© 2001 by the American Diabetes Association, Inc. Modest Overexpression of Neuropeptide Y in the Brain Leads to Obesity After High-Sucrose Feeding
1 Second Department of Internal Medicine, Kobe University School of Medicine, Kobe
Neuropeptide Y (NPY), one of the most abundant peptide transmitters in the mammalian brain, is assumed to play an important role in feeding and body weight regulation. However, there is little genetic evidence that overexpression or knockout of the NPY gene leads to altered body weight regulation. Previously, we developed NPY-overexpressing mice by using the Thy-1 promoter, which restricts NPY expression strictly within neurons in the central nervous system, but we failed to observe the obese phenotype in the heterozygote. Here we report that in the homozygous mice, overexpression of NPY leads to an obese phenotype, but only after appropriate dietary exposure. NPY-overexpressing mice exhibited significantly increased body weight gain with transiently increased food intake after 50% sucroseloaded diet, and later they developed hyperglycemia and hyperinsulinemia without altered glucose excursion during 1 year of our observation period.
Neuropeptide Y (NPY) is a member of the peptide family, which includes the endocrine peptides, pancreatic polypeptide, and peptide YY. A significant body of literature argues that the arcuate nucleus NPY neurons are involved in energy homeostasis (12345). NPY potently stimulates food intake when introduced into the hypothalamus in the vicinity of the paraventricular nucleus and perifornical area (12345). NPY alters body temperature and peripheral energy metabolism and increases plasma insulin levels. Many syndromes of obesity in animals are characterized either by an increase in NPY levels in the hypothalamus or by an increase in NPY receptor abundance and sensitivity (3). When NPY was chronically administered into the ventricle or the hypothalamus, it was found to produce an obesity syndrome with salient features of hyperphagia, increased body weight, hyperinsulinemia, hypercorticosteronemia, muscle insulin resistance, and increased fat storage (6). However, because both NPY-overexpressing and knockout mice showed almost normal patterns of food intake and body weight (5,789), it is still an open question as to whether NPY has an essential role in increased body weight. Here we show that sucrose feeding renders seemingly normal NPY-overexpressing mice to exhibit part of the obesity syndrome. NPY-overexpressing mice (homozygous) were developed by using the Thy-1 promoter, which localized transgene expression within the central nervous system (7). A modest overexpression of NPY was confirmed in these animals (Fig. 1), which were housed per group and examined in a less stressful condition than those housed individually. There were no apparent differences in body weight between NPY-overexpressing mice and their littermates when normal diet was provided. However, shortly after 50% sucrose was loaded on the diet, the homozygous mice became heavier than controls and maintained their increased body weight (Fig. 2A). This was accompanied by an increase in food intake that continued for several weeks after the high-sucrose feeding (Fig. 2B). The increased food intake in transgenic mice was significantly inhibited by the treatment with the NPY Y-1 receptor antagonist BIBO3304, but not the Y-5 receptor antagonist L-152,804 (Fig. 3). There was a significant difference in energetic efficiency, shown by a smaller decrease in body weight in response to 48-h fasting observed in transgenic mice compared with the control mice (13.4 ± 0.34 vs. 13.1 ± 0.96 and 8.11 ± 0.33 vs. 9.31 ± 0.41% body wt change at 024 h and 2448 h, respectively; P < 0.05, n = 7). The mice did not show any change in oxygen consumption (7.07 ± 0.31 vs. 7.38 ± 0.39 and 5.33 ± 0.24 vs. 5.62 ± 0.38 ml/g body wt/h at 01 h and 12 h, respectively, for transgenic versus control mice; n = 9), which was determined with an O2/CO2 metabolismmeasuring system at 25°C. NPY-overexpressing mice showed increased plasma levels of glucose and insulin (Figs. 1C and D), although they did not show impaired glucose tolerance after per os glucose loading (2 g/kg, area under curve [AUC] for glucose 369.0 ± 19.6 vs. 393.6 ± 35.8 mg · h · dl1 for transgenic versus control mice; n = 9). The levels of total cholesterol and triglycerides did not differ appreciably between transgenic and control animals (not shown).
NPY mediates its effects through binding to the Y-type receptors, and the well-characterized effect on food intake may involve the Y-1 and Y-5 receptor subtypes (5,10). Both receptors are coupled to G proteins and are expressed in the brain, including the hypothalamus. The Y-5 receptor has pharmacological properties most closely matching a proposed feeding receptor (11). However, recent progress in the development of nonpeptide antagonists indicates that the Y-1 receptor mediates the appetite-stimulating effect of NPY (10). In the present study, the levels of Y-1 and Y-5 receptor mRNA expression were found to be affected in the brains of NPY-overexpressing mice, with a significant increase of Y-1 and a decrease of Y-5 receptor expression. Previous studies demonstrated that appetite evoked by either fasting or food restriction increased expression of Y-1 mRNA but did not increase Y-5 mRNA expression (3,12). A significant decrease of Y-5 receptor mRNA expression was found in the ventromedial, dorsomedial, and arcuate hypothalamic nuclei of the obese mouse brain using in situ hybridization (13). Downregulation of the Y-5 receptor in the obese Zucker rat was also demonstrated by competition binding assays (14). These results, together with a significant inhibition of food intake by Y-1 receptor antagonist in the present study, suggest a key role of the Y-1 receptor in the regulation of NPY-induced feeding. It may be possible that the modest overexpression of NPY modifies the balance between Y-1 and Y-5 receptors, resulting in a higher expression of Y-1 receptors. Very recently, transgenic rats were generated by using the NPY genomic sequence (15). They exhibited behavioral insensitivity to stress and fear and impaired spatial learning, but they exhibited neither feeding nor body weight abnormalities, presumably because of the limited overexpression of NPY to areas outside those controlling feeding behaviors (15). Energy balance could be well maintained when NPY was removed or mildly overexpressed from early development of the nervous system (789). Moreover, mutant mice that lacked either the Y-1 or Y-5 receptor paradoxically developed obesity, despite the decrease in the effects of NPY and analogs on food intake (16,17). The unexpected phenotype of Y-1 and Y-5 knockouts developed later and was associated with decreased locomotor activity and increased food intake, respectively. In contrast, a recent study indicated an inhibitory role for the Y-2 receptor in the control of food intake and body weight. The Y-2 receptordeficient mice developed obesity, especially when fed a high-fat diet (14). This may be in keeping with a presynaptic function for the Y-2 receptor in the hypothalamus (i.e., an inhibition of NPY release) (18), although Y-2 receptor agonists have the least effect on feeding when injected into the cerebral ventricle (10). As a whole, these studies present a complex picture for the role of NPY and its receptors in feeding and body weight regulation. The present study provides direct genetic evidence that overexpression of NPY leads to obesity in the presence of a palatable diet, with increased food intake and energetic efficiency. The obesity phenotype in transgenic animals may be critically dependent on environmental (dietary) exposure. Previous studies in diet-induced obesity models suggested that there may be some central "set point" for the regulation of body weight that is genetically predetermined but expressed only in the presence of appropriate dietary exposure (19,20). Altered NPY regulation in the hypothalamus may be a characteristic feature, and the NPY system can respond both to the metabolic needs of the animals and the palatability of the food (20). Our model may thus be particularly useful for the study of diet-induced obesity, which is seen in most human obesity and diabetes.
Generation of transgenic mice. We generated mice (homozygous) that overexpress mouse NPY exclusively in the central nervous system (7). We used a 3.6-kb human Thy-1 DNA spanning the 5'-flanking region containing the first intron (21), which is a major component of the surface of mature neurons in the brain. The DNA fragment (Thy-1-NPY cDNA) was microinjected into the pronuclei of fertilized eggs obtained from BDF1 female mice using standard techniques, as previously reported (7,22).
Northern blot analysis.
The plasmid containing cDNA for NPY was supplied by Dr. Steven L. Sabol (National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD). The plasmid containing cDNA of Y-1 and Y-5 was obtained from Dr. Herbert Herzog (The Garvan Institute of Medical Research, Sydney, Australia), and the ß-actin cDNA was from Dr. S.S. Liu (Department of Microbiology and Immunology, National Cheng Kung University, Taiwan City, Taiwan, China). Probes were labeled with [ NPY expression was examined twice (in the 12th and 64th week of each mouses life), and the animals were killed at 10:00 A.M. by cervical dislocation. Essentially, the same results were obtained in each study and the latter data were presented.
Behavioral analysis.
NPY receptor antagonist experiments. NPY-overexpressing and control mice at 12 weeks of age were fed on 50% sucroseloaded powder diet after implantation. Cumulative food intake was measured before and after ICV injection of NPY Y-1 and Y-5 receptor antagonists. BIBO3304 was supplied by Boeringer-Ingelheim Pharma (Grenzach-wyhlen, Germany), and L-152,804 was from Banyu Pharmaceutical (Tokyo). Just before administration, each drug was diluted in 4 µl of artificial cerebrospinal fluid, which also served as the control solution, and ICV was administered at a dose of 2 nmol/mouse every 12 h. The dosage of the antagonists was chosen based on our previous study (25) to rule out potential nonspecific effects.
Energy expenditure.
Glucose tolerance.
This study was supported in part by Grants-in-Aid for Scientific Research (09671057), Developmental Scientific Research (08559012), and International Scientific Research (80168418) from the Ministry of Education, Science, Sports, and Culture of Japan (to A.I.). The authors sincerely thank Dr. Steven L. Sabol (National Heart, Lung, and Blood Institute, National Institutes of Health) and Dr. Herbert Herzog (The Garvan Institute of Medical Research) for kindly supplying us the plasmid containing cDNA for NPY and for Y-1 and Y-5 receptors, respectively.
Address correspondence and reprint requests to Akio Inui, MD, Associate Professor, Second Department of Internal Medicine, Kobe University School of Medicine, 7-5-2 Kusunoki-cho, Chuo-ku, Kobe 650-0017, Japan. E-mail: inui{at}med.kobe-u.ac.jp. Received for publication 16 March 2000 and accepted in revised form 16 January 2001. AUC, area under curve; ICV, intrathird cerebroventricular; NPY, neuropeptide Y; SSC, sodium saline citrate.
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