Temporal and regional onset of leptin resistance in diet-induced obese mice

Dietary wheat gluten induces astro- and microgliosis in the hypothalamus of male mice

Gluten, which is found in cereals such as wheat, rye and barley, makes up a major dietary component in most western nations, and has been shown to promote body mass gain and peripheral inflammation in mice. In the current study, we investigated the impact of gluten on central inflammation that is typically associated with diet-induced obesity. While we found no effect of gluten when added to a low-fat diet (LFD), male mice fed high fat diet (HFD) enriched with gluten increased body mass and adiposity compared with mice fed HFD without gluten. We furthermore found that gluten, when added to the LFD, increases circulating C-reactive protein levels. Gluten regardless of whether it was added to LFD or HFD led to a profound increase in the number of microglia and astrocytes in the arcuate nucleus of the hypothalamus, as detected by immunohistochemistry for ionised calcium binding adaptor molecule 1 (Iba-1) and glial fibrillary acidic protein (GFAP), respectively. In mice fed LFD, gluten mimicked the immunogenic effects of HFD exposure and when added to HFD led to a further increase in the number of immunoreactive cells. Taken together, our results confirm a moderate obesogenic effect of gluten when fed to mice exposed to HFD and for the first-time report gluten-induced astro- and microgliosis suggesting the development of hypothalamic injury in rodents.

Impaired leptin responsiveness in the nucleus accumbens of leptin-overexpressing transgenic mice with dysregulated sucrose and lipid preference independent of obesity

While hypothalamic leptin resistance can occur prior to establishment of obesity, clarification is needed as to whether the impaired response to leptin in the reward-related nuclei occurs independently of obesity. To answer this question, we attempted to dissociate the normally coexisting leptin resistance from obesity. We investigated phenotypes of leptin-overexpressing transgenic mice fed for 1 week with 60 % high-fat diet (HFD) (LepTg-HFD1W mice). After 1 week, we observed that LepTg-HFD1W mice weighed as same as wild type (WT) mice fed standard chow diet (CD) for 1 week (WT-CD1W mice). However, compared to WT-CD1W mice, LepTg-HFD1W mice exhibited attenuated leptin-induced anorexia, decreased leptin-induced c-fos immunostaining in nucleus accumbens (NAc), one of important site of reward system, decreased leptin-stimulated pSTAT3 immunostaining in hypothalamus. Furthermore, neither sucrose nor lipid preference was suppressed by leptin in LepTg-HFD1W mice. On the contrary, leptin significantly suppressed both preferences in WT mice fed HFD (WT-HFD1 W mice). These results indicate that leptin responsiveness decreases in NAc independently of obesity. Additionally, in this situation, suppressive effect of leptin on the hedonic feeding results in impaired regulation. Such findings suggest the impaired leptin responsiveness in NAc partially contributes to dysregulated hedonic feeding behavior independently of obesity.

Role of Leptin in the Digestive System

Leptin is a pluripotent peptide hormone produced mainly by adipocytes, as well as by other tissues such as the stomach. Leptin primarily acts on the central nervous system, particularly the hypothalamus, where this hormone regulates energy homeostasis and neuroendocrine function. Owing to this, disruption of leptin signaling has been linked with numerous pathological conditions. Recent studies have also highlighted the diverse roles of leptin in the digestive system including immune regulation, cell proliferation, tissue healing, and glucose metabolism. Of note, leptin acts differently under physiological and pathological conditions. Here, we review the current knowledge on the functions of leptin and its downstream signaling in the gastrointestinal tract and accessory digestive organs, with an emphasis on its physiological and pathological implications. We also discuss the current therapeutic uses of recombinant leptin, as well as its limitations.

Disordered Leptin signaling in the retrotrapezoid nucleus is associated with the impaired hypercapnic ventilatory response in obesity

Sleep-disordered breathing is characterized by disruptions of normal breathing patterns during sleep. Obesity is closely related to hypoventilation or apnea and becomes a primary risk factor for sleep-disordered breathing. Leptin, a peptide secreted by adipose tissue, has been implicated in central control of breathing. Activation of the retrotrapezoid nucleus (RTN) neurons, a critical central respiratory chemoreceptor candidate, potentiates a central drive to breathing. Here, we ask whether the disordered leptin signaling in the RTN is responsible for obesity-related hypoventilation. In a diet induced obesity (DIO) mouse model, the hypercapnic ventilatory response (HCVR) was assessed and the cellular leptin signaling in the RTN was examined. Our main findings demonstrate that DIO mice exhibit overweight, hypercapnia, high levels of serum and cerebrospinal leptin. During exposure to room air, DIO mice manifest basal hypoventilation with a rapid and shallow breathing pattern. Exposure to CO₂ elicits the impaired HCVR in DIO mice. In addition, both the number of CO₂-activated neurons and expression of TASK-2 channels in the RTN are dramatically reduced in DIO mice. Moreover, there is leptin signaling disorder in RTN neurons in DIO mice, including a significant decrease in leptin-activated RTN neurons, downregulation of phosphorylated STAT3 and upregulation of SOCS3. Altogether, we suggest that the disordered leptin/STAT3/SOCS3 signaling pathway in the RTN plays a role in obesity-related hypoventilation.

Loss of leptin receptor-expressing cells in the hindbrain decreases forebrain leptin sensitivity

Previous studies indicate that inhibition of food intake by leptin is mediated by an integrated response to activation of hypothalamic and hindbrain receptors. This study tested whether loss of hindbrain leptin receptor signaling changed sensitivity to forebrain leptin. Injections of leptin-conjugated saporin (Lep-Sap) into the medial nucleus of the solitary tract (NTS) were used to destroy hindbrain leptin receptor-expressing neurons of male Sprague-Dawley rats. Controls were injected with saporin conjugated with a nonsense peptide (Blk-Sap). Lep-Sap had no effect on daily food intake or body weight, but expression of phosphorylated signal transducer and activator of transcription 3 (pSTAT3) in the NTS following a peripheral injection of leptin was abolished 26 days after Lep-Sap injections. To test forebrain leptin sensitivity, Lep-Sap and Blk-Sap rats received third-ventricle injections of 0, 0.05, 0.1, 0.25, or 0.5 μg leptin. Food intake was inhibited by 0.25 and 0.5 μg leptin in Blk-Sap rats, but there was no significant effect of leptin on food intake of Lep-Sap rats. There was no difference in hypothalamic pSTAT3 in unstimulated conditions, but pSTAT3 was lower in the dorsomedial region of the ventromedial nucleus of the hypothalamus (VMH) of Lep-Sap rats compared with Blk-Sap rats following a third-ventricle injection of 0.25 μg leptin. These results are consistent with previous data showing that loss of VMH leptin receptor-expressing cells prevents weight loss caused by fourth-ventricle leptin infusion and show that the integrated response between the hindbrain and forebrain is heavily dependent on leptin activity in the VMH.

Intranasal Leptin Relieves Sleep-disordered Breathing in Mice with Diet-induced Obesity

RATIONALE

Leptin treats upper airway obstruction and alveolar hypoventilation in leptin-deficient ob/ob mice. However, obese humans and mice with diet-induced obesity (DIO) are resistant to leptin because of poor permeability of the blood-brain barrier. We propose that intranasal leptin will bypass leptin resistance and treat sleep-disordered breathing in obesity.

OBJECTIVES

To assess if intranasal leptin can treat obesity hypoventilation and upper airway obstruction during sleep in mice with DIO.

METHODS

Male C57BL/6J mice were fed with a high-fat diet for 16 weeks. A single dose of leptin (0.4 mg/kg) or BSA (vehicle) were administered intranasally or intraperitoneally, followed by either sleep studies (n = 10) or energy expenditure measurements (n = 10). A subset of mice was treated with leptin daily for 14 days for metabolic outcomes (n = 20). In a separate experiment, retrograde viral tracers were used to examine connections between leptin receptors and respiratory motoneurons.

MEASUREMENTS AND MAIN RESULTS

Acute intranasal, but not intraperitoneal, leptin decreased the number of oxygen desaturation events in REM sleep, and increased ventilation in non-REM and REM sleep, independently of metabolic effects. Chronic intranasal leptin decreased food intake and body weight, whereas intraperitoneal leptin had no effect. Intranasal leptin induced signal transducer and activator of transcription 3 phosphorylation in hypothalamic and medullary centers, whereas intraperitoneal leptin had no effect. Leptin receptor-positive cells were synaptically connected to respiratory motoneurons.

CONCLUSIONS

In mice with DIO, intranasal leptin bypassed leptin resistance and significantly attenuated sleep-disordered breathing independently of body weight.

Attenuation of Inflammation and Leptin Resistance by Pyrogallol-Phloroglucinol-6,6-Bieckol on in the Brain of Obese Animal Models

Obesity induces inflammation both in the adipose tissue and the brain. Activated macrophage infiltration, polarization of macrophages to a more inflammatory type (M1), and increased levels of pro-inflammatory cytokines are related to brain inflammation, which induces leptin resistance in the brain. Pyrogallol-phloroglucinol-6,6-bieckol (PPB), a compound from Ecklonia cava, has anti-inflammatory effects. In this study, we evaluated the effects of PPB effect M1 polarization and inflammation and its ability to restore the effects of leptin, such as a decrease in appetite and body weight. We administered PPB to diet-induced obesity (DIO) and leptin-deficient (ob/ob) mice, evaluated macrophage activation, polarization, and changes of inflammatory cytokine level in adipose tissue and brain, and determined the effect of PPB on leptin resistance or leptin sensitivity in the brain. The levels of activated macrophage marker, M1/M2, and pro-inflammatory cytokines were increased in the adipose tissue and brain of DIO and ob/ob mice than control. TLR4 expression, endoplasmic reticulum (ER) stress, and NF-κB expression in the brain of DIO and ob/ob mice were also increased; this increase was related to the upregulation of SOCS3 and decreased phosphorylated STAT3, which decreased leptin sensitivity in the brain. PPB decreased inflammation in the brain, restored leptin sensitivity, and decreased food intake and weight gain in both DIO and ob/ob mice.

Hypothalamic Neuropeptides Expression and Hypothalamic Inflammation in Adult Rats that Were Exposed to Tobacco Smoke during Breastfeeding: Sex-Related Differences

The hypothalamus controls food intake and energy expenditure. In rats, maternal exposure to nicotine during breastfeeding alters the hypothalamic circuitry of the adult offspring, resulting in leptin resistance, neuropeptides changes and gliosis. Tobacco smoke exposure during lactation causes greater adiposity, hyperphagia and hyperleptinemia in the adult progeny. To understand the central mechanisms underlying the obese phenotype of adult rats that were directly and indirectly exposed to cigarette smoke during lactation, we investigated leptin signaling, orexigenic and anorexigenic neuropeptides expression, as well as astrocyte and microglia markers in hypothalamus. From postnatal day (PND) 3 to 21, Wistar lactating rat dams and their pups were divided into two groups: SE, smoke-exposed in a cigarette-smoking machine (four times/day); Crtl, exposed to filtered air. Offspring of both sexes were euthanized at PND180. The leptin pathway was not altered in SE animals from both sexes. SE males showed increased NPY (arcuate nucleus, ARC), CRH (paraventricular nucleus, PVN), as well as higher GFAP fiber density (ARC and PVN) and IL6 protein content. TRH (PVN) immunohistochemistry was reduced. SE females had lower CART-positive cells (ARC) and lower α-MSH immunostaining intensity (PVN and lateral hypothalamus), with no change of GFAP or IL-6. The protein contents of CX3CR1 (marker of activated microglia) and α7nAChR (anti-inflammatory marker) were not altered in both SE males and females. Neonatal cigarette smoke is deleterious to the hypothalamic circuitry, inducing changes in energy homeostasis favoring hyperphagia and decreased energy expenditure at adulthood in both sexes; however sex-dependent mechanisms were observed.

Phlorotannins from Ecklonia cava Attenuates Palmitate-Induced Endoplasmic Reticulum Stress and Leptin Resistance in Hypothalamic Neurons

Leptin resistance in the hypothalamus has an essential role in obesity. Saturated fatty acids such as palmitate bind to Toll-like receptor 4 (TLR4) and lead to endoplasmic reticulum (ER) stress and leptin resistance. In this study, we evaluated whether extracts of Ecklonia cava would attenuate the ER stress induced by palmitate and reduce leptin resistance in hypothalamic neurons and microglia. We added palmitate to these cells to mimic the environment induced by high-fat diet in the hypothalamus and evaluated which of the E. cava phlorotannins-dieckol (DK), 2,7-phloroglucinol-6,6-bieckol (PHB), pyrogallol-phloroglucinol-6,6-bieckol (PPB), or phlorofucofuroeckol-A (PFFA)-had the most potent effect on attenuating leptin resistance. TLR4 and NF-κB expression induced by palmitate was attenuated most effectively by PPB in both hypothalamic neurons and microglia. ER stress markers were increased by palmitate and were attenuated by PPB in both hypothalamic neurons and microglia. Leptin resistance, which was evaluated as an increase in SOCS3 and a decrease in STAT3 with leptin receptor expression, was increased by palmitate and was decreased by PPB in hypothalamic neurons. The culture medium from palmitate-treated microglia increased leptin resistance in hypothalamic neurons and this resistance was attenuated by PPB. In conclusion, PPB attenuated leptin resistance by decreasing ER stress in both hypothalamic neurons and microglia.

Chronic Light Cycle Disruption Alters Central Insulin and Leptin Signaling as well as Metabolic Markers in Male Mice

Recent evidence suggests that the circadian timing system plays a role in energy and glucose homeostasis, and disruptions to this system are a risk factor for the development of metabolic disorders. We exposed animals to a constantly shifting lighting environment comprised of a 6-hour advance, occurring every 6 days, to chronically disrupt their circadian timing system. This treatment caused a gradual increase in body weight of 12 ± 2% after 12 phase shifts, compared with a 6 ± 1% increase in mice under control lighting conditions. Additionally, after the fifth phase shift, light cycle-disrupted (CD) animals showed a reversal in their diurnal pattern of energy homeostasis and locomotor activity, followed by a subsequent loss of this rhythm. To investigate potential molecular mechanisms mediating these metabolic alterations, we assessed central leptin and insulin sensitivity. We discovered that CD mice had a decrease in central leptin signaling, as indicated by a reduction in the number of phosphorylated signal transducer and activator of transcription 3 immunoreactive cells in the arcuate nucleus of the hypothalamus. Furthermore, CD animals exhibited a marked increase in fasting blood glucose (269.4 ± 21.1 mg/dL) compared with controls (108.8 ± 21.3 mg/dL). This dramatic increase in fasting glucose levels was not associated with an increase in insulin levels, suggesting impairments in pancreatic insulin release. Peripheral hyperglycemia was accompanied by central alterations in insulin signaling at the level of phospho Akt and insulin receptor substrate 1, suggesting that light cycle disruption alters central insulin signaling. These results provide mechanistic insights into the association between light cycle disruption and metabolic disease.

Hypothalamic leptin sensitivity and health benefits of time-restricted feeding are dependent on the time of day in male mice

Synchronization between biologic clocks and metabolism is crucial for most species. Here, we examined the ability of leptin, important in the control of energy metabolism, to induce leptin signaling at the molecular as well as the behavioral level throughout the 24-h day in mice fed either a control or a high-fat diet (HFD). Furthermore, we investigated the effects of time-restricted feeding (TRF; a limitation of HFD access to 6 h each day) on energy metabolism during different periods throughout the 24-h day. In control mice, molecular leptin sensitivity was highest at zeitgeber time (ZT)0 (lights on), declining during the light phase, and increasing during the dark phase. Surprisingly, leptin resistance in HFD-fed mice was only present from the middle of the dark to the middle of the light period. Specifically, when TRF occurred from ZT21 to ZT3 (when leptin resistance in HFD-fed mice was most profound), it resulted in a disruption of the daily rhythms of locomotor activity and energy expenditure and in increased plasma insulin levels compared with other TRF periods. These data provide evidence that leptin sensitivity is controlled by the circadian rhythm and that TRF periods may be most efficient when aligned with the leptin-sensitive period.-Boucsein, A., Rizwan, M. Z., Tups, A. Hypothalamic leptin sensitivity and health benefits of time-restricted feeding are dependent on the time of day in male mice.

Leptin resensitisation: a reversion of leptin-resistant states

Leptin resistance refers to states in which leptin fails to promote its anticipated effects, frequently coexisting with hyperleptinaemia. Leptin resistance is closely associated with obesity and also observed in physiological situations such as pregnancy and in seasonal animals. Leptin resensitisation refers to the reversion of leptin-resistant states and is associated with improvement in endocrine and metabolic disturbances commonly observed in obesity and a sustained decrease of plasma leptin levels, possibly below a critical threshold level. In obesity, leptin resensitisation can be achieved with treatments that reduce body adiposity and leptinaemia, or with some pharmacological compounds, while physiological leptin resistance reverts spontaneously. The restoration of leptin sensitivity could be a useful strategy to treat obesity, maintain weight loss and/or reduce the recidivism rate for weight regain after dieting. This review provides an update and discussion about reversion of leptin-resistant states and modulation of the molecular mechanisms involved in each situation.

"Insulin-like" effects of palmitate compromise insulin signalling in hypothalamic neurons

Saturated fatty acids are implicated in the development of metabolic diseases, including obesity and type 2 diabetes. There is evidence, however, that polyunsaturated fatty acids can counteract the pathogenic effects of saturated fatty acids. To gain insight into the early molecular mechanisms by which fatty acids influence hypothalamic inflammation and insulin signalling, we performed time-course experiments in a hypothalamic cell line, using different durations of treatment with the saturated fatty acid palmitate, and the omega-3 polyunsaturated fatty acid, docosahexaenoic acid (DHA). Western blot analysis revealed that palmitate elevated the protein levels of phospho(p)AKT in a time-dependent manner. This effect is involved in the pathogenicity of palmitate, as temporary inhibition of the PI3K/AKT pathway by selective PI3K inhibitors prevented the palmitate-induced attenuation of insulin signalling. Similar to palmitate, DHA also increased levels of pAKT, but to a weaker extent. Co-administration of DHA with palmitate decreased pAKT close to the basal level after 8 h, and prevented the palmitate-induced reduction of insulin signalling after 12 h. The monounsaturated fatty acid oleate had a similar effect on the palmitate-induced attenuation of insulin signalling, the polyunsaturated fatty acid linoleate had no effect. Measurement of the inflammatory markers pJNK and pNFκB-p65 revealed tonic elevation of both markers in the presence of palmitate alone. DHA alone transiently induced elevation of pJNK, returning to basal levels by 12 h treatment. Co-administration of DHA with palmitate prevented palmitate-induced inflammation after 12 h, but not at earlier timepoints.

Giving a good start to a new life via maternal brain allostatic adaptations in pregnancy

Successful pregnancy requires adjustments to multiple maternal homeostatic mechanisms, governed by the maternal brain to support and enable survival of the growing fetus and placenta. Such adjustments fit the concept of allostasis (stability through change) and have a cost: allostatic load. Allostasis is driven by ovarian, anterior pituitary, placental and feto-placental hormones acting on the maternal brain to promote adaptations that support the pregnancy and protect the fetus. Many women carry an existing allostatic load into pregnancy, from socio-economic circumstances, poor mental health and in 'developed' countries, also from obesity. These pregnancies have poorer outcomes indicating negative interactions (failing allostasis) between pre-pregnancy and pregnancy allostatic loads. Use of animal models, such as adult prenatally stressed female offspring with abnormal neuroendocrine, metabolic and behavioural phenotypes, to probe gene expression changes, and epigenetic mechanisms in the maternal brain in adverse pregnancies are discussed, with the prospect of ameliorating poor pregnancy outcomes.

Effects of Leptin on the Skeleton

Leptin originates in adipocytes, including those in bone marrow, and circulates in concentrations 20 to 90 times higher than those in the cerebrospinal fluid. It has direct anabolic effects on osteoblasts and chondrocytes, but it also influences bone indirectly, via the hypothalamus and sympathetic nervous system, via changes in body weight, and via effects on the production of other hormones (e.g., pituitary). Leptin's role in bone physiology is determined by the balance of these conflicting effects. Reflecting this inconsistency, the leptin-deficient mouse has reduced length and bone mineral content of long bones but increased vertebral trabecular bone. A consistent bone phenotype in human leptin deficiency has not been established. Systemic leptin administration in animals and humans usually exerts a positive effect on bone mass, and leptin administration into the cerebral ventricles usually normalizes the bone phenotype in leptin-deficient mice. Reflecting the role of the sympathetic nervous system in mediating the central catabolic effects of leptin on the skeleton, β-adrenergic agonists and antagonists have major effects on bone in mice, but this is not consistently seen in humans. The balance of the central and peripheral effects of leptin on bone remains an area of substantial controversy and might vary between species and according to other factors such as body weight, baseline circulating leptin levels, and the presence of specific pathologies. In humans, leptin is likely to contribute to the positive relationship observed between adiposity and bone density, which allows the skeleton to respond appropriately to changes in soft tissue mass.

Programming of central and peripheral insulin resistance by low birthweight and postnatal catch-up growth in male mice

AIMS

Intra-uterine growth restriction (IUGR) followed by accelerated postnatal growth is associated with an increased risk of obesity and type 2 diabetes. We aimed to determine central and peripheral insulin sensitivity in mice that underwent IUGR followed by postnatal catch-up growth and investigate potential molecular mechanisms underpinning their physiology.

METHODS

We used a C57BL/6J mouse model of maternal diet-induced IUGR (maternal diet, 8% protein) followed by cross-fostering to a normal nutrition dam (maternal diet, 20% protein) and litter size manipulation to cause accelerated postnatal catch-up growth. We performed intracerebroventricular insulin injection and hyperinsulinaemic-euglycaemic clamp studies to examine the effect of this early nutritional manipulation on central and peripheral insulin resistance. Furthermore, we performed quantitative real-time PCR and western blotting to examine the expression of key insulin-signalling components in discrete regions of the hypothalamus.

RESULTS

IUGR followed by accelerated postnatal growth caused impaired glucose tolerance and peripheral insulin resistance. In addition, these 'recuperated' animals were resistant to the anorectic effects of central insulin administration. This central insulin resistance was associated with reduced protein levels of the p110β subunit of phosphoinositide 3-kinase (PI3K) and increased serine phosphorylation of IRS-1 in the arcuate nucleus (ARC) of the hypothalamus. Expression of the gene encoding protein tyrosine phosphatase 1B (PTP1B; Ptpn1) was also increased specifically in this region of the hypothalamus.

CONCLUSIONS/INTERPRETATION

Mice that undergo IUGR followed by catch-up growth display peripheral and central insulin resistance in adulthood. Recuperated offspring show changes in expression/phosphorylation of components of the insulin signalling pathway in the ARC. These defects may contribute to the resistance to the anorectic effects of central insulin, as well as the impaired glucose homeostasis seen in these animals.

Adipokines in neurovascular diseases

Adipose tissue is now described as an endocrine organ secreting a number of adipokines contributing to the development of inflammation and metabolic imbalance, but also endothelial dysfunction, vascular remodeling, atherosclerosis, and ischemic stroke. Leptin, adiponectin, and resistin are the most studied adipokines which play important roles in the regulation of cardiovascular homeostasis. Leptin and adiponectin mediate both proatherogenic and antiatherogenic responses. Leptin and adiponectin have been linked to the development of coronary heart disease and may be involved in the underlying biological mechanism of ischemic stroke. Resistin, a pro-inflammatory cytokine, is predictive of atherosclerosis and poor clinical outcomes in patients with coronary artery disease and ischemic stroke. The changes in serum levels of novel adipokines apelin, visfatin are also associated with acute ischemic stroke. These adipokines have been proposed as potential prognostic biomarkers of cardiovascular mortality/morbidity and therapeutic targets in patients with cardiometabolic diseases. In this article, we summarize the biologic role of the adipokines and discuss the link between dysfunctional adipose tissue and metabolic/inflammation imbalance, consequently endothelial damage, progression of atherosclerotic disease, and the occurrence of ischemic stroke.