Null mutation of chloride channel 7 (Clcn7) impairs dental root formation but does not affect enamel mineralization.

ClC-7, located in late endosomes and lysosomes, is critical for the function of osteoclasts. Secretion of Cl(-) by the ruffled border of osteoclasts enables H(+) secretion by v-H(+)-ATPases to dissolve bone mineral. Mice lacking ClC-7 show altered lysosomal function that leads to severe lysosomal storage. Maturation ameloblasts are epithelial cells with a ruffled border that secrete Cl(-) as well as endocytose and digest large quantities of enamel matrix proteins during formation of dental enamel. We tested the hypothesis that ClC-7 in maturation ameloblasts is required for intracellular digestion of matrix fragments to complete enamel mineralization. Craniofacial bones and developing teeth in Clcn7(-/-) mice were examined by micro-CT, immunohistochemistry, quantified histomorphometry and electron microscopy. Osteoclasts and ameloblasts in wild-type mice stained intensely with anti-ClC-7 antibody but not in Clcn7(-/-) mice. Craniofacial bones in Clcn7(-/-) mice were severely osteopetrotic and contained 1.4- to 1.6-fold more bone volume, which was less mineralized than the wild-type littermates. In Clcn7(-/-) mice maturation ameloblasts and osteoclasts highly expressed Ae2 as in wild-type mice. However, teeth failed to erupt, incisors were much shorter and roots were disfigured. Molars formed a normal dental crown. In compacted teeth, dentin was slightly less mineralized, enamel did not retain a matrix and mineralized fairly normal. We concluded that ClC-7 is essential for osteoclasts to resorb craniofacial bones to enable tooth eruption and root development. Disruption of Clcn7 reduces bone and dentin mineral density but does not affect enamel mineralization.

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.