Serotonin Secretion by Human Carcinoid BON Cells

Elevated sortilin expression discriminates functional from non-functional neuroendocrine tumors and enables therapeutic targeting

A subset of neuroendocrine tumors (NETs) can cause an excessive secretion of hormones, neuropeptides, and biogenic amines into the bloodstream. These so-called functional NETs evoke a hormone-related disease and lead to several different syndromes, depending on the factors released. One of the most common functional syndromes, carcinoid syndrome, is characterized mainly by over-secretion of serotonin. However, what distinguishes functional from non-functional tumors on a molecular level remains unknown. Here, we demonstrate that the expression of sortilin, a widely expressed transmembrane receptor involved in intracellular protein sorting, is significantly increased in functional compared to non-functional NETs and thus can be used as a biomarker for functional NETs. Furthermore, using a cell line model of functional NETs, as well as organoids, we demonstrate that inhibition of sortilin reduces cellular serotonin concentrations and may therefore serve as a novel therapeutic target to treat patients with carcinoid syndrome.

Three-Dimensional-Printed Electrochemical Multiwell Plates for Monitoring Food Intolerance from Intestinal Organoids

Common symptoms of food intolerance are caused by chemical components within food that have a pharmacological activity to alter the motility of the gastrointestinal tract. Food intolerance is difficult to diagnose as it requires a long-term process of eliminating foods that are responsible for gastrointestinal symptoms. Enterochromaffin (EC) cells are key intestinal epithelium cells that respond to luminal chemical stimulants by releasing 5-HT. Changes in 5-HT levels have been shown to directly alter the motility of the intestinal tract. Therefore, a rapid approach for monitoring the impact of chemicals in food components on 5-HT levels can provide a personalized insight into food intolerance and help stratify diets. Within this study, we developed a three-dimensional (3D)-printed electrochemical multiwell plate to determine changes in 5-HT levels from intestinal organoids that were exposed to varying chemical components found in food. The carbon black/poly-lactic acid (CB/PLA) electrodes had a linear range in physiological concentrations of 5-HT (0.1-2 μM) with a limit of detection of 0.07 μM. The electrodes were stable for monitoring 5-HT overflow from intestinal organoids. Using the electrochemical multiwell plate containing intestinal organoids, increases in 5-HT were observed in the presence of 0.1 mM cinnamaldehyde and 10 mM quercetin but reduction in 5-HT levels was observed in 1 mM sorbitol when compared to control. These changes in the presence of chemicals commonly found in food were verified with ex vivo ileum tissue measurements using chromatography and amperometry with boron-doped diamond electrodes. Overall, our 3D electrochemical multiwell plate measurements with intestinal organoids highlight an approach that can be a high-throughput platform technology for rapid screening of food intolerance to provide personalized nutritional diet.

Effect of the Tryptophan Hydroxylase Inhibitor Telotristat on Growth and Serotonin Secretion in 2D and 3D Cultured Pancreatic Neuroendocrine Tumor Cells

Serotonin, a biologically active amine, is related to carcinoid syndrome in functioning neuroendocrine tumors (NETs). Telotristat ethyl is a novel inhibitor of the tryptophan hydroxylase (TPH), a key enzyme in the production of serotonin. While its use in patients with carcinoid syndrome and uncontrolled diarrhea under somatostatin analogs (SSAs) has been recently approved, in vitro data evaluating its effectiveness are lacking. For this reason, we aimed to evaluate the effect of telotristat as monotherapy, and in combination with SSAs, on proliferation and secretion in a NET cell line model. The human pancreatic NET cell lines BON-1/QGP-1 were used as 2D and 3D cultured models; somatostatin receptor and TPH mRNA expression, as well as the potential autocrine effect of serotonin on tumor cell proliferation using a 3D culture system were evaluated. Telotristat decreased serotonin production in a dose-dependent manner at a clinically feasible concentration, without affecting cell proliferation. Its combination with pasireotide, but not with octreotide, had an additive inhibitory effect on serotonin secretion. The effect of telotristat was slightly less potent, when BON-1 cells were co-treated with octreotide. Octreotide and pasireotide had no effect on the expression of TPH. Telotristat did not have an effect on mRNA expression of somatostatin receptor subtypes. Finally, we showed that serotonin did not have an autocrine effect on NET cell proliferation on the 3D cell model. These results suggest that telotristat is an effective drug for serotonin inhibition, but the effectiveness of its combination with SST2 (somatostatin receptor subtype 2)-preferring SSA should be evaluated in more detail.

Structural and Functional Characterization of the Interaction of Snapin with the Dopamine Transporter: Differential Modulation of Psychostimulant Actions

The importance of dopamine (DA) neurotransmission is emphasized by its direct implication in several neurological and psychiatric disorders. The DA transporter (DAT), target of psychostimulant drugs, is the key protein that regulates spatial and temporal activity of DA in the synaptic cleft via the rapid reuptake of DA into the presynaptic terminal. There is strong evidence suggesting that DAT-interacting proteins may have a role in its function and regulation. Performing a two-hybrid screening, we identified snapin, a SNARE-associated protein implicated in synaptic transmission, as a new binding partner of the carboxyl terminal of DAT. Our data show that snapin is a direct partner and regulator of DAT. First, we determined the domains required for this interaction in both proteins and characterized the DAT-snapin interface by generating a 3D model. Using different approaches, we demonstrated that (i) snapin is expressed in vivo in dopaminergic neurons along with DAT; (ii) both proteins colocalize in cultured cells and brain and, (iii) DAT and snapin are present in the same protein complex. Moreover, by functional studies we showed that snapin produces a significant decrease in DAT uptake activity. Finally, snapin downregulation in mice produces an increase in DAT levels and transport activity, hence increasing DA concentration and locomotor response to amphetamine. In conclusion, snapin/DAT interaction represents a direct link between exocytotic and reuptake mechanisms and is a potential target for DA transmission modulation.

High Throughput NPY-Venus and Serotonin Secretion Assays for Regulated Exocytosis in Neuroendocrine Cells

Here we describe two assays to measure dense core vesicle (DCV) exocytosis-mediated cargo secretion in neuroendocrine cells. To conduct siRNA screens for novel genes in regulated DCV exocytosis, we developed a plate reader-based secretion assay using DCV cargo, NPY-Venus, and an orthogonal ³H-serotonin secretion assay. The NPY-Venus secretion assay was successfully used for a high throughput siRNA screen, and the serotonin secretion assay was used to validate hits identified from the screen (Sorensen, 2017; Zhang et al., 2017).

Identification of a Novel Allosteric Inhibitory Site on Tryptophan Hydroxylase 1 Enabling Unprecedented Selectivity Over all Related Hydroxylases

Pulmonary arterial hypertension (PAH) has demonstrated multi-serotonin receptor dependent pathologies, characterized by increased tone (5-HT_1B receptor) and complex lesions (SERT, 5-HT_1B, 5-HT_2B receptors) of the pulmonary vasculature together with right ventricular hypertrophy, ischemia and fibrosis (5-HT_2B receptor). Selective inhibitors of individual signaling elements – SERT, 5-HT_2A, 5HT_2B, and combined 5-HT2_A/B receptors, have all been tested clinically and failed. Thus, inhibition of tryptophan hydroxylase 1 (TPH1), the rate limiting step in 5-HT synthesis, has been suggested as a more broad, and thereby more effective, mode of 5-HT inhibition. However, selectivity over non-pathogenic enzyme family members, TPH2, phenylalanine hydroxylase, and tyrosine hydroxylase has hampered therapeutic development. Here we describe the site/sequence, biochemical, and biophysical characterization of a novel allosteric site on TPH1 through which selectivity over TPH2 and related aromatic amino acid hydroxylases is achieved. We demonstrate the mechanism of action by which novel compounds selectively inhibit TPH1 using surface plasma resonance and enzyme competition assays with both tryptophan ligand and BH4 co-factor. We demonstrate 15-fold greater potency within a human carcinoid cell line versus the most potent known TPH1/2 non-specific inhibitor. Lastly, we detail a novel canine in vivo system utilized to determine effective biologic inhibition of newly synthesized 5-HT. These findings are the first to demonstrate TPH1-selective inhibition and may pave the way to a truly effective means to reduce pathologic 5-HT and thereby treat complex remodeling diseases such as PAH.

Gut microbes promote colonic serotonin production through an effect of short-chain fatty acids on enterochromaffin cells

Gut microbiota alterations have been described in several diseases with altered gastrointestinal (GI) motility, and awareness is increasing regarding the role of the gut microbiome in modulating GI function. Serotonin [5-hydroxytryptamine (5-HT)] is a key regulator of GI motility and secretion. To determine the relationship among gut microbes, colonic contractility, and host serotonergic gene expression, we evaluated mice that were germ-free (GF) or humanized (HM; ex-GF colonized with human gut microbiota). 5-HT reduced contractile duration in both GF and HM colons. Microbiota from HM and conventionally raised (CR) mice significantly increased colonic mRNAs Tph1 [(tryptophan hydroxylase) 1, rate limiting for mucosal 5-HT synthesis; P < 0.01] and chromogranin A (neuroendocrine secretion; P < 0.01), with no effect on monoamine oxidase A (serotonin catabolism), serotonin receptor 5-HT4, or mouse serotonin transporter. HM and CR mice also had increased colonic Tph1 protein (P < 0.05) and 5-HT concentrations (GF, 17 ± 3 ng/mg; HM, 25 ± 2 ng/mg; and CR, 35 ± 3 ng/mg; P < 0.05). Enterochromaffin (EC) cell numbers (cells producing 5-HT) were unchanged. Short-chain fatty acids (SCFAs) promoted TPH1 transcription in BON cells (human EC cell model). Thus, gut microbiota acting through SCFAs are important determinants of enteric 5-HT production and homeostasis.

Imatinib attenuates hypoxia-induced pulmonary arterial hypertension pathology via reduction in 5-hydroxytryptamine through inhibition of tryptophan hydroxylase 1 expression

RATIONALE

Whether idiopathic, familial, or secondary to another disease, pulmonary arterial hypertension (PAH) is characterized by increased vascular tone, neointimal hyperplasia, medial hypertrophy, and adventitial fibrosis. Imatinib, a potent receptor tyrosine kinase inhibitor, reverses pulmonary remodeling in animal models of PAH and improves hemodynamics and exercise capacity in selected patients with PAH.

OBJECTIVES

Here we use both imatinib and knockout animals to determine the relationship between platelet-derived growth factor receptor (PDGFR) and serotonin signaling and investigate the PAH pathologies each mediates.

METHODS

We investigated the effects of imatinib (100 mg/kg) on hemodynamics, vascular remodeling, and downstream molecular signatures in the chronic hypoxia/SU5416 murine model of PAH.

MEASUREMENTS AND MAIN RESULTS

Treatment with imatinib reduced all measures of PAH pathology observed in hypoxia/SU5416 mice. In addition, 5-hydroxytryptamine (5-HT) and tryptophan hydroxylase 1 (Tph1) expression were reduced compared with the normoxia/SU5416 control group. Imatinib attenuated hypoxia-induced increases in Tph1 expression in pulmonary endothelial cells in vitro via inhibition of the PDGFR-β pathway. To better understand the consequences of this novel mode of action for imatinib, we examined the development of PAH after hypoxic/SU5416 exposure in Tph1-deficient mice (Tph1(-/-)). The extensive changes in pulmonary vascular remodeling and hemodynamics in response to hypoxia/SU5416 were attenuated in Tph1(-/-) mice and further decreased after imatinib treatment. However, imatinib did not significantly further impact collagen deposition and collagen 3a1 expression in hypoxic Tph1(-/-) mice. Post hoc subgroup analysis suggests that patients with PAH with greater hemodynamic impairment showed significantly reduced 5-HT plasma levels after imatinib treatment compared with placebo.

CONCLUSIONS

We report a novel mode of action for imatinib, demonstrating TPH1 down-regulation via inhibition of PDGFR-β signaling. Our data reveal interplay between PDGF and 5-HT pathways within PAH, demonstrating TPH1-dependent imatinib efficacy in collagen-mediated mechanisms of fibrosis.

Activation of Type 1 CRH receptor isoforms induces serotonin release from human carcinoid BON-1N cells: an enterochromaffin cell model

CRH and 5-hydroxytryptamine (5-HT) are expressed in human colonic enterochromaffin (EC) cells, but their interactions at the cellular level remain largely unknown. The mechanistic and functional relationship between CRH and 5-HT systems in EC cells was investigated in a human carcinoid cloned BON cell line (BON-1N), widely used as an in vitro model of EC cell function. First, we identified multiple CRH(1) splice variants, including CRH(1a), CRH(1c), CRH(1f), and a novel form lacking exon 4, designated here as CRH(1i), in the BON-1N cells. The expression of CRH(1i) was also confirmed in human brain cortex, pituitary gland, and ileum. Immunocytochemistry and immunoblot analysis confirmed that BON-1N cells were CRH(1) and 5-HT positive. CRH, urocortin (Ucn)-1, and cortagine, a selective CRH(1) agonist, all increased intracellular cAMP, and this concentration-dependent response was inhibited by CRH(1)-selective antagonist NBI-35965. CRH and Ucn-1, but not Ucn-2, stimulated significant ERK1/2 phosphorylation. In transfected human embryonic kidney-293 cells, CRH(1i) isoforms produced a significant increase in pERK1/2 in response to CRH(1) agonists that was sensitive to NBI-35965. CRH and Ucn-1 stimulated 5-HT release that reached a maximal increase of 3.3- and 4-fold at 10(-8) m over the basal level, respectively. In addition, exposure to CRH for 24-h up-regulated tryptophan hydroxylase-1 mRNA levels in the BON-1N cells. These findings define the expression of EC cell-specific CRH(1) isoforms and activation of CRH(1)-dependent pathways leading to 5-HT release and synthesis; thus, providing functional evidence of a link exists between CRH and 5-HT systems, which have implications in stress-induced CRH(1) and 5-HT-mediated stimulation of lower intestinal function.

The cornucopia of intestinal chemosensory transduction

The chemosensory transduction mechanisms that the gastrointestinal (GI) tract uses to detect chemical and nutrient stimuli are poorly understood. The GI tract is presented with a wide variety of stimuli including potentially harmful chemicals or toxins as well as 'normal' stimuli including nutrients, bacteria and mechanical forces. Sensory transduction is at its simplest the conversion of these stimuli into a neural code in afferent nerves. Much of the information encoded is used by the enteric nervous system to generate local reflexes while complementary information is sent to the central nervous system via afferents or by release of hormones to affect behaviour. This review focuses on the chemosensory transduction mechanisms present in the GI tract. It examines the expression and localisation of the machinery for chemosensory transduction. It summarises the types of cells which might be involved in detecting stimuli and releasing neuroactive transmitters. Finally, it highlights the idea that chemosensory transduction mechanisms in the GI tract utilise many overlapping and complementary mechanisms for detecting and transducing stimuli into reflex action.

PACAP induces signaling and stimulation of 5-hydroxytryptamine release and growth in neuroendocrine tumor cells

Neuroendocrine tumors, although rare, are currently diagnosed with increasing frequency, owing to improved imaging techniques and a greater clinical awareness of this condition. To date, BON is a very well established and characterized human pancreatic neuroendocrine tumor cell line used to study the signal transduction and genetic regulation of neuroendocrine tumors secretion and growth. The secretory activity of BON cells is known to release peptides, such as chromogranin A, neurotensin, and biogenic amines, as 5-HT, permitting an assessment of their biological activity. The neuropeptide pituitary adenylate cyclase activating polypeptide (PACAP), released from the enteric neurons in the gastrointestinal tract by binding to its high affinity receptor PAC1, has been previously shown to regulate the secretory activity and growth of the neuroendocrine-derived enterochromaffin-like cells in the stomach. This led us to speculate that PACAP might also play an important role in regulating the growth of human neuroendocrine tumors. Accordingly, in the current study, we have shown that BON cells express PAC1 receptors, which are rapidly internalized upon PACAP activation. Furthermore, PAC1 receptor activation, in BON cells, couple to intracellular Ca(2+) as well as cAMP responses and induce the release of intracellular 5-HT, activate mitogen activated protein kinases, and stimulate cellular growth. These data indicate that PACAP functionally can stimulate 5-HT release and promote the growth of the BON neuroendocrine tumor cell line. Therefore, PACAP and its receptors regulate neuroendocrine tumor secretory activity and growth in vivo, and this knowledge will permit the development of novel diagnostic and therapeutic targets for managing neuroendocrine tumors in humans.

Serotonin release and uptake in the gastrointestinal tract

The afferent innervation of the gastrointestinal (GI) tract consists of intrinsic and extrinsic sensory neurons that respond to nutrients, chemicals or mechanical stimuli within the gut lumen. Most stimuli do not interact directly with the afferent nerves but instead activate specialised cells in the epithelium in a process of sensory transduction. It is thought that one of the first steps in this process is the release of serotonin (5-HT) from the enterochromaffin (EC) cells. The EC cells are a sub-type of enteroendocrine (EE) cells which are found among the enterocytes of the intestinal epithelium. The EC cells are responsible for the production and storage of the largest pool of 5 HT in the body. Released 5-HT can act on the intrinsic nerves and vagal endings. This review will focus on the role of 5-HT in sensory transduction and examine how the EC cell produces and releases 5-HT. We will explore recent developments that have helped to elucidate some of the proteins that allow EC cells to sense the luminal environment. Finally, we will highlight some of the findings from new studies using electrochemical techniques which allow the real-time recording of 5-HT concentrations near to the EC cell.

Gene expression in midgut carcinoid tumors: potential targets for immunotherapy

Classical midgut carcinoids are serotonin-secreting tumors derived from enterochromaffin cells in the gut. Metastatic disease represents a therapeutic challenge and immunotherapy implies a novel approach for treatment. In order to define antigens suitable for T-cell therapy with a preferential expression in midgut carcinoid tissue a broad screening of genes with preferential neuroendocrine restriction, genes described as over-expressed in various malignancies, and genes encoding cancer-testis associated antigens was performed. The expression of 32 genes was analyzed by reverse transcription polymerase chain reaction (RT-PCR) in 28 midgut carcinoid specimens, in the cell line BON and in normal tissues. Immunohistochemistry (IHC) was used to evaluate protein expression. Expression is shown of genes that have previously not been observed in midgut carcinoid tumors, such as Survivin and GAGEs. Also the expression is confirmed of genes that encode pivotal proteins in enterochromaffin cells, such as TPH1 and VMAT1, and their tissue-restricted expression is indicated. In addition, gene expression of IA-2 and CDX-2 in normal gastrointestinal (GI) tract and in tumor is shown. Protein expression of TPH, VMAT1, and Survivin was detected in tumor tissue. This study elucidates that TPH1, VMAT1, and Survivin should be further investigated as potential target antigens for T cell-mediated immunotherapy of midgut carcinoids.

A 20-nm step toward the cell membrane preceding exocytosis may correspond to docking of tethered granules

In endocrine cells, plasma membrane (PM)-bound secretory granules must undergo a number of maturation stages (i.e., priming) to become fusion-competent. Despite identification of several molecules involved in binding granules to the PM and priming them, the exact nature of events occurring at the PM still largely remains a mystery. In stimulated BON cells, we used evanescent wave microscopy to study trajectories of granules shortly before their exocytoses, which provided a physical description of vesicle-PM interactions at an unprecedented level of detail, and directly lead to an original mechanistic model. In these cells, tethered (T), nonfusogenic, vesicles are prevented from converting to fusogenic, docked (D) ones in resting conditions. Upon elevation of calcium, T-vesicles perform a 21-nm step toward the PM to become D, and fuse approximately 3 s thereafter. Our ability to directly visualize different modes of PM-attachment paves the way for clarifying the exact role of various molecules implicated in attachment and priming of granules in future studies.

Myosin va mediates docking of secretory granules at the plasma membrane

Myosin Va (MyoVa) is a prime candidate for controlling actin-based organelle motion in neurons and neuroendocrine cells. Its function in secretory granule (SG) trafficking was investigated in enterochromaffin cells by wide-field and total internal reflection fluorescence microscopy. The distribution of endogenous MyoVa partially overlapped with SGs and microtubules. Impairing MyoVa function by means of a truncated construct (MyoVa tail) or RNA interference prevented the formation of SG-rich regions at the cell periphery and reduced SG density in the subplasmalemmal region. Individual SG trajectories were tracked to analyze SG mobility. A wide distribution of their diffusion coefficient, D(xy), was observed. Almost immobile SGs (D(xy) < 5 x 10(-4) microm2 x s(-1)) were considered as docked at the plasma membrane based on two properties: (1) SGs that undergo exocytosis have a D(xy) below this threshold value for at least 2 s before fusion; (2) a negative autocorrelation of the vertical motion was found in subtrajectories with a D(xy) below the threshold. Using this criterion of docking, we found that the main effect of MyoVa inhibition was to reduce the number of docked granules, leading to reduced secretory responses. Surprisingly, this reduction was not attributable to a decreased transport of SGs toward release sites. In contrast, MyoVa silencing reduced the occurrence of long-lasting, but not short-lasting, docking periods. We thus propose that, despite its known motor activity, MyoVa directly mediates stable attachment of SGs at the plasma membrane.

Enterochromaffin cells of the human gut: sensors for spices and odorants

BACKGROUND & AIMS

Release of serotonin from mucosal enterochromaffin cells triggered by luminal substances is the key event in the regulation of gut motility and secretion. We were interested to know whether nasal olfactory receptors are also expressed in the human gut mucosa by enterochromaffin cells and whether their ligands and odorants present in spices, fragrances, detergents, and cosmetics cause serotonin release.

METHODS

Receptor expression was studied by the reverse-transcription polymerase chain reaction method in human mucosal enterochromaffin cells isolated by laser microdissection and in a cell line derived from human enterochromaffin cells. Activation of the cells by odorants was investigated by digital fluorescence imaging using the fluorescent Ca(2+) indicator Fluo-4. Serotonin release was measured in culture supernatants by a serotonin enzyme immunoassay and amperometry using carbon fiber microelectrodes placed on single cells.

RESULTS

We found expression of 4 olfactory receptors in microdissected human mucosal enterochromaffin cells and in a cell line derived from human enterochromaffin cells. Ca(2+) imaging studies revealed that odorant ligands of the identified olfactory receptors cause Ca(2+) influx, elevation of intracellular free Ca(2+) levels, and, consequently, serotonin release.

CONCLUSIONS

Our results show that odorants present in the luminal environment of the gut may stimulate serotonin release via olfactory receptors present in human enterochromaffin cells. Serotonin controls both gut motility and secretion and is implicated in pathologic conditions such as vomiting, diarrhea, and irritable bowel syndrome. Thus, olfactory receptors are potential novel targets for the treatment of gastrointestinal diseases and motility disorders.

Characterization of sequential exocytosis in a human neuroendocrine cell line using evanescent wave microscopy and “virtual trajectory” analysis

Secretion of hormones and other bioactive substances is a fundamental process for virtually all multicellular organisms. Using total internal reflection fluorescence microscopy (TIRFM), we have studied the calcium-triggered exocytosis of single, fluorescently labeled large, dense core vesicles in the human neuroendocrine BON cell line. Three types of exocytotic events were observed: (1) simple fusions (disappearance of a fluorescent spot by rapid diffusion of the dye released to the extracellular space), (2) "orphan" fusions for which only rapid dye diffusion, but not the parent vesicle, could be detected, and (3) events with incomplete or multi-step disappearance of a fluorescent spot. Although all three types were reported previously, only the first case is clearly understood. Here, thanks to a combination of two-color imaging, variable angle TIRFM, and novel statistical analyses, we show that the latter two types of events are generated by the same basic mechanism, namely shape retention of fused vesicle ghosts which become targets for sequential fusions with deeper lying vesicles. Overall, approximately 25% of all exocytotic events occur via sequential fusion. Secondary vesicles, located 200-300 nm away from the cell membrane are as fusion ready as primary vesicles located very near the cell membrane. These findings call for a fundamental shift in current models of regulated secretion in endocrine cells. Previously, sequential fusion had been studied mainly using two-photon imaging. To the best of our knowledge, this work constitutes the first quantitative report on sequential fusion using TIRFM, despite its long running and widespread use in studies of secretory mechanisms.

5-Hydroxytryptamine contributes significantly to a reflex pathway by which the duodenal mucosa protects itself from gastric acid injury

Although duodenal mucosal bicarbonate secretion (DMBS) is currently accepted as an important defense mechanism against acid-induced duodenal injury, the mechanism and the regulation of DMBS are largely unknown. 5-HT may regulate DMBS, but little is known about its physiological relevance in DMBS and the underlying mechanism(s). Thus, the aims of the present study were to demonstrate the role of 5-HT in acid-stimulated DMBS and to further elucidate the precise mechanisms involved in this process. Luminal acid stimulation significantly increased 5-HT release from the duodenal mucosa (P<0.01). SB204070, a selective 5-HT4 receptor antagonist, dose-dependently reduced luminal acid-stimulated HCO3(-) secretion of mice in vivo. In Ussing chamber studies, 5-HT-induced I(SC) and DMBS were abolished by removal of extracellular Ca2+, and significantly attenuated by pharmacological blockade of the Na+/Ca2+ exchanger (NCX), intermediate Ca2+-activated K+ channels (IK(Ca)), or cystic fibrosis transmembrane conductance regulator (CFTR). 5-HT increased cytoplasmic free calcium ([Ca2+]cyt) in SCBN cells, a duodenal epithelial cell line, and knockdown of NCX1 proteins with a specific siRNA greatly decreased this 5-HT-mediated Ca2+ signaling. Taken together, our data suggest that 5-HT plays a physiological role in acid-stimulated DMBS via a Ca2+ signaling pathway, in which the plasma membrane NCX transporter as well as IK(Ca) and CFTR channels may be involved.

Real-time measurement of serotonin release and motility in guinea pig ileum

Enterochromaffin (EC) cells are sensors that detect chemical or mechanical stimuli and respond with release of serotonin (5-HT). 5-HT activates local motor reflexes, but whether local motor reflexes also evoke 5-HT release is unknown. The aim of the present study was to establish the relationship between the release of 5-HT and the enteric neural circuits controlling the movements of the intestine. Recordings were made from full-thickness preparations of guinea pig ileum using electrochemical techniques with carbon fibre electrodes to measure local concentrations of 5-HT. The tension in the circular muscle (CM) and longitudinal muscle (LM) was recorded with force transducers. The release of 5-HT from the EC cells was detected selectively and the timing of the events quantified. Pressure-evoked peristalsis caused detectable 5-HT release only when the recording site was invaded by a ring of CM contraction. Spontaneous and stretch-evoked reflex contraction of the CM and LM occurred simultaneously with 5-HT release. Paralysis of the smooth muscle significantly reduced the stretch-evoked release. Muscarinic agonists evoked reflexes that were associated with increases in tension in CM and LM simultaneous with 5-HT release. Tetrodotoxin abolished the coordination between the CM contraction and 5-HT release but not the direct activation of the CM and EC cells by the agonists. In conclusion, the correlation between local motor reflexes and 5-HT release observed in the present study is caused primarily by the contraction of the smooth muscle and subsequent deformation of the mucosa. The EC cell is, thus, a site of convergence for mechanical forces that contribute to the release of 5-HT during motor reflexes.

Conservation of the Notch1 signaling pathway in gastrointestinal carcinoid cells

Gastrointestinal (GI) carcinoid cells secrete multiple neuroendocrine (NE) markers and hormones including 5-hydroxytryptamine and chromogranin A. We were interested in determining whether activation of the Notch1 signal transduction pathway in carcinoid cells could modulate production of NE markers and hormones. Human pancreatic carcinoid cells (BON cells) were stably transduced with an estrogen-inducible Notch1 construct, creating BON-NIER cells. In the present study, we found that Notch1 is not detectable in human GI carcinoid tumor cells. The induction of Notch1 in human BON carcinoid cells led to high levels of functional Notch1, as measured by CBF-1 binding studies, resulting in activation of the Notch1 pathway. Similar to its developmental role in the GI tract, Notch1 pathway activation led to an increase in hairy enhancer of split 1 (HES-1) protein and a concomitant silencing of human Notch1/HES-1/achaete-scute homolog 1. Furthermore, Notch1 activation led to a significant reduction in NE markers. Most interestingly, activation of the Notch1 pathway caused a significant reduction in 5-hydroxytryptamine, an important bioactive hormone in carcinoid syndrome. In addition, persistent activation of the Notch1 pathway in BON cells led to a notable reduction in cellular proliferation. These results demonstrate that the Notch1 pathway, which plays a critical role in the differentiation of enteroendocrine cells, is highly conserved in the gut. Therefore, manipulation of the Notch1 signaling pathway may be useful for expanding the targets for therapeutic and palliative treatment of patients with carcinoid tumors.