Nuclear Motility in Glioma Cells Reveals a Cell-Line Dependent Role of Various Cytoskeletal Components

Nuclear migration is a general term for the movement of the nucleus towards a specific site in the cell. These movements are involved in a number of fundamental biological processes, such as fertilization, cell division, and embryonic development. Despite of its importance, the mechanism of nuclear migration is still poorly understood in mammalian cells. In order to shed light on the mechanical processes underlying nuclear movements, we adapted a micro-patterning based assay. C6 rat and U87 human glioma cells seeded on fibronectin patterns - thereby forced into a bipolar morphology - displayed oscillatory movements of the nucleus or the whole cell, respectively. We found that both the actomyosin system and microtubules are involved in the nuclear/cellular movements of both cell lines, but their contributions are cell-/migration-type specific. Dynein activity was necessary for nuclear migration of C6 cells but active myosin-II was dispensable. On the other hand, coupled nuclear and cellular movements of U87 cells were driven by actomyosin contraction. We explain these cell-line dependent effects by the intrinsic differences in the overall mechanical tension due to the various cytoskeletal elements inside the cell. Our observations showed that the movements of the nucleus and the centrosome are strongly correlated and display large variation, indicating a tight but flexible coupling between them. The data also indicate that the forces responsible for nuclear movements are not acting directly via the centrosome. Based on our observations, we propose a new model for nuclear oscillations in C6 cells in which dynein and microtubule dynamics are the main drivers of nuclear movements. This mechanism is similar to the meiotic nuclear oscillations of Schizosaccharomyces pombe and may be evolutionary conserved.细胞核迁移（nuclear migration）是描述细胞核向细胞内特定位点移动的通用术语。这类迁移参与了诸多基础生物学过程，例如受精、细胞分裂与胚胎发育。尽管其功能至关重要，但目前学界对哺乳动物细胞内细胞核迁移的分子机制仍知之甚少。为阐明细胞核迁移背后的力学过程，我们改良了基于微图案化的检测方法。将接种于纤连蛋白（fibronectin）图案表面的C6大鼠胶质瘤细胞与U87人类胶质瘤细胞分别诱导为双极形态后，前者呈现细胞核的振荡运动，后者则展现整体细胞的振荡运动。我们发现，肌动蛋白-肌球蛋白系统（actomyosin system）与微管（microtubules）均参与了两种细胞系的核/细胞运动，但二者的贡献具有细胞系与运动类型特异性。动力蛋白（dynein）的活性是C6细胞完成核迁移的必要条件，而活化的肌球蛋白II（myosin-II）则并非必需。而U87细胞的细胞核与细胞运动偶联现象，则由肌动蛋白-肌球蛋白收缩所驱动。我们通过细胞内不同细胞骨架元件所导致的整体机械张力固有差异，解释了这种细胞系依赖性的实验现象。我们的观测结果显示，细胞核与中心体（centrosome）的运动具有高度相关性且波动幅度较大，表明二者之间存在紧密但灵活的偶联关系。实验数据同时表明，驱动细胞核迁移的作用力并非直接通过中心体传递。基于上述观测结果，我们提出了一种针对C6细胞细胞核振荡的全新模型：该模型中，动力蛋白与微管动力学是细胞核迁移的主要驱动因素。该机制与粟酒裂殖酵母（Schizosaccharomyces pombe）的减数分裂细胞核振荡机制相似，且可能在进化过程中具有保守性。