Featured ArticlesVolume 12 | June 2018
|The role of genetic variation of human metabolism for BMI, mental traits and mental disorders A substantial amount of evidence links obesity with mental health. Genome-wide association studies (GWAS) and meta-analyses (GWAMA) have offered insights into the genetic contributors of BMI and of several categorically defined mental disorders as well as dimensional mental phenotypes. Hebebrand, Peters et al. have now studied loci associated with levels of blood or urine metabolites for their additional association with body mass index (BMI) and several mental traits or disorders. 19 SNPs located in nine independent loci appeared significant when strictly corrected for multiple testing.|
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Objective: The aim was to assess whether loci associated with metabolic traits also have a significant role in BMI and mental traits/disorders
Methods: We first assessed the number of single nucleotide polymorphisms (SNPs) with genome-wide significance for human metabolism (NHGRI-EBI Catalog). These 516 SNPs (216 independent loci) were looked-up in genome-wide association studies for association with body mass index (BMI) and the mental traits/disorders educational attainment, neuroticism, schizophrenia, well-being, anxiety, depressive symptoms, major depressive disorder, autism-spectrum disorder, attention-deficit/hyperactivity disorder, Alzheimer's disease, bipolar disorder, aggressive behavior, and internalizing problems. A strict significance threshold of p < 6.92 × 10−6 was based on the correction for 516 SNPs and all 14 phenotypes, a second less conservative threshold (p < 9.69 × 10−5) on the correction for the 516 SNPs only.
Results: 19 SNPs located in nine independent loci revealed p-values < 6.92 × 10−6; the less strict criterion was met by 41 SNPs in 24 independent loci. BMI and schizophrenia showed the most pronounced genetic overlap with human metabolism with three loci each meeting the strict significance threshold. Overall, genetic variation associated with estimated glomerular filtration rate showed up frequently; single metabolite SNPs were associated with more than one phenotype. Replications in independent samples were obtained for BMI and educational attainment.
Conclusions: Approximately 5–10% of the regions involved in the regulation of blood/urine metabolite levels seem to also play a role in BMI and mental traits/disorders and related phenotypes. If validated in metabolomic studies of the respective phenotypes, the associated blood/urine metabolites may enable novel preventive and therapeutic strategies.[Hide abstract]
|Targeting Hepatic PDKs Restores Insulin Signaling and Mitigates Lipogenesis The mitochondrial pyruvate dehydrogenase complex (PDC), which converts pyruvate to acetyl-CoA, is regulated by pyruvate dehydrogenase kinases (PDKs 1-4) and pyruvate dehydrogenase phosphatases (PDPs). PDKs play a pivotal role in maintaining energy homeostasis and contribute to metabolic flexibility. Wu et al. hypothesized that PDKs1-4 are a pharmacological target for obesity and type 2 diabetes. They tested recently developed novel pan-PDK inhibitors for improved glucose tolerance and reduced hepatic steatosis. Their results show that the liver-specific inhibitor PS10 lowers plasma glucose concentration via reduced hepatic gluconeogenesis.|
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Objective: Mitochondrial pyruvate dehydrogenase kinases 1–4 (PDKs1–4) negatively regulate activity of the pyruvate dehydrogenase complex (PDC) by reversible phosphorylation. PDKs play a pivotal role in maintaining energy homeostasis and contribute to metabolic flexibility by attenuating PDC activity in various mammalian tissues. Cumulative evidence has shown that the up-regulation of PDK4 expression is tightly associated with obesity and diabetes. In this investigation, we test the central hypothesis that PDKs1-4 are a pharmacological target for lowering glucose levels and restoring insulin sensitivity in obesity and type 2 diabetes (T2D).
Methods: Diet-induced obese (DIO) mice were treated with a liver-specific pan-PDK inhibitor 2-[(2,4-dihydroxyphenyl) sulfonyl]isoindoline-4,6-diol (PS10) for four weeks, and results compared with PDK2/PDK4 double knockout (DKO) mice on the same high fat diet (HFD).
Results: Both PS10-treated DIO mice and HFD-fed DKO mice showed significantly improved glucose, insulin and pyruvate tolerance, compared to DIO controls, with lower plasma insulin levels and increased insulin signaling in liver. In response to lower glucose levels, phosphorylated AMPK in PS10-treated DIO and HFD-fed DKO mice is upregulated, accompanied by decreased nuclear carbohydrate-responsive element binding protein (ChREBP). The reduced ChREBP signaling correlates with down-regulation of hepatic lipogenic enzymes (ACC1, FAS, and SCD1), leading to markedly diminished hepatic steatosis in both study groups, with lower circulating cholesterol and triacylglyceride levels as well as reduced fat mass. PS10-treated DIO as well as DKO mice showed predominant fatty acid over glucose oxidation. However, unlike systemic DKO mice, increased hepatic PDC activity alone in PS10-treated DIO mice does not raise the plasma total ketone body level.
Conclusions: Our findings establish that specific targeting of hepatic PDKs with the PDK inhibitor PS10 is an effective therapeutic approach to maintaining glucose and lipid homeostasis in obesity and T2D, without the harmful ketoacidosis associated with systemic inhibition of PDKs.[Hide abstract]
|Phosphorylation of Beta-3 Adrenergic Receptor Drives Lipolysis Circulating free fatty acid (FFA) levels are predominantly derived from stored triglycerides that are actively broken down through enzymatic lipolysis in white adipose tissue. Signals that promote lipolysis involve activation of the β-adrenergic receptor (βAR). Hong et al. find that in vivo inhibition of the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (MEK/ERK) pathway alters lipolysis in adipose tissue, by decreasing β3AR phosphorylation at serine 247 and subsequent downstream phosphorylation events that control release of FFA.|
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Objective: The inappropriate release of free fatty acids from obese adipose tissue stores has detrimental effects on metabolism, but key molecular mechanisms controlling FFA release from adipocytes remain undefined. Although obesity promotes systemic inflammation, we find activation of the inflammation-associated Mitogen Activated Protein kinase ERK occurs specifically in adipose tissues of obese mice, and provide evidence that adipocyte ERK activation may explain exaggerated adipose tissue lipolysis observed in obesity.
Methods and Results: We provide genetic and pharmacological evidence that inhibition of the MEK/ERK pathway in human adipose tissue, mice, and flies all effectively limit adipocyte lipolysis. In complementary findings, we show that genetic and obesity-mediated activation of ERK enhances lipolysis, whereas adipose tissue specific knock-out of ERK2, the exclusive ERK1/2 protein in adipocytes, dramatically impairs lipolysis in explanted mouse adipose tissue. In addition, acute inhibition of MEK/ERK signaling also decreases lipolysis in adipose tissue and improves insulin sensitivity in obese mice. Mice with decreased rates of adipose tissue lipolysis in vivo caused by either MEK or ATGL pharmacological inhibition were unable to liberate sufficient White Adipose Tissue (WAT) energy stores to fuel thermogenesis from brown fat during a cold temperature challenge. To identify a molecular mechanism controlling these actions, we performed unbiased phosphoproteomic analysis of obese adipose tissue at different time points following acute pharmacological MEK/ERK inhibition. MEK/ERK inhibition decreased levels of adrenergic signaling and caused de-phosphorylation of the β3-adrenergic receptor (β3AR) on serine 247. To define the functional implications of this phosphorylation, we showed that CRISPR/Cas9 engineered cells expressing wild type β3AR exhibited β3AR phosphorylation by ERK2 and enhanced lipolysis, but this was not seen when serine 247 of β3AR was mutated to alanine.
Conclusions: Taken together, these data suggest that ERK activation in adipocytes and subsequent phosphorylation of the β3AR on S247 are critical regulatory steps in the enhanced adipocyte lipolysis of obesity.[Hide abstract]
|Resveratrol improves ex vivo mitochondrial function in first degree relatives of T2D patientsResveratrol, a polyphenol present in various foods, has been proposed as a promising treatment in the prevention of type 2 diabetes (T2D). In a placebo-controlled, crossover study, de Ligt and colleagues investigated if 30 days of resveratrol supplementation can improve insulin sensitivity and muscle mitochondrial function in males at increased risk of developing T2D. Their results indicate that resveratrol supplementation does not affect insulin sensitivity, substrate utilization, energy expenditure, ectopic fat accumulation, or cardiac function and does not stimulate brown adipose tissue in these subjects. However, the data do support previous findings that low-dose resveratrol supplementation can improve muscle mitochondrial oxidative capacity.
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Objective: Resveratrol supplementation improves metabolic health in healthy obese men, but not in patients with type 2 diabetes (T2D) when given as add-on therapy. Therefore, we examined whether resveratrol can enhance metabolic health in men at risk of developing T2D. Additionally, we examined if resveratrol can stimulate brown adipose tissue (BAT).
Methods: Thirteen male first degree relatives (FDR) of patients with T2D received resveratrol (150 mg/day) and placebo for 30 days in a randomized, placebo controlled, cross-over trial.
Results: Resveratrol significantly improved ex vivo muscle mitochondrial function on a fatty acid-derived substrate. However, resveratrol did not improve insulin sensitivity, expressed as the rate of glucose disposal during a two-step hyperinsulinemic-euglycemic clamp. Also, intrahepatic and intramyocellular lipid content, substrate utilization, energy metabolism, and cold-stimulated 18F-FDG glucose uptake in BAT (n = 8) remained unaffected by resveratrol. In vitro experiments in adipocytes derived from human BAT confirmed the lack of effect on BAT.
Conclusions: Resveratrol stimulates muscle mitochondrial function in FDR males, which is in concordance with previous results. However, no other metabolic benefits of resveratrol were found in this group. This could be attributed to subject characteristics causing alterations in metabolism of resveratrol and thereby affecting resveratrol's effectiveness.[Hide abstract]
|Antiretroviral therapy potentiates obesity and glucose intoleranceThe advent of antiretroviral therapy (ART) has significantly decreased mortality in HIV-infected patients. However, long-term administration of ART medications is associated with a disproportionate rise in non-AIDS-related mortality. HIV-1 patients often present with cardiovascular diseases, obesity, non-alcoholic fatty liver disease, and type-2 diabetes. Pepin et al. illustrate that ART worsens high-fat diet-induced metabolic derangements and exaggerates diet-induced adiposity. Combined transcriptomic and kinomic analyses of adipose tissue identified a diet-drug synergism that augments pro-inflammatory signaling within adipose tissue consistent with macrophage infiltration.
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Objective: Breakthroughs in HIV treatment, especially combination antiretroviral therapy (ART), have massively reduced AIDS-associated mortality. However, ART administration amplifies the risk of non-AIDS defining illnesses including obesity, diabetes, and cardiovascular disease, collectively known as metabolic syndrome. Initial reports suggest that ART-associated risk of metabolic syndrome correlates with socioeconomic status, a multifaceted finding that encompasses income, race, education, and diet. Therefore, determination of causal relationships is extremely challenging due to the complex interplay between viral infection, ART, and the many environmental factors.
Methods: In the current study, we employed a mouse model to specifically examine interactions between ART and diet that impacts energy balance and glucose metabolism. Previous studies have shown that high-fat feeding induces persistent low-grade systemic and adipose tissue inflammation contributing to insulin resistance and metabolic dysregulation via adipose-infiltrating macrophages. Studies herein test the hypothesis that ART potentiates the inflammatory effects of a high-fat diet (HFD). C57Bl/6J mice on a HFD or standard chow containing ART or vehicle, were subjected to functional metabolic testing, RNA-sequencing of epididymal white adipose tissue (eWAT), and array-based kinomic analysis of eWAT-infiltrating macrophages.
Results: ART-treated mice on a HFD displayed increased fat mass accumulation, impaired glucose tolerance, and potentiated insulin resistance. Gene set enrichment and kinomic array analyses revealed a pro-inflammatory transcriptional signature depicting granulocyte migration and activation.
Conclusions: The current study reveals a HFD-ART interaction that increases inflammatory transcriptional pathways and impairs glucose metabolism, energy balance, and metabolic dysfunction.[Hide abstract]
|Profiling of G Protein-Coupled Receptors in Vagal Afferents G protein-coupled receptors (GPCRs) act as transmembrane molecular sensors of neurotransmitters, hormones, nutrients, and metabolites. Because unmyelinated vagal afferents richly innervate the gastrointestinal mucosa, gut-derived molecules might directly modulate the activity of vagal afferents through GPCRs. However, the distribution of GPCR expression in vagal afferents is largely unknown. Egerod and colleagues determined the expression profile of all GPCRs in the vagal afferents of the mouse using a combination of quantitative PCR (qPCR), RNA sequencing, and double in situ hybridization studies.|
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Objective: G protein-coupled receptors (GPCRs) act as transmembrane molecular sensors of neurotransmitters, hormones, nutrients, and metabolites. Because unmyelinated vagal afferents richly innervate the gastrointestinal mucosa, gut-derived molecules may directly modulate the activity of vagal afferents through GPCRs. However, the types of GPCRs expressed in vagal afferents are largely unknown. Here, we determined the expression profile of all GPCRs expressed in vagal afferents of the mouse, with a special emphasis on those innervating the gastrointestinal tract.
Methods: Using a combination of high-throughput quantitative PCR, RNA sequencing, and in situ hybridization, we systematically quantified GPCRs expressed in vagal unmyelinated Nav1.8-expressing afferents.
Results: GPCRs for gut hormones that were the most enriched in Nav1.8-expressing vagal unmyelinated afferents included NTSR1, NPY2R, CCK1R, and to a lesser extent, GLP1R, but not GHSR and GIPR. Interestingly, both GLP1R and NPY2R were coexpressed with CCK1R. In contrast, NTSR1 was coexpressed with GPR65, a marker preferentially enriched in intestinal mucosal afferents. Only few microbiome-derived metabolite sensors such as GPR35 and, to a lesser extent, GPR119 and CaSR were identified in the Nav1.8-expressing vagal afferents. GPCRs involved in lipid sensing and inflammation (e.g. CB1R, CYSLTR2, PTGER4), and neurotransmitters signaling (CHRM4, DRD2, CRHR2) were also highly enriched in Nav1.8-expressing neurons. Finally, we identified 21 orphan GPCRs with unknown functions in vagal afferents.
Conclusions: Overall, this study provides a comprehensive description of GPCR-dependent sensing mechanisms in vagal afferents, including novel coexpression patterns, and conceivably coaction of key receptors for gut-derived molecules involved in gut-brain communication.[Hide abstract]
|Neuraminidase 1 activates insulin receptor and reverses insulin resistance Neuraminidases catalyze the removal of sialic acid residues from glycan chains of glycoproteins. Mammalian neuraminidase 1 (NEU1) modulates cellular receptors involved in diverse signaling pathways, one of which is the insulin receptor (IR). Fougerat et al. reveal the molecular mechanism by which NEU1 desialylation activates the IR. They also show that acute pharmacological increase of NEU1 activity in insulin target tissues may reverse insulin resistance and glucose intolerance induced in mice by obesity. Thus, selective targeting of NEU1 may represent a novel therapeutic strategy for restoring insulin sensitivity and resolving hyperglycemia associated with type 2 diabetes.|
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Objective: Neuraminidase 1 (NEU1) cleaves terminal sialic acids of glycoconjugates during lysosomal catabolism. It also modulates the structure and activity of cellular surface receptors affecting diverse pathways. Previously we demonstrated that NEU1 activates the insulin receptor (IR) and that NEU1-deficient CathAS190A-Neo mice (hypomorph of the NEU1 activator protein, cathepsin A/CathA) on a high-fat diet (HFD) develop hyperglycaemia and insulin resistance faster than wild-type animals. The major objective of the current work was to reveal the molecular mechanism by which NEU1 desialylation activates the IR and to test if increase of NEU1 activity in insulin target tissues reverses insulin resistance and glucose intolerance.
Methods: To test if desialylation causes a conformational change in the IR dimer we measured interaction between the receptor subunits by Bioluminescence Resonance Energy Transfer in the HEK293T cells either overexpressing NEU1 or treated with the NEU1 inhibitor. The influence of NEU1 overexpression on insulin resistance was studied in vitro in palmitate-treated HepG2 cells transduced with NEU1-expressing lentivirus and in vivo in C57Bl6 mice treated with HFD and either pharmacological inducer of NEU1, Ambroxol or NEU1-expressing adenovirus. NEU1-deficient CathAS190A-Neo mice were used as a control.
Results: By desialylation of IR, NEU1 induced formation of its active dimer leading to insulin signaling. Overexpression of NEU1 in palmitate-treated HepG2 cells restored insulin signaling, suggesting that increased NEU1 levels may reverse insulin resistance. Five-day treatment of glycemic C57Bl6 mice receiving HFD with the activator of the lysosomal gene network, Ambroxol, increased NEU1 expression and activity in muscle tissue, normalized fasting glucose levels, and improved physiological and molecular responses to glucose and insulin. Ambroxol did not improve insulin sensitivity in obese insulin-resistant CathAS190A-Neo mice indicating that the Ambroxol effect is mediated through NEU1 induction. Sustained increase of liver NEU1 activity through adenovirus-based gene transfer failed to attenuate insulin resistance most probably due to negative feedback regulation of IR expression.
Conclusions: Together our results demonstrate that increase of NEU1 activity in insulin target tissues reverses insulin resistance and glucose intolerance suggesting that a pharmacological modulation of NEU1 activity may be potentially explored for restoring insulin sensitivity and resolving hyperglycemia associated with T2DM.[Hide abstract]
|Female sex hormones are necessary for the metabolic effects mediated by loss of Interleukin 18 signalingInterleukin (IL)-18 is a cytokine that plays a crucial role in maintaining metabolic homeostasis. IL-18 is both necessary and sufficient for the maintenance of metabolic homeostasis. IL-18 levels are influenced by gender, age, and sex hormones. Lindegaard, Abildgaard and colleagues hypothesized that the presence of female sex hormones could preserve the metabolic phenotype of IL-18R-/- mice. They show that the combination of a lack of IL-18 signaling and female sex hormones, but not IL-18 or female sex hormone per se, leads to systemic insulin resistance and impaired insulin signaling in the liver.
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Objective: Interleukin (IL)-18 plays a crucial role in maintaining metabolic homeostasis and levels of this cytokine are influenced by gender, age, and sex hormones. The role of gender on IL-18 signaling, however, is unclear. We hypothesized that the presence of female sex hormone could preserve the metabolic phenotype of the IL-18R−/− animals.
Methods: We studied female mice with a global deletion of the α isoform of the IL-18 receptor (IL-18R−/−) and littermates control. Three studies were done: 1) animals fed a high fat diet (HFD) for 16 weeks; 2) animals fed chow diet for 72 weeks and 3) animals (3 weeks-old) randomized to either bilateral ovariectomy (OVX) or control surgery (SHAM) and followed for 16 weeks.
Results: Female IL-18R−/− mice gained less weight and maintained glucose homeostasis on a chow diet compared with HFD, but no differences between genotypes were observed. The maintenance of body weight and glucose homeostasis in IL-18R−/− mice was lost with aging. By 72 weeks of age, IL-18R−/− mice became heavier compared with WT mice due to an increase in both visceral and subcutaneous adiposity and displayed glucose intolerance. OVX did not affect body weight in IL-18R−/− mice but exacerbated glucose intolerance and impaired liver insulin signaling when compared with SHAM mice.
Conclusions: Female mice harboring a global deletion of the IL-18R, only present the same phenotype as reported in male IL-18R−/− mice if they are aged or have undergone OVX, in which circulating estrogen is likely to be blunted. The role of estrogen signaling in the protection against altered metabolic homeostasis in IL-18R−/− mice appears to be mediated by liver insulin signaling. We therefore suggest that the metabolic effects mediated by loss of IL-18 signaling are only present in a female sex hormone free environment.[Hide abstract]
|mTORC1-dependent increase in oxidative metabolism in POMC neurons regulates food intake and action of leptinReactive oxygen species (ROS) modify the activity of proopiomelanocortin (POMC) expressing neurons of the hypothalamic arcuate nucleus (ARC), which control feeding behavior and peripheral metabolism. Haissaguerre et al. hypothesized that the mammalian target of rapamycin complex 1 (mTORC1) pathway mediates ROS-dependent responses in POMC neurons. Their study reveals that ROS require a functional mTORC1 pathway in POMC neurons to decrease food intake and that increased ROS and mTORC1 activity in POMC neurons are needed in order to observe the appetite suppressant action of leptin.
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Objective: Nutrient availability modulates reactive oxygen species (ROS) production in the hypothalamus. In turn, ROS regulate hypothalamic neuronal activity and feeding behavior. The mechanistic target of rapamycin complex 1 (mTORC1) pathway is an important cellular integrator of the action of nutrients and hormones. Here we tested the hypothesis that modulation of mTORC1 activity, particularly in Proopiomelanocortin (POMC)-expressing neurons, mediates the cellular and behavioral effects of ROS.
Methods: C57BL/6J mice or controls and their knockout (KO) littermates deficient either for the mTORC1 downstream target 70-kDa ribosomal protein S6 kinase 1 (S6K1) or for the mTORC1 component Rptor specifically in POMC neurons (POMC-rptor-KO) were treated with an intracerebroventricular (icv) injection of the ROS hydrogen peroxide (H2O2) or the ROS scavenger honokiol, alone or, respectively, in combination with the mTORC1 inhibitor rapamycin or the mTORC1 activator leptin. Oxidant-related signal in POMC neurons was assessed using dihydroethidium (DHE) fluorescence.
Results: Icv administration of H2O2 decreased food intake, while co-administration of rapamycin, whole-body deletion of S6K1, or deletion of rptor in POMC neurons impeded the anorectic action of H2O2. H2O2 also increased oxidant levels in POMC neurons, an effect that hinged on functional mTORC1 in these neurons. Finally, scavenging ROS prevented the hypophagic action of leptin, which in turn required mTORC1 to increase oxidant levels in POMC neurons and to inhibit food intake.
Conclusions: Our results demonstrate that ROS and leptin require mTORC1 pathway activity in POMC neurons to increase oxidant levels in POMC neurons and consequently decrease food intake.[Hide abstract]
|The Role of IL-1 in Postprandial FatigueAfter ingesting a large meal, it is common to experience postprandial fatigue. Perception of fatigue has been linked to interleukin-1 (IL-1) - a family of 11 cytokines playing an important role in the initiation and regulation of the inflammatory response. The underlying mechanisms leading to postprandial fatigue are not well understood. Lehrskov et al. studied a cohort of both lean and obese individuals to investigate the role of IL-1 in postprandial fatigue. Their results indicate that IL-1 is at least partly driving postprandial fatigue in both lean and obese individuals.|
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Cytokines such as IL-1 seems to play a role in the pathogenesis of fatigue associated with some chronic diseases and anti-inflammatory treatment has been shown to reduce these symptoms.
Methods: In a double-blind, crossover study in 8 lean and 8 obese male subjects, randomized to receive either saline (placebo) or the IL-1 receptor antagonist anakinra, we investigated whether postprandial fatigue was regulated by IL-1. To promote postprandial fatigue, subjects ran 30 min prior to a high-fat, high-carbohydrate meal. Fatigue was determined using the Stanford Sleepiness Scale and blood samples were drawn at baseline and after the intervention.
Results: IL-1 antagonism led to a reduction in postprandial fatigue and this effect was more pronounced in obese than lean individuals.
Conclusions: We conclude that IL-1 is involved in the regulation of postprandial fatigue under physiologic conditions in lean and obese individuals. It remains to be shown whether this effect translates into clinical relevant effects.[Hide abstract]
|Regional differences in brain glucose metabolism Glucose is the main energy substrate for the brain, which accounts for about 20% of whole-body glucose utilization. Glucose is metabolized in cells either by glycolysis or the pentose phosphate pathway (PPP). Though each cell can use glucose for either glycolysis or the PPP, different cell populations favor distinct metabolic pathways. To investigate the regional fate of glucose, Kleinridders, Ferris, Reyzer et al. assessed multiple steps within the glycolytic pathway and PPP using a combination of gene and protein expression, protein activity assays, and imaging mass spectrometry (IMS). They demonstrate that IMS provides a direct measurement of the metabolites generated in these pathways in specific brain regions, including those which are otherwise difficult to assess.
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Objective: Glucose is the major energy substrate of the brain and crucial for normal brain function. In diabetes, the brain is subject to episodes of hypo- and hyperglycemia resulting in acute outcomes ranging from confusion to seizures, while chronic metabolic dysregulation puts patients at increased risk for depression and Alzheimer's disease. In the present study, we aimed to determine how glucose is metabolized in different regions of the brain using imaging mass spectrometry (IMS).
Methods: To examine the relative abundance of glucose and other metabolites in the brain, mouse brain sections were subjected to imaging mass spectrometry at a resolution of 100 μm. This was correlated with immunohistochemistry, qPCR, western blotting and enzyme assays of dissected brain regions to determine the relative contributions of the glycolytic and pentose phosphate pathways to regional glucose metabolism.
Results: In brain, there are significant regional differences in glucose metabolism, with low levels of hexose bisphosphate (a glycolytic intermediate) and high levels of the pentose phosphate pathway (PPP) enzyme glucose-6-phosphate dehydrogenase (G6PD) and PPP metabolite hexose phosphate in thalamus compared to cortex. The ratio of ATP to ADP is significantly higher in white matter tracts, such as corpus callosum, compared to less myelinated areas. While the brain is able to maintain normal ratios of hexose phosphate, hexose bisphosphate, ATP, and ADP during fasting, fasting causes a large increase in cortical and hippocampal lactate.
Conclusions: These data demonstrate the importance of direct measurement of metabolic intermediates to determine regional differences in brain glucose metabolism and illustrate the strength of imaging mass spectrometry for investigating the impact of changing metabolic states on brain function at a regional level with high resolution.[Hide abstract]