A novel peptidoglycan recognition protein containing a goose-type lysozyme domain from the Pacific oyster, Crassostrea gigas

Toxicity response to benzo[α]pyrene exposure: Modulation of immune parameters of the bay scallop, Argopectenirradians

Bay scallops were exposed to four BaP concentrations (0.5, 1.0, 10 and 50 μg/L) for 72 h to elucidate their immune response. Immune parameters were evaluated by measuring nitric oxide (NO) levels in hemolymph. Additionally, we measured peptidoglycan recognition proteins (PGRP), fibrinogen-domain-containing protein (FReDC1), metallothionein (MT), and heat shock protein (HSP) 70 mRNA expression in digestive diverticula. NO as well as FReDC1 and MT expression in each BaP group increased significantly over time except for the BaP 0.5 group. The PGRP and HSP70 mRNA expression in the BaP 50 group increased in the range 6-24 h and then decreased. In situ hybridization also confirmed that there was higher MT mRNA expression in the BaP 50 group than in the control group at 72 h. Our results suggest that higher levels of BaP dampened scallop immune responses, while simultaneously reducing their ability to cope with oxidative stress and DNA damage. BaP exposure can be considered a potential immune inducer in bay scallop.

Expression and functional characterization of peptidoglycan recognition protein-S6 involved in antibacterial responses in the razor clam Sinonovacula constricta

It has been recognized that peptidoglycan recognition proteins (PGRPs), structurally conserved molecules, play crucial roles in the innate immunity of invertebrate. However, few studies have been taken to explore their potential functions. In this study, a novel PGRP from the razor clam Sinonovacula constrict designated as ScPGRP-S6 was identified and characterized. The open reading frame (ORF) of ScPGRP-S6 was 666 bp in length, encoding a protein of 221 amino acid with a signal peptide (1-30) and a typical PGRP domain (39-187). The sequence alignment combined with phylogenetic analysis collectively confirmed that ScPGRP-S6 was a novel member belonging to PGRP-S family. The mRNA transcript of ScPGRP-S6 in the hepatopancreases was significantly up-regulated after peptidoglycan (PGN) stimulation, while it was moderately up-regulated after lipopolysaccharide (LPS) stimulation. The result of immunofluorescence detection demonstrated that the positive signal enhanced obviously after Vibrio parahaemolyticus challenge. Notably, the recombinant protein of ScPGRP-S6 (designed as rScPGRP-S6) exhibited high agglutination activity towards V. parahaemolyticus but weak to Staphylococcus aureus. Furthermore, rScPGRP-S6 showed strong amidase and antibacterial activity in the presence of Zn²⁺. Collectively, our results manifested that ScPGRP-S6 could act as a scavenger in the innate immune response of S. constricta.

Apostichopus japonicus matrix metalloproteinase-16 might act as a pattern recognition receptor

Matrix metalloproteinases (MMPs) are an important family of proteinases involved in various physiological processes and associated with the immune response. However, the role of MMPs in the immune response remains unclear. To explore the possible role of MMPs in innate immunity, this study selected the MMP-16 gene encoding peptidoglycan (PGN) binding domain identified in the sea cucumber Apostichopus japonicus (named AjMMP-16, GenBank accession No. AQT26486) for microbial polysaccharide-induced transcriptional expression analysis by quantitative real-time PCR, correlation analysis with nine representative genes from A. japonicus immune pathways in microbial polysaccharide-induced transcriptional expression by using Pearson's correlation test, and prokaryotic recombinant expression. Next, its recombinant protein was employed for microbial polysaccharide-binding analysis with ELISA and bacterial binding analysis with the indirect immunofluorescence method. The results showed that AjMMP-16 was significantly induced by diaminopimelic acid (DAP)-type PGN, lipopolysaccharide, mannan, and β-1,3-glucan and was closely correlated with myeloid differentiation factor 88 (MyD88) in microbial polysaccharide-induced transcriptional expression. In addition, recombinant AjMMP-16 bound to lysine-type PGN, DAP-type PGN, lipopolysaccharide, mannan, β-1,3-glucan, Vibrio splendidus, Pseudoalteromonas nigrifaciens, Shewanella baltica, Bacillus cereus, Escherichia coli, and Staphylococcus aureus. These results suggest that AjMMP-16 might act as a pattern recognition receptor in innate immunity and play an important role in initiating the MyD88-dependent Toll-like receptor signaling pathway.

Genome-wide identification and characteristic analysis of PGRP gene family in Tegillarca granosa reveals distinct immune response of the invasive pathogen

The peptidoglycan recognition proteins (PGRPs) are conserved innate immune molecular in invertebrates and vertebrates, which play important roles in immune system by recognize the peptidoglycans of bacterial cell walls. Although PGRPs have been extensively characterized in insects, a systematic analysis of PGRPs in bivalves is lacking. In the present study, the phylogenic relationships, gene structures and expression profiles of PGRPs in marine bivalves were analyzed. The results indicated that the most PGRPs of bivalves were predicted to degrade the peptidoglycans and prevent excessive immunostimulation of bacteria. In addition, the results of the present study showed that the protein diversity of PGRPs in most marine bivalves was mainly generated by the alternative splicing of genes, however the alternative splicing of PGRP gene family was absent in Tegillarca granosa. The differences of PGRPs might be related to the genetic and environmental differences of marine bivalves. Spatiotemporal expression profiling in T. granosa suggested that PGRPs play important roles in the immune response of invasive pathogens. The present study describes a comprehensive view of PGRPs in the blood clam T. granosa and provides a foundation for functional characterization of this gene family in innate immune of marine bivalves.

Identification of peptidoglycan recognition proteins in hemocytes and kidney of common periwinkle Littorinalittorea

Peptidoglycan Recognition Proteins (PGRPs) are a diverse group of proteins involved in innate immunity. In particular, PGRPs have been shown to participate in immune pattern recognition in various mollusks. However, they have not been described in Caenogastropoda, a large molluscan group comprising sea, freshwater and land snails. In this study, four short PGRPs with molecular weights ranging from 21 to 34 kDa and their isoforms were identified and structurally characterized in the kidney and hemocytic transcriptomes of a caenogastropod mollusk Littorina littorea. All of them (LlPGRP1-4) are secretory, possess a signal peptide and a characteristic N-terminal N-acetylmuramoyl-l-alanine amidase (Ami) domain with conserved Zn²⁺ binding- and amidase catalytic sites. The shortest proteins, LlPGRP1 and LlPGRP2, have no additional conserved motifs on the N-terminus. In longer and most abundantly expressed LlPGRP3 and LlPGRP4 the Ami-domain is combined with an N-terminal SH3-domain and a cysteine-rich motif, respectively. Expression analysis showed that LlPGRPs of the common periwinkle were uninvolved in the immune response to infection with trematode Himasthla elongata though they might act in antibacterial defense.

Lysozymes in Fish

In recent years, the deterioration of the aquaculture ecological environment has led to a high incidence of fish diseases. Lysozymes, important antimicrobial enzymes, play an important role in the innate immune system of fish. The studies of fish lysozymes benefit the control of fish infections caused by pathogens. In this review, we reviewed recent progress in fish lysozymes, including their classification, structural characteristics, biological functions and mechanisms, tissue distributions, and properties of their recombinant proteins, which will help us to systematically understand the fish lysozymes and facilitate their applications in the fields of food and agriculture.

Fish lysozyme gene family evolution and divergent function in early development

Lysozymes are an ancient group of antimicrobial enzymes of the innate immune system. Here we provide a comparative analysis of the evolution and function of lysozymes during early development in fish, the most speciose vertebrate group. In fishes, lineage and species-specific evolution of both C-type (chicken or conventional) and G-type (goose type) genes occurred. Phylogenetic analysis revealed that the teleost lysozyme G-type members group with the tetrapod homologues but the teleost C-type form three different clusters with the tetrapods. Most of the teleost C-type cluster with tetrapod Lyz but there are some that group with the mammalian Lyzl1/2 and LALBA. This suggests that early in gnathostome evolution these genes already existed and that lyzl1/2 and lalba genes are present in fish and tetrapods. Gene synteny analysis to confirm sequence orthologies failed to identify conserved genome regions between teleosts and other vertebrates lysozyme gene regions suggesting that in the ancestral bony fish genome lyz, lyzl1/2, lalba and lyg precursor genes were transposed to different chromosome regions. The homologue of the mammalian lactalbumin (LALBA) gene was identified for the first time in teleosts and was expressed in skin and during egg and larval development. Lysozyme activity was detected in teleost eggs and varied between species and in the gilthead sea bream lyg and lalba transcript abundance differed in eggs and larvae from different brood stock suggesting differences exist in maternal innate immune protection.

Surviving under pollution stress: Antibacterial and antifungal activities of the Oyster species (Magallana bilineata and Magallana cuttackensis)

Antibiotic effectively controls the bacterial and fungal infections. Pathogens developing multi-drug resistance are a global health concern, which necessitate developing new molecules to overcome the resistance dilemma. This study explores the intrinsic ability of marine oysters synthesizing bioactive compounds. The tissue extracts prepared in n-hexane from two oysters, Magallana bilineata and Magallana cuttackensis compared for broad spectral antimicrobial activates against the fungal, Gram positive and negative pathogens. Regardless, both the species tolerated the same pollution indices; the M. bilineata exhibits stronger antimicrobial activities as compared to M. cuttackensis. M. bilineata potentially inhibited the bacterial growth with minimal inhibitory concentration (0.75-20 μg/ml) and fungal pathogens (0.75-5 μg/ml) as compared to ciprofloxacin and miconazole. Inhibitory potential complimented with reduce bactericidal and fungicidal concentrations required to observed susceptible zone of inhibition (ZOI). The inhibition augmented with increased antimicrobial index (AMI) and total activity index (TAI) against the human pathogen than those of M. cuttackensis. It is widely acknowledged that there is a need to develop novel antimicrobial agents to minimize the threat of emerging multiple antimicrobial resistant pathogens. Therefore, the oysters surviving in the pollution stress modulate the physiological and immune response may exploit to develop potential antibiotics.

Identification of immune-related proteins of Dreissena polymorpha hemocytes and plasma involved in host-microbe interactions by differential proteomics

Biological responses of zebra mussel Dreissena polymorpha are investigated to assess the impact of contaminants on aquatic organisms and ecosystems. In addition to concentrate chemical contaminants in their tissues, zebra mussels accumulate several microorganisms such as viruses, protozoa and bacteria. In order to understand the molecular mechanisms involved in the defence against microorganisms this study aims at identifying immune proteins from D. polymorpha hemolymph involved in defence against protozoa and viruses. For this purpose, hemolymph were exposed ex vivo to Cryptosporidium parvum and RNA poly I:C. Differential proteomics on both hemocytes and plasma revealed immune proteins modulated under exposures. Different patterns of response were observed after C. parvum and RNA poly I:C exposures. The number of modulated proteins per hemolymphatic compartments suggest that C. parvum is managed in cells while RNA poly I:C is managed in plasma after 4 h exposure. BLAST annotation and GO terms enrichment analysis revealed further characteristics of immune mechanisms. Results showed that many proteins involved in the recognition and destruction of microorganisms were modulated in both exposure conditions, while proteins related to phagocytosis and apoptosis were exclusively modulated by C. parvum. This differential proteomic analysis highlights in zebra mussels modulated proteins involved in the response to microorganisms, which reflect a broad range of immune mechanisms such as recognition, internalization and destruction of microorganisms. This study paves the way for the identification of new markers of immune processes that can be used to assess the impact of both chemical and biological contaminations on the health status of aquatic organisms.

Is pallial mucus involved in Ostrea edulis defenses against the parasite Bonamia ostreae?

Bonamia ostreae is an intrahemocytic parasite that has been responsible for severe mortalities in the flat oyster Ostrea edulis since the 1970́s. The Pacific oyster Crassostrea gigas is considered to be resistant to the disease and appears to have mechanisms to avoid infection. Most studies carried out on the invertebrate immune system focus on the role of hemolymph, although mucus, which covers the body surface of molluscs, could also act as a barrier against pathogens. In this study, the in vitro effect of mucus from the oyster species Ostrea edulis and C. gigas on B. ostreae was investigated using flow cytometry. Results showed an increase in esterase activities and mortality rate of parasites exposed to mucus from both oyster species. In order to better understand the potential role of mucus in the defense of the oyster against parasites such as B. ostreae, liquid chromatography and tandem mass spectrometry were used to describe and compare mucus protein composition from both species. In all oyster species, pallial mucus contains a high level of proteins; however, O. edulis mucus produced a variety of proteins that could be involved in the immune response against the parasite, including Cu/Zn extracellular superoxide dismutase, thioxiredoxin, peroxiredon VI, heat shock protein 90 as well as several hydrolases. Conversely, a different set of antioxidant proteins, hydrolases and stress related proteins were identified in mucus from C. gigas. Our results suggest an innate immunity adaptation of oysters to develop a specific response against their respective pathogens. The mucosal protein composition also provides new insights for further investigations into the immune response in oysters.

The immune system of the freshwater zebra mussel, Dreissena polymorpha, decrypted by proteogenomics of hemocytes and plasma compartments

The immune system of bivalves is of great interest since it reflects the health status of these organisms during stressful conditions. While immune molecular responses are well documented for marine bivalves, few information is available for continental bivalves such as the zebra mussel, Dreissena polymorpha. A proteogenomic approach was conducted on both hemocytes and plasma to identified immune proteins of this non-model species. Combining transcriptomic sequences with mass spectrometry data acquired on proteins is a relevant strategy since 3020 proteins were identified, representing the largest protein inventory for this sentinel organism. Functional annotation and gene ontology (GO) analysis performed on the identified proteins described the main molecular players of hemocytes and plasma in immunity. GO analysis highlights the complementary immune functions of these two compartments in the management of micro-organisms. Functional annotation revealed new mechanisms in the immune defence of the zebra mussel. Proteins rarely observed in the hemolymph of bivalves were pinpointed such as natterin-like and thaumatin-like proteins. Furthermore, the high abundance of complement-related proteins observed in plasma suggested a strong implication of the complement system in the immune defence of D. polymorpha. This work brings a better understanding of the molecular mechanisms involved in zebra mussel immunity. SIGNIFICANCE: Although the molecular mechanisms of marine bivalves are widely investigated, little information is known for continental bivalves. Moreover, few proteomic studies described the complementarity of both hemolymphatic compartments (cellular and plasmatic) in the immune defence of invertebrates. The recent proteogenomics concept made it possible to discover proteins in non-model organisms. Here, we propose a proteogenomic strategy with the zebra mussel, a key sentinel species for biomonitoring of freshwater, to identify and describe the molecular actors involved in the immune system in both hemocytes and plasma compartments. More widely, this study provided new insight into bivalve immunity.

Molecular cloning and functional characterization of a short peptidoglycan recognition protein from triangle-shell pearl mussel (Hyriopsis cumingii)

Peptidoglycan (PGN) is an important target of recognition in invertebrate innate immunity. PGN recognition proteins (PGRPs) are responsible for PGN recognition. In this study, we cloned and functionally analyzed a short PGRP (HcPGRP2) from the triangle-shell pearl mussel Hyriopsis cumingii. The full-length cDNA sequence of HcPGRP2 gene was 1185 bp containing an open reading frame of 882 bp encoding a 293 amino acid protein. HcPGRP2 was predicted to have two SH3b domains and a conserved C-terminal PGRP domain. Quantitative real-time RT-PCR showed that HcPGRP2 was expressed in all examined tissues and its expression was induced most significantly by Staphylococcus aureus and Vibrio parahaemolyticus in the hepatopancreas and gills. RNA interference by siRNA results revealed that HcPGRP2 was involved in the regulation of whey acidic protein, theromacin, and defensin expression. As a pattern-recognition receptor, recombinant HcPGRP2 (rHcPGRP2) protein can bind and agglutinate (Ca²⁺ dependent) all tested bacteria. rHcPGRP2 exhibited specific binding to PGN but not to lipopolysaccharide. Moreover, rHcPGRP2 inhibited the growth activities of S. aureus and V. parahaemolyticus in vitro and accelerated the clearance of V. parahaemolyticus in vivo. Overall, our results indicated that HcPGRP2 may play an important role in the antibacterial immune mechanisms of H. cumingii.

Two Novel Short Peptidoglycan Recognition Proteins (PGRPs) From the Deep Sea Vesicomyidae Clam Archivesica packardana: Identification, Recombinant Expression and Bioactivity

Vesicomyidae clams are common species living in cold seeps, which incorporates symbiotic bacteria into their body maintaining endosymbiosis relationship. As members of pattern recognition receptor (PRR) family, peptidoglycan recognition proteins (PGRPs) recognize pathogen associated molecular patterns and play an important role in innate immunity. In present study, two short PGRPs (ApPGRP-1 and -2) were first identified from Vesicomyidae clam Archivesica packardana. Sequences analysis showed that they have both conserved Zn²⁺ binding sites (H-H-C) and amidase catalytic sites (H-Y-H-T-C), and phylogenetic tree indicated that they clustered with short PGRPs of other molluscs. PGN assay showed that ApPGRPs could bind Lys-type PGN from Staphylococcus aureus and Dap-type PGN from Bacillus subtilis, and revealed amidase activity with selective zinc ion dependence. rApPGRP-1 and -2 (recombinant ApPGRP-1 and -2) could bind six bacteria with a broad spectrum and had both zinc-dependent and -independent bactericidal activity. ApPGRPs had the complete functions of effectors and partial functions of receptors from PGRPs. Further analyses showed that ApPGRPs from A. packardana might be involved in the endosymbiosis relationship between the host clam and endosymbiotic bacteria as a regulator. The results of these experiments suggested that ApPGRPs were involved in cold seep clams’ immune response. This study provides basic information for further research on the immune mechanisms of deep sea organisms.

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.

Two short peptidoglycan recognition proteins from Crassostrea gigas with similar structure exhibited different PAMP binding activity

Peptidoglycan recognition protein (PGRP) is an essential molecule in innate immunity for both invertebrates and vertebrates, owing to its prominent ability in specifically recognizing bacterial peptidoglycan (PGN) and eliminating the invading bacteria. In the present study, the full length cDNA of two PGRP genes, CgPGRPS2 and CgPGRPS4, were cloned from oyster Crassostrea gigas. Their amino acid sequences both contained one signal peptide, one typical PGRP/amidase domain with conserved catalytic residues responsible for amidase activity (55H, 90Y, 164H, 172C in CgPGRPS2, and 98H, 133Y, 207H, 215C in CgPGRPS4), and specific PGN recognition (84R, 85W, 104R, 109V in CgPGRPS2, and 127G, 128W, 147R, 152V in CgPGRPS4), and they shared 55.9% sequence similarity. The mRNA transcripts of CgPGRPS2 and CgPGRPS4 were constitutively expressed in all the examined tissues, including haemocytes, hepatopancreas, mantle, gonad, heart, adductor muscle and gill, with the highest expression level in adductor muscle and hepatopancreas, respectively. Both CgPGRPS2 and CgPGRPS4 proteins were mainly localized in the cytoplasma. The recombinant protein of CgPGRPS2 (rCgPGRPS2) could bind lipopolysaccharide (LPS), PGN and mannan (Man), as well as various microorganisms including Gram-negative bacteria Escherichia coli, Vibrio anguillarum, Gram-positive bacteria Staphylococcus aureus and fungi Yarrowia lipolytica. The recombinant protein of CgPGRPS4 (rCgPGRPS4) exhibited higher binding affinity to PGN, lower binding affinity to LPS, while no binding activity to Man and Y. lipolytica. The results indicated that CgPGRPS2 and CgPGRPS4 could function as pattern recognition receptors (PRR) in the innate immune response of oyster, and they exhibited a certain degree of functional differentiation in recognition of Man.

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.

Molecular Basis for Adaptation of Oysters to Stressful Marine Intertidal Environments

Oysters that occupy estuarine and intertidal habitats have well-developed stress tolerance mechanisms to tolerate harsh and dynamically changing environments. In this review, we summarize common pathways and genomic features in oyster that are responsive to environmental stressors such as temperature, salinity, hypoxia, air exposure, pathogens, and anthropogenic pollutions. We first introduce the key genes involved in several pathways, which constitute the molecular basis for adaptation to stress. We use genome analysis to highlight the strong cellular homeostasis system, a unique adaptive characteristic of oysters. Next, we provide a global view of features of the oyster genome that contribute to stress adaptation, including oyster-specific gene expansion, highly inducible expression, and functional divergence. Finally, we review the consequences of interactions between oysters and the environment from ecological and evolutionary perspectives by discussing mass mortality and adaptive divergence among populations and related species of the genus Crassostrea. We conclude with prospects for future study.

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.

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.

Identification and characterization of a goose-type lysozyme from sewage snail Physa acuta

Freshwater snail Physa acuta has been considered as an important invasive species and medical mollusc. Field investigation has shown that this snail could survive better than other snails in polluted water bodies. To understand the immune mechanisms of P. acuta, suppression subtractive hybridization hepatopancreas cDNA library has been constructed with bacterial challenge. In this study, a full-length cDNA of a novel goose-type lysozyme (PALysG) has been identified from P. acuta by EST and RACE technique. The conservative structure domains share high homology with other molluscan g-type lysozymes including the SLT domain, the substrate binding sites, the catalytic residues, three alpha-helices structures and six molluscan specific cysteines. Meanwhile, PALysG is the first record of goose-type lysozyme in Gastropoda. Real-time PCR indicated that PALysG mRNA had been expressed significantly at high levels in hepatopancreas for 8-48 h. PALysG recombinant protein displayed the lytic activity of g-type lysozyme with other organisms against Micrococcus lysodikicus.

Identification of a novel molluscan short-type peptidoglycan recognition protein in disk abalone (Haliotis discus discus) involved in host antibacterial defense

Peptidoglycan recognition proteins (PGRPs) are a widely studied group of pattern recognition receptors found in invertebrate as well as vertebrate lineages, and are involved in bacterial pathogen sensing. However, in addition to this principal role, they can also function in multiple host defense processes, including cell phagocytosis and hydrolysis of peptidoglycans (PGNs). In this study, a novel invertebrate short-type PGRP was identified in disk abalone (Haliotis discus discus) designated as AbPGRP. The complete coding sequence of AbPGRP was 534 bp, encoding a 178-amino acid protein with a predicted molecular mass of 20 kDa. The AbPGRP gene had a bipartite arrangement consisting of two exons separated by a single intron. Homology analysis revealed that AbPGRP shares conserved features, including amino acid residues critical for substrate and ion binding as well as for its amidase activity, with homologs of other species. Phylogenetic analysis of AbPGRP revealed that it likely evolved from a common ancestor of invertebrates, having significant homology with other molluscan PGRPs. Recombinant AbPGRP exhibited detectable, dose-dependent PGN-hydrolyzing activity with the presence of Zn(2+), and strong antibacterial activity against Vibrio tapetis, consistent with the functional properties previously reported for PGRPs in other mollusks. Moreover, AbPGRP transcription was induced upon treatment of healthy abalones with bacterial peptidoglycan and lipopolysaccharide, although the expression profiles differed with treatment, suggesting a capacity for discriminating between bacterial pathogens through molecular pattern recognition. Collectively, the findings of this study indicate that AbPGRP is a true homolog of invertebrate PGRPs and likely plays an indispensable role in host immunity.

Involvement of Pacific oyster CgPGRP-S1S in bacterial recognition, agglutination and granulocyte degranulation

Peptidoglycan recognition protein (PGRP) recognizes invading bacteria through their peptidoglycans (PGN), a component of the bacterial cell wall. Insect PGRPs contribute to effective immune systems as inducers of other host defense responses, while this function has not been reported from PGRP of bivalves. In this study, recombinant CgPGRP-S1S (rCgPGRP-S1S), produced in the mantle and the gill, was synthesized and used to elucidate the immunological function of CgPGRP-S1S. rCgPGRP-S1S bound specifically to DAP-type PGN and to Escherichia coli cells, but not to other DAP-type PGN-containing bacterial species, Vibrio anguillarum, or Bacillus subtilis. Antibacterial activity was not detected, but E. coli cells were agglutinated. Moreover, in addition to these direct interactions with bacterial cells, rCgPGRP-S1S induced secretion of granular contents by hemocyte degranulation. Taken together, these results suggest for the first time that a PGRP of bivalves is, just as in insects, involved in host defense, not only by direct interaction with bacteria, but also by triggering other defense pathways.

Responses of Mytilus galloprovincialis to bacterial challenges by metabolomics and proteomics

Pathogens can cause diseases and lead to massive mortalities of aquaculture animals and substantial economic loss. In this work, we studied the responses induced by Micrococcus luteus and Vibrio anguillarum in gill of mussel Mytilus galloprovincialis at protein and metabolite levels. Metabolic biomarkers (e.g., amino acids, betaine, ATP) suggested that both M. luteus and V. anguillarum induced disturbances in energy metabolism and osmotic regulation. The unique and some more remarkably altered metabolic biomarkers (threonine, alanine, aspartate, taurine, succinate) demonstrated that V. anguillarum could cause more severe disturbances in osmotic regulation and energy metabolism. Proteomic biomarkers (e.g., goose-type lysozyme 2, matrilin, ependymin-related protein, peptidyl-prolyl cis-trans isomerases) indicated that M. luteus caused immune stress, and disturbances in signaling pathways and protein synthesis. However, V. anguillarum mainly induced oxidative stress and disturbance in energy metabolism in mussel gills indicated by altered procollagen-proline dioxygenase, protein disulfide isomerase, nucleoside diphosphate kinases, electron transfer flavoprotein and glutathione S-transferase. This work confirmed that an integration of proteomics and metabolomics could provide an insightful view into the effects of pathogens to the marine mussel M. galloprovincialis.

The first evidence of positive selection in peptidoglycan recognition protein (PGRP) genes of Crassostrea gigas

The oyster Crassostrea gigas is thought to have developed effective immunity to potentially harmful pathogens while under continuous exposure to marine microorganisms; however, the evolutionary mechanisms by which such immunity developed has not been understood. To understand the evolution of immunity, we characterized the family of peptidoglycan recognition proteins in the oyster (CgPGRPs). These proteins are crucial pattern recognition receptors for peptidoglycans (PGNs) and thereby, for activating the innate immune response of host. Herein, we identify seven new CgPGRP genes. Phylogenetic analysis of the seven new and five previously reported CgPRGP genes reveals that the CgPRGP gene family can be clustered into two groups, CgPRGPS and CgPRGPL. Moreover, the CgPRGPS group can be further divided into five subgroups. A codon-substitution model and three likelihood ratio tests (LRTs) suggest that seven sites in the CgPGRP family of genes have been subjected to strong positive selection (ω = 3.035-4.143), Three dimensional modeling revealed that these sites are found primarily at the periphery of coils and α-helices rather than in β-strands, perhaps allowing PGRP to adapt to, and recognize, variability of PGN structure. In conclusion, our studies provide the first evidence of positive Darwinian selection in the CgPGRP family, contributing to a better understanding of the adaptive mechanism of host-pathogens interaction in marine mollusks.

Four invertebrate-type lysozyme genes from triangle-shell pearl mussel (Hyriopsis cumingii)

Lysozymes in animals have three types, namely chicken-type, goose-type, and invertebrate-type (i-type) lysozymes and all these 3 types have been found in bivalve mollusks. The i-type lysozymes in mollusks are involved in digestion and innate immunity. In this study, four different lysozyme genes that belong to i-type were identified from Hyriopsis cumingii. The HcLyso1 to HcLyso4 genes encode proteins with 144, 144, 161, and 228 amino acids, respectively, and contain a destabilase domain. HcLyso4 also contains SH3b domain in addition to its destabilase domain. Multiple alignments showed that two catalytic residues of Glu and Asp which were necessary for enzyme activity were present in i-type lysozymes. Phylogenetic analysis using CDS sequences of i-type lysozymes showed that these lysozymes can be divided into mollusk and crustacean clades, and that HcLyso1 to HcLyso4 all belong to the mollusk clades. Although there was no positive selection predicted in i-type lysozymes, some branches suffered rapid evolution. HcLyso1 is mainly expressed in hepatopancreas and can be detected in hemocytes. HcLyso2 is primarily expressed in hepatopancreas and can be detected in hemocytes Whereas, HcLyso3 can be detected mainly in hemocytes, hepatopancreas, gills, and mantle. HcLyso4 is expressed in hemocytes and hepatopancreas. qRT-PCR analysis showed that HcLyso1 to HcLyso4 were all nearly down-regulated by Vibrio or Staphylococcus aureus challenge. Moreover, our research indicated that HcLyso1 to HcLyso4 might play a key role in the innate immunity of mussel.

The Antimicrobial Defense of the Pacific Oyster, Crassostrea gigas. How Diversity may Compensate for Scarcity in the Regulation of Resident/Pathogenic Microflora

Healthy oysters are inhabited by abundant microbial communities that vary with environmental conditions and coexist with immunocompetent cells in the circulatory system. In Crassostrea gigas oysters, the antimicrobial response, which is believed to control pathogens and commensals, relies on potent oxygen-dependent reactions and on antimicrobial peptides/proteins (AMPs) produced at low concentrations by epithelial cells and/or circulating hemocytes. In non-diseased oysters, hemocytes express basal levels of defensins (Cg-Defs) and proline-rich peptides (Cg-Prps). When the bacterial load dramatically increases in oyster tissues, both AMP families are driven to sites of infection by major hemocyte movements, together with bactericidal permeability/increasing proteins (Cg-BPIs) and given forms of big defensins (Cg-BigDef), whose expression in hemocytes is induced by infection. Co-localization of AMPs at sites of infection could be determinant in limiting invasion as synergies take place between peptide families, a phenomenon which is potentiated by the considerable diversity of AMP sequences. Besides, diversity occurs at the level of oyster AMP mechanisms of action, which range from membrane lysis for Cg-BPI to inhibition of metabolic pathways for Cg-Defs. The combination of such different mechanisms of action may account for the synergistic activities observed and compensate for the low peptide concentrations in C. gigas cells and tissues. To overcome the oyster antimicrobial response, oyster pathogens have developed subtle mechanisms of resistance and evasion. Thus, some Vibrio strains pathogenic for oysters are equipped with AMP-sensing systems that trigger resistance. More generally, the known oyster pathogenic vibrios have evolved strategies to evade intracellular killing through phagocytosis and the associated oxidative burst.

Massively parallel RNA sequencing identifies a complex immune gene repertoire in the lophotrochozoan Mytilus edulis

The marine mussel Mytilus edulis and its closely related sister species are distributed world-wide and play an important role in coastal ecology and economy. The diversification in different species and their hybrids, broad ecological distribution, as well as the filter feeding mode of life has made this genus an attractive model to investigate physiological and molecular adaptations and responses to various biotic and abiotic environmental factors. In the present study we investigated the immune system of Mytilus, which may contribute to the ecological plasticity of this species. We generated a large Mytilus transcriptome database from different tissues of immune challenged and stress treated individuals from the Baltic Sea using 454 pyrosequencing. Phylogenetic comparison of orthologous groups of 23 species demonstrated the basal position of lophotrochozoans within protostomes. The investigation of immune related transcripts revealed a complex repertoire of innate recognition receptors and downstream pathway members including transcripts for 27 toll-like receptors and 524 C1q domain containing transcripts. NOD-like receptors on the other hand were absent. We also found evidence for sophisticated TNF, autophagy and apoptosis systems as well as for cytokines. Gill tissue and hemocytes showed highest expression of putative immune related contigs and are promising tissues for further functional studies. Our results partly contrast with findings of a less complex immune repertoire in ecdysozoan and other lophotrochozoan protostomes. We show that bivalves are interesting candidates to investigate the evolution of the immune system from basal metazoans to deuterostomes and protostomes and provide a basis for future molecular work directed to immune system functioning in Mytilus.

Molecular cloning and mRNA expression of two peptidoglycan recognition protein (PGRP) genes from mollusk Solen grandis

Peptidoglycan recognition proteins (PGRPs) play crucial role in innate immunity for both invertebrates and vertebrates, owing to their prominent ability in detecting and eliminating invading bacteria. In the present study, two short PGRPs from mollusk Solen grandis (designated as SgPGRP-S1 and SgPGRP-S2) were identified, and their expression patterns, both in tissues and toward three PAMPs stimulation, were then characterized. The full-length cDNA of SgPGRP-S1 and SgPGRP-S2 was 1672 and 1285 bp, containing an open reading frame (ORF) of 813 and 426 bp, respectively, and deduced amino acid sequences showed high similarity to other members of PGRP superfamily. Both SgPGRP-S1 and SgPGRP-S2 encoded a PGRP domain. The motif of Zn(2+) binding sites and amidase catalytic sites were well conserved in SgPGRP-S1, but partially conserved in SgPGRP-S2. The two PGRPs exhibited different tissue expression pattern. SgPGRP-S1 was highly expressed in muscle and hepatopancreas, while SgPGRP-S2 was highly in gill and mantle. The mRNA expression of SgPGRP-S1 could be induced acutely by stimulation of PGN, and also moderately by β-1,3-glucan, but not by LPS, while expression of SgPGRP-S2 was significantly up-regulated (P < 0.01) when S. grandis was stimulated by all the three PAMPs, though the expression levels were relatively lower than SgPGRP-S1. Our results suggested SgPGRP-S1 and SgPGRP-S2 could serve as pattern recognition receptors (PRRs) involved in the immune recognition of S. grandis, and they might perform different functions in the immune defense against invaders.

Effect of acclimatization on hemocyte functional characteristics of the Pacific oyster (Crassostrea gigas) and carpet shell clam (Ruditapes decussatus)

Most experimental procedures on molluscs are done after acclimatization of wild animals to lab conditions. Similarly, short-term acclimation is often unavoidable in a field survey when biological analysis cannot be done within the day of sample collection. However, acclimatization can affect the general physiological condition and particularly the immune cell responses of molluscs. Our aim was to study the changes in the hemocyte characteristics of the Pacific oyster Crassostrea gigas and the carpet shell clam Ruditapes decussatus acclimated 1 or 2 days under emersed conditions at 14 ± 1 °C and for 1, 2, 7, or 10 days to flowing seawater conditions (submerged) at 9 ± 1 °C, when compared to hemolymph withdrawn from organisms sampled in the field and immediately analyzed in the laboratory (unacclimated). The hemocyte characteristics assessed by flow cytometry were the total (THC) and differential hemocyte count, percentage of dead cells, phagocytosis, and reactive oxygen species (ROS) production. Dead hemocytes were lower in oysters acclimated both in emersed and submerged conditions (1%-5%) compared to those sampled in the field (7%). Compared to oysters, the percentage of dead hemocytes was lower in clams (0.4% vs. 1.1%) and showed a tendency to decrease during acclimatization in both emersed and submerged conditions. In comparison to organisms not acclimated, the phagocytosis of hemocytes decreased in both oysters and clams acclimated under submerged conditions, but was similar in those acclimated in emersed conditions. The ROS production remained stable in both oysters and clams acclimated in emersed conditions, whereas in submerged conditions ROS production did not change in both the hyalinocytes and granulocytes of oysters, but increased in clams. In oysters, the THC decreased when they were acclimated 1 and 2 days in submerged conditions and was mainly caused by a decrease in granulocytes, but the decrease in THC in oysters acclimated 2 days in emersed conditions was caused by a decrease in hyalinocytes and small agranular cells. In clams, the THC was significantly lower in comparison to those not acclimated, regardless of the conditions of the acclimatization. These findings demonstrate that hemocyte characteristics were differentially affected in both species by the tested conditions of acclimatization. The phagocytosis and ROS production in clams and phagocytosis in oysters were not different in those acclimated for 1 day under both conditions, i.e. emersed and submerged, and those sampled in the field (unacclimated). The THC was significantly affected by acclimatization conditions, so the differences between clams and oysters should be considered in studies where important concentrations of hemocytes are required. The difference in the immune response between both species could be related to their habitat (epifaunal vs. infaunal) and their ability of resilience to manipulation and adaptation to captivity. Our results suggest that functional characteristics of hemocytes should be analyzed in both oysters and clams during the first 1 or 2 days, preferably acclimated under emersed rather than submerged conditions.

Gene discovery, comparative analysis and expression profile reveal the complexity of the Crassostrea gigas apoptosis system

Apoptosis system was reported to play important role in organism immunity, but it was a currently understudied respect in molluscan immunity studies. Base on the recent generation of ESTs in the pacific oyster, Crassostrea gigas, a survey of apoptosis-related molecules was conducted in the assembled unigenes, we found that the basic genes and domains in apoptosis-associated proteins were conserved, the overall apoptotic machinery was complex in C. gigas and that the organism had an expanded number of putative baculovirus inhibitor of apoptosis repeat domains. Moreover, four typical apoptosis-related genes were cloned in C. gigas and compared with the sequences of these genes in Drosophila melanogaster and Homo sapiens. The expression level of these four apoptosis-related genes in the hemolymph increased dramatically in the presence of the bacteria, Vibrio anguillarum, indicating their role in bacterial defense. Our results suggest that the oyster apoptosis system is not simple and cannot be represented by model invertebrates.

A Crassostrea gigas Toll-like receptor and comparative analysis of TLR pathway in invertebrates

Toll-like receptor (TLR) signaling pathway was an important and evolutionarily conserved innate immune pathway. Evolutionary lineage of this pathway in the Lophotrochozoans is still less understood. In this study, we cloned a novel TLR, a key component of TLR pathway, from Crassostrea gigas, and named it CgToll-1. The 4343 base pairs full-length cDNA was assembled with the 3' and 5' RACE (rapid amplification of cDNA ends) PCR results, and containing a 3540 bp open reading frame, which encoding a putative TLR protein of 1179 amino acid residues. Real-time reverse transcription polymerase chain reaction analysis revealed that the highest CgToll-1 expression level was in hemolymph, and the expression pattern in hemolymph dramatically increased in the presence of bacteria Vibrio anguillarum. Furthermore, TLR pathway core genes of mollusks were searched and compared with model invertebrates. Phylogenetic trees of two downstream genes (IκB, Rel) showed that mollusks genes were closer to Drosophila melanogaster than Strongylocentrotus purpuratus, while three upstream genes (MyD88, IRAK, TRAF6) showed the opposite propensity. We have also detected that these two downstream genes were significantly more conservative than the three upstream genes based on amino acid sequence alignment. We found no significant difference between the codon usage biases of TLR pathway genes. This study suggests that CgToll-1 was a constitutive and inducible protein and thus could play an important role in the immune responses against bacterium infection. Besides, comparative analysis of TLR pathway showed that gene loss and divergence might exist during evolution in invertebrate.

Peptidoglycan recognition protein of Chlamys farreri (CfPGRP-S1) mediates immune defenses against bacterial infection

Peptidoglycan recognition protein (PGRP) is an essential molecule in innate immunity for both invertebrates and vertebrates, owing to its prominent ability in detecting and eliminating the invading bacteria. Several PGRPs have been identified from mollusk, but their functions and the underlined mechanism are still unclear. In the present study, the mRNA expression profiles, location, and possible functions of PGRP-S1 from Zhikong scallop Chlamys farreri (CfPGRP-S1) were analyzed. The CfPGRP-S1 protein located in the mantle, gill, kidney and gonad of the scallops. Its mRNA expression in hemocytes was up-regulated extremely after PGN stimulation (P<0.01), while moderately after the stimulations of LPS (P<0.01) and β-glucan (P<0.05). The recombinant protein of CfPGRP-S1 (designated as rCfPGRP-S1) exhibited high affinity to PGN and moderate affinity to LPS, but it did not bind β-glucan. Meanwhile, rCfPGRP-S1 also exhibited strong agglutination activity to Gram-positive bacteria Micrococcus luteus and Bacillus subtilis and weak activity to Gram-negative bacteria Escherichia coli. More importantly, rCfPGRP-S1 functioned as a bactericidal amidase to degrade PGN and strongly inhibit the growth of E. coli and Staphyloccocus aureus in the presence of Zn(2+). These results indicated that CfPGRP-S1 could not only serve as a pattern recognition receptor recognizing bacterial PGN and LPS, but also function as a scavenger involved in eliminating response against the invaders.

Presence and characterization of multiple mantle lysozymes in the Pacific oyster, Crassostrea gigas

Mantle tissue extracts from the Pacific oyster, Crassostrea gigas, exhibited anti-Gram-positive bacterial and lysozyme activities over a wide pH range, suggesting that multiple defensive mantle lysozymes were present. Degenerated reverse-transcription PCR detected the expression of two mantle lysozymes, CGL-1 and a novel lysozyme CGL-3, confirming the presence of multiple lysozymes in the mantle. Since CGL-3 is a cognate protein of the digestive lysozyme CGL-2, it is assumed that CGL-3 has evolved specifically a defensive function. Functional assays using recombinant CGL-1 and CGL-3 suggested that CGL-1 and CGL-3 play a major defensive role in the mantle tissue, and that they are responsible for lysozyme activity under different pH, ionic strength and temperature conditions. Based on these observations, we conclude that multiple mantle lysozymes in the Pacific oyster are better for host-defense under broader conditions than a single lysozyme.

Identification and characterization of multiple beta-glucan binding proteins in the Pacific oyster, Crassostrea gigas

The present study reports on the characterization of two cDNAs coding beta-glucan binding proteins (betaGBPs), designated as Cg-betaGBP-1 and Cg-betaGBP-2, from the Pacific oyster, Crassostrea gigas. Cg-betaGBP-1 consists of 555 amino acid residues and possesses two possible integrin recognition sites. The other protein, Cg-betaGBP-2, is composed of 447 amino acid residues without integrin recognition sites. Domain structures of both Cg-betaGBPs are similar to other invertebrate betaGBPs, but phylogenetic positions and major expression tissues for these proteins are different. Cg-betaGBP-1 is expressed in circulatory hemocytes and Cg-betaGBP-2 in digestive glands. Functional assays using recombinant proteins revealed that Cg-betaGBP-2 enhanced the phenoloxidase (PO) activity of hemocyte suspensions under the presence of laminarin, but Cg-betaGBP-1 did not show this enhancement. It is suggested that Cg-betaGBPs in the Pacific oyster have evolved to obtain different immunological functions. Cg-betaGBP-1 possibly evolved for hemocyte-related functions through integrin, and Cg-betaGBP-2 for the PO activation system.