Dissecting Force Transmission across SUN Proteins Using Nuclear Tension Sensors

Forces on the linker of the nucleoskeleton and cytoskeleton (LINC) complex control nuclear mechano-sensing and mechano-adaptation. However, the force transmission dynamics across the LINC complex are not fully understood, mainly because of the lack of imaging tools. We developed a set of genetically-encoded fluorescence resonance energy transfer (FRET)-based nuclear tension sensors (NuTS) that measure tension forces across SUN1/2 proteins in living cells with high sensitivity. SUN2-based NuTS (NuTS2) responded rapidly to mechanical changes in cell contractility and matrix stiffness. Notably, NuTS2 dynamically showed force transmission with high spatiotemporal resolution during cell adhesion, migration and squeeze. We also used NuTS2 to monitor tension force changes as the notochord matures in zebrafish development. NuTS2 detected a gradient tension force that increased from the posterior tail buds to the anterior as vacuoles expanded in notochord cells. Force reduction affected notochord maturation and zebrafish embryo development. Our results provide a biophysical cue for dissecting how force transmission on SUN2 protein regulates embryo development.