How does the desmosome-actin crosstalk regulate desmosome function?

暴露于压力的组织中的细胞，例如心肌和身体表面的覆盖物，通过称为桥粒的微小结构结合在一起，桥粒的异常功能导致疾病，例如心力衰竭，伤口愈合不良，癌症扩散以及皮肤和口腔的起泡疾病。桥粒对正常发育也很重要，它们稳定发育中的组织。因此，了解桥粒功能是如何调节的是至关重要的。我们已经表明，有助于组织韧性的一个重要因素是桥粒能够采取高度粘附状态，称为超粘附。超粘附对组织强度很重要，但也会将细胞锁定在一起，限制它们的运动。在伤口愈合期间，表皮细胞迁移以闭合伤口。癌细胞的侵入性扩散也需要细胞迁移，并且在发育中细胞运动产生正确的组织结构。当细胞移动和生长以建立细胞片层时，它们形成新的桥粒，这些桥粒成熟以变得高度粘附。当细胞片层受伤时，它们迅速失去超粘附性，并通过内化它们来下调桥粒。关于桥粒如何组装、改变它们的粘附状态以及它们在需要时如何被下调知之甚少。桥粒具有由几种组分组成的特征结构，我们发现这些组分中的大多数稳定地整合到结构中。然而，其中一种称为plakophilin（Pkp）的蛋白质从细胞的外围快速移动到中心部分，反之亦然。我们认为，这种动态行为有助于在细胞内传递信息（信号），并导致细胞行为的变化。正常情况下桥粒出现在细胞之间的连接处，但研究表明整个桥粒都在细胞内，就好像一个细胞“吃掉”了桥粒一样！我们现在已经在培养中诱导细胞分离，并表明它们确实吞噬整个桥粒。这是令人兴奋的，因为它使我们能够研究正常和病变组织中发生的过程背后的机制。桥粒吞噬类似于一种称为吞噬作用的过程，免疫系统的细胞通过吞噬细胞外颗粒，例如细菌。吞噬作用需要吞噬细胞的主动收缩活动，以便包围颗粒并将其吸入。这种收缩需要依赖于称为肌动蛋白和肌球蛋白的丝状蛋白质的行动，类似于那些参与肌肉收缩。功能性肌动蛋白和相关蛋白对桥粒调节也很重要。桥粒通常在内部连接到称为中间丝（IF）的其他纤维，而不是肌动蛋白。桥粒将IF从细胞连接到细胞，形成一个支架，为组织提供力量。然而，IF不具有收缩活性。因此，当需要收缩时，肌动蛋白如何与桥粒连接以调节其功能的问题就出现了。我们有初步的数据显示，一些已知与肌动蛋白相互作用的蛋白质与桥粒非常接近，我们认为这些蛋白质参与了桥粒粘附和吞噬的调节。我们的数据还表明，肌动蛋白细胞骨架的影响Pkp信号。我们将使用最先进的显微镜来研究桥粒如何与收缩机制相关联，因为它们组装并从超粘附切换到吞噬，并使用质谱法来确定肌动蛋白结合蛋白如何参与调节桥粒功能。最后，我们将使用分子细胞生物学来确定PKP在这一过程中的作用和信号细胞行为。我们的研究结果将进一步加深我们对正常发育的理解，并为主要健康问题（如慢性伤口，某些类型的心力衰竭，皮肤起泡疾病）的新疗法提供基础，并可能限制癌症的传播。

Cells in stress-exposed tissues, e.g. heart muscle and coverings of body surfaces, are bound together by tiny structures called desmosomes, aberrant function of which causes diseases such as heart failure, defective wound healing, cancer spread and blistering diseases of the skin and oral cavity. Desmosomes are also important for normal development, where they stabilise developing tissues. It is therefore essential to understand how desmosome function is regulated.We have shown that an important factor contributing to tissue toughness is that the ability of desmosomes to adopt a highly adhesive state known as hyper-adhesion. Hyper-adhesion is important for tissue strength, but also locks cells together restricting their movement. During wound healing, epidermal cells migrate to close the wound. The invasive spread of cancer cells also requires cell migration and in development cell movement generates the correct architecture of tissues. When cells move and grow to establish cell sheets, they form new desmosomes that mature to become highly adhesive. When cell sheets are wounded they rapidly lose hyper-adhesion and downregulate desmosomes by internalising them. Little is known about how desmosomes assemble, change their adhesive state and how they are downregulated when this is needed.Desmosomes have a characteristic structure made up of a few components and we have found that most of these components are stably integrated into the structure. However, one of them, called plakophilin (Pkp), moves rapidly from the periphery to central parts of the cell and vice versa. We think that this dynamic behaviour serves to transmit information (signals) within the cell and leads to changes in cell behaviour. Normally desmosomes appear at the junctions between cells but studies have shown whole desmosome inside cells, as though one cell has "eaten" the desmosome! We have now induced cell separation in culture and shown that they do indeed engulf whole desmosomes. This is exciting because it enables us to investigate the mechanism behind a process that occurs in normal and diseased tissues.Desmosome engulfment resembles a process called phagocytosis whereby cells of the immune system engulf extracellular particles, e.g. bacteria. Phagocytosis requires active contractile activity by the engulfing cell so as to surround the particle and draw it inside. Such contractility requires the action depends upon filamentous proteins called actin and myosin, similar to those involved in muscle contraction. Functional actin and associated proteins are also important for desmosome regulation. Desmosomes are normally internally linked to other filaments called intermediate filaments (IF), rather than actin. Desmosomes link IF from cell-to-cell by, forming a scaffolding that gives strength to tissues. However, IF possess nor contractile activity Hence the question that arises of how actins link up to desmosome to regulate their function when contractility is required? We have pilot data showing that a number of proteins known to interact with actin are very close to desmosomes and we think that these are involved in regulation of desmosome adhesion and engulfment. Our data also suggest that the actin cytoskeleton influences signalling by Pkp. We will use state-of the-art microscopy to study how desmosomes become associated with the contractile machinery as they assemble and switch from hyper-adhesion to engulfment, and mass spectrometry to identify how actin binding proteins are involved in regulating desmosome function. Finally, we will use molecular cell biology to determine the role Pkp in this process and in signalling cell behaviour. Our results will both further our understanding of normal development and provide a basis for new therapies for major health problems such as chronic wounds, some types of heart failure, skin blistering diseases and, potentially, for limiting the spread of cancer.