Unraveling Population Dynamics in Adult Mouse Incisor Regeneration

Objectives: The adult mouse incisor continuously grows throughout the lifespan of the animal and provides a useful model to study adult stem cell-based tissue renewal. Recently, single cell RNA-sequencing coupled with computational and in vivo lineage tracing analyses revealed that a group of actively cycling epithelial progenitors located in the inner enamel epithelium gives rise to all differentiated populations in the incisor. These analyses also drew attention to the quiescent outer enamel epithelium (OEE), which contains several subpopulations of cells expressing canonical stemness markers. Here, we examine the role of the OEE during homeostasis and injury repair.
Methods: We lineage traced Lrig1 and Lgr5-expressing populations in adult mouse incisors. Then, we used two injury models and monitored incisor regeneration though histological analyses. Cytotoxic injury consisted in injecting 5-fluorouracil (5FU) chemotherapeutic drug to the animals, while another cohort was exposed to x-ray radiations targeting their head and neck. In addition, we started investigating the contribution of Igfbp5-expressing subpopulation to injury repair, having previously characterized its role during homeostasis. Upper and lower incisors were collected, fixed and sectioned for subsequent immunofluorescence and single molecule in situ hybridization assays and confocal microscopy imaging.
Results: We demonstrated that during homeostasis, Lrig1 and Lgr5-expressing cells generated progeny intercalating into the stratum intermedium and ameloblast layer. Recovery from 5FU-mediated injury was achieved faster than recovery from x-ray irradiation. We also found position-specific differences between the upper and lower incisor recovery stages. Our preliminary investigations into the contribution of specific subpopulations highlighted differential participation in overall incisor regeneration.
Conclusions: Our work has yielded important knowledge on the differential regulation of mouse incisor population dynamics in both homeostatic and regenerative conditions. Moreover, we have documented innovative methods to scrutinize the existence of stem-like cell reservoirs in these teeth. Further analyses into subpopulation-specific contribution to incisor repair are ongoing and currently focusing on the possible downregulation of Wnt signals.