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

Abstract

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 the nucleus and decreased deformation recovery rate. Recovery lasted minutes rather than seconds due to physiological damping of the released mechanical energy thus protecting nuclear integrity and preventing chromatin damage.

Lamin regulates the dietary restriction response via the mTOR pathway in Caenorhabditis elegans

“…This is likely the result of altered nuclear stiffness and lower elasticity, suggesting that aged nuclei are under constant self-induced stretch. A similar phenomena is observed when the LINC complex is impaired ( Zuela-Sopilniak et al, 2020 ). When nuclei are exposed to outside forces that deform the nuclear envelope morphology, NPC are redistributed and stretched, increasing transcription factor nuclear import ( Elosegui-Artola et al, 2017 ; Hoffman et al, 2020 ).…”

“…Nuclear deformations are also found in aged nuclei ( Haithcock et al, 2005 ; Pérez-Jiménez et al, 2014 ), along with compromised nucleocytoplasmic transportation ( D'Angelo et al, 2009 ). Recently, it was shown that nuclei of aged C. elegans are more susceptible to damaging effects from outside stretch forces ( Zuela-Sopilniak et al, 2020 ). This is likely the result of altered nuclear stiffness and lower elasticity, suggesting that aged nuclei are under constant self-induced stretch.…”

Lamin A/C mediated invaginations in the nuclear surface allow the nucleus to pass unimpeded through a dense array of fiber-like obstacles

“…[35] It is well known that on short time scales, nuclei can behave visco-elastically. [12,15,18,27,[36][37][38][39][40][41] These measurements are typically on the time scale of several seconds, while our observations are on the order of minutes to hours. Thus, it is difficult to extrapolate the shortterm measurements to the long-timescale behavior.…”

A survey of physical methods for studying nuclear mechanics and mechanobiology

“… 249 Furthermore, many of the methods describe previously, such as AFM and MM, simply cannot access individual nuclei in vivo . More broad-scale deformation techniques such as substrate stretching have been able to circumvent this, 250 but there still exists a dearth of techniques capable of measuring nuclear mechanical properties within tissue samples. This motivates a new frontier of tool development research aimed at combining novel force probes with advanced microscopy to better enable studies of the physical role of the nucleus in tissue morphogenesis and embryogenesis.…”