Encyclopedia of Signaling Molecules

2018 Edition
| Editors: Sangdun Choi

GLP-1R

Reference work entry
DOI: https://doi.org/10.1007/978-3-319-67199-4_101967

Synonyms

Historical Background

In the 1960s, it was shown that orally administered glucose induces a much stronger insulin response than that induced by intravenously administered glucose, despite the similar resulting plasma glucose levels; this was termed the “incretin effect” (Creutzfeldt 2005; Graaf et al. 2016). Gastric inhibitory peptide (GIP) was the first incretin hormone to be discovered in 1975, which is produced by K cells of the small intestine (Creutzfeldt 2005). It was then observed in 1981 that antibodies against GIP did not abolish the incretin effect which led to the discovery of glucagon-like peptide-1 (GLP-1) in the translational products of mRNAs isolated from pancreatic islets of anglerfish (Shields et al. 1981; Graaf et al. 2016). Subsequently, it was shown that hamster and human preproglucagon cDNAs encode GLP-1 and 2, but only GLP-1 possessed incretin activity (Graaf et al. 2016). After the discovery of GLP-1, research was undertaken to identify its receptor. The GLP-1 receptor (GLP-1R) was first cloned from a cDNA library derived from rat pancreatic islets in 1992, and in 1993 the human receptor was successfully cloned (Donnelly 2012; Graaf et al. 2016).

Introduction

Promoting metabolic homeostasis in humans is essential to ensure that sufficient energy (in the form of adenosine triphosphate [ATP]) is provided to all biological processes (Voet and Voet 2011). When appropriate, pancreatic islet alpha and beta-cells produce the hormones glucagon and insulin, respectively (Aronoff et al. 2004). Insulin is produced after nutrient ingestion and promotes peripheral uptake of nutrients (carbohydrates, fats, and amino acids) from the bloodstream into various tissues such as skeletal muscle etc. (Aronoff et al. 2004; Voet and Voet 2011). Glucagon is produced when blood glucose levels become too low and promotes gluconeogenesis in the liver to raise the plasma glucose levels (Aronoff et al. 2004). Insulin and glucagon are pivotal to maintaining metabolic homeostasis – the activity of both hormones are reciprocally correlated in order to achieve normoglycemia (Aronoff et al. 2004; Voet and Voet 2011). However, it has been elucidated that other hormones are also crucial for metabolic homeostasis. The discovery of the incretin effect lead to the identification of two gut hormones possessing incretin activity: GLP-1 and GIP (Graaf et al. 2016). GLP-1 is a gastrointestinal incretin hormone produced by enteroendocrine L-cells in response to postprandial nutrient loads (Holst 2007). The general consensus in the literature is that the incretin effect accounts for 60–70% of insulin secretion after glucose ingestion (Salehi et al. 2012). GIP and GLP-1 account for approximately 60 and 40% of the incretin effect, respectively (Goldstein and Wieland 2007). All of the aforementioned hormones exert their effects by binding to specific receptors on the surface of cells located in target tissues (Voet and Voet 2011). GLP-1 binds to its receptor GLP-1R on target cells, which induces the desired physiological response by activating intracellular pathways leading to altered cellular activity (Thompson and Kanamarlapudi 2013). GLP-1R is a G-protein coupled receptor (GPCR) consisting of 463 amino acid residues in humans (Graaf et al. 2016). GPCRs are of great interest from a clinical perspective given that they are targets for >50% of therapeutic drugs available in the market (Thompson and Kanamarlapudi 2013). GLP-1R consists of a large hydrophilic N-terminal extracellular domain containing a signal peptide, seven hydrophobic transmembrane alpha-helices (TM1–7) joined by three hydrophilic extracellular loops (ECL1–3) and intracellular loops (ICL1–3), and the C-terminal domain (which is located intracellularly and interacts with heterotrimeric G-proteins that consist of alpha, beta, and gamma subunits) (Table 1) (Voet and Voet 2011; Thompson and Kanamarlapudi 2013).
GLP-1R, Table 1

The number of amino acid residues that each domain of human GLP-1R consists of (Adapted from Thompson and Kanamarlapudi (2013))

Amino acid residues start-finish (length)

Description

Amino acid residues (start-finish) length

Description

1–23 (23)

Signal peptide

24–145 (122)

N-terminus

146–168 (23)

TM1

169–176 (8)

ICL1

177–196 (20)

TM2

197–227 (31)

ECL1

228–252 (25)

TM3

253–264 (12)

ICL2

265–288 (24)

TM4

289–303 (15)

ECL2

304–329 (26)

TM5

330–351 (22)

ICL3

352–372 (21)

TM6

373–387 (15)

ECL3

388–408 (21)

TM7

409–463 (55)

C-terminus

The Gα subunit has been categorized into four families based on differences within their primary sequence and functions: Gαs, Gαi/o, Gαq/11, and Gα12/13 (Thompson and Kanamarlapudi 2013). GLP-1R activates members of the Gαs, Gαi/o, and Gαq/11 families (Thompson and Kanamarlapudi 2013; Graaf et al. 2016). Gβγ associates with Gα in the inactive state. Upon ligand binding, the GPCR undergoes conformational change allowing the receptor to act as a guanine nucleotide exchange factor (GEF), which results in activation of the Gα subunit by exchange of bound GDP for GTP, after which Gβγ dissociates from Gα (Fig. 1). The Gα and Gβγ subunits can activate downstream signalling cascades to induce the desired physiological response (Thompson and Kanamarlapudi 2013). The Gαs family members activate adenylyl cyclase (AC) which raises intracellular cAMP levels (Thompson and Kanamarlapudi 2013). After ligand stimulation, most GPCRs internalize from the cell surface to dampen the biological response, but after a period of time the receptor is re-expressed back onto the cell surface from the cytosol. Additionally, the active Gα subunit hydrolyzes bound GTP to GDP due to its intrinsic GTPase activity, which then allows the reassociation of the Gα and Gβγ subunits with GLP-1R on the cell surface, ready for the arrival of a new ligand (Thompson and Kanamarlapudi 2013; Graaf et al. 2016).
GLP-1R, Fig. 1

GPCRs-mediated assembly/disassembly of heterotrimeric G-proteins. In the absence of an agonist, the Gβγ subunits are bound together with inactive (GDP bound) Gα. When an agonist binds to its receptor, it induces dissociation of the G-protein subunits and Gα subunit activation by exchange of GDP for GTP. Both the active Gα and Gβγ subunits activate downstream effectors to propagate GPCR signalling. The intrinsic GTPase activity converts the Gα subunit bound GTP to GDP and the G-protein subunits then reassociate ready for arrival of a new agonist (This figure and information in its legend are adapted from Thompson and Kanamarlapudi (2013))

GPCRs are categorized as either class A, B, or C based on their composition homology and functional or structural similarities (Donnelly 2012; Thompson and Kanamarlapudi 2013). GLP-1R is classified as a class B GPCR due to its large N-terminal extracellular domain which consists of >100 amino acid residues, and additionally, the B family has several disulfide bridges in the N-terminal domain which stabilizes the structure (Thompson and Kanamarlapudi 2013). The GLP-1R gene is found on chromosome 6p21 spanning 40 KB and consists of approximately 13 introns and exons (Lin et al. 2015; Graaf et al. 2016). The ability of GLP-1R to augment insulin production in response to binding of GLP-1 to it during the postprandial period is its best characterized and most-studied physiological effect and is a source of interest from a clinical perspective given the role of GLP-1 analogues (liraglutide and exenatide) in treatment of type 2 diabetes (Holst 2007; Thompson and Kanamarlapudi 2013).

GLP-1R Location, Synthesis, Structure, and Compositional Relevance to Activity

GLP-1R expression has been detected in the mouse and human pancreas, brain, heart, kidney, stomach, and lung using RNA protection techniques (Graaf et al. 2016). GLP-1R knockout and knockdown studies in mice have demonstrated that the ability of GLP-1 to act as an incretin hormone is dependent on the presence of its receptor in islet beta-cells (Lamont et al. 2012; Smith et al. 2014). Studies have also demonstrated altered GLP-1 induced intracellular responses upon GLP-1R knockout in other tissues (Graaf et al. 2016). Interestingly, GLP-1R has not been detected in tissues involving glucose metabolism such as the liver, skeletal muscle, and adipose tissue, which is surprising as GLP-1 is important for promoting metabolic homeostasis in those tissues (Voet and Voet 2011; Graaf et al. 2016). However, studies have reported that GLP-1 has insulin-like effects on the liver, skeletal muscle, and adipose tissue in experimental settings, so it is possible that GLP-1 may bind to a currently unidentified receptor in these tissues (Graaf et al. 2016). Like other GPCRs, GLP-1R is produced by the secretory pathway (Holst 2007; Graaf et al. 2016). The crystal structure of GLP-1R has not yet been determined but based on its amino acid sequence it is thought to have a structure similar to that of most GPCRs (Donnelly 2012; Thompson and Kanamarlapudi 2013). The transcriptional regulatory mechanisms of GLP-1R are still largely elusive, although there is evidence to suggest that transcription factors Sp1 and Sp3 have important roles in regulating its transcription (Graaf et al. 2016). The human GLP-1R amino acid sequence and predicted structure are shown in Fig. 2.
GLP-1R, Fig. 2

The amino acid sequence and speculated structure of the human GLP-1R. Residues 1–23 are the signal peptide (highlighted in red). Residues highlighted in blue, yellow, orange, green, purple, and gray indicate amino acids crucial for agonist binding, conserved cysteine residues that form disulfide bonds, amino acids that have a structural role, amino acids involved in G-protein activation, amino acids important for receptor internalization, and amino acids that have glycosylation sites, respectively (This figure and information in its legend are adapted from Thompson and Kanamarlapudi 2013; Graaf et al. 2016)

  1. 1.

    The newly synthesized GLP-1R is targeted to the endoplasmic reticulum (ER) by the signal peptide which is about 20 amino acids in length and is located at the N-terminus of the receptor (Thompson and Kanamarlapudi 2013). Studies have shown that cleavage of the signal peptide is essential for expression of GLP-1R on the cell surface: preventing cleavage of the signal peptide results in retention of GLP-1R in the ER (Thompson and Kanamarlapudi 2014; Graaf et al. 2016). GLP-1R has six highly conserved cysteine residues in its N-terminal domain: disulfide bonds form between Cys46 and Cys71, Cys62 and Cys104, and Cys85 and Cys126 (Thompson and Kanamarlapudi 2013). Additionally, the Asp67, Trp72, Pro86, Arg102, Gly108, and Trp110 residues are highly conserved across class B GPCRs, and Trp72 and Trp110 have been shown to be important in GLP-1R for agonist binding, and the crystal structure of the extracellular domain of GLP-1R has shown that these conserved residues are centrally positioned (Thompson and Kanamarlapudi 2013). Interactions between these residues in GLP-1R play an important role in stabilizing the N-terminal domain and the Pro86 residue is also important for agonist binding (Thompson and Kanamarlapudi 2014). Tryptophan residues 39, 72, 91, 110, and 120 in GLP-1R are crucial for successful agonist binding as substituting these amino acids for alanine results in abolished GLP-1 binding in rats (Graaf et al. 2016). Residues Thr29-Val30-Ser31-Lys32 located within the N-terminal domain of GLP-1R have also been shown to promote agonist binding – mutating this region results in a sevenfold decrease in its affinity to GLP-1 (Thompson and Kanamarlapudi 2013). GLP-1R has been shown to undergo N-linked glycosylation which is thought to occur in the ER (Thompson and Kanamarlapudi 2014; Graaf et al. 2016). Glycopeptidase F treatment has been shown to reduce the molecular weight of GLP-1R from 63 to 51 kDa, suggesting that this protein is a glycoprotein (Donnelly 2012). The N-terminal domain of the human GLP-1R contains three N-linked glycosylation sites: Asn63, Asn82, and Asn115 – mutation of any two or all three residues prevents cell surface expression and retention of GLP-1R in the Golgi and ER (Thompson and Kanamarlapudi 2014). Both glycosylation of GLP-1R and cleavage of the signal peptide are essential for successful trafficking of the receptor to the cell surface (Graaf et al. 2016). Interestingly, Trp39, 69, and 88 residues located in the N-terminal domain have also been shown to be crucial for cell surface expression of GLP-1R, even though mutation of these residues do not interfere with cleavage of the signal peptide or glycosylation of the receptor (Thompson and Kanamarlapudi 2014).

     
  2. 2.

    The ICLs of GPCRs are known to activate heterotrimeric G-proteins and this is the case with GLP-1R: ICLs 1 and 3 have been shown to modulate GLP-1R-mediated Gαs signalling, whereas ICL2 activates Gαs, Gαi/o, and Gαq/11 (Thompson and Kanamarlapudi 2013). Substituting specific residues in ICL1 resulted in decreased cAMP production in response to ligand binding to GLP-1R but there was no effect on the receptor cell surface expression or internalization (Thompson and Kanamarlapudi 2013). Certain residues within ICL3 are responsible for conferring the Gαs and Gαi/o activation, but all these domains prefer Gαs over Gαi/o subtypes (Thompson and Kanamarlapudi 2013). Residues located in ICL3 and where TM5 meets ICL3 have been shown to be important for G-protein coupling: substitutions of residues Val327, Ile328, or Val331 (where TM5 is adjacent to ICL3) with alanine resulted in significantly decreased cAMP production but there was no alteration in GLP-1R expression (Thompson and Kanamarlapudi 2013). A deletion of Lys334-Leu335-Lys336 residues (located in the N-terminal half of ICL3) also resulted in a significant decrease in cAMP production after GLP-1 binding (Takhar et al. 1996). When Arg348 (located near the C-terminal end of ICL3) was substituted with glycine in GLP-1R, GLP-1 induced cAMP production was almost completely blunted (Thompson and Kanamarlapudi 2013). These observations demonstrate that these residues are essential for coupling the G-protein activity to GLP-1R.

     
  3. 3.

    The ECLs of GPCRs have been demonstrated to play an important role in agonist binding and receptor trafficking: studies have shown that mutation of residues in both ECL1 and 2 result in a significant loss of GLP-1R binding to GLP-1 and signalling (Thompson and Kanamarlapudi 2013; Graaf et al. 2016). Interestingly, when Cys296, Trp297, Arg299, Asn300, Asn302, Tyr305, and Leu307 (located on ECL2) were substituted with alanine, intracellular signalling was altered by increasing signal bias towards extracellular signal-regulated kinase (ERK) activation (Thompson and Kanamarlapudi 2013). Mutation studies have also demonstrated that different agonists (GLP-1, exendin-4 and oxyntomodulin) activate GLP-1R differently as the same mutations in ECL2 resulted in different responses to these agonists (Thompson and Kanamarlapudi 2013). ECL3 is important for agonist action of several members of the B family of GPCRs, suggesting it is likely important for the same purpose in GLP-1R as well (Thompson and Kanamarlapudi 2013).

     
  4. 4.

    Residues in TM1-4 have been shown to be important for the receptor function. A missense mutation of Thr149 in TM1 reduced agonist binding and substitution of His180 by arginine in TM2 resulted in decreased agonist affinity and cAMP production (Thompson and Kanamarlapudi 2013). Substitution of Lys288 in TM4 by neutral leucine or alanine also reduced the affinity of the receptor for GLP-1, but substitution with a positively charged arginine had minimal effect on the affinity, suggesting that a positively charged amino acid at this position is crucial for agonist binding (Al-Sabah and Donnelly 2003). Serine and threonine residues in TM3 and the cytoplasmic C-terminal domain have been shown to be essential for GLP-1R internalization (Thompson and Kanamarlapudi 2013).

     
  5. 5.

    There are three regions of GLP-1R that are involved in internalization: the very end of the C-terminus, the region just downstream of TM7, and the region between these two regions (Thompson and Kanamarlapudi 2013). The region just downstream of TM7 is called the helix-8 which is an α-helix that terminates with palmitoylated cysteine residues which associates with a number of proteins, and the region between helix-8 and the very end of the C-terminus is where the G-proteins are attached to the receptor (Thompson and Kanamarlapudi 2013). In response to agonist binding, the receptor is phosphorylated and arrestins bind, mediating receptor internalization and uncoupling from G-proteins (Thompson and Kanamarlapudi 2013). GPCRs are phosphorylated at serine residues 441/442, 444/445, and 451/452 of the C-terminal domain in response to agonist binding, which is required for efficient receptor activation and subsequent internalization (Thompson and Kanamarlapudi 2013). A recent study demonstrated that GLP-1R has distinct regions within the C-terminus required for its cell surface expression, activity, and agonist-induced internalization (Thompson and Kanamarlapudi 2015a). The results of this study revealed that the residues 411–418 within the GLP-1R C-terminus are critical in targeting the newly synthesised receptor to the plasma membrane. The residues 419–430 are important for cAMP producing activity of the receptor, most likely by coupling to Gαs. However, the residues 431–450 within the C-terminus are essential for agonist-induced human GLP-1R internalization.

     
Agonist-induced GPCR internalization typically occurs in a clathrin-dependent fashion via GPCR kinases (GRKs), β-arrestins, and ADP-ribosylation factor (ARF) proteins (Kanamarlapudi et al. 2012; Thompson and Kanamarlapudi 2013). It has been reported that clathrin-coated vesicles mediate agonist-induced GLP-1R internalization but it is currently a matter of debate since GLP-1R has also shown to be internalized by caveolae-mediated endocytosis upon agonist stimulation (Fig. 3) (Thompson and Kanamarlapudi 2013). GPCRs that bind caveolin-1 can undergo endocytosis via caveolae (Thompson and Kanamarlapudi 2013). Caveolin-1 is the principle component of caveolae and can interact with a number of signalling molecules, including receptor tyrosine kinases, G proteins, and GPCRs such as GLP-1R, via a common caveolin-binding motif that exists in those signalling molecules (Thompson and Kanamarlapudi 2013). Endocytosis in this manner can lead to fission of caveolae enriched vesicles and then fusion with caveosomes (Thompson and Kanamarlapudi 2013). The Gαq pathway has recently been shown to be important for the agonist-induced GLP-1R internalization (Thompson and Kanamarlapudi 2015b). GLP-1R resensitization is not well studied but it has been shown that GLP-1R–mediated calcium signalling resensitizes within 1 h after agonist removal (Graaf et al. 2016).
GLP-1R, Fig. 3

The proposed models for GLP-1R internalization. (a) The clathrin-dependent internalization of GPCRs. Agonist binding induces phosphorylation of GPCRs by GRKs, which then leads to the recruitment of arrestin and subsequent ARF6 activation. ARF6 activation results in the promotion of clathrin, AP-2, and Src to form clathrin-coated pits. Finally, dynamin causes the “pinching off” of vesicles from the plasma membrane into the cytosol. (b) Caveolae-dependent internalization of GPCRs. After agonist binding, a number of signalling cascades are activated that results in the recruitment of caveolin, and a vesicle then forms on the plasma membrane which enters into the cytosol (This figure and information in its legend are adapted from Thompson and Kanamarlapudi (2013))

The GLP-1R has been shown to form a homodimer through an interface along TM4 which is required for the receptor signalling (Thompson and Kanamarlapudi 2013). Alanine substitutions to Leu256, Val259, or Gly252 in GLP-1R abolished GLP-1 binding, reduced cAMP levels and ERK signalling, and abolished calcium signalling (Thompson and Kanamarlapudi 2013).

GLP-1R Downstream Effectors

In all of the tissue types that GLP-1R is expressed in, GLP-1 binding triggers G-protein activation and thereby the G-protein coupled intracellular pathways. However, the activated signalling pathways are different as GLP-1R is coupled to a range of varying downstream signalling pathways in different tissues, each of which can impact on the physiologic response elicited by the receptor activation (Holst 2007). A common pathway activated by GLP-1R in all of these tissues is the Gαs pathway, which activates AC and thereby increases cAMP levels, which in turn activates both exchange protein activated by cAMP (EPAC) and protein kinase A (PKA), the subsequent downstream targets then differ (Thompson and Kanamarlapudi 2013; Graaf et al. 2016). It is generally accepted that the main function of GLP-1 is to act as an incretin hormone, and the intracellular signalling pathways coupled to GLP-1R are well characterized in islet beta-cells unlike in other tissues (Meloni et al. 2013). GLP-1 does not stimulate insulin secretion at low glucose levels; hence, the insulinotropic activity of GLP-1 is dependent on high levels of glucose in the blood (Graaf et al. 2016). GLP-1 also reduces blood glucose levels by inhibiting glucagon secretion from islet alpha-cells – how GLP-1 achieves this mechanistically is currently unclear (Donath and Burcelin 2013). Figure 4 summarizes how GLP-1 binding to GLP-1R promotes the incretin effect.
GLP-1R, Fig. 4

A summary of the processes in islet beta-cells resulting in insulin secretion. Glucose transporter 2 (GLUT2) is an insulin-insensitive glucose transporter which allows glucose to enter islet beta-cells. When glucose concentration in circulation is 5 mM or higher, glucokinase initiates glucose catabolism. This leads to an increase in the cell’s ATP/ADP ratio which leads to closure of ATP-sensitive potassium channels, causing membrane depolarization (due to potassium becoming sequestered in the cell) and subsequent calcium influx. The resulting calcium influx then triggers insulin secretion. The binding of GLP-1 to GLP-1R activates the Gα subunit which enhances glucose-stimulated insulin secretion by initiating cAMP production through AC which, in turn, activates PKA and Epac. PKA and Epac further induce potassium channel closure which indirectly assists with extracellular calcium influx, and Epac also promotes release of calcium from the endoplasmic reticulum. There is also evidence that PKA increases the permeability of calcium channels to allow for a faster influx of calcium. Then the raised calcium levels further promote exocytosis of insulin vesicles. GLP-1R activation also induces transcription of the preproinsulin, glucokinase, and GLUT2 genes allowing for further insulin production, glucose catabolism, and glucose uptake, respectively, via activation of the PDX-1 transcription factor and its translocation to the nucleus. PDX-1 activation also induces transcription of genes involved in proliferation, neogenesis, and apoptotic resistance. In addition, the lipolytic actions of GLP-1R activation are thought to provide the mitochondria with more metabolic fuel to further raise the ATP/ADP ratio, which is needed for both phases of insulin secretion (This figure and information in its legend are adapted from: (Brissova M 2002; Holst 2007); Wang and Thurmond 2009; Voet and Voet 2011; Meloni et al. 2013; Graaf et al. 2016))

It is not well studied how downstream GLP-1R signalling cascades mediate physiological effects on the cardiovascular system, kidney, lungs, gastrointestinal tract (GIT), and brain, but numerous studies have demonstrated physiological responses of these tissues upon GLP-1 binding (Graaf et al. 2016). In the GIT, GLP-1 decreases gastric motility and inhibits postprandial gastric acid secretion (Holst 2007). GLP-1 also inhibits smooth muscle activity in the small intestine resulting in an overall reduced digestion of nutrients form the GIT, and additionally, GLP-1 inhibits meal-induced pancreatic secretion (Holst 2007). GLP-1R ligand binding has been shown to have physiologically beneficial effects on the cardiovascular system, kidney, and the brain in experimental settings (Graaf et al. 2016). GLP-1R is expressed in the hypothalamic areas that control energy homeostasis and studies have shown that administration of GLP-1 induces satiety even in the absence of food in the GIT and when gastric empting has been inhibited; thus, GLP-1 can induce satiety via its effects on neurons in the caudal brainstem (Graaf et al. 2016). The effect of GLP-1R agonist binding in the lung is not well studied but one study has shown that GLP-1 enhances macromolecule secretion from neuroendocrine cells in the lungs (Graaf et al. 2016).

GLP-1R Current Therapeutic Applications

Modulating GLP-1R activity is currently used in treatment of type 2 diabetes. Administration of GLP-1 analogues (exenatide and liraglutide) to type 2 diabetic patients improves glycemic control by augmenting insulin secretion and dampening glucagon secretion, as well as delaying gastric emptying (Holst 2007; Graaf et al. 2016). Exenatide and liraglutide mimic endogenous GLP-1 activity by binding to GLP-1R on various tissues, but these analogues are resistant to degradation (Graaf et al. 2016). GLP-1 has a half-life of 1–2 min, whereas exenatide has a half-life of 3.4–4 h and liraglutide’s half-life is 11–13 h. Hence, these GLP-1 analogues vastly prolong the GLP-1 response promoting normoglycemia in type 2 diabetic patients during fasting and after nutrient ingestion (Thompson and Kanamarlapudi 2013). Recently, one study has observed that GLP-1R expression is decreased in the hypothalamus of type 2 diabetic patients in comparison to healthy controls, suggesting that the reduced ability of GLP-1 to induce satiety may contribute to the dysfunctional feeding behaviors and metabolic homeostasis observed in these patients (ten Kulve et al. 2015).

GLP-1R Allosteric Sites

Studies have demonstrated that GLP-1R has allosteric agonist binding sites and that these sites are distinct from the orthosteric agonist (GLP-1)-binding site (Thompson and Kanamarlapudi 2013). The first allosteric agonist identified for GLP-1R was compound 1 which had a low affinity and a low potency for GLP-1R (Thompson and Kanamarlapudi 2013). Compound 2 was then produced which is a more potent agonist, and this molecule also increases the affinity of GLP-1R for GLP-1 (Thompson and Kanamarlapudi 2013). However, compound 2 does not stimulate insulin secretion to the same degree as GLP-1, liraglutide, or exenatide, and combining liraglutide or exenatide with compound 2 did not result in enhanced insulin secretion (Thompson and Kanamarlapudi 2013). Compounds A and B have also demonstrated ago-allosteric properties and have induced cAMP signalling and increased insulin secretion in rat islets and animal studies (Thompson and Kanamarlapudi 2013; Thompson et al. 2016). One study showed that compound B almost normalized insulin secretion in human islets isolated from a donor with type 2 diabetes (Thompson and Kanamarlapudi 2013). Compounds 2 and B bind to a distinct site from the orthosteric binding site as GLP-1 antagonists Ex(9–39) and JANT4 as well as the V36A mutation failed to inhibit cAMP production upon compound 2 or B administration (Thompson et al. 2016). Compounds 2 and B induced cAMP production in human GLP-1R expressing cells but caused no intracellular Ca2+ accumulation, ERK phosphorylation, or receptor internalization (Thompson et al. 2016). The K334A mutation (which affects Gαs coupling) of GLP-1R inhibited both GLP-1 and compounds 2 and B induced cAMP production; this indicates that GLP-1 and both compounds 2 and B induce similar conformational changes for G-protein activation (Thompson et al. 2016). A recent study showed that compounds 2 and B bind and covalently modify Cys347 in ICL3 of GLP-1R (Nolte et al. 2014).

Summary

This review has discussed the discovery, synthesis, structure, functional domains, therapeutic applications, and allosteric sites of GLP-1R. GLP-1R is a class B GPCR, and the ability of GLP-1R to augment insulin production in response to binding of GLP-1 during the postprandial period is its best characterized physiological effect (Graaf et al. 2016). The structural relevance of various domains of GLP-1R has been elucidated by numerous studies (Thompson and Kanamarlapudi 2013, 2014, 2015a). The pathway(s) required for agonist-induced GLP-1R internalization has also been elucidated (Thompson and Kanamarlapudi 2015b). GLP-1 analogues are currently used to treat type 2 diabetic subjects as they promote normoglycemia in patients. The allosteric sites on GLP-1R and their agonists are currently being examined for their therapeutic potential (Thompson et al. 2016).

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© Springer International Publishing AG 2018

Authors and Affiliations

  1. 1.Institute of Life Science 1, School of MedicineSwansea UniversitySwanseaUK