The cholesterol-dependent cytolysin pneumolysin from Streptococcus pneumoniae binds to lipid raft microdomains in human corneal epithelial cells

Negative Cross-Talk between TLR2/4-Independent AMPKα1 and TLR2/4-Dependent JNK Regulates S. pneumoniae-Induced Mucosal Innate Immune Response

Streptococcus pneumoniae is major cause of otitis media (OM) and life-threatening pneumonia. Overproduction of mucin, the major component of mucus, plays a critical role in the pathogenesis of both OM and pneumonia. However, the molecular mechanisms underlying the tight regulation of mucin upregulation in the mucosal epithelium by S. pneumoniae infection remain largely unknown. In this study, we show that S. pneumoniae pneumolysin (PLY) activates AMP-activated protein kinase α1 (AMPKα1), the master regulator of energy homeostasis, which is required for S. pneumoniae-induced mucin MUC5AC upregulation in vitro and in vivo. Moreover, we found that PLY activates AMPKα1 via cholesterol-dependent membrane binding of PLY and subsequent activation of the Ca2+- Ca2+/calmodulin-dependent kinase kinase β (CaMKKβ) and Cdc42-mixed-lineage protein kinase 3 (MLK3) signaling axis in a TLR2/4-independent manner. AMPKα1 positively regulates PLY-induced MUC5AC expression via negative cross-talk with TLR2/4-dependent activation of MAPK JNK, the negative regulator of MUC5AC expression. Moreover, pharmacological inhibition of AMPKα1 suppressed MUC5AC induction in the S. pneumoniae-induced OM mouse model, thereby demonstrating its therapeutic potential in suppressing mucus overproduction in OM. Taken together, our data unveil a novel mechanism by which negative cross-talk between TLR2/4-independent activation of AMPKα1 and TLR2/4-dependent activation of JNK tightly regulates the S. pneumoniae PLY-induced host mucosal innate immune response.

Pneumolysin boosts the neuroinflammatory response to Streptococcus pneumoniae through enhanced endocytosis

In pneumococcal meningitis, bacterial growth in the cerebrospinal fluid results in lysis, the release of toxic factors, and subsequent neuroinflammation. Exposure of primary murine glia to Streptococcus pneumoniae lysates leads to strong proinflammatory cytokine and chemokine production, blocked by inhibition of the intracellular innate receptor Nod1. Lysates enhance dynamin-dependent endocytosis, and dynamin inhibition reduces neuroinflammation, blocking ligand internalization. Here we identify the cholesterol-dependent cytolysin pneumolysin as a pro-endocytotic factor in lysates, its elimination reduces their proinflammatory effect. Only pore-competent pneumolysin enhances endocytosis in a dynamin-, phosphatidylinositol-3-kinase- and potassium-dependent manner. Endocytic enhancement is limited to toxin-exposed parts of the membrane, the effect is rapid and pneumolysin permanently alters membrane dynamics. In a murine model of pneumococcal meningitis, mice treated with chlorpromazine, a neuroleptic with a complementary endocytosis inhibitory effect show reduced neuroinflammation. Thus, the dynamin-dependent endocytosis emerges as a factor in pneumococcal neuroinflammation, and its enhancement by a cytolysin represents a proinflammatory control mechanism.

Membrane Cholesterol Content and Lipid Organization Influence Melittin and Pneumolysin Pore-Forming Activity

Melittin, the main toxic component in the venom of the European honeybee, interacts with natural and artificial membranes due to its amphiphilic properties. Rather than interacting with a specific receptor, melittin interacts with the lipid components, disrupting the lipid bilayer and inducing ion leakage and osmotic shock. This mechanism of action is shared with pneumolysin and other members of the cholesterol-dependent cytolysin family. In this manuscript, we investigated the inverse correlation for cholesterol dependency of these two toxins. While pneumolysin-induced damage is reduced by pretreatment with the cholesterol-depleting agent methyl-β-cyclodextrin, the toxicity of melittin, after cholesterol depletion, increased. A similar response was also observed after a short incubation with lipophilic simvastatin, which alters membrane lipid organization and structure, clustering lipid rafts. Therefore, changes in toxin sensitivity can be achieved in cells by depleting cholesterol or changing the lipid bilayer organization.

The Yin and Yang of Pneumolysin During Pneumococcal Infection

Pneumolysin (PLY) is a pore-forming toxin produced by the human pathobiont Streptococcus pneumoniae, the major cause of pneumonia worldwide. PLY, a key pneumococcal virulence factor, can form transmembrane pores in host cells, disrupting plasma membrane integrity and deregulating cellular homeostasis. At lytic concentrations, PLY causes cell death. At sub-lytic concentrations, PLY triggers host cell survival pathways that cooperate to reseal the damaged plasma membrane and restore cell homeostasis. While PLY is generally considered a pivotal factor promoting S. pneumoniae colonization and survival, it is also a powerful trigger of the innate and adaptive host immune response against bacterial infection. The dichotomy of PLY as both a key bacterial virulence factor and a trigger for host immune modulation allows the toxin to display both "Yin" and "Yang" properties during infection, promoting disease by membrane perforation and activating inflammatory pathways, while also mitigating damage by triggering host cell repair and initiating anti-inflammatory responses. Due to its cytolytic activity and diverse immunomodulatory properties, PLY is integral to every stage of S. pneumoniae pathogenesis and may tip the balance towards either the pathogen or the host depending on the context of infection.

Absence of Streptococcus pneumoniae Capsule Increases Bacterial Binding, Persistence, and Inflammation in Corneal Infection

The role of the pneumococcal polysaccharide capsule is largely unclear for Streptococcus pneumoniae keratitis, an ocular inflammatory disease that develops as a result of bacterial infection of the cornea. In this study, capsule-deficient strains were compared to isogenic parent strains in their ability to adhere to human corneal epithelial cells. One isogenic pair was further used in topical ocular infection of mice to assess the contribution of the capsule to keratitis. The results showed that non-encapsulated pneumococci were significantly more adherent to cells, persisted in significantly higher numbers on mouse corneas in vivo, and caused significant increases in murine ocular IL9, IL10, IL12-p70, MIG, and MIP-1-gamma compared to encapsulated S. pneumoniae. These findings indicate that the bacterial capsule impedes virulence and the absence of capsule impacts inflammation following corneal infection.

A semisynthetic glycoconjugate provides expanded cross-serotype protection against Streptococcus pneumoniae

Streptococcus pneumoniae (S. pneumoniae)infections are the leading cause of child mortality globally. Currentvaccines fail to induceaprotective immune response towards a conserved part of the pathogen,resulting in newserotypescausing disease. Therefore, new vaccinestrategies are urgently needed.Described is atwo-pronged approach combiningS. pneumoniaeproteins, pneumolysin (Ply) and pneumococcal surface protein A (PspA),with aprecisely defined synthetic oligosaccharide,wherebythe carrier protein actsas a serotype-independent antigen to provideadditional protection. Proof of concept in mice and swine modelsrevealed thatthe conjugatesinhibited colonization of the nasopharynx, decreased the bacterial load and reduced disease severity in the bacteria challenge model. Immunization of piglets provided the first evidence for the immunogenicity and protective potential of synthetic glycoconjugate vaccine in a large animal model.Acombination of synthetic oligosaccharides with proteins from the target pathogen opens the path to create broadly cross-protective ("universal") pneumococcal vaccines.

Cholesterol-enriched membrane micro-domaindeficiency induces doxorubicin resistancevia promoting autophagy in breast cancer

Drug resistance has become one of the largest challenges for cancer chemotherapies. Under certain conditions, cancer cells hijack autophagy to cope with therapeutic stress, which largely undermines the chemo-therapeutic efficacy. Currently, biomarkers indicative of autophagy-derived drug resistance remain largely inclusive. Here, we report a novel role of lipid rafts/cholesterol-enriched membrane micro-domains (CEMMs) in autophagosome biogenesis and doxorubicin resistance in breast tumors. We showed that CEMMs are required for the interaction of VAMP3 with syntaxin 6 (STX6, a cholesterol-binding SNARE protein). Upon disruption of CEMM, VAMP3 is released from STX6, resulting in the trafficking of ATG16L1-containing vesicles to recycling endosomes and subsequent autophagosome biogenesis. Furthermore, we found that CEMM marker CAV1 is decreased in breast cancer patients and that the CEMM deficiency-induced autophagy is related to doxorubicin resistance, which is overcome by autophagy inhibition. Taken together, we propose a novel model whereby CEMMs in recycling endosomes support the VAMP3 and STX6 interaction and function as barriers to limit the activity of VAMP3 in autophagic vesicle fusion, thus CEMM deficiency promotes autophagosome biogenesis and doxorubicin resistance in breast tumors.

Streptococcus pneumoniae pneumolysin and neuraminidase A convert high-density lipoproteins into pro-atherogenic particles

High-density lipoproteins (HDLs) are a group of different subpopulations of sialylated particles that have an essential role in the reverse cholesterol transport (RCT) pathway. Importantly, changes in the protein and lipid composition of HDLs may lead to the formation of particles with reduced atheroprotective properties. Here, we show that Streptococcus pneumoniae pneumolysin (PLY) and neuraminidase A (NanA) impair HDL function by causing chemical and structural modifications of HDLs. The proteomic, lipidomic, cellular, and biochemical analysis revealed that PLY and NanA induce significant changes in sialic acid, protein, and lipid compositions of HDL. The modified HDL particles have reduced cholesterol acceptor potential from activated macrophages, elevated levels of malondialdehyde adducts, and show significantly increased complement activating capacity. These results suggest that accumulation of these modified HDL particles in the arterial intima may present a trigger for complement activation, inflammatory response, and thereby promote atherogenic disease progression.

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S. pneumoniae molecules PLY and NanA target human high-density lipoprotein (HDL).

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These interactions result in major modifications in the HDL proteome and lipidome.

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Microbially modified HDL activates humoral and cell-mediated innate immune responses.

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The activated immune response mediates formation of pro-atherogenic epitopes on HDL.

Structural insights into loss of function of a pore forming toxin and its role in pneumococcal adaptation to an intracellular lifestyle

The opportunistic pathogen Streptococcus pneumoniae has dual lifestyles: one of an asymptomatic colonizer in the human nasopharynx and the other of a deadly pathogen invading sterile host compartments. The latter triggers an overwhelming inflammatory response, partly driven via pore forming activity of the cholesterol dependent cytolysin (CDC), pneumolysin. Although pneumolysin-induced inflammation drives person-to-person transmission from nasopharynx, the primary reservoir for pneumococcus, it also contributes to high mortality rates, creating a bottleneck that hampers widespread bacterial dissemination, thus acting as a double-edged sword. Serotype 1 ST306, a widespread pneumococcal clone, harbours a non-hemolytic variant of pneumolysin (Ply-NH). Performing crystal structure analysis of Ply-NH, we identified Y150H and T172I as key substitutions responsible for loss of its pore forming activity. We uncovered a novel inter-molecular cation-π interaction, governing formation of the transmembrane β-hairpins (TMH) in the pore state of Ply, which can be extended to other CDCs. H150 in Ply-NH disrupts this interaction, while I172 provides structural rigidity to domain-3, through hydrophobic interactions, inhibiting TMH formation. Loss of pore forming activity enabled improved cellular invasion and autophagy evasion, promoting an atypical intracellular lifestyle for pneumococcus, a finding that was corroborated in in vivo infection models. Attenuation of inflammatory responses and tissue damage promoted tolerance of Ply-NH-expressing pneumococcus in the lower respiratory tract. Adoption of this altered lifestyle may be necessary for ST306 due to its limited nasopharyngeal carriage, with Ply-NH, aided partly by loss of its pore forming ability, facilitating a benign association of SPN in an alternative, intracellular host niche.

Lipid rafts and pathogens: the art of deception and exploitation

Lipid rafts, solid regions of the plasma membrane enriched in cholesterol and glycosphingolipids, are essential parts of a cell. Functionally, lipid rafts present a platform that facilitates interaction of cells with the outside world. However, the unique properties of lipid rafts required to fulfill this function at the same time make them susceptible to exploitation by pathogens. Many steps of pathogen interaction with host cells, and sometimes all steps within the entire lifecycle of various pathogens, rely on host lipid rafts. Such steps as binding of pathogens to the host cells, invasion of intracellular parasites into the cell, the intracellular dwelling of parasites, microbial assembly and exit from the host cell, and microbe transfer from one cell to another all involve lipid rafts. Interaction also includes modification of lipid rafts in host cells, inflicted by pathogens from both inside and outside the cell, through contact or remotely, to advance pathogen replication, to utilize cellular resources, and/or to mitigate immune response. Here, we provide a systematic overview of how and why pathogens interact with and exploit host lipid rafts, as well as the consequences of this interaction for the host, locally and systemically, and for the microbe. We also raise the possibility of modulation of lipid rafts as a therapeutic approach against a variety of infectious agents.

More than one way to bind to cholesterol: atypical variants of membrane-binding domain of perfringolysin O selected by ribosome display

Herein, we report a high-throughput approach for the selection of peripheral protein domains that bind specifically to cholesterol in lipid membranes. We discovered variants of perfringolysin O, with non-conserved amino acid substitutions at regions crucial for cholesterol recognition, demonstrating an unprecedented amino acid sequence variability with binding ability for cholesterol. The developed approach provides an effective platform for a comprehensive study of protein lipid interactions.

By using developed ribosomal display, we discovered variants of perfringolysin O, a pore forming toxin from bacteria Clostridium perfringens, with non-conserved amino acid substitutions at regions crucial for cholesterol recognition.

Membrane deformation and layer-by-layer peeling of giant vesicles induced by the pore-forming toxin pneumolysin

Membranes under attack by the pore-forming toxin pneumolysin reveal a hitherto unknown layer-by-layer peeling mechanism and disclose the multilamellar structure.

The Role of Pneumococcal Virulence Factors in Ocular Infectious Diseases

Streptococcus pneumoniae is a gram-positive, facultatively anaerobic pathogen that can cause severe infections such as pneumonia, meningitis, septicemia, and middle ear infections. It is also one of the top pathogens contributing to bacterial keratitis and conjunctivitis. Though two pneumococcal vaccines exist for the prevention of nonocular diseases, they do little to fully prevent ocular infections. This pathogen has several virulence factors that wreak havoc on the conjunctiva, cornea, and intraocular system. Polysaccharide capsule aids in the evasion of host complement system. Pneumolysin (PLY) is a cholesterol-dependent cytolysin that acts as pore-forming toxin. Neuraminidases assist in adherence and colonization by exposing cell surface receptors to the pneumococcus. Zinc metalloproteinases contribute to evasion of the immune system and disease severity. The main purpose of this review is to consolidate the multiple studies that have been conducted on several pneumococcal virulence factors and the role each plays in conjunctivitis, keratitis, and endophthalmitis.

Missing elimination via membrane vesicle shedding contributes to the diminished calcium sensitivity of listeriolysin O

The lytic capacity of cholesterol-dependent cytolysins is enhanced in the extracellular calcium-free environment through a combination of limited membrane repair and diminished membrane toxin removal. For a typical neurotoxin of the group, pneumolysin, this effect has already been observed at reduced (1 mM) calcium conditions, which are pathophysiologically relevant. Here, we tested another neurotoxin of the group, listeriolysin O from L. monocytogenes, active in the primary vacuole after bacterium phagocytosis in host cells. Reduced calcium did not increase the lytic capacity of listeriolysin (in contrast to pneumolysin), while calcium-free conditions elevated it 2.5 times compared to 10 times for pneumolysin (at equivalent hemolytic capacities). To clarify these differences, we analyzed membrane vesicle shedding, known to be a calcium-dependent process for toxin removal from eukaryotic cell membranes. Both pneumolysin and listeriolysin initiated vesicle shedding, which was completely blocked by the lack of extracellular calcium. Lack of calcium, however, elevated the toxin load per a cell only for pneumolysin and not for listeriolysin. This result indicates that vesicle shedding does not play a role in the membrane removal of listeriolysin and outlines a major difference between it and other members of the CDC group. Furthermore, it provides new tools for studying membrane vesicle shedding.

Lipids in the cell: organisation regulates function

Lipids are fundamental building blocks of all cells and play important roles in the pathogenesis of different diseases, including inflammation, autoimmune disease, cancer, and neurodegeneration. The lipid composition of different organelles can vary substantially from cell to cell, but increasing evidence demonstrates that lipids become organised specifically in each compartment, and this organisation is essential for regulating cell function. For example, lipid microdomains in the plasma membrane, known as lipid rafts, are platforms for concentrating protein receptors and can influence intra-cellular signalling. Lipid organisation is tightly regulated and can be observed across different model organisms, including bacteria, yeast, Drosophila, and Caenorhabditis elegans, suggesting that lipid organisation is evolutionarily conserved. In this review, we summarise the importance and function of specific lipid domains in main cellular organelles and discuss recent advances that investigate how these specific and highly regulated structures contribute to diverse biological processes.

Cholesterol-dependent cytolysins impair pro-inflammatory macrophage responses

Necrotizing soft tissue infections are lethal polymicrobial infections. Two key microbes that cause necrotizing soft tissue infections are Streptococcus pyogenes and Clostridium perfringens. These pathogens evade innate immunity using multiple virulence factors, including cholesterol-dependent cytolysins (CDCs). CDCs are resisted by mammalian cells through the sequestration and shedding of pores during intrinsic membrane repair. One hypothesis is that vesicle shedding promotes immune evasion by concomitantly eliminating key signaling proteins present in cholesterol-rich microdomains. To test this hypothesis, murine macrophages were challenged with sublytic CDC doses. CDCs suppressed LPS or IFNγ-stimulated TNFα production and CD69 and CD86 surface expression. This suppression was cell intrinsic. Two membrane repair pathways, patch repair and intrinsic repair, might mediate TNFα suppression. However, patch repair did not correlate with TNFα suppression. Intrinsic repair partially contributed to macrophage dysfunction because TLR4 and the IFNγR were partially shed following CDC challenge. Intrinsic repair was not sufficient for suppression, because pore formation was also required. These findings suggest that even when CDCs fail to kill cells, they may impair innate immune signaling responses dependent on cholesterol-rich microdomains. This is one potential mechanism to explain the lethality of S. pyogenes and C. perfringens during necrotizing soft tissue infections.

Activation-dependent modulation of Streptococcus pneumoniae-mediated death in human lymphocytes

Streptococcus pneumoniae (Spn) is a leading cause of community-acquired pneumonia, with infants and the elderly exhibiting significant susceptibility to the development of severe disease. A growing body of evidence supports the ability of Spn to negatively regulate the host response to infection, e.g. the capacity to induce death in numerous cell types. However, our understanding of the ability of Spn to directly impact lymphocytes remains limited. In this study, we tested the hypothesis that lymphocyte type and activation state influences the susceptibility to pneumococcus-mediated death. We show that in the resting state, CD4+ T cells exhibit a modestly increased susceptibility to Spn-induced death compared to CD8+ T cells or NK cells. In the presence of activating stimuli, the situation most reflective of what would occur in vivo during infection, all subsets demonstrated a significant increase in sensitivity to Spn-mediated death. Importantly, the activated subsets diverged dramatically in susceptibility with natural killer cells exhibiting an 8.6-fold greater sensitivity to pneumococcal components compared to the T-cell subsets. These results significantly expand our understanding of the capacity for pneumococcus to negatively regulate lymphocytes.

The STX6-VTI1B-VAMP3 complex facilitates xenophagy by regulating the fusion between recycling endosomes and autophagosomes

Macroautophagy/autophagy plays a critical role in immunity by directly degrading invading pathogens such as Group A Streptococcus (GAS), through a process that has been named xenophagy. We previously demonstrated that autophagic vacuoles directed against GAS, termed GAS-containing autophagosome-like vacuoles (GcAVs), use recycling endosomes (REs) as a membrane source. However, the precise molecular mechanism that facilitates the fusion between GcAVs and REs remains unclear. Here, we demonstrate that STX6 (syntaxin 6) is recruited to GcAVs and forms a complex with VTI1B and VAMP3 to regulate the GcAV-RE fusion that is required for xenophagy. STX6 targets the GcAV membrane through its tyrosine-based sorting motif and transmembrane domain, and localizes to TFRC (transferrin receptor)-positive punctate structures on GcAVs through its H2 SNARE domain. Knockdown and knockout experiments revealed that STX6 is required for the fusion between GcAVs and REs to promote clearance of intracellular GAS by autophagy. Moreover, VAMP3 and VTI1B interact with STX6 and localize on the TFRC-positive puncta on GcAVs, and are also involved in the RE-GcAV fusion. Furthermore, knockout of RABGEF1 impairs the RE-GcAV fusion and STX6-VAMP3 interaction. These findings demonstrate that RABGEF1 mediates RE fusion with GcAVs through the STX6-VAMP3-VTI1B complex, and reveal the SNARE dynamics involved in autophagosome formation in response to bacterial infection.

Red Blood Cell Susceptibility to Pneumolysin: CORRELATION WITH MEMBRANE BIOCHEMICAL AND PHYSICAL PROPERTIES

This study investigated the effect of the biochemical and biophysical properties of the plasma membrane as well as membrane morphology on the susceptibility of human red blood cells to the cholesterol-dependent cytolysin pneumolysin, a key virulence factor of Streptococcus pneumoniae, using single cell studies. We show a correlation between the physical properties of the membrane (bending rigidity and surface and dipole electrostatic potentials) and the susceptibility of red blood cells to pneumolysin-induced hemolysis. We demonstrate that biochemical modifications of the membrane induced by oxidative stress, lipid scrambling, and artificial cell aging modulate the cell response to the toxin. We provide evidence that the diversity of response to pneumolysin in diabetic red blood cells correlates with levels of glycated hemoglobin and that the mechanical properties of the red blood cell plasma membrane are altered in diabetes. Finally, we show that diabetic red blood cells are more resistant to pneumolysin and the related toxin perfringolysin O relative to healthy red blood cells. Taken together, these studies indicate that the diversity of cell response to pneumolysin within a population of human red blood cells is influenced by the biophysical and biochemical status of the plasma membrane and the chemical and/or oxidative stress pre-history of the cell.

Proteomics in the Study of Bacterial Keratitis

Bacterial keratitis is a serious ocular infection that can cause severe visual loss if treatment is not initiated at an early stage. It is most commonly caused by Staphylococcus aureus, Pseudomonas aeruginosa, Streptococcus pneumoniae, or Serratia species. Depending on the invading organism, bacterial keratitis can progress rapidly, leading to corneal destruction and potential blindness. Common risk factors for bacterial keratitis include contact lens wear, ocular trauma, ocular surface disease, ocular surgery, lid deformity, chronic use of topical steroids, contaminated ocular medications or solutions, and systemic immunosuppression. The pathogenesis of bacterial keratitis, which depends on the bacterium-host interaction and the virulence of the invading bacterium, is complicated and not completely understood. This review highlights some of the proteomic technologies that have been used to identify virulence factors and the host response to infections of bacterial keratitis in order to understand the disease process and develop improved methods of diagnosis and treatment. Although work in this field is not abundant, proteomic technologies have provided valuable information toward our current knowledge of bacterial keratitis. More studies using global proteomic approaches are warranted because it is an important tool to identify novel targets for intervention and prevention of corneal damage caused by these virulent microorganisms.

The cytolytic activity of vaginolysin strictly depends on cholesterol and is potentiated by human CD59

Gardnerella vaginalis produces cytolysin vaginolysin (VLY), which has been suggested to be a contributor to bacterial vaginosis pathogenesis. VLY along with intermedilysin (ILY) from Streptococcus intermedius have been attributed to a group of cholesterol-dependent cytolysins (CDCs) whose pore-forming activity depends on human CD59 (hCD59). Here, we show that different types of cells lacking hCD59 are susceptible to VLY-mediated lysis, albeit to different extents. We analyze the effects of both hCD59 and cholesterol on VLY cytolytic activity. We show that VLY binds to cholesterol-rich membranes of non-human cells, while VLY with an impaired cholesterol recognition site retains binding to the hCD59-containing cells. We further demonstrate that cholesterol binding by VLY is sufficient to trigger the formation of oligomeric complexes on cholesterol rich-liposomes lacking hCD59. Thus, VLY may induce cell lysis following two alternative pathways. One requires only cholesterol and does not depend on hCD59. The second pathway involves hCD59 contribution similarly to ILY. Apparently, under physiological conditions VLY acts in the most effective way by accepting the assistance of hCD59.

Fluorescence imaging of MACPF/CDC proteins: new techniques and their application

Structural and biochemical investigations have helped illuminate many of the important details of MACPF/CDC pore formation. However, conventional techniques are limited in their ability to tackle many of the remaining key questions, and new biophysical techniques might provide the means to improve our understanding. Here we attempt to identify the properties of MACPF/CDC proteins that warrant further study, and explore how new developments in fluorescence imaging are able to probe these properties.