The quantification of mechanical force and analysis of its conversion into biochemically relevant information (mechanotransduction) is inherently challenging. The recent application of FRET-based sensors that report tension through adhesive proteins have greatly aided in vitro studies but are yet to become widely used in vivo.

It has long been appreciated that physical forces contribute to vascular development. At the adherens junctions, VE-cadherin is the major junctional cadherin, mechanically coupling the endothelial cells within vascular tubes. Here, we generated a zebrafish VE-cadherin FRET-based Tension Sensor (TS) transgenic strain to visualize and investigate forces in the developing vasculature in vivo. We expressed the VE-cadherin TS module under the control of VE-cadherin’s own regulatory elements at endothelial cell junctions allowing live-imaging of protein and junctional dynamics. We show that the VE-cadherin TS protein is functional in vivo and capable of rescuing VE-cadherin loss-of-function phenotypes to adulthood. Quantification of the energy transfer using both ratio-metric FRET imaging and Fluorescent lifetime measurements revealed that changes within and between junctions reflect changes in intra-molecular tension through VE-cadherin.

Applying this novel tool, we have characterised tension changes that occur during arterial maturation, upon altered Vegfr2 signalling, altered endothelial calcium signalling and in a novel model of vascular malformation. This study reveals the capacity of TS approaches to study forces in vivo in developing organisms, the pitfalls associated with the approach and care needed to correctly interpret the data.