Inflammatory immune response by lipopolysaccharide-responsive nucleotide binding oligomerization domain (NOD)-like receptors in the Japanese pufferfish (Takifugu rubripes)

In vitro analysis of the expression of inflammasome, antiviral, and immune genes in an Oreochromis niloticus liver cell line following stimulation with bacterial ligands and infection with tilapia lake virus

The inflammasome is a multimeric protein complex that plays a vital role in the defence against pathogens and is therefore considered an essential component of the innate immune system. In this study, the expression patterns of inflammasome genes (NLRC3, ASC, and CAS-1), antiviral genes (IFNγ and MX), and immune genes (IL-1β and IL-18) were analysed in Oreochromis niloticus liver (ONIL) cells following stimulation with the bacterial ligands peptidoglycan (PGN) and lipopolysaccharide (LPS) and infection with TiLV. The cells were stimulated with PGN and LPS at concentrations of 10, 25, and 50 µg/ml. For viral infection, 106 TCID50 of TiLV per ml was used. After LPS stimulation, all seven genes were found to be expressed at specific time points at each of the three doses tested. However, at even higher doses of LPS, NLRC3 levels decreased. Following TiLV infection, all of the genes showed significant upregulation, especially at early time points. However, the gene expression pattern was found to be unique in PGN-treated cells. For instance, NLRC3 and ASC did not show any response to PGN stimulation, and the expression of IFNγ was downregulated at 25 and 50 µg of PGN per ml. CAS-1 and IL-18 expression was downregulated at 25 µg of PGN per ml. At a higher dose (50 µg/ml), IL-1β showed downregulation. Overall, our results indicate that these genes are involved in the immune response to viral and bacterial infection and that the degree of response is ligand- and dose-dependent.

Regulation of NF-κB signaling by NLRC (NLRC3-like) gene in the common carp (Cyprinus carpio)

Among teleost NLRs, NLR-C subfamily is a large group of proteins that were teleost-specific and evolution analysis showed that NLR-Cs are most likely to evolve from NLRC3 gene (thus also called as NLRC3Ls). Presently, although there have been rich studies investigating teleost NLRC3 and NLRC3L, the data on the regulatory mechanism was limited. In this study, immune regulation of inflammatory signaling pathway mediated by common carp NLRC3L gene (CcNLRC) has been investigated. Confocal microscopy analysis showed that CcNLRC was located in cytoplasm, and in HEK293T cells, dual-luciferase reporter assay showed the regulation of NF-κB signaling by CcNLRC, in which CcNLRC could alter/decrease RIPK2-induced activation of NF-κB. These results indicated that CcNLRC may function as a negative NLR in the regulation of inflammatory response in common carp. Our data will allow to gain more insights into the molecular mechanism of teleost specific NLR (NLRC3L).

Copper induced intestinal inflammation response through oxidative stress induced endoplasmic reticulum stress in Takifugu fasciatus

Copper (Cu) pollution in aquaculture water has seriously threatened the healthy and sustainable development of the aquaculture industry. Recently, many researchers have studied the toxic effects of Cu exposure on fish. However, the relationship between endoplasmic reticulum stress (ERS) and the inflammatory response, as well as its possible mechanisms, remain unclear. Particularly, information related to fish intestines must be expanded. Our study initially investigated the mechanisms underlying intestinal toxicity and inflammation resulting from Cu-induced ERS in vivo and in vitro in Takifugu fasciatus. In vivo study, T. fasciatus were treated with different concentrations (control, 20, and 100 µg/L) of Cu exposure for 28 days, causing intestinal oxidative stress, ERS, inflammatory responses, and histopathological and ultrastructural damage. Transcriptomic data further showed that Cu exposure caused ERS, as well as inflammatory responses, in the intestinal tracts of T. fasciatus. In vitro experiments on the intestinal cells of T. fasciatus showed that Cu exposure treatment (7.5 µg/mL) for 24 h induced ERS and increased mitochondrial numbers and inflammatory responses. In contrast, the addition of 4-phenylbutyric acid (4-PBA) alleviated ERS and inflammatory response in the Cu-exposed group. Furthermore, the reactive oxygen species (ROS) inhibitor, N-Acetyl-l-cysteine (NAC), effectively alleviated Cu-induced ERS. In conclusion, our in vivo and in vitro studies have confirmed that oxidative stress triggers the ERS pathway, which is involved in the intestinal inflammatory response. Our study provides new insights into the relationship among Cu-induced oxidative stress, ERS, and inflammatory responses in fish, as well as for the healthy culture of fish in aqueous environments.

Investigation of the efficacy of diosmin against organ damage caused by bendiocarb in male Wistar albino rats

Bendiocarb is a carbamate insecticide, which is used more in indoor areas, especially against scorpions, spiders, flies, mosquitoes and cockroaches. Diosmin is an antioxidant flavonoid found mostly in citrus fruits. In this study, the efficacy of diosmin against the adverse effects of bendiocarb was investigated in rats. For this purpose, 60, 2-3 month-old male Wistar albino rats, weighing 150-200 g, were used. The animals were assigned to six groups, one of which was maintained for control purposes and five of which were trial groups. The control rats received only corn oil, which was used as a vehicle for diosmin administration in the trial groups. Groups 2, 3, 4, 5 and 6 were administered with 10 mg/kg.bw bendiocarb, 10 mg/kg.bw diosmin, 20 mg/kg.bw diosmin, 2 mg/kg.bw bendiocarb plus 10 mg/kg.bw diosmin, and 2 mg/kg.bw bendiocarb plus 20 mg/kg.bw diosmin, respectively, using an oral catheter, for 28 days. At the end of the study period, blood and organ (liver, kidneys, brain, testes, heart and lungs) samples were collected. Body weight and organ weights were determined. Compared to the control group, in the group given bendiocarb alone, firstly, body weight and liver, lung and testicular weights decreased. Secondly, tissue/plasma malondialdehyde (MDA) and nitric oxide (NO) levels increased, and glutathione (GSH) levels and superoxide dismutase (SOD), glutathione peroxidase (GSH-Px) (except for lung tissue), glutathione reductase (GR), and glucose-6-phosphate dehydrogenase (G6PD) activities decreased in all tissues and erythrocytes. Thirdly, catalase (CAT) activity decreased in erythrocytes and the kidney, brain, heart and lung tissues and increased in the liver and testes. Fourthly, while GST activity decreased in the kidneys, testes, lung and erythrocytes, an increase was observed in the liver and heart tissues. Fifthly, while serum triglyceride levels and lactate dehydrogenase (LDH), alkaline phosphatase (ALP) and pseudo-cholinesterase (PchE) activities decreased, aspartate aminotransferase (AST) and alanine aminotransferase (ALT) activities and blood urea nitrogen (BUN), creatinine and uric acid levels increased. Lastly, liver caspase 3, caspase 9 and p53 expression levels significantly increased. When compared to the control group, the groups treated with diosmin alone showed no significant difference for the parameters investigated. On the other hand, it was observed that the values of the groups treated with a combination of bendiocarb and diosmin were closer to the values of the control group. In conclusion, while exposure to bendiocarb at a dose of 2 mg/kg.bw for 28 days caused oxidative stress/organ damage, diosmin administration at doses of 10 and 20 mg/kg.bw reduced this damage. This demonstrated that diosmin has pharmaceutical benefits, when used for supportive treatment as well as radical treatment, against the potential adverse effects of bendiocarb.

Nigericin treatment activates endoplasmic reticulum apoptosis pathway in goldfish kidney leukocytes

Nigericin has been reported to induce apoptosis and pyroptosis in mammalian models. However, the effects and mechanism underlying the immune responses of teleost HKLs induced by nigericin remain enigmatic. To decipher the mechanism after nigericin treatment, the transcriptomic profile of goldfish HKLs was analyzed. The results demonstrated that a total of 465 differently expressed genes (DEGs) with 275 up-regulated and 190 down-regulated genes were identified between the control and nigericin treated groups. Among them, the top 20 DEG KEGG enrichment pathways were observed including apoptosis pathways. In addition, the expression level of selected genes (ADP4, ADP5, IRE1, MARCC, ALR1, DDX58) by quantitative real-time PCR showed a significant change after treatment with nigericin, which was generally identical to the expression patterns of the transcriptomic data. Furthermore, the treatment could induce cell death of HKLs, which was confirmed by LDH release and annexin V-FITC/PI assays. Taken together, our results support the idea that nigericin treatment might activate the IRE1-JNK apoptosis pathway in goldfish HKLs, which will provide insights into the mechanisms underlying HKLs immunity towards apoptosis or pyroptosis regulation in teleosts.

Recurrent expansions of B30.2-associated immune receptor families in fish

B30.2 domains, also known as PRY/SPRY, are key components of specific subsets of two large families of proteins involved in innate immunity: the tripartite motif proteins (TRIMs) and the Nod-like receptors (NLRs). TRIM proteins are important, often inducible factors of antiviral innate immunity, targeting multiple steps of viral cycles through a variety of mechanisms. NLRs prime and regulate systemic innate defenses, especially against bacteria, and control inflammation. Large TRIM and NLR subsets characterized by the presence of a B30.2 domain have been reported from a few fish species including zebrafish and seem to be strongly prone to gene duplication/expansion. Here, we performed a large-scale survey of these receptors across about 150 fish genomes, focusing on ray-finned fishes. We assessed the number and genomic distribution of domains and domain combinations associated with TRIMs, NLRs, and other genes containing B30.2 domains and looked for gene expansion patterns across fish groups. We then used a model to test the impact of taxonomy, genome size, and environmental variables on the copy numbers of these genes. Our findings reveal novel domain structures, clade-specific gains and losses. They also assist with the timing of the gene expansions, reveal patterns associated with the MHC, and lay the groundwork for further studies delving deeper into the forces that drive the copy number variation of immune genes on a species level.

Alterations in hematological and biochemical parameters, oxidative stress, and immune response in Takifugu rubripes under acute ammonia exposure

Ammonia is a major pollutant in aquatic environments and poses a considerable threat to the survival of fish. In this study, we investigated the toxic effects of ammonia on the hematological and biochemical parameters, oxidative stress, and immune responses in Takifugu rubripes. Juvenile T. rubripes (average weight 246.17 ± 3.54 g) were exposed to different concentrations of ammonia (0, 5, 50, 100, and 150 mg/L) for 96 h. The results showed that the hematological parameters (hemoglobin, hematocrit, red blood cell, and white blood cell count) were significantly reduced in response to ammonia exposure. Of the plasma components, such as serum total protein, albumin, glucose, glutamic-oxalacetic transaminase, and glutamic-pyruvic transaminase, were significantly altered in response to ammonia exposure. Additionally, the levels of superoxide dismutase (SOD), catalase (CAT), glutathione (GSH), and glutathione peroxidase (GPx) were increased after exposure to low concentration ammonia exposure. However, when fish were exposed to a high concentration of ammonia, these parameters showed the opposite trend, suggesting that an increase in antioxidant enzymes during the early stages of ammonia exposure may contribute to the removal of the induced reactive oxygen species (ROS) and protect the cells from oxidative damage. However, as the ammonia concentration and exposure time increased, the overproduction of ROS accelerated the depletion of antioxidant enzymes. Ammonia exposure significantly increased the expression of heat shock proteins (HSP70 and HSP90). Ammonia poisoning elevated gene expressions of TLR-3, TNF-α, IL-6, IL-12, and IL-1β in the gills, causing an inflammatory response. Our findings provide new insights into the mechanisms involved in ammonia-induced aquatic toxicology in marine fish, which may aid in their captive management.

Inflammasomes in Teleosts: Structures and Mechanisms That Induce Pyroptosis during Bacterial Infection

Pattern recognition receptors (PRRs) play a crucial role in inducing inflammatory responses; they recognize pathogen-associated molecular patterns, damage-associated molecular patterns, and environmental factors. Nucleotide-binding oligomerization domain-leucine-rich repeat-containing receptors (NLRs) are part of the PRR family; they form a large multiple-protein complex called the inflammasome in the cytosol. In mammals, the inflammasome consists of an NLR, used as a sensor molecule, apoptosis-associated speck-like protein containing a caspase recruitment domain (ASC) as an adaptor protein, and pro-caspase1 (Casp1). Inflammasome activation induces Casp1 activation, promoting the maturation of proinflammatory cytokines, such as interleukin (IL)-1β and IL-18, and the induction of inflammatory cell death called pyroptosis via gasdermin D cleavage in mammals. Inflammasome activation and pyroptosis in mammals play important roles in protecting the host from pathogen infection. Recently, numerous inflammasome-related genes in teleosts have been identified, and their conservation and/or differentiation between their expression in mammals and teleosts have also been elucidated. In this review, we summarize the current knowledge of the molecular structure and machinery of the inflammasomes and the ASC-spec to induce pyroptosis; moreover, we explore the protective role of the inflammasome against pathogenic infection in teleosts.

Isomer-Specific Effects of cis-9,trans-11- and trans-10,cis-12-CLA on Immune Regulation in Ruminal Epithelial Cells

Simple Summary

The significant contribution of rumen microbiota to the balance of the innate immunity of rumen epithelium has been extensively verified. As the natural rumen microbial metabolites, information regarding the immunoprotective effects of different conjugated linoleic acid (CLA) isomers on ruminal epithelial cells (RECs) is limited. In this study, the 100 μM trans-10,cis-12-CLA exerted better anti-inflammatory effects than the cis-9,trans-11-CLA by significantly downregulating the expression of genes related to inflammation, cell proliferation and migration in RECs upon lipopolysaccharide (LPS) stimulation. The trans-10,cis-12-CLA, but not cis-9,trans-11-CLA, significantly suppressed the biological signals of gene ontology (GO) terms’ response to lipopolysaccharide, the regulation of signal transduction and cytokine production and the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways NF-κB, chemokine, NOD-like receptor, Hippo, PI3K-Akt, TGF-β and Rap1 signaling in RECs upon LPS stimulation. Furthermore, pretreatment with trans-10,cis-12-CLA significantly reduced the expression of lipogenic genes and the biosynthesis of the unsaturated fatty acid pathway in RECs compared with the LPS group, however, cis-9,trans-11-CLA exhibited the opposite results. These results suggest the distinct isomer differences of CLA in the regulation of inflammatory responses and adipocytokine signaling in RECs and will provide important references for determining their target use in the future.

Abstract

In this study, we used transcriptomics and qPCR to investigate the potential immunoprotective effects of different conjugated linoleic acid (CLA) isomers, the natural rumen microbial metabolites, on lipopolysaccharide (LPS)-induced inflammation of ruminal epithelial cells (RECs) in vitro. The results showed that 100 μM trans-10,cis-12-CLA exerted higher anti-inflammatory effects than cis-9,trans-11-CLA by significantly downregulating the expression of genes related to inflammation, cell proliferation and migration in RECs upon LPS stimulation. Transcriptomic analyses further indicated that pretreatment with trans-10,cis-12-CLA, but not cis-9,trans-11-CLA, significantly suppressed the biological signals of GO terms’ response to LPS, the regulation of signal transduction and cytokine production and KEGG pathways NF-κB, chemokine, NOD-like receptor, Hippo, PI3K-Akt, TGF-β and Rap1 signaling in RECs upon LPS stimulation. Furthermore, pretreatment with trans-10,cis-12-CLA significantly reduced the expression of lipogenic genes and the biosynthesis of the unsaturated fatty acid pathway in RECs compared with the LPS group, however, cis-9,trans-11-CLA exhibited the opposite results. These results suggest the distinct isomer differences of CLA in the regulation of inflammatory responses and adipocytokine signaling in RECs and will provide important references for determining their target use in the future.

microRNA-210 and microRNA-3570 Negatively Regulate NF-κB-Mediated Inflammatory Responses by Targeting RIPK2 in Teleost Fish

Pathogen infection can cause the production of inflammatory cytokines, which are key mediators that cause the host’s innate immune response. Therefore, proper regulation of immune genes associated with inflammation is essential for immune response. Among them, microRNAs (miRNAs) as gene regulator have been widely reported to be involved in the innate immune response of mammals. However, the regulatory network in which miRNAs are involved in the development of inflammation is largely unknown in lower vertebrates. Here, we identified two miRNAs from miiuy croaker (Miichthys miiuy), miR-210 and miR-3570, which play a negative regulatory role in host antibacterial immunity. We found that the expressions of miR-210 and miR-3570 were significantly upregulated under the stimulation of Gram-negative bacterium vibrio harveyi and LPS (lipopolysaccharide). Induced miR-210 and miR-3570 inhibit inflammatory cytokine production by targeting RIPK2, thereby avoiding excessive inflammation. In particular, we found that miR-210 and miR-3570 negatively regulate antimicrobial immunity by regulating the RIPK2-mediated NF-κB signaling pathway. The collective results indicated that both miRNAs are used as negative feedback regulators to regulate RIPK2-mediated NF-κB signaling pathway and thus play a regulatory role in bacteria-induced inflammatory response.

Characterization and expression analysis of tandemly-replicated asc genes in the Japanese medaka, Oryzias latipes

ASC is a component of the inflammasome playing crucial roles in the inflammatory response. In mammals, ASC induces pyroptosis and inflammatory cytokine production. In this study, three asc genes (asc1, asc2, and asc3) from the Japanese medaka (Oryzias latipes) were identified and characterized. These asc genes were tandem replicates on chromosome 16, and their exon-intron structures differed between them. All three ASCs conserved the pyrin and caspase-recruitment domains, which are important for inflammasome formation. In phylogenetic analysis, all ASCs clustered with those of other teleosts. The asc1 expression levels were significantly higher in several organs than those of asc2 and asc3, suggesting that asc1 may act as a dominant asc in the Japanese medaka. Expression of the three asc genes showed different patterns during Aeromonas hydrophila and Edwardsiella piscicida infections. Furthermore, their expression was adequately down-regulated in the medaka fin-derived cells stimulated with ATP for 12 h, while asc2 expression was statistically up-regulated after nigericin stimulation for 24 h. Moreover, the expression of asc2 and asc3 was significantly higher in the skin of ASC-1-knockout medaka than in that of the wild type medaka during A. hydrophila infection.

Characterization and expression analysis of Nod-like receptor 3 (NLRC3) against infection with Piscirickettsia salmonis in Atlantic salmon

The nucleotide binding oligomerization domain like receptors, or NOD like receptors (NLRs), are intracellular receptors responsible for recognizing pathogens in vertebrates. Several NLR mammalian models have been characterized and analyzed but few studies have been performed with teleost species. In this study, we analyzed the nucleotide sequence of six mRNA variants of NLRC3 in Atlantic salmon (SsNLRC3), and we deduced the amino acid sequence coding for two different isoforms with a total length of 1135 amino acids and 1093 amino acids. We analyzed the phylogeny of all variants, including a Piscirickettsia salmonis infection in Atlantic salmon. All variants and their expression pattern during infection were analyzed using real-time qPCR. One of the analyzed variants was over-expressed during the early stages of Piscirickettsia salmonis infection, and we were able to identify two different SsNLRC3 isoforms. Lastly, we observed that an alteration in the amino acid sequence of one of the isoforms can directly affect the pathogen recognition function.

Full-length transcriptome sequencing combined with RNA-seq analysis revealed the immune response of fat greenling (Hexagrammos otakii) to Vibrio harveyi in early infection

Fat greenling (Hexagrammos otakii) is an important commercial marine fish species cultured in northeast Asia, but its available gene sequences are limited. Vibrio harveyi is a causative agent of vibriosis in fat greenling and also causes severe losses to the aquaculture industry in China. In order to obtain more high-quality transcript information and investigate the early immune response of fat greenling against V. harveyi, the fish were artificially infected with V. harveyi, and five sampling points were set within 48 h. Iso-Seq combined with RNA-Seq were applied in the comprehensive transcriptome analysis of V. harveyi-infected fat greenling. Total 42,225 consensus isoforms were successfully extracted from the result of Iso-Seq, and more than 19,000 ORFs were predicted. In addition, total three modules were identified by WGCNA which significantly positive correlated to the infection time, and the KEGG analysis showed that the immune-related genes in these modules mainly enriched in TLR signaling pathway, NF-κB signaling pathway and Endocytosis. The activation of inflammation and endocytosis was the most significant characteristics of fat greenling immune response during the early infection. Based on the WGCNA, a series of high-degree nodes in the networks were identified as hub genes. The protein structures of cold-inducible RNA-binding protein (CIRBP), poly [ADP-ribose] polymerase 1 (PARP1) and protein arginine N-methyl transferase 1 (PRMT1) were subsequently found to be highly conserved in vertebrate, and the gene expression pattern of CIRBP, PARP1, PRMT1 and a part of TLR/NF-κB pathway-related genes indicated that these proteins might have similar biological functions in regulation of inflammatory response in teleost fish. The results of this study provided the first systematical full-length transcriptome profile of fat greenling and characterized its immune responses in early infection of V. harvey, which will serve as the foundation for further exploring the molecular mechanism of immune defense against bacterial infection in fat greenling.

Modulation of the Expression of Immune-related Gene in Atlantic and Coho Salmon during Infestation with the Sea lice Caligus rogercresseyi

Caligus rogercresseyi, a marine ectoparasite, causes notable economic losses for the Chilean salmonid industry. Nevertheless, the immunological responses of infected fish remain poorly understood, including proinflammatory cytokine generation and the respective modulatory effects of various cytokine receptors. This study evaluated mRNA expression of the NLRC5, major histocompatibility complex (MHC) class II, I-kappa-B-alpha, a regulatory that inhibits NF-kappa-B, and proinflammatory cytokines (IL-1β and IL-18) in the liver and muscle of Atlantic salmon (Salmo salar) and Coho salmon (Oncorhynchus kisutch) during a time-course C. rogercresseyi infestation trial. All assessed mRNA were strongly regulated during infestation, but S. salar showed up-regulated expression, possibly accounting for the high infestation vulnerability of this salmonid. In conclusion, this work helps to understand the modulation of the expression of different transcripts involved over short periods of C. rogercresseyi infestation in two salmonid species (S. salar and O. kisutch).

Transcriptome analysis of immune response in fat greenling (Hexagrammos otakii) against Vibrio harveyi infection

Fat greenling (Hexagrammos otakii) is an important aquaculture fish species in northern China. Unfortunately, Vibrio infections have caused considerable losses to the fat greenling aquaculture industry. However, the study on immune response of fat greenling against Vibrio species has not been reported yet. In this paper, the immune response of fat greenling against V. harveyi at gene expression level was studied by transcriptome analysis. A total of 189753 high-quality unigenes with a N50 length of 672bp were obtained by transcriptome profiling, which provided abundant data for the future study of fat greenling. Comparative analysis showed that 5425 differentially expressed genes (DEGs) were identified on day 3 post-infection (3dpi), containing 1837 up-regulated and 3588 down-regulated genes. Further annotation and analysis revealed that the DEGs were enriched in complement and coagulation cascades, ribosome, oxidative phosphorylation, glycine, serine and threonine metabolism and peroxisome proliferator-activated receptor (PPAR) signaling pathway. These pathways were mainly associated with phagocytosis and pathogen clearance, rarely involved in bacteria adhesion and pathogen identification, which suggested that the host might begin to clear and kill the invading bacteria on 3dpi. The research might provide a valuable resource to further study immune response and suggest strategies against V. harveyi infection in fat greenling.

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.

Mechanisms of Fish Macrophage Antimicrobial Immunity

Overcrowding conditions and temperatures shifts regularly manifest in large-scale infections of farmed fish, resulting in economic losses for the global aquaculture industries. Increased understanding of the functional mechanisms of fish antimicrobial host defenses is an important step forward in prevention of pathogen-induced morbidity and mortality in aquaculture setting. Like other vertebrates, macrophage-lineage cells are integral to fish immune responses and for this reason, much of the recent fish immunology research has focused on fish macrophage biology. These studies have revealed notable similarities as well as striking differences in the molecular strategies by which fish and higher vertebrates control their respective macrophage polarization and functionality. In this review, we address the current understanding of the biological mechanisms of teleost macrophage functional heterogeneity and immunity, focusing on the key cytokine regulators that control fish macrophage development and their antimicrobial armamentarium.

Transcriptome-wide mapping of signaling pathways and early immune responses in lumpfish leukocytes upon in vitro bacterial exposure

We performed RNA sequencing, identified components of the immune system and mapped early immune responses of lumpfish (Cyclopterus lumpus) leukocytes following in vitro exposure to the pathogenic bacterium Vibrio anguillarum O1. This is the first characterization of immune molecules in lumpfish at the gene level. In silico analyses revealed that genes encoding proteins involved in pathogen recognition, cell signaling and cytokines in mammals and teleosts are conserved in lumpfish. Unique molecules were also identified. Pathogen recognition components include 13 TLRs, several NLRs and complement factors. Transcriptome-wide analyses of immune responses 6 and 24 hours post bacterial exposure revealed differential expression of 9033 and 15225 genes, respectively. These included TLR5S, IL-1β, IL-8, IL-6, TNFα, IL-17A/F3, IL-17C and several components of the complement system. The data generated will be valuable for comparative studies and make an important basis for further functional analyses of immune and pathogenicity mechanisms. Such knowledge is also important for design of immunoprophylactic measures in lumpfish, a species of fish now farmed intensively for use as cleaner-fish in Atlantic salmon (Salmo salar) aquaculture.

Characterization and functional assessment of the NLRC3-like molecule of the goldfish (Carassius auratus L.)

The NLRC3-like (NLRC3L) molecule from the goldfish transcriptome database was identified and characterized. Quantitative gene expression analysis revealed the highest mRNA levels of NLRC3L were in the spleen and intestine, with lower mRNA levels observed in muscle and liver. Goldfish NLRC3L was differentially expressed in goldfish immune cell populations with highest mRNA levels measured in PBLs and macrophages. We generated a recombinant form of the molecule (rgfNLRC3L) and an anti-CT-NLRC3L IgG. Treatment of goldfish primary kidney macrophages in vitro with ATP, LPS and heat-killed Aeromonas salmonicida up-regulated the NLRC3L mRNA and protein. Confocal microscopy and co-immunoprecipitation assays indicated that goldfish rgfNLRC3L interacted with apoptosis-associated spec-like protein (ASC) in eukaryotic cells, indicating that NLRC3L may participate in the regulation of the inflammasome responses. The dual-luciferase reporter assay showed that NLRC3L over-expression did not cause the activation of NF-κB, but that it cooperated with RIP2 to down-regulate NF-κB activation. Our results indicate that the NLRC3L may function as a regulator of NLR pathways in teleosts.

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.

Transcriptome analysis of immune response against Vibrio harveyi infection in orange-spotted grouper (Epinephelus coioides)

Vibrio harveyi is a gram-negative bacterium reported as found in many aquaculture species. To increase knowledge of the immune response against V. harveyi, in this study we performed transcriptome analysis of head kidney and spleen in orange-spotted grouper (Epinephelus coioides) at 1 and 2 days post-infection (dpi), using the Illumina sequencing platform. After de novo assembly, a total of 79,128 unigenes was detected with an N50 of 2511 bp. After alignments with sequences recorded in the major databases (NT, NR, Swiss-Prot COG, KEGG, Interpro and GO), based on sequence similarity, 61,208 (77.4%) of the unigene total could be annotated using at least one database. Comparison of gene expression levels between V. harveyi and a control group at each time point revealed differentially expressed genes (DEGs) (P < 0.05): a total of 7918 (5536 upregulated and 2282 downregulated genes) from head kidney at 1 day post infection (dpi), 4260 (1444 upregulated and 2816 downregulated genes) from head kidney at 2 dpi, 7887 (4892 upregulated and 2995 downregulated genes) from spleen at 1 dpi, and 8952 (7388 upregulated and 1564 downregulated genes) from spleen at 2 dpi. The DEGs were mainly annotated into signal transduction and immune system categories, based on the KEGG database. The DEGs were enriched in immune-related pathway functions, NOD-like receptor signaling pathways, Toll-like receptor signaling pathways, NF-κB signaling pathways, and Jak-STAT signaling pathways. Additionally, we selected several DEGs and validated their expression level by RT-qPCR. The data generated in this study may provide a valuable resource for further immune response research and offer improved strategies against V. harveyi infection in teleost fishes.

Identification and expressional analysis of NLRC5 inflammasome gene in smolting Atlantic salmon (Salmo salar)

The NOD-like receptors (NLRs) were recently identified as an intracellular pathogen recognition receptor family in vertebrates. While the immune system participation of NLRs has been characterized and analyzed in various mammalian models, few studies have considered NLRs in teleost species. Therefore, this study analyzed the Atlantic salmon (Salmo salar) NLRC5. Structurally, Atlantic salmon NLRC5 presented leucine-rich repeat subfamily genes. Phylogenetically, NLRC5 was moderately conserved between S. salar and other species. Real-time quantitative PCR revealed NLRC5 expression in almost all analyzed organs, with greatest expressions in the head kidney, spleen, and hindgut. Furthermore, NLRC5 gene expression decreased during smolt stage. These data suggest that NLRC5 participates in the Atlantic salmon immune response and is regulated, at least partly, by the smoltification process, suggesting that there is a depression of immune system from parr at smolt stage. This is the first report on the NLRC5 gene in salmonid smolts.

A genome-wide survey of expansive NLR-C subfamily in miiuy croaker and characterization of the NLR-B30.2 genes

NOD-like receptors (NLRs) are essential intracellular pattern-recognition receptors that respond to pathogens and regulate innate immunity. NLRs include three distinct subfamilies: NLR-A, NLR-B and NLR-C, thereinto, NLR-C as a large subfamily is unique to bony fish and little research about it has been done. In the current study, we identified the members of NLR-B and NLR-C subfamilies containing 2 and 48 genes respectively in miiuy croaker. Compared with other teleosts except for zebrafish, NLR-C subfamily genes occurred expansion in miiuy croaker. The gene expansions of NLR-C subfamily may illustrate adaptive genome evolution in response to specific aquatic environments. Structural analysis showed that the N-terminus of NLR-C subfamily receptors has different characteristics of the domains including RING domain, FISNA domain or PYRIN domain. Interestingly, the C-terminus of 18 NLR-C subfamily members contains an extra B30.2 domain (named NLR-B30.2 genes) which plays an important role in antiviral immune recognition. Simultaneously, molecular evolutionary analysis indicated that the positively sites in miiuy croaker are mainly located in NACHT domain which was the vital region for signal transduction in immune response. Significantly, pathogens challenge in spleen and macrophages demonstrated that NLR-B30.2 genes exhibited more sensitive response to virus than bacteria, suggesting these genes play enhanced roles in innate antiviral immunity, which may represent a new family used for antiviral infection.

Cloning and characterization of apoptosis-associated speck-like protein containing a CARD domain (ASC) gene from Japanese flounder Paralichthys olivaceus

Apoptosis-associated speck-like protein containing a CARD domain (ASC) is a critical adaptor molecule in multiple inflammasome protein complexes that mediate inflammation and host defense. However, few studies have been performed in lower vertebrates such as in teleost. Here we identified and characterized a novel ASC gene (namely PoASC) from Japanese flounder Paralichthys olivaceus. The complete cDNA sequence of PoASC contains a 22 bp 5'-untranslated sequence, a 612 bp open reading frame, and a 438 bp 3'-untranslated sequence. The deduced PoASC protein is comprised of 203 amino acids with a conserved N-terminal PYD domain and a C-terminal CARD domain and shows 35-62% sequence identity with other vertebrate ASC proteins. PoASC mRNA transcripts was detected in various Japanese flounder tissues and is dominantly expressed in hepatopancreas. Oligomeric speck-like structures were observed when PoASC was exogenously expressed in Japanese flounder FG-9307 cells. Immune challenge experiments revealed that PoASC gene expression was significantly induced in the Japanese flounder head kidney macrophages and peripheral blood leukocytes by the canonical TLR ligands LPS, Poly(I:C) and zymosan stimulations. In addition, the induction of PoASC was also observed in Edwardsiella tarda challenged head kidney and gill tissues. Furthermore, we for the first time showed that extracellular ATP, an important signaling molecule in triggering innate immune response and activation of NLR inflammasome, significantly up-regulates PoASC expression in the Japanese flounder head kidney macrophages in a dose-dependent manner. Together, these findings addressed the involvement of PoASC in TLR and extracellular ATP-mediated innate immune signaling in the Japanese flounders.

Immune responses in the Japanese pufferfish (Takifugu rubripes) head kidney cells stimulated with particulate silica

Studies on immune response to crystal silica in mammals indicate immune stimulation effect of environmental parameters including silica or asbestos, but there is no information on this aspect in lower vertebrates. Therefore, we examined expression of cytokine genes related to innate immunity in the Japanese pufferfish, Fugu (Takifugu rubripes) head kidney (HK) cells stimulated with particulate silica at 10 and 50 μg mL(-1). Expression of eleven cytokine genes was analyzed by the multiplex RT-PCR method (GenomeLab Genetic Analysis System, GeXPS; Beckman Coulter Inc.). Additionally, to confirm functionality of activated inflammatory immunity, we assessed phagocytic activity. Expression of NLR family genes as potential sensor molecules of inflammasome and inflammasome-associated genes (ASC and caspase-1) was also confirmed in HK cells by quantitative real-time PCR (qRT-PCR). As a result, an increased gene expression of pro-inflammatory cytokines (IL-6, IL-17A/F3, TNF-α, TNF-β and IFN-γ) and other cytokines (IL-4/13A, IL-4/13B, Type I-IFN) was recorded in particulate silica stimulated HK cells. Moreover, phagocytic activity showed a tendency to significantly increase in stimulated monocyte of HK cells after 6 h. Expression of NLR-C9 and NLR-C12 genes significantly increased in silica-stimulated HK cells. The particulate silica also significantly induced expression of inflammosome-associated genes, which may relate to the induced NLR-Cs.