A cell-free lipid sterosomal therapeutic enables bone regeneration via USP18-mediated noncanonical S1P signaling

Effective treatment of critical-sized bone defects remains challenging due to impaired osteogenesis, insufficient vascularization and oxidative stress-induced cell loss. Here, we report a chemically defined, cell-free sterosomal nanoplatform composed of sphingosine-1-phosphate (S1P) and 20(S)-hydroxycholesterol (Oxy) that integrates cytoprotective, pro-angiogenic, and osteogenic functions. S1P/Oxy sterosomes simultaneously enhance osteogenic differentiation and endothelial angiogenic activity while protecting resident cells from oxidative stress. In a rat critical-sized calvarial defect model, local administration of sterosomes promoted robust and spatially organized bone regeneration, without exogenous cells or growth factors. Mechanistically, transcriptomic and functional analyses identified ubiquitin-specific peptidase 18 (USP18) as a novel and essential mediator of sterosomal S1P-induced osteogenesis, independent of canonical S1P receptors. Notably, this effect is observed only when S1P is presented in a sterosomal assembly rather than in its free molecular form. Our work establishes a cell-free sterosomal strategy for effective bone regeneration accompanied by functional neovascularization and discovered a novel intracellular actuator for S1P.