Novel Ca2+ -independent carbohydrate recognition of the C-type lectins, SPL-1 and SPL-2, from the bivalve Saxidomus purpuratus

A C-type lectin (CTL2) mediated both humoral and cellular immunity against bacterial infection in Tribolium castaneum

C-type lectins (CTLs) play essential roles in humoral and cellular immune responses of invertebrates. Previous studies have demonstrated the involvement of CTLs in the humoral immunity of Tribolium castaneum, a worldwide pest in stored products. However, the function of CTLs in cellular immunity remains unclear. Here, we identified a CTL gene located on chromosome X and designated it as CTL2 (TcCTL2) from T. castaneum. It encodes a protein of 305 amino acids with a secretion signal peptide and a carbohydrate-recognition domain. TcCTL2 was mainly expressed in the early pupae and primarily distributed in the hemocytes in the late larvae. It was significantly upregulated after larvae were infected with Escherichia coli or Staphylococcus aureus, while knockdown of TcCTL2 exacerbates larval mortality and bacterial colonization after infection. The purified recombinant TcCTL2 (rTcCTL2) can bind to pathogen-associated molecular patterns and microbes and promote hemocyte-mediated encapsulation, melanization and phagocytosis in vitro. rTcCTL2 also induced bacterial agglutination in a Ca2+-dependent manner. Knockdown of TcCTL2 drastically suppressed encapsulation, melanization, and phagocytosis. Furthermore, silencing of TcCTL2 followed by bacterial infection significantly decreased the expression of transcription factors in Toll and IMD pathways, antimicrobial peptides, and prophenoloxidases and phenoloxidase activity. These results unveiled that TcCTL2 mediates both humoral and cellular immunity to promote bacterial clearance and protect T. castaneum from infectious microbes, which will deepen the understanding of the interaction between CTLs and innate immunity in T. castaneum and permit the optimization of pest control strategies by a combination of RNAi technology and bacterial infection.

A C-type lectin with a single carbohydrate-recognition domain (CRD) containing unique QPN/WDD motifs from Tegillarca granosa is involved in the innate immune defense

C-type lectins (CTLs), a superfamily of Ca2+-dependent carbohydrate-recognition proteins, serve as pattern recognition receptors (PRRs) in the immune response of many species. However, little is currently known about the CTLs of the commercially and ecologically important bivalve species, blood clam (Tegillarca granosa). In this study, a CTL (designated as TgCTL-1) with a single carbohydrate-recognition domain (CRD) containing unique QPN/WDD motifs was identified in the blood clam through transcriptome and whole-genome searching. Multiple alignment and phylogenetic analysis strongly suggested that TgCTL-1 was a new member of the CTL superfamily. Expression analysis demonstrated that TgCTL-1 was highly expressed in the hemocytes and visceral mass of the clam under normal condition. In addition, the expression of TgCTL-1 was shown to be significantly up-regulated upon pathogen challenge. Moreover, the recombinant TgCTL-1 (rTgCTL-1) displayed agglutinating and binding activities against both the gram-positive and gram-negative bacteria tested in a Ca2+-dependent manner. Furthermore, it was found that the in vitro phagocytic activity of hemocytes was significantly enhanced by rTgCTL-1. In general, our results showed that TgCTL-1 was an inducible acute-phase secretory protein, playing crucial roles in recognizing, agglutinating, and binding to pathogenic bacteria as well as modulating phagocytic activity of hemocytes in the innate immune defense of blood clam.

Functional Diversity of Novel Lectins with Unique Structural Features in Marine Animals

Due to their remarkable structural diversity, glycans play important roles as recognition molecules on cell surfaces of living organisms. Carbohydrates exist in numerous isomeric forms and can adopt diverse structures through various branching patterns. Despite their relatively small molecular weights, they exhibit extensive structural diversity. On the other hand, lectins, also known as carbohydrate-binding proteins, not only recognize and bind to the diverse structures of glycans but also induce various biological reactions based on structural differences. Initially discovered as hemagglutinins in plant seeds, lectins have been found to play significant roles in cell recognition processes in higher vertebrates. However, our understanding of lectins in marine animals, particularly marine invertebrates, remains limited. Recent studies have revealed that marine animals possess novel lectins with unique structures and glycan recognition mechanisms not observed in known lectins. Of particular interest is their role as pattern recognition molecules in the innate immune system, where they recognize the glycan structures of pathogens. Furthermore, lectins serve as toxins for self-defense against foreign enemies. Recent discoveries have identified various pore-forming proteins containing lectin domains in fish venoms and skins. These proteins utilize lectin domains to bind target cells, triggering oligomerization and pore formation in the cell membrane. These findings have spurred research into the new functions of lectins and lectin domains. In this review, we present recent findings on the diverse structures and functions of lectins in marine animals.

A Genomic and Transcriptomic Analysis of the C-Type Lectin Gene Family Reveals Highly Expanded and Diversified Repertoires in Bivalves

C-type lectins belong to a widely conserved family of lectins characterized in Metazoa. They show important functional diversity and immune implications, mainly as pathogen recognition receptors. In this work, C-type lectin-like proteins (CTLs) of a set of metazoan species were analyzed, revealing an important expansion in bivalve mollusks, which contrasted with the reduced repertoires of other mollusks, such as cephalopods. Orthology relationships demonstrated that these expanded repertoires consisted of CTL subfamilies conserved within Mollusca or Bivalvia and of lineage-specific subfamilies with orthology only between closely related species. Transcriptomic analyses revealed the importance of the bivalve subfamilies in mucosal immunity, as they were mainly expressed in the digestive gland and gills and modulated with specific stimuli. CTL domain-containing proteins that had additional domains (CTLDcps) were also studied, revealing interesting gene families with different conservation degrees of the CTL domain across orthologs from different taxa. Unique bivalve CTLDcps with specific domain architectures were revealed, corresponding to uncharacterized bivalve proteins with putative immune function according to their transcriptomic modulation, which could constitute interesting targets for functional characterization.

Molecular and functional characterization of a ladderlectin-like molecule from ayu (Plecoglossus altivelis)

Ladderlectin is a member of C-type lectins (CTLs) in teleost fish and involved in innate immune defense. In this study, ayu (Plecoglossus altivelis) ladderlecin-like (PaLL-like) sequence was cloned, which encodes a polypeptide of 172 amino acids that includes a signal peptide and characteristic C-type lectin-like domains (CTLDs). Phylogenetically, PaLL-like was most closely related to its teleost counterpart from shishamo smelt (Spirinchus lanceolatus). Expression analysis revealed a ubiquitous expression profile, with highest expression detected in liver and its expression was up-regulated following Vibiro anguillarum infection. Similar to canonical CTLs, PaLL-like exhibited carbohydrate-binidng capacities to a wide range of well-defined mono-/di-saccharides and likely confer PaLL-like the ability to agglutinate all tested bacterial, including three Gram-positive species (i.e., Listeria monocytogenes, Staphylococcus aureus and Streptococcus iniae) and eight Gram-negative species (i.e., Edwardsiella tarda, Aeromonas (A.) hydrophila, Escherichia coli, Vibrio (V.) harveyi, V. anguillarum, V. parahemolyticus, A. versoni and V. vulnificus), in a calcium-dependent manner. Further functional studies revealed that PaLL-like displayed immunomodulatory activities leading to enhanced bactericidal activity of serum, pathogen opsonization and macrophage activation with increased expression of pro-inflammatory cytokines (i.e., PaIL-1β and PaTNF-α). Collectively, these immunomodulatory activities of PaLL-like suppressed proliferations of V. anguillarum in targeted tissued in vivo and likely contributed to the increased survival rate of infected-fish. Overall, our results demonstrated PaLL-like is a critical component of innate immunity and provides protective effects against bacterial infection.

C-type lectin-(like) fold - Protein-protein interaction patterns and utilization

The C-type lectin-like fold (CTL fold) is a building block of many proteins, including saccharide-binding lectins, natural killer cell receptors, macrophage mannose receptor, selectins, collectins, snake venoms and others. Some are important players in innate immunity and are involved in the first-line response to virally infected cells or cancer cells, some play a role in antimicrobial defense, and some are potential targets for fight against problems connected with allergies, obesity, and autoimmunity. The structure of a CTL domain typically contains two α-helices, two small β-sheets and a long surface loop, with two or three disulfide bridges stabilizing the structure. This small domain is often involved in interactions with a target molecule, however, utilizing varied parts of the domain surface, with or without structural modifications. More than 500 three-dimensional structures of CTL fold-containing proteins are available in the Protein Data Bank, including a significant number of complexes with their key interacting partners (protein:protein complexes). The amount of available structural data enables a detailed analysis of the rules of interaction patterns utilized in activation, inhibition, attachment and other pathways or functionalities. Interpretation of known CTL receptor structures and all other CTL-containing proteins and complexes with described three-dimensional structures, complemented with sequence/structure/interaction correlation analysis offers a comprehensive view of the rules of interaction patterns of the CTL fold. The results are of value for prediction of interaction behavior of so far not understood CTL-containing proteins and development of new protein binders based on this fold, with applications in biomedicine or biotechnologies. It follows from the available structural data that almost the whole surface of the CTL fold is utilized in protein:protein interactions, with the heaviest frequency of utilization in the canonical interaction region. The individual categories of interactions differ in the interface buildup strategy. The strongest CTL binders rely on interfaces with large interaction area, presence of hydrophobic core, or high surface complementarity. The typical interaction surfaces of the fold are not conserved in amino acid sequence and can be utilized in design of new binders for biotechnological applications.

CTL10 has multiple functions in the innate immune responses of the silkworm, Bombyx mori

Insect C-type lectins (CTLs) play crucial roles in modulating the humoral and cellular immune responses. In the domesticated silkworm Bombyx mori L., BmCTL10 gene encodes an immulectin containing two carbohydrate recognition domains (CRDs). The phylogenetic analysis showed that BmCTL10 didn't cluster with other immulectin homologs in B. mori. BmCTL10 was mainly expressed in second to fifth instar larvae, wandering stage larvae, prepupa, and adults. In naïve fifth instar larvae, BmCTL10 was predominantly expressed in the fat body and epidermis. In second instar larvae, the topical application of Beauveria bassiana by immersion caused down-regulation of BmCTL10. The intra-hemocoel injection of E. coli, S. aureus, B. bassiana, and 20-hydroxyecdysone in fifth instar larvae caused tissue and time-specific inductions. The recombinant protein (rBmCTL10) can bind to larval hemocytes and various pathogen-associated molecular patterns to enhance hemocyte-mediated nodulation, phagocytosis, and encapsulation. rBmCTL10 caused significant upregulation of most antimicrobial peptides and nitric oxide synthase 1 in hemocytes in vivo. Yeast two-hybrid demonstrated that integrin β3 and β4 subunits can interact with BmCTL10. Furthermore, only CRD2 can interact with the β3, while both CRD1 and CRD2 can interact with the β4. Taken together, this study showed that BmCTL10 has multiple functions in the innate immune responses of B. mori and two integrin β subunits are their potential receptors.

Identification of 35 C-Type Lectins in the Oriental Armyworm, Mythimna separata (Walker)

Simple Summary

The oriental armyworm Mythimna separata is a lepidopteral agricultural pest that causes serious damage to many crops, such as maize, wheat, and sorghum. To control this pest, it is advisable to take comprehensive measures, including the use of chemical pesticides, microbial pesticides, and cultural practices. However, microbial pesticides (entomopathogens) can be eliminated by the insect immune system. C-type lectins (CTLs) are a family of pattern-recognition receptors that recognize carbohydrates and mediate immune responses. C-type lectins in the oriental armyworm have not yet been identified and characterized. In this study, a transcriptome of M. separata larvae was constructed and a total of 35 CTLs containing single or dual carbohydrate-recognition domains (CRDs) were identified from unigenes. Phylogenetic analyses, sequence alignments and structural predictions were performed. Gene expression profiles in different developmental stages, naïve larval tissues, and bacteria/fungi-challenged larvae were analyzed. Overall, our findings indicate that most dual-CRD CTLs are expressed in mid-late-stage larvae, pupae, and adults. Bacterial and fungal challenges can stimulate the expression of many CTLs in larval hemocytes, fat body, and midgut. Our data suggest the importance of CTLs in immune responses of M. separata.

Abstract

Insect C-type lectins (CTLs) play vital roles in modulating humoral and cellular immune responses. The oriental armyworm, Mythimna separata (Walker) (Lepidoptera: Noctuidae) is a migratory pest that causes significant economic loss in agriculture. CTLs have not yet been systematically identified in M. separata. In this study, we first constructed a transcriptome of M. separata larvae, generating a total of 45,888 unigenes with an average length of 910 bp. Unigenes were functionally annotated in six databases: NR, GO, KEGG, Pfam, eggNOG, and Swiss-Prot. Unigenes were enriched in functional pathways, such as those of signal transduction, endocrine system, cellular community, and immune system. Thirty-five unigenes encoding C-type lectins were identified, including CTL-S1~CTL-S6 (single CRD) and IML-1~IML-29 (dual CRD). Phylogenetic analyses showed dramatic lineage-specific expansions of IMLs. Sequence alignment and structural modeling identified potential ligand-interacting residues. Real-time qPCR revealed that CTL-Ss mainly express in eggs and early stage larvae, while IMLs mainly express in mid-late-stage larvae, pupae, and adults. In naïve larvae, hemocytes, fat body, and epidermis are the major tissues that express CTLs. In larvae challenged by Escherichia coli, Staphylococcus aureus, or Beauveria bassiana, the expression of different CTLs was stimulated in hemocytes, fat body and midgut. The present study will help further explore functions of M. separata CTLs.

Cytochrome c nitrite reductase from the bacterium Geobacter lovleyi represents a new NrfA subclass

Cytochrome c nitrite reductase (NrfA) catalyzes the reduction of nitrite to ammonium in the dissimilatory nitrate reduction to ammonium (DNRA) pathway, a process that competes with denitrification, conserves nitrogen, and minimizes nutrient loss in soils. The environmental bacterium Geobacter lovleyi has recently been recognized as a key driver of DNRA in nature, but its enzymatic pathway is still uncharacterized. To address this limitation, here we overexpressed, purified, and characterized G. lovleyi NrfA. We observed that the enzyme crystallizes as a dimer but remains monomeric in solution. Importantly, its crystal structure at 2.55-Å resolution revealed the presence of an arginine residue in the region otherwise occupied by calcium in canonical NrfA enzymes. The presence of EDTA did not affect the activity of G. lovleyi NrfA, and site-directed mutagenesis of this arginine reduced enzymatic activity to <3% of the WT levels. Phylogenetic analysis revealed four separate emergences of Arg-containing NrfA enzymes. Thus, the Ca²⁺-independent, Arg-containing NrfA from G. lovleyi represents a new subclass of cytochrome c nitrite reductase. Most genera from the exclusive clades of Arg-containing NrfA proteins are also represented in clades containing Ca²⁺-dependent enzymes, suggesting convergent evolution.

Novel carbohydrate-recognition mode of the invertebrate C-type lectin SPL-1 fromSaxidomus purpuratusrevealed by the GlcNAc-complex crystal in the presence of Ca2+

The C-type lectins SPL-1 and SPL-2 from the bivalveSaxidomus purpuratusare composed of A and B chains and of two B chains, respectively. They bind specific carbohydrates containing acetamido groups, such asN-acetylglucosamine (GlcNAc) andN-acetylgalactosamine (GalNAc), in a Ca2+-independent manner. Unlike ordinary C-type lectins, which require Ca2+ions for carbohydrate recognition, these lectins recognize specific carbohydrates mainly through interactions with the acetamido group without Ca2+ions, even though Ca2+enhances the binding affinity of these lectins, especially SPL-1. In the present study, the crystal structure of the SPL-1–GlcNAc complex in the presence of Ca2+revealed that the binding of SPL-1 to GlcNAc is stabilized by hydrogen bonds to the water molecule(s) coordinating Ca2+, whereas in ordinary C-type lectins Ca2+directly forms coordinate bonds to the hydroxy groups of carbohydrates. These differences may also allow SPL-1 and SPL-2 to recognize both GlcNAc and GalNAc, which have different orientations of the 4-hydroxy group.

Two c-type lectins from Venerupis philippinarum: Possible roles in immune recognition and opsonization

In the study, two c-type lectins were identified and characterized from the manila clam Venerupis philippinarum (designed as VpClec-1 and VpClec-2, respectively). Multiple alignments and phylogenetic analysis strongly suggested that they were new members of the c-type lectin superfamily. In normal tissue of clams, both VpClec-1 and VpClec-2 transcripts were highly expressed in the tissue of hepatopancreas. After Vibrio anguillarum challenge, the temporal expression of both VpClec-1 and VpClec-2 transcripts was up-regulated in the hemocytes of manila clams. The recombinant protein VpClec-1 (rVpClec-1) showed obvious binding activities to lipopolysaccharide (LPS), peptidoglycan (PGN), glucan and zymosan in vitro, while the recombinant protein VpClec-2 (rVpClec-2) could only bind LPS, glucan and zymosan. Coinciding with the PAMPs binding assay, both rVpClec-1 and rVpClec-2 displayed broad agglutination and antibacterial activities towards Vibrio harveyi, Vibrio splendidus, Vibrio anguillarum, Enterobacter cloacae and Aeromonas hydrophila. Moreover, the phagocytosis and encapsulation ability of hemocytes could be significantly enhanced by rVpClec-1 and rVpClec-2. Notably, the rVpClec-1 but not rVpClec-2 elicited a chemotactic response from hemocytes. All the results showed that VpClec-1 and VpClec-2 functioned as pattern recognition receptors (PRRs) with distinct recognition spectrum, and involved in the innate immune responses of manila clams.

Structural Aspects of Carbohydrate Recognition Mechanisms of C-Type Lectins

Carbohydrate recognition is an essential function occurring in all living organisms. Lectins are carbohydrate-binding proteins and are classified into several families. In mammals, Ca²⁺-dependent C-type lectins, such as β-galactoside-binding galectin and sialic acid-binding siglec, play crucial roles in the immune response and homeostasis. C-type lectins are abundant and diverse in animals. Their immunological activities include lymphocyte homing, pathogen recognition, and clearance of apoptotic bodies. C-type lectin domains are composed of 110-130 amino acid residues with highly conserved structural folds. Remarkably, individual lectins can accept a wide variety of sugar ligands and can distinguish subtle structural differences in closely related ligands. In addition, several C-type lectin-like proteins specifically bind to carbohydrate ligands in Ca²⁺-independent ways. The accumulated 3D structural evidence clarifies the unexpected structural versatility of C-type lectins underlying the variety of ligand binding modes. In this issue, we focus on the structural aspects of carbohydrate recognition mechanisms of C-type lectins and C-type lectin-like proteins.