Axin2+ PDL Cells Directly Contribute to New Alveolar Bone Formation in Response to Orthodontic Tension Force

Wnt–β-catenin signaling plays a key role in orthodontic tooth movement (OTM), a common clinical practice for malocclusion correction. However, its targeted periodontal ligament (PDL) progenitor cells remain largely unclear. In this study, we first showed a synchronized increase in Wnt–β-catenin levels and Axin2+ PDL progenitor cell numbers during OTM using immunostaining of β-catenin in wild-type mice and X-gal staining in the Axin2-LacZ knock-in line. Next, we demonstrated time-dependent increases in Axin2+ PDL progenitors and their progeny cell numbers within PDL and alveolar bones during OTM using a one-time tamoxifen-induced Axin2 tracing line (Axin2CreERT2/+; R26RtdTomato/+). Coimmunostaining images displayed both early and late bone markers (such as RUNX2 and DMP1) in the Axin2Lin PDL cells. Conversely, ablation of Axin2+ PDL cells via one-time tamoxifen-induced diphtheria toxin subunit A (DTA) led to a drastic decrease in osteogenic activity (as reflected by alkaline phosphatase) in PDL and alveolar bone. There was also a decrease in new bone mass and a significant reduction in the mineral apposition rate on both the control side (to a moderate degree) and the OTM side (to a severe degree). Thus, we conclude that the Axin2+ PDL cells (the Wnt-targeted key cells) are highly sensitive to orthodontic tension force and play a critical role in OTM-induced PDL expansion and alveolar bone formation. Future drug development targeting the Axin2+ PDL progenitor cells may accelerate alveolar bone formation during orthodontic treatment.