Hiding in the yolk: A unique feature of Legionella pneumophila infection of zebrafish.

The zebrafish has become a powerful model organism to study host-pathogen interactions. Here, we developed a zebrafish model to dissect the innate immune response to Legionella pneumophila during infection. We show that L. pneumophila cause zebrafish larvae death in a dose dependent manner. Additionally, we show that macrophages are the first line of defence and cooperate with neutrophils to clear the infection. Immunocompromised humans have an increased propensity to develop pneumonia, similarly, when either macrophages or neutrophils are depleted, these "immunocompromised" larvae become lethally sensitive to L. pneumophila. Also, as observed in human infections, the adaptor signalling molecule Myd88 is not required to control disease in the larvae. Furthermore, proinflammatory cytokine genes il1ß and tnf-α were upregulated during infection, recapitulating key immune responses seen in human infection. Strikingly, we uncovered a previously undescribed infection phenotype in zebrafish larvae, whereby bloodborne, wild type L. pneumophila invade and grow in the larval yolk region, a phenotype not observed with a type IV secretion system deficient mutant that cannot translocate effectors into its host cell. Thus, zebrafish larva represents an innovative L. pneumophila infection model that mimics important aspects of the human immune response to L. pneumophila infection and will allow the elucidation of mechanisms by which type IV secretion effectors allow L. pneumophila to cross host cell membranes and obtain nutrients from nutrient rich environments.

Septins restrict inflammation and protect zebrafish larvae from Shigella infection.

Shigella flexneri, a Gram-negative enteroinvasive pathogen, causes inflammatory destruction of the human intestinal epithelium. Infection by S. flexneri has been well-studied in vitro and is a paradigm for bacterial interactions with the host immune system. Recent work has revealed that components of the cytoskeleton have important functions in innate immunity and inflammation control. Septins, highly conserved cytoskeletal proteins, have emerged as key players in innate immunity to bacterial infection, yet septin function in vivo is poorly understood. Here, we use S. flexneri infection of zebrafish (Danio rerio) larvae to study in vivo the role of septins in inflammation and infection control. We found that depletion of Sept15 or Sept7b, zebrafish orthologs of human SEPT7, significantly increased host susceptibility to bacterial infection. Live-cell imaging of Sept15-depleted larvae revealed increasing bacterial burdens and a failure of neutrophils to control infection. Strikingly, Sept15-depleted larvae present significantly increased activity of Caspase-1 and more cell death upon S. flexneri infection. Dampening of the inflammatory response with anakinra, an antagonist of interleukin-1 receptor (IL-1R), counteracts Sept15 deficiency in vivo by protecting zebrafish from hyper-inflammation and S. flexneri infection. These findings highlight a new role for septins in host defence against bacterial infection, and suggest that septin dysfunction may be an underlying factor in cases of hyper-inflammation.

The zebrafish as a new model for the in vivo study of Shigella flexneri interaction with phagocytes and bacterial autophagy.

Autophagy, an ancient and highly conserved intracellular degradation process, is viewed as a critical component of innate immunity because of its ability to deliver cytosolic bacteria to the lysosome. However, the role of bacterial autophagy in vivo remains poorly understood. The zebrafish (Danio rerio) has emerged as a vertebrate model for the study of infections because it is optically accessible at the larval stages when the innate immune system is already functional. Here, we have characterized the susceptibility of zebrafish larvae to Shigella flexneri, a paradigm for bacterial autophagy, and have used this model to study Shigella-phagocyte interactions in vivo. Depending on the dose, S. flexneri injected in zebrafish larvae were either cleared in a few days or resulted in a progressive and ultimately fatal infection. Using high resolution live imaging, we found that S. flexneri were rapidly engulfed by macrophages and neutrophils; moreover we discovered a scavenger role for neutrophils in eliminating infected dead macrophages and non-immune cell types that failed to control Shigella infection. We observed that intracellular S. flexneri could escape to the cytosol, induce septin caging and be targeted to autophagy in vivo. Depletion of p62 (sequestosome 1 or SQSTM1), an adaptor protein critical for bacterial autophagy in vitro, significantly increased bacterial burden and host susceptibility to infection. These results show the zebrafish larva as a new model for the study of S. flexneri interaction with phagocytes, and the manipulation of autophagy for anti-bacterial therapy in vivo.

Strategies of professional phagocytes in vivo: unlike macrophages, neutrophils engulf only surface-associated microbes.

The early control of potentially invading microbes by our immune system primarily depends on its main professional phagocytes - macrophages and neutrophils. Although the different functions of these two cell types have been extensively studied, little is known about their respective contributions to the initial control of invading microorganisms before the onset of adaptive immune responses. The naturally translucent zebrafish larva has recently emerged as a powerful model vertebrate in which to visualise the dynamic interactions between leukocytes and microbes in vivo. Using high-resolution live imaging, we found that whereas macrophages efficiently engulf bacteria from blood or fluid-filled body cavities, neutrophils barely do so. By contrast, neutrophils very efficiently sweep up surface-associated, but not fluid-borne, bacteria. Thus the physical presentation of unopsonised microbes is a crucial determinant of neutrophil phagocytic ability. Neutrophils engulf microbes only as they move over them, in a 'vacuum-cleaner' type of behaviour. This context-dependent nature of phagocytosis by neutrophils should be of particular relevance to human infectious diseases, especially for the early phase of encounter with microbes new to the host.

Whole-body analysis of a viral infection: vascular endothelium is a primary target of infectious hematopoietic necrosis virus in zebrafish larvae.

The progression of viral infections is notoriously difficult to follow in whole organisms. The small, transparent zebrafish larva constitutes a valuable system to study how pathogens spread. We describe here the course of infection of zebrafish early larvae with a heat-adapted variant of the Infectious Hematopoietic Necrosis Virus (IHNV), a rhabdovirus that represents an important threat to the salmonid culture industry. When incubated at 24 °C, a permissive temperature for virus replication, larvae infected by intravenous injection died within three to four days. Macroscopic signs of infection followed a highly predictable course, with a slowdown then arrest of blood flow despite continuing heartbeat, followed by a loss of reactivity to touch and ultimately by death. Using whole-mount in situ hybridization, patterns of infection were imaged in whole larvae. The first infected cells were detectable as early as 6 hours post infection, and a steady increase in infected cell number and staining intensity occurred with time. Venous endothelium appeared as a primary target of infection, as could be confirmed in fli1:GFP transgenic larvae by live imaging and immunohistochemistry. Disruption of the first vessels took place before arrest of blood circulation, and hemorrhages could be observed in various places. Our data suggest that infection spread from the damaged vessels to underlying tissue. By shifting infected fish to a temperature of 28 °C that is non-permissive for viral propagation, it was possible to establish when virus-generated damage became irreversible. This stage was reached many hours before any detectable induction of the host response. Zebrafish larvae infected with IHNV constitute a vertebrate model of an hemorrhagic viral disease. This tractable system will allow the in vivo dissection of host-virus interactions at the whole organism scale, a feature unrivalled by other vertebrate models.

NAD kinase promotes Staphylococcus aureus pathogenesis by supporting production of virulence factors and protective enzymes.

Nicotinamide adenine dinucleotide phosphate (NADPH) is the primary electron donor for reductive reactions that are essential for the biosynthesis of major cell components in all organisms. Nicotinamide adenine dinucleotide kinase (NADK) is the only enzyme that catalyzes the synthesis of NADP(H) from NAD(H). While the enzymatic properties and physiological functions of NADK have been thoroughly studied, the role of NADK in bacterial pathogenesis remains unknown. Here, we used CRISPR interference to knock down NADK gene expression to address the role of this enzyme in Staphylococcus aureus pathogenic potential. We find that NADK inhibition drastically decreases mortality of zebrafish infected with S. aureus. Furthermore, we show that NADK promotes S. aureus survival in infected macrophages by protecting bacteria from antimicrobial defense mechanisms. Proteome-wide data analysis revealed that production of major virulence-associated factors is sustained by NADK. We demonstrate that NADK is required for expression of the quorum-sensing response regulator AgrA, which controls critical S. aureus virulence determinants. These findings support a key role for NADK in bacteria survival within innate immune cells and the host during infection.

PTEN inhibits AMPK to control collective migration.

Pten is one of the most frequently mutated tumour suppressor gene in cancer. PTEN is generally altered in invasive cancers such as glioblastomas, but its function in collective cell migration and invasion is not fully characterised. Herein, we report that the loss of PTEN increases cell speed during collective migration of non-tumourous cells both in vitro and in vivo. We further show that loss of PTEN promotes LKB1-dependent phosphorylation and activation of the major metabolic regulator AMPK. In turn AMPK increases VASP phosphorylation, reduces VASP localisation at cell-cell junctions and decreases the interjunctional transverse actin arcs at the leading front, provoking a weakening of cell-cell contacts and increasing migration speed. Targeting AMPK activity not only slows down PTEN-depleted cells, it also limits PTEN-null glioblastoma cell invasion, opening new opportunities to treat glioblastoma lethal invasiveness.

Spatiotemporal analysis of mycolactone distribution in vivo reveals partial diffusion in the central nervous system.

Mycobacterium ulcerans, the causative agent of Buruli ulcer (BU) disease, is unique amongst human pathogens in its capacity to produce a lipid toxin called mycolactone. While previous studies have demonstrated that bacterially-released mycolactone diffuses beyond infection foci, the spatiotemporal distribution of mycolactone remained largely unknown. Here, we used the zebrafish model to provide the first global kinetic analysis of mycolactone's diffusion in vivo, and multicellular co-culture systems to address the critical question of the toxin's access to the brain. Zebrafish larvae were injected with a fluorescent-derivative of mycolactone to visualize the in vivo diffusion of the toxin from the peripheral circulation. A rapid, body-wide distribution of mycolactone was observed, with selective accumulation in tissues near the injection site and brain, together with an important excretion through the gastro-intestinal tract. Our conclusion that mycolactone reached the central nervous system was reinforced by an in cellulo model of human blood brain barrier and a mouse model of M. ulcerans-infection. Here we show that mycolactone has a broad but heterogenous profile of distribution in vivo. Our investigations in vitro and in vivo support the view that a fraction of bacterially-produced mycolactone gains access to the central nervous system. The relative persistence of mycolactone in the bloodstream suggests that assays of circulating mycolactone are relevant for BU disease monitoring and treatment optimization.

ADP ribosylation factor 6 is activated and controls membrane delivery during phagocytosis in macrophages.

Engulfment of particles by phagocytes is induced by their interaction with specific receptors on the cell surface, which leads to actin polymerization and the extension of membrane protrusions to form a closed phagosome. Membrane delivery from internal pools is considered to play an important role in pseudopod extension during phagocytosis. Here, we report that endogenous ADP ribosylation factor 6 (ARF6), a small GTP-binding protein, undergoes a sharp and transient activation in macrophages when phagocytosis was initiated via receptors for the Fc portion of immunoglobulins (FcRs). A dominant-negative mutant of ARF6 (T27N mutation) dramatically affected FcR-mediated phagocytosis. Expression of ARF6-T27N lead to a reduction in the focal delivery of vesicle-associated membrane protein 3+ endosomal recycling membranes at phagocytosis sites, whereas actin polymerization was unimpaired. This resulted in an early blockade in pseudopod extension and accumulation of intracellular vesicles, as observed by electron microscopy. We conclude that ARF6 is a major regulator of membrane recycling during phagocytosis.

Ultraspecific live imaging of the dynamics of zebrafish neutrophil granules by a histopermeable fluorogenic benzochalcone probe.

Neutrophil granules (NGs) are key components of the innate immune response and mark the development of neutrophilic granulocytes in mammals. However, there has been no specific fluorescent vital stain up to now to monitor their dynamics within a whole live organism. We rationally designed a benzochalcone fluorescent probe (HAB) featuring high tissue permeability and optimal photophysics such as elevated quantum yield, pronounced solvatochromism and target-induced fluorogenesis. Phenotypic screening identified HAB as the first cell- and organelle-specific small-molecule fluorescent tracer of NGs in live zebrafish larvae, with no labeling of other cell types or organelles. HAB staining was independent of the state of neutrophil activation, labeling NGs of both resting and phagocytically active neutrophils with equal specificity. By high-resolution live imaging, we documented the dynamics of HAB-stained NGs during phagocytosis. Upon zymosan injection, labeled NGs were rapidly recruited to the forming phagosomes. Despite being a reversible ligand, HAB could not be displaced by high concentrations of pharmacologically relevant competing chalcones, indicating that this specific labeling was the result of the HAB's precise physicochemical signature rather than a general feature of chalcones. However, one of the competitors was discovered as a promising interstitial fluorescent tracer illuminating zebrafish histology, similarly to BODIPY-ceramide. As a yellow-emitting histopermeable vital stain, HAB functionally and spectrally complements most genetically incorporated fluorescent tags commonly used in live zebrafish biology, holding promise for the study of neutrophil-dependent responses relevant to human physiopathology such as developmental defects, inflammation and infection. Furthermore, HAB intensely labeled isolated live human neutrophils at the level of granulated subcellular structures consistent with human NGs, suggesting that the labeling of NGs by HAB is not restricted to the zebrafish model but also relevant to mammalian systems.

A role for mammalian diaphanous-related formins in complement receptor (CR3)-mediated phagocytosis in macrophages.

Macrophages, dendritic cells, and neutrophils use phagocytosis to capture and clear off invading pathogens. The process is triggered by the interaction of ligands on the pathogens' surface with specific phagocytic receptors, including immunoglobulin (FcR) and complement C3bi (CR3) receptors (integrin alpha(M)beta2, Mac1) . Localized actin-filament assembly that acts as the driving force for particle engulfment is controlled by Rho-family small GTPases . RhoA regulates CR3-mediated phagocytosis through a mechanism that is still unclear . Mammalian Diaphanous-related (mDia) formins participate in the generation of a diverse set of actin-remodeling events downstream of RhoA , and mDia1 is recruited around fibronectin-coated beads in a RhoA-dependent manner in fibroblasts . Here, we set out to examine whether mDia proteins are involved in CR3-mediated phagocytosis in macrophages. We show that the RhoA effector mDia1 is recruited early during CR3-mediated phagocytosis and colocalizes with polymerized actin in the phagocytic cup. Interfering with mDia activity inhibits CR3-mediated phagocytosis while having no effect on FcR-mediated phagocytosis. These results indicate a new function for mDia proteins in the regulation of actin polymerization during CR3-mediated phagocytosis.

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.

A Model of Superinfection of Virus-Infected Zebrafish Larvae: Increased Susceptibility to Bacteria Associated With Neutrophil Death.

Enhanced susceptibility to bacterial infection in the days following an acute virus infection such as flu is a major clinical problem. Mouse models have provided major advances in understanding viral-bacterial superinfections, yet interactions of the anti-viral and anti-bacterial responses remain elusive. Here, we have exploited the transparency of zebrafish to study how viral infections can pave the way for bacterial co-infections. We have set up a zebrafish model of sequential viral and bacterial infection, using sublethal doses of Sindbis virus and Shigella flexneri bacteria. This virus induces a strong type I interferons (IFN) response, while the bacterium induces a strong IL1ß and TNFα-mediated inflammatory response. We found that virus-infected zebrafish larvae showed an increased susceptibility to bacterial infection. This resulted in the death with concomitant higher bacterial burden of the co-infected fish compared to the ones infected with bacteria only. By contrast, infecting with bacteria first and virus second did not lead to increased mortality or microbial burden. By high-resolution live imaging, we showed that neutrophil survival was impaired in Sindbis-then-Shigella co-infected fish. The two types of cytokine responses were strongly induced in co-infected fish. In addition to type I IFN, expression of the anti-inflammatory cytokine IL10 was induced by viral infection before bacterial superinfection. Collectively, these observations suggest the zebrafish larva as a useful animal model to address mechanisms underlying increased bacterial susceptibility upon viral infection.

Protective role of phosphorylation in turnover of glial fibrillary acidic protein in mice.

Glial fibrillary acidic protein (GFAP), the principal intermediate filament (IF) protein of mature astrocytes in the CNS, plays specific roles in astrocyte functions. GFAP has multiple phosphorylation sites at its N-terminal head domain. To examine the role of phosphorylation at these sites, we generated a series of substitution mutant mice in which phosphorylation sites (Ser/Thr) were replaced by Ala, in different combinations. Gfap(hm3/hm3) mice carrying substitutions at all five phosphorylation sites showed extensive decrease in both filament formation and amounts of GFAP. Gfap(hm1/hm1) and Gfap(hm2/hm2) mice, which carry substitutions at three of five sites and in different combinations, showed differential phenotypes. Although Gfap(hm3/hm3) mice retained GFAP filaments in Bergmann glia in the cerebellum, the (Gfap(hm3/hm3):Vim(-/-)) mice lacked GFAP filaments. Pulse-chase experiments of cultured astrocytes indicated that the Hm3-GFAP encoded by Gfap(hm3) was unstable particularly in the absence of vimentin, another IF protein. These results revealed the role of phosphorylation in turnover of GFAP and a synergistic role of GFAP and vimentin in the dynamics of glial filaments. The data further suggest that each of the phosphorylated sites has a distinct impact on the dynamics of GFAP.

Use of Shigella flexneri to study autophagy-cytoskeleton interactions.

Shigella flexneri is an intracellular pathogen that can escape from phagosomes to reach the cytosol, and polymerize the host actin cytoskeleton to promote its motility and dissemination. New work has shown that proteins involved in actin-based motility are also linked to autophagy, an intracellular degradation process crucial for cell autonomous immunity. Strikingly, host cells may prevent actin-based motility of S. flexneri by compartmentalizing bacteria inside 'septin cages' and targeting them to autophagy. These observations indicate that a more complete understanding of septins, a family of filamentous GTP-binding proteins, will provide new insights into the process of autophagy. This report describes protocols to monitor autophagy-cytoskeleton interactions caused by S. flexneri in vitro using tissue culture cells and in vivo using zebrafish larvae. These protocols enable investigation of intracellular mechanisms that control bacterial dissemination at the molecular, cellular, and whole organism level.

Origins and unconventional behavior of neutrophils in developing zebrafish.

The first leukocytes that arise in the development of vertebrate embryos are the primitive macrophages, which differentiate in the yolk sac and then quickly invade embryonic tissues. These macrophages have been considered to constitute a separate lineage, giving rise to no other cell type. Using an in vivo photoactivatable cell tracer in the transparent zebrafish (Danio rerio) embryo, we demonstrated that this lineage also gave rise to an equal or higher number of neutrophilic granulocytes. We were surprised to find that the differentiation of these primitive neutrophils occurs only after primitive myeloid progenitors have dispersed in the tissues. By 2 days after fertilization, these neutrophils have become the major leukocyte type found wandering in the epidermis and mesenchyme. Like the primitive macrophages, all primitive and larval neutrophils express PU.1 and L-plastin and they are highly attracted to local infections, yet only a small fraction of them phagocytose microbes, and to a much lesser extent per cell than the macrophages. They are also attracted to variously stressed or malformed tissues, suggesting a wider role than antimicrobial defense.

Dynamics of mutated GFAP aggregates revealed by real-time imaging of an astrocyte model of Alexander disease.

Alexander disease (AxD) is a rare neurodegenerative disorder characterized by large cytoplasmic aggregates in astrocytes and myelin abnormalities and caused by dominant mutations in the gene encoding glial fibrillary acidic protein (GFAP), the main intermediate filament protein in astrocytes. We tested the effects of three mutations (R236H, R76H and L232P) associated with AxD in cells transiently expressing mutated GFAP fused to green fluorescent protein (GFP). Mutated GFAP-GFP expressed in astrocytes formed networks or aggregates similar to those found in the brains of patients with the disease. Time-lapse recordings of living astrocytes showed that aggregates of mutated GFAP-GFP may either disappear, associated with cell survival, or coalesce in a huge juxtanuclear structure associated with cell death. Immunolabeling of fixed cells suggested that this gathering of aggregates forms an aggresome-like structure. Proteasome inhibition and immunoprecipitation assays revealed mutated GFAP-GFP ubiquitination, suggesting a role of the ubiquitin-proteasome system in the disaggregation process. In astrocytes from wild-type-, GFAP-, and vimentin-deficient mice, mutated GFAP-GFP aggregated or formed a network, depending on qualitative and quantitative interactions with normal intermediate filament partners. Particularly, vimentin displayed an anti-aggregation effect on mutated GFAP. Our data indicate a dynamic and reversible aggregation of mutated GFAP, suggesting that therapeutic approaches may be possible.

Keratin 17 null mice exhibit age- and strain-dependent alopecia.

Onset of type I keratin 17 (K17) synthesis marks the adoption of an appendageal fate within embryonic ectoderm, and its expression persists in specific cell types within mature hair, glands, and nail. We report that K17 null mice develop severe alopecia during the first week postbirth, correlating with hair fragility, alterations in follicular histology, and apoptosis in matrix cells. These alterations are incompletely penetrant and normalize starting with the first postnatal cycle. Absence of a hair phenotype correlates with a genetic strain-dependent compensation by related keratins, including K16. These findings reveal a crucial role for K17 in the structural integrity of the first hair produced and the survival of hair-producing cells. Given that identical inherited mutations in this gene can cause either pachyonychia congenita or steatocystoma multiplex, the features of this mouse model suggest that this clinical heterogeneity arises from a cell type-specific, genetically determined compensation by related keratins.

Recovery of Na-glucose cotransport activity after renal ischemia is impaired in mice lacking vimentin.

Vimentin, an intermediate filament protein mainly expressed in mesenchyma-derived cells, is reexpressed in renal tubular epithelial cells under many pathological conditions, characterized by intense cell proliferation. Whether vimentin reexpression is only a marker of cell dedifferentiation or is instrumental in the maintenance of cell structure and/or function is still unknown. Here, we used vimentin knockout mice (Vim(-/-)) and an experimental model of acute renal injury (30-min bilateral renal ischemia) to explore the role of vimentin. Bilateral renal ischemia induced an initial phase of acute tubular necrosis that did not require vimentin and was similar, in terms of morphological and functional changes, in Vim(+/+) and Vim(-/-) mice. However, vimentin was essential to favor Na-glucose cotransporter 1 localization to brush-border membranes and to restore Na-glucose cotransport activity in regenerating tubular cells. We show that the effect of vimentin inactivation is specific and results in persistent glucosuria. We propose that vimentin is part of a structural network that favors carrier localization to plasma membranes to restore transport activity in injured kidneys.

Identification of Fyn-binding proteins in MC/9 mast cells using mass spectrometry.

Fyn is a Src kinase known to have an essential role in mast cell degranulation induced following aggregation of the high affinity IgE-receptor. Although Fyn possesses SH2 and SH3 protein binding domains, the molecules that interact with Fyn have not been characterized in mast cells. We thus analyzed Fyn-binding proteins in MC/9 mast cells to explore the Fyn-mediated signaling pathway. On mass spectrometric analysis of proteins binding to the SH2 and SH3 domains of Fyn, we identified six proteins that bind to Fyn including vimentin, pyruvate kinase, p62 ras-GAP associated phosphoprotein, SLP-76, HS-1, and FYB. Among these proteins, vimentin and pyruvate kinase have not been shown to bind to Fyn. After IgE-receptor mediated stimulation, binding of vimentin to Fyn was increased; and this interaction was via binding to the SH2, but not the SH3, domain of Fyn. Mast cells from vimentin-deficient mice showed enhanced mediator release and tyrosine phosphorylation of intracellular proteins including NTAL and LAT. The observation that vimentin and pyruvate kinase bind to Fyn provides additional insight into Fyn-mediated signaling pathways, and suggests a critical role for Fyn in mast cell degranulation in interacting with both cytosolic and structural proteins.