NOD1 and NOD2 receptors in mrigal (Cirrhinus mrigala): Inductive expression and downstream signalling in ligand stimulation and bacterial infections

The interactions between the host immunity and intestinal microorganisms in fish

There is a huge quantity of microorganisms in the gut of fish, which exert pivotal roles in maintaining host intestinal and general health. The fish immunity can sense and shape the intestinal microbiota and maintain the intestinal homeostasis. In the meantime, the intestinal commensal microbes regulate the fish immunity, control the extravagant proliferation of pathogenic microorganisms, and ensure the intestinal health of the host. This review summarizes developments and progress on the known interactions between host immunity and intestinal microorganisms in fish, focusing on the recent advances in zebrafish (Danio rerio) showing the host immunity senses and shapes intestinal microbiota, and intestinal microorganisms tune host immunity. This review will offer theoretical references for the development, application, and commercialization of intestinal functional microorganisms in fish. KEY POINTS: • The interactions between the intestinal microorganisms and host immunity in zebrafish • Fish immunity senses and shapes the microbiota • Intestinal microbes tune host immunity in fish.

Exendin-4 blockade of T1R2/T1R3 activation improves Pseudomonas aeruginosa-related pneumonia in an animal model of chemically induced diabetes

OBJECTIVE

Poorly controlled diabetes frequently exacerbates lung infection, thereby complicating treatment strategies. Recent studies have shown that exendin-4 exhibits not only hypoglycemic but also anti-inflammatory properties. This study aimed to explore the role of exendin-4 in lung infection with diabetes, as well as its association with NOD1/NF-κB and the T1R2/T1R3 sweet taste receptor.

METHODS

16HBE human bronchial epithelial cells cultured with 20 mM glucose were stimulated with lipopolysaccharide (LPS) isolated from Pseudomonas aeruginosa (PA). Furthermore, Sprague‒Dawley rats were fed a high-fat diet, followed by intraperitoneal injection of streptozotocin and intratracheal instillation of PA. The levels of TNF-α, IL-1β and IL-6 were evaluated using ELISAs and RT‒qPCR. The expression of T1R2, T1R3, NOD1 and NF-κB p65 was assayed using western blotting and immunofluorescence staining. Pathological changes in the lungs of the rats were observed using hematoxylin and eosin (H&E) staining.

RESULTS

At the same dose of LPS, the 20 mM glucose group produced more proinflammatory cytokines (TNF-α, IL-1β and IL-6) and had higher levels of T1R2, T1R3, NOD1 and NF-κB p65 than the normal control group (with 5.6 mM glucose). However, preintervention with exendin-4 significantly reduced the levels of the aforementioned proinflammatory cytokines and signaling molecules. Similarly, diabetic rats infected with PA exhibited increased levels of proinflammatory cytokines in their lungs and increased expression of T1R2, T1R3, NOD1 and NF-κB p65, and these effects were reversed by exendin-4.

CONCLUSIONS

Diabetic hyperglycemia can exacerbate inflammation during lung infection, promote the increase in NOD1/NF-κB, and promote T1R2/T1R3. Exendin-4 can ameliorate PA-related pneumonia with diabetes and overexpression of NOD1/NF-κB. Additionally, exendin-4 suppresses T1R2/T1R3, potentially through its hypoglycemic effect or through a direct mechanism. The correlation between heightened expression of T1R2/T1R3 and an intensified inflammatory response in lung infection with diabetes requires further investigation.

Antiviral effect of miR-206 in rainbow trout (Oncorhynchus mykiss) against infectious hematopoietic necrosis virus infection

Infectious hematopoietic necrosis (IHN), caused by IHN virus, is a highly contagious and lethal disease that seriously hampers the development of rainbow trout (Oncorhynchus mykiss) aquaculture. However, the immune response mechanism of rainbow trout underlying IHNV infection remains largely unknown. MicroRNAs act as post-transcriptional regulators of gene expression and perform a crucial role in fish immune response. Herein, the regulatory mechanism and function of miR-206 in rainbow trout resistance to IHNV were investigated by overexpression and silencing. The expression analysis showed that miR-206 and its potential target receptor-interacting serine/threonine-protein kinase 2 (RIP2) exhibited significant time-dependent changes in headkidney, spleen and rainbow trout primary liver cells infected with IHNV and their expression displayed a negative correlation. In vitro, the interaction between miR-206 and RIP2 was verified by luciferase reporter assay, and miR-206 silencing in rainbow trout primary liver cells markedly increased RIP2 and interferon (IFN) expression but significantly decreased IHNV copies, and opposite results were obtained after miR-206 overexpression or RIP2 knockdown. In vivo, overexpressed miR-206 with agomiR resulted in a decrease in the expression of RIP2 and IFN in liver, headkidney and spleen. This study revealed the key role of miR-206 in anti-IHNV, which provided potential for anti-viral drug screening in rainbow trout.

Functional characterization of NOD1 from golden pompano Trachinotus ovatus

Fish rely on innate immune system for immunity, and nucleotide-binding oligomerization domain-like receptors (NLRs) are a vital group of receptor for recognition. In the present study, NOD1 gene was cloned and characterized from golden pompano Trachinotus ovatus, a commercially important aquaculture fish species. The ORF of T. ovatus NOD1 was 2820 bp long, encoding 939 amino acid residues with a highly conserved domains containing CARD-NACHT-LRRs. Phylogenetic analysis revealed that the T. ovatus NOD1 clustered with those of fish and separated from those of birds and mammals. T. ovatus NOD1 has wide tissue distribution with the highest expression in gills. Bacterial challenges (Streptococcus agalactiae and Vibrio alginolyticus) significantly up-regulated the expression of NOD1 with different response time. The results of T. ovatus NOD1 ligand recognition and signaling pathway analysis revealed that T. ovatus NOD1 could recognize iE-DAP at the concentration of ≧ 100 ng/mL and able to activate NF-κB signaling pathway. This study confirmed that NOD1 play a crucial role in the innate immunity of T. ovatus. The findings of this study improve our understanding on the immune function of NOD1 in teleost, especially T. ovatus.

Short peptidoglycan recognition protein 5 modulates immune response to bacteria in Indian major carp, Cirrhinusmrigala

Peptidoglycan recognition proteins (PGRPs) function in host antibacterial responses by recognizing bacterial peptidoglycan (PGN). In the present study, a short pgrp5 (named mpgrp5) was identified in Cirrhinus mrigala (mrigal). The full-length cDNA of the mpgrp5 gene was 1255 bp, containing an open reading frame of 746 bp encoding a protein of 248 amino acids. The predicted protein contained the typical Pgrp/amidase domain, conserved Zn2+, and PGN binding residues. The phylogenetic analysis revealed that the mpgrp5 is closely related to Pgrps reported in Labeo rohita, Cyrinus carpio, and Ctenopharyngodon idella. The ontogenetic expression of mpgrp5 was highest at 7 days post-hatching (dph) and its possible maternal transfer. mpgrp5 was constitutively expressed in all tissues examined, with the highest expression observed in the intestine. Furthermore, mpgrp5 was found upregulated in mrigal post-challenge in a time-dependent manner at 6hpi in the liver (3.16 folds, p < 0.05) and kidney (2.79 folds, p < 0.05) and at 12hpi in gill (1.90 folds, p < 0.01), skin (1.93 folds, p < 0.01), and intestine, (2.71 folds, p < 0.05) whereas at 24hpi in spleen (4.0 folds, p < 0.01). Our results suggest that mpgrp5 may play an important role in antibacterial immune response from early life stages in mrigal.

Identification, characterization and the inflammatory regulating effect of NOD1/2 in sturgeon

As an ancient species with both conservation and commercial value, Sturgeon's inflammatory regulation mechanism is a research point. Nucleotide-binding and oligomerization domain-containing proteins 1 and 2 (NOD1/2) are classical intracellular pattern recognition receptors (PRRs) in immunity of anti-bacterial infection. However, the characterization and function of NOD1/2 in Sturgeon are still unclear. In this study, we analyzed the synteny relationship of NOD1/2 genes between Acipenser ruthenus and representative fishes at the genome-level. Results showed that the ArNOD2 collinear genes pair was present in all representative fishes. The duplicated ArNOD1/2 genes were under purifying selection during evolution as indicated by their Ka/Ks values. To explore the function of NOD1/2, we further investigated their expression patterns and the effects of pathogenic infection, PAMPs treatment, and siRNA interference in Acipenser baerii, the sibling species of A. ruthenus. Results showed that both AbNOD1/2 were expressed at early developmental stages and in different tissues. Pathogenic infection in vivo and PAMPs treatment in vitro demonstrated that AbNOD1/2 could respond to pathogen stimulation. siRNA interference with AbNOD1/2 inhibited expression levels of RIPK2 and inflammatory cytokines compared to the control group after iE-DAP or MDP treatment. This study hinted that the AbNOD1/2 could stimulate the inflammatory cytokines response during evolutionary processes.

Recombinant Attenuated Edwardsiella piscicida Vaccine Displaying Regulated Lysis to Confer Biological Containment and Protect Catfish against Edwardsiellosis

We implemented a unique strategy to construct a recombinant attenuated Edwardsiella vaccine (RAEV) with a biological containment phenotype that causes regulated bacterial cell wall lysis. This process ensures that the vaccine strain is not able to persist in the environment. The murA gene is responsible for the catalysis of one of the first steps in the biosynthesis of muramic acid, which is a crucial component of the bacterial cell wall. The regulated lysis phenotype was achieved by inserting the tightly regulated araC ParaBAD cassette in place of the chromosomal murA promoter. Strains with this mutation require growth media supplemented with arabinose in order to survive. Without arabinose, they are unable to synthesize the peptidoglycan cell wall. Following the colonization of fish lymphoid tissues, the murA protein is no longer synthesized due to the lack of arabinose. Lysis is subsequently achieved in vivo, thus preventing the generation of disease symptoms and the spread of the strain into the environment. Vaccine strain χ16016 with the genotype ΔPmurA180::TT araC ParaBADmurA is attenuated and shows a higher LD50 value than that of the wild-type strain. Studies have demonstrated that χ16016 induced TLR4, TLR5, TLR8, TLR9, NOD1 and NOD2-mediated NF-κB pathways and upregulated the gene expression of various cytokines, such as il-8, il-1β, tnf-a, il-6 and ifn-γ in catfish. We observed significant upregulation of the expression profiles of cd4, cd8 and mhc-II genes in different organs of vaccinated catfish. Vaccine strain χ16016 induced systemic and mucosal IgM titers and conferred significant protection to catfish against E. piscicida wild-type challenge. Our lysis RAEV is the first live attenuated vaccine candidate designed to be used in the aquaculture industry that displays this biological containment property.

A Comparative Analysis on the Innate Immune Responses of Cirrhinus mrigala Challenged with Pseudomonas aeruginosa and Fusarium oxysporum

Microbes are the most significant ubiquitous pathogens that cause serious infections in freshwater fish, leading to tremendous economic losses. The present study was designed to investigate the extent of changes in cytokine expression, hemato-biochemical parameters, and tissue histology of Cirrhinus mrigala (C. mrigala) challenged with Pseudomonas aeruginosa (P. aeruginosa) and Fusarium oxysporum (F. oxysporum). Fish were divided into three major groups: control, P. aeruginosa-challenged, and F. oxysporum-challenged. The infection in both challenge assays was allowed to progress until 7 days post infection. Upregulated expression of TNF-α and IL-1β was found in blood, gills, livers, and kidneys of the challenged fish. Significant differences were noted in hematological parameters of challenged fish. Alanine aminotransferase, aspartate aminotransferase, and alkaline aminotransferase levels also showed significant differences in infected and control groups. An increase in serum albumin and globulin and a decrease in total protein were noted in infected groups as compared to the control group. Severe histological alterations were noted in gill, liver, and kidney tissues of the infected groups as compared to control. The order of histological alteration index for P. aeruginosa challenge was liver > kidney > gills, and for F. oxysporum challenge it was kidney > liver > gills. These changes in fish infected by P. aeruginosa and F. oxysporum can be used as an effective and subtle index to monitor the physiological and pathological conditions of fish.

Identification and expression profiling of receptor-interacting serine/threonine-protein kinase 2 in Siberian sturgeon (Acipenser baerii)

Receptor-interacting serine/threonine-protein kinase 2 (RIPK2) is an adaptor protein of the pattern recognition receptors NOD1 and NOD2 involved in regulating inflammatory response and resisting pathogenic microbial infection. In this study, Acipenser baerii RIPK2 (AbRIPK2) was identified. The open reading frame of AbRIPK2 was 1815 bp encoding 604 amino acids. AbRIPK2 possessed the typical N-terminal kinase domain (KD) and C-terminal caspase recruitment domain (CARD). The phylogenetic tree analysis revealed that AbRIPK2 shared a relatively high identity with bony fish. Real-time fluorescence quantitative PCR (qRT-PCR) results indicated that AbRIPK2 was highly expressed in the gill, followed by muscle, liver and heart. AbRIPK2 was significantly induced in the spleen and valvular intestine after Streptococcus iniae and Aeromonas hydrophila infection. AbRIPK2 was significantly upregulated after peptidoglycan (PGN) treatment in the splenic leukocytes. This study indicated that AbRIPK2 played a potential role in resisting the pathogenic infection of Siberian sturgeon by responding to bacteria.

Non-coding RNAs targeting NF-κB pathways in aquatic animals: A review

Nuclear factor-kappa B (NF-κB) pathways have a close relationship with many diseases, especially in terms of the regulation of inflammation and the immune response. Non-coding RNAs (ncRNAs) are a heterogeneous subset of endogenous RNAs that directly affect cellular function in the absence of proteins or peptide products; these include microRNAs (miRNAs), long noncoding RNAs (lncRNAs), circular RNAs (circRNAs), etc. Studies on the roles of ncRNAs in targeting the NF-κB pathways in aquatic animals are scarce. A few research studies have confirmed detailed regulatory mechanisms among ncRNAs and the NF-κB pathways in aquatic animals. This comprehensive review is presented concerning ncRNAs targeting the NF-κB pathway in aquatic animals and provides new insights into NF-κB pathways regulatory mechanisms of aquatic animals. The review discusses new possibilities for developing non-coding-RNA-based antiviral applications in fisheries.

Construction and Evaluation of Recombinant Attenuated Edwardsiella piscicida Vaccine (RAEV) Vector System Encoding Ichthyophthirius multifiliis (Ich) Antigen IAG52B

We have successfully designed and constructed a RAEV vector system with regulated-delayed attenuation in vivo attributes that synthesizes Ichthyophthirius multifiliis (Ich) protective antigen IAG52B to enable vaccination of fish susceptible to edwardsiellosis and white spot disease. The first feature of this vaccine delivery system is an Edwardsiella piscicida strain carrying genomic deletions of asdA. AsdA is an enzyme necessary for the synthesis of diaminopimelic acid (DAP), which is an essential component of the peptidoglycan layer of the cell wall of Gram-negative bacteria. asdA mutant strains have obligate growth requirements for DAP in the medium or a plasmid vector with the wild-type asdA gene enabling synthesis of DAP. This balanced-lethal plasmid vector-host system in E. piscicida enables as a second feature the synthesis of recombinant antigens to induce protective immunity against fish pathogens. Recombinant protective antigen IAG52B from the fish pathogen I. multifiliis was synthesized by RAEV strains harboring the AsdA⁺ plasmid pG8R8029. The third feature of this vaccine strain is a regulated-delayed attenuation in vivo phenotype that is based on the replacement of an arabinose-regulated araC P_araBAD cassette for the promoters of the fur and crp genes of E. piscicida such that the expression of these genes is dependent on arabinose provided during growth. Thus, following colonization, the Fur and Crp proteins stop being synthesized due to the lack of arabinose and attenuation is progressively achieved in vivo to prevent generation of diseases symptoms. Our vaccine strain χ16022 with the genotype ΔasdA10 ΔP_fur170::TT araC P_araBAD fur ΔP_crp68::TT araC P_araBAD crp contains the AsdA⁺ plasmid, pG8R8029, which encodes the IAG52B antigen. Vaccine strain χ16022(pG8R8029) is attenuated and induces systemic and mucosal IgM titer against E. piscicida and Ich in zebrafish. In addition, transcript levels of tnf-α, il-1β, il-6 and il-8 were significantly increased in different tissues of vaccinated zebrafish compared to unimmunized fish. Zebrafish vaccinated with χ16022(pG8R8029) showed 60% survival upon intracoelomic (i.c.) challenge with a lethal dose of virulent E. piscicida strain J118. Our RAEV system could be used as a generalized vaccine-vector system to protect teleost fish against multiple bacterial, viral and parasitic infectious diseases.

A genome-wide survey of NOD-like receptors in Chinese tongue sole (Cynoglossus semilaevis): Identification, characterization and expression analysis in response to bacterial infection

As intracellular pathogen recognition receptors (PRRs), nucleotide-binding domain, leucine-rich repeat containing receptors (NLRs, NOD-like receptors) are involved in innate immune responses in vertebrates. However, there is no systemic study on NLRs in Chinese tongue sole (Cynoglossus semilaevis), a popular maricultured fish in China. In the present study, a genome-wide survey of NLRs was performed in C. semilaevis, with the identification of 29 NLRs, including five genes from the NLR-A subfamily (referred to as CsNOD1-5), two genes from the NLR-B subfamily, 18 genes from the NLR-C subfamily (referred to as CsNLR-C1 to 18) and four other NLR genes. Phylogenetic analysis implied that CsNOD1-5 contained conserved functional domains and had orthologous relationships with human NOD1-5. Moreover, CsNLR-C genes all possessed the FISHNA domain, which is a fish-specific NACHT subdomain. Expression analysis showed that CsNOD1-5 and CsNLR-C1/2 were ubiquitously expressed in various normal tissues. Bacterial infection with Vibro harveyi revealed distinct expression patterns of all the tested CsNLRs in gill, intestine, trunk kidney, liver and spleen. In particular, CsNOD1-4 and CsNLR-C2 were significantly upregulated in gills at 48 h post bacterial infection. In addition, CsNOD3 and CsNOD4 were significantly elevated in infectious intestine, trunk kidney, liver and spleen, revealing that their expressions were more sensitive to bacterial infection than other CsNLRs. Together with the computational protein-protein interaction network of CsNLRs, it was suggested that individual NLR genes had different roles in the innate immune cascades of C. semilaevi against bacterial infection. This study provides valuable information for further studies on CsNLR immune function.

Identification and functional analysis of NOD2 and its two splicing variants associated with a novel pattern of signal regulation in teleost fishes

The nucleotide-binding oligomerization domain 2 (NOD2) has been identified as an important sensor for microorganic invasion in both mammals and teleost fishes. In this study, two splicing variants of NOD2 (NOD2-v1 and NOD2-v2) were identified as truncating the functional domains of wild-type NOD2 in the teleost fish Schizothorax prenanti. NOD2-v1 included an intron sequence that terminated within the third leucine-rich repeat (LRR) domain, while NOD2-v2 incorporated an insertion of one and half intron sequences and truncated within the second caspase activation and recruitment domain (CARD). NOD2, NOD2-v1 and NOD2-v2 genes were ubiquitously expressed. All three genes positively responded to exposure of Aeromonas hydrophila and lipopolysaccharide stimulation in varying degrees. Using luciferase activity assays in HEK293T cells, our results revealed that NOD2 activated the NF-κB signal and recognized muramyl dipeptide (MDP). NOD2-v1 exhibited deficiency in the LRR domains and could not sense MDP, but maintained the ability to activate NF-κB and enhanced NOD2-mediated MDP recognition. Given the significant change to the functional structure, NOD2-v2 lost its capacity for NF-κB activation, but interestingly repressed NOD2-mediated MDP sensing and NF-κB activation, and even NOD2-v1-induced NF-κB activation. Altogether, our study reveals a novel pattern of signal regulation by splicing variants in teleost fishes.

Genome-wide identification of NOD-like receptors and their expression profiling in mucosal tissues of turbot (Scophthalmus maximus L.) upon bacteria challenge

The innate immune system plays an important role in host defense against pathogenic infections. In the innate immune system, several families of innate pattern recognition receptors, including Toll-like receptors, RIG-I-like receptors, NOD-like receptors (NLRs), and DNA receptors (cytosolic sensors for DNA), are known to play vital roles in detecting and responding to various pathogens. In this study, we identified 29 NLRs in turbot including 4 NLRs from subfamily A: NOD1, NOD2, CIITA, NLRC5, 1 NLR from subfamily B: NLRB1, 21 NLRs from subfamily C: NLR-C3.1∼NLRC3.21, 1 from NLRX subfamily, and two that do not fall within these subfamilies: APAF1, NWD1. Phylogenetic analysis showed that these NLR genes were clearly divided into five subfamilies. Protein-protein interaction network analysis showed that some of these NLR genes shared same interacting genes and might participate in signal transductions associated with immunity. The evolutionary pressure selection analysis showed that the Ka/Ks ratios for all detected NLR genes were much less than one, implying more synonymous changes than non-synonymous changes. In addition, tissue expression analysis showed that the relative higher expression levels were observed in gill, skin and intestine. Meanwhile, NLR genes expression after bacterial infection results showed that most NLR genes participated in the process of defense of V. anguillarum and A. salmonicida infections in mucosal tissues. Taken together, identification and expression profiling analysis of NLR genes can provide valuable information for further functional characterization of these genes in turbot.

Recognition of DAP and activation of NF-κB by cytosolic sensor NOD1 in Oreochromis niloticus

As a lower vertebrate, the immune defense mechanism of fish mainly depends on the innate immune system. Nucleotide-binding oligomerization domain-like receptors (NLRs) are an important class of pattern recognition receptors in the innate immune system. In this study, NOD1 gene was cloned and characterized in Nile tilapia (Oreochromis niloticus). The ORF of Nile tilapia NOD1 gene was 2826 bp long and encoded 941 amino acid residues with a structure of CARD-NACHT-LRRs that was similar to the other counterparts in mammals and fishes. Phylogenetic and synteny analysis showed that NOD1 was conserved among different fishes and existed at least in the early stage of fish evolution. Expression pattern revealed that NOD1 mRNA was constitutively expressed in the tested tissues, while had high expression level in main immune organs and mucosal immune tissues (liver, head kidney, spleen, blood, gill, and intestine). Following Streptococcus agalactiae challenge, Nile tilapia NOD1 mRNA expression levels were altered in immune organs (liver, head kidney, spleen, blood), and the expression pattern was similar in liver, spleen and blood. Furthermore, the ligand recognition and signaling pathway of Nile tilapia NOD1 were also analyzed, it showed that NOD1 could recognize Tri-DAP intracellularly and activated NF-κB signaling pathway. In summary, our results indicated that the Nile tilapia NOD1 may play an important role in innate immune system and provided a basis for the functional study of NOD1 in teleost.

The long noncoding RNA NARL regulates immune responses via microRNA-mediated NOD1 downregulation in teleost fish

Increasing evidence shows that the long noncoding RNA (lncRNA) is a major regulator and participates in the regulation of various physiological and pathological processes, such as cell proliferation, differentiation, metastasis, and apoptosis. Unlike mammals, however, the study of lncRNA in lower invertebrates is just beginning and the extent of lncRNA-mediate regulation remains unclear. Here, we for the first time identify an lncRNA, termed nucleotide oligomerization domain 1 (NOD1) antibacterial and antiviral-related lncRNA (NARL), as a key regulator for innate immunity in teleost fish. We found that NOD1 plays an important role in the antibacterial and antiviral process in fish and that the microRNA miR-217-5p inhibits NOD1 expression and thus weakens the NF-κB and the IRF3-driven signaling pathway. Furthermore, our results indicated that NARL functions as a competing endogenous RNA (ceRNA) for miR-217-5p to regulate protein abundance of NOD1; thus, invading microorganisms are eliminated and immune responses are promoted. Our study also demonstrates the regulation mechanism that lncRNA NARL can competitive adsorption miR-217-5p to regulate the miR-217-5p/NOD1 axis is widespread in teleost fish. Taken together, our results reveal that NARL in fish is a critical positive regulator of innate immune responses to viral and bacterial infection by suppressing a feedback to NOD1-NF-κB/IRF3-mediated signaling.

NOD-like receptor signaling pathway activation: A potential mechanism underlying negative effects of benzo(α)pyrene on zebrafish

Benzo(α)pyrene (BaP) is one of typical polycyclic aromatic hydrocarbons (PAHs) in aquatic environments and has been shown to cause toxic effects to aquatic animals. Although the negative effects of BaP have been investigated, the potential toxic mechanisms remain uncharacterized. To explore the potential mechanisms mediating the toxic effects of BaP, zebrafish (Danio rerio) were exposed to BaP for 15 days and the toxic effects of BaP in zebrafish liver were investigated using physiological and transcriptomic analyses. After 15-day BaP exposure, zebrafish liver exhibited abnormalities including increased cytoplasmic vacuolation, inflammatory cell infiltration, swelled nuclei and irregular pigmentation. BaP exposure also induced oxidative stress to the liver of zebrafish. Transcriptomic profiles revealed 5129 differentially expressed genes (DEGs) after 15-days of BaP exposure, and the vast majority of DEGs were up-regulated under BaP treatment. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses suggest that genes related to immune response were significantly dysregulated. Furthermore, the nucleotide-binding, oligomerization domain (NOD)-like receptor signaling pathway was significantly enriched and most of the genes in this pathway exhibited enhanced expression after BaP exposure. These results partially explained the mechanisms underlying the toxic effects of BaP on zebrafish liver. In conclusion, BaP has the potential to induce physiological responses in zebrafish liver through altering associated genes.

Effect of chrysophanic acid on immune response and immune genes transcriptomic profile in Catla catla against Aeromonas hydrophila

The effect of chrysophanic acid (CA) (2, 4, and 8 mg kg-1) on the immunity and immune-related gene profile of Catla catla against Aeromonas hydrophila is reported. In both control and treated groups fed with 2 mg kg-1 (2 CA), the phagocytosis, hemolytic, myeloperoxidase content, and superoxide anion production decreased significantly between 6th and 8th weeks, whereas when fed with 4 mg kg-1 CA (4 CA) the H2O2 production and nitric oxide synthase increased significantly between 4th and 8th week. When fed with 2 CA and 4 CA diets, the total protein, bactericidal, and antibody titer increased significantly from the 4th week onwards. When fed with 2 CA, the IL-1β and IL-10 mRNA expression of head kidney leucocytes were significant between weeks 6 and 8. The expressions of toll-like receptors significantly increased when fed with a 4 CA diet from 4th week onwards. The 4 CA group significantly increased in TNF-α, TNF receptor-associated factor 6 (NOD), which influences protein expression, after the 4th week. The mRNA transcription of MHCI, lysozyme-chicken and goose type expressions significantly increased in 4 CA group within the 4th week. In summary, the dietary administration of 4 mg kg-1 of CA (4 CA) provides better immunity and enhances the up-regulation of immune-related genes in Catla against A. hydrophila.

Pathogenicity and immunogenicity of Edwardsiella piscicida ferric uptake regulator (fur) mutations in zebrafish

Edwardsiella piscicida is the etiological agent of edwardsiellosis in fish and causes severe economic losses in global aquaculture. Vaccination would be the most effective method to prevent infectious diseases and their associated economic losses. The ferric uptake regulator (Fur) is an important transcriptional global regulator of Gram-negative bacteria. In this study, we examined the regulatory function of Fur in E. piscicida. We designed a strain that displays features of the wild-type virulent strain of E. piscicida at the time of immunization to enable strains first to effectively colonize lymphoid tissues and then to exhibit a regulated delayed attenuation in vivo to preclude inducing disease symptoms. Regulated delayed attenuation in vivo is based on the substitution of a tightly regulated araC P_araBAD cassette for the promoter of the fur gene such that expression of this gene is dependent on arabinose provided during growth. Thus, following E. piscicida mutant colonization of lymphoid tissues, the Fur protein ceases to be synthesized due to the absence of arabinose such that attenuation is gradually manifest in vivo to preclude induction of diseases symptoms. We deleted the promoter, including all sequences that interact with activator or repressor proteins, for the fur gene, and substituted the improved araC P_araBAD cassette to yield an E. piscicida strain with the ΔP_fur170:TT araC P_araBADfur deletion-insertion mutation (χ16012). Compared to the wild-type strain J118, χ16012 exhibited retarded growth and enhanced siderophore production in the absence of arabinose. mRNA levels of Fur-regulated genes were analyzed in iron deplete or replete condition in wild-type and fur mutant strains. We observed zebrafish immunized with χ16012 showed better colonization and protection compared to the Δfur (χ16001). Studies showed that E. piscicida strain χ16012 is attenuated and induces systemic and mucosal IgM titer in zebrafish. In addition, we found an increase in transcript levels of tnf-α, il-1β, il-8 and ifn-γ in different tissues of zebrafish immunized with χ16012 compared to the unimmunized group. We conclude that, E. piscicida with regulated delayed attenuation could be an effective immersion vaccine for the aquaculture industry.

Structure of fish Toll-like receptors (TLR) and NOD-like receptors (NLR)

Innate immunity driven by pattern recognition receptor (PRR) protects the host from invading pathogens. Aquatic animals like fish where the adaptive immunity is poorly developed majorly rely on their innate immunity modulated by PRRs like toll-like receptors (TLR) and NOD-like receptors (NLR). However, current development to improve the fish immunity via TLR/NLR signaling is affected by a poor understanding of its mechanistic and structural features. This review discusses the structure of fish TLRs/NLRs and its interaction with pathogen associated molecular patterns (PAMPs) and downstream signaling molecules. Over the past one decade, significant progress has been done in studying the structure of TLRs/NLRs in higher eukaryotes; however, structural studies on fish innate immune receptors are undermined. Several novel TLR genes are identified in fish that are absent in higher eukaryotes, but the function is still poorly understood. Unlike the fundamental progress achieved in developing antagonist/agonist to modulate human innate immunity, analogous studies in fish are nearly lacking due to structural inadequacy. This underlies the importance of exploring the structural and mechanistic details of fish TLRs/NLRs at an atomic and molecular level. This review outlined the mechanistic and structural basis of fish TLR and NLR activation.

MicroRNA negatively regulates NF-κB-mediated immune responses by targeting NOD1 in the teleost fish Miichthys miiuy

Inflammation is a self-protection mechanism that can be triggered when innate immune cells detect infection. Eradication of pathogen infection requires appropriate immune and inflammatory responses, but excessive inflammatory responses can cause uncontrolled inflammation, autoimmune diseases, or pathogen dissemination. Mounting evidence has shown that microRNAs (miRNAs) in mammals act as important and versatile regulators of innate immunity and inflammation. However, miRNA-mediated regulation networks are largely unknown in inflammatory responses in lower vertebrates. Here miR-144 and miR-217 are identified as negative regulators in teleost inflammatory responses. We find that Vibrio harveyi and lipopolysaccharide (LPS) treatment significantly upregulate the expression of fish miR-144 and miR-217. Upregulated miR-144 and miR-217 suppress LPS-induced inflammatory cytokine expression by targeting nucleotide-binding oligomerization domain-containing protein 1 (NOD1), thereby avoiding excessive inflammatory responses. In addition, miR-144 and miR-217 regulate inflammatory responses through NOD1-induced nuclear factor kappa (NF-kB) signaling pathways. These findings demonstrate that miR-144 and miR-217 play regulatory roles in inflammatory responses by modulating the NOD1-induced NF-κB signaling pathway.

Transcriptome Analysis of Paralichthys olivaceus Erythrocytes Reveals Profound Immune Responses Induced by Edwardsiella tarda Infection

Unlike mammalian red blood cells (RBCs), fish RBCs are nucleated and thus capable of gene expression. Japanese flounder (Paralichthys olivaceus) is a species of marine fish with important economic values. Flounder are susceptible to Edwardsiella tarda, a severe bacterial pathogen that is able to infect and survive in flounder phagocytes. However, the infectivity of and the immune response induced by E. tarda in flounder RBCs are unclear. In the present research, we found that E. tarda was able to invade and replicate inside flounder RBCs in both in vitro and in vivo infections. To investigate the immune response induced by E. tarda in RBCs, transcriptome analysis of the spleen RBCs of flounder challenged with E. tarda was performed. Six sequencing libraries were constructed, and an average of 43 million clean reads per library were obtained, with 85% of the reads being successfully mapped to the genome of flounder. A total of 1720 differentially expressed genes (DEGs) were identified in E. tarda-infected fish. The DEGs were significantly enriched in diverse Gene Ontology (GO) terms and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways, especially those associated with immunity, disease, and infection. Ninety-one key DEGs involved in 12 immune-related pathways were found to form extensive interaction networks. Twenty-one genes that constituted the hub of the networks were further identified, which were highly regulated by E. tarda and involved in a number of immune processes, notably pathogen recognition and signal transduction, antigen processing and presentation, inflammation, and splicing. These results provide new insights into the immune role of flounder RBCs during bacterial infection.

IκBα phosphorylation and associated NF-κB activation are essential events in lymphocyte activation, proliferation, and anti-bacterial adaptive immune response of Nile tilapia

Inhibitory protein IκBα plays a crucial role in the inflammatory process and immune response by regulating the activity of transcription factor NF-κB. In teleost, great progress has been achieved regarding NF-κB signaling for innate immunity, but whether this pathway modulates adaptive immunity, and how, remains largely unclear. In this study, after characterizing the sequence, structure, and phylogeny of Nile tilapia Oreochromis niloticus IκBα (defined as On-IκBα), we investigated the association between IκBα-regulated NF-κB activation and the lymphocyte-mediated adaptive immune response in Nile tilapia. We found that On-IκBα was evolutionarily conserved, and its mRNA was expressed widely in various tissues, with most abundance in the trunk kidney. mRNA expression of On-IκBα was significantly upregulated in spleen at both innate and adaptive immune stages after Aeromonas hydrophila infection. Moreover, phosphorylation of On-IκBα and the downstream On-NF-κB p65 was obviously elevated in spleen leukocytes at 3, 5, or 8 days after A. hydrophila infection, indicating the activation of NF-κB signaling. Correlating with the augmented protein phosphorylation, leukocyte proliferation was enhanced during the same immune stage, suggesting the potential association of IκBα and IκBα-regulated NF-κB signaling in the primary adaptive immune response. Although lymphocyte activation by the T cell-specific mitogen PHA did not alter On-IκBα mRNA expression significantly, lymphocyte activation by the agonist PMA obviously elevated On-IκBα and OnNF-κB p65 phosphorylation in spleen leukocytes. Together, the results suggest that IκBα phosphorylation and its regulated NF-κB activation are essential events associated with lymphocyte activation, proliferation, and anti-bacterial adaptive immune response in Nile tilapia. Our study aids to understand the regulatory mechanism of adaptive immunity in teleost.

Identification of a fish-specific NOD-like receptor subfamily C (NLRC) gene from common carp (Cyprinus carpio L.): Characterization, ontogeny and expression analysis in response to immune stimulation

Nucleotide-binding oligomerization domain (NOD)-like receptors (NLRs) are a large group of cytoplasmic pattern recognition receptors (PRRs), which play an important role in pathogen recognition and regulation of innate immune response. In fish, NLRs are divided into three distinct subfamilies: NLR-A resembling mammalian NODs, NLR-B resembling mammalian NALPs and fish-specific NLR-C. Presently, no data is available about the common carp NLR gene, and meanwhile the studies concerning fish NLR-C subfamily genes are relatively poor. In the present study, we cloned and characterized a novel NLRC gene (CcNLRC) from common carp. The full-length cDNA of CcNLRC was 3642 bp, with an ORF of 3078 bp encoding 1025 amino acids. CcNLRC appears to be unique to fish, consisting of a fish-specific NACHT associated (FISNA) domain, a NACHT domain, three LRR motifs and an extra B30.2 domain at C-terminus. Expression analysis revealed that CcNLRC was constitutively expressed in various healthy tissues, and during early developmental stages CcNLRC had two expression peaks (1 dpf and 24 dpf). In vivo stimulation with polyI:C and V. anguillarum showed significant up-regulation of CcNLRC expression in some immune-related tissues including liver, spleen, foregut, hindgut and skin. Additionally, in vitro study in common carp PBLs and HKLs stimulated with different ligands such as polyI:C, flagellin and PGN showed enhanced gene expression of CcNLRC. These results suggested that CcNLRC might play an important role in the innate immune defense of common carp against pathogen invasion.

Nucleotide-binding and oligomerization domain (NOD)-like receptors in teleost fish: Current knowledge and future perspectives

Nucleotide-binding and oligomerization domain (NOD)-like receptors (NLRs) are a group of intracellular pathogen recognition receptors (PRRs) that play key roles in pathogen recognition and subsequent activation of innate immune signalling pathways. Expressions of several NLR subfamily members, including NOD1, NOD2, NLR-C3, NLR-C5 and NLR-X1 have been reported in many different teleost fish species. These receptors are activated by a variety of ligands, including lipopolysaccharides (LPS), peptidoglycans (PGN) and polyinosinic-polycytidylic acid [Poly(I:C)]. Synthetic dsRNA and bacterial or viral infections are known to stimulate these receptors both in vitro and in vivo. In this review, we focus on the identification, expression and function of teleost NLRs in response to bacterial or viral pathogens. Additionally, NLR ligand specificity and signalling pathways involved in the recognition of bacterial or viral stimuli are also summarized. This review focuses on current knowledge in this area and provides future perspectives regarding topics in need of additional investigation. Understanding the response of innate immune system to bacterial or viral infections in diverse species could inform the development of more effective therapies and vaccines.

The NOD1 and NOD2 in mandarinfish (Siniperca chuatsi): molecular characterization, tissue distribution, and expression analysis

Background

NOD-like receptors (NLRs) are a family of cytoplasmic pattern recognition receptors (PRRs), of which NOD1 and NOD2, are the main representative members. Many investigations have focused on the role of NOD1 and NOD2 in the innate immune response in Cypriniformes and Siluriformes. As an important economic fish in Perciformes, little is known about the function of NOD1 and NOD2 in mandarinfish (Siniperca chuatsi).

Results

The full-length NOD1 and NOD2 cDNA sequence was obtained using reverse transcription polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). The mandarinfish NOD1 and NOD2 cDNA sequences contain 3247 bp and 3257 bp, and encode 918 amino acids and 988 amino acids, respectively. Multiple sequence alignments showed that mNOD1 and mNOD2 share high similarity with that from other vertebrates. RT-PCR analysis revealed that relatively high levels of mNOD1 and mNOD2 mRNA were detected in gill and head kidney tissues, compared with the heart, spleen, liver, muscle, and intestine. In addition, the relative levels of mNOD1 and mNOD2 transcripts were significantly upregulated in three tissues when the fishes were challenged with LPS and Poly I:C, interestingly, the NOD1 mRNA got peaked earlier than NOD2 after LPS induction in the spleen, gill, and head kidney, and during Poly I:C treatment, the NOD2 mRNA got peaked at 8 h in spleen and gill, while NOD1 showed significant higher expression at 24 h post infection, besides, in head kidney, the NOD2 mRNA showed a great increasing trend and NOD1 got peaked at 16 h. Therefore the mNOD1 and mNOD2 may act differently within different tissues in different time during antiviral and antibacterial defense.

Conclusions

These results revealed the dynamic mNOD1 and mNOD2 expression during viral and bacterial infections, which suggested the NOD1 and NOD2 play important roles in innate immune of mandarinfish.

Electronic supplementary material

The online version of this article (10.1186/s12863-018-0667-y) contains supplementary material, which is available to authorized users.

Nanoconjugation of bicistronic DNA vaccine against Edwardsiella tarda using chitosan nanoparticles: Evaluation of its protective efficacy and immune modulatory effects in Labeo rohita vaccinated by different delivery routes

DNA-based immunization has proven to be an effective prophylactic measure to control aquatic animal diseases. In order to improve the efficiency of vaccine against fish pathogen, novel delivery mechanism needs to be adopted. In the present study we nanoconjugated the previously constructed DNA vaccine (pGPD + IFN) with chitosan nanoparticles (CNPs) by complex coacervation process. After construction of the vaccine, an in vivo vaccination trial was conducted in which 2 groups of rohu (L. rohita) fingerlings were vaccinated with CNPs-pGPD + IFN, one group by oral route (incorporated in feed for 14 days) and the other by immersion route (primary and booster immunised), whereas, a third group was intramuscularly (I/M) injected (initial and booster immunised) with naked pGPD + IFN and subsequently challenged with E. tarda (8.7 × 10⁴ CFU/fish) at 35-day post initial vaccination. The protective immune responses were determined in terms of relative percentage survival (RPS), specific antibody production, non-specific immune response, expression kinetics of immune-related genes and pathological manifestation. Evaluation of RPS analysis revealed that CNPs-pGPD + IFN groups recorded highest RPS (81.82% and 72.73% in oral and immersion vaccinated fish group respectively) while the naked pGPD + IFN injected group showed 63.62% RPS when compared with 55% cumulative mortality of control group. In addition, NBT, myeloperoxidase activity, serum lysozyme activity and specific antibody titre in case of CNPs-pGPD + IFN groups showed higher activities during all the time points. Furthermore, CNPs-pGPD + IFN groups showed significant (p < 0.05) upregulation of different immune gene transcripts (IgHC, iNOS, TLR22, NOD1 and IL-1β) in three immunologically important tissues post immunization (both primary and booster dose) as well as after challenge. Thus, from this study, we can conclude that oral or immersion vaccination with CNPs-pGPD + IFN can orchestrate an effective immunisation strategy in organizing a coordinative immune response against E. tarda in L. rohita exhibiting minimum stress to the host with maximum efficacy.

Molecular characterization, expression and functional analysis of NOD1, NOD2 and NLRC3 in Nile tilapia (Oreochromis niloticus)

The nucleotide-binding oligomerization domain proteins NOD1, NOD2 and NLRC3 are cytoplasmic pattern recognition receptors (PRRs) of the Nod-like receptor (NLR) family. In the present study, the Nile tilapia (Oreochromis niloticus) NOD1 (ntNOD1), NOD2 (ntNOD2) and NLRC3 (ntNLRC3) genes were cloned and characterized. The full-length ntNOD1, ntNOD2 and ntNLRC3 genes were 3924, 3886 and 4574 bp, encoding 941, 986 and 1130 amino acids, respectively. The three Nod-like receptors have a NACHT domain and a C-terminal leucine-rich repeat (LRR) domain. In addition, ntNOD1 and ntNOD2 have a N-terminal CARD domain (ntNOD2 has two). Phylogenetic analysis showed that the three NLRs are highly conserved. Tissue expression analysis of the three receptors revealed that the highest mRNA and protein levels of ntNOD1, ntNOD2 and ntNLRC3 were in the spleen. The expression patterns of NLRs during embryonic development showed that the expression levels of ntNOD2 and ntNLRC3 significantly increased from 2 to 8 days post-fertilization (dpf). The expression levels of ntNOD1 significantly increased from 2 to 6 dpf, decreased at 7 dpf and then increased at 8 dpf. Upon stimulation with an intraperitoneal injection of Streptococcus agalactiae, expression levels of the ntNOD1, ntNOD2 and ntNLRC3 mRNA and protein were clearly altered in the blood, spleen, kidney, intestine and gill. Furthermore, after cotransfection with an NF-κB reporter plasmid, NF-κB activation in ntNOD1-overexpressing 293T cells significantly increased compared with that in control cells, before or after i-EDPA-stimulation. By contrast, compared with control, ntNOD2 and ntNLRC3 had no effect on NF-κB activation in 293T cells, when their potential ligands were not stimulated. However, after MDP-stimulation, ntNOD2 and ntNLRC3 overexpression increased NF-κB activation in 293T cells. NOD1 and NLRC3 were uniformly distributed throughout the cytoplasm in 293T cells, whereas NOD2 was distributed throughout the cytoplasm and nucleus. Our results indicate that the three Nod-like receptors are functionally conserved and may play pivotal roles in defense against pathogens such as Streptococcus agalactiae.

NOD1 is the innate immune receptor for iE-DAP and can activate NF-κB pathway in teleost fish

The innate immune system is the first line for organisms defense against microbial infection, and NOD-like receptors (NLRs) protein family is an important member of innate immunity effector molecules. It has been proved that NLRs are located in the endochylema and can senses of microbial products. NOD1 is one of the representatives of this family, it has been proved that in mammals, NOD1 can distinguish a specific muropeptide (G-d-glutamyl-meso-diaminopimelic acid, iE-DAP) which was derived from bacterial peptidoglycans. However, the NOD-mediated intracellular recognition of microorganisms remains largely uncharacterized in teleost fishes. In this study, we use miiuy croaker (Miichthys miiuy) as a model to determine NOD1 can response to the infection of Gram-negative bacteria and it is the receptor that can recognize of iE-DAP by LRRs domain, it can activate the NF-κB signaling pathway through recruit RIP2 to induce inflammatory response in teleost fishes. Results showed that NOD1 can recognize the components of Gram-negative bacteria and activate inflammatory response to resistance of bacterial infection. Our study can improve the knowledge on immune system of fishes and provide a theoretical basis for the study of prevention and treatment of fish diseases.

Tissue specific expression profile of some immune related genes in Labeo rohita to Edwardsiella tarda infection

Rohu (Labeo rohita), an Indian Major Carp (IMC) is an economically important aquaculture species in India. Inspite of the technological advances, infectious diseases caused by viruses, bacteria and parasites have been a major limiting factor in the development and profitability of fish farms. At present, information regarding the immune status of the Indian major carps is limited. This lack of knowledge is a major impediment for establishment of effective preventive measures against broad spectrum of infectious agents. The present study was undertaken to examine the modulation of few immune-regulatory genes: IgHC, NOD 1, TLR 22, iNOS and IL-1β during experimental infection of E. tarda in L. rohita to understand their role in pathogenesis. Rohu fingerlings were intra-peritoneally injected with Edwardsiella tarda (LD₅₀ dose of 8.7 × 10⁴ CFU/fish) and sampled for three immunologically important organs (kidney, liver and spleen) at different time intervals (zero hour or pre-challenge and 6 h, 12 h, 24 h, 48 h and 96 h post challenge). For absolute quantification of genes by real time RT-PCR, all the genes transcript were amplified from Poly I:C induced rohu lymphocytes and cloned in pTZ57R/T plasmid. Standard curves for each gene was generated from serially diluted plasmid bearing respective genes. Evaluation of copy number of different genes present in the tissue showed that the expression of IgHC, iNOS and IL-1β was highest in kidney followed by spleen and least in liver. While for NOD 1 and TLR 22 gene, liver showed higher expression than kidney and spleen. Further, the expression of IgHC, INOS, TLR 22, NOD 1 and IL-1β genes significantly differed (P < 0.05) in the E. tarda challenged fish when compared with pre-challenged control fish. Among the five genes we studied, the basal expression of TLR 22 gene was highest. The result also depicts that iNOS and NOD 1 are immediate responsive genes as their expression reached maximum level at 6-24 h post infection (hpi) after which the expression declined. In contrast, TLR 22 and IgHC gene transcript showed enhanced expression during the late phase of with maximum expression observed after 48 hpi and 96 hpi respectively. IL-1β, being the exception, showed high expression both at 24 hpi and 96 hpi. From this study, we conclude that these five immune genes have a definite role to play in the defense mechanism of host (L. rohita) against E. tarda.

Mutual Regulation of NOD2 and RIG-I in Zebrafish Provides Insights into the Coordination between Innate Antibacterial and Antiviral Signaling Pathways

Nucleotide-binding oligomerization domain-containing protein 2 (NOD2) and retinoic acid-inducible gene I (RIG-I) are two important cytosolic pattern recognition receptors (PRRs) in the recognition of pathogen-associated molecular patterns (PAMPs), initiating innate antibacterial and antiviral signaling pathways. However, the relationship between these PRRs, especially in teleost fish models, is rarely reported. In this article, we describe the mutual regulation of zebrafish NOD2 (DrNOD2) and RIG-I (DrRIG-I) in innate immune responses. Luciferase assays were conducted to determine the activation of NF-κB and interferon signaling. Morpholino-mediated knockdown and mRNA-mediated rescue were performed to further confirm the regulatory roles between DrNOD2 and DrRIG-I. Results showed that DrNOD2 and DrRIG-I shared conserved structural hallmarks with their mammalian counterparts, and activated DrRIG-I signaling can induce DrNOD2 production. Surprisingly, DrNOD2-initiated signaling can also induce DrRIG-I expression, indicating that a mutual regulatory mechanism may exist between them. Studies conducted using HEK293T cells and zebrafish embryos showed that DrRIG-I could negatively regulate DrNOD2-activated NF-κB signaling, and DrNOD2 could inhibit DrRIG-I-induced IFN signaling. Moreover, knocking down DrRIG-I expression by morpholino could enhance DrNOD2-initiated NF-κB activation, and vice versa, which could be rescued by their corresponding mRNAs. Results revealed a mutual feedback regulatory mechanism underlying NOD2 and RIG-I signaling pathways in teleosts. This mechanism reflects the coordination between cytosolic antibacterial and antiviral PRRs in the complex network of innate immunity.

A NLRC3-like gene from blunt snout bream (Megalobrama amblycephala): Molecular characterization, expression and association with resistance to Aeromonas hydrophila infection

NLRC (the nucleotide-oligomerization domain (NOD)-like receptor subfamily C) consists of teleost-specific NLRs (NOD-like receptors) and plays pivotal roles in microbial recognition and regulation of innate immune response. In this study, we cloned and characterized a NLRC3-like gene (MamNLRC3-like) from blunt snout bream (Megalobrama amblycephala) by using the quantitative real-time PCR method, and analyzed the correlation between its polymorphisms and resistance to Aeromonas hydrophila infection. The full length cDNA of MamNLRC3-like was 2863 bp, with a 5'-UTR of 169 bp, ORF of 2301 bp and 3'-UTR of 393 bp, encoding 766 amino acid residues. MamNLRC3-like consisted of a conserved NACHT domain, a fish-specific NACHT associated domain (FISNA) and a PYRIN effective domain. During early developmental stages, MamNLRC3-like was most highly expressed at 39.4 hpf (hours post fertilization, hatching stage) and constitutively detected in various healthy tissues. The expression of MamNLRC3-like was strongly induced by A. hydrophila infection and peaked in the head kidney, liver, intestine, and trunk kidney at 12 h post infection. Six SNPs (single nucleotide polymorphisms) from MamNLRC3-like, with 2 in introns and 4 in exons, were identified by direct sequencing, in which 6515C/T was a novel non-synonymous mutation. HRM (high resolution melting) genotyping of the 6 loci showed that locus 6515C/T SNP was associated with the resistance/susceptibility of blunt snout bream to A. hydrophila infection (P < 0.01). The CC genotype fish were more resistant than CT carriers (P < 0.01). This study suggests that the MamNLRC3-like gene might play an important role in the innate immunity of blunt snout bream and could be used as a candidate marker for genetic selection of A. hydrophila-resistant blunt snout bream.

Inductive expression of the NOD1 signalling pathway in chickens infected with Salmonella pullorum

1. The aim of this study was to describe the role of Nucleotide-binding oligomerization domain-containing protein 1 (NOD1) receptor signalling in chicken. 2. Tissue-specific expression analysis of NOD1, receptor-interacting serine-threonine kinase 2 (RIPK2), nuclear factor kappa B (NF-κB) and mitogen-activated protein kinase 11 (MAPK11 or p38) by quantitative real-time PCR (qRT-PCR) revealed their wide distribution in various organs and tissues. 3. Salmonella pullorum infection activated NOD1 receptor signalling in vivo and in vitro, resulting in significant induction of downstream signalling molecules RIPK2, NF-κB/p65, MAPK11/p38 and the effector molecules IL-1b and IL-8. 4. Activation of NOD1 by its agonist bacterial γ-D-glutamyl-meso-diaminopimelic acid (iE-DAP) in HD11 cells induced the adapter molecular RIPK2 and activated the NF-κB/p65 and MAPK11/p38 pathways, resulting in an increase in IL-8 but not IL-1β. Additionally, inhibition of NOD1 using NOD1-shRNA resulted in downregulation of RIPK2, MAPK11 and IL-8, while NF-κB/p65 and IL-1β were unaltered. 5. These results highlight the important role of NOD1 receptors in eliciting the innate immune response following pathogenic invasion in chicken.

Identification of Nod like receptor C3 (NLRC3) in Asian seabass, Lates calcarifer: Characterisation, ontogeny and expression analysis after experimental infection and ligand stimulation

Nod like receptors (NLRs) are a large group of cytoplasmic PRRs believed to play an important role in bacterial recognition in higher vertebrates. In this study, a novel Nod like receptor C3 (AsNLRC3) has been identified, cloned and characterised from Asian seabass, Lates calcarifer. The full-length AsNLRC3 transcript composed of a 4142 bp nucleic acid sequence encode for a protein of 1134 deduced amino acids. Three signature domains identified are conserved NACHT-domain, C-terminal LLR domain and N-terminal CARD effector domain. From the domain architecture and phylogenetic analysis, it was quite evident that AsNLRC3 is different from the NLR subfamily C of other teleosts. AsNLRC3 expressed in all the 11 tissues tested but highly expressed in tissues facing external environment such as gill, hindgut and midgut. The ontogenic expression profile of this receptor showed constitutive expression throughout the embryonic and larval developmental stages, which could be an innate immune strategy against different marine pathogens for larval survival. Infection with Vibrio alginolyticus and poly I:C induction showed an alteration of expression pattern in different tissues but did not show significant alteration in expression with Staphylococcus aureus infection. In vitro study in Asian seabass kidney cell line (SISK) stimulated with different ligands such as LPS, PGN and poly I:C showed considerable up-regulation at some of the time-points tested. These results suggest that AsNLRC3 can be a pivotal cytosolic innate immune receptor for recognizing wide array of pathogens in a euryhaline teleost model like Asian seabass in diverse environmental conditions.

Molecular cloning and functional analysis of nucleotide-binding oligomerization domain-containing protein 1 in rainbow trout, Oncorhynchus mykiss

NOD1 has important roles in innate immunity as sensor of microbial components derived from bacterial peptidoglycan. In this study, we identified genes encoding components of the NOD1 signaling pathway, including NOD1 (OmNOD1) and RIP2 (OmRIP2) from rainbow trout, Oncorhynchus mykiss, and investigated whether OmNOD1 has immunomodulating activity in a rainbow trout hepatoma cell line RTH-149 treated with NOD1-specific ligand (iE-DAP). The deduced amino acid sequence of OmNOD1 contained conserved CARD, NOD and LRR domains. Loss-of-function and gain-of-function experiments indicated that OmNOD1 is involved in the expression of pro-inflammatory cytokines. Silencing of OmNOD1 in RTH-149 cells treated with iE-DAP decreased the expression of IL-1β, IL-6, IL-8 and TNF-α. Conversely, overexpression of OmNOD1 resulted in up-regulation of IL-1β, IL-6, IL-8 and TNF-α expression. In addition, RIP2 inhibitor (gefitinib) significantly decreased the expression of these pro-inflammatory cytokines induced by iE-DAP in RTH-149 cells. These findings highlight the important role of NOD1 signaling pathway in fish in eliciting innate immune response.

Modulation of TLR2, TLR4, TLR5, NOD1 and NOD2 receptor gene expressions and their downstream signaling molecules following thermal stress in the Indian major carp catla (Catla catla)

Toll-like receptors (TLRs) and nucleotide binding and oligomerization domain (NOD) receptors are pattern recognition receptors (PRRs) that recognize pathogen-associated molecular patterns (PAMPs) and play crucial role in innate immunity. In addition to PAMPs, PRRs recognize endogenous molecules released from damaged tissue or dead cells [damage-associated molecular patterns (DAMPs)] and activate signaling cascades to induce inflammatory processes. In the aquatic environment, large variation in seasonal and diurnal water temperature causes heat and cold stresses in fish, resulting in tissue injury and mortality of fish. In the Indian subcontinent, catla (Catla catla) is an economically important freshwater fish species and is prone to thermal stresses. To investigate the response of pattern recognition receptors in thermal stress, we analyzed TLRs (TLR2, TLR4 and TLR5) and NOD (NOD1 and NOD2) receptors gene expression in catla following heat and cold stress. Analysis of tissue samples (gill, liver, kidney and blood) of the thermal stressed and control fish by quantitative real-time PCR (qRT-PCR) assay revealed significant (p < 0.05) induction of TLR2, TLR4 and NOD2 gene expression in majority of the tested tissues of the treated fish as compared to the control. The expression of TLR5 and NOD1 gene was also induced in the heat and cold stressed fish, but mostly restricted in the blood. The downstream signaling molecule of TLR and NOD signaling pathway viz., MyD88 (myeloid differentiation primary response gene 88) and RICK (receptor interacting serine-threonine protein kinase-2) was also induced in the thermal stressed fish suggesting the engagement of TLR and NOD signaling pathway during thermal stress.

Repeated handling compromises the immune suppression and improves the disease resistance in overwintering channel catfish (Ictalurus punctatus)

In winter, fish have suppressed immune functions and are susceptible to bacteria or virus which may lead to a high mortality. It is necessary to improve the immune response and disease resistance for overwintering fish. A recent study suggested that repeated handling increased the innate immune mechanisms and disease resistance in Senegalese sole. Therefore, the present study tested the hypothesis that appropriate repeated handling could compromise the immune depression and increase the disease resistance in channel catfish over winter. The experiment was carried out in field cages from Nov. 2012 to April 2013. Before the experiment, 35 fish with an average weight of 188.4 g were randomly assigned to one of six cages (2 m × 2 m × 2 m). Three cages were designed as the control group and did not receive any interfere. Fish in the other three cages received a weekly repeated handling of an air exposure for 5 min. Fish were not fed over winter. At the end of the trial, fish were challenged with Aeromonas hydrophila at a dose of 5.6 × 10(8) CFU ml(-1). The results showed that final body weight was not changed between groups. The spleen somatic index increased while the head kidney somatic index decreased in the unhandled fish after bacterial challenge, but these indices were not significantly changed in the repeated handled fish. Plasma cortisol levels in the control fish were induced at 6 h post challenge and then declined to the normal levels. However, plasma cortisol levels in the repeated handled fish did not show any significant change after bacteria challenge. The reduced inducement of heat shock protein 70 (HSP70) expression by repeated handling was observed in fish post bacterial challenge. After overwintering, repeated handled fish exhibited increased catalase (CAT) activities and reduced malondialdehyde (MDA) contents. Plasma total antioxidant capacity (TAOC), CAT and superoxide dismutase (SOD) activities of channel catfish were enhanced by repeated handling post bacterial challenge. The enhanced up-regulation of Interleukin 8 (IL8), IL1β-a, IL1β-b together with the immune related genes of Toll-like receptor 2 (TLR2), TLR3, nucleotide-binding oligomerization domain 1 (NOD1) and NOD2 by repeated handling was found in catfish after bacterial challenge. The present results indicated that a combination of signaling pathways through TLRs and NODs was involved in the innate immune response of the overwintering repeated handled channel catfish against invading bacteria.

Characterization of NLR-A subfamily members in miiuy croaker and comparative genomics revealed NLRX1 underwent duplication and lose in actinopterygii

The NOD-like receptors (NLRs, nucleotide-binding domain and leucine-rich repeat containing receptors) are a recently identified family of intracellular pathogen recognition receptors in vertebrates. Several subfamilies of NLRs have been characterized in mammals and implicated in immunity and apoptosis, but studies of NLRs in teleost species have been lacking. Here we analyzed three NLR-A subfamily members from miiuy croaker: NLRC3, NLRC5, and NLRX1. Structural analysis showed that miiuy croaker NLR-A subfamily members own the feature of 5'UTR intron which may influence their role in enhancing translation level. Comparative analysis revealed NLRX1 duplicated into NLRX1a and NLRX1b, then NLRX1a was lost in actinopterygii and NLRX1b formed NLRX1 that now we called. Simultaneously, molecular evolutionary analysis indicated that the ancestral lineages of NLRX1 in tetrapod and actinopterygii under positive selection pressure. The positively sites in actinopterygii are mainly located in NACHT domain which was the critical region for signal transduction, suggesting that the evolution of NLRX1 gene in the ancestor of actinopterygii is beneficial in immune response. Pathogens challenge demonstrated that the expressions of NLRC3 and NLRC5 in miiuy croaker were induced not only by Vibrio anguillarum but also by poly (I:C), whereas NLRX1 exhibited more sensitive response to bacteria than virus.

Comparative genomic and evolution of vertebrate NOD1 and NOD2 genes and their immune response in miiuy croaker

The nucleotide-binding oligomerization domain proteins NOD1 and NOD2 are important cytoplasmic pathogen recognition receptors which sense microbial infections molecules to induce innate immune response. In this study, the sequence analysis showed that NOD1 and NOD2 genes in miiuy croaker (miichthys miiuy, mmiNOD1 and mmiNOD2) share some highly conserved motifs that crucial for recognizing the bacterial and viral components. Quantitative expression analysis revealed mmiNOD1 and mmiNOD2 had the highest level of expression in liver. Induction experiments with Vibrio anguillarum indicated the different expression levels of mmiNOD1 and mmiNOD2 in liver, spleen and kidney. The expressions of mmiNOD1 and mmiNOD2 increased more significantly after Poly(I:C) stimulation, meanwhile, we carried out the expression analysis at the transcriptome level and the regulation of microRNAs. In addition, the evolutionary analysis showed that the ancestral lineages of NOD1 in bony fish detected one positively selected site, however, both the current lineages of NOD1 and NOD2 genes in bony fish underwent purifying selection indicating that NOD1 gene in the ancestor of bony fish experienced positive selection. To further understand the evolutionary pattern of NOD1 and NOD2 in vertebrates, we were the first to conduct comparative genomic analysis by comparing the number and synteny of NOD1 and NOD2. Combining the duplication of NOD1, the lost of NOD2 and the more conserved synteny of NOD2 than NOD1, we proposed that the hypothetical evolutionary pattern is different between NOD1 and NOD2.

Asparagine attenuates hepatic injury caused by lipopolysaccharide in weaned piglets associated with modulation of Toll-like receptor 4 and nucleotide-binding oligomerisation domain protein signalling and their negative regulators

Pro-inflammatory cytokines play a key role in many models of hepatic damage. In addition, asparagine (Asn) plays an important role in immune function. We aimed to investigate whether Asn could attenuate lipopolysaccharide (LPS)-induced liver damage. Forty-eight castrated barrows were allotted to four groups including: (1) non-challenged control; (2) LPS-challenged control; (3) LPS + 0.5% Asn; and (4) LPS + 1.0% Asn. After 19 d feeding with control, 0.5 or 1.0% Asn diets, pigs were injected with LPS or saline. Blood and liver samples were obtained at 4 h (early stage) and 24 h (late stage) post-injection. Asn alleviated liver injury, indicated by reduced serum aspartate aminotransferase and alkaline phosphatase activities linearly and quadratically; it increased claudin-1 protein expression linearly and quadratically at 24 h, and less severe liver morphological impairment at 4 or 24 h. In addition, Asn decreased mRNA expression of TNF-α and heat shock protein 70 (HSP70) linearly and quadratically at 4 h; it increased TNF-α mRNA expression, and HSP70 protein expression linearly and quadratically at 24 h. Moreover, Asn increased inducible NO synthase activity linearly and quadratically. Finally, Asn down-regulated the mRNA expression of Toll-like receptor 4 (TLR4) signalling molecules (TLR4, IL-1 receptor-associated kinase 1 (IRAK1), TNF-α receptor-associated factor 6), nucleotide-binding oligomerisation domain protein (NOD) signalling molecules (NOD1, NOD2 and their adaptor molecule receptor-interacting serine/threonine-protein kinase 2 (RIPK2)), and NF-κB p65 linearly or quadratically at 4 h. Oppositely, Asn up-regulated mRNA expressions of TLR4 and NOD signalling molecules (TLR4, myeloid differentiation factor 88, IRAK1, NOD2 and RIPK2), and their negative regulators (radioprotective 105, single Ig IL-1R-related molecule, Erbb2 interacting protein and centaurin β1) linearly or quadratically at 24 h. These results indicate that, in early and late stages of LPS challenge, Asn improves liver integrity and exerts different regulatory effects on mRNA expression of TLR4 and NOD signalling molecules.

Characterization and Inductive Expression Analysis of Interferon Gamma-Related Gene in the Indian Major Carp, Rohu (Labeo rohita)

Interferon gamma (IFN-γ) is one of the key cytokines that plays a major role against viral and intracellular bacterial infection. In addition to the IFN-γ gene, teleost fish possess a second copy known as IFN-γ-related (IFN-γrel) gene. This report describes structural and functional properties of IFN-γrel gene in the Indian major carp, rohu (Labeo rohita), a commercially important freshwater fish species in the Indian subcontinent. The rohu IFN-γrel gene consisted of four exons with three intervening introns and phylogenetically closely related to grass carp. The full-length IFN-γrel cDNA comprised 927 bp nucleotides with a single open reading frame of 504 bp, encoding 167 amino acids (aa) polypeptide with a signal peptide of 24 aa. The mature rohu IFN-γrel protein was 143 aa with a predicted molecular weight of 16.85 kDa. Basal expression analysis of IFN-γrel showed its wide range of expression in all examined tissues: The highest was in the skin and the lowest was in the liver. In response to LPS, poly I:C, iE-DAP, muramyl dipeptide stimulations, and bacterial infections, IFN-γrel gene expression was significantly (p<0.05) induced in treated fish tissues as compared with their control. The IFN-γrel was expressed as recombinant protein (rIFN-γrel) and confirmed through western blot. Stimulation of peripheral blood leukocytes with rIFN-γrel protein resulted in the activation of IFN-γ receptor and marked induction of inducible nitric oxide synthase gene expression. These results together may suggest the important role of IFN-γrel as an antimicrobial cytokine in fish.