Cloning, characterization and immunolocalization of human ameloblastin

Ameloblastin and its multifunctionality in amelogenesis: A review

Extracellular matrix proteins play crucial roles in the formation of mineralized tissues like bone and teeth via multifunctional mechanisms. In tooth enamel, ameloblastin (Ambn) is one such multifunctional extracellular matrix protein implicated in cell signaling and polarity, cell adhesion to the developing enamel matrix, and stabilization of prismatic enamel morphology. To provide a perspective for Ambn structure and function, we begin this review by describing dental enamel and enamel formation (amelogenesis) followed by a description of enamel extracellular matrix. We then summarize the established domains and motifs in Ambn protein, human amelogenesis imperfecta cases, and genetically engineered mouse models involving mutated or null Ambn. We subsequently delineate in silico, in vitro, and in vivo evidence for the amphipathic helix in Ambn as a proposed cell-matrix adhesive and then more recent in vitro evidence for the multitargeting domain as the basis for dynamic interactions of Ambn with itself, amelogenin, and membranes. The multitargeting domain facilitates tuning between Ambn-membrane interactions and self/co-assembly and supports a likely overall role for Ambn as a matricellular protein. We anticipate that this review will enhance the understanding of multifunctional matrix proteins by consolidating diverse mechanisms through which Ambn contributes to enamel extracellular matrix mineralization.

Proteolytic profiles of two isoforms of human AMBN expressed in E. coli by MMP-20 and KLK-4 proteases

Ameloblastin is a protein in biomineralization of tooth enamel. However recent results indicate that this is probably not its only role in an organism. Enamel matrix formation represents a complex process enabled via specific crosslinking of two proteins - the most abundant amelogenin and the ameloblastin (AMBN). The human AMBN (hAMBN) gene possesses 13 protein coding exons with alternatively spliced transcripts and the longest isoform about 447 amino acid residues. It has been described that AMBN molecules in vitro assemble into oligomers via a sequence encoded by exon 5. Enamel is formed by the processing of enamel proteins by two specific proteases - enamelysin (MMP-20) and kallikrein 4 (KLK-4). The scaffold made of AMEL and non-amelogenin proteins is cleaved and removed from the developed tooth enamel. The hAMBN is expressed in two isoforms (ISO I and II), which could lead to their different utilization determined by distinct proteolytic profiles. In this study, we compared proteolytic profiles of both isoforms of hAMBN expressed in E. coli after proteolysis by MMP-20, KLK-4, and their 1:2 mixture. Proteolysis products were analysed and cleavage sites were identified by mass spectrometry. The proteolytic profiles of two AMBN isoforms showed different results, although we have to determine that the analysed AMBN was not post-translationally modified as expressed in prokaryotic cells. These results may lead to the suggestion of potentially divergent roles of AMBN isoforms cleavage products in various cell signalling pathways such as calcium buffering or signalling cascades.

Novel Ameloblastin Variants, Contrasting Amelogenesis Imperfecta Phenotypes

Amelogenesis imperfecta (AI) comprises a group of rare, inherited disorders with abnormal enamel formation. Ameloblastin (AMBN), the second most abundant enamel matrix protein (EMP), plays a critical role in amelogenesis. Pathogenic biallelic loss-of-function AMBN variants are known to cause recessive hypoplastic AI. A report of a family with dominant hypoplastic AI attributed to AMBN missense change p.Pro357Ser, together with data from animal models, suggests that the consequences of AMBN variants in human AI remain incompletely characterized. Here we describe 5 new pathogenic AMBN variants in 11 individuals with AI. These fall within 3 groups by phenotype. Group 1, consisting of 6 families biallelic for combinations of 4 different variants, have yellow hypoplastic AI with poor-quality enamel, consistent with previous reports. Group 2, with 2 families, appears monoallelic for a variant shared with group 1 and has hypomaturation AI of near-normal enamel volume with pitting. Group 3 includes 3 families, all monoallelic for a fifth variant, which are affected by white hypoplastic AI with a thin intact enamel layer. Three variants, c.209C>G; p.(Ser70*) (groups 1 and 2), c.295T>C; p.(Tyr99His) (group 1), and c.76G>A; p.(Ala26Thr) (group 3) were identified in multiple families. Long-read AMBN locus sequencing revealed these variants are on the same conserved haplotype, implying they originate from a common ancestor. Data presented therefore provide further support for possible dominant as well as recessive inheritance for AMBN-related AI and for multiple contrasting phenotypes. In conclusion, our findings suggest pathogenic AMBN variants have a more complex impact on human AI than previously reported.

Expression of 20 SCPP genes during tooth and bone mineralization in Senegal bichir

The African bichir (Polypterus senegalus) is a living representative of Polypteriformes. P. senegalus possesses teeth composed of dentin covered by an enameloid cap and a layer of collar enamel on the tooth shaft, as in lepisosteids. A thin layer of enamel matrix can also be found covering the cap enameloid after its maturation and during the collar enamel formation. Teleosts fish do not possess enamel; teeth are protected by cap and collar enameloid, and inversely in sarcopterygians, where teeth are only covered by enamel, with the exception of the cap enameloid in teeth of larval urodeles. The presence of enameloid and enamel in the teeth of the same organism is an opportunity to solve the evolutionary history of the presence of enamel/enameloid in basal actinopterygians. In silico analyses of the jaw transcriptome of a juvenile bichir provided twenty SCPP transcripts. They included enamel, dentin, and bone-specific SCPPs known in sarcopterygians and several actinopterygian-specific SCPPs. The expression of these 20 genes was investigated by in situ hybridizations on jaw sections during tooth and dentary bone formation. A spatiotemporal expression patterns were established and compared with previous studies of SCPP gene expression during enamel/enameloid and bone formation. Similarities and differences were highlighted, and several SCPP transcripts were found specifically expressed during tooth or bone formation suggesting either conserved or new functions of these SCPPs.

The Dynamic Interactions of a Multitargeting Domain in Ameloblastin Protein with Amelogenin and Membrane

The enamel matrix protein Ameloblastin (Ambn) has critical physiological functions, including regulation of mineral formation, cell differentiation, and cell-matrix adhesion. We investigated localized structural changes in Ambn during its interactions with its targets. We performed biophysical assays and used liposomes as a cell membrane model. The xAB2N and AB2 peptides were rationally designed to encompass regions of Ambn that contained self-assembly and helix-containing membrane-binding motifs. Electron paramagnetic resonance (EPR) on spin-labeled peptides showed localized structural gains in the presence of liposomes, amelogenin (Amel), and Ambn. Vesicle clearance and leakage assays indicated that peptide-membrane interactions were independent from peptide self-association. Tryptophan fluorescence and EPR showed competition between Ambn-Amel and Ambn-membrane interactions. We demonstrate localized structural changes in Ambn upon interaction with different targets via a multitargeting domain, spanning residues 57 to 90 of mouse Ambn. Structural changes of Ambn following its interaction with different targets have relevant implications for the multifunctionality of Ambn in enamel formation.

Production of recombinant human ameloblastin by a fully native purification pathway

Ameloblastin (Ambn) is an intrinsically disordered protein (IDP) with a specific function of forming heterogenous homooligomers. The oligomeric function is led through a specific sequence encoded by exon 5 of Ambn. Due to the IDP character of Ambn to form oligomers, protein purification is subject to many challenges. Human ameloblastin (AMBN) and its two isoforms, I and II have already been purified as a recombinant protein in a bacterial expression system and functionally characterized in vitro. However, here we present a new purification protocol for the production of native AMBN in its original formation as a homooligomeric heterogeneous IDP. The purification process consists of three chromatographic steps utilizing His-tag and Twin Strep-tag affinity chromatography, along with size exclusion and reverse affinity chromatography. The presented workflow offers the production of AMBN in sufficient yield for in vitro protein characterizations and can be used to produce both AMBN isoforms I and II.

Ameloblastin promotes polarization of ameloblast cell lines in a 3-D cell culture system

Studies on animal models with mutations in ameloblastin gene have suggested that the extracellular matrix protein ameloblastin (AMBN) plays important roles in controlling cell-matrix adhesion and ameloblast polarization during amelogenesis. In order to examine the function of AMBN in cell polarization and morphology, we developed an in vitro 3D cell culture model to examine the effect of AMBN and amelogenin (AMEL) addition on ameloblast cell lines. We further used high resolution confocal microscopy to detect expression of polarization markers in response to AMBN addition. Addition of AMBN to the 3D culture matrix resulted in the clustering and elongation (higher aspect ratio) of ALC in a dose dependent manner. The molar concentration of AMEL required to exact this response from ALC was 2.75- times greater than that of AMBN. This polarization effect of ameloblastin was attributable directly to an evolutionary conserved domain within its exon 5-encoded region. The lack of exon 6-encoded region also influenced AMBN-cell interactions but to a lesser extent. The clusters formed with AMBN were polarized with expression of E-cadherin, Par3 and Cldn1 assembly at the nascent cell-cell junctions. The elongation effect was specific to epithelial cells of ameloblastic lineage ALC and LS8 cells. Our data suggest that AMBN may play critical signaling roles in the initiation of cell polarity by acting as a communicator between cell-cell and cell-matrix interactions. Our investigation has important implications for understanding the function of ameloblastin in enamel-cell matrix adhesion and the outcomes may contribute to efforts to develop strategies for enamel tissue regeneration.

Characterization of AMBN I and II Isoforms and Study of Their Ca2+-Binding Properties

Ameloblastin (Ambn) as an intrinsically disordered protein (IDP) stands for an important role in the formation of enamel—the hardest biomineralized tissue commonly formed in vertebrates. The human ameloblastin (AMBN) is expressed in two isoforms: full-length isoform I (AMBN ISO I) and isoform II (AMBN ISO II), which is about 15 amino acid residues shorter than AMBN ISO I. The significant feature of AMBN—its oligomerization ability—is enabled due to a specific sequence encoded by exon 5 present at the N-terminal part in both known isoforms. In this study, we characterized AMBN ISO I and AMBN ISO II by biochemical and biophysical methods to determine their common features and differences. We confirmed that both AMBN ISO I and AMBN ISO II form oligomers in in vitro conditions. Due to an important role of AMBN in biomineralization, we further addressed the calcium (Ca²⁺)-binding properties of AMBN ISO I and ISO II. The binding properties of AMBN to Ca²⁺ may explain the role of AMBN in biomineralization and more generally in Ca²⁺ homeostasis processes.

An ameloblastin C-terminus variant is present in human adipose tissue

Objective

Transcriptional regulatory elements in the ameloblastin (AMBN) promoter indicate that adipogenesis may influence its expression. The objective here was to investigate if AMBN is expressed in adipose tissue, and have a role during differentiation of adipocytes.

Design

AMBN expression was examined in adipose tissue and adipocytes by real-time PCR and ELISA. Distribution of ameloblastin was investigated by immunofluorescence in sections of human subcutaneous adipose tissue. The effect of recombinant proteins resembling AMBN and its processed products on proliferation of primary human pre-adipocytes and murine 3T3-L1 cell lines was measured by [³H]-thymidine incorporation. The effect on adipocyte differentiation was evaluated by the expression profile of the adipogenic markers PPARγ and leptin, and the content of lipids droplets (Oil-Red-O staining).

Results

AMBN was found to be expressed in human adipose tissue, human primary adipocytes, and in 3T3-L1 cells. The C-terminus of the AMBN protein and a 45 bp shorter splice variant was identified in human subcutaneous adipose tissue. The expression of AMBN was found to increase four-fold during differentiation of 3T3-L1 cells. Administration of recombinant AMBN reduced the proliferation, and enhanced the expression of PPARγ and leptin in 3T3-L1 and human pre-adipocytes, respectively.

Conclusions

The AMBN C-terminus variant was identified in adipocytes. This variant may be encoded from a short splice variant. Increased expression of AMBN during adipogenesis and its effect on adipogenic factors suggests that AMBN also has a role in adipocyte development.

Expression of ameloblastin in the human tooth germ and ameloblastoma

OBJECTIVE

To analyse the immunohistochemical expression of ameloblastin in the bell stage of tooth germ and compare with ameloblastoma to determine the level of differentiation of tumour cells.

STUDY DESIGN

This study included eleven human tooth germs with four in the early and seven in the late bell stage, and six selected archival tissue samples of ameloblastomas were studied using haematoxylin and eosin, Masson's trichrome and ameloblastin.

RESULTS

All eleven tooth germs reacted positively to ameloblastin with a characteristic inverted and sequential pattern of expression in the acellular zone of the dental papilla and enamel organ. Of the six cases of ameloblastoma, five cases showed a variable level of expression of ameloblastin in the tumour cells, whereas in one case, ameloblastin was negative in the tumour cells but positive in the stromal fibrous tissue collar.

CONCLUSION

Expression of ameloblastin in human tooth germ is related to differentiation and mineralization, and it correlates with the state of differentiation of the tumour cells in ameloblastoma.

Amelogenesis Imperfecta; Genes, Proteins, and Pathways

Amelogenesis imperfecta (AI) is the name given to a heterogeneous group of conditions characterized by inherited developmental enamel defects. AI enamel is abnormally thin, soft, fragile, pitted and/or badly discolored, with poor function and aesthetics, causing patients problems such as early tooth loss, severe embarrassment, eating difficulties, and pain. It was first described separately from diseases of dentine nearly 80 years ago, but the underlying genetic and mechanistic basis of the condition is only now coming to light. Mutations in the gene AMELX, encoding an extracellular matrix protein secreted by ameloblasts during enamel formation, were first identified as a cause of AI in 1991. Since then, mutations in at least eighteen genes have been shown to cause AI presenting in isolation of other health problems, with many more implicated in syndromic AI. Some of the encoded proteins have well documented roles in amelogenesis, acting as enamel matrix proteins or the proteases that degrade them, cell adhesion molecules or regulators of calcium homeostasis. However, for others, function is less clear and further research is needed to understand the pathways and processes essential for the development of healthy enamel. Here, we review the genes and mutations underlying AI presenting in isolation of other health problems, the proteins they encode and knowledge of their roles in amelogenesis, combining evidence from human phenotypes, inheritance patterns, mouse models, and in vitro studies. An LOVD resource (http://dna2.leeds.ac.uk/LOVD/) containing all published gene mutations for AI presenting in isolation of other health problems is described. We use this resource to identify trends in the genes and mutations reported to cause AI in the 270 families for which molecular diagnoses have been reported by 23rd May 2017. Finally we discuss the potential value of the translation of AI genetics to clinical care with improved patient pathways and speculate on the possibility of novel treatments and prevention strategies for AI.

Are Cementoblasts a Subpopulation of Osteoblasts or a Unique Phenotype?

Experimental studies have shown a great potential for periodontal regeneration. The limitations of periodontal regeneration largely depend on the regenerative potential at the root surface. Cellular intrinsic fiber cementum (CIFC), so-called bone-like tissue, may form instead of the desired acellular extrinsic fiber cementum (AEFC), and the interfacial tissue bonding may be weak. The periodontal ligament harbors progenitor cells that can differentiate into periodontal ligament fibroblasts, osteoblasts, and cementoblasts, but their precise location is unknown. It is also not known whether osteoblasts and cementoblasts arise from a common precursor cell line, or whether distinct precursor cell lines exist. Thus, there is limited knowledge about how cell diversity evolves in the space between the developing root and the alveolar bone. This review supports the hypothesis that AEFC is a unique tissue, while CIFC and bone share some similarities. Morphologically, functionally, and biochemically, however, CIFC is distinctly different from any bone type. There are several lines of evidence to propose that cementoblasts that produce both AEFC and CIFC are unique phenotypes that are unrelated to osteoblasts. Cementum attachment protein appears to be cementum-specific, and the expression of two proteoglycans, fibromodulin and lumican, appears to be stronger in CIFC than in bone. A theory is presented that may help explain how cell diversity evolves in the periodontal ligament. It proposes that Hertwig’s epithelial root sheath and cells derived from it play an essential role in the development and maintenance of the periodontium. The role of enamel matrix proteins in cementoblast and osteoblast differentiation and their potential use for tissue engineering are discussed.

Comparative expression of the four enamel matrix protein genes, amelogenin, ameloblastin, enamelin and amelotin during amelogenesis in the lizard Anolis carolinensis

Background

In a recent study, we have demonstrated that amelotin (AMTN) gene structure and its expression during amelogenesis have changed during tetrapod evolution. Indeed, this gene is expressed throughout enamel matrix deposition and maturation in non-mammalian tetrapods, while in mammals its expression is restricted to the transition and maturation stages of amelogenesis. Previous studies of amelogenin (AMEL) gene expression in a lizard and a salamander have shown similar expression pattern to that in mammals, but to our knowledge there are no data regarding ameloblastin (AMBN) and enamelin (ENAM) expression in non-mammalian tetrapods. The present study aims to look at, and compare, the structure and expression of four enamel matrix protein genes, AMEL, AMBN, ENAM and AMTN during amelogenesis in the lizard Anolis carolinensis.

Results

We provide the full-length cDNA sequence of A. carolinensisAMEL and AMBN, and show for the first time the expression of ENAM and AMBN in a non-mammalian species. During amelogenesis in A. carolinensis, AMEL, AMBN and ENAM expression in ameloblasts is similar to that described in mammals. It is noteworthy that AMEL and AMBN expression is also found in odontoblasts.

Conclusions

Our findings indicate that AMTN is the only enamel matrix protein gene that is differentially expressed in ameloblasts between mammals and sauropsids. Changes in AMTN structure and expression could be the key to explain the structural differences between mammalian and reptilian enamel, i.e. prismatic versus non-prismatic.

Evolutionary analysis of selective constraints identifies ameloblastin (AMBN) as a potential candidate for amelogenesis imperfecta

Background

Ameloblastin (AMBN) is a phosphorylated, proline/glutamine-rich protein secreted during enamel formation. Previous studies have revealed that this enamel matrix protein was present early in vertebrate evolution and certainly plays important roles during enamel formation although its precise functions remain unclear. We performed evolutionary analyses of AMBN in order to (i) identify residues and motifs important for the protein function, (ii) predict mutations responsible for genetic diseases, and (iii) understand its molecular evolution in mammals.

Results

In silico searches retrieved 56 complete sequences in public databases that were aligned and analyzed computationally. We showed that AMBN is globally evolving under moderate purifying selection in mammals and contains a strong phylogenetic signal. In addition, our analyses revealed codons evolving under significant positive selection. Evidence for positive selection acting on AMBN was observed in catarrhine primates and the aye-aye. We also found that (i) an additional translation initiation site was recruited in the ancestral placental AMBN, (ii) a short exon was duplicated several times in various species including catarrhine primates, and (iii) several polyadenylation sites are present.

Conclusions

AMBN possesses many positions, which have been subjected to strong selective pressure for 200 million years. These positions correspond to several cleavage sites and hydroxylated, O-glycosylated, and phosphorylated residues. We predict that these conserved positions would be potentially responsible for enamel disorder if substituted. Some motifs that were previously identified as potentially important functionally were confirmed, and we found two, highly conserved, new motifs, the function of which should be tested in the near future. This study illustrates the power of evolutionary analyses for characterizing the functional constraints acting on proteins with yet uncharacterized structure.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-015-0431-0) contains supplementary material, which is available to authorized users.

Influence of enamel matrix derivative on cells at different maturation stages of differentiation

Enamel matrix derivative (EMD), a porcine extract harvested from developing porcine teeth, has been shown to promote formation of new cementum, periodontal ligament and alveolar bone. Despite its widespread use, an incredibly large variability among in vitro studies has been observed. The aim of the present study was to determine the influence of EMD on cells at different maturation stages of osteoblast differentiation by testing 6 cell types to determine if cell phenotype plays a role in cell behaviour following treatment with EMD. Six cell types including MC3T3-E1 pre-osteoblasts, rat calvarial osteoblasts, human periodontal ligament (PDL) cells, ROS cells, MG63 cells and human alveolar osteoblasts were cultured in the presence or absence of EMD and proliferation rates were quantified by an MTS assay. Gene expression of collagen1(COL1), alkaline phosphate(ALP) and osteocalcin(OC) were investigated by real-time PCR. While EMD significantly increased cell proliferation of all cell types, its effect on osteoblast differentiation was more variable. EMD significantly up-regulated gene expression of COL1, ALP and OC in cells early in their differentiation process when compared to osteoblasts at later stages of maturation. Furthermore, the effect of cell passaging of primary human PDL cells (passage 2 to 15) was tested in response to treatment with EMD. EMD significantly increased cell proliferation and differentiation of cells at passages 2-5 however had completely lost their ability to respond to EMD by passages 10+. The results from the present study suggest that cell stimulation with EMD has a more pronounced effect on cells earlier in their differentiation process and may partially explain why treatment with EMD primarily favors regeneration of periodontal defects (where the periodontal ligament contains a higher number of undifferentiated progenitor cells) over regeneration of pure alveolar bone defects containing no periodontal ligament and a more limited number of osteoprogenitor 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.

Adipose tissue-derived stem cell in vitro differentiation in a three-dimensional dental bud structure

Tooth morphogenesis requires sequential and reciprocal interactions between the cranial neural crest-derived mesenchymal cells and the stomadial epithelium, which regulate tooth morphogenesis and differentiation. We show how mesenchyme-derived single stem cell populations can be induced to transdifferentiate in vitro in a structure similar to a dental bud. The presence of stem cells in the adipose tissue has been previously reported. We incubated primary cultures of human adipose tissue-derived stem cells in a dental-inducing medium and cultured the aggregates in three-dimensional conditions. Four weeks later, cells formed a three-dimensional organized structure similar to a dental bud. Expression of dental tissue-related markers was tested assaying lineage-specific mRNA and proteins by RT-PCR, immunoblot, IHC, and physical-chemical analysis. In the induction medium, cells were positive for ameloblastic and odontoblastic markers as both mRNAs and proteins. Also, cells expressed epithelial, mesenchymal, and basement membrane markers with a positional relationship similar to the physiologic dental morphogenesis. Physical-chemical analysis revealed 200-nm and 50-nm oriented hydroxyapatite crystals as displayed in vivo by enamel and dentin, respectively. In conclusion, we show that adipose tissue-derived stem cells in vitro can transdifferentiate to produce a specific three-dimensional organization and phenotype resembling a dental bud even in the absence of structural matrix or scaffold to guide the developmental process.

Ameloblastin expression and putative autoregulation in mesenchymal cells suggest a role in early bone formation and repair

Ameloblastin is mainly known as a dental enamel protein, synthesized and secreted into developing enamel matrix by the enamel-forming ameloblasts. The function of ameloblastin in tooth development remains unclear, but it has been suggested to be involved in processes varying from regulating crystal growth to activity as a growth factor or partaking in cell signaling. Recent studies suggest that some enamel matrix proteins also might have important functions outside enamel formation. In this context ameloblastin has recently been reported to induce dentin and bone repair, as well as being present in the early bone and cartilage extracellular matrices during embryogenesis. However, what cells express ameloblastin in these tissues still remains unclear. Thus, the expression of ameloblastin was examined in cultured primary mesenchymal cells and in vivo during healing of bone defects in a "proof of concept" animal study. Real time RT-PCR analysis revealed human ameloblastin (AMBN) mRNA expression in human mesenchymal stem cells and primary osteoblasts and chondrocytes. Expression of AMBN mRNA was also confirmed in human CD34 positive cells and osteoclasts. Western and dot blot analysis of cell lysates and medium confirmed the expression and secretion of ameloblastin from mesenchymal stem cells, primary human osteoblasts and chondrocytes. Expression of ameloblastin was also detected in newly formed bone in experimental bone defects in adult rats. Together these findings suggest a role for this protein in early bone formation and repair.

Ameloblastin promotes bone growth by enhancing proliferation of progenitor cells and by stimulating immunoregulators

In this study, we examined the role of the enamel matrix protein, ameloblastin, in bone growth and remodelling, and attempted to identify some of the molecular mechanisms involved in these processes. The effects of recombinant ameloblastin (rAmbn) were tested in vivo in rats, and in vitro in primary human mesenchymal stem cells, osteoblasts, chondrocytes, and osteoclasts. We used a microarray technique to identify genes that were regulated in human osteoblasts and verified our findings using multiplex protein analysis and real-time RT-PCR. Recombinant ameloblastin was found to stimulate bone healing in vivo, and to enhance the proliferation of mesenchymal stem cells and osteoblasts, as well as the differentiation of osteoclast precursor cells in vitro. The most profound effect was on the regulation of genes related to immune responses as well as on the expression of cytokines and markers of bone cell differentiation, indicating that ameloblastin has an effect on mesenchymal cell differentiation. A receptor has not yet been identified, but we found rAmbn to induce, directly and indirectly, signal transducer and activator of transcription (STAT) 1 and 2 and downstream factors in the interferon pathway.

Critical role of heparin binding domains of ameloblastin for dental epithelium cell adhesion and ameloblastoma proliferation

AMBN (ameloblastin) is an enamel matrix protein that regulates cell adhesion, proliferation, and differentiation of ameloblasts. In AMBN-deficient mice, ameloblasts are detached from the enamel matrix, continue to proliferate, and form a multiple cell layer; often, odontogenic tumors develop in the maxilla with age. However, the mechanism of AMBN functions in these biological processes remains unclear. By using recombinant AMBN proteins, we found that AMBN had heparin binding domains at the C-terminal half and that these domains were critical for AMBN binding to dental epithelial cells. Overexpression of full-length AMBN protein inhibited proliferation of human ameloblastoma AM-1 cells, but overexpression of heparin binding domain-deficient AMBN protein had no inhibitory effect. In full-length AMBN-overexpressing AM-1 cells, the expression of Msx2, which is involved in the dental epithelial progenitor phenotype, was decreased, whereas the expression of cell proliferation inhibitors p21 and p27 was increased. We also found that the expression of enamelin, a marker of differentiated ameloblasts, was induced, suggesting that AMBN promotes odontogenic tumor differentiation. Thus, our results suggest that AMBN promotes cell binding through the heparin binding sites and plays an important role in preventing odontogenic tumor development by suppressing cell proliferation and maintaining differentiation phenotype through Msx2, p21, and p27.

Effect of acid etching time on bond strength of an etch-and-rinse adhesive to primary tooth dentine affected by amelogenesis imperfecta

BACKGROUND

Dentine affected by amelogenesis imperfecta (AI) is histologically altered following loss of the hypoplastic enamel and becomes hypermineralized, which would make bonding less predictable.

AIM

This study examined the effect of etching time on the microtensile bond strength (microTBS) to AI-affected primary dentine.

DESIGN

Flat coronal dentine surface was obtained from extracted AI-affected and noncarious primary molars. Teeth were etched either for 15 or for 30 s using 34% phosphoric acid. Prime & Bond NT (Dentsply De Trey), an etch-and-rinse adhesive, was applied to dentine surfaces, air-dried and light-cured, followed by composite build-ups. The bonded teeth were sectioned into beams of 0.8 mm(2) and stressed to failure under tension. Representative fractured beams from each group were examined under scanning electron microscopy.

RESULTS

The extended etching time had an adverse effect on the microTBS for the normal dentine, while no significant difference was found for AI-affected dentine. When the AI-affected dentine was etched for 30 s, the fracture occurred in the demineralized dentine at the base of the hybrid layer.

CONCLUSION

Bonding to AI-affected dentine compromised the bonding of the etch-and-rinse adhesive. The bonding could not be improved by increasing etching time.

Bioinformatic analysis and molecular modelling of human ameloblastin suggest a two-domain intrinsically unstructured calcium-binding protein

Ameloblastin (AMBN) was originally believed to be an enamel-specific extracellular matrix glycoprotein secreted by ameloblasts. Recently, AMBN expression was also detected in developing mesenchymal dental hard tissues, in trauma-induced reparative dentin, and during early craniofacial bone formation. The function and structure of AMBN still remain ambiguous, and there are no known proteins with similar primary sequences. We therefore performed a bio-informatic analysis of AMBN to model ab initio the three-dimensional structure of the molecule. The results suggest that AMBN is a two-domain, intrinsically unstructured protein (IUP). The analysis did not reveal any regions with structural similarity to known receptor-ligand systems, and did not identify any higher-order structures similar to functional regions in other known sequences. The AMBN model predicts 11 defined regions exposed on the surface, internalizing the rest of the molecule including a human-specific insert. Molecular dynamics analysis identified one specific and several non-specific calcium-binding regions, mostly at the C-terminal part of the molecule. The model is supported by previous observations that AMBN is a bipolar calcium-binding molecule and hints at a possible role in protein-protein interactions. The model provides information useful for further studies on the function of AMBN.

Ameloblastin expression during craniofacial bone formation in rats

Based on previous results showing the expression of ameloblastin (Ambn; amelin) in the formation of mesenchymal dental hard tissues, we investigated its presence during bone development. Immunohistochemistry (IHC), in situ hybridization (ISH), and reverse transcription-polymerase chain reaction (RT-PCR) were used to investigate the expression of ameloblastin protein and mRNA during craniofacial development in rats. Tissue samples were collected on embryonic day 18 and from days 2-28 postnatally. IHC revealed the expression of ameloblastin during bone formation at embryonic and early postnatal stages with different patterns of expression in intramembranous and endochondral ossification. In intramembranous ossification, ameloblastin expression was detected in the superficial layer of the condensed vascularized primitive connective tissue and in the cellular layer covering the surface of the newly formed woven bone. In endochondral ossification, ameloblastin was expressed within the extracellular matrix of the cartilage templates and in the perichondrium. Between days 2 and 28 the expression decreased markedly, concordant with the maturation of the bone, and disappeared after completion of bone remodeling. The results obtained by IHC were confirmed by ISH and RT-PCR, showing the expression of ameloblastin mRNA during craniofacial bone formation. This study indicates the expression of the putative dental protein ameloblastin during craniofacial bone development in rats.

Ameloblastin fusion protein enhances pulpal healing and dentin formation in porcine teeth

Ameloblastin (Ambn, also named "amelin" or "sheathlin") is a protein participating in enamel formation and mesenchymal-ectodermal interaction during early dentin formation in developing teeth. Experiments have demonstrated an association between Ambn expression and healing of acute pulp wounds. The purpose of this study was to investigate if local application of recombinant fusion Ambn (rAmbn) could influence reparative dentin formation in pulpotomized teeth. In this randomized, double-blinded study, pulpotomy was performed in 28 lower central incisors in 17 adult miniature pigs. Following the surgical procedure, the exposed pulp tissue was covered either with rAmbn or with calcium hydroxide. After 2, 4, or 8 weeks, the teeth were extracted and examined by histomorphometry and immunohistochemistry using antibodies against porcine ameloblastin, collagen type I, and dentin sialoprotein (DSP). In rAmbn-treated teeth, a substantial amount of newly formed reparative dentin was observed at the application site, completely bridging the pulpal wound. Dentin formation was also observed in calcium hydroxide-treated teeth; however, the amount of reparative dentin was significantly smaller (P < 0.001) than after rAmbn treatment. Immunohistochemistry confirmed that the new hard tissue formed was similar to dentin. This is the first time a direct link between ameloblastin and dentin formation has been made in vivo. The results suggest potential for rAmbn as a biologically active pulp-dressing agent for enhanced pulpal wound healing and reparative dentin formation after pulpotomy procedures.

Essential Roles of Ameloblastin in Maintaining Ameloblast Differentiation and Enamel Formation

During tooth development, dental epithelial cells interact with extracellular matrix components, such as the basement membrane and enamel matrix. Ameloblastin, an enamel matrix protein, plays a crucial role in maintaining the ameloblast differentiation state and is essential for enamel formation. Ameloblastin-null mice developed severe enamel hypoplasia. In mutant mice, dental epithelial cells started to differentiate into ameloblasts, but ameloblasts soon lost cell polarity, proliferated, and formed multiple cell layers, indicative of some aspects of preameloblast phenotypes. In addition, the expression of amelogenin, another component of the enamel matrix, was specifically reduced in mutant ameloblasts. More than 20% of amelobastin-null mice developed odontogenic tumors. We also found that recombinant ameloblastin specifically bound to ameloblasts and inhibited proliferation of dental epithelial cells. These results suggest that ameloblastin is an important regulator to maintain the differentiation state of ameloblasts.

Genes and related proteins involved in amelogenesis imperfecta

Dental enamel formation is a remarkable example of a biomineralization process. The exact mechanisms involved in this process remain partly obscure. Some of the genes encoding specific enamel proteins have been indicated as candidate genes for amelogenesis imperfecta. Mutational analyses within studied families have supported this hypothesis. Mutations in the amelogenin gene (AMELX) cause X-linked amelogenesis imperfecta, while mutations in the enamelin gene (ENAM) cause autosomal-inherited forms of amelogenesis imperfecta. Recent reports involve kallikrein-4 (KLK4), MMP-20, and DLX3 genes in the etiologies of some cases. This paper focuses mainly on the candidate genes involved in amelogenesis imperfecta and the proteins derived from them, and reviews current knowledge on their structure, localization within the tissue, and correlation with the various types of this disorder.

Protein–Protein Interactions of the Developing Enamel Matrix

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

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

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

The genetics of amelogenesis imperfecta: a review of the literature

A melogenesis imperfecta (AI) is a group of inherited defects of dental enamel formation that show both clinical and genetic heterogeneity. Enamel findings in AI are highly variable, ranging from deficient enamel formation to defects in the mineral and protein content. Enamel formation requires the expression of multiple genes that transcribes matrix proteins and proteinases needed to control the complex process of crystal growth and mineralization. The AI phenotypes depend on the specific gene involved, the location and type of mutation, and the corresponding putative change at the protein level. Different inheritance patterns such as X-linked, autosomal dominant and autosomal recessive types have been reported. Mutations in the amelogenin, enamelin, and kallikrein-4 genes have been demonstrated to result in different types of AI and a number of other genes critical to enamel formation have been identified and proposed as candidates for AI. The aim of this article was to present an evaluation of the literature regarding role of proteins and proteinases important to enamel formation and mutation associated with AI.

Ameloblastin and amelogenin expression in posnatal developing mouse molars

Ameloblastin and amelogenin are structural proteins present in the enamel matrix of developing teeth. Here we report the results of in situ hybridization analyses with DNA probes of ameloblastin and amelogenin expression in the mandibular first molars of ICR/Jcl mice from postnatal day 1 to day 15. Ameloblastin mRNA expression was observed in ameloblasts at day 2 while amelogenin mRNA was detected in secretory ameloblasts at day 3. Significant expression of both molecules was observed at days 4, 5 and 6, after which the levels decreased. Amelogenin expression ended on day 10, while ameloblastin mRNA was only weakly detected on day 12. Neither amelogenin nor ameloblastin expression was observed in day 15 mouse molars. Amelogenin and ameloblastin mRNAs were restricted to ameloblasts. We conclude that amelogenin and ameloblastin expression is enamel-specific, and suggest that these genes might be involved in the mineralization of enamel. It is possible that ameloblastin could participate in the attachment of ameloblasts to the enamel surface. In this case, the downregulation of expression may indicate the beginning of the maturation stage in which the ameloblasts tend to detach from the enamel layer.

Review: extracellular matrix regulates tooth morphogenesis

Mineralized tissues are unique in that they use proteins to attract and organize calcium and phosphate ions into a structured mineral phase, thus precise knowledge of the expression and extracellular distribution of matrix proteins is very important to understand their function. Tooth development is regulated by sequential and reciprocal interactions between neural crest-derived mesenchymal cells and the oral environment. However, the precise molecular mechanisms that mediate interactions between epithelium and mesenchymal cells are not clear, although basement membrane (BM) components have been shown to play important roles in these regulatory events. In addition, the extracellular matrix layer, whose main components are laminin, collagen IV, nidogen, and sulfated proteoglycan, and the BM layer are both considered to be involved with cell proliferation and differentiation. During tooth morphogenesis, extracellular matrices are dramatically changed. Further, the BM components, laminin and collagen IV support dental epithelium; however, in the late stage, they begin the processes of enamel matrix secretion and calcification, after which the BM structure between the dental epithelium and mesenchyme disappears. In addition, tooth abnormalities associated with several kinds of human diseases that cause mutations in the extracellular matrix, as well as the molecular mechanisms of the basement membrane and enamel matrix during tooth morphogenesis, are not clearly understood. In our review, we discuss the role of the extracellular matrix, with focus on the BM and enamel matrix during tooth morphogenesis.

Immunohistochemical characterization of rapid dentin formation induced by enamel matrix derivative

The purpose of this study was to examine the pulpal expression of dentin-related proteins during enamel matrix derivative (EMD)-induced reparative dentin formation in a pulpotomy model in pig incisors. Pulpotomies were performed on 72 lower incisors in 24 adult miniature swine. The exposed pulp tissue was treated with EMD or covered with a calcium hydroxide paste (Dycal). At predefined time-points, ranging from 4 days to 12 weeks, experimental teeth were extracted and examined by use of light microscopy, and expression of dentin-related proteins in the pulps was investigated by immunohistochemistry, using antibodies against type I collagen, dentin sialoprotein (DSP), sheathlin, and EMD. In all EMD-treated teeth a substantial amount of reparative dentin formation was observed. The amount of reparative dentin in calcium hydroxide-treated teeth was significantly smaller than in EMD-treated teeth (P < 0.005) and was less effective in bridging the pulpal wounds. Immunohistochemistry demonstrated that enamel matrix proteins were present in detectable amounts at the application site for about 4 weeks. Moreover, the expression of proteins related to dentin formation in the wounded pulp tissue was about 2 weeks advanced in EMD-treated teeth. These findings demonstrate that enamel matrix molecules have the capacity to induce rapid pulpal wound healing in pulpotomized teeth, and suggest that the longevity and continued presence of enamel matrix macromolecules at the application site can be utilized to stimulate growth and repair of dentin over a period consistent with a favorable clinical outcome.

Dentine structure and mineralization in hypocalcified amelogenesis imperfecta: a quantitative X-ray histochemical study

OBJECTIVE

This study was undertaken in order to establish the structural and mineralization pattern of the response of dentine to alterations in enamel in hypocalcified amelogenesis imperfecta (AI).

DESIGN

The images and data obtained with scanning electron microscopy and electron probe X-ray microanalysis in enamel and dentine specimens from control and affected teeth were compared in this study.

PATIENTS AND METHODS

We compared 46 fragments of permanent teeth from patients with clinically diagnosed hypocalcified AI and 20 normal permanent teeth. All specimens were prepared for electron probe X-ray microanalysis.

RESULTS

Dentine is characterized by thickening of the peritubular dentine and partial obliteration of the dentinal tubules that does not give rise to a compact sclerotic cast. In dentine, calcium levels were significantly higher in teeth with clinically hypocalcified AI in relation with control teeth (P < 0.001).

CONCLUSIONS

Dentine is affected in hypocalcified AI increasing mineralization (narrower tubules and higher content of calcium) in response to enamel disorder.

Expression of amelogenin in odontoblasts

Amelogenin is the major enamel protein produced by ameloblasts. Its expression has been shown to be down-regulated in ameloblasts of vitamin-D-deficient (-D) rats. The potential expression and localization of amelogenin in odontoblasts and its regulation by vitamin D were investigated in this study. RT-PCR and semi-quantitative Northern blot analyses were performed using the odontoblast cell line MO6-G3 and microdissected dental pulp mesenchyme. Both in vitro and in vivo odontoblasts expressed various alternatively spliced amelogenin transcripts. In situ hybridization studies showed that amelogenin expression was restricted to young odontoblasts during mantle dentin deposition. Electron microscopy studies localized the amelogenin protein in the odontoblast cell process cytoplasm and mantle dentin. Amelogenin immunolabeling was stronger in -D rats, suggesting an inverse regulation by vitamin D in odontoblasts. Furthermore, amelogenin mRNA steady-state levels were significantly increased in -D dental pulp mesenchyme. In addition, a temporal-spatial lengthening of the mantle dentin stage was observed in -D animals, suggesting that developmental perturbations occur in relation to the vitamin D status and/or amelogenin expression. These data show that amelogenin is expressed by odontoblasts selectively during mantle dentin deposition. This developmental regulated expression pattern is enhanced under vitamin-D-deficiency status and in a broader context may play an important role during ameloblast and odontoblast differentiation and function.

Expression of a 1,25-dihydroxyvitamin D3 membrane-associated rapid-response steroid binding protein during human tooth and bone development and biomineralization

The calciotropic hormone 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] has been established to control skeletal tissue formation and biomineralization via the regulation of gene expression. This action involves the well-characterized nuclear 1,25(OH)2D3 receptor. However, it has been recognized that several cellular responses to 1,25(OH)2D3 may not to be related to the exclusive nuclear receptor. Indeed, this secosteroid is able to generate rapid responses that have been proposed to be mediated by interactions of the ligand, which is a putative cell membrane-associated rapid-response steroid (MARRS) binding protein for 1,25(OH)2D3 [1,25D3-MARRS]. The nongenomic pathway of 1,25(OH)2D3 was studied here in detail by immunolocalization of the 1,25D3-MARRS during the specific context of human prenatal development. Western blotting with proteins extracted from 4 week- to 27-week-old embryos was performed, evidencing a 65-kDa molecular species recognized by antibody Ab 099 generated against synthetic peptides corresponding to the N terminus of the 1,25D3-MARRS from chick intestinal basolateral membranes. Based on this biochemical conservation of protein in the human species, the temporospatial expression patterns were established in the craniofacial skeleton at the same ages. Comparative analysis was performed in teeth and bones from early morphogenesis to terminal cell differentiation and extracellular biomineralization. The data show the potential implication of 1,25D3-MARRS in the heterogeneous cell population including ameloblasts, odontoblasts, osteoblasts, and osteoclasts. The epithelial-mesenchymal cascade related to odontogenesis was coincident with a sequence of up- and down-regulation of immunoreactive 1,25D3-MARRS. Biomineralization was associated with a striking up-regulation in the adjoining secretory cells in all tissues. Finally, osteoclasts appeared also to express the 1,25D3-MARRS during these early phases of bone modeling. Previously obtained data of the nuclear vitamin D receptor (VDR) expression and this study on 1,25D3-MARRS suggest the existence of cross-talk between the genomic and nongenomic pathways during human development.

Is there more to enamel matrix proteins than biomineralization?

Enamel proteins are proteins synthesized by ameloblast cells. These proteins are secreted into the enamel extracellular matrix where they nucleate and regulate the growth of hydroxyapatite crystals to form the mineralized enamel covering the crown of the teeth. Although the exact role of these proteins in enamel mineralization is just beginning to be elucidated, new studies suggest that these proteins might have functions outside enamel formation. Furthermore, extracts of enamel proteins are currently being used to regenerate periodontal tissues destroyed by periodontal disease and new studies suggest that they might have chondrogenic and osteogenic properties. These new functions of enamel proteins will be the focus of this review.

Human ameloblastin gene: genomic organization and mutation analysis in amelogenesis imperfecta patients

A gene encoding the enamel protein ameloblastin (AMBN) was recently localized to a region on chromosome 4q21 containing a gene for the inherited enamel defect local hypoplastic amelogenesis imperfecta (AIH2). Ameloblastin protein is located at the Tomes processes of secretory ameloblasts and in the sheath space between rod-interrod enamel, and the AMBN gene therefore represents a viable candidate gene for local hypoplastic amelogenesis imperfecta (AI). In this study, the genomic organization of human AMBN was characterized. The gene was shown to consist of 13 exons and 12 introns. An alternatively spliced 45 bp sequence was shown not to represent a separate exon and is most likely spliced by the use of a cryptic splice site. The finding that there were no recombinations between an intragenic microsatellite and AIH2 encouraged us to evaluate this gene's potential role as a candidate gene for local hypoplastic AI. Mutation screening was performed on all 13 exons in 20 families and 8 sporadic cases with 6 different forms of AI. DNA variants were found but none that was associated exclusively with local hypoplastic AI or any of the other variants of AI in the identified Swedish families. This study excludes the coding regions and the splice sites of AMBN from a causative role in the pathogenesis of AIH2.