Synthesis, secretion, degradation, and fate of ameloblastin during the matrix formation stage of the rat incisor as shown by immunocytochemistry and immunochemistry using region-specific antibodies

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.

Ganoin and acrodin formation on scales and teeth in spotted gar: A vital role of enamelin in the unique process of enamel mineralization

Gars and bichirs develop scales and teeth with ancient actinopterygian characteristics. Their scale surface and tooth collar are covered with enamel, also known as ganoin, whereas the tooth cap is equipped with an enamel-like tissue, acrodin. Here, we investigated the formation and mineralization of the ganoin and acrodin matrices in spotted gar, and the evolution of the scpp5, ameloblastin (ambn), and enamelin (enam) genes, which encode matrix proteins of ganoin. Results suggest that, in bichirs and gars, all these genes retain structural characteristics of their orthologs in stem actinopterygians, presumably reflecting the presence of ganoin on scales and teeth. During scale formation, Scpp5 and Enam were initially found in the incipient ganoin matrix and the underlying collagen matrix, whereas Ambn was detected mostly in a surface region of the well-developed ganoin matrix. Although collagen is the principal acrodin matrix protein, Scpp5 was detected within the matrix. Similarities in timings of mineralization and the secretion of Scpp5 suggest that acrodin evolved by the loss of the matrix secretory stage of ganoin formation: dentin formation is immediately followed by the maturation stage. The late onset of Ambn secretion during ganoin formation implies that Ambn is not essential for mineral ribbon formation, the hallmark of the enamel matrix. Furthermore, Scpp5 resembles amelogenin that is not important for the initial formation of mineral ribbons in mammals. It is thus likely that the evolution of ENAM was vital to the origin of the unique mineralization process of the enamel matrix.

Regulation of Hydroxyapatite Nucleation In Vitro through Ameloblastin-Amelogenin Interactions

Amelogenin (Amel) and ameloblastin (Ambn) are two primary extracellular enamel matrix proteins that play crucial roles for proper thickness, prismatic structure, and robust mechanical properties. Previous studies have shown that Amel and Ambn bind to each other, but the effect of their coassembly on the nucleation of hydroxyapatite (HAP) is unclear. Here, we systematically investigated the coassembly of recombinant mouse Amel and Ambn in various ratios using in situ atomic force microscopy, dynamic light scattering, and transmission electron microscopy. The size of protein particles decreased as the Ambn:Amel ratio increased. To define the coassembly domain on Ambn, we used Ambn-derived peptides and Ambn variants to examine their effects on the amelogenin particle size distribution. We found that the peptide sequence encoded by exon 5 of Ambn affected Amel self-assembly but the variant lacking this sequence did not have any effect on Amel self-assembly. Furthermore, through monitoring the pH change in bulk mineralization solution, we tracked the nucleation behavior of HAP in the presence of Ambn and Amel and found that their coassemblies at different ratios showed varying abilities to stabilize amorphous calcium phosphate. These results demonstrated that Ambn and Amel coassemble with each other via a motif within the sequence encoded by exon 5 of Ambn and cooperate in regulating the nucleation of HAP crystals, enhancing our understanding of the important role of enamel matrix proteins in amelogenesis.

How prenatal environmental factors affect rat molar enamel formation?

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

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.

Biomineralization of Enamel and Dentin Mediated by Matrix Proteins

Biomineralization of enamel, dentin, and bone involves the deposition of apatite mineral crystals within an organic matrix. Bone and teeth are classic examples of biomaterials with unique biomechanical properties that are crucial to their function. The collagen-based apatite mineralization and the important function of noncollagenous proteins are similar in dentin and bone; however, enamel is formed in a unique amelogenin-containing protein matrix. While the structure and organic composition of enamel are different from those of dentin and bone, the principal molecular mechanisms of protein-protein interactions, protein self-assembly, and control of crystallization events by the organic matrix are common among these apatite-containing tissues. This review briefly summarizes enamel and dentin matrix components and their interactions with other extracellular matrix components and calcium ions in mediating the mineralization process. We highlight the crystallization events that are controlled by the protein matrix and their interactions in the extracellular matrix during enamel and dentin biomineralization. Strategies for peptide-inspired biomimetic growth of tooth enamel and bioinspired mineralization of collagen to stimulate repair of demineralized dentin and bone tissue engineering are also addressed.

Mapping the Tooth Enamel Proteome and Amelogenin Phosphorylation Onto Mineralizing Porcine Tooth Crowns

Tooth enamel forms in an ephemeral protein matrix where changes in protein abundance, composition and posttranslational modifications are critical to achieve healthy enamel properties. Amelogenin (AMELX) with its splice variants is the most abundant enamel matrix protein, with only one known phosphorylation site at serine 16 shown in vitro to be critical for regulating mineralization. The phosphorylated form of AMELX stabilizes amorphous calcium phosphate, while crystalline hydroxyapatite forms in the presence of the unphosphorylated protein. While AMELX regulates mineral transitions over space and time, it is unknown whether and when un-phosphorylated amelogenin occurs during enamel mineralization. This study aims to reveal the spatiotemporal distribution of the cleavage products of the most abundant AMLEX splice variants including the full length P173, the shorter leucine-rich amelogenin protein (LRAP), and the exon 4-containing P190 in forming enamel, all within the context of the changing enamel matrix proteome during mineralization. We microsampled permanent pig molars, capturing known stages of enamel formation from both crown surface and inner enamel. Nano-LC-MS/MS proteomic analyses after tryptic digestion rendered more than 500 unique protein identifications in enamel, dentin, and bone. We mapped collagens, keratins, and proteolytic enzymes (CTSL, MMP2, MMP10) and determined distributions of P173, LRAP, and P190 products, the enamel proteins enamelin (ENAM) and ameloblastin (AMBN), and matrix-metalloprotease-20 (MMP20) and kallikrein-4 (KLK4). All enamel proteins and KLK4 were near-exclusive to enamel and in excellent agreement with published abundance levels. Phosphorylated P173 and LRAP products decreased in abundance from recently deposited matrix toward older enamel, mirrored by increasing abundances of testicular acid phosphatase (ACPT). Our results showed that hierarchical clustering analysis of secretory enamel links closely matching distributions of unphosphorylated P173 and LRAP products with ACPT and non-traditional amelogenesis proteins, many associated with enamel defects. We report higher protein diversity than previously published and Gene Ontology (GO)-defined protein functions related to the regulation of mineral formation in secretory enamel (e.g., casein α-S1, CSN1S1), immune response in erupted enamel (e.g., peptidoglycan recognition protein, PGRP), and phosphorylation. This study presents a novel approach to characterize and study functional relationships through spatiotemporal mapping of the ephemeral extracellular matrix proteome.

Effect of canonical NF-κB signaling pathway on the differentiation of rat dental epithelial stem cells

BACKGROUND

Nuclear factor-κB (NF-κB), an important transcription factor, participates in many physiological and pathological processes such as growth, differentiation, organogenesis, apoptosis, inflammation, and immune response, including tooth development. However, it is still unknown whether NF-κB participates in the regulation of dental epithelial stem cells (DESCs) in postnatal rat incisors. Here, we investigated the specific differentiation regulatory mechanisms of the canonical NF-κB signaling pathway in DESCs and provided the mechanism of cross-talk involved in DESC differentiation.

METHODS

After adding the activator or inhibitor of the NF-κB signaling pathway, Western blot and quantitative real-time PCR were used to analyze the expressions of amelogenesis-related genes and proteins and canonical transforming growth factor-β (TGF-β) signaling. In addition, we used amelogenesis induction in vitro by adding the activator or inhibitor of the NF-κB signaling pathway to the amelogenesis-induction medium, respectively. Recombinant TGF-β was used to activate the TGF-β pathway, and SMAD7 siRNA was used to downregulate the expression of SMAD7 in DESCs.

RESULTS

We found that the expression of amelogenesis-related genes and proteins as well as TGF-β signaling were downregulated, while SMAD7 expression was increased in NF-κB-activated DESCs. In addition, NF-κB-inhibited DESCs exhibited opposite results compared with NF-κB-activated DESCs. Furthermore, the canonical NF-κB signaling pathway suppressed the canonical TGF-β-SMAD signaling by inducing SMAD7 expression involved in the regulation of DESC differentiation.

CONCLUSIONS

These results indicate that the canonical NF-κB signaling pathway participated in the regulation of DESC differentiation, which was through upregulating SMAD7 expression and further suppressing the canonical TGF-β-SMAD signaling pathway.

Ameloblastin Binds to Phospholipid Bilayers via a Helix-Forming Motif within the Sequence Encoded by Exon 5

Ameloblastin (Ambn), the most abundant non-amelogenin enamel protein, is intrinsically disordered and has the potential to interact with other enamel proteins and with cell membranes. Here, through multiple biophysical methods, we investigated the interactions between Ambn and large unilamellar vesicles (LUVs), whose lipid compositions mimicked cell membranes involved in epithelial cell-extracellular matrix adhesion. Using a series of Ambn Trp/Phe variants and Ambn mutants, we further showed that Ambn binds to LUVs through a highly conserved motif within the sequence encoded by exon 5. Synthetic peptides derived from different regions of Ambn confirmed that the sequence encoded by exon 5 is involved in LUV binding. Sequence analysis of Ambn across different species showed that the N-terminus of this sequence contains a highly conserved motif with a propensity to form an amphipathic helix. Mutations in the helix-forming sequence resulted in a loss of peptide binding to LUVs. Our in vitro data suggest that Ambn binds the lipid membrane directly through a conserved helical motif and have implications for biological events such as Ambn-cell interactions, Ambn signaling, and Ambn secretion via secretory vesicles.

The Expression and Purification of Recombinant Mouse Ameloblastin in E. coli

Ameloblastin is the second most abundant enamel matrix protein, and is thought to be essential for ameloblast cell polarization, cell adhesion, and enamel mineralization. However, studies of ameloblastin's function and its molecular mechanism have been limited due to difficulty in obtaining recombinant ameloblastin in vitro. Here, we present a protocol for successful ameloblastin expression and purification in E. coli.

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.

Msx2 Prevents Stratified Squamous Epithelium Formation in the Enamel Organ

Tooth enamel is manufactured by the inner enamel epithelium of the multilayered enamel organ. Msx2 loss-of-function mutation in a mouse model causes an abnormal accumulation of epithelial cells in the enamel organ, but the underlying mechanism by which Msx2 regulates amelogenesis is poorly understood. We therefore performed detailed histological and molecular analyses of Msx2 null mice. Msx2 null ameloblasts and stratum intermedium (SI) cells differentiated normally in the early stages of amelogenesis. However, during subsequent developmental stages, the outer enamel epithelium (OEE) became highly proliferative and transformed into a keratinized stratified squamous epithelium that ectopically expressed stratified squamous epithelium markers, including Heat shock protein 25, Loricrin, and Keratin 10. Moreover, expression of hair follicle-specific keratin genes such as Keratin 26 and Keratin 73 was upregulated in the enamel organ of Msx2 mutants. With the accumulation of keratin in the stellate reticulum (SR) region and subsequent odontogenic cyst formation, SI cells gradually lost the ability to differentiate, and the expression of Sox2 and Notch1 was downregulated, leading to ameloblast depolarization. As a consequence, the organization of the Msx2 mutant enamel organ became disturbed and enamel failed to form in the normal location. Instead, there was ectopic mineralization that likely occurred within the SR. In summary, we show that during amelogenesis, Msx2 executes a bipartite function, repressing the transformation of OEE into a keratinized stratified squamous epithelium while simultaneously promoting the development of a properly differentiated enamel organ competent for enamel formation.

Phosphorylation Modulates Ameloblastin Self-assembly and Ca 2+ Binding

Ameloblastin (AMBN), an important component of the self-assembled enamel extra cellular matrix, contains several in silico predicted phosphorylation sites. However, to what extent these sites actually are phosphorylated and the possible effects of such post-translational modifications are still largely unknown. Here we report on in vitro experiments aimed at investigating what sites in AMBN are phosphorylated by casein kinase 2 (CK2) and protein kinase A (PKA) and the impact such phosphorylation has on self-assembly and calcium binding. All predicted sites in AMBN can be phosphorylated by CK2 and/or PKA. The experiments show that phosphorylation, especially in the exon 5 derived part of the molecule, is inversely correlated with AMBN self-assembly. These results support earlier findings suggesting that AMBN self-assembly is mostly dependent on the exon 5 encoded region of the AMBN gene. Phosphorylation was significantly more efficient when the AMBN molecules were in solution and not present as supramolecular assemblies, suggesting that post-translational modification of AMBN must take place before the enamel matrix molecules self-assemble inside the ameloblast cell. Moreover, phosphorylation of exon 5, and the consequent reduction in self-assembly, seem to reduce the calcium binding capacity of AMBN suggesting that post-translational modification of AMBN also can be involved in control of free Ca²⁺ during enamel extra cellular matrix biomineralization. Finally, it is speculated that phosphorylation can provide a functional crossroad for AMBN either to be phosphorylated and act as monomeric signal molecule during early odontogenesis and bone formation, or escape phosphorylation to be subsequently secreted as supramolecular assemblies that partake in enamel matrix structure and mineralization.

Intrinsically disordered proteins drive enamel formation via an evolutionarily conserved self-assembly motif

The formation of mineralized tissues is governed by extracellular matrix proteins that assemble into a 3D organic matrix directing the deposition of hydroxyapatite. Although the formation of bones and dentin depends on the self-assembly of type I collagen via the Gly-X-Y motif, the molecular mechanism by which enamel matrix proteins (EMPs) assemble into the organic matrix remains poorly understood. Here we identified a Y/F-x-x-Y/L/F-x-Y/F motif, evolutionarily conserved from the first tetrapods to man, that is crucial for higher order structure self-assembly of the key intrinsically disordered EMPs, ameloblastin and amelogenin. Using targeted mutations in mice and high-resolution imaging, we show that impairment of ameloblastin self-assembly causes disorganization of the enamel organic matrix and yields enamel with disordered hydroxyapatite crystallites. These findings define a paradigm for the molecular mechanism by which the EMPs self-assemble into supramolecular structures and demonstrate that this process is crucial for organization of the organic matrix and formation of properly structured enamel.

Ameloblastin Peptides Modulates the Osteogenic Capacity of Human Mesenchymal Stem Cells

During amelogenesis the extracellular enamel matrix protein AMBN is quickly processed into 17 kDa (N-terminus) and 23 kDa (C-terminus) fragments. In particular, alternatively spliced regions derived by exon 5/6 within the N-terminus region are known to be critical in biomineralization. Human mesenchymal stem cells (hMSC) also express and secrete AMBN, but it is unclear if this expression has effects on the hMSC themselves. If, as suggested from previous findings, AMBN act as a signaling molecule, such effects could influence hMSC growth and differentiation, as well as promoting the secretion of other signaling proteins like cytokines and chemokines. If AMBN is found to modulate stem cell behavior and fate, it will impact our understanding on how extracellular matrix molecules can have multiple roles during development ontogenesis, mineralization and healing of mesenchymal tissues. Here we show that synthetic peptides representing exon 5 promote hMSC proliferation. Interestingly, this effect is inhibited by the application of a 15 aa peptide representing the alternatively spliced start of exon 6. Both peptides also influence gene expression of RUNX2 and osteocalcin, and promote calcium deposition in cultures, indicating a positive influence on the osteogenic capacity of hMSC. We also show that the full-length AMBN-WT and N-terminus region enhance the secretion of RANTES, IP-10, and IL-8. In contrast, the AMBN C-terminus fragment and the exon 5 deleted AMBN (DelEx5) have no detectable effects on any of the parameters investigated. These findings suggest the signaling effect of AMBN is conveyed by processed products, whereas the effect on proliferation is differentially modulated through alternative splicing during gene expression.

The Semaphorin 4D-RhoA-Akt Signal Cascade Regulates Enamel Matrix Secretion in Coordination With Cell Polarization During Ameloblast Differentiation

During tooth development, oral epithelial cells differentiate into ameloblasts in order to form the most mineralized tissue in the vertebrate body: enamel. During this process, ameloblasts directionally secrete enamel matrix proteins and morphologically change from low columnar cells to polarized tall columnar cells, both of which are essential for the proper formation of enamel. In this study, we elucidated the molecular mechanism that integrates ameloblast function and morphology. Immunohistochemistry revealed that the restricted expression of semaphorin 4D (Sema4D) and RhoA activation status are closely associated with ameloblast differentiation in mouse incisors. In addition, in vitro gain-of-function and loss-of-function experiments demonstrated that Sema4D acts upstream of RhoA to regulate cell polarity and amelogenin expression via the Plexin B1/Leukemia-associated RhoGEF (LARG) complex during ameloblast differentiation. Experiments in transgenic mice demonstrated that expression of a dominant-negative form of RhoA in dental epithelium hindered ameloblast differentiation and subsequent enamel formation, as well as perturbing the establishment of polarized cell morphology and vectorial amelogenin expression. Finally, we showed that spatially restricted Akt mediates between Sema4D-RhoA signaling and these downstream cellular events. Collectively, our results reveal a novel signaling network, the Sema4D-RhoA-Akt signal cascade, that coordinates cellular function and morphology and highlights the importance of specific spatiotemporally restricted components of a signaling pathway in the regulation of ameloblast differentiation. © 2016 American Society for Bone and Mineral Research.

Amelogenin-Ameloblastin Spatial Interaction around Maturing Enamel Rods

Amelogenin and ameloblastin are 2 extracellular matrix proteins that are essential for the proper development of enamel. We recently reported that amelogenin and ameloblastin colocalized during the secretory stage of enamel formation when nucleation of enamel crystallites occurs. Direct interactions between the 2 proteins have been also demonstrated in our in vitro studies. Here, we explore interactions between their fragments during enamel maturation. We applied in vivo immunofluorescence imaging, quantitative co-localization analysis, and a new FRET (fluorescence resonance energy transfer) technique to demonstrate ameloblastin and amelogenin interaction in the maturing mouse enamel. Using immunochemical analysis of protein samples extracted from 8-d-old (P8) first molars from mice as a model for maturation-stage enamel, we identified the ~17-kDa ameloblastin (Ambn-N) and the TRAP (tyrosine-rich amelogenin peptide) fragments. We used Ambn-N18 and Ambn-M300 antibodies raised against the N-terminal and C-terminal segments of ameloblastin, as well as Amel-FL and Amel-C19 antibodies against full-length recombinant mouse amelogenin (rM179) and C-terminal amelogenin, respectively. In transverse sections, co-localization images of N-terminal fragments of amelogenin and ameloblastin around the prism boundary revealed the "fish net" pattern of the enamel matrix. Using in vivo FRET microscopy, we further demonstrated spatial interactions between amelogenin and ameloblastin N-terminal fragments. In the maturing mouse enamel, the association of these residual protein fragments created a discontinuity between enamel rods, which we suggest is important for support and maintenance of enamel rods and eventual contribution to unique enamel mechanical properties. We present data that support cooperative functions of enamel matrix proteins in mediating the structural hierarchy of enamel and that contribute to our efforts to design and develop enamel biomimetic material.

Ameloblastin peptide encoded by exon 5 interacts with amelogenin N-terminus

Interactions between enamel matrix proteins are important for enamel biomineralization. In recent in situ studies, we showed that the N-terminal proteolytic product of ameloblastin co-localized with amelogenin around the prism boundaries. However, the molecular mechanisms of such interactions are still unclear. Here, in order to determine the interacting domains between amelogenin and ameloblastin, we designed four ameloblastin peptides derived from different regions of the full-length protein (AB1, AB2 and AB3 at N-terminus, and AB6 at C-terminus) and studied their interactions with recombinant amelogenin (rP172), and the tyrosine-rich amelogenin polypeptide (TRAP). A series of amelogenin Trp variants (rP172(W25), rP172(W45) and rP172(W161)) were also used for intrinsic fluorescence spectroscopy. Fluorescence spectra of rP172 titrated with AB3, a peptide encoded by exon 5 of ameloblastin, showed a shift in λ_max in a dose-dependent manner, indicating molecular interactions in the region encoded by exon 5 of ameloblastin. Circular dichroism (CD) spectra of amelogenin titrated with AB3 showed that amelogenin was responsible for forming α-helix in the presence of ameloblastin. Fluorescence spectra of amelogenin Trp variants as well as the spectra of TRAP titrated with AB3 showed that the N-terminus of amelogenin is involved in the interaction between ameloblastin and amelogenin. We suggest that macromolecular co-assembly between amelogenin and ameloblastin may play important roles in enamel biomineralization.

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An in vitro approach to study ameloblastin-amelogenin interactions is presented.

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Intrinsic fluorescence of tryptophan and Circular Dichroism were utilized.

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We report that amelogenin has ameloblastin-binding ability via its N-terminal close to Trp 25.

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We report that ameloblastin has amelogenin-binding ability via a peptide encoded by exon 5.

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Macromolecular co-assembly between amelogenin and ameloblastin may play important roles in enamel biomineralization.

Intrinsically disordered proteins and biomineralization

In vertebrates and invertebrates, biomineralization is controlled by the cell and the proteins they produce. A large number of these proteins are intrinsically disordered, gaining some secondary structure when they interact with their binding partners. These partners include the component ions of the mineral being deposited, the crystals themselves, the template on which the initial crystals form, and other intrinsically disordered proteins and peptides. This review speculates why intrinsically disordered proteins are so important for biomineralization, providing illustrations from the SIBLING (small integrin binding N-glycosylated) proteins and their peptides. It is concluded that the flexible structure, and the ability of the intrinsically disordered proteins to bind to a multitude of surfaces is crucial, but details on the precise-interactions, energetics and kinetics of binding remain to be determined.

MMP20, KLK4, and MMP20/KLK4 double null mice define roles for matrix proteases during dental enamel formation

Matrix metalloproteinase 20 (MMP20) and kallikrein‐related peptidase 4 (KLK4) are secreted proteinases that are essential for proper dental enamel formation. We characterized and compared enamel formed in wild‐type, Mmp20^−/−, Klk4^−/−, Mmp20^+/−Klk4^+/−, and Mmp20^−/−Klk4^−/− mice using dissecting and light microscopy, backscattered scanning electron microscopy (bSEM), SEM, microcomputed tomography (μCT), and energy‐dispersive X‐ray analysis (EDX). Following eruption, fractures were observed on Mmp20^−/−, Klk4^−/−, Mmp20^+/−Klk4^+/−, and Mmp20^−/−Klk4^−/− molars. Failure of the enamel in the Mmp20^+/−Klk4^+/− molars was unexpected and suggested that digenic effects could contribute to the etiology of amelogenesis imperfecta in humans. Micro‐CT analyses of hemimandibles demonstrated significantly reduced high‐density enamel volume in the Mmp20^−/− and Klk4^−/− mice relative to the wild‐type, which was further reduced in Mmp20^−/−Klk4^−/− mice. bSEM images of 7‐week Mmp20^−/− and Mmp20^−/−Klk4^−/− mandibular incisors showed rough, pitted enamel surfaces with numerous indentations and protruding nodules. The Mmp20^+/− and Mmp20^+/−Klk4^+/− incisors showed prominent, evenly spaced, horizontal ridges that were more distinct in Mmp20^+/−Klk4^+/− incisors relative to Mmp20^+/− incisors due to the darkening of the valleys between the ridges. In cross sections, the Mmp20^−/− and Mmp20^−/−Klk4^−/− exhibited three distinct layers. The outer layer exhibited a disturbed elemental composition and an irregular enamel surface covered with nodules. The Mmp20 null enamel was apparently unable to withstand the sheer forces associated with eruption and separated from dentin during development. Cells invaded the cracks and interposed between the dentin and enamel layers. MMP20 and KLK4 serve overlapping and complementary functions to harden enamel by removing protein, but MMP20 potentially serves multiple additional functions necessary for the adherence of enamel to dentin, the release of intercellular protein stores into the enamel matrix, the retreat of ameloblasts to facilitate thickening of the enamel layer, and the timely transition of ameloblasts to maturation.

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.

Protein Interaction between Ameloblastin and Proteasome Subunit α Type 3 Can Facilitate Redistribution of Ameloblastin Domains within Forming Enamel

Enamel is a bioceramic tissue composed of thousands of hydroxyapatite crystallites aligned in parallel within boundaries fabricated by a single ameloblast cell. Enamel is the hardest tissue in the vertebrate body; however, it starts development as a self-organizing assembly of matrix proteins that control crystallite habit. Here, we examine ameloblastin, a protein that is initially distributed uniformly across the cell boundary but redistributes to the lateral margins of the extracellular matrix following secretion thus producing cell-defined boundaries within the matrix and the mineral phase. The yeast two-hybrid assay identified that proteasome subunit α type 3 (Psma3) interacts with ameloblastin. Confocal microscopy confirmed Psma3 co-distribution with ameloblastin at the ameloblast secretory end piece. Co-immunoprecipitation assay of mouse ameloblast cell lysates with either ameloblastin or Psma3 antibody identified each reciprocal protein partner. Protein engineering demonstrated that only the ameloblastin C terminus interacts with Psma3. We show that 20S proteasome digestion of ameloblastin in vitro generates an N-terminal cleavage fragment consistent with the in vivo pattern of ameloblastin distribution. These findings suggest a novel pathway participating in control of protein distribution within the extracellular space that serves to regulate the protein-mineral interactions essential to biomineralization.

Analysis of co-assembly and co-localization of ameloblastin and amelogenin

Epithelially-derived ameloblasts secrete extracellular matrix proteins including amelogenin, enamelin, and ameloblastin. Complex intermolecular interactions among these proteins are believed to be important in controlling enamel formation. Here we provide in vitro and in vivo evidence of co-assembly and co-localization of ameloblastin with amelogenin using both biophysical and immunohistochemical methods. We performed co-localization studies using immunofluorescence confocal microscopy with paraffin-embedded tissue sections from mandibular molars of mice at 1, 5, and 8 days of age. Commercially-available ameloblastin antibody (M300) against mouse ameloblastin residues 107-407 and an antibody against full-length recombinant mouse (rM179) amelogenin were used. Ameloblastin-M300 clearly reacted along the secretory face of ameloblasts from days 1-8. Quantitative co-localization was analyzed (QCA) in several configurations by choosing appropriate regions of interest (ROIs). Analysis of ROIs along the secretory face of ameloblasts revealed that at day 1, very high percentages of both the ameloblastin and amelogenin co-localized. At day 8 along the ameloblast cells the percentage of co-localization remained high for the ameloblastin whereas co-localization percentage was reduced for amelogenin. Analysis of the entire thickness on day 8 revealed no significant co-localization of amelogenin and ameloblastin. With the progress of amelogenesis and ameloblastin degradation, there was a segregation of ameloblastin and co-localization with the C-terminal region decreased. CD spectra indicated that structural changes in ameloblastin occurred upon addition of amelogenin. Our data suggest that amelogenin-ameloblastin complexes may be the functional entities at the early stage of enamel mineralization.

Tracking endogenous amelogenin and ameloblastin in vivo

Research on enamel matrix proteins (EMPs) is centered on understanding their role in enamel biomineralization and their bioactivity for tissue engineering. While therapeutic application of EMPs has been widely documented, their expression and biological function in non-enamel tissues is unclear. Our first aim was to screen for amelogenin (AMELX) and ameloblastin (AMBN) gene expression in mandibular bones and soft tissues isolated from adult mice (15 weeks old). Using RT-PCR, we showed mRNA expression of AMELX and AMBN in mandibular alveolar and basal bones and, at low levels, in several soft tissues; eyes and ovaries were RNA-positive for AMELX and eyes, tongues and testicles for AMBN. Moreover, in mandibular tissues AMELX and AMBN mRNA levels varied according to two parameters: 1) ontogenic stage (decreasing with age), and 2) tissue-type (e.g. higher level in dental epithelial cells and alveolar bone when compared to basal bone and dental mesenchymal cells in 1 week old mice). In situ hybridization and immunohistodetection were performed in mandibular tissues using AMELX KO mice as controls. We identified AMELX-producing (RNA-positive) cells lining the adjacent alveolar bone and AMBN and AMELX proteins in the microenvironment surrounding EMPs-producing cells. Western blotting of proteins extracted by non-dissociative means revealed that AMELX and AMBN are not exclusive to mineralized matrix; they are present to some degree in a solubilized state in mandibular bone and presumably have some capacity to diffuse. Our data support the notion that AMELX and AMBN may function as growth factor-like molecules solubilized in the aqueous microenvironment. In jaws, they might play some role in bone physiology through autocrine/paracrine pathways, particularly during development and stress-induced remodeling.

Deletion of ameloblastin exon 6 is associated with amelogenesis imperfecta

Amelogenesis imperfecta (AI) describes a heterogeneous group of inherited dental enamel defects reflecting failure of normal amelogenesis. Ameloblastin (AMBN) is the second most abundant enamel matrix protein expressed during amelogenesis. The pivotal role of AMBN in amelogenesis has been confirmed experimentally using mouse models. However, no AMBN mutations have been associated with human AI. Using autozygosity mapping and exome sequencing, we identified genomic deletion of AMBN exon 6 in a second cousin consanguineous family with three of the six children having hypoplastic AI. The genomic deletion corresponds to an in-frame deletion of 79 amino acids, shortening the protein from 447 to 368 residues. Exfoliated primary teeth (unmatched to genotype) were available from family members. The most severely affected had thin, aprismatic enamel (similar to that reported in mice homozygous for Ambn lacking exons 5 and 6). Other teeth exhibited thicker but largely aprismatic enamel. One tooth had apparently normal enamel. It has been suggested that AMBN may function in bone development. No clinically obvious bone or other co-segregating health problems were identified in the family investigated. This study confirms for the first time that AMBN mutations cause non-syndromic human AI and that mouse models with disrupted Ambn function are valid.

Inhibitory effects of ameloblastin on epithelial cell proliferation

OBJECTIVE

Ameloblastin is an enamel matrix protein expressed in several tissues. Many potential mechanisms have been identified by which ameloblastin functions as an extracellular matrix protein. However, the biological effects of ameloblastin on gingival epithelial cells remain unclear. In the present study, we established a novel system to purify recombinant human ameloblastin and clarified its biological functions in epithelial cells in vitro.

DESIGN

Recombinant human ameloblastin was isolated from COS-7 cells overexpressing HaloTag-fused human ameloblastin by the HaloTag system and then purified further by reverse-phase high-performance liquid chromatography. SCC-25 cells, derived from human oral squamous cell carcinoma, were treated with recombinant ameloblastin and then cell survival was assessed by a WST-1 assay. Cell cycle analysis was performed by flow cytometry.

RESULTS

The novel purification system allowed effective recovery of the recombinant ameloblastin proteins at a high purity. Recombinant ameloblastin protein was found to suppress the proliferation of SCC-25 cells. Flow cytometric analysis showed that ameloblastin treatment induced cell cycle arrest G1 phase.

CONCLUSIONS

We developed a procedure for production of highly purified recombinant human ameloblastin. Biological analyses suggest that ameloblastin induces cell cycle arrest in epithelial cells and regulates the progression of periodontitis.

Dental enamel development: proteinases and their enamel matrix substrates

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

Intrinsically disordered enamel matrix protein ameloblastin forms ribbon-like supramolecular structures via an N-terminal segment encoded by exon 5

Tooth enamel, the hardest tissue in the body, is formed by the evolutionarily highly conserved biomineralization process that is controlled by extracellular matrix proteins. The intrinsically disordered matrix protein ameloblastin (AMBN) is the most abundant nonamelogenin protein of the developing enamel and a key element for correct enamel formation. AMBN was suggested to be a cell adhesion molecule that regulates proliferation and differentiation of ameloblasts. Nevertheless, detailed structural and functional studies on AMBN have been substantially limited by the paucity of the purified nondegraded protein. With this study, we have developed a procedure for production of a highly purified form of recombinant human AMBN in quantities that allowed its structural characterization. Using size exclusion chromatography, analytical ultracentrifugation, transmission electron, and atomic force microscopy techniques, we show that AMBN self-associates into ribbon-like supramolecular structures with average widths and thicknesses of 18 and 0.34 nm, respectively. The AMBN ribbons exhibited lengths ranging from tens to hundreds of nm. Deletion analysis and NMR spectroscopy revealed that an N-terminal segment encoded by exon 5 comprises two short independently structured regions and plays a key role in self-assembly of AMBN.

Ameloblastin in Hertwig's epithelial root sheath regulates tooth root formation and development

Tooth root formation begins after the completion of crown morphogenesis. At the end edge of the tooth crown, inner and outer enamel epithelia form Hertwig’s epithelial root sheath (HERS). HERS extends along with dental follicular tissue for root formation. Ameloblastin (AMBN) is an enamel matrix protein secreted by ameloblasts and HERS derived cells. A number of enamel proteins are eliminated in root formation, except for AMBN. AMBN may be related to tooth root formation; however, its role in this process remains unclear. In this study, we found AMBN in the basal portion of HERS of lower first molar in mice, but not at the tip. We designed and synthesized small interfering RNA (siRNA) targeting AMBN based on the mouse sequence. When AMBN siRNA was injected into a prospective mandibular first molar of postnatal day 10 mice, the root became shorter 10 days later. Furthermore, HERS in these mice revealed a multilayered appearance and 5-bromo-2′-deoxyuridine (BrdU) positive cells increased in the outer layers. In vitro experiments, when cells were compared with and without transiently expressing AMBN mRNA, expression of growth suppressor genes such as p21^Cip1 and p27^Kip1 was enhanced without AMBN and BrdU incorporation increased. Thus, AMBN may regulate differentiation state of HERS derived cells. Moreover, our results suggest that the expression of AMBN in HERS functions as a trigger for normal root formation.

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.

Ameloblastin upstream region contains structural elements regulating transcriptional activity in a stromal cell line derived from bone marrow

Ameloblastin (AMBN) was originally described as a tooth-specific extracellular matrix protein, but current data have shown that AMBN is present in many different tissues of mesenchymal origin. The identification of regulatory elements in the promoter region of the Ambn gene would assist in identifying potential mesenchymal-specific transcriptional factors. In this study we subcloned a 3,788-bp region upstream (and a 54-bp region downstream) of the mouse Ambn transcriptional start site into a LacZ reporter construct and called this construct 3788-Ambn-lacZ. In silico analysis of the 3,788-bp Ambn promoter region identified 50 potential cis-regulatory elements, 29 of which are known to be functional in cell populations of mesenchymal origin. The reporter construct was activated in transfected bone marrow cells, and the promoter activity was induced in cell cultures following addition of recombinant AMBN, interferon-γ, serotonin, or dexamethasone. We discuss the relative significance of the potential cis-acting gene-regulatory elements of Ambn in relation to bone morphogenesis. Knowledge of Ambn gene-regulatory elements will be of importance when developing strategies for bone repair and replacement in a clinical surgical setting.

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.

Tooth Tissue Engineering: Potential and Pitfalls

All over the world a large number of people suffer from tooth diseases like dental caries, tooth abscess, and plaques. Tooth loss or damage, which occurs frequently in our society are generally repaired by applying several conventional methods, such as root-canal treatment, direct pulp capping and dental implants. These methods are quite painful, create damage to the surrounding tooth tissues and also may at times have adverse side-effects. The limitations of the conventional methods can be overcome by applying the concept of tooth tissue engineering. Tooth tissue engineering is the application of biosciences and engineering to regenerate a biofunctional tooth, which can be used to replace the missing tooth or repair the damaged tooth. Tissue engineering involves three key elements - cell, scaffold and growth factors, which interact with each other to regenerate a specific tissue. The success of tissue engineering depends on the proper selection of these three key elements and understanding the interactions among them. To bring us close to the realization of a tissue-engineered tooth, immense progress is going on in understanding how tooth is first developed, and there is a good advancement in tooth regeneration. In this review, “tooth tissue engineering” will be discussed, along with the recent advancements and challenges in bring a biofunctional tooth from laboratory out into clinical use.

Functional role of Rho-kinase in ameloblast differentiation

During tooth development, inner enamel epithelial (IEE) cells differentiate into enamel-secreting ameloblasts, a polarized and elongated cellular population. The molecular underpinnings of this morphogenesis and cytodifferentiation, however, are not well understood. Here, we show that Rho-associated coiled-coil-containing protein kinase (ROCK) regulates ameloblast differentiation and enamel formation. In mouse incisor organ cultures, inhibition of ROCK, hindered IEE cell elongation and disrupted polarization of differentiated ameloblasts. Expression of enamel matrix proteins, such as amelogenin and ameloblastin, and formation of the terminal band structure of actin and E-cadherin were also perturbed. Cultures of dental epithelial cells revealed that ROCK regulates cell morphology and cell adhesion through localization of actin bundles, E-cadherin, and β-catenin to cell membranes. Moreover, inhibition of ROCK promoted cell proliferation. Small interfering RNA specific for ROCK1 and ROCK2 demonstrated that the ROCK isoforms performed complementary functions in the regulation of actin organization and E-cadherin-mediated cell-cell adhesion. Thus, our results have uncovered a novel role for ROCK in amelogenesis.

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.

Biochemical and immunohistochemical characterization of the amyloid in canine amyloid-producing odontogenic tumor

The amyloid of canine amyloid-producing odontogenic tumor (APOT) was evaluated biochemically and immunohistochemically. The N-terminal amino-acid sequence of purified amyloid protein from a canine APOT was strikingly similar to the sequence in both rat ameloblastin and porcine sheathlin. Immunohistochemically, the amyloid in APOT from 9 dogs was strongly reactive with anti-rat ameloblastin, anti-porcine sheathlin, and anti-canine APOT amyloid and weakly reactive with anti-porcine amelogenin but negative for antibodies to cytokeratins, vimentin, desmin, alpha-smooth muscle actin, amyloid A, glial fibrillary acidic protein, or S100 protein. The neoplastic epithelial cells of APOT were focally reactive with antibodies to ameloblastin, sheathlin, amelogenin, and canine APOT amyloid. The similarity in amino-acid sequence of the amyloid protein of canine APOT to that of enamel proteins, such as ameloblastin, sheathlin, and amelogenin, and the expression of these antigens in both APOT amyloid and in the neoplastic cells suggest that the amyloid of canine APOT is derived from enamel proteins secreted by ameloblasts.

Cleavage site specificity of MMP-20 for secretory-stage ameloblastin

Ameloblastin is processed by protease(s) during enamel formation. We tested the hypothesis that MMP-20 (enamelysin) catalyzes the cleavages that generate secretory-stage ameloblastin cleavage products. We isolated a 23-kDa ameloblastin cleavage product from developing enamel and determined its N-terminus sequence. Ameloblastin was stably expressed and secreted from HEK293-H cells, purified, and digested with MMP-20 or Klk4 (kallikrein 4). The digests were analyzed by SDS-PAGE and Western blotting, and cleavage products were characterized by N-terminal sequencing. Six fluorescent peptides were digested with MMP-20 and Klk4 and analyzed by RP-HPLC and by mass spectrometry. MMP-20 cleaved each peptide exactly at the sites corresponding to ameloblastin cleavages catalyzed in vivo. Klk4 cleaved ameloblastin and the fluorescent peptides at sites not observed in vivo, and cleaved at only a single correct site: before Leu(171). We conclude that MMP-20 is the enzyme that processes ameloblastin during the secretory stage of amelogenesis, and we present a hypothesis about the sequence of ameloblastin cleavages.

A mouse model expressing a truncated form of ameloblastin exhibits dental and junctional epithelium defects

Ameloblastin (AMBN) is the second most abundant extracellular matrix protein produced by the epithelial cells called ameloblasts and is found mainly in forming dental enamel. Inactivation of its expression by gene knockout results in absence of the enamel layer and its replacement by a thin layer of dysplastic mineralized matrix. The objective of this study was to further characterize the enamel organ and mineralized matrix produced in the AMBN knockout mouse. However, in the course of our study, we unexpectedly found that this mouse is in fact a mutant that does not express the full-length protein but that produces a truncated form of AMBN. Mandibles from wild type and mutant mice were processed for morphological analyses and immunolabeling. Microdissected enamel organs and associated matrix were also prepared for molecular and biochemical analyses. In incisors from mutants, ameloblasts lost their polarized organization and the enamel organ detached from the tooth surface and became disorganized. A thin layer of dysplastic mineralized material was deposited onto dentin, and mineralized masses were present within the enamel organ. These mineralized materials generated lower backscattered electron contrast than normal enamel, and immunocytochemistry with colloidal gold revealed the presence of amelogenin, bone sialoprotein and osteopontin. In addition, the height of the alveolar bone was reduced, and the junctional epithelium lost its integrity. Immunochemical and RT-PCR results revealed that the altered enamel organ in the mutant mice produced a shorter AMBN protein that is translated from truncated RNA missing exons 5 and 6. These results indicate that absence of full-length protein and/or expression of an incomplete protein have direct/indirect effects beyond structuring of mineral during enamel formation, and highlight potential functional regions on the AMBN molecule.

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.

Splicing determines the glycosylation state of ameloblastin

In developing porcine enamel, the space between enamel rods selectively binds lectins and ameloblastin (Ambn) N-terminal antibodies. We tested the hypothesis that ameloblastin N-terminal cleavage products are glycosylated. Assorted Ambn cleavage products showed positive lectin staining by peanut agglutinin (PNA), Maclura pomifera agglutinin (MPA), and Limulus polyphemus agglutinin (LPA), suggesting the presence of an O-linked glycosylation containing galactose (Gal), N-acetylgalactosamine (GalNAc), and sialic acid. Edman sequencing of the lectin-positive bands gave the Ambn N-terminal sequence: VPAFPRQPGTXGVASLXLE. The blank cycles for Pro(11) and Ser(17) confirmed that these residues are hydroxylated and phosphorylated, respectively. The O-glycosylation site was determined by Edman sequencing of pronase-digested Ambn, which gave HPPPLPXQPS, indicating that Ser(86) is the site of the O-linked glycosylation. This modification is within the 15-amino-acid segment (73-YEYSLPVHPPPLPSQ-87) deleted by splicing in the mRNA encoding the 380-amino-acid Ambn isoform. We conclude that only the N-terminal Ambn products derived from the 395-Ambn isoform are glycosylated.

Spatiotemporal expression of ameloblastin isoforms during murine tooth development

Ameloblasts synthesize and secrete the enamel matrix proteins (amelogenin, ameloblastin, and enamelin). This investigation examined the profiles of ameloblastin in the ameloblasts and in the enamel matrix during different postnatal (PN) days (days 0-9) of development of mouse molar, using an antibody specific for C-terminal sequence of ameloblastin (Ct; GNKVHQPQVHNAWRF). Ameloblastin is found in three different molecular sizes (37, 55, and 66 kDa) in both ameloblasts and enamel matrix during PN development. In the ameloblasts, the sequence of expression of these fractions varied. The 37-kDa fraction was observed (even before the appearances of mRNA of the proteases, enamelysin and kallikrein-4) on days 0 and 1, persisted until day 3, and was not found thereafter. Other isoforms (55 and 66 kDa) distinctly appeared in ameloblasts after day 1, reached a peak on day 5, and remained thereafter. The Ct-positive granules appeared beaded in the ameloblasts on day 3. In the extracellular matrix, a 37-kDa (but not 66- or 55-kDa) fraction was detected on days 0 and 1 and remained in the matrix throughout the PN days. The larger isoforms (55 and 66 kDa) appeared in the enamel matrix from day 3 onward. On days 0-3, but not later, the 37-kDa isoform co-localizes with amelogenin in Tomes' process and formative enamel, as revealed by laser scan confocal microscopy. Autoradiography confirmed accumulation of 3H-labeled amelogenin trityrosyl motif peptide in the region of Tomes' process and formative enamel from day 0 to 3. These observations suggest that the 37-kDa isoform interacts with amelogenin during early tooth development.

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.

Enamel matrix proteins bind to wound matrix proteins and regulate their cell-adhesive properties

Enamel matrix proteins (EMP) induce periodontal regeneration and accelerate dermal wound healing, but the cellular mechanisms of these processes are unclear. We investigated the binding of EMP to the wound matrix proteins fibronectin, laminin-1, collagen type I, and collagen type IV and analyzed the interaction of epithelial cells and periodontal ligament fibroblasts (PDLF) with EMP and composite matrices of EMP + fibronectin or EMP + collagen. The adhesion of PDLF to EMP was concentration- and integrin-dependent and did not require de novo protein synthesis. EMP supported PDLF migration. In contrast, keratinocytes did not adhere to EMP if their protein synthesis was blocked. EMP showed concentration-dependent binding of fibronectin, peaking at 100 microg ml(-1) (before the precipitation point) of EMP. Type I collagen binding to EMP peaked at a low (1 microg ml(-1)) and narrow concentration range. Neither laminin-1 nor type IV collagen bound to EMP. Collagen and fibronectin, bound to EMP, showed significantly reduced (> 50%) binding of both epithelial cells and PDLF compared with the equivalent concentration of these proteins alone. PDLF, but not epithelial cell, adhesion was rescued by increasing the EMP concentration. These findings show that EMP binds to wound extracellular matrix proteins and regulates their adhesive properties. Such interactions may favor fibroblast adhesion over epithelial cells, potentially promoting connective tissue regeneration.

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.

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.

Expression of ameloblastin during enamel formation in a crocodile

Ameloblastin is an enamel-specific protein that plays critical roles in enamel formation, as well as adhesion between ameloblasts and the enamel matrix, as shown by analyses of ameloblastin-null mice. In the present study, we produced two distinct antibodies that recognize the N-terminus and C-terminus regions of caiman ameloblastin, in order to elucidate the fate of ameloblastin peptides during tooth development. An immunohistochemical study using the antibodies showed that caiman ameloblastin was a tooth-specific matrix protein that may initially be cleaved into two groups, N- and C-terminal peptides, as shown in mammals. The distribution of the N-terminal peptides was much different from that of the C-terminal peptides during enamel formation; however, it was similar to that of mammalian ameloblastin. Although ameloblastin is thought to have a relationship with the enamel prismatic structure in mammals, in the caiman, which has non-prismatic enamel, functional ameloblastin has no relationship with any enamel structure. Consequently, it is suggested that ameloblastin has kept its original functions during the evolutionary transition from reptiles to mammals and that it has been conserved in both lineages during more than 200 million years of evolution. Our results support the notion that ameloblastin acts as a factor for ameloblast adhesion to enamel matrix, because distribution of the C-terminal peptides was consistently restricted on the surface layers of enamel matrix specimens ranging from immature to nearly completely mature. The principal molecules that provide the adhesive function are presumably C-terminal peptides.

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.