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

Identification and functional analysis of Toll-like receptor 2 from razor clam Sinonovacula constricta

Toll-like receptor 2 (TLR2) is a member of TLR family that plays important roles in the innate immune system, such as pathogen recognition and inflammation regulation. In this study, the TLR2 homologue was cloned from razor clam Sinonovacula constricta (denoted as ScTLR2) and its immune function was explored. The full-length cDNA of ScTLR2 comprised 2890 nucleotides with a 5'-UTR of 218 bp, an open reading frame of 2169 bp encoding 722 amino acids and a 3'-UTR of 503 bp. The deduced amino acid of ScTLR2 showed similar structure to TLR2 homologue with a conserved signal peptide, four LRR domains, one LRR-TYP domain, one LRR-CT domain, one transmembrane domain and a conserved TIR domain. ScTLR2 mRNA was detected in all examined tissues with the highest expression in the gill. After Vibrio parahaemolyticus challenge, the mRNA expression of ScTLR2 was significantly induced both in gill and haemocytes. The recombinant ScTLR2-LRR protein could bind all tested PAMPs including LPS, PGN and MAN. Bacterial agglutination assay showed that rScTLR2 could agglutinate the six tested bacteria with a calcium dependent manner. More importantly, ScTLR2 silencing by siRNA transfection could significantly depress the mRNA expression of Myd88, NF-κB, Tollip, IRF1, and IRF8. The survival rate of S. constricta was markedly decreased after V. parahaemolyticus challenge under this condition. Our current study demonstrated that ScTLR2 served as a pattern recognition receptor to induce immune response against invasive pathogen.

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

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

A fibrinogen-related Lectin from Echinometra lucunter represents a new FReP family in Echinodermata phylum

Fibrinogen-related proteins (FREPs) have been identified in several animals. They are involved in the body's defense, acting as mediators of phagocytosis. Ficolins and intelectins are some of the most studied Fibrinogen-related Domain (FReD)-containing lectins. In this work, we have isolated a singular FReD-containing lectin, which cannot be classified as ficolin or intelectin. ELL (Echinometra lucunter lectin) was isolated from coelomic plasma by affinity chromatography on xanthan gum. Primary structure was determined by tandem mass spectrometry. Moreover, antimicrobial activity of ELL was evaluated against planktonic cells and biofilm of Escherichia coli, Staphylococcus aureus and S. epidermidis. ELL showed hemagglutinating activity in Ca²⁺ presence, which was inhibited by glycoprotein mucin and thyroglobulin. Complete amino acid sequence consisted of 229 residues, including a FReD in the N-terminal. Searches for similarity found that ELL was very close to putative proteins from Strongylocentrotus purpuratus. ELL showed moderate similarity with uncharacterized sea stars proteins and protochordate intelectins. ELL was able to inhibit the planktonic growth of the Gram-positive bacteria and significantly reduce the biofilm formation of all bacteria tested. In conclusion, we identified a new type of FReP-containing lectin with some structural and functional conservation towards intelectins.

Crucial roles of a novel exoskeletal-derived lectin in innate immunity of the oriental river prawn, Macrobrachium nipponense

As important pattern recognition receptors (PRRs), C-type lectins play crucial roles in the crustacean innate immune system. In this study, a novel C-type lectin, designated as MnLec1, was obtained from the exoskeleton of the oriental river prawn Macrobrachium nipponense for the first time. The full-length cDNA of MnLec1 was 1329 bp with an open reading frame of 774 bp. The predicted MnLec1 protein contains a single carbohydrate-recognition domain with an EPN/LND motif and one Ca²⁺ binding site-2. MnLec1 transcripts were widely detected in the tested tissues of M. nipponense and significantly up-regulated after Aeromonas hydrophila challenge. The recombinant MnLec1 protein was found to have a wide spectrum of binding activities towards various microorganisms, agglutinate two kinds of Gram-negative bacteria (Escherichia coli and A. hydrophila) in a Ca²⁺ -independent manner. What's more, the survivability of prawns was significantly down-regulated after RNAi of MnLec1 when infected with A. hydrophila. Collectively, these findings suggest that MnLec1 from the exoskeleton might function as a PRR and play a crucial role in immune defense against invading pathogens in M. nipponense.

Characterization of a C-Type Lectin Domain-Containing Protein with Antibacterial Activity from Pacific Abalone (Haliotis discus hannai)

Genes that influence the growth of Pacific abalone (Haliotis discus hannai) may improve the productivity of the aquaculture industry. Previous research demonstrated that the differential expression of a gene encoding a C-type lectin domain-containing protein (CTLD) was associated with a faster growth in Pacific abalone. We analyzed this gene and identified an open reading frame that consisted of 145 amino acids. The sequence showed a significant homology to other genes that encode CTLDs in the genus Haliotis. Expression profiling analysis at different developmental stages and from various tissues showed that the gene was first expressed at approximately 50 days after fertilization (shell length of 2.47 ± 0.13 mm). In adult Pacific abalone, the gene was strongly expressed in the epipodium, gill, and mantle. Recombinant Pacific abalone CTLD purified from Escherichia coli exhibited antimicrobial activity against several Gram-positive bacteria (Bacillus subtilis, Streptococcus iniae, and Lactococcus garvieae) and Gram-negative bacteria (Vibrio alginolyticus and Vibrio harveyi). We also performed bacterial agglutination assays in the presence of Ca²⁺, as well as bacterial binding assays in the presence of the detergent dodecyl maltoside. Incubation with E. coli and B. subtilis cells suggested that the CTLD stimulated Ca²⁺-dependent bacterial agglutination. Our results suggest that this novel Pacific abalone CTLD is important for the pathogen recognition in the gastropod host defense mechanism.

Neuroinflammation: An Integrating Overview of Reactive-Neuroimmune Cell Interactions in Health and Disease

The concept of central nervous system (CNS) inflammation has evolved over the last decades. Neuroinflammation is the response of reactive CNS components to altered homeostasis, regardless of the cause to be endogenous or exogenous. Neurological diseases, whether traumatic, neoplastic, ischemic, metabolic, toxic, infectious, autoimmune, developmental, or degenerative, involve direct and indirect immune-related neuroinflammation. Brain infiltrates of the innate and adaptive immune system cells appear in response to an infective or otherwise noxious agent and produce inflammatory mediators. Mediators of inflammation include local and recruited cells and signals. Processes derived from extrinsic and intrinsic CNS diseases also elicit the CNS inflammatory response. A deeper understanding of immune-related inflammation in health and disease is necessary to find potential therapeutic targets for preventing or reducing CNS damage. This review is aimed at discussing the innate and adaptive immune system functions and their roles in regulating brain cell responses in disease and homeostasis maintenance.

MolluscDB: a genome and transcriptome database for molluscs

As sequencing becomes more accessible and affordable, the analysis of genomic and transcriptomic data has become a cornerstone of many research initiatives. Communities with a focus on particular taxa or ecosystems need solutions capable of aggregating genomic resources and serving them in a standardized and analysis-friendly manner. Taxon-focussed resources can be more flexible in addressing the needs of a research community than can universal or general databases. Here, we present MolluscDB, a genome and transcriptome database for molluscs. MolluscDB offers a rich ecosystem of tools, including an Ensembl browser, a BLAST server for homology searches and an HTTP server from which any dataset present in the database can be downloaded. To demonstrate the utility of the database and verify the quality of its data, we imported data from assembled genomes and transcriptomes of 22 species, estimated the phylogeny of Mollusca using single-copy orthologues, explored patterns of gene family size change and interrogated the data for biomineralization-associated enzymes and shell matrix proteins. MolluscDB provides an easy-to-use and openly accessible data resource for the research community. This article is part of the Theo Murphy meeting issue 'Molluscan genomics: broad insights and future directions for a neglected phylum'.

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.

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.

Serpin-type serine protease inhibitor mediates coelomocyte apoptosis in Apostichopus japonicus

Serine protease inhibitors (SPIs, serpins) are a protein superfamily involved in almost all physiological processes in all organisms. In this study, a novel serpin was identified from Apostichopus japonicus (Ajserpin) by using high-throughput sequencing and RACE approaches. The full-length cDNA of Ajserpin was 1893 bp with a 5'-untranslated region (UTR) of 130 bp, a 3'-UTR of 587 bp, and an open reading frame of 1176 bp encoding a polypeptide of 391 amino acids with a deduced molecular weight of 43.8 kDa. Ajserpin shares the standard structure of SPI, including three β-sheets and eight α-helices. The deduced amino acid sequences of Ajserpin had no nuclear location signal and signal peptide structure. The phylogenetic tree and immunofluorescence showed that Ajserpin belonged to the clade B subfamily and was mainly located in the cytoplasm and nucleus. Sequence comparison and protein inhibition experiments showed that the active site (P1-P1' site) of Ajserpin was Arginine and Serine, which displayed inhibitory activity toward trypsin in a dose-dependent manner. Tissue distribution analysis showed that Ajserpin transcripts were constitutively expressed in all examined tissues with the peak in the body wall. Ajserpin mRNA transcripts could be induced in Vibrio splendidus-challenged sea cucumber or lipopolysaccharide-exposed coelomocytes. Furthermore, Ajserpin knockdown by small interfering RNAs could inhibit coelomocytes apoptosis. All our results revealed that Ajserpin might serve as an immune regulator in sea cucumber.

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

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

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

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

A mild and efficient extraction method for polysaccharides from Sinonovacula constricta and study of their structural characteristic and antioxidant activities

The aim of this paper is to develop a mild and efficient extraction method for polysaccharides from Sinonovacula constricta (SCP) using enzyme extraction, and analyze the structural characteristics and antioxidant activities of the two purified polysaccharide fractions (SCP-1 and SCP-2). Firstly, enzyme extraction conditions were optimized, and the conditions were found to be, as follows: enzymolysis time 173.0 min, pH 8.2, enzymolysis temperature 50.0 ℃ and enzyme content 4.0%. Comparison between enzymatic extraction and water extraction was obtained from visual, UV-visible and IR spectrum images. The results clearly indicate that there is no significant difference between them with regard to the composition of the SCP fraction, but the polysaccharide content produced by enzymatic extraction is higher. Then, the physicochemical properties and structural characteristics of SCP-1 and SCP-2 were investigated using FT-TR, UV, GC and HPGPC. The carbohydrate content, sulfuric radicals and uronic acids of the two fractions were detected. Both SCP-1 and SCP-2 were mainly consisted of glucose, but their molecular weights were different. In addition, compared the Fe²⁺ chelating activity, ABTS⁺ radical and superoxide radical scavenging activity, and lipid peroxidation inhibition activity of SCP-1 and SCP-2, it turned out that SCP-2 had stronger antioxidant activity than SCP-1.

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

Novel Ca2+ -independent C-type lectins, SPL-1 and SPL-2, were purified from the bivalve Saxidomus purpuratus. They are composed of dimers with either identical (SPL-2 composed of two B-chains) or distinct (SPL-1 composed of A- and B-chains) polypeptide chains, and show affinity for N-acetylglucosamine (GlcNAc)- and N-acetylgalactosamine (GalNAc)-containing carbohydrates, but not for glucose or galactose. A database search for sequence similarity suggested that they belong to the C-type lectin family. X-ray crystallographic analysis revealed definite structural similarities between their subunits and the carbohydrate-recognition domain (CRD) of the C-type lectin family. Nevertheless, these lectins (especially SPL-2) showed Ca2+ -independent binding affinity for GlcNAc and GalNAc. The crystal structure of SPL-2/GalNAc complex revealed that bound GalNAc was mainly recognized via its acetamido group through stacking interactions with Tyr and His residues and hydrogen bonds with Asp and Asn residues, while widely known carbohydrate-recognition motifs among the C-type CRD (the QPD [Gln-Pro-Asp] and EPN [Glu-Pro-Asn] sequences) are not involved in the binding of the carbohydrate. Carbohydrate-binding specificities of individual A- and B-chains were examined by glycan array analysis using recombinant lectins produced from Escherichia coli cells, where both subunits preferably bound oligosaccharides having terminal GlcNAc or GalNAc with α-glycosidic linkages with slightly different specificities.