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 applied force ex vivo, the mechanisms governing nuclear mechanoadaptation in response to physiologic forces in vivo remain unclear. To investigate nuclear mechanoadaptation in cells, we developed an atomic force microscopy (AFM) based procedure to probe live nuclei isolated from mesenchymal stem cells (MSCs) following the application of low intensity vibration (LIV) to determine whether nuclear stiffness increases as a result of LIV. Results indicated that isolated nuclei were, on average, 30% softer than nuclei tested within intact MSCs prior to LIV. 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 non-LIV controls. LIV-induced nuclear stiffening required functional Linker of Nucleoskeleton and Cytoskeleton (LINC) complex, but was not accompanied by increased levels of the nuclear envelope proteins LaminA/C or Sun-2. While depleting LaminA/C or Sun-1&2 resulted in either a 47% or 39% increased heterochromatin to nuclear area ratio in isolated nuclei, the heterochromatin to nuclear area ratio was decreased by 25% in LIV-treated nuclei compared to controls, indicating LIV-induced changes in the heterochromatin structure. Overall, our findings indicate that increased apparent cell stiffness in response to exogenous mechanical challenge of MSCs in the form of LIV is in part retained by increased nuclear stiffness and changes in heterochromatin structure.

细胞核是所有细胞活动的核心，它依赖于直接机械输入及其分子传感器来感知和响应外部刺激。虽然已经表明孤立的核可以适应体外施加的力，但控制核机械适应响应体内生理力的机制。仍然不清楚。为了研究细胞中的核机械适应，我们开发了一种基于原子力显微镜 (AFM) 的程序，用于在应用低强度振动 (LIV) 后探测从间充质干细胞 (MSC) 中分离的活细胞核，以确定核刚度是否因以下原因而增加丽芙。结果表明，在 LIV 之前，分离的细胞核比在完整 MSC 内测试的细胞核平均软 30%。当在以 1 小时间隔应用四次 (4x) 的 LIV（0.7g，90Hz，20 分钟）后分离细胞核时，与非 LIV 对照相比，分离细胞核的刚度增加了 75%。LIV 诱导的核硬化需要核骨架和细胞骨架 (LINC) 复合物的功能性接头，但不伴随核膜蛋白 LaminA/C 或 Sun-2 水平的增加。在分离的细胞核中增加异染色质与核面积比，与对照相比，LIV 处理的细胞核的异染色质与核面积比降低了 25%，表明 LIV 诱导的异染色质结构发生了变化。总体而言，我们的研究结果表明，响应于 LIV 形式的 MSC 的外源性机械挑战而增加的表观细胞刚度部分通过增加的核刚度和异染色质结构的变化而得以保留。