Molecular cloning and expression analysis of mannose receptor in blunt snout bream (Megalobrama amblycephala)

Identification and functional analysis of Mannose receptor in Asian swamp eel (Monopterus albus) in response to bacterial infection

Mannose receptor (MR), as a member of the C-type lectin (CLEC) family, plays an important role in the internalize pathogen-associated ligands and activate immune response. In the present study, MR was identified and characterized from Asian swamp eel (Monopterus albus) (namely MaMR). The open reading frame of MaMR was 4311 bp in length encoding 1437 amino acids of a ∼162.308 kDa protein, including a cysteine-rich (CR) domain, a fibronectin type II (FNII) domain, eight C-type lectin-like domains (CTLDs), a transmembrane domain and a short cytoplasmic domain. Phylogenetic analysis indicated that MaMR shared the highest similarity with that of Paralichthys olivaceus. The expression of MaMR was found in all the examined tissues, with the highest expression in the spleen and kidney. After injection with Edwardsiella tarda, the transcript level of MaMR was initially reduced and then significantly elevated in the liver, spleen, foregut and hindgut. In the isolated peripheral blood leukocytes, the expression of MaMR was significantly induced post stimulated with LPS and LTA. Then the MaMR-CTLD4-8 recombinant protein was purified. Bacterial agglutination and binding assay showed that rMaMR-CTLD4-8 could bind with both Gram-positive and Gram-negative bacteria and agglutinate bacteria in the presence of calcium in vitro. Further analysis revealed that MaMR and TLR2 coordinately induced the expression of TRAF6 and promoted the phosphorylation level of p65, leading to the expression of proinflammatory cytokines il-1β and tnf-α in EPC cells. Taken together, these results reveal that MaMR plays an important role in the immune response of fish to pathogen infections.

Expression and functional characterization of the mannose receptor (MR) from Nile tilapia (Oreochromis niloticus) in response to bacterial infection

Mannose receptor (MR) as a member of the pattern recognition receptors (PRRs) plays an important role in the immune response. In mammals, the role of MR in the regulation of phagocytosis is clarified; however, its contribution to opsonize phagocytosis remains unclear in bony fish. In this study, the expression pattern of Nile tilapia mannose receptor gene (OnMR) was investigated and its regulation of the phagocytosis of monocytes/macrophages to pathogenic bacteria was identified. The full-length of OnMR open reading frame is 4314 bp, encoding a peptide containing 1437 amino acid residues. The deduced amino acid sequence revealed that OnMR contained a cysteine-rich domain, a fibronectin type II domain, multiple C-type lectin-like domains, a transmembrane domain and a short cytoplasmic domain. Tissue distribution analysis showed the OnMR transcripts was widely distribute in the ten detected tissues, and highly expressed in head kidney, hind kidney, intestine and spleen. After S. agalactiae and A. hydrophila infection, the expression of OnMR in head kidney and spleen increased significantly. Moreover, the expression of OnMR in MO/Mø were also upregulated post the infection of bacteria and mannose solutions in vitro. This suggested that MR, as a mannose receptor on macrophage surface, could respond strongly to the stimulation of pathogenic bacteria. In addition, the (r)OnMR protein could effectively bind and agglutinate S. agalactiae and A. hydrophila, and regulate the phagocytic ability of monocytes/macrophages to pathogenic bacteria. These results suggest that OnMR is involved in response against bacterial infection in Nile tilapia, and this study will help us better understand the function of MR in teleost fish.

Mannose Receptor Mediates the Activation of Chitooligosaccharides on Blunt Snout Bream (Megalobrama amblycephala) Macrophages

Chitooligosaccharide (COS) is an important immune enhancer and has been proven to have a variety of biological activities. Our previous research has established an M1 polarization mode by COS in blunt snout bream (Megalobrama amblycephala) macrophages, but the mechanism of COS activation of blunt snout bream macrophages remains unclear. In this study, we further explored the internalization mechanism and signal transduction pathway of chitooligosaccharide hexamer (COS6) in blunt snout bream macrophages. The results showed that mannose receptor C-type lectin-like domain 4-8 of M. amblycephala (MaMR CTLD4-8) could recognize and bind to COS6 and mediate COS6 into macrophages by both clathrin-dependent and caveolin-dependent pathways. In the inflammatory response of macrophages activated by COS6, the gene expression of tumor necrosis factor (TNF)-α, interleukin (IL)-1β, and nitric oxide synthase 2 (NOS2) was significantly inhibited after MaMR CTLD4-8-specific antibody blockade. However, even if it was blocked, the expression of these inflammation-related genes was still relatively upregulated, which suggested that there are other receptors involved in immune regulation. Further studies indicated that MaMR CTLD4-8 and Toll-like receptor 4 (TLR4) cooperated to regulate the pro-inflammatory response of macrophages caused by COS6. Taken together, these results revealed that mannose receptor (MR) CTLD4-8 is indispensable in the process of recognition, binding, internalization, and immunoregulation of COS in macrophages of blunt snout bream.

A C-type lectin with antibacterial activity in weather loach, Misgurnus anguillicaudatus

C-type lectins are carbohydrate-binding proteins that play important roles in immunity by serving as pattern recognition receptors. In the present study, a novel nattectin-like C-type lectin was obtained from the weather loach, Misgurnus anguillicaudatus, designated as MaCTL. MaCTL encodes a peptide with 165 amino acids, with a signal peptide and a single C-type lectin domain (CTLD), containing a galactose-specific QPD motif and a conserved Ca²⁺ -binding site. Transcripts of MaCTL were significantly upregulated after immune challenge with its pathogen A. hydrophila. In vitro assays with recombinant MaCTL protein revealed that it exhibited hemagglutinating and bacterial agglutinating activities, in a Ca²⁺ -dependent manner. MaCTL was found to bind to a wide range of bacteria, as well as bind to bacterial polysaccharides LPS and PGN. Moreover, MaCTL displayed antimicrobial activity by inhibiting the growth of bacteria. These results collectively suggest that MaCTL is involved in the antibacterial defence of weather loach.

Effects of oligochitosan on the growth, immune responses and gut microbes of tilapia (Oreochromis niloticus)

The immunomodulatory effects of oligochitosan have been demonstrated in several fish. However, the underlying mechanisms are not well characterized. The profound interplay between gut microbes and aquaculture has received much scientific attention but understanding the alternations of microbes populating in gut of tilapia (Oreochromis niloticus) fed with oligochitosan remains enigmatic. In this study, the effects of oligochitosan on the growth, immune responses and gut microbes of tilapia were investigated. The feeding trial was conducted in triplicates with the control diet supplemented with oligochitosan at different concentrations (0, 100, 200, 400 or 800 mg/kg). Following a six-week feeding trial, body weights of the fish supplemented with 200 mg/kg and 400 mg/kg oligochitosan were significantly higher than that of the control group. To address the immune responses stimulated by oligochitosan, by the quantitative real time PCR (qRT-PCR), the mRNA expression levels of CSF, IL-1β, IgM, TLR2 and TLR3 genes from head kidney were all significantly up-regulated in the 400 mg/kg group compared to the control. To characterize the gut microbes, bacterial samples were collected from the foregut, midgut, and hindgut, respectively and were subjected to high-throughput sequencing of 16S rDNA. The results showed that significantly lower abundance of Fusobacterium was detected in the hindgut of 400 mg/kg group compared to the control. Additionally, beta-diversity revealed that both gut habitat and oligochitosan had effects on the gut bacterial assembly. To further elucidate the mechanism underlying the effects of oligochitosan on bacterial assembly, the results showed that difference dosages of dietary oligochitosan could alter the specific metabolic pathways and functions of the discriminatory bacterial taxa, resulting in the different bacterial assemblies. To test the antibacterial ability of tilapia fed with oligochitosan, when the tilapias were challenged with Aeromonas hydrophila, the mortality of groups fed with dietary oligochitosan was significantly lower than that of the control. Taken together, appropriate dietary oligochitosan could improve growth, immune responses and alter the bacterial flora in the intestine of tilapia, so as to play a role in fighting against the bacterial infection.

Distribution of mannose receptor in blunt snout bream (Megalobrama amblycephala) during the embryonic development and its immune response to the challenge of Aeromonas hydrophila

The mannose receptor (MR) is a type I transmembrane protein. Its ectodomain has eight C-type lectin-like domains, which are able to recognize and mediate the phagocytosis of a wide range of pathogens. Comprehensive studies have revealed that mammalian MR is widely distributed in the mononuclear phagocyte system (MPS, previously known as the reticuloendothelial system) and play a key role both in the physiological clearance and cell activation. Hitherto, neither the MR distribution, nor the function of clearance and cell activation has been investigated in fish. In the previous study, we have reported the full-length cDNA of blunt snout bream MR, analyzed its structure and relative mRNA expression during embryogenesis and in the liver, head kidney, spleen and intestine of fish after stimulation with killed Aeromonas hydrophila. In the present study, we developed a rabbit polyclonal antibody against MR and undertook a systematic survey of the expression of MR at the protein level by immunohistochemistry. To get more information about MR function, the mRNA expression of MR, pro-inflammatory factor TNF-α and anti-inflammatory factor ARG2 genes was measured by qRT-PCR in the liver, head kidney, and spleen after A. hydrophila challenge. We first observed MR expression in the yolk sac at the fertilized egg stage and possibly MR was expressed by early macrophages. We also showed the MR distribution in head kidney, body kidney, spleen, liver, intestine, muscle, brain, heart, and gills. Following A. hydrophila challenge the MR immunoreactive cells became more widespread in head kidney and spleen, which are the major reticuloendothelial systems of fish. The quantitative studies at mRNA levels showed that there exists a high correlation between MR expression and immune cytokine expressions after bacteria challenge.

Molecular and functional characterization of a novel CD302 gene from ayu (Plecoglossus altivelis)

Recognizing the presence of invading pathogens by pattern recognition receptors (PRRs) is key to mounting an effective innate immune response. Mammalian CD302 is an unconventional C-type lectin like receptor (CTLR) involved in the functional regulation of immune cells. However, the role of CD302 in fish remains unclear. In this study, we characterized a novel CD302 gene from ayu (Plecoglossus altivelis), which was tentatively named PaCD302. The cDNA sequence of PaCD302 is 1893 nucleotides in length, and encodes a polypeptide of 241 amino acids with molecular weight 27.1 kDa and pI 4.69. Sequence comparison and phylogenetic tree analysis showed that PaCD302 is a type I transmembrane CTLR devoid of the known amino acid residues essential for Ca(2+)-dependent sugar binding. PaCD302 mRNA expression was detected in all tissues and cells tested, with the highest level in the liver. Following Vibrio anguillarum infection, PaCD302 mRNA expression was significantly upregulated in all tissues tested. For further functional analysis, we generated a recombinant protein for PaCD302 (rPaCD302) by prokaryotic expression and raised a specific antibody against rPaCD302. Western blot analysis revealed that the native PaCD302 is glycosylated. Refolded rPaCD302 was unable to bind to five monosaccharides (l-fucose, d-galactose, d-glucose, d-mannose and N-acetyl glucosamine) or two other polysaccharides (lipopolysaccharide and peptidoglycan). It was able to bind to three Gram-positive and seven Gram-negative bacteria, but show no bacterial agglutinating activity. PaCD302 function blocking using anti-PaCD302 IgG resulted in inhibition of phagocytosis and bactericidal activity of ayu monocytes/macrophages (MO/MΦ), suggesting that PaCD302 regulates the function of ayu MO/MΦ. In summary, our study demonstrates that PaCD302 may participate in the immune response of ayu against bacterial infection via modulation of MO/MΦ function.

Molecular Cloning and Functional Characterization of Mannose Receptor in Zebra Fish (Danio rerio) during Infection with Aeromonas sobria

Mannose receptor (MR) is a member of pattern-recognition receptors (PRRs), which plays a significant role in immunity responses. Much work on MR has been done in mammals and birds while little in fish. In this report, a MR gene (designated as zfMR) was cloned from zebra fish (Danio rerio), which is an attractive model for the studies of animal diseases. The full-length cDNA of zfMR contains 6248 bp encoding a putative protein of 1428 amino acids. The predicted amino acid sequences showed that zfMR contained a cysteine-rich domain, a single fibronectin type II (FN II) domain, eight C-type lectin-like domains (CTLDs), a transmembrane domain and a short C-terminal cytoplasmic domain, sharing highly conserved structures with MRs from the other species. The MR mRNA could be detected in all examined tissues with highest level in kidney. The temporal expression patterns of MR, IL-1β and TNF-α mRNAs were analyzed in the liver, spleen, kidney and intestine post of infection with Aeromonas sobria. By immunohistochemistry assay, slight enhancement of MR protein was also observed in the spleen and intestine of the infected zebra fish. The established zebra fish-A. sobria infection model will be valuable for elucidating the role of MR in fish immune responses to infection.

Mannose receptor mediated phagocytosis of bacteria in macrophages of blunt snout bream (Megalobrama amblycephala) in a Ca(2+)-dependent manner

Mannose receptor (MR) is an important pattern-recognition receptor in macrophages and plays a critical role in immune responses. It is has been reported that mammalian macrophages are able to engulf a wide range of microorganisms mediated by Ca(2+)-dependent MR binding to terminal mannose residues which are frequently found on the pathogen surfaces. However, little is known about the MR-mediated phagocytosis in macrophages of fish. In this report, the distributions of MR in the macrophage and head kidney tissue from blunt snout bream were examined using MaMR specific antibody generated in our lab. Mannan and MaMR specific antibody inhibition experiments results collectively showed that MR was involved in the GFP-expressed E. coli engulfed in the macrophages, resulting in respiratory burst, nitric oxide production as well as inflammatory cytokines secretion, and the MaMR-mediated phagocytosis was Ca(2+)-dependent. These results will shed a new light on the immune functions of teleost MRs.

Oligochitosan stimulated phagocytic activity of macrophages from blunt snout bream (Megalobrama amblycephala) associated with respiratory burst coupled with nitric oxide production

The immunostimulating effects of oligochitosan have been proven in several fish, however, the mechanisms underlying the stimulation are not characterized. In the present study, the effects of oligochitosan were investigated using macrophages isolated from blunt snout bream (Megalobrama amblycephala). The results showed that the phagocytic activity of the macrophages was enhanced by the addition of oligochitosan in vitro and in vivo. The two of the most important antimicrobial pathways of macrophages, NADPH oxidase and iNOS pathways were included for further studies. The amounts of superoxide anion and the mRNAs of the five subunits of NADPH oxidase genes were significantly enhanced in the oligochitosan-treated macrophages and macrophages isolated from fish fed with feed containing oligochitosan. In addition, the NO production, iNOS activity and iNOS gene expression were all significantly increased in the presence of oligochitosan. Furthermore, the mRNA levels of the TNF-α and IL-1β were also significantly increased in the macrophages derived from fish fed with oligochitosan. In conclusion, the stimulation effects of oligochitosan on the phagocytic activity of the fish macrophages were associated with respiratory burst coupled with nitric oxide production.