Osteopontin regulates biomimetic calcium phosphate crystallization from disordered mineral layers covering apatite crystallites

Abstract

Details of apatite formation and development in bone below the nanometer scale remain enigmatic. Regulation of mineralization was shown to be governed by the activity of non-collagenous proteins with many bone diseases stemming from improper activity of these proteins. Apatite crystal growth inhibition or enhancement is thought to involve direct interaction of these proteins with exposed faces of apatite crystals. However, experimental evidence of the molecular binding events that occur and that allow these proteins to exert their functions are lacking. Moreover, recent high-resolution measurements of apatite crystallites in bone have shown that individual crystallites are covered by a persistent layer of amorphous calcium phosphate. It is therefore unclear whether non-collagenous proteins can interact with the faces of the mineral crystallites directly and what are the consequences of the presence of a disordered mineral layer to their functionality. In this work, the regulatory effect of recombinant osteopontin on biomimetic apatite is shown to produce platelet-shaped apatite crystallites with disordered layers coating them. The protein is also shown to regulate the content and properties of the disordered mineral phase (and sublayers within it). Through solid-state NMR atomic carbon-phosphorous distance measurements, the protein is shown to be located in the disordered phases, reaching out to interact with the surfaces of the crystals only through very few sidechains. These observations suggest that non-phosphorylated osteopontin acts as regulator of the coating mineral layers and exerts its effect on apatite crystal growth processes mostly from afar with a limited number of contact points with the crystal.

Hyperphosphorylation of Human Osteopontin and Its Impact on Structural Dynamics and Molecular Recognition

“… 13 , 23 , 24 Its abnormal phosphorylation patterns are directly connected to Fam20C mutations. Furthermore, the phosphorylation of OPN regulates its binding interaction with hydroxyapatite and hence the formation and growth of the mineral phase in bone material, 25 − 31 as well as bone remodeling and calcification. 13 , 28 , 32 , 33 On top of that, ECM phosphoproteome homeostatis, in particular OPN phosphorylation, has been associated with tumor cell progression, 34 macrophage migration, 35 and host–cell interactions.…”

Binding Mode Characterization of Osteopontin on Hydroxyapatite by Solution NMR Spectroscopy

“…OPNp has been shown to be a natural inhibitor of calcification and mineral formation, growth and proliferation,[ 19 , 21 , 22 , 32 , 35 ] and to prevent crystal formation in soft tissues and biological fluids. [ 25 , 28 , 32 ] Whereas the non‐phosphorylated form always has shown less or no effect (e. g. recently, a weak binding of non‐phosphorylated quail OPN on HAP crystal surfaces has only mildly affected the crystallite properties [63] ). This difference in function becomes clear by considering the elongated structure (in the apo and HAP‐bound state) of OPNp binding over the full length hence covering a larger surface area.…”

Enhanced osseointegration of dental implants with reduced graphene oxide coating

“…Notably, the cells cultured on the surface of R-ST discs showed signi cantly increased mRNA expression of OCN, which was around 1.5-fold and 11.2-fold higher than those cultured on the BT-ST and ST discs, respectively. OPN, bone sialoprotein I, is recognized as one of the factors that strongly bind to various types of calcium-based biominerals in bones and teeth [35]. Signi cant upregulation of OPN expression was found up to 9.5-fold on the R-ST discs directly, compared to the control.…”