A piscine birnavirus induces inhibition of protein synthesis in CHSE-214 cells primarily through the induction of eIF2α phosphorylation.

Amr Gamil,Stephen Mutoloki,Øystein Evensen

doi:10.3390/v7041987

Amr Gamil, Stephen Mutoloki + Show 1 more

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https://doi.org/10.3390/v7041987

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Journal: Viruses	Publication Date: Apr 15, 2015
Citations: 59	License type: CC BY 4.0

Affiliation: Norwegian University of Life Sciences

Abstract

Inhibition of protein synthesis represents one of the antiviral mechanisms employed by cells and it is also used by viruses for their own propagation. To what extent members of the Birnaviridae family employ such strategies is not well understood. Here we use a type-strain of the Aquabirnavirus, infectious pancreatic necrosis virus (IPNV), to investigate this phenomenon in vitro. CHSE-214 cells were infected with IPNV and at 3, 12, 24, and 48 hours post infection (hpi) before the cells were harvested and labeled with S35 methionine to assess protein synthesis. eIF2α phosphorylation was examined by Western blot while RT-qPCR was used to assess virus replication and the expression levels of IFN-α, Mx1 and PKR. Cellular responses to IPNV infection were assessed by DNA laddering, Caspase-3 assays and flow cytometry. The results show that the onset and kinetics of eIF2α phosphorylation was similar to that of protein synthesis inhibition as shown by metabolic labeling. Increased virus replication and virus protein formation was observed by 12 hpi, peaking at 24 hpi. Apoptosis was induced in a small fraction (1−2%) of IPNV-infected CHSE cells from 24 hpi while necrotic/late apoptotic cells increased from 10% by 24 hpi to 59% at 48 hpi, as shown by flow cytometry. These results were in accordance with a small decline in cell viability by 24hpi, dropping below 50% by 48 hpi. IPNV induced IFN-α mRNA upregulation by 24 hpi while no change was observed in the expression of Mx1 and PKR mRNA. Collectively, these findings show that IPNV induces inhibition of protein synthesis in CHSE cells through phosphorylation of eIF2α with minimal involvement of apoptosis. The anticipation is that protein inhibition is used by the virus to evade the host innate antiviral responses.

Highlights

Cells employ inhibition of protein synthesis as a defensive mechanism in response to virus invasion, the aim being to limit the production of virus progeny and arrest the spread within the organism
2-alpha. eIF2α phosphorylation blocks eIF2B-mediated GDP−GTP exchange preventing the formation of GTP–eIF2–tRNAiMet ternary complex, which is crucial for translation initiation, and results in translation inhibition [2]. eIF2α phosphorylation can be induced through PERK which is activated due to the accumulation of unfolded protein in the ER lumen, as a result of high virus replication, leading to the initiation of what is known as the unfolded protein response (UPR) [3]
At 12 and 24 hpi, a progressive reduction in protein synthesis was observed in infected cells, initially with a moderate reduction at 12 h (80% protein synthesis compared to uninfected controls; p < 0.01) followed by a marked reduction at 24 hpi, 65% reduction compared to uninfected controls (Figure 2b, p < 0.001)

Summary

Introduction

Cells employ inhibition of protein synthesis as a defensive mechanism in response to virus invasion, the aim being to limit the production of virus progeny and arrest the spread within the organism. There are mainly three well-known mechanisms by which inhibition of protein synthesis is induced during virus infections: (1) through the activation of the interferon inducible, dsRNA-activated protein kinase R (PKR); (2) through the activation of the PKR-like endoplasmic reticulum (ER) kinase (PERK); and 3) through factors associated with the induction of apoptosis. EIF2α phosphorylation blocks eIF2B-mediated GDP−GTP exchange preventing the formation of GTP–eIF2–tRNAiMet ternary complex, which is crucial for translation initiation, and results in translation inhibition [2]. EIF2α phosphorylation can be induced through PERK which is activated due to the accumulation of unfolded protein in the ER lumen, as a result of high virus replication, leading to the initiation of what is known as the unfolded protein response (UPR) [3]. Some of the caspases executing apoptosis can target components of the translation machinery for proteolysis [5]

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