Dynamic Interaction Between Transit Amplifying and Mesenchymal Stem Cells During Mesenchymal Tissue Homeostasis

Objectives: The mouse incisor provides an excellent model for mesenchymal stem cell (MSC) study. Our previous studies have demonstrated that Gli1⁺ perivascular cells are typical MSCs in vivo. Here, we examined the dynamic interaction between MSCs and transit amplifying (TA) cells during mesenchymal tissue homeostasis by generating mice conditionally lacking several canonical Wnt signaling related genes because previous studies have shown that Wnt/β-catenin signaling plays critical roles in many aspects of stem cell biology.
Methods: We generated multiple transgenic animal models for our study: Gli1-CE;Beta-catenin^fl/fl, Gli1-CE;Beta-catenin^fl/fl;Gli1-LacZ, Axin2-CE;Tdt, Axin2-CE;Wntless^flox/flox, Axin2-CE;Wntless^flox/flox;Gli1-LacZ, Axin2-CE;Beta-catenin^flox/flox, Axin2-CE;Beta-catenin^flox/flox;Gli1-LacZ mice. CT scanning and histological analysis were performed to analyze their phenotypes.
Results: We found that canonical Wnt signaling regulates the MSC to TA cell transition in mouse incisors because TA cells are undetectable in Gli1-CE;Beta-catenin^fl/fl mice one week after tamoxifen injection. Lineage tracing experiments demonstrated that Axin2+ cells represent at least a subpopulation of TA cells in mouse incisors. Moreover, TA cells disappear in Axin2-CE;Wntless^flox/flox mice three weeks after tamoxifen injection, suggesting that Axin2+ TA cells are regulated via an autocrine Wnt signaling feedback loop. We also found that Axin2+ TA cells are canonical Wnt signaling dependent because Axin2-CE;Beta-catenin^flox/flox mice lack TA cells one week after tamoxifen induction. Finally, Gli1+ MSCs are reduced after TA cell loss in Axin2-CE;Wntless^flox/flox;Gli1-LacZ and Axin2-CE;Beta-catenin^flox/flox;Gli1-LacZ mice, indicating that there is feedback from Axin2+ TA cells to Gli1+ MSCs.
Conclusions: Axin2+ TA cells are regulated via an autocrine Wnt signaling feedback loop and Axin2+ TA cells and Gli1+ MSCs dynamically interact during mesenchymal tissue homeostasis.