CLU delays fracture healing and intercepts osteogenic differentiation by enhancing SMURF1-mediated DDX5 ubiquitination.

Nonunion means failed fracture healing. Clusterin (CLU) was reported to be upregulated in callus tissues of nonunion patients. This study aims to investigate the roles of CLU in fracture healing and its function details. Our clinical data exhibited that CLU was increased in serum and callus tissues from nonunion patients, compared with control cases with normal healing after fracture. Experimental nonunion was induced by a transverse osteotomy of the femoral shaft combined with periosteum removing in rats, and X-ray showed that silencing of CLU facilitated fracture healing. Overexpression of CLU delayed mineralization and osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs), and deactivated Wnt/β-catenin signaling in vivo and in vitro. Immunoprecipitation-liquid chromatography/mass spectrometry assay revealed that CLU bound to DEAD-box helicase 5 (DDX5), which was required for activation of Wnt/β-catenin signaling and lowly expressed in callus tissues from nonunion patients. IP results demonstrated that CLU acted as a chaperone to aggravate SMAD specific E3 ubiquitin protein ligase 1 (SMURF1)-mediated ubiquitination of DDX5. The regulation of DDX5 by CLU was abrogated when SMURF1 was silenced, and the effects of CLU on osteogenic differentiation and Wnt/β-catenin signaling was abolished by DDX5 overexpression. In conclusion, we demonstrate that CLU delayed fracture healing of rats and suppressed osteogenic differentiation of BMSCs by enhancing SMURF1-mediated DDX5 ubiquitination and degradation as a chaperone, and deactivating Wnt/β-catenin signaling. These findings may provide novel potential therapeutic targets fracture and nonunion in clinic.