Isolated Nucleus Stiffens in Response to Low Intensity Vibration

The nucleus, central to all cellular activity, relies on both direct mechanical input and its molecular transducers to sense and respond to external stimuli. While it has been shown that isolated nuclei can adapt to force directly ex vivo, nuclear mechanoadaptation in response to physiologic forces in vivo remains unclear. To gain more knowledge regarding nuclear mechanoadaptation in cells, we have developed an atomic force microscopy (AFM) based experimental procedure to isolate live nuclei and specifically test whether nuclear stiffness increases following the application of low intensity vibration (LIV) in mesenchymal stem cells (MSCs). Results indicated that isolated nuclei, on average, were 30% softer than nuclei tested within intact MSCs. When the nucleus was isolated following LIV (0.7g, 90Hz, 20min) applied four times (4x) separated by 1h intervals, stiffness of isolated nuclei increased 75% compared to controls. LIV-induced intact MSC and nuclear stiffening required functional Linker of Nucleoskeleton and Cytoskeleton (LINC) complex but was not accompanied by increased levels of nuclear envelope proteins LaminA/C or Sun-2. Indicating LIV-induced changes in the chromatin structure, while depleting LaminA/C or Sun-1&2 resulted in a 47% and 39% increased heterochromatin to nuclear area ratio in isolated nuclei, ratio of heterochromatin to nuclear area was decreased by 25% in LIV treated nuclei compared to controls. Overall, our findings indicate that increased apparent cell stiffness in response to exogenous mechanical challenge in the form of LIV are in-part retained by increased nuclear stiffness and changes in chromatin structure in MSCs.