ORIGINAL ARTICLE Spontaneous caloric restriction associated with increased leptin levels in obesity-resistant aMUPA mice O Froy 1 , H Sherman 1 , G Bhargava 1 , N Chapnik 1 , R Cohen 2 , R Gutman 1 , N Kronfeld-Schor 2 and R Miskin 3 1 Robert H. Smith Faculty of Agriculture, Food and Environment, Institute of Biochemistry, Food Science and Nutrition, The Hebrew University of Jerusalem, Rehovot, Israel; 2 Faculty of Life Sciences, Department of Zoology, Tel Aviv University, Tel Aviv, Israel and 3 Department of Biological Chemistry, Weizmann Institute of Science, Rehovot, Israel Background: aMUPA mice carry as a transgene the cDNA encoding urokinase-type plasminogen activator, a member of the plasminogen/plasmin system that functions in fibrinolysis and extracellular proteolysis. These mice spontaneously consume less food when fed ad libitum and live longer compared with wild-type (WT) control mice. aMUPA mice are obesity resistant and they share many similarities with calorically restricted animals. However, extensive metabolic characterization of this unique transgenic model has never been performed. Method: Metabolism of aMUPA mice was analyzed by measuring hormone, lipid and glucose levels in the serum, as well as gene and protein expression levels in the liver, hypothalamus and brainstem. Results: aMUPA mice were found to be leaner than WT mice mainly because of reduced fat depots. Serum analyses showed that aMUPA mice have high levels of the anorexigenic hormones insulin and leptin, and low levels of the orexigenic hormone ghrelin. Analyses of brain neuropeptides showed that the transcript of the anorexigenic neuropeptide Pomc is highly expressed in the brainstem, whereas the expression of the orexigenic neuropeptides Npy, Orexin and Mch is blunted in the hypothalamus of aMUPA mice. In addition, adenosine monophosphate (AMP)-activated protein kinase (AMPK) levels were higher in the liver and lower in the hypothalamus, thus promoting simultaneously central reduction in appetite and peripheral loss of fat. The levels of SIRT1 were low in the liver, but high in the hypothalamus, a feature that aMUPA mice share with calorically restricted animals. Conclusion: Taken together, aMUPA mice exhibit a unique metabolic phenotype of low-calorie intake and high leptin levels, and could serve as a model for both spontaneous calorie restriction and resistance to obesity. International Journal of Obesity (2011) 35, 226–235; doi:10.1038/ijo.2010.125; published online 15 June 2010 Keywords: aMUPA; satiety; calorie restriction; leptin; SIRT1 Introduction The hypothalamus has a crucial role in determining energy homeostasis. 1 It is influenced by nutrients and hormones, such as leptin, insulin, ghrelin and adiponectin. Leptin, a satiety signal secreted from the adipose tissue, stimulates anorexigenic neurons within the hypothalamic arcuate nu- cleus to express pro-opiomelanocortin (POMC) and cocaine- and amphetamine-regulated transcript (CART). POMC gives rise posttranslationally to a-melanocyte-stimulating hormone, which subsequently activates the melanocortin receptor in the paraventricular and lateral hypothalamus and results in decreased food intake and increased energy expenditure. 1 In parallel, leptin suppresses and ghrelin activates a distinct set of orexigenic neurons within the arcuate nucleus expressing neuropeptide Y (NPY) and agouti-related protein (AgRP). These peptides lead to the secretion of orexins and melanocyte- concentrating hormone (MCH) from the lateral hypothalamus and induce wakefulness and food-seeking behavior. Leptin and ghrelin also affect orexin- and MCH-expressing neurons directly. 2,3 Thus, low levels of leptin and insulin during fasting or starvation and high levels of ghrelin and adiponectin activate orexigenic and inhibit anorexigenic neurons, leading to food-seeking behavior and increased food intake. 1 Leptin, ghrelin and insulin all affect adenosine mono- phosphate (AMP)-activated protein kinase (AMPK) consi- dered as the cellular ‘fuel gauge’, switching off anabolic Received 17 January 2010; revised 11 April 2010; accepted 12 May 2010; published online 15 June 2010 Correspondence: Professor O Froy, Institute of Biochemistry, Food Sciences and Nutrition, Hebrew University of Jerusalem, Rehovot 76100, Israel. E-mail: froy@agri.huji.ac.il or Professor R Miskin, Department of Biological Chemistry, Weizmann Institute of Science, Rehovot 76100, Israel. E-mail: ruth.miskin@weizmann.ac.il International Journal of Obesity (2011) 35, 226–235 & 2011 Macmillan Publishers Limited All rights reserved 0307-0565/11 www.nature.com/ijo