Neural control of the anorexia-cachexia syndrome

The anorexia-cachexia syndrome is a debilitating clinical condition characterizing the course of chronic diseases, which heavily impacts on patients' morbidity and quality of life, ultimately accelerating death. The pathogenesis is multifactorial and reflects the complexity and redundancy of the mechanisms controlling energy homeostasis under physiological conditions. Accumulating evidence indicates that, during disease, disturbances of the hypothalamic pathways controlling energy homeostasis occur, leading to profound metabolic changes in peripheral tissues. In particular, the hypothalamic melanocortin system does not respond appropriately to peripheral inputs, and its activity is diverted largely toward the promotion of catabolic stimuli (i.e., reduced energy intake, increased energy expenditure, possibly increased muscle proteolysis, and adipose tissue loss). Hypothalamic proinflammatory cytokines and serotonin, among other factors, are key in triggering hypothalamic resistance. These catabolic effects represent the central response to peripheral challenges (i.e., growing tumor, renal, cardiac failure, disrupted hepatic metabolism) that are likely sensed by the brain through the vagus nerve. Also, disease-induced changes in fatty acid oxidation within hypothalamic neurons may contribute to the dysfunction of the hypothalamic melanocortin system. Ultimately, sympathetic outflow mediates, at least in part, the metabolic changes in peripheral tissues. Other factors are likely involved in the pathogenesis of the anorexia-cachexia syndrome, and their role is currently being elucidated. However, available evidence shows that the constellation of symptoms characterizing this syndrome should be considered, at least in part, as different phenotypes of common neurochemical/metabolic alterations in the presence of a chronic inflammatory state.     

Glucose Regulates Hypothalamic Long-chain Fatty Acid Metabolism via AMP-activated Kinase (AMPK) in Neurons and Astrocytes

Hypothalamic controls of energy balance rely on the detection of circulating nutrients such as glucose and long-chain fatty acids (LCFA) by the mediobasal hypothalamus (MBH). LCFA metabolism in the MBH plays a key role in the control of food intake and glucose homeostasis, yet it is not known if glucose regulates LCFA oxidation and esterification in the MBH and, if so, which hypothalamic cell type(s) and intracellular signaling mechanisms are involved. The aim of this study was to determine the impact of glucose on LCFA metabolism, assess the role of AMP-activated Kinase (AMPK), and to establish if changes in LCFA metabolism and its regulation by glucose vary as a function of the kind of LCFA, cell type, and brain region. We show that glucose inhibits palmitate oxidation via AMPK in hypothalamic neuronal cell lines, primary hypothalamic astrocyte cultures, and MBH slices ex vivo but not in cortical astrocytes and slice preparations. In contrast, oleate oxidation was not affected by glucose or AMPK inhibition in MBH slices. In addition, our results show that glucose increases palmitate, but not oleate, esterification into neutral lipids in neurons and MBH slices but not in hypothalamic astrocytes. These findings reveal for the first time the metabolic fate of different LCFA in the MBH, demonstrate AMPK-dependent glucose regulation of LCFA oxidation in both astrocytes and neurons, and establish metabolic coupling of glucose and LCFA as a distinguishing feature of hypothalamic nuclei critical for the control of energy balance.     

Quercetin inhibits AMPK/TXNIP activation and reduces inflammatory lesions to improve insulin signaling defect in the hypothalamus of high fructose-fed rats

Fructose is a nutritional composition of fruits and honey. Its excess consumption induces insulin resistance-associated metabolic diseases. Hypothalamic insulin signaling plays a pivotal role in controlling whole-body insulin sensitivity and energy homeostasis. Quercetin, a natural flavonoid, has been reported to ameliorate high fructose-induced rat insulin resistance and hyperlipidemia. In this study, we investigated its regulatory effects on the hypothalamus of high fructose-fed rats. Rats were fed 10% fructose in drinking water for 10 weeks. After 4 weeks, these animals were orally treated with quercetin (50 and 100 mg/kg), allopurinol (5 mg/kg) and water daily for the next 6 weeks, respectively. Quercetin effectively restored high fructose-induced hypothalamic insulin signaling defect by up-regulating the phosphorylation of insulin receptor and protein kinase B. Furthermore, quercetin was found to reduce metabolic nutrient sensors adenosine monophosphate-activated protein kinase (AMPK) activation and thioredoxin-interacting protein (TXNIP) overexpression, as well as the glutamine–glutamate cycle dysfunction in the hypothalamus of high fructose-fed rats. Subsequently, it ameliorated high fructose-caused hypothalamic inflammatory lesions in rats by suppressing the activation of hypothalamic nuclear factor κB (NF-κB) pathway and NOD-like receptor 3 (NLRP3) inflammasome with interleukin 1β maturation. Allopurinol had similar effects. These results provide in vivo evidence that quercetin-mediated down-regulation of AMPK/TXNIP and subsequent inhibition of NF-κB pathway/NLRP3 inflammasome activation in the hypothalamus of rats may be associated with the reduction of hypothalamic inflammatory lesions, contributing to the improvement of hypothalamic insulin signaling defect in this model. Thus, quercetin with the central activity may be a therapeutic for high fructose-induced insulin resistance and hyperlipidemia in humans.     

Control of metabolism by nutrient-regulated nuclear receptors acting in the brain

Today, we are witnessing a rising incidence of obesity worldwide. This increase is due to a sedentary life style, an increased caloric intake and a decrease in physical activity. Obesity contributes to the appearance of type 2 diabetes, dyslipidemia and cardiovascular complications due to atherosclerosis, and nephropathy. Therefore, the development of new therapeutic strategies may become a necessity. Given the metabolism controlling properties of nuclear receptors in peripheral organs (such as liver, adipose tissues, pancreas) and their implication in various processes underlying metabolic diseases, they constitute interesting therapeutic targets for obesity, dyslipidemia, cardiovascular disease and type 2 diabetes. The recent identification of the central nervous system as a player in the control of peripheral metabolism opens new avenues to our understanding of the pathophysiology of obesity and type 2 diabetes and potential novel ways to treat these diseases. While the metabolic functions of nuclear receptors in peripheral organs have been extensively investigated, little is known about their functions in the brain, in particular with respect to brain control of energy homeostasis. This review provides an overview of the relationships between nuclear receptors in the brain, mainly at the hypothalamic level, and the central regulation of energy homeostasis. In this context, we will particularly focus on the role of PPARα, PPARγ, LXR and Rev-erbα.     

Lactate rescues neuronal sodium homeostasis during impaired energy metabolism

Recently, we established that recurrent activity evokes network sodium oscillations in neurons and astrocytes in hippocampal tissue slices. Interestingly, metabolic integrity of astrocytes was essential for the neurons'' capacity to maintain low sodium and to recover from sodium loads, indicating an intimate metabolic coupling between the 2 cell types. Here, we studied if lactate can support neuronal sodium homeostasis during impaired energy metabolism by analyzing whether glucose removal, pharmacological inhibition of glycolysis and/or addition of lactate affect cellular sodium regulation. Furthermore, we studied the effect of lactate on sodium regulation during recurrent network activity and upon inhibition of the glial Krebs cycle by sodium-fluoroacetate. Our results indicate that lactate is preferentially used by neurons. They demonstrate that lactate supports neuronal sodium homeostasis and rescues the effects of glial poisoning by sodium-fluoroacetate. Altogether, they are in line with the proposed transfer of lactate from astrocytes to neurons, the so-called astrocyte-neuron-lactate shuttle.     

Lactate rescues neuronal sodium homeostasis during impaired energy metabolism

Recently, we established that recurrent activity evokes network sodium oscillations in neurons and astrocytes in hippocampal tissue slices. Interestingly, metabolic integrity of astrocytes was essential for the neurons' capacity to maintain low sodium and to recover from sodium loads, indicating an intimate metabolic coupling between the 2 cell types. Here, we studied if lactate can support neuronal sodium homeostasis during impaired energy metabolism by analyzing whether glucose removal, pharmacological inhibition of glycolysis and/or addition of lactate affect cellular sodium regulation. Furthermore, we studied the effect of lactate on sodium regulation during recurrent network activity and upon inhibition of the glial Krebs cycle by sodium-fluoroacetate. Our results indicate that lactate is preferentially used by neurons. They demonstrate that lactate supports neuronal sodium homeostasis and rescues the effects of glial poisoning by sodium-fluoroacetate. Altogether, they are in line with the proposed transfer of lactate from astrocytes to neurons, the so-called astrocyte-neuron-lactate shuttle.     

Increasing Fatty Acid Oxidation Remodels the Hypothalamic Neurometabolome to Mitigate Stress and Inflammation

Modification of hypothalamic fatty acid (FA) metabolism can improve energy homeostasis and prevent hyperphagia and excessive weight gain in diet-induced obesity (DIO) from a diet high in saturated fatty acids. We have shown previously that C75, a stimulator of carnitine palmitoyl transferase-1 (CPT-1) and fatty acid oxidation (FAOx), exerts at least some of its hypophagic effects via neuronal mechanisms in the hypothalamus. In the present work, we characterized the effects of C75 and another anorexigenic compound, the glycerol-3-phosphate acyltransferase (GPAT) inhibitor FSG67, on FA metabolism, metabolomics profiles, and metabolic stress responses in cultured hypothalamic neurons and hypothalamic neuronal cell lines during lipid excess with palmitate. Both compounds enhanced palmitate oxidation, increased ATP, and inactivated AMP-activated protein kinase (AMPK) in hypothalamic neurons in vitro. Lipidomics and untargeted metabolomics revealed that enhanced catabolism of FA decreased palmitate availability and prevented the production of fatty acylglycerols, ceramides, and cholesterol esters, lipids that are associated with lipotoxicity-provoked metabolic stress. This improved metabolic signature was accompanied by increased levels of reactive oxygen species (ROS), and yet favorable changes in oxidative stress, overt ER stress, and inflammation. We propose that enhancing FAOx in hypothalamic neurons exposed to excess lipids promotes metabolic remodeling that reduces local inflammatory and cell stress responses. This shift would restore mitochondrial function such that increased FAOx can produce hypothalamic neuronal ATP and lead to decreased food intake and body weight to improve systemic metabolism.     

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.     

Cortical Astrocytes Acutely Exposed to the Monomethylarsonous Acid (MMA<Superscript>III</Superscript>) Show Increased Pro-inflammatory Cytokines Gene Expression that is Consistent with APP and BACE-1: Over-expression

Long-term exposure to inorganic arsenic (iAs) through drinking water has been associated with cognitive impairment in children and adults; however, the related pathogenic mechanisms have not been completely described. Increased or chronic inflammation in the brain is linked to impaired cognition and neurodegeneration; iAs induces strong inflammatory responses in several cells, but this effect has been poorly evaluated in central nervous system (CNS) cells. Because astrocytes are the most abundant cells in the CNS and play a critical role in brain homeostasis, including regulation of the inflammatory response, any functional impairment in them can be deleterious for the brain. We propose that iAs could induce cognitive impairment through inflammatory response activation in astrocytes. In the present work, rat cortical astrocytes were acutely exposed in vitro to the monomethylated metabolite of iAs (MMA^III), which accumulates in glial cells without compromising cell viability. MMA^III LD₅₀ in astrocytes was 10.52 μM, however, exposure to sub-toxic MMA^III concentrations (50–1000 nM) significantly increased IL-1β, IL-6, TNF-α, COX-2, and MIF-1 gene expression. These effects were consistent with amyloid precursor protein (APP) and β-secretase (BACE-1) increased gene expression, mainly for those MMA^III concentrations that also induced TNF-α over-expression. Other effects of MMA^III on cortical astrocytes included increased proliferative and metabolic activity. All tested MMA^III concentrations led to an inhibition of intracellular lactate dehydrogenase (LDH) activity. Results suggest that MMA^III induces important metabolic and functional changes in astrocytes that may affect brain homeostasis and that inflammation may play a major role in cognitive impairment-related pathogenicity in As-exposed populations.     

A novel anti-inflammatory role for spleen-derived interleukin-10 in obesity-induced hypothalamic inflammation

Obesity can be associated with systemic low-grade inflammation that contributes to obesity-related metabolic disorders. Recent studies raise the possibility that hypothalamic inflammation contributes to the pathogenesis of diet-induced obesity (DIO), while another study reported that obesity decreases the expression of pro-inflammatory cytokines in spleen. The following study examines the hypothesis that obesity suppresses the splenic synthesis of the anti-inflammatory cytokine, interleukin (IL)-10, thereby resulting in chronic hypothalamic inflammation. The results showed that due to oxidative stress or apoptosis, the synthesis of splenic IL-10 was decreased in DIO when compared with non-obesity rats. Splenectomy (SPX) accelerated DIO-induced inflammatory responses in the hypothalamus. Interestingly, SPX suppressed the DIO-induced increases in food intake and body weight and led to a hypothalamic pro-inflammatory state that was similar to that produced by DIO, indicating that hypothalamic inflammation exerts a dual effect on energy metabolism. These SPX-induced changes were inhibited by the systemic administration of IL-10. Moreover, SPX had no effect on hypothalamic inflammatory responses in IL-10-deficient mice. These data suggest that spleen-derived IL-10 plays an important role in the prevention of hypothalamic inflammation and may be a therapeutic target for the treatment of obesity and hypothalamic inflammation.     

Defective hypothalamic autophagy directs the central pathogenesis of obesity via the IkappaB kinase beta (IKKbeta)/NF-kappaB pathway

Autophagy has been recently demonstrated to control cell and tissue homeostasis, including the functions of various metabolic tissues. However, it remains unclear whether autophagy is critical for the central nervous system and particularly the hypothalamus for exerting metabolic regulation. Using autophagy-related protein 7 (Atg7) as an autophagic marker, this work showed that autophagy was highly active in the mediobasal hypothalamus of normal mice. In contrast, chronic development of dietary obesity was associated with autophagic decline in the mediobasal hypothalamus. To investigate the potential role of autophagy in the hypothalamic control of metabolic physiology, a mouse model was developed with autophagic inhibition in the mediobasal hypothalamus using site-specific delivery of lentiviral shRNA against Atg7. This model revealed that hypothalamic inhibition of autophagy increased energy intake and reduced energy expenditure. These metabolic changes were sufficient to increase body weight gain under normal chow feeding and exacerbate the progression of obesity and whole-body insulin resistance under high-fat diet feeding. To explore the underlying mechanism, this study found that defective hypothalamic autophagy led to hypothalamic inflammation, including the activation of proinflammatory IκB kinase β pathway. Using brain-specific IκB kinase β knockout mice, it was found that the effects of defective hypothalamic autophagy in promoting obesity were reversed by IκB kinase β inhibition in the brain. In conclusion, hypothalamic autophagy is crucial for the central control of feeding, energy, and body weight balance. Conversely, decline of hypothalamic autophagy under conditions of chronic caloric excess promotes hypothalamic inflammation and thus impairs hypothalamic control of energy balance, leading to accelerated development of obesity and comorbidities.     

Glial Fatty Acid-Binding Protein 7 (FABP7) Regulates Neuronal Leptin Sensitivity in the Hypothalamic Arcuate Nucleus

The hypothalamus is involved in the regulation of food intake and energy homeostasis. The arcuate nucleus (ARC) and median eminence (ME) are the primary hypothalamic sites that sense leptin and nutrients in the blood, thereby mediating food intake. Recently, studies demonstrating a role for non-neuronal cell types, including astrocytes and tanycytes, in these regulatory processes have begun to emerge. However, the molecular mechanisms involved in these activities remain largely unknown. In this study, we examined in detail the localization of fatty acid-binding protein 7 (FABP7) in the hypothalamic ARC and sought to determine its role in the hypothalamus. We performed a phenotypic analysis of diet-induced FABP7 knockout (KO) obese mice and of FABP7 KO mice treated with a single leptin injection. Immunohistochemistry revealed that FABP7⁺ cells are NG2⁺ or GFAP⁺ in the ARC and ME. In mice fed a high-fat diet, weight gain and food intake were lower in FABP7 KO mice than in wild-type (WT) mice. FABP7 KO mice also had lower food intake and weight gain after a single injection of leptin, and we consistently confirmed that the number of pSTAT3⁺ cells in the ARC indicated that the leptin-induced activation of neurons was significantly more frequent in FABP7 KO mice than in WT mice. In FABP7 KO mice-derived primary astrocyte cultures, the level of ERK phosphorylation was lower after leptin treatment. Collectively, these results indicate that in hypothalamic astrocytes, FABP7 might be involved in sensing neuronal leptin via glia-mediated mechanisms and plays a pivotal role in controlling systemic energy homeostasis.     

Neuroprotection by estrogen in the brain: the mitochondrial compartment as presumed therapeutic target

Neuroprotection by estrogen in the CNS is well-documented and comprises the intricate regulation of cell–cell communication between neurons and supportive non-neuronal glial cells. It is assumed that these interactions are essential for cell survival under pathological and toxic conditions by regulating the allocation of trophic molecules, e.g., growth factors, controlling relevant intracellular anti-apoptotic and death cascades, and attenuating inflammatory processes. Malfunction and disturbance of mitochondria are doubtlessly associated with brain cell degeneration during neurotoxic and neurodegenerative processes. Estrogen has been documented as protective agent in the brain by stimulating growth factor supply and cell-intrinsic pro-/anti-apoptotic signaling pathways. In recent years, an additional estrogen-dependent safe-guarding strategy comes into the focus of neuronal protection. The mitochondrial compartment appears to be regulated by estrogen at the level of ATP and reactive oxygen species production as well as under a structural-functional viewpoint. In the present article, we would like to highlight recent data which demonstrate that sex steroids can directly and indirectly interfere with mitochondrial properties via non-nuclear, presumably mitochondria-intrinsic and nuclear signaling mechanisms. This enables mitochondria to cope with pathological processes and provide stabile local energy homeostasis and an anti-apoptotic base setting in the brain which, in turn, is a prerequisite for neuronal survival.     

Obesity induced by estrogen deficiency is associated with hypothalamic inflammation

Occurrence of obesity during the postmenopausal period is closely associated with inflammatory processes in multiple peripheral organs that are metabolically active. Hypothalamic inflammation has been recognized as one of the major underlying causes of various metabolic disorders, including obesity. The association between menopause-related obesity and hypothalamic inflammation remains poorly understood. We observed an elevation in hypothalamic inflammation in the ovariectomized mice, which displayed altered metabolic phenotypes and visceral obesity. Furthermore, we confirmed that ovariectomized mice displayed microglial activation accompanied by the upregulation of multiple genes involved in the inflammatory responses in hypothalamic microglia. Collectively, the current findings suggest that hypothalamic inflammation associated with microglial functioning could be a major pathogenic element in disruption of energy homeostasis during the postmenopausal period.     

Steroidogenic factor 1 regulates expression of the cannabinoid receptor 1 in the ventromedial hypothalamic nucleus

The nuclear receptor steroidogenic factor 1 (SF-1) plays essential roles in the development and function of the ventromedial hypothalamic nucleus (VMH). Considerable evidence links the VMH and SF-1 with the regulation of energy homeostasis. Here, we demonstrate that SF-1 colocalizes in VMH neurons with the cannabinoid receptor 1 (CB1R) and that a specific CB1R agonist modulates electrical activity of SF-1 neurons in hypothalamic slice preparations. We further show that SF-1 directly regulates CB1R gene expression via a SF-1-responsive element at -101 in its 5'-flanking region. Finally, we show that knockout mice with selective inactivation of SF-1 in the brain have decreased expression of CB1R in the region of the VMH and exhibit a blunted response to systemically administered CB1R agonists. These studies suggest that SF-1 directly regulates the expression of CB1R, which has been implicated in the regulation of energy homeostasis and anxiety-like behavior.     

Hypothalamic neural projections are permanently disrupted in diet-induced obese rats

The arcuate nucleus of the hypothalamus (ARH) is a key component of hypothalamic pathways regulating energy balance, and leptin is required for normal development of ARH projections. Diet-induced obesity (DIO) has a polygenic mode of inheritance, and DIO individuals develop the metabolic syndrome when a moderate amount of fat is added to the diet. Here we demonstrate that rats selectively bred to develop DIO, which are known to be leptin resistant before they become obese, have defective ARH projections that persist into adulthood. Furthermore, the ability of leptin to activate intracellular signaling in ARH neurons in vivo and to promote ARH neurite outgrowth in vitro is significantly reduced in DIO neonates. Thus, animals that are genetically predisposed toward obesity display an abnormal organization of hypothalamic pathways involved in energy homeostasis that may be the result of diminished responsiveness of ARH neurons to the trophic actions of leptin during postnatal development.     

Surfactant protein d deficiency in mice is associated with hyperphagia, altered fat deposition, insulin resistance, and increased basal endotoxemia

Pulmonary surfactant protein D (SP-D) is a host defence lectin of the innate immune system that enhances clearance of pathogens and modulates inflammatory responses. Recently it has been found that systemic SP-D is associated with metabolic disturbances and that SP-D deficient mice are mildly obese. However, the mechanism behind SP-D's role in energy metabolism is not known.Here we report that SP-D deficient mice had significantly higher ad libitum energy intake compared to wild-type mice and unchanged energy expenditure. This resulted in accumulation but also redistribution of fat tissue. Blood pressure was unchanged. The change in energy intake was unrelated to the basal levels of hypothalamic Pro-opiomelanocortin (POMC) and Agouti-related peptide (AgRP) gene expression. Neither short time systemic, nor intracereberoventricular SP-D treatment altered the hypothalamic signalling or body weight accumulation.In ad libitum fed animals, serum leptin, insulin, and glucose were significantly increased in mice deficient in SP-D, and indicative of insulin resistance. However, restricted diets eliminated all metabolic differences except the distribution of body fat. SP-D deficiency was further associated with elevated levels of systemic bacterial lipopolysaccharide.In conclusion, our findings suggest that lack of SP-D mediates modulation of food intake not directly involving hypothalamic regulatory pathways. The resulting accumulation of adipose tissue was associated with insulin resistance. The data suggest SP-D as a regulator of energy intake and body composition and an inhibitor of metabolic endotoxemia. SP-D may play a causal role at the crossroads of inflammation, obesity, and insulin resistance.     

Neuronal Expression of Glucosylceramide Synthase in Central Nervous System Regulates Body Weight and Energy Homeostasis

Hypothalamic neurons are main regulators of energy homeostasis. Neuronal function essentially depends on plasma membrane-located gangliosides. The present work demonstrates that hypothalamic integration of metabolic signals requires neuronal expression of glucosylceramide synthase (GCS; UDP-glucose:ceramide glucosyltransferase). As a major mechanism of central nervous system (CNS) metabolic control, we demonstrate that GCS-derived gangliosides interacting with leptin receptors (ObR) in the neuronal membrane modulate leptin-stimulated formation of signaling metabolites in hypothalamic neurons. Furthermore, ganglioside-depleted hypothalamic neurons fail to adapt their activity (c-Fos) in response to alterations in peripheral energy signals. Consequently, mice with inducible forebrain neuron-specific deletion of the UDP-glucose:ceramide glucosyltransferase gene (Ugcg) display obesity, hypothermia, and lower sympathetic activity. Recombinant adeno-associated virus (rAAV)-mediated Ugcg delivery to the arcuate nucleus (Arc) significantly ameliorated obesity, specifying gangliosides as seminal components for hypothalamic regulation of body energy homeostasis.