Harnessing protein unfolding and aggregation in mechanotransduction

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Mechanical forces shape how our bodies develop and function. For instance as our muscles enlarge and contract with greater force, a mechanism senses these forces and strengthens the attachment of muscle ends to tendons so they are strong enough to withstand the increased force. This process is called mechanotransduction and it is central to many of our body's functions. The proposed research focuses on the molecular machinery involved in how cells sense these mechanical forces.All cells in the human body are held in the correct place via adhesion to neighbouring cells, and to a dense meshwork of proteins that surround cells, the extracellular matrix. Cells adhere to this matrix via cell surface proteins called integrins. Talin is the main linker protein coupling integrins to the cell's force generating machinery, engaging integrin at one end and coupling it to the cytoskeleton. As the cytoskeleton pulls on the integrin anchors, talin stretches like a spring and unfolding of talin recruits the protein vinculin, which reinforces the adhesion in a force-dependent manner. While this paradigm provides a feasible mechanism for force to induce a chemical change, namely the redistribution of vinculin within the cell, it also raises many questions, which are the focus of this research. In this proposal we build on our recent discovery of two new and unexpected pieces of the puzzle of how mechanotransduction works. We have discovered that talin unfolding can lead to self-assembly of talin molecules by a process called aggregation. This is an unexpected discovery, as protein aggregates are best known for their role in disease, for instance dementia and Alzheimer's disease are both caused by accumulation of protein aggregates. Our cells protect themselves from such aggregates using "chaperone" proteins that dissolve and refold misfolded proteins. Our central hypothesis is that these two harmful processes, protein unfolding and protein aggregation, have been harnessed by the cell to serve as elegant force sensing mechanisms that enable the cell to sense forces and convert them into biological signals. The hypothesis that we would like to test is that a normal feature of anchor sites is the formation of a meshwork of stretched talin molecules, which provide a solid platform for the assembly of many additional components required for integrin adhesion. Our pilot data suggest that the formation and rearrangement of this meshwork involves specific chaperones to control this process and to ensure it does not go wrong. We will test this hypothesis by combining the expertise of our two labs. The Goult lab will use biochemical, biophysical and structural methods to characterize how the components work together, and to identify specific changes that can be made to the molecules to alter their activity. The Brown lab will exploit the powerful genetics and imaging approaches that can be used in the fruit fly Drosophila to test the importance of the formation and remodelling of the talin meshwork in different processes that require the integrin machinery within the organism, such as attachment of muscles and anchoring of stem cells.This research is important at several levels. Our discoveries will improve our understanding of how forces strengthen cell adhesion, and how pathological protein aggregation is avoided, with potential benefits to the understanding of human disease. Diseases caused by weakening of cell adhesion may be improved by interventions that mimic the force signal and strengthen adhesion. Similarly, movement of cancer cells, or metastasis, renders cancers much more difficult to treat, and strengthening adhesion will anchor cancer cells and restrain cell movement. The experimental advantages of Drosophila will allow us to investigate the role of specific protein-protein interactions within an organism throughout its life cycle, and this knowledge will then be applied to humans.