Targeting miR-337 mitigates disuse-induced bone loss.

Disuse-induced bone loss occurs in long-term bed-ridden patients and in astronauts during spaceflight. The underlying mechanisms are poorly understood. In a rodent model of disuse-induced bone loss (called hindlimb unloading (HU)), we observed that decreased numbers of leptin receptor (LepR) positive mesenchymal stem cells (MSCs) in adult bone marrow, contribute to bone loss. MicroRNA-337-3p (miR-337) was upregulated in MSCs upon HU and inhibited MSC proliferation by directly targeting IRS-1 to suppress the PI3kinase-Akt-mTOR pathway. Piezo1 was the upstream receptor for sensing mechanical stress and regulated miR-337 through the Hippo-YAP signaling pathway. Remarkably, the knockout of miR-337 significantly attenuated HU-induced, but not ovariectomy-induced, bone loss by increasing MSC proliferation and osteogenesis. Finally, the transplantation of miR-337^-/- MSCs into wild-type HU mice was sufficient to mitigate bone loss. These findings reveal the cellular and molecular mechanisms underlying disuse-induced bone loss and highlight a feasible therapeutic strategy to prevent disuse- or microgravity-induced bone loss on Earth and during spaceflight.