# Moonlighting cytosolic function of ACAD9: suppression of TRAF6-mediated osteoclastogenesis and protection against osteoporosis

## TL;DR

ACAD9, known for mitochondrial function, also suppresses osteoclast formation in the cytosol, offering new insights into preventing osteoporosis.

## Contribution

Discovery of ACAD9's cytosolic role in suppressing TRAF6-mediated osteoclastogenesis and protecting against osteoporosis.

## Key findings

- ACAD9 deficiency is a risk factor for fragility fractures and osteoporosis.
- ACAD9 inhibits TRAF6 signaling by blocking ubiquitination and promoting its degradation.
- Osteoclast-specific Acad9 knockout mice show increased osteoclasts and reduced bone mass.

## Abstract

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 chain supercomplexes. 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.