The influence of fluoride on the adsorption of proteoglycans and glycosaminoglycans to hydroxyapatite

The effect of water on the binding of glycosaminoglycan saccharides to hydroxyapatite surfaces: a molecular dynamics study

Classical molecular dynamics (MD) simulations have been employed to study the interaction of the saccharides glucuronic acid (GlcA) and N-acetylgalactosamine (GalNAc) with the (0001) and (011̄0) surfaces of the mineral hydroxyapatite (HAP). GlcA and GalNAc are the two constituent monosaccharides of the glycosaminoglycan chondroitin sulfate, which is commonly found in bone and cartilage and has been implicated in the modulation of the hydroxyapatite biomineralization process. MD simulations of the mineral surfaces and the saccharides in the presence of solvent water allowed the calculation of the adsorption energies of the saccharides on the HAP surfaces. The calculations show that GalNAc interacts with HAP principally through the sulfate and the carbonyl of acetyl amine groups, whereas the GlcA interacts primarily through the carboxylate functional groups. The mode and strength of the interaction depends on the orientation of the saccharide with respect to the surface and the level of disruption of the layer of water competing with the saccharide for adsorption sites on the HAP surface, suggesting that chondroitin 4-sulfate binds to the layer of solvent water rather than to HAP.

Binding of glycosaminoglycan saccharides to hydroxyapatite surfaces: A density functional theory study

Density functional theory calculations implemented by the SIESTA code are used to study the interactions of the saccharides N-acetylgalactosamine (GalNAc) and glucuronic acid (GlcA) with the (0001) and [Formula: see text] surfaces of the mineral hydroxyapatite (HAP). GalNAc and GlcA are the constituent monosaccharides of chondroitin, which is a glycosaminoglycan found in bone and cartilage, and whose interactions with HAP have been implicated as a controlling factor in the process of biomineralisation. Geometry optimisation calculations are used to identify low energy adsorption structures of the monosaccharides on the HAP surfaces, and to calculate the corresponding adsorption energies. The calculations show that GalNAc interacts with HAP principally through its hydroxy and acetyl amine functional groups, and deprotonated GlcA interacts principally through its hydroxy and carboxylate functional groups. The mode and strength of adsorption depends on the orientation of the saccharide with respect to the HAP surface, which has implications for the structural conformation of chondroitin chains in the presence of hydroxyapatite. Both monosaccharides bind more strongly to the [Formula: see text] surface than to the (0001) surface.

Altered cell motility and attachment with titanium surface modifications

BACKGROUND

Titanium implants are widely used in dentistry to replace lost teeth. Various surface modifications have been used to improve implant retention and osseointegration. This study is designed to compare the ability of three titanium surfaces to promote cell attachment and cell motility of cells relevant to periodontal tissues.

METHODS

Three clinically relevant surfaces were tested: 1) machined titanium; 2) a titanium surface roughened through acid etching (dual thermal-etched titanium [DTET]); and 3) a titanium surface roughened with nanometer-scale calcium phosphate deposition (nanoscale calcium phosphate-impregnated titanium [NCPIT]). Cell attachment and migration were examined for four cell types: rat osteosarcoma cells, human osteoblasts, and gingival and periodontal ligament (PDL) fibroblasts.

RESULTS

All four cell types attached to each of the three titanium surfaces equally by 2 hours, and the PDL and gingival fibroblasts generally displayed less attachment than the osteosarcoma cells and osteoblasts. The cells displayed differential motility and long-term attachment to each of the titanium surfaces. Osteosarcoma cells displayed preferential motility on NCPIT, whereas PDL fibroblasts were more motile on machined titanium, and gingival fibroblasts moved more rapidly on both DTET and NCPIT. Osteoblasts displayed little motility on any of the titanium surfaces and lost viability on NCPIT after 24 hours. Gingival fibroblasts lost attachment to machined titanium.

CONCLUSIONS

Periodontal cells displayed differential motility and long-term attachment to titanium surfaces. Selective modification of titanium surface properties in various regions of an implant may be useful in guiding specific cell populations to specific locations where they might best aid in osseointegration and soft tissue remodeling.

Towards a model of the mineral-organic interface in bone: NMR of the structure of synthetic glycosaminoglycan- and polyaspartate-calcium phosphate composites

We have synthesised three materials-chondroitin sulphate (ChS, a commercial product derived from shark cartilage and predominantly chondroitin-6-sulphate (Ch-6-S)) bound to pre-formed hydroxyapatite (HA, Ca(10)(PO(4))(6)(OH)(2)), HA formed in the presence of ChS and poly-Asp bound to pre-formed HA-to generate models for the mineral-organic interface in bone. The three materials have been investigated by (13)C cross polarisation magic-angle spinning (CPMAS) NMR, (13)C{(31)P} rotational echo double resonance (REDOR) and powder x-ray diffraction (XRD) in order to verify their composition and to determine the nature of their binding to HA. Our results show that for HA formed in the presence of Ch-6-S, all carbon atoms in the Ch-6-S having contact with mineral phosphate. We propose that HA in this case forms all around the Ch-6-S polymer rather than along one face of it as is more commonly supposed in cases of templating by organic molecules. However, Ch-6-S binding to pre-formed HA probably occurs via a surface layer of water on the mineral rather than to the mineral directly. In contrast, poly-Asp binds closely to the pre-formed HA surface and so is clearly able to displace at least some of the surface-bound water.

Kinetics and Template Nucleation of Self-Assembled Hydroxyapatite Nanocrystallites by Chondroitin Sulfate

Biomineralization is an important process, which is often assisted by biomolecules. In this paper, the effect of chondroitin sulfate on the crystallization of hydroxyapatite was examined quantitatively based on a generic heterogeneous nucleation model. It is found that chondroitin sulfate can suppress the supersaturation-driven interfacial structure mismatch between the hydroxyapatite crystal and the substrate and promote the formation of ordered hydroxyapatite nanocrystallite assemblies. The nucleation mechanism of self-aligned hydroxyapatite nanocrystallites was examined from the viewpoints of kinetics and interfacial structure and properties, which contributes to an understanding of the fundamentals of biomineralization of self-assembled structures. The results obtained from this study will provide a basic principle to design and fabricate highly orderly organic-inorganic hybrid materials.

Dentinal proteoglycans demonstrate an increasing order of affinity for hydroxyapatite crystals during the transition of predentine to dentine

The transition from an unmineralized predentine to a mineralized dentine involves a variety of molecular extracellular matrix interactions and protein degradation events. Previous studies have identified that different pools of proteoglycan (PG) species are present within the matrix of the predentine, the transitional phase at the predentine-dentine border, and the mineralized dentine. These PGs alter with respect to the chemical nature of the glycosaminoglycan (GAG) chain and as a result of extracellular processing of the macromolecule in the matrix. This study has examined the hydroxyapatite (HAP) binding affinity of the PGs isolated from these phases and the influence of the attached GAG chains upon their binding characteristics. PGs isolated from the three phases were characterized to contain a mixture of decorin and biglycan, substituted with chondroitin sulfate GAG chain(s). Maximal binding for dentine PGs onto HAP was achieved at 15.60 microg/ml protein and for predentine-dentine interface PGs at 0.125 mg/ml. A significantly increasing gradient of affinity was observed moving toward dentine, with dentine PGs exhibiting 19 times greater binding affinity for HAP than predentine PGs and 7.5 times greater affinity than predentine-dentine interface PGs. Removal of the GAG chains from dentine PGs significantly reduced binding affinity for HAP but did not influence the number of binding sites. The difference in binding ability observed for the different PG pools gives further support for the involvement of these macromolecules in regulating the transition from predentine to dentine and suggests key roles for the GAG chains in the mineralisation process.

The effect of fluoride on apatite structure and growth

Fluoride participates in many aspects of calcium phosphate formation in vivo and has enormous effects on the process and on the nature and properties of formed mineral. The most well-documented effect of fluoride is that this ion substitutes for a column hydroxyl in the apatite structure, giving rise to a reduction of crystal volume and a concomitant increase in structural stability. In the process of enamel mineralization during amelogenesis (a unique model for the cell-mediated formation of well-crystallized carbonatoapatite), free fluoride ions in the fluid phase are supposed to accelerate the hydrolysis of acidic precursor(s) and increase the driving force for the growth of apatitic mineral. Once fluoride is incorporated into the enamel mineral, the ion likely affects the subsequent mineralization process by reducing the solubility of the mineral and thereby modulating the ionic composition in the fluid surrounding the mineral, and enhancing the matrix protein-mineral interaction. But excess fluoride leads to anomalous enamel formation by retarding tissue maturation. It is worth noting that enameloid/enamel minerals found in vertebrate teeth have a wide range of CO3 and fluoride substitutions. In the evolutionary process from elasmobranch through enameloid to mammalian enamel, the biosystems appear to develop regulatory functions for limiting the fluoridation of the formed mineral, but this development is accompanied by an increase of carbonate substitution or defects in the mineral. In research on the cariostatic effect of fluoride, considerable emphasis is placed on the roles of free fluoride ions (i.e., preventing the dissolution and accelerating the kinetics of remineralization) in the oral fluid bathing tooth mineral. Fluoride also has been used for the treatment of osteoporosis, but much still remains to be learned about maximizing the benefit and minimizing the risk of fluoride when used as a public health measure.