Keval Chandarana

The role of peptide YY in appetite regulation and obesity

The last decade has witnessed a marked increase in our understanding of the importance of gut hormones in the regulation of energy homeostasis. In particular, the discovery that the gut hormone peptide YY 3–36 (PYY3–36) reduced feeding in obese rodents and humans fuelled interest in the role of PYY3–36 in body weight regulation. Pharmacological and genetic approaches have revealed that the Y2‐receptor mediates the anorectic effects of PYY3–36 whilst mechanistic studies in rodents identified the hypothalamus, vagus and brainstem regions as potential sites of action. More recently, using functional brain imaging techniques in humans, PYY3–36 was found to modulate neuronal activity within hypothalamic and brainstem, and brain regions involved in reward processing. Several lines of evidence suggest that low circulating PYY concentrations predispose towards the development and or maintenance of obesity. Subjects with reduced postprandial PYY release exhibit lower satiety and circulating PYY levels that correlate negatively with markers of adiposity. In addition, mice lacking PYY are hyperphagic and become obese. Conversely, chronic PYY3–36 administration to obese rodents reduces adiposity, and transgenic mice with increased circulating PYY are resistant to diet‐induced obesity. Moreover, there is emerging evidence that PYY3–36 may partly mediate the reduced appetite and weight loss benefits observed post‐gastric bypass surgery. Taken together these findings, coupled with the retained responsiveness of obese subjects to the effects of PYY3–36, suggest that targeting the PYY system may offer a therapeutic strategy to help treat obesity.

Dominant role of the p110β isoform of PI3K over p110α in energy homeostasis regulation by POMC and AgRP neurons

Summary PI3K signaling is thought to mediate leptin and insulin action in hypothalamic pro-opiomelanocortin (POMC) and agouti-related protein (AgRP) neurons, key regulators of energy homeostasis, through largely unknown mechanisms. We inactivated either p110α or p110β PI3K catalytic subunits in these neurons and demonstrate a dominant role for the latter in energy homeostasis regulation. In POMC neurons, p110β inactivation prevented insulin- and leptin-stimulated electrophysiological responses. POMCp110β null mice exhibited central leptin resistance, increased adiposity, and diet-induced obesity. In contrast, the response to leptin was not blocked in p110α-deficient POMC neurons. Accordingly, POMCp110α null mice displayed minimal energy homeostasis abnormalities. Similarly, in AgRP neurons, p110β had a more important role than p110α. AgRPp110α null mice displayed normal energy homeostasis regulation, whereas AgRPp110β null mice were lean, with increased leptin sensitivity and resistance to diet-induced obesity. These results demonstrate distinct metabolic roles for the p110α and p110β isoforms of PI3K in hypothalamic energy regulation.

Diet and Gastrointestinal Bypass-Induced Weight Loss The Roles of Ghrelin and Peptide YY

OBJECTIVE Bariatric surgery causes durable weight loss. Gut hormones are implicated in obesity pathogenesis, dietary failure, and mediating gastrointestinal bypass (GIBP) surgery weight loss. In mice, we determined the effects of diet-induced obesity (DIO), subsequent dieting, and GIBP surgery on ghrelin, peptide YY (PYY), and glucagon-like peptide-1 (GLP-1). To evaluate PYY’s role in mediating weight loss post-GIBP, we undertook GIBP surgery in PyyKO mice. RESEARCH DESIGN AND METHODS Male C57BL/6 mice randomized to a high-fat diet or control diet were killed at 4-week intervals. DIO mice underwent switch to ad libitum low-fat diet (DIO-switch) or caloric restriction (CR) for 4 weeks before being killed. PyyKO mice and their DIO wild-type (WT) littermates underwent GIBP or sham surgery and were culled 10 days postoperatively. Fasting acyl-ghrelin, total PYY, active GLP-1 concentrations, stomach ghrelin expression, and colonic Pyy and glucagon expression were determined. Fasting and postprandial PYY and GLP-1 concentrations were assessed 30 days postsurgery in GIBP and sham pair-fed (sham.PF) groups. RESULTS DIO progressively reduced circulating fasting acyl-ghrelin, PYY, and GLP-1 levels. CR and DIO-switch caused weight loss but failed to restore circulating PYY to weight-appropriate levels. After GIBP, WT mice lost weight and exhibited increased circulating fasting PYY and colonic Pyy and glucagon expression. In contrast, the acute effects of GIBP on body weight were lost in PyyKO mice. Fasting PYY and postprandial PYY and GLP-1 levels were increased in GIBP mice compared with sham.PF mice. CONCLUSIONS PYY plays a key role in mediating the early weight loss observed post-GIBP, whereas relative PYY deficiency during dieting may compromise weight-loss attempts.

Subject standardization, acclimatization, and sample processing affect gut hormone levels and appetite in humans.

BACKGROUND & AIMS Gut hormones represent attractive therapeutic targets for the treatment of obesity and type 2 diabetes. However, controversy surrounds the effects that adiposity, dietary manipulations, and bariatric surgery have on their circulating concentrations. We sought to determine whether these discrepancies are due to methodologic differences. METHODS Ten normal-weight males participated in a 4-way crossover study investigating whether fasting appetite scores, plasma acyl-ghrelin, active glucagon-like peptide-1 (GLP-1), and peptide YY3-36 (PYY3-36) levels are altered by study-induced stress, prior food consumption, and sample processing. RESULTS Study visit order affected anxiety, plasma cortisol, and temporal profiles of appetite and plasma PYY3-36, with increased anxiety and cortisol concentrations on the first study day. Plasma cortisol area under the curve (AUC) correlated positively with plasma PYY3-36 AUC. Despite a 14-hour fast, baseline hunger, PYY3-36 concentrations, temporal appetite profiles, PYY3-36 AUC, and active GLP-1 were affected by the previous evenings meal. Sample processing studies revealed that sample acidification and esterase inhibition are required when measuring acyl-ghrelin and dipeptidyl-peptidase IV inhibitor addition for active GLP-1. However, plasma PYY3-36 concentrations were unaffected by addition of dipeptidyl-peptidase IV. CONCLUSIONS Accurate assessment of appetite, feeding behavior, and gut hormone concentrations requires standardization of prior food consumption and subject acclimatization to the study protocol. Moreover, because of the labile nature of acyl-ghrelin and active GLP-1, specialized sample processing needs to be undertaken.

Peripheral activation of the Y2-receptor promotes secretion of GLP-1 and improves glucose tolerance

The effect of peptide tyrosine-tyrosine (PYY) on feeding is well established but currently its role in glucose homeostasis is poorly defined. Here we show in mice, that intraperitoneal (ip) injection of PYY3-36 or Y2R agonist improves nutrient-stimulated glucose tolerance and enhances insulin secretion; an effect blocked by peripheral, but not central, Y2R antagonist administration. Studies on isolated mouse islets revealed no direct effect of PYY3-36 on insulin secretion. Bariatric surgery in mice, enterogastric anastomosis (EGA), improved glucose tolerance in wild-type mice and increased circulating PYY and active GLP-1. In contrast, in Pyy-null mice, post-operative glucose tolerance and active GLP-1 levels were similar in EGA and sham-operated groups. PYY3-36 ip increased hepato-portal active GLP-1 plasma levels, an effect blocked by ip Y2R antagonist. Collectively, these data suggest that PYY3-36 therefore acting via peripheral Y2R increases hepato-portal active GLP-1 plasma levels and improves nutrient-stimulated glucose tolerance.

Regulation of hindbrain Pyy expression by acute food deprivation, prolonged caloric restriction, and weight loss surgery in mice

PYY is a gut-derived putative satiety signal released in response to nutrient ingestion and is implicated in the regulation of energy homeostasis. Pyy-expressing neurons have been identified in the hindbrain of river lamprey, rodents, and primates. Despite this high evolutionary conservation, little is known about central PYY neurons. Using in situ hybridization, PYY-Cre;ROSA-EYFP mice, and immunohistochemistry, we identified PYY cell bodies in the gigantocellular reticular nucleus region of the hindbrain. PYY projections were present in the dorsal vagal complex and hypoglossal nucleus. In the hindbrain, Pyy mRNA was present at E9.5, and expression peaked at P2 and then decreased significantly by 70% at adulthood. We found that, in contrast to the circulation, PYY-(1-36) is the predominant isoform in mouse brainstem extracts in the ad libitum-fed state. However, following a 24-h fast, the relative amounts of PYY-(1-36) and PYY-(3-36) isoforms were similar. Interestingly, central Pyy expression showed nutritional regulation and decreased significantly by acute starvation, prolonged caloric restriction, and bariatric surgery (enterogastroanastomosis). Central Pyy expression correlated with body weight loss and circulating leptin and PYY concentrations. Central regulation of energy metabolism is not limited to the hypothalamus but also includes the midbrain and the brainstem. Our findings suggest a role for hindbrain PYY in the regulation of energy homeostasis and provide a starting point for further research on gigantocellular reticular nucleus PYY neurons, which will increase our understanding of the brain stem pathways in the integrated control of appetite and energy metabolism.

Differential Pre-mRNA Splicing Regulates Nnat Isoforms in the Hypothalamus after Gastric Bypass Surgery in Mice

Background Neuronatin (NNAT) is an endoplasmic reticulum proteolipid implicated in intracellular signalling. Nnat is highly-expressed in the hypothalamus, where it is acutely regulated by nutrients and leptin. Nnat pre-mRNA is differentially spliced to create Nnat-α and -β isoforms. Genetic variation of NNAT is associated with severe obesity. Currently, little is known about the long-term regulation of Nnat. Methods Expression of Nnat isoforms were examined in the hypothalamus of mice in response to acute fast/feed, chronic caloric restriction, diet-induced obesity and modified gastric bypass surgery. Nnat expression was assessed in the central nervous system and gastrointestinal tissues. RTqPCR was used to determine isoform-specific expression of Nnat mRNA. Results Hypothalamic expression of both Nnat isoforms was comparably decreased by overnight and 24-h fasting. Nnat expression was unaltered in diet-induced obesity, or subsequent switch to a calorie restricted diet. Nnat isoforms showed differential expression in the hypothalamus but not brainstem after bypass surgery. Hypothalamic Nnat-β expression was significantly reduced after bypass compared with sham surgery (P = 0.003), and was positively correlated with post-operative weight-loss (R2 = 0.38, P = 0.01). In contrast, Nnat-α expression was not suppressed after bypass surgery (P = 0.19), and expression did not correlate with reduction in weight after surgery (R2 = 0.06, P = 0.34). Hypothalamic expression of Nnat-β correlated weakly with circulating leptin, but neither isoform correlated with fasting gut hormone levels post- surgery. Nnat expression was detected in brainstem, brown-adipose tissue, stomach and small intestine. Conclusions Nnat expression in hypothalamus is regulated by short-term nutrient availability, but unaltered by diet-induced obesity or calorie restriction. While Nnat isoforms in the hypothalamus are co-ordinately regulated by acute nutrient supply, after modified gastric bypass surgery Nnat isoforms show differential expression. These results raise the possibility that in the radically altered nutrient and hormonal milieu created by bypass surgery, resultant differential splicing of Nnat pre-mRNA may contribute to weight-loss.