Regulation of the ER Stress Response by the Ion Channel Activity of the Infectious Bronchitis Coronavirus Envelope Protein Modulates Virion Release, Apoptosis, Viral Fitness, and Pathogenesis.

Shumin Li,Rui Ai Chen,Lixia Yuan,Guo Dai,To Sing Fung,Ding Xiang Liu

doi:10.3389/fmicb.2019.03022

Abstract

Coronavirus (CoV) envelope (E) protein is a small structural protein critical for virion morphogenesis and release. The recently characterized E protein ion channel activity (EIC) has also been implicated in modulating viral pathogenesis. In this study, we used infectious bronchitis coronavirus (IBV) as a model to study EIC. Two recombinant IBVs (rIBVs) harboring EIC-inactivating mutations – rT16A and rA26F – were serially passaged, and several compensatory mutations were identified in the transmembrane domain (TMD). Two rIBVs harboring these putative EIC-reverting mutations – rT16A/A26V and rA26F/F14N – were recovered. Compared with the parental rIBV-p65 control, all four EIC mutants exhibited comparable levels of intracellular RNA synthesis, structural protein production, and virion assembly. Our results showed that the IBV EIC contributed to the induction of ER stress response, as up-regulation of ER stress-related genes was markedly reduced in cells infected with the EIC-defective mutants. EIC-defective mutants also formed smaller plaques, released significantly less infectious virions into the culture supernatant, and had lower levels of viral fitness in cell culture. Significantly, all these defective phenotypes were restored in cells infected with the putative EIC revertants. EIC mutations were also implicated in regulating IBV-induced apoptosis, induction of pro-inflammatory cytokines, and viral pathogenicity in vivo. Taken together, this study highlights the importance of CoV EIC in modulating virion release and various aspects of CoV – host interaction.

Highlights

Coronaviruses (CoVs) are a group of enveloped viruses with non-segmented, single-stranded, and positive-sense RNA genomes (Masters, 2006)
Taken together, using authentic infection systems, our data establish the important roles of E protein ion channel activity (EIC) in regulating CoV infection, CoV – host interactions, and viral pathogenesis
Plaque sizes of the putative ion channel (IC) revertants returned to similar levels as the parental recombinant IBVs (rIBVs)-p65 (Figures 1B,C)

Summary

Introduction

Coronaviruses (CoVs) are a group of enveloped viruses with non-segmented, single-stranded, and positive-sense RNA genomes (Masters, 2006). The first 2/3 of the genome encodes the viral replicase, while the remaining 1/3 contains coding sequence for the accessory proteins and the four structural proteins: spike (S), envelope (E), membrane (M), and nucleocapsid (N) protein (Masters, 2006). The S protein is responsible for binding to the cognate receptor and mediating membrane fusion (Yamada and Liu, 2009; Yamada et al, 2009; Zheng et al, 2018); the M protein is crucial for the assembly and morphogenesis of mature virions (De Haan et al, 2000; Liang et al, 2019); the N protein binds to the RNA genome in a beads-on-a-string fashion to form the helically symmetric nucleocapsid (Masters, 2006)

Methods

Results

Conclusion