MMP20 cleaves E-cadherin and influences ameloblast development

Abstract

Dental enamel development occurs in stages as observed by the changing morphology of the ameloblasts that are responsible for enamel formation. During the secretory stage of development, proteins including MMP20 are secreted into the enamel matrix. MMP20 is required for proper enamel formation as mutation of the Mmp20 gene causes autosomal recessive amelogenesis imperfecta. Here, we examined in detail the morphology of the Mmp20-null ameloblast cell layer. Intriguingly, we found that the Mmp20-null mouse secretory stage ameloblasts retract their Tomes' processes as if preparing to enter the maturation stage but later reextend their Tomes' processes as if resuming the secretory stage. We also demonstrated that MMP20 cleaves epithelial cadherin, i.e. E-cadherin. Cadherins are transmembrane proteins with extracellular domains that provide adhesive contacts between neighboring cells. Their intracellular domains bind to the cell cytoskeleton through catenins, including β-catenin. When specific MMPs cleave the cadherin extracellular domain, β-catenin is released and may locate to the cell nucleus as a transcription factor. Therefore, MMP20 may influence ameloblast developmental progression through hydrolysis of cadherin extracellular domains with associated release of transcription factor(s).

Fig. 1.

Mmp20-null mouse ameloblasts can initiate, terminate, and reinitiate the secretory stage of enamel development. a–d Successive sections of an Mmp20-null mouse incisor. White arrows point to prominent Tomes’ processes that disappear in b and c. d The arrowhead indicates the return of the Tomes’ processes, with arrows indicating prominent processes. It appears that signals necessary for the ameloblasts to progress through the secretory stage are absent. Am = Ameloblast; d = dentin; e = enamel.

Fig. 2.

Immunoblot showing that MMP20 cleaves the E-cadherin extracellular domain. The recombinant human E-cadherin/Fc chimera that links the E-cadherin extracellular domain to an Fc fragment was incubated with MMP20 that was purified from porcine enamel. Incubations were for 0 or 20 h followed by immunoblotting with primary antisera specific for E-cadherin or the linked Fc fragment. The secondary antiserum was from rabbit (for IgG-Fc Ab) or goat (for E-Cadherin Ab). Also shown is an SDS-PAGE gel stained for protein and 2 negative control immunoblots that were treated with only the secondary antisera.