OB protein: A peripheral signal linking adiposity and central neural networks

Gonadotropin Inhibitory Hormone and Its Receptor: Potential Key to the Integration and Coordination of Metabolic Status and Reproduction

Since its discovery as a novel gonadotropin inhibitory peptide in 2000, the central and peripheral roles played by gonadotropin-inhibiting hormone (GnIH) have been significantly expanded. This is highlighted by the wide distribution of its receptor (GnIH-R) within the brain and throughout multiple peripheral organs and tissues. Furthermore, as GnIH is part of the wider RF-amide peptides family, many orthologues have been characterized across vertebrate species, and due to the promiscuity between ligands and receptors within this family, confusion over the nomenclature and function has arisen. In this review, we intend to first clarify the nomenclature, prevalence, and distribution of the GnIH-Rs, and by reviewing specific localization and ligand availability, we propose an integrative role for GnIH in the coordination of reproductive and metabolic processes. Specifically, we propose that GnIH participates in the central regulation of feed intake while modulating the impact of thyroid hormones and the stress axis to allow active reproduction to proceed depending on the availability of resources. Furthermore, beyond the central nervous system, we also propose a peripheral role for GnIH in the control of glucose and lipid metabolism at the level of the liver, pancreas, and adipose tissue. Taken together, evidence from the literature strongly suggests that, in fact, the inhibitory effect of GnIH on the reproductive axis is based on the integration of environmental cues and internal metabolic status.

Inflammation and macrophage modulation in adipose tissues

The adipose tissue is an active endocrine organ that harbours not only mature and developing adipocytes but also a wide array of immune cells, including macrophages, a key immune cell in determining metabolic functionality. With adipose tissue expansion, M1 pro-inflammatory macrophage infiltration increases, activates other immune cells, and affects lipid trafficking and metabolism, in part via inhibiting mitochondrial function and increasing reactive oxygen species (ROS). The pro-inflammatory cytokines produced and released interfere with insulin signalling, while inhibiting M1 macrophage activation improves systemic insulin sensitivity. In healthy adipose tissue, M2 alternative macrophages predominate and associate with enhanced lipid handling and mitochondrial function, anti-inflammatory cytokine production, and inhibition of ROS. The sequence of events leading to macrophage infiltration and activation in adipose tissue remains incompletely understood but lipid handling of both macrophages and adipocytes appears to play a major role.

2-Methoxyestradiol ameliorates glucose tolerance with the increase in β-cell mass in db/db mice

Aims/Introduction:  2‐Methoxyestradiol (2ME) is an estradiol metabolite with little estrogenic activity. Previous data identified its anti‐carcinogenic properties and possible cardiovascular benefits. However, its effect on diabetes mellitus has not been fully elucidated. The aim of the present study was to determine the effects of 2ME on glucose metabolism in the diabetic state.

Materials and Methods:  To evaluate the effects of 2ME, pellets of two different doses of the drug were implanted into female db/db mice at the age of 5 weeks. Intraperitoneal glucose tolerance test and insulin tolerance test were carried out at the age of 8 weeks. The pancreas was harvested for morphological analysis and β‐cell function at the age of 9 weeks.

Results:  2ME improved random blood glucose levels and glucose tolerance with increases in insulin levels during an intraperitoneal glucose tolerance test. Insulin sensitivity judged by an insulin tolerance test was comparable in the low‐ and high‐dose 2ME groups and the control group. Although glucose‐stimulated insulin secretion in isolated islets was comparable among the three groups, β‐cell mass in 2ME‐treated groups was higher than the control group. In the 2ME‐treated groups, the number of Ki67‐positive cells in islets was higher, whereas the number of cleaved caspase‐3‐positive cells was comparable with the control.

Conclusions:  2ME ameliorates glucose tolerance by promoting the proliferation of β‐cell mass in db/db mice. Our data suggests its potential clinical usefulness as a disease‐modifying drug for type 2 diabetes mellitus. (J Diabetes Invest, doi: 10.1111/j.2040‐1124.2010.00087.x, 2011)

Effects of ileal interposition on glucose metabolism in obese rats with diabetes

BACKGROUND

Ileal interposition (IT), in which the distal ileum is transposed isoperistaltically into the proximal jejunum, is considered as a procedure for metabolic or antidiabetes surgery. Our aim was to study the effects of IT on glycemic control, fat metabolism, and hormonal changes in obese rats with spontaneous diabetes.

METHODS

Animals were divided into either an IT or a sham (SH) group. They underwent an oral glucose tolerance test (OGTT) before and 4 and 8 weeks after the operation. All animals were killed 10 weeks after operation for analyses of tissue weight (liver, pancreas, epididymal fat, brown fat), immunoblotting of uncoupling protein-1 (UCP1) protein in brown adipose tissue (BAT), and fasting plasma levels of glucose, insulin, glucagon-like peptide (GLP)-1, peptide YY (PYY), glucose-dependent insulinotropic polypeptide (GIP), and leptin.

RESULTS

Body weight increased postoperatively in both groups compared with preoperative weight, but it did not differ between the 2 groups. Eight weeks postoperatively, integrated blood glucose levels during the OGTT were decreased in IT compared with SH (P < .05). Fasting plasma levels of insulin, GLP-1, and GIP did not differ between the 2 groups, but PYY levels were higher in the IT animals (P < .01). The weight of epididymal and BATs, homeostasis model assessment insulin resistance, and fasting plasma leptin levels were decreased in the IT group (P < .05). Expression of UCP1 was higher in IT than SH animals (P < .05).

CONCLUSION

These results suggest that IT improves glucose and lipid metabolism by decreasing insulin resistance and epididymal fat, and increased expression of UCP1 in BAT might be among the mechanisms responsible.

Cracking taste codes by tapping into sensory neuron impulse traffic

Insights into the biological basis for mammalian taste quality coding began with electrophysiological recordings from "taste" nerves and this technique continues to produce essential information today. Chorda tympani (geniculate ganglion) neurons, which are particularly involved in taste quality discrimination, are specialists or generalists. Specialists respond to stimuli characterized by a single taste quality as defined by behavioral cross-generalization in conditioned taste tests. Generalists respond to electrolytes that elicit multiple aversive qualities. Na(+)-salt (N) specialists in rodents and sweet-stimulus (S) specialists in multiple orders of mammals are well characterized. Specialists are associated with species' nutritional needs and their activation is known to be malleable by internal physiological conditions and contaminated external caloric sources. S specialists, associated with the heterodimeric G-protein coupled receptor T1R, and N specialists, associated with the epithelial sodium channel ENaC, are consistent with labeled line coding from taste bud to afferent neuron. Yet, S-specialist neurons and behavior are less specific than T1R2-3 in encompassing glutamate and E generalist neurons are much less specific than a candidate, PDK TRP channel, sour receptor in encompassing salts and bitter stimuli. Specialist labeled lines for nutrients and generalist patterns for aversive electrolytes may be transmitting taste information to the brain side by side. However, specific roles of generalists in taste quality coding may be resolved by selecting stimuli and stimulus levels found in natural situations. T2Rs, participating in reflexes via the glossopharynygeal nerve, became highly diversified in mammalian phylogenesis as they evolved to deal with dangerous substances within specific environmental niches. Establishing the information afferent neurons traffic to the brain about natural taste stimuli imbedded in dynamic complex mixtures will ultimately "crack taste codes."

Leptin modulation of peripheral controls of meal size

Leptin reduces food intake through a specific effect on meal size. Investigations into how this within meal effect of leptin is mediated have demonstrated that leptin increases the ability of within meal inhibitory feedback signaling to limit intake and activate neurons within the nucleus of the solitary tract (NTS). Leptin's effects on neural activation can be demonstrated both as an increase in c-fos activation and as increase in electrophysiological activity in response to peripheral stimuli. Leptin can exert these effects through interactions at hypothalamic sites and activation of a descending pathway. NPY has opposite effect suggesting a role for reduced NPY signaling in the actions of leptin. Forebrain ventricular administration of a melanocortin agonist does not mimic the actions of leptin. As well as modulating within meal signaling through a descending pathway leptin, NPY and melanocortins could work directly at hindbrain integrative sites suggesting the possibility of distributed controls of meal size by anorexigenic and orexigenic signaling.

Prenatal programming of postnatal obesity: fetal nutrition and the regulation of leptin synthesis and secretion before birth

Exposure to either an increased or decreased level of intrauterine nutrition can result in an increase in adiposity and in circulating leptin concentrations in later life. In animals such as the sheep and pig in which fat is deposited before birth, leptin is synthesised in fetal adipose tissue and is present in the fetal circulation throughout late gestation. In the sheep a moderate increase or decrease in the level of maternal nutrition does not alter fetal plasma leptin concentrations, but there is evidence that chronic fetal hyperglycaemia and hyperinsulinaemia increase fetal fat mass and leptin synthesis within fetal fat depots. Importantly, there is a positive relationship between the relative mass of the 'unilocular' component of fetal perirenal and interscapular adipose tissue and circulating fetal leptin concentrations in the sheep. Thus, as in the neonate and adult, circulating leptin concentrations may be a signal of fat mass in fetal life. There is also evidence that leptin can act to regulate the lipid storage, leptin synthetic capacity and potential thermogenic functions of fat before birth. Thus, leptin may act as a signal of energy supply and have a 'lipostatic' role before birth. Future studies are clearly required to determine whether the intrauterine and early postnatal nutrient environment programme the endocrine feedback loop between adipose tissue and the central and peripheral neuroendocrine systems that regulate energy balance, resulting in an enhanced risk of obesity in adult life.

Gene and cell therapies for diabetes mellitus: strategies and clinical potential

The last 5 years have witnessed an explosion in the use of genes and cells as biomedicines. While primarily aimed at cancer, gene engineering and cell therapy strategies have additionally been used for Mendelian, neurodegenerative and metabolic disorders. The main focus of gene and cell therapy strategies in metabolism has been diabetes mellitus. This disease is a disorder of glucose homeostasis, either due to the immune-mediated eradication of pancreatic beta cells in the islets of Langerhans (type 1 diabetes) or resulting from insulin resistance and obesity syndromes where the insulin-producing capability of the beta cell is ultimately exhausted in the face of insensitivity to the effects of insulin in the peripheral glucose-utilising tissues (type 2 diabetes). A significant number of animal studies have demonstrated the potential in restoring normoglycaemia by islet transplantation in the context of immunoregulation achieved by gene transfer of immunoregulatory genes to allo- and xenogeneic islets ex vivo. Additionally, gene and cell therapy has also been used to induce tolerance to auto- and alloantigens and to generate the tolerant state in autoimmune rodent animal models of type 1 diabetes or rodent recipients of allogeneic/xenogeneic islet transplants. The achievements of gene and cell therapy in type 2 diabetes are less evident, but seminal studies promise that this modality can be relevant to treat and perhaps prevent the underlying causes of the disease. Here we present an overview of the current status of gene and cell therapy for type 1 and 2 diabetes and we propose potential therapeutic options that could be clinically useful. For type 1 diabetes, transplantation of islets engineered to evade or suppress the recipient immune response is the most readily-available technology today. A number of gene delivery vectors encoding proteins that impair a variety of immune cells have already been examined and proven versatile. More challenging but, nonetheless, just over the horizon are attempts to promote tolerance to islet allografts. Type 2 diabetes will likely require a better understanding of the processes that determine insulin sensitivity in the periphery. Targeting tissues such as muscle and fat with vectors encoding genes whose products promote insulin sensitivity and glucose uptake is an approach that does not carry with it the side-effects often associated with pharmacologic agents currently in use. In the end, progress in vector design, elucidation of antigen-specific immunity and insulin sensitivity will provide the framework for gene drug use in the treatment of type 1 and type 2 diabetes.

Acute changes in the response to peripheral leptin with alteration in the diet composition

Dietary induced obesity in rodents is associated with a resistance to leptin. We have investigated the hypothesis that dietary fat per se alters the feeding response to peripheral leptin in rats that were fed either their habitual high- or low-fat diet or were naively exposed to the alternative diet. Osborne-Mendel rats were adapted to either high- or low-fat diet. Food-deprived rats were given either leptin (0.5 mg/kg body wt ip) or saline, after which they were provided with either their familiar diet or the alternative diet. Food intake of rats adapted and tested with the low-fat diet was reduced 4 h after leptin injection, whereas rats adapted and tested with a high-fat diet did not respond to leptin. Leptin was injected again 1 and 5 days after the high-fat diet-adapted rats were switched to the low-fat diet. Leptin reduced the food intake on both days. In contrast, when low-fat diet-adapted rats were switched to a high-fat diet, the leptin inhibitory response was present on day 1 but not observed on day 5. Peripheral injection of leptin increased serum corticosterone level and decreased hypothalamic neuropeptide Y mRNA expression in rats fed the low-fat but not the high-fat diet for 20 days. The data suggest that dietary fat itself, rather than obesity, may induce leptin resistance within a short time of exposure to a high-fat diet.

Linkage analysis of candidate obesity genes among the Mexican-American population of Starr County, Texas

Recent advances in the molecular basis of body fat regulation have identified several genes in which genetic variation may influence obesity and related measures in human populations. Genes that have been shown to have a regulatory function in the control of body fat utilization, eating behavior, and/or metabolic rate in rodent models of obesity include leptin (LEP), leptin receptor (LEPR), neuropeptide Y (NPY), NPY Y1 receptor (NPYY1), glucagon-like peptide-1 (GLP-1), GLP-1 receptor (GLP1R), and uncoupling protein 1 (UCP1). We have typed microsatellite markers located within or near these seven candidate obesity genes in 302 non-diabetic individuals from 59 Mexican-American families from Starr County, Texas. Sib pair linkage analysis was used to examine linkage between these genes and obesity status (obese siblings only; n = 170 pairs) and several obesity-related quantitative variables (all siblings; n = 545 total sibling pairs). Significant linkage (P = 0.042) was found between obesity and NPY within the obese sibling pairs. No other candidate gene was linked to obesity status in this subsample. Consistent with the obese sib pair linkage results, NPY showed evidence of linkage to body weight (P = 0.020), abdominal circumference (P = 0.031), hip circumference (P = 0.012), diastolic blood pressure (P = 0.005), and a composite measure of body mass and size (P = 0.048) in the entire sibling sample. Other significant linkages observed were between LEP and waist/hip ratio (P = 0.010), total cholesterol (P = 0.030), and HDL cholesterol (P = 0.026) and between LEPR and fasting blood glucose (P = 0.018) and diastolic blood pressure (P = 0.003). These results support further investigation of NPY, LEP, and LEPR to identify genetic variation that may influence obesity status, glucose and lipid metabolism, and blood pressure in Mexican Americans.

Expression of OB receptor splice variants during prenatal development of the mouse

In adult animals, signaling through the leptin receptor (OB-R) has been shown to play a critical role in fat metabolism. However, it is not known when these receptors are first expressed and what their role may be during embryonic development. To date, at least 6 splice variants of the OB-R have been identified. Although the function of each of these individual splice variants are unknown, only one of them, ob-rL ,encodes a receptor with a long intracellular domain that is implicated in OB-R signaling. In this study we have used in situ hybridization to examine the localization of OB-R splice variants during embryonic development of C57B1/6J mice. Using a probe, ob-r, that recognizes all of the splice variants, ob-r mRNA was found to be distributed in developing bone, mesenchyme, notochord and liver. In addition, epithelial structures including leptomeninges, choroid plexi and hair follicles also expressed ob-r. No ob-r mRNA was detected in the CNS. ob-rL, expression was only detected in notochord, bone and mesenchyme. The differential expression of these two mRNA isoforms suggests that the extracellular and intracellular domains of the OB receptor perform different biological functions.

Splice variants of the OB receptor gene are differentially expressed in brain and peripheral tissues of mice

A high affinity receptor for OB protein was recently cloned from the choroid plexus of mice. At least six alternatively spliced forms of the OB receptor (OB-R) gene have been described, all of which encode proteins containing the OB-R extracellular domain. One splice variant encodes a receptor with a long intracellular domain, OB-RL, that has been implicated in OB-R signaling. Here, we have used in situ hybridization to examine the localization of OB-R splice variants in brain and peripheral tissues of adult and newborn mice. Using a probe hybridizing with all known splice variants, we confirmed that OB-R mRNA was widely distributed in the adult tissues. In the CNS, choroid plexus was the major site of expression. We now demonstrate that OB-R mRNA is expressed in peripheral tissues; primarily associated with connective tissues. In addition, OB-R mRNA was detected at higher levels in peripheral tissues of newborn mice than in adult mice. With a probe specific for OB-RL, we confirmed that high mRNA expression was detected in hypothalamic nuclei, while low levels were observed in choroid plexus. We now report that in peripheral tissues of adult mice, OB-RL mRNA expression was either very low or undetectable. In newborn mice, the pattern of OB-RL message expression in the CNS was similar to that of adult mice, while bone was the site of highest OB-RL message expression in the peripheral tissue. These data suggest different biological roles for OB-R splice variants encoding the short and long forms of OB-R. The localization of OB-RL to hypothalamic nuclei supports the idea that OB-RL is the brain receptor that mediates OB protein signaling and actions. In addition, the expression of OB-R message in newborn mice also suggests a biological role of OB-R during development in mice.

Brain administration of OB protein (leptin) inhibits neuropeptide-Y-induced feeding in ob/ob mice

OB protein (or leptin) administration causes a long-lasting reduction in food intake and body weight in obese ob/ob mice. Neuropeptide Y, a stimulator of feeding, has been proposed to be a major mediator of the biological actions of OB protein. To test this hypothesis, the interaction of brain administration of exogenous OB protein and NPY on the feeding behavior of ob/ob mice was examined. Human OB protein, in a dose-dependent manner, partially or completely blocked feeding induced by exogenous NPY. These results demonstrate that OB protein can functionally antagonize and dominate the actions of exogenous NPY on feeding.

Plasma leptin and hunger ratings in healthy humans

Leptin, the ob gene product, is considered as a signal involved in the regulation of energy stores through centrally mediated effects on ingestive behavior and metabolism. To investigate the relationships between appetite-related sensations and circulating leptin in humans, 12 healthy male volunteers were served two test lunches (1200 kcal, 38% carbohydrate, 50% fat, 12% protein) with comparable palatability but resulting in different postprandial hunger ratings, and plasma leptin concentrations were monitored during 8 h after meal ingestion. Whereas postprandial hunger ratings (assessed by visual analog scales) and plasma insulin patterns differed significantly between meals, no significant difference in postprandial plasma leptin concentrations was observed between the two meals. A moderate but significant increase in mean plasma leptin was found over time. We conclude that postprandial plasma leptin concentrations do not significantly differ between two meals inducing different effects on hunger subjective feelings. These data are in agreement with a role for leptin in the regulation of energy balance as a long-term adiposity-related signal rather than a short-term meal-related factor.