Molecular characterization and expression analysis of interferon-gamma-inducible lysosomal thiol reductase (GILT) gene from pearl oyster Pinctada fucata

Planarian gamma-interferon-inducible lysosomal thiol reductase (GILT) is required for gram-negative bacterial clearance

The powerful regenerative ability of planarians has long been a concern of scientists, but recently, their efficient immune system has attracted more and more attention from researchers. Gamma-interferon-inducible lysosomal thiol reductase (GILT) is related not only to antigen presentation but also to bacteria invasions. But the systematic studies are not yet to be conducted on the relationship between bacterial infection. Our study reveals for the first time that GILT of planarian (DjGILT) plays an essential role in the clearance of Gram-negative bacteria by conducting H₂O₂ concentration in planarians. In animals that DjGILT was silenced, it persisted for up to 9 days before all bacteria were cleared, compared with 6 days of the control group. When infected with E. coli and V. anguillarum, the level of H₂O₂ was significantly increased in DjGILT-silenced planarians, and concomitantly, mRNA level of C-type lectin DjCTL, which modulates agglutination and clearance efficiency of invading bacteria, was decreased. Further study showed that the decrease of H₂O₂ level led to a significant increase in DjCTL transcripts. Collectively, we proposed a mechanism model for the involvement of GILT gene in bacterial elimination. We have for the first time revealed the specific mechanism of GILT in innate immune response against bacterial infection.

Penaeus monodon GILT enzyme restricts WSSV infectivity by reducing disulfide bonds in WSSV proteins

Gamma-interferon-inducible lysosomal thiol reductase (GILT) is involved in the adaptive immune response via its effects on major histocompatibility complex (MHC)-restricted antigen presentation. In addition to antigen presentation, GILT exerts its antiviral activity by reducing disulfide bonds in proteins involved in viral infection and assembly, thereby inhibiting viral envelope-mediated infection and viral progeny production. In black tiger shrimp, Penaeus monodon GILT (PmGILT) was cloned and characterized, and found to be involved in the shrimp innate immune response and to exert neutralizing activity against white spot syndrome virus (WSSV) infection. However, the anti-WSSV mechanism of PmGILT in the shrimp innate immune response has not been defined. To explore the anti-WSSV activity of PmGILT, a yeast 2-hybrid (Y2H) assay was performed to identify WSSV proteins targeted by PmGILT. The assay revealed 4 potential PmGILT-interacting WSSV proteins: WSSV002, WSSV164, WSSV189, and WSSV471. Three of these 4 WSSV proteins (WSSV002, WSSV164 and WSSV189) were successfully produced and confirmed to interact with PmGILT in in vitro pull-down assays. WSSV189 and WSSV471 were previously identified as structural proteins, whereas WSSV164 is an immediate-early protein which has anti-melanization activity, and WSSV002 is an unknown. Because of the thiol reductase activity of PmGILT, WSSV164 and WSSV189, both of which are cysteine-containing WSSV proteins, were chosen for disulfide bond reduction assays. PmGILT reduced intrachain disulfide bonds in both WSSV proteins, suggesting that PmGILT exerts its anti-WSSV activity via its thiol reductase activity to disrupt the WSSV protein complex and restore the melanization activity of PmproPO1 and PmproPO2.

Identification and functional characterization of interferon-γ-inducible lysosomal thiol reductase (GILT) gene in common Chinese cuttlefish Sepiella japonica

Interferon-γ-inducible lysosomal thiol reductase (GILT) is a pivotal enzyme involved in the histocompatibility complex (MHC) class II-restricted antigen processing whereby it catalyzes the disulfide bond reduction in the endocytic pathway. Here, a novel GILT homologue termed as SjGILT firstly identified from common Chinese cuttlefish Sepiella japonica. SjGILT shared domain topology containing a signal peptide, a signature sequence CQHGX₂ECX₂NX₄C, an activate-site CXXC motif, two potential N-glycosylation sites and six conserved cysteins with its counterparts in other animals. SjGILT transcripts were constitutively expressed in all examined tissues in S. japonica, with the higher expression levels in immune-related tissues such as pancreas, intestines, liver and gills. Upon lipopolysaccharide (LPS) challenge, SjGILT transcripts were significantly induced in liver and gill tissues, and SjGILT protein transferred to late endosomes and lysosomes in HeLa cells. Further study showed that recombinant SjGILT had obvious thiol reductase activity demonstrated by reducing the interchain disulfide bonds of IgG under acidic conditions. Taken together, these results suggested that SjGILT may be involved in the immune response to bacteria challenge, and then might play an important role in the processing of MHC class II-restricted antigens in S. japonica.

Identification and characterization of the interferon-γ-inducible lysosomal thiol reductase gene in Chinese soft-shelled turtle, Pelodiscus sinensis

The reduction of disulfide bonds of exogenous antigens is crucial to the MHC-II class antigen processing and presenting pathway and is catalysed by interferon-γ-inducible lysosomal thiol reductase (GILT). In this study, a reptile GILT gene from Chinese soft-shelled turtle, Pelodiscus sinensis (PsGILT), was identified. The full-length cDNA of PsGILT is 1631 nucleotides (nt), including a 5'-untranslated region (UTR) of 3 nt, a 3'-UTR of 860 nt and an open reading frame (ORF) of 768 nt encoding 255 amino acids (aa). The conserved features in known GILTs, such as signal peptide, CXXC motif, GILT signature sequence, N-glycosylation site and conserved cysteines, were all found in the putative PsGILT protein. Genomic analysis revealed that PsGILT kept the "7 exons and 6 introns" structure of vertebrate GILT genes. PsGILT was expressed in all examined organs/tissues and was mainly expressed in spleen and blood. Increased mRNA expression levels of PsIFN-γ and PsGILT in PBLs were observed after induction with LPS, PolyI:C and recombinant IFN-γ (rIFN-γ). We also tested the reductase activity of rGILT in vitro and found that it could reduce intact human IgG into H chains and L chains. These above results implied that PsGILT may play an important role in resisting bacterial and viral infections, like other vertebrate GILTs.

Immune response of interferon-γ-inducible lysosomal thiol reductase (GILT) from Chinese sturgeon (Acipenser sinensis) to microbial invasion and its antioxdative activity in lipopolysaccharides-treated mammalian dentritic cells

Interferon-γ-inducible lysosomal thiol reductase (GILT) plays an important role in the major histocompatibility complex-restricted antigen processing of endocytosed proteins via catalyzing the disulfide bond reduction in the endocytic pathway. Here, the cDNA of Chinese sturgeon (Acipenser sinensis) GILT (CsGILT) was cloned. It contained an open reading frame of 762 nucleotides encoding a protein of 254 amino acids with an estimated molecular weight of 28.1 kDa. The characteristic structural features, including a signature sequence CQHGX2ECX2NX4C, a CXXC motif, two potential N-glycosylation sites, and eight conserved cysteines were detected in the deduced amino acid sequence of CsGILT. CsGILT was widely expressed in Chinese sturgeon with the highest expression in the spleen, and CsGILT mRNA expression was significantly up-regulated when Chinese sturgeons were challenged with polyinosinic polycytidylic acid or Vibrio anguillarum. The recombinant CsGILT displayed obvious thiol reductase activity demonstrated by catalyzing the reduction of mouse IgG(H+L) by dithiothreitol into heavy chain and light chain. CsGILT also displayed significant antioxidant activity in mouse dentritic cells as indicated by its increasing GSH level and GSH/GSSG ratio, decreasing intracellular reactive oxygen species and nitric oxide levels and lipid peroxidation, as well as enhancing the activities of the antioxidative redox enzymes including catalase and superoxide dismutase. Our results suggested an important role for CsGILT in the immune response in Chinese sturgeon to pathogen invasion possibly via a conserved functional mechanism throughout vertebrate evolution, contributing to our understanding the immune biology and protection of Chinese sturgeon.

Transcriptome profiling of red swamp crayfish (Procambarus clarkii) hepatopancreas in response to lipopolysaccharide (LPS) infection

The RNA-sequencing followed by de novo assembly generated 61,912 unigene sequences of P. clarkii hepatopancreas. Comparison of gene expression between LPS challenged and PBS control samples revealed 2552 differentially expressed genes (DEGs). Of these sequences, 1162 DEGs were differentially up-regulated and 1360 DEGs differentially down-regulated. The DEGs were then annotated against gene ontology (GO) database and Kyoto Encyclopedia of Genes and Genomes (KEGG) database. Some immune-related pathways such as PPAR signaling pathway, lysosome, Chemical carcinogenesis, Peroxisome were predicted by canonical pathways analysis. The reliability of transcriptome data was validated by quantitative real time polymerase chain reaction (qRT-PCR) for the selected genes. The data presented here shed light into antibacterial immune responses of crayfish. In addition, these results suggest that transcriptomic data provides valuable sequence resource for immune-related gene identification and helps to understand P. clarkii immune functions.

The first echinoderm gamma-interferon-inducible lysosomal thiol reductase (GILT) identified from sea cucumber (Stichopus monotuberculatus)

Gamma-interferon-inducible lysosomal thiol reductase (GILT) has been described as a key enzyme that facilitating the processing and presentation of major histocompatibility complex class II-restricted antigen in mammals. In this study, the first echinoderm GILT named StmGILT was identified from sea cucumber (Stichopus monotuberculatus). The StmGILT cDNA is 1529 bp in length, containing a 5'-untranslated region (UTR) of 87 bp, a 3'-UTR of 674 bp and an open reading frame (ORF) of 768 bp that encoding a protein of 255 amino acids with a deduced molecular weight of 27.82 kDa and a predicted isoelectric point of 4.73. The putative StmGILT protein possesses all the main characteristics of known GILT proteins, including a signature sequence, a reductase active site CXXC, twelve conserved cysteines, and two potential N-linked glycosylation sites. For the gene structure, StmGILT contains four exons separated by three introns. In the promoter region of StmGILT gene, an NF-κB binding site and an IFN-γ activation site were found. The thiol reductase activity of recombinant StmGILT protein was also demonstrated in this study. In addition, the highest level of mRNA expression was noticed in coelomocytes of S. monotuberculatus. In in vitro experiments performed in coelomocytes, the expression of StmGILT mRNA was significantly up-regulated by lipopolysaccharides (LPS), inactivated bacteria or polyriboinosinic polyribocytidylic acid [poly (I:C)] challenge, suggested that the sea cucumber GILT might play critical roles in the innate immune defending against bacterial and viral infections.

The invertebrate midintestinal gland ("hepatopancreas") is an evolutionary forerunner in the integration of immunity and metabolism

The immune system has an impact on the metabolic performance in vertebrates, thus the metabolic effects of immune cells are receiving intense attention today in the biomedical field. However, the evolutionary origin of the immunity-metabolism interaction is still uncertain. In this review, I show that mollusks and crustaceans integrate immune functions to a metabolic organ, the midintestinal gland ("hepatopancreas"). In these animals, the epithelial cells of the midintestinal gland are major sources of immune molecules, such as lectins, hemocyanin, ferritin, antibacterial and antiviral proteins, proteolytic enzymes and nitric oxide. There is crosstalk between midintestinal gland cells and phagocytes, which aids the initiation of the immune response and the clearance of pathogens. The midintestinal gland is thereby an integrated organ of immunity and metabolism. It is likely that immunity was the primary function of the midintestinal gland cells and that their role in the intermediate metabolism has evolved during the course of their further specialization.

Identification of gamma-interferon-inducible lysosomal thiol reductase (GILT) homologues in the fruit fly Drosophila melanogaster

Gamma-interferon-inducible lysosomal thiol reductase (GILT) has been demonstrated to be involved in the immune response to bacterial challenge in various organisms. However, little is known about GILT function in innate immunity. Drosophila has been commonly used as a model for the study of the innate immune response of invertebrates. Here, we identify the CG9796, CG10157, and CG13822 genes of fruit fly Drosophila melanogaster as GILT homologues. All deduced Drosophila GILT coding sequences contained the major characteristic features of the GILT protein family: the GILT signature CQHGX2ECX2NX4C sequence and the active site CXXC or CXXS motif. The mRNA transcript levels of the Drosophila GILT genes were up-regulated after Gram-negative bacteria Escherichia coli DH5α infection. Moreover, a bacterial load assay showed that over-expression of Drosophila GILT in fat body or hemocytes led to a low bacterial colony number whereas knock-down of Drosophila GILT in fat body or hemocytes led to a high bacterial colony number when compared to a wild-type control. These results indicate that the Drosophila GILTs are very likely to play a role in the innate immune response upon bacterial challenge of Drosophila host defense. This study may provide the basis for further study on GILT function in innate immunity.

Isolation of gamma-interferon-inducible lysosomal thiol reductase (GILT) from the Yangtze finless porpoise

In this study, we isolated the cDNA of a gamma-interferon-inducible lysosomal thiol reductase (GILT), which is critical for innate immune regulation, from the Yangtze finless porpoise (FpGILT). This gene encoded a protein with 244 amino acids and a predicted molecular weight of 28 kDa. The amino acid sequence of FpGILT includes an active-site CXXC motif, a GILT signature sequence, CQHGX2ECX2NX4C, and three N-linked glycosylation sites. Phylogenetic analysis showed that FpGILT and other GILT family members were derived from a common ancestor and finless porpoises are closely related to artiodactyla. Recombinant protein (FpsGILT) was then efficiently expressed and purified, and thiol reductase activity assays suggested that FpGILT catalyses disulfide bond reduction. These findings provide a basis for understanding the characteristics of immunity in the finless porpoise and other aquatic mammals.

Gigabase-scale transcriptome analysis on four species of pearl oysters

Pearl oysters have been found to secrete nacre and form pearls with good quality and significant commercial interest. However, the transcriptomic and genomic resources for pearl oysters are still limited. To improve this situation, transcriptome sequencing was conducted from four species of pearl oysters with Illumina HiSeq™ 2000. There were four gigabase-scale transcriptomes for four species of pearl oysters, ∼26.3 million reads with ∼2.37 gigabase base pairs (Gbp) in Pinctada fucata, ∼26.5 million reads with ∼2.39 Gbp in Pinctada margaritifera, ∼27.0 million reads with ∼2.43 Gbp in Pinctada maxima, and ∼25.9 million reads with ∼2.33 Gbp in Pteria penguin, respectively. After sequence assembly and blastx alignment, the numbers of annotated unigenes ≥200 bp were 33,882 in P. fucata, 30,666 in P. margaritifera, 26,420 in P. maxima, and 29,928 in P. penguin. Based on these annotated unigenes among four species of pearl oysters, CDSs were extracted and predicted and furthermore, analyses of GO and KEGG assignments were performed. In addition, 60 putative genes of growth factors and their receptors from four species of pearl oysters were predicted. This study established an excellent resource for gene discovery and expression in pearl oysters, but also offered a significant platform for functional genomics and comparative genomic studies for mollusks.

Molecular cloning and characterization of class I NF-κB transcription factor from pearl oyster (Pinctada fucata)

NF-κB transcription factors play central roles in many important physiological and pathological processes including innate immune responses. Here we report the cloning of an NF-κB transcription factor, PfRelish from pearl oyster Pinctada fucata, one of the most important bivalve mollusks for seawater pearl production. PfRelish full-length cDNA is 3916 bp with an open reading frame of 3558 bp encoding a putative protein of 1186 amino acids. The deduced PfRelish contains a N-terminal RHD, a nucleus localization signal, an IκB-like domain with six ankyrin repeats and a death domain at the C-terminus, which is similar to class I NF-κB transcription factors. Comparison and phylogenetic analysis revealed that class I NF-κBs in mollusks including PfRelish might have most distant relationship to the arthropod Relish. Further expression analysis showed that PfRelish was apparently upregulated after Vibrio alginolyticus injection, which suggested that PfRelish was involved in the immune response to V. alginolyticus.

cDNA cloning and mRNA expression of a tandem-repeat galectin (PoGal2) from the pearl oyster, Pinctada fucata

Galectins can recognize and specifically bind to β-galactoside residues, playing crucial roles in innate immune responses of vertebrates and invertebrates. We cloned the cDNA of a tandem-repeat galectin from the pearl oyster Pinctada fucata (designated as PoGal2). PoGal2 cDNA is 1347 bp long and consists of a 5'-untranslated region (UTR) of 3 bp, a 3'-UTR of 297 bp with one cytokine RNA instability motif (ATTTA), and an open reading frame of 1047 bp, encoding a polypeptide of 349 amino acids, with an estimated molecular mass of 38.1 kDa and a theoretical isoelectric point of 8.5. PoGal2 contains two carbohydrate recognition domains (CRDs); both have the conserved carbohydrate-binding motifs H-NPR and WG-EE. PoGal2 shares 50.6 and 50.9% identity with those of abalone (Haliotis discus) and the Manila clam (Venerupis philippinarum), respectively. Phylogenetic analysis revealed that the tandem-repeat galectins formed two clades for the different species. Molluscan tandem-repeat galectins were clustered into a single clade, and nematode tandem-repeat galectins were clustered into another single clade. In both clades, CRD-N and CRD-C were divided into different groups. PoGal2 mRNA was constitutively expressed in all tissues analyzed, and the expression level of PoGal2 mRNA was found to be significantly up-regulated in digestive glands, gills and hemocytes after Vibrio alginolyticus stimulation/infection. Expression profile analysis showed that the expression level of PoGal2 mRNA was significantly up-regulated at 8, 12 and 24 h after V. alginolyticus infection. These results suggest that PoGal2 is a constitutive and inducible acute-phase protein involved in the innate immune response of pearl oysters.

Molecular characterization and mRNA expression of catalase from pearl oyster Pinctada fucata

Catalase (EC 1.11.1.6) is an important antioxidant enzyme that protects aerobic organisms against oxidative damage by degrading hydrogen peroxide to water and oxygen. In the present study, a catalase cDNA of peal oyster Pincatada fucata (designated as PoCAT) is cloned and characterized by expressed sequence tag (EST) and rapid amplification of cDNA ends (RACE) methods. PoCAT is 2428 bp long and consists of a 5'-UTR of 140 bp, an unusually long 3'-UTR of 749 bp, and an open reading frame (ORF) of 1539 bp. The ORF of PoCAT encodes a polypeptide of 512 amino acids with molecular weight of 58.1 kDa and the theoretical isoelectric point of 8.4. PoCAT shares 62.3-82.2% identity and 73.0-92.0% similarity to other catalase amino acid sequences. Sequence alignment indicates that PoCAT contains the proximal heme-ligand signature sequence (R³⁵¹LFSYSDT³⁵⁸), the proximal active site signature (F⁶¹NRERIPERVVHAKGGGA⁷⁸), and the three catalytic amino acid residues (His⁷², Asn¹⁴⁵, and Tyr³⁵⁵). PoCAT has two potential glycosylation sites (N⁴³⁶YS⁴³⁸ and N⁴⁷⁸FS⁴⁸⁰) and a peroxisome targeting signal (ASL). PoCAT mRNA was ubiquitously expressed in all detected tissues, and the expression level of PoCAT mRNA was higher in intestine and mantle. The expression profile analysis showed that the expression level of PoCAT mRNA in intestine was significantly up-regulated at 2, 4 and 12 h after Vibrio alginolyticus stimulation. These results demonstrated that PoCAT is a typical member of catalase family and might be involved in innate immune responses of pearl oyster.

A macrophage migration inhibitory factor like oxidoreductase from pearl oyster Pinctada fucata involved in innate immune responses

Macrophage migration inhibitory factor (MIF) is an important cytokine and plays a crucial role as a pivotal regulator of innate immunity. In this study, a MIF cDNA was identified and characterized from the pearl oyster Pinctada fucata (designated as PoMIF). The full-length of PoMIF was 1544 bp and consisted of a 5'-untranslated region (UTR) of 45 bp, a 3'-UTR of 1139 bp with a polyadenylation signal (AATAAA) at 12 nucleotides upstream of the poly (A) tail. The open reading frame (ORF) of PoMIF was 360 bp which encoded a polypeptide of 120 amino acids with an estimated molecular mass of 13.3 kDa and a predicted pI of 6.1. SMART analysis showed that PoMIF contained the catalytic-sites P² and K³³ for tautomerase activity, a motif C⁵⁷GSV⁶⁰ for oxidoreductase activity and a MIF family signature D⁵⁵PCGSVEVYSIGALG⁶⁹. Homology analysis revealed that the PoMIF shared 40.3-65.5% similarity and 26.9-45.0% identity to other known MIF sequences. PoMIF mRNA was constitutively expressed in seven selected tissues of healthy pearl oysters, with the highest expression level in digestive gland. Eight hours after P. fucata was injected with Vibrio alginolyticus, the expression of PoMIF mRNA was significantly up-regulated in digestive gland, gills, hemocytes and intestine. The cDNA fragment encoding mature protein of PoMIF was subcloned to expression vector pRSET and transformed into Escherichia coli BL21 (DE3). The recombinant PoMIF (rPoMIF) was expressed and purified under optimized conditions. Function analysis showed that rPoMIF had oxidoreductase activity and could utilize dithiothreitol (DTT) as reductant to reduce insulin.

Disulfide reduction in the endocytic pathway: immunological functions of gamma-interferon-inducible lysosomal thiol reductase

Gamma-interferon-inducible lysosomal thiol reductase (GILT) is constitutively expressed in most antigen presenting cells and is interferon γ inducible in other cell types via signal transducer and activator of transcription 1. Normally, N- and C-terminal propeptides are cleaved in the early endosome, and the mature protein resides in late endosomes and lysosomes. Correspondingly, GILT has maximal reductase activity at an acidic pH. Monocyte differentiation via Toll-like receptor 4 triggers secretion of a disulfide-linked dimer of the enzymatically active precursor, which may contribute to inflammation. GILT facilitates major histocompatibility complex (MHC) class II-restricted processing through reduction of protein disulfide bonds in the endocytic pathway and is hypothesized to expose buried epitopes for MHC class II binding. GILT can also facilitate the transfer of disulfide-containing antigens into the cytosol, enhancing their cross-presentation by MHC class I. A variety of antigens are strongly influenced by GILT-mediated reduction, including hen egg lysozyme, melanocyte differentiation antigens, and viral envelope glycoproteins. In addition, GILT is conserved among lower eukaryotes and likely has additional functions. For example, GILT expression increases the stability of superoxide dismutase 2 and decreases reactive oxygen species, which correlates with decreased cellular proliferation. It is also a critical host factor for infection with Listeria monocytogenes.

A multidomain galectin involved in innate immune response of pearl oyster Pinctada fucata

Galectins could specifically bind to β-galactoside residues and play crucial roles in innate immune responses of vertebrates and invertebrates. In this study, the cDNA of a galectin with multiple carbohydrate-recognition domains (CRDs) was cloned from pearl oyster Pinctada fucata (designated as PoGal). PoGal cDNA was 2138bp long and consisted of a 5'-untranslated region (UTR) of 120bp, a 3'-UTR of 350bp with two cytokine RNA instability motifs (ATTTA), and an open reading frame (ORF) of 1668bp encoding a polypeptide of 555 amino acids with an estimated molecular mass of 63.4kDa and a theoretical isoelectric point of 4.8. PoGal contained four CRDs, each CRD of PoGal all had the conserved carbohydrate-binding motifs H-NPR and WG-ER. PoGal shared 43.7% and 62.9% identity to those of bay scallop and eastern oyster, respectively, which were only two galectins with four CRDs. The phylogenetic analysis revealed that all galectins with four CRDs formed a single clade. PoGal mRNA was constitutively expressed in all detected tissues, and the expression level of PoGal mRNA was significantly up-regulated in digestive gland, mantle, haemocyte, gonad and intestine after Vibrio alginolyticus stimulation. The expression profile analysis showed that the expression level of PoGal mRNA was significantly up-regulated at 4, 8 and 12h after V. alginolyticus stimulation. These results suggested that PoGal was a constitutive and inducible acute-phase protein that perhaps involved in innate immune response of pearl oyster.