Mitoxosome: a mitochondrial platform for cross-talk between cellular stress and antiviral signaling

Abstract

Evidence is accumulating that the mitochondria form an integral platform from which innate signaling takes place. Recent studies revealed that the mitochondria are shaping the innate response to intracellular pathogens, and mitochondrial function is modulating and being modulated by innate immune signaling. Further, cell biologic analyses have uncovered the dynamic relocalization of key components involved in cytosolic viral recognition and signaling to the mitochondria, as well as the mobilization of mitochondria to the sites of viral replication. In this review, we provide an integrated view of how cellular stress and signals following cytosolic viral recognition are intimately linked and coordinated at the mitochondria. We incorporate recent findings into our current understanding of the role of mitochondrial function in antiviral immunity and suggest the existence of a 'mitoxosome', a mitochondrial oxidative signalosome where multiple pathways of viral recognition and cellular stress converge on the surface of the mitochondria to facilitate a coordinated antiviral response.

Fig. 1. The mitochondria as a crossroads

Mitochondria are dynamic organelles that have the capacity to sense the energy status of the cell as well as a variety of cellular stresses and the power to respond to dire situations with the initiation of cell death. Mitochondria power the cell processes by producing energy in the form of ATP through oxidative phosphorylation, and the reactive oxygen species generated as a byproduct of mitochondrial respiration are potent signaling molecules that can modulate protein functions.

Fig. 2. MFN2 negatively regulates IPS-1 signaling

Mitochondrial dynamics affect IPS-1-dependent RLR-mediated production of IFNs and pro-inflammatory cytokines. Two proteins required for mitochondrial fusion, MFN1 and MFN2, have different roles in IPS-1 signaling. MFN2 is required for mitochondria-ER tethering and efficient mitochondrial uptake of Ca²⁺ from the ER. In the absence of MFN2, but not MFN1/MFN2 double ablation, IPS-1 dependent cytokine production is increased. We speculate that altered Ca²⁺ levels may play a role in IPS-1 signaling.

Fig. 3. Impact of mtROS on RLR and NLRP3

Mitochondrial ROS can increase IPS-1 signaling, resulting in expression of pro-IL-1β. Additionally, mitochondrial ROS enhances the activation of NLRP3 inflammasome elicited by LPS + ATP, which leads to the release of mtDNA that feeds forward to activate further NLRP3 inflammasomes.

Fig. 4. Gene dose-dependent regulation of mitophagy and IPS-1 signaling by Atg5

(A) Atg5^+/+, Atg5^+/− and Atg5^−/− MEFs were stained with 100nM Mitotracker Green (which stains the lipid membrane of the mitochondria) and 100nM Mitotracker Red (which is an indicator of membrane potential). Contour plots of FACS analysis are depicted. (B) Mitochondrial DNA copy number was measured by qPCR and normalized to nuclear DNA levels in a ratio of mtDNA COI over 18s. Relative mitochondrial DNA copy numbers are depicted. These data are representative of three similar experiments. (C) ROS levels within WT, Atg5^+/−, and ATG5^−/− MEFs were examined by staining cells with MitoSOX (which labels mitochondria-associated ROS) and analyzed by FACS. (D) WT, Atg5^+/−, and Atg5^−/− MEFs were infected with 1 MOI of VSV-GFP and the levels of infection were determined by measuring GFP viral titers in the supernatants by plaque assay at the indicated time points. These results are representative of three separate experiments.

Fig. 5. The mitoxosome

Recognition of viral ligands by RIG-I or MDA5 initiates their binding to mitochondrial IPS-1 and subsequent induction of IFNs and proinflammatory cytokines, a process augmented by mtROS. NOD2 and STING have both also been shown to contribute to IPS-1 production of IFNs, though not through direct binding of viral ligands. NFκB-dependent production of proinflammatory cytokines such as IL-6 involves the adapter protein CARD9. The inactive pro-inflammatory cytokine IL-1β can be induced through either TLR or RLR viral recognition pathways and is then subject to inflammasome dependent processing to release the active form of IL-1β. The NLRP3 inflammasome plays an important role in viral recognition and upon activation relocalizes to the MAM. HSP90 facilitates cytokine production induced by IPS-1 activation in response to viruses, as well as for NOD2 and NLRP3 in response to bacteria. Localization of these antiviral signaling pathways proximal to the mitochondria helps to centralize cellular stress signaling to an organelle that can integrate these stimuli and facilitate a coordinated response through redox modulation of IPS-1 and NLRP3 signaling. This could also allow the specific combinations of stimuli to affect the exact nature of the response as well as providing a centralized location from which such signals can be readily terminated.