MyD88 innate immune function in a zebrafish embryo infection model

Epithelial cell proliferation in the developing zebrafish intestine is regulated by the Wnt pathway and microbial signaling via Myd88

Rates of cell proliferation in the vertebrate intestinal epithelium are modulated by intrinsic signaling pathways and extrinsic cues. Here, we report that epithelial cell proliferation in the developing zebrafish intestine is stimulated both by the presence of the resident microbiota and by activation of Wnt signaling. We find that the response to microbial proliferation-promoting signals requires Myd88 but not TNF receptor, implicating host innate immune pathways but not inflammation in the establishment of homeostasis in the developing intestinal epithelium. We show that loss of axin1, a component of the β-catenin destruction complex, results in greater than WT levels of intestinal epithelial cell proliferation. Compared with conventionally reared axin1 mutants, germ-free axin1 mutants exhibit decreased intestinal epithelial cell proliferation, whereas monoassociation with the resident intestinal bacterium Aeromonas veronii results in elevated epithelial cell proliferation. Disruption of β-catenin signaling by deletion of the β-catenin coactivator tcf4 partially decreases the proliferation-promoting capacity of A. veronii. We show that numbers of intestinal epithelial cells with cytoplasmic β-catenin are reduced in the absence of the microbiota in both WT and axin1 mutants and elevated in animals' monoassociated A. veronii. Collectively, these data demonstrate that resident intestinal bacteria enhance the stability of β-catenin in intestinal epithelial cells and promote cell proliferation in the developing vertebrate intestine.

Transcriptome analysis of Traf6 function in the innate immune response of zebrafish embryos

TRAF6 is a key player at the cross-roads of development and immunity. The analysis of its in vivo molecular function is a great challenge since severe developmental defects and early lethality caused by Traf6 deficiency in knock-out mice interfere with analyses of the immune response. In this study we have used a new strategy to analyze the function of Traf6 in a zebrafish-Salmonella infectious disease model. In our approach the effect of a Traf6 translation-blocking morpholino was titrated down to avoid developmental defects and the response to infection under these conditions was studied using the combination of microarray analysis and whole transcriptome deep sequencing. Transcriptome profiling of the traf6 knock-down allowed the identification of a gene set whose responsiveness during infection is highly dependent on Traf6. Expression trend analysis based on the resulting datasets identified nine clusters of genes with characteristic transcription response profiles, demonstrating Traf6 has a dynamic role as a positive and negative regulator. Among the Traf6-dependent genes was a large set of well known anti-microbial and inflammatory genes. Additionally, we identified several genes which were not previously linked to a response to microbial infection, such as the fertility hormone gene gnrh2 and the DNA-damage regulated autophagy modulator 1 gene dram1. With the use of the zebrafish embryo model we have now analyzed the in vivo function of Traf6 in the innate immune response without interference of adaptive immunity.

Specific resistance to Pseudomonas aeruginosa infection in zebrafish is mediated by the cystic fibrosis transmembrane conductance regulator

Cystic fibrosis (CF) is a genetic disease caused by recessive mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene and is associated with prevalent and chronic Pseudomonas aeruginosa lung infections. Despite numerous studies that have sought to elucidate the role of CFTR in the innate immune response, the links between CFTR, innate immunity, and P. aeruginosa infection remain unclear. The present work highlights the zebrafish as a powerful model organism for human infectious disease, particularly infection by P. aeruginosa. Zebrafish embryos with reduced expression of the cftr gene (Cftr morphants) exhibited reduced respiratory burst response and directed neutrophil migration, supporting a connection between cftr and the innate immune response. Cftr morphants were infected with P. aeruginosa or other bacterial species that are commonly associated with infections in CF patients, including Burkholderia cenocepacia, Haemophilus influenzae, and Staphylococcus aureus. Intriguingly, the bacterial burden of P. aeruginosa was found to be significantly higher in zebrafish Cftr morphants than in controls, but this phenomenon was not observed with the other bacterial species. Bacterial burden in Cftr morphants infected with a P. aeruginosa ΔLasR mutant, a quorum sensing-deficient strain, was comparable to that in control fish, indicating that the regulation of virulence factors through LasR is required for enhancement of infection in the absence of Cftr. The zebrafish system provides a multitude of advantages for studying the pathogenesis of P. aeruginosa and for understanding the role that innate immune cells, such as neutrophils, play in the host response to acute bacterial infections commonly associated with cystic fibrosis.

A unique feature of Toll/IL-1 receptor domain-containing adaptor protein is partially responsible for lipopolysaccharide insensitivity in zebrafish with a highly conserved function of MyD88

MyD88 and Toll/IL-1R domain-containing adaptor protein (TIRAP) are required for the TLR4 response to LPS stimulation in mammals, but the functions of the two adaptors and their involvement in zebrafish insensitivity to LPS remains unknown. We present a functional analysis of zebrafish Myd88 and Tirap and suggest that Myd88 is more important than Tirap for the activation of Tlr-mediated NF-kappaB, which may be a novel mechanism of Myd88-dependent TLR signaling in teleosts. Zebrafish Tirap lacks the phosphatidylinositol 4,5-bisphosphate binding motif required for human TIRAP location and has leucine at position 233 rather than the conserved proline of human TIRAP, as well as 105 additional aa at the N terminus. Overexpression of zebrafish Tirap in HEK293T cells did not activate NF-kappaB and IFN-beta, but slightly activated NF-kappaB in carp leukocyte cells. Zebrafish Myd88 alone strongly induced the activation of NF-kappaB and IFN-beta both in HEK293T and carp leukocyte cells. The function of Myd88 was dependent on its cellular location and the proline in the Toll/IL-1R domain. Although zebrafish Tirap was distributed throughout the cell rather than localized to the cytoplasmic membrane, its impaired ability to activate downstream Tlr molecules was unlikely to be related to its location because chimera TIRAP with a human TIRAP N terminus and membrane-binding domain also did not activate NF-kappaB. However, the mutation of leucine to proline increased the ability of Tirap to activate NF-kappaB. We suggest that the zebrafish Tirap needs a longer N terminus to perform its function and could be partially responsible for the resistance to LPS in zebrafish.

Trolling for the ideal model host: zebrafish take the bait

As little as 10 years ago, murine models of infectious disease were the host of choice for analyzing interactions between the pathogen and host during infection. However, not all pathogens can infect mice, nor do they always replicate the clinical syndromes observed in human infections. Furthermore, in the current economic environment, using mammalian models for large-scale screens may be less economically feasible. The emergence of the zebrafish (Danio rerio) as an infectious disease host model, as well as a model for vertebrate immune system development, has provided new information and insights into pathogenesis that, in many instances, would not have been possible using a murine model host. In this article we highlight some of the key findings and the latest techniques along with the many advantages of using the zebrafish host model to gain new insights into pathogenic mechanisms in a live vertebrate host.

Burkholderia cenocepacia creates an intramacrophage replication niche in zebrafish embryos, followed by bacterial dissemination and establishment of systemic infection

Bacteria belonging to the "Burkholderia cepacia complex" (Bcc) often cause fatal pulmonary infections in cystic fibrosis patients, yet little is know about the underlying molecular mechanisms. These Gram-negative bacteria can adopt an intracellular lifestyle, although their ability to replicate intracellularly has been difficult to demonstrate. Here we show that Bcc bacteria survive and multiply in macrophages of zebrafish embryos. Local dissemination by nonlytic release from infected cells was followed by bacteremia and extracellular replication. Burkholderia cenocepacia isolates belonging to the epidemic electrophoretic type 12 (ET12) lineage were highly virulent for the embryos; intravenous injection of <10 bacteria of strain K56-2 killed embryos within 3 days. However, small but significant differences between the clonal ET12 isolates K56-2, J2315, and BC7 were evident. In addition, the innate immune response in young embryos was sufficiently developed to control infection with other less virulent Bcc strains, such as Burkholderia vietnamiensis FC441 and Burkholderia stabilis LMG14294. A K56-2 cepR quorum-sensing regulator mutant was highly attenuated, and its ability to replicate and spread to neighboring cells was greatly reduced. Our data indicate that the zebrafish embryo is an excellent vertebrate model to dissect the molecular basis of intracellular replication and the early innate immune responses in this intricate host-pathogen interaction.

Characterisation of expression patterns and functional role of Cactin in early zebrafish development

The immune system of teleost zebrafish (Danio rerio) shows high similarity to mammalian counterparts sharing many innate immune components including Toll-Like Receptors (TLRs), cytokines, chemokines and complement molecules. As in mammals, zebrafish also contains the transcription factor NF-kappaB that plays dualist roles in innate immunity and early development. Indeed NF-kappaB members are expressed in different temporal patterns during the early stages of zebrafish embryogenesis indicating that each molecule is involved in specific developmental events. In the present study we employ zebrafish as a model to characterise the expression pattern and role of a novel NF-kappaB regulator, termed Cactin, in early development. Cactin was first characterised in Drosophila as a new member of the Rel pathway that could affect the generation of dorsal-ventral polarity. To explore the potential developmental role of Cactin in zebrafish, we initially investigated its expression pattern and functional role during early embryonic developmental stages. We detect Cactin expression at all stages of early development and knockdown of Cactin by specific morpholino antisense oligonucleotides causes developmental abnormalities manifested by an overall dysmorphic cellular organisation. These results indicate that Cactin has been highly conserved during evolution and plays a key role in early embryonic development.

Host-microbe interactions in the developing zebrafish

The amenability of the zebrafish to in vivo imaging and genetic analysis has fueled expanded use of this vertebrate model to investigate the molecular and cellular foundations of host-microbe relationships. Study of microbial encounters in zebrafish hosts has concentrated on developing embryonic and larval stages, when the advantages of the zebrafish model are maximized. A comprehensive understanding of these host-microbe interactions requires appreciation of the developmental context into which a microbe is introduced, as well as the effects of that microbial challenge on host ontogeny. In this review, we discuss how in vivo imaging and genetic analysis in zebrafish has advanced our knowledge of host-microbe interactions in the context of a developing vertebrate host. We focus on recent insights into immune cell ontogeny and function, commensal microbial relationships in the intestine, and microbial pathogenesis in zebrafish hosts.

Non-mammalian animal models to study infectious disease: worms or fly fishing?

A major challenge in studying human infectious diseases is to understand in detail the molecular bases, including both pathogen and host-related factors, which contribute to disease development. Non-mammalian models have proven to be of great value for our understanding of disease and have shown conservation in fundamental virulence mechanisms for the infection of evolutionary divergent hosts. In this review we describe recent advances with three major non-mammalian models used for analysis of infectious disease in humans; the nematode Caenorhabditis elegans, the fruit fly Drosophila melanogaster and the zebrafish Danio rerio.

Streptolysin S inhibits neutrophil recruitment during the early stages of Streptococcus pyogenes infection

In contrast to infection of superficial tissues, Streptococcus pyogenes infection of deeper tissue can be associated with a significantly diminished inflammatory response, suggesting that this bacterium has the ability to both promote and suppress inflammation. To examine this, we analyzed the behavior of an S. pyogenes mutant deficient in expression of the cytolytic toxin streptolysin S (SLS-) and evaluated events that occur during the first few hours of infection by using several models including injection of zebrafish (adults, larvae, and embryos), a transepithelial polymorphonuclear leukocyte (PMN) migration assay, and two-photon microscopy of mice in vivo. In contrast to wild-type S. pyogenes, the SLS- mutant was associated with the robust recruitment of neutrophils and significantly reduced lethal myositis in adult zebrafish. Similarly, the mutant was attenuated in embryos in its ability to cause lethality. Infection of larva muscle allowed an analysis of inflammation in real time, which revealed that the mutant had recruited PMNs to the infection site. Analysis of transepithelial migration in vitro suggested that SLS inhibited the host cells' production of signals chemotactic for neutrophils, which contrasted with the proinflammatory effect of an unrelated cytolytic toxin, streptolysin O. Using two-photon microscopy of mice in vivo, we showed that the extravasation of neutrophils during infection with SLS- mutant bacteria was significantly accelerated compared to infection with wild-type S. pyogenes. Taken together, these data support a role for SLS in the inhibition of neutrophil recruitment during the early stages of S. pyogenes infection.

Oxazolone-induced enterocolitis in zebrafish depends on the composition of the intestinal microbiota

BACKGROUND & AIMS

The pathogenesis of inflammatory bowel disease involves dysfunctional mucosal immune responses to commensal bacteria in genetically predisposed hosts. Interactions between host cells and bacteria are complicated, making it a challenge to assess their relative contribution to intestinal pathology. We developed a zebrafish model of enterocolitis to study these interactions.

METHODS

Enterocolitis was induced by intrarectal administration of the hapten oxazolone in adult wild-type and myeloperoxidase-reporter transgenic zebrafish in the presence or absence of antibiotics. Intestinal inflammation was evaluated by histological and flow cytometry analyses and cytokine profiling with quantitative real-time polymerase chain reaction. Changes in the composition of the intestinal microbiota following antibiotic administration were assessed by 16SrRNA sequencing and bacterial load was quantified by culture on nonselective media (colony-forming units).

RESULTS

In zebrafish, the infiltrate and severity of oxazolone-induced enterocolitis are influenced by the composition of the microbiota. Inflammation is characterized by granulocyte influx; epithelial damage; goblet cell depletion; and increased expression of interleukin-1beta, tumor necrosis factor-alpha, and interleukin-10. Zebrafish given vancomycin had bacterial populations dominated by Fusobacteria and reduced enterocolitis scores, intestinal damage, and percentages of infiltrating neutrophils and eosinophils. In contrast, zebrafish given colistin sulphate had a predominance of proteobacteria and reduced eosinophil and lymphocyte infiltration, but enterocolitis scores were not reduced.

CONCLUSIONS

In zebrafish with oxazolone-induced enterocolitis, components of the intestinal microbiota affect the severity and composition of the intestinal infiltrate.

The gene history of zebrafish tlr4a and tlr4b is predictive of their divergent functions

Mammalian immune responses to LPS exposure are typified by the robust induction of NF-kappaB and IFN-beta responses largely mediated by TLR4 signal transduction pathways. In contrast to mammals, Tlr4 signal transduction pathways in nontetrapods are not well understood. Comprehensive syntenic and phylogenetic analyses support our hypothesis that zebrafish tlr4a and tlr4b genes are paralogous rather than orthologous to human TLR4. Furthermore, we provide evidence to support our assertion that the in vivo responsiveness of zebrafish to LPS exposure is not mediated by Tlr4a and Tlr4b paralogs because they fail to respond to LPS stimulation in vitro. Zebrafish Tlr4a and Tlr4b paralogs were also unresponsive to heat-killed Escherichia coli and Legionella pneumophila. Using chimeric molecules in which portions of the zebrafish Tlr4 proteins were fused to portions of the mouse TLR4 protein, we show that the lack of responsiveness to LPS was most likely due to the inability of the extracellular portions of zebrafish Tlr4a and Tlr4b to recognize the molecule, rather than to changes in their capacities to transduce signals through their Toll/IL-1 receptor (TIR) domains. Taken together, these findings strongly support the notion that zebrafish tlr4a and tlr4b paralogs have evolved to provide alternative ligand specificities to the Tlr immune defense system in this species. These data demonstrate that intensive examination of gene histories when describing the Tlr proteins of basally diverging vertebrates is required to obtain fuller appreciation of the evolution of their function. These studies provide the first evidence for the functional evolution of a novel Tlr.

Estrogen-responsive transient expression assay using a brain aromatase-based reporter gene in zebrafish (Danio rerio)

Whereas endogenous estrogens play an important role in the development, maintenance, and function of female and male reproductive organs, xenoestrogens present in the environment disrupt normal endocrine function in humans and wildlife. Various in vivo and in vitro assays have been developed to screen these xenoestrogens. However, traditional in vivo assays are laborious and unsuitable for large-scale screening, and in vitro assays do not necessarily replicate in vivo functioning. To overcome these limitations, we developed a transient expression assay in zebrafish, into which a brain aromatase (cyp19a1b)-based estrogen-responsive reporter gene was introduced. In response to 17beta-estradiol (10(-6) M) and heptachlor (10(-6) M), zebrafish embryos carrying the reporter construct expressed enhanced green fluorescent protein in the olfactory bulb, telencephalon, preoptic area, and mediobasal hypothalamus. This system will serve to model the in vivo conversion and breakdown of estrogenic compounds and thus provide a rapid preliminary screening method to estimate their estrogenicity.

The role of gamma interferon in innate immunity in the zebrafish embryo

The zebrafish genome contains ten genes that encode class II cytokine-like peptides, of which the two that are related most closely to mammalian interferon gamma (IFN-gamma) were named IFN-gamma1 and IFN-gamma2. Although the zebrafish has become a popular model system to study immune mechanisms, and although interferons are central regulators of immunity, which zebrafish cytokines correspond functionally to mammalian IFN-gamma has not been established. We used zebrafish embryos to assay the functions of IFN-gamma1 and IFN-gamma2, and have identified a subset of zebrafish homologs of the mammalian IFN-responsive genes as IFN-gamma targets in the zebrafish embryo: these genes are upregulated in response to raised levels of either IFN-gamma1 or IFN-gamma2. Infection studies using two different pathogens show that IFN-gamma signalling is required for resistance against bacterial infections in the young embryo and that the levels of IFN-gamma need to be regulated tightly: raising IFN-gamma levels sensitizes fish embryos against bacterial infection. Embryos injected with high doses of Escherichia coli are able to clear the bacteria within a day, and the gamma-interferons are necessary for this defence reaction. The protective response to Yersinia ruckeri, a natural fish pathogen that is lethal at low doses, also requires IFN-gamma. As in the induction of target genes, the two interferons act at least partly redundantly. Together with the previously demonstrated type III interferon response, these results show that the counterparts of the mammalian viral and bacterial interferon-dependent defence functions are in place in zebrafish embryos, and suggest that zebrafish IFN-gamma1 and IFN-gamma2 are functionally equivalent to mammalian IFN-gamma.

Transcriptome profiling and functional analyses of the zebrafish embryonic innate immune response to Salmonella infection

Due to the clear separation of innate immunity from adaptive responses, the externally developing zebrafish embryo represents a useful in vivo model for identification of innate host determinants of the response to bacterial infection. Here we performed a time-course transcriptome profiling study and gene ontology analysis of the embryonic innate immune response to infection with two model Salmonella strains that elicit either a lethal infection or an attenuated response. The transcriptional response to infection with both the lethal strain and the avirulent LPS O-Ag mutant strain showed clear conservation with host responses detected in other vertebrate models and human cells, including induction of genes encoding cell surface receptors, signaling intermediates, transcription factors, and inflammatory mediators. Furthermore, our study led to the identification of a large set of novel immune response genes and infection markers, the future functional characterization of which will support vertebrate genome annotation. From the time series and bacterial strain comparisons, matrix metalloproteinase genes, including mmp9, were among the most consistent infection-responsive genes. Purified Salmonella flagellin also strongly induced mmp9 expression. Using knockdown analysis, we showed that this gene was downstream of the zebrafish homologs of the flagellin receptor TLR5 and the adaptor MyD88. Additionally, flagellin-mediated induction of other inflammation markers, including il1b, il8, and cxcl-C1c, was reduced upon Tlr5 knockdown as well as expression of irak3, a putative negative TLR pathway regulator. Finally, we showed that induction of il1b, mmp9, and irak3 requires Myd88-dependent signaling, while ifn1 and il8 were induced Myd88 independently during Salmonella infection.

Evolution of lipopolysaccharide (LPS) recognition and signaling: fish TLR4 does not recognize LPS and negatively regulates NF-kappaB activation

It has long been established that lower vertebrates, most notably fish and amphibians, are resistant to the toxic effect of LPS. Furthermore, the lack of a TLR4 ortholog in some fish species and the lack of the essential costimulatory molecules for LPS activation via TLR4 (i.e., myeloid differentiation protein 2 (MD-2) and CD14) in all the fish genomes and expressed sequence tag databases available led us to hypothesize that the mechanism of LPS recognition in fish may be different from that of mammals. To shed light on the role of fish TLRs in LPS recognition, a dual-luciferase reporter assay to study NF-kappaB activation in whole zebrafish embryos was developed and three different bony fish models were studied: 1) the gilthead seabream (Sparus aurata, Perciformes), an immunological-tractable teleost model in which the presence of a TLR4 ortholog is unknown; 2) the spotted green pufferfish (Tetraodon nigroviridis, Tetraodontiformes), which lacks a TLR4 ortholog; and 3) the zebrafish (Danio rerio, Cypriniformes), which possesses two TLR4 orthologs. Our results show that LPS signaled via a TLR4- and MyD88-independent manner in fish, and, surprisingly, that the zebrafish TLR4 orthologs negatively regulated the MyD88-dependent signaling pathway. We think that the identification of TLR4 as a negative regulator of TLR signaling in the zebrafish, together with the absence of this receptor in most fish species, explains the resistance of fish to endotoxic shock and supports the idea that the TLR4 receptor complex for LPS recognition arose after the divergence of fish and tetrapods.

Transgenic zebrafish reporter lines reveal conserved Toll-like receptor signaling potential in embryonic myeloid leukocytes and adult immune cell lineages

The immune response of a host to an invading pathogen is dependent on the capacity of its immune cell compartment to recognize highly conserved pathogen components using an ancient class of pattern recognition receptors known as Toll-like receptors (TLRs). Initiation of TLR-mediated signaling results in the induction of proinflammatory cytokines that help govern the scale and duration of any ensuing response. Specificity for TLR signaling is, in part, a result of the differential recruitment of intracellular adaptor molecules. Of these, MyD88 is required for the majority of TLR signaling. Zebrafish have been shown to possess TLRs and adaptor molecules throughout early development, including MyD88, strongly suggesting conservation of this ancient defense mechanism. However, information about which embryonic cells/tissues possess this conserved signaling potential is lacking. To help define which embryonic cells, in particular, those of the innate immune system, have the potential for MyD88-dependent, TLR-mediated signaling, we generated transgenic reporter lines using regulatory elements of the myd88 gene to drive the fluorescent reporters enhanced GFP and Discosoma red fluorescent protein 2 within live zebrafish. These lines possess fluorescently marked cells/tissues consistent with endogenous myd88 expression, including a subset of myeloid leukocytes. These innate immune cells were confirmed to express other TLR adaptors including Mal, trif, and Sarm. Live wound-healing and infection assays validated the potential of these myd88-expressing leukocytes to participate in immune responses. These lines will provide a valuable resource for further resolving the contribution of MyD88 to early vertebrate immunity.

Molecular cloning and expression of MyD88 in large yellow croaker, Pseudosciaena crocea

Myeloid differentiation factor 88 (MyD88) is an adaptor protein involved in the interleukin-1 receptor and Toll-like receptor-induced activation of nuclear factor-kappaB (NF-kappaB). In this report, the full-length cDNA of MyD88 was cloned from the large yellow croaker, Pseudosciaena crocea. It was of 1574 bp, including a 5'-terminal untranslated region (UTR) of 89 bp, a 3'-terminal UTR of 621bp and an open reading frame (ORF) of 864 bp encoding a polypeptide of 287 amino acids. It contained a typical death domain at the N-terminal and a conservative Toll/IL-1R (TIR) domain structure at the C-terminal. The quantitative real-time reverse transcription PCR analysis revealed a broad expression of MyD88 with the highest expression in the spleen and the weakest expression in the muscle. The expression of MyD88 after challenge with formalin-inactivated Gram-negative bacterium Vibrio parahaemolyticus was tested in blood, spleen and liver. It suggested that the highest expression was in the spleen (p<0.05) with 1.9 times (at 48 h) as much as that in the control and the lowest expression of MyD88 was in the liver (p<0.05) with 0.29 times (at 3h) of that in the control. These results indicated that as a universal key adaptor in the Toll-like receptor pathway in mammals, MyD88 might play an important role in large yellow croaker defense against pathogenic infection.

Pseudomonas aeruginosa Type III secretion system interacts with phagocytes to modulate systemic infection of zebrafish embryos

Pseudomonas aeruginosa is an opportunistic human pathogen that can cause serious infection in those with deficient or impaired phagocytes. We have developed the optically transparent and genetically tractable zebrafish embryo as a model for systemic P. aeruginosa infection. Despite lacking adaptive immunity at this developmental stage, zebrafish embryos were highly resistant to P. aeruginosa infection, but as in humans, phagocyte depletion dramatically increased their susceptibility. The virulence of an attenuated P. aeruginosa strain lacking a functional Type III secretion system was restored upon phagocyte depletion, suggesting that this system influences virulence through its effects on phagocytes. Intravital imaging revealed bacterial interactions with multiple blood cell types. Neutrophils and macrophages rapidly phagocytosed and killed P. aeruginosa, suggesting that both cell types play a role in protection against infection. Intravascular aggregation of erythrocytes and other blood cells with resultant circulatory blockage was observed immediately upon infection, which may be relevant to the pathogenesis of thrombotic complications of human P. aeruginosa infections. The real-time visualization capabilities and genetic tractability of the zebrafish infection model should enable elucidation of molecular and cellular details of P. aeruginosa pathogenesis in conditions associated with neutropenia or impaired phagocyte function.

Live cell imaging of zebrafish leukocytes

Zebrafish are ideally suited for the live imaging of early immune cell compartments. Macrophages that initially appear on the yolk surface prior to the onset of circulation are the first functional immune cells within the embryo, predating the emergence of the first granulocytic cells-the heterophilic neutrophils. Both cell types have been shown in zebrafish to contribute to a robust early innate immune system, capable of clearing systemic infections and participating in wound healing. Early imaging of these cells within zebrafish relied on differential interference contrast (DIC) optics because of their superficial locations in the embryo and the optical transparency of embryonic tissues. Recently, the creation of a number of transgenic reporter lines possessing fluorescently marked myelomonocytic compartments provides the potential to live image these cells during the inflammatory response, in real-time, within a whole animal context. Live imaging during the different stages of inflammation using this expanding library of reporter lines, coupled with the ability to model aspects of human disease in the zebrafish system, have the potential to provide significant insights into inflammation and diseases associated with its dysregulation.

Application of the dual-luciferase reporter assay to the analysis of promoter activity in Zebrafish embryos

BACKGROUND

The dual-luciferase assay has been widely used in cell lines to determine rapidly but accurately the activity of a given promoter. Although this strategy has proved very useful, it does not allow the promoter and gene function to be analyzed in the context of the whole organism.

RESULTS

Here, we present a rapid and sensitive assay based on the classical dual-luciferase reporter technique which can be used as a new tool to characterize the minimum promoter region of a gene as well as the in vivo response of inducible promoters to different stimuli. We illustrate the usefulness of this system for studying both constitutive (telomerase) and inducible (NF-kappaB-dependent) promoters. The flexibility of this assay is demonstrated by induction of the NF-kappaB-dependent promoters using simultaneous microinjection of different pathogen-associated molecular patterns as well as with the use of morpholino-gene mediated knockdown.

CONCLUSION

This assay has several advantages compared with the classical in vitro (cell lines) and in vivo (transgenic mice) approaches. Among others, the assay allows a rapid and quantitative measurement of the effects of particular genes or drugs in a given promoter in the context of a whole organism and it can also be used in high throughput screening experiments.

Zebrafish as a model for infectious disease and immune function

The zebrafish, Danio rerio, has come to the forefront of biomedical research as a powerful model for the study of development, neurobiology, and genetics of humans. In recent years, use of the zebrafish system has extended into studies in behaviour, immunology and toxicology, retaining the concept that it will serve as a model for human disease. As one of the most thoroughly studied teleosts, with a wealth of genetic and genomic information available, the zebrafish is now being considered as a model for pathogen studies in finfishes. Its genome is currently being sequenced and annotated, and gene microarrays and insertional mutants are commercially available. The use of gene-specific knockdown of translation through morpholino oligonucleotides is widespread. As a result, several laboratories have developed bacterial and viral disease models with the zebrafish to study immune responses to infection. Although many of the zebrafish pathogen models were developed to address human infectious disease, the results of these studies should provide important clues for the development of effective vaccines and prophylactic measures against bacterial and viral pathogens in economically important fishes. In this review, the capabilities and potential of the zebrafish model system will be discussed and an overview of information on zebrafish infectious disease models will be presented.

A novel vertebrate model of Staphylococcus aureus infection reveals phagocyte-dependent resistance of zebrafish to non-host specialized pathogens

With the emergence of multiply resistant Staphylococcus aureus, there is an urgent need to better understand the molecular determinants of S. aureus pathogenesis. A model of staphylococcal pathogenesis in zebrafish embryos has been established, in which host phagocytes are able to mount an effective immune response, preventing overwhelming infection from small inocula. Myeloid cell depletion, by pu.1 morpholino-modified antisense injection, removes this immune protection. Macrophages and neutrophils are both implicated in this immune response, phagocytosing circulating bacteria. In addition, in vivo phagocyte/bacteria interactions can be visualized within transparent embryos. A preliminary screen for bacterial pathogenesis determinants has shown that strains bearing mutations in perR, pheP and saeR are attenuated. perR and pheP mutants are deficient in growth in vivo, and their virulence is not fully restored by myeloid cell depletion. On the other hand, saeR mutants are able to grow in vivo, and are completely restored to virulence by myeloid cell depletion. Thus specific pathogen gene function can be matched with particular facets of host response. Zebrafish are a new addition to the tools available for the study of S. aureus pathogenesis, and may provide insights into the interactions of bacterial and host genomes in determining the outcome of infection.

Conservation and divergence of gene families encoding components of innate immune response systems in zebrafish

Analysis of several fish genomes reveals components of the innate immune system and identifies orthologous relationships between gene families of fish and mammals.

Background

The zebrafish has become a widely used model to study disease resistance and immunity. Although the genes encoding many components of immune signaling pathways have been found in teleost fish, it is not clear whether all components are present or whether the complexity of the signaling mechanisms employed by mammals is similar in fish.

Results

We searched the genomes of the zebrafish Danio rerio and two pufferfish for genes encoding components of the Toll-like receptor and interferon signaling pathways, the NLR (NACHT-domain and leucine rich repeat containing) protein family, and related proteins. We find that most of the components known in mammals are also present in fish, with clearly recognizable orthologous relationships. The class II cytokines and their receptors have diverged extensively, obscuring orthologies, but the number of receptors is similar in all species analyzed. In the family of the NLR proteins, the canonical members are conserved. We also found a conserved NACHT-domain protein with WD40 repeats that had previously not been described in mammals. Additionally, we have identified in each of the three fish a large species-specific subgroup of NLR proteins that contain a novel amino-terminal domain that is not found in mammalian genomes.

Conclusion

The main innate immune signaling pathways are conserved in mammals and teleost fish. Whereas the components that act downstream of the receptors are highly conserved, with orthologous sets of genes in mammals and teleosts, components that are known or assumed to interact with pathogens are more divergent and have undergone lineage-specific expansions.

Conditional transgene and gene targeting methodologies in zebrafish

The zebrafish has become a powerful tool for dissecting vertebrate gene function during embryogenesis. Numerous molecular systems have been developed to examine gene function in zebrafish, including transgenics for creating lineage-tracer lines of zebrafish that express a fluorescent protein as a marker for specific populations of cells, and antisense strategies, primarily morpholinos, for knocking down gene function. The focus of this review is to summarize the pros and cons of the currently available systems for functional genomics in zebrafish, and to discuss the need for future methodologies.

Differential contribution of neutrophilic granulocytes and macrophages to nitrosative stress in a host-parasite animal model

Tyrosine nitration is a hallmark for nitrosative stress caused by the release of reactive oxygen and nitrogen species by activated macrophages and neutrophilic granulocytes at sites of inflammation and infection. In the first part of the study, we used an informative host-parasite animal model to describe the differential contribution of macrophages and neutrophilic granulocytes to in vivo tissue nitration. To this purpose common carp (Cyprinus carpio) were infected with the extracellular blood parasite Trypanoplasma borreli (Kinetoplastida). After infection, serum nitrite levels significantly increased concurrently to the upregulation of inducible nitric oxide synthase (iNOS) gene expression. Tyrosine nitration, as measured by immunohistochemistry using an anti-nitrotyrosine antibody, dramatically increased in tissues from parasite-infected fish, demonstrating that elevated NO production during T. borreli infection coincides with nitrosative stress in immunologically active tissues. The combined use of an anti-nitrotyrosine antibody with a panel of monoclonal antibodies specific for several carp leukocytes, revealed that fish neutrophilic granulocytes strongly contribute to in vivo tissue nitration most likely through both, a peroxynitrite- and an MPO-mediated mechanism. Conversely, fish macrophages, by restricting the presence of radicals and enzymes to their intraphagosomal compartment, contribute to a much lesser extent to in vivo tissue nitration. In the second part of the study, we examined the effects of nitrosative stress on the parasite itself. Peroxynitrite, but not NO donor substances, exerted strong cytotoxicity on the parasite in vitro. In vivo, however, nitration of T. borreli was limited if not absent despite the presence of parasites in highly nitrated tissue areas. Further, we investigated parasite susceptibility to the human anti-trypanosome drug Melarsoprol (Arsobal), which directly interferes with the parasite-specific trypanothione anti-oxidant system. Arsobal treatment strongly decreased T. borreli viability both, in vitro and in vivo. All together, our data suggest an evolutionary conservation in modern bony fish of the function of neutrophilic granulocytes and macrophages in the nitration process and support the common carp as a suitable animal model for investigations on nitrosative stress in host-parasite interactions. The potential of T. borreli to serve as an alternative tool for pharmacological studies on human anti-trypanosome drugs is discussed.

Toll-like receptors--taking an evolutionary approach

The Toll receptor was initially identified in Drosophila melanogaster for its role in embryonic development. Subsequently, D. melanogaster Toll and mammalian Toll-like receptors (TLRs) have been recognized as key regulators of immune responses. After ten years of intense research on TLRs and the recent accumulation of genomic and functional data in diverse organisms, we review the distribution and functions of TLRs in the animal kingdom. We provide an evolutionary perspective on TLRs, which sheds light on their origin at the dawn of animal evolution and suggests that different TLRs might have been co-opted independently during animal evolution to mediate analogous immune functions.

Comparative pathogenesis of Mycobacterium marinum and Mycobacterium tuberculosis

A thorough understanding of Mycobacterium tuberculosis pathogenesis in humans has been elusive in part because of imperfect surrogate laboratory hosts, each with its own idiosyncrasies. Mycobacterium marinum is the closest genetic relative of the M. tuberculosis complex and is a natural pathogen of ectotherms. In this review, we present evidence that the similar genetic programmes of M. marinum and M. tuberculosis and the corresponding host immune responses reveal a conserved skeleton of Mycobacterium host-pathogen interactions. While both species have made niche-specific refinements, an essential framework has persisted. We highlight genetic comparisons of the two organisms and studies of M. marinum in the developing zebrafish. By pairing M. marinum with the simplified immune system of zebrafish embryos, many of the defining mechanisms of mycobacterial pathogenesis can be distilled and investigated in a tractable host/pathogen pair.

Intestinal alkaline phosphatase detoxifies lipopolysaccharide and prevents inflammation in zebrafish in response to the gut microbiota

Vertebrates harbor abundant lipopolysaccharide (LPS) in their gut microbiota. Alkaline phosphatases can dephosphorylate and detoxify the endotoxin component of LPS. Here, we show that expression of the zebrafish intestinal alkaline phosphatase (Iap), localized to the intestinal lumen brush border, is induced during establishment of the gut microbiota. Iap-deficient zebrafish are hypersensitive to LPS toxicity and exhibit the excessive intestinal neutrophil influx characteristic of wild-type zebrafish exposed to LPS. Both of these Iap mutant phenotypes are dependent on Myd88 and Tumor Necrosis Factor Receptor (Tnfr), proteins also involved in LPS sensitivity in mammals. When reared germ-free, the intestines of Iap-deficient zebrafish are devoid of neutrophils. Together, these findings demonstrate that the endogenous microbiota establish the normal homeostatic level of neutrophils in the zebrafish intestine through a process involving Iap, Myd88, and Tnfr. Thus, by preventing inflammatory responses, Iap plays a crucial role in promoting mucosal tolerance to resident gut bacteria.

In vivo analysis of zebrafish innate immunity

Among vertebrate model species, the zebrafish embryo combines at an unprecedented level optical accessibility with easy genetic manipulation. As such, it is gaining recognition as a powerful model to study innate immunity. In this chapter, we provide a protocol for the generation of zebrafish embryos deficient in a protein of interest for innate immune signaling using antisense morpholino oligonucleotides, the systemic or local infection of these embryos with bacteria, and the assessment of various aspects of the following immune response with emphasis on microscopic observation. This example can be easily adapted to study the role of other genes, either knocked down or overexpressed, and in response to any other challenge, from purified microbial compounds to pathogenic viruses. This protocol is aimed at people not necessarily familiar with zebrafish biology and handling.

The zebrafish mutant lbk/vam6 resembles human multisystemic disorders caused by aberrant trafficking of endosomal vesicles

The trafficking of intracellular vesicles is essential for a number of cellular processes and defects in this process have been implicated in a wide range of human diseases. We identify the zebrafish mutant lbk as a novel model for such disorders. lbk displays hypopigmentation of skin melanocytes and the retinal pigment epithelium (RPE), an absence of iridophore reflections, defects in internal organs (liver, intestine) as well as functional defects in vision and the innate immune system (macrophages). Positional cloning, an allele screen, rescue experiments and morpholino knock-down reveal a mutation in the zebrafish orthologue of the vam6/vps39 gene. Vam6p is part of the HOPS complex, which is essential for vesicle tethering and fusion. Affected cells in the lbk RPE, liver, intestine and macrophages display increased numbers and enlarged intracellular vesicles. Physiological and behavioural analyses reveal severe defects in visual ability in lbk mutants. The present study provides the first phenotypic description of a lack of vam6 gene function in a multicellular organism. lbk shares many of the characteristics of human diseases and suggests a novel disease gene for pathologies associated with defective vesicle transport, including the arthrogryposis-renal dysfunction-cholestasis (ARC) syndrome, the Hermansky-Pudlak syndrome, the Chediak-Higashi syndrome and the Griscelli syndrome.