Periodontium is the supporting tissue for teeth and is composed of alveolar bone, periodontal ligament (PDL), gingiva and cementum. Periodontal tissues are known to undergo constant turnover supported by the stem cell population. However, this process remain poorly understood because of the failure to identify and locate periodontium mesenchymal stem cells (MSCs) in vivo. Advanced periodontitis, which results in periodontium impairment (including alveolar bone loss and PDL space enlargement), is a major cause of tooth loss in adults. However, why MSCs fail to maintain periodontium integrity in infectious conditions is largely unknown. The goal of my thesis was to test a dual hypothesis: the transcription factor glioma-associated (Gli1) + cells surrounding the neurovascular bundle (NVB) inside PDL are periodontium MSCs, which play a critical role in periodontal tissue turnover and injury repair under the control of Wnt-ß-catenin signaling. Secondly, during periodontitis pathogenesis, the bioactivity of Gli1+ periodontium MSCs is compromised. To test the hypothesis, we used newly developed tissue clearing and multiple imaging techniques in Gli1-CreERT2; Ai14 transgenic fluorescent reporter mice line, plus the conditional knockout strain in combination with the ligation-periodontitis model. Our key findings were: 1). The newly developed tissue clearing method revealed a three-dimensional view of Gli1+ MSCs distribution in adult mouse molar periodontium for the first time. 2). The Gli1+ cells surrounding NVB are periodontium MSCs, which actively maintain periodontium integrity during the animal`s adult stage. Likewise, to facilitate this integrity, 3). Wnt signaling is essential in the regulation of Gli1+ MSCs. However, we also discovered that a loss of ß-catenin within the Gli1+ MSCs (Gli1-CreERT2; ßcatenin loxP; Ai14 line) leads to severe periodontal tissue defects.
For example, 4). overactivated Wnt signaling within Gli1+ MSCs (Gli1-CreERT2; ß-catenin-(Exon3) loxP; Ai14 line) leads to periodontium overgrowth in vivo. 5). Periodontitis also inhibits Gli1+ MSC' activation and lineage commitment activity. 6). Accordingly, chronic periodontitis compromises Gli1+ MSC maintenance. In addition to the issues above, PDL vasculature is compromised in advanced periodontitis. 8). Lastly, we found out that Wnt signaling activity is downregulated during periodontitis while 9) over-activation of Wnt signaling within Gli1+ MSCs restores normal periodontal morphology. Overall, our study found a reliable in vivo marker to label adult mouse molar periodontium MSCs and successfully localized them for the first time. Therefore, our work provides an effective animal model to further study the in vivo response of periodontal MSCs on pathological conditions, thereby providing insight for treatment planning in dental clinics. We further demonstrated the impact of infectious periodontitis on Gli1+ MSCs and revealed part of the mechanism behind the persistence of advanced periodontitis
