Molecular cloning and expression of a C-type lectin-like protein from orange-spotted grouper Epinephelus coioides

A C-type lectin with a single CRD from Onychostoma macrolepis mediates immune recognition against bacterial challenge

C-type lectins (CTL) are a large group of pattern-recognition proteins and to play important roles in glycoprotein metabolism, multicellular integration, and immunity. Based on their overall domain structure, they can be classified as different groups that possess different physiological functions. A typical C-type lectin (named as OmLec1) was identified from the fish, Onychostoma macrolepis, an important cultured fish in China. Open reading frame of OmLec1 contains a 570 bp, encoding a protein of 189 amino acids that includes a signal peptide and a single carbohydrate-recognition domain. The phylogenetic analysis showed that OmLec1 could be grouped with C-type lectin from other fish. OmLec1 was expressed in all the tissues in our study, and the expression level was highest in liver. And its relative expression levels were significantly upregulated following infection with Aeromonas hydrophila. The recombinant OmLec1 protein (rOmLec1) could agglutinate some Gram-negative bacteria and Gram-positive bacteria in vitro in the presence of Ca²⁺, showing a typical Ca²⁺-dependent carbohydrate-binding protein. Furthermore, rOmLec1 purified from E. coli BL21 (DE3), strongly bound to LPS and PGN, as well as all tested bacteria in a Ca²⁺-dependent manner. These results indicate that OmLec1 plays a central role in the innate immune response and as a pattern recognition receptor that recognizes diverse pathogens among O. macrolepis.

In silico characterization and expression analysis of eight C-type lectins in obscure puffer Takifugu obscurus

C-type lectins (CTLs) are a group of carbohydrate-binding proteins that play crucial roles in innate immune defense against invading pathogens. CTLs have been extensively studied in lower vertebrates, such as fish, for their roles in eliminating pathogens; however, their homologs in pufferfish are not well known. In the present study, eight CTLs from obscure puffer Takifugu obscurus (designated as ToCTL3-10 according to the order they were discovered) were obtained. All predicted ToCTL proteins contained a single carbohydrate recognition domain (CRD). ToCTL7 also contained one calcium-binding epidermal growth factor (EGF)-like domain (EGF_CA) and a transmembrane region. ToCTL9 also contained an SCP domain, an EGF domain, and an EGF-like domain. Bioinformatics analysis revealed that ToCTL3-10 mainly clustered with the corresponding CTL homologs of other pufferfish species. Tissue distribution analysis detected ToCTL3-10 in all tissues examined, including kidneys, liver, gills, spleen, intestines, and heart. Moreover, the expressions of ToCTL3-10 were significantly induced in the kidneys of obscure puffer following challenges with three Gram-negative bacterial pathogens, namely, Vibrio harveyi, Aeromonas hydrophila, and Edwardsiella tarda, and a synthetic analog of double-stranded RNA poly(I:C). The expression patterns of ToCTL3-10 in response to different immune stimulants were different. Our results indicated that the eight ToCTLs obtained herein might be involved in host defense against bacterial and poly(I:C) infections in T. obscurus.

A C-type lectin (OmCTL) in Onychostoma macrolepis: Binding ability to LPS, PGN and agglutinating activity against bacteria

C-type lectins (CTLs) are calcium-dependent carbohydrate-binding proteins that mainly bind to carbohydrate-based or other ligands to mediate cell adhesion, recognize pathogens, and play important roles in the immune system. In the present study, a novel C-type lectin (OmCTL) isolated from Onychostoma macrolepis was investigated. The open reading frame of OmCTL comprises 468 bp, encoding a 155 amino acid polypeptide with an 18 amino acid putative signaling peptide. The predicted primary OmCTL structure contains a signal peptide, a single carbohydrate recognition domain (CRD) and an EPN/WND motif required for carbohydrate-binding specificity. Using tissue expression pattern analysis, OmCTL has been shownto be highly expressed in the liver, and is also detected in other tissues. OmCTL was significantly upregulated in the liver and spleen following infection with Aeromonas hydrophila, suggesting its involvement in immune response. The recombinant OmCTL protein (rOmCTL) agglutinated two gram-negative bacteria, Escherichia coli and A. hydrophila, in vitro in the presence of Ca²⁺, showing that it is a typical Ca²⁺-dependent carbohydrate-binding protein.Furthermore, rOmCTL purified from E. coli BL21 (DE3) strongly bound to LPS and PGN, as well as all tested bacteria in a Ca²⁺-independent manner. These results indicate that OmCTL plays a central role in the innate immune response and as a pattern recognition receptor that recognizes diverse pathogens among O. macrolepis.

A C-type lectin with antibacterial activity in weather loach, Misgurnus anguillicaudatus

C-type lectins are carbohydrate-binding proteins that play important roles in immunity by serving as pattern recognition receptors. In the present study, a novel nattectin-like C-type lectin was obtained from the weather loach, Misgurnus anguillicaudatus, designated as MaCTL. MaCTL encodes a peptide with 165 amino acids, with a signal peptide and a single C-type lectin domain (CTLD), containing a galactose-specific QPD motif and a conserved Ca²⁺ -binding site. Transcripts of MaCTL were significantly upregulated after immune challenge with its pathogen A. hydrophila. In vitro assays with recombinant MaCTL protein revealed that it exhibited hemagglutinating and bacterial agglutinating activities, in a Ca²⁺ -dependent manner. MaCTL was found to bind to a wide range of bacteria, as well as bind to bacterial polysaccharides LPS and PGN. Moreover, MaCTL displayed antimicrobial activity by inhibiting the growth of bacteria. These results collectively suggest that MaCTL is involved in the antibacterial defence of weather loach.

Exposure to Aeromonas hydrophila induces inflammation and increases expression of the gene encoding for a putative dual CTLD-containing lectin in milkfish liver

Milkfish (Chanos chanos Forsskal) is an important aquaculture product and is the sole extant species of the family Chanidae (order Gonorynchiformes). While there are already several reports regarding milkfish aquaculture, studies on milkfish immunity and gene expression are very limited. In this study, we showed that Aeromonas hydrophila induces inflammation in milkfish liver. We identified a milkfish C-type lectin-like domain containing proteins (CTLDcps) gene, designated as CcClec, which was upregulated in respond to A. hydrophila. Full-length sequencing was performed using Rapid Amplification of cDNA Ends (RACE PCR) to produce a complete Coding DNA Sequence (CDS) of the gene. The CcClec gene encoded a predicted protein of 340 amino acids containing two CTLDs that may potentially bind carbohydrates, especially sucrose and cellobiose. The CcClec mRNA transcript was expressed highest in the liver, followed by head kidney, brain, heart, gills, spleen, and midgut. CcClec transcripts were upregulated in damaged liver upon exposure to A. hydrophila. Overall, these findings demonstrated that CcClec is implicated in milkfish innate immunity, and is most highly expressed in the liver, suggesting a role of the liver in the milkfish immune system.

Black rockfish C-type lectin, SsCTL4: A pattern recognition receptor that promotes bactericidal activity and virus escape from host immune defense

C-type lectin (CTL) is an immune receptor and is received extensive attention of its important roles in immune response and immune escape. Some CTL, such as CTL4, has been well characterized in human and several other mammals, but much less documentation exists about the immunological function of CTL4 in lower vertebrates. In the present study, a C-type lectin domain family 4 member, SsCTL4, which is also high homology with CD209 antigen-like protein, from the teleost fish black rockfish (Sebastes schlegelii) was identified and examined at expression and functional levels. The open reading frame of SsCTL4 is 765 bp, and the deduced amino acid sequence of SsCTL4 shares 78%-84% overall identities with the C-type lectin of several fish species. In silico analysis identified several conserved C-type lectin features, including a carbohydrate-recognition domain and four disulfide bond-forming cysteine residues. Expression of SsCTL4 occurred in multiple tissues and was upregulated during bacterial and viral infection. Recombinant SsCTL4 (rSsCTL4) exhibited apparent binding activities against bacteria (Edwardsiella tarda and Vibrio anguillarum) and virus (infectious spleen and kidney necrosis virus, ISKNV). rSsCTL4 was able to agglutinate the Gram-negative and Gram-positive bacteria in a Ca²⁺-dependent manner. The agglutinating ability of rSsCTL4 was abolished in the absence of calcium or presence of mannose. rSsCTL4 also increased macrophage bactericidal activity. In the presence of rSsCTL4, fish exhibited enhanced resistance against bacterial infection but increased susceptibility to viral infections. Collectively, these results indicate that SsCTL4 serves as a pattern recognition receptor that not only promotes bactericidal activity, but may also serve as targets for virus manipulation of host defense system.

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.

A C-type lectin that inhibits bacterial infection and facilitates viral invasion in black rockfish, Sebastes schlegelii

C-type lectins (CTLs) are important pattern recognition receptors (PRRs) that play vital roles in innate immunity. In teleosts, a number of CTLs have been reported, but their in vivo effects on host defense are still limited. In this study, a CTL homolog (SsLec1) was identified from black rockfish, Sebastes schlegelii, and its structure, expression and biological function was analyzed. The open reading frame of SsLec1 is 633 bp, with a 5'- untranslated region (UTR) of 36 bp and a 3'- UTR of 117 bp. The deduced amino acid sequence of SsLec1 shares the highest overall identity (73.20%) with the CTL of Oplegnathus fasciatus. SsLec1 possesses conserved CTL features, including a carbohydrate-recognition domain, four disulfide bond-forming cysteine residues, the mannose-type carbohydrate-binding motif, the conserved calcium binding sites and a putative signal peptide. The expression of SsLec1 was highest in liver and could be induced by experimental infection with Listonella anguillarum. Recombinant SsLec1 (rSsLec1) purified from E. coli was able to bind and agglutinate the Gram-negative fish pathogens Vibrio ichthyoenteri and Vibrio vulnificus. The agglutinating ability of rSsLec1 was abolished in the presence of mannose or ethylenediaminetetraacetic acid. Further analysis showed that rSsLec1 could enhance phagocytosis by macrophages. In vivo experiments indicated that rSsLec1 could inhibit bacterial infection and promote viral invasion. Taken together, these results suggest that SsLec1 is a novel CTL that possesses apparent immunoregulation property and plays a critical role in host defense against pathogens invasion.

Molecular cloning and characterization of a C-type lectin in yellow catfish Tachysurus fulvidraco

This study represents the first report of a C-type lectin (ctl) in yellow catfish Tachysurus fulvidraco. The complete sequence of ctl complementary (c)DNA consisted of 685 nucleotides. The open reading frame potentially encoded a protein of 177 amino acids with a calculated molecular mass of c.y 20.204 kDa. The deduced amino-acid sequence contained a signal peptide and a single carbohydrate recognition domain with four cysteine residues and GlnProAsp (QPD) and TrpAsnAsp (WND) motifs. Ctl showed the highest identity (56.0%) to the predicted lactose binding lectin from channel catfish Ictalurus punctatus. Quantitative real-time (qrt)-PCR analysis showed that ctl messenger (m)RNA was constitutively expressed in all examined tissues in normal fish, with high expression in trunk kidney and head kidney, which was increased following Aeromonas hydrophila challenge in a duration-dependent manner. Purified recombinant Ctl (rCtl) from Escherichia coli BL21 was able to bind and agglutinate Gram-positive and Gram-negative bacteria in a calcium-dependent manner. These results suggested that Ctl might be a C-type lectin of T. fulvidraco involved in innate immune responses as receptors (PRR).

Molecular and functional characterization of a novel CD302 gene from ayu (Plecoglossus altivelis)

Recognizing the presence of invading pathogens by pattern recognition receptors (PRRs) is key to mounting an effective innate immune response. Mammalian CD302 is an unconventional C-type lectin like receptor (CTLR) involved in the functional regulation of immune cells. However, the role of CD302 in fish remains unclear. In this study, we characterized a novel CD302 gene from ayu (Plecoglossus altivelis), which was tentatively named PaCD302. The cDNA sequence of PaCD302 is 1893 nucleotides in length, and encodes a polypeptide of 241 amino acids with molecular weight 27.1 kDa and pI 4.69. Sequence comparison and phylogenetic tree analysis showed that PaCD302 is a type I transmembrane CTLR devoid of the known amino acid residues essential for Ca(2+)-dependent sugar binding. PaCD302 mRNA expression was detected in all tissues and cells tested, with the highest level in the liver. Following Vibrio anguillarum infection, PaCD302 mRNA expression was significantly upregulated in all tissues tested. For further functional analysis, we generated a recombinant protein for PaCD302 (rPaCD302) by prokaryotic expression and raised a specific antibody against rPaCD302. Western blot analysis revealed that the native PaCD302 is glycosylated. Refolded rPaCD302 was unable to bind to five monosaccharides (l-fucose, d-galactose, d-glucose, d-mannose and N-acetyl glucosamine) or two other polysaccharides (lipopolysaccharide and peptidoglycan). It was able to bind to three Gram-positive and seven Gram-negative bacteria, but show no bacterial agglutinating activity. PaCD302 function blocking using anti-PaCD302 IgG resulted in inhibition of phagocytosis and bactericidal activity of ayu monocytes/macrophages (MO/MΦ), suggesting that PaCD302 regulates the function of ayu MO/MΦ. In summary, our study demonstrates that PaCD302 may participate in the immune response of ayu against bacterial infection via modulation of MO/MΦ function.

Molecular cloning and characterization of a galectin-1 homolog in orange-spotted grouper, Epinephelus coioides

As a member of animal lectin family, galectin has the functions of pathogen recognition, anti-bacteria and anti-virus. In the present study, a galectin-1 homolog (EcGel-1) from grouper (Epinephelus coioides) was cloned and its possible role in fish immunity was analyzed. The full length cDNA of EcGel-1 is 504 bp, including a 408 bp open reading frame (ORF) which encodes 135 amino acids with a molecular mass of 15.19 kDa. Quantitative real-time PCR analysis indicated that EcGel-1 was constitutively expressed in all analyzed tissues of healthy grouper. The expression of EcGel-1 in the spleen of grouper was differentially up-regulated challenged with Singapore grouper iridovirus (SGIV), poly (I:C), and LPS. EcGel-1 was abundantly distributed in the cytoplasm in GS cells. Recombinant EcGel-1(rEcGel-1) protein can make chicken erythrocyte aggregation, and combine with gram negative bacteria and gram positive bacteria in the presence of 2-Mercaptoethanol (β-ME). Taken together, the results showed that EcGel-1 may be an important molecule involved in pathogen recognition and pathogen elimination in the innate immunity of grouper.

A novel C-type lectin, Nattectin-like protein, with a wide range of bacterial agglutination activity in large yellow croaker Larimichthys crocea

C-type lectins (CTLs) are generally recognized as a superfamily of Ca(2+)-dependent carbohydrate-binding proteins, which serve as pattern recognition receptors (PRRs) in innate immunity of vertebrates. In this study, the molecular characterization and immune roles of a novel CTL from Larimichthys crocea (designated as LcNTC) were investigated. LcNTC is a novel protein that shared 33%-49% homology with other teleosts CTLs. The full-length cDNA of LcNTC was composed of 859 bp with a 465 bp open reading frame encoding a putative protein of 154 residues. LcNTC contained a single CRD with four conserved disulfide-bonded cysteine residues (Cys(57)-Cys(148), Cys(126)-Cys(140)) and EPN/AND motifs instead of invariant EPN/WND motifs required for carbohydrate-binding specificity and constructing Ca(2+)-binding sites. LcNTC mRNA was detected in all examined tissues with the most abundant in the gill. After challenged with poly I:C and Vibrio parahaemolyticus, the temporal expression of LcNTC was significantly up-regulated in the liver, spleen and head-kidney. LcNTC transcripts were also induced in the gill, skin, spleen and head-kidney post-infection with Cryptocaryon irritans. The recombinant LcNTC (rLcNTC) purified from Escherichia coli BL21 (DE3) exhibited strong agglutination activity against erythrocytes from human, rabbit and large yellow croaker in a Ca(2+)-dependent manner, and the agglutination could be inhibited by D-Mannose, D-Glucose, D-Fructose, α-Lactose, D-Maltose and LPS. Positive microbial agglutination activities of rLcNTC were observed against all tested bacteria in the presence of Ca(2+), including Gram-positive bacteria (Bacillus subtilis, Staphylococcus aureus and Micrococcus lysoleikticus) and Gram-negative bacteria (E. coli, V. parahaemolyticus, Vibrio alginolyticus and Aeromonas hydrophila). These findings collectively indicated that LcNTC might be involved in the innate immunity of L. crocea as a PRR.

Molecular characterization of the lgals1 gene in large scale loach Paramisgurnus dabryanus

Galectins constitute a group of lectins with binding specificity for β-galactoside sugars. Galectin-1 is a prototype galectin and the multifunctionality of mammalian galectin-1s is well-known, but only a few of fish galectin-1s have been identified. In this study, we obtained the full-length cDNA and genomic sequence of the galectin-1 gene (designated as Pdlgals1) from large scale loach (Paramisgurnus dabryanus), performed phylogenetic analysis, and characterized the expression pattern and the transcriptional activity of its 5' flanking region. The Pdlgals1 gene contains 4 exons that encode a peptide of 132 amino acids with all the galectin signature motifs. Phylogenetic analysis and sequence alignment indicated that Pdlgals1 is a homologue of human LGALS1. RT-PCR and whole-mount in situ hybridization revealed that Pdlgals1 is mainly expressed in the skin, muscle, intestine and cavum oropharyngeum. Transcriptional activity assays demonstrated that the basal promoter of Pdlgals1 is located in a region from -500bp to its transcriptional start site. Potential binding sites for transcription factors including C/EBP, AP-1, GATA, Oct-1, δEF1, NF-κB, c-Myb, SP-1, AP-2, AML-1α, and AP-4 were identified in the basal promoter, suggesting that these factors are associated with the regulation of Pdlgals1. These results provided clues for further investigation of galectin-1 functions in loaches.

CsCTL1, a teleost C-type lectin that promotes antibacterial and antiviral immune defense in a manner that depends on the conserved EPN motif

Many C-type lectins (CTLs) have been identified in teleost, however, the in vivo function of fish CTLs is essentially unknown. In this study, we examined the function of a CTL (CsCTL1) from tongue sole. CsCTL1 possesses the conserved EPN motif required for mannose binding in mammals but unknown in function in fish. Recombinant CsCTL1 (rCsCTL1), but not the mutant rCsCTL1M bearing substitutions at EPN, interacted with and agglutinated a limited range of bacteria. The agglutinating ability of rCsCTL1 was abolished in the absence of calcium or presence of mannose. Binding of rCsCTL1 to bacteria promoted phagocytosis and antimicrobial activity of head kidney monocytes. Fish administered with rCsCTL1 exhibited enhanced resistance against bacterial and viral infections. These results provide the first evidence that the EPN site is essential to a fish CTL and that, in addition to antibacterial properties, a fish CTL promotes the immune defense against viral infection as well.