Regulation of epithelial apical membrane differentiation and function

Epithelia are continuous layers of cells that delineate our tissues and organs. Individual epithelial cells interact with each other via molecular complexes that mediate adhesion but also function as sensors that transmit information about the presence or absence of neighbouring cells to the cell interior. Integrity of epithelia is important for our organs to develop and function normally, and to protect us from our environment. For example, breaches in epithelial layers such as the skin or in the lining of the intestine can lead to serious infections and can occur due to chronic inflammations or acute infections by viruses and bacteria. Similarly, a characteristic of cancer cells is that they have lost the capability to sense the presence of neighbouring cells or how tightly they are packed, and hence continue to proliferate and migrate on top of their neighbours.

Epithelia are polarised, which means they have two cell surface domains that have different compositions and functions. The cell surface that faces the outside world or the internal lumen in our organs is called the apical cell surface. This apical domain often forms a highly specialised structure that mediates organ specific functions, such as digestion and nutrient absorption in the intestine or supportive functions for the cells that sense light in our eyes. Here, we propose to investigate a molecular mechanism that drives the formation of such specialised apical membranes in intestinal and retinal epithelial cells. This pathway is based on signalling proteins that we have recently identified and for which we have evidence that they form a mechanism that may also inhibit the function of proteins that have been linked to tumorigenesis and cancer by stimulating cells to differentiate and form tightly packed cellular sheets.

Knowledge of how epithelial cells differentiate to mediate organ specific functions and how such pathways inhibit mechanisms that can lead to cancer is important to understand how tissues behave in disease. Epithelial dysfunction has been linked to many diseases such as cancer, chronic inflammations in the intestine, and inherited and age-related diseases that lead to blindness. The expected results will help us to think of new ways how we can treat such diseases that lead to epithelial degeneration and cancer.

Epithelial cells form sheets of cells that are connected by junctional complexes. Individual epithelial cells in simple epithelia are polarised and form apical and basolateral cell surface domains that are biochemically and functionally distinct. The apical domain often differentiates into a highly specialised structure such as the brush border membranes in the intestine or the phagocytic apical membrane in retinal pigment epithelial cells. The signalling pathways that regulate junction formation, positioning and apical membrane differentiation are linked and make use of overlapping sets of signalling proteins; however, we understand only poorly how these proteins are activated in process-specific manners and how development of epithelial cell morphology is linked to apical differentiation and junctional positioning. This proposal focuses on the signalling mechanisms activated by DOME, a Cdc42 exchange factor that we have recently discovered. DOME associates with the apical membrane and regulates columnar morphogenesis, apical differentiation and junctional positioning once initial junction assembly occurred, as well as morphogenesis in 3D cultures. Our preliminary data suggest that DOME activates two Cdc42 effector pathways, the Par6/aPKC pathway and actomyosin via MRCKs. Our aims are to identify the molecular mechanisms by which DOME activated MRCK signalling drives actomyosin dynamics, epithelial morphogenesis, and apical differentiation, and to establish the importance of this signalling mechanism for the differentiation and function of retinal pigment epithelial cells in vitro and in vivo. The expected results will be important for our understanding of the signalling networks and dynamic processes that guide and mediate cell polarization and epithelial differentiation, and how such pathways interact with mechanisms that guide epithelial proliferation and tumorigenesis.

Who will benefit from this research?
The immediate beneficiaries will be basic and applied scientists working in related fields at Universities as well as in industry. This includes scientists working in areas such as epithelial biology, organ development, retinal function and physiology, tissue engineering, as well as chronic inflammation and cancer biology. The retinal pigment epithelium plays a crucial role in the retina and is the primary cell type affected in one of the major blinding diseases: age-related macula degeneration. Hence, insights into the biology and function of retinal pigment epithelial cells will benefit medical scientists, clinicians and, ultimately, patients as well as the NHS and the general public.

How will they benefit from this research?
The research will benefit allied scientists by providing them with the molecular details and functional principles of a new pathway that guides epithelial cell differentiation and function. This will benefit their research as such knowledge and tools generated can be applied to different epithelial cell types and organs. Translational and clinical scientists will benefit in a similar way, as, for example, scientists interested in cancer may be able to exploit the potential tumour suppressor properties of the DOME pathway. Similarly, scientists investigating retinal dysfunction can exploit our results to analyse inherited and age-related retinal diseases.

The project will also involve training of two postdoctoral fellows that can benefit the private sector as well as public services through the NHS as one of the fellows will be trained in techniques required for the development of treatments for retinal diseases involving gene therapy.

The expected results are likely to start to benefit other scientists within the lifetime of this grant.