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

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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.