Altered amelogenin self-assembly based on mutations observed in human X-linked amelogenesis imperfecta (AIH1)

Minimal amelogenin domain for enamel formation

Amelogenin is the most abundant matrix protein guiding hydroxyapatite formation in enamel, the durable bioceramic tissue that covers vertebrate teeth. Here, we sought to refine structure-function for an amelogenin domain based on in vitro data showing a 42 amino acid amelogenin-derived peptide (ADP7) mimicked formation of hydroxyapatite similar to that observed for the full-length mouse 180 amino acid protein. In mice, we used CRISPR-Cas9 to express only ADP7 by the native amelogenin promoter. Analysis revealed ADP7 messenger RNA expression in developing mouse teeth with the formation of a thin layer of enamel. In vivo, ADP7 peptide partially replaced the function of the full-length amelogenin protein and its several protein isoforms. Protein structure-function relationships identified through in vitro assays can be deployed in whole model animals using CRISPR-Cas9 to validate function of a minimal protein domain to be translated for clinical use as an enamel biomimetic.

Co-Immunoprecipitation Reveals Interactions Between Amelogenin and Ameloblastin via Their Self-Assembly Domains

Macromolecular assembly of extracellular enamel matrix proteins (EMPs) is intimately associated with the nucleation, growth, and maturation of highly organized hydroxyapatite crystals giving rise to healthy dental enamel. Although the colocalization of two of the most abundant EMPs amelogenin (Amel) and ameloblastin (Ambn) in molar enamel has been established, the evidence toward their interaction is scarce. We used co-immunoprecipitation (co-IP) to show evidence of direct molecular interactions between recombinant and native Amel and Ambn. Ambn fragments containing Y/F-x-x-Y/L/F-x-Y/F self-assembly motif were isolated from the co-IP column and characterized by mass spectroscopy. We used recombinant Ambn (rAmbn) mutants with deletion of exons 5 and 6 as well as Ambn derived synthetic peptides to demonstrate that Ambn binds to Amel via its previously identified Y/F-x-x-Y/L/F-x-Y/F self-assembly motif at the N-terminus of its exon 5 encoded region. Using an N-terminal specific anti-Ambn antibody, we showed that Ambn N-terminal fragments colocalized with Amel from secretory to maturation stages of enamel formation in a single section of developing mouse incisor, and closely followed mineral patterns in enamel rod interrod architecture. We conclude that Ambn self-assembly motif is involved in its interaction with Amel in solution and that colocalization between the two proteins persists from secretory to maturation stages of amelogenesis. Our in vitro and in situ data support the notion that Amel and Ambn may form heteromolecular assemblies that may perform important physiological roles during enamel formation.

The Unfolded Protein Response in Amelogenesis and Enamel Pathologies

During the secretory phase of their life-cycle, ameloblasts are highly specialized secretory cells whose role is to elaborate an extracellular matrix that ultimately confers both form and function to dental enamel, the most highly mineralized of all mammalian tissues. In common with many other “professional” secretory cells, ameloblasts employ the unfolded protein response (UPR) to help them cope with the large secretory cargo of extracellular matrix proteins transiting their ER (endoplasmic reticulum)/Golgi complex and so minimize ER stress. However, the UPR is a double-edged sword, and, in cases where ER stress is severe and prolonged, the UPR switches from pro-survival to pro-apoptotic mode. The purpose of this review is to consider the role of the ameloblast UPR in the biology and pathology of amelogenesis; specifically in respect of amelogenesis imperfecta (AI) and fluorosis. Some forms of AI appear to correspond to classic proteopathies, where pathological intra-cellular accumulations of protein tip the UPR toward apoptosis. Fluorosis also involves the UPR and, while not of itself a classic proteopathic disease, shares some common elements through the involvement of the UPR. The possibility of therapeutic intervention by pharmacological modulation of the UPR in AI and fluorosis is also discussed.

DENTAL ENAMEL FORMATION AND IMPLICATIONS FOR ORAL HEALTH AND DISEASE

Dental enamel is the hardest and most mineralized tissue in extinct and extant vertebrate species and provides maximum durability that allows teeth to function as weapons and/or tools as well as for food processing. Enamel development and mineralization is an intricate process tightly regulated by cells of the enamel organ called ameloblasts. These heavily polarized cells form a monolayer around the developing enamel tissue and move as a single forming front in specified directions as they lay down a proteinaceous matrix that serves as a template for crystal growth. Ameloblasts maintain intercellular connections creating a semi-permeable barrier that at one end (basal/proximal) receives nutrients and ions from blood vessels, and at the opposite end (secretory/apical/distal) forms extracellular crystals within specified pH conditions. In this unique environment, ameloblasts orchestrate crystal growth via multiple cellular activities including modulating the transport of minerals and ions, pH regulation, proteolysis, and endocytosis. In many vertebrates, the bulk of the enamel tissue volume is first formed and subsequently mineralized by these same cells as they retransform their morphology and function. Cell death by apoptosis and regression are the fates of many ameloblasts following enamel maturation, and what cells remain of the enamel organ are shed during tooth eruption, or are incorporated into the tooth's epithelial attachment to the oral gingiva. In this review, we examine key aspects of dental enamel formation, from its developmental genesis to the ever-increasing wealth of data on the mechanisms mediating ionic transport, as well as the clinical outcomes resulting from abnormal ameloblast function.

Mouse Amelogenin Exons 8 and 9: Sequence Analysis and Protein Distribution

Amelogenin is the major protein of the developing enamel. Two additional exons, termed 8 and 9, have been characterized in the rat. Our aim was: to identify the mouse amelogenin exons 8/9 sequences; to investigate the potential presence of the alternative spliced isoforms of amelogenin exons 8/9; and to immunolocalize proteins containing sequences encoded by exons 8/9 during odontogenesis. RT-PCR analysis with exon 9 anti-sense primer generated 2 major amplicons with the use of a mouse tooth cDNA library and dental cell lines. DNA sequence analysis showed 93% identify with the rat exons 8/9 sequence. Alternative splicing of exon 3 was also found, but only in cDNAs lacking exons 8 and 9. Immunohistochemistry localized exons 8/9-encoded proteins in ameloblasts, young odontoblasts, and stratum intermedium cells. Analysis of our data supports the hypothesis that: (1) AMELX contains 2 additional exons; (2) ameloblasts and odontoblasts synthesize amelogenin 8/9; and (3) amelogenin splice variants may have unique functions during tooth formation.

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.

Bioactive nanofibers enable the identification of thrombospondin 2 as a key player in enamel regeneration

Tissue regeneration and development involves highly synchronized signals both between cells and with the extracellular environment. Biomaterials can be tuned to mimic specific biological signals and control cell response(s). As a result, these materials can be used as tools to elucidate cell signaling pathways and candidate molecules involved with cellular processes. In this work, we explore enamel-forming cells, ameloblasts, which have a limited regenerative capacity. By exposing undifferentiated cells to a self-assembling matrix bearing RGDS epitopes, we elicited a regenerative signal at will that subsequently led to the identification of thrombospondin 2 (TSP2), an extracellular matrix protein that has not been previously recognized as a key player in enamel development and regeneration. Targeted disruption of the thrombospondin 2 gene (Thbs2) resulted in enamel formation with a disordered architecture that was highly susceptible to wear compared to their wild-type counterparts. To test the regenerative capacity, we injected the bioactive matrix into the enamel organ and discovered that the enamel organic epithelial cells in TSP-null mice failed to polarize on the surface of the artificial matrix, greatly reducing integrin β1 and Notch1 expression levels, which represent signaling pathways known to be associated with TSP2. These results suggest TSP2 plays an important role in regulating cell-matrix interactions during enamel formation. Exploiting the signaling pathways activated by biomaterials can provide insight into native signaling mechanisms crucial for tooth development and cell-based strategies for enamel regeneration.

Biomineralization of a self-assembled-, soft-matrix precursor: Enamel

Enamel is the bioceramic covering of teeth, a composite tissue composed of hierarchical organized hydroxyapatite crystallites fabricated by cells under physiologic pH and temperature. Enamel material properties resist wear and fracture to serve a lifetime of chewing. Understanding the cellular and molecular mechanisms for enamel formation may allow a biology-inspired approach to material fabrication based on self-assembling proteins that control form and function. Genetic understanding of human diseases expose insight from Nature's errors by exposing critical fabrication events that can be validated experimentally and duplicated in mice using genetic engineering to phenocopy the human disease so that it can be explored in detail. This approach led to assessment of amelogenin protein self-assembly which, when altered, disrupts fabrication of the soft enamel protein matrix. A misassembled protein matrix precursor results in loss of cell to matrix contacts essential to fabrication and mineralization.

Amelogenin in Enamel Tissue Engineering

In this chapter the basic premises, the recent findings and the future challenges in the use of amelogenin for enamel tissue engineering are being discoursed on. Results emerging from the experiments performed to assess the fundamental physicochemical mechanisms of the interaction of amelogenin, the main protein of the enamel matrix, and the growing crystals of apatite, are mentioned, alongside a moderately comprehensive literature review of the subject at hand. The clinical importance of understanding this protein/mineral interaction at the nanoscale are highlighted as well as the potential for tooth enamel to act as an excellent model system for studying some of the essential aspects of biomineralization processes in general. The dominant paradigm stating that amelogenin directs the uniaxial growth of apatite crystals in enamel by slowing down the growth of (hk0) faces on which it adheres is being questioned based on the results demonstrating the ability of amelogenin to promote the nucleation and crystal growth of apatite under constant titration conditions designed to mimic those present in the developing enamel matrix. The role of numerous minor components of the enamel matrix is being highlighted as essential and impossible to compensate for by utilizing its more abundant ingredients only. It is concluded that the three major aspects of amelogenesis outlined hereby--(1) the assembly of amelogenin and other enamel matrix proteins, (2) the proteolytic activity, and (3) crystallization--need to be in precise synergy with each other in order for the grounds for the proper imitation of amelogenesis in the lab to be created.

The flexible structure of the K24S28 region of Leucine-Rich Amelogenin Protein (LRAP) bound to apatites as a function of surface type, calcium, mutation, and ionic strength

Leucine-Rich Amelogenin Protein (LRAP) is a member of the amelogenin family of biomineralization proteins, proteins which play a critical role in enamel formation. Recent studies have revealed the structure and orientation of the N- and C-terminus of LRAP bound to hydroxyapatite (HAP), a surface used as an analog of enamel. The structure of one region, K24 to S28, was found to be sensitive to phosphorylation of S16, the only naturally observed site of serine phosphorylation in LRAP, suggesting that K24S28 may sit at a key region of structural flexibility and play a role in the protein's function. In this work, we investigated the sensitivity of the structure and orientation of this region when bound to HAP as a function of several factors which may vary during enamel formation to influence structure: the ionic strength (0.05, 0.15, 0.2 M), the calcium concentration (0.07 and 0.4 mM), and the surface to which it is binding [HAP and carbonated apatite (CAP), a more direct mimic of enamel]. A naturally occurring mutation found in amelogenin (T21I) was also investigated. The structure in the K24S28 region of the protein was found to be sensitive to these conditions, with the CAP surface and excess Ca(2+) (8:1 [Ca(2+)]:[LRAP-K24S28(+P)]) resulting in a tighter helix, while low ionic strength relaxed the helical structure. Higher ionic strength and the point mutation did not result in any structural change in this region. The distance of the backbone of K24 from the surface was most sensitive to excess Ca(2+) and in the T21I-mutation. Collectively, these data suggest that phosphorylated LRAP is able to accommodate structural changes while maintaining its interaction with the surface, and provides further evidence of the structural sensitivity of the K24S28 region, a sensitivity that may contribute to function in biomineralization.

Self-assembly of amelogenin proteins at the water-oil interface

Self-assembly of amelogenin plays a key role in controlling enamel biomineralization. Recently, we generated self-aligning nanoribbons of amelogenin in water-in-oil emulsions stabilized by the full-length protein (rH174). Here, we tested the hypothesis that the hydrophilic C-terminus is critical for self-assembly of amelogenin into nanoribbons. The self-assembled structures of two amelogenin cleavage products, rH163 and rH146, were compared with structures of rH174 at different pH values and degrees of saturation using atomic force microscopy, electron microscopy, and dynamic light scattering. We observed that the number density of rH174 nanoribbons increased significantly when the initial pH was raised from 4.5 to 5.6. Nanoribbons, as well as unique helical nanostructures, were also readily observed when amelogenin rH146 was used, but showed little tendency for parallel alignment and did not bundle into fibrils like rH174. In contrast, rH163 rarely formed nanoribbons but predominantly assembled into nanospheres under the same conditions. We conclude that the presence of a hydrophilic C-terminus may not be a prerequisite for nanoribbon formation but may be critical for ribbon alignment and subsequent fibril formation. These results highlight the contribution of the hydrophobic domain in the self-assembly of elongated structures of amelogenins. Molecular mechanisms governing these processes based on the formation of reverse micelles are discussed.

Probing the self-association, intermolecular contacts, and folding propensity of amelogenin

Amelogenins are an intrinsically disordered protein family that plays a major role in the development of tooth enamel, one of the most highly mineralized materials in nature. Monomeric porcine amelogenin possesses random coil and residual secondary structures, but it is not known which sequence regions would be conformationally attractive to potential enamel matrix targets such as other amelogenins (self-assembly), other matrix proteins, cell surfaces, or biominerals. To address this further, we investigated recombinant porcine amelogenin (rP172) using "solvent engineering" techniques to simultaneously promote native-like structure and induce amelogenin oligomerization in a manner that allows identification of intermolecular contacts between amelogenin molecules. We discovered that in the presence of 2,2,2-trifluoroethanol (TFE) significant folding transitions and stabilization occurred primarily within the N- and C-termini, while the polyproline Type II central domain was largely resistant to conformational transitions. Seven Pro residues (P2, P127, P130, P139, P154, P157, P162) exhibited conformational response to TFE, and this indicates these Pro residues act as folding enhancers in rP172. The remaining Pro residues resisted TFE perturbations and thus act as conformational stabilizers. We also noted that TFE induced rP172 self-association via the formation of intermolecular contacts involving P4-H6, V19-P33, and E40-T58 regions of the N-terminus. Collectively, these results confirm that the N- and C-termini of amelogenin are conformationally responsive and represent potential interactive sites for amelogenin-target interactions during enamel matrix mineralization. Conversely, the Pro, Gln central domain is resistant to folding and this may have important functional significance for amelogenin.

Folding, assembly, and aggregation of recombinant murine amelogenins with T21I and P41T point mutations

Two point mutations (T21I and P40T) within amelogenin have been identified from human DNA sequences in 2 instances of amelogenesis imperfecta. We studied the folding and self-assembly of recombinant amelogenin (rM180) compared to the T21I and P40T mutants analogs. At pH 5.8 and 25°C, rM180 and the P41T mutant existed as monomers, whereas the T21I mutant formed small oligomers. At pH 8 and 25°C, all of the amelogenin samples formed nanospheres with hydrodynamic radii (R(H)) of around 15-16 nm. Upon heating to 37°C, particles of P41T increased in size (R(H) = 18 nm). During thermal denaturation at pH 5.8, both of the mutant proteins refolded more slowly than the wild-type (WT) rM180. Variable temperature tryptophan fluorescence and dynamic light scattering studies showed that the WT transformed to a partially folded conformation upon heating and remained stable. Thermal denaturation and refolding studies indicated that the mutants were less stable and exhibit a greater ability to prematurely aggregate compared to the WT. Our data suggest that in the case of P41T, alterations in the self-assembly of amelogenin are a consequence of destabilization of the secondary structure, while in the case of T21I they are a consequence of change in the overall hydrophobicity at the N-terminal region. We propose that alterations in the assembly (i.e. premature aggregation) of mutant amelogenins may have a profound effect on intra- and extracellular processes such as amelogenin secretion, proteolysis, and its interactions with nonamelogenins as well as with the forming mineral.

Perturbed Amelogenin Protein Self-assembly Alters Nanosphere Properties Resulting in Defective Enamel Formation

Dental enamel is a unique composite bioceramic material that is the hardest tissue in the vertebrate body, containing long-, thin-crystallites of substituted hydroxyapatite. Enamel functions under immense loads in a bacterial-laden environment, and generally without catastrophic failure over a lifetime for the organism. Unlike all other biogenerated hard tissues of mesodermal origin, such as bone and dentin, enamel is produced by ectoderm-derived cells called ameloblasts. Recent investigations on the formation of enamel using cell and molecular approaches have been coupled to biomechanical investigations at the nanoscale and mesoscale levels. For amelogenin, the principle protein of forming enamel, two domains have been identified that are required for the proper assembly of multimeric units of amelogenin to form nanospheres. One domain is at the amino-terminus and the other domain in the carboxyl-terminal region. Amelogenin nanospheres are believed to influence the hydroxyapatite crystal habit. Both the yeast two-hybrid assay and surface plasmon resonance have been used to examine the assembly properties of engineered amelogenin proteins. Amelogenin protein was engineered using recombinant DNA techniques to contain deletions to either of the two self-assembly domains. Amelogenin protein was also engineered to contain single amino-acid mutations/substitutions in the amino-terminal self-assembly domain; and these amino-acid changes are based upon point mutations observed in humans affected with a hereditary disturbance of enamel formation. All of these alterations reveal significant defects in amelogenin self-assembly into nanospheres in vitro. Transgenic animals containing these same amelogenin deletions illustrate the importance of a physiologically correct bio-fabrication of the enamel protein extracellular matrix to allow for the organization of the enamel prismatic structure.

Association between AMELX polymorphisms and dental caries in Koreans

OBJECTIVES

Dental caries is greatly influenced disease by environmental factors, but recently there are increasing evidences for a genetic component in caries susceptibility. AMELX is the gene coding amelogenin, which is the most important factor for normal enamel development. The aim of this study was to examine the relationship between dental caries and single nucleotide polymorphisms (SNPs) in AMELX.

SUBJECTS AND METHODS

For this study, we used DNA samples collected from 120 unrelated individuals older than 12 years of age. All of them were examined for their oral and dental status under the WHO recommended criteria, and clinical information such as DMFT and DMFS were evaluated. Individuals whose DMFT and DMFS index lower than 2 were designated 'very low caries experience' and higher than 3 were designated 'higher caries experience'. Genomic DNA was extracted from hair samples, and single nucleotide polymorphisms of AMELX were genotyped. Genotyping of three SNPs (rs17878486, rs5933871, rs5934997, intron) in AMELX gene was determined by direct sequencing and analyzed with SNPStats.

RESULTS

There were significant associations between rs5933871 and rs5934997 SNP and caries susceptibility in the water fluoridation group.

CONCLUSIONS

These results suggest that SNPs of AMELX might be associated with dental caries susceptibility in Korean population.

Prospects and Pits on the Path of Biomimetics: The case of tooth enamel

This review presents a discourse on challenges in understanding and imitating the process of amelogenesis in vitro on the molecular scale. In light of the analysis of imitation of the growth of dental enamel, it also impends on the prospects and potential drawbacks of the biomimetic approach in general. As the formation of enamel proceeds with the protein matrix guiding the crystal growth, while at the same time conducting its own degradation and removal, it is argued that three aspects of amelogenesis need to be induced in parallel: a) crystal growth; b) protein assembly; c) proteolytic degradation. A particular emphasis is therefore placed on ensuring conditions for proteolysis-coupled protein-guided crystallization to occur. Discussed are structural and functional properties of the protein species involved in amelogenesis, mainly amelogenin and enamelysin, the main protein and the protease of the developing enamel matrix, respectively. A model of enamel growth based on controlled delivery of constituent ions or crystalline or amorphous building blocks by means of amelogenin is proposed. The importance of high viscosity of the enamel matrix and a more intricate role that water may play in such a gelatinous medium are also touched upon. The tendency of amelogenin to self-assemble into fibrous and rod-shaped morphologies is considered as potentially important in explaining the formation of elongated apatite crystals. The idea that a preassembling protein matrix serves as a template for the uniaxial growth of apatite crystals in enamel is finally challenged with the one based on co-assembly of the protein and the mineral phases.

Perturbed amelogenin secondary structure leads to uncontrolled aggregation in amelogenesis imperfecta mutant proteins

Mutations in amelogenin sequence result in defective enamel, and the diverse group of genetically altered conditions is collectively known as amelogenesis imperfecta (AI). Despite numerous studies, the detailed molecular mechanism of defective enamel formation is still unknown. In this study, we have examined the biophysical properties of a recombinant murine amelogenin (rM180) and two point mutations identified from human DNA sequences in two cases of AI (T21I and P41T). At pH 5.8 and 25 °C, wild type (WT) rM180 and mutant P41T existed as monomers, and mutant T21I formed lower order oligomers. CD, dynamic light scattering, and fluorescence studies indicated that rM180 and P41T can be classified as a premolten globule-like subclass protein at 25 °C. Thermal denaturation and refolding monitored by CD ellipticity at 224 nm indicated the presence of a strong hysteresis in mutants compared with WT. Variable temperature tryptophan fluorescence and dynamic light scattering studies showed that WT transformed to a partially folded conformation upon heating and remained stable. The partially folded conformation formed by P41T, however, readily converted into a heterogeneous population of aggregates. T21I existed in an oligomeric state at room temperature and, upon heating, rapidly formed large aggregates over a very narrow temperature range. Thermal denaturation and refolding studies indicated that the mutants are less stable and exhibit poor refolding ability compared with WT rM180. Our results suggest that alterations in self-assembly of amelogenin are a consequence of destabilization of the intrinsic disorder. Therefore, we propose that, like a number of other human diseases, AI appears to be due to the destabilization of the secondary structure as a result of amelogenin mutations.

Analysis of secondary structure and self-assembly of amelogenin by variable temperature circular dichroism and isothermal titration calorimetry

Amelogenin is a proline-rich enamel matrix protein known to play an important role in the oriented growth of enamel crystals. Amelogenin self-assembles to form nanospheres and higher order structures mediated by hydrophobic interactions. This study aims to obtain a better insight into the relationship between primary-secondary structure and self-assembly of amelogenin by applying computational and biophysical methods. Variable temperature circular dichroism studies indicated that under physiological pH recombinant full-length porcine amelogenin contains unordered structures in equilibrium with polyproline type II (PPII) structure, the latter being more populated at lower temperatures. Increasing the concentration of rP172 resulted in the promotion of folding to an ordered beta-structured assembly. Isothermal titration calorimetry dilution studies revealed that at all temperatures, self-assembly is entropically driven due to the hydrophobic effect and the molar heat of assembly (DeltaH(A)) decreases with temperature. Using a computational approach, a profile of domains in the amino acid sequence that have a high propensity to assemble and to have PPII structures has been identified. We conclude that the assembly properties of amelogenin are due to complementarity between the hydrophobic and PPII helix prone regions.

The tooth enamel protein, porcine amelogenin, is an intrinsically disordered protein with an extended molecular configuration in the monomeric form

Amelogenins make up a class of proteins associated with the formation of mineralized enamel in vertebrates, possess highly conserved N- and C-terminal sequence regions, and represent an interesting model protein system for understanding biomineralization and protein assembly. Using bioinformatics, we report here the identification of molecular traits that classify 12 amelogenin proteins as members of the intrinsically disordered or unstructured protein family (IDPs), a group of proteins that normally exist as unfolded species but are capable of transformation to a folded state as part of their overall function. Using biophysical techniques (CD and NMR), we follow up on our bioinformatics studies and confirm that one of the amelogenins, recombinant porcine rP172, exists in an extended, unfolded state in the monomeric form. This protein exhibits evidence of conformational exchange between two states, and this exchange may be mediated by Pro residues in the sequence. Although the protein is globally unfolded, we detect the presence of local residual secondary structure [alpha-helix, extended beta-strand, turn/loop, and polyproline type II (PPII)] that may serve several functional roles within the enamel matrix. The extended, labile conformation of rP172 amelogenin is compatible with the known functions of amelogenin in enamel biomineralization, i.e., self-assembly, associations with other enamel matrix proteins and with calcium phosphate biominerals, and interaction with cell receptors. It is likely that the labile structure of this protein facilitates interactions of amelogenin with other macromolecules or with minerals for achievement of internal protein stabilization.

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.

Morphogenetic activity of silica and bio-silica on the expression of genes controlling biomineralization using SaOS-2 cells

In a previous study (Schröder et al., J Biomed Mater Res B Appl Biomater 75:387-392, 2005) we demonstrated that human SaOS-2 cells, when cultivated on bio-silica matrices, respond with an increased hydroxyapatite deposition. In the present contribution we investigate if silica-based components (Na-silicate, tetraethyl orthosilicate [TEOS], silica-nanoparticles) (1) change the extent of biomineralization in vitro (SaOS-2 cells) and (2) cause an alteration of the expression of the genes amelogenin, ameloblastin, and enamelin, which are characteristic for an early stage of osteogenesis. We demonstrate that the viability of SaOS-2 cells was not affected by the silica-based components. If Na-silicate or TEOS was added together with ss-glycerophosphate, an organic phosphate donor, a significant increase in biomineralization was measured. Finally, expression levels of the amelogenin, ameloblastin, and enamelin genes were determined in SaOS-2 cells during exposure to the silica-based components. After exposure for 2 days, expression levels of amelogenin and enamelin strongly increased in response to the silica-based components, while no significant change was seen for ameloblastin. In contrast, exposure of SaOS-2 cells to ss-glycerophosphate resulted in increased expression of all three genes. We conclude that the levels of the structural molecules of the enamel matrix, amelogenin and enamelin, increase in the presence of silica-based components and substantially contribute to the extent of hydroxyapatite crystallite formation. These results demonstrate that silica-based components augment hydroxyapatite deposition in vitro and suggest that enzymatically synthesized bio-silica (via silicatein) might be a promising route for tooth reconstruction in vivo.

Polyelectrolyte-mediated adsorption of amelogenin monomers and nanospheres forming mono- or multilayers

We have applied optical waveguide lightmode spectroscopy combined with streaming potential measurements and Fourier-transformed infrared spectroscopy to investigate adsorption of amelogenin nanospheres onto polyelectrolytes. The long-term objective was to better understand the chemical nature of these assemblies and to gain further insight into the molecular mechanisms involved during self-assembly. It was found that monolayers of monomers and negatively charged nanospheres of a recombinant amelogenin (rM179) irreversibly adsorbed onto a positively charged polyelectrolyte multilayer films. On the basis of measurements performed at different temperatures, it was demonstrated that intermolecular interactions for the formation of nanospheres were not affected by their adsorption onto polyelectrolytes. Consecutive adsorption of nanospheres resulting in the formation of multilayer structures was possible by using cationic poly(l-lysine) as mediators. N-Acetyl-d-glucosamine (GlcNac) did not disturb the nanosphere-assembled protein's structure, and it only affected the adsorption of monomeric amelogenin. Infrared spectroscopy of adsorbed amelogenin revealed conformational differences between the monomeric and assembled forms of rM179. While there was a considerable amount of alpha-helices in the monomers, beta-turn and beta-sheet structures dominated the assembled proteins. Our work constitutes the first report on a structurally controlled in vitro buildup of an rM179 nanosphere monolayer-based matrix. Our data support the notion that amelogenin self-assembly is mostly driven by hydrophobic interactions and that amelogenin/PEM interactions are dominated by electrostatic forces. We suggest that similar forces can govern amelogenin interactions with non-amelogenins or the mineral phase during enamel biomineralization.

Altering biomineralization by protein design

To create a bioceramic with unique materials properties, biomineralization exploits cells to create a tissue-specific protein matrix to control the crystal habit, timing, and position of the mineral phase. The biomineralized covering of vertebrate teeth is enamel, a distinctive tissue of ectodermal origin that is collagen-free. In forming enamel, amelogenin is the abundant protein that undergoes self-assembly to contribute to a matrix that guides its own replacement by mineral. Conserved domains in amelogenin suggest their importance to biomineralization. We used gene targeting in mice to replace native amelogenin with one of two engineered amelogenins. Replacement changed enamel organization by altering protein-to-crystallite interactions and crystallite stacking while diminishing the ability of the ameloblast to interact with the matrix. These data demonstrate that ameloblasts must continuously interact with the developing matrix to provide amelogenin-specific protein to protein, protein to mineral, and protein to membrane interactions critical to biomineralization and enamel architecture while suggesting that mutations within conserved amelogenin domains could account for enamel variations preserved in the fossil record.

NF-Y and CCAAT/enhancer-binding protein alpha synergistically activate the mouse amelogenin gene

Amelogenin is the major protein component of the forming enamel matrix. In situ hybridization revealed a periodicity for amelogenin mRNA hybridization signals ranging from low to high transcript abundance on serial sections of developing mouse teeth. This in vivo observation led us to examine the amelogenin promoter for the activity of transcription factor(s) that account for this expression aspect of the regulation for the amelogenin gene. We have previously shown that CCAAT/enhancer-binding protein alpha (C/EBPalpha) is a potent transactivator of the mouse X-chromosomal amelogenin gene acting at the C/EBPalpha cis-element located in the -70/+52 minimal promoter. The minimal promoter contains a reversed CCAAT box (-58/-54) that is four base pairs downstream from the C/EBPalpha binding site. Similar to the C/EBPalpha binding site, the integrity of the reversed CCAAT box is also required for maintaining the activity of the basal promoter. We therefore focused on transcription factors that interact with the reversed CCAAT box. Using electrophoretic mobility shift assays we demonstrated that NF-Y was directly bound to this reversed CCAAT site. Co-transfection of C/EBPalpha and NF-Y synergistically increased the promoter activity. In contrast, increased expression of NF-Y alone had only marginal effects on the promoter. A dominant-negative DNA binding-deficient NF-Y mutant (NF-YAm29) dramatically decreased the promoter activity both in the absence or presence of exogenous expression of C/EBPalpha. We identified protein-protein interactions between C/EBPalpha and NF-Y by a co-immunoprecipitation analysis. These results suggest that C/EBPalpha and NF-Y synergistically activate the mouse amelogenin gene and can contribute to its physiological regulation during amelogenesis.

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.

Tooth developmental biology: disruptions to enamel-matrix assembly and its impact on biomineralization

Dental enamel is a composite bioceramic material that is the hardest tissue in the vertebrate body, containing long, thin crystallites of substituted hydroxyapatite (HAP). Over a lifetime of an organism, enamel functions under repeated and immense loads, generally without catastrophic failure. Enamel is a product of ectoderm-derived cells called ameloblasts. Recent investigations on the formation of enamel using cell and molecular approaches are now being coupled to biomechanical investigations at the nanoscale and mesoscale levels. For amelogenin, the principal structural protein for forming enamel, we have identified two domains that are required for its proper self-assembly into supramolecular structures referred to as nanospheres. Nanospheres are believed to control HAP crystal habit. Other structural proteins of the enamel matrix include ameloblastin and enamelin, but little is known about their biological importance. Transgenic animals have been prepared to investigate the effect of overexpression of wild-type or mutated enamel proteins on the developing enamel matrix. Amelogenin transgenes were engineered to contain deletions to either of the two self-assembly domains and these alterations produced significant defects in the enamel. Additional transgenic animal lines have been prepared and studied and each gives additional insights into the mechanisms for enamel biofabrication. This study summarizes the observed enamel phenotypes of recently derived transgenic animals. These data are being used to help define the role of each of the enamel structural proteins in enamel and study how each of these proteins impact on enamel biomineralization.

Overexpression of TRAP in the enamel matrix does not alter the enamel structural hierarchy

The secreted, full-length amelogenin is the dominant protein of the forming enamel organ. As enamel mineralization progresses, amelogenin is quickly subjected to proteolytic activity, and eliminated from the enamel environment. Mature enamel contains only traces of structural proteins, including enamelin and the sheath protein ameloblastin. In addition, a proteolytic fragment of amelogenin, known as the tyrosine-rich amelogenin peptide or TRAP, is present in low but isolatable quantities. By overexpressing TRAP during enamel development we sought to determine if such overexpression would result in structural alterations to the mature enamel. We reasoned that overexpressing a protein associated with enamel maturation, at an inappropriate developmental stage, would result in alterations to the enamel protein assembly and hence, alterations in enamel structure and morphology. As judged by transmission and scanning electron microscopy, the enamel formed by overexpressing TRAP showed little morphological differences when compared to the enamel of normal nontransgenic animals. Based on scanning electron-microscopic images, there was modest hypomineralization evident in the interrod enamel of the TRAP-overexpressing animals. However, this finding was inconsistent and inconsequential from a structural and functional perspective. From these results it appears that additional amounts of TRAP protein in the immature enamel matrix are not sufficient to alter the properties of the enamel extracellular matrix to an extent that the hierarchical structure of mature enamel is altered.

A survey of the year 2002 commercial optical biosensor literature

We have compiled 819 articles published in the year 2002 that involved commercial optical biosensor technology. The literature demonstrates that the technology's application continues to increase as biosensors are contributing to diverse scientific fields and are used to examine interactions ranging in size from small molecules to whole cells. Also, the variety of available commercial biosensor platforms is increasing and the expertise of users is improving. In this review, we use the literature to focus on the basic types of biosensor experiments, including kinetics, equilibrium analysis, solution competition, active concentration determination and screening. In addition, using examples of particularly well-performed analyses, we illustrate the high information content available in the primary response data and emphasize the impact of including figures in publications to support the results of biosensor analyses.

Functional domains for amelogenin revealed by compound genetic defects

We have previously used the yeast two-hybrid assay and multiple in vitro methodologies to show that amelogenin undergoes self-assembly involving two domains (A and B). Using transgenic animals, we show that unique enamel phenotypes result from disruptions to either the A- or B-domain, supporting the role of amelogenin in influencing enamel structural organization. By crossbreeding, animals bearing two defective amelogenin gene products have a more extreme enamel phenotype than the sum of the defects evident in the individual parental lines. At the nanoscale level, the forming matrix shows alteration in the size of the amelogenin nanospheres. At the mesoscale level of enamel structural hierarchy, 6-week-old enamel exhibits defects in enamel rod organization caused by perturbed organization of the precursor organic matrix. These studies reflect the critical dependency of amelogenin self-assembly to form a highly organized enamel organic matrix, and that amelogenins engineered to be defective in self-assembly produce compound defects in the structural organization of enamel.