Using Zebrafish Models of Human Influenza A Virus Infections to Screen Antiviral Drugs and Characterize Host Immune Cell Responses

Each year, seasonal influenza outbreaks profoundly affect societies worldwide. In spite of global efforts, influenza remains an intractable healthcare ...

National Institute of Environmental Health Sciences, NIH

The influenza viral membrane protein hemagglutinin (HA) is required at high concentrations on virion and host-cell membranes for infectivity. Because the role of actin in membrane organization is not completely understood, we quantified the relationship between HA and host-cell actin at the nanoscale. Results obtained using superresolution fluorescence photoactivation localization microscopy (FPALM) in nonpolarized cells show that HA clusters colocalize with actin-rich membrane regions (ARMRs). Individual molecular trajectories in live cells indicate restricted HA mobility in ARMRs, and actin disruption caused specific changes to HA clustering. Surprisingly, the actin-binding protein cofilin was excluded from some regions within several hundred nanometers of HA clusters, suggesting that HA clusters or adjacent proteins within the same clusters influence local actin structure. Thus, with the use of imaging, we demonstrate a dynamic relationship between glycoprotein membrane organization and the actin cytoskeleton at the nanoscale.

Actin mediates the nanoscale membrane organization of the clustered membrane protein influenza hemagglutinin.

Understanding spatial distribution and dynamics of receptors within unperturbed membranes is essential for elucidating their role in antiviral signaling, but conventional studies of detergent-resistant membrane fractions cannot provide this information. Caveolae are integral to numerous signaling pathways and these membrane domains have been previously implicated in viral entry but not antiviral defense. This study shows, for the first time, the importance of spatio-temporal regulation of signaling receptors and the importance of the regulation of clustering for downstream signaling. A novel mechanism for virus evasion of host cell defenses is demonstrated through disruption of clusters of signaling molecules organized within caveolin-rich domains. Viral infection leads to a downregulation in Caveolin-1b (Cav-1b), disrupting clusters of CRFB1, a zebrafish type I interferon receptor (-R) subunit. Super-resolution microscopy has enabled the first single-molecule imaging of CRFB1 association with cav-1b-containing membrane domains. Strikingly, downregulation of Cav-1b, the major protein component of caveolae, caused CRFB1 clusters to disperse. Dispersal of CRFB1 clusters led to a suppressed antiviral immune response both in vitro and in vivo, through abrogation of downstream signaling. This response strongly suggests that CRFB1 organization within cav-1b-containing membrane domains is critical for IFN-mediated antiviral defense and presents a previously undescribed antiviral evasion strategy to alter IFN signaling and the antiviral immune response.

Super Resolution Microscopy Reveals that Caveolin-1 Is Required for Spatial Organization of CRFB1 and Subsequent Antiviral Signaling in Zebrafish.

Seasonal influenza virus infections cause annual epidemics and sporadic pandemics. These present a global health concern, resulting in substantial morbidity, mortality and economic burdens. Prevention and treatment of influenza illness is difficult due to the high mutation rate of the virus, the emergence of new virus strains and increasing antiviral resistance. Animal models of influenza infection are crucial to our gaining a better understanding of the pathogenesis of and host response to influenza infection, and for screening antiviral compounds. However, the current animal models used for influenza research are not amenable to visualization of host-pathogen interactions or high-throughput drug screening. The zebrafish is widely recognized as a valuable model system for infectious disease research and therapeutic drug testing. Here, we describe a zebrafish model for human influenza A virus (IAV) infection and show that zebrafish embryos are susceptible to challenge with both influenza A strains APR8 and X-31 (Aichi). Influenza-infected zebrafish show an increase in viral burden and mortality over time. The expression of innate antiviral genes, the gross pathology and the histopathology in infected zebrafish recapitulate clinical symptoms of influenza infections in humans. This is the first time that zebrafish embryos have been infected with a fluorescent IAV in order to visualize infection in a live vertebrate host, revealing a pattern of vascular endothelial infection. Treatment of infected zebrafish with a known anti-influenza compound, Zanamivir, reduced mortality and the expression of a fluorescent viral gene product, demonstrating the validity of this model to screen for potential antiviral drugs. The zebrafish model system has provided invaluable insights into host-pathogen interactions for a range of infectious diseases. Here, we demonstrate a novel use of this species for IAV research. This model has great potential to advance our understanding of influenza infection and the associated host innate immune response.

Influenza A virus infection in zebrafish recapitulates mammalian infection and sensitivity to anti-influenza drug treatment.

Light microscopy enables noninvasive imaging of fluorescent species in biological specimens, but resolution is generally limited by diffraction to ~200-250 nm. Many biological processes occur on smaller length scales, highlighting the importance of techniques that can image below the diffraction limit and provide valuable single-molecule information. In recent years, imaging techniques have been developed which can achieve resolution below the diffraction limit. Utilizing one such technique, fluorescence photoactivation localization microscopy (FPALM), we demonstrated its ability to construct super-resolution images from single molecules in a living zebrafish embryo, expanding the realm of previous super-resolution imaging to a living vertebrate organism. We imaged caveolin-1 in vivo, in living zebrafish embryos. Our results demonstrate the successful image acquisition of super-resolution images in a living vertebrate organism, opening several opportunities to answer more dynamic biological questions in vivo at the previously inaccessible nanoscale.

Nanoscale imaging of caveolin-1 membrane domains in vivo.

Lipid raft membrane microdomains organize signaling by many prototypical receptors, including the Toll-like receptors (TLRs) of the innate immune system. Raft-localization of proteins is widely thought to be regulated by raft cholesterol levels, but this is largely on the basis of studies that have manipulated cell cholesterol using crude and poorly specific chemical tools, such as β-cyclodextrins. To date, there has been no proteome-scale investigation of whether endogenous regulators of intracellular cholesterol trafficking, such as the ATP binding cassette (ABC)A1 lipid efflux transporter, regulate targeting of proteins to rafts. Abca1(-/-) macrophages have cholesterol-laden rafts that have been reported to contain increased levels of select proteins, including TLR4, the lipopolysaccharide receptor. Here, using quantitative proteomic profiling, we identified 383 proteins in raft isolates from Abca1(+/+) and Abca1(-/-) macrophages. ABCA1 deletion induced wide-ranging changes to the raft proteome. Remarkably, many of these changes were similar to those seen in Abca1(+/+) macrophages after lipopolysaccharide exposure. Stomatin-like protein (SLP)-2, a member of the stomatin-prohibitin-flotillin-HflK/C family of membrane scaffolding proteins, was robustly and specifically increased in Abca1(-/-) rafts. Pursuing SLP-2 function, we found that rafts of SLP-2-silenced macrophages had markedly abnormal composition. SLP-2 silencing did not compromise ABCA1-dependent cholesterol efflux but reduced macrophage responsiveness to multiple TLR ligands. This was associated with reduced raft levels of the TLR co-receptor, CD14, and defective lipopolysaccharide-induced recruitment of the common TLR adaptor, MyD88, to rafts. Taken together, we show that the lipid transporter ABCA1 regulates the protein repertoire of rafts and identify SLP-2 as an ABCA1-dependent regulator of raft composition and of the innate immune response.

Proteomic Analysis of ABCA1-Null Macrophages Reveals a Role for Stomatin-Like Protein-2 in Raft Composition and Toll-Like Receptor Signaling.

It has become increasingly recognized that cholesterol and lipoproteins play significant roles in both lung physiology and the innate immune response. It is now known that the innate immune response and the cholesterol biosynthesis/trafficking network regulate one another, with important implications for pathogen invasion and host defense. The activation of pathogen recognition receptors and downstream cellular host defense functions are critically sensitive to cellular cholesterol. Conversely, microorganisms can co-opt the sterol/lipoprotein network in order to facilitate their own replication. Given that over 50% of adults in the U.S. have cholesterol abnormalities and pneumonia remains a leading cause of death, the potential impact of cholesterol on pulmonary host defense is of tremendous public health significance. This review addresses the emerging link between the innate immune response and sterol homeostasis, with a focus upon implications for respiratory infection. Clinical translations, including in the area of potential therapeutic development for infectious lung disease, are also discussed.

Roles of the Mevalonate Pathway and Cholesterol Trafficking in Pulmonary Host Defense.

MicroRNAs (miRNAs) are short non-coding RNAs that regulate gene expression by promoting degradation and/or repressing translation of specific target mRNAs. Several miRNAs have been identified that regulate the amplitude of the innate immune response by directly targeting Toll-like receptor (TLR) pathway members and/or cytokines. miR-33a and miR-33b (the latter present in primates but absent in rodents and lower species) are located in introns of the sterol regulatory element-binding protein (SREBP)-encoding genes and control cholesterol/lipid homeostasis in concert with their host gene products. These miRNAs regulate macrophage cholesterol by targeting the lipid efflux transporters ATP binding cassette (ABC)A1 and ABCG1. We and others have previously reported that Abca1(-/-) and Abcg1(-/-) macrophages have increased TLR proinflammatory responses due to augmented lipid raft cholesterol. Given this, we hypothesized that miR-33 would augment TLR signaling in macrophages via a raft cholesterol-dependent mechanism. Herein, we report that multiple TLR ligands down-regulate miR-33 in murine macrophages. In the case of lipopolysaccharide, this is a delayed, Toll/interleukin-1 receptor (TIR) domain-containing adapter-inducing interferon-β-dependent response that also down-regulates Srebf-2, the host gene for miR-33. miR-33 augments macrophage lipid rafts and enhances proinflammatory cytokine induction and NF-κB activation by LPS. This occurs through an ABCA1- and ABCG1-dependent mechanism and is reversible by interventions upon raft cholesterol and by ABC transporter-inducing liver X receptor agonists. Taken together, these findings extend the purview of miR-33, identifying it as an indirect regulator of innate immunity that mediates bidirectional cross-talk between lipid homeostasis and inflammation.

MicroRNA-33 Regulates the Innate Immune Response via ATP Binding Cassette Transporter-mediated Remodeling of Membrane Microdomains.

Tumor necrosis factor-α-induced protein 8 (TNFAIP8) is a stress-response gene that has been associated with cancer, but no studies have differentiated among or defined the regulation or function of any of its several recently described expression variants. We found that TNFAIP8 variant 2 (v2) is overexpressed in multiple human cancers, whereas other variants are commonly downregulated in cancer (v1) or minimally expressed in cancer or normal tissue (v3-v6). Silencing v2 in cancer cells induces p53-independent inhibition of DNA synthesis, widespread binding of p53, and induction of target genes and p53-dependent cell cycle arrest and DNA damage sensitization. Cell cycle arrest induced by v2 silencing requires p53-dependent induction of p21. In response to the chemotherapeutic agent doxorubicin, p53 regulates v2 through binding to an intragenic enhancer, together indicating that p53 and v2 engage in complex reciprocal regulation. We propose that TNFAIP8 v2 promotes human cancer by broadly repressing p53 function, in essence offsetting p53-dependent tumor suppression.

Kristin A. Gabor

Division of Intramural Research,
Immunity,
Inflammation and Disease Laboratory,
Division of Intramural Research, Immunity, Inflammation and Disease Laboratory

National Institute of Environmental Health Sciences, NIH

Using Zebrafish Models of Human Influenza A Virus Infections to Screen Antiviral Drugs and Characterize Host Immune Cell Responses

Kristin A. Gabor

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National Institute of Environmental Health Sciences, NIH

Using Zebrafish Models of Human Influenza A Virus Infections to Screen Antiviral Drugs and Characterize Host Immune Cell Responses

Division of Intramural Research,
Immunity,
Inflammation and Disease Laboratory,
Division of Intramural Research, Immunity, Inflammation and Disease Laboratory