Not just a pipe's dream : plumbing the contribution of cell diversity to oesophageal homeostasis

Abstract

Epithelial barrier tissues like the skin, intestine, and oesophagus form a physical barrier that protects our body from external threats. To accurately fulfil their function, epithelial barriers are subject to constant epithelial cell renewal throughout our lifespan. Recurrent tissue turnover requires the precise control of epithelial cell proliferation and differentiation to maintain homeostasis and health. Adult tissue stem cells residing in epithelial barriers are of pivotal importance for tissue homeostasis and repair. The stem cell niche, composed of a variety of cells, mechanical and chemical elements, provides decisive signalling cues that influence stem cell behaviour and fate.

The major part of this thesis investigates behaviour of the mouse oesophageal epithelial progenitor cell and sets out to uncover and characterise the contribution of cellular diversity to oesophageal homeostasis.

In paper III, we demonstrated that a subpopulation of oesophageal progenitor cells expressed Tnfrsf19 (Troy) and contributed long-term to oesophageal homeostasis. Using lineage tracing in combination with mathematical modelling we proposed that Troy progenitor cell fate is predominantly symmetrical. In addition, functional TROY knockout in vivo suggested that TROY regulates progenitor proliferation and facilitates differentiation. Thus, TROY might be involved in context dependent cellular decision making processes providing a basis for behavioural progenitor heterogeneity.

In paper IV, we characterise regional oesophageal cell composition utilising single cell RNA sequencing. Combining cell-cell communication inference and organoid culture we reveal regionally diverse contributions of fibroblasts and immune cells as well as signalling pathways such as BMP and IGF that differently influence epithelial cell behaviour. In paper II, we developed an organoid co-culture system of oesophageal epithelial cells and fibroblasts that allows for detailed functional investigations of cell-cell communication in vitro.

The generation of the stratified squamous epithelium of the skin is governed by intricate and interwoven processes of proliferation, cell cycle exit, differentiation, and stratification. In paper I, we probed the function of ID1 in epidermal development and demonstrated ID1 binding to the transcription factor TCF3. We propose that ID1-CEBPA crosstalk regulates epidermal cell fate decision within a ID1-TCF3-CEBPA axis.

The work provided within this thesis demonstrates molecular mechanisms and signalling cues that impinge on epithelial cell behaviour during homeostasis and development.