Tat-modified leptin is more accessible to hypothalamus through brain-blood barrier with a significant inhibition of body-weight gain in high-fat-diet fed mice

Novel mechanisms involved in leptin sensitization in obesity

Leptin is a hormone that is secreted by adipocytes in proportion to adipose tissue size, and that informs the brain about the energy status of the body. Leptin acts through its receptor LepRb, expressed mainly in the hypothalamus, and induces a negative energy balance by potent inhibition of feeding and activation of energy expenditure. These actions have led to huge expectations for the development of therapeutic targets for metabolic complications based on leptin-derived compounds. However, the majority of patients with obesity presents elevated leptin production, suggesting that in this setting leptin is ineffective in the regulation of energy balance. This resistance to the action of leptin in obesity has led to the development of "leptin sensitizers," which have been tested in preclinical studies. Much research has focused on generating combined treatments that act on multiple levels of the gastrointestinal-brain axis. The gastrointestinal-brain axis secretes a variety of different anorexigenic signals, such as uroguanylin, glucagon-like peptide-1, amylin, or cholecystokinin, which can alleviate the resistance to leptin action. Moreover, alternative mechanism such as pharmacokinetics, proteostasis, the role of specific kinases, chaperones, ER stress and neonatal feeding modifications are also implicated in leptin resistance. This review will cover the current knowledge regarding the interaction of leptin with different endocrine factors from the gastrointestinal-brain axis and other novel mechanisms that improve leptin sensitivity in 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.

Leptin, Obesity, and Leptin Resistance: Where Are We 25 Years Later?

Leptin, a hormone that is capable of effectively reducing food intake and body weight, was initially considered for use in the treatment of obesity. However, obese subjects have since been found to have high levels of circulating leptin and to be insensitive to the exogenous administration of leptin. The inability of leptin to exert its anorexigenic effects in obese individuals, and therefore, the lack of clinical utility of leptin in obesity, is defined as leptin resistance. This phenomenon has not yet been adequately characterized. Elucidation of the molecular mechanisms underlying leptin resistance is of vital importance for the application of leptin as an effective treatment for obesity. Leptin must cross the blood–brain barrier (BBB) to reach the hypothalamus and exert its anorexigenic functions. The mechanisms involved in leptin transportation across the blood–brain barrier continue to be unclear, thereby preventing the clinical application of leptin in the treatment of obesity. In recent years, new strategies have been developed to recover the response to leptin in obesity. We have summarized these strategies in this review.

The cellular and molecular bases of leptin and ghrelin resistance in obesity

Obesity, a major risk factor for the development of diabetes mellitus, cardiovascular diseases and certain types of cancer, arises from a chronic positive energy balance that is often due to unlimited access to food and an increasingly sedentary lifestyle on the background of a genetic and epigenetic vulnerability. Our understanding of the humoral and neuronal systems that mediate the control of energy homeostasis has improved dramatically in the past few decades. However, our ability to develop effective strategies to slow the current epidemic of obesity has been hampered, largely owing to the limited knowledge of the mechanisms underlying resistance to the action of metabolic hormones such as leptin and ghrelin. The development of resistance to leptin and ghrelin, hormones that are crucial for the neuroendocrine control of energy homeostasis, is a hallmark of obesity. Intensive research over the past several years has yielded tremendous progress in our understanding of the cellular pathways that disrupt the action of leptin and ghrelin. In this Review, we discuss the molecular mechanisms underpinning resistance to leptin and ghrelin and how they can be exploited as targets for pharmacological management of obesity.

Brain signaling systems in the Type 2 diabetes and metabolic syndrome: promising target to treat and prevent these diseases

The changes in the brain signaling systems play an important role in etiology and pathogenesis of Type 2 diabetes mellitus (T2DM) and metabolic syndrome (MS), being a possible cause of these diseases. Therefore, their restoration at the early stages of T2DM and MS can be regarded as a promising way to treat and prevent these diseases and their complications. The data on the functional state of the brain signaling systems regulated by insulin, IGF-1, leptin, dopamine, serotonin, melanocortins and glucagon-like peptide-1, in T2DM and MS, are analyzed. The pharmacological approaches to restoration of these systems and improvement of insulin sensitivity, energy expenditure, lipid metabolism, and to prevent diabetic complications are discussed.

Phenomenon of leptin resistance in seasonal animals: the failure of leptin action in the brain

The core of the leptin resistance hypothesis promulgated several years ago to explain obesity as a result of environmental causes consists of 2 tenets: the extinction of leptin-induced intracellular signaling downstream of leptin binding to the long form of the neuronal receptor LTRb in the hypothalamus and the impedance to leptin entry imposed at the blood-brain barrier (BBB). A recent comprehensive investigation concluded that a central leptin insufficiency associated with obesity can be attributed to a decreased efficiency of BBB leptin transport and not to leptin insensitivity within the hypothalamus. Interestingly, anorectic leptin's effects are counteracted in some individuals by a natural resistance associated with hyperleptinemia, which is related to changes in hypothalamic sensitivity to leptin (eg, due to malnutrition, obesity, or seasonal variations due to day-length-dependent reproduction changes). In sheep, it has been observed that the hypothalamus is resistant to leptin in some periods, which is related to the adaptation of these animals to annual changes in energy supply and demand. However, a broad range of ambiguities exists regarding the implications that the intracellular signaling of signal transducer and activator of transcription-2/suppressor of cytokine signaling 3 (STAT2/SOCS3) imparts central leptin resistance. Furthermore, several plausible alternative possibilities have been proposed, such as compensatory functional and anatomic reorganizations in the appetite regulating network, rearrangements in the afferent hormonal feedback signaling involved in weight homeostasis, and modifications in leptin transport to the hypothalamus across the BBB. Taken together, these observations suggest that the contention that impaired intracellular signaling downstream of leptin entry into the appetite regulating network expedites environmentally induced obesity remains unsubstantiated and requires further evidence. Furthermore, pregnancy decreases hypothalamic sensitivity to leptin (or other unknown mechanisms), and lactation can also alter the appetite-suppressing central activity of leptin. The objective of this review was to offer an approach to understanding (1) how information regarding nutritional status is transmitted to and interpreted within the hypothalamus in animals, with special attention on seasonally breeding animals and (2) whether central leptin resistance and/or leptin insufficiency in the hypothalamus favors the development of obesity.

The blood-brain barrier: structure, function and therapeutic approaches to cross it

The blood-brain barrier (BBB) is constituted by a specialized vascular endothelium that interacts directly with astrocytes, neurons and pericytes. It protects the brain from the molecules of the systemic circulation but it has to be overcome for the proper treatment of brain cancer, psychiatric disorders or neurodegenerative diseases, which are dramatically increasing as the population ages. In the present work we have revised the current knowledge on the cellular structure of the BBB and the different procedures utilized currently and those proposed to cross it. Chemical modifications of the drugs, such as increasing their lipophilicity, turn them more prone to be internalized in the brain. Other mechanisms are the use of molecular tools to bind the drugs such as small immunoglobulins, liposomes or nanoparticles that will act as Trojan Horses favoring the drug delivery in brain. This fusion of the classical pharmacology with nanotechnology has opened a wide field to many different approaches with promising results to hypothesize that BBB will not be a major problem for the new generation of neuroactive drugs. The present review provides an overview of all state-of-the-art of the BBB structure and function, as well as of the classic strategies and these appeared in recent years to deliver drugs into the brain for the treatment of Central Nervous System (CNS) diseases.

Agile delivery of protein therapeutics to CNS

A variety of therapeutic proteins have shown potential to treat central nervous system (CNS) disorders. Challenge to deliver these protein molecules to the brain is well known. Proteins administered through parenteral routes are often excluded from the brain because of their poor bioavailability and the existence of the blood-brain barrier (BBB). Barriers also exist to proteins administered through non-parenteral routes that bypass the BBB. Several strategies have shown promise in delivering proteins to the brain. This review, first, describes the physiology and pathology of the BBB that underscore the rationale and needs of each strategy to be applied. Second, major classes of protein therapeutics along with some key factors that affect their delivery outcomes are presented. Third, different routes of protein administration (parenteral, central intracerebroventricular and intraparenchymal, intranasal and intrathecal) are discussed along with key barriers to CNS delivery associated with each route. Finally, current delivery strategies involving chemical modification of proteins and use of particle-based carriers are overviewed using examples from literature and our own work. Whereas most of these studies are in the early stage, some provide proof of mechanism of increased protein delivery to the brain in relevant models of CNS diseases, while in few cases proof of concept had been attained in clinical studies. This review will be useful to broad audience of students, academicians and industry professionals who consider critical issues of protein delivery to the brain and aim developing and studying effective brain delivery systems for protein therapeutics.

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.

Leptin-based adjuvants: an innovative approach to improve vaccine response

Leptin is a pleiotropic hormone with multiple direct and regulatory immune functions. Leptin deficiency or resistance hinders the immunologic, metabolic, and neuroendocrinologic processes necessary to thwart infections and their associated complications, and to possibly protect against infectious diseases following vaccination. Circulating leptin levels are proportional to body fat mass. High circulating leptin concentrations, as observed in obesity, are indicative of the development of leptin transport saturation/signaling desensitization. Leptin bridges nutritional status and immunity. Although its role in vaccine response is currently unknown, over-nutrition has been shown to suppress vaccine-induced immune responses. For instance, obesity (BMI ≥30 kg/m(2)) is associated with lower antigen-specific antibody titers following influenza, hepatitis B, and tetanus vaccinations. This suggests that obesity, and possibly saturable leptin levels, are contributing factors to poor vaccine immunogenicity. While leptin-based therapies have not been investigated as vaccine adjuvants thus far, leptin's role in immunity suggests that application of these therapies is promising and worth investigation to enhance vaccine response in people with leptin signaling impairments. This review will examine the possibility of using leptin as a vaccine adjuvant by: briefly reviewing the distribution and signal transduction of leptin and its receptors; discussing the physiology of leptin with emphasis on its immune functions; reviewing the causes of attenuation of leptin signaling; and finally, providing plausible inferences for the innovative use of leptin-based pharmacotherapies as vaccine adjuvants.

High fat diet intake during pre and periadolescence impairs learning of a conditioned place preference in adulthood

BACKGROUND

Brain regions that mediate learning of a conditioned place preference (CPP) undergo significant development in pre and periadolescence. Consuming a high fat (HF) diet during this developmental period and into adulthood can lead to learning impairments in rodents. The present study tested whether HF diet intake, consumed only in pre and periadolescence, would be sufficient to cause impairments using a CPP procedure.

METHODS

Rats were randomly assigned to consume a HF or a low fat (LF) diet during postnatal days (PD) 21-40 and were then placed back on a standard lab chow diet. A 20-day CPP procedure, using HF Cheetos® as the unconditioned stimulus (US), began either the next day (PD 41) or 40 days later (PD 81). A separate group of adult rats were given the HF diet for 20 days beginning on PD 61, and then immediately underwent the 20-day CPP procedure beginning on PD 81.

RESULTS

Pre and periadolescent exposure to a LF diet or adult exposure to a HF diet did not interfere with the development of a HF food-induced CPP, as these groups exhibited robust preferences for the HF Cheetos® food-paired compartment. However, pre and periadolescent exposure to the HF diet impaired the development of a HF food-induced CPP regardless of whether it was assessed immediately or 40 days after the exposure to the HF diet, and despite showing increased consumption of the HF Cheetos® in conditioning.

CONCLUSIONS

Intake of a HF diet, consumed only in pre and periadolescence, has long-lasting effects on learning that persist into adulthood.

Leptin-based glycopeptide induces weight loss and simultaneously restores fertility in animal models

AIM

To design, manufacture and test a second generation leptin receptor (ObR) agonist glycopeptide derivative. The major drawback to current experimental therapies involving leptin protein is the appearance of treatment resistance. Our novel peptidomimetic was tested for efficacy and lack of resistance induction in rodent models of obesity and appetite reduction.

METHODS

The glycopeptide containing two additional non-proteinogenic amino acids was synthesized by standard solid-phase methods. Normal mice were fed with peanuts until their blood laboratory data and liver histology showed typical signs of obesity but not diabetes. The mice were treated with the peptidomimetic at 0.02, 0.1 or 0.5 mg/kg/day intraperitoneally side-by-side with 0.1 mg/kg/day leptin for 11 days. After termination of the assay, the blood cholesterol and glucose amounts were measured, the liver fat content was visualized and quantified and the remaining mice returned to normal diet and were allowed to mate. In parallel experiments normal rats were treated intranasally with the glycopeptide at 0.1 mg/kg/day for 10 days.

RESULTS

The 12-residue glycosylated leptin-based peptidomimetic E1/6-amino-hexanoic acid (Aca) was designed to target a principal leptin/ObR-binding interface. E1/Aca induced leptin effects in ObR-positive cell lines at picomolar concentrations and readily crossed the blood-brain barrier (BBB) following intraperitoneal administration. The peptide initiated typical leptin-dependent signal transduction pathways both in the presence and absence of leptin protein. The peptide also reduced weight gain in mice fed with high-fat peanut diet in a dose-dependent manner. Obese mice receiving peptide E1/Aca at a 0.5 mg/kg/day dose lost weight, corresponding to a net 6.5% total body weight loss, while similar mice treated with leptin protein did not. Upon cessation of the weight loss treatment, several obesity-related pathologies (i.e. abnormal metabolic profile and liver histology as well as infertility) normalized in peptide-, but not leptin-treated, mice. Peptide E1/Aca added intranasally to growing normal rats decelerated normal weight gain corresponding to a net 6.8% net total body weight loss with statistical significance.

CONCLUSIONS

No resistance induction to peptide E1/Aca or toxicity in either obese or healthy rodents was observed, indicating the potential for widespread utility of the peptidomimetic in the treatment of leptin-deficiency disorders. We provide additional proof for the hypothesis that difficulties in current leptin therapies reside at the BBB penetration stage, and we document that by either glycosylation or intranasal peptide administration we can overcome this limitation.

Decreased blood-brain leptin transfer in an ovine model of obesity and weight loss: resolving the cause of leptin resistance

OBJECTIVE

Hypothalamic resistance to the anorexigenic actions of the peripheral adipostat hormone leptin is characteristic of obesity. Here, we use an obese animal model of similar body weight to that of the human to test in vivo whether leptin resistance is due to decreased blood-brain leptin transport or intra-hypothalamic insensitivity, and whether sensitivity to leptin is restored by weight loss. For 40 weeks, adult sheep surgically prepared with intra-cerebroventricular (ICV) cannulae were given a complete natural diet ad libitum ('Obese' group) or in restricted quantities ('Lean' group), and then the dietary amounts were reversed for 16 weeks until mean group body weights converged ('Slimmers' and 'Fatteners', respectively).

RESULTS

ICV leptin injection (0.5 mg) at 8-week intervals acutely decreased voluntary food intake by approximately 35% in the 'Obese' group on each occasion and in 'Slimmers' and 'Fatteners' at the end, providing no evidence of intra-hypothalamic insensitivity. The ratio between endogenous leptin concentrations in ventricular cerebrospinal fluid (CSF) and peripheral blood decreased with increasing leptinaemia in 'Obese' sheep, indicating decreased efficiency of blood-brain leptin transport, whereas leptin concentrations remained low and the CSF:blood ratio remained high in 'Lean' sheep. Compared with 'Fatteners' of similar body weight, 'Slimmers' were hypoleptinaemic, but their CSF:blood leptin concentration ratio remained low. Thus, the obesity-induced impairment of leptin blood-brain transport was sustained despite an approximately 15% weight loss.

CONCLUSION

These results support the hypothesis that central resistance to leptin in obesity with associated peripheral hyperleptinaemia is attributable to decreased efficiency of leptin transport into the brain and not to intra-hypothalamic leptin insensitivity. However, leptin transport efficiency is not restored after weight loss by caloric restriction despite the prevailing hypoleptinaemia.