Intrinsic and extrinsic control of epithelial tissue stem cell activity

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PROJECT SUMMARY Tissue stem cells are rare, undifferentiated cells that are capable of self-renewal and are essential for fueling the homeostasis and regeneration of the tissue in which they reside. They are often quiescent, and when activated, they proliferate and differentiate to produce mature cell types with specialized functions. Stem cell activities are jointly controlled by the intrinsic gene expression program and the signals from the surrounding tissue microenvironment. Dissecting the intrinsic and extrinsic mechanisms that govern stem cell quiescence and activation is important not only for gaining fundamental knowledge of tissue and stem cell biology, but also for understanding how to manipulate cell fates in tissue engineering and regenerative medicine. Myriad regenerative epithelial tissues, such as mammary gland, skin, and prostate, house stem cells in their basal cell compartment. We use two mammalian tissues, mammary gland and skin, each with its unique advantages and clinical relevance, as complimentary model systems to study both general and tissue- specific mechanisms underlying the regulation of basal cell fate and stem cell activities. Our research has elucidated the function of key transcription and chromatin factors in mammary and skin basal/stem cell gene regulation, and how these factors interface with major signaling pathways to control the activation, proliferation, differentiation, and epithelial-mesenchymal plasticity of basal stem cells. The recent advent of single-cell sequencing technology has enabled us to systematically probe the cellular and molecular heterogeneities of mammary and skin basal cells, allowing a deeper and more comprehensive understanding of their compositions and characteristics as well as providing novel insights into the sequence of events in stem cell activation and differentiation. In the next five years, we will continue to employ a multi-disciplinary approach combining single-cell genomics and spatial gene expression mapping with tissue-specific gene knockout and lineage tracing, in vivo and ex vivo stem cell assays, as well as molecular studies to address two major knowledge gaps regarding mammary basal stem cells: how their quiescence is maintained and active expansion is achieved. Specifically, we will test the innovative hypothesis that a low level of Wnt/b-catenin signaling and molecular cross-talks between basal cells and specific macrophage subsets are critical for maintaining basal stem cell quiescence. We will also characterize the novel function and regulation of a newly discovered subset of basal cells as transit amplifying progenitor cells that serve as workhorses to drive basal cell expansion during mammary epithelial morphogenesis, homeostasis, and regeneration. When and where applicable, we will perform parallel analysis on skin in order to identify potentially general principles and strategies underlying basal cell-macrophage cross-talks. Our findings will expose novel intrinsic and extrinsic regulators of basal stem cell quiescence and active expansion. This knowledge is fundamental to preventing stem cell depletion and regenerative diseases as well as to understanding cancer cell dormancy.