Genome-wide RNAi screen reveals a new role of a WNT/CTNNB1 signaling pathway as negative regulator of virus-induced innate immune responses

β-Catenin Upregulates the Constitutive and Virus-Induced Transcriptional Capacity of the Interferon Beta Promoter through T-Cell Factor Binding Sites

Rapid upregulation of interferon beta (IFN-β) expression following virus infection is essential to set up an efficient innate antiviral response. Biological roles related to the antiviral and immune response have also been associated with the constitutive production of IFN-β in naive cells. However, the mechanisms capable of modulating constitutive IFN-β expression in the absence of infection remain largely unknown. In this work, we demonstrate that inhibition of the kinase glycogen synthase kinase 3 (GSK-3) leads to the upregulation of the constitutive level of IFN-β expression in noninfected cells, provided that GSK-3 inhibition is correlated with the binding of β-catenin to the IFN-β promoter. Under these conditions, IFN-β expression occurred through the T-cell factor (TCF) binding sites present on the IFN-β promoter independently of interferon regulatory factor 3 (IRF3). Enhancement of the constitutive level of IFN-β per se was able to confer an efficient antiviral state to naive cells and acted in synergy with virus infection to stimulate virus-induced IFN-β expression. Further emphasizing the role of β-catenin in the innate antiviral response, we show here that highly pathogenic Rift Valley fever virus (RVFV) targets the Wnt/β-catenin pathway and the formation of active TCF/β-catenin complexes at the transcriptional and protein level in RVFV-infected cells and mice.

Fine-Tuning of the RIG-I-Like Receptor/Interferon Regulatory Factor 3-Dependent Antiviral Innate Immune Response by the Glycogen Synthase Kinase 3/β-Catenin Pathway

Induction of an antiviral innate immune response relies on pattern recognition receptors, including retinoic acid-inducible gene 1-like receptors (RLR), to detect invading pathogens, resulting in the activation of multiple latent transcription factors, including interferon regulatory factor 3 (IRF3). Upon sensing of viral RNA and DNA, IRF3 is phosphorylated and recruits coactivators to induce type I interferons (IFNs) and selected sets of IRF3-regulated IFN-stimulated genes (ISGs) such as those for ISG54 (Ifit2), ISG56 (Ifit1), and viperin (Rsad2). Here, we used wild-type, glycogen synthase kinase 3α knockout (GSK-3α(-/-)), GSK-3β(-/-), and GSK-3α/β double-knockout (DKO) embryonic stem (ES) cells, as well as GSK-3β(-/-) mouse embryonic fibroblast cells in which GSK-3α was knocked down to demonstrate that both isoforms of GSK-3, GSK-3α and GSK-3β, are required for this antiviral immune response. Moreover, the use of two selective small-molecule GSK-3 inhibitors (CHIR99021 and BIO-acetoxime) or ES cells reconstituted with the catalytically inactive versions of GSK-3 isoforms showed that GSK-3 activity is required for optimal induction of antiviral innate immunity. Mechanistically, GSK-3 isoform activation following Sendai virus infection results in phosphorylation of β-catenin at S33/S37/T41, promoting IRF3 DNA binding and activation of IRF3-regulated ISGs. This study identifies the role of a GSK-3/β-catenin axis in antiviral innate immunity.

Functional Analysis of Chicken IRF7 in Response to dsRNA Analog Poly(I:C) by Integrating Overexpression and Knockdown

In order to develop novel strategies to protect against increasingly virulent bird-linked pathogens, a better understanding of the avian antiviral response mechanism is essential. Type I interferons (IFNs) are recognized as the first line of defense in a host's antiviral response; and it has been suggested that IRF7, a member of the IFN regulatory factor (IRF) family, plays an important role in modulating the immune response to avian influenza virus infection in chickens. The objective of this study was to identify candidate genes and pathways associated with IRF7 regulation at the transcriptome level as a first step towards elucidating the underlying cellular mechanisms of IRF7 modulation in the chicken antiviral response. IRF7 overexpression and knockdown DF-1 cell lines were established and stimulated by various pathogen-associated molecular patterns. Significant IRF7 and type I IFN expression changes were observed in both the IRF7 overexpression cell line and the IRF7 knockdown cell line upon exposure to the double stranded RNA (dsRNA) analog poly(I:C). Using RNA-seq based transcriptome analysis, we identified potential novel genes that IRF7 may help regulate as part of the host immune response to dsRNA; potential biomarkers and therapeutic targets revealed as a result of this study warrant further investigation. Based on our results, we suggest that IRF7 may have conserved functional activity in the avian antiviral response, and plays a crucial role in type I IFN regulation.

β-Catenin/TCF-4 signaling regulates susceptibility of macrophages and resistance of monocytes to HIV-1 productive infection

Cells of the monocyte/macrophage lineage are an important target for HIV-1 infection. They are often at anatomical sites linked to HIV-1 transmission and are an important vehicle for disseminating HIV-1 throughout the body, including the central nervous system. Monocytes do not support extensive productive HIV-1 replication, but they become more susceptible to HIV-1infection as they differentiate into macrophages. The mechanisms guiding susceptibility of HIV-1 replication in monocytes versus macrophages are not entirely clear. We determined whether endogenous activity of β-catenin signaling impacts differential susceptibility of monocytes and monocyte-derived macrophages (MDMs) to productive HIV-1 replication. We show that monocytes have an approximately 4-fold higher activity of β-catenin signaling than MDMs. Inducing β-catenin in MDMs suppressed HIV-1 replication by 5-fold while inhibiting endogenous β-catenin signaling in monocytes by transfecting with a dominant negative mutant for the downstream effector of β- catenin (TCF-4) promoted productive HIV-1 replication by 6-fold. These findings indicate that β-catenin/TCF-4 is an important pathway for restricted HIV-1 replication in monocytes and plays a significant role in potentiating HIV-1 replication as monocytes differentiate into macrophages. Targeting this pathway may provide a novel strategy to purge the latent reservoir from monocytes/macrophages, especially in sanctuary sites for HIV-1 such as the central nervous system.

LiCl inhibits PRRSV infection by enhancing Wnt/β-catenin pathway and suppressing inflammatory responses

Lithium chloride (LiCl) has been used as a mood stabilizer in the manic depressive disorders treatment. Recent studies show that LiCl is also a potent inhibitor for some DNA and RNA viruses. Porcine reproductive and respiratory syndrome virus (PRRSV) is an important viral pathogen in modern pig industry. In this study, we assessed the inhibitory effect of LiCl on PRRSV infection using plaque-formation assay, Q-PCR and Western blot analysis. Our results showed that LiCl could inhibit PRRSV infection in MARC-145 and PAM-CD163 cells. Previous reports have shown that LiCl could induce the Wnt pathway in the absence of Wnt ligands. In our studies, we demonstrated that LiCl activates the Wnt pathway in PRRSV infected cells. Additionally, the knockdown of β-catenin or the Wnt/β-catenin pathway inhibitor PNU74654 was able to reverse the antiviral effect of LiCl, which suggested that the inhibitory effect of LiCl against PRRSV replication might be associated with the activation of the Wnt/β-catenin pathway. We also found that lower viral replication after LiCl treatment was associated with the reduced mRNA levels of pro-inflammatory IL-8, IL-6, IL-1 β, tumor necrosis factor α and decreased NF-κB nuclear translocation. Collectively, our data demonstrated that LiCl inhibited PRRSV infection by enhancing Wnt/β-catenin pathway and suppressing pro-inflammatory responses.

Dynamic interaction between astrocytes and infiltrating PBMCs in context of neuroAIDS

HIV-mediated neuropathogenesis is a multifaceted process involving several players, including resident brain cells (neurons, astrocytes, and microglia) and infiltrating cells [peripheral blood mononuclear cells (PBMCs)]. We evaluated the dynamic interaction between astrocytes and infiltrating PBMCs as it impacts HIV in the CNS. We demonstrate that human primary-derived astrocytes (PDAs) predominantly secrete Wnt 1, 2b, 3, 5b, and 10b. Wnts are small secreted glycoproteins that initiate either β-catenin-dependent or independent signal transduction. The Wnt pathway plays a vital role in the regulation of CNS activities including neurogenesis, neurotransmitter release, synaptic plasticity, and memory consolidation. We show that HIV infection of PDAs altered astrocyte Wnt profile by elevating Wnts 2b and 10b. Astrocyte conditioned media (ACM) inhibited HIV replication in PBMCs by 50%. Removal of Wnts from ACM abrogated its ability to suppress HIV replication in PBMCs. Inversely, PBMCs supernatant activated PDAs, as demonstrated by a 10-fold increase in HLA-DR and a 5-fold increase in IFNγ expression, and enhanced astrocyte susceptibility to HIV by 2-fold, which was mediated by IFNγ in a Stat-3-dependent manner. Collectively, these data demonstrate a dynamic interaction between astrocytes and PBMCs, whereby astrocyte-secreted Wnts exert an anti-HIV effect on infected PBMCs and PBMCs, in turn, secrete IFNγ that enhance astrocyte susceptibility to productive HIV infection and mediate their activation.

β-catenin promotes the type I IFN synthesis and the IFN-dependent signaling response but is suppressed by influenza A virus-induced RIG-I/NF-κB signaling

BACKGROUND

The replication cycle of most pathogens, including influenza viruses, is perfectly adapted to the metabolism and signal transduction pathways of host cells. After infection, influenza viruses activate several cellular signaling cascades that support their propagation but suppress those that interfere with viral replication. Accumulation of viral RNA plays thereby a central role. Its sensing by the pattern recognition receptors of the host cells leads to the activation of several signal transduction waves that result in induction of genes, responsible for the cellular innate immune response. Type I interferon (IFN) genes and interferon-stimulated genes (ISG) coding for antiviral-acting proteins, such as MxA, OAS-1 or PKR, are primary targets of these signaling cascades. β- and γ-catenin are closely related armadillo repeat-containing proteins with dual roles. At the cell membrane they serve as adapter molecules linking cell-cell contacts to microfilaments. In the cytosol and nucleus, the proteins form a transcriptional complex with the lymphoid enhancer factor/T-cell factor (LEF/TCF), regulating the transcription of many genes, thereby controlling different cellular functions such as cell cycle progression and differentiation.

RESULTS

In this study, we demonstrate that β- and γ-catenin are important regulators of the innate cellular immune response to influenza A virus (IAV) infections. They inhibit viral replication in lung epithelial cells by enhancing the virus-dependent induction of the IFNB1 gene and interferon-stimulated genes. Simultaneously, the prolonged infection counteracts the antiviral effect of β- and γ-catenin. Influenza viruses suppress β-catenin-dependent transcription by misusing the RIG-I/NF-κB signaling cascade that is induced in the course of infection by viral RNA.

CONCLUSION

We identified β- and γ-catenin as novel antiviral-acting proteins. While these factors support the induction of common target genes of the cellular innate immune response, their functional activity is suppressed by pathogen evasion.

Integrative framework for identification of key cell identity genes uncovers determinants of ES cell identity and homeostasis

Identification of genes associated with specific biological phenotypes is a fundamental step toward understanding the molecular basis underlying development and pathogenesis. Although RNAi-based high-throughput screens are routinely used for this task, false discovery and sensitivity remain a challenge. Here we describe a computational framework for systematic integration of published gene expression data to identify genes defining a phenotype of interest. We applied our approach to rank-order all genes based on their likelihood of determining ES cell (ESC) identity. RNAi-mediated loss-of-function experiments on top-ranked genes unearthed many novel determinants of ESC identity, thus validating the derived gene ranks to serve as a rich and valuable resource for those working to uncover novel ESC regulators. Underscoring the value of our gene ranks, functional studies of our top-hit Nucleolin (Ncl), abundant in stem and cancer cells, revealed Ncl's essential role in the maintenance of ESC homeostasis by shielding against differentiation-inducing redox imbalance-induced oxidative stress. Notably, we report a conceptually novel mechanism involving a Nucleolin-dependent Nanog-p53 bistable switch regulating the homeostatic balance between self-renewal and differentiation in ESCs. Our findings connect the dots on a previously unknown regulatory circuitry involving genes associated with traits in both ESCs and cancer and might have profound implications for understanding cell fate decisions in cancer stem cells. The proposed computational framework, by helping to prioritize and preselect candidate genes for tests using complex and expensive genetic screens, provides a powerful yet inexpensive means for identification of key cell identity genes.

Systems biology unravels interferon responses to respiratory virus infections

Interferon production is an important defence against viral replication and its activation is an attractive therapeutic target. However, it has long been known that viruses perpetually evolve a multitude of strategies to evade these host immune responses. In recent years there has been an explosion of information on virus-induced alterations of the host immune response that have resulted from data-rich omics technologies. Unravelling how these systems interact and determining the overall outcome of the host response to viral infection will play an important role in future treatment and vaccine development. In this review we focus primarily on the interferon pathway and its regulation as well as mechanisms by which respiratory RNA viruses interfere with its signalling capacity.