Cells on Fire: Regulation of the NLRP3 Inflammasome via Endoplasmic Reticulum and Mitochondrial Crosstalk.

Abstract

Inflammation is a complex biological response to stress triggered by microbial infections. Although inflammation drives microbial clearance, if uncontrolled it leads to cellular damage and cell death. Previous studies have demonstrated that endoplasmic reticulum (ER) and mitochondrial stress programs can contribute to immunity by mediating production of inflammatory cytokines; however, the regulatory mechanisms are unclear. By utilizing Brucella abortus cattle vaccine strain RB51, we elucidated how ER and mitochondrial stress mediate inflammatory responses and the eventual fate of infected cells. Our initial findings revealed that RB51-induced programmed cell death (PCD), mediated by caspase-2-induced mitochondrial damage, led to activation of proapoptotic caspase-3 and -8. Notably, this caspase-2 mediated PCD was accompanied by the production of proinflammatory cytokines, TNFα and IL-1β, a phenomenon associated with pyroptosis, not PCD. These observations led us to further investigate how caspase-2 could regulate inflammation and proinflammatory cell death. As early as 2 hours post infection, RB51 triggered activation of ER stress sensor, IRE1, leading to increased ROS levels in the mitochondrial network. This sharp increase in mitochondrial ROS elicited the recruitment of inflammasome component NLRP3 and caspase-2 to the mitochondrial network, which triggered mitochondrial damage. Damage promoted release of mitochondrial danger signals into the cytosol and the activation of the NLRP3-ASC-caspase-1 inflammasome, which processes the inflammatory cytokine, IL-1β. Damaged mitochondria are usually tagged for removal by a mitochondria-specific form of autophagy called mitophagy. We observed that killed RB51 activated a second ER stress sensor, PERK, which suppressed IL-1β production. Upon PERK activation, the downstream transcription factor ATF4 initiated transcription of Parkin, a key regulator of mitophagy, which decreased mitochondrial damage signals and activation of the inflammasome. We have discovered that the partnership between the ER and mitochondria is essential for mediating inflammatory responses and determining cell fate. By fine-tuning the immune response via selective activation of ER stress sensors, we have laid the groundwork for therapeutic targeting of these regulators in inflammatory diseases associated with cellular stress.