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

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.

Pathogen of Vibrio harveyi infection and C-type lectin proteins in whiteleg shrimp (Litopenaeus vannamei)

Diseases caused by Vibrio harveyi in shrimps have gradually become one group of the most serious threats to shrimp production, while related molecular mechanisms of infections with Vibrio harveyi are still not known well in shrimps. Here, we performed proteomic sequencing of hepatopancreas in whiteleg shrimps (Litopenaeus vannamei) infected with exogenous Vibrio harveyi, and subsequent functional annotation and calculation of differentially expressed proteins (DEPs) in this study. A total of 145 DEPs were obtained, among them 36 were up-regulated and 109 were down-regulated after the infection. Meanwhile, our results showed that after the infection of Vibrio harveyi, expression levels of a variety of C-type lectins (CTLs) were changed significantly. In-depth functional domain analysis and spatial structure prediction of these CTLs revealed that amino acid sequences and spatial structures of the C-type lectin domain (CTLD) shared by the CTL-S and IML proteins were variant, suggesting differential functions between the two CTLs. In summary, various members of the CTL family have different epidemic responses to Vibrio harveyi infection, which provides a theoretical guidance for deep-going investigations on practical immunity reactions and pathogen infections in shrimps.

Antimicrobial properties and immune-related gene expression of a C-type lectin isolated from Pinctada fucata martensii

C-type lectins are carbohydrate-binding proteins that play important roles in the innate immune response to pathogen infections. Here, multi-step high performance liquid chromatography (HPLC), combined with mass spectrometry (MS), was used to isolate and identify proteins with antibacterial activity from the serum of Pinctada fucata martensii. Using this method, we obtained a novel isoform of C-type lectin (PmCTL-1). PmCTL-1 strongly inhibited gram-positive bacteria. The complete cDNA sequence of PmCTL-1 was 636 bp in length, and encoded a protein 149 amino acids long, containing a typical carbohydrate recognition domain (CRD). A phylogenetic analysis based on a multiple sequence alignment indicated that PmCTL-1 was highly similar to C-type lectins from other mollusks. Fluorescent quantitative real-time PCR analysis showed that PmCTL-1 mRNA was strongly upregulated in the mantle of healthy P.f. martensii, but was expressed only at low levels in the gill, gonad, hepatopancreas, adductor muscle, and hemocytes. PmCTL-1 expression levels in the mantle and hemocytes increased significantly in response to bacterial stimulation. This study provides a valuable framework for further explorations of innate immunity and the immune response in mollusks.

Involvement of a novel Ca2+-independent C-type lectin from Sinonovacula constricta in food recognition and innate immunity

Bivalve lectins perform a crucial function in recognition of foreign particles, such as microalgae and pathogenic bacteria. In this study, a novel C-type lectin form Sinonovacula constricta (ScCL) was characterized. The full-length cDNA of ScCL was 1645 bp, encoding a predicted polypeptide of 273 amino acids with one typical carbohydrate-recognition domain. ScCL has the highest similarity and closest phylogenetic relationship with the C-type lectin from Solen grandis. Real-time PCR analysis showed that ScCL was expressed in all tested tissues, with the highest expression in the foot and the lowest expression in hemocytes. Agglutination activity of ScCL was Ca²⁺-independent. ScCL showed the strongest agglutination on Chlorella vulgaris, the modest agglutination on Platymonas subcordiformis, Nannochloropsis sp., and Thalassiosira pseudonana, the weakest agglutination on Chaetoceros sp., and no agglutination on Isochrysis zhanjiangensis. Meanwhile, agglutination tests and western blot analysis revealed that the recombinant ScCL protein could agglutinate Staphylococcus aureus and Vibrio harveyi, but could not agglutinate Vibrio anguillarum, Bacillus cereus, or Vibrio parahaemolyticus. Furthermore, ScCL had a high binding activity with LPS and mannose, a low binding activity with LTA, and no binding activity with PGN. The expression of ScCL in the gill of S. constricta fed with C. vulgaris and T. pseudonana was significantly increased at 1 and/or 3 h. After injection with S. aureus, the expression of ScCL in the gill was significantly increased at 3, 6, and 24 h. These results indicated that ScCL was involved in food particle recognition and immunity of S. constricta.

Effect of pathogenic bacteria on a novel C-type lectin, hemocyte and superoxide dismutase/ alkaline phosphatase activity in Onchidium reevesii

Bacterial infection in the marine environment is a serious problem to maintain the stability of marine ecosystems. Nevertheless, there is little report so far for the biological effects of pathogenic bacteria in coastal ecosystems. Hence, we investigated the responses of shell-less Onchidium reevesii to resist against pathogenic bacterial infection. Analysis of data here could be used as fundamental information for assessment of innate immune response of O. reevesii. The full-length OrCTL cDNA was cloned and consists of 1849 base pair (bp) encoding protein of 192 amino acids. Constructing multiple alignments suggested that OrCTL has conserved carbohydrate recognition domain (CRD) of CTLs, containing an EPS (Glu-Pro-Ser) motif that may imply the function of recognition of carbohydrates like others invertebrate. OrCTL mRNAs were mainly detected in ganglion and hepatopancreas, and expression was highly up-regulated from 2 h after Vibrio harveyi challenge, rapidly decreased at 4 h, and significantly increased at 12 h. In addition, after challenge with Vibrio parahaemolytics, OrCTL gene expression was slightly up-regulated from 2 h, peaked at 12 h. Enzyme activity (in the hepatopancreas) and cell immune (in the hemolymph) were investigated along with Superoxide dismutase (SOD) activity, alkaline phosphatase (ALP) activity and cell cycle. SOD activities were significantly higher after V. harveyi and V. parahaemolytics challenge than that in the control group, respectively. By contrast, ALP activities were significantly inhibited after challenged with bacteria than that in the control group, respectively. Enzyme activities in the hepatopancreas obviously fluctuated, and ALP activity was more sensitive to bacteria. Cell responses illustrated that there were a significant higher percentage of cells in the S and G2/M phase in hemolymph after challenged with bacteria. Our results suggested that the immune response of O. reevesii could be activated by pathogenic bacteria, and the data will provide referent for the disease prevention of systematic investigation in aquatic animal.

Expression analyses of C-type lectins (CTLs) in Manila clam under cold stress provide insights for its potential function in cold resistance of Ruditapes philippinarum

Manila clam Ruditapes philippinarum is an economically and scientifically important marine bivalve species. C-type lectin acts as a pattern-recognition receptor (PPR), plays a crucial role in the innate immunity of invertebrates, and involves in pathogen recognition, and cell-cell interaction. In this study, six different types of C-type lectin genes, CTL-1, CTL-2, CTL-3, CTL-4, CTL-5, and CTL-6, were identified from the R. philippinarum. CTL amino acid sequence was highly conserved compare to other invertebrate CTL sequences. Also, the temporal expressions of CTLs mRNA were detected in R. philippinarum with higher expression level in hepatopancreas and gill, and with lower expression levels in other tissues. The expression pattern of CTL genes in hepatopancreas were investigated under low-temperature stress using real-time quantitative fluorescence PCR (RT-qPCR) and the results showed that the transcription of CTL mRNAs were induced after low-temperature challenge. Overall, the sequence analysis and the expression patterns of CTLs provide clues for understanding the response of the Manila clam to low-temperature stress.

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.

Cloning and functional analysis of scavenger receptor B gene from the sea cucumber Apostichopus japonicus

Scavenger receptor (SR) class B (SR-B) is a transmembrane protein that belongs to the SR family with a wide range of functions in innate immunity. Here, an SR-B homologue, designated as AjSR-B, was cloned from the sea cucumber Apostichopus japonicus. AjSR-B comprised 2519 nucleotides with a 5'-untranslated region (UTR) of 153 bp, an open reading frame of 1581 bp encoding a 526 amino acid protein, and a 3'-UTR of 785 bp. SMART analysis indicated that AjSR-B has two transmembrane regions and a cluster determinant 36 domain. Multiple alignments and phylogenetic analysis supported that AjSR-B is a novel member of the SR-B protein family. Moreover, AjSR-B was constitutively expressed in all detected tissues, with the highest levels recorded in the intestine. Both were significantly induced in coelomocytes and the intestine after Vibrio splendidus challenge. Functionally, the recombinant rAjSR-B that corresponds to a large extracellular loop can bind pathogen-associated molecular patterns (PAMPs), including lipopolysaccharide (LPS), peptidoglycan, and mannan, with a high binding affinity to LPS. Bacterial agglutination assay showed that rAjSR-B can agglutinate the four tested bacteria (Gram-negative and Gram-positive bacteria) with calcium dependence. However, the agglutination ability for Gram-negative bacteria completely disappeared in the presence of PAMPs but a weak ability to bind Gram-positive bacteria (Micrococcus luteus) was still exhibited, suggesting there might exist a competition between Gram-positive bacteria and PAMPs under same condition. Our current study indicated that AjSR-B is a PAMP that plays important roles in the innate immune process of sea cucumbers.

Differential proteomic profiles and characterizations between hyalinocytes and granulocytes in ivory shell Babylonia areolata

The haemocytes of the ivory shell, Babylonia areolata are classified by morphologic observation into the following types: hyalinocytes (H) and granulocytes (G). Haemocytes comprise diverse cell types with morphological and functional heterogene and play indispensable roles in immunological homeostasis of invertebrates. In the present study, two types of haemocytes were morphologically identified and separated as H and G by Percoll density gradient centrifugation. The differentially expressed proteins were investigated between H and G using mass spectrometry. The results showed that total quantitative proteins between H and G samples were 1644, the number of up-regulated proteins in G was 215, and the number of down-regulated proteins in G was 378. Among them, cathepsin, p38 MAPK, toll-interacting protein-like and beta-adrenergic receptor kinase 2-like were up-regulated in G; alpha-2-macroglobulin-like protein, C-type lectin, galectin-2-1, galectin-3, β-1,3-glucan-binding protein, ferritin, mega-hemocyanin, mucin-17-like, mucin-5AC-like and catalytic subunit of protein kinase A were down-regulated in G. The results showed that the most significantly enriched KEGG pathways were the pathways related to ribosome, phagosome, endocytosis, carbon metabolism, protein processing in endoplasmic reticulum and oxidative phosphorylation. For phagosome and endocytosis pathway, the number of down-regulation proteins in G was more than that of up-regulation proteins. For lysosome pathway, the number of up-regulation proteins in G was more than that of down-regulation proteins. These results suggested that two sub-population haemocytes perform the different immune functions in B. areolata.

Novel Ca2+-independent C-type lectin involved in immune defense of the razor clam Sinonovacula constricta

C-type lectins (CTLs) are important pattern recognition molecules that participate in bacterial binding and agglutination by specific recognition of carbohydrates from pathogens. In this study, a full-length cDNA of CTL was cloned from Sinonovacula constricta (designated ScCTL-2). ScCTL-2 has a length of 981 bp, a 5'-untranslated region (UTR) of 47 bp, a short 3'-UTR of 37 bp, and an open reading frame (ORF) of 894 bp, which encodes a polypeptide of 298 amino acid residues. The deduced amino acid of ScCTL-2 possesses a conserved carbohydrate-recognition domain (CRD) similar to that of C³¹-E¹⁷¹. Spatial distribution analysis demonstrated that ScCTL-2 was constitutively expressed in all tested tissues, with dominant expression in foot and siphon and weak expression in hepatopancreas. The mRNA expression level of ScCTL-2 in gills and hepatopancreas was significantly upregulated at 6 and 12 h after challenge with the pathogen Vibrio parahaemolyticus. The recombinant ScCTL-2 showed specific binding and agglutinate capacities to all examined Gram-negative bacterial species, namely, Escherichia coli, Vibro anguillarum, and V. parahaemolyticus in a Ca²⁺-independent manner. However, these binding activities were not detected in Gram-positive Micrococcus luteus. Our results indicated that ScCTL-2 could be a novel pattern recognition receptor that can specifically recognize Gram-negative microorganisms in the innate immunity of S. constricta.