Uniaxial Hydroxyapatite Growth on a Self-Assembled Protein Scaffold.

Biomineralization is a crucial process whereby organisms produce mineralized tissues such as teeth for mastication, bones for support, and shells for protection. Mineralized tissues are composed of hierarchically organized hydroxyapatite crystals, with a limited capacity to regenerate when demineralized or damaged past a critical size. Thus, the development of protein-based materials that act as artificial scaffolds to guide hydroxyapatite growth is an attractive goal both for the design of ordered nanomaterials and for tissue regeneration. In particular, amelogenin, which is the main protein that scaffolds the hierarchical organization of hydroxyapatite crystals in enamel, amelogenin recombinamers, and amelogenin-derived peptide scaffolds have all been investigated for in vitro mineral growth. Here, we describe uniaxial hydroxyapatite growth on a nanoengineered amelogenin scaffold in combination with amelotin, a mineral promoting protein present during enamel formation. This bio-inspired approach for hydroxyapatite growth may inform the molecular mechanism of hydroxyapatite formation in vitro as well as possible mechanisms at play during mineralized tissue formation.

Shielding characteristics of nanocomposites for protection against X- and gamma rays in medical applications: effect of particle size, photon energy and nano-particle concentration.

In recent decades, nanomaterials have been extensively investigated for many applications. Composites doped with different metal nanoparticles have been suggested as effective shielding materials to replace conventional lead-based materials. The use of concretes as structural and radiation protective material has been influenced by the addition of nanomaterials. Several elements with high atomic number and density, such as lead, bismuth, and tungsten, have the potential to form nanoparticles that offer significant enhancements in the shielding ability of composites. Their performance for a range of particle concentrations, particle sizes, and photon energies have been investigated. This review is an attempt to gather the data published in the literature about the application of nanomaterials in radiation shielding, including the use of polymer composites and concretes for protection against X-rays and gamma radiation.

Modulation of calcium oxalate dihydrate growth by phosphorylated osteopontin peptides.

Osteopontin (OPN) is a significant component of kidney stone matrix and a key modulator of stone formation. Here, we investigated the effects of different phosphorylated states of a synthesized peptide of OPN (the ASARM peptide; acidic, serine- and aspartate-rich motif) on calcium oxalate dihydrate (COD) crystals, a major mineral phase of kidney stones. In vitro, phosphorylated OPN-ASARM peptides strongly inhibited COD crystal growth in solution as compared to the nonphosphorylated state, with increasing inhibitory potency correlating with the degree of peptide phosphorylation. Scanning electron microscopy revealed that the inhibition from the phosphopeptides resulted in distinctive, rosette-like crystal aggregates called spherulites. The OPN-ASARM peptides preferentially bound and specifically inhibited the {1 1 0} crystallographic faces of COD, as identified by combining atomic force microscopy and computational simulation approaches. These {1 1 0} surfaces of COD have high lattice calcium occupancy (exposure), providing preferential binding sites for the highly acidic peptides; binding and inhibition by OPN-ASARM peptides at the {1 1 0} faces led to crystal aggregation and intergrowth. The crystal spherulite formations obtained in vitro when using the most phosphorylated form of OPN-ASARM peptide at a high concentration, resembled crystal morphologies observed in vivo in a rat model of urolithiasis, in which crystal deposits in the kidney contain abundant OPN as revealed by immunogold labeling. A mechanistic model for spherulite formation is proposed based on the symmetry and crystallographic structure of COD, where the phosphate groups of OPN-ASARM bind to calcium atoms at [1 1 1] step risers on the COD {1 1 0} surface, inducing the periodic emergence of new COD crystals to form spherulites.

Phosphorylation-dependent mineral-type specificity for apatite-binding peptide sequences.

Apatite-binding peptides discovered by phage display provide an alternative design method for creating functional biomaterials for bone and tooth tissue repair. A limitation of this approach is the absence of display peptide phosphorylation--a post-translational modification important to mineral-binding proteins. To refine the material specificity of a recently identified apatite-binding peptide, and to determine critical design parameters (net charge, charge distribution, amino acid sequence and composition) controlling peptide affinity for mineral, we investigated the effects of phosphorylation and sequence scrambling on peptide adsorption to four different apatites (bone-like mineral, and three types of apatite containing initially 0, 5.6 and 10.5% carbonate). Phosphorylation of the VTKHLNQISQSY peptide (VTK peptide) led to a 10-fold increase in peptide adsorption (compared to nonphosphorylated peptide) to bone-like mineral, and a 2-fold increase in adsorption to the carbonated apatite, but there was no effect of phosphorylation on peptide affinity to pure hydroxyapatite (without carbonate). Sequence scrambling of the nonphosphorylated VTK peptide enhanced its specificity for the bone-like mineral, but scrambled phosphorylated VTK peptide (pVTK) did not significantly alter mineral-binding suggesting that despite the importance of sequence order and/or charge distribution to mineral-binding, the enhanced binding after phosphorylation exceeds any further enhancement by altered sequence order. Osteoblast culture mineralization was dose-dependently inhibited by pVTK and to a significantly lesser extent by scrambled pVTK, while the nonphosphorylated and scrambled forms had no effect, indicating that inhibition of osteoblast mineralization is dependent on both peptide sequence and charge. Computational modeling of peptide-mineral interactions indicated a favorable change in binding energy upon phosphorylation that was unaffected by scrambling. In conclusion, phosphorylation of serine residues increases peptide specificity for bone-like mineral, whose adsorption is determined primarily by sequence composition and net charge as opposed to sequence order. However, sequence order in addition to net charge modulates the mineralization of osteoblast cultures. The ability of such peptides to inhibit mineralization has potential utility in the management of pathologic calcification.