Endocytosis mediated by megalin and cubilin is involved in enamel development

Endocytosis of enamel matrix proteins (EMPs) by ameloblasts is a key process in the mineralization of enamel during the maturation stage of amelogenesis. However, the relevant receptor mediating endocytosis of EMPs is still unclear. The aim of this study was to explore potential endocytic receptors involved in this process.

Two endocytic receptors, megalin, and cubilin, were found to be distributed in ameloblasts of mouse incisors and molars during the secretory and maturation stages. Megalin was located at the distal end of ameloblasts during the maturation stage when proteolysis and recycling were the most active. Megalin and cubilin were also expressed in an ameloblast-lineage cell (ALC) line. The immunoelectron microscopy results showed that megalin was positively labeled on the vesicle structures of ALC, where endocytosis happened. Immunofluorescence showed that megalin and cubilin were colocalized with amelogenin, and the absorption of amelogenin was significantly reduced when megalin and cubilin were inhibited by their inhibitor, receptor-associated protein (RAP). Knockdown of megalin and cubilin with siRNA also reduced the ability of ALC to absorb amelogenin.

The results of this study suggest that megalin and cubilin are involved in the absorption process of ameloblasts during amelogenesis.