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

High-resolution Raman spectroscopy reveals compositional differences between pigmented incisor enamel and unpigmented molar enamel in Rattus norvegicus

Dental enamel is a peculiar biological tissue devoid of any self-renewal capacity as opposed to bone. Thus, a thorough understanding of enamel composition is essential to develop novel strategies for dental enamel repair. While the mineral found in bone and dental enamel is generally viewed as the biologically-produced equivalent of hydroxy(l)apatite, the formation of these bioapatites is controlled by different organic matrix frameworks-mainly type-I collagen in bone and amelogenin in enamel. In lower vertebrates, such as rodents, two distinct types of enamel are produced. Iron-containing pigmented enamel protects the continuously growing incisor teeth while magnesium-rich unpigmented enamel covers the molar teeth. Using high-resolution Raman spectroscopy, scanning electron microscopy, and energy dispersive X-ray spectroscopy, this work explores the differences in acid phosphate (HPO₄^2-), carbonate (CO₃^2-), hydroxyl (OH^-), iron, and magnesium content of pigmented incisor enamel and unpigmented molar enamel of Sprague Dawley rats. Bundles of hydroxy(l)apatite nanowires comprise the enamel prisms, where prisms in pigmented enamel are wider and longer than those in unpigmented molars. In contrast to magnesium-rich unpigmented enamel, higher mineral crystallinity, and higher HPO₄^2- and OH^- levels are hallmark features of iron-rich pigmented enamel. Furthermore, the apparent absence of iron oxides or oxy(hydroxides) indicates that iron is introduced into the apatite lattice at the expense of calcium, albeit in amounts that do not alter the Raman signatures of the PO₄^3- internal modes. Compositional idiosyncrasies of iron-rich pigmented and nominally iron-free unpigmented enamel offer new insights into enamel biomineralisation supporting the notion that, in rodents, ameloblast function differs significantly between the incisors and the molars.

Sulphur dioxide and fluoride co-exposure cause enamel damage by disrupting the Cl-/HCO3- ion transport

OBJECTIVE

Although there is growing evidence linking the exposure to sulphur dioxide (SO₂) and fluoride to human diseases, there is little data on the co-exposure of SO₂ and fluoride. Moreover, literature on SO₂ and fluoride co-exposure to enamel damage is insufficient. In this work, we concentrate on the concurrent environmental issues of excessive SO₂ and fluoride in several coal-consuming regions.

METHOD

To identify the toxicity of SO₂ and fluoride exposure either separately or together, we used both ICR mice and LS8 cells, and factorial design was employed to assess the type of potential combined action.

RESULT

In this study, co-exposure to SO₂ and fluoride exacerbated enamel damage, resulting in more severe enamel defects of incisor and the damage occurred earlier. Cl^-/HCO₃^- exchanger expression is increased by SO₂ and fluoride in mouse incisor. Consistent with in vivo results, co-exposure of SO₂ and fluoride decreased pHi and increased [Cl^-]i level by increasing the expression of the Cl^-/HCO₃^- exchanger in LS8 cells. Furthermore, SO₂ and F may increase merlin protein expression, and merlin deficiency causes AE2 expression to decrease in vitro.

CONCLUSION

Overall, these results indicate that co-exposure to SO₂ and fluoride may result in more toxicity both in vitro and in vivo than a single exposure to SO₂ and fluoride, suggesting that residents in areas contaminated with SO₂ and fluoride may be more likely to suffer enamel damage.

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.

Highly acidic pH facilitates enamel protein self-assembly, apatite crystal growth and enamel protein interactions in the early enamel matrix

Tooth enamel develops within a pH sensitive amelogenin-rich protein matrix. The purpose of the present study is to shed light on the intimate relationship between enamel matrix pH, enamel protein self-assembly, and enamel crystal growth during early amelogenesis. Universal indicator dye staining revealed highly acidic pH values (pH 3-4) at the exocytosis site of secretory ameloblasts. When increasing the pH of an amelogenin solution from pH 5 to pH 7, there was a gradual increase in subunit compartment size from 2 nm diameter subunits at pH 5 to a stretched configuration at pH6 and to 20 nm subunits at pH 7. HSQC NMR spectra revealed that the formation of the insoluble amelogenin self-assembly structure at pH6 was critically mediated by at least seven of the 11 histidine residues of the amelogenin coil domain (AA 46-117). Comparing calcium crystal growth on polystyrene plates, crystal length was more than 20-fold elevated at pH 4 when compared to crystals grown at pH 6 or pH 7. To illustrate the effect of pH on enamel protein self-assembly at the site of initial enamel formation, molar teeth were immersed in phosphate buffer at pH4 and pH7, resulting in the formation of intricate berry tree-like assemblies surrounding initial enamel crystal assemblies at pH4 that were not evident at pH7 nor in citrate buffer. Amelogenin and ameloblastin enamel proteins interacted at the secretory ameloblast pole and in the initial enamel layer, and co-immunoprecipitation studies revealed that this amelogenin/ameloblastin interaction preferentially takes place at pH 4-pH 4.5. Together, these studies highlight the highly acidic pH of the very early enamel matrix as an essential contributing factor for enamel protein structure and self-assembly, apatite crystal growth, and enamel protein interactions.

Manifestation and Mechanisms of Abnormal Mineralization in Teeth

Tooth biomineralization is a dynamic and complicated process influenced by local and systemic factors. Abnormal mineralization in teeth occurs when factors related to physiologic mineralization are altered during tooth formation and after tooth maturation, resulting in microscopic and macroscopic manifestations. The present Review provides timely information on the mechanisms and structural alterations of different forms of pathological tooth mineralization. A comprehensive study of these alterations benefits diagnosis and biomimetic treatment of abnormal mineralization in patients.

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.

TRPM7-Mediated Calcium Transport in HAT-7 Ameloblasts

TRPM7 plays an important role in cellular Ca²⁺, Zn²⁺ and Mg²⁺ homeostasis. TRPM7 channels are abundantly expressed in ameloblasts and, in the absence of TRPM7, dental enamel is hypomineralized. The potential role of TRPM7 channels in Ca²⁺ transport during amelogenesis was investigated in the HAT-7 rat ameloblast cell line. The cells showed strong TRPM7 mRNA and protein expression. Characteristic TRPM7 transmembrane currents were observed, which increased in the absence of intracellular Mg²⁺ ([Mg²⁺]_i), were reduced by elevated [Mg²⁺]_i, and were inhibited by the TRPM7 inhibitors NS8593 and FTY720. Mibefradil evoked similar currents, which were suppressed by elevated [Mg²⁺]_i, reducing extracellular pH stimulated transmembrane currents, which were inhibited by FTY720. Naltriben and mibefradil both evoked Ca²⁺ influx, which was further enhanced by the acidic intracellular conditions. The SOCE inhibitor BTP2 blocked Ca²⁺ entry induced by naltriben but not by mibefradil. Thus, in HAT-7 cells, TRPM7 may serves both as a potential modulator of Orai-dependent Ca²⁺ uptake and as an independent Ca²⁺ entry pathway sensitive to pH. Therefore, TRPM7 may contribute directly to transepithelial Ca²⁺ transport in amelogenesis.

Overexpression of miR-1306-5p, miR-3195, and miR-3914 Inhibits Ameloblast Differentiation through Suppression of Genes Associated with Human Amelogenesis Imperfecta

Amelogenesis imperfecta is a congenital form of enamel hypoplasia. Although a number of genetic mutations have been reported in humans, the regulatory network of these genes remains mostly unclear. To identify signatures of biological pathways in amelogenesis imperfecta, we conducted bioinformatic analyses on genes associated with the condition in humans. Through an extensive search of the main biomedical databases, we found 56 genes in which mutations and/or association/linkage were reported in individuals with amelogenesis imperfecta. These candidate genes were further grouped by function, pathway, protein-protein interaction, and tissue-specific expression patterns using various bioinformatic tools. The bioinformatic analyses highlighted a group of genes essential for extracellular matrix formation. Furthermore, advanced bioinformatic analyses for microRNAs (miRNAs), which are short non-coding RNAs that suppress target genes at the post-transcriptional level, predicted 37 candidates that may be involved in amelogenesis imperfecta. To validate the miRNA-gene regulation association, we analyzed the target gene expression of the top seven candidate miRNAs: miR-3195, miR-382-5p, miR-1306-5p, miR-4683, miR-6716-3p, miR-3914, and miR-3935. Among them, miR-1306-5p, miR-3195, and miR-3914 were confirmed to regulate ameloblast differentiation through the regulation of genes associated with amelogenesis imperfecta in AM-1 cells, a human ameloblastoma cell line. Taken together, our study suggests a potential role for miRNAs in amelogenesis imperfecta.

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.