Cellular adhesion and communication are vital during the development of multicellular organisms. These processes use proteins on the surface of cells, which stick cells together or transmit signals from outside the cell to the interior, so that the cell can respond to its environment. Members of one family of cell surface receptors, called integrins, can perform both of these activities, and therefore provide a molecular link between cell adhesion and signalling. Our research is focused on determining how proteins inside and outside the cell assist the integrins in their developmental roles: mediating cell migration, adhesion between cell layers, and cell differentiation.

We use the genetics of the fruit fly Drosophila to elucidate integrin function within the developing animal, to identify the proteins that work with integrins and decipher how they function. In this way, we aim to discover how integrins perform such distinct roles at different times and places during development. In the developing embryo a major function of integrins is to attach the ends of the large multinucleate muscles to the epidermal tendon cells (Fig. 1). In the absence of certain integrin-associated proteins, such as talin, integrin-mediated attachment of the muscles fails (Fig. 2), while paxillin controls the number of cell fusions, thus dictating muscle size. In the follicular epithelia, integrins organise dynamic actin structures (Fig. 3) that drive changes in cell shape. Some integrin-associated proteins have multiple tasks, including those that are crucial for integrin function and those within other pathways.

We are dissecting out how these molecules act in diverse ways, such as mapping the part of talin that controls the expression of another cell adhesion molecule, which in turn is critical for the earliest polarity in the developing egg (Fig. 4).

Fig 1: Live imaging of muscles in the developing embryo. Their outer surfaces are outlined in green (CD8-GFP), and they use integrins and their associated proteins to regulate their size by controlling the number of nuclei (red, histone-RFP) in each muscle, and by generating specialised attachment sites at the muscle ends, with concentrated integrins (blue, integrin-linked-kinase-BFP).

Fig 2: Wild type embryo (top) and embryo lacking the integrin-associated protein talin (bottom). Talin is needed for the integrins (green) to attach the ends of the muscles (purple), but not for the concentration of integrins at muscle ends.

Plain English:
The mechanism that keeps the individual units that make up our body, or cells, attached together is known as cell adhesion. Cell adhesion is of two types. In the first type, cell adhesion proteins on the surface of one cell bind directly to similar proteins on the surface of the adjacent cell. In the second type, which is the focus of our research, cell adhesion proteins on the surface of the cell, called integrins, bind to a network of proteins outside the cell, the extracellular matrix. Extracellular matrix proteins are made by the surrounding cells, transported outside, and assembled into a stable network. In many cases this matrix forms between two layers of cells and is used to link the two together, as the integrins in each layer bind to the same intervening extracellular matrix. An example of this is the link between two layers in our skin, the epidermis and the dermis. If the adhesion mechanism is faulty, then the two layers separate, resulting in a blister. Not only do integrins need to bind tightly to the extracellular matrix, but they also must be connected to a complex of proteins within the cell, the cytoskeleton, that dictate the cell shape, like reinforcing rods within cement.

There are two general aims to our research. The first is to elucidate how integrins are connected to the cytoskeleton. The second is to discover how integrins are used in different ways in the development of an organism from a single cell, the fertilised egg. These different ways include directing cell movements around the developing embryo, permitting cells to take on special abilities, and forming stable points of strong adhesion between cell layers. As these are complex problems, we have chosen a simple animal to study, the fruit fly Drosophila, so that we have the best chance of solving them. Fruit flies use integrins in the same way as we do, as exemplified by the fact that faulty integrins in the fly also cause blisters. We aim to discover the basic mechanisms of integrin function that are shared between all animals. In future, we will be able to apply this knowledge to the treatment of medical conditions arising from defects in integrin function, which include skin blistering diseases, muscular dystrophies, neurogical disorders, and aberrant blood clotting.

Selected publications:

• Delon I and Brown NH (2007) Integrins and the actin
cytoskeleton Curr Opin Cell Biol 19, 43-50

• Delon I and Brown NH (2009) The integrin adhesion complex changes its composition and function during morphogenesis of an epithelium J Cell Sci 122, 4363-4374

• Bataillé L, Delon I, Da Ponte JP, Brown NH and Jagla K (2010) Downstream of identity genes : Muscle-type specific regulation of fusion process Dev Cell 19, 317-28

• Sabino D, Brown NH and Basto R (2011) Ajuba restricts Aurora A activity to the centrosome but it is not an Aurora-A activator J Cell Sci [in press]

Fig 3: Intergrins (blue) organise contractile stress fibres within the follicle cell epithelium, composed of actin filaments (green) and the motor protein myosin (red).

Fig 4: The lack of talin in a patch of follicle cells (green) from the ovarian egg chamber often causes the oocyte (red) to become attached to the mutant cells, instead of at its normal position at the posterior pole (right side, also visible in the smaller egg chamber). This occurs because the mutant cells increase their synthesis of the cell adhesion molecule cadherin, which sticks the cells to oocyte. Only a fraction of the talin molecule is required for this function. The DNA in the nuclei of the cells is shown in white.