Decreased transport of leptin across the blood-brain barrier in rats lacking the short form of the leptin receptor

Directional secretion and transport of leptin and expression of leptin receptor isoforms in human placental BeWo cells

Placental leptin secretion has important implications for maternal adaptation to pregnancy, fetal growth and development, and local autocrine/paracrine actions within trophoblast. In this study we used a cell culture insert model to examine directional secretion of leptin from the basal and apical surfaces of human choriocarcinoma BeWo cells, and to assess the effects of dexamethasone and syncytialization. Additionally, the effects of dexamethasone on transcellular passage of leptin across BeWo monolayers, and on expression of the leptin receptor isoforms Ob-Rs and Ob-RL were examined. Leptin was secreted into both the basal and apical chambers and was stimulated by dexamethasone. Treatment of BeWo cells with forskolin induced syncytialization and loss of monolayer integrity, but resulted in a marked increase in total leptin secretion, an effect further enhanced by co-treatment with dexamethasone. Bidirectional transfer of 125I-leptin between the apical and basal chambers of BeWo cell cultures was low but indicative of specific transcellular passage of leptin; transfer was unaffected by dexamethasone. Treatment of BeWo cells with forskolin increased Ob-Rs mRNA expression, whilst Ob-RL mRNA expression increased in response to forskolin only in the presence of dexamethasone. In conclusion, our data show that leptin is secreted from both the apical and basal surfaces of BeWo placental cells and is increased by both syncytialization and glucocorticoids. Moreover, transport of exogenous leptin occurred in both the apical to basal and reverse directions, suggesting the potential for maternal-fetal exchange of leptin across the human placenta.

Structural and functional evidence supporting a role for leptin in central neural pathways influencing blood pressure in rats

Leptin, a peptide hormone normally associated with body weight homeostasis, is implicated in the generation of obesity-induced hypertension. Administration of leptin increases sympathetic nerve activity and blood pressure; however, the neural circuity involved in this pressor effect is not clearly defined. In this review we describe experiments in which pseudorabies virus was injected into the heart, kidney and the vasculature within skeletal muscle to reveal the distribution of neurones in the hypothalamus that project to these cardiovascular tissues. This distribution is compared to the well-documented distribution of leptin receptors. Finally we discuss microinjection studies designed to examine the effect of leptin, in these regions, on sympathetic nerve discharge and arterial blood pressure. Leptin injected directly into the ventromedial hypothalamus, arcuate nucleus and lateral hypothalamic area (particularly the perifornical area) increased lumbar sympathetic nerve activity. In addition, microinjection into the ventromedial hypothalamus and parvocellular paraventricular nucleus increased blood pressure. Our results demonstrate a discrete set of hypothalamic pathways that may underlie the involvement of leptin in obesity-induced hypertension.

Expression of leptin receptors in mononuclear cells from myelodysplastic syndromes and acute myeloid leukemias

Leptin, the adipocyte hormone, and its receptor have been implicated in the differentiation/proliferation of hematopoietic cells. Given that the deregulated expression of a variety of growth factors and/or their receptors has been implicated in the pathogenesis of certain leukemias, we aimed to characterize the potential differences in the expression pattern of the two major leptin receptor transcript variants in peripheral blood mononuclear cells (PBMC) between different hematologic malignancies. Using RT-PCR and Southern blotting, we compared the expression levels of the two major leptin receptors, the longest (OB-R(L)) and the shortest (OB-R(S)) splice variants, in PBMC from patients with myelodysplastic syndrome (MDS), acute myeloid leukemia (AML), healthy individuals and two human hematopoietic cell lines (HL-60 and K562). Expression of the OB-R(S) transcript clearly exceeded that of OB-R(L) in all patients and controls and in the HL-60 cells, but this was reversed in the K562 cell line. However, the expression of the OB-R(L) was significantly lower in MDS compared to controls and tended to be so in AML, while OB-R(S) tended to be higher in MDS and AML patients compared to controls, but this difference was not significant. Serum leptin levels and circulating soluble leptin receptor levels were slightly but not significantly higher in AML and MDS. These alterations in the expression of the leptin receptor isoforms in MDS and AML patients could suggest a potential role of leptin and its signaling in hematopoietic malignancies, which requires further examination.

Acute third ventricular administration of leptin decreases protein and fat in self-selecting rats

The peripheral administration of leptin reduces food intake (FI) body weight gain (BWG) and modifies food choice. The aim of this study was to examine the effect of acute cerebral injections of leptin on food selection in rats. Male rats were first adapted to the food choice paradigm (protein, carbohydrate, fat) for 3 weeks. They were then implanted with a cannula in the third ventricle. Leptin (leptin group=L) or saline (control group=C) injections were performed at either the beginning or the end of the night at 4-day intervals. FI was recorded continuously, 3 days before, during and then after injections. Rats were sacrificed 86 h after the second injection. After both injections, BWG and FI were reduced. The reduction in FI concerned only nocturnal intake, whatever the timing of the injection. When the injection was given at the beginning of the night, the reductions after a 1-h latency period were -45% and -27.5% during the first and second days, respectively. Following the second injection, the same effects were observed immediately (-16% and -41%, respectively). Only the fat and protein intakes were significantly reduced. This lower FI was due to a reduction in meal size and duration. The reduction resulted in a lower BWG and total white adipose tissue mass. At the time of sacrifice, 6 h after food deprivation, leptinemia and insulinemia were reduced in leptin-treated rats. Glycemia values were identical. It was thus demonstrated that central leptin was a satiation factor rather than a satiety factor.

Inhibition of appetite by nasal leptin administration in rats

OBJECTIVE

Leptin inhibits appetite and reduces body weight. However, subcutaneous leptin administration is not very effective on weight reduction. The present studies were undertaken to test the hypotheses that nasally administered leptin effectively accesses to the brain and inhibits appetite.

METHODS

Recombinant leptin (0.5 mg/rat) was administered into the bilateral nasal spaces of rats (i.n.). Changes in serum immunoreactive leptin (IRL) and cerebrospinal fluid (CSF)-IRL concentrations after i.n. leptin administration were compared after intraperitoneal (i.p.) administration. The influence of 0.1 or 0.5% lysophosphatidylcholine (LPC) as an optimizer of leptin absorption was examined. The anorexic effects of i.n. leptin were compared with i.p. leptin in ad libitum fed rats.

RESULTS

The i.n. leptin increased CSF-IRL concentrations, although serum IRL concentrations of rats administered leptin i.n. were lower than those administered i.p. The addition of 0.1 and 0.5% LPC dose-dependently increased serum IRL concentrations, but did not modify CSF-IRL concentrations in i.n. leptin-treated rats. The i.n. leptin inhibited dark-time food consumption at 0-1 h and 3-6 h in ad libitum fed rats. In contrast, i.p. leptin reduced food consumption only for an hour. Phosphorylated signal transducer and activator of transcription (STAT) 3 immunoreactive cells increased in the arcuate nucleus (ARC) of the hypothalamus at 3 h only following i.n. leptin.

CONCLUSION

The present study demonstrated that i.n. leptin caused longer inhibition of appetite and phosphorylation of STAT3 in ARC. It is concluded that the trans-nasal route may be useful for the selective access of leptin to the brain in obese people.

Leptin: structure, function and biology

Leptin is an adipocyte-derived hormone that acts as a major regulator for food intake and energy homeostasis. Leptin deficiency or resistance can result in profound obesity, diabetes, and infertility in humans. Since its discovery, our understanding of leptin's biological functions has expanded from anti-obesity to broad effects on reproduction, hematopoiesis, angiogenesis, blood pressure, bone mass, lymphoid organ homeostasis, and T lymphocyte systems. Leptin orchestrates complex biological effects through its receptors, expressed both centrally and peripherally. Leptin receptor belongs to the class I cytokine receptor superfamily. At least five isoforms of leptin receptor exist, primarily because of alternate splicing. The longest form is capable of full signal transduction. The short forms may serve as leptin binding proteins and play a role in leptin transporting across the blood-brain barrier. In this review, we present the crystal structure of leptin and the structural comparison with other four-helical cytokines, discuss the leptin-receptor binding models based on other cytokine-receptor complex structures, and summarize the most recent progress on leptin signal transduction pathways--especially its link to peripheral lipid metabolism through AMP-activated protein kinase and hepatic stearoyl-CoA desaturase-1 pathways. Furthermore, we propose the structure based design of leptin analogs with increased stability, improved potency, enhanced blood-brain barrier transport, and extended time action for future therapeutic application.

Regulation of leptin distribution between plasma and cerebrospinal fluid by cholecystokinin receptors

Cholecystokinin (CCK) is a postprandial hormone that elicits a satiating effect and regulates feeding behaviour. CCK has been shown to enhance the effect of leptin in several experimental paradigms. The goal of this work was to characterize the effect of endogenous CCK on plasma leptin content by using CCK receptor antagonists. Therefore, we administered SR-27897, a selective CCK1 receptor antagonist, and L-365260, a selective CCK2 receptor antagonist, to fed and food-deprived rats and determined plasma leptin concentration by enzyme immunoassay. Plasma insulin and glucose concentration as well as food intake were also determined. Under our conditions, SR-27897 increased plasma concentration of leptin both in fed and food-deprived rats. It also increased food intake as well as plasma concentration of insulin in fed animals. L-365260 increased plasma leptin concentration only in fed rats. In animals receiving exogenous leptin, CCK-8 increased the ratio between the concentration of leptin in cerebrospinal fluid and plasma. These results show that CCK receptor antagonists increases plasma concentration of leptin and suggest that endogenous CCK may facilitate the uptake of plasma leptin to the cerebrospinal fluid.

Why study transport of peptides and proteins at the neurovascular interface

The blood-brain barrier (BBB) is an immense neurovascular interface. In neurodegenerative, ischemic, and traumatic disorders of the central nervous system (CNS), the BBB may hinder the delivery of many therapeutic peptides and proteins to the brain and spinal cord. Fortunately, the mistaken dogma that peptides and proteins do not cross the BBB has been corrected during the past two decades by the accumulating evidence that peptides and proteins in the periphery exert potent effects in the CNS. Not only can peptides and proteins serve as carriers for selective therapeutic agents, but they themselves may directly cross the BBB after delivery into the bloodstream. Their passage may be mediated by simple diffusion or specific transport, both of which can be affected by interactions in the blood compartment (outside the BBB) and within the endothelial cells (at the BBB level). Although the majority of current delivery strategies focuses on modification of the molecule to be delivered, understanding the mechanisms of transport will eventually facilitate regulation of the BBB directly. We review the different aspects of interactions and discuss recent advances in the cell biology of peptide/protein transport across the BBB. Better understanding of the nature and regulation of the transport systems at the BBB will provide a new direction to enhance the interactions of peripheral peptides and proteins with the CNS.

The many lives of leptin

Leptin is a 16,000-Da protein which is secreted by fat but acts within the brain to regulate adiposity. Our Peptides Classic addressed the mystery of how such a large molecule could negotiate the blood-brain barrier (BBB), a structure which normally excludes proteins from the brain. We found that leptin was transported across the BBB by a saturable transport system. This finding was important to understanding how satiety-related peptides and proteins worked, but it was also important to the concept that the BBB is a regulatory interface important in brain-body communication. Obesity in humans and many animals is associated with a leptin resistant state rather than a leptin deficiency. Subsequent work has shown that a defect in the BBB transport of leptin is key in producing and reinforcing this state of resistance. Leptin is pluripotent and the concept of it being primarily an adipostat is being discarded for more encompassing views. Consideration of the BBB data would favor the view that ancestral levels of leptin were much lower than those currently considered normal and are consistent with leptin acting as a metabolic switch, informing the brain when fat reserves are adequate to direct energy expenditures towards activities other than seeking calories.

The involvement of the blood–brain and the blood–cerebrospinal fluid barriers in the distribution of leptin into and out of the rat brain

Leptin is a 16 kDa hormone that is produced by adipose tissue and has a central effect on food intake and energy homeostasis. The ability of leptin to cross the blood-brain and blood-cerebrospinal fluid (CSF) barriers and reach or leave the CNS was studied by the bilateral in situ brain perfusion and isolated incubated choroid plexus techniques in the rat. Brain perfusion results indicated that [(125)I]leptin reached the CNS at higher concentrations than the vascular marker, confirming that [(125)I]leptin crossed the brain barriers. Leptin distribution varied between CNS regions and indicated that the blood-brain barrier, in contrast to the blood-CSF route, was the key pathway for [(125)I]leptin to reach the brain. Further perfusion studies revealed that [(125)I]leptin movement into the arcuate nucleus, thalamus, frontal cortex, choroid plexuses and CSF was unaffected by unlabelled human or murine leptin at a concentration that reflects the upper human and rat plasma leptin concentration (2.5 nM). In contrast, the cerebellum uptake of [(125)I]leptin was decreased by 73% with 2.5 nM human leptin. Thus, this site of dense leptin receptor expression would be sensitive to physiological changes in leptin plasma concentrations. The highest rate (K(in)) of [(125)I]leptin uptake was into the choroid plexuses (307.7+/-68.0 microl/min/g); however, this was not reflected in the CSF (8.9+/-4.1 microl/min/g) and indicates that this tissue tightly regulates leptin distribution. The multiple-time brain uptake of [(125)I]leptin was non-linear and suggested leptin could also be removed from the CNS. Studies using the incubated rat choroid plexus model found that [(125)I]leptin could cross the apical membrane of the choroid plexus to leave the CSF. However, this movement was not sensitive to unlabelled human leptin or specific transport inhibitors/modulators (including probenecid, digoxin, deltorphin II, progesterone and indomethacin).This study supports the concept of brain-barrier regulation of leptin distribution to the CNS, and highlights an important link between leptin and the cerebellum.

Study of the alteration of gene expression in adipose tissue of diet-induced obese mice by microarray and reverse transcription-polymerase chain reaction analyses

In the present study we developed a model of diet-induced obesity (DIO) in male C57 BL/6J mice using an 8-wk high fat diet. This model should better reflect the physiology of the majority of the human obese patients than mouse genetic models of obesity with defects in leptin or leptin signaling. At the end of the diet, DIO mice displayed an increased weight (20%) and higher leptin, insulin, glucose, and corticosterone plasma levels compared with mice fed a standard diet during the same period. Moreover, they became resistant to the central effect of peripheral administration of leptin. Oligonucleotide microarray studies were conducted in adipose tissue. They showed that a great number of genes are differentially expressed. The majority of these genes (69%) are down-regulated in DIO mice. Among those are genes encoding enzymes of the lipid metabolism or markers of adipocyte differentiation, enzymes involved in detoxification processes, as well as structural components of the cytoskeleton. Some other groups of genes displayed increased expression, such as those encoding inflammatory markers. The results of the microarray analysis were confirmed by semiquantitative RT-PCR studies run on a selected number of genes that were differentially expressed or not modified.

Leptin distribution and metabolism in the pregnant rat: transplacental leptin passage increases in late gestation but is reduced by excess glucocorticoids

Leptin is essential for the establishment of pregnancy and appears to promote fetal growth, but the mechanisms regulating fetal leptin exposure remain unclear. In rodents, indirect evidence suggests that fetal leptin is partly derived from the maternal circulation via transplacental passage. Indeed, the placenta expresses mRNA for Ob-Ra, one of the short forms of the leptin receptor (Ob-R(S)) important in leptin transport, and this expression increases markedly in late pregnancy. Therefore, we determined the transplacental passage of maternal leptin to the fetus in the rat and whether this transport increases near term in association with a rise in placental expression of Ob-R(S) protein. Because of the proposed role of leptin in promoting fetal growth, we also assessed the effect of glucocorticoid-induced fetal growth retardation on placental leptin transport. Anesthetized rats received a constant infusion of (125)I-leptin via a jugular cannula before and at d 16 and 22 of pregnancy (term = d 23); plasma samples were obtained at 10, 20, 40, 60, 80, and 100 min, and fetuses and placentas were collected at the time of the final sample. The metabolic clearance rate of leptin fell (P < 0.01) from 3.08 +/- 0.23 ml/min per kg in nonpregnant rats to 2.36 +/- 0.13 ml/min per kg by d 22. Transplacental passage of (125)I-leptin, estimated from its concentration in the whole fetus relative to maternal plasma, increased 10-fold (P < 0.005) between d 16 and d 22 of pregnancy. Over this same period, Ob-R(S) protein expression in the placental labyrinth zone increased by almost 2-fold. Transplacental leptin passage was reduced (P < 0.05) by 77% after maternal dexamethasone treatment, whereas suppression of endogenous glucocorticoid synthesis (by metyrapone) increased (P < 0.05) the transfer of maternal leptin to the fetus by 55%. These data show that transplacental passage of maternal leptin is a significant source of fetal leptin and increases markedly during late pregnancy. Consistent with the proposed role of leptin as a fetal growth factor, transplacental leptin passage is reduced in association with glucocorticoid-induced fetal growth retardation.

Impaired transport of leptin across the blood-brain barrier in obesity is acquired and reversible

Leptin resistance is a major cause of obesity in humans. A major component of this resistance is likely an impaired transport of leptin across the blood-brain barrier (BBB). The fattest subgroup of otherwise normal 12-mo-old CD-1 mice have severely impaired transport of leptin across the BBB. However, it is unknown whether these mice are born with a BBB impairment or acquire it with aging and obesity. Here, we found within an otherwise normal population of CD-1 mice that the 10% fattest mice gained weight throughout a 12-mo-life span, whereas the 10% thinnest mice gained little weight after 3 mo of age. The fattest mice acquired a progressive impairment in their ability to transport leptin across the BBB, whereas the thinnest mice had a rate of transport that did not change with age. Fasting fat mice for 24 h or treating them with leptin resulted in modest weight reduction and development of transport rates for leptin across the BBB similar to those of thin mice. These results show that, in obese CD-1 mice, the impaired transport of leptin across the BBB develops in tandem with obesity and is reversible with even modest weight reduction.

Functional expression of the short isoform of the murine leptin receptor Ob-Rc (muB1.219) inXenopus laevis oocytes

Leptin, a hormone mainly secreted by the adipose tissue, acts on the hypothalamus to regulate food intake and thermogenesis. Six leptin receptor isoforms have been identified and localized in different tissues. While it is clear that leptin action in the brain occurs by binding to the long receptor isoform, several studies have shown that the short isoforms could be involved in the transcellular transport of the hormone from the blood to the brain. Based on these works, we decided to investigate whether the murine short leptin receptor isoform Ob-Rc (muB1.219) could transport leptin when expressed in Xenopus laevis oocytes. MuB1.219 cRNA was injected into the oocytes and functional studies were performed by incubating the oocytes in the presence of 2.5 nM [125I]-leptin, under different conditions. Results showed that leptin binding to the injected oocytes was four to eight-fold higher than the binding to the non-injected oocytes. This was blocked by 250 nM of non-radiolabeled leptin, suggesting that the binding was specific. Leptin internalization was observed from 30 min incubation onwards. Coexpression of the human Na+/glucose cotransporter and the leptin receptor showed that leptin increased sugar uptake into the oocytes. These results demonstrate that the short leptin receptor Ob-Rc is able to mediate binding and internalization of the hormone when expressed in oocytes and that it may perform intracellular signaling.

Leptin receptor expression in the basolateral nucleus of amygdala of conditioned taste aversion rats

AIM: To determine whether serum leptin level and the leptin receptor (OB-R) expression in the basolateral amygdala (BLA) change following conditioned taste aversion (CTA) formation.

METHODS: The serum leptin concentration was measured by rat leptin RIA kit, long and short forms of leptin receptor (OB-Rb and OB-Ra) mRNA in the brain sections were examined by in situ hybridization (ISH) and the expression of OB-R was assessed by immunohistochemistry ABC method with a highly specific goat anti-OB-R antibody.

RESULTS: The level of serum leptin didn’t show significant difference between CTA and control group. Comparing with the control group, the CTA group had an increase on count of OB-R immunohistochemistry positive-stained cells in the BLA (127 ± 12 vs 48 ± 9 per 1 mm²). The OB-Rb mRNA expression level enhanced by 11.9% in the BLA, while OB-Ra mRNA level increased by 7.4% on the choroid plexus in CTA group. So BLA was supposed to be a region where interactions between gustatory and vagal signals take place.

CONCLUSION: BLA is one of the sites, which are responsible for CTA formation in the brain. Leptin and OB-R maybe involved in neuronal communication for CTA. So leptin and its receptors probably take part in CTA and integration of autonomic and extroceptive information.

The therapeutic potential of leptin

Many studies have reported the difficulty most subjects have in maintaining weight loss. Leptin is a cytokine-like protein made in adipose tissue and is transported into the brain by the blood-brain barrier where it inhibits food intake by altering the expression of hypothalamic neurotransmitters. The discovery of leptin raised the hope that a natural compound had been found that could cause weight loss without adverse effects. However, the majority of obese people have high levels of circulating leptin and it is not surprising that clinical trials published so far have shown that leptin only works effectively to suppress food intake in subjects who are hyperphagic as a result of low leptin levels. Obesity secondary to leptin deficiency is rare, most being associated with leptin insensitivity. To overcome leptin insensitivity, higher leptin levels in the CNS may be required. However, there is evidence that the leptin transport mechanism is saturated at low plasma leptin concentrations, limiting the effectiveness of peripherally-administered hormone. It is concluded that for leptin to have therapeutic potential, it either needs to be modified or the transport system by which leptin enters the brain needs to be upregulated to allow leptin to enter the brain more easily. To achieve effective weight loss, it may also be necessary to overcome central leptin insensitivity by developing agents that act downstream of leptin action.

Spontaneous nocturnal leptin secretion in children with myelomeningocele and growth hormone deficiency

OBJECTIVE

To examine the spontaneous leptin secretion in patients with myelomeningocele (MMC) and growth hormone deficiency (GHD).

METHODS

Serum leptin levels were studied in 10 prepubertal MMC patients with GHD (CA 6.2 +/- 0.5 years), 10 patients with idiopathic GHD (IGHD; CA 7.6 +/- 0.7 years) and 12 children with normal variant short stature (NVSS; CA 7.6 +/- 0.5 years). Mean BMI (kg/m(2)) values of the groups did not differ significantly. Nocturnal leptin levels were analyzed over 10 h (blood samples every 20 min) and measured by specific radioimmunoassay.

RESULTS

Mean leptin concentrations did not correlate with BMI in MMC patients. Nocturnal leptin secretion of MMC patients was significantly different to those of children with IGHD and NVSS. Morning leptin levels did not decline as observed in both other groups.

CONCLUSION

Since all groups were matched for BMI values, we suggest a hypothalamic dysregulation of leptin secretion in MMC patients.

Leptin and melanocortin signaling in the hypothalamus

The regulation of body weight in humans is coordinated by the interplay between food intake and energy expenditure. The identification of the adipocyte-secreted hormone leptin as a key regulator on both of these processes has shed new light on the pathways involved in their regulation. Indeed, mutations in the gene's encoding leptin and its cognate receptor cause severe obesity in humans. Leptin's actions are mediated principally by target neurons in the hypothalamus where it acts to alter food intake, energy expenditure, and neuroendocrine-function. Recently, it has become clear that a number of critical neuropeptides are regulated by leptin in the hypothalamus. Among these is the proopiomelanocortin (POMC)-derived peptide, alpha-melanocyte-stimulating hormone (alpha-MSH), which is produced in the arcuate nucleus and is a potent negative regulator of food intake. Like leptin, mutations in POMC or in central melanocortin receptors lead to obesity in humans. Thus, an understanding of the mechanisms by which the leptin and melanocortin pathways signal in the hypothalamus is critical in order to begin to clarify the pathways involved in regulating body weight in humans.

Novel peptide-peptide cooperation may transform feeding behavior

There is need for a new approach to the suppression of feeding. Here, we show that two of the most potent endogenous satiety peptides interact in a novel way to cross the blood-brain barrier (BBB) and to suppress food intake. Combined peripheral administration of leptin and urocortin (UCN) significantly decreased food intake, whereas neither one showed an effect when given alone in the same doses. We further provide a mechanism whereby this novel cooperativity can occur by demonstrating that UCN, which by itself does not cross the BBB, can readily enter the brain by associating with leptin. Such a novel interaction between two peptides at the BBB opens new approaches for general study of the dynamic regulatory role of the BBB in brain-body communication as well as the specific study of obesity.

Leptin: a review of its peripheral actions and interactions

Following the discovery of leptin in 1994, the scientific and clinical communities have held great hope that manipulation of the leptin axis may lead to the successful treatment of obesity. This hope is not yet dashed; however the role of the leptin axis is now being shown to be ever more complex than was first envisaged. It is now well established that leptin interacts with pathways in the central nervous system and through direct peripheral mechanisms. In this review, we consider the tissues in which leptin is synthesized and the mechanisms which mediate leptin synthesis, the structure of leptin and the knowledge gained from cloning leptin genes in aiding our understanding of the role of leptin in the periphery. The discoveries of expression of leptin receptor isotypes in a wide range of tissues in the body have encouraged investigation of leptin interactions in the periphery. Many of these interactions appear to be direct, however many are also centrally mediated. Discovery of the relative importance of the centrally mediated and peripheral interactions of leptin under different physiological states and the variations between species is beginning to show the complexity of the leptin axis. Leptin appears to have a range of roles as a growth factor in a range of cell types: as be a mediator of energy expenditure; as a permissive factor for puberty; as a signal of metabolic status and modulation between the foetus and the maternal metabolism; and perhaps importantly in all of these interactions, to also interact with other hormonal mediators and regulators of energy status and metabolism such as insulin, glucagon, the insulin-like growth factors, growth hormone and glucocorticoids. Surely, more interactions are yet to be discovered. Leptin appears to act as an endocrine and a paracrine factor and perhaps also as an autocrine factor. Although the complexity of the leptin axis indicates that it is unlikely that effective treatments for obesity will be simply derived, our improving knowledge and understanding of these complex interactions may point the way to the underlying physiology which predisposes some individuals to apparently unregulated weight gain.

Leptin transport across the blood-brain barrier of the Koletsky rat is not mediated by a product of the leptin receptor gene

Obesity in humans is thought to be caused by a resistance to leptin. Currently, the evidence suggests that this resistance is caused by an impaired transport of leptin across the blood-brain barrier (BBB). It has been assumed that the short form of the leptin receptor, which is a splice variant of the gene which produces all known leptin receptors, is the leptin transporter, but evidence for this is mixed. The Koletsky rat model should provide a clear answer as to whether transport is dependent on leptin receptors as it does not express any functional receptors. The transport of intravenous leptin across the BBB of the Koletsky rat has been found to be greatly reduced, but evidence for a residual of transport makes it unclear whether the transporter is essentially absent or simply saturated by the high levels of leptin in the serum. Here we used the brain perfusion method to negate the influence of serum levels. We found that, whereas no transport of intravenous leptin occurred in the obese Koletsky, the rate of transport was no different from controls when brain perfusion was used. Leptin was transported completely across the BBB, was saturable, and had the same distribution among brain regions as previously found in normal weight mice (highest transport into the hippocampus and hypothalamus, lowest in the frontal cortex). We conclude that a leptin transporter and possibly its gene have yet to be identified and that the short form likely plays a role in the modulation of transport activity.

Mutations in the human leptin and leptin receptor genes as models of serum leptin receptor regulation

A part of serum Ob leptin, an adipocyte-secreted peptide, is bound to a soluble Ob receptor (sObR). Immunoreactive sObR was measured in 125 lean or obese control subjects (group 1), 18 individuals with a mutation in the leptin gene impairing leptin secretion (group 2), and 10 individuals with a mutation in the ObR gene, leading to production of a truncated ObR not anchored to cell membranes (group 3). In group 1, sObR levels were negatively correlated with age and BMI in children and with BMI in adults. sObR levels were also negatively correlated with leptin levels. Leptin binding activity and sObR levels coeluted in gel-filtration chromatography. In group 2, sObR levels did not differ from those in lean control subjects and were not correlated with BMI. A single peak was detected in chromatographic fractions. In group 3, sObR levels were high and positively correlated with BMI. Immunoreactive sObR coeluted with leptin binding activity. These data demonstrate that leptin is not needed for ObR gene expression, and they suggest that leptin plays a role in receptor downregulation because sObR levels are negatively correlated with leptin levels and BMI in control subjects, whereas sObR levels are not depressed in obese leptin-deficient or leptin receptor-deficient individuals.

Positron emission tomography shows that intrathecal leptin reaches the hypothalamus in baboons

Human obesity may be caused by a resistance to circulating leptin. Evidence from rodents and humans suggests that a major component of this resistance is an impairment in the ability of the blood-brain barrier (BBB) to transport leptin from the blood to the brain. One potential way to bypass the BBB is by administering leptin into the intrathecal (i.t.) space. To be effective, i.t. leptin would have to move caudally from the site of injection, enter the cranium, and reach the hypothalamic arcuate nucleus at the base of the pituitary fossa. However, many substances, especially small, lipid-soluble molecules, do not diffuse far from the site of i.t. injection but are resorbed back into blood. To determine whether i.t. leptin can move caudally, we injected leptin conjugated to diethylenetriaminepentaacetic acid (DTPA) and labeled with (68)Ga (G-Ob) into the lumbar space of three baboons. We also studied unconjugated DTPA labeled with (68)Ga, which did not move up the spinal cord but rapidly appeared in blood after i.t. injection. In contrast, G-Ob steadily moved toward the cranium and had reached the hypothalamus 91 and 139 min after i.t. injection in two baboons. We estimated the concentration of leptin in the hypothalamic region to be at least 8 ng/ml, which is about 40 times higher than cerebrospinal fluid levels in normal weight humans and about 4 times higher than the highest level ever recorded after the peripheral administration of leptin. In a third baboon, the leptin neither moved caudally nor appeared in the blood. We conclude that leptin administered i.t. can reach the hypothalamus in therapeutic concentrations, although there is considerable individual variation.

Characterizaton of short isoforms of the leptin receptor in rat cerebral microvessels and of brain uptake of leptin in mouse models of obesity

Leptin deficiency causes obesity in rodents and humans, but circulating levels of leptin are paradoxically elevated in obesity. The mechanisms underlying this leptin resistance are unknown, but may involve reduced leptin transport across the blood-brain barrier via short isoforms of the leptin receptor (Ob-R). Here, we first quantified short Ob-R mRNA expression in isolated rat cerebral microvessels constituting the blood-brain barrier and found that Ob-Ra and Ob-Rc mRNA were abundantly expressed in similar amounts. Second, brain uptake of leptin was reduced in mice lacking Ob-R. Third, brain uptake of leptin in New Zealand Obese mice, a strain that responds to central, but not peripheral, leptin, was reduced, suggesting that their obesity is at least partly due to deficient leptin transport into the brain. Fourth, brain uptake of leptin was significantly reduced in diet-induced obese mice. Neither New Zealand Obese mice nor diet-induced obese mice exhibited significant decreases in Ob-R mRNA expression in isolated cerebral microvessels. These data support the ideas that short isoforms of Ob-R are involved in brain uptake of leptin and that impaired blood-brain barrier function contributes to the pathogenesis of obesity. However, the mechanisms by which obesity-related deficits in brain uptake of leptin occur remain to be defined.

Plasma glucagon and free fatty acid responses to a glucose load in the obese spontaneous hypertensive rat (SHROB) model of metabolic syndrome X

Metabolic Syndrome X is a cluster of abnormalities including insulin resistance, hyperlipidemia, hypertension, and obesity. We sought to determine if excess plasma glucagon and free fatty acids (FFA) might contribute to the insulin resistance in the obese spontaneous hypertensive rat (SHROB), a unique animal model of leptin resistance and metabolic Syndrome X. SHROB were extremely hyperinsulinemic and mildly glucose intolerant compared with lean SHR. SHROB had elevated fasting plasma glucagon and FFA, and showed paradoxical responses to an oral glucose challenge, with increased glucagon at 30 and 60 min postchallenge (200% plus minus 45% and 91% plus minus 13%, respectively; n = 9). In lean SHR, glucagon was nearly unchanged by glucose loading (<30% increase, P > 0.05; n = 5). Plasma FFA were not affected by a glucose load in SHROB, whereas SHR showed a decrease of 40% plus minus 6% (n = 5--9). The I/G molar ratio changed in opposite directions in the two genotypes, with a decrease in SHROB at 30 and 60 min, in contrast to the appropriate increase at 30 and 60 min postchallenge in the lean SHR (P < 0.01; n = 5--9). Administration of 500 ng/kg exogenous glucagon to SHR raised glucagon 56% plus minus 5% to a level that was similar to fasting SHROB. This level of circulating glucagon was sufficient to elevate glucose and insulin during the 7 hr of observation (n = 9). Based on these results, we suggest that fasting hyperglucagonemia and impaired suppression of glucagon secretion and FFA in response to an oral glucose load may contribute to insulin resistance and glucose intolerance in the SHROB model of metabolic Syndrome X.

Leptin regulation of the immune response and the immunodeficiency of malnutrition

Leptin is a 16 kDa protein mainly produced by adipose tissue in proportion to adipose tissue mass. Originally thought to be a satiety factor, leptin is a pleiotropic molecule. In addition to playing a role in energy regulation, leptin also regulates endocrine and immune functions. Both the structure of leptin and that of its receptor suggest that leptin might be classified as a cytokine. The secondary structure of leptin has similarities to the long-chain helical cytokines family, which includes interleukin 6 (IL-6), IL-11, CNTF, and LIF, and the leptin receptor is homologous to the gp-130 signal-transducing subunit of the IL-6-type cytokine receptors. Leptin plays a role in innate and acquired immunity. Leptin levels increase acutely during infection and inflammation, and may represent a protective component of the host response to inflammation. More important, leptin deficiency increases susceptibility to infectious and inflammatory stimuli and is associated with dysregulation of cytokine production. Leptin deficiency also causes a defect in hematopoiesis. Leptin regulates T cells responses, polarizing Th cells toward a Th1 phenotype. Low leptin levels occurring during starvation mediate the neuroendocrine and immune dysfunction of starvation.

Validity of multiple-time regression analysis in measurement of tritiated and iodinated leptin crossing the blood-brain barrier: meaningful controls

Multiple-time regression analysis has been used to study the influx of radiolabeled peptides and polypeptides across the blood-brain barrier (BBB). This study used both tritiated and iodinated leptin to clarify several issues associated with these measurements. Recombinant murine leptin was radiolabeled with 3H by derivatization or with 125I by the iodobead method and each studied separately in mice. Intact 3H-leptin had a higher apparent influx rate from blood to brain than did intact 125I-leptin, correlating with its higher proportion of reversible association with the capillary lumen that would misleadingly appear to reflect entry. Yet the majority of 3H-leptin and 125I-leptin reached brain parenchyma. There was no significant difference in the influx rate between cerebral cortex and the subcortical regions, thus ruling out a predominant contribution of simple diffusion through the circumventricular organs or choroid plexuses outside the BBB. The influx of radiolabeled leptin, especially 125I-leptin, was decreased by excess unlabeled leptin, supporting the presence of a saturable transport system for leptin at the BBB. To identify the specificity of the transport system and determine whether it is shared by 3H-leptin and 125I-leptin, these radioactively labeled leptins were heat-denatured. Denaturation had no effect on the fast influx of 3H-leptin, but abolished the entry of 125I-leptin into brain; excess denatured leptin failed to inhibit the influx of either 3H-leptin or 125I-leptin. This indicates that the conformation of 125I-leptin is similar to that of native unlabeled leptin, so that iodination would be the better choice for investigating the interaction of leptin with the BBB. However, 3H-leptin can use the same transport system, as shown by inhibition of its influx by unlabeled leptin, whereas the derivatization procedure altered its biophysical properties such that its non-saturated influx was greatly enhanced. Finally, the rapid influx of radioactively labeled leptin contrasted greatly with that of the reference compounds 99mTc-albumin and 3H-inulin which had no significant penetration of the BBB. Thus, with additional considerations such as stability and interactions with the vasculature, multiple-time regression analysis is sensitive and selective for study of the penetration of peptides across the BBB.

Leptin and the regulation of food intake, energy homeostasis and immunity with special focus on periparturient ruminants

The biology of leptin has been studied most extensively in rodents and in humans. Leptin is involved in the regulation of food intake, energy homeostasis and immunity. Leptin is primarily produced in white adipose tissue and acts via a family of membrane bound receptors, including an isoform with a long intracellular domain (OB-Rb), and many isoforms with short intracellular domains (Ob-Rs). OB-Rb is predominantly expressed in the hypothalamic regions involved in the regulation of food intake and energy homeostasis. The other isoforms are distributed ubiquitously and are found in most peripheral tissues in far greater abundance than OB-Rb. The effects of leptin on food intake and energy homeostasis are central and are mediated via a network of orexigenic neuropeptides (neuropeptide Y, galanin, galanin-like peptide, melanin-concentrating hormone, orexins, agouti-related peptide) and anorexigenic neuropeptides (corticotropin-releasing hormone, pro-opiomelanocortin, alpha-melanocyte stimulating hormone and cocaine- and amphetamine-regulated transcript). In addition, leptin acts directly on immune cells to stimulate hematopoesis, T-cell immunity, phagocytosis, cytokine production, and to attenuate susceptibility to infectious insults. Emerging data in ruminants suggest that leptin is dynamically regulated by many factors and physiological states. Thus, leptin is secreted in a pulsatile fashion, but without a marked diurnal rhythm. A positive relationship between adiposity and plasma leptin concentration exists in growing and lactating ruminants. The concentration of plasma leptin increases during pregnancy, starts to decline 1--2 wk before parturition, and reaches a nadir in early lactation. The reduction of plasma leptin at parturition is likely to promote centrally mediated adaptations required in periods of energy deficit, but could have negative effects on immune cell function. Future research is needed in ruminants to address the roles played by leptin and the central nervous system in orchestrating metabolism during the periparturient period and during infectious diseases.

Chronic loss of ovarian function decreases transport of leptin into mouse brain

Loss of ovarian function, such as occurs with menopause in human beings and ovariectomy in rodents, results in weight gain. Using multiple-time regression analysis, a sensitive technique for quantifying blood-to-brain transport of peptides and polypeptides, we found that mice ovariectomized for at least 5 weeks had markedly reduced entry of the satiety factor leptin into brain. The rate of entry of leptin into brain remained reduced half a year later. The results suggest that the weight gain resulting from loss of ovarian function could be explained by decreased transport of leptin into the brain.

Enhanced leptin transport across the blood-brain barrier by alpha 1-adrenergic agents

Leptin is a 16 kDa protein secreted by fat cells which regulates body weight and thermogenesis at sites within the brain. Blood-borne leptin reaches those brain sites because of a saturable transport system located at the blood-brain barrier (BBB). Impaired transport occurs in obese rodents and likely underlies the resistance to the actions of peripheral leptin seen in obesity. Here, we show that leptin transport into the brain is enhanced 2-3-fold by epinephrine and other agents which are more specific for the alpha1 adrenergic receptor. Epinephrine had no effect on the transport across the BBB of insulin or tumor necrosis factor, on BBB integrity, or on the size of the vascular space of the brain. Dopamine, acetylcholine, histamine, serotonin, thyroid hormones, and phentolamine were without effect. Of several amino acids tested, only the catecholamine precursor tyrosine had an effect on leptin transport. Epinephrine was effective after intravenous or intraperitoneal injection, but neither epinephrine nor any of the other monoamines given by intracerebroventricular injection had an effect on leptin transport. These results show that epinephrine likely acts at a site on the luminal surface of the BBB. In conclusion, epinephrine works at an alpha1-like adrenergic, luminal side to enhance the transport of leptin across the BBB.

Glucose and insulin increase the transport of leptin through the blood-brain barrier in normal mice but not in streptozotocin-diabetic mice

Since fasting is one of the few factors found to change the rate of entry of leptin into brain, we used multiple-time regression analysis to study the effects of pretreatment with glucose or insulin on leptin transport across the blood-brain barrier (BBB). Two hours after intraperitoneal injection of glucose (3 g/kg), there was a statistically significant increase in the entry rate (K(i)) of leptin in fasted (from 4.91 +/- 0.70 x 10(-4) ml/g x min to 9.03 +/- 1.00 x 10(-4) ml/g x min) but not (p = 0.15) in nonfasted normal (from 4.90 +/- 1.21 x 10(-4) ml/g x min to 6.42 +/- 1.79 x 10(-4) ml/g x min) or fasted streptozotocin (STZ)-treated diabetic mice (from 4.043 +/- 0.959 x 10(-4) ml/g min to 5.395 +/- 1.355 x 10(-4) ml/g min). Insulin (10 U/kg) increased leptin influx in fasted (from 4.77 +/- 0.26 x 10(-4) ml/g x min to 10.6 +/- 0.15 x 10(-4) ml/g x min at 0.5 h) and nonfasted (from 4.64 +/- 0.75 x 10(-4) ml/g x min to 7.46 +/- 1.48 x 10(-4) ml/g x min at 0.5 h) normal mice, but not in STZ-diabetic mice deficient in insulin (and leptin), even though basal concentrations of glucose were similarly increased in the nonfasted normal and STZ-treated mice. Moreover, the basal rate of leptin influx was the same in overnight fasted normal mice, nonfasted normal mice and STZ-diabetic mice. The results indicate that glucose and insulin can increase leptin transport, but they probably are not the principal factors responsible for the regulatory effect of the BBB on leptin entry into the brain.

Modulation of circulating leptin levels by its soluble receptor

Leptin is an adipocyte-derived hormone with potent weight reducing effects. Genetically obese rodents with mutations of leptin or the leptin receptor are defective in leptin signaling and develop morbid obesity and diabetes. Interestingly, the levels of both leptin mRNA and protein are increased by up to 20-fold in these animals, suggesting the existence of a feedback mechanism controlling the amount of leptin in circulation. In this report, we attempted to determine whether the up-regulation of circulating leptin in Zucker Diabetic Fatty rats, which are nonresponsive to leptin due to a receptor point mutation, is entirely due to increased expression of leptin. We demonstrate that the high level of circulating leptin in these rats is attributable to at least two factors: increased leptin expression by the adipose tissue and delayed clearance of leptin from circulation due to binding to its soluble receptor. The latter conclusion was supported by three lines of evidence: 1) The soluble leptin receptor is up-regulated by about 20-fold in Zucker Diabetic Fatty rats; 2) Adenovirus-mediated overexpression of the soluble leptin receptor results in a similar -fold increase of circulating leptin; 3) In ob/ob mice, which have no endogenous leptin, exogenously administered leptin reaches a higher level when the soluble leptin receptor is overexpressed. The weight-reducing effect of leptin is enhanced in C57Bl/6 ob/ob mice with overexpression of the soluble leptin receptor. Soluble leptin receptor may be a significant factor determining the amount of total leptin in circulation.

Activation of urocortin transport into brain by leptin

There are several transport systems for peptides and polypeptides at the blood-brain barrier (BBB) which facilitate the passage of bioactive substances from blood to brain or from brain to blood. Nonetheless, it would be a novel concept for one peptide or polypeptide to activate the transport of another peptide with a similar function but unrelated structure. In this study, we report the first observation of such a phenomenon: activation of a urocortin transport system at the BBB by leptin. Urocortin, a corticotropin-releasing factor (CRF)-related neuropeptide, is a more potent suppressor of food intake than leptin or CRF when injected peripherally. Radiolabeled urocortin ((125)I-urocortin) was used for these in vivo studies in mice; it remained stable and intact during the experimental period. Unlike CRF, urocortin was not saturably transported out of the brain. There was no substantial entry of (125)I-urocortin into brain as determined by sensitive multiple-time regression analysis after iv bolus injection. Addition of leptin, however, caused a dose-related increase in the influx of (125)I-urocortin and greatly facilitated its entry into brain parenchyma; this effect disappeared at higher doses of leptin. Moreover, in the presence of an activating dose of leptin, the entry of (125)I-urocortin into brain was saturable. The results indicate that the presence of leptin contributes to the potent satiety effects of urocortin after peripheral administration. Thus, the action of leptin in the periphery extends beyond its direct passage across the BBB and involves acute modulation of an inert transport system. We believe that these findings have broad physiological implications and indicate a unique function of the BBB as a regulatory interface.

Dynamic regulation of leptin entry into brain by the blood-brain barrier

Regulation of the transport of leptin across the blood-brain barrier (BBB) may be crucial for its effects on food ingestion and obesity and may be responsible for 'leptin resistance'. This review summarizes current studies of leptin indicating a dynamic role of the BBB. It includes evidence for its susceptibility to change by physiological stimuli such as starvation, refeeding, and time of day. Although the short form of the leptin receptor is involved in leptin transport, it appears that other mechanisms of entry also exist. Regardless, the BBB is intimately involved with the regulation of the actions of leptin.

Persistence of blood-to-brain transport of leptin in obese leptin-deficient and leptin receptor-deficient mice

In lean CD-1 mice, leptin is delivered into the brain by a saturable transport mechanism. Previous work has shown that obesity is associated with decreased leptin transport. Here, we investigated the transport of leptin across the blood-brain barrier (BBB) in two murine models of obesity. Radioiodinated leptin was intravenously injected into ob/ob (no leptin production) and db/db (high leptin levels, but no long-form leptin receptor) mutant mice and their lean controls. In all groups, the labeled polypeptide was transported across the BBB by a saturable mechanism. The rates of transport were not significantly different between the mutant strains and their lean controls. The results demonstrate that leptin transport persists in the absence of production of the endogenous polypeptide or its signal-transducing receptor and suggest that the impaired transport previously seen is not directly explained by only obesity or alterations in serum plasma levels.

Fasting, but not adrenalectomy, reduces transport of leptin into the brain

Food deprivation and adrenalectomy are associated with low concentrations of leptin in blood and the absence of obesity. Because leptin is known to cross the blood-brain barrier (BBB) by a saturable transport system, we examined whether fasting and adrenalectomy (ADX) also act at the BBB. Multiple-time regression analysis showed that fasting, but not ADX, significantly decreased the entry of leptin into mouse brain. After 3 days of food deprivation, the influx of leptin became indistinguishable from that of the vascular control (albumin); 5 h of refeeding significantly reversed this reduced rate of influx. Thus, the results indicate that the BBB provides a dynamic site for the regulation of physiological processes involving leptin.