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Ann. N.Y. Acad. Sci. ISSN 0077-8923

A N N A L S O F T H E N E W Y O R K A C A D E M Y O F S C I E N C E SIssue:The Year in Diabetes and Obesity

CNS leptin and insulin action in the control of energy

homeostasisBengt F. Belgardt and Jens C. Bruning

Department of Mouse Genetics and Metabolism, Institute for Genetics, Center for Molecular Medicine, Cologne Excellence

Cluster on Cellular Stress Responses in Aging-Associated Diseases, Second Department for Internal Medicine University of

Cologne, and Max Planck Institute for the Biology of Ageing, Cologne, Germany

Address for correspondence: Jens C. Bruning, M.D., Institute for Genetics and Center for Molecular Medicine (CMMC),

Department of Mouse Genetics and Metabolism, Zulpicher Str. 47a, 50674 Cologne, Germany. [email protected]

The obesity and diabetes pandemics have made it an urgent necessity to define the central nervous system (CNS)

pathways controlling body weight, energy expenditure, and fuel metabolism. The pancreatic hormone insulin and

the adipose tissuederived leptin are known to act on diverse neuronal circuits in the CNS to maintain body weight

and metabolism in a variety of species, including humans. Because these homeostatic circuits are disrupted during

the development of obesity, the pathomechanisms leading to CNS leptin and insulin resistance are a focal point of

research. In this review, we summarize the recent findings concerning the mechanisms and novel neuronal mediators

of both insulin and leptin action in the CNS.

Keywords: obesity; leptin; insulin; central nervous system (CNS); pancreas; diabetes

Leptin and insulin as messengers ofperipheral energy levels to the CNS

The circulating levels of leptin and insulin are pos-itively correlated with adiposity and body weight,1

and arenow broadly accepted to deliver informationon peripheral energy stores to the central nervoussystem (CNS) by acting on diverse neuron popu-lations. In line with this notion, intracerebroven-tricular (i.c.v.) injection of insulin or intranasal ap-plication of insulin, which selectively mirrors CNS

insulin concentration, decreases food intake andbody weight in mice,2 rats,3 baboons,4 and men,5

although a recent report failed to detect an effect onfood intake and body weight in rats.6 Comparablystronger than insulins effect, leptins ability to re-duce food intake and decrease body weight is wellestablished.79 It has been ascertained that insulinand leptin action in the CNS, and here especially onneurons, is essential for decreasing the food intake(anorexigenic) and eventually weight reducing ef-fects of these two hormones,10,11 although notably,

the role of other cell types present in the CNS such asastrocytes or microglia, which do express insulin re-ceptors (IR) and leptin receptors (LEPR),12,13 is still

poorly understood in this regard. In addition to in-sulins critical roles in glucose and lipid metabolismin the periphery, leptin has also direct effects on pe-ripheral tissues, and the interested reader is directedto a recent review on this topic.14

Hypothalamic mediators of insulinand leptin action

After the discovery that leptin and insulin mediatetheir effects on body weight and fuel metabolism

by acting on neurons, the specific neuronal pop-ulation impacted on by both hormones had tobe established. It was already known that in ro-dents, lesions in the hypothalamus could impactbody weight, either inducing weight gain or weightloss, depending on the specific region of the hy-pothalamus ablated. In line with this, both lep-tin and IR are strongly and broadly expressed inthe hypothalamus, insulin action in the hypothala-mus has been demonstrated to induce anorexia andweight loss,15 whereas inhibition of insulin signal-

ing has the opposite effect.16 Similarly, hypothala-mic signaling is necessary for leptins effects on bodyweight.17,18

doi: 10.1111/j.1749-6632.2010.05799.x

Ann. N.Y. Acad. Sci. 1212 (2010) 97113 c 2010 New York Academy of Sciences. 97


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CNS leptin and insulin action Belgardt & Bruning

Figure 1. Circuitry of leptin and insulin responsive neurons. Neuron populations of the arcuate nucleus (ARC), ventromedial

hypothalamus (VMH), paraventricular hypothalamus (PVH), lateral hypothalamus (LH), ventral tegmental area (VTA), raphe

nucleus (RAPHE), and nucleus tractus solitarius (NTS)which are responsive to leptin, insulin, or bothare depicted, as well as

relevant synaptic contact between these neuron populations. POMC and AGRP neurons in the ARC are first-order neurons of both

leptin and insulin. Whereas -aminobutric acid (GABA) released from AGRP neurons inhibits POMC neuron activation directly,

POMC and AGRP neurons release -MSH and AGRP respectively onto PVH neurons, which express thyroid-releasing hormone

(TRH) or corticotrophin-releasing hormone (CRH), which are involved in thermogenesis and feeding, respectively. In the VMH,

steroidogenic factor 1 (SF1) expressing neurons are bona fide leptin-sensitive neurons, and predominantly regulate food intake. In

the lateral hypothalamus, leptin acts on GABAergic neurons, which are in synaptic contact with dopaminergic neurons in the VTA.

Both leptin and insulin also regulate activity of orexin neurons in the LH through hyperpolarization (leptin) or transcriptional

repression of orexin (insulin). Dopaminergic neurons in the VTA, which release dopamine in the striatum to control activity and

reward, are also directly silenced by leptin signaling, whereas dopamine reuptake is under control of insulin throughtranscriptional

control of the dopamine transporter. Although serotonergic neurons in the raphe nucleus release serotonin (5-HT) onto POMC

neurons in the ARC (and striatal neurons), leptin acts directly onto 5-HT neurons as well. Finally, nucleus tractus solitarius (NTS)

neurons relay and compute signals from the gastrointestinal (GI) tract and exchange information with the parabrachial nucleus

(PBN). Additionally, GABA release in the PBN from AGRP neurons is essential for feeding. Note that some synaptic connections

and target regions are not depicted for optimal clarity. Region descriptions are in bold and underlined, neuron descriptions are in

bold, and neurotransmitters/neuropeptides are written in normal formatting. Leptin (L) or insulin (I) responsiveness is depicted

in bold and italic.

The first breakthrough in defining primary tar-get neurons of leptin and insulin occured in2001, when it was demonstrated that proopiome-

lanocortin (POMC)-expressing neurons are depo-larized by leptin treatment, which leads to an in-crease in neuronal activity.19 In the hypothalamic

melanocortin system, POMC-expressing neuronsrelease the POMC cleavage product -melanocytestimulating hormone (-MSH), which acts on

downstream target neurons (some of which locatedin the paraventricular hypothalamus, Figure 1) toreduce food intake, increase energy expenditure,

98 Ann. N.Y. Acad. Sci. 1212 (2010) 97113 c 2010 New York Academy of Sciences.


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targets, there are likely more hypothalamic neuronpopulations important for weight regulation, butthere are no marker genes available yet to studythem in detail.

In contrast, the role of hypothalamic insulin sig-

naling in control of peripheral glucose metabolismis well established,48,49 where electrical inhibitionof neurons expressing AGRP has been identified asa major component of hepatic glucose productionregulation likely through vagal innervations.31 Lep-tins well-known ability to improve systemic glu-cose metabolism has also been shown to dependon hypothalamic circuits.50,51 Indeed, there is nowconclusive evidence that leptin signaling in POMCneurons is predominantly necessary for regulation

of systemic glucose homeostasis. This notion wasdeducted from the finding that reexpression of theLEPR only in POMC neurons, whereas all othercells lack the LEPR, was sufficient to mostly normal-ize glucose homeostasis (but not weight), whereasdb/dbmice (lacking LEPR globally) suffer from earlyonset uncontrolled diabetes.52

Notably, it has not yet been proven that CNS in-sulin and leptinaction play thesame role in humans,because patients undergoing liver transplantationand therefore hepatic deenervation show only mod-

est changes in glucose production and metabolism,although these findings are hard to qualify due tothe pathologies leading to the liver transplantationitself, immunosuppression therapy, and other en-docrine abnormalities detected after the transplan-tation.53 Moreover, one group demonstrated thatin dogs, CNS insulin action appears to have only asmall effect on glucose metabolism, underlying theneed for further studies in nonrodent species.54

In addition to glucose metabolism, both CNS in-

sulin and leptin action has been demonstrated toregulate lipid uptake and/ormetabolism in thewhiteadipose tissue, highlighting the broad potency ofthese two hormones to coordinately orchestrate fuelpartitioning.5557

Leptin and insulin intracellular signalingcascades

Because leptin and insulin have profound effects ontranscriptional and electrical events in neurons, thesignaling events evoked by these hormones are of

high interest (Figure 2). Leptin activates intracel-lular signaling cascades through the recruitment ofthe Janus kinase (JAK) to the LEPR, where it phos-

phorylates several key residues on the LEPR. Signal

transducer and activator of transcription (STAT) 3proteins binds to the phosphorylated LEPR, and arethemselves phosphorylated by JAKs.58 This allowsfor dimerisation, and subsequent translocalization

into the nucleus, where the STAT3 proteins bind toand regulatetranscription of targetgenes.24 The roleof this pathway especially in POMC transcriptionis well defined because leptin treatment increasesPOMC transcription.59 On the other hand, leptin isable to stimulate the phosphatidylinositol-3-kinase(PI3K), which is also insulins main intracellularsignaling cascade.60 Here, insulin will stimulatebinding of the regulatory subunit of the PI3K tophosphorylated insulin receptor substrates (IRS),

which allows for activation of the catalytic subunitof the PI3K. PI3K catalyzes the phosphorylationof the membrane lipid phosphatidylinositol-4,5-bisphosphate (PIP2) and thus generatesphosphatidylinositol-3,4,5-trisphosphate (PIP3).PIP3 can bind to and activate ion channels, but isalso recognized by phosphatidylinositol-dependentkinase 1, which phosphorylates several proteinssuch as the kinase AKT to elicit downstreamsignaling events.61 Although it is widely acceptedthat leptin activates PI3K signaling at least in

specific neurons, the signaling cascade linkingleptin stimulation to PI3K activation has not yetbeen fully resolved. Leptin action may also reducethe degradation of PIP3to PIP2by phosphorylationand thus deactivation of the PIP3 dephosphatasephosphatase and tensin homologue (PTEN).62 Onthe other hand, the adapter protein SH2B1 hasbeen shown to recruit JAK and IRS proteins in a su-percomplex, thus allowing crosstalk between thesetwo pathways.63 Importantly, SH2B1 has also been

linked to human obesity.

64

Leptin-induced PI3Ksignalling in the hypothalamus has been linked toperipheral glucose homeostasis and food intake,60,65

although the neuron populations mediating botheffects are not completely elucidated.

Another molecular target of both leptin and in-sulin is the AMP-dependent kinase (AMPK). Lowcellular energy levels will increase the AMP/ATP ra-tio, which is sensed by AMPK and converted intoa cellular response to induce ATP generation andreduce ATP consumption. In the hypothalamus,

AMPK activation increases food intake, and bothleptin and insulin have been shown to decrease the

phosphorylation and thus activation of AMPK in



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Belgardt & Bruning CNS leptin and insulin action

Figure 2. Intracellular signaling cascades activated by insulin and leptin. Insulin and leptin are able to stimulate activity of

the phosphatidylinositol-3-kinase (PI3K), which subsequently results in phosphorylation and nuclear exclusion of the forkhead

transcription factors FOXO1 and FOXA2. FOXO1 is a negative regulator of carboxypeptidase E (CPE) expression in POMC

neurons, whereas it stimulates transcription of AGRP in AGRP neurons. Leptin-activated STAT3 binds to POMC and AGRP

promoters, where it stimulates (POMC) or inhibits (AGRP) expression via recruitment of histone modifying enzymes. FOXA2 was

demonstrated to bind to the orexin and MCH promoters, where it stimulates expression of these neuropeptides. Insulin stimulation

is known to increase expression of the dopamine transporter (DAT) gene in dopaminergic neurons. Both leptin and insulin are

able to hyperpolarize neurons by PI3K-mediated opening of ATP-dependent potassium channels (KATPchannels) and subsequent

potassium outflow. On the other hand, leptin is able to depolarize neurons and thus increase the firing rate by opening a nonspecific

cation channel by a PI3K and JAK-dependent pathway, and pharmacological manipulation has led to the conclusion that this

channel may be the transient receptor potential (TRP) channel. Note that for clarity, several other pathways have been omitted (see

text).

the hypothalamus,66 although it is unresolved howexactly activation of AMPK is blocked by both hor-mones. AMPK action in POMC and AGRP neuronsplays a role in the neurons response to ambient lev-els of glucose, whereas leptins and insulins effect onneuron firing is not affected.67 Besides these path-

ways, leptin (and insulin) are able to inducemitogen-activated protein kinases (MAPK) suchas extracellular signal-regulated kinase (ERK). Al-

though ERK signaling mediates some of the effectsof leptin on food intake in the hypothalamus,68 itis unknown if insulin uses this pathway to maintainbody weight.

Transcription, membrane potential,

and synaptologyAs true regulators of neuronal activity, leptinand in-sulin change membrane potential of target neurons



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to control firing rate and thus neuropeptide andneurotransmitter release. Both leptins and insulinseffect on POMC neuron firing has been extensivelystudied, with leptin depolarizing and insulin hyper-polarizing a subset of POMC neurons. With regards

to leptin, the cation-channel opened by leptin stim-ulation has been elusive for some years, with recentreports implicating both leptin-stimulated JAK andespecially PI3K signaling in opening of transient re-ceptor potential channels in POMC neurons, and itwill be highly informative to see if this holds truealso for other leptin-stimulated neurons, such asVMH neurons.69,70 On the other hand, both in-sulin and leptin have been shown to be able to ac-tivate ATP-dependent potassium (KATP) channels,

which leads to potassium outflow, hyperpolariza-tion and a reduction of the firing rate.71,72 This hasbeen well demonstrated for AGRP neurons.31,73,74

Mechanistically, it has been proposed that PI3K ac-tivation leads to local accumulation of PIP3, whichbinds to KATPchannels, increasing the probabilityfor an open channel, and reducing the affinity forATP.75 On the other hand, PIP3 generation alonemay not be sufficient, because actin filament sta-bilization prevents insulin-stimulated KATPchannelopening, whereas the mechanisms induced by in-

sulin (and leptin) to control actin filament dynam-ics are poorly understood.74 As the name suggests,KATPchannels are sensitive to cellular levels of ATP,that is, they are closed by intracellular rises in ATP.Glucose-sensitiveneurons (suchas POMCneurons)areable to sense a rise in ambient glucose concentra-tions, because the increase in cytoplasmic ATP in re-sponse to glycolysis closes the KATPchannels, whicheventually results in depolarization and an increasein firing rate.76 PI3K activation and insulin stimu-

lation have also been reported to depolarizeAGRPneurons, although the channels involved have notbeen identified, underscoring the diversity of find-ings regarding electrophysiological responses to in-sulin and leptin, as recently discussed.67,76,77 Takentogether, depending on the neuronal population,acute insulin and leptin application may depolar-ize or hyperpolarize target neurons, effects whichmay be accounted for by differential expression oftheir receptors, that of ion channels or intracellularsignaling intermediates determining the net result.

Both leptin and insulin directly control transcrip-tion of target genes, including neuropeptides. As

mentioned before, leptin-activated STAT3 signaling

controls POMC transcription in POMC neurons.59

In the same vein, leptin and insulin stimulationleads to phosphorylation and nuclear exclusion ofthe forkhead transcription factor FOXO1, allowingfor STAT3 binding to the promoter and transcrip-

tion of POMC.33,34,78,79 FOXO1 is also a negativeregulator of carboxypeptidase E (CPE) expression,which is important for a distal step in processingPOMC into its cleavage products, i.e., -MSH.80

Regarding expression of orexigenic neuropep-tides, FOXO1 and STAT3 again compete for bindingto the promoters of AGRP and NPY, with FOXO1being an activator of transcription of these orexi-genic neuropetides,and STAT3 being inhibitory.78,79

Indeed, leptins ability to reduce AGRP/NPY ex-

pression depends on PI3K signaling,81

and insulinstimulation excludes FOXO1 from the nucleus ofAGRP neurons.34 Because mice with constitutiveSTAT3 activation only in AGRP neurons are hyper-active and lean, STAT3 signaling in these neuronssurprisingly regulates locomotor activity, althoughthe downstream neurons mediating this phenotypeare unknown.24 Interestingly, mice with constitutiveSTAT3 signaling in POMC neurons are hyperphagicand mildly obese, due to suppressor of cytokine sig-naling (SOCS) 3 overexpression.82 SOCS3 inhibits

activation of the leptin signaling cascade at the levelof the receptor, and SOCS3 expression is under con-trol of STAT3 signaling, thus constituting a negativefeedback loop. Because leptin levels are chronicallyelevated in obesity, SOCS3 levels are increased inthe hypothalamus of obese mice,83 and ablation ofSOCS3 in the brain or POMC neurons ameliorateshigh fat diet- (HFD)induced obesity.84,85 Nonethe-less, hyperleptinemia alone does not induce SOCS3-mediated leptin resistance and consequently

obesity, because leptin-transgenic mice remainleptin sensitive.86 Interestingly though, hyper-leptinemia predisposed leptin-transgenic mice toobesity when challenged with a HFD. SOCS3 is alsoan inhibitor of insulin signaling through degrada-tion of IRS proteins, thus (at least in POMC neu-rons) hyperleptinemia or STAT3 overactivation willconcurrently lead to cellular insulin resistance.82,87

Leptin also controls expression of multiple neu-ropeptides in neurons downstream of POMC andAGRP/NPY neurons, for example in the paraven-

tricular neurons (PVN). Here, leptin has beenshown to increase expression of thyroid releas-

ing hormone (TRH), which is a positive regulator



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of energy expenditure.88 Moreover, leptin stimu-lation affects transcription of many more genes,nonetheless mice with leptin receptor deficiencyonly in the PVN have not been generated yet, thusthe direct and indirect targets (through neuropep-

tide/neurotransmitters released by upstream neu-rons) are currently indistinguishable.89

Besides direct transcriptional control and mem-brane potential regulation, leptin (and purport-edly insulin) signaling is able to change thesynaptic input onto neurons. For example, micelacking leptin show decreased numbers of gluta-matergic (=excitatory) synapses on POMC neuronsand increased glutamatergic input on NPY neurons,both of which is rapidly, that is in hours, normal-

ized upon leptin treatment.90

Other hormones, forexample estrogen, a known anorexigenic hormone,or ghrelin, which stimulates food intake, also regu-late synaptic input,90,91 thus synaptic rewiring maybe an important level of regulation used by manydifferent hormones involved in energy homeostasis.Nonetheless, it is still unresolved by which mech-anism and which attractant the synapses are re-cruited or repelled, and, if this is due to signaling onthe presynaptic or postsynaptic neurons. Althougha role for insulin in synaptic plasticity in neurons

directly linked to energy homeostasis has not beenreported, postsynaptic insulin action in hippocam-pal neurons is known to recruit GABA receptors andthussensitize cells for thisinhibitoryneurotransmit-ter,92 which at least opens up the question if insulinplays a similar role in hypothalamic neurons.

Extrahypothalamic neurons targetedby insulin and leptin

Clear evidence that hypothalamic neurons do not

contributealloftheweight-regulatingeffectsofbothleptin and insulin has opened the search for othernuclei expressing LEPR and IR, and analysis of theirimportance with regards to energy homeostasis. Allof these nuclei and neuropeptide circuits had beenpreviously shown to control body weight regula-tion, and it is now obvious that insulin and leptinact on almost all levels of feeding, including foodrecognition, food liking, and meal initiation.

Dopamine

The generation of mice lacking dopamine in thebrain led to the discovery that these mice show re-duced activity, less food intake, and would die if

not treated with l-dopa, which is metabolized todopamine.93 Notably, if these mice are crossed tomice lacking leptin, the resulting animals are alsohypophagic and die.94 The role of the dopaminer-gic circuit concerning addiction to drugs such as

alcohol, amphetamines or cocaine but also to therewarding aspect of food is well established,95 withdopaminergic neurons located in the substantia ni-gra (SN) and ventral tegmental area (VTA) pro-jecting to many brain nuclei involved in activity,decision-making and activity, such as the frontalcortex, hippocampus or the striatum. Intriguingly,there is now growing evidence that obesity is alsolinked to dysfunction of the dopaminergic system,as striatal dopamine D2receptor binding is reduced

in obese patients as measured by positron emis-sion tomography.96 Most importantly, leptin andinsulin impact on midbrain dopaminergic neuronsto regulate food-finding behavior and eventuallybody weight.95 Thus, LEPR are expressed on VTAdopaminergic neurons, which are hyperpolarizedafter stimulation with leptinex vivo.97,98 Moreover,leptin microinjection into the VTA reduces foodintake, whereas ablation of the receptor only inthe VTA increased the sensitivity of these mice tothe rewarding aspect of highly palatable food, such

as sucrose.98 The crucial role for VTA dopamineneurons in the regulation of energy homeostasisis underlined by the finding that direct leptin ac-tion on LH neurons also signals to VTA dopamineneurons by synaptic contact, decreasing food in-take and thus body weight.43 Besides LEPR, theIR is also expressed on VTA (and SN) dopaminer-gic neurons.99 Intracerebroventricular insulin treat-ment has been demonstrated to increase expressionthe dopamine transporter (DAT) in dopaminergic

neurons.

100

Dopamine from the synaptic cleft istaken up by DAT back into the presynaptic neu-ron, and thus stops to stimulate postsynaptic neu-rons. Hence, insulin may act through this cascadeto decrease the rewarding effect of food, which is inline with the findings from multiple experimentalparadigms.95

SerotoninIn the CNS, serotonin (5-Hydroxytryptamin) is aneuropeptide expressed only in the raphe nucleus

located in the midbrain. Besides being involved inmood regulation, serotonin clearly plays an impor-tant role in the control of weight. Thus, molecules



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Figure 3. Mechanisms implicated in CNS leptin and insulin resistance. Both leptin and insulin must pass the bloodbrain barrier

(BBB) by a saturable and active transport mechanism. Binding of C-reactive protein (CRP), whose expression is elevated in obesity

to leptin inhibits interaction of leptin withits receptors, and has been speculated to impair BBB transport of leptin. Hyperlipidemia,

hyperglycemia, and obesity by itself have also been shown to impair insulin transport across the BBB. Elevated circulating levels of

saturated fatty acids, such as palmitate, may activate toll-like receptor TLR / Myd88-dependent signaling, which results in inhibitor

of NFKB kinase activation, which is able to both directly impair leptin signaling, but may also upregulate expression of suppressor

of cytokine signaling (SOCS) 3, which then acts on the leptin receptor. IKK and SOCS3 are also involved in the induction of CNS

insulin resistance. Inflammatory signals, for example tumor necrosis factor-(TNF) , can activate inflammatory signalling cascades,such as IKK but also c-Jun N-terminal kinase (JNK)1. JNK1 has been implicated in blocking insulin signaling directly at insulin

receptor substrates throughinhibitory serine phosphorylation. Both IKK and JNK1 can be activated by high-fat diet (HFD) induced



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increasing the bioavailability of serotonin (by in-hibiting reuptake into the presynaptic neuron) suchas fenfluramin were very effective in reducing bodyweight in patients,although side effects partially dueto serotonin receptor activation in the heart led to

its withdrawal from the market. Interestingly, sero-tonin acts on the melanocortin system to reducehunger. Thus, serotonin nerve terminals are foundon POMC neurons, serotonin treatment increasesexcitation and thus the firing rate of POMC neu-rons,and mice with reexpression of serotonin recep-tor type 2 only on POMC neurons on a backgroundof serotonin type 2c receptor knockout greatly nor-malizes body weight compared to the obese HTR2creceptor knockout mice.101,102 Although previous

reports had already detected significant LEPR ex-pression in the (dorsal) raphe nucleus,103 recentdata indicates that leptin acts directly on serotoner-gic neurons to control food intake and body weight,because mice lacking LEPR only on serotonergiccells showed similar obesity compared to mice lack-ing all LEPR (i.e., db/dbmice).104 The authors couldfurthermore show that leptin treatment decreasedthe number of action potentials of serotonin neu-rons.104 Because serotonin release onto POMC neu-rons is thought to mediate its anorexigenic effects at

least in part, it is not immediately obvious how thisinhibitory effect of leptin would lead to an outcomeof reduced feeding and body weight.

Nucleus tractus solitarius (NTS)The NTS of the brainstem is a relay between affer-ent input from the gut, with synaptic connectionsto the hypothalamus, the parabrachialis nucleus,butalso higher brain regions such as the forebrain.105

Gut distention due to food intake in combination

with release of gut hormones such as glucagon-likepeptide (GLP) signals to the brainstem via the va-gus nerve to induce satiety, and this input is thenforwarded and computed in the aforementionednuclei. Leptin signaling in the brainstem is clearlyrelevant for meal size regulation, because LEPR ex-pression is found in the brainstem, injection of lep-tin into the brainstem parenchyma at very small

doses induces phosphorylation of STAT3, and isable to reduce meal size and thus food intake.106

Importantly, a subpopulation of NTS neurons ex-ists that both integrates leptin action (through theLEPR) and gastrointestinal input (through vagal

innervation).105 Moreover, leptin potentiates gas-trointestinal (GI) signals; thus when gastric disten-tion and leptin injection into the 4th ventricle (inclose contact with the NTS), at levels that do notreduce food intake, are combined, a significant re-duction in food intake is achieved.105 The NTS alsoharbors a population of POMC neurons, which areactivated by the anorexigenic gut hormone chole-cystokinine, and signal to MC4R in the brainstemto subsequently reduce food intake.107 Although it

is not clear of insulin has similar properties with re-spect to NTS-mediated food intake suppression, thesynergy between leptin (and possibly insulin) andGI signals is of high interest, because GI hormonessuch as GLP (and its derivates) or oxyntomodulinpositively affect glucose homeostasis and/or bodyweight, and importantly, obesity may not induceresistance against gut hormones to the same extentas against leptin and insulin.108

The mechanisms of CNS insulin

and leptin resistance

Frustratingly, leptin and insulins ability to controlenergy homeostasis is abrogated both in obese ani-mal models and individuals suffering from obesity.Because lifestyle interventions alone are not suffi-cient to normalize body weight in most individuals,therapeutic interventions appear to be necessary fortreatment of obese patients. Thus, to understandthe mechanisms by which the weight-reducing ef-fects of leptin and insulin are blunted is of utmost

importance (Figure 3).The first obstruction that both leptin and in-

sulin must surpass is the bloodbrain barrier (BBB),at least in brain regions where the BBB is tight(in the ARC, the BBB is only weakly developed).Both leptin and insulin are transported acrossthe BBB via saturable mechanisms, which are im-paired by obesity and associated pathologies such as

endoplasmatic reticulum (ER) stress, induced by accumulation of misfolded proteins in the ER, which leads to the unfolded protein

response (UPR). Palmitate also regulates localization and activation of protein kinase C theta (PKC

), which may induce insulinresistance through serine phosphorylation of IRS proteins. Low-grade inflammation will induce expression of protein phosphatase

1B (PTP1B), which dephosphorylates and thereby deactivates IR, IRS, LEPR, and JAK proteins. Finally, HFD induces apoptosis of

hypothalamic neurons in rats, and it is unknown if this holds true also for other model organisms.



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hypertriglyceridemia.109112 A more direct impacton leptin signaling has been linked to the low-levelchronic inflammation found in obesity, that is C-reactive protein (CRP), secreted by the liver andincreased in obese patients, binds to leptin and in-

hibits interaction with LEPR, thus leading to leptinresistance.113 Notably, leptin itself induces hepaticCRP expression, and as such hyperleptinemia it-self has been shown to be a negative regulator ofleptin sensitivity. Thus, chronic hyperleptinemia incombination with additional factors such as HFDwill lead to overexpression of SOCS3, which inducesboth a blockade of leptin signaling at the level of theleptin receptor,114 and insulin resistance, becauseSOCS3 targets IRS proteins for ubiquitin-mediated

degradation.87

Further to this, SOCS3 ablation fromall neurons strongly protects from diet-inducedobesity.84

Besides SOCS3, othernegative regulators of leptinandinsulinsignalinghavebeenshowntobeinvolvedintheinductionofresistancetothesetwohormones,such as protein phosphatase 1b (PTP1B).115 Hy-pothalamic PTP1B is increased upon obesity (andinterestingly aging), it may dephosphorylate JAKs,STATs, the IR as well as IRS proteins.116,117 BothSOCS3 and PTP1B are involved in inflammatory

processes. The notion that obesity induces low-levelinflammation in the periphery, especially the adi-pose tissue, has been known since the late 1980s,whenitwasdemonstratedthatadiposetissueofbothmurine obesity models and that of obese patientsexpresses higher levels of tumor-necrosis factor-(TNF-),118 which is released into circulation, andinduces insulin resistance in target tissues such asskeletal muscle and adipose tissue itself.119 Indeed,in that setting, macrophages invade the adipose tis-

sue, likely recruited to take up overloaded and thusdying adipocytes,120 and release proinflammatorycytokines which in a positive feedback loop will at-tract more macrophages.121

Interestingly also in the hypothalamus, increasedexpression of both TNF- and Interleukin 6 havebeen reported upon obesity development,122 ac-companied by activation of inflammation-sensitivekinases such as inhibitor of NFKB kinase (IKK) andc-Jun N-terminal kinase (JNK).122124 Moreover,inhibition of IKK signaling improves leptin and

insulin sensitivity, whereas JNK1 inhibition amelio-rates hypothalamic insulin resistance123125. Mech-

anistically and in line with the accepted role for

SOCS3 in leptin resistance,IKK activationmay posi-tively control SOCS3 expression, especially in AGRPneurons.124 JNK1 on the other hand, may phospho-rylate IRS proteins on serine residues, to inhibitactivating tyrosine phosphorylation.126 JNK1 may

also inhibit TRH expression and thus energy expen-diture, although it is unknown if this is due to JNK1action directly in the PVN neurons.123,127

In this context, it is notable that TNF- recep-tor knockout mice show increased energy expendi-ture potentially due to elevated TRH levels, whichmay suggest a TNF--> JNK1->TRH-regulatorycircuit.128 On first view, this data may indicate thatincreased TNF- and Interleukin 6 signaling maybe underlying the activation of the proinflamma-

tory signaling cascades found in the hypothalami ofobese animals, yet closer inspection may yield otherconclusions. First, mice lacking interleukin 6 de-velop adult-onset obesity,129 indicating that IL6 sig-naling is necessary for normal energy homeostasis.Second, it is unclear if in the mouse model systemcentrally applied TNF- is anorexigenic or orexi-genic,130,131 which may be dependent on the con-centration used. Third, TNF- signaling has beendemonstrated to protect brain tissue against multi-ple cellular stressors such as (glutamate) excitotox-

icity, axonal injury or oxidative stress.132,133 In thiscontext it is important to note, that hyperglycemia,a hallmark of diabetes and obesity, induces neu-ronal oxidative stress,134 mitochondrial abnormali-ties,135 and most importantly, diet-induced obesityleads to loss of hypothalamic neurons due to apop-tosis in rats.136 It has also been noted that chroniccilliary neurotrophic factor (CNTF) administrationreduces body weight dependent on neurogenesis inthe hypothalamus,137 whereas the acute anorectic

effect of CNTF is mediated via gp130 receptor sig-naling in POMC neurons,138 indicating that gainor loss of neurons known to be relevant in energyhomeostasis may be involved in the developmentand prevention of obesity. Finally, it has not yetbeen revealed if cytokines produced in peripheraltissues such as adipose tissue are the source of thelow-level inflammation in the hypothalamus, or ifmicroglia, astrocytes and/or neurons produce thesecytokines, which then act in an autocrine/paracrinemanner. It is likely, that both peripheral and locally

produced cytokines are involved in the inflamma-tory processes in hypothalamic tissue of obese an-

imals. Further decisive experiments regarding the



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diet-induced dysfunction and degeneration of neu-rons involved in energy homeostasis will be highlyinformative.

Besides cytokines, (saturated) fatty acids such aspalmitate have been directly implicated in both lep-

tin and insulin resistance by eliciting activation ofinflammatory kinases such as IKKs or JNKs. It haslong been acknowledged that hyperlipidemia in-duces peripheral insulin resistance, partially due toactivation of toll-like receptor (TLR) 4 signaling.139

In the hypothalamus, i.c.v. injection of palmitateat a dose which does not affect food intake par-tially blocks the ability of leptin to activate STAT3signaling and reduce food intake.140 This was notthe case in mice lacking neuronal MYD88, a scaf-

fold protein necessary for TLR2/4-mediated activa-tion of IKK and JNK signaling.140 Similar studies inrats could show that palmitate application inducedIKK activation, and subsequently hypothalamic in-sulin resistance.141 Notably, this study confirmedthat even when total caloric intake was similar be-tween groups, the group consuming HFD showedgreater CNS insulin resistance.141 Finally, fatty acidsmay induce local expression of cytokines, whichthen may lead to leptin resistance and obesity.142

Fatty acids also activate protein kinase (PKC) theta,

which might then translocate to the plasma mem-brane and induce inhibitory serine phosphorylationon IRS proteins.143 This report underlined the no-tion that unsaturated fatty acids, such as oleate, donot induce CNS insulin resistance,143 and in factmay have anorexigenic properties, by inhibition ofKATP channels of POMC neurons.

144 It should benoted, that as of now, there has been no conclusiveevidence that saturated fatty acids such as palmi-tate directly bind to TLRs, and moreover, there has

been the intriguing finding that under conditions ofobesity, small amounts of intestinal lipopolysaccha-ride (LPS, theprototypicalTLR activatingmolecule)may enter the circulation due to a leakage of the in-testinal barrier function, which then might inducelow-grade inflammation.145

Besides both low-grade inflammation and hyper-lipidemia, another crucial event in the pathology ofperipheral insulin resistance is endoplasmatic retic-ulum (ER) stress. Protein folding in the ER is neces-sary for normal cellular homeostasis, as misfolding

of nascent proteins can have deleterious results, ulti-mately compounding cellviability. In obeseanimals,

the need for translation is increased (at least in liver

and the pancreas), whereas at the same time fold-

ing capability is limited. Accumulation of unfoldedproteins is then sensed by specific receptors, whichwill initiate an adaptive program, termed unfoldedprotein response (UPR). During the UPR, the ex-

pression of chaperones, proteins assisting in proteinfolding will be increased, whereas global transla-tion is reduced to resolve the ER stress.146 Note thatupon enduring unresolved ER stress, the cell willinitiate an apoptosis program.146 In peripheral tis-sues, especially the liver, ER stress has been detectedin diabetic and obese mouse models, and appli-cation of chemical chaperones can greatly reduceglucose intolerance and insulin resistance in theseanimals.147,148 ER stress is not restricted to periph-

eral tissues; instead it appears to play a major rolein the induction of hypothalamic leptin and insulinresistance. Neuronal ablation of XBP-1, a transcrip-tion factor involved in the UPR, leads to massiveleptin resistance, and obesity.149 Hence, i.c.v. ap-plication of chemical chaperones ameliorates leptinresistance.149 Moreover, i.c.v. application of chem-ical reagents such as tunicamycin or thapsigarginknown to induce ER stress partially inhibits the ef-ficacy of i.c.v. leptin and insulin as well.150 In linewith the notion that hyperlipidemia, inflammation

and ER stress are interlinked, hyperlipidemia andinflammation may induce hypothalamic ER stress,and ER stress induces cytokine expression, whichmay be involved in a specific decrease of anorex-igenic POMC expression, possibly due to POMCneuron apoptosis.124,136,142,151 Although several re-ports now point to the induction of ER stress uponhyperlipidemic or inflammatory insults also in thehypothalamus, the mechanisms by which ER stressis induced, are less well defined (and vice versa,

the mechanisms by which chaperone treatment im-proves hypothalamic leptin sensitivity). Saturatedfatty acids may directly impair ER homeostasis, forexamplebychangingthelipidcompositionoftheERmembrane, which is followed by calcium depletionandbreakdown of ER function.152154 Notably, over-abundance of saturated fatty acids such as palmitatemay also induce generation of ceramides, a lipidspecies involved in insulin resistance and apopto-sis. Ceramide generation has been linked to insulinresistance in peripheral tissues such as liver and

skeletal muscle.155 Palmitate is necessary for the denovogeneration of ceramides, palmitatesupplement

acutely induces ceramide generation, and ceramide



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Table 1. Urgent questions

Are there leptin- but not insulin-responsive subpopulations in all nuclei involved in energy homeostasis? How can

we identify and differentiate between these subpopulationsin vivoby transgenesis to address their function?

Does insulin influence synaptic rewiring of neurons? Is this cell-intrinsic or postsynaptically controlled? Can we

determine the molecular pathways controlled by leptin and insulin to affect axon guidance and synaptic contact? If leptin-mediated regulation of serotonin is crucial for energy homeostasis, is insulin involved in this as well?

Does VTA/NTS/raphe leptin and insulin resistance exist? Do the same mechanisms occur as in the hypothalamus?

Can we identify further extrahypothalamic targets mediating leptins and insulins effects, such as olfactory

neurons?

How can we connect and integrate genes found by human genomewide association studies (GWAS) into the

known signaling pathways of leptin and insulin? Is there a connection to be found at all?

Can we identify marker for the neurons implicated in computing gastrointestinal signals and insulin/leptin

signaling? Are GI signals in the CNS modulated by insulin?

accumulation induces insulin resistance by block-ing AKT activation and dephosphorylation of acti-vated AKT in peripheral tissues or nonneuronal celllines.156158

Conclusion

In this review, we attempt to give an informingoverview on the current concepts of insulin andleptin targets, the intracellular cascades activated,

and the pathomechanisms leading to CNS resistanceagainst these two anorexigenic hormones. We havesummarized the most urgent questions in need ofclarification in Table 1. It is clear from the existingdata that although hypothalamic circuits are nec-essary for control of energy homeostasis, severalothers such as the dopaminergic and serotonergicneurons critically contribute to normal body weightregulation. It is also accepted that besides leptin andinsulin, there are dozens of other hormones such

as ghrelin, but also metabolites such as glucose andlipids, which directly impact weight regulation andglucose metabolism. Moreover, there is the worryingfact that obesity and diabetes, especially in the preg-nant mother, have negative and importantly long-term effects for the unborn child (see review in thisvolume by S. Ozanne, Ref. 159), which suggest thatin our rapidly aging and westernized societies, thediabesity pandemics will grow. Thus, although en-vironmental changes and lifestyle adaptation willhopefully be able to throttle the increase in obe-

sity and diabetes with beneficial effects on otheraging-related diseases, we have to try to developpotential treatment options, which may eventually

include intranasal delivery of neuropeptides,160 orself-inactivating gene therapy.161

We would like to stress that although one maythink that we have identified all major brain sub-

regions involved in energy homeostasis, we havelikely only identified a small proportion. Furtherprogress in the analysis of extra-hypothalamic nu-clei and integration of these findings including therole of clearly relevant genes discovered by humangenomewide association studies (GWAS) of hu-

man obesity (such as FTO)162,163 into our existingconcepts will eventually lead to novel therapeuti-cal targets and better treatments for the diabesitypandemic.

Acknowledgments

We apologizeto allcolleagues whose important con-tribution could not be cited due to space limitations.We thank G. Schmall andT. Rayleforexcellent secre-tarial assistance and all members of the Bruning labfor helpful discussion of the manuscript. This workwas supported by grants from the CMMC (TV1)and the DFG (Br. 1492/71) to J.C.B. CECAD isfunded by the DFG within the Excellence Initiativeby German Federal and State Governments.

Conflicts of interest

The authors declare no conflicts of interest.

References

1. Woods, S.C. & R.J. Seeley. 2000. Adiposity signals and the

control of energy homeostasis. Nutrition (Burbank, LosAn-geles County, Calif.)16:894902.

2. Brown, L.M., D.J. Clegg, S.C. Benoit & S.C. Woods. 2006.

Intraventricular insulin and leptin reduce food intake and



13/17


body weight in C57BL/6j mice. Physiol. Behav. 89: 687

691.

3. Chavez, M., K. Kaiyala, L.J. Madden,et al. 1995. Intraven-

tricular insulin and the level of maintained body weight in

rats.Behav. Neurosci.109:528531.

4. Woods, S.C., E.C. Lotter, L.D. McKay & D. Porte Jr. 1979.

Chronic intracerebroventricular infusionof insulin reducesfood intake and body weight of baboons.Nature282:503

505.

5. Hallschmid, M.et al. 2004. Intranasal insulin reduces body

fat in men but not in women. Diabetes53:30243029.

6. Jessen, L., D.J. Clegg & S.D. Bouman. 2010. Evaluation

of the lack of anorectic effect of intracerebroventricular

insulin in rats.Am. J. Physiol. Regul. Integr. Comp. Physiol.

298:R43R50.

7. Air, E.L., S.C. Benoit, D.J. Clegg,et al. 2002. Insulin and

leptin combine additively to reduce food intake and body

weight in rats.Endocrinology143:24492452.

8. Halaas, J.L. et al. 1995. Weight-reducing effects of theplasma protein encoded by the obese gene. Science 269:

543546.

9. Licinio, J.et al. 2004. Phenotypic effects of leptin replace-

ment on morbid obesity, diabetes mellitus, hypogonadism,

and behavior in leptin-deficient adults. Proc. Natl. Acad.

Sci. USA101:45314536.

10. Bruning, J.C. et al. 2000. Role of brain insulin receptor

in control of body weight and reproduction. Science289:

21222125.

11. Cohen, P.et al. 2001. Selective deletion of leptin receptor

in neurons leads to obesity.J. Clin. Invest.108:11131121.

12. Cheunsuang, O. & R. Morris. 2005. Astrocytes in the arcu-

atenucleus and median eminence that take up a fluorescentdye from the circulation express leptin receptors and neu-

ropeptide Y Y1 receptors.Glia52:228233.

13. Albrecht, J., B. Wroblewska & M.J. Mossakowski. 1982. The

binding of insulin to cerebral capillaries and astrocytes of

the rat.Neurochem. Res.7:489494.

14. Ahima, R.S. & M.A. Lazar. 2008. Adipokines and the pe-

ripheral and neural control of energy balance. Mol. En-

docrinol.22:10231031.

15. McGowan, M.K., K.M. Andrews & S.P. Grossman. 1992.

Chronic intrahypothalamic infusions of insulin or insulin

antibodies alter body weight and food intake in the rat.

Physiol. Behav.51:753766.

16. Obici, S., Z. Feng, G. Karkanias, et al. 2002. Decreasing

hypothalamic insulin receptors causes hyperphagia and in-

sulin resistance in rats.Nat. Neurosci.5:566572.

17. Satoh, N.et al. 1997. The arcuate nucleus as a primary site

ofsatietyeffectofleptininrats. Neurosci.Lett.224: 149152.

18. Ring, L.E. & L.M. Zeltser. Disruption of hypothalamic lep-

tin signaling in mice leads to early-onset obesity, but phys-

iological adaptations in mature animals stabilize adiposity

levels.J. Clin. Invest.120:29312941.

19. Cowley, M.A. et al. 2001. Leptin activates anorexigenic

POMC neurons through a neural network in the arcuate

nucleus.Nature411:480484.

20. Krude, H.et al. 1998. Severe early-onset obesity, adrenalinsufficiency and red hair pigmentation caused by POMC

mutations in humans.Nat. Genet.19:155157.

21. Yaswen, L., N. Diehl, M.B. Brennan & U. Hochgeschwen-

der. 1999. Obesity in the mouse model of pro-

opiomelanocortin deficiency responds to peripheral

melanocortin.Nat. Med.5:10661070.

22. Parton, L.E. etal. 2007. Glucose sensing by POMC neurons

regulates glucose homeostasis and is impaired in obesity.

Nature449:228232.23. Coll, A.P., I.S. Farooqi & B.G. Challis,et al. 2004. Proop-

iomelanocortin and energy balance: insights from human

and murine genetics.J. Clin. Endocrinol. Metab.89:2557

2562.

24. Mesaros, A. et al. 2008. Activation of Stat3 signaling in

AgRP neurons promotes locomotor activity.Cell Metab.7:

236248.

25. Gropp, E. et al. 2005. Agouti-related peptide-expressing

neurons are mandatory for feeding. Nat. Neurosci. 8: 1289

1291.

26. Luquet, S., F.A. Perez, T.S. Hnasko & R.D. Palmiter. 2005.

NPY/AgRP neurons are essential for feeding in adult micebut can be ablated in neonates.Science310:683685.

27. Tong, Q., C.P. Ye, J.E. Jones,et al. 2008. Synaptic release of

GABA by AgRP neurons is required for normal regulation

of energy balance.Nat. Neurosci.11:9981000.

28. Wu, Q., M.P. Boyle & R.D. Palmiter. 2009. Loss of GABAer-

gic signaling by AgRP neurons to the parabrachial nucleus

leads to starvation.Cell137:12251234.

29. Balthasar, N. etal. 2004.Leptin receptor signalingin POMC

neurons is required for normal body weight homeostasis.

Neuron42:983991.

30. van de Wall, E. et al. 2008. Collective and individual

functions of leptin receptor modulated neurons control-

ling metabolism and ingestion.Endocrinology.149:17731785.

31. Konner, A.C. etal. 2007. Insulin action in AgRP-expressing

neurons is required for suppression of hepatic glucose pro-

duction.Cell Metab.5:438449.

32. Benoit, S.C.et al. 2002. The catabolic action of insulin in

the brain is mediated by melanocortins. J. Neurosci. 22:

90489052.

33. Belgardt, B.F. et al. 2008. PDK1 deficiency in POMC-

expressing cells reveals FOXO1-dependent and -

independent pathways in control of energy homeostasis

and stress response.Cell Metab.7:291301.

34. Fukuda, M. et al. 2008. Monitoring FoxO1 localization

in chemically identified neurons. J. Neurosci. 28: 13640

13648.

35. Plum, L. et al. 2006. Enhanced PIP3 signaling in POMC

neurons causes KATP channel activation and leads to diet-

sensitive obesity.J. Clin. Invest.116:18861901.

36. Williams, K.W.et al. 2010. Segregation of acute leptin and

insulin effects in distinct populations of arcuate proopi-

omelanocortin neurons.J. Neurosci.30:24722479.

37. Hill, J.W. et al. Direct insulin and leptin action on pro-

opiomelanocortin neurons is required for normal glucose

homeostasis and fertility.Cell Metab.11:286297.

38. Padilla, S.L., J.S. Carmody & L.M. Zeltser. 2010. POMC-

expressing progenitors give rise to antagonistic neuronalpopulations in hypothalamic feeding circuits.Nat. Med.

16:403405.



14/17


39. Michaud, J.L.et al. 2001. Sim1 haploinsufficiency causes

hyperphagia, obesity and reduction of the paraventricular

nucleus of the hypothalamus.Hum. Mol. Genet.10:1465

1473.

40. Majdic, G. etal. 2002. Knockoutmicelacking steroidogenic

factor 1 are a novel genetic model of hypothalamic obesity.

Endocrinology143:607614.41. Cotero, V.E. & V.H. Routh. 2009. Insulin blunts the re-

sponse of glucose-excited neurons in the ventrolateral

ventromedial hypothalamic nucleus to decreased glucose.

Am. J. Physiol. Endocrinol. Metab.296:E1101E1109.

42. Dhillon, H. etal.2006.LeptindirectlyactivatesSF1neurons

in theVMH, andthis actionby leptinis required fornormal

body-weight homeostasis.Neuron49:191203.

43. Leinninger, G.M. etal. 2009.Leptin acts via leptinreceptor-

expressing lateral hypothalamic neurons to modulate the

mesolimbic dopamine system and suppress feeding. Cell

Metab.10:8998.

44. Lin, L.et al. 1999. The sleep disorder canine narcolepsy iscaused by a mutation in the hypocretin (orexin) receptor 2

gene.Cell98:365376.

45. Yamanaka, A. et al. 2003. Hypothalamic orexin neurons

regulate arousal according to energy balance in mice.Neu-

ron38:701713.

46. Silva, J.P. etal. 2009. Regulation of adaptive behaviour dur-

ing fasting by hypothalamic Foxa2.Nature462:646650.

47. Ma, X., L. Zubcevic, J.C. Bruning, et al. 2007. Electrical

inhibition of identified anorexigenic POMC neurons by

orexin/hypocretin.J. Neurosci.27:15291533.

48. Obici, S., B.B. Zhang, G. Karkanias & L. Rossetti. 2002.

Hypothalamic insulin signaling is required for inhibition

of glucose production.Nat. Med.8:13761382.49. Pocai, A.et al. 2005. Hypothalamic K(ATP) channels con-

trol hepatic glucose production.Nature434:10261031.

50. Morton, G.J. et al. 2005. Leptin regulates insulin sensi-

tivity via phosphatidylinositol-3-OH kinase signaling in

mediobasal hypothalamic neurons. Cell Metab.2: 411420.

51. Coppari, R.et al. 2005. The hypothalamic arcuate nucleus:

a key site for mediating leptins effects on glucose home-

ostasis and locomotor activity.Cell Metab.1:6372.

52. Huo, L.et al. 2009. Leptin-dependent control of glucose

balance and locomotor activity by POMC neurons. Cell

Metab9:537547.

53. Perseghin, G. etal. 1997. Regulation of glucose homeostasis

in humans with denervated livers.J. Clin. Invest.100:931

941.

54. Edgerton, D.S.et al. 2006. Insulins direct effects on the

liver dominate the control of hepatic glucose production.

J. Clin. Invest.116:521527.

55. Koch, L. et al. 2008. Central insulin action regulates pe-

ripheral glucose and fat metabolism in mice.J. Clin. Invest.

118:21322147.

56. Plum, L.et al. 2007. Enhanced leptin-stimulated Pi3k acti-

vation in the CNS promotes white adipose tissue transdif-

ferentiation.Cell Metab.6:431445.

57. Buettner, C. etal. 2008. Leptin controls adipose tissue lipo-

genesis via central, STAT3-independent mechanisms.Nat.Med.14:667675.

58. Vaisse, C.et al. 1996. Leptin activation of Stat3 in the hy-

pothalamus of wild-type and ob/ob mice but not db/db

mice.Nat. Genet.14:9597.

59. Xu, A.W., L. Ste, C.B. Kaelin & G.S. Barsh. 2007. Inactiva-

tion of signal transducer and activator of transcription 3

in proopiomelanocortin (Pomc) neurons causes decreased

pomc expression, mild obesity, and defects in compen-

satory refeeding.Endocrinology148:7280.60. Niswender, K.D.et al. 2001. Intracellular signalling. Key

enzyme in leptin-induced anorexia.Nature413:794795.

61. Plum, L.,B.F. Belgardt & J.C. Bruning. 2006.Central insulin

action in energy und glucose homeostasis. J. Clin. Invest.

116:17611766.

62. Ning, K.et al. 2006. A novel leptin signalling pathway via

PTEN inhibition in hypothalamic cell lines and pancreatic

beta-cells.EMBO J.25:23772387.

63. Duan, C., M. Li & L. Rui. 2004. SH2-B promotes insulin

receptor substrate 1 (IRS1)- and IRS2-mediated activation

of the phosphatidylinositol 3-kinase pathway in response

to leptin.J. Biol. Chem.279: 4368443691.64. Bochukova, E.G. etal. 2004. Large, rare chromosomal dele-

tionsassociated withsevere early-onsetobesity. Nature463:

666670.

65. Zhao, A.Z., J.N. Huan, S. Gupta, et al. 2002. A phos-

phatidylinositol 3-kinase phosphodiesterase 3B-cyclic

AMP pathway in hypothalamic action of leptin on feed-

ing.Nat. Neurosci.5:727728.

66. Minokoshi,Y.etal. 2004. AMP-kinase regulates food intake

by responding to hormonal and nutrient signals in the

hypothalamus.Nature428:569574.

67. Claret, M.et al. 2007. AMPK is essential for energy home-

ostasis regulation and glucose sensing by POMC and AgRP

neurons.J. Clin. Invest.117:23252336.68. Rahmouni, K., C.D. Sigmund, W.G. Haynes & A.L. Mark.

2009. Hypothalamic ERK mediates the anorectic and ther-

mogenic sympathetic effects of leptin. Diabetes58: 536

542.

69. Qiu, J., Fang, Y., Ronnekleiv, O.K. & Kelly, M.J. 2010. Lep-

tin excites proopiomelanocortin neurons via activation of

TRPC channels.J. Neurosci.30: 15601565.

70. Hill, J.W. et al. 2008. Acute effects of leptin require PI3K

signaling in hypothalamic proopiomelanocortin neurons

in mice.J. Clin. Invest.118:17961805.

71. Spanswick, D., M.A. Smith, V.E. Groppi,et al. 1997. Lep-

tin inhibits hypothalamic neurons by activation of ATP-

sensitive potassium channels.Nature390:521525.

72. Spanswick, D., M.A. Smith, S. Mirshamsi,et al. 2000. In-

sulin activates ATP-sensitive K+ channels in hypothalamic

neurons of lean, but not obese rats.Nat. Neurosci.3:757

758.

73. van den Top, M., K. Lee, A.D. Whyment, et al. 2004.

Orexigen-sensitive NPY/AgRP pacemaker neurons in the

hypothalamic arcuate nucleus. Nat. Neurosci. 7: 493

494.

74. Mirshamsi, S.et al. 2004. Leptin and insulin stimulation

of signalling pathways in arcuate nucleus neurones: PI3K

dependent actin reorganization and KATP channel activa-

tion.BMC Neurosci.5:54.75. MacGregor, G.G. et al . 2002. Nucleotides and phos-

pholipids compete for binding to the C terminus of



15/17


KATP channels. Proc. Natl. Acad. Sci. USA 99: 2726

2731.

76. Belgardt, B.F., T. Okamura & J.C. Bruning. 2009. Hor-

mone and glucose signalling in POMC and AgRP neurons.

J. Physiol.587:53055314.

77. Al-Qassab, H.et al. 2009. Dominant role of the p110beta

isoform of PI3K over p110alpha in energy homeostasisregulation by POMC and AgRP neurons.Cell Metab.10:

343354.

78. Kitamura, T.et al. 2006. Forkhead protein FoxO1 mediates

Agrp-dependent effects of leptin on food intake.Nat. Med.

12:534540.

79. Kim, M.S.et al. 2006. Role of hypothalamic Foxo1 in the

regulation of food intake and energy homeostasis. Nat.

Neurosci.9:901906.

80. Plum, L. et al . 2009. The obesity susceptibility gene

Cpe links FoxO1 signaling in hypothalamic pro-

opiomelanocortin neurons with regulation of food intake.

Nat. Med.15:11951201.81. Morrison, C.D., G.J. Morton, K.D. Niswender,et al. 2005.

Leptin inhibits hypothalamic Npy and Agrp gene expres-

sion via a mechanism that requires phosphatidylinositol

3-OH-kinase signaling.Am. J. Physiol. Endocrinol. Metab.

289:E1051E1057.

82. Ernst, M.B. etal. 2009.EnhancedStat3activation in POMC

neurons provokes negative feedback inhibition of leptin

and insulin signaling in obesity. J. Neurosci. 29: 11582

11593.

83. Munzberg, H., J.S. Flier & C. Bjorbaek. 2004. Region-

specific leptin resistance within the hypothalamus of diet-

induced obese mice.Endocrinology145:48804889.

84. Mori, H.et al. 2004. Socs3 deficiency in the brain elevatesleptin sensitivity and confers resistance to diet-induced

obesity.Nat. Med.10:739743.

85. Kievit, P.et al. 2006. Enhanced leptin sensitivity and im-

proved glucose homeostasis in mice lacking suppressor of

cytokine signaling-3 in POMC-expressing cells. Cell Metab.

4:123132.

86. Ogawa, Y. et al. 1999. Increased glucose metabolism and

insulin sensitivityin transgenic skinny mice overexpressing

leptin.Diabetes48:18221829.

87. Rui, L.,M. Yuan, D.Frantz, etal. 2002.SOCS-1 andSOCS-3

block insulin signaling by ubiquitin-mediated degradation

of IRS1 and IRS2.J. Biol. Chem.277:4239442398.

88. Guo, F., K. Bakal, Y. Minokoshi & A.N. Hollenberg. 2004.

Leptin signaling targets the thyrotropin-releasing hor-

mone gene promoter in vivo. Endocrinology 145: 2221

2227.

89. Tung, Y.C. etal. 2008. Novel leptin-regulatedgenes revealed

by transcriptional profiling of the hypothalamic paraven-

tricular nucleus.J. Neurosci.28:1241912426.

90. Pinto, S. et al. 2004. Rapid rewiring of arcuate nucleus

feeding circuits by leptin.Science304:110115.

91. Gao,Q. etal. 2007. Anorecticestrogenmimics leptins effect

on the rewiring of melanocortin cells and Stat3 signaling

in obese animals.Nat. Med.13:8994.

92. Wan, Q.et al. 1997. Recruitment of functional GABA(A)receptors to postsynaptic domains by insulin.Nature388:

686690.

93. Zhou, Q.Y. & R.D. Palmiter. 1995. Dopamine-deficient

mice are severely hypoactive, adipsic, and aphagic.Cell83:

11971209.

94. Szczypka, M.S., M.A. Rainey & R.D. Palmiter. 2000.

Dopamine is required for hyperphagia in Lep(ob/ob) mice.

Nat. Genet.25:102104.

95. Figlewicz,D.P. & S.C. Benoit. 2009.Insulin,leptin, andfoodreward: update 2008. Am. J. Physiol. Regul. Integr. Comp.

Physiol.296:R9R19.

96. Wang, G.J.et al. 2001. Brain dopamine and obesity.Lancet

357:354357.

97. Fulton, S.et al. 2006. Leptin regulation of the mesoaccum-

bens dopamine pathway.Neuron51:811822.

98. Hommel, J.D.et al. 2006. Leptin receptor signaling in mid-

braindopamineneuronsregulates feeding. Neuron51:801

810.

99. Figlewicz, D.P., S.B. Evans, J. Murphy, et al. 2003. Ex-

pression of receptors for insulin and leptin in the ventral

tegmental area/substantia nigra (VTA/SN) of the rat.BrainRes.964:107115.

100. Figlewicz, D.P., P. Szot, M. Chavez,et al. 1994. Intraven-

tricular insulin increases dopamine transporter mRNA in

rat VTA/substantia nigra.Brain Res.644:331334.

101. Xu, Y. et al . 2008. 5-HT2CRs expressed by pro-

opiomelanocortin neurons regulate energy homeostasis.

Neuron60:582589.

102. Heisler, L.K. etal. 2002. Activation of central melanocortin

pathways by fenfluramine. Science (New York, NY) 297:

609611.

103. Scott, M.M.et al. 2009. Leptin targets in the mouse brain.

J. Comp. Neurol.514:518532.

104. Yadav, V.K.et al. 2009. A serotonin-dependent mechanismexplains the leptin regulation of bone mass, appetite, and

energy expenditure.Cell138:976989.

105. Grill, H.J. 2010. Leptin and the systems neuroscience of

meal size control.Front. Neuroendocrinol.31: 6178.

106. Hosoi, T., T. Kawagishi, Y. Okuma,et al. 2002. Brain stem

is a direct target for leptins action in the central nervous

system.Endocrinology143:34983504.

107. Fan, W. et al. 2004. Cholecystokinin-mediated suppres-

sion of feeding involves the brainstem melanocortin sys-

tem.Nat. Neurosci.7:335336.

108. Karra, E. & R.L. Batterham. The role of gut hormones in

theregulation of body weight andenergyhomeostasis. Mol.

Cell. Endocrinol.316: 120128.

109. Banks, W.A., A.J. Kastin, W. Huang,et al. 1996. Leptin en-

ters the brain by a saturable system independent of insulin.

Peptides17:305311.

110. Nonaka, N., S.M. Hileman, S. Shioda,et al. 2004. Effects

of lipopolysaccharide on leptin transport acrossthe blood

brain barrier.Brain Res.1016:5865.

111. Banks, W.A. et al. 2004. Triglycerides induce leptin re-

sistance at the bloodbrain barrier. Diabetes 53: 1253

1260.

112. Banks, W.A.et al. 2004. The source of cerebral insulin.Eur.

J. Pharmacol.490:512.

113. Chen, K.et al. 2006. Induction of leptin resistance throughdirect interaction of C-reactive protein with leptin. Nat.

Med.12:425432.



16/17


114. Bjorbaek, C., J.K. Elmquist, J.D. Frantz,et al. 1998. Identi-

fication of SOCS-3 as a potential mediator of central leptin

resistance.Mol. Cell1:619625.

115. Zabolotny, J.M.et al. 2002. PTP1B regulates leptin signal

transduction in vivo.Dev. Cell2:489495.

116. Zabolotny, J.M.et al. 2008. Protein-tyrosine phosphatase

1B expression is induced by inflammation in vivo.Journal-Biol. Chem.283:1423014241.

117. Bourdeau, A., N. Dube & M.L. Tremblay. 2005. Cytoplas-

mic protein tyrosine phosphatases, regulation and func-

tion: the roles of PTP1B and TC-PTP. Curr. Opin. Cell Biol.

17:203209.

118. Hotamisligil, G.S., N.S. Shargill & B.M. Spiegelman. 1993.

Adipose expression of tumor necrosis factor-alpha: direct

rolein obesity-linked insulin resistance. Science259: 8791.

119. Hotamisligil, G.S., A. Budavari, D. Murray & B.M. Spiegel-

man. 1994. Reduced tyrosine kinase activity of the insulin

receptor in obesity-diabetes. Central role of tumor necrosis

factor-alpha.J. Clin. Invest.94:15431549.120. Cinti, S.et al. 2005. Adipocyte death defines macrophage

localization and function in adipose tissue of obese mice

and humans.J.Lipid Res.46:23472355.

121. Schenk, S., M. Saberi & J.M. Olefsky. 2008. Insulin sensi-

tivity: modulation by nutrients and inflammation. J. Clin.

Invest.118:29923002.

122. De Souza, C.T.et al. 2005. Consumption of a fat-rich diet

activates a proinflammatory response and induces insulin

resistance in the hypothalamus.Endocrinology146:4192

4199.

123. Belgardt, B.F. et al. 2010. Hypothalamic and pituitary c-

Jun N-terminal kinase 1 signaling coordinately regulates

glucose metabolism.Proc. Natl. Acad. Sci. USA107:60286033.

124. Zhang, X.et al. 2008. Hypothalamic IKKbeta/NF-kappaB

and ER stress link overnutrition to energy imbalance and

obesity.Cell135:6173.

125. Unger, E.K., M.L. Piper, L.E. Olofsson & A.W. Xu. 2010.

Functional role of c-Jun-N-terminal kinase in feeding reg-

ulation.Endocrinology151:671682.

126. Hirosumi, J.et al. 2002. A central role for JNK in obesity

and insulin resistance.Nature420:333336.

127. Sabio, G.et al. 2010. Role of the hypothalamicpituitary

thyroid axis in metabolic regulation by JNK1. Genes Dev.

24:256264.

128. Romanatto, T. et al. 2009. Deletion of tumor necrosis

factor-alpha receptor 1 (TNFR1) protects against diet-

induced obesity by means of increased thermogenesis.

J.Biol. Chem.284:3621336222.

129. Wallenius, V.et al. 2002. Interleukin-6-deficient mice de-

velop mature-onset obesity.Nat. Med.8:7579.

130. Fantino, M. & L. Wieteska. 1993. Evidence for a direct

central anorectic effect of tumor-necrosis-factor-alpha in

the rat.Physiol. Behav.53:477483.

131. Moraes, J.C.et al. 2006. Inducible-NOS but not neuronal-

NOS participate in the acute effect of TNF-alpha on hy-

pothalamic insulin-dependent inhibition of food intake.

FEBS Lett.580:46254631.132. Schwartz, M.et al. 1991. Tumor necrosis factor facilitates

regenerationof injured central nervous system axons. Brain

Res.545:334338.

133. Cheng, B., S. Christakos & M.P. Mattson. 1994. Tu-

mor necrosis factors protect neurons against metabolic-

excitotoxic insults and promote maintenance of calcium

homeostasis.Neuron12:139153.

134. Tomlinson, D.R. & N.J. Gardiner. 2008. Glucose neurotox-

icity.Nat. Rev. Neurosci.9:3645.

135. Edwards, J.L.et al. 2010. Diabetes regulates mitochondrialbiogenesis and fission in mouse neurons. Diabetologia53:

160169.

136. Moraes, J.C.et al. 2009. High-fat diet induces apoptosis of

hypothalamic neurons.PLoS One4:e5045.

137. Kokoeva, M.V., H. Yin & J.S. Flier. 2005. Neurogenesis in

the hypothalamus of adult mice: potential role in energy

balance.Science310:679683.

138. Janoschek, R. et al. 2006. gp130 signaling in proopiome-

lanocortin neurons mediates the acute anorectic response

to centrally applied ciliary neurotrophic factor. Proc. Natl.

Acad. Sci. USA103:1070710712.

139. Shi, H.et al. 2006. TLR4 links innate immunity and fattyacid-induced insulin resistance.J. Clin. Invest.116:3015

3025.

140. Kleinridders, A. et al. 2009. MyD88 signaling in the CNS

is required for development of fatty acid-induced leptin

resistance and diet-induced obesity. Cell Metab.10: 249

259.

141. Posey, K.A. et al. 2009. Hypothalamic proinflammatory

lipid accumulation, inflammation, and insulin resistance

in rats fed a high-fat diet. Am. J. Physiol. Endocrinol. Metab.

296:E1003E1012.

142. Milanski, M.et al. 2009. Saturated fatty acids produce an

inflammatory response predominantly through the activa-

tion of TLR4 signaling in hypothalamus: implications forthe pathogenesis of obesity.J. Neurosci.29:359370.

143. Benoit, S.C.et al. 2009. Palmitic acid mediates hypotha-

lamic insulin resistance by altering PKC-theta subcellu-

lar localization in rodents. J. Clin. Invest. 119: 2577

2589.

144. Jo,Y.H.,Y.Su,R.Gutierrez&S.ChuaJr.2009.Oleicaciddi-

rectly regulates POMC neuron excitability in the hypotha-

lamus.J. Neurophysiol.101:23052316.

145. Cani, P.D.et al. 2008. Changes in gut microbiota control

metabolic endotoxemia-induced inflammation in high-fat

diet-induced obesity and diabetes in mice. Diabetes 57:

14701481.

146. Kim, I., W. Xu & J.C. Reed. 2008. Cell death and endo-

plasmic reticulum stress: disease relevance and therapeutic

opportunities.Nat. Rev. Drug Discov.7:10131030.

147. Ozcan, U.et al. 2004. Endoplasmic reticulum stress links

obesity, insulin action, and type 2 diabetes. Science 306:

457461.

148. Ozcan, U.etal. 2006. Chemical chaperonesreduce ER stress

and restore glucose homeostasis in a mouse model of type

2 diabetes.Science313:11371140.

149. Ozcan, L.et al. 2009. Endoplasmic reticulum stress plays a

central role in development of leptin resistance. Cell Metab.

9:3551.

150. Won, J.C.et al. 2009. Central administration of an endo-plasmic reticulum stress inducer inhibits the anorexigenic

effects of leptin and insulin. Obesity (Silver Spring, Md.) 17:

18611865.



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