Moonlighting Cytosolic Function of ACAD9: Suppression of TRAF6-Mediated Osteoclastogenesis and Protection Against Osteoporosis

Acyl-CoA dehydrogenase-9 (ACAD9) is classically known for its role in mitochondrial fatty acid β-oxidation and complex I assembly. Here, we identify ACAD9 deficiency as a clinically relevant risk factor for fragility fractures and reveal a previously unrecognized cytosolic function of ACAD9 in suppressing osteoclast differentiation, thereby protecting against osteoporosis. Mechanistically, while preserving its canonical mitochondrial role in complex I assembly, we find that ACAD9 also facilitates the formation of respiratory supercomplexes (I+III+IV). Notably, in the cytosol, ACAD9 competitively binds to TRAF6, preventing its interaction with the E2 ubiquitin-conjugating complex UBC13/UEV1A, and thereby blocking K63-linked polyubiquitination and downstream activation of the RANK/TRAF6/TAK1/NFATc1 signaling cascade. Additionally, ACAD9 promotes K48-linked polyubiquitination of TRAF6, leading to its proteasomal degradation. Osteoclast-specific Acad9 knockout mice exhibit increased osteoclast numbers and decreased bone mass. These findings uncover a novel extramitochondrial function of ACAD9 in regulating osteoclast differentiation and maturation, and offer potential therapeutic insights for targeting osteoclast hyperactivity in osteoporosis.