Defining a gut-brain-liver axis

When we eat a meal, our gut releases the digested sugars, which must be quickly taken up into tissues to avoid the development of diabetes. This uptake is controlled by the hormone insulin which is released by the pancreas. At the same time, insulin stops the liver from producing more, unneeded sugar from its stores. Important new research in animals which are diabetic, because they have no insulin, has shown that their blood-sugar levels can be controlled equally as efficiently by activating pathways in the brain. The gut sends signals directly to the brain, which processes the information through uncharacterised pathways before sending messages back out to the liver to block sugar production. If normal animals or humans eat diets which are high in fat and carbohydrates, the brain pathways stop responding to signals from the gut and become dysfunctional. This can contribute to higher than normal blood-sugar levels and the development of diabetes. Interestingly, diabetic patients who are very obese and undergo gut surgery to control their weight, can see drastic improvement in their blood-sugar levels long before they actually lose any weight. This effect has been attributed to beneficial changes in gut to brain signalling. A better understanding of the pathways from gut to brain to liver will help us to understand some of the mechanisms which lead to the development of diabetes, and how gut surgery can help. Furthermore, this may allow us to identify points in the gut-brain-liver axis where drugs could act to improve blood-sugar levels without the need for reverting to surgery.

Using our expertise in gut-brain signalling, new mouse models and the latest scientific tools, we will define the different parts of the gut-brain-liver axis. Firstly, we will measure how activation of specific nerve cells, identified as responding to meals, affects sugar production by the liver. This can only be achieved by using mice which have been bred to express specific genes only in the nerve cells in which we are interested. This allows us to turn on just one type of nerve cell at a time and to measure what effect this has on blood-sugar levels in normally-behaving mice. As well as activating the nerve cells selectively, we can use newly-developed methods to follow connections the nerves make in the brain and, also, to find out how the nerves themselves react to hormones, like insulin, and nutrients, such as sugar. We assume that there will be more than one separate pathway in the brain, but that these will converge on a single output which regulates the liver.

We will learn about the different components of the gut-brain-liver axis and see how it normally acts in concert with insulin to control blood sugar. We can then investigate if we can stimulate the brain pathways selectively to improve blood-sugar levels in mice with diabetes. If so, this could provide important proof of principle for alternative targets to develop drugs to treat diabetes.

The brain controls hepatic glucose production through insulin-dependent and -independent pathways, including a gut-brain-liver axis which is activated following meals. Functioning of this axis is lost with high-fat feeding, contributing to the development of hyperglycaemia and type-2 diabetes (T2D). Interestingly, bariatric surgery can rapidly reverse diabetes, long before any effect on weight loss, and this has been attributed to alterations in gut-brain-liver signalling. Thus, bariatric surgery could be a cost-effective way to treat diabetes, even in non-life-threatening situations. The neuronal elements constituting the axis are unknown. A full understanding will elucidate mechanisms behind T2D and how its reversal by surgery. Furthermore, knowledge of intervention points may avoid a need for surgery. Using our expertise in gut-brain signalling, new transgenic mouse models and the latest virus technology, we will define the different components of the axis. We will measure how activation of brainstem neurones, identified as responding to meal-related stimuli, affects hepatic glucose production (which is uncontrolled in diabetes). This will be achieved by transfecting specific neurones with designer receptors, which can then be activated selectively with a designer drug whilst carrying out pancreatic insulin clamps. Then, utilising the latest viruses for anterograde tracing we will follow the route from the gut via the brainstem. We will investigate short-loop circuits within the dorsal vagal complex, but also longer loops which integrate neurones in the hypothalamus, culminating in autonomic regulation of the liver. Functionally distinct sub-populations will be characterised by their efferent connections and how they respond to endocrine and nutrient signals, through retrograde tracing and patch-clamp electrophysiology. Finally, we will identify synergy between central insulin signalling and independent neuronal pathways that together control glycaemia.

Over 2.6 million people in the UK have diabetes on whom the NHS spends approximately 10% of its budget. The major reason for hyperglycaemia in diabetes patients is uncontrolled hepatic glucose production, which is partly due to a loss of function in a gut-brain-liver pathway. The underlying causes of this are a major research area for academic and clinical research groups. Our laboratory has the necessary tools and mouse models in order to make a significant contribution to the understanding the gut-brain-liver axis and also to inform on the mechanisms behind rapid improvements in glycaemic control in patients undergoing bariatric surgery. Our findings will be disseminated at international conferences and by publication in high-impact journals during the grant's duration. Following publication, each of the mouse models we develop will be made freely available.
A conservative commercial estimate of the annual market opportunity for anti-diabetes drugs is close to $100 bn. This project will guide future development of drugs for glycaemic regulation and provide a sound knowledge environment to understand the mechanisms of action of bariatric surgery and, perhaps, offer alternative pharmaceutical targets to mitigate against surgery. The applicant has been involved previously in successful collaborative projects with a number of industrial partners, providing evidence for several novel targets for drug development. The project will begin at the same time as a BBSRC iCASE studentship in partnership with Novo Nordisk. Before the end of the project, we will be in a position to approach the company which may be interested in making peptide mimetics, which could provide composition of matter filings on novel therapeutics comprising long-lasting peptide derivatives.

During the lifetime of the grant, the basic research will be discussed at meetings organised by the Child Health Research Network, the Diabetes and Obesity Research Network and the Association for the Study of Obesity. These annual meetings are forums for basic researchers, psychologists, clinicians, community nurses and other health professionals, patient group representatives and policy makers. Outreach work will be encouraged at all levels within the laboratory. Over the three years, the applicant will lecture at two local schools and at a local Café Scientifique-type meeting. The PDRA will be strongly encouraged to follow the example set by previous lab members, to tutor for the Manchester Access and STEM programmes (aimed at helping under-privileged children into further education), and to complete both a Wellcome Trust Researchers in Residence Scheme and a UK GRADschool.

This project will provide strong training in both in vivo skills and specialist techniques in electrophysiology and metabolic research. In the last twelve years, the applicant has supervised thirteen PhD students, nineteen masters students and eleven PDRAs, all of whom have remained in science (some have their own independent research groups and others have moved into the commercial sector). The PI directs a cross-University IMB initiative to promote and expand research and training in in vivo biology. This problematic area is crucial to the UK economy and to the ambitions of Manchester to be a world-leading university. He is program director for the MRes in Integrative Biology at Manchester (which will benefit from three MRes projects on work derived from this grant) and external examiner on Integrative Biology courses at the Universities of Liverpool and Edinburgh.