Population and Individual Stem Cell Dynamics in the Olfactory Epithelium
Unlike the majority of the nervous system, the olfactory epithelium (OE) continuously produces new neurons throughout adulthood. Under homeostatic conditions, OE neurogenesis is fueled by the ongoing divisions of globose basal cells (GBCs), but following severe injury that destroys all mature cell types, the normally quiescent horizontal basal cells (HBCs) regenerate the entire tissue, including GBCs, neurons, and sustentacular cells. Moreover, conditional knockout (cKO) of the transcription factor, p63, induces HBCs to differentiate at steady-state (Fletcher et al. 2011). However, the cellular and transcriptional dynamics underlying HBC self-renewal and differentiation remain poorly characterized. To maintain tissue homeostasis, an appropriate balance of stem cell self-renewal and differentiation is achieved via invariantly asymmetric divisions in invertebrates (Doe and Bowerman 2001), and a mixture of symmetric and asymmetric divisions during vertebrate development (Lechler and Fuchs 2005; Morrison et al. 1995). While regulators of these types of mitoses are conserved in adult tissues, adult stem cell divisions are often modulated by the surrounding tissue, a phenomenon known as population asymmetry (Simons and Clevers 2011). Lineage tracing of stem cells can provide insights into population asymmetry in adult tissues (Ritsma et al. 2015; Bonaguidi et al. 2011), and single-cell RNA sequencing has recently emerged as a powerful tool for probing the transcriptional changes that occur in cycling and regenerating tissues (Trapnell et al. 2014; Shin et al. 2015). This dissertation explores mechanisms of HBC self-renewal and differentiation at steady-state and during regeneration using lineage tracing and single-cell RNA-seq. The following was determined: (1) Symmetric HBC divisions are balanced by population asymmetry, independent of spindle orientation and cell polarity biases in dividing HBCs, producing activated HBCs; (2) At steady-state, p63cKO HBCs differentiate directly into sustentacular cells without dividing, and give rise to proliferative neural progenitors that generate numerous neurons; (3) During regeneration, all HBCs are recruited into an activated state and enter the cell cycle prior to renewing or differentiating; and (4) The sustentacular lineage develops prior to the neuronal lineage and via fewer changes in transcriptional state. These results provide novel insights into the dynamics that regulate HBC renewal and differentiation, as well as avenues for further exploration of OE stem cell subtypes, and provide a methodological model for investigating adult stem cells in other tissues.