Regulation of epithelial and endothelial cell-cell junctions by mechanical forces

Most parts of our body are constantly changing in response to mechanical forces. For example, every time we breathe in, our lungs expand. Our gut muscles contract to push food through the gut after we eat. When we move our arms or legs, our skin needs to stretch or compress. Our blood vessels are exposed to pulsatile blood flow generated by our hearts.
Our lungs, gut and skin are lined by cells called epithelial cells, which provide the interface between the external environment and our body. Epithelial cells need to act as barriers: our skin, lung and gut epithelial cells protect us from bacteria and viruses, as well as toxic substances. Our blood vessels are lined by cells called endothelial cells, which mediate the transport of nutrients out of the blood stream to supply all our tissues and organs. They also allow Endothelial cells also need to form a barrier to stop the content of the blood leaking out into our tissues, yet still allow white blood cells into the tissues to fight infections and repair wounds. Epithelial cells and endothelial cells are normally tightly attached to each other to make a sealed barrier, similar to Velcro.
Despite all the rapidly changing mechanical forces that epithelial cells and endothelial cells are constantly exposed to, it is important that they are flexible enough to move yet maintain their barrier functions. In addition, as a baby gradually grows into an adult, these cells need to divide yet still form tight barriers. Furthermore, if they are exposed to a sustained change in mechanical force, such as an increase in blood pressure, the cells need to adapt to this change.
In our research, we aim to find out how epithelial cells and endothelial cells adapt to changes in mechanical forces. We will focus on studying how neighbouring epithelial or endothelial cells pass on messages about mechanical force to each other through their Velcro-like attachments. These attachments between cells contain thousands of different types of molecules. We will test which of these molecules are important for detecting messages from neighbouring cells, and how these molecules adapt to rapid changes in mechanical forces. We will also determine how cells respond to long-term changes to the level of mechanical forces, perhaps by altering the composition of their Velcro-like attachments to make them stronger or weaker.
Epithelial cells and endothelial cells experience different kinds of mechanical forces because of their different locations in our bodies. We will directly compare the molecules that are required for sensing mechanical forces in these two types of cells. This will provide new insight into how cells adapt to their environment and the stresses that they experience.
Through our work, we will generate important information about how our body forms and maintains barriers to the outside world, via epithelial cells, and between the blood and tissues, via endothelial cells. This will be useful to develop new ways to repair or replace damaged tissues, for example after operations, extensive wounding or severe infections.

Epithelial and endothelial tissues usually maintain their cell-cell junctions despite the large range of forces they experience. Compression, stretch and shear forces are transmitted into and between cells by transmembrane cell-cell adhesion molecules via their interacting proteins and the associated cytoskeleton. We will take an integrated multidisciplinary approach to identify and characterise the major force-sensing components of cell-cell junctions, and how they act from single molecules to cell and tissue models. We will determine how acute responses to mechanical forces are converted to long-term changes in cell behaviour, including transcriptional changes, cell extrusion and cell proliferation. Importantly we will directly compare junctional mechanosensing in epithelial versus endothelial cells, which are exposed to distinct types of forces in vivo, hence we predict will respond differently to mechanical forces.
We will deplete junctional adhesion molecules and their protein partners to determine how they contribute to mechanosensing. This will be complemented by RNAi screening to identify novel protein and lipid players involved in force sensing. We will carry out lipidomic analysis to delineate how membrane lipids contribute to junctional mechanosensing.
We will determine how force-induced unfolding of key junctional proteins contributes to mechanosignalling at the single molecule level and in cells. We will also develop DNA-based tension sensors to compare the junctional tension levels between endothelial and epithelial cells, and to identify which junctional proteins, membrane lipids and cytoskeletal components alters this tension.
We will compare how epithelial and endothelial cells in 2D and 3D model systems signal to their neighbours and how this varies with force levels, using optogenetic probes. In parallel, the effects of compression versus stretch forces will be compared in epithelial and endothelial cells of the lung in mouse models in vivo.

Academic impact
Our programme will benefit researchers working on mechanosensing, cell-cell adhesions, cell interactions, signal transduction, and animal models for tissue development and repair. By bridging across scales from single molecules to whole animals, our results will be important to a wide range of bioscience researchers including biophysicists, biochemists, chemical biologists and cell biologists. The research will benefit these scientists by providing (1) information on how individual proteins respond to mechanical force, important for structure-function analysis or proteins, (2) an understanding of how lipids influence membrane protein mechanosensing, which has so far received little attention, (3) new tools and methodologies for analysing mechanical forces in and across cells, (4) new insight into how forces affect cell behaviour, using innovative in vitro and in vivo cellular models, which will be important for researchers aiming to optimize tissue regeneration and repair.
The postdoctoral researchers employed on the programme will benefit from training in a wide range of skills and approaches. By being part of an interdisciplinary network of groups, they will gain an in-depth understanding of how research is improved by combining the expertise of different specialities. They will have the opportunity to cross disciplines and work in different groups to extend their skills portfolio and lead to more impactful publications. Their career progression will be strongly enhanced by their interdisciplinary training, whether in academia, industry or another sector.

Economic impact
The new tools and methodologies we generate in this research have strong potential to be commercialized either through partnering with biotech and pharmaceutical companies or through a spin-out company from one of the three universities involved in the programme. Examples of tools are the mechanosensing molecules and biosensors that we develop. Methodologies include ways to combine lipids and proteins to measure their combined roles in mechanical force sensing, and techniques for measuring forces in 3D endothelial/epithelial tubes.
The results of our research will enhance the quality of life in the future by improving methods for repairing and replacing damaged tissues. For example, by exposing endothelial tubes to mechanical forces prior to implantation could improve their function in vivo. In addition, optimal stretching of skin epithelia can enhance skin grafting.
Our work on how mechanical forces act on signalling networks will identify potential new targets for therapeutic intervention in human diseases. These include genetically inherited diseases that are exacerbated by mechanical forces and affect epithelia (e.g. Epidermolysis bullosa) or blood vessels (e.g. cavernous malformations). They also include common diseases such as high blood pressure leading to heart failure, or cancer growth that is influenced by tissue stiffness.
We will raise the profile of our research through a variety of public engagement activities. The six group leaders and their group members are actively involved in engaging with the public. They visit local schools to talk about their research and discuss the most important issues to address in the future. They also participate in University-led outreach programmes. At the University of Bristol, we contribute to events for members of the public, including talks at the Pint of Science Festival or Science Cafés. We contribute hands on activities at the annual Big Bang South West Fair, which encourages young people to study science and find out about science careers. Researchers at King's College London works actively with the new Science Gallery London, a public outreach venue located on the Guy's Campus at London Bridge. UCL has an extensive long-term pre-16 programme to encourage young people to come to University, including an exciting range of science activities.