The eponymous member of the interferon regulatory factor (IRF) family, IRF1, was originally identified as a nuclear factor that binds and activates the promoters of type I interferon genes. However, ...subsequent studies using genetic knockouts or RNAi-mediated depletion of IRF1 provide a much broader view, linking IRF1 to a wide range of functions in protection against invading pathogens. Conserved throughout vertebrate evolution, IRF1 has been shown in recent years to mediate constitutive as well as inducible host defenses against a variety of viruses. Fine-tuning of these ancient IRF1-mediated host defenses, and countering strategies by pathogens to disarm IRF1, play crucial roles in pathogenesis and determining the outcome of infection.
Although epidemic jaundice was well known to physicians of antiquity, it is only in recent years that medical science has begun to unravel the origins of hepatitis A virus (HAV) and the unique ...pathobiology underlying acute hepatitis A in humans. Improvements in sanitation and the successful development of highly efficacious vaccines have markedly reduced the worldwide occurence of this enterically-transmitted infection over the past quarter century, yet the virus persists in vulnerable populations and those without HAV immunity and remains a common cause of food-borne disease outbreaks in economically-advantaged societies. Reductions in HAV incidence have led to increases in the median age at which infection occurs, often resulting in more severe disease in affected persons and paradoxical increases in disease burden in some developing nations. Here, we summarize recent advances in the molecular virology and epidemiology of HAV, an atypical member of the Picornaviridae family, survey what is known of the pathogenesis of hepatitis A in humans and the host-pathogen interactions that typify the infection. The article also reviews medical and public health aspects of HAV vaccination and disease prevention.
Relatively little is known of the mechanisms underlying hepatitis A virus (HAV) genome replication. Unlike other well-studied picornaviruses, HAV RNA replication requires the zinc finger protein ...ZCCHC14 and non-canonical TENT4 poly(A) polymerases with which it forms a complex. The ZCCHC14-TENT4 complex binds to a stem-loop located within the internal ribosome entry site (IRES) in the 5' untranslated RNA (5'UTR) and is essential for viral RNA synthesis, but the underlying mechanism is unknown. Here, we describe how different ZCCHC14 domains contribute to its RNA-binding, TENT4-binding, and HAV host factor activities. We show that the RNA-binding activity of ZCCHC14 requires both a sterile alpha motif (SAM) and a downstream unstructured domain (D4) and that ZCCHC14 contains two TENT4-binding sites: one at the N-terminus and the other around D4. Both RNA-binding and TENT4-binding are required for HAV host factor activity of ZCCHC14. We also demonstrate that the location of the ZCCHC14-binding site within the 5'UTR is critical for its function. Our study provides a novel insight into the function of ZCCHC14 and helps elucidate the mechanism of the ZCCHC14-TENT4 complex in HAV replication.IMPORTANCEThe zinc finger protein ZCCHC14 is an essential host factor for both hepatitis A virus (HAV) and hepatitis B virus (HBV). It recruits the non-canonical TENT4 poly(A) polymerases to viral RNAs and most likely also a subset of cellular mRNAs. Little is known about the details of these interactions. We show here the functional domains of ZCCHC14 that are involved in binding to HAV RNA and interactions with TENT4 and describe previously unrecognized peptide sequences that are critical for the HAV host factor activity of ZCCHC14. Our study advances the understanding of the ZCCHC14-TENT4 complex and how it functions in regulating viral and cellular RNAs.
Persistent hepatitis C virus infection is associated with progressive hepatic fibrosis and liver cancer. Acute infection evokes several distinct innate immune responses, but these are partially or ...completely countered by the virus. Hepatitis C virus proteins serve dual functions in replication and immune evasion, acting to disrupt cellular signaling pathways leading to interferon synthesis, subvert Jak-STAT signaling to limit expression of interferon-stimulated genes, and block antiviral activities of interferon-stimulated genes. The net effect is a multilayered evasion of innate immunity, which negatively influences the subsequent development of antigen-specific adaptive immunity, thereby contributing to virus persistence and resistance to therapy.
The metabolic oxidative degradation of cellular lipids severely restricts replication of hepatitis C virus (HCV), a leading cause of chronic liver disease, but little is known about the factors ...regulating this process in infected cells. Here we show that HCV is restricted by an iron-dependent mechanism resembling the one triggering ferroptosis, an iron-dependent form of non-apoptotic cell death, and mediated by the non-canonical desaturation of oleate to Mead acid and other highly unsaturated fatty acids by fatty acid desaturase 2 (FADS2). Genetic depletion and ectopic expression experiments show FADS2 is a key determinant of cellular sensitivity to ferroptosis. Inhibiting FADS2 markedly enhances HCV replication, whereas the ferroptosis-inducing compound erastin alters conformation of the HCV replicase and sensitizes it to direct-acting antiviral agents targeting the viral protease. Our results identify FADS2 as a rate-limiting factor in ferroptosis, and suggest the possibility of pharmacologically manipulating the ferroptosis pathway to attenuate viral replication.
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•Iron reacts with FADS2-catalyzed HUFAs to promote endogenous lipid peroxidation•HCV is restricted by lipid peroxidation driven primarily by the Fenton reaction•FADS2 expression level determines the cellular sensitivity to ferroptosis•Promoting ferroptosis sensitizes the HCV replicase to a viral protease inhibitor
Yamane et al. demonstrate that cellular metabolic processes that regulate ferroptosis noncytolytically restrict replication of hepatitis C virus in hepatocytes. The authors show that the ferroptosis-like signals are regulated primarily by canonical and non-canonical fatty acid desaturation catalyzed by FADS2 and iron-dependent oxidation of highly unsaturated fatty acids.
Understanding the negative regulators of antiviral immune responses will be critical for advancing immune-modulated antiviral strategies. NLRX1, an NLR protein that negatively regulates innate ...immunity, was previously identified in an unbiased siRNA screen as required for HIV infection. We find that NLRX1 depletion results in impaired nuclear import of HIV-1 DNA in human monocytic cells. Additionally, NLRX1 was observed to reduce type-I interferon (IFN-I) and cytokines in response to HIV-1 reverse-transcribed DNA. NLRX1 sequesters the DNA-sensing adaptor STING from interaction with TANK-binding kinase 1 (TBK1), which is a requisite for IFN-1 induction in response to DNA. NLRX1-deficient cells generate an amplified STING-dependent host response to cytosolic DNA, c-di-GMP, cGAMP, HIV-1, and DNA viruses. Accordingly, Nlrx1−/− mice infected with DNA viruses exhibit enhanced innate immunity and reduced viral load. Thus, NLRX1 is a negative regulator of the host innate immune response to HIV-1 and DNA viruses.
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•NLRX1 inhibits HIV-1 cDNA-induced innate immune response and enhances HIV-1 infection•NLRX1 interacts with the DNA-sensing adaptor STING to disrupt STING-TBK1 signaling•STING deficiency abrogates the enhancement of HIV-1 infection by NLRX1•Nlrx1−/− mice show enhanced innate immunity and are more resistant to DNA viruses
Negative regulation of innate immunity is critical to maintain homeostasis, but it can have negative consequences in an infection context. Guo et al. demonstrate that NLRX1 sequesters the DNA-sensing adaptor STING to disrupt STING-TBK1 signaling and inhibit innate immunity to HIV-1 and DNA viruses. Thus, NLRX1 facilitates virus infection.
Hepatitis C virus (HCV) replication is dependent on microRNA 122 (miR-122), a liver-specific microRNA that recruits Argonaute 2 to the 5′ end of the viral genome, stabilizing it and slowing its decay ...both in cell-free reactions and in infected cells. Here we describe the RNA degradation pathways against which miR-122 provides protection. Transfected HCV RNA is degraded by both the 5′ exonuclease Xrn1 and 3′ exonuclease exosome complex, whereas replicating RNA within infected cells is degraded primarily by Xrn1 with no contribution from the exosome. Consistent with this, sequencing of the 5′ and 3′ ends of RNA degradation intermediates in infected cells confirmed that 5′ decay is the primary pathway for HCV RNA degradation. Xrn1 knockdown enhances HCV replication, indicating that Xrn1 decay and the viral replicase compete to set RNA abundance within infected cells. Xrn1 knockdown and miR-122 supplementation have equal, redundant, and nonadditive effects on the rate of viral RNA decay, indicating that miR-122 protects HCV RNA from 5′ decay. Nevertheless, Xrn1 knockdown does not rescue replication of a viral mutant defective in miR-122 binding, indicating that miR-122 has additional yet uncharacterized function(s) in the viral life cycle.
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