Insulin-like growth factor 1-mediated hyperthermia involves anterior hypothalamic insulin receptors

The neurobiology of insulin-like growth factor I: From neuroprotection to modulation of brain states

After decades of research in the neurobiology of IGF-I, its role as a prototypical neurotrophic factor is undisputed. However, many of its actions in the adult brain indicate that this growth factor is not only involved in brain development or in the response to injury. Following a three-layer assessment of its role in the central nervous system, we consider that at the cellular level, IGF-I is indeed a bona fide neurotrophic factor, modulating along ontogeny the generation and function of all the major types of brain cells, contributing to sculpt brain architecture and adaptive responses to damage. At the circuit level, IGF-I modulates neuronal excitability and synaptic plasticity at multiple sites, whereas at the system level, IGF-I intervenes in energy allocation, proteostasis, circadian cycles, mood, and cognition. Local and peripheral sources of brain IGF-I input contribute to a spatially restricted, compartmentalized, and timed modulation of brain activity. To better define these variety of actions, we consider IGF-I a modulator of brain states. This definition aims to reconcile all aspects of IGF-I neurobiology, and may provide a new conceptual framework in the design of future research on the actions of this multitasking neuromodulator in the brain.

Potential role of IGF-1/GLP-1 signaling activation in intracerebral hemorrhage

IGF-1 and GLP-1 receptors are essential in all tissues, facilitating defense by upregulating anabolic processes. They are abundantly distributed throughout the central nervous system, promoting neuronal proliferation, survival, and differentiation. IGF-1/GLP-1 is a growth factor that stimulates neurons' development, reorganization, myelination, and survival. In primary and secondary brain injury, the IGF-1/GLP-1 receptors are impaired, resulting in further neuro complications such as cerebral tissue degradation, neuroinflammation, oxidative stress, and atrophy. Intracerebral hemorrhage (ICH) is a severe condition caused by a stroke for which there is currently no effective treatment. While some pre-clinical studies and medications are being developed as symptomatic therapies in clinical trials, there are specific pharmacological implications for improving post-operative conditions in patients with intensive treatment. Identifying the underlying molecular process and recognizing the worsening situation can assist researchers in developing effective therapeutic solutions to prevent post-hemorrhagic symptoms and the associated neural dysfunctions. As a result, in the current review, we have addressed the manifestations of the disease that are aggravated by the downregulation of IGF-1 and GLP-1 receptors, which can lead to ICH or other neurodegenerative disorders. Our review summarizes that IGF-1/GLP-1 activators may be useful for treating ICH and its related neurodegeneration.

Homeostatic Regulation of Glucose Metabolism by the Central Nervous System

Evidence for involvement of the central nervous system (CNS) in the regulation of glucose metabolism dates back to the 19th century, although the majority of the research on glucose metabolism has focused on the peripheral metabolic organs. Due to recent advances in neuroscience, it has now become clear that the CNS is indeed vital for maintaining glucose homeostasis. To achieve normoglycemia, specific populations of neurons and glia in the hypothalamus sense changes in the blood concentrations of glucose and of glucoregulatory hormones such as insulin, leptin, glucagon-like peptide 1, and glucagon. This information is integrated and transmitted to other areas of the brain where it eventually modulates various processes in glucose metabolism (i.e., hepatic glucose production, glucose uptake in the brown adipose tissue and skeletal muscle, pancreatic insulin and glucagon secretion, renal glucose reabsorption, etc.). Errors in these processes lead to hyper- or hypoglycemia. We here review the current understanding of the brain regulation of glucose metabolism.

Modulation of Glucose Production by Central Insulin Requires IGF-1 Receptors in AgRP Neurons

Similar to insulin, central administration of IGF-1 can suppress hepatic glucose production (HGP), but it is unclear whether this effect is mediated via insulin receptors (InsRs) or IGF-1 receptors (IGF-1Rs) in the brain. To this end, we used pharmacologic and genetic approaches in combination with hyperinsulinemic-euglycemic clamps to decipher the role of these receptors in mediating central effects of IGF-1 and insulin on HGP. In rats, we observed that intracerebroventricular (ICV) administration of IGF-1 or insulin markedly increased the glucose infusion rate (GIR) by >50% and suppressed HGP (P < 0.001). However, these effects were completely prevented by preemptive ICV infusion with an IGF-1R and InsR/IGF-1R hybrid (HybridR) blocking antibody. Likewise, ICV infusion of the InsR antagonist, S961, which also can bind HybridRs, interfered with the ability of central insulin, but not IGF-1, to increase the GIR. Furthermore, hyperinsulinemic clamps in mice lacking IGF-1Rs in AgRP neurons revealed ∼30% reduction in the GIR in knockout animals, which was explained by an impaired ability of peripheral insulin to completely suppress HGP (P < 0.05). Signaling studies further revealed an impaired ability of peripheral insulin to trigger ribosomal S6 phosphorylation or phosphatidylinositol (3,4,5)-trisphosphate production in AgRP neurons lacking IGF-1Rs. In summary, these data suggest that attenuation of IGF-1R signaling in the mediobasal hypothalamus, and specifically in AgRP neurons, can phenocopy impaired regulation of HGP as previously demonstrated in mice lacking InsRs in these cells, suggesting a previously unappreciated role for IGF-1Rs and/or HybridRs in the regulation of central insulin/IGF-1 signaling in glucose metabolism.

The brain as an insulin-sensitive metabolic organ

BACKGROUND

The brain was once thought of as an insulin-insensitive organ. We now know that the insulin receptor is present throughout the brain and serves important functions in whole-body metabolism and brain function. Brain insulin signaling is involved not only in brain homeostatic processes but also neuropathological processes such as cognitive decline and Alzheimer's disease.

SCOPE OF REVIEW

In this review, we provide an overview of insulin signaling within the brain and the metabolic impact of brain insulin resistance and discuss Alzheimer's disease, one of the neurologic diseases most closely associated with brain insulin resistance.

MAJOR CONCLUSIONS

While brain insulin signaling plays only a small role in central nervous system glucose regulation, it has a significant impact on the brain's metabolic health. Normal insulin signaling is important for mitochondrial functioning and normal food intake. Brain insulin resistance contributes to obesity and may also play an important role in neurodegeneration.

Brain insulin signalling in metabolic homeostasis and disease

Insulin signalling in the central nervous system regulates energy homeostasis by controlling metabolism in several organs and by coordinating organ crosstalk. Studies performed in rodents, non-human primates and humans over more than five decades using intracerebroventricular, direct hypothalamic or intranasal application of insulin provide evidence that brain insulin action might reduce food intake and, more importantly, regulates energy homeostasis by orchestrating nutrient partitioning. This Review discusses the metabolic pathways that are under the control of brain insulin action and explains how brain insulin resistance contributes to metabolic disease in obesity, the metabolic syndrome and type 2 diabetes mellitus.

Central actions of insulin during pregnancy and lactation

Pregnancy and lactation are highly metabolically demanding states. Maternal glucose is a key fuel source for the growth and development of the fetus, as well as for the production of milk during lactation. Hence, the maternal body undergoes major adaptations in the systems regulating glucose homeostasis to cope with the increased demand for glucose. As part of these changes, insulin levels are elevated during pregnancy and lower in lactation. The increased insulin secretion during pregnancy plays a vital role in the periphery; however, the potential effects of increased insulin action in the brain have not been widely investigated. In this review, we consider the impact of pregnancy on brain access and brain levels of insulin. Moreover, we explore the hypothesis that pregnancy is associated with site-specific central insulin resistance that is adaptive, allowing for the increases in peripheral insulin secretion without the consequences of increased central and peripheral insulin functions, such as to stimulate glucose uptake into maternal tissues or to inhibit food intake. Conversely, the loss of central insulin actions may impair other functions, such as insulin control of the autonomic nervous system. The potential role of low insulin in facilitating adaptive responses to lactation, such as hyperphagia and suppression of reproductive function, are also discussed. We end the review with a list of key research questions requiring resolution.

Leptin Modulates the Response of Brown Adipose Tissue to Negative Energy Balance: Implication of the GH/IGF-I Axis

The growth hormone (GH)/insulin-like growth factor I (IGF-I) axis is involved in metabolic control. Malnutrition reduces IGF-I and modifies the thermogenic capacity of brown adipose tissue (BAT). Leptin has effects on the GH/IGF-I axis and the function of BAT, but its interaction with IGF-I and the mechanisms involved in the regulation of thermogenesis remains unknown. We studied the GH/IGF-I axis and activation of IGF-I-related signaling and metabolism related to BAT thermogenesis in chronic central leptin infused (L), pair-fed (PF), and control rats. Hypothalamic somatostatin mRNA levels were increased in PF and decreased in L, while pituitary GH mRNA was reduced in PF. Serum GH and IGF-I concentrations were decreased only in PF. In BAT, the association between suppressor of cytokine signaling 3 and the IGF-I receptor was reduced, and phosphorylation of the IGF-I receptor increased in the L group. Phosphorylation of Akt and cyclic AMP response element binding protein and glucose transporter 4 mRNA levels were increased in L and mRNA levels of uncoupling protein-1 (UCP-1) and enzymes involved in lipid anabolism reduced in PF. These results suggest that modifications in UCP-1 in BAT and changes in the GH/IGF-I axis induced by negative energy balance are dependent upon leptin levels.

Effects of adipokine administration to the hypothalamic preoptic area on body temperature in rats

The effects of adipokine administration to the hypothalamic preoptic area (POA), which is one of the body temperature (BT) regulation centers in the central nervous system, on BT were investigated in male Wistar rats. BT was measured in conscious rats using telemetry. Insulin-like growth factor-1 (IGF-1), interleukin-1β (IL-1β), monocyte chemoattractant protein-1 and lipocalin-2 produced hyperthermia, and the effects induced by IL-1β (25 ng) and IGF-1 (5 μg) were sustainable and remarkable. IL-6 did not show any significant effect. The IGF-1-induced effect was inhibited by pretreatment with IGF binding protein 3 (IGFBP3) or NVP-AEW541 (NVP, a selective inhibitor of type 1 IGF receptor tyrosine kinase, IGF1R TK). NVP-induced inhibition was observed only in the early phase of IGF-1-induced hyperthermia. In addition, IGF-1 increased the IL-1β concentration in the microdialysate of POA perfusion, but did not increase the IL-1β concentration in the plasma or the PGE₂ concentration in the microdialysate. These findings suggested that IGF-1 produced hyperthermia, which was mediated, at least a part, through an increased IL-1β concentration after activation of IGF1R TK in the POA, and the IGF-IGFBP system possibly participates in BT homeostasis in the POA.

Insulin signaling in the hippocampus and amygdala regulates metabolism and neurobehavior

Previous studies have shown that insulin and IGF-1 signaling in the brain, especially the hypothalamus, is important for regulation of systemic metabolism. Here, we develop mice in which we have specifically inactivated both insulin receptors (IRs) and IGF-1 receptors (IGF1Rs) in the hippocampus (Hippo-DKO) or central amygdala (CeA-DKO) by stereotaxic delivery of AAV-Cre into IR^lox/lox/IGF1R^lox/lox mice. Consequently, both Hippo-DKO and CeA-DKO mice have decreased levels of the GluA1 subunit of glutamate AMPA receptor and display increased anxiety-like behavior, impaired cognition, and metabolic abnormalities, including glucose intolerance. Hippo-DKO mice also display abnormal spatial learning and memory whereas CeA-DKO mice have impaired cold-induced thermogenesis. Thus, insulin/IGF-1 signaling has common roles in the hippocampus and central amygdala, affecting synaptic function, systemic glucose homeostasis, behavior, and cognition. In addition, in the hippocampus, insulin/IGF-1 signaling is important for spatial learning and memory whereas insulin/IGF-1 signaling in the central amygdala controls thermogenesis via regulation of neural circuits innervating interscapular brown adipose tissue.

Insulin regulates astrocyte gliotransmission and modulates behavior

Complications of diabetes affect tissues throughout the body, including the central nervous system. Epidemiological studies show that diabetic patients have an increased risk of depression, anxiety, age-related cognitive decline, and Alzheimer's disease. Mice lacking insulin receptor (IR) in the brain or on hypothalamic neurons display an array of metabolic abnormalities; however, the role of insulin action on astrocytes and neurobehaviors remains less well studied. Here, we demonstrate that astrocytes are a direct insulin target in the brain and that knockout of IR on astrocytes causes increased anxiety- and depressive-like behaviors in mice. This can be reproduced in part by deletion of IR on astrocytes in the nucleus accumbens. At a molecular level, loss of insulin signaling in astrocytes impaired tyrosine phosphorylation of Munc18c. This led to decreased exocytosis of ATP from astrocytes, resulting in decreased purinergic signaling on dopaminergic neurons. These reductions contributed to decreased dopamine release from brain slices. Central administration of ATP analogs could reverse depressive-like behaviors in mice with astrocyte IR knockout. Thus, astrocytic insulin signaling plays an important role in dopaminergic signaling, providing a potential mechanism by which astrocytic insulin action may contribute to increased rates of depression in people with diabetes, obesity, and other insulin-resistant states.

Insulin enhances GABAA receptor-mediated inhibitory currents in rat central amygdala neurons

Insulin, a pancreatic hormone, can access the central nervous system, activate insulin receptors distributed in selective brain regions and affect various cellular functions such as neurotransmission. We have previously shown that physiologically relevant concentration of insulin potentiates the GABA_A receptor-mediated tonic inhibition and reduces excitability of rat hippocampal CA1 neurons. The central nucleus of the amygdala (CeA) comprises heterogeneous neuronal populations that can respond to hormonal stimulus. Using quantitative PCR and immunofluorescent labeling, we report that the mRNA and protein of the insulin receptor are abundantly expressed in the rat CeA. The insulin receptor mRNA is also detected in the CeA from post-mortem human brain samples. Furthermore, our whole-cell patch-clamp recordings show that the application of insulin (5 and 50 nM) selectively enhances the frequency and amplitude of spontaneous inhibitory postsynaptic currents (sIPSCs) in rat CeA neurons. Our findings reveal that GABAergic synaptic transmission is a target in the CeA for insulin receptor signaling that may underlie insulin modulation of emotion- and feeding-related behaviors.

Molecular basis of central thermosensation

Classic lesion and physiology experiments identified the hypothalamic preoptic area as a pivotal region in the regulation of temperature homeostasis. The preoptic area can sense changes in local temperature, receives information about ambient temperature, contributes to fever, and can affect thermoregulation in response to several biologic signals. Electrophysiologic studies indicate that these actions are mediated by a neuronal circuitry that comprises temperature-sensitive as well as temperature-insensitive neurons. Little is known on the molecules that may be required for central thermosensation and much of the efforts towards their identification was done for warm-sensitive neurons. Here we summarize the current knowledge on the subject as well as what the search for these molecules revealed about warm-sensitive neurons.

Insulin-like growth factor 1 receptor regulates hypothermia during calorie restriction

When food resources are scarce, endothermic animals can lower core body temperature (T_b). This phenomenon is believed to be part of an adaptive mechanism that may have evolved to conserve energy until more food becomes available. Here, we found in the mouse that the insulin-like growth factor 1 receptor (IGF-1R) controls this response in the central nervous system. Pharmacological or genetic inhibition of IGF-1R enhanced the reduction of temperature and of energy expenditure during calorie restriction. Full blockade of IGF-1R affected female and male mice similarly. In contrast, genetic IGF-1R dosage was effective only in females, where it also induced transient and estrus-specific hypothermia in animals fed ad libitum. These effects were regulated in the brain, as only central, not peripheral, pharmacological activation of IGF-1R prevented hypothermia during calorie restriction. Targeted IGF-1R knockout selectively in forebrain neurons revealed that IGF signaling also modulates calorie restriction-dependent T_b regulation in regions rostral of the canonical hypothalamic nuclei involved in controlling body temperature. In aggregate, these data identify central IGF-1R as a mediator of the integration of nutrient and temperature homeostasis. They also show that calorie restriction, IGF-1R signaling, and body temperature, three of the main regulators of metabolism, aging, and longevity, are components of the same pathway.

Neuro-psychopharmacological perspective of Orphan receptors of Rhodopsin (class A) family of G protein-coupled receptors

BACKGROUND

In the central nervous system (CNS), G protein-coupled receptors (GPCRs) are the most fruitful targets for neuropsychopharmacological drug development. Rhodopsin (class A) is the most studied class of GPCR and includes orphan receptors for which the endogenous ligand is not known or is unclear. Characterization of orphan GPCRs has proven to be challenging, and the production pace of GPCR-based drugs has been incredibly slow.

OBJECTIVE

Determination of the functions of these receptors may provide unexpected insight into physiological and neuropathological processes. Advances in various methods and techniques to investigate orphan receptors including in situ hybridization and knockdown/knockout (KD/KO) showed extensive expression of these receptors in the mammalian brain and unmasked their physiological and neuropathological roles. Due to these rapid progress and development, orphan GPCRs are rising as a new and promising class of drug targets for neurodegenerative diseases and psychiatric disorders.

CONCLUSION

This review presents a neuropsychopharmacological perspective of 26 orphan receptors of rhodopsin (class A) family, namely GPR3, GPR6, GPR12, GPR17, GPR26, GPR35, GPR39, GPR48, GPR49, GPR50, GPR52, GPR55, GPR61, GPR62, GPR63, GPR68, GPR75, GPR78, GPR83, GPR84, GPR85, GPR88, GPR153, GPR162, GPR171, and TAAR6. We discussed the expression of these receptors in mammalian brain and their physiological roles. Furthermore, we have briefly highlighted their roles in neurodegenerative diseases and psychiatric disorders including Alzheimer's disease, Parkinson's disease, neuroinflammation, inflammatory pain, bipolar and schizophrenic disorders, epilepsy, anxiety, and depression.

Deciphering Brain Insulin Receptor and Insulin-Like Growth Factor 1 Receptor Signalling

Insulin receptor (IR) and insulin-like growth factor 1 receptor (IGF1R) are highly conserved receptor tyrosine kinases that share signalling proteins and are ubiquitously expressed in the brain. Central application of insulin or IGF1 exerts several similar physiological outcomes, varying in strength, whereas disruption of the corresponding receptors in the brain leads to remarkably different effects on brain size and physiology, thus highlighting the unique effects of the corresponding hormone receptors. Central insulin/IGF1 resistance impacts upon various levels of the IR/IGF1R signalling pathways and is a feature of the metabolic syndrome and neurodegenerative diseases such as Alzheimer's disease. The intricacy of brain insulin and IGF1 signalling represents a challenge for the identification of specific IR and IGF1R signalling differences in pathophysiological conditions. The present perspective sheds light on signalling differences and methodologies for specifically deciphering brain IR and IGF1R signalling.

Insulin in the nervous system and the mind: Functions in metabolism, memory, and mood

BACKGROUND

Insulin, a pleotrophic hormone, has diverse effects in the body. Recent work has highlighted the important role of insulin's action in the nervous system on glucose and energy homeostasis, memory, and mood.

SCOPE OF REVIEW

Here we review experimental and clinical work that has broadened the understanding of insulin's diverse functions in the central and peripheral nervous systems, including glucose and body weight homeostasis, memory and mood, with particular emphasis on intranasal insulin.

MAJOR CONCLUSIONS

Implications for the treatment of obesity, type 2 diabetes, dementia, and mood disorders are discussed in the context of brain insulin action. Intranasal insulin may have potential in the treatment of central nervous system-related metabolic disorders.

Transcriptome composition of the preoptic area in mid-age and escitalopram treatment in male mice

The decrease in serotonergic neurotransmission during aging can increase the risk of neuropsychiatric diseases such as depression in elderly population and decline the reproductive system. Therefore, it is important to understand the age-associated molecular mechanisms of brain aging. In this study, the effect of aging and chronic escitalopram (antidepressant) treatment to admit mice was investigated by comparing transcriptomes in the preoptic area (POA) which is a key nucleus for reproduction. In the mid-aged brain, the immune system-related genes were increased and hormone response-related genes were decreased. In the escitalopram treated brains, transcription-, granule cell proliferation- and vasoconstriction-related genes were increased and olfactory receptors were decreased. Since homeostasis and neuroprotection-related genes were altered in both of mid-age and escitalopram treatment, these genes could be important for serotonin related physiologies in the POA.

Single-cell transcriptomics and functional target validation of brown adipocytes show their complex roles in metabolic homeostasis

Brown adipocytes (BAs) are specialized for adaptive thermogenesis and, upon sympathetic stimulation, activate mitochondrial uncoupling protein (UCP)-1 and oxidize fatty acids to generate heat. The capacity for brown adipose tissue (BAT) to protect against obesity and metabolic disease is recognized, yet information about which signals activate BA, besides β3-adrenergic receptor stimulation, is limited. Using single-cell transcriptomics, we confirmed the presence of mRNAs encoding traditional BAT markers (i.e., UCP1, expressed in 100% of BAs Adrb3, expressed in <50% of BAs) in mouse and have shown single-cell variability (>1000-fold) in their expression at both the mRNA and protein levels. We further identified mRNAs encoding novel markers, orphan GPCRs, and many receptors that bind the classic neurotransmitters, neuropeptides, chemokines, cytokines, and hormones. The transcriptome variability between BAs suggests a much larger range of responsiveness of BAT than previously recognized and that not all BAs function identically. We examined the in vivo functional expression of 12 selected receptors by microinjecting agonists into live mouse BAT and analyzing the metabolic response. In this manner, we expanded the number of known receptors on BAs at least 25-fold, while showing that the expression of classic BA markers is more complex and variable than previously thought.

Knock-down of IL-1Ra in obese mice decreases liver inflammation and improves insulin sensitivity

Interleukin 1 Receptor antagonist (IL-1Ra) is highly elevated in obesity and is widely recognized as an anti-inflammatory cytokine. While the anti-inflammatory role of IL-1Ra in the pancreas is well established, the role of IL-1Ra in other insulin target tissues and the contribution of systemic IL-1Ra levels to the development of insulin resistance remains to be defined. Using antisense knock down of IL-1Ra in vivo, we show that normalization of IL-1Ra improved insulin sensitivity due to decreased inflammation in the liver and improved hepatic insulin sensitivity and these effects were independent of changes in body weight. A similar effect was observed in IL1-R1 KO mice, suggesting that at high concentrations of IL-1Ra typically observed in obesity, IL-1Ra can contribute to the development of insulin resistance in a mechanism independent of IL-1Ra binding to IL-1R1. These results demonstrate that normalization of plasma IL-1Ra concentration improves insulin sensitivity in diet- induced obese mice.

Insulin action in brain regulates systemic metabolism and brain function

Insulin receptors, as well as IGF-1 receptors and their postreceptor signaling partners, are distributed throughout the brain. Insulin acts on these receptors to modulate peripheral metabolism, including regulation of appetite, reproductive function, body temperature, white fat mass, hepatic glucose output, and response to hypoglycemia. Insulin signaling also modulates neurotransmitter channel activity, brain cholesterol synthesis, and mitochondrial function. Disruption of insulin action in the brain leads to impairment of neuronal function and synaptogenesis. In addition, insulin signaling modulates phosphorylation of tau protein, an early component in the development of Alzheimer disease. Thus, alterations in insulin action in the brain can contribute to metabolic syndrome, and the development of mood disorders and neurodegenerative diseases.

Single-cell transcriptomics for drug target discovery

Single cell sequencing is currently in its relative infancy although an unprecedented amount of information is already being generated. These techniques are providing new insight into intercellular variability as well as identification of previously unrecognized drug targets. As more groups are gaining an interest in this fruitful technique, new sample preparation techniques, sequencing platforms, and bioinformatics tools are being developed which only improve the quantity and quality of data generated in these studies. Great advancements in harvest (in vivo pipette), sample preparation, and sequencing (Illumina HiSeq 2500/MiSeq, Ion Torrent PGM, Pacific Biosciences RS) are allowing for previously untestable questions to be answered and for expanded accessibility of these technologies.

Peripheral signalling involved in energy homeostasis control

The alarming prevalence of obesity has led to a better understanding of the molecular mechanisms controlling energy homeostasis. Regulation of energy intake and expenditure is more complex than previously thought, being influenced by signals from many peripheral tissues. In this sense, a wide variety of peripheral signals derived from different organs contributes to the regulation of body weight and energy expenditure. Besides the well-known role of insulin and adipokines, such as leptin and adiponectin, in the regulation of energy homeostasis, signals from other tissues not previously thought to play a role in body weight regulation have emerged in recent years. The role of fibroblast growth factor 21 (FGF21), insulin-like growth factor 1 (IGF-I), and sex hormone-binding globulin (SHBG) produced by the liver in the regulation of body weight and insulin sensitivity has been recently described. Moreover, molecules expressed by skeletal muscle such as myostatin have also been involved in adipose tissue regulation. Better known is the involvement of ghrelin, cholecystokinin, glucagon-like peptide 1 (GLP-1) and PYY(3-36), produced by the gut, in energy homeostasis. Even the kidney, through the production of renin, appears to regulate body weight, with mice lacking this hormone exhibiting resistance to diet-induced obesity. In addition, the skeleton has recently emerged as an endocrine organ, with effects on body weight control and glucose homeostasis through the actions of bone-derived factors such as osteocalcin and osteopontin. The comprehension of these signals will help in a better understanding of the aetiopathology of obesity, contributing to the potential development of new therapeutic targets aimed at tackling excess body fat accumulation.

Neuronal Sirt1 deficiency increases insulin sensitivity in both brain and peripheral tissues

Sirt1 is a NAD(+)-dependent class III deacetylase that functions as a cellular energy sensor. In addition to its well-characterized effects in peripheral tissues, emerging evidence suggests that neuronal Sirt1 activity plays a role in the central regulation of energy balance and glucose metabolism. To assess this idea, we generated Sirt1 neuron-specific knockout (SINKO) mice. On both standard chow and HFD, SINKO mice were more insulin sensitive than Sirt1(f/f) mice. Thus, SINKO mice had lower fasting insulin levels, improved glucose tolerance and insulin tolerance, and enhanced systemic insulin sensitivity during hyperinsulinemic euglycemic clamp studies. Hypothalamic insulin sensitivity of SINKO mice was also increased over controls, as assessed by hypothalamic activation of PI3K, phosphorylation of Akt and FoxO1 following systemic insulin injection. Intracerebroventricular injection of insulin led to a greater systemic effect to improve glucose tolerance and insulin sensitivity in SINKO mice compared with controls. In line with the in vivo results, insulin-induced AKT and FoxO1 phosphorylation were potentiated by inhibition of Sirt1 in a cultured hypothalamic cell line. Mechanistically, this effect was traced to a reduced effect of Sirt1 to directly deacetylate and repress IRS-1 function. The enhanced central insulin signaling in SINKO mice was accompanied by increased insulin receptor signal transduction in liver, muscle, and adipose tissue. In summary, we conclude that neuronal Sirt1 negatively regulates hypothalamic insulin signaling, leading to systemic insulin resistance. Interventions that reduce neuronal Sirt1 activity have the potential to improve systemic insulin action and limit weight gain on an obesigenic diet.

Molecules affecting hypothalamic control of core body temperature in response to calorie intake

Core body temperature (CBT) and calorie intake are main components of energy homeostasis and two important regulators of health, longevity, and aging. In homeotherms, CBT can be influenced by calorie intake as food deprivation or calorie restriction (CR) lowers CBT whereas feeding has hyperthermic effects. The finding that in mice CBT prolonged lifespan independently of CR, suggested that the mechanisms modulating CBT may represent important regulators of aging. Here we summarize the current knowledge on the signaling molecules and their receptors that participate in the regulation of CBT responses to calorie intake. These include hypothalamic neuropeptides regulating feeding but also energy expenditure via modulation of thermogenesis. We also report studies indicating that nutrient signals can contribute to regulation of CBT by direct action on hypothalamic preoptic warm-sensitive neurons that in turn regulate adaptive thermogenesis and hence CBT. Finally, we show the role played by two orphans G protein-coupled receptor: GPR50 and GPR83, that were recently demonstrated to regulate temperature-dependent energy expenditure.

Downregulation of GPR83 in the hypothalamic preoptic area reduces core body temperature and elevates circulating levels of adiponectin

The G protein-coupled receptor 83 (GPR83) was recently demonstrated in warm sensitive neurons (WSN) of the hypothalamic preoptic area (POA) that participate in temperature homeostasis. Thus, we investigated whether GPR83 may have a role in regulating core body temperature (CBT) by reducing its expression in the POA. Dissipation of energy in the form of heat is the primary mode of energy expenditure in mammals and can ultimately affect energy homeostasis. Thus, we also measured the level of important regulators of metabolism. Downregulation of GPR83 was obtained by lentiviral short-hairpin RNAs (shGPR83) vectors designed and selected for their ability to reduce GPR83 levels in vitro. Mice received POA injection of shGPR83 or non-silencing vectors and were monitored for CBT, motor activity, food intake body weight and circulating levels of IGF-1, insulin, leptin and adiponectin. Down-regulation of GPR83 in the POA resulted in a small (0.15°C) but significant reduction of CBT during the dark/active cycle of the day. Temperature reduction was followed by increased body weight gain independent of caloric intake. shGPR83 mice also had increased level of circulating adiponectin (31916±952 pg/mL vs. 23474±1507 pg/mL, P<.01) while no change was observed for insulin, IGF-1 or leptin. GPR83 may participate in central thermoregulation and the central control of circulating adiponectin. Further work is required to determine how GPR83 can affect POA WSN and what are the long term metabolic consequences of its down-regulation.

G protein-coupled receptor 21 deletion improves insulin sensitivity in diet-induced obese mice

Obesity-induced inflammation is a key component of systemic insulin resistance, which is a hallmark of type 2 diabetes. A major driver of this inflammation/insulin resistance syndrome is the accumulation of proinflammatory macrophages in adipose tissue and liver. We found that the orphan GPCR Gpr21 was highly expressed in the hypothalamus and macrophages of mice and that whole-body KO of this receptor led to a robust improvement in glucose tolerance and systemic insulin sensitivity and a modest lean phenotype. The improvement in insulin sensitivity in the high-fat diet-fed (HFD-fed) Gpr21 KO mouse was traced to a marked reduction in tissue inflammation caused by decreased chemotaxis of Gpr21 KO macrophages into adipose tissue and liver. Furthermore, mice lacking macrophage expression of Gpr21 were protected from HFD-induced inflammation and displayed improved insulin sensitivity. Results of in vitro chemotaxis studies in human monocytes suggested that the defect in chemotaxis observed ex vivo and in vivo in mice is also translatable to humans. Cumulatively, our data indicate that GPR21 has a critical function in coordinating macrophage proinflammatory activity in the context of obesity-induced insulin resistance.

Insulin resistance in the brain: an old-age or new-age problem?

Life expectancy is rising however with more people living longer there is a concomitant rise in the incidence of dementia. In addition to age-related cognitive decline there is a higher risk of going on to develop vascular dementia and Alzheimer's disease associated with aspects of modern lifestyle. Most worryingly, recent data reports accelerated cognitive decline in adolescents associated with poor diet (high fat and calorie intake). Thus the increase in dementia in 'old-age' may have as much to do with 'new-age' lifestyle as it does with normal ageing. It would seem wise therefore to investigate the molecular connections between lifestyle and cognitive decline in more detail. Epidemiological evidence suggests an increased risk of developing dementia (including Alzheimer's disease) in individuals with obesity and type 2 diabetes but also in those with poor insulin sensitivity without diabetes, implicating a mechanistic link between adiposity, insulin sensitivity and dementia. Insulin receptors are expressed in the brain and physiological roles for insulin in the CNS are starting to be delineated. Indeed disrupted neuronal insulin action may underlie the link between diabetes and neurodegenerative disorders. This review discusses the difficulties in quantifying insulin sensitivity of the brain and why it is vital that we develop technology for this purpose so that we can establish its role in this 'new-age' dementia. This has particular relevance to the design and interpretation of clinical trials in progress to assess potential benefits of insulin and insulin sensitisers on prevention of cognitive decline.

Drug targets: single-cell transcriptomics hastens unbiased discovery

Drug discovery in neuro- and psychopharmacology is lagging, and the most commonly mentioned cause is the scarcity of drug targets. Using NextGen 'sequencing based single-cell transcriptomics' (SBSCT), several hundred different receptors and channels can be identified in individual neurons, and the functional gene product can subsequently be validated. The use of single-cell transcriptome data to reveal the entire receptor repertoire is crucial, as the copy numbers of mRNAs encoding receptors are low, and when cells are pooled dilution of rare mRNAs leads to loss of signal. These overlooked receptors on key neurons often mediate robust effects that may be therapeutically useful. SBSCT also enables the identification of orphan receptors and can provide strong evidence for receptor heterodimers. Here, we compare SBSCT to other single-cell profiling methods. We argue that the unbiased nature of SBSCT makes it a powerful tool for the identification of new drug targets.

Seasonal regulation of the growth hormone-insulin-like growth factor-I axis in the American black bear (Ursus americanus)

The American black bear maintains lean body mass for months without food during winter denning. We asked whether changes in the growth hormone/insulin-like growth factor-I (GH-IGF-I) axis may contribute to this remarkable adaptation to starvation. Serum IGF-I levels were measured by radioimmunoassay, and IGF-binding proteins (IGFBPs) were analyzed by ligand blotting. Initial studies in bears living in the wild showed that IGF-I levels are highest in summer and lowest in early winter denning. Detailed studies in captive bears showed that IGF-I levels decline in autumn when bears are hyperphagic, continue to decline in early denning, and later rise above predenning levels despite continued starvation in the den. IGFBP-2 increased and IGFBP-3 decreased in early denning, and these changes were also reversed in later denning. Treatment with GH (0.1 mg·kg(-1)·day(-1) × 6 days) during early denning increased serum levels of IGF-I and IGFBP-3 and lowered levels of IGFBP-2, indicating that denning bears remain responsive to GH. GH treatment lowered blood urea nitrogen levels, reflecting effects on protein metabolism. GH also accelerated weight loss and markedly increased serum levels of free fatty acids and β-hydroxybutyrate, resulting in a ketoacidosis (bicarbonate decreased to 15 meq/l), which was reversed when GH was withdrawn. These results demonstrate seasonal regulation of GH/IGF-I axis activity in black bears. Diminished GH activity may promote fat storage in autumn in preparation for denning and prevent excessive mobilization and premature exhaustion of fat stores in early denning, whereas restoration of GH/IGF activity in later denning may prepare the bear for normal activity outside the den.