Phosvitin plays a critical role in the immunity of zebrafish embryos via acting as a pattern recognition receptor and an antimicrobial effector

Zebrafish phosvitin is an antioxidant with non-cytotoxic activity

Antioxidants, or anti-oxidant agents, have attracted a great deal of attention in recent years because of their roles in prevention of chronic diseases and utilization as preservatives in food and cosmetics. In this study, we clearly demonstrated that zebrafish recombinant phosvitin (rPv) is an antioxidant agent capable of inhibiting the oxidation of the linoleic acid, and scavenging the 2,2-diphenyl-1-picrylhydrazyl radical. We also showed that zebrafish rPv is a cellular antioxidant capable of protecting radical-mediated oxidation of cellular biomolecules. Importantly, zebrafish rPv is non-cytotoxic to murine macrophage RAW264.7 cells. It is the first report that showed the antioxidant activities of Pv in fishes, suggesting that zebrafish Pv can be an important antioxidant, which can be used as preservatives in food and cosmetics and even as supplementary mediator in different diseased states.

Maternal immune transfer in mollusc

Maternal immunity refers to the immunity transferred from mother to offspring via egg, playing an important role in protecting the offspring at early life stages and contributing a trans-generational effect on offspring's phenotype. Because fertilization is external in most of the molluscs, oocytes and early embryos are directly exposed to pathogens in the seawater, and thus maternal immunity could provide a better protection before full maturation of their immunological systems. Several innate immune factors including pattern recognition receptors (PRRs) like lectins, and immune effectors like lysozyme, lipopolysaccharide binding protein/bacterial permeability-increasing proteins (LBP/BPI) and antioxidant enzymes have been identified as maternally derived immune factors in mollusc eggs. Among these immune factors, some maternally derived lectins and antibacterial factors have been proved to endue mollusc eggs with effective defense ability against pathogen infection, while the roles of other factors still remain untested. The physiological condition of mollusc broodstock has a profound effect on their offspring fitness. Many other factors such as nutrients, pathogens, environment conditions and pollutants could exert considerable influence on the maternal transfer of immunity. The parent molluscs which have encountered an immune stimulation endow their offspring with a trans-generational immune capability to protect them against infections effectively. The knowledge on maternal transfer of immunity and the trans-generational immune effect could provide us with an ideal management strategy of mollusc broodstock to improve the immunity of offspring and to establish a disease-resistant family for a long-term improvement of cultured stocks.

Functional characterization of chitinase-3 reveals involvement of chitinases in early embryo immunity in zebrafish

The function and mechanism of chitinases in early embryonic development remain largely unknown. We show here that recombinant chitinase-3 (rChi3) is able to hydrolyze the artificial chitin substrate, 4-methylumbelliferyl-β-D-N,N',N″-triacetylchitotrioside, and to bind to and inhibit the growth of the fungus Candida albicans, implicating that Chi3 plays a dual function in innate immunity and chitin-bearing food digestion in zebrafish. This is further corroborated by the expression profile of Chi3 in the liver and gut, which are both immune- and digestion-relevant organs. Compared with rChi3, rChi3-CD lacking CBD still retains partial capacity to bind to C. albicans, but its enzymatic and antifungal activities are significantly reduced. By contrast, rChi3-E140N with the putative catalytic residue E140 mutated shows little affinity to chitin, and its enzymatic and antifungal activities are nearly completely lost. These suggest that both enzymatic and antifungal activities of Chi3 are dependent on the presence of CBD and E140. We also clearly demonstrate that in zebrafish, both the embryo extract and the developing embryo display antifungal activity against C. albicans, and all the findings point to chitinase-3 (Chi3) being a newly-identified factor involved in the antifungal activity. Taken together, a dual function in both innate immunity and food digestion in embryo is proposed for zebrafish Chi3. It also provides a new angle to understand the immune role of chitinases in early embryonic development of animals.

Functional characterization of mannose-binding lectin in zebrafish: implication for a lectin-dependent complement system in early embryos

The lectin pathway involves recognition of pathogen-associated molecular patterns by mannose-binding lectin (MBL), and the subsequent activation of associated enzymes, termed MBL-associated serine proteases (MASPs). In this study, we demonstrate that the transcript of MBL gene is present in the early embryo of zebrafish, and MBL protein is also present in the embryo. In addition, we show that recombinant zebrafish MBL was able to bind the Gram-negative bacterium Escherichia coli and the Gram-positive bacterium Staphylococcus aureus, and rMBL was able to promote the phagocytosis of E. coli and S. aureus by macrophages, indicating that like mammalian MBL, zebrafish MBL performs a dual function in both pattern recognition and opsonization. Importantly, we show that microinjection of anti-MBL antibody into the early developing embryos resulted in a significantly increased mortality in the embryos challenged with Aeromonas hydrophila (pathogenic to zebrafish); and injection of rMBL into the embryos (resulting in increase in MBL in the embryo) markedly promoted their resistance to A. hydrophila; and this promoted bacterial resistance was significantly reduced by the co-injection of anti-MBL antibody with rMBL but not by the injection of anti-actin antibody with rMBL. These suggest that the lectin pathway may be already functional in the early embryos in zebrafish before their immune system is fully matured, protecting the developing embryos from microbial infection. This work provides a new angle to understand the immune role of the lectin pathway in early development of animals.

Adaptive and innate immune molecules in developing rainbow trout, Oncorhynchus mykiss eggs and larvae: expression of genes and occurrence of effector molecules

The ontogenetic development of the immune system was studied during the egg phase and the early post-hatch period of rainbow trout. Quantitative real-time PCR (qPCR) was used to assess the timing and degree of expression of 9 important immune relevant genes and EF1-α. Further, immunohistochemical staining using monoclonal antibodies was applied on rainbow trout embryos and larvae in order to localize five different protein molecules (MHCII, CD8, IgM, IgT and SAA) in the developing tissue and immune organs. Maternally transferred transcripts of EF1-α mRNA were detected in the unfertilized egg. Early onset of expression was seen for all immune genes at very low levels. The amount of mRNA slowly increased and peaked around and after hatching. The highest increases were seen for MHCII, C3, C5 and SAA. Immunohistochemistry using five monoclonal antibodies showed positive staining from day 84 post fertilization. Skin, gills, intestine, pseudobranch and thymus showed reactivity for MHCII, thymus for CD8, gill mucus for IgT and pseudobranch and cartilage associated tissue for SAA. The importance of detected factors for early protection of eggs and larvae is discussed.

Functional analysis of domain of unknown function (DUF) 1943, DUF1944 and von Willebrand factor type D domain (VWD) in vitellogenin2 in zebrafish

Vitellogenin (Vg), the precursor of egg-yolk proteins in all oviparous organisms, shares a similar domain structure combination. In most cases, Vg contains the Vitellogenin_N domain, the domain of unknown function (DUF) 1943, and the von Willebrand factor type D domain (VWD), which are present in different forms of Vg from both vertebrates and invertebrates. Occasionally, a DUF1944 domain is also present in between DUF1943 and VWD in some Vg proteins of vertebrates. Recent studies have shown that Vg participates in immune defense of host with multiple functions. However, whether all Vg proteins encoded by different vg genes play an immune role is unknown. In addition, the correlation of different domains in Vg with the multiple immune functions remains completely unclear. Here we demonstrated clearly that recombinant proteins, rDUF1943, rDUF1944 and rVWD from zebrafish Vg2 interacted with both the Gram-positive bacteria Staphylococcus aureus, Bacillus subtilis and Micrococcus luteus and the Gram-negative bacteria Escherichia coli and Vibrio anguillarum, as well as their signature components LTA and LPS. Moreover, both rDUF1943 and rDUF1944 promoted the phagocytosis of E. coli and S. aureus by carp macrophages. These suggest that both DUF1943 and DUF1944 as well as VWD may contribute to the function of Vg as a pattern recognition receptor, and DUF1943 and DUF1944 also contribute to the function of Vg as an opsonin. This study also opens a new angle for identification of function of genes of unknown function, which have the domains DUF1943 and DUF1944.

Zebrafish CD59 has both bacterial-binding and inhibiting activities

CD59, known as protectin, usually plays roles as a regulatory inhibitor of complement, but it also exhibits activities independent of its function as a complement inhibitor. This study reported the identification and characterization of an ortholog of mammalian cd59 from zebrafish Danio rerio, which is similar to known cd59 in terms of both amino acid sequence and genomic structure as well as synteny conservation. We showed that zebrafish cd59 was maternally expressed in early embryos and expressed in a tissue-specific manner, with most abundant expression in the brain. We further showed that recombinant zebrafish CD59 was capable of binding to both the Gram-negative and Gram-positive bacteria as well as the microbial signature molecules LPS and LTA. In addition we demonstrated that recombinant zebrafish CD59 displayed slight antimicrobial activity capable of inhibiting the growth of E. coli and S. aureus. All these data indicate that zebrafish CD59 can not only binds to the bacteria and their signature molecules LPS and LTA but can also inhibit their growth, a novel role assigned to CD59.

Vitellogenin from the silkworm, Bombyx mori: an effective anti-bacterial agent

Silkworm, Bombyx mori, vitellogenin (Vg) was isolated from perivisceral fat body of day 3 of pupa. Both Vg subunits were co-purified as verified by mass spectrometry and immunoblot. Purified Vg responded to specific tests for major posttranslational modifications on native gels indicating its nature as lipo-glyco-phosphoprotein. The Vg fraction had strong antibacterial activity against Gram negative bacterium Escherichia coli and Gram positive bacterium Bacillus subtilis. Microscopic images showed binding of Vg to bacterial cells and their destruction. When infected silkworm larvae were treated with purified Vg they survived the full life cycle in contrast to untreated animals. This result showed that Vg has the ability to inhibit the proliferation of bacteria in the silkworm fluid system without disturbing the regular metabolism of the host.

Lipopolysaccharide neutralization by a novel peptide derived from phosvitin

Lipopolysaccharide (LPS), also known as endotoxin, is the primary trigger of sepsis, which is associated with high mortality in patients. No therapeutic agents are currently efficacious enough to protect patients from sepsis characterized by LPS-mediated tissue damage and organ failure. Previously, a phosvitin-derived peptide, Pt5, which consists of the C-terminal 55 residues of zebrafish phosvitin, has been shown to function as an antibacterial agent. In this study, we have generated six mutants by site-directed mutagenesis based on the sequence of Pt5, and found that one of the six mutants, Pt5e, showed the strongest bactericidal activities against Escherichia coli and Staphylococcus aureus. We then demonstrated that Pt5e was able to bind to LPS and lipoteichoic acid (LTA). More importantly, we showed that Pt5e significantly inhibited LPS-induced tumor-necrosis factor (TNF)-α and interleukin (IL)-1β release from murine RAW264.7 cells and considerably reduced serum TNF-α and IL-1β levels in mice. Additionally, Pt5e protected the liver from damage by LPS, and remarkably promoted the survival rate of the endotoxemia mice. Furthermore, Pt5e displayed no cytotoxicity to murine RAW264.7 macrophages and no hemolytic activity toward human red blood cells. These data together indicate that Pt5e is an endotoxin-neutralizing agent with a therapeutic potential in clinical treatment of LPS-induced sepsis.

Vitellogenin recognizes cell damage through membrane binding and shields living cells from reactive oxygen species

Large lipid transfer proteins are involved in lipid transportation and diverse other molecular processes. These serum proteins include vitellogenins, which are egg yolk precursors and pathogen pattern recognition receptors, and apolipoprotein B, which is an anti-inflammatory cholesterol carrier. In the honey bee, vitellogenin acts as an antioxidant, and elevated vitellogenin titer is linked to prolonged life span in this animal. Here, we show that vitellogenin has cell and membrane binding activity and that it binds preferentially to dead and damaged cells. Vitellogenin binds directly to phosphatidylcholine liposomes and with higher affinity to liposomes containing phosphatidylserine, a lipid of the inner leaflet of cell membranes that is exposed in damaged cells. Vitellogenin binding to live cells, furthermore, improves cell oxidative stress tolerance. This study can shed more light on why large lipid transfer proteins have a well conserved α-helical domain, because we locate the lipid bilayer-binding ability of vitellogenin largely to this region. We suggest that recognition of cell damage and oxidation shield properties are two mechanisms that allow vitellogenin to extend honey bee life span.

Antiviral activity of phosvitin from zebrafish Danio rerio

Lymphocystis disease virus (LCDV), a virus of Iridoviridae, can infect numerous teleost species, causing serious losses of aquaculture industry, and thus effective ways of prophylaxis and treatment are demanded. Previous studies have shown that phosvitin (Pv) is an antimicrobial agent in zebrafish, and vitellogenin, the precursor of yolk proteins including Pv, is able to neutralize virus, we thus hypothesize that Pv may have an antiviral activity. Here we clearly demonstrated that recombinant Pv (rPv) purified was capable of inhibiting the cytopathic effect in LCDV-infected cells and reducing the virus quantities in the infected cells as well as in the infected zebrafish. These data indicate that Pv possesses an antiviral activity and participates in immune defense of host against the infection by viruses like LCDV.

Maternal immunity in fish

Both innate and adaptive immune-relevant factors are transferred from mother to offspring in fishes. These maternally-transferred factors include IgM, lysozymes, lectin, cathelicidin and complement components. Recently, yolk proteins, phosvitin and lipovitellin, have been shown to be maternally-transferred factors, functioning in the defense of teleost larvae against pathogens. Among these factors, the mode of action of complement components and yolk proteins has been explored, whereas that of all the other factors remains elusive. At present, the transfer mechanisms of maternally-derived immune factors are largely unknown although those of IgM and yolk protein transmission from mother to offspring have been reported in some fishes. Maternal transfer of immunity is affected by many elements, including biological factors, such as age and maturation, and environmental conditions experienced by brood fish, such as pathogens and nutritional supply. Practically, the manipulation of maternal immunity transfer can be used to enhance the survival rate of fish larvae.

Antimicrobial-immunomodulatory activities of zebrafish phosvitin-derived peptide Pt5

A phosvitin (Pv)-derived peptide, Pt5, which consists of the C-terminal 55 residues of Pv in zebrafish, has been shown to function as an antimicrobial agent capable of killing microbes in vitro. However, its in vivo role in zebrafish remains unknown. In this study, we clearly demonstrated that Pt5 protected adult zebrafish from pathogenic Aeromonas hydrophila attack, capable of significantly enhancing the survival rate of zebrafish after the pathogenic challenge. Pt5 also caused a marked decrease in the numbers of A. hydrophila in the blood, spleen, kidney, liver and muscle, suggesting that Pt5 was able to block multiplication/dissemination of A. hydrophila in zebrafish. Additionally, Pt5 markedly suppressed the expression of the proinflammatory cytokine genes IL-1β, IL-6, TNF-α and IFN-γ in the spleen and head kidney of A. hydrophila-infected zebrafish, but it considerably enhanced the expressions of the antiinflammatory cytokine genes IL-10 and IL-4 in the same tissues. Taken together, these data indicate that Pt5 plays a dual role in zebrafish as an antimicrobial and immunomodulatory agent, capable of protecting zebrafish against pathogenic A. hydrophila through its antimicrobial activity as well as preventing zebrafish from the detrimental effects of an excessive inflammatory response via modulating immune functions.

A novel short peptidoglycan recognition protein in amphioxus: identification, expression and bioactivity

Peptidoglycan recognition proteins (PGRPs) are widely distributed in invertebrates and vertebrates, and structure-activity relationship of insect and mammalian PGRPs has been well characterized, but functional and structural insights into PGRPs in other species are rather limited. Here we identified a novel short PGRP gene from the amphioxus Branchiostoma japonicum, named pgrp-s, which possesses a domain combination of ChtBD1 domain-PGRP domain, which is unique to all known PGRPs. Amphioxus pgrp-s was predominantly expressed in the hepatic caecum, hind-gut and muscle in a tissue-specific manner. Recombinant PGRP-S, rPGRP-S, and truncated protein with ChtBD1 domain deleted, rP86/250, both showed affinity to Dap-type PGN, Lys-type PGN and chitin. Consistently, they were also able to bind to Escherichia coli, Staphylococcus aureus and Pichia pastoris. Moreover, both rPGRP-S and rP86/250 had amidase enzymatic activity, capable of hydrolyzing Dap-type and Lys-type PGNs. Like vertebrate PGRPs, rPGRP-S was directly microbicidal, capable of killing E. coli, S. aureus and P. pastoris, whereas rP86/250 only inhibited the growth of E. coli and S. aureus, and its anti-P. pastoris activity was significantly reduced. It is clear that neither the binding of amphioxus PGRP-S nor its amidase enzymatic activity depend on the N-terminal ChtBD1 domain, but its antifungal activity does. Collectively, these data suggested that amphioxus PGRP-S may function as a multivalent pattern recognition receptor, capable of recognizing PGN and chitin, a microbicidal agent, capable of killing bacteria such as E. coli and S. aureus and fungus like P. pastoris, and probably a PGN scavenger, capable of hydrolyzing PGN.

Zebrafish (Danio rerio) larvae as a system to test the efficacy of polysaccharides as immunostimulants

The present study was carried out to examine the use of zebrafish (Danio rerio) as a preliminary screening model for testing the effect of potential immunostimulant substances on the innate immune system. β-Glucan, a polysaccharide used widely as an immunostimulant, was used as a representative molecule and tested on zebrafish embryos and larvae. The efficacy of the molecule was evaluated by determining the differential expression of some selected genes related to the immune system by RT-qPCR. Larvae from 72 hours post fertilization were found at the optimal developmental stage for assessing the expression of the selected genes. To verify if the β-glucan entered the larvae and therefore was responsible for the effects produced, the molecule was labeled fluorescently to check its localization by using microscopy. For estimating the effects of β-glucan on gene expression, zebrafish embryos and larvae were immersed in three different concentrations of β-glucan (50, 100, and 150 μg/mL) using five different exposure times. A stronger gene induction was observed when longer times of exposure and older larvae were used. The most evident effects of β-glucan were the overexpression of the genes TNFα, MPO, TRF, and LYZ. Moreover, slight changes in MPO expression were detected using a transgenic line of zebrafish (MPO::GFP), and a temporal increase in resistance against Vibrio anguillarum was found after β-glucan immersion. The assay used in this study permits the testing potential of immunostimulants in a simple and cost-effective way.