Electronic Thesis and Dissertation Repository

Degree

Doctor of Philosophy

Program

Biomedical Engineering

Supervisor

Harvey A. Goldberg

Abstract

Bone development and repair involve complex processes that include interaction between cells and their surrounding matrix. In the body, bone sialoprotein (BSP) expression is up-regulated at the onset of mineralization. BSP is a multifunctional acidic phosphoprotein with collagen-binding, hydroxyapatite nucleating, and integrin recognition (RGD sequence, which is important for cell-attachment and signaling) regions. Mice lacking BSP expression (Bsp-/-), exhibit a bone phenotype with reductions in bone mineral density, bone length, osteoclast activation, and impaired bone healing. This thesis examined the role of BSP in tooth development and also its potential use as a therapeutic reagent for bone repair. MicroCT and histological analysis of Bsp-/-mice revealed significant periodontal tissue breakdown marked by defective acellular cementum formation leading to periodontal ligament detachment, extensive alveolar bone and tooth root resorption, and tooth malocclusion. Substituting hard to soft diet, which minimized applied stress during mastication, did not reduce extent of periodontal tissue breakdown. However, soft diets eliminated the incidence of severe incisor malocclusion and the Bsp-/- mice featured normal body weight, long-bone length, and serum alkaline phosphatase activity, suggesting that tooth dysfunction and malnutrition contribute to growth and skeletal defects previously reported.

In the bone repair studies, the effectiveness of BSP-treated nano-hydroxyapatite/poly(ester-urethane) (nHA/PU) scaffolds in promoting bone regeneration was determined using a rat calvarial defect model. Recombinant human bone morphogenetic protein (rhBMP-2), which is a potent growth factor was used as the positive control for repair. Addition of BSP to nHA/PU scaffolds improved cell attachment and differentiation, and the consequent osteogenic mineralization in vitro. In vivo, at 6 weeks, microCT and histological analysis indicated that the rhBMP-2 treated nHA/PU scaffolds promoted significant new bone repair, in which approximately 70% of the defect, based on bone volume to total defect volume, was filled with new bone. In contrast, both BSP-treated nHA/PU scaffolds and non-treated scaffolds resulted in approximately 20% new bone formed. These findings suggest that BSP plays a non-redundant role in cementum formation, likely involved in initiating mineralization on the root surface. The effectiveness of nHA/PU scaffold as a carrier for rhBMP-2 and BSP was verified. However, in our system, BSP by itself was not a potent promoter of bone regeneration in vivo.

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