Bilateral BBSRC-SFI: The role of hypothalamic neuropeptide network in regulating tissue sizes in response to diet energy content and composition

If you go on a diet to lose weight then you don't only lose body fat. You also lose muscle mass and your vital organs like your liver, heart and brain also shrink a little. If you put on weight however you generally put on disproportionately more fat tissue, but you may also deposit fat into your liver and muscles. It is thought by some scientists that this deposition of fat into the liver and muscle may be a primary reason why some people with obesity also develop type 2 diabetes. Another situation involving body composition changes is if you are unlucky enough to have a heart attack or develop a serious disorder like chronic kidney disease or cancer. An unfortunate side effect of these conditions in some people is that you may lose appetite and in these situations when you lose weight you lose lots of muscle as well as reduced vital organ sizes. These changes can have a major impact on quality of life of the chronically sick patient, and they greatly increase the risk of dying. Finally as we age some people go into a negative energy balance state where they also lose muscle mass over longer periods of time which leads to weakness and contributes to frailty which is a major risk factor for mortality in later life.

At present we know very little about how the system that regulates these changes in our body composition actually works. We have some evidence that a small area of the brain that is known to regulate how hungry we are, called the hypothalamus, may play an important role in co-ordinating the responses of the individual tissues to overall changes in energy balance. The aim of this work is to find out the key genes in the hypothalamus that may be involved in this regulation. The first step in doing this is to explore how the global pattern of gene expression in this brain area changes when we perturb the system in different ways - giving different diets and restricting energy supply by different amounts. We will then be able to correlate the changes in the brain to the patterns of tissue use. To show that the genes are causally related to the tissue size changes we will manipulate the genes directly and then see how that alters the response to a change in energy balance. For example, from the correlations we may find that the expression of gene 'x' is strongly linked to the increased use of skeletal muscle when we are in negative energy balance. So we will knock that gene out and then see how not having this gene affects the muscle use. If, when we knock out the gene, there is no muscle loss, then we will know that the gene we manipulated has a direct effect on that part of the system.

Our primary aim in the grant is to find the key genes that are involved in regulating the system. These genes might then become targets for the development of pharmaceuticals that might be able to affect our tissue utilisation patterns. For example it may be possible to use this information to develop drugs that can be given to patients with chronic diseases like cancer to prevent their weight loss. This would potentially have a large impact on both quality of life and mortality of chronically ill patients. Moreover, since we will be manipulating diets to find out how the system works this may also allow us to make dietary recommendations to achieve the same ends, both for chronically ill patients but also for people engaged in weight loss strategies to alleviate obesity. Ultimately we consider it may be possible to devise nutritional interventions that maximise fat loss and minimise loss of other tissues when dieting, and conversely prevent fat gain and maximise tissue recovery when a diet ends. This grant will provide the first steps towards making that happen.

During periods of perturbed energy balance individuals must supply shortfalls in energy by extracting energy from tissues, or during energy surplus must store energy by expanding tissue sizes. Our recent BBSRC supported work (BB/G009953/1) has shown that the responses are complex and tissue specific. For example under caloric restriction some tissues (eg alimentary tract) increase in size, while others decrease enormously (adipose tissue), and yet others are relatively protected. During energy surplus however the expansion is predominantly in adipose tissue. The regulation of these complex changes in body composition is only poorly understood. Preliminary data suggests that the network of gene expression in the hypothalamus which is responsive to different levels of energy imbalance probably plays a key role in co-ordinating the tissue level responses. The primary aim here is to identify and validate the most important hypothalamic genes that regulate the tissue level responses. The identification stage will involve manipulating animals using a range of perturbations of energy supply, dietary macronutrient composition and the source of dietary protein and looking for correlations to the impact on the tissue sizes in the hypothalamic gene expression network. This work will use a pathway correlation and gene network analysis approach based on mutual information that we have recently published. We will then validate the importance of the identified target genes by using gene knockouts and gene knockdowns based on adeno-associated viral constructs. The final outcome will be a range of hypothalamic genes that may provide suitable targets for pharmaceutical developments aimed at manipulating body composition in conditions of energy imbalance. In addition we will be able to formulate optimal nutritional combinations that minimise lean tissue loss under negative energy balance (during dieting for example) and maximise lean tissue regain during post-diet feeding.

The intellectual property (IP) connected with this research will comprise two main areas: knowledge of the hypothalamic genes that regulate tissue size and knowledge of diets that impact body composition during periods of energy imbalance. In addition to opportunities for public outreach detailed in the pathways to impact attachment, we see three potential areas where this IP will be of potential commercial impact: diet formulations in relation to weight loss, dietary treatments for sarcopenia, and treatment options for cachexia.

Diet formulations in relation to weight loss. The world is in the middle of an obesity crisis. Dieting is the primary self and physician prescribed treatment. There are numerous commercial companies that fulfil the demand for such programs in a market measured in billions of dollars annually. We feel that there is opportunity to refine the existing commercial dietary formulations utilising the data that we will generate. We have liaised with a commercial diet company in the USA called the 'idiet' (www.idiet.com). Dr Susan Roberts, the founder of idiet has expressed an interest in our work, and the potential for our studies to help them fine tune the packages they sell. The most suitable way for them to become involved would be for us first to show that the findings of our studies translate to humans. Hence the primary pathway to impact would be to perform this translational work, possibly with joint funding from the company, and then licence the IP to the company for exploitation.

Treatment options for age associated sarcopenia. During ageing there is a progressive loss of muscle mass in some individuals (sarcopenia) which is a major cause for late life frailty. Our work will provide an understanding of the factors that cause sarcopenia and indicate potential nutritional interventions that may stem the rate of muscle loss. In this respect we see a clear route to a marketable product because the co-applicant's affiliated organisation (Teagasc , Ireland) has recently signed a licence agreement with Dairy Concepts IRL to develop snack products based around the use of whey protein isolate to minimise muscle tissue loss in the elderly. The current study will contribute to this development by providing information on the optimum nutritional context in which such a product should be delivered.

Treatment options for cachexia. Cachexia develops during multiple chronic diseases involving loss of appetite and negative energy balance, accompanied by a disproportional loss of skeletal muscle. Developing a treatment for cachexia would have enormous impacts on patients with chronic illnesses. It is likely that a major cause of the pathology of cachexia is a regulatory dysfunction that prioritises lean tissue for use during energy imbalance. Our work could result in two potential outcomes that would be of commercial relevance for developing treatments for cachexia. The first concerns drugs that target specific hypothalamic genes that we will identify to be causally linked to lean tissue utilisation. It is unlikely that simply knowing that these genes are involved in regulation of tissue size would be sufficient for a pharmaceutical company to become interested. We would envisage therefore that the next logical step in the pathway to impact would be to apply for a follow-on fund grant which would allow us to show that these same hypothalamic targets are modulated in mouse models of cachexia. If successful, we consider there would be good opportunities to then approach a pharmaceutical company to take over and develop the idea into a possible pharmacological treatment option. The second potential is to develop specific dietary formulations that will affect gene expression of these same hypothalamic targets. Again we would envisage the best route would be to apply for follow-up funding to make this a more commercially attractive package.