Effects of saturated versus unsaturated fatty acids on metabolism, gliosis, and hypothalamic leptin sensitivity in male mice.

BACKGROUND: Development of obesity and its comorbidities is not only the result of excess energy intake, but also of dietary composition. Understanding how hypothalamic metabolic circuits interpret nutritional signals is fundamental to advance towards effective dietary interventions. OBJECTIVE: We aimed to determine the metabolic response to diets enriched in specific fatty acids. METHODS: Male mice received a diet enriched in unsaturated fatty acids (UOLF) or saturated fatty acids (SOLF) for 8 weeks. RESULTS: UOLF and SOLF mice gained more weight and adiposity, but with no difference between these two groups. Circulating leptin levels increased on both fatty acid-enriched diet, but were higher in UOLF mice, as were leptin mRNA levels in visceral adipose tissue. In contrast, serum non-esterified fatty acid levels only rose in SOLF mice. Hypothalamic mRNA levels of NPY decreased and of POMC increased in both UOLF and SOLF mice, but only SOLF mice showed signs of hypothalamic astrogliosis and affectation of central fatty acid metabolism. Exogenous leptin activated STAT3 in the hypothalamus of all groups, but the activation of AKT and mTOR and the decrease in AMPK activation in observed in controls and UOLF mice was not found in SOLF mice. CONCLUSIONS: Diets rich in fatty acids increase body weight and adiposity even if energy intake is not increased, while increased intake of saturated and unsaturated fatty acids differentially modify metabolic parameters that could underlie more long-term comorbidities. Thus, more understanding of how specific nutrients affect metabolism, weight gain, and obesity associated complications is necessary.

Long-Term Outcomes of Biological Therapy in Crohn's Disease Complicated With Internal Fistulizing Disease: BIOSCOPE Study From GETECCU.

INTRODUCTION: The prevalence of penetrating complications in Crohn's disease (CD) increases progressively over time, but evidence on the medical treatment in this setting is limited. The aim of this study was to evaluate the effectiveness of biologic agents in CD complicated with internal fistulizing disease. METHODS: Adult patients with CD-related fistulae who received at least 1 biologic agent for this condition from the prospectively maintained ENEIDA registry were included. Exclusion criteria involved those receiving biologics for perianal disease, enterocutaneous, rectovaginal, anastomotic, or peristomal fistulae. The primary end point was fistula-related surgery. Predictive factors associated with surgery and fistula closure were evaluated by multivariate logistic regression and survival analyses. RESULTS: A total of 760 patients from 53 hospitals (673 receiving anti-tumor necrosis factors, 69 ustekinumab, and 18 vedolizumab) were included. After a median follow-up of 56 months (interquartile range, 26-102 months), 240 patients required surgery, with surgery rates of 32%, 41%, and 24% among those under anti-tumor necrosis factor, vedolizumab, or ustekinumab, respectively. Fistula closure was observed in 24% of patients. Older patients, ileocolonic disease, entero-urinary fistulae, or an intestinal stricture distal to the origin of the fistula were associated with a higher risk of surgery, whereas nonsmokers and combination therapy with an immunomodulator reduced this risk. DISCUSSION: Biologic therapy is beneficial in approximately three-quarters of patients with fistulizing CD, achieving fistula closure in 24%. However, around one-third still undergo surgery due to refractory disease. Some patient- and lesion-related factors can identify patients who will obtain more benefit from these drugs.

Regional specific effect of saturated vs unsaturated fat on leptin receptor signalling in mice brain areas regulating feeding.

Leptin receptors (LepR) are expressed in brain areas controlling food intake homeostasis, such as the hypothalamus, the hippocampus and the prefrontal cortex. In a previous study we reported that long-term intake of saturated and monounsaturated fat alters hypothalamic LepR signalling. The current study aims at investigating the effect of foods high in either saturated (SOLF) or monounsaturated fat (UOLF) on LepR functionality in the hippocampus and the prefrontal cortex. Male mice were placed on SOLF/UOLF (eight weeks), then treated with recombinant murine leptin (1 mg/kg). After 60 min, brain regions were dissected and processed for western blot of phosphorylated STAT3 (pSTAT3), Akt (pAkt) and AMPK (pAMPK). Levels of SOCS3 were also quantified. SOLF itself increased basal levels of pSTAT3, while UOLF impaired leptin-induced phosphorylation of both Akt and AMPK. SOCS3 levels were specifically increased by UOLF within the prefrontal cortex. Our results show that SOLF and UOLF differently affect LepR signalling within the hippocampus and the prefrontal cortex, which points to the complex effect of saturated and unsaturated fat on brain function, particularly in areas regulating food intake.

Saturated and unsaturated triglyceride-enriched diets modify amino acid content in the mice hippocampus.

Elevated intake of fat modulates l-glutamate (l-Glu) turnover within the hippocampus (HIP). Our aim has been to investigate the effect of saturated vs unsaturated fat on the content of l-Glu and other amino acids involved in synaptic transmission within the HIP. The study was carried out in male mice fed (2 h or 8 weeks) with standard chow or with diets enriched either with saturated (SOLF) or unsaturated triglycerides (UOLF). An in vitro assay was performed in HIP slices incubated with palmitic (PA), oleic (OA), or lauric acid (LA). Amino acids were quantified by capillary electrophoresis. While both diets increased the amount of l-Glu and l-aspartate and decreased l-glutamine levels, only UOLF affected d-serine and taurine levels. γ-Aminobutyric acid was specifically decreased by SOLF. In vitro assays revealed that PA and OA modified l-Glu, glycine, l-serine and d-serine concentration. Our results suggest that fatty acids contained in SOLF and UOLF have an impact on HIP amino acid turnover that may account, at least partially, for the functional changes evoked by these diets.

Effect of Lauric vs. Oleic Acid-Enriched Diets on Leptin Autoparacrine Signalling in Male Mice.

High-fat diets enriched with lauric acid (SOLF) do not enhance leptin production despite expanding white adipose tissue (WAT). Our study aimed at identifying the influence of SOLF vs. oleic acid-enriched diets (UOLF) on the autoparacrine effect of leptin and was carried out on eight-week-old mice consuming control chow, UOLF or SOLF. Phosphorylation of kinases integral to leptin receptor (LepR) signalling pathways (705Tyr-STAT3, 473Ser-Akt, 172Thr-AMPK), adipocyte-size distribution, fatty acid content, and gene expression were analyzed in WAT. SOLF enhanced basal levels of phosphorylated proteins but reduced the ability of leptin to enhance kinase phosphorylation. In contrast, UOLF failed to increase basal levels of phosphorylated proteins and did not modify the effect of leptin. Both SOLF and UOLF similarly affected adipocyte-size distribution, and the expression of genes related with adipogenesis and inflammation. WAT composition was different between groups, with SOLF samples mostly containing palmitic, myristic and lauric acids (>48% w/w) and UOLF WAT containing more than 80% (w/w) of oleic acid. In conclusion, SOLF appears to be more detrimental than UOLF to the autoparacrine leptin actions, which may have an impact on WAT inflammation. The effect of SOLF and UOLF on WAT composition may affect WAT biophysical properties, which are able to condition LepR signaling.

Butyric Acid Precursor Tributyrin Modulates Hippocampal Synaptic Plasticity and Prevents Spatial Memory Deficits: Role of PPARγ and AMPK.

BACKGROUND: Short chain fatty acids (SCFA), such as butyric acid (BA), derived from the intestinal fermentation of dietary fiber and contained in dairy products, are gaining interest in relation to their possible beneficial effects on neuropsychological disorders. METHODS: C57BL/6J male mice were used to investigate the effect of tributyrin (TB), a prodrug of BA, on hippocampus (HIP)-dependent spatial memory, HIP synaptic transmission and plasticity mechanisms, and the expression of genes and proteins relevant to HIP glutamatergic transmission. RESULTS: Ex vivo studies, carried out in HIP slices, revealed that TB can transform early-LTP into late-LTP (l-LTP) and to rescue LTP-inhibition induced by scopolamine. The facilitation of l-LTP induced by TB was blocked both by GW9662 (a PPARγ antagonist) and C-Compound (an AMPK inhibitor), suggesting the involvement of both PPARγ and AMPK on TB effects. Moreover, 48-hour intake of a diet containing 1% TB prevented, in adolescent but not in adult mice, scopolamine-induced impairment of HIP-dependent spatial memory. In the adolescent HIP, TB upregulated gene expression levels of Pparg, leptin, and adiponectin receptors, and that of the glutamate receptor subunits AMPA-2, NMDA-1, NMDA-2A, and NMDA-2B. CONCLUSIONS: Our study shows that TB has a positive influence on LTP and HIP-dependent spatial memory, which suggests that BA may have beneficial effects on memory.

Specific Deletion of the Astrocyte Leptin Receptor Induces Changes in Hippocampus Glutamate Metabolism, Synaptic Transmission and Plasticity.

The aim of this study was to indentify the involvement of leptin receptors (LepR) in astrocytes in hippocampal synaptic transmission and plasticity and metabolism. To this end we used a genetic mouse model (GFAP-LepR-/-) of specific LepR ablation in GFAP positive cells and recorded excitatory postsynaptic potentials (fEPSPs) within the CA1 area. Glutamate (Glu) uptake and the expression of Glu transporters (EEAT3, GLT-1 and GLAST) and enzymes involved in Glu metabolism (glutamine synthase, GABA decarboxylase 65 and 67) were quantified. Modifications in the expression of GFAP, the glucose transporter (GLUT)-1, and the monocarboxylate transporters MCT-2 and MCT-4, were also analyzed. The results show that depletion of LepR in GFAP positive cells reduced basal synaptic transmission within the CA1 area and impaired N-methyl-d-aspartate (NMDA)-evoked long-term depression (NMDA-LTD). Hippocampal slices from GFAP-LepR-/- mice displayed lower Glu uptake efficacy together with up-regulation of GLT-1, glutamine synthase, GFAP and GLUT-1. In conclusion, astrocyte LepRs are involved in the maintenance of Glu homeostasis and Glu neurotransmission within the hippocampus. Our findings support a role of hippocampal LepRs in synaptic plasticity, which could have an impact on memory and learning processes.

Inflammatory stress and altered angiogenesis evoked by very high-fat diets in mouse liver.

BACKGROUND: Non-alcoholic fatty liver disease (NAFLD), a condition that leads to fibrosis, is caused by intake of very high-fat diets (HFDs). However, while the negative impact on the liver of these diets has been an issue of interest, systematic research on the effect of HFDs are lacking. OBJECTIVE: To characterize the overall impact of HFDs on both molecular and morphological signs of liver remodeling. METHODS: A study was conducted on male C57BL/6J mice to assess the effect of 4- and 8-week HFDs (60% kcal from fat) on (i) liver steatosis and fibrosis, and (ii) expression of factors involved in inflammation and angiogenesis. RESULTS: After an 8-week HFD, vascular endothelial growth factor type-2 receptor (VEGF-R2) and fatty acid translocase/trombospondin-1 receptor (CD36) were overexpressed in liver tissue of mice given HFDs. These changes suggest impaired liver angiogenesis and occurred together with (i) increased GPR78-BiP and EIF2α phosphorylation, suggesting endoplasmic reticulum stress, (ii) induction of Col1a1 gene expression, a marker of fibrosis, and (iii) increased CD31 immunolabeling, consistent with active angiogenesis and fibrosis. CONCLUSION: Our data show that very HFDs promote a rapid inflammatory response, as well as deregulation of angiogenesis, both consistent with development of liver fibrosis.

Cholecystokinin is involved in triglyceride fatty acid uptake by rat adipose tissue.

The incorporation of plasma triglyceride (TG) fatty acids to white adipose tissue (WAT) depends on lipoprotein lipase (LPL), which is regulated by angiopoietin-like protein-4 (ANGPTL-4), an unfolding molecular chaperone that converts active LPL dimers into inactive monomers. The production of ANGPTL-4 is promoted by fasting and repressed by feeding. We hypothesized that the postprandial hormone cholecystokinin (CCK) facilitates the storage of dietary TG fatty acids in WAT by regulating the activity of the LPL/ANGPTL-4 axis and that it does so by acting directly on CCK receptors in adipocytes. We report that administration of CCK-8 (a bioactive fragment of CCK) to rats: (i) reduces plasma ANGTPL-4 levels; (ii) represses Angptl-4 expression in WAT and (iii) simultaneously enhances LPL activity in this tissue without inducing Lpl expression. In vivo CCK-8 effects are specifically antagonized by the CCK-2 receptor (CCK-2R) antagonist, L-365,260. Moreover, CCK-8 downregulates Angptl-4 expression in wild-type pre-adipocytes, an effect that is not observed in engineered pre-adipocytes lacking CCK-2R. These effects have functional consequences as CCK-8 was found to promote the uptake of dietary fatty acids by WAT, as demonstrated by means of proton nuclear magnetic resonance (1H-NMR). The efficacy of acute CCK-8 administration was not reduced after chronic CCK-8 treatment. Moreover, the effects of CCK-8 on WAT were not associated to the increase of circulating insulin. Our results show that cholecystokinin promotes lipid storage in WAT by acting on adipocyte CCK-2R, suggesting a pivotal role for CCK in TG homeostasis.

Free-choice high-fat diet alters circadian oscillation of energy intake in adolescent mice: role of prefrontal cortex.

PURPOSE: Our aim was to characterize the effect of an unfamiliar high-fat diet (HFD) on circadian feeding behaviour, plasma parameters, body weight (BW), and gene expression in the prefrontal cortex (PFC) of adolescent male mice. To this end, mice were allowed to consume a HFD during 48 h, but one group was allowed a free choice of HFD or normal chow (FC-HFD), while the other was restricted to a non-optional unfamiliar HFD feeding (NOP-HFD). METHODS: Energy intake was monitored at 6-h intervals during 48 h. Mice cohorts were killed at 6-h intervals after 48-h dietary treatment, and PFC samples dissected for RT-PCR analysis. RESULTS: Mice on the FC-HFD protocol avoided eating the standard chow, showed lower energy intake and lower BW increase than NOP-HFD mice. All animals with access to HFD exhibited nocturnal overeating, but diurnal hyperphagia was more prominent in the FC-HFD cohort. A robust increase in tyrosine hydroxylase (Th) gene expression was detected specifically during the light period of the circadian cycle in FC-HFD mice. In contrast, both protocols similarly up-regulated the expression of cytosolic malic enzyme (Me1), which is very sensitive to HFD. CONCLUSION: Our data show that the PFC participates in driving motivational feeding during HFD-evoked hyperphagia and also suggest that sensory neural pathways might be relevant for the onset of eating disorders and overweight. Moreover, we have observed that animals that had the possibility of choosing between standard chow and HFD were more hyperphagic and specifically displayed an overexpression of the tyrosine hydroxylase gene.

Deciphering tissue-induced Klebsiella pneumoniae lipid A structure.

The outcome of an infection depends on host recognition of the pathogen, hence leading to the activation of signaling pathways controlling defense responses. A long-held belief is that the modification of the lipid A moiety of the lipopolysaccharide could help Gram-negative pathogens to evade innate immunity. However, direct evidence that this happens in vivo is lacking. Here we report the lipid A expressed in the tissues of infected mice by the human pathogen Klebsiella pneumoniae. Our findings demonstrate that Klebsiella remodels its lipid A in a tissue-dependent manner. Lipid A species found in the lungs are consistent with a 2-hydroxyacyl-modified lipid A dependent on the PhoPQ-regulated oxygenase LpxO. The in vivo lipid A pattern is lost in minimally passaged bacteria isolated from the tissues. LpxO-dependent modification reduces the activation of inflammatory responses and mediates resistance to antimicrobial peptides. An lpxO mutant is attenuated in vivo thereby highlighting the importance of this lipid A modification in Klebsiella infection biology. Colistin, one of the last options to treat multidrug-resistant Klebsiella infections, triggers the in vivo lipid A pattern. Moreover, colistin-resistant isolates already express the in vivo lipid A pattern. In these isolates, LpxO-dependent lipid A modification mediates resistance to colistin. Deciphering the lipid A expressed in vivo opens the possibility of designing novel therapeutics targeting the enzymes responsible for the in vivo lipid A pattern.

Klebsiella pneumoniae survives within macrophages by avoiding delivery to lysosomes.

Klebsiella pneumoniae is an important cause of community-acquired and nosocomial pneumonia. Evidence indicates that Klebsiella might be able to persist intracellularly within a vacuolar compartment. This study was designed to investigate the interaction between Klebsiella and macrophages. Engulfment of K. pneumoniae was dependent on host cytoskeleton, cell plasma membrane lipid rafts and the activation of phosphoinositide 3-kinase (PI3K). Microscopy studies revealed that K. pneumoniae resides within a vacuolar compartment, the Klebsiella-containing vacuole (KCV), which traffics within vacuoles associated with the endocytic pathway. In contrast to UV-killed bacteria, the majority of live bacteria did not co-localize with markers of the lysosomal compartment. Our data suggest that K. pneumoniae triggers a programmed cell death in macrophages displaying features of apoptosis. Our efforts to identify the mechanism(s) whereby K. pneumoniae prevents the fusion of the lysosomes to the KCV uncovered the central role of the PI3K-Akt-Rab14 axis to control the phagosome maturation. Our data revealed that the capsule is dispensable for Klebsiella intracellular survival if bacteria were not opsonized. Furthermore, the environment found by Klebsiella within the KCV triggered the down-regulation of the expression of cps. Altogether, this study proves evidence that K. pneumoniae survives killing by macrophages by manipulating phagosome maturation that may contribute to Klebsiella pathogenesis.

Involvement of the dorsomedial prefrontal cortex in high-fat food conditioning in adolescent mice.

Our hypothesis is that direct targeting of brain areas involved in the perception of food as a rewarding stimulus accounts for initial hyperphagia caused by high-fat food (HFD). Because adolescents are more sensitive than adults to HFD, studies were performed in five-week old male mice. We analyzed the effect of acute exposition to HFD on c-Fos immunolabeling and we observed that this diet selectively increased c-Fos immunolabeling in the dorsomedial prefrontal cortex (PFC). Furthermore HFD triggered strong and long-lasting conditioned place-preference (CPP) behavior. We also found that the strength of conditioning correlated with the up-regulation of the expression of genes involved in dopaminergic transmission together with a decreased expression of the Per2 gene in the CPF. Our data are coherent with the involvement of the dorsomedial PFC in the perception of HFD as a positive reinforcer and suggest that sensory stimuli activate this brain area after HFD intake.

Morphological changes in glial fibrillary acidic protein immunopositive astrocytes in the hippocampus of dietary-induced obese mice.

Long-term consumption of a high-fat diet (HFD) has been shown to trigger both metabolic and cardiovascular diseases. In contrast, the effect of this type of dietary regime on the central nervous system, particularly outside the hypothalamus, has been investigated poorly. Astrocytes, the most abundant population of glial cells in the brain, are pivotal in regulating glutamatergic transmission as they are responsible for most of the glutamate uptake and metabolism. Mice on an HFD show deficits in learning and memory, together with neurochemical and electrophysiological changes compatible with the impairment in hippocampal glutamatergic activity. Because astrocyte function and morphology have been shown to be interdependent, we speculated whether HFD would trigger changes in astrocyte morphology. For this purpose, we have used a model of diet-induced obesity in mice. We have analyzed astrocyte morphology and density by glial fibrillary acidic protein immunohistochemistry, as well as the expression of the glutamate transporters, GLT-1 (glutamate transporter type-1), and GLAST (astrocyte glutamate transporter), in the CA3 area of the hippocampus. We found that astrocytes from HFD mice showed longer and less abundant projections. These changes were accompanied by the upregulation of both GLT-1 and GLAST. Our data show that the functional impairment detected previously in HFD mice is concomitant with morphological changes within the hippocampus.

Role of bacterial surface structures on the interaction of Klebsiella pneumoniae with phagocytes.

Phagocytosis is a key process of the immune system. The human pathogen Klebsiella pneumoniae is a well known example of a pathogen highly resistant to phagocytosis. A wealth of evidence demonstrates that the capsule polysaccharide (CPS) plays a crucial role in resistance to phagocytosis. The amoeba Dictyostelium discoideum shares with mammalian macrophages the ability to phagocytose and kill bacteria. The fact that K. pneumoniae is ubiquitous in nature and, therefore, should avoid predation by amoebae, poses the question whether K. pneumoniae employs similar means to counteract amoebae and mammalian phagocytes. Here we developed an assay to evaluate K. pneumoniae-D. discoideum interaction. The richness of the growth medium affected the threshold at which the cps mutant was permissive for Dictyostelium and only at lower nutrient concentrations the cps mutant was susceptible to predation by amoebae. Given the critical role of bacterial surface elements on host-pathogen interactions, we explored the possible contribution of the lipopolysaccharide (LPS) and outer membrane proteins (OMPs) to combat phagoyctosis by D. discoideum. We uncover that, in addition to the CPS, the LPS O-polysaccharide and the first core sugar participate in Klebsiella resistance to predation by D. discoideum. K. pneumoniae LPS lipid A decorations are also necessary to avoid predation by amoebae although PagP-dependent palmitoylation plays a more important role than the lipid A modification with aminoarabinose. Mutants lacking OMPs OmpA or OmpK36 were also permissive for D. discoideium growth. Except the LPS O-polysaccharide mutants, all mutants were more susceptible to phagocytosis by mouse alveolar macrophages. Finally, we found a correlation between virulence, using the pneumonia mouse model, and resistance to phagocytosis. Altogether, this work reveals novel K. pneumoniae determinants involved in resistance to phagocytosis and supports the notion that Dictyostelium amoebae might be useful as host model to measure K. pneumoniae virulence and not only phagocytosis.

Host cell kinases, α5 and ß1 integrins, and Rac1 signalling on the microtubule cytoskeleton are important for non-typable Haemophilus influenzae invasion of respiratory epithelial cells.

Non-typable Haemophilus influenzae (NTHi) is a common commensal of the human nasopharynx, but causes opportunistic infection when the respiratory tract is compromised by infection or disease. The ability of NTHi to invade epithelial cells has been described, but the underlying molecular mechanisms are poorly characterized. We previously determined that NTHi promotes phosphorylation of the serine-threonine kinase Akt in A549 human lung epithelial cells, and that Akt phosphorylation and NTHi cell invasion are prevented by inhibition of phosphoinositide 3-kinase (PI3K). Because PI3K-Akt signalling is associated with several host cell networks, the purpose of the current study was to identify eukaryotic molecules important for NTHi epithelial invasion. We found that inhibition of Akt activity reduced NTHi internalization; differently, bacterial entry was increased by phospholipase Cγ1 inhibition but was not affected by protein kinase inhibition. We also found that α5 and ß1 integrins, and the tyrosine kinases focal adhesion kinase and Src, are important for NTHi A549 cell invasion. NTHi internalization was shown to be favoured by activation of Rac1 guanosine triphosphatase (GTPase), together with the guanine nucleotide exchange factor Vav2 and the effector Pak1. Also, Pak1 might be associated with inactivation of the microtubule destabilizing agent Op18/stathmin, to facilitate microtubule polymerization and NTHi entry. Conversely, inhibition of RhoA GTPase and its effector ROCK increased the number of internalized bacteria. Src and Rac1 were found to be important for NTHi-triggered Akt phosphorylation. An increase in host cyclic AMP reduced bacterial entry, which was linked to protein kinase A. These findings suggest that NTHi finely manipulates host signalling molecules to invade respiratory epithelial cells.

Shift of circadian feeding pattern by high-fat diets is coincident with reward deficits in obese mice.

Recent studies provide evidence that high-fat diets (HF) trigger both i) a deficit of reward responses linked to a decrease of mesolimbic dopaminergic activity, and ii) a disorganization of circadian feeding behavior that switch from a structured meal-based schedule to a continuous snacking, even during periods normally devoted to rest. This feeding pattern has been shown to be a cause of HF-induced overweight and obesity. Our hypothesis deals with the eventual link between the rewarding properties of food and the circadian distribution of meals. We have investigated the effect of circadian feeding pattern on reward circuits by means of the conditioned-place preference (CPP) paradigm and we have characterized the rewarding properties of natural (food) and artificial (cocaine) reinforcers both in free-feeding ad libitum HF mice and in HF animals submitted to a re-organized feeding schedule based on the standard feeding behavior displayed by mice feeding normal chow ("forced synchronization"). We demonstrate that i) ad libitum HF diet attenuates cocaine and food reward in the CPP protocol, and ii) forced synchronization of feeding prevents this reward deficit. Our study provides further evidence that the rewarding impact of food with low palatability is diminished in mice exposed to a high-fat diet and strongly suggest that the decreased sensitivity to chow as a positive reinforcer triggers a disorganized feeding pattern which might account for metabolic disorders leading to obesity.

High-fat diets induce changes in hippocampal glutamate metabolism and neurotransmission.

Obesity and high-fat (HF) diets have a deleterious impact on hippocampal function and lead to impaired synaptic plasticity and learning deficits. Because all of these processes need an adequate glutamatergic transmission, we have hypothesized that nutritional imbalance triggered by these diets might eventually concern glutamate (Glu) neural pathways within the hippocampus. Glu is withdrawn from excitatory synapses by specific uptake mechanisms involving neuronal (EAAT-3) and glial (GLT-1, GLAST) transporters, which regulate the time that synaptically released Glu remains in the extracellular space and, consequently, the duration and location of postsynaptic receptor activation. The goal of the present study was to evaluate in mouse hippocampus the effect of a short-term high-fat dietary treatment on 1) Glu uptake kinetics, 2) the density of Glu carriers and Glu-degrading enzymes, 3) the density of Glu receptor subunits, and 4) synaptic transmission and plasticity. Here, we show that HF diet triggers a 50% decrease of the Michaelis-Menten constant together with a 300% increase of the maximal velocity of the uptake process. Glial Glu carriers GLT-1 and GLAST were upregulated in HF mice (32 and 27%, respectively), whereas Glu-degrading enzymes glutamine synthase and GABA-decarboxilase appeared to be downregulated in these animals. In addition, HF diet hippocampus displayed diminished basal synaptic transmission and hindered NMDA-induced long-term depression (NMDA-LTD). This was coincident with a reduced density of the NR2B subunit of NMDA receptors. All of these results are compatible with the development of leptin resistance within the hippocampus. Our data show that HF diets upregulate mechanisms involved in Glu clearance and simultaneously impair Glu metabolism. Neurochemical changes occur concomitantly with impaired basal synaptic transmission and reduced NMDA-LTD. Taken together, our results suggest that HF diets trigger neurochemical changes, leading to a desensitization of NMDA receptors within the hippocampus, which might account for cognitive deficits.

Circadian feeding drive of metabolic activity in adipose tissue and not hyperphagia triggers overweight in mice: is there a role of the pentose-phosphate pathway?

High-fat (HF) diets trigger an increase in adipose tissue and body weight (BW) and disordered eating behavior. Our study deals with the hypothesis that circadian distribution of energy intake is more relevant for BW dynamics than diet composition. Four-week-old mice were exposed for 8 wk to a HF diet and compared with animals receiving control chow. HF mice progressively increased BW, decreased the amount of nocturnal (1800-0900 h) calories (energy or food intake) (30%) and increased diurnal (0900-1800 h) caloric intake (energy or food intake), although total daily intake was identical between groups. Animals were killed at 3-h intervals and plasma insulin, leptin, corticosterone, glucose, and fatty acid levels quantified. Adipose tissue was weighed, and enzymatic activities integral to the pentose phosphate pathway (PPP) assayed in lumbar adipose tissue. Phosphorylated AMP-dependent protein kinase and fatty acid synthase were quantified by Western blotting. In HF mice, there was a shift in the circadian oscillations of plasma parameters together with an inhibition of PPP activity and a decrease in phosphorylated AMP-dependent protein kinase and fatty acid synthase. In a second experiment, HF mice were forced to adhere to a circadian pattern of food intake similar to that in control animals. In this case, BW, adipose tissue, morning plasma parameters and PPP activity appeared to be normal. These data indicate that disordered feeding behavior can trigger BW gain independently of food composition and daily energy intake. Because PPP is the main source of reduced nicotinamide adenine dinucleotide phosphate, we suggest that PPP inhibition might be an early marker of adipose dysfunction in diet-induced obesity.

Leptin resistance develops spontaneously in mice during adult life in a tissue-specific manner. Consequences for hepatic steatosis.

Leptin is an adipocyte-derived hormone which stimulates ß-oxidation in peripheral tissues and prevents steatosis. Because leptin production naturally increases during adult life, we have hypothesized that leptin receptors might undergo a physiological and gradual desensitization during ageing. Therefore we have characterized in three- five- and ten-month old mice i) the weight of different white adipose pads, heart and liver, ii) lipid content in these tissues/organs, and iii) responsiveness to acute leptin, measured in terms of phosphorylation of signal transducer and activator of transcription 3 (STAT3) and protein kinase B (Akt). In this study we have detected that leptin-mediated STAT3 phosphorylation appears to be preserved in cardiac tissue even in 10-month old animals but not in adipose tissue and liver of five- and ten-month old mice, respectively. Nevertheless, leptin increased pAkt content in the liver of these mice. In a parallel study we have analyzed the functionality of leptin signalling pathways in 10-month old obese mice and we have observed that the STAT3 pathway appears to be only operative in the heart whereas the Akt pathway remains functional both in heart and liver. Nevertheless, hepatic lipids increased almost 300% compared to age-matched lean controls. Our data demonstrate that during adult life there is a lost of leptin receptor functionality which is tissue-dependent and mainly affects the STAT3 pathway. Otherwise we demonstrate that the antisteatotic effect of leptin is independent of the Akt signalling pathway.