Hepatic interoception in health and disease

The liver is a large organ with crucial functions in metabolism and immune defense, as well as blood homeostasis and detoxification, and it is clearly in bidirectional communication with the brain and rest of the body via both neural and humoral pathways. A host of neural sensory mechanisms have been proposed, but in contrast to the gut-brain axis, details for both the exact site and molecular signaling steps of their peripheral transduction mechanisms are generally lacking. Similarly, knowledge about function-specific sensory and motor components of both vagal and spinal access pathways to the hepatic parenchyma is missing. Lack of progress largely owes to controversies regarding selectivity of vagal access pathways and extent of hepatocyte innervation. In contrast, there is considerable evidence for glucose sensors in the wall of the hepatic portal vein and their importance for glucose handling by the liver and the brain and the systemic response to hypoglycemia. As liver diseases are on the rise globally, and there are intriguing associations between liver diseases and mental illnesses, it will be important to further dissect and identify both neural and humoral pathways that mediate hepatocyte-specific signals to relevant brain areas. The question of whether and how sensations from the liver contribute to interoceptive self-awareness has not yet been explored.

Sensory spinal interoceptive pathways and energy balance regulation

Interoception plays an important role in homeostatic regulation of energy intake and metabolism. Major interoceptive pathways include gut-to-brain and adipose tissue-to brain signaling via vagal sensory nerves and hormones, such as leptin. However, signaling via spinal sensory neurons is rapidly emerging as an additional important signaling pathway. Here we provide an in-depth review of the known anatomy and functions of spinal sensory pathways and discuss potential mechanisms relevant for energy balance homeostasis in health and disease. Because sensory innervation by dorsal root ganglia (DRG) neurons goes far beyond vagally innervated viscera and includes adipose tissue, skeletal muscle, and skin, it is in a position to provide much more complete metabolic information to the brain. Molecular and anatomical identification of function specific DRG neurons will be important steps in designing pharmacological and neuromodulation approaches to affect energy balance regulation in disease states such as obesity, diabetes, and cancer.

Regulation of body weight: Lessons learned from bariatric surgery

BACKGROUND

Bariatric or weight loss surgery is currently the most effective treatment for obesity and metabolic disease. Unlike dieting and pharmacology, its beneficial effects are sustained over decades in most patients, and mortality is among the lowest for major surgery. Because there are not nearly enough surgeons to implement bariatric surgery on a global scale, intensive research efforts have begun to identify its mechanisms of action on a molecular level in order to replace surgery with targeted behavioral or pharmacological treatments. To date, however, there is no consensus as to the critical mechanisms involved.

SCOPE OF REVIEW

The purpose of this non-systematic review is to evaluate the existing evidence for specific molecular and inter-organ signaling pathways that play major roles in bariatric surgery-induced weight loss and metabolic benefits, with a focus on Roux-en-Y gastric bypass (RYGB) and vertical sleeve gastrectomy (VSG), in both humans and rodents.

MAJOR CONCLUSIONS

Gut-brain communication and its brain targets of food intake control and energy balance regulation are complex and redundant. Although the relatively young science of bariatric surgery has generated a number of hypotheses, no clear and unique mechanism has yet emerged. It seems increasingly likely that the broad physiological and behavioral effects produced by bariatric surgery do not involve a single mechanism, but rather multiple signaling pathways. Besides a need to improve and better validate surgeries in animals, advanced techniques, including inducible, tissue-specific knockout models, and the use of humanized physiological traits will be necessary. State-of-the-art genetically-guided neural identification techniques should be used to more selectively manipulate function-specific pathways.

Galanin Regulates Myocardial Mitochondrial ROS Homeostasis and Hypertrophic Remodeling Through GalR2

The regulatory peptide galanin is broadly distributed in the central nervous systems and peripheral tissues where it modulates numerous physiological and pathological processes through binding to its three G-protein-coupled receptors, GalR1-3. However, the function and identity of the galaninergic system in the heart remain unclear. Therefore, we investigated the expression of the galanin receptors in cardiac cells and tissues and found that GalR2 is the dominant receptor subtype in adult mouse hearts, cardiomyocytes and H9C2 cardiomyoblasts. In vivo, genetic suppression of GalR2 promotes cardiac hypertrophy, fibrosis and mitochondrial oxidative stress in the heart. In vitro, GalR2 silencing by siRNA abolished the beneficial effects of galanin on cell hypertrophy and mitochondrial reactive oxygen species (ROS) production. These findings unravel new insights into the role of galaninergic system in the heart and suggest novel therapeutic strategies in heart disease.

Lateral hypothalamic galanin neurons are activated by stress and blunt anxiety-like behavior in mice

Despite the prevalence of anxiety disorders, the molecular identity of neural circuits underlying anxiety remains unclear. The lateral hypothalamus (LH) is one brain region implicated in the regulation of anxiety, and our recent data found that chemogenetic activation of LH galanin neurons attenuated the stress response to a novel environment as measured by the marble burying test. Thus, we hypothesize that LH galanin neurons may contribute to anxiety-related behavior. We used chemogenetics and fiber photometry to test the ability of LH galanin neurons to influence anxiety and stress-related behavior. Chemogenetic activation of LH galanin neurons significantly decreased anxiety-like behavior in the elevated plus maze, open field test, and light dark test. However, LH galanin activation did not alter restraint stress induced HPA activation or freezing behavior in the fear conditioning paradigm. In vivo calcium monitoring by fiber photometry indicated that LH galanin neurons were activated by anxiogenic and/or stressful stimuli including tail suspension, novel mouse interaction, and predator odor. Further, in a fear conditioning task, calcium transients strongly increased during foot shock, but were not affected by the unconditioned stimulus tone. These data indicate that LH galanin neurons both respond to and modulate anxiety, with no influence on stress induced HPA activation or fear behaviors. Further investigation of LH galanin circuitry and functional mediators of behavioral output may offer a more refined pharmacological target as an alternative to first-line broad pharmacotherapies such as benzodiazepines.

Organization of sympathetic innervation of interscapular brown adipose tissue in the mouse

The interscapular brown adipose tissue (iBAT) is under sympathetic control, and recent studies emphasized the importance of efferent sympathetic and afferent sensory or humoral feedback systems to regulate adipose tissue function and overall metabolic health. However, functional studies of the sympathetic nervous system in the mouse are limited, because details of anatomy and fine structure are lacking. Here, we used reporter mice for tyrosine hydroxylase expressing neurons (TH:tomato mice), iDISCO tissue clearance, confocal, lightsheet, and electron microscopy to clarify that (a) iBAT receives sympathetic input via dorsal rami (instead of often cited intercostal nerves); (b) dorsal rami T1-T5 correspond to the postganglionic input from sympathetic chain ganglia (stellate/T1-T5); (c) dorsal rami serve as conduits for sympathetic axons that branch off in finer nerve bundles to enter iBAT; (d) axonal varicosities show strong differential innervation of brown (dense innervation) versus white (sparse innervation) adipocytes, that surround the core iBAT in the mouse and are intermingled in human adipose tissues, (e) axonal varicosities can form neuro-adipocyte junctions with brown adipocytes. Taken together, we demonstrate that sympathetic iBAT innervation is organized by specific nerves and terminal structures that can be surgically and genetically accessed for neuromodulatory purposes.

Protein Appetite at the Interface between Nutrient Sensing and Physiological Homeostasis

Feeding behavior is guided by multiple competing physiological needs, as animals must sense their internal nutritional state and then identify and consume foods that meet nutritional needs. Dietary protein intake is necessary to provide essential amino acids and represents a specific, distinct nutritional need. Consistent with this importance, there is a relatively strong body of literature indicating that protein intake is defended, such that animals sense the restriction of protein and adaptively alter feeding behavior to increase protein intake. Here, we argue that this matching of food consumption with physiological need requires at least two concurrent mechanisms: the first being the detection of internal nutritional need (a protein need state) and the second being the discrimination between foods with differing nutritional compositions. In this review, we outline various mechanisms that could mediate the sensing of need state and the discrimination between protein-rich and protein-poor foods. Finally, we briefly describe how the interaction of these mechanisms might allow an animal to self-select between a complex array of foods to meet nutritional needs and adaptively respond to changes in either the external environment or internal physiological state.

Sympathetic innervation of the mouse kidney and liver arising from prevertebral ganglia

The sympathetic nervous system (SNS) plays a crucial role in the regulation of renal and hepatic functions. Although sympathetic nerves to the kidney and liver have been identified in many species, specific details are lacking in the mouse. In the absence of detailed information of sympathetic prevertebral innervation of specific organs, selective manipulation of a specific function will remain challenging. Despite providing major postganglionic inputs to abdominal organs, limited data are available about the mouse celiac-superior mesenteric complex. We used tyrosine hydroxylase (TH) and dopamine β-hydroxylase (DbH) reporter mice to visualize abdominal prevertebral ganglia. We found that both the TH and DbH reporter mice are useful models for identification of ganglia and nerve bundles. We further tested if the celiac-superior mesenteric complex provides differential inputs to the mouse kidney and liver. The retrograde viral tracer, pseudorabies virus (PRV)-152 was injected into the cortex of the left kidney or the main lobe of the liver to identify kidney-projecting and liver-projecting neurons in the celiac-superior mesenteric complex. iDISCO immunostaining and tissue clearing were used to visualize unprecedented anatomical detail of kidney-related and liver-related postganglionic neurons in the celiac-superior mesenteric complex and aorticorenal and suprarenal ganglia compared with TH-positive neurons. Kidney-projecting neurons were restricted to the suprarenal and aorticorenal ganglia, whereas only sparse labeling was observed in the celiac-superior mesenteric complex. In contrast, liver-projecting postganglionic neurons were observed in the celiac-superior mesenteric complex and aorticorenal and suprarenal ganglia, suggesting spatial separation between the sympathetic innervation of the mouse kidney and liver.

Maternal cold exposure induces distinct transcriptome changes in the placenta and fetal brown adipose tissue in mice

Background

Brown adipose tissue (BAT) is specialized to dissipate energy in the form of heat. BAT-mediated heat production in rodents and humans is critical for effective temperature adaptation of newborns to the extrauterine environment immediately after birth. However, very little is known about whether and how fetal BAT development is modulated in-utero in response to changes in maternal thermal environment during pregnancy. Using BL6 mice, we evaluated the impact of different maternal environmental temperatures (28 °C and 18 °C) on the transcriptome of the placenta and fetal BAT to test if maternal cold exposure influences fetal BAT development via placental remodeling.

Results

Maternal weight gain during pregnancy, the average number of fetuses per pregnancy, and placental weight did not differ between the groups at 28 °C and 18 °C. However, the average fetal weight at E18.5 was 6% lower in the 18 °C-group compared to the 28 °C-group. In fetal BATs, cold exposure during pregnancy induced increased expression of genes involved in de novo lipogenesis and lipid metabolism while decreasing the expression of genes associated with muscle cell differentiation, thus suggesting that maternal cold exposure may promote fetal brown adipogenesis by suppressing the myogenic lineage in bidirectional progenitors. In placental tissues, maternal cold exposure was associated with upregulation of genes involved in complement activation and downregulation of genes related to muscle contraction and actin-myosin filament sliding. These changes may coordinate placental adaptation to maternal cold exposure, potentially by protecting against cold stress-induced inflammatory damage and modulating the vascular and extravascular contractile system in the placenta.

Conclusions

These findings provide evidence that environmental cold temperature sensed by the mother can modulate the transcriptome of placental and fetal BAT tissues. The ramifications of the observed gene expression changes warrant future investigation.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-021-07825-6.

Sympathetic Innervation of White Adipose Tissue: to Beige or Not to Beige?

Obesity research progresses in understanding neuronal circuits and adipocyte biology to regulate metabolism. However, the interface of neuro-adipocyte interaction is less studied. We summarize the current knowledge of adipose tissue innervation and interaction with adipocytes and emphasize adipocyte transitions from white to brown adipocytes and vice versa. We further highlight emerging concepts for the differential neuronal regulation of brown/beige versus white adipocyte and the interdependence of both for metabolic regulation.

Sympathetic innervation of inguinal white adipose tissue in the mouse

Adipose tissue plays an important role in metabolic homeostasis and its prominent role as endocrine organ is now well recognized. Adipose tissue is controlled via the sympathetic nervous system (SNS). New viral, molecular-genetic tools will soon allow a more detailed study of adipose tissue innervation in metabolic function, yet, the precise anatomical extent of preganglionic and postganglionic inputs to the inguinal white adipose tissue (iWAT) is limited. Furthermore, several viral, molecular-genetic tools will require the use of cre/loxP mouse models, while the available studies on sympathetic iWAT innervation were established in larger species. In this study, we generated a detailed map for the sympathetic innervation of iWAT in male and female mice. We adapted iDISCO tissue clearing to process large, whole-body specimens for an unprecedented view of the natural abdominal SNS. Combined with pseudorabies virus retrograde tracing from the iWAT, we defined the preganglionic and postganglionic sympathetic input to iWAT. We used fluorescence-guided anatomical dissections of sympathetic nerves in reporter mice to further clarify that postganglionic axons connect to iWAT via lateral cutaneous rami (dorsolumbar iWAT portion) and the lumbar plexus (inguinal iWAT portion). Importantly, these rami carry axons that branch to iWAT, as well as axons that travel further to innervate the skin and vasculature, and their functional impact will require consideration in denervation studies. Our study may serve as a comprehensive map for future experiments that employ virally driven neuromodulation techniques to predict anatomy-based viral labeling.

IGFBP-2 partly mediates the early metabolic improvements caused by bariatric surgery

Insulin-like growth factor-binding protein (IGFBP)-2 is a circulating biomarker of cardiometabolic health. Here, we report that circulating IGFBP-2 concentrations robustly increase after different bariatric procedures in humans, reaching higher levels after biliopancreatic diversion with duodenal switch (BPD-DS) than after Roux-en-Y gastric bypass (RYGB) and sleeve gastrectomy (SG). This increase is closely associated with insulin sensitization. In mice and rats, BPD-DS and RYGB operations also increase circulating IGFBP-2 levels, which are not affected by SG or caloric restriction. In mice, Igfbp2 deficiency significantly impairs surgery-induced loss in adiposity and early improvement in insulin sensitivity but does not affect long-term enhancement in glucose homeostasis. This study demonstrates that the modulation of circulating IGFBP-2 may play a role in the early improvement of insulin sensitivity and loss of adiposity brought about by bariatric surgery.

FGF21 Signals Protein Status to the Brain and Adaptively Regulates Food Choice and Metabolism

Reduced dietary protein intake induces adaptive physiological changes in macronutrient preference, energy expenditure, growth, and glucose homeostasis. We demonstrate that deletion of the FGF21 co-receptor βKlotho (Klb) from the brain produces mice that are unable to mount a physiological response to protein restriction, an effect that is replicated by whole-body deletion of FGF21. Mice forced to consume a low-protein diet exhibit reduced growth, increased energy expenditure, and a resistance to diet-induced obesity, but the loss of FGF21 signaling in the brain completely abrogates that response. When given access to a higher protein alternative, protein-restricted mice exhibit a shift toward protein-containing foods, and central FGF21 signaling is essential for that response. FGF21 is an endocrine signal linking the liver and brain, which regulates adaptive, homeostatic changes in metabolism and feeding behavior during protein restriction.

Recent advances in understanding the role of leptin in energy homeostasis

The hormone leptin plays a critical role in energy homeostasis, although our overall understanding of acutely changing leptin levels still needs improvement. Several developments allow a fresh look at recent and early data on leptin action. This review highlights select recent publications that are relevant for understanding the role played by dynamic changes in circulating leptin levels. We further discuss the relevance for our current understanding of leptin signaling in central neuronal feeding and energy expenditure circuits and highlight cohesive and discrepant findings that need to be addressed in future studies to understand how leptin couples with physiological adaptations of food intake and energy expenditure.

The obesity epidemic in the face of homeostatic body weight regulation: What went wrong and how can it be fixed?

Ever since the pioneering discoveries in the mid nineteen hundreds, the hypothalamus was recognized as a crucial component of the neural system controlling appetite and energy balance. The new wave of neuron-specific research tools has confirmed this key role of the hypothalamus and has delineated many other brain areas to be part of an expanded neural system sub serving these crucial functions. However, despite significant progress in defining this complex neural circuitry, many questions remain. One of the key questions is why the sophisticated body weight regulatory system is unable to prevent the rampant obesity epidemic we are experiencing. Why are pathologically obese body weight levels defended, and what can we do about it? Here we try to find answers to these questions by 1) reminding the reader that the neural controls of ingestive behavior have evolved in a demanding, restrictive environment and encompass much of the brain's major functions, far beyond the hypothalamus and brainstem, 2) hypothesizing that the current obesogenic environment impinges mainly on a critical pathway linking hypothalamic areas with the motivational and reward systems to produce uncompensated hyperphagia, and 3) proposing adequate strategies for prevention and treatment.

FGF21 and the Physiological Regulation of Macronutrient Preference

The ability to respond to variations in nutritional status depends on regulatory systems that monitor nutrient intake and adaptively alter metabolism and feeding behavior during nutrient restriction. There is ample evidence that the restriction of water, sodium, or energy intake triggers adaptive responses that conserve existing nutrient stores and promote the ingestion of the missing nutrient, and that these homeostatic responses are mediated, at least in part, by nutritionally regulated hormones acting within the brain. This review highlights recent research that suggests that the metabolic hormone fibroblast growth factor 21 (FGF21) acts on the brain to homeostatically alter macronutrient preference. Circulating FGF21 levels are robustly increased by diets that are high in carbohydrate but low in protein, and exogenous FGF21 treatment reduces the consumption of sweet foods and alcohol while alternatively increasing the consumption of protein. In addition, while control mice adaptively shift macronutrient preference and increase protein intake in response to dietary protein restriction, mice that lack either FGF21 or FGF21 signaling in the brain fail to exhibit this homeostatic response. FGF21 therefore mediates a unique physiological niche, coordinating adaptive shifts in macronutrient preference that serve to maintain protein intake in the face of dietary protein restriction.

Combined loss of GLP-1R and Y2R does not alter progression of high-fat diet-induced obesity or response to RYGB surgery in mice

OBJECTIVE

Understanding the mechanisms underlying the remarkable beneficial effects of gastric bypass surgery is important for the development of non-surgical therapies or less invasive surgeries in the fight against obesity and metabolic disease. Although the intestinal L-cell hormones glucagon-like peptide-1 (GLP-1) and peptide tyrosine-tyrosine (PYY) have attracted the most attention, direct tests in humans and rodents with pharmacological blockade or genetic deletion of either the GLP1-receptor (GLP1R) or the Y2-receptor (Y2R) were unable to confirm their critical roles in the beneficial effects gastric bypass surgery on body weight and glucose homeostasis. However, new awareness of the power of combinatorial therapies in the treatment of metabolic disease would suggest that combined blockade of more than one signaling pathway may be necessary to reverse the beneficial effects of bariatric surgery.

METHODS

The metabolic effects of high-fat diet and the ability of Roux-en-Y gastric bypass surgery to lower food intake and body weight, as well as improve glucose handling, was tested in GLP1R and Y2R-double knockout (GLP1RKO/Y2RKO) and C57BL6J wildtype (WT) mice.

RESULTS

GLP1RKO/Y2RKO and WT mice responded similarly for up to 20 weeks on high-fat diet and 16 weeks after RYGB. There were no significant differences in loss of body and liver weight, fat mass, reduced food intake, relative increase in energy expenditure, improved fasting insulin, glucose tolerance, and insulin tolerance between WT and GLP1RKO/Y2RKO mice after RYGB.

CONCLUSIONS

Combined loss of GLP1R and Y2R-signaling was not able to negate or attenuate the beneficial effects of RYGB on body weight and glucose homeostasis in mice, suggesting that a larger number of signaling pathways is involved or that the critical pathway has not yet been identified.

Sympathetic innervation of the interscapular brown adipose tissue in mouse

The recent discovery of significant brown fat depots in adult humans has revived discussion of exploiting brown fat thermogenesis in the control of energy balance and body weight. The sympathetic nervous system (SNS) has a key role in the activation of brown fat and functional mapping of its components will be crucial for the development of specific neuromodulation techniques. The mouse is an important species used for molecular genetic modulations, but its small size is not ideal for anatomical dissections, thus brown fat innervation studies are mostly available in larger rodents such as rats and hamsters. Here, we use pseudorabies virus retrograde tracing, whole tissue clearing, and confocal/light sheet microscopy to show the location of pre- and postganglionic neurons selectively innervating the interscapular brown adipose tissue (iBAT) in the mouse. Using iDISCO whole tissue clearing, we identified iBAT projecting postganglionic neurons in the caudal parts of the ipsilateral fused stellate/T1, as well as the T2-T5 sympathetic chain ganglia and preganglionic neurons between levels T2 and T6 of the ipsilateral spinal cord. The methodology enabled high-resolution imaging and 3D rendering of the specific SNS innervation of iBAT and will be helpful to discern peripheral nervous system innervation of other organs and tissues.

Gastric bypass surgery in lean adolescent mice prevents diet-induced obesity later in life

Gastric bypass surgery is the most effective treatment and is often the only option for subjects with severe obesity. However, investigation of critical molecular mechanisms involved has been hindered by confounding of specific effects of surgery and side effects associated with acute surgical trauma. Here, we dissociate the two components by carrying out surgery in the lean state and testing its effectiveness to prevent diet-induced obesity later in life. Body weight and composition of female mice with RYGB performed at 6 weeks of age were not significantly different from sham-operated and age-matched non-surgical mice at the time of high-fat diet exposure 12 weeks after surgery. These female mice were completely protected from high-fat diet-induced obesity and accompanying metabolic impairments for up to 50 weeks. Similar effects were seen in male mice subjected to RYGB at 5-6 weeks, although growth was slightly inhibited and protection from diet-induced obesity was less complete. The findings confirm that RYGB does not indiscriminately lower body weight but specifically prevents excessive diet-induced obesity and ensuing metabolic impairments. This prevention of obesity model should be crucial for identifying the molecular mechanisms underlying gastric bypass surgery.

New Insights into the Regulation of Leptin Gene Expression

Leptin is a key hormone in the homeostatic regulation of body weight. While past research focused mainly on overall leptin actions, a recent study by Dallner et al. (2019) takes a fresh look at the regulatory elements of the leptin gene locus, providing new insights into processes that modulate leptin levels.

The PYY/Y2R-Deficient Mouse Responds Normally to High-Fat Diet and Gastric Bypass Surgery

BACKGROUND/GOALS

The gut hormone peptide YY (PYY) secreted from intestinal L-cells has been implicated in the mechanisms of satiation via Y2-receptor (Y2R) signaling in the brain and periphery and is a major candidate for mediating the beneficial effects of bariatric surgery on appetite and body weight.

METHODS

Here we assessed the role of Y2R signaling in the response to low- and high-fat diets and its role in the effects of Roux-en-Y gastric bypass (RYGB) surgery on body weight, body composition, food intake, energy expenditure and glucose handling, in global Y2R-deficient (Y2RKO) and wildtype (WT) mice made obese on high-fat diet.

RESULTS

Both male and female Y2RKO mice responded normally to low- and high-fat diet in terms of body weight, body composition, fasting levels of glucose and insulin, as well as glucose and insulin tolerance for up to 30 weeks of age. Contrary to expectations, obese Y2RKO mice also responded similarly to RYGB compared to WT mice for up to 20 weeks after surgery, with initial hypophagia, sustained body weight loss, and significant improvements in fasting insulin, glucose tolerance, insulin resistance (HOMA-IR), and liver weight compared to sham-operated mice. Furthermore, non-surgical Y2RKO mice weight-matched to RYGB showed the same improvements in glycemic control as Y2RKO mice with RYGB that were similar to WT mice.

CONCLUSIONS

PYY signaling through Y2R is not required for the normal appetite-suppressing and body weight-lowering effects of RYGB in this global knockout mouse model. Potential compensatory adaptations of PYY signaling through other receptor subtypes or other gut satiety hormones such as glucagon-like peptide-1 (GLP-1) remain to be investigated.

Modulation of Feeding and Associated Behaviors by Lateral Hypothalamic Circuits

Our ability to modulate and observe neuronal activity in defined neurons in freely moving animals has revolutionized neuroscience research in recent years. Findings in the lateral hypothalamus (LHA) highlighted the existence of many neuronal circuits that regulate distinct phenotypes of feeding behavior, emotional valence, and locomotor activity. Several of these neuronal circuits do not fit into a common model of neuronal integration and highlight the need to improve working models for complex behaviors. This review will specifically focus on recent literature that distinguishes LHA circuits based on their molecular and anatomical characteristics and studies their role in feeding, associated behaviors (e.g., arousal and locomotion), and emotional states (e.g., emotional valences).

Homeostatic sensing of dietary protein restriction: A case for FGF21

Restriction of dietary protein intake increases food intake and energy expenditure, reduces growth, and alters amino acid, lipid, and glucose metabolism. While these responses suggest that animals 'sense' variations in amino acid consumption, the basic physiological mechanism mediating the adaptive response to protein restriction has been largely undescribed. In this review we make the case that the liver-derived metabolic hormone FGF21 is the key signal which communicates and coordinates the homeostatic response to dietary protein restriction. Support for this model centers on the evidence that FGF21 is induced by the restriction of dietary protein or amino acid intake and is required for adaptive changes in metabolism and behavior. FGF21 occupies a unique endocrine niche, being induced when energy intake is adequate but protein and carbohydrate are imbalanced. Collectively, the evidence thus suggests that FGF21 is the first known endocrine signal of dietary protein restriction.

Roux-en-Y Gastric Bypass Surgery-Induced Weight Loss and Metabolic Improvements Are Similar in TGR5-Deficient and Wildtype Mice

BACKGROUND AND PURPOSE

Roux-en-Y gastric bypass surgery (RYGB) remains one of the most effective treatments for obesity and type 2 diabetes. Despite this, the mechanisms through which it acts are still not well understood. Bile acid signaling through the transmembrane G-protein-coupled receptor TGR5 has been shown to have significant effects on metabolism and has recently been reported to be necessary for the full effects of vertical sleeve gastrectomy (VSG), a bariatric surgery with similar effects to RYGB. The goal of the current study is therefore to investigate the role of bile acid signaling through TGR5 to see if it is necessary to obtain the full effects of RYGB.

METHODS

High-fat diet-induced obese TGR5-/- and wildtype mice (WT) were subjected to RYGB, sham surgery, or weight matching (WM) to RYGB mice via caloric restriction. Body weight, body composition, food intake, energy expenditure, glucose tolerance, insulin sensitivity, and liver weight were measured.

RESULTS

Although the difference in fat mass 20 weeks after surgery between RYGB and sham-operated mice was slightly reduced in TGR5-/- mice when compared to wildtype mice, loss of body weight and fat mass from preoperative levels, reduction of food intake, increase of energy expenditure, and improvement in glycemic control were similar in the two genotypes. Furthermore, improvements in glycemic control were similar in non-surgical mice weight-matched to RYGB.

CONCLUSIONS

We conclude that bile acid signaling through TGR5 is not required for the beneficial effects of RYGB in the mouse and that RYGB and VSG may achieve their similar beneficial effects through different mechanisms.

Androgen excess in pancreatic β cells and neurons predisposes female mice to type 2 diabetes

Androgen excess predisposes women to type 2 diabetes (T2D), but the mechanism of this is poorly understood. We report that female mice fed a Western diet and exposed to chronic androgen excess using dihydrotestosterone (DHT) exhibit hyperinsulinemia and insulin resistance associated with secondary pancreatic β cell failure, leading to hyperglycemia. These abnormalities are not observed in mice lacking the androgen receptor (AR) in β cells and partially in neurons of the mediobasal hypothalamus (MBH) as well as in mice lacking AR selectively in neurons. Accordingly, i.c.v. infusion of DHT produces hyperinsulinemia and insulin resistance in female WT mice. We observe that acute DHT produces insulin hypersecretion in response to glucose in cultured female mouse and human pancreatic islets in an AR-dependent manner via a cAMP- and mTOR-dependent pathway. Acute DHT exposure increases mitochondrial respiration and oxygen consumption in female cultured islets. As a result, chronic DHT exposure in vivo promotes islet oxidative damage and susceptibility to additional stress induced by streptozotocin via AR in β cells. This study suggests that excess androgen predisposes female mice to T2D following AR activation in neurons, producing peripheral insulin resistance, and in pancreatic β cells, promoting insulin hypersecretion, oxidative injury, and secondary β cell failure.

Preoptic leptin signaling modulates energy balance independent of body temperature regulation

The adipokine leptin acts on the brain to regulate energy balance but specific functions in many brain areas remain poorly understood. Among these, the preoptic area (POA) is well known to regulate core body temperature by controlling brown fat thermogenesis, and we have previously shown that glutamatergic, long-form leptin receptor (Lepr)-expressing neurons in the POA are stimulated by warm ambient temperature and suppress energy expenditure and food intake. Here we further investigate the role of POA leptin signaling in body weight regulation and its relationship to body temperature regulation in mice. We show that POA Lepr signaling modulates energy expenditure in response to internal energy state, and thus contributes to body weight homeostasis. However, POA leptin signaling is not involved in ambient temperature-dependent metabolic adaptations. Our study reveals a novel cell population through which leptin regulates body weight.

Testing Effects of Chronic Chemogenetic Neuronal Stimulation on Energy Balance by Indirect Calorimetry

The fundamental of neuroscience is to connect the firing of neurons to physiological and behavioral outcomes. Chemogenetics enables researchers to control the activity of a genetically defined population of neurons in vivo through the expression of designer receptor exclusively activated by designer drug (DREADD) in specific neurons and the administration of its synthetic ligand clozapine N-oxide (CNO) (Sternson and Roth, 2014). Using stimulatory Gq-coupled DREADD (hM3Dq) in mice, we showed that leptin receptor (LepRb)-expressing neurons in the preoptic area (POA) of the hypothalamus are warm-sensitive neurons that mediate warm-responsive metabolic and behavioral adaptations by reducing energy expenditure and food intake (Yu et al., 2016). We also used DREADD technology to test effects of chronic stimulation of POA LepRb neurons on energy expenditure, food intake, and body weight with the TSE indirect calorimetry system. Here we describe the detailed protocol of how we used indirect calorimetry to study the outcome of chronic stimulation of POA LepRb neurons. This protocol can be adapted to study long-term metabolic and behavioral consequences of other neuronal modulations, with possible modifications to the type of DREADD, duration of CNO treatment, or method of CNO delivery.

The Hypothalamic Preoptic Area and Body Weight Control

The preoptic area (POA) of the hypothalamus is involved in many physiological and behavioral processes thanks to its interconnections to many brain areas and ability to respond to diverse humoral factors. One main function of the POA is to manage body temperature homeostasis, e.g. in response to ambient temperature change, which is achieved in part by controlling brown adipose tissue thermogenesis. The POA is also importantly involved in modulating food intake in response to temperature change, thus making it relevant for body weight homeostasis and obesity research. POA function in body weight control is highly unexplored, and a better understanding of POA circuits and their integration into classic hypothalamic circuits that regulate energy homeostasis is expected to provide new opportunities for the scientific basis and treatment of obesity and comorbidities.

RYGB Produces more Sustained Body Weight Loss and Improvement of Glycemic Control Compared with VSG in the Diet-Induced Obese Mouse Model

BACKGROUND

Weight regain and type-2 diabetes relapse has been reported in a significant proportion of vertical sleeve gastrectomy (VSG) patients in some studies, but definitive conclusions regarding the long-term comparative effectiveness of VSG and Roux-en-Y gastric bypass (RYGB) surgery are lacking both in humans and rodent models. This study's objective was to compare the effects of murine models of VSG and RYGB surgery on body weight, body composition, food intake, energy expenditure, and glycemic control.

METHODS

VSG, RYGB, and sham surgery was performed in high-fat diet-induced obese mice, and the effects on body weight and glycemic control were observed for a period of 12 weeks.

RESULTS

After the initial weight loss, VSG mice regained significant amounts of body weight and fat mass that were only marginally lower than in sham-operated mice. In contrast, RYGB produced sustained loss of body weight and fat mass up to 12 weeks and drastically improved fasting insulin and HOMA-IR compared with sham-operated mice. Using weight-matched control groups, we also found that the adaptive hypometabolic response to weight loss was blunted by both VSG and RYGB, and that despite large weight/fat regain, fasting insulin and HOMA-IR were markedly improved, but not reversed, in VSG mice.

CONCLUSIONS

VSG is less effective to lastingly suppress body weight and improve glycemic control compared with RYGB in mice. Given similar observations in many human studies, the run towards replacing RYGB with VSG is premature and should await carefully controlled randomized long-term trials with VSG and RYGB.

Genetics-based manipulation of adipose tissue sympathetic innervation

There is renewed interest in leveraging the thermogenic capacity of brown adipose tissue (BAT) and browning of white adipose tissue (WAT) to improve energy balance and prevent obesity. In addition to these effects on energy expenditure, both BAT and WAT secrete large numbers of hormones and cytokines that play important roles in maintaining metabolic health. Both BAT and WAT are densely innervated by the sympathetic nervous system (SNS) and this innervation is crucial for BAT thermogenesis and WAT browning, making it a potentially interesting target for manipulating energy balance and treatment of obesity and metabolic disease. Peripheral neuromodulation in the form of electrical manipulation of the SNS and parasympathetic nervous system (PSNS) has been used for the management of pain and many other conditions, but progress is hampered by lack of detailed knowledge of function-specific neurons and nerves innervating particular organs and tissues. Therefore, the goal of the National Institutes of Health (NIH) Common Fund project "Stimulating Peripheral Activity to Relieve Conditions (SPARC)" is to comprehensively map both anatomical and neurochemical aspects of the peripheral nervous system in animal model systems to ultimately guide optimal neuromodulation strategies in humans. Compared to electrical manipulation, neuron-specific opto- and chemogenetic manipulation, now being extensively used to decode the function of brain circuits, will further increase the functional specificity of peripheral neuromodulation.

Low protein-induced increases in FGF21 drive UCP1-dependent metabolic but not thermoregulatory endpoints

Dietary protein restriction increases adipose tissue uncoupling protein 1 (UCP1), energy expenditure and food intake, and these effects require the metabolic hormone fibroblast growth factor 21 (FGF21). Here we test whether the induction of energy expenditure during protein restriction requires UCP1, promotes a resistance to cold stress, and is dependent on the concomitant hyperphagia. Wildtype, Ucp1-KO and Fgf21-KO mice were placed on control and low protein (LP) diets to assess changes in energy expenditure, food intake and other metabolic endpoints. Deletion of Ucp1 blocked LP-induced increases in energy expenditure and food intake, and exacerbated LP-induced weight loss. While LP diet increased energy expenditure and Ucp1 expression in an FGF21-dependent manner, neither LP diet nor the deletion of Fgf21 influenced sensitivity to acute cold stress. Finally, LP-induced energy expenditure occurred even in the absence of hyperphagia. Increased energy expenditure is a primary metabolic effect of dietary protein restriction, and requires both UCP1 and FGF21 but is independent of changes in food intake. However, the FGF21-dependent increase in UCP1 and energy expenditure by LP has no effect on the ability to acutely respond to cold stress, suggesting that LP-induced increases in FGF21 impact metabolic but not thermogenic endpoints.

Phenotyping of nNOS neurons in the postnatal and adult female mouse hypothalamus

Neurons expressing nitric oxide (NO) synthase (nNOS) and thus capable of synthesizing NO play major roles in many aspects of brain function. While the heterogeneity of nNOS-expressing neurons has been studied in various brain regions, their phenotype in the hypothalamus remains largely unknown. Here we examined the distribution of cells expressing nNOS in the postnatal and adult female mouse hypothalamus using immunohistochemistry. In both adults and neonates, nNOS was largely restricted to regions of the hypothalamus involved in the control of bodily functions, such as energy balance and reproduction. Labeled cells were found in the paraventricular, ventromedial, and dorsomedial nuclei as well as in the lateral area of the hypothalamus. Intriguingly, nNOS was seen only after the second week of life in the arcuate nucleus of the hypothalamus (ARH). The most dense and heavily labeled population of cells was found in the organum vasculosum laminae terminalis (OV) and the median preoptic nucleus (MEPO), where most of the somata of the neuroendocrine neurons releasing GnRH and controlling reproduction are located. A great proportion of nNOS-immunoreactive neurons in the OV/MEPO and ARH were seen to express estrogen receptor (ER) α. Notably, almost all ERα-immunoreactive cells of the OV/MEPO also expressed nNOS. Moreover, the use of EYFP^Vglut2 , EYFP^Vgat , and GFP^Gad67 transgenic mouse lines revealed that, like GnRH neurons, most hypothalamic nNOS neurons have a glutamatergic phenotype, except for nNOS neurons of the ARH, which are GABAergic. Altogether, these observations are consistent with the proposed role of nNOS neurons in physiological processes.

Blaming the Brain for Obesity: Integration of Hedonic and Homeostatic Mechanisms

The brain plays a key role in the controls of energy intake and expenditure, and many genes associated with obesity are expressed in the central nervous system. Technological and conceptual advances in both basic and clinical neurosciences have expanded the traditional view of homeostatic regulation of body weight by mainly the hypothalamus to include hedonic controls of appetite by cortical and subcortical brain areas processing external sensory information, reward, cognition, and executive functions. Hedonic controls interact with homeostatic controls to regulate body weight in a flexible and adaptive manner that takes environmental conditions into account. This new conceptual framework has several important implications for the treatment of obesity. Because much of this interactive neural processing is outside awareness, cognitive restraint in a world of plenty is made difficult and prevention and treatment of obesity should be more rationally directed to the complex and often redundant mechanisms underlying this interaction.

Does gastric bypass surgery change body weight set point?

The relatively stable body weight during adulthood is attributed to a homeostatic regulatory mechanism residing in the brain which uses feedback from the body to control energy intake and expenditure. This mechanism guarantees that if perturbed up or down by design, body weight will return to pre-perturbation levels, defined as the defended level or set point. The fact that weight re-gain is common after dieting suggests that obese subjects defend a higher level of body weight. Thus, the set point for body weight is flexible and likely determined by the complex interaction of genetic, epigenetic and environmental factors. Unlike dieting, bariatric surgery does a much better job in producing sustained suppression of food intake and body weight, and an intensive search for the underlying mechanisms has started. Although one explanation for this lasting effect of particularly Roux-en-Y gastric bypass surgery (RYGB) is simple physical restriction due to the invasive surgery, a more exciting explanation is that the surgery physiologically reprograms the body weight defense mechanism. In this non-systematic review, we present behavioral evidence from our own and other studies that defended body weight is lowered after RYGB and sleeve gastrectomy. After these surgeries, rodents return to their preferred lower body weight if over- or underfed for a period of time, and the ability to drastically increase food intake during the anabolic phase strongly argues against the physical restriction hypothesis. However, the underlying mechanisms remain obscure. Although the mechanism involves central leptin and melanocortin signaling pathways, other peripheral signals such as gut hormones and their neural effector pathways likely contribute. Future research using both targeted and non-targeted 'omics' techniques in both humans and rodents as well as modern, genetically targeted, neuronal manipulation techniques in rodents will be necessary.

Reprogramming of defended body weight after Roux-En-Y gastric bypass surgery in diet-induced obese mice

OBJECTIVE

Roux-en-Y gastric bypass surgery (RYGB) results in sustained lowering of body weight in most patients, but the mechanisms involved are poorly understood. The aim of this study was to obtain support for the notion that reprogramming of defended body weight, rather than passive restriction of energy intake, is a fundamental mechanism of RYGB.

METHODS

Male C57BL6J mice reaching different degrees of obesity on a high-fat diet either with ad libitum access or with caloric restriction (weight-reduced) were subjected to RYGB.

RESULTS

RYGB-induced weight loss and fat mass loss were proportional to pre-surgical levels, with moderately obese mice losing less body weight and fat compared with very obese mice. Remarkably, mice that were weight-reduced to the level of chow controls before surgery immediately gained weight after surgery, exclusively accounted for by lean mass gain.

CONCLUSIONS

The results provide additional evidence for reprogramming of a new defended body weight as an important principle by which RYGB lastingly suppresses body weight. RYGB appears to selectively abolish defense of a higher fat mass level, while remaining sensitive to the defense of lean mass. The molecular and physiological mechanisms underlying this reprogramming remain to be elucidated.

Leptin modulates nutrient reward via inhibitory galanin action on orexin neurons

Objective

Leptin modulates food reward via central leptin receptor (LepRb) expressing neurons. Food reward requires stimulation of midbrain dopamine neurons and is modulated by central leptin action, but the exact central mechanisms remain unclear. Stimulatory and inhibitory leptin actions on dopamine neurons have been reported, e.g. by indirect actions on orexin neurons or via direct innervation of dopamine neurons in the ventral tegmental area.

Methods

We showed earlier that LepRb neurons in the lateral hypothalamus (LHA) co-express the inhibitory acting neuropeptide galanin (GAL-LepRb neurons). We studied the involvement of GAL-LepRb neurons to regulate nutrient reward in mice with selective LepRb deletion from galanin neurons (GAL-LepRb^KO mice).

Results

We found that the rewarding value and preference for sucrose over fat was increased in GAL-LepRb^KO mice compared to controls. LHA GAL-LepRb neurons innervate orexin neurons, but not the VTA. Further, expression of galanin and its receptor GalR1 are decreased in the LHA of GAL-LepRb^KO mice, resulting in increased activation of orexin neurons.

Conclusion

We suggest galanin as an important mediator of leptin action to modulate nutrient reward by inhibiting orexin neurons.

•

GAL-LepRb^KO shows ↓ galanin and ↓ GalR1 mRNA, ↑ body weight gain.

•

GAL-LepRb^KO shows ↑ orexin/hypocretin neuronal activation.

•

GAL-LepRb neurons innervate local orexin/hypocretin and noradrenergic locus coeruleus neurons.

•

Leptin regulates natural reward and body weight via GAL-LepRb neurons.

Leptin and insulin act on POMC neurons to promote the browning of white fat

The primary task of white adipose tissue (WAT) is the storage of lipids. However, "beige" adipocytes also exist in WAT. Beige adipocytes burn fat and dissipate the energy as heat, but their abundance is diminished in obesity. Stimulating beige adipocyte development, or WAT browning, increases energy expenditure and holds potential for combating metabolic disease and obesity. Here, we report that insulin and leptin act together on hypothalamic neurons to promote WAT browning and weight loss. Deletion of the phosphatases PTP1B and TCPTP enhanced insulin and leptin signaling in proopiomelanocortin neurons and prevented diet-induced obesity by increasing WAT browning and energy expenditure. The coinfusion of insulin plus leptin into the CNS or the activation of proopiomelanocortin neurons also increased WAT browning and decreased adiposity. Our findings identify a homeostatic mechanism for coordinating the status of energy stores, as relayed by insulin and leptin, with the central control of WAT browning.

Appetite and body weight regulation after bariatric surgery

Bariatric surgery continues to be remarkably efficient in treating obesity and type 2 diabetes mellitus and a debate has started whether it should remain the last resort only or also be used for the prevention of metabolic diseases. Intense research efforts in humans and rodent models are underway to identify the critical mechanisms underlying the beneficial effects with a view towards non-surgical treatment options. This non-systematic review summarizes and interprets some of this literature, with an emphasis on changes in the controls of appetite. Contrary to earlier views, surgery-induced reduction of energy intake and subsequent weight loss appear to be the main drivers for rapid improvements of glycaemic control. The mechanisms responsible for suppression of appetite, particularly in the face of the large weight loss, are not well understood. Although a number of changes in food choice, taste functions, hedonic evaluation, motivation and self-control have been documented in both humans and rodents after surgery, their importance and relative contribution to diminished appetite has not yet been demonstrated. Furthermore, none of the major candidate mechanisms postulated in mediating surgery-induced changes from the gut and other organs to the brain, such as gut hormones and sensory neuronal pathways, have been confirmed yet. Future research efforts should focus on interventional rather than descriptive approaches in both humans and rodent models.

Structure, production and signaling of leptin

The cloning of leptin in 1994 was an important milestone in obesity research. In those days obesity was stigmatized as a condition caused by lack of character and self-control. Mutations in either leptin or its receptor were the first single gene mutations found to cause morbid obesity, and it is now appreciated that obesity is caused by a dysregulation of central neuronal circuits. From the first discovery of the leptin deficient obese mouse (ob/ob), to the cloning of leptin (ob aka lep) and leptin receptor (db aka lepr) genes, much has been learned about leptin and its action in the central nervous system. The initial high hopes that leptin would cure obesity were quickly dampened by the discovery that most obese humans have increased leptin levels and develop leptin resistance. Nevertheless, leptin target sites in the brain represent an excellent blueprint for distinct neuronal circuits that control energy homeostasis. A better understanding of the regulation and interconnection of these circuits will further guide and improve the development of safe and effective interventions to treat obesity. This review will highlight our current knowledge about the hormone leptin, its signaling pathways and its central actions to mediate distinct physiological functions.

Regulation of leptin receptor-expressing neurons in the brainstem by TRPV1

The central nervous system plays a critical role in the regulation of feeding behavior and whole‐body metabolism via controlling the autonomic output to the visceral organs. Activity of the parasympathetic neurons in the dorsal motor nucleus of the vagus (DMV) determines the vagal tone and thereby modulates the function of the subdiaphragmatic organs. Leptin is highly involved in the regulation of food intake and alters neuronal excitability of brainstem neurons. Transient receptor potential vanilloid type 1 (TRPV1) has also been shown to increase neurotransmission in the brainstem and we tested the hypothesis that TRPV1 regulates presynaptic neurotransmitter release to leptin receptor‐expressing (LepRb^EGFP) DMV neurons. Whole‐cell patch‐clamp recordings were performed to determine the effect of TRPV1 activation on excitatory and inhibitory postsynaptic currents (EPSC, IPSC) of LepRb^EGFP neurons in the DMV. Capsaicin, a TRPV1 agonist increased the frequency of miniature EPSCs in 50% of LepRb^EGFP neurons without altering the frequency of miniature IPSCs in the DMV. Stomach‐projecting LepRb^EGFP neurons were identified in the DMV using the transsynaptic retrograde viral tracer PRV‐614. Activation of TRPV1 increased the frequency of mEPSC in ~50% of stomach‐related LepRb^EGFP DMV neurons. These data demonstrate that TRPV1 increases excitatory neurotransmission to a subpopulation of LepRb^EGFP DMV neurons via presynaptic mechanisms and suggest a potential interaction between TRPV1 and leptin signaling in the DMV.

e12160

Our data demonstrate that TRPV1 is involved in the regulation of a subpopulation of leptin receptor‐expressing neurons in the dorsal motor nucleus of the vagus via presynaptic mechanisms.

FGF21 is an endocrine signal of protein restriction

Enhanced fibroblast growth factor 21 (FGF21) production and circulation has been linked to the metabolic adaptation to starvation. Here, we demonstrated that hepatic FGF21 expression is induced by dietary protein restriction, but not energy restriction. Circulating FGF21 was increased 10-fold in mice and rats fed a low-protein (LP) diet. In these animals, liver Fgf21 expression was increased within 24 hours of reduced protein intake. In humans, circulating FGF21 levels increased dramatically following 28 days on a LP diet. LP-induced increases in FGF21 were associated with increased phosphorylation of eukaryotic initiation factor 2α (eIF2α) in the liver, and both baseline and LP-induced serum FGF21 levels were reduced in mice lacking the eIF2α kinase general control nonderepressible 2 (GCN2). Finally, while protein restriction altered food intake, energy expenditure, and body weight gain in WT mice, FGF21-deficient animals did not exhibit these changes in response to a LP diet. These and other data demonstrate that reduced protein intake underlies the increase in circulating FGF21 in response to starvation and a ketogenic diet and that FGF21 is required for behavioral and metabolic responses to protein restriction. FGF21 therefore represents an endocrine signal of protein restriction, which acts to coordinate metabolism and growth during periods of reduced protein intake.

Leptin receptor neurons in the dorsomedial hypothalamus are key regulators of energy expenditure and body weight, but not food intake

OBJECTIVE

Leptin responsive neurons play an important role in energy homeostasis, controlling specific autonomic, behavioral, and neuroendocrine functions. We have previously identified a population of leptin receptor (LepRb) expressing neurons within the dorsomedial hypothalamus/dorsal hypothalamic area (DMH/DHA) which are related to neuronal circuits that control brown adipose tissue (BAT) thermogenesis. Intra-DMH leptin injections also activate sympathetic outflow to BAT, but whether such effects are mediated directly via DMH/DHA LepRb neurons and whether this is physiologically relevant for whole body energy expenditure and body weight regulation has yet to be determined.

METHODS

We used pharmacosynthetic receptors (DREADDs) to selectively activate DMH/DHA LepRb neurons. We further deleted LepRb with virally driven cre-recombinase from DMH/DHA neurons and determined the physiological importance of DMH/DHA LepRb neurons in whole body energy homeostasis.

RESULTS

Neuronal activation of DMH/DHA LepRb neurons with DREADDs promoted BAT thermogenesis and locomotor activity, which robustly induced energy expenditure (p < 0.001) and decreases body weight (p < 0.001). Similarly, intra-DMH/DHA leptin injections normalized hypothermia and attenuated body weight gain in leptin-deficient ob/ob mice. Conversely, ablation of LepRb from DMH/DHA neurons remarkably drives weight gain (p < 0.001) by reducing energy expenditure (p < 0.001) and locomotor activity (p < 0.001). The observed changes in body weight were largely independent of food intake.

CONCLUSION

Taken together, our data highlight that DMH/DHA LepRb neurons are sufficient and necessary to regulate energy expenditure and body weight.

Reversible hyperphagia and obesity in rats with gastric bypass by central MC3/4R blockade

OBJECTIVE

To test the commonly held assumption that gastric bypass surgery lowers body weight because it limits the ability to eat large amounts of food.

METHODS

Central melanocortin signaling was blocked by ICV infusion of the melanocortin-3/4 receptor antagonist SHU9119 for 14 days in rats whose high-fat diet-induced obesity had been reversed by Roux-en-Y gastric bypass surgery.

RESULTS

SHU9119 increased daily food intake (+ 100%), body weight (+30%), and fat mass (+50%) in rats with RYGB, surpassing the presurgical body weight and that of saline-treated sham-operated rats. Doubling of food intake was entirely due to increased meal frequency, but not meal size. After termination of SHU9119, body weight promptly returned to near preinfusion levels. In sham-operated rats, SHU9119 produced even larger increases in food intake and body weight.

CONCLUSIONS

RYGB rats do not settle at a lower level of body weight because they cannot eat more food as they can easily double food intake by increasing meal frequency. The reversible obesity suggests that RYGB rats actively defend the lower body weight. However, because both RYGB and sham-operated rats responded to SHU9119, central melanocortin signaling is not the critical mechanism in RYGB rats responsible for this defense.

Leucine acts in the brain to suppress food intake but does not function as a physiological signal of low dietary protein

Intracerebroventricular injections of leucine are sufficient to suppress food intake, but it remains unclear whether brain leucine signaling represents a physiological signal of protein balance. We tested whether variations in dietary and circulating levels of leucine, or all three branched-chain amino acids (BCAAs), contribute to the detection of reduced dietary protein. Of the essential amino acids (EAAs) tested, only intracerebroventricular injection of leucine (10 μg) was sufficient to suppress food intake. Isocaloric low- (9% protein energy; LP) or normal- (18% protein energy) protein diets induced a divergence in food intake, with an increased consumption of LP beginning on day 2 and persisting throughout the study (P < 0.05). Circulating BCAA levels were reduced the day after LP diet exposure, but levels subsequently increased and normalized by day 4, despite persistent hyperphagia. Brain BCAA levels as measured by microdialysis on day 2 of diet exposure were reduced in LP rats, but this effect was most prominent postprandially. Despite these diet-induced changes in BCAA levels, reducing dietary leucine or total BCAAs independently from total protein was neither necessary nor sufficient to induce hyperphagia, while chronic infusion of EAAs into the brain of LP rats failed to consistently block LP-induced hyperphagia. Collectively, these data suggest that circulating BCAAs are transiently reduced by dietary protein restriction, but variations in dietary or brain BCAAs alone do not explain the hyperphagia induced by a low-protein diet.

Central mechanisms of adiposity in adult female mice with androgen excess

OBJECTIVE

Androgen excess in women is associated with visceral adiposity. However, little is known on the mechanism through which androgen promotes visceral fat accumulation.

METHODS

To address this issue, female mice to chronic androgen excess using 5α-dihydrotestosterone (DHT) and studied the regulation of energy homeostasis was exposed.

RESULTS

DHT induced a leptin failure to decrease body weight associated with visceral adiposity but without alterations in leptin anorectic action. This paralleled leptin's failure to upregulate brown adipose tissue expression of uncoupling protein-1, associated with decreased energy expenditure (EE). DHT decreased hypothalamic proopiomelanocortin (pomc) mRNA expression and increased POMC intensity in neuronal bodies of the arcuate nucleus while simultaneously decreasing the intensity of POMC projections to the dorsomedial hypothalamus (DMH). This was associated with a failure of the melanocortin 4 receptor agonist melanotan-II to suppress body weight.

CONCLUSION

Taken together, these data indicate that androgen excess promotes visceral adiposity with reduced POMC neuronal innervation in the DMH, reduced EE but without hyperphagia.

GLP-1 receptor signaling is not required for reduced body weight after RYGB in rodents

Exaggerated GLP-1 and PYY secretion is thought to be a major mechanism in the reduced food intake and body weight after Roux-en-Y gastric bypass surgery. Here, we use complementary pharmacological and genetic loss-of-function approaches to test the role of increased signaling by these gut hormones in high-fat diet-induced obese rodents. Chronic brain infusion of a supramaximal dose of the selective GLP-1 receptor antagonist exendin-9-39 into the lateral cerebral ventricle significantly increased food intake and body weight in both RYGB and sham-operated rats, suggesting that, while contributing to the physiological control of food intake and body weight, central GLP-1 receptor signaling tone is not the critical mechanism uniquely responsible for the body weight-lowering effects of RYGB. Central infusion of the selective Y2R-antagonist BIIE0246 had no effect in either group, suggesting that it is not critical for the effects of RYGB on body weight under the conditions tested. In a recently established mouse model of RYGB that closely mimics surgery and weight loss dynamics in humans, obese GLP-1R-deficient mice lost the same amount of body weight and fat mass and maintained similarly lower body weight compared with wild-type mice. Together, the results surprisingly provide no support for important individual roles of either gut hormone in the specific mechanisms by which RYGB rats settle at a lower body weight. It is likely that the beneficial effects of bariatric surgeries are expressed through complex mechanisms that require combination approaches for their identification.