Showing papers by "Joel K. Elmquist published in 2008"

Synaptic release of GABA by AgRP neurons is required for normal regulation of energy balance

Abstract: The physiologic importance of GABAergic neurotransmission in hypothalamic neurocircuits is unknown. To examine the importance of GABA release from agouti-related protein (AgRP) neurons (which also release AgRP and neuropeptide Y), we generated mice with an AgRP neuron-specific deletion of vesicular GABA transporter. These mice are lean, resistant to obesity and have an attenuated hyperphagic response to ghrelin. Thus, GABA release from AgRP neurons is important in regulating energy balance.

The orexigenic hormone ghrelin defends against depressive symptoms of chronic stress

Abstract: The peptide hormone ghrelin has previously been linked to the regulation of metabolism. This study in mice finds that increasing levels of ghrelin, either through subcutaneous injections or calorie restriction, has an anxiolytic and antidepressive effect. This reveals a previously unknown function for ghrelin. We found that increasing ghrelin levels, through subcutaneous injections or calorie restriction, produced anxiolytic- and antidepressant-like responses in the elevated plus maze and forced swim test. Moreover, chronic social defeat stress, a rodent model of depression, persistently increased ghrelin levels, whereas growth hormone secretagogue receptor (Ghsr) null mice showed increased deleterious effects of chronic defeat. Together, these findings demonstrate a previously unknown function for ghrelin in defending against depressive-like symptoms of chronic stress.

Hypothalamic pathways linking energy balance and reproduction.

Abstract: During periods of metabolic stress, animals must channel energy toward survival and away from processes such as reproduction. The reproductive axis, therefore, has the capacity to respond to changing levels of metabolic cues. The cellular and molecular mechanisms that link energy balance and reproduction, as well as the brain sites mediating this function, are still not well understood. This review focuses on the best characterized of the adiposity signals: leptin and insulin. We examine their reproductive role acting on the classic metabolic pathways of the arcuate nucleus, NPY/AgRP and POMC/CART neurons, and the newly identified kisspeptin network. In addition, other hypothalamic nuclei that may play a role in linking metabolic state and reproductive function are discussed. The nature of the interplay between these elements of the metabolic and reproductive systems presents a fascinating puzzle, whose pieces are just beginning to fall into place.

Acute effects of leptin require PI3K signaling in hypothalamic proopiomelanocortin neurons in mice

Abstract: Normal food intake and body weight homeostasis require the direct action of leptin on hypothalamic proopiomelanocortin (POMC) neurons. It has been proposed that leptin action requires PI3K activity. We therefore assessed the contribution of PI3K signaling to leptin's effects on POMC neurons and organismal energy balance. Leptin caused a rapid depolarization of POMC neurons and an increase in action potential frequency in patch-clamp recordings of hypothalamic slices. Pharmacologic inhibition of PI3K prevented this depolarization and increased POMC firing rate, indicating a PI3K-dependent mechanism of leptin action. Mice with genetically disrupted PI3K signaling in POMC cells failed to undergo POMC depolarization or increased firing frequency in response to leptin. Insulin's ability to hyperpolarize POMC neurons was also abolished in these mice. Moreover, targeted disruption of PI3K blunted the suppression of feeding elicited by central leptin administration. Despite these differences, mice with impaired PI3K signaling in POMC neurons exhibited normal long-term body weight regulation. Collectively, these results suggest that PI3K signaling in POMC neurons is essential for leptin-induced activation and insulin-induced inhibition of POMC cells and for the acute suppression of food intake elicited by leptin, but is not a major contributor to the regulation of long-term organismal energy homeostasis.

Collective and Individual Functions of Leptin Receptor Modulated Neurons Controlling Metabolism and Ingestion

Abstract: Two known types of leptin-responsive neurons reside within the arcuate nucleus: the agouti gene-related peptide (AgRP)/neuropeptide Y (NPY) neuron and the proopiomelanocortin (POMC) neuron. By deleting the leptin receptor gene (Lepr) specifically in AgRP/NPY and/or POMC neurons of mice, we examined the several and combined contributions of these neurons to leptin action. Body weight and adiposity were increased by Lepr deletion from AgRP and POMC neurons individually, and simultaneous deletion in both neurons (A+P LEPR-KO mice) further increased these measures. Young (periweaning) A+P LEPR-KO mice exhibit hyperphagia and decreased energy expenditure, with increased weight gain, oxidative sparing of triglycerides, and increased fat accumulation. Interestingly, however, many of these abnormalities were attenuated in adult animals, and high doses of leptin partially suppress food intake in the A+P LEPR-KO mice. Although mildly hyperinsulinemic, the A+P LEPR-KO mice displayed normal glucose tolerance and fertility. Thus, AgRP/NPY and POMC neurons each play mandatory roles in aspects of leptin-regulated energy homeostasis, high leptin levels in adult mice mitigate the importance of leptin-responsiveness in these neurons for components of energy balance, suggesting the presence of other leptin-regulated pathways that partially compensate for the lack of leptin action on the POMC and AgRP/NPY neurons.

Brain SIRT1: anatomical distribution and regulation by energy availability.

Abstract: SIRT1 is a nicotinamide adenosine dinucleotide-dependent deacetylase that orchestrates key metabolic adaptations to nutrient deprivation in peripheral tissues. SIRT1 is induced also in the brain by reduced energy intake. However, very little is known about SIRT1 distribution and the biochemical phenotypes of SIRT1-expressing cells in the neuraxis. Unknown are also the brain sites in which SIRT1 is regulated by energy availability and whether these regulations are altered in a genetic model of obesity. To address these issues, we performed in situ hybridization histochemistry analyses and found that Sirt1 mRNA is highly expressed in metabolically relevant sites. These include, but are not limited to, the hypothalamic arcuate, ventromedial, dorsomedial, and paraventricular nuclei and the area postrema and the nucleus of the solitary tract in the hindbrain. Of note, our single-cell reverse transcription-PCR analyses revealed that Sirt1 mRNA is expressed in pro-opiomelanocortin neurons that are critical for normal body weight and glucose homeostasis. We also found that SIRT1 protein levels are restrictedly increased in the hypothalamus in the fasted brain. Of note, we found that this hypothalamic-specific, fasting-induced SIRT1 regulation is altered in leptin-deficient, obese mice. Collectively, our findings establish the distribution of Sirt1 mRNA throughout the neuraxis and suggest a previously unrecognized role of brain SIRT1 in regulating energy homeostasis.

Monitoring FoxO1 localization in chemically identified neurons.

Abstract: The PI3K-Akt-FoxO1 pathway contributes to the actions of insulin and leptin in several cell types, including neurons in the CNS. However, identifying these actions in chemically identified neurons has proven difficult. To address this problem, we have developed a reporter mouse for monitoring PI3K-Akt signaling in specific populations of neurons, based on FoxO1 nucleocytoplasmic shuttling. The reporter, FoxO1 fused to green fluorescent protein (FoxO1GFP), is expressed under the control of a ubiquitous promoter that is silenced by a loxP flanked transcriptional blocker. Thus, the expression of the reporter in selected cells is dependent on the action of Cre recombinase. Using this model, we found that insulin treatment resulted in the nuclear exclusion of FoxO1GFP within POMC and AgRP neurons in a dose- and time-dependent manner. FoxO1GFP nuclear exclusion was also observed in POMC neurons following in vivo administration of insulin. In addition, leptin induced transient nuclear export of FoxO1GFP in POMC neurons in a dose dependent manner. Finally, insulin-induced nuclear export was impaired in POMC neurons by pretreatment with free fatty acids, a paradigm known to induce insulin resistance in peripheral insulin target tissues. Thus, our FoxO1GFP mouse provides a tool for monitoring the status of PI3K-Akt signaling in a cell-specific manner under physiological and pathophysiological conditions.

PANIC-ATTAC: A Mouse Model for Inducible and Reversible β-Cell Ablation

Abstract: OBJECTIVE—Islet transplantations have been performed clinically, but their practical applications are limited. An extensive effort has been made toward the identification of pancreatic β-cell stem cells that has yielded many insights to date, yet targeted reconstitution of β-cell mass remains elusive. Here, we present a mouse model for inducible and reversible ablation of pancreatic β-cells named the PANIC-ATTAC (pancreatic islet β-cell apoptosis through targeted activation of caspase 8) mouse. RESEARCH DESIGN AND METHODS—We efficiently induce β-cell death through apoptosis and concomitant hyperglycemia by administration of a chemical dimerizer to the transgenic mice. In contrast to animals administered streptozotocin, the diabetes phenotype and β-cell loss are fully reversible in the PANIC-ATTAC mice, and we find significant β-cell recovery with normalization of glucose levels after 2 months. RESULTS—The rate of recovery can be enhanced by various pharmacological interventions with agents acting on the glucagon-like peptide 1 axis and agonists of peroxisome proliferator–activated receptor-γ. During recovery, we find an increased population of GLUT2+/insulin− cells in the islets of PANIC-ATTAC mice, which may represent a novel pool of potential β-cell precursors. CONCLUSIONS—The PANIC-ATTAC mouse may be used as an animal model of inducible and reversible β-cell ablation and therefore has applications in many areas of diabetes research that include identification of β-cell precursors, evaluation of glucotoxicity effects in diabetes, and examination of pharmacological interventions.