Summary
Despite the efficacy of vaccines, severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has killed over 5 million individuals worldwide and continues to spread in countries where the vaccines are not yet widely available or its citizens are hesitant to become vaccinated. Therefore, it is critical to unravel the molecular mechanisms that allow SARS-CoV-2 and other coronaviruses to infect and overtake the host machinery of human cells. Coronavirus replication triggers endoplasmic reticulum (ER) stress and activation of the unfolded protein response (UPR), a key host cell pathway widely believed essential for viral replication. We examined the activation status and requirement of the master UPR sensor IRE1α kinase/RNase and its downstream transcription factor effector XBP1s, which is processed through an IRE1α-mediated mRNA splicing event, in human lung-derived cells infected with betacoronaviruses. We found human respiratory coronavirus OC43 (HCoV-OC43), Middle East respiratory syndrome coronavirus (MERS-CoV), and the murine coronavirus (MHV) all induce ER stress and strongly trigger the kinase and RNase activities of IRE1α as well as XBP1 splicing. In contrast, SARS-CoV-2 only partially activates IRE1α whereby it autophosphorylates, but its RNase fails to splice XBP1. Moreover, IRE1α was dispensable for optimal replication in human cells for all coronaviruses tested. Our findings demonstrate that IRE1α activation status differs upon infection with distinct betacoronaviruses and is not essential for efficient replication of any of them. Our data suggest that SARS-CoV-2 actively inhibits the RNase of autophosphorylated IRE1α through an unknown mechanism, perhaps as a strategy to eliminate detection by the host immune system.
Importance SARS-CoV-2 is the third lethal respiratory coronaviruses after MERS-CoV and SARS-CoV to emerge this century, causing millions of deaths world-wide. Other common coronaviruses such as OC43 cause less severe respiratory disease. Thus, it is imperative to understand the similarities and differences among these viruses in how each interacts with host cells. We focused here on the inositol-requiring enzyme 1α (IRE1α) pathway, part of the host unfolded protein response to virus-induced stress. We found that while MERS-CoV and OC43 fully activate the IRE1α kinase and RNase activites, SARS-CoV-2 only partially activates IRE1α, promoting its kinase activity but not RNase activity. We propose that SARS-CoV-2 prevents IRE1α RNase activation as a strategy to eliminate detection by the host immune system.
Competing Interest Statement
Susan R Weiss. is on the Scientific Advisory Boards of Immunome, Inc and Ocugen, Inc. Scott A. Oakes is a cofounder and consultant at OptiKira., L.L.C. (Cleveland, OH)
