DLX homeoproteins physiological implications in enamel formation

Objectives: Tooth development is a complex process including successive stages of initiation, morphogenesis and histogenesis. The role of the Dlx family of homeobox genes during the early stages of tooth development has been widely analyzed, while little data has been reported on their role in dental histogenesis. The expression pattern of Dlx2 has been described in the mouse incisor; an inverse linear relationship exists between the level of Dlx2 expression and enamel thickness, suggesting a role for Dlx2 in regulation of ameloblast differentiation and activity. The aim of the present study was to analyze the expression and function of Dlx genes during amelogenesis.

Methods: In situ hybridization and Dlx2/LacZ transgenic mice were used to establish Dlx genes expression patterns. Dlx2 and Dlx1/Dlx2 null mutant mice were analyzed to characterize Dlx2 functions. Amelogenin promoter studies were realized to evidence its control by DLX homeoproteins.

Results: Analysis of transgenic mice identified spatial variations in Dlx2 expression within molar tooth germs and suggests a role for Dlx2 in the organization of pre-ameloblastic cells in molars. Later, during the secretory and maturation stages of amelogenesis, Dlx2 expression pattern in molars was found to be similar to that previously described in incisors. The expression patterns of the other Dlx genes were examined in incisors and compared to Dlx2. Within ameloblasts Dlx3 and Dlx6 are expressed constantly throughout presecretory, secretory and maturation stages; during the secretory phase when Dlx2 is transitorily switched off, Dlx1 expression is upregulated. Sequence analysis of amelogenin gene promoter revealed five potential responsive elements for DLX proteins that are shown to be functional for DLX2.

Conclusion: This study establishes supplementary functions of Dlx family members during tooth development: the participation in establishment of dental epithelial functional organization and the control of enamel morphogenesis via regulation of amelogenin expression.