How do astrotactin-1 and astrotactin-2 act in the determination of mammalian cell polarity?

大脑的生长、发展和修复需要神经元的迁移，以便它们能够塑造其子结构，建立新的连接，并修复旧的连接。同样，其他组织和器官的生长需要细胞以极化的方式生长，以赋予身体平面和其中各个器官独特的形状特征。最近发现，一对被称为星形蛋白(ASTN)的蛋白质在这两个过程中都发挥了关键作用。在大脑中，ASTN1存在于神经元的表面，它与神经元沿其移动的引导轨道(“胶质纤维”)相连接。然而，为了让神经元移动，它们需要打破旧的连接，然后形成新的连接，为了做到这一点，需要将ASTN1介导的接触点回收到迁移细胞的前沿。使这种情况发生的关键分子是ASTN1，它是一种与ASTN1非常相似的蛋白质，但它是在细胞内由膜定义的小隔间中发现的，而不是像ASTN1那样从细胞膜表面伸出。在其他组织方面，我们在美国约翰·霍普金斯大学医学院的合作者最近发现，ASTN2有助于控制一种被称为平面细胞极性的东西，它支撑着神经系统和身体其他部位器官和组织的形成。临床和遗传数据表明，ASTN1和ASTN2的突变形式与一系列疾病有关，包括注意力缺陷多动障碍(ADHD)、自闭症谱系障碍、精神分裂症和各种形式的神经发育延迟，如学习语言困难。最近的另一项研究表明，不同形式的ASTN1可以将阿尔茨海默氏症的发病年龄提前约5年，同时ASTN对免疫系统和神经系统外的一般组织形态的影响也已被注意到。我们将使用给我们提供ASTN1和ASTN2结构的3D原子分辨率图像的方法来研究它们如何进行控制正常人类发育的活动，当组织受损时进行修复，以及对阿尔茨海默氏症等条件的耐受性。我们实验室和其他实验室的初步数据表明，关键是ASTN1和ASTN2在更酸性条件下行为的变化-细胞内不同的隔室具有不同的酸度，这调节了细胞内的事件。我们有一个很好的起点，因为我们已经以两种不同的形式结晶了ASTN1，在中性pH和酸性pH下。我们还对它们如何相互作用进行了初步研究，但将能够从我们现在计划的工作中提供更多细节。显然，我们还需要观察活细胞，看看ASTN1和ASTN2如何在细胞内移动，影响彼此的活动和位置。我们也会这样做，这将为我们在蛋白质原子结构上所做的工作提供一个功能背景。我们将使用的方法是基于荧光标记蛋白质，这将使我们能够看到ASTN蛋白质的位置并测量它们之间的相互作用。我们的工作将从与约翰·霍普金斯大学的小组和牛津大学当地同事的持续合作中受益匪浅。一个特别令人兴奋的领域将进一步探索ASTN1和ASTN2与已知在决定平面呼叫极性方面起主导作用的蛋白质相互作用的方式，例如细胞表面受体蛋白质Frizzled3、Frizzled6和Celsr1。

The growth, development and repair of the brain requires the migration of neurons so they can shape its sub-structures, make new connections, and repair old ones. Similarly, the growth of other tissues and organs requires the growth of cells in a polarised fashion, to give the distinctive shapes characteristic of body plans and individual organs within them. It has recently been found that a pair of proteins called astrotactins (ASTNs) play critical roles in both these processes.In the brain, ASTN1 is found on the surface of neurons where it makes connections to the guide tracks ("glial fibres") along which they move. In order for the neurons to move, however, they need to break old connections and then form new ones, and in order to do that ASTN1-mediated contact sites need to be recycled towards the leading edge of the migrating cell. The key molecule enabling that to happen is ASTN2, which is a very similar protein to ASTN1 but which is found in small membrane-defined compartments within the cell rather than sticking out of the cell membrane at its surface, like ASTN1 does. In other tissues, our collaborators at Johns Hopkins University School of Medicine in the United States recently found that ASTN2 helps control something known as planar cell polarity, which underpins the formation of organs and tissues both in the nervous system and elsewhere in the body.Clinical and genetic data indicate a key involvement of mutant forms of ASTN1 and ASTN2 in a host of conditions including attention deficit-hyperactivity disorder (ADHD), autism spectrum disorders, schizophrenia and forms of neurodevelopmental delay such as difficulty in learning language. Another recent study showing that different forms of ASTN2 could bring forward by ~5 years the age of onset of Alzheimer's, while effects of ASTNs have also been noted on the immune system and general tissue morphology outside the nervous system.We are going to use methods which give us a 3D atomic-resolution picture of the structure of ASTN1 and ASTN2 to work out how they carry out their activities controlling normal human development, repair when tissues are damaged, and the suceptibility to conditions like Alzheimer's. Preliminary data from our lab and others suggests that what is critical is a change in the behaviour of ASTN1 and ASTN2 in more acidic conditions - different compartments within the cell have differing acidities and this regulates events within them. We have an excellent starting point because we have crystallised ASTN1 in two different forms, at neutral pH and at acidic pH. We have also made preliminary studies of how they interact but will be able to provide much greater detail from the work we are planning now.Clearly, we also need to look at living cells and see how ASTN1 and ASTN2 move about within them, affecting each other's activity and location. We will do this as well, which will give a functional context to the work we are doing on the atomic structures of the proteins. The approach we will use is based on fluorescently labelling the proteins and this will enable us both to see the location of the ASTN proteins and to measure the interactions they undergo.Our work will benefit greatly from ongoing collaboration both with the group at Johns Hopkins and with colleagues local to Oxford. One particularly exciting area to explore further will be the way in which ASTN1 and ASTN2 interact with proteins already known to play leading roles in determining planar call polarity, such as the cell surface receptor proteins Frizzled-3 and Frizzled-6 and Celsr1.