BSP RGD Domain has a Role in Alveolar Bone Modulation

Objectives: Bone sialoprotein (BSP), a multifunctional extracellular matrix protein found in bone and cementum, is thought to be involved in promoting mineralization. The RGD integrin-binding domain is considered to be responsible for promoting cell migration, adhesion and signaling. Previously we reported Ibsp knockout (Ibsp^-/-) mice exhibit a periodontal phenotype that includes inhibition of acellular cementum formation, delayed alveolar bone mineralization, and disruption of periodontal ligament (PDL) attachment to the tooth root. As a step in defining the role of the RGD region, mice were generated where the RGD domain was replaced by a non-functional KAE sequence (Ibsp^KAE/KAE).
Methods: Mandibles from wild type (WT), Ibsp^-/-, and Ibsp^KAE/KAE mice were compared at 26 and 120 days postnatal (dpn) by micro-computed tomography (µCT) and histology.
Results: By µCT analysis, Ibsp^-/- mice exhibited 20-30% reduction in mandibular bone volume at 26 and 120dpn vs. WT. Compared to WT at 26dpn, Ibsp^KAE/KAE mice exhibited no difference in mandibular bone volume. However, at day 120dpn Ibsp^KAE/KAE mice exhibited 10% increased (p=0.009) mandibular bone volume vs. WT. Histologically, while Ibsp^-/- mice featured progressive periodontal breakdown, Ibsp^KAE/KAE mice displayed normal cementum, alveolar bone, and a functional PDL. TRAP staining for osteoclast-like cells revealed significantly higher TRAP-positive cells on alveolar bone surfaces of Ibsp^KAE/KAE mice at 26dpn (2.1-fold) and 120dpn (2.6-fold) vs. WT. No significant differences were noted in TRAP-positive cells on alveolar bone surfaces of Ibsp^KAE/KAE vs. Ibsp^-/- mice at 26 or 120dpn.
Conclusions: Absence of the RGD domain in BSP did not inhibit cementum formation. However, the increase in osteoclast-like cells in Ibsp^KAE/KAE mice coupled with the increased alveolar bone volume suggests that the RGD region has a role in regulating alveolar bone volume and osteoclast behavior. K. Nagasaki and M. B. Chavez contributed equally to this work.