Cloning of rat amelotin and localization of the protein to the basal lamina of maturation stage ameloblasts and junctional epithelium

Enamel Hypomineralization and Structural Defects in Amelotin-deficient Mice

Amelotin (AMTN) is a relatively recently discovered enamel protein that is predominantly expressed by ameloblasts during the maturation stage of amelogenesis and is present at lower levels in the junctional epithelium of erupted teeth. Previous studies have suggested a function of this protein in enamel mineralization and cell attachment. Genetic mouse models have been instrumental in defining the role of many enamel-related proteins, but a genetic mouse model lacking the Amtn gene has not been reported. Here, we describe the generation of amelotin-deficient mice and the analysis of their enamel phenotype in comparison with that of wild-type animals. Ablation of AMTN expression resulted in mechanically inferior enamel of mandibular incisors that showed chipping and fractures at the incisal edge. Enamel mineralization was delayed, resulting in hypomineralized inner enamel and structural defects in the outer enamel. Erupted enamel close to the gingival margin showed increased surface roughness. The expression levels of the enamel matrix proteins AMEL, AMBN, ENAM, and ODAM and the enamel proteases MMP-20 and KLK-4 were not significantly altered, although the expression of KLK-4 was delayed. The morphology of ameloblasts showing prominent Tomes' processes during the secretory stage was not altered, and there was no indication of disruption of cell structures or activities, but a residual layer, presumably consisting of organic material, remained at the enamel surface close to the gingival margin. The integrity of the dentogingival attachment at the junctional epithelium appeared unaffected by AMTN deficiency. These observations indicate that AMTN plays a subtle yet critical role in enamel biomineralization, particularly during the establishment of the outer and surface enamel layers. This role appears to be largely independent of other enamel proteins.

Maturation and beyond: proteins in the developmental continuum from enamel epithelium to junctional epithelium

Enamel, covering the surface of teeth, is the hardest substance in mammals. It is designed to last a lifetime in spite of severe environmental challenges. Enamel is formed in a biomineralization process that is essentially divided into secretory and maturation stages. While the molecular events of enamel formation during the secretory stage have been elucidated to some extent, the mechanisms of enamel maturation are less defined, and little is known about the molecules present beyond the maturation stage. Several genes, all located within the secreted calcium-binding phosphoprotein (SCPP) gene cluster, were recently shown to be expressed during the developmental continuum from maturation stage ameloblasts to junctional epithelium (JE). This review introduces four such genes and their protein products, and presents our current state of knowledge on their roles, primarily in enamel formation and JE biology. The discovery of these proteins, and a more detailed analysis of their biological functions, will likely contribute to a more thorough understanding of the molecular mechanisms of enamel maturation and dentogingival attachment.

Characterisation of secretory calcium-binding phosphoprotein-proline-glutamine-rich 1: a novel basal lamina component expressed at cell-tooth interfaces

Functional genomic screening of the rat enamel organ (EO) has led to the identification of a number of secreted proteins expressed during the maturation stage of amelogenesis, including amelotin (AMTN) and odontogenic ameloblast-associated (ODAM). In this study, we characterise the gene, protein and pattern of expression of a related protein called secretory calcium-binding phosphoprotein-proline-glutamine-rich 1 (SCPPPQ1). The Scpppq1 gene resides within the secretory calcium-binding phosphoprotein (Scpp) cluster. SCPPPQ1 is a highly conserved, 75-residue, secreted protein rich in proline, leucine, glutamine and phenylalanine. In silico data mining has revealed no correlation to any known sequences. Northern blotting of various rat tissues suggests that the expression of Scpppq1 is restricted to tooth and associated tissues. Immunohistochemical analyses show that the protein is expressed during the late maturation stage of amelogenesis and in the junctional epithelium where it localises to an atypical basal lamina at the cell-tooth interface. This discrete localisation suggests that SCPPPQ1, together with AMTN and ODAM, participates in structuring the basal lamina and in mediating attachment of epithelia cells to mineralised tooth surfaces.

Topological mapping of BRIL reveals a type II orientation and effects of osteogenesis imperfecta mutations on its cellular destination

BRIL/IFITM5 is a membrane protein present almost exclusively in osteoblasts, which is believed to adopt a type III (N-out/C-out) topology. Mutations in IFITM5 cause OI type V, but the characteristics of the mutant protein and the mechanism involved are still unknown. The purpose of the current study was to re-assess the topology, localization, and biochemical properties of BRIL and compare it to the OI type V mutant in MC3T3 osteoblasts. Immunofluorescence labeling was performed with antibodies directed against BRIL N- or C-terminus. In intact cells, BRIL labeling was conspicuously detected at the plasma membrane only with the anti-C antibody. Detection of BRIL N-terminus was only possible after cell permeabilization, revealing both plasma membrane and Golgi labeling. Trypsinization of live cells expressing BRIL only cleaved off the C-terminus, confirming that it is a type II protein and that its N-terminus is intracellular. A truncated form of BRIL lacking the last 18 residues did not appear to affect localization, whereas mutation of a single leucine to arginine within the transmembrane segment abolished plasma membrane targeting. BRIL is first targeted to the endoplasmic reticulum as the entry point to the secretory pathway and rapidly traffics to the Golgi via a COPII-dependent pathway. BRIL was found to be palmitoylated and two conserved cysteine residues (C52 and C53) were critical for targeting to the plasma membrane. The OI type V mutant BRIL, having a five residue extension (MALEP) at its N-terminus, presented with exactly the same topological and biochemical characteristics as wild type BRIL. In contrast, the S42 > L mutant BRIL was trapped intracellularly in the Golgi. BRIL proteins and transcripts were equally detected in bone from a patient with OI type V, suggesting that the cause of the disease is a gain of function mediated by a faulty intracellular activity of the mutant BRIL.

Transcriptional regulation of amelotin gene by proinflammatory cytokines in gingival fibroblasts

Amelotin (AMTN) is a secreted protein expressed during the late stages of enamel formation and in the junctional epithelium. Among many differentially expressed genes, we found significantly increased AMTN mRNA level in inflamed gingiva by DNA microarray. The inductions of AMTN mRNA expressions in inflamed gingiva and human gingival fibroblasts (HGF) were confirmed by real-time polymerase chain reaction. To determine the molecular basis of the expression of AMTN and its regulation by proinflammatory cytokines, we have isolated and characterized the promoter region of mouse AMTN gene. Transient transfection assays were performed using luciferase constructs including mouse AMTN gene promoter. Interleukin-1β, Interleukin-6 and tumor necrosis factor-α induced AMTN mRNA levels in HGF. These cytokines increased the luciferase activities of the AMTN promoter constructs in HGF. The results suggest that proinflammatory cytokines induce AMTN gene transcription and a role for AMTN in gingival inflammation.

Expression and localization of laminin 5, laminin 10, type IV collagen, and amelotin in adult murine gingiva

The biochemical composition of the internal and external basal laminae in the junctional epithelium differs significantly, and the precise cellular origin of their respective molecules remains to be determined. In the present study, the expression and localization of three basement membrane-specific molecules-laminin 5 (γ2 chain), type IV collagen (α1 chain), and laminin 10 (α5 chain)-and one tooth-specific molecule, amelotin, was analyzed in adult murine gingiva by using in situ hybridization and immunohistochemistry. The results showed that the outermost cells in junctional epithelium facing the tooth enamel strongly expressed laminin 5 mRNA, supporting the immunohistochemical staining data. This suggests that laminin 5 is actively synthesized in junctional epithelial cells and that the products are incorporated into the internal basal lamina to maintain firm epithelial adhesion to the tooth enamel throughout life. Conversely, no amelotin mRNA signals were detected in the junctional epithelial cells, suggesting that the molecules localized on the internal basal lamina are mainly derived from maturation-stage ameloblasts. Weak and sporadic expression of type IV collagen in addition to laminin 10 in the gingiva indicates that these molecules undergo turnover less frequently in adult animals.

The junctional epithelium originates from the odontogenic epithelium of an erupted tooth

The junctional epithelium (JE) is an epithelial component that is directly attached to the tooth surface and has a protective function against periodontal diseases. In this study, we determined the origin of the JE using a bioengineered tooth technique. We transplanted the bioengineered tooth germ into the alveolar bone with an epithelial component that expressed green fluorescence protein. The reduced enamel epithelium from the bioengineered tooth fused with the oral epithelium, and the JE was apparently formed around the bioengineered tooth 50 days after transplantation. Importantly, the JE exhibited green fluorescence for at least 140 days after transplantation, suggesting that the JE was not replaced by oral epithelium. Therefore, our results demonstrated that the origin of the JE was the odontogenic epithelium, and odontogenic epithelium-derived JE was maintained for a relatively long period.

The specific role of FAM20C in amelogenesis

Previously, we showed that Sox2-Cre;Fam20C(fl/fl) mice in which Fam20C was ubiquitously inactivated had severe defects in dentin, enamel, and bone, along with hypophosphatemia. It remains to be determined if the enamel defects in the mice with universal inactivation of Family with sequence similarity 20-C (FAM20C) were associated with the dentin defects and whether hypophosphatemia in the knockout mice contributed to the enamel defects. In this study, we crossed Fam20C(fl/fl) mice with keratin 14-Cre (K14-Cre) transgenic mice to specifically inactivate Fam20C in the epithelial cells, including the dental epithelial cells that are responsible for forming tooth enamel. X-ray, backscattered scanning electron microscopic, and histological analyses showed that the K14-Cre;Fam20C(fl/fl) mice had severe enamel and ameloblast defects, while their dentin and alveolar bone were not significantly affected. Accordingly, serum biochemistry of the K14-Cre;Fam20C(fl/fl) mice showed normal phosphate and FGF23 levels in the circulation. Analysis of these data indicates that, while FAM20C is a molecule essential to amelogenesis, its inactivation in the dental epithelium does not significantly affect dentinogenesis. Hypophosphatemia makes no significant contribution to the enamel defects in the mice with the ubiquitous deletion of Fam20C.

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.

A post-classical theory of enamel biomineralization… and why we need one

Enamel crystals are unique in shape, orientation and organization. They are hundreds of thousands times longer than they are wide, run parallel to each other, are oriented with respect to the ameloblast membrane at the mineralization front and are organized into rod or interrod enamel. The classical theory of amelogenesis postulates that extracellular matrix proteins shape crystallites by specifically inhibiting ion deposition on the crystal sides, orient them by binding multiple crystallites and establish higher levels of crystal organization. Elements of the classical theory are supported in principle by in vitro studies; however, the classical theory does not explain how enamel forms in vivo. In this review, we describe how amelogenesis is highly integrated with ameloblast cell activities and how the shape, orientation and organization of enamel mineral ribbons are established by a mineralization front apparatus along the secretory surface of the ameloblast cell membrane.

Evolutionary analysis suggests that AMTN is enamel-specific and a candidate for AI

Molecular evolutionary analysis is an efficient method to predict and/or validate amino acid substitutions that could lead to a genetic disease and to highlight residues and motifs that could play an important role in the protein structure and/or function. We have applied such analysis to amelotin (AMTN), a recently identified enamel protein in the rat, mouse, and humans. An in silico search for AMTN provided 42 new mammalian sequences that were added to the 3 published sequences with which we performed the analysis using a dataset representative of all lineages (circa 220 million years of evolution), including 2 enamel-less species, sloth and armadillo. During evolution, of the 209 residues of human AMTN, 17 were unchanged and 34 had conserved their chemical properties. Substituting these important residues could lead to amelogenesis imperfecta (AI). Also, AMTN possesses a well-conserved signal peptide, 2 conserved motifs whose function is certainly important but unknown, and a putative phosphorylation site (SXE). In addition, the sequences of the 2 enamel-less species display mutations revealing that AMTN underwent pseudogenization, which suggests that AMTN is an enamel-specific protein.

FAM20C plays an essential role in the formation of murine teeth

FAM20C is highly expressed in bone and tooth. Previously, we showed that Fam20C conditional knock-out (KO) mice manifest hypophosphatemic rickets, which highlights the crucial roles of this molecule in promoting bone formation and mediating phosphate homeostasis. In this study, we characterized the dentin, enamel, and cementum of Sox2-Cre-mediated Fam20C KO mice. The KO mice exhibited small malformed teeth, severe enamel defects, very thin dentin, less cementum than normal, and overall hypomineralization in the dental mineralized tissues. In situ hybridization and immunohistochemistry analyses revealed remarkable down-regulation of dentin matrix protein 1 (DMP1) and dentin sialophosphoprotein in odontoblasts, along with a sharply reduced expression of ameloblastin and amelotin in ameloblasts. Collectively, these data indicate that FAM20C is essential to the differentiation and mineralization of dental tissues through the regulation of molecules critical to the differentiation of tooth-formative cells.

Protein-mediated enamel mineralization

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

Targeted overexpression of amelotin disrupts the microstructure of dental enamel

We have previously identified amelotin (AMTN) as a novel protein expressed predominantly during the late stages of dental enamel formation, but its role during amelogenesis remains to be determined. In this study we generated transgenic mice that produce AMTN under the amelogenin (Amel) gene promoter to study the effect of AMTN overexpression on enamel formation in vivo. The specific overexpression of AMTN in secretory stage ameloblasts was confirmed by Western blot and immunohistochemistry. The gross histological appearance of ameloblasts or supporting cellular structures as well as the expression of the enamel proteins amelogenin (AMEL) and ameloblastin (AMBN) was not altered by AMTN overexpression, suggesting that protein production, processing and secretion occurred normally in transgenic mice. The expression of Odontogenic, Ameloblast-Associated (ODAM) was slightly increased in secretory stage ameloblasts of transgenic animals. The enamel in AMTN-overexpressing mice was much thinner and displayed a highly irregular surface structure compared to wild type littermates. Teeth of transgenic animals underwent rapid attrition due to the brittleness of the enamel layer. The microstructure of enamel, normally a highly ordered arrangement of hydroxyapatite crystals, was completely disorganized. Tomes' process, the hallmark of secretory stage ameloblasts, did not form in transgenic mice. Collectively our data demonstrate that the overexpression of amelotin has a profound effect on enamel structure by disrupting the formation of Tomes' process and the orderly growth of enamel prisms.

Odontogenic ameloblast-associated and amelotin are novel basal lamina components

Odontogenic ameloblast-associated (ODAM) and amelotin (AMTN) are secreted by maturation stage ameloblasts and accumulate at the interface with enamel where an atypical basal lamina (BL) is present. This study aimed at determining and quantifying the ultrastructural distribution of ODAM and AMTN at the cell-tooth interface. Ultrathin sections of enamel organs from the early to mid- and late maturation stage of amelogenesis were processed for immunogold labeling with antibodies against ODAM, AMTN or with the lectins wheat germ agglutinin, Helix pomatia agglutinin (HPA) and Ricinus communis I agglutinin. Immunolabeling showed that both ODAM and AMTN localized to the BL. Quantitative analyses indicated that at the beginning of maturation there is a concentration of ODAM on the cell side of the BL while AMTN appears more concentrated on the enamel side. In the late maturation stage, such differential distribution is no longer apparent. All three lectins are bound to the BL. Competitive incubation with native lectins did not affect the binding efficiency of ODAM; however, AMTN binding was significantly reduced after incubation with HPA. In conclusion, ODAM and AMTN are bona fide components of the BL associated with maturation stage ameloblasts and they organize into different subdomains during the early maturation stage. The data also suggest that the BL is a dynamic structure that rearranges its organization as enamel maturation advances. Finally, the abrogation of AMTN antibody labeling by HPA supports the presence of O-linked sugars in the molecule and/or its close association with other O-glycosylated molecules.

Possible linkage of SP6 transcriptional activity with amelogenesis by protein stabilization

Ameloblasts produce enamel matrix proteins such as amelogenin, ameloblastin, and amelotin during tooth development. The molecular mechanisms of ameloblast differentiation (amelogenesis) are currently not well understood. SP6 is a transcription factor of the Sp/KLF family that was recently found to regulate cell proliferation in a cell-type-specific manner. Sp6-deficient mice demonstrate characteristic tooth anomalies such as delayed eruption of the incisors and supernumerary teeth with disorganized amelogenesis. However, it remains unclear how Sp6 controls amelogenesis. In this study, we used SP6 high producer cells to identify SP6 target genes. Based on the observations that long-term culture of SP6 high producer cells reduced SP6 protein expression but not Sp6 mRNA expression, we found that SP6 is short lived and specifically degraded through a proteasome pathway. We established an in vitro inducible SP6 expression system coupled with siRNA knockdown and found a possible linkage between SP6 and amelogenesis through the regulation of amelotin and Rock1 gene expression by microarray analysis. Our findings suggest that the regulation of SP6 protein stability is one of the crucial steps in amelogenesis.

Adenovirus gene transfer to amelogenesis imperfecta ameloblast-like cells

To explore gene therapy strategies for amelogenesis imperfecta (AI), a human ameloblast-like cell population was established from third molars of an AI-affected patient. These cells were characterized by expression of cytokeratin 14, major enamel proteins and alkaline phosphatase staining. Suboptimal transduction of the ameloblast-like cells by an adenovirus type 5 (Ad5) vector was consistent with lower levels of the coxsackie-and-adenovirus receptor (CAR) on those cells relative to CAR-positive A549 cells. To overcome CAR -deficiency, we evaluated capsid-modified Ad5 vectors with various genetic capsid modifications including “pK7” and/or “RGD” motif-containing short peptides incorporated in the capsid protein fiber as well as fiber chimera with the Ad serotype 3 (Ad3) fiber “knob” domain. All fiber modifications provided an augmented transduction of AI-ameloblasts, revealed following vector dose normalization in A549 cells with a superior effect (up to 404-fold) of pK7/RGD double modification. This robust infectivity enhancement occurred through vector binding to both α_vβ3/α_vβ5 integrins and heparan sulfate proteoglycans (HSPGs) highly expressed by AI-ameloblasts as revealed by gene transfer blocking experiments. This work thus not only pioneers establishment of human AI ameloblast-like cell population as a model for in vitro studies but also reveals an optimal infectivity-enhancement strategy for a potential Ad5 vector-mediated gene therapy for AI.

Expression of odontogenic ameloblast-associated protein, amelotin, ameloblastin, and amelogenin in odontogenic tumors: immunohistochemical analysis and pathogenetic considerations

Screening for expression of amelogenesis-related proteins represents a powerful molecular approach to characterize odontogenic tumors and investigate their pathogenesis. In this study, we have examined the presence and distribution of odontogenic ameloblast-associated protein (ODAM), amelotin (AMTN), ameloblastin (AMBN), and amelogenin (AMEL) by immunohistochemistry in samples of adenomatoid odontogenic tumor (AOT), calcifying epithelial odontogenic tumor (CEOT), developing odontoma, ameloblastoma, calcifying cystic odontogenic tumor (CCOT), ameloblastic fibroma (AF), myxoma, odontogenic fibroma (OF), and reduced enamel epithelia (REE). Positive results were obtained in those tumors with epithelial component, except for AF, OF, and ameloblastoma. ODAM was found around mineralized structures (dystrophic calcifications) and CEOT's amyloid, whereas AMTN stained the eosinophilic material of AOTs. The CCOT transitory cells to ghost cells were strongly positive with all proteins except AMEL, and the REE as well as odontomas showed immunoexpression for ODAM, AMTN, AMBN, and AMEL similar to those found in normal rat tooth germs. Based on these results, some histopathogenetic theories were formulated.

Comparative temporospatial expression profiling of murine amelotin protein during amelogenesis

Tooth enamel is formed in a typical biomineralization process under the guidance of specific organic components. Amelotin (AMTN) is a recently identified, secreted protein that is transcribed predominantly during the maturation stage of enamel formation, but its protein expression profile throughout amelogenesis has not been described in detail. The main objective of this study was to define the spatiotemporal expression profile of AMTN during tooth development in comparison with other known enamel proteins. A peptide antibody against AMTN was raised in rabbits, affinity purified and used for immunohistochemical analyses on sagittal and transverse paraffin sections of decalcified mouse hemimandibles. The localization of AMTN was compared to that of known enamel proteins amelogenin, ameloblastin, enamelin, odontogenic ameloblast-associated/amyloid in Pindborg tumors and kallikrein 4. Three-dimensional images of AMTN localization in molars at selected ages were reconstructed from serial stained sections, and transmission electron microscopy was used for ultrastructural localization of AMTN. AMTN was detected in ameloblasts of molars in a transient fashion, declining at the time of tooth eruption. Prominent expression in maturation stage ameloblasts of the continuously erupting incisor persisted into adulthood. In contrast, amelogenin, ameloblastin and enamelin were predominantly found during the early secretory stage, while odontogenic ameloblast-associated/amyloid in Pindborg tumors and kallikrein 4 expression in maturation stage ameloblasts paralleled that of AMTN. Secreted AMTN was detected at the interface between ameloblasts and the mineralized enamel. Recombinant AMTN protein did not mediate cell attachment in vitro. These results suggest a primary role for AMTN in the late stages of enamel mineralization.

Chymotrypsin C (caldecrin) is associated with enamel development

Two main proteases cleave enamel extracellular matrix proteins during amelogenesis. Matrix metalloprotease-20 (Mmp20) is the predominant enzyme expressed during the secretory stage, while kallikrein-related peptidase-4 (Klk4) is predominantly expressed during maturation. Mutations to both Mmp20 and Klk4 result in abnormal enamel phenotypes. During a recent whole-genome microarray analysis of rat incisor enamel organ cells derived from the secretory and maturation stages of amelogenesis, the serine protease chymotrypsin C (caldecrin, Ctrc) was identified as significantly up-regulated (> 11-fold) during enamel maturation. Prior reports indicate that Ctrc expression is pancreas-specific, albeit low levels were also noted in brain. We here report on the expression of Ctrc in the enamel organ. Quantitative PCR (qPCR) and Western blot analysis were used to confirm the expression of Ctrc in the developing enamel organ. The expression profile of Ctrc is similar to that of Klk4, increasing markedly during the maturation stage relative to the secretory stage, although levels of Ctrc mRNA are lower than for Klk4. The discovery of a new serine protease possibly involved in enamel development has important implications for our understanding of the factors that regulate enamel biomineralization.

PERP regulates enamel formation via effects on cell-cell adhesion and gene expression

Little is known about the role of cell-cell adhesion in the development of mineralized tissues. Here we report that PERP, a tetraspan membrane protein essential for epithelial integrity, regulates enamel formation. PERP is necessary for proper cell attachment and gene expression during tooth development, and its expression is controlled by P63, a master regulator of stratified epithelial development. During enamel formation, PERP is localized to the interface between the enamel-producing ameloblasts and the stratum intermedium (SI), a layer of cells subjacent to the ameloblasts. Perp-null mice display dramatic enamel defects, which are caused, in part, by the detachment of ameloblasts from the SI. Microarray analysis comparing gene expression in teeth of wild-type and Perp-null mice identified several differentially expressed genes during enamel formation. Analysis of these genes in ameloblast-derived LS8 cells upon knockdown of PERP confirmed the role for PERP in the regulation of gene expression. Together, our data show that PERP is necessary for the integrity of the ameloblast-SI interface and that a lack of Perp causes downregulation of genes that are required for proper enamel formation.

Regulation of dental enamel shape and hardness

Epithelial-mesenchymal interactions guide tooth development through its early stages and establish the morphology of the dentin surface upon which enamel will be deposited. Starting with the onset of amelogenesis beneath the future cusp tips, the shape of the enamel layer covering the crown is determined by five growth parameters: the (1) appositional growth rate, (2) duration of appositional growth (at the cusp tip), (3) ameloblast extension rate, (4) duration of ameloblast extension, and (5) spreading rate of appositional termination. Appositional growth occurs at a mineralization front along the ameloblast distal membrane in which amorphous calcium phosphate (ACP) ribbons form and lengthen. The ACP ribbons convert into hydroxyapatite crystallites as the ribbons elongate. Appositional growth involves a secretory cycle that is reflected in a series of incremental lines. A potentially important function of enamel proteins is to ensure alignment of successive mineral increments on the tips of enamel ribbons deposited in the previous cycle, causing the crystallites to lengthen with each cycle. Enamel hardens in a maturation process that involves mineral deposition onto the sides of existing crystallites until they interlock with adjacent crystallites. Neutralization of acidity generated by hydroxyapatite formation is a key part of the mechanism. Here we review the growth parameters that determine the shape of the enamel crown as well as the mechanisms of enamel appositional growth and maturation.

Distribution of amelotin in mouse tooth development

Amelotin is expressed and secreted by ameloblasts in tooth development, but amelotin distribution during enamel development is not clear. In this report, we first investigated amelotin expression in developing teeth by immunohistochemistry. Amelotin was detected in the enamel matrix at the secretion and maturation stages of enamel development. Amelotin was also observed at Tomes' processes on the apical ends of secretory ameloblasts. We then compared amelotin gene expression with those of amelogenin, enamelin, and ameloblastin in the mandibles of postnatal mice by RT-PCR. The expression of amelotin was detected as early as in postnatal day 0 mandibles and amelotin was coexpressed with amelogenin, ameloblastin, and enamelin during tooth development. These data strongly suggest that amelotin is an enamel matrix protein expressed at the secretion and maturation stages of enamel development.

Regulation of pH During Amelogenesis

During amelogenesis, extracellular matrix proteins interact with growing hydroxyapatite crystals to create one of the most architecturally complex biological tissues. The process of enamel formation is a unique biomineralizing system characterized first by an increase in crystallite length during the secretory phase of amelogenesis, followed by a vast increase in crystallite width and thickness in the later maturation phase when organic complexes are enzymatically removed. Crystal growth is modulated by changes in the pH of the enamel microenvironment that is critical for proper enamel biomineralization. Whereas the genetic bases for most abnormal enamel phenotypes (amelogenesis imperfecta) are generally associated with mutations to enamel matrix specific genes, mutations to genes involved in pH regulation may result in severely affected enamel structure, highlighting the importance of pH regulation for normal enamel development. This review summarizes the intra- and extracellular mechanisms employed by the enamel-forming cells, ameloblasts, to maintain pH homeostasis and, also, discusses the enamel phenotypes associated with disruptions to genes involved in pH regulation.

Identification of a fibronectin interaction site in the extracellular matrix protein ameloblastin

Mammalian teeth are composed of hydroxyapatite crystals that are embedded in a rich extracellular matrix. This matrix is produced by only two cell types, the mesenchymal odontoblasts and the ectodermal ameloblasts. Ameloblasts secrete the enamel proteins amelogenin, ameloblastin, enamelin and amelotin. Odontoblasts secrete collagen type I and several calcium-binding phosphoproteins including dentin sialophosphoprotein, dentin matrix protein, bone sialoprotein and osteopontin. The latter four proteins have recently been grouped in the family of the SIBLINGs (small integrin-binding ligand, N-linked glycoproteins) because they display similar gene structures and because they contain an RGD tripeptide sequence that binds to integrin receptors and thus mediates cell adhesion. We have prepared all the other tooth-specific proteins in recombinant form and examined whether they might also promote cell adhesion similar to the SIBLINGs. We found that only ameloblastin consistently mediated adhesion of osteoblastic and fibroblastic cells to plastic or titanium surfaces. The activity was dependent on the intact three-dimensional structure of ameloblastin and required de novo protein synthesis of the adhering cells. By deletion analysis and in vitro mutagenesis, the active site could be narrowed down to a sequence of 13 amino acid residues (VPIMDFADPQFPT) derived from exon 7 of the rat ameloblastin gene or exons 7-9 of the human gene. Kinetic studies and RNA interference experiments further demonstrated that this sequence does not directly bind to a cell surface receptor but that it interacts with cellular fibronectin, which in turn binds to integrin receptors. The identification of a fibronectin-binding domain in ameloblastin might permit interesting applications for dental implantology. Implants could be coated with peptides containing the active sequence, which in turn would recruit fibronectin from the patient's blood. The recruited fibronectin should then promote cell adhesion on the implant surface, thereby accelerating osseointegration of the implant.

Consequences for enamel development and mineralization resulting from loss of function of ameloblastin or enamelin

Although the nonamelogenin proteins, ameloblastin and enamelin, are both low-abundance and rapidly degrading components of forming enamel, they seem to serve essential developmental functions, as suggested by findings that an enamel layer fails to appear on teeth of mice genetically engineered to produce either a truncated form of ameloblastin (exons 5 and 6 deleted) or no enamelin at all (null). The purpose of this study was to characterize, by direct micro weighing, changes in enamel mineralization occurring on maxillary and mandibular incisors of mice bred for these alterations in nonamelogenin function (Ambn(+/+, +/-5,6, -5,6/-5,6), Enam(+/+, +/- ,-/-)). The results indicated similar changes to enamel-mineralization patterns within the altered genotypes, including significant decreases by as much as 50% in the mineral content of maturing enamel from heterozygous mice and the formation of a thin, crusty, and disorganized mineralized layer, rather than true enamel, on the labial (occlusal) surfaces of incisors and molars along with ectopic calcifications within enamel organ cells in Ambn(-5,6/-5,6) and Enam(-/-) homozygous mice. These findings confirm that both ameloblastin and enamelin are required by ameloblasts to create an enamel layer by appositional growth as well as to assist in achieving its unique high level of mineralization.

Mutations in the beta propeller WDR72 cause autosomal-recessive hypomaturation amelogenesis imperfecta

Healthy dental enamel is the hardest and most highly mineralized human tissue. Though acellular, nonvital, and without capacity for turnover or repair, it can nevertheless last a lifetime. Amelogenesis imperfecta (AI) is a collective term for failure of normal enamel development, covering diverse clinical phenotypes that typically show Mendelian inheritance patterns. One subset, known as hypomaturation AI, is characterised by near-normal volumes of organic enamel matrix but with weak, creamy-brown opaque enamel that fails prematurely after tooth eruption. Mutations in genes critical to enamel matrix formation have been documented, but current understanding of other key events in enamel biomineralization is limited. We investigated autosomal-recessive hypomaturation AI in a consanguineous Pakistani family. A whole-genome SNP autozygosity screen identified a locus on chromosome 15q21.3. Sequencing candidate genes revealed a point mutation in the poorly characterized WDR72 gene. Screening of WDR72 in a panel of nine additional hypomaturation AI families revealed the same mutation in a second, apparently unrelated, Pakistani family and two further nonsense mutations in Omani families. Immunohistochemistry confirmed intracellular localization in maturation-stage ameloblasts. WDR72 function is unknown, but as a putative β propeller is expected to be a scaffold for protein-protein interactions. The nearest homolog, WDR7, is involved in vesicle mobilization and Ca²⁺-dependent exocytosis at synapses. Vesicle trafficking is important in maturation-stage ameloblasts with respect to secretion into immature enamel and removal of cleaved enamel matrix proteins via endocytosis. This raises the intriguing possibility that WDR72 is critical to ameloblast vesicle turnover during enamel maturation.

Fam83h is associated with intracellular vesicles and ADHCAI

Defects in FAM83H on human chromosome 8q24.3 cause autosomal-dominant hypocalcified amelogenesis imperfecta (ADHCAI). FAM83H does not encode a recognizable signal peptide, so we predicted that the Fam83h protein functions within the cell. We tested this hypothesis by constitutively expressing mouse Fam83h with green fluorescent protein (GFP) fused to its C-terminus in HEK293 and HeLa cell lines. Green fluorescent signal from the Fam83h-GFP fusion protein was associated with perinuclear vesicles, usually in the vicinity of the Golgi apparatus. No signal was observed within the nucleus. In addition, we identified FAM83H nonsense mutations in Hispanic (C1330C>T; p.Q444X) and Caucasian (c.1192C>T; p.Q398X) families with ADHCAI. We conclude that Fam83h localizes in the intracellular environment, is associated with vesicles, and plays an important role in dental enamel formation. FAM83H is the first gene involved in the etiology of amelogenesis imperfecta (AI) that does not encode a secreted protein.

Molecular genetics of ameloblast cell lineage

Late tooth morphogenesis is characterized by a series of events that determine crown morphogenesis and the histodifferentiation of epithelial cells into enamel-secreting ameloblasts and of mesenchymal cells into dentin-secreting odontoblasts. Functional ameloblasts are tall, columnar, polarized cells that synthesize and secrete a number of enamel-specific proteins. After depositing the full thickness of enamel matrix, ameloblasts shrink in size and regulate enamel maturation. Amelogenesis imperfecta (AI) is a heterogeneous group of inherited defects in enamel formation. Clinically, AI presents as a spectrum of enamel malformations that are categorized as hypoplastic, hypocalcified, or hypomaturation types, based upon the thickness and hardness of the enamel. The different types of AI are inherited, either as X-linked, autosomal-dominant, or autosomal-recessive traits. Recently, several gene mutations have been identified to cause the subtypes of AI. How these genes, however, coordinate their function to control amelogenesis is not understood. In this review, we discuss the role of genes that play definitive role on the determination of ameloblast cell fate and life cycle based on studies in transgenic animals.

The SCPP gene repertoire in bony vertebrates and graded differences in mineralized tissues

The vertebrate tooth is covered with enamel in most sarcopterygians or enameloid in chondrichthyans and actinopterygians. The evolutionary relationship among these two tissues, the hardest tissue in the body, and other mineralized tissues has long been controversial. We have recently reported that specific combinations of secretory calcium-binding phosphoprotein (SCPP) genes are involved in the mineralization of bone, dentin, enameloid, and enamel. Thus, the early repertoire of SCPP genes would elucidate the evolutionary relationship across these tissues. However, the diversity of SCPP genes in teleosts and tetrapods and the roles of these genes in distinct tissues have remained unclear, mainly because many SCPP genes are lineage-specific. In this study, I show that the repertoire of SCPP genes in the zebrafish, frog, and humans includes many lineage-specific genes and some widely conserved genes that originated in stem osteichthyans or earlier. Expression analysis demonstrates that some frog and zebrafish SCPP genes are used primarily in bone, but also in dentin, while the reverse is true of other genes, similar to some mammalian SCPP genes. Dentin and enameloid initially use shared genes in the matrix, but enameloid is subsequently hypermineralized. Notably, enameloid and enamel use an orthologous SCPP gene in the hypermineralization process. Thus, the hypermineralization machinery ancestral to both enameloid and enamel arose before the actinopterygian-sarcopterygian divergence. However, enamel employs specialized SCPPs as structuring proteins, not used in enameloid, reflecting the divergence of enamel from enameloid. These results show graded differences in mineralized dental tissues and reinforce the hypothesis that bone-dentin-enameloid-enamel constitutes an evolutionary continuum.

Functions of KLK4 and MMP-20 in dental enamel formation

Two proteases are secreted into the enamel matrix of developing teeth. The early protease is enamelysin (MMP-20). The late protease is kallikrein 4 (KLK4). Mutations in MMP20 and KLK4 both cause autosomal recessive amelogenesis imperfecta, a condition featuring soft, porous enamel containing residual protein. MMP-20 is secreted along with enamel proteins by secretory-stage ameloblasts. Enamel protein-cleavage products accumulate in the space between the crystal ribbons, helping to support them. MMP-20 steadily cleaves accumulated enamel proteins, so their concentration decreases with depth. KLK4 is secreted by transition- and maturation-stage ameloblasts. KLK4 aggressively degrades the retained organic matrix following the termination of enamel protein secretion. The principle functions of MMP-20 and KLK4 in dental enamel formation are to facilitate the orderly replacement of organic matrix with mineral, generating an enamel layer that is harder, less porous, and unstained by retained enamel proteins.

Expression of odontogenic ameloblast-associated protein (ODAM) in dental and other epithelial neoplasms

We previously have communicated our discovery that the amyloid associated with calcifying epithelial odontogenic tumors is composed of N-terminal fragments of the structurally novel odontogenic ameloblast-associated protein designated ODAM. Subsequently, it was shown by other investigators that ODAM is expressed in rodent enamel organ and is likely involved in dental development. We now report that this molecule also is found in certain human tissues, principally the salivary gland and trachea, as evidenced by RNA array analysis and immunohistochemistry-utilizing antibodies prepared against synthetic ODAM-related peptides and recombinant protein. Notably, these reagents immunostained normal and malignant ameloblasts and other types of human neoplastic cells, including those of gastric, lung, and breast origin where the presence in the latter was confirmed by in situ hybridization using gene-specific molecular probes. Moreover, significant titers of anti-ODAM IgG antibodies were detected in the sera of patients with these malignancies. Our studies have provided the first evidence in humans for the cellular expression of ODAM in normal and diseased states. Based on our findings, we posit that ODAM is a developmental antigen that has an essential role in tooth maturation and in the pathogenesis of certain odontogenic and other epithelial neoplasms; further, we suggest that ODAM may serve as a novel prognostic biomarker, as well as a potential diagnostic and therapeutic target for patients with breast and other epithelial forms of cancer.

SCPP gene evolution and the dental mineralization continuum

Many genes critical to vertebrate skeletal mineralization are members of the secretory calcium-binding phosphoprotein (SCPP) gene family, which has evolved by gene duplication from a single ancestral gene. In humans, mutations in some of these SCPP genes have been associated with various diseases related to dentin or enamel hypoplasia. Recently, systematic searches for SCPP genes of various species have allowed us to investigate the history of phylogenetically variable dental tissues as a whole. One important conclusion is that not all disease-associated SCPP genes are present in tetrapods, and teleost fish probably have none, even in toothed species, having acquired their complement of SCPP genes through an independent duplication history. Here, we review comparative analyses of mineralized dental tissues, with particular emphasis on the use of SCPPs, within and between tetrapods and teleosts. Current knowledge suggests a close relationship among bone, dentin, teleost fish enameloid (enamel-like hard tissue), and tetrapod enamel. These tissues thus form a mineralized-tissue continuum. Contemporary dental tissues have evolved from an ancestral continuum through lineage-specific modifications.

Characterization of Apin, a secreted protein highly expressed in tooth-associated epithelia

We previously reported expression of a protein by enamel organ (EO) cells in rat incisors, originally isolated from the amyloid of Pindborg odontogenic tumors called Apin. The aim of the present study was to further characterize the Apin gene and its protein in various species, assess tissue specificity, and clarify its localization within the EO. Northern blotting and RT-PCR revealed that expression of Apin was highest in the EO and gingiva, moderate in nasal and salivary glands, and lowest in the epididymis. The protein sequences deduced from the cloned cDNA for rat, mouse, pig, and human were aligned together with those obtained from four other mammal genomes. Apin is highly conserved in mammals but is absent in fish, birds, and amphibians. Comparative SDS-PAGE analyses of the protein obtained from bacteria, transfected cells, and extracted from EOs all indicated that Apin is post-translationally modified, a finding consistent with the presence of predicted sites for phosphorylation and O-linked glycosylation. In rodent incisors, Apin was detected only in the ameloblast layer of the EO, starting at post-secretory transition and extending throughout the maturation stage. Intense labeling was visible over the Golgi region as well as on the apices of ameloblasts abutting the enamel matrix. Apin was also immunodetected in epithelial cells of the gingiva which bind it to the tooth surface (junctional epithelium). The presence of Apin at cell-tooth interfaces suggests involvement in adhesive mechanisms active at these sites, but its presence among other epithelial tissues indicates Apin likely possesses broader physiological roles.

Identification and characterization of a novel mouse peroxisome proliferator-activated receptor alpha-regulated and starvation-induced gene, Ppsig

The peroxisome proliferator-activated receptor alpha (PPARalpha) has been known to play a pivotal role in maintaining the energy balance during fasting; however, the battery of PPARalpha target genes involved in this metabolic response is still not fully characterized. Here, we report the identification and characterization of Ppsig (for PPARalpha-regulated and starvation-induced gene) with unknown biological function from mouse liver. Multiple Ppsig cDNAs which differed in the 3'-untranslated regions were identified. The open reading frame of Ppsig cDNA is 1830 bp which encodes a protein of 67.33 kDa. Ppsig contains 11 exons spanning at least 10 kb. Although the exact biological function of Ppsig is still not known, we found that Ppsig mRNA transcript was dramatically up-regulated during 72 h fasting and following treatment with a potent PPARalpha agonist, in a tissue-specific and PPARalpha-dependent manner. A functional peroxisome proliferator-response element was found in the intron 1 of Ppsig, thus confirming that Ppsig is a novel direct mouse PPARalpha target gene. This finding might help in elucidating the transcriptional regulatory mechanism of Ppsig in the cellular response to fasting.

Ultrastructure and composition of basement membrane separating mature ameloblasts from enamel

At a late stage of amelogenesis, a basement-membrane-like (BML) structure appears between mature ameloblasts and the enamel surface. Although this BML structure is known to contain certain basement membrane components, its detailed nature and role were not well defined. As such, this study examined the BML structure using high-resolution electron microscopy combined with immunohistochemical staining. Mandibular rat incisors were processed for the preparation of Epon sections for ultrastructural observations, and frozen sections were used for immunostaining laminin, heparan sulphate proteoglycan (HSPG) and type IV collagen. The BML structure was characterized by the presence of abundant ribbon-like 'double tracks', 4.5-5.0 nm wide; the form known to be taken by HSPG in basal laminae. The main ultrastructural component of basal laminae, known as 'cords', was replaced by fine filaments of type IV collagen. Immunohistochemical staining of the BML structure showed an intense reaction for HSPG, moderate staining for type IV collagen and negligible staining for laminin. These observations indicate that this structure is an atypical basement membrane in which the cord network is replaced by type IV collagen filaments. However, the BML structure was found to be unusually rich in HSPG, similar to kidney glomerular basement membrane. It is likely that this specialized basement membrane mediates firm attachment of mature ameloblasts to the enamel surface, and filters the influx and efflux of materials to and from enamel during maturation.

Identification of novel genes expressed during mouse tooth development by microarray gene expression analysis

To identify genes heretofore undiscovered as critical players in the biogenesis of teeth, we have used microarray gene expression analysis of the developing mouse molar tooth (DMT) between postnatal day (P) 1 and P10 to identify genes differentially expressed when compared with 16 control tissues. Of the top 100 genes exhibiting increased expression in the DMT, 29 were found to have been previously associated with tooth development. Differential expression of the remaining 71 genes not previously associated with tooth development was confirmed by quantitative reverse transcription-polymerase chain reaction analysis. Further analysis of seven of the latter genes by mRNA in situ hybridization found that five were specific to the developing tooth in the craniofacial region (Rspo4, Papln, Amtn, Gja1, Maf). Of the remaining two, one was found to be more widely expressed (Sp7) and the other was found to be specific to the nasal serous gland, which is close to, but distinct from, the developing tooth (Vrm).

Enamel formation and amelogenesis imperfecta

Dental enamel is the epithelial-derived hard tissue covering the crowns of teeth. It is the most highly mineralized and hardest tissue in the body. Dental enamel is acellular and has no physiological means of repair outside of the protective and remineralization potential provided by saliva. Enamel is comprised of highly organized hydroxyapatite crystals that form in a defined extracellular space, the contents of which are supplied and regulated by ameloblasts. The entire process is under genetic instruction. The genetic control of amelogenesis is poorly understood, but requires the activities of multiple components that are uniquely important for dental enamel formation. Amelogenesis imperfecta (AI) is a collective designation for the variety of inherited conditions displaying isolated enamel malformations, but the designation is also used to indicate the presence of an enamel phenotype in syndromes. Recently, genetic studies have demonstrated the importance of genes encoding enamel matrix proteins in the etiology of isolated AI. Here we review the essential elements of dental enamel formation and the results of genetic analyses that have identified disease-causing mutations in genes encoding enamel matrix proteins. In addition, we provide a fresh perspective on the roles matrix proteins play in catalyzing the biomineralization of dental enamel.

Rat wct mutation induces a hypo-mineralization form of amelogenesis imperfecta and cyst formation in molar teeth

Our previous findings have demonstrated that the rat autosomal-recessive mutation, whitish chalk-like teeth (wct), induces enamel defects resembling those of human amelogenesis imperfecta (AI) in continuously growing incisor teeth. The present study clarifies the effect of the wct mutation on the morphogenesis and calcification of rat molar teeth. Formalin-fixed maxillae obtained from animals aged 4-30 days were examined by electron probe micro-analysis (EPMA) and by immunocytochemistry for amelogenin, ameloblastin, and enamelin. There were no distinct differences in the calcium and phosphorous contents and the amount of enamel between homozygous mutant and wild-type teeth during postnatal days 4-11. Although the mineral density in the enamel matrix considerably increased in the wild-type teeth until day 15, no changes occurred in mutant teeth during days 11-30. The immunoreactivity for enamel proteins in the secretory-stage ameloblasts in mutant teeth was similar to that in the wild-type teeth, and subsequently mutant maturation-stage ameloblasts became detached from the enamel surface, resulting in odontogenic cyst formation between the enamel organ and matrix until day 7 and the expansion of the cyst around the whole tooth crown on day 15. On day 30, the erupted mutant teeth presented morphological changes such as enamel destruction and tertiary dentin formation in addition to low mineral density in the enamel. Thus, the wct mutation prevents mineral transport without disturbing the synthesis of enamel proteins in molar teeth because of the absence of maturation-stage ameloblasts, in addition to the occurrence of odontogenic cysts.

Rat wct mutation prevents differentiation of maturation-stage ameloblasts resulting in hypo-mineralization in incisor teeth

A recent study provided genetic and morphological evidence that rat autosomal-recessive mutation, whitish chalk-like teeth (wct), induced tooth enamel defects resembling those of human amelogenesis imperfecta (AI). The wct locus maps to a specific interval of rat chromosome 14 corresponding to human chromosome 4q21 where the ameloblastin and enamelin genes exist, although these genes are not included in the wct locus. The effect of the wct gene mutation on the enamel matrix synthesis and calcification remains to be elucidated. This study clarifies how the wct gene mutation influences the synthesis of enamel matrix and its calcification by immunocytochemistry for amelogenin, ameloblastin and enamelin, and by electron probe micro-analysis (EPMA). The immunoreactivity for enamel proteins such as amelogenin, ameloblastin, and enamelin in the ameloblasts in the homozygous teeth was the same as that in the heterozygous teeth from secretory to transitional stages, although the homozygous ameloblasts became detached from the enamel matrix in the transitional stage. The flattened ameloblasts in the maturation stage of the homozygous samples contained enamel proteins in their cytoplasm. Thus, the wct mutation was found to prevent the morphological transition of ameloblasts from secretory to maturation stages without disturbing the synthesis of enamel matrix proteins, resulting in the hypo-mineralization of incisor enamel and cyst formation between the enamel organ and matrix. This mutation also prevents the transfer of iron into the enamel.