Controlled proteolysis of amelogenins reveals exposure of both carboxy- and amino-terminal regions

Changes in the C-terminal, N-terminal, and histidine regions of amelogenin reveal the role of oligomer quaternary structure on adsorption and hydroxyapatite mineralization

Adsorption interactions between amelogenin and calcium phosphate minerals are believed to be important to amelogenin's function in enamel formation, however, the role of specific amino acid residues and domains within the protein in controlling adsorption is not well known. We synthesized "mechanistic probes" by systematically removing charged regions of amelogenin in order to elucidate their roles. The probes included amelogenin without the charged residues in the N-terminus (SEKR), without two, three, or eight histidines (H) in the central protein region (H2, H3, H8), or without the C-terminal residues (Delta). In-situ atomic force microscopy (AFM) adsorption studies onto hydroxyapatite (HAP) single crystals confirmed that the C-terminus was the dominant domain in promoting adsorption. We propose that subtle changes in protein-protein interactions for proteins with histidines and N-terminal residues removed resulted in changes in the oligomer quaternary size and structure that also affected protein adsorption. HAP mineralization studies revealed that the oligomer-HAP binding energy and protein layer thickness were factors in controlling the amorphous calcium phosphate (ACP) to HAP induction time. Our studies with mechanistic probes reveal the importance of the oligomer quaternary structure in controlling amelogenin adsorption and HAP mineralization.

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 Presence of MMP-20 Reinforces Biomimetic Enamel Regrowth

Biomimetic synthesis of artificial enamel is a promising strategy for the prevention and restoration of defective enamel. We have recently reported that a hydrogel system composed of chitosan-amelogenin (CS-AMEL) and calcium phosphate is effective in forming an enamel-like layer that has a seamless interface with natural tooth surfaces. Here, to improve the mechanical system function and to facilitate the biomimetic enamel regrowth, matrix metalloproteinase-20 (MMP-20) was introduced into the CS-AMEL hydrogel. Inspired by our recent finding that MMP-20 prevents protein occlusion inside enamel crystals, we hypothesized that addition of MMP-20 to CS-AMEL hydrogel could reinforce the newly grown layer. Recombinant human MMP-20 was added to the CS-AMEL hydrogel to cleave full-length amelogenin during the growth of enamel-like crystals on an etched enamel surface. The MMP-20 proteolysis of amelogenin was studied, and the morphology, composition, and mechanical properties of the newly grown layer were characterized. We found that amelogenin was gradually degraded by MMP-20 in the presence of chitosan. The newly grown crystals in the sample treated with MMP-20-CS-AMEL hydrogel showed more uniform orientation and greater crystallinity than the samples treated with CS-AMEL hydrogel without MMP-20. Stepwise processing of amelogenin by MMP-20 in the CS-AMEL hydrogel prevented undesirable protein occlusion within the newly formed crystals. As a result, both the modulus and hardness of the repaired enamel were significantly increased (1.8- and 2.4-fold, respectively) by the MMP-20-CS-AMEL hydrogel. Although future work is needed to further incorporate other enamel matrix proteins into the system, this study brings us one step closer to biomimetic enamel regrowth.

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.

Immunolocalization of Enamelysin (Matrix Metalloproteinase-20) in the Forming Rat Incisor

In the rat model, we used the continuously growing incisor to study the expression pattern of matrix metalloproteinase-20 (MMP-20) during the formation of mineralized dental tissues. Casein zymography analysis of extracts of the forming part of the incisor revealed lysis bands corresponding to both the latent form at 57 kD and the active 46- and 41-kD forms, whereas omission of proteinase inhibitors during protein extraction resulted in a single band at 21 kD. A higher molecular weight form of 78 kD was also stained with MMP-20 and TIMP-2 antibodies in Western blotting, and was therefore believed to correspond to an MMP-20/TIMP-2 complex. Immunohistochemical and immunogold electron microscopic results demonstrated strong MMP-20 staining in the forming outer enamel, which diminished near the dentino-enamel junction, but dentin and predentin were unstained. A strong concentration of MMP-20 was seen in the stratum intermedium (SI), particularly at the earlier stages of enamel development. Our results confirm the presence of MMP-20 protein in ameloblasts and odontoblasts of rat incisor and show it to be localized in the same sites of the forming enamel as amelogenin. Their expression is transient in odontoblasts but persists in ameloblasts, and in both cases the expression of amelogenin preceded that of MMP-20 suggesting a developmentally controlled regulation.

Matrix metalloproteinase-20 mediates dental enamel biomineralization by preventing protein occlusion inside apatite crystals

Reconstruction of enamel-like materials is a central topic of research in dentistry and material sciences. The importance of precise proteolytic mechanisms in amelogenesis to form a hard tissue with more than 95% mineral content has already been reported. A mutation in the Matrix Metalloproteinase-20 (MMP-20) gene results in hypomineralized enamel that is thin, disorganized and breaks from the underlying dentin. We hypothesized that the absence of MMP-20 during amelogenesis results in the occlusion of amelogenin in the enamel hydroxyapatite crystals. We used spectroscopy and electron microscopy techniques to qualitatively and quantitatively analyze occluded proteins within the isolated enamel crystals from MMP-20 null and Wild type (WT) mice. Our results showed that the isolated enamel crystals of MMP-20 null mice had more organic macromolecules occluded inside them than enamel crystals from the WT. The crystal lattice arrangements of MMP-20 null enamel crystals analyzed by High Resolution Transmission Electron Microscopy (HRTEM) were found to be significantly different from those of the WT. Raman studies indicated that the crystallinity of the MMP-20 null enamel crystals was lower than that of the WT. In conclusion, we present a novel functional mechanism of MMP-20, specifically prevention of unwanted organic material entrapped in the forming enamel crystals, which occurs as the result of precise amelogenin cleavage. MMP-20 action guides the growth morphology of the forming hydroxyapatite crystals and enhances their crystallinity. Elucidating such molecular mechanisms can be applied in the design of novel biomaterials for future clinical applications in dental restoration or repair.

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.

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 and enamel biomimetics

Mature tooth enamel is acellular and does not regenerate itself.

Preferential and selective degradation and removal of amelogenin adsorbed on hydroxyapatites by MMP20 and KLK4 in vitro

The hardest tooth enamel tissue develops from a soft layer of protein-rich matrix, predominated by amelogenin that is secreted by epithelial ameloblasts in the secretory stage of tooth enamel development. During enamel formation, a well-controlled progressive removal of matrix proteins by resident proteases, Matrix metalloproteinase 20 (MMP20), and kallikrein 4 (KLK4), will provide space for the apatite crystals to grow. To better understand the role of amelogenin degradation in enamel biomineralization, the present study was conducted to investigate how the adsorption of amelogenin to hydroxyapatite (HAP) crystals affects its degradation by enamel proteinases, MMP20 and KLK4. Equal quantities of amelogenins confirmed by protein assays before digestions, either adsorbed to HAP or in solution, were incubated with MMP20 or KLK4. The digested samples collected at different time points were analyzed by spectrophotometry, SDS-PAGE, high performance liquid chromatography (HPLC), and LC-MALDI MS/MS. We found that majority of amelogenin adsorbed on HAP was released into the surrounding solution by enzymatic processing (88% for MMP20 and 98% for KLK4). The results show that as compared with amelogenin in solution, the HAP-bound amelogenin was hydrolyzed by both MMP20 and KLK4 at significantly higher rates. Using LC-MALDI MS/MS, more accessible cleavage sites and hydrolytic fragments from MMP20/KLK4 digestion were identified for the amelogenin adsorbed on HAP crystals as compared to the amelogenin in solution. These results suggest that the adsorption of amelogenin to HAP results in their preferential and selective degradation and removal from HAP by MMP20 and KLK4 in vitro. Based on these findings, a new degradation model related to enamel crystal growth is proposed.

Dental enamel development: proteinases and their enamel matrix substrates

This review focuses on recent discoveries and delves in detail about what is known about each of the proteins (amelogenin, ameloblastin, and enamelin) and proteinases (matrix metalloproteinase-20 and kallikrein-related peptidase-4) that are secreted into the enamel matrix. After an overview of enamel development, this review focuses on these enamel proteins by describing their nomenclature, tissue expression, functions, proteinase activation, and proteinase substrate specificity. These proteins and their respective null mice and human mutations are also evaluated to shed light on the mechanisms that cause nonsyndromic enamel malformations termed amelogenesis imperfecta. Pertinent controversies are addressed. For example, do any of these proteins have a critical function in addition to their role in enamel development? Does amelogenin initiate crystallite growth, does it inhibit crystallite growth in width and thickness, or does it do neither? Detailed examination of the null mouse literature provides unmistakable clues and/or answers to these questions, and this data is thoroughly analyzed. Striking conclusions from this analysis reveal that widely held paradigms of enamel formation are inadequate. The final section of this review weaves the recent data into a plausible new mechanism by which these enamel matrix proteins support and promote enamel development.

Phosphorylation and ionic strength alter the LRAP-HAP interface in the N-terminus

The conditions present during enamel crystallite development change dramatically as a function of time, including the pH, protein concentration, surface type, and ionic strength. In this work, we investigate the role that two of these changing conditions, pH and ionic strength, have in modulating the interaction of the amelogenin, LRAP, with hydroxyapatite (HAP). Using solid-state NMR dipolar recoupling and chemical shift data, we investigate the structure, orientation, and dynamics of three regions in the N-terminus of the protein: L(15) to V(19), V(19) to L(23), and K(24) to S(28). These regions are also near the only phosphorylated residue in the protein pS(16); therefore, changes in the LRAP-HAP interaction as a function of phosphorylation (LRAP(-P) vs LRAP(+P)) were also investigated. All of the regions and conditions studied for the surface immobilized proteins showed restricted motion, with indications of slightly more mobility under all conditions for L(15)(+P) and K(24)(-P). The structure and orientation of the LRAP-HAP interaction in the N-terminus of the phosphorylated protein is very stable to changing solution conditions. From REDOR dipolar recoupling data, the structure and orientation in the region L(15)V(19)(+P) did not change significantly as a function of pH or ionic strength. The structure and orientation of the region V(19)L(23)(+P) were also stable to changes in pH, with the only significant change observed at high ionic strength, where the region becomes extended, suggesting this may be an important region in regulating mineral development. Chemical shift studies also suggest minimal changes in all three regions studied for both LRAP(-P) and LRAP(+P) as a function of pH or ionic strength, and also reveal that K(24) has multiple resolvable resonances, suggestive of two coexisting structures. Phosphorylation also alters the LRAP-HAP interface. All of the three residues investigated (L(15), V(19), and K(24)) are closer to the surface in LRAP(+P), but only K(24)S(28) changes structure as a result of phosphorylation, from a random coil to a largely helical structure, and V(19)L(23) becomes more extended at high ionic strength when phosphorylated. These observations suggest that ionic strength and dephosphorylation may provide switching mechanisms to trigger a change in the function of the N-terminus during enamel development.

Amelogenin processing by MMP-20 prevents protein occlusion inside calcite crystals

Calcite crystals were grown in the presence of full-length amelogenin and during its proteolysis by recombinant human matrix metalloproteinase 20 (rhMMP-20). Recombinant porcine amelogenin (rP172) altered the shape of calcite crystals by inhibiting the growth of steps on the {104} faces and became occluded inside the crystals. Upon co-addition of rhMMP-20, the majority of the protein was digested resulting in a truncated amelogenin lacking the C-terminal segment. In rP172-rhMMP-20 samples, the occlusion of amelogenin into the calcite crystals was drastically decreased. Truncated amelogenin (rP147) and the 25-residue C-terminal domain produced crystals with regular shape and less occluded organic material. Removal of the C-terminal diminished the affinity of amelogenin to the crystals and therefore prevented occlusion. We hypothesize that HAP and calcite interact with amelogenin in a similar manner. In the case of each material, full-length amelogenin binds most strongly, truncated amelogenin binds weakly and the C-terminus alone has the weakest interaction. Regarding enamel crystal growth, the prevention of occlusion into maturing enamel crystals might be a major benefit resulting from the selective cleavage of amelogenin at the C-terminus by MMP-20. Our data have important implications for understanding the hypomineralized enamel phenotype in cases of amelogenesis imperfecta resulting from MMP-20 mutations and will contribute to the design of enamel inspired biomaterials.

Matrix metalloproteinase 20 promotes a smooth enamel surface, a strong dentino-enamel junction, and a decussating enamel rod pattern

Mutations of the matrix metalloproteinase 20 (MMP20, enamelysin) gene cause autosomal-recessive amelogenesis imperfecta, and Mmp20 ablated mice also have malformed dental enamel. Here we showed that Mmp20 null mouse secretory-stage ameloblasts maintain a columnar shape and are present as a single layer of cells. However, the maturation-stage ameloblasts from null mouse cover extraneous nodules of ectopic calcified material formed at the enamel surface. Remarkably, nodule formation occurs in null mouse enamel when MMP20 is normally no longer expressed. The malformed enamel in Mmp20 null teeth was loosely attached to the dentin and the entire enamel layer tended to separate from the dentin, indicative of a faulty dentino-enamel junction (DEJ). The enamel rod pattern was also altered in Mmp20 null mice. Each enamel rod is formed by a single ameloblast and is a mineralized record of the migration path of the ameloblast that formed it. The enamel rods in Mmp20 null mice were grossly malformed or absent, indicating that the ameloblasts do not migrate properly when backing away from the DEJ. Thus, MMP20 is required for ameloblast cell movement necessary to form the decussating enamel rod patterns, for the prevention of ectopic mineral formation, and to maintain a functional DEJ.

Hydrolysis of amelogenin by matrix metalloprotease-20 accelerates mineralization in vitro

In the following respects, tooth enamel is a unique tissue in the mammalian body: (a) it is the most mineralized and hardest tissue in it comprising up to 95 wt% of apatite; (b) its microstructure is dominated by parallel rods composed of bundles of 40-60 nm wide apatite crystals with aspect ratios reaching up to 1:10,000 and (c) not only does the protein matrix that gives rise to enamel guides the crystal growth, but it also conducts its own degradation and removal in parallel. Hence, when mimicking the process of amelogenesis in vitro, crystal growth has to be coupled to proteolytic digestion of the amelogenin assemblies that are known to play a pivotal role in conducting the proper crystal growth. Experimental settings based on controlled and programmable titration of amelogenin sols digested by means of MMP-20 with buffered calcium and phosphate solutions were employed to imitate the formation of elongated, plate-shaped crystals. Whilst amelogenin can act as a promoter of nucleation and crystal growth alone, in this study we show that proteolysis exerts an additional nucleation- and growth-promoting effect. Hydrolysis of full-length amelogenin by MMP-20 decreases the critical time needed for the protein and peptides to adhere and to cover the substrate. The formation and immobilization of a protein layer subsequently reduces the time for calcium phosphate crystallization. Coupling the proteolytic reaction to titration in the presence of 0.4 mg/ml rH174 has been shown to have the same effect on the crystal growth promotion as quadrupling the concentration of rH174 to 1.6 mg/ml. Controlling the rate and the extent of the proteolytic cleavage can thus be used to control the nucleation and growth rates in a protein-guided crystallization system.

Amelogenin "nanorods" formation during proteolysis by Mmp-20

Amelogenin is cleaved by enamelysin (Mmp-20) soon after its secretion, and the cleavage products accumulate in specific locations during enamel formation, suggesting that parent amelogenin proteolysis is necessary for activating its functions. To investigate the precise roles of Mmp-20 and its influence on the assembly of amelogenin, an in vitro enzymatic digestion process mimicking the initial stages of amelogenin proteolysis was investigated at near-physiological conditions using recombinant porcine amelogenin (rP172) and enamelysin. Hierarchically organized nanorod structures formed during different digestion stages were detected by TEM. At the earliest stage, uniformly dispersed parent amelogenin spherical particles, mixed with some darker stained smaller spheres, and accompanying elongated chain-like nanostructures were observed. Cylindrical nanorods, which appeared to be the result of tight assembly of thin subunit cylindrical discs with thicknesses ranging from ∼2.5 to ∼6.0nm, were formed after an hour of proteolysis. These subunit building blocks stacked to form nanorods with maximum length of ∼100nm. With the production of more cleavage products, additional morphologies spontaneously evolved from the cylindrical nanorods. Larger ball-like aggregates ultimately formed at the end of proteolysis. The uniform spherical particles, nanorods, morphological patterns evolved from nanorods, and globular aggregated microstructures were successively formed by means of co-assembly of amelogenin and its cleavage products during a comparatively slow proteolysis process. We propose that, following the C-terminal cleavage of amelogenin, co-assembly with its fragments leads to formation of nanorod structures whose properties eventually dictate the super-structural organization of enamel matrix, controlling the elongated growth of enamel apatite crystals.

How amelogenin orchestrates the organization of hierarchical elongated microstructures of apatite

Amelogenin (Amel) accelerates the nucleation of hydroxyapatite (HAP) in supersaturated solutions of calcium phosphate (Ca-P), shortening the induction time (delay period), under near-physiological conditions of pH, temperature, and ionic strength. Hierarchically organized Amel and amorphous calcium phosphate (ACP) nanorod microstructures are formed involving a coassembly of Amel-ACP particles at low supersaturations and low protein concentrations in a slow, well-controlled, constant composition (CC) crystallization system. At the earliest nucleation stages, the CC method allows the capture of prenucleation clusters and intermediate nanoclusers, spherical nanoparticles, and nanochains prior to enamel-like nanorod microstructure formations at later maturation stages. Amel-ACP nanoscaled building blocks are formed spontaneously by synergistic interactions between flexible Amel protein molecules and Ca-P prenucleation clusters, and these spherical nanoparticles evolve by orientated aggregation to form nanochains. Our results suggest that, in vivo, Amel may determine the structure of enamel by controlling prenucleation cluster aggregation at the earliest stages by forming stable Amel-ACP microstructures prior to subsequent crystal growth and mineral maturation.

Elongated polyproline motifs facilitate enamel evolution through matrix subunit compaction

How does proline-repeat motif length in the proteins of teeth and bones relate to the evolution of vertebrates? Counterintuitively, longer repeat stretches are associated with smaller aggregated subunits within a supramolecular matrix, resulting in enhanced crystal length in mammalian versus amphibian tooth enamel.

Vertebrate body designs rely on hydroxyapatite as the principal mineral component of relatively light-weight, articulated endoskeletons and sophisticated tooth-bearing jaws, facilitating rapid movement and efficient predation. Biological mineralization and skeletal growth are frequently accomplished through proteins containing polyproline repeat elements. Through their well-defined yet mobile and flexible structure polyproline-rich proteins control mineral shape and contribute many other biological functions including Alzheimer's amyloid aggregation and prolamine plant storage. In the present study we have hypothesized that polyproline repeat proteins exert their control over biological events such as mineral growth, plaque aggregation, or viscous adhesion by altering the length of their central repeat domain, resulting in dramatic changes in supramolecular assembly dimensions. In order to test our hypothesis, we have used the vertebrate mineralization protein amelogenin as an exemplar and determined the biological effect of the four-fold increased polyproline tandem repeat length in the amphibian/mammalian transition. To study the effect of polyproline repeat length on matrix assembly, protein structure, and apatite crystal growth, we have measured supramolecular assembly dimensions in various vertebrates using atomic force microscopy, tested the effect of protein assemblies on crystal growth by electron microscopy, generated a transgenic mouse model to examine the effect of an abbreviated polyproline sequence on crystal growth, and determined the structure of polyproline repeat elements using 3D NMR. Our study shows that an increase in PXX/PXQ tandem repeat motif length results (i) in a compaction of protein matrix subunit dimensions, (ii) reduced conformational variability, (iii) an increase in polyproline II helices, and (iv) promotion of apatite crystal length. Together, these findings establish a direct relationship between polyproline tandem repeat fragment assemblies and the evolution and the design of vertebrate mineralized tissue microstructures. Our findings reveal that in the greater context of chordate evolution, the biological control of apatite growth by polyproline-based matrix assemblies provides a molecular basis for the evolution of the vertebrate body plan.

The microstructure of vertebrate bones and teeth is controlled by polyproline-rich protein matrices (such as amelogenin) that serve as a scaffold to control the assembly of biological apatites. In tooth enamel, amphibians have large amelogenin subunits and thin enamel while mammals have smaller amelogenin subunits in tandem with elongated crystals and complex prismatic organization. Using specific peptides and frog amelogenin overexpressed in mice, we confirmed the effect of the length of the elongated polyproline repeat on reduced matrix subunit dimensions and enhanced apatite crystal length. Three-dimensional structures solved by NMR (nuclear magnetic resonance) and surface modeling algorithms indicate that elongated polyproline repeat stretches in amelogenins affect the dimensions of the supramolecular matrix through an increase in polyproline II helices, resulting in a compaction of supramolecular subunit dimensions. We propose that the availability of readily shaped apatites and innovative mechanisms based on amelogenin-repeat motifsthat compartmentalize and shape biological minerals was essential for the rise of early vertebrates, enabling the manufacture of strong teeth and backbones that might have given vertebrates a decisive survival advantage in the competition for food and in the sophistication of locomotion.

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.

A solution NMR investigation into the early events of amelogenin nanosphere self-assembly initiated with sodium chloride or calcium chloride

Using solution-state NMR spectroscopy, new insights into the early events governing amelogenin supramolecular self-assembly have been identified using sodium chloride and calcium chloride to trigger the association. Two-dimensional 1H-15N HSQC spectra were recorded for 15N- and 13C-labeled murine amelogenin as a function of increasing NaCl and CaCl2 concentration beginning with solution conditions of 2% acetic acid at pH 3.0, where amelogenin was monomeric. Residue specific changes in molecular dynamics, manifested by the reduction in intensity and disappearance of 1H-15N HSQC cross-peaks, were observed with the addition of either salt to the protein. With increasing NaCl concentrations, residues between T21 and R31 near the N-terminus were affected first, suggesting that these residues may initiate amelogenin dimerization, the first step in nanosphere assembly. At higher NaCl concentrations, more residues near the N-terminus (Y12-I51) were affected, and with further additions of NaCl, residues near the C-terminus (L141-T171) began to show a similar change in molecular dynamics. With increasing CaCl2 concentrations, a similar stepwise change in molecular dynamics involving essentially the same set of amelogenin residues was observed. As the concentration of either salt was increased, a concomitant increase in the estimated overall rotational correlation time (tau(c)) was observed, consistent with assembly. Self-assembly into a dimer or trimer was established with dynamic light scattering studies under similar conditions that showed an increase in diameter of the smallest species from 4.1 nm in the absence of salt to 10 nm in the presence of salt. These results suggest a possible stepwise interaction mechanism, starting with the N-terminus and followed by the C-terminus, leading to amelogenin nanosphere assembly.

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.

Structure, orientation, and dynamics of the C-terminal hexapeptide of LRAP determined using solid-state NMR

Amelogenin is the predominant protein found during enamel development and has been shown to be essential to proper enamel formation. Leucine-rich amelogenin peptide (LRAP) is a naturally occurring splice variant that preserves the charged N- and C-termini of full length amelogenin, regions thought to be crucial in interacting with hydroxaypatite. Particularly, the highly charged C-terminal hexapeptide (KREEVD) is thought to be the region most intimately interacting with hydroxyapatite (HAP). The structure of this charged region was investigated, along with the proximity to the surface and the mobility of two of the residues. The structure was found to be consistent with a random coil or more extended structure, as has been seen for more internalized residues in the C-terminus. The backbone K54(13C'), V58(13C'), and V58(15N) were all found to be close to the surface of HAP, approximately 6.0 angstroms from the nearest 31P atom, suggesting a strong interaction and emphasizing the importance of these residues in interacting with HAP. However, both ends of the hexapeptide at residues K54 and V58 experience significant mobility under hydrated conditions, implying that another portion of the protein helps to stabilize the strong LRAP-HAP interaction. Interestingly, the backbone of the C-terminal third of the protein is consistently 6.0 angstroms from the HAP surface, providing a model in this region of the protein lying flat on the surface with no three-dimensional folding. The combination of these features, that is, a random coil structure, a significant mobility, and a lack of three-dimensional folding in this region of the protein, may have an important functional role, possibly allowing maximum crystal inhibition at low protein concentrations.

Functional roles of prolines at amelogenin C terminal during tooth enamel formation

Amelogenins, the chief proteins in enamel matrix, undergo progressive degradation by matrix metalloproteinase-20 (MMP-20) to facilitate crystal growth. Proline is the most abundant residue in amelogenin and is located upstream to all MMP-20 cleavage sites in the amelogenin sequence. Pro(41) is critical for amelogenin N-terminal processing, while the role of prolines at the amelogenin C terminus have not been determined. This study sought to elucidate the effect of the C-terminal prolines on apatite binding and MMP-20 hydrolysis. To compare apatite affinity, recombinant full-length human amelogenin (rh174) and mutated variants (P156T and P164T) were incubated with hydroxyapatite (HAP), and the unbound proteins were quantified by the Bradford assay. rh174 and mutants, as well as 3 oligopeptides, including wild-type peptide and peptides containing 2 mutations, were incubated with MMP-20 both in solution and on HAP. The digested products were analyzed by SDS-PAGE, reverse-phase high-performance liquid chromatography and mass spectrometry. Mutated amelogenins displayed a significantly lower affinity to HAP than the wild type (P156T < P164T < rH174). The proline mutation at amino acid location 164 significantly reduced the initial hydrolysis of either the amelogenins in solution or the proteins bound on HAP, which was confirmed by amelogenin oligopeptide assays. It was concluded that prolines at the amelogenin C terminus are essential for the initial processing of amelogenin and amelogenin-mineral interactions.

pH triggered self-assembly of native and recombinant amelogenins under physiological pH and temperature in vitro

Self-assembly of the extracellular matrix protein amelogenin is believed to play an essential role in regulating the growth and organization of enamel crystals during enamel formation. This study examines the effect of temperature and pH on amelogenin self-assembly under physiological pH conditions in vitro, using dynamic light scattering, turbidity measurements, and transmission electron microscopy. Full-length recombinant amelogenins from mouse (rM179) and pig (rP172) were investigated, along with proteolytic cleavage products (rM166 and native P148) lacking the hydrophilic C-terminus of parent molecules. Results indicated that the self-assembly of full-length amelogenin is primarily triggered by pH in the temperature range from 13 to 37 degrees C and not by temperature. Furthermore, very large assemblies of all proteins studied formed through the rearrangement of similarly sized nanospherical particles, although at different pH values: pH 7.7 (P148), pH 7.5 (rM166), pH 7.2 (rP172), and pH 7.2 (rM179). Structural differences were also observed. The full-length molecules formed apparently tightly connected elongated, high-aspect ratio assemblies comprised of small spheres, while the amelogenin cleavage products appeared as loosely associated spherical particles, suggesting that the hydrophilic C-terminus plays an essential role in higher-order amelogenin assembly. Hence, tightly controlled pH values during secretory amelogenesis may serve to regulate the functions of both full-length and cleaved amelogenins.

Assembly and processing of an engineered amelogenin proteolytic product (rP148)

The purpose of this study was to express, characterize, and investigate the self-assembly of a recombinant porcine amelogenin lacking the hydrophilic 24 C-terminal amino acids (rP148). To gain further insight into the function of amelogenin processing during enamel mineralization, this protein was also used as a substrate to examine the action of matrix metalloproteinase-20 (MMP-20). The assembly properties of rP148 were monitored by dynamic light scattering (DLS). In general, rP148 molecules assemble into monomers, dimers, oligomers, and some nanosphere-like particles. Depending on the solution conditions, large aggregates were also observed. Matrix metalloproteinase-20 cleaved the rP148 molecule at a few sites, creating a number of different products, including the tyrosine-rich amelogenin polypeptide (TRAP). Our data suggest that although rP148 self-assembles into small particles, its assembly properties are different from those of the full-length rP172, indicating that the C-terminal 24 amino acids play a critical role in nanosphere assembly. We further demonstrate that MMP-20 digests rP148 in a manner that generates a similar proteolytic pattern, as would be expected to occur in vivo.

Histidine tag fusion increases expression levels of active recombinant amelogenin in Escherichia coli

Amelogenin is a dental enamel matrix protein involved in formation of dental enamel. In this study, we have expressed two different recombinant murine amelogenins in Escherichia coli: the untagged rM179, and the histidine tagged rp(H)M180, identical to rM179 except that it carries the additional N-terminal sequence MRGSHHHHHHGS. The effects of the histidine tag on expression levels, and on growth properties of the amelogenin expressing cells were studied. Purification of a crude protein extract containing rp(H)M180 was also carried out using IMAC and reverse-phase HPLC. The results of this study showed clearly that both growth properties and amelogenin expression levels were improved for E. coli cells expressing the histidine tagged amelogenin rp(H)M180, compared to cells expressing the untagged amelogenin rM179. The positive effect of the histidine tag on amelogenin expression is proposed to be due to the hydrophilic nature of the histidine tag, generating a more hydrophilic amelogenin, which is more compatible with the host cell. Human osteoblasts treated with the purified rp(H)M180 showed increased levels of secreted osteocalcin, compared to untreated cells. This response was similar to cells treated with enamel matrix derivate, mainly composed by amelogenin, suggesting that the recombinant protein is biologically active. Thus, the histidine tag favors expression and purification of biologically active recombinant amelogenin.

On the formation of amelogenin microribbons

We recently reported the remarkable spontaneous self-assembly and hierarchical organization of amelogenin 'microribbons' and their ability to facilitate oriented growth of apatite crystals in vitro. In a letter of correction we communicated the finding that the X-ray diffraction pattern reported in our original report was that of cellulose contaminant and not amelogenin microribbon. We have re-evaluated our data and confirmed the protein nature of the microribbons using Fourier transform infrared and Raman microspectroscopy. Some microribbons were remarkably similar in their morphology to that of cellulose fibers. The size distribution of amelogenin microribbons was wider, particularly in width and length, and generally smaller than those originally reported. Here we present additional detailed information on the formation of a series of intermediate hierarchical structures of amelogenin assemblies prior to the formation of microribbon. The most significant finding was that full-length amelogenin nanospheres had a tendency to assemble into collinear arrays whose function is assumed to be critical at the initial stage of enamel mineral deposition. The present data gives an insight into the step-by-step assembly process of amelogenin from nanometer scale molecules to micrometer scale organized structures that can be used as templates for controlled and oriented growth of apatite mineralization in vitro.

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.

Comparative calcium binding of leucine-rich amelogenin peptide and full-length amelogenin

Leucine-rich amelogenin peptide (LRAP) is an alternately spliced amelogenin. LRAP is known to bind to hydroxyapatite, and has been shown to signal mesenchymal cells to proliferate, but its function in enamel formation is unclear. The purpose of this study was to determine the calcium-binding properties and structure of recombinant human LRAP (rLRAP) compared with full-length amelogenin (rH174). rLRAP and rH174 were synthesized in Escherichia coli and purified by affinity chromatography and reverse-phase high-performance liquid chromatography. Calcium binding was measured by isothermal titration calorimetry (ITC) at pH 7.5 and 25 degrees C, and raw data were analyzed by origin 7.0 software. The structure of rLRAP was analyzed by nuclear magnetic resonance (NMR) and circular dichroism (CD) in the absence or presence of Ca2+, pH 7.5 and 4.0, at 25 degrees C. Thermodynamic values showed that rLRAP had a Ca2+-binding affinity approximately 6.4-times greater than rH174. NMR and CD data revealed that rLRAP was randomly coiled, and that this structure was not altered by Ca2+, which bound to rLRAP and rH174 via ionic interactions. Unlike r174 (beta-spiral), rLRAP had a random-coiled structure. The calcium binding and structural differences between rLRAP and rH174 suggest that these proteins have different functions in enamel biomineralization.

Formation of the dentino-enamel interface in enamelysin (MMP-20)-deficient mouse incisors

An anomalous dentino-enamel junction (DEJ), manifested by delamination of the enamel layer, was reported in enamelysin [matrix metalloproteinase-20 (MMP-20)] knockout (KO) mice. To better understand the possible role of MMP-20 in the formation of the DEJ, we performed transmission electron microscopy (TEM) studies of the DEJ at early stages of tooth morphogenesis in KO mice. Our TEM analysis revealed that in the incisors from KO mice the mantle dentin is hypomineralized at the onset of enamel mineralization. At this early stage, TEM revealed no apparent differences in nascent aprismatic enamel between the KO mice and the controls. Hypomineralized mantle dentin was also observed in the incisors from KO mice, as assessed by back-scattered SEM at the secretory and early maturation stages, but not in the late-maturation stage, suggesting that the mineralization of mantle dentin is not completely arrested, but rather postponed. Histological studies indicate that the organic content in the initial enamel layer remains very high throughout amelogenesis. These results imply that MMP-20 is involved in the regulation of mineralization in mantle dentin and demonstrate the complex nature of DEJ formation. They also suggest that the structural and functional properties of the DEJ are determined during the initial mineralization stages.

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.

Matrix metalloproteinase inhibition impairs the processing, formation and mineralization of dental tissues during mouse molar development

Organotypic cultures of embryonic mouse tooth germs were used to investigate the developmental expression and roles of MMPs in the formation and mineralization of dentin and enamel matrices. The spatially and temporally regulated expression of MMP-2, MMP-9 and MMP-20 in developing mouse tooth germs in vivo was maintained in culture. The inhibition of metalloproteinases activity by marimastat altered the morphogenesis and mineralization of the tooth germs associated with an inhibition of the activation of both MMP-20 and MMP-2. MMP inhibition deregulated the molecular processing of two major dental matrix proteins, amelogenin and dentin sialoprotein (DSP). This coincided with their accumulation and the loss of their normal distribution within the extracellular matrix, resulting in a defective mineralization of dentin and enamel matrices. These findings demonstrate the critical role of MMPs in the processing and maturation of the dental matrix.

The effect of recombinant mouse amelogenins on the formation and organization of hydroxyapatite crystals in vitro

Amelogenin is the most abundant protein in developing dental enamel. It is believed to play an important role in the regulation of the growth and organization of enamel crystals. Amelogenin, unlike many other proteins found in biominerals, is mostly hydrophobic except for a 13 amino acid hydrophilic C-terminal domain. To clarify the role of amelogenin in enamel mineralization, we designed calcium phosphate crystal growth experiments in the presence of recombinant amelogenins with or without the charged C-terminal domain. The shape and organization of the crystals were examined by TEM in bright field and diffraction modes. It was found that both full-length and truncated amelogenin inhibit crystal growth in directions normal to the c-axis. At the same time, crystallites organized into parallel arrays only in the presence of the full-length amelogenin in monomeric form. Pre-assembled amelogenins had no effect on crystals organization. These results imply that the hydrophobic portion of amelogenin plays a role in an inhibition of crystal growth, whereas the C-terminal domain is essential for the alignment of crystals into parallel arrays. Our data also suggest that nascent enamel structure emerges as a result of cooperative interactions between forming crystals and assembling proteins.

Limited proteolysis of amelogenin: toward understanding the proteolytic processes in enamel extracellular matrix

This article is a short review of our recent study on controlled proteolysis of amelogenins by a series of commercially available proteinases as well as the tooth-specific metalloproteinase enamelysin. A limited proteolysis approach and mass spectrometry were applied in order to determine the surface accessibility of conserved domains of amelogenin nanospheres. Furthermore, this study was aimed at exploring the factors that affect the activity of enamel proteases to process amelogenins and at providing insight into the mechanisms of amelogenin degradation during amelogenesis. We found that, under limited conditions, certain amino acid residues at both the C- and N-termini of amelogenin are accessible to proteolytic action by a series of proteinases, suggesting that these regions are exposed on the surface of amelogenin nanospheres. Recombinant enamelysin cleaved amelogenin at the C-terminal region, showing a preference of the enzyme to cleave the S/M and F/S bonds. This result of enamelysin activity on amelogenin explains the abundance of the p148 (20k) pig amelogenin during the secretory stage of amelogenesis.

Assessment of enamelysin (MMP-20) selectivity to three peptide bonds on amelogenin sequence

Recent studies have highlighted the potential role of the metalloproteinase enamelysin (MMP-20) in controlling some of the most critical stages during enamel development. This study was aimed to assess the selectivity of enamelysin to the three most abundant cleavage sites on the amelogenin sequence, and to gain insight into the factors that control the pattern of amelogenin processing during enamel mineralization. Three deca-peptides with sequences based on pig amelogenin and including the proteolytic cleavage sites W/L, S/M, and P/A were synthesized as substrates. Statistical analysis revealed no significant differences in the rates of cleavage among the three peptides, indicating comparable selectivity of enamelysin for these peptide bonds. Considering the selective appearance of amelogenin proteolytic products, we suggest that amelogenin folding and assembly are the primary factors in controlling the pattern of its proteolysis during the secretory stage of enamel development.

Analysis of self-assembly and apatite binding properties of amelogenin proteins lacking the hydrophilic C-terminal

Amelogenins, the major protein component of the mineralizing enamel extracellular matrix, are critical for normal enamel formation as documented in the linkage studies of a group of inherited disorders, with defective enamel formation, called Amelogenesis imperfecta. Recent cases of Amelogenesis imperfecta include mutations that resulted in truncated amelogenin protein lacking the hydrophilic C-terminal amino acids. Current advances in knowledge on amelogenin structure, nanospheres assembly and their effects on crystal growth have supported the hypothesis that amelogenin nanospheres provide the organized microstructure for the initiation and modulated growth of enamel apatite crystals. In order to evaluate the function of the conserved hydrophilic C-terminal telopeptide during enamel biomineralization, the present study was designed to analyze the self-assembly and apatite binding behavior of amelogenin proteins and their isoforms lacking the hydrophilic C-terminal. We applied dynamic light scattering to investigate the size distribution of amelogenin nanospheres formed by a series of native and recombinant proteins. In addition, the apatite binding properties of these amelogenins were examined using commercially available hydroxyapatite crystals. Amelogenins lacking the carboxy-terminal (native P161 and recombinant rM166) formed larger nanospheres than those formed by their full-length precursors: native P173 and recombinant rM179. These data suggest that after removal of the hydrophilic carboxy-terminal segment further association of the nanospheres takes place through hydrophobic interactions. The affinity of amelogenins lacking the carboxy-terminal regions to apatite crystals was significantly lower than their parent amelogenins. These structure-functional analyses suggest that the hydrophilic carboxy-terminal plays critical functional roles in mineralization of enamel and that the lack of this segment causes abnormal mineralization.

Amelogenins: assembly, processing and control of crystal morphology

The remarkable properties of enamel crystals and their arrangements in an extraordinary micro-architecture are clear indications that the processes of crystal nucleation and growth in the extracellular matrix are highly controlled. The major extracellular events involved in enamel formation are: (a) delineation of space by the secretory ameloblasts and the dentino-enamel junction; (b) self-assembly of amelogenin proteins to form the supramolecular structural framework; (c) transportation of calcium and phosphate ions by the ameloblasts resulting in a supersaturated solution; (d) nucleation of apatite crystallites; and (e) elongated growth of the crystallites. Finally, during the 'maturation' step, rapid growth and thickening of the crystallites take place, which is concomitant with progressive degradation and eventual removal of the enamel extracellular matrix components (mainly amelogenins). This latter stage during which physical hardening of enamel occurs is perhaps unique to dental enamel. We have focused our in vitro studies on three major extracellular events: matrix assembly, matrix processing and control of crystal growth. This paper summarizes current knowledge on the assembly, processing and effect on crystal morphology by amelogenin proteins. The correlation between these three events and putative functional roles for amelogenin protein are discussed.