Measuring nucleus mechanics within a living multicellular organism: Physical decoupling and attenuated recovery rate are physiological protective mechanisms of the cell nucleus under high mechanical load

Nuclei within cells are constantly subjected to compressive, tensile and shear forces, which regulate nucleoskeletal and cytoskeletal remodeling, activate signaling pathways and direct cell-fate decisions. Multiple rheological methods have been adapted for characterizing the response to applied forces of isolated nuclei and nuclei within intact cells. However, in vitro measurements fail to capture the viscoelastic modulation of nuclear stress-strain relationships by the physiological tethering to the surrounding cytoskeleton, extracellular matrix and cells, and tissue-level architectures. Using an equiaxial stretching apparatus, we applied a step stress and measured nucleus deformation dynamics within living C. elegans nematodes. Nuclei deformed non-monotonically under constant load. Non-monotonic deformation was conserved across tissues and robust to nucleoskeletal and cytoskeletal perturbations, but it required intact Linker of Nucleoskeleton and Cytoskeleton (LINC) complex attachments. The transition from creep to strain recovery fits a tensile-compressive linear viscoelastic model that is indicative of nucleoskeletal-cytoskeletal decoupling under high load. Ce-lamin (lmn-1) knockdown softened the nucleus whereas nematode ageing stiffened equilibrium elasticity and decreased deformation recovery rate. Recovery lasted minutes due to physiological damping of the released mechanical energy thus protecting nuclear integrity and preventing chromatin damage.