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.

中文翻译：

---

响应低强度振动的孤立核刚度

细胞核是所有细胞活动的核心，它依赖于直接的机械输入及其分子传感器来感应和响应外部刺激。虽然已经显示分离的核可以适应直接来自离体的力，但是响应于体内生理力的核机械适应仍然不清楚。为了获得有关细胞核机械适应的更多知识，我们开发了一种基于原子力显微镜（AFM）的实验程序来分离活核，并专门测试在间充质干细胞（MSCs）中应用低强度振动（LIV）后核硬度是否增加）。结果表明，分离出的细胞核平均比完整MSC中测试的细胞核软30％。当LIV（0.7g，90Hz，间隔4小时（4x）间隔1h间隔20分钟），与对照相比，分离核的硬度提高了75％。LIV诱导的完整MSC和核硬化需要核骨架和细胞骨架（LINC）复合物的功能连接子，但并不伴随着核被膜蛋白LaminA / C或Sun-2水平的增加。指示LIV诱导的染色质结构变化，而消耗LaminA / C或Sun-1＆2则导致分离核中异染色质与核面积的比率增加47％和39％，LIV中异染色质与核面积的比率减少25％处理的细胞核与对照相比。总体，