The transcriptional control of human epidermal stemness.

In everyday life our skin has to cope with a lot of wear and tear. The epidermis forms the surface of the skin and is made up of several layers of cells. The epidermis needs to be renewed constantly to keep our skin in good condition. What's more, trauma, disease or ageing induce tissue damage, so the epidermis needs to be able to repair itself efficiently to keep doing its job - protecting our body from the outside world. Epidermal stem cells make all this possible. They are responsible for constant renewal (regeneration) of our epidermis, and for repairing the epidermis after skin wounding. Epidermal stem cells are one of the few types of stem cells already used to treat patients. They can be taken from a patient, multiplied and used to grow sheets of epidermis in the lab. The new epidermis can then be transplanted back onto the patient as a skin graft to permanently restore massive epidermal defects in patients suffering from severe burn wounds or hereditary skin blistering diseases.

Most of the work in the cells of our body - including epidermal stem cells - is done by proteins, which are a huge, varied group of molecules. Therefore, the thousands of genes expressed in a particular cell determine what that cell can do. The instructions for how to make proteins comes from genes. The process by which the information in a gene is turned into a functional protein is called gene expression. Gene expression is a tightly regulated process that allows a cell to respond to its changing environment. Only a fraction of the genes in a cell are expressed at any one time, and differences in gene expression programmes determine the distinct functions of different cell types.

The physiological process that maintains a constant number of cells in renewing organs is called tissue homeostasis. How a stable pool of epidermal stem cells is maintained during homeostasis and in response to tissue damage remains a fundamental open question. Two functionally redundant proteins called YAP and TAZ are absolutely essential for sustaining human epidermal stem cells. YAP/TAZ work in the epidermal stem cell's nucleus where they interact with many other proteins to promote the expression of genes that allow the stem cells to self-renew in order to maintain stem cell numbers during homeostasis and tissue repair. However, we know only very little about which genes are expressed by YAP/TAZ, and which cellular processes these YAP/TAZ-regulated genes control. We also don't know the identity of the various nuclear proteins that interact with YAP/TAZ to help them control gene expression and sustain epidermal stem cells.

In this proposal, we will use modern technologies to identify all the genes that are regulated by YAP/TAZ to control epidermal stem cell self-renewal. By using computational tools, we can then categorise these genes into gene expression programmes. In addition, we will also obtain a global picture of the various nuclear proteins that interact with YAP/TAZ to control the gene expression programmes that facilitate epidermal stem cell self-renewal. We will assess the physiological roles of these important YAP/TAZ-associated proteins by using human skin equivalents that we can grow outside of the human body in a culture dish.

This project will provide new and significant insights into the control of human epidermal stem cell self-renewal, and enable us to better understand how the epidermis is able to constantly renew itself.

Technical Summary
Trauma, disease or ageing induce tissue damage, which requires the activation of regenerative responses to restore organ function. The skin epidermis is subjected to daily assaults from the external environment. Constant regeneration and repair of human epidermis is endowed by long-lived stem cells (SCs), which generate short-lived, transient-amplifying progenitors that undergo terminal differentiation.

How a stable pool of epidermal SCs is maintained during homeostasis and in response to tissue defects remains a fundamental open question. The functionally redundant transcriptional co-activators YAP and TAZ are absolutely essential for sustaining human epidermal SCs. YAP/TAZ are essential for homeostasis and repair not only in the epidermis but also in a variety of other epithelial tissues. However, we have only a rudimentary understanding of the molecular processes involved.

In this proposal, we will characterise the molecular mechanisms whereby YAP/TAZ control self-renewal of human epidermal SCs. By mapping the genome-wide binding profiles of YAP/TAZ and their essential co-factor WBP2, as well as of histone modifications defining promoters, enhancers, and super-enhancers using a parallel ChIP-sequencing approach, and by computationally integrating these data with transcriptomics data we will comprehensively define the transcriptional programmes controlled by YAP/TAZ/WBP2 that sustain the SC state in human epidermis. In addition, by using two complementary proteomics approaches we will also obtain a global picture of the transcriptional binding partners of YAP/TAZ that facilitate epidermal SC self-renewal. We will assess their physiological roles in sustaining epidermal stemness in the tissue context using state-of-the-art 3D organotypic human skin models that recapitulate both tissue regeneration and homeostasis.

This project will provide new and significant insights into the transcriptional control of human SC self-renewal.