Role of brain insulin receptor in control of body weight and reproduction

Decreasing hypothalamic insulin receptors causes hyperphagia and insulin resistance in rats

We investigated the role of hypothalamic insulin signaling in the regulation of energy balance and insulin action in rats through selective decreases in insulin receptor expression in discrete hypothalamic nuclei. We generated an antisense oligodeoxynucleotide directed against the insulin receptor precursor protein and administered this directly into the third cerebral ventricle. Immunostaining of rat brains after 7-day administration of the oligodeoxynucleotide showed a selective decrease of insulin receptor protein within cells in the medial portion of the arcuate nucleus (decreased by approximately 80% as compared to rats treated with a control oligodeoxynucleotide). Insulin receptors in other hypothalamic and extra-hypothalamic areas were not affected. This selective decrease in hypothalamic insulin receptor protein was accompanied by rapid onset of hyperphagia and increased fat mass. During insulin-clamp studies, physiological hyperinsulinemia decreased glucose production by 55% in rats treated with control oligodeoxynucleotides but by only 25% in rats treated with insulin receptor antisense oligodeoxynucleotides. Thus, insulin receptors in discrete areas of the hypothalamus have a physiological role in the control of food intake, fat mass and hepatic action of insulin.

Aging is a deprivation syndrome driven by a germ-soma conflict

Evolution through natural selection can be described as driven by a perpetual conflict of individuals competing for limited resources. Recently, I postulated that the shortage of resources godfathered the evolutionary achievements of the differentiation-apoptosis programming [Rev. Neurosci. 12 (2001) 217]. Unicellular deprivation-induced differentiation into germ cell-like spores can be regarded as the archaic reproduction events which were fueled by the remains of the fratricided cells of the apoptotic fruiting body. Evidence has been accumulated suggesting that conserved through the ages as the evolutionary legacy of the germ-soma conflict, the somatic loss of immortality during the ontogenetic segregation of primordial germ cells recapitulates the archaic fate of the fruiting body. In this heritage, somatic death is a germ cell-triggered event and has been established as evolutionary-fixed default state following asymmetric reproduction in a world of finite resources. Aging, on the other hand, is the stress resistance-dependent phenotype of the somatic resilience that counteracts the germ cell-inflicted death pathway. Thus, aging is a survival response and, in contrast to current beliefs, is antagonistically linked to death that is not imposed by group selection but enforced upon the soma by the selfish genes of the "enemy within". Environmental conditions shape the trade-off solutions as compromise between the conflicting germ-soma interests. Mechanistically, the neuroendocrine system, particularly those components that control energy balance, reproduction and stress responses, orchestrate these events. The reproductive phase is a self-limited process that moulds onset and progress of senescence with germ cell-dependent factors, e.g. gonadal hormones. These degenerate the regulatory pacemakers of the pineal-hypothalamic-pituitary network and its peripheral, e.g. thymic, gonadal and adrenal targets thereby eroding the trophic milieu. The ensuing cellular metabolic stress engenders adaptive adjustments of the glucose-fatty acid cycle, responses that are adequate and thus fitness-boosting under fuel shortage (e.g. during caloric restriction) but become detrimental under fuel abundance. In a Janus-faced capacity, the cellular stress response apparatus expresses both tolerogenic and mutagenic features of the social and asocial deprivation responses [Rev. Neurosci. 12 (2001) 217]. Mediated by the derangement of the energy-Ca(2+)-redox homeostatic triangle, a mosaic of dedifferentiation/apoptosis and mutagenic responses actuates the gradual exhaustion of functional reserves and eventually results in a multitude of aging-related diseases. This scenario reconciles programmed and stochastic features of aging and resolves the major inconsistencies of current theories by linking ultimate and proximate causes of aging. Reproduction, differentiation, apoptosis, stress response and metabolism are merged into a coherent regulatory network that stages aging as a naturally selected, germ cell-triggered and reproductive phase-modulated deprivation response.

Mouse models of insulin resistance

The hallmarks of type 2 diabetes are impaired insulin action in peripheral tissues and decreased pancreatic beta-cell function. Classically, the two defects have been viewed as separate entities, with insulin resistance arising primarily from impaired insulin-dependent glucose uptake in skeletal muscle, and beta-cell dysfunction arising from impaired coupling of glucose sensing to insulin secretion. Targeted mutagenesis and transgenesis involving components of the insulin action pathway have changed our understanding of these phenomena. It appears that the role of insulin signaling in the pathogenesis of type 2 diabetes has been overestimated in classic insulin target tissues, such as skeletal muscle, whereas it has been overlooked in liver, pancreatic beta-cells, and brain, which had been thought not to be primary insulin targets. We review recent progress and try to reconcile areas of apparent controversy surrounding insulin signaling in skeletal muscle and pancreatic beta-cells.

Genetic analysis of insulin signaling in Drosophila

Studies in the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans have revealed that components of the insulin signaling pathway have been highly conserved during evolution. Genetic analysis in Drosophila suggests that structural conservation also extends to the functional level. Flies carrying mutations that reduce insulin signaling have a growth deficiency phenotype similar to that seen in mice with disruptions of genes encoding insulin-like growth factors (IGFs) or the IGF-I receptor. Recent studies in flies have demonstrated a role for the insulin signaling pathway in the regulation of metabolism, reproduction and lifespan via modulation of central neuroendocrine pathways. Similarly, mice with loss of brain insulin receptors or insulin receptor substrate 2 deficiency exhibit neuroendocrine defects and female infertility. These parallels suggest that the insulin system has multiple conserved roles, acting directly to modulate growth and indirectly, via the neuroendocrine system, to modulate peripheral physiology in response to changes in nutrient availability.

Functional integrity of ErbB-4/-2 tyrosine kinase receptor complex in the hypothalamus is required for maintaining normal reproduction in young adult female rats

ErbB-1 tyrosine kinase receptors are necessary for maintaining female reproduction by modulating the release of LH-releasing hormone (LHRH). Changes in ErbB-1 signaling capacity in aging rats are linked to compromised reproduction. The interactive and synergistic nature of different members of ErbB receptors in mediating signal transduction exists in many cellular systems. Particularly, the interactions among ErbB-1 and ErbB-2 or ErbB-4 and ErbB-2 are known to be involved in the stimulation of LHRH secretion during sexual maturation. Thus, ErbB-4/-2 receptors may also play a role in maintaining reproduction during adulthood, and consequently, alteration in ErbB-4/-2 signaling capacity may contribute to compromised reproductive competence during aging. By in situ hybridization histochemistry, ErbB-4/-2 mRNAs were detected in the preoptic area (POA) and arcuate nucleus, which are important areas involved in the control of LHRH neuronal activity. RT-PCR analyses showed that levels of ErbB-4/-2 mRNA increased to a maximal value in the POA of young adult animals before the LH surge. However, no such increase was found in middle-aged female rats. The timing of the decrease in ErbB-4 mRNA in the median eminence-arcuate nucleus of middle-aged rats was delayed compared with that in young adult animals. Disruption of functional ErbB-4/-2 receptor complex by blocking ErbB-2 receptor synthesis in the hypothalamus via an infusion of ErbB-2 antisense oligodeoxynucleotide resulted in an estrous acyclicity in young adult rats. These results indicate that changes in ErbB-4/-2 gene expression and functional integrity of this ErbB-4/-2 receptor complex in the hypothalamus of middle-aged female animals may lead to an altered preovulatory LH release. Thus, the ErbB-4/-2 receptor complex is a physiological component necessary for maintaining female reproduction.

Reproductive abnormalities in human IGF binding protein-1 transgenic female mice

The mechanisms responsible for reproductive abnormalities in transgenic female mice overexpressing human IGF binding protein-1 (IGFBP-1) in the liver have been investigated. At 2 months of age, none of these transgenic mice exhibited ovarian cyclicity. Genital tract and ovary tissue weights were reduced in transgenic mice, this weight reduction being disproportionate with the reduction of body weight. Examination of ovarian follicular population revealed a marked decrease in the number of corpora lutea and gonadotropin-dependent follicles, suggesting an alteration of terminal follicular growth and ovulation. Stimulation of ovaries by exogenous gonadotropins revealed that ovaries from transgenic mice ovulated less oocytes than nontransgenic mice. This lower responsiveness of ovaries from transgenic mice to gonadotropins was not associated with a decrease in FSH-, LH- or IGF-I receptor expression. Transgenic and nontransgenic mice have similar circulating LH and FSH concentrations at dioestrus, after castration, 46 h after equine CG administration, or 15 min after GnRH injection. However, LH concentrations were 8-fold higher in pituitaries from transgenic vs. nontransgenic mice. Moreover, the size of LH-immunoreactive cells was reduced and their number was increased, suggesting a subtle alteration of LH secretion. Overall, these data indicate that reduced fertility in transgenic female mice overexpressing human IGFBP-1 are mainly due to an alteration of terminal follicular growth leading to a decrease in natural and induced ovulation rate, likely due to an impairment of IGF-I action on follicular cells. Increased circulating IGFBP-1 concentrations may additionally lead to altered GnRH and LH pulsatility and thereby exacerbate the ovulation defect.

Insulin resistance in the sisters of women with polycystic ovary syndrome: association with hyperandrogenemia rather than menstrual irregularity

This study was performed to determine whether the sisters of women with polycystic ovary syndrome (PCOS) have evidence for insulin resistance. Three hundred and thirty-six women with PCOS, 307 sisters of these probands, and 47 control women were studied. The sisters were grouped by phenotypes: PCOS [hyperandrogenemia (HA) with chronic oligo- or amenorrhea, n = 39], HA with regular menses (n = 36), unaffected (UA; n = 122), and unknown (n = 110). The analyses were adjusted for age and body mass index. PCOS and HA sisters of women with PCOS had similar and significantly elevated fasting insulin levels (P = 0.001) as well as similar and significantly decreased fasting glucose/insulin ratios (P < 0.001) suggestive of insulin resistance compared with UA sisters and control women. Markers of insulin resistance were associated with hyperandrogenemia and not with menstrual irregularity. PCOS sisters also had decreased levels of SHBG (P = 0.02) suggestive of higher ambient insulin levels. PCOS sisters had increased levels of proinsulin (P = 0.04) compared with control women, which suggested pancreatic beta-cell dysfunction in this group of sisters. The magnitude of obesity also differed significantly among the groups of sisters. The PCOS sisters were significantly more obese than all the other groups, and the HA sisters were more obese than the UA sisters. We conclude that markers of insulin resistance are associated with hyperandrogenemia rather than menstrual irregularity in the sisters of women with PCOS. Menstrual irregularity may be related to the magnitude of insulin sensitivity or insulin secretion or to other factors associated with obesity.

Brain-derived neurotrophic factor (BDNF) regulates glucose and energy metabolism in diabetic mice

Neurotrophins are important regulators in the embryogenesis, development and functioning of nervous systems. In addition to the efficacy of brain-derived neurotrophic factor (BDNF) in neurological disorders, we have found that BDNF demonstrates endocrinological functions and reduces food intake and blood glucose concentration in rodent obese diabetic models, such as C57BL/KsJ-db/db mice. The hypoglycemic effect of BDNF was found to be stronger in younger db/db mice with hyperinsulinemia than in older mice. While BDNF itself did not alter blood glucose in normal mice and streptozotocin (STZ)-treated mice, BDNF enhanced the hypoglycemic effect of insulin in STZ-treated mice. These data indicate that BDNF needs endogenous or exogenous insulin to show hypoglycemic action. In addition, BDNF treatment enhanced energy expenditure in db/db mice. The efficacy of BDNF in regulating glucose and energy metabolism was reproduced through intracerebroventricular administration, suggesting that BDNF acted directly on the hypothalamus, the autonomic center of the brain.

Insulin resistance: concepts, controversies, and the role of nutrition

Insulin resistance is a prevalent condition, in which insulin loses its normal physiological action. Since people were first classified as insulin resistant over 60 years ago, one of the main discoveries has been that insulin resistance clusters with other risk factors such as obesity, elevated triglycerides, and low high-density lipoprotein cholesterol, increasing cardiovascular disease risk. Although insulin resistance appears to manifest first in the periphery and then in the liver, other sites, such as the brain and the pancreatic &b.beta-cell, may play pathogenic roles. Factors contributing to insulin resistance at these sites include perturbations in free fatty acids, glucose, and hormone-signalling, some of which may be linked to various genetic polymorphisms. Appropriate nutritional treatment for insulin resistance is controversial. Two main approaches are drawn from diabetes recommendations: i) a high-carbohydrate, low-fat, high-fibre diet emphasizing low glycemic-index foods and ii) sharing calories between monounsaturated fat and complex carbohydrate at the expense of saturated fat. Recent interest in insulin resistance has prompted the development of new guidelines. Promising data have also emerged, showing that a high-carbohydrate, high-fibre, low-fat diet plus exercise programs maintained through intensive counselling can decrease diabetes risk by over 40%. Additional research is required to confirm the sustainability of this approach and sort out the determinants of insulin resistance so that more effective nutritional interventions will result.

Genetic manipulation of insulin signaling, action and secretion in mice. Insights into glucose homeostasis and pathogenesis of type 2 diabetes

Non-insulin-dependent diabetes mellitus (NIDDM) is a complex heterogeneous polygenic disease characterized mainly by insulin resistance and pancreatic beta-cell dysfunction. In recent years, several genetically engineered mouse models have been developed for the study of the pathophysiological consequences of defined alterations in a single gene or in a set of candidate diabetogenes. These represent new tools that are providing invaluable insights into NIDDM pathogenesis. In this review, we highlight the lessons emerging from the study of some of the transgenic or knockout mice in which the expression of key actors in insulin signaling, action or secretion has been manipulated. In addition to contributing to our knowledge of the specific roles of individual genes in the control of glucose homeostasis, these studies have made it possible to address several crucial issues in NIDDM that have remained controversial or unanswered for a number of years.

Ups and downs for neuropeptides in body weight homeostasis: pharmacological potential of cocaine amphetamine regulated transcript and pre-proglucagon-derived peptides

Although most humans experience an underlying upwards drift of the body-weight set-point, body weight appears tightly regulated throughout life. The present review describes the structural basis of the adipostat and hypothesise, which components may constitute available targets for pharmacotherapy of excess body weight. Hypothalamic neurones constitute the major components of the body weight homeostasis maintaining device. Together with neurones of the nucleus of the solitary tract, neurones of the hypothalamic arcuate nucleus constitute the sensory components of the adipostat. The arcuate nucleus neurones respond to circulating levels of leptin and insulin, both of which reflect the levels of energy stored as triacylglycerol in adipocytes. The arcuate nucleus projects heavily to the hypothalamic paraventricular nucleus. Neurones of the hypothalamic paraventricular nucleus are hypothesised to constitute, at least partly, the adipostat motor pattern generator, which upon stimulation activates either net anabolic or catabolic physiological responses. The overall sensitivity of the adipostat is influenced by gain setting neurones hypothesised to be located in the dorsomedial hypothalamic nucleus and lateral hypothalamic area. Cocaine amphetamine regulated transcript (CART) peptides and pre-proglucagon derived peptides, glucagon-like peptide-1 (GLP-1) and glucagon-like peptide-2 (GLP-2) are catabolic neurotransmitters synthesised in neurones of the arcuate nucleus and the nucleus of the solitary tract, respectively. The present review summarises the available evidence that both families of peptides constitute endogenous transmitters mediating satiety and touch upon potential pharmacological exploitation of this knowledge.

Attenuation of leptin action and regulation of obesity by protein tyrosine phosphatase 1B

Common obesity is primarily characterized by resistance to the actions of the hormone leptin. Mice deficient in protein tyrosine phosphatase 1B (PTP1B) are resistant to diabetes and diet-induced obesity, prompting us to further define the relationship between PTP1B and leptin in modulating obesity. Leptin-deficient (Lep(ob/ob)) mice lacking PTP1B exhibit an attenuated weight gain, a decrease in adipose tissue, and an increase in resting metabolic rate. Furthermore, PTP1B-deficient mice show an enhanced response toward leptin-mediated weight loss and suppression of feeding. Hypothalami from these mice also display markedly increased leptin-induced Stat3 phosphorylation. Finally, substrate-trapping experiments demonstrate that leptin-activated Jak2, but not Stat3 or the leptin receptor, is a substrate of PTP1B. These results suggest that PTP1B negatively regulates leptin signaling, and provide one mechanism by which it may regulate obesity.

PTP1B regulates leptin signal transduction in vivo

Mice lacking the protein-tyrosine phosphatase PTP1B are hypersensitive to insulin and resistant to obesity. However, the molecular basis for resistance to obesity has been unclear. Here we show that PTP1B regulates leptin signaling. In transfection studies, PTP1B dephosphorylates the leptin receptor-associated kinase, Jak2. PTP1B is expressed in hypothalamic regions harboring leptin-responsive neurons. Compared to wild-type littermates, PTP1B(-/-) mice have decreased leptin/body fat ratios, leptin hypersensitivity, and enhanced leptin-induced hypothalamic Stat3 tyrosyl phosphorylation. Gold thioglucose treatment, which ablates leptin-responsive hypothalamic neurons, partially overcomes resistance to obesity in PTP1B(-/-) mice. Our data indicate that PTP1B regulates leptin signaling in vivo, likely by targeting Jak2. PTP1B may be a novel target to treat leptin resistance in obesity.

Modulation of insulin transport in rat brain microvessel endothelial cells by an ecto-phosphatase activity

The physiological function of alkaline phosphatase (ALP) remains controversial. It was recently suggested that this membrane-bound enzyme has a role in the modulation of transmembranar transport systems into hepatocytes and Caco-2 cells. ALP activity expressed on the apical surface of blood-brain barrier cells, and its relationship with (125)I-insulin internalization were investigated under physiological conditions using p-nitrophenylphosphate (p-NPP) as substrate. For this, an immortalized cell line of rat capillary cerebral endothelial cells (RBE4 cells) was used. ALP activity and (125)I-insulin internalization were evaluated in these cells. The results showed that RBE4 cells expressed ALP, characterized by an ecto-oriented active site which was functional at physiological pH. Orthovanadate (100 microM), an inhibitor of phosphatase activities, decreased both RBE4-ALP activity and (125)I-insulin internalization. In the presence of L-arginine (1 mM) or adenosine (100 microM) RBE4-ALP activity and (125)I-insulin, internalization were significantly reduced. However, D-arginine (1 mM) had no significant effect. Additionally, RBE4-ALP activity and (125)I-insulin internalization significantly increased in the presence of the bioflavonoid kaempferol (100 microM), of the phorbol ester PMA (80 nM), IBMX (1 mM), progesterone (200 microM and 100 microM), beta-estradiol (100 microM), iron (100 microM) or in the presence of all-trans retinoic acid (RA) (10 microM). The ALP inhibitor levamisole (500 microM) was able to reduce (125)I-insulin internalization to 69.1 +/- 7.1% of control. Our data showed a positive correlation between ecto-ALP activity and (125)I-insulin incorporation (r = 0.82; P < 0.0001) in cultured rat brain endothelial cells, suggesting that insulin entry into the blood-brain barrier may be modulated through ALP.

The leaves of green plants as well as a cyanobacterium, a red alga, and fungi contain insulin-like antigens

We report the detection of insulin-like antigens in a large range of species utilizing a modified ELISA plate assay and Western blotting. We tested the leaves or aerial parts of species of Rhodophyta (red alga), Bryophyta (mosses), Psilophyta (whisk ferns), Lycopodophyta (club mosses), Sphenopsida (horsetails), gymnosperms, and angiosperms, including monocots and dicots. We also studied species of fungi and a cyanobacterium, Spirulina maxima. The wide distribution of insulin-like antigens, which in some cases present the same electrophoretic mobility as bovine insulin, together with results recently published by us on the amino acid sequence of an insulin isolated from the seed coat of jack bean (Canavalia ensiformis) and from the developing fruits of cowpea (Vigna unguiculata), suggests that pathways depending on this hormone have been conserved through evolution.

Leptin and reproduction: a review

OBJECTIVE

To review recent advances in understanding the role of leptin in the physiology and pathophysiology of reproduction, with a focus on relevant clinical situations.

DESIGN

A MEDLINE computer search was performed to identify relevant articles.

RESULT(S)

Leptin, an adipocyte hormone important in regulating energy homeostasis, interacts with the reproductive axis at multiple sites, with stimulatory effects at the hypothalamus and pituitary and inhibitory actions at the gonads. More recently, leptin has been shown to play a role in other target reproductive organs, such as the endometrium, placenta, and mammary gland, with corresponding influences on important physiologic processes such as menstruation, pregnancy, and lactation. As a marker of whether nutritional stores are adequate, leptin may act in concert with gonadotropins and the growth hormone axis to initiate the complex process of puberty. Conditions in which nutritional status is suboptimal, such as eating disorders, exercise-induced amenorrhea, and functional hypothalamic amenorrhea, are associated with low serum leptin levels; and conditions with excess energy stores or metabolic disturbances, such as obesity and polycystic ovarian syndrome, often have elevated serum or follicular fluid leptin levels, raising the possibility that relative leptin deficiency or resistance may be at least partly responsible for the reproductive abnormalities that occur with these conditions.

CONCLUSION(S)

Leptin may act as the critical link between adipose tissue and the reproductive system, indicating whether adequate energy reserves are present for normal reproductive function. Future interventional studies involving leptin administration are expected to further clarify this role of leptin and may provide new therapeutic options for the reproductive dysfunction associated with states of relative leptin deficiency or resistance.

Insulin signaling coordinately regulates cardiac size, metabolism, and contractile protein isoform expression

To investigate the role of insulin signaling on postnatal cardiac development, physiology, and cardiac metabolism, we generated mice with a cardiomyocyte-selective insulin receptor knockout (CIRKO) using cre/loxP recombination. Hearts of CIRKO mice were reduced in size by 20-30% due to reduced cardiomyocyte size and had persistent expression of the fetal beta-myosin heavy chain isoform. In CIRKO hearts, glucose transporter 1 (GLUT1) expression was reduced by about 50%, but there was a twofold increase in GLUT4 expression as well as increased rates of cardiac glucose uptake in vivo and increased glycolysis in isolated working hearts. Fatty acid oxidation rates were diminished as a result of reduced expression of enzymes that catalyze mitochondrial beta-oxidation. Although basal rates of glucose oxidation were reduced, insulin unexpectedly stimulated glucose oxidation and glycogenolysis in CIRKO hearts. Cardiac performance in vivo and in isolated hearts was mildly impaired. Thus, insulin signaling plays an important developmental role in regulating postnatal cardiac size, myosin isoform expression, and the switching of cardiac substrate utilization from glucose to fatty acids. Insulin may also modulate cardiac myocyte metabolism through paracrine mechanisms by activating insulin receptors in other cell types within the heart.

A variation in 3' UTR of hPTP1B increases specific gene expression and associates with insulin resistance

Protein tyrosine phosphatase 1B (PTP1B) inhibits insulin signaling and, when overexpressed, plays a role in insulin resistance (Ahmad et al. 1997). We identified, in the 3' untranslated region of the PTP1B gene, a 1484insG variation that, in two different populations, is associated with several features of insulin resistance: among male individuals, higher values of the insulin resistance HOMA(IR) index (P=.006), serum triglycerides (P=.0002), and total/HDL cholesterol ratio (P=.025) and, among female individuals, higher blood pressure (P=.01). Similar data were also obtained in a family-based association study by use of sib pairs discordant for genotype (Gu et al. 2000). Subjects carrying the 1484insG variant showed also PTP1B mRNA overexpression in skeletal muscle (6,166 plus minus 1,879 copies/40 ng RNA vs. 2,983 plus minus 1,620; P<.01). Finally, PTP1B mRNA stability was significantly higher (P<.01) in human embryo kidney 293 cells transfected with 1484insG PTP1B, as compared with those transfected with wild-type PTP1B. Our data indicate that the 1484insG allele causes PTP1B overexpression and plays a role in insulin resistance. Therefore, individuals carrying the 1484insG variant might particularly benefit from PTP1B inhibitors, a promising new tool for treatment of insulin resistance (Kennedy and Ramachandran 2000).

Altered gene activity of epidermal growth factor receptor (ErbB-1) in the hypothalamus of aging female rat is linked to abnormal estrous cycles

Activation of the ErbB-1 receptor is necessary for initiating mammalian female puberty by stimulating the release of LH-releasing hormone. It remains unclear whether ErbB-1 is also required in governing reproduction during adulthood and whether altered ErbB-1 signaling is linked to changes in gonadotropin secretion in aging females. The present study examined these issues. RT-PCR was employed to determine changes in ErbB-1 mRNA levels during proestrus in both young adult (YA) and middle-aged (MA) female rats. Before the LH surge, expression levels in the preoptic area of YA rats increased to a maximal value. No such increase in ErbB-1 mRNA was found in MA rats. This difference was confirmed by the analysis of in situ hybridization histochemistry, where a stronger mRNA signal was observed in the preoptic area of YA rats compared with MA females. ErbB-1 protein levels measured by Western blot reflected this difference. A peak level of ErbB-1 mRNA in the median eminence-arcuate nucleus was detected at 0800 h in YA rats, but it was delayed in MA animals. There were intense ErbB-1 mRNA-positive cells in the arcuate nucleus. Pharmacological blockade of ErbB-1 receptor-mediated signal transduction resulted in the disruption of estrous cyclicity in YA rats. These results indicate that ErbB-1 receptors are necessary for maintaining normal estrous cycles. Consequently, age-related alterations in hypothalamic ErbB-1 gene activity may contribute to a delayed preovulatory LH secretion in aging females. Thus, the ErbB-1 signaling system plays an important role in the control of female reproduction during adulthood.

Small molecule insulin mimetics reduce food intake and body weight and prevent development of obesity

Obesity and insulin resistance are major risk factors for a number of metabolic disorders, such as type 2 diabetes mellitus. Insulin has been suggested to function as one of the adiposity signals to the brain for modulation of energy balance. Administration of insulin into the brain reduces food intake and body weight, and mice with a genetic deletion of neuronal insulin receptors are hyperphagic and obese. However, insulin is also an anabolic factor; when administered systemically, pharmacological levels of insulin are associated with body weight gain in patients. In this study, we investigated the efficacy and feasibility of small molecule insulin mimetic compounds to regulate key parameters of energy homeostasis. Central intracerebroventricular (i.c.v.) administration of an insulin mimetic resulted in a dose-dependent reduction of food intake and body weight in rats, and altered the expression of hypothalamic genes known to regulate food intake and body weight. Oral administration of a mimetic in a mouse model of high-fat diet-induced obesity reduced body weight gain, adiposity and insulin resistance. Thus, insulin mimetics have a unique advantage over insulin in the control of body weight and hold potential as a novel anti-obesity treatment.

Glucose relays information regarding nutritional status to the neural circuits that control the somatotropic, corticotropic, and gonadotropic axes in adult male rhesus macaques

In male mammals, the neuroendocrine responses to fasting include increased GH and cortisol secretion and suppressed LH and T levels. Because blood glucose levels fall during fasting, we hypothesized that this modest, but consistent, change in blood glucose was a metabolic signal for the neuroendocrine adjustments of reproductive and metabolic hormones. Glucose (D-dextrose, 480 kcal/d) was infused into fasted (48 h) adult male rhesus macaques; and LH, cortisol, and GH were measured in plasma from samples collected at 15-min intervals for the last 15 h of the fast. We analyzed hormone secretion by deconvolution analysis, and the orderliness of release patterns by the approximate entropy statistic. Circulating blood glucose was 76 +/- 7 mg/dl in the fed control group, significantly higher (P < 0.01) than the level of 56 +/- 3 mg/dl in the fasted group. The increase in GH pulsatility and the 2-fold elevation in cortisol levels observed in the fasted male macaques were prevented by parenteral glucose delivery. The suppression of LH in fasted animals was not relieved by glucose infusions but seemed to be partially prevented in three of the animals. These findings are consistent with the hypothesis that glucose serves as a signal of nutritional status controlling adaptive neuroendocrine responses to fasting in the primate.

Olf-1/early B cell factor is a regulator of glut4 gene expression in 3T3-L1 adipocytes

A negative regulatory element in the 5'-flanking region of the murine glut4 gene mediates chronic insulin- and cAMP-induced repression in 3T3-L1 adipocytes. Previous work demonstrated that members of the nuclear factor 1 (NF1) family of transcription factors and an unidentified factor bind to and mediate repression from this regulatory element. By using a yeast one-hybrid screen, Olf-1/Early B cell factor (O/E-1) was isolated as a candidate for this unidentified factor. A protein complex from 3T3-L1 adipocyte nuclear extract that bound the negative regulatory element was recognized by O/E-specific antiserum, and binding activity was competed effectively by distinct O/E-binding sequences. O/E binding activity was also detected in nuclear extracts from insulin-responsive, GLUT4-expressing tissues including adipose, skeletal muscle, and heart. Mutations within the negative regulatory element that abolish binding of O/E proteins concomitantly blocked insulin-induced repression in reporter gene assays. These results suggest that one or more members of the O/E transcription factor family function as important regulators of glut4 gene expression and therefore may play a heretofore unanticipated role in glucose homeostasis and insulin signaling.

The genetics of aging

The genetic analysis of life span has only begun in mammals, invertebrates, such as Caenorhabditis elegans and Drosophila, and yeast. Even at this primitive stage of the genetic analysis of aging, the physiological observations that rate of metabolism is intimately tied to life span is supported. In many examples from mice to worms to flies to yeast, genetic variants that affect life span also modify metabolism. Insulin signaling regulates life span coordinately with reproduction, metabolism, and free radical protective gene regulation in C. elegans. This may be related to the findings that caloric restriction also regulates mammalian aging, perhaps via the modulation of insulin-like signaling pathways. The nervous system has been implicated as a key tissue where insulin-like signaling and free radical protective pathways regulate life span in C. elegans and Drosophila. Genes that determine the life span could act in neuroendocrine cells in diverse animals. The involvement of insulin-like hormones suggests that the plasticity in life spans evident in animal phylogeny may be due to variation in the timing of release of hormones that control vitality and mortality as well as variation in the response to those hormones. Pedigree analysis of human aging may reveal variations in the orthologs of the insulin pathway genes and coupled pathways that regulate invertebrate aging. Thus, genetic approaches may identify a set of circuits that was established in ancestral metazoans to regulate their longevity.

The neuroanatomical axis for control of energy balance

The hypothalamic feeding-center model, articulated in the 1950s, held that the hypothalamus contains the interoceptors sensitive to blood-borne correlates of available or stored fuels as well as the integrative substrates that process metabolic and visceral afferent signals and issue commands to brainstem mechanisms for the production of ingestive behavior. A number of findings reviewed here, however, indicate that sensory and integrative functions are distributed across a central control axis that includes critical substrates in the basal forebrain as well as in the caudal brainstem. First, the interoceptors relevant to energy balance are distributed more widely than had been previously thought, with a prominent brainstem complement of leptin and insulin receptors, glucose-sensing mechanisms, and neuropeptide mediators. The physiological relevance of this multiple representation is suggested by the demonstration that similar behavioral effects can be obtained independently by stimulation of respective forebrain and brainstem subpopulations of the same receptor types (e.g., leptin, CRH, and melanocortin). The classical hypothalamic model is also challenged by the integrative achievements of the chronically maintained, supracollicular decerebrate rat. Decerebrate and neurologically intact rats show similar discriminative responses to taste stimuli and are similarly sensitive to intake-inhibitory feedback from the gut. Thus, the caudal brainstem, in neural isolation from forebrain influence, is sufficient to mediate ingestive responses to a range of visceral afferent signals. The decerebrate rat, however, does not show a hyperphagic response to food deprivation, suggesting that interactions between forebrain and brainstem are necessary for the behavioral response to systemic/ metabolic correlates of deprivation in the neurologically intact rat. At the same time, however, there is evidence suggesting that hypothalamic-neuroendocrine responses to fasting depend on pathways ascending from brainstem. Results reviewed are consistent with a distributionist (as opposed to hierarchical) model for the control of energy balance that emphasizes: (i) control mechanisms endemic to hypothalamus and brainstem that drive their unique effector systems on the basis of local interoceptive, and in the brainstem case, visceral, afferent inputs and (ii) a set of uni- and bidirectional interactions that coordinate adaptive neuroendocrine, autonomic, and behavioral responses to changes in metabolic status.

The tubby-like proteins, a family with roles in neuronal development and function

The identification of a mutation at the tubby (Tub) locus, which causes obesity and neurosensory degeneration, led to the discovery of the tubby-like proteins (TULPs). Tub and the genes that encode three tubby-like proteins (TULP1- TULP3) form a novel, small gene family that plays an important role in maintenance and function of neuronal cells during development and post-differentiation. Although exploration of the molecular function of these genes is still in its infancy, recent biochemical studies have provided ‘entry points’ into pathways whose elucidation will further our understanding of TULP action. In addition, mRNA expression and translocation of the TUB protein have been shown to be regulated by thyroid hormone and by G-protein-coupled receptor signaling, respectively. These latter findings may help to link the cellular function of TUB to known mechanisms for energy homeostasis.

Insulin signalling and the regulation of glucose and lipid metabolism

The epidemic of type 2 diabetes and impaired glucose tolerance is one of the main causes of morbidity and mortality worldwide. In both disorders, tissues such as muscle, fat and liver become less responsive or resistant to insulin. This state is also linked to other common health problems, such as obesity, polycystic ovarian disease, hyperlipidaemia, hypertension and atherosclerosis. The pathophysiology of insulin resistance involves a complex network of signalling pathways, activated by the insulin receptor, which regulates intermediary metabolism and its organization in cells. But recent studies have shown that numerous other hormones and signalling events attenuate insulin action, and are important in type 2 diabetes.

Peripheral signals conveying metabolic information to the brain: short-term and long-term regulation of food intake and energy homeostasis

Numerous peripheral signals contribute to the regulation of food intake and energy homeostasis. Mechano- and chemoreceptors signaling the presence and energy density of food in the gastrointestinal (GI) tract contribute to satiety in the immediate postprandial period. Changes in circulating glucose concentrations appear to elicit meal initiation and termination by regulating activity of specific hypothalamic neurons that respond to glucose. Other nutrients (e.g., amino acids and fatty acids) and GI peptide hormones, most notably cholecystokinin, are also involved in short-term regulation of food intake. However, the energy density of food and short-term hormonal signals by themselves are insufficient to produce sustained changes in energy balance and body adiposity. Rather, these signals interact with long-term regulators (i.e., insulin, leptin, and possibly the orexigenic gastric peptide, ghrelin) to maintain energy homeostasis. Insulin and leptin are transported into the brain where they modulate expression of hypothalamic neuropeptides known to regulate feeding behavior and body weight. Circulating insulin and leptin concentrations are proportional to body fat content; however, their secretion and circulating levels are also influenced by recent energy intake and dietary macronutrient content. Insulin and leptin concentrations decrease during fasting and energy-restricted diets, independent of body fat changes, ensuring that feeding is triggered before body energy stores become depleted. Dietary fat and fructose do not stimulate insulin secretion and leptin production. Therefore, attenuated production of insulin and leptin could lead to increased energy intake and contribute to weight gain and obesity during long-term consumption of diets high in fat and/or fructose. Transcription of the leptin gene and leptin secretion are regulated by insulin-mediated increases of glucose utilization and appear to require aerobic metabolism of glucose beyond pyruvate. Other adipocyte-derived hormones and proteins that regulate adipocyte metabolism, including acylation stimulating protein, adiponectin, diacylglycerol acyltransferase, and perilipin, are likely to have significant roles in energy homeostasis.

Distinct and overlapping functions of insulin and IGF-I receptors

Targeted gene mutations have established distinct, yet overlapping, developmental roles for receptors of the insulin/IGF family. IGF-I receptor mediates IGF-I and IGF-II action on prenatal growth and IGF-I action on postnatal growth. Insulin receptor mediates prenatal growth in response to IGF-II and postnatal metabolism in response to insulin. In rodents, unlike humans, insulin does not participate in embryonic growth until late gestation. The ability of the insulin receptor to act as a bona fide IGF-II-dependent growth promoter is underscored by its rescue of double knockout Igf1r/Igf2r mice. Thus, IGF-II is a true bifunctional ligand that is able to stimulate both insulin and IGF-I receptor signaling, although with different potencies. In contrast, the IGF-II/cation-independent mannose-6-phosphate receptor regulates IGF-II clearance. The growth retardation of mice lacking IGF-I and/or insulin receptors is due to reduced cell number, resulting from decreased proliferation. Evidence from genetically engineered mice does not support the view that insulin and IGF receptors promote cellular differentiation in vivo or that they are required for early embryonic development. The phenotypes of insulin receptor gene mutations in humans and in mice indicate important differences between the developmental roles of insulin and its receptor in the two species.

Brain pathways controlling food intake and body weight

Evidence has existed for more than 50 years in support of the hypothesis that body energy stored in the form of fat is homeostatically regulated. Implicit in this concept is the existence of a biological system that operates dynamically over time to match cumulative energy intake to energy expenditure. For example, to compensate for weight loss induced by energy restriction, animals must enter a period of positive energy balance (i.e., energy intake greater than energy expenditure) that is sustained for as long as it takes to correct the deficit in body fat stores. Having reached this point, the animal must return to a state of neutral energy balance if stable fat mass is to be maintained. The identification of neuronal circuits in the hypothalamus that, when activated, exert potent, unidirectional effects on energy balance provides a cornerstone of support for this model. The additional finding that these central effector pathways are regulated by humoral signals generated in proportion to body fat stores, including the hormones insulin and leptin, helps to round out the picture of how energy homeostasis is achieved. The goal of this overview is to highlight the evidence that specific subsets of hypothalamic neurons containing specific signaling molecules participate in this dynamic regulatory process, and to put these observations in the larger context of a biological system that controls body adiposity.

A hormonal signaling pathway influencing C. elegans metabolism, reproductive development, and life span

During C. elegans development, animals must choose between reproductive growth or dauer diapause in response to sensory cues. Insulin/IGF-I and TGF-beta signaling converge on the orphan nuclear receptor daf-12 to mediate this choice. Here we show that daf-9 acts downstream of these inputs but upstream of daf-12. daf-9 and daf-12 mutants have similar larval defects and modulate insulin/IGF-I and gonadal signals that regulate adult life span. daf-9 encodes a cytochrome P450 related to vertebrate steroidogenic hydroxylases, suggesting that it could metabolize a DAF-12 ligand. Sterols may be the daf-9 substrate and daf-12 ligand because cholesterol deprivation phenocopies mutant defects. Sensory neurons, hypodermis, and somatic gonadal cells expressing daf-9 identify potential endocrine tissues. Evidently, lipophilic hormones influence nematode metabolism, diapause, and life span.

Is obesity an inflammatory condition?

Obesity may be a low-grade systemic inflammatory disease. Overweight and obese children and adults have elevated serum levels of C-reactive protein, interleukin-6, tumor necrosis factor-alpha, and leptin, which are known markers of inflammation and closely associated with cardiovascular risk factors and cardiovascular and non-cardiovascular causes of death. This may explain the increased risk of diabetes, heart disease, and many other chronic diseases in the obese. The complex interaction between several neurotransmitters such as dopamine, serotonin, neuropeptide Y, leptin, acetylcholine, melanin-concentrating hormone, ghrelin, nitric oxide, and cytokines and insulin and insulin receptors in the brain ultimately determines and regulates food intake. Breast-feeding of more than 12 mo is associated with decreased incidence of obesity. Breast milk is a rich source of long-chain polyunsaturated fatty acids (LCPUFAs) and brain is especially rich in these fatty acids. LCPUFAs inhibit the production of proinflammatory cytokines and enhance the number of insulin receptors in various tissues and the actions of insulin and several neurotransmitters. LCPUFAs may enhance the production of bone morphogenetic proteins, which participate in neurogenesis, so these fatty acids might play an important role in brain development and function. It is proposed that obesity is a result of inadequate breast feeding, which results in marginal deficiency of LCPUFAs during the critical stages of brain development. This results in an imbalance in the structure, function, and feedback loops among various neurotransmitters and their receptors, which ultimately leads to a decrease in the number of dopamine and insulin receptors in the brain. Hence, promoting prolonged breast feeding may decrease the prevalence of obesity. Exercise enhances parasympathetic tone, promotes antiinflammation, and augments brain acetylcholine and dopamine levels, events that suppress appetite. Acetylcholine and insulin inhibit the production of proinflammatory cytokines and provide a negative feedback loop for postprandial inhibition of food intake, in part, by regulating leptin action. Statins, peroxisome proliferator-activated receptor-gamma binding agents, non-steroidal antiinflammatory drugs, and infant formulas supplemented with LCPUFAs, and LCPUFAs themselves, which suppress inflammation, may be beneficial in obesity.

Estrogen deficiency causes central leptin insensitivity and increased hypothalamic neuropeptide Y

OBJECTIVE

Altered fat distribution is a consequence of menopause, but the mechanisms responsible are unknown. Estrogen insufficiency in humans can be modeled using ovariectomized rats. We have shown that increased adiposity in these rats is due to reduced physical activity and transient hyperphagia, and can be reversed with 17beta-estradiol treatment. The aims of this study were to examine whether this altered energy balance is associated with circulating leptin insufficiency, central leptin insensitivity, decreased hypothalamic leptin receptor (Ob-Rb) expression, and/or increased hypothalamic neuropeptide Y (NPY).

METHODS

Plasma leptin levels, adipose tissue ob gene expression, energy balance responses to i.c.v. leptin, hypothalamic Ob-Rb expression and NPY concentration in five separate hypothalamic regions were measured in adult female rats after either ovariectomy or sham operations.

RESULTS

Obesity was not associated with hypoleptinemia or decreased ob gene expression in ovariectomized rats; however, it was associated with insensitivity to central leptin administration. Food intake was less suppressed and spontaneous physical activity was less stimulated by leptin. This was not due to decreased hypothalamic Ob-Rb expression. NPY concentration in the paraventricular nucleus of the hypothalamus was elevated in the ovariectomized rats, consistent with leptin insensitivity; however this effect was transient and disappeared as body fat and leptin levels increased further and hyperphagia normalized.

CONCLUSION

Impaired central leptin sensitivity and overproduction of NPY may contribute to excess fat accumulation caused by estrogen deficiency.

Increased expression of the sterol regulatory element-binding protein-1 gene in insulin receptor substrate-2(-/-) mouse liver

Insulin receptor substrate (IRS)-2(-/-) mice develop diabetes because of insulin resistance in the liver and failure to undergo beta-cell hyperplasia. Here we show by DNA chip microarray analysis that expression of the sterol regulatory element-binding protein (SREBP)-1 gene, a downstream target of insulin, was paradoxically increased in 16-week-old IRS-2(-/-) mouse liver, where insulin-mediated intracellular signaling events were substantially attenuated. The expression of SREBP-1 downstream genes, such as the spot 14, ATP citrate-lyase, and fatty acid synthase genes, was also increased. Increased liver triglyceride content in IRS-2(-/-) mice assures the physiological importance of SREBP-1 gene induction. IRS-2(-/-) mice showed leptin resistance; low dose leptin administration, enough to reduce food intake and body weight in wild-type mice, failed to do so in IRS-2(-/-) mice. Interestingly, high dose leptin administration reduced SREBP-1 expression in IRS-2(-/-) mouse liver. Thus, IRS-2 gene disruption results in leptin resistance, causing an SREBP-1 gene induction, obesity, fatty liver, and diabetes.

Brown adipose tissue-specific insulin receptor knockout shows diabetic phenotype without insulin resistance

Although insulin regulates metabolism in both brown and white adipocytes, the role of these tissues in energy storage and utilization is quite different. Recombination technology using the Cre-loxP approach allows inactivation of the insulin receptor in a tissue-specific manner. Mice lacking insulin receptors in brown adipocytes show an age-dependent loss of interscapular brown fat but increased expression of uncoupling protein-1 and -2. In parallel, these mice develop an insulin-secretion defect resulting in a progressive glucose intolerance, without insulin resistance. This model provides direct evidence for not only a role for the insulin receptors in brown fat adipogenesis, the data also suggest a novel role of brown adipose tissue in the regulation of insulin secretion and glucose homeostasis.

The role of carbohydrates in insulin resistance

Insulin resistance is a metabolic disorder that is increasing worldwide and is associated with some of the most common diseases affecting modern societies including diabetes, hypertension, obesity and coronary heart disease. Although pharmacologic approaches to managing insulin resistance are being advocated by some, public health approaches involving changes in diet and physical activity are attractive because of their lower cost and risk. We briefly summarize some new information on the mechanisms that mediate insulin's many biological actions and examine the effects of dietary carbohydrates on insulin sensitivity. Specifically, we summarize some of the information available on the effects of simple sugars, complex carbohydrates including fiber, slowly digested starch and the general concept of glycemic index. The available data support the idea that consumption of diets high in total carbohydrate does not adversely affect insulin sensitivity compared with high fat diets. Animal data suggest that simple sugars, in particular fructose, have adverse effects on insulin action, but adverse effects have not been shown conclusively in humans. Increased intake of dietary fiber appears to improve insulin action and may protect against the development of diabetes. The effects of diets with high or low glycemic index on insulin action are controversial at this time. For firm conclusions to be reached, future studies must be of reasonable duration, be in defined populations and compare the effects of relevant doses of nutrients on specific endpoints of insulin action.

Drug treatment of obesity

At present only two drugs are approved for long-term treatment of obesity. Sibutramine inhibits the reuptake of serotonin and norepinephrine. In clinical trials it produces a dose-dependent 5-10% decrease in body weight. Its side effects include dry mouth, insomnia, asthenia, and constipation. In addition, sibutramine produces a small increase in blood pressure and pulse that is a contraindication to the use of this drug in some individuals with heart disease. Xenical is the other drug approved for long-term use in the treatment of obesity. It works by blocking lipase and thus increasing the fecal loss of triglyceride. One valuable consequence of this mechanism of action is the reduction of serum cholesterol that averages about 5% more than can be accounted for by weight loss alone. In clinical trials it produces a 5-10% loss of weight. Its side effects are entirely due to undigested fat in the intestine that can lead to increased frequency and change in the character of stools. It can also lower fat-soluble vitamins. The ingestion of a vitamin supplement before bedtime is a reasonable treatment strategy. The effect on weight loss during long-term trials with these two drugs is shown in Figs 7 and 8 above. Also in this figure is data on phentermine used in trials of six months or more. Although there were differences in mean weight losses with these drugs, when the placebo effect was taken into account they all had a surprisingly similar magnitude of weight loss.

Neurohumoral regulation of body weight gain

The regulation of body weight is a complex process which relies on a balance between supply of nutrients and demand on these nutrients in the form of energy expenditure. Various central and peripheral mechanisms play a crucial role in maintaining this balance. While various neuropeptides in the central nervous system (CNS), particularly in the hypothalamus, maintain the necessary harmony between hyperphagia and anorexia, peripheral signals arising from the gastrointestinal tract (cholecystokinin-8 [CCK-8], amylin), pancreas (insulin) and adipose tissue (leptin) provide the necessary stimuli or a feedback inhibition for the synthesis and secretion of these hypothalamic neuropeptides. Various metabolites of the carbohydrate and fat metabolism are also involved in regulating the neuronal activity in the hypothalamus which ultimately leads to a release of key neuropeptides. In addition to the central mechanisms, peripheral mechanisms that regulate energy expenditure, particularly in the brown adipose tissue and skeletal muscle, are critical in maintaining the overall balance. Insight into these mechanisms sets the stage for developing novel strategies in the treatment of emerging childhood diseases such as obesity, anorexia nervosa, and bulimia. Further, delineation of these processes in the fetus and newborn sets the stage for investigating their role in molding the adult phenotype due to intrauterine adaptations.

Peptide derived from insulin with regulatory activity of dopamine transporter

A nonapeptide derived from the C terminus of the insulin B chain, H(2)N-Arg-Gly-Phe-Phe-Tyr-Thr-Pro-Lys-Ala-COOH, was found to strongly inhibit dopamine (DA) uptake by rat dopamine transporter (DAT) stably expressed in CHO cells (designated D8 cells). The kinetic experiments on D8 cells gave a curve typical of competitive inhibition with an IC(50)=6.9 microM. This inhibitory effect was also confirmed by experiments on striatal synaptosomes. The rat administered with the nonapeptide unilaterally into substantia nigra showed dose-dependent velocity and duration of the round movement contralateral to the nonapeptide-injected side. In addition, the nonapeptide dose-dependently reduced the binding of the tritium-labeled cocaine analog (-)-2 beta-carbomethoxy-3 beta-(4-fluorophenyl)tropane (WIN35,428) to DAT of D8 cells, which suggests that the nonapeptide may inhibit the transport activity of DAT in the way as cocaine does. Meanwhile, the peptide DOI (insulin with 8 amino acid residues deleted at the C terminus of the B chain) shows a significantly stimulating effect on DAT uptake activity in D8 cells. So insulin is proposed as a kind of neuropeptide precursor in the brain and insulin-derived peptides may be involved in the process of regulating the DA system, and these peptides may be developed into new medicines for disorders concerning the DA system such as Parkinson's disease and cocaine addiction.

The gut and food intake: an update for surgeons

Food intake is the simplest and most obvious measure of gastrointestinal function, yet it rarely receives more than cursory attention from surgeons. In this review we cover recent findings on relationships between gut function and appetite regulation mediated via neuropeptides influenced by afferent and efferent vagal activity. Evidence from the new discipline known as neurogastroenterology elucidates gastric and intestinal signals involved in the elicitation of hunger, satiety, and aversion. Discovery of the adipose-tissue-derived hormone, leptin, has energized the field of metabolism spawning increasing numbers of publications related to interactions between leptin and insulin release and glucose disposal, as well as appetitive behavior. Peptides such as cholecystokinin (CCK), the proglucagon-derived peptides, glucagon-like peptides 1 and 2 (GLP-1 and GLP-2), and the recently identified powerful intake-stimulating molecule, orexin, are examples of potential targets for drug development and studies of surgical pathophysiology. A major conclusion of this work is that the considerable redundancy and overlap between mediators of caloric intake subserving survival of the species, while beneficial after foregut surgery, contribute to the complexity of treating the global epidemic of obesity. Possibly knowledge derived from basic research in neurogastroenterology can translate into advances in surgical treatment of obesity.

The neuroendocrinology of obesity

The regulation of energy balance is enormously complex, with numerous genetic, hormonal, neural/behavioral, and societal influences. Although the current epidemic of obesity has its underpinnings in the changes in culture during the last half century, the role of the neuroendocrine system in the genesis of obesity is physiologically and therapeutically unavoidable. Increased understanding of this system has suggested organic etiologies (and therapies) for some rare and not-so-rare forms of obesity. With so many inputs, it is not implausible that dysfunction of other parts of this feedback system will be found to explain other forms of obesity in the future. Fortunately or unfortunately, diet and exercise remain the mainstays of obesity therapy. Most diet-exercise programs result in an acute 11-kg weight loss in adults; the question is whether it can be sustained without significant long-term behavior modification. In the European Sibutramine Trial of Obesity Reduction and Maintenance (STORM), 42% of treated patients dropped out; of those remaining, 77% of subjects lost more than 5% of initial body weight, but only 43% of these individuals maintained greater than 80% of this loss over 2 years. Could there be an organic component in persons who do not respond? Obesity pharmacotherapies sometimes have beneficial acute effects, but these effects are impermanent; discontinuation tends to result in a rebound weight gain, suggesting that the etiology of the obesity is still present. A useful guiding principle is that patients who do not respond to diet and exercise should undergo an initial medical evaluation, including assessments of birth weight, past medical history, weight history, family history, diet, exercise, and fasting insulin and thyroid levels. As the nosology of obesity improves, diagnostic efficiency and therapeutic success should increase, leading to a decrease in associated morbidity, mortality, and socioeconomic ramifications.

Serum leptin levels and anthropometric correlates in Ache Amerindians of eastern Paraguay

Leptin is a recently discovered peptide hormone secreted primarily from adipocytes in humans and other mammals; it is a reflection of fat stores, and has been associated with reproductive function. However, few leptin measurements are available from nonindustrialized populations, including contemporary hunter/gatherer communities undergoing the transition to sedentary agriculture. This investigation reports single-sample serum leptin measurements in healthy Ache Amerindian males (n = 21; average age, 32.8 +/- 3.4 SE) and females (n = 12; average age, 31.3 +/- 4.3) in eastern Paraguay. Ache leptin concentrations were much lower than in industrialized populations, although significant sexual dimorphism was evident (female 5.64 ng/ml +/- 0.91 SE vs. male 1.13 ng/ml +/- 0.08; P < 0.0001). Indeed, female leptin levels were similar to those of anorexic women, despite apparently adequate adiposity. Controlling for fat percentage, no significant sex difference was evident, suggesting that adiposity was the primary source of leptin variation. Body fat percentage was highly correlated with leptin in females (r2 = 0.72; P < 0.0005) but not males, who exhibited a modest negative correlation (r2 = 0.25; P < 0.03). Weight (r2 = 0.45; P = 0.02) and BMI (kg/m2) (r2 = 0.81; P < 0.0001) were also significantly correlated in females but not males. These results suggest that: 1) clinical leptin norms based on industrialized populations may represent the highest range of human variation and may not be representative of most human populations; 2) hormonal priming may underlie population variation in leptin profiles; and 3) the relative importance of leptin as a proximate mechanism regulating reproductive effort during human evolution may have been modest.

The neuroendocrinology of childhood obesity

The regulation of energy balance is enormously complex, with numerous genetic, hormonal, neural and behavioral, and societal influences. Although the current epidemic of obesity clearly has its underpinnings in the changes in culture during the past half-century (see other articles in this issue), the role of the neuroendocrine system in the genesis of obesity, as described in this article, is physiologically and therapeutically unavoidable. An understanding of this system has suggested organic causes (and therapies) for some rare and not-so-rare forms of obesity. With so many inputs, it is not far-fetched to assume that dysfunction of other parts of this feedback system will be found to explain other forms of obesity in the future. What does this mean for obese children entering the pediatrician's office? Fortunately or unfortunately, diet and exercise are the mainstays of obesity therapy for children and adults. Most diet-exercise programs result in an acute 11-kg weight loss in adults; the question is whether it can be sustained without significant long-term behavioral modification. For instance, the European Sibutramine Trial of Obesity Reduction and Maintenance trial showed that 42% of treated subjects drop out; of those remaining, 77% of subjects lost more than 5% of initial body weight, but only 43% of those maintained more than 80% of this over 2 years. Could there be an organic component in those who do not respond? Of course, obesity pharmacotherapies sometimes have beneficial acute effects, but these drugs work for only as long as they are consumed; discontinuation tends to result in a "rebound" weight gain, suggesting that the cause of the obesity is still present. Furthermore, in 2001, there are no obesity drugs approved for children. A useful guiding principle is that children deserve at the minimum an initial medical evaluation, including birth weight, medical history, family history, dietary evaluation, and exercise assessment. Perhaps the most important feature that can distinguish "organic" from "behavioral" weight gain in childhood is the age of the "adiposity rebound." The Centers for Disease Control and Prevention now supplies BMI charts for boys and girls at www.cdc.gov/growthcharts. Plotting of the BMI versus age allows pediatricians to determine the age at which the BMI starts to increase (mean, 5.5 years). The earlier the adiposity rebound, the more likely the child will be obese as an adult, and the more likely that an organic cause can be determined. In such patients, thyroid levels and fasting insulin and leptin levels should be measured. An initial attempt at diet and exercise is essential; patients who do not respond with BMI stabilization should be investigated for a more ominous cause of their obesity. As the nosology of obesity improves, pediatricians will be able to increase the diagnostic efficiency and therapeutic success of this unfortunate, debilitating, and expensive epidemic.

Control of oligodendrocyte differentiation by the Nkx2.2 homeodomain transcription factor

Oligodendrocytes are derived from glial precursors that arise from the ventral neural tube early in development. In the developing chicken CNS, oligodendrocyte progenitors selectively express Nkx2.2 homeodomain transcription factor, raising the possibility that Nkx2.2 may directly regulate oligogliogenesis. In this study, we have examined Nkx2.2 expression in rodent glial precursors and studied the effect of a loss of Nkx2.2 on oligodendrocyte and astrocyte differentiation. We show that Nkx2.2 is also expressed in mammalian oligodendrocyte progenitors and that the differentiation of MBP-positive and PLP-DM20-positive oligodendrocytes is dramatically retarded in Nkx2.2-null mutants along the entire rostrocaudal axis. In contrast, no effect is seen on astrocytic differentiation. Interestingly, absence of Nkx2.2 expression leads to a ventral expansion of the Olig1/Olig2 expression in neuroepithelial cells into the Nkx2.2 domain and a consequent increase in the production of Olig1/Olig2-positive and platelet-derived growth factor receptor α-positive oligodendrocyte progenitors. These results strongly suggest that Nkx2.2 regulates the differentiation and/or maturation, but not the initial specification, of oligodendrocyte progenitors. Consistent with this suggestion, overproduction of Nkx2.2 protein in fibroblast cells can induce gene expression from the proteolipid protein promoter.