Enamel Mineral Acquisition Rates in Kallekrein 4 Null Mice

Mice deficient in kallekrein 4 (Klk4^-/-) produce enamel that appears normal in thickness and rod arrangements but is noticeably softer and less resistant to abrasion, and prone to fracture near the boundary between initial and inner enamel layers. This contrasts with enamelysin (Mmp20^-/-) null mice which have thin and soft enamel showing poorly organized rod patterns and a tendency to delaminate easily from the dentino-enamel junction. Objective: To quantify the amount of mineral acquired during enamel development on incisors from Klk4^-/- mice as compared to wild type and Mmp20^-/- mice. Methods: Hemi–maxillae and –mandibles were removed from 6-7 week old mice of each genotype, rapidly frozen, and lyophilized. A series of 1-mm-long enamel strips were removed from the incisors, individually weighed, and ashed for 18 h at 575°C. The residues were then reweighed to determine the amount of mineral present in each strip. Results: The total dry weights of enamel strips taken from Klk4^-/- incisors unexpectedly were normal and the enamel became very hard at its surface at the same position within the maturation stage as occurs in wild type mice. The enamel underneath the surface, however, was organic-rich and contained less mineral allowing strips to be removed all the way to the incisal tips of these incisors. Mineral acquisition rates at their peak in the maturation stage were ~25% below normal on Klk4^-/- incisors compared to ~70% below normal on Mmp20-/- incisors. Data adjusted for enamel thickness indicated that mineral acquisition rates on Mmp20^-/- incisors were relatively normal up to early maturation but still 60% below normal in mid maturation where acquisition rates peak. Conclusions: Enamel proteases are essential for development of proper thickness (Mmp20) and for ultimate removal of organic material from the innermost regions of the enamel layer (Mmp20+Klk4). Supported by NIDCR Grants DE016276 and DE015846.