Even receptors get stressed

“mechanical stress across the plasma membrane of endothelial cells can directly activate G-protein-coupled receptors.”

We all have our own way of dealing with stress — but how do our cells deal with it at a molecular level? Chachisvilis and colleagues now report that mechanical stress across the plasma membrane of endothelial cells can directly activate G-protein-coupled receptors (GPCRs).

It is known that blood pressure can be regulated by shear stress. An increase in blood volume, for example, causes pressure against the endothelial cell lining of blood vessels that activates signalling pathways. These pathways control the level of contraction in the vascular smooth muscle and, in the longer term, contribute to changes in the structure of the vasculature observed during some pathologies. However, the precise mechanisms by which the endothelial cells convert the mechanical stimulus into a biochemical response have been difficult to establish.

Chachisvilis and colleagues studied the B₂ bradykinin GPCR that, upon binding to bradykinin, undergoes a conformational change in its structure that activates intracellular heterotrimeric G-proteins. The authors studied the activation of the B₂ receptor using intramolecular fluorescence resonance energy transfer (FRET).

Chachisvilis and colleagues first confirmed that a B₂ 'chameleon' receptor, named because of its colour-changing properties associated with FRET, was correctly expressed at the plasma membrane in cultured endothelial cells. Stimulation of the receptor with the agonist bradykinin caused changes in the FRET signal, due to the movement of helices in the receptor as it switched from its inactive to active conformation. By contrast, application of a receptor antagonist induced no change in the receptor. These experiments confirm that the chameleon receptor can be used to report GPCR activation.

The authors then stimulated the cells with shear stress by loading the cells in a chamber and altering the flow of fluid across the surface of the cells. The cells were stimulated with two other types of mechanical stress; membrane stretch that was induced by changing the osmolarity of the bathing media and changes in the fluidity of the plasma membrane. All three types of stress caused a conformational change in the receptor, similar to that observed during the activation of the receptor by bradykinin.

Previous studies have been unable to confirm whether mechanical stress can directly activate the receptors or whether the mechanism of activation is indirect by, instead, stimulating the release of autocrine-signalling molecules from the cells. In this study, however, this is unlikely because the chameleon receptors require higher concentrations of agonist than the native receptors to be activated. In addition, an inhibitor that prevents the production of a possible autocrine molecule had no significant effect on receptor activation. Chachisvilis and colleagues are confident therefore that their observed effects of mechanical stress are due to the direct activation of the membrane receptor. If so, this study provides a starting point to understand how factors such as membrane composition and tension might influence the dynamic conformation of GPCRs.