Rescue of the murine amelogenin null phenotype with two amelogenin transgenes

The anatomy, neurophysiology, and cellular mechanisms of intradental sensation

Somatosensory innervation of the oral cavity enables the detection of a range of environmental stimuli including minute and noxious mechanical forces. The trigeminal sensory neurons underlie sensation originating from the tooth. Prior work has provided important physiological and molecular characterization of dental pulp sensory innervation. Clinical dental experiences have informed our conception of the consequence of activating these neurons. However, the biological role of sensory innervation within the tooth is yet to be defined. Recent transcriptomic data, combined with mouse genetic tools, have the capacity to provide important cell-type resolution for the physiological and behavioral function of pulp-innervating sensory neurons. Importantly, these tools can be applied to determine the neuronal origin of acute dental pain that coincides with tooth damage as well as pain stemming from tissue inflammation (i.e., pulpitis) toward developing treatment strategies aimed at relieving these distinct forms of pain.

Enamel biomineralization under the effects of indomethacin and celecoxib non-steroidal anti-inflammatory drugs

The aim of this study was to explore the effects of nonsteroidal anti-inflammatory drugs on biomineralization of enamel. Sixty C57Bl6 male mice were used, which were assigned into three groups: celecoxib (n = 20) or indomethacin (n = 20) treatment for a period of 28 days or received no medication (control group, n = 20). Visual inspection and microcomputed tomography were used to analyze enamel morphology. Scanning electron microscopy–Energy dispersive X-ray and Knoop microhardness test were used to quantify chemical element content (Ca, P, C, O) and enamel microhardness, respectively. Tissues were collected to investigate the synthesis, activity or nuclear translocation of metalloproteinase-20, transcription factor Runx2, dentin sialoprotein and cyclooxygenase-2 enzyme by means of immunohistochemistry, in situ zymography and indirect immunofluorescence. Treatment with indomethacin and celecoxib reduced the Ca and P content, microhardness and mineral density in enamel. Treatment with nonsteroidal anti-inflammatory drugs caused an accumulation of metalloproteinase-20 and overall increased enzymatic activity in enamel matrix, while the synthesis of the transcription factor Runx2 was inhibited by these drugs. Interestingly, indomethacin inhibited Runx2 translocation to the nucleus whereas celecoxib did not. Those findings show that non-steroidal anti-inflammatory drugs impact the enamel biomineralization and could be involved in the etiology tooth enamel defects if used during the period of tooth formation and mineralization.

How prenatal environmental factors affect rat molar enamel formation?

Amelogenin (AMELX) and ameloblastin (AMBN) are crucial for enamel formation, and interruptions in the production of these proteins may cause enamel defects. We investigated how prenatal environmental factors (chronic stress, bisphenol A (BPA), amoxicillin, and lipopolysaccharide (LPS)) affect AMELX and AMBN production of ameloblasts. Fifteen pregnant Sprague-Dawley rats were divided into four experimental groups and a control group. Chronic-stress group rats were exposed to a 12:12 light/light cycle (LL) from day E18 until delivery. BPA group rats were orally administered 5 μg/kg BPA daily from day E1 until delivery. Amoxicillin group rats were injected 100 mg/kg amoxicillin daily from day E18 until delivery. LPS-infection group rats were injected 125 μg/kg bacterial LPS once on day E18. Seven pups from the control group and ten pups from the experimental groups were euthanized on P10. Sections were stained with hematoxylin and eosin (H&E) and Gomori's one-step trichrome staining (GT) and incubated with rabbit polyclonal antibodies to AMELX and AMBN, to evaluate staining intensity at ameloblast stages. The surface morphology was evaluated with a stereomicroscope. AMELX (p = 0.008, p = 0.0001, p = 0.009) and AMBN (p = 0.002, p = 0.001, p = 0.0001) staining of all groups were significantly lower than that of the control group in the secretory, transitional, and maturation stages. Abnormal enamel matrix formation was observed in the H&E and GT staining sections of all experimental groups. Yellowish coloration of the amoxicillin group was observed in morphologic evaluation.

Tooth Enamel and its Dynamic Protein Matrix

Tooth enamel is the outer covering of tooth crowns, the hardest material in the mammalian body, yet fracture resistant. The extremely high content of 95 wt% calcium phosphate in healthy adult teeth is achieved through mineralization of a proteinaceous matrix that changes in abundance and composition. Enamel-specific proteins and proteases are known to be critical for proper enamel formation. Recent proteomics analyses revealed many other proteins with their roles in enamel formation yet to be unraveled. Although the exact protein composition of healthy tooth enamel is still unknown, it is apparent that compromised enamel deviates in amount and composition of its organic material. Why these differences affect both the mineralization process before tooth eruption and the properties of erupted teeth will become apparent as proteomics protocols are adjusted to the variability between species, tooth size, sample size and ephemeral organic content of forming teeth. This review summarizes the current knowledge and published proteomics data of healthy and diseased tooth enamel, including advancements in forensic applications and disease models in animals. A summary and discussion of the status quo highlights how recent proteomics findings advance our understating of the complexity and temporal changes of extracellular matrix composition during tooth enamel formation.

Phosphorylated and Non-phosphorylated Leucine Rich Amelogenin Peptide Differentially Affect Ameloblast Mineralization

The Leucine Rich Amelogenin Peptide (LRAP) is a product of alternative splicing of the amelogenin gene. As full length amelogenin, LRAP has been shown, in precipitation experiments, to regulate hydroxyapatite (HAP) crystal formation depending on its phosphorylation status. However, very few studies have questioned the impact of its phosphorylation status on enamel mineralization in biological models. Therefore, we have analyzed the effect of phosphorylated (+P) or non-phosphorylated (−P) LRAP on enamel formation in ameloblast-like cell lines and ex vivo cultures of murine postnatal day 1 molar germs. To this end, the mineral formed was analyzed by micro-computed tomography, Field Emission Scanning Electron Microscopy, Transmission Electron Microscopy, Selected Area Electon Diffraction imaging. Amelogenin gene transcription was evaluated by qPCR analysis. Our data show that, in both cells and germ cultures, LRAP is able to induce an up-regulation of amelogenin transcription independently of its phosphorylation status. Mineral formation is promoted by LRAP(+P) in all models, while LRAP(–P) essentially affects HAP crystal formation through an increase in crystal length and organization in ameloblast-like cells. Altogether, these data suggest a differential effect of LRAP depending on its phosphorylation status and on the ameloblast stage at the time of treatment. Therefore, LRAP isoforms can be envisioned as potential candidates for treatment of enamel lesions or defects and their action should be further evaluated in pathological models.

Dose-Dependent Rescue of KO Amelogenin Enamel by Transgenes in Vivo

Mice lacking amelogenin (KO) have hypoplastic enamel. Overexpression of the most abundant amelogenin splice variant M180 and LRAP transgenes can substantially improve KO enamel, but only ~40% of the incisor thickness is recovered and the prisms are not as tightly woven as in WT enamel. This implies that the compositional complexity of the enamel matrix is required for different aspects of enamel formation, such as organizational structure and thickness. The question arises, therefore, how important the ratio of different matrix components, and in particular amelogenin splice products, is in enamel formation. Can optimal expression levels of amelogenin transgenes representing both the most abundant splice variants and cleavage product at protein levels similar to that of WT improve the enamel phenotype of KO mice? Addressing this question, our objective was here to understand dosage effects of amelogenin transgenes (Tg) representing the major splice variants M180 and LRAP and cleavage product CTRNC on enamel properties. Amelogenin KO mice were mated with M180Tg, CTRNCTg and LRAPTg mice to generate M180Tg and CTRNCTg double transgene and M180Tg, CTRNCTg, LRAPTg triple transgene mice with transgene hemizygosity (on one allelle) or homozygosity (on both alleles). Transgene homo- vs. hemizygosity was determined by qPCR and relative transgene expression confirmed by Western blot. Enamel volume and mineral density were analyzed by microCT, thickness and structure by SEM, and mechanical properties by Vickers microhardness testing. There were no differences in incisor enamel thickness between amelogenin KO mice with three or two different transgenes, but mice homozygous for a given transgene had significantly thinner enamel than mice hemizygous for the transgene (p < 0.05). The presence of the LRAPTg did not improve the phenotype of M180Tg/CTRNCTg/KO enamel. In the absence of endogenous amelogenin, the addition of amelogenin transgenes representing the most abundant splice variants and cleavage product can rescue abnormal enamel properties and structure, but only up to a maximum of ~80% that of molar and ~40% that of incisor wild-type enamel.

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.

Abrogation of epithelial BMP2 and BMP4 causes Amelogenesis Imperfecta by reducing MMP20 and KLK4 expression

Amelogenesis Imperfecta (AI) can be caused by the deficiencies of enamel matrix proteins, molecules responsible for the transportation and secretion of enamel matrix components, and proteases processing enamel matrix proteins. In the present study, we discovered the double deletion of bone morphogenetic protein 2 (Bmp2) and bone morphogenetic protein 4 (Bmp4) in the dental epithelium by K14-cre resulted in hypoplastic enamel and reduced density in X-ray radiography as well as shortened enamel rods under scanning electron microscopy. Such enamel phenotype was consistent with the diagnosis of hypoplastic amelogenesis imperfecta. Histological and molecular analyses revealed that the removal of matrix proteins in the mutant enamel was drastically delayed, which was coincided with the greatly reduced expression of matrix metalloproteinase 20 (MMP20) and kallikrein 4 (KLK4). Although the expression of multiple enamel matrix proteins was down-regulated in the mutant ameloblasts, the cleavage of ameloblastin was drastically impaired. Therefore, we attributed the AI primarily to the reduction of MMP20 and KLK4. Further investigation found that BMP/Smad4 signaling pathway was down-regulated in the K14-cre;Bmp2(f/f);Bmp4(f/f)ameloblasts, suggesting that the reduced MMP20 and KLK4 expression may be due to the attenuated epithelial BMP/Smad4 signaling.

Truncated amelogenin and LRAP transgenes improve Amelx null mouse enamel

Amelogenin is the most abundant enamel protein involved in enamel mineralization. Our goal was to determine whether all three regions of amelogenin (N-terminus, C-terminus, central core) are required for enamel formation. Amelogenin RNA is alternatively spliced, resulting in at least 16 different amelogenin isoforms in mice, with M180 and LRAP expressed most abundantly. Soon after secretion by ameloblasts, M180 is cleaved by MMP20 resulting in C-terminal truncated (CTRNC) amelogenin. We aimed to determine whether the 2 transgenes (Tg), LRAP and CTRNC together, can improve LRAPTg/Amelx-/- and CTRNCTg/Amelx-/- enamel thickness and prism organization, which were not rescued in Amelx-/- enamel. We generated CTRNCTg/LRAPTg/Amelx-/- mice and analyzed developing and mature incisor and molar enamel histologically, by microCT, SEM and microhardness testing. CTRNCTg and LRAPTg overexpression together significantly improved the enamel phenotype of LRAPTg/Amelx-/- and CTRNCTg/Amelx-/- mouse enamel, however enamel microhardness was recovered only when M180Tg was expressed, alone or with LRAPTg. We determined that both LRAP and CTRNC, which together express all three regions of the amelogenin protein (N-terminus, C-terminus and hydrophobic core) contribute to the final enamel thickness and prism organization in mice.

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.

Bmp2 deletion causes an amelogenesis imperfecta phenotype via regulating enamel gene expression

Although Bmp2 is essential for tooth formation, the role of Bmp2 during enamel formation remains unknown in vivo. In this study, the role of Bmp2 in regulation of enamel formation was investigated by the Bmp2 conditional knock out (Bmp2 cKO) mice. Teeth of Bmp2 cKO mice displayed severe and profound phenotypes with asymmetric and misshaped incisors as well as abrasion of incisors and molars. Scanning electron microscopy analysis showed that the enamel layer was hypoplastic and enamel lacked a typical prismatic pattern. Teeth from null mice were much more brittle as tested by shear and compressive moduli. Expression of enamel matrix protein genes, amelogenin, enamelin, and enamel-processing proteases, Mmp-20 and Klk4 was reduced in the Bmp2 cKO teeth as reflected in a reduced enamel formation. Exogenous Bmp2 up-regulated those gene expressions in mouse enamel organ epithelial cells. This result for the first time indicates Bmp2 signaling is essential for proper enamel development and mineralization in vivo.

Exon4 amelogenin transcripts in enamel biomineralization

Amelogenins are proteins formed by alternative splicing of the amelogenin gene, and are essential for tooth enamel formation. However, the unique functions of various alternatively spliced amelogenins in enamel formation are not well understood. In this study, we determined the spatiotemporal location of amelogenins derived from transcripts containing exon4 (AMG+4) in the enamel matrix, and the relative binding of recombinant AMG+4 to hydroxyapatite (HAP). Immunohistochemistry and mass spectrometry analyses showed that AMG+4 proteins were secreted into the enamel matrix at the early maturation stage. A stage-specific increase in the synthesis of AMG+4 was further supported by our observation that in mice overexpressing leucine-rich amelogenin peptide (TgLRAP), in which ameloblasts differentiate earlier, AMG+4 transcripts were also upregulated earlier. In vitro binding studies, supported by in silico modeling of protein binding to calcium and phosphate, showed that more recombinant AMG+4 bound to hydroxyapatite (HAP) as compared with recombinant AMG-4. The temporal and spatial localization of amelogenins containing exon4 peptide, and their functional differences in HAP binding, suggests that the unique properties of amelogenins containing exon4 cause a specific enhancement of biomineralization related to stabilization of early-formed HAP at the maturation stage.

Weaker dental enamel explains dental decay

Dental caries continues to be the most prevalent bacteria-mediated non-contagious disease of humankind. Dental professionals assert the disease can be explained by poor oral hygiene and a diet rich in sugars but this does not account for caries free individuals exposed to the same risk factors. In order to test the hypothesis that amount of amelogenin during enamel development can influence caries susceptibility, we generated multiple strains of mice with varying levels of available amelogenin during dental development. Mechanical tests showed that dental enamel developed with less amelogenin is "weaker" while the dental enamel of animals over-expressing amelogenin appears to be more resistant to acid dissolution.

The leucine-rich amelogenin protein (LRAP) is primarily monomeric and unstructured in physiological solution

Amelogenin proteins are critical to the formation of enamel in teeth and may have roles in controlling growth and regulating microstructures of the intricately woven hydroxyapatite (HAP). Leucine-rich amelogenin protein (LRAP) is a 59-residue splice variant of amelogenin and contains the N- and C-terminal charged regions of the full-length protein thought to control crystal growth. Although the quaternary structure of full-length amelogenin in solution has been well studied and can consist of self-assemblies of monomers called nanospheres, there is limited information on the quaternary structure of LRAP. Here, sedimentation velocity analytical ultracentrifugation (SV) and small angle neutron scattering (SANS) were used to study the tertiary and quaternary structure of LRAP at various pH values, ionic strengths, and concentrations. We found that the monomer is the dominant species of phosphorylated LRAP (LRAP(+P)) over a range of solution conditions (pH 2.7-4.1, pH 4.5-8, 50 mmol/L(mM) to 200 mM NaCl, 0.065-2 mg/mL). The monomer is also the dominant species for unphosphorylated LRAP (LRAP(-P)) at pH 7.4 and for LRAP(+P) in the presence of 2.5 mM calcium at pH 7.4. LRAP aggregates in a narrow pH range near the isoelectric point of pH 4.1. SV and SANS show that the LRAP monomer has a radius of ∼2.0 nm and an asymmetric structure, and solution NMR studies indicate that the monomer is largely unstructured. This work provides new insights into the secondary, tertiary, and quaternary structure of LRAP in solution and provides evidence that the monomeric species may be an important functional form of some amelogenins.

Analysis of enamel development using murine model systems: approaches and limitations

A primary goal of enamel research is to understand and potentially treat or prevent enamel defects related to amelogenesis imperfecta (AI). Rodents are ideal models to assist our understanding of how enamel is formed because they are easily genetically modified, and their continuously erupting incisors display all stages of enamel development and mineralization. While numerous methods have been developed to generate and analyze genetically modified rodent enamel, it is crucial to understand the limitations and challenges associated with these methods in order to draw appropriate conclusions that can be applied translationally, to AI patient care. We have highlighted methods involved in generating and analyzing rodent enamel and potential approaches to overcoming limitations of these methods: (1) generating transgenic, knockout, and knockin mouse models, and (2) analyzing rodent enamel mineral density and functional properties (structure and mechanics) of mature enamel. There is a need for a standardized workflow to analyze enamel phenotypes in rodent models so that investigators can compare data from different studies. These methods include analyses of gene and protein expression, developing enamel histology, enamel pigment, degree of mineralization, enamel structure, and mechanical properties. Standardization of these methods with regard to stage of enamel development and sample preparation is crucial, and ideally investigators can use correlative and complementary techniques with the understanding that developing mouse enamel is dynamic and complex.

Mouse genetic background influences the dental phenotype

Dental enamel covers the crown of the vertebrate tooth and is considered to be the hardest tissue in the body. Enamel develops during secretion of an extracellular matrix by ameloblast cells in the tooth germ, prior to eruption of the tooth into the oral cavity. Secreted enamel proteins direct mineralization patterns during the maturation stage of amelogenesis as the tooth prepares to erupt. The amelogenins are the most abundant enamel proteins and are required for normal enamel development. Phenotypic differences were observed between incisors from individual Amelx (amelogenin) null mice that had a mixed 129xC57BL/6J genetic background and between inbred wild-type (WT) mice with different genetic backgrounds (C57BL/6J, C3H/HeJ, FVB/NJ). We hypothesized that this could be due to modifier genes, as human patients with a mutation in an enamel protein gene causing the enamel defect amelogenesis imperfecta (AI) can also have varied appearance of dentitions within a kindred. Enamel density measurements varied for all WT inbred strains midway during incisor development. Enamel thickness varied between some WT strains, and, unexpectedly, dentin density varied extensively between incisors and molars of all WT and Amelx null strains studied. WTFVB/NJ incisors were more similar to those of Amelx null mice than to those of the other WT strains in terms of incisor height/width ratio and pattern of enamel mineralization. Strain-specific differences led to the conclusion that modifier genes may be implicated in determining both normal development and severity of enamel appearance in AI mouse models and may in future studies be related to phenotypic heterogeneity within human AI kindreds reported in the literature.

Enamelin is critical for ameloblast integrity and enamel ultrastructure formation

Mutations in the human enamelin gene cause autosomal dominant hypoplastic amelogenesis imperfecta in which the affected enamel is thin or absent. Study of enamelin knockout NLS-lacZ knockin mice revealed that mineralization along the distal membrane of ameloblast is deficient, resulting in no true enamel formation. To determine the function of enamelin during enamel formation, we characterized the developing teeth of the Enam^−/− mice, generated amelogenin-driven enamelin transgenic mouse models, and then introduced enamelin transgenes into the Enam^−/− mice to rescue enamel defects. Mice at specific stages of development were subjected to morphologic and structural analysis using β-galactosidase staining, immunohistochemistry, and transmission and scanning electron microscopy. Enamelin expression was ameloblast-specific. In the absence of enamelin, ameloblasts pathology became evident at the onset of the secretory stage. Although the aggregated ameloblasts generated matrix-containing amelogenin, they were not able to create a well-defined enamel space or produce normal enamel crystals. When enamelin is present at half of the normal quantity, enamel was thinner with enamel rods not as tightly arranged as in wild type suggesting that a specific quantity of enamelin is critical for normal enamel formation. Enamelin dosage effect was further demonstrated in transgenic mouse lines over expressing enamelin. Introducing enamelin transgene at various expression levels into the Enam^−/− background did not fully recover enamel formation while a medium expresser in the Enam^+/− background did. Too much or too little enamelin abolishes the production of enamel crystals and prism structure. Enamelin is essential for ameloblast integrity and enamel formation.

M180 amelogenin processed by MMP20 is sufficient for decussating murine enamel

Amelogenin (AMELX) and matrix metalloproteinase-20 (MMP20) are essential for proper enamel development. Amelx and Mmp20 mutations cause amelogenesis imperfecta. MMP20, a protease secreted by ameloblasts, is responsible for processing enamel proteins, including AMELX, during the secretory stage of enamel formation. Of at least 16 different amelogenin splice products, the most abundant isoform found in murine ameloblasts and developing enamel is the M180 protein. To understand the role of MMP20 processing of M180 AMELX, we generated AmelxKO/Mmp20KO (DKO) mice with an amelogenin (M180Tg) transgene. We analyzed the enamel phenotype by SEM to determine enamel structure and thickness, µCT, and by nanoindentation to quantify enamel mechanical properties. M180Tg/DKO mouse enamel had 37% of the hardness of M180Tg/AmelxKO teeth and demonstrated a complete lack of normal prismatic architecture. Although molar enamel of M180Tg/AmelxKO mice was thinner than WT, it had similar mechanical properties and decussating enamel prisms, which were abolished by the loss of MMP20 in the M180Tg/DKO mice. Retention of the C-terminus or complete lack of this domain is unable to rescue amelogenin null enamel. We conclude that among amelogenins, M180 alone is sufficient for normal enamel mechanical properties and prism patterns, but that additional amelogenin splice products are required to restore enamel thickness.

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.

Neutron reflectometry studies of the adsorbed structure of the amelogenin, LRAP

Amelogenins make up over 90% of the protein present during enamel formation and have been demonstrated to be critical in proper enamel development, but the mechanism governing this control is not well understood. Leucine-rich amelogenin peptide (LRAP) is a 59-residue splice variant of amelogenin and contains the charged regions from the full protein thought to control crystal regulation. In this work, we utilized neutron reflectivity (NR) to investigate the structure and orientation of LRAP adsorbed from solutions onto molecularly smooth COOH-terminated self-assembled monolayer (SAM) surfaces. Sedimentation velocity (SV) experiments revealed that LRAP is primarily a monomer in saturated calcium phosphate (SCP) solutions (0.15 M NaCl) at pH 7.4. LRAP adsorbed as ∼32 Å thick layers at ∼70% coverage as determined by NR. Rosetta simulations of the dimensions of LRAP in solution (37 Å diameter) indicate that the NR determined z dimension is consistent with an LRAP monomer. SV experiments and Rosetta simulations show that the LRAP monomer has an extended, asymmetric shape in solution. The NR data suggests that the protein is not completely extended on the surface, having some degree of structure away from the surface. A protein orientation with the C-terminal and inner N-terminal regions (residues ∼8-24) located near the surface is consistent with the higher scattering length density (SLD) found near the surface by NR. This work presents new information on the tertiary and quaternary structure of LRAP in solution and adsorbed onto surfaces. It also presents further evidence that the monomeric species may be an important functional form of amelogenin proteins.

Gene evolution and functions of extracellular matrix proteins in teeth

The extracellular matrix (ECM) not only provides physical support for tissues, but it is also critical for tissue development, homeostasis and disease. Over 300 ECM molecules have been defined as comprising the "core matrisome" in mammals through the analysis of whole genome sequences. During tooth development, the structure and functions of the ECM dynamically change. In the early stages, basement membranes (BMs) separate two cell layers of the dental epithelium and the mesenchyme. Later in the differentiation stages, the BM layer is replaced with the enamel matrix and the dentin matrix, which are secreted by ameloblasts and odontoblasts, respectively. The enamel matrix genes and the dentin matrix genes are each clustered in two closed regions located on human chromosome 4 (mouse chromosome 5), except for the gene coded for amelogenin, the major enamel matrix protein, which is located on the sex chromosomes. These genes for enamel and dentin matrix proteins are derived from a common ancestral gene, but as a result of evolution, they diverged in terms of their specific functions. These matrix proteins play important roles in cell adhesion, polarity, and differentiation and mineralization of enamel and dentin matrices. Mutations of these genes cause diseases such as odontogenesis imperfect (OI) and amelogenesis imperfect (AI). In this review, we discuss the recently defined terms matrisome and matrixome for ECMs, as well as focus on genes and functions of enamel and dentin matrix proteins.

The role of amelogenin during enamel-crystallite growth and organization in vivo

Amelogenin is critical for enamel formation, and human amelogenin gene (AMELX) mutations cause hypoplastic and/or hypomaturation enamel phenotypes. The Amelx null (AKO) mouse has a severe hypoplastic phenotype. This study evaluated the effect of amelogenin loss on enamel formation and crystallite morphology. Enamel from AKO and wild-type (WT) mice was used. The AKO mice were mated with transgenic mice expressing the most abundant known amelogenin isoform, TgM180-87, to rescue (KOM180-87) the enamel crystallite phenotype. Molar enamel was embedded, sectioned with a diamond microtome, and images were obtained by transmission electron microscopy. The crystallite sizes from multiple sections were measured using Image J. The mean thicknesses (WT = 26 nm, AKO = 16 nm, and KOM180-87 = 25 nm) and the mean widths (WT = 96 nm, AKO = 59 nm, KOM180-87 = 85 nm) of crystallites were measured. Despite a complete loss of amelogenin in AKO mice, a mineralized enamel layer with well-defined and organized crystallites is formed. In the absence of amelogenin, enamel crystallites were reduced in thickness and width. For the first time we show that introduction of the m180 amelogenin isoform into the AKO mouse through cross-breeding rescues the crystallite phenotype. We conclude that amelogenin is essential for the development of normal crystallite size.