Region-Specific Cell Membrane N-Glycome of Functional Mouse Brain Areas Revealed by nanoLC-MS Analysis

N-glycosylation is a ubiquitous posttranslational modification that affects protein structure and function, including those of the central nervous system. N-glycans attached to cell membrane proteins play crucial roles in all aspects of biology, including embryogenesis, development, cell-cell recognition and adhesion, and cell signaling and communication. Although brain function and behavior are known to be regulated by the N-glycosylation state of numerous cell surface glycoproteins, our current understanding of brain glycosylation is limited, and glycan variations associated with functional brain regions remain largely unknown. In this work, we used a well-established cell surface glycomic nanoLC-Chip-Q-TOF platform developed in our laboratory to characterize the N-glycome of membrane fractions enriched in cell surface glycoproteins obtained from specific functional brain areas. We report the cell membrane N-glycome of two major developmental divisions of mice brain with specific and distinctive functions, namely the forebrain and hindbrain. Region-specific glycan maps were obtained with ∼120 N-glycan compositions in each region, revealing significant differences in "brain-type" glycans involving high mannose, bisecting, and core and antenna fucosylated species. Additionally, the cell membrane N-glycome of three functional regions of the forebrain and hindbrain, the cerebral cortex, hippocampus, and cerebellum, was characterized. In total, 125 N-glycan compositions were identified, and their region-specific expression profiles were characterized. Over 70 N-glycans contributed to the differentiation of the cerebral cortex, hippocampus, and cerebellum N-glycome, including bisecting and branched glycans with varying degrees of core and antenna fucosylation and sialylation. This study presents a comprehensive spatial distribution of the cell-membrane enriched N-glycomes associated with five discrete anatomical and functional brain areas, providing evidence for the presence of a previously unknown brain glyco-architecture. The region-specific molecular glyco fingerprints identified here will enable a better understanding of the critical biological roles that N-glycans play in the specialized functional brain areas in health and disease.

Myelin as a regulator of development of the microbiota-gut-brain axis

Myelination in the peripheral and central nervous systems is critical in regulating motor, sensory, and cognitive functions. As myelination occurs rapidly during early life, neonatal gut dysbiosis during early colonization can potentially alter proper myelination by dysregulating immune responses and neuronal differentiation. Despite common usage of antibiotics (Abx) in children, the impact of neonatal Abx-induced dysbiosis on the development of microbiota, gut, brain (MGB) axis, including myelination and behavior, is unknown. We hypothesized that neonatal Abx-induced dysbiosis dysregulates host-microbe interactions, impairing myelination in the brain, and altering the MGB axis. Neonatal C57BL/6 mice were orally gavaged daily with an Abx cocktail (neomycin, vancomycin, ampicillin) or water (vehicle) from postnatal day 7 (P7) until weaning (P23) to induce gut dysbiosis. Behavior (cognition; anxiety-like behavior), microbiota sequencing, and qPCR (ileum, colon, hippocampus and pre-frontal cortex [PFC]) were performed in adult mice (6-8 weeks). Neonatal Abx administration led to intestinal dysbiosis in adulthood, impaired intestinal physiology, coupled with perturbations of bacterial metabolites and behavioral alterations (cognitive deficits and anxiolytic behavior). Expression of myelin-related genes (Mag, Mog, Mbp, Mobp, Plp) and transcription factors (Sox10, Myrf) important for oligodendrocytes were significantly increased in the PFC region of Abx-treated mice. Increased myelination was confirmed by immunofluorescence imaging and western blot analysis, demonstrating increased expression of MBP, SOX10 and MYRF in neonatally Abx-treated mice compared to sham controls in adulthood. Finally, administration of the short chain fatty acid butyrate following completion of the Abx treatment restored intestinal physiology, behavior, and myelination impairments, suggesting a critical role for the gut microbiota in mediating these effects. Taken together, we identified a long-lasting impact of neonatal Abx administration on the MGB axis, specifically on myelin regulation in the PFC region, potentially contributing to impaired cognitive function and bacterial metabolites are effective in reversing this altered phenotype.

Metabolic Responses to Butyrate Supplementation in LF- and HF-Fed Mice Are Cohort-Dependent and Associated with Changes in Composition and Function of the Gut Microbiota

The gut microbiota and associated metabolites have emerged as potential modulators of pathophysiological changes in obesity and related metabolic disorders. Butyrate, a product of bacterial fermentation, has been shown to have beneficial effects in obesity and rodent models of diet-induced obesity. Here, we aimed to determine the beneficial effects of butyrate (as glycerol ester of butyrate monobutyrin, MB) supplementation on metabolic phenotype, intestinal permeability and inflammation, feeding behavior, and the gut microbiota in low-fat (LF)- and high-fat (HF)-fed mice. Two cohorts (separated by 2 weeks) of male C57BL/6J mice (n = 24 in each cohort, 6/group/cohort; 6 weeks old) were separated into four weight-matched groups and fed either a LF (10 % fat/kcal) or HF (45% fat/kcal) with or without supplementation of MB (LF/MB or HF/MB) at 0.25% (w/v) in drinking water for 6 weeks. Metabolic phenotypes (body weight and adiposity), intestinal inflammation, feeding behavior, and fecal microbiome and metabolites were measured. Despite identical genetic and experimental conditions, we found marked differences between cohorts in the response (body weight gain, adiposity, and intestinal permeability) to HF-diet and MB. Notably, the composition of the gut microbiota was significantly different between cohorts, characterized by lower species richness and differential abundance of a large number of taxa, including subtaxa from five phyla, including increased abundance of the genera Bacteroides, Proteobacteria, and Parasutterella in cohort 2 compared to cohort 1. These differences may have contributed to the differential response in intestinal permeability to the HF diet in cohort 2. MB supplementation had no significant effect on metabolic phenotype, but there was a trend to protect from HF-induced impairments in intestinal barrier function in cohort 1 and in sensitivity to cholecystokinin (CCK) in both cohorts. These data support the concept that microbiota composition may have a crucial effect on metabolic responses of a host to dietary interventions and highlight the importance of taking steps to ensure reproducibility in rodent studies.

Specific ablation of the NCoR corepressor δ splice variant reveals alternative RNA splicing as a key regulator of hepatic metabolism

The NCoR corepressor plays critical roles in mediating transcriptional repression by both nuclear receptors and non-receptor transcription factors. Alternative mRNA splicing of NCoR produces a series of variants with differing molecular and biological properties. The NCoRω splice-variant inhibits adipogenesis whereas the NCoRδ splice-variant promotes it, and mice bearing a splice-specific knockout of NCoRω display enhanced hepatic steatosis and overall weight gain on a high fat diet as well as a greatly increased resistance to diet-induced glucose intolerance. We report here that the reciprocal NCoRδ splice-specific knock-out mice display the contrary phenotypes of reduced hepatic steatosis and reduced weight gain relative to the NCoRω-/- mice. The NCoRδ-/- mice also fail to demonstrate the strong resistance to diet-induced glucose intolerance exhibited by the NCoRω-/- animals. The NCoR δ and ω variants possess both unique and shared transcriptional targets, with expression of certain hepatic genes affected in opposite directions in the two mutants, others altered in one but not the other genotype, and yet others changed in parallel in both NCoRδ-/- and NCoRω-/- animals versus WT. Gene set expression analysis (GSEA) identified a series of lipid, carbohydrate, and amino acid metabolic pathways that are likely to contribute to their distinct steatosis and glucose tolerance phenotypes. We conclude that alternative-splicing of the NCoR corepressor plays a key role in the regulation of hepatic energy storage and utilization, with the NCoRδ and NCoRω variants exerting both opposing and shared functions in many aspects of this phenomenon and in the organism as a whole.

A murine model of pediatric inflammatory bowel disease causes microbiota-gut-brain axis deficits in adulthood

Inflammatory bowel diseases (IBDs) are chronic intestinal diseases, frequently associated with comorbid psychological and cognitive deficits. These neuropsychiatric effects include anxiety, depression, and memory impairments that can be seen both during active disease and following remission and are more frequently seen in pediatric patients. The mechanism(s) through which these extraintestinal deficits develop remain unknown, and the study of these phenomenon is hampered by a lack of murine pediatric IBD models. Herein we describe microbiota-gut-brain (MGB) axis deficits following induction of colitis in a pediatric setting. Acute colitis was induced by administration of 2% dextran sodium sulfate (DSS) for 5 days starting at weaning [postnatal day (P)21] causing reduced weight gain, colonic shortening, and colonic inflammation by 8 days post-DSS (P29), which were mostly resolved in adult (P56) mice. Despite resolution of acute disease, cognitive deficits (novel object recognition task) and anxiety-like behavior (light/dark box) were identified in the absence of changes in exploratory behavior (open field test) in P56 mice previously treated with DSS at weaning. Behavioral deficits were found in conjunction with neuroinflammation, decreased neurogenesis, and altered expression of pattern recognition receptor genes in the hippocampus. Additionally, persistent alterations in the gut microbiota composition were observed at P56, including reduced butyrate-producing species. Taken together, these results describe for the first time the presence of MGB axis deficits following induction of colitis at weaning, which persist in adulthood.NEW & NOTEWORTHY Here we describe long-lasting impacts on the microbiota-gut-brain (MGB) axis following administration of low-dose dextran sodium sulfate (DSS) to weaning mice (P21), including gut dysbiosis, colonic inflammation, and brain/behavioral deficits in adulthood (P56). Early-life DSS leads to acute colonic inflammation, similar to adult mice; however, it results in long-lasting deficits in the MGB axis in adulthood (P56), in contrast to the transient deficits seen in adult DSS. This model highlights the unique features of pediatric inflammatory bowel disease.

Sex-specific alterations in whole body energetics and voluntary activity in heterozygous R163C malignant hyperthermia-susceptible mice

Malignant hyperthermia (MH) is characterized by induction of skeletal muscle hyperthermia in response to a dysregulated increase in myoplasmic calcium. Although altered energetics play a central role in MH, MH‐susceptible humans and mouse models are often described as having no phenotype until exposure to a triggering agent. The purpose of this study was to determine the influence of the R163C ryanodine receptor 1 mutation, a common MH mutation in humans, on energy expenditure, and voluntary wheel running in mice. Energy expenditure was measured by indirect respiration calorimetry in wild‐type (WT) and heterozygous R163C (HET) mice over a range of ambient temperatures. Energy expenditure adjusted for body weight or lean mass was increased (P < .05) in male, but not female, HET mice housed at 22°C or when housed at 28°C with a running wheel. In female mice, voluntary wheel running was decreased (P < .05) in the HET vs WT animals when analyzed across ambient temperatures. The thermoneutral zone was also widened in both male and female HET mice. The results of the study show that the R163C mutations alters energetics even at temperatures that do not typically induce MH.

Blood cytokine patterns suggest a modest inflammation phenotype in subjects with long-chain fatty acid oxidation disorders

Excessive cellular accumulation or exposure to lipids such as long‐chain acylcarnitines (LCACs), ceramides, and others is implicated in cell stress and inflammation. Such a situation might manifest when there is a significant mismatch between long‐chain fatty acid (LCFA) availability versus storage and oxidative utilization; for example, in cardiac ischemia, increased LCACs may contribute to tissue cell stress and infarct damage. Perturbed LCFAβ‐oxidation is also seen in fatty acid oxidation disorders (FAODs). FAODs typically manifest with fasting‐ or stress‐induced symptoms, and patients can manage many symptoms through control of diet and physical activity. However, episodic clinical events involving cardiac and skeletal muscle myopathies are common and can present without an obvious molecular trigger. We have speculated that systemic or tissue‐specific lipotoxicity and activation of inflammation pathways contribute to long‐chain FAOD pathophysiology. With this in mind, we characterized inflammatory phenotype (14 blood plasma cytokines) in resting, overnight‐fasted (~10 h), or exercise‐challenged subjects with clinically well‐controlled long‐chain FAODs (n = 12; 10 long‐chain 3‐hydroxyacyl‐CoA dehydrogenase [LCHAD]; 2 carnitine palmitoyltransferase 2 [CPT2]) compared to healthy controls (n = 12). Across experimental conditions, concentrations of three cytokines were modestly but significantly increased in FAOD (IFNγ, IL‐8, and MDC), and plasma levels of IL‐10 (considered an inflammation‐dampening cytokine) were significantly decreased. These novel results indicate that while asymptomatic FAOD patients do not display gross body‐wide inflammation even after moderate exercise, β‐oxidation deficiencies might be associated with chronic and subtle activation of “sterile inflammation.” Further studies are warranted to determine if inflammation is more apparent in poorly controlled long‐chain FAOD or when long‐chain FAOD‐associated symptoms are present.

MeCP2 isoform e1 mutant mice recapitulate motor and metabolic phenotypes of Rett syndrome

Mutations in the X-linked gene MECP2 cause the majority of Rett syndrome (RTT) cases. Two differentially spliced isoforms of exons 1 and 2 (MeCP2-e1 and MeCP2-e2) contribute to the diverse functions of MeCP2, but only mutations in exon 1, not exon 2, are observed in RTT. We previously described an isoform-specific MeCP2-e1-deficient male mouse model of a human RTT mutation that lacks MeCP2-e1 while preserving expression of MeCP2-e2. However, RTT patients are heterozygous females that exhibit delayed and progressive symptom onset beginning in late infancy, including neurologic as well as metabolic, immune, respiratory and gastrointestinal phenotypes. Consequently, we conducted a longitudinal assessment of symptom development in MeCP2-e1 mutant females and males. A delayed and progressive onset of motor impairments was observed in both female and male MeCP2-e1 mutant mice, including hind limb clasping and motor deficits in gait and balance. Because these motor impairments were significantly impacted by age-dependent increases in body weight, we also investigated metabolic phenotypes at an early stage of disease progression. Both male and female MeCP2-e1 mutants exhibited significantly increased body fat compared to sex-matched wild-type littermates prior to weight differences. Mecp2e1-/y males exhibited significant metabolic phenotypes of hypoactivity, decreased energy expenditure, increased respiratory exchange ratio, but decreased food intake compared to wild-type. Untargeted analysis of lipid metabolites demonstrated a distinguishable profile in MeCP2-e1 female mutant liver characterized by increased triglycerides. Together, these results demonstrate that MeCP2-e1 mutation in mice of both sexes recapitulates early and progressive metabolic and motor phenotypes of human RTT.

Conservation Implications of Shifting Gut Microbiomes in Captive-Reared Endangered Voles Intended for Reintroduction into the Wild

The Amargosa vole is a highly endangered rodent endemic to a small stretch of the Amargosa River basin in Inyo County, California. It specializes on a single, nutritionally marginal food source in nature. As part of a conservation effort to preserve the species, a captive breeding population was established to serve as an insurance colony and a source of individuals to release into the wild as restored habitat becomes available. The colony has successfully been maintained on commercial diets for multiple generations, but there are concerns that colony animals could lose gut microbes necessary to digest a wild diet. We analyzed feces from colony-reared and recently captured wild-born voles on various diets, and foregut contents from colony and wild voles. Unexpectedly, fecal microbial composition did not greatly differ despite drastically different diets and differences observed were mostly in low-abundance microbes. In contrast, colony vole foregut microbiomes were dominated by Allobaculum sp. while wild foreguts were dominated by Lactobacillus sp. If these bacterial community differences result in beneficial functional differences in digestion, then captive-reared Amargosa voles should be prepared prior to release into the wild to minimize or eliminate those differences to maximize their chance of success.

Reduced cognitive function, increased blood-brain-barrier transport and inflammatory responses, and altered brain metabolites in LDLr -/-and C57BL/6 mice fed a western diet

Recent work suggests that diet affects brain metabolism thereby impacting cognitive function. Our objective was to determine if a western diet altered brain metabolism, increased blood-brain barrier (BBB) transport and inflammation, and induced cognitive impairment in C57BL/6 (WT) mice and low-density lipoprotein receptor null (LDLr -/-) mice, a model of hyperlipidemia and cognitive decline. We show that a western diet and LDLr -/- moderately influence cognitive processes as assessed by Y-maze and radial arm water maze. Also, western diet significantly increased BBB transport, as well as microvessel factor VIII in LDLr -/- and microglia IBA1 staining in WT, both indicators of activation and neuroinflammation. Interestingly, LDLr -/- mice had a significant increase in ¹⁸F- fluorodeoxyglucose uptake irrespective of diet and brain ¹H-magnetic resonance spectroscopy showed increased lactate and lipid moieties. Metabolic assessments of whole mouse brain by GC/MS and LC/MS/MS showed that a western diet altered brain TCA cycle and β-oxidation intermediates, levels of amino acids, and complex lipid levels and elevated proinflammatory lipid mediators. Our study reveals that the western diet has multiple impacts on brain metabolism, physiology, and altered cognitive function that likely manifest via multiple cellular pathways.

Unique plasma metabolomic signatures of individuals with inherited disorders of long-chain fatty acid oxidation

Blood and urine acylcarnitine profiles are commonly used to diagnose long-chain fatty acid oxidation disorders (FAOD: i.e., long-chain hydroxy-acyl-CoA dehydrogenase [LCHAD] and carnitine palmitoyltransferase 2 [CPT2] deficiency), but the global metabolic impact of long-chain FAOD has not been reported. We utilized untargeted metabolomics to characterize plasma metabolites in 12 overnight-fasted individuals with FAOD (10 LCHAD, two CPT2) and 11 healthy age-, sex-, and body mass index (BMI)-matched controls, with the caveat that individuals with FAOD consume a low-fat diet supplemented with medium-chain triglycerides (MCT) while matched controls consume a typical American diet. In plasma 832 metabolites were identified, and partial least squared-discriminant analysis (PLS-DA) identified 114 non-acylcarnitine variables that discriminated FAOD subjects and controls. FAOD individuals had significantly higher triglycerides and lower specific phosphatidylethanolamines, ceramides, and sphingomyelins. Differences in phosphatidylcholines were also found but the directionality differed by metabolite species. Further, there were few differences in non-lipid metabolites, indicating the metabolic impact of FAOD specifically on lipid pathways. This analysis provides evidence that LCHAD/CPT2 deficiency significantly alters complex lipid pathway flux. This metabolic signature may provide new clinical tools capable of confirming or diagnosing FAOD, even in subjects with a mild phenotype, and may provide clues regarding the biochemical and metabolic impact of FAOD that is relevant to the etiology of FAOD symptoms.

Acylcarnitines--old actors auditioning for new roles in metabolic physiology

Perturbations in metabolic pathways can cause substantial increases in plasma and tissue concentrations of long-chain acylcarnitines (LCACs). For example, the levels of LCACs and other acylcarnitines rise in the blood and muscle during exercise, as changes in tissue pools of acyl-coenzyme A reflect accelerated fuel flux that is incompletely coupled to mitochondrial energy demand and capacity of the tricarboxylic acid cycle. This natural ebb and flow of acylcarnitine generation and accumulation contrasts with that of inherited fatty acid oxidation disorders (FAODs), cardiac ischaemia or type 2 diabetes mellitus. These conditions are characterized by very high (FAODs, ischaemia) or modestly increased (type 2 diabetes mellitus) tissue and blood levels of LCACs. Although specific plasma concentrations of LCACs and chain-lengths are widely used as diagnostic markers of FAODs, research into the potential effects of excessive LCAC accumulation or the roles of acylcarnitines as physiological modulators of cell metabolism is lacking. Nevertheless, a growing body of evidence has highlighted possible effects of LCACs on disparate aspects of pathophysiology, such as cardiac ischaemia outcomes, insulin sensitivity and inflammation. This Review, therefore, aims to provide a theoretical framework for the potential consequences of tissue build-up of LCACs among individuals with metabolic disorders.

Long-chain acylcarnitines activate cell stress and myokine release in C2C12 myotubes: calcium-dependent and -independent effects

Acylcarnitines, important lipid biomarkers reflective of acyl-CoA status, are metabolites that possess bioactive and inflammatory properties. This study examined the potential for long-chain acylcarnitines to activate cellular inflammatory, stress, and death pathways in a skeletal muscle model. Differentiated C2C12 myotubes treated with l-C14, C16, C18, and C18:1 carnitine displayed dose-dependent increases in IL-6 production with a concomitant rise in markers of cell permeability and death, which was not observed for shorter chain lengths. l-C16 carnitine, used as a representative long-chain acylcarnitine at initial extracellular concentrations ≥25 μM, increased IL-6 production 4.1-, 14.9-, and 31.4-fold over vehicle at 25, 50, and 100 μM. Additionally, l-C16 carnitine activated c-Jun NH2-terminal kinase, extracellular signal-regulated kinase, and p38 mitogen-activated protein kinase between 2.5- and 11-fold and induced cell injury and death within 6 h with modest activation of the apoptotic caspase-3 protein. l-C16 carnitine rapidly increased intracellular calcium, most clearly by 10 μM, implicating calcium as a potential mechanism for some activities of long-chain acylcarnitines. The intracellular calcium chelator BAPTA-AM blunted l-C16 carnitine-mediated IL-6 production by >65%. However, BAPTA-AM did not attenuate cell permeability and death responses, indicating that these outcomes are calcium independent. The 16-carbon zwitterionic compound amidosulfobetaine-16 qualitatively mimicked the l-C16 carnitine-associated cell stress outcomes, suggesting that the effects of high experimental concentrations of long-chain acylcarnitines are through membrane disruption. Herein, a model is proposed in which acylcarnitine cell membrane interactions take place along a spectrum of cellular concentrations encountered in physiological-to-pathophysiological conditions, thus regulating function of membrane-based systems and impacting cell biology.

Whey protein supplementation does not alter plasma branched-chained amino acid profiles but results in unique metabolomics patterns in obese women enrolled in an 8-week weight loss trial

BACKGROUND

It has been suggested that perturbations in branched-chain amino acid (BCAA) catabolism are associated with insulin resistance and contribute to elevated systemic BCAAs. Evidence in rodents suggests dietary protein rich in BCAAs can increase BCAA catabolism, but there is limited evidence in humans.

OBJECTIVE

We hypothesize that a diet rich in BCAAs will increase BCAA catabolism, which will manifest in a reduction of fasting plasma BCAA concentrations.

METHODS

The metabolome of 27 obese women with metabolic syndrome before and after weight loss was investigated to identify changes in BCAA metabolism using GC-time-of-flight mass spectrometry. Subjects were enrolled in an 8-wk weight-loss study including either a 20-g/d whey (whey group, n = 16) or gelatin (gelatin group, n = 11) protein supplement. When matched for total protein by weight, whey protein has 3 times the amount of BCAAs compared with gelatin protein.

RESULTS

Postintervention plasma abundances of Ile (gelatin group: 637 ± 18, quantifier ion peak height ÷ 100; whey group: 744 ± 65), Leu (gelatin group: 1210 ± 33; whey group: 1380 ± 79), and Val (gelatin group: 2080 ± 59; whey group: 2510 ± 230) did not differ between treatment groups. BCAAs were significantly correlated with homeostasis model assessment of insulin resistance at baseline (r = 0.52, 0.43, and 0.49 for Leu, Ile, and Val, respectively; all, P < 0.05), but correlations were no longer significant at postintervention. Pro- and Cys-related pathways were found discriminant of whey protein vs. gelatin protein supplementation in multivariate statistical analyses.

CONCLUSIONS

These findings suggest that BCAA metabolism is, at best, only modestly affected at a whey protein supplementation dose of 20 g/d. Furthermore, the loss of an association between postintervention BCAA and homeostasis model assessment suggests that factors associated with calorie restriction or protein intake affect how plasma BCAAs relate to insulin sensitivity. This trial was registered at clinicaltrials.gov as NCT00739479.

Acylcarnitines: potential implications for skeletal muscle insulin resistance

Insulin resistance may be linked to incomplete fatty acid β-oxidation and the subsequent increase in acylcarnitine species in different tissues including skeletal muscle. It is not known if acylcarnitines participate in muscle insulin resistance or simply reflect dysregulated metabolism. The aims of this study were to determine whether acylcarnitines can elicit muscle insulin resistance and to better understand the link between incomplete muscle fatty acid β-oxidation, oxidative stress, inflammation, and insulin-resistance development. Differentiated C2C12, primary mouse, and human myotubes were treated with acylcarnitines (C4:0, C14:0, C16:0) or with palmitate with or without carnitine acyltransferase inhibition by mildronate. Treatment with C4:0, C14:0, and C16:0 acylcarnitines resulted in 20-30% decrease in insulin response at the level of Akt phosphorylation and/or glucose uptake. Mildronate reversed palmitate-induced insulin resistance concomitant with an ∼25% decrease in short-chain acylcarnitine and acetylcarnitine secretion. Although proinflammatory cytokines were not affected under these conditions, oxidative stress was increased by 2-3 times by short- or long-chain acylcarnitines. Acylcarnitine-induced oxidative stress and insulin resistance were reversed by treatment with antioxidants. Results are consistent with the conclusion that incomplete muscle fatty acid β-oxidation causes acylcarnitine accumulation and associated oxidative stress, raising the possibility that these metabolites play a role in muscle insulin resistance.

Acylcarnitines activate proinflammatory signaling pathways

Incomplete β-oxidation of fatty acids in mitochondria is a feature of insulin resistance and type 2 diabetes mellitus (T2DM). Previous studies revealed that plasma concentrations of medium- and long-chain acylcarnitines (by-products of incomplete β-oxidation) are elevated in T2DM and insulin resistance. In a previous study, we reported that mixed D,L isomers of C12- or C14-carnitine induced an NF-κB-luciferase reporter gene in RAW 264.7 cells, suggesting potential activation of proinflammatory pathways. Here, we determined whether the physiologically relevant L-acylcarnitines activate classical proinflammatory signaling pathways and if these outcomes involve pattern recognition receptor (PRR)-associated pathways. Acylcarnitines induced the expression of cyclooxygenase-2 in a chain length-dependent manner in RAW 264.7 cells. L-C14 carnitine (5-25 μM), used as a representative acylcarnitine, stimulated the expression and secretion of proinflammatory cytokines in a dose-dependent manner. Furthermore, L-C14 carnitine induced phosphorylation of JNK and ERK, common downstream components of many proinflammatory signaling pathways including PRRs. Knockdown of MyD88, a key cofactor in PRR signaling and inflammation, blunted the proinflammatory effects of acylcarnitine. While these results point to potential involvement of PRRs, L-C14 carnitine promoted IL-8 secretion from human epithelial cells (HCT-116) lacking Toll-like receptors (TLR)2 and -4, and did not activate reporter constructs in TLR overexpression cell models. Thus, acylcarnitines have the potential to activate inflammation, but the specific molecular and tissue target(s) involved remain to be identified.

Increased phosphoenolpyruvate carboxykinase gene expression and steatosis during hepatitis C virus subgenome replication: role of nonstructural component 5A and CCAAT/enhancer-binding protein β

Chronic hepatitis C virus (HCV) infection greatly increases the risk for type 2 diabetes and nonalcoholic steatohepatitis; however, the pathogenic mechanisms remain incompletely understood. Here we report gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK) transcription and associated transcription factors are dramatically up-regulated in Huh.8 cells, which stably express an HCV subgenome replicon. HCV increased activation of cAMP response element-binding protein (CREB), CCAAT/enhancer-binding protein (C/EBPβ), forkhead box protein O1 (FOXO1), and peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α) and involved activation of the cAMP response element in the PEPCK promoter. Infection with dominant-negative CREB or C/EBPβ-shRNA significantly reduced or normalized PEPCK expression, with no change in PGC-1α or FOXO1 levels. Notably, expression of HCV nonstructural component NS5A in Huh7 or primary hepatocytes stimulated PEPCK gene expression and glucose output in HepG2 cells, whereas a deletion in NS5A reduced PEPCK expression and lowered cellular lipids but was without effect on insulin resistance, as demonstrated by the inability of insulin to stimulate mobilization of a pool of insulin-responsive vesicles to the plasma membrane. HCV-replicating cells demonstrated increases in cellular lipids with insulin resistance at the level of the insulin receptor, increased insulin receptor substrate 1 (Ser-312), and decreased Akt (Ser-473) activation in response to insulin. C/EBPβ-RNAi normalized lipogenic genes sterol regulatory element-binding protein-1c, peroxisome proliferator-activated receptor γ, and liver X receptor α but was unable to reduce accumulation of triglycerides in Huh.8 cells or reverse the increase in ApoB expression, suggesting a role for increased lipid retention in steatotic hepatocytes. Collectively, these data reveal an important role of NS5A, C/EBPβ, and pCREB in promoting HCV-induced gluconeogenic gene expression and suggest that increased C/EBPβ and NS5A may be essential components leading to increased gluconeogenesis associated with HCV infection.

Lipid-induced insulin resistance mediated by the proinflammatory receptor TLR4 requires saturated fatty acid-induced ceramide biosynthesis in mice

Obesity is associated with an enhanced inflammatory response that exacerbates insulin resistance and contributes to diabetes, atherosclerosis, and cardiovascular disease. One mechanism accounting for the increased inflammation associated with obesity is activation of the innate immune signaling pathway triggered by TLR4 recognition of saturated fatty acids, an event that is essential for lipid-induced insulin resistance. Using in vitro and in vivo systems to model lipid induction of TLR4-dependent inflammatory events in rodents, we show here that TLR4 is an upstream signaling component required for saturated fatty acid-induced ceramide biosynthesis. This increase in ceramide production was associated with the upregulation of genes driving ceramide biosynthesis, an event dependent of the activity of the proinflammatory kinase IKKβ. Importantly, increased ceramide production was not required for TLR4-dependent induction of inflammatory cytokines, but it was essential for TLR4-dependent insulin resistance. These findings suggest that sphingolipids such as ceramide might be key components of the signaling networks that link lipid-induced inflammatory pathways to the antagonism of insulin action that contributes to diabetes.

Increased expression of receptors for orexigenic factors in nodose ganglion of diet-induced obese rats

The vagal afferent pathway is important in short-term regulation of food intake, and decreased activation of this neural pathway with long-term ingestion of a high-fat diet may contribute to hyperphagic weight gain. We tested the hypothesis that expression of genes encoding receptors for orexigenic factors in vagal afferent neurons are increased by long-term ingestion of a high-fat diet, thus supporting orexigenic signals from the gut. Obesity-prone (DIO-P) rats fed a high-fat diet showed increased body weight and hyperleptinemia compared with low-fat diet-fed controls and high-fat diet-induced obesity-resistant (DIO-R) rats. Expression of the type I cannabinoid receptor and growth hormone secretagogue receptor 1a in the nodose ganglia was increased in DIO-P compared with low-fat diet-fed controls or DIO-R rats. Shifts in the balance between orexigenic and anorexigenic signals within the vagal afferent pathway may influence food intake and body weight gain induced by high fat diets.

Gamma-synuclein is an adipocyte-neuron gene coordinately expressed with leptin and increased in human obesity

Recently, we characterized tumor suppressor candidate 5 (Tusc5) as an adipocyte-neuron PPARgamma target gene. Our objective herein was to identify additional genes that display distinctly high expression in fat and neurons, because such a pattern could signal previously uncharacterized functional pathways shared in these disparate tissues. gamma-Synuclein, a marker of peripheral and select central nervous system neurons, was strongly expressed in white adipose tissue (WAT) and peripheral nervous system ganglia using bioinformatics and quantitative PCR approaches. Gamma-synuclein expression was determined during adipogenesis and in subcutaneous (SC) and visceral adipose tissue (VAT) from obese and nonobese humans. Gamma-synuclein mRNA increased from trace levels in preadipocytes to high levels in mature 3T3-L1 adipocytes and decreased approximately 50% following treatment with the PPARgamma agonist GW1929 (P < 0.01). Because gamma-synuclein limits growth arrest and is implicated in cancer progression in nonadipocytes, we suspected that expression would be increased in situations where WAT plasticity/adipocyte turnover are engaged. Consistent with this postulate, human WAT gamma-synuclein mRNA levels consistently increased in obesity and were higher in SC than in VAT; i.e. they increased approximately 1.7-fold in obese Pima Indian adipocytes (P = 0.003) and approximately 2-fold in SC and VAT of other obese cohorts relative to nonobese subjects. Expression correlated with leptin transcript levels in human SC and VAT (r = 0.887; P < 0.0001; n = 44). Gamma-synuclein protein was observed in rodent and human WAT but not in negative control liver. These results are consistent with the hypothesis that gamma-synuclein plays an important role in adipocyte physiology.

Characterization of Tusc5, an adipocyte gene co-expressed in peripheral neurons

Tumor suppressor candidate 5 (Tusc5, also termed brain endothelial cell derived gene-1 or BEC-1), a CD225 domain-containing, cold-repressed gene identified during brown adipose tissue (BAT) transcriptome analyses was found to be robustly-expressed in mouse white adipose tissue (WAT) and BAT, with similarly high expression in human adipocytes. Tusc5 mRNA was markedly increased from trace levels in pre-adipocytes to significant levels in developing 3T3-L1 adipocytes, coincident with several mature adipocyte markers (phosphoenolpyruvate carboxykinase 1, GLUT4, adipsin, leptin). The Tusc5 transcript levels were increased by the peroxisome proliferator activated receptor-gamma (PPARgamma) agonist GW1929 (1microg/mL, 18h) by >10-fold (pre-adipocytes) to approximately 1.5-fold (mature adipocytes) versus controls (p<0.0001). Taken together, these results suggest an important role for Tusc5 in maturing adipocytes. Intriguingly, we discovered robust co-expression of the gene in peripheral nerves (primary somatosensory neurons). In light of the marked repression of the gene observed after cold exposure, these findings may point to participation of Tusc5 in shared adipose-nervous system functions linking environmental cues, CNS signals, and WAT-BAT physiology. Characterization of such links is important for clarifying the molecular basis for adipocyte proliferation and could have implications for understanding the biology of metabolic disease-related neuropathies.

Lipid mediators of insulin resistance

Lipid abnormalities such as obesity, increased circulating free fatty acid levels, and excess intramyocellular lipid accumulation are frequently associated with insulin resistance. These observations have prompted investigators to speculate that the accumulation of lipids in tissues not suited for fat storage (e.g., skeletal muscle and liver) is an underlying component of insulin resistance and the metabolic syndrome. We review the metabolic fates of lipids in insulin-responsive tissues and discuss the roles of specific lipid metabolites (e.g., ceramides, GM3 ganglioside, and diacylglycerol) as antagonists of insulin signaling and action.

Inhibition of ceramide synthesis ameliorates glucocorticoid-, saturated-fat-, and obesity-induced insulin resistance

Insulin resistance occurs in 20%-25% of the human population, and the condition is a chief component of type 2 diabetes mellitus and a risk factor for cardiovascular disease and certain forms of cancer. Herein, we demonstrate that the sphingolipid ceramide is a common molecular intermediate linking several different pathological metabolic stresses (i.e., glucocorticoids and saturated fats, but not unsaturated fats) to the induction of insulin resistance. Moreover, inhibition of ceramide synthesis markedly improves glucose tolerance and prevents the onset of frank diabetes in obese rodents. Collectively, these data have two important implications. First, they indicate that different fatty acids induce insulin resistance by distinct mechanisms discerned by their reliance on sphingolipid synthesis. Second, they identify enzymes required for ceramide synthesis as therapeutic targets for combating insulin resistance caused by nutrient excess or glucocorticoid therapy.

Increased P85alpha is a potent negative regulator of skeletal muscle insulin signaling and induces in vivo insulin resistance associated with growth hormone excess

Insulin resistance is a cardinal feature of normal pregnancy and excess growth hormone (GH) states, but its underlying mechanism remains enigmatic. We previously found a significant increase in the p85 regulatory subunit of phosphatidylinositol kinase (PI 3-kinase) and striking decrease in IRS-1-associated PI 3-kinase activity in the skeletal muscle of transgenic animals overexpressing human placental growth hormone. Herein, using transgenic mice bearing deletions in p85alpha, p85beta, or insulin-like growth factor-1, we provide novel evidence suggesting that overexpression of p85alpha is a primary mechanism for skeletal muscle insulin resistance in response to GH. We found that the excess in total p85 was entirely accounted for by an increase in the free p85alpha-specific isoform. In mice with a liver-specific deletion in insulin-like growth factor-1, excess GH caused insulin resistance and an increase in skeletal muscle p85alpha, which was completely reversible using a GH-releasing hormone antagonist. To understand the role of p85alpha in GH-induced insulin resistance, we used mice bearing deletions of the genes coding for p85alpha or p85beta, respectively (p85alpha (+/-) and p85beta(-/-)). Wild type and p85beta(-/-) mice developed in vivo insulin resistance and demonstrated overexpression of p85alpha and reduced insulin-stimulated PI 3-kinase activity in skeletal muscle in response to GH. In contrast, p85alpha(+/-)mice retained global insulin sensitivity and PI 3-kinase activity associated with reduced p85alpha expression. These findings demonstrated the importance of increased p85alpha in mediating skeletal muscle insulin resistance in response to GH and suggested a potential role for reducing p85alpha as a therapeutic strategy for enhancing insulin sensitivity in skeletal muscle.

A role for ceramide, but not diacylglycerol, in the antagonism of insulin signal transduction by saturated fatty acids

Multiple studies suggest that lipid oversupply to skeletal muscle contributes to the development of insulin resistance, perhaps by promoting the accumulation of lipid metabolites capable of inhibiting signal transduction. Herein we demonstrate that exposing muscle cells to particular saturated free fatty acids (FFAs), but not mono-unsaturated FFAs, inhibits insulin stimulation of Akt/protein kinase B, a serine/threonine kinase that is a central mediator of insulin-stimulated anabolic metabolism. These saturated FFAs concomitantly induced the accumulation of ceramide and diacylglycerol, two products of fatty acyl-CoA that have been shown to accumulate in insulin-resistant tissues and to inhibit early steps in insulin signaling. Preventing de novo ceramide synthesis negated the antagonistic effect of saturated FFAs toward Akt/protein kinase B. Moreover, inducing ceramide buildup recapitulated and augmented the inhibitory effect of saturated FFAs. By contrast, diacylglycerol proved dispensable for these FFA effects. Collectively these results identify ceramide as a necessary and sufficient intermediate linking saturated fats to the inhibition of insulin signaling.

Identification of a phosphorylation site in the hinge region of the human progesterone receptor and additional amino-terminal phosphorylation sites

We have previously reported the identification of seven in vivo phosphorylation sites in the amino-terminal region of the human progesterone receptor (PR). From our previous in vivo studies, it was evident that several phosphopeptides remained unidentified. In particular, we wished to determine whether human PR contains a phosphorylation site in the hinge region, as do other steroid receptors including chicken PR, human androgen receptor, and mouse estrogen receptor. Previously, problematic trypsin cleavage sites hampered our ability to detect phosphorylation sites in large incomplete tryptic peptides. Using a combination of mass spectrometry and in vitro phosphorylation, we have identified six previously unidentified phosphorylation sites in human PR. Using nanoelectrospray ionization mass spectrometry, we have identified two new in vivo phosphorylation sites, Ser(20) and Ser(676), in baculovirus-expressed human PR. Ser(676) is analogous to the hinge site identified in other steroid receptors. Additionally, precursor ion scans identified another phosphopeptide that contains Ser(130)-Pro(131), a likely candidate for phosphorylation. In vitro phosphorylation of PR with Cdk2 has revealed five additional in vitro Cdk2 phosphorylation sites: Ser(25), Ser(213), Thr(430), Ser(554), and Ser(676). At least two of these, Ser(213) and Ser(676), are authentic in vivo sites. We confirmed the presence of the Cdk2-phosphorylated peptide containing Ser(213) in PR from in vivo labeled T47D cells, indicating that this is an in vivo site. Our combined studies indicate that most, if not all, of the Ser-Pro motifs in human PR are sites for phosphorylation. Taken together, these data indicate that the phosphorylation of PR is highly complex, with at least 14 phosphorylation sites.

The nuclear corepressors NCoR and SMRT are key regulators of both ligand- and 8-bromo-cyclic AMP-dependent transcriptional activity of the human progesterone receptor

Previously, we defined a novel class of ligands for the human progesterone receptor (PR) which function as mixed agonists. These compounds induce a conformational change upon binding the receptor that is different from those induced by agonists and antagonists. This establishes a correlation between the structure of a ligand-receptor complex and its transcriptional activity. In an attempt to define the cellular components which distinguish between different ligand-induced PR conformations, we have determined, by using a mammalian two-hybrid assay, that the nuclear receptor corepressor (NCoR) and the silencing mediator for retinoid and thyroid hormone receptor (SMRT) differentially associate with PR depending upon the class of ligand bound to the receptor. Specifically, we observed that the corepressors preferentially associate with antagonist-occupied PR and that overexpression of these corepressors suppresses the partial agonist activity of antagonist-occupied PR. Binding studies performed in vitro, however, reveal that recombinant SMRT can interact with PR in a manner which is not influenced by the nature of the bound ligand. Thus, the inability of SMRT or NCoR to interact with agonist-activated PR when assayed in vivo may relate more to the increased affinity of PR for coactivators, with a subsequent displacement of corepressors, than to an inherent low affinity for the corepressor proteins. Previous work from other groups has shown that 8-bromo-cyclic AMP (8-bromo-cAMP) can convert the PR antagonist RU486 into an agonist and, additionally, can potentiate the transcriptional activity of agonist-bound PR. In this study, we show that exogenous expression of NCoR or SMRT suppresses all 8-bromo-cAMP-mediated potentiation of PR transcriptional activity. Further analysis revealed that 8-bromo-cAMP addition decreases the association of NCoR and SMRT with PR. Thus, we propose that 8-bromo-cAMP-mediated potentiation of PR transcriptional activity is due, at least in part, to a disruption of the interaction between PR and the corepressors NCoR and SMRT. Cumulatively, these results suggest that NCoR and SMRT expression may play a pivotal role in PR pharmacology.