Abstract

MMP20 (matrix metalloproteinase-20; enamelysin) and KLK4 (Kallikrein-4) are the two major, and perhaps only, proteinases present in developing dental enamel. MMP20 cleaves enamel resident proteins during the early developmental stages when the enamel grows in thickness and KLK4 cleaves enamel proteins prior to their absorption by the ameloblasts that sit atop full thickness enamel. KLK4 is expressed when the enamel starts to harden into its final mature form. Mutations in humans that eliminate the function of either MMP20 or KLK4 cause non-syndromic enamel malformations termed amelogenesis imperfecta. Homozygous deletion of either of these proteinases from the mouse genome will also result in severely malformed dental enamel. With the exception of malformed enamel, no other phenotype is yet known to occur when either of these proteinases are rendered functionless in humans or mice. Therefore, both MMP20 and KLK4 are essential for enamel formation. This chapter will provide historical insights into the discovery of these proteinases and will provide insights into the mechanistic function of how these proteinases act to direct enamel development.