Proteinases in Developing Dental Enamel

Abstract

For almost three decades, proteinases have been known to reside within developing dental enamel. However, identification and characterization of these proteinases have been slow and difficult, because they are present in very small quantities and they are difficult to purify directly from the mineralizing enamel. Enamel matrix proteins such as amelogenin, ameloblastin, and enamelin are cleaved by proteinases soon after they are secreted, and their cleavage products accumulate in the deeper, more mature enamel layers, while the full-length proteins are observed only at the surface. These results suggest that proteinases are necessary for "activating" enamel proteins so the parent proteins and their cleavage products may perform different functions. A novel matrix metalloproteinase named enamelysin (MMP-20) was recently cloned from tooth tissues and was later shown to localize primarily within the most recently formed enamel. Furthermore, recombinant porcine enamelysin was demonstrated to cleave recombinant porcine amelogenin at virtually all of the sites that have previously been described in vivo. Therefore, enamelysin is at least one enzyme that may be important during early enamel development. As enamel development progresses to the later stages, a profound decrease in the enamel protein content is observed. Proteinases have traditionally been assumed to degrade the organic matrix prior to its removal from the enamel. Recently, a novel serine proteinase named enamel matrix serine proteinase-l (EMSPI) was cloned from enamel organ epithelia. EMSP1 localizes primarily to the early maturation stage enamel and may, therefore, be involved in the degradation of proteins prior to their removal from the maturing enamel. Other, as yet unidentified, proteinases and proteinase inhibitors are almost certainly present within the forming enamel and await discovery.

Evolution of Nacre: Biochemistry and Proteomics of the Shell Organic Matrix of the Cephalopod Nautilus macromphalus

“…The degradation of a long precursor into shorter peptides of well-defined size is largely documented for enamel proteins, in particular for amelogenin and MEPE. [54,55] The proteomic approach that we used allowed 38 amino acid sequences, of six to 18 residues long, to be obtained. We are fully aware that this sequence pool represents only a small part of the whole sequences of the shell matrix proteins, thus giving a rather partial picture of the complete shell proteome of Nautilus macromphalus.…”

Altered Amelogenin Self-assembly Based on Mutations Observed in Human X-linked Amelogenesis Imperfecta (AIH1)

“…In contrast, domain B cannot be separated as an interacting domain in isolation, suggesting that conformation is of significance. The previous observation that a truncated amelogenin lacking domain B can no longer interact with domain A (7) could explain in part matrix disassembly following proteolytic cleavage at the carboxyl end of M180 amelogenin by a specific enamel protease (17).…”

“…HBS-EP buffer at pH 7.4 (including150 mM NaCl) approximates closely these ionic values previously determined for maturing enamel (pH 7.26, with 140 mM Na ϩ and 150 mM Cl Ϫ ) (18). Although the recombinant proteins prepared for the SPR studies are non-phosphorylated, a characteristic apparent in some animal species (19), the use of recombinant proteins to study amelogenin biochemistry is widely accepted and well documented (4,11,12,17).…”

Amelogenin-deficient Mice Display an Amelogenesis Imperfecta Phenotype

“…Processing of the amelogenins by enamel proteases, which are also products of ameloblasts, begins shortly after secretion (2,28,29) and, by the end of the maturation stage, results in enamel with ϳ95% mineral and Ͻ1% protein (1,30). Experimental approaches have indicated that the amelogenins play an important role in amelogenesis as antisense inhibition of amelogenin mRNA in tooth organ culture led to disrupted mineral formation (31) and amelogenin ribozyme injection into developing murine mandibles produced a phenotype with enamel mineral abnormalities (32).…”