Identifying components of the NF-kappaB pathway in the beneficial Euprymna scolopes-Vibrio fischeri light organ symbiosis

Impact of modeled microgravity stress on innate immunity in a beneficial animal-microbe symbiosis

The innate immune response is the first line of defense for all animals to not only detect invading microbes and toxins but also sense and interface with the environment. One such environment that can significantly affect innate immunity is spaceflight. In this study, we explored the impact of microgravity stress on key elements of the NFκB innate immune pathway. The symbiosis between the bobtail squid Euprymna scolopes and its beneficial symbiont Vibrio fischeri was used as a model system under a simulated microgravity environment. The expression of genes associated with the NFκB pathway was monitored over time as the symbiosis progressed. Results revealed that although the onset of the symbiosis was the major driver in the differential expression of NFκB signaling, the stress of simulated low-shear microgravity also caused a dysregulation of expression. Several genes were expressed at earlier time points suggesting that elements of the E. scolopes NFκB pathway are stress-inducible, whereas expression of other pathway components was delayed. The results provide new insights into the role of NFκB signaling in the squid-vibrio symbiosis, and how the stress of microgravity negatively impacts the host immune response. Together, these results provide a foundation to develop mitigation strategies to maintain host-microbe homeostasis during spaceflight.

Modeled microgravity alters apoptotic gene expression and caspase activity in the squid-vibrio symbiosis

Background

Spaceflight is a novel and profoundly stressful environment for life. One aspect of spaceflight, microgravity, has been shown to perturb animal physiology thereby posing numerous health risks, including dysregulation of normal developmental pathways. Microgravity can also negatively impact the interactions between animals and their microbiomes. However, the effects of microgravity on developmental processes influenced by beneficial microbes, such as apoptosis, remains poorly understood. Here, the binary mutualism between the bobtail squid, Euprymna scolopes, and the gram-negative bacterium, Vibrio fischeri, was studied under modeled microgravity conditions to elucidate how this unique stressor alters apoptotic cell death induced by beneficial microbes.

Results

Analysis of the host genome and transcriptome revealed a complex network of apoptosis genes affiliated with extrinsic/receptor-mediated and intrinsic/stress-induced apoptosis. Expression of apoptosis genes under modeled microgravity conditions occurred earlier and at high levels compared to gravity controls, in particular the expression of genes encoding initiator and executioner caspases. Functional assays of these apoptotic proteases revealed heightened activity under modeled microgravity; however, these increases could be mitigated using caspase inhibitors.

Conclusions

The outcomes of this study indicated that modeled microgravity alters the expression of both extrinsic and intrinsic apoptosis gene expression and that this process is mediated in part by caspases. Modeled microgravity-associated increases of caspase activity can be pharmacologically inhibited suggesting that perturbations to the normal apoptosis signaling cascade can be mitigated, which may have broader implications for maintaining animal-microbial homeostasis in spaceflight.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12866-022-02614-x.

Transcriptome profiling based on larvae at different time points after hatching provides a core set of gene resource for understanding the immune response mechanisms of the egg-protecting behavior against Vibrio anguillarum infection in Amphioctopus fangsiao

Mollusks have recently received increasing attention because of their unique immune systems. Mollusks such as Amphioctopus fangsiao are economically important cephalopods, and the effects of their egg-protecting behavior on the larval immune response are unclear. Meanwhile, little research has been done on the resistance response of cephalopod larvae infected with pathogenic bacteria such as Vibrio anguillarum. In this study, V. anguillarum was used to infect the primary hatching A. fangsiao larvae under different egg-protecting behaviors for 24 h, and a total of 7156 differentially expressed genes (DEGs) were identified at four time points after hatching based on transcriptome analysis. GO and KEGG enrichment analyses showed that multiple immune-related GO terms and KEGG signaling pathways were enriched. Protein-protein interaction networks (PPI networks) were used to search functional relationships between immune-related DEGs. Finally, 20 hub genes related to multiple gene functions or involved in multiple signaling pathways were identified, and their accuracy was verified using quantitative RT-PCR. PPI networks were first used to study the effects A. fangsiao larvae after infection with V. anguillarum under different egg-protecting behaviors. The results provide significant genetic resources for exploring invertebrate larval immune processes. The data lays a foundation for further study the immune response mechanisms for invertebrates after infection.

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.

A lasting symbiosis: how the Hawaiian bobtail squid finds and keeps its bioluminescent bacterial partner

For more than 30 years, the association between the Hawaiian bobtail squid, Euprymna scolopes, and the bioluminescent bacterium Vibrio fischeri has been studied as a model system for understanding the colonization of animal epithelia by symbiotic bacteria. The squid-vibrio light-organ system provides the exquisite resolution only possible with the study of a binary partnership. The impact of this relationship on the partners' biology has been broadly characterized, including their ecology and evolutionary biology as well as the underlying molecular mechanisms of symbiotic dynamics. Much has been learned about the factors that foster initial light-organ colonization, and more recently about the maturation and long-term maintenance of the association. This Review synthesizes the results of recent research on the light-organ association and also describes the development of new horizons for E. scolopes as a model organism that promises to inform biology and biomedicine about the basic nature of host-microorganism interactions.

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

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

Molecular features of interleukin-1 receptor-associated kinase-b and a in Mytilus coruscus, regulating their function by infection of aquatic pathogens and the expression of their serine/threonine protein kinase functional domains

Interleukin-1 receptor-associated kinases (IRAKs) play important roles in the innate immune system of TLR (Toll-like receptor) signaling pathway. In this paper, interleukin-1 receptor-associated kinase-b (designated as McIRAK-b) and interleukin-1 receptor-associated kinase-a (named as McIRAK-a) were obtained based on the transcriptome data, the full length of McIRAK-b was 1815 bp and McIRAK-a was 3168bp, encoding 532 and 978 amino acids, respectively. BLASTp analysis and phylogenetic relationship strongly suggested that the deduced amino acid sequence of McIRAK-b had high homology with IRAK-4 and McIRAK-a was similar to IRAK-1 of other mollusks, especially at their function domains. The expressions of McIRAK-b and McIRAK-a were detected in six tissues including adductor muscle, hemocyte, gills, gonad and hepatopancreas, and the highest expressions appeared both in gills. The expressions of McIRAK-b and McIRAK-a in gills were observed with time-dependent manners after bacterial infections. After being challenged with Vibrio alginolyticus, McIRAK-b expressed significantly and got the peak at 8 h (9.47 times compared with the control group), but the peak appeared at 4 h by being infected with Vibrio parahaemolyticus (12.02 times compared with the control group). The highest point of McIRAK-a mRNA appeared at 12 h (5.16 times) after being challenged with V.alginolyticus and 8 h (4.21 times) for V.parahaemolyticus challenge. The results suggested that IRAK-b and IRAK-a might be important in immune signaling pathway of mussels. The kinase functional domain sequences (S_TKc) of McIRAK-b and McIRAK-a expressed in BL21(DE3) and purified by Ni-NAT Superflow resin conforming to the expected molecular weight with many active sites for their conferring protein-protein interaction functions. This study may provide some further understandings of the regulatory mechanisms in the bivalve innate immune system for IRAKs family.

Molecular characterization and functional analysis of a Rel gene in the Pacific oyster

The nuclear factor-κB (NF-κB) signaling pathway plays a crucial role in regulating many physiological processes such as development, inflammation, apoptosis, cell proliferation, differentiation and immune responses. And the NF-κB/Rel family members were considered as the most important transcription factors in the NF-κB signaling pathway. In this study, we cloned a Rel homolog gene (named as CgRel2) from the Pacific oyster, Crassostrea gigas. The 2115-bp open reading frame (ORF) encodes 704 amino acids and CgRel2 possesses a conserved Rel Homology Domain (RHD) at the N-terminus. Phylogenetic analysis revealed that CgRel2 is most closely related to Pinctada fucata dorsal protein. CgRel2 transcripts are widely expressed in all tested tissues, with the highest expression observed in the labial palp and the gill. Moreover, the expression of CgRel2 is significantly upregulated after lipopolysaccharide (LPS), peptidoglycan (PGN), and polyinosinic-polycytidylic acid [poly(I:C)] challenge. CgRel2 transfection into human cell lines activated NF-κB, TNFα and oyster IL-17 (CgIL-17) reporter genes in a dose-dependent manner, while CgRel2 overexpression cannot induce ISRE (Interferon stimulation response element) reporter gene's transcriptional activity. Additionally, the results of co-immunoprecipitation showed that CgRel2 or CgRel1 could interact with oyster IκB1, IκB2 and IκB3 proteins strongly, which may be critical for the immune signaling transduction and the regulation of its immune functions. Together, these results suggest that CgRel2 could respond to pathogenic infection, participate in the immune signal transduction and activate NF-κB, TNFα and CgIL-17 reporter genes. Thus, CgRel2 could play an important role in the oyster immune system.

Roles of Carotenoids in Invertebrate Immunology

Carotenoids are biologically active pigments that are well-known to enhance the defense and immunity of the vertebrate system. However, in invertebrates, the role of carotenoids in immunity is not clear. Therefore, this study aims to review the scientific evidence for the role of carotenoids in invertebrate immunization. From the analysis of published literatures and recent studies from our laboratory, it is obvious that carotenoids are involved in invertebrate immunity in two ways. On the one hand, carotenoids can act as antioxidant enzymes to remove singlet oxygen, superoxide anion radicals, and hydroxyl radicals, thereby reducing SOD activity and reducing the cost of immunity. In some organisms, carotenoids have been shown to promote SOD activity by up-regulating the expression of the ZnCuSOD gene. Carotenoids, on the other hand, play a role in the expression and regulation of many genes involved in invertebrate immunity, including thioredoxins (TRX), peptidoglycan recognition receptor proteins (PGRPs), ferritins, prophenoloxidase (ProPO), vitellogenin (Vg), toll-like receptor (TLRs), heat shock proteins (HSPs), and CuZnSOD gene. The information in this review is very useful for updating our understanding of the progress of carotenoid research in invertebrate immunology and to help identify topics for future topics.

Identification of cells expressing two peptidoglycan recognition proteins in the gill of the vent mussel, Bathymodiolus septemdierum

In symbiotic systems in which symbionts are transmitted horizontally, hosts must accept symbionts from the environment while defending themselves against invading pathogenic microorganisms. How they distinguish pathogens from symbionts and how the latter evade host immune defences are not clearly understood. Recognition of foreign materials is one of the most critical steps in stimulating immune responses, and pattern recognition receptors (PRRs) play vital roles in this process. In this study, we focused on a group of highly conserved PRRs, peptidoglycan recognition proteins (PGRPs), in the deep-sea mussel, Bathymodiolus septemdierum, which harbours chemosynthetic bacteria in their gill epithelial cells. We isolated B. septemdierum PGRP genes BsPGRP-S and BsPGRP-L, which encode a short- and a long-type PGRP, respectively. The short-type PGRP has a signal peptide and was expressed in the asymbiotic goblet mucous cells in the gill epithelium, whereas the long-type PGRP was predicted to include a transmembrane domain and was expressed in gill bacteriocytes. Based on these findings, we hypothesize that the secreted and transmembrane PGRPs are engaged in host defence against pathogenic bacteria and/or in the regulation of symbiosis via different cellular localizations and mechanisms.

Peptidoglycan Muropeptides: Release, Perception, and Functions as Signaling Molecules

Peptidoglycan (PG) is an essential molecule for the survival of bacteria, and thus, its biosynthesis and remodeling have always been in the spotlight when it comes to the development of antibiotics. The peptidoglycan polymer provides a protective function in bacteria, but at the same time is continuously subjected to editing activities that in some cases lead to the release of peptidoglycan fragments (i.e., muropeptides) to the environment. Several soluble muropeptides have been reported to work as signaling molecules. In this review, we summarize the mechanisms involved in muropeptide release (PG breakdown and PG recycling) and describe the known PG-receptor proteins responsible for PG sensing. Furthermore, we overview the role of muropeptides as signaling molecules, focusing on the microbial responses and their functions in the host beyond their immunostimulatory activity.

Integrative transcriptome analysis and discovery of genes involving in immune response of hypoxia/thermal challenges in the small abalone Haliotis diversicolor

In recent years, the abalone aquaculture industry has been threatened by the deteriorating environmental conditions, such as hypoxia and thermal stress in the hot summers. It is necessary to investigate the molecular mechanism in response to these environmental challenges, and subsequently understand the immune defense system. In this study, the transcriptome profiles by RNA-seq of hemocytes from the small abalone Haliotis diversicolor after exposure to hypoxia, thermal stress, and hypoxia plus thermal stress were established. A total of 103,703,074 clean reads were obtained and 99,774 unigenes were assembled. Of the 99,774 unigenes, 47,154 and 20,455 had homologous sequences in the Nr and Swiss-Prot protein databases, while 16,944 and 10,840 unigenes could be classified by COG or KEGG databases, respectively. RNAseq analysis revealed that the differentially expressed genes (DEGs) after challenges of hypoxia, thermal stress, or hypoxia plus thermal stress were 24,189, 29,165 and 23,665, among which more than 3000 genes involved in at least 230 pathways, including several classical immune-related pathways. The genes and pathways that were involved in immune response to hypoxia/thermal challenges were identified by transcriptome analysis and further validated by quantitative real-time PCR and RNAi technology. The findings in this study can provide information on H. diversicolor innate immunity to improve the abalone aquaculture industry, and the analysis of the potential immune-related genes in innate immunity signaling pathways and the obtained transcriptome data can provide an invaluable genetic resource for the study of the genome and functional genes.

A novel toll-like receptor from Mytilus coruscus is induced in response to stress

Toll-like receptor (TLR) is considered to be an evolutionarily conserved transmembrane protein which promotes the Toll signal pathway to active the expression of transcription factors in the innate immunity of the organism. In this study, a full length of TLR homologue of 2525bp in Mytilus coruscus (named as McTLR-a, GenBank accession no: KY940571) was characterized. Its ORF was 1815 bp with a 5'untranslated region (UTR) of 128 bp and a 3'UTR of 582 bp, encoding 602 amino acid residues with a calculated molecular weight of 70.870 kDa (pI = 6.10). BLASTn analysis and phylogenetic relationship strongly suggested that this cDNA sequence was a member of TLR family. Quantitative real time RT-PCR showed that constitutive expression of McTLR-a was occurred, with increasing order in hemocyte, gonad, mantle, adducter, gill and hepatopancreas. Bacterial infection and heavy metals stimulation up-regulated the expression of McTLR-a mRNA in hepatopancreas with time-dependent manners. The maximum expression appeared at 12 h after pathogenic bacteria injection, with approximately 22-fold in Aeromonas hydrophila and 17-fold in Vibrio parahemolyticus higher than that of the blank group. In heavy metals stress group, they all reached peaks at 3d, while the diverse concentration caused the maximum expression were different. The highest expression reached approximately 7-fold higher than the blank in low concentration of Pb²⁺ exposure. In Cu²⁺ treated group, it reached the peak (approximately 12-fold higher than the blank)in middle concentration. These results indicated that McTLR-a might be involved in the defense response and had a significant role in mediating the environmental stress.

Cloning, expression pattern and functional characterization of interleukin-1 receptor-associated kinase 4, an important mediator of the Toll-like receptor signaling pathway, from Artemia sinica

As a crucial component of Toll-like receptor (TLR)/interleukin-1 receptor (IL-1R) signaling pathways, IL-1R--associated kinase 4 (IRAK-4) plays a central role in innate immunity and embryonic development. Herein, we have characterized the full length cDNA of IRAK4 from Artemia sinica. Molecular characterization revealed that the sequence includes a 2550 bp open reading frame, encoding a predicted protein of 849 amino acids. The predicted protein contains a death domain in the N-terminus and two serine/threonine/tyrosine protein kinasedomains. Bioinformatics analysis showed that it belonged to a new member of the IRAK-4 family. The expression of AsIRAK-4 was researched in various stages during embryonic development by several molecular biology methods including real time PCR, Western blotting and immunohistochemistry. The results showed that AsIRAK-4 was constitutively expressed at all developmental stages from embryo to adult, and it was mainly expressed in the head and thorax at the early stages and on the surface of the alimentary canal at later stages. The highest expression level was at the 0 h, 15 h and 5 d stages of A. sinica. While challenged by Gram-positive and Gram-negative bacteria, AsIRAK-4 was remarkably upregulated with the rising concentration of bacteria. Our research revealed that AsIRAK-4 plays a vital role in growth, development and innate immunity of A. sinica.

Transcriptome mining of immune-related genes in the muricid snail Concholepas concholepas

The population of the Chilean endemic marine gastropod Concholepas concholepas locally called "loco" has dramatically decreased in the past 50 years as a result of intense activity of local fisheries and high environmental variability observed along the Chilean coast, including episodes of hypoxia, changes in sea surface temperature, ocean acidification and diseases. In this study, we set out to explore the molecular basis of C. concholepas to cope with biotic stressors such as exposure to the pathogenic bacterium Vibrio anguillarum. Here, 454pyrosequencing was conducted and 61 transcripts related to the immune response in this muricid species were identified. Among these, the expression of six genes (CcNFκβ, CcIκβ, CcLITAF, CcTLR, CcCas8 and CcCath) involved in the regulation of inflammatory, apoptotic and immune processes upon stimuli, were evaluated during the first 33 h post challenge (hpc). The results showed that CcTLR, CcCas8 and CcCath have an initial response at 4 hpc, evidencing an up-regulation from 4 to 24 hpc. Notably, the response of CcNFKB occurred 2 h later with a statistically significant up-regulation at 6 hpc and 10 hpc. Furthermore, the challenge with V. anguillarum induced a statistically significant down-regulation of CcIKB between 2 and 10 hpc as well as a down-regulation of CcLITAF between 2 and 4 hpc followed in both cases by an up-regulation between 24 and 33 hpc. This work describes the first transcriptomic effort to characterize the immune response of C. concholepas and constitutes a valuable transcriptomic resource for future efforts to develop sustainable aquaculture and conservations tools for this endemic marine snail species.

Characterization of the cell polarity gene crumbs during the early development and maintenance of the squid-vibrio light organ symbiosis

The protein Crumbs is a determinant of apical-basal cell polarity and plays a role in apoptosis of epithelial cells and their protection against photodamage. Using the squid-vibrio system, a model for development of symbiotic partnerships, we examined the modulation of the crumbs gene in host epithelial tissues during initiation and maintenance of the association. The extracellular luminous symbiont Vibrio fischeri colonizes the apical surfaces of polarized epithelia in deep crypts of the Euprymna scolopes light organ. During initial colonization each generation, symbiont harvesting is potentiated by the biochemical and biophysical activity of superficial ciliated epithelia, which are several cell layers from the crypt epithelia where the symbionts reside. Within hours of crypt colonization, the symbionts induce the cell death mediated regression of the remote superficial ciliated fields. However, the crypt cells directly interacting with the symbiont are protected from death. In the squid host, we characterized the gene and encoded protein during light organ morphogenesis and in response to symbiosis. Features of the protein sequence and structure, phylogenetic relationships, and localization patterns in the eye supported assignment of the squid protein to the Crumbs family. In situ hybridization revealed that the crumbs transcript shows opposite expression at the onset of symbiosis in the two different regions of the light organ: elevated levels in the superficial epithelia were attenuated whereas low levels in the crypt epithelia were turned up. Although a rhythmic association in which the host controls the symbiont population over the day-night cycle begins in the juvenile upon colonization, cycling of crumbs was evident only in the adult organ with peak expression coincident with maximum symbiont population and luminescence. Our results provide evidence that crumbs responds to symbiont cues that induce developmental apoptosis and to symbiont population dynamics correlating with luminescence-based stress throughout the duration of the host-microbe association.

Comparative immunogenomics of molluscs

Comparative immunology, studying both vertebrates and invertebrates, provided the earliest descriptions of phagocytosis as a general immune mechanism. However, the large scale of animal diversity challenges all-inclusive investigations and the field of immunology has developed by mostly emphasizing study of a few vertebrate species. In addressing the lack of comprehensive understanding of animal immunity, especially that of invertebrates, comparative immunology helps toward management of invertebrates that are food sources, agricultural pests, pathogens, or transmit diseases, and helps interpret the evolution of animal immunity. Initial studies showed that the Mollusca (second largest animal phylum), and invertebrates in general, possess innate defenses but lack the lymphocytic immune system that characterizes vertebrate immunology. Recognizing the reality of both common and taxon-specific immune features, and applying up-to-date cell and molecular research capabilities, in-depth studies of a select number of bivalve and gastropod species continue to reveal novel aspects of molluscan immunity. The genomics era heralded a new stage of comparative immunology; large-scale efforts yielded an initial set of full molluscan genome sequences that is available for analyses of full complements of immune genes and regulatory sequences. Next-generation sequencing (NGS), due to lower cost and effort required, allows individual researchers to generate large sequence datasets for growing numbers of molluscs. RNAseq provides expression profiles that enable discovery of immune genes and genome sequences reveal distribution and diversity of immune factors across molluscan phylogeny. Although computational de novo sequence assembly will benefit from continued development and automated annotation may require some experimental validation, NGS is a powerful tool for comparative immunology, especially increasing coverage of the extensive molluscan diversity. To date, immunogenomics revealed new levels of complexity of molluscan defense by indicating sequence heterogeneity in individual snails and bivalves, and members of expanded immune gene families are expressed differentially to generate pathogen-specific defense responses.

Genome-wide identification, characterization and expression analyses of TLRs in Yesso scallop (Patinopecten yessoensis) provide insight into the disparity of responses to acidifying exposure in bivalves

Toll-like receptors (TLRs) play a crucial role in innate immunity by recognizing specific pathogen-associated molecular patterns, including lipoproteins, lipopeptides, lipopolysaccharide, flagellin, dsRNA, ssRNA and CpG DNA motifs. Although significant effects of TLRs on immunity have been reported in most vertebrates and some invertebrates, the complete TLR superfamily has not been systematically characterized in scallops. In this study, 18 TLR genes were identified from Yesso scallop (Patinopecten yessoensis) using whole-genome scanning. Phylogenetic and protein structural analyses were performed to determine the identities and evolutionary relationships of the 18 genes. Extensive expansion of TLR genes from the Yesso scallop genome indicated gene duplication events. In addition, expression profiling of PyTLRs was performed at different acidifying exposure levels (pH = 6.50, 7.50) with different challenge durations (3, 6, 12 and 24 h) via in silico analysis using transcriptome and genome databases. Our results confirmed the inducible expression patterns of PyTLRs under acidifying exposure, and the responses to immune stress may have arisen through adaptive recruitment of tandem duplications of TLR genes. Collectively, this study provides novel insight into PyTLRs as well as the specific role and response of TLR signaling pathways in host immune responses against acidifying exposure in bivalves.

Cloning and differential expression of a novel toll-like receptor gene in noble scallop Chlamys nobilis with different total carotenoid content

To investigate whether toll like receptors (TLRs) genes do have an immune influence on noble scallop Chlamys nobilis under pathogen stress, acute challenges lasting 48 h to Vibrio parahaemolyticus, lipopolysaccharide (LPS), polyinosinic polycytidylic acid (Poly I:C), and PBS were conducted in two scallop stains of orange and brown with different carotenoids content. A novel toll-like receptor gene called CnTLR-1 was cloned and its transcripts under different challenges were determined. Meantime, total carotenoids content (TCC) of different immune responses were determined to investigate whether there was a relationship between gene expression and carotenoids content. The full length cDNA of CnTLR-1 is 2982 bp with an open reading frame (ORF) of 1920 bp encoding 639-deduced amino acids, which contains five leucine-rich repeats (LRR), two LRR-C-terminal (LRRCT) motifs and a LRR-N-terminal (LRRNT) motif in the extracellular domain, a transmembrane domain and a Toll/Interleukin-1 Receptor (TIR) of 138-amino acids in the cytoplasmic region. Phylogenetic tree analysis showed that CnTLR-1 could be clustered with mollusk TLRs into one group and especially was related closely to Crassostrea gigas and Mytilus galloprovincialis TLRs. CnTLR-1 transcripts were detected in decreasing levels in the mantle, hemocytes, gill, kidney, gonad, hepatopancreas, intestines and adductor. Compared with PBS control group, CnTLR-1 transcripts were up-regulated in V. parahaemolyticus, LPS and Poly I:C groups. Further, CnTLR-1 transcripts were significantly higher in orange scallops than that of brown ones with and without pathogenic challenges. TCC, which is higher in orange scallops, was initially increased and then decreased during a 48 h immune challenge in the hemocytes. The present results indicate that CnTLR-1 is an important factor involved in the immune defense against pathogens in the noble scallop.

Identification of nuclear factor kappaB (NF-κB) binding motifs in Biomphalaria glabrata

Biomphalaria glabrata acts as the intermediate host to the parasite, Schistosoma mansoni, and for this reason, the immune system of B. glabrata has been researched extensively. Several studies have demonstrated that the transcriptome profile of B. glabrata changes following exposure to a variety of pathogens, yet very little is known regarding the regulation of gene expression in this species. Nuclear factor kappaB (NF-κB) homologues have recently been identified in B. glabrata but few functional studies have been carried out on this family of transcription factors. The aims of this study therefore were to identify NF-κB binding sites (κB motifs) in B. glabrata and examine them via functional assays. Two different κB motifs were predicted. Furthermore, the Rel homology domain (RHD) of a B. glabrata NF-κB was able to bind these κB motifs in EMSAs, as well as a vertebrate κB motif.

The Toll/NF-κB pathway in cuttlefish symbiotic accessory nidamental gland

The female genital apparatus of decapod cephalopods contains a symbiotic accessory nidamental gland (ANG) that harbors bacterial symbionts. Although the ANG bacterial consortium is now well described, the impact of symbiosis on Sepia officinalis innate immunity pathways remains unknown. In silico analysis of the de novo transcriptome of ANG highlighted for the first time the existence of the NF-κB pathway in S. officinalis. Several signaling components were identified, i.e. five Toll-like receptors, eight signaling cascade features, and the immune response target gene iNOS, previously described as being involved in the initiation of bacterial symbiosis in a cephalopod gland. This work provides a first key for studying bacterial symbiosis and its impact on innate immunity in S. officinalis ANG.

CgIκB3, the third novel inhibitor of NF-kappa B (IκB) protein, is involved in the immune defense of the Pacific oyster, Crassostrea gigas

Inhibitor of NF-κB (IκB), the important regulator of NF-κB/Rel signaling pathway, plays the crucial role in immune response of both vertebrates and invertebrates. Here, a novel homologue of IκB was cloned from Crassostrea gigas, and designated as CgIκB3. The complete CgIκB3 cDNA was 1282 bp in length, including a 942 bp open reading frame (ORF), a 51 bp 5' UTR and a 289 bp 3' UTR. The ORF encodes a putative protein of 313 amino acids with a predicted molecular weight of approximately 34.7 kDa. Sequence analysis reveals that CgIκB3 contains a conserved degradation motif but with only five ankyrin repeats. Neither a PEST domain nor a C-terminal casein kinase II phosphorylation site was identified through either alignment or bioinformatic prediction. Phylogenetic analysis suggested that CgIκB3 shares common ancestor with CgIκB1 rather CgIκB2, and theoretically it may originate from one duplication event prior to divergence of CgIκB1 and CgIκB2. Tissue expression analyses demonstrated that CgIκB3 mRNA is the most abundant in gills and heart. The expression following PAMP infection showed that CgIκB3 was significantly up-regulated in a similar pattern when challenged with LPS, HKLM or HKVA, respectively. Moreover, similar to CgIκB1 and CgIκB2, CgIκB3 can also inhibit Rel dependent NF-κB activation in HEK293 cells in a dose-dependent manner. In summary, these findings suggest that CgIκB3 can be as the functional inhibitor of NF-κB/Rel and involved in the host defense of C. gigas. The discovery of the third IκB emphasizes the complexity and importance of the regulation on NF-κB activation.

Understanding the cephalopod immune system based on functional and molecular evidence

Cephalopods have the most advanced circulatory and nervous system among the mollusks. Recently, they have been included in the European directive which state that suffering and pain should be minimized in cephalopods used in experimentation. The knowledge about cephalopod welfare is still limited and several gaps are yet to be filled, especially in reference to pathogens, pathologies and immune response of these mollusks. In light of the requirements of the normative, in addition to the ecologic and economic importance of cephalopods, in this review we update the work published to date concerning cephalopod immune system. Significant advances have been reached in relation to the characterization of haemocytes and defensive mechanisms comprising cellular and humoral factors mainly, but not limited, in species of high economic value like Sepia officinalis and Octopus vulgaris. Moreover, the improvement of molecular approaches has helped to discover several immune-related genes/proteins. These immune genes/proteins include antimicrobial peptides, phenoloxidases, antioxidant enzymes, serine protease inhibitor, lipopolysaccharide-induced TNF-α factor, Toll-like receptors, lectins, even clusters of differentiation among others. Most of them have been found in haemocytes but also in gills and digestive gland, and the characterization as well as their precise role in the immune response of cephalopods is still pending to be elucidated. The assessment of immune parameters in cephalopods exposed to contaminants is just starting, but the negative impact of some pollutants on the immune response of the common octopus has been reported. This review summarizes the current status of our knowledge about the cephalopod immune system that seems to be far from simply. On the contrary, the advances gained to date point out a complex innate immunity in cephalopods.

The immune response of cephalopods from head to foot

Cephalopods are a diverse group of marine molluscs that have proven their worth in a vast array of ways, ranging from their importance within ecological settings and increasing commercial value, to their recent use as model organisms in biological research. However, despite their acknowledged importance, our understanding of basic cephalopod biology does not equate their ecological, societal, and scientific significance. Among these undeveloped research areas, cephalopod immunology stands out because it encompasses a wide variety of scientific fields including many within the biological and chemical sciences, and because of its potential biomedical and commercial relevance. This review aims to address the current knowledge on the topic of cephalopod immunity, focusing on components and functions already established as part of the animals' internal defense mechanisms, as well as identifying gaps that would benefit from future research. More specifically, the present review details both cellular and humoral defenses, and organizes them into sensor, signaling, and effector components. Molluscan, and particularly cephalopod immunology has lagged behind many other areas of study, but thanks to the efforts of many dedicated researchers and the assistance of modern technology, this gap is steadily decreasing. A better understanding of cephalopod immunity will have a positive impact on the health and survival of one of the most intriguing and unique animal groups on the planet, and will certainly influence many other areas of human interest such as ecology, evolution, physiology, symbiosis, and aquaculture.

First description and expression analysis of tumor necrosis factor receptor-associated factor 6 (TRAF6) from the swimming crab, Portunus trituberculatus

Tumor necrosis factor receptor-associated factor 6 (TRAF6) is a cytoplasmic adapter protein that mediates signals induced by the tumor necrosis factor receptor (TNFR) superfamily and the interleukin-1 receptor (IL-1R). In the present study, the full-length cDNA of TRAF6 (Pt-TRAF6) was identified in a marine crab, Portunus trituberculatus. Pt-TRAF6 ORF is predicted to encode a 599-amino acid protein, including a RING type zinc finger, two TRAF-type zinc fingers, and a meprin and TRAF homology (MATH) domain. The overall amino acid sequence identity between Pt-TRAF6 and other TRAF6s ranged from 50.9 to 51.3% for shrimp and from 16.1 to 19.4% for insects. The Pt-TRAF6 gene contains six exons and five introns, which is different from the organization of the insect TRAF6 gene. Pt-TRAF6 transcripts were broadly expressed in all tissues tested, and their expression was higher in hemocytes, gills, the intestine, and heart than in muscle. Interestingly, the level of Pt-TRAF6 transcript differed between male and female crabs. After Vibrio alginolyticus or lipopolysaccharide (LPS) challenge, the Pt-TRAF6 transcript was down-regulated in hemocytes and up-regulated in gills. Moreover, Pt-TRAF6 expression was altered sooner in the LPS challenge group than in the V. alginolyticus challenge group. These results indicate that Pt-TRAF6 may respond to Gram-negative bacterial infections.

Insights into molecular profiles and genomic evolution of an IRAK4 homolog from rock bream (Oplegnathus fasciatus): immunogen- and pathogen-induced transcriptional expression

As a pivotal signaling mediator of toll-like receptor (TLR) and interleukin (IL)-1 receptor (IL-1R) signaling cascades, the IL-1R-associated kinase 4 (IRAK4) is engaged in the activation of host immunity. This study investigates the molecular and expressional profiles of an IRAK4-like homolog from Oplegnathus fasciatus (OfIRAK4). The OfIRAK4 gene (8.2 kb) was structured with eleven exons and ten introns. A putative coding sequence (1395bp) was translated to the OfIRAK protein of 464 amino acids. The deduced OfIRAK4 protein featured a bipartite domain structure composed of a death domain (DD) and a kinase domain (PKc). Teleost IRAK4 appears to be distinct and divergent from that of tetrapods in terms of its exon-intron structure and evolutionary relatedness. Analysis of the sequence upstream of translation initiation site revealed the presence of putative regulatory elements, including NF-κB-binding sites, which are possibly involved in transcriptional control of OfIRAK4. Quantitative real-time PCR (qPCR) was employed to assess the transcriptional expression of OfIRAK4 in different juvenile tissues and post-injection of different immunogens and pathogens. Ubiquitous basal mRNA expression was widely detected with highest level in liver. In vivo flagellin (FLA) challenge significantly intensified its mRNA levels in intestine, liver and head kidney indicating its role in FLA-induced signaling. Meanwhile, up-regulated expression was also determined in liver and head kidney of animals challenged with potent immunogens (LPS and poly I:C) and pathogens (Edwardsiella tarda and Streptococcus iniae and rock bream iridovirus (RBIV)). Taken together, these data implicate that OfIRAK4 might be engaged in antibacterial and antiviral immunity in rock bream.

Transcriptome analysis of the white body of the squid Euprymna tasmanica with emphasis on immune and hematopoietic gene discovery

In the mutualistic relationship between the squid Euprymna tasmanica and the bioluminescent bacterium Vibrio fischeri, several host factors, including immune-related proteins, are known to interact and respond specifically and exclusively to the presence of the symbiont. In squid and octopus, the white body is considered to be an immune organ mainly due to the fact that blood cells, or hemocytes, are known to be present in high numbers and in different developmental stages. Hence, the white body has been described as the site of hematopoiesis in cephalopods. However, to our knowledge, there are no studies showing any molecular evidence of such functions. In this study, we performed a transcriptomic analysis of white body tissue of the Southern dumpling squid, E. tasmanica. Our primary goal was to gain insights into the functions of this tissue and to test for the presence of gene transcripts associated with hematopoietic and immune processes. Several hematopoiesis genes including CPSF1, GATA 2, TFIID, and FGFR2 were found to be expressed in the white body. In addition, transcripts associated with immune-related signal transduction pathways, such as the toll-like receptor/NF-κβ, and MAPK pathways were also found, as well as other immune genes previously identified in E. tasmanica's sister species, E. scolopes. This study is the first to analyze an immune organ within cephalopods, and to provide gene expression data supporting the white body as a hematopoietic tissue.

Diversity of animal immune receptors and the origins of recognition complexity in the deuterostomes

Invertebrate animals are characterized by extraordinary diversity in terms of body plan, life history and life span. The past impression that invertebrate immune responses are controlled by relatively simple innate systems is increasingly contradicted by genomic analyses that reveal significant evolutionary novelty and complexity. One accessible measure of this complexity is the multiplicity of genes encoding homologs of pattern recognition receptors. These multigene families vary significantly in size, and their sequence character suggests that they vary in function. At the same time, certain aspects of downstream signaling appear to be conserved. Here, we analyze five major classes of immune recognition receptors from newly available animal genome sequences. These include the Toll-like receptors (TLR), Nod-like receptors (NLR), SRCR domain scavenger receptors, peptidoglycan recognition proteins (PGRP), and Gram negative binding proteins (GNBP). We discuss innate immune complexity in the invertebrate deuterostomes, which was first recognized in sea urchins, within the wider context of emerging genomic information across animal phyla.

Identification and expression analysis of immune-related genes linked to Rel/NF-κB signaling pathway under stresses and bacterial challenge from the small abalone Haliotis diversicolor

Inhibitor of NF-κB (IκB), nuclear factor-κB (NF-κB), and Akirin2 are all important members of Rel/NF-κB signaling pathway, which plays a pivotal role in regulating the innate immune response of vertebrates and invertebrates. In this study, the IκB (SaIκB) and Akirin2 (SaAkirin2) cDNAs of small abalone Haliotis diversicolor were cloned and characterized. The full length cDNA of SaIκB and SaAkirin2 were 1748 bp and 1452 bp respectively, encoding a protein of 401 aa and 187 aa respectively. A conserved degradation motif (DS56GIYS60) and six ankyrin repeats were identified in the SaIκB by SMART analysis. Meanwhile, a typical nuclear localization signal (NLS) was found at the N-terminal region of the SaAkirin2 protein. Also, the mRNA expression level of SaIκB, SaAkirin2, and AbNF-κB were detected by quantitative real-time PCR. The results revealed that all these three genes were ubiquitously expressed in 7 selected tissues. The expression level of SaIκB in gills was higher than that in other tissues (P < 0.05) while the expression level of AbNF-κB was significantly higher in hepatopancreas and haemocytes. The highest expression level of SaAkirin2 was detected in hepatopancreas, followed by mantle. The mRNA expression levels in either gills or haemocytes of SaIκB, SaAkirin2, and AbNF-κB were significantly up-regulated (P < 0.05) post thermal stress, hypoxia exposure, thermal plus hypoxia stress and the injection of Vibrio parahaemolyticus. These results indicated that these three NF-κB signaling pathway-related genes are involved in response to bacterial infection and play essential roles in response to thermal and hypoxia stress.

De novo transcriptome sequencing of the Octopus vulgaris hemocytes using Illumina RNA-Seq technology: response to the infection by the gastrointestinal parasite Aggregata octopiana

Background

Octopus vulgaris is a highly valuable species of great commercial interest and excellent candidate for aquaculture diversification; however, the octopus’ well-being is impaired by pathogens, of which the gastrointestinal coccidian parasite Aggregata octopiana is one of the most important. The knowledge of the molecular mechanisms of the immune response in cephalopods, especially in octopus is scarce. The transcriptome of the hemocytes of O. vulgaris was de novo sequenced using the high-throughput paired-end Illumina technology to identify genes involved in immune defense and to understand the molecular basis of octopus tolerance/resistance to coccidiosis.

Results

A bi-directional mRNA library was constructed from hemocytes of two groups of octopus according to the infection by A. octopiana, sick octopus, suffering coccidiosis, and healthy octopus, and reads were de novo assembled together. The differential expression of transcripts was analysed using the general assembly as a reference for mapping the reads from each condition. After sequencing, a total of 75,571,280 high quality reads were obtained from the sick octopus group and 74,731,646 from the healthy group. The general transcriptome of the O. vulgaris hemocytes was assembled in 254,506 contigs. A total of 48,225 contigs were successfully identified, and 538 transcripts exhibited differential expression between groups of infection. The general transcriptome revealed genes involved in pathways like NF-kB, TLR and Complement. Differential expression of TLR-2, PGRP, C1q and PRDX genes due to infection was validated using RT-qPCR. In sick octopuses, only TLR-2 was up-regulated in hemocytes, but all of them were up-regulated in caecum and gills.

Conclusion

The transcriptome reported here de novo establishes the first molecular clues to understand how the octopus immune system works and interacts with a highly pathogenic coccidian. The data provided here will contribute to identification of biomarkers for octopus resistance against pathogens, which could improve octopus farming in the near future.

Repurposed transcriptomic data facilitate discovery of innate immunity toll-like receptor (TLR) Genes across Lophotrochozoa

The growing volume of genomic data from across life represents opportunities for deriving valuable biological information from data that were initially collected for another purpose. Here, we use transcriptomes collected for phylogenomic studies to search for toll-like receptor (TLR) genes in poorly sampled lophotrochozoan clades (Annelida, Mollusca, Brachiopoda, Phoronida, and Entoprocta) and one ecdysozoan clade (Priapulida). TLR genes are involved in innate immunity across animals by recognizing potential microbial infection. They have an extracellular leucine-rich repeat (LRR) domain connected to a transmembrane domain and an intracellular toll/interleukin-1 receptor (TIR) domain. Consequently, these genes are important in initiating a signaling pathway to trigger defense. We found at least one TLR ortholog in all but two taxa examined, suggesting that a broad array of lophotrochozoans may have innate immune systems similar to those observed in vertebrates and arthropods. Comparison to the SMART database confirmed the presence of both the LRR and the TIR protein motifs characteristic of TLR genes. Because we looked at only one transcriptome per species, discovery of TLR genes was limited for most taxa. However, several TRL-like genes that vary in the number and placement of LRR domains were found in phoronids. Additionally, several contigs contained LRR domains but lacked TIR domains, suggesting they were not TLRs. Many of these LRR-containing contigs had other domains (e.g., immunoglobin) and are likely involved in innate immunity.

Molecular cloning and mRNA expression of the peptidoglycan recognition protein gene HcPGRP1 and its isoform HcPGRP1a from the freshwater mussel Hyriopsis cumingi

Peptidoglycan recognition proteins (PGRPs) are innate immune molecules that have been structurally conserved throughout evolution in invertebrates and vertebrates. In this study, peptidoglycan recognition protein HcPGRP1 and its isoform HcPGRP1a were identified in the freshwater mussel Hyriopsis cumingii. The full-length cDNAs of HcPGRP1 (973 bp) and HcPGRP1a (537 bp) encoded polypeptides with 218 and 151 amino acids, respectively. Sequence analysis showed that HcPGRP1 had one C-terminal PGRP domain that was conserved throughout evolution. Phylogenetic analysis showed that HcPGRP1 clustered closely with EsPGRP4 of Euprymna scolopes. Real-time PCR showed that the mRNA transcripts of HcPGRP1 and HcPGRP1a were constitutively expressed in various tissues, with the highest level in hepatopancreas. Stimulation with lipopolysaccharide (LPS) and peptidoglycan (PGN) significantly up-regulated HcPGRP1 mRNA expression in hepatopancreas and foot, but not in gill, whereas HcPGRP1a expression was significantly up-regulated in all three tissues. Our results indicate that HcPGRP1 is both a constitutive and inducible protein that may be involved in immune responses (recognition and defense) against invaders.

A model symbiosis reveals a role for sheathed-flagellum rotation in the release of immunogenic lipopolysaccharide

Bacterial flagella mediate host–microbe interactions through tissue tropism during colonization, as well as by activating immune responses. The flagellar shaft of some bacteria, including several human pathogens, is encased in a membranous sheath of unknown function. While it has been hypothesized that the sheath may allow these bacteria to evade host responses to the immunogenic flagellin subunit, this unusual structural feature has remained an enigma. Here we demonstrate that the rotation of the sheathed flagellum in both the mutualist Vibrio fischeri and the pathogen Vibrio cholerae promotes release of a potent bacteria-derived immunogen, lipopolysaccharide, found in the flagellar sheath. We further present a new role for the flagellar sheath in triggering, rather than circumventing, host immune responses in the model squid-vibrio symbiosis. Such an observation not only has implications for the study of bacterial pathogens with sheathed flagella, but also raises important biophysical questions of sheathed-flagellum function.

DOI:http://dx.doi.org/10.7554/eLife.01579.001

While a few of the bacteria that live in and on the bodies of humans and other animals are harmful and can cause disease, most others can offer benefits to their hosts. Many bacteria—including some important human pathogens—have tails called flagella that rotate to move the bacteria inside its host. However, the immune system can detect parts of these flagella and eliminate the pathogen.

Bacterial flagella are made from filaments of proteins, and some flagella are also enclosed by a sheath that is similar to the outer membrane that encloses certain bacteria. The function of this sheath is unclear, although some researchers have suggested that it might prevent the immune system from detecting the proteins in the flagellum. Now, by studying the interactions between the Hawaiian bobtail squid and a marine bacterium, Brennan et al. show that the sheath can actually alert the host that the bacteria are around.

The Hawaiian bobtail squid collects bioluminescent bacteria within a so-called ‘light organ’. This organ undergoes a number of developmental changes to house the bacteria, and the squid then uses the light from the bacteria to mask its own shadow, which helps it to avoid being detected by predators. Brennan et al. compared how wild-type bacteria and mutant bacteria that either had no flagella, or had flagella that did not rotate, interacted with young squid. Only bacteria with working flagella were able to trigger the normal development of the squid’s light organ, which suggests that the rotating flagella are releasing the signal that tells the squid that the beneficial bacteria are present.

Brennan et al. demonstrated that the rotation of sheathed flagella led to the release of a molecule called lipopolysaccharide. This molecule is known to activate the immune system in animals, and it is one of the bacterial signals that the squid responds to. Moreover, when the flagella of other bacteria with sheaths—such as those that cause cholera—are rotating, there is also an increase in the release of lipopolysaccharide. However, rotation of the flagella of bacteria without sheaths has no such effect. The next challenge will be to test the importance of this release of lipopolysaccharide from rotating flagella on the outcome of bacterial diseases of humans and other animals.

DOI:http://dx.doi.org/10.7554/eLife.01579.002

Eye-specification genes in the bacterial light organ of the bobtail squid Euprymna scolopes, and their expression in response to symbiont cues

The squid Euprymna scolopes has evolved independent sets of tissues capable of light detection, including a complex eye and a photophore or 'light organ', which houses the luminous bacterial symbiont Vibrio fischeri. As the eye and light organ originate from different embryonic tissues, we examined whether the eye-specification genes, pax6, eya, six, and dac, are shared by these two organs, and if so, whether they are regulated in the light organ by symbiosis. We obtained sequences of the four genes with PCR, confirmed orthology with phylogenetic analysis, and determined that each was expressed in the eye and light organ. With in situ hybridization (ISH), we localized the gene transcripts in developing embryos, comparing the patterns of expression in the two organs. The four transcripts localized to similar tissues, including those associated with the visual system ∼1/4 into embryogenesis (Naef stage 18) and the light organ ∼3/4 into embryogenesis (Naef stage 26). We used ISH and quantitative real-time PCR to examine transcript expression and differential regulation in postembryonic light organs in response to the following colonization conditions: wild-type, luminescent V. fischeri; a mutant strain defective in light production; and as a control, no symbiont. In ISH experiments light organs showed down regulation of the pax6, eya, and six transcripts in response to wild-type V. fischeri. Mutant strains also induced down regulation of the pax6 and eya transcripts, but not of the six transcript. Thus, luminescence was required for down regulation of the six transcript. We discuss these results in the context of symbiont-induced light-organ development. Our study indicates that the eye-specification genes are expressed in light-interacting tissues independent of their embryonic origin and are capable of responding to bacterial cues. These results offer evidence for evolutionary tinkering or the recruitment of eye development genes for use in a light-sensing photophore.

Innate immune recognition of the microbiota promotes host-microbial symbiosis

Pattern-recognition receptors (PRRs) are traditionally known to sense microbial molecules during infection to initiate inflammatory responses. However, ligands for PRRs are not exclusive to pathogens and are abundantly produced by the resident microbiota during normal colonization. Mechanism(s) that underlie this paradox have remained unclear. Recent studies reveal that gut bacterial ligands from the microbiota signal through PRRs to promote development of host tissue and the immune system, and protection from disease. Evidence from both invertebrate and vertebrate models reveals that innate immune receptors are required to promote long-term colonization by the microbiota. This emerging perspective challenges current models in immunology and suggests that PRRs may have evolved, in part, to mediate the bidirectional cross-talk between microbial symbionts and their hosts.

Characteristic and functional analysis of toll-like receptors (TLRs) in the lophotrocozoan, Crassostrea gigas, reveals ancient origin of TLR-mediated innate immunity

The evolution of TLR-mediated innate immunity is a fundamental question in immunology. Here, we report the characterization and functional analysis of four TLR members in the lophotrochozoans Crassostreagigas (CgTLRs). All CgTLRs bear a conserved domain organization and have a close relationship with TLRs in ancient non-vertebrate chordates. In HEK293 cells, every CgTLR could constitutively activate NF-κB responsive reporter, but none of the PAMPs tested could stimulate CgTLR-activated NF-κB induction. Subcellular localization showed that CgTLR members have similar and dual distribution on late endosomes and plasma membranes. Moreover, CgTLRs and CgMyD88 mRNA show a consistent response to multiple PAMP challenges in oyster hemocytes. As CgTLR-mediated NF-κB activation is dependent on CgMyD88, we designed a blocking peptide for CgTLR signaling that would inhibit CgTLR-CgMyD88 dependent NF-κB activation. This was used to demonstrate that a Vibrio parahaemolyticus infection-induced enhancement of degranulation and increase of cytokines TNF mRNA in hemocytes, could be inhibited by blocking CgTLR signaling. In summary, our study characterized the primitive TLRs in the lophotrocozoan C. gigas and demonstrated a fundamental role of TLR signaling in infection-induced hemocyte activation. This provides further evidence for an ancient origin of TLR-mediated innate immunity.

De novo whole transcriptome analysis of the fish louse, Argulus siamensis: first molecular insights into characterization of Toll downstream signalling molecules of crustaceans

Argulus siamensis is a major ectoparasitic pathogen of freshwater fish capable of causing substantial economic loss. None of the available control measures have been able to address the problem of argulosis resourcefully. To combat this pathogen effectively, it is necessary to have a comprehensive understanding of its life processes with information on various genes involved. The transcriptome studies can generate introductory information about genes participating in physiological processes of the parasite which could be targeted for their control. In this study, the transcriptome sequencing of A. siamensis was performed on Illumina HiSeq 2000 platform which generated 75,126,957 high quality reads. A total of 46,352 transcript contigs were assembled with average length of 1211bp and N50 length of 2302bp. In total, 19,290 CDS including 184 novel CDS and 59,019 open reading frames (ORFs) were identified from the assembled contigs. Gene ontology and Kyoto Encylopedia of Genes and Genomes pathway analysis were performed to classify contigs into their functional categories and regulation pathways. Additionally, 1171 simple sequence repeats were identified from the assembled contigs. Further, twelve contigs with high similarity with downstream molecules of the mammalian toll like receptor (TLR) pathway were validated by their inductive expressions in response to lipopolysaccharide (LPS) of Gram negative bacteria, Escherichia coli and Gram positive bacteria, Staphylococcus aureus. The transcriptome of an ectoparasite A. siamensis was sequenced, assembled, annotated, and the downstream signalling molecules of Toll pathway characterized. The transcriptome data generated will facilitate studies on functional genomics that will subsequently be applied for vaccine development and other control strategies against the parasite.

Impact of simulated microgravity on the normal developmental time line of an animal-bacteria symbiosis

The microgravity environment during space flight imposes numerous adverse effects on animal and microbial physiology. It is unclear, however, how microgravity impacts those cellular interactions between mutualistic microbes and their hosts. Here, we used the symbiosis between the host squid Euprymna scolopes and its luminescent bacterium Vibrio fischeri as a model system. We examined the impact of simulated microgravity on the timeline of bacteria-induced development in the host light organ, the site of the symbiosis. To simulate the microgravity environment, host squid and symbiosis-competent bacteria were incubated together in high-aspect ratio rotating wall vessel bioreactors and examined throughout the early stages of the bacteria-induced morphogenesis. The host innate immune response was suppressed under simulated microgravity; however, there was an acceleration of bacteria-induced apoptosis and regression in the host tissues. These results suggest that the space flight environment may alter the cellular interactions between animal hosts and their natural healthy microbiome.

Functional characterization of a short peptidoglycan recognition protein, PGRP5 in grass carp Ctenopharyngodon idella

Peptidoglycan recognition proteins (PGRPs), which are evolutionarily conserved from insects to mammals, recognize bacterial peptidoglycan (PGN) and function in antibacterial innate immunity. In this study, a short-form PGRP, designated as gcPGRP5 was identified from grass carp Ctenopharyngodon idella. The deduced amino acid sequence of gcPGRP5 is composed of 180 residues with a conserved PGRP domain at the C-terminus. The gcPGRP5 gene consists of four exons and three introns, spacing approximately 2.3 kb in genomic sequence. Phylogenetic analysis demonstrated that the gcPGRP5 is clustered with other PGRP-S identified in teleost fish. The gcPGRP5 is constitutively expressed in all organs/tissues examined, and its expression was significantly induced in CIK cells treated with lipoteichoic acid (LTA), polyinosinic polycytidylic acid (Poly I:C) and PGN. Fluorescence analysis showed that gcPGRP5 is distributed in cytoplasm of CIK cells, and cell lysates from CIK cells transfected with pTurbo-gcPGRP5-GFP and ptGFP1-gcPGRP5 plasmids display the binding activity and peptidoglycan-lytic amidase activity toward Lys-PGN from Staphylococcus aureus and Dap-PGN from Bacillus subtilis. Furthermore, heat-shock protein70 (Hsp70), and MyD88, an adaptor molecule in Toll-like receptor pathway, had an increased expression in CIK cells overexpressed with gcPGRP5. It is thus indicated that gcPGRP5 exhibits amidase activity, and also possesses roles in anti-stress, and in Toll-like receptor signaling pathway.

Toll-like receptors and MyD88 adaptors in Mytilus: complete cds and gene expression levels

TLR- and MyD88-related sequences have been previously investigated in Mytibase and then in new transcript reads obtained by Illumina technology from the mussel, Mytilus galloprovincialis. Based on full cds and domain organizations of virtual translations, we identified 23 Toll-like receptors (TLRs) and 3 MyD88 adaptors. MgTLRs can be arranged in 4 clusters according to extra-cellular LRR domain content. MgTLR-b, -i and -k were the only ones containing a multiple cysteine cluster (mccTLR), a domain composition also found in Drosophila Toll-1 and 18-wheeler. The 3 MyD88 we identified in M. galloprovincialis were also retrieved from Mytilus edulis, as well as MgTLR-b and -i. All MgTLRs were constitutively expressed in digestive gland whereas only 4 of them were also present in hemocytes. On the opposite, the 3 MgMyD88s were constitutively expressed in all the tissues. In vivo challenge of M. galloprovincialis with bacteria caused the up regulation of only MgTLR-i, but of all the 3 MgMyD88s. Highest response was induced by Gram-negative Vibrio anguillarum at 9h p.i. Injection of filamentous fungus, Fusarium oxysporum, resulted in up regulation of MgTLR-i and MgMyD88-c at 9h p.i. Such similar pattern of responses suggested MgMyD88-c represents the intra cytoplasm partner of MgTLR-i. Their interaction constituted the first cellular event revealing the existence of a Toll-signaling pathway in Lophotrochozoa.

Molecular characterization and expression analysis of IκB from Haliotis discus discus

Innate immune system relies on the recognition of pathogen associated molecular patterns present in the microbes by the pattern recognition receptors leading to the activation of signaling cascade and subsequent synthesis of cytokines. NF-κB is a major stimulus activated transcription factor, which regulates the expression of a diverse array of genes. IκB is an inhibitor of NF-κB, retaining NF-κB in an inactive state in the cytoplasm. In this study, we have reported the characterization of first abalone IκB (HdIκB). The cDNA possessed an ORF of 1200 bp coding for a protein of 400 amino acids with molecular mass of 45 kDa and isoelectric point of 4.7. HdIκB protein possessed a conserved phosphorylation site (58)DSGIFS(63) in the N-terminal region, six ankyrin repeats, and a PEST sequence in the C-terminal region. A casein kinase II phosphorylation site could also be observed in the PEST sequence. Constitutive expression of HdIκB revealed its physiological significance since NF-κB is known to be activated by various stimuli. Elevated expression of HdIκB transcripts could be observed in abalones challenged with various mitogens and live microbes. This novel characterization of abalone IκB would further be a positive approach in the affirmation of evolutionary conservation and significance of this protein as a repressor/inhibitor of a pleiotropic transcription factor like NF-κB.

Identification and expression analysis of two Toll-like receptor genes from sea cucumber (Apostichopus japonicus)

Toll-like receptors (TLRs) are a family of type I integral membrane glycoproteins which play pivotal roles in innate immunity. In this study, two TLRs named AjTLR3 and AjToll were cloned from sea cucumber (Apostichopus japonicus). The full-length cDNA sequences of AjTLR3 and AjToll are 3484 bp and 4211 bp, with an open reading frame (ORF) of 2679 bp and 2853 bp, encoding 892 and 950 amino acids, respectively. Both AjTLR3 and AjToll are composed of a leucine-rich repeat (LRR) domain, a transmembrane (TM) domain and an intracellular Toll/interleukin-1 receptor (TIR) domain. Evolution analysis revealed that AjTLR3 and AjToll were clustered with the vertebrate-like TLRs (V-TLRs) and the protostome-like TLRs (P-TLRs), respectively. These two genes were widely expressed in all five tested tissues (body wall, coelomocytes, tube feet, intestine and respiratory tree), but showed different expression patterns. The significantly up-regulated expressions of AjTLR3 and AjToll after peptidoglycan (PGN), lipopolysaccharides (LPS), Zymosan A and polyinosinic-polycytidylic acid (PolyI:C) challenges suggested that they were functionally involved in the immune responses to the Cram-positive bacteria, Gram-negative bacteria, fungi and double-stranded RNA (dsRNA) viruses, respectively.

Identification of genes potentially related to biomineralization and immunity by transcriptome analysis of pearl sac in pearl oyster Pinctada martensii

Pearl oyster Pinctada martensii is cultured for production of pearl in China. It needs to implant a mantle graft cut from a donor oyster and a seed nucleus into the gonad of the host oyster to produce a pearl. Pearl sac surrounding the nucleus is formed by the proliferation of the implanted mantle graft from the outer mantle epithelial cells in the host oyster. The pearl sac is responsible for production of a cultured pearl. A comprehensive transcriptome analysis on pearl sac will help to understand the mechanism on pearl formation and immune response of host oyster after nucleus implantation. In the present study, 39,400,004 reads were produced from the pearl sac using RNA-sequence technology and then assembled into 102,762 unigenes. More than 22.4% of these unigenes were possibly involved in approximately 219 known signaling pathways. A total of 37,188 unigenes were annotated based on sequences similarities with known proteins. Fifty-one biomineralization-related unigenes and 268 immune-related unigenes were not previously detected in P. martensii. The un-annotated unigenes may be some genes specifically existed in P. martensii. These annotated or un-annotated unigenes in the present studies were valuable for the future investigation on molecular mechanism of pearl formation and immune response of the species.

Knowing your friends: invertebrate innate immunity fosters beneficial bacterial symbioses

The innate immune system is present in all animals and is a crucial first line of defence against pathogens. However, animals also harbour large numbers of beneficial microorganisms that can be housed in the digestive tract, in specialized organs or on tissue surfaces. Although invertebrates lack conventional antibody-based immunity, they are capable of eliminating pathogens and, perhaps more importantly, discriminating them from other microorganisms. This Review examines the interactions between the innate immune systems of several model invertebrates and the symbionts of these organisms, and addresses the central question of how these long-lived and specific associations are established and maintained.

Immune role of MrNFκBI-α, an IκB family member characterized in prawn M. rosenbergii

NF kappa B inhibitor alpha (MrNFκBI-α) was sequenced from a freshwater prawn Macrobrachium rosenbergii. The MrNFκBI-α protein contains a long ankyrin repeat region circular domain between 193 and 413 along with its 6 repeats (ankyrin repeat 1,2,3,4,5 and 6). An IκB degradation motif and a putative PEST motif is present at 37-64 and 418-471 of the N- and C-terminal regions of MrNFκBI-α respectively. The gene expressions of MrNFκBI-α in healthy and infectious hematopoietic and hypodermal necrosis virus (IHHNV), poly I:C, Aeromonas hydrophila and Enterococcus faecium injected M. rosenbergii were examined using quantitative real time PCR. The MrNFκBI-α is expressed in all the tissues taken for examination and the highest is observed in hemocytes. The MrNFκBI-α gene expression is strongly up-regulated in hemocytes of prawn after IHHNV, poly I:C, A. hydrophila and E. faecium infection. This result indicates an important role of MrNFκBI-α in M. rosenbergii immune system. This, however, remains to be verified by further studies.

Molecular cloning and characterization of class I NF-κB transcription factor from pearl oyster (Pinctada fucata)

NF-κB transcription factors play central roles in many important physiological and pathological processes including innate immune responses. Here we report the cloning of an NF-κB transcription factor, PfRelish from pearl oyster Pinctada fucata, one of the most important bivalve mollusks for seawater pearl production. PfRelish full-length cDNA is 3916 bp with an open reading frame of 3558 bp encoding a putative protein of 1186 amino acids. The deduced PfRelish contains a N-terminal RHD, a nucleus localization signal, an IκB-like domain with six ankyrin repeats and a death domain at the C-terminus, which is similar to class I NF-κB transcription factors. Comparison and phylogenetic analysis revealed that class I NF-κBs in mollusks including PfRelish might have most distant relationship to the arthropod Relish. Further expression analysis showed that PfRelish was apparently upregulated after Vibrio alginolyticus injection, which suggested that PfRelish was involved in the immune response to V. alginolyticus.