Stress granules in antiviral immunity and the integrated stress response

Abstract

Adequate defense against microbial infection depends on the controlled activation of the immune system. This is particularly important for the RIG-I-like receptors (RLRs), which recognize viral dsRNA and initiate antiviral innate immune responses with the potential of triggering systemic inflammation and immunopathology. Foreign dsRNA also triggers other cellular changes, including the activation of the integrated stress response through PKR. This leads to inhibition of translation and subsequently the assembly of phase-separated condensates known as stress granules (SGs). Physiological functions of SGs in the innate immune response have been debated. Our results showed that SGs play a key role in regulating activation of RLR signaling in response to dsRNA. Without the SG nucleators, G3BP1/G3BP2 or UBAP2L, transfected dsRNA or viral infection triggers excessive innate immune activation and consequent apoptosis. Additionally, SGs can mitigate the toxic immune response to endogenous dsRNA accumulated in pathologic conditions, such as ADAR1 deficiency. Intriguingly, we found that SGs can also directly inhibit viral replication independently of their functions in regulating antiviral immune response. These results underscore the multifaceted functions of SGs, acting as cellular shock absorbers that maintain cellular homeostasis by dampening both toxic immune response and viral replication. More recently, we found that the function of SGs in cell homeostasis goes beyond their involvement in antiviral immunity. We found that SGs also safeguard cells against non-immunological stressors, such as ER stress and nutrient starvation. Knocking out G3BP1 and G3BP2 compromised cellular ability to control the integrated stress response triggered by ER stressor or nutrient starvation, leading to cell death. Notably, cell death under these conditions depended on MAVS, but not upstream RLRs, revealing a previously unappreciated role of MAVS in the integrated stress response. To understand the mechanism behind this novel function of MAVS, we performed a CRISPR-Cas9 screen using a cell fitness assay, identifying two factors, VHL and NF2, playing key roles in MAVS-dependent cell death in response to ER stress. Future work will include determining the precise roles of MAVS, VHL, and NF2 in the integrated stress response and cell death, and the mechanism by which SGs suppress this pathway. In conclusion, our work led to the discovery of novel functions of SGs in antiviral defense and the integrated stress response. We found that SGs protect cells against excessive innate immune signaling and the toxic integrated stress response, while also suppressing viral replication in a manner independent of antiviral immunity. Our work also revealed the importance of MAVS in amplifying the integrated stress response aside from its well-established function in antiviral immune signaling. Future work in mechanistic dissection of these novel functions of SGs and MAVS will reshape our understanding of innate immunity, the integrated stress response and phase-separation.