A Galectin of Unique Domain Organization from Hemocytes of the Eastern Oyster (Crassostrea virginica) Is a Receptor for the Protistan Parasite Perkinsus marinus

Characterisation and functional comparison of single-CRD and multidomain containing galectins CgGal-2 and CgGal-3 from oyster Crassostrea gigas

Galectins are β-galactoside binding lectins that play crucial roles in innate immunity in vertebrates and invertebrates through their conserved carbohydrate-recognition domains (CRDs). In the present study, single- and four-CRD-containing galectins were identified in oyster Crassostrea gigas (designated CgGal-2 and CgGal-3). The open reading frames (ORFs) of CgGal-2 and CgGal-3 encode polypeptides of 200 and 555 amino acids, respectively. All CRDs of CgGal-3 include two consensus motifs essential for ligand-binding, and a novel motif is present in CgGal-2. Pathogen-associated molecular pattern (PAMP) profiles were determined for recombinant rCgGal-2 and rCgGal-3, and rCgGal-2 displayed low binding affinity for PAMPs, while rCgGal-3 bound various PAMPs including glucan, lipopolysaccharide (LPS), and peptidoglycan (PGN) with relatively high affinity. Furthermore, rCgGal-2 and rCgGal-3 exhibited different microbe binding profiles; rCgGal-2 bound to Gram-negative bacteria (Escherichia coli and Vibrio vulnificus) and fungi (Saccharomyces cerevisiae and Pichia pastoris), while rCgGal-3 bound to these microbes but also to Gram-positive bacteria (Micrococcus luteus). In addition, rCgGal-3 possessed microbial agglutinating activity and coagulation activity against fungi and erythrocytes, respectively, but rCgGal-2 lacked any agglutinating activity. Carbohydrate binding specificity analysis showed that rCgGal-3 specifically bound D-galactose. Furthermore, rCgGal-2 and rCgGal-3 functioned as opsonin participating in the clearance against invaders in C. gigas. Thus, CgGal-2 with one CRD and CgGal-3 with four CRDs are new members of the galectin family involved in immune responses against bacterial infection. Differences in the organisation and amino acid sequences of CRDs may affect their specificity and affinity for nonself substances.

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.

Three newly identified galectin homologues from triangle sail mussel (Hyriopsis cumingii) function as potential pattern-recognition receptors

Galactoside-binding lectins, also known as galectins, play crucial roles in innate immune response in invertebrates. In this study, three cDNA sequences from Hyriopsis cumingii were identified and collectively called HcGalec genes. Each of the three deduced HcGalec proteins contained a galactose-binding lectin domain or a GLECT domain. All the three HcGalec genes are mainly present in the hepatopancreas and gills, and their expression is induced at 24 h after bacterial challenge. Three recombinant HcGalec proteins can bind and agglutinate (Ca²⁺-dependent) various microorganisms, including Gram-positive and Gram-negative bacteria. These proteins can attach to mannan and peptidoglycan. Meanwhile, the expression of the three HcGalec genes in the gills were significantly down-regulated after dsRNA interference (HcGalec1-RNAi, HcGalec2-RNAi, and HcGalec3-RNAi) and Vibrio parahaemolyticus injection. The expression levels of some antimicrobial peptides, including lysozyme 1 and lysozyme 2, were also markedly decreased after dsRNA interference. Overall, these results suggested that these three HcGalec proteins may function as potential receptors participating in the innate immune responses of H. cumingii against bacterial infection.

Screening of binding proteins that interact with Chinese sacbrood virus VP3 capsid protein in Apis cerana larvae cDNA library by the yeast two-hybrid method

Chinese sacbrood virus (CSBV) causes larval death and apiary collapse of Apis cerana. VP3 is a capsid protein of CSBV but its function is poorly understood. To determine the function of VP3 and screen for novel binding proteins that interact with VP3, we conducted yeast two-hybrid screening, glutathione S-transferase pull-down, and co-immunoprecipitation assays. Galectin (GAL) is a protein involved in immune regulation and host-pathogen interactions. The yeast two-hybrid screen implicated GAL as a major VP3-binding candidate. The assays showed that the VP3 interacted with GAL. Identification of these cellular targets and clarifying their contributions to the host-pathogen interaction may be useful for the development of novel therapeutic and prevention strategies against CSBV infection.

Pathogen-Derived Carbohydrate Recognition in Molluscs Immune Defense

Self-nonself discrimination is a common theme for all of the organisms in different evolutionary branches, which is also the most fundamental step for host immune protection. Plenty of pattern recognition receptors (PRRs) with great diversity have been identified from different organisms to recognize various pathogen-associated molecular patterns (PAMPs) in the last two decades, depicting a complicated scene of host-pathogen interaction. However, the detailed mechanism of the complicate PAMPs–PRRs interactions at the contacting interface between pathogens and hosts is still not well understood. All of the cells are coated by glycosylation complex and thick carbohydrates layer. The different polysaccharides in extracellular matrix of pathogen-host are important for nonself recognition of most organisms. Coincidentally, massive expansion of PRRs, majority of which contain recognition domains of Ig, leucine-rich repeat (LRR), C-type lectin (CTL), C1q and scavenger receptor (SR), have been annotated and identified in invertebrates by screening the available genomic sequence. The phylum Mollusca is one of the largest groups in the animal kingdom with abundant biodiversity providing plenty of solutions about pathogen recognition and immune protection, which might offer a suitable model to figure out the common rules of immune recognition mechanism. The present review summarizes the diverse PRRs and common elements of various PAMPs, especially focusing on the structural and functional characteristics of canonical carbohydrate recognition proteins and some novel proteins functioning in molluscan immune defense system, with the objective to provide new ideas about the immune recognition mechanisms.

The oyster immunity

Oysters, the common name for a number of different bivalve molluscs, are the worldwide aquaculture species and also play vital roles in the function of ecosystem. As invertebrate, oysters have evolved an integrated, highly complex innate immune system to recognize and eliminate various invaders via an array of orchestrated immune reactions, such as immune recognition, signal transduction, synthesis of antimicrobial peptides, as well as encapsulation and phagocytosis of the circulating haemocytes. The hematopoietic tissue, hematopoiesis, and the circulating haemocytes have been preliminary characterized, and the detailed annotation of the Pacific oyster Crassostrea gigas genome has revealed massive expansion and functional divergence of innate immune genes in this animal. Moreover, immune priming and maternal immune transfer are reported in oysters, suggesting the adaptability of invertebrate immunity. Apoptosis and autophagy are proved to be important immune mechanisms in oysters. This review will summarize the research progresses of immune system and the immunomodulation mechanisms of the primitive catecholaminergic, cholinergic, neuropeptides, GABAergic and nitric oxidase system, which possibly make oysters ideal model for studying the origin and evolution of immune system and the neuroendocrine-immune regulatory network in lower invertebrates.

Functions of galectins as 'self/non-self'-recognition and effector factors

Carbohydrate structures on the cell surface encode complex information that through specific recognition by carbohydrate-binding proteins (lectins) modulates interactions between cells, cells and the extracellular matrix, or mediates recognition of potential microbial pathogens. Galectins are a family of ß-galactoside-binding lectins, which are evolutionary conserved and have been identified in most organisms, from fungi to invertebrates and vertebrates, including mammals. Since their discovery in the 1970s, their biological roles, initially understood as limited to recognition of endogenous carbohydrate ligands in embryogenesis and development, have expanded in recent years by the discovery of their roles in tissue repair and regulation of immune homeostasis. More recently, evidence has accumulated to support the notion that galectins can also bind glycans on the surface of potentially pathogenic microbes, and function as recognition and effector factors in innate immunity, thus establishing a new paradigm. Furthermore, some parasites 'subvert' the recognition roles of the vector/host galectins for successful attachment or invasion. These recent findings have revealed a striking functional diversification in this structurally conserved lectin family.

The roles of galectins in parasitic infections

Galectins is a family of multifunctional lectins. Fifteen galectins have been identified from a variety of cells and tissues of vertebrates and invertebrates. Galectins have been shown to play pivotal roles in host-pathogen interaction such as adhesion of pathogens to host cells and activation of host innate and adaptive immunity. In recent years, the roles of galectins during parasite infections have gained increasing attention. Galectins produced by different hosts can act as pattern recognition receptors detecting conserved pathogen-associated molecular patterns of parasites, while galectins produced by parasites can modulate host responses. This review summarizes some recent studies on the roles of galectins produced by parasitic protozoa, nematodes, and trematodes and their hosts. Understanding the roles of galectins in host-parasite interactions may provide targets for immune intervention and therapies of parasitic infections.

Unique phagocytic properties of hemocytes of Pacific oyster Crassostrea gigas against yeast and yeast cell-wall derivatives

For a marine bivalve mollusk such as Pacific oyster Crassostrea gigas, the elimination of foreign particles via hemocyte phagocytosis plays an important role in host defense mechanisms. The hemocytes of C. gigas have a high phagocytic ability for baker's yeast (Saccharomyces cerevisiae) and its cell-wall product zymosan. C. gigas hemocytes might phagocytose yeast cells after binding to polysaccharides on the cell-wall surface, but it is unknown how and what kinds of polysaccharide molecules are recognized. We conducted experiments to determine differences in the phagocytic ability of C. gigas hemocytes against heat-killed yeast (HK yeast), zymosan and zymocel, which are similarly sized and shaped but differ in the polysaccharide composition of their particle surface. We found that both the agranulocytes and granulocytes exerted strong phagocytic ability on all tested particles. The phagocytic index (PI) of granulocytes for zymosan was 9.4 ± 1.7, which significantly differed with that for HK yeast and zymocel (P < 0.05). To evaluate the PI for the three types of particles, and especially to understand the outcome of the much higher PI for zymosan, PI was gauged in increments of 5 (1-5, 6-10, 11-15, and ≥16), and the phagocytic frequencies were compared according to these increments. The results show that a markedly high PI of ≥16 was exhibited by 18.1% of granulocytes for zymosan, significantly higher than 1.7% and 3.9% shown for HK yeast and zymocel, respectively (P < 0.05). These findings indicate that the relatively high PI for zymosan could not be attributed to a situation wherein all phagocytic hemocytes shared a high mean PI, but rather to the ability of some hemocytes to phagocytose a larger portion of zymosan. To determine whether the phagocytosis of these respective particles depended on the recognition of specific polysaccharide receptors on the hemocyte surface, C. gigas hemocytes were pretreated with soluble α-mannan or β-laminarin and then allowed to phagocytose the three types of the particles. The percentage of phagocytic cells of β-laminarin-treated granulocytes decreased significantly for zymosan and zymocel, but not for yeast. These results suggest that C. gigas might possess at least two types of hemocytes, and that one type of the hemocytes (granulocytes) is more active for phagocytosis. The granulocytes were found to have multiple subtypes with different phagocytic abilities and multiple phagocytic receptors. Some of the granulocyte subtypes revealed a much stronger phagocytic ability, depending on the presence of β-glucan receptors for phagocytosis.

A GalNAc/Gal-specific lectin from the sea mussel Crenomytilus grayanus modulates immune response in macrophages and in mice

A GalNAc/Gal-specific lectin (CGL) from the edible mussel Crenomytilus grayanus has been demonstrated to exhibit antibacterial properties. However, the mechanism of immune modulation by CGL in mammalian cells remains unclear. Here, we demonstrated that CGL can activate immune responses in macrophages and in mice. In the in vitro cell models, CGL induced tumour necrosis factor-α and interleukin-6 secretion in mouse RAW264.7 macrophages, mouse bone marrow-derived macrophages, human THP-1 macrophages, human peripheral blood mononuclear cells and human blood monocyte-derived macrophages. The CGL-mediated cytokine production was regulated by reactive oxygen species, mitogen-activated protein kinases, protein kinase C-α/δ and NF-κB. Interestingly, in lipopolysaccharide-activated macrophages, CGL induced endotoxin tolerance (characterized by the downregulation of nitric oxide, inducible nitric oxide synthase, interleukin-6 and cyclooxygenase II) via the downregulation of IRAK2 expression, JNK1/2 phosphorylation and NF-κB activation. CGL also slightly increased the bactericidal activity of macrophages and induced cytokine production in mouse models. Overall, our data indicate that CGL has the potential to be used as an immune modulator in mammals.

Lectin CGL from the sea mussel Crenomytilus grayanus induces Burkitt's lymphoma cells death via interaction with surface glycan

Marine organisms are rich sources of lectins. Lectins are able to bind specifically and reversibly to different types of carbohydrates or glycoproteins. The present study reports the evaluation of glycan binding profile and anti-tumor potential of lectin CGL from the sea mussel Crenomytilus grayanus. Glycan array assay revealed that CGL was able to bind both α and β anomer of galactose, but interaction with the αGal-terminated glycans was stronger. Analysis of most common glycan motifs for CGL showed high affinity to Galα1-4Galβ1-4GlcNAc motif similar to globotriose structure (Gb3: Galα1-4Galβ1-4Glc), the epitope of globotriaosylceramide. CGL recognized Gb3 on the surface of Burkitt's lymphoma Raji cells (high Gb3 expression), leading to dose-dependent cytotoxic effect, G2/M phase cell cycle arrest and apoptosis. Lectin had no effect on erythroleukemia K562 cells (no Gb3 expression). The activity of CGL was specifically blocked by α-galactoside. Our findings suggest the use of CGL in cancer diagnosis and treatment.

A proto-type galectin-2 from rock bream (Oplegnathus fasciatus): Molecular, genomic, and expression analysis, and recognition of microbial pathogens by recombinant protein

A β-galactoside binding lectin, designated as galectin-2, was identified and characterized from rock bream Oplegnathus fasciatus (OfGal-2). The cDNA of OfGal-2 comprised of 692 bp with a coding sequence of 396 bp, encoding a putative polypeptide of 131 amino acids. Gene structure analysis of OfGal-2 revealed a four exon-three intron organization. A single carbohydrate-binding domain containing all seven important residues for carbohydrate binding was located in the third exon, which formed a carbohydrate-binding pocket. Homology screening and sequence analysis demonstrated that OfGal-2 is an evolutionarily conserved proto-type galectin. OfGal-2 transcripts were detected in several healthy fish tissues, with the highest level observed in the intestine, followed by the liver. The expression of OfGal-2 was elevated upon the injection of various mitogenic stimulants and pathogens in a time-dependent manner. Upregulated expression in the liver after tissue injury suggested its role as a damage-associated molecular pattern. Recombinant OfGal-2 protein had hemagglutinating potential and possessed affinity towards lactose and galactose. Moreover, the recombinant protein agglutinated and bound potential pathogenic bacteria and a ciliate. The results of this study indicate that the galectin-2 from rock bream has a potential role in immunity, particularly in the recognition of invading pathogens.

Infectious diseases of marine molluscs and host responses as revealed by genomic tools

More and more infectious diseases affect marine molluscs. Some diseases have impacted commercial species including MSX and Dermo of the eastern oyster, QPX of hard clams, withering syndrome of abalone and ostreid herpesvirus 1 (OsHV-1) infections of many molluscs. Although the exact transmission mechanisms are not well understood, human activities and associated environmental changes often correlate with increased disease prevalence. For instance, hatcheries and large-scale aquaculture create high host densities, which, along with increasing ocean temperature, might have contributed to OsHV-1 epizootics in scallops and oysters. A key to understanding linkages between the environment and disease is to understand how the environment affects the host immune system. Although we might be tempted to downplay the role of immunity in invertebrates, recent advances in genomics have provided insights into host and parasite genomes and revealed surprisingly sophisticated innate immune systems in molluscs. All major innate immune pathways are found in molluscs with many immune receptors, regulators and effectors expanded. The expanded gene families provide great diversity and complexity in innate immune response, which may be key to mollusc's defence against diverse pathogens in the absence of adaptive immunity. Further advances in host and parasite genomics should improve our understanding of genetic variation in parasite virulence and host disease resistance.

Crystallization of Galectin-8 Linker Reveals Intricate Relationship between the N-terminal Tail and the Linker

Galectin-8 (Gal-8) plays a significant role in normal immunological function as well as in cancer. This lectin contains two carbohydrate recognition domains (CRD) connected by a peptide linker. The N-terminal CRD determines ligand binding specificity, whereas the linker has been proposed to regulate overall Gal-8 function, including multimerization and biological activity. Here, we crystallized the Gal-8 N-terminal CRD with the peptide linker using a crystallization condition that contains Ni²⁺. The Ni²⁺ ion was found to be complexed between two CRDs via crystal packing contacts. The coordination between Ni²⁺ and Asp25 plays an indirect role in determining the structure of β-strand F0 and in influencing the linker conformation which could not be defined due to its dynamic nature. The linker was also shortened in situ and crystallized under a different condition, leading to a higher resolution structure refined to 1.08 Å. This crystal structure allowed definition of a short portion of the linker interacting with the Gal-8 N-terminal tail via ionic interactions and hydrogen bonds. Observation of two Gal-8 N-terminal CRD structures implies that the N-terminal tail and the linker may influence each other’s conformation. In addition, under specific crystallization conditions, glycerol could replace lactose and was observed at the carbohydrate binding site. However, glycerol did not show inhibition activity in hemagglutination assay.

Innate immunity against molecular mimicry: Examining galectin-mediated antimicrobial activity

Adaptive immunity provides the unique ability to respond to a nearly infinite range of antigenic determinants. Given the inherent plasticity of the adaptive immune system, a series of tolerance mechanisms exist to reduce reactivity toward self. While this reduces the probability of autoimmunity, it also creates an important gap in adaptive immunity: the ability to recognize microbes that look like self. As a variety of microbes decorate themselves in self-like carbohydrate antigens and tolerance reduces the ability of adaptive immunity to react with self-like structures, protection against molecular mimicry likely resides within the innate arm of immunity. In this review, we will explore the potential consequences of microbial molecular mimicry, including factors within innate immunity that appear to specifically target microbes expressing self-like antigens, and therefore provide protection against molecular mimicry.

Sequencing and de novo assembly of visceral mass transcriptome of the critically endangered land snail Satsuma myomphala: Annotation and SSR discovery

Satsuma myomphala is critically endangered through loss of natural habitats, predation by natural enemies, and indiscriminate collection. It is a protected species in Korea but lacks genomic resources for an understanding of varied functional processes attributable to evolutionary success under natural habitats. For assessing the genetic information of S. myomphala, we performed for the first time, de novo transcriptome sequencing and functional annotation of expressed sequences using Illumina Next-Generation Sequencing (NGS) platform and bioinformatics analysis. We identified 103,774 unigenes of which 37,959, 12,890, and 17,699 were annotated in the PANM (Protostome DB), Unigene, and COG (Clusters of Orthologous Groups) databases, respectively. In addition, 14,451 unigenes were predicted under Gene Ontology functional categories, with 4581 assigned to a single category. Furthermore, 3369 sequences with 646 having Enzyme Commission (EC) numbers were mapped to 122 pathways in the Kyoto Encyclopedia of Genes and Genomes Pathway database. The prominent protein domains included the Zinc finger (C2H2-like), Reverse Transcriptase, Thioredoxin-like fold, and RNA recognition motif domain. Many unigenes with homology to immunity, defense, and reproduction-related genes were screened in the transcriptome. We also detected 3120 putative simple sequence repeats (SSRs) encompassing dinucleotide to hexanucleotide repeat motifs from >1kb unigene sequences. A list of PCR primers of SSR loci have been identified to study the genetic polymorphisms. The transcriptome data represents a valuable resource for further investigations on the species genome structure and biology. The unigenes information and microsatellites would provide an indispensable tool for conservation of the species in natural and adaptive environments.

Human galectin-2 interacts with carbohydrates and peptides non-classically: new insight from X-ray crystallography and hemagglutination

Galectin-2 (Gal-2) plays a role in cancer, myocardial infarction, immune response, and gastrointestinal tract diseases. The only reported crystal structure of Gal-2 shows that it is a dimer in which the monomer subunits have almost identical structures, each binding with one molecule of lactose. In this study, we crystallized Gal-2 under new conditions that produced three crystal structures. In each Gal-2 dimer structure, lactose was shown to be bound to only one of the carbohydrate recognition domain subunits. In solution studies, the thermal shift assay demonstrated that inequivalent monomer subunits in the Gal-2 dimer become equivalent upon ligand binding. In addition, galectin-mediated erythrocyte agglutination assays using lactose and larger complex polysaccharides as inhibitors showed the structural differences between Gal-1 and Gal-2. Overall, our results reveal some novel aspects to the structural differentiation in Gal-2 and expand the potential for different types of molecular interactions that may be specific to this lectin.

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 of a dual-CRD galectin in the silkworm Bombyx mori

Galectins (S-type lectins) are an ancient family of lectins with the β-galactoside binding activity. In mammals, galectins play essential roles in many biological processes, such as development, immune homeostasis and tumor progression. However, few studies have been devoted to their functions in insects. Here, we characterized the only dual-CRD galectin in the silkworm Bombyx mori (BmGalectin-4). BmGalectin-4 cDNA possesses an open reading frame of 1089 bp, which encodes a putative galectin of 363 amino acids containing tandem carbohydrate recognition domains (CRDs). BmGalectin-4 was expressed in various tissues but the protein was most abundant in fertilized eggs. Its transcript level in fertilized eggs was upregulated upon bacterial challenge. Recombinant BmGalectin-4 purified from Escherichia coli bound to bacterial cell wall components and bacterial cells. In addition, the recombinant protein induced bacterial agglutination, but did not have antibacterial activity against selected microorganisms. Taken together, our results suggest that BmGalectin-4 may function as a pattern recognition receptor primarily in silkworm fertilized eggs.

Identification and characterization of pufflectin from the grass pufferfish Takifugu niphobles and comparison of its expression with that of Takifugu rubripes

Pufflectin found in Takifugu rubripes (Tr pufflectin) is the first animal lectin reported to show sequence similarity to monocotyledonous plant lectins. In the present study, we identified and characterized an orthologous lectin from Takifugu niphobles (Tn pufflectin), a species closely related to T. rubripes. Tn pufflectin exhibits 86% identity to Tr pufflectin with two conserved mannose-binding domains. Tn pufflectin was mainly expressed in the skin, gills, brain, and muscles; however, it was expressed at a lower level in the other examined tissues. Recombinant Tn pufflectin, expressed by Escherichia coli, exhibited binding activity specific for d-mannose. The expression of pufflectin in the gills was much lower in T. niphobles than in T. rubripes; notably, the former and latter are resistant and susceptible, respectively, to the monogenean parasite Heterobothrium okamotoi, which parasitizes gills. This suggests that pufflectin might be utilized by the parasite for host recognition.

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.

Epizootiology of Perkinsus sp. inCrassostrea gasar oysters in polyculture with shrimps in northeastern Brazil

Abstract Bivalve culture is of considerable economic and social interest in northeastern (NE) Brazil. The polyculture is an alternative approach to traditional monoculture for reducing the environmental impact of shrimp farming and improving oyster culture. Perkinsus marinus andPerkinsus olseni were found infecting oysters in NE Brazil and can threaten oyster production. This study evaluatedPerkinsus spp. occurrence in Crassostrea gasar during all production stages. Oyster spats were produced in a hatchery and grown in shrimp ponds in Rio Grande do Norte state.Perkinsus spp. were surveyed by Ray’s fluid thioglycollate medium and confirmed by polymerase chain reaction. Prevalence and intensity of infection were determined in oysters until they reached 7 cm. Results showed that the broodstock was already infected by Perkinsus (60%), but the derived spats were Perkinsus-free. Oyster spats acquired Perkinsus infection when transferred to ponds. The prevalence gradually increased in the seven months following placement in ponds (73%), and then decreased to 17% by the tenth month. The infections were initially mild, but intensity increased at the final growth stage. In conclusion, it is possible to produce Perkinsus-free C. gasar oyster spats from infected broodstock, and their culture in shrimp ponds is feasible.

A galectin related protein from Oplegnathus fasciatus: Genomic, molecular, transcriptional features and biological responses against microbial pathogens

Galectins, a family of β-galactoside-binding lectins, are pattern recognition receptors that recognize pathogen-associated molecular patterns and are subsequently involved in the opsonization, phagocytosis, complement activation, and killing of microbes. Here, we report a novel galectin related protein (GRP) identified from rock bream (Oplegnathus fasciatus), designated OfGal like B. The cDNA of OfGal like B is 517 bp with an open reading frame (ORF) of 438 bp, encoding 145 amino acids, with a single carbohydrate recognition domain (CRD). However, only two of the seven critical residues responsible for carbohydrate recognition were identified in the CRD. There was no signal peptide identified in the OfGal like B protein. The genomic structure of OfGal like B, determined using a bacterial artificial chromosome (BAC) genomic library, consists of four exons and three introns. Homology assessment, multiple sequence alignment, and phylogenetic analysis indicated that OfGal like B is an evolutionarily conserved lectin that is closely related to the proto-type galectins. OfGal like B mRNA was constitutively expressed in a wide range of tissues in healthy rock breams. When challenged with bacterial or viral stimulants, OfGal like B was up-regulated in the gills and spleen of rock breams, indicating that it likely plays an important role during bacterial and viral infections. Furthermore, recombinant OfGal like B (rOfGal like B) lacked carbohydrate-binding activity but was able to recognize and agglutinate bacteria, including Streptococcus iniae, Listeria monocytogenes, Vibrio tapetis, Escherichia coli, and Edwardsiella tarda, and a ciliate parasite, Miamiensis avidus. These results collectively suggest that OfGal like B is involved in pathogen recognition and plays a significant role(s) in the innate defense mechanism of rock bream.

Antimicrobial response of galectin-1 from rock bream Oplegnathus fasciatus: Molecular, transcriptional, and biological characterization

In this study, we describe the identification and characterization of a proto type galectin, galectin-1, from rock bream Oplegnathus fasciatus (OfGal-1). Galectins are evolutionarily conserved carbohydrate binding lectins that show a wide range of functions related to development and immune physiology. They have been identified as pattern recognition receptors of innate immune system that recognize a broad range of microbes. OfGal-1 cDNA comprised of 993 bp with an open reading frame of 408 bp that encodes 135 amino acids. A single carbohydrate recognition domain was present in the OfGal-1 amino acid sequence. The sequence comparison by multiple and pairwise alignments and the phylogenetic tree emphasized the strong evolutionary conservation of Gal-1. The typical β-sandwich structure was identified from the predicted tertiary structure. The constitutive expression of mRNA transcripts was detected in a wide range of tissues examined, with the highest expression in the heart. Immune challenges with live bacteria (Edwardsiella tarda and Streptococcus iniae), rock bream irido virus, and mitogens (lipopolysaccharide and poly I:C) modulated the expression of OfGal-1 mRNAs in the gills, head kidney, and liver. The recombinant OfGal-1 (rOfGal-1) strongly agglutinatinated the human erythrocytes, and this hemagglutination was inhibited by lactose and D-galactose. A wide range of bacteria (S. iniae, S. parauberis, Escherichia coli, Edwardsiella tarda, Vibrio anguillarum, Vibrio harveyi, and Vibrio tapetis) and a ciliate (Miamiensis avidus) were also effectively recognized by rOfGal-1. Significant antiviral activity against rock bream irido virus was also demonstrated by rOfGal-1. Collectively, results from the present study indicate that OfGal-1 can recognize a wide range of microbes and is a vital pattern recognition receptor in the innate immune system of rock bream.

Effects of air-exposure gradients on spatial infection patterns of Perkinsus marinus in the eastern oyster Crassostrea virginica

Spatial distributions of species can be shaped by factors such as parasites, mortality, and reproduction, all of which may be influenced by differences in physical factors along environmental gradients. In nearshore tidal waters, an elevational gradient in aerial exposure during low tide can shape the spatial distributions of benthic marine organisms. The eastern oyster Crassostrea virginica is an ecologically and economically important species that can dominate both subtidal and intertidal habitats along the east coast of the USA. Our goal was to determine whether prevalence and intensity of Perkinsus marinus (the causative agent of Dermo disease) infections vary along intertidal to subtidal gradients during summer. We used (1) field experiments conducted at 4 sites in the Chesapeake Bay and a Virginia coastal bay, (2) a controlled air-exposure experiment, and (3) field surveys from 7 sites ranging from Maine to North Carolina to test for effects of tidal exposure on infection. Results from our field surveys suggested that high intertidal oysters tend to have higher infection prevalence than subtidal oysters, but there was no effect on infection intensity. Field experiments rarely yielded significant effects of tidal exposure on infection prevalence and intensity. Overall, our study shows that exposure to air may not be a strong driver of infection patterns in this host-parasite system.

The zebrafish galectins Drgal1-L2 and Drgal3-L1 bind in vitro to the infectious hematopoietic necrosis virus (IHNV) glycoprotein and reduce viral adhesion to fish epithelial cells

The infectious hematopoietic necrosis virus (IHNV; Rhabdoviridae, Novirhabdovirus) infects teleost fish, such as salmon and trout, and is responsible for significant losses in the aquaculture industry and in wild fish populations. Although IHNV enters the host through the skin at the base of the fins, the viral adhesion and entry mechanisms are not fully understood. In recent years, evidence has accumulated in support of the key roles played by protein-carbohydrate interactions between host lectins secreted to the extracellular space and virion envelope glycoproteins in modulating viral adhesion and infectivity. In this study, we assessed in vitro the potential role(s) of zebrafish (Danio rerio) proto type galectin-1 (Drgal1-L2) and a chimera galectin-3 (Drgal3-L1) in IHNV adhesion to epithelial cells. Our results suggest that the extracellular Drgal1-L2 and Drgal3-L1 interact directly and in a carbohydrate-dependent manner with the IHNV glycosylated envelope and glycans on the epithelial cell surface, significantly reducing viral adhesion.

Alterations of the immune transcriptome in resistant and susceptible hard clams (Mercenaria mercenaria) in response to Quahog Parasite Unknown (QPX) and temperature

Quahog Parasite Unknown (QPX) is a fatal protistan parasite that causes severe losses in the hard clam (Mercenaria mercenaria) fisheries along the northeastern coast of the US. Field and laboratory studies of QPX disease have demonstrated a major role for water temperature and M. mercenaria genetic origin in disease development. Infections are more likely to occur at cold temperatures, with clam stocks originating from southern states being more susceptible than clams from northern origin where disease is enzootic. Even though the influence of temperature on QPX infection have been examined in susceptible and resistant M. mercenaria at physiological and cellular scales, the underlying molecular mechanisms associated with host-pathogen interactions remain largely unknown. This study was carried out to explore the molecular changes in M. mercenaria in response to temperature and QPX infection on the transcriptomic level, and also to compare molecular responses between susceptible and resistant clam stocks. A M. mercenaria oligoarray (15 K Agilent) platform was produced based on our previously generated transcriptomic data and was used to compare gene expression profiles in naive and QPX-infected susceptible (Florida stock) and resistant (Massachusetts) clams maintained at temperatures favoring disease development (13 °C) or clam healing (21 °C). In addition, transcriptomic changes reflecting focal (the site of infection, mantle) and systemic (circulating hemocytes) responses were also assessed using the oligoarray platform. Results revealed significant regulation of multiple biological pathways by temperature and QPX infection, mainly associated with immune recognition, microbial killing, protein synthesis, oxidative protection and metabolism. Alterations were widely systemic with most changes in gene expression revealed in hemocytes, highlighting the role of circulating hemocytes as the first line of defense against pathogenic stress. A large number of complement-related recognition molecules with fibrinogen or C1q domains were shown to be specially induced following QPX challenge, and the expression of these molecules was significantly higher in resistant clams as compared to susceptible ones. These highly variable immune proteins may be potent candidate molecular markers for future study of M. mercenaria resistance against QPX. Beyond the specific case of clam response to QPX, this study also provides insights into the primitive complement-like system in the hard clam.

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

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

Multistep Fractionation and Mass Spectrometry Reveal Zwitterionic and Anionic Modifications of the N- and O-glycans of a Marine Snail

Various studies in the past have revealed that molluscs can produce a wide range of rather complex N-glycan structures, which vary from those occurring in other invertebrate animals; particularly methylated glycans have been found in gastropods, and there are some reports of anionic glycans in bivalves. Due to the high variability in terms of previously described structures and methodologies, it is a major challenge to establish glycomic workflows that yield the maximum amount of detailed structural information from relatively low quantities of sample. In this study, we apply differential release with peptide:N-glycosidases F and A followed by solid-phase extraction on graphitized carbon and reversed-phase materials to examine the glycome of Volvarina rubella (C. B. Adams, 1845), a margin snail of the clade Neogastropoda. The resulting four pools of N-glycans were fractionated on a fused core RP-HPLC column and subject to MALDI-TOF MS and MS/MS in conjunction with chemical and enzymatic treatments. In addition, selected N-glycan fractions, as well as O-glycans released by β-elimination, were analyzed by porous graphitized carbon-LC-MS and MS(n). This comprehensive approach enabled us to determine a number of novel modifications of protein-linked glycans, including N-methyl-2-aminoethylphosphonate on mannose and N-acetylhexosamine residues, core β1,3-linked mannose, zwitterionic moieties on core Galβ1,4Fuc motifs, additional mannose residues on oligomannosidic glycans, and bisubstituted antennal fucose; furthermore, typical invertebrate N-glycans with sulfate and core fucose residues are present in this gastropod.

Immune and stress responses in oysters with insights on adaptation

Oysters are representative bivalve molluscs that are widely distributed in world oceans. As successful colonizers of estuaries and intertidal zones, oysters are remarkably resilient against harsh environmental conditions including wide fluctuations in temperature and salinity as well as prolonged air exposure. Oysters have no adaptive immunity but can thrive in microbe-rich estuaries as filter-feeders. These unique adaptations make oysters interesting models to study the evolution of host-defense systems. Recent advances in genomic studies including sequencing of the oyster genome have provided insights into oyster's immune and stress responses underlying their amazing resilience. Studies show that the oyster genomes are highly polymorphic and complex, which may be key to their resilience. The oyster genome has a large gene repertoire that is enriched for immune and stress response genes. Thousands of genes are involved in oyster's immune and stress responses, through complex interactions, with many gene families expanded showing high sequence, structural and functional diversity. The high diversity of immune receptors and effectors may provide oysters with enhanced specificity in immune recognition and response to cope with diverse pathogens in the absence of adaptive immunity. Some members of expanded immune gene families have diverged to function at different temperatures and salinities or assumed new roles in abiotic stress response. Most canonical innate immunity pathways are conserved in oysters and supported by a large number of diverse and often novel genes. The great diversity in immune and stress response genes exhibited by expanded gene families as well as high sequence and structural polymorphisms may be central to oyster's adaptation to highly stressful and widely changing environments.

The immune system and its modulation mechanism in scallop

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

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.

Transcriptome analysis reveals strong and complex antiviral response in a mollusc

Viruses are highly abundant in the oceans, and how filter-feeding molluscs without adaptive immunity defend themselves against viruses is not well understood. We studied the response of a mollusc Crassostrea gigas to Ostreid herpesvirus 1 µVar (OsHV-1μVar) infections using transcriptome sequencing. OsHV-1μVar can replicate extremely rapidly after challenge of C. gigas as evidenced by explosive viral transcription and DNA synthesis, which peaked at 24 and 48 h post-inoculation, respectively, accompanied by heavy oyster mortalities. At 120 h post-injection, however, viral gene transcription and DNA load, and oyster mortality, were greatly reduced indicating an end of active infections and effective control of viral replication in surviving oysters. Transcriptome analysis of the host revealed strong and complex responses involving the activation of all major innate immune pathways that are equipped with expanded and often novel receptors and adaptors. Novel Toll-like receptor (TLR) and MyD88-like genes lacking essential domains were highly up-regulated in the oyster, possibly interfering with TLR signal transduction. RIG-1/MDA5 receptors for viral RNA, interferon-regulatory factors, tissue necrosis factors and interleukin-17 were highly activated and likely central to the oyster's antiviral response. Genes related to anti-apoptosis, oxidation, RNA and protein destruction were also highly up-regulated, while genes related to anti-oxidation were down-regulated. The oxidative burst induced by the up-regulation of oxidases and severe down-regulation of anti-oxidant genes may be important for the destruction of viral components, but may also exacerbate oyster mortality. This study provides unprecedented insights into antiviral response in a mollusc. The mobilization and complex regulation of expanded innate immune-gene families highlights the oyster genome's adaptation to a virus-rich marine environment.

DNA methylation is associated with expression level changes of galectin gene in mantle wound healing process of pearl oyster, Pinctada fucata

Galectin is one important member of pattern recognition proteins that plays a pivotal role in regulating innate immune response of invertebrates. In this study, we cloned the promoter sequence of a tandem-repeat galectin from the pearl oyster Pinctada fucata (P. fucata). The quantitative real-time PCR analysis revealed that galectin mRNA expression in mantle tissues were firstly up-regulated from time points of 2 h-24 h, and then down-regulated from time points of 24 h-168 h after mantle injury. The genome methylation level of mantle tissue was inversely related to galectin mRNA expression (Pearson correlation: -0.554, P: 0.154). The "CpG4-6" methylation level in promoter region of galectin was significant positive correlated with the mRNA expression (Pearson correlation: 0.313, P: 0.049). The results indicated that galectin gene may be involved in immune response in mantle wound healing process of P. fucata, and DNA methylation may be a regulation factor of gene expression.

Galectin CvGal2 from the Eastern Oyster (Crassostrea virginica) Displays Unique Specificity for ABH Blood Group Oligosaccharides and Differentially Recognizes Sympatric Perkinsus Species

Galectins are highly conserved lectins that are key to multiple biological functions, including pathogen recognition and regulation of immune responses. We previously reported that CvGal1, a galectin expressed in phagocytic cells (hemocytes) of the eastern oyster (Crassostrea virginica), is hijacked by the parasite Perkinsus marinus to enter the host, where it causes systemic infection and death. Screening of an oyster hemocyte cDNA library revealed a novel galectin, which we designated CvGal2, with four tandemly arrayed carbohydrate recognition domains (CRDs). Phylogentic analysis of the CvGal2 CRDs suggests close relationships with homologous CRDs from CvGal1. Glycan array analysis, however, revealed that, unlike CvGal1 which preferentially binds to the blood group A tetrasaccharide, CvGal2 recognizes both blood group A and B tetrasaccharides and related structures, suggesting that CvGal2 has broader binding specificity. Furthermore, SPR analysis demonstrated significant differences in the binding kinetics of CvGal1 and CvGal2, and structural modeling revealed substantial differences in their interactions with the oligosaccharide ligands. CvGal2 is homogeneously distributed in the hemocyte cytoplasm, is released to the extracellular space, and binds to the hemocyte surface. CvGal2 binds to P. marinus trophozoites in a dose-dependent and β-galactoside-specific manner. Strikingly, negligible binding of CvGal2 was observed for Perkinsus chesapeaki, a sympatric parasite species mostly prevalent in the clams Mya arenaria and Macoma balthica. The differential recognition of Perkinsus species by the oyster galectins is consistent with their relative prevalence in oyster and clam species and supports their role in facilitating parasite entry and infectivity in a host-preferential manner.

Oral immunostimulation of the oyster Ostrea edulis: Impacts on the parasite Bonamia ostreae

Bioactive compounds were orally administered to the native European oyster Ostrea edulis to evaluate the immune response and the progression of infection of the protozoan parasite Bonamia ostreae. The immunostimulants lipopolysaccharide and zymosan directly administrated to the water column induced an increase in lysozyme activity and the percentage of granulocytes in naïve oysters over a period of 7 days. In another set of experiments, zymosan and curdlan were microencapsulated in alginate and also administered to the water column to naïve and B. ostreae infected O. edulis. Oyster mortality, prevalence and intensity of infection and several immune parameters were evaluated up to 28 days post-administration. Lysozyme activity, nitric oxide production and the expression of galectin, lysozyme and superoxide dismutase increased after 24 h in both infected and uninfected oysters. Zymosan immunostimulated oysters displayed a decrease in the prevalence of B. ostreae infection not attributed to mortalities but which could be associated to the enhanced ability of immunostimulants to evoke an enhanced immune response in the oysters and reduce infection.

The use of -omic tools in the study of disease processes in marine bivalve mollusks

Our understanding of disease processes and host-pathogen interactions in model species has benefited greatly from the application of medium and high-throughput genomic, metagenomic, epigenomic, transcriptomic, and proteomic analyses. The rate at which new, low-cost, high-throughput -omic technologies are being developed has also led to an expansion in the number of studies aimed at gaining a better understanding of disease processes in bivalves. This review provides a catalogue of the genetic and -omic tools available for bivalve species and examples of how -omics has contributed to the advancement of marine bivalve disease research, with a special focus in the areas of immunity, bivalve-pathogen interactions, mechanisms of disease resistance and pathogen virulence, and disease diagnosis. The analysis of bivalve genomes and transcriptomes has revealed that many immune and stress-related gene families are expanded in the bivalve taxa examined thus far. In addition, the analysis of proteomes confirms that responses to infection are influenced by epigenetic, post-transcriptional, and post-translational modifications. The few studies performed in bivalves show that epigenetic modifications are non-random, suggesting a role for epigenetics in regulating the interactions between bivalves and their environments. Despite the progress -omic tools have enabled in the field of marine bivalve disease processes, there is much more work to be done. To date, only three bivalve genomes have been sequenced completely, with assembly status at different levels of completion. Transcriptome datasets are relatively easy and inexpensive to generate, but their interpretation will benefit greatly from high quality genome assemblies and improved data analysis pipelines. Finally, metagenomic, epigenomic, proteomic, and metabolomic studies focused on bivalve disease processes are currently limited but their expansion should be facilitated as more transcriptome datasets and complete genome sequences become available for marine bivalve species.

Immune responses to infectious diseases in bivalves

Many species of bivalve mollusks (phylum Mollusca, class Bivalvia) are important in fisheries and aquaculture, whilst others are critical to ecosystem structure and function. These crucial roles mean that considerable attention has been paid to the immune responses of bivalves such as oysters, clams and mussels against infectious diseases that can threaten the viability of entire populations. As with many invertebrates, bivalves have a comprehensive repertoire of immune cells, genes and proteins. Hemocytes represent the backbone of the bivalve immune system. However, it is clear that mucosal tissues at the interface with the environment also play a critical role in host defense. Bivalve immune cells express a range of pattern recognition receptors and are highly responsive to the recognition of microbe-associated molecular patterns. Their responses to infection include chemotaxis, phagolysosomal activity, encapsulation, complex intracellular signaling and transcriptional activity, apoptosis, and the induction of anti-viral states. Bivalves also express a range of inducible extracellular recognition and effector proteins, such as lectins, peptidoglycan-recognition proteins, thioester bearing proteins, lipopolysaccharide and β1,3-glucan-binding proteins, fibrinogen-related proteins (FREPs) and antimicrobial proteins. The identification of FREPs and other highly diversified gene families in bivalves leaves open the possibility that some of their responses to infection may involve a high degree of pathogen specificity and immune priming. The current review article provides a comprehensive, but not exhaustive, description of these factors and how they are regulated by infectious agents. It concludes that one of the remaining challenges is to use new "omics" technologies to understand how this diverse array of factors is integrated and controlled during infection.

Screening of candidate genes encoding proteins expressed in pectoral fins of fugu Takifugu rubripes, in relation to habitat site of parasitic copepod Caligus fugu, using suppression subtractive hybridization

Caligus fugu is a parasitic copepod specific to the tetraodontid genus Takifugu including the commercially important Takifugu rubripes. Despite the rapid accumulation of knowledge on other aspects of its biology, the host and settlement-site recognition mechanisms of this parasite are not yet well understood. Since the infective copepodid stage shows preferential site selection in attaching to the fins, we considered it likely that the copepodid recognizes chemical cues released or leaking from the fins, and/or transmembrane protein present on the fins. To isolate molecules potentially related to attachment site specificity, we applied suppression subtractive hybridization (SSH) PCR by identifying genes expressed more highly in pectoral fins of T. rubripes than in the body surface skin. We sequenced plasmid DNA from 392 clones in a SSH library. The number of non-redundant sequences was 276, which included 135 sequences located on 117 annotated genes and 141 located in positions where no genes had been annotated. We characterized those annotated genes on the basis of gene ontology terms, and found that 46 of the identified genes encode secreted proteins, enzymes or membrane proteins. Among them nine showed higher expression in the pectoral fins than in the skin. These could be candidate genes for involvement in behavioral mechanisms related to the site specificity shown by the infective copepodids of C. fugu.

Survey for protozoan parasites in Eastern oysters (Crassostrea virginica) from the Gulf of Maine using PCR-based assays

Protozoan pathogens represent a serious threat to oyster aquaculture, since they can lead to significant production loses. Moreover, oysters can concentrate human pathogens through filter feeding, thus putting at risk raw oyster consumers' health. Using PCR-based assays in oysters (Crassostrea virginica) from Maine, we expand the Northeast range in the USA for the protozoans Perkinsus marinus, Perkinsus chesapeaki, and Haplosporidium nelsoni, and report for the first time the detection of the human pathogens Toxoplasma gondii and Cryptosporidium parvum. Oysters hosting both P. marinus and P. chesapeaki were more than three times as likely to be infected by a non-Perkinsus than those free of Perkinsus infections.

An immune-enriched oligo-microarray analysis of gene expression in Manila clam (Venerupis philippinarum) haemocytes after a Perkinsus olseni challenge

Parasites of the genus Perkinsus cause high mortality and economic losses in bivalves commonly produced in global aquaculture. Although the immune responses of oysters and clams naturally infected with Perkinsus marinus or Perkinsus olseni have been extensively studied, there is not much information on host response at the early stages of infection. In this study, we analysed how P. olseni influences the gene expression profiles of haemocytes from the Manila clam (Venerupis philippinarum) using temporal experimental infections and an immune-enriched microarray. We identified an early phase of infection that was characterised by no mortality and by the increased expression of genes associated with pathogen recognition, production of nitrogen radicals and antimicrobial activity. Cellular processes such as inhibition of serine proteases and proliferation were also involved in this early response. This phase was followed by an intermediate stage, when the pathogen was most likely multiplying and infecting new areas of the body, and animals began to die. In this stage, many genes related to cell movement were over-expressed. Thirty days after infection metabolic pathway genes were the most affected. Apoptosis appears to be important during pathogenesis. Our results provide novel observations of the broader innate immune response triggered by P. olseni at different infection stages.

Physiological and pathological changes in the eastern oyster Crassostrea virginica infested with the trematode Bucephalus sp. and exposed to the toxic dinoflagellate Alexandrium fundyense

Effects of experimental exposure to Alexandrium fundyense, a Paralytic Shellfish Toxin (PST) producer known to affect bivalve physiological condition, upon eastern oysters, Crassostrea virginica with a variable natural infestation of the digenetic trematode Bucephalus sp. were determined. After a three-week exposure to cultured A. fundyense or to a control algal treatment with a non-toxic dinoflagellate, adult oysters were assessed for a suite of variables: histopathological condition, hematological variables (total and differential hemocyte counts, morphology), hemocyte functions (Reactive Oxygen Species (ROS) production and mitochondrial membrane potential), and expression in gills of genes involved in immune responses and cellular protection (MnSOD, CAT, GPX, MT-IV, galectin CvGal) or suspected to be (Dominin, Segon). By comparing individual oysters infested heavily with Bucephalus sp. and uninfested individuals, we found altered gonad and digestive gland tissue and an inflammatory response (increased hemocyte concentration in circulating hemolymph and hemocyte infiltrations in tissues) associated with trematode infestation. Exposure to A. fundyense led to a higher weighted prevalence of infection by the protozoan parasite Perkinsus marinus, responsible for Dermo disease. Additionally, exposure to A. fundyense in trematode-infested oysters was associated with the highest prevalence of P. marinus infection. These observations suggest that the development of P. marinus infection was advanced by A. fundyense exposure, and that, in trematode-infested oysters, P. marinus risk of infection was higher when exposed to A. fundyense. These effects were associated with suppression of the inflammatory response to trematode infestation by A. fundyense exposure. Additionally, the combination of trematode infestation and A. fundyense exposure caused degeneration of adductor muscle fibers, suggesting alteration of valve movements and catch state, which could increase susceptibility to predation. Altogether, these results suggest that exposure of trematode-infested oysters to A. fundyense can lead to overall physiological weakness that decrease oyster defense mechanisms.

A lectin with antifungal activity from the mussel Crenomytilus grayanus

Lectins (carbohydrate-binding proteins) are well known to actively participate in the defense functions of vertebrates and invertebrates where they play an important role in the recognition of foreign particles. In this study, we investigated of in vitro antifungal activity of lectin from the mussel Crenomytilus grayanus (CGL). Enzyme-linked immunosorbent assay indicated that CGL was predominantly detectable in tissues of mantle and to a lesser degree in the tissues of muscle, hepatopancreas, gill and hemocytes. After challenged by Pichia pastoris the level of CGL was upregulated and reached the maximum level at 12 h post challenge and recovered to the original level at 24 h. The lectin was capable of inhibiting the germination of spores and hyphal growth in the fungi. All these results indicated that CGL is involved in the innate immune response in mollusc animals.