Dynamics and Buffering Functions of Nuclear Actin-Cofilin Assemblies in Cellular Stress Response

肌动蛋白是真核细胞中最丰富的蛋白质，以其在细胞质中聚合成动态的半柔性细丝的能力而闻名。这些纤维是细胞骨架的一个组成部分，在决定细胞形状、分裂和运动方面是必不可少的。肌动蛋白现在也被认为是细胞核中的一个关键角色，其中肌动蛋白单体在必要的基因表达过程中起作用。动态核肌动蛋白丝也被证明在特定基因调控和dna损伤修复机制中发挥关键作用。因此，肌动蛋白及其调控伙伴cofilin进出细胞核并维持其稳态水平对正常细胞功能具有重要意义。几十年来，人们已经知道肌动蛋白和cofilin也可以在细胞核内组装成大规模、稳定的“棒”，这是压力条件下形成的结果。虽然杆状细胞的短暂形成被认为是正常细胞反应的一部分，可能具有保护功能，但杆状细胞的持续存在与多种病理状况有关，例如某些肌病和一些神经系统疾病。然而，由于难以在活细胞中可视化内源性核肌动蛋白，人们仍然不清楚到底是什么触发了核内棒的形成，棒的组装或拆卸的潜在机制是什么，以及它们在细胞应激反应中的功能是什么。有趣的是，细胞生物学中新出现的液-液相分离过程概念，由于其对细胞内物理和化学扰动的敏感性以及快速重组细胞功能的能力，涉及许多细胞应激反应机制。在这里，我们推测核肌动蛋白-cofilin棒的动态组装是通过单体相分离到凝聚状态的细胞扰动触发的。这些组合成熟后形成稳定的液晶棒，将大部分游离肌动蛋白隔离在细胞核中。稳定的组装可能具有缓冲功能，影响作为应激反应一部分所需的单体肌动蛋白依赖基因表达过程。我们建议研究核actin-cofilin组装和拆卸的相分离动力学，阐明它们在完整细胞中的结构基础，以及它们与其他相分离核室的关系。

Actin is the most abundant protein in eukaryotic cells and is well known for its capability to polymerize into dynamic, semi-flexible filaments in the cytoplasm. These filaments are an integral part of the cytoskeleton and are essential in determining cell shape, division and motility. Actin is now also recognized as a key player in the cell nucleus, where the actin monomers function in essential gene-expression processes. Dynamic nuclear actin filaments have also been shown to play a critical role in specific gene regulation and DNA-damage repair mechanisms. Therefore, shuttling of actin in and out of the nucleus, together with its regulatory partner cofilin, and maintaining their steady-state levels are of importance to normal cellular function. It has been known for decades that actin and cofilin can also assemble into large-scale, stable ‘rods’ within the nucleus, which form as a consequence of stress conditions. While the transient formation of rods is thought to be part of a normal cell response and may have protective functions, persistence of rods is implicated in a variety of pathological conditions, such as certain myopathies and some neurological disorders. However, due to the difficulty in visualizing endogenous nuclear actin in live cells, it is still not well understood what exactly triggers the formation of intranuclear rods, what are the underlying mechanisms of rod assembly or disassembly, and what is their function in the cellular stress response. Intriguingly, the emerging concept of liquid-liquid phase separation processes in cell biology is implicated in many cellular stress response mechanisms due to its exquisite sensitivity to physical and chemical intracellular perturbations and capability to rapidly reorganize cellular functions. Here we speculate that the dynamic assembly of nuclear actin-cofilin rods is triggered upon cellular perturbation through phase separation of monomers into a condensed state. The assemblies mature into stable, liquid-crystalline rods that sequester most of the free actin in the nucleus. The stable assembly may serve a buffering function affecting monomeric actin-dependent gene expression processes that are required as part of the stress response. We propose to study the phase separation dynamics of nuclear actin-cofilin assembly and disassembly, elucidate their structural basis in intact cells, and their relation to other phase separated nuclear compartments.