Defining the functional roles of the enigmatic G protein-coupled receptor GPR35

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G protein-coupled receptors (GPCRs) are cell surface proteins that play key roles in allowing cells to respond to external cues and signals. They are routinely considered to do so by activating one or more of a group of so called G proteins. Because of these key roles in controlling responses to many hormones and neurotransmitters a substantial number of GPCRs are the molecular targets for currently employed medicines and, in general, GPCRs are considered 'tractable' i.e. that molecules that activate or block their activation can be found and developed as potential new medicines. However, there are a number of GPCRs that are poorly characterised and the roles they play in the control of physiological functions are unknown or uncertain. In such cases the potential for them to be targets for new medicines in the longer term is often assessed partially by the effects produced in mice in which the gene encoding the GPCR has been eliminated (knocked-out) or, where such evidence is available, in studies in which variations in the sequence of the gene encoding the GPCR in humans is linked to the potential to develop a disease. GPR35 is such a GPCR. It is known to be present in the colon and variations in sequence of the protein have been linked to the potential development of inflammatory diseases of the lower gut including Ulcerative Colitis. There may also be a more general role for GPR35 in the regulation of inflammation. For example, some ligands that are able to activate GPR35 have been useful medicines for the treatment of airway/lung inflammatory disease asthma. There are two major challenges to better understand if GPR35 could be a useful target for new medicines. Firstly, although there are two chemicals available that can block the function of the human form of GPR35, neither of them work at either the rat or mouse versions of GPR35. This means that it is very difficult to explore the function of GPR35 in rodent models of physiology and disease although such models are vital to provide support to build a case that this might be worth testing in human patients. Secondly, as the mechanisms of signal transduction by GPR35 are both unusual and poorly explored it is unclear which of the various signals generated by GPR35 might be most appropriate to mimic or block to treat disease. In the proposed studies we plan to use a variety of highly innovative approaches to overcome these challenges. We have used a technique called 'gene-editing' to develop a range of cell lines in which only subsets of signals that can be generated by a GPCR are actually induced. These will allow us to assess if different activators of GPR35 are likely to cause different effects in physiological systems. The second key approach will involve the production of transgenic (i.e. genetically modified) mice. Overall, mice and humans have a very similar set of genes. However, in some circumstances ligands that can activate or block a human receptor do not have the same effect at the mouse receptor. This is the case for GPR35. As such, we plan to 'humanise' mice, by replacing the mouse gene for GPR35 with the equivalent gene from humans. This will produce mice in which the responses to GPR35 ligands will be akin to those we would anticipate if we activated or blocked GPR35 in humans and will provide a much clearer picture of how GPR35 ligands might affect the development of treatment of diseases in humans. Although the key objectives of the application are to develop approaches that will deepen our understanding of the roles of GPR35 and how it functions, the results obtained will greatly influence future decisions on whether this receptor might become a new target for the development of novel medicines.