Pharmacology and physiology of gastrointestinal enteroendocrine cells

Abstract

Gastrointestinal (GI) polypeptides are secreted from enteroendocrine cells (EECs). Recent technical advances and the identification of endogenous and synthetic ligands have enabled exploration of the pharmacology and physiology of EECs. Enteroendocrine signaling pathways stimulating hormone secretion involve multiple nutrient transporters and G protein-coupled receptors (GPCRs), which are activated simultaneously under prevailing nutrient conditions in the intestine following a meal. The majority of studies investigate hormone secretion from EECs in response to single ligands and although the mechanisms behind how individual signaling pathways generate a hormonal output have been well characterized, our understanding of how these signaling pathways converge to generate a single hormone secretory response is still in its infancy. However, a picture is beginning to emerge of how nutrients and full, partial, or allosteric GPCR ligands differentially regulate the enteroendocrine system and its interaction with the enteric and central nervous system. So far, activation of multiple pathways underlies drug discovery efforts to harness the therapeutic potential of the enteroendocrine system to mimic the phenotypic changes observed in patients who have undergone Roux-en-Y gastric surgery. Typically obese patients exhibit ∼30% weight loss and greater than 80% of obese diabetics show remission of diabetes. Targeting combinations of enteroendocrine signaling pathways that work synergistically may manifest with significant, differentiated EEC secretory efficacy. Furthermore, allosteric modulators with their increased selectivity, self-limiting activity, and structural novelty may translate into more promising enteroendocrine drugs. Together with the potential to bias enteroendocrine GPCR signaling and/or to activate multiple divergent signaling pathways highlights the considerable range of therapeutic possibilities available. Here, we review the pharmacology and physiology of the EEC system.

Keywords: Chemosensing; GLP-1; GPCR; diabetes; enteroendocrine; intestine.

Figure 1

Sensing by the enteroendocrine system. Digestion products enter the small intestine and stimulate enteroendocrine cells (EECs) to secrete hormones which modulate gastrointestinal (GI) secretion, insulin secretion, gastric and GI motility, and satiety. Open type EECs have processes that extend to reach into the lumen to detect nutrients. Glucose, amino acids, and peptides are sensed by EECs via nutrient transporters (A). Nutrient transport depolarizes the plasma membrane (PM). Nutrient uptake signaling converges on voltage-gated Ca²⁺ channels (VGCCs). Plasma membrane depolarisation (ΔΨ) opens VGCCs allowing entry of extracellular Ca²⁺ to raise intracellular Ca²⁺ levels [Ca²⁺]_I and stimulate the secretion of hormones from EECs. Glucose is sensed by electrogenic Na⁺-coupled uptake by sodium-coupled glucose transporter (SGLT1) to trigger membrane depolarization and the entry of extracellular Ca²⁺ via VGCCs. Intracellular metabolism of glucose or fructose via glucokinase, and closure of ATP-sensitive K⁺ channels also causes membrane depolarization and opening of VGCCs. Electrogenic uptake of certain amino acids via H⁺ or Na⁺ coupled amino acid transporters or peptides via the H⁺ coupled peptide transporter-1 (PepT1) can also trigger membrane depolarization and hormone secretion. Nutrients are also sensed by GPCRs (B). GPCR-mediated nutrient sensing in EECs stimulate the release of hormones via coupling to Gα_s and Gα_i that promote or inhibit adenylate cyclase activity, respectively, altering intracellular cAMP levels [cAMP]_I. Gα_q/11-coupled GPCRs stimulate phospholipase C activity to breakdown PIP₂ into IP₃ and DAG. Intracellular stores release Ca²⁺ in response to activation of IP₃ receptors. Protein kinase C is activated by Ca²⁺ and DAG. Gα_gustducin couples to TRPM5 via phospholipase Cβ2 and Ca²⁺ to cause membrane depolarization and open VGCCs. For example, fatty acids activate FFAR1 – 4 which mobilize Ca²⁺ while CB1 inhibits cAMP production. Products of triacylglycerol digestion, including oleoylethanolamide and monoacylglycerols, activate GPR119 to increase cAMP levels. Amino acids and oligopeptides also activate the CasR to trigger hormone secretion. Nutrient transporters are shown to highlight that these signaling pathways also operate in the presence of nutrient. Enteroendocrine signaling is integrated through GPCR signaling cascades. Hormones secreted from EECs may mediate effects locally or systemically. For example, hormones may enter the systemic circulation and the hepatic portal vein to activate receptors in other tissues via endocrine pathways (C). These hormones may activate receptors on enterocytes for example, PYY may activate NPY1R which increases cAMP levels and inhibits Cl⁻ secretion or VIP may activate VPAC to decrease cAMP and stimulate Cl⁻ secretion (D) or EECs in the vicinity, for example, CCK may activate CCK1R, GIP may activate GIPR, and Sst may activate SSTR5 (E). Hormones may activate GPCRs on vagal afferent neurones, for example, PYY may activate NPY1R, CCK may activate CCK1R, and GLP-1 or 2 may activate GLP1R or GLP2R, respectively (F). The enteric nervous system (ENS) also modulates EEC activity through the release of hormones and neurotransmitters including Ach (M2R), GRP/NMB (BBS2), PACAP (VPAC1R), galanin (GAL1), or α-MSH (MC4R) (G). The ENS can also detect absorbed nutrients through GPCRs including FFAR3 (H).

Figure 2

Representative structures of G protein-coupled receptors from Class A, B, and C. As there are no actual structures that exist with the N-terminal domain and 7TM domain connected together, the ribbon diagrams show the size of the N-terminal domain relative to the 7 TM region. The 7TM domain of the Class A example is Orexin. Generally, amine receptor ligands of Class A bind between the TM domains of the receptor while peptide and glycoprotein hormone receptors of Class A bind between the N-terminal domain, extracellular loops, and upper part of the TM domain. The N-terminal domain of the Class B example is from the GLP-1 receptor and the 7TM section from the glucagon receptor. The peptide ligands of Class B generally bind to the extracellular region and reach into the lower part of the TM domain. The N-terminal domain of the Class C example is from mGluR1 and the 7TM section from mGluR5. For clarity, the Cystein-rich region of Class C is missing from between the N-terminal and 7TM domain. Class C GPCRs exhibit a large amino-terminal domain, which binds orthosteric agonists, while allosteric modulators generally bind to the 7TM domain.

Figure 3

The pharmacology of known G protein-coupled receptors that regulate gut hormone secretion from EECs. The in vitro properties (pEC₅₀) of some ligands from Class A (3a) B (3b) and C (3c) are shown from functional (readouts being cAMP, CRE, IP₁, Ca²⁺, GLP-1 secretion) cell-based assays (cells lines used include HEK, GLUTag, NCl-H716, STC-1) and the effects of these on gut hormone secretion in ex vivo intestinal tissue, in vivo rodent models or in humans are summarized.