Convergent trafficking pattern of leptin after endocytosis mediated by ObRa-ObRd

Leptin deficiency impairs adipogenesis and browning response in mouse mesenchymal progenitors

Although phenotypically different, brown adipose tissue (BAT) and inguinal white adipose tissue (iWAT) are able to produce heat through non-shivering thermogenesis due to the presence of mitochondrial uncoupling protein 1 (UCP1). The appearance of thermogenically active beige adipocytes in iWAT is known as browning. Both brown and beige cells originate from mesenchymal stem cells (MSCs), and in culture conditions a browning response can be induced with hypothermia (i.e. 32 °C) during which nuclear leptin immunodetection was observed. The central role of leptin in regulating food intake and energy consumption is well recognised, but its importance in the browning process at the cellular level is unclear. Here, immunocytochemical analysis of MSC-derived adipocytes established nuclear localization of both leptin and leptin receptor suggesting an involvement of the leptin pathway in the browning response. In order to elucidate whether leptin modulates the expression of brown and beige adipocyte markers, BAT and iWAT samples from leptin-deficient (ob/ob) mice were analysed and exhibited reduced brown/beige marker expression compared to wild-type controls. When MSCs were isolated and differentiated into adipocytes, leptin deficiency was observed to induce a white phenotype, especially when incubated at 32 °C. These adaptations were accompanied with morphological signs of impaired adipogenic differentiation. Overall, our results indicate that leptin supports adipocyte browning and suggest a potential role for leptin in adipogenesis and browning.

Peptide-Based Agents for Cancer Treatment: Current Applications and Future Directions

Peptide-based strategies have received an enormous amount of attention because of their specificity and applicability. Their specificity and tumor-targeting ability are applied to diagnosis and treatment for cancer patients. In this review, we will summarize recent advancements and future perspectives on peptide-based strategies for cancer treatment. The literature search was conducted to identify relevant articles for peptide-based strategies for cancer treatment. It was performed using PubMed for articles in English until June 2023. Information on clinical trials was also obtained from ClinicalTrial.gov. Given that peptide-based strategies have several advantages such as targeted delivery to the diseased area, personalized designs, relatively small sizes, and simple production process, bioactive peptides having anti-cancer activities (anti-cancer peptides or ACPs) have been tested in pre-clinical settings and clinical trials. The capability of peptides for tumor targeting is essentially useful for peptide-drug conjugates (PDCs), diagnosis, and image-guided surgery. Immunomodulation with peptide vaccines has been extensively tested in clinical trials. Despite such advantages, FDA-approved peptide agents for solid cancer are still limited. This review will provide a detailed overview of current approaches, design strategies, routes of administration, and new technological advancements. We will highlight the success and limitations of peptide-based therapies for cancer treatment.

Deletion of endothelial leptin receptors in mice promotes diet-induced obesity

Obesity promotes endothelial dysfunction. Endothelial cells not only respond, but possibly actively promote the development of obesity and metabolic dysfunction. Our aim was to characterize the role of endothelial leptin receptors (LepR) for endothelial and whole body metabolism and diet-induced obesity. Mice with tamoxifen-inducible, Tie2.Cre-ER^T2-mediated deletion of LepR in endothelial cells (End.LepR knockout, KO) were fed high-fat diet (HFD) for 16 weeks. Body weight gain, serum leptin levels, visceral adiposity and adipose tissue inflammation were more pronounced in obese End.LepR-KO mice, whereas fasting serum glucose and insulin levels or the extent of hepatic steatosis did not differ. Reduced brain endothelial transcytosis of exogenous leptin, increased food intake and total energy balance were observed in End.LepR-KO mice and accompanied by brain perivascular macrophage accumulation, whereas physical activity, energy expenditure and respiratory exchange rates did not differ. Metabolic flux analysis revealed no changes in the bioenergetic profile of endothelial cells from brain or visceral adipose tissue, but higher glycolysis and mitochondrial respiration rates in those isolated from lungs. Our findings support a role for endothelial LepRs in the transport of leptin into the brain and neuronal control of food intake, and also suggest organ-specific changes in endothelial cell, but not whole-body metabolism.

Enhancing early life nutrition alters the hepatic transcriptome of Angus × Holstein-Friesian heifer calves

Early life nutrition has a major influence on subsequent lifetime performance in cattle. The aim of this experiment was to investigate the effect of plane of nutrition from 3 to 21 weeks of age on the liver transcriptome. Holstein-Friesian × Angus heifer calves with a mean (±SD) age and BW of 19 (±5) days and 51.2 (±7.8) kg, respectively, were assigned to either a high-energy diet to support a mean average daily gain (ADG) of 1.2 kg/day (HI; n = 15) or a moderate diet (MOD; n = 15) to support a mean ADG of 0.5 kg/day. At 145 ± 3 days of age, all calves were euthanised, liver tissue samples collected and flash-frozen in liquid nitrogen. Following RNA sequence analysis, the total number of differentially expressed genes (DEGs) (at false discovery rate (FDR) > 0.05) was 537; 308 upregulated and 229 downregulated in HI compared to MOD. The number of DEGs mapped to IPA (at FDR > 0.05) was 460; 264 upregulated and 196 downregulated. There was greater expression of genes associated with cellular development and metabolism in heifers on the HI compared to the MOD diet. The genes (fold change) of the somatotrophic axis; IGF1 (3.7), IGFALS (2.6) and GHR (1.5) were upregulated in the HI compared to MOD diet. The cytokine receptor genes, IL17RB (1.7) and IL20RA (3.3), were upregulated in the HI heifers, which were detected in a network interacting with metabolically regulated genes. The potential enhanced cell-to-cell communication evident from DEGs would increase the calves' ability to combat health challenges. The findings of this study indicate that enhancing the early life plane of nutrition in heifer calves results in the upregulation of genes that are associated with increased metabolic activity and thus metabolic capacity. Moreover, the interaction between metabolic and immune communication genes indicates that enhanced nutrition has the potential to improve the immune response in the liver which will play a central role in ensuring optimal lifetime performance.

Tissue-Specific Effects of Leptin on Glucose and Lipid Metabolism

The discovery of leptin was intrinsically associated with its ability to regulate body weight. However, the effects of leptin are more far-reaching and include profound glucose-lowering and anti-lipogenic effects, independent of leptin's regulation of body weight. Regulation of glucose metabolism by leptin is mediated both centrally and via peripheral tissues and is influenced by the activation status of insulin signaling pathways. Ectopic fat accumulation is diminished by both central and peripheral leptin, an effect that is beneficial in obesity-associated disorders. The magnitude of leptin action depends upon the tissue, sex, and context being examined. Peripheral tissues that are of particular relevance include the endocrine pancreas, liver, skeletal muscle, adipose tissues, immune cells, and the cardiovascular system. As a result of its potent metabolic activity, leptin is used to control hyperglycemia in patients with lipodystrophy and is being explored as an adjunct to insulin in patients with type 1 diabetes. To fully understand the role of leptin in physiology and to maximize its therapeutic potential, the mechanisms of leptin action in these tissues needs to be further explored.

Blood-brain barrier shuttle peptides: an emerging paradigm for brain delivery

Brain delivery is one of the major challenges in drug development because of the high number of patients suffering from neural diseases and the low efficiency of the treatments available. Although the blood-brain barrier (BBB) prevents most drugs from reaching their targets, molecular vectors - known as BBB shuttles - offer great promise to safely overcome this formidable obstacle. In recent years, peptide shuttles have received growing attention because of their lower cost, reduced immunogenicity, and higher chemical versatility than traditional Trojan horse antibodies and other proteins.

Leptin stimulates migration and invasion and maintains cancer stem-like properties in ovarian cancer cells: an explanation for poor outcomes in obese women

The evidence linking obesity with ovarian cancer remains controversial. Leptin is expressed at higher levels in obese women and stimulates cell migration in other epithelial cancers. Here, we explored the clinical impact of overweight/obesity on patient prognosis and leptin's effects on the metastatic potential of ovarian cancer cells. We assessed clinical outcomes in 70 ovarian cancer patients (33 healthy weight and 37 overweight) that were validated with an external cohort from The Cancer Genome Atlas (TCGA) database. Progression-free and overall survival rates were significantly decreased in overweight patients. Similarly, a worse overall survival rate was found in TCGA patients expressing higher leptin/OB-Rb levels. We explored serum and ascites leptin levels and OB-Rb expression in our cohort. Serum and ascites leptin levels were higher in overweight patients experiencing worse survival. OB-Rb was more highly expressed in ascites and metastases than in primary tumors. Leptin exposure increased cancer cell migration/invasion through leptin-mediated activation of JAK/STAT3, PI3/AKT and RhoA/ROCK and promoted new lamellipodial, stress-fiber and focal adhesion formation. Leptin also contributed to the maintenance of stemness and the mesenchymal phenotype in ovarian cancer cells. Our findings demonstrate that leptin stimulated ovarian cancer cell migration and invasion, offering a potential explanation for the poor prognosis among obese women.

Pharmacokinetics of leptin in female mice

Pharmacokinetics of leptin in mammals has received limited attention and only one study has examined more than two time points and this was in ob/ob mice. This study is the first to observe the distribution of leptin over a time course in female mice. A physiologic dose (12 ng) of radiolabelled leptin was injected in adult female mice via the lateral tail vein and tissues were dissected out and measured for radioactivity over a time course up to two hours. Major targets for administered leptin included the liver, kidneys, gastrointestinal tract and the skin while the lungs had high concentrations of administered leptin per gram of tissue. Leptin was also found to enter the lumen of the digestive tract intact from the plasma. Very little of the dose (<1 %) was recovered from the brain at any time. Consequently we confirm that the brain is not a major target for leptin from the periphery, although it may be very sensitive to leptin that does get to the hypothalamus. Several of the major targets (GI tract, skin and lungs) for leptin form the interface for the body with the environment, and given the ability of leptin to modulate immune function, this may represent a priming effect for tissues to respond to damage and infection.

Endothelial cell leptin receptor mutant mice have hyperleptinemia and reduced tissue uptake

Hyperleptinemia is usually associated with obesity and leptin resistance. Endothelial cell leptin receptor knockout (ELKO) mice without a signaling membrane-bound leptin receptor in endothelia, however, have profound hyperleptinemia without signs of leptin resistance. Leptin mRNA in adipose tissue was unchanged. To test the hypothesis that the ELKO mutation results in delayed degradation and slowed excretion, we determined the kinetics of leptin transfer in groups of ELKO and wildtype mice after intravenous bolus injection of (125) I-leptin and the reference substance (131) I-albumin. The degradation pattern of (125) I-leptin in serum and brain homogenates at different time points between 10 and 60 min was measured by HPLC and acid precipitation. Although ELKO mice had reduced uptake of (125) I-leptin uptake by the brain and several peripheral organs, leptin was more stable in blood and tissue. There was no change in the rate of renal excretion. ELISA showed that serum soluble leptin receptor, known to antagonize leptin transport, had a 400-fold increase, probably contributing to the hyperleptinemia and reduced tissue uptake. Thus, the ELKO mutation unexpectedly increased the stability of leptin but suppressed its tissue uptake. These changes probably contribute to the known partial resistance of the ELKO mice to diet-induced obesity.

Saturable leptin transport across the BBB persists in EAE mice

We have shown that mice with experimental autoimmune encephalomyelitis (EAE), a model of multiple sclerosis, have upregulated leptin receptor expression in reactive astrocytes of the hippocampus, a region involved in sickness behavior. Leptin can exacerbate EAE when its serum concentration is high. Although leptin receptors in astrocytes modulate leptin transport across cultured endothelial cell monolayers, it is not known how leptin transport in EAE mice is regulated. Here, we determined brain and cervical spinal cord uptake of leptin in early and recovery stages of EAE, after either intravenous delivery or in situ brain perfusion of (125)I-leptin and the vascular marker (131)I-albumin. While increased vascular space and general blood-brain barrier (BBB) permeability after EAE were expected, the specific saturable transport system for leptin crossing the BBB also persisted. Moreover, there was upregulation of leptin transport in hippocampus and cervical spinal cord in the early stage of EAE, shown by higher leptin uptake in these regions and by competitive inhibition with coadministered excess unlabeled leptin. We conclude that EAE induced a time- and region-specific increase of leptin transport. The results provide a link between circulating leptin and enhanced leptin signaling that may play a crucial role in disease progression.

Cytokine signaling modulates blood-brain barrier function

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

Protective role of astrocytic leptin signaling against excitotoxicity

Both proconvulsive and anticonvulsive roles of leptin have been reported, suggesting cell-specific actions of leptin in different models of seizure and epilepsy. The goal of our study was to determine the regulation and function of astrocytic leptin receptors in a mouse model of epilepsy and glutamate-induced cytotoxicity. We show that in pilocarpine-challenged mice developing epilepsy with recurrent seizures after a latent period of 2 weeks, hippocampal leptin receptor (ObR) immunofluorescence was increased at 6 weeks. This was more pronounced in astrocytes than in neurons. In cultured astrocytes, glutamate increased ObRa and ObRb expression, whereas leptin pretreatment attenuated glial cytotoxicity by excess glutamate, reflected by better preserved adenosine triphosphate production. The protective role of astrocytic leptin signaling is further supported by the higher lethality of the astrocyte-specific leptin receptor knockout mice in the initial phase of seizure production. Thus, leptin signaling in astrocytes plays a protective role against seizure, and the effects are at least partially mediated by attenuation of glutamate toxicity. Astrocytic leptin signaling, therefore, may be a novel therapeutic target.

C-reactive protein increases BBB permeability: implications for obesity and neuroinflammation

BACKGROUND/AIMS

Acute phase C-reactive protein (CRP), elevated in obesity and inflammation, is a major binding protein for leptin. It is thought that CRP contributes to leptin resistance by preventing leptin from crossing the blood-brain barrier (BBB). Here we determined how CRP interacts with the BBB and whether it deters leptin from reaching CNS targets.

METHODS

BBB permeability, compartmental distribution, tracer stability, and expression of tight junction protein and inflammatory marker were determined.

RESULTS

CRP was stable in blood, but did not permeate the BBB in trace amounts. However, it increased paracellular permeability at a higher dose. Agouti viable (A(vy)) mice with adult-onset obesity show higher CRP entry into the brain. CRP did not permeate hCMEC/D3 cells nor change zona occludin-1 or cyclooxygenase-2 expression. An intermediate dose of CRP had no effect on leptin transport across the BBB after co-treatment. Thus, acute interactions between CRP and leptin at the BBB level were negligible and did not explain the leptin resistance seen in obesity.

CONCLUSIONS

The interactions of CRP and the BBB are a two-phase process, with increased paracellular permeability at a high dose that enables its entry into the CNS and serves to induce reactive gliosis and impair CNS function.

Upregulation of astrocytic leptin receptor in mice with experimental autoimmune encephalomyelitis

The detrimental role of leptin in experimental autoimmune encephalomyelitis (EAE) is opposite to its neuroprotective role in other neuropathologies. We hypothesize that a shifted cellular distribution of leptin receptors underlies the differential effects of leptin. A robust increase of ObR immunoreactivity was seen along glial fibrillary acidic protein (GFAP)(+) intermediate filaments in reactive astrocytes in the hippocampus and hypothalamus of mice with EAE. Although astrocyte-specific GFAP mRNA and protein were both increased, ObRa mRNA was elevated only after resolution of EAE symptoms, and ObRb mRNA was even decreased at the peak time of symptoms of EAE. A cell type-specific action of leptin may underlie its differential effects.

Leptin action on nonneuronal cells in the CNS: potential clinical applications

Leptin, an adipocyte-derived cytokine, crosses the blood–brain barrier to act on many regions of the central nervous system (CNS). It participates in the regulation of energy balance, inflammatory processes, immune regulation, synaptic formation, memory condensation, and neurotrophic activities. This review focuses on the newly identified actions of leptin on astrocytes. We first summarize the distribution of leptin receptors in the brain, with a focus on the hypothalamus, where the leptin receptor is known to mediate essential feeding suppression activities, and on the hippocampus, where leptin facilitates memory, reduces neurodegeneration, and plays a dual role in seizures. We will then discuss regulation of the nonneuronal leptin system in obesity. Its relationship with neuronal leptin signaling is illustrated by in vitro assays in primary astrocyte culture and by in vivo studies on mice after pretreatment with a glial metabolic inhibitor or after cell-specific deletion of intracellular signaling leptin receptors. Overall, the glial leptin system shows robust regulation and plays an essential role in obesity. Strategies to manipulate this nonneuronal leptin signaling may have major clinical impact.

Can leptin-derived sequence-modified nanoparticles be suitable tools for brain delivery?

Aim: In order to increase the knowledge on the use of nanoparticles (NPs) in brain targeting, this article describes the conjugation of the sequence 12–32 (g21) of leptin to poly-lactide-co-glycolide NPs. The capability of these modified NPs to reach the brain was evaluated in rats after intravenous administration. Materials & Methods: The g21 was linked on the surface of NPs labeled with tetramethylrhodamine by means of the Avidin-Biotin technology. The g21-labeled NPs were injected into the tail vein of rats and, after animal sacrifice, the brain localization was evaluated by confocal microscopy, fluorescence microscopy and electron microscopy. Studies to evaluate the biodistribution of the g21-modified NPs in comparison to the unmodified NPs were also carried out. Moreover, to confirm the absence of any anorectic effect of g21 linked on the surface of NPs, appropriate studies were used to assess the rats. Results: After intravenous administration, the g21-modified NPs were able to cross the blood–brain barrier and to enter the brain parenchyma. The biodistribution studies of both unmodified and modified NPs pointed out an uptake at liver and spleen level, whereas only the g21-modified NPs showed brain localization. The food-intake experiments pointed out that the intravenous administration of g21 conjugated to the NP surface did not produce any anorectic effect in the rats. Conclusion: g21-modified NPs were able to cross the blood–brain barrier. These new modified NPs could be effectively considered as useful carrier systems for brain drug delivery.

Original submitted: 27/11/2010; Revised submitted: 09/03/2011

Endothelial leptin receptor mutation provides partial resistance to diet-induced obesity

Leptin, a polypeptide hormone produced mainly by adipocytes, has diverse effects in both the brain and peripheral organs, including suppression of feeding. Other than mediating leptin transport across the blood-brain barrier, the role of the endothelial leptin receptor remains unclear. We recently generated a mutant mouse strain lacking endothelial leptin receptor signaling, and showed that there is an increased uptake of leptin by brain parenchyma after its delivery by in situ brain perfusion. Here, we tested the hypothesis that endothelial leptin receptor mutation confers partial resistance to diet-induced obesity. These ELKO mice had similar body weight and percent fat as their wild-type littermates when fed with rodent chow, but blood concentrations of leptin were significantly elevated. In response to a high-fat diet, wild-type mice had a greater gain of body weight and fat than ELKO mice. As shown by metabolic chamber measurement, the ELKO mice had higher oxygen consumption, carbon dioxide production, and heat dissipation, although food intake was similar to that of the wild-type mice and locomotor activity was even reduced. This indicates that the partial resistance to diet-induced obesity was mediated by higher metabolic activity in the ELKO mice. Since neuronal leptin receptor knockout mice show obesity and diabetes, the results suggest that endothelial leptin signaling shows opposite effects from that of neuronal leptin signaling, with a facilitatory role in diet-induced obesity.

Effects of cell-type specific leptin receptor mutation on leptin transport across the BBB

The functions of leptin receptors (LRs) are cell-type specific. At the blood-brain barrier, LRs mediate leptin transport that is essential for its CNS actions, and both endothelial and astrocytic LRs may be involved. To test this, we generated endothelia specific LR knockout (ELKO) and astrocyte specific LR knockout (ALKO) mice. ELKO mice were derived from a cross of Tie2-cre recombinase mice with LR-floxed mice, whereas ALKO mice were generated by a cross of GFAP-cre with LR-floxed mice, yielding mutant transmembrane LRs without signaling functions in endothelial cells and astrocytes, respectively. The ELKO mutation did not affect leptin half-life in blood or apparent influx rate to the brain and spinal cord, though there was an increase of brain parenchymal uptake of leptin after in situ brain perfusion. Similarly, the ALKO mutation did not affect blood-brain barrier permeation of leptin or its degradation in blood and brain. The results support our observation from cellular studies that membrane-bound truncated LRs are fully efficient in transporting leptin, and that basal levels of astrocytic LRs do not affect leptin transport across the endothelial monolayer. Nonetheless, the absence of leptin signaling at the BBB appears to enhance the availability of leptin to CNS parenchyma. The ELKO and ALKO mice provide new models to determine the dynamic regulation of leptin transport in metabolic and inflammatory disorders where cellular distribution of LRs is shifted.

Effects of diet-induced obesity on colitis-associated colon tumor formation in A/J mice

OBJECTIVE

Studies have indicated that obesity is associated with a higher risk of colorectal cancer. This study was performed to determine the effect of diet-induced obesity on the formation of azoxymethane (AOM)/dextran sodium sulfate (DSS)-induced colon tumors and to identify adiposity-related mechanisms.

METHODS

Male A/J mice were placed on either a high-fat diet (HFD; 45% of total calories from fat) or a normal diet (ND; 15% of calories from fat) for 12 weeks. To induce colon tumors, AOM was administered at a dose of 10 mg/kg body weight, followed by two cycles of DSS supply.

RESULTS

Study results indicated that the HFD group had twofold higher numbers of colonic tumors, as compared with the ND group. The HFD group also had significantly increased body weight and epididymal fat weight, which were associated with increases of serum insulin, insulin-like growth factor-1, leptin, epididymal fat pad leptin mRNA and colonic leptin receptor (Ob-R) mRNA. Animals on HFD showed higher expressions of Ob-R, insulin receptor, phosphorylated Akt, phosphorylated extracellular signal-regulated kinases, Bcl-xL and Cyclin D1 proteins in the colon.

CONCLUSION

The results suggest that HFD-induced obesity facilitates colon tumor formation, possibly by regulating downstream targets of circulating adiposity-related factors via receptor-mediated signaling of the phosphatidylinositol 3-kinase/Akt pathway.

Concepts for biologically active peptides

Here we review a unique aspect of CNS research on biologically active peptides that started against a background of prevalent dogmas but ended by exerting considerable influence on the field. During the course of refuting some doctrines, we introduced several concepts that were unconventional and paradigm-shifting at the time. We showed that (1) hypothalamic peptides can act 'up' on the brain as well as 'down' on the pituitary, (2) peripheral peptides can affect the brain, (3) peptides can cross the blood-brain barrier, (4) the actions of peptides can persist longer than their half-lives in blood, (5) perinatal administration of peptides can exert actions persisting into adulthood, (6) a single peptide can have more than one action, (7) dose-response relationships of peptides need not be linear, (8) the brain produces antiopiate as well as opiate peptides, (9) there is a selective high affinity endogenous peptide ligand for the mu-opiate receptor, (10) a peptide's name does not restrict its effects, and (11) astrocytes assume an active role in response to metabolic disturbance and hyperleptinemia. The evolving questions in our laboratories reflect the diligent effort of the neuropeptide community to identify the roles of peptides in the CNS. The next decade is expected to see greater progress in the following areas: (a) interactions of peptides with other molecules in the CNS; (b) peptide involvement in cell-cell interactions; and (c) peptides in neuropsychiatric, autoimmune, and neurodegenerative diseases. The development of peptidomics and gene silencing approaches will expedite the formation of many new concepts in a new era.

Expression of leptin and leptin receptor isoforms in the rat and human carotid body

Leptin is known to play a role in the modulation of metabolism and control of breathing acting mainly on central nervous structures, although additional actions on peripheral arterial chemoreceptors have also been suggested in the literature. We therefore examined by means of immunohistochemistry the expression of leptin and leptin receptors in the carotid bodies of rats and humans. Leptin expression and relative expression of leptin receptor isoforms were also studied in rats by real-time PCR. No leptin or leptin receptor immunoreactivities were visible in the type II cells of either series. In rat carotid bodies, diffuse positive stainings for leptin and leptin receptors (both with antibody recognizing all receptor isoforms and antibody specific for Ob-Rb) were observed in type I cells. In human carotid bodies, the mean percentage (±standard error) of leptin immunoreactive type I cells was 39.4%±5.1% and the percentages of leptin receptor immunoreactive type I cells were 57.3%±3.9% with antibody recognizing all receptor isoforms and 33.3%±4.2% with antibody specific for isoform Ob-Rb. Double immunofluorescences with anti-tyrosine hydroxylase (type I cell marker) and anti-glial fibrillary acidic protein (type II cell markers) confirmed the selective location of leptin and Ob-Rb in type I cells. Real-time PCR showed the expression of leptin and Ob-Ra, Ob-Rb, Ob-Rc and Ob-Rf isoform mRNA in the rat carotid body, levels of expression being Ob-Rf>Ob-Rc>>Ob-Ra>>Ob-Rb. Ob-Re mRNA was not detected. The above findings suggest a role of circulating or locally produced leptin in the regulation of chemoreceptor discharge and/or metabolic sensing function, by means of direct action on type I cells.

Rapid endocytosis of interleukin-15 by cerebral endothelia

Interleukin (IL)-15 receptors are present in the cerebral endothelia composing the blood-brain barrier where they show robust up-regulation by neuroinflammation. To determine how IL15 receptor subunits participate in the endocytosis and intracellular trafficking of IL15, we performed confocal microscopic imaging and radioactive tracer uptake assays in primary brain microvessel endothelial cells and related cell lines transfected with modulatory molecules. By immunostaining and co-localization studies with organelle markers, we showed that IL15 was rapidly endocytosed via lipid rafts and was directed to diverse intracellular pathways. During the course of intracellular trafficking, Alexa dye-conjugated IL15 was partially co-localized with both the specific receptor IL15Rα and the co-receptor IL2Rγ. However, deletion of one of the receptor subunits had only a minor effect in slowing IL15 uptake when primary brain microvessel endothelial cells from the receptor knockout mice were compared with those from wildtype mice. IL15 was trafficked to early, recycling, and late endosomes, to the Golgi, and to lysosomes. The diffuse distribution suggests that IL15 activates multiple endothelial signaling events.

Role of astrocytic leptin receptor subtypes on leptin permeation across hCMEC/D3 human brain endothelial cells

Astrocytic leptin receptors (ObR) can be up-regulated in conditions such as adult-onset obesity. To determine whether the levels and subtypes of astrocytic ObR modulate leptin transport, we co-cultured hCMEC/D3 human brain endothelial cells and C6 astrocytoma cells in the Transwell system, and tested leptin permeation from apical to basolateral chambers. In comparison with hCMEC alone, co-culture of C6 cells reduced the permeability of paracellular markers and leptin. Unexpectedly, ObRb over-expression in C6 cells increased leptin permeation whereas ObRa over-expression showed no effect when compared with the control group of pcDNA-transfected C6 cells. By contrast, the paracellular permeability to the sodium fluorescein control was unchanged by over-expression of ObR subtypes. Leptin remained intact after crossing the monolayer as shown by HPLC and acid precipitation, and this was not affected by C6 cell co-culture or the over-expression of different ObR subtypes. Thus, increased expression of ObRb (and to a lesser extent ObRe) in C6 cells specifically increased the permeation of leptin across the hCMEC monolayer. Consistent with the evidence that the most apparent regulatory changes of ObR during obesity and inflammation occur in astrocytes, the results indicate that astrocytes actively regulate leptin transport across the blood-brain barrier, a mechanism independent of reduction of paracellular permeability.

Reduced body weight and increased energy expenditure in transgenic mice over-expressing soluble leptin receptor

Background

Soluble leptin receptor (OBRe), one of several leptin receptor isoforms, is the only bona fide leptin binding protein in plasma. Our earlier studies demonstrated that OBRe modulates leptin levels in circulation. Both clinical and in vitro studies have shown that OBRe expression is inversely correlated to body weight and leptin levels. However, it is not clear whether OBRe plays an active role, either in collaboration with leptin or independently, in the maintenance of body weight.

Methodology/Principal Findings

To investigate the function of OBRe in the regulation of energy homeostasis, we generated transgenic mice that express OBRe under the control of human serum amyloid P (hSAP) component gene promoter. The transgene led to approximately doubling of OBRe in circulation in the transgenic mice than in wild type control mice. Transgenic mice exhibited lower body weight at 4 weeks of age, and slower rate of weight gain when compared with control mice. Furthermore, transgenic mice had lower body fat content. Indirect calorimetry revealed that transgenic mice had reduced food intake, increased basal metabolic rate, and increased lipid oxidation, which could account for the differences in body weight and body fat content. Transgenic mice also showed higher total circulating leptin, with the majority of it being in the bound form, while the amount of free leptin is comparable between transgenic and control mice.

Conclusions

These results are consistent with the role of OBRe as a leptin binding protein in regulating leptin's bioavailability and activity.

Cross-talk between adipose and gastric leptins for the control of food intake and energy metabolism

The understanding of the regulation of food intake has become increasingly complex. More than 20 hormones, both orexigenic and anorexigenic, have been identified. After crossing the blood-brain barrier, they reach their main site of action located in several hypothalamic areas and interact to balance satiety and hunger. One of the most significant advances in this matter has been the discovery of leptin. This hormone plays fundamental roles in the control of appetite and in regulating energy expenditure. In accordance with the lipostatic theory stated by Kennedy in 1953, leptin was originally discovered in white adipose tissue. Its expression by other tissues was later established. Among them, the gastric mucosa has been shown to secrete large amounts of leptin. Both the adipose and the gastric tissues share similar characteristics in the synthesis and storage of leptin in granules, in the formation of a complex with the soluble receptor and a secretion modulated by hormones and energy substrates. However while adipose tissue secretes leptin in a slow constitutive endocrine way, the gastric mucosa releases leptin in a rapid regulated exocrine fashion into the gastric juice. Exocrine-secreted leptin survives the extreme hydrolytic conditions of the gastric juice and reach the duodenal lumen in an intact active form. Scrutiny into transport mechanisms revealed that a significant amount of the exocrine leptin crosses the intestinal wall by active transcytosis. Leptin receptors, expressed on the luminal and basal membrane of intestinal epithelial cells, are involved in the control of nutrient absorption by enterocytes, mucus secretion by goblet cells and motility, among other processes, and this control is indeed different depending upon luminal or basal stimulus. Gastric leptin after transcytosis reaches the central nervous system, to control food intake. Studies using the Caco-2, the human intestinal cell line, in vitro allowed analysis of the mechanisms of leptin actions on the intestinal mucosa, identification of the mechanisms of leptin transcytosis and understanding the modulation of leptin receptors by nutrients and hormones. Exocrine-secreted gastric leptin thus participates in a physiological axis independent in terms of time and regulation from that of adipose tissue to rapidly control food intake and nutrient absorption. Adipocytes and gastric epithelial cells are two cell types the metabolism of which is closely linked to food intake and energy storage. The coordinated secretion of adipose and gastric leptins ensures proper management of food processing and energy storage.

Unique leptin trafficking by a tailless receptor

Impairment in blood-to-brain transport of leptin is a major cause as well as consequence of obesity. Leptin crosses the blood-brain barrier by transcytosis rather than undergoing intracellular degradation. Results from previous studies have indicated that the membrane juxtapositional cytoplasmic sequence of the leptin receptor ObR is responsible for leptin transport. To identify the specific structural domains, we generated a series of ObR truncates with different lengths of the intracellular sequence, overexpressed them in 3 types of mammalian cells including cerebral endothelia, and quantified leptin binding and endocytosis. All mutant ObRs were able to bind and mediate the internalization of leptin. Surprisingly, ObR860, a construct with no cytoplasmic sequence, could act like the classical ObRa transporter in internalizing leptin. There were some cell type-dependent variations in the intracellular trafficking of Alexa-labeled leptin when mediated by ObR860 or ObRa because of differential involvement of membrane microdomains, as shown by use of the clathrin inhibitor chlorpromazine and the dynamin inhibitor Dynasore. The clathrin- and dynamin-mediated endocytosis of leptin contrasts with the lack of effect of the caveolae inhibitors nystatin and filipin. Thus, leptin-induced internalization of the ligand-receptor complex can occur without specific sorting signals in the cytoplasmic region of ObR. This novel finding may have significant implications for leptin transport.

Leptin-derived peptide, a targeting ligand for mouse brain-derived endothelial cells via macropinocytosis

Leptin is an appetite regulatory hormone that is secreted into the blood circulation by adipose tissue, and functions in the central nerve system (i.e. hypothalamus) by crossing the blood brain barrier (BBB). In the present study, we investigated the function of a leptin-derived peptide (Lep(70-89)) as a ligand for mouse brain-derived endothelial cells (MBEC4). Lep(70-89)-modified liposomes, prepared with a polyethyleneglycol (PEG) spacer (Lep(70-89)-PEG-LPs) exhibited a significantly higher cellular uptake than peptide-unmodified liposomes (PEG-LPs). Furthermore, cellular uptake was inhibited by amiloride, while no significant inhibitory effect was observed by the presence of chlorpromazine and filipin III, suggesting that macropinocytosis largely contributed to the cellular uptake of Lep(70-89)-PEG-LPs. Imaging studies revealed that Lep(70-89)-PEG-LPs were not colocalized with endosome/lysosomes, whereas neutral dextran (70 kDa) was predominantly colocalized with these compartments. This indicates that Lep(70-89)-PEG-LPs are taken up via macropinocytosis and are subject to non-classical intracellular trafficking, resulting in the circumvention of lysosomal degradation in endothelial cells.

Corticotropin-releasing hormone receptor-1 in cerebral microvessels changes during development and influences urocortin transport across the blood-brain barrier

In this study we tested the hypothesis that receptor-mediated transport of urocortin across the blood-brain barrier (BBB) undergoes developmental changes. Urocortin is a peptide produced by both selective brain regions and peripheral organs, and it is involved in feeding, memory, mood, cardiovascular functions, and immune regulation. In BBB studies with multiple-time regression analysis, we found that neonatal mice had a significant influx of (125)I-urocortin. By contrast, adult mice did not transport urocortin across the BBB. Quantitative RT-PCR showed that corticotropin-releasing hormone receptor (CRHR)-1 was developmentally regulated in enriched cerebral microvessels as well as hypothalamus, being significantly higher in neonatal than adult mice. This change was less dramatic in agouti viable yellow mice, a strain that develops adult-onset obesity. The level of expression of CRHR1 mRNA was 33-fold higher in the microvessels than in hypothalamic homogenates. The mRNA for CRHR2 was less abundant in both regions and less prone to changes with development or the agouti viable yellow mutation. Supported by previous findings of receptor-mediated endocytosis of urocortin, these results suggest that permeation of urocortin across the BBB is dependent on the level of CRHR1 expression in cerebral microvessels. These novel findings of differential regulation of CRH receptor subtypes help elucidate developmental processes in the brain, particularly for the urocortin system.

Cerebral microvascular IL15 is a novel mediator of TNF action

The blood-brain barrier is a gatekeeper and modulatory interface for the CNS. Cerebral endothelial cells are the major component of the blood-brain barrier, and they modify inflammatory signals from the circulation to the CNS by production and secretion of secondary substances. The inflammatory mediators induced by tumor necrosis factor alpha (TNF) were determined by microarray analysis of RBE4 cerebral endothelial cells, at 0, 6, 12, or 24 h after TNF treatment. Interleukin (IL)-15 and its receptors were among the most robustly up-regulated genes. This was confirmed by real-time RT-PCR and western blotting. The three subunits of the IL15 receptor complex (IL15Ralpha, IL2Rbeta, and IL2Rgamma) showed differential regulation by TNF in their time course and amplitude of increased expression. Consistent with increased expression of the specific high affinity receptor IL15Ralpha, TNF increased cellular uptake of (125)I-IL15 and enhanced the fluorescent intensity of Alexa568-IL15 in RBE4 cells. TNF treatment in mice also increased the level of expression of IL15 receptors in enriched cerebral microvessels. We conclude that the cerebral microvascular IL15 system is a novel inflammatory mediator that transduces the actions of TNF.

Melanocortin potentiates leptin-induced STAT3 signaling via MAPK pathway

The co-existence of receptors for leptin and melanocortin in cerebral microvessels suggests possible interactions between leptin and alpha-melanocyte stimulating hormone (MSH) signaling. In this study, we showed that ObRb and melanocortin receptor MC3R and MC4R were present in enriched cerebral microvessels. To test the interactions between ObRb and MC3R or MC4R-mediated cellular signaling, we over-expressed these plasmids in RBE4 cerebral microvascular endothelial cells and HEK293 cells in culture. Activation of signal transducers and activators of transcription-3 (STAT3) in response to leptin was determined by western blotting and luciferase reporter assays. Production of cAMP downstream to melanocortin receptors was determined with a chemiluminescent ELISA kit. alphaMSH, which increased intracellular cAMP, also potentiated leptin-induced STAT3 activation. This potentiation was abolished by a specific MEK inhibitor, indicating the involvement of the mitogen-activated kinase pathway. Reversely, the effect of leptin on alphaMSH-induced cAMP production was minimal. Thus, melanocortin specifically potentiated STAT3 signaling downstream to ObRb by cross-talk with mitogen-activated kinase. The cooperation of ObRb and G protein-coupled receptors in cellular signaling may have considerable biological implications not restricted to feeding and obesity.

Obesity induces functional astrocytic leptin receptors in hypothalamus

The possible role of astrocytes in the regulation of feeding has been overlooked. It is well-established that the endothelial cells constituting the blood-brain barrier transport leptin from blood to brain and that hypothalamic neurons respond to leptin to induce anorexic signaling. However, few studies have addressed the role of astrocytes in either leptin transport or cellular activation. We recently showed that the obese agouti viable yellow mouse has prominent astrocytic expression of the leptin receptor. In this study, we test the hypothesis that diet-induced obesity increases astrocytic leptin receptor expression and function in the hypothalamus. Double-labelling immunohistochemistry and confocal microscopic analysis showed that all astrocytes in the hypothalamus express leptin receptors. In adult obese mice, 2 months after being placed on a high-fat diet, there was a striking increase of leptin receptor (+) astrocytes, most prominent in the dorsomedial hypothalamus and arcuate nucleus. Agouti viable yellow mice with their adult-onset obesity showed similar changes, but the increase of leptin receptor (+) astrocytes was barely seen in ob/ob or db/db mice with their early-onset obesity and defective leptin systems. The marked leptin receptor protein expression in the astrocytes, shown with several antibodies against different receptor epitopes, was supported by RT-PCR detection of leptin receptor-a and -b mRNAs in primary hypothalamic astrocytes. Unexpectedly, the protein expression of GFAP, a marker of astrocytes, was also increased in adult-onset obesity. Real-time confocal imaging showed that leptin caused a robust increase of calcium signalling in primary astrocytes from the hypothalamus, confirming their functionality. The results indicate that metabolic changes in obese mice can rapidly alter leptin receptor expression and astrocytic activity, and that leptin receptor is responsible for leptin-induced calcium signalling in astrocytes. This novel and clinically relevant finding opens new avenues in astrocyte biology.

The Cdk5/p35 kinases modulate leptin-induced STAT3 signaling

Cyclin-dependent kinase (Cdk) 5 is ubiquitously expressed in the brain and plays an essential role in central nervous system development and synaptic plasticity. The p35 kinase is a neuronal specific activator of Cdk5. Here, we show for the first time that Cdk5 activation modulates leptin signaling. P35 and its metabolite p25 were colocalized with the leptin receptor ObR in selective neurons in the hypothalamus. Overexpression of p35 alone was sufficient to induce the transcriptional activation of signal transducer and activator of transcription 3 (STAT3) in a cellular model. In retinoic acid-differentiated SH-SY5Y neuronal cells where ObRb was induced, leptin increased the expression of Cdk5, p35, and p25 kinases. The time course of induction coincided with that of phosphorylated (p)-STAT3. When Cdk5 activity was inhibited, either by roscovitine or overexpression of dominant negative Cdk5, there was a reduction of pSTAT3 activation. The results show that the activation of Cdk5 by p35 sustained leptin-induced pSTAT3 at 3-6 h. Thus, p35 is a novel modulator of leptin-induced STAT3 signaling.

Blood-brain barrier and feeding: regulatory roles of saturable transport systems for ingestive peptides

The two main ways for peptides in the peripheral body to enter the brain are by either saturable transport or passive diffusion across the blood-brain barrier (BBB). Saturable transport systems have the advantage of being responsive to physiological and pathological stimuli. Since saturable systems can regulate peptide entry into the brain, they have the potential to play controlling roles in feeding behavior. For therapeutic applications, however, saturable systems have the disadvantage of functioning as a threshold to limit access of large amounts of peptides into the brain. This pharmacological problem presumably would not be encountered for peptides crossing the BBB by passive diffusion, a process dependent on physicochemical properties. Thus, the gatekeeper function of the BBB can be expanded to a primary governing role, especially for entry of ingestive peptides subject to their respective saturable transport systems.

Insulin regulates hepatic leptin receptor expression in early lactating dairy cows

Energy balance controls the expression of the leptin receptor (Lepr) in the ruminant hypothalamus but whether similar regulation occurs in peripheral tissues is unknown. To address this issue, we measured Lepr expression in the liver and adipose tissue of dairy cows during the transition from late pregnancy (LP) to early lactation (EL). This period is characterized by the development of a profound state of energy insufficiency and is associated with reduced plasma insulin and leptin and with increased plasma growth hormone. Hepatic expression of the short (Lepr-a) and long (Lepr-b) isoforms was 40% higher during EL (8 days postpartum) than LP (30 days prepartum). A similar effect was observed when negative energy balance was induced in nonpregnant, late-lactation dairy cows by food restriction, implicating energy insufficiency as a specific cause in EL. The stimulation of hepatic Lepr expression was reversed after a 48-h period of hyperinsulinemic euglycemia in EL. Changes in hepatic Lepr expression during chronic elevation of plasma leptin in EL or plasma growth hormone in nonpregnant, late-lactation cows did not support a role for these hormones in mediating the effects of energy insufficiency on hepatic Lepr expression. In adipose tissue, Lepr expression was increased 10-fold during the transition from LP to EL. Overall, these data indicate that hypoinsulinemia is partly responsible for the induction of Lepr expression in the liver, and perhaps adipose tissue, of energy-deficient dairy cows.

Permeation of blood-borne IL15 across the blood-brain barrier and the effect of LPS

Interleukin15 (IL 15) is a proinflammatory cytokine with elevated concentrations in autoimmune diseases involving the periphery (e.g. rheumatoid arthritis) and CNS (e.g. multiple sclerosis). Its interactions with the blood-brain barrier (BBB) were studied in normal and lipopolysaccharide (LPS)-treated mice. (125)I-IL15 remained intact for at least 10 min after i.v. injection and reached CNS parenchyma with regional differences between brain and spinal cord. Both in vivo and in situ brain perfusion of (125)I-IL15 showed that its permeation of the BBB was non-saturable. LPS induced a significant increase of IL15 uptake by the brain and spinal cord, partly related to a higher general permeability of the BBB. The results suggest that the BBB is an interface for blood-borne IL15 to interact with the CNS in the basal state and during inflammation.

Astrocyte leptin receptor (ObR) and leptin transport in adult-onset obese mice

The agouti viable yellow (A vy) spontaneous mutation generates an unusual mouse phenotype of agouti-colored coat and adult-onset obesity with metabolic syndrome. Persistent production of agouti signaling protein in A vy mice antagonizes melanocortin receptors in the hypothalamus. To determine how this disruption of neuroendocrine circuits affects leptin transport across the blood-brain barrier (BBB), we measured leptin influx in A vy and B6 control mice after the development of obesity, hyperleptinemia, and increased adiposity. After iv bolus injection, (125)I-leptin crossed the BBB significantly faster in young (2 month old) B6 mice than in young A vy mice or in older (8 month old) mice of either strain. This difference was not observed by in situ brain perfusion studies, indicating the cause being circulating factors, such as elevated leptin levels or soluble receptors. Thus, A vy mice showed peripheral leptin resistance. ObRa, the main transporting receptor for leptin at the BBB, showed no change in mRNA expression in the cerebral microvessels between the age-matched (2 month old) A vy and B6 mice. Higher ObRb mRNA was seen in the A vy microvasculature with unknown significance. Immunofluorescent staining unexpectedly revealed that many of the ObR(+) cells were astrocytes and that the A vy mice showed significantly more ObR(+) astrocytes in the hypothalamus than the B6 mice. Although leptin permeation from the circulation was slower in the A vy mice, the increased ObR expression in astrocytes and increased ObRb mRNA in microvessels suggest the possibility of heightened central nervous system sensitivity to circulating leptin.

Developmental changes of leptin receptors in cerebral microvessels: unexpected relation to leptin transport

The adipokine leptin participates not only in the regulation of feeding and obesity in adults but also in neonatal development. It crosses the blood-brain barrier (BBB) by receptor-mediated transport. Leptin concentrations in blood differ between neonates and adults. We determined the developmental changes of leptin receptor subtypes in the cerebral microvessels composing the BBB and examined their expected correlation with leptin transport across the BBB. Total RNA was extracted from enriched cerebral microvessels of mice 1, 7, 14, and 60 d of age for real-time RT-PCR analysis of leptin receptor subtypes. In cerebral microvessels from neonates, ObRa, ObRb, ObRc, and ObRe mRNA were all higher than in adults, but ObRd was not detectable. Hypothalamus showed similar age-related changes except for ObRb, which was higher in adults. The homologous receptor gp130 did not show significant age-related changes in either region. Despite the increase of leptin receptors, leptin permeation across the BBB after iv injection was less in the neonates. In situ brain perfusion with blood-free buffer showed no significant difference in the brain uptake of leptin between neonates and adults, indicating an antagonistic role of leptin-binding proteins in the circulation, especially the soluble receptor ObRe. The results are consistent with our previous finding that ObRe antagonizes leptin endocytosis in cultured endothelia and transport from blood to brain in mice. Overall, the developmental changes observed for leptin receptors unexpectedly failed to correlate with the entry of leptin into brain, and this may indicate different functions of the receptors in neonates and adults.

Differential role of TNF receptors in cellular trafficking of intact TNF

BACKGROUND/AIMS

Although ligand signaling and degradation within the cell have received much attention, few studies have quantified the role of receptors on the transcytosis of ligand into and out of the cell in intact form. Accordingly, we determined the differential role of the two receptors for tumor necrosis factor alpha (TNFR1, TNFR2) on cellular transcytosis.

METHODS

TNFR1 and TNFR2 were overexpressed in HEK293 cells by transient transfection. Cell surface binding, endocytosis, and exocytosis of (125)I-TNF were quantified. Degradation was determined by acid precipitation and size-exclusion chromatography.

RESULTS

TNFR1- mediated uptake of TNF was faster than TNFR2-mediated uptake of TNF. TNFR2, however, exhibited greater capacity, leading to a higher percentage release of TNF into the exocytosis medium. Rather than being degraded, most of the TNF inside the cell remained intact for 1 h. Both receptors exerted protective roles against degradation, but there was no cooperativity between them.

CONCLUSION

The effects of TNFR1 and TNFR2 in shepherding TNF across the cell illustrate the differential roles of receptors on the cellular trafficking of the ligand in intact form so as to facilitate its biological effects.

Adipokines and the blood-brain barrier

Just as the blood-brain barrier (BBB) is not a static barrier, the adipocytes are not inert storage depots. Adipokines are peptides or polypeptides produced by white adipose tissue; they play important roles in normal physiology as well as in the metabolic syndrome. Adipokines secreted into the circulation can interact with the BBB and exert potent CNS effects. The specific transport systems for two important adipokines, leptin and tumor necrosis factor alpha, have been characterized during the past decade. By contrast, transforming growth factor beta-1 and adiponectin do not show specific permeation across the BBB, but modulate endothelial functions. Still others, like interleukin-6, may reach the brain but are rapidly degraded. This review summarizes current knowledge and recent findings of the rapidly growing family of adipokines and their interactions with the BBB.

Soluble receptor inhibits leptin transport

Evidence both from mice and cultured cells suggests an important role of soluble leptin receptors in obesity and leptin signaling. However, the direct effects of soluble receptors on leptin uptake by cells are not clear. This study shows that soluble leptin receptors antagonize the permeation of leptin across the mouse blood-brain barrier by reducing the binding and endocytosis of leptin. This is illustrated by analysis of radioactively labeled and fluorescent-tagged leptin in normal mice and in cultured cells overexpressing various forms of leptin receptors. Three constructs of soluble leptin receptors were generated in this study: ObRe (805 aa), ObR839, and ObR852. (125)I-leptin was injected intravenously and its influx rate from blood to brain determined by multiple-time regression analysis. Pre-incubation with ObR839 caused a significant reduction of leptin influx across the blood-brain barrier. Endocytosis assays and fluorescent image analysis further showed that ObRe, ObR839, and ObR852 failed to mediate leptin internalization and trafficking within the cells. Instead, these soluble receptors inhibited surface binding and endocytosis of leptin. Thus, we provide novel direct evidence both in vivo and in vitro that soluble receptors of leptin serve as antagonists of the transport of leptin.