Regulation of glucose homeostasis in humans with denervated livers

Signaling through the nicotinic acetylcholine receptor in the liver protects against the development of metabolic dysfunction-associated steatohepatitis

Metabolic dysfunction-associated steatohepatitis (MASH) is the progressive form of liver steatosis, the most common liver disease, and substantially increases the mortality rate. However, limited therapies are currently available to prevent MASH development. Identifying potential pharmacological treatments for the condition has been hampered by its heterogeneous and complex nature. Here, we identified a hepatic nonneuronal cholinergic signaling pathway required for metabolic adaptation to caloric overload. We found that cholinergic receptor nicotinic alpha 2 subunit (CHRNA2) is highly expressed in hepatocytes of mice and humans. Further, CHRNA2 is activated by a subpopulation of local acetylcholine-producing macrophages during MASH development. The activation of CHRNA2 coordinates defensive programs against a broad spectrum of MASH-related pathogenesis, including steatosis, inflammation, and fibrosis. Hepatocyte-specific loss of CHRNA2 signaling accelerates the disease onset in different MASH mouse models. Activation of this pathway via pharmacological inhibition of acetylcholine degradation protects against MASH development. Our study uncovers a hepatic nicotinic cholinergic receptor pathway that constitutes a cell-autonomous self-defense route against prolonged metabolic stress and holds therapeutic potential for combatting human MASH.

The Sympathetic-Immune Milieu in Metabolic Health and Diseases: Insights from Pancreas, Liver, Intestine, and Adipose Tissues

Sympathetic innervation plays a crucial role in maintaining energy balance and contributes to metabolic pathophysiology. Recent evidence has begun to uncover the innervation landscape of sympathetic projections and sheds light on their important functions in metabolic activities. Additionally, the immune system has long been studied for its essential roles in metabolic health and diseases. In this review, the aim is to provide an overview of the current research progress on the sympathetic regulation of key metabolic organs, including the pancreas, liver, intestine, and adipose tissues. In particular, efforts are made to highlight the critical roles of the peripheral nervous system and its potential interplay with immune components. Overall, it is hoped to underscore the importance of studying metabolic organs from a comprehensive and interconnected perspective, which will provide valuable insights into the complex mechanisms underlying metabolic regulation and may lead to novel therapeutic strategies for metabolic diseases.

Bariatric Surgery Alters the Postprandial Recovery From Hypoglycemia, Mediated by Cholinergic Signal

Roux-en-Y gastric bypass (GB) and sleeve gastrectomy (SG) surgeries increase prandial insulin and glucagon secretion but reduce the endogenous glucose production (EGP) response to hypoglycemia in comparison with control subjects who had not undergone gastric surgery (CN), suggesting that parasympathetic nervous system (PNS) plays a role. Here, we investigated the effect of acute PNS blockade on the post-meal counterregulatory response to insulin-induced hypoglycemia in GB and SG compared with CN. Glucose kinetics and islet cell secretion were measured in nine subjects without diabetes with GB and seven with SG and five CN during hyperinsulinemic-hypoglycemic clamp (∼3.2 mmol/L) combined with meal ingestion on two separate days with and without intravenous atropine infusion. Glucose and hormonal levels were similar at baseline and during steady-state hypoglycemia before meal ingestion in three groups and unaffected by atropine. Atropine infusion diminished prandial systemic appearance of ingested glucose (RaO) by 30%, EGP by 40%, and glucagon response to hypoglycemia by 90% in CN. In GB or SG, blocking PNS had no effect on the RaO or meal-induced hyperglucagonemia but increased EGP in SG without any effect in GB (P < 0.05 interaction). These findings indicate that cholinergic signal contributes to the recovery from hypoglycemia by meal consumption in humans. However, bariatric surgery dissipates PNS-mediated physiologic responses to hypoglycemia in the fed state.

ARTICLE HIGHLIGHTS

Rerouted gut after Roux-en-Y gastric bypass (GB) and, to a lesser degree, after sleeve gastrectomy (SG) leads to larger glucose excursion and lower nadir glucose, predisposing individuals to hypoglycemia. Despite prandial hyperglucagonemia, endogenous glucose production response to hypoglycemia is reduced after GB or SG. Parasympathetic nervous system (PNS) activity plays a key role in regulation of glucose kinetics and islet cell function. We examined the effect of acute PNS blockade on counterregulatory glucose and islet cell response to meal ingestion during insulin-induced hypoglycemia among GB, SG, and control subjects who had not had gastric surgery. Our findings demonstrate that cholinergic signal is critical in the recovery from hypoglycemia by meal ingestion in humans who have not had gastric surgery, although prandial PNS-mediated physiologic responses to hypoglycemia are differentially changed by GB and SG.

Short-term fasting lowers glucagon levels under euglycemic and hypoglycemic conditions in healthy humans

Fasting is associated with increased susceptibility to hypoglycemia in people with type 1 diabetes, thereby making it a significant health risk. To date, the relationship between fasting and insulin-induced hypoglycemia has not been well characterized, so our objective was to determine whether insulin-independent factors, such as counterregulatory hormone responses, are adversely impacted by fasting in healthy control individuals. Counterregulatory responses to insulin-induced hypoglycemia were measured in 12 healthy people during 2 metabolic studies. During one study, participants ate breakfast and lunch, after which they underwent a 2-hour bout of insulin-induced hypoglycemia (FED). During the other study, participants remained fasted prior to hypoglycemia (FAST). As expected, hepatic glycogen concentrations were lower in FAST, and associated with diminished peak glucagon levels and reduced endogenous glucose production (EGP) during hypoglycemia. Accompanying lower EGP in FAST was a reduction in peripheral glucose utilization, and a resultant reduction in the amount of exogenous glucose required to maintain glycemia. These data suggest that whereas a fasting-induced lowering of glucose utilization could potentially delay the onset of insulin-induced hypoglycemia, subsequent reductions in glucagon levels and EGP are likely to encumber recovery from it. As a result of this diminished metabolic flexibility in response to fasting, susceptibility to hypoglycemia could be enhanced in patients with type 1 diabetes under similar conditions.

Hypoglycemic Effect of Exopolysaccharide from Lactiplantibacillus plantarum JLAU103 on Streptozotocin and High-Fat Diet-Induced Type 2 Diabetic Mice

Two doses (300 mg/kg bw and 600 mg/kg bw) of the Lactiplantibacillus plantarum JLAU103 exopolysaccharide (EPS103) were orally administered to a type 2 diabetic (T2DM) mouse model induced by streptozotocin and a high-fat diet. The hypoglycemic, hypolipidemic and neuroprotective effects of EPS103 on T2DM mice were evaluated. The results indicated that administration of EPS103 could alleviate insulin resistance, reduce the levels of fasting blood glucose, glycosylated hemoglobin A1c, leptin and fasting serum insulin, improve glucose tolerance, protect pancreas and liver, and modulate blood lipid disorders. EPS103 promoted hepatic glycogen synthesis by upregulating the phosphorylation of GSK3β. Meanwhile, it upregulated the phosphorylation of IRS-1, PI3K and Akt, as well as the expression of IRS-2 and GLUT4, and downregulated the expression of PEPCK, G6Pase and PGC-1α, indicating that EPS103 promotes the uptake and transport of glucose and inhibits gluconeogenesis, which might be related to the activation of the IRS-1/PI3K/Akt pathway. Additionally, EPS103 can protect against brain nerve damage through improving oxidative stress injury, restoring the expression of IRS-2, alleviating neuronal apoptosis and inhibiting inflammation in the hippocampus of T2DM mice. Taken together, our results demonstrated that EPS103 may be a potential therapeutic agent for the treatment of T2DM.

Targeting the liver in dementia and cognitive impairment: Dietary macronutrients and diabetic therapeutics

Many people living with dementia and cognitive impairment have dysfunctional mitochondrial and insulin-glucose metabolism resembling type 2 diabetes mellitus and old age. Evidence from human trials shows that nutritional interventions and anti-diabetic medicines that target nutrient-sensing pathways overcome these deficits in glucose and energy metabolism and can improve cognition and/or reduce symptoms of dementia. The liver is the main organ that mediates the systemic effects of diets and many diabetic medicines; therefore, it is an intermediate target for such dementia interventions. A challenge is the efficacy of these treatments in older age. Solutions include the targeted hepatic delivery of diabetic medicines using nanotechnologies and titration of macronutrients to optimize hepatic energy metabolism.

Hepatic Nervous System in Development, Regeneration, and Disease

The liver is innervated by autonomic and sensory fibers of the sympathetic and parasympathetic nervous systems that regulate liver function, regeneration, and disease. Although the importance of the hepatic nervous system in maintaining and restoring liver homeostasis is increasingly appreciated, much remains unknown about the specific mechanisms by which hepatic nerves both influence and are influenced by liver diseases. While recent work has begun to illuminate the developmental mechanisms underlying recruitment of nerves to the liver, evolutionary differences contributing to species-specific patterns of hepatic innervation remain elusive. In this review, we summarize current knowledge on the development of the hepatic nervous system and its role in liver regeneration and disease. We also highlight areas in which further investigation would greatly enhance our understanding of the evolution and function of liver innervation.

Brain insulin signalling in metabolic homeostasis and disease

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

Insulin on the brain: The role of central insulin signalling in energy and glucose homeostasis

Insulin signals to the brain where it coordinates multiple physiological processes underlying energy and glucose homeostasis. This review explores where and how insulin interacts within the brain parenchyma, how brain insulin signalling functions to coordinate energy and glucose homeostasis and how this contributes to the pathogenesis of metabolic disease.

Combined renal and common hepatic artery denervation as a novel approach to reduce cardiometabolic risk: technical approach, feasibility and safety in a pre-clinical model

Background

Cardiovascular and metabolic regulation is governed by neurohumoral signalling in relevant organs such as kidney, liver, pancreas, duodenum, adipose tissue, and skeletal muscle. Combined targeting of relevant neural outflows may provide a unique therapeutic opportunity for cardiometabolic disease.

Objectives

We aimed to investigate the feasibility, safety, and performance of a novel device-based approach for multi-organ denervation in a swine model over 30 and 90 days of follow-up.

Methods

Five Yorkshire cross pigs underwent combined percutaneous denervation in the renal arteries and the common hepatic artery (CHA) with the iRF Denervation System. Control animals (n = 3) were also studied. Specific energy doses were administered in the renal arteries and CHA. Blood was collected at 30 and 90 days. All animals had a pre-terminal procedure angiography. Tissue samples were collected for norepinephrine (NEPI) bioanalysis. Histopathological evaluation of collateral structures and tissues near the treatment sites was performed to assess treatment safety.

Results

All animals entered and exited the study in good health. No stenosis or vessel abnormalities were present. No significant changes in serum chemistry occurred. NEPI concentrations were significantly reduced in the liver (− 88%, p = 0.005), kidneys (− 78%, p < 0.001), pancreas (− 78%, p = 0.018) and duodenum (− 95%, p = 0.028) following multi-organ denervation treatment compared to control animals. Histologic findings were consistent with favourable tissue responses at 90 days follow-up.

Conclusions

Significant and sustained denervation of the treated organs was achieved at 90 days without major safety events. Our findings demonstrate the feasibility of multi-organ denervation using a novel iRF Denervation System in a single procedure.

Brain control of blood glucose levels: implications for the pathogenesis of type 2 diabetes

Despite a rapidly growing literature, the role played by the brain in both normal glucose homeostasis and in type 2 diabetes pathogenesis remains poorly understood. In this review, we introduce a framework for understanding the brain's essential role in these processes based on evidence that the brain, like the pancreas, is equipped to sense and respond to changes in the circulating glucose level. Further, we review evidence that glucose sensing by the brain plays a fundamental role in establishing the defended level of blood glucose, and that defects in this control system contribute to type 2 diabetes pathogenesis. We also consider the possibility that the close association between obesity and type 2 diabetes arises from a shared defect in the highly integrated neurocircuitry governing energy homeostasis and glucose homeostasis. Thus, whereas obesity is characterised by an increase in the defended level of the body's fuel stores (e.g. adipose mass), type 2 diabetes is characterised by an increase in the defended level of the body's available fuel (e.g. circulating glucose), with the underlying pathogenesis in each case involving impaired sensing of (or responsiveness to) relevant humoral negative feedback signals. This perspective is strengthened by growing preclinical evidence that in type 2 diabetes the defended level of blood glucose can be restored to normal by therapies that restore the brain's ability to properly sense the circulating glucose level. Graphical abstract.

Noradrenergic uptake in the liver on 123 I-mIBG imaging: Influence of heart failure and diabetes

BACKGROUND AND AIM

Imaging noradrenergic uptake in the liver with norepinephrine analog ¹²³ I-meta-iodobenzylguanidine (mIBG) was explored in normal controls and patients with heart failure (HF).

METHODS

A total of 961 HF (343 with diabetes mellitus [DM]) and 94 control subjects underwent anterior planar mIBG images including upper abdomen at 15 min (early) and 3 h 50 min (late) post-injection. Decay-corrected liver activity normalized to injected activity and body surface area (counts/pixel [cpp]/MBq/m² ) was compared in three groups: HF with DM; HF without DM; and controls. Associations with plasma norepinephrine, liver function tests, and level of cardiac innervation were explored.

RESULTS

In controls, liver mIBG activity decreased over time (early: 2.78 vs late: 2.43 cpp/MBq/m² , P < 0.0001); in HF subjects, activity increased during this interval (HF without DM: 2.85 vs 2.93 [P = 0.005]; HF with DM: 2.37 vs 2.43 [P = 0.054]). Early liver activity was lower in HF with DM subjects than in the other groups (P < 0.001); late liver activity was higher in HF without DM than in the other two groups (P < 0.01). Subjects with elevated plasma norepinephrine (> 520 pg/mL) or ≥ 1 abnormal liver function test had lower early and late liver activity. In subjects with preserved cardiac mIBG uptake, HF subjects had higher and control subjects lower liver activity than comparable subjects with decreased cardiac innervation.

CONCLUSIONS

In HF subjects, liver mIBG activity increased over time, reversing the normal washout pattern, suggesting a compensatory change in sympathetic nerve function. DM, abnormal liver function tests, and decreased cardiac innervation were associated with decreased liver mIBG uptake in HF.

Antibody-negative autoimmune encephalitis as a complication of long-term immune-suppression for liver transplantation

Autoimmune encephalitis is a rare spectrum of disease that can be a complication of chronic immunosuppression. Diagnosis often requires the presence of antineuronal antibodies, but many causative antibodies have not yet been identified. Antibody-negative autoimmune encephalitis (AbNAE) is especially difficult to diagnose and must rely largely on exclusion of other causes. In chronically immune-suppressed transplant recipients, the differential is broad, likely resulting in underdiagnosis and worse outcomes. Here, we present a 58-year-old liver transplant recipient taking tacrolimus for prevention of chronic rejection who presented with 5 days of confusion, lethargy and lightheadedness. He was diagnosed with AbNAE after an extensive workup and recovered fully after high-dose corticosteroids. Our case highlights the importance of recognising the association between chronic immunosuppression and autoimmune encephalitis. Autoimmune encephalitis, even in the absence of characterised antibodies, should be considered when transplant recipients present with central neurologic symptoms.

Sympathetic Denervation of the Common Hepatic Artery Lessens Glucose Intolerance in the Fat- and Fructose-Fed Dog

This study assessed the effectiveness of surgical sympathetic denervation of the common hepatic artery (CHADN) in improving glucose tolerance. CHADN eliminated norepinephrine content in the liver and partially decreased it in the pancreas and the upper gut. We assessed oral glucose tolerance at baseline and after 4 weeks of high-fat high-fructose (HFHF) feeding. Dogs were then randomized to sham surgery (SHAM) (n = 9) or CHADN surgery (n = 11) and retested 2.5 or 3.5 weeks later while still on the HFHF diet. CHADN improved glucose tolerance by ∼60% in part because of enhanced insulin secretion, as indicated by an increase in the insulinogenic index. In a subset of dogs (SHAM, n = 5; CHADN, n = 6), a hyperinsulinemic-hyperglycemic clamp was used to assess whether CHADN could improve hepatic glucose metabolism independent of a change in insulin release. CHADN reduced the diet-induced defect in net hepatic glucose balance by 37%. In another subset of dogs (SHAM, n = 4; CHADN, n = 5) the HFHF diet was continued for 3 months postsurgery and the improvement in glucose tolerance caused by CHADN continued. In conclusion, CHADN has the potential to enhance postprandial glucose clearance in states of diet-induced glucose intolerance.

The role of insulin at brain-liver axis in the control of glucose production

Glucose is an essential metabolic substrate for all mammalian cells, and its availability in the circulation is carefully controlled to avoid wide variations. Different mechanisms are involved in the glucose disposal, such as an adequate pancreatic and hepatic function. Insulin is the main hormone in glycemic control, and its action occurs directly in the cells, as well as in the liver, in an indirect way, to ultimately control the glycemia. Insulin has also an important action within the central nervous system, more precisely in the hypothalamus that projects directly to preautonomic nuclei in the brain stem to control hepatic glucose production. The central action of insulin relies on autonomic outflow through the vagal innervation of the liver, where insulin is able to modulate the production of glucose at this organ level. In this way, responses generated in the CNS reach the effector organs by autonomic efferent pathways as part of an important brain-organ axis in the control of glycemia. The purpose of this minireview is to shed light on the brain-liver axis in the control of hepatic glucose by central action of insulin via the autonomic nervous system.

Insulin action in the brain: Roles in energy and glucose homeostasis

A growing body of evidence from research in rodents and humans has identified insulin as an important neuoregulatory peptide in the brain, where it coordinates diverse aspects of energy balance and peripheral glucose homeostasis. This review discusses where and how insulin interacts within the brain and evaluates the physiological and pathophysiological consequences of central insulin signalling in metabolism, obesity and type 2 diabetes.

Acute Effects of Morning Light on Plasma Glucose and Triglycerides in Healthy Men and Men with Type 2 Diabetes

Ambient light intensity is signaled directly to hypothalamic areas that regulate energy metabolism. Observational studies have shown associations between ambient light intensity and plasma glucose and lipid levels, but human data on the acute metabolic effects of light are scarce. Since light is the main signal indicating the onset of the diurnal phase of physical activity and food intake in humans, we hypothesized that bright light would affect glucose and lipid metabolism. Therefore, we determined the acute effects of bright light on plasma glucose and lipid concentrations in 2 randomized crossover trials: (1) in 8 healthy lean men and (2) in 8 obese men with type 2 diabetes. From 0730 h, subjects were exposed to either bright light (4000 lux) or dim light (10 lux) for 5 h. After 1 h of light exposure, subjects consumed a 600-kcal mixed meal. Primary endpoints were fasting and postprandial plasma glucose levels. In healthy men, bright light did not affect fasting or postprandial plasma glucose levels. However, bright light increased fasting and postprandial plasma triglycerides. In men with type 2 diabetes, bright light increased fasting and postprandial glucose levels. In men with type 2 diabetes, bright light did not affect fasting triglyceride levels but increased postprandial triglyceride levels. We show that ambient light intensity acutely affects human plasma glucose and triglyceride levels. Our findings warrant further research into the consequences of the metabolic effects of light for the diagnosis and prevention of hyperglycemia and dyslipidemia.

Autonomic Nervous System and the Liver

The liver is innervated by both the sympathetic and the parasympathetic nerve systems. These nerves are derived from the splanchnic and vagal nerves that surround the portal vein, hepatic artery, and bile duct. The afferent fiber delivers information regarding osmolality, glucose level, and lipid level in the portal vein to the central nervous system (CNS). In contrast, the efferent fiber is crucial in the regulation of metabolism, blood flow, and bile secretion. Furthermore, liver innervation has been associated with hepatic fibrosis, regeneration, and circadian rhythm. Knowledge of these mechanisms can be applied for potential liver disease treatment.

Autonomic nervous system-mediated effects of galanin-like peptide on lipid metabolism in liver and adipose tissue

Galanin-like peptide (GALP) is a neuropeptide involved in the regulation of feeding behavior and energy metabolism in mammals. While a weight loss effect of GALP has been reported, its effects on lipid metabolism have not been investigated. The aim of this study was to determine if GALP regulates lipid metabolism in liver and adipose tissue via an action on the sympathetic nervous system. The respiratory exchange ratio of mice administered GALP intracerebroventricularly was lower than that of saline-treated animals, and fatty acid oxidation-related gene mRNA levels were increased in the liver. Even though the respiratory exchange ratio was reduced by GALP, this change was not significant when mice were treated with the sympatholytic drug, guanethidine. Lipolysis-related gene mRNA levels were increased in the adipose tissue of GALP-treated mice compared with saline-treated animals. These results show that GALP stimulates fatty acid β-oxidation in liver and lipolysis in adipose tissue, and suggest that the anti-obesity effect of GALP may be due to anorexigenic actions and improvement of lipid metabolism in peripheral tissues via the sympathetic nervous system.

Alterations in carbohydrate metabolism in cirrhotic patients before and after liver transplant

AIM

The main objective of this study is to demonstrate whether carbohydrate metabolism alterations identified in patients with advanced cirrhosis show any improvement after liver transplant.

METHODS

The study included 86 patients who underwent liver transplant between March 2010 and February 2011. An oral glucose tolerance test was performed before the liver transplant, and 6 and 12 months after. Beta cell function and insulin resistance were also calculated, applying formulae that use basal plasma glycaemia and insulin, and plasma glycaemia and insulin during an oral glucose tolerance test. Risk factors for pre- and post-transplant diabetes were also studied. The diagnosis of diabetes was based on an OGTT.

RESULTS

The proportion of patients with diabetes before transplant, and at month 6 and 12 after transplant were 70.9%, 48.8% and 39.2%, respectively. Compared to baseline, at month 6 the odds ratio of having diabetes was 0.39 (IC 95% [0.21, 0.73]) and at month 12 it was 0.26 (IC 95% [0.14, 0.50]). The composite insulin sensitivity index values at 6 and 12 months were 1.72 units higher (IC 95% [0.84, 2.58]) and 1.58 units higher (IC 95% [0.68, 2.44)] than baseline. A statistically significant association was found between high MELD values and high body mass index, and risk of pre-transplant diabetes (p=0.001 and p=0.033, respectively). Cirrhosis aetiology did not influence the risk of diabetes.

CONCLUSIONS

In this study, we were able to ascertain that alterations in carbohydrate metabolism typical of advanced cirrhosis improve after liver transplant. This improvement is mainly due to an improvement in insulin resistance.

Glucoregulatory Relevance of Small Intestinal Nutrient Sensing in Physiology, Bariatric Surgery, and Pharmacology

Emerging evidence suggests the gastrointestinal tract plays an important glucoregulatory role. In this perspective, we first review how the intestine senses ingested nutrients, initiating crucial negative feedback mechanisms through a gut-brain neuronal axis to regulate glycemia, mainly via reduction in hepatic glucose production. We then highlight how intestinal energy sensory mechanisms are responsible for the glucose-lowering effects of bariatric surgery, specifically duodenal-jejunal bypass, and the antidiabetic agents metformin and resveratrol. A better understanding of these pathways lays the groundwork for intestinally targeted drug therapy for the treatment of diabetes.

Hypothalamic neurones governing glucose homeostasis

The notion that the brain directly controls the level of glucose in the blood (glycaemia) independent of its known action on food intake and body weight has been known ever since 1849. That year, the French physiologist Dr Claude Bernard reported that physical puncture of the floor of the fourth cerebral ventricle rapidly leads to an increased level of sugar in the blood (and urine) in rabbits. Despite this important discovery, it took approximately 150 years before significant efforts aimed at understanding the underlying mechanism of brain-mediated control of glucose metabolism were made. Technological developments allowing for genetically-mediated manipulation of selected molecular pathways in a neurone-type-specific fashion unravelled the importance of specific molecules in specific neuronal populations. These neuronal pathways govern glucose metabolism in the presence and even in the absence of insulin. Also, a peculiarity of these pathways is that certain biochemically-defined neurones govern glucose metabolism in a tissue-specific fashion.

Peripheral and central glucose sensing in hypoglycemic detection

Hypoglycemia poses a serious threat to the integrity of the brain, owing to its reliance on blood glucose as a fuel. Protecting against hypoglycemia is an extended network of glucose sensors located within the brain and in the periphery that serve to mediate responses restoring euglycemia, i.e., counterregulatory responses. This review examines the various glucose sensory loci involved in hypoglycemic detection, with a particular emphasis on peripheral glucose sensory loci and their contribution to hypoglycemic counterregulation.

Subdiaphragmatic vagus nerve activity and hepatic venous glucose are differentially regulated by the central actions of insulin in Wistar and SHR

Glucose is the most important energy substrate for the maintenance of tissues function. The liver plays an essential role in the control of glucose production, since it is able to synthesize, store, and release glucose into the circulation under different situations. Hormones like insulin and catecholamines influence hepatic glucose production (HGP), but little is known about the role of the central actions of physiological doses of insulin in modulating HGP via the autonomic nervous system in nonanesthetized rats especially in SHR where we see a high degree of insulin resistance and metabolic dysfunction. Wistar and SHR received ICV injection of insulin (100 nU/μL) and hepatic venous glucose concentration (HVGC) was monitored for 30 min, as an indirect measure of HGP. At 10 min after insulin injection, HVGC decreased by 27% in Wistar rats, with a negligible change (3%) in SHR. Pretreatment with atropine totally blocked the reduction in HVGC, while pretreatment with propranolol and phentolamine induced a decrease of 8% in HVGC after ICV insulin injection in Wistar. Intracarotid infusion of insulin caused a significant increase in subdiaphragmatic vagus nerve (SVN) activity in Wistar (12 ± 2%), with negligible effects on the lumbar splanchnic sympathetic nerve (LSSN) activity (−6 ± 3%). No change was observed in SVN (−2 ± 2%) and LSSN activities (2 ± 3%) in SHR after ICA insulin infusion. Taken together, these results show, in nonanesthetized animals, the importance of the parasympathetic nervous system in controlling HVGC, and subdiaphragmatic nerve activity following central administration of insulin; a mechanism that is impaired in the SHR.

Intranasal Neuropeptide Administration To Target the Human Brain in Health and Disease

Central nervous system control of metabolic function relies on the input of endocrine messengers from the periphery, including the pancreatic hormone insulin and the adipokine leptin. This concept primarily derives from experiments in animals where substances can be directly applied to the brain. A feasible approach to study the impact of peptidergic messengers on brain function in humans is the intranasal (IN) route of administration, which bypasses the blood-brain barrier and delivers neuropeptides to the brain compartment, but induces considerably less, if any, peripheral uptake than other administration modes. Experimental IN insulin administration has been extensively used to delineate the role of brain insulin signaling in the control of energy homeostasis, but also cognitive function in healthy humans. Clinical pilot studies have found beneficial effects of IN insulin in patients with memory deficits, suggesting that the IN delivery of this and other peptides bears some promise for new, selectively brain-targeted pharmaceutical approaches in the treatment of metabolic and cognitive disorders. More recently, experiments relying on the IN delivery of the hypothalamic hormone oxytocin, which is primarily known for its involvement in psychosocial processes, have provided evidence that oxytocin influences metabolic control in humans. The IN administration of leptin has been successfully tested in animal models but remains to be investigated in the human setting. We briefly summarize the literature on the IN administration of insulin, leptin, and oxytocin, with a particular focus on metabolic effects, and address limitations and perspectives of IN neuropeptide administration.

Central nervous insulin administration does not potentiate the acute glucoregulatory impact of concurrent mild hyperinsulinemia

Experiments in rodents suggest that hypothalamic insulin signaling essentially contributes to the acute control of peripheral glucose homeostasis. Against this background, we investigated in healthy humans whether intranasal (IN) insulin, which is known to effectively reach the brain compartment, impacts systemic glucose metabolism. Twenty overnight-fasted healthy, normal-weight men were IN administered 210 and 420 international units [IU] (10 and 20 IU every 15 min) of the insulin analog aspart (ins-asp) and placebo, respectively, during experimental sessions lasting 6 h. The use of ins-asp rather than human insulin enabled us to disentangle exogenous and endogenous insulin kinetics. IN insulin dose-dependently decreased plasma glucose concentrations while reducing C-peptide and attenuating endogenous insulin levels. However, we also observed a slight dose-dependent permeation of ins-asp into the circulation. In control experiments mimicking the systemic but not the central nervous uptake of the IN 210 IU dose via intravenous infusion of ins-asp at a dose of 0.12 IU/kg/24 h (n = 10), we obtained essentially identical effects on fasting plasma glucose concentrations. This pattern indicates that sustained IN insulin administration to the human brain to enhance central nervous insulin signaling does not acutely alter systemic glucose homeostasis beyond effects accounted for by concurrent mild hyperinsulinemia.

The regulation of glucose metabolism: implications and considerations for the assessment of glucose homeostasis in rodents

The incidence of insulin resistance and type 2 diabetes (T2D) is increasing at alarming rates. In the quest to understand the underlying causes of and to identify novel therapeutic targets to treat T2D, scientists have become increasingly reliant on the use of rodent models. Here, we provide a discussion on the regulation of rodent glucose metabolism, highlighting key differences and similarities that exist between rodents and humans. In addition, some of the issues and considerations associated with assessing glucose homeostasis and insulin action are outlined. We also discuss the role of the liver vs. skeletal muscle in regulating whole body glucose metabolism in rodents, emphasizing the importance of defective hepatic glucose metabolism in the development of impaired glucose tolerance, insulin resistance, and T2D.

Hepatic nervous system and neurobiology of the liver

The liver has a nervous system containing both afferent and efferent neurons that are involved in a number of processes. The afferent arm includes the sensation of lipids, glucose, and metabolites (after eating and drinking) and triggers the nervous system to make appropriate physiological changes. The efferent arm is essential for metabolic regulation, modulation of fibrosis and biliary function and the control of a number of other processes. Experimental models have helped us to establish how: (i) the liver is innervated by the autonomic nervous system; and (ii) the cell types that are involved in these processes. Thus, the liver acts as both a sensor and effector that is influenced by neurological signals and ablation. Understanding these processes hold significant implications in disease processes such as diabetes and obesity, which are influenced by appetite and hormonal signals.

Enhanced insulin clearance in mice lacking TRPM8 channels

Blood glucose concentration is tightly regulated by the rate of insulin secretion and clearance, a process partially controlled by sensory neurons serving as metabolic sensors in relevant tissues. The activity of these neurons is regulated by the products of metabolism which regulate transmitter release, and recent evidence suggests that neuronally expressed ion channels of the transient receptor potential (TRP) family function in this critical process. Here, we report the novel finding that the cold and menthol-gated channel TRPM8 is necessary for proper insulin homeostasis. Mice lacking TRPM8 respond normally to a glucose challenge while exhibiting prolonged hypoglycemia in response to insulin. Additionally, Trpm8-/- mice have increased rates of insulin clearance compared with wild-type animals and increased expression of insulin-degrading enzyme in the liver. TRPM8 channels are not expressed in the liver, but TRPM8-expressing sensory afferents innervate the hepatic portal vein, suggesting a TRPM8-mediated neuronal control of liver insulin clearance. These results demonstrate that TRPM8 is a novel regulator of serum insulin and support the role of sensory innervation in metabolic homeostasis.

CNS insulin signaling in the control of energy homeostasis and glucose metabolism - from embryo to old age

Central nervous system (CNS) insulin signaling regulates energy and glucose homeostasis by acting on hypothalamic neurocircuits and higher brain circuits such as the dopaminergic system. However, overnutrition, obesity, and type 2 diabetes mellitus (T2DM) induce insulin resistance selectively in different regions of the brain, thereby impairing energy homeostasis and augmenting disease progression. Moreover, fetal hyperinsulinemia in response to maternal overnutrition, obesity, and diabetes disrupts hypothalamic neurocircuit development and predisposes to metabolic disorders later in life. In light of the current obesity and diabetes epidemic, we review the molecular basis of insulin action and resistance in the CNS, mechanisms which are causal to the development of these metabolic disorders, both in the neonate and in the adult.

Interaction between the central and peripheral effects of insulin in controlling hepatic glucose metabolism in the conscious dog

The importance of hypothalamic insulin action to the regulation of hepatic glucose metabolism in the presence of a normal liver/brain insulin ratio (3:1) is unknown. Thus, we assessed the role of central insulin action in the response of the liver to normal physiologic hyperinsulinemia over 4 h. Using a pancreatic clamp, hepatic portal vein insulin delivery was increased three- or eightfold in the conscious dog. Insulin action was studied in the presence or absence of intracerebroventricularly mediated blockade of hypothalamic insulin action. Euglycemia was maintained, and glucagon was clamped at basal. Both the molecular and metabolic aspects of insulin action were assessed. Blockade of hypothalamic insulin signaling did not alter the insulin-mediated suppression of hepatic gluconeogenic gene transcription but blunted the induction of glucokinase gene transcription and completely blocked the inhibition of glycogen synthase kinase-3β gene transcription. Thus, central and peripheral insulin action combined to control some, but not other, hepatic enzyme programs. Nevertheless, inhibition of hypothalamic insulin action did not alter the effects of the hormone on hepatic glucose flux (production or uptake). These data indicate that brain insulin action is not a determinant of the rapid (<4 h) inhibition of hepatic glucose metabolism caused by normal physiologic hyperinsulinemia in this large animal model.

Selective insulin and leptin resistance in metabolic disorders

Obesity represents a major risk factor for the development of insulin and leptin resistance, ultimately leading to a pleiotropic spectrum of metabolic alterations. However, resistance to both hormones does not uniformly affect all target cells and intracellular signaling pathways. In contrast, numerous clinical phenotypes arise from selective hormone resistance, leading to inhibition of defined intracellular signaling pathways in some tissues, while in other cell types hormone action is maintained or even overactivated. Here, we review the molecular mechanisms and clinical outcomes resulting from selective insulin and leptin resistance, which should ultimately guide future strategies for the treatment of obesity-associated diseases.

Evidence against a physiologic role for acute changes in CNS insulin action in the rapid regulation of hepatic glucose production

This Perspective will discuss the physiologic relevance of data that suggest CNS insulin action is required for the rapid suppression of hepatic glucose production. It will also review data from experiments on the conscious dog, which show that although the canine brain can sense insulin and, thereby, regulate hepatic glucoregulatory enzyme expression, CNS insulin action is not essential for the rapid suppression of glucose production caused by the hormone. Insulin's direct hepatic effects are dominant, thus it appears that insulin's central effects are redundant in the acute regulation of hepatic glucose metabolism.

Hypothalamic inflammation: a double-edged sword to nutritional diseases

The hypothalamus is one of the master regulators of various physiological processes, including energy balance and nutrient metabolism. These regulatory functions are mediated by discrete hypothalamic regions that integrate metabolic sensing with neuroendocrine and neural controls of systemic physiology. Neurons and nonneuronal cells in these hypothalamic regions act supportively to execute metabolic regulations. Under conditions of brain and hypothalamic inflammation, which may result from overnutrition-induced intracellular stresses or disease-associated systemic inflammatory factors, extracellular and intracellular environments of hypothalamic cells are disrupted, leading to central metabolic dysregulations and various diseases. Recent research has begun to elucidate the effects of hypothalamic inflammation in causing diverse components of metabolic syndrome leading to diabetes and cardiovascular disease. These new understandings have provocatively expanded previous knowledge on the cachectic roles of brain inflammatory response in diseases, such as infections and cancers. This review describes the molecular and cellular characteristics of hypothalamic inflammation in metabolic syndrome and related diseases as opposed to cachectic diseases, and also discusses concepts and potential applications of inhibiting central/hypothalamic inflammation to treat nutritional diseases.

Nutritional sensing and its utility in treating obesity

Obesity remains a major worldwide health problem, with current medical treatments being poorly effective. Nutrient sensing allows organs such as the GI tract and the brain to recognize and respond to fuel substrates such as carbohydrates, protein and fats. Specialized neural and hormonal pathways exist to facilitate and regulate these chemosensory mechanisms. Manipulation of factors involved in either central or peripheral chemosensory pathways may provide possible targets for the manipulation of appetite. However, further research is required to assess the utility of this approach to developing novel anti-obesity agents.

Liver afferents contribute to water drinking-induced sympathetic activation in human subjects: a clinical trial

Water drinking acutely increases sympathetic activity in human subjects. In animals, the response appears to be mediated through transient receptor potential channel TRPV4 activation on osmosensitive hepatic spinal afferents, described as osmopressor response. We hypothesized that hepatic denervation attenuates water drinking-induced sympathetic activation. We studied 20 liver transplant recipients (44±2.6 years, 1.2±0.1 years post transplant) as model of hepatic denervation and 20 kidney transplant recipients (43±2.6 years, 0.8±0.1 years post transplant) as immunosuppressive drug matched control group. Before and after 500 ml water ingestion, we obtained venous blood samples for catecholamine analysis. We also monitored brachial and finger blood pressure, ECG, and thoracic bioimpedance. Plasma norepinephrine concentration had changed by 0.01±0.07 nmol/l in liver and by 0.21±0.07 nmol/l in kidney transplant recipients (p<0.05 between groups) after 30–40 minutes of water drinking. While blood pressure and systemic vascular resistance increased in both groups, the responses tended to be attenuated in liver transplant recipients. Our findings support the idea that osmosensitive hepatic afferents are involved in water drinking-induced sympathetic activation in human subjects.

Metabolic clearance rate is a more robust and physiological parameter for insulin sensitivity than glucose infusion rate in the isoglycemic glucose clamp technique

BACKGROUND

The metabolic clearance rate (MCR) of glucose has been defined as the value of the glucose infusion rate (GIR) divided by the glucose concentration and could be thus expected to be a robust marker at various glucose concentrations.

METHODS

We evaluated the validity of MCR compared with GIR in 15 healthy subjects and 38 type 2 diabetes patients. The glucose clamp technique was performed at two different glucose levels-isoglycemia (fasting plasma glucose [FPG]) of each subject and euglycemia (100 mg/dL), consecutively. GIR and MCR were obtained at both glucose levels, and ratios of those at isoglycemia to euglycemia were calculated.

RESULTS

Although there was no obvious relationship between FPG levels and GIR ratio, the MCR ratio showed a good linear regression with FPG levels (r=-0.652, P<0.0001). Furthermore, MCR at FPG was excellently (r=0.955) correlated with that at euglycemia in comparison with the modest correlation of GIR between the two plasma glucose levels (r=0.876).

CONCLUSIONS

GIR was rather variable and not proportional to clamped plasma glucose levels. MCR was a less variable parameter than GIR at various plasma glucose levels, and MCR at FPG in the isoglycemic clamp study could be substituted for GIR at the euglycemic clamp study.

Brain insulin action augments hepatic glycogen synthesis without suppressing glucose production or gluconeogenesis in dogs

In rodents, acute brain insulin action reduces blood glucose levels by suppressing the expression of enzymes in the hepatic gluconeogenic pathway, thereby reducing gluconeogenesis and endogenous glucose production (EGP). Whether a similar mechanism is functional in large animals, including humans, is unknown. Here, we demonstrated that in canines, physiologic brain hyperinsulinemia brought about by infusion of insulin into the head arteries (during a pancreatic clamp to maintain basal hepatic insulin and glucagon levels) activated hypothalamic Akt, altered STAT3 signaling in the liver, and suppressed hepatic gluconeogenic gene expression without altering EGP or gluconeogenesis. Rather, brain hyperinsulinemia slowly caused a modest reduction in net hepatic glucose output (NHGO) that was attributable to increased net hepatic glucose uptake and glycogen synthesis. This was associated with decreased levels of glycogen synthase kinase 3β (GSK3β) protein and mRNA and with decreased glycogen synthase phosphorylation, changes that were blocked by hypothalamic PI3K inhibition. Therefore, we conclude that the canine brain senses physiologic elevations in plasma insulin, and that this in turn regulates genetic events in the liver. In the context of basal insulin and glucagon levels at the liver, this input augments hepatic glucose uptake and glycogen synthesis, reducing NHGO without altering EGP.

Peripheral glucose homeostasis: does brain insulin matter?

Much controversy surrounds the relative role of insulin signaling in the brain in the control of hepatic glucose metabolism. In this issue of the JCI, Ramnanan and colleagues demonstrate that arterial infusion of insulin into the brains of dogs reduces net hepatic glucose output without altering endogenous glucose production. However, this effect was modest and required both prolonged fasting and prolonged exposure of the brain to insulin, raising doubts about the overall physiological relevance of insulin action in the brain on hepatic glucose metabolism. Given the dominant direct role that insulin plays in inhibiting glucose production in the liver, we suggest that the main effect of central insulin on hepatic glucose metabolism may be more chronic and assume greater significance either when portal insulin is deficient, as occurs during exogenous insulin treatment of type 1 diabetes, or when chronic hyperinsulinemia and central insulin resistance develops, as occurs in type 2 diabetes.

Why does NAFLD predict type 2 diabetes?

Based on the "lipotoxic" hypothesis, the free fatty acid flux from the excessive amount of adipose tissue toward the peripheral tissues would induce the development of insulin resistance especially when the triglyceride storage or the concentration of intermediate fat metabolites (diacylglycerides, ceramides) within the cytoplasm of these cells become excessive. Nonalcoholic fatty liver disease (NAFLD) includes a wide spectrum of liver damage, ranging from simple steatosis to steatohepatitis and advanced fibrosis. NAFLD is associated with general and intra-abdominal obesity and with a reduced ability of insulin to stimulate metabolic pathways in the liver itself and in other tissues. There are animal models and models in human diseases sustaining the hypothesis that a primary hepatic disease may determine the development of type 2 diabetes (T2DM). Epidemiologic data generated on surrogate markers of NAFLD (transaminases and γ-glutamyltransferase), semiquantitative assessment of fatty liver (ultrasound), and surrogate algorithms of NAFLD also support a causative effect of NAFLD on the risk to develop T2DM. In spite of the presence of these indirect associations, a clear-cut link between NAFLD and abnormal β-cell function is yet to be reported. Therefore, more data are warranted to prove what is considered a likely causative relationship between NAFLD and T2DM.

CNS leptin and insulin action in the control of energy homeostasis

The obesity and diabetes pandemics have made it an urgent necessity to define the central nervous system (CNS) pathways controlling body weight, energy expenditure, and fuel metabolism. The pancreatic hormone insulin and the adipose tissue-derived leptin are known to act on diverse neuronal circuits in the CNS to maintain body weight and metabolism in a variety of species, including humans. Because these homeostatic circuits are disrupted during the development of obesity, the pathomechanisms leading to CNS leptin and insulin resistance are a focal point of research. In this review, we summarize the recent findings concerning the mechanisms and novel neuronal mediators of both insulin and leptin action in the CNS.

The role of the autonomic nervous liver innervation in the control of energy metabolism

Despite a longstanding research interest ever since the early work by Claude Bernard, the functional significance of autonomic liver innervation, either sympathetic or parasympathetic, is still ill defined. This scarcity of information not only holds for the brain control of hepatic metabolism, but also for the metabolic sensing function of the liver and the way in which this metabolic information from the liver affects the brain. Clinical information from the bedside suggests that successful human liver transplantation (implying a complete autonomic liver denervation) causes no life threatening metabolic derangements, at least in the absence of severe metabolic challenges such as hypoglycemia. However, from the benchside, data are accumulating that interference with the neuronal brain-liver connection does cause pronounced changes in liver metabolism. This review provides an extensive overview on how metabolic information is sensed by the liver, and how this information is processed via neuronal pathways to the brain. With this information the brain controls liver metabolism and that of other organs and tissues. We will pay special attention to the hypothalamic pathways involved in these liver-brain-liver circuits. At this stage, we still do not know the final destination and processing of the metabolic information that is transferred from the liver to the brain. On the other hand, in recent years, there has been a considerable increase in the understanding which brain areas are involved in the control of liver metabolism via its autonomic innervation. However, in view of the ever rising prevalence of type 2 diabetes, this potentially highly relevant knowledge is still by far too limited. Thus the autonomic innervation of the liver and its role in the control of metabolism needs our continued and devoted attention.

Insulin action on the liver in vivo

Insulin has a potent inhibitory effect on hepatic glucose production by direct action at hepatic receptors. The hormone also inhibits glucose production by suppressing both lipolysis in the fat cell and secretion of glucagon by the alpha-cell. Neural sensing of insulin levels appears to participate in control of hepatic glucose production in rodents, but a role for brain insulin sensing has not been documented in dogs or humans. The primary effect of insulin on the liver is its direct action.

Brain glucose sensing, counterregulation, and energy homeostasis

Neuronal circuits in the central nervous system play a critical role in orchestrating the control of glucose and energy homeostasis. Glucose, beside being a nutrient, is also a signal detected by several glucose-sensing units that are located at different anatomical sites and converge to the hypothalamus to cooperate with leptin and insulin in controlling the melanocortin pathway.

The effect of vagal cooling on canine hepatic glucose metabolism in the presence of hyperglycemia of peripheral origin

We examined the role of vagus nerves in the transmission of the portal glucose signal in conscious dogs. At time 0, somatostatin infusion was started along with intraportal insulin and glucagon at 4-fold basal and basal rates, respectively. Glucose was infused via a peripheral vein to create hyperglycemia ( approximately 2 fold basal). At t = 90, hollow coils around the vagus nerves were perfused with -10 degrees C or 37 degrees C solution in the vagally cooled (COOL) and sham-cooled (SHAM) groups, respectively (n = 6 per group). Effectiveness of vagal blockade was demonstrated by increase in heart rate during perfusion in the COOL vs SHAM groups (183 +/- 3 vs 102 +/- 5 beats per minute, respectively) and by prolapse of the third eyelid in the COOL group. Arterial plasma insulin (22 +/- 2 and 24 +/- 3 micro U/mL) and glucagon (37 +/- 5 and 40 +/- 4 pg/mL) concentrations did not change significantly between the first experimental period and the coil perfusion period in either the SHAM or COOL group, respectively. The hepatic glucose load throughout the entire experiment was 46 +/- 1 and 50 +/- 2 mg . kg(-1) . min(-1) in the SHAM and COOL groups, respectively. Net hepatic glucose uptake (NHGU) did not differ in the SHAM and COOL groups before (2.2 +/- 0.5 and 2.9 +/- 0.8 mg . kg(-1) . min(-1), respectively) or during the cooling period (3.0 +/- 0.5 and 3.4 +/- 0.6 mg . kg(-1) . min(-1), respectively). Likewise, net hepatic glucose fractional extraction and nonhepatic glucose uptake and clearance were not different between groups during coil perfusion. Interruption of vagal signaling in the presence of hyperinsulinemia and hyperglycemia resulting from peripheral glucose infusion did not affect NHGU, further supporting our previous suggestion that vagal input to the liver is not a primary determinant of NHGU.