Multiscale analysis of extracellular regulation of BMP signalling

The Bone Morphogenetic Proteins (BMPs) are powerful "growth factors" which are essential for development of nearly all organs and tissues, as well as subsequently maintaining normal tissue structure and function. During development, cells sense signals from their environment that are converted into cellular responses, resulting in a change in their behaviour and often the type of cell they will become. The research proposed here focuses on one major type of signal, the BMP signal, which is critically important in early embryos, where it controls cell fates along the front to back axis.

The action of BMPs is controlled by regulator proteins that can inhibit or enhance the BMP signal, although how they do this is not yet understood. The inhibitors are large proteins that bind to the BMPs, thereby preventing them from interacting with their receptors and sending a message to the cell. We are interested in two BMP regulators, Chordin and Tsg, which are necessary for correct embryonic development. However, there are currently few details of these protein complexes and their interactions which presents a major obstacle to understanding BMP regulation. The main aim of our research therefore is to understand the mechanisms by which these regulators modulate BMP signals. We will achieve this by visualising the location of Chordin and Tsg using microscopy and analysing their structures to determine how these regulator proteins interact with each other and BMPs.

Chordin and Tsg control BMP signalling during development of embryos from simple organisms, such as fruitflies, up to humans. Therefore, we will use the fruitfly Drosophila as a model for this research as it has many advantages as a research organism, including a rapid life cycle, ease of maintenance and amenability to genetics and genome editing. Given that Chordin and Tsg regulate BMP signalling in both fruitflies and humans, our findings will be directly relevant to human biology.

Our results will allow us to gain a better understanding of how BMP regulation occurs and how these interactions underpin the important roles that Chordin and Tsg play in tissue assembly and embryo development. Results from this study will not only be important for understanding embryonic development, but will ultimately allow the design of new therapeutics modulating BMPs, as many human diseases arise when cells receive excessive or insufficient BMP signal. Moreover, a major goal in the stem cell field is to be able to efficiently differentiate stem cells into a particular cell type. Often this process is achieved by adding or inhibiting BMPs. Therefore, by increasing understanding of precisely how to manipulate BMP activity, our results will also be of major benefit to the stem cell field.

Bone morphogenetic proteins (BMPs) are powerful signalling molecules that are essential for embryonic dorsal-ventral axis patterning, and development and homeostasis of nearly all organs and tissues. Additionally, BMP regulation is of therapeutic interest for a broad range of pathologies including cancer, vascular disease and arthritis. BMP activity is modulated by extracellular proteins including Chordin (called Sog in Drosophila) and Tsg. Together these extracellular regulators bind BMPs and inhibit signalling, moreover Tsg is unusual in that it can also promote BMP signalling. Currently we lack an understanding of how these regulators control BMP activity and the dynamics of their localisation in vivo. Therefore, the overall aim of this proposal is to determine the molecular mechanisms that modulate extracellular BMP signalling by combining synergistic structural, molecular, developmental and imaging approaches. Using cryoEM, we will determine the structure of Chordin/Sog alone and in complex with its binding partner Tsg. In parallel, we will use live and super-resolution imaging to visualise the localisation of Sog and Tsg in the early embryo. We will then use mutational analysis and Affimers, small non-antibody binding proteins, to determine the effect of disabling particular protein interactions on extracellular function in vivo. The Drosophila embryo is the ideal model for this study, due to its rapid development, amenability to manipulation and genome engineering, and extremely well characterised BMP responses that will allow signalling levels to be sensitively assessed. The findings from this study will not only demonstrate how a signalling ligand is regulated extracellularly, shifting the emphasis away from events downstream of receptor activation, but also reveal new strategies for enhancing and inhibiting BMP signalling, both of which represent major goals therapeutically.