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.     

Obesity: the role of hypothalamic AMP-activated protein kinase in body weight regulation

Obesity is rapidly increasing and is of great public health concern worldwide. Although there have been remarkable developments in obesity research over the past 10 years, the molecular mechanism of obesity is still not completely understood. Body weight results from the balance between food intake and energy expenditure. Recent studies have found that hypothalamic AMP-activated protein kinase plays a key role in regulating these processes. Leptin, insulin, glucose and alpha-lipoic acid have been shown to reduce food intake by lowering hypothalamic AMP-activated protein kinase activity, whereas ghrelin and glucose depletion increase food intake by increasing hypothalamic AMP-activated protein kinase activity. In addition, this enzyme plays a role in the central regulation of energy expenditure. These findings indicate that hypothalamic AMP-activated protein kinase is an important signal molecule, which integrates nutritional and hormonal signals and modulates feeding behavior and energy expenditure.     

Insulin Secretion in Hypothalamic Obesity: Diurnal Variation and the Effect of Naloxone

This paper has tested the hypothesis that patients with hypothalamic obesity have altered mechanisms controlling insulin secretion when compared to obese patients without hypothalamic injury. Fasting glucose and insulin values were significantly higher in the morning than in the afternoon in the six control obese patients, but there was no diurnal difference in the six patients with hypothalamic obesity (n=6). The control obese subjects showed a diurnal variation in glucose-stimulated insulin secretion, whereas the patients with hypothalamic obesity did not, suggesting that hypothalamic injury had destroyed diurnal rhythms. Naloxone, an opioid antagonist, acutely suppressed fasting insulin in the six patients with essential obesity but had little effect on fasting insulin in the three patients with hypothalamic obesity or in five normal-weight controls. Naloxone increased insulin sensitivity in the obese control patients, but did not affect either insulin secretion or insulin sensitivity in patients with hypothalamic obesity or in normal weight subjects. Our results support the conclusion that hypothalamic obesity disrupts diurnal rhythms, with the suggestion that opioid peptides affect insulin secretion differently in patients with essential obesity as compared to normal weight subjects or those with hypothalamic obesity.     

Saturated Fatty Acids Modulate Autophagy’s Proteins in the Hypothalamus

Autophagy is an important process that regulates cellular homeostasis by degrading dysfunctional proteins, organelles and lipids. In this study, the hypothesis that obesity could lead to impairment in hypothalamic autophagy in mice was evaluated by examining the hypothalamic distribution and content of autophagic proteins in animal with obesity induced by 8 or 16 weeks high fat diet to induce obesity and in response to intracerebroventricular injections of palmitic acid. The results showed that chronic exposure to a high fat diet leads to an increased expression of inflammatory markers and downregulation of autophagic proteins. In obese mice, autophagic induction leads to the downregulation of proteins, such as JNK and Bax, which are involved in the stress pathways. In neuron cell- line, palmitate has a direct effect on autophagy even without inflammatory activity. Understanding the cellular and molecular bases of overnutrition is essential for identifying new diagnostic and therapeutic targets for obesity.     

Adaptive responses in hypothalamic neuropeptide Y in the face of prolonged high-fat feeding in the rat

While a dysregulation in neuropeptide Y (NPY) signaling has been described in rodent models of obesity, few studies have investigated the time-course of changes in NPY content and responsiveness during development of diet-induced obesity. Therefore we investigated the effect of differing lengths (2-17 weeks) of high-fat diet on hypothalamic NPY peptide content, release and NPY-induced hyperphagia. Male Sprague-Dawley rats (211 +/- 3 g) were fed either a high-fat diet (30% fat) or laboratory chow (5% fat). Animals were implanted with intracerebroventricular cannulae to investigate feeding responses to NPY (0.5 nmol, 1 nmol) after 4 or 12 weeks of diet. At the earlier stage of obesity, NPY-induced hyperphagia was not altered; however, animals maintained on the high-fat diet for the longer duration were hyper-responsive to NPY, compared to chow-fed control rats (p < 0.05). Overall, hypothalamic NPY peptide content tended to be decreased from 9 to 17 weeks of diet (p < 0.05). Total hypothalamic NPY content was negatively correlated with plasma leptin concentration (p < 0.05), suggesting the hypothalamic NPY system remains responsive to leptin's inhibitory signal. In addition, hypothalamic NPY overflow was significantly reduced in high-fat fed animals (p < 0.05). Together these results suggest a reduction in hypothalamic NPY activity in high-fat fed animals, perhaps in an attempt to restore energy balance.     

Hypothalamic regulation of adiposity: the role of 11beta-hydroxysteroid dehydrogenase type 1.

Following extensive suprasellar operations for excision of hypothalamic tumors, some patients develop morbid obesity despite receiving replacement doses of glucocorticoids. Urine analysis of cortisol and cortisone metabolites show that 11-OH/11-oxo ratios are significantly higher in patients with hypothalamic obesity, indicating enhanced 11beta-HSD1 activity. This correlates with the visceral-to-subcutaneous fat ratio. The consequence of increased 11beta-HSD1 activity and a shift of the steroid inter-conversion towards cortisol may contribute to the effects of the latter in adipose tissue. The message from the hypothalamus to adipocyte 11beta-HSD-1 involves hormones, the sympathetic nervous system and cytokines. CRH and ACTH downregulate 11beta-HSD-1 activity and induce lipolysis. Tumor necrosis factor-alpha and interleukin-1beta upregulate 11beta-HSD-1 expression and activity, while enhancing lipolysis. The sympathetic nervous system exerts its effects through beta-adrenergic upregulation and alpha-adrenergic downregulation of 11beta-HSD-1 activity. Inhibition of 11beta-HSD-1 suppresses preadipocyte differentiation into mature adipocytes, and may provide a therapeutic tool.     

Leptin physiology: a second look

It is widely believed that the primary physiologic role of leptin is to prevent obesity by regulating food intake and thermogenesis through actions on hypothalamic centers. Here we sugest that the first premise, the anti-obesity role, is untenable, and present evidence for an alternative physiologic role, namely antisteatotic activity in which fatty acid overaccumulation in nonadipose tissues is prevented by leptin-mediated regulation of beta-oxidation. The second premise, namely that leptin acts exclusively on the hypothalamus, is confirmed in normal lean animals with plasma leptin concentrations below 5 ng/ml; their correlation with cerebrospinal fluid levels supports the classical concept of leptin-mediated hypothalamic regulation of food intake. However, when chronic hyperleptinemia exceeds 15 ng/ml, as in obesity, a further rise in plasma leptin does not raise cerebrospinal leptin levels or reduce food intake. Nevertheless, the peripheral antisteatotic action of leptin in acquired obesity continues, suggesting that at chronically hyperleptinemic levels the hormone acts primarily on peripheral tissues and that its hypothalamic action has reached a plateau.     

Circadian intervention of obesity development via resting-stage feeding manipulation or oxytocin treatment

The obesity pandemic can be viewed as a result of an imbalanced reaction to changing environmental factors. Recent research has linked circadian arrhythmicity to obesity and related diseases; however, the underlying mechanisms are still unclear. In this study, we found that high-fat diet (HFD) feeding strikingly promoted daytime rather than nighttime caloric intake in mice, leading to feeding circadian arrhythmicity. Using scheduled feeding with a defined amount of daily HFD intake, we found that an increase in the ratio of daytime to nighttime feeding promoted weight gain, whereas a decrease of this ratio rebalanced energy expenditure to counteract obesity. In identifying the underlying mechanism, we found that hypothalamic release of anorexigenic neuropeptide oxytocin displayed a diurnal rhythm of daytime rise and nighttime decline, which negatively correlated with the diurnal feeding activities of normal chow-fed mice. In contrast, chronic HFD feeding abrogated oxytocin diurnal rhythmicity, primarily by suppressing daytime oxytocin rise. Using pharmacological experiments with hypothalamic injection of oxytocin or oxytocin antagonist, we showed that daytime manipulation of oxytocin can change feeding circadian patterns to reprogram energy expenditure, leading to attenuation or induction of obesity independently of 24-h caloric intake. Also importantly, we found that peripheral injection of oxytocin activated hypothalamic oxytocin neurons to release oxytocin, and exerted metabolic effects similar to central oxytocin injection, thus offering a practical clinical avenue to use oxytocin in obesity control. In conclusion, resting-stage oxytocin release and feeding activity represent a critical circadian mechanism and therapeutic target for obesity.     

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.     

Hypothalamic IKKbeta/NF-kappaB and ER stress link overnutrition to energy imbalance and obesity

Overnutrition is associated with chronic inflammation in metabolic tissues. Whether metabolic inflammation compromises the neural regulatory systems and therefore promotes overnutrition-associated diseases remains unexplored. Here we show that a mediator of metabolic inflammation, IKKbeta/NF-kappaB, normally remains inactive although enriched in hypothalamic neurons. Overnutrition atypically activates hypothalamic IKKbeta/NF-kappaB at least in part through elevated endoplasmic reticulum stress in the hypothalamus. While forced activation of hypothalamic IKKbeta/NF-kappaB interrupts central insulin/leptin signaling and actions, site- or cell-specific suppression of IKKbeta either broadly across the brain or locally within the mediobasal hypothalamus, or specifically in hypothalamic AGRP neurons significantly protects against obesity and glucose intolerance. The molecular mechanisms involved include regulation by IKKbeta/NF-kappaB of SOCS3, a core inhibitor of insulin and leptin signaling. Our results show that the hypothalamic IKKbeta/NF-kappaB program is a general neural mechanism for energy imbalance underlying obesity and suggest that suppressing hypothalamic IKKbeta/NF-kappaB may represent a strategy to combat obesity and related diseases.     

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.     

Mechanisms of Weight Control by Primary Cilia

A primary cilium, a hair-like protrusion of the plasma membrane, is a pivotal organelle for sensing external environmental signals and transducing intracellular signaling. An interesting linkage between cilia and obesity has been revealed by studies of the human genetic ciliopathies Bardet-Biedl syndrome and Alström syndrome, in which obesity is a principal manifestation. Mouse models of cell type-specific cilia dysgenesis have subsequently demonstrated that ciliary defects restricted to specific hypothalamic neurons are sufficient to induce obesity and hyperphagia. A potential mechanism underlying hypothalamic neuron cilia-related obesity is impaired ciliary localization of G protein-coupled receptors involved in the regulation of appetite and energy metabolism. A well-studied example of this is melanocortin 4 receptor (MC4R), mutations in which are the most common cause of human monogenic obesity. In the paraventricular hypothalamus neurons, a blockade of ciliary trafficking of MC4R as well as its downstream ciliary signaling leads to hyperphagia and weight gain. Another potential mechanism is reduced leptin signaling in hypothalamic neurons with defective cilia. Leptin receptors traffic to the periciliary area upon leptin stimulation. Moreover, defects in cilia formation hamper leptin signaling and actions in both developing and differentiated hypothalamic neurons. The list of obesity-linked ciliary proteins is expending and this supports a tight association between cilia and obesity. This article provides a brief review on the mechanism of how ciliary defects in hypothalamic neurons facilitate obesity.     

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.     

Effects of chronic administration of sibutramine on body weight, food intake and motor activity in neonatally monosodium glutamate-treated obese female rats: relationship of antiobesity effect with monoamines.

When the hypothalamic ventromedial nucleus and arcuate nucleus were destroyed in rats by treatment with monosodium glutamate in the neonatal stage, increase in the Lee index (body weight 1/3/body length) and in retroperitoneal fat as well as decreases in spontaneous motor activity, food consumption and growth hormone secretion function associated with hypothalamic low body length obesity (monosodium glutamate-treated obesity; MSG-OB) were observed as these rats grew. Treatment with sibutramine at 3 and 10 mg/kg p.o. once a day continuously for 14 days improved these parameters, and the degree of improvement was dose related. The plasma lipid values in MSG-OB rats, which were the same as those in normal rats, were decreased by consecutive administration of sibutramine. Levels of hypothalamic monoamines (MAs) such as norepinephrine, 5-HT (serotonin) and dopamine and their metabolites DOPAC, HVA and 5-HIAA were decreased in MSG-OB rats, and further decrease in them, though slight, was observed with consecutive daily administration of sibutramine, probably as a result of the feedback attributable to an increase in MA in synapses caused by inhibition of MA uptake by sibutramine. These results suggest that sibutramine can activate the MA nervous system by MA uptake inhibition in regions of the brain such as the lateral hypothalamic area and the paraventricular nucleus, which control food intake and sympathetic nerve activity, and the nigrostriatal area related to the extrapyramidal motor system, and thereby exhibit anti-obesity effects in the MSG-OB rat.     

PTP1B regulates leptin signal transduction in vivo

Mice lacking the protein-tyrosine phosphatase PTP1B are hypersensitive to insulin and resistant to obesity. However, the molecular basis for resistance to obesity has been unclear. Here we show that PTP1B regulates leptin signaling. In transfection studies, PTP1B dephosphorylates the leptin receptor-associated kinase, Jak2. PTP1B is expressed in hypothalamic regions harboring leptin-responsive neurons. Compared to wild-type littermates, PTP1B(-/-) mice have decreased leptin/body fat ratios, leptin hypersensitivity, and enhanced leptin-induced hypothalamic Stat3 tyrosyl phosphorylation. Gold thioglucose treatment, which ablates leptin-responsive hypothalamic neurons, partially overcomes resistance to obesity in PTP1B(-/-) mice. Our data indicate that PTP1B regulates leptin signaling in vivo, likely by targeting Jak2. PTP1B may be a novel target to treat leptin resistance in obesity.     

Adrenalectomy stimulates hypothalamic proopiomelanocortin expression but does not correct diet-induced obesity

Background  

Elevated glucocorticoid production and reduced hypothalamic POMC mRNA can cause obese phenotypes. Conversely, adrenalectomy can reverse obese phenotypes caused by the absence of leptin, a model in which glucocorticoid production is elevated. Adrenalectomy also increases hypothalamic POMC mRNA in leptin-deficient mice. However most forms of human obesity do not appear to entail elevated plasma glucocorticoids. It is therefore not clear if reducing glucocorticoid production would be useful to treat these forms of obesity. We hypothesized that adrenalectomy would increase hypothalamic POMC mRNA and reverse obese phenotypes in obesity due to a high-fat diet as it does in obesity due to leptin deficiency.     

Hypothalamic inflammation and thermogenesis: the brown adipose tissue connection

Hypothalamic inflammation and dysfunction are common features of experimental obesity. An imbalance between caloric intake and energy expenditure is generated as a consequence of this inflammation, leading to the progressive increase of body adiposity. Thermogenesis, is one of the main functions affected by obesity-linked hypothalamic dysfunction and the complete characterization of the mechanisms involved in this process may offer new therapeutic perspectives for obesity. The brown adipose tissue is an important target for hypothalamic action in thermogenesis. This tissue has been thoroughly studied in rodents and hibernating mammals; however, until recently, its advocated role in human thermogenesis was neglected due to the lack of substantial evidence of its presence in adult humans. The recent demonstration of the presence of functional brown adipose tissue in adult humans has renovated the interest in this tissue. Here, we review some of the work that shows how inflammation and dysfunction of the hypothalamus can control brown adipose tissue activity and how this can impact on whole body thermogenesis and energy expenditure.     

Uncoupling the mechanisms of obesity and hypertension by targeting hypothalamic IKK-β and NF-κB

Obesity-related hypertension has become an epidemic health problem and a major risk factor for the development of cardiovascular disease (CVD). Recent research on the pathophysiology of obesity has implicated a role for the hypothalamus in the pathogenesis of this condition. However, it remains unknown whether the often-seen coupling of hypertension with obesity can also be explained by hypothalamic dysfunction, despite the emerging appreciation that many forms of hypertension are neurogenic in origin. Our studies here revealed that acute activation of the proinflammatory protein nuclear factor κB (NF-κB) and its upstream activator IκB kinase-β (IKK-β, encoded by Ikbkb) in the mediobasal hypothalamus rapidly elevated blood pressure in mice independently of obesity. This form of hypothalamic inflammation-induced hypertension involved the sympathetic upregulation of hemodynamics and was reversed by sympathetic suppression. Loss-of-function studies further showed that NF-κB inhibition in the mediobasal hypothalamus counteracted obesity-related hypertension in a manner that was dissociable from changes in body weight. In addition, we found that pro-opiomelanocortin (POMC) neurons were crucial for the hypertensive effects of the activation of hypothalamic IKK-β and NF-κB, which underlie obesity-related hypertension. In conclusion, obesity-associated activation of IKK-β and NF-κB in the mediobasal hypothalamus--particularly in the hypothalamic POMC neurons--is a primary pathogenic link between obesity and hypertension. Breaking this pathogenic link may represent an avenue for controlling obesity-related hypertension and CVD without requiring obesity control.     

Diacylglycerol kinase zeta in hypothalamus interacts with long form leptin receptor. Relation to dietary fat and body weight regulation

Leptin and its long form receptor, Ob-Rb, in hypothalamic nuclei play a key role in regulating energy balance. The mutation of Ob-Rb into one of its natural variants, Ob-Ra, results in severe obesity in rodents. We demonstrate here that diacylglycerol kinase zeta (DGKzeta) interacts, via its ankyrin repeats, with the cytoplasmic portion of Ob-Rb in yeast two-hybrid systems, in protein precipitation experiments in vitro and in vivo. It does not interact, however, with the short form, Ob-Ra, which mediates the entry of leptin into the brain. Furthermore, we show by in situ hybridization that DGKzeta is expressed in neurons of hypothalamic nuclei known to synthesize Ob-Rb and to participate in energy homeostasis. The mutant ob-/ob- and db-/db- mice exhibit increased hypothalamic DGKzeta mRNA level compared with their wild-type controls, suggesting a role for the leptin/OB-Rb system in regulating DGKzeta expression. Further experiments show that hypothalamic DGKzeta mRNA level is stimulated by the consumption of a high-fat diet. In addition, DGKzeta mRNA is statistically significantly lower in rats and inbred mice that become obese on a high-fat diet compared with their lean counterparts. In fact, it is strongly, negatively correlated with both body fat and circulating levels of leptin. Taken together, our evidence suggests that DGKzeta constitutes a downstream component of the leptin signaling pathway and that reduced hypothalamic DGKzeta mRNA, and possibly activity, is associated with obesity.