A galectin with quadruple-domain from red abalone Haliotis rufescens involved in the immune innate response against to Vibrio anguillarum

Antimicrobial Functions of Galectins from Fish, Mollusks, and Crustaceans: A Review

Galectins are a member of the β-galactoside binding protein family, which play a pivotal role in the immune defense of vertebrates as a pattern recognition receptor and occupy an important position in the innate immune system of invertebrates. The study of galectins in aquatic organisms has only recently emerged. Galectins in aquatic animals exhibit agglutination activity toward bacteria, inhibit bacterial growth, and enhance phagocytosis of immune cells. Additionally, some galectins contribute to the antiviral immune defenses of aquatic animals. This review aims to review recent advancements in the antimicrobial mechanisms, molecular structures, and evolution of galectins from fish, mollusks, and crustaceans. The antimicrobial galectins, as crucial components in the innate immune defense, pave new avenues for developing innovative disease control strategies in aquaculture.

Tandem-repeat lectins: structural and functional insights

Multivalency in lectins plays a pivotal role in influencing glycan cross-linking, thereby affecting lectin functionality. This multivalency can be achieved through oligomerization, the presence of tandemly repeated carbohydrate recognition domains, or a combination of both. Unlike lectins that rely on multiple factors for the oligomerization of identical monomers, tandem-repeat lectins inherently possess multivalency, independent of this complex process. The repeat domains, although not identical, display slightly distinct specificities within a predetermined geometry, enhancing specificity, affinity, avidity and even oligomerization. Despite the recognition of this structural characteristic in recently discovered lectins by numerous studies, a unified criterion to define tandem-repeat lectins is still necessary. We suggest defining them multivalent lectins with intrachain tandem repeats corresponding to carbohydrate recognition domains, independent of oligomerization. This systematic review examines the folding and phyletic diversity of tandem-repeat lectins and refers to relevant literature. Our study categorizes all lectins with tandemly repeated carbohydrate recognition domains into nine distinct folding classes associated with specific biological functions. Our findings provide a comprehensive description and analysis of tandem-repeat lectins in terms of their functions and structural features. Our exploration of phyletic and functional diversity has revealed previously undocumented tandem-repeat lectins. We propose research directions aimed at enhancing our understanding of the origins of tandem-repeat lectin and fostering the development of medical and biotechnological applications, notably in the design of artificial sugars and neolectins.

Molecular characterization and functional analysis of galectin-1 from silver pomfret (Pampus argenteus)

Galectins, as members of lectin families, exhibit a high affinity for β-galactosides and play diverse roles in biological processes. They function as pattern recognition receptors (PRRs) with important roles in immune defense. In this study, galectin-1, designated as SpGal-1, was identified and characterized from silver pomfret (Pampus argenteus). The SpGal-1 comprises an open reading frame (ORF) spanning 396 base pairs (bp) and encodes a deduced amino acid (aa) sequence containing a single carbohydrate recognition domain (CRD). Sublocalization analysis revealed that SpGal-1 was mainly expressed in the cytoplasm. The mRNA transcripts of SpGal-1 were ubiquitously detected in various tissues, with a higher expression level in the intestine. In addition, when exposed to Photobacterium damselae subsp. damselae (PDD) infection, both the liver and head kidney exhibited significantly increased SpGal-1 mRNA expression. The recombinant protein of SpGal-1 (named as rSpGal-1) demonstrated hemagglutination against red blood cells (RBCs) from Larimichthys crocea and P. argenteus in a Ca2+ or β-Mercaptoethanol (β-ME)-independent manner. Notably, rSpGal-1 could bind with various pathogen-associated molecular patterns (PAMPs) including D-galactose, D-mannose, lipopolysaccharide (LPS), and peptidoglycan (PGN), with highest affinity to PGN. Moreover, rSpGal-1 effectively interacted with an array of bacterial types encompassing Gram-positive bacteria (Staphylococcus aureus and Nocardia seriolae) and Gram-negative bacteria (PDD and Escherichia coli, among others), with the most robust binding affinity towards PDD. Collectively, these findings highlight that SpGal-1 is a crucial PRR with involvement in the host immune defense of silver pomfret.

Structural and functional analysis of a tandem repeat galacturonic acid-binding lectin from the sea hare Aplysia californica

A d-galacturonic acid-specific lectin, named AcL, was purified from the sea hare Aplysia californica by galactose-agarose affinity chromatography. AcL has a molecular mass of 27.5 kDa determined by MALDI-TOF mass spectrometry. This lectin shows a good affinity for d-galacturonic acid and a lower affinity for galactosides: raffinose, melibiose, α and β-lactose, and d-galactose. We determined the amino acid sequence of AcL by trypsin digestion and subsequent peptide analysis by mass spectrometry, resulting in a 238 amino acid protein with a theoretical molecular mass of 26.4 kDa. The difference between the theoretical and experimental values can be attributed to post-translational modifications. Thiol-disulfide quantification discerned five disulfide bonds and three free cysteines. The structure of Acl is mainly comprised of beta sheets, determined by circular dichroism, and predicted with AlphaFold. Theoretical models depict three nearly identical tandem domains consisting of two beta sheets each. From docking analysis, we identified AcL glycan-binding sites as multiple conserved motifs in each domain. Furthermore, phylogenetic analysis based on its structure and sequence showed that AcL and its closest homologues (GalULs) form a clear monophyletic group, distinct from other glycan-binding proteins with a jelly-roll fold: lectins of types F and H. GalULs possess four conserved sequence regions that distinguish them and are either ligand-binding motifs or stabilizing network hubs. We suggest that this new family should be referred to as GalUL or D-type, following the traditional naming of lectins; D standing for depilans, the epithet for the species (Aplysia depilans) from which a lectin of this family was first isolated and described.

Structural and evolutionary insights into the multidomain galectin from the red abalone Haliotis rufescens with specificity for sulfated glycans

Galectins are an evolutionarily ancient family of lectins characterized by their affinity for β-galactosides and a conserved binding site in the carbohydrate recognition domain (CRD). These lectins are involved in multiple physiological functions, including the recognition of glycans on the surface of viruses and bacteria. This feature supports their role in innate immune responses in marine mollusks. Here, we identified and characterized a galectin, from the mollusk Haliotis rufescens (named HrGal), with four CRDs that belong to the tandem-repeat type. HrGal was purified by affinity chromatography in a galactose-agarose resin and exhibited a molecular mass of 64.11 kDa determined by MALDI-TOF mass spectrometry. The identity of HrGal was verified by sequencing, confirming that it is a 555 amino acid protein with a mass of 63.86 kDa. This protein corresponds to a galectin reported in GenBank with accession number AHX26603. HrGal is stable in the presence of urea, reducing agents, and ions such as Cu²⁺ and Zn²⁺. The recombinant galectin (rHrGal) was purified from inclusion bodies in the presence of these ions. A theoretical model obtained with the AlphaFold server exhibits four non-identical CRDs, with a β sandwich folding and the representative motifs for binding β-galactosides. This allows us to classify HrGal within the tandem repeat galectin family. On the basis of a phylogenetic analysis, we found that the mollusk sequences form a monophyletic group of tetradomain galectins unrelated to vertebrate galectins. HrGal showed specificity for galactosides and glucosides but only the sulfated sugars heparin and ι-carrageenan inhibited its hemagglutinating activity with a minimum inhibitory concentration of 4 mM and 6.25 X 10^-5% respectively. The position of the sulfate groups seemed crucial for binding, both by carrageenans and heparin.

Identification and Characterization of a Novel Lectin from the Clam Glycymeris yessoensis and Its Functional Characterization under Microbial Stimulation and Environmental Stress

Lectin from the bivalve Glycymeris yessoensis (GYL) was purified by affinity chromatography on porcine stomach mucin-Sepharose. GYL is a dimeric protein with a molecular mass of 36 kDa, as established by SDS-PAGE and MALDI-TOF analysis, consisting of 18 kDa subunits linked by a disulfide bridge. According to circular dichroism data, GYL is a β/α-protein with the predominance of β-structure. GYL preferentially agglutinates enzyme-treated rabbit erythrocytes and recognizes glycoproteins containing O-glycosidically linked glycans, such as porcine stomach mucin (PSM), fetuin, thyroglobulin, and ovalbumin. The amino acid sequences of five segments of GYL were acquired via mass spectrometry. The sequences have no homology with other known lectins. GYL is Ca2+-dependent and stable over a range above a pH of 8 and temperatures up to 20 °C for 30 min. GYL is a pattern recognition receptor, as it binds common pathogen-associated molecular patterns, such as peptidoglycan, LPS, β-1,3-glucan and mannan. GYL possesses a broad microbial-binding spectrum, including Gram-positive (Bacillus subtilis, Staphylococcus aureus) and Gram-negative bacteria (Escherichia coli, Vibrio proteolyticus), but not the fungus Candida albicans. Expression levels of GYL in the hemolymph were significantly upregulated after bacterial challenge by V. proteolyticus plus environmental stress (diesel fuel). Results indicate that GYL is probably a new member of the C-type lectin family, and may be involved in the immune response of G. yessoensis to bacterial attack.

Quadruple domain-containing galectin from marine invertebrate disk abalone (Haliotis discus discus): Molecular perspectives in early development, immune expression, and potent antiviral responses

Galectins are well-known β-galactoside-binding proteins, which play vital roles in innate immune responses of both vertebrates and invertebrates. However, knowledge regarding invertebrate galectins is still in its infancy. With the intention of filling the knowledge gap, here we identified a quadruple domain-containing galectin from marine invertebrate disk abalone, Haliotis discus discus (AbGalec), and characterized it. AbGalec consisted of four distinct carbohydrate-recognition domains (CRDs) and lacked a signal peptide. Expression analysis revealed AbGalec to be ubiquitously expressed in all the examined early embryonic stages of abalone, with highest expression in the 16-cell stage, suggesting the importance of AbGalec in early developmental processes. Tissue distribution analysis revealed the highest expression of AbGalec in abalone mantle, followed by that in gills and hemocytes. Immune challenge experiments revealed significant upregulation of AbGalec at 24 h and 48 h post injection (p.i.) with bacterial and viral components. These results suggested the possible involvement of AbGalec in host defense mechanisms. Polyinosinic: polycytidylic acid (Poly I:C) and viral hemorrhagic septicemia virus (VHSV) injections were capable of inducing AbGalec transcript expression more prominently than bacterial stimulants, thus providing evidence for its role in viral infections. We determined the virus-neutralizing ability of a quadruple domain-containing galectin for the first time, by analyzing the downregulation of VHSV transcripts during the overexpression of AbGalec. Significant downregulation of VHSV transcripts was observed after 24 h and 48 h of post infection. Collectively, our findings reveal the potent antiviral responses of molluscan quadruple domain-containing galectin, AbGalec, along with its involvement in innate immunity.

Effects of high stocking density on the galectin gene expression in noble scallop Chalmys nobilis under bacterial infection

High stocking densities have been shown to have adverse effects on the physiology of bivalves. The noble scallop Chlamys nobilis is one of the most important cultured shellfish in Southern China. However, the effects of scallop stocking density on its immunity is not well understood. In this context, this study was conducted to assess the effect of high stocking density on the galectin (an important protein in innate immunity) gene expression of C. nobilis during bacterial infection. A full-length galectin (CnGal) gene was cloned. The ORF of the CnGal cDNA encodes a predicted protein containing 549 aa with four CRDs and no signal peptide. Our results reveal that high stocking density in the scallop not only led to high mortality and slow growth, but also changed tissue distribution of the CnGal expression. The individuals from the high stocking density group exhibited more differences among tissues than those from the control group, but the highest expression were both recorded in hemolymph. After the Vibrio parahaeomlyticus challenge, the gene's expression levels were all significantly up-regulated in the hemolymph and gill, but the time up to peak was different between the two tissues. The findings of this study could fill a gap in knowledge about how high stocking density affect scallop immunity at the molecular level.

Molecular characterization and biological function of a tandem-repeat galectin-9 in Qihe crucian carp Carassius auratus

Galectin-9, as one of the important PRRs in host, could initiate the immune defense responses through recognizing and binding PAMPs on the surface of invading microorganisms. In this study, a new galectin-9 cDNA was identified and characterized in Qihe crucian carp Carassius auratus (named as CaGal-9). The complete cDNA sequence of CaGal-9 was 1318 bp, with an open reading frame (ORF) of 963 bp encoding 320 amino acids. The predicted CaGal-9 protein contained two non-identical carbohydrate recognition domains (CRDs), which possessed the representative motifs H-NPR and WG-EER to bind with β-galactoside. Based on the RT-qPCR detection, CaGal-9 was ubiquitously expressed at mRNA level in various tested tissues, and predominately expressed in spleen. Upon Aeromonas hydrophila and poly I: C challenge, the expressions of CaGal-9 were remarkably up-regulated in liver, spleen, kidney and head kidney in a time-depended manner. The recombinant CaGal-9 (rCaGal-9), purified from Escherichia coli BL21 (DE3), exhibited strong binding ability with lipopolysaccharide (LPS), peptidoglycan (PGN) and β-Glucan, as well as the examined microorganisms including fungus, Gram-negative bacteria, and Gram-positive bacteria. With regard to the agglutinating activity of rCaGal-9, it could agglutinate erythrocytes of rabbit and crucian carp, and the examined microorganisms. Taken together, in this study, it was suggested that CaGal-9 could play an important role in immune defense against pathogenic microorganisms in C. auratus, which functions as an important PRR to recognize PAMPs and agglutinate pathogenic microorganisms.

-Omic Analysis of the Sepia officinalis White Body: New Insights into Multifunctionality and Haematopoiesis Regulation

Cephalopods, like other protostomes, lack an adaptive immune system and only rely on an innate immune system. The main immune cells are haemocytes (Hcts), which are able to respond to pathogens and external attacks. First reports based on morphological observations revealed that the white body (WB) located in the optic sinuses of cuttlefish was the origin of Hcts. Combining transcriptomic and proteomic analyses, we identified several factors known to be involved in haematopoiesis in vertebrate species in cuttlefish WB. Among these factors, members of the JAK-STAT signaling pathway were identified, some of them for the first time in a molluscan transcriptome and proteome. Immune factors, such as members of the Toll/NF-κB signaling pathway, pattern recognition proteins and receptors, and members of the oxidative stress responses, were also identified, and support an immune role of the WB. Both transcriptome and proteome analyses revealed that the WB harbors an intense metabolism concurrent with the haematopoietic function. Finally, a comparative analysis of the WB and Hct proteomes revealed many proteins in common, confirming previous morphological studies on the origin of Hcts in cuttlefish. This molecular work demonstrates that the WB is multifunctional and provides bases for haematopoiesis regulation in cuttlefish.

A tandem-repeat galectin-1 from Apostichopus japonicus with broad PAMP recognition pattern and antibacterial activity

Galectins belong to the family of carbohydrate-binding proteins and play major roles in the immune and inflammatory responses of both vertebrates and invertebrates. In the present study, one novel galectin-1 protein named AjGal-1 was identified from Apostichopus japonicas with an open reading frame of 1179 bp encoding a polypeptide of 392 amino acids. The deduced amino acids sequence of AjGal-1 contained three carbohydrate recognition domains (CRDs) which shared 34-37% identity with that of other galectin proteins from echinodermata, fishes, and birds. In the phylogenetic tree, AjGal-1 was closely clustered with galectins from Mesocentrotus nudus and Paracentrotus lividus. The mRNA transcripts of AjGal-1 were ubiquitously expressed in all the detected tissues, including gut, longitudinal muscle, gonad, coelomocytes, respiratory tree, tentacle and body wall, with the highest expression level in coelomocytes. After Vibrio splendidus stimulation, the mRNA expression levels of AjGal-1 in coelomocytes were significantly increased at 6 and 12 h (P < 0.01) compared with that in control group, and went back to normal level at 72 h. The recombinant protein of AjGal-1 (rAjGal-1) could bind various PAMPs including d-galactose, lipopolysaccharide (LPS), peptidoglycan (PGN) and mannose (Man), and exhibited the highest affinity to d-galactose. Meanwhile, rAjGal-1 could also bind and agglutinate different kinds of microorganisms, including gram-negative bacteria (V. splendidus and Escherichia coli), gram-positive bacteria (Micrococus leteus), and fungi (Pichia pastoris). rAjGal-1 also exhibited anti-microbial activity against V. splendidus and E. coli. All these results suggested that AjGal-1 could function as an important PRR with broad spectrum of microbial recognition and anti-microbial activity against the invading pathogen in A. japonicas.

Adaptation evolution and bioactivity of galectin from the deep sea Vesicomyidae clam Archivesica packardana

Hydrothermal vents and cold seep zones are two special habitats in the deep sea. These habitats are always dark, and have extreme temperatures (low or high), heavy metals and toxic substances (sulfide, methane). Vesicomyidae clams, which maintain endosymbionts in their gills, are common species in these two special zones and are thought to develop an efficacious immune system against unusual habitats. In the present study, a novel galectin (Apgalectin) was identified from the Vesicomyidae clam Archivesica packardana. The phylogenetic tree indicated that Apgalectin had two CRDs and was closely clustered with galectins from invertebrates, especially mollusks. A branch-site model showed that 9 positively selected sites (ω2 = 6.83950) were identified comparing to galectins from the Order Veneroida, implying a different function of Vesicomyidae galectins. A microbe binding assay showed that rApgalectin could bind to gram-positive bacteria, gram-negative bacteria and fungi. A PAMP binding assay indicated that Apgalectin could bind LPS, PGN, β-1,3-glucan, glucan from yeast and Poly I:C in dose-dependent manner. Apgalectin only agglutinated Micrococcus luteus and agglutination could be inhibited by galactose which demonstrated that Apgalectin might be involved in immune defense by recognizing and binding bacteria in a β-galactoside manner. Further experiments showed that Apgalectin might play an indirect effector role in the immune response because of its limited antibacterial spectrum. All analyses validated that Apgalectin from Archivesica packardana plays a variety of functions in immune responses and provided basal information for the immune study of deep-sea mollusks.

Activity Dependence of a Novel Lectin Family on Structure and Carbohydrate-Binding Properties

A GalNAc/Gal-specific lectins named CGL and MTL were isolated and characterized from the edible mussels Crenomytilus grayanus and Mytilus trossulus. Amino acid sequence analysis of these lectins showed that they, together with another lectin MytiLec-1, formed a novel lectin family, adopting β-trefoil fold. In this mini review we discuss the structure, oligomerization, and carbohydrate-binding properties of a novel lectin family. We describe also the antibacterial, antifungal, and antiproliferative activities of these lectins and report about dependence of activities on molecular properties. Summarizing, CGL, MTL, and MytiLec-1 could be involved in the immunity in mollusks and may become a basis for the elaboration of new diagnostic tools or treatments for a variety of cancers.

Identification and characterisation of a novel small galectin in razor clam (Sinonovacula constricta) with multiple innate immune functions

Galectins are lectins possessing an evolutionarily conserved carbohydrate recognition domain (CRD) with affinity for β-galactoside. The key role played by innate immunity in invertebrates has recently become apparent. Herein, a full-length galectin (ScGal) was identified in razor clam (Sinonovacula constricta). The 528 bp open reading frame encodes a polypeptide of 176 amino acids with a single CRD and no signal peptide. ScGal mRNA transcripts were mainly expressed in hemolymph and gill, and were significantly up-regulated following bacterial challenge. Recombinant rScGal protein binds to and aggregates various bacteria, and has affinity for peptidoglycan, lipoteichoic acid and _d-galactose. The protein also stimulates hemocytes to phagocytose invading bacterial pathogens. ScGal is an important immune factor in innate immunity, and a small protein with multiple important functions.

Nucleus-translocated matrix metalloprotease 1 regulates innate immune response in Pacific abalone (Haliotis discus hannai)

As an important economical shellfish in coastal area of China, abalone is susceptible to bacterial infection, especially Vibiro parahemolyticus (V. parahemolyticus). Matrix metalloproteinases (MMPs) have been extensively investigated in the immune response of mammals. However, little is known about the involvement of MMP in abalone innate immune system against pathogen infection. In this study, the role of MMP-1 in the immune response of Pacific abalone (Haliotis discus hannai) was explored. The results showed that V. parahemolyticus infection induced significantly elevated expression of MMP-1 as well as immune related genes including allograft inflammatory factor 1 (AIF-1), macrophage expressed gene 1 (MPEG-1) and TPA-inducible sequence 11 family protein (Tis11FP). Notably, silencing of MMP-1 reduced the expression of these genes, suggesting that MMP-1 was an upstream regulatory factor in V. parahemolyticus infection. Further analysis showed that MMP-1 was engaged in the regulation of cellular (phagocytosis, apoptosis) and humoral [superoxide dismutase (SOD), alkaline phosphatase (ALP), acid phosphatase (ACP)] immunity. Interestingly, the extracellularly distributed MMP-1 could be translocated to the nuclei of hemocytes, thereby functioning as a transcriptional regulator or by selectively activating or inactivating other components through proteolysis. Hence, our study established an important role of MMP-1 in abalone innate immunity against V. parahemolyticus infection and it represented the first report on the investigation of MMP in abalone.

Galactoside-binding lectin in Solen grandis as a pattern recognition receptor mediating opsonization

Galactoside-binding lectin (galectin) is a type of pathogen recognition molecule that occupies an important position in the invertebrate innate immunity system. Our previous study has identified a galectin gene in mollusk Solen grandis (SgGal-1) and illustrated its potential roles in innate immunity. By the functional study using recombinant protein and specific antibody, here, we confirmed the pivotal roles of SgGal-1 in immune defense of S. grandis. SgGal-1 protein was expressed in many tested tissues including gill, mantle, hepatopancreas and gonad, except hemocytes and muscle. The recombinant SgGal-1 (rSgGal-1) bound PGN and β-glucan instead of LPS in vitro, and it further caused significant agglutination of five different microbes, suggesting SgGal-1 served as a pattern recognition receptor (PRR) involved in immune defense of mollusk. Furthermore, SgGal-1 recruited hemocytes to encapsulate, which was blocked by anti-rSgGal-1 serum. In the meantime, rSgGal-1 as well as promoted the phagocytosis of hemocytes against Escherichia coli in vitro. All these results suggested that SgGal-1 in S. grandis not only acted as a PRR recognizing microbes but also directly participated in the process of immune opsonization to protect the host from pathogenic infection.

Molecular cloning, characterization and expression profiling of galectin-9 gene from Labeo rohita (Hamilton, 1822)

Galectin-9 is a b-galactoside-binding tandem repeat galectin that regulates many cellular functions, ranging from cell adhesion to pathogen recognition. In spite of extensive study of mammalian galectin importance in immune system, little is known about that of fish. To study the normal expression and immune response of Labeo rohita to pathogens, a tandem-repeat galectin-9 from Labeo rohita was identified and named LrGal-9. Its full-length cDNA was 1534 bp encoded 291 amino acids (35.12 KDa), shared the highest 81% identity with the galectin-9 of Danio rerio. LrGal-9 identified in this study lacked signal peptide and a transmembrane domain like galectin-9 members reported in other fishes. Quantitative PCR showed that LrGal-9 was lowly expressed in gill, muscle, heart, highly expressed in tested immune tissues (intestine, kidney, liver, spleen) in normal body. After Aeromonas hydrophila challenge, LrGal-9 was remarkably increased in all tested immune tissues in a time-dependent manner. These results suggest that LrGal-9 plays a role in innate immunity in Labeo rohita.

A galectin contributes to the innate immune recognition and elimination of pathogens in the freshwater mussel Hyriopsis cumingii

Galectins are members of the lectin superfamily. They function as pattern recognition receptors in the innate immune system of vertebrates and invertebrates. A galectin homolog from the triangle sail mussel Hyriopsis cumingii (HcGal2) was cloned and characterized. HcGal2 mRNA was expressed in all tissues examined, displaying particular enrichment in mantle tissue. Interestingly, rHcGAL2 protein was only detected in the mantle, hemocytes, and gills, suggesting that post-transcriptional regulation may occur. HcGal2 expression was induced in the mantle, liver, and hemocytes after exposure to lipopolysaccharides, Gram-negative bacteria (Aeromonas hydrophila), and Gram-positive bacteria (Staphylococcus aureus). The transcript significant upregulated was also detected after implantation in the mantle, pearl sac, liver, and hemocytes. Recombinant HcGAL2 protein (rHcGAL2) agglutinated Gram-positive and Gram-negative bacteria. In addition, rHcGAL2 promoted phagocytosis by hemocytes in vivo. Our data suggest that HcGal2 functioned as a pattern recognition receptor in against the pathogenic microbes and contributed to the "non-self" recognition and elimination in H. cumingii.

A galectin from Eriocheir sinensis functions as pattern recognition receptor enhancing microbe agglutination and haemocytes encapsulation

Galectins are a family of β-galactoside binding lectins that function as pattern recognition receptors (PRRs) in innate immune system of both vertebrates and invertebrates. The cDNA of Chinese mitten crab Eriocheir sinensis galectin (designated as EsGal) was cloned via rapid amplification of cDNA ends (RACE) technique based on expressed sequence tags (ESTs) analysis. The full-length cDNA of EsGal was 999 bp. Its open reading frame encoded a polypeptide of 218 amino acids containing a GLECT/Gal-bind_lectin domain and a proline/glycine rich low complexity region. The deduced amino acid sequence and domain organization of EsGal were highly similar to those of crustacean galectins. The mRNA transcripts of EsGal were found to be constitutively expressed in a wide range of tissues and mainly in hepatopancreas, gill and haemocytes. The mRNA expression level of EsGal increased rapidly and significantly after crabs were stimulated by different microbes. The recombinant EsGal (rEsGal) could bind various pathogen-associated molecular patterns (PAMPs), including lipopolysaccharide (LPS), peptidoglycan (PGN) and glucan (GLU), and exhibited strong activity to agglutinate Escherichia coli, Vibrio anguillarum, Bacillus subtilis, Micrococcus luteus, Staphylococcus aureus and Pichia pastoris, and such agglutinating activity could be inhibited by both d-galactose and α-lactose. The in vitro encapsulation assay revealed that rEsGal could enhance the encapsulation of haemocytes towards agarose beads. These results collectively suggested that EsGal played crucial roles in the immune recognition and elimination of pathogens and contributed to the innate immune response against various microbes in crabs.

Characterization and functional analysis of a tandem-repeat galectin-9 in large yellow croaker Larimichthys crocea

Galectins are a family of endogenous lectins with β-galactosides affinity, playing significant roles in the innate immunity of vertebrates and invertebrates. In this report, a new galectin-9 cDNA was identified and characterized in large yellow croaker Larimichthys crocea (designated as LcGal-9). The complete cDNA sequence of LcGal-9 was 1795 bp, with an open reading frame (ORF) of 1032 bp encoding 343 amino acids. The putative LcGal-9 protein contained two carbohydrate recognition domains (CRDs) connected by a linker peptide, with each carrying two conserved β-galactoside binding motifs H-NPR and WG-EE-, and it possessed neither a signal peptide nor a transmembrane domain. LcGal-9 protein shared 43-74% identity with galectin-9 sequences from other species. The qRT-PCR analysis revealed that LcGal-9 mRNA was constitutively expressed in all tissues examined, predominately expressed in liver, spleen, gill, kidney, head-kidney and intestine. Western blot analysis showed that LcGal-9 protein was highly expressed in liver, spleen, intestine, kidney, head-kidney, skin, gill, and heart, but not detected in muscle and plasma. LcGal-9 mRNA transcripts were induced by poly I:C in the liver (from 6 h to 48 h), spleen (at 12 h) and head-kidney (at 12 h and 24 h). In contrast, Vibrio parahaemolyticus caused a significant down-regulation in these three tissues, except for in spleen of 48 h and head-kidney of 3 h. Post-infection with Cryptocaryon irritans, the transcripts were dramatically up-regulated in gill, skin, spleen and head-kidney during initial infection period, while significant down-regulation afterward was also observed both in spleen and head-kidney. The recombinant LcGal-9 (named as rLcGal-9) purified from Escherichia coli BL21 (DE3) demonstrated hemagglutination against human, rabbit and L. crocea in a Ca(2+)-independent manner, which was inhibited by α-Lactose and LPS. The results of bacterial agglutination assays showed that rLcGal-9 was able to agglutinate Gram-negative bacteria V. alginolyticus and Aeromonas hydrophila in a Ca(2+)-independent manner. By immunohistochemistry assay, significant increases of LcGal-9 protein appeared in the spleen stimulated with poly I:C (for 12 h) and V. parahaemolyticus (for 48 h) compared with the control. Based on the collective data, LcGal-9 might play an important role in innate immune responses, especially defense against Gram-negative bacteria in L. crocea.

А new Gal/GalNAc-specific lectin from the mussel Mytilus trossulus: Structure, tissue specificity, antimicrobial and antifungal activity

In the present study, a new Gal/GalNAc specific lectin from the mussel Mytilus trossulus (designated as MTL) was identified, and its expression levels, both in tissues and toward pathogen stimulation, were then characterized. The MTL primary structure was determined via cDNA sequencing. Deduced sequence of 150 amino acid residues showed 89% similarity to lectins from the mussels Crenomytilus grayanus and Mytilus galloprovincialis that were the first members of a new family of zoolectins. The results indicated that the MTL might be involved in immune response toward pathogen infection, and it might perform different recognition specificity toward bacteria or fungi.

Structural, functional, and evolutionary aspects of galectins in aquatic mollusks: From a sweet tooth to the Trojan horse

Galectins constitute a conserved and widely distributed lectin family characterized by their binding affinity for β-galactosides and a unique binding site sequence motif in the carbohydrate recognition domain (CRD). In spite of their structural conservation, galectins display a remarkable functional diversity, by participating in developmental processes, cell adhesion and motility, regulation of immune homeostasis, and recognition of glycans on the surface of viruses, bacteria and protozoan parasites. In contrast with mammals, and other vertebrate and invertebrate taxa, the identification and characterization of bona fide galectins in aquatic mollusks has been relatively recent. Most of the studies have focused on the identification and domain organization of galectin-like transcripts or proteins in diverse tissues and cell types, including hemocytes, and their expression upon environmental or infectious challenge. Lectins from the eastern oyster Crassostrea virginica, however, have been characterized in their molecular, structural and functional aspects and some notable features have become apparent in the galectin repertoire of aquatic mollusks. These including less diversified galectin repertoires and different domain organizations relative to those observed in vertebrates, carbohydrate specificity for blood group oligosaccharides, and up regulation of galectin expression by infectious challenge, a feature that supports their proposed role(s) in innate immune responses. Although galectins from some aquatic mollusks have been shown to recognize microbial pathogens and parasites and promote their phagocytosis, they can also selectively bind to phytoplankton components, suggesting that they also participate in uptake and intracellular digestion of microalgae. In addition, the experimental evidence suggests that the protozoan parasite Perkinsus marinus has co-evolved with the oyster host to be selectively recognized by the oyster hemocyte galectins over algal food or bacterial pathogens, thereby subverting the oyster's innate immune/feeding recognition mechanisms to gain entry into the host cells.

Molecular characterization of collagen IV evidences early transcription expression related to the immune response against bacterial infection in the red abalone (Haliotis rufescens)

Collagen IV has been described as a structural protein of the basement membrane, which as a whole forms a specialized extracellular matrix. Recent studies have indicated a possible relationship between collagen IV and the innate immune response of invertebrate organisms. The present study characterized the alpha-1 chain of collagen IV in the red abalone Haliotis rufescens (Hr-ColIV) and evaluated its association with the innate immune response against Vibrio anguillarum. To further evidence the immune response, the matrix metalloproteinase-1 (Hr-MMP-1) and C-type lectin (Hr-CLEC) genes were also assessed. The complete sequence of Hr-ColIV was composed of 6658 bp, with a 5'UTR of 154 bp, a 3'UTR of 1177 bp, and an ORF of 5327 bp that coded for 1776 amino acids. The innate immune response generated against V. anguillarum resulted in a significant increase in the transcript levels of Hr-ColIV between 3 and 6 hpi, whereas Hr-MMP-1 and Hr-CLEC had the highest transcript activity 6 and 12 hpi, respectively. The results obtained in this study propose a putative biological function for collagen IV involved in the early innate immune response of the red abalone H. rufescens.