How does bone sialoprotein promote the nucleation of hydroxyapatite? A molecular dynamics study using model peptides of different conformations

Biopolymer/Calcium phosphate scaffolds for bone tissue engineering

With nearly 30 years of progress, tissue engineering has shown promise in developing solutions for tissue repair and regeneration. Scaffolds, together with cells and growth factors, are key components of this development. Recently, an increasing number of studies have reported on the design and fabrication of scaffolding materials. In particular, inspired by the nature of bone, polymer/ceramic composite scaffolds have been studied extensively. The purpose of this paper is to review the recent progress of the naturally derived biopolymers and the methods applied to generate biomimetic biopolymer/calcium phosphate composites as well as their biomedical applications in bone tissue engineering.

The effect of five proteins on stem cells used for osteoblast differentiation and proliferation: a current review of the literature

Bone-tissue engineering is a therapeutic target in the field of dental implant and orthopedic surgery. It is therefore essential to find a microenvironment that enhances the growth and differentiation of osteoblasts both from mesenchymal stem cells (MSCs) and those derived from dental pulp. The aim of this review is to determine the relationship among the proteins fibronectin (FN), osteopontin (OPN), tenascin (TN), bone sialoprotein (BSP), and bone morphogenetic protein (BMP2) and their ability to coat different types of biomaterials and surfaces to enhance osteoblast differentiation. Pre-treatment of biomaterials with FN during the initial phase of osteogenic differentiation on all types of surfaces, including slotted titanium and polymers, provides an ideal microenvironment that enhances adhesion, morphology, and proliferation of pluripotent and multipotent cells. Likewise, in the second stage of differentiation, surface coating with BMP2 decreases the diameter and the pore size of the scaffold, causing better adhesion and reduced proliferation of BMP-MSCs. Coating oligomerization surfaces with OPN and BSP promotes cell adhesion, but it is clear that the polymeric coating material BSP alone is insufficient to induce priming of MSCs and functional osteoblastic differentiation in vivo. Finally, TN is involved in mineralization and can accelerate new bone formation in a multicellular environment but has no effect on the initial stage of osteogenesis.

Assembly of amorphous clusters under floating monolayers: a comparison of in situ and ex situ techniques

We report synchrotron X-ray scattering studies of biomimetic crystallization of hydroxyapatite (the primary constituent of bone), using monolayers of fatty acid molecules floating on simulated body fluid (SBF) as well as aqueous solutions of calcium phosphate. A ∼10 Å thick film of amorphous material is observed to form immediately at the molecular monolayer, consistent with the proposed formation of "Posner clusters". This layer becomes denser but not significantly thicker as the subphase concentration and the temperature approach physiological conditions. The amorphous films do not crystallize within 24 h, in contrast to prior reports of more rapid crystallization using electron microscopy on ex situ samples. However, crystallization occurs almost immediately after our films are transferred onto solid substrates. These results illustrate the importance of in situ measurements for model biomineralization experiments.

Role of bioactive 3D hybrid fibrous scaffolds on mechanical behavior and spatiotemporal osteoblast gene expression

Three-dimensional (3D) bioactive organic-inorganic (O/I) hybrid fibrous scaffolds are attractive extracellular matrix (ECM) surrogates for bone tissue engineering. With the aim of regulating osteoblast gene expression in 3D, a new class of hybrid fibrous scaffolds with two distinct fiber diameters (260 and 600 nm) and excellent physico-mechanical properties were fabricated from tertiary (SiO2-CaO-P2O5) bioactive glass (BG) and poly (ε-caprolactone) (PCL) by in situ sol-gel and electrospinning process. The PCL/BG hybrid fibrous scaffolds exhibited accelerated wetting properties, enhanced pore sizes and porosity, and superior mechanical properties that were dependent on fiber diameter. Contrary to control PCL fibrous scaffolds that were devoid of bonelike apatite particles, incubating PCL/BG hybrid fibrous scaffolds in simulated body fluid (SBF) revealed bonelike apatite deposition. Osteoblast cells cultured on PCL/BG hybrid fibrous scaffolds spread with multiple attachments and actively proliferated suggesting that the low temperature in situ sol-gel and electrospinning process did not have a detrimental effect. Targeted bone-associated gene expressions by rat calvarial osteoblasts seeded on these hybrid scaffolds demonstrated remarkable spatiotemporal gene activation. Transcriptional-level gene expressions for alkaline phosphatase (ALP), osteopontin (OPN), bone sialoprotein (BSP), and osteocalcin (OCN) were significantly higher on the hybrid fibrous scaffolds (p < 0.001) that were largely dependent on fiber diameter compared. Taken together, our results suggest that PCL/BG fibrous scaffolds may accelerate bone formation by providing a favorable microenvironment.

Single-crystal apatite nanowires sheathed in graphitic shells: synthesis, characterization, and application

Vertically aligned one-dimensional hybrid structures, which are composed of apatite and graphitic structures, can be beneficial for orthopedic applications. However, they are difficult to generate using the current method. Here, we report the first synthesis of a single-crystal apatite nanowire encapsulated in graphitic shells by a one-step chemical vapor deposition. Incipient nucleation of apatite and its subsequent transformation to an oriented crystal are directed by derived gaseous phosphorine. Longitudinal growth of the oriented apatite crystal is achieved by a vapor-solid growth mechanism, whereas lateral growth is suppressed by the graphitic layers formed through arrangement of the derived aromatic hydrocarbon molecules. We show that this unusual combination of the apatite crystal and the graphitic shells can lead to an excellent osteogenic differentiation and bony fusion through a programmed smart behavior. For instance, the graphitic shells are degraded after the initial cell growth promoted by the graphitic nanostructures, and the cells continue proliferation on the bare apatite nanowires. Furthermore, a bending experiment indicates that such core-shell nanowires exhibited a superior bending stiffness compared to single-crystal apatite nanowires without graphitic shells. The results suggest a new strategy and direction for bone grafting materials with a highly controllable morphology and material conditions that can best stimulate bone cell differentiation and growth.

A structural and functional model for human bone sialoprotein

Human bone sialoprotein (BSP) is an essential component of the extracellular matrix of bone. It is thought to be the primary nucleator of hydroxyapatite crystallization, and is known to bind to hydroxyapatite, collagen, and cells. Mature BSP shows extensive post-translational modifications, including attachment of glycans, sulfation, and phosphorylation, and is highly flexible with no specific 2D or 3D structure in solution or the solid state. These features have severely limited the experimental characterization of the structure of this protein. We have therefore developed a 3D structural model for BSP, based on the available literature data, using molecular modelling techniques. The complete model consists of 301 amino acids, including six phosphorylated serines and two sulfated tyrosines, plus 92 N- and O-linked glycan residues. A notable feature of the model is a large acidic patch that provides a surface for binding Ca(2+) ions. Density functional theory quantum calculations with an implicit solvent model indicate that Ca(2+) ions are bound most strongly by the phosphorylated serines within BSP, along with reasonably strong binding to Asp and Glu, but weak binding to His and sulfated tyrosine. The process of early hydroxyapatite nucleation has been studied by molecular dynamics on an acidic surface loop of the protein; the results suggest that the cationic nature of the loop promotes nucleation by attracting Ca(2+) ions, while its flexibility allows for their rapid self-assembly with PO(4)(3-) ions, rather than providing a regular template for crystallization. The binding of a hydroxyapatite crystal at the protein's acidic patch has also been modelled. The relationships between hydroxyapatite, collagen and BSP are discussed.

A review of protein adsorption on bioceramics

Bioceramics, because of its excellent biocompatible and mechanical properties, has always been considered as the most promising materials for hard tissue repair. It is well know that an appropriate cellular response to bioceramics surfaces is essential for tissue regeneration and integration. As the in vivo implants, the implanted bioceramics are immediately coated with proteins from blood and body fluids, and it is through this coated layer that cells sense and respond to foreign implants. Hence, the adsorption of proteins is critical within the sequence of biological activities. However, the biological mechanisms of the interactions of bioceramics and proteins are still not well understood. In this review, we will recapitulate the recent studies on the bioceramic-protein interactions.

Prediction of calcite morphology from computational and experimental studies of mutations of a de novo-designed peptide

Many organisms use macromolecules, often proteins or peptides, to control the growth of inorganic crystals into complex materials. The ability to model peptide-mineral interactions accurately could allow for the design of novel peptides to produce materials with desired properties. Here, we tested a computational algorithm developed to predict the structure of peptides on mineral surfaces. Using this algorithm, we analyzed energetic and structural differences between a 16-residue peptide (bap4) designed to interact with a calcite growth plane and single- and double-point mutations of the charged residues. Currently, no experimental method is available to resolve the structures of proteins on solid surfaces, which precludes benchmarking for computational models. Therefore, to test the models, we chemically synthesized each peptide and analyzed its effects on calcite crystal growth. Whereas bap4 affected the crystal growth by producing heavily stepped corners and edges, point mutants had variable influences on morphology. Calculated residue-specific binding energies correlated with experimental observations; point mutations of residues predicted to be crucial to surface interactions produced morphologies most similar to unmodified calcite. These results suggest that peptide conformation plays a role in mineral interactions and that the computational model supplies valid energetic and structural data that can provide information about expected crystal morphology.

AFM study of morphology and mechanical properties of a chimeric spider silk and bone sialoprotein protein for bone regeneration

Atomic force microscopy (AFM) was used to assess a new chimeric protein consisting of a fusion protein of the consensus repeat for Nephila clavipes spider dragline protein and bone sialoprotein (6mer+BSP). The elastic modulus of this protein in film form was assessed through force curves, and film surface roughness was also determined. The results showed a significant difference among the elastic modulus of the chimeric silk protein, 6mer+BSP, and control films consisting of only the silk component (6mer). The behavior of the 6mer+BSP and 6mer proteins in aqueous solution in the presence of calcium (Ca) ions was also assessed to determine interactions between the inorganic and organic components related to bone interactions, anchoring, and biomaterial network formation. The results demonstrated the formation of protein networks in the presence of Ca(2+) ions, characteristics that may be important in the context of controlling materials assembly and properties related to bone formation with this new chimeric silk-BSP protein.