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Gregory J Morton

Publications and source records attributed to Gregory J Morton.

15 recordsLinked to original sources
PubMed2025

A consensus guide to preclinical indirect calorimetry experiments.

Understanding the complex factors influencing mammalian metabolism and body weight homeostasis is a long-standing challenge requiring knowledge of energy intake, absorption and expenditure. Using measurements of respiratory gas exchange, indirect calorimetry can provide non-invasive estimates of whole-body energy expenditure. However, inconsistent measurement units and flawed data normalization methods have slowed progress in this field. This guide aims to establish consensus standards to unify indirect calorimetry experiments and their analysis for more consistent, meaningful and reproducible results. By establishing community-driven standards, we hope to facilitate data comparison across research datasets. This advance will allow the creation of an in-depth, machine-readable data repository built on shared standards. This overdue initiative stands to markedly improve the accuracy and depth of efforts to interrogate mammalian metabolism. Data sharing according to established best practices will also accelerate the translation of basic findings into clinical applications for metabolic diseases afflicting global populations.

Calorimetry, Indirect↗
PubMed2006

Thermoregulatory and metabolic defects in Huntington's disease transgenic mice implicate PGC-1alpha in Huntington's disease neurodegeneration.

Huntington's disease (HD) is a fatal, dominantly inherited disorder caused by polyglutamine repeat expansion in the huntingtin (htt) gene. Here, we observe that HD mice develop hypothermia associated with impaired activation of brown adipose tissue (BAT). Although sympathetic stimulation of PPARgamma coactivator 1alpha (PGC-1alpha) was intact in BAT of HD mice, uncoupling protein 1 (UCP-1) induction was blunted. In cultured cells, expression of mutant htt suppressed UCP-1 promoter activity; this was reversed by PGC-1alpha expression. HD mice showed reduced food intake and increased energy expenditure, with dysfunctional BAT mitochondria. PGC-1alpha is a known regulator of mitochondrial function; here, we document reduced expression of PGC-1alpha target genes in HD patient and mouse striatum. Mitochondria of HD mouse brain show reduced oxygen consumption rates. Finally, HD striatal neurons expressing exogenous PGC-1alpha were resistant to 3-nitropropionic acid treatment. Altered PGC-1alpha function may thus link transcription dysregulation and mitochondrial dysfunction in HD.

Adipose Tissue, Brown↗
PubMed2006

Signal transducer and activator of transcription (stat) binding sites but not stat3 are required for fasting-induced transcription of agouti-related protein messenger ribonucleic acid.

Energy homeostasis depends on the regulation of hypothalamic neurons by leptin, an adipocyte hormone whose circulating levels communicate body energy stores. Leptin activates the transcription factor signal transducer and activator of transcription 3 (Stat3) in hypothalamic neurons, including neuronal subtypes producing Agouti-related protein (Agrp), a neuropeptide that stimulates feeding. Previous studies have suggested a model in which high levels of Agrp transcription during fasting represent a default state that is actively repressed by phospho-Stat3 induced by leptin signaling in the fed state. We identify putative Stat3 binding elements in the Agrp promoter that have been highly conserved during vertebrate evolution. Using a reporter assay in transgenic mice that faithfully recapitulates normal regulation of Agrp, we show that these sites are required, but in a way opposite to that predicted by the existing model: mutation of the sites leads to a default state characterized by a low level of Agrp transcription and insensitivity to fasting. We also find that removing activatable Stat3 from Agrp neurons has no detectable effect on steady-state levels of Agrp mRNA in the fed or fasted state. These results suggest a new model for transcriptional regulation of orexigenic neuropeptides in which the default level of expression is low in the fed state, and transcriptional activation in response to fasting is mediated by factors other than Stat3.

Agouti-Related Protein↗
PubMed2006

Insulin action in the brain contributes to glucose lowering during insulin treatment of diabetes.

To investigate the role of brain insulin action in the pathogenesis and treatment of diabetes, we asked whether neuronal insulin signaling is required for glucose-lowering during insulin treatment of diabetes. Hypothalamic signaling via the insulin receptor substrate-phosphatidylinositol 3-kinase (IRS-PI3K) pathway, a key intracellular mediator of insulin action, was reduced in rats with uncontrolled diabetes induced by streptozotocin (STZ-DM). Further, infusion of a PI3K inhibitor into the third cerebral ventricle of STZ-DM rats prior to peripheral insulin injection attenuated insulin-induced glucose lowering by approximately 35%-40% in both acute and chronic insulin treatment paradigms. Conversely, increased PI3K signaling induced by hypothalamic overexpression of either IRS-2 or protein kinase B (PKB, a key downstream mediator of PI3K action) enhanced the glycemic response to insulin by approximately 2-fold in STZ-DM rats. We conclude that hypothalamic insulin signaling via the IRS-PI3K pathway is a key determinant of the response to insulin in the management of uncontrolled diabetes.

Animals↗
PubMed2005

Effects of hypothalamic neurodegeneration on energy balance.

Normal aging in humans and rodents is accompanied by a progressive increase in adiposity. To investigate the role of hypothalamic neuronal circuits in this process, we used a Cre-lox strategy to create mice with specific and progressive degeneration of hypothalamic neurons that express agouti-related protein (Agrp) or proopiomelanocortin (Pomc), neuropeptides that promote positive or negative energy balance, respectively, through their opposing effects on melanocortin receptor signaling. In previous studies, Pomc mutant mice became obese, but Agrp mutant mice were surprisingly normal, suggesting potential compensation by neuronal circuits or genetic redundancy. Here we find that Pomc-ablation mice develop obesity similar to that described for Pomc knockout mice, but also exhibit defects in compensatory hyperphagia similar to what occurs during normal aging. Agrp-ablation female mice exhibit reduced adiposity with normal compensatory hyperphagia, while animals ablated for both Pomc and Agrp neurons exhibit an additive interaction phenotype. These findings provide new insight into the roles of hypothalamic neurons in energy balance regulation, and provide a model for understanding defects in human energy balance associated with neurodegeneration and aging.

Agouti-Related Protein↗
PubMed2005

Leptin inhibits hypothalamic Npy and Agrp gene expression via a mechanism that requires phosphatidylinositol 3-OH-kinase signaling.

Phosphatidylinositol 3-OH-kinase (PI3K) and STAT3 are signal transduction molecules activated by leptin in brain areas controlling food intake. To investigate their role in leptin-mediated inhibition of hypothalamic neuropeptide Y (Npy) and agouti-related peptide (Agrp) gene expression, male Sprague-Dawley rats (n = 5/group) were either fed ad libitum or subjected to a 52-h fast. At 12-h intervals, the PI3K inhibitor LY-294002 (LY, 1 nmol) or vehicle was injected intracerebroventricularly (ICV) as a pretreatment, followed 1 h later by leptin (3 microg icv) or vehicle. Fasting increased hypothalamic Npy and Agrp mRNA levels (P < 0.05), and ICV leptin administration prevented this increase. As predicted, LY pretreatment blocked this inhibitory effect of leptin, such that Npy and Agrp levels in LY-leptin-treated animals were similar to fasted controls. By comparison, leptin-mediated activation of hypothalamic STAT3 signaling, as measured by induction of both phospho-STAT3 immunohistochemistry and suppressor of cytokine signaling-3 (Socs3) mRNA, was not significantly attenuated by ICV LY pretreatment. Because NPY/AgRP neurons project to the hypothalamic paraventricular nucleus (PVN), we next investigated whether leptin activation of PVN neurons is similarly PI3K dependent. Compared with vehicle, leptin increased the number of c-Fos positive cells within the parvocellular PVN (P = 0.001), and LY pretreatment attenuated this effect by 35% (P = 0.043). We conclude that leptin requires intact PI3K signaling both to inhibit hypothalamic Npy and Agrp gene expression and activate neurons within the PVN. In addition, these data suggest that leptin activation of STAT3 is insufficient to inhibit expression of Npy or Agrp in the absence of PI3K signaling.

Agouti-Related Protein↗
PubMed2005

Src homology 3-domain growth factor receptor-bound 2-like (endophilin) interacting protein 1, a novel neuronal protein that regulates energy balance.

To identify genes involved in the central regulation of energy balance, we compared hypothalamic mRNA from lean and obese Psammomys obesus, a polygenic model of obesity, using differential display PCR. One mRNA transcript was observed to be elevated in obese, and obese diabetic, P. obesus compared with lean animals and was subsequently found to be increased 4-fold in the hypothalamus of lethal yellow agouti (A(y)/a) mice, a murine model of obesity and diabetes. Intracerebroventricular infusion of antisense oligonucleotide targeted to this transcript selectively suppressed its hypothalamic mRNA levels and resulted in loss of body weight in both P. obesus and Sprague Dawley rats. Reductions in body weight were mediated by profoundly reduced food intake without a concomitant reduction in metabolic rate. Yeast two-hybrid screening, and confirmation in mammalian cells by bioluminescence resonance energy transfer analysis, demonstrated that the protein it encodes interacts with endophilins, mediators of synaptic vesicle recycling and receptor endocytosis in the brain. We therefore named this transcript Src homology 3-domain growth factor receptor-bound 2-like (endophilin) interacting protein 1 (SGIP1). SGIP1 encodes a large proline-rich protein that is expressed predominantly in the brain and is highly conserved between species. Together these data suggest that SGIP1 is an important and novel member of the group of neuronal molecules required for the regulation of energy homeostasis.

Adaptor Proteins, Signal Transducing↗
PubMed2005

Leptin regulates insulin sensitivity via phosphatidylinositol-3-OH kinase signaling in mediobasal hypothalamic neurons.

To investigate whether phosphatidylinositol-3 kinase (PI3K) signaling mediates the metabolic effects of hypothalamic leptin action, adenoviral gene therapy was used to direct expression of leptin receptors to the area of the hypothalamic arcuate nucleus (ARC). This intervention markedly improved insulin sensitivity in genetically obese, leptin-receptor-deficient Koletsky (fa(k)/fa(k)) rats via a mechanism that was not dependent on reduced food intake but was attenuated by approximately 44% by third-ventricular infusion of the PI3K inhibitor LY294002. Conversely, ARC-directed expression of a constitutively active mutant of protein kinase B (PKB/Akt, an enzyme activated by PI3K) mimicked the insulin-sensitizing effect of restored hypothalamic leptin signaling in these animals, despite having no effect on food intake or body weight. These findings suggest that hypothalamic leptin signaling is an important determinant of glucose metabolism and that the underlying neuronal mechanism involves PI3K.

Adenoviridae↗
PubMed2005

Leptin action in the forebrain regulates the hindbrain response to satiety signals.

The capacity to adjust energy intake in response to changing energy requirements is a defining feature of energy homeostasis. Despite the identification of leptin as a key mediator of this process, the mechanism whereby changes of body adiposity are coupled to adaptive, short-term adjustments of energy intake remains poorly understood. To investigate the physiological role of leptin in the control of meal size and the response to satiety signals, and to identify brain areas mediating this effect, we studied Koletsky (fa(k)/fa(k)) rats, which develop severe obesity due to the genetic absence of leptin receptors. Our finding of markedly increased meal size and reduced satiety in response to the gut peptide cholecystokinin (CCK) in these leptin receptor-deficient animals suggests a critical role for leptin signaling in the response to endogenous signals that promote meal termination. To determine if the hypothalamic arcuate nucleus (ARC) (a key forebrain site of leptin action) mediates this leptin effect, we used adenoviral gene therapy to express either functional leptin receptors or a reporter gene in the area of the ARC of fa(k)/fa(k) rats. Restoration of leptin signaling to this brain area normalized the effect of CCK on the activation of neurons in the nucleus of the solitary tract and area postrema, key hindbrain areas for processing satiety-related inputs. This intervention also reduced meal size and enhanced CCK-induced satiety in fa(k)/fa(k) rats. These findings demonstrate that forebrain signaling by leptin, a long-term regulator of body adiposity, limits food intake on a meal-to-meal basis by regulating the hindbrain response to short-acting satiety signals.

Animals↗
PubMed2004

Physiological regulation of hypothalamic IL-1beta gene expression by leptin and glucocorticoids: implications for energy homeostasis.

Interleukin-1beta (IL-1beta) is synthesized in a variety of tissues, including the hypothalamus, where it is implicated in the control of food intake. The current studies were undertaken to investigate whether hypothalamic IL-1beta gene expression is subject to physiological regulation by leptin and glucocorticoids (GCs), key hormones involved in energy homeostasis. Adrenalectomy (ADX) increased hypothalamic IL-1beta mRNA levels twofold, measured by real-time PCR (P < 0.05 vs. sham-operated controls), and this effect was blocked by subcutaneous infusion of a physiological dose of corticosterone. Conversely, hypothalamic IL-1beta mRNA levels were reduced by 30% in fa/fa (Zucker) rats, a model of genetic obesity caused by leptin receptor mutation (P = 0.01 vs. lean littermates), and the effect of ADX to increase hypothalamic IL-1beta mRNA levels in fa/fa rats (P = 0.02) is similar to that seen in normal animals. Moreover, fasting for 48 h (which lowers leptin and raises corticosterone levels) reduced hypothalamic IL-1beta mRNA levels by 30% (P = 0.02), and this decrease was fully reversed by refeeding for 12 h. Thus leptin and GCs exert opposing effects on hypothalamic IL-1beta gene expression, and corticosterone plays a physiological role to limit expression of this cytokine in both the presence and absence of intact leptin signaling. Consistent with this hypothesis, systemic leptin administration to normal rats (2 mg/kg ip) increased hypothalamic IL-1beta mRNA levels twofold (P < 0.05 vs. vehicle), an effect similar to that of ADX. These data support a model in which expression of hypothalamic IL-1beta is subject to opposing physiological regulation by corticosterone and leptin.

Adrenalectomy↗
PubMed2004

Effect of uncontrolled diabetes on plasma ghrelin concentrations and ghrelin-induced feeding.

Plasma levels of the orexigenic hormone, ghrelin, decrease rapidly on nutrient ingestion and yet are paradoxically elevated in rats with hyperphagia induced by streptozotocin-induced diabetes (STZ-DM). In the current work, we investigated the mechanisms underlying the relationships among uncontrolled diabetes, food intake, and plasma ghrelin concentrations in an effort to clarify whether increased ghrelin signaling contributes to diabetic hyperphagia. Whereas food intake did not increase until d 3 after STZ administration, plasma ghrelin levels were increased by more than 2-fold (P < 0.05) on d 1. As hyperphagia developed, however, plasma ghrelin levels declined steadily. Because this reduction of plasma ghrelin levels was reversed by matching food intake of STZ-DM rats to that of nondiabetic controls, our results demonstrated that the effect of uncontrolled diabetes to increase plasma ghrelin levels is partially offset by hyperphagic feeding. In addition, we found that although intragastric nutrient infusion rapidly and comparably decreased plasma ghrelin levels in both groups (by 46-49%; P < 0.05), this effect was short lived in STZ-DM rats relative to nondiabetic controls (60 min vs. 120 min; P < 0.05). We further demonstrated that in rats with STZ-DM, food intake increased by 357% (P < 0.05) in response to intracerebroventricular administration of ghrelin at a dose that was subthreshold for feeding effects in nondiabetic controls. Collectively, these findings demonstrate that uncontrolled diabetes increases both circulating ghrelin levels and behavioral sensitivity to ghrelin. Although plasma ghrelin levels fall in response to hyperphagic feeding, these findings support the hypothesis that increased ghrelin signaling contributes to the pathogenesis of diabetic hyperphagia.

Animal Nutritional Physiological Phenomena↗
PubMed2004

Neuropeptide Y is required for hyperphagic feeding in response to neuroglucopenia.

To investigate the role played by the orexigenic peptide, neuropeptide Y (NPY), in adaptive responses to insulin-induced hypoglycemia, we measured hypothalamic, feeding, and hormonal responses to this stimulus in both wild-type (Npy+/+) and NPY-deficient (Npy-/-) mice. After administration of insulin at a dose (60 mU ip) sufficient to cause moderate hypoglycemia (plasma glucose levels, 40 +/- 3 and 37 +/- 2 mg/dl for Npy+/+ and Npy-/- mice, respectively; P = not significant), 4-h food intake was increased 2.5-fold in Npy+/+ mice relative to saline-injected controls. By comparison, the increase of intake in Npy-/- mice was far smaller (45%) and did not achieve statistical significance (P = 0.08). Hyperphagic feeding in response to insulin-induced hypoglycemia was therefore markedly attenuated in mice lacking NPY, and a similar feeding deficit was detected in these animals after neuroglucopenia induced by 2-deoxyglucose (500 mg/kg ip). A role for NPY in glucoprivic feeding is further supported by our finding that Npy mRNA content (measured by real-time PCR) increased 2.4-fold in the hypothalamus of Npy+/+ mice by 7 h after insulin injection. Unlike the feeding deficits observed in mice lacking NPY, the effect of hypoglycemia to increase plasma glucagon and corticosterone levels was fully intact in these animals, as were both the nadir glucose value and time to recovery of euglycemia after insulin injection (P = not significant). We conclude that NPY signaling is required for hyperphagic feeding, but not neuroendocrine responses to moderate hypoglycemia.

Animals↗
PubMed2004

Increased hypothalamic melanin concentrating hormone gene expression during energy restriction involves a melanocortin-independent, estrogen-sensitive mechanism.

Increased expression of melanin concentrating hormone (MCH), an orexigenic neuropeptide produced by neurons in the lateral hypothalamic area (LHA), is implicated in the effect of energy restriction to increase food intake. Since melanocortins inhibit Mch gene expression, this effect of energy restriction to increase Mch signaling may involve reduced hypothalamic melanocortin signaling. Consistent with this hypothesis, we detected increased hypothalamic Mch mRNA levels in agouti (Ay) mice (by 102%; P < 0.05), a model of genetic obesity resulting from impaired melanocortin signaling, compared to wild-type controls. If reduced melanocortin signaling mediates the effect of energy restriction, hypothalamic Mch gene expression in Ay mice should not be increased further by energy restriction, since melanocortin signaling is impaired in these animals regardless of nutritional state. We therefore investigated the effects of energy restriction on hypothalamic Mch gene expression in both Ay mice and in wild-type mice with diet-induced obesity (DIO). Responses in these mice were compared to those induced by administration of 17beta-estradiol (E2) at a dose previously shown to reduce food intake and Mch expression in rats. In both Ay and DIO mice, energy restriction increased hypothalamic Mch mRNA levels (P < 0.05 for each) via a mechanism that was fully blocked by E2. However, E2 did not lower levels of Mch mRNA below basal values in Ay mice, whereas it did so in DIO mice. Thus, the effect of energy restriction to increase hypothalamic Mch gene expression involves an E2-sensitive mechanism that is not altered by impaired melanocortin signaling. By comparison, impaired melanocortin signaling increases hypothalamic Mch gene expression via a mechanism that is insensitive to E2. These findings suggest that while both energy restriction and reduced melanocortin signaling stimulate hypothalamic Mch gene expression, they do so via distinct mechanisms.

Animals↗
PubMed2003

Arcuate nucleus-specific leptin receptor gene therapy attenuates the obesity phenotype of Koletsky (fa(k)/fa(k)) rats.

Leptin signaling in the hypothalamic arcuate nucleus (ARC) is hypothesized to play an important role in energy homeostasis. To investigate whether leptin signaling limited to this brain area is sufficient to reduce food intake and body weight, we used adenoviral gene therapy to express the signaling isoform of the leptin receptor, lepr(b), in the ARC of leptin receptor-deficient Koletsky (fa(k)/fa(k)) rats. Successful expression of adenovirus containing lepr(b) (Ad-lepr(b)) selectively in the ARC was documented by in situ hybridization. Using real-time PCR, we further demonstrated that bilateral microinjection of Ad-lepr(b) into the ARC restored low hypothalamic levels of lepr(b) mRNA to values approximating those of wild-type (Fa(k)/Fa(k)) controls. Restored leptin receptor expression in the ARC reduced both mean daily food intake (by 13%) and body weight gain (by 33%) and increased hypothalamic proopiomelanocortin mRNA by 65% while decreasing neuropeptide Y mRNA levels by 30%, relative to fa(k)/fa(k) rats injected with a control adenovirus (Ad-lacZ) (P < 0.05 for each comparison). In contrast, Ad-lepr(b) delivery to either the lateral hypothalamic area of fa(k)/fa(k) rats or to the ARC of wild-type Fa(k)/Fa(k) rats had no effect on any of these parameters. These findings collectively support the hypothesis that leptin receptor signaling in the ARC is sufficient to mediate major effects of leptin on long-term energy homeostasis. Adenoviral gene therapy is thus a viable strategy with which to study the physiological importance of specific molecules acting in discrete brain areas.

Adenoviridae↗