Melatonin Counteracts Mechanical Unloading-Induced Bone Loss Through YTHDF3-Mediated m⁶A Modification of Dapk2 mRNA

Disuse osteoporosis, a consequence of prolonged mechanical unloading, is characterized by bone loss and elevated fracture susceptibility. Although melatonin exhibits bone‑anabolic properties, its mechanistic role in the context of mechanical unloading remains elusive. Our findings demonstrate that melatonin promotes osteogenic differentiation and suppresses osteoblast apoptosis, collectively mitigating unloading‑induced osteoporotic bone loss in hindlimb unloading (HLU) mice. Moreover, unloading suppressed YTHDF3 expression in osteoblasts and bone tissue, which was effectively rescued by melatonin administration. Functionally, YTHDF3 potentiated osteoblast differentiation and matrix mineralization while inhibiting apoptotic cell death. At the molecular level, YTHDF3 directly recognized m⁶A‑modified Dapk2 transcripts and promoted their decay. DAPK2 was characterized as a negative regulator that impedes osteoblast differentiation and survival. Genetic analyses established that melatonin‑driven suppression of DAPK2 and functional recovery of osteoblasts are contingent upon YTHDF3. In summary, we delineate a melatonin/YTHDF3/DAPK2 protective axis that safeguards against unloading‑induced bone deterioration via post‑transcriptional regulation of Dapk2, thereby unveiling new mechanistic perspectives and therapeutic opportunities for disuse osteoporosis.