Excess fluoride interferes with chloride-channel-dependent endocytosis in ameloblasts

Molecular-based phenotype variations in amelogenesis imperfecta

Amelogenesis imperfecta (AI) is one of the typical dental genetic diseases in human. It can occur isolatedly or as part of a syndrome. Previous reports have mainly clarified the types and mechanisms of nonsyndromic AI. This review aimed to compare the phenotypic differences among the hereditary enamel defects with or without syndromes and their underlying pathogenic genes. We searched the articles in PubMed with different strategies or keywords including but not limited to amelogenesis imperfecta, enamel defects, hypoplastic/hypomaturation/hypocalcified, syndrome, or specific syndrome name. The articles with detailed clinical information about the enamel and other phenotypes and clear genetic background were used for the analysis. We totally summarized and compared enamel phenotypes of 18 nonsyndromic AI with 17 causative genes and 19 syndromic AI with 26 causative genes. According to the clinical features, radiographic or ultrastructural changes in enamel, the enamel defects were basically divided into hypoplastic and hypomineralized (hypomaturated and hypocalcified) and presented a higher heterogeneity which were closely related to the involved pathogenic genes, types of mutation, hereditary pattern, X chromosome inactivation, incomplete penetrance, and other mechanisms.The gene-specific enamel phenotypes could be an important indicator for diagnosing nonsyndromic and syndromic AI.

Exogenous transforming growth factor-β1 prevents the inflow of fluoride to ameleoblasts through regulation of voltage-gated chloride channels 5 and 7

Dental fluorosis is a global issue. Although there are multiple causes of dental fluorosis, the precise mechanism remains controversial. Previous studies have demonstrated that extracellular fluoride may promote an accumulation of fluoride ions in ameloblasts, which may induce oxidative and endoplasmic reticulum stresses, leading to dental fluorosis. However, the exact process by which fluoride ions enter cells has not been determined. In the present study, intracellular fluoride concentration was determined using a newly developed specific fluorescent probe called probe 1. Under high extracellular fluoride concentrations, the fluorescence intensity of the ameloblasts increased, however, exogenous transforming growth factor-β1 (TGF-β1) was able to inhibit the increase. Furthermore, changes in the expression of the voltage-gated chloride channels 5 and 7 (ClC5 and ClC-7), which are responsible for the transport of fluoride were investigated. The results indicated that fluoride reduced the expression of endogenous TGF-β1 and increased the expression of ClC-5 and ClC-7. Additionally, exogenous TGF-β1 reduced the expression of ClC-5 and ClC-7. The results of the present study indicate that exogenous TGF-β1 may prevent accumulation of fluoride in ameloblasts through the regulation of ClC-5 and ClC-7 under high extracellular fluoride concentrations.

Fluoride exposure alters Ca2+ signaling and mitochondrial function in enamel cells

Fluoride ions are highly reactive, and their incorporation in forming dental enamel at low concentrations promotes mineralization. In contrast, excessive fluoride intake causes dental fluorosis, visually recognizable enamel defects that can increase the risk of caries. To investigate the molecular bases of dental fluorosis, we analyzed the effects of fluoride exposure in enamel cells to assess its impact on Ca2+ signaling. Primary enamel cells and an enamel cell line (LS8) exposed to fluoride showed decreased internal Ca2+ stores and store-operated Ca2+ entry (SOCE). RNA-sequencing analysis revealed changes in gene expression suggestive of endoplasmic reticulum (ER) stress in fluoride-treated LS8 cells. Fluoride exposure did not alter Ca2+ homeostasis or increase the expression of ER stress-associated genes in HEK-293 cells. In enamel cells, fluoride exposure affected the functioning of the ER-localized Ca2+ channel IP3R and the activity of the sarco-endoplasmic reticulum Ca2+-ATPase (SERCA) pump during Ca2+ refilling of the ER. Fluoride negatively affected mitochondrial respiration, elicited mitochondrial membrane depolarization, and disrupted mitochondrial morphology. Together, these data provide a potential mechanism underlying dental fluorosis.

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.

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 overview of channels, transporters, and calcium signaling molecules during amelogenesis

Enamel is a highly calcified tissue. Its formation requires a progressive and dynamic system for the regulation of electrolyte concentration by enamel epithelia. A critical function of enamel epithelial cells, ameloblasts, is the secretion and movement of electrolytes via various channels and transporters to develop the enamel tissue. Enamel formation generates protons, which need to be neutralised. Thus, ameloblasts possess a buffering system to sustain mineral accretion. Normal tooth formation involves stage-dependent net fluctuations in pH during amelogenesis. To date, all of our information about ion transporters in dental enamel tissue is based solely on immunostaining-expression techniques. This review critically evaluates the current understanding and recent discoveries and physiological role of ion channels and transporters, Mg²⁺ transporters, and Ca²⁺ regulatory proteins during amelogenesis in enamel formation. The ways in which ameloblasts modulate ions are discussed in the context of current research for developing a novel morphologic-functional model of enamel maturation.

Mutant Runx2 regulates amelogenesis and osteogenesis through a miR-185-5p-Dlx2 axis

Regulation of microRNAs (miRNA) has been extensively investigated in diseases; however, little is known about the roles of miRNAs in cleidocranial dysplasia (CCD). The aim of the present study was to investigate the potential involvement of miRNAs in CCD. In vitro site-directed mutagenesis was performed to construct three mutant Runx2 expression vectors, which were then transfected into LS8 cells and MC3T3-E1 cells, to determine the impact on amelogenesis and osteogenesis, respectively. miRCURY LNA miRNA microarray identify miR-185-5p as a miRNA target commonly induced by all three Runx2 mutants. Real-time quantitative PCR was applied to determine the expression of miR-185-5p and Dlx2 in samples. Dual-luciferase reporter assays were conducted to confirm Dlx2 as a legitimate target of miR-185-5p. The suppressive effect of miR-185-5p on amelogenesis and osteogenesis of miR-185-5p was evaluated by RT-PCR and western blot examination of Amelx, Enam, Klk4, and Mmp20 gene and protein expression, and by Alizarin Red stain. We found that mutant Runx2 suppressed amelogenesis and osteogenesis. miR-185-5p, induced by Runx2, suppressed amelogenesis and osteogenesis. Furthermore, we identified Dlx2 as direct target of miR-185-5p. Consistently, Dlx2 expression was inversely correlated with miR-185-5p levels. This study highlights the molecular etiology and significance of miR-185-5p in CCD, and suggests that targeting miR-185-5p may represent a new therapeutic strategy in prevention or intervention of CCD.

Bicarbonate Transport During Enamel Maturation

Amelogenesis (tooth enamel formation) is a biomineralization process consisting primarily of two stages (secretory stage and maturation stage) with unique features. During the secretory stage, the inner epithelium of the enamel organ (i.e., the ameloblast cells) synthesizes and secretes enamel matrix proteins (EMPs) into the enamel space. The protein-rich enamel matrix forms a highly organized architecture in a pH-neutral microenvironment. As amelogenesis transitions to maturation stage, EMPs are degraded and internalized by ameloblasts through endosomal-lysosomal pathways. Enamel crystallite formation is initiated early in the secretory stage, however, during maturation stage the more rapid deposition of calcium and phosphate into the enamel space results in a rapid expansion of crystallite length and mineral volume. During maturation-stage amelogenesis, the pH value of enamel varies considerably from slightly above neutral to acidic. Extracellular acid-base balance during enamel maturation is tightly controlled by ameloblast-mediated regulatory networks, which include significant synthesis and movement of bicarbonate ions from both the enamel papillary layer cells and ameloblasts. In this review we summarize the carbonic anhydrases and the carbonate transporters/exchangers involved in pH regulation in maturation-stage amelogenesis. Proteins that have been shown to be instrumental in this process include CA2, CA6, CFTR, AE2, NBCe1, SLC26A1/SAT1, SLC26A3/DRA, SLC26A4/PDS, SLC26A6/PAT1, and SLC26A7/SUT2. In addition, we discuss the association of miRNA regulation with bicarbonate transport in tooth enamel formation.

Deletion of Slc26a1 and Slc26a7 Delays Enamel Mineralization in Mice

Amelogenesis features two major developmental stages—secretory and maturation. During maturation stage, hydroxyapatite deposition and matrix turnover require delicate pH regulatory mechanisms mediated by multiple ion transporters. Several members of the Slc26 gene family (Slc26a1, Slc26a3, Slc26a4, Slc26a6, and Slc26a7), which exhibit bicarbonate transport activities, have been suggested by previous studies to be involved in maturation-stage amelogenesis, especially the key process of pH regulation. However, details regarding the functional role of these genes in enamel formation are yet to be clarified, as none of the separate mutant animal lines demonstrates any discernible enamel defects. Continuing with our previous investigation of Slc26a1^−/− and Slc26a7^−/− animal models, we generated a double-mutant animal line with the absence of both Slc26a1 and Slc26a7. We showed in the present study that the double-mutant enamel density was significantly lower in the regions that represent late maturation-, maturation- and secretory-stage enamel development in wild-type mandibular incisors. However, the “maturation” and “secretory” enamel microstructures in double-mutant animals resembled those observed in wild-type secretory and/or pre-secretory stages. Elemental composition analysis revealed a lack of mineral deposition and an accumulation of carbon and chloride in double-mutant enamel. Deletion of Slc26a1 and Slc26a7 did not affect the stage-specific morphology of the enamel organ. Finally, compensatory expression of pH regulator genes and ion transporters was detected in maturation-stage enamel organs of double-mutant animals when compared to wild-type. Combined with the findings from our previous study, these data indicate the involvement of SLC26A1and SLC26A7 as key ion transporters in the pH regulatory network during enamel maturation.

SLC26A Gene Family Participate in pH Regulation during Enamel Maturation

The bicarbonate transport activities of Slc26a1, Slc26a6 and Slc26a7 are essential to physiological processes in multiple organs. Although mutations of Slc26a1, Slc26a6 and Slc26a7 have not been linked to any human diseases, disruption of Slc26a1, Slc26a6 or Slc26a7 expression in animals causes severe dysregulation of acid-base balance and disorder of anion homeostasis. Amelogenesis, especially the enamel formation during maturation stage, requires complex pH regulation mechanisms based on ion transport. The disruption of stage-specific ion transporters frequently results in enamel pathosis in animals. Here we present evidence that Slc26a1, Slc26a6 and Slc26a7 are highly expressed in rodent incisor ameloblasts during maturation-stage tooth development. In maturation-stage ameloblasts, Slc26a1, Slc26a6 and Slc26a7 show a similar cellular distribution as the cystic fibrosis transmembrane conductance regulator (Cftr) to the apical region of cytoplasmic membrane, and the distribution of Slc26a7 is also seen in the cytoplasmic/subapical region, presumably on the lysosomal membrane. We have also examined Slc26a1 and Slc26a7 null mice, and although no overt abnormal enamel phenotypes were observed in Slc26a1-/- or Slc26a7-/- animals, absence of Slc26a1 or Slc26a7 results in up-regulation of Cftr, Ca2, Slc4a4, Slc4a9 and Slc26a9, all of which are involved in pH homeostasis, indicating that this might be a compensatory mechanism used by ameloblasts cells in the absence of Slc26 genes. Together, our data show that Slc26a1, Slc26a6 and Slc26a7 are novel participants in the extracellular transport of bicarbonate during enamel maturation, and that their functional roles may be achieved by forming interaction units with Cftr.

Fluorescent sensing of fluoride in cellular system

Fluoride ions have the important roles in a lot of physiological activities related with biological and medical system, such as water fluoridation, caries treatment, and bone disease treatment. Great efforts have been made to develop new methods and strategies for F(-) detection in the past decades. Traditional methods for the detection of F(-) including ion chromatography, ion-selective electrodes, and spectroscopic techniques have the limitations in the biomedicine research. The fluorescent probes for F(-) are very promising that overcome some drawbacks of traditional fluoride detection methods. These probes exhibit high selectivity, high sensitivity as well as quick response to the detection of fluoride anions. The review commences with a brief description of photophysical mechanisms for fluorescent probes for fluoride, including photo induced electron transfer (PET), intramolecular charge transfer (ICT), fluorescence resonance energy transfer (FRET), and excited-state intramolecular proton transfer (ESIPT). Followed by a discussion about common dyes for fluorescent fluoride probes, such as anthracene, naphalimide, pyrene, BODIPY, fluorescein, rhodamine, resorufin, coumarin, cyanine, and near-infrared (NIR) dyes. We divide the fluorescent probes for fluoride in cellular application systems into nine groups, for example, type of hydrogen bonds, type of cleavage of Si-O bonds, type of Si-O bond cleavage and cylization reactions, etc. We also review the recent reported carriers in the delivery of fluorescent fluoride probes. Seventy-four typical fluorescent fluoride probes are listed and compared in detail, including quantum yield, reaction medium, excitation and emission wavelengths, linear detection range, selectivity for F(-), mechanism, and analytical applications. Finally, we discuss the future challenges of the application of fluorescent fluoride probes in cellular system and in vivo. We wish that more and more excellent fluorescent fluoride probes will be developed and applied in the biomedicine field in the future.

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.

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.

Ion channels, channelopathies, and tooth formation

The biological functions of ion channels in tooth development vary according to the nature of their gating, the species of ions passing through those gates, the number of gates, localization of channels, tissue expressing the channel, and interactions between cells and microenvironment. Ion channels feature unique and specific ion flux in ameloblasts, odontoblasts, and other tooth-specific cell lineages. Both enamel and dentin have active chemical systems orchestrating a variety of ion exchanges and demineralization and remineralization processes in a stage-dependent manner. An important role for ion channels is to regulate and maintain the calcium and pH homeostasis that are critical for proper enamel and dentin biomineralization. Specific functions of chloride channels, TRPVs, calcium channels, potassium channels, and solute carrier superfamily members in tooth formation have been gradually clarified in recent years. Mutations in these ion channels or transporters often result in disastrous changes in tooth development. The channelopathies of tooth include altered eruption (CLCN7, KCNJ2, TRPV3), root dysplasia (CLCN7, KCNJ2), amelogenesis imperfecta (KCNJ1, CFTR, AE2, CACNA1C, GJA1), dentin dysplasia (CLCN5), small teeth (CACNA1C, GJA1), tooth agenesis (CLCN7), and other impairments. The mechanisms leading to tooth channelopathies are primarily related to pH regulation, calcium homeostasis, or other alterations of the niche for tooth eruption and development.