CAREER: Computational and Theoretical Investigation of Actomyosin Contraction Systems

生物体的功能和发育取决于其细胞的不断活动。当我们从一个小胚胎开始生长时，细胞不断生长，分裂，迁移，改变形状并相互拉扯。即使是成年人，我们的细胞也会继续这些过程来维持身体功能，并对损伤做出反应。所有这些活动都依赖于细胞产生和传递力的能力。肌动蛋白细胞骨架（actin cytoskeleton）是一种存在于每个细胞内的小的动态细丝（actin）和分子马达的网络，是细胞内以及组织和器官之间力产生和传播的主要驱动力。所有细胞都需要功能性肌动蛋白细胞骨架来维持它们的形状，分裂成新的细胞并在发育过程中驱动形态发生。如果没有一个最佳功能的肌动蛋白细胞骨架，细胞的迁移，分化，分裂和对损伤的反应能力受到损害，导致出生缺陷，癌症，纤维化，免疫缺陷等，要充分了解这些过程，我们需要首先了解肌动蛋白细胞骨架如何作为其组件（电机，细丝和交联剂）的功能，以及它如何在细胞内和细胞之间产生和传递力。肌动蛋白细胞骨架活性的分子解析（自下而上）的观点，是势在必行的，以便提供对其所有相关细胞过程的机械理解，并提供与细胞骨架功能障碍相关的新的治疗靶点。实验细胞生物学和体外研究揭示了肌动蛋白细胞骨架收缩的许多方面，如分子马达的运动，细胞膜（皮层）下的肌动蛋白和运动组织，以及它们在细胞分裂中的作用。然而，许多必须解决的重要问题是极其困难的，如果不是不可能的，单独用实验来探索。这些问题包括：1）肌动蛋白细胞骨架收缩的多种机制是什么？2)是什么使一种收缩机制比其他收缩机制占优势？3)一个网络能产生多大的力量？4）相邻网络如何相互作用？幸运的是，最近在计算机能力和模拟方法上有了巨大的进步，现在允许我们在硅片上进行计算实验，绕过湿实验室的限制，开辟新的方法来探测和发现肌动蛋白动力学的隐藏方面以前无法访问。该项目利用这些改进，采用最先进的模拟技术和主要研究者实验室开发的新方法，对细胞骨架动力学进行了一系列新颖，系统的理论和计算研究。该项目分为两个科学任务，利用我们小组在物理学方面的专业知识。第一个目标是模拟所有三个主要的肌动蛋白收缩机制（丝屈曲，极性分选，解聚末端跟踪），并询问在何种条件下，每个成为主导的其他和协同和拮抗作用时出现。这项任务的结果将作为指导，以确定不同细胞中的潜在收缩机制以及不同扰动增强或赋予其功能的程度。在第二个目标中，首席研究员将研究肌动蛋白网络可以产生多少力，随着时间的推移，并将其传递到相邻的网络。目前的研究主要集中在收缩率，这并没有提供一个足够的图片肌动蛋白收缩的长期影响。这种新方法将有助于填补细胞骨架的内部动力学和大规模细胞行为之间的差距。此外，首席研究员还将实施一项教育计划，其中包括针对不同群体的活动：（i）为整个科学界开设的细胞骨架系统建模夏季讲习班;（ii）为北卡罗来纳州州立大学的高年级学生和研究生开设的细胞骨架物理学专题课程;（iii）K-12外联活动，利用射箭教授STEM概念。这三个相互关联的任务旨在解决生命系统物理学中高度相关的基本问题。这些任务的完成将为肌动蛋白细胞骨架的内部运作提供坚实的理论基础，并为体内系统的现实模型铺平道路，使我们能够从机械的角度研究肌动蛋白相关的细胞过程和疾病。该奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

The function and development of organisms depends on the incessant activities of its cells. As we growth from a small embryo, cells are constantly growing, dividing, migrating, changing shapes and pulling on each other. Even as adults, our cells continue these processes to maintain body functions and heal in response to injury. All these activities rely on the cells’ ability to generate and transmit forces. The actin cytoskeleton, a meshwork of small, dynamic filaments (actin) and molecular motors that exists inside each cell, is the main driver of force generation and propagation within cells and across tissues and organs.All cells need a functional actin cytoskeleton to maintain their shapes, divide into new cells and drive morphogenesis during development. Without an optimally functioning actin cytoskeleton, cells’ ability to migrate, differentiate, divide and respond to injury is compromised, leading to birth defects, cancers, fibrosis, immunodeficiencies, etc. To fully understand these processes, we need to first understand how the actin cytoskeleton works as a function of its components (motors, filaments and crosslinkers), and how it generates and transmits forces within and between cells. Elucidation of the actin cytoskeleton’s activities from a molecular (bottom up) view, is imperative in order to provide a mechanistic understanding of all its associated cell processes and provide new therapeutic targets related to cytoskeletal dysfunction.Experimental cell biology and in vitro studies have revealed many aspects of actin cytoskeleton contraction, such as the movement of molecular motors, the actin and motor organization underneath cell membranes (the cortex), and their role during cell division. However, many important questions that must be addressed are extremely difficult, if not impossible, to probe with experiments alone. These include: 1) What are the multiple mechanisms that the actin cytoskeleton use to contract?; 2) What makes one contraction mechanism dominant over the others?; 3) How much force can a network produce?; and 4) How do adjacent networks interact?Fortunately, there have been dramatic recent improvements in computer power and simulation methodologies that now allow us to conduct in silico, computational experiments that circumvent wet lab constraints, opening novel ways to probe and uncover hidden aspects of actin dynamics not previously accessible. This project takes advantage of these improvements with a novel, systematic series of theoretical and computational studies of cytoskeletal dynamics using state-of-the-art simulation techniques and new approaches developed in the Principal Investigator laboratory. This project is divided into two scientific tasks that leverage our group’s expertise in physics. The first aim is to model all three main actin contraction mechanisms (filament buckling, polarity sorting, and depolymerization end-tracking) and ask under which conditions each becomes dominant over the others and when synergistic and antagonistic effects arise. Results from this task will serve as a guide to determine the underlying contraction mechanism in different cells and the degree to which different perturbations would enhance or impart its function. In the second aim the Principal Investigator will investigate how much force actin networks can produce, sustain over time, and transmit to adjacent networks. Current studies focus mostly on contraction rates, which does not provide a sufficient picture for the long-term effects of actin contraction. This new approach will help fill the gap between the cytoskeleton’s internal dynamics and large-scale cell behaviors. In addition, the Principal Investigator will also implement an educational plan consisting activities targeted to different groups: (i) a summer workshop on modelling cytoskeletal systems opened for the whole scientific community; (ii) a special topics course on physics of the cytoskeletal for seniors and graduate students at North Carolina State University; and (iii) a K-12 outreach activity to teach STEM concepts using archery. These three interrelated tasks are designed to address fundamental questions of high relevance in the physics of living systems. Completion of these tasks will provide a solid theoretical foundation of the inner workings of the actin cytoskeleton and pave the way for realistic models of in vivo systems, allowing us to study actin-associated cellular processes and diseases from a mechanistic point of view.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.