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

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.

CFTR induces extracellular acid sensing in Xenopus oocytes which activates endogenous Ca²⁺-activated Cl⁻ conductance

The cystic fibrosis transmembrane conductance regulator (CFTR) produces a cyclic adenosine monophosphate (cAMP)-dependent Cl⁻ conductance of distinct properties that is essential for electrolyte secretion in human epithelial tissues. However, the functional consequences of CFTR expression are multifaceted, encompassing much more than simply supplying a cellular cAMP-regulated Cl⁻ conductance. When we expressed CFTR in Xenopus oocytes, we found that extracellular acidic pH activates a Ca²⁺-dependent outwardly rectifying Cl⁻ conductance that does not reflect CFTR activity. The proton-activated Cl⁻ conductance showed biophysical and pharmacological features of a Ca²⁺-dependent Cl⁻ conductance, most likely mediated by Xenopus TMEM16A. In contrast to the effects of extracellular acidification, intracellular acidification did not activate an endogenous Cl⁻ conductance. Proton/CFTR-mediated activation of human TMEM16A was also detected in HEK293 cells. The gating mutant G551D-CFTR conferred proton sensitivity, while deltaF508-CFTR enabled proton activation of TMEM16A only in Xenopus oocytes, which, unlike HEK293 cells, allow deltaF508-CFTR to be trafficked to the cell membrane. Activation of TMEM16A by lysophosphatidic acid was enhanced in the presence of CFTR but was additive with activation by extracellular protons. Because expression of CFTR-E1474X did not confer proton sensitivity, we propose that CFTR translocates a proton receptor to the plasma membrane via its PDZ-binding domain.

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.

NBCe1 in mouse and human ameloblasts may be indirectly regulated by fluoride

Enamel biomineralization results in a release of protons into the enamel matrix, causing an acidification of the local microenvironment. This acidification, which may be enhanced by more rapid mineral deposition in the presence of fluoride, must be neutralized by the overlying ameloblasts. The electrogenic sodium bicarbonate co-transporter NBCe1 has been localized in mouse ameloblasts, and has been proposed to have a role in matrix pH regulation. In this study, transcript analysis by PCR showed NBCe1-A present in human ameloblasts, whereas mouse ameloblasts expressed NBCe1-B. In situ hybridization and qPCR in mouse and fetal human incisors showed that NBCe1 mRNA was up-regulated as ameloblasts differentiated. Ingestion of 50 ppm fluoride resulted in an up-regulation of NBCe1 mRNA in maturation-stage mouse ameloblasts in vivo, as compared with controls. NBCe1 expression was up-regulated by low pH, but not by fluoride, in human ameloblast-lineage cells in vitro. The up-regulation of NBCe1 in vivo as enamel maturation and mineralization progressed provides evidence that NBCe1 participates in pH modulation during enamel formation. Up-regulation of NBCe1 in fluorosed maturation ameloblasts in vivo, with no effect of fluoride in vitro, supports the hypothesis that fluoride-enhanced mineral deposition results in acidification of the mineralizing enamel matrix.

Elevated incidence of dental caries in a mouse model of cystic fibrosis

BACKGROUND

Dental caries is the single most prevalent and costly infectious disease worldwide, affecting more than 90% of the population in the U.S. The development of dental cavities requires the colonization of the tooth surface by acid-producing bacteria, such as Streptococcus mutans. Saliva bicarbonate constitutes the main buffering system which neutralizes the pH fall generated by the plaque bacteria during sugar metabolism. We found that the saliva pH is severely decreased in a mouse model of cystic fibrosis disease (CF). Given the close relationship between pH and caries development, we hypothesized that caries incidence might be elevated in the mouse CF model.

METHODOLOGY/PRINCIPAL FINDINGS

We induced carious lesions in CF and wildtype mice by infecting their oral cavity with S. mutans, a well-studied cariogenic bacterium. After infection, the mice were fed a high-sucrose diet for 5 weeks (diet 2000). The mice were then euthanized and their jaws removed for caries scoring and bacterial counting. A dramatic increase in caries and severity of lesions scores were apparent in CF mice compared to their wildtype littermates. The elevated incidence of carious lesions correlated with a striking increase in the S. mutans viable population in dental plaque (20-fold increase in CF vs. wildtype mice; p value < 0.003; t test). We also found that the pilocarpine-stimulated saliva bicarbonate concentration was significantly reduced in CF mice (16 ± 2 mM vs. 31 ± 2 mM, CF and wildtype mice, respectively; p value < 0.01; t test).

CONCLUSIONS/SIGNIFICANCE

Considering that bicarbonate is the most important pH buffering system in saliva, and the adherence and survival of aciduric bacteria such as S. mutans are enhanced at low pH values, we speculate that the decrease in the bicarbonate content and pH buffering of the saliva is at least partially responsible for the increased severity of lesions observed in the CF mouse.

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.

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.