Molecular characterization and expression analysis of galectins in Japanese pufferfish (Takifugu rubripes) in response to Vibrio harveyi infection

A review of soluble factors and receptors involved in fish skin immunity: The tip of the iceberg

The immune system of fish possesses soluble factors, receptors, pathways and cells very similar to those of the other vertebrates' immune system. Throughout evolutionary history, the exocrine secretions of organisms have accumulated a large reservoir of soluble factors that serve to protect organisms from microbial pathogens that could disrupt mucosal barrier homeostasis. In parallel, a diverse set of recognition molecules have been discovered that alert the organism to the presence of pathogens. The known functions of both the soluble factors and receptors mentioned above encompass critical aspects of host defense, such as pathogen binding and neutralization, opsonization, or modulation of inflammation if present. The molecules and receptors cooperate and are able to initiate the most appropriate immune response in an attempt to eliminate pathogens before host infection can begin. Furthermore, these recognition molecules, working in coordination with soluble defence factors, collaboratively erect a robust and perfectly coordinated defence system with complementary specificity, activity and tissue distribution. This intricate network constitutes an immensely effective defence mechanism for fish. In this context, the present review focuses on some of the main soluble factors and recognition molecules studied in the last decade in the skin mucosa of teleost fish. However, knowledge of these molecules is still very limited in all teleosts. Therefore, further studies are suggested throughout the review that would help to better understand the functions in which the proteins studied are involved.

Molecular characterization and functional study of a galectin-9 from a teleost fish, Boleophthalmus pectinirostris

Galectin-9, a tandem-repeat galectin, plays an important role in the regulation of innate immune response against various microbial infections. Here, galectin-9 from mudskipper (Boleophthalmus pectinirostris) was identified and named as BpGal-9. Putative BpGal-9 contains two conserved carbohydrate recognition domains (CRDs), one CRD within N-terminal (N-CRD) and the other one within C-terminal (C-CRD). Multi-alignment analysis indicated that BpGal-9 shared the highest amino acid sequence identity of 64.3 % with that of Southern platyfish (Xiphophorus maculatus). Phylogenetic analysis showed that BpGal-9 grouped tightly with other teleosts galectin-9 and was most closely related to that of Southern platyfish. BpGal-9 transcripts were more abundant in the intestine, and its expression upregulated significantly in the intestine, kidney, spleen, gills, and skin after Edwardsiella tarda infection. Meanwhile, BpGal-9 expression significantly increased in hemocytes and serum of mudskipper infected by E. tarda. The recombinant BpGal-9 (rBpGal-9) and rBpGal-9C-CRD could agglutinate all tested bacteria, whereas rBpGal-9N-CRD could only agglutinate three kinds of bacteria. When targeting the same bacteria, rBpGal-9 showed stronger agglutinating activities than rBpGal-9C-CRD or rBpGal-9N-CRD. In addition, the induction effect of three recombinant proteins on the mRNA expression of anti-inflammatory cytokines (BpIL-10 and BpTGF-β) was better than that on the pro-inflammatory cytokines (BpIL-1β and BpTNF-α). Our result suggested that the N-CRD and C-CRD of galectin-9 contribute differently to its multiple functions in innate immunity in teleosts.

Molecular Characterization of Galectin-3 in Large Yellow Croaker Larimichthys crocea Functioning in Antibacterial Activity

Galectins are proteins that play a crucial role in the innate immune response against pathogenic microorganisms. Previous studies have suggested that Galectin-3 could be a candidate gene for antibacterial immunity in the large yellow croaker Larimichthys crocea. In this study, we cloned the Galectin-3 gene in the large yellow croaker, and named it LcGal-3. The deduced amino acid sequence of LcGal-3 contains a carbohydrate recognition domain with two conserved β-galactoside binding motifs. Quantitative reverse transcription PCR (qRT-PCR) analysis revealed that LcGal-3 was expressed in all the organs/tissues that were tested, with the highest expression level in the gill. In Larimichthys crocea kidney cell lines, LcGal-3 protein was distributed in both the cytoplasm and nucleus. Moreover, we found that the expression of LcGal-3 was significantly upregulated upon infection with Pseudomonas plecoglossicida, as demonstrated by qRT-PCR analyses. We also purified the LcGal-3 protein that was expressed in prokaryotes, and found that it has the ability to agglutinate large yellow croaker red blood cells in a Ca²⁺-independent manner. The agglutination activity of LcGal-3 was inhibited by lipopolysaccharides (LPS) in a concentration-dependent manner, as shown in the sugar inhibition test. Additionally, LcGal-3 exhibited agglutination and antibacterial activities against three Gram-negative bacteria, including P. plecoglossicida, Vibrio parahaemolyticus, and Vibrio harveyi. Furthermore, we studied the agglutination mechanism of the LcGal-3 protein using blood coagulation tests with LcGal-3 deletion and point mutation proteins. Our results indicate that LcGal-3 protein plays a critical role in the innate immunity of the large yellow croaker, providing a basis for further studies on the immune mechanism and disease-resistant breeding in L. crocea and other marine fish.

Molecular Cloning and Functional Characterization of Galectin-1 in Yellow Drum (Nibea albiflora)

Galectins are proteins that are involved in the innate immune response against pathogenic microorganisms. In the present study, the gene expression pattern of galectin-1 (named as NaGal-1) and its function in mediating the defense response to bacterial attack were investigated. The tertiary structure of NaGal-1 protein consists of homodimers and each subunit has one carbohydrate recognition domain. Quantitative RT-PCR analysis indicated that NaGal-1 was ubiquitously distributed in all the detected tissues and highly expressed in the swim-bladder of Nibea albiflora, and its expression could be upregulated by the pathogenic Vibrio harveyi attack in the brain. Expression of NaGal-1 protein in HEK 293T cells was distributed in the cytoplasm as well as in the nucleus. The recombinant NaGal-1 protein by prokaryotic expression could agglutinate red blood cells from rabbit, Larimichthys crocea, and N. albiflora. The agglutination of N. albiflora red blood cells by the recombinant NaGal-1 protein was inhibited by peptidoglycan, lactose, D-galactose, and lipopolysaccharide in certain concentrations. In addition, the recombinant NaGal-1 protein agglutinated and killed some gram-negative bacteria including Edwardsiella tarda, Escherichia coli, Photobacterium phosphoreum, Aeromonas hydrophila, Pseudomonas aeruginosa, and Aeromonas veronii. These results set the stage for further studies of NaGal-1 protein in the innate immunity of N. albiflora.

Involvement of galectin-9 from koi carp (Cyprinus carpio) in the immune response against Aeromonas veronii infection

Galectins are β-galactoside sugar binding proteins which function as important pattern recognition receptors (PRRs) in innate immunity. Here, we identified a galectin-9 gene from koi carp (Cyprinus carpio), named kGal-9. The ORF of kGal-9 is 963 bp in length, which encodes a polypeptide of 320 amino acids without either signal peptide. The predicted molecular weight is 36.25 kDa, and the isoelectric point is 8.3. Multiple sequence alignment showed that the putative kGal-9 contains two carbohydrate recognition domains (CRD), which are conserved in Galectin-9s. The phylogenetic tree showed that kGal-9 clustered to Galectin-9s from other teleosts, and shared the highest identity of 87.5% with Qihe crucian (Carassius auratus). kGal-9 mRNA was abundant in head kidney, gills, and gut, but low in liver and muscle. Further, the expression level of kGal-9 in the head kidney and liver increased significantly after Aeromonas veronii (abbreviated A.v) infection. Unexpectedly, kGal-9 showed a remarkable downregulation in the spleen at various time points post A.v infection. Intramuscular injection of pckGal-9 not merely reduced the bacterial load of spleen tissue, but also improved the survival rate of koi carp post A.v challenge. Besides, administration of pckGal-9 stimulated the expression of several immuno-related genes including proinflammatory cytokines (IL-1β, IL-6), anti-inflammatory cytokine (IL-10), complement components (C4, C9), with fluctuation in spleen and head kidney. Taken together, the obtained results suggest that kGal-9 occupies an important role in innate immunity and defense against bacterial infection in koi carp.

Characteristics of cathepsin members and expression responses to poly I:C challenge in Pacific cod (Gadus macrocephalus)

Cathepsins are major lysosomal enzymes that participate in necessary physiological processes, including protein degradation, tissue differentiation, and innate or adaptive immune responses. According to their proteolytic activity, vertebrate cathepsins are classified as cysteine proteases (cathepsins B, C, F, H, K, L, O, S, V, W, and X or Z), aspartic proteases (cathepsin D and E), and serine proteases (cathepsin A and G). Several cathepsins were reported in teleosts, however, no cathepsin gene has been identified from Pacific cod so far. In the present study, a total of 13 cathepsin genes were identified for Pacific cod. The evolutionary path of each cathepsin gene was demonstrated via analysis of phylogenetic trees, multiple alignments, conserved domains, motif compositions, and tertiary structures. Tissue distribution analysis showed that all cathepsin genes were ubiquitously expressed in eight healthy tissues but they exhibited diverse levels of expression. Several cathepsin genes were found to be highly expressed in the kidney, spleen, head kidney and liver, whereas low or modest levels were detected in the gills, skin, intestines, and heart. Temporal-specific expression of cathepsins in early developmental stages of Pacific cod were also conducted. CTSK, S, F, and Z were highly expressed at 1 dph and 5 dph and decreased later, while CTSL, L1, and L.1 transcript levels gradually increased in a time-dependent manner. Additionally, the expression profiles of cathepsin genes in Pacific cod were evaluated in the spleen and liver after poly I:C challenge. The results indicated that all cathepsin genes were significantly upregulated upon poly I:C stimulation, suggesting that they play key roles in antiviral immune responses in Pacific cod. Our findings establish a foundation for future exploration of the molecular mechanisms of cathepsins in modulating antiviral immunity in Pacific cod.

fliA, flrB, and fliR regulate adhesion by controlling the expression of critical virulence genes in Vibrio harveyi

Bacteria adhesion to fish mucus is a crucial virulence mechanism. As the initial step of bacterial infection, adhesion is impacted by bacterial motility and environmental conditions. However, its molecular mechanism is yet unclear. In this study, a significant decrease in gene expression of adhesion-deficient Vibrio harveyi was observed when the bacteria were subjected by Cu²⁺(50 mg/L), Pb²⁺(100 mg/L), Hg²⁺(25 mg/L), and Zn²⁺(50 mg/L). The genes fliA, fliR, and flrB were responsible for flagellation; being crucial for adhesion, these genes were identified and silenced via RNAi. After silencing of these genes by RNAi technology, the ability of adhesion, biofilm formation, motility, and flagella synthesis of V. harveyi were considerably reduced. Compared with the control group, it was observed that the expression levels of fliS, fliD, flgH, and flrC were significant down-regulated in fliR-RNAi, flrB-RNAi, and fliA-RNAi. This data indicates that the expression levels of most virulence genes are affected by fliA, fliR, and flrB. Also, the expression of fliA, fliR, and flrB can be influenced by the salinity, temperature, and pH. The results show that: (1) fliA, fliR, and flrB have important roles in the adhesion of V. harveyi; (2) fliA, fliR, and flrB can regulate bacterial adhesion by affecting its motility, and biofilm formation; (3) fliA, fliR, and flrB can regulate adhesion ability of V. harveyi in different environments.

Galectins in turbot (Scophthalmus maximus L.): Characterization and expression profiling in mucosal tissues

Galectins, a family of evolutionary conserved β-galactoside-binding proteins, have been characterized in a wide range of species. Many reports have indicated vital roles of galectins in innate immunity, especially in the mucosal tissues against infection. However, the systematic identification of galectin gene family is still lacking in teleost. Here, we characterized the galectin gene family and investigated their expression profiles post bacterial challenge in turbot (Scophthalmus maximus L.). In this study, a total of 13 galectin genes were characterized in turbot, phylogenetic analyses revealed their strong relationships to half smooth tongue sole and puffer fish, and syntenic analyses confirmed the orthology suggested by the phylogenetic analysis. In addition, the copy number of galectin genes is similar across a broad spectrum of species from fish to amphibians, birds, and mammals, ranging from 8 to 16 genes. Furthermore, the galectin genes were widely expressed in all the examined turbot tissues, and most of the galectin genes were strongly expressed in mucosal tissues (skin, gill and intestine). Moreover, majority of the galectin genes were significantly regulated after Vibrio anguillarum infection in the intestine, gill and skin, suggesting that galectins were involved in the mucosal immune response to V. anguillarum infection in turbot. In addition, subcellular localization analysis showed lgals3a was distributed in the cytoplasm and nucleus. However, the knowledge of galectins are still limited in teleost species, further studies should be carried out to better characterize its detailed roles in teleost mucosal immunity.

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.

Identification and characterization of a novel galectin from the mud crab Scylla paramamosain

Galectins are a family of β-galactoside-binding lectins that play key roles in the invertebrate innate immunity system, but no galectin genes have been identified in the mud crab (Scylla paramamosain) so far. The present study is the first to clone a galectin gene (SpGal) from S. paramamosain, by the rapid amplification of cDNA ends technique based on expressed sequence tags. The full-length cDNA of SpGal was 3142 bp. Its open reading frame encoded a polypeptide of 280 amino acids containing a GLECT/Gal-bind lectin domain and a potential N-glycosylation site. The deduced amino acid sequence and multi-domain organization of SpGal were highly similar to those of invertebrate galectins, and phylogenetic analysis showed that SpGal was closely related to galectin isolated from Portunus trituberculatus. The mRNA transcripts of SpGal were found to be constitutively expressed in a wide range of tissues, with its expression level being higher in the hepatopancreas, gill, and hemocytes. The mRNA expression level of SpGal increased rapidly after the crabs were stimulated by Vibrio alginolyticus, and the maximum expression appeared at 6 h after the challenge. The lipopolysaccharide-binding ability of SpGal was dependent on its concentration, and it also exhibited agglutination activity with three Gram-negative (Aeromonas hydrophila, Chryseobacterium indologenes and Vibrio alginolyticus) and three Gram-positive (Bacillus aquimaris, Staphylococcus aureus and Micrococcus lysodeik) bacterial strains. In addition, hemagglutination activity with rabbit erythrocytes was observed in the absence of d-galactose. These results indicate that SpGal in S. paramamosain acts as a pattern recognition receptor to recognize a broad spectrum of microbes. The findings together indicate that SpGal plays an important role in the innate immune mechanisms of S. paramamosain against pathogenic infection.

Molecular characterization, tissue distribution and functional analysis of galectin 1-like 2 in grass carp (Ctenopharyngodon idella)

Galectins, as an evolutionary conserved group of lectin superfamily, has the functions of pathogen recognition, anti-bacteria and anti-virus. In this study, a 405 bp cDNA sequence of galectin 1-like 2 (CiGal1-L2) was obtained from grass carp (Ctenopharyngodon idella), which encoded 134 amino acids with a predicted molecular mass of 15.143 kDa and an isoelectric point of 5.33. The sugar binding motifs (H-N-R, V-N and W--E-R) were detected in carbohydrate-binding domain (CRD). The amino acid sequence similarity showed that CiGal1-L2 was 40.30-42.54% and 66.42-81.20% similarity to mammalian and fish counterparts, respectively. The phylogenetic tree showed that CiGal1-L2 was clustered with fish galectin-1s and closely related to Cyprinus carpio. Real-time quantitative PCR (RT-qPCR) analysis revealed that CiGal1-L2 was widely expressed in all tested tissues. In addition, the expression of CiGal1-L2 was differentially up-regulated challenged with grass carp reovirus (GCRV), lipopolysaccharide (LPS) and polyinosinic:polycytidylic acid (poly I:C). The fluorescence of CiGal1-L2-GFP was distributed in the cytoplasm and nucleus of HEK 293T cells and showed a trend of nuclear translocation after LPS and poly I:C treatment. Finally, the recombinant CiGal1-L2 (rCiGal1-L2) protein showed strong binding ability to LPS. In conclusion, the results provided further insight into the immune roles of galectin-1 in teleost.