Characterization of beta-glucan recognition site on C-type lectin, dectin 1

Solvent stabilizing effects on the order-disorder transition of schizophyllan in aqueous mixtures of carboxylic acids

Schizophyllan is a triple helical β-1,3-D-glucan, and shows the cooperative order-disorder transition in the aqueous solution at the triple helix state. In this paper, the solvent stabilizing effects of two carboxylic acids, acetic acid and citric acid, on the cooperative order-disorder transition of aqueous schizophyllan solution were investigated from DSC and SEC-MALS measurements. The transition temperature (Tr) was shifted to higher temperature with increasing the molar fraction of carboxylic acid in the mixture (x). The transition enthalpy (ΔHr) was increased with increasing x. These solvent stabilizing effects indicate that these carboxylic acid molecules were selectively associated with the branched side chains of schizophyllan to stabilize the ordered state. The composition dependencies of Tr and ΔHr were analyzed by the linear cooperative transition theory to estimate the association parameters between the side chains and carboxylic acid. The theoretical parameters obtained were compared with those for the other active substances for the transition to discuss the molecular interactions between the triple helix and carboxylic acid.

Tissue-resident trained immunity in hepatocytes protects against septic liver injury in zebrafish

Trained immunity is classically characterized by long-term functional reprogramming of innate immune cells to combat infectious diseases. Infection-induced organ injury is a common clinical severity phenotype of sepsis. However, whether the induction of trained immunity plays a role in protecting septic organ injury remains largely unknown. Here, through establishing an in vivo β-glucan training and lipopolysaccharide (LPS) challenge model in zebrafish larvae, we observe that induction of trained immunity could inhibit pyroptosis of hepatocytes to alleviate septic liver injury, with an elevated trimethyl-histone H3 lysine 4 (H3K4me3) modification that targets mitophagy-related genes. Moreover, we identify a C-type lectin domain receptor in zebrafish, named DrDectin-1, which is revealed as the orchestrator in gating H3K4me3 rewiring-mediated mitophagy activation and alleviating pyroptosis-engaged septic liver injury in vivo. Taken together, our results uncover tissue-resident trained immunity in maintaining liver homeostasis at the whole-animal level and offer an in vivo model to efficiently integrate trained immunity for immunotherapies.

Secondary Sites of the C-type Lectin-Like Fold

C-type lectins are a large superfamily of proteins involved in a multitude of biological processes. In particular, their involvement in immunity and homeostasis has rendered them attractive targets for diverse therapeutic interventions. They share a characteristic C-type lectin-like domain whose adaptability enables them to bind a broad spectrum of ligands beyond the originally defined canonical Ca2+-dependent carbohydrate binding. Together with variable domain architecture and high-level conformational plasticity, this enables C-type lectins to meet diverse functional demands. Secondary sites provide another layer of regulation and are often intricately linked to functional diversity. Located remote from the canonical primary binding site, secondary sites can accommodate ligands with other physicochemical properties and alter protein dynamics, thus enhancing selectivity and enabling fine-tuning of the biological response. In this review, we outline the structural determinants allowing C-type lectins to perform a large variety of tasks and to accommodate the ligands associated with it. Using the six well-characterized Ca2+-dependent and Ca2+-independent C-type lectin receptors DC-SIGN, langerin, MGL, dectin-1, CLEC-2 and NKG2D as examples, we focus on the characteristics of non-canonical interactions and secondary sites and their potential use in drug discovery endeavors.

Self-recognition through Dectin-1 exacerbates liver inflammation

Dectin-1 is a well-characterized C-type lectin receptor involved in anti-fungal immunity through the recognition of polysaccharides; however, molecular mechanisms and outcomes initiated through self-recognition have not been fully understood. Here, we purified a water-soluble fraction from mouse liver that acts as a Dectin-1 agonist. To address the physiological relevance of this recognition, we utilized sterile liver inflammation models. The CCl4-induced hepatitis model showed that Dectin-1 deficiency led to reduced inflammation through decreased inflammatory cell infiltration and lower pro-inflammatory cytokine levels. Moreover, in a NASH model induced by streptozotocin and a high-fat diet, hepatic inflammation and fibrosis were ameliorated in Dectin-1-deficient mice. The Dectin-1 agonist activity was increased in the water-soluble fraction from NASH mice, suggesting a potential pathogenic cycle between Dectin-1 activation and hepatitis progression. In vivo administration of the fraction into mice induced hepatic inflammation. These results highlight a role of self-recognition through Dectin-1 that triggers hepatic innate immune responses and contributes to the exacerbation of inflammation in pathogenic settings. Thus, the blockade of this axis may provide a therapeutic option for liver inflammatory diseases.

A convenient assay for soluble Dectin-1 lectin domain binding to insoluble β-glucans

β-Glucan is a homopolymer with a backbone of β-1,3-linked glucose residues. The solubility and biological activity of β-glucan can be influenced by the length of the backbone and the length/interval of the β-1,6 branches. Dectin-1 is crucial in innate immunity through its binding to exogenous β-glucans. However, there are few quantitative binding affinities available and there is no comprehensive comparative analysis of the binding of Dectin-1 to insoluble β-glucans. Here, we have developed a simple binding assay for the interaction between Dectin-1 lectin domain (Dectin-1 CTLD) and insoluble β-glucans. We utilized the paramylon particle as a model of insoluble β-glucans. Dectin-1 CTLD bound to paramylon (particle size 3.1 μm) was separated from unbound Dectin-1 CTLD by centrifugation using a membrane filter (pore size 0.2 μm). The protein in the filtrate was quantified by SDS-PAGE and densitometry. The amount decreased in proportion to the amount of paramylon in the mixture. A control experiment using the Dectin-1 CTLD inactive mutant W221A showed that the mutant passes through the filter without binding paramylon. These results are evidence of site-specific binding of Dectin-1 CTLD to paramylon and demonstrate that the separation of paramylon-bound/unbound Dectin-1 CTLD is achievable through centrifugation using a filter. The assay was extended to other insoluble β-glucans including curdlan. Additionally, it can be utilized in competitive inhibition experiments with soluble short-chain β-glucans such as laminarin. The assay system allows for quantitative comparison of the affinities between insoluble and soluble β-glucans and Dectin-1 CTLD, and should be useful because of its low-tech convenience.

Development of Negative Controls for Fc-C-Type Lectin Receptor Probes

Fc-C-type lectin receptor (Fc-CTLRs) probes are soluble chimeric proteins constituted of the extracellular domain of a CTLR fused with the constant fraction (Fc) of the human IgG. These probes are useful tools to study the interaction of CTLRs with their ligands, with applications similar to those of antibodies, often in combination with widely available fluorescent antibodies targeting the Fc fragment (anti-hFc). In particular, Fc-Dectin-1 has been extensively used to study the accessibility of β-glucans at the surface of pathogenic fungi. However, there is no universal negative control for Fc-CTLRs, making the distinction of specific versus nonspecific binding difficult. We describe here 2 negative controls for Fc-CTLRs: a Fc-control constituting of only the Fc portion, and a Fc-Dectin-1 mutant predicted to be unable to bind β-glucans. Using these new probes, we found that while Fc-CTLRs exhibit virtually no nonspecific binding to Candida albicans yeasts, Aspergillus fumigatus resting spores strongly bind Fc-CTLRs in a nonspecific manner. Nevertheless, using the controls we describe here, we were able to demonstrate that A. fumigatus spores expose a low amount of β-glucan. Our data highlight the necessity of appropriate negative controls for experiments involving Fc-CTLRs probes. IMPORTANCE While Fc-CTLRs probes are useful tools to study the interaction of CTLRs with ligands, their use is limited by the lack of appropriate negative controls in assays involving fungi and potentially other pathogens. We have developed and characterized 2 negative controls for Fc-CTLRs assays: Fc-control and a Fc-Dectin-1 mutant. In this manuscript, we characterize the use of these negative controls with zymosan, a β-glucan containing particle, and 2 human pathogenic fungi, Candida albicans yeasts and Aspergillus fumigatus conidia. We show that A. fumigatus conidia nonspecifically bind Fc-CTLRs probes, demonstrating the need for appropriate negative controls in such assays.

Refinement and complete solution NMR analysis of the structure of a 6-branched 1,3-β-D-glucan (OL-2) isolate from Omphalialapidescens

OL-2 is a water-soluble β-glucan produced by Omphalia lapidescens. This versatile glucan has potential applications in various industries, including food, cosmetics, and pharmaceuticals. In addition, OL-2 is known for its promising applications as a biomaterial and drug, owing to its reported antitumor and antiseptic properties. Although the biological activities of β-glucans vary depending on their primary structure, holistic clarification of OL-2 via solution NMR spectroscopy to ascertain its complete and unambiguous structure has not yet been achieved. In this study, a chain of solution NMR techniques, such as correlation spectroscopy, total correlation spectroscopy (TOCSY), nuclear Overhauser effect and exchange spectroscopy, ¹³C-edited heteronuclear single quantum coherence (HSQC), HSQC-TOCSY, heteronuclear multiple bond correlation, and heteronuclear 2-bond correlation pulse sequences were used to unambiguously assign all ¹H and ¹³C atoms in OL-2. Based on our investigation, OL-2 consists of a 1,3-β-glucan backbone chain decorated with a single 6-branched β-glucosyl side unit on every fourth residue.

β-glucans: a potential source for maintaining gut microbiota and the immune system

The human gastrointestinal (GI) tract holds a complex and dynamic population of microbial communities, which exerts a marked influence on the host physiology during homeostasis and disease conditions. Diet is considered one of the main factors in structuring the gut microbiota across a lifespan. Intestinal microbial communities play a vital role in sustaining immune and metabolic homeostasis as well as protecting against pathogens. The negatively altered gut bacterial composition has related to many inflammatory diseases and infections. β-glucans are a heterogeneous assemblage of glucose polymers with a typical structure comprising a leading chain of β-(1,4) and/or β-(1,3)-glucopyranosyl units with various branches and lengths as a side chain. β-glucans bind to specific receptors on immune cells and initiate immune responses. However, β-glucans from different sources differ in their structures, conformation, physical properties, and binding affinity to receptors. How these properties modulate biological functions in terms of molecular mechanisms is not known in many examples. This review provides a critical understanding of the structures of β-glucans and their functions for modulating the gut microbiota and immune system.

Lentinan: An unexplored novel biomaterial in drug and gene delivery applications

Recently, lentinan (LNT) has been utilized for its diversified potential in research with an extended role from nutritional or medicinal applications to a novel biomaterial. LNT is a biocompatible, multifunctional polysaccharide employed as a pharmaceutical additive in engineering customized drug or gene carriers with an improved safety profile. Its triple helical structure containing hydrogen bonding offers more extraordinary binding sites for the attachments of dectin-1 receptors and polynucleotide sequences (poly(dA)). Hence, the diseases expressing dectin-1 receptors can be specifically targeted through so-designed LNT-engineered drug carriers. Gene delivery using poly(dA)-s-LNT complexes and composites has exhibited greater targetability and specificity. The achievement of such gene applications is assessed through the pH and redox potential of the extracellular cell membrane. The steric hindrance-acquiring behavior of LNT shows promise as a system stabilizer in drug carrier engineering. LNT shows viscoelastic gelling behavior temperature-dependently and therefore needs to explore more to meet topical disease applications. The immunomodulatory and vaccine adjuvant properties of LNT help in mitigating viral infections too. This review highlights the new role of LNT as a novel biomaterial, particularly in drug delivery and gene delivery applications. In addition, its importance in achieving various biomedical applications is also discussed.

Binding assay of human Dectin-1 variants for DNA/ β-glucan complex for active-targeting delivery of antisense DNA: Part II

A β-1,3-glucan binding receptor called Dectin-1 is mainly expressed on antigen-presenting immunocytes. Dectin-1 may be a target molecule for receptor-mediated and active-targeting delivery of drugs to regulate or interfere with the immune system. Therapeutic oligonucleotides are one such drug of interest. To this end, we have been studying the complex of schizophyllan (SPG, one of the linear (1,3)-β-ᴅ-glucan family) with oligonucleotide and its delivery mechanism to the Dectin-1 expressing cells. There are at least six types of human Dectin-1 expressed on the cell surface (designated V-1, V-2, etc.), with V-1 having a complete carbohydrate recognition domain (CRD) and stalk, V-2 having a complete CRD but no stalk, and other variants having an incomplete CRD due to exon skipping. Our previous studies have shown that SPG binds only to V-1 and V-2. By contrast, SPG/oligonucleotide complexes bind both V-1 and V-2 more strongly than SPG itself and show a certain affinity, for other variants. As a continuing work, the present paper discusses the structure and nature of all human Dectin-1 variants expressed on the cellular surface. we found that (1) a new N-linked glycosylation site is present in some variants, (2) the glycosylation of Dectin-1 plays an important role in the fate of Dectin-1 and its localization in the cells, and (3) the glycosylation is related to the amount of ingestion of the complex. The present findings suggest that, in addition to V-1 and V-2, two other variants that are highly expressed at the plasma membrane and stabilized by the glycosylation may also be targets of the complex.

The phagocytic receptors of β-glucan

Phagocytosis is a cellular process maintaining tissue balance and plays an essential role in initiating the innate immune response. The process of phagocytosis was triggered by the binding of pathogen-associated molecular patterns (PAMP) with their cell surface receptors on the phagocytes. These receptors not only perform phagocytic functions, but also bridge the gap between extracellular and intracellular communication, leading to signal transduction and the production of inflammatory mediators, which are crucial for clearing the invading pathogens and maintaining cell homeostasis. For the past few years, the application of β-glucan comes down to immunoregulation and anti-tumor territory. As a well-known PAMP, β-glucan is one of the most abundant polysaccharides in nature. By binding to specific receptors on immune cells and activating intracellular signal transduction pathways, it causes phagocytosis and promotes the release of cytokines. Further retrieval and straightening out literature related to β-glucan phagocytic receptors will help better elucidate their immunomodulatory functions. This review attempts to summarize physicochemical properties and specific processes involved in β-glucan induced phagocytosis, its phagocytic receptors, and cascade events triggered by β-glucan at the cellular and molecular levels.

Association with Imidazole in the Cooperative Order-Disorder Transition in Aqueous Solution of Schizophyllan

Schizophyllan, a triple helical polysaccharide, exhibits cooperative order-disorder transition (CODT) in aqueous solutions. The transition transforms the ordered structure (triple helix I) formed between the branched side chains and solvent molecules into the disordered structure (triple helix II) without dissociation of the triple helix. The CODT behaviors in H₂O-imidazole mixtures containing HCl with different molar ratios of imidazole/HCl were investigated by adiabatic calorimetry and differential scanning calorimetry on two schizophyllan solutions with different molar masses. The transition temperature (T_r) and the transition enthalpy (ΔH_r) significantly depended on both of the mole fractions of imidazole and imidazole/HCl. The composition dependences of T_r and ΔH_r in H₂O-imidazole mixtures were analyzed with linear cooperative transition theory for the solvent-stabilizing effect in the mixture with active compounds. Theoretical analyses confirmed that both imidazole and imidazolium ions in the solutions competitively interact with the side chain of the triple helix.

A Meloidogyne incognita C-type lectin effector targets plant catalases to promote parasitism

Root-knot nematodes, Meloidogyne spp., secrete effectors to modulate plant immune responses and establish a parasitic relationship with host plants. However, the functions and plant targets of C-type lectin (CTL)-like effectors of Meloidogyne incognita remain unknown. Here, we characterized a CTL-like effector of M. incognita, MiCTL1a, and identified its target and role in nematode parasitism. In situ hybridization demonstrated the expression of MiCTL1 in the subventral glands; and in planta, immunolocalization showed its secretion during M. incognita parasitism. Virus-induced gene silencing of the MiCTL1 reduced the infection ability of M. incognita in Nicotiana benthamiana. The ectopic expression in Arabidopsis not only increased susceptibility to M. incognita but also promoted root growth. Yeast two-hybrid and co-immunoprecipitation assays revealed that MiCTL1a interacts with Arabidopsis catalases, which play essential roles in hydrogen peroxide homeostasis. Knockout or overexpression of catalases showed either increased or reduced susceptibility to M. incognita, respectively. Moreover, MiCTL1a not only reduced catalase activity in vitro and in planta but also modulated stress-related gene expressions in Arabidopsis. Our data suggest that MiCTL1a interacts with plant catalases and interferes with catalase activity, allowing M. incognita to establish a parasitic relationship with its host by fine-tuning responses mediated by reactive oxygen species.

Interaction between polysaccharides and toll-like receptor 4: Primary structural role, immune balance perspective, and 3D interaction model hypothesis

Various structural types of polysaccharides are recognized by toll-like receptor 4 (TLR4). However, the mechanism of interaction between the polysaccharides with different structures and TLR4 is unclarified. This review summarized the primary structure of polysaccharides related to TLR4, mainly including molecular weight, monosaccharide composition, glycosidic bonds, functional groups, and branched-chain structure. The optimal primary structure for interacting with TLR4 was obtained by the statistical analysis. Besides, the dual-directional regulation of TLR4 signaling cascade by polysaccharides was also elucidated from an immune balance perspective. Finally, the 3D interaction model of polysaccharides to TLR4-myeloid differentiation factor 2 (MD2) complex was hypothesized according to the LPS-TLR4-MD2 dimerization model and the polysaccharides solution conformation. The essence of polysaccharides binding to TLR4-MD2 complex is a multivalent non-covalent bond interaction. All the arguments summarized in this review are intended to provide some new insights into the interaction between polysaccharides and TLR4.

A novel β-glucan-oligonucleotide complex selectively delivers siRNA to APCs via Dectin-1

Delivering therapeutic nucleic acids to targeted cells and organs has been a challenge for decades. A novel technology to deliver oligonucleotide therapeutics to immune cells is here described. In this approach, a macromolecular complex of oligonucleotides and the β-1,3-glucan schizophyllan (SPG) is selectively delivered to cells expressing a lectin receptor, Dectin-1, via SPG-Dectin-1 interaction. Detailed investigation of Dectin-1-expressing cells revealed that Dectin-1 is expressed in all subsets of monocytes as well as dendritic cell (DC) populations, including conventional DCs (cDCs) and plasmacytoid DCs (pDCs), in humans. The expression patterns in mice and humans are comparable, except for the expression in pDCs. The results indicate that Dectin-1 is expressed on cells capable of professional antigen presentation, except for B cells. We chose CD40 as a target gene for small interfering RNA (siRNA) as CD40 expression in antigen-presenting cells (APCs), particularly in DCs, plays critical roles in regulating immune responses. Dose-dependent cellular uptake of siCD40-SPG complexes was confirmed in cells expressing Dectin-1. Gene silencing activity was confirmed in vitro by the reduction of CD40 mRNA and by the site-specific cleavage of CD40 mRNA as determined by the 5' RNA ligase-mediated rapid amplification of cDNA ends (5'RLM-RACE) technique. In vivo activity of siCD40-SPG complexes was demonstrated as the reduced CD40 protein expression in monocytes and DCs in mice. Furthermore, the in vivo activity of siCD40-SPG targeting human CD40 was confirmed in cynomolgus monkeys by the 5'RLM-RACE technique. In conclusion, we have demonstrated the receptor-ligand binding-mediated delivery of siRNA targeting immune-regulating monocytes and DCs via the interaction of SPG and its receptor, Dectin-1. As monocytes and DCs play central roles in inducing and controlling immune responses, Dectin-1-targeted delivery of nucleic acids should provide a useful tool for developing drugs to treat a wide range of diseases, including autoimmune diseases, allergy, and cancer, as well as transplantation.

[Fungal Beta-glucans: Structure and Effect on Host Immune Responses]

Fungi are eukaryotic microorganisms that show complex life cycles, including both anamorph and teleomorph stages. Beta-1,3-1,6-glucans (BGs) are major cell wall components in fungi. BGs are also found in a soluble form and are secreted by fungal cells. Studies of fungal BGs extensively expanded from 1960 to 1990 due to their applications in cancer immunotherapy. However, progress in this field slowed down due to the low efficacy of such therapies. In the early 21st century, the discovery of C-type lectin receptors significantly enhanced the molecular understanding of innate immunity. Moreover, pathogen-associated molecular patterns (PAMPs) and pattern recognition receptors (PRRs) were also discovered. Soon, dectin-1 was identified as the PRR of BGs, whereas BGs were established as PAMPs. Then, studies on fungal BGs focused on their participation in the development of deep-seated mycoses and on their role as a source of functional foods. Fungal BGs may have numerous and complex linkages, making it difficult to systematize them even at the primary structure level. Moreover, elucidating the structure of BGs is largely hindered by the multiplicity of genes involved in cell wall biosynthesis, including those for BGs, and by fungal diversity. The present review mainly focused on the characteristics of fungal BGs from the viewpoint of structure and immunological activities.

Comparative transcriptome analysis of the gills of Procambarus clarkii provide novel insights into the response mechanism of ammonia stress tolerance

Procambarus clarkii is an important model crustacean organism in many researches. Ammonia nitrogen is one of common contaminants in aquatic environment, influencing the health of aquatic organisms. The primary objective of this study was to investigate molecular mechanisms on ammonia stress in gills of P. clarkii to provide new insights into the strategies of aquatic animals in responding to high concentration of ammonia in the environment. Procambarus clarkii were randomly assigned into two groups (ammonia stress group, AG; control group, CG), and gill samples were dependently excised from AG and CG. Then response mechanisms on ammonia stress were investigated based on transcriptome data of P. clarkii. 9237 differentially expressed genes were identified in ammonia stress group. The genes of ion transport enzymes (NKA and SLC6A5S) were significantly up-regulated. Whereas the immune-related genes (e.g. MAP3K7, HSP70, HSP90A, CTSF, CTSL1, CHI and CTL4) and pathways were significantly up-regulated, which played an important role in reacting to ammonia stress. Procambarus clarkii may enhance immune defense to counteract ammonia toxicity by the up-regulation of immune-related genes and signaling pathways. The activities of ion transport enzymes are changed to mobilise signal transduction and ion channel regulation for adapting to ammonia environment. These previous key genes play an important role in resistance to ammonia stress to better prepare for survival in high concentration of ammonia.

The role of dectin-1 in health and disease

Dendritic cell-associated C-type lectin-1 (Dectin-1), also known as β-glucan receptor is an emerging pattern recognition receptor (PRR) which belongs to the family of C-type lectin receptor (CLR). This CLR identifies ligands independently of Ca²⁺ and is majorly involved in coupling of innate with adaptive immunity. Formerly, Dectin-1 was best known for its role in anti-fungal defense only. However, recent explorations suggested its wider role in defense against variety of infectious diseases caused by pathogens including bacteria, parasites and viruses. In fact, Dectin-1 signaling axis has been suggested to be targeted as an effective therapeutic strategy for cancers. Dectin-1 has also been elucidated ascetically in the heart, respiratory, intestinal, neurological and developmental disorders. Being a defensive PRR, Dectin-1 results in optimal immune responses in collaboration with other PRRs, but the overall evaluation reinforces the hypothesis of disease development on dis-regulation of Dectin-1 activity. This underscores the impact of Dectin-1 polymorphisms in modulating protein expression and generation of non-optimal immune responses through defective collaborations, further underlining their therapeutic potential. To add on, Dectin-1 influence autoimmunity and severe inflammation accredited to recognition of self T cells and apoptotic cells through unknown ligands. Few reports have also testified its redundant role in infections, which makes it a complicated molecule to be fully resolved. Thus, Dectin-1 is a hub that runs a complex collaborative network, whose interactive wire connections to different PRRs are still pending to be revealed. Alternatively, so far focus of almost all the researchers was the two major cell surface isoforms of Dectin-1, despite the fact that its soluble functional intracellular isoform (Dectin-1E) has already been dissected but is indefinable. Therefore, this review intensely recommends the need of future research to resolve the un-resolved and treasure the comprehensive role of Dectin-1 in different clinical outcomes, before determining its therapeutic prospective.

3D Structural Insights into β-Glucans and Their Binding Proteins

β(1,3)-glucans are a component of fungal and plant cell walls. The β-glucan of pathogens is recognized as a non-self-component in the host defense system. Long β-glucan chains are capable of forming a triple helix structure, and the tertiary structure may profoundly affect the interaction with β-glucan-binding proteins. Although the atomic details of β-glucan binding and signaling of cognate receptors remain mostly unclear, X-ray crystallography and NMR analyses have revealed some aspects of β-glucan structure and interaction. Here, we will review three-dimensional (3D) structural characteristics of β-glucans and the modes of interaction with β-glucan-binding proteins.

Can SARS-CoV-2 Virus Use Multiple Receptors to Enter Host Cells?

The occurrence of the novel severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2), responsible for coronavirus disease 2019 (COVD-19), represents a catastrophic threat to global health. Protruding from the viral surface is a densely glycosylated spike (S) protein, which engages angiotensin-converting enzyme 2 (ACE2) to mediate host cell entry. However, studies have reported viral susceptibility in intra- and extrapulmonary immune and non-immune cells lacking ACE2, suggesting that the S protein may exploit additional receptors for infection. Studies have demonstrated interactions between S protein and innate immune system, including C-lectin type receptors (CLR), toll-like receptors (TLR) and neuropilin-1 (NRP1), and the non-immune receptor glucose regulated protein 78 (GRP78). Recognition of carbohydrate moieties clustered on the surface of the S protein may drive receptor-dependent internalization, accentuate severe immunopathological inflammation, and allow for systemic spread of infection, independent of ACE2. Furthermore, targeting TLRs, CLRs, and other receptors (Ezrin and dipeptidyl peptidase-4) that do not directly engage SARS-CoV-2 S protein, but may contribute to augmented anti-viral immunity and viral clearance, may represent therapeutic targets against COVID-19.

Latent 1,3-β-D-glucan acts as an adjuvant for allergen-specific IgE production induced by Japanese cedar pollen exposure

BACKGROUND

The pollen grains of several plant species contain 1,3-β-D-glucan (BG). BG activates dendritic cells (DCs) and subsequently regulates the innate immune responses. Within Japan, the most common disease associated with type-I hypersensitivity is Japanese cedar pollinosis. However, the role of BG in Japanese cedar pollen (JCP) remains unclear. This study examined the localization and immunological effects of BG in JCP.

METHODS

The localization of BG in JCP grain was determined by immunohistochemical staining using a soluble dectin-1 protein probe and a BG recognition protein (BGRP). The content of BG extracted from JCP was measured by a BGRP-based ELISA-like assay. The cytokine production by bone marrow-derived DCs (BMDCs) obtained from wild-type and BG receptor (dectin-1) knock-out mice was examined in vitro. The mice were intranasally administered JCP grains and the specific serum Ig levels were then quantified.

RESULTS

BG was detected in the exine and cell wall of the generative cell and tube cell of the JCP grain. Moreover, BG in the exine stimulated production of TNF-α and IL-6 in the BMDCs via a dectin-1-dependent mechanism. Meanwhile, JCP-specific IgE and IgG were detected in the serum of wild-type mice that had been intranasally administered with JCP grains. These mice also exhibited significantly enhanced sneezing behavior. However, dectin-1 knock-out mice exhibited significantly lower JCP-specific IgE and IgG levels compared to wild-type mice.

CONCLUSIONS

Latent BG in JCP can act as an adjuvant to induce JCP-specific antibody production via dectin-1.

Binding assay of human Dectin-1 variants to DNA/β-glucan complex for active-targeting delivery of antisense DNA

The lectin Dectin-1 is a good target for β-glucan-mediated drug delivery. Although many murine studies of Dectin-1 have been performed, its human analog has not been studied well in terms of being a drug delivery target. We thus analyzed human Dectin-1 cDNA obtained from chronic myelogenous leukemia-derived cells, CML-1, and confirmed the findings of previous studies that there are many isoforms of human Dectin-1 due to exon skipping, although murine Dectin-1 has only two forms. When we transfected the Dectin-1 gene into a non-Dectin-1-expressing cell line and examined cellular uptake of the antisense DNA/β-glucan complex, we confirmed that expression of the target gene was effectively suppressed through β-glucan/Dectin-1-mediated uptake. The present results suggest that the β-glucan complex would be an effective tool to deliver antisense oligonucleotide (AS-ODN) to Dectin-1-expressing cells not only for mice but also for humans.

Macroalgal dietary glycans: potential source for human gut bacteria and enhancing immune system for better health

Macroalgae are the diverse group of photosynthetic algae found at the intertidal regions of oceans. Recent advances suggest that macroalgal derived glycans have tremendous potential to maintain gut microbiome and immune system. The human gut bacteria harbor unique arsenals for utilizing a variety of macroalgal glycans, and produce a variety of oligosaccharides in vivo. Those oligosaccharides interact with immune cell receptors, and also are available for microbial fermentation, thus play magnificent roles in balancing the gut homeostasis. However, this area of research is still in infancy condition in term to understand their molecular interactions. For wooing this area, we urge to emphasize more studies on mechanistic level sympathetic of depolymerizing marine dietary glycans by gut bacteria and elucidating molecular aspect of glycans to cell receptors interactions. This will invent new nutraceutical strategies to purposefully manipulate the microbial composition to improve health. Therefore, review focuses on the recent development of mechanistic understanding of human gut bacterial communities for utilizing macroalgal derived glycans. Recent trends of application of glycans in modulating immune system at mechanistic level and their available evidences are discussed.

Chemical Synthesis of Residue-Selectively 13C and 2H Double-Isotope-Labeled Oligosaccharides as Chemical Probes for the NMR-Based Conformational Analysis of Oligosaccharides

The conformational analysis of oligosaccharide is a fundamental issue in glycobiology. NMR measurements of atom-selectively ¹³C-labeled oligosaccharides have provided valuable information concerning their conformation, which would not be possible using nonlabeled oligosaccharides. The amount of accessible information from an atom-selectively labeled molecule, however, is limited. In this work, we report on the chemical synthesis of residue-selectively ¹³C- and ²H-labeled oligosaccharides and their use in conformational analysis. ¹H NMR measurements of such double isotope-labeled compounds can provide a great deal of information on the dihedral angles across glycosidic linkages. We demonstrated this method in the conformational analyses of some linear and branched β(1,3)-glucan oligosaccharides.

Biological response modifier glucan through balancing of blood glucose may have a prophylactic potential in COVID-19 patients

With the COVID-19 pandemic causing huge threat to public health and definite treatment modalities and preventive vaccines yet to be arrived at, some of the key indicators of relevance to its prognosis have started emerging. One such independent predictor of outcome has been fasting plasma glucose (FPG) at the time of admission. Earlier, co-morbidities such as diabetes also have been reported to have a risk of relatively increased mortality due to COVID-19. In this background, we herein report on the beneficial effects of Biological response modifier glucan (BRMG) secreted by the black yeast Aureobasidium pullulans AFO-202 which has been proven to bring under control blood sugar levels in human subjects and also has potential in enhancing & regulating the immune parameters in relevance to COVID-19. We further recommend that this BRMG be tried in clinical studies of COVID-19 to provide a prophylactic effect for validation.

Induction of potent cell growth inhibition by schizophyllan/K-ras antisense complex in combination with gemcitabine

Antisense oligonucleotides (AS-ODNs) specifically hybridize with target mRNAs, resulting in interference with the splicing mechanism or the regulation of protein translation. In our previous reports, we demonstrated that β-glucan schizophyllan (SPG) can form a complex with AS-ODNs attached with oligo deoxyadenosine dA₄₀ (AS-ODN-dA₄₀/SPG), and that this complex can be recognized by β-glucan receptor Dectin-1 on antigen presenting cells and lung cancer cells. In many types of cancer cell, activating K-ras mutations related to malignancy are frequently observed. In this study, we first designed 78 AS-ODNs for K-ras to optimize the sequence for highly efficient gene suppression. The selected AS-ODN (K-AS07) having dA₄₀ made a complex with SPG. The resultant complex (K-AS07-dA₄₀/SPG) showed an effect of silencing the ras gene in the cells (PC9: human adenocarcinoma differentiated from lung tissue) expressing Dectin-1, leading to the suppression of cell growth. Furthermore, the cytotoxic effect was enhanced when used in combination with the anticancer drug gemcitabine. Gemcitabine, a derivative of cytidine, was shown to interact with dA₄₀ in a sequence-dependent manner. This interaction did not appear to be so strong, with the gemcitabine being released from the complex after internalization into the cells. SPG and the dA₄₀ part of K-AS07-dA₄₀ play roles in carriers for K-AS07 and gemcitabine, respectively, resulting in a strong cytotoxic effect. This combination effect is a novel feature of the AS-ODN-dA₄₀/SPG complexes. These results could facilitate the clinical application of these complexes for cancer treatment.

iTRAQ-Based Comparative Proteomic Analysis of Larval Midgut From the Beet Armyworm, Spodoptera exigua (Hübner) (Lepidoptera: Noctuidae) Challenged With the Entomopathogenic Bacteria Serratia marcescens

Entomopathogenic bacteria Serratia marcescens is widely used as an environmentally friendly biocontrol agent against various pests, including Spodoptera exigua. Understanding the immune defense mechanism of S. exigua through comparative proteomic analysis can identify the key proteins expressed in response to the microbial infection. Here, we employed the as isobaric tags for relative and absolute quantification (iTRAQ) technique to investigate the effects of S. marcescens on the proteomic expression of S. exigua. Based on the molecular functional analysis, the differentially expressed proteins (DEPs) were mainly involved in the binding process and catalytic activities. Further bioinformatics analysis revealed important DEPs that played a crucial role in innate immunity of S. exigua with recognition (C-type lectin), melanization (propanol oxidase 3, serine protease, Serine-type carboxypeptidase activity, clip domain serine protease 4), antimicrobial activity (lysozyme, lysozyme-like, gloverin, cecropin B), detoxification (acetyl-CoA C-acetyltransferase, 3-dehydroecdysone 3-alpha-reductase, glucuronosyltransferase, glutathione S-transferase) and others. The Quantitative real-time PCR (qRT-PCR) results further indicated the significant upregulation of the immune-related genes in Spodoptera exigua following S. marcescens infection. To the best of our knowledge, this is the first iTRAQ based study to characterize S. marcescens mediated proteomic changes in S. exigua and identified important immune-related DEPs. The results of this study will provide an essential resource for understanding the host-pathogen interactions and the development of novel microbial biopesticides against various pests.

Topical Therapy with Antisense Tumor Necrosis Factor Alpha Using Novel β-Glucan-Based Drug Delivery System Ameliorates Intestinal Inflammation

Anti-tumor necrosis factor alpha (TNF-α) antibodies are effective in patients with inflammatory bowel disease (IBD). However, the effect is not optimal because a sufficient concentration of antibodies cannot be maintained at the site of inflammation. Thus, a macromolecular complex was developed with schizophyllan (SPG) and antisense oligonucleotides. In the present study, an SPG-antisense TNF-α complex was prepared, and its therapeutic efficacy was examined using a dextran sodium sulfate (DSS)-induced colitis model. The TNF-α production in CD11b+ macrophages significantly increased in the colon of DSS-treated mice. Dectin-1, a receptor of SPG, binds with SPG and is subsequently taken into the cells via phagocytosis. The expression of dectin-1 by CD11b+ macrophages significantly increased in DSS-treated mice. Flow cytometry revealed that the uptake of SPG-antisense TNF-α in the macrophages was efficient. TNF-α production was suppressed significantly by SPG-antisense TNF-α in vitro, which was administered via enema to evaluate its efficacy. The intrarectal administration of SPG-antisense TNF-α ameliorated the intestinal inflammation. In this study, we showed that the delivery system that conjugates SPG and antisense can have higher therapeutic efficacy. Thus, the new therapeutic approach presented in this study may be used in the management of IBD.

Schizophyllum commune induces IL-17-mediated neutrophilic airway inflammation in OVA-induced asthma model mice

Schizophyllum commune is a ubiquitous basidiomycetous fungus typically found across the world, which has been detected in indoor and outdoor air. Some studies indicated that sensitization to S. commune is correlated with asthma severity in patients. Patients with chronic severe or acute fatal asthma have neutrophil-dominant airway inflammation. We hypothesized that S. commune can exacerbate asthma. To test this hypothesis, we evaluated the direct immunomodulatory activities of S. commune in allergic airway inflammation induced by non-fungal sensitization. Ovalbumin (OVA)-induced asthma model mice were generated using wild-type (WT) and Il-17a^-/-Il-17f^-/- mice that were intratracheally exposed to S. commune, then immune responses in the lungs were assessed after 24 h. Intratracheal administration of S. commune in OVA-induced asthma model mice enhanced neutrophilic airway inflammation, increased the mRNA expression of CXCL1 and CXCL2 in the lungs, and provoked IL-17A, and IL-17F production in BAL fluid. In addition, neutrophilic airway inflammation was significantly inhibited in Il-17a^-/-Il-17f^-/- mice compared with those found in WT mice. We demonstrated that S. commune induces neutrophilic airway inflammation in OVA-induced asthma model mice, and IL-17A and IL-17F had central roles in this activity. As S. commune inhabits the general environment, including indoor and outdoor air, our results suggested that S. commune is a causative agent of asthma exacerbation. This study has provided clues regarding the mechanisms behind fungi and asthma exacerbation.

The Core Fucose on an IgG Antibody is an Endogenous Ligand of Dectin-1

The core fucose, a major modification of N-glycans, is implicated in immune regulation, such as the attenuation of the antibody-dependent cell-mediated cytotoxicity of antibody drugs and the inhibition of anti-tumor responses via the promotion of PD-1 expression on T cells. Although the core fucose regulates many biological processes, no core fucose recognition molecule has been identified in mammals. Herein, we report that Dectin-1, a known anti-β-glucan lectin, recognizes the core fucose on IgG antibodies. A combination of biophysical experiments further suggested that Dectin-1 recognizes aromatic amino acids adjacent to the N-terminal asparagine at the glycosylation site as well as the core fucose. Thus, Dectin-1 appears to be the first lectin-like molecule involved in the heterovalent and specific recognition of characteristic N-glycans on antibodies.

N-Terminal (1→3)-β-d-Glucan Recognition Proteins from Insects Recognize the Difference in Ultra-Structures of (1→3)-β-d-Glucan

Recognition of (1→3)-β-d-glucans (BGs) by invertebrate β-1,3-d-glucan recognition protein (BGRP) plays a significant role in the activation of Toll pathway and prophenoloxidase systems in insect host defense against fungal invasion. To examine the structure diversity of BGRPs for the recognition of physiochemically different BGs, the binding specificity of BGRPs cloned from four different insects to structure different BGs was characterized using ELISA. Recombinant BGRPs expressed as Fc-fusion proteins of human IgG1 bound to the solid phase of BGs. Based on the binding specificities, the BGRPs were categorized into two groups with different ultrastructures and binding characters; one group specifically binds BGs with triple-helical conformation, while the other group recognizes BGs with disordered conformations like single-helical or partially opened triple helix. The BGRPs from the silkworm and the Indian meal moth bound to the BGs with a triple-helical structure, whereas BGRPs from the red flour beetle and yellow mealworm beetle showed no binding to triple-helical BGs, but bound to alkaline-treated BGs that have a partially opened triple-helical conformation. This evidence suggests that the insect BGRPs can distinguish between different conformations of BGs and are equipped for determining the diversity of BG structures.

C-type lectin receptor-mediated immune recognition and response of the microbiota in the gut

C-type lectin receptors (CLRs) are powerful pattern-recognition receptors that discern ‘self’ and ‘non-self’ in our body and protect us from invasive pathogens by mediating immune recognition and response. The gastrointestinal tract is very important for the maintenance of homeostasis; it is the largest shelter for the billions of microorganisms in the body and CLRs play a crucial regulatory role in this system. This study focuses on several CLRs, including Dectin-1, Dectin-2, Dectin-3 and Mincle. We summarize the roles of CLRs in maintaining gastrointestinal immune-system homeostasis, especially their functions in mediating immune recognition and responses in the gut, discuss their relationships to some diseases, highlight the significance of CLR-mediated sensing of microbial and non-microbial compounds in the gut immune system and identify new therapeutic targets.

Consumption of β-glucans to spice up T cell treatment of tumors: a review

INTRODUCTION

Adoptive T-cell treatments of solid cancers have evolved into a robust therapy with objective response rates surpassing those of standardized treatments. Unfortunately, only a limited fraction of patients shows durable responses, which is considered to be due to a T cell-suppressive tumor microenvironment (TME). Here we argue that naturally occurring β-glucans can enable reversion of such T cell suppression by engaging innate immune cells and enhancing numbers and function of lymphocyte effectors.

AREAS COVERED

This review summarizes timely reports with respect to absorption, trafficking and immune stimulatory effects of β-glucans, particularly in relation to innate immune cells. Furthermore, we list effects toward well-being and immune functions in healthy subjects as well as cancer patients treated with orally administered β-glucans, extended with effects of β-glucan treatments in mouse cancer models.

EXPERT OPINION

Beta-glucans, when present in food and following uptake in the proximal gut, stimulate immune cells present in gut-associated lymphoid tissue and initiate highly conserved pro-inflammatory pathways. When tested in mouse cancer models, β-glucans result in better control of tumor growth and shift the TME toward a T cell-sensitive environment. Along these lines, we advocate that intake of β-glucans provides an accessible and immune-potentiating adjuvant when combined with adoptive T-cell treatments of cancer.

Studies Into β-Glucan Recognition in Fish Suggests a Key Role for the C-Type Lectin Pathway

Immune-modulatory effects of β-glucans are generally considered beneficial to fish health. Despite the frequent application of β-glucans in aquaculture practice, the exact receptors and downstream signalling remains to be described for fish. In mammals, Dectin-1 is a member of the C-type lectin receptor (CLR) family and the best-described receptor for β-glucans. In fish genomes, no clear homologue of Dectin-1 could be identified so far. Yet, in previous studies we could activate carp macrophages with curdlan, considered a Dectin-1-specific β-(1,3)-glucan ligand in mammals. It was therefore proposed that immune-modulatory effects of β-glucan in carp macrophages could be triggered by a member of the CLR family activating the classical CLR signalling pathway, different from Dectin-1. In the current study, we used primary macrophages of common carp to examine immune modulation by β-glucans using transcriptome analysis of RNA isolated 6 h after stimulation with two different β-glucan preparations. Pathway analysis of differentially expressed genes (DEGs) showed that both β-glucans regulate a comparable signalling pathway typical of CLR activation. Carp genome analysis identified 239 genes encoding for proteins with at least one C-type Lectin Domains (CTLD). Narrowing the search for candidate β-glucan receptors, based on the presence of a conserved glucan-binding motif, identified 13 genes encoding a WxH sugar-binding motif in their CTLD. These genes, however, were not expressed in macrophages. Instead, among the β-glucan-stimulated DEGs, a total of six CTLD-encoding genes were significantly regulated, all of which were down-regulated in carp macrophages. Several candidates had a protein architecture similar to Dectin-1, therefore potential conservation of synteny of the mammalian Dectin-1 region was investigated by mining the zebrafish genome. Partial conservation of synteny with a region on the zebrafish chromosome 16 highlighted two genes as candidate β-glucan receptor. Altogether, the regulation of a gene expression profile typical of a signalling pathway associated with CLR activation and, the identification of several candidate β-glucan receptors, suggest that immune-modulatory effects of β-glucan in carp macrophages could be a result of signalling mediated by a member of the CLR family.

β-Glucan-induced cooperative oligomerization of Dectin-1 C-type lectin-like domain

Dectin-1 is a C-type lectin-like pattern recognition receptor that recognizes β(1-3)-glucans present on non-self pathogens. It is of great importance in innate immunity to understand the mechanism whereby Dectin-1 senses β(1-3)-glucans and induces intracellular signaling. In this study, we characterize the ligand binding and ligand-induced oligomerization of murine Dectin-1 using its C-type lectin-like domain (CTLD). Interaction of CTLD with laminarin, a β-glucan ligand, induced a tetramer of CTLD, as evidenced by size exclusion chromatography and multi-angle light scattering. Component analysis suggested a stoichiometry of four CTLD molecules bound to four laminarin molecules. Dimers and trimers of CTLD were not detected suggesting cooperative oligomerization. In order to map the amino acid residues of CTLD involved in β-glucan binding and domain oligomerization, we performed site-directed mutagenesis on surface-exposed and most conserved amino acid residues. Among the mutants examined, W221A, H223A and Y228A abolished oligomer formation. Since these residues are spatially arranged to form a hydrophobic groove, it is likely that W221, H223 and Y228 are directly involved in β-glucan binding. Interestingly, mutation of the residues on the other side of the hydrophobic groove, including Y141, R145 and E243, also exhibited reduced oligomer formation, suggesting involvement in protein-protein interactions guided by laminarin. Ligand titration using intrinsic tryptophan fluorescence revealed that wild-type CTLD binds laminarin cooperatively with a Hill coefficient of ~3, while the oligomer-reducing mutations, inside and outside the putative binding site abolish or decrease cooperativity. We suggest that the ligand-induced cooperative oligomer formation of Dectin-1 is physiologically relevant in sensing exogenous β-glucan and triggering intracellular signaling.

Complex consisting of antisense DNA and β-glucan promotes internalization into cell through Dectin-1 and hybridizes with target mRNA in cytosol

Antisense oligonucleotides (AS-ODNs) hybridize with specific mRNAs, resulting in interference with the splicing mechanism or the regulation of protein translation. We previously demonstrated that the β-glucan schizophyllan (SPG) can form a complex with AS-ODNs with attached dA₄₀ (AS-ODNs/SPG), and this complex can be incorporated into cells, such as macrophages and dendritic cells, expressing the β-glucan receptor Dectin-1. We have achieved efficient gene silencing in animal models, but the uptake mechanism and intracellular distribution are unclear. In this study, we prepared the complex consisting of SPG and AS-ODNs (AS014) for Y-box binding protein-1 (YB-1). After treatment with endocytosis inhibitor Pitstop 2 and small interfering RNA targeting Dectin-1, we found that AS014/SPG complexes are incorporated into cells by Dectin-1-mediated endocytosis and inhibit cell growth in a Dectin-1 expression level-dependent manner. After treatment with AS014/SPG complexes, we separated the cell lysate into endosomal and cytoplasmic components by ultracentrifugation and directly determined the distribution of AS014 by reverse transcription PCR using AS014 ODNs as a template or a reverse transcription primer. In the cytoplasm, AS014 clearly hybridized with YB-1 mRNAs. This is the first demonstration of the distinct distribution of the complex in cells. These results could facilitate the clinical application of the complex.

The Interaction of Human Pathogenic Fungi With C-Type Lectin Receptors

Fungi, usually present as commensals, are a major cause of opportunistic infections in immunocompromised patients. Such infections, if not diagnosed or treated properly, can prove fatal. However, in most cases healthy individuals are able to avert the fungal attacks by mounting proper antifungal immune responses. Among the pattern recognition receptors (PRRs), C-type lectin receptors (CLRs) are the major players in antifungal immunity. CLRs can recognize carbohydrate ligands, such as β-glucans and mannans, which are mainly found on fungal cell surfaces. They induce proinflammatory immune reactions, including phagocytosis, oxidative burst, cytokine, and chemokine production from innate effector cells, as well as activation of adaptive immunity via Th17 responses. CLRs such as Dectin-1, Dectin-2, Mincle, mannose receptor (MR), and DC-SIGN can recognize many disease-causing fungi and also collaborate with each other as well as other PRRs in mounting a fungi-specific immune response. Mutations in these receptors affect the host response and have been linked to a higher risk in contracting fungal infections. This review focuses on how CLRs on various immune cells orchestrate the antifungal response and on the contribution of single nucleotide polymorphisms in these receptors toward the risk of developing such infections.

ABO Antigens Active Tri- and Disaccharides Microarray to Evaluate C-type Lectin Receptor Binding Preferences

Understanding blood group antigen binding preferences for C-type lectin receptors holds promise for modulating immune responses, since several Gram-negative bacteria express blood group antigens as molecular mimicry to evade immune responses. Herein, we report the synthesis of ABO blood group antigen active tri and disaccharides to investigate the binding specificity with various C-type lectin receptors using glycan microarray. The results of binding preferences show that distinct glycosylation on the galactose and fucose motifs are key for C-type lectin receptor binding and that these interactions occur in a Ca²⁺-dependent fashion.

iTRAQ-based comparative proteome analysis for molecular mechanism of defense against acute ammonia toxicity in Pacific White shrimp Litopenaeus vannamei

In the practical farming of Litopenaeus vannamei, the intensive culture system and environmental pollution usually results in a high concentration of ammonia, which brings large detrimental effects to shrimp, such as increasing the susceptibility to pathogens and even causing high mortality. We have revealed that the survival time under acute ammonia stress varied substantially among different families and obtained ammonia-tolerant (LV_T) and ammonia-sensitive (LV_S) families. In order to understand the molecular mechanism of defense against ammonia toxicity in shrimp, we performed iTRAQ LC-MS/MS proteomic analysis between LV_T and LV_S groups of L. vannamei under acute ammonia stress to identify the key proteins and pathways that play an effective role for against ammonia toxicity. By comparative proteome analysis, 202 significantly differentially proteins (DEPs) were identified in LV_T compared to LV_S, and most of the DEPs (60%) were up-regulated. Excepting for the proteins without function reporting, the meaningful finding is that 77.8% of the DEPs have been reported mainly involving in immune defense and stress tolerant in crustacean species, such as hemocyanin, Rab7, Rab GTPase, Rac1, alpha 2 macroglobulin, Bip, peroxiredoxin, Cu/Zn SOD, glutathione peroxidase, thioredoxin, calreticulin, and Elongation Factor 1-alpha, etc. These DEPs might potentially play important role in against ammonia toxicity, and it also reflected a relation between ammonia tolerance and pathogen resistance. In addition, a total of 10 significantly changed KEGG pathways were detected, and the network diagram of these KEGG pathways showed that more critical nodes were up-regulated, which involved in protein synthesis and transport, and against stress stimuli. This study provided important information for understanding the molecular mechanism of defense against ammonia toxicity in shrimp at whole protein level.

Contact, Collaboration, and Conflict: Signal Integration of Syk-Coupled C-Type Lectin Receptors

Several spleen tyrosine kinase-coupled C-type lectin receptors (CLRs) have emerged as important pattern recognition receptors for infectious danger. Because encounter with microbial pathogens leads to the simultaneous ligation of several CLRs and TLRs, the signals emanating from different pattern recognition receptors have to be integrated to achieve appropriate biological responses. In this review, we briefly summarize current knowledge about ligand recognition and core signaling by Syk-coupled CLRs. We then address mechanisms of synergistic and antagonistic crosstalk between different CLRs and with TLRs. Emerging evidence suggests that signal integration occurs through 1) direct interaction between receptors, 2) regulation of expression levels and localization, and 3) collaborative or conflicting signaling interference. Accordingly, we aim to provide a conceptual framework for the complex and sometimes unexpected outcome of CLR ligation in bacterial and fungal infection.

Dectin-1 intracellular domain determines species-specific ligand spectrum by modulating receptor sensitivity

C-type lectin receptors (CLRs) comprise a large family of immunoreceptors that recognize polysaccharide ligands exposed on pathogen surfaces and are conserved among mammals. However, interspecies differences in their ligand spectrums are not fully understood. Dectin-1 is a well-characterized CLR that recognizes β-glucan. We report here that seaweed-derived fucan activates cells expressing human Dectin-1 but not mouse Dectin-1. Low-valency β-glucan components within fucan appeared to be responsible for this activation, as the ligand activity was eliminated by β-glucanase treatment. The low-valency β-glucan laminarin also acted as an agonist for human Dectin-1 but not for mouse Dectin-1, whereas the high-valency β-glucan curdlan activated both human and mouse Dectin-1. Reciprocal mutagenesis analysis revealed that the ligand-binding domain of human Dectin-1 does not determine its unique sensitivity to low-valency β-glucan. Rather, we found that its intracellular domain renders human Dectin-1 reactive to low-valency β-glucan ligand. Substitution with two amino acids, Glu² and Pro⁵, located in the human Dectin-1 intracellular domain was sufficient to confer sensitivity to low-valency β-glucan in mouse Dectin-1. Conversely, the introduction of mouse-specific amino acids, Lys² and Ser⁵, to human Dectin-1 reduced the reactivity to low-valency β-glucan. Indeed, low-valency ligands induced a set of proinflammatory genes in human but not mouse dendritic cells. These results suggest that the intracellular domain, not ligand-binding domain, of Dectin-1 determines the species-specific ligand profile.