Immunoresponsive Gene 1 Augments Bactericidal Activity of Macrophage-Lineage Cells by Regulating β-Oxidation-Dependent Mitochondrial ROS Production

Abstract

Evidence suggests the bactericidal activity of mitochondria-derived reactive oxygen species (mROS) directly contributes to killing phagocytozed bacteria. Infection-responsive components that regulate this process remain incompletely understood. We describe a role for the mitochondria-localizing enzyme encoded by Immunoresponsive gene 1 (IRG1) during the utilization of fatty acids as a fuel for oxidative phosphorylation (OXPHOS) and associated mROS production. In a zebrafish infection model, infection-responsive expression of zebrafish irg1 is specific to macrophage-lineage cells and is regulated cooperatively by glucocorticoid and JAK/STAT signaling pathways. Irg1-depleted macrophage-lineage cells are impaired in their ability to utilize fatty acids as an energy substrate for OXPHOS-derived mROS production resulting in defective bactericidal activity. Additionally, the requirement for fatty acid β-oxidation during infection-responsive mROS production and bactericidal activity toward intracellular bacteria is conserved in murine macrophages. These results reveal IRG1 as a key component of the immunometabolism axis, connecting infection, cellular metabolism, and macrophage effector function.

Deciphering the complexity of macrophage biology and function with the zebrafish model

“…Immunoresponsive gene 1 (irg1) encodes a mitochondria-localizing enzyme that is involved in bactericidal activity of Macrophages. Irg1 is specifically induced in macrophages following injection of live Salmonella enterica serovar Typhimurium into the hindbrain ventricle, showing that irg1 is a responsive gene to macrophage activation [52] . The transgenic line Tg(irg1:EGFP) in which EGFP expression is driven by irg1 promoter, can be combined with transgenic lines that constitutively mark macrophages, allowing thus real time visualization of macrophage activation in response to LPS and xenografted human cancer cells (Table 1) [50] .…”

Live imaging epidermal cell metabolism during inflammation: a new immunometabolic role for epidermal keratinocytes?

“…Once considered an insignificant byproduct of OXPHOS, it is now clear that ETC-coupled mROS contributes to a number of physiological processes, including the immune response. In macrophages, LPS treatment has been shown to augment mROS production while IFN- stimulation increases expression of specific OXPHOS components [13][14][15] . It is also established that mitochondrial respiratory capacity increases within infected macrophages to help fuel bactericidal mROS production [12-14 ,16] .…”

“…Live imaging these cells using neutrophil-and macrophage-marking transgenic zebrafish lines has provided a window to explore their function within an intact transparent animal model [34][35][36][37][38][39][40][41] . In addition, this transparency has been exploited to directly visualize the metabolic fates of fluorescent FA analogs [14,[42][43][44] . Using the zebrafish system to explore the function of immunoresponsive gene 1-like (Irg1l), a paralog of Irg1 (a mitochondrial enzyme that we previously showed connected metabolism and immune cell function within macrophages [14] ), we uncovered a new mechanism where Irg1l-dependent -oxidation-fueled mROS production within epidermal cells helps drive mmp9 expression and leukocyte migration during inflammation ( Figure 1B) [31] .…”

“…In addition, this transparency has been exploited to directly visualize the metabolic fates of fluorescent FA analogs [14,[42][43][44] . Using the zebrafish system to explore the function of immunoresponsive gene 1-like (Irg1l), a paralog of Irg1 (a mitochondrial enzyme that we previously showed connected metabolism and immune cell function within macrophages [14] ), we uncovered a new mechanism where Irg1l-dependent -oxidation-fueled mROS production within epidermal cells helps drive mmp9 expression and leukocyte migration during inflammation ( Figure 1B) [31] . We also confirmed increased abundance of IRG1 (the murine homolog of Irg1l) within the mitochondria of keratinocytes in a mouse model of cutaneous inflammation, supporting conservation of this mechanism in mammals.…”

“…It is also established that mitochondrial respiratory capacity increases within infected macrophages to help fuel bactericidal mROS production [12-14 ,16] . This mROS, that is delivered to intracellular bacteria through the active clustering of mitochondria around phagosomes containing bacteria, is believed to act in concert with nicotinamide adenine dinucleotide phosphate-oxidase-generated ROS to significantly augment phagosome bactericidal function [13,14] . In addition to providing bactericidal activity, mROS can also directly influence expression of pro-inflammatory cytokines and other immune modulators [17,18] .…”