Expression, structure, and function of enamel proteinases.

Proteinases serve two important functions during dental enamel formation: They (a) process and (b) degrade enamel proteins. Different enzymes carry out these functions. Enamelysin (MMP-20) is the foremost enamel matrix-processing enzyme. Its expression initiates prior to the onset of dentin mineralization and continues throughout the secretory stage of amelogenesis. In vitro, enamelysin catalyzes all of the amelogenin cleavages that are known to occur during the secretory stage in vivo, and it is probably the enzyme responsible for the processing of all enamel proteins. There is evidence suggesting that enamelysin activity is critical for proper enamel formation. Uncleaved and processed enamel proteins often segregate into different compartments within the developing enamel layer, suggesting that they may have different functions. Intact ameloblastin and its C-terminal cleavage products localize in the superficial rod and interrod enamel, while its N-terminal cleavage products congregate in the sheath space. Intact enamelin is only present at the mineralization front within a micrometer of the enamel surface, while its cleavage products concentrate in the rod and interrod enamel. Processed enamel proteins accumulate during the secretory stage, but disappear early in the maturation stage. Enamel matrix serine proteinase 1 (EMSP1), now officially designated kallikrein 4 (KLK4), is believed to be the predominant degradative enzyme that clears enamel proteins from the matrix during maturation. KLK4 expression initiates during the transition stage and continues throughout maturation. KLK4 concentrates at the enamel surface when the enamel matrix disappears, and aggressively degrades amelogenin in vitro. During tooth development, proteinases are secreted by ameloblasts into the extracellular space, where they cleave enamel proteins by catalyzing the hydrolysis of peptide bonds. Enamel proteinases are present in low abundance and are not likely to participate directly in the mineralization process. Two major enamel proteinases have been identified: enamelysin (MMP20) and kallikrein 4 (KLK4). These proteinases are expressed at different times and have different functions. Their roles are to modify and/or to eliminate enamel matrix proteins, which affects the way enamel proteins interact with each other and with the developing enamel crystallites. A brief review of dental enamel formation is presented, followed by a more detailed analysis of enamelysin and KLK4 expression, structure, and function.