Carbonic anhydrase XII in enamel maturation: TGF-β-responsive expression and association with enamel mineralization

Carbonic anhydrases (CAs) contribute to acid-base regulation during enamel mineralization; however, their regulation and roles in enamel maturation are unclear. Transforming growth factor (TGF)-β signaling may contribute to acid-base regulatory mechanisms during enamel formation, but its relationship with CA isoforms has not been fully elucidated. Therefore, TGF-β-regulated CA isozymes and the role of CA12 in enamel formation were investigated.

CA isozyme expression in TGF-β-treated dental epithelial cells (mHAT9d) was analyzed using RNA sequencing and quantitative polymerase chain reaction, and CA12 localization was examined by immunohistochemistry. Enamel morphology, volume, thickness, mineral composition, and density in Car12-knockout mice were evaluated by micro-computed tomography, electron probe microanalysis, and histomorphometry.

Among the CA family genes, Car12 was most markedly induced by TGF-β1 and TGF-β3. CA12 was localized to the distal region of maturation-stage ameloblasts, but it was not detected in secretory-stage ameloblasts. There was no significant reduction in enamel volume on days 5 or 11 in Car12^-/- mice; however, there were surface irregularities, enamel defects, and reduced enamel volume and thickness by day 70. There were reduced Ca/P ratios and highly mineralized enamel regions in Car12^+/- and Car12^-/- mice, with more pronounced changes in Car12^-/- mice.

TGF-β1 and TGF-β3 induce Car12 expression, and CA12 is specifically localized to maturation-stage ameloblasts. CA12 may contribute to optimal enamel mineralization and long-term structural stability during enamel maturation. It may be involved in local acid-base homeostasis because of its known biochemical function and stage-specific localization, but it has not been directly demonstrated in ameloblasts.