Analysis of hydroxyapatite surface coverage by amelogenin nanospheres following the Langmuir model for protein adsorption

Adsorption of Biomineralization Protein Mms6 on Magnetite (Fe3O4) Nanoparticles

Biomineralization is an elaborate process that controls the deposition of inorganic materials in living organisms with the aid of associated proteins. Magnetotactic bacteria mineralize magnetite (Fe₃O₄) nanoparticles with finely tuned morphologies in their cells. Mms6, a magnetosome membrane specific (Mms) protein isolated from the surfaces of bacterial magnetite nanoparticles, plays an important role in regulating the magnetite crystal morphology. Although the binding ability of Mms6 to magnetite nanoparticles has been speculated, the interactions between Mms6 and magnetite crystals have not been elucidated thus far. Here, we show a direct adsorption ability of Mms6 on magnetite nanoparticles in vitro. An adsorption isotherm indicates that Mms6 has a high adsorption affinity (Kd = 9.52 µM) to magnetite nanoparticles. In addition, Mms6 also demonstrated adsorption on other inorganic nanoparticles such as titanium oxide, zinc oxide, and hydroxyapatite. Therefore, Mms6 can potentially be utilized for the bioconjugation of functional proteins to inorganic material surfaces to modulate inorganic nanoparticles for biomedical and medicinal applications.

La3+ and F- dual-doped multifunctional hydroxyapatite nanoparticles: Synthesis and characterization

Hydroxyapatite (HA) co-doped with La³⁺ and F^- ions were synthesized by the precipitation method and sintered at 1,100°C for 1 hr. Samples were characterized by the standard experimental methods including the density, X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), and Scanning Electron Microscopy (SEM) to investigate their microstructure, phase formation, and bonding characteristics in detail. Moreover, the materials produced were identified using the microhardness tests. It was observed that in the most of materials, the hydroxyapatite was found to be the main phase with a minor amount of β-tricalcium phosphate (β-TCP). Furthermore, the presence of fluoride and small amount of β-TCP was verified with all the characteristic FTIR bands of hydroxyapatite for the majority of samples studied. The result in SEM evaluation is that the produced HA powders have less deformed, uniformly distributed, and regularly shaped particles. Here, the material density has changed towards a less dense state with the increasing rate of La doping, but statistically significant difference was not obtained (p, .1942 > .05) with increase of the F doping. A significant difference was obtained the microhardness values between La³⁺ and F^- ions co-doped HA materials and pure HA (p [.0053] < .05). Accordingly, this study confirmed that since the La³⁺ and F^- ions can potentially increase the efficacy of HA. According to the spectral, mechanical, and microstructure analysis result, this material can be as a good candidate product for use as an occluding material for dental application.

Controls of nature: Secondary, tertiary, and quaternary structure of the enamel protein amelogenin in solution and on hydroxyapatite

Amelogenin, a protein critical to enamel formation, is presented as a model for understanding how the structure of biomineralization proteins orchestrate biomineral formation. Amelogenin is the predominant biomineralization protein in the early stages of enamel formation and contributes to the controlled formation of hydroxyapatite (HAP) enamel crystals. The resulting enamel mineral is one of the hardest tissues in the human body and one of the hardest biominerals in nature. Structural studies have been hindered by the lack of techniques to evaluate surface adsorbed proteins and by amelogenin's disposition to self-assemble. Recent advancements in solution and solid state nuclear magnetic resonance (NMR) spectroscopy, atomic force microscopy (AFM), and recombinant isotope labeling strategies are now enabling detailed structural studies. These recent studies, coupled with insights from techniques such as CD and IR spectroscopy and computational methodologies, are contributing to important advancements in our structural understanding of amelogenesis. In this review we focus on recent advances in solution and solid state NMR spectroscopy and in situ AFM that reveal new insights into the secondary, tertiary, and quaternary structure of amelogenin by itself and in contact with HAP. These studies have increased our understanding of the interface between amelogenin and HAP and how amelogenin controls enamel formation.

Unraveling the mechanism for an amelogenin-derived peptide regulated hydroxyapatite mineralization via specific functional domain identification

Amelogenin and its various derived peptides play important roles in promoting biomimetic mineralization of enamel. Previously, an amelogenin-derived peptide named QP5 was proved to be able to repair demineralized enamel. The objective here was to interpret the mechanism of QP5 by elucidating the specific function of each domain for further sequence and efficacy improvement. Peptide QP5 was separated into domains (QPX)5 and C-tail. (QPX)3 was also synthesized to investigate how QPX repeats affect the mineralization process. Circular dichroism spectroscopy showed that two (QPX) repeats adopted a β-sheet structure, while C-tail exhibited a disordered structure. (QPX)5 showed more absorption in confocal laser scanning microscopy observation and a higher K value in Langmuir adsorption isotherms compared to C-tail, while (QPX)3 with better hydropathy had greater adsorption capability than (QPX)5. Meanwhile, calcium consumption kinetics, transmission electron microscopy and selected area electron diffraction indicated that (QPX)5, C-tail and (QPX)3 had similar inhibitory effects on the spontaneous calcium consumption and the morphology of their nucleation products were alike, while QP5 had a greater inhibitory effect than them and induced elongated plate-like crystals. X-Ray diffraction further showed that both C-tail and (QPX)3 had greater potential in improving the apatite crystal orientation degree. In conclusion, (QPX)5 was the major adsorption region, both (QPX)5 and C-tail inhibited the nucleation, and C-tail contributed more to improve the HAP orientation degree, so QP5 could exert a significant remineralization effect. By reducing two repeats, (QPX)3 showed higher hydropathicity than (QPX)5 and achieved higher binding affinity, and it was more potential in improving the HAP orientation degree with lower economic cost.

The energetic basis for hydroxyapatite mineralization by amelogenin variants provides insights into the origin of amelogenesis imperfecta

Small variations in the primary amino acid sequence of extracellular matrix proteins can have profound effects on the biomineralization of hard tissues. For example, a change in one amino acid within the amelogenin protein can lead to drastic changes in enamel phenotype, resulting in amelogenesis imperfecta, enamel that is defective and easily damaged. Despite the importance of these undesirable phenotypes, there is very little understanding of how single amino acid variation in amelogenins can lead to malformed enamel. Here, we aim to develop a thermodynamic understanding of how protein variants can affect steps of the biomineralization process. High-resolution, in situ atomic force microscopy (AFM) showed that altering one amino acid within the murine amelogenin sequence (natural variants T21 and P41T, and experimental variant P71T) resulted in an increase in the quantity of protein adsorbed onto hydroxyapatite (HAP) and the formation of multiple protein layers. Quantitative analysis of the equilibrium adsorbate amounts revealed that the protein variants had higher oligomer-oligomer binding energies. MMP20 enzyme degradation and HAP mineralization studies showed that the amino acid variants slowed the degradation of amelogenin by MMP20 and inhibited the growth and phase transformation of HAP. We propose that the protein variants cause malformed enamel because they bind excessively to HAP and disrupt the normal HAP growth and enzymatic degradation processes. The in situ methods applied to determine the energetics of molecular level processes are powerful tools toward understanding the mechanisms of biomineralization.

The innovative applications of therapeutic nanostructures in dentistry

Nanotechnology has paved multiple ways in preventing, reversing or restoring dental caries which is one of the major health care problems. Nanotechnology aided in processing variety of nanomaterials with innovative dental applications. Some showed antimicrobial effect helping in the preventive stage. Others have remineralizing potential intercepting early lesion progression as nanosized calcium phosphate, carbonate hydroxyapatite nanocrystals, nanoamorphous calcium phosphate and nanoparticulate bioactive glass particularly with provision of self-assembles protein that furnish essential role in biomimetic repair. The unique size of nanomaterials makes them fascinating carriers for dental products. Thus, it is recentlyclaimedthat fortifying the adhesives with nanomaterials that possess biological meritsdoes not only enhance the mechanical and physical properties of the adhesives, but also help to attain and maintain a durable adhesive joint and enhanced longevity. Accordingly, this review will focus on the current status and the future implications of nanotechnology in preventive and adhesive dentistry.

Clinoptilolite/PCL–PEG–PCL composite scaffolds for bone tissue engineering applications

The aim of this study was to prepare and characterize highly porous clinoptilolite/poly(ε-caprolactone)-poly(ethylene glycol)-poly(ε-caprolactone) composite scaffolds. Scaffolds with different clinoptilolite contents (10% and 20%) were fabricated with reproducible solvent-free powder compression/particulate leaching technique. The scaffolds had interconnective porosity in the range of 55–76%. Clinoptilolite/poly(ε-caprolactone)-poly(ethylene glycol)-poly(ε-caprolactone) scaffolds showed negligible degradation within eight weeks and displayed less water uptake and higher bioactivity than poly(ε-caprolactone)-poly(ethylene glycol)-poly(ε-caprolactone) scaffolds. The presence of clinoptilolite improved the mechanical properties. Highest compressive strength (5.6 MPa) and modulus (114.84 MPa) were reached with scaffold group containing 20% clinoptilolite. In vitro protein adsorption capacity of the scaffolds was also higher for clinoptilolite/poly(ε-caprolactone)-poly(ethylene glycol)-poly(ε-caprolactone) scaffolds. These scaffolds had 0.95 mg protein/g scaffold adsorption capacity and also higher osteoinductivity in terms of enhanced ALP, OSP activities and intracellular calcium deposition. Stoichiometric apatite deposition (Ca/P=1.686) was observed during cellular proliferation analysis with human fetal osteoblasts cells. Thus, it can be suggested that clinoptilolite/poly(ε-caprolactone)-poly(ethylene glycol)-poly(ε-caprolactone) composite scaffolds could be promising carriers for enhancement of bone regeneration in bone tissue engineering applications.

Dynamic interactions of amelogenin with hydroxyapatite surfaces are dependent on protein phosphorylation and solution pH

Amelogenin, the predominant extracellular matrix protein secreted by ameloblasts, has been shown to be essential for proper tooth enamel formation. In this study, amelogenin adsorption to hydroxyapatite (HAP) surfaces, a prototype for enamel mineral, has been studied using a quartz crystal microbalance (QCM) to interrogate effects of protein phosphorylation and solution pH. Dynamic flow-based experiments were conducted at pH 7.4 and 8.0 using native phosphorylated porcine amelogenin (P173) and recombinant non-phosphorylated porcine amelogenin (rP172). Loading capacities (μmol/m²) on HAP surfaces were calculated under all conditions and adsorption affinities (K_ad) were calculated when Langmuir isotherm conditions appeared to be met. At pH 8.0, binding characteristics were remarkably similar for the two proteins. However, at pH 7.4 a higher affinity and lower surface loading for the phosphorylated P173 was found compared to any other set of conditions. This suggests that phosphorylated P173 adopts a more extended conformation than non-phosphorylated full-length amelogenin, occupying a larger footprint on the HAP surface. This surface-induced structural difference may help explain why P173 is a more effective inhibitor of spontaneous HAP formation in vitro than rP172. Differences in the viscoelastic properties of P173 and rP172 in the adsorbed state were also observed, consistent with noted differences in HAP binding. These collective findings provide new insight into the important role of amelogenin phosphorylation in the mechanism by which amelogenin regulates enamel crystal formation.

Sequence-Defined Energetic Shifts Control the Disassembly Kinetics and Microstructure of Amelogenin Adsorbed onto Hydroxyapatite (100)

The interactions between proteins and surfaces are critical to a number of important processes including biomineralization, the biocompatibility of biomaterials, and the function of biosensors. Although many proteins exist as monomers or small oligomers, amelogenin is a unique protein that self-assembles into supramolecular structures called "nanospheres," aggregates of hundreds of monomers that are 20-60 nm in diameter. The nanosphere quaternary structure is observed in solution; however, the quaternary structure of amelogenin adsorbed onto hydroxyapatite (HAP) surfaces is not known even though it may be important to amelogenin's function in forming highly elongated and intricately assembled HAP crystallites during enamel formation. We report studies of the interactions of the enamel protein, amelogenin (rpM179), with a well-defined (100) face prepared by the synthesis of large crystals of HAP. High-resolution in situ atomic force microscopy (AFM) was used to directly observe protein adsorption onto HAP at the molecular level within an aqueous solution environment. Our study shows that the amelogenin nanospheres disassemble onto the HAP surface, breaking down into oligomeric (25-mer) subunits of the larger nanosphere. In some cases, the disassembly event is directly observed by in situ imaging for the first time. Quantification of the adsorbate amounts by size analysis led to the determination of a protein binding energy (17.1k(b)T) to a specific face of HAP (100). The kinetics of disassembly are greatly slowed in aged solutions, indicating that there are time-dependent increases in oligomer-oligomer binding interactions within the nanosphere. A small change in the sequence of amelogenin by the attachment of a histidine tag to the N-terminus of rpM179 to form rp(H)M180 results in the adsorption of a complete second layer on top of the underlying first layer. Our research elucidates how supramolecular protein structures interact and break down at surfaces and how small changes in the primary sequence of amelogenin can affect the disassembly process.

Solid-state NMR studies of proteins immobilized on inorganic surfaces

Solid state NMR is the primary tool for studying the quantitative, site-specific structure, orientation, and dynamics of biomineralization proteins under biologically relevant conditions. Two calcium phosphate proteins, statherin (43 amino acids) and leucine rich amelogenin protein (LRAP; 59 amino acids), have been studied in depth and have different dynamic properties and 2D- and 3D-structural features. These differences make it difficult to extract design principles used in nature for building materials with properties such as high strength, unusual morphologies, or uncommon phases. Consequently, design principles needed for developing synthetic materials controlled by proteins are not clear. Many biomineralization proteins are much larger than statherin and LRAP, necessitating the study of larger biomineralization proteins. More recent studies of the significantly larger full-length amelogenin (180 residues) represent a significant step forward to ultimately investigate the full diversity of biomineralization proteins. Interactions of amino acids, a silaffin derived peptide, and the model LK peptide with silica are also being studied, along with qualitative studies of the organic matrices interacting with calcium carbonate. Dipolar recoupling techniques have formed the core of the quantitative studies, yet the need for isolated spin pairs makes this approach costly and time intensive. The use of multi-dimensional techniques to study biomineralization proteins is becoming more common, methodology which, despite its challenges with these difficult-to-study proteins, will continue to drive future advancements in this area.

Tooth enamel protein amelogenin binds to ameloblast cell membrane-mimicking vesicles via its N-terminus

We have recently reported that the extracellular enamel protein amelogenin has affinity to interact with phospholipids and proposed that such interactions may play key roles in enamel biomineralization as well as reported amelogenin signaling activities. Here, in order to identify the liposome-interacting domains of amelogenin we designed four different amelogenin mutants containing only a single tryptophan at positions 25, 45, 112 and 161. Circular dichroism studies of the mutants confirmed that they are structurally similar to the wild-type amelogenin. Utilizing the intrinsic fluorescence of single tryptophan residue and fluorescence resonance energy transfer [FRET], we analyzed the accessibility and strength of their binding with an ameloblast cell membrane-mimicking model membrane (ACML) and a negatively charged liposome used as a membrane model. We found that amelogenin has membrane-binding ability mainly via its N-terminal, close to residues W25 and W45. Significant blue shift was also observed in the fluorescence of a N-terminal peptide following addition of liposomes. We suggest that, among other mechanisms, enamel malformation in cases of Amelogenesis Imperfecta (AI) with mutations at the N-terminal may be the result of defective amelogenin-cell interactions.

Amelogenin and Enamel Biomimetics

Mature tooth enamel is acellular and does not regenerate itself. Developing technologies that rebuild tooth enamel and preserve tooth structure is therefore of great interest. Considering the importance of amelogenin protein in dental enamel formation, its ability to control apatite mineralization in vitro, and its potential to be applied in fabrication of future bio-inspired dental material this review focuses on two major subjects: amelogenin and enamel biomimetics. We review the most recent findings on amelogenin secondary and tertiary structural properties with a focus on its interactions with different targets including other enamel proteins, apatite mineral, and phospholipids. Following a brief overview of enamel hierarchical structure and its mechanical properties we will present the state-of-the-art strategies in the biomimetic reconstruction of human enamel.

The enamel protein amelotin is a promoter of hydroxyapatite mineralization

Amelotin (AMTN) is a recently discovered protein that is specifically expressed during the maturation stage of dental enamel formation. It is localized at the interface between the enamel surface and the apical surface of ameloblasts. AMTN knock-out mice have hypomineralized enamel, whereas transgenic mice overexpressing AMTN have a compact but disorganized enamel hydroxyapatite (HA) microstructure, indicating a possible involvement of AMTN in regulating HA mineralization directly. In this study, we demonstrated that recombinant human (rh) AMTN dissolved in a metastable buffer system, based on light scattering measurements, promotes HA precipitation. The mineral precipitates were characterized by scanning and transmission electron microscopy and electron diffraction. Colloidal gold immunolabeling of AMTN in the mineral deposits showed that protein molecules were associated with HA crystals. The binding affinity of rh-AMTN to HA was found to be comparable to that of amelogenin, the major protein of the forming enamel matrix. Overexpression of AMTN in mouse calvaria cells also increased the formation of calcium deposits in the culture medium. Overexpression of AMTN during the secretory stage of enamel formation in vivo resulted in rapid and uncontrolled enamel mineralization. Site-specific mutagenesis of the potential serine phosphorylation motif SSEEL reduced the in vitro mineral precipitation to less than 25%, revealing that this motif is important for the HA mineralizing function of the protein. A synthetic short peptide containing the SSEEL motif was only able to facilitate mineralization in its phosphorylated form ((P)S(P) SEEL), indicating that this motif is necessary but not sufficient for the mineralizing properties of AMTN. These findings demonstrate that AMTN has a direct influence on biomineralization by promoting HA mineralization and suggest a critical role for AMTN in the formation of the compact aprismatic enamel surface layer during the maturation stage of amelogenesis.

Kinetic analysis of the weak affinity interaction between tris and lysozyme

The biosensor based on total internal reflection imaging ellipsometry (TIRIE), regarded as an automotive real-time research approach for biomolecular interaction, is introduced to analyze the kinetic process of the weak interaction between tris and lysozyme. The experiment is performed by delivering lysozyme solution diluted to different concentrations to the biosensor substrate interface immobilized with tris. By applying pseudo-first-order interaction kinetics model, we are able to obtain the kinetic parameters from fitting experimental data. The calculated association rate constant and dissociation rate constant of tris and lysozyme interaction are in 10(-2) mol(-1) s(-1) and 10(3)s(-1) magnitude, respectively. To further improve TIRIE's ability for kinetically characterizing biomolecular interaction, a theoretical method to deduce associate rate constant before experiment is proposed.

Amelogenin and enamel biomimetics

Mature tooth enamel is acellular and does not regenerate itself.

New developments in polymer-controlled, bioinspired calcium phosphate mineralization from aqueous solution

The polymer-controlled and bioinspired precipitation of inorganic minerals from aqueous solution at near-ambient or physiological conditions avoiding high temperatures or organic solvents is a key research area in materials science. Polymer-controlled mineralization has been studied as a model for biomineralization and for the synthesis of (bioinspired and biocompatible) hybrid materials for a virtually unlimited number of applications. Calcium phosphate mineralization is of particular interest for bone and dental repair. Numerous studies have therefore addressed the mineralization of calcium phosphate using a wide variety of low- and high-molecular-weight additives. In spite of the growing interest and increasing number of experimental and theoretical data, the mechanisms of polymer-controlled calcium phosphate mineralization are not entirely clear to date, although the field has made significant progress in the last years. A set of elegant experiments and calculations has shed light on some details of mineral formation, but it is currently not possible to preprogram a mineralization reaction to yield a desired product for a specific application. The current article therefore summarizes and discusses the influence of (macro)molecular entities such as polymers, peptides, proteins and gels on biomimetic calcium phosphate mineralization from aqueous solution. It focuses on strategies to tune the kinetics, morphologies, final dimensions and crystal phases of calcium phosphate, as well as on mechanistic considerations.

Structural changes in amelogenin upon self-assembly and mineral interactions

Amelogenin, the major protein of forming dental enamel, plays a crucial role in the biomineralization of this tissue. Amelogenin is soluble at low pH and self-assembles to form higher order structures at physiological pH. To understand the mechanisms of its assembly and interactions with calcium phosphate mineral, we conducted FTIR spectroscopy (FTIRS) studies of pH-triggered assembly of recombinant porcine amelogenin rP172 and its interactions with mature hydroxyapatite and apatitic mineral formed in situ. Analysis of our data indicated that rP172 at pH 3.0 exists in an unfolded disordered state, while increases in pH led to structural ordering, manifested by increases in intra- and intermolecular β-sheet structures and a decrease in random coil and β-turns. Amelogenin assembled at pH 7.2 was also found to contain large portions of extended intramolecular β-sheet and PPII. These FTIRS findings are consistent with those previously obtained with other techniques, thus verifying the validity of our experimental approach. Interestingly, interactions with mineral led to a reduction in protein structural organization. The findings obtained show that amelogenin has intrinsic structural flexibility to accommodate interactions with both forming and mature calcium phosphate mineral phases, providing new insights into the potential importance of amelogenin-mineral interactions in enamel biomineralization.

Protein-mediated enamel mineralization

Enamel is a hard nanocomposite bioceramic with significant resilience that protects the mammalian tooth from external physical and chemical damages. The remarkable mechanical properties of enamel are associated with its hierarchical structural organization and its thorough connection with underlying dentin. This dynamic mineralizing system offers scientists a wealth of information that allows the study of basic principels of organic matrix-mediated biomineralization and can potentially be utilized in the fields of material science and engineering for development and design of biomimetic materials. This chapter will provide a brief overview of enamel hierarchical structure and properties and the process and stages of amelogenesis. Particular emphasis is given to current knowledge of extracellular matrix protein and proteinases, and the structural chemistry of the matrix components and their putative functions. The chapter will conclude by discussing the potential of enamel for regrowth.

Enamel matrix derivative: a review of cellular effects in vitro and a model of molecular arrangement and functioning

BACKGROUND

Enamel matrix derivative (EMD), the active component of Emdogain®, is a viable option in the treatment of periodontal disease owing to its ability to regenerate lost tissue. It is believed to mimic odontogenesis, though the details of its functioning remain the focus of current research.

OBJECTIVE

The aim of this article is to review all relevant literature reporting on the composition/characterization of EMD as well as the effects of EMD, and its components amelogenin and ameloblastin, on the behavior of various cell types in vitro. In this way, insight into the underlying mechanism of regeneration will be garnered and utilized to propose a model for the molecular arrangement and functioning of EMD.

METHODS

A review of in vitro studies of EMD, or components of EMD, was performed using key words "enamel matrix proteins" OR "EMD" OR "Emdogain" OR "amelogenin" OR "ameloblastin" OR "sheath proteins" AND "cells." Results of this analysis, together with current knowledge on the molecular composition of EMD and the structure and regulation of its components, are then used to present a model of EMD functioning.

RESULTS

Characterization of the molecular composition of EMD confirmed that amelogenin proteins, including their enzymatically cleaved and alternatively spliced fragments, dominate the protein complex (>90%). A small presence of ameloblastin has also been reported. Analysis of the effects of EMD indicated that gene expression, protein production, proliferation, and differentiation of various cell types are affected and often enhanced by EMD, particularly for periodontal ligament and osteoblastic cell types. EMD also stimulated angiogenesis. In contrast, EMD had a cytostatic effect on epithelial cells. Full-length amelogenin elicited similar effects to EMD, though to a lesser extent. Both the leucine-rich amelogenin peptide and the ameloblastin peptides demonstrated osteogenic effects. A model for molecular structure and functioning of EMD involving nanosphere formation, aggregation, and dissolution is presented.

CONCLUSIONS

EMD elicits a regenerative response in periodontal tissues that is only partly replicated by amelogenin or ameloblastin components. A synergistic effect among the various proteins and with the cells, as well as a temporal effect, may prove important aspects of the EMD response in vivo.

Dissecting amelogenin protein nanospheres: characterization of metastable oligomers

Amelogenin self-assembles to form an extracellular protein matrix, which serves as a template for the continuously growing enamel apatite crystals. To gain further insight into the molecular mechanism of amelogenin nanosphere formation, we manipulated the interactions between amelogenin monomers by altering pH, temperature, and protein concentration to create isolated metastable amelogenin oligomers. Recombinant porcine amelogenins (rP172 and rP148) and three different mutants containing only a single tryptophan (Trp(161), Trp(45), and Trp(25)) were used. Dynamic light scattering and fluorescence studies demonstrated that oligomers were metastable and in constant equilibrium with monomers. Stable oligomers with an average hydrodynamic radius (R(H)) of 7.5 nm were observed at pH 5.5 between 4 and 10 mg · ml(-1). We did not find any evidence of a significant increase in folding upon self-association of the monomers into oligomers, indicating that they are disordered. Fluorescence experiments with single tryptophan amelogenins revealed that upon oligomerization the C terminus of amelogenin (around residue Trp(161)) is exposed at the surface of the oligomers, whereas the N-terminal region around Trp(25) and Trp(45) is involved in protein-protein interaction. The truncated rP148 formed similar but smaller oligomers, suggesting that the C terminus is not critical for amelogenin oligomerization. We propose a model for nanosphere formation via oligomers, and we predict that nanospheres will break up to form oligomers in mildly acidic environments via histidine protonation. We further suggest that oligomeric structures might be functional components during maturation of enamel apatite.

Effect of proteins on the synthesis and assembly of calcium phosphate nanomaterials

Recent developments on biomineralization and biomaterials have demonstrated that proteins play an important role in the formation of biominerals in the body, which induce or inhibit mineralization of calcium phosphate together with modulation of the mineral phase structure. Many efforts have been made to understand the mechanism of this process and mimic the exquisite structure of biominerals in biomimetic methodologies. This review is focused on recent advances in the synthesis of calcium phosphate materials by taking advantage of protein assemblies. We try to review the examples of templates based on proteins and polypeptides that have been successfully employed to manufacture calcium phosphate nanomaterials.

The leucine rich amelogenin protein (LRAP) adsorbs as monomers or dimers onto surfaces

Amelogenin is believed to be involved in controlling the formation of the highly anisotropic and ordered hydroxyapatite crystallites that form enamel. The adsorption behavior of amelogenin proteins onto substrates is very important because protein-surface interactions are critical to its function. We have previously used LRAP, a splice variant of amelogenin, as a model protein for the full-length amelogenin in solid-state NMR and neutron reflectivity studies at interfaces. In this work, we examined the adsorption behavior of LRAP in greater detail using model self-assembled monolayers containing COOH, CH(3), and NH(2) end groups as substrates. Dynamic light scattering (DLS) experiments indicated that LRAP in phosphate buffered saline and solutions containing low concentrations of calcium and phosphate consisted of aggregates of nanospheres. Null ellipsometry and atomic force microscopy (AFM) were used to study protein adsorption amounts and quaternary structures on the surfaces. Relatively high amounts of adsorption occurred onto the CH(3) and NH(2) surfaces from both buffer solutions. Adsorption was also promoted onto COOH surfaces only when calcium was present in the solutions suggesting an interaction that involves calcium bridging with the negatively charged C-terminus. The ellipsometry and AFM studies revealed that LRAP adsorbed onto the surfaces as small subnanosphere-sized structures such as monomers or dimers. We propose that the monomers/dimers were present in solution even though they were not detected by DLS or that they adsorbed onto the surfaces by disassembling or "shedding" from the nanospheres that are present in solution. This work reveals the importance of small subnanosphere-sized structures of LRAP at interfaces.

A study in situ of the effect of metallo- and serine proteinase inhibitors on the birefringence of the secretory stage enamel organic extracellular matrix

Dental enamel formation occurs extracellularly and establishment of an ordered enamel organic extracellular matrix (ECM) seems to be crucial for proper construction of the enamel mineral phase. Polarizing microscopy shows that the ordered supramolecular structure of the secretory stage enamel organic ECM exhibits strong birefringence. We reported earlier that this birefringence is lost in unfixed specimens, probably due to extensive proteolytic cleavage of enamel proteins. Therefore, we investigated the association between enamel proteinase activities by analyzing the effects of metallo- and serine proteinase inhibitors in situ on the birefringence of the secretory stage enamel organic ECM. Male rats were used in the present study. After sacrifice, distal 10 mm fragments of upper incisors were removed and immersed for 15 h under continuous shaking at 37°C in one of the following solutions: 1) 10 mM Tris, pH 8.0; 150 mM NaCl (negative control, n = 8); 2) 2% paraformaldehyde and 0.5% glutaraldehyde in 0.2 M phosphate-buffered saline (PBS), pH 7.2 (positive control, n = 5); 3) 10 mM Tris, pH 8.0; 150 mM NaCl; 2 mM 1,10-phenanthroline (n = 9); 4) 10 mM Tris, pH 8.0; 150 mM NaCl; 2 mM phenylmethyl-sulfonyl fluoride (PMSF) (n = 8); 5) 10 mM Tris, pH 8.0; 150 mM NaCl; 2 mM 1,10-phenanthroline; 2 mM PMSF (n = 9). Samples then were immersed in fixative solution for 24 h and processed to obtain 5 μm thick longitudinal sections of the secretory stage enamel organic ECM. The sections were immersed in 80% glycerin for 30 min and analyzed by transmitted polarizing light microscopy. 1,10-Phenanthroline (inhibitor of metalloproteinases) and 1,10-phenanthroline + PMSF (inhibitor of serine proteinases) clearly prevented a decrease in the optical retardation of birefringence brightness from the tissue. PMSF alone promoted a slight preservation of the birefringence exhibited by the secretory stage enamel organic ECM. Rapid loss of birefringence in secretory stage enamel organic ECM that is not fixed immediately is caused by enamel proteinases and the activity of metalloproteinases seems to lead to preliminary degradation of the enamel organic ECM, which in turn facilitates subsequent serine proteinase activity.

Specific binding and mineralization of calcified surfaces by small peptides

Several small (<25aa) peptides have been designed based on the sequence of the dentin phosphoprotein, one of the major noncollagenous proteins thought to be involved in the mineralization of the dentin extracellular matrix during tooth development. These peptides, consisting of multiple repeats of the tripeptide aspartate-serine-serine (DSS), bind with high affinity to calcium phosphate compounds and, when immobilized, can recruit calcium phosphate to peptide-derivatized polystyrene beads or to demineralized human dentin surfaces. The affinity of binding to hydroxyapatite surfaces increases with the number of (DSS)(n) repeats, and though similar repeated sequences-(NTT)(n), (DTT)(n), (ETT)(n), (NSS)(n), (ESS)(n), (DAA)(n), (ASS)(n), and (NAA)(n)-also showed HA binding activity, it was generally not at the same level as the natural sequence. Binding of the (DSS)(n) peptides to sectioned human teeth was shown to be tissue-specific, with high levels of binding to the mantle dentin, lower levels of binding to the circumpulpal dentin, and little or no binding to healthy enamel. Phosphorylation of the serines of these peptides was found to affect the avidity, but not the affinity, of binding. The potential utility of these peptides in the detection of carious lesions, the delivery of therapeutic compounds to mineralized tissues, and the modulation of remineralization is discussed.

Adsorption of amelogenin onto self-assembled and fluoroapatite surfaces

The interactions of proteins at surfaces are of great importance to biomineralizaton processes and to the development and function of biomaterials. Amelogenin is a unique biomineralization protein because it self-assembles to form supramolecular structures called "nanospheres", spherical aggregates of monomers that are 20-60 nm in diameter. Although the nanosphere quaternary structure has been observed in solution, the quaternary structure of amelogenin adsorbed onto surfaces is also of great interest because the surface structure is critical to its function. We report studies of the adsorption of the amelogenin onto self-assembled monolayers (SAMs) with COOH and CH(3) end group functionality and single crystal fluoroapatite (FAP). Dynamic light scattering (DLS) experiments showed that the solutions contained nanospheres and aggregates of nanospheres. Protein adsorption onto the various substrates was evidenced by null ellipsometry, X-ray photoelectron spectroscopy (XPS), and external reflectance Fourier transform infrared spectroscopy (ERFTIR). Although only nanospheres were observed in solution, ellipsometry and atomic force microscopy (AFM) indicated that the protein adsorbates were much smaller structures than the original nanospheres, from monomers to small oligomers in size. Monomer adsorption was promoted onto the CH(3) surfaces, and small oligomer adsorption was promoted onto the COOH and FAP substrates. In some cases, remnants of the original nanospheres adsorbed as multilayers on top of the underlying subnanosphere layers. Although the small structures may be present in solution even though they are not detected by DLS, we also propose that amelogenin may adsorb by the "shedding" or disassembling of substructures from the nanospheres onto the substrates. This work suggests that amelogenin may have a range of possible quaternary structures that interact with surfaces.

Changes in the quaternary structure of amelogenin when adsorbed onto surfaces

Amelogenin is a unique protein that self-assembles into spherical aggregates called "nanospheres" and is believed to be involved in controlling the formation of the highly anisotropic and ordered hydroxyapatite crystallites that form enamel. The adsorption behavior of amelogenin onto substrates is of great interest because protein-surface interactions are critical to its function. We report studies of the adsorption of amelogenin onto self-assembled monolayers containing COOH end group functionality as well as single crystal fluoroapatite, a biologically relevant surface. We found that although our solutions contained only nanospheres of narrow size distribution, smaller structures such as dimers or trimers were observed on the hydrophilic surfaces. This suggests that amelogenin can adsorb onto surfaces as small structures that "shed" or disassemble from the nanospheres that are present in solution.

Enamel proteases reduce amelogenin-apatite binding

Organic matrix degradation and crystal maturation are extracellular events that occur simultaneously during enamel biomineralization. We hypothesized that enamel proteases control amelogenin-mineral interaction, which, in turn can affect crystal nucleation, organization, and growth. We used a recombinant amelogenin (rP172), a homolog of its major cleavage product (rP148), and a native amelogenin lacking both N- and C-termini (13k). We compared apatite binding affinity between amelogenins and their digest products during proteolysis. We further compared binding affinity among the 3 amelogenins using a Langmuir model for protein adsorption. Amelogenin-apatite binding affinity was progressively reduced with the proteolysis at the C- and N- termini by recombinant pig MMP-20 (rpMMP20) and recombinant human kallikrein-4 (rhKLK4), respectively. The binding affinity of amelogenin to apatite was found to be in the descending order of rP172, rP148, and 13k. Analysis of our data suggests that, before its complete degradation during enamel maturation, stepwise processing of amelogenin by MMP-20 and then KLK4 reduces amelogenin-apatite interaction.

Anisotropic demineralization and oriented assembly of hydroxyapatite crystals in enamel: smart structures of biominerals

It is interesting to note that the demineralization of natural enamel does not happen as readily as that of the synthesized hydroxyapatite (HAP), although they share a similar chemical composition. We suggest that the hierarchical structure of enamel is an important factor in the preservation of the natural material against dissolution. The anisotropic demineralization of HAP is revealed experimentally, and this phenomenon is understood by the different interfacial structures of HAP-water at the atomic level. It is found that HAP {001} facets can be more resistant against dissolution than {100} under acidic conditions. Although {100} is the largest surface of the typical HAP crystal, it is {001}, the smallest habit face, that is chosen by the living organisms to build the outer surface of enamel by an oriented assembly of the rodlike crystals. We reveal that such a biological construction can confer on enamel protections against erosion, since {001} is relatively dissolution-insensitive. Thus, the spontaneous dissolution of enamel surface can be retarded in biological milieu by such a smart construction. The current study demonstrates the importance of hierarchical structures in the functional biomaterials.

The structure and orientation of the C-terminus of LRAP

Amelogenin is the predominant protein found during enamel development and is thought to be the biomineralization protein controlling the unique elongated hydroxyapatite crystals that constitute enamel. The secondary structure of biomineralization proteins is thought to be important in the interaction with hydroxyapatite. Unfortunately, very little data are available on the structure or the orientation of amelogenin, either in solution or bound to hydroxyapatite. The C-terminus contains the majority of the charged residues and is predicted to interact with hydroxyapatite; thus, we used solid-state NMR dipolar recoupling techniques to investigate the structure and orientation of the C-terminus of LRAP, a naturally occurring splice variant of full-length amelogenin. Using (13)C{(15)N} Rotational Echo DOuble Resonance (REDOR), the structure of the C-terminus was found to be largely random coil, both on the surface of hydroxyapatite as well as lyophilized from solution. The orientation of the C-terminal region with respect to hydroxyapatite was investigated for two alanine residues (Ala(46) and Ala(49)) using (13)C{(31)P} REDOR and one lysine residue (Lys(52)) using (15)N{(31)P} REDOR. The residues examined were found to be 7.0, 5.7, and 5.8 A from the surface of hydroxyapatite for Ala(46), Ala(49), and Lys(52), respectively. This provides direct evidence that the charged C-terminus is interacting closely with hydroxyapatite, positioning the acidic amino acids to aid in controlling crystal growth. However, solid-state NMR dynamics measurements also revealed significant mobility in the C-terminal region of the protein, in both the side chains and the backbone, suggesting that this region alone is not responsible for binding.

Understanding adsorption-desorption dynamics of BMP-2 on hydroxyapatite (001) surface

The interaction between protein molecules and the hydroxyapatite (HAP) crystal is an important research topic in many fields. However, the nature of their noncovalent bonding is still not clear at the atomic level. In this work, molecular dynamics simulation, steered molecular dynamics simulation, and quantum chemistry calculations were used to study the adsorption-desorption dynamics of BMP-2 on HAP (001) surface. The results suggest that there are three types of functional groups through which BMP-2 can interact with HAP crystallite, and they are -OH, -NH(2), and -COO(-). Based on the different orientations of protein, each might interact with HAP crystallite individually, or, two or three of them can work cooperatively. Concerning the mechanisms of interaction, it is found that the water-bridged H-bond plays an important role, which is the main force for groups without net charges. If there were more than one set of adsorption groups for a certain orientation of protein, the adsorption-desorption process would likely be stepwise. On the contrary, if there were only one set, there would be only the key-adsorption period. The results of density functional theory calculations confirm the actual existence of this type of water-bridged H-bond. Furthermore, it is also found that the CHARMM27 force field could provide correct structural information qualitatively, although the data are slightly different from those obtained by UB3LYP/6-31G* method.

'Magic bullets' for bone diseases: progress in rational design of bone-seeking medicinal agents

An ideal therapeutic agent for bone diseases should act solely on bone tissue with no pharmacological activity at other anatomical sites. Current therapeutic agents, however, do not usually display a preferential affinity to bones and non-specifically distribute throughout the body after administration. Attempts to design bone-specific agents have relied on engineering a desired therapeutic agent with bone-seeking molecules so that the latter delivers the therapeutic agents specifically to bones. In this critical review, we summarize the latest attempts to engineer bone-seeking therapeutic agents based on formulating therapeutic agents with bisphosphonates, a class of compounds with high affinity to biological apatite. We first provide a relevant summary of the structure of bone mineral and bisphosphonates, highlighting the mode of interaction between these two entities. The use of bisphosphonates in the diagnosis of bone diseases is then presented, since this application helps us to understand the bone-carrier properties of bisphosphonates under physiological conditions. A summary of recent attempts to formulate bisphosphonates with traditional therapeutic agents to restrict their activities to bone tissues is then provided, with special emphasis on the structure-function relationships of the engineered compounds. Finally, attempts to use bisphosphonates to deliver macromolecular therapeutics (i.e., proteins) are summarized, based on recent data from the authors' lab. The collective research into bone-seeking medicinal agents is progressively laying the foundation for next-generation 'magic bullets' that display desirable activities at the disease sites with no undesirable activity on other organ systems. (164 references.).

The effects of fluoride on the nanostructure and surface pK of enamel crystals: an atomic force microscopy study of human and rat enamel

Atomic force microscopy (AFM) studies have revealed 30-40 nm-wide regular positively charged bands across maturation-stage rat enamel crystals. Low pH resolved these into positively charged spherical domains of approximately 30 nm diameter. Crystal surface pK values from adhesion force titrations were approximately 6.5. The effect of fluoride on this pK value and on the nanostructure of fluorosed human enamel crystals has not been reported. The nanostructure and surface chemistry (pK) of normal and fluorotic human and of fluoride-treated rat maturing enamel crystals was examined. Enamel was sectioned and polished, prior to examination, using AFM in height and friction modes. High-resolution height images revealed 30 nm-diameter spherical domains within crystals, arranged as layers of hexagons or as a shallow spiral. Fluorotic enamel showed similar, but less well ordered, nanodomains. These could represent an arrangement of original initiation sites or binding sites for modulating matrix proteins. Surface pK was derived from adhesion-force measurements between functionalized tips (OH or COOH) and crystal surfaces between pH 2 and pH 10. pK values of approximately 6.5 for normal crystals were reduced to approximately 5.5 after fluoride treatment. Reduction in surface pK by fluoride might indicate lowered protonation with possible effects on matrix protein binding.

Intracellular nanosphere subunit assembly as revealed by amelogenin molecular cross-linking studies

Enamel matrix comprises nanospheres predominantly composed of amelogenin. Studies have shown that recombinant amelogenin forms nanospheres similar to those formed in vivo, but it is unclear exactly how nanospheres assemble in vivo. Are amelogenin monomers secreted into the enamel matrix where they then self-assemble to form nanospheres, or does nanosphere assembly actually occur intracellularly? The aim of this study was to attempt to answer this question. Rat enamel organs were treated with the bifunctional cross-linker, dithio bis (succinimidyl propionate) (DSP), which cross-links primary amines lying in close molecular proximity. The key to this technique is the fact that DSP cross-links are later sensitive to reductive cleavage. The cross-linked proteins were first subjected to non-reducing sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) in the first dimension and then to reducing SDS-PAGE in the second dimension (so-called diagonal electrophoresis) followed by western blot probing with anti-amelogenin. The results indicated that intracellular amelogenin monomers are in close neighbor contact, forming complexes comprising up to six individual amelogenin monomers. We suggest that these initial complexes are prefabricated intracellularly before secretion. Once secreted, these prefabricated subunits assemble further to form the mature full-size nanospheres containing hundreds of individual amelogenins characteristic of enamel matrix.

Amelogenin Supra-Molecular Assembly in vitro Compared with the Architecture of the Forming Enamel Matrix

Tooth enamel is formed in the extracellular space within an organic matrix enriched in amelogenin proteins. Amelogenin nanosphere assembly is a key factor in controlling the oriented and organized growth of enamel apatite crystals. Recently, we have reported the formation of higher ordered structures resulting from organized association and self-orientation of amelogenin nanospheres in vitro. This remarkable hierarchical organization includes self-assembly of amelogenin molecules into subunits of 4-6 nm in diameter followed by their assembly to form nanospheres of 15-25 nm in radii. Chains of >100 nm length are then formed as the result of nanosphere association. These linear arrays of nanospheres assemble to form the microribbons that are hundreds of microns in length, tens of microns in width, and a few microns in thickness. Here, we review the step by step process of amelogenin self-assembly during the formation of microribbon structures in vitro. Assembly properties of selected amelogenins lacking the hydrophilic C terminus will then be reviewed. We will consider amelogenin as a template for the organized growth of crystals in vitro. Finally, we will compare the structures formed in vitro with globular and periodic structures observed earlier, in vivo, by different sample preparation conditions. We propose that the alignment of amelogenin nanospheres into long chains is evident in vivo, and is an important indication for the function of this protein in controlling the oriented and elongated growth of apatite crystals during enamel biomineralization.

Protein–Protein Interactions of the Developing Enamel Matrix

Extracellular matrix proteins control the formation of the inorganic component of hard tissues including bone, dentin, and enamel. The structural proteins expressed primarily in the enamel matrix are amelogenin, ameloblastin, enamelin, and amelotin. Other proteins, like biglycan, are also present in the enamel matrix as well as in other mineralizing and nonmineralizing tissues of mammals. In addition, the presence of sulfated enamel proteins, and "tuft" proteins has been examined and discussed in relation to enamel formation. The structural proteins of the enamel matrix must have specific protein-protein interactions to produce a matrix capable of directing the highly ordered structure of the enamel crystallites. Protein-protein interactions are also likely to occur between the secreted enamel proteins and the plasma membrane of the enamel producing cells, the ameloblasts. Such protein-protein interactions are hypothesized to influence the secretion of enamel proteins, establish short-term order of the forming matrix, and to mediate feedback signals to the transcriptional machinery of these cells. Membrane-bound proteins identified in ameloblasts, and which interact with the structural enamel proteins, include Cd63 (cluster of differentiation 63 antigen), annexin A2 (Anxa2), and lysosomal-associated glycoprotein 1 (Lamp1). These and related data help explain the molecular and cellular mechanisms responsible for the removal of the organic enamel matrix during the events of enamel mineralization, and how the enamel matrix influences its own fate through signaling initiated at the cell surface. The knowledge gained from enamel developmental studies may lead to better dental and nondental materials, or materials inspired by Nature. These data will be critical to scientists, engineers, and dentists in their pursuits to regenerate an entire tooth. For tooth regeneration to become a reality, the protein-protein interactions involving the key dental proteins must be identified and understood. The scope of this review is to discuss the current understanding of protein-protein interactions of the developing enamel matrix, and relate this knowledge to enamel biomineralization.

Reuptake of extracellular amelogenin by dental epithelial cells results in increased levels of amelogenin mRNA through enhanced mRNA stabilization

Amelogenin is an extracellular matrix protein secreted by ameloblasts and is a major component of enamel matrix. Recently, in addition to their role in enamel formation, the biological activity of enamel proteins in the process of cell differentiation has recently become widely appreciated. In this study, we examined the biological activity of amelogenin on ameloblast differentiation. Recombinant mouse amelogenin (rm-amelogenin) enhanced the expression of endogenous amelogenin mRNA in a cultured dental epithelial cell line (HAT-7), despite a lack of increased amelogenin promoter activity. To solve this discrepancy, we analyzed the effects of rm-amelogenin on the stability of amelogenin mRNA. The half-life of amelogenin mRNA is extremely short, but in the presence of rm-amelogenin its half-life was extended three times longer than the control. Furthermore, we showed the entry of exogenous fluorescein isothiocyanate-conjugated rm-amelogenin into the cytoplasm of HAT-7 cells. It follows from our results that exogenous amelogenin increases amelogenin mRNA levels through stabilization of mRNA in the cytoplasm of HAT-7 cells. Here we speculated that during differentiation, dental epithelial cells utilize a unique mechanism for increasing the production of amelogenin, the reuptake of secreted amelogenin.

Control of apatite crystal growth in a fluoride containing amelogenin-rich matrix

To study how crystal growth in dental enamel is controlled by the components of the extracellular matrix, we investigated the functional roles of amelogenins and fluoride ions in apatite formation occurring through an octacalcium phosphate (OCP)-precursor pathway. Using a cation selective membrane system as a model of tooth enamel formation, we evaluated the resulting mineral habit grown in native porcine amelogenins and fluoride ions. In the absence of amelogenin and in the presence of 1 or 2 ppm F, we obtained OCP + apatite and apatite, respectively. Without amelogenins, the crystals were hexagonal prisms and cones with diameters of approximately 100-200 nm. In the presence of 10% amelogenins and in the absence of fluoride, rod-like OCP with a diameter of 35 nm were obtained. Remarkably, a combination of amelogenin and fluoride created the formation of rod-like apatite crystals with dimensions similar to the former crystals. These observations indicate a cooperative role of amelogenin and fluoride in the regulation of habit, size orientation and phase of the calcium-phosphate crystals, resulting in the formation of fine rod-like apatite whose habit and orientation were similar to that of authentic tooth enamel crystals. The significant modulating effect of the amelogenin matrix combined with fluoride ions suggests the potential for this artificial system to contribute to the engineering of novel enamel-like biomaterials in vitro.

Competitive ion-exchange adsorption of proteins: Competitive isotherms with controlled competitor concentration

The competitive adsorption processes inevitably present in chromatographic separations of complex mixtures have not been extensively studied. This is partly due to the difficulty of measuring true competitive isotherms, in which all system parameters (including competitor concentrations) are held constant. We report a novel approach to determining competitive protein adsorption isotherms in which the competitor concentration is held constant across the entire isotherm. By using the heme prosthetic group in cytochrome b5 as a quantitative spectrophotometric label, competitive isotherms between cytochrome b5 and alpha-lactalbumin can be constructed. Similarly, manganese-substituted protoporphyrin IX heme replacement allows the non-perturbing labeling of individual cytochrome b5 conservative surface charge mutants by replacement of a single atom in the interior of the protein. This labeling allows the study of competition between cytochrome b5 charge mutants of identical size and shape, which differ only in charge arrangement. Using these techniques, the effect of competing species on equilibrium behavior and the apparent heterogeneity of anion-exchange adsorbents in the presence of competitors can be quantitatively studied by fitting the data to two popular single-component binding models, the Temkin and the Langmuir-Freundlich (L-F) isotherms.