Featured ArticlesVolume 6 | No. 6 | June 2017
|Four new genes contributing to human insulin secretionType 2 diabetes (T2D) is a complex, multifactorial disorder with an estimated heritability ranging between 40% and 70%. Genome-wide association studies (GWAS) have shown that common T2D is highly polygenic. Ndiaye, Ortalli, Canouil et al. performed a comprehensive expression study of candidate T2D susceptibility genes closest to all GWAS-identified T2D single nucleotide polymorphisms (SNPs) in a large panel of human organs, tissues, and cells, followed by the functional analysis of the knockdown of these genes in human pancreatic beta-cell lines. They found that the expression of tested candidate T2D susceptibility genes was significantly and specifically enriched in pancreatic beta cells, and they report functional evidence for a role in insulin secretion of four T2D susceptibility genes (PRC1, SRR, ZFAND3, and ZFAND6).|
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Objectives: Genome-wide association studies (GWAS) have identified >100 loci independently contributing to type 2 diabetes (T2D) risk. However, translational implications for precision medicine and for the development of novel treatments have been disappointing, due to poor knowledge of how these loci impact T2D pathophysiology. Here, we aimed to measure the expression of genes located nearby T2D associated signals and to assess their effect on insulin secretion from pancreatic beta cells.
Methods: The expression of 104 candidate T2D susceptibility genes was measured in a human multi-tissue panel, through PCR-free expression assay. The effects of the knockdown of beta-cell enriched genes were next investigated on insulin secretion from the human EndoC-βH1 beta-cell line. Finally, we performed RNA-sequencing (RNA-seq) so as to assess the pathways affected by the knockdown of the new genes impacting insulin secretion from EndoC-βH1, and we analyzed the expression of the new genes in mouse models with altered pancreatic beta-cell function.
Results: We found that the candidate T2D susceptibility genes' expression is significantly enriched in pancreatic beta cells obtained by laser capture microdissection or sorted by flow cytometry and in EndoC-βH1 cells, but not in insulin sensitive tissues. Furthermore, the knockdown of seven T2D-susceptibility genes (CDKN2A, GCK, HNF4A, KCNK16, SLC30A8, TBC1D4, and TCF19) with already known expression and/or function in beta cells changed insulin secretion, supporting our functional approach. We showed first evidence for a role in insulin secretion of four candidate T2D-susceptibility genes (PRC1, SRR, ZFAND3, and ZFAND6) with no previous knowledge of presence and function in beta cells. RNA-seq in EndoC-βH1 cells with decreased expression of PRC1, SRR, ZFAND6, or ZFAND3 identified specific gene networks related to T2D pathophysiology. Finally, a positive correlation between the expression of Ins2 and the expression of Prc1, Srr, Zfand6, and Zfand3 was found in mouse pancreatic islets with altered beta-cell function.
Conclusions: This study showed the ability of post-GWAS functional studies to identify new genes and pathways involved in human pancreatic beta-cell function and in T2D pathophysiology.[Hide abstract]
|FGF21 does not require AMPK or the phosphorylation of ACC to mediate improvements in glucose homeostasisFibroblast growth factor 21 (FGF21) is an endocrine factor that exerts potent anti-obesity and anti-diabetic effects. FGF21 administration activates the energy sensor AMP-activated protein kinase (AMPK) to promote mitochondrial biogenesis and greater mitochondrial oxidative function. Mottillo, Desjardin et al. demonstrate that unlike metformin, the beneficial metabolic effects of pharmacological administration of FGF21 on energy metabolism, fuel selection, body weight, insulin tolerance, and liver lipids are not mediated through adipocyte AMPK or the AMPK/ACC pathway.|
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Objective: Fibroblast growth factor 21 (FGF21) shows great potential for the treatment of obesity and type 2 diabetes, as its long-acting analogue reduces body weight and improves lipid profiles of participants in clinical studies; however, the intracellular mechanisms mediating these effects are poorly understood. AMP-activated protein kinase (AMPK) is an important energy sensor of the cell and a molecular target for anti-diabetic medications. This work examined the role of AMPK in mediating the glucose and lipid-lowering effects of FGF21.
Methods: Inducible adipocyte AMPK β1β2 knockout mice (iβ1β2AKO) and littermate controls were fed a high fat diet (HFD) and treated with native FGF21 or saline for two weeks. Additionally, HFD-fed mice with knock-in mutations on the AMPK phosphorylation sites of acetyl-CoA carboxylase (ACC)1 and ACC2 (DKI mice) along with wild-type (WT) controls received long-acting FGF21 for two weeks.
Results: Consistent with previous studies, FGF21 treatment significantly reduced body weight, adiposity, and liver lipids in HFD fed mice. To add, FGF21 improved circulating lipids, glycemic control, and insulin sensitivity. These effects were independent of adipocyte AMPK and were not associated with changes in browning of white (WAT) and brown adipose tissue (BAT). Lastly, we assessed whether FGF21 exerted its effects through the AMPK/ACC axis, which is critical in the therapeutic benefits of the anti-diabetic medication metformin. ACC DKI mice had improved glucose and insulin tolerance and a reduction in body weight, body fat and hepatic steatosis similar to WT mice in response to FGF21 administration.
Conclusions: These data illustrate that the metabolic improvements upon FGF21 administration are independent of adipocyte AMPK, and do not require the inhibitory action of AMPK on ACC. This is in contrast to the anti-diabetic medication metformin and suggests that the treatment of obesity and diabetes with the combination of FGF21 and AMPK activators merits consideration.[Hide abstract]
|Vaspin expression is increased after obesogenic diets and cold exposureVisceral adipose tissue-derived serine protease inhibitor (vaspin) has anti-diabetic and anti-obesogenic properties. Weiner et al. investigated the effects of brown adipose tissue (BAT) activating physiological stimuli, such as high caloric diets or cold exposure, on vaspin gene expression and secretion in mice. They report a novel BAT-specific regulation of vaspin gene expression upon BAT activating physiological stimuli in vivo with epigenetic changes. Their data suggest that vaspin gene expression is specifically increased in activated BAT, indicating functional relevance and potentially beneficial effects of vaspin on BAT function.|
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Objective: Several studies have demonstrated anti-diabetic and anti-obesogenic properties of visceral adipose tissue-derived serine protease inhibitor (vaspin) and so evoked its potential use for treatment of obesity-related diseases. The aim of the study was to unravel physiological regulators of vaspin expression and secretion with a particular focus on its role in brown adipose tissue (BAT) biology.
Methods: We analyzed the effects of obesogenic diets and cold exposure on vaspin expression in liver and white and brown adipose tissue (AT) and plasma levels. Vaspin expression was analyzed in isolated white and brown adipocytes during adipogenesis and in response to adrenergic stimuli. DNA-methylation within the vaspin promoter was analyzed to investigate acute epigenetic changes after cold-exposure in BAT.
Results: Our results demonstrate a strong induction of vaspin mRNA and protein expression specifically in BAT of both cold-exposed and high-fat (HF) or high-sugar (HS) fed mice. While obesogenic diets also upregulated hepatic vaspin mRNA levels, cold exposure tended to increase vaspin gene expression of inguinal white adipose tissue (iWAT) depots. Concomitantly, vaspin plasma levels were decreased upon obesogenic or thermogenic triggers. Vaspin expression was increased during adipogenesis but unaffected by sympathetic activation in brown adipocytes. Analysis of vaspin promoter methylation in AT revealed lowest methylation levels in BAT, which were acutely reduced after cold exposure.
Conclusions: Our data demonstrate a novel BAT-specific regulation of vaspin gene expression upon physiological stimuli in vivo with acute epigenetic changes that may contribute to cold-induced expression in BAT. We conclude that these findings indicate functional relevance and potentially beneficial effects of vaspin in BAT function.[Hide abstract]
|Rasal2 deficiency reduces adipogenesis and obesity-related disordersRas proteins are known to play fundamental roles in cell growth and proliferation. Rasal2 (Ras protein activator like 2) is a Ras-GTPase-activating protein (Ras-GAP). A single nucleotide polymorphism of its gene is associated with increased body mass index. Zhu et al. demonstrate that Rasal2 deficiency leads to lower adiposity and an increase in resistance to HFD-induced obesity. In addition, Rasal2 deficiency ameliorates numerous obesity-related metabolic disorders, including glucose intolerance, insulin resistance, and hepatic steatosis. These findings implicate Rasal2 as a potential therapeutic target to combat obesity-related disorders.|
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Objective: Identification of additional regulatory factors involved in the onset of obesity is important to understand the mechanisms underlying this prevailing disease and its associated metabolic disorders and to develop therapeutic strategies. Through isolation and analysis of a mutant, we aimed to uncover the function of a Ras-GAP gene, Rasal2 (Ras protein activator like 2), in the development of obesity and related metabolic disorders and to obtain valuable insights regarding the mechanism underlying the function.
Methods: An obesity-based genetic screen was performed to identify an insertional mutation that disrupts the expression of Rasal2 (Rasal2PB/PB mice). Important metabolic parameters, such as fat mass and glucose tolerance, were measured in Rasal2PB/PB mice. The impact of Rasal2 on adipogenesis was evaluated in the mutant mice and in 3T3-L1 preadipocytes treated with Rasal2 siRNA. Ras and ERK activities were then evaluated in Rasal2-deficient preadipocytes or mice, and their functional relationships with Rasal2 on adipogenesis were investigated by employing Ras and MEK inhibitors.
Results: Rasal2PB/PB mice showed drastic decrease in Rasal2 expression and a lean phenotype. The mutant mice displayed decreased adiposity and resistance to high-fat diet induced metabolic disorders. Further analysis indicated that Rasal2 deficiency leads to impaired adipogenesis in vivo and in vitro. Moreover, while Rasal2 deficiency resulted in increased activity of both Ras and ERK in preadipocytes, reducing Ras, but not ERK, suppressed the impaired adipogenesis.
Conclusions: Rasal2 promotes adipogenesis, which may critically contribute to its role in the development of obesity and related metabolic disorders and may do so by repressing Ras activity in an ERK-independent manner.[Hide abstract]
|Glucagon-like peptide-2 promotes gallbladder refillingThe physiological cycle of gallbladder (GB) filling and emptying dictates the flow of the bile into the intestine, enabling the enterohepatic circulation of bile acids (BA). Unexpectedly, reports of gallbladder disease have been associated with the use of both glucagon-like peptide-1 receptor (GLP-1R) and GLP-2R agonists and after bariatric surgery. To explore whether GLP-1 and/or GLP-2 influence GB physiology, Yusta et al. have studied the acute actions of these peptides on GB activity in mice. They delineate a Tgr5-independent, GLP-2R-dependent pathway that counteracts the actions of cholecystokinin and promotes GB filling. |
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Objective: Glucagon-like peptides (GLPs) are secreted from enteroendocrine cells in response to nutrients and bile acids and control metabolism via actions on structurally-related yet distinct G protein coupled receptors. GLP-1 regulates gut motility, appetite, islet function, and glucose homeostasis, whereas GLP-2 enhances intestinal nutrient absorption. GLP-1R agonists are used to treat diabetes and obesity, and a GLP-2R agonist is approved to treat short bowel syndrome. Unexpectedly, reports of gallbladder disease have been associated with the use of both GLP-1R and GLP-2R agonists and after bariatric surgery, although the mechanisms remain unknown.
Methods: We investigated whether GLP-1 or GLP-2 acutely controls gallbladder (GB) volume and whether GLP-2 regulates GB muscle activity in mice. The expression of Tgr5, Glp2r, and Glp1r was assessed in mouse GB, and the effects of GLP-2 on hepatic bile acid (BA) flow, intestinal and liver BA uptake, and GB gene expression were determined. GLP-2 regulation of GB volume was assessed in wildtype, Glp2r−/− and Tgr5−/− mice. The effect of GLP-2 on GB smooth muscle (GBSM) calcium transients was characterized ex vivo.
Results: Acute administration of the GLP-1R agonist exendin-4 lowered glucose but had no effect on GB volume in mice. In contrast, GLP-2 rapidly enhanced GB filling in a dose-dependent manner, actions maintained in the presence of cholecystokinin, and mediated through the canonical GLP-2R. GLP-2 also rapidly induced immediate early gene expression in GB, consistent with detection of the endogenous Glp2r in GB RNA. The ability of GLP-2 to increase GB volume was not abrogated by systemic administration of hexamethonium, propranolol, a vasoactive peptide receptor antagonist or N-Nitroarginine methyl ester, and was maintained in Tgr5−/− mice. In contrast, lithocholic acid, a Tgr5 agonist, increased GB filling in Glp2r−/− but not in Tgr5−/− mice. GLP-2 had no effect on ileal uptake or hepatic clearance of taurocholic acid or on hepatic bile flow, yet reduced the frequency of spontaneous calcium transients in murine GBSM ex vivo, in a tetrodotoxin-sensitive manner.
Conclusions: Our data extend endocrine concepts of regulation of GB filling beyond FXR-FGF15/19 and the direct effects of BA via Tgr5, to encompass a novel BA-Tgr5-L cell GLP-2 axis providing nutrient-mediated feedback from BA to terminate meal-related GB contraction. These findings have implications for conditions characterized by elevated circulating levels of GLP-2 such as after bariatric surgery and the development and use of agents that promote Tgr5 activation, L cell secretion, or GLP-2R agonism for the treatment of metabolic disease.[Hide abstract]
|Hepatic gene therapy rescues high-fat diet responses in circadian Clock mutant miceEndogenous circadian clocks are based on transcriptional-translational feedback loops built from a set of clock genes/proteins including the transcription factor circadian locomotor output cycles kaput (CLOCK). Mice carrying a dominant negative mutation in the gene encoding CLOCK are overweight and, under high-fat diet (HFD) conditions, develop symptoms of the metabolic syndrome. Meyer-Kovac, Kolbe, et al. show that liver gene therapy may partially restore metabolic responses to HFD in circadian Clock mutant mice, indicating an important role of liver clock function in energy homeostasis and appetite regulation.
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Objective: Circadian Clock gene mutant mice show dampened 24-h feeding rhythms and an increased sensitivity to high-fat diet (HFD) feeding. Restricting HFD access to the dark phase counteracts its obesogenic effect in wild-type mice. The extent to which altered feeding rhythms are causative for the obesogenic phenotype of Clock mutant mice, however, remains unknown.
Methods: Metabolic parameters of wild-type (WT) and ClockΔ19 mutant mice (MT) were investigated under ad libitum and nighttime restricted HFD feeding. Liver circadian clock function was partially rescued by hydrodynamic tail vein delivery of WT-Clock DNA vectors in mutant mice and transcriptional, metabolic, endocrine and behavioral rhythms studied.
Results: Nighttime-restricted feeding restored food intake, but not body weight regulation in MT mice under HFD, suggesting Clock-dependent metabolic dysregulation downstream of circadian appetite control. Liver-directed Clock gene therapy partially restored liver circadian oscillator function and transcriptome regulation without affecting centrally controlled circadian behaviors. Under HFD, MT mice with partially restored liver clock function (MT-LR) showed normalized body weight gain, rescued 24-h food intake rhythms, and WT-like energy expenditure. This was associated with decreased nighttime leptin and daytime ghrelin levels, reduced hepatic lipid accumulation, and improved glucose tolerance. Transcriptome analysis revealed that hepatic Clock rescue in MT mice affected a range of metabolic pathways.
Conclusions: Liver Clock gene therapy improves resistance against HFD-induced metabolic impairments in mice with circadian clock disruption. Restoring or stabilizing liver clock function might be a promising target for therapeutic interventions in obesity and metabolic disorders.[Hide abstract]
|Foxa2 and Pdx1 cooperatively regulate postnatal maturation of pancreatic β-cellsThe transcription factors (TFs) forkhead box protein A2 (Foxa2) and pancreatic and duodenal homeobox 1 (Pdx1) share several important functions, such as regulation of insulin production and modulation of the function of other transcription factors. In diabetic models, the levels of both TFs appear to be reduced in dedifferentiated β-cells, suggesting that these TFs act alone or in combination to induce or maintain β-cell identity. Bastidas-Ponce and colleagues generated a reporter mouse line expressing TF fusion proteins that develops hyperglycemia postnatally. This phenotype occurs due to the failure to generate and preserve mature β-cells. The reduction in Pdx1 levels along with the possible destruction of the cooperative function of this TF with Foxa2 are the responsible mechanisms underlying loss of β-cell maturation, identity, and function.|
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Objective: The transcription factors (TF) Foxa2 and Pdx1 are key regulators of beta-cell (β-cell) development and function. Mutations of these TFs or their respective cis-regulatory consensus binding sites have been linked to maturity diabetes of the young (MODY), pancreas agenesis, or diabetes susceptibility in human. Although Foxa2 has been shown to directly regulate Pdx1 expression during mouse embryonic development, the impact of this gene regulatory interaction on postnatal β-cell maturation remains obscure.
Methods: In order to easily monitor the expression domains of Foxa2 and Pdx1 and analyze their functional interconnection, we generated a novel double knock-in homozygous (FVFPBFDHom) fluorescent reporter mouse model by crossing the previously described Foxa2-Venus fusion (FVF) with the newly generated Pdx1-BFP (blue fluorescent protein) fusion (PBF) mice.
Results: Although adult PBF homozygous animals exhibited a reduction in expression levels of Pdx1, they are normoglycemic. On the contrary, despite normal pancreas and endocrine development, the FVFPBFDHom reporter male animals developed hyperglycemia at weaning age and displayed a reduction in Pdx1 levels in islets, which coincided with alterations in β-cell number and islet architecture. The failure to establish mature β-cells resulted in loss of β-cell identity and trans-differentiation towards other endocrine cell fates. Further analysis suggested that Foxa2 and Pdx1 genetically and functionally cooperate to regulate maturation of adult β-cells.
Conclusions: Our data show that the maturation of pancreatic β-cells requires the cooperative function of Foxa2 and Pdx1. Understanding the postnatal gene regulatory network of β-cell maturation will help to decipher pathomechanisms of diabetes and identify triggers to regenerate dedifferentiated β-cell mass.[Hide abstract]
|NNT reverse mode of operation mediates glucose control of mitochondrial redox state The glucose stimulation of insulin secretion (GSIS) by pancreatic β-cells depends on the increased flux of glucose metabolism through glycolysis and the mitochondrial Krebs cycle. The mitochondrial NAD(P)H and glutathione redox state may play a role in GSIS. Mitochondrial NADPH is typically produced by Nicotinamide nucleotide transhydrogenase (NNT) and some other enzymes. Santos, Muller et al. show that NNT mediates the effects of glucose on islet NADPH and mitochondrial glutathione redox state, but, contrary to current views on NNT in β-cells, it does so by reducing its reverse mode of operation, which consumes NADPH, from non-stimulating to stimulating glucose concentrations.|
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Objective: The glucose stimulation of insulin secretion (GSIS) by pancreatic β-cells critically depends on increased production of metabolic coupling factors, including NADPH. Nicotinamide nucleotide transhydrogenase (NNT) typically produces NADPH at the expense of NADH and ΔpH in energized mitochondria. Its spontaneous inactivation in C57BL/6J mice was previously shown to alter ATP production, Ca2+ influx, and GSIS, thereby leading to glucose intolerance. Here, we tested the role of NNT in the glucose regulation of mitochondrial NADPH and glutathione redox state and reinvestigated its role in GSIS coupling events in mouse pancreatic islets.
Methods: Islets were isolated from female C57BL/6J mice (J-islets), which lack functional NNT, and genetically close C57BL/6N mice (N-islets). Wild-type mouse NNT was expressed in J-islets by adenoviral infection. Mitochondrial and cytosolic glutathione oxidation was measured with glutaredoxin 1-fused roGFP2 probes targeted or not to the mitochondrial matrix. NADPH and NADH redox state was measured biochemically. Insulin secretion and upstream coupling events were measured under dynamic or static conditions by standard procedures.
Results: NNT is largely responsible for the acute glucose-induced rise in islet NADPH/NADP+ ratio and decrease in mitochondrial glutathione oxidation, with a small impact on cytosolic glutathione. However, contrary to current views on NNT in β-cells, these effects resulted from a glucose-dependent reduction in NADPH consumption by NNT reverse mode of operation, rather than from a stimulation of its forward mode of operation. Accordingly, the lack of NNT in J-islets decreased their sensitivity to exogenous H2O2 at non-stimulating glucose. Surprisingly, the lack of NNT did not alter the glucose-stimulation of Ca2+ influx and upstream mitochondrial events, but it markedly reduced both phases of GSIS by altering Ca2+-induced exocytosis and its metabolic amplification.
Conclusions: These results drastically modify current views on NNT operation and mitochondrial function in pancreatic β-cells.[Hide abstract]
|Opposing effects of prostaglandin E2 receptors EP3 and EP4 on mouse and human β-cell survival and proliferation The receptors for the endogenous lipid signaling molecule prostaglandin E2 (PGE2), called E-Prostanoid Receptor 1-4 (EP1-4), are expressed in rodent and human islets. Little is known about the role of PGE2 and the EP receptors in regulating β-cell proliferation and survival, which can affect β-cell mass dynamics. Carboneau and colleagues identify EP3 and EP4 as novel regulators of β-cell proliferation and survival in mouse and human islets ex vivo. Their data suggest that inhibition of EP3 and/or activation of EP4 may improve outcomes in the settings of obesity and T2D by enhancing β-cell proliferation, survival and glucose stimulation of insulin secretion.|
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Objective: Hyperglycemia and systemic inflammation, hallmarks of Type 2 Diabetes (T2D), can induce the production of the inflammatory signaling molecule Prostaglandin E2 (PGE2) in islets. The effects of PGE2 are mediated by its four receptors, E-Prostanoid Receptors 1-4 (EP1-4). EP3 and EP4 play opposing roles in many cell types due to signaling through different G proteins, Gi and GS, respectively. We previously found that EP3 and EP4 expression are reciprocally regulated by activation of the FoxM1 transcription factor, which promotes β-cell proliferation and survival. Our goal was to determine if EP3 and EP4 regulate β-cell proliferation and survival and, if so, to elucidate the downstream signaling mechanisms.
Methods: β-cell proliferation was assessed in mouse and human islets ex vivo treated with selective agonists and antagonists for EP3 (sulprostone and DG-041, respectively) and EP4 (CAY10598 and L-161,982, respectively). β-cell survival was measured in mouse and human islets treated with the EP3- and EP4-selective ligands in conjunction with a cytokine cocktail to induce cell death. Changes in gene expression and protein phosphorylation were analyzed in response to modulation of EP3 and EP4 activity in mouse islets.
Results: Blockade of EP3 enhanced β-cell proliferation in young, but not old, mouse islets in part through phospholipase C (PLC)-γ1 activity. Blocking EP3 also increased human β-cell proliferation. EP4 modulation had no effect on ex vivo proliferation alone. However, blockade of EP3 in combination with activation of EP4 enhanced human, but not mouse, β-cell proliferation. In both mouse and human islets, EP3 blockade or EP4 activation enhanced β-cell survival in the presence of cytokines. EP4 acts in a protein kinase A (PKA)-dependent manner to increase mouse β-cell survival. In addition, the positive effects of FoxM1 activation on β-cell survival are inhibited by EP3 and dependent on EP4 signaling.
Conclusions: Our results identify EP3 and EP4 as novel regulators of β-cell proliferation and survival in mouse and human islets ex vivo.[Hide abstract]
|Role of nutrients and mTOR signaling in the regulation of pancreatic progenitors developmentThe “thrifty phenotype hypothesis” proposes that poor fetal and infant growth increases the risk of type 2 diabetes and metabolic syndrome. Although the mechanisms are not completely clear, nutrient availability is essential for proper cell growth and survival, and nutritional insults early in life are believed to have a deleterious effect on the developmental program of the pancreas and ultimately impair islet function. Elghazi et al. determine the effect of different nutrients and growth factors on the control of embryonic pancreatic growth and development. In addition, they identify an important role of mTOR signaling on these processes.|
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Objective: Poor fetal nutrition increases the risk of type 2 diabetes in the offspring at least in part by reduced embryonic β-cell growth and impaired function. However, it is not entirely clear how fetal nutrients and growth factors impact β-cells during development to alter glucose homeostasis and metabolism later in life. The current experiments aimed to test the impact of fetal nutrients and growth factors on endocrine development and how these signals acting on mTOR signaling regulate β-cell mass and glucose homeostasis.
Method: Pancreatic rudiments in culture were used to study the role of glucose, growth factors, and amino acids on β-cell development. The number and proliferation of pancreatic and endocrine progenitor were assessed in the presence or absence of rapamycin. The impact of mTOR signaling in vivo on pancreas development and glucose homeostasis was assessed in models deficient for mTOR or Raptor in Pdx1 expressing pancreatic progenitors.
Results: We found that amino acid concentrations, and leucine in particular, enhance the number of pancreatic and endocrine progenitors and are essential for growth factor induced proliferation. Rapamycin, an mTORC1 complex inhibitor, reduced the number and proliferation of pancreatic and endocrine progenitors. Mice lacking mTOR in pancreatic progenitors exhibited hyperglycemia in neonates, hypoinsulinemia and pancreatic agenesis/hypoplasia with pancreas rudiments containing ductal structures lacking differentiated acinar and endocrine cells. In addition, loss of mTORC1 by deletion of raptor in pancreatic progenitors reduced pancreas size with reduced number of β-cells.
Conclusions: Together, these results suggest that amino acids concentrations and in particular leucine modulates growth responses of pancreatic and endocrine progenitors and that mTOR signaling is critical for these responses. Inactivation of mTOR and raptor in pancreatic progenitors suggested that alterations in some of the components of this pathway during development could be a cause of pancreatic agenesis/hypoplasia and hyperglycemia.[Hide abstract]
|Insulin controls food intake and energy balance via NPY neurons Insulin plays a critical role in the short- and long-term control of glucose and energy homeostasis. While the functions of insulin signaling in peripheral tissues have been studied extensively, its actions in the central nervous system are far less understood. Loh et al. generated a variety of models by selectively removing insulin signaling in NPY neurons in flies and mice and tested the consequences on energy homeostasis. Their results suggest that insulin signaling in NPY neurons is required for maintaining normal energy homeostasis and that an impaired insulin-NPY signaling pathway may contribute to the development of metabolic diseases.|
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Objective: Insulin signaling in the brain has been implicated in the control of satiety, glucose homeostasis and energy balance. However, insulin signaling is dispensable in energy homeostasis controlling AgRP or POMC neurons and it is unclear which other neurons regulate these effects. Here we describe an ancient insulin/NPY neuronal network that governs energy homeostasis across phyla.
Methods: To address the role of insulin action specifically in NPY neurons, we generated a variety of models by selectively removing insulin signaling in NPY neurons in flies and mice and testing the consequences on energy homeostasis.
Results: By specifically targeting the insulin receptor in both fly and mouse NPY expressing neurons, we found NPY-specific insulin signaling controls food intake and energy expenditure, and lack of insulin signaling in NPY neurons leads to increased energy stores and an obese phenotype. Additionally, the lack of insulin signaling in NPY neurons leads to a dysregulation of GH/IGF-1 axis and to altered insulin sensitivity.
Conclusions: Taken together, these results suggest that insulin actions in NPY neurons is critical for maintaining energy balance and an impairment of this pathway may be causally linked to the development of metabolic diseases.[Hide abstract]
|Restoration of Lepr in β cells does not prevent hyperinsulinemia and hyperglycemiaIn vitro studies have demonstrated that Leptin receptor (Lepr) is expressed in murine and human pancreatic β cells, as well as in β cell lines. Previous studies have attempted to assess whether leptin has a direct action on β cell function, but the results are contradictory. D’souza and Kieffer selectively restored Lepr in pancreatic β cells of Lepr knockout mice. This did not prevent the development of hyperinsulinemia and β cell expansion. These findings suggest that leptin receptor signaling directly in β cells does not regulate β cell function. The authors propose that actions of leptin on β cells are secondary to action of leptin signaling in other tissues.|
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Objective: The adipose-derived hormone leptin plays an important role in regulating body weight and glucose homeostasis. Leptin receptors are expressed in the central nervous system as well as peripheral tissues involved in regulating glucose homeostasis, including insulin-producing β cells of the pancreas. Previous studies assessing the role of leptin receptors in β cells used Cre-loxP to disrupt the leptin receptor gene (Lepr) in β cells, but variable results were obtained. Furthermore, recombination of Lepr was observed in the hypothalamus or exocrine pancreas, in addition to the β cells, and Lepr in non-β cells may have compensated for the loss of Lepr in β cells, thus making it difficult to assess the direct effects of Lepr in β cells. To determine the significance of Lepr exclusively in β cells, we chose to selectively restore Lepr in β cells of Lepr null mice (LeprloxTB/loxTB).
Materials and methods: We used a mouse model in which endogenous expression of Lepr was disrupted by a loxP-flanked transcription blocker (LeprloxTB/loxTB), but was restored by Cre recombinase knocked into the Ins1 gene, which is specifically expressed in β cells (Ins1Cre). We bred LeprloxTB/loxTB and Ins1Cre mice to generate LeprloxTB/loxTB and LeprloxTB/loxTB Ins1Cre mice, as well as Leprwt/wt and Leprwt/wt Ins1Cre littermate mice. Male and female mice were weighed weekly between 6 and 11 weeks of age and fasting blood glucose was measured during this time. Oral glucose was administered to mice aged 7–12 weeks to assess glucose tolerance and insulin secretion. Relative β and α cell area and islet size were also assessed by immunostaining and analysis of pancreas sections of 12–14 week old mice.
Results: Male and female LeprloxTB/loxTB mice, lacking whole-body expression of Lepr, had a phenotype similar to db/db mice characterized by obesity, hyperinsulinemia, glucose intolerance, and impaired glucose stimulated insulin secretion. Despite restoring Lepr in β cells of LeprloxTB/loxTB mice, fasting insulin levels, blood glucose levels and body weight were comparable between LeprloxTB/loxTB Ins1Cre mice and LeprloxTB/loxTB littermates. Furthermore, glucose tolerance and insulin secretion in male and female LeprloxTB/loxTB Ins1Cre mice were similar to that observed in LeprloxTB/loxTB mice. Analysis of pancreatic insulin positive area revealed that restoration of Lepr in β cells of LeprloxTB/loxTB mice did not prevent hyperplasia of insulin positive cells nor did it rescue Glut-2 expression.
Conclusion: Collectively, these data suggest that direct action of leptin on β cells is insufficient to restore normal insulin secretion and glucose tolerance in mice without leptin receptor signaling elsewhere.[Hide abstract]
|Endocannabinoid-dependent disinhibition of orexinergic neuronsOrexin (OX) neurons have been implicated in a variety of functions, including energy balance and food seeking and intake. It was recently demonstrated that chronic leptin deficiency or resistance in ob/ob and in high-fat diet (HFD) fed mice, respectively, causes a morphological switch from predominantly excitatory to predominantly inhibitory innervation of OX neurons. As a consequence, OX neurons of ob/ob and HFD animals are disinhibited and release more OX in target brain area. Becker and colleagues show that the functional excitatory innervation is reduced, although modestly, in ob/ob compared to wild type mice. In ob/ob mice, chronic absence of leptin induces a 2-arachidonoylglycerol (2-AG) mediated functional disinhibition of OX neurons. This helps explain the increase of OX production and, consequently, the excessive food intake of ob/ob mice.
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Objectives: In the ob/ob mouse model of obesity, chronic absence of leptin causes a significant increase of orexin (OX) production by hypothalamic neurons and excessive food intake. The altered OX level is linked to a dramatic increase of the inhibitory innervation of OX producing neurons (OX neurons) and the over expression of the endocannabinoid 2-arachidonoylglycerol (2-AG) by OX neurons of ob/ob mice. Little is known about the function of the excitatory synapses of OX neurons in ob/ob mice, and their modulation by 2-AG. In the present study, we fill this gap and provide the first evidence of the overall level of activation of OX neurons in the ob/ob mice.
Methods: We performed in vitro whole-cell patch-clamp recordings on OX neurons located in the perifornical area of the lateral hypothalamus in acute brain slices of wt and ob/ob mice. We identified OX neurons on the basis of their electrophysiological membrane properties, with 96% of concordance with immunohistochemisty.
Results: We found that OX neurons of ob/ob mice are innervated by less efficient and fewer excitatory synapses than wt mice. Consequently, ob/ob OX neurons show more negative resting membrane potential and lower action potential firing frequency than wt. The bath application of the cannabinoid type-1 receptor agonist WIN55,212-2, depresses both the excitatory and the inhibitory synapses in ob/ob animals, but only the excitatory synapses in wt animals. Finally, the physiologic release of 2-AG induces a prevalent depression of inhibition (disinhibition) of OX neurons in ob/ob animals but not in wt.
Conclusions: In ob/ob mice, chronic absence of leptin induces a 2-AG mediated functional disinhibition of OX neurons. This helps explain the increase of OX production and, consequently, the excessive food intake of ob/ob mice.[Hide abstract]
|FGF21 resistance is not mediated by downregulation of beta-klotho expressionFibroblast growth factor 21 (FGF21) is an important regulator of nutrient and energy homeostasis. FGF21 signals to target cells through a receptor complex consisting of the FGF receptor (FGFR), FGFR1c, and a co-receptor termed β-klotho. Elevated FGF21 levels during obesity have led to the postulation that obesity is a “FGF21-resistant state”. Markan and colleagues show that while β-klotho expression in white adipose tissue is markedly reduced during diet-induced obesity, maintenance of β-klotho expression in adipose tissue does not increase FGF21 sensitivity or significantly improve metabolic parameters during obesity.|
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Objective: Fibroblast growth factor 21 (FGF21) is an endocrine hormone that regulates metabolic homeostasis. Previous work has suggested that impairment of FGF21 signaling in adipose tissue may occur through downregulation of the obligate FGF21 co-receptor, β-klotho, which leads to “FGF21 resistance” during the onset of diet-induced obesity. Here, we sought to determine whether maintenance of β-klotho expression in adipose tissue prevents FGF21 resistance and whether other mechanisms also contribute to FGF21 resistance in vivo.
Methods: We generated adipose-specific β-klotho transgenic mice to determine whether maintenance of β-klotho expression in adipose tissue prevents FGF21 resistance in vivo.
Results: β-klotho protein levels are markedly decreased in white adipose tissue, but not liver or brown adipose tissue, during diet-induced obesity. Maintenance of β-klotho protein expression in adipose tissue does not alleviate impaired FGF21 signaling in white adipose or increase FGF21 sensitivity in vivo.
Conclusions: In white adipose tissue, downregulation of β-klotho expression is not the major mechanism contributing to impaired FGF21 signaling in white adipose tissue.[Hide abstract]
|β3-Adrenergically induced glucose uptake in brown adipose tissue is independent of UCP1
Brown adipose tissue (BAT) can combust (surplus) energy through uncoupled respiration mediated by uncoupling protein 1 (UCP1). A strong pattern of BAT glucose uptake is seen in positron emission scanning tomography (PET) scans after activation of β3-adrenergic receptors. Olsen, Csikasz, and colleagues found that acute β3-adrenergic stimulation induces glucose uptake in BAT independently of the presence and activation of UCP1 and thus independently of thermogenesis. These results demonstrate that acute glucose uptake is a mechanism separate from thermogenesis.|
Abstract | PDF
Objective: Today, the presence and activity of brown adipose tissue (BAT) in adult humans is generally equated with the induced accumulation of [2-18F]2-fluoro-2-deoxy-d-glucose ([18F]FDG) in adipose tissues, as investigated by positron emission tomography (PET) scanning. In reality, PET-FDG is currently the only method available for in vivo quantification of BAT activity in adult humans. The underlying assumption is that the glucose uptake reflects the thermogenic activity of the tissue.
Methods: To examine this basic assumption, we here followed [18F]FDG uptake by PET and by tissue [3H]-2-deoxy-d-glucose uptake in wildtype and UCP1(−/−) mice, i.e. in mice that do or do not possess the unique thermogenic and calorie-consuming ability of BAT.
Results: Unexpectedly, we found that β3-adrenergically induced (by CL-316,243) glucose uptake was UCP1-independent. Thus, whereas PET-FDG scans adequately reflect glucose uptake, this acute glucose uptake is not secondary to thermogenesis but is governed by an independent cellular signalling, here demonstrated to be mediated via the previously described KU-0063794-sensitive mTOR pathway.
Conclusions: Thus, PET-FDG scans do not exclusively reveal active BAT deposits but rather any tissue possessing an adrenergically-mediated glucose uptake pathway. In contrast, we found that the marked glucose uptake-ameliorating effect of prolonged β3-adrenergic treatment was UCP1 dependent. Thus, therapeutically, UCP1 activity is required for any anti-diabetic effect of BAT activation.[Hide abstract]