Neutralizing hepatic apolipoprotein E enhances aged bone fracture healing

Advanced age impairs bone fracture healing; the underlying mechanism of this phenomenon remains unknown. We determined that apolipoprotein E (ApoE) increases with age and causes poor fracture healing. After deletion of hepatic ApoE expression (ΔApoE), 24-month-old ΔApoE mice displayed a 95% reduction in circulating ApoE levels and significantly improved fracture healing. ApoE treatment of aged BMSCs inhibited osteoblast differentiation in tissue culture models; RNA-seq, Western blot, immunofluorescence, and RT-PCR analyses indicated that the Wnt/β-catenin pathway is the target of this inhibition. Indeed, we showed that ApoE had no effect on cultures with stabilized β-catenin levels. Next, we determined that Lrp4 serves as the osteoblast cell surface receptor to ApoE, as expression of Lrp4 is required in ApoE-based inhibition of Wnt/β-catenin signaling and osteoblast differentiation. Importantly, we validated this ApoE-Lrp4-Wnt/β-catenin molecular mechanism in human osteoblast differentiation. Finally, we identified an ApoE-neutralizing antibody (NAb) and used it to treat aged, wildtype mice 3 days after fracture surgery resulting in fracture calluses with 35% more bone deposition. Our work here identifies novel liver-to-bone cross-talk and a noninvasive, translatable therapeutic intervention for aged bone regeneration.