Investigating the Role of Paired-related Homeobox-1 Expressing Cells in Periodontal Formation, Homeostasis, and Regeneration

Abstract

Objectives: The first specific aim of this thesis was to define the presence, assess the distribution, and characterize PRX1 expressing cells within the periodontium. The second specific aim was to assess the contribution of PRX1 expressing cells to the periodontal formation, post-natal periodontal hemostasis, and periodontal regeneration. Methods: To assess the significance of the expression of PRX1 in human periodontium, we tested the expression of the Prx1 gene in samples of human bone, periodontal ligament (PDL), and periodontal stem cells (PDLSCs). The presence of PRX1 expressing cells (PRX1+ cells) in the mouse periodontium was explored by means of in vivo imaging. The distribution of PRX1+ cells in mouse periodontium was investigated in inducible lineage tracing analysis performed postnatally (Prx1-CreER-EGFP+/-;Rosa26-tdTOMATO+/-). PRX1+ cells also were isolated from periodontal ligament of mouse mandibular incisors and their gene expression pattern was explored. The contribution of PRX1+ cells to the formation of the mouse periodontium was investigated by constitutive lineage tracing analysis performed during embryogenesis (Prx1-cre+/-;Rosa26-tdTOMATO+/-). By means of diphtheria toxin (DTA)-mediated conditional ablation of PRX1+ cells, we also explored their contribution to periodontal homeostasis by assessing the width of PDL in Prx1-CreER-EGFP+/-;Rosa26-DTA+/- mice (ablation mouse model) (n=5) and their non-ablating littermates. The contribution of PRX1+ cells to periodontal regeneration was assessed by creating sub-critical size periodontal fenestration defects by the mandibular incisor and first molar teeth. The healing of defects was compared in ablation mouse model (test group) and non-ablation mouse model (control group) (n=5). Results: Gene expression analysis of human samples confirmed that PRX1 is significantly expressed in human PDL and PDLSCs. The gene expression signature of PRX1+ cells demonstrated the expression mesenchymal stem cells markers, up-regulation of Notch signaling, and down-regulation of Wnt signaling. Lineage tracing analyses showed that PRX1+ cell are present in high abundance within the PDL of mouse incisor teeth, and PRX1-lineage cells contribute to the formation of the mouse incisor’s periodontium. Post-natal ablation of PRX1+ cells resulted in a significant enlargement of the periodontal ligament space and lack of periodontal regeneration in sub-critical size periodontal defects in the continuously regenerating periodontal ligament of mouse incisor. Conclusions: PRX1 expressing cells are present in high abundance within the continuously regenerating periodontal ligament of mouse incisor teeth and at such location they contribute to periodontal formation, periodontal homeostasis, and periodontal regeneration. This study, for the first time, reports the presence of PRX1 expressing cells within human periodontal ligament. Thus, studying mouse periodontal PRX1+ cells may provide significant information for the development of novel and more effective therapeutic approaches to periodontal regeneration in humans.