Molecular evolution of IRG1 shapes itaconate production in metazoans and alleviates the 'double-edged dilemma' of innate immune defense

Itaconate is an innate immune metabolite specifically produced in activated immune cells via the decarboxylation of cis-aconitate, an intermediate of the TCA cycle. By inhibiting succinate-related metabolism, itaconate exerts antimicrobial properties at the expense of potentially disrupting the hosts own central energy metabolism, a double-edged dilemma of immunometabolism. To explore the evolutionary logic of itaconate biosynthesis, we investigated the evolutionary trajectory of IRG1, which encodes for cis-aconitate decarboxylase (CAD), the enzyme responsible for itaconate production. Phylogenetic analysis reveals a putative independent acquisition of metazoan and fungal IRG1 from prokaryotic sources. In metazoans, IRG1 underwent gene duplication and subsequently lost the mitochondrial targeting sequence (MTS), relocating CAD outside the mitochondrial matrix and therefore preventing direct inhibition of energy metabolism. In basal metazoans that contain IRG1, oysters and amphioxus, primitive IRG1 expression is also induced by innate immune stimuli, suggesting an already specialized role of itaconate for innate immune defense in early bilaterians. Our integrated in silico and experimental analysis highlight the molecular adaptations in IRG1, including subcellular relocation, that optimize itaconate production for innate immunity in resolving a fundamental trade-off in immunometabolism.