Physical forces arising in the extra-cellular environment have a profound impact on cell fate and gene regulation; however the underlying biophysical mechanisms that control this sensitivity remain elusive. It is hypothesized that gene expression may be influenced by the physical deformation of the nucleus in response to force. Here, using 3T3s as a model, we demonstrate that extra-cellular forces cause cell nuclei to rapidly deform (<1 s) preferentially along their shorter nuclear axis, in an anisotropic manner. Nuclear anisotropy is shown to be regulated by the cytoskeleton within intact cells, with actin and microtubules resistant to orthonormal strains. Importantly, nuclear anisotropy is intrinsic, and observed in isolated nuclei. The sensitivity of this behaviour is influenced by chromatin organization and lamin-A expression. An anisotropic response to force was also highly conserved amongst an array of examined nuclei from differentiated and undifferentiated cell types. Although the functional purpose of this conserved material property remains elusive, it may provide a mechanism through which mechanical cues in the microenvironment are rapidly transmitted to the genome.

Physical forces arising in the extra-cellular environment have a profound impact on cell fate and gene regulation; however the underlying biophysical mechanisms that control this sensitivity remain elusive. It is hypothesized that gene expression may be influenced by the physical deformation of the nucleus in response to force. Here, using 3T3s as a model, we demonstrate that extra-cellular forces cause cell nuclei to rapidly deform (<1 s) preferentially along their shorter nuclear axis, in an anisotropic manner. Nuclear anisotropy is shown to be regulated by the cytoskeleton within intact cells, with actin and microtubules resistant to orthonormal strains. Importantly, nuclear anisotropy is intrinsic, and observed in isolated nuclei. The sensitivity of this behaviour is influenced by chromatin organization and lamin-A expression. An anisotropic response to force was also highly conserved amongst an array of examined nuclei from differentiated and undifferentiated cell types. Although the functional purpose of this conserved material property remains elusive, it may provide a mechanism through which mechanical cues in the microenvironment are rapidly transmitted to the genome.Mechanical forces transmitted through the cell directly affect nuclear shape and function, have been implicated in altered gene expression 1,2 , and affect numerous processes at the cellular level 3,4 . Such forces result in internal remodelling of the nuclear cytoarchitecture and chromatin 3,5,6 , leading to alterations in transcriptional activity 7,8 . How nuclei respond to physical cues depends on their inherent material properties, which alone direct diverse biological functions. Mechanosensitive proteins, such as lamins, are known to control and regulate these properties by physically coupling the inner nucleus with the cell's cytoskeleton, focal adhesions and integrins 3,5 . Mutations in these proteins result in a number of diseased states, and manifest in misshapen nuclei and increased nuclear fragility, particularly under applied strain [9][10][11] . The importance of nuclear mechanics is evidenced during differentiation and development wherein mechanical forces are necessary 12,13 to direct deformable undifferentiated stem cells into committed cell types 14,15 . Matrix stiffness also influences lamin-A expression 16 thereby governing nuclear resistance to force [17][18][19] . Clearly, characterizing how the nucleus deforms and remodels in response to force is of critical importance.A large body of research involves examination of isolated nuclei 10,20,21 , as well as characterization of nuclei within their cyto-architecture 9,14,15,22,23 . Importantly, nuclei, and their constituents are often denoted as homogeneous, or isotropic materials 9,14,20,24,25 , despite displaying anisotropic material properties during physical perturbation 15,22,23 . Laser ablation studies have shown that the disruption of heterochromatin nodes causes elliptical nuclei to undergo anisotropic shrinkage in which they collapse significantly more along their ...

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