Temporospatial expression of neurogranin in the rat molar development

Differentiation and proliferation of odontogenic cells are regulated by specific genes, driving the morphological changes during tooth germ development. While substantial data have been gathered on genes involved in early tooth development, knowledge about the terminal differentiation of odontogenic cells-particularly ameloblasts-remains limited. To identify molecules involved in cytodifferentiation, gene expression profiles in maxillary cap stage and root stage molar germs of rats at postnatal day 9, respectively were compared using differential display-PCR. This analysis unexpectedly revealed the upregulation of neurogranin, a brain-specific calmodulin-binding protein, at the cap stage. The expression of neurogranin at both transcriptional and translational levels progressively decreased from the early bell stage, through the crown stage, to the root stage in molar germs. Similar temporal expression patterns were observed for other calmodulin-binding proteins, including neuromodulin and PEP-19. Immunofluorescence confirmed the presence of neurogranin in the inner enamel epithelium of cap and early bell stage molar germs. Correspondingly, the expression levels of calmodulin and PKCγ mRNA displayed dynamic changes during the developmental timeline. Notably, the ablation of neurogranin in SF2 ameloblastic cells enhanced ameloblast differentiation and mineralization. These findings, for the first time, suggest that neurogranin may play a critical role in the temporospatial regulation of the differentiation of inner enamel epithelial cells into ameloblasts.