The P2X7 Interactome: Protein interactions in the P2X7 C-terminus and their role in inflammatory signalling

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ATP is released by cells under a variety of conditions, including mechanical stress, infection or tissue injury, where it activates the P2X7 receptor, initiating an inflammatory signalling cascade involving calcium influx, opening of a dye-permeable plasma membrane pore, reactive oxygen species generation, inflammasome activation, cytokine release and cell death via apoptosis or necrosis. Many of these events are regulated by the 250 amino-acid C-terminal domain, yet surprisingly little is known about the structure, organisation and role of the P2X7 C-terminal domain in macrophage signalling, and the key first step in pro-inflammatory signalling following P2X7 activation remains to be elucidated.

Recently, we have been able to express the C-terminal domain of rat P2X7 in yeast, purify folded protein, and demonstrate that it forms known interactions. We aim to exploit this important breakthrough to study the structure, organisation and interacting partners of the mouse P2X7 C-terminus. We will use the purified C-terminus in pull-down assays using mouse bone marrow-derived macrophage lysates, to discover novel intra- and inter-molecular interactions (the P2X7 Interactome). We will then feed this data into co-expression studies in HEK cells, to confirm interactions by immunoprecipitation and proximity ligation assay, and measure the effects of P2X7 C-terminal truncations and mutants on both ion channel function (using patch clamp electrophysiology) and signalling pathways (using dye uptake assays). We will then translate our studies into mouse macrophages, using si-RNA knockdown of interacting partners, multiplex assays for cytokine release and assays for cell death via apoptosis (Annexin V labelling, cytochrome c release) or necrosis (lactate dehydrogenase release). In this way we will not only define the structural organisation of the P2X7 C-terminal domain, but also identify the key first step in pro-inflammatory signalling following P2X7 activation.

Relevance to BBSRC strategy
Research towards a molecular understanding of inflammation and inflammatory signalling is of great impact to human health worldwide. While inflammation is a healthy response to tissue infection or injury, older people tend to be more likely to suffer from pain and a reduced quality of life with conditions of chronic inflammation such as rheumatoid arthritis, which currently affects around 400000 people in the UK alone. Research into inflammation and its causes is therefore a direct fit with the BBSRC strategic priority of ageing research: lifelong health and wellbeing.
The aim of the 'P2X7 Interactome' project is to understand the molecular basis for pro-inflammatory signalling via P2X7 receptor activation. We still do not understand how activated P2X7 receptors are able to initiate a cascade of downstream events, such as the release of pro-inflammatory cytokines and cell death. Finding out which regions within the P2X7 intracellular C-terminal domain are important for signalling, and what signalling proteins they interact with, will close a significant gap in our knowledge, and be of major impact to both the P2X and inflammation research fields. We will discover new protein-protein interaction targets, which will enable the screening of small molecules designed to uncouple of P2X7 activation from pro-inflammatory signalling, which may be of therapeutic benefit in the future.

Who will benefit from this research?
The main non-academic beneficiaries from the 'P2X7 Interactome' research include the pharmaceutical industry and the general public. The research detailed in this proposal will lead to both a greater understanding of how P2X7 activation couples to inflammation, and also the identification of potential targets for therapeutic intervention. Several selective P2X7 antagonists have already been developed and are in clinical trials for the treatment of inflammatory diseases. These molecules all target the extracellular, ligand-binding portion of the molecule, and block the function of the ion channel as well as downstream signalling. Drugs designed to target protein-protein interactions in the C-terminal domain will have the advantage that ion channel function is uncompromised, while signalling is blocked, and so may have fewer side-effects and enhanced specificity.

How will they benefit from this research?
1. Knowledge
At the end of this project, we aim to understand how P2X7 activation leads to downstream signalling. We will have made a major advance in our understanding of inflammation, and provided new drug targets which we will hope to exploit in collaboration with the pharmaceutical industry. In addition, we will set up and maintain a dedicated 'P2X7 interactome' website, which will contain information about known P2X7 interacting proteins and their roles in health and disease, which will be fully accessible to and understandable by non-scientists.
2. People
The PDRA employed on this project will receive a comprehensive training in protein expression, purification, biochemistry and cell biology and this will contribute greatly to their career development. During the course of the project, they will also disseminate their knowledge to both undergraduate and postgraduate students working in the laboratory. The PI has recently secured a BBSRC Quota PhD student working on a similar protein expression project, whose training will also benefit from working closely with the PDRA.
3. Improvement of health and quality of life
The results of this research will lead to greater understanding of the role of P2X7 receptors in inflammation and human health. We hope that, by providing novel targets and ideas for therapeutic intervention, the development of P2X7-selective anti-inflammatory drugs will be accelerated, which will provide significant benefits for human health, particularly in the ageing population, reducing the burden of age-related disease on health services worldwide.