Title: Identification and characterization of epithelial progenitor niches in skin
Description: The skin contains a highly regionalized stem and progenitor cell niche system, where a particular epithelial progenitor population primarily displays unipotent or bipotent behavior by contributing to their local niche under homeostatic conditions. Some of these populations, however, can also contribute to lineages beyond their local niche in response to wounding or other traumas. The goal of this thesis was to identify additional epithelial progenitor niches in the skin and characterize them. I focused on identifying and investigating the role of epithelial progenitors residing in two discrete compartments, the upper isthmus (UI) of the hair follicle and the touch dome (TD) in the epidermis, during skin development and homeostasis. The UI niche was identified by immunofluorescence studies based on the expression profile of a6 integrin low, Sca-1 negative, CD34 negative cells. The progenitor characteristics of UI keratinocytes were confirmed using skin reconstitution and in vitro colony formation assays.

The transcriptional profiling of UI cells led to the identification of Tbc1d10c, one of the uniquely upregulated genes in UI cells. The comprehensive characterization of the UI niche by immunofluorescence led to the identification of a neural complex called palisade nerve endings (PN) present outside of the UI. Further investigation of PN using immunofluorescence and animal models revealed the presence of a glutamate transport to the HF via sensory nerve fibers. I elucidated that the presence of glutamate was critical for the organization of Schwann cells in the developing HF. Although the relationship between UI keratinocytes and PN still needs to be investigated, the presence of PN in the UI niche implicates that there is more function ascribed to UI cells in addition to serving as epithelial progenitors. The next epithelial progenitor niche I identified and characterized was the touch dome (TD) in the epidermis of hairy skin. The TD is a specialized niche, in which both epidermal Merkel cells and TD keratinocytes reside.

The unexpected and exclusive presence of both Tbc1d10c and CD200 proteins in TD keratinocytes eventually led us to find that they were epidermal Merkel cell progenitors. Their progenitor characteristics were validated using skin reconstitution assay and in vivo lineage tracing by EdU incorporation. Our results collectively suggested that TD keratinocytes were bipotent progenitors that gave rise to both squamous and neuroendocrine epidermal lineages. Lastly, I generated a UI-specific Cre transgenic mouse line driven by the Tbc1d10c promoter, which will be utilized for further characterization of UI keratinocytes, including in vivo lineage tracing. I also generated Tbc1d10c complete knockout mice, which I will investigate to determine whether Tbc1d10c plays a role during skin development and homeostasis.