Frog amelogenin overexpression in mice disrupts enamel prism formation

Amelogenin, the major protein of the developing enamel matrix, controls enamel crystal growth and habit. We have determined that enamel hardness and elasticity and the number of Exon 6 PXQ repeats are increased in mammalian enamel as compared to frog enamel. In addition, mouse enamel contains prisms, while frog enamel does not. Objective: To test and compare the effect of amphibian amelogenins in a mouse model, transgenic mouse overexpressing amphibian amelogenin recombinant protein were generated. Methods: A transgenic mouse model was generated in which the mouse amelogenin promoter drove a Rana pipiens amelogenin construct. For histological analysis, jaws were fixed, decalcified, and processed for paraffin sections. For scanning microscopy analysis, jaws were dehydrated and teeth sectioned in half. Tooth enamel structure was analyzed after samples were coated with a thin layer of gold-palladium. Results: Our data suggest that transgenic overexpression of Rana pipiens amelogenin results in significant changes in tooth enamel structure. In eruption-stage mouse incisors, transgenic mice lacked a transparent zone at the tip of the enamel, featured a reduced pigment zone and an Amelogenesis imperfecta resembling enamel surface. In transgenic mouse molars overexpressing frog amelogenin, the enamel layer was divided equally into a basal prismatic and a superficial prism-less zone. Moreover, transgenic animals featured decreased enamel thickness by 9 percent and a sharply separated dentin-enamel junction. Conclusion: Our findings suggest that the evolution of vertebrate enamel from amphibians to mammals involved the introduction of prismatic enamel, increased enamel thickness, and an interconnected dentin-enamel junction. Funding by NIH grant DE15425 and NSF grant MCB-0236226 is gratefully acknowledged.