Lamia, Katja A.; Evans, Ronald M.
Metabolism Tick, tock, a β-cell clock
The pancreas is a mosaic organ: its different cell types regulate sugar and fat metabolism through controlled production of digestive enzymes and hormones in response to food and physiological demand. To maintain normal blood glucose levels after a meal, for instance, β-cells in pancreatic islets produce insulin, which then stimulates glucose uptake and storage by the muscle and fat cells, and stops glucose production and secretion by the liver. It is increasingly being appreciated that circadian clocks — sets of genes through which organisms keep track of time — regulate these processes. But exactly how they function is not known. On page 627 of this issue, Marcheva et al . report that pancreatic islets have a circadian clock that regulates insulin secretion, and that disruption of this clock causes greatly increased blood glucose due to impaired insulin secretion, a hallmark of diabetes. Circadian biological phenomena — from the daily movements of plant leaves to human sleep–wake cycles — have been recognized for centuries, but their underlying physical and biochemical mechanisms have remained mysterious. In 1972, a study demonstrated that a brain region in the hypothalamus called the suprachiasmatic nucleus, which sits just above the optic nerves, is required
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