Natural bone tissue is primarily composed of hydroxyapatite (HAP), which accounts for 70-90% of the mass of bone. The remaining 10-30% is composed of protein, of which ~90% has been shown to be collagenous. During natural bone formation cells first lay down a collagenous matrix composed of primarily type 1 collagen. After this collagen network is formed, proteins bind to the collagen matrix and then the matrix is mineralized to form woven bone. While a number of proteins have been located within bone, only osteopontin (OPN) and bone sialoprotein (BSP) have been localized in the collagenous matrix ahead of the mineralization front. Additionally, both of these proteins have been found to be enriched at bone-implant interfaces. Based on this, these two proteins are hypothesized to play an important role in cellular adhesion at these interfaces. This work details efforts to understand the roles of OPN and BSP in cellular binding to engineered systems mimicking the collagen and mineralized interfaces of bone.

In the first part of this work, we compared the cell binding abilities of OPN and BSP when specifically bound to collagen. Both of these proteins have been shown to have a specific protein-protein interaction with collagen and this was confirmed through the development of radio-labeled protein adsorption isotherms for each of the proteins. These isotherms were used to determine adsorption parameters that resulted in identical amounts of adsorbed protein in order to directly compare the cell binding properties of these two proteins when specifically bound to collagen. The results indicate that OPN promotes a greater amount of cell binding to the collagen interface than BSP.

The second part of this work compared the cell binding abilities of OPN and BSP when they are specifically bound to HAP, mimicking mineralized bone interfaces. HAP was formed from a simulated body fluid and was characterized by scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDX), and electron spectroscopy for chemical analysis (ESCA) before use. Both OPN and BSP have been shown to have a specific HAP binding domain and this was confirmed through the development of radio-labeled protein adsorption isotherms. The cellular adhesion to HAP was then compared with identical amounts of adsorbed proteins. The results of this work indicate that BSP has a more favorable orientation or conformation for cellular adhesion as compared to OPN. However, this influence on cell binding is eliminated when the surface roughness of the underlying HAP substrate becomes too great.