Physiological response to long-term peripheral and central leptin infusion in lean and obese mice

Role of hypothalamic autophagy in the control of whole body energy balance

Autophagy is a catabolic process involving the rearrangement of subcellular membranes to sequester cytoplasm and organelles for delivery to lysosomes, where the sequestered material is degraded and recycled. Autophagy is important for maintenance of intracellular energy homeostasis and the quality control of organelles such as the endoplasmic reticulum (ER) and mitochondria, which suggests that dysregulated autophagy might play a role in the pathogenesis of metabolic disorders and diabetes. In an attempt to elucidate the role of autophagy in metabolic disorders, diverse in vivo and in vitro models have been employed. Site-specific autophagy knockout models that are autophagy-deficient specifically in pancreatic β-cells, skeletal muscle, adipose tissues or liver have been produced. These models have generated valuable information regarding the role of autophagy in body metabolism. The role of autophagy in the hypothalamus, which controls whole body energy balance, appetite and energy expenditure, has also been investigated. Thus, mice with autophagy deficiency in the hypothalamus have shown diverse phenotypes (lean vs. obese) depending on the site of autophagy deficiency or the method of autophagy abrogation.

The gut microbiota reduces leptin sensitivity and the expression of the obesity-suppressing neuropeptides proglucagon (Gcg) and brain-derived neurotrophic factor (Bdnf) in the central nervous system

The gut microbiota contributes to fat mass and the susceptibility to obesity. However, the underlying mechanisms are not completely understood. To investigate whether the gut microbiota affects hypothalamic and brainstem body fat-regulating circuits, we compared gene expression of food intake-regulating neuropeptides between germ-free and conventionally raised (CONV-R) mice. We found that CONV-R mice had decreased expression of the antiobesity neuropeptide glucagon-like peptide-1 (GLP-1) precursor proglucagon (Gcg) in the brainstem. Moreover, in both the hypothalamus and the brainstem, CONV-R mice had decreased expression of the antiobesity neuropeptide brain-derived neurotrophic factor (Bdnf). CONV-R mice had reduced expression of the pro-obesity peptides neuropeptide-Y (Npy) and agouti-related protein (Agrp), and increased expression of the antiobesity peptides proopiomelanocortin (Pomc) and cocaine- and amphetamine-regulated transcript (Cart) in the hypothalamus. The latter changes in neuropeptide expression could be secondary to elevated fat mass in CONV-R mice. Leptin treatment caused less weight reduction and less suppression of orexigenic Npy and Agrp expression in CONV-R mice compared with germ-free mice. The hypothalamic expression of leptin resistance-associated suppressor of cytokine signaling 3 (Socs-3) was increased in CONV-R mice. In conclusion, the gut microbiota reduces the expression of 2 genes coding for body fat-suppressing neuropeptides, Gcg and Bdnf, an alteration that may contribute to fat mass induction by the gut microbiota. Moreover, the presence of body fat-inducing gut microbiota is associated with hypothalamic signs of Socs-3-mediated leptin resistance, which may be linked to failed compensatory body fat reduction.

Uncovering novel roles of nonneuronal cells in body weight homeostasis and obesity

Glial cells, which constitute more than 50% of the mass of the central nervous system and greatly outnumber neurons, are at the vanguard of neuroendocrine research in metabolic control and obesity. Historically relegated to roles of structural support and protection, diverse functions have been gradually attributed to this heterogeneous class of cells with their protagonism in crescendo in all areas of neuroscience during the past decade. However, this dramatic increase in attention bestowed upon glial cells has also emphasized our vast lack of knowledge concerning many aspects of their physiological functions, let alone their participation in numerous pathologies. This minireview focuses on the recent advances in our understanding of how glial cells participate in the physiological regulation of appetite and systemic metabolism as well as their role in the pathophysiological response to poor nutrition and secondary complications associated with obesity. Moreover, we highlight some of the existing lagoons of knowledge in this increasingly important area of investigation.

Sex differences in the physiology of eating

Hypothalamic-pituitary-gonadal (HPG) axis function fundamentally affects the physiology of eating. We review sex differences in the physiological and pathophysiological controls of amounts eaten in rats, mice, monkeys, and humans. These controls result from interactions among genetic effects, organizational effects of reproductive hormones (i.e., permanent early developmental effects), and activational effects of these hormones (i.e., effects dependent on hormone levels). Male-female sex differences in the physiology of eating involve both organizational and activational effects of androgens and estrogens. An activational effect of estrogens decreases eating 1) during the periovulatory period of the ovarian cycle in rats, mice, monkeys, and women and 2) tonically between puberty and reproductive senescence or ovariectomy in rats and monkeys, sometimes in mice, and possibly in women. Estrogens acting on estrogen receptor-α (ERα) in the caudal medial nucleus of the solitary tract appear to mediate these effects in rats. Androgens, prolactin, and other reproductive hormones also affect eating in rats. Sex differences in eating are mediated by alterations in orosensory capacity and hedonics, gastric mechanoreception, ghrelin, CCK, glucagon-like peptide-1 (GLP-1), glucagon, insulin, amylin, apolipoprotein A-IV, fatty-acid oxidation, and leptin. The control of eating by central neurochemical signaling via serotonin, MSH, neuropeptide Y, Agouti-related peptide (AgRP), melanin-concentrating hormone, and dopamine is modulated by HPG function. Finally, sex differences in the physiology of eating may contribute to human obesity, anorexia nervosa, and binge eating. The variety and physiological importance of what has been learned so far warrant intensifying basic, translational, and clinical research on sex differences in eating.

Selective leptin resistance revisited

In addition to effects on appetite and metabolism, leptin influences many neuroendocrine and physiological systems, including the sympathetic nervous system. Building on my Carl Ludwig Lecture of the American Physiological Society, I review the sympathetic and cardiovascular actions of leptin. The review focuses on a critical analysis of the concept of selective leptin resistance (SLR) and the role of leptin in the pathogenesis of obesity-induced hypertension in both experimental animals and humans. We introduced the concept of SLR in 2002 to explain how leptin might increase blood pressure (BP) in obese states, such as diet-induced obesity (DIO), that are accompanied by partial leptin resistance. This concept, analogous to selective insulin resistance in the metabolic syndrome, holds that in several genetic and acquired models of obesity, there is preservation of the renal sympathetic and pressor actions of leptin despite attenuation of the appetite and weight-reducing actions. Two potential overlapping mechanisms of SLR are reviewed: 1) differential leptin molecular signaling pathways that mediate selective as opposed to universal leptin action and 2) brain site-specific leptin action and resistance. Although the phenomenon of SLR in DIO has so far focused on preservation of sympathetic and BP actions of leptin, consideration should be given to the possibility that this concept may extend to preservation of other actions of leptin. Finally, I review perplexing data on the effects of leptin on sympathetic activity and BP in humans and its role in human obesity-induced hypertension.

Differential associations of leptin with adiposity across early childhood

OBJECTIVE

To examine associations of perinatal and 3-year leptin with weight gain and adiposity through 7 years.

DESIGN AND METHODS

In Project Viva, plasma leptin from mothers at 26-28 weeks' gestation (n = 893), umbilical cord vein at delivery (n = 540), and children at 3 years (n = 510) was assessed in relation to BMI z-score, waist circumference, skinfold thicknesses, and dual X-ray absorptiometry body fat.

RESULTS

50.1% of children were male and 29.5% non-white. Mean (SD) maternal, cord, and age 3 leptin concentrations were 22.9 (14.2), 8.8 (6.4), and 1.8 (1.7) ng/ml, respectively, and 3- and 7-year BMI z-scores were 0.46 (1.00) and 0.35 (0.97), respectively. After adjusting for parental and child characteristics, higher maternal and cord leptin were associated with less 3-year adiposity. For example, mean 3-year BMI z-score was 0.5 lower (95% CI: -0.7, -0.2; P-trend = 0.003) among children whose mothers' leptin concentrations were in the top versus bottom quintile. In contrast, higher age 3 leptin was associated with greater weight gain and adiposity through age 7 [e.g., change in BMI z-score from 3 to 7 years was 0.2 units (95% CI: -0.0, 0.4; P-trend =0.05)].

CONCLUSION

Higher perinatal leptin was associated with lower 3-year adiposity, whereas higher age 3 leptin was associated with greater weight gain and adiposity by 7 years.

Limits to sustained energy intake. XVIII. Energy intake and reproductive output during lactation in Swiss mice raising small litters

Limits to sustained energy intake (SusEI) during lactation in Swiss mice have been suggested to reflect the secretory capacity of the mammary glands. However, an alternative explanation is that milk production and food intake are regulated to match the limited growth capacity of the offspring. In the present study, female Swiss mice were experimentally manipulated in two ways – litter sizes were adjusted to be between 1 and 9 pups and mice were exposed to either warm (21°C) or cold (5°C) conditions from day 10 of lactation. Energy intake, number of pups and litter mass, milk energy output (MEO), thermogenesis, mass of the mammary glands and brown adipose tissue cytochrome c oxidase activity of the mothers were measured. At 21 and 5°C, pup mass at weaning was almost independent of litter size. Positive correlations were observed between the number of pups, litter mass, asymptotic food intake and MEO. These data were consistent with the suggestion that in small litters, pup requirements may be the major factor limiting milk production. Pups raised at 5°C had significantly lower body masses than those raised at 21°C. This was despite the fact that milk production and energy intake at the same litter sizes were both substantially higher in females raising pups at 5°C. This suggests that pup growth capacity is lower in the cold, perhaps due to pups allocating ingested energy to fuel thermogenesis. Differences in observed levels of milk production under different conditions may then reflect a complex interplay between factors limiting maternal performance (peripheral limitation and heat dissipation: generally better when it is cooler) and factors influencing maximum pup growth (litter size and temperature: generally better when it is hotter), and may together result in an optimal temperature favouring reproduction.

A superactive leptin antagonist alters metabolism and locomotion in high-leptin mice

Transgenic alpha murine urokinase-type plasminogen activator (αMUPA) mice are resistant to obesity and their locomotor activity is altered. As these mice have high leptin levels, our objective was to test whether leptin is responsible for these characteristics. αMUPA, their genetic background control (FVB/N), and C57BL mice were injected s.c. every other day with 20  mg/kg pegylated superactive mouse leptin antagonist (PEG-SMLA) for 6 weeks. We tested the effect of PEG-SMLA on body weight, locomotion, and bone health. The antagonist led to a rapid increase in body weight and subsequent insulin resistance in all treated mice. Food intake of PEG-SMLA-injected animals increased during the initial period of the experiment but then declined to a similar level to that of the control animals. Interestingly, αMUPA mice were found to have reduced bone volume (BV) than FVB/N mice, although PEG-SMLA increased bone mass in both strains. In addition, PEG-SMLA led to disrupted locomotor activity and increased corticosterone levels in C57BL but decreased levels in αMUPA or FVB/N mice. These results suggest that leptin is responsible for the lean phenotype and reduced BV in αMUPA mice; leptin affects corticosterone levels in mice in a strain-specific manner; and leptin alters locomotor activity, a behavior determined by the central circadian clock.

Ion channels in the central regulation of energy and glucose homeostasis

Ion channels are critical regulators of neuronal excitability and synaptic function in the brain. Recent evidence suggests that ion channels expressed by neurons within the brain are responsible for regulating energy and glucose homeostasis. In addition, the central effects of neurotransmitters and hormones are at least in part achieved by modifications of ion channel activity. This review focuses on ion channels and their neuronal functions followed by a discussion of the identified roles for specific ion channels in the central pathways regulating food intake, energy expenditure, and glucose balance.

Direct and indirect effects of leptin on adipocyte metabolism

Leptin is hypothesized to function as a negative feedback signal in the regulation of energy balance. It is produced primarily by adipose tissue and circulating concentrations correlate with the size of body fat stores. Administration of exogenous leptin to normal weight, leptin responsive animals inhibits food intake and reduces the size of body fat stores whereas mice that are deficient in either leptin or functional leptin receptors are hyperphagic and obese, consistent with a role for leptin in the control of body weight. This review discusses the effect of leptin on adipocyte metabolism. Because adipocytes express leptin receptors there is the potential for leptin to influence adipocyte metabolism directly. Adipocytes also are insulin responsive and receive sympathetic innervation, therefore leptin can also modify adipocyte metabolism indirectly. Studies published to date suggest that direct activation of adipocyte leptin receptors has little effect on cell metabolism in vivo, but that leptin modifies adipocyte sensitivity to insulin to inhibit lipid accumulation. In vivo administration of leptin leads to a suppression of lipogenesis, an increase in triglyceride hydrolysis and an increase in fatty acid and glucose oxidation. Activation of central leptin receptors also contributes to the development of a catabolic state in adipocytes, but this may vary between different fat depots. Leptin reduces the size of white fat depots by inhibiting cell proliferation both through induction of inhibitory circulating factors and by contributing to sympathetic tone which suppresses adipocyte proliferation. This article is part of a Special Issue entitled: Modulation of Adipose Tissue in Health and Disease.

Neuronal expression of glucosylceramide synthase in central nervous system regulates body weight and energy homeostasis

Body weight and energy homeostasis are regulated by leptin receptor interactions with gangliosides, a class of plasma membrane lipids, in forebrain neurons of mice.

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

Obesity is a growing health threat that affects nearly half a billion people worldwide, and its incidence rates in lower income countries are rising dramatically. As obesity is a major risk factor for type II diabetes and cardiovascular disease, significant effort has been put into the exploration of causes, prevention, and potential treatment. Recent research has demonstrated that a region of the brain called the hypothalamus is a major integrator of metabolic and nutrient signals, adapting food intake and energy expenditure to current metabolic needs. Leptin or insulin receptors located in the plasma cell membrane of neurons sense energy signals from the body. They transmit this information inside the cell, which then regulates neuronal function. In this study, we show that leptin receptors interact with gangliosides, a class of plasma membrane lipids. This interaction is a prerequisite for proper receptor activation. Consequently, ganglioside loss in hypothalamic neurons inhibits leptin receptor signal transduction in response to energy metabolites. Furthermore, mice lacking gangliosides in distinct forebrain areas, amongst them the hypothalamus, develop progressive obesity and hypothermia. Our results suggest a previously unknown regulatory mechanism of plasma membrane lipids for hypothalamic control of body weight.

Centrally administered urocortin 2 decreases gorging on high-fat diet in both diet-induced obesity-prone and -resistant rats

Objective

Obesity is a costly, deadly public health problem for which new treatments are needed. Individual differences in meal pattern have been proposed to play a role in obesity risk. The present study tested the hypothesis that i) the microstructure of chronic high-fat diet intake differs between genetically selected Diet-Induced Obesity (DIO) and Diet Resistant (DR) rats, and ii) central administration of urocortin 2 (Ucn 2), a corticotropin-releasing factor type 2 (CRF₂) agonist, decreases high-fat diet intake not only in lean DR rats, but also in obese DIO rats.

Design

Male, selectively bred DIO and DR rats (n=10/genotype) were chronically fed a high-fat diet. Food and water intake as well as ingestion microstructure were then compared under baseline conditions and following third intracerebroventricular injection of Ucn 2 (0, 0.1, 0.3, 1, 3 µg).

Results

Irrespective of genotype, Ucn 2 reduced nocturnal food intake with a minimum effective dose of 0.3 µg, suppressing high-fat diet intake by ~40% at the 3 µg dose. Ucn 2 also made rats of both genotypes eat smaller and briefer meals, including at doses that did not reduce drinking. Obese DIO rats ate fewer but larger meals than DR rats, which they ate more quickly and consumed with 2/3^rd less water.

Conclusions

Unlike leptin and insulin, Ucn 2 retains its full central anorectic efficacy to reduce high-fat diet intake even in obese, genetically-prone DIO rats, which otherwise show a “gorging” meal pattern. These results open new opportunities of investigation towards treating some forms of diet-induced obesity.

Dietary components in the development of leptin resistance

Classically, leptin resistance has been associated with increased body fat and circulating leptin levels, and the condition is believed to contribute to the onset and/or maintenance of obesity. Although a great deal is known about the central nervous system mechanisms mediating leptin resistance, considerably less is known about the role of diet in establishing and maintaining this altered hormonal state. An exciting new finding has recently been published demonstrating the existence of leptin resistance in normal-weight rats with lean leptin levels by feeding them a high-concentration-fructose diet. This finding has opened the possibility that specific macronutrients may be capable of inducing leptin resistance, independently of the amount of body fat or circulating leptin present in the treated animals. This review describes several lines of research that have recently emerged indicating that specific types of dietary sugars and fats are capable of inducing leptin resistance in experimental rodent models. The results further show that diet-induced leptin resistance is capable of increasing energy intake and elevating body weight gain under appropriate dietary challenges. It appears that biological mechanisms on multiple levels may underlie the dietary induction of leptin resistance, including alterations in the leptin blood-to-brain transport system, in peripheral glucose metabolism, and in central leptin receptor signaling pathways. What is clear from the findings reviewed here is that diet-induced leptin resistance can occur in the absence of elevated circulating leptin levels and body weight, rendering it a potential cause and/or predisposing factor to excess body weight gain and obesity.

Contribution made by parabiosis to the understanding of energy balance regulation

Parabiosis is a chronic preparation that allows exchange of whole blood between two animals. It has been used extensively to test for involvement of circulating factors in feedback regulation of physiological systems. The total blood volume of each animal exchanges approximately ten times each day, therefore, factors that are rapidly cleared from the circulation do not reach equilibrium across the parabiotic union whereas those with a long half-life achieve a uniform concentration and bioactivity in both members of a pair. Involvement of a circulating factor in the regulation of energy balance was first demonstrated when one member of a pair of parabiosed rats became hyperphagic and obese following bilateral lesioning of the ventromedial hypothalamus. The non-lesioned partner stopped eating, lost a large amount of weight and appeared to be responding to a circulating "satiety" factor released by the obese rat. These results were confirmed using different techniques to induce obesity in one member of a pair. Studies with phenotypically similar ob/ob obese and db/db diabetic mice indicated that the obese mouse lacked a circulating signal that regulated energy balance, whereas the diabetic mouse appeared insensitive to such a signal. Positional cloning studies identified leptin as the circulating factor and subsequent parabiosis studies confirmed leptin's ability to exchange effectively between parabionts. These studies also suggest the presence of additional unidentified factors that influence body composition. This article is part of a Special Issue entitled: Animal Models of Disease.

Role of reactive oxygen species in hypothalamic regulation of energy metabolism

To understand the etiology of metabolic disorders, including obesity and type II diabetes, it is essential to gain better insight into how stored and available energy sources are monitored by the central nervous system. In particular, a comprehension of the fine cellular interplay and intracellular mechanisms that enable appropriate hypothalamic and consequent endocrine and behavioral responses to both circulating hormonal and nutrient signals remains elusive. Recent data, including those from our laboratories, raised the notion that reactive oxygen species (ROS) generation is not merely a by-product of substrate oxidation, but it plays a crucial role in modulating cellular responses involved in the regulation of energy metabolism. These review summarizes the published recent data on the effect of ROS levels in the regulation of neuronal function, including that of hypothalamic melanocortin neurons, pro-opiomelanocortin and neuropeptide Y-/agouti related peptide-neurons, in the modulation of food intake.

Cytokine signaling modulates blood-brain barrier function

The blood-brain barrier (BBB) provides a vast interface for cytokines to affect CNS function. The BBB is a target for therapeutic intervention. It is essential, therefore, to understand how cytokines interact with each other at the level of the BBB and how secondary signals modulate CNS functions beyond the BBB. The interactions between cytokines and lipids, however, have not been fully addressed at the level of the BBB. Here, we summarize current understanding of the localization of cytokine receptors and transporters in specific membrane microdomains, particularly lipid rafts, on the luminal (apical) surface of the microvascular endothelial cells composing the BBB. We then illustrate the clinical context of cytokine effects on the BBB by neuroendocrine regulation and amplification of inflammatory signals. Two unusual aspects discussed are signaling crosstalk by different classes of cytokines and genetic regulation of drug efflux transporters. We also introduce a novel area of focus on how cytokines may act through nuclear hormone receptors to modulate efflux transporters and other targets. A specific example discussed is the ATP-binding cassette transporter-1 (ABCA-1) that regulates lipid metabolism. Overall, cytokine signaling at the level of the BBB is a crucial feature of the dynamic regulation that can rapidly change BBB function and affect brain health and disease.

Sweet tea leaves extract improves leptin resistance in diet-induced obese rats

AIM OF THE STUDY

Dietary obesity is usually characterized by leptin resistance and abnormal lipid metabolism. Lithocarpus polystachyus Rehd.(Sweet Tea) leaf is a kind of Chinese folkloric medicine, and it has been widely used for obesity, diabetes, and hypertension in South China. The present study is aimed at investigating the pharmacological mechanism of the anti-hyperleptinaemia effects of Sweet Tea leaves extract in high fat diet-induced obese rats.

MATERIALS AND METHODS

We induced high fat diet obesity for 14 weeks to test the corrective effects of three ST doses (75, 150 and 300 mg/kg per day) for 8 weeks. At the end of the experiment, body weight, fasting blood glucose and serum lipids, superoxide dismutase (SOD), malondialdehyde (MDA), fasting serum insulin and leptin, C-reactive protein, adiponectin and resistin levels were measured, Homeostasis Model Assessment for Insulin Resistance (HOMA-IR) was also calculated. mRNA gene expression of PPARγ (peroxisome proliferator-activated receptor γ) and C/EBPα(CCAAT/enhancer-binding protein α) in epididymal adipose tissue of DIO control and experimental groups were evaluated.

RESULTS

Sweet Tea leaves extract could significantly decrease the levels of serum lipids, attenuate body weight gain and lower circulating leptin and insulin levels, ameliorate the state of oxidative stress, raise serum adiponectin, reduce circulating CRP and resistin levels, and depress the expression of PPARγ and C/EBPα in epididymal adipose tissue of obese rats.

CONCLUSION

The present findings suggest that ST can effectively attenuate the leptin resistance at least through anti-hyperlipidemic activity and thus has the therapeutic potential in treating hyperlipidemia and hyperleptinaemia related to dietary obesity.

Rapid onset and reversal of peripheral and central leptin resistance in rats offered chow, sucrose solution, and lard

We previously reported that rats offered choice diet (chow, 30% sucrose solution, lard) increase body fat by 130% within 3 weeks. We tested the effects of choice diet on the development of leptin resistance in rats. Intraperitoneal injection of 2mg/kg leptin inhibited 14 h food intake and weight gain of all rats after 2 days and 4 days of diet. On day 8, choice rats were leptin insensitive and by day 16 they were resistant. Chow rats remained leptin responsive. A second study showed that on day 16 choice, but not chow rats, were centrally leptin resistant (1.5 μg leptin, 3rd ventricle). In both studies, rats were switched back to chow only after approximately 3 weeks on choice diet and were leptin responsive after 4 days. A third study showed that carcass fat was reduced by 30% 4 days after switching back to chow. A final experiment showed that leptin responsive chow rats, but not leptin resistant choice rats, increased energy expenditure by 12% during the 2.6h after a central leptin injection. Thus, choice diet rapidly induces leptin resistance, but leptin responsiveness is quickly restored when choice is replaced with chow. This rapid onset and reversal of leptin resistance may be associated with changes in either substrate metabolism or adiposity.

Melanocortin-3 receptors and metabolic homeostasis

Attenuated activity of the central nervous melanocortin system causes obesity and insulin resistance. Obese rodents treated with melanocortins exhibit improvements in obesity and metabolic homeostasis that are not mutually dependent, suggesting metabolic actions that are independent of weight changes. These responses are generally thought to involve G-protein-coupled receptors expressed in the brain. Melanocortin-4 receptors (MC4Rs) regulate satiety and autonomic nervous system and thyroid function. MC3Rs are expressed in hypothalamic and limbic regions involved in controlling ingestive behaviors and autonomic function. Mc3r-/- mice exhibit increased adiposity and an accelerated diet-induced obesity. While this phenotype is not dependent on hyperphagia, data on the regulation of food intake by MC3Rs are inconsistent. Recent investigations by our laboratory suggest a unique combination of behavioral and metabolic disorders in Mc3r-/- mice. MC3Rs are critical for the expression of the anticipatory response and metabolic homeostasis when food intake occurs outside the normal voluntary rhythms driven by photoperiod. Using a Cre-Lox strategy, we can now investigate MC3Rs expressed in different brain regions and organ systems in the periphery. While focusing on the functions of neural MC3Rs, early results suggest an additional layer of complexity with central and peripheral MC3Rs involved in the defense of body weight.

Markers in nonalcoholic steatohepatitis

Nonalcoholic fatty liver disease (NAFLD), the most common liver disorder worldwide, encompasses a spectrum of abnormal liver histology ranging from simple steatosis to nonalcoholic steatohepatitis (NASH) and cirrhosis. Population studies show that NAFLD is strongly associated with insulin resistance, obesity, type 2 diabetes mellitus, and lipid abnormalities. In the context of hepatic steatosis, factors that promote cell injury, inflammation, and fibrosis include oxidative stress, early mitochondrial dysfunction, endoplasmic reticulum stress, iron accumulation, apoptosis, adipocytokines, and stellate cell activation. The exact NASH prevalence is unknown because of the absence of simple noninvasive diagnostic tests. Although liver biopsy is the "gold standard" for the diagnosis of NASH, other tests are needed to facilitate the diagnosis and greatly reduce the requirement for invasive liver biopsy. In addition, the development of new fibrosis markers in NASH is needed to facilitate the assessment of its progression and the effectiveness of new therapies. The aim of this chapter, which is overview of biomarkers in NASH, is to establish a systematic approach to laboratory findings of the disease.

Comparison of Two New Mouse Models of Polygenic Type 2 Diabetes at the Jackson Laboratory, NONcNZO10Lt/J and TALLYHO/JngJ

This review compares two novel polygenic mouse models of type 2 diabetes (T2D), TALLYHO/JngJ and NONcNZO10/LtJ, and contrasts both with the well-known C57BLKS/J-Lepr(db) (db/db) monogenic diabesity model. We posit that the new polygenic models are more representative of the "garden variety" obesity underlying human T2D in terms of their polygenetic rather than monogenic etiology. Moreover, the clinical phenotypes in these new models are less extreme, for example, more moderated development of obesity coupled with less extreme endocrine disturbances. The more progressive development of obesity produces a maturity-onset development of hyperglycemia in contrast to the juvenile-onset diabetes observed in the morbidly obese db/db model. Unlike the leptin receptor-deficient db/db models with central leptin resistance, the new models develop a progressive peripheral leptin resistance and are able to maintain reproductive function. Although the T2D pathophysiology in both TALLYHO/JngJ and NONcNZO10/LtJ is remarkably similar, their genetic etiologies are clearly different, underscoring the genetic heterogeneity underlying T2D in humans.

Combined treatment of betulinic acid, a PTP1B inhibitor, with Orthosiphon stamineus extract decreases body weight in high-fat-fed mice

Leptin resistance is a common feature of obesity and is accompanied by hyperleptinemia. Although leptin sensitizers improve leptin resistance, they also decrease plasma leptin levels that attenuate the leptin-associated antiobesity effect. We hypothesized that the combinational treatment of leptin sensitizer and endogenous leptin expression stimulant would synergistically induce an antiobesity effect in high-fat-fed obese animals. Betulinic acid (BA) isolated from Saussurea lappa suppressed the hypothalamic protein tyrosine phosphatase 1B in mice and enhanced the antiobesity effect of leptin in obese rats. Ethanol extract of Orthosiphon stamineus (OS) induced leptin expressions in both 3T3-L1 adipocytes and mice in a dose-dependent manner. To evaluate our hypothesis, we treated obese mice induced by 6 weeks of high-fat-diet feeding with BA and OS for 2 weeks. Although BA or OS alone did not decrease body weight in obese mice, the combinational treatment of BA and OS decreased body weight significantly compared to either BA- or OS-treated obese mice. These results suggest that combinational treatment of BA and OS would be effective for the treatment of obesity.

Leptin signaling and Alzheimer's disease

Leptin, an adipocytokine produced in the peripheral system as well as in the brain, is implicated in obesity, food intake, glucose homeostasis, and energy expenditure. Leptin expression levels and signaling pathways may also be linked to the pathophysiology of neurodegenerative diseases including Alzheimer's disease. Epidemiological studies have demonstrated that higher circulating leptin levels are associated with lower risk of dementia including Alzheimer's disease, and lower circulating levels of leptin have been reported in patients with Alzheimer's disease. Leptin receptors are highly expressed in the hippocampus, a brain area involved in learning and memory and severely affected during the course of Alzheimer's disease. In laboratory studies, several in vivo and in vitro studies have shown that leptin supplementation decreases amyloid-β (Aβ) production and tau phosphorylation, two major biochemical events that play a key role in the pathogenesis of Alzheimer's disease. In this review, we will review the structure of leptin, the type of receptors of leptin in the brain, the various biological functions attributed to this adipocytokine, the signaling pathways that govern leptin actions, and the potential role of leptin in the pathophysiology of Alzheimer's disease. Leptin exerts its functions by binding to the leptin receptor (ObR). This binding can involve several signaling pathways including JAK/STAT pathway, ERK pathway and the PI3K/Akt/mTOR Pathway. Modulation of these pathways leads to the regulation of a multitude of functions that define the intricate involvement of leptin in various physiological tasks. In this review, we will specifically relate the potential involvement of leptin signaling in Alzheimer's disease based on work published by several laboratories including ours. All this work points to leptin as a possible target for developing supplementation therapies for reducing the progression of Alzheimer's disease.

Influence of the core circadian gene "Clock" on obesity and leptin resistance in mice

Alterations in metabolism could be due to cell-autonomous effects associated with altered expression of Clock in central nervous system feeding centers and/or peripheral tissues involved in metabolism. Clock mutant mice are hyperphagic and obese, which indicates that Clock is related to obesity. In the present study, we used intracerebroventricular injection of recombinant adenoviral vector harboring Clock genes to explore the role of Clock on diet induced obesity and the mechanisms involved in leptin resistance and leptin signaling in mice. The results demonstrated that expression of Clock in the arcuate nucleus of diet induced obesity mice was down-regulated. The recombinant adenoviral vector harboring Clock genes could reduce obesity indexes of diet induced obesity mice including body weight, BMI and total fat mass, attenuate hyperleptinemia, increase leptin sensitivity and decrease accumulated suppressor of cytokine signaling-3 in the arcuate nucleus. These results indicate that Clock plays an important role on obesity, which may be involved in leptin resistance and regulation of suppressor of cytokine signaling-3 in arcuate nucleus.

Intranasal treatment of central nervous system dysfunction in humans

One of the most challenging problems facing modern medicine is how to deliver a given drug to a specific target at the exclusion of other regions. For example, a variety of compounds have beneficial effects within the central nervous system (CNS), but unwanted side effects in the periphery. For such compounds, traditional oral or intravenous drug delivery fails to provide benefit without cost. However, intranasal delivery is emerging as a noninvasive option for delivering drugs to the CNS with minimal peripheral exposure. Additionally, this method facilitates the delivery of large and/or charged therapeutics, which fail to effectively cross the blood-brain barrier (BBB). Thus, for a variety of growth factors, hormones, neuropeptides and therapeutics including insulin, oxytocin, orexin, and even stem cells, intranasal delivery is emerging as an efficient method of administration, and represents a promising therapeutic strategy for the treatment of diseases with CNS involvement, such as obesity, Alzheimer’s disease, Parkinson’s disease, Huntington’s disease, depression, anxiety, autism spectrum disorders, seizures, drug addiction, eating disorders, and stroke.

Effect of electroacupuncture on leptin resistance in rats with diet-induced obesity

In the present study, the effects of electroacupuncture (EA) on body weight and sensitivity of leptin in diet-induced obese rats were examined and the underlying mechanisms were explored. After feeding with high-fat (HIF) diet for 12 weeks, the diet-induced obese rats received electroacupuncture stimulation three times per week for four weeks. The expression of the leptin receptor in the hypothalamus was measured using immunohistochemistry. The plasma leptin was detected with ELISA. The leptin and leptin receptor mRNA was examined with real-time PCR. Results showed that electroacupuncture treatment led to a reduction of body weight, decrease in the plasma leptin levels, and an increase in leptin receptor expression in the hypothalamus. Our results suggested that regulating the expression of leptin and the leptin receptor might be one of the molecular mechanisms underlying the reduction of body weight in diet-induced obese rats by electroacupuncture treatment.

The use of animal models in diabetes research

Diabetes is a disease characterized by a relative or absolute lack of insulin, leading to hyperglycaemia. There are two main types of diabetes: type 1 diabetes and type 2 diabetes. Type 1 diabetes is due to an autoimmune destruction of the insulin-producing pancreatic beta cells, and type 2 diabetes is caused by insulin resistance coupled by a failure of the beta cell to compensate. Animal models for type 1 diabetes range from animals with spontaneously developing autoimmune diabetes to chemical ablation of the pancreatic beta cells. Type 2 diabetes is modelled in both obese and non-obese animal models with varying degrees of insulin resistance and beta cell failure. This review outlines some of the models currently used in diabetes research. In addition, the use of transgenic and knock-out mouse models is discussed. Ideally, more than one animal model should be used to represent the diversity seen in human diabetic patients.

Nutrition-induced differences in body composition, compensatory growth and endocrine status in growing pigs

In this experiment, we assessed the effect of amino acid (AA) intake restriction in entire male Yorkshire pigs between 15 and 38 kg BW (restriction phase) on BW gain, body composition and plasma levels of blood urea nitrogen (BUN), cortisol, insulin-like growth factor I (IGF-I), growth hormone (GH) and leptin during the subsequent re-alimentation phase. During the restriction phase, 36 pigs were allotted to one of two dietary treatments: adequate AA intake (control) or AA-limiting diets (AA-30%). Thereafter, pigs were fed common non-limiting diets up to 110 kg BW. Throughout the experiment, pigs were scale-fed at 90% of the estimated voluntary daily digestible energy intake. At the end of the restriction phase, pigs on AA-30% had lesser BW gain (650 v. 784 g/day; P < 0.001), loin area (LA; 12.2 v. 14.2 cm2; P < 0.001), BUN (4.6 v. 6.3 mg/dl; P < 0.02), lesser plasma levels of IGF-I (440 v. 640 ng/m; P < 0.001) and cortisol (8.2 v. 19.2 μg/dl; P < 0.001), greater backfat thickness (BF; 7.56 v. 6.56 mm; P < 0.02), and greater plasma levels of leptin (2.7 v. 1.8 ng/ml; P = 0.027) and GH (3.3 v. 2.0 ng/ml; P = 0.05) than pigs on control. During the re-alimentation phase, previously restricted pigs showed full compensatory growth (CG) in terms of BW gain (1170 v. 1077 g/day; P < 0.002), whole-body protein deposition (Pd) (179 v. 163 g/day; P < 0.001) as well as physical and chemical body composition (whole-body lipid to body protein mass ratio, LB/PB; 1.14 v. 1.15; P > 0.10). Besides GH at 45 kg BW (4.2 v. 2.4 ng/ml; P = 0.066), there were no effects of previous AA intake restriction on leptin, IGF-I and BUN during the re-alimentation phase (P > 0.10). Plasma cortisol and IGF-I levels may act as an indicator of AA-induced restriction in Pd in growing pigs. Plasma BUN level does not appear as a sensitive indicator for compensatory Pd. Plasma leptin and GH levels allow for the involvement of the brain in controlling chemical body composition. Full CG was observed during the energy-dependent phase of Pd in growing pigs and might be driven by a target LB/PB, possibly mediated via plasma leptin, IGF-I and GH levels.

Leptin activates hepatic 5'-AMP-activated protein kinase through sympathetic nervous system and α1-adrenergic receptor: a potential mechanism for improvement of fatty liver in lipodystrophy by leptin

BACKGROUND

AMPK activation promotes glucose and lipid metabolism.

RESULTS

Hepatic AMPK activities were decreased in fatty liver from lipodystrophic mice, and leptin activated the hepatic AMPK via the α-adrenergic effect.

CONCLUSION

Leptin improved the fatty liver possibly by activating hepatic AMPK through the central and sympathetic nervous systems.

SIGNIFICANCE

Hepatic AMPK plays significant roles in the pathophysiology of lipodystrophy and metabolic action of leptin. Leptin is an adipocyte-derived hormone that regulates energy homeostasis. Leptin treatment strikingly ameliorates metabolic disorders of lipodystrophy, which exhibits ectopic fat accumulation and severe insulin-resistant diabetes due to a paucity of adipose tissue. Although leptin is shown to activate 5'-AMP-activated protein kinase (AMPK) in the skeletal muscle, the effect of leptin in the liver is still unclear. We investigated the effect of leptin on hepatic AMPK and its pathophysiological relevance in A-ZIP/F-1 mice, a model of generalized lipodystrophy. Here, we demonstrated that leptin activates hepatic AMPK through the central nervous system and α-adrenergic sympathetic nerves. AMPK activities were decreased in the fatty liver of A-ZIP/F-1 mice, and leptin administration increased AMPK activities in the liver as well as in skeletal muscle with significant reduction in triglyceride content. Activation of hepatic AMPK with A769662 also led to a decrease in hepatic triglyceride content and blood glucose levels in A-ZIP/F-1 mice. These results indicate that the down-regulation of hepatic AMPK activities plays a pathophysiological role in the metabolic disturbances of lipodystrophy, and the hepatic AMPK activation is involved in the therapeutic effects of leptin.

Brain meets body: the blood-brain barrier as an endocrine interface

The blood-brain barrier (BBB) separates the central nervous system (CNS) from the peripheral tissues. However, this does not prevent hormones from entering the brain, but shifts the main control of entry to the BBB. In general, steroid hormones cross the BBB by transmembrane diffusion, a nonsaturable process resulting in brain levels that reflect blood levels, whereas thyroid hormones and many peptides and regulatory proteins cross using transporters, a saturable process resulting in brain levels that reflect blood levels and transporter characteristics. Protein binding, brain-to-blood transport, and pharmacokinetics modulate BBB penetration. Some hormones have the opposite effect within the CNS than they do in the periphery, suggesting that these hormones cross the BBB to act as their own counterregulators. The cells making up the BBB are also endocrine like, both responding to circulating substances and secreting substances into the circulation and CNS. By dividing a hormone's receptors into central and peripheral pools, the former of which may not be part of the hormone's negative feed back loop, the BBB fosters the development of variable hormone resistance syndromes, as exemplified by evidence that altered insulin action in the CNS can contribute to Alzheimer's disease. In summary, the BBB acts as a regulatory interface in an endocrine-like, humoral-based communication between the CNS and peripheral tissues.

The receptive function of hypothalamic and brainstem centres to hormonal and nutrient signals affecting energy balance

The hypothalamic arcuate nucleus (ARC) and the area postrema (AP) represent targets for hormonal and metabolic signals involved in energy homoeostasis, e.g. glucose, amylin, insulin, leptin, peptide YY (PYY), glucagon-like peptide 1 (GLP-1) and ghrelin. Orexigenic neuropeptide Y expressing ARC neurons are activated by food deprivation and inhibited by feeding in a nutrient-dependent manner. PYY and leptin also reverse or prevent fasting-induced activation of the ARC. Interestingly, hypothalamic responses to fasting are blunted in different models of obesity (e.g. diet-induced obesity (DIO) or late-onset obesity). The AP also responds to feeding-related signals. The pancreatic hormone amylin acts via the AP to control energy intake. Amylin-sensitive AP neurons are also glucose-responsive. Furthermore, diet-derived protein attenuates amylin responsiveness suggesting a modulation of AP sensitivity by macronutrient supply. This review gives an overview of the receptive function of the ARC and the AP to hormonal and nutritional stimuli involved in the control of energy balance and the possible implications in the context of obesity. Collectively, there is consistency between the neurophysiological actions of these stimuli and their effects on energy homoeostasis under experimental conditions. However, surprisingly little progress has been made in the development of effective pharmacological approaches against obesity. A promising way to improve effectiveness involves combination treatments (e.g. amylin/leptin agonists). Hormonal alterations (e.g. GLP-1 and PYY) are also considered to mediate body weight loss observed in obese patients receiving bariatric surgery. The effects of hormonal and nutritional signals and their interactions might hold the potential to develop poly-mechanistic therapeutic strategies against obesity.

Is leptin the parabiotic "satiety" factor? Past and present interpretations

In 1959 Hervey hypothesized that a circulating feedback signal informed the hypothalamus of the size of fat stores and initiated appropriate corrections to energy balance. The hypothesis resulted from a parabiosis study in which one animal became obese following lesioning of the ventromedial hypothalamus. The partner of the lesioned rat was hypophagic and lost a large amount of body fat. Similar results came from parabiosis studies with obese Zucker rats and rats that overate due to stimulation of the lateral hypothalamus. In studies in which one parabiont was made obese by overfeeding the non-overfed partners lost substantial amounts of fat with a minimal reduction in food intake and no loss of lean tissue. The loss of fat was due to inhibition of adipose lipogenesis and other metabolic adjustments typical of food restriction. Parabiosis with genetically obese mice implied that ob/ob mice did not produce the feedback signal and subsequently the mutant ob protein, leptin, was identified. This paper provides a review and interpretation of parabiosis work that preceded the discovery of leptin, an evaluation of leptin in relation to its function as the circulating feedback signal and evidence for additional circulating factors involved in the control of adipose tissue mass.

High calorie diet triggers hypothalamic angiopathy

Obesity, type 2 diabetes, and related diseases represent major health threats to modern society. Related pathophysiology of impaired neuronal function in hypothalamic control centers regulating metabolism and body weight has been dissected extensively and recent studies have started focusing on potential roles of astrocytes and microglia. The hypothalamic vascular system, however, which maintains the microenvironment necessary for appropriate neuronal function, has been largely understudied. We recently discovered that high fat/high sucrose diet exposure leads to increased hypothalamic presence of immunoglobulin G (IgG1). Investigating this phenomenon further, we have discovered a significant increase in blood vessel length and density in the arcuate nucleus (ARC) of the hypothalamus in mice fed a high fat/high sucrose diet, compared to matched controls fed standard chow diet. We also found a clearly increased presence of α-smooth muscle actin immunoreactive vessels, which are rarely present in the ARC and indicate an increase in the formation of new arterial vessels. Along the blood brain barrier, an increase of degenerated endothelial cells are observed. Moreover, such hypothalamic angiogenesis was not limited to rodent models. We also found an increase in the number of arterioles of the infundibular nucleus (the human equivalent of the mouse ARC) in patients with type 2 diabetes, suggesting angiogenesis occurs in the human hypothalamus of diabetics. Our discovery reveals novel hypothalamic pathophysiology, which is reminiscent of diabetic retinopathy and suggests a potential functional involvement of the hypothalamic vasculature in the later stage pathogenesis of metabolic syndrome.

The prolonged survival of fibroblasts with forced lipid catabolism in visceral fat following encapsulation in alginate-poly-L-lysine

Although alginate-poly-L-lysine (AP(L)) encapsulation of cells producing bioactive peptides has been widely tested, it is unknown whether AP(L) supports lasting catabolic functions of encapsulated cells in adipose tissue, which are required for obesity reduction. We tested functions of AP(L)-encapsulated fibroblasts isolated from wild-type (WT) and aldehyde dehydrogenase 1a1 knockout mice (KO), which resist obesity on a high-fat (HF) diet, have a higher metabolic rate, and express increased levels of thermogenic uncoupling protein-1 (Ucp1) in their deleterious visceral fat depots compared to WT mice. To enable in vivo detection and quantification, fibroblasts were stably transfected with green-fluorescent protein. WT- or KO-containing microcapsules were injected into two visceral depots of WT mice fed an HF diet. Eighty days after transplantation, microcapsules were located in vivo using magnetic resonance imaging. KO microcapsules prevented weight gain in obese WT mice compared to a mock- and WT capsule-injected groups on an HF diet. The weight loss in KO-treated mice corresponded to lipid reduction and induction of thermogenesis in the injected visceral fat. The non-treated subcutaneous fat was not altered. Our data suggest that the AP(L) polymer supports long-term catabolic functions of genetically-modified fibroblasts, which can be potentially used for depot-specific obesity treatment.

Role of the blood-brain barrier in the evolution of feeding and cognition

The blood-brain barrier (BBB) regulates the blood-to-brain passage of gastrointestinal hormones, thus informing the brain about feeding and nutritional status. Disruption of this communication results in dysregulation of feeding and body weight control. Leptin, which crosses the BBB to inform the CNS about adiposity, provides an example. Impaired leptin transport, especially coupled with central resistance, results in obesity. Various substances/conditions regulate leptin BBB transport. For example, triglycerides inhibit leptin transport. This may represent an evolutionary adaptation in that hypertriglyceridemia occurs during starvation. Inhibition of leptin, an anorectic, during starvation could have survival advantages. The large number of other substances that influence feeding is explained by the complexity of feeding. This complexity includes cognitive aspects; animals in the wild are faced with cost/benefit analyses to feed in the safest, most economical way. This cognitive aspect partially explains why so many feeding substances affect neurogenesis, neuroprotection, and cognition. The relation between triglycerides and cognition may be partially mediated through triglyceride's ability to regulate the BBB transport of cognitively active gastrointestinal hormones such as leptin, insulin, and ghrelin.

Leptin, a neuroendocrine mediator of immune responses, inflammation, and sickness behaviors

Effective immune responses are coordinated by interactions among the nervous, endocrine, and immune systems. Mounting immune, inflammatory, and sickness responses requires substantial energetic investments, and as such, an organism may need to balance energy allocation to these processes with the energetic demands of other competing physiological systems. The metabolic hormone leptin appears to be mediating trade-offs between the immune system and other physiological systems through its actions on immune cells and the brain. Here we review the evidence in both mammalian and non-mammalian vertebrates that suggests leptin is involved in regulating immune responses, inflammation, and sickness behaviors. Leptin has also been implicated in the regulation of seasonal immune responses, including sickness; however, the precise physiological mechanisms remain unclear. Thus, we discuss recent data in support of leptin as a mediator of seasonal sickness responses and provide a theoretical model that outlines how seasonal cues, leptin, and proinflammatory cytokines may interact to coordinate seasonal immune and sickness responses.

Insulin resistance in the defense against obesity

In the face of the current obesity epidemic, the nature of the relationship between overnutrition and type 2 diabetes is of great importance. Obesity can be considered a state of excessive insulin action that elicits a series of cellular homeostatic responses, producing systemic insulin resistance. These responses occur in four steps: homologous desensitization to insulin action, leptin secretion, inflammation, and, finally, a counter-inflammatory phase that serves to conserve energy storage. The molecular mechanisms underlying these steps are discussed in the context of potential new therapeutic approaches.

AgRP and NPY expression in the human hypothalamic infundibular nucleus correlate with body mass index, whereas changes in αMSH are related to type 2 diabetes

CONTEXT

Rodent data show that altered hypothalamic signaling contributes to the development of obesity and insulin resistance.

OBJECTIVE

To determine differences in hypothalamic expression levels of neuropeptide Y (NPY), agouti-related peptide (AgRP), and αMSH in the infundibular nucleus, the human equivalent of the arcuate nucleus, in relation to body mass index (BMI). In addition, the expression in the infundibular nucleus of eight subjects diagnosed with type 2 diabetes was measured to determine possible interference of type 2 diabetes with the association observed between neuropeptides and BMI.

DESIGN

We studied AgRP, NPY, and αMSH expression by means of quantitative immunocytochemistry in postmortem hypothalami of 30 subjects with known BMI. In separate experiments, we compared neuropeptide expression in eight subjects with type 2 diabetes with eight matched controls.

RESULTS

We found that AgRP immunoreactivity showed a U-shaped correlation with BMI. No evidence was found for possible influences of corticosteroid treatment. NPY immunoreactivity was significantly lower in overweight and obese subjects. αMSH did not correlate with BMI but was significantly lower in subjects with type 2 diabetes compared with controls. By contrast, NPY and AgRP expression was not affected in type 2 diabetes.

CONCLUSION

Our results indicate that the expression of AgRP and NPY are correlated with body weight changes, rather than the presence of type 2 diabetes, whereas changes in αMSH immunoreactivity are related to the presence of type 2 diabetes, indicating separate hypothalamic mechanisms.

Restoration of leptin responsiveness in diet-induced obese mice using an optimized leptin analog in combination with exendin-4 or FGF21

The identification of leptin as a mediator of body weight regulation provided much initial excitement for the treatment of obesity. Unfortunately, leptin monotherapy is insufficient in reversing obesity in rodents or humans. Recent findings suggest that amylin is able to restore leptin sensitivity and when used in combination with leptin enhances body weight loss in obese rodents and humans. However, as the uniqueness of this combination therapy remains unclear, we assessed whether co-administration of leptin with other weight loss-inducing hormones equally restores leptin responsiveness in diet-induced obese (DIO) mice. Accordingly, we report here the design and characterization of a series of site-specifically enhanced leptin analogs of high potency and sustained action that, when administered in combination with exendin-4 or fibroblast growth factor 21 (FGF21), restores leptin responsiveness in DIO mice after an initial body weight loss of 30%. Using either combination, body weight loss was enhanced compared with either exendin-4 or FGF21 monotherapy, and leptin alone was sufficient to maintain the reduced body weight. In contrast, leptin monotherapy proved ineffective when identical weight loss was induced by caloric restriction alone over a comparable time. Accordingly, we find that a hypothalamic counter-regulatory response to weight loss, assessed using changes in hypothalamic agouti related peptide (AgRP) levels, is triggered by caloric restriction, but blunted by treatment with exendin-4. We conclude that leptin re-sensitization requires pharmacotherapy but does not appear to be restricted to a unique signaling pathway. Our findings provide preclinical evidence that high activity, long-acting leptin analogs are additively efficacious when used in combination with other weight-lowering agents.

Suppressor of cytokine signaling 3 in the human hypothalamus

In rodents, the mediobasal hypothalamus and the hypothalamic paraventricular nucleus (PVN) are implicated in leptin signaling. Surprisingly little data is available on the human hypothalamus. We set out to study the expression of suppressor-of-cytokine-signaling 3 (SOCS3), α-melanocyte stimulating hormone (αMSH) and agouti-related protein (AgRP) in the infundibular nucleus (IFN) and to investigate the relationship between these neuropeptide expressions and serum leptin concentrations in a blood sample taken within 24h before death. We studied post-mortem human brain material by means of quantitative immunocytochemistry. We found that SOCS3 immunoreactivity was widely distributed throughout the hypothalamus, and most prominent in the PVN, whereas expression levels in the IFN were low. Surprisingly, SOCS3 expression in the PVN was inversely related to serum leptin. A significant positive correlation was observed between AgRP and NPY expression in the IFN. The inverse correlation between SOCS3 expression in the PVN and serum leptin was unexpected and may be related to the hypothalamic adaptation to fatal illness rather than to nutritional status, or may represent an interspecies difference.

Role of hypothalamic proopiomelanocortin neuron autophagy in the control of appetite and leptin response

Autophagy is a catabolic cellular process involving the degradation of the cell's own components. Although the role of autophagy of diverse tissues in body metabolism has been investigated, the importance of autophagy in hypothalamic proopiomelanocortin (POMC) neurons, key regulators of energy balance, has not been addressed. The role of autophagy in leptin sensitivity that is critical for the control of body weight and appetite has also not been investigated. We produced mice with specific deletion of autophagy-related 7 (Atg7), an essential autophagy gene, in hypothalamic POMC neurons (Atg7(ΔPOMC) mice). Atg7 expression was deficient in the arcuate nucleus of the hypothalamus of Atg7(ΔPOMC) mice. p62, a specific substrate of autophagy, accumulated in the hypothalamus of Atg7(ΔPOMC) mice, which colocalized with ubiquitin. Atg7(ΔPOMC) mice had increased body weight due to increased food intake and decreased energy expenditure. Atg7(ΔPOMC) mice were not more prone to diet-induced obesity compared with control mice but more susceptible to hyperglycemia after high-fat diet. The ability of leptin to suppress fasting-elicited hyperphagia and weight gain during refeeding was attenuated in Atg7(ΔPOMC) mice. Deficient autophagy did not significantly affect POMC neuron number but impaired leptin-induced signal transducer and activation of transcription 3 activation. Our findings indicate a critical role for autophagy of POMC neurons in the control of energy homeostasis and leptin signaling.

The utility of animal models to evaluate novel anti-obesity agents

The global incidence of obesity continues to rise and is a major driver of morbidity and mortality through cardiovascular and cerebrovascular diseases. Animal models used in the discovery of novel treatments for obesity range from straightforward measures of food intake in lean rodents to long-term studies in animals exhibiting obesity due to the continuous access to diets high in fat. The utility of these animal models can be extended to determine, for example, that weight loss is due to fat loss and/or assess whether beneficial changes in key plasma parameters (e.g. insulin) are evident. In addition, behavioural models such as the behavioural satiety sequence can be used to confirm that a drug treatment has a selective effect on food intake. Typically, animal models have excellent predictive validity whereby drug-induced weight loss in rodents subsequently translates to weight loss in man. However, despite this, at the time of writing orlistat (Europe; USA) remains the only drug currently marketed for the treatment of obesity, with sibutramine having recently been withdrawn from sale globally due to the increased incidence of serious, non-fatal cardiovascular events. While the utility of rodent models in predicting clinical weight loss is detailed, the review also discusses whether animals can be used to predict adverse events such as those seen with recent anti-obesity drugs in the clinic.

Functional heterogeneity of arcuate nucleus pro-opiomelanocortin neurons: implications for diverging melanocortin pathways

Arcuate nucleus (ARC) pro-opiomelanocortin (POMC) neurons are essential regulators of food intake, energy expenditure, and glucose homeostasis. POMC neurons integrate several key metabolic signals that include neurotransmitters and hormones. The change in activity of POMC neurons is relayed to melanocortin receptors in distinct regions of the central nervous system. This review will summarize the role of leptin and serotonin receptors in regulating the activity of POMC neurons and provide a model in which different melanocortin pathways regulate energy and glucose homeostasis.

In vivo effects of leptin on lymphocyte subpopulations in mice

Leptin, a hormone-cytokine mainly produced by the adipose tissue, has pleitropic effects on many biological system including metabolic, endocrine, and immune system. Although it is well known that leptin controls food intake on hypothalamic regions of brain, the role of leptin in hematopoietic and immune processes has been mainly investigated with in vitro and transgenic mouse studies. The aim of this study was to investigate the effects of peripheral leptin on lymphocyte subpopulation. Initially forty male Swiss albino mice were divided into five groups. Mice in group I (Control) were given serum physiologic (SP) and group L100, group L250, group L500, and group L1000 were given 100, 250, 500 and 1000 μg/kg/day recombinant mouse leptin, respectively. Leptin or SP was injected subcutaneously for the next 6 days. Daily food/water intake was recorded for each group. At the end of the study, whole blood samples (500 μl) were obtained via intracardiac punction in anesthetized mice. Leptin levels and lymphocyte subpopulations in blood samples were analyzed. We show that no in vivo dose-dependent effect of leptin is existed on lymphocyte subpopulations count in mice. Treatment of mice with high-dose leptin led to increase only CD4+ cells (P<0.05). In addition, high-dose leptin slightly increased CD3+ cells but this was not statistically confirmed (P=0.08). Notably, it was found that leptin caused insignificant changes on body weight and food intake in normal body weight mice. The data support that high-dose leptin has proliferative effect on CD4+ cells in vivo. However, more in vivo study needs to be examined to clarify how leptin affect lymphocyte subpopulations.

Leptin in anorexia and cachexia syndrome

Leptin is a product of the obese (OB) gene secreted by adipocytes in proportion to fat mass. It decreases food intake and increases energy expenditure by affecting the balance between orexigenic and anorexigenic hypothalamic pathways. Low leptin levels are responsible for the compensatory increase in appetite and body weight and decreased energy expenditure (EE) following caloric deprivation. The anorexia-cachexia syndrome is a complication of many chronic conditions including cancer, chronic obstructive pulmonary disease, congestive heart failure, chronic kidney disease, and aging, where the decrease in body weight and food intake is not followed by a compensatory increase in appetite or decreased EE. Crosstalk between leptin and inflammatory signaling known to be activated in these conditions may be responsible for this paradox. This manuscript will review the evidence and potential mechanisms mediating changes in the leptin pathway in the setting of anorexia and cachexia associated with chronic diseases.