Featured ArticlesVolume 4 | No. 10 | October 2015
|E4orf1 induction in adipose tissue promotes insulin-independent signaling in the adipocyteKusminski and colleagues study the different branches of the insulin signaling pathway by generating and characterizing an inducible mouse model of E4orf1 expression in adipocytes. Expression of the 14-kDa adenoviral polypeptide results in 'insulin-sparing' characteristics during glucose tolerance tests. This phenomenon arises from activation of a Ras-ERK-MAPK signaling pathway that enhances insulin-independent p-Akt activation.|
Abstract | PDF
Background/Purpose: Type 2 diabetes remains a worldwide epidemic with major pathophysiological changes as a result of chronic insulin resistance. Insulin regulates numerous biochemical pathways related to carbohydrate and lipid metabolism.
Methods: We have generated a novel mouse model that allows us to constitutively activate, in an inducible fashion, the distal branch of the insulin signaling transduction pathway specifically in adipocytes.
Results: Using the adenoviral 36 E4orf1 protein, we chronically stimulate locally the Ras-ERK-MAPK signaling pathway. At the whole body level, this leads to reduced body-weight gain under a high fat diet challenge. Despite overlapping glucose tolerance curves, there is a reduced requirement for insulin action under these conditions. The mice further exhibit reduced circulating adiponectin levels that ultimately lead to impaired lipid clearance, and inflamed and fibrotic white adipose tissues. Nevertheless, they are protected from diet-induced hepatic steatosis. As we observe constitutively elevated p-Akt levels in the adipocytes, even under conditions of low insulin levels, this pinpoints enhanced Ras-ERK-MAPK signaling in transgenic adipocytes as a potential alternative route to bypass proximal insulin signaling events.
Conclusions: We conclude that E4orf1 expression in the adipocyte leads to enhanced baseline activation of the distal insulin signaling node, yet impaired insulin receptor stimulation in the presence of insulin, with important implications for the regulation of adiponectin secretion. The resulting systemic phenotype is complex, yet highlights the powerful nature of manipulating selective branches of the insulin signaling network within the adipocyte.[Hide abstract]
|NF-κB drives survival of adipose tissue macrophages in an obesogenic environmentDuring obesity, macrophages accumulate in adipose tissue (AT) and correlate with adiposity, systemic inflammation and insulin resistance. Hill and colleagues now establish a recruitment-independent mechanism contributing to this accumulation. They demonstrate that during obesity, activation of NF-κB in AT macrophages promotes cell survival.|
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Objective: Macrophage accumulation in adipose tissue (AT) during obesity contributes to inflammation and insulin resistance. Recruitment of monocytes to obese AT has been the most studied mechanism explaining this accumulation. However, recent evidence suggests that recruitment-independent mechanisms may also regulate pro-inflammatory AT macrophage (ATM) numbers. The role of increased ATM survival during obesity has yet to be explored.
Results: We demonstrate that activation of apoptotic pathways is significantly reduced in ATMs from diet-induced and genetically obese mice. Concurrently, pro-survival Bcl-2 family member protein levels and localization to the mitochondria is elevated in ATMs from obese mice. This increased pro-survival signaling was associated with elevated activation of the transcription factor, NF-κB, and increased expression of its pro-survival target genes. Finally, an obesogenic milieu increased ATM viability only when NF-κB signaling pathways were functional.
Conclusions: Our data demonstrate that obesity promotes survival of inflammatory ATMs, possibly through an NF-κB-regulated mechanism.[Hide abstract]
|The role of GluN2A and GluN2B NMDA Receptor Subunits in AgRP and POMC Neurons on Body Weight and Glucose HomeostasisHypothalamic agouti-related peptide (AgRP) and pro-opiomelanocortin (POMC) producing neurons play important roles in the regulation of body weight homeostasis and get glutamatergic input via n-methyl-D-aspartate (NMDA) receptors. Üner and colleagues investigate the role of the specific NMDA receptor subunits GluN2A and GluN2B. They show that GluN2B expression in hypothalamic AgRP neurons is required for normal control of caloric intake and body weight in lean animals and can influence leptin sensitivity and glucose balance under obese and diabetic conditions.|
Abstract | PDF
Objective: Hypothalamic agouti-related peptide (AgRP) and pro-opiomelanocortin (POMC) expressing neurons play critical roles in control of energy balance. Glutamatergic input via n-methyl-D-aspartate receptors (NMDARs) is pivotal for regulation of neuronal activity and is required in AgRP neurons for normal body weight homeostasis. NMDARs typically consist of the obligatory GluN1 subunit and different GluN2 subunits, the latter exerting crucial differential effects on channel activity and neuronal function. Currently, the role of specific GluN2 subunits in AgRP and POMC neurons on whole body energy and glucose balance is unknown.
Methods: We used the cre-lox system to genetically delete GluN2A or GluN2B only from AgRP or POMC neurons in mice. Mice were then subjected to metabolic analyses and assessment of AgRP and POMC neuronal function through morphological studies.
Results: We show that loss of GluN2B from AgRP neurons reduces body weight, fat mass, and food intake, whereas GluN2B in POMC neurons is not required for normal energy balance control. GluN2A subunits in either AgRP or POMC neurons are not required for regulation of body weight. Deletion of GluN2B reduces the number of AgRP neurons and decreases their dendritic length. In addition, loss of GluN2B in AgRP neurons of the morbidly obese and severely diabetic leptin-deficient Lepob/ob mice does not affect body weight and food intake but, remarkably, leads to full correction of hyperglycemia. Lepob/ob mice lacking GluN2B in AgRP neurons are also more sensitive to leptin’s anti-obesity actions.
Conclusions: GluN2B-containing NMDA receptors in AgRP neurons play a critical role in central control of body weight homeostasis and blood glucose balance via mechanisms that likely involve regulation of AgRP neuronal survival and structure, and modulation of hypothalamic leptin action.[Hide abstract]
|Moderate voluntary exercise attenuates the metabolic syndrome in melanocortin-4 receptor-deficient ratsMelanocortin-4 receptor (MC4R) loss-of-function mutations are the most commonly observed monogenetic associations with the metabolic syndrome in humans. Obici and colleagues demonstrate in rats that MC4R-deficiency results in reduced voluntary wheel running, associated with dysregulation of the mesolimbic dopamine system. They also show that voluntary wheel running can normalize several characteristic phenotypes of MC4R deficiency, including body weight gain, hyperphagia, insulin insensitivity, hypercholesterolemia, and hepatic steatosis.|
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Objective: Melanocortin-4 receptors (MC4Rs) are highly expressed by dopamine-secreting neurons of the mesolimbic tract, but their functional role has not been fully resolved. Voluntary wheel running (VWR) induces adaptations in the mesolimbic dopamine system and has a myriad of long-term beneficial effects on health. In the present experiments we asked whether MC4R function regulates the effects of VWR, and whether VWR ameliorates MC4R-associated symptoms of the metabolic syndrome.
Methods: Electrically evoked dopamine release was measured in slice preparations from sedentary wild-type and MC4R-deficient Mc4rK314X (HOM) rats. VWR was assessed in wild-type and HOM rats, and in MC4R-deficient loxTBMc4r mice, wild-type mice body weight-matched to loxTBMc4r mice, and wild-type mice with intracerebroventricular administration of the MC4R antagonist SHU9119. Mesolimbic dopamine system function (gene/protein expression) and metabolic parameters were examined in wheel-running and sedentary wild-type and HOM rats.
Results: Sedentary obese HOM rats had increased electrically evoked dopamine release in several ventral tegmental area (VTA) projection sites compared to wild-type controls. MC4R loss-of-function decreased VWR, and this was partially independent of body weight. HOM wheel-runners had attenuated markers of intracellular D1-type dopamine receptor signaling despite increased dopamine flux in the VTA. VWR increased and decreased ΔFosB levels in the nucleus accumbens (NAc) of wild-type and HOM runners, respectively. VWR improved metabolic parameters in wild-type wheel-runners. Finally, moderate voluntary exercise corrected many aspects of the metabolic syndrome in HOM runners.
Conclusions: Central dopamine dysregulation during VWR reinforces the link between MC4R function and molecular and behavioral responding to rewards. The data also suggest that exercise can be a successful lifestyle intervention in MC4R-haploinsufficient individuals despite reduced positive reinforcement during exercise training.[Hide abstract]
|Leptin modulates nutrient reward via inhibitory galanin action on orexin neuronsLaque and colleagues uncover a novel neuronal circuit where leptin receptor (LepRb) and galanin (GAL) co-expressing neurons in the lateral hypothalamus directly innervate orexin as well as noradrenergic locus coeruleus neurons. Dysregulation of this system by deleting LepRb from GAL neurons causes increased nutrient reward value and consumption for sucrose, while fat consumption is decreased.|
Abstract | PDF
Objective: Leptin modulates food reward via central leptin receptor (LepRb) expressing neurons. Food reward requires stimulation of midbrain dopamine neurons and is modulated by central leptin action, but the exact central mechanisms remain unclear. Stimulatory and inhibitory leptin actions on dopamine neurons have been reported, e.g. by indirect actions on orexin neurons or via direct innervation of dopamine neurons in the ventral tegmental area.
Methods: We showed earlier that LepRb neurons in the lateral hypothalamus (LHA) co-express the inhibitory acting neuropeptide galanin (GAL-LepRb neurons). We studied the involvement of GAL-LepRb neurons to regulate nutrient reward in mice with selective LepRb deletion from galanin neurons (GAL-LepRbKO mice).
Results: We found that the rewarding value and preference for sucrose over fat was increased in GAL-LepRbKO mice compared to controls. LHA GAL-LepRb neurons innervate orexin neurons, but not the VTA. Further, expression of galanin and its receptor GalR1 are decreased in the LHA of GAL-LepRbKO mice, resulting in increased activation of orexin neurons.
Conclusion: We suggest galanin as an important mediator of leptin action to modulate nutrient reward by inhibiting orexin neurons.[Hide abstract]
|Distribution and characterisation of Glucagon-like peptide-1 receptor expressing cells in the mouse brainCork and colleagues detail the distribution of Glucagon-like peptide-1 receptor (GLP-1R) expressing cells throughout the mouse brain using a novel transgenic model, in which cre-recombinase is expressed under the control of the Glp1r gene. This mouse model can also be used to manipulate specific subsets of GLP-1R expressing cells with cre-dependent viral gene transfer in vivo.|
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Objective: Although Glucagon-like peptide 1 is a key regulator of energy metabolism and food intake, the precise location of GLP-1 receptors and the physiological relevance of certain populations is debatable. This study investigated the novel GLP-1R-Cre mouse as a functional tool to address this question.
Methods: Mice expressing Cre-recombinase under the Glp1r promoter were crossed with either a ROSA26 eYFP or tdRFP reporter strain to identify GLP-1R expressing cells. Patch-clamp recordings were performed on tdRFP-positive neurons in acute coronal brain slices from adult mice and selective targeting of GLP-1R cells in vivo was achieved using viral gene delivery.
Results: Large numbers of eYFP or tdRFP immunoreactive cells were found in the circumventricular organs, amygdala, hypothalamic nuclei and the ventrolateral medulla. Smaller numbers were observed in the nucleus of the solitary tract and the thalamic paraventricular nucleus. However, tdRFP positive neurons were also found in areas without preproglucagon-neuronal projections like hippocampus and cortex. GLP-1R cells were not immunoreactive for GFAP or parvalbumin although some were catecholaminergic. GLP-1R expression was confirmed in whole-cell recordings from BNST, hippocampus and PVN, where 100 nM GLP-1 elicited a reversible inward current or depolarisation. Additionally, a unilateral ste-reotaxic injection of a cre-dependent AAV into the PVN demonstrated that tdRFP-positive cells express cre-recombinase facilitating virally-mediated eYFP expression.
Conclusions: This study is a comprehensive description and phenotypic analysis of GLP-1R expression in the mouse CNS. We demonstrate the power of combining the GLP-1R-CRE mouse with a virus to generate a selective molecular handle enabling future in vivo investigation as to their physiological importance.[Hide abstract]
|Molecular regulation of urea cycle function by the liver glucocorticoid receptorGlucocorticoid (GC) treatment is an effective and thus often prescribed immunosuppressive therapy, but also has negative side effects such as lean tissue wasting. Okun and colleagues now demonstrate that the liver glucocorticoid receptor (GR) coordinates proper urea cycle function. In particular, the liver GR upregulates the expression of Arginase I during dexamethasone treatment, aiding in the disposal of excess amino acid N yielded from lean tissue wasting and preventing accumulation of ammonia and associated neuromuscular dysfunction.|
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Objective: One of the major side effects of glucocorticoid (GC) treatment is lean tissue wasting, indicating a prominent role in systemic aminoacid metabolism. In order to uncover a novel aspect of GCs and their intracellular-receptor, the glucocorticoid receptor (GR), on metabolic control, we conducted amino acid and acylcarnitine profiling in human and mouse models of GC/GR gain- and loss-of-function.
Methods: Blood serum and tissue metabolite levels were determined in Human Addison’s disease (AD) patients as well as in mouse models of systemic and liver-specific GR loss-of-function (AAV-miR-GR) with or without dexamethasone (DEX) treatments. Body composition and neuro-muscular and metabolic function tests were conducted in vivo and ex vivo, the latter using precision cut liver slices.
Results: A serum metabolite signature of impaired urea cycle function (i.e. higher [ARG]:[ORN + CIT]) was observed in human (CTRL:0.45 ± 0.03, AD: 1.29 ± 0.04; p < 0.001) and mouse (AAV-miR-NC: 0.97 ± 0.13, AAV-miR-GR: 2.20 ± 0.19; p < 0.001) GC/GR loss-of-function, with similar patterns also observed in liver. Serum urea levels were consistently affected by GC/GR gain- (~ +32%) and loss (~ −30%) -of-function. Combined liver-specific GR loss-of-function with DEX treatment revealed a tissue-autonomous role for the GR to coordinate an upregulation of liver urea production rate in vivo and ex vivo, and prevent hyperammonaemia and associated neuromuscular dysfunction in vivo. Liver mRNA expression profiling and GR-cistrome mining identified Arginase I (ARG1) a urea cycle gene targeted by the liver GR.
Conclusions: The liver GR controls systemic and liver urea cycle function by transcriptional regulation of ARG1 expression.[Hide abstract]
|TIMP3 interplays with apelin to regulate cardiovascular metabolism in hypercholesterolemic miceTissue inhibitor of metalloproteinase 3 (TIMP3) is an extracellular matrix bound protein, which is downregulated in human subjects and experimental models with cardiometabolic disorders, such as type 2 diabetes mellitus, hypertension and atherosclerosis. Stöhr and colleagues report that ApoE-/-TIMP3-/- mice show decreased lifespan and reduced resistance to cardiac stress. Based on metabolite profiling and gene/protein expression analyses, they conclude that loss of TIMP3 results in a perturbation of metabolic flexibility in the heart and arrhythmias, phenomena that may underlie the increased rates of cardiovascular death in this model.|
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Objective: Tissue inhibitor of metalloproteinase 3 (TIMP3) is an extracellular matrix (ECM) bound protein, which has been shown to be downregulated in human subjects and experimental models with cardiometabolic disorders, including type 2 diabetes mellitus, hypertension and atherosclerosis. The aim of this study was to investigate the effects of TIMP3 on cardiac energy homeostasis during increased metabolic stress conditions.
Methods: ApoE−/−TIMP3−/− and ApoE−/− mice on a C57BL/6 background were subjected to telemetric ECG analysis and experimental myocardial infarction as models of cardiac stress induction. We used Western blot, qRT-PCR, histology, metabolomics, RNA-sequencing and in vivo phenotypical analysis to investigate the molecular mechanisms of altered cardiac energy metabolism.
Results: ApoE−/−TIMP3−/− revealed decreased lifespan. Telemetric ECG analysis showed increased arrhythmic episodes, and experimental myocardial infarction by left anterior descending artery (LAD) ligation resulted in increased peri-operative mortality together with increased scar formation, ventricular dilatation and a reduction of cardiac function after 4 weeks in the few survivors. Hearts of ApoE−/−TIMP3−/− exhibited accumulation of neutral lipids when fed a chow diet, which was exacerbated by a high fat, high cholesterol diet. Metabolomics analysis revealed an increase in circulating markers of oxidative stress with a reduction in long chain fatty acids. Using whole heart mRNA sequencing, we identified apelin as a putative modulator of these metabolic defects. Apelin is a regulator of fatty acid oxidation, and we found a reduction in the levels of enzymes involved in fatty acid oxidation in the left ventricle of ApoE−/−TIMP3−/− mice. Injection of apelin restored the hitherto identified metabolic defects of lipid oxidation.
Conclusion: TIMP3 regulates lipid metabolism as well as oxidative stress response via apelin. These findings therefore suggest that TIMP3 maintains metabolic flexibility in the heart, particularly during episodes of increased cardiac stress.[Hide abstract]