Alpl ablation in dental epithelium disrupts ameloblasts and incisor enamel mineralization in male mice

Abstract

Tissue-nonspecific alkaline phosphatase (gene: ALPL; protein: TNAP) is a mineralization-associated enzyme that catalyzes several physiological substrates and is widely expressed in skeletal and nonskeletal tissues indicating diverse physiological actions. Loss-of-function mutations in ALPL cause hypophosphatasia (HPP), an inherited skeletal dysplasia featuring rickets and osteomalacia. Hypophosphatasia is also associated with dentoalveolar abnormalities, including dentin defects and acellular cementum hypoplasia, resulting in premature tooth loss. Some reports indicate HPP causes enamel defects. We conditionally ablated Alpl in enamel organ by crossing Krt14^Cre and Alpl^fl/fl mice to generate Krt14^Cre ;Alpl^fl/fl conditional KO (cKO) mice. Control (CTR) and cKO mice were analyzed at 14 and 60 d postnatal using micro-CT, histology, scanning electron microscopy, and an ex vivo caries induction model. In situ hybridization of the incisor revealed Alpl mRNA was expressed in stratum intermedium and maturation stage ameloblasts, but not preameloblasts or secretory stage ameloblasts. Compared to CTR, cKO mice showed no differences in body weight, circulating alkaline phosphatase, or cranial and appendicular bone parameters. By gross observation, cKO incisors showed discoloration, surface irregularities, and blunted tips; however, these alterations were evident in only male cKO mice. Micro-CT analysis indicated enamel hypomineralization in male cKO vs CTR incisors. Scanning electron microscopy showed intact rod-interrod pattern but less mineralized, thinner crystals in male cKO vs CTR incisors. Histology of male cKO incisors revealed impaired prematuration stage ameloblasts due to ameloblast/enamel detachment near the enamel-dentin junction, aberrant proliferating cell nuclear antigen immunostaining, and disorganized multicellular layers. Immunohistochemistry revealed disrupted enamel matrix proteins in cKO incisors. Conditional Alpl ablation in enamel epithelium revealed enamel phenotypes and suggests potential role(s) in maintaining ameloblast integrity, proper initial enamel matrix secretion, and enamel mineralization. Collectively, these findings confirm a role for TNAP in amelogenesis and point to direct effects of HPP on enamel formation, improving our understanding of amelogenesis.

Keywords: ameloblasts; enamel defects; hypophosphatasia; mineralized tissue/development; tissue-nonspecific alkaline phosphatase.