Ultrastructure of the membrane attack complex of complement. Heterogeneity of the complex caused by different degree of C9 polymerization

Liposomes with Low Levels of Grafted Poly(ethylene glycol) Remain Susceptible to Destabilization by Anti-Poly(ethylene glycol) Antibodies

Binding of anti-PEG antibodies to poly(ethylene glycol) (PEG) on the surface of PEGylated liposomal doxorubicin (PLD) in vitro and in rats can activate complement and cause the rapid release of doxorubicin from the liposome interior. Here, we find that irinotecan liposomes (IL) and L-PLD, which have 16-fold lower levels of 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (DSPE)-PEG2000 in their liposome membrane as compared to PLD, generate less complement activation but remain sensitive to destabilization and drug release by anti-PEG antibodies. Complement activation and liposome destabilization correlated with the theoretically estimated number of antibody molecules bound per liposome. Drug release from liposomes proceeded through the alternative complement pathway but was accelerated by the classical complement pathway. In contrast to PLD destabilization by anti-PEG immunoglobulin G (IgG), which proceeded by the insertion of membrane attack complexes in the lipid bilayer of otherwise intact PLD, anti-PEG IgG promoted the fusion of L-PLD, and IL to form unilamellar and oligo-vesicular liposomes. Anti-PEG immunoglobulin M (IgM) induced drug release from all liposomes (PLD, L-PLD, and IL) via the formation of unilamellar and oligo-vesicular liposomes. Anti-PEG IgG destabilized both PLD and L-PLD in rats, indicating that the reduction of PEG levels on liposomes is not an effective approach to prevent liposome destabilization by anti-PEG antibodies.

The role of properdin and Factor H in disease

The complement system consists of three pathways (alternative, classical, and lectin) that play a fundamental role in immunity and homeostasis. The multifunctional role of the complement system includes direct lysis of pathogens, tagging pathogens for phagocytosis, promotion of inflammatory responses to control infection, regulation of adaptive cellular immune responses, and removal of apoptotic/dead cells and immune complexes from circulation. A tight regulation of the complement system is essential to avoid unwanted complement-mediated damage to the host. This regulation is ensured by a set of proteins called complement regulatory proteins. Deficiencies or malfunction of these regulatory proteins may lead to pro-thrombotic hematological diseases, renal and ocular diseases, and autoimmune diseases, among others. This review focuses on the importance of two complement regulatory proteins of the alternative pathway, Factor H and properdin, and their role in human diseases with an emphasis on: (a) characterizing the main mechanism of action of Factor H and properdin in regulating the complement system and protecting the host from complement-mediated attack, (b) describing the dysregulation of the alternative pathway as a result of deficiencies, or mutations, in Factor H and properdin, (c) outlining the clinical findings, management and treatment of diseases associated with mutations and deficiencies in Factor H, and (d) defining the unwanted and inadequate functioning of properdin in disease, through a discussion of various experimental research findings utilizing in vitro, mouse and human models.

On-target IgG hexamerisation driven by a C-terminal IgM tail-piece fusion variant confers augmented complement activation

The majority of depleting monoclonal antibody (mAb) drugs elicit responses via Fc-FcγR and Fc-C1q interactions. Optimal C1q interaction is achieved through hexameric Fc:Fc interactions at the target cell surface. Herein is described an approach to exploit the tailpiece of the naturally multimeric IgM to augment hexamerisation of IgG. Fusion of the C-terminal tailpiece of IgM promoted spontaneous hIgG hexamer formation, resulting in enhanced C1q recruitment and complement-dependent cytotoxicity (CDC) but with off-target complement activation and reduced in-vivo efficacy. Mutation of the penultimate tailpiece cysteine to serine (C575S) ablated spontaneous hexamer formation, but facilitated reversible hexamer formation after concentration in solution. C575S mutant tailpiece antibodies displayed increased complement activity only after target binding, in-line with the concept of 'on-target hexamerisation', whilst retaining efficient in-vivo efficacy and augmented target cell killing in the lymph node. Hence, C575S-tailpiece technology represents an alternative format for promoting on-target hexamerisation and enhanced CDC.

Structures and functions of the membrane-damaging pore-forming proteins

Pore-forming proteins (PFPs) of the diverse life forms have emerged as the potent cell-killing entities owing to their specialized membrane-damaging properties. PFPs have the unique ability to perforate the plasma membranes of their target cells, and they exert this functionality by creating oligomeric pores in the membrane lipid bilayer. Pathogenic bacteria employ PFPs as toxins to execute their virulence mechanisms, whereas in the higher vertebrates PFPs are deployed as the part of the immune system and to generate inflammatory responses. PFPs are the unique dimorphic proteins that are generally synthesized as water-soluble molecules, and transform into membrane-inserted oligomeric pore assemblies upon interacting with the target membranes. In spite of sharing very little sequence similarity, PFPs from diverse organisms display incredible structural similarity. Yet, at the same time, structure-function mechanisms of the PFPs document remarkable versatility. Such notions establish PFPs as the fascinating model system to explore variety of unsolved issues pertaining to the structure-function paradigm of the proteins that interact and act in the membrane environment. In this article, we discuss our current understanding regarding the structural basis of the pore-forming functions of the diverse class of PFPs. We attempt to highlight the similarities and differences in their structures, membrane pore-formation mechanisms, and their implications for the various biological processes, ranging from the bacterial virulence mechanisms to the inflammatory immune response generation in the higher animals.

Electron microscopy as a critical tool in the determination of pore forming mechanisms in proteins

Electron microscopy has consistently played an important role in the description of pore-forming protein systems. The discovery of pore-forming proteins has depended on visualization of the structural pores formed by their oligomeric protein complexes, and as electron microscopy has advanced technologically so has the degree of insight it has been able to give. This review considers a large number of published studies of pore-forming complexes in prepore and pore states determined using single-particle cryo-electron microscopy. Sample isolation and preparation, imaging and image analysis, structure determination and optimization of results are all discussed alongside challenges which pore-forming proteins particularly present. The review also considers the use made of cryo-electron tomography to study pores within their membrane environment and which will prove an increasingly important approach for the future.

Modeling the activation of the alternative complement pathway and its effects on hemolysis in health and disease

The complement system is a powerful mechanism of innate immunity poised to eliminate foreign cells and pathogens. It is an intricate network of >35 proteins, which, once activated, leads to the tagging of the surface to be eliminated, produces potent chemoattractants to recruit immune cells, and inserts cytotoxic pores into nearby lipid surfaces. Although it can be triggered via different pathways, its net output is largely based on the direct or indirect activation of the alternative pathway. Complement dysregulation or deficiencies may cause severe pathologies, such as paroxysmal nocturnal hemoglobinuria (PNH), where a lack of complement control proteins leads to hemolysis and life-threatening anemia. The complexity of the system poses a challenge for the interpretation of experimental data and the design of effective pharmacological therapies. To address this issue, we developed a mathematical model of the alternative complement pathway building on previous modelling efforts. The model links complement activation to the hemolytic activity of the terminal alternative pathway, providing an accurate description of pathway activity as observed in vitro and in vivo, in health and disease. Through adjustment of the parameters describing experimental conditions, the model was capable of reproducing the results of an array of standard assays used in complement research. To demonstrate its clinical applicability, we compared model predictions with clinical observations of the recovery of hematological biomarkers in PNH patients treated with the complement inhibiting anti-C5 antibody eculizumab. In conclusion, the model can enhance the understanding of complement biology and its role in disease pathogenesis, help identifying promising targets for pharmacological intervention, and predict the outcome of complement-targeting pharmacological interventions.

Premature Drug Release from Polyethylene Glycol (PEG)-Coated Liposomal Doxorubicin via Formation of the Membrane Attack Complex

Anti-polyethylene glycol (PEG) antibodies are present in many healthy individuals as well as in patients receiving polyethylene glycol-functionalized drugs. Antibodies against PEG-coated nanocarriers can accelerate their clearance, but their impact on nanodrug properties including nanocarrier integrity is unclear. Here, we show that anti-PEG IgG and IgM antibodies bind to PEG molecules on the surface of PEG-coated liposomal doxorubicin (Doxil, Doxisome, LC-101, and Lipo-Dox), resulting in complement activation, formation of the membrane attack complex (C5b-9) in the liposomal membrane, and rapid release of encapsulated doxorubicin from the liposomes. Drug release depended on both classical and alternative pathways of complement activation. Doxorubicin release of up to 40% was also observed in rats treated with anti-PEG IgG and PEG-coated liposomal doxorubicin. Our results demonstrate that anti-PEG antibodies can disrupt the membrane integrity of PEG-coated liposomal doxorubicin through activation of complement, which may alter therapeutic efficacy and safety in patients with high levels of pre-existing antibodies against PEG.

Vascular endothelial cells evade complement-mediated membrane injury via Weibel-Palade body mobilization

BACKGROUND

Defective complement inhibition can lead to the formation of membrane attack complexes (MAC; C5b-9) on the plasma membranes of vascular endothelial cells, resulting in injury that drives the progression of thrombotic microangiopathy (TMA), a key pathology in kidney disease.

OBJECTIVE/METHODS

We examined the response of human endothelial cells to complement-mediated damage using blood outgrowth endothelial cells (BOECs) derived from healthy donors. BOECs were sensitized to complement factors present in normal human serum to induce the formation of C5b-9 on their plasma membranes.

RESULTS

This triggered an expected abrupt rise in intracellular Ca2+ reflecting membrane leakage. Remarkably, while intracellular Ca2+ remained elevated, membrane leakage ceased within 30 minutes, and cells did not show significant death. Extensive mobilization of Weibel-Palade bodies (WPBs) was observed along with secretion of von Willebrand factor (VWF). The potential role of WPBs and VWF in mitigating complement-mediated damage was examined by comparing the effects of C5b-9 on BOECs derived from von Willebrand disease (VWD) patients expressing reduced amounts of VWF, lacking expression of functional VWF, or lacking both VWF and WPBs. BOECs lacking WPBs were not resistant to complement-mediated damage, but became resistant when transfected to express VWF (and thus WPBs).

CONCLUSION

We conclude that BOECs exposed to C5b-9 attack respond by mobilizing WPBs, which mitigate and repair damage by fusing with the plasma membrane. We propose that a similar cell-specific response may protect the vascular endothelium from complement-mediated damage in vivo.

Transcriptome analysis and histopathology of black carp (Mylopharyngodon piceus) spleen infected by Aeromonas hydrophila

Aeromonas hydrophila causes serious economic losses to the black carp (Mylopharyngodon piceus) industry. In this study, we analyzed the spleen of disease-resistant and susceptible black carp by RNA-seq. Overall, a total of 5243 terms were enriched in the gene ontology (GO) analysis, and 323 related pathways were found in the Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis. A total of 1935 differentially expressed genes were found and were primarily involved in cell adhesion, pathogen recognition, cellular immunity, cytokines, complement systems, and iron transport. Sixteen of the differently expressed genes involved in the immune response and the accuracy of the transcriptome data were further validated by quantitative real-time PCR (qRT-PCR). We observed Tissue sections of the spleen infected with A. hydrophila and the control group and found that the spleen of the infected group had necrosis.

Membrane attack complex-associated molecules from redlip mullet (Liza haematocheila): Molecular characterization and transcriptional evidence of C6, C7, C8β, and C9 in innate immunity

The redlip mullet (Liza haematocheila) is one of the most economically important fish in Korea and other East Asian countries; it is susceptible to infections by pathogens such as Lactococcus garvieae, Argulus spp., Trichodina spp., and Vibrio spp. Learning about the mechanisms of the complement system of the innate immunity of redlip mullet is important for efforts towards eradicating pathogens. Here, we report a comprehensive study of the terminal complement complex (TCC) components that form the membrane attack complex (MAC) through in-silico characterization and comparative spatial and temporal expression profiling. Five conserved domains (TSP1, LDLa, MACPF, CCP, and FIMAC) were detected in the TCC components, but the CCP and FIMAC domains were absent in MuC8β and MuC9. Expression analysis of four TCC genes from healthy redlip mullets showed the highest expression levels in the liver, whereas limited expression was observed in other tissues; immune-induced expression in the head kidney and spleen revealed significant responses against Lactococcus garvieae and poly I:C injection, suggesting their involvement in MAC formation in response to harmful pathogenic infections. Furthermore, the response to poly I:C may suggest the role of TCC components in the breakdown of the membrane of enveloped viruses. These findings may help to elucidate the mechanisms behind the complement system of the teleosts innate immunity.

Advances in structure determination by cryo-EM to unravel membrane-spanning pore formation

The beta pore-forming proteins (β-PFPs) are a large class of polypeptides that are produced by all Kingdoms of life to contribute to their species' own survival. Pore assembly is a sophisticated multi-step process that includes receptor/membrane recognition and oligomerization events, and is ensued by large-scale structural rearrangements, which facilitate maturation of a prepore into a functional membrane spanning pore. A full understanding of pore formation, assembly, and maturation has traditionally been hindered by a lack of structural data; particularly for assemblies representing differing conformations of functional pores. However, recent advancements in cryo-electron microscopy (cryo-EM) techniques have provided the opportunity to delineate the structures of such flexible complexes, and in different states, to near-atomic resolution. In this review, we place a particular emphasis on the use of cryo-EM to uncover the mechanistic details including architecture, activation, and maturation for some of the prominent members of this family.

Review: Towards antibacterial strategies: studies on the mechanisms of interaction between antibacterial peptides and model membranes

Lipopolysaccharides (LPSs) play a dual role as inflammation-inducing and as membrane-forming molecules. The former role attracts significantly more attention from scientists, possibly because it is more closely related to sepsis and septic shock. This review aims to focus the reader's attention to the other role, the function of LPS as the major constituent of the outer layer of the outer membrane of Gram-negative bacteria, in particular those of enterobacterial strains. In this function, LPS is a necessary component of the cell envelope and guarantees survival of the bacterial organism. At the same time, it represents the first target for attacking molecules which may either be synthesized by the host's innate or adaptive immune system or administered to the human body. The interaction of these molecules with the outer membrane may not only directly cause the death of the bacterial organism, but may also lead to the release of LPS into the circulation. Here, we review membrane model systems and their application for the study of molecular mechanisms of interaction of peptides such as those of the human complement system, the bactericidal/permeability-increasing protein (BPI), cationic antibacterial peptide 18 kDa (CAP18) as an example of cathelicidins, defensins, and polymyxin B (PMB). Emphasis is on electrical measurements with a reconstitution system of the lipid matrix of the outer membrane which was established in the authors' laboratory as a planar asymmetric bilayer with one leaflet being composed solely of LPS and the other of the natural phospholipid mixture. The main conclusion, which can be drawn from these investigations, is that LPS and in general its negative charges are the dominant determinants for specific peptide—membrane interactions. However, the detailed mechanisms of interaction, which finally lead to bacterial killing, may involve further steps and differ for different antibacterial peptides.

Heterogeneous MAC Initiator and Pore Structures in a Lipid Bilayer by Phase-Plate Cryo-electron Tomography

Pore formation in membranes is important for mammalian immune defense against invading bacteria. Induced by complement activation, the membrane attack complex (MAC) forms through sequential binding and membrane insertion of C5b6, C7, C8, and C9. Using cryo-electron tomography with a Volta phase plate and subtomogram averaging, we imaged C5b-7, C5b-8, and C5b-9 complexes and determined the C5b-9 pore structure in lipid bilayers. The in situ C5b-9 pore structure at 2.3-nm resolution reveals a 10- to 11.5-nm cone-shaped pore starting with C5b678 and multiple copies of C9 that is poorly closed, yielding a seam between C9 and C6 substituting for the shorter β strands in C6 and C7. However, large variations of composite pore complexes are apparent in subtomograms. Oligomerized initiator complexes C5b-7 and C5b-8 show stages of membrane binding, deformation, and perforation that yield ∼3.5-nm-wide pores. These data indicate a dynamic process of pore formation that likely adapts to biological membranes under attack.

Measuring kinetic drivers of pneumolysin pore structure

Most membrane attack complex-perforin/cholesterol-dependent cytolysin (MACPF/CDC) proteins are thought to form pores in target membranes by assembling into pre-pore oligomers before undergoing a pre-pore to pore transition. Assembly during pore formation is into both full rings of subunits and incomplete rings (arcs). The balance between arcs and full rings is determined by a mechanism dependent on protein concentration in which arc pores arise due to kinetic trapping of the pre-pore forms by the depletion of free protein subunits during oligomerization. Here we describe the use of a kinetic assay to study pore formation in red blood cells by the MACPF/CDC pneumolysin from Streptococcus pneumoniae. We show that cell lysis displays two kinds of dependence on protein concentration. At lower concentrations, it is dependent on the pre-pore to pore transition of arc oligomers, which we show to be a cooperative process. At higher concentrations, it is dependent on the amount of pneumolysin bound to the membrane and reflects the affinity of the protein for its receptor, cholesterol. A lag occurs before cell lysis begins; this is dependent on oligomerization of pneumolysin. Kinetic dissection of cell lysis by pneumolysin demonstrates the capacity of MACPF/CDCs to generate pore-forming oligomeric structures of variable size with, most likely, different functional roles in biology.

Structural basis of complement membrane attack complex formation

In response to complement activation, the membrane attack complex (MAC) assembles from fluid-phase proteins to form pores in lipid bilayers. MAC directly lyses pathogens by a 'multi-hit' mechanism; however, sublytic MAC pores on host cells activate signalling pathways. Previous studies have described the structures of individual MAC components and subcomplexes; however, the molecular details of its assembly and mechanism of action remain unresolved. Here we report the electron cryo-microscopy structure of human MAC at subnanometre resolution. Structural analyses define the stoichiometry of the complete pore and identify a network of interaction interfaces that determine its assembly mechanism. MAC adopts a 'split-washer' configuration, in contrast to the predicted closed ring observed for perforin and cholesterol-dependent cytolysins. Assembly precursors partially penetrate the lipid bilayer, resulting in an irregular β-barrel pore. Our results demonstrate how differences in symmetric and asymmetric components of the MAC underpin a molecular basis for pore formation and suggest a mechanism of action that extends beyond membrane penetration.

Protein-lipid interactions and non-lamellar lipidic structures in membrane pore formation and membrane fusion

Pore-forming proteins and peptides act on their targeted lipid bilayer membranes to increase permeability. This approach to the modulation of biological function is relevant to a great number of living processes, including; infection, parasitism, immunity, apoptosis, development and neurodegeneration. While some pore-forming proteins/peptides assemble into rings of subunits to generate discrete, well-defined pore-forming structures, an increasing number is recognised to form pores via mechanisms which co-opt membrane lipids themselves. Among these, membrane attack complex-perforin/cholesterol-dependent cytolysin (MACPF/CDC) family proteins, Bax/colicin family proteins and actinoporins are especially prominent and among the mechanisms believed to apply are the formation of non-lamellar (semi-toroidal or toroidal) lipidic structures. In this review I focus on the ways in which lipids contribute to pore formation and contrast this with the ways in which lipids are co-opted also in membrane fusion and fission events. A variety of mechanisms for pore formation that involve lipids exists, but they consistently result in stable hybrid proteolipidic structures. These structures are stabilised by mechanisms in which pore-forming proteins modify the innate capacity of lipid membranes to respond to their environment, changing shape and/or phase and binding individual lipid molecules directly. In contrast, and despite the diversity in fusion protein types, mechanisms for membrane fusion are rather similar to each other, mapping out a pathway from pairs of separated compartments to fully confluent fused membranes. Fusion proteins generate metastable structures along the way which, like long-lived proteolipidic pore-forming complexes, rely on the basic physical properties of lipid bilayers. Membrane fission involves similar intermediates, in the reverse order. I conclude by considering the possibility that at least some pore-forming and fusion proteins are evolutionarily related homologues. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

Supported Lipid Bilayer Platform To Test Inhibitors of the Membrane Attack Complex: Insights into Biomacromolecular Assembly and Regulation

Complement activation plays an important role in innate immune defense by triggering formation of the membrane attack complex (MAC), which is a biomacromolecular assembly that exhibits membrane-lytic activity against foreign invaders including various pathogens and biomaterials. Understanding the details of MAC structure and function has been the subject of extensive work involving bulk liposome and erythrocyte assays. However, it is difficult to characterize the mechanism of action of MAC inhibitor drug candidates using the conventional assays. To address this issue, we employ a biomimetic supported lipid bilayer platform to investigate how two MAC inhibitors, vitronectin and clusterin, interfere with MAC assembly in a sequential addition format, as monitored by the quartz crystal microbalance-dissipation (QCM-D) technique. Two experimental strategies based on modular assembly were selected, precincubation of inhibitor and C5b-7 complex before addition to the lipid bilayer or initial addition of inhibitor followed by the C5b-7 complex. The findings indicate that vitronectin inhibits membrane association of C5b-7 via a direct interaction with C5b-7 and via competitive membrane association onto the supported lipid bilayer. On the other hand, clusterin directly interacts with C5b-7 such that C5b-7 is still able to bind to the lipid bilayer, and clusterin affects the subsequent binding of other complement proteins involved in the MAC assembly. Taken together, the findings in this study outline a biomimetic approach based on supported lipid bilayers to explore the interactions between complement proteins and inhibitors, thereby offering insight into MAC assembly and regulation.

Genetic Association of the Porcine C9 Complement Component with Hemolytic Complement Activity

The complement system is a part of the natural immune regulation mechanism against invading pathogens. Complement activation from three different pathways (classical, lectin, and alternative) leads to the formation of C5-convertase, an enzyme for cleavage of C5 into C5a and C5b, followed by C6, C7, C8, and C9 in membrane attack complex. The C9 is the last complement component of the terminal lytic pathway, which plays an important role in lysis of the target cells depending on its self-polymerization to form transmembrane channels. To address the association of C9 with traits related to disease resistance, the complete porcine C9 cDNA was comparatively sequenced to detect single nucleotide polymorphisms (SNPs) in pigs of the breeds Hampshire (HS), Duroc (DU), Berlin miniature pig (BMP), German Landrace (LR), Pietrain (PIE), and Muong Khuong (Vietnamese potbelly pig). Genotyping was performed in 417 F2 animals of a resource population (DUMI: DU×BMP) that were vaccinated with Mycoplasma hyopneumoniae, Aujeszky diseases virus and porcine respiratory and reproductive syndrome virus at 6, 14 and 16 weeks of age, respectively. Two SNPs were detected within the third exon. One of them has an amino acid substitution. The European porcine breeds (LR and PIE) show higher allele frequency of these SNPs than Vietnamese porcine breed (MK). Association of the substitution SNP with hemolytic complement activity indicated statistically significant differences between genotypes in the classical pathway but not in the alternative pathway. The interactions between eight time points of measurement of complement activity before and after vaccinations and genotypes were significantly different. The difference in hemolytic complement activity in the both pathways depends on genotype, kind of vaccine, age and the interaction to the other complement components. These results promote the porcine C9 (pC9) as a candidate gene to improve general animal health in the future.

CD59 signaling and membrane pores drive Syk-dependent erythrocyte necroptosis

Mature erythrocytes (red blood cells (RBCs)) undergo the programmed cell death (PCD) pathway of necroptosis in response to bacterial pore-forming toxins (PFTs) that target human CD59 (hCD59) but not hCD59-independent PFTs. Here, we investigate the biochemical mechanism of RBC necroptosis with a focus on the mechanism of induction and the minimal requirements for such RBC death. Binding or crosslinking of the hCD59 receptor led to Syk-dependent induction of vesiculated morphology (echinocytes) that was associated with phosphorylation of Band 3 and was required for Fas ligand (FasL) release. FasL-dependent phosphorylation of receptor-interacting protein kinase 1 (RIP1) in combination with plasma membrane pore formation was required for execution of RBC necroptosis. RIP1 phosphorylation led to the phosphorylation of RIP3, which was also critical for RBC necroptosis. Notably, RBC necroptosis was mediated by FasL and not by other candidate inducers, including tumor necrosis factor alpha (TNF-α) and TNF-related apoptosis-inducing ligand (TRAIL). Other types of RBC damage, such as eryptotic damage, failed to induce necroptosis when combined with hCD59 crosslinking. This work sheds light on the requirements for this recently discovered PCD in RBCs and provides a clear picture of the biochemical mechanism of induction of RBC necroptosis.

The assembly dynamics of the cytolytic pore toxin ClyA

Pore-forming toxins are protein assemblies used by many organisms to disrupt the membranes of target cells. They are expressed as soluble monomers that assemble spontaneously into multimeric pores. However, owing to their complexity, the assembly processes have not been resolved in detail for any pore-forming toxin. To determine the assembly mechanism for the ring-shaped, homododecameric pore of the bacterial cytolytic toxin ClyA, we collected a diverse set of kinetic data using single-molecule spectroscopy and complementary techniques on timescales from milliseconds to hours, and from picomolar to micromolar ClyA concentrations. The entire range of experimental results can be explained quantitatively by a surprisingly simple mechanism. First, addition of the detergent n-dodecyl-β-D-maltopyranoside to the soluble monomers triggers the formation of assembly-competent toxin subunits, accompanied by the transient formation of a molten-globule-like intermediate. Then, all sterically compatible oligomers contribute to assembly, which greatly enhances the efficiency of pore formation compared with simple monomer addition.

Protein arcs may form stable pores in lipid membranes

Electron microscopy and atomic force microscopy images of cholesterol-dependent cytolysins and related proteins that form large pores in lipid membranes have revealed the presence of incomplete rings, or arcs. Some evidence indicates that these arcs are inserted into the membrane and induce membrane leakage, but other experiments seem to refute that. Could such pores, only partially lined by protein, be kinetically and thermodynamically stable? How would the lipids be structured in such a pore? Using the antimicrobial peptide protegrin-1 as a model, we test the stability of pores only partially lined by peptide using all-atom molecular dynamics simulations in POPC and POPE/POPG membranes. The data show that, whereas pure lipid pores close rapidly, pores partially lined by protegrin arcs are stable for at least 300 ns. Estimates of the thermodynamic stability of these arcs using line tension data and implicit solvent calculations show that these arcs can be marginally stable in both zwitterionic and anionic membranes. Arcs provide an explanation for the observed ion selectivity in protegrin electrophysiology experiments and could possibly be involved in other membrane permeabilization processes where lipids are thought to participate, such as those induced by antimicrobial peptides and colicins, as well as the Bax apoptotic pore.

The making of a macromolecular machine: assembly of the membrane attack complex

The complement terminal pathway clears pathogens by generating cytotoxic membrane attack complex (MAC) pores on target cells. For more than 40 years, biochemical and cellular assays have been used to characterize the lytic nature of the MAC and to define its protein composition. Although models for pore formation have been inferred from structures of bacterial cytolysins, it was only recently that we were able to visualize how complement components come together during MAC assembly. This review highlights structural analyses of terminal pathway complexes to explore molecular mechanisms underlying MAC formation.

Structural biology of the membrane attack complex

The complement system is an intricate network of serum proteins that mediates humoral innate immunity through an amplification cascade that ultimately leads to recruitment of inflammatory cells or opsonisation or killing of pathogens. One effector arm of this network is the terminal pathway of complement, which leads to the formation of the membrane attack complex (MAC) composed of complement components C5b, C6, C7, C8 and C9. Upon formation of C5 convertases via the classical or alternative pathways of complement activation, C5b is generated from C5 by proteolytic cleavage, nucleating a series of association and polymerisation reactions of the MAC-constituting complement components that culminate in pore formation of pathogenic membranes. Recent structures of MAC components and homologous proteins significantly increased our understanding of oligomerisation, membrane association and integration, shedding light onto the molecular mechanism of this important branch of the innate immune system.

Assembly and regulation of the membrane attack complex based on structures of C5b6 and sC5b9

Activation of the complement system results in formation of membrane attack complexes (MACs), pores that disrupt lipid bilayers and lyse bacteria and other pathogens. Here, we present the crystal structure of the first assembly intermediate, C5b6, together with a cryo-electron microscopy reconstruction of a soluble, regulated form of the pore, sC5b9. Cleavage of C5 to C5b results in marked conformational changes, distinct from those observed in the homologous C3-to-C3b transition. C6 captures this conformation, which is preserved in the larger sC5b9 assembly. Together with antibody labeling, these structures reveal that complement components associate through sideways alignment of the central MAC-perforin (MACPF) domains, resulting in a C5b6-C7-C8β-C8α-C9 arc. Soluble regulatory proteins below the arc indicate a potential dual mechanism in protection from pore formation. These results provide a structural framework for understanding MAC pore formation and regulation, processes important for fighting infections and preventing complement-mediated tissue damage.

Structure of complement C6 suggests a mechanism for initiation and unidirectional, sequential assembly of membrane attack complex (MAC)

The complement membrane attack complex (MAC) is formed by the sequential assembly of C5b with four homologous proteins as follows: one copy each of C6, C7, and C8 and 12-14 copies of C9. Together these form a lytic pore in bacterial membranes. C6 through C9 comprise a MAC-perforin domain flanked by 4-9 "auxiliary" domains. Here, we report the crystal structure of C6, the first and longest of the pore proteins to be recruited by C5b. Comparisons with the structures of the C8αβγ heterodimer and perforin show that the central domain of C6 adopts a "closed" (perforin-like) state that is distinct from the "open" conformations in C8. We further show that C6, C8α, and C8β contain three homologous subdomains ("upper," "lower," and "regulatory") related by rotations about two hinge points. In C6, the regulatory segment includes four auxiliary domains that stabilize the closed conformation, inhibiting release of membrane-inserting elements. In C8β, rotation of the regulatory segment is linked to an opening of the central β-sheet of its clockwise partner, C8α. Based on these observations, we propose a model for initiation and unidirectional propagation of the MAC in which the auxiliary domains play key roles: in the assembly of the C5b-8 initiation complex; in driving and regulating the opening of the β-sheet of the MAC-performin domain of each new recruit as it adds to the growing pore; and in stabilizing the final pore. Our model of the assembled pore resembles those of the cholesterol-dependent cytolysins but is distinct from that recently proposed for perforin.

Structure of C8alpha-MACPF reveals mechanism of membrane attack in complement immune defense

Membrane attack is important for mammalian immune defense against invading microorganisms and infected host cells. Proteins of the complement membrane attack complex (MAC) and the protein perforin share a common MACPF domain that is responsible for membrane insertion and pore formation. We determined the crystal structure of the MACPF domain of complement component C8alpha at 2.5 angstrom resolution and show that it is structurally homologous to the bacterial, pore-forming, cholesterol-dependent cytolysins. The structure displays two regions that (in the bacterial cytolysins) refold into transmembrane beta hairpins, forming the lining of a barrel pore. Local hydrophobicity explains why C8alpha is the first complement protein to insert into the membrane. The size of the MACPF domain is consistent with known C9 pore sizes. These data imply that these mammalian and bacterial cytolytic proteins share a common mechanism of membrane insertion.

Direct and Simultaneous Identification of Mycobacterium tuberculosis complex (MTBC) and Mycobacterium tuberculosis (MTB) by Rapid Multiplex nested PCR-ICT assay

The Mycobacterium tuberculosis (MTB) shows different virulence and host infection range from other members of the M. tuberculosis complex (MTBC). Differential identification of MTB from MTBC is thus important in certain occasions. The currently commercially available molecular assays which use either IS6110 or 16S rDNA fragment as identification targets are mainly designed for identifying MTBC but not for MTB. Comparative genomic DNA analysis has provided valuable information on regions of difference (RD) present in MTB but not in other members of the MTBC. RD9 region is further suggested to be a potential target for differential identification of MTB from MTBC. In this study, using IS6110 and Rv3618 (belong to RD9) as the specific identification targets for MTBC and MTB, respectively, we developed and tested a multiplex nested PCR-ICT (immuno-chromatography test) assay for simultaneously and directly detecting not only MTBC but also MTB from 1500 clinical sputum specimens. The results were compared with traditional culture and biochemical identification results together with patients' clinical assessments. This assay showed a 95.5% sensitivity, 97.9% specificity, 2.1% false positive rate and 4.5% false negative rate towards detection of MTBC, and a 93.0% sensitivity, 99.8% specificity, 0.2% false positive rate and 7.0% false negative rate for detection of MTB. This detection system shows great potential in clinical application.

Development of an improved PCR-ICT hybrid assay for direct detection of Legionellae and Legionella pneumophila from cooling tower water specimens

A novelly improved polymerase chian reaction and immunochromatography test (PCR-ICT) hybrid assay comprising traditional multiplex-nested PCR and ICT, (a lateral-flow device) was developed for direct detection of Legionella bacteria from environmental cooling tower samples. The partial 16S rDNA (specific for Legionella spp.) and dnaJ (specific for Legionella pneumophila) genes from Legionella chromosome were first specifically amplified by multiplex-nested PCR, respectively, followed by detection using ICT strip. Reading of results was based on presence or absence of the two test lines on the strips. Presence of test line 1 indicated existence of Legionella spp. specific 16S rDNA and identified Legionella spp. Presence of test line 2 further indicated existence of dnaJ and thus specifically identified L. pneumophila. In contrast, for non-Legionellae bacteria no test line formation was observed. Results of direct detection of Legionella bacteria and L. pneumophila from water tower specimens by this assay showed 100% sensitivity, and 96.6% and 100% specificity, respectively compared with traditional culture, biochemical and serological identification methods. The PCR-ICT hybrid assay does not require sophisticated equipment and was proved to be practically useful in rapid and direct Legionellae detection from environmental water samples.

Inactivation and Activity of Cholesterol-Dependent Cytolysins: What Structural Studies Tell Us

The homologous bacterially expressed cholesterol-dependent cytolysins (CDCs) form pores via oligomerization; this must occur preferentially once the target membrane has been engaged. Conformational changes in CDCs then drive partition from an aqueous environment to a lipidic one. This review addresses how premature oligomerization is prevented, how conformational changes are triggered, and how cooperativity between subunits brings about new functionality absent from isolated protomers. Variations are found in the answers provided by the CDCs to these issues. Some toxins use pH as a trigger of activity, but recent results have shown that dimerization in solution is an alternative way of preventing premature oligomerization, in particular for the CDC from Clostridium perfringens, perfringolysin. More controversially, there is still no resolution to the debate as to whether incomplete (arciform) oligomers form pores: recent results again suggest that they do.

Cardiac involvement in non-human primates infected with the Lyme disease spirochete Borrelia burgdorferi

To investigate cardiac involvement in the non-human primate (NHP) model of Lyme disease, we inoculated 39 adult Macaca mulatta with Borrelia burgdorferi sensu stricto strains N40 (BbN40) by needle (N=22, 14 immunocompetent (IC), seven permanently immunosuppressed (IS), and four transiently immunosuppressed (TISP)) or by tick-bite (N=4, all TISP) or strain 297 (Bb297) by needle (N=2 IS), or with B. garinii strains Pbi (N=4, 2 TISP and 2 IS), 793 (N=2, TISP) or Pli (N=2, TISP). Five uninfected NHPs were used as controls. Infection and inflammation was studied in the hearts and the aorta removed at necropsy 2-32 months after inoculation by (1) H&E and trichrome-staining; (2) immunohistochemistry and digital image analysis; (3) Western blot densitometry; and (4) TaqMan RT-PCR. All NHPs inoculated with BbN40 became infected and showed carditis at necropsy. The predominant cells were T cells, plasma cells, and macrophages. There was increased IgG and IgM in the heart independent of immunosuppression. The B-cell chemokine BLC was significantly increased in IS-NHPs. There was increased deposition of the complement membrane attack complex (MAC) in TISP and IS-NHPs. The spirochetal load was very high in all BbN40-inoculated IS-NHPs but minimal if any in IC or TISP NHPs. Double-immunostaining revealed that many spirochetes in the heart of BbN40-IS NHPs had MAC on their membranes. We conclude that carditis in NHPs infected with B. burgdorferi is frequent and can persist for years but is mild unless they are immunosupressed.

The family of thiol-activated, cholesterol-binding cytolysins

Several species of both pathogenic and non-pathogenic grampositive bacteria within the genera Streptococcus, Clostridium and Bacillus secrete cytolytic proteins that belong to a single, highly homologous family. The most widely known members of this family are streptolysin O, listeriolysin, perfringolysin, and pneumolysin. These toxins specifically require membrane cholesterol but, apparently, do not depend on any other specific cell surface receptor, so that they are able to lyse the cytoplasmic membranes of virtually any animal cell. Upon binding as monomers, they oligomerize to form large pores with up to 30 nm internal diameter. These are the largest pores known, permitting permeation not only of ions and small metabolites but also of macromolecules. The latter property renders these toxins useful tools in cell biology. While several of these cytolysins have been shown to be determinants of bacterial pathogenicity, their biological roles may vary, as do the lifestyles of the bacteria secreting them. A unique function is surely fulfilled by listeriolysin O, which helps the intracellular pathogen Listeria monocytogenes escape from phagolysosomes and then spread to adjacent host cells.

Human complement protein C8 gamma

Human C8 gamma is a 22 kDa subunit of complement component C8, which is one of five components (C5b, C6, C7, C8, C9) that interact to form the cytolytic membrane attack complex (MAC) of complement. C8 contains three nonidentical subunits (alpha, beta, gamma) that are products of different genes. These subunits are arranged asymmetrically to form a disulfide-linked C8 alpha-gamma dimer that is noncovalently associated with C8 beta. C8 alpha and C8 beta are homologous to C6, C7 and C9 and together these proteins comprise what is referred to as the 'MAC protein family'. By comparison, C8 gamma is distinct in that it belongs to the lipocalin family of small, secreted proteins which have the common ability to bind small hydrophobic ligands. While specific roles have been identified for C8 alpha and C8 beta in the formation and function of the MAC, a function for C8 gamma and the identity of its ligand are unknown. This review summarizes the current status of C8 gamma structure and function and the progress made from efforts to determine its role in the complement system.

The four terminal components of the complement system are C-mannosylated on multiple tryptophan residues

C-Mannosylation is a unique form of protein glycosylation, involving the C-glycosidic attachment of a mannosyl residue to the indole moiety of Trp. In the two examples found so far, human RNase 2 and interleukin-12, only the first Trp in the recognition motif WXXW is specifically C-mannosylated. To establish the generality of protein C-mannosylation, and to learn more about its mechanism, the terminal components of the human complement system (C6, C7, C8,and C9), which contain multiple and complex recognition motifs, were examined. Together with C5b they form the cytolytic agent, the membrane attack complex. These are the first proteins that are C-mannosylated on more than one Trp residue as follows: six in C6, four in C7, C8alpha, and C8beta, and two in C9. Thus, from the 113 Trp residues in the complete membrane attack complex, 50 were found to undergo C-mannosylation. The other important finding is that in C6, C7, C8, and C9 Trp residues without a second Trp (or another aromatic residue) at the +3 position can be C-mannosylated. This shows that they must contain an additional C-mannosylation signal. Whether this is encoded in the primary or tertiary structure is presently unknown. Finally, all modified Trp residues are part of the highly conserved core of the thrombospondin type 1 repeats present in these proteins. Since this module has been found in a large number of other proteins, the results suggest further candidates for C-mannosylation.

The dual role of lipopolysaccharide as effector and target molecule

Lipopolysaccharides (LPS) are major integral components of the outer membrane of Gram-negative bacteria being exclusively located in its outer leaflet facing the bacterial environment. Chemically they consist in different bacterial strains of a highly variable O-specific chain, a less variable core oligosaccharide, and a lipid component, termed lipid A, with low structural variability. LPS participate in the physiological membrane functions and are, therefore, essential for bacterial growth and viability. They contribute to the low membrane permeability and increase the resistance towards hydrophobic agents. They are also the primary target for the attack of antibacterial drugs and proteins such as components of the host's immune response. When set free LPS elicit, in higher organisms, a broad spectrum of biological activities. They play an important role in the manifestation of Gram-negative infection and are therefore termed endotoxins. Physico-chemical parameters such as the molecular conformation and the charges of the lipid A portion, which is responsible for endotoxin-typical biological activities and is therefore termed the 'endotoxic principle' of LPS, are correlated with the biological activity of chemically different LPS.

The membrane attack complex of complement. Assembly, structure and cytotoxic activity

The membrane attack complex of complement is formed by the molecular fusion of the five terminal complement proteins, C5, C6, C7, C8, and C9. While the assembly process on a target membrane and its modulation by restriction factors present on host cells is now quite well understood the molecular details of the architecture of the complex still need much further clarification. This is especially true for the interaction of the last acting protein C9, which provides the cytotoxic action of the complex, with the precursor C5b-8 complex. Because of this lack of structural details the molecular mechanisms that lead to complement-mediated cell death remain cryptic, however, it is hoped that recent advances in controlling the assembly process and in site-specific modification of the terminal complement proteins by recombinant DNA techniques should change this predicament quickly.

The size, shape and stability of complement component C9

Electron microscopy of specimens of C9 tilted through 90 degrees visualized this protein to be a globular ellipsoid with dimensions of 77 x 70 x 52 A. To check the congruence of this observation with physical properties of the molecule, hydrodynamic parameters for C9 were determined. From this work a frictional ratio of 1.32 was calculated. C9 was compared with several other proteins of similar frictional ratios whose tertiary structures are known. All examples found of such proteins whose frictional ratios were between 1.26 and 1.37 are either heart-shaped or globular ellipsoids, but none are prolate ellipsoids. By comparison the size and shape of C9 determined by electron microscopy are congruent with its hydrodynamic parameters. Both electron microscopy and physical measurements suggest that the length (110-120 A) of C9 determined by neutron and X-ray scattering experiments is an overestimate. The source of the discrepancy was identified by the demonstration that the high concns of C9 employed in neutron and X-ray scattering work lead to aggregation of the protein. Thus, investigations involving neutron and X-ray scattering were measuring polydisperse solutions of C9. The deduced value of the radius of gyration from that work (33-35 A) is now recognized as being statistical and significantly higher than the correct value of monomeric C9 (26 A), which was calculated from electron microscopy measurements. Also high-resolution electron microscopy clearly visualized poly(C9) to be a barrel-stave construct. These results suggest that monomeric C9 must undergo a major conformational alteration to extend by 55-70 A in order to self-associate laterally in order to fashion the cylindrical poly(C9).

Membrane damage: common mechanisms of induction and prevention

Common features in the induction of pores by various agents are as follows: induction is stochastic and progressive; damage by different agents is often synergistic and limited. The prevention of membrane damage is affected by trivalent and divalent cations, by low pH, by low ionic strength and by high osmotic pressure. The inhibitory role of protons and divalent cations is considered in greater detail: pore-forming agents can be classified into two groups: channels across planar lipid bilayers induced by the first group display voltage-sensitive, reversible inhibition by divalent cations; channels of the second group show voltage-insensitive, irreversible inhibition by divalent cations. A search for the ligands to which divalent cations and protons bind has proved elusive. Comparison with the phenomenon of 'surface conductance' through narrow apertures, that is manifest in the absence of any pore-forming agent, may prove fruitful.

Assembly of macromolecular pores by immune defense systems

Immune defence systems (complement, cytolytic lymphocytes) make use of transmembrane pores assembled from up to 20 soluble monomers in a highly regulated process to induce cell death. Inhibitors of pore formation have been found which protect blood, endothelial and epithelial cells from the destructive effect of complement lesions. Recently, a pore-forming protein showing immunological crossreactivity to complement C9 has been found in the protozoan parasite Trypanosoma cruzi, thereby extending this protein family and generalizing its means of generating non-selective membrane permeability.

Small colloidal gold conjugated to Fab fragments or to immunoglobulin G as high-resolution labels for electron microscopy: a technical overview

Fab-colloidal gold labelling in conjunction with negative staining and high-resolution electron microscopy was used for targeting single protein units in regular arrays. These were bacteriophage T4 polyheads with Fab-Au2.5, and a specific antibody binding site on the haemagglutinin polypeptide of influenza virus with Fab-Au3, Fab-Au2.5, and Fab-Au1-2. For the latter, IgG-Au3 was also used. Experimental details are summarized to provide generally applicable methods for the preparation of small gold colloids Fab-Au and of labelling. The putative mechanism of protein-gold complex formation and adsorption to preferred sites on Fab and IgG, most probably to sulphur-rich regions, is discussed. The influence of pH during complex formation was found to be of minor importance in the samples investigated. Reported experimental details and our own experiences suggest that the importance of a protein's pI relative to its optimum gold complexing pH critically depends on the nature of the protein in question rather than being of general importance for protein-gold complex stability.

Several epitopes on native human complement C9 are involved in interaction with the C5b-8 complex and other C9 molecules

Ten monoclonal antibodies (mAb) against native human C9 exhibiting various inhibitory effects on the hemolytic activity of C9 (Bausback, J., Kontermann, R. and Rauterberg, E. W., Immunobiology 1988. 178: 58) were further analyzed regarding their reactivities with monomeric C9 (mC9), polymerized C9 (pC9), and the non-lytic SC5b-9 complex in enzyme-linked immunosorbent assay and with the membrane attack complex (MAC) generated on rabbit erythrocytes analyzed by flow cytometry. In addition, the inhibitory effects of mAb on zinc-induced C9 polymerization were investigated. One epitope of the C-terminal half of C9b exposed on the surface of pC9 and the MAC seems not to participate directly in lytic function or polymerization since no inhibitory effect of the respective mAb was observed. The nine other mAb directed against epitopes of the C9a part exhibit various inhibitory potentials. The mAb inhibit either hemolysis or polymerization, or both processes. Due to the reactivity with the tested antigens the mAb can be divided into two groups. mAb of the first group bind with nearly the same affinity to all four antigens, whereas mAb of the second group react preferentially with mC9 while their affinity to pC9, SC5b-9 and the MAC is reduced. Comparison of reaction patterns and inhibitory effects strongly suggest that different epitopes on the surface of native C9 are involved in interaction of C9 with C5b-8 and/or in C9-C9 interaction. The finding that mAb inhibiting polymerization of C9 in vitro have no inhibitory effect on hemolysis confirms that C9 polymers are no prerequisite for lysis.

N-deglycosylation of human complement component C9 reduces its hemolytic activity

The effect of enzymatic deglycosylation of human complement component C9 on its hemolytic activity was investigated. Treatment of native C9 (Mr 71,000) with glyocpeptidase F (PNGase F) results in a stepwise decrease of the mol. wt. The formation of an Mr 67,000 peptide which is further converted to Mr 63,000 suggests that there are two N-linked carbohydrate chains per C9 polypeptide. Removal of approximately 88% of the N-linked oligosaccharides results in 80% reduction of the hemolytic activity (CH50). The completely N-deglycosylated Mr 63,000 peptide contains a remaining amount of 25% of the total carbohydrates of native C9. These glycans are assumed to be O-linked and predominantly attached to the C9a part of C9. The electrophoretic mobility of C9 is not affected by endoglycosidase F or H treatments revealing that the two N-linked glycans are of the tri- or tetra-antennary complex type. Cleavage of terminal sialic acids from native C9 by neuraminidase results in an Mr 67,000 product with nearly unaltered hemolytic activity. In contrast to other glycoproteins in which deglycosylation remained without major effects on their functional activity, our findings suggest that the N-linked carbohydrates are required for full expression of hemolytic activity of C9.

Amphotericin B and Nystatin show different activities on sterol-free vesicles

It has generally been assumed that the polyene antibiotics Nystatin and Amphotericin B cause membrane damage by the same mechanism. However, using kinetic fluorescence methods we have found that AmB and Nystatin have very different activities on sterol-free dioleoyl phosphatidylcholine and egg phosphatidylcholine small unilamellar vesicles. At very low AmB concentrations (less than 1/1000 lipids in egg phosphatidylcholine) significant K+ permeability enhancement is observed. However, even at very high Nystatin to lipid ratios (1/100) very little K+ current is induced, particularly in dioleoyl phosphatidylcholine vesicles. The novel technique described here uses a K+/H+ exchange mechanism to detect minute transmembrane K+ currents by monitoring internal membrane vesicle pH changes with pyranine.