A proto-type galectin-2 from rock bream (Oplegnathus fasciatus): Molecular, genomic, and expression analysis, and recognition of microbial pathogens by recombinant protein

Antimicrobial Functions of Galectins from Fish, Mollusks, and Crustaceans: A Review

Galectins are a member of the β-galactoside binding protein family, which play a pivotal role in the immune defense of vertebrates as a pattern recognition receptor and occupy an important position in the innate immune system of invertebrates. The study of galectins in aquatic organisms has only recently emerged. Galectins in aquatic animals exhibit agglutination activity toward bacteria, inhibit bacterial growth, and enhance phagocytosis of immune cells. Additionally, some galectins contribute to the antiviral immune defenses of aquatic animals. This review aims to review recent advancements in the antimicrobial mechanisms, molecular structures, and evolution of galectins from fish, mollusks, and crustaceans. The antimicrobial galectins, as crucial components in the innate immune defense, pave new avenues for developing innovative disease control strategies in aquaculture.

Impact of mutations in carbohydrate binding sites of tandem-repeat type galectin from Takifugu obscurus on its antimicrobial activity

Galectins belong to a family of galactoside-binding proteins and exhibit diverse biological functions. In the present research, a tandem-repeat type galectin (named ToGalectin) was identified from obscure puffer Takifugu obscurus. The 296 amino acids ToGalectin contained two carbohydrate recognition domains (CRDs), one of which possessed two conserved carbohydrate binding motifs. Phylogenetic analysis showed that ToGalectin clustered tightly with other galectin-8 proteins from teleost fish. ToGalectin transcripts were ubiquitously expressed in all tissues examined and its expression was significantly upregulated in the liver, kidney, and intestine after Vibrio harveyi or Staphylococcus aureus infection. To investigate the effect of carbohydrate binding sites on biological activity, ToGalectin and its mutant (MUT-ToGalectin) were expressed and purified. The recombinant ToGalectin and MUT-ToGalectin proteins showed strong agglutinating activity against both V. harveyi and S. aureus. rToGalectin could bind to all tested carbohydrates and bacteria, whereas rMUT-ToGalectin bound to some carbohydrates and bacteria with specific and relatively strong affinity. rToGalectin significantly suppressed the growth of all six bacteria detected and promoted bacterial clearance in vivo, whereas MUT-ToGalectin inhibited the growth of only two bacterial species, which could be attributed to the differences in conserved motifs within the CRDs. Our results suggested that ToGalectin is involved in the immune response against bacterial infection and the clearance of pathogens in T. obscurus.

Molecular cloning and functional characterization of eight galectin genes from Takifugu obscurus

Galectins are a family of animal lectins involved in the immune response against pathogens. However, the roles of fish galectins during pathogen infection require comprehensive studies. In the present research, eight different galectin genes from Takifugu obscurus (named ToGalec1-8) were identified and characterized. ToGalec1-8 encoded proteins of 240, 182, 373, 145, 452, 135, 359 and 346 amino acids, respectively. All predicted ToGalec1-8 proteins possessed one or more conserved carbohydrate recognition domains (CRDs). Phylogenetic analysis revealed that ToGalec1-8 were evolutionarily closely related to their counterparts in other selected vertebrates, hinting their genetic relationship. Tissue distribution analysis showed that most ToGalec genes were distributed ubiquitously in all detected tissues, with relatively high expression in immune tissues. After stimulation by Vibrio harveyi and Staphylococcus aureus, the mRNA transcripts of ToGalec1-8 in liver and kidney were significantly upregulated. In addition, RNA interference experiments indicated that knockdown of ToGalec1 and ToGalec7 inhibited the clearance of bacteria in vivo. Taken together, these obtained results suggested that ToGalec1-8 play an important role in innate immunity and defense against bacterial infection in T. obscurus.

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.

A Fish Galectin-8 Possesses Direct Bactericidal Activity

Galectins are a family of animal lectins with high affinity for β-galactosides. Galectins are able to bind to bacteria, and a few mammalian galectins are known to kill the bound bacteria. In fish, no galectins with direct bactericidal effect have been reported. In the present study, we identified and characterized a tandem repeat galectin-8 from tongue sole Cynoglossus semilaevis (designated CsGal-8). CsGal-8 possesses conserved carbohydrate recognition domains (CRDs), as well as the conserved HXNPR and WGXEE motifs that are critical for carbohydrate binding. CsGal-8 was constitutively expressed in nine tissues of tongue sole and up-regulated in kidney, spleen, and blood by bacterial challenge. When expressed in HeLa cells, CsGal-8 protein was detected both in the cytoplasm and in the micro-vesicles secreted from the cells. Recombinant CsGal-8 (rCsGal-8) bound to lactose and other carbohydrates in a dose dependent manner. rCsGal-8 bound to a wide range of gram-positive and gram-negative bacteria and was co-localized with the bound bacteria in animal cells. Lactose, fructose, galactose, and trehalose effectively blocked the interactions between rCsGal-8 and different bacteria. Furthermore, rCsGal-8 exerted potent bactericidal activity against some gram-negative bacterial pathogens by directly damaging the membrane and structure of the pathogens. Taken together, these results indicate that CsGal-8 likely plays an important role in the immune defense against some bacterial pathogens by direct bacterial interaction and killing.

Cherry Valley Duck Galectin-2 Plays an Essential Role in Avian Pathogenic Escherichia coli Infection-Induced Innate Immune Response

Galectins play important roles in the host's innate immunity as pattern recognition receptors. In this study, the coding sequences of galectin-2 were identified from Cherry Valley ducks. Tissue distribution of duck galectin-2 (duGal-2) in healthy ducks and ducks infected with avian pathogenic Escherichia coli (APEC) was studied, respectively. The results showed that duGal-2 expression was higher in the gut, kidney, and liver tissue, and weakly expressed in the lung and brain, in healthy ducks; however, the expression level of duGal-2 was detected as being up-regulated after infection with APEC. In addition, knockdown or overexpression of duGal-2 in DEFs was achieved by small interference RNA (siRNA) transfection and plasmid transduction, respectively. The knockdown of duGal-2 led to a decrease in the expression of some inflammatory cytokines such as IL-1β, IL-6, and IL-8, while the expression levels of anti-inflammatory factor IL-10 were up-regulated. At the same time, the bacterial load of APEC was increased after knockdown of duGal-2 in vitro. However, the opposite results were obtained in the duGal-2 overexpression group. Taken together, duGal-2 plays an important role in the host against APEC infection.

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, transcriptional and functional delineation of Galectin-8 from black rockfish (Sebastes schlegelii) and its potential immunological role

Galectins are β-galactoside-binding lectins, which are involved in pattern recognition, cell adhesion, and stimulation of the host innate immune responses against microbial pathogens. In spite of several functional studies on different galectins isolated from vertebrates and invertebrates, this is the first report to present functional studies for galectin-8 from the marine teleost tissues. In the present study, we characterized galectin-8 homolog from black rockfish (Sebastes schlegelii), in molecular and functional aspects. Rockfish galectin-8 (SsGal8) was found to consist of a 969 bp long open reading frame (ORF), encoding a protein of 322 amino acids and the predicted molecular weight was 35.82 kDa. In silico analysis of SsGal8 revealed the presence of two carbohydrate binding domains (CRDs), at both N and C-termini and a linker peptide of 40 amino acids, in between the two domains. As expected, the phylogenetic tree categorized SsGal8 as a tandem-repeat galectin, and ultimately positioned it in the sub-clade of fish galectin-8. rSsGal8 was able to strongly agglutinate fish erythrocytes and the inhibition of agglutination was successfully exhibited by lactose and d-galactose. Bacterial agglutination assay resulted in agglutination of both Gram (+) and Gram (-) bacteria, including Escherichia coli, Vibrio harveyi, Vibrio parahaemolyticus, Streptococcus parauberis, Lactococcus garvieae, Streptococcus iniae and Vibrio tapetis. The tissue distribution analysis based on qPCR assays, revealed a ubiquitous tissue expression of SsGal8 for the examined rockfish tissues, with the most pronounced expression in blood, followed by brain, intestine, head kidney and kidney. Furthermore, the mRNA transcription level of SsGal8 was significantly up-regulated in spleen, liver and head kidney, upon immune challenges with Streptococcus iniae, LPS and poly I:C, in a time dependent manner. Taken together, these findings strongly suggest the contribution of SsGal8 in regulating innate immune responses to protect the rockfish from bacterial infections.

Characterization of Galectin-2 from Nile tilapia (Oreochromis niloticus) involved in the immune response to bacterial infection

galectin-2 plays important roles in innate and adaptive immunity. In this study, galectin-2 (OnGal-2) was identified from Nile tilapia (Oreochromis niloticus). Its tissue distribution and expression patterns following bacterial infection were also investigated. OnGal-2 is widely distributed in various tissues of healthy tilapia. After Streptococcus agalactiae challenge, OnGal-2 expressions were significantly up-regulated in all tested tissues. Meanwhile, the recombinant OnGal-2 (rOnGal-2) protein showed strong agglutinating activities against both Gram-negative bacteria and Gram-positive bacteria. Moreover, rOnGal-2 could promote phagocytosis of macrophages. Taken together, the present study indicated that OnGal-2 might play roles in the immune responses of Nile tilapia against bacterial pathogens.

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

Galectins are a family of proteins with conserved carbohydrate recognition domains (CRDs) that bind to specific glycans, including the glycans on the surface of pathogens, and therefore play a role in cytokine secretion, cell activation, migration, adhesion and apoptosis. Currently, galectins have been extensively studied in mammalian species but rarely studied in teleost fish species. In this study, a total of 12 galectin genes were characterized to understand the molecular mechanisms of galectin function in Japanese pufferfish (Takifugu rubripes). Phylogenetic analyses and syntenic analyses confirmed their correct annotation and suggested the strongest relationships to tetraodon. Furthermore, expression analyses were conducted in healthy tissues of Japanese pufferfish and after infection with Vibrio harveyi in the intestine, liver and spleen. The results showed that galectin genes were widely expressed in all examined tissues; however, most of the galectin genes were highly expressed in mucosal tissues (skin, gill and intestine). Moreover, majority of the galectin genes were significantly regulated after V. harveyi infection in the intestine, liver and spleen, suggesting that galectins were involved in the immune response to V. harveyi infection in Japanese pufferfish. This study established the foundation for future studies of galectin gene functions.