Itaconate restrains acute proinflammatory activation of microglia after traumatic brain injury in mice

Traumatic brain injury (TBI) rapidly triggers proinflammatory activation of microglia, contributing to secondary brain damage post-TBI. Although the governing role of energy metabolism in shaping the inflammatory phenotype and function of immune cells has been increasingly recognized, the specific alterations in microglial bioenergetics post-TBI remain poorly understood. Itaconate, a metabolite produced by the enzyme aconitate decarboxylase 1 [IRG1; encoded by immune responsive gene 1 (Irg1)], is a pivotal metabolic regulator in immune cells, particularly in macrophages. Because microglia are macrophages of the brain parenchyma, the IRG1/itaconate pathway likely modulates microglial inflammatory responses. In this study, we explored the role of the IRG1/itaconate pathway in regulating microglial bioenergetics and inflammatory activation post-TBI using a mouse controlled cortical impact (CCI) model. We isolated microglia before and 4 and 12 hours after TBI and observed a swift but transient increase in glycolysis coupled with a prolonged disruption of mitochondrial metabolism after injury. Despite an up-regulation of Irg1 expression, itaconate in microglia declined after TBI. Microglia-specific Irg1 gene knockout (Irg1-Mi-KO) exacerbated metabolic changes, intensified proinflammatory activation and neurodegeneration, and worsened certain long-term neurological deficits. Supplementation with 4-octyl itaconate (OI) reinstated the use and oxidative metabolism of glucose, glutamine, and fatty acid, thereby enhancing microglial bioenergetics post-TBI. OI supplementation also attenuated proinflammatory activation and neurodegeneration and improved long-term neurological outcomes. These results suggest that therapeutically targeting the itaconate pathway could improve microglial energy metabolism and neurological outcomes after TBI.