Isolation and function analysis of apolipoprotein A-I gene response to virus infection in grouper

Singapore grouper iridovirus VP12 evades the host antiviral immune response by targeting the cGAS-STING signalling pathway

The emergence of Singapore grouper iridovirus (SGIV) has caused huge losses to grouper farming. SGIV is a DNA virus and belongs to the genus Ranavirus. Groupers infected with SGIV showed haemorrhaging and swelling of the spleen, with a mortality rate of more than 90% within a week. Therefore, it is of great significance to study the escape mechanism of SGIV from host innate immunity for the prevention and treatment of viral diseases in grouper. In this study, the viral proteins that interact with EccGAS were identified by mass spectrometry, and the SGIV VP12 protein that inhibits cyclic GMP-AMP synthase (cGAS)-stimulator of interferon genes (STING)-mediated antiviral innate immunity was screened by the dual-luciferase reporter gene assay. VP12 belongs to the late gene of the virus. The immunofluorescence analysis demonstrated that VP12 was aggregated and distributed in the cytoplasm during the early stage of virus infection and translocated into the nucleus at the late stage of virus infection. VP12 inhibited the activation of IFN3, ISRE and NF-κB promoter activities mediated by cGAS-STING, EcTBK1 and EcIRF3. Quantitative real-time PCR analysis showed that VP12 inhibited the expression of interferon-related genes, including those mediated by cGAS-STING. VP12 enhanced the inhibition of IFN3, ISRE and NF-κB promoter activity by EccGAS, EccGAS-mab-21 and EccGAS-delete-mab21. The interaction between VP12 and EccGAS was found to be domain independent. The immunoprecipitation results demonstrated that VP12 interacted and co-localized with EccGAS, EcTBK1 and EcIRF3. VP12 degraded the protein levels of EcTBK1 and EcIRF3 and degraded EcIRF3 through the protease pathway. These results suggest that SGIV VP12 protein escapes the cGAS-STING signalling pathway and degrades EcIRF3 protein expression through the protease pathway.

Grouper cGAS is a negative regulator of STING-mediated interferon response

Cyclic GMP-AMP synthase (cGAS) is one of the classical pattern recognition receptors that recognizes mainly intracytoplasmic DNA. cGAS induces type I IFN responses to the cGAS-STING signaling pathway. To investigate the roles of cGAS-STING signaling pathway in grouper, a cGAS homolog (named EccGAS) was cloned and identified from orange-spotted grouper (Epinephelus coioides). The open reading frame (ORF) of EccGAS is 1695 bp, encodes 575 amino acids, and contains a Mab-21 typical structural domain. EccGAS is homologous to Sebastes umbrosus and humans at 71.8% and 41.49%, respectively. EccGAS mRNA is abundant in the blood, skin, and gills. It is uniformly distributed in the cytoplasm and colocalized in the endoplasmic reticulum and mitochondria. Silencing of EccGAS inhibited the replication of Singapore grouper iridovirus (SGIV) in grouper spleen (GS) cells and enhanced the expression of interferon-related factors. Furthermore, EccGAS inhibited EcSTING-mediated interferon response and interacted with EcSTING, EcTAK1, EcTBK1, and EcIRF3. These results suggest that EccGAS may be a negative regulator of the cGAS-STING signaling pathway of fish.

Differential Expression of the Apolipoprotein AI Gene in Spotnape Ponyfish (Nuchequula nuchalis) Inhabiting Different Salinity Ranges at the Top of the Estuary and in the Deep-Bay Area of Gwangyang Bay, South Korea

Spotnape ponyfish (Nuchequula nuchalis) is a dominant species that is broadly distributed from estuarine to deep-bay areas, reflecting a euryhaline habitat. Apolipoprotein AI (ApoAI) is a main component of plasma lipoproteins and has crucial roles in lipid metabolism and the defense immune system. In this study, we characterized the N. nuchalis ApoAI gene and analyzed the expression of the ApoAI transcript in N. nuchalis collected at various sites in the estuary and the deep-bay area which have different salinities. Owing to the fish’s mobility, we conducted stable isotope analyses to confirm the habitat characteristics of N. nuchalis. Carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N) from N. nuchalis indicated different feeding sources and trophic levels in the estuarine and deep-bay habitats. The characterized N. nuchalis ApoAI displayed residual repeats that formed a pair of alpha helices, indicating that the protein belongs to the apolipoprotein family. In the phylogenetic analysis, there was no sister group of N. nuchalis ApoAI among the large clades of fish species. The transcriptional expression level of ApoAI was higher in N. nuchalis inhabiting the deep-bay area with a high salinity (over 31 psu) than in N. nuchalis inhabiting the top of the estuary with a low salinity (6~15 psu). In addition, the expression patterns of N. nuchalis ApoAI were positively correlated with environmental factors (transparency, pH, TC, and TIC) in the high salinity area. These results suggest that ApoAI gene expression can reflect habitat characteristics of N. nuchalis which traverse broad salinity ranges and is associated with functional roles of osmoregulation and lipid metabolism for fish growth and development.

Polystyrene nanoplastics alter virus replication in orange-spotted grouper (Epinephelus coioides) spleen and brain tissues and spleen cells

Polystyrene nanoplastics (PS-NPs) are known to impair the function of the digestive system, intestinal flora, immune system, and nervous system of marine organisms. We tested whether PS-NPs influence viral infection of orange-spotted grouper (Epinephelus coioides). We found that grouper spleen (GS) cells took up PS-NPs at exposure concentrations of 5, 50, and 500 μg/mL and experienced cytotoxicity at 50 and 500 μg/mL concentrations. At 12 h after exposure to 50 μg/mL of PS-NPs, the replication of Singapore grouper iridovirus (SGIV) and red-spotted grouper nervous necrosis virus (RGNNV) increased in GS cells after their invasion. Juvenile fish exposed to 300 and 3000 μg/L of PS-NPs for 7 d showed PS-NPs uptake to the spleen and vacuole formation in brain tissue. Moreover, PS-NPs exposure accelerated SGIV replication in the spleen and RGNNV replication in the brain. PS-NP exposure also decreased the expression of toll-like receptor genes and interferon-related genes before and after virus invasion in vitro and in vivo, thus reducing the resistance of cells and tissues to viral replication. This is the first report that PS-NPs have toxic effects on GS cells and spleen and brain tissues, and it provides new insights into assessing the impact of PS-NPs on marine fish.

Plasma Proteome Responses in Salmonid Fish Following Immunization

Vaccination plays a critical role in the protection of humans and other animals from infectious diseases. However, the same vaccine often confers different protection levels among individuals due to variation in genetics and/or immunological histories. While this represents a well-recognized issue in humans, it has received little attention in fish. Here we address this knowledge gap in a proteomic study of rainbow trout (Oncorhynchus mykiss, Walbaum), using non-lethal repeated blood sampling to establish the plasma protein response of individual fish following immunization. Six trout were immunized with adjuvanted hen egg-white lysozyme (HEL) and peripheral blood sampled at ten time points from day 0 to day 84 post-injection. We confirm that an antigen-specific antibody response to HEL was raised, showing differences in timing and magnitude among individuals. Using label-free liquid chromatography-mass spectrometry, we quantified the abundance of 278 plasma proteins across the timecourse. As part of the analysis, we show that this approach can distinguish many (but not all) duplicated plasma proteins encoded by paralogous genes retained from the salmonid-specific whole genome duplication event. Global variation in the plasma proteome was predominantly explained by individual differences among fish. However, sampling day explained a major component of variation in abundance for a statistically defined subset of 41 proteins, representing 15% of those detected. These proteins clustered into five groups showing distinct temporal responses to HEL immunization at the population level, and include classical immune (e.g. complement system members) and acute phase molecules (e.g. apolipoproteins, haptoglobins), several enzymes and other proteins supporting the immune response, in addition to evolutionarily conserved molecules that are as yet uncharacterized. Overall, this study improves our understanding of the fish plasma proteome, provides valuable marker proteins for different phases of the immune response, and has implications for vaccine development and the design of immune challenge experiments.

Molecular Evolution of Apolipoprotein Multigene Family and the Original Functional Properties of Serum Apolipoprotein (LAL2) in Lampetra japonica

Apolipoprotein (APO) genes represent a large family of genes encoding various binding proteins associated with plasma lipid transport. Due to the long divergence history, it remains to be confirmed whether these genes evolved from a common ancestor through gene duplication and original function, and how this evolution occurred. In this study, based on the phylogenetic tree, sequence alignment, motifs, and evolutionary analysis of gene synteny and collinearity, APOA, APOC, and APOE in higher vertebrates may have a common ancestor, lamprey serum apolipoprotein LAL1 or LAL2, which traces back to 360 million years ago. Moreover, the results of immunofluorescence, immunohistochemistry, and flow cytometry show that LAL2 is primarily distributed in the liver, kidney, and blood leukocytes of lampreys, and specifically localized in the cytoplasm of liver cells and leukocytes, as well as secreted into sera. Surface plasmon resonance technology demonstrates that LAL2 colocalizes to breast adenocarcinoma cells (MCF-7) or chronic myeloid leukemia cells (K562) associated with lamprey immune protein (LIP) and further enhances the killing effect of LIP on tumor cells. In addition, using quantitative real-time PCR (Q-PCR) and western blot methods, we found that the relative mRNA and protein expression of lal2 in lamprey leukocytes and sera increased significantly at different times after stimulating with Staphylococcus aureus, Vibrio anguillarum, and Polyinosinic-polycytidylic acid (Poly I:C). Moreover, LAL2 was found to recognize and bind to gram-positive bacteria (Staphylococcus aureus and Bacillus cereus) and gram-negative bacteria (Escherichia coli) and play an important role in the antibacterial process. All in all, our data reveals a long, complex evolutionary history for apolipoprotein genes under different selection pressures, confirms the immune effect of LAL2 in lamprey sera against pathogens, and lays the foundation for further research regarding biological functions of lamprey immune systems.

Tissue distribution of transcription for 29 lipid metabolism-related genes in Takifugu rubripes, a marine teleost storing lipid predominantly in liver

The tissue distribution pattern of lipid is highly diverse among different fish species. Tiger puffer has a special lipid storage pattern, storing lipid predominantly in liver. In order to better understand the lipid physiology in fish storing lipid in liver, the present study preliminarily investigated the tissue distribution of transcription for 29 lipid metabolism-related genes in tiger puffer, which are involved in lipogenesis, fatty acid oxidation, biosynthesis and hydrolysis of glycerides, lipid transport, and relevant transcription regulation. Samples of eight tissues, brain, eye, heart, spleen, liver, intestine, skin, and muscle, from fifteen juvenile tiger puffer were used in the qRT-PCR analysis. The intestine and brain had high transcription of lipogenic genes, whereas the liver and muscle had low expression levels. The intestine also had the highest transcription level of most apolipoproteins and lipid metabolism-related transcription factors. The transcription of fatty acid β-oxidation-related genes was low in the muscle. The peroxisomal fatty acid oxidation may dominate over mitochondrial β-oxidation in the liver and intestine of tiger puffer, and the MAG pathway probably predominates over the G3P pathway in re-acylation of absorbed lipids in the intestine. The intracellular glyceridases were highly transcribed in the brain, eye, and heart. In conclusion, in tiger puffer, the intestine could be a center of lipid metabolism whereas the liver is more likely a pure storage organ for lipid. The lipid metabolism in the muscle could also be inactive, possibly due to the very low level of intramuscular lipid.

Transcriptomics of Cherax quadricarinatus hepatopancreas during infection with Decapod iridescent virus 1 (DIV1)

Cherax quadricarinatus is a large-sized, highly fecund, and fast-growing species of freshwater crayfish, and has become one of the world's most intensely studied crustaceans. Decapod iridescent virus 1 (DIV1), a newly described species in the family Iridoviridae, is known to infect various crustaceans, including C. quadricarinatus, and may pose a new threat in the shrimp-farming industry. The present study performed de novo transcriptome sequencing of C. quadricarinatus hepatopancreas during DIV1 infection. A total of 114,784 transcripts and 56,418 genes were obtained; 1070 genes were upregulated and 775 genes were downregulated when compared with the uninfected samples (controls). Three pattern recognition receptor genes (fibrinogen-related protein, C-type lectin, and beta-1,3-glucan-binding protein) were upregulated during DIV1 infection. Among the top-30 upregulated unigenes, 9 unigenes were identified as vitellogenin (Vg) genes, and the top-3 upregulated unigenes were identified as involved in Vg lipid transport, lipid localization, and lipid transporter activity, which were all significantly over-representative GO terms in the GO enrichment analysis of total and upregulated differentially expressed genes (DEGs). Many genes associated with Jak-STAT signaling pathway, Endocytosis, Phagosome, MAPK signaling pathway, Apoptosis and Lysosome were positively modified after DIV1 infection. The predicted protein-protein interaction (PPI) analysis showed NF1 and TUBA, CRM1 and TUBB were involved in protein interactions. This research showed that DIV1 infection has a significant impact on the transcriptome profile of C. quadricarinatus hepatopancreas, and the results enhance our understanding of virus-host interactions. Furthermore, the high number of transcripts generated in the present study will provide information for identifying novel genes in the absence of a full C. quadricarinatus genome sequence.

Analysis of apolipoprotein multigene family in spotted sea bass (Lateolabrax maculatus) and their expression profiles in response to Vibrio harveyi infection

Apolipoproteins (Apos), which are the protein components of plasma lipoproteins, play important roles in lipid transport in vertebrates. It has been demonstrated that in teleosts, several Apos display antimicrobial activity and play crucial roles in innate immunity. Despite their importance, apo genes have not been systematically characterized in many aquaculture fish species. In our study, a complete set of 23 apo genes was identified and annotated from spotted sea bass (Lateolabrax maculatus). Phylogenetic and homology analyses provided evidence for their annotation and evolutionary relationships. To investigate their potential roles in the immune response, the expression patterns of 23 apo genes were determined in the liver and intestine by qRT-PCR after Vibrio harveyi infection. After infection, a total of 20 differentially expressed apo genes were observed, and their expression profiles varied among the genes and tissues. 5 apo genes (apoA1, apoA4a.1, apoC2, apoF and apoO) were dramatically induced or suppressed (log2 fold change >4, P < 0.05), suggesting their involvement in the immune response of spotted sea bass. Our study provides a valuable foundation for future studies aimed at uncovering the specific roles of each apo gene during bacterial infection in spotted sea bass and other teleost species.

Plasma proteomic analysis of zebrafish following spring viremia of carp virus infection

To better understand spring viremia of carp virus (SVCV) pathogenesis in zebrafish proteomic analysis was used to examine the plasma protein profile in SVCV-infected zebrafish. A total of 3062 proteins were identified. Of those 137, 63 and 31 proteins were enriched in blood samples harvested at 1, 2 and 5 days post SVCV infection, respectively. These altered host proteins were classified based on their biological function: 23 proteins under the response to stimulus term were identified. Interestingly, at the top of the up-regulated proteins during SVCV infection were the proteins of the vitellogenin family (Vtg) and the grass carp reovirus-induced gene (Gig) proteins. Real-time RT-PCR evaluation of samples from internal organs verified that SVCV infection induced vtg and gig2 gene expression already at day 1 post-infection. Western blot analysis revealed the presence of Vtg protein only in blood of SVCV-infected fish. This is the first proteomic study to reveal the involvement of Vtg proteins in adult fish response to viral challenge. It also highlights the role of Gig proteins as important factors in antiviral response in fish. This work provides valuable relevant insight into virus-host interaction and the identification of molecular markers of fish response to virus.

Nannochloropsis oceania-derived defatted meal as an alternative to fishmeal in Atlantic salmon feeds

Defatted microalgal biomass derived from biorefinery can be potential feed ingredients for carnivorous fish. The present study investigated the growth, feed intake:gain and health parameters in Atlantic salmon fed for 84 days with defatted Nannochloropsis oceania as a fishmeal replacer. Fish fed feeds containing the algal biomass (at 10 and 20% inclusion, alga groups) were compared with groups that consumed alga-devoid feeds (control group). The fish that received 20% alga tended to have reduced weight gain and specific growth rate. Condition factor, feed conversion ratio and feed intake of this fish group were significantly different when compared with the control group. Hepatosomatic and viscerosomatic indices, whole body and fillet proximate composition were not affected by the dietary treatments. Digestibility of dry matter, protein, lipid, ash and energy, as well as retention of lipid and energy of the fish that received feed with 20% alga meal were also significantly different from those of the control group. Serum superoxide dismutase activity of the 10% alga-fed fish was significantly higher compared with the control fish. Although alga feeding did not cause any distal intestinal inflammation, the intestinal proteins that were altered upon feeding 20% algal meal might be pointing to systemic physiological disturbances. In conclusion, feeds with 20% alga had a negative effect on feed intake, FCR, lipid and energy retention and health of the fish. The defatted Nannochloropsis oceania can be used at modest inclusion levels, around 10%, without negative effects on the performance of Atlantic salmon.

Molecular cloning, genomic structure, polymorphism analysis and recombinant expression of a α1-antitrypsin like gene from swamp eel, Monopterus albus

Alpha-1-antitrypsin (AAT) is a highly polymorphic glycoprotein antiprotease, involved in the regulation of human immune response. Beyond some genomic characterization and a few protein characterizations, the function of teleost AAT remains uncertain. In this study we cloned an AAT-like gene from a swamp eel liver identifying four exons and three introns, and the full-length cDNA. The elucidated swamp eel AAT amino acid sequence showed high homology with known AATs from other teleosts. The swamp eel AAT was examined both in ten healthy tissues and in four bacterially-stimulated tissues resulting in up-regulation of swamp eel AAT at different times. Swamp eel AAT transcripts were ubiquitously but unevenly expressed in ten tissues. Further, the mature peptide sequence of swamp eel AAT was subcloned and transformed into E. coli with the recombinant proteins successfully inhibiting bovine trypsin activity. Analysis of recombinant AAT showed equimolar formation of irreversible complexes with proteinases, high stability at pH 7.0-10.0 and temperatures below 55 °C. Serum AAT protein level significantly increased in response to inflammation with AAT anti-sera, and, NF-κB, apolipoprotein A1 and transferrin gene expression were dramatically decreased over 72 h post recombinant AAT injection. Lastly, examination of swamp eel AAT allelic polymorphism identified all alleles in both healthy and diseased stock except allele*g, found only in diseased stock, but without statistical difference between the distribution frequency of allele*g in the two stocks. These results are crucial to our ongoing study of the role of teleost AAT in the innate immune system.

Apolipoprotein A-I in Labeo rohita: Cloning and functional characterisation reveal its broad spectrum antimicrobial property, and indicate significant role during ectoparasitic infection

Apolipoprotein A-I (ApoA-I) is the most abundant and multifunctional high-density lipoprotein (HDL) having a major role in lipid transport and potent antimicrobial activity against a wide range of microbes. In this study, a complete CDS of 771 bp of Labeo rohita (rohu) ApoA-I (LrApoA-I) encoding a protein of 256 amino acids was amplified, cloned and sequenced. Tissue specific transcription analysis of LrApoA-I revealed its expression in a wide range of tissues, with a very high level of expression in liver and spleen. Ontogenic study of LrApoA-I expression showed presence of transcripts in milt and 3 h post-fertilization onwards in the larvae. The expression kinetics of LrApoA-I was studied upon infection with three different types of pathogens to elucidate its functional significance. Its expression was found to be up-regulated in the anterior kidney of L. rohita post-infection with Aeromonas hydrophila. Similarly following poly I:C (poly inosinic:cytidylic) stimulation, the transcript levels increased in both the anterior kidney and liver tissues. Significant up-regulation of LrApoA-I expression was observed in skin, mucous, liver and anterior kidney of the fish challenged with the ectoparasite Argulus siamensis. Immunomodulatory effect of recombinant LrApoA-I (rApoA-I) produced in Escherichia coli was demonstrated against A. hydrophila challenge in vivo. L. rohita administered with rApoA-I at a dose of 100 μg exhibited significantly higher protection (∼55%) upon challenge with A. hydrophila 12 h post-administration of the protein, in comparison to that observed in control group, along with higher level of expression of immune-related genes. The heightened expression of ApoA-I observed post-infection reflected its involvement in immune responses against a wide range of infections including bacterial, viral as well as parasitic pathogens. Our results also suggest the possibility of using rApoA-I as an immunostimulant, particularly rendering protection against A. hydrophila.

TTRAP is a critical factor in grouper immune response to virus infection

TTRAP (TRAF and TNF receptor-associated protein) is latest identified cytosolic protein that serves as a negative regulator for TNF signaling pathway. In this study, a member of TNF superfamily, TTRAP gene (designed as EcTTRAP) was cloned from grouper, Epinephelus coioides. There was an Exo_endo_phos type domain in EcTTRAP, and it was well conserved when compared with other TTRAPs, especially the endonuclease activity related motifs. EcTTRAP exhibited prominent endonuclease activity against the genome DNA from Escherichia coli, Vibrio vulnificus and E. coli JM109. Intracellular localization revealed that EcTTRAP expression distributed in both cytoplasm and nucleus. Real-time PCR analysis indicates that EcTTRAP is expressed in all selective grouper tissues, with the higher expression level in muscle, skin and gills. EcTTRAP was identified as a remarkably (P < 0.01) up-regulated protein responding to Singapore grouper iridovirus (SGIV) infection. Overexpression of EcTTRAP inhibited NF-κB activation, meanwhile the C terminal portion of the protein was found to be responsive domain for the inhibition. Stable transfection of FHM cells with EcTTRAP inhibited apoptosis induced by SGIV. Overexpression of EcTTRAP in grouper spleen (GS) cells inhibited the replication of SGIV. The present results provided new evidences for the potential roles of such molecule in E. coioides, and further confirmed the existence of TTRAP modulated TNF signaling pathway in grouper.