ADAM10 May Facilitate Stem Cell, Pre-ameloblast and Ameloblast Cell Movement

Objectives: Previously we implicated matrix metalloproteinase-20 (MMP20) in cleaving cell-cell contacts to allow ameloblast migration to form the characteristic rodent decussating rod pattern. However, MMP20 is not expressed in the apical loop of mouse incisors, yet this is where the stem cell niche resides and these stem cells can migrate and give rise to pre-ameloblasts. We therefore sought to identify a proteinase that may facilitate cell movement in the apical loop.
Methods: Quantitative PCR was performed to identify candidate gene expression in enamel organs. Expression within the apical loop was identified via in situ hybridization. Western blots and immunohistochemistry confirmed that the proteinase transcripts were translated. And, an in vitro migration/invasion assay with and without proteinase specific inhibitors confirmed that the proteinase facilitated cell movement.
Results: The enamel matrix has been thoroughly assessed over the years and only MMP20 and Kallikrein-4 are known to function within the matrix. Therefore, we focused on membrane bound proteinases with catalytic domains that face the enamel matrix. We screened for ADAM (A Disintegrin And Metalloproteinase) proteinases and discovered that ADAM10 is expressed in the apical loop, in pre-ameloblasts and in secretory stage ameloblasts, but not in maturation stage ameloblasts. Previously ADAM10 was demonstrated to cleave E-cadherin, N-cadherin, and nectin-1. Each of these proteins are involved in cell-cell attachments. Furthermore, removal of cadherins from epithelial cell surfaces or ablation of nectin-1 each cause enamel malformation. Strikingly, ablation of ADAM10 in epithelial tissues also causes tooth malformation. Our in vitro migration/invasion assays demonstrated that ADAM10 facilities migration of ameloblast-derived cells through a Matrigel filter.
Conclusions: We conclude that ADAM10 may facilitate stem cell, pre-ameloblast and ameloblast cell movement perhaps through cleavage of cell-cell contacts. However, ADAM10 is also capable of releasing cell surface signaling molecules and this may also aid cell migration.