Targeting skeletal endothelium to ameliorate bone loss

Recent studies have identified a specialized subset of CD31(hi)endomucin(hi) (CD31(hi)EMCN(hi)) vascular endothelium that positively regulates bone formation. However, it remains unclear how CD31(hi)EMCN(hi) endothelium levels are coupled to anabolic bone formation. Mice with an osteoblast-specific deletion of Shn3, which have markedly elevated bone formation, demonstrated an increase in CD31(hi)EMCN(hi) endothelium. Transcriptomic analysis identified SLIT3 as an osteoblast-derived, SHN3-regulated proangiogenic factor. Genetic deletion of Slit3 reduced skeletal CD31(hi)EMCN(hi) endothelium, resulted in low bone mass because of impaired bone formation and partially reversed the high bone mass phenotype of Shn3(-/-) mice. This coupling between osteo-blasts and CD31(hi)EMCN(hi) endothelium is essential for bone healing, as shown by defective fracture repair in SLIT3-mutant mice and enhanced fracture repair in SHN3-mutant mice. Finally, administration of recombinant SLIT3 both enhanced bone fracture healing and counteracted bone loss in a mouse model of postmenopausal osteoporosis. Thus, drugs that target the SLIT3 pathway may represent a new approach for vascular-targeted osteoanabolic therapy to treat bone loss.