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.
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