Anion Exchanger Ae2 is Essential for Mouse Incisor Enamel Maturation

Objectives: Na+-independent anion exchangers (AE), responsible for exchange of Cl^- for HCO₃^- ions across plasma membranes, are essential for homeostasis of intracellular pH. Recent studies suggest the presence of a bicarbonate release mechanism in maturation stage ameloblasts to control excess acid arising from mineral formation (Smith et al., JBMR 20:240,2004). The aim of this study was to investigate if the ubiquitous anion exchanger-2 (Ae2) is responsible for this function and thus involved in enamel mineralisation.

Methods: Ae2_a,b^-/- mice were generated as described previously (Medina et al., PNAS 100:15847, 2003).Lower jaws of Ae2_a,b^-/- mice (n=5), Ae2_a,b^+/- (n=4) and Ae2_a,b^+/+ (n=4) mice were fixed in buffered formaldehyde. One hemimandible was decalcified, embedded in epoxy resin and used for histology. The other one was embedded undecalcified in methylmethacrylate and the early maturation stage incisor enamel analysed for Ca, P, S and Mg by quantitative X-ray microprobe.

Results: Macroscopically, the erupted incisor enamel of the Ae2_a,b^-/- mice either was absent or highly eroded. In the Ae2_a,b^-/-mice, but not in the Ae2_a,b^+/+, the maturation stage ameloblasts were disorganised with ample evidence of organic matrix and cellular debris retention in the maturation stage enamel. Quantitative X-ray microprobe data indicated that Ca and P were significantly decreased while S, a marker for amelogenins, and Mg were significantly increased in enamel of Ae2_a,b^-/- mice compared to Ae2_a,b^+/+ mice. No differences were found between Ae2_a,b^+/+and Ae2_a,b^+/- mice.

Conclusion: The results indicate that Ae2_a,b is essential for proper enamel maturation and organic matrix resorption probably through control of acidification of the extracellular enamel compartment adjacent to the ruffle-ended ameloblasts. The significance of the elevated Mg content in the Ae2_a,b^-/- mice is not known.