A metabolic balance of GLP-1 and NMDA receptors in the brain.

Glucagon-like peptide-1 (GLP-1) and N-methyl-D-aspartate (NMDA) receptors in the brain regulate metabolic homeostasis. In a paper published in Nature, Petersen et al. describe a bimodal molecule that conjugates a GLP-1 analog with MK-801 (NMDA receptor antagonist), which lowers feeding and body weight to a greater extent than the GLP-1R agonist alone.

A glucose-sensing mechanism with glucose transporter 1 and pyruvate kinase in the area postrema regulates hepatic glucose production in rats.

The area postrema (AP) of the brain is exposed to circulating metabolites and hormones. However, whether AP detects glucose changes to exert biological responses remains unknown. Its neighboring nuclei, the nucleus tractus solitarius (NTS), responds to acute glucose infusion by inhibiting hepatic glucose production, but the mechanism also remains elusive. Herein, we characterized AP and NTS glucose-sensing mechanisms. Infusion of glucose into the AP, like the NTS, of chow rats suppressed glucose production during the pancreatic (basal insulin)-euglycemic clamps. Glucose transporter 1 or pyruvate kinase lentiviral-mediated knockdown in the AP negated AP glucose infusion to lower glucose production, while the glucoregulatory effect of NTS glucose infusion was also negated by knocking down glucose transporter 1 or pyruvate kinase in the NTS. Furthermore, we determined that high-fat (HF) feeding disrupts glucose infusion to lower glucose production in association with a modest reduction in the expression of glucose transporter 1, but not pyruvate kinase, in the AP and NTS. However, pyruvate dehydrogenase activator dichloroacetate infusion into the AP or NTS that enhanced downstream pyruvate metabolism and recapitulated the glucoregulatory effect of glucose in chow rats still failed to lower glucose production in HF rats. We discovered that a glucose transporter 1- and pyruvate kinase-dependent glucose-sensing mechanism in the AP (as well as the NTS) lowers glucose production in chow rats and that HF disrupts the glucose-sensing mechanism that is downstream of pyruvate metabolism in the AP and NTS. These findings highlight the role of AP and NTS in mediating glucose to regulate hepatic glucose production.

The Gut Microbiome: Connecting Diet, Glucose Homeostasis, and Disease.

Type 2 diabetes rates continue to rise unabated, underscoring the need to better understand the etiology and potential therapeutic options available for this disease. The gut microbiome plays a role in glucose homeostasis, and diabetes is associated with alterations in the gut microbiome. Given that consumption of a Western diet is associated with increased metabolic disease, and that a Western diet alters the gut microbiome, it is plausible that changes in the gut microbiota mediate the dysregulation in glucose homeostasis. In this review, we highlight a few of the most significant mechanisms by which the gut microbiome can influence glucose regulation, including changes in gut permeability, gut-brain signaling, and production of bacteria-derived metabolites like short-chain fatty acids and bile acids. A better understanding of these pathways could lead to the development of novel therapeutics to target the gut microbiome in order to restore glucose homeostasis in metabolic disease.

Human gut microbiota after bariatric surgery alters intestinal morphology and glucose absorption in mice independently of obesity.

OBJECTIVE: Bariatric surgery is an effective treatment for type 2 diabetes (T2D) that changes gut microbial composition. We determined whether the gut microbiota in humans after restrictive or malabsorptive bariatric surgery was sufficient to lower blood glucose. DESIGN: Women with obesity and T2D had biliopancreatic diversion with duodenal switch (BPD-DS) or laparoscopic sleeve gastrectomy (LSG). Faecal samples from the same patient before and after each surgery were used to colonise rodents, and determinants of blood glucose control were assessed. RESULTS: Glucose tolerance was improved in germ-free mice orally colonised for 7 weeks with human microbiota after either BPD-DS or LSG, whereas food intake, fat mass, insulin resistance, secretion and clearance were unchanged. Mice colonised with microbiota post-BPD-DS had lower villus height/width and crypt depth in the distal jejunum and lower intestinal glucose absorption. Inhibition of sodium-glucose cotransporter (Sglt)1 abrogated microbiota-transmissible improvements in blood glucose control in mice. In specific pathogen-free (SPF) rats, intrajejunal colonisation for 4 weeks with microbiota post-BPD-DS was sufficient to improve blood glucose control, which was negated after intrajejunal Sglt-1 inhibition. Higher Parabacteroides and lower Blautia coincided with improvements in blood glucose control after colonisation with human bacteria post-BPD-DS and LSG. CONCLUSION: Exposure of rodents to human gut microbiota after restrictive or malabsorptive bariatric surgery improves glycaemic control. The gut microbiota after bariatric surgery is a standalone factor that alters upper gut intestinal morphology and lowers Sglt1-mediated intestinal glucose absorption, which improves blood glucose control independently from changes in obesity, insulin or insulin resistance.

FXR in the dorsal vagal complex is sufficient and necessary for upper small intestinal microbiome-mediated changes of TCDCA to alter insulin action in rats.

OBJECTIVE: Conjugated bile acids are metabolised by upper small intestinal microbiota, and serum levels of taurine-conjugated bile acids are elevated and correlated with insulin resistance in people with type 2 diabetes. However, whether changes in taurine-conjugated bile acids are necessary for small intestinal microbiome to alter insulin action remain unknown. DESIGN: We evaluated circulating and specifically brain insulin action using the pancreatic-euglycaemic clamps in high-fat (HF) versus chow fed rats with or without upper small intestinal healthy microbiome transplant. Chemical and molecular gain/loss-of-function experiments targeting specific taurine-conjugated bile acid-induced changes of farnesoid X receptor (FXR) in the brain were performed in parallel. RESULTS: We found that short-term HF feeding increased the levels of taurochenodeoxycholic acid (TCDCA, an FXR ligand) in the upper small intestine, ileum, plasma and dorsal vagal complex (DVC) of the brain. Transplantation of upper small intestinal healthy microbiome into the upper small intestine of HF rats not only reversed the rise of TCDCA in all reported tissues but also enhanced the ability of either circulating hyperinsulinaemia or DVC insulin action to lower glucose production. Further, DVC infusion of TCDCA or FXR agonist negated the enhancement of insulin action, while genetic knockdown or chemical inhibition of FXR in the DVC of HF rats reversed insulin resistance. CONCLUSION: Our findings indicate that FXR in the DVC is sufficient and necessary for upper small intestinal microbiome-mediated changes of TCDCA to alter insulin action in rats, and highlight a previously unappreciated TCDCA-FXR axis linking gut microbiome and host insulin action.

Peripheral and central regulation of insulin by the intestine and microbiome.

Blood glucose and insulin homeostasis is disrupted during the progression of type 2 diabetes. Insulin levels and action are regulated by both peripheral and central responses that involve the intestine and microbiome. The intestine and its microbiota process nutrients and generate molecules that influence blood glucose and insulin. Peripheral insulin regulation is regulated by gut-segment-dependent nutrient sensing and microbial factors such as short-chain fatty acids and bile acids that engage G-protein-coupled receptors. Innate immune sensing of gut-derived bacterial cell wall components and lipopolysaccharides also alter insulin homeostasis. These bacterial metabolites and postbiotics influence insulin secretion and insulin clearance in part by altering endocrine responses such as glucagon-like peptide-1. Gut-derived bacterial factors can promote inflammation and insulin resistance, but other postbiotics can be insulin sensitizers. In parallel, activation of small intestinal sirtuin 1 increases insulin sensitivity by reversing high fat-induced hypothalamic insulin resistance through a gut-brain neuronal axis, whereas high fat-feeding alters small intestinal microbiome and increases taurochenodeoxycholic acid in the plasma and the dorsal vagal complex to induce insulin resistance. In summary, emerging evidence indicates that intestinal molecular signaling involving nutrient sensing and the host-microbe symbiosis alters insulin homeostasis and action. Gut-derived host endocrine and paracrine factors as well as microbial metabolites act on the liver, pancreas, and the brain, and in parallel on the gut-brain neuronal axis. Understanding common nodes of peripheral and central insulin homeostasis and action may reveal new ways to target the intestinal host-microbe relationship in obesity, metabolic disease, and type 2 diabetes.

Leptin enhances hypothalamic lactate dehydrogenase A (LDHA)-dependent glucose sensing to lower glucose production in high-fat-fed rats.

The responsiveness of glucose sensing per se to regulate whole-body glucose homeostasis is dependent on the ability of a rise in glucose to lower hepatic glucose production and increase peripheral glucose uptake in vivo In both rodents and humans, glucose sensing is lost in diabetes and obesity, but the site(s) of impairment remains elusive. Here, we first report that short-term high-fat feeding disrupts hypothalamic glucose sensing to lower glucose production in rats. Second, leptin administration into the hypothalamus of high-fat-fed rats restored hypothalamic glucose sensing to lower glucose production during a pancreatic (basal insulin)-euglycemic clamp and increased whole-body glucose tolerance during an intravenous glucose tolerance test. Finally, both chemical inhibition of hypothalamic lactate dehydrogenase (LDH) (achieved via hypothalamic LDH inhibitor oxamate infusion) and molecular knockdown of LDHA (achieved via hypothalamic lentiviral LDHA shRNA injection) negated the ability of hypothalamic leptin infusion to enhance glucose sensing to lower glucose production in high fat-fed rats. In summary, our findings illustrate that leptin enhances LDHA-dependent glucose sensing in the hypothalamus to lower glucose production in high-fat-fed rodents in vivo.

Activation of Short and Long Chain Fatty Acid Sensing Machinery in the Ileum Lowers Glucose Production in Vivo.

Evidence continues to emerge detailing the myriad of ways the gut microbiota influences host energy homeostasis. Among the potential mechanisms, short chain fatty acids (SCFAs), the byproducts of microbial fermentation of dietary fibers, exhibit correlative beneficial metabolic effects in humans and rodents, including improvements in glucose homeostasis. The underlying mechanisms, however, remain elusive. We here report that one of the main bacterially produced SCFAs, propionate, activates ileal mucosal free fatty acid receptor 2 to trigger a negative feedback pathway to lower hepatic glucose production in healthy rats in vivo We further demonstrate that an ileal glucagon-like peptide-1 receptor-dependent neuronal network is necessary for ileal propionate and long chain fatty acid sensing to regulate glucose homeostasis. These findings highlight the potential to manipulate fatty acid sensing machinery in the ileum to regulate glucose homeostasis.

Glucagon signalling in the dorsal vagal complex is sufficient and necessary for high-protein feeding to regulate glucose homeostasis in vivo.

High-protein feeding acutely lowers postprandial glucose concentration compared to low-protein feeding, despite a dichotomous rise of circulating glucagon levels. The physiological role of this glucagon rise has been largely overlooked. We here first report that glucagon signalling in the dorsal vagal complex (DVC) of the brain is sufficient to lower glucose production by activating a Gcgr-PKA-ERK-KATP channel signalling cascade in the DVC of rats in vivo. We further demonstrate that direct blockade of DVC Gcgr signalling negates the acute ability of high- vs. low-protein feeding to reduce plasma glucose concentration, indicating that the elevated circulating glucagon during high-protein feeding acts in the brain to lower plasma glucose levels. These data revise the physiological role of glucagon and argue that brain glucagon signalling contributes to glucose homeostasis during dietary protein intake.

Glucagon action in the brain.

In recent years, novel discoveries have reshaped our understanding of the biology of brain glucagon in the regulation of peripheral homeostasis. Here we compare and contrast brain glucagon action in feeding vs glucose regulation and depict the physiological relevance of brain glucagon by reviewing their actions in two key regions of the central nervous system: the mediobasal hypothalamus and the dorsal vagal complex. These novel findings pave the way to future therapeutic strategies aimed at enhancing brain glucagon action for the treatment of diabetes and obesity. This review summarises a presentation given at the 'Novel data on glucagon' symposium at the 2015 annual meeting of the EASD. It is accompanied by two other reviews on topics from this symposium (by Young Lee and colleagues, DOI: 10.1007/s00125-016-3965-9 ), and by Russell Miller and Morris Birnbaum, DOI: 10.1007/s00125-016-3955-y ) and an overview by the Session Chair, Isabel Valverde (DOI: 10.1007/s00125-016-3946-z ).

Hormonal signaling in the gut.

The gut is anatomically positioned to play a critical role in the regulation of metabolic homeostasis, providing negative feedback via nutrient sensing and local hormonal signaling. Gut hormones, such as cholecystokinin (CCK) and glucagon-like peptide-1 (GLP-1), are released following a meal and act on local receptors to regulate glycemia via a neuronal gut-brain axis. Additionally, jejunal nutrient sensing and leptin action are demonstrated to suppress glucose production, and both are required for the rapid antidiabetic effect of duodenal jejunal bypass surgery. Strategies aimed at targeting local gut hormonal signaling pathways may prove to be efficacious therapeutic options to improve glucose control in diabetes.

FFA-induced hepatic insulin resistance in vivo is mediated by PKCδ, NADPH oxidase, and oxidative stress.

Fat-induced hepatic insulin resistance plays a key role in the pathogenesis of type 2 diabetes in obese individuals. Although PKC and inflammatory pathways have been implicated in fat-induced hepatic insulin resistance, the sequence of events leading to impaired insulin signaling is unknown. We used Wistar rats to investigate whether PKCδ and oxidative stress play causal roles in this process and whether this occurs via IKKß- and JNK-dependent pathways. Rats received a 7-h infusion of Intralipid plus heparin (IH) to elevate circulating free fatty acids (FFA). During the last 2 h of the infusion, a hyperinsulinemic-euglycemic clamp with tracer was performed to assess hepatic and peripheral insulin sensitivity. An antioxidant, N-acetyl-L-cysteine (NAC), prevented IH-induced hepatic insulin resistance in parallel with prevention of decreased IκBα content, increased JNK phosphorylation (markers of IKKß and JNK activation, respectively), increased serine phosphorylation of IRS-1 and IRS-2, and impaired insulin signaling in the liver without affecting IH-induced hepatic PKCδ activation. Furthermore, an antisense oligonucleotide against PKCδ prevented IH-induced phosphorylation of p47(phox) (marker of NADPH oxidase activation) and hepatic insulin resistance. Apocynin, an NADPH oxidase inhibitor, prevented IH-induced hepatic and peripheral insulin resistance similarly to NAC. These results demonstrate that PKCδ, NADPH oxidase, and oxidative stress play a causal role in FFA-induced hepatic insulin resistance in vivo and suggest that the pathway of FFA-induced hepatic insulin resistance is FFA → PKCδ → NADPH oxidase and oxidative stress → IKKß/JNK → impaired hepatic insulin signaling.

Glucagon and lipid signaling in the hypothalamus.

Hyperglycemia, caused in part by elevated hepatic glucose production (GP), is a hallmark feature of diabetes and obesity. The hypothalamus responds to hormones and nutrients to regulate hepatic GP and glucose homeostasis. This invited perspective focuses on the molecular signaling and biochemical pathways involved in the gluco-regulatory action of hypothalamic glucagon signaling and lipid sensing in health and disease. Recent evidence generated via genetic, molecular and chemical experimental approaches indicates that glucagon and lipid signaling independently trigger complementary hypothalamic mechanisms to lower GP. Thus, targeting hypothalamic glucagon or lipid signaling may have therapeutic potential in diabetes and obesity.

Insulin action in the hypothalamus and dorsal vagal complex.

Insulin resistance is a hallmark feature of type 2 diabetes and obesity. In addition to the classical view that insulin resistance in the liver, muscle and fat disrupts glucose homeostasis, studies in the past decade have illustrated that insulin resistance in the hypothalamus dysregulates hepatic glucose production and food intake, leading to type 2 diabetes and obesity. This invited review argues that in addition to the hypothalamus, insulin signalling in the dorsal vagal complex regulates hepatic glucose production and food intake. A thorough understanding of the physiological and pathophysiological mechanisms of insulin action in the hypothalamus and dorsal vagal complex is necessary in order to identify therapeutic targets for obesity and type 2 diabetes.

Glycine normalizes hepatic triglyceride-rich VLDL secretion by triggering the CNS in high-fat fed rats.

RATIONALE: Dysregulation of hepatic triglyceride (TG)-rich very low-density lipoproteins (VLDL-TG) in obesity and type 2 diabetes contributes to the dyslipidemia that leads to cardiovascular morbidity. The central nervous system (CNS), particularly the hypothalamus, regulates hepatic lipid metabolism. Although the underlying neurocircuitry remains elusive, glycine has been documented to enhance CNS N-methyl-d-aspartate (NMDA) receptor-mediated transmission. OBJECTIVE: We tested the hypothesis that glycine regulates hepatic VLDL-TG secretion by potentiating NMDA receptor-mediated transmission in the CNS. METHODS AND RESULTS: Using 10-hour fasted male Sprague-Dawley rats implanted with stereotaxic cannulae into an extrahypothalamic region termed the dorsal vagal complex (DVC) and vascular catheters to enable direct DVC infusion and blood sampling, respectively, the rate of hepatic VLDL-TG secretion was measured following tyloxapol (an inhibitor of lipoprotein lipase) injection. Direct DVC infusion of glycine lowered VLDL-TG secretion, whereas NMDA receptor blocker MK-801 fully negated glycine's effect. NR1 subunit of NMDA receptor antagonist 7-chlorokynurenic acid, adenoviral injection of NR1 short hairpin RNA (shRNA), and hepatic vagotomy also nullified glycine's effect. Finally, DVC glycine normalized the hypersecretion of VLDL-TG induced by high-fat feeding. CONCLUSIONS: Molecular and pharmacological inhibition of the NR1-containing NMDA receptors in the DVC negated the ability of glycine to inhibit hepatic secretion of VLDL-TG in vivo. Importantly, the hypersecretion of VLDL-TG from the liver induced by a model of high-fat feeding was restored by the hepatic lipid control of CNS glycine sensing. These findings collectively suggest that glycine or glycine analogues may have therapeutic benefits in lowering plasma lipid levels in diabetes and obesity by triggering the CNS.

Brain glucose metabolism controls the hepatic secretion of triglyceride-rich lipoproteins.

Increased production of very low-density lipoprotein (VLDL) is a critical feature of the metabolic syndrome. Here we report that a selective increase in brain glucose lowered circulating triglycerides (TG) through the inhibition of TG-VLDL secretion by the liver. We found that the effect of glucose required its conversion to lactate, leading to activation of ATP-sensitive potassium channels and to decreased hepatic activity of stearoyl-CoA desaturase-1 (SCD1). SCD1 catalyzed the synthesis of oleyl-CoA from stearoyl-CoA. Curtailing the liver activity of SCD1 was sufficient to lower the hepatic levels of oleyl-CoA and to recapitulate the effects of central glucose administration on VLDL secretion. Notably, portal infusion of oleic acid restored hepatic oleyl-CoA to control levels and negated the effects of both central glucose and SCD1 deficiency on TG-VLDL secretion. These central effects of glucose (but not those of lactate) were rapidly lost in diet-induced obesity. These findings indicate that a defect in brain glucose sensing could play a critical role in the etiology of the metabolic syndrome.

Upper intestinal lipids trigger a gut-brain-liver axis to regulate glucose production.

Energy and glucose homeostasis are regulated by food intake and liver glucose production, respectively. The upper intestine has a critical role in nutrient digestion and absorption. However, studies indicate that upper intestinal lipids inhibit food intake as well in rodents and humans by the activation of an intestine-brain axis. In parallel, a brain-liver axis has recently been proposed to detect blood lipids to inhibit glucose production in rodents. Thus, we tested the hypothesis that upper intestinal lipids activate an intestine-brain-liver neural axis to regulate glucose homeostasis. Here we demonstrate that direct administration of lipids into the upper intestine increased upper intestinal long-chain fatty acyl-coenzyme A (LCFA-CoA) levels and suppressed glucose production. Co-infusion of the acyl-CoA synthase inhibitor triacsin C or the anaesthetic tetracaine with duodenal lipids abolished the inhibition of glucose production, indicating that upper intestinal LCFA-CoAs regulate glucose production in the preabsorptive state. Subdiaphragmatic vagotomy or gut vagal deafferentation interrupts the neural connection between the gut and the brain, and blocks the ability of upper intestinal lipids to inhibit glucose production. Direct administration of the N-methyl-d-aspartate ion channel blocker MK-801 into the fourth ventricle or the nucleus of the solitary tract where gut sensory fibres terminate abolished the upper-intestinal-lipid-induced inhibition of glucose production. Finally, hepatic vagotomy negated the inhibitory effects of upper intestinal lipids on glucose production. These findings indicate that upper intestinal lipids activate an intestine-brain-liver neural axis to inhibit glucose production, and thereby reveal a previously unappreciated pathway that regulates glucose homeostasis.

Obesity and the kidney: mechanistic links and therapeutic advances.

Obesity is strongly associated with the development of diabetes mellitus and chronic kidney disease (CKD), but there is evidence for a bidirectional relationship wherein the kidney also acts as a key regulator of body weight. In this Review, we highlight the mechanisms implicated in obesity-related CKD, and outline how the kidney might modulate feeding and body weight through a growth differentiation factor 15-dependent kidney-brain axis. The favourable effects of bariatric surgery on kidney function are discussed, and medical therapies designed for the treatment of diabetes mellitus that lower body weight and preserve kidney function independent of glycaemic lowering, including sodium-glucose cotransporter 2 inhibitors, incretin-based therapies and metformin, are also reviewed. In summary, we propose that kidney function and body weight are related in a bidirectional fashion, and that this interrelationship affects human health and disease.

Acute Activation of GFRAL in the Area Postrema Contributes to Glucose Regulation Independent of Weight.

GDF15 regulates energy balance and glucose homeostasis in rodents by activating its receptor GFRAL, expressed in the area postrema of the brain. However, whether GDF15-GFRAL signaling in the area postrema regulates glucose tolerance independent of changes in food intake and weight and contributes to the glucose-lowering effect of metformin remain unknown. Herein, we report that direct, acute GDF15 infusion into the area postrema of rats fed a high-fat diet increased intravenous glucose tolerance and insulin sensitivity to lower hepatic glucose production independent of changes in food intake, weight, and plasma insulin levels under conscious, unrestrained, and nonstressed conditions. In parallel, metformin infusion concurrently increased plasma GDF15 levels and glucose tolerance. Finally, a knockdown of GFRAL expression in the area postrema negated administration of GDF15, as well as metformin, to increase glucose tolerance independent of changes in food intake, weight, and plasma insulin levels. In summary, activation of GFRAL in the area postrema contributes to glucose regulation of GDF15 and metformin in vivo.