Biological age and environmental risk factors for dementia and stroke: Molecular mechanisms

Since the development of antibiotics and vaccination, as well as major improvements in public hygiene, the main risk factors for morbidity and mortality are age and chronic exposure to environmental factors, both of which can interact with genetic predispositions. As the average age of the population increases, the prevalence and costs of chronic diseases, especially neurological conditions, are rapidly increasing. The deleterious effects of age and environmental risk factors, develop chronically over relatively long periods of time, in contrast to the relatively rapid deleterious effects of infectious diseases or accidents. Of particular interest is the hypothesis that the deleterious effects of environmental factors may be mediated by acceleration of biological age. This hypothesis is supported by evidence that dietary restriction, which universally delays age-related diseases, also ameliorates deleterious effects of environmental factors. Conversely, both age and environmental risk factors are associated with the accumulation of somatic mutations in mitotic cells and epigenetic modifications that are a measure of "biological age", a better predictor of age-related morbidity and mortality than chronological age. Here we review evidence that environmental risk factors such as smoking and air pollution may also drive neurological conditions, including Alzheimer's Disease, by the acceleration of biological age, mediated by cumulative and persistent epigenetic effects as well as somatic mutations. Elucidation of such mechanisms could plausibly allow the development of interventions which delay deleterious effects of both aging and environmental risk factors.

Serotonin and Dopamine Mimic Glucose-Induced Reinforcement in C. elegans: Potential Role of NSM Neurons and the Serotonin Subtype 4 Receptor

Food produces powerful reinforcement that can lead to overconsumption and likely contributes to the obesity epidemic. The present studies examined molecular mechanisms mediating food-induced reinforcement in the model system C. elegans. After a 1-h training session during which food (bacteria) is paired with the odorant butanone, odor preference for butanone robustly increased. Glucose mimicked this effect of bacteria. Glucose-induced odor preference was enhanced similarly by prior food withdrawal or blocking glucose metabolism in the presence of food. Food- and glucose-induced odor preference was mimicked by serotonin signaling through the serotonin type-4 (5-HT4) receptor. Dopamine (thought to act primarily through a D1-like receptor) facilitated, whereas the D2 agonist bromocriptine blocked, food- and glucose-induced odor preference. Furthermore, prior food withdrawal similarly influenced reward produced by serotonin, dopamine, or food, implying post-synaptic enhancement of sensitivity to serotonin and dopamine. These results suggest that glucose metabolism plays a key role in mediating both food-induced reinforcement and enhancement of that reinforcement by prior food withdrawal and implicate serotonergic signaling through 5-HT4 receptor in the re-enforcing properties of food.

Biopsy During Minimally Invasive Intracerebral Hemorrhage Clot Evacuation

BACKGROUND

The safety and efficacy of brain parenchyma biopsy during minimally invasive (MIS) intracerebral hemorrhage (ICH) clot evacuation has not been previously reported. The objective of this study was to establish the safety and diagnostic efficacy of brain biopsy during MIS ICH clot evacuation and to validate the modified Boston criteria as a predictor of cerebral amyloid angiopathy (CAA) in this cohort.

METHODS

From October 2016 to March 2018, superficial and perihematomal biopsies were collected for 40 patients undergoing MIS ICH clot evacuation and analyzed by the pathology department to assess for various ICH etiologies. Additionally, the admission magnetic resonance imaging or computed tomography scan of each patient was analyzed and evaluated for the likelihood of a CAA etiology based on the modified Boston criteria. Student t test was used to analyze intergroup differences in continuous variables, and a 2-tailed Fisher exact test was used to determine intergroup differences of categorical variables, with significance set at P < 0.05.

RESULTS

Two of the 40 patients (5%) experienced postoperative rebleed. Four of the 40 patients (10%) had evidence of CAA on biopsy. Patients with CAA on biopsy were older (P = 0.005) and had a higher prevalence of parietal lobe (P = 0.02) and occipital lobe (P = 0.001) hemorrhage. The modified Boston criteria had a sensitivity of 100% (95% confidence interval [CI], 39.6%-100%) and a specificity of 72.2% (95% CI, 54.6%-84.2%) for predicting CAA on biopsy.

CONCLUSIONS

Brain biopsy in MIS ICH clot evacuation is safe and allows for the diagnosis of various ICH etiologies.

Dietary Restriction and Glycolytic Inhibition Reduce Proteotoxicity and Extend Lifespan via NHR-49

Mechanisms mediating protective effects of dietary restriction during aging are of great interest since activating such mechanisms protect against a wide range of age-related diseases. In mammals key metabolic responses to nutritional deprivation are mediated by the transcription factor PPAR-alpha, which is activated by free fatty acids and promotes lipid metabolism while inhibiting glucose metabolism. The C. elegans gene nhr-49 appears to function similarly in C. elegans. Here we report that protective effects of dietary restriction and inhibition of glucose metabolism to increase lifespan wild-type C. elegans and reduce toxicity in a polyQ model of Huntington's disease in C. elegans are dependent on NHR-49 and its co-activator CREB-Binding Protein (CBP). We have previously demonstrated that inhibition of cbp blocks protective effects of dietary restriction and blocks the molecular switch from glucose metabolism to alternative substrates. Conversely, increased glucose concentration and inhibition of cbp reduce lifespan and increase proteotoxicity. Lactate and inhibition of ETC complex II mimicked toxic effects of glucose on proteotoxicity whereas pyruvate and inhibition of ETC complex I protected against glucose-enhanced proteotoxicity. These results support that PPAR-alpha-like activity mediates protective effects of dietary restriction by reducing glucose metabolism via reducing production of NADH, and corroborate and extend recent studies demonstrating that PPPAR-alpha agonists increase lifespan in C. elegans dependent on NHR-49.

Glucose-Induced Transcriptional Hysteresis: Role in Obesity, Metabolic Memory, Diabetes, and Aging

During differentiation transient, inducers produce permanent changes in gene expression. A similar phenomenon, transcriptional hysteresis, produced by transient or prolonged exposure to glucose, leads to cumulative, persistent, and largely irreversible effects on glucose-regulated gene expression, and may drive key aspects of metabolic memory, obesity, diabetes, and aging, and explain the protective effects of dietary restriction during aging. The most relevant effects of glucose-induced transcriptional hysteresis are the persistent effects of elevated glucose on genes that control glucose metabolism itself. A key observation is that, as with the lac operon, glucose induces genes that promote glycolysis and inhibits gene expression of alternative metabolic pathways including the pentose pathway, beta oxidation, and the TCA cycle. A similar pattern of metabolic gene expression is observed during aging, suggesting that cumulative exposure to glucose during aging produces this metabolic shift. Conversely, dietary restriction, which increases lifespan and delays age-related impairments, produces the opposite metabolic profile, leading to a shift away from glycolysis and toward the use of alternative substrates, including lipid and ketone metabolisms. The effect of glucose on gene expression leads to a positive feedback loop that leads to metastable persistent expression of genes that promote glycolysis and inhibit alternative pathways, a phenomenon first observed in the regulation of the lac operon. On the other hand, this pattern of gene expression can also be inhibited by activation of peroxisome proliferator activating receptor transcription factors that promote beta oxidation and inhibit metabolism of glucose-derived carbon bonds in the TCA cycle. Several pathological consequences may arise from glucose-induced transcriptional hysteresis. First, elevated glucose induces glycolytic genes in pancreatic beta cells, which induces a semi-stable persistent increase in insulin secretion, which could drive obesity and insulin resistance, and also due to glucose toxicity could eventually lead to beta-cell decompensation and diabetes. Diabetic complications persist even after complete normalization of glucose, a phenomenon known as metabolic memory. This too can be explained by persistent bistable expression of glucose-induced glycolytic genes.

Epigenetic mechanisms underlying lifespan and age-related effects of dietary restriction and the ketogenic diet

Aging constitutes the central risk factor for major diseases including many forms of cancer, neurodegeneration, and cardiovascular diseases. The aging process is characterized by both global and tissue-specific changes in gene expression across taxonomically diverse species. While aging has historically been thought to entail cell-autonomous, even stochastic changes, recent evidence suggests that modulation of this process can be hierarchal, wherein manipulations of nutrient-sensing neurons (e.g., in the hypothalamus) produce peripheral effects that may modulate the aging process itself. The most robust intervention extending lifespan, plausibly impinging on the aging process, involves different modalities of dietary restriction (DR). Lifespan extension by DR is associated with broad protection against diseases (natural and engineered). Here we review potential epigenetic processes that may link lifespan to age-related diseases, particularly in the context of DR and (other) ketogenic diets, focusing on brain and hypothalamic mechanisms.

Mechanisms and significance of brain glucose signaling in energy balance, glucose homeostasis, and food-induced reward

The concept that hypothalamic glucose signaling plays an important role in regulating energy balance, e.g., as instantiated in the so-called "glucostat" hypothesis, is one of the oldest in the field of metabolism. However the mechanisms by which neurons in the hypothalamus sense glucose, and the function of glucose signaling in the brain, has been difficult to establish. Nevertheless recent studies probing mechanisms of glucose signaling have also strongly supported a role for glucose signaling in regulating energy balance, glucose homeostasis, and food-induced reward.

Role of Hypothalamic Creb-Binding Protein in Obesity and Molecular Reprogramming of Metabolic Substrates

We have reported a correlation between hypothalamic expression of Creb-binding protein (Cbp) and lifespan, and that inhibition of Cbp prevents protective effects of dietary restriction during aging, suggesting that hypothalamic Cbp plays a role in responses to nutritional status and energy balance. Recent GWAS and network analyses have also implicated Cbp as the most connected gene in protein-protein interactions in human Type 2 diabetes. The present studies address mechanisms mediating the role of Cbp in diabetes by inhibiting hypothalamic Cbp using a Cre-lox strategy. Inhibition of hypothalamic Cbp results in profound obesity and impaired glucose homeostasis, increased food intake, and decreased body temperature. In addition, these changes are accompanied by molecular evidence in the hypothalamus for impaired leptin and insulin signaling, a shift from glucose to lipid metabolism, and decreased Pomc mRNA, with no effect on locomotion. Further assessment of the significance of the metabolic switch demonstrated that enhanced expression of hypothalamic Cpt1a, which promotes lipid metabolism, similarly resulted in increased body weight and reduced Pomc mRNA.

Discover the network underlying the connections between aging and age-related diseases

Although our knowledge of aging has greatly expanded in the past decades, it remains elusive why and how aging contributes to the development of age-related diseases (ARDs). In particular, a global mechanistic understanding of the connections between aging and ARDs is yet to be established. We rely on a network modelling named "GeroNet" to study the connections between aging and more than a hundred diseases. By evaluating topological connections between aging genes and disease genes in over three thousand subnetworks corresponding to various biological processes, we show that aging has stronger connections with ARD genes compared to non-ARD genes in subnetworks corresponding to "response to decreased oxygen levels", "insulin signalling pathway", "cell cycle", etc. Based on subnetwork connectivity, we can correctly "predict" if a disease is age-related and prioritize the biological processes that are involved in connecting to multiple ARDs. Using Alzheimer's disease (AD) as an example, GeroNet identifies meaningful genes that may play key roles in connecting aging and ARDs. The top modules identified by GeroNet in AD significantly overlap with modules identified from a large scale AD brain gene expression experiment, supporting that GeroNet indeed reveals the underlying biological processes involved in the disease.

Protection by dietary restriction in the YAC128 mouse model of Huntington's disease: Relation to genes regulating histone acetylation and HTT

Huntington's disease (HD) is a fatal neurodegenerative disease characterized by metabolic, cognitive, and motor deficits. HD is caused by an expanded CAG repeat in the first exon of the HTT gene, resulting in an expanded polyglutamine section. Dietary restriction (DR) increases lifespan and ameliorates age-related pathologies, including in a model of HD, but the mechanisms mediating these protective effects are unknown. We report metabolic and behavioral effects of DR in the full-length YAC128 HD mouse model, and associated transcriptional changes in hypothalamus and striatum. DR corrected many effects of the transgene including increased body weight, decreased blood glucose, and impaired motor function. These changes were associated with reduced striatal human (but not mouse) HTT expression, as well as alteration in gene expression regulating histone acetylation modifications, particularly Hdac2. Other mRNAs related to Huntington's pathology in striatal tissue showed significant modulation by the transgene, dietary restriction or both. These results establish a protective role of DR in a transgenic model that contains the complete human HTT gene and for the first time suggest a role for DR in lowering HTT level, which correlates with severity of symptoms.

Metabolic mystery: aging, obesity, diabetes, and the ventromedial hypothalamus

We propose that energy balance, glucose homeostasis, and aging are all regulated largely by the same nutrient-sensing neurons in the ventromedial hypothalamus (VMH). Although the central role of these neurons in regulating energy balance is clear, their role in regulating glucose homeostasis has only recently become more clear. This latter function may be most relevant to aging and lifespan by controlling the rate of glucose metabolism. Specifically, glucose-sensing neurons in VMH promote peripheral glucose metabolism, and dietary restriction, by reducing glucose metabolism in these neurons, reduces glucose metabolism of the rest of the body, thereby increasing lifespan. Here we discuss recent studies demonstrating the key role of hypothalamic neurons in driving aging and age-related diseases.

Glucose-induced metabolic memory in Schwann cells: prevention by PPAR agonists

A major barrier in reversing diabetic complications is that molecular and pathologic effects of elevated glucose persist despite normalization of glucose, a phenomenon referred to as metabolic memory. In the present studies we have investigated the effects of elevated glucose on Schwann cells, which are implicated in diabetic neuropathy. Using quantitative PCR arrays for glucose and fatty acid metabolism, we have found that chronic (>8 wk) 25 mM high glucose induces a persistent increase in genes that promote glycolysis, while inhibiting those that oppose glycolysis and alternate metabolic pathways such as fatty acid metabolism, the pentose phosphate pathway, and trichloroacetic acid cycle. These sustained effects were associated with decreased peroxisome proliferator-activated receptor (PPAR)γ binding and persistently increased reactive oxygen species, cellular NADH, and altered DNA methylation. Agonists of PPARγ and PPARα prevented select effects of glucose-induced gene expression. These observations suggest that Schwann cells exhibit features of metabolic memory that may be regulated at the transcriptional level. Furthermore, targeting PPAR may prevent metabolic memory and the development of diabetic complications.

Treatment of diabetes and diabetic complications with a ketogenic diet

Accumulating evidence suggests that low-carbohydrate, high-fat diets are safe and effective to reduce glycemia in diabetic patients without producing significant cardiovascular risks. Most of these studies have been carried out specifically restricting carbohydrates, which tends to lead to increased protein intake, thus reducing the ketosis. However, diets that limit protein as well as carbohydrates, entailing a composition very high in fat, appear even more effective to reduce glucose and whole-body glucose metabolism in humans. In animal models, low-carbohydrate, high-protein diets do not produce ketosis or reduce glycemia but rather cause obesity. However, limiting both protein and carbohydrates as in a classic ketogenic diet remarkably reduces blood glucose in animal models of type 1 and type 2 diabetes and reverses diabetic nephropathy. Future studies should assess if ketogenic diets would be effective to reverse diabetic complications in humans.

Role of the hypothalamus in mediating protective effects of dietary restriction during aging

Dietary restriction (DR) can extend lifespan and reduce disease burden across a wide range of animals and yeast but the mechanisms mediating these remarkably protective effects remain to be elucidated despite extensive efforts. Although it has generally been assumed that protective effects of DR are cell-autonomous, there is considerable evidence that many whole-body responses to nutritional state, including DR, are regulated by nutrient-sensing neurons. In this review, we explore the hypothesis that nutrient sensing neurons in the ventromedial hypothalamus hierarchically regulate the protective responses of dietary restriction. We describe multiple peripheral responses that are hierarchically regulated by the hypothalamus and we present evidence for non-cell autonomous signaling of dietary restriction gathered from a diverse range of models including invertebrates, mammalian cell culture, and rodent studies.

Regulation of peripheral metabolism by substrate partitioning in the brain

All organisms must adapt to changing nutrient availability, with nutrient surplus promoting glucose metabolism and nutrient deficit promoting alternative fuels (in mammals, mainly free fatty acids). A major function of glucose-sensing neurons in the hypothalamus is to regulate blood glucose. When these neurons sense glucose levels are too low, they activate robust counterregulatory responses to enhance glucose production, primarily from liver, and reduce peripheral metabolism. Some hypothalamic neurons can metabolize free fatty acids via β-oxidation, and β-oxidation generally opposes effects of glucose on hypothalamic neurons. Thus hypothalamic β-oxidation promotes obese phenotypes, including enhanced hepatic glucose output.

Comparative proteomic analysis of the ATP-sensitive K+ channel complex in different tissue types

ATP-sensitive K(+) (K(ATP)) channels are expressed ubiquitously, but have diverse roles in various organs and cells. Their diversity can partly be explained by distinct tissue-specific compositions of four copies of the pore-forming inward rectifier potassium channel subunits (Kir6.1 and/or Kir6.2) and four regulatory sulfonylurea receptor subunits (SUR1 and/or SUR2). Channel function and/or subcellular localization also can be modified by the proteins with which they transiently or permanently interact to generate even more diversity. We performed a quantitative proteomic analysis of K(ATP) channel complexes in the heart, endothelium, insulin-secreting min6 cells (pancreatic β-cell like), and the hypothalamus to identify proteins with which they interact in different tissues. Glycolysis is an overrepresented pathway in identified proteins of the heart, min6 cells, and the endothelium. Proteins with other energy metabolic functions were identified in the hypothalamic samples. These data suggest that the metabolo-electrical coupling conferred by K(ATP) channels is conferred partly by proteins with which they interact. A large number of identified cytoskeletal and trafficking proteins suggests endocytic recycling may help control K(ATP) channel surface density and/or subcellular localization. Overall, our data demonstrate that K(ATP) channels in different tissues may assemble with proteins having common functions, but that tissue-specific complex organization also occurs.

Hypothalamic Fkbp51 is induced by fasting, and elevated hypothalamic expression promotes obese phenotypes

To discover hypothalamic genes that might play a role in regulating energy balance, we carried out a microarray screen for genes induced by a 48-h fast in male C57Bl/6J mouse hypothalamus. One such gene was Fkbp51 (FK506 binding protein 5; Locus NP_034350). The product of this gene is of interest because it blocks glucocorticoid action, suggesting that fasting-induced elevation of this gene in the hypothalamus may reduce glucocorticoid negative feedback, leading to elevated glucocorticoid levels, thus promoting obese phenotypes. Subsequent analysis demonstrated that a 48-h fast induces Fkbp51 in ventromedial, paraventricular, and arcuate hypothalamic nuclei of mice and rats. To assess if hypothalamic Fkbp51 promotes obesity, the gene was transferred to the hypothalamus via an adeno-associated virus vector. Within 2 wk following Fkbp51 overexpression, mice on a high-fat diet exhibited elevated body weight, without hyperphagia, relative to mice receiving the control mCherry vector. Body weight remained elevated for more than 8 wk and was associated with elevated corticosterone and impaired glucose tolerance. These studies suggest that elevated hypothalamic Fkbp51 promotes obese phenotypes.

Peroxisome proliferation-associated control of reactive oxygen species sets melanocortin tone and feeding in diet-induced obesity

Previous studies have proposed roles for hypothalamic reactive oxygen species (ROS) in the modulation of circuit activity of the melanocortin system. Here we show that suppression of ROS diminishes pro-opiomelanocortin (POMC) cell activation and promotes the activity of neuropeptide Y (NPY)- and agouti-related peptide (AgRP)-co-producing (NPY/AgRP) neurons and feeding, whereas ROS-activates POMC neurons and reduces feeding. The levels of ROS in POMC neurons were positively correlated with those of leptin in lean and ob/ob mice, a relationship that was diminished in diet-induced obese (DIO) mice. High-fat feeding resulted in proliferation of peroxisomes and elevated peroxisome proliferator-activated receptor γ (PPAR-γ) mRNA levels within the hypothalamus. The proliferation of peroxisomes in POMC neurons induced by the PPAR-γ agonist rosiglitazone decreased ROS levels and increased food intake in lean mice on high-fat diet. Conversely, the suppression of peroxisome proliferation by the PPAR antagonist GW9662 increased ROS concentrations and c-fos expression in POMC neurons. Also, it reversed high-fat feeding-triggered elevated NPY/AgRP and low POMC neuronal firing, and resulted in decreased feeding of DIO mice. Finally, central administration of ROS alone increased c-fos and phosphorylated signal transducer and activator of transcription 3 (pStat3) expression in POMC neurons and reduced feeding of DIO mice. These observations unmask a previously unknown hypothalamic cellular process associated with peroxisomes and ROS in the central regulation of energy metabolism in states of leptin resistance.

Reversal of diabetic nephropathy by a ketogenic diet

Intensive insulin therapy and protein restriction delay the development of nephropathy in a variety of conditions, but few interventions are known to reverse nephropathy. Having recently observed that the ketone 3-beta-hydroxybutyric acid (3-OHB) reduces molecular responses to glucose, we hypothesized that a ketogenic diet, which produces prolonged elevation of 3-OHB, may reverse pathological processes caused by diabetes. To address this hypothesis, we assessed if prolonged maintenance on a ketogenic diet would reverse nephropathy produced by diabetes. In mouse models for both Type 1 (Akita) and Type 2 (db/db) diabetes, diabetic nephropathy (as indicated by albuminuria) was allowed to develop, then half the mice were switched to a ketogenic diet. After 8 weeks on the diet, mice were sacrificed to assess gene expression and histology. Diabetic nephropathy, as indicated by albumin/creatinine ratios as well as expression of stress-induced genes, was completely reversed by 2 months maintenance on a ketogenic diet. However, histological evidence of nephropathy was only partly reversed. These studies demonstrate that diabetic nephropathy can be reversed by a relatively simple dietary intervention. Whether reduced glucose metabolism mediates the protective effects of the ketogenic diet remains to be determined.

Naloxone, but not valsartan, preserves responses to hypoglycemia after antecedent hypoglycemia: role of metabolic reprogramming in counterregulatory failure

OBJECTIVE

Hypoglycemia-associated autonomic failure (HAAF) constitutes one of the main clinical obstacles to optimum treatment of type 1 diabetes. Neurons in the ventromedial hypothalamus are thought to mediate counterregulatory responses to hypoglycemia. We have previously hypothesized that hypoglycemia-induced hypothalamic angiotensin might contribute to HAAF, suggesting that the angiotensin blocker valsartan might prevent HAAF. On the other hand, clinical studies have demonstrated that the opioid receptor blocker naloxone ameliorates HAAF. The goal of this study was to generate novel hypothalamic markers of hypoglycemia and use them to assess mechanisms mediating HAAF and its reversal.

RESEARCH DESIGN AND METHODS

Quantitative PCR was used to validate a novel panel of hypothalamic genes regulated by hypoglycemia. Mice were exposed to one or five episodes of insulin-induced hypoglycemia, with or without concurrent exposure to valsartan or naloxone. Corticosterone, glucagon, epinephrine, and hypothalamic gene expression were assessed after the final episode of hypoglycemia.

RESULTS

A subset of hypothalamic genes regulated acutely by hypoglycemia failed to respond after repetitive hypoglycemia. Responsiveness of a subset of these genes was preserved by naloxone but not valsartan. Notably, hypothalamic expression of four genes, including pyruvate dehydrogenase kinase 4 and glycerol 3-phosphate dehydrogenase 1, was acutely induced by a single episode of hypoglycemia, but not after antecedent hypoglycemia; naloxone treatment prevented this failure. Similarly, carnitine palmitoyltransferase-1 was inhibited after repetitive hypoglycemia, and this inhibition was prevented by naloxone. Repetitive hypoglycemia also caused a loss of hypoglycemia-induced elevation of glucocorticoid secretion, a failure prevented by naloxone but not valsartan.

CONCLUSIONS

Based on these observations we speculate that acute hypoglycemia induces reprogramming of hypothalamic metabolism away from glycolysis toward β-oxidation, HAAF is associated with a reversal of this reprogramming, and naloxone preserves some responses to hypoglycemia by preventing this reversal.

Hypothalamic responses to fasting indicate metabolic reprogramming away from glycolysis toward lipid oxidation

Nutrient-sensitive hypothalamic neurons regulate energy balance and glucose homeostasis, but the molecular mechanisms mediating hypothalamic responses to nutritional state remain incompletely characterized. To address these mechanisms, the present studies used quantitative PCR to characterize the expression of a panel of genes the hypothalamic expression by nutritional status of which had been suggested by DNA microarray studies. Although these genes regulate a variety of function, the most prominent set regulate intermediary metabolism, and the overall pattern clearly indicated that a 48-h fast produced a metabolic reprogramming away from glucose metabolism and toward the utilization of alternative fuels, particularly lipid metabolism. This general reprogramming of intermediary metabolism by fasting was observed both in cortex and hypothalamus but most prominently in hypothalamus. The effect of fasting on the expression of these genes may be mediated by reduction in plasma glucose or glucose metabolism, rather than leptin, because they were generally recapitulated by hypoglycemia even in the presence of elevated insulin and in vitro by low glucose but were not recapitulated in ob/ob mice. These studies suggest that fasting reduces glucose metabolism and thus minimizes the production of hypothalamic malonyl-coenzyme A. However, because the reprogramming of glucose metabolism by fasting was also observed in cortex, this apparent substrate competition may mediate more general responses to nutritional deprivation, including those responsible for the protective effects of dietary restriction. The present studies also provide a large panel of novel glucose-regulated genes that can be used as markers of glucose action to address mechanisms mediating hypothalamic responses to nutritional state.

Evidence for only two independent pathways for decreasing senescence in Caenorhabditis elegans

Cold temperature, dietary restriction, reduced insulin/insulin-like growth factor signaling, and mutations in mitochondrial genes have all been shown to extend the lifespan of Caenorhabditis elegans (Kenyon et al., Nature 366:461-464, 1993; Klass, Mech Ageing Dev 6:413-429, 1977; Lakowski and Hekimi, Science 272:1010-1013, 1996). Additionally, all of them extend the lifespan of mice (Bluher et al., Science 299:572-574, 2003; Conti et al., Science 314:825-828, 2006; Holzenberger et al., Nature 421:182-187, 2003; Liu et al., Genes Dev 19:2424-2434, 2005; Weindruch and Walford, Science 215:1415-1418, 1982). The mechanism by which these treatments extend lifespan is currently unknown, but our study uses an epistatic approach to show that these four manipulations are mainly additive in terms of lifespan. Classical interpretation of this data suggests that these manipulations are independent of each other. However, using a Gompertz mortality rate analysis, the maximum mortality rate doubling time can be achieved through the use of only dietary restriction and cold temperature, suggesting that the mechanisms by which cold temperature and caloric restriction extend lifespan are the only independent mechanisms.

Role of CBP and SATB-1 in aging, dietary restriction, and insulin-like signaling

Increased transcriptional complex activity, or pharmacological mimics of increased complex activity, predict lifespan in mice and mediate the protective effects of dietary restriction during aging.

How dietary restriction (DR) increases lifespan and decreases disease burden are questions of major interest in biomedical research. Here we report that hypothalamic expression of CREB-binding protein (CBP) and CBP-binding partner Special AT-rich sequence binding protein 1 (SATB-1) is highly correlated with lifespan across five strains of mice, and expression of these genes decreases with age and diabetes in mice. Furthermore, in Caenorhabditis elegans, cbp-1 is induced by bacterial dilution DR (bDR) and the daf-2 mutation, and cbp-1 RNAi specifically in adults completely blocks lifespan extension by three distinct protocols of DR, partially blocks lifespan extension by the daf-2 mutation but not of cold, and blocks delay of other age-related pathologies by bDR. Inhibiting the C. elegans ortholog of SATB-1 and CBP-binding partners daf-16 and hsf-1 also attenuates lifespan extension by bDR, but not other protocols of DR. In a transgenic Aβ42 model of Alzheimer's disease, cbp-1 RNAi prevents protective effects of bDR and accelerates Aβ42-related pathology. Furthermore, consistent with the function of CBP as a histone acetyltransferase, drugs that enhance histone acetylation increase lifespan and reduce Aβ42-related pathology, protective effects completely blocked by cbp-1 RNAi. Other factors implicated in lifespan extension are also CBP-binding partners, suggesting that CBP constitutes a common factor in the modulation of lifespan and disease burden by DR and the insulin/IGF1 signaling pathway.

The simple manipulation of dietary restriction (DR) (reduction of caloric intake by about 30% in rodents) produces robust increases in lifespan and slows the development of almost all age-related diseases, including cancer and neurological diseases. This relationship between dietary restriction and longevity is observed in most models in which the effect of DR has been tested. Thus, understanding how DR produces its protective mechanisms would have potentially profound implications for the treatment of age-related diseases, including possibly the development of a “magic bullet” for these diseases. In the present study we have discovered that DR induces a transcription factor, CBP, and additional factors that work with CBP to control the expression of other genes involved in determination of lifespan. When we blocked the DR-mediated increase in CBP and associated factors, we blocked all the protective effects of DR on lifespan extension, on the slowed rate of aging, and on protection against pathology in a model of Alzheimer's disease. Further, in mice expression of CBP and a CBP-interacting factor positively predicted lifespan, and expression of both factors decreased with age and in diabetes. Finally, pharmacological manipulations that mimicked enhanced CBP activity increased lifespan and reduced pathology in a model of Alzheimer's disease.

Chemosensory and caloric mechanisms influence distinct components of mortality rate

Both caloric and chemosensory mechanisms influence lifespan, but the relative importance of each of these mechanisms in mediating effects of dietary restriction on lifespan has been unclear. Here we demonstrate that chemosensory mechanisms consistently influence initial mortality rate, whereas caloric mechanisms consistently influence age-dependent acceleration of mortality rate. Based on this analysis, life-extending effects of dietary restriction are mediated primarily by caloric mechanisms in rats and nematodes and by both mechanisms in mice and flies.

Inhibition of agouti-related peptide expression by glucose in a clonal hypothalamic neuronal cell line is mediated by glycolysis, not oxidative phosphorylation

The regulation of neuroendocrine electrical activity and gene expression by glucose is mediated through several distinct metabolic pathways. Many studies have implicated AMP and ATP as key metabolites mediating neuroendocrine responses to glucose, especially through their effects on AMP-activated protein kinase (AMPK), but other studies have suggested that glycolysis, and in particular the cytoplasmic conversion of nicotinamide adenine dinucleotide (NAD+) to reduced NAD (NADH), may play a more important role than oxidative phosphorylation for some effects of glucose. To address these molecular mechanisms further, we have examined the regulation of agouti-related peptide (AgRP) in a clonal hypothalamic cell line, N-38. AgRP expression was induced monotonically as glucose concentrations decreased from 10 to 0.5 mm glucose and with increasing concentrations of glycolytic inhibitors. However, neither pyruvate nor 3-beta-hydroxybutyrate mimicked the effect of glucose to reduce AgRP mRNA, but on the contrary, produced the opposite effect of glucose and actually increased AgRP mRNA. Nevertheless, 3beta-hydroxybutyrate mimicked the effect of glucose to increase ATP and to decrease AMPK phosphorylation. Similarly, inhibition of AMPK by RNA interference increased, and activation of AMPK decreased, AgRP mRNA. Additional studies demonstrated that neither the hexosamine nor the pentose/carbohydrate response element-binding protein pathways mediate the effects of glucose on AgRP expression. These studies do not support that either ATP or AMPK mediate effects of glucose on AgRP in this hypothalamic cell line but support a role for glycolysis and, in particular, NADH. These studies support that cytoplasmic or nuclear NADH, uniquely produced by glucose metabolism, mediates effects of glucose on AgRP expression.

Gene memory in neuroendocrine and behavioural systems

Several examples of sex steroid hormone actions on rat brain and behaviour show that initial hormone exposures may be followed by enduring neuronal alterations, apparent long after the hormone itself has disappeared. Precedents from non-neuronal systems led to the concept of 'gene memory'. We are studying genomic structural alterations in rat hypothalamic neurons to account for these effects. The preproenkephalin gene is turned on by oestradiol in rat brain neurons in a tissue-specific and genetic sex-specific manner. Levels of preproenkephalin mRNA in the ventromedial hypothalamus correlate tightly with oestradiol-dependent reproductive behaviour. Our results indicate a tissue-specific pattern of DNA methylation in the enkephalin promoter. Putative binding sites for several transcription factors have been described in the preproenkephalin gene promoter; a role for some of these factors in regulating expression of the gene has been demonstrated.

Glucokinase regulates reproductive function, glucocorticoid secretion, food intake, and hypothalamic gene expression

Because appetite, hypothalamic gene expression, reproductive function, and adrenal function are highly sensitive to acute changes in plasma glucose levels, it has been hypothesized hypothalamic neurons sensitive to glucose play a role in regulating these functions. To assess this hypothesis, we examined these neuronendocrine functions in mice in which the glucokinase gene, which plays an essential role in neuroendocrine glucose sensing, has been ablated. Haploinsufficiency in heterozygous glucokinase knockout mice produced effects similar to those produced by hypoglycemia: impaired reproductive function, elevated plasma corticosterone, increased food intake, and hypothalamic gene expression similar to that observed in fasted or leptin-deficient obese mice (increased hypothalamic neuropeptide Y mRNA and reduced hypothalamic proopiomelanocortin mRNA). Plasma glucose was elevated 2-fold in glucokinase knockout mice, consistent with a maturity-onset diabetes of the young phenotype, but plasma insulin and leptin levels were normal. These data support the hypothesis that glucokinase plays a key role in the neuroendocrine regulation of metabolic economy.

Silencing of estrogen receptor alpha in the ventromedial nucleus of hypothalamus leads to metabolic syndrome

Estrogen receptor alpha (ERalpha) plays a pivotal role in the regulation of food intake and energy expenditure by estrogens. Although it is well documented that a disruption of ERalpha signaling in ERalpha knockout (ERKO) mice leads to an obese phenotype, the sites of estrogen action and mechanisms underlying this phenomenon are still largely unknown. In the present study, we exploited RNA interference mediated by adeno-associated viral vectors to achieve focused silencing of ERalpha in the ventromedial nucleus of the hypothalamus, a key center of energy homeostasis. After suppression of ERalpha expression in this nucleus, female mice and rats developed a phenotype characteristic for metabolic syndrome and marked by obesity, hyperphagia, impaired tolerance to glucose, and reduced energy expenditure. This phenotype persisted despite normal ERalpha levels elsewhere in the brain. Although an increase in food intake preceded weight gain, our data suggest that a leading factor of obesity in this model is likely a decline in energy expenditure with all three major constituents being affected, including voluntary activity, basal metabolic rate, and diet-induced thermogenesis. Together, these findings indicate that ERalpha in the ventromedial nucleus of the hypothalamus neurons plays an essential role in the control of energy balance and the maintenance of normal body weight.

The "domino theory" of hunger: the hypothalamus is hot

Mechanisms by which the hypothalamus senses nutritional status are important for many metabolic diseases, including obesity and diabetes. Now, report that hypothalamic neurons sense nutritional deficit through a cascade of events involving leptin, corticosterone, and glial production of thyroid hormone, leading to neuronal induction of uncoupling protein.

Low-carbohydrate diets cause obesity, low-carbohydrate diets reverse obesity: a metabolic mechanism resolving the paradox

High-fat diets produce obesity in part because, per calorie, glucose produces greater post-prandial thermogenesis than lipids, an effect probably mediated by glucose-sensing neurons. A very low-carbohydrate/high-fat/high-protein Atkins-type diet produces obesity but is marginally ketogenic in mice. In contrast, high-sucrose/low-fat diets, and very low-carbohydrate/high-fat/low-protein (anti-epileptic) ketogenic diets reverse diet-induced obesity independent of caloric intake. We propose that a non-ketogenic high-fat diet reduces glucose metabolism and signaling in glucose-sensing neurons, thereby reducing post-prandial thermogenesis, and that a ketogenic high-fat diet does not reduce glucose signaling, thereby preventing and/or reversing obesity.

Secrets of the lac operon. Glucose hysteresis as a mechanism in dietary restriction, aging and disease

Elevated blood glucose associated with diabetes produces progressive and apparently irreversible damage to many cell types. Conversely, reduction of glucose extends life span in yeast, and dietary restriction reduces blood glucose. Therefore it has been hypothesized that cumulative toxic effects of glucose drive at least some aspects of the aging process and, conversely, that protective effects of dietary restriction are mediated by a reduction in exposure to glucose. The mechanisms mediating cumulative toxic effects of glucose are suggested by two general principles of metabolic processes, illustrated by the lac operon but also observed with glucose-induced gene expression. First, metabolites induce the machinery of their own metabolism. Second, induction of gene expression by metabolites can entail a form of molecular memory called hysteresis. When applied to glucose-regulated gene expression, these two principles suggest a mechanism whereby repetitive exposure to postprandial excursions of glucose leads to an age-related increase in glycolytic capacity (and reduction in beta-oxidation of free fatty acids), which in turn leads to an increased generation of oxidative damage and a decreased capacity to respond to oxidative damage, independent of metabolic rate. According to this mechanism, dietary restriction increases life span and reduces pathology by reducing exposure to glucose and therefore delaying the development of glucose-induced glycolytic capacity.

VGF ablation blocks the development of hyperinsulinemia and hyperglycemia in several mouse models of obesity

Targeted deletion of the gene encoding the neuronal and endocrine secreted peptide precursor called VGF (nonacronymic) produces a lean, hypermetabolic, hyperactive mouse. Because VGF mutant mice are resistant to specific forms of diet-, lesion-, and genetically induced obesity, we investigated the role that this polypeptide plays in glucose homeostasis. We report that VGF mutant mice have increased insulin sensitivity by hyperinsulinemic euglycemic clamp analysis, and by insulin and glucose tolerance testing. Blunted counterregulatory responses in VGF-deficient mice were likely influenced by their significantly lower liver glycogen levels. VGF deficiency lowered circulating glucose and insulin levels in several murine models of obesity that are also susceptible to adult onset diabetes mellitus, including A(y)/a agouti, ob/ob, and MC4R(-)/MC4R(-) mice. Interestingly, ablation of Vgf in ob/ob mice decreased circulating glucose and insulin levels but did not affect adiposity, whereas MC4R(-)/MC4R(-) mice that are additionally deficient in VGF have improved insulin responsiveness at 7-8 wk of age, when lean MC4R(-)/MC4R(-) mice already have impaired insulin tolerance but are not yet obese. VGF mutant mice also resisted developing obesity and hyperglycemia in response to a high-fat/high-carbohydrate diet, and after gold thioglucose treatment, which is toxic to hypothalamic glucose-sensitive neurons. Lastly, circulating adiponectin, an adipose-synthesized protein the levels of which are correlated with improved insulin sensitivity, increased in VGF mutant compared with wild-type mice. Modulation of VGF levels and/or VGF signaling may consequently represent an alternative means to regulate circulating glucose levels and insulin sensitivity.

Impaired glucose signaling as a cause of obesity and the metabolic syndrome: the glucoadipostatic hypothesis

Since nutrition-sensitive feedback signals normally act to maintain relatively stable levels of both available and stored nutritional resources, failure in one or more of these feedback signals could plausibly lead to obese phenotypes. The glucostatic hypothesis in its original form posited that glucose serves as a physiological satiety factor (in the sense that post-prandial increases in plasma glucose cause meal termination), but in this form the hypothesis has been difficult to prove, and, especially since the discovery of leptin, the glucostatic hypothesis has largely been abandoned. Nevertheless, reduction of plasma glucose levels or glucose signaling produces a profile of neuroendocrine responses similar to those produced by leptin deficiency. Since leptin is not a physiological satiety factor (because it does not increase before meal termination), yet leptin deficiency causes obesity, we suggest that the glucostatic hypothesis be re-formulated without reference to satiety (i.e., short-term effects on food intake). Instead we argue that like leptin signaling, glucose signaling regulates long-term energy balance, in part by regulating metabolic rate but also by chronically regulating food intake. We further speculate that high-fat diets produce obesity in part because carbohydrates are, per calorie, more effective than lipids to reduce food intake and increase metabolic rate. In support of this glucoadipostatic hypothesis, the 5 present review examines evidence that obesity and the metabolic syndrome may be due to reduction in neuroendocrine sensitivity to glucose leading to increased metabolic efficiency.

Acute induction of gene expression in brain and liver by insulin-induced hypoglycemia

The robust neuroendocrine counterregulatory responses induced by hypoglycemia protect the brain by restoring plasma glucose, but little is known about molecular responses to hypoglycemia that may also be neuroprotective. To clarify these mechanisms, we examined gene expression in hypothalamus, cortex, and liver 3 h after induction of mild hypoglycemia by a single injection of insulin, using cDNA microarray analysis and quantitative real-time PCR. Real-time PCR corroborated the induction of six genes (angiotensinogen, GLUT-1, inhibitor of kappaB, inhibitor of DNA binding 1 [ID-1], Ubp41, and mitogen-activated protein kinase phosphatase-1 [MKP-1]) by insulin-induced hypoglycemia in the hypothalamus: five of these six genes in cortex and three (GLUT-1, angiotensinogen, and MKP-1) in liver. The induction was due to hypoglycemia and not hyperinsulinemia, since fasting (characterized by low insulin and glucose) also induced these genes. Four of these genes (angiotensinogen, GLUT-1, ID-1, and MKP-1) have been implicated in enhancement of glucose availability, which could plausibly serve a neuroprotective role during acute hypoglycemia but, if persistent, could also cause glucose-sensing mechanisms to overestimate plasma glucose levels, potentially causing hypoglycemia-induced counterregulatory failure. Although using cDNA microarrays with more genes, or microdissection, would presumably reveal further responses to hypoglycemia, these hypoglycemia-induced genes represent useful markers to assess molecular mechanisms mediating cellular responses to hypoglycemia.

Not wisely but too well: aging as a cost of neuroendocrine activity

Progressive decline of some neuroendocrine signaling systems has long been assumed to cause age-related physiological impairments and limit life span. However, hypophysectomy--removal of the pituitary gland--can delay many aspects of the aging process, and recent genetic studies have confirmed that reducing the secretion of pituitary hormones can increase the life span of laboratory organisms. Most strikingly, reducing activity of the insulin/insulin-like growth factor-1 signaling system substantially increases life span. Conversely, activity of the reproductive system or activation of stress responses can curtail life span. Because caloric restriction also reduces the activity of several neuroendocrine systems while increasing life span, it now appears that the aging process is driven, at least in part, by neuroendocrine activity rather than by its decline with age.

Mining Microarrays for Metabolic Meaning: Nutritional Regulation of Hypothalamic Gene Expression

DNA microarray analysis has been used to investigate relative changes in the level of gene expression in the CNS, including changes that are associated with disease, injury, psychiatric disorders, drug exposure or withdrawal, and memory formation. We have used oligonucleotide microarrays to identify hypothalamic genes that respond to nutritional manipulation. In addition to commonly used microarray analysis based on criteria such as fold-regulation, we have also found that simply carrying out multiple t tests then sorting by P value constitutes a highly reliable method to detect true regulation, as assessed by real-time polymerase chain reaction (PCR), even for relatively low abundance genes or relatively low magnitude of regulation. Such analyses directly suggested novel mechanisms that mediate effects of nutritional state on neuroendocrine function and are being used to identify regulated gene products that may elucidate the metabolic pathology of obese ob/ob, lean Vgf-/Vgf-, and other models with profound metabolic impairments.

Specific preservation of biosynthetic responses to insulin in adipose tissue may contribute to hyperleptinemia in insulin-resistant obese mice

Obesity is characterized by whole-body insulin resistance, yet the expression of many insulin-stimulated genes, including leptin, is elevated in obesity. These observations suggest that insulin resistance may depend on tissue type and gene. To address this hypothesis, we examined the regulation of immediate-early gene expression in liver and adipose tissue after injection of insulin and glucose, in lean insulin-sensitive, and in A(y)/a obese insulin-sensitive and obese insulin-resistant mice. Expression of hepatic jun-B mRNA was robustly increased after insulin injection in lean insulin-sensitive a/a mice and insulin-sensitive A(y)/a mice. In contrast, induction of hepatic jun-B and c-fos gene expression by insulin was markedly attenuated in obese insulin-resistant mice. Surprisingly, induction of adipose jun-B and c-fos gene expression by insulin was markedly enhanced in obese insulin-resistant mice. Furthermore, the expressions of jun-B and leptin were also enhanced in insulin-resistant mice after injection of glucose. Leptin mRNA was positively correlated with blood glucose levels and jun-B mRNA in lean but not insulin-resistant mice. Multiple regression analysis indicated that the correlation between leptin mRNA and jun-B mRNA was significant even after removing the effect of blood glucose, but the correlation between leptin mRNA and glucose was no longer significant after removing the effect of jun-B mRNA. These data suggest that some impairments in biosynthetic responses to insulin are manifest primarily in the liver, leading to hyperinsulinemia and stimulating the expression of some adipose insulin-stimulated genes, including leptin. These studies demonstrate the utility of immediate-early gene expression in the analysis of biosynthetic mechanisms of insulin resistance.

Diet-induced insulin resistance promotes amyloidosis in a transgenic mouse model of Alzheimer's disease

Recent epidemiological evidence indicates that insulin resistance, a proximal cause of Type II diabetes [a non-insulin dependent form of diabetes mellitus (NIDDM)], is associated with an increased relative risk for Alzheimer's disease (AD). In this study we examined the role of dietary conditions leading to NIDDM-like insulin resistance on amyloidosis in Tg2576 mice, which model AD-like neuropathology. We found that diet-induced insulin resistance promoted amyloidogenic beta-amyloid (Abeta) Abeta1-40 and Abeta1-42 peptide generation in the brain that corresponded with increased gamma-secretase activities and decreased insulin degrading enzyme (IDE) activities. Moreover, increased Abeta production also coincided with increased AD-type amyloid plaque burden in the brain and impaired performance in a spatial water maze task. Further exploration of the apparent interrelationship of insulin resistance to brain amyloidosis revealed a functional decrease in insulin receptor (IR)-mediated signal transduction in the brain, as suggested by decreased IR beta-subunit (IRbeta) Y1162/1163 autophosphorylation and reduced phosphatidylinositol 3 (PI3)-kinase/pS473-AKT/Protein kinase (PK)-B in these same brain regions. This latter finding is of particular interest given the known inhibitory role of AKT/PKB on glycogen synthase kinase (GSK)-3alpha activity, which has previously been shown to promote Abeta peptide generation. Most interestingly, we found that decreased pS21-GSK-3alpha and pS9-GSK-3beta phosphorylation, which is an index of GSK activation, positively correlated with the generation of brain C-terminal fragment (CTF)-gamma cleavage product of amyloid precursor protein, an index of gamma-secretase activity, in the brain of insulin-resistant relative to normoglycemic Tg2576 mice. Our study is consistent with the hypothesis that insulin resistance may be an underlying mechanism responsible for the observed increased relative risk for AD neuropathology, and presents the first evidence to suggest that IR signaling can influence Abeta production in the brain.

Lobotomy of genes: use of RNA interference in Neuroscience

Galen of Pergamon studied nerve function by shearing nerves in various species including monkeys, dogs, bulls and even elephants (humans being off limits to researchers; Sartan, 1954). An analogous strategy to determine gene function by ablating gene expression has recently been developed. RNA interference (RNAi) is a cellular response to double-stranded RNA (dsRNA) apparently as a defense against viral or transposon activity (Denli and Hannon, 2003; Dykxhoorn et al., 2003; Plasterk, 2002; Zamore, 2002). By activating this ancient defense mechanism through the introduction of artificial dsRNA, it is now possible to inhibit expression of almost any gene in almost any cell type, among them neuronal cells. In mammalian cells the active RNAi species must be short, approximately 21 nucleotide RNAs; these 21-bp species are called short interfering RNA (siRNA; Fig 1).

Metabolic pathways that mediate inhibition of hypothalamic neurons by glucose

Neurons in the ventromedial hypothalamus mediate some counterregulatory responses to hypoglycemia and 2-deoxyglucose, but the mechanisms that mediate these responses to glucose are unclear. In the present study, ventromedial hypothalamus neurons were identified on the basis of their inhibition by the transition from 5 to 20 mmol/l glucose. Tolbutamide, which activates glucose-stimulated neurons, failed to inhibit or activate glucose-inhibited neurons. Inhibitors of glucose transport and glycolysis, in particular by the glucokinase inhibitor glucosamine, blocked the effect of glucose on glucose-inhibited neurons. Furthermore, the glucose-inhibited neurons were activated by 2-deoxyglucose, which also activates counterregulatory responses. Conversely, glucose-inhibited neurons were inhibited by glycolytic metabolites, including lactate, but not by pyruvate. These data indicate that hypoglycemia induces electrical activity in glucose-inhibited neurons by attenuating glycolysis in those neurons. Thus, counterregulatory failure could be due to relatively enhanced glycolysis in glucose-stimulated neurons during hypoglycemia and attenuation of glycolysis in glucose-inhibited neurons might reverse counterregulatory failure.

Transgenic neuronal expression of proopiomelanocortin attenuates hyperphagic response to fasting and reverses metabolic impairments in leptin-deficient obese mice

Hypothalamic proopiomelanocortin (POMC) gene expression is reduced in many forms of obesity and diabetes, particularly in those attributable to deficiencies in leptin or its receptor. To assess the functional significance of POMC in mediating metabolic phenotypes associated with leptin deficiency, leptin-deficient mice bearing a transgene expressing the POMC gene under control of the neuron-specific enolase promoter were produced. The POMC transgene attenuated fasting-induced hyperphagia in wild-type mice. Furthermore, the POMC transgene partially reversed obesity, hyperphagia, and hypothermia and effectively normalized hyperglycemia, glucosuria, glucose intolerance, and insulin resistance in leptin-deficient mice. Effects of the POMC transgene on glucose homeostasis were independent of the partial correction of hyperphagia and obesity. Furthermore, the POMC transgene normalized the profile of hepatic and adipose gene expression associated with gluconeogenesis, glucose output, and insulin sensitivity. These results indicate that central POMC is a key modulator of glucose homeostasis and that agonists of POMC products may provide effective therapy in treating impairments in glucose homeostasis when hypothalamic POMC expression is reduced, as occurs with leptin deficiency, hypothalamic damage, and aging.

The fatty acid synthase inhibitor cerulenin and feeding, like leptin, activate hypothalamic pro-opiomelanocortin (POMC) neurons

Hypothalamic POMC neurons mediate catabolic responses such as decreased food intake and increased energy expenditure by, in part, monitoring levels of metabolic factors such as glucose, insulin and leptin. Recently, fatty acid synthase inhibitors were reported to reduce body weight, inhibit food intake, and increase metabolic rate, possibly by acting on hypothalamic neurons through a mechanism involving malonyl-CoA accumulation. Given the observation that leptin mediates similar catabolic effects by, in part, activating hypothalamic POMC neurons, it is possible that other catabolic signals such as feeding and fatty acid synthase inhibition may also activate POMC neurons. To test this hypothesis, hypothalamic sections from mice that were fed or injected with the fatty acid synthase inhibitor cerulenin were examined for Fos (a marker for neuronal activation) and POMC product immunoreactivity and compared with similarly processed sections from leptin-injected mice. Feeding increased Fos immunoreactivity in the lateral peri-arcuate area of the hypothalamus of both wild-type and leptin-deficient ob/ob mice (P<0.05), indicating that nutritional activation of the hypothalamus can be leptin-independent. Furthermore, feeding significantly induced Fos immunoreactivity in neurons expressing POMC (P<0.003), indicating that feeding, like leptin, activates POMC neurons. Injection with cerulenin, like feeding and leptin, also increased Fos immunoreactivity in the lateral peri-arcuate area (P<0.03) and, more specifically, in neurons expressing POMC. In contrast, injection with cerulenin had no grossly observable effects on cortical Fos immunoreactivity and appeared to suppress fasting-induced Fos immunoreactivity by about 35% (although the decrease did not reach statistical significance) in the medial arcuate nucleus, an area associated with anabolic responses such as increased food intake. Injection with cerulenin also decreased Fos immunoreactivity in the granular layer of the dentate gyrus of the hippocampus by about 30% (P<0.05), further suggesting that cerulenin does not non-specifically activate wide varieties of neurons. These results suggest that activation of hypothalamic POMC neurons may help to mediate some of the catabolic effects associated with feeding, cerulenin and leptin.

Central pro-opiomelanocortin gene delivery results in hypophagia, reduced visceral adiposity, and improved insulin sensitivity in genetically obese Zucker rats

Zucker (fa/fa) rats with defective leptin receptors are obese, hyperphagic, and hyperinsulinemic. For testing whether chronic activation of the central melanocortin pathway can bypass the defective leptin signaling and normalize altered energy homeostasis in these rats, recombinant adeno-associated virus encoding pro-opiomelanocortin (rAAV-POMC) or control vector was delivered bilaterally into the basal hypothalamus with coordinates targeting the arcuate nucleus. Thirty-eight days after POMC gene delivery, hypothalamic POMC expression increased fourfold and melanocortin signaling (indicated by phosphorylation of CREB) increased by 62% with respect to controls. There was a sustained reduction in food intake, a moderate but significant attenuation of weight gain, and a 24% decrease in visceral adiposity in rAAV-POMC rats. POMC gene delivery enhanced uncoupling protein 1 in brown adipose tissue (BAT) by more than fourfold. Fasting serum leptin, insulin, and cholesterol levels were also significantly reduced by rAAV-POMC treatment. This study demonstrates that targeted POMC gene delivery in the hypothalamus suppresses food intake and weight gain and reduces visceral adiposity and hyperinsulinemia in leptin-resistant obese Zucker rats. The mechanisms may involve the sustained hypophagia and the augmentation of thermogenesis in BAT.

Role of glucocorticoids in mediating effects of fasting and diabetes on hypothalamic gene expression

BACKGROUND

Fasting and diabetes are characterized by elevated glucocorticoids and reduced insulin, leptin, elevated hypothalamic AGRP and NPY mRNA, and reduced hypothalamic POMC mRNA. Although leptin replacement can reverse changes in hypothalamic gene expression associated with fasting and diabetes, leptin also normalizes corticosterone; therefore the extent to which the elevated corticosterone contributes to the regulation of hypothalamic gene expression in fasting and diabetes remains unclear. To address if elevated corticosterone is necessary for hypothalamic responses to fasting and diabetes, we assessed the effects of adrenalectomy on hypothalamic gene expression in 48-hour-fasted or diabetic mice. To assess if elevated corticosterone is sufficient for the hypothalamic responses to fasting and diabetes, we assessed the effect of corticosterone pellets implanted for 48 hours on hypothalamic gene expression.

RESULTS

Fasting and streptozotocin-induced diabetes elevated plasma glucocorticoid levels and reduced serum insulin and leptin levels. Adrenalectomy prevented the rise in plasma glucocorticoids associated with fasting and diabetes, but not the associated reductions in insulin or leptin. Adrenalectomy blocked the effects of fasting and diabetes on hypothalamic AGRP, NPY, and POMC expression. Conversely, corticosterone implants induced both AGRP and POMC mRNA (with a non-significant trend toward induction of NPY mRNA), accompanied by elevated insulin and leptin (with no change in food intake or body weight).

CONCLUSION

These data suggest that elevated plasma corticosterone mediate some effects of fasting and diabetes on hypothalamic gene expression. Specifically, elevated plasma corticosterone is necessary for the induction of NPY mRNA with fasting and diabetes; since corticosterone implants only produced a non-significant trend in NPY mRNA, it remains uncertain if a rise in corticosterone may be sufficient to induce NPY mRNA. A rise in corticosterone is necessary to reduce hypothalamic POMC mRNA with fasting and diabetes, but not sufficient for the reduction of hypothalamic POMC mRNA. Finally, elevated plasma corticosterone is both necessary and sufficient for the induction of hypothalamic AGRP mRNA with fasting and diabetes.