How the brain controls food intake: the emerging role of the brain GLP-1 system in energy balance and autonomic control

Obesity, diabetes and co-morbidities, such as hypertension, are a serious health burden for the patient and a strain on public resources. Promising drugs that might be beneficial for the treatment of these conditions are glucagon-like peptide 1 (GLP-1) analogues. GLP-1 is a hormone that is secreted from the gut after a meal. It is also produced in the brain. Its principal role is to improve the digestion of sugars, and to generate the sensation of fullness, or satiety. The aim of our project is to improve our knowledge of the relative importance of the brain and gut GLP-1 signalling in the control of blood sugar and food intake, and to clarify whether GLP-1 analogues, which are already in clinical use, activate the brain GLP-1 system.
Neurons that produce GLP-1 have been found in the brainstem. These cells send a network of nerve fibres to many parts of the brain, including the hypothalamus, a brain region implicated in the regulation of appetite. We hypothesise that it is GLP-1 released from these brainstem cells that mediates the satiety effects, some of the beneficial effects on blood glucose control, and further functions that are less well understood, such as in cardiovascular control, developing of hypertension, nausea and vomiting, and learning and memory. The proposed research will clarify the exact role of GLP-1 neurons in these effects and thus validate their importance as a drug target for treatment of metabolic disease.
We will use a gene therapy approach where new genes are delivered to GLP-1 cells in specific areas of the brain. These genes will produce proteins allowing cell-specific suppression (or alternatively increases) of the activity of GLP-1 cells with unprecendented temporal and spatial resolution. By measuring the effects of inhibiting or increasing activity of specific GLP-1-expressing cells on blood glucose control, food intake and behaviour, we will be able to tease out the exact physiological function played by GLP-1-expressing central neurons.
The primary aim of our project is to improve knowledge on the interaction between peripheral and central GLP-1 receptor activation. From this work we will learn whether interference with brain GLP-1 release is an important target for the treatment of obesity, diabetes and co-morbidities. These data will provide further insight into the clinical benefit of GLP-1-based treatments, possibly not only for patients with diabetes, but more generally in metabolic disease. Furthermore, it will provide information on the cause of undesired side effects such as nausea. Since GLP-1 analogues are already on the market for the treatment of type 2 diabetes in overweight patients, we expect that the knowledge from this project will feed into drug development and formulation, and thus an influence on therapy could be hoped for within 10 years.

This proposal aims at characterising the exact physiological role of GLP-1 expressing neurons in the control of energy balance and autonomic functions. We will experimentally silence or activate GLP-1-expressing neuronal populations in vivo by using viral gene transfer. Specific targeting will be achieved by combining a transgenic mouse line that expresses Cre-recombinase selectively in cells that express GLP-1 or glucagon, with viral expression vectors that are only active in cells expressing Cre-recombinase (FLEX-switch technique). The adeno-associated virus (AAV)-based vectors are injected stereotaxically into the brain region containing GLP-1-expressing neurons and 2-5 weeks after the infections the animals will be tested for effects of GLP-1 neuron modulation on food intake, blood glucose control, cardiovascular and circadian activities. AAV vectors produced for this purpose will include those that inactivate or ablate GLP-1 cells permanently (Tetanus toxin light chain; Diphteria toxin A subunit), and those that inactivate GLP-1 cells transiently (Allatostatin receptor; DREADD hM4D) or activate GLP-1 neurons transiently (Channelrhodopsin2, DREADD hM3Dq), when the appropriate agonist is applied (allatostatin, clozapine-N-oxide or light, respectively). Generation of these tools will also greatly facilitate physiological in vivo studies of other specific cell populations in the CNS.
Food and water intake will be monitored using metabolic cages, blood glucose control will be evaluated with oral and intraperitoneal glucose tolerances tests, cardiovascular and respiratory activities will be assessed using standard physiological monitoring in anesthetised animals.
Our results will further define the physiological role of the GLP-1 neurons and provide crucial data on the usefulness of a GLP-1 based therapy for metabolic disease. They will also address the question whether overactivation of this system leads to emesis as observed with GLP-1 injection.

The basic research we propose here is likely to significantly increase understanding of how GLP-1 affects energy balance and cardiovascular control. We expect these results to be of considerable nterest to the pharmaceutical industry. This is particularly likely becauseGLP-1 analogues are already in clinical use for the treatment of overweight diabetics. Indeed, Astra Zeneca agreed to sponsor a symposium on GLP-1 at the recent FEPS meeting in Istanbul that ST organised in collaboration with Frank Reimann from Cambridge University. Any intellectual property arising from this research grant will be exploited through Imperial College Innovations Ltd, the College's technology transfer company.
A benefit to the general population, in terms of improvements in healthcare, might be possible over a time-course of 10 years if our findings influence the availability of GLP-1 therapy for diabetic and for obese patients. Additionally, our research might identify new treatment strategies for metabolic disease, which could hopefully enter clinical practice over a 10-15 year time-course. This again would elicit interest from the pharmaceutical industry. Currently ~10% of the NHS budget (£9billion) is spent on the 2.6 million diabetics in the UK. Diabetes UK predicts this number to rise to 4 million by 2025, mainly due to an increased incidence of obesity, driven by increasingly sedentary lifestyles. In 2009 more than 60% of the adult UK population were overweight. The complications of these diseases include stroke, retinopathy, neuropathy, renal failure, cardiovascular disease and cancer. The increased prevalence of diabetes, obesity and co-morbidities was recently predicted to contribute to a lowered overall life expectancy in the UK (http://www.independent.co.uk/life-style/health-and-wellbeing/health-news/diabetes-may-cause-first-fall-in-life-expectancy-for-200-years-966914.html) for the first time in 200 years. GLP-1 analogues show great potential in the treatment of type 2 diabetics mainly due to their actions as insulin secretagogues and positive trophic effect on beta-cell mass. The anorexic effect of GLP-1 is mediated by central actions and so might be some of the insulin secretagogue effect. Since GLP-1 has various central effects (including induction of nausea and vomiting, in addition to satiety) exogenous application might be counterproductive. Our study of the endogenous source of GLP-1 in brain will educate us about the different pathways involved and thus clarify the role of central GLP-1 in satiety and blood glucose control. In particular, this work will address roadblocks in diabetes research as identified recently by the European Commission's Support Action "DIAMAP: A Road Map for Diabetes in Europe" (http://www.diamap.eu/) including "Determine central nervous master switches that control food intake and energy expenditure", which is identified as lacking "collaboration of researchers in related fields" and "available in vivo methodology and imaging techniques", both of which we are addressing with this proposal.