The Role of Aconitate Decarboxylase 1 in Inflammation and Disease

Abstract

Immune cell metabolism, or immunometabolism, has recently become of interest for its role in inflammation and disease. A growing field of research has identified that metabolic rewiring and immune cell activation are intimately connected, however the mechanisms driving these connections have remained poorly understood. The tricarboxylic acid cycle and its intermediates have become recognized as major players in disease and inflammation. The immunometabolite itaconate has been identified as a potent immunomodulator produced in high quantities in activated macrophages. Itaconate is produced by the enzyme aconitate decarboxylase 1 (Acod1), which is highly upregulated in proinflammatory macrophages. Although itaconate and Acod1 have been found to be upregulated in macrophages under stimulated conditions, the potential role of itaconate production in other non-immune cells remains poorly understood. Itaconate and its exogenous derivative forms have been found to be potent mediators of inflammation, and specifically have been found to decrease proinflammatory cytokine production in cultured macrophages. In this dissertation, we sought to identify the role of itaconate in three separate murine models of disease: cerebral ischemia/reperfusion injury, diet-induced obesity, and ulcerative colitis. We hypothesized that deletion of Acod1 would lead to greater disease severity in these models and that macrophages would be the primary cell type responsible. To understand the role of endogenously produced itaconate, mice lacking Acod1 (Acod1-/-) were used. We demonstrate that global Acod1 deletion leads to significantly worsened disease severity in all three models studied. Specifically, Acod1 deletion leads to increased lesion volume size compared to wild type (WT) mice in a model of ischemia/reperfusion stroke. The observed increased lesion volume did not appear to be caused by increased inflammation, indicating a separate potential mechanism driving these changes. In a model of diet-induced obesity, mice lacking Acod1 showed similar weight gain compared to WT mice, however, Acod1-/- mice had elevated blood glucose levels after 12 weeks of high fat diet. Acod1-/- mice also had elevated inflammatory gene expression. Furthermore, naïve Acod1-/- mice had significant increases in fat deposition when on chow diet at 3 and 6 months of age. Lastly, Acod1-/- mice exposed to an acute ulcerative colitis model induced by dextran sulfate sodium (DSS) treatment showed increased disease severity with more severe and sustained body weight loss and increased inflammatory gene expression. Importantly, cell specific knockout of Acod1 in myeloid cells (MyAcod1-/-) with LysM-Cre did not phenocopy disease severity in any of the three in vivo models. This suggests that myeloid cells, specifically macrophages, are not the primary cell type responsible for the observed phenotypes seen in the global Acod1-/- studies. These data define a novel role for Acod1 in transient ischemia/reperfusion occlusion stroke, diet-induced obesity, and ulcerative colitis. Furthermore, these differences do not appear to be regulated by Acod1 and itaconate in macrophage/myeloid cells. These findings suggest that Acod1 and itaconate are likely working through other cell types.