S100a6 knockdown promotes the differentiation of dental epithelial cells toward the epidermal lineage instead of the odontogenic lineage

Tooth development is a complex process involving various signaling pathways and genes. Recent findings suggest that ion channels and transporters, including the S100 family of calcium-binding proteins, may be involved in tooth formation. However, our knowledge in this regard is limited. Therefore, this study aimed to investigate the expression of S100 family members and their functions during tooth formation. Tooth germs were extracted from the embryonic and post-natal mice and the expression of S100a6 was examined. Additionally, the effects of S100a6 knockdown and calcium treatment on S100a6 expression and the proliferation of SF2 cells were examined. Microarrays and single-cell RNA-sequencing indicated that S100a6 was highly expressed in ameloblasts. Immunostaining of mouse tooth germs showed that S100a6 was expressed in ameloblasts but not in the undifferentiated dental epithelium. Additionally, S100a6 was localized to the calcification-forming side in enamel-forming ameloblasts. Moreover, siRNA-mediated S100a6 knockdown in ameloblasts reduced intracellular calcium concentration and the expression of ameloblast marker genes, indicating that S100a6 is associated with ameloblast differentiation. Furthermore, S100a6 knockdown inhibited the ERK/PI3K signaling pathway, suppressed ameloblast proliferation, and promoted the differentiation of the dental epithelium toward epidermal lineage. Conclusively, S100a6 knockdown in the dental epithelium suppresses cell proliferation via calcium and intracellular signaling and promotes differentiation of the dental epithelium toward the epidermal lineage.

Sodium fluoride-induced autophagy of ameloblast-like cells via the p-ULk1/ATG13/LC3B pathway in vitro

OBJECTIVE

To investigate the effects of sodium fluoride on the ameloblast and reveal the mechanism of dental fluorosis.

MATERIALS AND METHODS

Mouse ameloblast-like cell line (ALC) cells were treated with various concentrations of NaF, and subjected to Incucyte, fluorescence immunoassay, transmission electron microscopy, reverse transcription quantitative polymerase chain reaction (RT-qPCR), western blot for autophagy examination, alkaline phosphatase and alizarin red staining for mineralization after osteogenic induction.

RESULTS

NaF exerts a dose-dependent inhibitory effect on ALC cell growth. TEM and fluorescence immunoassay showed that 1.5 mM or higher concentrations of NaF could induce a fusion of lysosome and mitochondria, finally increasing the number of autophagosome. RT-qPCR and western blot showed that the upregulation of autophagy related gene 13 (ATG13), downregulation of phosphorylated Unc-51-like kinase 1 (p-ULK1) were found in NaF-induced autophagy of ALC cells. The knockdown of ATG13 could rescue it as well as the expression of p-ULK1 and LC3B. Besides, alizarin red staining showed that fluoride under these concentrations could promote the mineralization of ALC.

CONCLUSIONS

The data show that fluoride in higher concentration can induce autophagy via the p-ULk1/ATG13/LC3B pathway of ALCs than lower ones promote mineralization in vitro, which provides insight into the function of NaF in the autophagy and mineralization of ameloblast.

Single-cell census of human tooth development enables generation of human enamel

Tooth enamel secreted by ameloblasts (AMs) is the hardest material in the human body, acting as a shield to protect the teeth. However, the enamel is gradually damaged or partially lost in over 90% of adults and cannot be regenerated due to a lack of ameloblasts in erupted teeth. Here, we use single-cell combinatorial indexing RNA sequencing (sci-RNA-seq) to establish a spatiotemporal single-cell census for the developing human tooth and identify regulatory mechanisms controlling the differentiation process of human ameloblasts. We identify key signaling pathways involved between the support cells and ameloblasts during fetal development and recapitulate those findings in human ameloblast in vitro differentiation from induced pluripotent stem cells (iPSCs). We furthermore develop a disease model of amelogenesis imperfecta in a three-dimensional (3D) organoid system and show AM maturation to mineralized structure in vivo. These studies pave the way for future regenerative dentistry.

Fabrication of functional ameloblasts from hiPSCs for dental application

Tooth formation relies on two types of dental cell populations, namely, the dental epithelium and dental mesenchyme, and the interactions between these cell populations are important during tooth development. Although human-induced pluripotent stem cells (hiPSCs) can differentiate into dental epithelial and mesenchymal cells, organoid research on tooth development has not been established yet. This study focused on the hiPSC-derived human ameloblast organoid (hAO) using a three-dimensional (3D) culture system. hAOs had similar properties to ameloblasts, forming enamel in response to calcium and mineralization by interaction with the dental mesenchyme. hAOs simultaneously had osteogenic and odontogenic differentiation potential. Furthermore, hAOs demonstrated tooth regenerative potential upon interaction with the mouse dental mesenchyme. Our findings provide new insights into a suitable hiPSC-derived dental source and demonstrate that hAOs can be beneficial not only for tooth regeneration but also for the study of various dental diseases for which treatment has not been developed yet.

Deficiency of G protein-coupled receptor Gpr111/Adgrf2 causes enamel hypomineralization in mice by alteration of the expression of kallikrein-related peptidase 4 (Klk4) during pH cycling process

Enamel is formed by the repetitive secretion of a tooth-specific extracellular matrix and its decomposition. Calcification of the enamel matrix via hydroxyapatite (HAP) maturation requires pH cycling to be tightly regulated through the neutralization of protons released during HAP synthesis. We found that Gpr115, which responds to changes in extracellular pH, plays an important role in enamel formation. Gpr115-deficient mice show partial enamel hypomineralization, suggesting that other pH-responsive molecules may be involved. In this study, we focused on the role of Gpr111/Adgrf2, a duplicate gene of Gpr115, in tooth development. Gpr111 was highly expressed in mature ameloblasts. Gpr111-KO mice showed enamel hypomineralization. Dysplasia of enamel rods and high carbon content seen in Gpr111-deficient mice suggested the presence of residual enamel matrices in enamel. Depletion of Gpr111 in dental epithelial cells induced the expression of ameloblast-specific protease, kallikrein-related peptidase 4 (Klk4), suggesting that Gpr111 may act as a suppressor of Klk4 expression. Moreover, reduction of extracellular pH to 6.8 suppressed the expression of Gpr111, while the converse increased Klk4 expression. Such induction of Klk4 was synergistically enhanced by Gpr111 knockdown, suggesting that proper enamel mineralization may be linked to the modulation of Klk4 expression by Gpr111. Furthermore, our in vitro suppression of Gpr111 and Gpr115 expression indicated that their suppressive effect on calcification was additive. These results suggest that both Gpr111 and Gpr115 respond to extracellular pH, contribute to the expression of proteolytic enzymes, and regulate the pH cycle, thereby playing important roles in enamel formation.

Deciphering the functions of Stromal Interaction Molecule-1 in amelogenesis using AmelX-iCre mice

Introduction: The intracellular Ca2+ sensor stromal interaction molecule 1 (STIM1) is thought to play a critical role in enamel development, as its mutations cause Amelogenesis Imperfecta (AI). We recently established an ameloblast-specific (AmelX-iCre) Stim1 conditional deletion mouse model to investigate the role of STIM1 in controlling ameloblast function and differentiation in vivo (Stim1 cKO). Our pilot data (Said et al., J. Dent. Res., 2019, 98, 1002-1010) support our hypothesis for a broad role of Stim1 in amelogenesis. This paper aims to provide an in-depth characterization of the enamel phenotype observed in our Stim1 cKO model. Methods: We crossed AmelX-iCre mice with Stim1-floxed animals to develop ameloblast-specific Stim1 cKO mice. Scanning electron microscopy, energy dispersive spectroscopy, and micro- CT were used to study the enamel phenotype. RNAseq and RT-qPCR were utilized to evaluate changes in the gene expression of several key ameloblast genes. Immunohistochemistry was used to detect the amelogenin, matrix metalloprotease 20 and kallikrein 4 proteins in ameloblasts. Results: Stim1 cKO animals exhibited a hypomineralized AI phenotype, with reduced enamel volume, diminished mineral density, and lower calcium content. The mutant enamel phenotype was more severe in older Stim1 cKO mice compared to younger ones and changes in enamel volume and mineral content were more pronounced in incisors compared to molars. Exploratory RNAseq analysis of incisors' ameloblasts suggested that ablation of Stim1 altered the expression levels of several genes encoding enamel matrix proteins which were confirmed by subsequent RT-qPCR. On the other hand, RT-qPCR analysis of molars' ameloblasts showed non-significant differences in the expression levels of enamel matrix genes between control and Stim1-deficient cells. Moreover, gene expression analysis of incisors' and molars' ameloblasts showed that Stim1 ablation caused changes in the expression levels of several genes associated with calcium transport and mitochondrial kinetics. Conclusions: Collectively, these findings suggest that the loss of Stim1 in ameloblasts may impact enamel mineralization and ameloblast gene expression.

Elucidating the role of keratin 75 in enamel using Krt75tm1Der knock-in mouse model

Keratin 75 (K75) was recently discovered in ameloblasts and enamel organic matrix. Carriers of A161T substitution in K75 present with the skin condition Pseudofollicullitis barbae. This mutation is also associated with high prevalence of caries and compromised structural and mechanical properties of enamel. Krt75^tm1Der knock-in mouse (KI) with deletion of Asn159, located two amino acids away from KRT75^A161T, can be a potential model for studying the role of K75 in enamel and the causes of the higher caries susceptibility associated with KRT75^A161T mutation. To test the hypotheses that KI enamel is more susceptible to a simulated acid attack (SAA), and has altered structural and mechanical properties, we conducted in vitro SAA experiments, microCT, and microhardness analyses on 1st molars of one-month-old WT and KI mice. KI and WT hemimandibles were subjected to SAA and contralateral hemimandibles were used as controls. Changes in enamel porosity were assessed by immersion of the hemimandibles in rhodamine, followed by fluorescent microscopy analysis. Fluorescence intensity of KI enamel after SSA was significantly higher than in WT, indicating that KI enamel is more susceptible to acid attack. MicroCT analysis of 1st molars revealed that while enamel volumes were not significantly different, enamel mineral density was significantly lower in KI, suggesting a potential defect of enamel maturation. Microhardness tests revealed that in KI enamel is softer than in WT, and potentially less resilient to damages. These results suggest that the KI enamel can be used as a model to study the role of K75 in enamel.

Energy metabolic shift contributes to the phenotype modulation of maturation stage ameloblasts

Maturation stage ameloblasts (M-ABs) are responsible for terminal enamel mineralization in teeth and undergo characteristic cyclic changes in both morphology and function between ruffle-ended ameloblasts (RA) and smooth-ended ameloblasts (SA). Energy metabolism has recently emerged as a potential regulator of cell differentiation and fate decisions; however, its implication in M-ABs remains unclear. To elucidate the relationship between M-ABs and energy metabolism, we examined the expression pattern of energy metabolic enzymes in M-ABs of mouse incisors. Further, using the HAT7 cell line with M-AB characteristics, we designed experiments to induce an energy metabolic shift by changes in oxygen concentration. We revealed that RA preferentially utilizes oxidative phosphorylation, whereas SA depends on glycolysis-dominant energy metabolism in mouse incisors. In HAT7 cells, hypoxia induced an energy metabolic shift toward a more glycolytic-dominant state, and the energy metabolic shift reduced alkaline phosphatase (ALP) activity and calcium transport and deposition with a change in calcium-related gene expression, implying a phenotype shift from RA to SA. Taken together, these results indicate that the energy metabolic state is an important determinant of the RA/SA phenotype in M-ABs. This study sheds light on the biological significance of energy metabolism in governing M-ABs, providing a novel molecular basis for understanding enamel mineralization and elucidating the pathogenesis of enamel hypomineralization.

Ameloblastic carcinoma in horses: case report and literature review

Ameloblastic carcinoma is a malignant odontogenic neoplasm that has been reported only rarely in veterinary species. A 16-y-old Arabian crossbred mare was presented for evaluation of a hard mass on the body of the mandible, with evidence of osteolysis on radiographs. Incisional biopsies revealed an invasive neoplasm comprised of spindloid epithelial cells with a high mitotic count and partial dual cytokeratin-vimentin immunoreactivity. The horse was euthanized because of rapid tumor progression 3 mo after presentation. Postmortem evaluation revealed partial obliteration of the mandible by a large, firm-to-hard, tan, locally destructive and invasive mass with no gross or histologic evidence of metastasis. Postmortem histology revealed a poorly differentiated epithelial neoplasm with variably prominent features suggestive of odontogenic histogenesis: a plexiform ribbon architecture, infrequent basilar palisading with antibasilar nuclei, rare basilar cytoplasmic clearing, subepithelial matrix hyalinization, and partial dual cytokeratin-vimentin immunoreactivity. Features of malignancy included regions of necrosis, pronounced cellular atypia, a high mitotic count, extensive tissue invasion and local tissue destruction, and extension of neoplastic cells beyond the margins of the mandibular bone. Collectively, these features are most consistent with mandibular ameloblastic carcinoma. Including our case described here, ameloblastic carcinoma has been reported in only 5 horses. The microscopic features reported most consistently are dual cytokeratin-vimentin immunoreactivity, a high mitotic count, and basilar palisading. Ameloblastic carcinoma should be considered as a differential diagnosis for rapidly growing, locally invasive masses arising from the dentate jaw of horses.

An miRNA derived from amelogenin exon4 regulates expression of transcription factor Runx2 by directly targeting upstream activators Nfia and Prkch

Amel, the gene encoding the amelogenin protein involved in enamel formation, is highly alternatively spliced. When exon4 is excised, it can form a mature miRNA (miR-exon4) that has previously been suggested to indirectly regulate expression of the Runt-related transcription factor 2 (Runx2) involved in bone development in ameloblasts and osteoblasts. However, the precise mechanism of this regulation is unclear. In this study, we aimed to identify direct targets of miR-exon4. The transcription factor family nuclear factor I/A (NFI/A) is known to negatively regulate expression of Runx2 and is among the most highly predicted direct targets of miR-exon4 that link to Runx2. Immunostaining detected NFI/A in osteoblasts and ameloblasts in vivo, and reporter assays confirmed direct interaction of the Nfia 3'-UTR and miR-exon4. In addition, silencing of Nfia in MC3T3-E1-M14 osteoblasts resulted in subsequent downregulation of Runx2. In a monoclonal subclone (mi2) of MC3T3-E1 cells wherein mature miR-exon4 was functionally inhibited, we observed significantly downregulated Runx2 expression. We showed that NFI/A was significantly upregulated in mi2 cells at both mRNA and protein levels. Furthermore, quantitative proteomics and pathway analysis of gene expression in mi2 cells suggested that miR-exon4 could directly target Prkch (protein kinase C-eta), possibly leading to RUNX2 regulation through mechanistic target of rapamycin kinase activation. Reporter assays also confirmed the direct interaction of miR-exon4 and the 3'-UTR of Prkch, and Western blot analysis confirmed significantly upregulated mechanistic target of rapamycin kinase phosphorylation in mi2 cells. Taken together, we conclude that Nfia and Prkch expression negatively correlates with miR-exon4-mediated Runx2 regulation in vivo and in vitro, suggesting miR-exon4 directly targets Nfia and Prkch to regulate Runx2.

Partial epithelial-mesenchymal transition during enamel development

Objectives

We set out to investigate whether a hybrid stem‐like p‐EMT phenotype develops during murine molar enamel development in vivo.

Setting and Sample Population

Histology specimens incorporating molar tooth buds harvested from mice at post‐natal day 4 (P4) were included in our studies.

Materials and Methods

We employed double immunofluorescence staining to analyze the simultaneous expression of the epithelial marker E‐cadherin and the mesenchymal marker N‐cadherin in histology specimens with tooth buds harvested from P4 mice. Moreover, we evaluated the expression of the core master stem cell markers Oct4 and Sox2, as well as the mature ameloblast marker amelogenin.

Results

Here we document the co‐expression of E‐cadherin and N‐cadherin in a sub‐population of pre‐ameloblasts in the inner enamel epithelium suggestive of the presence of a hybrid epithelial/mesenchymal phenotype resulting from p‐EMT. Moreover, the core stem cell factors Oct4 and Sox2 colocalized with E‐cadherin expressing pre‐ameloblasts, whereas the mesenchymal marker N‐cadherin was expressed specifically by amelogenin–positive mature secretory ameloblasts.

Conclusions

The differentiation of E‐cadherin–positive pre‐ameloblasts towards N‐cadherin–positive mature secretory ameloblasts transition through a previously unidentified epithelial/mesenchymal stage derived through p‐EMT, co‐expressing the master transcription factors Oct4 and Sox2.

Connexin 43-Mediated Gap Junction Communication Regulates Ameloblast Differentiation via ERK1/2 Phosphorylation

Connexin 43 (Cx43) is an integral membrane protein that forms gap junction channels. These channels mediate intercellular transport and intracellular signaling to regulate organogenesis. The human disease oculodentodigital dysplasia (ODDD) is caused by mutations in Cx43 and is characterized by skeletal, ocular, and dental abnormalities including amelogenesis imperfecta. To clarify the role of Cx43 in amelogenesis, we examined the expression and function of Cx43 in tooth development. Single-cell RNA-seq analysis and immunostaining showed that Cx43 is highly expressed in pre-secretory ameloblasts, differentiated ameloblasts, and odontoblasts. Further, we investigated the pathogenic mechanisms of ODDD by analyzing Cx43-null mice. These mice developed abnormal teeth with multiple dental epithelium layers. The expression of enamel matrix proteins such as ameloblastin (Ambn), which is critical for enamel formation, was significantly reduced in Cx43-null mice. TGF-β1 induces Ambn transcription in dental epithelial cells. The induction of Ambn expression by TGF-β1 depends on the density of the cultured cells. Cell culture at low densities reduces cell–cell contact and reduces the effect of TGF-β1 on Ambn induction. When cell density was high, Ambn expression by TGF-β1 was enhanced. This induction was inhibited by the gap junction inhibitors, oleamide, and 18α-grycyrrhizic acid and was also inhibited in cells expressing Cx43 mutations (R76S and R202H). TGF-β1-mediated phosphorylation and nuclear translocation of ERK1/2, but not Smad2/3, were suppressed by gap junction inhibitors. Cx43 gap junction activity is required for TGF-β1-mediated Runx2 phosphorylation through ERK1/2, which forms complexes with Smad2/3. In addition to its gap junction activity, Cx43 may also function as a Ca²⁺ channel that regulates slow Ca²⁺ influx and ERK1/2 phosphorylation. TGF-β1 transiently increases intracellular calcium levels, and the increase in intracellular calcium over a short period was not related to the expression level of Cx43. However, long-term intracellular calcium elevation was enhanced in cells overexpressing Cx43. Our results suggest that Cx43 regulates intercellular communication through gap junction activity by modulating TGF-β1-mediated ERK signaling and enamel formation.

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.

The absence of muscle segment homeobox 2 leads to the pyroptosis of ameloblasts by inducing squamous epithelial hyperplasia in the enamel organ

Muscle segment homeobox 2 (MSX2) has been confirmed to be involved in the regulation of early tooth development. However, the role of MSX2 has not been fully elucidated in enamel development. To research the functions of MSX2 in enamel formation, we used a Msx2-/- (KO) mouse model with no full Msx2 gene. In the present study, the dental appearance and enamel microstructure were detected by scanning electron microscopy and micro-computed tomography. The results showed that the absence of Msx2 resulted in enamel defects, leading to severe tooth wear in KO mice. To further investigate the mechanism behind the phenotype, we performed detailed histological analyses of the enamel organ in KO mice. We discovered that ameloblasts without Msx2 could secrete a small amount of enamel matrix protein in the early stage. However, the enamel epithelium occurred squamous epithelial hyperplasia and partial keratinization in the enamel organ during subsequent developmental stages. Ameloblasts depolarized and underwent pyroptosis. Overall, during the development of enamel, MSX2 affects the formation of enamel by regulating the function of epithelial cells in the enamel organ.

Three-Dimensional Culture of Ameloblast-Originated HAT-7 Cells for Functional Modeling of Defective Tooth Enamel Formation

Background: Amelogenesis, the formation of dental enamel, is well understood at the histomorphological level but the underlying molecular mechanisms are poorly characterized. Ameloblasts secrete enamel matrix proteins and Ca²⁺, and also regulate extracellular pH as the formation of hydroxyapatite crystals generates large quantities of protons. Genetic or environmental impairment of transport and regulatory processes (e.g. dental fluorosis) leads to the development of enamel defects such as hypomineralization.

Aims: Our aims were to optimize the culture conditions for the three-dimensional growth of ameloblast-derived HAT-7 cells and to test the effects of fluoride exposure on HAT-7 spheroid formation.

Methods: To generate 3D HAT-7 structures, cells were dispersed and plated within a Matrigel extracellular matrix scaffold and incubated in three different culture media. Spheroid formation was then monitored over a two-week period. Ion transporter and tight-junction protein expression was investigated by RT-qPCR. Intracellular Ca²⁺ and pH changes were measured by microfluorometry using the fluorescent dyes fura-2 and BCECF.

Results: A combination of Hepato-STIM epithelial cell differentiation medium and Matrigel induced the expansion and formation of 3D HAT-7 spheroids. The cells retained their epithelial cell morphology and continued to express both ameloblast-specific and ion transport-specific marker genes. Furthermore, like two-dimensional HAT-7 monolayers, the HAT-7 spheroids were able to regulate their intracellular pH and to show intracellular calcium responses to extracellular stimulation. Finally, we demonstrated that HAT-7 spheroids may serve as a disease model for studying the effects of fluoride exposure during amelogenesis.

Conclusion: In conclusion, HAT-7 cells cultivated within a Matrigel extracellular matrix form three-dimensional, multi-cellular, spheroidal structures that retain their functional capacity for pH regulation and intracellular Ca²⁺ signaling. This new 3D model will allow us to gain a better understanding of the molecular mechanisms involved in amelogenesis, not only in health but also in disorders of enamel formation, such as those resulting from fluoride exposure.

The Resistance of Succedaneous Teeth to Periapical Infection: A Series of Seven Cases

Pediatric dental procedures are carried out largely to prevent the adverse effects of primary teeth infection on succedaneous teeth (ST). The existing literature on how and when periapical infection (PI) from primary teeth affect ST is conflicted. A series of seven case reports on PI (abscesses and radicular cysts) from primary molars, until the eruption of the premolars, have been illustrated. The resistance of ST to PI based on their developmental stages is also discussed.

ODAM promotes junctional epithelium-related gene expression via activation of WNT1 signaling pathway in an ameloblast-like cell line ALC

OBJECTIVE

In this study, we investigated the potential and mechanism of odontogenic ameloblast-associated protein (ODAM) in the promoting junctional epithelium-related gene expression in an ameloblast-like cell line ALC.

BACKGROUND

ODAM is expressed in ameloblasts and JE and acts as a component of the inner basal lamina (IBL) and intercellular matrix of JE. ODAM KO mice showed destruction of the integrity of the JE, which detaches from teeth. ODAM was confirmed to regulate the cytoskeleton through the ODAM-ARHGEF5-RhoA signaling pathway of the JE. Whether ODAM contributes to the regulation of ameloblast differentiation in JE remains unclear. After the formation of enamel, the ameloblast undergoes a series of morphological changes. Whether ODAM will affect the biological behavior of ameloblasts making them have the characteristics of JE is unclear.

METHODS

A murine ameloblast-like cell line, ALC, was used to investigate the effects of ODAM on the JE-like changes of ALC cells in an epithelium-induced environment by generating ODAM overexpression and ODAM knockdown cells through a lentivirus transduction approach. The biomarkers of junctional epithelium CK19, SLPI, and ODAM and the potential regulatory gene WNT1 were investigated by real-time PCR, western blot, immunocytochemistry, immunostaining, luciferase reporter, and rescue assays.

RESULTS

ODAM, CK19, and SLPI were significantly upregulated after epithelial induction. Overexpression of ODAM in ALC cells markedly increased CK19 and SLPI expression, while knockdown of ODAM in ALC cells clearly decreased CK19 and SLPI expression. A reporter luciferase assay showed that ODAM activated the WNT signaling pathway, especially through WNT1. Exogenous overexpression of ODAM upregulated WNT1 expression, while knockdown of ODAM reversed this effect. The WNT1 inhibition assay further confirmed the above results and showed that the WNT1 pathway was positively correlated with biomarkers of junctional epithelium CK19 and SLPI expression. Rescue studies showed that knocking down WNT1 in the ODAM-overexpressing ALC cells decreased the expression of CK19 and SLPI. Immunocytochemistry showed that ODAM colocalized with CK19, SLPI, and WNT1 in the cells.

CONCLUSION

In conclusion, the research work showed that ODAM promotes junctional epithelium-related gene expression in ALC via the ODAM-WNT1 axis, which may provide new insight into the function of ODAM and JE formation.

Single-Cell RNA-Sequencing From Mouse Incisor Reveals Dental Epithelial Cell-Type Specific Genes

Dental epithelial stem cells give rise to four types of dental epithelial cells: inner enamel epithelium (IEE), outer enamel epithelium (OEE), stratum intermedium (SI), and stellate reticulum (SR). IEE cells further differentiate into enamel-forming ameloblasts, which play distinct roles, and are essential for enamel formation. These are conventionally classified by their shape, although their transcriptome and biological roles are yet to be fully understood. Here, we aimed to use single-cell RNA sequencing to clarify the heterogeneity of dental epithelial cell types. Unbiased clustering of 6,260 single cells from incisors of postnatal day 7 mice classified them into two clusters of ameloblast, IEE/OEE, SI/SR, and two mesenchymal populations. Secretory-stage ameloblasts expressed Amel and Enam were divided into Dspp + and Ambn + ameloblasts. Pseudo-time analysis indicated Dspp + ameloblasts differentiate into Ambn + ameloblasts. Further, Dspp and Ambn could be stage-specific markers of ameloblasts. Gene ontology analysis of each cluster indicated potent roles of cell types: OEE in the regulation of tooth size and SR in the transport of nutrients. Subsequently, we identified novel dental epithelial cell marker genes, namely Pttg1, Atf3, Cldn10, and Krt15. The results not only provided a resource of transcriptome data in dental cells but also contributed to the molecular analyses of enamel formation.

G protein-coupled receptor Gpr115 (Adgrf4) is required for enamel mineralization mediated by ameloblasts

Dental enamel, the hardest tissue in the human body, is derived from dental epithelial cell ameloblast-secreted enamel matrices. Enamel mineralization occurs in a strictly synchronized manner along with ameloblast maturation in association with ion transport and pH balance, and any disruption of these processes results in enamel hypomineralization. G protein-coupled receptors (GPCRs) function as transducers of external signals by activating associated G proteins and regulate cellular physiology. Tissue-specific GPCRs play important roles in organ development, although their activities in tooth development remain poorly understood. The present results show that the adhesion GPCR Gpr115 (Adgrf4) is highly and preferentially expressed in mature ameloblasts and plays a crucial role during enamel mineralization. To investigate the in vivo function of Gpr115, knockout (Gpr115-KO) mice were created and found to develop hypomineralized enamel, with a larger acidic area because of the dysregulation of ion composition. Transcriptomic analysis also revealed that deletion of Gpr115 disrupted pH homeostasis and ion transport processes in enamel formation. In addition, in vitro analyses using the dental epithelial cell line cervical loop-derived dental epithelial (CLDE) cell demonstrated that Gpr115 is indispensable for the expression of carbonic anhydrase 6 (Car6), which has a critical role in enamel mineralization. Furthermore, an acidic condition induced Car6 expression under the regulation of Gpr115 in CLDE cells. Thus, we concluded that Gpr115 plays an important role in enamel mineralization via regulation of Car6 expression in ameloblasts. The present findings indicate a novel function of Gpr115 in ectodermal organ development and clarify the molecular mechanism of enamel formation.

Calcium Sets the Clock in Ameloblasts

Background

Stromal interaction molecule 1 (STIM1) is one of the main components of the store operated Ca²⁺ entry (SOCE) signaling pathway. Individuals with mutated STIM1 present severely hypomineralized enamel characterized as amelogenesis imperfecta (AI) but the downstream molecular mechanisms involved remain unclear. Circadian clock signaling plays a key role in regulating the enamel thickness and mineralization, but the effects of STIM1-mediated AI on circadian clock are unknown.

Objectives

The aim of this study is to examine the potential links between SOCE and the circadian clock during amelogenesis.

Methods

We have generated mice with ameloblast-specific deletion of Stim1 (Stim1^fl/fl/Amelx-iCre^+/+, Stim1 cKO) and analyzed circadian gene expression profile in Stim1 cKO compared to control (Stim1^fl/fl/Amelx-iCre^–/–) using ameloblast micro-dissection and RNA micro-array of 84 circadian genes. Expression level changes were validated by qRT-PCR and immunohistochemistry.

Results

Stim1 deletion has resulted in significant upregulation of the core circadian activator gene Brain and Muscle Aryl Hydrocarbon Receptor Nuclear Translocation 1 (Bmal1) and downregulation of the circadian inhibitor Period 2 (Per2). Our analyses also revealed that SOCE disruption results in dysregulation of two additional circadian regulators; p38α mitogen-activated protein kinase (MAPK14) and transforming growth factor-beta1 (TGF-β1). Both MAPK14 and TGF-β1 pathways are known to play major roles in enamel secretion and their dysregulation has been previously implicated in the development of AI phenotype.

Conclusion

These data indicate that disruption of SOCE significantly affects the ameloblasts molecular circadian clock, suggesting that alteration of the circadian clock may be partly involved in the development of STIM1-mediated AI.

Functional Expression of Sodium-Dependent Glucose Transporter in Amelogenesis

Glucose is an essential source of energy for mammalian cells and is transported into the cells by glucose transporters. There are 2 types of glucose transporters: one is a passive glucose transporter, GLUT (SLC2A), and the other is a sodium-dependent active glucose transporter, SGLT (SLC5A). We previously reported that the expression of GLUTs during tooth development is precisely and spatiotemporally controlled and that the glucose uptake mediated by GLUT1 plays a crucial role in early tooth morphogenesis and tooth size determination. This study aimed to clarify the localization and roles of SGLT1 and SGLT2 in murine ameloblast differentiation by using immunohistochemistry, immunoelectron microscopy, an in vitro tooth organ culture experiment, and in vivo administration of an inhibitor of SGLT1/2, phloridzin. SGLT1, which has high affinity with glucose, was immunolocalized in the early secretory ameloblasts and the ruffle-ended ameloblasts in the maturation stage. However, SGLT2, which has high glucose transport capacity, was observed in the stratum intermedium, papillary layer, and ameloblasts at the maturation stage and colocalized with Na⁺-K⁺-ATPase. The inhibition of SGLT1/2 by phloridzin in the tooth germs induced the disturbance of ameloblast differentiation and enamel matrix formation both in vitro (organ culture) and in vivo (mouse model). The expression of SGLT1 and SGLT2 was significantly upregulated in hypoxic conditions in the ameloblast-lineage cells. These findings suggest that the active glucose uptake mediated by SGLT1 and SGLT2 is strictly regulated and dependent on the intra- and extracellular microenvironments during tooth morphogenesis and that the appropriate passive and active glucose transport is an essential event in amelogenesis.

ATG7 is essential for secretion of iron from ameloblasts and normal growth of murine incisors during aging

The incisors of rodents comprise an iron-rich enamel and grow throughout adult life, making them unique models of iron metabolism and tissue homeostasis during aging. Here, we deleted Atg7 (autophagy related 7) in murine ameloblasts, i.e. the epithelial cells that produce enamel. The absence of ATG7 blocked the transport of iron from ameloblasts into the maturing enamel, leading to a white instead of yellow surface of maxillary incisors. In aging mice, lack of ATG7 was associated with the growth of ectopic incisors inside severely deformed primordial incisors. These results suggest that 2 characteristic features of rodent incisors, i.e. deposition of iron on the enamel surface and stable growth during aging, depend on autophagic activity in ameloblasts. Abbreviations: ATG5: autophagy related 5; ATG7: autophagy related 7; CMV: cytomegalovirus; Cre: Cre recombinase; CT: computed tomography; FTH1: ferritin heavy polypeptide 1; GFP: green fluorescent protein; KRT5: keratin 5; KRT14: keratin 14; LGALS3: lectin, galactose binding, soluble 3; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; NCOA4: nuclear receptor coactivator 4; NRF2: nuclear factor, erythroid 2 like 2; SQSTM1: sequestosome 1.

Trafficking and secretion of keratin 75 by ameloblasts in vivo

A highly specialized cytoskeletal protein, keratin 75 (K75), expressed primarily in hair follicles, nail beds, and lingual papillae, was recently discovered in dental enamel, the most highly mineralized hard tissue in the human body. Among many questions this discovery poses, the fundamental question regarding the trafficking and secretion of this protein, which lacks a signal peptide, is of an utmost importance. Here, we present evidence that K75 is expressed during the secretory stage of enamel formation and is present in the forming enamel matrix. We further show that K75 is secreted together with major enamel matrix proteins amelogenin and ameloblastin, and it was detected in Golgi and the endoplasmic reticulum (ER)-Golgi intermediate compartment (ERGIC) but not in rough ER (rER). Inhibition of ER-Golgi transport by brefeldin A did not affect the association of K75 with Golgi, whereas ameloblastin accumulated in rER, and its transport from rER into Golgi was disrupted. Together, these results indicate that K75, a cytosolic protein lacking a signal sequence, is secreted into the forming enamel matrix utilizing portions of the conventional ER-Golgi secretory pathway. To the best of our knowledge, this is the first study providing insights into mechanisms of keratin secretion.

Defense Mechanisms Against Acid Exposure by Dental Enamel Formation, Saliva and Pancreatic Juice Production

The pancreas, the salivary glands and the dental enamel producing ameloblasts have marked developmental, structural and functional similarities. One of the most striking similarities is their bicarbonate-rich secretory product, serving acid neutralization. An important difference between them is that while pancreatic juice and saliva are delivered into a lumen where they can be collected and analyzed, ameloblasts produce locally precipitating hydroxyapatite which cannot be easily studied. Interestingly, the ion and protein secretion by the pancreas, the salivary glands, and maturation ameloblasts are all two-step processes, of course with significant differences too. As they all have to defend against acid exposure by producing extremely large quantities of bicarbonate, the failure of this function leads to deteriorating consequences. The aim of the present review is to describe and characterize the defense mechanisms of the pancreas, the salivary glands and enamel-producing ameloblasts against acid exposure and to compare their functional capabilities to do this by producing bicarbonate.

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.

Identification of the Novel Tooth-Specific Transcription Factor AmeloD

Basic-helix-loop-helix (bHLH) transcription factors play an important role in various organs' development; however, a tooth-specific bHLH factor has not been reported. In this study, we identified a novel tooth-specific bHLH transcription factor, which we named AmeloD, by screening a tooth germ complementary DNA (cDNA) library using a yeast 2-hybrid system. AmeloD was mapped onto the mouse chromosome 1q32. Phylogenetic analysis showed that AmeloD belongs to the achaete-scute complex-like ( ASCL) gene family and is a homologue of ASCL5. AmeloD was uniquely expressed in the inner enamel epithelium (IEE), but its expression was suppressed after IEE cell differentiation into ameloblasts. Furthermore, AmeloD expression showed an inverse expression pattern with the epithelial cell-specific cell-cell adhesion molecule E-cadherin in the dental epithelium. Overexpression of AmeloD in dental epithelial cell line CLDE cells resulted in E-cadherin suppression. We found that AmeloD bound to E-box cis-regulatory elements in the proximal promoter region of the E-cadherin gene. These results reveal that AmeloD functions as a suppressor of E-cadherin transcription in IEE cells. Our study demonstrated that AmeloD is a novel tooth-specific bHLH transcription factor that may regulate tooth development through the suppression of E-cadherin in IEE cells.

How pH is regulated during amelogenesis in dental fluorosis

Amelogenesis is a complicated process that concerns the interaction between growing hydroxyapatite crystals and extracellular proteins, which requires the tight regulation of pH. In dental fluorosis, the balance of pH regulation is broken, leading to abnormal mineralization. The current review focuses on the electrolyte transport processes associated with pH homeostasis, particularly regarding the changes in ion transporters that occur during amelogenesis, following exposure to excessive fluoride. Furthermore, the possible mechanism of fluorosis is discussed on the basis of acid hypothesis. There are two main methods by which F^- accelerates crystal formation in ameloblasts. Firstly, it induces the release of protons, lowering the pH of the cell microenvironment. The decreased pH stimulates the upregulation of ion transporters, which attenuates further declines in the pH. Secondly, F^- triggers an unknown signaling pathway, causing changes in the transcription of ion transporters and upregulating the expression of bicarbonate transporters. This results in the release of a large amount of bicarbonate from ameloblasts, which may neutralize the pH to form a microenvironment that favors crystal nucleation. The decreased pH stimulates the diffusion of F^- into the cytoplasm of amelobalsts along the concentration gradient formed by the release of protons. The retention of F^- causes a series of pathological changes, including oxidative and endoplasmic reticulum stress. If the buffering capacity of ameloblasts facing F^- toxicity holds, normal mineralization occurs; however, if F^- levels are high enough to overwhelm the buffering capacity of ameloblasts, abnormal mineralization occurs, leading to dental fluorosis.

The Importance of Connexin 43 in Enamel Development and Mineralization

During enamel development, formation of hydroxyapatite crystals and regulation of pH in the enamel matrix require massive transport of ions. Both ameloblasts and adjacent dental epithelial cells in the stellate reticulum co-express several transmembrane cotransporters/ion-exchangers for transport of ions across plasma membranes. Gap junctions (GJs) enable intercellular exchanges of ions between neighboring cells. This suggests that the ameloblasts and other cell layers of the enamel organ, form a functional unit. During the bell stage of tooth formation, the non-ameloblast dental epithelium highly expresses the Na-K-Cl cotransporter (Nkcc1). Nkcc1-null mice are associated with enamel hypomineralization and increased expression of GJ protein connexin 43 (Cx43), suggesting that reduced ion transport in the Nkcc1-null mouse is in part compensated by increased intercellular ion transport through GJs. To understand the role of GJs in ion transport and its effect on pH regulation, we examined in a mouse strain in which Cx43 was ablated selectively in DMP1 expressing cells (Cx43^flox/flox mice crossed with DMP1-8kb-Cre mice), including ameloblasts. Micro-CT analysis showed that the mineral density at late maturation stage incisal enamel of the Cx43-null mice was 10% less than in controls, whereas that in dentin was unchanged. Maturation stage ameloblasts of mice lacking the pH regulating sodium/bicarbonate transporter NBCe1 (Nbce1-null), or chloride channel Cftr (Cftr-null) were found to have increased Cx43-immunostaining. These results support the possibility that GJs in the ameloblast-papillary complex at the maturation stage contribute to ion transport by enabling passage of ions directly from cells of the papillary layer into ameloblast layer. Increasing the number of GJs may partly compensate the reduction of ion-cotransporters and ion exchangers in dental epithelium.

Bioengineered Tooth Buds Exhibit Features of Natural Tooth Buds

Tooth loss is a significant health issue currently affecting millions of people worldwide. Artificial dental implants, the current gold standard tooth replacement therapy, do not exhibit many properties of natural teeth and can be associated with complications leading to implant failure. Here we propose bioengineered tooth buds as a superior alternative tooth replacement therapy. We describe improved methods to create highly cellularized bioengineered tooth bud constructs that formed hallmark features that resemble natural tooth buds such as the dental epithelial stem cell niche, enamel knot signaling centers, transient amplifying cells, and mineralized dental tissue formation. These constructs were composed of postnatal dental cells encapsulated within a hydrogel material that were implanted subcutaneously into immunocompromised rats. To our knowledge, this is the first report describing the use of postnatal dental cells to create bioengineered tooth buds that exhibit evidence of these features of natural tooth development. We propose future bioengineered tooth buds as a promising, clinically relevant tooth replacement therapy.

The effect of a single injection of irinotecan on the development of enamel in the Wistar rats

Cancer is the second most frequent cause of death in children. Because the prognosis for childhood malignancies has improved, attention has now focused on long-term consequences of cancer treatment. The immediate effects of chemotherapy on soft tissues have been well described; however, there is less information about long-term effects of chemotherapy on the development of dental tissues. To test the association between the effect of chemotherapy on enamel development, we examined two groups of rats: one that had received an intraperitoneal dose of 200 mg/kg of irinotecan, whereas the other (control) group had received vehicle only. Rats were killed at 6, 48 and 96 hr post-injection; the mandibles dissected out, fixed for histological evaluation and scanned for mineralization defects by Micro-CT. Our results showed structural changes in the ameloblast layer along with a significant reduction in mineralization and thickness of enamel at 96 hr after chemotherapy. These data demonstrate that irinotecan induces structural changes in forming enamel that become apparent after anticancer chemotherapy treatment.

No Change in Bicarbonate Transport but Tight-Junction Formation Is Delayed by Fluoride in a Novel Ameloblast Model

We have recently developed a novel in vitro model using HAT-7 rat ameloblast cells to functionally study epithelial ion transport during amelogenesis. Our present aims were to identify key transporters of bicarbonate in HAT-7 cells and also to examine the effects of fluoride exposure on vectorial bicarbonate transport, cell viability, and the development of transepithelial resistance. To obtain monolayers, the HAT-7 cells were cultured on Transwell permeable filters. We monitored transepithelial resistance (TER) as an indicator of tight junction formation and polarization. We evaluated intracellular pH changes by microfluorometry using the fluorescent indicator BCECF. Activities of ion transporters were tested by withdrawal of various ions from the bathing medium, by using transporter specific inhibitors, and by activation of transporters with forskolin and ATP. Cell survival was estimated by alamarBlue assay. Changes in gene expression were monitored by qPCR. We identified the activity of several ion transporters, NBCe1, NHE1, NKCC1, and AE2, which are involved in intracellular pH regulation and vectorial bicarbonate and chloride transport. Bicarbonate secretion by HAT-7 cells was not affected by acute fluoride exposure over a wide range of concentrations. However, tight-junction formation was inhibited by 1 mM fluoride, a concentration which did not substantially reduce cell viability, suggesting an effect of fluoride on paracellular permeability and tight-junction formation. Cell viability was only reduced by prolonged exposure to fluoride concentrations greater than 1 mM. In conclusion, cultured HAT-7 cells are functionally polarized and are able to transport bicarbonate ions from the basolateral to the apical fluid spaces. Exposure to 1 mM fluoride has little effect on bicarbonate secretion or cell viability but delays tight-junction formation, suggesting a novel mechanism that may contribute to dental fluorosis.

Intravesicular Phosphatase PHOSPHO1 Function in Enamel Mineralization and Prism Formation

The transport of mineral ions from the enamel organ-associated blood vessels to the developing enamel crystals involves complex cargo packaging and carriage mechanisms across several cell layers, including the ameloblast layer and the stratum intermedium. Previous studies have established PHOSPHO1 as a matrix vesicle membrane-associated phosphatase that interacts with matrix vesicles molecules phosphoethanolamine and phosphocholine to initiate apatite crystal formation inside of matrix vesicles in bone. In the present study, we sought to determine the function of Phospho1 during amelogenesis. PHOSPHO1 protein localization during amelogenesis was verified using immunohistochemistry, with positive signals in the enamel layer, ameloblast Tomes' processes, and in the walls of ameloblast secretory vesicles. These ameloblast secretory vesicle walls were also labeled for amelogenin and the exosomal protein marker HSP70 using immunohistochemistry. Furthermore, PHOSPHO1 presence in the enamel organ was confirmed by Western blot. Phospho1^−/− mice lacked sharp incisal tips, featured a significant 25% increase in total enamel volume, and demonstrated a significant 2-fold reduction in silver grain density of von Kossa stained ground sections indicative of reduced mineralization in the enamel layer when compared to wild-type mice (p < 0.001). Scanning electron micrographs of Phospho1^−/− mouse enamel revealed a loss of the prominent enamel prism “picket fence” structure, a loss of parallel crystal organization within prisms, and a 1.56-fold increase in enamel prism width (p < 0.0001). Finally, EDS elemental analysis demonstrated a significant decrease in phosphate incorporation in the enamel layer when compared to controls (p < 0.05). Together, these data establish that the matrix vesicle membrane-associated phosphatase PHOSPHO1 is essential for physiological enamel mineralization. Our findings also suggest that intracellular ameloblast secretory vesicles have unexpected compositional similarities with the extracellular matrix vesicles of bone, dentin, and cementum in terms of vesicle membrane composition and intravesicular ion assembly.

Importance of bicarbonate transport in pH control during amelogenesis - need for functional studies

Dental enamel, the hardest mammalian tissue, is produced by ameloblasts. Ameloblasts show many similarities to other transporting epithelia although their secretory product, the enamel matrix, is quite different. Ameloblasts direct the formation of hydroxyapatite crystals, which liberate large quantities of protons that then need to be buffered to allow mineralization to proceed. Buffering requires a tight pH regulation and secretion of bicarbonate by ameloblasts. Many investigations have used immunohistochemical and knockout studies to determine the effects of these genes on enamel formation, but up till recently very little functional data were available for mineral ion transport. To address this, we developed a novel 2D in vitro model using HAT-7 ameloblast cells. HAT-7 cells can be polarized and develop functional tight junctions. Furthermore, they are able to accumulate bicarbonate ions from the basolateral to the apical fluid spaces. We propose that in the future, the HAT-7 2D system along with similar cellular models will be useful to functionally model ion transport processes during amelogenesis. Additionally, we also suggest that similar approaches will allow a better understanding of the regulation of the cycling process in maturation-stage ameloblasts, and the pH sensory mechanisms, which are required to develop sound, healthy enamel.

The Unfolded Protein Response in Amelogenesis and Enamel Pathologies

During the secretory phase of their life-cycle, ameloblasts are highly specialized secretory cells whose role is to elaborate an extracellular matrix that ultimately confers both form and function to dental enamel, the most highly mineralized of all mammalian tissues. In common with many other “professional” secretory cells, ameloblasts employ the unfolded protein response (UPR) to help them cope with the large secretory cargo of extracellular matrix proteins transiting their ER (endoplasmic reticulum)/Golgi complex and so minimize ER stress. However, the UPR is a double-edged sword, and, in cases where ER stress is severe and prolonged, the UPR switches from pro-survival to pro-apoptotic mode. The purpose of this review is to consider the role of the ameloblast UPR in the biology and pathology of amelogenesis; specifically in respect of amelogenesis imperfecta (AI) and fluorosis. Some forms of AI appear to correspond to classic proteopathies, where pathological intra-cellular accumulations of protein tip the UPR toward apoptosis. Fluorosis also involves the UPR and, while not of itself a classic proteopathic disease, shares some common elements through the involvement of the UPR. The possibility of therapeutic intervention by pharmacological modulation of the UPR in AI and fluorosis is also discussed.

Ras Signaling Regulates Stem Cells and Amelogenesis in the Mouse Incisor

The role of Ras signaling during tooth development is poorly understood. Ras proteins-which are activated by many upstream pathways, including receptor tyrosine kinase cascades-signal through multiple effectors, such as the mitogen-activated protein kinase (MAPK) and PI3K pathways. Here, we utilized the mouse incisor as a model to study how the MAPK and PI3K pathways regulate dental epithelial stem cells and amelogenesis. The rodent incisor-which grows continuously throughout the life of the animal due to the presence of epithelial and mesenchymal stem cells-provides a model for the study of ectodermal organ renewal and regeneration. Utilizing models of Ras dysregulation as well as inhibitors of the MAPK and PI3K pathways, we found that MAPK and PI3K regulate dental epithelial stem cell activity, transit-amplifying cell proliferation, and enamel formation in the mouse incisor.

Heterogeneity and Developmental Connections between Cell Types Inhabiting Teeth

Every tissue is composed of multiple cell types that are developmentally, evolutionary and functionally integrated into the unit we call an organ. Teeth, our organs for biting and mastication, are complex and made of many different cell types connected or disconnected in terms of their ontogeny. In general, epithelial and mesenchymal compartments represent the major framework of tooth formation. Thus, they give rise to the two most important matrix–producing populations: ameloblasts generating enamel and odontoblasts producing dentin. However, the real picture is far from this quite simplified view. Diverse pulp cells, the immune system, the vascular system, the innervation and cells organizing the dental follicle all interact, and jointly participate in transforming lifeless matrix into a functional organ that can sense and protect itself. Here we outline the heterogeneity of cell types that inhabit the tooth, and also provide a life history of the major populations. The mouse model system has been indispensable not only for the studies of cell lineages and heterogeneity, but also for the investigation of dental stem cells and tooth patterning during development. Finally, we briefly discuss the evolutionary aspects of cell type diversity and dental tissue integration.

[Expression and function of microRNAs in enamel development]

microRNAs (miRNAs) are endogenous short, noncoding RNAs that can negatively regulate gene expression post-transcriptionally. miRNAs are involved in multiple developmental events in various tissues and organs, including dental enamel development. Any disruption during enamel development may result in inherited enamel malformations. This article reviews the expression and function of miRNAs in enamel development.

4-phenylbutyrate Mitigates Fluoride-Induced Cytotoxicity in ALC Cells

Chronic fluoride over-exposure during pre-eruptive enamel development can cause dental fluorosis. Severe dental fluorosis is characterized by porous, soft enamel that is vulnerable to erosion and decay. The prevalence of dental fluorosis among the population in the USA, India and China is increasing. Other than avoiding excessive intake, treatments to prevent dental fluorosis remain unknown. We previously reported that high-dose fluoride induces endoplasmic reticulum (ER) stress and oxidative stress in ameloblasts. Cell stress induces gene repression, mitochondrial damage and apoptosis. An aromatic fatty acid, 4-phenylbutyrate (4PBA) is a chemical chaperone that interacts with misfolded proteins to prevent ER stress. We hypothesized that 4PBA ameliorates fluoride-induced ER stress in ameloblasts. To determine whether 4PBA protects ameloblasts from fluoride toxicity, we analyzed gene expression of Tgf-β1, Bcl2/Bax ratio and cytochrome-c release in vitro. In vivo, we measured fluorosis levels, enamel hardness and fluoride concentration. Fluoride treated Ameloblast-lineage cells (ALC) had decreased Tgf-β1 expression and this was reversed by 4PBA treatment. The anti-apoptotic Blc2/Bax ratio was significantly increased in ALC cells treated with fluoride/4PBA compared to fluoride treatment alone. Fluoride treatment induced cytochrome-c release from mitochondria into the cytosol and this was inhibited by 4PBA treatment. These results suggest that 4PBA mitigates fluoride-induced gene suppression, apoptosis and mitochondrial damage in vitro. In vivo, C57BL/6J mice were provided fluoridated water for six weeks with either fluoride free control-chow or 4PBA-containing chow (7 g/kg 4PBA). With few exceptions, enamel microhardness, fluorosis levels, and fluoride concentrations of bone and urine did not differ significantly between fluoride treated animals fed with control-chow or 4PBA-chow. Although 4PBA mitigated high-dose fluoride toxicity in vitro, a diet rich in 4PBA did not attenuate dental fluorosis in rodents. Perhaps, not enough intact 4PBA reaches the rodent ameloblasts necessary to reverse the effects of fluoride toxicity. Further studies will be required to optimize protocols for 4PBA administration in vivo in order to evaluate the effect of 4PBA on dental fluorosis.

Angled Growth of the Dental Lamina Is Accompanied by Asymmetrical Expression of the WNT Pathway Receptor Frizzled 6

Frizzled 6 (FZD6) belongs to a family of proteins that serve as receptors in the WNT signaling pathway. FZD6 plays an important role in the establishment of planar cell polarity in many embryonic processes such as convergent extension during gastrulation, neural tube closure, or hair patterning. Based on its role during hair development, we hypothesized that FZD6 may have similar expression pattern and function in the dental lamina, which is also a distinct epithelial protrusion growing characteristically angled into the mesenchyme. Diphyodont minipig was selected as a model species because its dentition closely resemble human ones with successional generation of teeth initiated from the dental lamina. We revealed asymmetrical expression of FZD6 in the dental lamina of early as well as late stages during its regression with stronger expression located on the labial side of the dental lamina. During lamina regression, FZD6-positive cells were found in its superficial part and the signal coincided with the upregulation of molecules involved in epithelial-mesenchymal transition and increased migratory potential of epithelial cells. FZD6-expression was also turned on during differentiation of cells producing hard tissues, in which mature odontoblasts, ameloblasts, or surrounding osteoblasts were FZD6-positive. On the other hand, the tip of successional lamina and its lingual part, in which progenitor cells are located, exhibited FZD6-negativity. In conclusion, asymmetrical expression of FZD6 correlates with the growth directionality and side-specific morphological differences in the dental lamina of diphyodont species. Based on observed expression pattern, we propose that the dental lamina is other epithelial tissue, where planar cell polarity signaling is involved during its asymmetrical growth.

Loss of Function of Evc2 in Dental Mesenchyme Leads to Hypomorphic Enamel

Ellis-van Creveld (EvC) syndrome is an autosomal-recessive skeletal dysplasia, characterized by short stature and postaxial polydactyly. A series of dental abnormalities, including hypomorphic enamel formation, has been reported in patients with EvC. Despite previous studies that attempted to uncover the mechanism leading to abnormal tooth development, little is known regarding how hypomorphic enamel is formed in patients with EvC. In the current study, using Evc2/ Limbin mutant mice we recently generated, we analyzed enamel formation in the mouse incisor. Consistent with symptoms in human patients, we observed that Evc2 mutant mice had smaller incisors with enamel hypoplasia. Histologic observations coupled with ameloblast marker analyses suggested that Evc2 mutant preameloblasts were capable of differentiating to secretory ameloblasts; this process, however, was apparently delayed, due to delayed odontoblast differentiation, mediated by a limited number of dental mesenchymal stem cells in Evc2 mutant mice. This concept was further supported by the observation that dental mesenchymal-specific deletion of Evc2 phenocopied the tooth abnormalities in Evc2 mutants. Overall, our findings suggest that mutations in Evc2 affect dental mesenchymal stem cell homeostasis, which further leads to hypomorphic enamel formation.

Expression of Nerve Growth Factor (NGF), TrkA, and p75(NTR) in Developing Human Fetal Teeth

Nerve growth factor (NGF) is important for the development and the differentiation of neuronal and non-neuronal cells. NGF binds to specific low- and high-affinity cell surface receptors, respectively, p75^NTR and TrkA. In the present study, we examined by immunohistochemistry the expression patterns of the NGF, p75^NTR, and TrkA proteins during human fetal tooth development, in order to better understand the mode of NGF signaling action in dental tissues. The results obtained show that these molecules are expressed in a wide range of dental cells of both epithelial and mesenchymal origin during early stages of odontogenesis, as well as in nerve fibers that surround the developing tooth germs. At more advanced developmental stages, NGF and TrkA are localized in differentiated cells with secretory capacities such as preameloblasts/ameloblasts secreting enamel matrix and odontoblasts secreting dentine matrix. In contrast, p75^NTR expression is absent from these secretory cells and restricted in proliferating cells of the dental epithelium. The temporospatial distribution of NGF and p75^NTR in fetal human teeth is similar, but not identical, with that observed previously in the developing rodent teeth, thus indicating that the genetic information is well-conserved during evolution. The expression patterns of NGF, p75^NTR, and TrkA during odontogenesis suggest regulatory roles for NGF signaling in proliferation and differentiation of epithelial and mesenchymal cells, as well as in attraction and sprouting of nerve fibers within dental tissues.

Keratoameloblastoma or Kerato-odontoameloblastoma: report of its soft tissue recurrence with literature review

Keratoameloblastoma (KA) is a rare histological variant of the ameloblastoma with extensive keratin production within the odontogenic islands as well as in the fibrous stroma. Pindborg first reported it in 1970, since then only 18 cases have been reported in the literature. We report a soft tissue recurrence of KA, involving right posterior region of the lower jaw in a 27-year-old female.

Diseases caused by mutations in ORAI1 and STIM1

Ca(2+) release-activated Ca(2+) (CRAC) channels mediate a specific form of Ca(2+) influx called store-operated Ca(2+) entry (SOCE) that contributes to the function of many cell types. CRAC channels are composed of ORAI1 proteins located in the plasma membrane, which form its ion-conducting pore. ORAI1 channels are activated by stromal interaction molecule (STIM) 1 and STIM2 located in the endoplasmic reticulum. Loss- and gain-of-function gene mutations in ORAI1 and STIM1 in human patients cause distinct disease syndromes. CRAC channelopathy is caused by loss-of-function mutations in ORAI1 and STIM1 that abolish CRAC channel function and SOCE; it is characterized by severe combined immunodeficiency (SCID)-like disease, autoimmunity, muscular hypotonia, and ectodermal dysplasia, with defects in sweat gland function and dental enamel formation. The latter defect emphasizes an important role of CRAC channels in tooth development. By contrast, autosomal dominant gain-of-function mutations in ORAI1 and STIM1 result in constitutive CRAC channel activation, SOCE, and increased intracellular Ca(2+) levels that are associated with an overlapping spectrum of diseases, including nonsyndromic tubular aggregate myopathy (TAM) and York platelet and Stormorken syndromes. The latter two syndromes are defined, besides myopathy, by thrombocytopenia, thrombopathy, and bleeding diathesis. The fact that myopathy results from both loss- and gain-of-function mutations in ORAI1 and STIM1 highlights the importance of CRAC channels for Ca(2+) homeostasis in skeletal muscle function. The cellular dysfunction and clinical disease spectrum observed in mutant patients provide important information about the molecular regulation of ORAI1 and STIM1 proteins and the role of CRAC channels in human physiology.

Hypoxia increases the expression of enamel genes and cytokines in an ameloblast-derived cell line

The aim of this study was to investigate the effect of hypoxic conditions on the expression of enamel genes and on the secretion of alkaline phosphatase (ALP), lactate dehydrogenase (LDH), cytokines, and interleukins by an ameloblast-derived cell line. Murine ameloblast-derived cells (LS-8 cells) were exposed to 1% oxygen for 24 and 48 h and harvested after 1, 2, 3, and 7 d. The effect of culture in hypoxic conditions on the expression of structural enamel matrix genes and on the secretion of cytokines and interleukins, as well as ALP and LDH, into the cell-culture medium was calculated relative to the expression and secretion of these factors by untreated cells (controls) at each time point. Hypoxia increased expression of the structural enamel matrix genes amelogenin (Amelx), ameloblastin (Ambn), and enamelin (Enam), and the enamel protease matrix metalloproteinase-20 (Mmp20). Expression of hypoxia-inducible factor 1-alpha (Hif1α), and secretion of several vascularization factors and pro-inflammatory factors, were increased after 24 and 48 h of hypoxia. The ALP activity was reduced after 24 and 48 h of hypoxia, whereas the LDH level in the cell-culture medium was higher after 24 h of hypoxic conditions compared with 48 h. In conclusion, hypoxic exposure may disrupt the controlled fine-tuned expression and processing of enamel genes, and promote the secretion of pro-inflammatory factors.

Localization of Core Planar Cell Polarity Proteins, PRICKLEs, in Ameloblasts of Rat Incisors: Possible Regulation of Enamel Rod Decussation

To confirm the possible involvement of planar cell polarity proteins in odontogenesis, one group of core proteins, PRICKLE1, PRICKLE2, PRICKLE3, and PRICKLE4, was examined in enamel epithelial cells and ameloblasts by immunofluorescence microscopy. PRICKLE1 and PRICKLE2 showed similar localization in the proliferation and secretory zones of the incisor. Immunoreactive dots and short rods in ameloblasts and stratum intermedium cells were evident in the proliferation to differentiation zone, but in the secretion zone, cytoplasmic dots decreased and the distal terminal web was positive for PRICKLE1 and PRICKLE2. PRICKLE3 and PRICKLE4 showed cytoplasmic labeling in ameloblasts and other enamel epithelial cells. Double labeling of PRICKLE2 with VANGL1, which is another planar cell polarity protein, showed partial co-localization. To examine the transport route of PRICKLE proteins, PRICKLE1 localization was examined after injection of a microtubule-disrupting reagent, colchicine, and was compared with CX43, which is a membrane protein transported as vesicles via microtubules. The results confirmed the retention of immunoreactive dots for PRICKLE1 in the cytoplasm of secretory ameloblasts of colchicine-injected animals, but fewer dots were observed in control animals. These results suggest that PRICKLE1 and PRICKLE2 are transported as vesicles to the junctional area, and are involved in pattern formation of distal junctional complexes and terminal webs of ameloblasts, further implying a role in the formed enamel rod arrangement.

Dental and Cranial Pathologies in Mice Lacking the Cl(-) /H(+) -Exchanger ClC-7

ClC-7 is a 2Cl(-) /1H(+) -exchanger expressed at late endosomes and lysosomes, as well as the ruffled border of osteoclasts. ClC-7 deficiencies in mice and humans lead to impaired osteoclast function and therefore osteopetrosis. Failure of tooth eruption is also apparent in ClC-7 mutant animals, and this has been attributed to the osteoclast dysfunction and the subsequent defect in alveolar bone resorptive activity surrounding tooth roots. Ameloblasts also express ClC-7, and this study aims to determine the significance of ClC-7 in enamel formation by examining the dentitions of ClC-7 mutant mice. Micro-CT analysis revealed that the molar teeth of 3-week old ClC-7 mutant mice had no roots, and the incisors were smaller than their age-matched controls. Despite these notable developmental differences, the enamel and dentin densities of the mutant mice were comparable to those of the wild-type littermates. Scanning electron microscopy showed normal enamel crystallite and prismatic organization in the ClC-7 mutant mice, although the enamel was thinner (hypoplastic) than in controls. These results suggested that ClC-7 was not critical to enamel and dentin formation, and the observed tooth defects may be related more to a resulting alveolar bone phenotype. Micro-CT analysis also revealed abnormal features in the calvarial bones of the mutant mice. The cranial sutures in ClC-7 mutant mice remained open compared to the closed sutures seen in the control mice at 3 weeks. These data demonstrate that ClC-7 deficiency impacts the development of the dentition and calvaria, but does not significantly disrupt amelogenesis.

Distribution of syndecan-1 protein in developing mouse teeth

Syndecan-1 is a cell surface proteoglycan involved in the regulation of various biological processes such as proliferation, migration, condensation and differentiation of cells, intercellular communication, and morphogenesis. The extracellular domain of syndecan-1 can bind to extracellular matrix components and signaling molecules, while its intracellular domain interacts with cytoskeletal proteins, thus allowing the transfer of information about extracellular environment changes into the cell that consequently affect cellular behavior. Although previous studies have shown syndecan-1 expression during precise stages of tooth development, there is no equivalent study regrouping the expression patterns of syndecan-1 during all stages of odontogenesis. Here we examined the distribution of syndecan-1 protein in embryonic and post-natal developing mouse molars and incisors. Syndecan-1 distribution in mesenchymal tissues such as dental papilla and dental follicle was correlated with proliferating events and its expression was often linked to stem cell niche territories. Syndecan-1 was also expressed in mesenchymal cells that will differentiate into the dentin producing odontoblasts, but not in differentiated functional odontoblasts. In the epithelium, syndecan-1 was detected in all cell layers, by the exception of differentiated ameloblasts that form the enamel. Furthermore, syndecan-1 was expressed in osteoblast precursors and osteoclasts of the alveolar bone that surrounds the developing tooth germs. Taken together these results show the dynamic nature of syndecan-1 expression during odontogenesis and suggest its implication in various processes of tooth development and homeostasis.

MSX2 in ameloblast cell fate and activity

While many effectors have been identified in enamel matrix and cells via genetic studies, physiological networks underlying their expression levels and thus the natural spectrum of enamel thickness and degree of mineralization are now just emerging. Several transcription factors are candidates for enamel gene expression regulation and thus the control of enamel quality. Some of these factors, such as MSX2, are mainly confined to the dental epithelium. MSX2 homeoprotein controls several stages of the ameloblast life cycle. This chapter introduces MSX2 and its target genes in the ameloblast and provides an overview of knowledge regarding its effects in vivo in transgenic mouse models. Currently available in vitro data on the role of MSX2 as a transcription factor and its links to other players in ameloblast gene regulation are considered. MSX2 modulations are relevant to the interplay between developmental, hormonal and environmental pathways and in vivo investigations, notably in the rodent incisor, have provided insight into dental physiology. Indeed, in vivo models are particularly promising for investigating enamel formation and MSX2 function in ameloblast cell fate. MSX2 may be central to the temporal-spatial restriction of enamel protein production by the dental epithelium and thus regulation of enamel quality (thickness and mineralization level) under physiological and pathological conditions. Studies on MSX2 show that amelogenesis is not an isolated process but is part of the more general physiology of coordinated dental-bone complex growth.

Comparison of two mouse ameloblast-like cell lines for enamel-specific gene expression

Ameloblasts are ectoderm-derived cells that produce an extracellular enamel matrix that mineralizes to form enamel. The development and use of immortalized cell lines, with a stable phenotype, is an important contribution to biological studies as it allows for the investigation of molecular activities without the continuous need for animals. In this study we compare the expression profiles of enamel-specific genes in two mouse derived ameloblast-like cell lines: LS8 and ALC cells. Quantitative PCR analysis indicates that, relative to each other, LS8 cells express greater mRNA levels for genes that define secretory-stage activities (Amelx, Ambn, Enam, and Mmp20), while ALC express greater mRNA levels for genes that define maturation-stage activities (Odam and Klk4). Western blot analyses show that Amelx, Ambn, and Odam proteins are detectable in ALC, but not LS8 cells. Unstimulated ALC cells form calcified nodules, while LS8 cells do not. These data provide greater insight as to the suitability of both cell lines to contribute to biological studies on enamel formation and biomineralization, and highlight some of the strengths and weaknesses when relying on enamel epithelial organ-derived cell lines to study molecular activities of amelogenesis.