Featured ArticlesVolume 16 | October 2018
|Human metabolomics reveal daily variations under nutritional challengesMetabolomics is increasingly playing a role in tracking and diagnosing an individual’s health status. The identification of specific metabolomics ‘signatures’ in response to challenges such as a high fat diet, exercise, drug abuse, or diseases is important for the discovery of biomarkers. Sato, Parr, et al. analyzed the metabolome levels of human serum and skeletal muscle in the morning and evening in response to nutritional challenges in order to identify unique signatures. Their findings illustrate how the changes in metabolomics in response to nutritional challenges are dependent on the time of the day.
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Objective: Advances in the field of metabolomics and the concomitant development of bioinformatics tools constitute a promising avenue towards the development of precision medicine and personalized profiling for numerous disease states. Studies in animal models have strengthened this concept, but the application in human subjects is scarce.
Methods: Utilizing high-throughput metabolomics, we have analyzed the metabolome levels of human serum and skeletal muscle in the morning and evening in response to divergent nutritional challenges in order to identify unique signatures present in serum and muscle.
Results: We reveal dynamic daily variation of human metabolome unique to serum and muscle. The overall effect of nutritional challenges on the serum and muscle metabolome results in a profound rewiring of morning-evening metabolic profiles in human participants in response to the timing and type of dietary challenge.
Conclusion: We highlight time-of-day and meal-composition dependence of reprogramming of human metabolome by nutritional challenges.[Hide abstract]
|AMPK activation negatively regulates GDAP1, which influences skeletal muscle
Skeletal muscle is a major regulator of whole-body metabolic homeostasis. AMP-activated protein kinase (AMPK) is of particular importance for metabolic regulation in skeletal muscle. Transcriptomic studies reveal that ganglioside-induced differentiation-associated protein 1 (GDAP1) mRNA is inversely associated with AMPK activity. Lassiter et al. reveal a role for GDAP1 in modulating mitochondrial protein abundance, lipid oxidation, and nonmitochondrial respiration. Their results show that GDAP1 influences metabolic processes and circadian gene expression in skeletal muscle.
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Objective: We sought to identify AMPK-regulated genes via bioinformatic analysis of microarray data generated from skeletal muscle of animal models with genetically altered AMPK activity. We hypothesized that such genes would play a role in metabolism. Ganglioside-induced differentiation-associated protein 1 (GDAP1), a gene which plays a role in mitochondrial fission and peroxisomal function in neuronal cells but whose function in skeletal muscle is undescribed, was identified and further validated. AMPK activation reduced GDAP1 expression in skeletal muscle. GDAP1 expression was elevated in skeletal muscle from type 2 diabetic patients but decreased after acute exercise.
Methods: The metabolic impact of GDAP1 silencing was determined in primary skeletal muscle cells via siRNA-transfections. Confocal microscopy was used to visualize whether silencing GDAP1 impacted mitochondrial network morphology and membrane potential.
Results: GDAP1 silencing increased mitochondrial protein abundance, decreased palmitate oxidation, and decreased non-mitochondrial respiration. Mitochondrial morphology was unaltered by GDAP1 silencing. GDAP1 silencing and treatment of cells with AMPK agonists altered several genes in the core molecular clock machinery.
Conclusion: We describe a role for GDAP1 in regulating mitochondrial proteins, circadian genes, and metabolic flux in skeletal muscle. Collectively, our results implicate GDAP1 in the circadian control of metabolism.[Hide abstract]
|Exercise-induced mechanisms promoting glycogen supercompensation in muscleThe majority of glycogen is stored in skeletal muscle and the “set-point” for glycogen concentration in human skeletal muscle appears to be tightly controlled. However, when a single bout of exercise is followed by intake of carbohydrates, the muscle cell favors glycogen synthesis far beyond resting levels. The molecular mechanisms of this phenomenon, known as glycogen supercompensation, are unknown. Hingst and colleagues used an invasive study in man and advanced muscle proteomics to come up with a model of the molecular mechanisms of glycogen supercompensation.|
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Objective: A single bout of exercise followed by intake of carbohydrates leads to glycogen supercompensation in prior exercised muscle. Our objective was to illuminate molecular mechanisms underlying this phenomenon in skeletal muscle of man.
Methods: We studied the temporal regulation of glycogen supercompensation in human skeletal muscle during a 5 day recovery period following a single bout of exercise. Nine healthy men depleted (day 1), normalized (day 2) and supercompensated (day 5) muscle glycogen in one leg while the contralateral leg served as a resting control. Euglycemic hyperinsulinemic clamps in combination with leg balance technique allowed for investigating insulin-stimulated leg glucose uptake under these 3 experimental conditions. Cellular signaling in muscle biopsies was investigated by global proteomic analyses and immunoblotting. We strengthened the validity of proposed molecular effectors by follow-up studies in muscle of transgenic mice.
Results: Sustained activation of glycogen synthase (GS) and AMPK in combination with elevated expression of proteins determining glucose uptake capacity were evident in the prior exercised muscle. We hypothesize that these alterations offset the otherwise tight feedback inhibition of glycogen synthesis and glucose uptake by glycogen. In line with key roles of AMPK and GS seen in the human experiments we observed abrogated ability for glycogen supercompensation in muscle with inducible AMPK deletion and in muscle carrying a G6P-insensitive form of GS in muscle.
Conclusion: Our study demonstrates that both AMPK and GS are key regulators of glycogen supercompensation following a single bout of glycogen-depleting exercise in skeletal muscle of both man and mouse.[Hide abstract]
|Maternal exercise intervention in obese pregnancy improves offspring cardiovascular health The risk of cardiovascular disease is elevated in individuals whose mothers were obese during pregnancy. Disease risk could be reduced prior to birth through targeted interventions to the mother before and during pregnancy. However, the long-term results of lifestyle intervention trials in humans will only become available many years in the future. Therefore, Beeson, Blackmore, and colleagues used a murine model of maternal diet-induced obesity to test the effectiveness of exercise intervention during obese pregnancy at preventing programmed cardiovascular dysfunction in the adult offspring. They demonstrate that offspring cardiac hypertrophy and dysfunction can be programmed independently of hypertension by adverse intrauterine conditions.|
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Objective: Obesity during pregnancy is associated with an elevated risk of cardiovascular disease in the offspring. With increased numbers of women entering pregnancy overweight or obese, there is a requirement for targeted interventions to reduce disease risk in future generations. Using an established murine model of maternal obesity during pregnancy, we investigated if a treadmill exercise intervention in the mother could improve offspring cardiac health and explored potential underlying mechanisms.
Methods: A 20-minute treadmill exercise intervention protocol was performed 5 days a week in diet-induced obese female C57BL/6 mice 1 week prior to, and up to E17 of pregnancy. All male offspring were weaned onto a control diet and studied at 8 weeks of age when their cardiovascular physiology was assessed by in vivo echocardiography and non-invasive tail cuff plethysmography. Cardiomyocyte cell area, re-expression of fetal genes and the expression of calcium handling and sympathetic activation proteins were determined.
Results: At 8 weeks, there was no difference in bodyweight or fat mass between groups. Offspring of obese dams developed pathologic cardiac hypertrophy, hypertension and cardiac dysfunction characterized by reduced ejection fraction (p < 0.001). Maternal exercise prevented cardiac hypertrophy and dysfunction but failed to prevent hypertension. These offspring of exercised dams also had enhanced (p < 0.001) levels of calcium handling proteins and a sympathetic-activated inotropic response.
Conclusions: Exercise in obese pregnancy was beneficial to offspring cardiac function and structure but did not influence hypertension suggesting they are programmed by separate mechanistic pathways. These data suggest combination interventions in obese pregnancies will be required to improve all aspects of the cardiovascular health of the next generation.[Hide abstract]
|GLP-2 receptor signaling is dispensable for the metabolic benefits of vertical sleeve gastrectomyGlucagon-like peptide 2 (GLP-2), together with GLP-1, controls the absorption of nutrients through reduction of gut motility, upregulation of nutrient transport and via optimization of mucosal surface area and gut integrity. Patel, Yusta, et al. hypothesized that elevated levels of GLP-2 may contribute to the metabolic benefits arising following reduction of glucagon receptor (GCGR) signaling or metabolic surgery. However, their data using two different experimental models failed to demonstrate a key role for the GLP-2 receptor in regulation of glucose control or body weight evident after disruption of GCGR signaling, or following vertical sleeve gastrectomy.|
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Objective: Therapeutic interventions that improve glucose homeostasis such as attenuation of glucagon receptor (Gcgr) signaling and bariatric surgery share common metabolic features conserved in mice and humans. These include increased circulating levels of bile acids (BA) and the proglucagon-derived peptides (PGDPs), GLP-1 and GLP-2. Whether BA acting through TGR5 (Gpbar1) increases PGDP levels in these scenarios has not been examined. Furthermore, although the importance of GLP-1 action has been interrogated in Gcgr−/− mice and after bariatric surgery, whether GLP-2 contributes to the metabolic benefits of these interventions is not known.
Methods: To assess whether BA acting through Gpbar1 mediates improved glucose homeostasis in Gcgr−/− mice we generated and characterized Gcgr−/−:Gpbar1−/− mice. The contribution of GLP-2 receptor (GLP-2R) signaling to intestinal and metabolic adaptation arising following loss of the Gcgr was studied in Gcgr−/−:Glp2r−/− mice. The role of the GLP-2R in the metabolic improvements evident after bariatric surgery was studied in high fat-fed Glp2r−/− mice subjected to vertical sleeve gastrectomy (VSG).
Results: Circulating levels of BA were markedly elevated yet similar in Gcgr−/−:Gpbar1+/+ vs. Gcgr−/−:Gpbar1−/− mice. Loss of GLP-2R lowered levels of BA in Gcgr−/− mice. Gcgr−/−:Glp2r−/− mice also exhibited shifts in the proportion of circulating BA species. Loss of Gpbar1 did not impact body weight, intestinal mass, or glucose homeostasis in Gcgr−/− mice. In contrast, small bowel growth was attenuated in Gcgr−/−:Glp2r−/− mice. The improvement in glucose tolerance, elevated circulating levels of GLP-1, and glucose-stimulated insulin levels were not different in Gcgr−/−:Glp2r+/+ vs. Gcgr−/−:Glp2r−/− mice. Similarly, loss of the GLP-2R did not attenuate the extent of weight loss and improvement in glucose control after VSG.
Conclusions: These findings reveal that GLP-2R controls BA levels and relative proportions of BA species in Gcgr−/− mice. Nevertheless, the GLP-2R is not essential for i) control of body weight or glucose homeostasis in Gcgr−/− mice or ii) metabolic improvements arising after VSG in high fat-fed mice. Furthermore, despite elevations of circulating levels of BA, Gpbar1 does not mediate elevated levels of PGDPs or major metabolic phenotypes in Gcgr−/− mice. Collectively these findings refine our understanding of the relationship between Gpbar1, elevated levels of BA, PGDPs, and the GLP-2R in amelioration of metabolic derangements arising following loss of Gcgr signaling or after vertical sleeve gastrectomy.[Hide abstract]
|Reduction of lysophospholipids in the digestive tract promotes diabetes remissionThe most effective obesity intervention currently is bariatric surgery. A recent survey of metabolomic and lipidomic changes associated with bariatric surgery in humans revealed that successful diabetes remission is associated with reduced sphingomyelin and lysophospholipid levels. Cash et al. find that a 1B phospholipase A2 (PLA2G1B) intervention has similar metabolic benefits as bariatric surgery with normalization of plasma lipid, glucose, insulin, and trimethylamine N-oxide levels. PLA2G1B inhibitors may be an alternative option to bariatric surgery for management of obesity and obesity-related diseases.|
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Objective: Obesity and obesity-related metabolic disorders are major health problems worldwide. The most effective obesity intervention is bariatric surgery. This study tested the hypothesis that bariatric surgery alters phospholipid metabolism in the gastrointestinal tract to favor a metabolically healthy gut microbiota profile and therapeutic intervention of phospholipid metabolism in the gastrointestinal may have similar metabolic benefits.
Methods: The first study compared plasma levels of the bioactive lipid metabolites lysophospholipid and trimethylamine N-oxide (TMAO) as well as gut microbiota profile in high fat/carbohydrate (HFHC) diet-fed C57BL/6 mice with or without vertical sleeve gastrectomy (VSG) and in Pla2g1b−/− mice with group 1B phospholipase A2 gene inactivation. The second study examined the effectiveness of the non-absorbable secretory phospholipase A2 inhibitor methyl indoxam to reverse hyperglycemia and hyperlipidemia in HFHC diet-fed C57BL/6 mice after diabetes onset.
Results: Both bariatric surgery and PLA2G1B inactivation were shown to reduce lysophospholipid content in the gastrointestinal tract, resulting in resistance to HFHC diet-induced alterations of the gut microbiota, reduction of the cardiovascular risk factors hyperlipidemia and TMAO, decreased adiposity, and prevention of HFHC diet-induced diabetes. Importantly, treatment of wild type mice with methyl indoxam after HFHC diet-induced onset of hyperlipidemia and hyperglycemia effectively restored normal plasma lipid and glucose levels and replicated the metabolic benefits of VSG surgery with diabetes remission and TMAO reduction.
Conclusion: These results provided pre-clinical evidence that PLA2G1B inhibition in the digestive tract may be a viable alternative option to bariatric surgery for obesity and obesity-related cardiometabolic disorder intervention.[Hide abstract]
|Co-storage and release of INSL5, GLP-1 and peptideYY from colonic cellsInsulin-like-5 (INSL5) is a peptide hormone found to be highly expressed in enteroendocrine cells also producing GLP-1 and PYY in the colon and rectum. However, INSL5 production is increased by calorie restriction, contrasting with plasma GLP-1 and PYY concentrations that are elevated after food ingestion. Whether INSL5 secretion is triggered by the same stimuli as GLP-1 and PYY is unknown. The opposing behaviors of these peptides could be explained at the level of tissue exposure or single cell responsiveness. Billing, Smith, Larraufie, Goldspink, and colleagues found that INSL5 is co-packaged with GLP-1 and PYY in secretory vesicles of colonic L-cells, and that all three peptides are co-released in response to a broad range of stimuli.|
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Objective: Insulin-like peptide-5 (INSL5) is an orexigenic gut hormone found in a subset of colonic and rectal enteroendocrine L-cells together with the anorexigenic hormones glucagon-like peptide-1 (GLP-1) and peptideYY (PYY). Unlike GLP-1 and PYY, INSL5 levels are elevated by calorie restriction, raising questions about how these hormones respond to different stimuli when they arise from the same cell type. The aim of the current study was to identify whether and how INSL5, GLP-1 and PYY are co-secreted or differentially secreted from colonic L-cells.
Methods: An inducible reporter mouse (Insl5-rtTA) was created to enable selective characterisation of Insl5-expressing cells. Expression profiling and Ca2+-dynamics were assessed using TET-reporter mice. Secretion of INSL5, PYY, and GLP-1 from murine and human colonic crypt cultures was quantified by tandem mass spectrometry. Vesicular co-localisation of the three hormones was analysed in 3D-SIM images of immunofluorescently-labelled murine colonic primary cultures and tissue sections.
Results: INSL5-producing cells expressed a range of G-protein coupled receptors previously identified in GLP-1 expressing L-cells, including Ffar1, Gpbar1, and Agtr1a. Pharmacological or physiological agonists for these receptors triggered Ca2+ transients in INSL5-producing cells and stimulated INSL5 secretion. INSL5 secretory responses strongly correlated with those of PYY and GLP-1 across a range of stimuli. The majority (>80%) of secretory vesicles co-labelled for INSL5, PYY and GLP-1.
Conclusions: INSL5 is largely co-stored with PYY and GLP-1 and all three hormones are co-secreted when INSL5-positive cells are stimulated. Opposing hormonal profiles observed in vivo likely reflect differential stimulation of L-cells in the proximal and distal gut.[Hide abstract]
|Intestinal bitter taste receptor activation alters hormone secretion and imparts metabolic benefitsExtracts of the hops plant have been shown to reduce weight and improve glucose homeostasis in rodents and humans. Thus, it is of considerable interest to identify the molecular mechanisms that mediate these benefits. A promising candidate is signaling via bitter taste receptors present in extraoral tissues, particularly the gastrointestinal (GI) tract. Kok et al. demonstrate that specific modulation of bitter taste receptor signaling in extraoral tissues improves multiple features of metabolic disease. In the gut, bitter taste agonism alters enteroendocrine hormone secretion. Therefore, bitter taste agonists may prove useful in conditions characterized by insulin resistance, dyslipidemia, and inflammation|
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Objectives: Extracts of the hops plant have been shown to reduce weight and insulin resistance in rodents and humans, but elucidation of the mechanisms responsible for these benefits has been hindered by the use of heterogeneous hops-derived mixtures. Because hop extracts are used as flavoring agents for their bitter properties, we hypothesized that bitter taste receptors (Tas2rs) could be mediating their beneficial effects in metabolic disease. Studies have shown that exposure of cultured enteroendocrine cells to bitter tastants can stimulate release of hormones, including glucagon-like peptide 1 (GLP-1). These findings have led to the suggestion that activation of Tas2rs may be of benefit in diabetes, but this tenet has not been tested. Here, we have assessed the ability of a pure derivative of a hops isohumulone with anti-diabetic properties, KDT501, to signal through Tas2rs. We have further used this compound as a tool to systematically assess the impact of bitter taste receptor activation in obesity-diabetes.
Methods: KDT501 was tested in a panel of bitter taste receptor signaling assays. Diet-induced obese mice (DIO) were dosed orally with KDT501 and acute effects on glucose homeostasis determined. A wide range of metabolic parameters were evaluated in DIO mice chronically treated with KDT501 to establish the full impact of activating gut bitter taste signaling.
Results: We show that KDT501 signals through Tas2r108, one of 35 mouse Tas2rs. In DIO mice, acute treatment stimulated GLP-1 secretion and enhanced glucose tolerance. Chronic treatment caused weight and fat mass loss, increased energy expenditure, enhanced glucose tolerance and insulin sensitivity, normalized plasma lipids, and induced broad suppression of inflammatory markers. Chronic KDT501 treatment altered enteroendocrine hormone levels and bile acid homeostasis and stimulated sustained GLP-1 release. Combined treatment with a dipeptidyl peptidase IV inhibitor amplified the incretin-based benefits of this pure isohumulone.
Conclusion: Activation of Tas2r108 in the gut results in a remodeling of enteroendocrine hormone release and bile acid metabolism that ameliorates multiple features of metabolic syndrome. Targeting extraoral bitter taste receptors may be useful in metabolic disease.[Hide abstract]
|PID1 regulates endocytosis of postprandial triglyceride-rich lipoproteinsThe accumulation of plasma triglyceride-rich lipoproteins (TRL) and their remnants is an important risk factor for cardiovascular disease. The LDLR-related protein 1 (LRP1) is an important endocytic receptor that is especially relevant for the uptake of postprandial TRL remnants. The protein phosphotyrosine interacting domain containing 1 (PID1) is an intracellular binding partner of LRP1. Fischer, Albers, and colleagues show that PID1 serves as an intracellular retention adaptor protein for LRP1 in hepatocytes. By controlling LRP1 localization, PID1 determines hepatic clearance and consequently plasma levels of pro-atherogenic TRL remnants in the postprandial phase.|
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Objective: Insulin resistance is associated with impaired receptor dependent hepatic uptake of triglyceride-rich lipoproteins (TRL), promoting hypertriglyceridemia and atherosclerosis. Next to low-density lipoprotein (LDL) receptor (LDLR) and syndecan-1, the LDLR-related protein 1 (LRP1) stimulated by insulin action contributes to the rapid clearance of TRL in the postprandial state. Here, we investigated the hypothesis that the adaptor protein phosphotyrosine interacting domain-containing protein 1 (PID1) regulates LRP1 function, thereby controlling hepatic endocytosis of postprandial lipoproteins.
Methods: Localization and interaction of PID1 and LRP1 in cultured hepatocytes was studied by confocal microscopy of fluorescent tagged proteins, by indirect immunohistochemistry of endogenous proteins, by GST-based pull down and by immunoprecipitation experiments. The in vivo relevance of PID1 was assessed using whole body as well as liver-specific Pid1-deficient mice on a wild type or Ldlr-deficient (Ldlr−/−) background. Intravital microscopy was used to study LRP1 translocation in the liver. Lipoprotein metabolism was investigated by lipoprotein profiling, gene and protein expression as well as organ-specific uptake of radiolabelled TRL.
Results: PID1 co-localized in perinuclear endosomes and was found associated with LRP1 under fasting conditions. We identified the distal NPxY motif of the intracellular C-terminal domain (ICD) of LRP1 as the site critical for the interaction with PID1. Insulin-mediated NPxY-phosphorylation caused the dissociation of PID1 from the ICD, causing LRP1 translocation to the plasma membrane. PID1 deletion resulted in higher LRP1 abundance at the cell surface, higher LDLR protein levels and, paradoxically, reduced total LRP1. The latter can be explained by higher receptor shedding, which we observed in cultured Pid1-deficient hepatocytes. Consistently, PID1 deficiency alone led to increased LDLR-dependent endocytosis of postprandial lipoproteins and lower plasma triglycerides. In contrast, hepatic PID1 deletion on an Ldlr−/− background reduced lipoprotein uptake into liver and caused plasma TRL accumulation.
Conclusion: By acting as an insulin-dependent retention adaptor, PID1 serves as a regulator of LRP1 function controlling the disposal of postprandial lipoproteins. PID1 inhibition provides a novel approach to lower plasma levels of pro-atherogenic TRL remnants by stimulating endocytic function of both LRP1 and LDLR in the liver.[Hide abstract]
|Intracellular lipids are an independent cause of liver injury and chronic kidney diseaseThe rise in global epidemics of obesity and type 2 diabetes in westernized countries contributes to a worrying increase in the incidence of non-alcoholic fatty liver disease and chronic kidney disease. A common metabolic feature of these complications is ectopic lipid accumulation, which is suspected to induce organ damage and dysfunction. Monteillet, Gjorgjieva, et al. show that the development of these hepatic and renal diseases is mainly initiated by altered lipid metabolism resulting in lipid accumulation, independently of excessive glycogen contents in these organs.|
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Objective: Ectopic lipid accumulation in the liver and kidneys is a hallmark of metabolic diseases leading to non-alcoholic fatty liver disease (NAFLD) and chronic kidney disease (CKD). Moreover, recent data have highlighted a strong correlation between NAFLD and CKD incidences. In this study, we use two mouse models of hepatic steatosis or CKD, each initiated independently of the other upon the suppression of glucose production specifically in the liver or kidneys, to elucidate the mechanisms underlying the development of CKD in the context of NAFLD-like pathology.
Methods: Mice with a deletion of G6pc, encoding glucose-6 phosphatase catalytic subunit, specifically in the liver (L.G6pc−/− mice) or the kidneys (K.G6pc−/− mice), were fed with either a standard diet or a high fat/high sucrose (HF/HS) diet during 9 months. These mice represent two original models of a rare metabolic disease named Glycogen Storage Disease Type Ia (GSDIa) that is characterized by both NAFLD-like pathology and CKD. Two other groups of L.G6pc−/− and K.G6pc−/− mice were fed a standard diet for 6 months and then treated with fenofibrate for 3 months. Lipid and glucose metabolisms were characterized, and NAFLD-like and CKD damages were evaluated.
Results: Lipid depot exacerbation upon high-calorie diet strongly accelerated hepatic and renal pathologies induced by the G6pc-deficiency. In L.G6pc−/− mice, HF/HS diet increased liver injuries, characterized by higher levels of plasmatic transaminases and increased hepatic tumor incidence. In K.G6pc−/− mice, HF/HS diet increased urinary albumin and lipocalin 2 excretion and aggravated renal fibrosis. In both cases, the worsening of NAFLD-like injuries and CKD was independent of glycogen content. Furthermore, fenofibrate, via the activation of lipid oxidation significantly decreased the hepatic or renal lipid accumulations and prevented liver or kidney damages in L.G6pc−/− and K.G6pc−/− mice, respectively. Finally, we show that L.G6pc−/− mice and K.G6pc−/− mice developed NAFLD-like pathology and CKD independently.
Conclusions: This study highlights the crucial role that lipids play in the independent development of both NAFLD and CKD and demonstrates the importance of lipid-lowering treatments in various metabolic diseases featured by lipid load, from the “rare” GSDIa to the “epidemic” morbid obesity or type 2 diabetes.[Hide abstract]
|Neuronal modulation of brown adipose activity through perturbation of white adipocyte lipogenesisAdipocytes of different types (white, brown or beige) may interconvert and change their behavior depending on local sympathetic nerve activity. De novo lipogenesis (DNL) within adipocytes may be linked to control of local sympathetic nerve activity. Guilherme, Pedersen, et al. investigated the role of DNL in adipose tissue under extreme temperature conditions. Their data indicate that DNL activity in white adipocytes can initiate long distance signaling to brown adipose tissue that enhances its thermoregulation program.
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Objective: Crosstalk between adipocytes and local neurons may be an important regulatory mechanism to control energy homeostasis. We previously reported that perturbation of adipocyte de novo lipogenesis (DNL) by deletion of fatty acid synthase (FASN) expands sympathetic neurons within white adipose tissue (WAT) and stimulates the appearance of “beige” adipocytes. Here we tested whether WAT DNL activity can also influence neuronal regulation and thermogenesis in brown adipose tissue (BAT).
Methods and results: Induced deletion of FASN in all adipocytes in mature mice (iAdFASNKO) enhanced sympathetic innervation and neuronal activity as well as UCP1 expression in both WAT and BAT. This increased sympathetic innervation could be observed at both 22 °C and 30 °C, indicating it is not a response to heat loss but rather adipocyte signaling. In contrast, selective ablation of FASN in brown adipocytes of mice (iUCP1FASNKO) failed to modulate sympathetic innervation and the thermogenic program in BAT. Surprisingly, DNL in brown adipocytes was also dispensable in maintaining euthermia when UCP1FASNKO mice were cold-exposed.
Conclusion: These results indicate that DNL in white adipocytes influences long distance signaling to BAT, which can modify BAT sympathetic innervation and expression of genes involved in thermogenesis.[Hide abstract]
|FGF21 regulates insulin sensitivity following chronic stressProlonged exposure to stress and posttraumatic stress disorders (PTSD) increase the risk for developing type 2 diabetes, cardiovascular disease, and premature death. Adaptive mechanisms and associated biomarkers need to be explored in order to develop therapeutic options for the prevention of stress-induced insulin resistance. Jelenik, Dille, Müller-Lühlhoff, Kabra, et al. identify a molecular network in which fibroblast growth factor 21 (FGF21) regulates late-onset metabolic responses to stress and may account for the physiological memory mechanism to achieve metabolic homeostasis again.|
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Objective: Post-traumatic stress disorder (PTSD) increases type 2 diabetes risk, yet the underlying mechanisms are unclear. We investigated how early-life exposure to chronic stress affects long-term insulin sensitivity.
Methods: C57Bl/6J mice were exposed to chronic variable stress for 15 days (Cvs) and then recovered for three months without stress (Cvs3m).
Results: Cvs mice showed markedly increased plasma corticosterone and hepatic insulin resistance. Cvs3m mice exhibited improved whole-body insulin sensitivity along with enhanced adipose glucose uptake and skeletal muscle mitochondrial function and fatty acid oxidation. Plasma FGF21 levels were substantially increased and associated with expression of genes involved in fatty acid oxidation and formation of brown-like adipocytes. In humans, serum FGF21 levels were associated with stress coping long time after the exposure.
Conclusions: Early-life exposure to chronic stress leads to long term improvements in insulin sensitivity, oxidative metabolism and adipose tissue remodeling. FGF21 contributes to a physiological memory mechanism to maintain metabolic homeostasis.[Hide abstract]
|Loss of Tbk1 kinase activity protects from metabolic dysfunctionA primary pathology of obesity and metabolic disorders is the induction of chronic low-grade inflammation. TANK binding kinase 1 (TBK1) is a non-canonical activator of nuclear factor kappa B (NF-κB) that is thought to play an important role in obesity-associated inflammation. Cruz et al. studied the function of TBK1 using a Tbk1 mutant mouse that harbors two copies of a null Tbk1 allele. They report that these mice are resistant to high fat diet-induced weight gain and pancreatic islet hyperplasia and hypertrophy. This suggests that TBK1 can be a therapeutic target in the prevention and treatment of type 2 diabetes and obesity.|
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Objective: TANK Binding Kinase 1 (TBK1) has been implicated in the regulation of metabolism through studies with the drug amlexanox, an inhibitor of the IκB kinase (IKK)-related kinases. Amlexanox induced weight loss, reduced fatty liver and insulin resistance in high fat diet (HFD) fed mice and has now progressed into clinical testing for the treatment and prevention of obesity and type 2 diabetes. However, since amlexanox is a dual IKKε/TBK1 inhibitor, the specific metabolic contribution of TBK1 is not clear.
Methods: To distinguish metabolic functions unique to TBK1, we examined the metabolic profile of global Tbk1 mutant mice challenged with an obesogenic diet and investigated potential mechanisms for the improved metabolic phenotype.
Results and conclusion: We report that systemic loss of TBK1 kinase function has an overall protective effect on metabolic readouts in mice on an obesogenic diet, which is mediated by loss of an inhibitory interaction between TBK1 and the insulin receptor.
|Individual islet respirometry reveals functional diversity within the islet populationPancreatic islets rely on mitochondrial respiration to secrete insulin. Islets and β-cells within islets are known to show remarkable variability in glucose sensitivity, size, architecture, and cell composition. Islet heterogeneity seems to play a critical role in pathogenesis of metabolic disease. However, the high-throughput study of individual islet oxygen consumption has not been possible so far. Taddeo et al. have established a non-invasive method that enables high-throughput measurement of oxygen consumption in large-sized individual islets from mice and humans. This is possible using a special microplate. The authors found heterogeneous glucose sensitivity in large individual islets. Their method might accelerate islet research and help elucidate mechanisms underlying the metabolic heterogeneity in pancreatic islets.|
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Objective: Islets from the same pancreas show remarkable variability in glucose sensitivity. While mitochondrial respiration is essential for glucose-stimulated insulin secretion, little is known regarding heterogeneity in mitochondrial function at the individual islet level. This is due in part to a lack of high-throughput and non-invasive methods for detecting single islet function.
Methods: We have developed a novel non-invasive, high-throughput methodology capable of assessing mitochondrial respiration in large-sized individual islets using the XF96 analyzer (Agilent Technologies).
Results: By increasing measurement sensitivity, we have reduced the minimal size of mouse and human islets needed to assess mitochondrial respiration to single large islets of >35,000 μm2 area (∼210 μm diameter). In addition, we have measured heterogeneous glucose-stimulated mitochondrial respiration among individual human and mouse islets from the same pancreas, allowing population analyses of islet mitochondrial function for the first time.
Conclusions: We have developed a novel methodology capable of analyzing mitochondrial function in large-sized individual islets. By highlighting islet functional heterogeneity, we hope this methodology can significantly advance islet research.[Hide abstract]
|TGF-β receptor 1 regulates progenitors that promote browning of white fatBeige (or brite) adipocytes have therapeutic potential to combat metabolic diseases. They appear in response to cold exposure or upon stimulation by β-adrenergic pathways and might be derived from specialized progenitors. Wankhade et al. describe an important role of transforming growth factor β receptor I (TβRI) in regulating glucose and energy homeostasis via targeting of putative beige stem cells in white fat. These progenitor cells express key beige/brown signature markers and upon transplantation demonstrate the capacity to undergo beige adipogenesis.|
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Objective: Beige/brite adipose tissue displays morphological characteristics and beneficial metabolic traits of brown adipose tissue. Previously, we showed that TGF-β signaling regulates the browning of white adipose tissue. Here, we inquired whether TGF-β signals regulated presumptive beige progenitors in white fat and investigated the TGF-β regulated mechanisms involved in beige adipogenesis.
Methods: We deleted TGF-β receptor 1 (TβRI) in adipose tissue (TβRIAdKO mice) and, using flow-cytometry based assays, identified and isolated presumptive beige progenitors located in the stromal vascular cells of white fat. These cells were molecularly characterized to examine beige/brown marker expression and to investigate TGF-β dependent mechanisms. Further, the cells were transplanted into athymic nude mice to examine their adipogenesis potential.
Results: Deletion of TβRI promotes beige adipogenesis while reducing the detrimental effects of high fat diet feeding. Interaction of TGF-β signaling with the prostaglandin pathway regulated the appearance of beige adipocytes in white fat. Using flow cytometry techniques and stromal vascular fraction from white fat, we isolated presumptive beige stem/progenitor cells (iBSCs). Upon genetic or pharmacologic inhibition of TGF-β signaling, these cells express high levels of predominantly beige markers. Transplantation of TβRI-deficient stromal vascular cells or iBSCs into athymic nude mice followed by high fat diet feeding and stimulation of β-adrenergic signaling via CL316,243 injection or cold exposure promoted robust beige adipogenesis in vivo.
Conclusions: TβRI signals target the prostaglandin network to regulate presumptive beige progenitors in white fat capable of developing into beige adipocytes with functional attributes. Controlled inhibition of TβRI signaling and concomitant PGE2 stimulation has the potential to promote beige adipogenesis and improve metabolism.[Hide abstract]
|Cartilage oligomeric matrix protein is expressed in adipose tissue and regulates adipogenesisRemodeling of the extracellular matrix (ECM) is crucial for adipogenesis. Cartilage oligomeric matrix protein (COMP) exhibits a striking depot-specific expression pattern between subcutaneous abdominal and gluteal adipose tissue (AT). Denton and colleagues report that COMP expression in abdominal and gluteal AT as well as circulating COMP levels are positively correlated with adiposity but not with regional AT distribution. Also, exogenous COMP protein promoted adipogenesis. These results highlight the important yet poorly understood role of the ECM in AT.|
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Objective: The composition of the extracellular matrix (ECM) impacts adipocyte function and might determine adipose tissue (AT) function and distribution. Cartilage oligomeric matrix protein (COMP), a matricellular protein usually studied in bone and cartilage, is highly differentially expressed between subcutaneous abdominal and gluteal AT. This study aimed to explore COMP's role in human subcutaneous abdominal and gluteal AT and preadipocyte biology.
Methods: COMP mRNA levels were measured in whole AT and immortalised preadipocytes via quantitative (q)-PCR. Tissue and cellular COMP protein were measured via Western blot and immunohistochemistry; plasma COMP was measured by ELISA. The effect of COMP on adipogenesis in immortalised preadipocytes was evaluated by qPCR of adipogenic markers and cellular triacylglycerol (TAG) accumulation.
Results: qPCR analysis of paired subcutaneous abdominal and gluteal AT biopsies (n = 190) across a range of BMI (20.7–45.5 kg/m2) indicated ∼3-fold higher COMP expression in gluteal AT (P = 1.7 × 10−31); protein levels mirrored this. Immunohistochemistry indicated COMP was abundant in gluteal AT ECM and co-localised with collagen-1. AT COMP mRNA levels and circulating COMP protein levels were positively associated with BMI/adiposity but unrelated to AT distribution. COMP expression changed dynamically during adipogenesis (time × depot, P = 0.01). Supplementation of adipogenic medium with exogenous COMP protein (500 ng/ml) increased PPARG2 expression ∼1.5-fold (P = 0.0003) and TAG accumulation ∼1.25-fold in abdominal and gluteal preadipocytes (P = 0.02).
Conclusions: We confirmed that COMP is an ECM protein which is differentially expressed between subcutaneous abdominal and gluteal AT. Despite its depot-specific expression pattern, however, AT COMP mRNA levels and plasma COMP concentration correlated positively with overall obesity but not body fat distribution. Exogenous COMP enhanced adipogenesis. These data identify COMP as a novel regulator of AT and highlight the importance of the ECM to AT biology.[Hide abstract]
|Metabolic syndrome and adipose tissue inflammation in obese Göttingen minipigsIn obesity, a deeper understanding of inflammatory triggers and mechanisms may uncover novel therapeutic targets for treatment. In this respect, animal models are important for increasing our knowledge of pathological processes and assessment of drug candidates. Rodent models are the first-line option for pharmacological evaluation, but their predictive value for drug efficacy and safety in humans is limited. Given the strong similarity of human anatomy and physiology to pigs, these can serve as a model bridging the gap between mouse and man. Renner, Blutke, and colleagues have established a diet-induced obesity (DIO) model in Göttingen minipigs (GM). Munich DIO-GM showed phenotypic signatures representative of the metabolic syndrome, as well as extensive fat tissue inflammation.
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Objective: The worldwide prevalence of obesity has increased to 10% in men and 15% in women and is associated with severe comorbidities such as diabetes, cancer, and cardiovascular disease. Animal models of obesity are central to experimental studies of disease mechanisms and therapeutic strategies. Diet-induced obesity (DIO) models in rodents have provided important insights into the pathophysiology of obesity and, in most instances, are the first in line for exploratory pharmacology studies. To deepen the relevance towards translation to human patients, we established a corresponding DIO model in Göttingen minipigs (GM).
Methods: Young adult female ovariectomized GM were fed a high-fat/high-energy diet for a period of 70 weeks. The ration was calculated to meet the requirements and maintain body weight (BW) of lean adult minipigs (L-GM group) or increased stepwise to achieve an obese state (DIO-GM group). Body composition, blood parameters and intravenous glucose tolerance were determined at regular intervals. A pilot chronic treatment trial with a GLP1 receptor agonist was conducted in DIO-GM. At the end of the study, the animals were necropsied and a biobank of selected tissues was established.
Results: DIO-GM developed severe subcutaneous and visceral adiposity (body fat >50% of body mass vs. 22% in L-GM), increased plasma cholesterol, triglyceride, and free fatty acid levels, insulin resistance (HOMA-IR >5 vs. 2 in L-GM), impaired glucose tolerance and increased heart rate when resting and active. However, fasting glucose concentrations stayed within normal range throughout the study. Treatment with a long-acting GLP1 receptor agonist revealed substantial reduction of food intake and body weight within four weeks, with increased drug sensitivity relative to observations in other DIO animal models. Extensive adipose tissue inflammation and adipocyte necrosis was observed in visceral, but not subcutaneous, adipose tissue of DIO-GM.
Conclusions: The Munich DIO-GM model resembles hallmarks of the human metabolic syndrome with extensive adipose tissue inflammation and adipocyte necrosis reported for the first time. DIO-GM may be used for evaluating novel treatments of obesity and associated comorbidities. They may help to identify triggers and mechanisms of fat tissue inflammation and mechanisms preventing complete metabolic decompensation despite morbid obesity.[Hide abstract]
|Chronic D-serine supplementation impairs insulin secretionSerine racemase (SRR) catalyzes the conversion from L-serine to D-serine, an important co-agonist of N-methyl-D-aspartate (NMDA) receptors. SRR was recently suggested to regulate insulin secretion from pancreatic beta cells. Suwandhi et al. report positive and negative consequences of long-term treatment with D-serine. Importantly, it impairs glucose tolerance due to impaired insulin secretion from pancreatic beta cells. This depends on increased sympathetic activity in the islets of Langerhans rather than direct action on beta cells. Thus, alterations in D-serine levels could contribute to the development of T2D through the modulation of insulin secretion, especially in genetically predisposed subjects.|
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Objective: The metabolic role of d-serine, a non-proteinogenic NMDA receptor co-agonist, is poorly understood. Conversely, inhibition of pancreatic NMDA receptors as well as loss of the d-serine producing enzyme serine racemase have been shown to modulate insulin secretion. Thus, we aim to study the impact of chronic and acute d-serine supplementation on insulin secretion and other parameters of glucose homeostasis.
Methods: We apply MALDI FT-ICR mass spectrometry imaging, NMR based metabolomics, 16s rRNA gene sequencing of gut microbiota in combination with a detailed physiological characterization to unravel the metabolic action of d-serine in mice acutely and chronically treated with 1% d-serine in drinking water in combination with either chow or high fat diet feeding. Moreover, we identify SNPs in SRR, the enzyme converting L-to d-serine and two subunits of the NMDA receptor to associate with insulin secretion in humans, based on the analysis of 2760 non-diabetic Caucasian individuals.
Results: We show that chronic elevation of d-serine results in reduced high fat diet intake. In addition, d-serine leads to diet-independent hyperglycemia due to blunted insulin secretion from pancreatic beta cells. Inhibition of alpha 2-adrenergic receptors rapidly restores glycemia and glucose tolerance in d-serine supplemented mice. Moreover, we show that single nucleotide polymorphisms (SNPs) in SRR as well as in individual NMDAR subunits are associated with insulin secretion in humans.
Conclusion: Thus, we identify a novel role of d-serine in regulating systemic glucose metabolism through modulating insulin secretion.[Hide abstract]
|The role of kidney in the coordination of glucose production during fastingGluconeogenesis is performed in three organs: liver, kidney, and intestine. While the liver is the main producer of glucose at the onset of fasting, gluconeogenesis from the kidney and intestine becomes increasingly important during prolonged fasting. Kaneko et al. find that a diminution of renal gluconeogenesis promotes a novel repartition of glucose production during fasting, featured by increased intestinal gluconeogenesis that leads to sparing glycogen stores in the liver. Their data shed light on the inter-organ crosstalk among glucose-producing organs, which might take place in the situations of deficient renal glucose production, such as chronic kidney disease.|
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Objective: The respective contributions to endogenous glucose production (EGP) of the liver, kidney and intestine vary during fasting. We previously reported that the deficiency in either hepatic or intestinal gluconeogenesis modulates the repartition of EGP via glucagon secretion (humoral factor) and gut–brain–liver axis (neural factor), respectively. Considering renal gluconeogenesis reportedly accounted for approximately 50% of EGP during fasting, we examined whether a reduction in renal gluconeogenesis could promote alterations in the repartition of EGP in this situation.
Methods: We studied mice whose glucose-6-phosphatase (G6Pase) catalytic subunit (G6PC) is specifically knocked down in the kidneys (K-G6pc-/- mice) during fasting. We also examined the additional effects of intestinal G6pc deletion, renal denervation and vitamin D administration on the altered glucose metabolism in K-G6pc-/- mice.
Results: Compared with WT mice, K-G6pc-/- mice exhibited (1) lower glycemia, (2) enhanced intestinal but not hepatic G6Pase activity, (3) enhanced hepatic glucokinase (GK encoded by Gck) activity, (4) increased hepatic glucose-6-phosphate and (5) hepatic glycogen spared from exhaustion during fasting. Increased hepatic Gck expression in the post-absorptive state could be dependent on the enhancement of insulin signal (AKT phosphorylation) in K-G6pc-/- mice. In contrast, the increase in hepatic GK activity was not observed in mice with both kidney- and intestine-knockout (KI-G6pc-/- mice). Hepatic Gck gene expression and hepatic AKT phosphorylation were reduced in KI-G6pc-/- mice. Renal denervation by capsaicin did not induce any effect on glucose metabolism in K-G6pc-/- mice. Plasma level of 1,25 (OH)2 D3, an active form of vitamin D, was decreased in K-G6pc-/- mice. Interestingly, the administration of 1,25 (OH)2 D3 prevented the enhancement of intestinal gluconeogenesis and hepatic GK activity and blocked the accumulation of hepatic glycogen otherwise observed in K-G6pc-/- mice during fasting.
Conclusion: A diminution in renal gluconeogenesis that is accompanied by a decrease in blood vitamin D promotes a novel repartition of EGP among glucose producing organs during fasting, featured by increased intestinal gluconeogenesis that leads to sparing glycogen stores in the liver. Our data suggest a possible involvement of a crosstalk between the kidneys and intestine (via the vitamin D system) and the intestine and liver (via a neural gut-brain axis), which might take place in the situations of deficient renal glucose production, such as chronic kidney disease.[Hide abstract]