Notch signalling controls tooth mesenchymal stem cell activation

Objectives: Crosstalk between quiescent stem cells (QSCs) and transit amplifying cells (TACs) determine tissue development and homeostasis. The molecular evidence for that communication is still missing. The continuously growing mouse incisor provides an excellent model for studying molecular mechanisms of stem cell fate regulation. This work aims to use the mouse incisor mesenchyme to study the mechanism of QSC maintenance and activation, as well as crosstalk with TACs.
Methods: Laser Capture Microdissection was used to isolate regions of interest within the mouse incisor mesenchyme. Real-time RT-PCR screening of cDNA from these samples was performed with probes targeting established mesenchymal QSC and TAC markers. Corresponding protein expression levels were evaluated using immunofluorescent microscopy. Furthermore, cell culture of dissected incisor mesenchyme containing QSCs was performed and stem cell status manipulation was attempted. Transgenic mice with knocking down or overexpression of key Notch molecules were analysed. Three-dimensional reconstruction was applied to study the QSCs and TACs region volume vs. cell number. Rat and clinical human tooth capping samples were evaluated for stem cell status and activation.
Results: We identified populations of QSCs and TACs within the mouse incisor mesenchyme. Notch signalling appears to be a key regulator of cellular quiescence and activation. Notch ligands, which are abundant in the TACs, could promote QSC activation into the transit amplifying stage. Conversely, epigenetic changes within QSCs leads to TAC activation. Similar findings were observed in clinical samples.
Conclusions: Our study showed that Notch signalling plays a key role in tooth stem cell maintenance and differentiation. It is expected that the molecular mechanisms driving dental stem cell behaviour, can be applied to direct differentiation of stem cells into dental tissues and ultimately be utilised in clinical applications.