Laminin 332 is Critical for Secretory Stage Enamel Formation

Objectives: Early during odontogenesis, a basement membrane (BM) separates the oral epithelium-derived enamel organ and the underlying cranial neural crest-derived mesenchyme. As the inner enamel epithelium differentiates into secretory ameloblasts this BM degrades, and proteins normally associated with BM-attachment function in enamel mineral deposition. Biallelic defects in LAMA3 and LAMB3 cause junctional epidermolysis bullosa, featuring broad skin fragility and enamel hypoplasia. Single allelic LAMA3 and LAMB3 defects only cause isolated enamel pits and grooves. We intended to reveal the underlying mechanisms of amelogenesis imperfecta associated with laminin 332 (the heterotrimer of LAMA3, LAMB3, and LAMC2) defects.
Methods: We localized laminin 332 during amelogenesis and characterized the structural and molecular defects in Amelx-iCre;Lama3^fl/fl mice.
Results: Laminin 332 localized along the ameloblast Tomes’ process adjacent to the forming enamel where no BM structure is present, but was not present in the presecretory BM. Amelx-iCre mice expressed CRE recombinase specifically in ameloblasts immediately following the degradation of presecretory BM. No LAMA3 was detected in Amelx-iCre;Lama3^fl/fl ameloblasts. Developing Amelx-iCre;Lama3^fl/fl molars showed no normal enamel deposition, only porous ectopic mineral nodules on the surface of coronal dentin. Following eruption, these molars underwent severe attrition and numerous cracks were observed in the enamel malformations. In Amelx-iCre;Lama3^fl/fl incisors, a thin layer of decussating enamel sometimes formed adjacent to the dentinoenamel junction that became covered by layers of mineral of varied density and thickness. Amelx-iCre;Lama3^fl/fl ameloblasts detached from the forming enamel soon after the onset of secretory stage. These detached ameloblasts remained polarized but denuded of Tomes’ processes. LAMB3 and LAMC2 proteins accumulated in the endoplasmic reticulum (ER), but below the threshold of triggering ER stress. Enamel of Amelx-iCre;Lama3^+/fl mice was comparable to that of the wild-type.
Conclusions: Our studies demonstrate the critical role of laminin 332 during secretory stage amelogenesis.