MMP20-generated amelogenin cleavage products prevent formation of fan-shaped enamel malformations.

Dental enamel forms extracellularly as thin ribbons of amorphous calcium phosphate (ACP) that initiate on dentin mineral in close proximity to the ameloblast distal membrane. Secreted proteins are critical for this process. Enam-/- and Ambn-/- mice fail to form enamel. We characterize enamel ribbon formation in wild-type (WT), Amelx-/- and Mmp20-/- mouse mandibular incisors using focused ion beam scanning electron microscopy (FIB-SEM) in inverted backscatter mode. In Amelx-/- mice, initial enamel mineral ribbons extending from dentin are similar in form to those of WT mice. As early enamel development progresses, the Amelx-/- mineral ribbons develop multiple branches, resembling the staves of a Japanese fan. These striking fan-shaped structures cease growing after attaining ~ 20 µm of enamel thickness (WT is ~ 120 µm). The initial enamel mineral ribbons in Mmp20-/- mice, like those of the Amelx-/- and WT, extend from the dentin surface to the ameloblast membrane, but appear to be fewer in number and coated on their sides with organic material. Remarkably, Mmp20-/- mineral ribbons also form fan-like structures that extend to ~ 20 µm from the dentin surface. However, these fans are subsequently capped with a hard, disorganized outer mineral layer. Amelogenin cleavage products are the only matrix components absent in both Amelx-/- and Mmp20-/- mice. We conclude that MMP20 and amelogenin are not critical for enamel mineral ribbon initiation, orientation, or initial shape. The pathological fan-like plates in these mice may form from the lack of amelogenin cleavage products, which appear necessary to form ordered hydroxyapatite.

An Evolutionarily Conserved Helix Mediates Ameloblastin-Cell Interaction.

Ameloblastin (Ambn) has the potential to regulate cell-matrix adhesion through familiar cell-binding domains, but the proposed sequence motifs are not highly conserved across species. Here, we report that Ambn binds to ameloblast-like cell membranes through a highly evolutionary conserved amphipathic helix-forming (AH) motif encoded by exon 5. We applied high-resolution confocal microscopy to show colocalization of Ambn with ameloblast membrane surfaces in developing mouse incisors. Using a series of Ambn-derived peptides and Ambn variants, we showed that Ambn binds to cell membranes through a motif within the sequence encoded by exon 5. Using peptides derived from the N- or C-termini of this sequence, as well as Ambn variants that lacked or had a disrupted AH motif, we demonstrated that the AH motif located at the N-terminus of the sequence is involved in cell-Ambn adhesion. Sequence analysis revealed that this highly conserved AH motif is absent from other enamel matrix proteins, including amelogenin, enamelin, and amelotin. Collectively, these data suggest that Ambn binds to the cell surface membrane via a helix-forming motif and provide insight into the molecular mechanism and function of Ambn in enamel cell-matrix interaction.

Analysis of the Yp11.2 Deletion Region of Phenotypically Normal Males with an AMELY-Null Allele in the Chinese Han Population.

Aim: The amelogenin gene is a widely used gender marker for forensic DNA profiling. Males who have the amelogenin Y (AMELY) allele deletion can be mistakenly identified as females if genotyping is performed only on the amelogenin gene. The aim of this study was to investigate the frequency of the AMELY allele deletion in the Chinese Han population and analyze the possible genetic variation on the Y chromosome. Materials and Methods: The amelogenin gene of 12,735 unrelated males from the Chinese Han population were genotyped using common forensic short tandem repeat (STR) kits. The AMELY allele deletion was verified by redesigned primers and sequencing. Eighteen Y-specific sequence tagged sites (STSs) on the Yp11.2 region were selected to delineate the deletion breakpoints on the Y chromosome. Results: Three males were confirmed to have no AMELY allele. The frequency rate of the AMELY-null allele was 0.236% (3/12,735) in the Chinese Han population of Central China; 2.73 Mb of sequence on the Y chromosome were absent in all the AMELY-negative samples. The deleted region was mapped using SRY, AMELY, 5 Y-STRs, and 18 STSs, which belong to the class I deleted pattern. The three unrelated males shared the same Y-STR haplotype with four males from other Chinese populations, all of whom have the AMELY-null allele. The haplogroup of these males was identified as the O3 haplogroup. Conclusion: The AMELY allele deletion in the Chinese population was accompanied by the deletion of the Y-STR loci on the Yp11.2 region. Therefore, another Y-specific marker should be tested simultaneously when unknown samples are examined as part of a criminal investigation.

Recombinant amelogenin regulates the bioactivity of mouse cementoblasts in vitro.

Amelogenin (AMG) is a cell adhesion molecule that has an important role in the mineralization of enamel and regulates events during dental development and root formation. The purpose of the present study was to investigate the effects of recombinant human AMG (rhAMG) on mineralized tissue-associated genes in cementoblasts. Immortalized mouse cementoblasts (OCCM-30) were treated with different concentrations (0.1, 1, 10, 100, 1000, 10,000, 100,000 ng · mL-1) of recombinant human AMG (rhAMG) and analyzed for proliferation, mineralization and mRNA expression of bone sialoprotein (BSP), osteocalcin (OCN), collagen type I (COL I), osteopontin (OPN), runt-related transcription factor 2 (Runx2), cementum attachment protein (CAP), and alkaline phosphatase (ALP) genes using quantitative RT-PCR. The dose response of rhAMG was evaluated using a real-time cell analyzer. Total RNA was isolated on day 3, and cell mineralization was assessed using von Kossa staining on day 8. COL I, OPN and lysosomal-associated membrane protein-1 (LAMP-1), which is a cell surface binding site for amelogenin, were evaluated using immunocytochemistry. F-actin bundles were imaged using confocal microscopy. rhAMG at a concentration of 100,000 ng · mL-1 increased cell proliferation after 72 h compared to the other concentrations and the untreated control group. rhAMG (100,000 ng · mL-1) upregulated BSP and OCN mRNA expression levels eightfold and fivefold, respectively. rhAMG at a concentration of 100,000 ng · mL-1 remarkably enhanced LAMP-1 staining in cementoblasts. Increased numbers of mineralized nodules were observed at concentrations of 10,000 and 100,000 ng · mL-1 rhAMG. The present data suggest that rhAMG is a potent regulator of gene expression in cementoblasts and support the potential application of rhAMG in therapies aimed at fast regeneration of damaged periodontal tissue.

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.

Full-length amelogenin influences the differentiation of human dental pulp stem cells.

BACKGROUND: Amelogenin is an extracellular matrix protein well known for its role in the organization and mineralization of enamel. Clinically, it is used for periodontal regeneration and, due to its finding also in predentin and intercellular spaces of dental pulp cells, it has recently been suggested for pulp capping procedures. The aim of this study was to analyse in vitro the effect of the recombinant human full-length amelogenin on the growth and differentiation of human dental pulp stem cells (hDPSCs). METHODS: Human DPSCs were treated with a supplement of amelogenin at a concentration of 10 ng/ml, 100 ng/ml and 1000 ng/ml. The groups were compared to the unstimulated control in terms of cell morphology and proliferation, mineralization and gene expression for ALP (alkaline phosphatase), DMP1 (dentin matrix protein-1) and DSPP (dentin sialophosphoprotein). RESULTS: Amelogenin affects hDPSCs differently than PDL (periodontal ligament) cells and other cell lines. The proliferation rate at two weeks is significantly reduced in presence of the highest concentration of amelogenin as compared to the unstimulated control. hDPSCs treated with low concentrations present a downregulation of DMP1 and DSPP, which is significant for DSPP (p = 0.011), but not for DMP1 (p = 0.395). CONCLUSIONS: These finding suggest that the role of full-length amelogenin is not restricted to participation in tooth structure. It influences the differentiation of hDPSC according to various concentrations and this might impair the clinical results of pulp capping.

Expression of amelogenin and effects of cyclosporin A in developing hair follicles in rats.

Amelogenin, an enamel matrix protein has been considered to be exclusively expressed by ameloblasts during odontogenesis. However, burgeoning evidence indicates that amelogenin is also expressed in non-mineralizing tissues. Under the hypothesis that amelogenin may be a functional molecule in developing hair follicles which share developmental features with odontogenesis, this study for the first time elucidated the presence and functional changes of amelogenin and its receptors during rat hair follicle development. Amelogenin was specifically localized in the outer epithelial root sheath of hair follicles. Its expression appeared in the deeper portion of hair follicles, i.e. the bulbar and suprabulbar regions rather than the superficial region. Lamp-1, an amelogenin receptor, was localized in either follicular cells or outer epithelial sheath cells, reflecting functional changes during development. The expression of amelogenin splicing variants increased in a time-dependent manner during postnatal development of hair follicles. Amelogenin expression was increased by treatment with cyclosporin A, which is an inducer of anagen in the hair follicle, whereas the level of Lamp-1 and -2 was decreased by cyclosporin A treatment. These results suggest that amelogenin may be a functional molecule involved in the development of the hair follicle rather than an inert hair shaft matrix protein.

Grp78 Is Critical for Amelogenin-Induced Cell Migration in a Multipotent Clonal Human Periodontal Ligament Cell Line.

Periodontal ligament stem cells (PDLSCs) are known to play a pivotal role in regenerating the periodontium. Amelogenin, which belongs to a family of extracellular matrix (ECM) proteins, is a potential bioactive molecule for periodontal regenerative therapy. However, its downstream target molecules and/or signaling patterns are still unknown. Our recent proteomic study identified glucose-regulated protein 78 (Grp78) as a new amelogenin-binding protein. In this study, we demonstrate, for the first time, the cellular responses induced by the biological interaction between amelogenin and Grp78 in the human undifferentiated PDL cell line 1-17, which possesses the most typical characteristics of PDLSCs. Confocal co-localization experiments revealed the internalization of recombinant amelogenin (rM180) via binding to cell surface Grp78, and the endocytosis was inhibited by the silencing of Grp78 in 1-17 cells. Microarray analysis indicated that rM180 and Grp78 regulate the expression profiles of cell migration-associated genes in 1-17 cells. Moreover, Grp78 overexpression enhanced rM180-induced cell migration and adhesion without affecting cell proliferation, while silencing of Grp78 diminished these activities. Finally, binding of rM180 to Grp78 promoted the formation of lamellipodia, and the simultaneous activation of Rac1 was also demonstrated by NSC23766, a widely accepted Rac1 inhibitor. These results suggest that Grp78 is essential for enhancing amelogenin-induced migration in 1-17 cells. The biological interaction of amelogenin with Grp78 offers significant therapeutic potential for understanding the biological components and specific functions involved in the signal transduction of amelogenin-induced periodontal tissue regeneration.

Amelogenins as potential buffers during secretory-stage amelogenesis.

Amelogenins are the most abundant protein species in forming dental enamel, taken to regulate crystal shape and crystal growth. Unprotonated amelogenins can bind protons, suggesting that amelogenins could regulate the pH in enamel in situ. We hypothesized that without amelogenins the enamel would acidify unless ameloblasts were buffered by alternative ways. To investigate this, we measured the mineral and chloride content in incisor enamel of amelogenin-knockout (AmelX(-/-)) mice and determined the pH of enamel by staining with methyl-red. Ameloblasts were immunostained for anion exchanger-2 (Ae2), a transmembrane pH regulator sensitive for acid that secretes bicarbonate in exchange for chloride. The enamel of AmelX(-/-) mice was 10-fold thinner, mineralized in the secretory stage 1.8-fold more than wild-type enamel and containing less chloride (suggesting more bicarbonate secretion). Enamel of AmelX(-/-) mice stained with methyl-red contained no acidic bands in the maturation stage as seen in wild-type enamel. Secretory ameloblasts of AmelX(-/-) mice, but not wild-type mice, were immunopositive for Ae2, and stained more intensely in the maturation stage compared with wild-type mice. Exposure of AmelX(-/-) mice to fluoride enhanced the mineral content in the secretory stage, lowered chloride, and intensified Ae2 immunostaining in the enamel organ in comparison with non-fluorotic mutant teeth. The results suggest that unprotonated amelogenins may regulate the pH of forming enamel in situ. Without amelogenins, Ae2 could compensate for the pH drop associated with crystal formation.

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.

Amelogenin splice isoforms stimulate chondrogenic differentiation of ATDC5 cells.

OBJECTIVE: Amelogenins are the most abundant matrix proteins in enamel. Among the amelogenin isoforms, full-length amelogenin (M180) and leucine-rich amelogenin peptide (LRAP) are expressed in various tissues and are implicated as signalling molecules in mesenchymal cells. Here, we examined the effects of M180 and LRAP on a chondrogenic cell line, ATDC5, to investigate the role of amelogenins in chondrogenesis. MATERIALS AND METHODS: Recombinant mouse M180- or LRAP-protein-containing medium or control medium was mixed with a chondrogenesis-stimulating medium, and changes in the phenotype, gene expression levels and cell proliferation of cultured ATDC5 cells were analysed. RESULTS: The addition of amelogenins increased alkaline phosphatase activity and glycosaminoglycan secretion at 14 and 21 days of culture, respectively, as compared with the control. Quantitative PCR (Q-PCR) analysis revealed that LRAP increased the gene expression levels of Runx2, Col2a1 and Aggrecan at 7 days of differentiation. Moreover, both M180 and LRAP significantly increased the gene expression levels of ALP, Aggrecan, Col10a1 and osteopontin at 28 days of culture. Bromodeoxyuridine assay and Q-PCR analysis for Wnt signalling indicated that both M180 and LRAP reduced proliferation, but induced the cell differentiation possibly through altered non-canonical Wnt signalling. CONCLUSION: M180 and LRAP accelerate chondrogenic differentiation and maturation of ATDC5 cells.

Analogous effects of recombinant human full-length amelogenin expressed by Pichia pastoris yeast and enamel matrix derivative in vitro.

OBJECTIVES: Amelogenins are proposed to be responsible for enamel matrix derivative (EMD)-induced periodontal regeneration; however, heterogeneity of amelogenins makes it challenging to purify the full-length proteins. This study has been carried out to express and purify a recombinant full-length human amelogenin protein (rHhAm175) in the eukaryotic yeast Pichia pastoris, and further compare biological responses of human periodontal ligament fibroblasts (PDLFs) to rHhAm175 and porcine EMD (pEMD). MATERIALS AND METHODS: Human cDNA encoding a 175-amino acid amelogenin was subcloned into the pPIC3.5K vector. The rHhAm175 expressed in P. pastoris GS115 (Mut+) was purified and characterized. We examined cell attachment, migration and proliferation responses of human PDLFs to rHhAm175 and pEMD respectively, and characterized associated changes of proliferation-related intracellular signalling molecules, including extracellular signal response kinase (ERK) and Akt kinases/protein kinase B (Akt/PKB) kinases. RESULTS: The purified rHhAm175 was confirmed to be molecular mass 22 021.13 Da, phosphorylated human amelogenin, and alone significantly promoted proliferation and migration of human PDLFs to an extent comparable to that of pEMD. Cell attachment was increased over the first 60 min incubation with rHhAm175 or pEMD. Both rHhAm175 and pEMD induced PDLF mitogenesis via extracellular signal response kinase (ERK1/2), but not by Akt kinases/protein kinase B (Akt/PKB). CONCLUSIONS: rHhAm175 modulated cell activities of human PDLFs, to a comparable extent as porcine EMD. These data suggest that rHhAm175 might be used to induce periodontal tissue regeneration.

Cementomimetics-constructing a cementum-like biomineralized microlayer via amelogenin-derived peptides.

Cementum is the outer-, mineralized-tissue covering the tooth root and an essential part of the system of periodontal tissue that anchors the tooth to the bone. Periodontal disease results from the destructive behavior of the host elicited by an infectious biofilm adhering to the tooth root and left untreated, may lead to tooth loss. We describe a novel protocol for identifying peptide sequences from native proteins with the potential to repair damaged dental tissues by controlling hydroxyapatite biomineralization. Using amelogenin as a case study and a bioinformatics scoring matrix, we identified regions within amelogenin that are shared with a set of hydroxyapatite-binding peptides (HABPs) previously selected by phage display. One 22-amino acid long peptide regions referred to as amelogenin-derived peptide 5 (ADP5) was shown to facilitate cell-free formation of a cementum-like hydroxyapatite mineral layer on demineralized human root dentin that, in turn, supported attachment of periodontal ligament cells in vitro. Our findings have several implications in peptide-assisted mineral formation that mimic biomineralization. By further elaborating the mechanism for protein control over the biomineral formed, we afford new insights into the evolution of protein-mineral interactions. By exploiting small peptide domains of native proteins, our understanding of structure-function relationships of biomineralizing proteins can be extended and these peptides can be utilized to engineer mineral formation. Finally, the cementomimetic layer formed by ADP5 has the potential clinical application to repair diseased root surfaces so as to promote the regeneration of periodontal tissues and thereby reduce the morbidity associated with tooth loss.

Amelogenins modulate cytokine expression in LPS-challenged cultured human macrophages.

Amelogenins are enamel matrix proteins with a proven ability to restore tissues in patients with advanced periodontitis and chronic skin wounds. To explore the mechanisms of action of amelogenins in wound inflammation, the in vitro effect on the expression of selected cell mediators involved in inflammation and tissue repair from human monocyte-derived macrophages was studied. Macrophages were treated with amelogenins in serum-enriched medium with simultaneous lipopolysaccharide (LPS) stimulation, for 6, 24 and 72 h, and the conditioned culture medium was analysed for 28 different cytokines. Amelogenin treatment directed the LPS-induced release of both pro- and anti-inflammatory cytokines towards an alternatively activated macrophage phenotype. This change in activation was also demonstrated by the amelogenin-induced secretion of alternative macrophage activation-associated CC chemokine-1 (AMAC-1, also known as CCL18; p<0.001), a well-documented marker of alternative activation. Amelogenins were also shown significantly to increase the macrophage expression of vascular endothelial growth factor and, to a lesser but significant extent, insulin-like growth factor-1 after 24h of culture. The results of the present in vitro study show that monocyte-derived macrophages stimulated by inflammatory agonist LPS respond to the treatment with amelogenins by reducing the pro-inflammatory activity and increasing the expression of tissue repair mediators.

Differential effects of amelogenin on mineralization of cementoblasts and periodontal ligament cells.

BACKGROUND: Amelogenin is a major component of developing extracellular enamel matrix proteins and plays a crucial role during the formation of tooth enamel. In addition, amelogenins are suggested to exert biologic functions as signaling molecules through cell-surface receptors. The purpose of this study is to examine the effect of recombinant human full-length amelogenin (rh174) on the mineralization of human cementoblasts (HCEMs) and human periodontal ligament cells (HPDLs). METHODS: HCEMs, namely, a cell line immortalized by transfection of human telomerase reverse transcription gene, and HPDLs isolated from human first premolars were cultured and treated with 0 to 1,000 ng/mL rh174. The messenger ribonucleic acid (mRNA) levels of alkaline phosphatase (ALP), osteocalcin (OCN), and bone sialoprotein (BSP) were examined by real-time polymerase chain reaction analysis. The protein levels of OCN and BSP were examined by Western blot analysis. ALP activity and calcium deposition of cell cultures were also determined. Mineralization of cells was evaluated by red dye staining. RESULTS: The treatment of HCEMs with rh174 upregulated the ALP, OCN, and BSP mRNA levels. In addition, the protein levels of OCN and BSP, ALP activity, and calcium deposition were enhanced, resulting in enhanced mineralization. Conversely, there were no significant effects of rh174 on the mineralization of HPDLs. CONCLUSION: The present study shows that rh174 enhances mineralization accompanied by upregulation of mineralization markers in HCEMs, whereas it has no effect on that in HPDLs, suggesting different effects of amelogenin on PDL and cementum.

The use of mouse models to investigate shear bond strength in amelogenesis imperfecta.

Patients with amelogenesis imperfecta (AI) have defective enamel; therefore, bonded restorations of patients with AI have variable success rates. To distinguish which cases of AI may have good clinical outcomes with bonded materials, we evaluated etching characteristics and bond strength of enamel in mouse models, comparing wild-type (WT) with those having mutations in amelogenin (Amelx) and matrix metalloproteinase-20 (Mmp20), which mimic 2 forms of human AI. Etched enamel surfaces were compared for roughness by scanning electron microscopy (SEM) images. Bonding was compared through shear bond strength (SBS) studies with 2 different systems (etch-and-rinse and self-etch). Etched enamel surfaces of incisors from Amelx knock-out (AmelxKO) mice appeared randomly organized and non-uniform compared with WT. Etching of Mmp20KO surfaces left little enamel, and the etching pattern was indistinguishable from unetched surfaces. SBS results were significantly different when AmelxKO and Mmp20KO enamel surfaces were compared. A significant increase in SBS was measured for all samples when the self-etch system was compared with the etch-and-rinse system. We have developed a novel system for testing shear bond strength of mouse incisors with AI variants, and analysis of these data may have important clinical implications for the treatment of patients with AI.

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

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

Regulation of calcium phosphate formation by amelogenins under physiological conditions.

Amelogenin is essential for proper enamel formation. The present in vitro study extends our previous work at low (10 mM) ionic strength (IS) by examining the effect of amelogenin on mineralization under higher (162 mM) IS conditions found in developing enamel. Full-length phosphorylated (P173) and non-phosphorylated (rP172) amelogenins were examined, along with P148 and rP147 that lack the hydrophilic C-terminus. Calcium phosphate formation was assessed by pH change, while the minerals formed were characterized using transmission electron microscopy (TEM) and Fourier transform infrared spectroscopy. Amelogenin self-assembly was also studied using dynamic light scattering and TEM. The results indicate that IS does not influence the effects of rP147, rP172, and P173 on mineralization. However, in contrast to the findings for low IS, where both P173 and P148 stabilize initially formed amorphous calcium phosphate (ACP) nanoparticles for >1 d, elongated hydroxyapatite crystals were observed after 24 h using P148 at high IS, unlike that seen with P173. Differences in self-assembly help explain these findings, which suggest that P173 and P148 may play different roles in regulating enamel mineral formation. The present data support the notion that proteolytic processing of P173 is required in vivo to induce the transformation of initial ACP phases to apatitic enamel crystals.

Ameloblast differentiation in the human developing tooth: effects of extracellular matrices.

Tooth enamel is formed by epithelially-derived cells called ameloblasts, while the pulp dentin complex is formed by the dental mesenchyme. These tissues differentiate with reciprocal signaling interactions to form a mature tooth. In this study we have characterized ameloblast differentiation in human developing incisors, and have further investigated the role of extracellular matrix proteins on ameloblast differentiation. Histological and immunohistochemical analyses showed that in the human tooth, the basement membrane separating the early developing dental epithelium and mesenchyme was lost shortly before dentin deposition was initiated, prior to enamel matrix secretion. Presecretary ameloblasts elongated as they came into contact with the dentin matrix, and then shortened to become secretory ameloblasts. In situ hybridization showed that the presecretory stage of odontoblasts started to express type I collagen mRNA, and also briefly expressed amelogenin mRNA. This was followed by upregulation of amelogenin mRNA expression in secretory ameloblasts. In vitro, amelogenin expression was upregulated in ameloblast lineage cells cultured in Matrigel, and was further up-regulated when these cells/Matrigel were co-cultured with dental pulp cells. Co-culture also up-regulated type I collagen expression by the dental pulp cells. Type I collagen coated culture dishes promoted a more elongated ameloblast lineage cell morphology and enhanced cell adhesion via integrin alpha2beta1. Taken together, these results suggest that the basement membrane proteins and signals from underlying mesenchymal cells coordinate to initiate differentiation of preameloblasts and regulate type I collagen expression by odontoblasts. Type I collagen in the dentin matrix then anchors the presecretary ameloblasts as they further differentiate to secretory cells. These studies show the critical roles of the extracellular matrix proteins in ameloblast differentiation.

Different effects of 25-kDa amelogenin on the proliferation, attachment and migration of various periodontal cells.

Previous studies have assumed that amelogenin is responsible for the therapeutic effect of the enamel matrix derivative (EMD) in periodontal tissue healing and regeneration. However, it is difficult to confirm this hypothesis because both the EMD and the amelogenins are complex mixtures of multiple proteins. Further adding to the difficulties is the fact that periodontal tissue regeneration involves various types of cells and a sequence of associated cellular events including the attachment, migration and proliferation of various cells. In this study, we investigated the potential effect of a 25-kDa recombinant porcine amelogenin (rPAm) on primarily cultured periodontal ligament fibroblasts (PDLF), gingival fibroblasts (GF) and gingival epithelial cells (GEC). The cells were treated with 25-kDa recombinant porcine amelogenin at a concentration of 10 microg/mL. We found that rPAm significantly promoted the proliferation and migration of PDLF, but not their adhesion. Similarly, the proliferation and adhesion of GF were significantly enhanced by treatment with rPAm, while migration was greatly inhibited. Interestingly, this recombinant protein inhibited the growth rate, cell adhesion and migration of GEC. These data suggest that rPAm may play an essential role in periodontal regeneration through the activation of periodontal fibroblasts and inhibition of the cellular behaviors of gingival epithelial cells.