No abstract

Tick-borne encephalitis virus (TBEV) causes a severe and potentially fatal neuroinfection in humans. Despite its high medical relevance, no specific antiviral therapy is currently available. Here we demonstrate that treatment with a nucleoside analog, 7-deaza-2' --methyladenosine (7-deaza-2' -CMA), substantially improved disease outcome, increased survival, and reduced signs of neuroinfection and viral titers in the brains of mice infected with a lethal dose of TBEV. To investigate the mechanism of action of 7-deaza-2' -CMA, two drug-resistant TBEV clones were generated and characterized. The two clones shared a signature amino acid substitution, S603T, in the viral NS5 RNA-dependent RNA polymerase (RdRp) domain. This mutation conferred resistance to various 2' --methylated nucleoside derivatives, but no cross-resistance was seen to other nucleoside analogs, such as 4' -azidocytidine and 2' -deoxy-2' -beta-hydroxy-4' -azidocytidine (RO-9187). All-atom molecular dynamics simulations revealed that the S603T RdRp mutant repels a water molecule that coordinates the position of a metal ion cofactor as 2' --methylated nucleoside analogs approach the active site. To investigate its phenotype, the S603T mutation was introduced into a recombinant TBEV (Oshima-IC) generated from an infectious cDNA clone and into a TBEV replicon that expresses a reporter luciferase gene (Oshima-REP-luc2A). The mutants were replication-impaired, showing reduced growth and small plaque size in mammalian cell culture and reduced levels of neuroinvasiveness and neurovirulence in rodent models. These results indicate that TBEV resistance to 2' --methylated nucleoside inhibitors is conferred by a single conservative mutation that causes a subtle atomic effect within the active site of viral NS5 RdRp and is associated with strong attenuation of the virus. This study found that the nucleoside analog 7-deaza-2' --methyladenosine (7-deaza-2' -CMA) has high antiviral activity against tick-borne encephalitis virus (TBEV), a pathogen that causes severe human neuroinfections in large areas of Europe and Asia and for which there is currently no specific therapy. Treating mice infected with a lethal dose of TBEV with 7-deaza-2' -CMA resulted in significantly higher survival rates, reduced the severity of neurological signs of the disease. Thus, this compound shows promise for further development as an anti-TBEV drug. It is important to generate drug-resistant mutants to understand how the drug works and to develop guidelines for patient treatment. We generated TBEV mutants that were resistant not only to 7-deaza-2' -CMA but also to a broad range of other 2' --methylated antiviral medications. Our findings suggest that combination therapy could be used to improve treatment and reduce the emergence of drug-resistant viruses during nucleoside analog therapy for TBEV infection.

West Nile virus (WNV) belongs to the Flaviviridae family and has emerged as a significant cause of viral encephalitis in birds and animals including humans. WNV replication directly induces neuronal injury, followed by neuronal cell death. We previously showed that accumulation of ubiquitinated protein aggregates was involved in neuronal cell death in the WNV-infected mouse brain. In this study, we attempted to elucidate the mechanisms of the accumulation of protein aggregates in the WNV-infected cells. To identify the viral factor inducing the accumulation of ubiquitinated proteins, intracellular accumulation of ubiquitinated proteins was examined in the cells expressing the viral protein. Expression of capsid (C) protein induced the accumulation, while mutations at residues L51 and A52 in C protein abrogated the accumulation. Wild-type (WT) or mutant WNV in which mutations were introduced into the residues was inoculated into human neuroblastoma cells. The expression levels of LC3-II, an autophagy-related protein, and AMP-activated protein kinase (AMPK), an autophagy inducer, were reduced in the cells infected with WT WNV, while the reduction was not observed in the cells infected with WNV with the mutations in C protein. Similarly, ubiquitination and degradation of AMPK were only observed in the cells infected with WT WNV. In the cells expressing C protein, AMPK was co-precipitated with C protein and mutations in L51 and A52 reduced the interaction. Although the viral replication was not affected, the accumulation of ubiquitinated proteins in brain and neurological symptoms were attenuated in the mouse inoculated with WNV with the mutations in C protein as compared with that with WT WNV. Taken together, ubiquitination and degradation of AMPK by C protein resulted in the inhibition of autophagy and the accumulation of protein aggregates, which contributes to the development of neurological disease.