Investigation of role of hypothalamic glucokinase in the control of food intake

Food intake is regulated in part by the brain and in particular part of the brain called the hypothalamus. It has for a long time been thought that glucose has a specific role in the regulation of food intake, known as the glucostatic regulation of food intake. However until now the mechanism by which this occurs has been unclear. We have recently found that an enzyme called glucokinase which is present in the hypothalamus may be an important part of this mechanism. We have found that in fasting rats levels of glucokinase activity in a specific part of the hypothalamus called the arcuate nucleus are increased. When we mimicked this increase in glucokinase activity using a type of virus (rAAV) to over-express glucokinase in the arcuate nucleus we found it caused a decrease in food intake. This effect seems to be due to glucose in the diet since when rats were given access to a glucose solution; those in which we had increased glucokinase activity in the arcuate consumed less glucose than those with normal levels of glucokinase. This effect appears to be specific for glucose since when rats were fed a closely related sugar fructose there was no difference in intake between the two groups. We then investigated whether this change in glucokinase could explain the glucostatic regulation of food intake. To investigate this, rats were given a fixed amount of glucose solution and after which there food intake was measured. We found that in rats in which we had increased glucokinase activity in their arcuate nucleus food intake was reduced compared to control rats with normal glucokinase levels. This suggests that glucokinase in the hypothalamus plays an important role in the regulation of food intake in response to intake of glucose. We now plan to investigate whether glucokinase plays a role in the regulation of long-term food intake and weight gain. We will use rAAV to both over-express and decrease expression of glucokinase in the arcuate nucleus and examine the effect on long-term food intake and weight gain. The effect of supplementing the diet with glucose on food intake and body weight gain will be examined. We will also examine whether increased or decreased levels of glucokinase in the arcuate nucleus alter the response to a high fat diet. In addition we will examine whether supplementation of a high fat diet with glucose alters food intake and weight gain. We will examine the interaction between glucokinase and other known modulators of energy homeostasis in the hypothalamus. Together this work will allow us to investigate the physiological role of glucokinase in the arcuate nucleus in the regulation of food intake and determine if it is a possible mechanism by which the glucostatic regulation of food intake occurs. This will establish whether hypothalamic glucokinase may be a target of interest to the pharmaceutical industry.

Within the CNS glucokinase (GK) is expressed at high levels in the hypothalamus, notably the paraventricular nucleus (PVN), ventromedial nucleus (VMN) and arcuate nucleus. We have conducted pilot experiments to investigate the physiological role of GK in the hypothalamus. We hypothesised GK may play a role in the regulation of food intake. This is supported by our finding that fasting increased GK activity specifically in the arcuate. To investigate the physiological importance of this change we injected AAV expressing GK (AAV-GK) into the arcuate nucleus this produced an increase in GK activity similar to that seen following a fast. This increase in GK activity resulted in a decrease in food intake. This effect on food intake may be due to an effect on glucose appetite as rats with increased arcuate GK had reduced intake of glucose compared to controls, whilst intake of fructose was unaffected. This decrease in glucose intake suggests GK may be the mechanism underlying the glucostatic regulation of food intake. This is supported by our finding a preload of glucose reduced food intake in rats with increased arcuate GK compared to controls. We will now investigate the physiological role of GK in the control of long term food intake. We will do this by injecting AAV encoding GK or anti-sense GK into the arcuate of groups of adult rats. Their food intake and body weight will be measured. The effect of addition of glucose to the diet on food intake and body weight will be investigated. This will be performed using both normal and high fat diet. This will investigate whether GK has a physiological role in the glucostatic regulation of food intake. Possible mechanisms by which GK influences food intake will be investigated. Together this data will examine the physiological role of GK in the regulation of energy homeostasis. It will establish the utility of hypothalamic GK as a pharmaceutical target.

This research will benefit a number of groups both directly and indirectly. It will be of direct benefit to the pharmaceutical industry who are currently developing glucokinase activators for therapeutic use. It will determine whether targeting of glucokinase within the hypothalamus is a useful therapeutic goal. Alternatively it could demonstrate that glucokinase activators should be developed which do not have access to the CNS. This would have direct effects on the competitiveness of the activators. The research could also benefit the general public who might be treated with glucokinase activators. They would benefit if the activators were designed that were more efficient or had a reduced side effect profile. The pharmaceutical industry is likely to benefit immediately from published data as it will inform further development of glucokinase activators or choice of developmental pharmaceutical candidate compound. The benefit to the general public will occur over a longer timeframe the exact timeframe is difficult to predict as it will depend on the rate of development of glucokinase activators. In addition to publication in peer reviewed journals the data will be publicised via other media sources. The Department of Investigative Medicine has a strong commitment to public engagement with science. The Department has a track record of drawing public attention to its scientific findings through both printed and broadcast media. In addition, the Department is involved in a popular Imperial College Outreach scheme which organises science education events for pre-university students, their teachers and their parents or guardians, introducing them to the science of Endocrinology and discussing the Department's latest discoveries. The Department also collaborates with the Dana Centre to promote public awareness of science. The Dana Centre is an organisation and resource designed as a forum for non-scientists to take part in exciting, informative and innovative debates about contemporary science, technology and culture. Thus we intend to publicise the results of our studies in the media in addition to specialist scientific journals, and to organise presentations on our findings via both the Outreach scheme and the Dana Centre. The department has a record of maximising the impact and benefit of its research. The department was the first to demonstrate that GLP-1 acted as an incretin in man and reduced food intake. GLP-1 analogues were later developed as pharmaceutical agents for use in type 2 diabetes mellitus. The department also established a spin out company Thiakis based upon the finding that a gut hormone oxyntomodulin inhibited food intake and increased energy expenditure. Thiakis developed analogues of oxyntomodulin as potential therapeutic agents which had an improved pharmacokinetic profile. Subsequently Thiakis was sold to Wyeth pharmaceuticals for £20 million, with £80 million potential performance related payments.