More than a gut feeling: mechanical control of epithelial homeostasis

All organs in the body are covered by a layer of tightly connected cells: the epithelium. Epithelia form protective barriers while performing essential physiological functions, such as nutrient absorption in the intestine and gas exchange in the lungs. Due to constant exposure to stresses and damage, old or non-functioning cells must continuously be replaced through division of stem cells, which then give rise to the specialized cell types that control tissue function. To maintain a healthy tissue, epithelia tightly balance their rate of division, fate specification and cell loss. Traditionally, cells were shown to do this by adjusting their behavior to biochemical signals, such as growth factors. It is now clear that mechanical forces also play an essential role in this. Epithelial cells experience a variety of mechanical forces, originating from their neighbors, tissue deformations, and external sources. Cells convert these forces into intracellular biochemical responses through specialized mechanosensor proteins. In this thesis, we investigate how epithelial cells sense and respond to mechanical signals, and how mechanical control of cell behavior ensures epithelial tissue homeostasis.

We first investigated the molecular mechanisms of mechanical force transduction in simple cultured epithelia. Through controlled stretch application to epithelial monolayers, we identified the intercellular signaling pathways regulated by mechanical strain. We show that forces transduced by E-cadherin-based cell-cell adhesions activate EGFR-ERK signaling by increasing ligand shedding. This demonstrates how mechanical and biochemical signals can operate within a linear cascade to regulate epithelial responses. We further uncover that epithelial tissues adjust their rate of cell division in response to fluctuations in cell density through a mechanosensitive cell cycle checkpoint. Densely packed cells experience low tensile forces, which arrests a population of cells in G2-phase of the cell cycle. These cells are readily triggered to divide when tension increases, for instance when cells are lost during wounding. These studies identify how intercellular force transduction ensures intracellular signaling regulation and homeostatic control of cell proliferation.

We next extended our insight to more complex epithelia by investigating the role of forces in the homeostatic regulation of the intestinal epithelium. The intestinal epithelium is highly organized in crypt- and villus compartments respectively containing proliferating cell types and diverse specialized cell types that are renewed every 3-5 days. By controlled stretch application to intestinal organoids, we show that mechanical strain regulates intestinal cell type composition by shifting the absorptive lineage to a less mature state. Moreover, we identified that intestinal cell populations elicit distinct responses to mechanical input depending on their position in the tissue. Specifically, mechanical signals exclusively trigger calcium influx through mechanosensitive calcium channels in the crypt cells. Finally, we demonstrate that the mechanosensitive calcium channel PIEZO is essential for intestinal stem cell maintenance and function, thus playing a crucial role in preserving intestinal tissue homeostasis.

Together, this work reveals how mechanical forces regulate epithelial behavior through integration of molecular, cellular, and tissue-level processes. These findings advance our understanding of epithelial homeostasis and provide a conceptual framework for investigating how dysregulation of mechanoresponses may contribute to disease.