Co‐opting Lef‐1 and miR‐26bactivities to regulate dental epithelial stem cells and supporting tissues
Authors: Su Dan , Eliason Steve , Leonard Riley , Shao Fan , Amendt Brad 
Published: May 2022

ObjectiveStem cells are potent to self‐renew and differentiate which contribute to the regenerative process of various mature tissue types. It is important to understand how stem cell behaviors are regulated to give further insights to the application of stem cell therapies in tissue regeneration and repair. We use the developing murine lower incisor as a model to study stem cell maintenance and differentiation, which are housed in a niche and are fated to only a handful of differentiated cell types. Tooth development is a complex process involving rigid temporal and reciprocal regulation of transcription factors including Sox2 and Lef‐1, and signaling pathways including Wnt. We are particularly interested in the regulation between Lef‐1 and miR‐26b due to their inverse expression profiles during tooth development. Lef1 harbors a miR‐26b binding site in its 3’‐UTR and is negatively regulated by miR‐26b. In this study, we aim to understand the role of Lef‐1 and miR‐26b in dental epithelial stem cell (DESC) differentiation, determine the molecular mechanism of miR‐26b regulated Lef‐1 expression and identify genetic pathways for tooth development and regeneration.MethodsWith our newly developed miR‐26b over‐expression (miR‐26b OE) and Lef‐1 conditional over‐expression (COEL) mouse models, we determined the expression of miR‐26b and Lef‐1 by microarray and qPCR. To evaluate the phenotype of tooth development of these models, we performed H&E and IF staining during embryogenesis. Knowing the regulatory relationship between miR‐26b and Lef‐1, we crossed COEL and miR‐26b OE mice and evaluated their phenotype by microCT, H&E and IF staining to validate that COEL mouse phenotype could be rescued by miR‐26b OE. Because Lef‐1 is a Wnt signaling target, a Wnt signaling array was performed in miR‐26b OE E18.5 mandibles.ResultsThe COEL mouse exhibits tusk‐like incisors, a new stem cell compartment in the LaCL and increased DESC proliferation, whereas the miR‐26b OE mice do not have teeth. The expression of miR‐26b is decreased in the COEL mouse and Lef‐1 is decreased in miR‐26b OE mouse. The incisor phenotype observed in the COEL mouse can be rescued by crossing to the miR‐26b OE mouse. Other Wnt related genes including Wnt5a are downregulated whereas some Wnt related genes including Nanog are upregulated in miR‐26b OE mouse.ConclusionCOEL and miR‐26b OE mice have very different incisor phenotypes and crossing COEL and miR‐26b OE mouse rescues the phenotypes. These results further validate that miR‐26b regulates the dental epithelial stem cell niche by negatively regulating Lef‐1, which also indicate their important roles in tooth development. Wnt signaling participates in this regulatory network.SignificanceWith the first miR‐26b OE mouse model and COEL mouse model, we are the first to demonstrate a molecular mechanism for how stage specific miR‐26b expression acts to compartmentalize the stem cell niche through regulating the known developmental transcription factor Lef‐1. Our future direction is to determine the transcriptome and epigenome profiles of the DESC niche in WT, COEL and COEL x miR‐26b OE mice. We expect to identify new regulatory pathways using Lef‐1 and miR‐26b that can be used in bioengineering to repair or regenerate tissues.