title: Ameloblast Modulation and Transport of Cl⁻, Na⁺, and K⁺ during Amelogenesis.
abstract: Ameloblasts express transmembrane proteins for transport of mineral ions and regulation of pH in the enamel space. Two major transporters recently identified in ameloblasts are the Na(+)K(+)-dependent calcium transporter NCKX4 and the Na(+)-dependent HPO4 (2-) (Pi) cotransporter NaPi-2b. To regulate pH, ameloblasts express anion exchanger 2 (Ae2a,b), chloride channel Cftr, and amelogenins that can bind protons. Exposure to fluoride or null mutation of Cftr, Ae2a,b, or Amelx each results in formation of hypomineralized enamel. We hypothesized that enamel hypomineralization associated with disturbed pH regulation results from reduced ion transport by NCKX4 and NaPi-2b. This was tested by correlation analyses among the levels of Ca, Pi, Cl, Na, and K in forming enamel of mice with null mutation of Cftr, Ae2a,b, and Amelx, according to quantitative x-ray electron probe microanalysis. Immunohistochemistry, polymerase chain reaction analysis, and Western blotting confirmed the presence of apical NaPi-2b and Nckx4 in maturation-stage ameloblasts. In wild-type mice, K levels in enamel were negatively correlated with Ca and Cl but less negatively or even positively in fluorotic enamel. Na did not correlate with P or Ca in enamel of wild-type mice but showed strong positive correlation in fluorotic and nonfluorotic Ae2a,b- and Cftr-null enamel. In hypomineralizing enamel of all models tested, 1) Cl(-) was strongly reduced; 2) K(+) and Na(+) accumulated (Na(+) not in Amelx-null enamel); and 3) modulation was delayed or blocked. These results suggest that a Na(+)K(+)-dependent calcium transporter (likely NCKX4) and a Na(+)-dependent Pi transporter (potentially NaPi-2b) located in ruffle-ended ameloblasts operate in a coordinated way with the pH-regulating machinery to transport Ca(2+), Pi, and bicarbonate into maturation-stage enamel. Acidification and/or associated physicochemical/electrochemical changes in ion levels in enamel fluid near the apical ameloblast membrane may reduce the transport activity of mineral transporters, which results in hypomineralization.
authors.affiliation: Department of Oral Cell Biology, Academic Centre for Dentistry Amsterdam, University of Amsterdam, and MOVE Research Institute, VU University Amsterdam, Amsterdam, Netherlands a.bronckers@acta.nl.
authors.forename: A L J J
authors.initials: AL
authors.lastname: Bronckers
authors.affiliation: Department of Oral Cell Biology, Academic Centre for Dentistry Amsterdam, University of Amsterdam, and MOVE Research Institute, VU University Amsterdam, Amsterdam, Netherlands.
authors.forename: D
authors.initials: D
authors.lastname: Lyaruu
authors.affiliation: Department of Oral Cell Biology, Academic Centre for Dentistry Amsterdam, University of Amsterdam, and MOVE Research Institute, VU University Amsterdam, Amsterdam, Netherlands.
authors.forename: R
authors.initials: R
authors.lastname: Jalali
authors.affiliation: Division of Gene Therapy and Hepatology, School of Medicine/CIMA, University of Navarra, and CIBERehd, Pamplona, Spain.
authors.forename: J F
authors.initials: JF
authors.lastname: Medina
authors.affiliation: Department of Oral Cell Biology, Academic Centre for Dentistry Amsterdam, University of Amsterdam, and MOVE Research Institute, VU University Amsterdam, Amsterdam, Netherlands.
authors.forename: B
authors.initials: B
authors.lastname: Zandieh-Doulabi
authors.affiliation: Department of Oral Sciences, University of California, San Francisco, CA, USA.
authors.forename: P K
authors.initials: PK
authors.lastname: DenBesten
mesh_terms: D000565:Ameloblasts / Q000378:metabolism / Q000502:physiology*; D000566:Amelogenesis / Q000502:physiology*; D000818:Animals; D017920:Antiporters / Q000502:physiology; D015153:Blotting, Western; D002113:Calcification, Physiologic / Q000502:physiology; D027963:Chloride-Bicarbonate Antiporters / Q000502:physiology; D002712:Chlorides / Q000378:metabolism; D019005:Cystic Fibrosis Transmembrane Conductance Regulator / Q000502:physiology; D003743:Dental Enamel / Q000254:growth & development; D004577:Electron Probe Microanalysis; D051379:Mice; D011188:Potassium / Q000378:metabolism; D012964:Sodium / Q000378:metabolism; D050614:Sodium-Phosphate Cotransporter Proteins, Type IIb / Q000502:physiology
chemical_list: D017920:Antiporters; D027963:Chloride-Bicarbonate Antiporters; D002712:Chlorides; C569957:Slc24a4 protein, mouse; C493558:Slc34a2 protein, mouse; D050614:Sodium-Phosphate Cotransporter Proteins, Type IIb; D019005:Cystic Fibrosis Transmembrane Conductance Regulator; D012964:Sodium; D011188:Potassium
keywords: SLC24A4; SLC34A2; chloride-bicarbonate exchangers; dental enamel hypomaturation; electron probe microanalysis; fluorosis