Coordinating the remodelling of cell polarity to form a functional organ.

The cells that make up our tissues adopt complex shapes. In most cases, each cell is able to do part of this process autonomously by defining an intrinsic polarity axis, so they know which end is up and which end is down. For epithelial cells, which line most of our organs, one end must point to the inside and one to the outside. This is true for all organs, and in epithelia is important because it underpins their function in absorbing nutrients (intestine), oxygen (lung) or filtering out salts and toxins (kidney or liver). Loss of polarity affects tissue health and the ability of epithelial cells to adhere to one another, which in turn can lead to cancer cells escaping a tissue to invade another through the process of metastasis. The aim of this programme is to figure out the mechanisms that induce polarised organisation of individual cells, and to determine how these cells work together to form complex multicellular structures in 3D. These structures can consist of cells that arrange themselves into layers found one on top of another, as in the skin or oesophagus, for example. How such tissue organisation is created is not well understood and will be studied with this research grant. Recent work in our laboratory has identified factors that are required for epithelial polarity, some of which have been previously linked to human pathologies. Our results also indicate that mechanical forces might play an essential role in coordinating polarity between cells, as they assemble into a tissue. In summary, we aim to elucidate how cells work together to induce polarised organisation of a complex animal tissue in health and disease.

The objective of this new research grant is to elucidate how polarised tissue organisation arises from the collective action of individual cells. While a small set of conserved cell-intrinsic polarity regulators has been identified through genetic studies across a range of systems, we do not yet have a good understanding of the general logic by which these proteins break cellular symmetry to polarise an entire cell. Nor do we understand how this information is combined with extrinsic cues, both soluble and mechanical, to coordinate the formation of a complex 3D multicellular tissue during development. The developing fly eye is one of the best systems to study this in a living animal. Building upon unexpected findings in our laboratory that alternative mechanisms control polarity in different retinal epithelial cell types, we will reveal i) the different pathways by which polarity is induced in individual cells, and ii) how interactions between cells coordinate this polarity across a tissue in space and time. We expect that the principles we uncover will be of general importance, since cell polarity is critical for epithelia to function as selective barriers that mediate exchange between the outside and inside of an organism. By focusing on a genetically tractable, physiological animal model system of organogenesis, this proposal aims to elucidate how polarity machineries working within cells and communication between cell types come together to induce the functional polarised organisation of complex animal tissues, in health and disease.

Our proposal aims to address fundamental questions concerning the biology of epithelial cells and organs, and will be of broad interest to the biomedical community.

- Fundamental biology and biomedical community: Epithelial cells are the building blocks of most of our organs, and their polarity is essential for their function as selective barriers. To function they have to coordinate their polarity, so tissue organisation is polarised. Complex multicellular patterning in 3D, as seen in most organs, has not been extensively studied. Therefore, our work has great potential to open new avenues of research and to have significant impact in biology and medicine by discovering new processes and mechanisms. Elucidating how cells work together to generate a complex tissue architecture during organogenesis will further our understanding of how defects in this process might impact on pathogenesis and will help us to better understand the mechanisms of organ development, repair and regeneration. We anticipate our work to reveal mechanisms that will be broadly relevant for epithelial and neuroepithelial tissue development in animals. Loss of cell polarity is a hallmark of cancer. Mutations in Rap1 are found in many epithelial cancers (e.g. breast, pancreas), and loss of Afadin/Cno is associated with poor prognostics of breast cancer. By elucidating how these factors induce polarised cell organisation, our work will help further our understanding of how their deregulation might contribute to pathogenesis. In addition, our work on machine learning to improve segmentation of large electron microscopy data sets should have a significant impact on the way researchers process these data by making data segmentation less labor intensive. In summary, we fully expect our work to impact on academic scientists outside of our immediate community. These include medical scientists interested in the transfer of basic research to the clinic. Additionally, we hope that our work will be of interest to the general public.

- Industrial partnership: We are collaborating with two global players in microscopy, Zeiss and 3i. These collaborations aim to address the technological challenges around imaging epithelial cells at ultra-high spatial and temporal resolution in a living animal, during organ development. With this new research grant, we will combine lattice light sheet and scanning array tomography electron microscopy to examine retinal cells as they work together to generate the eye. This part of our work will require protocol development and optimisation (e.g. correlative lattice/3D EM reconstruction). These will be shared with the community and will open paths to novel approaches to studying animal tissues across scales.

- Public engagement: We will continue to take advantage of UCL Open Days and MRC led events to present our work to the general public and school pupils. We will continue to contribute to the annual school visit at the LMCB, to familiarise young students with the higher education sector and to encourage them to consider a career in life sciences and medicine.