Cellular responses to hydrostatic pressure and the role of the hippo pathway

Abstract

The ability of cells to respond to changes in mechanical forces in their microenvironment is critical for development, to respond to insults as well as for maintaining homeostasis. The Hippo Pathway and its downstream effectors, the transcriptional co-activators YAP and TAZ, are central to transducing mechanical forces into cellular effects. It is well recognised that various types of mechanical stimuli, such as shear stress and stiffness of the extracellular environment, determine YAP/TAZ activity through regulating their subcellular localisation. However, the role of the Hippo Pathway and YAP/TAZ in cellular responses to hydrostatic pressure has not been established to date. Investigating the role of compressive fluid pressure in determining cellular responses may enable better understanding of the effects of mechanical forces in diseased states, such as elevated interstitial fluid pressure within solid tumours. Moreover, it is not clear whether cellular components responsible for detecting external forces differ depending on the type of mechanical stress applied. The aim of this project was to delineate the mechanism underlying cellular responses to hydrostatic pressure with a focus on the role of the Hippo Pathway and its downstream effectors YAP/TAZ. By using a bespoke hydrostatic pump setup and digital holographic microscopy in conjunction with various biochemical assays, it was identified that elevated hydrostatic pressure activates YAP/TAZ activity and induces expression of pro-proliferative YAP/TAZ target genes (CYR61 and CTGF).

Furthermore, YAP/TAZ expression are critical to regulation of cell volume at steadystate and in response to dynamic fluid pressure. Mechanistically, unlike caveolae implicated in cell response to shear stress, oscillating hydrostatic pressure induces clathrin-dependent internalisation. Importantly, inactivation of Hippo Pathway kinases LATS1/2 and MST1/2 are central to this response. These results support a mechanism where dynamic changes in extracellular fluid pressure likely lowers the energy barrier required to initiate clathrin-dependent internalisation and downstream signalling events.