A four-CRD C-type lectin from Chlamys farreri mediating nonself-recognition with broader spectrum and opsonization

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.

A C-type lectin from Crassostrea gigas with novel EFG/FVN motif involved in recognition of various PAMPs and induction of interleukin expression

C-type lectins (CTLs) are a superfamily of Ca²⁺-dependent carbohydrate-recognition proteins, which participate in the nonself-recognition and triggering the transduction pathways in the innate immunity. In the present study, a novel CTL (designated as CgCLEC-TM2) with a carbohydrate-recognition domain (CRD) and a transmembrane domain (TM) was identified from the Pacific oyster Crassostrea gigas. Two novel EFG and FVN motifs were found in Ca²⁺-binding site 2 of CgCLEC-TM2. The mRNA transcripts of CgCLEC-TM2 were detected in all tested tissues with the highest expression level in haemocytes, which was 94.41-fold (p < 0.01) of that in adductor muscle. The relative expression level of CgCLEC-TM2 in haemocytes significantly up-regulated at 6 h and 24 h after the stimulation of Vibrio splendidus, which was 4.94- and 12.77-fold of that in control group (p < 0.01), respectively. The recombinant CRD of CgCLEC-TM2 (rCRD) was able to bind lipopolysaccharide (LPS), mannose (MAN), peptidoglycan (PGN), and poly (I: C) in a Ca²⁺-dependent manner. The rCRD exhibited binding activity to V. anguillarum, Bacillus subtilis, V. splendidus, Escherichia coli, Pichia pastoris, Staphylococcus aureus and Micrococcus luteus in a Ca²⁺-dependent manner. The rCRD also exhibited agglutination activity to E. coli, V. splendidus, S. aureus, M. luteus and P. pastoris in a Ca²⁺-dependent manner. The phagocytosis rate of haemocytes towards V. splendidus significantly down-regulated from 27.2% to 20.9% after treatment of anti-CgCLEC-TM2-CRD antibody, while the growth of V. splendidus and E. coli was inhibited compared with the TBS and rTrx groups. After the expression of CgCLEC-TM2 was inhibited by RNAi, the expression level of phospho-extracellular regulated protein kinases (p-CgERK) in haemocytes, and the mRNA expressions of interleukin17s (CgIL17-1 and CgIL17-4) decreased significantly after V. splendidus stimulation, compared with that in EGFP-RNAi oysters, respectively. These results suggested that CgCLEC-TM2 with novel motifs served as a pattern recognition receptor (PRR) involved in the recognition of microorganisms, and induction of CgIL17s expression in the immune response of oysters.

C-type lectin (CTL) and sialic acid-binding lectin (SABL) from Venerupis philippinarum: Function on PAMP binding and opsonic activities in immune responses

Lectins are a superfamily of carbohydrate-recognition proteins that bind to specific carbohydrate structures and play significant roles in immune recognition and clearance of invaders. In the study, we investigated the potential mechanisms of PAMP binding and opsonic activities of a c-type lectin and a sialic acid-binding lectin from manila clam Venerupis philippinarum (designed as VpCTL and VpSABL). Both recombinant proteins (rVpCTL and rVpSABL) could bind LPS, PGN, glucan and zymosan in vitro. Coinciding with the PAMPs binding assay, a broad agglutination spectrum was displayed by rVpSABL including gram-positive bacteria Staphyloccocus aureus, gram-negative bacteria Escherichia coli, Vibrio parahaemolyticus, Vibrio harveyi, Pseudomonas putida, Proteus mirabilis and fungi Pichia pastoris, while no agglutinative activities on P. mirabilis and P. putida was observed in rVpCTL. Moreover, the phagocytosis and encapsulation ability of hemocytes could be significantly enhanced by rVpCTL and rVpSABL. More remarkable, VpCTL and VpSABL were highly detected in all the examined tissues, especially in gills and hepatopancreas. All the results showed that VpCTL and VpSABL could function as pattern recognition receptors (PRRs) with distinct recognition spectrum, perhaps involved in the innate immune responses of V. philippinarum.

Molecular Cloning and Characteristics of a Lectin from the Bivalve Glycymeris yessoensis

C-type lectins (CTLs) are a family of carbohydrate-binding proteins that mediate multiple biological events, including adhesion between cells, the turnover of serum glycoproteins, and innate immune system reactions to prospective invaders. Here, we describe the cDNA cloning of lectin from the bivalve Glycymeris yessoensis (GYL), which encodes 161 amino acids and the C-type carbohydrate recognition domain (CRD) with EPN and WND motifs. The deduced amino acid sequence showed similarity to other CTLs. GYL is a glycoprotein containing two N-glycosylation sites per subunit. N-glycans are made up of xylose, mannose, D-glucosamine, 3-O-methylated galactose, D-quinovoses, and 3-O-methylated 6-deoxy-D-glucose. The potential CRD tertiary structure of the GYL adopted CTL-typical long-form double-loop structure and included three disulfide bridges at the bases of the loops. Additionally, when confirming the GYL sequence, eight isoforms of this lectin were identified. This fact indicates the presence of a multigene family of GYL-like C-type lectins in the bivalve G. yessoensis. Using the glycan microarray approach, natural carbohydrate ligands were established, and the glycotope for GYL was reconstructed as "Galβ1-4GlcNAcβ obligatory containing an additional fragment", like a sulfate group or a methyl group of fucose or N-acetylgalactosamine residues.

β-1,3-Glucan/chitin unmasking in the Saccharomyces cerevisiae mutant, Δmnn9, promotes immune response and resistance of the Pacific oyster (Crassostrea gigas) to Vibrio coralliilyticus infection

Yeast cells can play a crucial role in immune activation in fish and shellfish predominantly due to the cell wall component β-1,3-glucan, providing protection against bacterial or viral infections. However, the immunostimulatory capacity of dietary yeast cells remains poorly studied in bivalves. To understand the role of yeast cell wall components (mannan, β-glucan and chitin) as immune activators, this study characterized the surface carbohydrate exposure of the wild-type baker's yeast Saccharomyces cerevisiae (WT) and its Δmnn9 mutant, which presents a defective mannan structure, and compared these profiles with that of β-glucan particles, using fluorescein isothiocyanate (FITC)-labeled lectin binding analysis. Then, a first trial evaluated the immunological response in Crassostrea gigas juveniles after being fed for 24 h with an algae-based diet (100A) and its 50% substituted version (based on dry weight) with WT (50A50WT) and Δmnn9 (50A50Y), and the posterior resistance of the juveniles against Vibrio coralliilyticus infection (trial 1). The mRNA expression was measured for β-glucan-binding protein (CgβGBP), Toll-like receptor 4 (CgTLR4), C-type lectin receptor 3 (CgCLec-3), myeloid differentiation factor 88 (CgMyD88), nuclear factor-kappa B (CgNFκB), lysozyme (CgLys), interleukin 17-5 (CgIL17-5), and superoxide dismutase (CgSOD), in oysters, before and 24 h after the bacterial inoculation. A second trial tested the effect of incorporating Δmnn9 into the 100A diet for 24 h at different substitution levels: 0, 5, 10, 25, and 50% (100A, 95A5Y, 90A10Y, 75A25Y, and 50A50Y), followed by the bacterial challenge with V. coralliilyticus (trial 2). Our findings showed that the outer cell wall surface of WT is largely composed of mannan, while Δmnn9 presents high exposure of β-glucan and chitin, exhibiting similar FITC-lectin binding profiles (fluorescence intensity) to β-glucan particles. A significantly higher survival after the bacterial challenge was observed in oysters fed on 50A50Y compared to those fed 50A50WT and 100A in trial 1. This better performance of 50A50Y was supported by significantly higher gene expressions of CgLys, CgSOD, CgMyD88, and CgβGBP compared to 100A, and CgSOD and CgNFκB in relation to those fed on 50A50WT, prior to the bacterial inoculation. Furthermore, improved survival was observed in oysters fed 50A50Y compared to those offered lower Δmnn9 levels and 100A in trial 2. The superior performance of Δmnn9-fed oysters is mostly associated with the elevated presence of unmasked β-glucans on Δmnn9 cell wall surface, facilitating their interactions with oyster hemocytes. Further studies are needed to evaluate administration dose and frequency of Δmnn9 to develop strategies for long-term feeding.

A C-type lectin containing two carbohydrate recognition domains participates in the antibacterial response by regulating the JNK pathway and promoting phagocytosis

C-type lectins (CTLs) are important immune-related molecules in crustaceans. However, the immunologic mechanism by which CTLs eliminate invading pathogens is still unclear. In this study, we studied the antimicrobial mechanism of a CTL containing two carbohydrate recognition domains (DClec). After Aeromonas hydrophila challenge, several antimicrobial peptides (ALF1, ALF4, ALF5 and lys-i2) were upregulated. The transcript levels of ALF1, ALF4 and ALF5 were downregulated after A. hydrophila challenge in groups with DClec interference or inhibition compared with the control group. Similar results were obtained after c-Jun N-terminal kinase (JNK) interference. This finding indicates that DClec might regulate the JNK signalling pathway and subsequently adjust antimicrobial peptide (AMP) expression. Additionally, we found that DClec was secreted into the hemolymph. Recombinant protein DClec (rDClec) agglutinated gram-positive or gram-negative bacteria. Both rDClec and the native DClec in hemolymph bound to different bacteria. In this process, Ca²⁺ promoted the rDClec bacterial binding ability. After DClec interference, the phagocytosis ability of hemocytes was lower than that of the control group. Therefore, DClec can facilitate bacterial elimination by promoting AMPs expression and hemocyte phagocytosis.

Pomacea canaliculata Ampullar Proteome: A Nematode-Based Bio-Pesticide Induces Changes in Metabolic and Stress-Related Pathways

Pomacea canaliculata is a freshwater gastropod known for being both a highly invasive species and one of the possible intermediate hosts of the mammalian parasite Angiostrongylus cantonensis. With the aim of providing new information concerning P. canaliculata biology and adaptability, the first proteome of the ampulla, i.e., a small organ associated with the circulatory system and known as a reservoir of nitrogen-containing compounds, was obtained. The ampullar proteome was derived from ampullae of control snails or after exposure to a nematode-based molluscicide, known for killing snails in a dose- and temperature-dependent fashion. Proteome analysis revealed that the composition of connective ampulla walls, cell metabolism and oxidative stress response were affected by the bio-pesticide. Ultrastructural investigations have highlighted the presence of rhogocytes within the ampullar walls, as it has been reported for other organs containing nitrogen storage tissue. Collected data suggested that the ampulla may belong to a network of organs involved in controlling and facing oxidative stress in different situations. The response against the nematode-based molluscicide recalled the response set up during early arousal after aestivation and hibernation, thus encouraging the hypothesis that metabolic pathways and antioxidant defences promoting amphibiousness could also prove useful in facing other challenges stimulating an oxidative stress response, e.g., immune challenges or biocide exposure. Targeting the oxidative stress resistance of P. canaliculata may prove helpful for increasing its susceptibility to bio-pesticides and may help the sustainable control of this pest's diffusion.

A C-Type Lectin Highly Expressed in Portunus trituberculatus Intestine Functions in AMP Regulation and Prophenoloxidase Activation

A C-type lectin (PtCLec2) from Portunus trituberculatus was identified for characterization of its role in defense and innate immunity. PtCLec2 contains a single carbohydrate-recognition domain (CRD) with a conserved QPD motif, which was predicted to have galactose specificity. The mRNA expression of PtCLec2 was predominantly detected in intestine and increased rapidly and significantly upon pathogen challenge. The recombinant PtCLec2 (rPtCLec2) could bind various microorganisms and PAMPs with weak binding ability to yeast and PGN. It agglutinated the tested Gram-negative bacteria (Vibrio alginolyticus and Pseudomonas aeruginosa), Gram-positive bacteria (Staphylococcus aureus and Micrococcus luteus), and rabbit erythrocytes in the presence of exogenous Ca²⁺, and these agglutination activities were suppressed by LPS, d-galactose, and d-mannose. Further, rPtCLec2 enhanced phagocytosis and clearance of V. alginolyticus, and displayed inhibitory activities against the tested bacteria. Knockdown of PtCLec2 decreased the transcription of two phagocytosis genes (PtArp and PtMyosin), three prophenoloxidase (proPO) system-related genes (PtPPAF, PtcSP1, and PtproPO), six antimicrobial peptides (AMPs) (PtALF4-7, PtCrustin1, and PtCrustin3), and PtRelish but upregulated the expression levels of PtJNK, PtPelle, and PtTLR. These results collectively indicate that PtCLec2 might perform its immune recognition function via binding and agglutination, and mediate pathogen elimination via regulating hemocyte phagocytosis, AMP synthesis, and proPO activation.

Identification of a Novel Pattern Recognition Receptor DM9 Domain Containing Protein 4 as a Marker for Pro-Hemocyte of Pacific Oyster Crassostrea gigas

DM9 refers to an uncharacterized protein domain that is originally discovered in Drosophila melanogaster. Two proteins with DM9 repeats have been recently identified from Pacific oyster Crassostrea gigas as mannose-specific binding pattern-recognition receptors (PRRs). In the present study, a novel member of DM9 domain containing protein (designated as CgDM9CP-4) was identified from C. gigas. CgDM9CP-4, about 16 kDa with only two tandem DM9 domains, was highly enriched in hemocytes and gill. The transcripts level of CgDM9CP-4 in circulating hemocytes were decreased after LPS, PGN and Vibrio splendidus stimulations. The recombinant protein of CgDM9CP-4 (rCgDM9CP-4) displayed a broad binding spectrum towards various pathogen-associated molecular patterns (PAMPs) (LPS, PGN, β-glucan and Mannose) and microorganisms (Staphylococcus aureus, Micrococcus luteus, V. splendidus, V. anguillarum, Escherichia coli, Pichia pastoris and Yarrowia lipolytica). CgDM9CP-4 was mostly expressed in gill and some of the hemocytes. Flow cytometry analysis demonstrated that the CgDM9CP-4-positive hemocytes accounted for 7.3% of the total hemocytes, and they were small in size and less in granularity. CgDM9CP-4 was highly expressed in non-phagocytes (~82% of total hemocytes). The reactive oxygen species (ROS) and the expression levels of cytokines in CgDM9CP-4-positive hemocytes were much lower than that in CgDM9CP-4-negative hemocytes. The mRNA expression level of CgDM9CP-4 in hemocytes was decreased after RNAi of hematopoietic-related factors (CgGATA, CgRunt, CgSCL, and CgNotch). In addition, CgDM9CP-4-positive cells were found to be much more abundant in hemocytes from gill than that from hemolymph, with most of them located in the gill filament. All these results suggested that CgDM9CP-4 was a novel member of PRR that expressed in undifferentiated pro-hemocytes to mediate immune recognition of pathogens.

Molecular characterization, expression and immune functions of two C-type lectin from Venerupis philippinarum

In the present study, two C-type lectins (designated as VpClec-3 and VpClec-4) were identified and characterized from the manila clam Venerupis philippinarum. Multiple alignment and phylogenetic relationship analysis strongly suggested that VpClec-3 and VpClec-4 belong to the C-type lectin family. In nonstimulated clams, the VpClec-3 transcript was dominantly expressed in the hepatopancreas, while the VpClec-4 transcript was mainly expressed in gill tissues. Both VpClec-3 and VpClec-4 mRNA expression was significantly upregulated following Vibrio anguillarum challenge. Recombinant VpClec-4 (rVpClec-4) was shown to bind lipopolysaccharide (LPS) and glucan in vitro, whereas recombinant VpClec-3 (rVpClec-3) only bound to glucan. In addition, rVpClec-3 and rVpClec-4 displayed broad agglutination activities towards Vibrio harveyi, Vibrio splendidus and V. anguillarum, while no agglutination activities towards Enterobacter cloacae or Aeromonas hydrophila were observed in rVpClec-3. Moreover, hemocyte phagocytosis was significantly enhanced by rVpClec-3 and rVpClec-4. All the results showed that VpClecs function as pattern recognition receptors (PRRs) with distinct recognition spectra and are potentially involved in the innate immune responses of V. philippinarum.

CgRel involved in antibacterial immunity by regulating the production of CgIL17s and CgBigDef1 in the Pacific oyster Crassostrea gigas

The NF-κB/Rel transcription factors play essential roles in the induction and regulation of innate immune responses. In the present study, the full-length cDNA of CgRel from the Pacific oyster Crassostrea gigas was of 2,647 bp with an RHD and an IPT domain. The mRNA of CgRel was found to be constitutively expressed in all the tested tissues including gills, hepatopancreas, gonad, adductor muscle, labial palps, mantle, hemocytes, and ganglion. After lipopolysaccharide (LPS) stimulation, the expression level of CgRel mRNA in hemocytes was up-regulated to the first peak at 3 h (3.06-fold compared to the control group, p < 0.001) and second peak at 48 h (1.96-fold, p < 0.05). It increased significantly at 3 h (7.68-fold compared to the control group, p < 0.001), 24 h (3.63-fold, p < 0.05) and 48 h (1.99-fold, p < 0.05) post Vibrio splendidus stimulation, respectively. The protein of CgRel was translocated from cytoplasm into nucleus of oyster hemocytes after LPS stimulation. The mRNA expression levels of interleukin17s (CgIL17s) and big defensin (CgBigDef1) in hemocytes were examined after the expression of CgRel was silenced by RNAi. The transcripts of CgIL17-1 (0.25-fold of the control group, p < 0.01), CgIL17-2 (0.12-fold, p < 0.01), CgIL17-4 (0.33-fold, p < 0.01), CgIL17-6 (0.27-fold, p < 0.05) and CgBigDef1 (0.38-fold, p < 0.01) in CgRel-knockdown oysters decreased significantly at 12 h after LPS stimulation. The results indicated that CgRel played important roles in the immune defense against bacteria by regulating the expression of CgIL17 and CgBigDef1.

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.

A novel C-type lectin with microbiostatic and immune regulatory functions from Litopenaeus vannamei

C-type lectins (CTLs) are a group of lectins with at least one carbohydrate recognition domain (CRD), the binding of which to carbohydrates requires the presence of calcium ions. CTLs generally function as pattern recognition receptors (PRRs), essentially participating in innate immunity. In the current study, a novel CTL termed LvCTL5 was identified from Pacific white shrimp Litopenaeus vannamei, which shared sequence identities with other crustacean CTLs. LvCTL5 was highly expressed in hepatopancreas and could be activated by infection with bacteria, virus and fungi. The recombinant LvCTL5 protein purified from E. coli showed microbiostatic and agglutination activities against bacteria and fungi in vitro. Silencing of LvCTL5 in vivo could significantly affect expression of a series of immune effector genes and down-regulate the phagocytic activity of hemocytes. Compared with controls, the LvCTL5-silenced shrimp were highly susceptible to Vibrio parahaemolyticus and white spot syndrome virus (WSSV) infections. These suggest that LvCTL5 has microbiostatic and immune regulatory activities and is implicated in antiviral and antibacterial responses.

Immune recognition, antimicrobial and opsonic activities mediated by a sialic acid binding lectin from Ruditapes philippinarum

In the present study, a sialic acid-binding lectin was identified and characterized from Manila clam Ruditapes philippinarum (designed as RpSABL-1). Multiple alignments strongly suggested that RpSABL-1 was a new member of the sialic acid-binding lectin family. In non-stimulated clams, RpSABL-1 transcripts were constitutively expressed in all five tested tissues, especially in hepatopancreas. After Vibrio anguillarum challenge, the expression of RpSABL-1 mRNA was significantly up-regulated at 6 h (P < 0.05), 12 h (P < 0.01) and 24 h (P < 0.01). Recombinant RpSABL-1 protein (rRpSABL-1) displayed apparent binding activities towards lipopolysaccharides (LPS) and peptidoglycan (PGN), but not to glucan or chitin in vitro. Coinciding with the PAMPs binding assay, rRpSABL-1 exhibited obvious agglutination activities against Gram-positive bacterium Staphyloccocus aureus, Gram-negative bacteria Escherichia coli, V. anguillarum and Vibrio harveyi. Meanwhile, rRpSABL-1 showed antibacterial activities against E. coli, and biofilm formation of E. coli could also be inhibited after incubated with rRpSABL-1. Moreover, the encapsulation, phagocytosis and chemotactic ability of hemocytes could be enhanced by rRpSABL-1. All these results suggested that RpSABL-1 could function as a pattern recognition receptor with versatile functions in the innate immune responses of R. philippinarum.

A single-CRD C-type lectin (CgCLec-3) with novel DIN motif exhibits versatile immune functions in Crassostrea gigas

C-type lectins (CTLs), as important pattern recognition receptors (PRRs), are a superfamily of Ca²⁺-dependent carbohydrate-recognition proteins which participate in nonself-recognition and eliminating pathogens. In the present study, a novel CTL (designated as CgCLec-3) was identified from the Pacific oyster Crassostrea gigas. There was only one carbohydrate-recognition domain (CRD) of 151 amino acid residues within the deduced amino acid sequence of CgCLec-3. The deduced amino acid sequence of CgCLec-3 CRD shared low homology with the CRDs of other CTLs in oyster with the identities ranging from 12% to 22%. A novel DIN motif was found in Ca²⁺-binding site 2 of CgCLec-3. The relative expression level of CgCLec-3 in hemocytes was up-regulated significantly after the stimulations of bacteria and Pathogen Associated Molecular Patterns (PAMPs). Immunohistochemistry assay showed that CgCLec-3 protein was mainly distributed in gill and mantle, less in gonad, and could not be detected in adductor muscle and hepatopancreas. The recombinant protein (rCgCLec-3) could bind lipopolysaccharide (LPS), mannose (MAN) and peptidoglycan (PGN), but not poly (I:C). rCgCLec-3 exihibited strong binding ability to Vibrio anguillarum and V. splendidus, moderate binding activities to Escherichia coli, Pichia pastoris and Yarrowia lipolytica, weak binding affinity to Staphylococcus aureus and Micrococcus luteus. rCgCLec-3 could agglutinate microorganisms, in a Ca²⁺-dependent manner and its activity to agglutinate V. splendidus was remarkably higher than that to agglutinate E. coli, S. aureus and P. pastoris. The phagocytic activity of oyster hemocytes was significantly enhanced after incubation with rCgCLec-3. rCgCLec-3 also exhibited antibacterial activity against E. coli and S. aureus. The results clearly suggested that CgCLec-3 in Pacific oyster C. gigas not only served as a PRR involved in the PAMPs recognition and microbes binding, but also functioned as an immune effector participating in the clearance of invaders.

A SPRY domain-containing SOCS box protein 3 (SPSB3) involved in the regulation of cytokine production in granulocytes of Crassostrea gigas

The sp1A/ryanodine receptor (SPRY) family members have been reported to involve in important biological pathways, including innate immune signaling, cytokine signaling suppression, development, cell growth, and retroviral restriction. In the present study, a SPRY domain-containing SOCS box protein (named as CgSPSB3) was identified and characterized from oyster Crassostrea gigas. The open reading frame of CgSPSB3 gene was of 699 bp, encoding a polypeptide of 232 amino acid residues with a central SPRY domain and a C-terminal SOCS box motif. CgSPSB3 mRNA transcripts could be detected in all the examined tissues with the highest level in hemocytes, which was about 82.72-fold (p < 0.05) of that in gonad. Furthermore, the expression level of CgSPSB3 mRNA in granulocytes was significantly higher than that in semi-granulocytes and agranulocytes, which was about 2.04-fold (p < 0.05) of the average level of hemocytes. Immunofluorescence assay further revealed that CgSPSB3 protein was mainly distributed in the cytoplasm of granulocytes. The mRNA expression of CgSPSB3 in hemocytes was up-regulated after lipopolysaccharide (LPS) and Vibrio splendidus stimulations. The mRNA expression of CgIFNLP, CgIL17-5 and CgTNF-1 decreased significantly (p < 0.05) at 24 h after the CgSPSB3 mRNA was knocked down by RNAi. These results collectively indicated that CgSPSB3 might play an important role in regulating cytokines production in granulocytes of C. gigas.

HcCUB-Lec, a newly identified C-type lectin that contains a distinct CUB domain and participates in the immune defense of the triangle sail mussel Hyriopsis cumingii

As pattern recognition receptors (PRRs), C-type lectins (CTLs) play crucial roles in recognizing and eliminating pathogens in innate immunity. In this study, a novel CTL (HcCUB-Lec) was identified from the triangle sail mussel Hyriopsis cumingii. The full-length of HcCUB-Lec cDNA was 1558 bp with an open reading frame of 1281 bp that encodes a putative protein of 426 amino acid residues, including an N-terminal signal peptide, a complement Uegf Bmp1 (CUB) domain, a single carbohydrate recognition domain (CRD), and a transmembrane domain. Quantitative real-time PCR analysis revealed that HcCUB-Lec transcript was distributed in all examined tissues with the highest levels in hepatopancreas and was significantly upregulated in gills and hepatopancreas after immune challenge with Staphyloccocus aureus and Vibrio parahaemolyticus. When HcCUB-Lec was silenced by RNAi, the expression levels of three antimicrobial peptides, including whey acidic protein (HcWAP), defensin (HcDef), and lysozyme (HcLyso), were dramatically decreased in gills. The recombinant HcCUB-Lec and its individual CUB and CRD domains can bind with Gram-positive bacteria (S. aureus and Bacillus subtilis), Gram-negative bacteria (V. parahaemolyticus and Aeromonas hydrophila), and polysaccharides (lipopolysaccharide and peptidoglycan). Moreover, rHcCUB-Lec and its domains could also agglutinate S. aureus and V. parahaemolyticus in the presence of Ca²⁺ and can clear V. parahaemolyticus in H. cumingii. Results of this study suggest that HcCUB-Lec acts as an antimicrobial PRR that participates in the innate immune responses of H. cumingii.

Novel C-type lectin from razor clam Sinonovacula constricta agglutinates bacteria and erythrocytes in a Ca2+-dependent manner

Among its other physiological roles, C-type lectins functioned as pattern recognition receptors (PRR) in innate immunity received much attention. In the present study, a novel C-type lectin was identified and characterized from the invertebrate razor clam Sinonovacula constrict and designated as ScCTL. The complete cDNA sequence of ScCTL was 828 bp in length and coded a secreted polypeptide of 158 amino acids with a typical CRD domain. Multiple sequence alignments combined with phylogenetic analysis both collectively confirmed that ScCTL was a novel member belong to lectin family. Spatial expression distribution analysis revealed that ScCTL was extensively expressed in all of the examined tissues, and the highest expression was detected in the hepatopancreas. After 1 × 10⁷ CFU/mL Vibrio parahaemolyticus challenge by immersion infection, the ScCTL transcript in hepatopancreas and gill were markedly upregulated and arrived the maximum levels at 24 or 12 h after challenge, respectively. Recombinant ScCTL could agglutinate not only all tested bacteria but sheep and mouse erythrocyte in the presence of Ca²⁺. All of our studies suggested that ScCTL performed important roles in protecting cells from pathogenic infection in S. constrict.

A hypervariable immunoglobulin superfamily member from Crassostrea gigas functions as pattern recognition receptor with opsonic activity

Immunoglobulin superfamily (IgSF), an extensive collection of proteins possessing at least one immunoglobulin-like (Ig-like) domain, performs a wide range of functions in recognition, binding or adhesion process of cells. In the present study, a cysteine-rich motif associated immunoglobulin domain containing protein (designated CgCAICP-1) was identified in Pacific oyster Crassostrea gigas. The deduced protein sequence of CgCAICP-1 contained 534 amino acidresidues, with three Ig domains which were designated as IG1, IG2 and IG3, and a cysteine-rich motif between the first and second Ig domain. The mRNA transcripts of CgCAICP-1 were highly expressed in hemocytes and up-regulated significantly (p < 0.05) after the stimulation of lipopolysaccharides (LPS), but not peptidoglycan (PGN). The recombinant CgCAICP-1 protein (rCgCAICP-1) exhibited binding activity to various pathogen-associated molecular patterns (PAMPs) including LPS, PGN, mannose (Man) and D-galactose (D-Gal), and microorganisms including Vibrio splendidus, Escherichia coli, Staphylococcus aureus, Micrococcus luteus and Pichia pastoris. The phagocytic rates of oyster hemocytes towards Gram-negative bacteria V. splendidus and Gram-positive bacteria M. luteus were significantly enhanced (p < 0.05) after pre-incubation of microbes with rCgCAICP-1. Furthermore, the transcripts of CgCAICP-1 exhibited high level of polymorphism among individuals. The ratio of nonsynonymous and synonymous distances (dN/dS) for AA'BCC'D strands of IG1 (the possible binding sites 1, pbs1) across all allelic variants was 2.09 (p < 0.05), while the ratio for the non-pbs regions was less than 1.0. The 1248 bp fragment amplified from the 5' end of CgCAICP-1 open reading frame (ORF) from 24 transcript variants could be divided artificially into seven regions of 50 elements, and all of the allelic variants might be derived from these elements by point mutation and recombination processes. These results collectively suggested that CgCAICP-1 might function as an important pattern recognition receptor (PRR) to recognize various PAMPs and facilitated the phagocytosis of oyster hemocytes towards both Gram-positive and Gram-negative bacteria. Diverse isoforms of CgCAICP-1 were generated through point mutation and recombination processes and maintained by balancing selection, which would provide a broader spectrum of interaction surface and be associated with immune resistance of oysters to infectious pathogens.

A conserved interferon regulation factor 1 (IRF-1) from Pacific oyster Crassostrea gigas functioned as an activator of IFN pathway

Interferon regulatory factors (IRFs), a family of transcription factors with a novel helix-turn-helix DNA-binding motif, play important roles in regulating the expression of interferons (IFNs) and IFN-stimulated genes. In the present study, an interferon regulation factor 1 was identified from oyster Crassostrea gigas (designated CgIRF-1), and its immune function was characterized to understand the regulatory mechanism of interferon system against viral infection in invertebrates. The open reading frame (ORF) of CgIRF-1 was 990 bp, encoding a polypeptide of 329 amino acids with a typical IRF domain (also known as DNA-binding domain). The mRNA transcripts of CgIRF-1 were detected in all the tested tissues with the highest expression level in hemocyte. CgIRF-1 protein was distributed in both nucleus and cytoplasm of the oyster hemocyte. The mRNA expression of CgIRF-1 in hemocytes was significantly up-regulated at 48 h after poly (I:C) stimulation (p < 0.05). The recombinant CgIRF-1 (rCgIRF-1) could interact with classically IFN-stimulated response elements (ISRE) in vitro. The relative luciferase activity of interferon-like protein promotor reporter gene (pGL-CgIFNLP promotor) was significantly (p < 0.05) enhanced in HEK293T cell after transfection of CgIRF-1. These results indicated that CgIRF-1 could bind ISRE and regulate the expression of CgIFNLP as a transcriptional regulatory factor, and participated in the antiviral immune response of oysters.

The functional characterization and comparison of two single CRD containing C-type lectins with novel and typical key motifs from Portunus trituberculatus

C-type lectins are a superfamily of Ca²⁺-dependent carbohydrate-recognition proteins, which play crucial roles in innate immunity including nonself-recognition and pathogen elimination. In the present study, two single-CRD containing C-type lectins were identified from swimming crab Portunus trituberculatus (designated as PtCTL-2 and PtCTL-3). The open reading frame (ORF) of PtCTL-2 encoded polypeptides of 485 amino acids with a signal peptide and a single carbohydrate-recognition domain (CRD), while PtCTL-3's ORF encoded polypeptides of 241 amino acids with a coiled-coil region and a single-CRD. The key motifs determining carbohydrate binding specificity in PtCTL-2 and PtCTL-3 were EPR (Glu-Pro-Arg) and QPD (Gln-Pro-Asp). EPR is a motif being identified for the first time, whereas QPD is a typical motif in C-type lectins. Different PAMPs binding features of the two recombinant proteins - PtCTL-2 (rPtCTL-2) and PtCTL-3 (rPtCTL-3) have been observed in our experiments. rPtCTL-2 could bind three pathogen-associated molecular patterns (PAMPs) with relatively high affinity, including glucan, lipopolysaccharide (LPS) and peptidoglycan (PGN), while rPtCTL-3 could barely bind any of them. However, rPtCTL-2 could bind seven kinds of microbes and rPtCTL-3 could bind six kinds in microbe binding assay. Moreover, rPtCTL-2 and rPtCTL-3 exhibited similar agglutination activity against Gram-positive bacteria, Gram-negative bacteria and fungi in agglutination assay. All these results illustrated that PtCTL-2 and PtCTL-3 could function as important pattern-recognition receptors (PRR) with broad nonself-recognition spectrum involved in immune defense against invaders. In addition, the results of carbohydrate binding specificity showed that PtCTL-2 with novel key motif had broad carbohydrate binding specificity, while PtCTL-3 with typical key motif possessed different carbohydrate binding specificity from the classical binding rule. Furthermore, PtCTL-2 and PtCTL-3 could also function as opsonin to enhance encapsulation of hemocytes against Ni-NTA beads.

A C-type lectin, Nattectin-like protein (CaNTC) in Qihe crucian carp Carassius auratus: Binding ability with LPS, PGN and various bacteria, and agglutinating activity against bacteria

C-type lectins (CTLs), as the members of pattern-recognition receptors (PRRs), play the significant roles in innate immunity through binding with pathogen-associated molecular patterns (PAMPs) on the surface of microbe. In the present study, a novel CTL, Nattectin-like protein (named as CaNTC), was investigated in Qihe crucian carp Carassius auratus. The full-length cDNA of CaNTC was composed of 776 bp, with a 152 bp 5'-untranslated region (UTR), a 492 bp ORF encoding a 163-aa protein, and a 132 bp 3'-UTR with a polyadenylation signal sequence AATAAA and a poly(A) tail. The deduced amino acid sequence of CaNTC contained a signal peptide, a single carbohydrate recognition domain (CRD) which had four conserved disulfide-bonded cysteine residues (Cys⁵⁷-Cys¹⁵⁰, Cys¹²⁶-Cys¹⁴²), and an EPN/WND motif required for carbohydrate-binding specificity. With regard to the mRNA transcript of CaNTC, it was predominately expressed in liver. The temporal expressions of CaNTC were obviously up-regulated in liver, spleen and head-kidney after challenged by Aeromonas hydrophila and poly I: C, respectively, and the change pattern was in the time-depended manner. The recombinant CaNTC (rCaNTC) purified from Escherichia coli BL21 (DE3), exhibited strong binding ability with LPS and PGN, as well as all tested bacteria in a Ca²⁺-independent manner. With regard to the agglutinating activity of rCaNTC, rCaNTC was able to agglutinate rabbit erythrocytes and three kinds of bacteria (Gram-negative bacteria, Escherichia coli and A. hydrophila, and Gram-positive bacteria Staphylococcus aureus) in a Ca²⁺-dependent manner. These findings collectively demonstrated that CaNTC, as a PRR, could be involved in the innate immunity and play an important role in immune defense of C. auratus.

Microbial Diseases of Bivalve Mollusks: Infections, Immunology and Antimicrobial Defense

A variety of bivalve mollusks (phylum Mollusca, class Bivalvia) constitute a prominent commodity in fisheries and aquacultures, but are also crucial in order to preserve our ecosystem’s complexity and function. Bivalve mollusks, such as clams, mussels, oysters and scallops, are relevant bred species, and their global farming maintains a high incremental annual growth rate, representing a considerable proportion of the overall fishery activities. Bivalve mollusks are filter feeders; therefore by filtering a great quantity of water, they may bioaccumulate in their tissues a high number of microorganisms that can be considered infectious for humans and higher vertebrates. Moreover, since some pathogens are also able to infect bivalve mollusks, they are a threat for the entire mollusk farming industry. In consideration of the leading role in aquaculture and the growing financial importance of bivalve farming, much interest has been recently devoted to investigate the pathogenesis of infectious diseases of these mollusks in order to be prepared for public health emergencies and to avoid dreadful income losses. Several bacterial and viral pathogens will be described herein. Despite the minor complexity of the organization of the immune system of bivalves, compared to mammalian immune systems, a precise description of the different mechanisms that induce its activation and functioning is still missing. In the present review, a substantial consideration will be devoted in outlining the immune responses of bivalves and their repertoire of immune cells. Finally, we will focus on the description of antimicrobial peptides that have been identified and characterized in bivalve mollusks. Their structural and antimicrobial features are also of great interest for the biotechnology sector as antimicrobial templates to combat the increasing antibiotic-resistance of different pathogenic bacteria that plague the human population all over the world.

The sequence variation and functional differentiation of CRDs in a scallop multiple CRDs containing lectin

A C-type lectin of multiple CRDs (CfLec-4) from Chlamys farreri was selected to investigate the sequence variation and functional differentiation of its CRDs. Its four CRDs with EPD/LSD, EPN/FAD, EPN/LND and EPN/YND key motifs were recombined separately. The recombinant proteins of CRD1 and CRD2 (designated as rCRD1 and rCRD2) could bind LPS and mannan, while the recombinant proteins of CRD3 and CRD4 (designated as rCRD3 and rCRD4) could bind LPS, PGN, mannan and glucan. Moreover, rCRD3 displayed broad microbe binding spectrum towards Gram-positive bacteria Staphylococcus aureus and Micrococcus luteus, Gram-negative bacteria Escherichia coli and Vibrio anguillarum, as well as fungi Pichia pastoris and Yarrowia lipolytica. These results indicated CRD3 contributed more to CfLec-4's nonself-recognition ability. Furthermore, CRD1, CRD3 and CRD4 functioned as opsonin participating in the clearance against invaders in scallops. The sequence variation in Ca²⁺ binding site 2 among CRDs was suspected to be associated with such functional differentiation.

A novel C-type lectin with four CRDs is involved in the regulation of antimicrobial peptide gene expression in Hyriopsis cumingii

C-type lectins (CTLs) are found in a wide number of invertebrates, and have been reported to participate in immune responses, such as the activation of prophenoloxidase, cell adhesion, bacterial clearance and phagocytosis. Previous studies on CTLs focused on the function of their carbohydrate recognition domains (CRDs). Currently, studies on lectins with multi-CRDs are limited. In this study, a lectin with four CRDs was cloned from Hyriopsis cumingii, and called HcLec4. HcLec4 was widely distributed in several tissues and was significantly down-regulated at the early stage (2 h) of bacterial infection. We further analyzed the bacteria and carbohydrate binding activities of HcLec4. The results showed that HcLec4 could bind to several bacteria, lipopolysaccharide (LPS) and peptidoglycan (PGN). In HcLec4 knockdown mussels, the bacterial clearance rate was increased, and the expression level of antimicrobial peptides (AMPs) was up-regulated. This study reveals that HcLec4 exerts its antibacterial effect by regulating the expression of AMPs at the early stage of bacterial infection.

A single CRD C-type lectin from Eriocheir sinensis (EsLecB) with microbial-binding, antibacterial prophenoloxidase activation and hem-encapsulation activities

C-type lectins (CTLs) exist widely in crustaceans. To date, thirteen CTLs have been reported in crustaceans, and play significant roles in pathogen recognition, encapsulation of hemocytes and antimicrobial activity in the innate immune response. Based on the initial expressed sequence tags (EST) of a hepatopancreatic cDNA library, a novel CTL, designated as EsLecB, with a 470 bp open reading frame encodes a polypeptide of 156 amino acids, including a signal peptide of 19 amino acid residues and one carbohydrate-recognition domain of 131 aa residues, was cloned from the crustacean Eriocheir sinensis. By qRT-PCR analysis, EsLecB was detected in all tested tissues, and showed highest expression in hemocytes, hepatopancreas and heart. The expression of EsLecB was up-regulated following injections of PAMPs or bacteria. The recombinant protein (rEsLecB) expressed in Escherichia coli had a calcium-independent but carbohydrate-dependent microbial-binding and microbial-agglutinating, microorganism growth inhibitory and hem-encapsulation activities. Moreover, the rEsLecB could stimulate the activation of prophenoloxidase in vitro. These results indicated that EsLecB, as an antibacterial pattern recognition receptor is involved in innate immunity, and may act as an upstream detector of the prophenoloxidase activating system, which can detect pathogen invasion in E. sinensis.

Bivalve-specific gene expansion in the pearl oyster genome: implications of adaptation to a sessile lifestyle

INTRODUCTION

Bivalve molluscs have flourished in marine environments, and many species constitute important aquatic resources. Recently, whole genome sequences from two bivalves, the pearl oyster, Pinctada fucata, and the Pacific oyster, Crassostrea gigas, have been decoded, making it possible to compare genomic sequences among molluscs, and to explore general and lineage-specific genetic features and trends in bivalves. In order to improve the quality of sequence data for these purposes, we have updated the entire P. fucata genome assembly.

RESULTS

We present a new genome assembly of the pearl oyster, Pinctada fucata (version 2.0). To update the assembly, we conducted additional sequencing, obtaining accumulated sequence data amounting to 193× the P. fucata genome. Sequence redundancy in contigs that was caused by heterozygosity was removed in silico, which significantly improved subsequent scaffolding. Gene model version 2.0 was generated with the aid of manual gene annotations supplied by the P. fucata research community. Comparison of mollusc and other bilaterian genomes shows that gene arrangements of Hox, ParaHox, and Wnt clusters in the P. fucata genome are similar to those of other molluscs. Like the Pacific oyster, P. fucata possesses many genes involved in environmental responses and in immune defense. Phylogenetic analyses of heat shock protein70 and C1q domain-containing protein families indicate that extensive expansion of genes occurred independently in each lineage. Several gene duplication events prior to the split between the pearl oyster and the Pacific oyster are also evident. In addition, a number of tandem duplications of genes that encode shell matrix proteins are also well characterized in the P. fucata genome.

CONCLUSIONS

Both the Pinctada and Crassostrea lineages have expanded specific gene families in a lineage-specific manner. Frequent duplication of genes responsible for shell formation in the P. fucata genome explains the diversity of mollusc shell structures. These duplications reveal dynamic genome evolution to forge the complex physiology that enables bivalves to employ a sessile lifestyle in the intertidal zone.

Proteomic characterization of mucosal secretions in the eastern oyster, Crassostrea virginica

The soft body surface of marine invertebrates is covered by a layer of mucus, a slippery gel secreted by mucocytes lining epithelia. The functions of this gel are diverse including locomotion, cleansing, food particles processing and defense against physicochemical injuries and infectious agents. In oysters, mucus covering pallial organs has been demonstrated to have a major importance in the processing of food particles and in the interactions with waterborne pathogens. Given the limited information available on mucus in bivalves and the apparent wide spectra of activity of bioactive molecules present in this matrix, the characterization of these mucosal secretions has become a research priority. In this study, mucus was separately collected from the mantle, gills and labial palps of the eastern oyster (Crassostrea virginica) and analyzed by liquid chromatography and tandem mass spectrometry. Results showed the presence of a wide variety of molecules involved in host-microbe interactions, including putative adhesion molecules (e.g. c-type lectins) confirming that transcripts previously identified in epithelial cells are translated into proteins secreted in mucus. Mucus composition was different among samples collected from different organs. These results generate a reference map for C. virginica pallial mucus to better characterize the various physiological functions of mucosal secretions.

The immune system and its modulation mechanism in scallop

Scallops are a cosmopolitan family of bivalves, and some of them are highly prized as dominant aquaculture species. In the past decades, there have been increasing studies on the basic biology and immunology of scallops, and this review summarizes the research progresses of immune system and its modulation mechanism in scallop. As invertebrate, scallops lack adaptive immunity and they have evolved an array of sophisticated strategies to recognize and eliminate various invaders by employing a set of molecules and cells. It is evident that basic immune reactions such as immune recognition, signal transduction, and effector synthesis involved in immune response are accomplished in a variety of ways. They rely upon an extensive repertoire of phagocytosis, apoptosis and encapsulation of the circulating hemocytes for eliminating invasive pathogens, as well as the production of immune effectors that are active against a large range of pathogens or sensitive for the environmental stress. Furthermore, the molecular constitutions, metabolic pathways and immunomodulation mechanisms of the primitive catecholaminergic, cholinergic, enkephalinergic system and NO system in scallop are also discussed, which can be taken as an entrance to better understand the origin and evolution of the neuroendocrine-immune regulatory network in lower invertebrates.

A single-CRD C-type lectin from oyster Crassostrea gigas mediates immune recognition and pathogen elimination with a potential role in the activation of complement system

C-type lectins (CTLs), serving as pattern recognition receptors (PRRs), are a superfamily of Ca(2+)-dependent carbohydrate-recognition proteins that participate in nonself-recognition and pathogen elimination. In the present study, a single carbohydrate-recognition domain (CRD) CTL was identified from oyster Crassostrea gigas (designated as CgCLec-2). There was only one CRD within the deduced amino acid sequence of CgCLec-2 consisting of 129 amino acid residues. A conserved EPN (Glu246-Pro247-Asn248) motif was found in Ca(2+)-binding site 2 of CgCLec-2. The CgCLec-2 mRNA could be detected in all the examined tissues at different expression levels in oysters. The mRNA expression of CgCLec-2 in hemocytes was up-regulated significantly at 6 h post Vibrio splendidus challenge. The recombinant CgCLec-2 (rCgCLec-2) could bind various Pathogen-Associated Molecular Patterns (PAMPs), including lipopolysaccharide, mannan and peptidoglycan, and displayed strong binding abilities to Vibrio anguillarum, V. splendidus and Yarrowiali polytica and week binding ability to Staphylococcus aureus. It could also enhance the phagocytic activity of oyster hemocytes to V. splendidus and exhibited growth suppression activity against gram-positive bacteria S. aureus but no effect on gram-negative bacteria V. splendidus. Furthermore, the interaction between rCgCLec-2 and rCgMASPL-1 was confirmed by GST Pull down. The results suggested that CgCLec-2 served as not only a PRR in immune recognition but also a regulatory factor in pathogen elimination, and played a potential role in the activation of complement system.

CfLec-3 from scallop: an entrance to non-self recognition mechanism of invertebrate C-type lectin

A C-type lectin (CfLec-3) from Chlamys farreri with three carbohydrate-recognition domains (CRDs) was selected to dissect the possible mechanisms of PAMP binding and functional differentiation of invertebrate lectins. CfLec-3 distributed broadly, and its mRNA expression in hemocytes increased significantly after stimulations with LPS, PGN or β-glucan, but not poly(I:C). The recombinant CfLec-3 (rCfLec-3) could bind PAMPs and several microbes. rCfLec-3 mediated hemocytes phagocytosis against Escherichia coli and encapsulation towards agarose beads. Obvious functional differentiation occurred among the three CRDs, as CRD1 exhibited higher activity to bind PAMPs, while CRD2/3 were expert in promoting hemocyte mediated opsonisation. The tertiary structural differences were suspected to be associated with such functional differentiation. PAMP binding abilities of CfLec-3 were determined by Ca(2+)-binding site 2 motif. When Pro in this motif of each CRD was mutated into Ser, their PAMP binding abilities were deprived absolutely. rCRD2 acquired mannan binding capability when its EPD was replaced by EPN, but lost when EPN in rCRD3 was changed into EPD. The Pro in Ca(2+)-binding site 2 was indispensable for PAMPs binding, while Asn was determinant for specific binding to mannan. It shed new insight into PAMPs binding mechanism of invertebrate C-type lectins and their functional differentiation.

An EPD/WSD motifs containing C-type lectin from Argopectens irradians recognizes and binds microbes with broad spectrum

C-type lectins are a superfamily of Ca(2+)-dependent carbohydrate-recognition proteins consisting of at least one carbohydrate-recognition domain (CRD), which play significant roles in nonself-recognition and clearance of invaders. The immune function of a C-type lectin (AiCTL-7) with EPD/WSD motifs from Argopectens irradians was investigated in the present study. The recombinant protein of AiCTL-7 (rAiCTL-7) could bind LPS, PGN, mannan, yeast glucan and poly I:C in vitro, and displayed a broader microbes binding spectrum towards Gram-positive bacteria Staphylococcus aureus, Gram-negative bacteria Escherichia coli, Vibrio anguillarum, as well as fungi Pichia pastoris and Yarrowia lipolytica. Moreover, it could also inhibit the growth of E. coli and significantly (P < 0.01) mediate the cell-cell adhesion in vitro. The results clearly suggested that EPD/WSD motifs containing lectin AiCTL-7 could serve as PRR with wider recognition spectrum, and function both as collectin and selectin participating in the immunity against invaders in scallops. It could be inferred that the diversity and complexity of motifs in Ca(2+) binding site 2 in CRDs endowed C-type lectins with comprehensive recognition spectrum and multiple immune functions against complex living environment.

A novel phagocytic receptor (CgNimC) from Pacific oyster Crassostrea gigas with lipopolysaccharide and gram-negative bacteria binding activity

Phagocytosis is an evolutionarily conserved process to ingest the invading microbes and apoptotic or necrotic corpses, playing vital roles in defensing invaders and maintenance of normal physiological conditions. In the present study, a new Nimrod family phagocytic receptor with three EGF-like domains was identified in Pacific oyster Crassostrea gigas (designated CgNimC). CgNimC shared homology with other identified multiple EGF-like domain containing proteins. The mRNA transcripts of CgNimC were mainly distributed in mantle and hemocytes. Its relative expression level in hemocytes was significantly (P < 0.01) up-regulated after the injection of bacteria Vibrio anguillarum. Different to the NimC in Drosophila and Anopheles gambiae, the recombinant protein of CgNimC (rCgNimC) could bind directly to two gram-negative bacteria V. anguillarum and Vibrio splendidus, but not to gram-positive bacteria Staphylococci aureus, Micrococcus luteus or fungi Yarrowia lipolytica and Pichia pastoris. The affinity of rCgNimC toward M. luteus and Y. lipolytica was enhanced when the microorganisms were pre-incubated with the cell free hemolymph. rCgNimC exhibited higher affinity to lipopolysaccharide (LPS) and relatively lower affinity to peptidoglycan (PGN), while no affinity to glucan (GLU). After the CgNimC receptor was blocked by anti-rCgNimC antibody in vitro, the phagocytic rate of hemocytes toward two gram-negative bacteria V. anguillarum and V. splendidus was reduced significantly (P < 0.05), but no significant change of phagocytic rate was observed toward M. luteus and Y. lipolytica. All these results implied that CgNimC, with significant binding capability to LPS and gram-negative bacteria, was a novel phagocytic receptor involved in immune response of Pacific oyster. Further, it was speculated that receptors of Nimrod family might function as a phagocytic receptor to recognize PAMPs on the invaders and its recognition could be promoted by opsonization of molecules in hemolymph.