The complement regulator C4b-binding protein (C4BP) interacts with both the C4c and C4dg subfragments of the parent C4b ligand: evidence for synergy in C4BP subsite binding

An in-silico receptor-pharmacophore based multistep molecular docking and simulation study to evaluate the inhibitory potentials against NS1 of DENV-2

DENV-2 strain is the most fatal and infectious of the five dengue virus serotypes. The non-structural protein NS1 encoded by its genome is the most significant protein required for viral pathogenesis and replication inside the host body. Thus, targeting the NS1 protein and designing an inhibitor to limit its stability and secretion is a propitious attempt in our fight against dengue. Four novel inhibitors are designed to target the conserved cysteine residues (C55, C313, C316, and C329) and glycosylation sites (N130 and N207) of the NS1 protein in an attempt to halt the spread of the dengue infection in the host body altogether. Numerous computer-aided drug designing techniques including molecular docking, molecular dynamics simulation, virtual screening, principal component analysis, and dynamic cross-correlation matrix were employed to determine the structural and functional activity of the NS1-inhibitor complexes. From our analysis, it was evident that the extent of structural and atomic level fluctuations of the ligand-bound protein exhibited a declining trend in contrast to unbound protein which was prominently noticeable through the RMSD, RMSF, Rg, and SASA graphs. The ADMET analysis of the four ligands revealed a promising pharmacokinetics and pharmacodynamic profile, along with good bioavailability and toxicity properties. The proposed drugs when bound to the targeted cavities resulted in stable conformations in comparison to their unbound state, implying they have good affinity promising effective drug action. Thus, they can be tested in vitro and used as potential anti-dengue drugs.Communicated by Ramaswamy H. Sarma.

Multivalent Scaffolds to Promote B cell Tolerance

Autoimmune diseases are characterized by aberrant immune responses toward self-antigens. Current treatments lack specificity, promoting adverse effects by broadly suppressing the immune system. Therapies that specifically target the immune cells responsible for disease are a compelling strategy to mitigate adverse effects. Multivalent formats that display numerous binding epitopes off a single scaffold may enable selective immunomodulation by eliciting signals through pathways unique to the targeted immune cells. However, the architecture of multivalent immunotherapies can vary widely, and there is limited clinical data with which to evaluate their efficacy. Here, we set forth to review the architectural properties and functional mechanisms afforded by multivalent ligands and evaluate four multivalent scaffolds that address autoimmunity by altering B cell signaling pathways. First, we address both synthetic and natural polymer backbones functionalized with a variety of small molecule, peptide, and protein ligands for probing the effects of valency and costimulation. Then, we review nanoparticles composed entirely from immune signals which have been shown to be efficacious. Lastly, we outline multivalent liposomal nanoparticles capable of displaying high numbers of protein antigens. Taken together, these examples highlight the versatility and desirability of multivalent ligands for immunomodulation and illuminate strengths and weaknesses of multivalent scaffolds for treating autoimmunity.

Novel structure of the N-terminal helical domain of BibA, a group B streptococcus immunogenic bacterial adhesin

BibA, a group B streptococcus (GBS) surface protein, has been shown to protect the pathogen from phagocytic killing by sequestering a complement inhibitor: C4b-binding protein (C4BP). Here, the X-ray crystallographic structure of a GBS BibA fragment (BibA126-398) and a low-resolution small-angle X-ray scattering (SAXS) structure of the full-length N-terminal domain (BibA34-400) are described. The BibA126-398 fragment crystal structure displayed a novel and predominantly helical structure. The tertiary arrangement of helices forms four antiparallel three-helix-bundle-motif repeats, with one long helix from a bundle extending into the next. Multiple mutations on recombinant BibA34-400 delayed the degradation of the protein, and circular dichroism spectroscopy of BibA34-400 suggested a similar secondary-structure composition to that observed in the crystallized BibA126-398 fragment. A model was generated for the 92 N-terminal residues (BibA34-125) using structural similarity prediction programs, and a BibA34-400 model was generated by combining the coordinates of BibA34-126 and BibA126-398. The X-ray structure of BibA126-398 and the model of BibA34-400 fitted well into the calculated SAXS envelope. One possible binding site for the BibA N-terminal domain was localized to the N-terminal CCP (complement-control protein) domains of the C4BP α-chain, as indicated by the decreased binding of BibA to a ΔCCP1 C4BP α-chain mutant. In summary, it is suggested that the GBS surface protein BibA, which consists of three antiparallel α-helical-bundle motifs, is unique and belongs to a new class of Gram-positive surface adhesins.

Hepatitis B virus X protein up-regulates C4b-binding protein α through activating transcription factor Sp1 in protection of hepatoma cells from complement attack

Hepatitis B virus X protein (HBx) plays crucial roles in the development of hepatocellular carcinoma (HCC). We previously showed that HBx protected hepatoma cells from complement attack by activation of CD59. Moreover, in this study we found that HBx protected hepatoma cells from complement attack by activation of C4b-binding protein α (C4BPα), a potent inhibitor of complement system. We observed that HBx were positively correlated with those of C4BPα in clinical HCC tissues. Mechanistically, HBx activated the promoter core region of C4BPα, located at -1199/-803nt, through binding to transcription factor Sp1. In addition, chromatin immunoprecipitation (ChIP) assays showed that HBx was able to bind to the promoter of C4BPα, which could be blocked by Sp1 silencing. Functionally, knockdown of C4BPα obviously increased the deposition of C5b-9, a complex of complement membrane attack, and remarkably abolished the HBx-induced resistance of hepatoma cells from complement attack in vitro and in vivo. Thus, we conclude that HBx up-regulates C4BPα through activating transcription factor Sp1 in protection of liver cancer cells from complement attack. Our finding provides new insights into the mechanism by which HBx enhances protection of hepatoma cells from complement attack.

C4b-binding protein: The good, the bad and the deadly. Novel functions of an old friend

C4b-binding protein (C4BP) is best known as a potent soluble inhibitor of the classical and lectin pathways of the complement system. This large 500 kDa multimeric plasma glycoprotein is expressed mainly in the liver but also in lung and pancreas. It consists of several identical 75 kDa α-chains and often also one 40 kDa β-chain, both of which are mainly composed of complement control protein (CCP) domains. Structure-function studies revealed that one crucial binding site responsible for inhibition of complement is located to CCP1-3 of the α-chain. Binding of anticoagulant protein S to the CCP1 of the β-chain provides C4BP with the ability to strongly bind apoptotic and necrotic cells in order to prevent inflammation arising from activation of complement by these cells. Further, C4BP interacts strongly with various types of amyloid and enhances fibrillation of islet amyloid polypeptide secreted from pancreatic beta cells, which may attenuate pro-inflammatory and cytotoxic effects of this amyloid. Full deficiency of C4BP has not been identified but non-synonymous alterations in its sequence have been found in haemolytic uremic syndrome and recurrent pregnancy loss. Furthermore, C4BP is bound by several bacterial pathogens, notably Streptococcus pyogenes, which due to inhibition of complement and enhancement of bacterial adhesion to endothelial cells provides these bacteria with a survival advantage in the host. Thus, depending on the context, C4BP has a protective or detrimental role in the organism.

Structural Basis for the Function of Complement Component C4 within the Classical and Lectin Pathways of Complement

Complement component C4 is a central protein in the classical and lectin pathways within the complement system. During activation of complement, its major fragment C4b becomes covalently attached to the surface of pathogens and altered self-tissue, where it acts as an opsonin marking the surface for removal. Moreover, C4b provides a platform for assembly of the proteolytically active convertases that mediate downstream complement activation by cleavage of C3 and C5. In this article, we present the crystal and solution structures of the 195-kDa C4b. Our results provide the molecular details of the rearrangement accompanying C4 cleavage and suggest intramolecular flexibility of C4b. The conformations of C4b and its paralogue C3b are shown to be remarkably conserved, suggesting that the convertases from the classical and alternative pathways are likely to share their overall architecture and mode of substrate recognition. We propose an overall molecular model for the classical pathway C5 convertase in complex with C5, suggesting that C3b increases the affinity for the substrate by inducing conformational changes in C4b rather than a direct interaction with C5. C4b-specific features revealed by our structural studies are probably involved in the assembly of the classical pathway C3/C5 convertases and C4b binding to regulators.

Complement-related proteins control the flavivirus infection of Aedes aegypti by inducing antimicrobial peptides

The complement system functions during the early phase of infection and directly mediates pathogen elimination. The recent identification of complement-like factors in arthropods indicates that this system shares common ancestry in vertebrates and invertebrates as an immune defense mechanism. Thioester (TE)-containing proteins (TEPs), which show high similarity to mammalian complement C3, are thought to play a key role in innate immunity in arthropods. Herein, we report that a viral recognition cascade composed of two complement-related proteins limits the flaviviral infection of Aedes aegypti. An A. aegypti macroglobulin complement-related factor (AaMCR), belonging to the insect TEP family, is a crucial effector in opposing the flaviviral infection of A. aegypti. However, AaMCR does not directly interact with DENV, and its antiviral effect requires an A. aegypti homologue of scavenger receptor-C (AaSR-C), which interacts with DENV and AaMCR simultaneously in vitro and in vivo. Furthermore, recognition of DENV by the AaSR-C/AaMCR axis regulates the expression of antimicrobial peptides (AMPs), which exerts potent anti-DENV activity. Our results both demonstrate the existence of a viral recognition pathway that controls the flaviviral infection by inducing AMPs and offer insights into a previously unappreciated antiviral function of the complement-like system in arthropods.

Hosts are equipped with sophisticated machineries for detecting and eliminating invading viruses before they cause significant physiological damage. Unlike mammals which have both innate and adaptive immune systems, insects rely solely on the innate immune system to limit viral infection. Mosquitoes are natural vectors for many human pathogenic viruses, such as dengue virus (DENV) and yellow fever virus. Despite lacking an immunoglobulin-based humoral response, mosquitoes arm themselves with a functional complement-like system to ward off invading pathogens. Herein, we show that a system composed of complement-related factors recognizes and limits flaviviruses by inducing antimicrobial peptides expression. Understanding antiviral mechanisms in arthropods may provide novel strategies for limiting arboviral transmission in nature.

Multivalent ligand: design principle for targeted therapeutic delivery approach

Multivalent interactions of biological molecules play an important role in many biochemical events. A multivalent ligand comprises of multiple copies of ligands conjugated to scaffolds, allowing the simultaneous binding of multivalent ligands to multiple binding sites or receptors. Many research groups have successfully designed and synthesized multivalent ligands to increase the binding affinity, avidity and specificity of the ligand to the receptor. A multimeric ligand is a promising option for the specific treatment of diseases. In this review, the factors affecting multivalent interactions, including the size and shape of the ligand, geometry and an arrangement of ligands on the scaffold, linker length, thermodynamic, and kinetics of the interactions are discussed. Examples of the multivalent ligand applications for therapeutic delivery are also summarized.