Active response of metabolically intact isolated nuclei to compression

The nucleus is generally considered to be the stiffest part of the cell with an apparent Young's modulus of ~10 kPa, but cells deform their nuclei as they move through much softer matrices and apply <kPa stresses. Metabolically active nuclei were produced from cells by a centrifugation process that enucleates the cell, leaving behind a cytoplast and producing a nucleus that is wrapped with a plasma membrane and a few hundred nanometers of cytosol (a karyoplast) but no discernible cytoskeleton, endoplasmic reticulum, ribosomes or other large organelles. Karyoplasts have a wrinkled nuclear lamina, as expected since the nuclei transform from flattened structure to spheres as they are centrifuged out of the cell spread on a substrate. If the nucleus is modeled as an elastic object, force-indentation relations lead to an apparent Young's modulus of 5-10 kPa when measured at low indentations that increases with larger indentations. However, new data show that most of the work of compressing the nucleus is dissipated rather than elastically stored, even though nuclei maintain volume and recover shape after large deformations. Dissipation is driven by active mechanisms since depleting ATP by inhibiting glycolysis eliminates most of the dissipation and increases nuclear stiffness. The ATPase motor BRG1 in the chromatin remodeling BAF complex appears to generate the active motions since its inhibition mimics global ATP depletion.