM occupied by a small PEX5 protein. Interestingly, the PEX5(1-125)-DTM interaction was inhibited by a polypeptide comprising PEX5 residues 138-639. Apparently, the DTM can recruit soluble PEX5 through interactions with different PEX5 domains, suggesting that the PEX5-DTM interactions are to some degree fuzzy. Finally, we found that the interaction between PEX5 and PEX14, a major DTM component, is stable at pH 11.5. Thus, there is no reason to assume that the hitherto intriguing resistance of DTM-bound PEX5 to alkaline extraction reflects its direct contact with the peroxisomal lipid bilayer. Collectively, these results suggest that the DTM is best described as a large cavity-forming protein assembly into which cytosolic PEX5 can enter to release its cargo.

Multiple Pseudomonas species secrete exolysin-like toxins and provoke Caspase-1-dependent macrophage death

Pathogenic bacteria secrete protein toxins that provoke apoptosis or necrosis of eukaryotic cells. Here, we developed a live-imaging method, based on incorporation of a DNA-intercalating dye into membrane-damaged host cells, to study the kinetics of primary bone marrow-derived macrophages (BMDMs) mortality induced by opportunistic pathogen Pseudomonas aeruginosa expressing either Type III Secretion System (T3SS) toxins or the pore-forming toxin, Exolysin (ExlA). We found that ExlA promotes the activation of Caspase-1 and maturation of interleukin-1β. BMDMs deficient for Caspase-1 and Caspase-11 were resistant to ExlA-induced death. Furthermore, by using KO BMDMs, we determined that the upstream NLRP3/ASC complex leads to the Caspase-1 activation. We also demonstrated that Pseudomonas putida and Pseudomonas protegens and the Drosophila pathogen Pseudomonas entomophila, which naturally express ExlA-like toxins, are cytotoxic toward macrophages and provoke the same type of pro-inflammatory death as does ExlA⁺ P. aeruginosa. These results demonstrate that ExlA-like toxins of two-partner secretion systems from diverse Pseudomonas species activate the NLRP3 inflammasome and provoke inflammatory pyroptotic death of macrophages.

Multiomics analysis of the giant triton snail salivary gland, a crown-of-thorns starfish predator

The giant triton snail (Charonia tritonis) is one of the few natural predators of the adult Crown-of-Thorns starfish (COTS), a corallivore that has been damaging to many reefs in the Indo-Pacific. Charonia species have large salivary glands (SGs) that are suspected to produce either a venom and/or sulphuric acid which can immobilize their prey and neutralize the intrinsic toxic properties of COTS. To date, there is little information on the types of toxins produced by tritons. In this paper, the predatory behaviour of the C. tritonis is described. Then, the C. tritonis SG, which itself is made up of an anterior lobe (AL) and posterior lobe (PL), was analyzed using an integrated transcriptomics and proteomics approach, to identify putative toxin- and feeding-related proteins. A de novo transcriptome database and in silico protein analysis predicts that ~3800 proteins have features consistent with being secreted. A gland-specific proteomics analysis confirmed the presence of numerous SG-AL and SG-PL proteins, including those with similarity to cysteine-rich venom proteins. Sulfuric acid biosynthesis enzymes were identified, specific to the SG-PL. Our analysis of the C. tritonis SG (AL and PL) has provided a deeper insight into the biomolecular toolkit used for predation and feeding by C. tritonis.

Impact of an arbuscular mycorrhizal fungus versus a mixed microbial inoculum on the transcriptome reprogramming of grapevine roots

Grapevine, cultivated for both fruit and beverage production, represents one of the most economically important fruit crops worldwide. With the aim of better understanding how grape roots respond to beneficial microbes, a transcriptome sequencing experiment has been performed to evaluate the impact of a single arbuscular mycorrhizal (AM) fungal species (Funneliformis mosseae) versus a mixed inoculum containing a bacterial and fungal consortium, including different AM species, on Richter 110 rootstock. Results showed that the impact of a single AM fungus and of a complex microbial inoculum on the grapevine transcriptome differed. After 3 months, roots exclusively were colonized after the F. mosseae treatment and several AM marker genes were found to be upregulated. The mixed inoculum led only to traces of colonization by AM fungi, but elicited an important transcriptional regulation. Additionally, the expression of genes belonging to categories such as nutrient transport, transcription factors, and cell wall-related genes was significantly altered in both treatments, but the exact genes affected differed in the two conditions. These findings advance our understanding about the impact of soil beneficial microbes on the root system of a woody plant, also offering the basis for novel approaches in grapevine cultivation.

Study on the Association among Mycotoxins and other Variables in Children with Autism

Environmental factors and genetic susceptibility are implicated in the increased risk of autism spectrum disorder (ASD). Mycotoxins are agricultural contaminants of fungal origin that represent real risk factors for human health and especially for children. Thus, the main hypothesis of this work is that the deterioration of the clinical manifestation of autism in children may result from the exposure to mycotoxins through the consumption of contaminated food. Within a cross-sectional study, a group of autistic children (n = 172) and a group of controls (n = 61) (siblings and non-parental) were recruited in North and South Italy. All children had blood and urine samples taken, for testing some mycotoxins by a LC-MS/MS validated method. Blood samples were also tested for assessing specific IgG against food and fungal antigens and cytokines. The analyses outputs highlighted statistically significant differences comparing mycotoxins levels between (i) children groups both in urine (deoxynivalenol and de-epoxydeoxynivalenol, p = 0.0141 and p = 0.0259, respectively) and serum (aflatoxin M1, ochratoxin A and fumonisin B1, p = 0.0072, p = 0.0141 and p = 0.0061, respectively); (ii) a group of selected fungal IgGs, and IgGs against wheat and gluten and (iii) cytokines. These results suggest the need for a deeper examination of the role that mycotoxins may have on the etiology of ASD.

NMR structure of the Bacillus cereus hemolysin II C-terminal domain reveals a novel fold

In addition to multiple virulence factors, Bacillus cereus a pathogen that causes food poisoning and life-threatening wound infections, secretes the pore-forming toxin hemolysin II (HlyII). The HlyII toxin has a unique 94 amino acid C-terminal domain (HlyIIC). HlyIIC exhibits splitting of NMR resonances due to cis/trans isomerization of a single proline near the C-terminus. To overcome heterogeneity, we solved the structure of P405M-HlyIIC, a mutant that exclusively stabilizes the trans state. The NMR structure of HlyIIC reveals a novel fold, consisting of two subdomains αA-β1-β2 and β3-β4-αB-β5, that come together in a barrel-like structure. The barrel core is fastened by three layers of hydrophobic residues. The barrel end opposite the HlyIIC-core has a positively charged surface, that by binding negatively charged moieties on cellular membranes, may play a role in target-cell surface recognition or stabilization of the heptameric pore complex. In the WT domain, dynamic flexibility occurs at the N-terminus and the first α-helix that connects the HlyIIC domain to the HlyII-core structure. In the destabilizing P405M mutant, increased flexibility is evident throughout the first subdomain, suggesting that the HlyIIC structure may have arisen through gene fusion.

Anticancer effects of the microbiome and its products

The human gut microbiome modulates many host processes, including metabolism, inflammation, and immune and cellular responses. It is becoming increasingly apparent that the microbiome can also influence the development of cancer. In preclinical models, the host response to cancer treatment has been improved by modulating the gut microbiome; this is known to have an altered composition in many diseases, including cancer. In addition, cancer treatment with microbial agents or their products has the potential to shrink tumours. However, the microbiome could also negatively influence cancer prognosis through the production of potentially oncogenic toxins and metabolites by bacteria. Thus, future antineoplastic treatments could combine the modulation of the microbiome and its products with immunotherapeutics and more conventional approaches that directly target malignant cells.

Identification of a target protein of Hydra actinoporin-like toxin-1 (HALT-1) using GST affinity purification and SILAC-based quantitative proteomics

Hydra actinoporin-like toxin-1 (HALT-1) is a 20.8 kDa pore-forming toxin isolated from Hydra magnipapillata. HALT-1 shares structural similarity with actinoporins, a family that is well known for its haemolytic and cytolytic activity. However, the precise pore-forming mechanism of HALT-1 remains an open question since little is known about the specific target binding for HALT-1. For this reason, a comprehensive proteomic analysis was performed using affinity purification and SILAC-based mass spectrometry to identify potential protein-protein interactions between mammalian HeLa cell surface proteins and HALT-1. A total of 4 mammalian proteins was identified, of which only folate receptor alpha was further verified by ELISA. Our preliminary results highlight an alternative-binding mode of HALT-1 to the human plasma membrane. This is the first evidence showing that HALT-1, an actinoporin-like protein, binds to a membrane protein, the folate receptor alpha. This study would advance our understanding of the molecular basis of toxicity of pore-forming toxins and provide new insights in the production of more potent inhibitors for the toxin-membrane receptor interactions.

A Novel Role of Listeria monocytogenes Membrane Vesicles in Inhibition of Autophagy and Cell Death

Bacterial membrane vesicle (MV) production has been mainly studied in Gram-negative species. In this study, we show that Listeria monocytogenes, a Gram-positive pathogen that causes the food-borne illness listeriosis, produces MVs both in vitro and in vivo. We found that a major virulence factor, the pore-forming hemolysin listeriolysin O (LLO), is tightly associated with the MVs, where it resides in an oxidized, inactive state. Previous studies have shown that LLO may induce cell death and autophagy. To monitor possible effects of LLO and MVs on autophagy, we performed assays for LC3 lipidation and LDH sequestration as well as analysis by confocal microscopy of HEK293 cells expressing GFP-LC3. The results revealed that MVs alone did not affect autophagy whereas they effectively abrogated autophagy induced by pure LLO or by another pore-forming toxin from Vibrio cholerae, VCC. Moreover, Listeria monocytogenes MVs significantly decreased Torin1-stimulated macroautophagy. In addition, MVs protected against necrosis of HEK293 cells caused by the lytic action of LLO. We explored the mechanisms of LLO-induced autophagy and cell death and demonstrated that the protective effect of MVs involves an inhibition of LLO-induced pore formation resulting in inhibition of autophagy and the lytic action on eukaryotic cells. Further, we determined that these MVs help bacteria to survive inside eukaryotic cells (mouse embryonic fibroblasts). Taken together, these findings suggest that intracellular release of MVs from L. monocytogenes may represent a bacterial strategy to survive inside host cells, by its control of LLO activity and by avoidance of destruction from the autophagy system during infection.

Pore-forming toxin-mediated ion dysregulation leads to death receptor-independent necroptosis of lung epithelial cells during bacterial pneumonia

We report that pore-forming toxins (PFTs) induce respiratory epithelial cell necroptosis independently of death receptor signaling during bacterial pneumonia. Instead, necroptosis was activated as a result of ion dysregulation arising from membrane permeabilization. PFT-induced necroptosis required RIP1, RIP3 and MLKL, and could be induced in the absence or inhibition of TNFR1, TNFR2 and TLR4 signaling. We detected activated MLKL in the lungs from mice and nonhuman primates experiencing Serratia marcescens and Streptococcus pneumoniae pneumonia, respectively. We subsequently identified calcium influx and potassium efflux as the key initiating signals responsible for necroptosis; also that mitochondrial damage was not required for necroptosis activation but was exacerbated by MLKL activation. PFT-induced necroptosis in respiratory epithelial cells did not involve CamKII or reactive oxygen species. KO mice deficient in MLKL or RIP3 had increased survival and reduced pulmonary injury during S. marcescens pneumonia. Our results establish necroptosis as a major cell death pathway active during bacterial pneumonia and that necroptosis can occur without death receptor signaling.

Staphylococcus aureus pore-forming toxins: The interface of pathogen and host complexity

Staphylococcus aureus is a prominent human pathogen capable of infecting a variety of host species and tissue sites. This versatility stems from the pathogen's ability to secrete diverse host-damaging virulence factors. Among these factors, the S. aureus pore-forming toxins (PFTs) α-toxin and the bicomponent leukocidins, have garnered much attention for their ability to lyse cells at low concentrations and modulate disease severity. Although many of these toxins were discovered nearly a century ago, their host cell specificities have only been elucidated over the past five to six years, starting with the discovery of the eukaryotic receptor for α-toxin and rapidly followed by identification of the leukocidin receptors. The identification of these receptors has revealed the species- and cell type-specificity of toxin binding, and provided insight into non-lytic effects of PFT intoxication that contribute to disease pathogenesis.

Leukocidins: staphylococcal bi-component pore-forming toxins find their receptors

Staphylococcus aureus is a major bacterial pathogen that causes disease worldwide. The emergence of strains that are resistant to commonly used antibiotics and the failure of vaccine development have resulted in a renewed interest in the pathophysiology of this bacterium. Staphylococcal leukocidins are a family of bi-component pore-forming toxins that are important virulence factors. During the past five years, cellular receptors have been identified for all of the bi-component leukocidins. The identification of the leukocidin receptors explains the cellular tropism and species specificity that is exhibited by these toxins, which has important biological consequences. In this Review, we summarize the recent discoveries that have reignited interest in these toxins and provide an outlook for future research.

Validation of ADAM10 metalloprotease as a Bacillus thuringiensis Cry3Aa toxin functional receptor in Colorado potato beetle (Leptinotarsa decemlineata)

Bacillus thuringiensis parasporal crystal proteins (Cry proteins) are insecticidal pore-forming toxins that bind to specific receptor molecules on the brush border membrane of susceptible insect midgut cells to exert their toxic action. In the Colorado potato beetle (CPB), a coleopteran pest, we previously proposed that interaction of Cry3Aa toxin with a CPB ADAM10 metalloprotease is an essential part of the mode of action of this toxin. Here, we annotated the gene sequence encoding an ADAM10 metalloprotease protein (CPB-ADAM10) in the CPB genome sequencing project, and using RNA interference gene silencing we demonstrated that CPB-ADAM10 is a Cry3Aa toxin functional receptor in CPB. Cry3Aa toxicity was significantly lower in CPB-ADAM10 silenced larvae and in vitro toxin pore-forming ability was greatly diminished in lipid planar bilayers fused with CPB brush border membrane vesicles (BBMVs) prepared from CPB-ADAM10 silenced larvae. In accordance with our previous data that indicated this toxin was a substrate of ADAM10 in CPB, Cry3Aa toxin membrane-associated proteolysis was altered when CPB BBMVs lacked ADAM10. The functional validation of CPB-ADAM10 as a Cry3Aa toxin receptor in CPB expands the already recognized role of ADAM10 as a pathogenicity determinant of pore-forming toxins in humans to an invertebrate species.

The Molecular Basis of Toxins' Interactions with Intracellular Signaling via Discrete Portals

An understanding of the molecular mechanisms by which microbial, plant or animal-secreted toxins exert their action provides the most important element for assessment of human health risks and opens new insights into therapies addressing a plethora of pathologies, ranging from neurological disorders to cancer, using toxinomimetic agents. Recently, molecular and cellular biology dissecting tools have provided a wealth of information on the action of these diverse toxins, yet, an integrated framework to explain their selective toxicity is still lacking. In this review, specific examples of different toxins are emphasized to illustrate the fundamental mechanisms of toxicity at different biochemical, molecular and cellular- levels with particular consideration for the nervous system. The target of primary action has been highlighted and operationally classified into 13 sub-categories. Selected examples of toxins were assigned to each target category, denominated as portal, and the modulation of the different portal’s signaling was featured. The first portal encompasses the plasma membrane lipid domains, which give rise to pores when challenged for example with pardaxin, a fish toxin, or is subject to degradation when enzymes of lipid metabolism such as phospholipases A₂ (PLA₂) or phospholipase C (PLC) act upon it. Several major portals consist of ion channels, pumps, transporters and ligand gated ionotropic receptors which many toxins act on, disturbing the intracellular ion homeostasis. Another group of portals consists of G-protein-coupled and tyrosine kinase receptors that, upon interaction with discrete toxins, alter second messengers towards pathological levels. Lastly, subcellular organelles such as mitochondria, nucleus, protein- and RNA-synthesis machineries, cytoskeletal networks and exocytic vesicles are also portals targeted and deregulated by other diverse group of toxins. A fundamental concept can be drawn from these seemingly different toxins with respect to the site of action and the secondary messengers and signaling cascades they trigger in the host. While the interaction with the initial portal is largely determined by the chemical nature of the toxin, once inside the cell, several ubiquitous second messengers and protein kinases/ phosphatases pathways are impaired, to attain toxicity. Therefore, toxins represent one of the most promising natural molecules for developing novel therapeutics that selectively target the major cellular portals involved in human physiology and diseases.

ESX-1 and phthiocerol dimycocerosates of Mycobacterium tuberculosis act in concert to cause phagosomal rupture and host cell apoptosis

Although phthiocerol dimycocerosates (DIM) are major virulence factors of Mycobacterium tuberculosis (Mtb), the causative agent of human tuberculosis, little is known about their mechanism of action. Localized in the outer membrane of mycobacterial pathogens, DIM are predicted to interact with host cell membranes. Interaction with eukaryotic membranes is a property shared with another virulence factor of Mtb, the early secretory antigenic target EsxA (also known as ESAT-6). This small protein, which is secreted by the type VII secretion system ESX-1 (T7SS/ESX-1), is involved in phagosomal rupture and cell death induced by virulent mycobacteria inside host phagocytes. In this work, by the use of several knock-out or knock-in mutants of Mtb or Mycobacterium bovis BCG strains and different cell biological assays, we present conclusive evidence that ESX-1 and DIM act in concert to induce phagosomal membrane damage and rupture in infected macrophages, ultimately leading to host cell apoptosis. These results identify an as yet unknown function for DIM in the infection process and open up a new research field for the study of the interaction of lipid and protein virulence factors of Mtb.

Recent Insights into the Molecular Mechanisms Underlying Pyroptosis and Gasdermin Family Functions

Pyroptosis is an inflammatory form of cell death that not only protects multicellular organisms from invading pathogenic bacteria and microbial infections, but can also lead to sepsis and lethal septic shock if overactivated. Here, we present an overview of recent developments within the pyroptosis field, beginning with the discovery of Gasdermin D (GSDMD) as a substrate of caspase-1 and caspase-11 upon detection of cytosolic lipopolysaccharide (LPS). Cleavage releases the N-terminal domain of GSDMD, causing it to form cytotoxic pores in the plasma membrane of cells. We further discuss the implications for the rest of the gasdermin (GSDM) family, which are emerging as mediators of programmed cell death in a variety of processes that regulate cellular differentiation and proliferation.

Electron Cryo-microscopy as a Tool for Structure-Based Drug Development

For decades, X-ray crystallography and NMR have been the most important techniques for studying the atomic structure of macromolecules. However, as a result of size, instability, low yield, and other factors, many macromolecules are difficult to crystallize or unsuitable for NMR studies. Electron cryo-microscopy (cryo-EM) does not depend on crystals and has therefore been the method of choice for many macromolecular complexes that cannot be crystallized, but atomic resolution has mostly been beyond its reach. A new generation of detectors that are capable of sensing directly the incident electrons has recently revolutionized the field, with structures of macromolecules now routinely being solved to near-atomic resolution. In this review, we summarize some of the most recent examples of high-resolution cryo-EM structures. We put particular emphasis on proteins with pharmacological relevance that have traditionally been inaccessible to crystallography. Furthermore, we discuss examples where interactions with small molecules have been fully characterized at atomic resolution. Finally, we stress the current limits of cryo-EM, and methodological issues related to its usage as a tool for drug development.

Damage of eukaryotic cells by the pore-forming toxin sticholysin II: Consequences of the potassium efflux

Pore-forming toxins (PFTs) form holes in membranes causing one of the most catastrophic damages to a target cell. Target organisms have evolved a regulated response against PFTs damage including cell membrane repair. This ability of cells strongly depends on the toxin concentration and the properties of the pores. It has been hypothesized that there is an inverse correlation between the size of the pores and the time required to repair the membrane, which has been for long a non-intuitive concept and far to be completely understood. Moreover, there is a lack of information about how cells react to the injury triggered by eukaryotic PFTs. Here, we investigated some molecular events related with eukaryotic cells response against the membrane damage caused by sticholysin II (StII), a eukaryotic PFT produced by a sea anemone. We evaluated the change in the cytoplasmic potassium, identified the main MAPK pathways activated after pore-formation by StII, and compared its effect with those from two well-studied bacterial PFTs: aerolysin and listeriolysin O (LLO). Strikingly, we found that membrane recovery upon StII damage takes place in a time scale similar to LLO in spite of the fact that they form pores by far different in size. Furthermore, our data support a common role of the potassium ion, as well as MAPKs in the mechanism that cells use to cope with these toxins injury.

Pseudomonas aeruginosa Pore-Forming Exolysin and Type IV Pili Cooperate To Induce Host Cell Lysis

Clinical strains of Pseudomonas aeruginosa lacking the type III secretion system genes employ a toxin, exolysin (ExlA), for host cell membrane disruption. Here, we demonstrated that ExlA export requires a predicted outer membrane protein, ExlB, showing that ExlA and ExlB define a new active two-partner secretion (TPS) system of P. aeruginosa In addition to the TPS signals, ExlA harbors several distinct domains, which include one hemagglutinin domain, five arginine-glycine-aspartic acid (RGD) motifs, and a C-terminal region lacking any identifiable sequence motifs. However, this C-terminal region is important for the toxic activity, since its deletion abolishes host cell lysis. Using lipid vesicles and eukaryotic cells, including red blood cells, we demonstrated that ExlA has a pore-forming activity which precedes cell membrane disruption of nucleated cells. Finally, we developed a high-throughput cell-based live-dead assay and used it to screen a transposon mutant library of an ExlA-producing P. aeruginosa clinical strain for bacterial factors required for ExlA-mediated toxicity. The screen resulted in the identification of proteins involved in the formation of type IV pili as being required for ExlA to exert its cytotoxic activity by promoting close contact between bacteria and the host cell. These findings represent the first example of cooperation between a pore-forming toxin of the TPS family and surface appendages in host cell intoxication.

IMPORTANCE

The course and outcome of acute, toxigenic infections by Pseudomonas aeruginosa clinical isolates rely on the deployment of one of two virulence strategies: delivery of effectors by the well-known type III secretion system or the cytolytic activity of the recently identified two-partner secreted toxin, exolysin. Here, we characterize several features of the mammalian cell intoxication process mediated by exolysin. We found that exolysin requires the outer membrane protein ExlB for export into extracellular medium. Using in vitro recombinant protein and ex vivo assays, we demonstrated a pore-forming activity of exolysin. A cellular cytotoxicity screen of a transposon mutant library, made in an exolysin-producing clinical strain, identified type IV pili as bacterial appendages required for exolysin toxic function. This work deciphers molecular mechanisms underlying the activity of novel virulence factors used by P. aeruginosa clinical strains lacking the type III secretion system, including a requirement for the toxin-producing bacteria to be attached to the targeted cell to induce cytolysis, and defines new targets for developing antivirulence strategies.

Above and beyond C5a Receptor Targeting by Staphylococcal Leucotoxins: Retrograde Transport of Panton-Valentine Leucocidin and γ-Hemolysin

Various membrane receptors associated with the innate immune response have recently been identified as mediators of the cellular action of Staphylococcus aureus leucotoxins. Two of these, the Panton–Valentine leucotoxin LukS-PV/LukF-PV and the γ-hemolysin HlgC/HlgB, bind the C5a complement-derived peptide receptor. These leucotoxins utilize the receptor to induce intracellular Ca²⁺ release from internal stores, other than those activated by C5a. The two leucotoxins are internalized with the phosphorylated receptor, but it is unknown whether they divert retrograde transport of the receptor or follow another pathway. Immunolabeling and confocal microscopic techniques were used to analyze the presence of leucotoxins in endosomes, lysosomes, endoplasmic reticulum, and Golgi. The two leucotoxins apparently followed retrograde transport similar to that of the C5a peptide-activated receptor. However, HlgC/HlgB reached the Golgi network very early, whereas LukS-PV/LukF-PV followed slower kinetics. The HlgC/HlgB leucotoxin remained in neutrophils 6 h after a 10-min incubation of the cells in the presence of the toxin with no signs of apoptosis, whereas apoptosis was observed 3 h after neutrophils were incubated with LukS-PV/LukF-PV. Such retrograde transport of leucotoxins provides a novel understanding of the cellular effects initiated by sublytic concentrations of these toxins.

Distinct Neurotoxicity Profile of Listeriolysin O from Listeria monocytogenes

Cholesterol-dependent cytolysins (CDCs) are protein toxins that originate from Gram-positive bacteria and contribute substantially to their pathogenicity. CDCs bind membrane cholesterol and build prepores and lytic pores. Some effects of the toxins are observed in non-lytic concentrations. Two pathogens, Streptococcus pneumoniae and Listeria monocytogenes, cause fatal bacterial meningitis, and both produce toxins of the CDC family-pneumolysin and listeriolysin O, respectively. It has been demonstrated that pneumolysin produces dendritic varicosities (dendrite swellings) and dendritic spine collapse in the mouse neocortex, followed by synaptic loss and astrocyte cell shape remodeling without elevated cell death. We utilized primary glial cultures and acute mouse brain slices to examine the neuropathological effects of listeriolysin O and to compare it to pneumolysin with identical hemolytic activity. In cultures, listeriolysin O permeabilized cells slower than pneumolysin did but still initiated non-lytic astrocytic cell shape changes, just as pneumolysin did. In an acute brain slice culture system, listeriolysin O produced dendritic varicosities in an NMDA-dependent manner but failed to cause dendritic spine collapse and cortical astrocyte reorganization. Thus, listeriolysin O demonstrated slower cell permeabilization and milder glial cell remodeling ability than did pneumolysin and lacked dendritic spine collapse capacity but exhibited equivalent dendritic pathology.

Cleavage of DFNA5 by caspase-3 during apoptosis mediates progression to secondary necrotic/pyroptotic cell death

Apoptosis is a genetically regulated cell suicide programme mediated by activation of the effector caspases 3, 6 and 7. If apoptotic cells are not scavenged, they progress to a lytic and inflammatory phase called secondary necrosis. The mechanism by which this occurs is unknown. Here we show that caspase-3 cleaves the GSDMD-related protein DFNA5 after Asp270 to generate a necrotic DFNA5-N fragment that targets the plasma membrane to induce secondary necrosis/pyroptosis. Cells that express DFNA5 progress to secondary necrosis, when stimulated with apoptotic triggers such as etoposide or vesicular stomatitis virus infection, but disassemble into small apoptotic bodies when DFNA5 is deleted. Our findings identify DFNA5 as a central molecule that regulates apoptotic cell disassembly and progression to secondary necrosis, and provide a molecular mechanism for secondary necrosis. Because DFNA5-induced secondary necrosis and GSDMD-induced pyroptosis are dependent on caspase activation, we propose that they are forms of programmed necrosis.

DFNA5 is related to the caspase-dependent pyroptosis inducer gasdermin D. Here the authors find that DFNA5 is cleaved by caspase 3 and show this cleavage skews cells away from apoptosis into secondary necrosis, a form of cell death characterized by membrane ballooning similar to pyroptosis.

Structure and Function of the Two-Component Cytotoxins of Staphylococcus aureus - Learnings for Designing Novel Therapeutics

Staphylococcus aureus can produce up to five different bi-component cytotoxins: two gamma-hemolysins HlgAB and HlgCB, and leukocidins SF-PV (Panton Valentine leukocidin), ED (LukED) and GH (LukGH, also called LukAB). Their major function in S. aureus pathogenesis is to evade innate immunity by attacking phagocytic cells and to support bacterial growth by lysing red blood cells. The five cytotoxins display different levels of amino acid sequence conservation (30-82%), but all form a remarkably similar beta-barrel type pore structure (greatly resembling the mono-component toxin alpha-hemolysin) that inserts into the target cell membrane leading to necrotic cell death. This review provides an overview of the culmination of decades of research on the structure of these toxins, their unique sequence and structural features that helps to explain the observed functional differences, such as toxin potency towards different cell types and species, receptor specificity and formation of functional non-cognate toxin pairs. The vast knowledge accumulated in this field supports novel approaches and the design of therapeutics targeting these cytotoxins to tame virulence and fight S. aureus infections.

Non-hemolytic enterotoxin of Bacillus cereus induces apoptosis in Vero cells

Bacillus cereus is an opportunistic pathogen that often causes foodborne infectious diseases and food poisoning. Non-hemolytic enterotoxin (Nhe) is the major toxin found in almost all enteropathogenic B. cereus and B. thuringiensis isolates. However, little is known about the cellular response after Nhe triggered pore formation on cell membrane. Here, we demonstrate that Nhe induced cell cycle arrest at G₀ /G₁ phase and provoked apoptosis in Vero cells, most likely associated with mitogen-activated protein kinase (MAPK) and death receptor pathways. The influx of extracellular calcium ions and increased level of reactive oxygen species in cytoplasm were sensed by apoptosis signal-regulating kinase 1 (ASK1) and p38 MAPK. Extrinsic death receptor Fas could also promote the activation of p38 MAPK. Subsequently, ASK1 and p38 MAPK triggered downstream caspase-8 and 3 to initiate apoptosis. Our results clearly demonstrate that ASK1, and Fas-p38 MAPK-mediated caspase-8 dependent pathways are involved in apoptotic cell death provoked by the pore-forming enterotoxin Nhe.

Vibrio cholerae hemolysin: The β-trefoil domain is required for folding to the native conformation

Vibrio cholerae cytolysin/hemolysin (VCC) is a 65 kDa β-pore-forming toxin causing lysis and death of eukaryotic cells. Apart from the core cytolysin domain, VCC has two lectin domains with β-trefoil and β-prism folds. The β-prism domain binds to cell surface carbohydrate receptors; the role of the β-trefoil domain is unknown. Here, we show that the pro-VCC mutant without the β-trefoil domain formed aggregates highly susceptible to proteolysis, suggesting lack of a properly folded compact structure. The VCC variants with Trp532Ala or Trp534Ala mutation in the β-trefoil domain formed hemolytically inactive, protease-resistant, ring-shaped SDS-labile oligomers with diameters of ~19 nm. The Trp mutation induced a dramatic change in the global conformation of VCC, as indicated by: (a) the change in surface polarity from hydrophobic to hydrophilic; (b) movement of core Trp residues to the protein-water interface; and (c) decrease in reactivity to the anti-VCC antibody by >100-fold. In fact, the mutant VCC had little similarity to the wild toxin. However, the association constant for the carbohydrate-dependent interaction mediated by the β-prism domain decreased marginally from ~3×108 to ~5×107 M-1. We interpret the observations by proposing: (a) the β-trefoil domain is critical to the folding of the cytolysin domain to its active conformation; (b) the β-prism domain is an autonomous folding unit.

Alteration of epithelial cell lysosomal integrity induced by bacterial cholesterol-dependent cytolysins

Bacterial pathogens can interfere during infection with host cell organelles, such as mitochondria, the endoplasmic reticulum‐Golgi system or nuclei. As important cellular functions are often compartmentalized in these organelles, their targeting allows pathogens to manipulate key host functions during infection. Here, we identify lysosomes as a new class of organelles targeted by the pathogenic bacterium Listeria monocytogenes. We demonstrate that extracellular Listeria, via secretion of the pore‐forming toxin listeriolysin O, alters lysosomal integrity in epithelial cells but not in macrophages. Listeriolysin O induces lysosomal membrane permeabilization and release of lysosomal content, such as cathepsins proteases, which remain transiently active in the host cytosol. We furthermore show that other bacterial pore‐forming toxins, such as perfringolysin O and pneumolysin, also induce lysosomes alteration. Together, our data unveil a novel activity of bacterial cholesterol‐dependent cytolysins.

Disulphide bond restrains the C-terminal region of thermostable direct hemolysin during folding to promote oligomerization

Pore-forming toxins (PFTs) are typically produced as water-soluble monomers, which upon interacting with target cells assemble into transmembrane oligomeric pores. Vibrio parahaemolyticus thermostable direct hemolysin (TDH) is an atypical PFT that exists as a tetramer in solution, prior to membrane binding. The TDH structure highlights a core β-sandwich domain similar to those found in the eukaryotic actinoporin family of PFTs. However, the TDH structure harbors an extended C-terminal region (CTR) that is not documented in the actinoporins. This CTR remains tethered to the β-sandwich domain through an intra-molecular disulphide bond. Part of the CTR is positioned at the inter-protomer interface in the TDH tetramer. Here we show that the truncation, as well as mutation, of the CTR compromise tetrameric assembly, and the membrane-damaging activity of TDH. Our study also reveals that intra-protomer disulphide bond formation during the folding/assembly process of TDH restrains the CTR to mediate its participation in the formation of inter-protomer contact, thus facilitating TDH oligomerization. However, once tetramerization is achieved, disruption of the disulphide bond does not affect oligomeric assembly. Our study provides critical insights regarding the regulation of the oligomerization mechanism of TDH, which has not been previously documented in the PFT family.

Super-resolution Stimulated Emission Depletion-Fluorescence Correlation Spectroscopy Reveals Nanoscale Membrane Reorganization Induced by Pore-Forming Proteins

Membrane-protein interactions play a central role in membrane mediated cellular processes ranging from signaling, budding, and fusion, to transport across the cell membrane. Of particular significance is the process of efficient protein olgomerization and transmembrane pore formation on the membrane surface; the primary virulent pathway for the action of antimicrobial peptides and pore forming toxins (PFTs). The suggested nanoscopic length scales and dynamic nature of such membrane lipid-protein interactions makes their detection extremely challenging. Using a combination of super-resolution stimulated emission depletion nanoscopy with fluorescence correlation spectroscopy (STED-FCS) we unravel the emergence of nanoscale lateral heterogeneity in supported bilayer membranes made up of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) and cholesterol upon interaction with the PFT, listeriolysin O (LLO). A distinct length scale-dependent dynamical crossover (<200 nm) from a Brownian diffusive regime is observed at 33 and 50% cholesterol compositions, indicating the partitioning of lipids into domains with variable cholesterol content. At 25% cholesterol content, this dyamical crossover is observed only in bilayers incubated with LLO providing evidence for the existence of sub ∼100 nm dynamical lipid nanodomains bound to LLO pore assemblies. By introducing asymmetry in cholesterol composition across the bilayer leaflets we infer that this domain formation is driven largely due to active cholesterol sequestration and transient trapping of lipids to the membrane bound motifs present in the toxins, en route to LLO oligomerization and subsequent pore formation. Bilayers prepared with labeled lipids present in either the proximal or distal leaflet allow us to track the dynamical perturbation in a leaflet-dependent manner upon LLO incubation. From the differences in the extent and intensity of the dynamical crossover as observed with STED-FCS, these experiments reveal that the affinity for cholesterol in the membrane binding motifs of the LLO subdomains induce cholesterol and lipid reorganization to a greater extent in the distal (upper) leaflet when compared with the proximal (lower) leaflet. The observed length scale-dependent membrane reorganization that occurs due to invasion by LLO could be generalized to other cholesterol-dependent cytolysins and emphasizes the significant advantage of using super-resolution STED nanoscopy to unravel complex lipid-protein interactions in membrane and cellular biophysics.

Engineered transmembrane pores

Today, hundreds of researchers are working on nanopores, making an impact in both basic science and biotechnology. Proteins remain the most versatile sources of nanopores, based on our ability to engineer them with sub-nanometer precision. Recent work aimed at the construction and discovery of novel pores has included unnatural amino acid mutagenesis and the application of selection techniques. The diversity of structures has now been increased through the development of helix-based pores as well as the better-known β barrels. New developments also include truncated pores, which pierce bilayers through lipid rearrangement, and hybrid pores, which do away with bilayers altogether. Pore dimers, which span two lipid bilayers, have been constructed and pores based on DNA nanostructures are gaining in importance. While nanopore DNA sequencing has received enthusiastic attention, protein pores have a wider range of potential applications, requiring specifications that will require engineering efforts to continue for years to come.

The Colicin E1 TolC-Binding Conformer: Pillar or Pore Function of TolC in Colicin Import?

The mechanism by which the drug export protein TolC is utilized for import of the cytotoxin colicin E1 across the outer membrane and periplasmic space is addressed. Studies of the initial binding of colicin E1 with TolC, occlusion of membrane-incorporated TolC ion channels, and the structure underlying the colicin-TolC complex were based on the interactions with TolC of individual colicin translocation domain (T-domain) peptides from a set of 19 that span different segments of the T-domain. These studies led to identification of a short 20-residue segment 101-120, a "TolC box", located near the center of the colicin T-domain, which is necessary for binding of colicin to TolC. Omission of this segment eliminated the ability of the T-domain to occlude TolC channels and to co-elute with TolC on a size-exclusion column. Far-ultraviolet circular dichroism spectral and thermal stability analysis of the structure of T-domain peptides implies (i) a helical hairpin conformation of the T-domain, (ii) the overlap of the TolC-binding site with a hinge of the helical hairpin, and (iii) a TolC-dependent stage of colicin import in which a central segment of the T-domain in a helical hairpin conformation binds to the TolC entry port following initial binding to the BtuB receptor. These studies provide the first structure-based information about the interaction of colicin E1 with the unique TolC protein. The model inferred for binding of the T-domain to TolC implies reservations about the traditional model for colicin import in which TolC functions to provide a channel for translocation of the colicin in an unfolded state across the bacterial outer membrane and a large part of the periplasmic space.

Proteomics Analysis Reveals Previously Uncharacterized Virulence Factors in Vibrio proteolyticus

Members of the genus Vibrio include many pathogens of humans and marine animals that share genetic information via horizontal gene transfer. Hence, the Vibrio pan-genome carries the potential to establish new pathogenic strains by sharing virulence determinants, many of which have yet to be characterized. Here, we investigated the virulence properties of Vibrio proteolyticus, a Gram-negative marine bacterium previously identified as part of the Vibrio consortium isolated from diseased corals. We found that V. proteolyticus causes actin cytoskeleton rearrangements followed by cell lysis in HeLa cells in a contact-independent manner. In search of the responsible virulence factor involved, we determined the V. proteolyticus secretome. This proteomics approach revealed various putative virulence factors, including active type VI secretion systems and effectors with virulence toxin domains; however, these type VI secretion systems were not responsible for the observed cytotoxic effects. Further examination of the V. proteolyticus secretome led us to hypothesize and subsequently demonstrate that a secreted hemolysin, belonging to a previously uncharacterized clan of the leukocidin superfamily, was the toxin responsible for the V. proteolyticus-mediated cytotoxicity in both HeLa cells and macrophages. Clearly, there remains an armory of yet-to-be-discovered virulence factors in the Vibrio pan-genome that will undoubtedly provide a wealth of knowledge on how a pathogen can manipulate host cells.

The pan-genome of the genus Vibrio is a potential reservoir of unidentified toxins that can provide insight into how members of this genus have successfully risen as emerging pathogens worldwide. We focused on Vibrio proteolyticus, a marine bacterium that was previously implicated in virulence toward marine animals, and characterized its interaction with eukaryotic cells. We found that this bacterium causes actin cytoskeleton rearrangements and leads to cell death. Using a proteomics approach, we identified a previously unstudied member of the leukocidin family of pore-forming toxins as the virulence factor responsible for the observed cytotoxicity in eukaryotic cells, as well as a plethora of additional putative virulence factors secreted by this bacterium. Our findings reveal a functional new clan of the leukocidin toxin superfamily and establish this pathogen as a reservoir of potential toxins that can be used for biomedical applications.

Identification of a Membrane-bound Prepore Species Clarifies the Lytic Mechanism of Actinoporins

Pore-forming toxins (PFTs) are cytolytic proteins belonging to the molecular warfare apparatus of living organisms. The assembly of the functional transmembrane pore requires several intermediate steps ranging from a water-soluble monomeric species to the multimeric ensemble inserted in the cell membrane. The non-lytic oligomeric intermediate known as prepore plays an essential role in the mechanism of insertion of the class of β-PFTs. However, in the class of α-PFTs, like the actinoporins produced by sea anemones, evidence of membrane-bound prepores is still lacking. We have employed single-particle cryo-electron microscopy (cryo-EM) and atomic force microscopy to identify, for the first time, a prepore species of the actinoporin fragaceatoxin C bound to lipid vesicles. The size of the prepore coincides with that of the functional pore, except for the transmembrane region, which is absent in the prepore. Biochemical assays indicated that, in the prepore species, the N terminus is not inserted in the bilayer but is exposed to the aqueous solution. Our study reveals the structure of the prepore in actinoporins and highlights the role of structural intermediates for the formation of cytolytic pores by an α-PFT.

Host cell remodelling in malaria parasites: a new pool of potential drug targets

When in their human hosts, malaria parasites spend most of their time housed within vacuoles inside erythrocytes and hepatocytes. The parasites extensively modify their host cells to obtain nutrients, prevent host cell breakdown and avoid the immune system. To perform these modifications, malaria parasites export hundreds of effector proteins into their host cells and this process is best understood in the most lethal species to infect humans, Plasmodium falciparum. The effector proteins are synthesized within the parasite and following a proteolytic cleavage event in the endoplasmic reticulum and sorting of mature proteins into the correct vesicular trafficking pathway, they are transported to the parasite surface and released into the vacuole. The effector proteins are then unfolded before extrusion across the vacuole membrane by a unique translocon complex called Plasmodium translocon of exported proteins. After gaining access to the erythrocyte cytoplasm many effector proteins continue their journey to the erythrocyte surface by utilising various membranous structures established by the parasite. This complex trafficking pathway and a large number of the effector proteins are unique to Plasmodium parasites. This pathway could, therefore, be developed as new drug targets given that protein export and the functional role of these proteins are essential for parasite survival. This review explores known and potential drug targetable steps in the protein export pathway and strategies for discovering novel drug targets.

GsdmD p30 elicited by caspase-11 during pyroptosis forms pores in membranes

Gasdermin-D (GsdmD) is a critical mediator of innate immune defense because its cleavage by the inflammatory caspases 1, 4, 5, and 11 yields an N-terminal p30 fragment that induces pyroptosis, a death program important for the elimination of intracellular bacteria. Precisely how GsdmD p30 triggers pyroptosis has not been established. Here we show that human GsdmD p30 forms functional pores within membranes. When liberated from the corresponding C-terminal GsdmD p20 fragment in the presence of liposomes, GsdmD p30 localized to the lipid bilayer, whereas p20 remained in the aqueous environment. Within liposomes, p30 existed as higher-order oligomers and formed ring-like structures that were visualized by negative stain electron microscopy. These structures appeared within minutes of GsdmD cleavage and released Ca(2+) from preloaded liposomes. Consistent with GsdmD p30 favoring association with membranes, p30 was only detected in the membrane-containing fraction of immortalized macrophages after caspase-11 activation by lipopolysaccharide. We found that the mouse I105N/human I104N mutation, which has been shown to prevent macrophage pyroptosis, attenuated both cell killing by p30 in a 293T transient overexpression system and membrane permeabilization in vitro, suggesting that the mutants are actually hypomorphs, but must be above certain concentration to exhibit activity. Collectively, our data suggest that GsdmD p30 kills cells by forming pores that compromise the integrity of the cell membrane.

Crystal structure of an invertebrate cytolysin pore reveals unique properties and mechanism of assembly

The invertebrate cytolysin lysenin is a member of the aerolysin family of pore-forming toxins that includes many representatives from pathogenic bacteria. Here we report the crystal structure of the lysenin pore and provide insights into its assembly mechanism. The lysenin pore is assembled from nine monomers via dramatic reorganization of almost half of the monomeric subunit structure leading to a β-barrel pore ∼10 nm long and 1.6–2.5 nm wide. The lysenin pore is devoid of additional luminal compartments as commonly found in other toxin pores. Mutagenic analysis and atomic force microscopy imaging, together with these structural insights, suggest a mechanism for pore assembly for lysenin. These insights are relevant to the understanding of pore formation by other aerolysin-like pore-forming toxins, which often represent crucial virulence factors in bacteria.

Pore-forming toxins act by forming oligomeric pores in lipid membranes. Here the authors report the crystal structure of the lysenin pore, providing insights into the assembly and function of the pore in addition to suggesting that its properties make lysenin potentially well-suited for nanopore sensing applications.