Mapping of binding site III in the leptin receptor and modeling of a hexameric leptin.leptin receptor complex

Rare Antagonistic Leptin Variants and Severe, Early-Onset Obesity

Hormone absence or inactivity is common in congenital disease, but hormone antagonism remains controversial. Here, we characterize two novel homozygous leptin variants that yielded antagonistic proteins in two unrelated children with intense hyperphagia, severe obesity, and high circulating levels of leptin. Both variants bind to the leptin receptor but trigger marginal, if any, signaling. In the presence of nonvariant leptin, the variants act as competitive antagonists. Thus, treatment with recombinant leptin was initiated at high doses, which were gradually lowered. Both patients eventually attained near-normal weight. Antidrug antibodies developed in the patients, although they had no apparent effect on efficacy. No severe adverse events were observed. (Funded by the German Research Foundation and others.).

A Small Contribution to a Large System: The Leptin Receptor Complex

Obesity is a classified epidemic, increasing the risk of secondary diseases such as diabetes, inflammation, cardiovascular disease, and cancer. The pleiotropic hormone leptin is the proposed link for the gut-brain axis controlling nutritional status and energy expenditure. Research into leptin signaling provides great promise toward discovering therapeutics for obesity and its related diseases targeting leptin and its cognate leptin receptor (LEP-R). The molecular basis underlying the human leptin receptor complex assembly remains obscure, due to the lack of structural information regarding the biologically active complex. In this work, we investigate the proposed receptor binding sites in human leptin utilizing designed antagonist proteins combined with AlphaFold predictions. Our results show that binding site I has a more intricate role in the active signaling complex than previously described. We hypothesize that the hydrophobic patch in this region engages a third receptor forming a higher-order complex, or a new LEP-R binding site inducing allosteric rearrangement.

Mechanism of receptor assembly via the pleiotropic adipokine Leptin

The adipokine Leptin activates its receptor LEP-R in the hypothalamus to regulate body weight and exerts additional pleiotropic functions in immunity, fertility and cancer. However, the structure and mechanism of Leptin-mediated LEP-R assemblies has remained unclear. Intriguingly, the signaling-competent isoform of LEP-R is only lowly abundant amid several inactive short LEP-R isoforms contributing to a mechanistic conundrum. Here we show by X-ray crystallography and cryo-EM that, in contrast to long-standing paradigms, Leptin induces type I cytokine receptor assemblies featuring 3:3 stoichiometry and demonstrate such Leptin-induced trimerization of LEP-R on living cells via single-molecule microscopy. In mediating these assemblies, Leptin undergoes drastic restructuring that activates its site III for binding to the Ig domain of an adjacent LEP-R. These interactions are abolished by mutations linked to obesity. Collectively, our study provides the structural and mechanistic framework for how evolutionarily conserved Leptin:LEP-R assemblies with 3:3 stoichiometry can engage distinct LEP-R isoforms to achieve signaling.

Recent Advances in the Knowledge of the Mechanisms of Leptin Physiology and Actions in Neurological and Metabolic Pathologies

Excess body weight is frequently associated with low-grade inflammation. Evidence indicates a relationship between obesity and cancer, as well as with other diseases, such as diabetes and non-alcoholic fatty liver disease, in which inflammation and the actions of various adipokines play a role in the pathological mechanisms involved in these disorders. Leptin is mainly produced by adipose tissue in proportion to fat stores, but it is also synthesized in other organs, where leptin receptors are expressed. This hormone performs numerous actions in the brain, mainly related to the control of energy homeostasis. It is also involved in neurogenesis and neuroprotection, and central leptin resistance is related to some neurological disorders, e.g., Parkinson's and Alzheimer's diseases. In peripheral tissues, leptin is implicated in the regulation of metabolism, as well as of bone density and muscle mass. All these actions can be affected by changes in leptin levels and the mechanisms associated with resistance to this hormone. This review will present recent advances in the molecular mechanisms of leptin action and their underlying roles in pathological situations, which may be of interest for revealing new approaches for the treatment of diseases where the actions of this adipokine might be compromised.

Atomistic mechanism of leptin and leptin-receptor association

The leptin-leptin receptor complex is at the very core of energy homeostasis and immune system regulation, among many other functions. In this work, we built homology models of leptin and the leptin binding domain (LBD) of the receptor from humans and mice. Docking analyses were used to obtain the coordinates of the native leptin-LBD complexes and a mixed heterodimer formed by human leptin and mouse LBD. Molecular dynamics (MD) simulations were performed using all models (monomers and heterodimers) as initial coordinates and the GROMACS program. The overall structural and dynamical behaviors are similar for the three complexes. Upon MD simulations, several new interactions appear. In particular, hydrophobic interactions, with more than 90% persistence, seem to be the most relevant for the stability of the dimers, as well as the pair formed by Asp85_Lep and Arg468_LBD. This in silico analysis provides structural and dynamical information, at the atomistic level, about the mechanism of leptin-LBD complex formation and leptin receptor activation. This knowledge might be used in the rational drug design of therapeutics to modulate leptin signaling.Communicated by Ramaswamy H. Sarma.

Leptin-Activity Modulators and Their Potential Pharmaceutical Applications

Leptin, a multifunctional hormone primarily, but not exclusively, secreted in adipose tissue, is implicated in a wide range of biological functions that control different processes, such as the regulation of body weight and energy expenditure, reproductive function, immune response, and bone metabolism. In addition, leptin can exert angiogenic and mitogenic actions in peripheral organs. Leptin biological activities are greatly related to its interaction with the leptin receptor. Both leptin excess and leptin deficiency, as well as leptin resistance, are correlated with different human pathologies, such as autoimmune diseases and cancers, making leptin and leptin receptor important drug targets. The development of leptin signaling modulators represents a promising strategy for the treatment of cancers and other leptin-related diseases. In the present manuscript, we provide an update review about leptin-activity modulators, comprising leptin mutants, peptide-based leptin modulators, as well as leptin and leptin receptor specific monoclonal antibodies and nanobodies.

Leptin Receptor Compound Heterozygosity in Humans and Animal Models

Leptin and its receptor are essential for regulating food intake, energy expenditure, glucose homeostasis and fertility. Mutations within leptin or the leptin receptor cause early-onset obesity and hyperphagia, as described in human and animal models. The effect of both heterozygous and homozygous variants is much more investigated than compound heterozygous ones. Recently, we discovered a spontaneous compound heterozygous mutation within the leptin receptor, resulting in a considerably more obese phenotype than described for the homozygous leptin receptor deficient mice. Accordingly, we focus on compound heterozygous mutations of the leptin receptor and their effects on health, as well as possible therapy options in human and animal models in this review.

Leptin signalling in teleost fish with emphasis in food intake regulation

Leptin, the product of the obese (ob or Lep) gene, was first cloned in teleost fish in 2005, more than a decade after its identification in mammals. This was because bony fish and mammalian leptins share a very low amino acid sequence identity, which suggests different functionality of the leptin system in fish compared to that of mammals. Indeed, major differences are evident between the mammalian and fish leptin system. Thus, for instance, mammalian leptin is synthesized and released by the adipose tissue in response to the amount of fat depots, while several tissues (mainly the liver) are the main sources of leptin in fish, whose determining factors of production are still unclear. In mammals, the main physiological role for leptin is its involvement in the maintenance of energy balance by decreasing food intake and increasing energy expenditure, although a wide variety of actions have been attributed to this hormone (e.g., regulation of lipid and carbohydrate metabolism, reproduction and immune functions). In fish, available literature also points towards a multifunctional nature for leptin, although knowledge on its functions is limited. In this review, we offer an overview of teleostean leptin structure and mechanism of action, and discuss the available knowledge on the role of this hormone in food intake regulation in teleost fish, aiming to provide a comparative overview between the functioning of the teleostean and mammalian leptin systems.

Pegylated Human Leptin D23L Mutant-Preparation and Biological Activity In Vitro and In Vivo in Male ob/ob Mice

Recombinant monomeric human leptin (hLEP) and its D23L mutant were prepared in Escherichia coli and pegylated at their N-terminus using 20-kDa methoxy pegylated (PEG)-propionylaldehyde. As determined by both SDS-PAGE and size-exclusion chromatography, the pegylated proteins consisted of >90% monopegylated and <10% double-pegylated species. Circular dichroism spectra showed that their secondary structure, characteristic of all four α-helix bundle cytokines, was not affected by either the D23L mutation or pegylation. Because of the D23L mutation, affinity for hLEP receptor increased 25- and 40-fold for the pegylated and nonpegylated mutant, respectively. However, whereas the proliferation-promoting activity in vitro of nonmutated and mutated nonpegylated hLEP was identical, that of the respective pegylated mutant was approximately sixfold higher compared with the pegylated nonmutated hLEP. This difference was also seen in vivo. Both pegylated hLEPs at all doses significantly decreased body weight and food consumption, as compared with the vehicle-treated control. Once-daily administration of pegylated hLEP D23L at doses of 0.1, 0.3, and 1 mg/kg for 14 consecutive days in ob/ob mice resulted in significantly decreased body weight and food consumption as compared with respective pegylated hLEP-treated animals, with the biggest difference observed at 0.1 mg/kg. Repeated administration of either pegylated hLEP D23L or pegylated hLEP significantly decreased blood glucose levels compared with the control before glucose challenge and after oral glucose tolerance test, but with no difference between the two treatments. The pegylated hLEP D23L mutant seems to be a more potent reagent suitable for in vivo studies than the pegylated nonmutated hLEP.

Functional and Phenotypic Characteristics of Human Leptin Receptor Mutations

Several case series of extreme early-onset obesity due to mutations in the human leptin receptor (LEPR) gene have been reported. In this review we summarize published functional and phenotypic data on mutations in the human LEPR gene causing severe early-onset obesity. Additionally, we included data on six new cases from our obesity center. Literature research was performed using PubMed and OMIM. Functional relevance of mutations was estimated based on reported functional analysis, mutation size, and location, as well as phenotypic characteristics of affected patients. We identified 57 cases with 38 distinct LEPR mutations. We found severe early-onset obesity, hyperphagia, and hypogonadotropic hypogonadism as cardinal features of a complete loss of LEPR function. Other features, for example, metabolic disorders and recurring infections, were variable in manifestation. Obesity degree or other manifestations did not aggregate by genotype. Few patients underwent bariatric surgery with variable success. Most mutations occurred in the fibronectin III and cytokine receptor homology II domains, whereas none was found in cytoplasmic domain. In silico data were available for 25 mutations and in vitro data were available for four mutations, revealing residual activity in one case. By assessing provided information on the clinical phenotype, functional analysis, and character of the 38 mutations, we assume residual LEPR activity for five additional mutations. Functional in vitro analysis is necessary to confirm this assumption.

Leptin and Leptin receptor polymorphisms, plasma Leptin levels and obesity in Tunisian volunteers

Adipose tissue is an important endocrine organ that secretes a number of adipokines, like Leptin (LEP). The aim this study was to investigate the prevalence of single nucleotide polymorphisms in LEP gene (LEP 3'UTR A/C, -2548 G/A) and LEPR (K109R and Q223R) and their association with Leptin level and obesity. We recruited 169 non-obese (body mass index [BMI] = 24.51-3.69 kg/m² ) and 160 obese (BMI = 36-4.78 kg/m² ) patients. Genotyping was performed using polymerase chain reaction-restriction fragment length polymorphism, BMI was calculated, and Leptin level was measured by ELISA. Statistical analyses were performed by spss19.0. According to LEP 3'UTR A/C polymorphism, AC and CC genotype carriers had higher Leptin levels than AA genotype carriers, respectively, 31[0.05-148.8] (P = .008) vs 41[0.05-111.6] (P = .003). The K109R polymorphism was associated with obesity (P = .025) and seems to significantly decrease the LEP levels (P < .001). Concerning LEP G2548A polymorphism, our results showed that the OR of obesity associated with 2548 AA/GG was 1.87[1.106-2.78] P = .028 vs 1.41[1.035-1.85] P = .045 for 223AA/GG polymorphism. In our haplotype analysis, one haplotype seems to be the more protective and one other seems to be the highest risk to obesity. LEP 3'UTR A/C and LEPR K109R polymorphisms were associated with Leptin level and obesity.

In silico design of small peptides antagonist against leptin receptor for the treatment of obesity and its associated immune-mediated diseases

Excess adiposity in obese inhibits negatively impacts immune function and host defence. Obesity is characterized by a state of low-grade, chronic inflammation in addition to disturbed levels of circulating nutrients and metabolic hormones. The impact of metabolic abnormalities on obesity-related co-morbidities has undergone intense scrutiny over the past decades. Thus, treatment of obesity and its associated immune-mediated diseases is challenging due to impaired function of leptin system. These disorders are managed through antibiotics and by cytokines replacement. However, the effectiveness of cytokines coupled to the complexity of the cytokine network leads to severe side-effects, which can still occur after careful preclinical evaluation. In addition, synthetic immunotherapeutics carry a degree of risk, time-consuming and expensive. Hence, the complexity of existing therapy and adverse effects emphasizes the need for an alternative approach for the management of immune dysfunction associated with obesity. Computer-aided small molecule antibody technology has been successful in the design of novel biologicals for the diagnosis of diseases and therapeutic interventions. In this study, the crystal structure of leptin receptor (LEPR) complex with monoclonal antibody (9F8 Fab) was explored to predict Ag-Ab interactions using bioinformatics tools. The LEPR of complementarity-determining region (CDR) loops were mutated with published positive control residues of Ser, Thr, Tyr, Trp, and Phe to design a set of 678 peptides which were evaluated through Ag-peptide docking, binding free-energies, and interaction energies. Thus, hypothesized novel peptides can be explored as clinically applicable antagonists for the treatment of obesity and associated immune-mediated diseases.

Targeting the leptin receptor: To evaluate therapeutic efficacy and anti-tumor effects of Doxil, in vitro and in vivo in mice bearing C26 colon carcinoma tumor

Leptin is an appetite regulatory hormone that is secreted into the blood circulation by the adipose tissue and it functions via its over expressed receptors (Ob-R) in a wide variety of cancers. In the present study, the function of a leptin-derived peptide (LP16, 91-110 of Leptin) was investigated as a targeting ligand to decorate PEGylated liposomal doxorubicin (PLD, Doxil^®) surface and the anti-tumor activity and therapeutic efficacy of Doxil in C26 (Colon Carcinoma) tumor model were also evaluated. As a result of this, Doxil with different LP16 peptide density (25, 50, 100 and 200 peptide on the surface of each liposome) was successfully prepared and characterized. In vitro results showed significant enhanced cytotoxicity and cellular binding and uptake of LP16-targeted Doxil formulations (LP16-Doxil) in C26 cells as compared to Doxil. In BALB/c mice bearing C26 murine carcinoma, at a dose of 15 mg/kg, LP16-Doxil groups (100 ligand) significantly suppressed the growth of the tumor and showed higher inclination to tumor as compared to non-targeted Doxil. This study revealed that the potential of LP16 peptide targeting increased the therapeutic efficacy of Doxil and highlighted the importance of optimizing the ligand density to maximize the targeting ability of the nanocarriers and merits further investigations.

Estimated prevalence of potentially damaging variants in the leptin gene

Background

Mutations in the leptin gene (LEP) can alter the secretion or interaction of leptin with its receptor, leading to extreme early-onset obesity. The purpose of this work was to estimate the prevalence of heterozygous and homozygous mutations in the leptin gene with the help of the Exome Aggregation Consortium (ExAC) database (http://exac.broadinstitute.org/about).

Results

The ExAC database encompasses exome sequencing data from 60,706 individuals. We searched for listed leptin variants and identified 36 missense, 1 in-frame deletion, and 3 loss-of-function variants. The functional relevance of these variants was assessed by the in silico prediction tools PolyPhen-2, Sorting Intolerant from Tolerant (SIFT), and Loss-Of-Function Transcript Effect Estimator (LOFTEE). PolyPhen-2 predicted 7 of the missense variants to be probably damaging and 10 to be possibly damaging. SIFT predicted 7 of the missense variants to be deleterious. Three loss-of-function variants were predicted by LOFTEE. Excluding double counts, we can summarize 21 variants as potentially damaging. Considering the allele count, we identified 31 heterozygous but no homozygous subjects with at least probably damaging variants. In the ExAC population, the estimated prevalence of heterozygous carriers of these potentially damaging variants was 1:2000. The probability of homozygosity was 1:15,000,000.

We furthermore tried to assess the functionality of ExAC-listed leptin variants by applying a knowledge-driven approach. By this approach, additional 6 of the ExAC-listed variants were considered potentially damaging, increasing the number of heterozygous subjects to 58, the prevalence of heterozygosity to 1:1050, and the probability of homozygosity to 1:4,400,000.

Conclusion

Using exome sequencing data from ExAC, in silico prediction tools and by applying a knowledge-driven approach, we identified 27 probably damaging variants in the leptin gene of 58 heterozygous subjects. With this information, we estimate the prevalence for heterozygosity at 1:1050 corresponding to an prevalence of homozygosity of 1:4,400,000 in this large pluriethnic cohort.

Electronic supplementary material

The online version of this article (10.1186/s40348-017-0074-x) contains supplementary material, which is available to authorized users.

The Leptin Receptor Complex: Heavier Than Expected?

Under normal physiological conditions, leptin and the leptin receptor (ObR) regulate the body weight by balancing food intake and energy expenditure. However, this adipocyte-derived hormone also directs peripheral processes, including immunity, reproduction, and bone metabolism. Leptin, therefore, can act as a metabolic switch connecting the body's nutritional status to high energy consuming processes. We provide an extensive overview of current structural insights on the leptin-ObR interface and ObR activation, coupling to signaling pathways and their negative regulation, and leptin functioning under normal and pathophysiological conditions (obesity, autoimmunity, cancer, … ). We also discuss possible cross-talk with other receptor systems on the receptor (extracellular) and signaling cascade (intracellular) levels.

Leptin and leptin-receptor polymorphisms in fertile and infertile men

The association of leptin (LEP) -2548G/A and/or leptin receptor (LEPR) Gln223Arg polymorphisms with male infertility and plasma FSH, LH, and testosterone (T) levels was examined. The genotypes and allele frequency distributions of LEP -2548G/A and LEPR Gln223Arg polymorphisms were investigated in 150 fertile and 150 infertile men by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). Also, plasma levels of FSH, LH, and T were measured using commercial ELISA kits. Frequencies of AA, AG and GG genotypes of LEP-2548G/A polymorphism were statistically different in fertile and infertile men (p=0.012). The AG genotype showed a protective effect which could decrease risk of male infertility about 3 fold (p = 0.004). We did not observe any differences in frequencies of LEPR Gln223Arg alleles and genotypes between groups (p > 0.05). Sperm counts from infertile men with the AG and GG genotypes of the LEP polymorphism were significantly higher than AA genotype (p<0.05). Moreover, infertile men who carried the RR genotype of LEPR showed a statistically higher percentage of sperm with progressive motility than individuals with other genotypes (p = 0.004). There was no correlation between different combinations of LEP and LEPR genotypes and LH, FSH, and T levels (p > 0.05). Our study suggests that the LEP -2548G/A polymorphism may play a role in male fertility and the AG genotype may have a protective effect through increasing sperm counts. The distribution of genotypes of LEP -2548G/A polymorphism are different in fertile and infertile males and may be a useful tool in evaluation of male infertility.

ABBREVIATIONS

LEP: leptin; LEPR: leptin receptor; T: testosterone; FSH: follicle-stimulating hormone; LH: luteinizing hormone.

Regulation of intestinal SGLT1 by catestatin in hyperleptinemic type 2 diabetic mice

The small intestine is the major site for nutrient absorption that is critical in maintenance of euglycemia. Leptin, a key hormone involved in energy homeostasis, directly affects nutrient transport across the intestinal epithelium. Catestatin (CST), a 21-amino acid peptide derived from proprotein chromogranin A, has been shown to modulate leptin signaling. Therefore, we reasoned that leptin and CST could modulate intestinal Na(+)-glucose transporter 1 (SGLT1) expression in the context of obesity and diabetes. We found that hyperleptinemic db/db mice exhibit increased mucosal mass, associated with an enhanced proliferative response and decreased apoptosis in intestinal crypts, a finding absent in leptin-deficient ob/ob mice. Intestinal SGLT1 abundance was significantly decreased in hyperleptinemic but not leptin-deficient mice, indicating leptin regulation of SGLT1 expression. Phlorizin, a SGLT1/2 inhibitor, was without effect in an oral glucose tolerance test in db/db mice. The alterations in architecture and SGLT1 abundance were not accompanied by changes in the localization of intestinal alkaline phosphatase, indicating intact differentiation. Treatment of db/db mice with CST restored intestinal SGLT1 abundance and intestinal turnover, suggesting a cross-talk between leptin and CST, without affecting plasma leptin levels. Consistent with this hypothesis, we identified structural homology between CST and the AB-loop of leptin and protein-protein docking revealed binding of CST and leptin with the Ig-like binding site-III of the leptin receptor. In summary, downregulation of SGLT1 in an obese type 2 diabetic mouse model with hyperleptinemia is presumably mediated via the short form of the leptin receptor and reduces overt hyperglycemia.

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.

PASylation of Murine Leptin Leads to Extended Plasma Half-Life and Enhanced in Vivo Efficacy

Leptin plays a central role in the control of energy homeostasis and appetite and, thus, has attracted attention for therapeutic approaches in spite of its limited pharmacological activity owing to the very short circulation in the body. To improve drug delivery and prolong plasma half-life, we have fused murine leptin with Pro/Ala/Ser (PAS) polypeptides of up to 600 residues, which adopt random coil conformation with expanded hydrodynamic volume in solution and, consequently, retard kidney filtration in a similar manner as polyethylene glycol (PEG). Relative to unmodified leptin, size exclusion chromatography and dynamic light scattering revealed an approximately 21-fold increase in apparent size and a much larger molecular diameter of around 18 nm for PAS(600)-leptin. High receptor-binding activity for all PASylated leptin versions was confirmed in BIAcore measurements and cell-based dual-luciferase assays. Pharmacokinetic studies in mice revealed a much extended plasma half-life after ip injection, from 26 min for the unmodified leptin to 19.6 h for the PAS(600) fusion. In vivo activity was investigated after single ip injection of equimolar doses of each leptin version. Strongly increased and prolonged hypothalamic STAT3 phosphorylation was detected for PAS(600)-leptin. Also, a reduction in daily food intake by up to 60% as well as loss in body weight of >10% lasting for >5 days was observed, whereas unmodified leptin was merely effective for 1 day. Notably, application of a PASylated superactive mouse leptin antagonist (SMLA) led to the opposite effects. Thus, PASylated leptin not only provides a promising reagent to study its physiological role in vivo but also may offer a superior drug candidate for clinical therapy.

A novel leptin antagonist peptide inhibits breast cancer growth in vitro and in vivo

The role of the obesity cytokine leptin in breast cancer progression has raised interest in interfering with leptin's actions as a valuable therapeutic strategy. Leptin interacts with its receptor through three different binding sites: I–III. Site I is crucial for the formation of an active leptin–leptin receptor complex and in its subsequent activation. Amino acids 39-42 (Leu-Asp-Phe-Ile- LDFI) were shown to contribute to leptin binding site I and their mutations in alanine resulted in muteins acting as typical antagonists. We synthesized a small peptide based on the wild-type sequence of leptin binding site I (LDFI) and evaluated its efficacy in antagonizing leptin actions in breast cancer using in vitro and in vivo experimental models. The peptide LDFI abolished the leptin-induced anchorage-dependent and -independent growth as well as the migration of ERα-positive (MCF-7) and -negative (SKBR3) breast cancer cells. These results were well correlated with a reduction in the phosphorylation levels of leptin downstream effectors, as JAK2/STAT3/AKT/MAPK. Importantly, the peptide LDFI reversed the leptin-mediated up-regulation of its gene expression, as an additional mechanism able to enhance the peptide antagonistic activity. The described effects were specific for leptin signalling, since the developed peptide was not able to antagonize the other growth factors' actions on signalling activation, proliferation and migration. Finally, we showed that the LDFI pegylated peptide markedly reduced breast tumour growth in xenograft models. The unmodified peptide LDFI acting as a full leptin antagonist could become an attractive option for breast cancer treatment, especially in obese women.

Functional human antibody CDR fusions as long-acting therapeutic endocrine agonists

On the basis of the 3D structure of a bovine antibody with a well-folded, ultralong complementarity-determining region (CDR), we have developed a versatile approach for generating human or humanized antibody agonists with excellent pharmacological properties. Using human growth hormone (hGH) and human leptin (hLeptin) as model proteins, we have demonstrated that functional human antibody CDR fusions can be efficiently engineered by grafting the native hormones into different CDRs of the humanized antibody Herceptin. The resulting Herceptin CDR fusion proteins were expressed in good yields in mammalian cells and retain comparable in vitro biological activity to the native hormones. Pharmacological studies in rodents indicated a 20- to 100-fold increase in plasma circulating half-life for these antibody agonists and significantly extended in vivo activities in the GH-deficient rat model and leptin-deficient obese mouse model for the hGH and hLeptin antibody fusions, respectively. These results illustrate the utility of antibody CDR fusions as a general and versatile strategy for generating long-acting protein therapeutics.

20 years of leptin: insights into signaling assemblies of the leptin receptor

Leptin plays a central role in the control of body weight and energy homeostasis, but is a pleiotropic cytokine with activities on many peripheral cell types. In this review, we discuss the interaction of leptin with its receptor, and focus on the structural and mechanistic aspects of the extracellular aspects of leptin receptor (LR) activation. We provide an extensive overview of all structural information that has been obtained for leptin and its receptor via X-ray crystallography, electron microscopy, small-angle X-ray scattering, homology modeling, and mutagenesis studies. The available knowledge is integrated into putative models toward a recapitulation of the LR activation mechanism.

Hypomorphism for RPGRIP1L, a ciliary gene vicinal to the FTO locus, causes increased adiposity in mice

Common polymorphisms in the first intron of FTO are associated with increased body weight in adults. Previous studies have suggested that a CUX1-regulatory element within the implicated FTO region controls expression of FTO and the nearby ciliary gene, RPGRIP1L. Given the role of ciliary genes in energy homeostasis, we hypothesized that mice hypomorphic for Rpgrip1l would display increased adiposity. We find that Rpgrip1l⁺/⁻ mice are hyperphagic and fatter, and display diminished suppression of food intake in response to leptin administration. In the hypothalamus of Rpgrip1l⁺/⁻ mice, and in human fibroblasts with hypomorphic mutations in RPGRIP1L, the number of AcIII-positive cilia is diminished, accompanied by impaired convening of the leptin receptor to the vicinity of the cilium, and diminished pStat3 in response to leptin. These findings suggest that RPGRIP1L may be partly or exclusively responsible for the obesity susceptibility signal at the FTO locus.

Kinetics of leptin binding to the Q223R leptin receptor

Studies in human populations and mouse models of disease have linked the common leptin receptor Q223R mutation to obesity, multiple forms of cancer, adverse drug reactions, and susceptibility to enteric and respiratory infections. Contradictory results cast doubt on the phenotypic consequences of this variant. We set out to determine whether the Q223R substitution affects leptin binding kinetics using surface plasmon resonance (SPR), a technique that allows sensitive real-time monitoring of protein-protein interactions. We measured the binding and dissociation rate constants for leptin to the extracellular domain of WT and Q223R murine leptin receptors expressed as Fc-fusion proteins and found that the mutant receptor does not significantly differ in kinetics of leptin binding from the WT leptin receptor. (WT: k_a 1.76×10⁶±0.193×10⁶ M⁻¹ s⁻¹, k_d 1.21×10⁻⁴±0.707×10⁻⁴ s⁻¹, K_D 6.47×10⁻¹¹±3.30×10⁻¹¹ M; Q223R: k_a 1.75×10⁶±0.0245×10⁶ M⁻¹ s⁻¹, k_d 1.47×10⁻⁴±0.0505×10⁻⁴ s⁻¹, K_D 8.43×10⁻¹¹±0.407×10⁻¹¹ M). Our results support earlier findings that differences in affinity and kinetics of leptin binding are unlikely to explain mechanistically the phenotypes that have been linked to this common genetic variant. Future studies will seek to elucidate the mechanism by which this mutation influences susceptibility to metabolic, infectious, and malignant pathologies.

Leptin receptor somatic mutations are frequent in HCV-infected cirrhotic liver and associated with hepatocellular carcinoma

BACKGROUND & AIMS

Hepatocellular carcinoma develops in patients with chronic hepatitis or cirrhosis via a stepwise accumulation of various genetic alterations. To explore the genetic basis of development of hepatocellular carcinoma in hepatitis C virus (HCV)-associated chronic liver disease, we evaluated genetic variants that accumulate in nontumor cirrhotic liver.

METHODS

We determined the whole exome sequences of 7 tumors and background cirrhotic liver tissues from 4 patients with HCV infection. We then performed additional sequencing of selected exomes of mutated genes, identified by whole exome sequencing, and of representative tumor-related genes on samples from 22 cirrhotic livers with HCV infection. We performed in vitro and in vivo functional studies for one of the mutated genes.

RESULTS

Whole exome sequencing showed that somatic mutations accumulated in various genes in HCV-infected cirrhotic liver tissues. Among the identified genes, the leptin receptor gene (LEPR) was one of the most frequently mutated in tumor and nontumor cirrhotic liver tissue. Selected exome sequencing analyses detected LEPR mutations in 12 of 22 (54.5%) nontumorous cirrhotic livers. In vitro, 4 of 7 (57.1%) LEPR mutations found in cirrhotic livers reduced phosphorylation of STAT3 to inactivate LEPR-mediated signaling. Moreover, 40% of Lepr-deficient (C57BL/KsJ-db/db) mice developed liver tumors after administration of thioacetamide compared with none of the control mice.

CONCLUSIONS

Based on analysis of liver tissue samples from patients, somatic mutations accumulate in LEPR in cirrhotic liver with chronic HCV infection. These mutations could disrupt LEPR signaling and increase susceptibility to hepatocarcinogenesis.

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.

Design and validation of a homogeneous time-resolved fluorescence-based leptin receptor binding assay

The pleiotropic cytokine hormone leptin, by activating its receptor OB-R, plays a major role in many biological processes, including energy homeostasis, immune function, and cell survival and proliferation. Abnormal leptin action is associated with obesity, autoimmune diseases, and cancer. The pharmacological characterization of OB-R and the development of synthetic OB-R ligands are still in their infancy because currently available binding assays are not compatible with ligand saturation binding experiments and high-throughput screening (HTS) approaches. We have developed here a novel homogeneous time-resolved fluorescence-based binding assay that overcomes these limitations. In this assay, fluorescently labeled leptin or leptin antagonist binds to the SNAP-tagged OB-R covalently labeled with terbium cryptate (Tb). Successful binding is monitored by measuring the energy transfer between the Tb energy donor and the fluorescently labeled leptin energy acceptor. Ligand binding saturation experiments revealed high-affinity dissociation constants in the subnanomolar range with an excellent signal-to-noise ratio. The assay performed in a 384-well format shows high specificity and reproducibility, making it perfectly compatible with HTS applications to identify new OB-R agonists or antagonists. In addition, fluorescently labeled leptin and SNAP-tagged OB-R will be valuable tools for monitoring leptin and OB-R trafficking in cells and tissues.

Leptin and leptin receptor: analysis of a structure to function relationship in interaction and evolution from humans to fish

Leptin is a circulating protein which regulates dietary intake through binding the leptin receptor. Numerous labs have used known structures and mutagenesis to study this binding process in common animal models (human, mouse and rat). Understanding this binding process in other vertebrate species will allow for a better understanding of leptin and leptin receptor function. The binding site between leptin and leptin receptor is highly conserved in mammals as confirmed through sequence alignments mapped onto structures of both leptin and leptin receptor. More variation in this interaction is found in lizard and frog sequences. Using our models, we show that the avian leptin sequences have far less variation in the binding site than does the leptin receptor. This analysis further suggests that avian leptins are artifactual. In fish, gene duplication events have led to the expression of multiple leptin proteins. These multiple leptin proteins have variation in the regions interacting with leptin receptor. In zebrafish and the Japanese rice fish, we propose that leptin A has a higher binding energy than does B. Differing binding energies are evidence of either divergent functions, different binding confirmations, or other protein partners of leptin B.

Ligand-induced architecture of the leptin receptor signaling complex

Despite the crucial impact of leptin signaling on metabolism and body weight, little is known about the structure of the liganded leptin receptor (LEP-R) complex. Here, we applied single-particle electron microscopy (EM) to characterize the architecture of the extracellular region of LEP-R alone and in complex with leptin. We show that unliganded LEP-R displays significant flexibility in a hinge region within the cytokine homology region 2 (CHR2) that is connected to rigid membrane-proximal FnIII domains. Leptin binds to CHR2 in order to restrict the flexible hinge and the disposition of the FnIII "legs." Through a separate interaction, leptin engages the Ig-like domain of a second liganded LEP-R, resulting in the formation of a quaternary signaling complex. We propose that the membrane proximal domain rigidification in the context of a liganded cytokine receptor dimer is a key mechanism for the transactivation of Janus kinases (Jaks) bound at the intracellular receptor region.

Structure of the human obesity receptor leptin-binding domain reveals the mechanism of leptin antagonism by a monoclonal antibody

Leptin regulates energy homeostasis, fertility, and the immune system, making it an important drug target. However, due to a complete lack of structural data for the obesity receptor (ObR), leptin's mechanism of receptor activation remains poorly understood. We have crystallized the Fab fragment of a leptin-blocking monoclonal antibody (9F8), both in its uncomplexed state and bound to the leptin-binding domain (LBD) of human ObR. We describe the structure of the LBD-9F8 Fab complex and the conformational changes in 9F8 associated with LBD binding. A molecular model of the putative leptin-LBD complex reveals that 9F8 Fab blocks leptin binding through only a small (10%) overlap in their binding sites, and that leptin binding is likely to involve an induced fit mechanism. This crystal structure of the leptin-binding domain of the obesity receptor will facilitate the design of therapeutics to modulate leptin signaling.

Leptin protects host cells from Entamoeba histolytica cytotoxicity by a STAT3-dependent mechanism

The adipocytokine leptin links nutritional status to immune function. Leptin signaling protects from amebiasis, but the molecular mechanism is not understood. We developed an in vitro model of ameba-host cell interaction to test the hypothesis that leptin prevents ameba-induced apoptosis in host epithelial cells. We demonstrated that activation of mammalian leptin signaling increased cellular resistance to amebic cytotoxicity, including caspase-3 activation. Exogenous expression of the leptin receptor conferred resistance in susceptible cells, and leptin stimulation enhanced protection. A series of leptin receptor signaling mutants showed that resistance to amebic cytotoxicity was dependent on activation of STAT3 but not the Src homology-2 domain-containing tyrosine phosphatase (SHP-2) or STAT5. A common polymorphism in the leptin receptor (Q223R) that increases susceptibility to amebiasis in humans and mice was found to increase susceptibility to amebic cytotoxicity in single cells. The Q223R polymorphism also decreased leptin-dependent STAT3 activation by 21% relative to that of the wild-type (WT) receptor (P = 0.035), consistent with a central role of STAT3 signaling in protection. A subset of genes uniquely regulated by STAT3 in response to leptin was identified. Most notable were the TRIB1 and suppressor of cytokine signaling 3 (SOCS3) genes, which have opposing roles in the regulation of apoptosis. Overall apoptotic genes were highly enriched in this gene set (P < 1E-05), supporting the hypothesis that leptin regulation of host apoptotic genes via STAT3 is responsible for protection. This is the first demonstration of a mammalian signaling pathway that restricts amebic pathogenesis and represents an important advance in our mechanistic understanding of how leptin links nutrition and susceptibility to infection.

Selection of non-competitive leptin antagonists using a random nanobody-based approach

The adipocyte-derived cytokine leptin acts as a metabolic switch, connecting the body's metabolism to high-energy consuming processes such as reproduction and immune responses. Accumulating evidence suggests that leptin plays a role in human pathologies, such as autoimmune diseases and cancer, thus providing a rationale for the development of leptin antagonists. In the present study, we generated and evaluated a panel of neutralizing nanobodies targeting the LR (leptin receptor). A nanobody comprises the variable domain of the naturally occurring single-chain antibodies found in members of the Camelidae family. We identified three classes of neutralizing nanobodies targeting different LR subdomains: i.e. the CRH2 (cytokine receptor homology 2), Ig-like and FNIII (fibronectin type III) domains. Only nanobodies directed against the CRH2 domain inhibited leptin binding. We could show that a nanobody that targets the Ig-like domain potently interfered with leptin-dependent regulation of hypothalamic NPY (neuropeptide Y) expression. As a consequence, daily intraperitoneal injection increased body weight, body fat content, food intake, liver size and serum insulin levels. All of these characteristics resemble the phenotype of leptin and LR-deficient animals. The results of the present study support proposed models of the activated LR complex, and demonstrate that it is possible to block LR signalling without affecting ligand binding. These nanobodies form new tools to study the mechanisms of BBB (blood-brain barrier) leptin transport and the effect of LR inhibition in disease models.

Evidence for positive selection on the leptin gene in Cetacea and Pinnipedia

The leptin gene has received intensive attention and scientific investigation for its importance in energy homeostasis and reproductive regulation in mammals. Furthermore, study of the leptin gene is of crucial importance for public health, particularly for its role in obesity, as well as for other numerous physiological roles that it plays in mammals. In the present work, we report the identification of novel leptin genes in 4 species of Cetacea, and a comparison with 55 publicly available leptin sequences from mammalian genome assemblies and previous studies. Our study provides evidence for positive selection in the suborder Odontoceti (toothed whales) of the Cetacea and the family Phocidae (earless seals) of the Pinnipedia. We also detected positive selection in several leptin gene residues in these two lineages. To test whether leptin and its receptor evolved in a coordinated manner, we analyzed 24 leptin receptor gene (LPR) sequences from available mammalian genome assemblies and other published data. Unlike the case of leptin, our analyses did not find evidence of positive selection for LPR across the Cetacea and Pinnipedia lineages. In line with this, positively selected sites identified in the leptin genes of these two lineages were located outside of leptin receptor binding sites, which at least partially explains why co-evolution of leptin and its receptor was not observed in the present study. Our study provides interesting insights into current understanding of the evolution of mammalian leptin genes in response to selective pressures from life in an aquatic environment, and leads to a hypothesis that new tissue specificity or novel physiologic functions of leptin genes may have arisen in both odontocetes and phocids. Additional data from other species encompassing varying life histories and functional tests of the adaptive role of the amino acid changes identified in this study will help determine the factors that promote the adaptive evolution of the leptin genes in marine mammals.

Nonsynonymous natural genetic polymorphisms in the bovine leptin gene affect biochemical and biological characteristics of the mature hormone

Leptin (LEP) is a cytokine-like hormone proven to be involved in diverse biological processes. In livestock, it regulates feed intake, BW homeostasis, and energy balance, among other traits. Natural nonsynonymous genetic polymorphisms in the ovine leptin (oLEP) alter the biochemical and physiological characteristics of its gene products. Here we studied in vitro and in vivo the biochemical and physiological characteristics of recombinant hormones representing the oLEP and bovine leptin (bLEP) reference sequences of wild-type (WT) leptins (GenBank accession No. U84247 and U50365, respectively), oLEP and bLEP recombinant muteins carrying the R4C mutation, and oLEP recombinant hormones carrying the A59V and Q62R mutations, which were detected in bLEP. All proteins were purified to homogeneity as monomers and formed 1:1 molar ratio complexes with the chicken leptin-binding domain (LBD). Surface plasmon resonance experiments revealed that all protein variants exhibit reduced (P < 0.05) affinity to chicken (ch) and human (h) LBD compared with the WT oLEP and bLEP recombinant proteins. The ovine and bovine R4C muteins exhibited significantly (P < 0.05) greater induction of cell proliferation in a Baf/3 cell line bioassay, despite lower affinity toward both hLBD and chLBD. Intra-third cerebral ventricle infusion of oLEP and its 3 muteins in sheep resulted in reduced feed intake. However, the 3 tested muteins had a decreased (P < 0.05) inhibitory effect than the WT LEP. It was concluded that natural genetic polymorphisms in the bLEP are associated with variation in the biochemical and physiological properties of the protein.

Biochemical and in vitro biological significance of natural sequence variation in the ovine leptin gene

The hormone leptin is involved in diverse biological processes, including regulation of food intake, body-weight homeostasis and energy balance. Sequence variation in the bovine leptin gene has been found to be associated with variations in carcass fat content and average daily gain, as well as in milk yield, milk somatic cell count and several traits governing reproduction. We sequenced genomic DNA and cDNA samples of individuals from three divergent sheep breeds and revealed synonymous as well as novel non-synonymous allelic variation at the third exon of the ovine leptin gene (oLEP) as compared to the sequence published at Accession No. U84247 (reference sequence). In addition, two alternatively spliced oLEP transcripts were found in the abdominal fat tissue. The biochemical and the in vitro biological significance of the sequence variation in the oLEP was examined by generating recombinant oLEP-protein variants namely: p.Q28del, p.N78S, p.R84Q, p.P99Q, p.V123L and p.R138Q, carrying the corresponding sequence variation. Surface plasmon resonance experiments revealed, in most cases, reduced affinity of the oLEP protein variants examined, to human leptin-binding domain (hLBD), relative to the reference variant, being 0.75, 0.60, 0.60, 0.89, 0.92 and 1.03, respectively. In competitive binding assays between biotinylated oLEP and the recombinant leptin protein variants, p.N78S and p.R84Q variants exhibited the lowest affinity to hLBD (0.18 and 0.41, respectively) as compared to the reference hormone. We then tested the protein variants' ability to induce proliferation in Baf-3 cells stably expressing the long form of the human leptin receptor: significant differences in proliferative activity were only found for p.N78S (1.8-fold higher) and p.R138Q (4.2-fold lower) relative to the reference oLEP variant.

Toward understanding the role of leptin and leptin receptor antagonism in preclinical models of rheumatoid arthritis

A potential link between obesity, circulating leptin levels and autoimmune disease symptoms suggests that targeting the leptin receptor (ObR) might be a viable novel strategy to combat rheumatoid arthritis (RA). However, studies in animal models and evaluation of clinical cases did not provide clear view on leptin's involvement in RA. To validate ObR as RA target, we used our peptide-based ObR agonists and antagonist in different in vitro and in vivo models of the disease. In human peripheral blood mononuclear cells, leptin and its agonist fragment, desI(2)-E1/Aca, moderately induced constitutive activation of a major proinflammatory transcription factor, NF-κB, while the ObR antagonist peptide Allo-aca inhibited the process. Leptin administration itself did not induce arthritis in rats, but worsened the clinical condition of mice given K/BxN serum transfer arthritis. Simultaneous administration of Allo-aca reduced leptin-dependent increase in disease severity by more than 50%, but the antagonist was ineffective when injected with a 3-day delay. In rats inflicted with mild adjuvant-induced arthritis, both leptin and Allo-aca reduced the extent of joint swelling and the number of arthritic joints. In a more aggressive disease stage, Allo-aca decreased the number of arthritic joints in a dose-dependent manner but did not affect other arthritis markers. In summary, leptin exerts diverse effects on RA depending on the experimental model. This might reflect the heterogeneous character of RA, which is differently impacted by leptin and is unmasked by ObR antagonism. Nevertheless, the results suggest that ObR antagonists might become useful therapeutics in leptin-sensitive early stages of RA.

Leptin-derived peptides that stimulate food intake and increase body weight following peripheral administration

We previously showed that peptides containing leptin sequences 1-33 or 61-90 are taken up by the rat brain. We now report the effects of these peptides on food intake and body weight in mature rats. Peptides were infused intravenously for 4weeks, using Alzet minipumps. Dosages were 20μg/kg/day in experiment I, and 60μg/kg/day in experiment 2. In experiment 1, female rats receiving peptides 1-33 and 61-90 each underwent an approximate doubling of the weight gain of control rats. These peptides also increased food intake in female rats. Peptide 15-32, which has a lesser degree of brain uptake, gave a smaller weight gain. Peptide 83-108, which is not taken up by the brain, had no effect on weight gain or food intake. Similar results were obtained in experiment 2. In male rats, however, none of the peptides caused significant changes in food intake or body weight. This was at least partly due to the fact that all male rats underwent vigorous weight increases. We conclude that peptides 1-33 and 61-90 acted as leptin antagonists, stimulating food intake and body weight increases, at least in female rats. These peptides may lead to clinical applications in conditions such as anorexia and cachexia.

Leptin receptor Lys109Arg and Gln223Arg polymorphisms are associated with early atherosclerosis

BACKGROUND

Leptin is a hormone expressed by the leptin gene, primarily in adipocytes, controlling food intake and energy expenditure. The effects of leptin are mediated by its receptor (LEPR) located in the central nervous system and other tissues, including adipocytes and endothelial cells. The aim of this study was to characterize two polymorphisms of LEPR, Lys109Arg (rs1137100) and Gln223Arg (rs1137101), as risk factors for early atherosclerosis. This connection has not been studied before.

METHODS

This study was performed in the randomly selected, middle-aged control subjects (n=526) from our well-defined OPERA (Oulu Project Elucidating Risk of Atherosclerosis) study. Analysis of covariance (ANCOVA) was performed to study the associations between genotypes, intima media thickness (IMT) measurements, and risk factors for atherosclerosis.

RESULTS

Subjects with the genotype Lys109Arg had the lowest body mass index (BMI) (P = 0.035), whereas Arg109Arg homozygotes had the highest total cholesterol (P=0.021) when adjusted for sex and age. Gln223Arg associated independently with systolic blood pressure (P=0.036). There were no differences in leptin concentrations between the genotypes. The adjusted (sex, age, BMI, smoking status, low-density lipoprotein cholesterol, systolic blood pressure, and fasting blood glucose) means for the IMT measurements were lowest in the Arg109 and Arg223 homozygotes (P=0.042 and P=0.041, ANCOVA, respectively).

CONCLUSIONS

The variations in the LEPR gene are independently associated with early atherosclerosis and some of its risk factors. These variations could possibly affect leptin signaling and thereby modify the effects of leptin on the atherosclerotic process.

Evolution of leptin structure and function

Leptin, the protein product of the obese(ob or Lep) gene, is a hormone synthesized by adipocytes that signals available energy reserves to the brain, and thereby influences development, growth, metabolism and reproduction. In mammals, leptin functions as an adiposity signal: circulating leptin fluctuates in proportion to fat mass, and it acts on the hypothalamus to suppress food intake. Orthologs of mammalian Lep genes were recently isolated from several fish and two amphibian species, and here we report the identification of two Lep genes in a reptile, the lizard Anolis carolinensis. While vertebrate leptins show large divergence in their primary amino acid sequence, they form similar tertiary structures, and may have similar potencies when tested in vitro on heterologous leptin receptors (LepRs). Leptin binds to LepRs on the plasma membrane, activating several intracellular signaling pathways. Vertebrate LepRs signal via the Janus kinase (Jak) and signal transducer and activator of transcription (STAT) pathway. Three tyrosine residues located within the LepR cytoplasmic domain are phosphorylated by Jak2 and are required for activation of SH2-containing tyrosine phosphatase-2, STAT5 and STAT3 signaling. These tyrosines are conserved from fishes to mammals, demonstrating their critical role in signaling by the LepR. Leptin is anorexigenic in representatives of all vertebrate classes, suggesting that its role in energy balance is ancient and has been evolutionarily conserved. In addition to its integral role as a regulator of appetite and energy balance, leptin exerts pleiotropic actions in development, physiology and behavior.

Development and characterization of high affinity leptins and leptin antagonists

Leptin is a pleiotropic hormone acting both centrally and peripherally. It participates in a variety of biological processes, including energy metabolism, reproduction, and modulation of the immune response. So far, structural elements affecting leptin binding to its receptor remain unknown. We employed random mutagenesis of leptin, followed by selection of high affinity mutants by yeast surface display and discovered that replacing residue Asp-23 with a non-negatively charged amino acid leads to dramatically enhanced affinity of leptin for its soluble receptor. Rational mutagenesis of Asp-23 revealed the D23L substitution to be most effective. Coupling the Asp-23 mutation with alanine mutagenesis of three amino acids (L39A/D40A/F41A) previously reported to convert leptin into antagonist resulted in potent antagonistic activity. These novel superactive mouse and human leptin antagonists (D23L/L39A/D40A/F41A), termed SMLA and SHLA, respectively, exhibited over 60-fold increased binding to leptin receptor and 14-fold higher antagonistic activity in vitro relative to the L39A/D40A/F41A mutants. To prolong and enhance in vivo activity, SMLA and SHLA were monopegylated mainly at the N terminus. Administration of the pegylated SMLA to mice resulted in a remarkably rapid, significant, and reversible 27-fold more potent increase in body weight (as compared with pegylated mouse leptin antagonist), because of increased food consumption. Thus, recognition and mutagenesis of Asp-23 enabled construction of novel compounds that induce potent and reversible central and peripheral leptin deficiency. In addition to enhancing our understanding of leptin interactions with its receptor, these antagonists enable in vivo study of the role of leptin in metabolic and immune processes and hold potential for future therapeutic use in disease pathologies involving leptin.

Cut-like homeobox 1 (CUX1) regulates expression of the fat mass and obesity-associated and retinitis pigmentosa GTPase regulator-interacting protein-1-like (RPGRIP1L) genes and coordinates leptin receptor signaling

The first intron of FTO contains common single nucleotide polymorphisms associated with body weight and adiposity in humans. In an effort to identify the molecular basis for this association, we discovered that FTO and RPGRIP1L (a ciliary gene located in close proximity to the transcriptional start site of FTO) are regulated by isoforms P200 and P110 of the transcription factor, CUX1. This regulation occurs via a single AATAAATA regulatory site (conserved in the mouse) within the FTO intronic region associated with adiposity in humans. Single nucleotide polymorphism rs8050136 (located in this regulatory site) affects binding affinities of P200 and P110. Promoter-probe analysis revealed that binding of P200 to this site represses FTO, whereas binding of P110 increases transcriptional activity from the FTO as well as RPGRIP1L minimal promoters. Reduced expression of Fto or Rpgrip1l affects leptin receptor isoform b trafficking and leptin signaling in N41 mouse hypothalamic or N2a neuroblastoma cells in vitro. Leptin receptor clusters in the vicinity of the cilium of arcuate hypothalamic neurons in C57BL/6J mice treated with leptin, but not in fasted mice, suggesting a potentially important role of the cilium in leptin signaling that is, in part, regulated by FTO and RPGRIP1L. Decreased Fto/Rpgrip1l expression in the arcuate hypothalamus coincides with decreased nuclear enzymatic activity of a protease (cathepsin L) that has been shown to cleave full-length CUX1 (P200) to P110. P200 disrupts (whereas P110 promotes) leptin receptor isoform b clustering in the vicinity of the cilium in vitro. Clustering of the receptor coincides with increased leptin signaling as reflected in protein levels of phosphorylated Stat3 (p-Stat3). Association of the FTO locus with adiposity in humans may reflect functional consequences of A/C alleles at rs8050136. The obesity-risk (A) allele shows reduced affinity for the FTO and RPGRIP1L transcriptional activator P110, leading to the following: 1) decreased FTO and RPGRIP1L mRNA levels; 2) reduced LEPR trafficking to the cilium; and, as a consequence, 3) a diminished cellular response to leptin.