Synergistic roles of amelogenin and ameloblastin.

Amelogenin and ameloblastin, the major enamel matrix proteins, are important for enamel mineralization. To identify their synergistic roles in enamel development, we generated Amel X(-/-)/Ambn(-/-) mice. These mice showed additional enamel defects in comparison with Amel X(-/-) or Ambn(-/-) mice. In 7-day-old Amel X(-/-)/Ambn(-/-) mice, not only was the ameloblast layer irregular and detached from the enamel surface, as in Ambn(-/-), but also, the enamel width was significantly reduced in the double-null mice as compared with Amel X(-/-) or Ambn(-/-) mice. Proteomic analysis of the double-null teeth revealed increased levels of RhoGDI (Arhgdia), a Rho-family-specific guanine nucleotide dissociation inhibitor, which is involved in important cellular processes, such as cell attachment. Both Amel X(-/-)/Ambn(-/-) mice and Ambn(-/-) mice displayed positive staining with RhoGDI antibody in the irregularly shaped ameloblasts detached from the matrix. Ameloblastin-regulated expression of RhoGDI suggests that Rho-mediated signaling pathway might play a role in enamel formation.