Influenza virus-induced caspase-dependent enlargement of nuclear pores promotes nuclear export of viral ribonucleoprotein complexes.

UNLABELLED: Influenza A viruses (IAV) replicate their segmented RNA genome in the nucleus of infected cells and utilize caspase-dependent nucleocytoplasmic export mechanisms to transport newly formed ribonucleoprotein complexes (RNPs) to the site of infectious virion release at the plasma membrane. In this study, we obtained evidence that apoptotic caspase activation in IAV-infected cells is associated with the degradation of the nucleoporin Nup153, an integral subunit of the nuclear pore complex. Transmission electron microscopy studies revealed a distinct enlargement of nuclear pores in IAV-infected cells. Transient expression and subcellular accumulation studies of multimeric marker proteins in virus-infected cells provided additional evidence for increased nuclear pore diameters facilitating the translocation of large protein complexes across the nuclear membrane. Furthermore, caspase 3/7 inhibition data obtained in this study suggest that active, Crm1-dependent IAV RNP export mechanisms are increasingly complemented by passive, caspase-induced export mechanisms at later stages of infection. IMPORTANCE: In contrast to the process seen with most other RNA viruses, influenza virus genome replication occurs in the nucleus (rather than the cytoplasm) of infected cells. Therefore, completion of the viral replication cycle critically depends on intracellular transport mechanisms that ensure the translocation of viral ribonucleoprotein (RNP) complexes across the nuclear membrane. Here, we demonstrate that virus-induced cellular caspase activities cause a widening of nuclear pores, thereby facilitating nucleocytoplasmic translocation processes and, possibly, promoting nuclear export of newly synthesized RNPs. These passive transport mechanisms are suggested to complement Crm1-dependent RNP export mechanisms known to occur at early stages of the replication cycle and may contribute to highly efficient production of infectious virus progeny at late stages of the viral replication cycle. The report provides an intriguing example of how influenza virus exploits cellular structures and regulatory pathways, including intracellular transport mechanisms, to complete its replication cycle and maximize the production of infectious virus progeny.

Polymorphisms in melanoma differentiation-associated gene 5 link protein function to clearance of hepatitis C virus.

UNLABELLED: Among patients newly infected with hepatitis C virus (HCV), only 20-30% clear the infection spontaneously. In the remaining 70% the infection persists, causing chronic liver inflammation and disease. It is well established that polymorphisms in host genes, especially in components of the innate immune response, contribute to the phenomenon of spontaneous HCV clearance. Retinoic acid inducible gene-I (RIG-I)-like helicases such as melanoma differentiation-associated gene 5 (MDA-5) are cytoplasmic sensors of viral RNA that are critical for triggering innate immune responses after infection with RNA viruses. We analyzed 14 nonsynonymous single-nucleotide polymorphisms in RIG-I-like helicase-pathway-genes comparing European patients who spontaneously cleared HCV (n = 285) or had persistent infection (n = 509). We found that polymorphic haplotypes in the MDA-5 gene IFIH1 encoding histidine at position 843 and threonine at position 946 strongly correlate with the resolution of HCV infection (odds ratio [OR]: 16.23; 95% confidence interval [CI]: 3.67-71.87; P = 1.1 × 10(-6) ). Overexpression of MDA-5 genetic variants in HEK 293 cells and in a tissue culture model of HCV infection revealed that the histidine 843/threonine 946 variant leads to increased baseline and ligand-induced expression of interferon-induced genes and confers an increased ability to suppress HCV replication. CONCLUSION: These data suggest that MDA-5 plays a significant role in the defense against HCV and that polymorphisms in MDA-5 can influence the outcome of HCV infection.

Interferon-induced antiviral protein MxA interacts with the cellular RNA helicases UAP56 and URH49.

Mx proteins are a family of large GTPases that are induced exclusively by interferon-α/ß and have a broad antiviral activity against several viruses, including influenza A virus (IAV). Although the antiviral activities of mouse Mx1 and human MxA have been studied extensively, the molecular mechanism of action remains largely unsolved. Because no direct interaction between Mx proteins and IAV proteins or RNA had been demonstrated so far, we addressed the question of whether Mx protein would interact with cellular proteins required for efficient replication of IAV. Immunoprecipitation of MxA revealed its association with two closely related RNA helicases, UAP56 and URH49. UAP56 and its paralog URH49 play an important role in IAV replication and are involved in nuclear export of IAV mRNAs and prevention of dsRNA accumulation in infected cells. In vitro binding assays with purified recombinant proteins revealed that MxA formed a direct complex with the RNA helicases. In addition, recombinant mouse Mx1 was also able to bind to UAP56 or URH49. Furthermore, the complex formation between cytoplasmic MxA and UAP56 or URH49 occurred in the perinuclear region, whereas nuclear Mx1 interacted with UAP56 or URH49 in distinct dots in the nucleus. Taken together, our data reveal that Mx proteins exerting antiviral activity can directly bind to the two cellular DExD/H box RNA helicases UAP56 and URH49. Moreover, the observed subcellular localization of the Mx-RNA helicase complexes coincides with the subcellular localization, where human MxA and mouse Mx1 proteins act antivirally. On the basis of these data, we propose that Mx proteins exert their antiviral activity against IAV by interfering with the function of the RNA helicases UAP56 and URH49.

The cellular RNA helicase UAP56 is required for prevention of double-stranded RNA formation during influenza A virus infection.

The cellular DEAD box RNA helicase UAP56 plays a pivotal role in the efficient transcription/replication of influenza A virus. UAP56 is recruited by the nucleoprotein (NP) of influenza A viruses, and recent data revealed that the RNA helicase is required for the nuclear export of a subset of spliced and unspliced viral mRNAs. The fact that influenza viruses do not produce detectable amounts of double-stranded RNA (dsRNA) intermediates during transcription/replication suggests the involvement of cellular RNA helicases. Hence, we examined whether the RNA-unwinding activity of UAP56 or its paralog URH49 plays a role in preventing the accumulation of dsRNA during infection. First, our data showed that not only UAP56 but also its paralog URH49 can interact with NPs of avian and human influenza A viruses. The small interfering RNA (siRNA)-mediated depletion of either RNA helicase reduced the transport of M1 and hemagglutinin (HA) mRNAs and, to a lesser extent, NP and NS1 mRNAs into the cytoplasm. Moreover, we found that virus infection of UAP56-depleted cells leads to the rapid accumulation of dsRNA in the perinuclear region. In parallel, we observed a robust virus-mediated activation of dsRNA-dependent protein kinase R (PKR), indicating that the cellular RNA helicase UAP56 may be recruited by influenza virus to prevent dsRNA formation. The accumulation of dsRNA was blocked when actinomycin D or cycloheximide was used to inhibit viral transcription/replication or translation, respectively. In summary, we demonstrate that UAP56 is utilized by influenza A viruses to prevent the formation of dsRNA and, hence, the activation of the innate immune response.