Hypothalamic sensing of fatty acids

Nutritional sensing and its utility in treating obesity

Obesity remains a major worldwide health problem, with current medical treatments being poorly effective. Nutrient sensing allows organs such as the GI tract and the brain to recognize and respond to fuel substrates such as carbohydrates, protein and fats. Specialized neural and hormonal pathways exist to facilitate and regulate these chemosensory mechanisms. Manipulation of factors involved in either central or peripheral chemosensory pathways may provide possible targets for the manipulation of appetite. However, further research is required to assess the utility of this approach to developing novel anti-obesity agents.

Metabolic syndrome as a risk factor for neurological disorders

The metabolic syndrome is a cluster of common pathologies: abdominal obesity linked to an excess of visceral fat, insulin resistance, dyslipidemia and hypertension. At the molecular level, metabolic syndrome is accompanied not only by dysregulation in the expression of adipokines (cytokines and chemokines), but also by alterations in levels of leptin, a peptide hormone released by white adipose tissue. These changes modulate immune response and inflammation that lead to alterations in the hypothalamic 'bodyweight/appetite/satiety set point,' resulting in the initiation and development of metabolic syndrome. Metabolic syndrome is a risk factor for neurological disorders such as stroke, depression and Alzheimer's disease. The molecular mechanism underlying the mirror relationship between metabolic syndrome and neurological disorders is not fully understood. However, it is becoming increasingly evident that all cellular and biochemical alterations observed in metabolic syndrome like impairment of endothelial cell function, abnormality in essential fatty acid metabolism and alterations in lipid mediators along with abnormal insulin/leptin signaling may represent a pathological bridge between metabolic syndrome and neurological disorders such as stroke, Alzheimer's disease and depression. The purpose of this review is not only to describe the involvement of brain in the pathogenesis of metabolic syndrome, but also to link the pathogenesis of metabolic syndrome with neurochemical changes in stroke, Alzheimer's disease and depression to a wider audience of neuroscientists with the hope that this discussion will initiate more studies on the relationship between metabolic syndrome and neurological disorders.

Hypothalamic lipophagy and energetic balance

Autophagy is a conserved cellular turnover process that degrades unwanted cytoplasmic material within lysosomes. Through “in bulk” degradation of cytoplasmic proteins and organelles, including lipid droplets, autophagy helps provide an alternative fuel source, in particular, when nutrients are scarce. Recent work demonstrates a role for autophagy in hypothalamic agouti-related peptide (AgRP) neurons in regulation of food intake and energy balance. The induction of autophagy in hypothalamic neurons during starvation mobilizes neuronal neutral lipids to generate neuron-intrinsic free fatty acids that serve to upregulate fasting-induced AgRP levels. Blocking autophagy in AgRP neurons in mice reduces fasting-induced food intake, and increases constitutive levels of anorexigenic hypothalamic proopiomelanocortin and its cleavage product α-melanocyte stimulating hormone. The energetic consequences of these molecular events are decreased body weight and reduced adiposity. The present article discusses this recent finding, as well as considers possible future directions that may help better understand how neuronal autophagy, and its possible reduction during aging, may affect whole body energy balance.

Opposing effects of fructokinase C and A isoforms on fructose-induced metabolic syndrome in mice

Fructose intake from added sugars correlates with the epidemic rise in obesity, metabolic syndrome, and nonalcoholic fatty liver disease. Fructose intake also causes features of metabolic syndrome in laboratory animals and humans. The first enzyme in fructose metabolism is fructokinase, which exists as two isoforms, A and C. Here we show that fructose-induced metabolic syndrome is prevented in mice lacking both isoforms but is exacerbated in mice lacking fructokinase A. Fructokinase C is expressed primarily in liver, intestine, and kidney and has high affinity for fructose, resulting in rapid metabolism and marked ATP depletion. In contrast, fructokinase A is widely distributed, has low affinity for fructose, and has less dramatic effects on ATP levels. By reducing the amount of fructose for metabolism in the liver, fructokinase A protects against fructokinase C-mediated metabolic syndrome. These studies provide insights into the mechanisms by which fructose causes obesity and metabolic syndrome.

Obesity and adipokines: effects on sympathetic overactivity

Excess body weight is a major risk factor for cardiovascular disease, increasing the risk of hypertension, hyperglycaemia and dyslipidaemia, recognized as the metabolic syndrome. Adipose tissue acts as an endocrine organ by producing various signalling cytokines called adipokines (including leptin, free fatty acids, tumour necrosis factor-α, interleukin-6, C-reactive protein, angiotensinogen and adiponectin). A chronic dysregulation of certain adipokines can have deleterious effects on insulin signalling. Chronic sympathetic overactivity is also known to be present in central obesity, and recent findings demonstrate the consequence of an elevated sympathetic outflow to organs such as the heart, kidneys and blood vessels. Chronic sympathetic nervous system overactivity can also contribute to a further decline of insulin sensitivity, creating a vicious cycle that may contribute to the development of the metabolic syndrome and hypertension. The cause of this overactivity is not clear, but may be driven by certain adipokines. The purpose of this review is to summarize how obesity, notably central or visceral as observed in the metabolic syndrome, leads to adipokine expression contributing to changes in insulin sensitivity and overactivity of the sympathetic nervous system.

Activation of central orexin/hypocretin neurons by dietary amino acids

Hypothalamic orexin/hypocretin (orx/hcrt) neurons regulate energy balance, wakefulness, and reward; their loss produces narcolepsy and weight gain. Glucose can lower the activity of orx/hcrt cells, but whether other dietary macronutrients have similar effects is unclear. We show that orx/hcrt cells are stimulated by nutritionally relevant mixtures of amino acids (AAs), both in brain slice patch-clamp experiments, and in c-Fos expression assays following central or peripheral administration of AAs to mice in vivo. Physiological mixtures of AAs electrically excited orx/hcrt cells through a dual mechanism involving inhibition of K(ATP) channels and activation of system-A amino acid transporters. Nonessential AAs were more potent in activating orx/hcrt cells than essential AAs. Moreover, the presence of physiological concentrations of AAs suppressed the glucose responses of orx/hcrt cells. These results suggest a new mechanism of hypothalamic integration of macronutrient signals and imply that orx/hcrt cells sense macronutrient balance, rather than net energy value, in extracellular fluid.

Fasting and postprandial conditions affect both fatty acids and lipid compositions in the hypothalamus and fat-soluble vitamins in the serum of male rats

The purpose of this study was to examine the effects of fasting and re-feeding on lipid derivative patterns in the hypothalamus, and on vitamin, cholesterol and malondialdehyde (MDA) levels in serum. Adult male Wistar albino rats were assigned (n = 6/group) as follows: normally-fed controls (CON), fasted for 24 h (24F), fasted for 48 h (48F), and fed normally for 2 d after fasting for 48 h (FAF). Biochemical measures were analyzed by gas chromatography (GC) and high performance liquid chromatography (HPLC). Our results demonstrate that hypothalamic saturated fatty acid (C16:0, C18:0) levels were lower, and unsaturated fatty acid (C22:6 n-3, C22:4 n-6, C20:4 n-6) levels were higher, in the 48F and FAF groups than in CON (). In addition, hypothalamic monounsaturated fatty acid (MUFA) levels were lower, and polyunsaturated fatty acids (PUFA) levels were higher, in 24F than in CON. Total hypothalamic lipids in both 24F and 48F were higher than CON, and cholesterol was elevated in the hypothalami of all experimental groups as compared to CON. Serum malondialdehyde was higher in fasted and FAF groups; 24F and 48F also had higher serum cholesterol levels, than CON (, ). Serum α-tocoferol, retinol and vitamin C values were lower in 48F than CON (). In conclusion, we demonstrate that feeding state can significantly alter brain fatty acid and lipid derivative levels, and serum concentrations of cholesterol and vitamins. These changes may consequently influence lipid peroxidation, fatty acid synthase or desaturase system in hypothalamic fields.

The role of the endocannabinoid system in the neuroendocrine regulation of energy balance

Animal and human studies carried out so far have established a role for the endocannabinoid system (ECS) in the regulation of energy balance. Here we critically discuss the role of the endocannabinoid signalling in brain structures, such as the hypothalamus and reward-related areas, and its interaction with neurotransmitter and neuropeptide systems involved in the regulation of food intake and body weight. The ECS has been found to interact with peripheral signals, like leptin, insulin, ghrelin and satiety hormones and the resulting effects on both central and peripheral mechanisms affecting energy balance and adiposity will be described. Furthermore, ECS dysregulation has been associated with the development of dyslipidemia, glucose intolerance and obesity; phenomena that are often accompanied by a plethora of neuroendocrine alterations which might play a causal role in determining ECS dysregulation. Despite the withdrawal of the first generation of cannabinoid type 1 receptor (CB1) antagonists from the pharmaceutical market due to the occurrence of psychiatric adverse events, new evidence suggests that peripherally restricted CB1 antagonists might be efficacious for the treatment of obesity and its associated metabolic disorders. Thus, a perspective on new promising strategies to selectively target the ECS in the context of energy balance regulation is given.

The metabolic regulator PGC-1α directly controls the expression of the hypothalamic neuropeptide oxytocin

The transcriptional coactivator PGC-1α is a key regulator of cellular energy expenditure in peripheral tissues. Recent studies report that PGC-1α-null mice develop late-onset obesity and that the neuronal inactivation of PGC-1α causes increased food intake. However, the exact role of PGC-1α in the CNS remains unclear. Here we show that PGC-1α directly regulates the expression of the hypothalamic neuropeptide oxytocin, a known central regulator of appetite. We developed a unique genetic approach in the zebrafish, allowing us to monitor and manipulate PGC-1α activity in oxytocinergic neurons. We found that PGC-1α is coexpressed with oxytocin in the zebrafish hypothalamus. Targeted knockdown of the zebrafish PGC-1α gene activity caused a marked decrease in oxytocin mRNA levels and inhibited the expression of a transgenic GFP reporter driven by the oxytocin promoter. The effect of PGC-1α loss of function on oxytocin gene activity was rescued by tissue-specific re-expression of either PGC-1α or oxytocin precursor in zebrafish oxytocinergic neurons. PGC-1α activated the oxytocin promoter in a heterologous cell culture system, and overexpression of PGC-1α induced ectopic expression of oxytocin in muscles and neurons. Finally, PGC-1α forms an in vivo complex with the oxytocin promoter in fed but not fasted animals. These findings demonstrate that PGC-1α is both necessary and sufficient for the production of oxytocin, implicating hypothalamic PGC-1α in the direct activation of a hypothalamic hormone known to control energy intake.

The regulation of food intake in mammalian hibernators: a review

One of the most profound hallmarks of mammalian hibernation is the dramatic reduction in food intake during the winter months. Several species of hibernator completely cease food intake (aphagia) for nearly 7 months regardless of ambient temperature and in many cases, whether or not food is available to them. Food intake regulation has been studied in mammals that hibernate for over 50 years and still little is known about the physiological mechanisms that control this important behavior in hibernators. It is well known from lesion experiments in non-hibernators that the hypothalamus is the main brain region controlling food intake and therefore body mass. In hibernators, the regulation of food intake and body mass is presumably governed by a circannual rhythm since there is a clear seasonal rhythm to food intake: animals increase food intake in the summer and early autumn, food intake declines in autumn and actually ceases in winter in many species, and resumes again in spring as food becomes available in the environment. Changes in circulating hormones (e.g., leptin, insulin, and ghrelin), nutrients (glucose, and free fatty acids), and cellular enzymes such as AMP-activated protein kinase (AMPK) have been shown to determine the activity of neurons involved in the food intake pathway. Thus, it appears likely that the food intake pathway is controlled by a variety of inputs, but is also acted upon by upstream regulators that are presumably rhythmic in nature. Current research examining the molecular mechanisms and integration of environmental signals (e.g., temperature and light) with these molecular mechanisms will hopefully shed light on how animals can turn off food intake and survive without eating for months on end.

Involvement of the long-chain fatty acid receptor GPR40 as a novel pain regulatory system

G-protein receptor (GPR) 40 is known to be activated by docosahexaenoic acid (DHA). However, reports studying the role and functions (including pain regulation) of GPR40 in the brain are lacking. We investigated the involvement of GPR40 in the brain on DHA-induced antinociceptive effects. Expression of GPR40 protein was observed in the olfactory bulb, striatum, hippocampus, midbrain, hypothalamus, medulla oblongata, cerebellum and cerebral cortex in the brain as well as the spinal cord, whereas GPR120 protein expression in these areas was not observed. Intracerebroventricular (i.c.v.), but not intrathecal (i.t.) injection of DHA (25 and 50μg/mouse) and GW9508 (a GPR40- and GPR120-selective agonist; 0.1 and 1.0μg/mouse) significantly reduced formalin-induced pain behavior. These effects were inhibited by pretreatment with the μ opioid receptor antagonist β-funaltrexamine (β-FNA), naltrindole (δ opioid receptor antagonist) and anti-β-endorphin antiserum. The κ opioid receptor antagonist norbinaltorphimine (nor-BNI) did not affect the antinociception of DHA or GW9508. Furthermore, the immunoreactivity of β-endorphin in the hypothalamus increased at 10 and 20min after i.c.v. injection of DHA and GW9508. These findings suggest that DHA-induced antinociception via β-endorphin release may be mediated (at least in part) through GPR40 signaling in the supraspinal area, and may provide valuable information on a novel therapeutic approach for pain control.

Expanding neurotransmitters in the hypothalamic neurocircuitry for energy balance regulation

The current epidemic of obesity and its associated metabolic syndromes impose unprecedented challenges to our society. Despite intensive research on obesity pathogenesis, an effective therapeutic strategy to treat and cure obesity is still lacking. Exciting studies in last decades have established the importance of the leptin neural pathway in the hypothalamus in the regulation of body weight homeostasis. Important hypothalamic neuropeptides have been identified as critical neurotransmitters from leptin-sensitive neurons to mediate leptin action. Recent research advance has significantly expanded the list of neurotransmitters involved in body weight-regulating neural pathways, including fast-acting neurotransmitters, gamma-aminobutyric acid (GABA) and glutamate. Given the limited knowledge on the leptin neural pathway for body weight homeostasis, understanding the function of neurotransmitters released from key neurons for energy balance regulation is essential for delineating leptin neural pathway and eventually for designing effective therapeutic drugs against the obesity epidemic.

Central apelin controls glucose homeostasis via a nitric oxide-dependent pathway in mice

AIMS

Apelin and its receptor have emerged as promising targets for the treatment of insulin resistance. Indeed, peripheral administration of apelin stimulates glucose utilization and insulin sensitivity via a nitric oxide (NO) pathway. In addition to being expressed on peripheral metabolically active adipose tissues, apelin is also found in the brain. However, no data are available on the role of central effects of apelin on metabolic control. We studied glucose metabolism in response to acute and chronic intracerebroventricular (i.c.v.) injection of apelin performed in normal and obese/diabetic mice.

RESULTS

We demonstrate that i.c.v. injection of apelin into fed mice improves glucose control via NO-dependent mechanisms. These results have been strengthened by transgenic (eNOS-KO mice), pharmacological (L-NMMA i.c.v. treated mice), and real-time measurement of NO release with amperometric probes detection. High-fat diet-fed mice displayed a severely blunted response to i.c.v. apelin associated with a lack of NO response by the hypothalamus. Moreover, central administration of high dose apelin in fasted normal mice provoked hyperinsulinemia, hyperglycemia, glucose intolerance, and insulin resistance.

CONCLUSION

These data provide compelling evidence that central apelin participates in the regulation of glucose homeostasis and suggest a novel pathophysiological mechanism involved in the transition from normal to diabetic state.

Autophagy in hypothalamic AgRP neurons regulates food intake and energy balance

Macroautophagy is a lysosomal degradative pathway that maintains cellular homeostasis by turning over cellular components. Here we demonstrate a role for autophagy in hypothalamic agouti-related peptide (AgRP) neurons in the regulation of food intake and energy balance. We show that starvation-induced hypothalamic autophagy mobilizes neuron-intrinsic lipids to generate endogenous free fatty acids, which in turn regulate AgRP levels. The functional consequences of inhibiting autophagy are the failure to upregulate AgRP in response to starvation, and constitutive increases in hypothalamic levels of pro-opiomelanocortin and its cleavage product α-melanocyte-stimulating hormone that typically contribute to a lean phenotype. We propose a conceptual framework for considering how autophagy-regulated lipid metabolism within hypothalamic neurons may modulate neuropeptide levels to have immediate effects on food intake, as well as long-term effects on energy homeostasis. Regulation of hypothalamic autophagy could become an effective intervention in conditions such as obesity and the metabolic syndrome.

Association between plasma metabolites and gene expression profiles in five porcine endocrine tissues

Background

Endocrine tissues play a fundamental role in maintaining homeostasis of plasma metabolites such as non-esterified fatty acids and glucose, the levels of which reflect the energy balance or the health status of animals. However, the relationship between the transcriptome of endocrine tissues and plasma metabolites has been poorly studied.

Methods

We determined the blood levels of 12 plasma metabolites in 27 pigs belonging to five breeds, each breed consisting of both females and males. The transcriptome of five endocrine tissues i.e. hypothalamus, adenohypophysis, thyroid gland, gonads and backfat tissues from 16 out of the 27 pigs was also determined. Sex and breed effects on the 12 plasma metabolites were investigated and associations between genes expressed in the five endocrine tissues and the 12 plasma metabolites measured were analyzed. A probeset was defined as a quantitative trait transcript (QTT) when its association with a particular metabolic trait achieved a nominal P value < 0.01.

Results

A larger than expected number of QTT was found for non-esterified fatty acids and alanine aminotransferase in at least two tissues. The associations were highly tissue-specific. The QTT within the tissues were divided into co-expression network modules enriched for genes in Kyoto Encyclopedia of Genes and Genomes or gene ontology categories that are related to the physiological functions of the corresponding tissues. We also explored a multi-tissue co-expression network using QTT for non-esterified fatty acids from the five tissues and found that a module, enriched in hypothalamus QTT, was positioned at the centre of the entire multi-tissue network.

Conclusions

These results emphasize the relationships between endocrine tissues and plasma metabolites in terms of gene expression. Highly tissue-specific association patterns suggest that candidate genes or gene pathways should be investigated in the context of specific tissues.

RGS6 variants are associated with dietary fat intake in Hispanics: the IRAS Family Study

Recently, a genome-wide association scan was completed in the IRAS (Insulin Resistance Atherosclerosis Study) Family Study (IRASFS) Hispanic-American cohort. Multiple single-nucleotide polymorphisms (SNPs) in the G-protein signaling 6 (RGS6) gene were found to be associated with adiposity phenotypes. RGS6 has shown downstream antagonistic interplay with opioid receptors, targets of fatty/sugary food agonists. The possibility that RGS6 promotes tolerance and tachyphylaxis among the opioid receptor is a plausible pathway for overconsuming fat/sugar-laden food. Therefore, we hypothesized that RGS6 variants are associated with intake of fatty/sugary foods. In 932 Hispanics from San Antonio and San Luis Valley, CO, the following dietary intake variables were assessed using the Block Brief 2000 food frequency questionnaire: total calories, total fat, % calories from fat, % calories from saturated fat, protein, % calories from protein, carbohydrates, % calories from carbohydrates, and daily frequency of servings of fats/oils/sweets. We tested for association between 23 SNPs in RGS6 and dietary intake using a variance components measured genotype approach. All models were adjusted for gender, recruitment site, admixture, BMI, and age. Using an additive genetic model, rs1402064 was associated with higher intake of fats/oils/sweets, total calories, total fat and saturated fat (P = 0.0007, 0.026, 0.023, and 0.024). SNPs rs847330 and rs847354 were associated with higher intake of fats/oils/sweets (P = 0.002 and 0.018), total fat (P = 0.040 and 0.048) and saturated fat (P = 0.044 and 0.041). Finally, rs769148 was associated with higher intake of fats/oils/sweets (P = 0.002). RGS6 is a new candidate gene for adiposity traits that may be associated with a behavioral tendency toward fat-laden food intake.

Carnitine palmitoyltransferase-1c gain-of-function in the brain results in postnatal microencephaly

Carnitine palmitoyltransferase-1c (CPT1c) is a newly identified and poorly understood brain-specific CPT1 homologue. Here, we have generated a new animal model that allows the conditional expression of CPT1c in a tissue specific and/or temporal manner via Cre-lox mediated recombination. Brain-specific, exogenous expression of CPT1c was achieved by crossing transgenic CPT1c mice to Nestin-Cre mice. The resulting double transgenic mice (CPT1c-TgN) displayed severe growth retardation in the postnatal period with a stunted development at 2 weeks of age. CPT1c-TgN mice had a greater than 2.3-fold reduction in brain weight. Even with this degree of microencephaly, CPT1c-TgN mice were viable and fertile and exhibited normal post-weaning growth. When fed a high fat diet CPT1c-TgN mice were protected from weight gain and the difference in body weight between CPT1c-TgN and control mice was further exaggerated. Conversely, low fat, high carbohydrate feeding partially reversed the body weight defects in CPT1c-TgN mice. Analysis of total brain lipids of low fat fed mice revealed a depletion of total very long chain fatty acids in adult CPT1c-TgN mice which was not evident in high fat fed CPT1c-TgN mice. These data show that CPT1c can elicit profound effects on brain physiology and total fatty acid profiles, which can be modulated by the nutritional composition of the diet.

Lipid-induced peroxidation in the intestine is involved in glucose homeostasis imbalance in mice

Background

Daily variations in lipid concentrations in both gut lumen and blood are detected by specific sensors located in the gastrointestinal tract and in specialized central areas. Deregulation of the lipid sensors could be partly involved in the dysfunction of glucose homeostasis. The study aimed at comparing the effect of Medialipid (ML) overload on insulin secretion and sensitivity when administered either through the intestine or the carotid artery in mice.

Methodology/Principal Findings

An indwelling intragastric or intracarotid catheter was installed in mice and ML or an isocaloric solution was infused over 24 hours. Glucose and insulin tolerance and vagus nerve activity were assessed. Some mice were treated daily for one week with the anti-lipid peroxidation agent aminoguanidine prior to the infusions and tests. The intestinal but not the intracarotid infusion of ML led to glucose and insulin intolerance when compared with controls. The intestinal ML overload induced lipid accumulation and increased lipid peroxidation as assessed by increased malondialdehyde production within both jejunum and duodenum. These effects were associated with the concomitant deregulation of vagus nerve. Administration of aminoguanidine protected against the effects of lipid overload and normalized glucose homeostasis and vagus nerve activity.

Conclusions/Significance

Lipid overload within the intestine led to deregulation of gastrointestinal lipid sensing that in turn impaired glucose homeostasis through changes in autonomic nervous system activity.

Circadian rhythms, sleep, and metabolism

The discovery of the genetic basis for circadian rhythms has expanded our knowledge of the temporal organization of behavior and physiology. The observations that the circadian gene network is present in most living organisms from eubacteria to humans, that most cells and tissues express autonomous clocks, and that disruption of clock genes results in metabolic dysregulation have revealed interactions between metabolism and circadian rhythms at neural, molecular, and cellular levels. A major challenge remains in understanding the interplay between brain and peripheral clocks and in determining how these interactions promote energy homeostasis across the sleep-wake cycle. In this Review, we evaluate how investigation of molecular timing may create new opportunities to understand and develop therapies for obesity and diabetes.

Expression of fatty acid-binding proteins in adult hippocampal neurogenic niche of postischemic monkeys

Intracellular fatty acid (FA) chaperones known as FA-binding proteins (FABPs) are a group of molecules known to participate in cellular metabolic processes such as lipid storage, membrane synthesis, and β-oxidation or to coordinate transcriptional programs. However, their role in adult neurogenesis still remains obscure. The FABPs expressed in the central nervous system (CNS) are heart-type (FABP3), epidermal-type (FABP5), and brain-type (FABP7). These three FABPs possess a differential affinity for polyunsaturated fatty acids (PUFAs). Recently, we reported that GPR40, a receptor for free FAs and particularly for PUFAs, is expressed in the CNS of adult monkeys and upregulated after transient global brain ischemia in the hippocampal subgranular zone (SGZ), a neurogenic niche in adulthood. The SGZ showed a peak proliferation of progenitor cells and maximal expression of GPR40 during the second week after ischemia. As both FABPs and GPR40 might be closely related to the adult neurogenesis, here, we studied the expression of FABP 3, 5, and 7 in the SGZ, comparing normal and postischemic adult monkeys. Immunoblotting revealed that FABP5 and FABP7, but not FABP3, were significantly increased on day 15 after ischemia when compared with the nonischemic control. Immunohistochemistry showed that FABP5 was almost undetectable in the control SGZ but was abundant on day 15 after ischemia. FABP 3, 5, and 7 were expressed in S-100β-positive astrocytes and nestin-positive neural progenitors. However, only FABP 5 and 7 were found in bromodeoxyuridine (BrdU)-positive newly generated cells. FABPs were most frequently coexpressed with the S-100β-positive astrocytes, whereas βIII-tubulin-or polysialylated neural cell-adhesion molecule (PSA-NCAM)-positive newborn neurons in the vicinity of the astrocytes expressed none of the three FABPs. These results support a role of astrocyte- and/or neural progenitor-derived FABPs as components of the molecular machine regulating the progenitor cell niche in the adult primate brain.

Role of lipids in the control of food intake

PURPOSE OF REVIEW

The classical hypothesis that a decrease in glucose utilization, registered by specific sensors in the brain, leads to hunger was challenged as lipids and amino acids also provide energy for cell metabolism. This review presents recent progress in the knowledge on lipid signals associated with feeding initiation or inhibition.

RECENT FINDINGS

Although the brain does not utilize fatty acids as primary energy sources, recent evidence indicates that intermediates of fatty acids metabolism in the hypothalamus serve as sensors of energy status. Pharmacological or genetic inhibition of brain fatty-acid synthase and carnitine palmitoyltransferase-1c results in profound decreases in feeding and body weight in rodents. These effects depend on changes in the intracytoplasmic pool of long-chain fatty-acyl-CoA and malonyl-CoA, an intermediate of the de-novo fatty acid synthesis in neurons that integrate hormonal and nutrient-derived signals to control feeding behaviour. The central regulatory enzyme is AMP-activated protein kinase, which reversibly phosphorylates acetyl-CoA carboxylase and malonyl-CoA decarboxylase, two enzymes that increase the level of malonyl-CoA in the cells when phosphorylated in response to a low intracellular ratio of AMP/ATP due to decrease in glucose oxidation.

SUMMARY

Such a mechanism is compatible with a central energostatic control of feeding based on glucose utilization by neurons and opens new therapeutic perspectives to develop pharmacological compounds that act at the level of pivotal enzymes such as AMP-activated protein kinase, acetyl-CoA carboxylase, malonyl-CoA decarboxylase, and module malonyl-CoA level in cells, to favour central inhibition of feeding.

Gut-brain signalling: how lipids can trigger the gut

The gut plays a unique role in the metabolic defence against energy excess and glucose imbalance. Nutrients, such as lipids, enter the small intestine and activate sensing mechanisms to maintain energy and glucose homeostasis. It is clear that a lipid-induced gut-brain axis exists and that cholecystokinin and a neuronal network are involved, yet the underlying mechanisms in gut lipid sensing that regulate homeostasis remain largely unknown. In parallel, studies underscore the importance of enzymes involved in lipid metabolism within the brain, such as adenosine monophosphate -activated protein kinase, to maintain homeostasis. In this review, we will first examine what is known regarding the mechanisms involved in this lipid-induced gut-brain neuronal axis that regulate food intake and hepatic glucose production. We will also discuss how enzymes that govern brain lipid metabolism could potentially reveal how lipids trigger the gut, and that both the gut and brain may share common biochemical pathways to sense lipids.

Hypothalamic control of lipid metabolism: focus on leptin, ghrelin and melanocortins

The hypothalamus plays a crucial role in the regulation of food intake and energy expenditure. One of the main regulatory factors within the hypothalamus is AMP-activated protein kinase (AMPK), which is involved in a large number of biological actions including the modulation of energy balance. Leptin and ghrelin-induced changes in hypothalamic AMPK lead to important alterations in hypothalamic fatty acid metabolism. Furthermore, it is well known that the hypothalamus controls peripheral lipid metabolism through the sympathetic nervous system, and those actions are independent of food intake. In this short review, we highlight the main molecular pathways triggered by leptin and ghrelin altering both central and peripheral lipid metabolism and, therefore, controlling feeding behavior and energy expenditure.

Gustatory and reward brain circuits in the control of food intake

Gustation is a multisensory process allowing for the selection of nutrients and the rejection of irritating and/or toxic compounds. Since obesity is a highly prevalent condition that is critically dependent on food intake and energy expenditure, a deeper understanding of gustatory processing is an important objective in biomedical research. Recent findings have provided evidence that central gustatory processes are distributed across several cortical and subcortical brain areas. Furthermore, these gustatory sensory circuits are closely related to the circuits that process reward. Here, we present an overview of the activation and connectivity between central gustatory and reward areas. Moreover, and given the limitations in number and effectiveness of treatments currently available for overweight patients, we discuss the possibility of modulating neuronal activity in these circuits as an alternative in the treatment of obesity.

Hypothalamic malonyl-CoA and CPT1c in the treatment of obesity

Metabolic integration of nutrient sensing in the central nervous system has been shown to be an important regulator of adiposity by affecting food intake and peripheral energy expenditure. Modulation of de novo fatty acid synthetic flux by cytokines and nutrient availability plays an important role in this process. Inhibition of hypothalamic fatty acid synthase by pharmacologic or genetic means leads to an increased malonyl-CoA level and suppression of food intake and adiposity. Conversely, the ectopic expression of malonyl-CoA decarboxylase in the hypothalamus is sufficient to promote feeding and adiposity. Based on these and other findings, metabolic intermediates in fatty acid biogenesis, including malonyl-CoA and long-chain acyl-CoAs, have been implicated as signaling mediators in the central control of body weight. Malonyl-CoA has been hypothesized to mediate its effects in part through an allosteric interaction with an atypical and brain-specific carnitine palmitoyltransferase-1 (CPT1c). CPT1c is expressed in neurons and binds malonyl-CoA, however, it does not perform the same biochemical function as the prototypical CPT1 enzymes. Mouse knockout models of CPT1c exhibit suppressed food intake and smaller body weight, but are highly susceptible to weight gain when fed a high-fat diet. Thus, the brain can directly sense and respond to changes in nutrient availability and composition to affect body weight and adiposity.

Brain lipid sensing and nervous control of energy balance

Nutrient sensitive neurons (glucose and fatty acids (FA)) are present in many sites throughout the brain, including the hypothalamus and brainstem, and play a key role in the neural control of energy and glucose homeostasis. Through neuronal output, FA may modulate feeding behaviour as well as both insulin secretion and action. For example, central administration of oleate inhibits food intake and glucose production in rats. This suggests that daily variations in plasma FA concentrations might be detected by the central nervous system as a signal which contributes to the regulation of energy balance. At the cellular level, subpopulations of neurons in the ventromedial and arcuate hypothalamic nuclei are selectively either inhibited or activated by FA. Possible molecular effectors of these FA effects likely include chloride or potassium ion channels. While intracellular metabolism and activation of the ATP-sensitive K(+) channel appear to be necessary for some of the signaling effects of FA, at least half of the FA responses in ventromedial hypothalamic neurons are mediated by interaction with FAT/CD36, a FA transporter/receptor that does not require intracellular metabolism to activate downstream signaling. Thus, FA or their metabolites can modulate neuronal activity as a means of directly monitoring ongoing fuel availability by brain nutrient-sensing neurons involved in the regulation of energy and glucose homeostasis. Besides these physiological effects, FA overload or metabolic dysfunction might impair neural control of energy homeostasis and contribute to obesity and/or type 2 diabetes in predisposed subjects.

Dioxins, the aryl hydrocarbon receptor and the central regulation of energy balance

Dioxins are ubiquitous environmental contaminants that have attracted toxicological interest not only for the potential risk they pose to human health but also because of their unique mechanism of action. This mechanism involves a specific, phylogenetically old intracellular receptor (the aryl hydrocarbon receptor, AHR) which has recently proven to have an integral regulatory role in a number of physiological processes, but whose endogenous ligand is still elusive. A major acute impact of dioxins in laboratory animals is the wasting syndrome, which represents a puzzling and dramatic perturbation of the regulatory systems for energy balance. A single dose of the most potent dioxin, TCDD, can permanently readjust the defended body weight set-point level thus providing a potentially useful tool and model for physiological research. Recent evidence of response-selective modulation of AHR action by alternative ligands suggests further that even therapeutic implications might be possible in the future.

Hypothalamic AMP-activated protein kinase regulates glucose production

OBJECTIVE

The fuel sensor AMP-activated protein kinase (AMPK) in the hypothalamus regulates energy homeostasis by sensing nutritional and hormonal signals. However, the role of hypothalamic AMPK in glucose production regulation remains to be elucidated. We hypothesize that bidirectional changes in hypothalamic AMPK activity alter glucose production.

RESEARCH DESIGN AND METHODS

To introduce bidirectional changes in hypothalamic AMPK activity in vivo, we first knocked down hypothalamic AMPK activity in male Sprague-Dawley rats by either injecting an adenovirus expressing the dominant-negative form of AMPK (Ad-DN AMPKα2 [D(157)A]) or infusing AMPK inhibitor compound C directly into the mediobasal hypothalamus. Next, we independently activated hypothalamic AMPK by delivering either an adenovirus expressing the constitutive active form of AMPK (Ad-CA AMPKα1(312) [T172D]) or the AMPK activator AICAR. The pancreatic (basal insulin)-euglycemic clamp technique in combination with the tracer-dilution methodology was used to assess the impact of alternations in hypothalamic AMPK activity on changes in glucose kinetics in vivo.

RESULTS

Injection of Ad-DN AMPK into the hypothalamus knocked down hypothalamic AMPK activity and led to a significant suppression of glucose production with no changes in peripheral glucose uptake during the clamps. In parallel, hypothalamic infusion of AMPK inhibitor compound C lowered glucose production as well. Conversely, molecular and pharmacological activation of hypothalamic AMPK negated the ability of hypothalamic nutrients to lower glucose production.

CONCLUSIONS

These data indicate that changes in hypothalamic AMPK activity are sufficient and necessary for hypothalamic nutrient-sensing mechanisms to alter glucose production in vivo.

Sensing the fuels: glucose and lipid signaling in the CNS controlling energy homeostasis

The central nervous system (CNS) is capable of gathering information on the body's nutritional state and it implements appropriate behavioral and metabolic responses to changes in fuel availability. This feedback signaling of peripheral tissues ensures the maintenance of energy homeostasis. The hypothalamus is a primary site of convergence and integration for these nutrient-related feedback signals, which include central and peripheral neuronal inputs as well as hormonal signals. Increasing evidence indicates that glucose and lipids are detected by specialized fuel-sensing neurons that are integrated in these hypothalamic neuronal circuits. The purpose of this review is to outline the current understanding of fuel-sensing mechanisms in the hypothalamus, to integrate the recent findings in this field, and to address the potential role of dysregulation in these pathways in the development of obesity and type 2 diabetes mellitus.

Increased brain fatty acid uptake in metabolic syndrome

OBJECTIVE

To test whether brain fatty acid uptake is enhanced in obese subjects with metabolic syndrome (MS) and whether weight reduction modifies it.

RESEARCH DESIGN AND METHODS

We measured brain fatty acid uptake in a group of 23 patients with MS and 7 age-matched healthy control subjects during fasting conditions using positron emission tomography (PET) with [(11)C]-palmitate and [(18)F]fluoro-6-thia-heptadecanoic acid ([(18)F]-FTHA). Sixteen MS subjects were restudied after 6 weeks of very low calorie diet intervention.

RESULTS

At baseline, brain global fatty acid uptake derived from [(18)F]-FTHA was 50% higher in patients with MS compared with control subjects. The mean percentage increment was 130% in the white matter, 47% in the gray matter, and uniform across brain regions. In the MS group, the nonoxidized fraction measured using [(11)C]-palmitate was 86% higher. Brain fatty acid uptake measured with [(18)F]-FTHA-PET was associated with age, fasting serum insulin, and homeostasis model assessment (HOMA) index. Both total and nonoxidized fractions of fatty acid uptake were associated with BMI. Rapid weight reduction decreased brain fatty acid uptake by 17%.

CONCLUSIONS

To our knowledge, this is the first study on humans to observe enhanced brain fatty acid uptake in patients with MS. Both fatty acid uptake and accumulation appear to be increased in MS patients and reversed by weight reduction.

Hypothalamic AMPK and fatty acid metabolism mediate thyroid regulation of energy balance

Thyroid hormones have widespread cellular effects; however it is unclear whether their effects on the central nervous system (CNS) contribute to global energy balance. Here we demonstrate that either whole-body hyperthyroidism or central administration of triiodothyronine (T3) decreases the activity of hypothalamic AMP-activated protein kinase (AMPK), increases sympathetic nervous system (SNS) activity and upregulates thermogenic markers in brown adipose tissue (BAT). Inhibition of the lipogenic pathway in the ventromedial nucleus of the hypothalamus (VMH) prevents CNS-mediated activation of BAT by thyroid hormone and reverses the weight loss associated with hyperthyroidism. Similarly, inhibition of thyroid hormone receptors in the VMH reverses the weight loss associated with hyperthyroidism. This regulatory mechanism depends on AMPK inactivation, as genetic inhibition of this enzyme in the VMH of euthyroid rats induces feeding-independent weight loss and increases expression of thermogenic markers in BAT. These effects are reversed by pharmacological blockade of the SNS. Thus, thyroid hormone-induced modulation of AMPK activity and lipid metabolism in the hypothalamus is a major regulator of whole-body energy homeostasis.

Hyperleptinemia is required for the development of leptin resistance

Leptin regulates body weight by signaling to the brain the availability of energy stored as fat. This negative feedback loop becomes disrupted in most obese individuals, resulting in a state known as leptin resistance. The physiological causes of leptin resistance remain poorly understood. Here we test the hypothesis that hyperleptinemia is required for the development of leptin resistance in diet-induced obese mice. We show that mice whose plasma leptin has been clamped to lean levels develop obesity in response to a high-fat diet, and the magnitude of this obesity is indistinguishable from wild-type controls. Yet these obese animals with constant low levels of plasma leptin remain highly sensitive to exogenous leptin even after long-term exposure to a high fat diet. This shows that dietary fats alone are insufficient to block the response to leptin. The data also suggest that hyperleptinemia itself can contribute to leptin resistance by downregulating cellular response to leptin as has been shown for other hormones.

Functions for pro-opiomelanocortin-derived peptides in obesity and diabetes

Melanocortin peptides, derived from POMC (pro-opiomelanocortin) are produced in the ARH (arcuate nucleus of the hypothalamus) neurons and the neurons in the commissural NTS (nucleus of the solitary tract) of the brainstem, in anterior and intermediate lobes of the pituitary, skin and a wide range of peripheral tissues, including reproductive organs. A hypothetical model for functional roles of melanocortin receptors in maintaining energy balance was proposed in 1997. Since this time, there has been an extraordinary amount of knowledge gained about POMC-derived peptides in relation to energy homoeostasis. Development of a Pomc-null mouse provided definitive proof that POMC-derived peptides are critical for the regulation of energy homoeostasis. The melanocortin system consists of endogenous agonists and antagonists, five melanocortin receptor subtypes and receptor accessory proteins. The melanocortin system, as is now known, is far more complex than most of us could have imagined in 1997, and, similarly, the importance of this system for regulating energy homoeostasis in the general human population is much greater than we would have predicted. Of the known factors that can cause human obesity, or protect against it, the melanocortin system is by far the most significant. The present review is a discussion of the current understanding of the roles and mechanism of action of POMC, melanocortin receptors and AgRP (agouti-related peptide) in obesity and Type 2 diabetes and how the central and/or peripheral melanocortin systems mediate nutrient, leptin, insulin, gut hormone and cytokine regulation of energy homoeostasis.

Glucose prevents the fall in ventromedial hypothalamic GABA that is required for full activation of glucose counterregulatory responses during hypoglycemia

Local delivery of glucose into a critical glucose-sensing region within the brain, the ventromedial hypothalamus (VMH), can suppress glucose counterregulatory responses to systemic hypoglycemia. Here, we investigated whether this suppression was accomplished through changes in GABA output in the VMH. Sprague-Dawley rats had catheters and guide cannulas implanted. Eight to ten days later, microdialysis-microinjection probes were inserted into the VMH, and they were dialyzed with varying concentrations of glucose from 0 to 100 mM. Two groups of rats were microdialyzed with 100 mM glucose and microinjected with either the K(ATP) channel opener diazoxide or a GABA(A) receptor antagonist. These animals were then subjected to a hyperinsulinemic-hypoglycemic glucose clamp. As expected, perfusion of glucose into the VMH suppressed the counterregulatory responses. Extracellular VMH GABA levels positively correlated with the concentration of glucose in the perfusate. In turn, extracellular GABA concentrations in the VMH were inversely related to the degree of counterregulatory hormone release. Of note, microinjection of either diazoxide or the GABA(A) receptor antagonist reversed the suppressive effects of VMH glucose delivery on counterregulatory responses. Some GABAergic neurons in the VMH respond to changes in local glucose concentration. Glucose in the VMH dose-dependently stimulates GABA release, and this in turn dose-dependently suppresses the glucagon and epinephrine responses to hypoglycemia. These data suggest that during hypoglycemia a decrease in glucose concentration within the VMH may provide an important signal that rapidly inactivates VMH GABAergic neurons, reducing inhibitory GABAergic tone, which in turn enhances the counterregulatory responses to hypoglycemia.

Regulation of lipid metabolism by energy availability: a role for the central nervous system

The central nervous system (CNS) is crucial in the regulation of energy homeostasis. Many neuroanatomical studies have shown that the white adipose tissue (WAT) is innervated by the sympathetic nervous system, which plays a critical role in adipocyte lipid metabolism. Therefore, there are currently numerous reports indicating that signals from the CNS control the amount of fat by modulating the storage or oxidation of fatty acids. Importantly, some CNS pathways regulate adipocyte metabolism independently of food intake, suggesting that some signals possess alternative mechanisms to regulate energy homeostasis. In this review, we mainly focus on how neuronal circuits within the hypothalamus, such as leptin- ghrelin-and resistin-responsive neurons, as well as melanocortins, neuropeptide Y, and the cannabinoid system exert their actions on lipid metabolism in peripheral tissues such as WAT, liver or muscle. Dissecting the complicated interactions between peripheral signals and neuronal circuits regulating lipid metabolism might open new avenues for the development of new therapies preventing and treating obesity and its associated cardiometabolic sequelae.

Integrative neurobiology of energy homeostasis-neurocircuits, signals and mediators

Body weight is tightly controlled in a species-specific range from insects to vertebrates and organisms have developed a complex regulatory network in order to avoid either excessive weight gain or chronic weight loss. Energy homeostasis, a term comprising all processes that aim to maintain stability of the metabolic state, requires a constant communication of the different organs involved; i.e. adipose tissue, skeletal muscle, liver, pancreas and the central nervous system (CNS). A tight hormonal network ensures rapid communication to control initiation and cessation of eating, nutrient processing and partitioning of the available energy within different organs and metabolic pathways. Moreover, recent experiments indicate that many of these homeostatic signals modulate the neural circuitry of food reward and motivation. Disturbances in each individual system can affect the maintenance and regulation of the others, making the analysis of energy homeostasis and its dysregulation highly complex. Though this cross-talk has been intensively studied for many years now, we are far from a complete understanding of how energy balance is maintained and multiple key questions remain unanswered. This review summarizes some of the latest developments in the field and focuses on the effects of leptin, insulin, and nutrient-related signals in the central regulation of feeding behavior. The integrated view, how these signals interact and the definition of functional neurocircuits in control of energy homeostasis, will ultimately help to develop new therapeutic interventions within the current obesity epidemic.

Effect of high-fat feeding on expression of genes controlling availability of dopamine in mouse hypothalamus

Objective

Hypothalamic centers integrate external signals of nutrient availability and energy status and initiate responses to maintain homeostasis. Quantifying changes in hypothalamic gene expression in the presence of nutrient excess may identify novel responsive elements.

Methods

Affymetrix Mouse Genome 430 2.0 oligonucleotide microarrays containing 45 102 probe sets were used to interrogate differential expression of genes in dietary-induced obesity model C57BL6 inbred mice fed a high-fat (35% fat; n = 8) or standard (4% fat; n = 6) diet from 3 to 15 wk of age. Ontologies of regulated genes were examined and expression of selected genes was validated by quantitative real-time polymerase chain reaction.

Results

One thousand two hundred twelve unique gene transcripts showed altered expression on the microarrays. Gene ontology analysis revealed changes in neuropeptide genes responding to leptin, Pomc, Cart, Npy, and Agrp, compatible with a homeostatic response to high-fat intake, although mean weight increased 2.3-fold compared with standard fed mice (P < 0.001). Neurotransmitter system ontologies revealed upregulation of five genes controlling availability of dopamine. Changes in Th tyrosine hydroxylase (2.1-fold) and Slc18a2 solute carrier family 18 (vesicular monoamine), member 2 (4.4-fold) controlling synthesis and release, and Slc6a3 solute carrier family 6 (neurotransmitter transporter, dopamine), member 3 (4.8-fold), Snca α-synuclein (1.3-fold), and Maoa monoamine oxidase (1.9-fold) limiting availability were confirmed by quantitative real-time polymerase chain reaction.

Conclusion

Expression of five genes involved in availability of dopamine was increased after a high-fat diet. Failure to reduce dopamine availability sufficiently, to counter the feeding reward effect, could contribute to diet-induced obesity in these mice.

Non-esterified fatty acids increase arterial pressure via central sympathetic activation in humans

Previous studies have shown that acute increases in plasma NEFAs (non-esterified fatty acids) raise SVR (systemic vascular resistance) and BP (blood pressure). However, these studies have failed to distinguish between CNS (central nervous system) mechanisms that raise sympathetic activity and paracrine mechanisms that increase SVR directly, independent of CNS involvement. The aim of the present study was to directly determine whether the sympathetic nervous system contributes to the pressor response to NEFAs. On 2 days separated by at least 2 weeks, 17 lean healthy volunteers (ten male/seven female; age, 22±1 years; body mass index, 23±1 kg/m2; values are means±S.E.M.) received a 4-h intravenous infusion of 20% Intralipid® or placebo (in a single-blind randomized balanced order). MSNA (muscle sympathetic nerve activity), HR (heart rate), BP (oscillometric brachial measurement) and (cardiac output; acetylene rebreathing) were measured before and throughout infusion. The change in HR (+8.2±1.0 and +2.4±1.2 beats/min), systolic BP (+14.0±1.6 and +3.2±2.5 mmHg) and diastolic BP (+8.2±1.0 and −0.1±1.7 mmHg) were significantly greater after the 4-h infusion of Intralipid® compared with placebo (P<0.001). The change in BP with Intralipid® resulted from an increase in SVR (/mean arterial pressure; P<0.001) compared with baseline, without a change in . MSNA burst frequency increased during Intralipid® infusion compared with baseline (+4.9±1.3 bursts/min; P<0.05), and total MSNA (frequency×amplitude) was augmented 65% (P<0.001), with no change during placebo infusion. Lipid infusion increased insulin, aldosterone and F2-isoprostane, but not leptin, concentrations. On the basis of the concomitant increase in BP, MSNA and SVR, we conclude that central sympathetic activation contributes to the pressor response to NEFAs.

Hypothalamic lipotoxicity and the metabolic syndrome

Ectopic accumulation of lipids in peripheral tissues, such as pancreatic beta cells, liver, heart and skeletal muscle, leads to lipotoxicity, a process that contributes substantially to the pathophysiology of insulin resistance, type 2 diabetes, steatotic liver disease and heart failure. Current evidence has demonstrated that hypothalamic sensing of circulating lipids and modulation of hypothalamic endogenous fatty acid and lipid metabolism are two bona fide mechanisms modulating energy homeostasis at the whole body level. Key enzymes, such as AMP-activated protein kinase (AMPK) and fatty acid synthase (FAS), as well as intermediate metabolites, such as malonyl-CoA and long-chain fatty acids-CoA (LCFAs-CoA), play a major role in this neuronal network, integrating peripheral signals with classical neuropeptide-based mechanisms. However, one key question to be addressed is whether impairment of lipid metabolism and accumulation of specific lipid species in the hypothalamus, leading to lipotoxicity, have deleterious effects on hypothalamic neurons. In this review, we summarize what is known about hypothalamic lipid metabolism with focus on the events associated to lipotoxicity, such as endoplasmic reticulum (ER) stress in the hypothalamus. A better understanding of these molecular mechanisms will help to identify new drug targets for the treatment of obesity and metabolic syndrome.