Role of Discoidin Domain Receptor 2 in Craniofacial Bone Regeneration

Bone loss caused by trauma, neoplasia, congenital defects, or periodontal disease is a major cause of disability and human suffering. Skeletal progenitor cell-extracellular matrix interactions are critical for bone regeneration. Discoidin domain receptor 2 (DDR2), an understudied collagen receptor, plays an important role in skeletal development. Ddr2 loss-of-function mutations in humans and mice cause severe craniofacial and skeletal defects, including altered cranial shape, dwarfing, reduced trabecular and cortical bone, alveolar bone/periodontal defects, and altered dentition. However, the role of this collagen receptor in craniofacial regeneration has not been examined. To address this, calvarial subcritical-size defects were generated in wild-type (WT) and Ddr2-deficient mice. The complete bridging seen in WT controls at 4 wk postsurgery was not observed in Ddr2-deficient mice even after 12 wk. Quantitation of defect bone area by micro-computed tomography also revealed a 50% reduction in new bone volume in Ddr2-deficient mice. Ddr2 expression during calvarial bone regeneration was measured using Ddr2-LacZ knock-in mice. Expression was restricted to periosteal surfaces of uninjured calvarial bone and, after injury, was detected in select regions of the defect site by 3 d postsurgery and expanded during the healing process. The impaired bone healing associated with Ddr2 deficiency may be related to reduced osteoprogenitor or osteoblast cell proliferation and differentiation since knockdown/knockout of Ddr2 in a mesenchymal cell line and primary calvarial osteoblast cultures reduced osteoblast differentiation while Ddr2 overexpression was stimulatory. In conclusion, Ddr2 is required for cranial bone regeneration and may be a novel target for therapy.