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Abstract

Immune cells undergo metabolic reprogramming to rapidly support functional activation and cytokine protein in response to inflammatory stimuli. The immuneresponsive gene 1 (Irg1) and its encoded enzyme aconitate decarboxylase (ACOD1) play a central role in this process. By catalyzing the production of itaconate, Irg1/ACOD1 is able to break the tricarboxylic acid (TCA) cycle to accumulate succinate. While ACOD1-dependent metabolic reprogramming has been extensively studied in macrophages, its role in osteoclast differentiation remains to be investigated. In this thesis, the molecular mechanism of how ACOD1-mediated metabolic reprogramming impacts on osteoclastogenesis was revealed. ACOD1 expression was found to be induced in RANKL-stimulated osteoclast precursors, suggesting its regulatory role during osteoclastogenesis. Stable overexpression of ACOD1 in RAW 264.7 cells impaired osteoclast formation and suppressed osteoclast-associated gene expression. However, the master regulator of osteoclast differentiation, NFATc1, was pre-activated independent of RANKL stimulation. Mechanistic analyses demonstrated that ACOD1 overexpression promotes succinate secretion and activation of succinate receptor GPR91 signaling, which leads to pre-activation of NFATc1 that interferes with canonical RANKL-induced osteoclastogenic signaling. In addition, this work also demonstrates that ACOD1 overexpression enhances global protein lysine succinylation, which disrupts the tightly regulated RANKL-dependent succinylation dynamics that are fine-tuned by ACOD1 and desuccinylase SIRT5. This dysregulated succinylation pattern contributes to impaired mitochondrial function and further limits osteoclast differentiation. Together, my data identify succinate as a key metabolic regulator of osteoclastogenesis through both extracellular succinate-GPR91 signaling and intracellular succinylation. This highlights the multifunctional role of succinate in both immunometabolism and signaling activation. These findings also suggest succinate signaling pathways might become a potential target for metabolic disorders and bone-associated diseases.