In vitro apatite induction by osteopontin: Interfacial energy for hydroxyapatite nucleation on osteopontin

Abstract

Noncollagenous phosphoproteins that interact with type I collagen are thought to nucleate the mineral phase to the collagen fibril network of mineralized tissues. We previously reported that a specific dentin phosphoprotein, phosphophoryn, crosslinked to an insoluble substrate such as type I collagen fibrils, was an effective nucleator of apatite. In this study, we investigated the capacity of another phosphoprotein in dentin, osteopontin, for apatite nucleation in vitro. Osteopontin purified from bovine milk was either absorbed to agarose beads or crosslinked to agarose beads. Each preparation was incubated at 37 degrees C in metastable solutions. Apatite was induced by osteopontin that was crosslinked to agarose beads, whereas osteopontin adsorbed to agarose beads failed to induce apatite crystal formation. Using classical nucleation theory, the interfacial energy for hydroxyapatite nucleation on osteopontin crosslinked to agarose beads was determined to be 94.7 ergs/cm(2). This value is larger than that for phosphophoryn, though it is similar to that for hydroxyapatite. The present study indicates that osteopontin, like phosphophoryn, has a high potential to induce apatite formation when it is covalently bound to some substrate in vitro, and suggests the possibility that osteopontin bound to type I collagen may induce apatite formation in vivo.

Apatite plaque particles in inner medulla of kidneys of calcium oxalate stone formers: Osteopontin localization

“…Exogenous osteopontin limited the extent of apatite formation in cultured bovine smooth muscle cells [23]. On the other hand, when immobilized, osteopontin can nucleate hydroxyapatite crystallization [24]. Possibly both occur.…”

Phosphorylated Proteins and Control over Apatite Nucleation, Crystal Growth, and Inhibition

“…Recent studies have shown that the function of OPN is dependent on the extent of post-translational modifications133. In-vitro experiments have demonstrated that OPN is a potent inhibitor of de novo HAP formation134,135,136,137. Analysis of the amino acid sequence in terms of sequences responsible for mineralization has identified conserved acidic domains such as an extended stretch of aspartic acid residues, DDDDDDDDD in the rat and DDEDDDD in humans63.…”

“…Nucleation processes usually present an activation energy barrier before allowing the formation of a solid phase from a supersaturated metastable solution. In biological micro-environments, the activation energy for nucleation can be reduced by lowering of the interfacial energy136. DPP immobilized on a collagen surface can lower the interfacial energy for HAP nucleation, indicating that DPP bound on collagen template had a high capacity to nucleate HAP136.…”

The importance of amino acid interactions in the crystallization of hydroxyapatite

“…In general, it is believed that negatively charged AAs, containing carboxylate and phosphorylated residues, play a key role in controlling HA mineralization in bone [7,[20][21][22][23]. However, contradictory results reported by different authors make it difficult to draw a comprehensive conclusion about the effect of AAs on HA crystallization.…”