Hypothalamic control of energy and glucose metabolism

Sex differences in metabolic phenotype and hypothalamic inflammation in the 3xTg-AD mouse model of Alzheimer's disease

BACKGROUND

Alzheimer's disease (AD) is notably associated with cognitive decline resulting from impaired function of hippocampal and cortical areas; however, several other domains and corresponding brain regions are affected. One such brain region is the hypothalamus, shown to atrophy and develop amyloid and tau pathology in AD patients. The hypothalamus controls several functions necessary for survival, including energy and glucose homeostasis. Changes in appetite and body weight are common in AD, often seen several years prior to the onset of cognitive symptoms. Therefore, altered metabolic processes may serve as a biomarker for AD, as well as a target for treatment, considering they are likely both a result of pathological changes and contributor to disease progression. Previously, we reported sexually dimorphic metabolic disturbances in ~ 7-month-old 3xTg-AD mice, accompanied by differences in systemic and hypothalamic inflammation.

METHODS

In the current study, we investigated metabolic outcomes and hypothalamic inflammation in 3xTg-AD males and females at 3, 6, 9, and 12 months of age to determine when these sex differences emerge.

RESULTS

In agreement with our previous study, AD males displayed less weight gain and adiposity, as well as reduced blood glucose levels following a glucose challenge, compared to females. These trends were apparent by 6-9 months of age, coinciding with increased expression of inflammatory markers (Iba1, GFAP, TNF-α, and IL-1β) in the hypothalamus of AD males.

CONCLUSIONS

These findings provide additional evidence for sex-dependent effects of AD pathology on energy and glucose homeostasis, which may be linked to hypothalamic inflammation.

Neuregulin4 Acts on Hypothalamic ErBb4 to Excite Oxytocin Neurons and Preserve Metabolic Homeostasis

Neuregulin 4 (Nrg4) is an adipose tissue-enriched secreted factor that modulates glucose and lipid metabolism. Nrg4 is closely associated with obesity and preserves diet-induced metabolic disorders. However, the specific mechanisms via which Nrg4 regulates metabolic homeostasis remain incompletely understood. Here, this work finds that the Nrg4 receptor, ErbB4, is highly expressed in the hypothalamus, and the phosphorylation of hypothalamic ErbB4 is reduced in diet-induced obesity (DIO) mice. Peripheral Nrg4 can act on ErbB4 via blood circulation and excite neurons in the paraventricular nucleus of hypothalamus (PVN). Central administration of recombinant Nrg4 protein (rNrg4) reduces obesity and related metabolic disorders by influencing energy expenditure and intake. Overexpression of ErbB4 in the PVN protects against obesity, whereas its knock down in oxytocin (Oxt) neuron accelerates obesity. Furthermore, Nrg4-ErbB4 signaling excites Oxt release, and ablation of Oxt neuron considerably attenuates the effect of Nrg4 on energy balance. These data suggest that the hypothalamus is a key target of Nrg4, which partially explains the multifaceted roles of Nrg4 in metabolism.

Paraventricular Vitamin D Receptors Are Required for Glucose Tolerance in Males but Not Females

When delivered directly into the brain, vitamin D, can improve glucose levels in male mice. Additionally, the loss of the vitamin D receptor (VDR) in male mice's paraventricular hypothalamus (PVH) results in impaired glucose tolerance. Data in humans shows that low vitamin D levels are detrimental to glucose homeostasis, an effect that may be more prominent in men. However, it is unknown if vitamin D action in the brain is required for normal glucose regulation in female mice. This study shows that in both viral and genetic models, male mice with obesity and PVH VDR loss have impaired glucose tolerance while female mice are unaffected. Weights were unaltered in both sexes by PVH VDR loss. Additionally, PVH VDR loss did not cause any glucose abnormalities in either sex when the mice were on a chow diet. Utilizing electrophysiology studies, we show PVH VDR loss resulted in decreased baseline firing frequency and resting membrane potential in males, but not females. Additionally, male mice with PVH VDR loss had impaired miniature excitatory postsynaptic currents (mEPSC), while females were unaffected. Interestingly, the PVH neurons of both sexes were activated by exogenous vitamin D (1,25-dihydroxyvitamin D3), an effect dependent upon the VDR. Thus, there is sexual dimorphism, for the actions of the PVH VDR on glucose regulation. PVH VDRs are necessary for normal glucose homeostasis in males but not females and this may be secondary to actions of the VDR on neuronal activity.

Modulating effects of capsaicin on glucose homeostasis and the underlying mechanism

Abnormal glucose homeostasis is linked to a variety of metabolic syndromes, such as insulin resistance, obesity, type-2 diabetes mellitus, hypertension and cardiovascular diseases. Maintenance of normal glucose homeostasis is important for the body to keep normal biological functions. As the major bioactive ingredient in chili peppers responsible for the pungent flavor, capsaicin has been reported to effectively improve glucose homeostasis with low cytotoxicity. In this review, the modulating effects of capsaicin on glucose homeostasis in cell models, animal models and human trials are summarized through both TRPV1 dependent and TRPV1 independent pathways. The relevant molecular mechanisms underlying its regulatory effects are also evaluated. Understanding the effects and mechanisms of capsaicin on glucose metabolism could provide theoretical evidence for its application in the food and pharmaceutical industries.

Satb2 regulates the development of dopaminergic neurons in the arcuate nucleus by Dlx1

Dopaminergic (DA) neurons in the arcuate nucleus (ARC) of the hypothalamus play essential roles in the secretion of prolactin and the regulation of energy homeostasis. However, the gene regulatory network responsible for the development of the DA neurons remains poorly understood. Here we report that the transcription factor special AT-rich binding protein 2 (Satb2) is required for the development of ARC DA neurons. Satb2 is expressed in a large proportion of DA neurons without colocalization with proopiomelanocortin (POMC), orexigenic agouti-related peptide (AgRP), neuropeptide-Y (NPY), somatostatin (Sst), growth hormone-releasing hormone (GHRH), or galanin in the ARC. Nestin-Cre;Satb2flox/flox (Satb2 CKO) mice show a reduced number of ARC DA neurons with unchanged numbers of the other types of ARC neurons, and exhibit an increase of serum prolactin level and an elevated metabolic rate. The reduction of ARC DA neurons in the CKO mice is observed at an embryonic stage and Dlx1 is identified as a potential downstream gene of Satb2 in regulating the development of ARC DA neurons. Together, our study demonstrates that Satb2 plays a critical role in the gene regulatory network directing the development of DA neurons in ARC.

Metabolic Consequences of Neuronal HIF1α-Deficiency in Mediobasal Hypothalamus in Mice

OBJECTIVE

This study aims to investigate whether hypoxia-inducible factor 1α (HIF1α) in the neurons of the mediobasal hypothalamus is involved in the regulation of body weight, glucose, and lipid metabolism in mice and to explore the underlying molecular mechanisms.

METHODS

HIF1α ^flox/flox mice were used. The adeno-associated virus that contained either cre, GFP and syn, or GFP and syn (controls) was injected into the mediobasal hypothalamus to selectively knock out HIF1α in the neurons of the mediobasal hypothalamus. The body weight and food intake were weighed daily. The levels of blood glucose, insulin, total cholesterol (TC), triglyceride (TG), free fatty acid (FFA), high-density lipoprotein (HDL), and low-density lipoprotein (LDL)were tested. Intraperitoneal glucose tolerance test (IPGTT) was performed. The insulin-stimulated Akt phosphorylation in the liver, epididymal fat, and skeletal muscle were examined. Also, the mRNA expression levels of HIF1α, proopiomelanocortin (POMC), neuropeptide Y (NPY), and glucose transporter protein 4 (Glut4) in the hypothalamus were checked.

RESULTS

After selectively knocking out HIF1α in the neurons of the mediobasal hypothalamus (HIF1αKOMBH), the body weights and food intake of mice increased significantly compared with the control mice (p < 0.001 at 4 weeks). Compared with that of the control group, the insulin level of HIF1αKOMBH mice was 3.5 times higher (p < 0.01). The results of the IPGTT showed that the blood glucose level of the HIF1αKOMBH group at 20-120 min was significantly higher than that of the control group (p < 0.05). The serum TC, FFA, HDL, and LDL content of the HIF1αKOMBH group was significantly higher than those of the control group (p < 0.05). Western blot results showed that compared with those in the control group, insulin-induced AKT phosphorylation levels in liver, epididymal fat, and skeletal muscle in the HIF1αKOMBH group were not as significantly elevated as in the control group. Reverse transcription-polymerase chain reaction (RT-PCR) results in the whole hypothalamus showed a significant decrease in Glut4 mRNA expression. And the mRNA expression levels of HIF1α, POMC, and NPY of the HIF1αKOMBH group decreased significantly in ventral hypothalamus.

CONCLUSIONS

The hypothalamic neuronal HIF1α plays an important role in the regulation of body weight balance in mice under normoxic condition. In the absence of hypothalamic neuronal HIF1α, the mice gained weight with increased appetite, accompanied with abnormal glucose and lipid metabolism. POMC and Glut4 may be responsible for this effect of HIF1α.

Characterization of mWake expression in the murine brain

Structure-function analyses of the mammalian brain have historically relied on anatomically-based approaches. In these investigations, physical, chemical, or electrolytic lesions of anatomical structures are applied, and the resulting behavioral or physiological responses assayed. An alternative approach is to focus on the expression pattern of a molecule whose function has been characterized and then use genetic intersectional methods to optogenetically or chemogenetically manipulate distinct circuits. We previously identified WIDE AWAKE (WAKE) in Drosophila, a clock output molecule that mediates the temporal regulation of sleep onset and sleep maintenance. More recently, we have studied the mouse homolog, mWAKE/ANKFN1, and our data suggest that its basic role in the circadian regulation of arousal is conserved. Here, we perform a systematic analysis of the expression pattern of mWake mRNA, protein, and cells throughout the adult mouse brain. We find that mWAKE labels neurons in a restricted, but distributed manner, in multiple regions of the hypothalamus (including the suprachiasmatic nucleus, dorsomedial hypothalamus, and tuberomammillary nucleus region), the limbic system, sensory processing nuclei, and additional specific brainstem, subcortical, and cortical areas. Interestingly, mWAKE is also observed in non-neuronal ependymal cells. In addition, to describe the molecular identities and clustering of mWake⁺ cells, we provide detailed analyses of single cell RNA sequencing data from the hypothalamus, a region with particularly significant mWAKE expression. These findings lay the groundwork for future studies into the potential role of mWAKE⁺ cells in the rhythmic control of diverse behaviors and physiological processes.

Neuronal cAMP/PKA Signaling and Energy Homeostasis

The brain plays a key role in the regulation of body weight and glucose metabolism. Peripheral signals including hormones, metabolites, and neural afferent signals are received and processed by the brain which in turn elicits proper behavioral and metabolic responses for maintaining energy and glucose homeostasis. The cAMP/protein kinase A (PKA) pathway acts downstream G-protein-coupled receptors (GPCR) to mediate the physiological effects of many hormones and neurotransmitters. Activated PKA phosphorylates various proteins including ion channels, enzymes, and transcription factors and regulates their activity. Recent studies have shown that neuronal cAMP/PKA activity in multiple brain regions are involved in the regulation of feeding, energy expenditure, and glucose homeostasis. In this chapter I summarize recent genetic and pharmacological studies concerning the regulation of body weight and glucose homeostasis by cAMP/PKA signaling in the brain.

Overweight Mice Show Coordinated Homeostatic and Hedonic Transcriptional Response across Brain

Obesogenic diets lead to overeating and obesity by inducing the expression of genes involved in hedonic and homeostatic responses in specific brain regions. However, how the effects on gene expression are coordinated in the brain so far remains largely unknown. In our study, we provided mice with access to energy-dense diet, which induced overeating and overweight, and we explored the transcriptome changes across the main regions involved in feeding and energy balance: hypothalamus, frontal cortex, and striatum. Interestingly, we detected two regulatory processes: a switch-like regulation with differentially expressed (DE) genes changing over 1.5-fold and “fine-tuned” subtler changes of genes whose levels correlated with body weight and behavioral changes. We found that genes in both categories were positioned within specific topologically associated domains (TADs), which were often differently regulated across different brain regions. These TADs were enriched in genes relevant for the physiological and behavioral observed changes. Our results suggest that chromatin structure coordinates diet-dependent transcriptional regulation.

p53 in AgRP neurons is required for protection against diet-induced obesity via JNK1

p53 is a well-known tumor suppressor that has emerged as an important player in energy balance. However, its metabolic role in the hypothalamus remains unknown. Herein, we show that mice lacking p53 in agouti-related peptide (AgRP), but not proopiomelanocortin (POMC) or steroidogenic factor-1 (SF1) neurons, are more prone to develop diet-induced obesity and show reduced brown adipose tissue (BAT) thermogenic activity. AgRP-specific ablation of p53 resulted in increased hypothalamic c-Jun N-terminal kinase (JNK) activity before the mice developed obesity, and central inhibition of JNK reversed the obese phenotype of these mice. The overexpression of p53 in the ARC or specifically in AgRP neurons of obese mice decreased body weight and stimulated BAT thermogenesis, resulting in body weight loss. Finally, p53 in AgRP neurons regulates the ghrelin-induced food intake and body weight. Overall, our findings provide evidence that p53 in AgRP neurons is required for normal adaptations against diet-induced obesity.

Emerging studies suggest that p53 is an important regulator of energy metabolism, yet there is little known about the metabolic function of this tumor suppressor in the hypothalamus. Here, authors illustrate that p53, specifically in AgRP neurons, is required for adaptation to diet-induced obesity.

Vitamin D and the paraventricular nucleus: Relevance for type 2 diabetes

Vitamin D deficiency is linked to type 2 diabetes and we recently showed this may be through action of vitamin D in the paraventricular nuclei (PVN) in the hypothalamus of the brain. This review focuses on the known roles of the PVN in glucose control and how previously discovered actions of vitamin D in other tissues may translate to action in the PVN. Specifically, we focus on the role of insulin and inflammation in the hypothalamus and how these may be modified through vitamin D action.

Hypothalamic sensing of ketone bodies after prolonged cerebral exposure leads to metabolic control dysregulation

Ketone bodies have been shown to transiently stimulate food intake and modify energy homeostasis regulatory systems following cerebral infusion for a moderate period of time (<6 hours). As ketone bodies are usually enhanced during episodes of fasting, this effect might correspond to a physiological regulation. In contrast, ketone bodies levels remain elevated for prolonged periods during obesity, and thus could play an important role in the development of this pathology. In order to understand this transition, ketone bodies were infused through a catheter inserted in the carotid to directly stimulate the brain for a period of 24 hours. Food ingested and blood circulating parameters involved in metabolic control as well as glucose homeostasis were determined. Results show that ketone bodies infusion for 24 hours increased food intake associated with a stimulation of hypothalamic orexigenic neuropeptides. Moreover, insulinemia was increased and caused a decrease in glucose production despite an increased resistance to insulin. The present study confirms that ketone bodies reaching the brain stimulates food intake. Moreover, we provide evidence that a prolonged hyperketonemia leads to a dysregulation of energy homeostasis control mechanisms. Finally, this study shows that brain exposure to ketone bodies alters insulin signaling and consequently glucose homeostasis.

Hypothalamic Vitamin D Improves Glucose Homeostasis and Reduces Weight

Despite clear associations between vitamin D deficiency and obesity and/or type 2 diabetes, a causal relationship is not established. Vitamin D receptors (VDRs) are found within multiple tissues, including the brain. Given the importance of the brain in controlling both glucose levels and body weight, we hypothesized that activation of central VDR links vitamin D to the regulation of glucose and energy homeostasis. Indeed, we found that small doses of active vitamin D, 1α,25-dihydroxyvitamin D3 (1,25D3) (calcitriol), into the third ventricle of the brain improved glucose tolerance and markedly increased hepatic insulin sensitivity, an effect that is dependent upon VDR within the paraventricular nucleus of the hypothalamus. In addition, chronic central administration of 1,25D3 dramatically decreased body weight by lowering food intake in obese rodents. Our data indicate that 1,25D3-mediated changes in food intake occur through action within the arcuate nucleus. We found that VDR colocalized with and activated key appetite-regulating neurons in the arcuate, namely proopiomelanocortin neurons. Together, these findings define a novel pathway for vitamin D regulation of metabolism with unique and divergent roles for central nervous system VDR signaling. Specifically, our data suggest that vitamin D regulates glucose homeostasis via the paraventricular nuclei and energy homeostasis via the arcuate nuclei.

Brain GLP-1 and insulin sensitivity

Type 2 diabetes is often treated with a class of drugs referred to as glucagon-like peptide-1 (GLP-1) analogs. GLP-1 is a peptide secreted by the gut that acts through only one known receptor, the GLP-1 receptor. The primary function of GLP-1 is thought to be lowering of postprandial glucose levels. Indeed, medications utilizing this system, including the long-acting GLP-1 analogs liraglutide and exenatide, are beneficial in reducing both blood sugars and body weight. GLP-1 analogs were long presumed to affect glucose control through their ability to increase insulin levels through peripheral action on beta cells. However, multiple lines of data point to the ability of GLP-1 to act within the brain to alter glucose regulation. In this review we will discuss the evidence for a central GLP-1 system and the effects of GLP-1 in the brain on regulating multiple facets of glucose homeostasis including glucose tolerance, insulin production, insulin sensitivity, hepatic glucose production, muscle glucose uptake, and connections of the central GLP-1 system to the gut. Although the evidence indicates that GLP-1 receptors in the brain are not necessary for physiologic control of glucose regulation, we discuss the research showing a strong effect of acute manipulation of the central GLP-1 system on glucose control and how it is relevant to type 2 diabetic patients.

Can Brown Fat Win the Battle Against White Fat?

A rapid growth in the overweight and obese population in the last few decades suggest that the current diet, exercise, awareness or drug strategies are still not effectively restraining the obesity epidemic. Obesity results from increased energy intake, and the body's energy balance shifts towards energy abundance. Therefore, current research is focused on developing new strategies aimed at increasing energy expenditure. As a result, brown adipose tissue (BAT) is receiving tremendous attention since the major function of BAT is to dissipate energy as heat. For example, mouse models that have increased BAT activity or increased numbers of brown-like adipocytes within the white adipose tissue (WAT) are lean and protected from obesity. Alternatively, mouse models that lack BAT activity are more susceptible to age and diet-induced obesity. However, a significant loss of BAT mass during the natural growth process in humans has created enormous challenges in effectively utilizing this tissue to increase energy expenditure. New strategies are primarily focused on expanding the BAT mass and/or activating the existing BAT. In this regard, recent finding that expression of early B cell factor-2 (Ebf2) reprograms the white pre-adipocytes into brown adipocytes is a significant break-through in developing BAT-mediated strategies to treat obesity. Here we review the major biological functions of WAT and BAT, which play critical but opposing roles in the energy spectrum, energy storage versus energy expenditure, and we evaluate whether activation and/or expansion of BAT is practically achievable to treat obesity in humans.

S-resistin, a non secretable resistin isoform, impairs the insulin signalling pathway in 3T3-L1 adipocytes

S-resistin is a non-secretable resistin spliced variant, which is expressed mainly in the white adipose tissue from Wistar rats. Previous results confirmed that 3T3-L1 cells expressing s-resistin (3T3-L1-s-res) showed an inflammatory response and exhibited a decrease in glucose transport, both basal and insulin-stimulated. Here we present evidences demonstrating for the first time that s-resistin, like resistin, blocks insulin signalling pathway by inhibiting insulin receptor, insulin receptor substrate 1, protein kinase B/Akt and the mammalian target of rapamycin phosphorylation, and increasing the suppressor of cytokine signalling 3 levels being the later probably due to augmented of leptin expression. Thus, our data suggest that s-resistin could act by a still unknown intracrine pathway as an intracellular sensor, regulating the adipocyte insulin sensitivity.

A potential role for the midbrain in integrating fat-free mass determined energy needs: An H2 (15) O PET study

Little is known on how sensing of energy needs is centrally represented, integrated, and translated into the behavioral aspects of energy homeostasis. Fat free mass (FFM) is the major determinant of energy expenditure. We investigated how interindividual variances in FFM relate to neuronal activity in humans. Healthy adults (n = 64, 21F/43M; age 31.3 ± 9.1y; percentage of body fat [PFAT] 25.6 ± 10.7%; BMI 30.4 ± 9) underwent a 36h fast and subsequent H(2) (15) O positron emission tomographic (PET) measurement of regional cerebral blood flow (rCBF). Multiple variable regression analysis revealed significant associations of FFM with rCBF within the midbrain [including parts of the periaqueductal gray (PAG), ventral tegmental area (VTA), thalamic and hypothalamic regions], the bilateral parahippocampal region, left anterior cingulate, left insular cortex, right cerebellum, and distinct regions within the temporal and occipital cortex. In contrast, no significant associations were found for fat mass (FM). We investigated the potential functional-anatomical link between FFM and central regulation of food intake by performing a conjunction analysis of FFM and the perceived hunger feelings. This showed a significant overlap within the midbrain PAG. Mediation analysis demonstrated a significant indirect effect of FFM on hunger with PAG rCBF as mediator. Most regions we found to be associated with FFM form part in ascending homeostatic pathways and cortical circuitries implicated in the regulation of basic bodily functions indicating a potential role of these central networks in the integration of FFM determined energy needs.

Insulin transport into the brain and cerebrospinal fluid

The pancreatic hormone insulin plays a well-described role in the periphery, based principally on its ability to lower circulating glucose levels via activation of glucose transporters. However, insulin also acts within the central nervous system (CNS) to alter a number of physiological outcomes ranging from energy balance and glucose homeostasis to cognitive performance. Insulin is transported into the CNS by a saturable receptor-mediated process that is proposed to be dependent on the insulin receptor. Transport of insulin into the brain is dependent on numerous factors including diet, glycemia, a diabetic state and notably, obesity. Obesity leads to a marked decrease in insulin transport from the periphery into the CNS and the biological basis of this reduction of transport remains unresolved. Despite decades of research into the effects of central insulin on a wide range of physiological functions and its transport from the periphery to the CNS, numerous questions remain unanswered including which receptor is responsible for transport and the precise mechanisms of action of insulin within the brain.

Chronic alcohol consumption from adolescence to adulthood in mice--hypothalamic gene expression changes in insulin-signaling pathway

Adolescence is a developmental stage vulnerable to alcohol drinking-related problems, and alcohol exposure during adolescence may lead to long-lasting consequences. The hypothalamus is a key brain region for food and water intake regulation as well as weight control, and is one of the alcohol-sensitive brain regions. However, it is not known what the alcohol effect is on the hypothalamus following adolescent alcohol intake, chronically over adolescent development, at moderate levels. We employed a model of chronic moderate alcohol intake from adolescence to adulthood in mice, and analyzed the effect of alcohol on growth and weight gain, as well as hypothalamic gene expression patterns. The results indicated that chronic alcohol consumption during adolescence, even at moderate levels, led to both a reduction in weight gain in mice, and considerable gene expression changes in the hypothalamus. Pathway analysis and real-time PCR identified the type II diabetes mellitus and the insulin-signaling pathways as being the hypothalamic pathways affected by chronic alcohol. Our findings from the mouse alcohol consumption study therefore serve as a potential warning against alcohol consumption during adolescence, such as in teens and college students.

Differences in acute anorectic effects of long-acting GLP-1 receptor agonists in rats

Long-acting glucagon-like peptide-1 receptor (GLP-1R) agonists have both glucose- and weight-lowering effects. The brain is poised to mediate both of these actions since GLP-1Rs are present in key areas known to control weight and glucose. Although some research has been performed on the effects of exendin-4 in the brain, little data exists on the central effects of liraglutide, a long-acting GLP-1R agonist with much closer structural homology to native GLP-1. In lean, Long-Evans rats, we found that direct intra-third cerebroventricular (i3vt) administration of 0.26 nmol liraglutide caused a 50% reduction in food intake. However, exendin-4 produced the same reduction in food intake with 10-fold greater potency (0.02 nmol). These data are supported by similar c-Fos immunoreactivity in the hypothalamic paraventricular nuclei by exendin-4 as compared to liraglutide despite differing doses. The anorectic effects of both drugs were blocked with i3vt pre-treatment of a GLP-1R competitive antagonist, exendin(9-39), indicating that both drugs required the GLP-1R for their effects. Exendin-4, and not liraglutide, caused hyperglycemia when given i3vt prior to an oral glucose tolerance test, although liraglutide did not lower glucose. Thus, these data show that GLP-1R agonists have differing anorectic potencies in the CNS, which may account for some of their clinical differences. Additionally, we show here that the glucose lowering properties of acute administration of GLP-1R agonists are not accounted for by their central effects.

Mitofusin 2 in POMC neurons connects ER stress with leptin resistance and energy imbalance

Mitofusin 2 (MFN2) plays critical roles in both mitochondrial fusion and the establishment of mitochondria-endoplasmic reticulum (ER) interactions. Hypothalamic ER stress has emerged as a causative factor for the development of leptin resistance, but the underlying mechanisms are largely unknown. Here, we show that mitochondria-ER contacts in anorexigenic pro-opiomelanocortin (POMC) neurons in the hypothalamus are decreased in diet-induced obesity. POMC-specific ablation of Mfn2 resulted in loss of mitochondria-ER contacts, defective POMC processing, ER stress-induced leptin resistance, hyperphagia, reduced energy expenditure, and obesity. Pharmacological relieve of hypothalamic ER stress reversed these metabolic alterations. Our data establish MFN2 in POMC neurons as an essential regulator of systemic energy balance by fine-tuning the mitochondrial-ER axis homeostasis and function. This previously unrecognized role for MFN2 argues for a crucial involvement in mediating ER stress-induced leptin resistance.

GRP78 overproduction in pancreatic beta cells protects against high-fat-diet-induced diabetes in mice

AIMS/HYPOTHESIS

Endoplasmic reticulum (ER) stress has been detected in pancreatic beta cells and in insulin-sensitive tissues, such as adipose and liver, in obesity-linked rodent models of type 2 diabetes. The contribution of ER stress to pancreatic beta cell dysfunction in type 2 diabetes is unclear. We hypothesised that increased chaperone capacity protects beta cells from ER stress and dysfunction caused by obesity and improves overall glucose homeostasis.

METHODS

We generated a mouse model that overproduces the resident ER chaperone GRP78 (glucose-regulated protein 78 kDa) in pancreatic beta cells under the control of a rat insulin promoter. These mice were subjected to high-fat diet (HFD) feeding for 20 weeks and metabolic variables and markers of ER stress in islets were measured.

RESULTS

As expected, control mice on the HFD developed obesity, glucose intolerance and insulin resistance. In contrast, GRP78 transgenic mice tended to be leaner than their non-transgenic littermates and were protected against development of glucose intolerance, insulin resistance and ER stress in islets. Furthermore, islets from transgenic mice had a normal insulin content and normal levels of cell-surface GLUT2 (glucose transporter 2) and the transgenic mice were less hyperinsulinaemic than control mice on the HFD.

CONCLUSIONS/INTERPRETATION

These data show that increased chaperone capacity in beta cells provides protection against the pathogenesis of obesity-induced type 2 diabetes by maintaining pancreatic beta cell function, which ultimately improves whole-body glucose homeostasis.

Increasing muscle mass to improve metabolism

Skeletal muscle insulin resistance is a predictor of the development of type 2 diabetes and maintenance of adequate muscle glucose disposal in muscle may help to prevent diabetes. Lipodystrophy is a type of diabetes caused by a reduction of white adipose tissue and the adipokine leptin. Lipidemia, insulin resistance and hyperphagia develop as a consequence. In a recent study, we showed that increasing skeletal muscle mass by inhibiting signaling of myostatin, a transforming growth factor β (TGFβ) family member that negatively regulates muscle growth, prevents the development of diabetes in a mouse model of lipodystrophy. Muscle-specific myostatin inhibition also prevented hyperphagia suggesting muscle may regulate food intake. Here we discuss these results in the context of strategies to increase muscle insulin sensitivity as well as new findings about the effects of myostatin and other TGFβ family members on similar metabolic processes.

Interactions between the central nervous system and pancreatic islet secretions: a historical perspective

The endocrine pancreas is richly innervated with sympathetic and parasympathetic projections from the brain. In the mid-20th century, it was established that α-adrenergic activation inhibits, whereas cholinergic stimulation promotes, insulin secretion; this demonstrated the importance of the sympathetic and parasympathetic systems in pancreatic endocrine function. It was later established that insulin injected peripherally could act within the brain, leading to the discovery of insulin and insulin receptors within the brain and the receptor-mediated transport of insulin into the central nervous system from endothelial cells. The insulin receptor within the central nervous system is widely distributed, reflecting insulin's diverse range of actions, including acting as an adiposity signal to reduce food intake and increase energy expenditure, regulation of systemic glucose responses, altering sympathetic activity, and involvement in cognitive function. As observed with central insulin administration, the pancreatic hormones glucagon, somatostatin, pancreatic polypeptide, and amylin can each also reduce food intake. Pancreatic and also gut hormones are released cephalically, in what is an important mechanism to prepare the body for a meal and prevent excessive postprandial hyperglycemia.

Olanzapine-induced hyperglycemia: possible involvement of histaminergic, dopaminergic and adrenergic functions in the central nervous system

BACKGROUND/AIMS

Atypical antipsychotic drugs such as olanzapine are known to induce metabolic disturbance. We have already shown that olanzapine induces hepatic glucose production through the activation of hypothalamic adenosine 5'-monophosphate-activated protein kinase (AMPK). However, it is unclear how olanzapine activates hypothalamic AMPK. Since olanzapine is known to antagonize several receptors, including histaminergic, muscarinic, serotonergic, dopaminergic and adrenergic receptors, we examined the effect of each receptor antagonist on blood glucose levels in mice. Moreover, we also investigated whether these antagonists activate hypothalamic AMPK.

METHODS

Male 6-week-old ICR mice were used. Blood glucose levels were determined by the glucose oxidase method. AMPK expression was measured by Western blotting.

RESULTS

Central administration of olanzapine (5-15 nmol i.c.v.) dose-dependently increased blood glucose levels in mice, whereas olanzapine did not change blood insulin levels. Histamine H1 receptor antagonist chlorpheniramine (1-10 μg i.c.v.), dopamine D2 receptor antagonist L-sulpiride (1-10 μg i.c.v.) and α1-adrenoceptor antagonist prazosin (0.3-3 μg i.c.v.) also significantly increased blood glucose levels in mice. In contrast, the blood glucose levels were not affected by muscarinic M1 receptor antagonist dicyclomine (1-10 μg i.c.v.) or serotonin 5-HT2A receptor antagonist M100907 (1-10 ng i.c.v.). Olanzapine-induced hyperglycemia was inhibited by the AMPK inhibitor compound C, and AMPK activator AICAR (10 ng to 1 μg i.c.v.) significantly increased blood glucose levels. Olanzapine (15 nmol), chlorpheniramine (10 μg), L-sulpiride (10 μg) and prazosin (3 μg) significantly increased phosphorylated AMPK in the hypothalamus of mice.

CONCLUSION

These results suggest that olanzapine activates hypothalamic AMPK by antagonizing histamine H1 receptors, dopamine D2 receptors and α1-adrenoceptors, which induces hyperglycemia.

Deletion of miRNA processing enzyme Dicer in POMC-expressing cells leads to pituitary dysfunction, neurodegeneration and development of obesity

MicroRNAs (miRNAs) have recently emerged as key regulators of metabolism. However, their potential role in the central regulation of whole-body energy homeostasis is still unknown. In this study we show that the expression of Dicer, an essential endoribonuclease for miRNA maturation, is modulated by nutrient availability and excess in the hypothalamus. Conditional deletion of Dicer in POMC-expressing cells resulted in obesity, characterized by hyperphagia, increased adiposity, hyperleptinemia, defective glucose metabolism and alterations in the pituitary-adrenal axis. The development of the obese phenotype was paralleled by a POMC neuron degenerative process that started around 3 weeks of age. Hypothalamic transcriptomic analysis in presymptomatic POMCDicerKO mice revealed the downregulation of genes implicated in biological pathways associated with classical neurodegenerative disorders, such as MAPK signaling, ubiquitin-proteosome system, autophagy and ribosome biosynthesis. Collectively, our results highlight a key role for miRNAs in POMC neuron survival and the consequent development of neurodegenerative obesity.