Identification of AgRP cells in the murine hindbrain that drive feeding

OBJECTIVE

The central melanocortin system is essential for the regulation of food intake and body weight. Agouti-related protein (AgRP) is the sole orexigenic component of the central melanocortin system and is conserved across mammalian species. AgRP is currently known to be expressed exclusively in the mediobasal hypothalamus, and hypothalamic AgRP-expressing neurons are essential for feeding. Here we characterized a previously unknown population of AgRP cells in the mouse hindbrain.

METHODS

Expression of AgRP in the hindbrain was investigated using gene expression analysis, single-cell RNA sequencing, immunofluorescent analysis and multiple transgenic mice with reporter expressions. Activation of AgRP neurons was achieved by Designer Receptors Exclusively Activated by Designer Drugs (DREADD) and by transcranial focal photo-stimulation using a step-function opsin with ultra-high light sensitivity (SOUL).

RESULTS

AgRP expressing cells were present in the area postrema (AP) and the adjacent subpostrema area (SubP) and commissural nucleus of the solitary tract (cNTS) of the mouse hindbrain (termed AgRPHind herein). AgRPHind cells consisted of locally projecting neurons as well as tanycyte-like cells. Food deprivation stimulated hindbrain Agrp expression as well as neuronal activity of subsets of AgRPHind cells. In adult mice that lacked hypothalamic AgRP neurons, chemogenetic activation of AgRP neurons resulted in hyperphagia and weight gain. In addition, transcranial focal photo-stimulation of hindbrain AgRP cells increased food intake in adult mice with or without hypothalamic AgRP neurons.

CONCLUSIONS

Our study indicates that the central melanocortin system in the hindbrain possesses an orexigenic component, and that AgRPHind neurons stimulate feeding independently of hypothalamic AgRP neurons.

Exercise-induced hypothalamic neuroplasticity: Implications for energy and glucose metabolism

BACKGROUND

Neuroplasticity refers to the brain's ability to undergo functional and structural changes in response to diverse challenges. Converging evidence supports the notion that exercise serves as a metabolic challenge, triggering the release of multiple factors both in the periphery and within the brain. These factors actively contribute to plasticity in the brain, and in turn, regulate energy and glucose metabolism.

SCOPE OF REVIEW

The primary focus of this review is to explore the impact of exercise-induced plasticity in the brain on metabolic homeostasis, with an emphasis on the role of the hypothalamus in this process. Additionally, the review provides an overview of various factors induced by exercise that contribute to energy balance and glucose metabolism. Notably, these factors exert their effects, at least in part, through actions within the hypothalamus and more broadly in the central nervous system.

MAJOR CONCLUSIONS

Exercise elicits both transient and sustained changes in metabolism, accompanied by changes in neural activity within specific brain regions. Importantly, the contribution of exercise-induced plasticity and the underlying mechanisms by which it influences the beneficial effects of exercise are not well understood. Recent work has begun to overcome this gap in knowledge by examining the complex interactions of exercise-induced factors which alter neural circuit properties to influence metabolism.

Sustained inhibition of NPY/AgRP neuronal activity by FGF1

In rodent models of type 2 diabetes (T2D), central administration of FGF1 normalizes elevated blood glucose levels in a manner that is sustained for weeks or months. Increased activity of NPY/AgRP neurons in the hypothalamic arcuate nucleus (ARC) is implicated in the pathogenesis of hyperglycemia in these animals, and the ARC is a key brain area for the antidiabetic action of FGF1. We therefore sought to determine whether FGF1 inhibits NPY/AgRP neurons and, if so, whether this inhibitory effect is sufficiently durable to offer a feasible explanation for sustained diabetes remission induced by central administration of FGF1. Here, we show that FGF1 inhibited ARC NPY/AgRP neuron activity, both after intracerebroventricular injection in vivo and when applied ex vivo in a slice preparation; we also showed that the underlying mechanism involved increased input from presynaptic GABAergic neurons. Following central administration, the inhibitory effect of FGF1 on NPY/AgRP neurons was also highly durable, lasting for at least 2 weeks. To our knowledge, no precedent for such a prolonged inhibitory effect exists. Future studies are warranted to determine whether NPY/AgRP neuron inhibition contributes to the sustained antidiabetic action elicited by intracerebroventricular FGF1 injection in rodent models of T2D.

Activating Connexin43 gap junctions primes adipose tissue for therapeutic intervention

Adipose tissue is a promising target for treating obesity and metabolic diseases. However, pharmacological agents usually fail to effectively engage adipocytes due to their extraordinarily large size and insufficient vascularization, especially in obese subjects. We have previously shown that during cold exposure, connexin43 (Cx43) gap junctions are induced and activated to connect neighboring adipocytes to share limited sympathetic neuronal input amongst multiple cells. We reason the same mechanism may be leveraged to improve the efficacy of various pharmacological agents that target adipose tissue. Using an adipose tissue-specific Cx43 overexpression mouse model, we demonstrate effectiveness in connecting adipocytes to augment metabolic efficacy of the β 3-adrenergic receptor agonist Mirabegron and FGF21. Additionally, combing those molecules with the Cx43 gap junction channel activator danegaptide shows a similar enhanced efficacy. In light of these findings, we propose a model in which connecting adipocytes via Cx43 gap junction channels primes adipose tissue to pharmacological agents designed to engage it. Thus, Cx43 gap junction activators hold great potential for combination with additional agents targeting adipose tissue.

Leptin-inhibited neurons in the lateral parabrachial nucleus do not alter food intake or glucose balance

The lateral parabrachial nucleus (LPBN) has been shown to be involved in the suppression of appetite at the pharmacological, optogenetic and chemogenetic levels. However, the signalling that mediates activation of these neurons in physiological conditions has been hindered by difficulties in segregating different cell populations in this region. Using reporter mice, we identify at the electrophysiological level the effects of an anorexic hormone, leptin, on leptin receptor (ObR)-expressing neurons in the LPBN (LPBNObR neurons). Application of leptin caused inhibition in a subpopulation of LPBNObR neurons. This effect was mediated by an increased potassium conductance and was also accompanied by a decrease in excitatory synaptic input onto these neurons. However, mimicking the inhibitory effects of leptin on LPBNObR neurons through chemogenetics led to no changes in feeding or glucose levels, which suggests that leptin action on LPBNObR neurons may not be sufficient to regulate these metabolic aspects.

Time and metabolic state-dependent effects of GLP-1R agonists on NPY/AgRP and POMC neuronal activity in vivo

OBJECTIVE

Long-acting glucagon-like peptide-1 receptor agonists (GLP-1RAs), like liraglutide and semaglutide, are viable treatments for diabetes and obesity. Liraglutide directly activates hypothalamic proopiomelanocortin (POMC) neurons while indirectly inhibiting Neuropeptide Y/Agouti-related peptide (NPY/AgRP) neurons ex vivo. While temporal control of GLP-1R agonist concentration as well as accessibility to tissues/cells can be achieved with relative ease ex vivo, in vivo this is dependent upon the pharmacokinetics of these agonists and relative penetration into structures of interest. Thus, whether liraglutide or semaglutide modifies the activity of POMC and NPY/AgRP neurons in vivo as well as mechanisms required for any changes in cellular activity remains undefined.

METHODS

In order to resolve this issue, we utilized neuron-specific transgenic mouse models to examine changes in the activity of POMC and NPY/AgRP neurons after injection of either liraglutide or semaglutide (intraperitoneal - I.P. and subcutaneous - S·C.). POMC and NPY/AgRP neurons were targeted for patch-clamp electrophysiology as well as in vivo fiber photometry.

RESULTS

We found that liraglutide and semaglutide directly activate and increase excitatory tone to POMC neurons in a time-dependent manner. This increased activity of POMC neurons required GLP-1Rs in POMC neurons as well as a downstream mixed cation channel comprised of TRPC5 subunits. We also observed an indirect upregulation of excitatory input to POMC neurons originating from glutamatergic cells that also required TRPC5 subunits. Conversely, GLP-1Ra's decreased excitatory input to and indirectly inhibited NPY/AgRP neurons through activation of K-ATP and TRPC5 channels in GABAergic neurons. Notably, the temporal activation of POMC and inhibition of NPY/AgRP neuronal activity after liraglutide or semaglutide was injected [either intraperitoneal (I.P.) or subcutaneous (S·C.)] was dependent upon the nutritional state of the animals (fed vs food-deprived).

CONCLUSIONS

Our results support a mechanism of liraglutide and semaglutide in vivo to activate POMC while inhibiting NPY/AgRP neurons, which depends upon metabolic state and mirrors the pharmacokinetic profile of these compounds in vivo.

PERK in POMC neurons connects celastrol with metabolism

ER stress and activation of the unfolded protein response in the periphery as well as the central nervous system have been linked to various metabolic abnormalities. Chemically lowering protein kinase R–like ER kinase (PERK) activity within the hypothalamus leads to decreased food intake and body weight. However, the cell populations required in this response remain undefined. In the current study, we investigated the effects of proopiomelanocortin-specific (POMC-specific) PERK deficiency on energy balance and glucose metabolism. Male mice deficient for PERK in POMC neurons exhibited improvements in energy balance on a high-fat diet, showing decreased food intake and body weight, independent of changes in glucose and insulin tolerances. The plant-based inhibitor of PERK, celastrol, increases leptin sensitivity, resulting in decreased food intake and body weight in a murine model of diet-induced obesity (DIO). Our data extend these observations by demonstrating that celastrol-induced improvements in leptin sensitivity and energy balance were attenuated in mice with PERK deficiency in POMC neurons. Altogether, these data suggest that POMC-specific PERK deficiency in male mice confers protection against DIO, possibly providing a new therapeutic target for the treatment of diabetes and metabolic syndrome.

Glucagon-like peptide-1 (GLP-1) signalling in the brain: From neural circuits and metabolism to therapeutics

Glucagon‐like‐peptide‐1 (GLP‐1) derived from gut enteroendocrine cells and a discrete population of neurons in the caudal medulla acts through humoral and neural pathways to regulate satiety, gastric motility and pancreatic endocrine function. These physiological attributes contribute to GLP‐1 having a potent therapeutic action in glycaemic regulation and chronic weight management. In this review, we provide an overview of the neural circuits targeted by endogenous versus exogenous GLP‐1 and related drugs. We also highlight candidate subpopulations of neurons and cellular mechanisms responsible for the acute and chronic effects of GLP‐1 and GLP‐1 receptor agonists on energy balance and glucose metabolism. Finally, we present potential future directions to translate these findings towards the development of effective therapies for treatment of metabolic disease.

POMC neuronal heterogeneity in energy balance and beyond: an integrated view

Hypothalamic AgRP and POMC neurons are conventionally viewed as the yin and yang of the body's energy status, since they act in an opposite manner to modulate appetite and systemic energy metabolism. However, although AgRP neurons' functions are comparatively well understood, a unifying theory of how POMC neuronal cells operate has remained elusive, probably due to their high level of heterogeneity, which suggests that their physiological roles might be more complex than initially thought. In this Perspective, we propose a conceptual framework that integrates POMC neuronal heterogeneity with appetite regulation, whole-body metabolic physiology and the development of obesity. We highlight emerging evidence indicating that POMC neurons respond to distinct combinations of interoceptive signals and food-related cues to fine-tune divergent metabolic pathways and behaviours necessary for survival. The new framework we propose reflects the high degree of developmental plasticity of this neuronal population and may enable progress towards understanding of both the aetiology and treatment of metabolic disorders.

Electrophysiological Properties of Genetically Identified Histaminergic Neurons

Histaminergic neurons of the tuberomammillary nucleus (TMN) are important regulators of behavioral and homeostatic processes. Previous work suggested that histaminergic neurons exhibit a characteristic electrophysiological signature, allowing for their identification in brain slice preparations. However, these previous investigations focused on neurons in the ventral subregion of the TMN of rats. Consequently, it remains unclear whether such electrophysiological properties extend to mice, including other subregions of the TMN, and the potential for differences between males and females. To further characterize the electrophysiological properties of histaminergic neurons, we performed whole-cell patch-clamp recordings on transgenic mice expressing Cre recombinase in histidine decarboxylase (HDC)-expressing cells; the sole enzyme for histamine synthesis (Hdc-cre::tdTomato). Despite similarities with the electrophysiological properties reported in rats, we observed considerable variability in mouse HDC neuron passive membrane properties, action potential firing, and intrinsic subthreshold active membrane properties. Overall, the electrophysiological properties of HDC neurons appeared similar across subregions of the TMN, consistent with a lack of topographical organization in this nucleus. Moreover, we found no obvious sex differences in the electrical excitability of HDC neurons. However, our data reveal a diversity in the electrophysiological properties of genetically identified histaminergic neurons from mice not previously appreciated from rat studies. Thus, these data highlight the utility of mouse genetics to target the widespread histaminergic neuronal population within the TMN and support the idea that histaminergic neurons are a heterogeneous neuronal population.

Beneficial metabolic role of β-arrestin-1 expressed by AgRP neurons

β-Arrestin-1 and β-arrestin-2 have emerged as important signaling molecules that modulate glucose fluxes in several peripheral tissues. The potential roles of neuronally expressed β-arrestins in regulating glucose homeostasis remain unknown. We here report that mice lacking β-arrestin-1 (barr1) selectively in AgRP neurons displayed impaired glucose tolerance and insulin sensitivity when consuming an obesogenic diet, while mice overexpressing barr1 selectively in AgRP neurons were protected against obesity-associated metabolic impairments. Additional physiological, biochemical, and electrophysiological data indicated that the presence of barr1 is essential for insulin-mediated hyperpolarization of AgRP neurons. As a result, barr1 expressed by AgRP neurons regulates efferent neuronal pathways that suppress hepatic glucose production and promote lipolysis in adipose tissue. Mice lacking β-arrestin-2 (barr2) selectively in AgRP neurons showed no substantial metabolic phenotypes. Our data suggest that agents able to enhance the activity of barr1 in AgRP neurons may prove beneficial as antidiabetic drugs.

LEAP2 changes with body mass and food intake in humans and mice

Acyl-ghrelin administration increases food intake, body weight, and blood glucose. In contrast, mice lacking ghrelin or ghrelin receptors (GHSRs) exhibit life-threatening hypoglycemia during starvation-like conditions, but do not consistently exhibit overt metabolic phenotypes when given ad libitum food access. These results, and findings of ghrelin resistance in obese states, imply nutritional state dependence of ghrelin's metabolic actions. Here, we hypothesized that liver-enriched antimicrobial peptide-2 (LEAP2), a recently characterized endogenous GHSR antagonist, blunts ghrelin action during obese states and postprandially. To test this hypothesis, we determined changes in plasma LEAP2 and acyl-ghrelin due to fasting, eating, obesity, Roux-en-Y gastric bypass (RYGB), vertical sleeve gastrectomy (VSG), oral glucose administration, and type 1 diabetes mellitus (T1DM) using humans and/or mice. Our results suggest that plasma LEAP2 is regulated by metabolic status: its levels increased with body mass and blood glucose and decreased with fasting, RYGB, and in postprandial states following VSG. These changes were mostly opposite of those of acyl-ghrelin. Furthermore, using electrophysiology, we showed that LEAP2 both hyperpolarizes and prevents acyl-ghrelin from activating arcuate NPY neurons. We predict that the plasma LEAP2/acyl-ghrelin molar ratio may be a key determinant modulating acyl-ghrelin activity in response to body mass, feeding status, and blood glucose.

Melanocortin regulation of histaminergic neurons via perifornical lateral hypothalamic melanocortin 4 receptors

OBJECTIVE

Histaminergic neurons of the tuberomammillary nucleus (TMN) are wake-promoting and contribute to the regulation of energy homeostasis. Evidence indicates that melanocortin 4 receptors (MC4R) are expressed within the TMN. However, whether the melanocortin system influences the activity and function of TMN neurons expressing histidine decarboxylase (HDC), the enzyme required for histamine synthesis, remains undefined.

METHODS

We utilized Hdc-Cre mice in combination with whole-cell patch-clamp electrophysiology and in vivo chemogenetic techniques to determine whether HDC neurons receive metabolically relevant information via the melanocortin system.

RESULTS

We found that subsets of HDC-expressing neurons were excited by melanotan II (MTII), a non-selective melanocortin receptor agonist. Use of melanocortin receptor selective agonists (THIQ, [D-Trp8]-γ-MSH) and inhibitors of synaptic transmission (TTX, CNQX, AP5) indicated that the effect was mediated specifically by MC4Rs and involved a glutamatergic dependent presynaptic mechanism. MTII enhanced evoked excitatory post-synaptic currents (EPSCs) originating from electrical stimulation of the perifornical lateral hypothalamic area (PeFLH), supportive of melanocortin effects on the glutamatergic PeFLH projection to the TMN. Finally, in vivo chemogenetic inhibition of HDC neurons strikingly enhanced the anorexigenic effects of intracerebroventricular administration of MTII, suggesting that MC4R activation of histaminergic neurons may restrain the anorexigenic effects of melanocortin system activation.

CONCLUSIONS

These experiments identify a functional interaction between the melanocortin and histaminergic systems and suggest that HDC neurons act naturally to restrain the anorexigenic effect of melanocortin system activation. These findings may have implications for the control of arousal and metabolic homeostasis, especially in the context of obesity, in which both processes are subjected to alterations.

Metabolic insights from a GHSR-A203E mutant mouse model

Objective

Binding of ghrelin to its receptor, growth hormone secretagogue receptor (GHSR), stimulates GH release, induces eating, and increases blood glucose. These processes may also be influenced by constitutive (ghrelin-independent) GHSR activity, as suggested by findings in short people with naturally occurring GHSR-A204E mutations and reduced food intake and blood glucose in rodents administered GHSR inverse agonists, both of which impair constitutive GHSR activity. In this study, we aimed to more fully determine the physiologic relevance of constitutive GHSR activity.

Methods

We generated mice with a GHSR mutation that replaces alanine at position 203 with glutamate (GHSR-A203E), which corresponds to the previously described human GHSR-A204E mutation, and used them to conduct ex vivo neuronal electrophysiology and in vivo metabolic assessments. We also measured signaling within COS-7 and HEK293T cells transfected with wild-type GHSR (GHSR-WT) or GHSR-A203E constructs.

Results

In COS-7 cells, GHSR-A203E resulted in lower baseline IP₃ accumulation than GHSR-WT; ghrelin-induced IP₃ accumulation was observed in both constructs. In HEK293T cells co-transfected with voltage-gated Ca_V2.2 calcium channel complex, GHSR-A203E had no effect on basal Ca_V2.2 current density while GHSR-WT did; both GHSR-A203E and GHSR-WT inhibited Ca_V2.2 current in the presence of ghrelin. In cultured hypothalamic neurons from GHSR-A203E and GHSR-deficient mice, native calcium currents were greater than those in neurons from wild-type mice; ghrelin inhibited calcium currents in cultured hypothalamic neurons from both GHSR-A203E and wild-type mice. In brain slices, resting membrane potentials of arcuate NPY neurons from GHSR-A203E mice were hyperpolarized compared to those from wild-type mice; the same percentage of arcuate NPY neurons from GHSR-A203E and wild-type mice depolarized upon ghrelin exposure. The GHSR-A203E mutation did not significantly affect body weight, body length, or femur length in the first ∼6 months of life, yet these parameters were lower in GHSR-A203E mice after 1 year of age. During a 7-d 60% caloric restriction regimen, GHSR-A203E mice lacked the usual marked rise in plasma GH and demonstrated an exaggerated drop in blood glucose. Administered ghrelin also exhibited reduced orexigenic and GH secretagogue efficacies in GHSR-A203E mice.

Conclusions

Our data suggest that the A203E mutation ablates constitutive GHSR activity and that constitutive GHSR activity contributes to the native depolarizing conductance of GHSR-expressing arcuate NPY neurons. Although the A203E mutation does not block ghrelin-evoked signaling as assessed using in vitro and ex vivo models, GHSR-A203E mice lack the usual acute food intake response to administered ghrelin in vivo. The GHSR-A203E mutation also blunts GH release, and in aged mice leads to reduced body length and femur length, which are consistent with the short stature of human carriers of the GHSR-A204E mutation.

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We generated mice with a GHSR mutation replacing Ala at position 203 with Glu.

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The A203E mutation ablates constitutive GHSR activity & hyperpolarizes NPY neurons.

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GHSR-A203E mice lack the usual orexigenic response to administered ghrelin.

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The GHSR-A203E mutation blunts GH release and causes reduced body length.

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This finding is consistent with short stature in human carriers of the GHSR-A204E mutation.

A neural basis for tonic suppression of sodium appetite

Sodium appetite is a powerful form of motivation that can drive ingestion of high, yet aversive concentrations of sodium in animals that are depleted of sodium. However, in normal conditions, sodium appetite is suppressed to prevent homeostatic deviations. Although molecular and neural mechanisms underlying the stimulation of sodium appetite have received much attention recently, mechanisms that inhibit sodium appetite remain largely obscure. Here we report that serotonin 2c receptor (Htr2c)-expressing neurons in the lateral parabrachial nucleus (LPBNHtr2c neurons) inhibit sodium appetite. Activity of these neurons is regulated by bodily sodium content, and their activation can rapidly suppress sodium intake. Conversely, inhibition of these neurons specifically drives sodium appetite, even during euvolemic conditions. Notably, the physiological role of Htr2c expressed by LPBN neurons is to disinhibit sodium appetite. Our results suggest that LPBNHtr2c neurons act as a brake against sodium appetite and that their alleviation is required for the full manifestation of sodium appetite.

Lztfl1/BBS17 controls energy homeostasis by regulating the leptin signaling in the hypothalamic neurons

Leptin receptor (LepRb) signaling pathway in the hypothalamus of the forebrain controls food intake and energy expenditure in response to an altered energy state. Defects in the LepRb signaling pathway can result in leptin-resistance and obesity. Leucine zipper transcription factor like 1 (Lztfl1)/BBS17 is a member of the Bardet-Biedl syndrome (BBS) gene family. Human BBS patients have a wide range of pathologies including obesity. The cellular and molecular mechanisms underlying Lztfl1-regulated obesity are unknown. Here, we generated Lztfl1f/f mouse model in which Lztfl1 can be deleted globally and in tissue-specific manner. Global Lztfl1 deficiency resulted in pleiotropic phenotypes including obesity. Lztfl1-/- mice are hyperphagic and showed similar energy expenditure as WT littermates. The obese phenotype of Lztfl1-/- mice is caused by the loss of Lztfl1 in the brain but not in the adipocytes. Lztfl1-/- mice are leptin-resistant. Inactivation of Lztfl1 abolished phosphorylation of Stat3 in the LepRb signaling pathway in the hypothalamus upon leptin stimulation. Deletion of Lztfl1 had no effect on LepRb membrane localization. Furthermore, we observed that Lztfl1-/- mouse embryonic fibroblasts (MEFs) have significantly longer cilia than WT MEFs. We identified several proteins that potentially interact with Lztfl1. As these proteins are known to be involved in regulation of actin/cytoskeleton dynamics, we suggest that Lztfl1 may regulate leptin signaling and ciliary structure via these proteins. Our study identified Lztfl1 as a novel player in the LepRb signaling pathway in the hypothalamus that controls energy homeostasis.

Partial Leptin Reduction as an Insulin Sensitization and Weight Loss Strategy

The physiological role of leptin is thought to be a driving force to reduce food intake and increase energy expenditure. However, leptin therapies in the clinic have failed to effectively treat obesity, predominantly due to a phenomenon referred to as leptin resistance. The mechanisms linking obesity and the associated leptin resistance remain largely unclear. With various mouse models and a leptin neutralizing antibody, we demonstrated that hyperleptinemia is a driving force for metabolic disorders. A partial reduction of plasma leptin levels in the context of obesity restores hypothalamic leptin sensitivity and effectively reduces weight gain and enhances insulin sensitivity. These results highlight that a partial reduction in plasma leptin levels leads to improved leptin sensitivity, while pointing to a new avenue for therapeutic interventions in the treatment of obesity and its associated comorbidities.

Gut-derived GIP activates central Rap1 to impair neural leptin sensitivity during overnutrition

Nutrient excess, a major driver of obesity, diminishes hypothalamic responses to exogenously administered leptin, a critical hormone of energy balance. Here, we aimed to identify a physiological signal that arises from excess caloric intake and negatively controls hypothalamic leptin action. We found that deficiency of the gastric inhibitory polypeptide receptor (Gipr) for the gut-derived incretin hormone GIP protected against diet-induced neural leptin resistance. Furthermore, a centrally administered antibody that neutralizes GIPR had remarkable antiobesity effects in diet-induced obese mice, including reduced body weight and adiposity, and a decreased hypothalamic level of SOCS3, an inhibitor of leptin actions. In contrast, centrally administered GIP diminished hypothalamic sensitivity to leptin and increased hypothalamic levels of Socs3. Finally, we show that GIP increased the active form of the small GTPase Rap1 in the brain and that its activation was required for the central actions of GIP. Altogether, our results identify GIPR/Rap1 signaling in the brain as a molecular pathway linking overnutrition to the control of neural leptin actions.

Direct and indirect effects of liraglutide on hypothalamic POMC and NPY/AgRP neurons - Implications for energy balance and glucose control

Objective

The long-acting glucagon-like peptide-1 receptor (GLP-1R) agonist, liraglutide, stimulates insulin secretion and efficiently suppresses food intake to reduce body weight. As such, liraglutide is growing in popularity in the treatment of diabetes and chronic weight management. Within the brain, liraglutide has been shown to alter the activity of hypothalamic proopiomelanocortin (POMC) and Neuropeptide Y/Agouti-related peptide (NPY/AgRP) neurons. Moreover, the acute activities of POMC and NPY neurons have been directly linked to feeding behavior, body weight, and glucose metabolism. Despite the increased usage of liraglutide and other GLP-1 analogues as diabetic and obesity interventions, the cellular mechanisms by which liraglutide alters the activity of metabolically relevant neuronal populations are poorly understood.

Methods

In order to resolve this issue, we utilized neuron-specific transgenic mouse models to identify POMC and NPY neurons for patch-clamp electrophysiology experiments.

Results

We found that liraglutide directly activated arcuate POMC neurons via TrpC5 channels, sharing a similar mechanistic pathway to the adipose-derived peptide leptin. Liraglutide also indirectly increases excitatory tone to POMC neurons. In contrast, liraglutide inhibited NPY/AgRP neurons through post-synaptic GABA_A receptors and enhanced activity of pre-synaptic GABAergic neurons, which required both TrpC5 subunits and K-ATP channels. In support of an additive role of leptin and liraglutide in suppressing food intake, leptin potentiated the acute effects of liraglutide to activate POMC neurons. TrpC5 subunits in POMC neurons were also required for the intact pharmacological effects of liraglutide on food intake and body weight. Thus, the current study adds to recent work from our group and others, which highlight potential mechanisms to amplify the effects of GLP-1 agonists in vivo. Moreover, these data highlight multiple sites of action (both pre- and post-synaptic) for GLP-1 agonists on this circuit.

Conclusions

Taken together, our results identify critical molecular mechanisms linking GLP-1 analogues in arcuate POMC and NPY/AgRP neurons with metabolism.

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Liraglutide directly activates arcuate POMC neurons, while also increasing pre-synaptic excitatory inputs to POMC neurons.

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Leptin potentiates the acute effects of liraglutide to activate POMC neurons.

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Liraglutide indirectly inhibits arcuate NPY/AgRP neurons via presynaptic TrpC 5 subunits and K_ATP channels.

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TrpC5 subunits in POMC neurons are required for the intact pharmacological effects of liraglutide.

Long-Acting Neurotensin Synergizes With Liraglutide to Reverse Obesity Through a Melanocortin-Dependent Pathway

Neurotensin (NT), a gut hormone and neuropeptide, increases in circulation after bariatric surgery in rodents and humans and inhibits food intake in mice. However, its potential to treat obesity and the subsequent metabolic dysfunctions have been difficult to assess owing to its short half-life in vivo. Here, we demonstrate that a long-acting, pegylated analog of the NT peptide (P-NT) reduces food intake, body weight, and adiposity in diet-induced obese mice when administered once daily for 6 days. Strikingly, when P-NT was combined with the glucagon-like peptide 1 mimetic liraglutide, the two peptides synergized to reduce food intake and body weight relative to each monotherapy, without inducing a taste aversion. Further, P-NT and liraglutide coadministration improved glycemia and reduced steatohepatitis. Finally, we show that the melanocortin pathway is central for P-NT-induced anorexia and necessary for the full synergistic effect of P-NT and liraglutide combination therapy. Overall, our data suggest that P-NT and liraglutide combination therapy could be an enhanced treatment for obesity with improved tolerability compared with liraglutide monotherapy.

Acute effects of zinc and insulin on arcuate anorexigenic proopiomelanocortin neurons

BACKGROUND AND PURPOSE

Acute insulin administration hyperpolarized, with concomitant decrease of firing rate, a subpopulation of arcuate proopiomelanocortin (POMC) and neuropeptide Y/agouti-related peptide cells. This rapid effect on cellular activity has been proposed as a cellular correlate of insulin effects on energy balance and glucose homoeostasis. Recent evidence suggests that zinc in mammalian insulin formulations is required for the insulin-induced inhibition of arcuate POMC neurons, while guinea pig insulin, which fails to bind zinc, activates POMC neurons in mice. Here, we tested the effects of zinc and insulin formations on arcuate POMC neurons.

EXPERIMENTAL APPROACH

Effects of zinc and insulin formulations were assessed through whole-cell patch clamp recordings on transgenic mice in vitro.

KEY RESULTS

Insulin formulations containing zinc hyperpolarized POMC neurons. Zinc also hyperpolarized arcuate POMC neurons, albeit at much higher concentration than found in various insulin formulations. Chelation of zinc inhibited the zinc-induced hyperpolarization of POMC neurons, whereas effects of insulin on POMC cellular activity were unchanged after chelation. Zinc-free insulin also hyperpolarized arcuate POMC neurons. Insulin failed to hyperpolarize POMC neurons deficient for insulin receptors, suggesting that insulin receptors are required for these effects. Activation of POMC neurons by guinea pig insulin was independent of insulin receptors but was inhibited by PDGF receptor antagonism or loss of TRPC5 channel subunits.

CONCLUSIONS AND IMPLICATIONS

Together, these findings suggest that insulin inhibited arcuate POMC neurons independent of zinc and highlights a possible role of putative PDGF receptors in the acute effects of guinea pig insulin.

Cellular and synaptic reorganization of arcuate NPY/AgRP and POMC neurons after exercise

OBJECTIVE

Hypothalamic Pro-opiomelanocortin (POMC) and Neuropeptide Y/Agouti-Related Peptide (NPY/AgRP) neurons are critical nodes of a circuit within the brain that sense key metabolic cues as well as regulate metabolism. Importantly, these neurons retain an innate ability to rapidly reorganize synaptic inputs and electrophysiological properties in response to metabolic state. While the cellular properties of these neurons have been investigated in the context of obesity, much less is known about the effects of exercise training.

METHODS

In order to further investigate this issue, we utilized neuron-specific transgenic mouse models to identify POMC and NPY/AgRP neurons for patch-clamp electrophysiology experiments.

RESULTS

Using whole-cell patch-clamp electrophysiology, we found exercise depolarized and increased firing rate of arcuate POMC neurons. The increased excitability of POMC neurons was concomitant with increased excitatory inputs to these neurons. In agreement with recent work suggesting leptin plays an important role in the synaptic (re)organization of POMC neurons, POMC neurons which express leptin receptors were more sensitive to exercise-induced changes in biophysical properties. Opposite to effects observed in POMC neurons, NPY neurons were shunted toward inhibition following exercise.

CONCLUSIONS

Together, these data support a rapid reorganization of synaptic inputs and biophysical properties in response to exercise, which may facilitate adaptations to altered energy balance and glucose metabolism.

Phosphoinositide 3-Kinase Is Integral for the Acute Activity of Leptin and Insulin in Male Arcuate NPY/AgRP Neurons

Neuropeptide Y (NPY)/Agouti-related protein (AgRP) neurons in the arcuate nucleus of the hypothalamus are part of a neuroendocrine feedback loop that regulates feeding behavior and glucose homeostasis. NPY/AgRP neurons sense peripheral signals (including the hormones leptin, insulin, and ghrelin) and integrate those signals with inputs from other brain regions. These inputs modify both long-term changes in gene transcription and acute changes in the electrical activity of these neurons, leading to a coordinated response to maintain energy and glucose homeostasis. However, the mechanisms by which the hormones insulin and leptin acutely modify the electrical activity of these neurons remain unclear. In this study, we show that loss of the phosphoinositide 3-kinase catalytic subunits p110α and p110β in AgRP neurons abrogates the leptin- and insulin-induced inhibition of AgRP neurons. Moreover, continual disruption of p110α and p110β in AgRP neurons results in increased weight gain. The increased adiposity was concomitant with a hypometabolic phenotype: decreased energy expenditure independent of changes in food intake. Deficiency of p110α and p110β in AgRP neurons also impaired glucose homeostasis and insulin sensitivity. In summary, these data highlight the requirement of both p110α and p110β in AgRP neurons for the proper regulation of energy balance and glucose homeostasis.

Genetic Mouse Models: The Powerful Tools to Study Fat Tissues

Obesity and Type 2 diabetes (T2D) are associated with a variety of comorbidities that contribute to mortality around the world. Although significant effort has been expended in understanding mechanisms that mitigate the consequences of this epidemic, the field has experienced limited success thus far. The potential ability of brown adipose tissue (BAT) to counteract obesity and metabolic disease in rodents (and potentially in humans) has been a topical realization. Recently, there is also another thermogenic fat cell called beige adipocytes, which are located among white adipocytes and share similar activated responses to cyclic AMP as classical BAT. In this chapter, we review contemporary molecular strategies to investigate the role of adipose tissue depots in metabolism. In particular, we will discuss the generation of adipose tissue-specific knockout and overexpression of target genes in various mouse models. We will also discuss how to use different Cre (cyclization recombination) mouse lines to investigate diverse types of adipocytes.

TrpC5 Mediates Acute Leptin and Serotonin Effects via Pomc Neurons

The molecular mechanisms underlying acute leptin and serotonin 2C receptor-induced hypophagia remain unclear. Here, we show that neuronal and pro-opiomelanocortin (Pomc)-specific loss of transient receptor potential cation 5 (TrpC5) subunits is sufficient to decrease energy expenditure and increase food intake resulting in elevated body weight. Deficiency of Trpc5 subunits in Pomc neurons is also sufficient to block the anorexigenic effects of leptin and serotonin 2C receptor (Ht2Cr) agonists. The loss of acute anorexigenic effects of these receptors is concomitant with a blunted electrophysiological response to both leptin and Ht2Cr agonists in arcuate Pomc neurons. We also demonstrate that the Ht2Cr agonist lorcaserin-induced improvements in glucose and insulin tolerance are blocked by TrpC5 deficiency in Pomc neurons. Together, our results link TrpC5 subunits in the brain with leptin- and serotonin 2C receptor-dependent changes in neuronal activity, as well as energy balance, feeding behavior, and glucose metabolism.

Corrigendum: MC4R-dependent suppression of appetite by bone-derived lipocalin 2

This corrects the article DOI: 10.1038/nature21697.

MC4R-dependent suppression of appetite by bone-derived lipocalin 2

Bone has recently emerged as a pleiotropic endocrine organ that secretes at least two hormones, FGF23 and osteocalcin, which regulate kidney function and glucose homeostasis, respectively. These findings have raised the question of whether other bone-derived hormones exist and what their potential functions are. Here we identify, through molecular and genetic analyses in mice, lipocalin 2 (LCN2) as an osteoblast-enriched, secreted protein. Loss- and gain-of-function experiments in mice demonstrate that osteoblast-derived LCN2 maintains glucose homeostasis by inducing insulin secretion and improves glucose tolerance and insulin sensitivity. In addition, osteoblast-derived LCN2 inhibits food intake. LCN2 crosses the blood-brain barrier, binds to the melanocortin 4 receptor (MC4R) in the paraventricular and ventromedial neurons of the hypothalamus and activates an MC4R-dependent anorexigenic (appetite-suppressing) pathway. These results identify LCN2 as a bone-derived hormone with metabolic regulatory effects, which suppresses appetite in a MC4R-dependent manner, and show that the control of appetite is an endocrine function of bone.

Ire1α in Pomc Neurons Is Required for Thermogenesis and Glycemia

Whether neuronal inositol-requiring enzyme 1 (Ire1) is required for the proper regulation of energy balance and glucose homeostasis is unclear. We found that pro-opiomelanocortin (Pomc)-specific deficiency of Ire1α accelerated diet-induced obesity concomitant with a decrease in energy expenditure. This hypometabolic phenotype included deficits in thermogenic responses to diet and cold exposure as well as "beiging" of white adipose tissue. We also demonstrate that loss of Ire1α in Pomc neurons impaired whole-body glucose and insulin tolerance as well as hepatic insulin sensitivity. At the cellular level, deletion of Ire1α in Pomc neurons elevated hypothalamic endoplasmic reticulum (ER) stress and predisposed Pomc neurons to leptin and insulin resistance. Together, the current studies extend and confirm conclusions that Ire1α-Xbp1s and associated molecular targets link ER stress in arcuate Pomc neurons to aspects of normal energy and glucose homeostasis.

Activation of murine pre-proglucagon-producing neurons reduces food intake and body weight

Peptides derived from pre-proglucagon (GCG peptides) act in both the periphery and the CNS to change food intake, glucose homeostasis, and metabolic rate while playing a role in anxiety behaviors and physiological responses to stress. Although the actions of GCG peptides produced in the gut and pancreas are well described, the role of glutamatergic GGC peptide-secreting hindbrain neurons in regulating metabolic homeostasis has not been investigated. Here, we have shown that chemogenetic stimulation of GCG-producing neurons reduces metabolic rate and food intake in fed and fasted states and suppresses glucose production without an effect on glucose uptake. Stimulation of GCG neurons had no effect on corticosterone secretion, body weight, or conditioned taste aversion. In the diet-induced obese state, the effects of GCG neuronal stimulation on gluconeogenesis were lost, while the food intake-lowering effects remained, resulting in reductions in body weight and adiposity. Our work suggests that GCG peptide-expressing neurons can alter feeding, metabolic rate, and glucose production independent of their effects on hypothalamic pituitary-adrenal (HPA) axis activation, aversive conditioning, or insulin secretion. We conclude that GCG neurons likely stimulate separate populations of downstream cells to produce a change in food intake and glucose homeostasis and that these effects depend on the metabolic state of the animal.

Connexin 43 Mediates White Adipose Tissue Beiging by Facilitating the Propagation of Sympathetic Neuronal Signals

"Beige" adipocytes reside in white adipose tissue (WAT) and dissipate energy as heat. Several studies have shown that cold temperature can activate pro-opiomelanocortin-expressing (POMC) neurons and increase sympathetic neuronal tone to regulate WAT beiging. WAT, however, is traditionally known to be sparsely innervated. Details regarding the neuronal innervation and, more importantly, the propagation of the signal within the population of "beige" adipocytes are sparse. Here, we demonstrate that beige adipocytes display an increased cell-to-cell coupling via connexin 43 (Cx43) gap junction channels. Blocking of Cx43 channels by 18α-glycyrrhetinic acid decreases POMC-activation-induced adipose tissue beiging. Adipocyte-specific deletion of Cx43 reduces WAT beiging to a level similar to that observed in denervated fat pads. In contrast, overexpression of Cx43 is sufficient to promote beiging even with mild cold stimuli. These data reveal the importance of cell-to-cell communication, effective in cold-induced WAT beiging, for the propagation of limited neuronal inputs in adipose tissue.

Adiponectin potentiates the acute effects of leptin in arcuate Pomc neurons

Objective

Adiponectin receptors (AdipoRs) are located on neurons of the hypothalamus involved in metabolic regulation – including arcuate proopiomelanocortin (Pomc) and Neuropeptide Y/Agouti-related peptide (NPY/AgRP) neurons. AdipoRs play a critical role in regulating glucose and fatty acid metabolism by initiating several signaling cascades overlapping with Leptin receptors (LepRs). However, the mechanism by which adiponectin regulates cellular activity in the brain remains undefined.

Methods

In order to resolve this issue, we utilized neuron-specific transgenic mouse models to identify Pomc and NPY/AgRP neurons which express LepRs for patch-clamp electrophysiology experiments.

Results

We found that leptin and adiponectin synergistically activated melanocortin neurons in the arcuate nucleus. Conversely, NPY/AgRP neurons were inhibited in response to adiponectin. The adiponectin-induced depolarization of arcuate Pomc neurons occurred via activation of Phosphoinositide-3-kinase (PI3K) signaling, independent of 5′ AMP-activated protein kinase (AMPK) activity. Adiponectin also activated melanocortin neurons at various physiological glucose levels.

Conclusions

Our results demonstrate a requirement for PI3K signaling in the acute adiponectin-induced effects on the cellular activity of arcuate melanocortin neurons. Moreover, these data provide evidence for PI3K as a substrate for both leptin and adiponectin to regulate energy balance and glucose metabolism via melanocortin activity.

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Adiponectin activates arcuate Pomc neurons.

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Adiponectin-induced activation of Pomc neurons requires PI3K (independent of AMPK).

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Adiponectin inhibits adjacent NPY/AgRP neurons (disinhibiting arcuate Pomc neurons).

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Leptin potentiates the effects of adiponectin arcuate Pomc neurons.

Leptin and insulin engage specific PI3K subunits in hypothalamic SF1 neurons

Objective

The ventromedial hypothalamic nucleus (VMH) regulates energy balance and glucose homeostasis. Leptin and insulin exert metabolic effects via their cognate receptors expressed by the steroidogenic factor 1 (SF1) neurons within the VMH. However, detailed cellular mechanisms involved in the regulation of these neurons by leptin and insulin remain to be identified.

Methods

We utilized genetically-modified mouse models and performed patch-clamp electrophysiology experiments to resolve this issue.

Results

We identified distinct populations of leptin-activated and leptin-inhibited SF1 neurons. In contrast, insulin uniformly inhibited SF1 neurons. Notably, we found that leptin-activated, leptin-inhibited, and insulin-inhibited SF1 neurons are distinct subpopulations within the VMH. Leptin depolarization of SF1 neuron also required the PI3K p110β catalytic subunit. This effect was mediated by the putative transient receptor potential C (TRPC) channel. On the other hand, hyperpolarizing responses of SF1 neurons by leptin and insulin required either of the p110α or p110β catalytic subunits, and were mediated by the putative ATP-sensitive K⁺ (K_ATP) channel.

Conclusions

Our results demonstrate that specific PI3K catalytic subunits are responsible for the acute effects of leptin and insulin on VMH SF1 neurons, and provide insights into the cellular mechanisms of leptin and insulin action on VMH SF1 neurons that regulate energy balance and glucose homeostasis.

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Leptin recruits p110β/TRPC channels to depolarize/activate SF1 neurons.

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Leptin recruits p110α/p110β/K_ATP channels to hyperpolarize/inhibit SF1 neurons.

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Insulin recruits p110α/p110β/K_ATP channels to hyperpolarize/inhibit SF1 neurons.

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Acute leptin and insulin responses are segregated to distinct subsets of VMH SF1 neurons.

Motivation to Eat-AgRP Neurons and Homeostatic Need

Activation of AgRP neurons potently induces feeding behaviors; however, whether this activity is involved in motivations of feeding behavior is unclear. A recent study in Nature (Betley et al., 2015) reports that AgRP neuron activity conditions learned behavior by transmitting a negative-valence signal: linking AgRP neurons to the preference of environmental cues associated with homeostatic need.

Neural control of energy balance: translating circuits to therapies

Recent insights into the neural circuits controlling energy balance and glucose homeostasis have rekindled the hope for development of novel treatments for obesity and diabetes. However, many therapies contribute relatively modest beneficial gains with accompanying side effects, and the mechanisms of action for other interventions remain undefined. This Review summarizes current knowledge linking the neural circuits regulating energy and glucose balance with current and potential pharmacotherapeutic and surgical interventions for the treatment of obesity and diabetes.

Melanocortin 4 receptors in autonomic neurons regulate thermogenesis and glycemia

Melanocortin 4 receptors (Mc4rs) are expressed by extra-hypothalamic neurons including cholinergic autonomic pre-ganglionic neurons. However, whether Mc4rs in these neurons are required to control energy and glucose homeostasis is unclear. Here we report that Mc4rs in sympathetic, but not parasympathetic, pre-ganglionic neurons are required to regulate energy expenditure and body weight including brown and white adipose tissue thermogenic responses to diet and cold exposure. In addition, deletion of Mc4rs in both sympathetic and parasympathetic cholinergic neurons impairs glucose homeostasis.

Estradiol modulates Kiss1 neuronal response to ghrelin

Ghrelin is a metabolic signal regulating energy homeostasis. Circulating ghrelin levels rise during starvation and fall after a meal, and therefore, ghrelin may function as a signal of negative energy balance. Ghrelin may also act as a modulator of reproductive physiology, as acute ghrelin administration suppresses gonadotropin secretion and inhibits the neuroendocrine reproductive axis. Interestingly, ghrelin's effect in female metabolism varies according to the estrogen milieu predicting an interaction between ghrelin and estrogens, likely at the hypothalamic level. Here, we show that ghrelin receptor (GHSR) and estrogen receptor-α (ERα) are coexpressed in several hypothalamic sites. Higher levels of circulating estradiol increased the expression of GHSR mRNA and the coexpression of GHSR mRNA and ERα selectively in the arcuate nucleus (ARC). Subsets of preoptic and ARC Kiss1 neurons coexpressed GHSR. Increased colocalization was observed in ARC Kiss1 neurons of ovariectomized estradiol-treated (OVX + E₂; 80%) compared with ovariectomized oil-treated (OVX; 25%) mice. Acute actions of ghrelin on ARC Kiss1 neurons were also modulated by estradiol; 75 and 22% of Kiss1 neurons of OVX + E₂ and OVX mice, respectively, depolarized in response to ghrelin. Our findings indicate that ghrelin and estradiol may interact in several hypothalamic sites. In the ARC, high levels of E₂ increase GHSR mRNA expression, modifying the colocalization rate with ERα and Kiss1 and the proportion of Kiss1 neurons acutely responding to ghrelin. Our findings indicate that E₂ alters the responsiveness of kisspeptin neurons to metabolic signals, potentially acting as a critical player in the metabolic control of the reproductive physiology.

Serotonin 2C receptors in pro-opiomelanocortin neurons regulate energy and glucose homeostasis

Energy and glucose homeostasis are regulated by central serotonin 2C receptors. These receptors are attractive pharmacological targets for the treatment of obesity; however, the identity of the serotonin 2C receptor-expressing neurons that mediate the effects of serotonin and serotonin 2C receptor agonists on energy and glucose homeostasis are unknown. Here, we show that mice lacking serotonin 2C receptors (Htr2c) specifically in pro-opiomelanocortin (POMC) neurons had normal body weight but developed glucoregulatory defects including hyperinsulinemia, hyperglucagonemia, hyperglycemia, and insulin resistance. Moreover, these mice did not show anorectic responses to serotonergic agents that suppress appetite and developed hyperphagia and obesity when they were fed a high-fat/high-sugar diet. A requirement of serotonin 2C receptors in POMC neurons for the maintenance of normal energy and glucose homeostasis was further demonstrated when Htr2c loss was induced in POMC neurons in adult mice using a tamoxifen-inducible POMC-cre system. These data demonstrate that serotonin 2C receptor-expressing POMC neurons are required to control energy and glucose homeostasis and implicate POMC neurons as the target for the effect of serotonin 2C receptor agonists on weight-loss induction and improved glycemic control.

Mrap2: an accessory protein linked to obesity

Melanocortin receptors are critical modulators of energy balance and glucose homeostasis. Companion studies published in Science (Asai et al., 2013; Sebag et al., 2013) establish a role for melanocortin receptor accessory protein 2 (Mrap2) in regulating melanocortin receptor activity and in the development of obesity in zebrafish, rodents, and humans.

Neuronal circuits that regulate feeding behavior and metabolism

Neurons within the central nervous system receive humoral and central (neurotransmitter or neuropeptide) signals that ultimately regulate ingestive behavior and metabolism. Recent advances in mouse genetics combined with neuroanatomical and electrophysiological techniques have contributed to a better understanding of these central mechanisms. This review integrates recently defined cellular mechanisms and neural circuits relevant to the regulation of feeding behavior, energy expenditure, and glucose homeostasis by metabolic signals.

Neuroanatomy of melanocortin-4 receptor pathway in the lateral hypothalamic area

The central melanocortin system regulates body energy homeostasis including the melanocortin-4 receptor (MC4R). The lateral hypothalamic area (LHA) receives dense melanocortinergic inputs from the arcuate nucleus of the hypothalamus and regulates multiple processes including food intake, reward behaviors, and autonomic function. By using a mouse line in which green fluorescent protein (GFP) is expressed under control of the MC4R gene promoter, we systemically investigated MC4R signaling in the LHA by combining double immunohistochemistry, electrophysiology, and retrograde tracing techniques. We found that LHA MC4R-GFP neurons coexpress neurotensin as well as the leptin receptor but do not coexpress other peptide neurotransmitters found in the LHA including orexin, melanin-concentrating hormone, and nesfatin-1. Furthermore, electrophysiological recording demonstrated that leptin, but not the MC4R agonist melanotan II, hyperpolarizes the majority of LHA MC4R-GFP neurons in an ATP- sensitive potassium channel-dependent manner. Retrograde tracing revealed that LHA MC4R-GFP neurons do not project to the ventral tegmental area, dorsal raphe nucleus, nucleus accumbens, and spinal cord, and only limited number of neurons project to the nucleus of the solitary tract and parabrachial nucleus. Our findings provide new insights into MC4R signaling in the LHA and its potential implications in homeostatic regulation of body energy balance.

Melanocortin 4 receptors reciprocally regulate sympathetic and parasympathetic preganglionic neurons

Melanocortin 4 receptors (MC4Rs) in the central nervous system are key regulators of energy and glucose homeostasis. Notably, obese patients with MC4R mutations are hyperinsulinemic and resistant to obesity-induced hypertension. Although these effects are probably dependent upon the activity of the autonomic nervous system, the cellular effects of MC4Rs on parasympathetic and sympathetic neurons remain undefined. Here, we show that MC4R agonists inhibit parasympathetic preganglionic neurons in the brainstem. In contrast, MC4R agonists activate sympathetic preganglionic neurons in the spinal cord. Deletion of MC4Rs in cholinergic neurons resulted in elevated levels of insulin. Furthermore, re-expression of MC4Rs specifically in cholinergic neurons (including sympathetic preganglionic neurons) restores obesity-associated hypertension in MC4R null mice. These findings provide a cellular correlate of the autonomic side effects associated with MC4R agonists and demonstrate a role for MC4Rs expressed in cholinergic neurons in the regulation of insulin levels and in the development of obesity-induced hypertension.

Leptin signaling in Kiss1 neurons arises after pubertal development

The adipocyte-derived hormone leptin is required for normal pubertal maturation in mice and humans and, therefore, leptin has been recognized as a crucial metabolic cue linking energy stores and the onset of puberty. Several lines of evidence have suggested that leptin acts via kisspeptin expressing neurons of the arcuate nucleus to exert its effects. Using conditional knockout mice, we have previously demonstrated that deletion of leptin receptors (LepR) from kisspeptin cells cause no puberty or fertility deficits. However, developmental adaptations and system redundancies may have obscured the physiologic relevance of direct leptin signaling in kisspeptin neurons. To overcome these putative effects, we re-expressed endogenous LepR selectively in kisspeptin cells of mice otherwise null for LepR, using the Cre-loxP system. Kiss1-Cre LepR null mice showed no pubertal development and no improvement of the metabolic phenotype, remaining obese, diabetic and infertile. These mice displayed decreased numbers of neurons expressing Kiss1 gene, similar to prepubertal control mice, and an unexpected lack of re-expression of functional LepR. To further assess the temporal coexpression of Kiss1 and Lepr genes, we generated mice with the human renilla green fluorescent protein (hrGFP) driven by Kiss1 regulatory elements and crossed them with mice that express Cre recombinase from the Lepr locus and the R26-tdTomato reporter gene. No coexpression of Kiss1 and LepR was observed in prepubertal mice. Our findings unequivocally demonstrate that kisspeptin neurons are not the direct target of leptin in the onset of puberty. Leptin signaling in kisspeptin neurons arises only after completion of sexual maturation.

From neuroanatomy to behavior: central integration of peripheral signals regulating feeding behavior

Over the past two centuries, prevalent models of energy and glucose homeostasis have emerged from careful anatomical descriptions in tandem with an understanding of cellular physiology. More recent technological advances have culminated in the identification of peripheral and central factors that influence neural circuits regulating metabolism. This Review highlights contributions to our understanding of peripheral and central factors regulating food intake and energy expenditure.

Functional heterogeneity of arcuate nucleus pro-opiomelanocortin neurons: implications for diverging melanocortin pathways

Arcuate nucleus (ARC) pro-opiomelanocortin (POMC) neurons are essential regulators of food intake, energy expenditure, and glucose homeostasis. POMC neurons integrate several key metabolic signals that include neurotransmitters and hormones. The change in activity of POMC neurons is relayed to melanocortin receptors in distinct regions of the central nervous system. This review will summarize the role of leptin and serotonin receptors in regulating the activity of POMC neurons and provide a model in which different melanocortin pathways regulate energy and glucose homeostasis.

Serotonin 2C receptor activates a distinct population of arcuate pro-opiomelanocortin neurons via TRPC channels

VIDEO ABSTRACT

Serotonin 2C receptors (5-HT(2C)Rs) expressed by pro-opiomelanocortin (POMC) neurons of hypothalamic arcuate nucleus regulate food intake, energy homeostasis and glucose metabolism. However, the cellular mechanisms underlying the effects of 5-HT to regulate POMC neuronal activity via 5-HT(2C)Rs have not yet been identified. In the present study, we found the putative transient receptor potential C (TRPC) channels mediate the activation of a subpopulation of POMC neurons by mCPP (a 5-HT(2C)R agonist). Interestingly, mCPP-activated POMC neurons were found to be a distinct population from those activated by leptin. Together, our data suggest that 5-HT(2C)R and leptin receptors are expressed by distinct subpopulations of arcuate POMC neurons and that both 5-HT and leptin exert their actions in POMC neurons via TRPC channels.

High-fat feeding promotes obesity via insulin receptor/PI3K-dependent inhibition of SF-1 VMH neurons

SF-1-expressing neurons of the ventromedial hypothalamus (VMH) control energy homeostasis, but the role of insulin action in these cells remains undefined. We show that insulin activates PI3-kinase (PI3k) signaling in SF-1 neurons and reduces firing frequency in these cells via activation of K_ATP-channels. These effects are abrogated in mice with insulin receptor (IR) deficiency restricted to SF-1 neurons (SF-1^ΔIR-mice). While body weight and glucose homeostasis remain unaltered in SF-1^ΔIR-mice under normal chow diet, they exhibit protection from diet-induced leptin resistance, weight gain, adiposity and impaired glucose tolerance. High-fat feeding activates PI3k signaling in SF-1 neurons of control mice, and this response is attenuated in the VMH of SF-1^ΔIR-mice. Mimicking diet-induced overactivation of PI3k signaling by disruption of the PIP₃-phosphatase PTEN leads to increased body weight and hyperphagia under normal chow diet. Collectively, our experiments reveal a critical role for HFD-induced, insulin-dependent PI3k activation in VMH neurons to control energy homeostasis.

Leptin receptor expression in hindbrain Glp-1 neurons regulates food intake and energy balance in mice

Leptin is an adipose-derived hormone that signals to inform the brain of nutrient status; loss of leptin signaling results in marked hyperphagia and obesity. Recent work has identified several groups of neurons that contribute to the effects of leptin to regulate energy balance, but leptin receptors are distributed throughout the brain, and the function of leptin signaling in discrete neuronal populations outside of the hypothalamus has not been defined. In the current study, we produced mice in which the long form of the leptin receptor (Lepr) was selectively ablated using Cre-recombinase selectively expressed in the hindbrain under control of the paired-like homeobox 2b (Phox2b) promoter (Phox2b Cre Lepr(flox/flox) mice). In these mice, Lepr was deleted from glucagon-like 1 peptide-expressing neurons resident in the nucleus of the solitary tract. Phox2b Cre Lepr(flox/flox) mice were hyperphagic, displayed increased food intake after fasting, and gained weight at a faster rate than wild-type controls. Paradoxically, Phox2b Cre Lepr(flox/flox) mice also exhibited an increased metabolic rate independent of a change in locomotor activity that was dependent on food intake, and glucose homeostasis was normal. Together, these data support a physiologically important role of direct leptin action in the hindbrain.