Examination of the cellular stress response and post-transcriptional regulation of RNA during Ebola virus infection
Nelson, Emily Victoria
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Ebola virus (EBOV) causes severe disease in humans characterized by high case fatality rates and significant immune dysfunction. A hallmark of EBOV infection is the formation of viral inclusions in the cytoplasm of infected cells. These inclusions contain the EBOV nucleocapsids and are sites of viral replication and nucleocapsid maturation. Although there is growing evidence that viral inclusions create a protected environment that fosters EBOV gene expression and genome replication, little is known about their role in the host response to infection. The cellular stress response is an antiviral strategy that leads to stress granule (SG) formation and translational arrest mediated by the phosphorylation of the alpha subunit of eukaryotic translation initiation factor 2 (eIF2α). Related to this response is the post-transcriptional regulation of RNA mediated by stability elements called AU-rich elements (AREs) and their associated binding proteins (ARE-BPs), many of which are found in SGs. Because these processes have antiviral implications, many viruses have evolved strategies to interfere with SG formation, or appropriate ARE-BPs to benefit viral replication. However, it is unknown if EBOV interacts with these cellular systems. Here, we show that SG proteins were sequestered within EBOV inclusions where they formed distinct granules that colocalized with viral RNA. The inclusion-bound aggregates were not canonical SGs, and did not lead to translational arrest in infected cells. EBOV did not induce cytoplasmic SGs at any time post infection, but was unable to overcome SG formation induced by additional stressors. Despite the sequestration of SG proteins, canonical SGs did not form within inclusions. At high levels of expression, viral protein 35 (VP35), the viral polymerase co-factor that also mediates various immune evasion functions, disrupted SGs formation independently of eIF2α phosphorylation. Finally, we found that the cellular ARE-BP tristetraprolin (TTP) specifically targeted the 3’untranslated region (UTR) of the viral nucleoprotein (NP) mRNA and promoted its degradation. Interestingly, TTP was not found within viral inclusions, leading us to speculate that inclusions might serve to prevent viral RNA from encountering TTP. These results indicate that EBOV interacts with the cellular stress response and associated RNA regulatory proteins in ways that promote viral replication.