Osteoblast differentiation of Gli1⁺ cells via Wnt and BMP signaling pathways during orthodontic tooth movement

Objectives

Factors that induce bone formation during orthodontic tooth movement (OTM) remain unclear. Gli1 was recently identified as a stem cell marker in the periodontal ligament (PDL). Therefore, we evaluated the mechanism of differentiation of Cre/LoxP-mediated Gli1/Tomato⁺ cells into osteoblasts during OTM.

Methods

After the final administration of tamoxifen to 8-week-old Gli1-Cre^ERT2/ROSA26-loxP-stop-loxP-tdTomato mice for 2 days, nickel–titanium closed coil springs were attached between the upper anterior alveolar bone and the first molar. Immunohistochemical localizations of β-catenin, Smad4, and Runx2 were observed in the PDL on 2, 5, and 10 days after OTM initiation.

Results

In the untreated tooth, few Gli1/Tomato⁺ cells were detected in the PDL. Two days after OTM initiation, the number of Gli1/Tomato⁺ cells increased in the PDL on the tension side. On this side, 49.3 ± 7.0% of β-catenin⁺ and 48.7 ± 5.7% of Smad4⁺ cells were found in the PDL, and Runx2 expression was detected in some Gli1/Tomato⁺ cells apart from the alveolar bone. The number of positive cells in the PDL reached a maximum on day 5. In contrast, on the compression side, β-catenin and Smad4 exhibited less immunoreactivity. On day 10, Gli1/Tomato⁺ cells were aligned on the alveolar bone on the tension side, with some expressing Runx2.

Conclusions

Gli1⁺ cells in the PDL differentiated into osteoblasts during OTM. Wnt and bone morphogenetic proteins signaling pathways may be involved in this differentiation.