Mitochondrial DNA Sensing in Type I Interferon Responses and Disease

Abstract

Mitochondria are pleiotropic organelles central to a wide array of cellular pathways including metabolism, signal transduction, and programmed cell death. Mitochondria also play key roles in mammalian immune responses, functioning as scaffolds for innate immune signaling, governing metabolic switches required for immune cell activation, and releasing agonists that promote inflammation. Due to their ancestral bacterial origin, mitochondria possess prokaryotic features, including a double-membrane structure with distinct lipid composition, N-formylated proteins, and circular, hypomethylated mitochondrial DNA (mtDNA). These mitochondrial molecules are often referred to as mitochondrial damage-associated molecular patterns (DAMPs), and they can potently activate pattern-recognition receptor (PRR) signaling when liberated from the organelle during cellular stress. mtDNA is a particularly immunostimulatory DAMP, eliciting innate immune responses and contributing to human disease. In this dissertation, I provide molecular evidence linking mitochondria-innate immune crosstalk to the pathobiology of mitochondrial disorders, aging, and heart failure in cell and mouse models of mtDNA instability.

I first employed the polymerase gamma (POLG) mutator model and report that aberrant activation of the cyclic GMP-AMP synthase (cGAS)-type I interferon (IFN-I) innate immune axis potentiates immunometabolic dysfunction, reduces health span, and accelerates aging in mutator mice. Mechanistically, elevated IFN-I signaling suppresses activation of nuclear factor erythroid 2–related factor 2 (NRF2), which increases oxidative stress, enhances proinflammatory cytokine responses, and accelerates metabolic dysfunction. Ablation of IFN-I signaling attenuates hyperinflammatory phenotypes by restoring NRF2 activity and reducing aerobic glycolysis, which combine to lessen tissue pathologies in aged mutator mice. Secondly, I show that mtDNA instability leads to the accumulation of Z-form DNA, which is stabilized by Z-DNA binding protein 1 (ZBP1). I identified a novel cytosolic DNA sensing complex of ZBP1 and cGAS, which recruits the receptor-interacting serine/threonine-protein kinases RIPK1 and RIPK3 to sustain IFN-I responses downstream of mtDNA stress. Furthermore, my research has uncovered that ZBP1 is a novel regulator of IFN-I-mediated disease pathology, working in tandem with the cGAS-STING pathway to sense mtDNA instability and sustain IFN-I signaling that contributes to cardiac remodeling and heart failure. Overall, my findings advance knowledge of how mitochondrial dysfunction shapes innate immune responses and highlight important roles for mtDNA stress and downstream IFN-I signaling in disease and aging.