Sensing of glucose in the gastrointestinal tract

Neuroimmune modulation in liver pathophysiology

The liver, the largest organ in the human body, plays a multifaceted role in digestion, coagulation, synthesis, metabolism, detoxification, and immune defense. Changes in liver function often coincide with disruptions in both the central and peripheral nervous systems. The intricate interplay between the nervous and immune systems is vital for maintaining tissue balance and combating diseases. Signaling molecules and pathways, including cytokines, inflammatory mediators, neuropeptides, neurotransmitters, chemoreceptors, and neural pathways, facilitate this complex communication. They establish feedback loops among diverse immune cell populations and the central, peripheral, sympathetic, parasympathetic, and enteric nervous systems within the liver. In this concise review, we provide an overview of the structural and compositional aspects of the hepatic neural and immune systems. We further explore the molecular mechanisms and pathways that govern neuroimmune communication, highlighting their significance in liver pathology. Finally, we summarize the current clinical implications of therapeutic approaches targeting neuroimmune interactions and present prospects for future research in this area.

Brain-Body Control of Glucose Homeostasis-Insights From Model Organisms

Tight regulation of blood glucose is essential for long term health. Blood glucose levels are defended by the correct function of, and communication between, internal organs including the gastrointestinal tract, pancreas, liver, and brain. Critically, the brain is sensitive to acute changes in blood glucose level and can modulate peripheral processes to defend against these deviations. In this mini-review we highlight select key findings showcasing the utility, strengths, and limitations of model organisms to study brain-body interactions that sense and control blood glucose levels. First, we discuss the large platform of genetic tools available to investigators studying mice and how this field may yet reveal new modes of communication between peripheral organs and the brain. Second, we discuss how rats, by virtue of their size, have unique advantages for the study of CNS control of glucose homeostasis and note that they may more closely model some aspects of human (patho)physiology. Third, we discuss the nascent field of studying the CNS control of blood glucose in the zebrafish which permits ease of genetic modification, large-scale measurements of neural activity and live imaging in addition to high-throughput screening. Finally, we briefly discuss glucose homeostasis in drosophila, which have a distinct physiology and glucoregulatory systems to vertebrates.

The gut microbiota to the brain axis in the metabolic control

The regulation of glycemia is under a tight neuronal detection of glucose levels performed by the gut-brain axis and an efficient efferent neuronal message sent to the peripheral organs, as the pancreas to induce insulin and inhibit glucagon secretions. The neuronal detection of glucose levels is performed by the autonomic nervous system including the enteric nervous system and the vagus nerve innervating the gastro-intestinal tractus, from the mouth to the anus. A dysregulation of this detection leads to the one of the most important current health issue around the world i.e. diabetes mellitus. Furthemore, the consequences of diabetes mellitus on neuronal homeostasis and activities participate to the aggravation of the disease establishing a viscious circle. Prokaryotic cells as bacteria, reside in our gut. The strong relationship between prokaryotic cells and our eukaryotic cells has been established long ago, and prokaryotic and eukaryotic cells in our body have evolved synbiotically. For the last decades, studies demonstrated the critical role of the gut microbiota on the metabolic control and how its shift can induce diseases such as diabetes. Despite an important increase of knowledge, few is known about 1) how the gut microbiota influences the neuronal detection of glucose and 2) how the diabetes mellitus-induced gut microbiota shift observed participates to the alterations of autonomic nervous system and the gut-brain axis activity.

Sugars, Sweet Taste Receptors, and Brain Responses

Sweet taste receptors are composed of a heterodimer of taste 1 receptor member 2 (T1R2) and taste 1 receptor member 3 (T1R3). Accumulating evidence shows that sweet taste receptors are ubiquitous throughout the body, including in the gastrointestinal tract as well as the hypothalamus. These sweet taste receptors are heavily involved in nutrient sensing, monitoring changes in energy stores, and triggering metabolic and behavioral responses to maintain energy balance. Not surprisingly, these pathways are heavily regulated by external and internal factors. Dysfunction in one or more of these pathways may be important in the pathogenesis of common diseases, such as obesity and type 2 diabetes mellitus.

Serotonin-secreting enteroendocrine cells respond via diverse mechanisms to acute and chronic changes in glucose availability

Background

Enteroendocrine cells collectively constitute our largest endocrine tissue, with serotonin (5-HT) secreting enterochromaffin (EC) cells being the largest component (~50 %). This gut-derived 5-HT has multiple paracrine and endocrine roles. EC cells are thought to act as nutrient sensors and luminal glucose is the major absorbed form of carbohydrate in the gut and activates secretion in an array of cell types. It is unknown whether EC cells release 5-HT in response to glucose in primary EC cells. Furthermore, fasting augments 5-HT synthesis and release into the circulation. However, which nutrients cause fasting-induced synthesis of EC cell 5-HT is unknown. Here we examine the effects of acute and chronic changes in glucose availability on 5-HT release from intact tissue and single EC cells.

Methods

We utilised established approaches in our laboratories measuring 5-HT release in intact mouse colon with amperometry. We then examined single EC cells function using our published protocol in guinea-pig colon. Single cell Ca²⁺ imaging and amperometry were used with these cells. Real-time PCR was used along with amperometry, on primary EC cells cultured for 24 h in 5 or 25 mM glucose.

Results

We demonstrate that acute increases in glucose, at levels found in the gut lumen rather than in plasma, trigger 5-HT release from intact colon, and cause Ca²⁺ entry and 5-HT release in primary EC cells. Single cell amperometry demonstrates that high glucose increases the amount of 5-HT released from individual vesicles as they undergo exocytosis. Finally, 24 h incubation of EC cells in low glucose causes an increase in the transcription of the 5-HT synthesising enzyme Tph1 as well as increasing in 5-HT secretion in EC cells.

Conclusions

We demonstrate that primary EC cells respond to acute changes in glucose availability through increases in intracellular Ca²⁺ the activation of 5-HT secretion, but respond to chronic changes in glucose levels through the transcriptional regulation of Tph1 to alter 5-HT synthesis.

Hypothalamic glucose sensing: making ends meet

The neuroendocrine system governs essential survival and homeostatic functions. For example, growth is needed for development, thermoregulation maintains optimal core temperature in a changing environment, and reproduction ensures species survival. Stress and immune responses enable an organism to overcome external and internal threats while the circadian system regulates arousal and sleep such that vegetative and active functions do not overlap. All of these functions require a significant portion of the body's energy. As the integrator of the neuroendocrine system, the hypothalamus carefully assesses the energy status of the body in order to appropriately partition resources to provide for each system without compromising the others. While doing so the hypothalamus must ensure that adequate glucose levels are preserved for brain function since glucose is the primary fuel of the brain. To this end, the hypothalamus contains specialized glucose sensing neurons which are scattered throughout the nuclei controlling distinct neuroendocrine functions. We hypothesize that these neurons play a key role in enabling the hypothalamus to partition energy to meet these peripheral survival needs without endangering the brain's glucose supply. This review will first describe the varied mechanisms underlying glucose sensing in neurons within discrete hypothalamic nuclei. We will then evaluate the way in which peripheral energy status regulates glucose sensitivity. For example, during energy deficit such as fasting specific hypothalamic glucose sensing neurons become sensitized to decreased glucose. This increases the gain of the information relay when glucose availability is a greater concern for the brain. Finally, changes in glucose sensitivity under pathological conditions (e.g., recurrent insulin-hypoglycemia, diabetes) will be addressed. The overall goal of this review is to place glucose sensing neurons within the context of hypothalamic control of neuroendocrine function.

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.

Physiological roles of dietary glutamate signaling via gut-brain axis due to efficient digestion and absorption

Dietary glutamate (Glu) stimulates to evoke the umami taste, one of the five basic tastes, enhancing food palatability. But it is also the main gut energy source for the absorption and metabolism for each nutrient, thus, only a trace amount of Glu reaches the general circulation. Recently, we demonstrated a unique gut sensing system for free Glu (glutamate signaling). Glu is the only nutrient among amino acids, sugars and electrolytes that activates rat gastric vagal afferents from the luminal side specifically via metabotropic Glu receptors type 1 on mucosal cells releasing mucin and nitrite mono-oxide (NO), then NO stimulates serotonin (5HT) release at the enterochromaffin cell. Finally released 5HT stimulates 5HT3 receptor at the nerve end of the vagal afferent fiber. Functional magnetic resonance imaging (f-MRI, 4.7 T) analysis revealed that luminal sensing with 1 % (w/v) monosodium L-glutamate (MSG) in rat stomach activates both the medial preoptic area (body temperature controller) and the dorsomedial hypothalamus (basic metabolic regulator), resulting in diet-induced thermogenesis during mealing without changes of appetite for food. Interestingly, rats were forced to eat a high fat and high sugar diet with free access to 1 % (w/w) MSG and water in a choice paradigm and showed the strong preference for the MSG solution and subsequently, they displayed lower fat deposition, weight gain and blood leptin. On the other hand, these brain functional changes by the f-MRI signal after 60 mM MSG intubation into the stomach was abolished in the case of total vagotomized rats, suggesting that luminal glutamate signaling contributes to control digestion and thermogenesis without obesity.

Glucose sensing neurons in the ventromedial hypothalamus

Neurons whose activity is regulated by glucose are found in a number of brain regions. Glucose-excited (GE) neurons increase while glucose-inhibited (GI) neurons decrease their action potential frequency as interstitial brain glucose levels increase. We hypothesize that these neurons evolved to sense and respond to severe energy deficit (e.g., fasting) that threatens the brains glucose supply. During modern times, they are also important for the restoration of blood glucose levels following insulin-induced hypoglycemia. Our data suggest that impaired glucose sensing by hypothalamic glucose sensing neurons may contribute to the syndrome known as hypoglycemia-associated autonomic failure in which the mechanisms which restore euglycemia following hypoglycemia become impaired. On the other hand, increased responses of glucose sensing neurons to glucose deficit may play a role in the development of Type 2 Diabetes Mellitus and obesity. This review will discuss the mechanisms by which glucose sensing neurons sense changes in interstitial glucose and explore the roles of these specialized glucose sensors in glucose and energy homeostasis.

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.

New therapeutic strategy for amino acid medicine: effects of dietary glutamate on gut and brain function

The gustatory and visceral stimulation from food regulates digestion and nutrient utilization, and free glutamate (Glu) release from food is responsible for the umami taste perception that increases food palatability. The results of recent studies reveal a variety of physiological roles for Glu. For example, luminal applications of Glu into the mouth, stomach, and intestine increase the afferent nerve activities of the glossopharyngeal nerve, the gastric branch of the vagus nerve, and the celiac branch of the vagus nerve, respectively. Additionally, luminal Glu evokes efferent nerve activation of each branch of the abdominal vagus nerve. The intragastric administration of Glu activates several brain areas (e.g., insular cortex, limbic system, and hypothalamus) and has been shown to induce flavor-preference learning in rats. Functional magnetic resonance imaging of rats has shown that the intragastric administration of Glu activates the nucleus tractus solitarius, amygdala, and lateral hypothalamus. In addition, Glu may increase flavor preference as a result of its postingestive effect. Considering these results, we propose that dietary Glu functions as a signal for the regulation of the gastrointestinal tract via the gut-brain axis and contributes to the maintenance of a healthy life.

Rapid post-oral stimulation of intake and flavor conditioning by glucose and fat in the mouse

Although widely assumed to have only satiating actions, nutrients in the gut can also condition increases in intake in some cases. Here we studied the time course of post-oral nutrient stimulation of ingestion in food-restricted C57BL/6J mice. In experiment 1, mice adapted to drink a 0.8% sucralose solution 1 h/day, rapidly increased their rate of licking (within 4-6 min) when first tested with an 8% glucose solution and even more so in tests 2 and 3. Other mice decreased their licking rate when switched from sucralose to 8% fructose, a sugar that is sweet like glucose but lacks positive post-oral effects in mice. The glucose-stimulated drinking is due to the sugar's post-oral rather than taste properties, because sucralose is highly preferred to glucose and fructose in brief choice tests. A second experiment showed that the glucose-stimulated ingestion is associated with a conditioned flavor preference in both intact and capsaicin-treated mice. This indicates that the post-oral stimulatory action of glucose is not mediated by capsaicin-sensitive visceral afferents. In experiment 3, mice consumed flavored saccharin solutions as they self-infused water or glucose via an intragastric (IG) catheter. The glucose self-infusion stimulated ingestion within 13-15 min in test 1 and produced a conditioned increase in licking that was apparent in the initial minute of tests 2 and 3. Experiment 4 revealed that IG self-infusions of a fat emulsion also resulted in post-oral stimulation of licking in test 1 and conditioned increases in tests 2 and 3. These findings indicate that glucose and fat can generate stimulatory post-oral signals early in a feeding session that increase ongoing ingestion and condition increases in flavor acceptance and preference revealed in subsequent feeding sessions. The test procedures developed here can be used to investigate the peripheral and central processes involved in stimulation of intake by post-oral nutrients.

Intestinal glucose-induced calcium-calmodulin kinase signaling in the gut-brain axis in awake rats

BACKGROUND

Lumenal glucose initiates changes in gastrointestinal (GI) function, including inhibition of gastric emptying, stimulation of pancreatic exocrine and endocrine secretion, and intestinal fluid secretion. Glucose stimulates the release of GI hormones and 5-hydroxytryptamine (5-HT), and activates intrinsic and extrinsic neuronal pathways to initiate changes in GI function. The precise mechanisms involved in luminal glucose-sensing are not clear; studying gut endocrine cells is difficult due to their sparse and irregular localization within the epithelium.

METHODS

Here we show a technique to determine activation of gut epithelial cells and the gut-brain pathway in vivo in rats using immunohistochemical detection of the activated, phosphorylated, form of calcium-calmodulin kinase II (pCaMKII).

KEY RESULTS

Perfusion of the gut with glucose (60 mg) increased pCaMKII immunoreactivity in 5-HT-expressing enterochromaffin (EC) cells, cytokeratin-18 immunopositive brush cells, but not in enterocytes or cholecystokinin-expressing cells. Lumenal glucose increased pCaMKII in neurons in the myenteric plexus and nodose ganglion, nucleus of the solitary tract, dorsal motor nucleus of the vagus and the arcuate nucleus. pCaMKII expression in neurons, but not in EC cells, was significantly attenuated by pretreatment with the 5-HT(3) R antagonist ondansetron. Deoxynojirimycin, a selective agonist for the putative glucose sensor, sodium-glucose cotransporter-3 (SGLT-3), mimicked the effects of glucose with increased pCaMKII in ECs and neurons; galactose had no effect.

CONCLUSIONS & INFERENCES

The data suggest that native EC cells in situ respond to glucose, possibly via SGLT-3, to activate intrinsic and extrinsic neurons and thereby regulate GI function.

Starch with high amylose and low in vitro digestibility increases short-chain fatty acid absorption, reduces peak insulin secretion, and modulates incretin secretion in pigs

Diets containing different starch types affect peripheral glucose and insulin responses. However, the role of starch chemistry in kinetics of nutrient absorption and insulin and incretin secretion is poorly understood. Four portal vein-catheterized pigs (35.0 ± 0.2 kg body weight) consumed 4 diets containing 70% purified starch [0-63.2% amylose content and 0.22 (slowly) to 1.06%/min (rapidly) maximum rate of in vitro digestion] for 7-d periods in a 4 × 4 Latin square. On d 7, blood was collected for 12 h postprandial with simultaneous blood flow measurement for determining the net portal appearance (NPA) of nutrients and hormones. The NPA of glucose, insulin, C-peptide, and glucose-dependent insulinotropic polypeptide (GIP) during 0-4 h postprandial were lower (P < 0.05) and those of butyrate and total SCFA were higher (P < 0.05) when pigs consumed the diet containing slowly digestible compared with rapidly digestible starch. The peak NPA of insulin occurred prior to that of glucose when pigs consumed diets containing rapidly digestible starch. The kinetics of insulin secretion had a linear positive relation with kinetics of NPA of glucose (R(2) = 0.50; P < 0.01). In conclusion, starch with high amylose and low in vitro digestibility decreases the kinetics of glucose absorption and insulin and GIP secretion and increases SCFA absorption and glucagon-like peptide-1 secretion. In conclusion, starch with high amylose content and a lower rate and extent of in vitro digestion decreased glucose absorption and insulin secretion and increased SCFA absorption.

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.

Post-oral infusion sites that support glucose-conditioned flavor preferences in rats

Rats learn to prefer a flavored solution (CS+) paired with a gastrointestinal glucose infusion over an alternate flavor (CS-) paired with a non-caloric infusion. Prior work implicates a post-gastric site of glucose action, which is the focus of this study. In Exp. 1, male rats (8-10/group) were infused in the duodenum (ID), mid-jejunum (IJ), or distal ileum (II) with 8% glucose or water as they drank saccharin-sweetened CS+ and CS- solutions, respectively, in one-bottle 30-min sessions. Two-bottle tests (no infusions) were followed by a second train-test cycle. By the second test, the ID and IJ groups preferred the CS+ (69%, 67%) to the CS- but the II group did not (48%). Satiation tests showed that ID and IJ infusions of glucose reduced intake of a palatable solution similarly, while II infusions were ineffective. In Exp. 2, rats (10/group) drank CS solutions in one-bottle, 30-min sessions and were given 2-h ID or hepatic portal vein (HP) infusions. The CS+ and CS- were paired with 10 ml infusions of 10% glucose and 0.9% saline, respectively. Following 8 training sessions, the ID group preferred the CS+ (67%) to the CS- but the HP group did not (47%) in a two-bottle test. The similar CS+ preferences displayed by ID and IJ, but not II groups implicate the jejunum as a critical site for glucose-conditioned preferences. A pre-absorptive glucose action is indicated by the CS+ preference displayed by ID but not HP rats in Exp. 2. Our data were obtained with non-nutritive CS solutions. HP glucose infusions are reported to condition preferences for a flavored food that itself has pre- and post-absorptive actions. Thus, there may be multiple sites for glucose conditioning with the upper or mid-intestines being the first site of action.

Gut chemosensing: interactions between gut endocrine cells and visceral afferents

Chemosensing in the gastrointestinal tract is less well understood than many aspects of gut mechanosensitivity; however, it is important in the overall function of the GI tract and indeed the organism as a whole. Chemosensing in the gut represents a complex interplay between the function of enteroendocrine (EEC) cells and visceral (primarily vagal) afferent neurons. In this brief review, I will concentrate on a new data on endocrine cells in chemosensing in the GI tract, in particular on new findings on glucose-sensing by gut EEC cells and the importance of incretin peptides and vagal afferents in glucose homeostasis, on the role of G protein coupled receptors in gut chemosensing, and on the possibility that gut endocrine cells may be involved in the detection of a luminal constituent other than nutrients, the microbiota. The role of vagal afferent pathways as a downstream target of EEC cell products will be considered and, in particular, exciting new data on the plasticity of the vagal afferent pathway with respect to expression of receptors for GI hormones and how this may play a role in energy homeostasis will also be discussed.

Nutrient sensing in the gastrointestinal tract: possible role for nutrient transporters

Although it is well established that the presence of nutrients in the gut lumen can bring about changes in GI function, the mechanisms and pathways by which these changes occur has not been fully elucidated. It has been known for many years that luminal nutrients stimulate the release of hormones and regulatory peptides from gut endocrine cells and that luminal nutrients activate intrinsic and extrinsic neural pathways innervating the gut. Activation of gut endocrine cells and neural pathways by nutrients in the gut lumen is key in coordination of postprandial GI function and also in the regulation of food intake. Recent evidence suggests that these pathways can be modified by long term changes in diet or by inflammatory processes in the gut wall. Thus it is important to determine the cellular and molecular mechanisms underlying these processes not only to increase our understanding of as part of basic physiology but also to understand changes in these pathways that occur in the presence of pathophysiology and disease. This review summarizes some of the latest data that we have obtained, together with information from the other laboratories, which have elucidated some of the mechanisms involved in nutrient detection in the gut wall. The focus is on monosaccharides and protein hydrolysates as there is some evidence for a role for nutrient transporters in detection of these nutrients.

The locus for hypoglycemic detection shifts with the rate of fall in glycemia: the role of portal-superior mesenteric vein glucose sensing

OBJECTIVE

To ascertain whether portal glucose sensing extends beyond the portal vein to the superior mesenteric vein and then test whether the role of portal-superior mesenteric glucose sensors varies with the rate of fall in glycemia.

RESEARCH DESIGN AND METHODS

Chronically cannulated rats underwent afferent ablation of the portal vein (PV) or portal and superior mesenteric veins (PMV) or sham operation (control). One week later, animals underwent hyperinsulinemic-hypoglycemic clamps in which the hypoglycemic nadir, 2.48 +/- 0.06 mmol/l, was reached at a rate of decline in glucose of -0.09 or -0.21 mmol x l(-1) x min(-1) (PMV and control only). Additional PMV and control animals received an intravenous injection of the glucopenic agent 2-deoxyglucose. RESULTS; Inducing hypoglycemia slowly, at a rate of -0.09 mmol x l(-1) x min(-1), resulted in a 26-fold increase in epinephrine (23.39 +/- 0.62 nmol/l) and 12-fold increase in norepinephrine (11.42 +/- 0.92 nmol/l) for controls (P < 0.001). The epinephrine response to hypoglycemia was suppressed by 91% in PMV (2.09 +/- 0.07 nmol/l) vs. 61% in PV (9.05 +/- 1.59 nmol/l) (P < 0.001). The norepinephrine response to hypoglycemia was suppressed by 94 and 80% in PMV and PV, respectively, compared with that in controls. In contrast, when arterial glucose was lowered to 2.49 +/- 0.06 mmol/l within 20 min, no significant differences were observed in the catecholamine responses for PMV and controls over the first 45 min of hypoglycemia (20-65 min). Only at min 105 were catecholamines significantly lower for PMV vs. controls. Injection of 2-deoxyglucose induced a very rapid sympathoadrenal response with no significant differences between PMV and controls.

CONCLUSIONS

The critical locus for hypoglycemic detection shifts away from the portal-mesenteric vein to some other loci (e.g., the brain) when hypoglycemia develops rapidly.