Non-invasive Chromatin Deformation and Measurement of Differential Mechanical Properties in the Nucleus
The nucleus is highly organized to facilitate coordinated gene transcription. Measuring the rheological properties of the nucleus and its sub-compartments will be crucial to understand the principles underlying nuclear organization. Here, we show that strongly localized temperature gradients (approaching 1{degrees}C /m) can lead to substantial intra-nuclear chromatin displacements (>1 m), while nuclear area and lamina shape remain unaffected. Using particle image velocimetry (PIV), intra-nuclear displacement fields can be calculated and converted into spatio-temporally resolved maps of various strain components. Using this approach, we show that chromatin displacements are highly reversible, indicating that elastic contributions are dominant in maintaining nuclear organization on the time scale of seconds. In genetically inverted nuclei, centrally compacted heterochromatin displays high resistance to deformation, giving a rigid, solid-like appearance. Correlating spatially resolved strain maps with fluorescent reporters in conventional interphase nuclei reveals that various nuclear compartments possess distinct mechanical identities. Surprisingly, both densely and loosely packed chromatin showed high resistance to deformation, compared to medium dense chromatin. Equally, nucleoli display particularly high rigidity and strong local anchoring to heterochromatin. Our results establish how localized temperature gradients can be used to drive nuclear compartments out of mechanical equilibrium to obtain spatial maps of their material responses. Main FindingsO_LINovel non-invasive active micro-rheology method to probe spatial intranuclear material responses, unhindered by the nuclear lamina, using strongly localized temperature gradients C_LIO_LIChromatin shows both elastic and viscous properties at the mesoscale with a retardation time of {tau} [~] 1s C_LIO_LICompacted heterochromatin in a model of nuclear inversion shows high resistance to deformation, suggesting dominantly solid-like behavior C_LIO_LIThe nucleus displays spatially distinct material properties for different compartments C_LIO_LIThe nucleolus shows high resistance to deformation on the time scale of seconds C_LIO_LIImmobile nucleoli appear solidly anchored to and retain the deformation of surrounding chromatin C_LI