Respiratory Syncytial Virus and Cellular Stress Responses: Impact on Replication and Physiopathology

Abstract

Human respiratory syncytial virus (RSV), a member of the Paramyxoviridae family, is a major cause of severe acute lower respiratory tract infection in infants, elderly and immunocompromised adults. Despite decades of research, a complete integrated picture of RSV-host interaction is still missing. Several cellular responses to stress are involved in the host-response to many virus infections. The endoplasmic reticulum stress induced by altered endoplasmic reticulum (ER) function leads to activation of the unfolded-protein response (UPR) to restore homeostasis. Formation of cytoplasmic stress granules containing translationally stalled mRNAs is a means to control protein translation. Production of reactive oxygen species is balanced by an antioxidant response to prevent oxidative stress and the resulting damages. In recent years, ongoing research has started to unveil specific regulatory interactions of RSV with these host cellular stress responses. Here, we discuss the latest findings regarding the mechanisms evolved by RSV to induce, subvert or manipulate the ER stress, the stress granule and oxidative stress responses. We summarize the evidence linking these stress responses with the regulation of RSV replication and the associated pathogenesis.

Keywords: ER stress; RSV; endoplasmic reticulum; inclusion bodies; oxidative stress; reactive oxygen species; respiratory syncytial virus; stress granule; stress response; virus.

Figure 1

RSV Induces a Non-Canonical UPR Response. Endoplasmic Reticulum (ER) stress induction upon Respiratory Syncytial Virus (RSV) infection, possibly as a result of ER overload due to glycoprotein RSV F, G and SH transit, is associated with activation of the inositol-requiring enzyme 1 (IRE1) and activated transcription factor 6 (ATF6) branches of the unfolded protein response (UPR). Interaction of F with the binding immunoglobulin protein (BiP/GRP78) chaperone could be an initiating event of the UPR response. Increased protein folding and degradation capacity are induced as a result of X-box binding protein 1 (XBP1) splicing variant XBP1s production by IRE1 endonuclease activity and ATF6f fragment formation. IRE1 acts as a restriction factor against RSV independently of XBP1s. IRE1 kinase activity and IRE1-endonuclease activity involved in selective mRNA degradation, a process called IRE1-dependent decay (RIDD), might contribute to IRE1-dependent inhibition of RSV replication through mechanisms that remain to be characterized. Autophagy was shown to promote negative feedback on the IRE1 pathway.

Figure 2

Crosstalk between SGs and IBs during RSV infection. Respiratory syncytial Virus (RSV) infection triggers the formation of cytoplasmic G3BP1/TIA1/eIf3η positive stress granules (SGs) and inclusion bodies (IBs). Viral genomic ssRNA transiently localizes within SGs, but is mainly associated in IBs. Colocalization of RSV ribonucleoprotein complex, composed of N, P and L, and of the M2-1 protein in IBs suggests that IBs favor RSV replication and transcription. IBs also contain the pattern recognition receptors, retinoic acid-inducible gene I (RIG-I) and melanoma differentiation-associated protein 5 (MDA5), which sense viral replication intermediates to trigger the interferon (IFN) antiviral response. Interaction of N with MDA5 is thought to counteract the antiviral response. Formation of SGs containing stalled mRNA to inhibit their translation is initiated upon activation of dsRNA-dependent protein kinase R (PKR) by dsRNA intermediates and downstream eIF2α phosphorylation. However, several mechanisms limit the formation of SGs. First, PKR-dependent eIF2α phosphorylation is limited by the interaction of RSV N with PKR. Second, the 5’ extragenic trailer sequence (5’ Tr) of the RSV genome interferes with SG assembly possibly through interaction with TIA1-related (TIAR) protein, as it was shown during SeV infection. Finally, RSV induces the redistribution of the O-linked N-acetylglucosamine transferase (OGT), an enzyme critical for assembly of stalled RNA into SGs, into IBs.

Figure 3

Regulation of the Oxidative Stress Response during RSV Infection. Nuclear factor erythroid 2-related factor 2 (Nrf2) is the main transcriptional activator of antioxidant genes through binding to the antioxidant responsive elements (ARE). In basal conditions, Nrf2 is sequestered in the cytoplasm by interaction with kelch-like ECH-associated protein 1 (Keap1). Keap1 also binds the E3 ubiquitin-protein ligase complex Cul3/Rbx1, which ubiquitinates Nrf2, leading to its degradation by the proteasome. RSV infection leads to production of reactive oxygen species (ROS), likely by the NADPH oxidase 2 (NOX2), which positively regulates the signaling pathways leading to expression of antiviral and proinflammatory cytokines. Additionally, ROS induce Nrf2 release from interaction with Keap1. Stabilized Nrf2 is subjected to additional regulation by phosphorylation and acetylation. RSV increases histone deacetylase (HDAC) activity leading to nuclear deacetylation of Nrf2 thereby inducing dissociation from ARE and inhibition of antioxidant genes.