SURVIVING METABOLISM: acid handling and signalling
We study how small ions trigger and modulate the function of cells in human tissue. By understanding their effects on the body’s physiology, we’re able to understand the fundamental role they play in health and disease.
In particular, our research focuses on the roles of pH and calcium on the physiology of the heart and cancer. These two tissues are similar in that they both have high metabolic rates, complex signalling pathways and major implications on human health. Both also experience high acid loads, thanks to their high metabolic rate, and are strongly influenced by the presence of calcium, as its role in signalling controls heartbeat and tumour progression.
We use molecular imaging to study the role of hydrogen and calcium ions in these tissues. Crucially, though, we take a bottom-to-top approach: from using confocal fluorescence microscopy to understand sub-cellular behaviour, all the way up to employing magnetic resonance to learn about the the whole-organ response. Based on our experimental work, we are able to build mathematical models that can be used to predict how cellular behaviour varies depending on the presence of small ions. Our approach is, necessarily, recursive: our observations feed our models, which are then validated against further experiments.
In our approach to validation, however, we differ from many similar groups around the world. We are pioneering the use of 3D clusters of cells, known as spheroids, in the lab, which allows us to mimic tissue much more accurately. While it makes our work more time-consuming and complex, it provides much more useful results than single, isolated cell preparations ever can. Currently we’re using these approaches to understand the importance of mitochondria in cardiac cell physiology and the links between pH and cancer development.
Ultimately, our research programme, built upon understanding the role of small ions in cardiac and tumour tissue, helps provides a more complete understanding of physiology, highlights different physiological needs and challenges, and fuels technological progress.
Our research interests are broad because (i) multiple biological systems are strongly modulated by acid-base chemistry, (ii) pH changes are strongly related to metabolic and perfusion status, (iii) many disease states involve changes in pH homeostasis or signalling. The projects that we are currently pursuing include:
- Characterising the molecules involved in shaping the acid-base landscape of tumours, with a particular focus on colorectal cancer (CRC) and pancreatic ductal adenocarcinoma (PDAC);
- Investigating the effect of stromal acid-handling mechanisms in supporting the growth of CRCs, and exploiting these interactions for therapy;
- Understanding the mechanisms by which sustained changes in pH affect cancer cell phenotype via altered gene expression programmes;
- Defining the molecular characteristics of cellular acid-fitness in selecting aggressive forms of cancer;
- Characterising the mechanisms that regulate cardiomyocyte nuclear pH and how these affect gene expression;
- Studying the effect of metabolic acidoses associated with inborn errors of metabolism on cardiac physiology;
- Understanding the importance of diffusivity and shape for efficient gas exchange by red blood cells.
E.R.C. CONSOLIDATOR AWARD
Metabolism generates vast quantities of acid (protons). Essentially all biological processes are pH-sensitive, therefore the regulation of acid/base chemistry is a fundamental homeostatic priority. However, controlled intracellular pH (pHi) dynamics are, potentially, a versatile form of cell signalling with a broad remit of targets because protonation of proteins is an enzyme-independent post-translational modification. Indeed, many examples of orchestrated spatio-temporal changes in pHi have been demonstrated to take place inside cells, yielding the concept of protons as bona fide signals.
We and others have now made a compelling case for studying acid handling and signalling in cancer. Acidity is an established chemical signature of the tumour microenvironment. It arises because cancer metabolism releases an exceptionally large acid-load into the extracellular space. Due to abnormal vascular function, this acid-load is not promptly washed away; instead, it produces the low extracellular pH (pHe) measured reproducibly in solid tumours in vivo. Extracellular acidity is not merely a chemical consequence of metabolism, but a biological signal that feeds back on tumour biology somewhat analogously to hypoxia.
Carefully exercised acid handling is pivotal for cancer survival because it aims to maintain a favourable combination of pHi and pHe. Essentially all cells devolve a substantial fraction of their energetic and synthetic resources to keeping pHi within a narrow range that is conducive for biological activity, although a degree of cell-to-cell variation in pHi control is normally observed within a population of cells. Dysregulated acid-base balance has been shown to perturb or even kill cancer cells therefore each cell, based on its acid handling phenotype, can be ascribed a fitness to survive at a particular microenvironmental pHe. An important pHi-regulating process is acid-extrusion by membrane-bound proteins that export H+ ions (e.g. Na+/H+ exchangers) or import base (e.g. Na+-HCO3- cotransporters), but in the diffusion-limited tumour microenvironment, acid handling must also consider the diffusive transport of protons across the intra- and extracellular fluids and the role of non-cancer cells present in the tumour stroma, such as fibroblasts.
Proton signalling underlies the cellular responses to changes in acid/base chemistry. The majority of proton targets are intracellular and many examples of proton sensors have been reported, mostly on the basis of acute readouts. The longer-term effects of protons, such as on gene expression, are highly relevant to cancer cells living under acid-stress, but remain poorly characterised, despite evidence for proton-sensing transcription factors. Extracellular acidity has been proposed to exert a Darwinian selection pressure that favours a sub-population of cancer cells bearing a compatible acid handling and signalling phenotype. An analogy can be drawn to hypoxic-selection, although acid-selection has the added complexity of an intricately regulated pHe/pHi relationship. On the premise that more fit ‘pH phenotypes’ are more aggressive (e.g. are associated with cancer stem cells, CSC), acid-selection could play a major role in cancer progression. However, the definition of ‘pH-fitness’ and its relationship with stemness remain unclear.
To summarise: acidity is a potent, endogenous and broad-spectrum modulator of biological function that is regulated by a relatively small number of proteins. In principle, these characteristics should make acidity an ideal candidate for the therapeutic management of tumour growth. In reality, translating the sum of our understanding of acid handling and signalling into therapy is not trivial, and none of the major approved therapies are based explicitly on disrupting acid handling and/or signalling. Reasons for this paradox relate to inadequacies in our understanding of pH handling and signalling in cancer, exacerbated by the experimental challenges associated with pH studies.