ACOD1-dependent succinate signalling acts as a metabolic checkpoint for osteoclast differentiation

Immune cells undergo metabolic reprogramming in response to inflammatory stimuli. The immuneresponsive gene 1 (Irg1) encodes aconitate decarboxylase (ACOD1), which generates itaconate from cis-aconitate in the TCA cycle. Itaconate inhibits succinate dehydrogenase, resulting in succinate accumulation. Stable ACOD1 overexpression in RAW264.7 cells shifted cellular metabolism towards glycolysis, as indicated by enhanced mTOR activation, increased 4E-BP1 phosphorylation, and reduced ATP levels. ACOD1 cells displayed impaired osteoclastogenesis with reduced expression of osteoclast-associated genes and fewer TRAP-positive multinucleated osteoclasts. Unexpectedly, NFATc1 was constitutively present in the nucleus of untreated ACOD1 cells, resulting in residual NFAT activity and induction of inflammatory genes. Upon RANKL stimulation, these pre-activated cells showed delayed osteoclastogenic signalling accompanied by sustained expression of the transcriptional repressors BCL6, MafB, and IRF8. Using a GPR91 antagonist and a Gαq inhibitor, we demonstrate that extracellular succinate activates NFATc1 via GPR91-Gαq signalling. RNA sequencing further revealed that ACOD1 overexpression promotes an innate immune transcriptional program rather than osteoclast differentiation. Together, our findings identify succinate-GPR91 signalling as a regulator of the transition between inflammatory activation and osteoclastogenesis.