Role of the biological clock system in the pathophysiology of type 2 diabetes

Type 2 diabetes is a worldwide health threat. At present, type 2 diabetes is considered the consequence of increased calorie intake and decreased physical activity in susceptible individuals. Recent scientific work revealed the importance of a third factor: circadian desynchrony. Most creatures, ranging from bacteria to humans, possess a molecular clock system consisting of several transcriptional-translational feedback loops. In humans, the biological master clock is located in the suprachiasmatic nucleus (SCN) in the anterior hypothalamus. The SCN is synchronised with the environmental light-dark cycle by direct afferent projections from the retina. The SCN signals to other hypothalamic areas, including orexin neurons that control food intake behaviour, and to peripheral clocks in metabolic tissues such as liver, pancreas and adipose tissue. These peripheral clocks regulate the transcription of key genes involved in carbohydrate and lipid metabolism. Thus, the clock system serves to synchronise daily cycles of energy storage and energy use with daily cycles of sleep and activity (or fasting and feeding). In modern society, however, circadian synchrony is easily disturbed due to the around-the-clock availability of artificial light and high caloric food. Three recent lines of evidence point to circadian desynchrony as a possible cause of the increasing incidence of type 2 diabetes. First, genetic disruption of the molecular clock machinery in mice leads to obesity and diabetes. Second, glucose tolerance strongly decreases in humans whose food intake is out of phase with their internal clock rhythm. Finally, epidemiological evidence shows a clear association between type 2 diabetes and shift work. At present, two clinically relevant observations on the circadian system of patients with type 2 diabetes have been made: 1) patients with type 2 diabetes appear to have an altered rhythm of glucose tolerance and 2) in contrast to non diabetic patients, patients with type 2 diabetes do not show the morning surge in the occurrence of cardiovascular events, which could be due to either altered rhythms of circulating hemostatic factors or sympathovagal balance. Currently, the role of the SCN in the pathophysiology of type 2 diabetes remains to be determined. This knowledge is essential in order to target the circadian system to prevent or treat type 2 diabetes. We expect that the orexin neurons in the lateral hypothalamus are a potential target for intervention, since orexin release is under direct control of the SCN, and orexin both induces food intake behaviour and affects glucose metabolism. Our overall hypothesis is that altered SCN output due to misaligned circadian environmental input leads to obesity and type 2 diabetes via altered signalling to orexin neurons and subsequent derangements in the rhythms of food intake, autonomic activity, peripheral clock functioning, and glucose metabolism. We will address this hypothesis with four specific objectives: 1. To determine whether patients with type 2 diabetes show altered day-night rhythms of glucose meal responses, hemostatic factors, autonomic activity and adipose gene expression. 2. To determine the contribution of the SCN to the regulation of the day-night rhythms under objective 1 in healthy humans and in patients with type 2 diabetes. 3. To determine the effect of a decreased amplitude of the light-dark cycle (by exposure to light at night) on food intake, body weight and glucose tolerance in rats. 4. To investigate the causal role of rhythmic orexin signalling in the metabolic changes induced by a decreased amplitude of the light-dark cycle in rats. Human experiments: In experiment 1, we will study the metabolic day-night rhythms of patients with type 2 diabetes. Patients and healthy controls will take three identical meals at fixed time points. We will determine the rhythms of glucose tolerance, hemostatic factors, autonomic activity, and adipose tissue expression of clock genes and metabolic genes. In experiment 2 we will determine the role of the SCN by inducing a phase shift of the SCN with bright light and determine if this leads to shifts in the rhythms of glucose tolerance and the observed altered rhythms in type 2 diabetes. Animal experiments: When mice are subjected to dim light during the dark period (the active period for nocturnal rodents), they partly shift their food intake towards the light (inactive) period. This protocol has been shown to induce excessive weight gain and decreased glucose tolerance despite similar total food intake. In experiment 3, we will subject male Wistar rats to a 12:12 light:dim light cycle or a 12:12 light:dark cycle. We will determine the effects on the rhythm of food intake, weight gain, and intravenous glucose tolerance. In experiment 4 we will assess the causal role of orexin rhythmicity in the development of obesity and diabetes using intracerebral microdialysis.