Patterns of Wnt pathway activity in the mouse incisor indicate absence of Wnt/β‐catenin signaling in the epithelial stem cells

Abstract

The Wnt pathway is crucial for tooth development as shown by dental defects caused by impaired Wnt signaling in mouse and human. We investigated Wnt signaling in continuously growing mouse incisors focusing on epithelial stem cells. Ten Wnt ligands were expressed both in the dental epithelium and mesenchyme, and were associated mainly with odontoblast and ameloblast differentiation. Wnt/b-catenin activity was detected in mesenchyme in BATgal and TOPgal reporter mice while Axin2, also a reporter of Wnt/b-catenin signaling, was expressed additionally in the epithelium. Axin2 was, however, excluded from the epithelial stem cells in the cervical loop. Interestingly, these cells expressed specifically Lgr5, a Wnt target gene and stem cell marker in the intestine, suggesting that Lgr5 is a marker of incisor stem cells but is not regulated by Wnt signaling in the incisor. We conclude that epithelial stem cells in the mouse incisors are not regulated directly by Wnt/b-catenin signaling. Developmental Dynamics 239:364-372,

Roles of Heparan Sulfate Sulfation in Dentinogenesis

“…However, there are some discrepancies among developmentally dynamic Sulf mRNA expression, 10E4 immunoreactivity, and TOPGAL reporter expression (23). Indeed, Wnt10a mRNA expression and TOPGAL reporter expression are only localized in the odontoblast layer.…”

“…Mouse Sulf1 and Sulf2 probes were generated from a 461-bp Sulf1 fragment spanning the region between 124 and 584 in accession number AY101178 and a 459-bp Sulf2 fragment spanning the region between 3056 and 3514 in accession number AY101178. The preparation of Wnt10a, Dspp, and Axin2 RNA probes has been described previously (23,26).…”

“…Recent TOP-GAL staining data and Axin2 expression data suggested that the Wnt canonical pathway is activated in dentinogenesis (23,24). This pathway is known to be promoted through Sulf activity (38).…”

“…In early tooth development, various Wnt genes are expressed from the bud stage to the early bell stage (22). A recent study revealed that TOPGAL and Axin2 reporter activity are high in the odontoblast layer (23,24), suggesting that dentinogenesis is associated with the activation of the canonical Wnt signaling pathway. These findings are in line with the human tooth phenotypes observed after the heterozygotic loss of Axin2 function, which causes tooth agenesis and/or hypodontia (25).…”

Immunolocalization of Axin2 and p-Smad3 in Developing Rat Molar Germ

“…On the other hand, in the late bell stage in which dentin and enamel are formed, Wnt-10a mRNA is expressed in preodontoblasts and odontoblasts and dentin sialophosphoprotein (Dspp) mRNA is expressed by odontoblasts, on the basis of which Wnt-10a has been reported to be involved in odontoblast differentiation and dentin formation by regulating Dspp 10) . It has been reported that Wnt-10a and Axin2 appeared in the incisor tooth germ ameloblasts 11) , Wnt10, Dvl, inactive-type GSK-3β and β-catenin distributed in secretory-stage ameloblast and progenitors of the more tooth germ 12) , and β-catenin/LEF1 activates the appearance of enamelin in ameloblast-like cells 13) , indicating involvement of Figure 1. Immunohistochemical observation of the rat first molar germ on embryonic day 19. a Hematoxylin and eosin (H-E) staining.…”