Interferon Inducer IFI35 regulates RIG-I-mediated innate antiviral response through mutual antagonism with Influenza protein NS1

Interferon-stimulated genes (ISGs) create multiple lines of defense against viral infection. Here we show that interferon induced protein 35 (IFI35) inhibits swine (H3N2) influenza virus replication by directly interacting with the viral protein NS1. IFI35 binds more preferentially to the effector domain of NS1 (128-207aa) than to the viral RNA sensor RIG-I. This promotes mutual antagonism between IFI35 and NS1, and frees RIG-I from IFI35-mediated K48-linked ubiquitination and degradation. However, IFI35 does not interact with the NS1 encoded by avian (H7N9) influenza virus, resulting in IFI35 playing an opposite virus enabling role during highly pathogenic H7N9 virus infection. Notably, replacing the 128-207aa region of NS1-H7N9 with the corresponding region of NS1-H3N2 results in the chimeric NS1 acquiring the ability to bind to and mutually antagonize IFI35. IFI35 deficient mice accordingly exhibit more resistance to lethal H7N9 infection than their wild-type control exhibit. Our data uncover a novel mechanism by which IFI35 regulates RIG-I-mediated anti-viral immunity through mutual antagonism with influenza protein NS1.IMPORTANCEIAV infection poses a global health threat, and is among the most common contagious pathogens to cause severe respiratory infections in humans and animals. ISGs play a key role in host defense against IAV infection. In line with others, we show IFI35-mediated ubiquitination of RIG-I to be involved in innate immunity. Moreover, we define a novel role of IFI35 in regulating the type I IFN pathway during IAV infection. We found that IFI35 regulates RIG-I mediated antiviral signaling by interacting with IAV-NS1. H3N2 NS1, but notably not H7N9 NS1, interacts with IFI35 and efficiently suppresses IFI35-dependent ubiquitination of RIG-I. IFI35 deficiency protected mice from H7N9 virus infection. Therefore, manipulation of the IFI35-NS1 provides a new approach for the development of anti-IAV treatments.

Single-cell studies of IFN-β promoter activation by wild-type and NS1-defective influenza A viruses

Deletion or truncation of NS1, the principal IFN antagonist of influenza viruses, leads to increased IFN induction during influenza virus infection. We have studied activation of the IFN induction cascade by both wild-type and NS1-defective viruses at the single-cell level using a cell line expressing GFP under the control of the IFN-β promoter and by examining MxA expression. The IFN-β promoter was not activated in all infected cells even during NS1-defective virus infections. Loss of NS1 expression is therefore insufficient per se to induce IFN in an infected cell, and factors besides NS1 expression status must dictate whether the IFN response is activated. The IFN response was efficiently stimulated in these cells following infection with other viruses; the differential IFN response we observe with influenza viruses is therefore not cell specific but is likely due to differences in the nature of the infecting virus particles and their subsequent replication.

Viral genes as oncolytic agents for cancer therapy

Many viruses have the ability to modulate the apoptosis, and to accomplish it; viruses encode proteins which specifically interact with the cellular signaling pathways. While some viruses encode proteins, which inhibit the apoptosis or death of the infected cells, there are viruses whose encoded proteins can kill the infected cells by multiple mechanisms, including apoptosis. A particular class of these viruses has specific gene(s) in their genomes which, upon ectopic expression, can kill the tumor cells selectively without affecting the normal cells. These genes and their encoded products have demonstrated great potential to be developed as novel anticancer therapeutic agents which can specifically target and kill the cancer cells leaving the normal cells unharmed. In this review, we will discuss about the viral genes having specific cancer cell killing properties, what is known about their functioning, signaling pathways and their therapeutic applications as anticancer agents.

Mutations within the conserved NS1 nuclear export signal lead to inhibition of influenza A virus replication

BACKGROUND

The influenza A virus NS1 protein is a virulence factor and an antagonist of host cell innate immune responses. During virus infection NS1 protein has several functions both in the nucleus and in the cytoplasm and its intracellular localization is regulated by one or two nuclear localization signals (NLS) and a nuclear export signal (NES).

METHODS

In order to investigate the role of NS1 NES in intracellular localization, virus life cycle and host interferon responses, we generated recombinant A/Udorn/72 viruses harboring point mutations in the NES sequence.

RESULTS

NS1 NES was found to be inactivated by several of the mutations resulting in nuclear retention of NS1 at late stages of infection confirming that this sequence is a bona fide functional NES. Some of the mutant viruses showed reduced growth properties in cell culture, inability to antagonize host cell interferon production and increased p-IRF3 levels, but no clear correlation between these phenotypes and NS1 localization could be made. Impaired activation of Akt phosphorylation by the replication-deficient viruses indicates possible disruption of NS1-p85β interaction by mutations in the NES region.

CONCLUSION

We conclude that mutations within the NS1 NES result in impairment of several NS1 functions which extends further from the NES site being only involved in regulating the nuclear-cytoplasmic trafficking of NS1.

Nuclear imprisonment: viral strategies to arrest host mRNA nuclear export

Viruses possess many strategies to impair host cellular responses to infection. Nuclear export of host messenger RNAs (mRNA) that encode antiviral factors is critical for antiviral protein production and control of viral infections. Several viruses have evolved sophisticated strategies to inhibit nuclear export of host mRNAs, including targeting mRNA export factors and nucleoporins to compromise their roles in nucleo-cytoplasmic trafficking of cellular mRNA. Here, we present a review of research focused on suppression of host mRNA nuclear export by viruses, including influenza A virus and vesicular stomatitis virus, and the impact of this viral suppression on host antiviral responses.

The distinct binding properties between avian/human influenza A virus NS1 and Postsynaptic density protein-95 (PSD-95), and inhibition of nitric oxide production

BACKGROUND

The NS1 protein of influenza A virus is able to bind with many proteins that affect cellular signal transduction and protein synthesis in infected cells. The NS1 protein consists of approximately 230 amino acids and the last 4 amino acids of the NS1 C-terminal form a PDZ binding motif. Postsynaptic Density Protein-95 (PSD-95), which is mainly expressed in neurons, has 3 PDZ domains. We hypothesise that NS1 binds to PSD-95, and this binding is able to affect neuronal function.

RESULT

We conducted a yeast two-hybrid analysis, GST-pull down assays and co-immunoprecipitations to detect the interaction between NS1 and PSD-95. The results showed that NS1 of avian influenza virus H5N1 (A/chicken/Guangdong/1/2005) is able to bind to PSD-95, whereas NS1 of human influenza virus H1N1 (A/Shantou/169/2006) is unable to do so. The results also revealed that NS1 of H5N1 significantly reduces the production of nitric oxide (NO) in rat hippocampal neurons.

CONCLUSION

In summary, our study indicates that NS1 of influenza A virus can bind with neuronal PSD-95, and the avian H5N1 and human H1N1 influenza A viruses possess distinct binding properties.

Innate immune evasion strategies of influenza viruses

Influenza viruses are globally important human respiratory pathogens. These viruses cause seasonal epidemics and occasional worldwide pandemics, both of which can vary significantly in disease severity. The virulence of a particular influenza virus strain is partly determined by its success in circumventing the host immune response. This article briefly reviews the innate mechanisms that host cells have evolved to resist virus infection, and outlines the plethora of strategies that influenza viruses have developed in order to counteract such powerful defences. The molecular details of this virus-host interplay are summarized, and the ways in which research in this area is being applied to the rational design of protective vaccines and novel antivirals are discussed.