A 35 amino acid fragment of leptin inhibits feeding in the rat

Turning the clock forward: New pharmacological and non pharmacological targets for the treatment of obesity

AIMS

Obesity and its main metabolic complication, type 2 diabetes, have attained the status of a global pandemic; there is need for novel strategies aimed at treating obesity and preventing the development of diabetes. A healthy diet and exercise are basic for treatment of obesity but often not enough. Pharmacotherapy can be helpful in maintaining compliance, ameliorating obesity-related health risks, and improving quality of life. In the last two decades, the knowledge of central and peripheral mechanisms underlying homeostatic and hedonic aspects of food intake has significantly increased. Dysregulation of one or more of these components could lead to obesity.

DATA SYNTHESIS

In order to better understand how potential innovative treatment options can affect obesity, homeostatic and reward mechanisms that regulate energy balance has been firstly illustrated. Then, an overview of potential therapeutic targets for obesity, distinguished according to the level of regulation of feeding behavior, has been provided. Moreover, several non-drug therapies have been recently tested in obesity, such as non-invasive neurostimulation: Transcranial Magnetic Stimulation or Transcranial Direct Current Stimulation. All of them are promising for obesity treatment and are almost devoid of side effects, constituting a potential resource for the prevention of metabolic diseases.

CONCLUSIONS

The plethora of current anti-obesity therapies creates the unique challenge for physicians to customize the intervention, according to the specific obesity characteristics and the intervention side effect profiles; moreover, it allows multimodal approaches addressed to treat obesity and metabolic adaptation with complementary mechanisms.

An overview of obesity mechanisms in humans: Endocrine regulation of food intake, eating behaviour and common determinants of body weight

Obesity is one of the biggest health challenges of the 21st century, already affecting close to 700 million people worldwide, debilitating and shortening lives and costing billions of pounds in healthcare costs and loss of workability. Body weight homeostasis relies on complex biological mechanisms and the development of obesity occurs on a background of genetic susceptibility and an environment promoting increased caloric intake and reduced physical activity. The pathophysiology of common obesity links neuro-endocrine and metabolic disturbances with behavioural changes, genetics, epigenetics and cultural habits. Also, specific causes of obesity exist, including monogenetic diseases and iatrogenic causes. In this review, we provide an overview of obesity mechanisms in humans with a focus on energy homeostasis, endocrine regulation of food intake and eating behavior, as well as the most common specific causes of obesity.

Potential Leptin Receptor Response Modifier Peptides

Drug targets for the treatment of obesity and comorbidities represent an ever-renewable source of research opportunities worldwide. One of the earliest is the leptin–leptin receptor system that was discovered in the mid-1990s. Leptin, a satiety hormone, is overproduced in overweight patients but the protein is unable to cross the blood–brain barrier and remains inactive. Circulating high levels of leptin induces a series of conditions that would not be manifested without leptin overproduction, including various forms of cancer and inflammatory and cardiovascular diseases. Current pharmaceutical research focuses on improving the blood–brain barrier penetration of leptin receptor agonists and the development of monofunctional antagonists with broad spectrum therapeutic efficacies but without unwanted side effects. Designer peptides with their expanded chemical space as well as well controllable receptor binding and elimination properties slowly replace full-sized leptin products in the drug development pipeline.

A Leptin Fragment Mirrors the Cognitive Enhancing and Neuroprotective Actions of Leptin

A key pathology of Alzheimer's disease (AD) is amyloid β (Aβ) accumulation that triggers synaptic impairments and neuronal death. Metabolic disruption is common in AD and recent evidence implicates impaired leptin function in AD. Thus the leptin system may be a novel therapeutic target in AD. Indeed, leptin has cognitive enhancing properties and it prevents the aberrant effects of Aβ on hippocampal synaptic function and neuronal viability. However, as leptin is a large peptide, development of smaller leptin-mimetics may be the best therapeutic approach. Thus, we have examined the cognitive enhancing and neuroprotective properties of known bioactive leptin fragments. Here we show that the leptin (116-130) fragment, but not leptin (22-56), mirrored the ability of leptin to promote AMPA receptor trafficking to synapses and facilitate activity-dependent hippocampal synaptic plasticity. Administration of leptin (116-130) also mirrored the cognitive enhancing effects of leptin as it enhanced performance in episodic-like memory tests. Moreover, leptin (116-130) prevented hippocampal synaptic disruption and neuronal cell death in models of amyloid toxicity. These findings establish further the importance of the leptin system as a therapeutic target in AD.

Central injection of a synthetic chicken partial leptin peptide does not affect food intake in chicks

Leptin is an adipose tissue-derived hormone in mammals that plays an important role in whole body energy balance via its inhibitory effects on food intake mediated through the hypothalamus. Chicken leptin has a low sequence homology to mammalian leptin and its role in appetite regulation is not reported; hence the objective of this study was to determine effects of central injection of chicken leptin on food and water intake and associated behaviors in chicks. Chicks were intracerebroventricularly injected with 0 (vehicle), 0.3, 1.0, or 3.0 nmol of a synthetic chicken leptin partial peptide and food and water intake were monitored. There were no effects observed and a second experiment was conducted to evaluate food and water intake at higher doses; after injection of 0, 2.5, 5.0, or 10.0 nmol leptin. Again, there were no effects on food or water intake. In the third experiment, behaviors were analyzed during the first 30 min post-injection of vehicle or 10 nmol leptin. At 5 min post-injection, vehicle-injected chicks spent more time sitting than leptin-injected chicks. A wide dose range was evaluated however, the absence of an effect on food intake or behavior suggests that the chicken leptin peptide that was tested does not mediate effects on appetite in the brain and that chicken leptin likely has a different physiological role in birds than in mammals.

Leptin: From structural insights to the design of antagonists

After its discovery in 1994, it soon became clear that leptin acts as an adipocyte-derived hormone with a central role in the control of body weight and energy homeostasis. However, a growing body of evidence has revealed that leptin is a pleiotropic cytokine with activities on many peripheral cell types. Inappropriate leptin signaling can promote autoimmunity, certain cardiovascular diseases, elevated blood pressure and cancer, which makes leptin and the leptin receptor interesting targets for antagonism. Profound insights in the leptin receptor (LR) activation mechanisms are a prerequisite for the rational design of these antagonists. In this review, we focus on the molecular mechanisms underlying leptin receptor activation and signaling. We also discuss the current strategies to interfere with leptin signaling and their therapeutic potential.

New pharmacological perspectives for the leptin receptor in the treatment of obesity

After its discovery in 1994, leptin became the great hope as an anti-obesity treatment based on its ability to reduce food intake and increase energy expenditure. However, treating obese people with exogenous leptin was unsuccessful in most cases since most of them present already high circulating leptin levels to which they do not respond anymore defining the so-called state of "leptin resistance." Indeed, leptin therapy is unsuccessful to lower body weight in commonly obese people but effective in people with rare single gene mutations of the leptin gene. Consequently, treatment of obese people with leptin was given less attention and the focus of obesity research shifted toward the prevention and reversal of the state of leptin resistance. Many of these new promising approaches aim to restore or sensitize the impaired function of the leptin receptor by pharmacological means. The current review will focus on the different emerging therapeutic strategies in obesity research that are related to leptin and its receptor.

G-CSF receptor-binding cyclic peptides designed with artificial amino-acid linkers

Designing small molecules that mimic the receptor-binding local surface structure of large proteins such as cytokines or growth factors is fascinating and challenging. In this study, we designed cyclic peptides that reproduce the receptor-binding loop structures of G-CSF. We found it is important to select a suitable linker to join two or more discontinuous sequences and both termini of the peptide corresponding to the receptor-binding loop. Structural simulations based on the crystallographic structure of KW-2228, a stable and potent analog of human G-CSF, led us to choose 4-aminobenzoic acid (Abz) as a part of the linker. A combination of 4-Abz with beta-alanine or glycine, and disulfide bridges between cysteins or homocysteins, gave a structure suitable for receptor binding. In this structure, the side-chains of several amino acids important for the interactions with the receptor are protruding from one side of the peptide ring. This artificial peptide showed G-CSF antagonistic activity in a cell proliferation assay.

Tertiary windowing to detect positive diversifying selection

As a protein-encoding gene evolves, different selective pressures act on the gene temporally and spatially. An examination of the ratio of nonsynonymous-to-synonymous nucleotide substitution rate ratios (K(a)/K(s)) has proven to be a valuable method to examine selective pressures on protein encoding genes, including detecting positive diversifying selection. To gain power over averaging all sites in a gene together, examination of sites in primary sequence windows has frequently been employed. However, selection acts on folded proteins and sites that are close in tertiary space may not be close in primary sequence. A new method for the examination of K(a)/K(s) ratios based upon windows in tertiary structure is introduced and applied to the leptin gene family in mammals. Tertiary sequence windowing detects new sites under positive diversifying selection and detects positive diversifying selection with a more significant signal along various branches of the leptin gene family tree.

Leptin as a Potential Treatment for Obesity

Despite significant reductions in the consumption of dietary fat, the prevalence of obesity is steadily rising in western civilization. Of particular concern is the recent epidemic of childhood obesity, which is expected to increase the incidence of obesity-related disorders. The obese gene (ob) protein product leptin is a hormone that is secreted from adipocytes and functions to suppress appetite and increase energy expenditure. Leptin is an attractive candidate for the treatment of obesity as it is an endogenous protein and has been demonstrated to have potent effects on bodyweight and adiposity in rodents. Whereas leptin has been successfully used in the treatment of leptin-deficient obese patients, trials in hyperleptinemic obese patients have yielded variable results. Long-acting leptins have been tried but with no greater success. Other strategies including the use of leptin analogs and other factors that bypass normal leptin delivery systems are being developed. Identifying the mechanisms at the molecular level by which leptin functions will create new avenues for pharmaceutical targeting to simulate the intracellular effects of leptin.

Stimulation of prolactin secretion by chronic, but not acute, administration of leptin in the rat

Leptin, the product of obese (ob) gene, has been reported to affect the secretion of all six anterior pituitary hormones, but data are especially scarce regarding the interplay between leptin and prolactin (PRL). Thus, in this study we examined and compared in vivo the effects of acute and chronic administrations of recombinant mouse leptin on PRL secretion in male rats. Normally-fed and 3-day-fasted rats received an intraperitoneal bolus injection of leptin [1.0 mg/kg body weight (BW)] or vehicle only. The leptin treatment was without effect on plasma PRL levels up to 5 h postadministration. Food deprivation for 3 days significantly decreased both PRL and leptin levels. This decrease in plasma PRL was prevented by a 3-day constant infusion of 75 microg/kg BW/day of leptin, which maintained plasma leptin levels similar to those of normally-fed rats. The administration of three times the higher dose of leptin (225 microg/kg BW/day) to fasted rats led to further increases in both PRL and leptin in the plasma. Thus, a dose-dependent stimulatory effect of chronic leptin treatment on PRL secretion was indicated. This study demonstrates that chronic, but not acute, administration of leptin stimulates PRL secretion in the rat.

Cardiovascular regulatory actions of the hypocretins in brain

The hypocretins, also known as the orexins, are alternate translation products of a single gene. The recognition of their production in neurons of the rostral diencephalon, and their axonal localization in brain sites known to be important in the control of appetite, led to the demonstration of their orexogenic actions. However, these peptides are not as potent as other appetite stimulating neuropeptides and they have been localized in areas of brain more related to cardiovascular function. We verified the orexogenic actions of hypocretin-1 (Hcrt-1) and hypocretin-2 (Hcrt-2) in an ad libitum feeding model and identified the threshold dose to be 1 nmol when given into the lateral cerebroventricle (i.c. v.). Even at threshold doses for feeding, both Hcrt-1 and Hcrt-2 given i.c.v. into conscious, unrestrained rats stimulated significant elevations in mean arterial blood pressure, that appeared dose related. These elevations were relatively long lasting, mirroring the time course of a pressor dose of angiotensin II (0.1 nmol i.c.v.); however, the magnitude of blood pressure elevation to hypocretin did not equal that of A II. These data suggest an additional, non-appetitive action of the hypocretins and indicate that the peptide and receptor mapping studies may have predicted important roles for the peptides in the central nervous system control of cardiovascular function.

Intracerebroventricular administration of mouse leptin does not reduce food intake in the chicken

Recently, it has been suggested that leptin plays an important role in regulation of food intake and metabolism in rats and mice, however, the effect of central administration of leptin on food intake in chicks has not been reported. We have investigated the anorexigenic effect of leptin administered by intracerebroventricular (i.c.v.) injection in chicks using mouse leptin, which shows 97% homology to chicken leptin. Three experiments were conducted. After being deprived of food for 3 h, male broiler chicks were administered leptin by i.c.v. injection at dose levels of 0, 0.2, 1.0 and 5.0 microg (Experiment 1) or 0, 2.5 and 5.0 microg (Experiment 2). The birds were allowed free access to the diet for 2 h (Experiment 1) and 24 h (Experiment 2) after treatment. Male Single Comb White Leghorn chicks were used in Experiment 3 and were treated in the same manner as in Experiment 1. In all experiments, central administration of mouse leptin did not influence food intake in the time periods examined. It appears that either mouse leptin does not bind to the chicken leptin receptor or in the chicken brain the leptin receptor may be absent.

Structure-function studies of human leptin

To elucidate the structural requirement of human leptin for its functions, the wild-type, mutant-type, C-terminal deletion, and N-terminal deletion were expressed in Escherichia coli and purified in soluble forms. These leptin analogs were intracerebroventrically injected into C57BL/6J ob/ob mice, and their in vivo biological activities were evaluated. The mutant-type leptin lacking a C-terminal disulfide bond reduced food intake at doses of more than 15 pmol/mouse, which was as effective as the wild-type leptin. C-terminal deletion without the loop structure, also significantly, but to a lesser extent, reduced food intake at doses of more than 90 pmol/mouse. However, N-terminal deletions showed no effect on food intake. We also evaluated the effects of the leptin analogs on radiolabeled leptin binding to its receptor in the choroid plexus using autoradiography. An excess of unlabeled mutant-type leptin as well as wild-type leptin led to complete inhibition of binding. C-terminal deletions led to weak inhibitory activity, whereas N-terminal deletions caused no inhibitory activity. These results clearly demonstrate that the N-terminal region of leptin is essential for both its biological and receptor binding activities. The amino acid sequence of the C-terminal loop structure is also important for enhancing these actions, whereas the C-terminal disulfide bond is not needed.

Effects of various N-terminal fragments of glucagon-like peptide-1(7-36) on food intake in the neonatal chick

Recently, the suppressive effect on food intake by the central administration of glucagon-like peptide-1 (GLP-1) has been confirmed in both rats and chicks. The importance of the N-terminal amino acid, histidine, for the bioactivity of GLP-1(7-36) in the central nervous system was suggested, though the role for C-terminal amino acids in the central nervous system has not been reported. The present study was done to elucidate the central effect of N-terminal fragments of GLP-1(7-36) on food intake of the neonatal chick. Intracerebroventricular (i.c.v.) administration of mammalian GLP-1(7-36) inhibited food intake of chicks, but the fragments of GLP-1(7-16) and GLP-1(7-26) did not show the suppressive effect on food intake. Furthermore, the extended fragments, GLP-1(7-30) and GLP-1(7-33), also had no effects on food intake. It is concluded that C-terminal amino acids of GLP-1(7-36) have an important role for the bioactivity in the central nervous system with special reference to feeding behavior.

Neural site of leptin influence on neuropeptide Y signaling pathways altering feeding and uncoupling protein

Inhibition of a signal that produces positive energy balance involving neuropeptide Y (NPY) projection from arcuate nucleus (Arc; site of NPY synthesis) to paraventricular nucleus (PVN; site of NPY release) is one potential mechanism of leptin action. NPY in the PVN increases feeding and decreases uncoupling protein (UCP) activity in brown fat, whereas leptin decreases NPY biosynthesis in the Arc, which presumably decreases PVN NPY. It is hypothesized that decreased NPY activity is necessary for the satiety and thermogenic effects of leptin. To test this, we first determined the effect of leptin on feeding in two paradigms: satiated rats and food-deprived rats. Leptin was effective in decreasing feeding in the satiated rats but ineffective in the food-deprived rats. Next, we determined that leptin decreases NPY and increases UCP gene expression. Finally, we injected leptin intracerebroventricularly before specific PVN NPY microinjection. We found that repletion of NPY in PVN by specific NPY microinjection reverses the feeding-inhibitory and thermogenic effects of centrally administered leptin, the first functional evidence indicating that leptin acts on the Arc-PVN feeding-regulatory pathway.

Leptin depolarizes rat hypothalamic paraventricular nucleus neurons

Leptin, the protein product of the ob/ob gene, is thought to have a central site of action, presumably within the hypothalamus, through which it regulates feeding behavior. THe paraventricular nucleus (PVN) is one structure that has been implicated in regulating feeding behavior. Using patch-clamp recording techniques, this study examines the direct membrane effects of leptin on neurons in a coronal PVN slice. Bath application of the physiologically active leptin fragment (amino acids 22-56) elicited dose-related depolarizations in 82% of the type I cells tested (n = 17) and 67% of the type II cells tested (n = 9). By contrast, the physiologically inactive leptin fragment (amino acids 57-92) had no discernible effect on membrane potential (n = 7). The effects of this peptide were unaffected following synaptic isolation of the cells by bath application of the sodium channel blocker tetrodotoxin (n = 5). Voltage clamp recordings in six cells demonstrated that leptin increased a nonspecific cation conductance with a reversal potential near -30 mV. These findings suggest that neurons in PVN may play an important role in the central neuronal circuitry involved in the physiological response to leptin.

In vivo effects of leptin-related synthetic peptides on body weight and food intake in female ob/ob mice: localization of leptin activity to domains between amino acid residues 106-140

In C57BL/6J ob/ob mice, a single base mutation of the ob gene in codon 105 results in the replacement of arginine by a premature stop codon and production of a truncated inactive form of leptin. These observations suggest that leptin activity may be localized, at least in part, to domains distal to amino acid residue 104. To investigate this possibility, we synthesized six overlapping peptide amides corresponding to residues 106-167 of leptin, and examined their effects on body weight and food intake in female C57BL/6J ob/ob mice. When compared with vehicle-injected control mice, weight gain by mice receiving 28 daily 1-mg i.p. injections of LEP-(106-120), LEP-(116-130), or LEP-(126-140) was significantly (P < 0.01) reduced with no apparent toxicity. Weight gain by mice receiving LEP-(136-150), LEP-(146-160), or LEP-(156-167) was not significantly different from that of vehicle-injected control mice. The effects of LEP-(106-120), LEP-(116-130), or LEP-(126-140) were most pronounced during the first week of peptide treatment. Within 7 days, mice receiving these peptides lost 12.3%, 13.8%, and 9.8%, respectively, of their initial body weights. After 28 days, mice given vehicle alone, LEP-(136-150), LEP-(146-160), or LEP-(156-167) were 14.7%, 20.3%, 25.0%, and 24.8% heavier, respectively, than they were at the beginning of the study. Mice given LEP-(106-120) or LEP-(126-140) were only 1.8% and 4.2% heavier, respectively, whereas mice given LEP-(116-130) were 3.4% lighter. Food intake by mice receiving LEP-(106-120), LEP-(116-130), or LEP-(126-140), but not by mice receiving LEP-(136-150), LEP-(146-160), or LEP-(156-167), was reduced by 15%. The results of this study indicate 1) that leptin activity is localized, at least in part, in domains between residues 106-140; 2) that leptin-related peptides have in vivo effects similar to those of native leptin; and 3) offer hope for development of peptide analogs of leptin having potential application in human or veterinary medicine.