A genome wide RNA interference screening method to identify host factors that modulate influenza A virus replication

Emerging insights on the role of V-ATPase in human diseases: Therapeutic challenges and opportunities

The control of the intracellular pH is vital for the survival of all organisms. Membrane transporters, both at the plasma and intracellular membranes, are key players in maintaining a finely tuned pH balance between intra- and extracellular spaces, and therefore in cellular homeostasis. V-ATPase is a housekeeping ATP-driven proton pump highly conserved among prokaryotes and eukaryotes. This proton pump, which exhibits a complex multisubunit structure based on cell type-specific isoforms, is essential for pH regulation and for a multitude of ubiquitous and specialized functions. Thus, it is not surprising that V-ATPase aberrant overexpression, mislocalization, and mutations in V-ATPase subunit-encoding genes have been associated with several human diseases. However, the ubiquitous expression of this transporter and the high toxicity driven by its off-target inhibition, renders V-ATPase-directed therapies very challenging and increases the need for selective strategies. Here we review emerging evidence linking V-ATPase and both inherited and acquired human diseases, explore the therapeutic challenges and opportunities envisaged from recent data, and advance future research avenues. We highlight the importance of V-ATPases with unique subunit isoform molecular signatures and disease-associated isoforms to design selective V-ATPase-directed therapies. We also discuss the rational design of drug development pipelines and cutting-edge methodological approaches toward V-ATPase-centered drug discovery. Diseases like cancer, osteoporosis, and even fungal infections can benefit from V-ATPase-directed therapies.

Functional Genomic Strategies for Elucidating Human-Virus Interactions: Will CRISPR Knockout RNAi and Haploid Cells?

Over the last several years a wealth of transformative human–virus interaction discoveries have been produced using loss-of-function functional genomics. These insights have greatly expanded our understanding of how human pathogenic viruses exploit our cells to replicate. Two technologies have been at the forefront of this genetic revolution, RNA interference (RNAi) and random retroviral insertional mutagenesis using haploid cell lines (haploid cell screening), with the former technology largely predominating. Now the cutting edge gene editing of the CRISPR/Cas9 system has also been harnessed for large-scale functional genomics and is poised to possibly displace these earlier methods. Here we compare and contrast these three screening approaches for elucidating host–virus interactions, outline their key strengths and weaknesses including a comparison of an arrayed multiple orthologous RNAi reagent screen to a pooled CRISPR/Cas9 human rhinovirus 14–human cell interaction screen, and recount some notable insights made possible by each. We conclude with a brief perspective on what might lie ahead for the fast evolving field of human–virus functional genomics.

Virus-Host Interactions: From Unbiased Genetic Screens to Function

Deciphering the many interactions that occur between a virus and host cell over the course of infection is paramount to understanding mechanisms of pathogenesis and to the future development of antiviral therapies. Over the past decade, researchers have started to understand these complicated relationships through the development of methodologies, including advances in RNA interference, proteomics, and the development of genetic tools such as haploid cell lines, allowing high-throughput screening to identify critical contact points between virus and host. These advances have produced a wealth of data regarding host factors hijacked by viruses to promote infection, as well as antiviral factors responsible for subverting viral infection. This review highlights findings from virus-host screens and discusses our thoughts on the direction of screening strategies moving forward.

Caveolin-1 limits human influenza A virus (H1N1) propagation in mouse embryo-derived fibroblasts

Caveolin expression supports the multiplication of retro-, ortho- and paramyxoviruses in susceptible cells. However, human influenza A virus (IAV), an orthomyxovirus, does not multiply efficiently in mouse embryo fibroblasts (MEFs), which are abundant in caveolin-1 (Cav-1). Surprisingly, the absence of Cav-1 in a MEF cell line removed the block for IAV replication and raised the infectious titer 250-fold, whereas the re-introduction of Cav-1 reversed the effect. The monitoring of cellular pathways revealed that Cav-1 loss considerably increased activities of p53. Furthermore, infection of MEF Cav-1 (-/-) induced reactive oxygen species (ROS) and pronounced apoptosis in the late phase of viral multiplication, but no type I IFN response. Strikingly, pharmacological inactivation showed that the elevated levels of ROS together with apoptosis caused the increase of virus yield. Thus, Cav-1 represents a new negative regulator of IAV infection in MEF that diminishes IAV infectious titer by controlling virus-supportive pathways.

New-generation screening assays for the detection of anti-influenza compounds targeting viral and host functions

Current options for influenza antiviral therapy are limited to the neuraminidase inhibitors, and knowledge that high levels of oseltamivir resistance have been seen among previously circulating H1N1 viruses increases the urgency to find new influenza therapeutics. To feed this pipeline, assays that are appropriate for use in high-throughput screens are being developed and are discussed in this review. Particular emphasis is placed on cell-based assays that capture both inhibitors of viral functions as well as the host functions that facilitate optimal influenza virus replication. Success in this area has been fueled by a greater understanding of the genome structure of influenza viruses and the ability to generate replication-competent recombinant viruses that carry a reporter gene, allowing for easy monitoring of viral infection in a high-throughput setting. This article forms part of a symposium in Antiviral Research on "Treatment of influenza: targeting the virus or the host."

Inhibitory and combinatorial effect of diphyllin, a v-ATPase blocker, on influenza viruses

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Diphyllin inhibits endosomal acidification in MDCK cells and A549 cells.

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Treatment with diphyllin alters the cellular susceptibility to the influenza virus.

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Diphyllin demonstrated a broad-spectrum antiviral activity.

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The combination of diphyllin and other drugs showed an enhanced antiviral effect.

An influenza pandemic poses a serious threat to humans and animals. Conventional treatments against influenza include two classes of pathogen-targeting antivirals: M2 ion channel blockers (such as amantadine) and neuraminidase inhibitors (such as oseltamivir). Examination of the mechanism of influenza viral infection has shown that endosomal acidification plays a major role in facilitating the fusion between viral and endosomal membranes. This pathway has led to investigations on vacuolar ATPase (v-ATPase) activity, whose role as a regulating factor on influenza virus replication has been verified in extensive genome-wide screenings. Blocking v-ATPase activity thus presents the opportunity to interfere with influenza viral infection by preventing the pH-dependent membrane fusion between endosomes and virions. This study aims to apply diphyllin, a natural compound shown to be as a novel v-ATPase inhibitor, as a potential antiviral for various influenza virus strains using cell-based assays. The results show that diphyllin alters cellular susceptibility to influenza viruses through the inhibition of endosomal acidification, thus interfering with downstream virus replication, including that of known drug-resistant strains. In addition, combinatorial treatment of the host-targeting diphyllin with pathogen-targeting therapeutics (oseltamivir and amantadine) demonstrates enhanced antiviral effects and cell protection in vitro.