Identification and activity of a paralog of cathepsin S from yellow catfish (Pelteobagrus fulvidraco) involved in immune response

Transcriptome analysis in the spleen of Northern Snakehead (Channa argus) challenged with Nocardia seriolae

Northern snakehead (Channa argus) is an indigenous fish species and is one of popularly cultured snakeheads in China and other Asian countries. Unfortunately, Nocardia seriolae infections have caused considerable losses in the snakehead aquaculture industry. However, the infectivity and the immune response induced by N. seriolae in snakehead are unclear. In order to better understand the immune response of Northern snakehead in a series of time points after N. seriolae challenge, we conducted the transcriptomic comparison in snakehead spleen at 48, 96, and 144 h after the challenge of N. seriola against their control counterparts. Gene annotation and pathway analysis of differentially expressed genes (DEGs) were carried out to understand the functions of the DEGs. Additionally, protein-protein interaction networks were conducted to obtain the interaction relationships of immune-related DEGs. These results revealed the expression changes of multiple DEGs and signaling pathways involved in immunity during N. seriolae infection, which will facilitate our comprehensive understanding of the mechanisms involved in the immune response to bacterial infection in the northern snakehead.

Integrated transcriptomics and metabolomics analyses reveal benzo[a]pyrene enhances the toxicity of mercury to the Manila clam, Ruditapes philippinarum

Mercury (Hg²⁺) and benzo[a]pyrene (BaP) are ubiquitous and persistent pollutants with multiple toxicities in bivalve molluscs. Here, the toxicological responses in the gills of Manila clams, Ruditapes philippinarum, to Hg²⁺ (10 μg L^-1), BaP (3 μg L^-1), and their mixture were analysed using transcriptomics and metabolomics approaches. Comparisons of the transcriptomes and metabolomes of Hg²⁺-and/or BaP-treated clams with control animals revealed the involvement of the detoxification metabolism, immune defence, energy-related pathways, and osmotic regulation in the stress response of R. philippinarum. Exposure to Hg²⁺ alone primarily enhanced the detoxification and energy metabolic pathways by significantly increasing the expression of genes associated with heat-shock proteins and oxidative phosphorylation. However, co-exposure to Hg²⁺ and BaP caused greater immunotoxicity and disrupted detoxification metabolism, the TCA cycle, glycolysis, and ATP generation. The expression levels of cytochrome P450 1A1 (CYP1A1), multidrug resistance-associated protein 1 (MRP1), and myosin (MYO), and the activity of electron transport system (ETS) in gills were detected, supporting the underlying toxic mechanisms of Hg²⁺ and BaP. We suggest that the presence of BaP enhances the toxicity of Hg²⁺ by 1) hampering the detoxification of Hg²⁺, 2) increasing the immunotoxicity of Hg²⁺, and 3) constraining energy availability for clams.

An identified PfHMGB1 promotes microcystin-LR-induced liver injury of yellow catfish (Pelteobagrus fulvidraco)

Microcystin-LR (MC-LR) is a potent hepatotoxin that can cause liver inflammation and injury. However, the mode of action of related inflammatory factors is not fully understood. PfHMGB1 is an inflammatory factor induced at the mRNA level in the liver of juvenile yellow catfish (Pelteobagrus fulvidraco) that were intraperitoneally injected with 50 μg/kg MC-LR. The PfHMGB1 mRNA level was highest in the liver and muscle among 11 tissues examined. The full-length cDNA sequence of PfHMGB1 was cloned and overexpressed in E. coli, and the purified protein rPfHMGB1 demonstrated DNA binding affinity. Endotoxin-free rPfHMGB1 (6-150 μg/mL) also showed dose-dependent hepatotoxicity and induced inflammatory gene expression of primary hepatocytes. PfHMGB1 antibody (anti-PfHMGB1) in vitro reduced MC-LR (30 and 50 μmol/L)-induced hepatotoxicity, suggesting PfHMGB1 is important in the toxic effects of MC-LR. In vivo study showed that MC-LR upregulated PfHMGB1 protein in the liver. The anti-PfHMGB1 blocked its counterpart and reduced ALT/AST activities after MC-LR exposure. Anti-PfHMGB1 partly neutralized MC-LR-induced hepatocyte disorganization, nucleus shrinkage, mitochondria, and rough endoplasmic reticula destruction. These findings suggest that PfHMGB1 promotes MC-LR-induced liver damage in the yellow catfish. HMGB1 may help protect catfish against widespread microcystin pollution.

Genome-wide identification, expression signature and immune functional analysis of two cathepsin S (CTSS) genes in turbot (Scophthalmus maximus L.)

Cathepsins, a superfamily of hydrolytic enzymes produced and enclosed within lysosomes, play multiple roles at physiological and pathological states. Cathepsin S is a lysosomal cysteine endopeptidase of the papain family, and exerts critical roles in the regulation of MHC class II immune responses. In the present study, we captured two Cathepsin S genes in turbot (SmCTSS1 and SmCTSS2.1), characterized their expression patterns following V. anguillarum and S. iniae infections, and explored their binding ability and agglutination capability. Firstly, the SmCTSS1 contained a 990 bp ORF encoding 329 amino acids, while SmCTSS2.1 contained a 1,014 bp ORF encoding 337 amino acids. The phylogenetic analysis revealed that both genes showed the closest relationship to their counterparts of Japanese flounder (Paralichthys olivaceus). In addition, both genes were ubiquitously expressed in all examined healthy tissues, with the highest expression level observed in spleen and intestine, respectively, while the lowest expression level both observed in liver. Both SmCTSS1 and SmCTSS2.1 were significantly differentially expressed, and exhibited general down-regulations at most time points in skin and intestine after two bacterial infections. Finally, both rSmCTSS1 and rSmCTSS2.1 showed significant binding ability to three examined microbial ligands (LPS, PGN and LTA), and strong agglutination effect to different bacteria (E. tarda, S. agalactiae, S. aureus and V. anguillarum). Collectively, this study provided valuable data for understanding the roles of CTSS in the host defense against bacterial infections in turbot, and indicated the potential vital roles of CTSS in innate immune responses of teleost species.

Characterization, expression, enzymatic activity, and functional identification of cathepsin S from black rockfish Sebastes schlegelii

Cathepsin S belong to the cathepsin L-like family of cysteine cathepsins. It is well known that Cathepsin S participate in various physiological processes and host immune defense in mammals. However, in teleost fish, the function of cathepsin S is less investigated. In the present study, a cathepsin S homologue (SsCTSS) from the teleost fish black rockfish (Sebastes schlegelii) were identified and examined at expression and functional levels. In silico analysis showed that three domains, including signal peptide, cathepsin propeptide inhibitor I29 domain, and functional domain Pept_C1, were existed in the cathepsin. SsCTSS possesses a peptidase domain with three catalytically essential residues (Cys25, His162, and Asn183). Phylogenetic profiling indicated that SsCTSS are evolutionally close to the cathepsin S of other teleost fish. The expression of SsCTSS in immune-related tissues was upregulated in a time-dependent manner upon bacterial pathogen infection. Purified recombinant SsCTSS (rSsCTSS) exhibited apparent peptidase activity, which was remarkably declined in the presence of the cathepsin inhibitor E-64. rSsCTSS showed strong binding ability to LPS and PGN, the major constituents of the outer membranes of Gram-negative and Gram-positive bacteria, respectively. rSsCTSS also exhibited the capability of agglutination to different bacteria. The knockdown of SsCTSS attenuated the ability of host to eliminate pathogenic bacteria. Taken together, our results suggested that SsCTSS functions as cysteine protease which might be involved in the antibacterial immunity of black rockfish.

PfHMGB2 protects yellow catfish (Pelteobagrus fulvidraco) from bacterial infection by promoting phagocytosis and proliferation of PBL

HMGB2, a member of the high mobility group box family, plays an important role in host immune responses. However, the mechanism of action of HMGB2 is not well understood. Herein, a homologue from yellow catfish (Pelteobagrus fulvidraco) was cloned and named PfHMGB2. The deduced amino acid sequence of PfHMGB2 possessed a typical tripartite structure (two DNA binding boxes and an acid tail) and shared 90% identity with the predicted HMGB2 from I. punctatus. The mRNA of PfHMGB2 was widely distributed in all 11 tested tissues in healthy fish bodies and was significantly induced in the liver and head kidney when yellow catfish were injected with inactivated Aeromonas hydrophila. Consistently, PfHMGB2 mRNA could also be induced in yellow catfish peripheral blood leucocytes (PBL) by lipopolysaccharide. The recombinant PfHMGB2 protein was purified from E. coli BL21 (DE3):pET-28a/PfHMGB2 and showed DNA-binding affinity. Moreover, rPfHMGB2 improved the phagocytosis and proliferation activity and upregulated the mRNA expression of the pro-inflammatory cytokine TNFα in yellow catfish PBL. These results indicated that PfHMGB2 could protect yellow catfish from pathogen infection by activating PBL.

Identification and expression of cathepsin B from the freshwater mussel Cristaria plicata

Cathepsin B plays crucial roles in host immune defense against pathogen infection. In present study, a cathepsin B gene from the freshwater mussel, Cristaria plicata (CpCathB) was cloned and characterized. The full-length cDNA of CpCathB was 1825 bp, and contained a 5' untranslated region (UTR) of 36 nucleotides, an open reading frame (ORF) of 1044 bp and a 3' UTR of 745 bp with a poly (A) tail. The deduced CpCathB protein was encoded as a preproenzyme with 347 amino acid residues and predicted molecular weight of 38.55 kDa. Sequence alignment revealed that CpCathB protein shared 56% - 60.7% identity comparison with other species. The predicted tertiary structure of CpCathB protein was highly similar to that of human. The CpCathB mRNA was expressed in hemocytes, hepatopancreas, adductor muscle, gills and mantle tissues of healthy mussels, and the highest expression level was in hepatopancreas. The transcripts of CpCathB were increased in hemocytes and hepatopancreas from mussels after Aeromonas hydrophila challenge. Moreover, the recombinant CpCathB was expressed in the Escherichia coli Rosetta-gami (DE3) strain. The maximum titer of the anti-CpCathB polyclonal antibodies was 1:640,000.The CpCathB protein had a higher expression in hepatopancreas and mantle and a lower level in hemocytes.

Brain Transcriptome Profiling Analysis of Nile Tilapia (Oreochromis niloticus) Under Long-Term Hypersaline Stress

The fish brain plays an important role in controlling growth, development, reproduction, and adaptation to environmental change. However, few studies stem from the perspective of whole transcriptome change in a fish brain and its response to long-term hypersaline stress. This study compares the differential transcriptomic responses of juvenile Nile tilapia (Oreochromis niloticus) maintained for 8 weeks in brackish water (16 practical salinity units, psu) and in freshwater. Fish brains from each treatment were collected for RNA-seq analysis to identify potential genes and pathways responding to hypersaline stress. A total of 27,089 genes were annotated, and 391 genes were expressed differently in the salinity treatment. Ten pathways containing 40 differentially expressed genes were identified in the tilapia brain. Antigen processing and presentation and phagosome were the two principally affected pathways in the immune system. Thirty-one of 40 genes were involved in various expressions associated with environmental information processing pathways such as neuroactive ligand-receptor interaction, cytokine-cytokine receptor interaction, the Jak-STAT signaling pathway, cell adhesion molecules (CAMs), and the PI3K-Akt signaling pathway, which are the upstream pathways for modulation of immunity and osmoregulation. The most-changed genes (>5-fold) were all down-regulated, including four growth hormone/prolactin gene families, i.e., prolactin precursor (−10.62), prolactin-1 (−11), somatotropin (−10.15), somatolactin-like (−6.18), and two other genes [thyrotropin subunit beta (−7.73) and gonadotropin subunit beta-2 (−5.06)] that stimulated prolactin release in tilapia. The downregulation pattern of these genes corroborates the decrease in tilapia immunity with increasing salinity and reveals an adaptive mechanism of tilapia to long-term hypersaline stress. Ovarian steroidogenesis, isoquinoline alkaloid biosynthesis, and phenylalanine metabolism are the three important pathways in the response of the fish to long-term hypersaline stress. This study has identified several pathways and relevant genes that are involved in salinity regulation in a euryhaline fish and provides insight into understanding regulatory mechanisms of fish to salinity change.

Deciphering transcriptome profile of the yellow catfish (Pelteobagrus fulvidraco) in response to Edwardsiella ictaluri

Edwardsiella ictaluri is one of the most important pathogens posing a serious threat for yellow catfish (Pelteobagrus fulvidraco), a highly valuable fish species of increasing commercial interest in China. Here, a transcriptomic strategy was undertaken to investigate the yellow catfish gene expression profile against infection by the bacterial pathogen E. ictaluri. Comparison of the transcriptome profiles between the infected and uninfected samples showed that a massive gene expression change occurred in yellow catfish following bacterial exposure. A total of 5527 differentially expressed genes (DEGs) were detected, of which 2265 showed up-regulation and 3262 down-regulation. Gene set enrichment analysis revealed the presence of canonical pathways directly linked to innate and adaptive immune response, such as pattern recognition receptor (PRR) signaling pathways, complement and coagulation cascades, as well as T-cell receptor (TCR) and B-cell receptor (BCR) signaling pathways. Additionally, 47,526 putative EST-liked simple sequence repeats (SSRs) markers were retrieved for use in genetic studies. This study establishes the first molecular clues to understand the potential mechanisms of yellow catfish resistance to E. ictaluri, thus enabling future efforts on disease control programs in this valuable aquaculture species.

Transcriptomic variation of locally-infected skin of Epinephelus coioides reveals the mucosal immune mechanism against Cryptocaryon irritans

Fish skin is the largest immunologically active mucosal organ, providing first-line defense against external pathogens. However, the skin-associated immune mechanisms of fish are still unclear. Cryptocaryon irritans is an obligate ectoparasitic ciliated protozoan that infects almost all marine fish, and is believed to be an excellent pathogen model to study fish mucosal immunity. In this study, a de novo transcriptome assembly of Epinephelus coioides skin post C. irritans tail-infection was performed for the first time using the Illumina HiSeq™ 2500 system. Comparative analyses of infected skin (group Isk) and uninfected skin (group Nsk) from the same challenged fish and control skin (group C) from uninfected control fish were conducted. As a result, a total of 91,082 unigenes with an average length of 2880 base pairs were obtained and among them, 38,704 and 48,617 unigenes were annotated based on homology with matches in the non-redundant and zebrafish database, respectively. Pairwise comparison resulted in 10,115 differentially-expressed genes (DEGs) in the Isk/C group comparison (4,983 up-regulated and 5,132 down-regulated), 2,275 DEGs in the Isk/Nsk group comparison (1,319 up-regulated and 956 down-regulated) and 4,566 DEGs in the Nsk/C group comparison (1,534 up-regulated and 3,032 down-regulated). Seven immune-related categories including 91 differentially-expressed immune genes (86 up-regulated and 5 down-regulated) were scrutinized. Both DEGs and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis and immune-related gene expression analysis were used, and both analyses showed that the genes were more significantly altered in the locally-infected skin than in the uninfected skin of the same challenged fish. This suggests the skin's local immune response is important for host defense against this ectoparasite infection. Innate immune molecules, including hepcidin, C-type lectin, transferrin, transferrin receptor protein, serum amyloid A, cathepsin and complement components were significantly up-regulated (fold-change ranged from 3.3 to 12,944) in infected skin compared with control skin. The up-regulation of chemokines and chemokine receptors and activation of the leukocyte transendothelial migration pathway suggested that leucocytes intensively migrated to the local infected sites to mount a local immune defense. Toll-like receptors (TLRs) 1, 2, 5 and 5S were most significantly up-regulated in the infected skin, suggesting that these TLRs may be involved in parasite pathogen-associated molecular pattern (PAMPs) recognition. Up-regulation of the dendritic cell markers CD209 and CD83 and other antigen presentation pathway molecules provided evidence for skin local antigen presentation. Up-regulation of the T cell markers CD4 and CD48, B cell markers CD22 and CD81 and B cell receptor signaling kinase Lyn, showed the presence and population expansion of T/B cells at locally-infected sites, which suggested possible activation of a local specific immune response in the skin. Our results will facilitate in-depth understanding of local immune defense mechanisms in fish skin against ectoparasite infection.