A novel model of AMPK-mediated obesity involving the gamma2 subunit

In common with much of the rest of the world, the UK is facing a massive increase in the prevalence of obesity and type 2 diabetes mellitus despite growing societal and medical awareness of the importance of weight gain in driving metabolic disease. In 2010, just over a quarter of adults in the UK were classed as obese, with government predictions of over half of all UK adults becoming obese by 2050. The number of people diagnosed in the UK with diabetes has increased from 1.4 million in 1996 to 2.9 million today, and is projected to reach over 4 million by 2025. The complications of obesity and diabetes include some of the most prevalent and serious diseases affecting industrialised societies today, including heart disease, hypertension, stroke, cancer, visual impairment and joint disease, with potential associated disability, loss of earnings, reduction in life expectancy (by an estimated 9 years) and huge impact on the healthcare system.

Both obesity and type 2 diabetes are chronic metabolic diseases reflecting a complex interaction between an individual's genetics, behaviour and environment (e.g. food intake and physical activity). Typical weight-loss strategies include an attempt at modification of lifestyle through dieting and exercise, but these can be frustratingly difficult to implement and even harder to maintain, prompting the use of additional, more effective treatment strategies. The latter has included medication as well as weight-loss (bariatric) surgery. Despite the increasingly clear prospect of disease alleviation or even resolution with bariatric surgery for selected patients, the limited capacity of current healthcare systems to offer it to more than a small proportion of all those affected (with 6000 procedures performed annually in the UK), coupled with the poor long-term efficacy and safety of established drug treatments for obesity, has prompted an intense search for alternative drug therapeutic options.

One increasingly attractive molecular target is the protein AMP-activated protein kinase (AMPK), which has now been recognised as being acted upon, albeit indirectly, by major classes of antidiabetes drugs, including metformin and the thiazolidinediones. Activation of AMPK leads to multiple metabolic effects and is thought to underlie much of the aforementioned drug's benefits. Accordingly, there is growing intense interest in the possibility that direct activators of AMPK may provide much needed, effective treatment strategies for the worldwide epidemic of obesity and diabetes.

To realise this aim and design rationally-based, novel therapies for the treatment of these metabolic disorders, there is a growing imperative to better understand the biology of AMPK with regard to the regulation of whole-body energy balance, in particular its role in appetite regulation by the brain. A further key unanswered question is the potential long-term risk-benefit profile conferred by activating AMPK across the entire body. Existing scientific models involving AMPK have generally adopted a 'loss of function' or gene 'knock-out' approach, whereby a component of the AMPK enzyme complex is genetically deleted. In contrast, we have developed a 'gain of function' model utilising gene-targeting to induce a precise activating genetic alteration in the energy-sensor subunit of AMPK, namely the gamma2 subunit. Our initial findings suggest that chronic, whole-body AMPK activation via the gamma2 subunit has both beneficial and adverse effects in key metabolic organs, including the brain and pancreas. Building on these results, the detailed application of a range of molecular, cellular and physiological techniques to this model is expected to yield new insights into the long-term risk-benefit profile of chronic AMPK activation, vital to the design of rational metabolic therapies based on the AMPK system.

Obesity is a global public health problem. Current pharmacological options for obesity are limited by poor efficacy in terms of weight loss and a preponderance of adverse reactions when used chronically. An unprecedented need now exists to better understand the biological mechanisms of obesity and identify suitable molecular substrates for safe and efficacious long-term pharmacotherapeutics.

An attractive target in this regard is the mammalian AMP-activated protein kinase (AMPK). AMPK is a conserved serine-threonine kinase that acts as an energy sensor (through binding of adenine nucleotides via its gamma subunit) and integrator of metabolic signals at cellular and whole-body level. Activation of AMPK switches on ATP-generating catabolic processes (e.g. fatty acid oxidation), whilst repressing anabolic ATP-consuming pathways (e.g. lipid synthesis). Accordingly, short-term in vivo administration of AMPK activators is associated with beneficial metabolic effects in experimental obesity models. However, the chronic sequelae of these drugs remain unexplored.

We previously identified activating mutations in AMPK's regulatory gamma2 subunit as the cause of a rare human cardiomyopathy. In seeking to better understand the nature of these mutations, we generated the first gene-targeted murine model bearing an activating mutation in gamma2. Surprisingly, whilst inducing only subtle cardiac abnormalities, mutant mice develop striking adult-onset obesity associated with hyperphagia on a normal diet. In this proposal we seek to define the mechanisms underlying the systemic phenotype of this model and, in doing so, aim to elucidate the diverse consequences of chronic in vivo AMPK activation. Given intense academic and pharmaceutical interest into the potential of AMPK activation as a viable therapeutic strategy for the epidemic of metabolic disease, a better understanding of the long-term risk-benefit profile of chronic AMPK activation, as exemplified herein, is essential.

Who outside of academia will benefit from this research?

Outside of sharing research outcomes with academic audiences, other principle stakeholders include: those working within the pharmaceutical and biotechnology industry on novel drug targets for treatment of obesity and diabetes; charities focusing on diabetes and obesity, e.g. Diabetes UK; and the wider public interested in how the brain regulates energy intake and the metabolic consequences of when this goes wrong.

How will they benefit?

The central role of AMPK in regulating cellular and whole-body energy homeostasis, global surge in burden of metabolic-related disease and the identification of AMPK as an (indirect) target of current widely prescribed metabolic therapies, has led to increasing interest in the development of drugs targeting AMPK specifically. Whilst adding to the emerging complexity of AMPK biology, findings from the research are expected to provide a greater impetus for metabolic research and development teams in the pharmaceutical industry to focus on gamma isoform-specific modulators of AMPK function, in particular highlighting the potential importance of the gamma2 subunit in energy sensing and regulation of central appetite. Development of such agents will not only have great utility for academics in future scientific studies looking at AMPK's role in physiology and models of disease, but also potentially hold promise as novel therapies for metabolic disorders, thereby being of relevance to the wider public.

What will be done to ensure they have the opportunity to benefit?

Whilst it is not envisaged that the research project will lead directly to a novel drug therapy, findings clearly implicating Prkag2 as a novel regulator of food intake or pancreatic endocrine function will be valuable in establishing a basis with which to pursue the gamma2 subunit as a novel drug target for obesity and/or diabetes. Our collaborators at the Clore Laboratory have first-hand experience of working for leading pharmaceutical industries and - in maintaining close links with industry - are well-placed to enable us to take the first steps in achieving this long-term and, at present speculative, impact.

We have members of our group active in engaging the local community with our science as part of the Wellcome Trust Centre for Human Genetics' own organised public engagement sessions. As well as continuing this with specific reference to this research project, in partnership with other groups working on systemic metabolism described earlier, we will seek to publicise our work to a much wider audience by contributing a health and wellbeing workshop event on the regulation of appetite and its relation to obesity at the annual Oxfordshire Science Festival.

In addition, we have successful experience of achieving public impact as a result of our research findings (http://www.ox.ac.uk/media/news_releases_for_journalists/120306.html). The Press Offices of the Universities of Oxford and Buckingham will be notified early in the event of a high-profile publication to aid in drafting a press release for the media. Postings on websites will also be made, including the web pages of Oxford's Wellcome Trust Centre for Human Genetics and the Clore Laboratory.