Ca2+ transport and signalling in enamel cells

Association of defects of enamel with polymorphisms in the vitamin D receptor and parathyroid hormone genes

This cross-sectional study aimed to investigate the association between developmental defects of enamel (DDE) and single nucleotide polymorphisms (SNPs) in the genes encoding the vitamin D receptor (VDR) and parathyroid hormone (PTH). Orthodontic patients receiving treatment at a dental school were selected through convenience sampling. Intra-oral photographs were used to assess DDE, which were classified according to the criteria proposed by Ghanim et al. (2015) by a single calibrated examiner (Kappa>0.80). Enamel hypoplasia, molar-incisor hypomineralization (MIH), hypomimineralized second primary molar (HSPM), and non-MIH/HSPM demarcated opacities were considered for the analysis. Genomic DNA was extracted from buccal cells. The SNPs in VDR (rs7975232) and PHT (rs694, rs6256, and rs307247) were genotyped using real-time polymerase chain reactions (PCR). Statistical analyses were performed using the PLINK software (version 1.03, designed by Shaun Purcell, EUA). Chi-square or Fisher's exact tests were performed at a significance level of 5%. Ninety-one (n=91) patients (49 females and 42 males) (mean age of 14.1±5.8 years) were included. The frequency of DDE was 38.5% (35 patients). Genotype distributions were in Hardy-Weinberg equilibrium. No significant statistical association was found between DDE and the SNPs evaluated. A borderline association (p=0.09) was observed between DDE and the CC haplotype for SNP rs7975232 in VDR. In conclusion, the selected SNPs in VDR and PTH genes were not associated with DDE in the studied samples.

Natal factors affecting developmental defects of enamel in preterm infants: a prospective cohort study

This study investigated natal factors influencing developmental defects of enamel (DDE) in premature infants using a newly refined preterm developmental defects of enamel (PDDE) index. Dental examinations were conducted on a cohort of 118 preterm infants (average age 3.5 ± 1.4 years) to record PDDE scores, while reviewing their medical records to examine natal factors. According to the logistic regression analysis, factors related to DDE prevalence were advanced maternal age, gestational age < 28 weeks, birth weight < 1000 g, 1 min APGAR score < 7, and hospitalization period > 2 months, which were significantly higher by 2.91, 5.53, 7.63, 10.02, and 4.0 times, respectively. According to regression analysis with generalized linear models, the PDDE scores were approximately 7.65, 4.96, and 15.0 points higher in premature children diagnosed with bronchopulmonary dysplasia, intraventricular hemorrhage, and necrotizing enterocolitis, respectively. When endotracheal intubation was performed, the PDDE score was 5.06 points higher. The prevalence of PDDE was primarily observed bilaterally in the maxillary anterior teeth. Extremely preterm infants showed significantly delayed tooth eruption, suggesting that the influence of gestational age on dental development rates. Identifying the factors related to DDE in premature children can inform early dental interventions to support the oral health of high-risk children.

Na+/Ca2+ exchange in enamel cells is dominated by the K+-dependent NCKX exchanger

Calcium (Ca2+) extrusion is an essential function of the enamel-forming ameloblasts, providing Ca2+ for extracellular mineralization. The plasma membrane Ca2+ ATPases (PMCAs) remove cytosolic Ca2+ (cCa2+) and were recently shown to be efficient when ameloblasts experienced low cCa2+ elevation. Sodium-calcium (Na+/Ca2+) exchange has higher capacity to extrude cCa2+, but there is limited evidence on the function of the two main families of Na+/Ca2+ exchangers in enamel formation. The purpose of this study was to analyze the function of the NCX (coded by SLC8) and the K+-dependent NCKX (coded by SLC24) exchangers in rat ameloblasts and to compare their efficacy in the two main stages of enamel formation: the enamel forming secretory stage and the mineralizing or maturation stage. mRNA expression profiling confirmed the expression of Slc8 and Slc24 genes in enamel cells, Slc24a4 being the most highly upregulated transcript during the maturation stage, when Ca2+ transport increases. Na+/Ca2+ exchange was analyzed in the Ca2+ influx mode in Fura-2 AM-loaded ameloblasts. We show that maturation-stage ameloblasts have a higher Na+/Ca2+ exchange capacity than secretory-stage cells. We also show that Na+/Ca2+ exchange in both stages is dominated by NCKX over NCX. The importance of NCKX function in ameloblasts may partly explain why mutations in the SLC24A4 gene, but not in SLC8 genes, result in enamel disease. Our results demonstrate that Na+/Ca2+ exchangers are fully operational in ameloblasts and that their contribution to Ca2+ homeostasis increases in the maturation stage, when Ca2+ transport need is higher.

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.

Sodium/(calcium + potassium) exchanger NCKX4 optimizes KLK4 activity in the enamel matrix microenvironment to regulate ECM modeling

Enamel development is a process in which extracellular matrix models from a soft proteinaceous matrix to the most mineralized tissue in vertebrates. Patients with mutant NCKX4, a gene encoding a K+-dependent Na+/Ca2+-exchanger, develop a hypomineralized and hypomature enamel. How NCKX4 regulates enamel protein removal to achieve an almost protein-free enamel is unknown. We characterized the upregulation pattern of Nckx4 in the progressively differentiating enamel-forming ameloblasts by qPCR, and as well as confirmed NCKX4 protein to primarily localize at the apical surface of wild-type ruffle-ended maturation ameloblasts by immunostaining of the continuously growing mouse incisors, posing the entire developmental trajectory of enamel. In contrast to the normal mature enamel, where ECM proteins are hydrolyzed and removed, we found significant protein retention in the maturation stage of Nckx4 -/- mouse enamel. The Nckx4 -/- enamel held less Ca2+ and K+ but more Na+ than the Nckx4 +/+ enamel did, as measured by EDX. The alternating acidic and neutral pH zones at the surface of mineralizing Nckx4 +/+ enamel were replaced by a largely neutral pH matrix in the Nckx4 -/- enamel. In situ zymography revealed a reduced kallikrein-related peptidase 4 (KLK4) activity in the Nckx4 -/- enamel. We showed that KLK4 took on 90% of proteinase activity in the maturation stage of normal enamel, and that recombinant KLK4 as well as native mouse enamel KLK4 both performed less effectively in a buffer with increased [Na+] and pH, conditions found in the Nckx4 -/- developing enamel. This study, for the first time to our knowledge, provides evidence demonstrating the impaired in situ KLK4 activity in Nckx4 -/- enamel and suggests a novel function of NCKX4 in facilitating KLK4-mediated hydrolysis and removal of ECM proteins, warranting the completion of enamel matrix modeling.

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.

Crucial Role of microRNAs as New Targets for Amelogenesis Disorders Detection

INTRODUCTION

Amelogenesis imperfecta (AI) refers to a heterogeneous group of conditions with multiple factors which contribute to the hypomineralisation of enamel. Preventive measures are necessary to predict this pathology. Prospects for preventive medicine are closely related to the search for new informative methods for diagnosing a human disease. MicroRNAs are prominent for the non-invasive diagnostic platform.

THE AIM OF THE STUDY

The aim of the review is to review the heterogeneous factors involved in amelogenesis and to select the microRNA panel associated with the AI type.

METHODS

We used DIANA Tools (algorithms, databases and software) for interpreting and archiving data in a systematic framework ranging from the analysis of expression regulation from deep sequencing data to the annotation of miRNA regulatory elements and targets (https://dianalab. e-ce.uth.gr/). In our study, based on a gene panel associated with the AI types, twenty-four miRNAs were identified for the hypoplastic type (supplement), thirty-five for hypocalcified and forty-- nine for hypomaturation AI. The selection strategy included the microRNA search with multiple targets using the AI type's gene panel.

RESULTS

Key proteins, calcium-dependent and genetic factors were analysed to reveal their role in amelogenesis. The role of extracellular non-coding RNA sequences with multiple regulatory functions seems to be the most attractive. We chose the list of microRNAs associated with the AI genes. We found four microRNAs (hsa-miR-27a-3p, hsa-miR-375, hsa-miR-16-5p and hsamiR- 146a-5p) for the gene panel, associated with the hypoplastic type of AI; five microRNAs (hsa- miR-29c-3p, hsa-miR-124-3p, hsa-miR-1343-3p, hsa-miR-335-5p, and hsa-miR-16-5p - for hypocalcified type of AI, and seven ones (hsa-miR-124-3p, hsa-miR-147a, hsa-miR-16-5p, hsamiR- 429, hsa-let-7b-5p, hsa-miR-146a-5p, hsa-miR-335-5p) - for hypomaturation. It was revealed that hsa-miR-16-5p is included in three panels specific for both hypoplastic, hypocalcified, and hypomaturation types. Hsa-miR-146a-5p is associated with hypoplastic and hypomaturation type of AI, which is associated with the peculiarities of the inflammatory response immune response. In turn, hsa-miR-335-5p associated with hypocalcified and hypomaturation type of AI.

CONCLUSION

Liquid biopsy approaches are a promising way to reduce the economic cost of treatment for these patients in modern healthcare. Unique data exist about the role of microRNA in regulating amelogenesis. The list of microRNAs that are associated with AI genes and classified by AI types has been uncovered. The target gene analysis showed the variety of functions of selected microRNAs, which explains the multiple heterogeneous mechanisms in amelogenesis. Predisposition to mineralisation problems is a programmed event. Many factors determine the manifestation of this problem. Additionally, it is necessary to remember the variable nature of the changes, which reduces the prediction accuracy. Therefore, models based on liquid biopsy and microRNAs make it possible to take into account these factors and their influence on the mineralisation. The found data needs further investigation.

PMCA Ca2+ clearance in dental enamel cells depends on the magnitude of cytosolic Ca2

Enamel formation (amelogenesis) is a two-step process whereby crystals partially grow during the secretory stage followed by a significant growth expansion during the maturation stage concurrent with an increase in vectorial Ca2+ transport. This requires tight regulation of cytosolic Ca2+ (c Ca2+ ) concentration in the enamel forming ameloblasts by controlling Ca2+ influx (entry) and Ca2+ extrusion (clearance). Gene and protein expression studies suggest that the plasma membrane Ca2+ -ATPases (PMCA1-4) are likely involved in c Ca2+ extrusion in ameloblasts, yet no functional analysis of these pumps has been reported nor whether their activity changes across amelogenesis. PMCAs have high Ca2+ affinity and low Ca2+ clearance which may be a limiting factor in their contribution to enamel formation as maturation stage ameloblasts handle high Ca2+ loads. We analyzed PMCA function in rat secretory and maturation ameloblasts by blocking or potentiating these pumps. Low/moderate elevations in c Ca2+ measured using the Ca2+ probe Fura-2-AM show that secretory ameloblasts clear Ca2+ faster than maturation stage cells through PMCAs. This process was completely inhibited by an external alkaline (pH 9.0) solution or was significantly delayed by the PMCA blockers vanadate and caloxin 1b1. Eliciting higher c Ca2+ transients via the activation of the ORAI1 Ca2+ channel showed that the PMCAs of maturation ameloblasts were more efficient. Inhibiting PMCAs decreased the rate of Ca2+ influx via ORAI1 but potentiation with forskolin had no effect. Our findings suggest that PMCAs are functional Ca2+ pumps during amelogenesis regulating c Ca2+ upon low and/or moderate Ca2+ stimulus in secretory stage, thus participating in amelogenesis.

Identification by whole-exome sequencing of new single-nucleotide polymorphisms associated with molar-incisor hypomineralisation among the Lebanese population

OBJECTIVE

Molar-incisor hypomineralization (MIH) is a developmental qualitative enamel defect, causing a worldwide challenging dental problem. The etiology of this defect remains unclear. Here we identify by whole-exome sequencing (WES) new single-nucleotide polymorphisms (SNPs) in genes expressed during enamel mineralization and in those modulating prenatal, natal and postnatal risk factors among the Lebanese MIH children: immune system and xenobiotic detoxification.

DESIGN

Dental examination for MIH was performed based on the MIH index for diagnostic criteria. Saliva samples were collected from 37 non-related, MIH-diagnosed subjects for DNA extraction. WES was performed on the Illumina HiSeq2000 platform. The χ² test and Fisher's exact test were used to determine relationship between SNPs frequencies and MIH. OR and its 95% CI were used to report the strength of association. The significance threshold was set at 0.05.

RESULTS

Among the Lebanese population, 37 SNPs presented a significant association with MIH in the following genes: AMTN, MMP-20, STIM1, STIM2, ORAI1, SLC34A2, SLC34A3, VDR, PVALB, HSP90B1, TRPM7, SLC24A4, CA6, SLC4A2, TNFRSF11A, IL10RB, ARNT, ESR1 and CYP1B1.

CONCLUSION

This is the first WES study conducted in patients with MIH. Yet, interactions between polymorphisms in different gene categories are to be investigated for a better assessment of MIH susceptibility.

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.

Organoids from human tooth showing epithelial stemness phenotype and differentiation potential

Insight into human tooth epithelial stem cells and their biology is sparse. Tissue-derived organoid models typically replicate the tissue’s epithelial stem cell compartment. Here, we developed a first-in-time epithelial organoid model starting from human tooth. Dental follicle (DF) tissue, isolated from unerupted wisdom teeth, efficiently generated epithelial organoids that were long-term expandable. The organoids displayed a tooth epithelial stemness phenotype similar to the DF’s epithelial cell rests of Malassez (ERM), a compartment containing dental epithelial stem cells. Single-cell transcriptomics reinforced this organoid-ERM congruence, and uncovered novel, mouse-mirroring stem cell features. Exposure of the organoids to epidermal growth factor induced transient proliferation and eventual epithelial-mesenchymal transition, highly mimicking events taking place in the ERM in vivo. Moreover, the ERM stemness organoids were able to unfold an ameloblast differentiation process, further enhanced by transforming growth factor-β (TGFβ) and abrogated by TGFβ receptor inhibition, thereby reproducing TGFβ's known key position in amelogenesis. Interestingly, by creating a mesenchymal-epithelial composite organoid (assembloid) model, we demonstrated that the presence of dental mesenchymal cells (i.e. pulp stem cells) triggered ameloblast differentiation in the epithelial stem cells, thus replicating the known importance of mesenchyme-epithelium interaction in tooth development and amelogenesis. Also here, differentiation was abrogated by TGFβ receptor inhibition. Together, we developed novel organoid models empowering the exploration of human tooth epithelial stem cell biology and function as well as their interplay with dental mesenchyme, all at present only poorly defined in humans. Moreover, the new models may pave the way to future tooth-regenerative perspectives.

Mitochondria modulate ameloblast Ca2+ signaling

The role of mitochondria in enamel, the most mineralized tissue in the body, is poorly defined. Enamel is formed by ameloblast cells in two main sequential stages known as secretory and maturation. Defining the physiological features of each stage is essential to understand mineralization. Here, we analyzed functional features of mitochondria in rat primary secretory and maturation-stage ameloblasts focusing on their role in Ca²⁺ signaling. Quantification of the Ca²⁺ stored in the mitochondria by trifluoromethoxy carbonylcyanide phenylhydrazone stimulation was comparable in both stages. The release of endoplasmic reticulum Ca²⁺ pools by adenosine triphosphate in rhod2AM-loaded cells showed similar mitochondrial Ca²⁺ (_m Ca²⁺ ) uptake. However, _m Ca²⁺ extrusion via Na⁺ -Li⁺ -Ca²⁺ exchanger was more prominent in maturation. To address if _m Ca²⁺ uptake via the mitochondrial Ca²⁺ uniporter (MCU) played a role in cytosolic Ca²⁺ (_c Ca²⁺ ) buffering, we stimulated Ca²⁺ influx via the store-operated Ca²⁺ entry (SOCE) and blocked MCU with the inhibitor Ru265. This inhibitor was first tested using the enamel cell line LS8 cells. Ru265 prevented _c Ca²⁺ clearance in permeabilized LS8 cells like ruthenium red, and it did not affect ΔΨm in intact cells. In primary ameloblasts, SOCE stimulation elicited a significantly higher _m Ca²⁺ uptake in maturation ameloblasts. The uptake of Ca²⁺ into the mitochondria was dramatically decreased in the presence of Ru265. Combined, these results suggest an increased mitochondrial Ca²⁺ handling in maturation but only upon stimulation of Ca²⁺ influx via SOCE. These functional studies provide insights not only on the role of mitochondria in ameloblast Ca²⁺ physiology, but also advance the concept that SOCE and _m Ca²⁺ uptake are complementary processes in biological mineralization.

Structural and functional relations between the connective tissue and epithelium of enamel organ and their role during enamel maturation

The morphological and possible functional interactions between the connective tissue and enamel organ cells were examined during the maturation phase of enamel formation, using immunohistochemical techniques. Decalcified mandibular sections (10 µm) including incisors were used from Wistar rats ages 10-12 weeks. Sections were incubated with one or two primary antibodies targeting cell cytoskeleton (vimentin, α-actin, α-tubulin), dendritic marker (OX6), gap junctions (cx-43), enzymes (nitric-oxide synthase (nos1) and cyclooxygenase (cox1)), and the ion transporters (Na+/H+ exchanger (NHE1) and Na+/Ca2+ exchanger (NCX)) for 24 h, before incubation with the appropriate conjugated fluorescent secondary antibodies. Sections were examined by fluorescence microscopy. Haematoxylin-eosin slides were also employed. Cellular heterogeneity and morphological modulations were identified within enamel organ cells and connective tissue covering suggesting complex cellular interactions and indicating a new functional concept and possible complementary role during enamel maturation. Also, some ion transportation activity, and nos1 and cox1 signalling pathways have been identified, indicating intercellular communication between these regions. A hypothesis is suggested, to explain the morphological modulation of ameloblasts and papillary cells during enamel maturation which functions to increase the transporting membrane surface area to accomplish faster and bulker ion transportation to achieve controlled pH and to direct Ca2+ towards enamel.

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.

Calcium Transport in Specialized Dental Epithelia and Its Modulation by Fluoride

Most cells use calcium (Ca2+) as a second messenger to convey signals that affect a multitude of biological processes. The ability of Ca2+ to bind to proteins to alter their charge and conformation is essential to achieve its signaling role. Cytosolic Ca2+ (cCa2+) concentration is maintained low at ~100 nM so that the impact of elevations in cCa2+ is readily sensed and transduced by cells. However, such elevations in cCa2+ must be transient to prevent detrimental effects. Cells have developed a variety of systems to rapidly clear the excess of cCa2+ including Ca2+ pumps, exchangers and sequestering Ca2+ within intracellular organelles. This Ca2+ signaling toolkit is evolutionarily adapted so that each cell, tissue, and organ can fulfill its biological function optimally. One of the most specialized cells in mammals are the enamel forming cells, the ameloblasts, which also handle large quantities of Ca2+. The end goal of ameloblasts is to synthesize, secrete and mineralize a unique proteinaceous matrix without the benefit of remodeling or repair mechanisms. Ca2+ uptake into ameloblasts is mainly regulated by the store operated Ca2+ entry (SOCE) before it is transported across the polarized ameloblasts to reach the insulated enamel space. Here we review the ameloblasts Ca2+ signaling toolkit and address how the common electronegative non-metal fluoride can alter its function, potentially addressing the biology of dental fluorosis.

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.

Epithelial loss of mitochondrial oxidative phosphorylation leads to disturbed enamel and impaired dentin matrix formation in postnatal developed mouse incisor

The formation of dentin and enamel matrix depends on reciprocal interactions between epithelial-mesenchymal cells. To assess the role of mitochondrial function in amelogenesis and dentinogenesis, we studied postnatal incisor development in K320E-Twinkle^Epi mice. In these mice, a loss of mitochondrial DNA (mtDNA), followed by a severe defect in the oxidative phosphorylation system is induced specifically in Keratin 14 (K14+) expressing epithelial cells. Histochemical staining showed severe reduction of cytochrome c oxidase activity only in K14+ epithelial cells. In mutant incisors, H&E staining showed severe defects in the ameloblasts, in the epithelial cells of the stratum intermedium and the papillary cell layer, but also a disturbed odontoblast layer. The lack of amelogenin in the enamel matrix of K320E-Twinkle^Epi mice indicated that defective ameloblasts are not able to form extracellular enamel matrix proteins. In comparison to control incisors, von Kossa staining showed enamel biomineralization defects and dentin matrix impairment. In mutant incisor, TUNEL staining and ultrastructural analyses revealed differentiation defects, while in hair follicle cells apoptosis is prevalent. We concluded that mitochondrial oxidative phosphorylation in epithelial cells of the developed incisor is required for Ca2+ homeostasis to regulate the formation of enamel matrix and induce the differentiation of ectomesenchymal cells into odontoblasts.

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.

Distinct and Overlapping Expression Patterns of the Homer Family of Scaffolding Proteins and Their Encoding Genes in Developing Murine Cephalic Tissues

In mammals Homer1, Homer2 and Homer3 constitute a family of scaffolding proteins with key roles in Ca²⁺ signaling and Ca²⁺ transport. In rodents, Homer proteins and mRNAs have been shown to be expressed in various postnatal tissues and to be enriched in brain. However, whether the Homers are expressed in developing tissues is hitherto largely unknown. In this work, we used immunohistochemistry and in situ hybridization to analyze the expression patterns of Homer1, Homer2 and Homer3 in developing cephalic structures. Our study revealed that the three Homer proteins and their encoding genes are expressed in a wide range of developing tissues and organs, including the brain, tooth, eye, cochlea, salivary glands, olfactory and respiratory mucosae, bone and taste buds. We show that although overall the three Homers exhibit overlapping distribution patterns, the proteins localize at distinct subcellular domains in several cell types, that in both undifferentiated and differentiated cells Homer proteins are concentrated in puncta and that the vascular endothelium is enriched with Homer3 mRNA and protein. Our findings suggest that Homer proteins may have differential and overlapping functions and are expected to be of value for future research aiming at deciphering the roles of Homer proteins during embryonic development.

Differential regulation of Ca2+ influx by ORAI channels mediates enamel mineralization

Store-operated Ca²⁺ entry (SOCE) channels are highly selective Ca²⁺ channels activated by the endoplasmic reticulum (ER) sensors STIM1 and STIM2. Their direct interaction with the pore-forming plasma membrane ORAI proteins (ORAI1, ORAI2, and ORAI3) leads to sustained Ca²⁺ fluxes that are critical for many cellular functions. Mutations in the human ORAI1 gene result in immunodeficiency, anhidrotic ectodermal dysplasia, and enamel defects. In our investigation of the role of ORAI proteins in enamel, we identified enamel defects in a patient with an ORAI1 null mutation. Targeted deletion of the Orai1 gene in mice showed enamel defects and reduced SOCE in isolated enamel cells. However, Orai2^-/- mice showed normal enamel despite having increased SOCE in the enamel cells. Knockdown experiments in the enamel cell line LS8 suggested that ORAI2 and ORAI3 modulated ORAI1 function, with ORAI1 and ORAI2 being the main contributors to SOCE. ORAI1-deficient LS8 cells showed altered mitochondrial respiration with increased oxygen consumption rate and ATP, which was associated with altered redox status and enhanced ER Ca²⁺ uptake, likely due to S-glutathionylation of SERCA pumps. Our findings demonstrate an important role of ORAI1 in Ca²⁺ influx in enamel cells and establish a link between SOCE, mitochondrial function, and redox homeostasis.

CRAC channels in dental enamel cells

Enamel mineralization relies on Ca2+ availability provided by Ca2+ release activated Ca2+ (CRAC) channels. CRAC channels are modulated by the endoplasmic reticulum Ca2+ sensor STIM1 which gates the pore subunit of the channel known as ORAI1, found the in plasma membrane, to enable sustained Ca2+ influx. Mutations in the STIM1 and ORAI1 genes result in CRAC channelopathy, an ensemble of diseases including immunodeficiency, muscular hypotonia, ectodermal dysplasia with defects in sweat gland function and abnormal enamel mineralization similar to amelogenesis imperfecta (AI). In some reports, the chief medical complain has been the patient's dental health, highlighting the direct and important link between CRAC channels and enamel. The reported enamel defects are apparent in both the deciduous and in permanent teeth and often require extensive dental treatment to provide the patient with a functional dentition. Among the dental phenotypes observed in the patients, discoloration, increased wear, hypoplasias (thinning of enamel) and chipping has been reported. These findings are not universal in all patients. Here we review the mutations in STIM1 and ORAI1 causing AI-like phenotype, and evaluate the enamel defects in CRAC channel deficient mice. We also provide a brief overview of the role of CRAC channels in other mineralizing systems such as dentine and bone.

Altered Ca2+ signaling in enamelopathies

Biomineralization requires the controlled movement of ions across cell barriers to reach the sites of crystal growth. Mineral precipitation occurs in aqueous phases as fluids become supersaturated with specific ionic compositions. In the biological world, biomineralization is dominated by the presence of calcium (Ca²⁺) in crystal lattices. Ca²⁺ channels are intrinsic modulators of this process, facilitating the availability of Ca²⁺ within cells in a tightly regulated manner in time and space. Unequivocally, the most mineralized tissue produced by vertebrates, past and present, is dental enamel. With some of the longest carbonated hydroxyapatite (Hap) crystals known, dental enamel formation is fully coordinated by specialized epithelial cells of ectodermal origin known as ameloblasts. These cells form enamel in two main developmental stages: a) secretory; and b) maturation. The secretory stage is marked by volumetric growth of the tissue with limited mineralization, and the opposite is found in the maturation stage, as enamel crystals expand in width concomitant with increased ion transport. Disruptions in the formation and/or mineralization stages result, in most cases, in permanent alterations in the crystal assembly. This introduces weaknesses in the material properties affecting enamel's hardness and durability, thus limiting its efficacy as a biting, chewing tool and increasing the possibility of pathology. Here, we briefly review enamel development and discuss key properties of ameloblasts and their Ca²⁺-handling machinery, and how alterations in this toolkit result in enamelopathies.

DENTAL ENAMEL FORMATION AND IMPLICATIONS FOR ORAL HEALTH AND DISEASE

Dental enamel is the hardest and most mineralized tissue in extinct and extant vertebrate species and provides maximum durability that allows teeth to function as weapons and/or tools as well as for food processing. Enamel development and mineralization is an intricate process tightly regulated by cells of the enamel organ called ameloblasts. These heavily polarized cells form a monolayer around the developing enamel tissue and move as a single forming front in specified directions as they lay down a proteinaceous matrix that serves as a template for crystal growth. Ameloblasts maintain intercellular connections creating a semi-permeable barrier that at one end (basal/proximal) receives nutrients and ions from blood vessels, and at the opposite end (secretory/apical/distal) forms extracellular crystals within specified pH conditions. In this unique environment, ameloblasts orchestrate crystal growth via multiple cellular activities including modulating the transport of minerals and ions, pH regulation, proteolysis, and endocytosis. In many vertebrates, the bulk of the enamel tissue volume is first formed and subsequently mineralized by these same cells as they retransform their morphology and function. Cell death by apoptosis and regression are the fates of many ameloblasts following enamel maturation, and what cells remain of the enamel organ are shed during tooth eruption, or are incorporated into the tooth's epithelial attachment to the oral gingiva. In this review, we examine key aspects of dental enamel formation, from its developmental genesis to the ever-increasing wealth of data on the mechanisms mediating ionic transport, as well as the clinical outcomes resulting from abnormal ameloblast function.

Stim1 Regulates Enamel Mineralization and Ameloblast Modulation

Loss-of-function mutations in the Ca²⁺ release-activated Ca²⁺ channel genes ORAI1 and STIM1 abolish store-operated Ca²⁺ entry (SOCE) and result in ectodermal dysplasia with amelogenesis imperfecta. However, because of the limited availability of patient tissue, analyses of enamel mineralization or possible changes in ameloblast function or morphology have not been possible. Here, we generated mice with ectodermal tissue-specific deletion of Stim1 ( Stim1 cKO [conditional knockout]), Stim2 ( Stim2 cKO), and Stim1 and Stim2 ( Stim1/2 cKO) and analyzed their enamel phenotypes as compared with those of control ( Stim1/2^fl/fl) animals. Ablation of Stim1 and Stim1/2 but not Stim2 expression resulted in chalky enamel and severe attrition at the incisor tips and molar cusps. Stim1 and Stim1/2 cKO, but not Stim2 cKO, demonstrated inferior enamel mineralization with impaired structural integrity, whereas the shape of the teeth and enamel thickness appeared to be normal in all animals. The gene expression levels of the enamel matrix proteins Amelx and Ambn and the enamel matrix proteases Mmp20 and Klk4 were not altered by the abrogation of SOCE in Stim1/2 cKO mice. The morphology of ameloblasts during the secretory and maturation stages was not significantly altered in either the incisors or molars of the cKO animals. However, in Stim1 and Stim1/2 cKO incisors, the alternating modulation of maturation-stage ameloblasts between the smooth- and ruffle-ended cell types continued beyond the regular cycle and extended to the areas corresponding to the zone of postmodulation ameloblasts in the teeth of control animals. These results indicate that SOCE is essential for proper enamel mineralization, in which Stim1 plays a critical role during the maturation process.

Enamel: Molecular identity of its transepithelial ion transport system

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

Multiple Calcium Export Exchangers and Pumps Are a Prominent Feature of Enamel Organ Cells

Calcium export is a key function for the enamel organ during all stages of amelogenesis. Expression of a number of ATPase calcium transporting, plasma membrane genes (ATP2B1-4/PMCA1-4), solute carrier SLC8A genes (sodium/calcium exchanger or NCX1-3), and SLC24A gene family members (sodium/potassium/calcium exchanger or NCKX1-6) have been investigated in the developing enamel organ in earlier studies. This paper reviews the calcium export pathways that have been described and adds novel insights to the spatiotemporal expression patterns of PMCA1, PMCA4, and NCKX3 during amelogenesis. New data are presented to show the mRNA expression profiles for the four Atp2b1-4 gene family members (PMCA1-4) in secretory-stage and maturation-stage rat enamel organs. These data are compared to expression profiles for all Slc8a and Slc24a gene family members. PMCA1, PMCA4, and NCKX3 immunolocalization data is also presented. Gene expression profiles quantitated by real time PCR show that: (1) PMCA1, 3, and 4, and NCKX3 are most highly expressed during secretory-stage amelogenesis; (2) NCX1 and 3, and NCKX6 are expressed during secretory and maturation stages; (3) NCKX4 is most highly expressed during maturation-stage amelogenesis; and (4) expression levels of PMCA2, NCX2, NCKX1, NCKX2, and NCKX5 are negligible throughout amelogenesis. In the enamel organ PMCA1 localizes to the basolateral membrane of both secretory and maturation ameloblasts; PMCA4 expression is seen in the basolateral membrane of secretory and maturation ameloblasts, and also cells of the stratum intermedium and papillary layer; while NCKX3 expression is limited to Tomes' processes, and the apical membrane of maturation-stage ameloblasts. These new findings are discussed in the perspective of data already present in the literature, and highlight the multiplicity of calcium export systems in the enamel organ needed to regulate biomineralization.

Store-operated Ca2+ entry controls ameloblast cell function and enamel development

Loss-of-function mutations in stromal interaction molecule 1 (STIM1) impair the activation of Ca²⁺ release-activated Ca²⁺ (CRAC) channels and store-operated Ca²⁺ entry (SOCE), resulting in a disease syndrome called CRAC channelopathy that is characterized by severe dental enamel defects. The cause of these enamel defects has remained unclear given a lack of animal models. We generated Stim1/2^K14cre mice to delete STIM1 and its homolog STIM2 in enamel cells. These mice showed impaired SOCE in enamel cells. Enamel in Stim1/2^K14cre mice was hypomineralized with decreased Ca content, mechanically weak, and thinner. The morphology of SOCE-deficient ameloblasts was altered, showing loss of the typical ruffled border, resulting in mislocalized mitochondria. Global gene expression analysis of SOCE-deficient ameloblasts revealed strong dysregulation of several pathways. ER stress genes associated with the unfolded protein response were increased in Stim1/2-deficient cells, whereas the expression of components of the glutathione system were decreased. Consistent with increased oxidative stress, we found increased ROS production, decreased mitochondrial function, and abnormal mitochondrial morphology in ameloblasts of Stim1/2^K14cre mice. Collectively, these data show that loss of SOCE in enamel cells has substantial detrimental effects on gene expression, cell function, and the mineralization of dental enamel.