De Belly, Henry; (2020) Plasma membrane and cell surface mechanics in embryonic stem cells. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Changes in cell shape frequently accompany cell fate transitions. Cell shape changes are regulated by cell surface mechanics. One of the main determinants of cell surface mechanics is membrane tension, which is regulated by the interaction between the plasma membrane and the cytoskeleton. Yet how mechanics, and in particular membrane tension, affects the regulatory pathways controlling cell fate is poorly understood. In my PhD, I investigated the role of cell surface mechanics in regulating cell fate transition in early development. In order, to probe the interplay between shape, mechanics and fate, I used mouse embryonic stem (ES) cells, which spread as they undergo early differentiation. In order to asses cell surface mechanical changes during exit form naïve pluripotency, I helped establish a membrane pulling assay using an optical tweezer. Using this assay, I found that cell spreading during exit from naïve pluripotency is regulated by a decrease in plasma membrane tension. Higher tension appears to be due to higher expression and activity of proteins regulating membrane-to-cortex attachment, such as Ezrin-Radixin- Moesin. Next I demonstrated using Ezrin mutants that preventing this decrease in membrane tension obstructs early differentiation of ES cells. I confirmed these results using micropatterning to physically prevent the cells from changing their shape and membrane tension. I next investigated which membrane tension-mediated mechanosensitive pathway could explain these results. I found that decrease in membrane tension results in an increase in endocytosis which is a major regulator of signalling events. Specifically, I found that if cell membrane tension is not decreased, endocytosis of FGF signalling components, which direct exit from the ES cell state, is significantly inhibited. This results in defects in exiting naïve pluripotency as the ERK pathway requires endocytosis for full activation. Strikingly, the early differentiation defects I observed can be rescued by increasing Rab5a-facilitated endocytosis. Thus, I show that a mechanically-triggered increase in endocytosis regulates fate transitions. My findings are of fundamental importance for understanding how cell mechanics regulates biochemical signaling during cell fate changes.

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