The mechanics of epithelial tissues

Many of the cavities and free surfaces of the human body (e.g. gut, lungs, blood vessels) are lined by tissues just a few cells thick. These epithelial tissues separate the body's internal environment from the external environment. As part of their normal function, epithelial tissues are continuously exposed to large mechanical deformations: lung alveoli deform during respiration, intestinal epithelia resist peristaltic movements in the gut, and endothelia are exposed to pulsatile fluid shear stresses in blood flow. The mechanical function of epithelia is particularly apparent in disease when mutations or pathogens affecting the cell skeleton (cytoskeleton) or junctions linking cells to one another result in fragile tissues that tear during routine function (e.g. epidermis bullosa, staphylococcus blistering). Cells within epithelial tissues are tightly connected to one another by intercellular junctions: some junctions form a barrier restricting the passage of solutes across the tissue whilst others integrate the cytoskeletons of neighbouring cells to form a strong multicellular tissue that can withstand mechanical stresses. Despite their clear mechanical role, little is currently known about the mechanics of epithelial tissues and how this derives from the mechanical properties of the cells that make up the tissue and the proteins that make up the cells. This is primarily due to the lack of specific experimental techniques to measure the intrinsic mechanical properties of tissues while monitoring cellular and subcellular traits.

We have developed a novel tool to quantify the mechanics of epithelial tissues by stretching cultured epithelia. During tissue deformation, the applied mechanical tension can be measured and the tissues can be simultaneously imaged at subcellular, cellular and tissue length scales, such that the architecture of the sub-cellular components, the shape of the cells and their eventual reorganisation can be accurately monitored as a function of the imposed force. To complement this experimental tool, we have developed a novel computational model of epithelial tissues that can serve as a means to interpret and refine our experiments.

We now propose to use our new techniques to understand what proteins play a role in setting the mechanical properties of epithelial tissues. To do this, we will focus on three aims:

1) Develop a systematic methodology for characterizing the mechanics of tissues
2) Discover what proteins set tissue mechanical properties
3) Incorporate our findings into a computational model of tissues.

Aim1 is geared at creating a systematic methodology for collecting all of the necessary information to fully characterise the mechanics of normal tissues.

In aim 2, we will ask how the absence of a given protein affects the mechanics of a tissue. To answer this question, we will reduce the level of expression of a chosen gene in the cells that make up the tissue and measure how this affects the mechanics of the tissue. We will also examine how gene depletion changes the organization of the tissue and the cells that compose it. We will pay particular attention to proteins identified in clinical studies of fragile epithelia, as they have direct relevance to patients and potential palliative therapies.
In aim 3, we will use computational and statistical approaches to identify what cellular structures are the most important for setting tissue mechanics using the results of aim 2. Moreover, this analysis and the experiments from aim 2 will directly support the development of a model specifically tailored to study tissue mechanics at large deformation and used to refine our understanding of how changes in protein expression within cells can lead to failure of epithelial sheets, as in clinical cases.

In summary, the proposed investigations will greatly enhance our understanding of the mechanics of epithelial tissues and how pathologies can affect tissue strength.

Exposure to mechanical stresses is a normal part of physiology for epithelial tissues. This mechanical function is particularly apparent in disease when mutations or pathogens affecting the cytoskeleton, adherens junctions, or desmosomes result in increased fragility of tissues. Despite clear physiological relevance, little is known about the mechanics of epithelia and how these relate to the mechanical properties of the tissue's constituent cells.

We have developed a new experimental tool for measuring the mechanical properties of cell monolayers which can be coupled to high magnification optical microscopy to image cellular phenotype and subcellular organization during mechanical testing. To complement this, we have developed a versatile new numerical modeling platform to serve as a means of interpreting and refining our experiments.

We propose to couple mechanical testing with chemical and genetic perturbations to understand what subcellular structures govern tissue mechanics. To do this, we will carry out a limited siRNA screen focusing on proteins that form or regulate subcellular structures thought to be important for tissue mechanics: adherens junctions, desmosomes, the apical actin cortex, intermediate filaments, contractile proteins and proteins identified in pathologies causing fragile epithelia. Statistical methods will be then used to cluster proteins in groups according to the changes their depletion induces on tissue mechanics. We expect that depletion of proteins participating to the assembly of the same substructures should result in similar mechanical phenotypes. These results will then be implemented into our computational model to tailor it for the study of tissues at large deformations. An iterative cycle of mechanical testing and simulations will be used to refine our understanding of how subcellular structures, cellular structures, and multicellular behaviours control the mechanics of epithelia in normal and pathological conditions.

The proposed research will seek to bridge the gap between molecular, cellular, and tissue-scales to understand the molecular and cellular determinants of epithelial tissue mechanics and investigate how tissues adapt to their mechanical environment. This will primarily benefit academics in the fields of tissue and developmental biology but, in the longer term, through comprehension of the determinants of tissue strength, we envisage that it will benefit tissue engineering startups in the UK and clinical medicine.

Academic impact

Academic advancement and innovation:
We expect our research to attract interest from many fields in the global scientific community such as developmental biology, cell biology, biophysics, bioengineering, and clinical medicine.
To ensure our findings have the highest possible impact, we will present our technological developments and preliminary results generated at high profile conferences that cover relevant topics including tissue engineering, biophysics, developmental biology and cell biology throughout the duration of the grant. Where possible we will disseminate our findings in general audience journals.

Training and professional development:
Both GC and AK are actively involved in interdisciplinary training activities at UCL and Cambridge University. GC participates in teaching in the CoMPLEX DTP and is a member of the new interdisciplinary BBSRC DTP. AK is an important contributor to the development of the Bioengineering curriculum in Cambridge, and teaches a number of relevant subjects ranging from material sciences to physiology. The project described here will be used to introduce students from different backgrounds to interdisciplinary research in the life sciences. Elements of the work will be used as exemplar projects for students in the CoMPLEX and BBSRC DTPs. The mechanical aspects of the project will also form the basis of a couple of 4th year engineering projects in Cambridge and are likely to attract students with a Mechanical/Bio Engineering background.

Throughout the course of the project, the post-docs involved will receive cross-disciplinary mentoring and benefit from regular interactions both in Cambridge and London. In addition, they will be involved in mentoring students and develop their own mentoring and leadership skills. This will aid their progression towards an independent group leader position.

Societal and economic impact

Commercialisation and exploitation:
We envisage that, in the longer term, our integrated mechanical testing and computational modeling approach will be of interest to clinical medicine, bioengineering startups, and the pharmaceutical industry. Indeed, we anticipate that our approach could be utilized to study the effect of pathologic genetic mutations on tissue mechanical properties and test the efficacy of palliative treatments in restoring the mechanical properties of diseased tissues. Should there be industrial interest, we will study the possibility of designing a new prototype in a format suited to high throughput screening.

Both the UCL and Cambridge University have efficient mechanisms to assist academics in the development of commercial applications of their research outputs and in the management of intellectual property rights (see for instance Cambridge Enterprise or UCL Business).

Increasing public engagement and understanding:
Previously members of the team have been involved in interactions with the wider community through public discussions and school visits. Through this type of outreach we expect this work to reach a wide audience, giving the public a better understanding of multidisciplinary research and an appreciation of the remarkable natural world in which we live. The co-Is each expect to participate in one public engagement event per year during the course of this project. We will use these opportunities to stress the important role played by basic research in driving societal advances.