The 2.0-A crystal structure of tachylectin 5A provides evidence for the common origin of the innate immunity and the blood coagulation systems

Crystal structures of human immune protein FIBCD1 suggest an extended binding site compatible with recognition of pathogen-associated carbohydrate motifs

Fibrinogen C domain-containing protein 1 (FIBCD1) is an immune protein proposed to be involved in host recognition of chitin on the surface of pathogens. As FIBCD1 readily binds acetylated molecules, we have determined the high-resolution crystal structures of a recombinant fragment of the FIBCD1 C-terminal domain complexed with small N-acetyl-containing ligands to determine the mode of recognition. All ligands bind at the conserved N-acetyl-binding site (S1) with galactose and glucose-derived ligands rotated 180° relative to each other. One subunit of a native structure derived from protein expressed in mammalian cells binds glycosylation from a neighboring subunit, in an extended binding site. Across the various structures, the primary S1 binding pocket is occupied by N-acetyl-containing ligands or acetate, with N-acetyl, acetate, or sulfate ion in an adjacent pocket S1(2). Inhibition binding studies of N-acetylglucosamine oligomers, (GlcNAc)n, n = 1, 2, 3, 5, 11, via ELISA along with microscale thermophoresis affinity assays indicate a strong preference of FIBCD1 for longer N-acetylchitooligosaccharides. Binding studies of mutant H396A, located beyond the S1(2) site, showed no significant difference from wildtype, but K381L, within the S1(2) pocket, blocked binding to the model ligand acetylated bovine serum albumin, suggesting that S1(2) may have functional importance in ligand binding. The binding studies, alongside structural definition of diverse N-acetyl monosaccharide binding in the primary S1 pocket and of additional, adjacent binding pockets, able to accommodate both carbohydrate and sulfate functional groups, suggest a versatility in FIBCD1 to recognize chitin oligomers and other pathogen-associated carbohydrate motifs across an extended surface.

Complement-pentraxins synergy: Navigating the immune battlefield and beyond

The complement is a crucial immune defense system that triggers rapid immune responses and offers efficient protection against foreign invaders and unwanted host elements, acting as a sentinel. Activation of the complement system occurs upon the recognition of pathogenic microorganisms or altered self-cells by pattern-recognition molecules (PRMs) such as C1q, collectins, ficolins, and pentraxins. Recent accumulating evidence shows that pentraxins establish a cooperative network with different classes of effector PRMs, resulting in synergistic effects in complement activation. This review describes the complex interaction of pentraxins with the complement system and the implications of this cooperative network for effective host defense during pathogen invasion.

Protein-protein interactions between tenascin-R and RPTPζ/phosphacan are critical to maintain the architecture of perineuronal nets

Neural plasticity, the ability to alter the structure and function of neural circuits, varies throughout the age of an individual. The end of the hyperplastic period in the central nervous system coincides with the appearance of honeycomb-like structures called perineuronal nets (PNNs) that surround a subset of neurons. PNNs are a condensed form of neural extracellular matrix that include the glycosaminoglycan hyaluronan and extracellular matrix proteins such as aggrecan and tenascin-R (TNR). PNNs are key regulators of developmental neural plasticity and cognitive functions, yet our current understanding of the molecular interactions that help assemble them remains limited. Disruption of Ptprz1, the gene encoding the receptor protein tyrosine phosphatase RPTPζ, altered the appearance of nets from a reticulated structure to puncta on the surface of cortical neuron bodies in adult mice. The structural alterations mirror those found in Tnr-/- mice, and TNR is absent from the net structures that form in dissociated cultures of Ptprz1-/- cortical neurons. These findings raised the possibility that TNR and RPTPζ cooperate to promote the assembly of PNNs. Here, we show that TNR associates with the RPTPζ ectodomain and provide a structural basis for these interactions. Furthermore, we show that RPTPζ forms an identical complex with tenascin-C, a homolog of TNR that also regulates neural plasticity. Finally, we demonstrate that mutating residues at the RPTPζ-TNR interface impairs the formation of PNNs in dissociated neuronal cultures. Overall, this work sets the stage for analyzing the roles of protein-protein interactions that underpin the formation of nets.

New insights into the hemolymph coagulation cascade of horseshoe crabs initiated by autocatalytic activation of a lipopolysaccharide-sensitive zymogen

The concept of a chain reaction of proteolytic activation of multiple protease zymogens was first proposed to explain the blood clotting system in mammals as an enzyme cascade. In multicellular organisms, similar enzyme cascades are widely present in signal transduction and amplification systems. The initiation step of the blood coagulation cascade often consists of autocatalytic activation of the corresponding zymogens located on the surfaces of host- or foreign-derived substances at injured sites. However, the molecular mechanism underlying the concept of autocatalytic activation remains speculative. In this review, we will focus on the autocatalytic activation of prochelicerase C on the surface of lipopolysaccharide as a potential initiator of hemolymph coagulation in horseshoe crabs. Prochelicerase C is presumed to have evolved from a common complement factor in Chelicerata; thus, evolutionary insights into the hemolymph coagulation and complement systems in horseshoe crabs will also be discussed.

Evolutionary Insight into Immunothrombosis as a Healing Mechanism

Both invertebrates and vertebrates possess a cluster of immediate and local wound-sealing, pathogen-killing, and tissue healing responses known as immunoclotting and immunothrombosis, respectively, to cope with two life-threatening emergencies, namely, bleeding and microbial invasion. Despite their convergence in function, immunoclotting and immunothrombosis are deployed by different blood cells and intravascular multidomain proteins. In vertebrates, these proteins share some domains with intrinsic chemical affinities useful in generating cooperative networks such as pathogen and damage pattern recognition molecules. Moreover, many of the proteins involved in coagulation and fibrinolysis in humans are multifunctional molecules playing roles in other processes from inflammation to healing and beyond. In our modern society, however, the interaction of activated intravascular allosteric proteins with one another and with blood cells entails vulnerabilities posing a biological paradox: intravascular proteins that locally operate as tissue repair enhancers can nevertheless generate pathogenic processes by acting systemically. In this manuscript, we contextualize and frame the coagulation system and hemostasis through an evolutionary time scale, illustrating their role as dual players in the defense against exsanguination and pathogens while significantly influencing wound healing.

Cross-species opsonic activity of zebrafish fish-egg lectin on mouse macrophages

Zebrafish Fish-egg lectin (zFEL) has been identified and proved to be a maternal factor with antibacterial and opsonic ability in fishes. In this study, we found that zFEL was capable of enhancing the phagocytosis of the bacteria by macrophages of mouse (RAW264.7 and mouse peritoneal macrophages), suggesting a cross-species function of zFEL in higher animals. Further studies showed that zFEL can active the antigen presentation ability by up-regulating the expression of CD80, CD86 and MHC II. Meanwhile, zFEL also promoted the polarization of macrophages to M1-type, which was confirmed by the increase of cytokines TNF-α and IL-6. The expression of p38 gene was up-regulated in macrophages preincubated with zFEL. Taken together, zFEL appears opsonic function in mammal macrophages and has potential application in immunomodulation.

A fibrinogen-related protein mediates the recognition of various bacteria and haemocyte phagocytosis in oyster Crassostrea gigas

The family of fibrinogen-related proteins (FREPs) is a group of proteins with fibrinogen-like (FBG) domains, which play important roles as pattern recognition receptors (PRRs) in the innate immune responses. In the present study, a fibrinogen-like protein was identified from the oyster Crassostrea gigas (defined as CgFREP1). The open reading frame of CgFREP1 was of 966 bp that encoded a predicted polypeptide of 321 amino acids comprising a signal peptide and a fibrinogen-like domain. The mRNA expression of CgFREP1 was detected in all the examined tissues. The recombinant CgFREP1 (rCgFREP1) displayed binding activities to lipopolysaccharide (LPS), mannose (MAN), as well as Gram-positive bacteria (Micrococcus luteus and Staphylococcus aureus) and Gram-negative bacteria (Vibrio splendidus and Escherichia coli). The rCgFREP1 displayed the agglutinating activity towards M. luteus, V. splendidus and E. coli in the presence of Ca²⁺. rCgFREP1 was able to enhance the phagocytic activity of haemocytes towards V. splendidus, and exhibited binding activity to the CUB domain of CgMASPL-1. These results suggest that CgFREP1 not only serves as a PRR to recognize and agglutinate different bacteria but also mediates the haemocytes phagocytosis towards V. splendidus.

Biochemical and structural characterization of a recombinant fibrinogen-related lectin from Penaeus monodon

Fibrinogen-related lectins are carbohydrate-binding proteins of the innate immune system that recognize glycan structures on microbial surfaces. These innate immune lectins are crucial for invertebrates as they do not rely on adaptive immunity for pathogen clearance. Here, we characterize a recombinant fibrinogen-related lectin PmFREP from the black tiger shrimp Penaeus monodon expressed in the Trichoplusia ni insect cell. Electron microscopy and cross-linking experiments revealed that PmFREP is a disulfide-linked dimer of pentamers distinct from other fibrinogen-related lectins. The full-length protein binds N-acetyl sugars in a Ca²⁺ ion-independent manner. PmFREP recognized and agglutinated Pseudomonas aeruginosa. Weak binding was detected with other bacteria, including Vibrio parahaemolyticus, but no agglutination activity was observed. The biologically active PmFREP will not only be a crucial tool to elucidate the innate immune signaling in P. monodon and other economically important species, but will also aid in detection and prevention of shrimp bacterial infectious diseases.

Immune Responses to Gram-Negative Bacteria in Hemolymph of the Chinese Horseshoe Crab, Tachypleus tridentatus

Chinese horseshoe crab, Tachypleus tridentatus, is an ancient marine arthropod with a long evolutionary history. As a kind of living fossil species, the pathogen defenses of horseshoe crabs entirely depend on the innate immune system. Although, there are abundant immune molecules found in the horseshoe crab hemolymph, the biological mechanisms underlying their abilities of distinguishing and defending against invading microbes are still unclear. In this study, we used high-throughput sequencing at mRNA and protein levels and bioinformatics analysis methods to systematically analyze the innate immune response to Gram-negative bacteria in hemolymph of Chinese horseshoe crab. These results showed that many genes in the complement and coagulation cascades, Toll, NF-κB, C-type lectin receptor, JAK-STAT, and MAPK signaling pathways, and antimicrobial substances were activated at 12 and 24 h post-infection, suggesting that Gram-negative bacteria could activate the hemolymph coagulation cascade and antibacterial substances release via the above pathways. In addition, we conjectured that Toll and NF-κB signaling pathway were most likely to participate in the immune response to Gram-negative bacteria in hemolymph of horseshoe crab through an integral signal cascade. These findings will provide a useful reference for exploring the ancient original innate immune mechanism.

Draft genome sequences of Hirudo medicinalis and salivary transcriptome of three closely related medicinal leeches

BACKGROUND

Salivary cell secretion (SCS) plays a critical role in blood feeding by medicinal leeches, making them of use for certain medical purposes even today.

RESULTS

We annotated the Hirudo medicinalis genome and performed RNA-seq on salivary cells isolated from three closely related leech species, H. medicinalis, Hirudo orientalis, and Hirudo verbana. Differential expression analysis verified by proteomics identified salivary cell-specific gene expression, many of which encode previously unknown salivary components. However, the genes encoding known anticoagulants have been found to be expressed not only in salivary cells. The function-related analysis of the unique salivary cell genes enabled an update of the concept of interactions between salivary proteins and components of haemostasis.

CONCLUSIONS

Here we report a genome draft of Hirudo medicinalis and describe identification of novel salivary proteins and new homologs of genes encoding known anticoagulants in transcriptomes of three medicinal leech species. Our data provide new insights in genetics of blood-feeding lifestyle in leeches.

Purification and Assays of Tachylectin-5

Tachylectin-5, a 41-kDa protein with a common fold of the C-terminal globular domain of the γ-chain of fibrinogen, is purified from horseshoe crab hemolymph plasma by affinity column chromatography, using acetyl-group-immobilized resin. Two types of isolectins, tachylectin-5A and tachylectin-5B, are obtained by stepwise elution with GlcNAc at 25 and 250 mM, respectively. Tachylectins-5A and -5B exhibit extraordinarily strong hemagglutinating activity against all types of human erythrocytes (the minimum agglutinating concentration of 0.004-0.008 μg/mL for tachylectin-5A and 0.077-0.27 μg/mL for tachylectin-5B). Their hemagglutinating activities are inhibited by acetyl group-containing sugars and noncarbohydrates such as sodium acetate, acetylcholine, and acetyl CoA (the minimum inhibitory concentrations of 1.3-1.6 mM), indicating that the acetyl group is required and sufficient for recognition by tachylectins-5A and -5B. EDTA inhibits their hemagglutinating activity, whereas the inhibition is overcome by adding an excess amount of Ca²⁺. Tachylectins-5A and -5B also exhibit bacterial agglutinating activity against both Gram-negative bacteria (the minimum agglutinating concentrations of 0.04-0.08 μg/mL for tachylectin-5A and 0.05-0.11 μg/mL for tachylectin-5B) and Gram-positive bacteria (the minimum agglutinating concentrations of 0.3-2.4 μg/mL for tachylectin-5A and 15.1-26.8 μg/mL for tachylectin-5B). Interestingly, tachylectins-5A and -5B enhance the antimicrobial activity of a hemocyte-derived peptide, big defensin.

Antitumor Potential of Marine and Freshwater Lectins

Often, even the most effective antineoplastic drugs currently used in clinic do not efficiently allow complete healing due to the related toxicity. The reason for the toxicity lies in the lack of selectivity for cancer cells of the vast majority of anticancer agents. Thus, the need for new potent anticancer compounds characterized by a better toxicological profile is compelling. Lectins belong to a particular class of non-immunogenic glycoproteins and have the characteristics to selectively bind specific sugar sequences on the surface of cells. This property is exploited to exclusively bind cancer cells and exert antitumor activity through the induction of different forms of regulated cell death and the inhibition of cancer cell proliferation. Thanks to the extraordinary biodiversity, marine environments represent a unique source of active natural compounds with anticancer potential. Several marine and freshwater organisms, ranging from the simplest alga to the most complex vertebrate, are amazingly enriched in these proteins. Remarkably, all studies gathered in this review show the impressive anticancer effect of each studied marine lectin combined with irrelevant toxicity in vitro and in vivo and pave the way to design clinical trials to assess the real antineoplastic potential of these promising proteins. It provides a concise and precise description of the experimental results, their interpretation as well as the experimental conclusions that can be drawn.

Surface loops of trypsin-like serine proteases as determinants of function

Trypsin and chymotrypsin-like serine proteases from family S1 (clan PA) constitute the largest protease group in humans and more generally in vertebrates. The prototypes chymotrypsin, trypsin and elastase represent simple digestive proteases in the gut, where they cleave nearly any protein. Multidomain trypsin-like proteases are key players in the tightly controlled blood coagulation and complement systems, as well as related proteases that are secreted from diverse immune cells. Some serine proteases are expressed in nearly all tissues and fluids of the human body, such as the human kallikreins and kallikrein-related peptidases with specialization for often unique substrates and accurate timing of activity. HtrA and membrane-anchored serine proteases fulfill important physiological tasks with emerging roles in cancer. The high diversity of all family members, which share the tandem β-barrel architecture of the chymotrypsin-fold in the catalytic domain, is conferred by the large differences of eight surface loops, surrounding the active site. The length of these loops alters with insertions and deletions, resulting in remarkably different three-dimensional arrangements. In addition, metal binding sites for Na+, Ca2+ and Zn2+ serve as regulatory elements, as do N-glycosylation sites. Depending on the individual tasks of the protease, the surface loops determine substrate specificity, control the turnover and allow regulation of activation, activity and degradation by other proteins, which are often serine proteases themselves. Most intriguingly, in some serine proteases, the surface loops interact as allosteric network, partially tuned by protein co-factors. Knowledge of these subtle and complicated molecular motions may allow nowadays for new and specific pharmaceutical or medical approaches.

A unique lectin composing of fibrinogen-like domain from Fenneropenaeus merguiensis contributed in shrimp immune defense and firstly found to mediate encapsulation

In invertebrates, both fibrinogen-related proteins (FREPs) and C-type lectins are acknowledged to act as pattern recognition receptors (PRRs) to participate particularly in an innate immunity. Hereby, a unique C-type lectin designated as FmLFd was isolated from the hemocytes of Fenneropenaeus merguiensis. FmLFd contained one open reading frame which encoding a peptide of 312 amino acid residues and a signal peptide of 18 amino acids. The primary sequence of FmLFd was composed of a fibrinogen-like domain (Fd) with a Ca²⁺-binding site and possessing specificity to bind N-acetyl glucosamine (GlcNAc). The FmLFd transcripts were detected mainly in hemocytes of healthy shrimp. The expression of FmLFd was significantly up-regulated upon challenge shrimp with Vibrio parahaemolyticus and Vibrio harveyi which more potent than by white spot syndrome virus (WSSV). The knocking down shrimp with FmLFd double-stranded RNA caused dramatical gene down-regulation. The gene silencing with co-injection of pathogens resulted in reduction of the shrimp survival rate. Recombinant protein of FmLFd (rFmLFd) could agglutinate and bind directly to both Gram-negative and Gram-positive bacteria in a Ca²⁺-dependent manner and showed the sugar specificity to GlcNAc and bacterial saccharides; peptidoglycan (PGN), lipopolysaccharide (LPS) and lipoteichoic acid (LTA). Recombinant protein of Fd domain (rFd) displayed the lower activity and specificity only to PGN. The binding between recombinant proteins of FmLFd and its domain confirming by ELISA demonstrated that both rFmLFd and rFd could bind to PGN, LPS and LTA with the highest affinity respected to PGN including a less extent of rFd. Besides, rFmLFd but not rFd could bind to WSSV proteins with the highest binding affinity to capsid VP15 and decreasing in order to envelope VP28 and tegument VP39A, respectively. It was presumed that entire molecule of FmLFd exhibited the antimicrobial ability by inhibiting the growth of pathogenic V. parahaemolyticus and this action was not affected by GlcNAc. Otherwise, FmLFd, a lectin containing fibrinogen-like domain, was firstly reported to be capable of promoting encapsulation by hemocytes. Altogether, we concluded that FmLFd belonged to a FREP family indentified by the existence of a conserved fibrinogen-like domain with possessing an ability to bind GlcNAc. It was a new C-type lectin existed in F. merguiensis and might presumably act as a kind of PRRs to participate in the shrimp immune defense towards bacterial and viral pathogens.

Ficolins and the Recognition of Pathogenic Microorganisms: An Overview of the Innate Immune Response and Contribution of Single Nucleotide Polymorphisms

Ficolins are innate pattern recognition receptors (PRR) and play integral roles within the innate immune response to numerous pathogens throughout the circulation, as well as within organs. Pathogens are primarily removed by direct opsonisation following the recognition of cell surface carbohydrates and other immunostimulatory molecules or via the activation of the lectin complement pathway, which results in the deposition of C3b and the recruitment of phagocytes. In recent years, there have been a number of studies implicating ficolins in the recognition and removal of numerous bacterial, viral, fungal, and parasitic pathogens. Moreover, there has been expanding evidence highlighting that mutations within these key immune proteins, or the possession of particular haplotypes, enhance susceptibility to colonization by pathogens and dysfunctional immune responses. This review will therefore encompass previous knowledge on the role of ficolins in the recognition of bacterial and viral pathogens, while acknowledging the recent advances in the immune response to fungal and parasitic infections. Additionally, we will explore the various genetic susceptibility factors that predispose individuals to infection.

A bite so sweet: the glycobiology interface of tick-host-pathogen interactions

Vector-borne diseases constitute 17% of all infectious diseases in the world; among the blood-feeding arthropods, ticks transmit the highest number of pathogens. Understanding the interactions between the tick vector, the mammalian host and the pathogens circulating between them is the basis for the successful development of vaccines against ticks or the tick-transmitted pathogens as well as for the development of specific treatments against tick-borne infections. A lot of effort has been put into transcriptomic and proteomic analyses; however, the protein-carbohydrate interactions and the overall glycobiology of ticks and tick-borne pathogens has not been given the importance or priority deserved. Novel (bio)analytical techniques and their availability have immensely increased the possibilities in glycobiology research and thus novel information in the glycobiology of ticks and tick-borne pathogens is being generated at a faster pace each year. This review brings a comprehensive summary of the knowledge on both the glycosylated proteins and the glycan-binding proteins of the ticks as well as the tick-transmitted pathogens, with emphasis on the interactions allowing the infection of both the ticks and the hosts by various bacteria and tick-borne encephalitis virus.

Structures of Angptl3 and Angptl4, modulators of triglyceride levels and coronary artery disease

Coronary artery disease is the most common cause of death globally and is linked to a number of risk factors including serum low density lipoprotein, high density lipoprotein, triglycerides and lipoprotein(a). Recently two proteins, angiopoietin-like protein 3 and 4, have emerged from genetic studies as being factors that significantly modulate plasma triglyceride levels and coronary artery disease. The exact function and mechanism of action of both proteins remains to be elucidated, however, mutations in these proteins results in up to 34% reduction in coronary artery disease and inhibition of function results in reduced plasma triglyceride levels. Here we report the crystal structures of the fibrinogen-like domains of both proteins. These structures offer new insights into the reported loss of function mutations, the mechanisms of action of the proteins and open up the possibility for the rational design of low molecular weight inhibitors for intervention in coronary artery disease.

Immunohistochemical Localization of Fibrinogen C Domain Containing 1 on Epithelial and Mucosal Surfaces in Human Tissues

Fibrinogen C domain containing 1 (FIBCD1) is a transmembrane receptor that binds chitin and other acetylated compounds with high affinity. FIBCD1 has previously been shown to be present in the epithelium of the gastrointestinal tract. In the present study, we performed a detailed analysis of normally structured human tissues for the expression of FIBCD1 by quantitative PCR and immunohistochemistry. We find that FIBCD1 is expressed in epithelial cells derived from all three germ layers. Endodermal-derived epithelial cells throughout the gastrointestinal tract and the respiratory system showed high expression of FIBCD1 and also mesodermal-derived cells in the genitourinary system and ectodermal-derived epidermis and sebaceous glands cells expressed FIBCD1. In some columnar epithelial cells, for example, in the salivary gland and gall bladder, the FIBCD1 expression was clearly polarized with strong apical reaction, while other columnar cells, for example, in small and large intestine and in bronchi, the staining was equally strong apically and basolaterally. In keratinocytes in skin, tongue, and oral cavity, the FIBCD1 staining was granular. This expression pattern together with the known binding properties supports that FIBCD1 plays a role in innate immunity in the skin and at mucosal surfaces.

Characterization and function of a tachylectin 5-like immune molecule in Penaeus monodon

Tachylectin5A and its homolog, tachylectin5B both contain a fibrinogen-related domain (FReD) and have been studied in horseshoe crabs, Tachypleus tridentatus and Carcinoscorpius rotundicauda and shown to be involved in host defense. Here, we demonstrate the presence of tachylectin5-like genes in shrimp, Penaeus monodon, designated as Penlectin5-1 (PL5-1) and Penlectin5-2 (PL5-2), which both contain a signal peptide and a single FReD with an acetyl group and a calcium binding sites and they are both structurally similar to horseshoe crab tachylectin/carcinolectin5. The PL5-1and PL5-2 transcript were expressed in various shrimp tissues in normal shrimp, and their expression was upregulated in tissues such as hemocytes and hindgut following challenge with pathogenic Vibrio harveyi. The PL5-2 protein was detected in various tissues as well as in cell-free hemolymph. The biological function of the PL5-2 protein is to recognize some Gram-positive and Gram-negative bacteria regardless whether they are non-pathogenic or pathogenic. They have hemagglutination activity on human erythrocyte and bacterial agglutination activity to both Gram negative and Gram positive bacteria. Possible binding sites of PL5-2 to bacteria could be at the N-acetyl moiety of the GlcNAc-MurNAc cell wall of the peptidoglycan since the binding could be inhibited by GlcNAc or GalNAC. The presence of PL5-2 protein in both circulating hemolymph and intestine, where host and microbes are usually interacting, may suggest that the physiological function of shrimp tachylectin-like proteins is to recognize and bind to invading bacteria to immobilize and entrap these microbes and subsequently clear them from circulation and the host body, and probably to control and maintain the normal flora in the intestine.

FBN30 in wild Anopheles gambiae functions as a pathogen recognition molecule against clinically circulating Plasmodium falciparum in malaria endemic areas in Kenya

Malaria is a worldwide health problem that affects two-thirds of the world population. Plasmodium invasion of anopheline mosquitoes is an obligatory step for malaria transmission. However, mosquito-malaria molecular interactions in nature are not clear. A genetic variation within mosquito fibrinogen related-protein 30 (FBN30) was previously identified to be associated with Plasmodium falciparum infection in natural Anopheles gambiae populations at malaria endemic areas in Kenya, and reducing FBN30 expression by RNAi makes mosquitoes more susceptible to P. berghei. New results show that FBN30 is a secreted octamer that binds to both P. berghei and clinically circulating P. falciparum from malaria endemic areas in Kenya, but not laboratory P. falciparum strain NF54. Moreover, the natural genetic mutation (T to C) within FBN30 signal peptide, which changes the position 10 amino acid from phenylalanine to leucine, reduces protein expression by approximately half. This change is consistent to more susceptible An. gambiae to P. falciparum infection in the field. FBN30 in natural An. gambiae is proposed to work as a pathogen recognition molecule in inhibiting P. falciparum transmission in malaria endemic areas.

An overview of the synergy and crosstalk between pentraxins and collectins/ficolins: their functional relevance in complement activation

The complement system is an innate immune defense machinery comprising components that deploy rapid immune responses and provide efficient protection against foreign invaders and unwanted host elements. The complement system is activated upon recognition of pathogenic microorganisms or altered self-cells by exclusive pattern recognition molecules (PRMs), such as collectins, ficolins and pentraxins. Recent accumulating evidence shows that the different classes of effector PRMs build up a co-operative network and exert synergistic effects on complement activation. In this review, we describe our updated view of the crosstalk between previously unlinked PRMs in complement activation and the potential pathogenic effects during infection and inflammation.

Identification of 10 transcripts of FREP in penaeid shrimp Litopenaeus vannamei

Fibrinogen-related proteins (FREPs) are widely distributed in vertebrates and invertebrates and known having Fibrinogen-related domains (FReDs) which function in multiple aspects, especially in innate immune response as pattern recognition receptors. However, there is no any report about FREP in penaeid shrimp Litopenaeus vannamei. Here, totally 10 transcripts of FREP were isolated and named LvFREP1.1, 1.2 until 1.10. All of the 10 transcripts have high identity in their 3' ends and encode conserved FReDs. Since the 10 transcripts are highly similar we could not design any primers that can amplify a single transcript. We chose a pair of primers corresponding to part of LvFREP1.1 and LvFREP1.5 to examine their expression. Tissue distribution indicated LvFREP1.1 and LvFREP1.5 mRNA locates in hemocytes, gills, intestine, hearts and slightly in nerve. The expression of LvFREP1.1 and LvFREP1.5 behaves differently post bacteria and virus infection. Besides, recombinant LvFReD could agglutinate bacteria Vibrio harveyi in the presence of Ca²⁺. These initial data presents the diversity of FREPs in penaeid shrimp and also push us to further explore their roles in shrimp immune response.

Molecular basis of non-self recognition by the horseshoe crab lectins

The target molecules of innate immunity are not proteins of direct gene products but the molecular arrays or patterns on pathogens, called pathogen-associated molecular patterns (PAMPs). The self/non-self discrimination by innate immunity through simple ligands universally expressed both on pathogens and hosts, such as monosaccharides and the acetyl group, probably depends on the density or clustering patterns of the ligands. The specific recognition by the horseshoe crab lectins, named tachylectins, with a propeller-like fold or a propeller-like oligomeric arrangement is reinforced by the short distance between the individual binding sites that interact with PAMPs. There is virtually no conformational change in the main or side chains of tachylectins upon binding with the ligands. This low structural flexibility of the propeller structures must be very important for specific interaction with PAMPs.

A fibrinogen-related protein identified from hepatopancreas of crayfish is a potential pattern recognition receptor

Fibrinogen-related protein (FREP) family is a large group of proteins containing fibrinogen-like (FBG) domain and plays multiple physiological roles in animals. However, their immune functions in crayfish are not fully explored. In the present study, a novel fibrinogen-like protein (designated as PcFBN1) was identified and characterized from hepatopancreas of red swamp crayfish Procambarus clarkii. The cDNA sequence of PcFBN1 contains an open reading frame (ORF) of 1353 bp encoding a protein of 450 amino acids. Sequence and structural analysis indicated that PcFBN1 contains an FBG domain in C-terminal and a putative signal peptide of 19 amino acids in N-terminal. Semi-quantitative PCR revealed that the main expression of PcFBN1 was observed in hepatopancreas and hemocyte. Temporal expression analysis exhibited that PcFBN1 expression could be significantly induced by heat-killed Aeromonas hydrophila. Tissue distribution and temporal change of PcFBN1 suggested that PcFBN1 may be involved in immune responses of red swamp crayfish. Recombinant PcFBN1 protein binds and agglutinates both gram-negative bacteria Escherichia coli and gram-positive bacteria Micrococcus lysodeikticus. Moreover, binding and agglutination is Ca(2+) dependent. Further analysis indicated that PcFBN1 recognizes some acetyl group-containing substance LPS and PGN. RNAi experiment revealed that PcFBN1 is required for bacterial clearance and survival from A. hydrophila infection. Reduction of PcFBN1 expression significantly decreased the survival and enhanced the number of A. hydrophila in the hemolymph. These results indicated that PcFBN1 plays an important role in the innate immunity of red swamp crayfish as a potential pattern recognition receptor.

The antimicrobial properties of C-reactive protein (CRP)

C-reactive protein, CRP, is a predominant pattern-recognition receptor (PRR) in the plasma of the horseshoe crab, which recognizes lipopolysaccharide (LPS). Native CRP2 has previously been shown to exhibit agglutination activity against the polysialic capsule of Escherichia coli K1 but its role in bacterial clearance is not well characterized. In this work, the antimicrobial activity of a recombinant CRP2 isoform (rCRP2) was tested against E. coli, Pseudomonas aeruginosa and Staphylococcus aureus. rCRP2 agglutinates bacteria and exhibits bactericidal activity against Gram-negative bacteria. In addition, the antimicrobial activity of rCRP2 is calcium-independent. GST pulldown experiments suggest that in the naïve physiological state, CRP2 interacts with hemocyanin, native CRPs, a 35-kDa plasma lectin and an as yet unidentified 40-kDa protein. This interaction was enhanced upon Pseudomonas infection. We propose that rCRP2 is a PRR with potent antimicrobial activity and its interacting partners contribute to effective bacterial clearance.

Characterization of Microfibrillar-associated Protein 4 (MFAP4) as a Tropoelastin- and Fibrillin-binding Protein Involved in Elastic Fiber Formation

MFAP4 (microfibrillar-associated protein 4) is an extracellular glycoprotein found in elastic fibers without a clearly defined role in elastic fiber assembly. In the present study, we characterized molecular interactions between MFAP4 and elastic fiber components. We established that MFAP4 primarily assembles into trimeric and hexameric structures of homodimers. Binding analysis revealed that MFAP4 specifically binds tropoelastin and fibrillin-1 and -2, as well as the elastin cross-linking amino acid desmosine, and that it co-localizes with fibrillin-1-positive fibers in vivo. Site-directed mutagenesis disclosed residues Phe(241) and Ser(203) in MFAP4 as being crucial for type I collagen, elastin, and tropoelastin binding. Furthermore, we found that MFAP4 actively promotes tropoelastin self-assembly. In conclusion, our data identify MFAP4 as a new ligand of microfibrils and tropoelastin involved in proper elastic fiber organization.

Fibrinogen-Related Proteins in Tissue Repair: How a Unique Domain with a Common Structure Controls Diverse Aspects of Wound Healing

Significance: Fibrinogen-related proteins (FRePs) comprise an intriguing collection of extracellular molecules, each containing a conserved fibrinogen-like globe (FBG). This group includes the eponymous fibrinogen as well as the tenascin, angiopoietin, and ficolin families. Many of these proteins are upregulated during tissue repair and exhibit diverse roles during wound healing. Recent Advances: An increasing body of evidence highlights the specific expression of a number of FRePs following tissue injury and infection. Upon induction, each FReP uses its FBG domain to mediate quite distinct effects that contribute to different stages of tissue repair, such as driving coagulation, pathogen detection, inflammation, angiogenesis, and tissue remodeling. Critical Issues: Despite a high degree of homology among FRePs, each contains unique sequences that enable their diversification of function. Comparative analysis of the structure and function of FRePs and precise mapping of regions that interact with a variety of ligands has started to reveal the underlying molecular mechanisms by which these proteins play very different roles using their common domain. Future Directions: Fibrinogen has long been used in the clinic as a synthetic matrix serving as a scaffold or a delivery system to aid tissue repair. Novel therapeutic strategies are now emerging that harness the use of other FRePs to improve wound healing outcomes. As we learn more about the underlying mechanisms by which each FReP contributes to the repair response, specific blockade, or indeed potentiation, of their function offers real potential to enable regulation of distinct processes during pathological wound healing.

Highly diverse fibrinogen-related proteins in the Pacific oyster Crassostrea gigas

Fibrinogen-related proteins (FREPs) are a family of proteins with high sequence diversity, and they play crucial roles in invertebrate immune response. However, few studies have characterized this diversity at the whole-genome level. In the present study, approximately 190 predicted FREPs with more than 200 fibrinogen-like (FBG) domains were identified in the genome of the Pacific oyster (Crassostrea gigas), suggesting a historical expansion of this protein family. A sequence analysis showed high numbers of polymorphisms in C. gigas FREP (CgFREP) genes, which may contribute to the versatile immune function of FREPs. A phylogenetic analysis of molluscan FREP sequences indicated lineage-specific duplication of these genes in C. gigas. Additionally, several CgFREP mRNAs were highly expressed in the gills, digestive glands, and hemocytes. Taken together, these findings will help elucidate FREP immune function and facilitate studies of the functional validation of this gene family.

New insights into the role of ficolins in the lectin pathway of innate immunity

In the innate immune system, a variety of recognition molecules provide the first-line host defense to prevent infection and maintain endogenous homeostasis. Ficolin is a soluble recognition molecule, which senses pathogen-associated molecular patterns on microbes and aberrant sugar structures on self-cells. It consists of a collagen-like stalk and a globular fibrinogen-like domain, the latter binding to carbohydrates such as N-acetylglucosamine. Ficolins have been widely identified in animals from higher invertebrates to mammals. In mammals, ficolins form complexes with mannose-binding lectin-associated serine proteases (MASPs), and ficolin-MASP complexes trigger complement activation via the lectin pathway. Once activated, complement mediates many immune responses including opsonization, phagocytosis, and cytokine production. Although the precise function of each ficolin is still under investigation, accumulating information suggests that ficolins have a crucial role in host defense by recognizing a variety of microorganisms and interacting with effector proteins.

A fibrinogen-related protein (FREP) is involved in the antibacterial immunity of Marsupenaeus japonicus

Fibrinogen-related proteins (FREPs) in invertebrates have important functions in innate immunity. In this study, the cDNA of FREP was identified from the kuruma shrimp Marsupenaeus japonicus (MjFREP2). The full-length cDNA of MjFREP2 is 1138 bp with an open reading frame of 954 bp that encodes a 317-amino acid protein comprising a signal peptide and a fibrinogen-like domain. MjFREP2 could be detected in hemocytes, heart, hepatopancreas, gills, stomach, and intestines. MjFREP2 could also be upregulated in hemocytes after Vibrio anguillarum and Staphylococcus aureus challenge. Agglutination and binding assay results revealed that the recombinant MjFREP2 bound to bacteria and polysaccharides. Immunocytochemical analysis results showed that MjFREP2 proteins were mainly distributed in the cytoplasm of hemocytes from unchallenged shrimp and transported to the membrane or secreted out of the cell after V. anguillarum or S. aureus challenge. The secreted MjFREP2 bound to the bacteria presented in shrimp hemolymph. The overexpression of MjFREP2 could enhance bacterial clearance by inducing the phagocytosis of hemocytes. This ability was impaired by knockdown of MjFREP2 with RNA interference. The cumulative mortality of MjFREP2-silenced shrimp was significantly higher than that of the control shrimp. These results suggested that MjFREP2 has an important function in the antibacterial immunity of M. japonicus.

A new fibrinogen-related protein from Argopecten irradians (AiFREP-2) with broad recognition spectrum and bacteria agglutination activity

Fibrinogen-related proteins (FREPs) are a kind of pattern recognition receptors (PRRs) containing fibrinogen-like (FBG) domains, and they play curial roles in the innate immune response. In the present study, a new FREP protein was identified from bay scallop Argopecten irradians (designated as AiFREP-2). The full-length cDNA of AiFREP-2 was of 1299 bp with an open reading frame of 762 bp encoding a polypeptide of 253 amino acids, including a signal sequence and an FBG domain. The FBG domain in AiFREP-2 was highly similar to those of ficolins, tenascins and other FREPs. The mRNA expression of AiFREP-2 could be detected in all the examined tissues with the highest level in gill. The mRNA expression of AiFREP-2 in hemocytes was significantly up-regulated post the stimulation of lipopolysaccharide (LPS), peptidoglycan (PGN) and β-glucan (GLU) (P < 0.01). The recombinant AiFREP-2 (rAiFREP-2) could bind not only different PAMP ligands including LPS, PGN and GLU, but also various microbes including Gram-negative bacteria (Vibrio anguillarum), Gram-positive bacteria (Staphylococcus aureus) and fungus (Pichia pastoris and Yarrowia lipolytica). Additionally, rAiFREP-2 exhibited obvious agglutination activity towards Gram-negative bacteria V. anguillarum and Gram positive bacteria S. aureus. The results indicated that AiFREP-2 was involved in the immune response against Gram-negative bacteria, Gram-positive bacteria and fungus as a PRR in bay scallop, and the information was helpful to understand the innate immune defense mechanisms of mollusks.

Variable immune molecules in invertebrates

Recently it has become evident that invertebrates may mount a highly variable immune response that is dependent on which pathogen is involved. The molecular mechanisms behind this diversity are beginning to be unravelled and in several invertebrate taxa immune proteins exhibiting a broad range of diversity have been found. In some cases, evidence has been gathered suggesting that this molecular diversity translates into the ability of an affected invertebrate to mount a defence that is specifically aimed at a particular pathogen.

Crystal structure of the tetrameric fibrinogen-like recognition domain of fibrinogen C domain containing 1 (FIBCD1) protein

The high resolution crystal structures of a recombinant fragment of the C-terminal fibrinogen-like recognition domain of FIBCD1, a vertebrate receptor that binds chitin, have been determined. The overall tetrameric structure shows similarity in structure and aggregation to the horseshoe crab innate immune protein tachylectin 5A. The high affinity ligand N-acetylmannosamine (ManNAc) binds in the S1 site, predominantly via the acetyl group with the oxygen and acetamide nitrogen hydrogen-bonded to the protein and the methyl group inserted into a hydrophobic pocket. The binding of the ManNAc pyranose ring differs markedly between the two independent subunits, but in all structures the binding of the N-acetyl group is conserved. In the native structure, a crystal contact results in one of the independent protomers binding the first GlcNAc of the Asn³⁴⁰N-linked glycan on the other independent protomer. In the ligand-bound structure this GlcNAc is replaced by the higher affinity ligand ManNAc. In addition, a sulfate ion has been modeled into the electron density at a location similar to the S3 binding site in L-ficolin, whereas in the native structure an acetate ion has been placed in the S1 N-acetyl binding site, and a sulfate ion has been placed adjacent to this site. These ion binding sites are ideally placed to receive the N-acetyl and sulfate groups of sulfated GalNAc residues of glycosaminoglycans such as chondroitin and dermatan sulfate. Together, these structures give insight into important determinants of ligand selectivity, demonstrating versatility in recognition and binding while maintaining conservation in N-acetyl and calcium binding.

Correlating structure and function during the evolution of fibrinogen-related domains

Fibrinogen-related domains (FReDs) are found in a variety of animal proteins with widely different functions, ranging from non-self recognition to clot formation. All appear to have a common surface where binding of one sort or other occurs. An examination of 19 completed animal genomes--including a sponge and sea anemone, six protostomes, and 11 deuterostomes--has allowed phylogenies to be constructed that show where various types of FReP (proteins containing FReDs) first made their appearance. Comparisons of sequences and structures also reveal particular features that correlate with function, including the influence of neighbor-domains. A particular set of insertions in the carboxyl-terminal subdomain was involved in the transition from structures known to bind sugars to those known to bind amino-terminal peptides. Perhaps not unexpectedly, FReDs with different functions have changed at different rates, with ficolins by far the fastest changing group. Significantly, the greatest amount of change in ficolin FReDs occurs in the third subdomain ("P domain"), the very opposite of the situation in most other vertebrate FReDs. The unbalanced style of change was also observed in FReDs from non-chordates, many of which have been implicated in innate immunity.

Characterization of fibrinogen-like protein 2 (FGL2): monomeric FGL2 has enhanced immunosuppressive activity in comparison to oligomeric FGL2

Fibrinogen-like protein 2 (FGL2), a novel effector molecule of CD4(+)CD25(+)Foxp3(+) regulatory T cells (Treg), mediates its suppressive activity through binding to low affinity Fcγ receptors expressed on antigen presenting cells (APCs). FGL2 has been implicated in the pathogenesis of viral hepatitis, xeno- and allotransplant rejection, and rheumatoid arthritis. Here we fully analyzed the structure-function relationships of recombinant murine FGL2 generated in COS-7 cells and identified the receptor binding domains. Native FGL2 exists as an oligomer with a molecular weight of approximately 260 kDa, while under reducing conditions, FGL2 has a molecular weight of 65 kDa suggesting that native FGL2 is composed of four monomers. By site-directed mutation, cysteines at positions 94, 97, 184 and 187, found in the coiled-coil domain were shown to be crucial for FGL2 oligomerization. Monomeric FGL2 had a lower affinity binding to APCs, but increased immunosuppressive activity compared to oligomeric FGL2. Deglycosylation demonstrated that sugar moieties are critical for maintaining solubility of FGL2. SWISS-MODEL analysis suggested that FGL2 has a similar tertiary structure with other members of the fibrinogen family such as fibrinogen and tachylectin. Mutational analysis of cysteine residues and Western blots suggested an asymmetric bouquet-shaped quaternary structure for oligomeric FGL2, resembling many pattern-recognition molecules in the lectin pathway of innate immunity. The functional motifs of FGL2 were mapped to the C terminal globular domain, using a peptide blockade assay. These results collectively define the biochemical and immunological determinants of FGL2, an important immunosuppressive molecule of Treg providing important insights for designing FGL2-related therapeutics.

Glycocodon theory--the first table of glycocodons

Hydrophobic cellular membranes separate cells from an environment that is generally based on water. Therefore, it is not surprising that hydrophilic glycans and glycoproteins are exposed on the lipidic surface of membranes and that the glycocalyx has evolved in all basic cell types. During the evolution of multicellular life, the surface exposed protein-glycan interactions were taken as the origin of the language of cell-cell communication. The bioinformatics analysis presented here reveals that the amino acid triplets, the glycocodons, can be deduced for each glycan letter (monosaccharide). This theory proposes to distinguish between the "sugar code" (the sugar sequence) and the "glycocode" (evolutionary selected amino acids recognising the mono-sugar). Similarly to genetic code, original glycocodons are related to G, A, V, and D amino acids. Modern glycocodons can be deduced from GAVD-glycocodons using hydropathic similarity. In general, the amino acid triplets can be assembled from one dipeptide that is specific to a monosaccharide plus a polar amino acid. This theory may shed a different light on the reason for WWD conservation in the active sites of oligosaccharyltransferases and for GGQ in the active sites of ribosomes.

Diversified carbohydrate-binding lectins from marine resources

Marine bioresources produce a great variety of specific and potent bioactive molecules including natural organic compounds such as fatty acids, polysaccharides, polyether, peptides, proteins, and enzymes. Lectins are also one of the promising candidates for useful therapeutic agents because they can recognize the specific carbohydrate structures such as proteoglycans, glycoproteins, and glycolipids, resulting in the regulation of various cells via glycoconjugates and their physiological and pathological phenomenon through the host-pathogen interactions and cell-cell communications. Here, we review the multiple lectins from marine resources including fishes and sea invertebrate in terms of their structure-activity relationships and molecular evolution. Especially, we focus on the unique structural properties and molecular evolution of C-type lectins, galectin, F-type lectin, and rhamnose-binding lectin families.

The protochordate Ciona intestinalis has a protein like full-length vertebrate fibrinogen

In a recent review, a putative fibrinogen-like protein in the protochordate Ciona intestinalis was noted. Unfortunately, computer-directed splicing had omitted several exons, mistakenly generating a single long polypeptide chain. In fact, 3 consecutive genes exist, the translated versions of which are homologous to individual vertebrate fibrinogen chains. The circulating form is likely a 6-chain covalent dimer, just as occurs in vertebrates.

Two novel ficolin-like proteins act as pattern recognition receptors for invading pathogens in the freshwater crayfish Pacifastacus leniusculus

To isolate pathogen-associated molecular patterns (PAMPs)-binding molecules, the bacterium, Staphylococcus aureus was used as an affinity matrix to find bacteria-binding proteins in the plasma of the freshwater crayfish, Pacifastacus leniusculus. Two new bacteria-binding ficolin-like proteins (FLPs) were identified by 2-DE and MS analysis. The FLPs have a fibrinogen-related domain (FReD) in their C-terminal and a repeat region in their N-terminal regions with putative structural similarities to the collagen-like domain of vertebrate ficolins and mannose binding lectins (MBLs). Phylogenetic analysis shows that the newly isolated crayfish FLP1 and FLP2 cluster separately from other FReD-containing proteins. A tissue distribution study showed that the mRNA expression of FLP occurred mainly in the hematopoietic tissue (Hpt) and in the hepatopancreas. Recombinant FLPs exhibited agglutination activity of Gram-negative bacteria Escherichia coli and Aeromonas hydrophila in the presence of Ca(2+) . The FLPs could bind to A. hydrophila, E. coli as well as S. aureus as judged by bacteria adsorption. Moreover, the FLPs may help crayfish to clear Gram-negative bacteria, but not Gram-positive bacteria which had been injected into the hemolymph. When Gram-negative bacteria coated with FLPs were incubated with Hpt cells, a lower death rate of the cells was found compared with control treatment. Our results suggest that FLPs function as pattern recognition receptors in the immune response of crayfish.

The role of ficolins in the lectin pathway of innate immunity

Ficolins are a family of oligomeric proteins consisting of an N-terminal collagen-like domain and a C-terminal globular fibrinogen-like domain. They are novel lectins that employ the fibrinogen-like domain as a functional domain. Ficolins specifically recognize N-acetyl compounds such as N-acetylglucosamine, components of bacterial and fungal cell walls, and certain bacteria. Like mannose-binding lectin (MBL), ficolins circulate in complexes with MBL-associated serine proteases (MASPs). MASP complexes form with ficolins and MBL, thereby activating the complement through the lectin pathway. Upon binding of ficolins and MBL to carbohydrates on pathogens, MASPs convert to active forms, and subsequently activate the complement. The activated complements lead to pathogen phagocytosis, aggregation and lysis. In humans, three ficolins (L-, M- and H-ficolins) have been identified, which exhibit differences in tissue expression, protein location site, ligand-binding and bacteria-recognition, suggesting a specific role of each ficolin. In addition, these ficolins form complexes with three MASPs (MASP-1, MASP-2 and MASP-3) and two nonenzymatic proteins (sMAP and MAP-1), suggesting a highly sophisticated organization and regulated activation of the ficolin-dependent lectin pathway. This review provides an overview of our current knowledge of ficolins, especially human ficolins and their mouse homologues. We also discuss their possible physiological roles in innate immunity, especially their defensive role against bacterial infection.

Ficolins and FIBCD1: soluble and membrane bound pattern recognition molecules with acetyl group selectivity

A network of molecules, which recognizes pathogens, work together to establish a quick and efficient immune response to infectious agents. Molecules containing a fibrinogen related domain in invertebrates and vertebrates have been implicated in immune responses against pathogens, and characterized as pattern recognition molecules. Ficolins are soluble oligomeric proteins composed of trimeric collagen-like regions linked to fibrinogen-related domains (FReDs) that have the ability to sense molecular patterns on both pathogens and apoptotic cell surfaces and activate the complement system. The ficolins have acetyl-binding properties, which have been localized to different binding sites in the FReD-region. A newly discovered tetrameric transmembrane protein, FIBCD1, likewise binds acetylated structures via the highly conserved FReD. This review presents current knowledge on acetyl binding FReD-containing molecules, and discusses structural resemblance but also diversity in recognition of acetylated ligands.

The primary role of fibrinogen-related proteins in invertebrates is defense, not coagulation

In vertebrates, the conversion of fibrinogen into fibrin is an essential process that underlies the establishment of the supporting protein framework required for coagulation. In invertebrates, fibrinogen-domain-containing proteins play a role in the defense response generated against pathogens; however, they do not function in coagulation, suggesting that this role has been recently acquired. Molecules containing fibrinogen motifs have been identified in numerous invertebrate organisms, and most of these molecules known to date have been linked to defense. Moreover, recent genome projects of invertebrate animals have revealed surprisingly high numbers of fibrinogen-like loci in their genomes, suggesting important and perhaps diverse functions of fibrinogen-like proteins in invertebrates. The ancestral role of molecules containing fibrinogen-related domains (FReDs) with immunity is the focus of this review, with emphasis on specific FReDs called fibrinogen-related proteins (FREPs) identified from the schistosome-transmitting mollusc Biomphalaria glabrata. Herein, we outline the range of invertebrate organisms FREPs can be found in, and detail the roles these molecules play in defense and protection against infection.

Mannose-binding lectin and its associated proteases (MASPs) mediate coagulation and its deficiency is a risk factor in developing complications from infection, including disseminated intravascular coagulation

The first line of host defense is the innate immune system that includes coagulation factors and pattern recognition molecules, one of which is mannose-binding lectin (MBL). Previous studies have demonstrated that MBL deficiency increases susceptibility to infection. Several mechanisms are associated with increased susceptibility to infection, including reduced opsonophagocytic killing and reduced lectin complement pathway activation. In this study, we demonstrate that MBL and MBL-associated serine protease (MASP)-1/3 together mediate coagulation factor-like activities, including thrombin-like activity. MBL and/or MASP-1/3 deficient hosts demonstrate in vivo evidence that MBL and MASP-1/3 are involved with hemostasis following injury. Staphylococcus aureus infected MBL null mice developed disseminated intravascular coagulation (DIC), which was associated with elevated blood IL-6 levels (but not TNF-α and multi-organ inflammatory responses). Infected MBL null mice also develop liver injury. These findings suggest that MBL deficiency may manifest into DIC and organ failure during infectious diseases.

Carbohydrate recognition properties of human ficolins: glycan array screening reveals the sialic acid binding specificity of M-ficolin

Ficolins are oligomeric innate immune recognition proteins consisting of a collagen-like region and a fibrinogen-like recognition domain that bind to pathogen- and apoptotic cell-associated molecular patterns. To investigate their carbohydrate binding specificities, serum-derived L-ficolin and recombinant H- and M-ficolins were fluorescently labeled, and their carbohydrate binding ability was analyzed by glycan array screening. L-ficolin preferentially recognized disulfated N-acetyllactosamine and tri- and tetrasaccharides containing terminal galactose or N-acetylglucosamine. Binding was sensitive to the position and orientation of the bond between N-acetyllactosamine and the adjacent carbohydrate. No significant binding of H-ficolin to any of the 377 glycans probed could be detected, providing further evidence for its poor lectin activity. M-ficolin bound preferentially to 9-O-acetylated 2-6-linked sialic acid derivatives and to various glycans containing sialic acid engaged in a 2-3 linkage. To further investigate the structural basis of sialic acid recognition by M-ficolin, point mutants were produced in which three residues of the fibrinogen domain were replaced by their counterparts in L-ficolin. Mutations G221F and A256V inhibited binding to the 9-O-acetylated sialic acid derivatives, whereas Y271F abolished interaction with all sialic acid-containing glycans. The crystal structure of the Y271F mutant fibrinogen domain was solved, showing that the mutation does not alter the structure of the ligand binding pocket. These analyses reveal novel ficolin ligands such as sulfated N-acetyllactosamine (L-ficolin) and gangliosides (M-ficolin) and provide precise insights into the sialic acid binding specificity of M-ficolin, emphasizing the essential role of Tyr(271) in this respect.

The recognition unit of FIBCD1 organizes into a noncovalently linked tetrameric structure and uses a hydrophobic funnel (S1) for acetyl group recognition

We have recently identified FIBCD1 (Fibrinogen C domain containing 1) as a type II transmembrane endocytic receptor located primarily in the intestinal brush border. The ectodomain of FIBCD1 comprises a coiled coil, a polycationic region, and a C-terminal FReD (fibrinogen-related domain) that assembles into disulfide-linked homotetramers. The FIBCD1-FReD binds Ca(2+) dependently to acetylated structures like chitin, N-acetylated carbohydrates, and amino acids. FReDs are present in diverse innate immune pattern recognition proteins including the ficolins and horseshoe crab TL5A. Here, we use chemical cross-linking, combined with analytical ultracentrifugation and electron microscopy of the negatively stained recombinant FIBCD1-FReD to show that it assembles into noncovalent tetramers in the absence of the coiled coil. We use surface plasmon resonance, carbohydrate binding, and pulldown assays combined with site-directed mutagenesis to define the binding site involved in the interaction of FIBCD1 with acetylated structures. We show that mutations of central residues (A432V and H415G) in the hydrophobic funnel (S1) abolish the binding of FIBCD1 to acetylated bovine serum albumin and chitin. The double mutations (D393N/D395A) at the putative calcium-binding site reduce the ability of FIBCD1 to bind ligands. We conclude that the FReDs of FIBCD1 forms noncovalent tetramers and that the acetyl-binding site of FReDs of FIBCD1 is homologous to that of tachylectin 5A and M-ficolin but not to the FReD of L-ficolin. We suggest that the spatial organization of the FIBCD1-FReDs determine the molecular pattern recognition specificity and subsequent biological functions.