Structure of human complement C8, a precursor to membrane attack

Combining sandblasting and pink anodisation of Ti implants as a promising method for improving fibroblast adhesion and immune response

This study examined the effect of combining the sandblasting and anodising of titanium alloys used in implants on the cell response and protein adsorption patterns. The titanium samples were divided into four groups depending on the surface treatment: machining (MC), pink anodisation (PA), sandblasting (MC04) and a combination of the last two (MC04 + PA). Their physicochemical properties were analysed by SEM/EDX, Raman, contact angle measurements and profilometry. In vitro responses were examined using human gingival fibroblastic (HGF) cells and THP-1 macrophages. Cytokine secretion, macrophage adhesion and gene expression were measured by ELISA, confocal microscopy and RT-PCR. Cell adhesion and collagen secretion were evaluated in HGF cultures. The adsorption of immune and regenerative proteins onto the surfaces was assessed employing nLC-MS/MS. MC04 + PA surfaces exhibited a change in the roughness, chemical composition and hydrophilicity of the material, showing more elongated HGF cells and a considerable increase in the area of cells exposed to the MC04 + PA surfaces. Moreover, cells cultured on MC04 + PA generally showed a reduction in the expression of proinflammatory genes (TNF-α, MCP-1, C5, NF-kB and ICAM-1) and an increase in the secretion of anti-inflammatory cytokines, such as IL-4. These results correlated with the proteomic data; we found preferential adsorption of proteins favouring cell adhesion, such as DSC1 and PCOC1. A considerable reduction in the adsorption of immunoglobulins and proteins associated with acute inflammatory response (including SAA4) was also observed. The study highlights the potential advantages of MC04 + PA surface treatment to modify dental implant abutments; it enhances their compatibility with soft tissues and reduces the inflammatory response.

Hearing loss and its association with the proteome of perilymph, cerebrospinal fluid, and tumor tissue in patients with vestibular schwannoma

This study examined the association between hearing loss in sporadic vestibular schwannoma patients and the proteome of perilymph (PL), cerebrospinal fluid (CSF), and vestibular schwannoma. Intraoperative sampling of PL and of CSF, and biopsy of vestibular schwannoma tissue, was performed in 32, 32, and 20 patients with vestibular schwannoma, respectively. Perilymph and CSF in three patients with meningioma and normal hearing were also sampled. The proteomes were identified by liquid chromatography coupled to high-resolution tandem mass spectrometry. Preoperative hearing function of the patients was evaluated with pure tone audiometry, with mean values at frequencies of 500, 1000, 2000, and 4000 Hz (PTA4) in the tumor-affected ear used to delineate three hearing groups. Analysis of the PL samples revealed significant upregulation of complement factor H-related protein 2 (CFHR2) in patients with severe to profound hearing loss after false discovery rate correction. Pathway analysis of biofunctions revealed higher activation scores in the severe/profound hearing loss group of leukocyte migration, viral infection, and migration of cells in PL. Upregulation of CFHR2 and activation of these pathways indicate chronic inflammation in the cochlea of vestibular schwannoma patients with severe to profound hearing loss compared with patients with normal hearing or mild hearing loss.

The Impact of Acute EBV Infection on Changes in the Serum Proteome in Children-A Pilot Study

This study investigates the impact of Epstein-Barr virus (EBV) infection on children's proteomes across different phases of the disease, utilising seventy-nine blood samples categorised into three groups: EBV-naive patients, acute infectious mononucleosis (IM) cases, and convalescents followed up for 12 months post-IM. The aim is to identify proteins influenced by EBV infection, shedding light on the chronic processes triggered by the virus. The results reveal thirty-nine proteins distinguishing between naive patients and those with IM, including actin, lumican, peroxiredoxin-2, fibulin-1, gelsolin, and alpha-2-macroglobulin, which are involved in immune responses, cell adhesion, and inflammation. Elevated oxidative stress markers like peroxiredoxin-2 in IM patients suggest potential links to EBV's induction of reactive oxygen species. Increased levels of apolipoproteins A-I, A-IV, C-IV, and M during IM imply associations with viral infection, while complement system proteins (C1q, C1r, and C8 gamma chain) are also elevated, reflecting their role in the immune response and viral clearance. This study's focus on children provides unique insights into EBV's impact on young populations, emphasising proteomics' role in uncovering protein associations and understanding the virus's long-term consequences. However, specific relationships between identified proteins and EBV infection require further investigation.

Bioreactor-based stem cell therapy for liver fibrosis

Stem cell therapy, achieved using mesenchymal stem cells (MSCs), has been highlighted for the treatment of liver fibrosis. Infusion into the circulatory system is a traditional application of MSCs; however, this approach is limited by phenotypic drift, stem cell senescence, and vascular embolism. Maintaining the therapeutic phenotype of MSCs while avoiding adverse infusion-related reactions is the key to developing next-generation stem cell therapy technologies. Here, we propose a bioreactor-based MSCs therapy to avoid cell infusion. In this scheme, 5% liver fibrosis serum was used to induce the therapeutic phenotype of MSCs, and a fluid bioreactor carrying a co-culture system of hepatocytes and MSCs was constructed to produce the therapeutic medium. In a rat model of liver fibrosis, the therapeutic medium derived from the bioreactor significantly alleviated liver fibrosis. Therapeutic mechanisms include immune regulation, inhibition of hepatic stellate cell activation, establishment of hepatocyte homeostasis, and recovery of liver stem cell subsets. Overall, the bioreactor-based stem cell therapy (scheme) described here represents a promising new strategy for the treatment of liver fibrosis and will be beneficial for the development of 'cell-free' stem cell therapy.

Systemic immune parameters after prior radiation therapy in patients receiving immune checkpoint inhibitors

INTRODUCTION

Preclinical studies have demonstrated the ability of radiation therapy (RT) to augment immune response and tumor control by immune checkpoint inhibitors (ICI). However, numerous clinical trials combining RT and ICI have yielded relatively disappointing results. To improve understanding of optimal use of these therapies, we assessed systemic immune effects of prior RT in patients receiving ICI.

METHODS AND MATERIALS

Pre- and post-ICI blood samples were collected from patients enrolled in a prospective immunotherapy biospecimen protocol. Mutiplex panels of 40 cytokines and 120 autoantibodies (Ab) were analyzed. We identified differences in these parameters according to receipt, timing, and type of prior RT. We calculated P values using the Pearson product-moment correlation coefficient and false discovery rate (FDR) using the Benjamini-Hochberg Procedure.

RESULTS

Among 277 total patients, 69 (25%) received RT in the 6 months prior to ICI initiation. Among RT-treated patients, 23 (33%) received stereotactic RT, and 33 (48%) received curative intent RT. There was no significant difference in demographics or type of immunotherapy between patients according to prior RT exposure. Baseline complement C8 Ab and MIP-1d/CCL15 were significantly higher among patients with prior RT. For MIP-1d/CCL15, only prior stereotactic RT was associated with significant differences.

CONCLUSIONS

Prior RT is associated with few changes in systemic immune parameters in patients receiving ICI. The underlying mechanisms and optimal approach to harnessing the potential synergy of RT and ICI require further prospective clinical investigation.

The impact of blood Transfusion on T Helper Cells and Cytokines in Transfusion-Refractory Patients: a Prospective Study

Red blood cell (RBC) transfusion can increase patients' hemoglobin levels and improve hypoxia. The factors affecting the transfusion efficacy include immune and nonimmune factors. The objective of this study was to explore the impact of blood transfusion on T helper (Th) cell ratios and levels of serum cytokines in RBC transfusion-refractory patients. In this prospective study, anemic patients receiving RBC transfusion were enrolled. Peripheral venous blood samples were extracted from patients before RBC transfusion and within 24 h after transfusion. Th cell ratios and levels of serum cytokines were detected by flow cytometry. Differences in Th cell ratios and levels of serum cytokines (IL-2, IL-4, IL-6, IL-10, TNF-α and IFN-γ) between pretransfusion and posttransfusion were compared. A total of 47 patients agreed to participate in this study. They were grouped according to incremental Hb levels, 20 (42.55%) patients were divided into the RBC transfusion refractory group, while 27 (57.45%) patients were in the validity group. The expected Hb increment was defined by a panel of Chinese experts. In RBC transfusion-refractory patients, Th1 and Th2 cell ratios increased while levels of serum IL-2 and IL-10 decreased after transfusion. In RBC transfusion validity patients, there were no significant changes in Th cell ratios or levels of serum cytokines between pretransfusion and posttransfusion. We found that Th1 and Th2 cell ratios increased while serum IL-2 and IL-10 levels decreased after transfusion in RBC-refractory patients.

Transcriptome analysis reveals similarities and differences in immune responses in the head and trunk kidneys of yellow catfish (Pelteobagrus fulvidraco) stimulated with Aeromonas hydrophila

Teleosts have a unique immune system because their head kidney (HK) and trunk kidney (TK) are sites for hematopoiesis. However, the immune functions of the HK and TKs require further elucidation in yellow catfish (Pelteobagrus fulvidraco). In the present study, imprints of the HK and TK were examined using the Wright's-Giemsa staining method. Morphological characteristics of the blood cell lineages revealed that the HK and TK were hematopoietic organs. To explore its immune function, transcriptome sequencing was performed after infection with Aeromonas hydrophila. A total of 1139 genes showed significant alterations in their expression in the kidney; these genes included 737 upregulated and 402 downregulated genes. Furthermore, 1117 differentially expressed genes were observed in the HK, which included 784 upregulated and 333 downregulated genes. Both organs showed 357 upregulated genes and 85 downregulated genes. Some immune-related genes were only expressed in the TK, such as ATP-dependent RNA helicase DDX58, the gene encoding the immunoglobulin heavy chain and light chain. The immune responses in the HK and TK were differential and the TK played a critical role in the mechanism underlying the immune response. The purpose of the present study was to facilitate the elucidation of the immune defense mechanism of yellow catfish and other teleosts.

Implications of the Essential Role of Small Molecule Ligand Binding Pockets in Protein-Protein Interactions

Protein-protein interactions (PPIs) and protein-metabolite interactions play a key role in many biochemical processes, yet they are often viewed as being independent. However, the fact that small molecule drugs have been successful in inhibiting PPIs suggests a deeper relationship between protein pockets that bind small molecules and PPIs. We demonstrate that 2/3 of PPI interfaces, including antibody-epitope interfaces, contain at least one significant small molecule ligand binding pocket. In a representative library of 50 distinct protein-protein interactions involving hundreds of mutations, >75% of hot spot residues overlap with small molecule ligand binding pockets. Hence, ligand binding pockets play an essential role in PPIs. In representative cases, evolutionary unrelated monomers that are involved in different multimeric interactions yet share the same pocket are predicted to bind the same metabolites/drugs; these results are confirmed by examples in the PDB. Thus, the binding of a metabolite can shift the equilibrium between monomers and multimers. This implicit coupling of PPI equilibria, termed "metabolic entanglement", was successfully employed to suggest novel functional relationships among protein multimers that do not directly interact. Thus, the current work provides an approach to unify metabolomics and protein interactomics.

A single-cell lung atlas of complement genes identifies the mesothelium and epithelium as prominent sources of extrahepatic complement proteins

To understand functional duality of the complement system in host defense and lung injury, a more comprehensive view of its localized production in the lung, and the impact of age on complement production are essential. Here, we explored the expression of complement genes through computational analysis of preexisting single cell RNA sequencing data from lung transcriptomes of healthy young (3 months) and old C57BL/6 mice (24 months), and humans. We characterized the distribution of 48 complement genes. Across 28 distinct immune and non-immune cell types in mice, mesothelial cells expressed the greatest number of complement genes (e.g., C1ra, C2, C3), and regulators (e.g., Serping1, Cfh). C5 was abundant in type II alveolar epithelial cells and C1q in interstitial lung macrophages. There were only moderate differences in gene expression between young and old mice. Among 57 human lung cell types, mesothelial cells showed abundant complement expression. A few differences in gene  expression (e.g., FCN1, CFI, C6, C7) were also evident between mice and human lung cells. Our findings present a novel perspective on the expression patterns of complement genes in normal lungs. These findings highlight the potential functions of complement in tissue-specific homeostasis and immunity and may foster a mechanistic understanding of its role in lung health and disease.

Quantitative proteomics reveal three potential biomarkers for risk assessment of acute myocardial infarction

Acute myocardial infarction (AMI) is the one of the main cause of death worldwide. Exosomes carry important information about intercellular communication and could be diagnostic marker for many diseases. Here, we aimed to find potential key proteins for the early diagnosis of AMI. A label free proteomics strategy was used to identify the differentially expressed proteins (DEPs) of AMI patients’ plasma exosome. By bioinformatics analysis and enzyme-linked immunosorbent assay to validate the candidate proteins. Compared to healthy control plasma exosome, we totally identified 72 differentially expressed proteins (DEPs) in AMI patients. Also, we found that complement and coagulation cascades was activated by KEGG analysis and GSEA. PLG, C8B and F2 were selected as candidate molecules for further study, and then validated another 40 plasma samples using enzyme-linked immunosorbent assay. Finally, we found that the expression levels of these three proteins (PLG, C8B and F2) were significantly higher than those of healthy controls (P < 0.05). ROC analysis revealed that PLG, C8B and F2 had potential value for AMI early diagnosis. In conclusion, our study identified three potential biomarkers for AMI diagnosis. But there remains a need to further study the mechanism of the biomarkers.

Comparative Analysis of Serum Proteins Between Hepatitis B Virus Genotypes B and C Infection by DIA-Based Quantitative Proteomics

Purpose

In clinical practice, the clinicopathological profiles and outcomes of patients infected with hepatitis B virus (HBV) are different between genotypes B and C. However, little is known about the potential mechanism and differences in specific biological pathways associated with the different genotype. This study aimed to compare the serum protein profile between patients infected with HBV genotype B and those infected with HBV genotype C.

Patients and Methods

A total of 54 serum samples from patients with chronic HBV genotype B infection and those with chronic HBV genotype C infection, and healthy controls were used for the proteomic analysis (n = 18 samples in per group). Serum proteomic profiles were analyzed using data-independent acquisition (DIA)-based liquid chromatography-mass spectrometry to identify differentially expressed proteins (up- or downregulation of at least 1.5-fold) between serum samples from HBV patients infected with HBV genotype B and those infected with genotype C.

Results

We identified 1010 proteins, 53 of which were differentially expressed between the serum samples of the healthy controls and those of HBV genotype B infected patients, and 59 that were differentially expressed between the samples of the healthy controls and those of HBV genotype C infected patients. Furthermore, our results indicated that two proteins identified as being differentially expressed (VWF and C8B) have potential as biomarkers for distinguishing genotype B infected HBV patients from those infected with genotype C.

Conclusion

The results of our DIA-based quantitative proteomic analysis revealed that HBV genotypes B and C are associated with different molecular profiles and may provide fundamental information for further detailed investigations of the molecular mechanism underlying these differences.

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.

Similarities and differences in the structures and proteoform profiles of the complement proteins C6 and C7

The human complement system provides a first line of defence against pathogens. It requires a well-orchestrated sequential assembly of an array of terminal complement components (C5, C6, C7, C8, and C9), ultimately forming the membrane attack complex (MAC). Although much information about MAC assembly is available, the structure of the soluble C7 has remained elusive. The complement proteins C7 and C6 share very high sequence homology and exhibit several conserved domains, disulphide bridges, and C-mannosylation sites. Here, we used an integrative structural MS-based approach combining native MS, glycopeptide-centric MS, in-gel cross-linking MS (IGX-MS) and structural modelling to describe structural features, including glycosylation, of human serum soluble C7. We compare this data with structural and glycosylation data for human serum C6. The new structural model for C7 shows that it adopts a compact conformation in solution. Although C6 and C7 share many similarities, our data reveals distinct O-, and N-linked glycosylation patterns in terms of location and glycan composition. Cumulatively, our data provide valuable new insight into the structure and proteoforms of C7, solving an essential piece of the puzzle in our understanding of MAC assembly.

Proteotyping of knockout mouse strains reveals sex- and strain-specific signatures in blood plasma

We proteotyped blood plasma from 30 mouse knockout strains and corresponding wild-type mice from the International Mouse Phenotyping Consortium. We used targeted proteomics with internal standards to quantify 375 proteins in 218 samples. Our results provide insights into the manifested effects of each gene knockout at the plasma proteome level. We first investigated possible contamination by erythrocytes during sample preparation and labeled, in one case, up to 11 differential proteins as erythrocyte originated. Second, we showed that differences in baseline protein abundance between female and male mice were evident in all mice, emphasizing the necessity to include both sexes in basic research, target discovery, and preclinical effect and safety studies. Next, we identified the protein signature of each gene knockout and performed functional analyses for all knockout strains. Further, to demonstrate how proteome analysis identifies the effect of gene deficiency beyond traditional phenotyping tests, we provide in-depth analysis of two strains, C8a-/- and Npc2+/-. The proteins encoded by these genes are well-characterized providing good validation of our method in homozygous and heterozygous knockout mice. Ig alpha chain C region, a poorly characterized protein, was among the differentiating proteins in C8a-/-. In Npc2+/- mice, where histopathology and traditional tests failed to differentiate heterozygous from wild-type mice, our data showed significant difference in various lysosomal storage disease-related proteins. Our results demonstrate how to combine absolute quantitative proteomics with mouse gene knockout strategies to systematically study the effect of protein absence. The approach used here for blood plasma is applicable to all tissue protein extracts.

To Kill But Not Be Killed: Controlling the Activity of Mammalian Pore-Forming Proteins

Pore-forming proteins (PFPs) are present in all domains of life, and play an important role in host-pathogen warfare and in the elimination of cancers. They can be employed to deliver specific effectors across membranes, to disrupt membrane integrity interfering with cell homeostasis, and to lyse membranes either destroying intracellular organelles or entire cells. Considering the destructive potential of PFPs, it is perhaps not surprising that mechanisms controlling their activity are remarkably complex, especially in multicellular organisms. Mammalian PFPs discovered to date include the complement membrane attack complex (MAC), perforins, as well as gasdermins. While the primary function of perforin-1 and gasdermins is to eliminate infected or cancerous host cells, perforin-2 and MAC can target pathogens directly. Yet, all mammalian PFPs are in principle capable of generating pores in membranes of healthy host cells which-if uncontrolled-could have dire, and potentially lethal consequences. In this review, we will highlight the strategies employed to protect the host from destruction by endogenous PFPs, while enabling timely and efficient elimination of target cells.

QTL Mapping of Intestinal Neutrophil Variation in Threespine Stickleback Reveals Possible Gene Targets Connecting Intestinal Inflammation and Systemic Health

Selection, via host immunity, is often required to foster beneficial microbial symbionts and suppress deleterious pathogens. In animals, the host immune system is at the center of this relationship. Failed host immune system-microbial interactions can result in a persistent inflammatory response in which the immune system indiscriminately attacks resident microbes, and at times the host cells themselves, leading to diseases such as Ulcerative Colitis, Crohn’s Disease, and Psoriasis. Host genetic variation has been linked to both microbiome diversity and to severity of such inflammatory disease states in humans. However, the microbiome and inflammatory states manifest as quantitative traits, which encompass many genes interacting with one another and the environment. The mechanistic relationships among all of these interacting components are still not clear. Developing natural genetic models of host-microbe interactions is therefore fundamental to understanding the complex genetics of these and other diseases. Threespine stickleback (Gasterosteus aculeatus) fish are a tractable model for attacking this problem because of abundant population-level genetic and phenotypic variation in the gut inflammatory response. Previous work in our laboratory identified genetically divergent stickleback populations exhibiting differences in intestinal neutrophil activity. We took advantage of this diversity to genetically map variation in an emblematic element of gut inflammation - intestinal neutrophil recruitment - using an F2-intercross mapping framework. We identified two regions of the genome associated with increased intestinal inflammation containing several promising candidate genes. Within these regions we found candidates in the Coagulation/Complement System, NFkB and MAPK pathways along with several genes associated with intestinal diseases and neurological diseases commonly accompanying intestinal inflammation as a secondary symptom. These findings highlight the utility of using naturally genetically diverse ‘evolutionary mutant models’ such as threespine stickleback to better understand interactions among host genetic diversity and microbiome variation in health and disease states.

Comparative proteomics identified immune response proteins involved in response to vaccination with heat-inactivated Mycobacterium bovis and mycobacterial challenge in cattle

There is an imperative need for effective control of bovine tuberculosis (BTB) on a global scale and vaccination of cattle may prove to be pivotal in achieving this. The oral and parenteral use of a heat-inactivated Mycobacterium bovis (M. bovis) vaccine has previously been found to confer partial protection against BTB in several species. A role for complement factor C3 has been suggested in wild boar, but the exact mechanism by which this vaccine provides protection remains unclear. In the present study, a quantitative proteomics approach was used to analyze the white blood cell proteome of vaccinated cattle in comparison to unvaccinated controls, prior (T0) and in response to vaccination, skin test and challenge (T9 and T12). The Fisher's exact test was used to compare the proportion of positive reactors to standard immunological assays for BTB (the BOVIGAM^® assay, IDEXX TB ELISA and skin test) between the vaccinated and control groups. Using reverse-phase liquid-chromatography tandem mass spectrometry (RP-LC-MS/MS), a total of 12,346 proteins were identified with at least two peptides per protein and the Chi²-test (P = 0.05) determined 1,222 to be differentially represented at the key time point comparisons. Gene ontology (GO) analysis was performed in order to determine the biological processes (BPs), molecular functions (MFs) and cell components (CCs) the proteins formed part of. The analysis was focused on immune system BPs, specifically. GO analysis revealed that the most overrepresented proteins in immune system BPs, were kinase activity and receptor activity molecular functions and extracellular, Golgi apparatus and endosome cell components and included complement factor C8α and C8β as well as toll-like receptors 4 (TLR4) and 9 (TLR9). Proteins of the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) (JAK-STAT) and protein kinase C (PKC) signaling pathways were furthermore found to potentially be involved in the immune response elicited by the inactivated vaccine. In conclusion, this study provides a first indication of the role of several immune system pathways in response to the heat-inactivated M. bovis vaccine and mycobacterial challenge.

Comprehensive Proteoform Characterization of Plasma Complement Component C8αβγ by Hybrid Mass Spectrometry Approaches

The human complement hetero-trimeric C8αβγ (C8) protein assembly (~ 150 kDa) is an important component of the membrane attack complex (MAC). C8 initiates membrane penetration and coordinates MAC pore formation. Here, we charted in detail the structural micro-heterogeneity within C8, purified from human plasma, combining high-resolution native mass spectrometry and (glyco)peptide-centric proteomics. The intact C8 proteoform profile revealed at least ~ 20 co-occurring MS signals. Additionally, we employed ion exchange chromatography to separate purified C8 into four distinct fractions. Their native MS analysis revealed even more detailed structural micro-heterogeneity on C8. Subsequent peptide-centric analysis, by proteolytic digestion of C8 and LC-MS/MS, provided site-specific quantitative profiles of different types of C8 glycosylation. Combining all this data provides a detailed specification of co-occurring C8 proteoforms, including experimental evidence on N-glycosylation, C-mannosylation, and O-glycosylation. In addition to the known N-glycosylation sites, two more N-glycosylation sites were detected on C8. Additionally, we elucidated the stoichiometry of all C-mannosylation sites in all the thrombospondin-like (TSP) domains of C8α and C8β. Lastly, our data contain the first experimental evidence of O-linked glycans located on C8γ. Albeit low abundant, these O-glycans are the first PTMs ever detected on this subunit. By placing the observed PTMs in structural models of free C8 and C8 embedded in the MAC, it may be speculated that some of the newly identified modifications may play a role in the MAC formation. Graphical Abstract ᅟ.

The membrane attack complex, perforin and cholesterol-dependent cytolysin superfamily of pore-forming proteins

The membrane attack complex and perforin proteins (MACPFs) and bacterial cholesterol-dependent cytolysins (CDCs) are two branches of a large and diverse superfamily of pore-forming proteins that function in immunity and pathogenesis. During pore formation, soluble monomers assemble into large transmembrane pores through conformational transitions that involve extrusion and refolding of two α-helical regions into transmembrane β-hairpins. These transitions entail a dramatic refolding of the protein structure, and the resulting assemblies create large holes in cellular membranes, but they do not use any external source of energy. Structures of the membrane-bound assemblies are required to mechanistically understand and modulate these processes. In this Commentary, we discuss recent advances in the understanding of assembly mechanisms and molecular details of the conformational changes that occur during MACPF and CDC pore formation.

Giant MACPF/CDC pore forming toxins: A class of their own

Pore Forming Toxins (PFTs) represent a key mechanism for permitting the passage of proteins and small molecules across the lipid membrane. These proteins are typically produced as soluble monomers that self-assemble into ring-like oligomeric structures on the membrane surface. Following such assembly PFTs undergo a remarkable conformational change to insert into the lipid membrane. While many different protein families have independently evolved such ability, members of the Membrane Attack Complex PerForin/Cholesterol Dependent Cytolysin (MACPF/CDC) superfamily form distinctive giant β-barrel pores comprised of up to 50 monomers and up to 300Å in diameter. In this review we focus on recent advances in understanding the structure of these giant MACPF/CDC pores as well as the underlying molecular mechanisms leading to their formation. Commonalities and evolved variations of the pore forming mechanism across the superfamily are discussed. This article is part of a Special Issue entitled: Pore-Forming Toxins edited by Mauro Dalla Serra and Franco Gambale.

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.

Fungal MACPF-like proteins and aegerolysins: bi-component pore-forming proteins?

Proteins with membrane-attack complex/perforin (MACPF) domains are found in almost all kingdoms of life, and they have a variety of biological roles, including defence and attack, organism development, and cell adhesion and signalling. The distribution of these proteins in fungi appears to be restricted to some Pezizomycotina and Basidiomycota species only, in correlation with another group of proteins with unknown biological function, known as aegerolysins. These two protein groups coincide in only a few species, and they might operate in concert as cytolytic bi-component pore-forming agents. Representative proteins here include pleurotolysin B, which has a MACPF domain, and the aegerolysin-like protein pleurotolysin A, and the very similar ostreolysin A, which have been purified from oyster mushroom (Pleurotus ostreatus). These have been shown to act in concert to perforate natural and artificial lipid membranes with high cholesterol and sphingomyelin content. The aegerolysin-like proteins provide the membrane cholesterol/sphingomyelin selectivity and recruit oligomerised pleurotolysin B molecules, to create a membrane-inserted pore complex. The resulting protein structure has been imaged with electron microscopy, and it has a 13-meric rosette-like structure, with a central lumen that is ~4-5 nm in diameter. The opened transmembrane pore is non-selectively permeable for ions and smaller neutral solutes, and is a cause of cytolysis of a colloid-osmotic type. The biological significance of these proteins for the fungal life-style is discussed.

Molecular cloning of the alpha subunit of complement component C8 (CpC8α) of whitespotted bamboo shark (Chiloscyllium plagiosum)

Complement-mediated cytolysis is the important effect of immune response, which results from the assembly of terminal complement components (C5b-9). Among them, α subunit of C8 (C8α) is the first protein that traverses the lipid bilayer, and then initiates the recruitment of C9 molecules to form pore on target membranes. In this article, a full-length cDNA of C8α (CpC8α) is identified from the whitespotted bamboo shark (Chiloscyllium plagiosum) by RACE. The CpC8α cDNA is 2183 bp in length, encoding a protein of 591 amino acids. The deduced CpC8α exhibits 89%, 49% and 44% identity with nurse shark, frog and human orthologs, respectively. Sequence alignment indicates that the C8α is well conserved during the evolution process from sharks to mammals, with the same modular architecture as well as the identical cysteine composition in the mature protein. Phylogenetic analysis places CpC8α and nurse shark C8α in cartilaginous fish clade, in parallel with the teleost taxa, to form the C8α cluster with higher vertebrates. Hydrophobicity analysis also indicates a similar hydrophobicity of CpC8α to mammals. Finally, expression analysis revealed CpC8α transcripts were constitutively highly expressed in shark liver, with much less expression in other tissues. The well conserved structure and properties suggests an analogous function of CpC8α to mammalian C8α, though it remains to be confirmed by further study.

Putting the structure into complement

In a field where structure has finally begun to have a real impact, a series of new structures over the last two years have further extended our understanding of some of the critical regulatory events of the complement system. Notably, information has begun to flow from larger assemblies of components which allow insight into the often transient assemblies critical to complement regulation at the cell surface. This review will summarise the key structures determined since the last International Complement Workshop and the insights these have given us, before highlighting some questions that still require molecular frameworks to drive understanding.

Membrane attack by complement: the assembly and biology of terminal complement complexes

Complement system activation plays an important role in both innate and acquired immunity. Activation of the complement and the subsequent formation of C5b-9 channels (the membrane attack complex) on the cell membranes lead to cell death. However, when the number of channels assembled on the surface of nucleated cells is limited, sublytic C5b-9 can induce cell cycle progression by activating signal transduction pathways and transcription factors and inhibiting apoptosis. This induction by C5b-9 is dependent upon the activation of the phosphatidylinositol 3-kinase/Akt/FOXO1 and ERK1 pathways in a Gi protein-dependent manner. C5b-9 induces sequential activation of CDK4 and CDK2, enabling the G1/S-phase transition and cellular proliferation. In addition, it induces RGC-32, a novel gene that plays a role in cell cycle activation by interacting with Akt and the cyclin B1-CDC2 complex. C5b-9 also inhibits apoptosis by inducing the phosphorylation of Bad and blocking the activation of FLIP, caspase-8, and Bid cleavage. Thus, sublytic C5b-9 plays an important role in cell activation, proliferation, and differentiation, thereby contributing to the maintenance of cell and tissue homeostasis.

Complement in cancer and cancer immunotherapy

Recently, there has been an increase of interest in the use of biological or immune-based therapies for patients with malignancies. This has been informed by the deeper understanding of the crosstalk between the host immune system and malignant tumours, as well as the potential advantages of immunotherapy-high specificity and less toxicity compared to standard approaches. The particular emphasis of this article is on the role of the complement system in tumour growth and antibody-based cancer immunotherapy. The functional consequences from overexpression of complement regulators by tumours and the development of strategies for overcoming this are discussed in detail. This review discusses these issues with a view to inspiring the development of new agents that could be useful for the treatment of cancer.