Dissecting the molecular organisation of Fat-Dachsous cadherin complexes to understand mechanisms of coordinated cell polarisation

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The cells that compose our bodies show great diversity, nevertheless some properties are shared by many cell types. These include the ability to communicate ('signal') to coordinate functions, and to adopt polarised shapes or behaviours. Thus, understanding cell signalling and cell polarisation are fundamental goals of cell biology. Notably, in some contexts, signalling and polarisation are coupled. An example is 'planar polarity', where cells in a sheet interact together so they all point in the same direction in the tissue plane. This results in hairs on the skin being able to point in one direction, and cilia lining the trachea and lungs to beat in the same direction.We will study two molecules that play important roles in establishing planar polarity in animal tissues, the cadherins Fat (Ft) and Dachsous (Ds). Cadherins mediate binding between cells, by sitting in the outer membrane and forming bonds with cadherins sitting on neighbouring cells. The best studied cadherin - Epithelial cadherin (E-cad) - binds between cells in epithelial sheets, and plays a structural role in holding tissues together.Ft and Ds cadherins also bind between cells, with Ft on one cell binding to Ds on the neighbouring cell. This 'heterophilic' binding between Ft and Ds mediates signalling between cells, as high Ds on one cell results in more Ft molecules being bound on the surface of a contacting cell. Furthermore, Ft-Ds heterodimers cluster together at cell-cell contacts, and within these clusters the molecules are 'sorted', such that clusters consist predominantly of Ft in one cell and Ds in the neighbouring cell. This local sorting appears to promote cell-level polarisation of Ft and Ds, such that most Ft goes to one cell end and most Ds goes to the other. These two properties of Ft and Ds - heterophilic binding between cells and segregation to opposite edges within cells - result in tissue-level planar polarisation of cell sheets, such that each cell has Ft at one edge and Ds at the other.We will investigate how Ft and Ds promote planar polarisation of tissues. We propose that their stable clustering at cell-cell contacts depends on a combination of cis- and trans-interactions between Ft and Ds, and we will test this by systematically mapping parts of Ft and Ds involved in binding to themselves and each other. Furthermore, we propose that local sorting into polarised clusters is due to a combination of stabilising interactions between Ft-Ds heterodimers of the same orientation, and destabilising interactions between heterodimers of the opposite orientation. We will seek to identify the molecular basis of such sorting interactions.To understand the behaviour of Ft and Ds, we will exploit advances in microscopy that allow imaging of individual molecules at high resolution. Moreover, as Ft and Ds are very large molecules, we will use leading-edge genetic engineering techniques to assist their fine-scale dissection. We will study the properties of Ft and Ds in a cultured cell system where they can be rapidly manipulated, and also in a simple animal model - the developing fruit fly wing - which is particularly amenable to genetic manipulation and imaging.This work will shed light on the molecular properties of a poorly studied group of 'atypical' cadherins which are required for human health. Disruption of Ft-Ds activity is associated with congenital diseases such as cardiac valve defects and Van Maldergem and Hennekam syndromes, as well as being implicated in cancer progression. Detailed knowledge of how Ft and Ds mediate cell signalling and cell polarisation will reveal mechanisms underlying fundamental aspects of cell function, and open the way to understanding how to therapeutically modulate their activities.