Featured ArticlesVolume 5 | No. 8 | August 2016
|An ancestral role for the mitochondrial pyruvate carrierMitochondrial pyruvate import is mediated by two proteins, the Mitochondrial Pyruvate Carrier 1 and 2 (MPC1 and MPC2), which form a hetero-oligomeric complex in the inner mitochondrial membrane. McCommis, Hodges, Bricker et al. demonstrate that the MPC plays a central role in glucose-stimulated insulin secretion (GSIS) and systemic glucose homeostasis. MPC deficiency in Drosophila or the β-cells of mice leads to elevated blood glucose concentrations, glucose intolerance, and reduced GSIS. Pancreas-specific MPC deficiency results in impaired islet glucose metabolism and KATP channel hyperactivity. The authors show that glucose increases MPC activity in cultured INS-1 cells in a concentration-dependent manner, suggesting that glucose sensing is coupled to mitochondrial pyruvate transport and utilization to support efficient GSIS.
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Objective: Transport of pyruvate into the mitochondrial matrix by the Mitochondrial Pyruvate Carrier (MPC) is an important and rate-limiting step in its metabolism. In pancreatic β-cells, mitochondrial pyruvate metabolism is thought to be important for glucose sensing and glucose-stimulated insulin secretion.
Methods: To evaluate the role that the MPC plays in maintaining systemic glucose homeostasis, we used genetically-engineered Drosophila and mice with loss of MPC activity in insulin-producing cells.
Results: In both species, MPC deficiency results in elevated blood sugar concentrations and glucose intolerance accompanied by impaired glucose-stimulated insulin secretion. In mouse islets, β-cell MPC-deficiency resulted in decreased respiration with glucose, ATP-sensitive potassium (KATP) channel hyperactivity, and impaired insulin release. Moreover, treatment of pancreas-specific MPC knockout mice with glibenclamide, a sulfonylurea KATP channel inhibitor, improved defects in islet insulin secretion and abnormalities in glucose homeostasis in vivo. Finally, using a recently-developed biosensor for MPC activity, we show that the MPC is rapidly stimulated by glucose treatment in INS-1 insulinoma cells suggesting that glucose sensing is coupled to mitochondrial pyruvate carrier activity.
Conclusions: Altogether, these studies suggest that the MPC plays an important and ancestral role in insulin-secreting cells in mediating glucose sensing, regulating insulin secretion, and controlling systemic glycemia.[Hide abstract]
|miR-125b affects mitochondrial biogenesis and impairs brite adipocyte formationBrown adipose tissue (BAT) dissipates energy in the form of heat by uncoupling the mitochondrial respiratory chain from ATP synthesis. Also, white adipose tissue contains thermogenic fat cells called “brown-inwhite” (“brite”), and “beige” or inducible brown adipocytes. Giroud and colleagues analyze the role of miR-125b-5p in the browning/britening of white adipocytes in human and murine cell models and tissues. The authors show that miR-125b-5p plays an important role in the modulation of brite and brown adipocyte function by targeting mitochondriogenesis and oxygen consumption.|
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Objective: In rodents and humans, besides brown adipose tissue (BAT), islands of thermogenic adipocytes, termed “brite” (brown-in-white) or beige adipocytes, emerge within white adipose tissue (WAT) after cold exposure or β3-adrenoceptor stimulation, which may protect from obesity and associated diseases. microRNAs are novel modulators of adipose tissue development and function. The purpose of this work was to characterize the role of microRNAs in the control of brite adipocyte formation.
Methods/Results: Using human multipotent adipose derived stem cells, we identified miR-125b-5p as downregulated upon brite adipocyte formation. In humans and rodents, miR-125b-5p expression was lower in BAT than in WAT. In vitro, overexpression and knockdown of miR-125b-5p decreased and increased mitochondrial biogenesis, respectively. In vivo, miR-125b-5p levels were downregulated in subcutaneous WAT and interscapular BAT upon β3-adrenergic receptor stimulation. Injections of an miR-125b-5p mimic and LNA inhibitor directly into WAT inhibited and increased β3-adrenoceptor-mediated induction of UCP1, respectively, and mitochondrial brite adipocyte marker expression and mitochondriogenesis.
Conclusions: Collectively, our results demonstrate that miR-125b-5p plays an important role in the repression of brite adipocyte function by modulating oxygen consumption and mitochondrial gene expression.[Hide abstract]
|Lipolysis sensation by white fat afferent nerves triggers brown fat thermogenesisAdipose tissue is connected to the central nervous system via afferent and efferent nerve fibers directly innervating each fat depot. Garretson and colleagues investigate if white adipose tissue (WAT) lipolysis and/or its products are sufficient to drive spinal afferent nerves receiving sensory input from subcutaneous inguinal WAT. Their findings illustrate a functional neural connectivity between WAT and brown adipose tissue (BAT) that acutely induces BAT thermogenesis.|
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Objective: Metabolic challenges, such as a cold environment, stimulate sympathetic neural efferent activity to white adipose tissue (WAT) to drive lipolysis, thereby increasing the availability of free fatty acids as one source of fuel for brown adipose tissue (BAT) thermogenesis. WAT is also innervated by sensory nerve fibers that network to metabolic brain areas; moreover, activation of these afferents is reported to increase sympathetic nervous system outflow. However, the endogenous stimuli sufficient to drive WAT afferents during metabolic challenges as well as their functional relation to BAT thermogenesis remain unknown.
Methods: We tested if local WAT lipolysis directly activates WAT afferent nerves, and then assessed whether this WAT sensory signal affected BAT thermogenesis in Siberian hamsters (Phodopus sungorus).
Results: 2-deoxyglucose, a sympathetic nervous system stimulant, caused β-adrenergic receptor dependent increases in inguinal WAT (IWAT) afferent neurophysiological activity. In addition, direct IWAT injections of the β3-AR agonist CL316,243 dose-dependently increased: 1) phosphorylation of IWAT hormone sensitive lipase, an indicator of SNS-stimulated lipolysis, 2) expression of the neuronal activation marker c-Fos in dorsal root ganglion neurons receiving sensory input from IWAT, and 3) IWAT afferent neurophysiological activity, an increase blocked by antilipolytic agent 3,5-dimethylpyrazole. Finally, we demonstrated that IWAT afferent activation by lipolysis triggers interscapular BAT thermogenesis through a neural link between these two tissues.
Conclusions: These data suggest IWAT lipolysis activates local IWAT afferents triggering a neural circuit from WAT to BAT that acutely induces BAT thermogenesis.[Hide abstract]
|Demonstration of a day-night rhythm in human skeletal muscle oxidative capacityMany metabolic processes are synchronized to day-night cycles by the circadian clock, thereby anticipating changes in metabolic activity associated with feeding or fasting and physical activity or rest. Van Moorsel, Hansen et al. demonstrate the presence of a profound day-night rhythm in human skeletal muscle mitochondrial oxidative capacity. Peak oxidative capacity and highest resting energy expenditure coincide at the end of the day, partly matching the time phase of peak physical performance as described in the literature. In addition, the authors found significant variations over time in proteins involved in mitochondrial dynamics, possibly linking the circadian clock with mitochondrial metabolism.|
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Objective: A disturbed day-night rhythm is associated with metabolic perturbations that can lead to obesity and type 2 diabetes mellitus (T2DM). In skeletal muscle, a reduced oxidative capacity is also associated with the development of T2DM. However, whether oxidative capacity in skeletal muscle displays a day-night rhythm in humans has so far not been investigated.
Methods: Lean, healthy subjects were enrolled in a standardized living protocol with regular meals, physical activity and sleep to reflect our everyday lifestyle. Mitochondrial oxidative capacity was examined in skeletal muscle biopsies taken at five time points within a 24-hour period.
Results: Core-body temperature was lower during the early night, confirming a normal day-night rhythm. Skeletal muscle oxidative capacity demonstrated a robust day-night rhythm, with a significant time effect in ADP-stimulated respiration (state 3 MO, state 3 MOG and state 3 MOGS, p < 0.05). Respiration was lowest at 1 PM and highest at 11 PM (state 3 MOGS: 80.6 ± 4.0 vs. 95.8 ± 4.7 pmol/mg/s). Interestingly, the fluctuation in mitochondrial function was also observed in whole-body energy expenditure, with peak energy expenditure at 11 PM and lowest energy expenditure at 4 AM (p < 0.001). In addition, we demonstrate rhythmicity in mRNA expression of molecular clock genes in human skeletal muscle.
Conclusions: Our results suggest that the biological clock drives robust rhythms in human skeletal muscle oxidative metabolism. It is tempting to speculate that disruption of these rhythms contribute to the deterioration of metabolic health associated with circadian misalignment.[Hide abstract]
|mTORC2 and AMPK differentially regulate muscle triglyceride content via Perilipin 3The mammalian target of rapamycin (mTOR) is a serine/threonine protein kinase that is found in two distinct mTOR complexes, mTOR complex 1 and mTOR complex 2 (mTORC2). Kleinert and colleagues characterize new aspects of mTORC2 and AMPK biology in muscle lipid metabolism by showing that mTORC2 and AMPK are negative and positive regulators of lipid storage, respectively, via Perilipin 3. In addition, lack of mTORC2 activity in muscle causes a greater reliance on fat as an energy substrate and repartitioning of lean to fat mass.|
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Objective: We have recently shown that acute inhibition of both mTOR complexes (mTORC1 and mTORC2) increases whole-body lipid utilization, while mTORC1 inhibition had no effect. Therefore, we tested the hypothesis that mTORC2 regulates lipid metabolism in skeletal muscle.
Methods: Body composition, substrate utilization and muscle lipid storage were measured in mice lacking mTORC2 activity in skeletal muscle (specific knockout of RICTOR (Ric mKO)). We further examined the RICTOR/mTORC2-controlled muscle metabolome and proteome; and performed follow-up studies in other genetic mouse models and in cell culture.
Results: Ric mKO mice exhibited a greater reliance on fat as an energy substrate, a re-partitioning of lean to fat mass and an increase in intramyocellular triglyceride (IMTG) content, along with increases in several lipid metabolites in muscle. Unbiased proteomics revealed an increase in the expression of the lipid droplet binding protein Perilipin 3 (PLIN3) in muscle from Ric mKO mice. This was associated with increased AMPK activity in Ric mKO muscle. Reducing AMPK kinase activity decreased muscle PLIN3 expression and IMTG content. AMPK agonism, in turn, increased PLIN3 expression in a FoxO1 dependent manner. PLIN3 overexpression was sufficient to increase triglyceride content in muscle cells.
Conclusions: We identified a novel link between mTORC2 and PLIN3, which regulates lipid storage in muscle. While mTORC2 is a negative regulator, we further identified AMPK as a positive regulator of PLIN3, which impacts whole-body substrate utilization and nutrient partitioning.[Hide abstract]
|The F-actin modifier villin regulates insulin granule dynamics and exocytosisThe islet cell autoantigen 512 (Ica512) tethers insulin secretory granules (SGs) to actin microfilaments. Mziaut and colleagues analyze the gene expression profile of Ica512-depleted mouse islets. They show that Ica512 depletion leads to downregulation of the F-actin modifier villin in β cells, increasing the size of actin cages surrounding cortical SGs and thus their motility and exocytosis in basal conditions while reducing glucose-stimulated insulin release. The data indicate that villin is key for tight control of insulin SG mobility and exocytosis.|
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Objective: Insulin release from pancreatic islet ß cells should be tightly controlled to avoid hypoglycemia and insulin resistance. The cortical actin cytoskeleton is a gate for regulated exocytosis of insulin secretory granules (SGs) by restricting their mobility and access to the plasma membrane. Prior studies suggest that SGs interact with F-actin through their transmembrane cargo islet cell autoantigen 512 (Ica512) (also known as islet antigen 2/Ptprn). Here we investigated how Ica512 modulates SG trafficking and exocytosis.
Methods: Transcriptomic changes in Ica512 -/- mouse islets were analyzed. Imaging as well as biophysical and biochemical methods were used to validate if and how the Ica512-regulated gene villin modulates insulin secretion in mouse islets and insulinoma cells.
Results: The F-actin modifier villin was consistently downregulated in Ica512 -/-mouse islets and in Ica512-depleted insulinoma cells. Villin was enriched at the cell cortex of ß cells and dispersed villin -/- islet cells were less round and less deformable. Basal mobility of SGs in villin-depleted cells was enhanced. Moreover, in cells depleted either of villin or Ica512 F-actin cages restraining cortical SGs were enlarged, basal secretion was increased while glucose-stimulated insulin release was blunted. The latter changes were reverted by overexpressing villin in Ica512-depleted cells, but not vice versa.
Conclusions: Our findings show that villin controls the size of the F-actin cages restricting SGs and, thus, regulates their dynamics and availability for exocytosis. Evidence that villin acts downstream of Ica512 also indicates that SGs directly influence the remodeling properties of the cortical actin cytoskeleton for tight control of insulin secretion.[Hide abstract]
|Leptin and insulin engage specific PI3K subunits in hypothalamic SF1 neuronsSteroidogenic factor 1 (SF1)-expressing neurons within the ventromedial nucleus of hypothalamus (VMH SF1 neurons) regulate energy balance and glucose homeostasis. Sohn and colleagues characterize the acute effects of leptin and insulin on VMH SF1 neurons and identify the role of specific phosphatidylinositol-3-kinase (PI3K) catalytic subunit isoforms (p110a and p110b) in this response. They provide further evidence that SF1 neurons are heterogeneous in their acute response to leptin and insulin and demonstrate specificity in receptor-enzyme-target ion channel. |
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Objective: The ventromedial hypothalamic nucleus (VMH) regulates energy balance and glucose homeostasis. Leptin and insulin exert metabolic effects via their cognate receptors expressed by the steroidogenic factor 1 (SF1) neurons within the VMH. However, detailed cellular mechanisms involved in the regulation of these neurons by leptin and insulin remain to be identified.
Methods: We utilized genetically-modified mouse models and performed patch-clamp electrophysiology experiments to resolve this issue.
Results: We identified distinct populations of leptin-activated and leptin-inhibited SF1 neurons. In contrast, insulin uniformly inhibited SF1 neurons. Notably, we found that leptin-activated, leptin-inhibited, and insulin-inhibited SF1 neurons are distinct subpopulations within the VMH. Leptin depolarization of SF1 neuron also required the PI3K p110β catalytic subunit. This effect was mediated by the putative transient receptor potential C (TRPC) channel. On the other hand, hyperpolarizing responses of SF1 neurons by leptin and insulin required either of the p110a or p110β catalytic subunits, and were mediated by the putative ATP-sensitive K+ (KATP) channel.
Conclusions: Our results demonstrate that specific PI3K catalytic subunits are responsible for the acute effects of leptin and insulin on VMH SF1 neurons, and provide insights into the cellular mechanisms of leptin and insulin action on VMH SF1 neurons that regulate energy balance and glucose homeostasis.[Hide abstract]
|Sexually dimorphic brain fatty acid composition in low and high fat diet-fed miceRodriguez-Navas and colleagues demonstrate that male and female brains of mice differ in their fatty acid composition even when fed a regular/chow diet. Additionally, when chronically exposed to a Western-style high fat diet, this sexual dimorphism continues to exist. Males have an increased percentage of saturated fatty acids and reductions in ω6-polyunsaturated fatty acids (ω6-PUFAs) when compared to females. The findings of the authors suggest that ω6-PUFA linoleic acid, which is higher in female brains, has an anti-inflammatory role and reduces palmitic acid-induced inflammation.|
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Objective: In this study, we analyzed the fatty acid profile of brains and plasma from male and female mice fed chow or a western-style high fat diet (WD) for 16 weeks to determine if males and females process fatty acids differently. Based on the differences in fatty acids observed in vivo, we performed in vitro experiments on N43 hypothalamic neuronal cells to begin to elucidate how the fatty acid milieu may impact brain inflammation.
Methods: Using a comprehensive mass spectrometry fatty acid analysis, which includes a profile for 52 different fatty acid isomers, we assayed the plasma and brain fatty acid composition of age-matched male and female mice maintained on chow or a WD. Additionally, using the same techniques, we determined the fatty acid composition of N43 hypothalamic cells following exposure to palmitic and linoleic acid, alone or in combination.
Results: Our data demonstrate there is a sexual dimorphism in brain fatty acid content both following the consumption of the chow diet, as well as the WD, with males having an increased percentage of saturated fatty acids and reductions in ω6-polyunsaturated fatty acids when compared to females. Interestingly, we did not observe a sexual dimorphism in fatty acid content in the plasma of the same mice. Furthermore, exposure of N43 cells to the ω6-PUFA linoleic acid, which is higher in female brains when compared to males, reduces palmitic acid-induced inflammation.
Conclusions: Our data suggest male and female brains, and not plasma, differ in their fatty acid profile. This is the first time, to our knowledge, lipidomic analyses has been used to directly test the hypothesis there is a sexual dimorphism in brain and plasma fatty acid composition following consumption of the chow diet, as well as following exposure to the WD.[Hide abstract]
|Fibroblast growth factor 21 has no direct role in regulating fertility in female miceFibroblast growth factor 21 (FGF21) has been proposed as a regulator of fertility in rodents based on the finding that female mice overexpressing FGF21 to supraphysiologic levels (FGF21-Tg) have delayed onset of puberty and remain infertile through adulthood. Singhal et al. confirm that FGF21-Tg mice consuming a chow diet are indeed infertile. In contrast to this finding, the authors observe that female mice consuming a ketogenic diet (KD), a model with dramatic increases in hepatic synthesis of FGF21 and elevated serum levels, demonstrate normal estrus cycles and remain fertile. Further, infusion of FGF21 into the lateral ventricle has no effect on fertility in wild type (WT) female mice. Mice lacking FGF21 (FGF21-KO) have the same response as WT animals to the fasting induced suppression of cycling. High fat diet feeding to FGF21-Tg mice restores fertility. Together, these results render unlikely the claim that FGF21 has a physiologically relevant influence on fertility in female mice via action on the gonadotropic axis.|
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Objective: Reproduction is an energetically expensive process. Insufficient calorie reserves, signaled to the brain through peripheral signals such as leptin, suppress fertility. Recently, fibroblast growth factor 21 (FGF21) was implicated as a signal from the liver to the hypothalamus that directly inhibits the hypothalamic–gonadotropin axis during fasting and starvation. However, FGF21 itself increases metabolic rate and can induce weight loss, which suggests that the effects of FGF21 on fertility may not be direct and may reflect changes in energy balance.
Methods: To address this important question, we evaluated fertility in several mouse models with elevated FGF21 levels including ketogenic diet fed mice, fasted mice, mice treated with exogenous FGF21 and transgenic mice over-expressing FGF21.
Results: We find that ketogenic diet fed mice remain fertile despite significant elevation in serum FGF21 levels. Absence of FGF21 does not alter transient infertility induced by fasting. Centrally infused FGF21 does not suppress fertility despite its efficacy in inducing browning of inguinal white adipose tissue. Furthermore, a high fat diet (HFD) can restore fertility of female FGF21-overexpressing mice, a model of growth restriction, even in the presence of supraphysiological serum FGF21 levels.
Conclusions: We conclude that FGF21 is not a direct physiological regulator of fertility in mice. The infertility observed in FGF21 overexpressing mice is likely driven by the increased energy expenditure and consequent excess calorie requirements resulting from high FGF21 levels.[Hide abstract]
|Male-lineage transmission of an acquired metabolic phenotypeOffspring phenotypes can be affected by parental health or parental environmental exposures, independent of variation in the inherited DNA sequence. Cropley, Eaton et al. demonstrate transgenerational inheritance of an acquired metabolic trait in mammals. The data of the authors implicate small RNAs in the mechanism of inheritance. If the phenomenon observed here translates to humans, the findings suggest that individuals with obese paternal ancestry could avoid the deleterious effects of metabolic programming with dietary intervention, yet still propagate the propensity for metabolic dysfunction to their offspring. |
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Objective: Parental obesity can induce metabolic phenotypes in offspring independent of the inherited DNA sequence. Here we asked whether such non-genetic acquired metabolic traits can be passed on to a second generation that has never been exposed to obesity, even as germ cells.
Methods: We examined the F1, F2, and F3 a/a offspring derived from F0 matings of obese prediabetic Avy/a sires and lean a/a dams. After F0, only lean a/a mice were used for breeding.
Results: We found that F1 sons of obese founder males exhibited defects in glucose and lipid metabolism, but only upon a post-weaning dietary challenge. F1 males transmitted these defects to their own male progeny (F2) in the absence of the dietary challenge, but the phenotype was largely attenuated by F3. The sperm of F1 males exhibited changes in the abundance of several small RNA species, including the recently reported diet-responsive tRNA-derived fragments.
Conclusions: These data indicate that induced metabolic phenotypes may be propagated for a generation beyond any direct exposure to an inducing factor. This non-genetic inheritance likely occurs via the actions of sperm noncoding RNA.[Hide abstract]
|Optogenetic activation of leptin- and glucose-regulated GABAergic neurons in dorsomedial hypothalamus promotes food intakeThe dorsomedial hypothalamus (DMH) is considered an orexigenic nucleus as a lesion to the DMH reduces food intake and body weight and induces resistance against diet-induced obesity. Otgon-Uul, Suyama et al. show that DMH GABAergic neurons are hyperpolarized by leptin and depolarized by lowering glucose. Further, the optogenetic activation of DMH GABAergic neurons elicits inhibitory synaptic transmission to the paraventricular nucleus of hypothalamus (PVN), where anorexigenic neurons are localized, and promotes food intake.|
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Objective: The dorsomedial hypothalamus (DMH) has been considered an orexigenic nucleus, since the DMH lesion reduced food intake and body weight and induced resistance to diet-induced obesity. The DMH expresses feeding regulatory neuropeptides and receptors including neuropeptide Y (NPY), cocaine- and amphetamine-regulated transcript (CART), cholecystokinin (CCK), leptin receptor, and melanocortin 3/4 receptors. However, the principal neurons generating the orexigenic function in the DMH remain to be defined. This study aimed to clarify the role of the DMH GABAergic neurons in feeding regulation by using optogenetics and electrophysiological techniques.
Methods: We generated the mice expressing ChRFR-C167A, a bistable chimeric channelrhodopsin, selectively in GABAergic neurons of DMH via locally injected adeno-associated virus 2. Food intake after optogenetic activation of DMH GABAergic neurons was measured. Electrophysiological properties of DMH GABAergic neurons were measured using slice patch clamp.
Results: Optogenetic activation of DMH GABAergic neurons promoted food intake. Leptin hyperpolarized and lowering glucose depolarized half of DMH GABAergic neurons, suggesting their orexigenic property. Optical activation of axonal terminals of DMH GABAergic neurons at the paraventricular nucleus of hypothalamus (PVN), where anorexigenic neurons are localized, increased inhibitory postsynaptic currents on PVN neurons and promoted food intake.
Conclusions: DMH GABAergic neurons are regulated by metabolic signals leptin and glucose and, once activated, promote food intake via inhibitory synaptic transmission to PVN.[Hide abstract]
|The role of autonomic efferents and uncoupling protein 1 in the glucose-lowering effect of leptin therapyLeptin can reverse hyperglycemia in rodent models of type 1 diabetes, independent of food intake and without raising circulating insulin levels. Denroche and colleagues report that neither subdiaphragmatic vagotomy nor partial chemical sympathectomy attenuates the glucose-lowering action of leptin. In addition, the authors demonstrate that although leptin induces Ucp1 expression and thermogenesis in BAT of STZ-diabetic mice, diabetic Ucp1 knockout mice are not refractory to leptin therapy.|
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Objective: Leptin reverses hyperglycemia in rodent models of type 1 diabetes (T1D). Direct application of leptin to the brain can lower blood glucose in diabetic rodents, and can activate autonomic efferents and non-shivering thermogenesis in brown adipose tissue (BAT). We investigated whether leptin reverses hyperglycemia through a mechanism that requires autonomic innervation, or uncoupling protein 1 (UCP1)-mediated thermogenesis.
Methods: To examine the role of parasympathetic and sympathetic efferents in the glucose-lowering action of leptin, mice with a subdiaphragmatic vagotomy or 6-hydroxydopamine induced chemical sympathectomy were injected with streptozotocin (STZ) to induce hyperglycemia, and subsequently leptin treated. To test whether the glucose-lowering action of leptin requires activation of UCP1-mediated thermogenesis in BAT, we administered leptin in STZ-diabetic Ucp1 knockout (Ucp1-/-) mice and wildtype controls.
Results: Leptin ameliorated STZ-induced hyperglycemia in both intact and vagotomised mice. Similarly, mice with a partial chemical sympathectomy did not have an attenuated response to leptin-mediated glucose lowering relative to sham controls, and showed intact leptin-induced Ucp1 expression in BAT. Although leptin activated BAT thermogenesis in STZ-diabetic mice, the anti-diabetic effect of leptin was not blunted in Ucp1-/- mice.
Conclusions: These results suggest that leptin lowers blood glucose in insulin-deficient diabetes through a manner that does not require parasympathetic or sympathetic innervation, and thus imply that leptin lowers blood glucose through an alternative CNS-mediated mechanism or redundant target tissues. Furthermore, we conclude that the glucose lowering action of leptin is independent of UCP1-dependent thermogenesis.[Hide abstract]
|Microbially produced glucagon-like peptide 1 improves glucose tolerance in miceGlucagon-like peptide 1 (GLP-1) is released from enteroendocrine L-cells after food intake. It stimulates insulin release, reduces appetite, and slows down gastric emptying. Arora and colleagues demonstrate the efficacy of a Lactococcus lactis strain that is genetically modified to produce GLP-1, stimulating insulin secretion in isolated mouse islets and improving glucose tolerance in mice fed either chow or a high-fat diet (HFD). In summary, these findings provide evidence that recombinant L. lactis strains may have potential clinical applications.|
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Objective: The enteroendocrine hormone glucagon-like peptide 1 (GLP-1) is an attractive anti-diabetic therapy. Here, we generated a recombinant Lactococcus lactis strain genetically modified to produce GLP-1 and investigated its ability to improve glucose tolerance in mice on chow or high-fat diet (HFD).
Methods: We transformed L. lactis FI5876 with either empty vector (pUK200) or murine GLP-1 expression vector to generate LL-UK200 and LL-GLP1, respectively, and determined their potential to induce insulin secretion by incubating primary islets from wild-type (WT) and GLP-1 receptor knockout (GLP1R-KO) mice with culture supernatant of these strains. In addition, we administered these strains to mice on chow or HFD. At the end of the study period, we measured plasma GLP-1 levels, performed intraperitoneal glucose tolerance and insulin tolerance tests, and determined hepatic expression of the gluconeogenic genes G6pc and Pepck.
Results: Insulin release from primary islets of WT but not GLP1R-KO mice was higher following incubation with culture supernatant from LL-GLP1 compared with LL-UK200. In mice on chow, supplementation with LL-GLP1 versus LL-UK200 promoted increased vena porta levels of GLP-1 in both WT and GLP1R-KO mice; however, LL-GLP1 promoted improved glucose tolerance in WT but not in GLP1R-KO mice, indicating a requirement for the GLP-1 receptor. In mice on HFD and thus with impaired glucose tolerance, supplementation with LL-GLP1 versus LL-UK200 promoted a pronounced improvement in glucose tolerance together with increased insulin levels. Supplementation with LL-GLP1 versus LL-UK200 did not affect insulin tolerance but resulted in reduced expression of G6pc in both chow and HFD-fed mice.
Conclusions: The L. lactis strain genetically modified to produce GLP-1 is capable of stimulating insulin secretion from islets and improving glucose tolerance in mice.[Hide abstract]
|Glucagon receptor gene deletion in insulin knockout mice modestly reduces blood glucose and ketonesGlucagon suppression can reverse or prevent type 1 diabetes in rodents suggesting that dysregulated glucagon is also required for development of diabetic symptoms. Neumann and colleagues show that the absence of glucagon signalling modestly improves diabetic symptoms in insulin knockout mice, but this improvement is insufficient to promote survival. The authors provide unequivocal evidence that it is not possible to protect from the catabolic consequences due to the complete loss of insulin by eliminating glucagon action.|
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Objective: It has been thought that the depletion of insulin is responsible for the catabolic consequences of diabetes; however, evidence suggests that glucagon also plays a role in diabetes pathogenesis. Glucagon suppression by glucagon receptor (Gcgr) gene deletion, glucagon immunoneutralization, or Gcgr antagonist can reverse or prevent type 1 diabetes in rodents suggesting that dysregulated glucagon is also required for development of diabetic symptoms. However, the models used in these studies were rendered diabetic by chemical- or immune-mediated β-cell destruction, in which insulin depletion is incomplete. Therefore, it is unclear whether glucagon suppression could overcome the consequence of the complete lack of insulin.
Methods: To directly test this we characterized mice that lack the Gcgr and both insulin genes (GcgrKO/InsKO).
Results: In both P1 pups and mice that were kept alive to young adulthood using insulin therapy, blood glucose and plasma ketones were modestly normalized; however, mice survived for only up to 6 days, similar to GcgrHet/InsKO controls. In addition, Gcgr gene deletion was unable to normalize plasma leptin levels, triglycerides, fatty acids, or hepatic cholesterol accumulation compared to GcgrHet/InsKO controls.
Conclusions: Therefore, the metabolic manifestations associated with a complete lack of insulin cannot be overcome by glucagon receptor gene inactivation.[Hide abstract]