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Keywords:

  • 5HT3 receptor;
  • brush cell;
  • enterochromaffin cell;
  • sodium-glucose cotransporter 3

Abstract

  1. Top of page
  2. Abstract
  3. Introduction
  4. Signaling of luminal glucose
  5. Detection of luminal glucose
  6. Acknowledgments
  7. References

Intestinal luminal exposure to glucose initiates changes in food intake and gastrointestinal (GI) motor and secretory function. It does this by stimulating the release of GI hormones and 5-hydroxytryptamine (5-HT) from enteroendocrine and enterochromaffin cells (EC), respectively, which in turn activate intrinsic and extrinsic neuronal pathways. An article in this issue of the journal provides new insight into the mechanisms involved in luminal glucose sensing. Vincent et al. have used a novel in vivo technique to determine activation of gut epithelial cells and vagal afferent pathways in rats by staining for activated calcium-calmodulin kinase II (pCaMKII) along the pathway. In the mucosa, they found that intraluminal glucose activated EC cells and brush cells. At the next stage, pCaMKII was seen in neurons of the myenteric plexus and vagal afferent neurons in the nodose ganglia. In the central nervous system (CNS), activation was seen in second- and higher-order neurons in the dorsal vagal complex and hypothalamus. They found that 5-HT3 receptors were involved in initiating neural signaling as activation of neurons, but not EC cells, was reduced by 5-HT3 receptor antagonism. Selectively stimulating the sodium-glucose cotransporter (SGLT-3) had similar effects to glucose. This suggests that SGLT-3 behaves as a glucose sensor, mainly on EC cells, inducing the release of 5-HT, which activates 5-HT3 receptors on vagal afferent endings nearby and in turn, their connections in the CNS. There is evidence elsewhere that other sensors and transmitter mechanisms are involved in this pathway, so the possibility exists of multiple redundant systems.


Introduction

  1. Top of page
  2. Abstract
  3. Introduction
  4. Signaling of luminal glucose
  5. Detection of luminal glucose
  6. Acknowledgments
  7. References

The detection of nutrients in the gut is of crucial importance in the control of motility, glycemia, and energy intake and yet until recently we have known little of even the most fundamental aspects of this process. In comparison with the mechanism of taste in the tongue, this deficit in understanding becomes even starker. Lingual taste mechanisms are well characterized and provide an excellent starting point to begin to understand nutrient detection in the intestine.

Type-2 diabetes and obesity represent two of the major health issues in the modern era, as a result of their comorbidities. The increasing prevalence of these diseases has focused attention on problems that are not related to famine or infection, but reflect the outcome of surplus energy production. Most developed societies are experiencing an epidemic of diabetes and obesity-related disorders. Type-2 diabetes and obesity are not independent, but linked through common causative factors and interdependent mechanisms; in particular, obesity is frequently associated with insulin resistance, which is a major pathophysiological mechanism in type-2 diabetes. Not surprisingly, there is intense interest in developing non-surgical, pharmacological therapies that lead to a reduction in energy intake by altering satiety and/or hunger and, thereby weight loss. A key factor predisposing to obesity is the acquired capacity of humans to disregard hunger and satiety cues, including those arising from the gastrointestinal (GI) tract.

Vagal afferent neurons play a key role in the regulation of energy intake and digestive processes; they are responsible for sensations such as fullness, bloating and nausea and for reflex control of gut function in response to mechanical and nutrient cues.1,2 These reflexes, in particular, govern the rate of gastric emptying in order to match the digestive and absorptive capacity of the intestine.3 It is now well recognized that gastric emptying accounts for at least 35% of the variance in peak postprandial glucose levels after oral glucose in both healthy individuals and patients with type-2 diabetes.4 Accordingly, it is likely that altered sensitivity of vagal feedback from the small intestine contributes to both abnormal gut motor function and GI symptoms in diabetes. Glucagon-like peptide-1 (GLP-1) mimetics, such as Exenatide and inhibitors of GLP-1 breakdown such as Sitagliptin, have recently become available for the management of type-2 diabetes and have been shown to improve glycaemia. Glucagon-like peptide-1 and Exenatide have numerous physiological effects, but the dominant effect accounting for the reduction in postprandial glycemia is almost certainly slowing of gastric emptying,5 which is known to be vagally-mediated.6

Signaling of luminal glucose

  1. Top of page
  2. Abstract
  3. Introduction
  4. Signaling of luminal glucose
  5. Detection of luminal glucose
  6. Acknowledgments
  7. References

Vagal sensory innervation of the intestine arises from afferent neurons with cell bodies in the nodose and jugular ganglia, with endings concentrated in the upper gut. Tracing studies have shown that vagal afferent endings are present within the parenchyma of mucosal villi in close contact with the basal lamina, but not with the epithelial surface.7 Detection of intestinal nutrients is therefore mediated by highly specialized epithelial cells, which sample largely at sensors on their microvilli and release neuroactive mediators into the subepithelial space, which activate vagal endings. The intestinal epithelium possesses several types of enteroendocrine cells, each with region-specific distribution, that contain a wide range of mediators that can activate vagal afferents.8 This diffuse endocrine system is in fact the largest endocrine organ of the body. A number of enteroendocrine cell mediators have been shown to regulate energy intake and gastric emptying via vagal pathways (for review, see 9) indicative of the involvement of common vagal afferent mechanisms in the regulation of both.

The presence of monosaccharides in the intestine causes the release of the incretin hormone GLP-1 from L-cells, which induces satiation and slows gastric emptying,10 as well as stimulating insulin and suppressing glucagon release. Vagal deafferentation effectively blocks the suppression of food intake and delay in gastric emptying induced by GLP-1,6 indicating the involvement of vagal afferent signals. Correspondingly, GLP-1 receptors are present on vagal afferent cell bodies in the nodose ganglia, 11 and GLP-1 has the capacity to excite them directly.12 Monosaccharides also stimulate the release of serotonin 5-hydroxytryptamine (5-HT) from enterochromaffin cells, 13 which activate 5-HT3 receptors on vagal afferent endings in the intestinal mucosa.14,15 Correspondingly, 5-HT3 receptor antagonists reduce vagally-mediated satiation in response to intestinal glucose.16 Separate populations of enteroendocrine cells express 5-HT and GLP-1 in the small intestine,17 suggesting that these two pathways are either more nutrient-specific than currently known, or mutually redundant.

Current knowledge of intestinal pathways of glucose sensing has been gained largely through direct recordings of electrical activity in enteric and vagal neurons following intestinal luminal exposure to glucose.15,18,19 These established the excitability of these nerve pathways and subsequently, their sensitivities to neuromediators released from intestinal enteroendocrine cells, such as GLP-1 and 5-HT. In the current issue of neurogastroenterology and motility, Vincent et al. investigated the pathway activated by intestinal glucose using a novel cellular approach. A number of approaches have been taken to show activation of various cell types along sensory pathways, including gene expression (c-Fos and others), receptor-induced enzyme activation (phospho-ERK), and in the case of Vincent et al. intracellular calcium-induced pathway activation. Each of these approaches provides unique advantages in terms of rapid activation and gradual decay (phospho-ERK), slow activation and slow decay (c-Fos) that lend their use to certain systems. What Vincent et al. have discovered here is a system that must not only have relatively rapid activation (less than 5 min) and gradual decay like phospho-ERK, but also with expression at several points along the pathway and therefore more sensitive than others. By comparison, our group showed a fourfold increase in specific phospho-ERK expression in distinct epithelial cells in the mouse intestinal mucosa by glucose.20 However, phospho-ERK did not specifically mark vagal afferents responsive to glucose.

Vincent et al. showed that in response to intestinal glucose, approximately half of EC/5-HT cells express pCaMKII at levels higher than 5% (mean 5.8%), while a non-EC cell population – the cytokeratin-positive brush cells expressed pCaMKII levels of 12% in response to glucose. Although they also noted that GLP-1- and glucose-dependent insulinotropic peptide (GIP)-containing populations increased pCaMKII levels in response to glucose, no quantitative data were presented. It will be interesting to know in future how these compare with gauge the importance of each cell type in glucose signaling and therefore the predominant mediator.

Systemic treatment with the 5-HT3 receptor antagonist ondansetron virtually abolished pCaMKII responses to glucose in neurons downstream in the vagal and enteric signaling pathways, but not in epithelial cells. This suggests that 5-HT is the predominant mediator of glucose-induced neuronal activation. In the light of evidence discussed above that other enteroendocrine mediators are involved, this may indicate that there is cooperativity and/or overlap in the actions of 5-HT and for example GLP-1, but this occurs downstream at the level of the afferent ending, not upstream in the epithelium.

Twenty percent of nodose ganglion neurons were immunopositive for pCaMKII after intestinal glucose, which constitutes a large proportion of all visceral afferents. There is therefore a possibility that glucose may activate vagal afferents other than those projecting to the intestinal mucosa. There are reports of central components of vagal afferent transmission that are sensitive to circulating levels of glucose,21 so some nodose and higher-order neurons may have been activated via this route. There is also emerging evidence that circulating 5-HT may interact with glucose at central vagal afferent endings.21

Detection of luminal glucose

  1. Top of page
  2. Abstract
  3. Introduction
  4. Signaling of luminal glucose
  5. Detection of luminal glucose
  6. Acknowledgments
  7. References

Vincent et al. found that selectively stimulating the sodium–glucose cotransporter SGLT-3 using intraluminal deoxynojirimycin (DNJ) had similar effects to glucose throughout the pathway, whereas the SGLT-3-inactive sugar galactose was not effective. This would indicate that SGLT-3 is the predominant mechanism for primary detection of luminal glucose in the intact system. This is emerging as a controversial area in gut physiology, as there are parallel data suggesting a different type of sensor – a G-protein coupled receptor (GPCR), is responsible for glucose detection. This notion is drawn from similarities between specialized intestinal epithelial cells and lingual taste cells. Both cell types are polarized, possess apical microvilli that are exposed to the chemical environment of the lumen and are equipped to release neuromediators. Heterodimeric sweet taste receptors comprising the GPCR – T1R2 and T1R3 – detect a wide range of sweet tastants in the intestinal lumen. Upon GPCR binding with sugar, the taste-specific G-protein gustducin is activated, liberating Gα and Gβ3 and Gγ13-subunits, which are thought to activate PLCβ2, leading to the release of intracellular calcium from IP3-sensitive stores. Rising intracellular calcium can then gate the taste-specific cation channel TRPM5 leading to Na+ influx, membrane depolarization, neuromediator release, and activation of adjacent nerve terminals.22–25 However, evidence for this system operating in enterochromaffin cells to induce 5-HT release is lacking. The strongest evidence for this system operating in the gut is in stimulation of GLP-1 release, which may be a parallel pathway to that involving 5-HT. Research in mice deficient in α-gustducin has shown that circulating GLP-1 levels do not rise in response to intragastric glucose gavage and the rise in insulin is delayed.26 These data strongly link intestinal taste molecules with GLP-1 release in vivo and are consistent with the hypothesis that lingual taste transduction pathways are conserved throughout the alimentary tract and employed in primary intestinal chemosensation.

It is interesting to note that Vincent et al. found the strongest pCamKII responses to glucose in intestinal brush cells. These cells are the predominant cell type displaying α-gustducin immunoreactivity,17 and may release nitric oxide or opioids (β-endorphin, Met-enkephalin) from the gut in response to glucose, 27,28 probably via opening of TRPM5.28 All of these observations point to the fact that multiple mechanisms involving multiple cell types are involved in the detection of glucose. However, the contribution of the signal initiated by activation of SGLT-3 is clearly strong, which is evident from the responses to DNJ, which was equipotent with glucose itself. Deoxynojirimycin is a putatively selective activator of SGLT-3 at the dose used, but has been reported to have other actions unrelated to this pathway.

When considered together, these observations illustrate that we are acquiring a better understanding of how intestinal carbohydrate is detected by epithelial cells and signaled by vagal sensory pathways and how the molecular components of this detection mechanism are regulated.

Acknowledgments

  1. Top of page
  2. Abstract
  3. Introduction
  4. Signaling of luminal glucose
  5. Detection of luminal glucose
  6. Acknowledgments
  7. References

LAB is supported by a National Health and Medical Research Council Principal Research Fellowship and RLY is a NHMRC New Investigator.

References

  1. Top of page
  2. Abstract
  3. Introduction
  4. Signaling of luminal glucose
  5. Detection of luminal glucose
  6. Acknowledgments
  7. References