Food Perception Primes Hepatic ER Homeostasis via Melanocortin-Dependent Control of mTOR Activation.

Adaptation of liver to the postprandial state requires coordinated regulation of protein synthesis and folding aligned with changes in lipid metabolism. Here we demonstrate that sensory food perception is sufficient to elicit early activation of hepatic mTOR signaling, Xbp1 splicing, increased expression of ER-stress genes, and phosphatidylcholine synthesis, which translate into a rapid morphological ER remodeling. These responses overlap with those activated during refeeding, where they are maintained and constantly increased upon nutrient supply. Sensory food perception activates POMC neurons in the hypothalamus, optogenetic activation of POMC neurons activates hepatic mTOR signaling and Xbp1 splicing, whereas lack of MC4R expression attenuates these responses to sensory food perception. Chemogenetic POMC-neuron activation promotes sympathetic nerve activity (SNA) subserving the liver, and norepinephrine evokes the same responses in hepatocytes in vitro and in liver in vivo as observed upon sensory food perception. Collectively, our experiments unravel that sensory food perception coordinately primes postprandial liver ER adaption through a melanocortin-SNA-mTOR-Xbp1s axis. VIDEO ABSTRACT.

AgRP Neurons Control Systemic Insulin Sensitivity via Myostatin Expression in Brown Adipose Tissue.

Activation of Agouti-related peptide (AgRP) neurons potently promotes feeding, and chronically altering their activity also affects peripheral glucose homeostasis. We demonstrate that acute activation of AgRP neurons causes insulin resistance through impairment of insulin-stimulated glucose uptake into brown adipose tissue (BAT). AgRP neuron activation acutely reprograms gene expression in BAT toward a myogenic signature, including increased expression of myostatin. Interference with myostatin activity improves insulin sensitivity that was impaired by AgRP neurons activation. Optogenetic circuitry mapping reveals that feeding and insulin sensitivity are controlled by both distinct and overlapping projections. Stimulation of AgRP → LHA projections impairs insulin sensitivity and promotes feeding while activation of AgRP → anterior bed nucleus of the stria terminalis (aBNST)vl projections, distinct from AgRP → aBNSTdm projections controlling feeding, mediate the effect of AgRP neuron activation on BAT-myostatin expression and insulin sensitivity. Collectively, our results suggest that AgRP neurons in mice induce not only eating, but also insulin resistance by stimulating expression of muscle-related genes in BAT, revealing a mechanism by which these neurons rapidly coordinate hunger states with glucose homeostasis.

A leptin-BDNF pathway regulating sympathetic innervation of adipose tissue.

Mutations in the leptin gene (ob) result in a metabolic disorder that includes severe obesity1, and defects in thermogenesis2 and lipolysis3, both of which are adipose tissue functions regulated by the sympathetic nervous system. However, the basis of these sympathetic-associated abnormalities remains unclear. Furthermore, chronic leptin administration reverses these abnormalities in adipose tissue, but the underlying mechanism remains to be discovered. Here we report that ob/ob mice, as well as leptin-resistant diet-induced obese mice, show significant reductions of sympathetic innervation of subcutaneous white and brown adipose tissue. Chronic leptin treatment of ob/ob mice restores adipose tissue sympathetic innervation, which in turn is necessary to correct the associated functional defects. The effects of leptin on innervation are mediated via agouti-related peptide and pro-opiomelanocortin neurons in the hypothalamic arcuate nucleus. Deletion of the gene encoding the leptin receptor in either population leads to reduced innervation in fat. These agouti-related peptide and pro-opiomelanocortin neurons act via brain-derived neurotropic factor-expressing neurons in the paraventricular nucleus of the hypothalamus (BDNFPVH). Deletion of BDNFPVH blunts the effects of leptin on innervation. These data show that leptin signalling regulates the plasticity of sympathetic architecture of adipose tissue via a top-down neural pathway that is crucial for energy homeostasis.

Adipocyte-specific disruption of the BBSome causes metabolic and autonomic dysfunction.

Obesity is a major public health issue due to its association with type 2 diabetes, hypertension, and other cardiovascular risks. The BBSome, a complex of eight conserved Bardet-Biedl syndrome (BBS) proteins, has emerged as a key regulator of energy and glucose homeostasis as well as cardiovascular function. However, the importance of adipocyte BBSome in controlling these physiological processes is not clear. Here, we show that adipocyte-specific constitutive disruption of the BBSome through selective deletion of the Bbs1 gene adiponectin (AdipoCre/Bbs1fl/fl mice) does not affect body weight under normal chow or high-fat and high-sucrose diet (HFHSD). However, constitutive BBSome deficiency caused impairment in glucose tolerance and insulin sensitivity. Similar phenotypes were observed after inducible adipocyte-specific disruption of the BBSome (AdipoCreERT2/Bbs1fl/fl mice). Interestingly, a significant increase in renal sympathetic nerve activity, measured using multifiber recording in the conscious state, was observed in AdipoCre/Bbs1fl/fl mice on both chow and HFHSD. A significant increase in tail-cuff arterial pressure was also observed in chow-fed AdipoCre/Bbs1fl/fl mice, but this was not reproduced when arterial pressure was measured by radiotelemetry. Moreover, AdipoCre/Bbs1fl/fl mice had no significant alterations in vascular reactivity. On the other hand, AdipoCre/Bbs1fl/fl mice displayed impaired baroreceptor reflex sensitivity when fed HFHSD, but not on normal chow. Taken together, these data highlight the relevance of the adipocyte BBSome for the regulation of glucose homeostasis and sympathetic traffic. The BBSome also contributes to baroreflex sensitivity under HFHSD, but not normal chow.NEW & NOTEWORTHY The current study show how genetic manipulation of fat cells impacts various functions of the body including sensitivity to the hormone insulin.

Melanocortin MC4R receptor is required for energy expenditure but not blood pressure effects of angiotensin II within the mouse brain.

The brain renin-angiotensin system (RAS) is implicated in control of blood pressure (BP), fluid intake, and energy expenditure (EE). Angiotensin II (ANG II) within the arcuate nucleus of the hypothalamus contributes to control of resting metabolic rate (RMR) and thereby EE through its actions on Agouti-related peptide (AgRP) neurons, which also contribute to EE control by leptin. First, we determined that although leptin stimulates EE in control littermates, mice with transgenic activation of the brain RAS (sRA) exhibit increased EE and leptin has no additive effect to exaggerate EE in these mice. These findings led us to hypothesize that leptin and ANG II in the brain stimulate EE through a shared mechanism. Because AgRP signaling to the melanocortin MC4R receptor contributes to the metabolic effects of leptin, we performed a series of studies examining RMR, fluid intake, and BP responses to ANG II in mice rendered deficient for expression of MC4R via a transcriptional block (Mc4r-TB). These mice were resistant to stimulation of RMR in response to activation of the endogenous brain RAS via chronic deoxycorticosterone acetate (DOCA)-salt treatment, whereas fluid and electrolyte effects remained intact. These mice were also resistant to stimulation of RMR via acute intracerebroventricular (ICV) injection of ANG II, whereas BP responses to ICV ANG II remained intact. Collectively, these data demonstrate that the effects of ANG II within the brain to control RMR and EE are dependent on MC4R signaling, whereas fluid homeostasis and BP responses are independent of MC4R signaling.

Hypothalamic MC4R regulates glucose homeostasis through adrenaline-mediated control of glucose reabsorption via renal GLUT2 in mice.

AIMS/HYPOTHESIS: Melanocortin 4 receptor (MC4R) mutation is the most common cause of known monogenic obesity in humans. Unexpectedly, humans and rodents with MC4R deficiency do not develop hyperglycaemia despite chronic obesity and insulin resistance. To explain the underlying mechanisms for this phenotype, we determined the role of MC4R in glucose homeostasis in the presence and absence of obesity in mice. METHODS: We used global and hypothalamus-specific MC4R-deficient mice to investigate the brain regions that contribute to glucose homeostasis via MC4R. We performed oral, intraperitoneal and intravenous glucose tolerance tests in MC4R-deficient mice that were either obese or weight-matched to their littermate controls to define the role of MC4R in glucose regulation independently of changes in body weight. To identify the integrative pathways through which MC4R regulates glucose homeostasis, we measured renal and adrenal sympathetic nerve activity. We also evaluated glucose homeostasis in adrenaline (epinephrine)-deficient mice to investigate the role of adrenaline in mediating the effects of MC4R in glucose homeostasis. We employed a graded [13C6]glucose infusion procedure to quantify renal glucose reabsorption in MC4R-deficient mice. Finally, we measured the levels of renal glucose transporters in hypothalamus-specific MC4R-deficient mice and adrenaline-deficient mice using western blotting to ascertain the molecular mechanisms underlying MC4R control of glucose homeostasis. RESULTS: We found that obese and weight-matched MC4R-deficient mice exhibited improved glucose tolerance due to elevated glucosuria, not enhanced beta cell function. Moreover, MC4R deficiency selectively in the paraventricular nucleus of the hypothalamus (PVH) is responsible for reducing the renal threshold for glucose as measured by graded [13C6]glucose infusion technique. The MC4R deficiency suppressed renal sympathetic nerve activity by 50% in addition to decreasing circulating adrenaline and renal GLUT2 levels in mice, which contributed to the elevated glucosuria. We further report that adrenaline-deficient mice recapitulated the increased excretion of glucose in urine observed in the MC4R-deficient mice. Restoration of circulating adrenaline in both the MC4R- and adrenaline-deficient mice reversed their phenotype of improved glucose tolerance and elevated glucosuria, demonstrating the role of adrenaline in mediating the effects of MC4R on glucose reabsorption. CONCLUSIONS/INTERPRETATION: These findings define a previously unrecognised function of hypothalamic MC4R in glucose reabsorption mediated by adrenaline and renal GLUT2. Taken together, our findings indicate that elevated glucosuria due to low sympathetic tone explains why MC4R deficiency does not cause hyperglycaemia despite inducing obesity and insulin resistance. Graphical abstract.

cAMP-inducible coactivator CRTC3 attenuates brown adipose tissue thermogenesis.

In response to cold exposure, placental mammals maintain body temperature by increasing sympathetic nerve activity in brown adipose tissue (BAT). Triggering of ß-adrenergic receptors on brown adipocytes stimulates thermogenesis via induction of the cAMP/PKA pathway. Although cAMP response element-binding protein (CREB) and its coactivators-the cAMP-regulated transcriptional coactivators (CRTCs)-mediate transcriptional effects of cAMP in most tissues, other transcription factors such as ATF2 appear critical for induction of thermogenic genes by cAMP in BAT. Brown adipocytes arise from Myf5-positive mesenchymal cells under the control of PRDM16, a coactivator that concurrently represses differentiation along the skeletal muscle lineage. Here, we show that the CREB coactivator CRTC3 is part of an inhibitory feedback pathway that antagonizes PRDM16-dependent differentiation. Mice with a knockout of CRTC3 in BAT (BKO) have increased cold tolerance and reduced adiposity, whereas mice overexpressing constitutively active CRTC3 in adipose tissue are more cold sensitive and have greater fat mass. CRTC3 reduced sympathetic nerve activity in BAT by up-regulating the expression of miR-206, a microRNA that promotes differentiation along the myogenic lineage and that we show here decreases the expression of VEGFA and neurotrophins critical for BAT innervation and vascularization. Sympathetic nerve activity to BAT was enhanced in BKO mice, leading to increases in catecholamine signaling that stimulated energy expenditure. As reexpression of miR-206 in BAT from BKO mice reversed the salutary effects of CRTC3 depletion on cold tolerance, our studies suggest that small-molecule inhibitors against this coactivator may provide therapeutic benefit to overweight individuals.

Activation of hypothalamic AgRP and POMC neurons evokes disparate sympathetic and cardiovascular responses.

The arcuate nucleus of the hypothalamus (ARC) plays a key role in linking peripheral metabolic status to the brain melanocortin system, which influences a wide range of physiological processes including the sympathetic nervous system and blood pressure. The importance of the activity of agouti-related peptide (AgRP)- and proopiomelanocortin (POMC)-expressing neurons, two molecularly distinct populations of ARC neurons, for metabolic regulation is well established, but their relevance for sympathetic and cardiovascular control remains unclear. We used designer receptors exclusively activated by designer drug (DREADD) technology to study how activation of AgRP and POMC neurons affect renal sympathetic nerve traffic and blood pressure. In addition to the drastic feeding-stimulatory effect, DREADD-mediated activation of AgRP, but not POMC neurons, induced an acute reduction in renal sympathetic nerve activity in conscious mice. Paradoxically, however, DREADD-mediated chronic activation of AgRP neurons caused a significant increase in blood pressure specifically in the inactive light phase. On the other hand, chronic activation of POMC neurons led to a significant reduction in blood pressure. These results bring new insights to a previously unappreciated role of ARC AgRP and POMC neuronal activity in autonomic and cardiovascular regulation.NEW & NOTEWORTHY Agouti-related peptide (AgRP)- and proopiomelanocortin (POMC)-expressing neurons of the arcuate nucleus are essential components of the brain melanocortin system that controls various physiological processes. Here, we tested the metabolic and cardiovascular effects of direct activation of these two populations of neurons. Our findings show that, in addition to stimulation of food intake, chemogenetic mediated activation of hypothalamic arcuate nucleus AgRP, but not POMC, neurons reduce renal sympathetic traffic. Despite this, chronic activation of AgRP neurons increased blood pressure. However, chronic activation of POMC neurons led to a significant reduction in blood pressure. Our findings highlight the importance of arcuate nucleus AgRP and POMC neuronal activity in autonomic and cardiovascular regulation.

Liver sympathetic denervation reverses obesity-induced hepatic steatosis.

KEY POINTS: Non-alcoholic fatty liver disease, characterized in part by elevated liver triglycerides (i.e. hepatic steatosis), is a growing health problem. In this study, we found that hepatic steatosis is associated with robust hepatic sympathetic overactivity. Removal of hepatic sympathetic nerves reduced obesity-induced hepatic steatosis. Liver sympathetic innervation modulated hepatic lipid acquisition pathways during obesity. ABSTRACT: Non-alcoholic fatty liver disease (NAFLD) affects 1 in 3 Americans and is a significant risk factor for type II diabetes mellitus, insulin resistance and hepatic carcinoma. Characterized in part by excessive hepatic triglyceride accumulation (i.e. hepatic steatosis), the incidence of NAFLD is increasing - in line with the growing obesity epidemic. The role of the autonomic nervous system in NAFLD remains unclear. Here, we show that chronic hepatic sympathetic overactivity mediates hepatic steatosis. Direct multiunit recordings of hepatic sympathetic nerve activity were obtained in high fat diet and normal chow fed male C57BL/6J mice. To reduce hepatic sympathetic nerve activity we utilized two approaches including pharmacological ablation of the sympathetic nerves and phenol-based hepatic sympathetic nerve denervation. Diet-induced NAFLD was associated with a nearly doubled firing rate of the hepatic sympathetic nerves, which was largely due to an increase in efferent nerve traffic. Furthermore, established high fat diet-induced hepatic steatosis was effectively reduced with pharmacological or phenol-based removal of the hepatic sympathetic nerves, independent of changes in body weight, caloric intake or adiposity. Ablation of liver sympathetic nerves was also associated with improvements in liver triglyceride accumulation pathways including free fatty acid uptake and de novo lipogenesis. These findings highlight an unrecognized pathogenic link between liver sympathetic outflow and hepatic steatosis and suggest that manipulation of the liver sympathetic nerves may represent a novel therapeutic strategy for NAFLD.

The BBSome Controls Energy Homeostasis by Mediating the Transport of the Leptin Receptor to the Plasma Membrane.

Bardet-Biedl syndrome (BBS) is a highly pleiotropic autosomal recessive disorder associated with a wide range of phenotypes including obesity. However, the underlying mechanism remains unclear. Here, we show that neuronal BBSome is a critical determinant of energy balance through its role in the regulation of the trafficking of the long signaling form of the leptin receptor (LRb). Targeted disruption of the BBSome by deleting the Bbs1 gene from the nervous system causes obesity in mice, and this phenotype is reproduced by ablation of the Bbs1 gene selectively in the LRb-expressing cells, but not from adipocytes. Obesity developed as a consequence of both increased food intake and decreased energy expenditure in mice lacking the Bbs1 gene in LRb-expressing cells. Strikingly, the well-known role of BBS proteins in the regulation of ciliary formation and function is unlikely to account for the obesogenic effect of BBS1 loss as disruption of the intraflagellar transport (IFT) machinery required for ciliogenesis by deleting the Ift88 gene in LRb-expressing cells caused a marginal increase in body weight and adiposity. Instead, we demonstrate that silencing BBS proteins, but not IFT88, impair the trafficking of the LRb to the plasma membrane leading to central leptin resistance in a manner independent of obesity. Our data also demonstrate that postnatal deletion of the Bbs1 gene in the mediobasal hypothalamus can cause obesity in mice, arguing against an early neurodevelopmental origin of obesity in BBS. Our results depict a novel mechanism underlying energy imbalance and obesity in BBS with potential implications in common forms of human obesity.

Leptin receptor signaling in the hypothalamus regulates hepatic autonomic nerve activity via phosphatidylinositol 3-kinase and AMP-activated protein kinase.

Leptin action in the brain has emerged as an important regulator of liver function independently from its effects on food intake and body weight. The autonomic nervous system plays a key role in the regulation of physiological processes by leptin. Here, we used direct recording of nerve activity from sympathetic or vagal nerves subserving the liver to investigate how brain action of leptin controls hepatic autonomic nerve activity. Intracerebroventricular (ICV) administration of leptin activated hepatic sympathetic traffic in rats and mice in dose- and receptor-dependent manners. The hepatic sympatho-excitatory effects of leptin were also observed when leptin was microinjected directly into the arcuate nucleus (ARC), but not into the ventromedial hypothalamus (VMH). Moreover, using pharmacological and genetic approaches, we show that leptin-induced increase in hepatic sympathetic outflow depends on PI3K but not AMP-activated protein kinase (AMPK), STAT3, or ERK1/2. Interestingly, ICV leptin also increased hepatic vagal nerve activity in rats. We show that this response is reproduced by intra-ARC, but not intra-VMH, leptin administration and requires PI3K and AMPK. We conclude that central leptin signaling conveys the information to the liver through the sympathetic and parasympathetic branches of the autonomic nervous system. Our data also provide important insight into the molecular events underlying leptin's control of hepatic autonomic nerve activity by implicating PI3K and AMPK pathways.

Neonatal growth restriction-related leptin deficiency enhances leptin-triggered sympathetic activation and central angiotensin II receptor-dependent stress-evoked hypertension.

BACKGROUND: Neonatal growth restriction (nGR) leads to leptin deficiency and increases the risk of hypertension. Previous studies have shown nGR-related hypertension is normalized by neonatal leptin (nLep) and exacerbated by psychological stress. With recent studies linking leptin and angiotensin signaling, we hypothesized that nGR-induced nLep deficiency increases adult leptin sensitivity; leading to leptin- or stress-induced hypertension, through a pathway involving central angiotensin II type 1 receptors. METHODS: We randomized mice with incipient nGR, by virtue of their presence in large litters, to vehicle or physiologic nLep supplementation (80 ng/g/d). Adult caloric intake and arterial pressure were monitored at baseline, during intracerebroventricular losartan infusion and during systemic leptin administration. RESULTS: nGR increased leptin-triggered renal sympathetic activation and hypertension with increased leptin receptor expression in the arcuate nucleus of the hypothalamus; all of those nGR-associated phenotypes were normalized by nLep. nGR mice also had stress-related hyperphagia and hypertension, but only the stress hypertension was blocked by central losartan infusion. CONCLUSION: nGR leads to stress hypertension through a pathway that involves central angiotensin II receptors, and nGR-associated leptin deficiency increases leptin-triggered hypertension in adulthood. These data suggest potential roles for preservation of neonatal growth and nLep supplementation in the prevention of nGR-related hypertension.

Central melanin-concentrating hormone influences liver and adipose metabolism via specific hypothalamic nuclei and efferent autonomic/JNK1 pathways.

BACKGROUND & AIMS: Specific neuronal circuits modulate autonomic outflow to liver and white adipose tissue. Melanin-concentrating hormone (MCH)-deficient mice are hypophagic, lean, and do not develop hepatosteatosis when fed a high-fat diet. Herein, we sought to investigate the role of MCH, an orexigenic neuropeptide specifically expressed in the lateral hypothalamic area, on hepatic and adipocyte metabolism. METHODS: Chronic central administration of MCH and adenoviral vectors increasing MCH signaling were performed in rats and mice. Vagal denervation was performed to assess its effect on liver metabolism. The peripheral effects on lipid metabolism were assessed by real-time polymerase chain reaction and Western blot. RESULTS: We showed that the activation of MCH receptors promotes nonalcoholic fatty liver disease through the parasympathetic nervous system, whereas it increases fat deposition in white adipose tissue via the suppression of sympathetic traffic. These metabolic actions are independent of parallel changes in food intake and energy expenditure. In the liver, MCH triggers lipid accumulation and lipid uptake, with c-Jun N-terminal kinase being an essential player, whereas in adipocytes MCH induces metabolic pathways that promote lipid storage and decreases lipid mobilization. Genetic activation of MCH receptors or infusion of MCH specifically in the lateral hypothalamic area modulated hepatic lipid metabolism, whereas the specific activation of this receptor in the arcuate nucleus affected adipocyte metabolism. CONCLUSIONS: Our findings show that central MCH directly controls hepatic and adipocyte metabolism through different pathways.

Bardet-Biedl syndrome 3 (Bbs3) knockout mouse model reveals common BBS-associated phenotypes and Bbs3 unique phenotypes.

Bardet-Biedl syndrome (BBS) is a heterogeneous disorder characterized by obesity, retinopathy, polydactyly, and congenital anomalies. The incidence of hypertension and diabetes are also increased in BBS patients. Mutation of 16 genes independently causes BBS, and seven BBS proteins form the BBSome that promotes ciliary membrane elongation. BBS3 (ARL6), an ADP ribosylation factor-like small GTPase, is not part of the BBSome complex. The in vivo function of BBS3 is largely unknown. Here we developed a Bbs3 knockout model and demonstrate that Bbs3(-/-) mice develop BBS-associated phenotypes, including retinal degeneration, male infertility, and increased body fat. Interestingly, Bbs3(-/-) mice develop some unique phenotypes not seen in other BBS knockout models: no overt obesity, severe hydrocephalus, and elevated blood pressure (shared by some but not all BBS gene knockout mice). We found that endogenous BBS3 and the BBSome physically interact and depend on each other for their ciliary localization. This finding explains the phenotypic similarity between Bbs3(-/-) mice and BBSome subunit knockout mice. Loss of Bbs3 does not affect BBSome formation but disrupts normal localization of melanin concentrating hormone receptor 1 to ciliary membranes and affects retrograde transport of Smoothened inside cilia. We also show that the endogenous BBSome and BBS3 associate with membranes and the membrane association of the BBSome and BBS3 are not interdependent. Differences between BBS mouse models suggest nonoverlapping functions to individual BBS protein.

A kidney-hypothalamus axis promotes compensatory glucose production in response to glycosuria.

The kidneys facilitate energy conservation through reabsorption of nutrients including glucose. Almost all of the filtered blood glucose is reabsorbed by the kidneys. Loss of glucose in urine (glycosuria) is offset by an increase in endogenous glucose production to maintain normal energy supply in the body. How the body senses this glucose loss and consequently enhances glucose production is unclear. Using renal Glut2 knockout mice, we demonstrate that elevated glycosuria activates the hypothalamic-pituitary-adrenal axis, which in turn drives endogenous glucose production. This phenotype was attenuated by selective afferent renal denervation, indicating the involvement of the afferent nerves in promoting the compensatory increase in glucose production. In addition, through plasma proteomics analyses we observed that acute phase proteins - which are usually involved in body's defense mechanisms against a threat - were the top candidates which were either upregulated or downregulated in renal Glut2 KO mice. Overall, afferent renal nerves contribute to promoting endogenous glucose production in response to elevated glycosuria and loss of glucose in urine is sensed as a biological threat in mice. These findings may be useful in improving efficiency of drugs like SGLT2 inhibitors that are intended to treat hyperglycemia by enhancing glycosuria, but are met with a compensatory increase in endogenous glucose production.

A kidney-hypothalamus axis promotes compensatory glucose production in response to glycosuria.

The kidneys facilitate energy conservation through reabsorption of nutrients including glucose. Almost all the filtered blood glucose is reabsorbed by the kidneys. Loss of glucose in urine (glycosuria) is offset by an increase in endogenous glucose production to maintain normal energy supply in the body. How the body senses this glucose loss and consequently enhances glucose production is unclear. Using renal Slc2a2 (also known as Glut2) knockout mice, we demonstrate that elevated glycosuria activates the hypothalamic-pituitary-adrenal axis, which in turn drives endogenous glucose production. This phenotype was attenuated by selective afferent renal denervation, indicating the involvement of the afferent nerves in promoting the compensatory increase in glucose production. In addition, through plasma proteomics analyses we observed that acute phase proteins - which are usually involved in the body's defense mechanisms against a threat - were the top candidates which were either upregulated or downregulated in renal Slc2a2 KO mice. Overall, afferent renal nerves contribute to promoting endogenous glucose production in response to elevated glycosuria and loss of glucose in urine is sensed as a biological threat in mice. These findings may be useful in improving the efficiency of drugs like SGLT2 inhibitors that are intended to treat hyperglycemia by enhancing glycosuria but are met with a compensatory increase in endogenous glucose production.

Overnutrition causes insulin resistance and metabolic disorder through increased sympathetic nervous system activity.

The mechanisms underlying obesity-induced insulin resistance remain incompletely understood, as impaired cellular insulin signaling, traditionally considered the primary driver of insulin resistance, does not always accompany impaired insulin action. Overnutrition rapidly increases plasma norepinephrine (NE), suggesting overactivation of the sympathetic nervous system (SNS). However, the role of the SNS in obesity is controversial, as both increased and decreased SNS activity (SNA) have been reported. Here, we show that reducing catecholamine (CA) release from the SNS protects against overnutrition-induced insulin resistance as well as hyperglucagonemia, adipose tissue dysfunction, and fatty liver disease, as we demonstrate utilizing a mouse model of inducible and peripherally restricted deletion of tyrosine hydroxylase (th; THΔper). A key mechanism through which heightened SNA induces insulin resistance is by triggering adipose tissue lipolysis. Increased SNA emerges as a critical driver in the pathogenesis of overnutrition-induced insulin resistance and metabolic disease independent of cellular insulin signaling.

Reciprocal activity of AgRP and POMC neurons governs coordinated control of feeding and metabolism.

Agouti-related peptide (AgRP)-expressing and proopiomelanocortin (POMC)-expressing neurons reciprocally regulate food intake. Here, we combine non-interacting recombinases to simultaneously express functionally opposing chemogenetic receptors in AgRP and POMC neurons for comparing metabolic responses in male and female mice with simultaneous activation of AgRP and inhibition of POMC neurons with isolated activation of AgRP neurons or isolated inhibition of POMC neurons. We show that food intake is regulated by the additive effect of AgRP neuron activation and POMC neuron inhibition, while systemic insulin sensitivity and gluconeogenesis are differentially modulated by isolated-versus-simultaneous regulation of AgRP and POMC neurons. We identify a neurocircuit engaging Npy1R-expressing neurons in the paraventricular nucleus of the hypothalamus, where activated AgRP neurons and inhibited POMC neurons cooperate to promote food consumption and activate Th+ neurons in the nucleus tractus solitarii. Collectively, these results unveil how food intake is precisely regulated by the simultaneous bidirectional interplay between AgRP and POMC neurocircuits.

Ablation of the leptin receptor in the hypothalamic arcuate nucleus abrogates leptin-induced sympathetic activation.

RATIONALE: The hypothalamic arcuate nucleus (ARC) is considered a major site for leptin signaling that regulates several physiological processes. OBJECTIVE: To test the hypothesis that leptin receptor in the ARC is required to mediate leptin-induced sympathetic activation. METHODS AND RESULTS: First, we used the ROSA Cre-reporter mice to establish the feasibility of driving Cre expression in the ARC in a controlled manner with bilateral microinjection of adenovirus-expressing Cre-recombinase (Ad-Cre). Ad-Cre microinjection into the ARC of ObR(flox/flox) mice robustly reduced ObR expression and leptin-induced Stat3 activation in the ARC but not in the adjacent nuclei, confirming the efficacy and selectivity of the ARC deletion of ObR. Critically, deletion of ObR in the ARC attenuated brown adipose tissue and renal sympathetic nerve responses to leptin. We also examined whether ObR in the ARC is required for the preserved leptin-induced increase in renal sympathetic activity in dietary obesity. We found that deletion of ARC ObR abrogated leptin-induced increases in renal sympathetic discharge and resolved arterial pressure elevation in diet-induced obese ObR(flox/flox) mice. CONCLUSIONS: These data demonstrate a critical role for ObR in the ARC in mediating the sympathetic nerve responses to leptin and in the adverse sympathoexcitatory effects of leptin in obesity.

Enhanced leptin-stimulated Pi3k activation in the CNS promotes white adipose tissue transdifferentiation.

The contribution of different leptin-induced signaling pathways in control of energy homeostasis is only partly understood. Here we show that selective Pten ablation in leptin-sensitive neurons (Pten(DeltaObRb)) results in enhanced Pi3k activation in these cells and reduces adiposity by increasing energy expenditure. White adipose tissue (WAT) of Pten(DeltaObRb) mice shows characteristics of brown adipose tissue (BAT), reflected by increased mitochondrial content and Ucp1 expression resulting from enhanced leptin-stimulated sympathetic nerve activity (SNA) in WAT. In contrast, leptin-deficient ob/ob-Pten(DeltaObRb) mice exhibit unaltered body weight and WAT morphology compared to ob/ob mice, pointing to a pivotal role of endogenous leptin in control of WAT transdifferentiation. Leanness of Pten(DeltaObRb) mice is accompanied by enhanced sensitivity to insulin in skeletal muscle. These data provide direct genetic evidence that leptin-stimulated Pi3k signaling in the CNS regulates energy expenditure via activation of SNA to perigonadal WAT leading to BAT-like differentiation of WAT.