Blockage of ATGL-mediated breakdown of lipid droplets in microglia alleviates neuroinflammatory and behavioural responses to lipopolysaccharides.

Lipid droplets (LD) are triglyceride storing organelles that have emerged as an important component of cellular inflammatory responses. LD lipolysis via adipose triglyceride lipase (ATGL), the enzyme that catalyses the rate-limiting step of triglyceride lipolysis, regulates inflammation in peripheral immune and non-immune cells. ATGL elicits both pro- and anti-inflammatory responses in the periphery in a cell-type dependent manner. The present study determined the impact of ATGL inhibition and microglia-specific ATGL genetic loss-of-function on acute inflammatory and behavioural responses to pro-inflammatory insult. First, we evaluated the impact of lipolysis inhibition on lipopolysaccharide (LPS)-induced expression and secretion of cytokines and phagocytosis in mouse primary microglia cultures. Lipase inhibitors (ORlistat and ATGListatin) and LPS led to LD accumulation in microglia. Pan-lipase inhibition with ORlistat alleviated LPS-induced expression of IL-1ß and IL-6. Specific inhibition of ATGL had a similar action on CCL2, IL-1ß and IL-6 expression in both neonatal and adult microglia cultures. CCL2 and IL-6 secretion were also reduced by ATGListatin or knockdown of ATGL. ATGListatin increased phagocytosis in neonatal cultures independently from LPS treatment. Second, targeted and untargeted lipid profiling revealed that ATGListatin reduced LPS-induced generation of pro-inflammatory prostanoids and modulated ceramide species in neonatal microglia. Finally, the role of microglial ATGL in neuroinflammation was assessed using a novel microglia-specific and inducible ATGL knockout mouse model. Loss of microglial ATGL in adult male mice dampened LPS-induced expression of IL-6 and IL-1ß and microglial density. LPS-induced sickness- and anxiety-like behaviours were also reduced in male mice with loss of ATGL in microglia. Together, our results demonstrate potent anti-inflammatory effects produced by pharmacological or genetic inhibition of ATGL-mediated triglyceride lipolysis and thereby propose that supressing microglial LD lipolysis has beneficial actions in acute neuroinflammatory conditions.

Central activation of the fatty acid sensor GPR120 suppresses microglia reactivity and alleviates sickness- and anxiety-like behaviors.

G protein-coupled receptor 120 (GPR120, Ffar4) is a sensor for long-chain fatty acids including omega-3 polyunsaturated fatty acids (n-3 PUFAs) known for beneficial effects on inflammation, metabolism, and mood. GPR120 mediates the anti-inflammatory and insulin-sensitizing effects of n-3 PUFAs in peripheral tissues. The aim of this study was to determine the impact of GPR120 stimulation on microglial reactivity, neuroinflammation and sickness- and anxiety-like behaviors by acute proinflammatory insults. We found GPR120 mRNA to be enriched in  both murine and human microglia, and in situ hybridization revealed GPR120 expression in microglia of the nucleus accumbens (NAc) in mice. In a manner similar to or exceeding n-3 PUFAs, GPR120 agonism (Compound A, CpdA) strongly attenuated lipopolysaccharide (LPS)-induced proinflammatory marker expression in primary mouse microglia, including tumor necrosis factor-α (TNF-α) and interleukin-1ß (IL-1ß), and inhibited nuclear factor-ĸB translocation to the nucleus. Central administration of CpdA to adult mice blunted LPS-induced hypolocomotion and anxiety-like behavior and reduced TNF-α, IL-1ß and IBA-1 (microglia marker) mRNA in the NAc, a brain region modulating anxiety and motivation and implicated in neuroinflammation-induced mood deficits. GPR120 agonist pre-treatment attenuated NAc microglia reactivity and alleviated sickness-like behaviors elicited by central injection TNF-α and IL-1ß. These findings suggest that microglial GPR120 contributes to neuroimmune regulation and behavioral changes in response to acute infection and elevated brain cytokines. GPR120 may participate in the protective action of n-3 PUFAs at the neural and behavioral level and offers potential as treatment target for neuroinflammatory conditions.

A novel role for central ACBP/DBI as a regulator of long-chain fatty acid metabolism in astrocytes.

Acyl-CoA-binding protein (ACBP) is a ubiquitously expressed protein that binds intracellular acyl-CoA esters. Several studies have suggested that ACBP acts as an acyl-CoA pool former and regulates long-chain fatty acids (LCFA) metabolism in peripheral tissues. In the brain, ACBP is known as Diazepam-Binding Inhibitor, a secreted peptide acting as an allosteric modulator of the GABAA receptor. However, its role in central LCFA metabolism remains unknown. In the present study, we investigated ACBP cellular expression, ACBP regulation of LCFA intracellular metabolism, FA profile, and FA metabolism-related gene expression using ACBP-deficient and control mice. ACBP was mainly found in astrocytes with high expression levels in the mediobasal hypothalamus. We demonstrate that ACBP deficiency alters the central LCFA-CoA profile and impairs unsaturated (oleate, linolenate) but not saturated (palmitate, stearate) LCFA metabolic fluxes in hypothalamic slices and astrocyte cultures. In addition, lack of ACBP differently affects the expression of genes involved in FA metabolism in cortical versus hypothalamic astrocytes. Finally, ACBP deficiency increases FA content and impairs their release in response to palmitate in hypothalamic astrocytes. Collectively, these findings reveal for the first time that central ACBP acts as a regulator of LCFA intracellular metabolism in astrocytes. Acyl-CoA-binding protein (ACBP) or diazepam-binding inhibitor is a secreted peptide acting centrally as a GABAA allosteric modulator. Using brain slices, cortical, and hypothalamic astrocyte cultures from ACBP KO mice, we demonstrate that ACBP mainly localizes in astrocytes and regulates unsaturated but not saturated long-chain fatty acids (LCFA) metabolism. In addition, ACBP deficiency alters FA metabolism-related genes and results in intracellular FA accumulation while affecting their release. Our results support a novel role for ACBP in brain lipid metabolism. FA, fatty acids; KO, knockout; PL, phospholipids; TAG, triacylglycerol.

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.

Neuronal lipid droplets play a conserved and sex-biased role in maintaining whole-body energy homeostasis.

Lipids are essential for neuron development and physiology. Yet, the central hubs that coordinate lipid supply and demand in neurons remain unclear. Here, we combine invertebrate and vertebrate models to establish the presence and functional significance of neuronal lipid droplets (LD) in vivo. We find that LD are normally present in neurons in a non-uniform distribution across the brain, and demonstrate triglyceride metabolism enzymes and lipid droplet-associated proteins control neuronal LD formation through both canonical and recently-discovered pathways. Appropriate LD regulation in neurons has conserved and male-biased effects on whole-body energy homeostasis across flies and mice, specifically neurons that couple environmental cues with energy homeostasis. Mechanistically, LD-derived lipids support neuron function by providing phospholipids to sustain mitochondrial and endoplasmic reticulum homeostasis. Together, our work identifies a conserved role for LD as the organelle that coordinates lipid management in neurons, with implications for our understanding of mechanisms that preserve neuronal lipid homeostasis and function in health and disease.

Role of astroglial ACBP in energy metabolism flexibility and feeding responses to metabolic challenges in male mice.

Acyl-CoA binding protein (ACBP), also known as diazepam binding inhibitor (DBI), has recently emerged as a hypothalamic and brainstem gliopeptide regulating energy balance. Previous work has shown that the ACBP-derived octadecaneuropeptide exerts strong anorectic action via proopiomelanocortin (POMC) neuron activation and the melanocortin-4 receptor. Importantly, targeted ACBP loss-of-function in astrocytes promotes hyperphagia and diet-induced obesity while its overexpression in arcuate astrocytes reduces feeding and body weight. Despite this knowledge, the role of astroglial ACBP in adaptive feeding and metabolic responses to acute metabolic challenges has not been investigated. Using different paradigms, we found that ACBP deletion in glial fibrillary acidic protein (GFAP)-positive astrocytes does not affect weight loss when obese male mice are transitioned from a high fat diet to a chow diet, nor metabolic parameters in mice fed with a normal chow diet (e.g., energy expenditure, body temperature) during fasting, cold exposure and at thermoneutrality. In contrast, astroglial ACBP deletion impairs meal pattern and feeding responses during refeeding after a fast and during cold exposure, thereby showing that ACBP is required to stimulate feeding in states of increased energy demand. These findings challenge the general view that astroglial ACBP exerts anorectic effects and suggest that regulation of feeding by ACBP is dependent on metabolic status.

The gliotransmitter ACBP controls feeding and energy homeostasis via the melanocortin system.

Glial cells have emerged as key players in the central control of energy balance and etiology of obesity. Astrocytes play a central role in neural communication via the release of gliotransmitters. Acyl-CoA binding protein (ACBP)-derived endozepines are secreted peptides that modulate the GABAA receptor. In the hypothalamus, ACBP is enriched in arcuate nucleus (ARC) astrocytes, ependymocytes and tanycytes. Central administration of the endozepine octadecaneuropeptide (ODN) reduces feeding and improves glucose tolerance, yet the contribution of endogenous ACBP in energy homeostasis is unknown. We demonstrated that ACBP deletion in GFAP+ astrocytes, but not in Nkx2.1-lineage neural cells, promoted diet-induced hyperphagia and obesity in both male and female mice, an effect prevented by viral rescue of ACBP in ARC astrocytes. ACBP-astrocytes were observed in apposition with proopiomelanocortin (POMC) neurons and ODN selectively activated POMC neurons through the ODN-GPCR but not GABAA, and supressed feeding while increasing carbohydrate utilization via the melanocortin system. Similarly, ACBP overexpression in ARC astrocytes reduced feeding and weight gain. Finally, the ODN-GPCR agonist decreased feeding and promoted weight loss in ob/ob mice. These findings uncover ACBP as an ARC gliopeptide playing a key role in energy balance control and exerting strong anorectic effects via the central melanocortin system.

DBI/ACBP loss-of-function does not affect anxiety-like behaviour but reduces anxiolytic responses to diazepam in mice.

Diazepam is well known for its anxiolytic properties, which are mediated via activation of the GABAA receptor. Diazepam Binding Inhibitor (DBI), also called acyl-CoA binding protein (ACBP), is a ubiquitously expressed protein originally identified based on its ability to displace diazepam from its binding site on the GABAA receptor. Central administration of ACBP or its cleaved fragment, commonly referred to as endozepines, induces proconflict and anxiety-like behaviour in rodents. For this reason, ACBP is known as an anxiogenic peptide. However, the role of endogenous ACBP in anxiety-like behaviour and anxiolytic responses to diazepam has not been investigated. To address this question, we assessed anxiety behaviour and anxiolytic responses to diazepam in two complementary loss-of-function mouse models including astrocyte-specific ACBP KO (ACBP(GFAP) KO) and whole-body KO (ACBP KO) mice. Male and female ACBP(GFAP) KO and ACBP KO mice do not show significant changes in anxiety-like behaviour compared to control littermates during elevated plus maze (EPM) and open field (OF) tests. Surprisingly, ACBP(GFAP) KO and ACBP KO mice were unresponsive to the anxiolytic effect of a low dose of diazepam during EPM tests. In conclusion, our experiments using genetic ACBP loss-of-function models suggest that endozepines deficiency does not affect anxiety-like behaviour in mice and impairs the anxiolytic action of diazepam.

Dampened Mesolimbic Dopamine Function and Signaling by Saturated but not Monounsaturated Dietary Lipids.

Overconsumption of dietary fat is increasingly linked with motivational and emotional impairments. Human and animal studies demonstrate associations between obesity and blunted reward function at the behavioral and neural level, but it is unclear to what degree such changes are a consequence of an obese state and whether they are contingent on dietary lipid class. We sought to determine the impact of prolonged ad libitum intake of diets rich in saturated or monounsaturated fat, separate from metabolic signals associated with increased adiposity, on dopamine (DA)-dependent behaviors and to identify pertinent signaling changes in the nucleus accumbens (NAc). Male rats fed a saturated (palm oil), but not an isocaloric monounsaturated (olive oil), high-fat diet exhibited decreased sensitivity to the rewarding (place preference) and locomotor-sensitizing effects of amphetamine as compared with low-fat diet controls. Blunted amphetamine action by saturated high-fat feeding was entirely independent of caloric intake, weight gain, and plasma levels of leptin, insulin, and glucose and was accompanied by biochemical and behavioral evidence of reduced D1R signaling in the NAc. Saturated high-fat feeding was also tied to protein markers of increased AMPA receptor-mediated plasticity and decreased DA transporter expression in the NAc but not to alterations in DA turnover and biosynthesis. Collectively, the results suggest that intake of saturated lipids can suppress DA signaling apart from increases in body weight and adiposity-related signals known to affect mesolimbic DA function, in part by diminishing D1 receptor signaling, and that equivalent intake of monounsaturated dietary fat protects against such changes.

Blockade of stress-induced but not cocaine-induced reinstatement by infusion of noradrenergic antagonists into the bed nucleus of the stria terminalis or the central nucleus of the amygdala.

Experiments in our laboratory have shown that central noradrenergic (NA) activation plays a major role in stress-induced reinstatement of drug seeking in rats. In the present experiments, we investigated the effects of blockade of beta-NA adrenoceptors in the bed nucleus of the stria terminalis (BNST) and in the region of the central nucleus of the amygdala (CeA) on footshock- and cocaine-induced reinstatement. Rats were trained to self-administer cocaine (0.5 mg/kg, i.v.) for 9 d and, after a 5-7 d drug-free period, were given extinction sessions followed by a test for footshock stress-induced (15 min of intermittent footshock, 0.8 mA) or cocaine-induced (20 mg/kg, i.p.) reinstatement. Before the test, different groups of rats were given bilateral infusions of one of four doses of a mixture of the beta(1)- and beta(2)-receptor antagonists betaxolol and ICI-118,551 (vehicle, 0.25, 0.5, and 1 nmol of each compound in 0.5 microliter) into either the BNST or CeA. We observed a dose-dependent reduction of stress-induced reinstatement after infusions into the BNST and a complete blockade of stress-induced reinstatement after infusions into the CeA at all doses tested. The same treatments did not block cocaine-induced reinstatement when given at either site. These data suggest that stress-induced NA activation in the BNST and in the region of the CeA is critical to relapse to drug seeking induced by stress but not to relapse induced by priming injections of cocaine, and we hypothesize that NA activity leads to activation of corticotropin-releasing factor neurons in these regions.

A role for the prefrontal cortex in stress- and cocaine-induced reinstatement of cocaine seeking in rats.

RATIONALE AND OBJECTIVE: It is well established that stress induces reinstatement of drug seeking in an animal model of relapse. Here we studied the role of the medial prefrontal cortex (mPFC) and orbitofrontal cortex (OFC) in foot-shock stress-induced reinstatement of cocaine seeking. METHODS: Groups of rats were trained to self-administer cocaine (0.5 mg/kg per infusion, i.v., 3 h/day for 9 days) and after ten drug-free days were exposed to extinction and reinstatement test sessions. Each 60 min of extinction was separated by a 30-min time-out period after which the lever and stimulus lights were reintroduced. Rats were given four 1-h extinction sessions on day 1 and then on subsequent days were given two to three 1-h extinction sessions that were followed by a 3-h test for reinstatement. Tests were run every 48 h. In one set of experiments, the effects of inactivation of the prelimbic (PL), infralimbic (IL) or OFC by tetrodotoxin (TTX, 5 ng/0.5 micro l per side) on reinstatement induced by foot shock (5 min, intermittent, 1 mA) or priming injections of cocaine (20 mg/kg, i.p.) were determined. In a second set, the effects of infusions of the D1-like and D2-like dopamine receptor antagonists (SCH 23390 and raclopride) were studied using the same methods. RESULTS: TTX infusions into the PL cortex blocked both foot shock and cocaine-induced reinstatement. TTX into OFC attenuated foot-shock-induced, but not cocaine-induced reinstatement. Infusions into IL were ineffective. Infusions of SCH 22390 (0.25 micro g/0.5 micro l per side) into either PL or OFC blocked foot-shock-induced reinstatement, but infusions into PL had no effect on cocaine-induced reinstatement. Raclopride (5 micro g/0.5 micro l per side) had no effect on foot-shock-induced reinstatement in either PL or OFC or on cocaine-induced reinstatement when infused into PL. Neither TTX nor SCH23390 infusions into PL or OFC had any effect on lever pressing for sucrose. CONCLUSIONS: These results suggest that the PL and OFC regions form part of the circuitry mediating the effects of foot shock stress on reinstatement of drug seeking and that the PL region may be a common pathway for cue, drug and foot-shock stress-induced reinstatement of drug seeking.

Hypoxia in early life is associated with lasting changes in left ventricular structure and function at maturity in the rat.

BACKGROUND: There is a growing population of adults with repaired cyanotic congenital heart disease. These patients have increased risk of impaired cardiac health and premature death. We hypothesized that hypoxia in early life before surgical intervention causes lasting changes in left ventricular structure and function with physiological implications in later life. METHODS: Sprague-Dawley rats reared initially hypoxic conditions (FiO(2)=0.12) for days 1-10 of life were compared to rats reared only in ambient air. Cellular morphology and viability were compared among LV cardiomyocytes and histological analyses were performed on LV myocardium and arterioles. Intracellular calcium transients and cell shortening were measured in freshly-isolated cardiomyocytes, and mitochondrial hexokinase 2 (HK2) expression and activity were determined. Transthoracic echocardiography was used to assess LV function in anesthetized animals. RESULTS: Cardiomyocytes from adult animals following hypoxia in early life had greater cellular volumes but significantly reduced viability. Echocardiographic analyses revealed LV hypertrophy and diastolic dysfunction, and alterations in cardiomyocyte calcium transients and cell shortening suggested impaired diastolic calcium reuptake. Histological analyses revealed significantly greater intima-media thickness and decreased lumen area in LV arterioles from hypoxic animals. Alterations in mitochondrial HK2 protein distribution and activity were also observed which may contribute to cardiomyocyte fragility. CONCLUSIONS: Hypoxia in early life causes lasting changes in left ventricular structure and function that may negatively influence myocardial and vascular responses to physiological stress in later life. These data have implications for the growing population of adults with repaired or palliated cyanotic congenital heart disease.

Systemic arterial pressure at maturity in rats following chronic hypoxia in early life.

BACKGROUND: The effect of prolonged hypoxemia in early life on systemic arterial blood pressure at maturity was assessed in Sprague-Dawley rats. METHODS: Animals hypoxic in early life (12 males, 10 females) were raised in hypoxia (FiO2 = 0.12) for the first 10 days of life and subsequently raised in normoxia, along with age-matched controls (11 males, 9 females). At 2 months of age, arterial blood pressure was recorded intravascularly using telemetry in awake and unrestrained animals over two 12-h night-time (active) and daytime (resting) periods. Aortic pulse wave velocity was assessed in six additional hypoxic pretreated and five control anesthetized 2-month-old male rats. RESULTS: Systolic, mean, and pulse pressures were significantly greater in the hypoxic pretreated group compared to the control group during resting and active periods in both sexes (P ≤ 0.05). Diastolic pressure and heart rate did not differ between the two groups. Hypoxic pretreated males displayed significantly increased blood pressure variability during the resting period. Aortic pulse wave velocity was also found to be elevated in the hypoxic pretreated rats. CONCLUSIONS: Prolonged hypoxic stress in early life in the rat is associated with increased systolic arterial pressure at maturity very likely due to decreased arterial compliance. These findings suggest that a nutrient-independent, postnatal stress may lead to long-lasting vascular alterations predisposing to increased arterial pressure at maturity. This raises the possibility that adult survivors of congenital cyanotic cardiac disease may be at risk for secondary cardiovascular morbidity unrelated to surgical repair or residual cardiac defects.

Association of neonatal hypoxia with lasting changes in left ventricular gene expression: an animal model.

OBJECTIVE: Innovations in pediatric cardiovascular surgery have resulted in significant improvements in survival for children with congenital heart disease. In adults with such disease, however, surgical morbidity and mortality remain significant. We hypothesized that hypoxemia in early life causes lasting changes in gene expression in the developing heart and that such changes may persist into later life, affecting the physiology of the adult myocardium. METHODS: Microarray expression analyses were performed with left ventricular tissue from 10- and 90-day-old rats exposed to hypoxia (inspired oxygen fraction 0.12) for the first 10 days after birth then subsequently reared in ambient air and with tissue from age-matched rats reared entirely in ambient air. Changes in expression of selected genes were confirmed with real-time reverse transcriptase polymerase chain reaction. Left ventricular cardiomyocytes were isolated from adult animals in both groups, and cellular morphology and viability were compared. RESULTS: Microarray analyses revealed significant changes in 1945 and 422 genes in neonates and adults, respectively. Changes in genes associated with adaptive vascular remodeling and energy homeostasis, as well as regulation of apoptosis, were confirmed by real-time reverse transcriptase polymerase chain reaction. The viability of cardiomyocytes isolated from hypoxic animals was significantly lower than in those from control animals (36.7% +/- 13.3% vs 85.0% +/- 2.9%, P = .024). CONCLUSIONS: Neonatal hypoxia is associated with significant changes in left ventricular gene expression in both neonatal and adult rats. This may have physiologic implications for the adult myocardium.