Altered pH regulation during enamel development in the cystic fibrosis mouse incisor

Association between respiratory diseases and molar-incisor hypomineralization: A systematic review and meta-analysis

The molar-incisor permineralização (MIH) is a qualitative enamel developing abnormality involving the occlusal and/or incisal third of one or more molars or permanent incisors, caused by systemic factors. Several systemic disorders and environmental factors, such as respiratory diseases, have been reported as probable causes of MIH. Thus, this work aimed to evaluate whether respiratory diseases and MIH are associated. The searches were carried out in electronic databases, including PubMed, Scopus, Web of Science, the Cochrane Library, LILACS, OpenGrey, and Google Scholar. The acronym PECO was used, in which the P (population) was humans in permanent dentition stage; (E-exposure) molar-incisor hypomineralization; (C-comparison) reference population and (O - outcome) respiratory diseases. After the search retrieval, the duplicates were removed, and the articles were evaluated by title and abstract; then, the papers were read and thoroughly assessed. After selection, the risk of bias assessment was performed using the Newcastle-Ottawa Scale (NOS) for observational studies. The Grading of Recommendations, Assessment, Development, and Evaluation (GRADE) tool was used to assess the level of evidence. Three thousand six hundred and sixty six were found through the searches. After exclusion by duplicates, title, abstract, and full-reading, 13 articles remained. The articles included in this review evaluated the association of MIH with asthma, tonsilitis, pneumonia, and bronchitis. Most reports showed a low risk of bias. When exploring through GRADE, a very low level of evidence was found. We observed that the included studies showed that children with MIH had more respiratory diseases than the group that did not have MIH.

Systematic review registration

https://osf.io/un76d.

Oral Care Recommendation for Cystic Fibrosis Patients-Recommendation for Dentists

Cystic fibrosis (CF) is a genetic disease that is caused by a defect in the gene coding for the transmembrane cystic fibrosis transmembrane conductance regulator (CFTR). Research papers published so far point out that despite the numerous dental treatment needs of CF patients, there are no oral care guidelines for this group of patients. The aim of the article is to propose standards of dental prophylactic and therapeutic procedures for CF patients in different age groups. Regardless of the CF patient's age, dental check-ups should be scheduled at least every 6 months. However, taking into account the actual condition of the individual CF patients, therapeutic visits may be scheduled for earlier dates, to provide well-fitting treatment, considering the level of risk of oral diseases. The described management standards may be helpful and may improve the quality of dental care provided to CF patients.

Molar Incisor Hypomineralization: Prevalence, Associated Risk Factors, Its Relation with Dental Caries and Various Enamel Surface Defects in 8–16-year-old Schoolchildren of Lucknow District

Introduction

Aim

Methodology

Results

Conclusion

Clinical significance

How to cite this article

Association between asthma and enamel defects in primary and young permanent teeth - A systematic review

Childhood respiratory diseases, such as asthma, are important public health problems worldwide and could be associated with tooth enamel defects. This study aimed to verify the relationship between asthma and enamel defects in teeth, to answer the following question: "Could asthma in children be significantly associated with enamel defects in deciduous dentition and young permanent teeth?." PUBMED-MEDLINE, EMBASE, LILACS, and COCHRANE databases were systematically searched and assessed articles (2000-2021) were cautiously scored according to a predetermined criterion. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses were considered. Twenty-two articles were critically appraised and used as a basis for conclusions. The relationship between asthma and enamel defects was confirmed in the majority of appraised papers, one with a high level of evidence, nine with a moderate level, and four with a low level. Out of the eight manuscripts investigating the influence of asthma medication on enamel defects, only three (one with high, one moderate, and another with a low level of evidence) suggested an association. It can be concluded that asthma is closely connected with enamel defects in young permanent teeth. However, as most of the papers appraised were of cross-sectional or case-control design, further well-designed clinical investigations with a prospective design are welcome to reinforce our findings.

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.

Estimation of Proinflammatory Factors in the Saliva of Adult Patients with Cystic Fibrosis and Dental Caries

Introduction: The available literature lacks data regarding the levels of resistin, lysozyme, lactoferrin, α-amylase activity, pH, and saliva buffer capacity, as well as oral health and hygiene in the group of adult patients with cystic fibrosis (CF). The aim of the research was to assess the selected saliva parameters in patients diagnosed with cystic fibrosis. Materials and methods: Examined group was composed of 40 patients diagnosed with CF, while the control group of 40 healthy individuals. Both groups underwent the same scheme of the assessment (DMT index, salivary pH, buffer capacity, analysis of total sialic acid, total protein estimation, lysozyme levels estimation, lactofferin levels measurement, α-amylase activity, estimation of the levels of resistin and TNF-α). Results: In the examined group, there were higher values of decayed teeth as well as values of sialic acid, total protein, lactoferrin, α-amylase, and TNF-α. However, mean lysozyme, and resistin levels, as well as pH and buffer capacity of the saliva, were lower. Conclusions: New diagnostic methods, including the evaluation of selected salivary biochemical parameters, may indicate the existence of factors predisposing to severe tooth decay in the study group. Appropriate preventive treatment to combat dental caries in adult patients with CF will significantly improve their comfort and life expectancy.

Prevalence and Clinical Characteristics of Molar-Incisor Hypomineralization in 8-16-year-old Children in Industrial Town of Solan District of Himachal Pradesh

Aim

The aim of the present study was to investigate the prevalence and etiology of molar–incisor hypomineralization (MIH) in 8- to 16-year-old children from town (Baddi) of Himachal Pradesh.

Materials and methods

A cross-sectional study was conducted with 2000 children from various schools of Baddi. Molar–incisor hypomineralization was diagnosed on the basis of EAPD 2003 criteria revised in 2009.

Results

Prevalence of MIH found was to be 2.9%. Demarcated opacities were predominant among all the defects. Postnatal illnesses were highly associated with MIH. The prevalence of dental caries reported in MIH patients was 82.7%.

Conclusion

Postnatal illnesses are significantly associated with MIH. Molar–incisor hypomineralization predisposes the tooth to dental caries.

How to cite this article

Thakur H, Kaur A, Singh R, et al. Prevalence and Clinical Characteristics of Molar–Incisor Hypomineralization in 8–16-year-old Children in Industrial Town of Solan District of Himachal Pradesh. Int J Clin Pediatr Dent 2020;13(3):230–234.

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.

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.

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.

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.

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.

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.

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.

DLX3-Dependent Regulation of Ion Transporters and Carbonic Anhydrases is Crucial for Enamel Mineralization

Patients with tricho-dento-osseous (TDO) syndrome, an ectodermal dysplasia caused by mutations in the homeodomain transcription factor DLX3, exhibit enamel hypoplasia and hypomineralization. Here we used a conditional knockout mouse model to investigate the developmental and molecular consequences of Dlx3 deletion in the dental epithelium in vivo. Dlx3 deletion in the dental epithelium resulted in the formation of chalky hypomineralized enamel in all teeth. Interestingly, transcriptomic analysis revealed that major enamel matrix proteins and proteases known to be involved in enamel secretion and maturation were not affected significantly by Dlx3 deletion in the enamel organ. In contrast, expression of several ion transporters and carbonic anhydrases known to play an important role in enamel pH regulation during maturation was significantly affected in enamel organs lacking DLX3. Most of these affected genes showed binding of DLX3 to their proximal promoter as evidenced by chromatin immunoprecipitation sequencing (ChIP-seq) analysis on rat enamel organ. These molecular findings were consistent with altered pH staining evidenced by disruption of characteristic pH oscillations in the enamel. Taken together, these results show that DLX3 is indispensable for the regulation of ion transporters and carbonic anhydrases during the maturation stage of amelogenesis, exerting a crucial regulatory function on pH oscillations during enamel mineralization. © 2016 American Society for Bone and Mineral Research.

Magnesium, pH regulation and modulation by mouse ameloblasts exposed to fluoride

Supraoptimal intake of fluoride (F) induces structural defects in forming enamel, dentin and bone and increases the risk of bone fractures. In comparison to bone and dentin is formation of enamel most sensitive to low levels of F and the degree of enamel fluorosis depends on the mouse strain. What molecular mechanism is responsible for these differences in sensitivity is unclear. Maturation ameloblasts transport bicarbonates into enamel in exchange for Cl- to buffer protons released by forming apatites. We proposed that F-enhanced mineral deposition releases excess of protons that will affect mineralization in forming enamel. In this study we tested the hypothesis that increased sensitivity to F is associated with a reduced capacity of ameloblasts to buffer acids. Quantified electron probe microanalysis showed that enamel of F-sensitive C57Bl mice contained the same levels of Cl- as enamel of F-resistant FVB mice. Enamel of C57Bl mice was less mineral dense, contained less Ca but more Mg and K. Ameloblast modulation was much more impaired than in FVB mice. In enamel of FVB mice the levels of Mg correlated negative with Ca (r=-0.57, p=0.01) and with the Ca/P molar ratio (r=-0.32, p=0.53). In moderate and high acidic enamel the correlations between Mg and Ca/P ratio were strong (r=-0.75, p=0.08) to very strong negative (r=-0.98, p=0.0020), respectively. Correlations in enamel between F and Ca were (weak) negative but between F and Ca/P very high positive (r=+0.95, p=0.003) in high acidic enamel and less positive (r=0.45, p=0.27) in moderate acidic fluorotic enamel (r=0.45, p=0.27). Similar correlations between Mg and Ca/P or F and Ca/P were found in dentin and bone of fluorotic and Cftr null mice. These data are consistent with the concept that Mg delays but F increases maturation of crystals particularly when enamel is acidic. The sensitivity of forming enamel to F likely is due to the sensitivity of pH cycling to acidification of enamel associated with F-induced release of protons.

Ion Transport by Ameloblasts during Amelogenesis

Hypomineralization of developing enamel is associated with changes in ameloblast modulation during the maturation stage. Modulation (or pH cycling) involves the cyclic transformation of ruffle-ended (RE) ameloblasts facing slightly acidic enamel into smooth-ended (SE) ameloblasts near pH-neutral enamel. The mechanism of ameloblast modulation is not clear. Failure of ameloblasts of Cftr-null and anion exchanger 2 ( Ae2)-null mice to transport Cl^- into enamel acidifies enamel, prevents modulation, and reduces mineralization. It suggests that pH regulation is critical for modulation and for completion of enamel mineralization. This report presents a review of the major types of transmembrane molecules that ameloblasts express to transport calcium to form crystals and bicarbonates to regulate pH. The type of transporter depends on the developmental stage. Modulation is proposed to be driven by the pH of enamel fluid and the compositional and/or physicochemical changes that result from increased acidity, which may turn RE ameloblasts into SE mode. Amelogenins delay outgrowth of crystals and keep the intercrystalline space open for diffusion of mineral ions into complete depth of enamel. Modulation enables stepwise removal of amelogenins from the crystal surface, their degradation, and removal from the enamel. Removal of matrix allows slow expansion of crystals. Modulation also reduces the stress that ameloblasts experience when exposed to high acid levels generated by mineral formation or by increased intracellular Ca²⁺. By cyclically interrupting Ca²⁺ transport by RE ameloblasts and their transformation into SE ameloblasts, proton production ceases shortly and enables the ameloblasts to recover. Modulation also improves enamel crystal quality by selectively dissolving immature Ca²⁺-poor crystals, removing impurities as Mg²⁺ and carbonates, and recrystallizing into more acid-resistant crystals.

Buffering of protons released by mineral formation during amelogenesis in mice

Regulation of pH by ameloblasts during amelogenesis is critical for enamel mineralization. We examined the effects of reduced bicarbonate secretion and the presence or absence of amelogenins on ameloblast modulation and enamel mineralization. To that end, the composition of fluorotic and non-fluorotic enamel of several different mouse mutants, including enamel of cystic fibrosis transmembrane conductance regulator-deficient (Cftr null), anion exchanger-2-deficient (Ae2a,b null), and amelogenin-deficient (Amelx null) mice, was determined by quantitative X-ray microanalysis. Correlation analysis was carried out to compare the effects of changes in the levels of sulfated-matrix (S) and chlorine (Cl; for bicarbonate secretion) on mineralization and modulation. The chloride (Cl^- ) levels in forming enamel determined the ability of ameloblasts to modulate, remove matrix, and mineralize enamel. In general, the lower the Cl^- content, the stronger the negative effects. In Amelx-null mice, modulation was essentially normal and the calcium content was reduced least. Retention of amelogenins in enamel of kallikrein-4-deficient (Klk4-null) mice resulted in decreased mineralization and reduced the length of the first acid modulation band without changing the total length of all acidic bands. These data suggest that buffering by bicarbonates is critical for modulation, matrix removal and enamel mineralization. Amelogenins also act as a buffer but are not critical for modulation.

Etiology of molar incisor hypomineralization - A systematic review

OBJECTIVES

Molar incisor hypomineralization (MIH) is a common developmental dental defect of permanent teeth, which can increase the risk of dental caries, infection and hospitalization. The etiology is currently unclear although prenatal or early childhood health factors are suspected. The aim of this systematic review was to assess the strength of evidence linking etiological factors with MIH.

METHODS

A systematic search was conducted using the Medline and Embase electronic databases for studies investigating environmental etiological factors of MIH. Two reviewers assessed the eligibility of studies. The level of evidence and bias was determined for all eligible studies according to Australian National Health and Medical Research Council guidelines for systematic reviews of etiology and the Newcastle-Ottawa Scale.

RESULTS

From a total of 2254 studies identified through electronic and hand searching, 28 were eligible for inclusion. Twenty-five of these investigated MIH and three investigated a related condition in primary teeth, hypomineralized second primary molars (HSPM), and these were analysed separately. A limited number of studies reported significant associations between MIH and pre- and perinatal factors such as maternal illness and medication use in pregnancy, prematurity and birth complications. Early childhood illness was implicated as an etiological factor in MIH in several studies, in particular fever, asthma and pneumonia. The studies investigating HSPM revealed an association with maternal alcohol consumption, infantile fever and ethnicity. However, the validity of these findings is impaired by study design, lack of adjustment for confounders, lack of detail and consistency of exposures investigated and poor reporting.

CONCLUSIONS

Childhood illness is likely to be associated with MIH. Further prospective studies of the etiology of MIH/HSPM are needed.

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.

Orai1 expression pattern in tooth and craniofacial ectodermal tissues and potential functions during ameloblast differentiation

BACKGROUND

Orai1 is a plasma membrane protein that forms the pore of the calcium release activated calcium channel. Humans with mutated Orai1 present with hereditary combined immunodeficiency, congenital myopathy and anhidrotic ectodermal dysplasia. Consistent with the ectodermal dysplasia phenotype, enamel formation and mineralization is also abnormal in Orai1 deficient patients. The expression pattern and potential functions of Orai1 in enamel formation remains unclear. To contribute toward understanding the role of Orai1 in amelogenesis we characterized ORAI1 protein developmental pattern in comparison with other ectodermal organs. We also examined the effects of Orai1 down-regulation in ameloblast cell proliferation and differentiation.

RESULTS

Our data show strong expression of ORAI1 protein during the ameloblast secretory stage, which weans at the end of the maturation stage. In salivary glands, ORAI1 is expressed mainly in acini cells. ORAI1 expression is also found in hair follicle and oral epithelium. Knockdown of Orai1 expression decreases cell proliferation and results in RNA expression levels changes of key ameloblast genes regulating enamel thickness and mineralization.

CONCLUSIONS

This study provides insights in the anhidrotic ectodermal dysplasia phenotype due to Orai1 mutation and highlights the importance of calcium signaling in controlling ameloblast differentiation and maturation during tooth development.

Composition of mineralizing incisor enamel in cystic fibrosis transmembrane conductance regulator-deficient mice

Formation of crystals in the enamel space releases protons that need to be buffered to sustain mineral accretion. We hypothesized that apical cystic fibrosis transmembrane conductance regulator (CFTR) in maturation ameloblasts transduces chloride into forming enamel as a critical step to secrete bicarbonates. We tested this by determining the calcium, chloride, and fluoride levels in developing enamel of Cftr-null mice by quantitative electron probe microanalysis. Maturation-stage enamel from Cftr-null mice contained less chloride and calcium than did wild-type enamel, was more acidic when stained with pH dyes ex vivo, and formed no fluorescent modulation bands after in vivo injection of the mice with calcein. To acidify the enamel further we exposed Cftr-null mice to fluoride in drinking water to stimulate proton release during formation of hypermineralized lines. In Cftr-deficient mice, fluoride further lowered enamel calcium without further reducing chloride levels. The data support the view that apical CFTR in maturation ameloblasts tranduces chloride into developing enamel as part of the machinery to buffer protons released during mineral accretion.

Genome-wide analysis of miRNA and mRNA transcriptomes during amelogenesis

BACKGROUND

In the rodent incisor during amelogenesis, as ameloblast cells transition from secretory stage to maturation stage, their morphology and transcriptome profiles change dramatically. Prior whole genome transcriptome analysis has given a broad picture of the molecular activities dominating both stages of amelogenesis, but this type of analysis has not included miRNA transcript profiling. In this study, we set out to document which miRNAs and corresponding target genes change significantly as ameloblasts transition from secretory- to maturation-stage amelogenesis.

RESULTS

Total RNA samples from both secretory- and maturation-stage rat enamel organs were subjected to genome-wide miRNA and mRNA transcript profiling. We identified 59 miRNAs that were differentially expressed at the maturation stage relative to the secretory stage of enamel development (False Discovery Rate (FDR)<0.05, fold change (FC)≥1.8). In parallel, transcriptome profiling experiments identified 1,729 mRNA transcripts that were differentially expressed in the maturation stage compared to the secretory stage (FDR<0.05, FC≥1.8). Based on bioinformatics analyses, 5.8% (629 total) of these differentially expressed genes (DEGS) were highlighted as being the potential targets of 59 miRNAs that were differentially expressed in the opposite direction, in the same tissue samples. Although the number of predicted target DEGs was not higher than baseline expectations generated by examination of stably expressed miRNAs, Gene Ontology (GO) analysis showed that these 629 DEGS were enriched for ion transport, pH regulation, calcium handling, endocytotic, and apoptotic activities. Seven differentially expressed miRNAs (miR-21, miR-31, miR-488, miR-153, miR-135b, miR-135a and miR298) in secretory- and/or maturation-stage enamel organs were confirmed by in situ hybridization. Further, we used luciferase reporter assays to provide evidence that two of these differentially expressed miRNAs, miR-153 and miR-31, are potential regulators for their predicated target mRNAs, Lamp1 (miR-153) and Tfrc (miR-31).

CONCLUSIONS

In conclusion, these data indicate that miRNAs exhibit a dynamic expression pattern during the transition from secretory-stage to maturation-stage tooth enamel formation. Although they represent only one of numerous mechanisms influencing gene activities, miRNAs specific to the maturation stage could be involved in regulating several key processes of enamel maturation by influencing mRNA stability and translation.

Factors associated with molar incisor hypomineralization in Thai children

Molar incisor hypomineralization (MIH) is a qualitative developmental enamel defect that affects one to four permanent first molars, with or without involvement of permanent incisors. Its etiology is of systemic origin, but is not well understood. Therefore, we conducted this cross-sectional study to examine pre-, peri-, and postnatal risk factors for MIH among children, 7-8 yr of age, in urban areas of Khon Kaen, Thailand. Molar incisor hypomineralization defects were diagnosed using the European Academy of Pediatric Dentistry criteria. Mothers or primary caregivers were interviewed on maternal medical history and habits during pregnancy, pregnancy and delivery complications, and the child's medical history. Molar incisor hypomineralization defects were observed in 78 (27.7%) of 282 children. Multiple logistic regression analysis showed a statistically significant association between the development of MIH and Cesarean section (adjusted OR = 2.0, 95% CI = 1.1-3.7), complications during vaginal delivery (adjusted OR = 4.5, 95% CI = 1.9-11.0), and severe/chronic illness when under 3 yr of age (adjusted OR = 2.9, 95% CI = 1.6-5.0). There was no association of preterm birth and low birth weight with MIH. The results suggest that Cesarean section, complications during vaginal delivery, and poor health during the first 3 yr of life are independent risk factors for MIH.

Molar incisor hypomineralization, prevalence, and etiology

Aim. To evaluate the prevalence and possible etiological factors associated with molar incisor hypomineralization (MIH) among a group of children in Jeddah, Saudi Arabia. Methods. A group of 8-12-year-old children were recruited (n = 267) from the Pediatric Dental Clinics at the Faculty of Dentistry, King Abdulaziz University. Children had at least one first permanent molar (FPM), erupted or partially erupted. Demographic information, children's medical history, and pregnancy-related data were obtained. The crowns of the FPM and permanent incisors were examined for demarcated opacities, posteruptive breakdown (PEB), atypical restorations, and extracted FPMs. Children were considered to have MIH if one or more FPM with or without involvement of incisors met the diagnostic criteria. Results. MIH showed a prevalence of 8.6%. Demarcated opacities were the most common form. Maxillary central incisors were more affected than mandibular (P = 0.01). The condition was more prevalent in children with history of illnesses during the first four years of life including tonsillitis (P = 0.001), adenoiditis (P = 0.001), asthma (P = 0.001), fever (P = 0.014), and antibiotics intake (P = 0.001). Conclusions. The prevalence of MIH is significantly associated with childhood illnesses during the first four years of life including asthma, adenoid infections, tonsillitis, fever, and antibiotics intake.

Stress response pathways in ameloblasts: implications for amelogenesis and dental fluorosis

Human enamel development of the permanent teeth takes place during childhood and stresses encountered during this period can have lasting effects on the appearance and structural integrity of the enamel. One of the most common examples of this is the development of dental fluorosis after childhood exposure to excess fluoride, an elemental agent used to increase enamel hardness and prevent dental caries. Currently the molecular mechanism responsible for dental fluorosis remains unknown; however, recent work suggests dental fluorosis may be the result of activated stress response pathways in ameloblasts during the development of permanent teeth. Using fluorosis as an example, the role of stress response pathways during enamel maturation is discussed.

Related factors of dental caries and molar incisor hypomineralisation in a group of children with cystic fibrosis

AIM

To investigate dental caries and molar incisor hypomineralisation (MIH)-related factors such as treatment, diet, brushing and salivary factors in children with cystic fibrosis (CF) compared with healthy peers.

STUDY DESIGN

A cohort study was performed.

METHODS

This study was performed on 30 CF children comprising patients at the Faculty of Medicine and 30 control children recruited from the Dental School. Salivary factors, dental caries, MIH, daily diet, brushing habits were analysed. Statistical analysis was calculated by SPSS for Windows.

RESULTS

Decay missing filled teeth (DMF-T) score was 4.6 ± 4.0 in CF and 7.7 ± 2.7 in control (p = 0.001). 43% of CF children with MIH were found to use antibiotics, but no significant difference in the caries experience was found with antibiotic usage (p > 0.05). DMF-T of CF adolescents (23%) who use Tobramycin was 7 ± 3.5. DMF-T of CF children (20%) who take other antibiotics was 2.5 ± 3.5, but no statistical difference was found (p = 0.054). Saliva pH, salivary flow rate, and buffering capacity were not found statistically significant (p > 0.05).

STATISTICS

Percentage arithmetic mean value, standard deviation, independent sample t test, Fisher's exact test, Chi-square test and Mann-Whitney U test were used, while a p value of <0.05 was considered statistically significant.

CONCLUSIONS

Medication and diet could be considered as a risk factor for dental caries and factors such as salivary pH, good oral hygiene could play a protective role for oral health CF children. MIH frequency and lower caries experience seen in CF children could be due to salivary factors or pharmacological treatment they take. The multidisciplinary approach team would be advantageous in the management of children with CF and oral health should be under control during early years of life by paediatric dentists.

Developmental expression of solute carrier family 26A member 4 (SLC26A4/pendrin) during amelogenesis in developing rodent teeth

Ameloblasts need to regulate pH during the formation of enamel crystals, a process that generates protons. Solute carrier family 26A member 4 (SLC26A4, or pendrin) is an anion exchanger for chloride, bicarbonate, iodine, and formate. It is expressed in apical membranes of ion-transporting epithelia in kidney, inner ear, and thyroid where it regulates luminal pH and fluid transport. We hypothesized that maturation ameloblasts express SLC26A4 to neutralize acidification of enamel fluid in forming enamel. In rodents, secretory and maturation ameloblasts were immunopositive for SLC26A4. Staining was particularly strong in apical membranes of maturation ameloblasts facing forming enamel. RT-PCR confirmed the presence of mRNA transcripts for Slc26a4 in enamel organs. SLC26A4 immunostaining was also found in mineralizing connective tissues, including odontoblasts, osteoblasts, osteocytes, osteoclasts, bone lining cells, cellular cementoblasts, and cementocytes. However, Slc26a4-null mutant mice had no overt dental phenotype. The presence of SLC26A4 in apical plasma membranes of maturation ameloblasts is consistent with a potential function as a pH regulator. SLC26A4 does not appear to be critical for ameloblast function and is probably compensated by other pH regulators.

Identification of novel candidate genes involved in mineralization of dental enamel by genome-wide transcript profiling

The gene repertoire regulating vertebrate biomineralization is poorly understood. Dental enamel, the most highly mineralized tissue in mammals, differs from other calcifying systems in that the formative cells (ameloblasts) lack remodeling activity and largely degrade and resorb the initial extracellular matrix. Enamel mineralization requires that ameloblasts undergo a profound functional switch from matrix-secreting to maturational (calcium transport, protein resorption) roles as mineralization progresses. During the maturation stage, extracellular pH decreases markedly, placing high demands on ameloblasts to regulate acidic environments present around the growing hydroxyapatite crystals. To identify the genetic events driving enamel mineralization, we conducted genome-wide transcript profiling of the developing enamel organ from rat incisors and highlight over 300 genes differentially expressed during maturation. Using multiple bioinformatics analyses, we identified groups of maturation-associated genes whose functions are linked to key mineralization processes including pH regulation, calcium handling, and matrix turnover. Subsequent qPCR and Western blot analyses revealed that a number of solute carrier (SLC) gene family members were up-regulated during maturation, including the novel protein Slc24a4 involved in calcium handling as well as other proteins of similar function (Stim1). By providing the first global overview of the cellular machinery required for enamel maturation, this study provide a strong foundation for improving basic understanding of biomineralization and its practical applications in healthcare.

Requirements for ion and solute transport, and pH regulation during enamel maturation

Transcellular bicarbonate transport is suspected to be an important pathway used by ameloblasts to regulate extracellular pH and support crystal growth during enamel maturation. Proteins that play a role in amelogenesis include members of the ABC transporters (SLC gene family and CFTR). A number of carbonic anhydrases (CAs) have also been identified. The defined functions of these genes are likely interlinked during enamel mineralization. The purpose of this study is to quantify relative mRNA levels of individual SLC, Cftr, and CAs in enamel cells obtained from secretory and maturation stages on rat incisors. We also present novel data on the enamel phenotypes for two animal models, a mutant porcine (CFTR-ΔF508) and the NBCe1-null mouse. Our data show that two SLCs (AE2 and NBCe1), Cftr, and Car2, Car3, Car6, and Car12 are all significantly up-regulated at the onset of the maturation stage of amelogenesis when compared to the secretory stage. The remaining SLCs and CA gene transcripts showed negligible expression or no significant change in expression from secretory to maturation stages. The enamel of CFTR-ΔF508 adult pigs was hypomineralized and showed abnormal crystal growth. NBCe1-null mice enamel was structurally defective and had a marked decrease in mineral content relative to wild-type. These data demonstrate the importance of many non-matrix proteins to amelogenesis and that the expression levels of multiple genes regulating extracellular pH are modulated during enamel maturation in response to an increased need for pH buffering during hydroxyapatite crystal growth.

Enamel pathology resulting from loss of function in the cystic fibrosis transmembrane conductance regulator in a porcine animal model

Cystic fibrosis (CF) is caused by mutations in the gene encoding the CF transmembrane conductance regulator (CFTR), a phosphorylation- and ATP-regulated anion channel. CFTR expression and activity is frequently associated with an anion exchanger (AE) such as AE2 coded by the Slc4a2 gene. Mice null for Cftr and mice null for Slc4a2 have enamel defects, and there are some case reports of enamel anomalies in patients with CF. In this study we demonstrate that both Cftr and AE2 expression increased significantly during the rat enamel maturation stage versus the earlier secretory stage (5.6- and 2.9-fold, respectively). These qPCR data im- ply that there is a greater demand for Cl(-) and bicarbonate (HCO₃⁻) transport during the maturation stage of enamel formation, and that this is, at least in part, provided by changes in Cftr and AE2 expression. In addition, the enamel phenotypes of 2 porcine models of CF, CFTR-null, and CFTR-ΔF508 have been examined using backscattered electron microscopy in a scanning electron microscope. The enamel of newborn CFTR-null and CFTR-ΔF508 animals is hypomineralized. Together, these data provide a molecular basis for interpreting enamel disease associated with disruptions to CFTR and AE2 expression.

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

Mutations in the cystic fibrosis transmembrane conductance regulator (CFTR) gene cause cystic fibrosis (CF). Both CF and dental fluorosis result in protein retention in mature enamel. We hypothesized that excess fluoride might cause protein retention by interfering with CFTR function, resulting in abnormal expression of proteases and pathological endocytosis. Millimolar concentrations of fluoride reduced uptake of Emdogain, an enamel matrix derivative, in ameloblast-like PABSo-E cells, while stimulating an acidic intracellular environment at the same time. When CFTR function was inhibited by either an siRNA or a chloride channel inhibitor, CFTRinh-172, fluoride's effect on Emdogain uptake was partially blocked. Treatment of cells with CFTR siRNA down-regulated expression of proteases MMP20 and KLK4 and increased intracellular pH. We conclude that excess fluoride inhibits endocytic activity of ameloblasts through the CFTR chloride channel or other chloride channels. The intracellular pH might be the key mechanism by which abnormal proteolytic activity and defective endocytosis cause the residual protein observed in enamel of patients with CF and dental fluorosis.

Defining a new candidate gene for amelogenesis imperfecta: from molecular genetics to biochemistry

Amelogenesis imperfecta is a group of genetic conditions that affect the structure and clinical appearance of tooth enamel. The types (hypoplastic, hypocalcified, and hypomature) are correlated with defects in different stages of the process of enamel synthesis. Autosomal dominant, recessive, and X-linked types have been previously described. These disorders are considered clinically and genetically heterogeneous in etiology, involving a variety of genes, such as AMELX, ENAM, DLX3, FAM83H, MMP-20, KLK4, and WDR72. The mutations identified within these causal genes explain less than half of all cases of amelogenesis imperfecta. Most of the candidate and causal genes currently identified encode proteins involved in enamel synthesis. We think it is necessary to refocus the search for candidate genes using biochemical processes. This review provides theoretical evidence that the human SLC4A4 gene (sodium bicarbonate cotransporter) may be a new candidate gene.

Ion transporters in secretory and cyclically modulating ameloblasts: a new hypothesis for cellular control of preeruptive enamel maturation

Mature enamel consists of densely packed and highly organized large hydroxyapatite crystals. The molecular machinery responsible for the formation of fully matured enamel is poorly described but appears to involve oscillative pH changes at the enamel surface. We conducted an immunohistochemical investigation of selected transporters and related proteins in the multilayered rat incisor enamel organ. Connexin 43 (Cx-43) is found in papillary cells and ameloblasts, whereas Na+-K+-ATPase is heavily expressed during maturation in the papillary cell layer only. Given the distribution of Cx-43 channels and Na+-K+-ATPase, we suggest that ameloblasts and the papillary cell layer act as a functional syncytium. During enamel maturation ameloblasts undergo repetitive cycles of modulation between ruffle-ended (RA) and smooth-ended (SA) ameloblast morphologies. Carbonic anhydrase II and vacuolar H+-ATPase are expressed simultaneously at the beginning of the maturation stage in RA cells. The proton pumps are present in the ruffled border of RA and appear to be internalized during the SA stage. Both papillary cells and ameloblasts express plasma membrane acid/base transporters (AE2, NBC, and NHE1). AE2 and NHE1 change position relative to the enamel surface as localization of the tight junctions changes during ameloblast modulation cycles. We suggest that the concerted action of the papillary cell layer and the modulating ameloblasts regulates the enamel microenvironment, resulting in oscillating pH fluctuations. The pH fluctuations at the enamel surface may be required to keep intercrystalline spaces open in the surface layers of the enamel, enabling degraded enamel matrix proteins to be removed while hydroxyapatite crystals grow as a result of influx of calcium and phosphate ions.

Altered ion-responsive gene expression in Mmp20 null mice

During enamel maturation, hydroxyapatite crystallites expand in volume, releasing protons that acidify the developing enamel. This acidity is neutralized by the buffering activity of carbonic anhydrases and ion transporters. Less hydroxyapatite forms in matrix metalloproteinase-20 null (Mmp20(-/-)) mouse incisors, because enamel thickness is reduced by approximately 50%. We therefore asked if ion regulation was altered in Mmp20(-/-) mouse enamel. Staining of wild-type and Mmp20(-/-) incisors with pH indicators demonstrated that wild-type mice had pronounced changes in enamel pH as development progressed. These pH changes were greatly attenuated in Mmp20(-/-) mice. Expression of 4 ion-regulatory genes (Atp2b4, Slc4a2, Car6, Cftr) was significantly decreased in enamel organs from Mmp20(-/-) mice. Notably, expression of secreted carbonic anhydrase (Car6) was reduced to almost undetectable levels in the null enamel organ. In contrast, Odam and Klk4 expression was unaffected. We concluded that a feedback mechanism regulates ion-responsive gene expression during enamel development.

Regulation of dental enamel shape and hardness

Epithelial-mesenchymal interactions guide tooth development through its early stages and establish the morphology of the dentin surface upon which enamel will be deposited. Starting with the onset of amelogenesis beneath the future cusp tips, the shape of the enamel layer covering the crown is determined by five growth parameters: the (1) appositional growth rate, (2) duration of appositional growth (at the cusp tip), (3) ameloblast extension rate, (4) duration of ameloblast extension, and (5) spreading rate of appositional termination. Appositional growth occurs at a mineralization front along the ameloblast distal membrane in which amorphous calcium phosphate (ACP) ribbons form and lengthen. The ACP ribbons convert into hydroxyapatite crystallites as the ribbons elongate. Appositional growth involves a secretory cycle that is reflected in a series of incremental lines. A potentially important function of enamel proteins is to ensure alignment of successive mineral increments on the tips of enamel ribbons deposited in the previous cycle, causing the crystallites to lengthen with each cycle. Enamel hardens in a maturation process that involves mineral deposition onto the sides of existing crystallites until they interlock with adjacent crystallites. Neutralization of acidity generated by hydroxyapatite formation is a key part of the mechanism. Here we review the growth parameters that determine the shape of the enamel crown as well as the mechanisms of enamel appositional growth and maturation.

The sodium bicarbonate cotransporter (NBCe1) is essential for normal development of mouse dentition

Proximal renal tubular acidosis (pRTA) is a syndrome caused by abnormal proximal tubule reabsorption of bicarbonate resulting in metabolic acidosis. Patients with mutations to the SLC4A4 gene (coding for the sodium bicarbonate cotransporter NBCe1), have pRTA, growth delay, ocular defects, and enamel abnormalities. In an earlier report, we provided the first evidence that enamel cells, the ameloblasts, express NBCe1 in a polarized fashion, thereby contributing to trans-cellular bicarbonate transport. To determine whether NBCe1 plays a critical role in enamel development, we studied the expression of NBCe1 at various stages of enamel formation in wild-type mice and characterized the biophysical properties of enamel in NBCe1(-/-) animals. The enamel of NBCe1(-/-) animals was extremely hypomineralized and weak with an abnormal prismatic architecture. The expression profile of amelogenin, a known enamel-specific gene, was not altered in NBCe1(-/-) animals. Our results show for the first time that NBCe1 expression is required for the development of normal enamel. This study provides a mechanistic model to account for enamel abnormalities in certain patients with pRTA.

The cystic fibrosis transmembrane conductance regulator (CFTR) is expressed in maturation stage ameloblasts, odontoblasts and bone cells

Patients with cystic fibrosis (CF) have mild defects in dental enamel. The gene mutated in these patients is CFTR, a Cl(-) channel involved in transepithelial salt and water transport and bicarbonate secretion. We tested the hypothesis that Cftr channels are present and operating in the plasma membranes of mouse ameloblasts. Tissue sections of young mouse jaws and fetal human jaws were immunostained with various anti-Cftr antibodies. Specificity of the antibodies was validated in Cftr-deficient murine and human tissues. Immunostaining for Cftr was obtained in the apical plasma membranes of mouse maturation ameloblasts of both incisor and molar tooth germs. A granular intracellular immunostaining of variable intensity was also noted in bone cells and odontoblasts. In Cftr-deficient mice the incisors were chalky white and eroded much faster than in wild type mice. Histologically, only maturation ameloblasts of incisors were structurally affected in Cftr-deficient mice. Some antibody species gave also a positive cytosolic staining in Cftr-deficient cells. Transcripts of Cftr were found in maturation ameloblasts, odontoblasts and bone cells. Similar data were obtained in forming human dentin and bone. We conclude that Cftr protein locates in the apical plasma membranes of mouse maturation ameloblasts. In mouse incisors Cftr is critical for completion of enamel mineralization and conceivably functions as a regulator of pH during rapid crystal growth. Osteopenia found in CF patients as well as in Cftr-deficient mice is likely associated with defective Cftr operating in bone cells.

Regulation of pH During Amelogenesis

During amelogenesis, extracellular matrix proteins interact with growing hydroxyapatite crystals to create one of the most architecturally complex biological tissues. The process of enamel formation is a unique biomineralizing system characterized first by an increase in crystallite length during the secretory phase of amelogenesis, followed by a vast increase in crystallite width and thickness in the later maturation phase when organic complexes are enzymatically removed. Crystal growth is modulated by changes in the pH of the enamel microenvironment that is critical for proper enamel biomineralization. Whereas the genetic bases for most abnormal enamel phenotypes (amelogenesis imperfecta) are generally associated with mutations to enamel matrix specific genes, mutations to genes involved in pH regulation may result in severely affected enamel structure, highlighting the importance of pH regulation for normal enamel development. This review summarizes the intra- and extracellular mechanisms employed by the enamel-forming cells, ameloblasts, to maintain pH homeostasis and, also, discusses the enamel phenotypes associated with disruptions to genes involved in pH regulation.

Systemic disorders and their influence on the development of dental hard tissues: a literature review

OBJECTIVES

This report highlights the influence of a number of disorders with systemic physiological effects that impact on the development of dental hard tissues. It focuses in particular, on the pathological effects of systemic conditions with less well recognised, but no less important, impacts on dental development. Such conditions, include cystic fibrosis, HIV/AIDS, leukaemia, Alstrom syndrome, hypophosphatasia, Prader-Willi syndrome, Tricho-dento-osseous syndrome, tuberous sclerosis, familial steroid dehydrogenase deficiency and epidermolysis bullosa. These, along with developmental and environmental causes of enamel and dentine defects, are discussed and the possible aetiology of such effects are proposed. Furthermore, the dental management and long-term dental care of these patients is outlined.

SOURCES

MEDLINE/PubMed.

CONCLUSIONS

Enamel and dentine defects can present with a wide spectrum of clinical features and may be caused by a variety of factors occurring throughout tooth development from before birth to adulthood. These may include host traits, genetic factors, immunological responses to cariogenic bacteria, saliva composition, environmental and behavioural factors and systemic diseases. These diseases and their spectrum of clinical manifestations on the organs affected (including the dentition) require an increased knowledge by dental practitioners of the disease processes, aetiology, relevant treatment strategies and prognosis, and must encompass more than simply the management of the dental requirements of the patient. It is important that the impact of the disease and its treatment, particularly in respect of immunosuppression where dental interventions may become life-threatening, is also taken into consideration.

Mutations in the beta propeller WDR72 cause autosomal-recessive hypomaturation amelogenesis imperfecta

Healthy dental enamel is the hardest and most highly mineralized human tissue. Though acellular, nonvital, and without capacity for turnover or repair, it can nevertheless last a lifetime. Amelogenesis imperfecta (AI) is a collective term for failure of normal enamel development, covering diverse clinical phenotypes that typically show Mendelian inheritance patterns. One subset, known as hypomaturation AI, is characterised by near-normal volumes of organic enamel matrix but with weak, creamy-brown opaque enamel that fails prematurely after tooth eruption. Mutations in genes critical to enamel matrix formation have been documented, but current understanding of other key events in enamel biomineralization is limited. We investigated autosomal-recessive hypomaturation AI in a consanguineous Pakistani family. A whole-genome SNP autozygosity screen identified a locus on chromosome 15q21.3. Sequencing candidate genes revealed a point mutation in the poorly characterized WDR72 gene. Screening of WDR72 in a panel of nine additional hypomaturation AI families revealed the same mutation in a second, apparently unrelated, Pakistani family and two further nonsense mutations in Omani families. Immunohistochemistry confirmed intracellular localization in maturation-stage ameloblasts. WDR72 function is unknown, but as a putative β propeller is expected to be a scaffold for protein-protein interactions. The nearest homolog, WDR7, is involved in vesicle mobilization and Ca²⁺-dependent exocytosis at synapses. Vesicle trafficking is important in maturation-stage ameloblasts with respect to secretion into immature enamel and removal of cleaved enamel matrix proteins via endocytosis. This raises the intriguing possibility that WDR72 is critical to ameloblast vesicle turnover during enamel maturation.

Molar-incisor-hypomineralisation (MIH). A retrospective clinical study in Greek children. II. Possible medical aetiological factors

AIM

This was to examine the potential medical aetiological factors involved in the development of MIH.

METHODS

During the years 2003--2005, all MIH cases diagnosed according to set criteria were selected from the new patients clinic of a Community Dental Centre for Children (Athens). The age, gender and teeth involved were recorded. A control group of socio-demographically matched controls was also identified. The potential aetiological factors were retrieved through personal interview with the parents and from each child and mother's medical book. Only verified aetiological factors were recorded. Evaluation of the correlation of affected teeth and the timing of the insult was performed in a separate group of 225 affected children aged 8-12 with their entire 12 'index' teeth erupted.

RESULTS

From the 3,518, 5.5 to 12 years old children examined, 360 (10.2%) had MIH. Aetiology of MIH: 44 children (12.2%), presented without any relevant medical history, the remaining 316 (87.8%) recorded various medical problems associated with MIH, compared with 18.9% for controls. Perinatal (163, 33.6%) and postnatal (162, 33.9%) problems were the most frequently found and prenatal the least (33, 8.6%). For 42 children (11.7%) problems occurred in more than one chronological period, mainly during both the perinatal and postnatal period (11.1%). The most common prenatal problem was repeated episodes of high fever (12/33), in the perinatal period birth by Caesarean section (92/163) and other birth complications (34/163). Various respiratory conditions (88/162), repeated episodes of high fever (31/162) and neonatal illness (28/162) were the commonly reported problems in the postnatal period. Many MIH cases presented with more than one medical problem during the peri-and postnatal period.

STATISTICAL ANALYSIS

Children with MIH recorded 68.9% more frequent medical problems than controls (p<0.0001). A positive correlation (p<0.001) between the total number and type of affected teeth with the timing of the insult was observed in the 225 MIH children with all their 'index' teeth erupted.

CONCLUSION

Children with MIH present with more medical problems than controls during their prenatal, perinatal and postnatal period. The majority of these illnesses may produce hypocalcaemia, hypoxia and pyrexia to the child or the mother. The number of affected teeth was associated with the timing of the possible insult; children with prenatal, perinatal and postnatal problems present more affected teeth in increasing order.