Strain-Activated Signaling within Cell Adhesions Dictates Cell Fate

就像钢梁支撑建筑物一样，细胞外基质纤维有助于将组织结合在一起。组织中的细胞的行为就像建筑物中的人：它们附着在细胞外基质上并在上面爬行，就像人在地板上行走一样。然而，与人类不同的是，细胞也会对它们的基质产生拉力，并能对相邻基质的僵硬做出反应。正常情况下，干细胞只需成熟为组织中已有的细胞即可做出反应。然而，疾病往往会导致这些纤维变得比正常组织更坚硬，这使得这些组织中的干细胞做出不适当的反应；通常成为肌肉的细胞成熟为骨骼。虽然我们已经做出了这样的观察，但我们对为什么会发生这种情况的理解是有限的。因此，系统地研究像僵硬这样的物理信号如何转化为干细胞可以解释的生化信号是非常必要的。细胞能够感觉到其周围组织的僵硬程度的方法之一是通过特定分子从细胞收缩的力量中展开。通过对已知蛋白质结构的计算机分析，研究人员确定了两个强有力的候选分子，这些分子将展开并允许细胞测量力。通过这项研究确定的细胞化学途径可能成为基因疗法的靶点，用于治疗癌症和心脏病等使组织僵硬的疾病。细胞通过收缩来感知细胞外基质的特性，并在一个称为机械转导的过程中将信息转换为生化读数。尽管了解这一过程的输入和输出，但对发生在这些观察之间的机械诱导信号传递知之甚少。干细胞非常适合于阐明这些分子细节，因为它们呈现出一张“白板”，成熟为特定组织可以描述物理-化学传感器的敏感性，即脂肪细胞的收缩能力不如肌肉和骨骼细胞。这项研究将结合目前的分子工具和工程方法来测试细胞黏附中的蛋白质是否起到“分子应变仪”的作用，即它在应变下暴露隐秘的激酶结合部位。一项基于生物信息学的筛选已经确定了3种具有隐含激酶结合位点的蛋白质，例如SORBS1、SORBS3和vinculin；项目目标将使用原位荧光共振能量转移分析来描述细胞应变诱导的这些蛋白质的构象变化，并验证用于信号传递的“分子应变计”模型。它还将证实新的机械传感器的功能，这些传感器可以作为治疗靶点，使干细胞对组织僵硬不敏感。

Just like steel girders hold up buildings, extracellular matrix fibers help hold tissues together. Cells within tissues act like people in buildings: they adhere to and crawl on extracellular matrix like people walk on the floors. Unlike people however, cells also pull against their matrix and can respond to the stiffness of this adjacent matrix. Normally, stem cells respond simply by maturing into the cells already present in the tissue. However diseases often cause these fibers to become stiffer than normal, which makes the stem cells within these tissues respond inappropriately; cells that typically become muscle instead mature into bone. While we have made such observations, our understanding of why this occurs is limited. Thus a systematic examination of how physical cues like stiffness are converted into biochemical cues that a stem cell can interpret is greatly needed. One of the ways that cells can sense the stiffness of the tissues that surround them is through the unfolding of particular molecules from the forces of cellular contraction. Using computer analysis of known protein structures, the investigator has identified two strong candidates for molecules that will unfold and allow the cells to measure force. Cell chemical pathways identified through this study could be targeted in genetic therapies to treat diseases that stiffen tissues such as cancer and heart disease. Cells sense extracellular matrix properties by contracting against it and converting that information into biochemical readouts in a process called mechanotransduction. Despite understanding the inputs and outputs of this process, little is known about mechanically induced signaling that occurs in between these observations. Stem cells are ideally suited to elucidate these molecular details because they present a "blank slate" where maturation into specific tissues can describe the sensitivity of a physical-to-chemical sensor, i.e. fat cells are less contractile than muscle and bone cells. This research will combine current molecular tools and engineering approaches to test whether proteins within a cell adhesion act as a "molecular strain gauge," i.e. it exposes cryptic kinase binding sites under strain. A bioinformatics-based screen has identified 3 proteins with cryptic kinase binding sites, e.g. SORBS1, SORBS3, and vinculin; project objectives will use in situ fluorescence resonance energy transfer assays to describe cellular strain-induced conformational changes in these proteins and validate the "molecular strain gauge" model for signaling. It will also confirm the function of new mechanosensors, which could be used as therapeutic targets to make stem cells insensitive to tissue stiffening.