Axin2-CreERT2 mediated knockout of bone morphogenetic protein receptor type 1A caused abnormal secondary dentine and altered cell fate of Axin2-expressing odontogenic cells.

Aim: Inducing odontogenic differentiation and tubular dentine formation is extremely important in dentine repair and tooth regeneration. Bone morphogenic proteins (BMPs) signalling plays a critical role in dentine development and tertiary dentine formation, whilst how BMPR1A-mediated signalling affects odontoblastic differentiation of Axin2-expressing (Axin2 ) odontogenic cells and tubular dentine formation remains largely unknown. This study aims to reveal the cellular and molecular mechanisms involved in the formation of secondary dentine.

Methodology: Axin2 mice harvested at post-natal 21 (P21) were used to map Axin2 mesenchymal cells. Axin2 ; R26R mice and Axin2 ; R26R ; R26R mice were generated to observe the tempo-spatial distribution pattern of Axin2-lineage cells and the effect of ablation of Axin2 cells on dentinogenesis, respectively. A loss-of-function model was established with Axin2 ; Bmpr1a ; R26R (cKO) mice to study the role of BMP signalling in regulating Axin2 cells. Micro-computed tomography, histologic and immunostainings, and other approaches were used to examine biological functions, including dentine formation, mineralization and cell differentiation in cKO mice.

Results: The results showed rich expression of Axin2 in odontoblasts at P21. Lineage tracing assay confirmed the wide distribution of Axin2 lineage cells in odontoblast layer and dental pulp during secondary dentine formation (P23 to P56), suggesting that Axin2 cells are important cell source of primary odontoblasts. Ablation of Axin2 cells (DTA mice) significantly impaired secondary dentine formation characterized with notably reduced dentine thickness (Mean of control: 54.11 μm, Mean of DTA: 27.79 μm, p = .0101). Furthermore, malformed osteo-dentine replaced the tubular secondary dentine in the absence of Bmpr1a with irregular cell morphology, abnormal cellular process formation and lack of cell-cell tight conjunction. Remarkably increased expression of osteogenic markers like Runx2 and DMP1 was detected, whilst DSP expression was observed in a dispersed manner, indicating an impaired odontogenic cell fate and failure in producing tubular dentine in cKO mice.

Conclusions: Axin2 cells are a critical population of primary odontoblasts which contribute to tubular secondary dentine formation, and BMP signalling pathway plays a vital role in maintaining the odontogenic fate of Axin2 cells.