MiR-1a-3p Inhibits Apoptosis in Fluoride-exposed LS8 Cells by Targeting Map3k1

Dental fluorosis is a common chemical disease. It is currently unclear how fluorosis occurs at the molecular level. We used miRNA-seq to look at the differences between miRNAs in the cell line of ameloblasts LS8 that had been treated with 3.2 mmol/L NaF. We also performed gene ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway enrichment analyses. miR-1a-3p levels were significantly lower in mouse LS8 cells treated with 3.2 mmol/L NaF, and miR-1a-3p-targeted genes were significantly enriched in the MAPK pathway. LS8 cells were divided into four groups: control, NaF, NaF+miR-1a-3p mimics, and NaF+miR-1a-3p mimics normal control groups. Cellular morphology was observed by an inverted microscope, and the proliferation activity of LS8 cells was assessed by Cell Counting Kit-8 (CCK-8). Using the real-time quantitative polymerase chain reaction (RT-qPCR), transcription levels of miR-1a-3p and Map3k1 were detected. The expressions of Bax, Bcl-2, Map3k1, p38MAPK, ERK1/2, p-p38MAPK, and p-ERK1/2 were measured by Western blot. After bioinformatics analysis, we used a luciferase reporter assay (LRA) to validate the target of miR-1a-3p, showing that miR-1a-3p could inhibit apoptosis while increasing proliferation in fluoride-exposed LS8 cells. Generally, miR-1a-3p might directly inhibit Map3k1, reduce MAPK signal pathway activation, and promote phosphorylation. Thus, our findings revealed that the interaction of miR-1a-3p with its target gene Map3k1 and MAPK signal pathway might decrease the apoptosis of LS8 cells treated with 3.2 mmol/L NaF.

A genetic model for the secretory stage of dental enamel formation

The revolution in genetics has rapidly increased our knowledge of human and mouse genes that are critical for the formation of dental enamel and helps us understand how enamel evolved. In this graphical review we focus on the roles of 41 genes that are essential for the secretory stage of amelogenesis when characteristic enamel mineral ribbons initiate on dentin and elongate to expand the enamel layer to the future surface of the tooth. Based upon ultrastructural analyses of genetically modified mice, we propose a molecular model explaining how a cell attachment apparatus including collagen 17, α6ß4 and αvß6 integrins, laminin 332, and secreted enamel proteins could attach to individual enamel mineral ribbons and mold their cross-sectional dimensions as they simultaneously elongate and orient them in the direction of the retrograde movement of the ameloblast membrane.

Sox2 controls asymmetric patterning of ameloblast lineage commitment by regulation of FGF signaling in the mouse incisor

Mouse incisors are covered by enamel only on the labial side and the lingual side is covered by dentin without enamel. This asymmetric distribution of enamel makes it possible to be abrased on the lingual side, generating the sharp cutting edge of incisor on the labial side. The abrasion of mouse incisors is compensated by the continuous growth throughout life. Epithelium stem cells responsible for its continuous growth are reported to localize within the labial cervical loop. The transcription factor Sox2 plays important roles in the maintenance of stem cell pluripotency and organ formation. We previously found that Sox2 mainly expressed in the dental epithelium. Besides, Sox2 has been reported to be a dental epithelium stem cell marker in the incisor. However, the exact mechanism of Sox2 controlling amelogenesis is still not quite clearly elucidated. Here we report that conditional deletion of Sox2 in the dental epithelium using Shhcre caused impaired ameloblast differentiation in the labial side and induced ectopic ameloblast-like cell differentiation on the lingual side. Abnormal FGF gene expression was detected by RNAscope in situ hybridization in the mutant incisor. Collectively, we speculate that asymmetric ameloblast lineage commitment of mouse incisor might be regulated by Sox2 through FGF signaling.

Stage-Specific Role of Amelx Activation in Stepwise Ameloblast Induction from Mouse Induced Pluripotent Stem Cells

Amelogenin comprises ~90% of enamel proteins; however, the involvement of Amelx transcriptional activation in regulating ameloblast differentiation from induced pluripotent stem cells (iPSCs) remains unknown. In this study, we generated doxycycline-inducible Amelx-expressing mouse iPSCs (Amelx-iPSCs). We then established a three-stage ameloblast induction strategy from Amelx-iPSCs, including induction of surface ectoderm (stage 1), dental epithelial cells (DECs; stage 2), and ameloblast lineage (stage 3) in sequence, by manipulating several signaling molecules. We found that adjunctive use of lithium chloride (LiCl) in addition to bone morphogenetic protein 4 and retinoic acid promoted concentration-dependent differentiation of DECs. The resulting cells had a cobblestone appearance and keratin14 positivity. Attenuation of LiCl at stage 3 together with transforming growth factor β1 and epidermal growth factor resulted in an ameloblast lineage with elongated cell morphology, positivity for ameloblast markers, and calcium deposition. Although stage-specific activation of Amelx did not produce noticeable phenotypic changes in ameloblast differentiation, Amelx activation at stage 3 significantly enhanced cell adhesion as well as decreased proliferation and migration. These results suggest that the combination of inducible Amelx transcription and stage-specific ameloblast induction for iPSCs represents a powerful tool to highlight underlying mechanisms in ameloblast differentiation and function in association with Amelx expression.

Transcriptional Regulation of Dental Epithelial Cell Fate

Dental enamel is hardest tissue in the body and is produced by dental epithelial cells residing in the tooth. Their cell fates are tightly controlled by transcriptional programs that are facilitated by fate determining transcription factors and chromatin regulators. Understanding the transcriptional program controlling dental cell fate is critical for our efforts to build and repair teeth. In this review, we describe the current understanding of these regulators essential for regeneration of dental epithelial stem cells and progeny, which are identified through transgenic mouse models. We first describe the development and morphogenesis of mouse dental epithelium in which different subpopulations of epithelia such as ameloblasts contribute to enamel formation. Then, we describe the function of critical factors in stem cells or progeny to drive enamel lineages. We also show that gene mutations of these factors are associated with dental anomalies in craniofacial diseases in humans. We also describe the function of the master regulators to govern dental lineages, in which the genetic removal of each factor switches dental cell fate to that generating hair. The distinct and related mechanisms responsible for the lineage plasticity are discussed. This knowledge will lead us to develop a potential tool for bioengineering new teeth.

MicroRNA-31 regulates dental epithelial cell proliferation by targeting Satb2

MicroRNAs (miRNAs) exhibit strong potential clinical application owing to their extensive regulation and flexible delivery properties. MicroRNA-31 (miR-31) is an evolutionarily conserved miRNA expressed during tooth development, and it is highly expressed in mouse incisor epithelium. The specific role of miR-31 in odontogenesis has not been elucidated comprehensively, and the aim of the present study was to investigate its activity. Our results showed that miR-31 suppressed LS8 cell proliferation by inhibiting the cell cycle at the G1/S transition. Mutation of Special AT-rich sequence-binding protein 2 (SATB2) gene is responsible for human SATB2-associated syndrome (SAS), which is often accompanied by dental abnormities. Here, it was identified as a direct target of miR-31 in LS8 cells and a promoter of cell proliferation. The expression and distribution of SATB2 in mouse molars and incisors were explored using immunofluorescence, which showed strong signals in the nuclei of incisor epithelial cells and weak signals in the cytoplasm of molar epithelial cells. Moreover, rescue experiments demonstrated that Satb2 could mitigate the inhibitory effect of miR-31 on cell proliferation by promoting the expression of CDK4. Collectively, our results suggested that miR-31 regulates dental epithelial cell proliferation by targeting Satb2, highlighting the biological importance of miR-31 in odontogenesis.

A large pool of actively cycling progenitors orchestrates self-renewal and injury repair of an ectodermal appendage

The classical model of tissue renewal posits that small numbers of quiescent stem cells (SCs) give rise to proliferating transit-amplifying cells before terminal differentiation. However, many organs house pools of SCs with proliferative and differentiation potentials that diverge from this template. Resolving SC identity and organization is therefore central to understanding tissue renewal. Here, using a combination of single-cell RNA sequencing (scRNA-seq), mouse genetics and tissue injury approaches, we uncover cellular hierarchies and mechanisms that underlie the maintenance and repair of the continuously growing mouse incisor. Our results reveal that, during homeostasis, a group of actively cycling epithelial progenitors generates enamel-producing ameloblasts and adjacent layers of non-ameloblast cells. After injury, tissue repair was achieved through transient increases in progenitor-cell proliferation and through direct conversion of Notch1-expressing cells to ameloblasts. We elucidate epithelial SC identity, position and function, providing a mechanistic basis for the homeostasis and repair of a fast-turnover ectodermal appendage.

MDM2-Mediated p21 Proteasomal Degradation Promotes Fluoride Toxicity in Ameloblasts

Fluoride overexposure is an environmental health hazard and can cause enamel and skeletal fluorosis. Previously we demonstrated that fluoride increased acetylated-p53 and its downstream target p21 in ameloblast-derived LS8 cells. However, p21 function in fluoride toxicity is not well characterized. This study seeks to gain a better understanding of how p53 down-stream mediators, p21 and MDM2, respond to fluoride toxicity. LS8 cells were treated with NaF with/without MG-132 (proteasome inhibitor) or Nutlin-3a (MDM2 antagonist). NaF treatment for 2-6 h increased phospho-p21, which can inhibit apoptosis. However, phospho-p21 and p21 were decreased by NaF at 24 h, even though p21 mRNA was significantly increased at this time point. MG-132 reversed the fluoride-mediated p21 decrease, indicating that fluoride facilitates p21 proteasomal degradation. MG-132 suppressed fluoride-induced caspase-3 cleavage, suggesting that the proteasome plays a pro-apoptotic role in fluoride toxicity. NaF increased phospho-MDM2 in vitro and in mouse ameloblasts in vivo. Nutlin-3a suppressed NaF-mediated MDM2-p21 binding to reverse p21 degradation which increased phospho-p21. This suppressed apoptosis after 24 h NaF treatment. These results suggest that MDM2-mediated p21 proteasomal degradation with subsequent phospho-p21 attenuation contributes to fluoride-induced apoptosis. Inhibition of MDM2-mediated p21 degradation may be a potential therapeutic target to mitigate fluoride toxicity.

NaF Reduces KLK4 Gene Expression by Decreasing Foxo1 in LS8 Cells

Decreased expression and increased phosphorylation of Forkhead box o1 (Foxo1) in ameloblasts were observed both in vivo and in vitro when treated by fluoride. The present study aims to investigate the possible relationship between Foxo1 and enamel matrix proteinases, matrix metalloproteinase 20 (MMP20), and kallikrein 4 (KLK4), in NaF-treated ameloblasts. Ameloblast-like cells (LS8 cells) were exposed to NaF at selected concentration (0/2 mM) for 24 h. Gene overexpression and silencing experiments were used to up- and down-regulate Foxo1 expression. The expression levels of Foxo1, MMP20, and KLK4 were detected by quantitative real-time PCR and western blot. Dual luciferase reporter assay was performed to evaluate the regulation of Foxo1 on the transcriptional activity of KLK4 promoter. The results showed that KLK4 expression was decreased in LS8 cells treated by NaF, while MMP20 expression was not changed. Foxo1 activation led to significantly up-regulation of KLK4 in LS8 cells under NaF condition. Knockout of Foxo1 markedly decreased klk4 expression in mRNA level, and intensified inhibition occurred in LS8 cells when combined with NaF treatment. However, the variation trend of MMP20 was not clear. Dual luciferase reporter assay showed that Foxo1 activation enhanced the transcriptional activity of KLK4 promoter. These findings suggest that the decrease of Foxo1 expression induced by high fluoride was a cause for low KLK4 expression.

Nanostructured Ti surfaces and retinoic acid/dexamethasone present a spatial framework for the maturation and amelogenesis of LS-8 cells

PURPOSE

To investigate the amelogenesis-inductive effects of surface structures at the nanoscale. For this purpose, variable nanostructured titanium dioxide (TiO2) surfaces were used as a framework to regulate the amelogenic behaviors of ameloblasts with the administration of retinoic acid (RA)/dexamethasone (DEX).

MATERIALS AND METHODS

TiO2 nanotubular (NT) surfaces were fabricated via anodization. Mouse ameloblast-like LS-8 cells were seeded and cultured on NT surfaces in the presence or absence of RA/DEX for 48 h. The amelogenic behaviors and extracellular matrix (ECM) mineralization of LS-8 cells on nanostructured Ti surfaces were characterized using field emission scanning electron microscope, laser scanning confocal microscope, quantitative polymerase chain reaction, MTT assay, and flow cytometry.

RESULTS

TiO2 NT surfaces (tube size ~30 and ~80 nm) were constructed via anodization at 5 or 20 V and denoted as NT5 and NT20, respectively. LS-8 cells exhibited significantly increased spread and proliferation, and lower rates of apoptosis and necrosis on NT surfaces. The amelogenic gene expression and ECM mineralization differed significantly on the NT20 and the NT5 and polished Ti sample surfaces in standard medium. The amelogenic behaviors of LS-8 cells were further changed by RA/DEX pretreatment, which directly drove maturation of LS-8 cells.

CONCLUSION

Controlling the amelogenic behaviors of ameloblast-like LS-8 cells by manipulating the nanostructure of biomaterials surfaces represents an effective tool for the establishment of a systemic framework for supporting enamel regeneration. The administration of RA/DEX is an effective approach for driving the amelogenesis and maturation of ameloblasts.

Effect of preameloblast-conditioned medium and CPNE7 on root surfaces in dogs: a histologic and histomorphometric evaluation

Preameloblast-conditioned medium (PACM) has been reported as a potent dentin regenerative material, but its effects as a mixture on periodontal regeneration and the role of CPNE7 in PACM are not known. The purpose of this study is to evaluate the histologic and histomorphometric effects of preameloblast-conditioned medium (PACM) and CPNE7 on periodontal tissue healing in dogs. Seventy-two mandibular premolar roots from ten dogs were extracted and randomly divided into six groups (n = 12 each): (1) positive control group; (2) negative control group; (3) cementum-removed and PACM-treated group; (4) cementum-preserved and PACM-treated group; (5) CPNE7-inactivated PACM-treated group; and (6) recombinant CPNE7-treated group. The extracted roots were replanted into extraction sockets for 4 and 8 weeks and analyzed histologically. Most of the root surfaces in the negative control group showed ankylosis; and those in the experimental groups showed newly formed PDL-like and cementum-like tissues. Histomorphometric analysis of horizontal sections showed that the mean length of the PDL on the roots of the positive controls was similar to those in cementum-removed or -preserved and PACM-treated group at 8 weeks (p = 1.08). Sagittal sections showed that the mean length of the new cementum on the roots in cementum-removed and PACM-treated group was significantly greater than that in CPNE7-inactivated PACM-treated group (p = 0.037). The mean length of the newly formed PDL on the roots in CPNE7- inactivated PACM-treated and rCPNE7-treated groups was significantly greater than that in the negative controls at 8 weeks (p = 0.037, p = 0.036). The use of PACM and CPNE7 in tooth replantation resulted in increased PDL and cementum formation, suggesting the beneficial role of PACM and CPNE7 in periodontal tissue healing.

Accelerated dentinogenesis by inhibiting the mitochondrial fission factor, dynamin related protein 1

Undifferentiated odontogenic epithelium and dental papilla cells differentiate into ameloblasts and odontoblasts, respectively, both of which are essential for tooth development. These differentiation processes involve dramatic functional and morphological changes of the cells. For these changes to occur, activation of mitochondrial functions, including ATP production, is extremely important. In addition, these changes are closely related to mitochondrial fission and fusion, known as mitochondrial dynamics. However, few studies have focused on the role of mitochondrial dynamics in tooth development. The purpose of this study was to clarify this role. We used mouse tooth germ organ cultures and a mouse dental papilla cell line with the ability to differentiate into odontoblasts, in combination with knockdown of the mitochondrial fission factor, dynamin related protein (DRP)1. In organ cultures of the mouse first molar, tooth germ developed to the early bell stage. The amount of dentin formed under DRP1 inhibition was significantly larger than that of the control. In experiments using a mouse dental papilla cell line, differentiation into odontoblasts was enhanced by inhibiting DRP1. This was associated with increased mitochondrial elongation and ATP production compared to the control. These results suggest that DRP1 inhibition accelerates dentin formation through mitochondrial elongation and activation. This raises the possibility that DRP1 might be a therapeutic target for developmental disorders of teeth.

Enamel: Molecular identity of its transepithelial ion transport system

Enamel is the most calcified tissue in vertebrates. It differs from bone in a number of characteristics including its origin from ectodermal epithelium, lack of remodeling capacity by the enamel forming cells, and absence of collagen. The enamel-forming cells known as ameloblasts, choreograph first the synthesis of a unique protein-rich matrix, followed by the mineralization of this matrix into a tissue that is ∼95% mineral. To do this, ameloblasts arrange the coordinated movement of ions across a cell barrier while removing matrix proteins and monitoring extracellular pH using a variety of buffering systems to enable the growth of carbonated apatite crystals. Although our knowledge of these processes and the molecular identity of the proteins involved in transepithelial ion transport has increased in the last decade, it remains limited compared to other cells. Here we present an overview of the evolution and development of enamel, its differences with bone, and describe the ion transport systems associated with ameloblasts.

The Influence of Sodium Fluoride on the Growth of Ameloblasts and Kidney Proximal Tubular Cells

Fluoride has toxic potential particularly for teeth, bones, and kidney. This study was aimed to investigate the NaF exposure effects on the growth of ameloblasts and kidney proximal tubular cells. Adult male healthy rats were used as experiment models, divided into control and NaF-induced groups. The expression of amelogenin, Bcl-2, and caspase-3 were significantly different in the control and NaF-induced group (P < 0.05). There was no correlation among these proteins in the control group but significant correlation in the NaF-induced group (r = 0.694). There was a significant correlation in proximal tubular cells, as seen from the increase of caspase-3 in the NaF-induced group (r = 0.715).

Characterization of Dental Epithelial Progenitor Cells Derived from Cervical-loop Epithelium in a Rat Lower Incisor

Dental epithelial progenitor cells differentiate into various cell types during development of tooth germs. To study this mechanism, we produced immortalized dental epithelial progenitor cells derived from the cervical-loop epithelium of a rat lower incisor. The expression patterns of cytokeratin 14, nerve growth factor receptor p75, amelogenin, Notch2, and alkaline phosphatase were examined by immnohistochemistry in both lower and higher cell densities. The patterns of each were compared in the dental epithelium of rat lower incisors. The results demonstrated that these cells could produce ameloblast lineage cells, stratum intermedium cells, stellate reticulum, and outer enamel epithelium. Furthermore, fibroblast growth factor 10 stimulated proliferation of dental progenitor cells and subsequently increased the number of cells expressing alkaline phosphatase. These results suggest that fibroblast growth factor 10 plays a role in coupling mitogenesis of the cervical-loop cells and the production of stratum intermedium cells in rat incisors.

Expression of Protein Kinases C βI, βII, and VEGF during the Differentiation of Enamel Epithelium in Tooth Development

Protein kinase C (PKC) is an important molecule involved in various cell function, and mediates induced secretion of vascular endothelial growth factor (VEGF). It is hypothesized that PKC and VEGF may be associated with tooth development. Using the laser microdissection method and real-time reverse-transcription-polymerase chain-reaction (RT-PCR), we investigated the expression of PKC βI and βII, VEGF, and amelogenin (used as a marker of differentiation to ameloblasts) in the inner and outer enamel epithelia, stellate reticulum, and dental papilla in each stage of the dental germ. We found that the expression levels of PKC βI and βII were increased in the inner enamel epithelium during the early bell stage. In addition, the increased expression levels of PKC βI and βII were accompanied by increased VEGF expression. These results indicate that PKC βI, βII, and VEGF are closely associated with the differentiation of the inner enamel epithelium to ameloblasts.

Circadian Rhythm Regulates Development of Enamel in Mouse Mandibular First Molar

Rhythmic incremental growth lines and the presence of melatonin receptors were discovered in tooth enamel, suggesting possible role of circadian rhythm. We therefore hypothesized that circadian rhythm may regulate enamel formation through melatonin receptors. To test this hypothesis, we examined expression of melatonin receptors (MTs) and amelogenin (AMELX), a maker of enamel formation, during tooth germ development in mouse. Using qRT-PCR and immunocytochemistry, we found that mRNA and protein levels of both MTs and AMELX in normal mandibular first molar tooth germs increased gradually after birth, peaked at 3 or 4 day postnatal, and then decreased. Expression of MTs and AMELX by immunocytochemistry was significantly delayed in neonatal mice raised in all-dark or all-light environment as well as the enamel development. Furthermore, development of tooth enamel was also delayed showing significant immature histology in those animals, especially for newborn mice raised in all daylight condition. Interestingly, disruption in circadian rhythm in pregnant mice also resulted in delayed enamel development in their babies. Treatment with melatonin receptor antagonist 4P-PDOT in pregnant mice caused underexpression of MTs and AMELX associated with long-lasting deficiency in baby enamel tissue. Electromicroscopic evidence demonstrated increased necrosis and poor enamel mineralization in ameloblasts. The above results suggest that circadian rhythm is important for normal enamel development at both pre- and postnatal stages. Melatonin receptors were partly responsible for the regulation.

Expression of phosphate transporters in optimized cell culture models for dental cells biomineralization

Phosphate is a key component of dental mineral composition. The physiological role of membrane proteins of dental cells is suspected to be crucial for mineralization mechanisms. Contrary to published data related to calcium, data on regulation of phosphate flux through membrane of mineralizing cells are scarce. To address this lack of data, we studied the expression of six membranous phosphate transporters in two dental cell lines: a rat odontoblastic cell line (M2H4) and a mouse ameloblastic cell line (ALC) for which we optimized the mineralizing culture conditions.

Preameloblast-Derived Factors Mediate Osteoblast Differentiation of Human Bone Marrow Mesenchymal Stem Cells by Runx2-Osterix-BSP Signaling

Epithelial-mesenchymal interaction occurs during development of various tissues, including teeth and bone. Recently, a preameloblast-conditioned medium (PA-CM) from mouse apical bud cells (ABCs), a type of dental epithelial cell, was found to induce odontogenic differentiation of dental pulp stem cells and promote dentin formation. The aims of the present study were to investigate the effects of PA-CM on human bone marrow mesenchymal stem cells (hBMSCs) in vitro, and to investigate the bone regenerative capacity in vivo through epithelial-mesenchymal interactions of developmental osteogenesis. Coculturing with ABCs and PA-CM treatment upregulated osteoblast differentiation markers of hBMSCs compared to cells cultured alone. PA-CM accelerated mineralized nodule formation and also increased bone sialoprotein promoter activity in hBMSCs. PA-CM facilitated the migration of hBMSCs, but did not significantly influence proliferation. PA-CM promoted bone formation of hBMSCs in vivo. Radiographic and histologic findings showed that PA-CM induced the bony regeneration at calvarial defects in rat. Taken together, these data show that PA-CM enhances the migration and osteogenic differentiation of hBMSCs in vitro and induces bone formation in vivo.

Tooth enamel protein amelogenin binds to ameloblast cell membrane-mimicking vesicles via its N-terminus

We have recently reported that the extracellular enamel protein amelogenin has affinity to interact with phospholipids and proposed that such interactions may play key roles in enamel biomineralization as well as reported amelogenin signaling activities. Here, in order to identify the liposome-interacting domains of amelogenin we designed four different amelogenin mutants containing only a single tryptophan at positions 25, 45, 112 and 161. Circular dichroism studies of the mutants confirmed that they are structurally similar to the wild-type amelogenin. Utilizing the intrinsic fluorescence of single tryptophan residue and fluorescence resonance energy transfer [FRET], we analyzed the accessibility and strength of their binding with an ameloblast cell membrane-mimicking model membrane (ACML) and a negatively charged liposome used as a membrane model. We found that amelogenin has membrane-binding ability mainly via its N-terminal, close to residues W25 and W45. Significant blue shift was also observed in the fluorescence of a N-terminal peptide following addition of liposomes. We suggest that, among other mechanisms, enamel malformation in cases of Amelogenesis Imperfecta (AI) with mutations at the N-terminal may be the result of defective amelogenin-cell interactions.

High-fluoride promoted phagocytosis-induced apoptosis in a matured ameloblast-like cell line

Endocytosis and phagocytosis are important physiologic activities occurring during ameloblast differentiation. We have previously found that excess fluoride inhibited ameloblasts endocytotic functions. Here, we hypothesized that increasing amounts of fluoride may affect ameloblast phagocytotic function during their differentiation. Using cell culture, we first induced maturation of the mouse ameloblast-like LS8 cells by treatment with exogenous retinoic acid (RA) and dexamethasone (DEX). We measured their phagocytotic activity by fluorescent microscopy using a live cell visualization station. We found that ameloblast-like LS8 cells matured with RA/DEX treatment and the increasing amounts of fluoride demonstrated the up-regulated expression of the phagocytotic marker proteins, LAMP1 and CD68. A connection between phagocytosis and apoptosis was confirmed by the increased number of phagocytotic vacuole-like structures and the heterochromatin margination phenomenon observed in the RA/DEX with NaF treatment group. The increase in albumin uptake by ameloblasts was confirmed using whole organ culture of incisor tooth germs. Here, in fluoride treated tooth germs, mature canonical ameloblasts showed greater amounts of albumin uptake, which was accompanied by decreased expression of the anti-apoptosis marker, Bcl-2 along with up-regulated expression of CD68. From these observations, we inferred that high doses of fluoride may cause apoptosis by increasing the phagocytosis of protein particles in mature-stage ameloblasts and loss of Bcl-2 signals might be involved in this process.

Estrogen and bisphenol A affect male rat enamel formation and promote ameloblast proliferation

Bisphenol A (BPA) is a widespread endocrine disrupting chemical (EDC) strongly suspected to have adverse health effects. Numerous tissues and cells are affected by BPA, and we showed recently that BPA targets include ameloblasts and enamel. We therefore investigated the effects of BPA on ameloblasts and the possible involvement of the estrogen signaling pathway. Rats were exposed daily to low-dose BPA, and developed enamel hypomineralization similar to human molar incisor hypomineralization (MIH). BPA increased ameloblast proliferation in vivo and in vitro. The proliferation of the rat dental epithelial cell line HAT-7 was also increased by estrogen (E2). Ameloblasts express ERα but not ERβ both in vivo and in vitro. The ER antagonist ICI 182,780 was used to inactivate ERα and abolished the effects of E2 on cell proliferation and transcription, but only partially reduced the effects of BPA. In conclusion, we show, for the first time, that: 1) BPA has ER-dependent and ER-independent effects on ameloblast proliferation and gene transcription; 2) the estrogen signaling pathway is involved in tooth development and the enamel mineralization process; and 3) BPA impacts preferentially amelogenesis in male rats. These results are consistent with the steroid hormones having effect on ameloblasts, raising the issues of the hormonal influence on amelogenesis and possible differences in enamel quality between sexes.

E-cadherin can replace N-cadherin during secretory-stage enamel development

BACKGROUND

N-cadherin is a cell-cell adhesion molecule and deletion of N-cadherin in mice is embryonic lethal. During the secretory stage of enamel development, E-cadherin is down-regulated and N-cadherin is specifically up-regulated in ameloblasts when groups of ameloblasts slide by one another to form the rodent decussating enamel rod pattern. Since N-cadherin promotes cell migration, we asked if N-cadherin is essential for ameloblast cell movement during enamel development.

METHODOLOGY/PRINCIPAL FINDINGS

The enamel organ, including its ameloblasts, is an epithelial tissue and for this study a mouse strain with N-cadherin ablated from epithelium was generated. Enamel from wild-type (WT) and N-cadherin conditional knockout (cKO) mice was analyzed. μCT and scanning electron microscopy showed that thickness, surface structure, and prism pattern of the cKO enamel looked identical to WT. No significant difference in hardness was observed between WT and cKO enamel. Interestingly, immunohistochemistry revealed the WT and N-cadherin cKO secretory stage ameloblasts expressed approximately equal amounts of total cadherins. Strikingly, E-cadherin was not normally down-regulated during the secretory stage in the cKO mice suggesting that E-cadherin can compensate for the loss of N-cadherin. Previously it was demonstrated that bone morphogenetic protein-2 (BMP2) induces E- and N-cadherin expression in human calvaria osteoblasts and we show that the N-cadherin cKO enamel organ expressed significantly more BMP2 and significantly less of the BMP antagonist Noggin than did WT enamel organ.

CONCLUSIONS/SIGNIFICANCE

The E- to N-cadherin switch at the secretory stage is not essential for enamel development or for forming the decussating enamel rod pattern. E-cadherin can substitute for N-cadherin during these developmental processes. Bmp2 expression may compensate for the loss of N-cadherin by inducing or maintaining E-cadherin expression when E-cadherin is normally down-regulated. Notably, this is the first demonstration of a natural endogenous increase in E-cadherin expression due to N-cadherin ablation in a healthy developing tissue.

FAM20C functions intracellularly within both ameloblasts and odontoblasts in vivo

FAM20C, also known as Golgi casein kinase (G-CK), is proposed to be the archetype for a family of secreted kinases that phosphorylate target proteins in the Golgi and in extracellular matrices, but FAM20C serving an extracellular function is controversial. FAM20C phosphorylates secretory calcium-binding phosphoproteins (SCPPs), which are associated with the evolution of biomineralization in vertebrates. Current models of biomineralization assume SCPP proteins are secreted as phosphoproteins and their phosphates are essential for protein conformation and function. It would be a radical departure from current theories if proteins in mineralizing matrices were dephosphorylated as part of the mineralization mechanism and rephosphorylated in the extracellular milieu by FAM20C using ATP. To see if such mechanisms are possible in the formation of dental enamel, we tested the hypothesis that FAM20C is secreted by ameloblasts and accumulates in the enamel extracellular matrix during tooth development. FAM20C localization was determined by immunohistochemistry in day 5 mouse incisors and molars and by Western blot analyses of proteins extracted from pig enamel organ epithelia (EOE) and enamel shavings. FAM20C localized intracellularly within ameloblasts and odontoblasts in a pattern consistent with Golgi localization. Western blots detected FAM20C in the EOE extracts but not in the enamel matrix. We conclude that FAM20C is not a constituent of the enamel extracellular matrix and functions intracellularly within ameloblasts.

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.

The circadian clock modulates enamel development

Fully mature enamel is about 98% mineral by weight. While mineral crystals appear very early during its formative phase, the newly secreted enamel is a soft gel-like matrix containing several enamel matrix proteins of which the most abundant is amelogenin (Amelx). Histological analysis of mineralized dental enamel reveals markings called cross-striations associated with daily increments of enamel formation, as evidenced by injections of labeling dyes at known time intervals. The daily incremental growth of enamel has led to the hypothesis that the circadian clock might be involved in the regulation of enamel development. To identify daily rhythms of clock genes and Amelx, we subjected murine ameloblast cells to serum synchronization to analyze the expression of the circadian transcription factors Per2 and Bmal1 by real-time PCR. Results indicate that these key genetic regulators of the circadian clock are expressed in synchronized murine ameloblast cell cultures and that their expression profile follows a circadian pattern with acrophase and bathyphase for both gene transcripts in antiphase. Immunohistological analysis confirms the protein expression of Bmal and Cry in enamel cells. Amelx expression in 2-day postnatal mouse molars dissected every 4 hours for a duration of 48 hours oscillated with an approximately 24-hour period, with a significant approximately 2-fold decrease in expression during the dark period compared to the light period. The expression of genes involved in bicarbonate production (Car2) and transport (Slc4a4), as well as in enamel matrix endocytosis (Lamp1), was greater during the dark period, indicating that ameloblasts express these proteins when Amelx expression is at the nadir. The human and mouse Amelx genes each contain a single nonconserved E-box element within 10 kb upstream of their respective transcription start sites. We also found that within 2 kb of the transcription start site of the human NFYA gene, which encodes a positive regulator of amelogenin, there is an E-box element that is conserved in rodents and other mammals. Moreover, we found that Nfya expression in serum-synchronized murine ameloblasts oscillated with a strong 24-hour rhythm. Taken together, our data support the hypothesis that the circadian clock temporally regulates enamel development.

Expression patterns of Sema3F, PlexinA4, -A3, Neuropilin1 and -2 in the postnatal mouse molar suggest roles in tooth innervation and organogenesis

OBJECTIVE

Semaphorins form a family of axon wiring molecules but still little is known about their role in tooth formation. A class 3 semaphorin, Semaphorin3F (Sema3F), besides acting as a chemorepellant for different types of axons, controls a variety of non-neuronal developmental processes.

MATERIALS AND METHODS

Cellular mRNA expression patterns of Sema3F as well as neuropilin 1 (Npn1), neuropilin 2 (Npn2), plexinA3 and plexinA4 receptors were analyzed by sectional in situ hybridization in the mouse molar tooth during postnatal days 0-7. The expression of the receptors was studied in PN5 trigeminal ganglia.

RESULTS

Sema3F, Npn1, -2 and plexinA4 exhibited distinct, spatiotemporally changing expression patterns, whereas plexinA3 was not observed in the tooth germs. Besides being expressed in the base of the dental mesenchyme Sema3F, like plexinA4, Npn1 and -2, was present in the ameloblast cell lineage. Npn1 and Npn2 were additionally seen in the pulp horns and endothelial cells and like PlexinA4 in the developing alveolar bone. Npn1, plexinA3 and -A4 were observed in trigeminal ganglion neurons.

CONCLUSIONS

Sema3F may act as a tooth target-derived axonal chemorepellant controlling establishment of the tooth nerve supply. Furthermore, Sema3F, like Npn1, -2 and plexinA4 may serve non-neuronal functions by controlling the development of the ameloblast cell lineage. Moreover, Npn1 and Npn2 may regulate dental vasculogenesis and, together with PlexinA4, alveolar bone formation. Further analyses such as investigation of transgenic mouse models will be required to elucidate in vivo signaling functions of Sema3F and the receptors in odontogenesis.

Developmentally regulated expression of intracellular Fgf11-13, hormone-like Fgf15 and canonical Fgf16, -17 and -20 mRNAs in the developing mouse molar tooth

OBJECTIVE

To investigate and compare the cellular expression of non-secreted Fgf11-14 and secreted Fgf15-18 and -20 mRNAs during tooth formation.

MATERIALS AND METHODS

mRNA expression was analyzed from the morphological initiation of the mouse mandibular first molar development to the onset of crown calcification using sectional in situ hybridization.

RESULTS

This study found distinct, differentially regulated expression patterns for the Fgf11-13, -15-17 and -20, in particular in the epithelial-mesenchymal interface, whereas Fgf14 and 18 mRNAs were not detected. Fgf11, -15, -16, -17 and -20 were seen in the epithelium, whereas Fgf12 and -13 signals were restricted to the mesenchymal tissue component of the tooth. Fgf11 was observed in the putative epithelial signaling areas, the tertiary enamel knots and enamel free areas of the calcifying crown. Fgf15, Fgf17 and -20 were transiently colocalized in the thickened dental epithelium at E11.5. Later Fgf15 and -20 were exclusively expressed in the epithelial enamel knot signaling centers. In contrast, Fgf13 was present in the dental mesenchyme including odontoblasts cell lineage, whereas Fgf12 appeared transiently in the preodontoblasts.

CONCLUSIONS

The expression of the Fgf11-13, -15, -17 and -20 in the epithelial signaling centers and/or epithelial-mesenchymal interfaces at key stages of the tooth formation suggest important functions in odontogenesis. Future analyses of the transgenic mice will help elucidate in vivo functions of the studied Fgfs during odontogenesis and whether any of the functions of the tooth expressed epithelial and mesenchymal Fgfs of different sub-families are redundant.

Induction of insulin-like growth factor 2 expression in a mesenchymal cell line co-cultured with an ameloblast cell line

Various growth factors have been implicated in the regulation of cell proliferation and differentiation during tooth development. It has been unclear if insulin-like growth factors (IGFs) participate in the epithelium–mesenchyme interactions of tooth development. We previously produced three-dimensional sandwich co-culture systems (SW) containing a collagen membrane that induce the differentiation of epithelial cells. In the present study, we used the SW system to analyze the expression of IGFs and IGFRs. We demonstrate that IGF2 expression in mesenchymal cells was increased by SW. IGF1R transduces a signal; however, IGF2R does not transduce a signal. Recombinant IGF2 induces IGF1R and IGF2R expression in epithelial cells. IGF1R expression is increased by SW; however, IGF2R expression did not increase by SW. Thus, IGF2 signaling works effectively in SW. These results suggest that IGF signaling acts through the collagen membrane on the interaction between the epithelium and mesenchyme. In SW, other cytokines may be suppressed to induce IGF2R induction. Our results suggest that IGF2 may play a role in tooth differentiation.

Expression of microRNAs in the stem cell niche of the adult mouse incisor

The mouse incisor is a valuable but under-utilized model organ for studying the behavior of adult stem cells. This remarkable tooth grows continuously throughout the animal's lifetime and houses two distinct epithelial stem cell niches called the labial and lingual cervical loop (laCL and liCL, respectively). These stem cells produce progeny that undergo a series of well-defined differentiation events en route to becoming enamel-producing ameloblasts. During this differentiation process, the progeny move out of the stem cell niche and migrate toward the distal tip of the tooth. Although the molecular pathways involved in tooth development are well documented, little is known about the roles of miRNAs in this process. We used microarray technology to compare the expression of miRNAs in three regions of the adult mouse incisor: the laCL, liCL, and ameloblasts. We identified 26 and 35 differentially expressed miRNAs from laCL/liCL and laCL/ameloblast comparisons, respectively. Out of 10 miRNAs selected for validation by qPCR, all transcripts were confirmed to be differentially expressed. In situ hybridization and target prediction analyses further supported the reliability of our microarray results. These studies point to miRNAs that likely play a role in the renewal and differentiation of adult stem cells during stem cell-fueled incisor growth.

[Relationship between Sp3 and the transcriptional regulation of enamelin gene]

OBJECTIVE

To investigate the role of Sp3 in the transcriptional regulation of enamelin gene.

METHODS

By bioinformatic analysis, a putative responsive element for Sp3 was identified. Electrophoretic mobility shift assay was used to examine the interaction between Sp3 and enamelin. 5'-flanking regulatory region of enamelin was cloned and ligated into pGL3-basic luciferase vector. Sp3 and the Enam-luc were cotransfected into mouse ameloblast-like cell line, and the activity of luciferase was examined.

RESULTS

The results showed that Sp3 could not directly bind to the enamelin regulation region and activate enamelin transcription.

CONCLUSIONS

Sp3 might not be involved in transcriptional regulation of enamelin gene via an indirect interaction.

Cooperation of nectin-1 and nectin-3 is required for normal ameloblast function and crown shape development in mouse teeth

Nectins are immunoglobulin-like cell adhesion proteins and their interactions recruit various cell-cell junctions. Mutations in human NECTIN-1 cause an ectodermal dysplasia syndrome, but Nectin-1 null mice have only slight defects in teeth, suggesting compensation by other nectin(s). We observed overlapping expression of nectin-3 with nectin-1 and enamel abnormality in the nectin-3 mutant. We, therefore, generated nectin-1;nectin-3 compound mutants. However, all teeth developed and no significant dental abnormalities were observed before birth. At postnatal day 10, the upper molars of compound mutants exhibited conical crown shape and retarded enamel maturation. Nectin-1 was expressed in ameloblasts whereas nectin-3 was expressed in neighboring stratum intermedium cells at this stage. The immunohistochemical localization and electron microscopical observations indicated that the desmosomal junctions between stratum intermedium and ameloblasts were significantly reduced. These results suggest that heterophilic interaction between nectin-1 and nectin-3 recruits desmosomal junctions, and that these are required for proper enamel formation.

MicroRNAs play a critical role in tooth development

MicroRNAs are known to regulate gene function in many tissues and organs, but their expression and function, if any, in tooth development are elusive. We sought to identify them by microRNA screening analyses and reveal their overall roles by inactivating Dicer1 in the dental epithelium and mesenchyme. Discrete sets of microRNAs are expressed in molars compared with incisors as well as epithelium compared with mesenchyme. Conditional knockout (cKO) of Dicer1 (mature microRNAs) in the dental epithelium of the Pitx2-Cre mouse results in multiple and branched enamel-free incisors and cuspless molars, and change in incisor patterning and in incisor and molar size and shape. Analyses of differentiating dental epithelial markers reveal a defect in ameloblast differentiation. Conversely, the cervical loop (stem cell niche) is expanded in Dicer1 cKO. These results demonstrate that tooth development is tightly controlled by microRNAs and that specific microRNAs regulate tooth epithelial stem cell differentiation.

New roles and mechanism of action of BMP4 in postnatal tooth cytodifferentiation

During the phase of overt tooth cytodifferentiation that occurs after birth in the mouse and using the 3.6Collagen1a-Cre and the BMP4 floxed and BMP4 knockout mice, the BMP4 gene was deleted in early collagen producing odontoblasts around postnatal day 1. BMP4 expression was reduced over 90% in alveolar osteoblasts and odontoblasts. There was decreased rate of predentin to dentin formation and decreased mature odontoblast differentiation reflected in reduced DMP1 expression and proper dentinal tubule formation, as well as reduced Collagen type I and Osteocalcin expression. We observed mutant dysmorphogenic odontoblasts that failed to properly elongate and differentiate. The consequence of this failed differentiation process leads to permanent loss of dentin thickness, apparent enlarged pulp chambers in the molars and reduced bone supporting the tooth structures in mice as old as 10-12 months. Deletion of the BMP4 gene in odontoblasts also indirectly disrupted the process of enamel formation that persisted throughout life. The mechanism for this altered differentiation program in the absence of the BMP4 gene in odontoblasts is from decreased BMP signaling, and decreased expression of three key transcription factors, Dlx3, Dlx5, and Osterix. BMP signaling, as well as Dlx3 and Amelogenin expression, is also indirectly reduced in the ameloblasts of the odontoblast BMP4 cKO mice. This supports a key paracrine or endocrine postnatal role of odontoblast derived BMP4 on the proper amelogenesis and formation of the enamel.

The acid test of fluoride: how pH modulates toxicity

Background

It is not known why the ameloblasts responsible for dental enamel formation are uniquely sensitive to fluoride (F⁻). Herein, we present a novel theory with supporting data to show that the low pH environment of maturating stage ameloblasts enhances their sensitivity to a given dose of F⁻. Enamel formation is initiated in a neutral pH environment (secretory stage); however, the pH can fall to below 6.0 as most of the mineral precipitates (maturation stage). Low pH can facilitate entry of F⁻ into cells. Here, we asked if F⁻ was more toxic at low pH, as measured by increased cell stress and decreased cell function.

Methodology/Principal Findings

Treatment of ameloblast-derived LS8 cells with F⁻ at low pH reduced the threshold dose of F⁻ required to phosphorylate stress-related proteins, PERK, eIF2α, JNK and c-jun. To assess protein secretion, LS8 cells were stably transduced with a secreted reporter, Gaussia luciferase, and secretion was quantified as a function of F⁻ dose and pH. Luciferase secretion significantly decreased within 2 hr of F⁻ treatment at low pH versus neutral pH, indicating increased functional toxicity. Rats given 100 ppm F⁻ in their drinking water exhibited increased stress-mediated phosphorylation of eIF2α in maturation stage ameloblasts (pH<6.0) as compared to secretory stage ameloblasts (pH∼7.2). Intriguingly, F⁻-treated rats demonstrated a striking decrease in transcripts expressed during the maturation stage of enamel development (Klk4 and Amtn). In contrast, the expression of secretory stage genes, AmelX, Ambn, Enam and Mmp20, was unaffected.

Conclusions

The low pH environment of maturation stage ameloblasts facilitates the uptake of F⁻, causing increased cell stress that compromises ameloblast function, resulting in dental fluorosis.

Possible involvement of melatonin in tooth development: expression of melatonin 1a receptor in human and mouse tooth germs

Melatonin is known to regulate a variety of physiological processes including control of circadian rhythms, regulation of seasonal reproductive function, regulation of body temperature, free radical scavenging, and so forth. Accumulating evidence from in vitro and in vivo experiments has also suggested that melatonin may have an influence on skeletal growth and bone formation. However, little is known about the effects of melatonin on tooth development and growth, which thus remain to be elucidated. This study was performed to examine the possibility that melatonin might exert its influence on tooth development as well as skeletal growth. Immunohistochemical analysis revealed that melatonin 1a receptor (Mel1aR) was expressed in secretory ameloblasts, the cells of the stratum intermedium and stellate reticulum, external dental epithelial cells, odontoblasts, and dental sac cells. Reverse transcription-polymerase chain reaction and Western blot analysis showed that HAT-7, a rat dental epithelial cell line, expressed Mel1aR and its expression levels increased after the cells reached confluence. These results strongly suggest that melatonin may play a physiological role in tooth development/growth by regulating the cellular function of odontogenic cells in tooth germs.

Type XVII collagen is a key player in tooth enamel formation

Inherited tooth enamel hypoplasia occurs due to mutations in genes that encode major enamel components. Enamel hypoplasia also has been reported in junctional epidermolysis bullosa, caused by mutations in the genes that encode type XVII collagen (COL17), a component of the epithelial-mesenchymal junction. To elucidate the pathological mechanisms of the enamel hypoplasia that arise from the deficiency of epithelial-mesenchymal junction molecules, such as COL17, we investigated tooth formation in our recently established Col17(-/-) and Col17 rescued mice. Compared with wild-type mice, the incisors of the Col17(-/-) mice exhibited reduced yellow pigmentation, diminished iron deposition, delayed calcification, and markedly irregular enamel prisms, indicating the presence of enamel hypoplasia. The molars of the Col17(-/-) mice demonstrated advanced occlusal wear. These abnormalities were corrected in the Col17 rescued humanized mice. Thus, the Col17(-/-) mice clearly reproduced the enamel hypoplasia in human patients with junctional epidermolysis bullosa. We were able to investigate tooth formation in the Col17(-/-) mice because the Col17(-/-) genotype is not lethal. Col17(-/-) mouse incisors had poorly differentiated ameloblasts that lacked enamel protein-secreting Tomes' processes and reduced mRNA expression of amelogenin, ameloblastin, and of other enamel genes. These findings indicated that COL17 regulates ameloblast differentiation and is essential for normal formation of Tomes' processes. In conclusion, COL17 deficiency disrupts the epithelial-mesenchymal interactions, leading to both defective ameloblast differentiation and enamel malformation.

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

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

Identification of the functional activity of the [A-4] amelogenin gene splice product in newborn mouse ameloblasts

In the mouse tooth organ, shortly after birth, ameloblasts acquire their secretory phenotype, which is characterized by the prominent expression and subsequent secretion of two isoforms of amelogenin, M180 and M59 (LRAP, [A-4]). Amelogenin deposition into the ameloblast extracellular matrix promotes enamel biomineralization. A complex set of intercellular signaling events, reciprocal communications between the developing oral epithelium and its underlying dental mesenchyme, guide the expression of amelogenin mRNA, and limit it to a defined period of tooth development. In tooth germ organ culture, addition of the [A-4] isoform, lacking amelogenin exon 4 and exon 6 segments a, b, c, was shown to affect ameloblast development. To understand the basis for this regulatory activity, we have studied the effects of r[A-4] on ameloblast-like LS8 cells, and the role of the putative [A-4] cell surface receptor, LAMP1, as well as the related receptor LAMP3. In the LS8 cells, the expression of the spliced isoforms of amelogenin, LAMP1, and LAMP3 were identified by RT-PCR, and real-time PCR semi-quantitative analysis assessed the modulation of M180 message. M180 mRNA was up-regulated by exogenous [A-4], and this was further increased by blockade of LAMP1, suggesting additive effects between the intracellular signaling pathways activated by the discrete agonists. Immunofluorescence staining identified the patterns of [A-4] and LAMP1 localization in LS8 cells. Internalized r[A-4] was co-localized with LAMP1 in late endosomal/lysosomal compartments. Thus, the LAMP1 and [A-4] intracellular sorting pathways are interrelated. The nitric oxide (NO) signaling pathway was activated by exogenous [A-4]. [A-4] modulated inducible nitric oxide synthase (iNOS, NOS2) and endothelial nitric oxide synthase (eNOS, NOS3) expression, albeit, to different extents. NOS2 was significantly up-regulated after 4 h, while NOS3 increased slightly after 24 h. Co-treatment of LS8 cells with r[A-4] and anti-LAMP1 antibodies further enhanced NOS2 expression. Anti-LAMP1 antibodies did not abrogate NO production in LS8 cells treated for 4 h with r[A-4], but the iNOS inhibitor, l-Nil, down-regulated both NO production and the expression of M180 mRNA. These data suggest that [A-4] modulates M180 mRNA expression, partly, via the NO signaling pathway.

A novel autosomal-recessive mutation, whitish chalk-like teeth, resembling amelogenesis imperfecta, maps to rat chromosome 14 corresponding to human 4q21

A rat mutant, whitish chalk-like teeth (wct), with white, chalk-like abnormal incisors, was discovered and morphologically and genetically characterized. The mutant rats showed tooth enamel defects that were similar to those of human amelogenesis imperfecta. The wct mutation was found to disturb the morphological transition of ameloblasts from secretory to maturation stages and to induce cyst formation. This mutation also disturbs the transfer of iron into the enamel, resulting in the whitish chalk-like incisors. A genetic linkage study indicated that the wct locus maps to a specific interval of rat chromosome 14 between D14Got13 and D14Wox2. Interestingly, the human chromosomal region orthologous to wct, a 5.5-Mb interval in human chromosome 4q21, is a critical region for the locus of human amelogenesis imperfecta AIH2. These results strongly suggest that this wct mutant is a useful model for the identification of genes responsible for amelogenesis imperfecta and molecular mechanisms of tooth development.

Establishment and characterization of rat dental epithelial derived ameloblast-lineage clones

Teeth are the hardest tissues covered with enamel produced by ameloblasts. The ameloblast differentiation is controlled by sequential epithelial-mesenchymal interactions during tooth morphogenesis. However, the molecular mechanism of ameloblast differentiation remains unclear. To address this question, we developed an in vitro assay system to evaluate the molecular mechanism of amelogenesis. First, we established dental epithelium-derived clones from 6-day-old rat incisors and established that cells of the clone SRE-G5 were the largest producers of amelogenin mRNA. Next, we analyzed the effects of several chemicals on the amelogenin expression in SRE-G5 cells. Only mitogen-activated protein kinase (MAPK) activators enhanced amelogenin mRNA expression. This finding corresponded to the immunohistochemical data showing the presence of phosphorylated forms of p38, c-Jun N-terminal kinase (JNK), and extracellular signal-regulated kinase (ERK) during ameloblast differentiation. To examine the roles of MAPK signals, we compared the effects of anisomycin and sodium salicylate on the expression of tooth-related differentiation markers. Both anisomycin and sodium salicylate induced amelogenin, Abcg2, and Bmp4 mRNA and down-regulated p75NGFR mRNA. On the other hand, ALP, ectodin, Bmp2 and Fgf8 mRNA were up-regulated only by anisomycin. These results indicate that MAPK signaling functions, at least in part, as the inducer of ameloblast differentiation.

JNK/c-Jun signaling pathway mediates the fluoride-induced down-regulation of MMP-20 in vitro

Delayed removal of amelogenins, which are initially hydrolyzed by matrix metalloproteinase MMP-20, is a characteristic of enamel fluorosis. In this study, we investigated the regulation of MMP-20 and possible effects of fluoride on MMP-20 expression in human ameloblast lineage cells. Protein expression and signaling pathways of human ameloblast lineage cells, exposed to 10 muM fluoride, were compared to control cells without fluoride exposure. The role of activator protein-1 in MMP-20 regulation was analyzed by DNA-protein affinity precipitation and luciferase reporter gene assays. MMP-20 protein levels in human ameloblast lineage cells decreased in the presence of fluoride, while amelogenin and TIMP-2 were not altered. Fluoride also decreased the transcription of a luciferase reporter gene driven by the MMP-20 promoter. Down-regulation of MMP-20 by fluoride was related to suppression of JNK/c-Jun phosphorylation. In contrast, the JNK activator elevated the expression of MMP-20. Three c-Jun binding sites on the MMP-20 promoter were identified for the first time, and were occupied by c-Jun as MMP-20 was induced. Deletion of any one of AP-1 binding sites on the MMP-20 promoter significantly reduced the transcription of downstream luciferase reporter. These in vitro findings suggest that c-Jun is a key regulatory element for MMP-20 expression, and human ameloblast lineage cells can respond to fluoride by down-regulating MMP-20 transcription through the JNK/c-Jun signaling pathway.

Dentin Matrix Protein 4, a Novel Secretory Calcium-binding Protein That Modulates Odontoblast Differentiation

Formation of calcified tissues is a well regulated process. In dentin, the odontoblasts synthesize several biomolecules that function as nucleators or inhibitors of mineralization. To identify genes that are odontoblast-specific, a subtractive hybridization technique was employed that resulted in the identification of a previously undescribed novel gene synthesized by the odontoblasts. Based on the nomenclature in our laboratory, this gene has been named dentin matrix protein 4 (DMP4). The protein encoded by mouse DMP4 cDNA contained 579 amino acids, including a 26-amino acid signal peptide. Analysis of the protein sequence demonstrated the presence of a Greek key calcium-binding domain and one conserved domain of unknown function in all the species examined thus far. Calcium binding property was confirmed by (45)Ca binding assays and the corresponding change in conformation by far-ultraviolet circular dichroism. Northern analysis demonstrated high expression levels of a single 3-kb mRNA transcript in tooth, whereas low expression levels were detected in other tissues. In situ hybridization analysis showed high expression levels of DMP4 in odontoblasts and low levels in osteoblasts and ameloblasts during tooth development. Gain and loss of function experiments demonstrated that DMP4 had the potential to differentiate mesenchymal precursor cells into functional odontoblast-like cells.

The amyloid protein APin is highly expressed during enamel mineralization and maturation in rat incisors

This study investigated the expression and localization of APin (which was previously identified and cloned from a rat odontoblast cDNA library), during ameloblast differentiation in rat incisors, by using in situ hybridization and immunohistochemistry. The subcellular localization of APin varied during ameloblast differentiation, but was stage-specific. APin mRNA was not expressed in pre-ameloblasts, was weakly expressed in secretory ameloblasts, and was strongly expressed in maturation-stage ameloblasts as well as in the junctional epithelium attached to the enamel of erupted molars. In the maturation-stage ameloblasts, APin protein was conspicuous in the supranuclear area (Golgi complex) of smooth-ended ameloblasts as well as in both the supranuclear area and the ruffle end of ruffle-ended ameloblasts. During ameloblast-lineage cell culture, APin was expressed at a low level in the early stages of culture, but at a high level in the late stage of culture, which was equivalent to the maturation stage. APin protein was efficiently secreted from transfected cells in culture. Furthermore, its overexpression and inactivation caused an increase and decrease in matrix metalloproteinase-20 (MMP-20) and tuftelin expression, respectively. These findings indicate a functional role for APin in the mineralization and maturation of enamel that is mediated by the expression of MMP-20 and tuftelin.

Endothelin receptors and endothelin-1 in developing rat teeth

INTRODUCTION

The endothelins are a family of small peptides with multiple roles in a variety of tissues. Signaling is mediated through two receptor subtypes, the endothelin A receptor (ET(A)) specific for Et-1 and the non-specific endothelin B receptor (ET(B)).

OBJECTIVE

Our goal was to determine the location of immunoreactivity (IR) for ET(A) and ET(B) in developing and mature rat teeth as indicators of endothelin (Et) regulatory sites and to compare this to the Et-1 (ligand)-IR expression patterns.

DESIGN

We used immunohistochemistry to study developing and mature rat molars and continuously developing incisors.

RESULTS

We demonstrate ET(A), ET(B), and Et-1 expression patterns in teeth, for the first time. ET(A) was found in developing molar root pulp, pulpal vasculature, and preodontoblasts, and then persisted in odontoblasts or cellular cementocytes at the root apices of mature teeth. ET(B) was found at the molar (Hertwig's) root sheath during root formation and in molar ameloblasts, nerve fibers and odontoblasts of immature and mature teeth. In incisors, ET(B)-IR was associated with ameloblasts and the stem cell niche of the cervical loop while ET(A) was located in the substratum layer. Et-1 was found throughout the dental and periodontal tissues with higher concentrations associated with odontoblasts, nerves and incisor layers that expressed ET(B).

CONCLUSION

The patterns of ET(A) and ET(B) in teeth differ from each other and from those of adjacent tissues suggesting multiple tooth-specific functions for endothelin during development and mature dental function.

Differentiation of an ameloblast-lineage cell line (ALC) is induced by Sonic hedgehog signaling

Sonic hedgehog (Shh), one of the essential molecules for embryogenesis and organogenesis, is strongly expressed in the enamel knot, which represents the signaling center for odontogenesis due to the presence of essential secretory molecules. Here, we investigated the effects of Shh on ameloblast differentiation using a mouse ameloblast-lineage cell line (ALC). In the presence of Shh, the expression of amelogenin and ameloblastin was up-regulated, and their promoter activities were also elevated. Subcutaneous implantation of Shh-treated ALC into the dorsa of C57/Bl6J mouse caused increased expression of amelogenin and ameloblastin. Forced expression of Gli1, a major transcription factor in Shh signaling, up-regulated both amelogenin and ameloblastin, while Gli1-specific RNAi blocked their expression. These results suggest that Shh directly promotes ALC differentiation, and that Gli1 may be responsible for the signal mediation of Shh.

The relationship between the termination of cell proliferation and expression of heat-shock protein-25 in the rat developing tooth germ

Odontoblast- and ameloblast-lineage cells acquire heat-shock protein (HSP)-25 immunoreactivity after they complete cell division during postnatal odontogenesis in rat molars. However, there are no data available concerning the relationship between the termination of cell proliferation and HSP-25 immunoreactivity during tooth morphogenesis. We compared the expression of HSP-25 in tooth germs with their proliferative activity in the rat prenatal to perinatal molar and postnatal incisor to clarify the functional significance of HSP-25 during tooth morphogenesis by immunohistochemistry using anti-HSP-25 and anti-Ki67/5-bromo-2'-deoxyuridine (BrdU). Numerous proliferating cells in developing molars were distributed throughout the tooth germ and HSP-25 immunoreactivity was recognizable in the dental epithelial and mesenchymal cells after they completed cell division. However, both cell proliferation and immunoreaction for HSP-25 are absent in the enamel knots. The distribution pattern of the proliferating cells in the incisors was basically identical to that in the prenatal molars except for the lack of non-proliferating secondary enamel knots and the sparse distribution of proliferating cells in the apical bud. Thus, HSP-25 protein is suggested to act as a switch between cell proliferation and terminal cyto-differentiation during odontogenesis.