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Polymer physics of metabolically active isolated nuclei

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SPLW01 - Building a bridge between non-equilibrium statistical physics and biology

Nuclei are generally the stiffest organelle in the cell, and their large elastic modulus is thought to be due to a combination of polymer networks formed by chromatin and the 2D lamin network underlying the nuclear membrane. Direct measurement of nuclear stiffness within the cell is complicated by the surrounding cytoskeleton, and isolating nuclei by disrupting the cell results in diffusion of solutes through the nuclear pores, loss of ATP -dependent active processes, and osmotic dysregulation that might affect nuclear mechanics.  Therefore, metabolically active isolated nuclei were produced from live cells by a centrifugation process that enucleates the cell. This process leaves behind a cytoplast and produces a nucleus that is wrapped by a plasma membrane and a thin layer of cytosol (a karyoplast), but no discernible cytoskeleton, endoplasmic reticulum, ribosomes or other large organelles. The metabolic activity within this membrane-wrapped nucleus remains intact for at least 12 hours after isolation. Force-indentation curves measured by atomic force microscopy show that the apparent Young’s modulus of these nuclei is on the order of 5 to 8 kilopascal. This large Young’s modulus contrasts with the DNA gels at the same concentration either made from purified DNA or within bacterial biofilms. The large Young’s modulus inferred from force-indentation curves assumes a purely elastic object, but comparison of the force-indentation and force-retraction curves from AFM studies shows that most of the work of nuclear indentation is dissipated.  Repeated deformations show little or no weakening of the nucleus and very little rate dependence, suggesting that the material is not a passive viscoelastic body.  In contrast, treatment with a glycolysis inhibitor nearly eliminates the dissipation, suggesting that it depends on active processes within the nucleus.  

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