Scanning of the glucagon-like peptide-1 receptor localizes G protein-activating determinants primarily to the N terminus of the third intracellular loop

Naturally occurring mutations in G protein-coupled receptors associated with obesity and type 2 diabetes mellitus

G protein-coupled receptors (GPCRs) are the largest family of membrane receptors involved in the regulation of almost all known physiological processes. Dysfunctions of GPCR-mediated signaling have been shown to cause various diseases. The prevalence of obesity and type 2 diabetes mellitus (T2DM), two strongly associated disorders, is increasing worldwide, with tremendous economical and health burden. New safer and more efficacious drugs are required for successful weight reduction and T2DM treatment. Multiple GPCRs are involved in the regulation of energy and glucose homeostasis. Mutations in these GPCRs contribute to the development and progression of obesity and T2DM. Therefore, these receptors can be therapeutic targets for obesity and T2DM. Indeed some of these receptors, such as melanocortin-4 receptor and glucagon-like peptide 1 receptor, have provided important new drugs for treating obesity and T2DM. This review will focus on the naturally occurring mutations of several GPCRs associated with obesity and T2DM, especially incorporating recent large genomic data and insights from structure-function studies, providing leads for future investigations.

The Role of ICL1 and H8 in Class B1 GPCRs; Implications for Receptor Activation

The first intracellular loop (ICL1) of G protein-coupled receptors (GPCRs) has received little attention, although there is evidence that, with the 8^th helix (H8), it is involved in early conformational changes following receptor activation as well as contacting the G protein β subunit. In class B1 GPCRs, the distal part of ICL1 contains a conserved R^12.48KLRCxR^2.46b motif that extends into the base of the second transmembrane helix; this is weakly conserved as a [R/H]^12.48KL[R/H] motif in class A GPCRs. In the current study, the role of ICL1 and H8 in signaling through cAMP, _iCa²⁺ and ERK1/2 has been examined in two class B1 GPCRs, using mutagenesis and molecular dynamics. Mutations throughout ICL1 can either enhance or disrupt cAMP production by CGRP at the CGRP receptor. Alanine mutagenesis identified subtle differences with regard elevation of _iCa²⁺, with the distal end of the loop being particularly sensitive. ERK1/2 activation displayed little sensitivity to ICL1 mutation. A broadly similar pattern was observed with the glucagon receptor, although there were differences in significance of individual residues. Extending the study revealed that at the CRF1 receptor, an insertion in ICL1 switched signaling bias between _iCa²⁺ and cAMP. Molecular dynamics suggested that changes in ICL1 altered the conformation of ICL2 and the H8/TM7 junction (ICL4). For H8, alanine mutagenesis showed the importance of E390^8.49b for all three signal transduction pathways, for the CGRP receptor, but mutations of other residues largely just altered ERK1/2 activation. Thus, ICL1 may modulate GPCR bias via interactions with ICL2, ICL4 and the Gβ subunit.

Current understanding of the structure and function of family B GPCRs to design novel drugs

Family B of G-protein-coupled receptors (GPCRs) and their ligands play a central role in a number of homeostatic mechanisms in the endocrine, gastrointestinal, skeletal, immune, cardiovascular and central nervous systems. Alterations in family B GPCR-regulated homeostatic mechanisms may cause a variety of potentially life-threatening conditions, signifying the necessity to develop novel ligands targeting these receptors. Obtaining structural and functional information on family B GPCRs will accelerate the development of novel drugs to target these receptors. Family B GPCRs are proteins that span the plasma membrane seven times, thus forming seven transmembrane domains (TM1-TM7) which are connected to each other by three extracellular (EL) and three intracellular (IL) loops. In addition, these receptors have a long extracellular N-domain and an intracellular C-tail. The upper parts of the TMs and ELs form the J-domain of receptors. The C-terminal region of peptides first binds to the N-domain of receptors. This 'first-step' interaction orients the N-terminal region of peptides towards the J-domain of receptors, thus resulting in a 'second-step' of ligand-receptor interaction that activates the receptor. Activation-associated structural changes of receptors are transmitted through TMs to their intracellular regions and are responsible for their interaction with the G proteins and activation of the latter, thus resulting in a biological effect. This review summarizes the current information regarding the structure and function of family B GPCRs and their physiological and pathophysiological roles.

Key interactions by conserved polar amino acids located at the transmembrane helical boundaries in Class B GPCRs modulate activation, effector specificity and biased signalling in the glucagon-like peptide-1 receptor

Class B GPCRs can activate multiple signalling effectors with the potential to exhibit biased agonism in response to ligand stimulation. Previously, we highlighted key TM domain polar amino acids that were crucial for the function of the GLP-1 receptor, a key therapeutic target for diabetes and obesity. Using a combination of mutagenesis, pharmacological characterisation, mathematical and computational molecular modelling, this study identifies additional highly conserved polar residues located towards the TM helical boundaries of Class B GPCRs that are important for GLP-1 receptor stability and/or controlling signalling specificity and biased agonism. This includes (i) three positively charged residues (R3.30227, K4.64288, R5.40310) located at the extracellular boundaries of TMs 3, 4 and 5 that are predicted in molecular models to stabilise extracellular loop 2, a crucial domain for ligand affinity and receptor activation; (ii) a predicted hydrogen bond network between residues located in TMs 2 (R2.46176), 6 (R6.37348) and 7 (N7.61406 and E7.63408) at the cytoplasmic face of the receptor that is important for stabilising the inactive receptor and directing signalling specificity, (iii) residues at the bottom of TM 5 (R5.56326) and TM6 (K6.35346 and K6.40351) that are crucial for receptor activation and downstream signalling; (iv) residues predicted to be involved in stabilisation of TM4 (N2.52182 and Y3.52250) that also influence cell signalling. Collectively, this work expands our understanding of peptide-mediated signalling by the GLP-1 receptor.

Molecular Characterisation of Small Molecule Agonists Effect on the Human Glucagon Like Peptide-1 Receptor Internalisation

The glucagon-like peptide receptor (GLP-1R), which is a G-protein coupled receptor (GPCR), signals through both Gαs and Gαq coupled pathways and ERK phosphorylation to stimulate insulin secretion. The aim of this study was to determine molecular details of the effect of small molecule agonists, compounds 2 and B, on GLP-1R mediated cAMP production, intracellular Ca2+ accumulation, ERK phosphorylation and its internalisation. In human GLP-1R (hGLP-1R) expressing cells, compounds 2 and B induced cAMP production but caused no intracellular Ca2+ accumulation, ERK phosphorylation or hGLP-1R internalisation. GLP-1 antagonists Ex(9-39) and JANT-4 and the orthosteric binding site mutation (V36A) in hGLP-1R failed to inhibit compounds 2 and B induced cAMP production, confirming that their binding site distinct from the GLP-1 binding site on GLP-1R. However, K334A mutation of hGLP-1R, which affects Gαs coupling, inhibited GLP-1 as well as compounds 2 and B induced cAMP production, indicating that GLP-1, compounds 2 and B binding induce similar conformational changes in the GLP-1R for Gαs coupling. Additionally, compound 2 or B binding to the hGLP-1R had significantly reduced GLP-1 induced intracellular Ca2+ accumulation, ERK phosphorylation and hGLP-1R internalisation. This study illustrates pharmacology of differential activation of GLP-1R by GLP-1 and compounds 2 and B.

Structural Determinants of Binding the Seven-transmembrane Domain of the Glucagon-like Peptide-1 Receptor (GLP-1R)

The glucagon-like peptide-1 receptor (GLP-1R) belongs to the secretin-like (class B) family of G protein-coupled receptors. Members of the class B family are distinguished by their large extracellular domain, which works cooperatively with the canonical seven-transmembrane (7TM) helical domain to signal in response to binding of various peptide hormones. We have combined structure-based site-specific mutational studies with molecular dynamics simulations of a full-length model of GLP-1R bound to multiple peptide ligand variants. Despite the high sequence similarity between GLP-1R and its closest structural homologue, the glucagon receptor (GCGR), nearly half of the 62 stably expressed mutants affected GLP-1R in a different manner than the corresponding mutants in GCGR. The molecular dynamics simulations of wild-type and mutant GLP-1R·ligand complexes provided molecular insights into GLP-1R-specific recognition mechanisms for the N terminus of GLP-1 by residues in the 7TM pocket and explained how glucagon-mimicking GLP-1 mutants restored binding affinity for (GCGR-mimicking) GLP-1R mutants. Structural analysis of the simulations suggested that peptide ligand binding mode variations in the 7TM binding pocket are facilitated by movement of the extracellular domain relative to the 7TM bundle. These differences in binding modes may account for the pharmacological differences between GLP-1 peptide variants.

Transmembrane signal transduction by peptide hormones via family B G protein-coupled receptors

Although family B G protein-coupled receptors (GPCRs) contain only 15 members, they play key roles in transmembrane signal transduction of hormones. Family B GPCRs are drug targets for developing therapeutics for diseases ranging from metabolic to neurological disorders. Despite their importance, the molecular mechanism of activation of family B GPCRs remains largely unexplored due to the challenges in expression and purification of functional receptors to the quantity for biophysical characterization. Currently, there is no crystal structure available of a full-length family B GPCR. However, structures of key domains, including the extracellular ligand binding regions and seven-helical transmembrane regions, have been solved by X-ray crystallography and NMR, providing insights into the mechanisms of ligand recognition and selectivity, and helical arrangements within the cell membrane. Moreover, biophysical and biochemical methods have been used to explore functions, key residues for signaling, and the kinetics and dynamics of signaling processes. This review summarizes the current knowledge of the signal transduction mechanism of family B GPCRs at the molecular level and comments on the challenges and outlook for mechanistic studies of family B GPCRs.

Distinct regions in the C-Terminus required for GLP-1R cell surface expression, activity and internalisation

The glucagon-like peptide-1 (GLP-1) receptor (GLP-1R), an important drug target in the treatment of type 2 diabetes, is a G-protein coupled receptor (GPCR) that mediates insulin secretion by GLP-1. The N-terminus controls GLP-1R biosynthetic trafficking to the cell surface but the C-terminus involvement in that trafficking is unknown. The aim of this study was to identify distinct regions within the C-terminal domain required for human GLP-1R (hGLP-1R) cell surface expression, activity and internalisation using a number of C-terminal deletions and site-directed mutations. The results of this study revealed that the residues 411-418 within the C-terminal domain of the hGLP-1R 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 hGLP-1R internalisation. In conclusion, these findings demonstrate the hGLP-1R has distinct regions within the C-terminal domain required for its cell surface expression, activity and agonist-induced internalisation.

Genetic variability in GLP-1 receptor is associated with inter-individual differences in weight lowering potential of liraglutide in obese women with PCOS: a pilot study

PURPOSE

The weight lowering potential of glucagon-like peptide 1 (GLP-1) receptor agonists (RAs) is inter-individually different and clinically unpredictable. The potential role of genetic variability of GLP-1R on body weight response to GLP-1 RAs in obese women with polycystic ovary syndrome (PCOS) has not yet been evaluated.

METHODS

Fifty-seven obese women with PCOS (aged 30.7 ± 7.0, BMI 38.6 ± 5.3 kg/m(2)) were assigned to liraglutide 1.2 mg QD s.c. for 12 weeks and classified as strong responders regarding weight loss if they lost 5% or more of their initial body weight. They were genotyped for common GLP-1R single nucleotide polymorphisms (SNPs) rs6923761 and rs10305420. Changes of measures of obesity were measured before and at the end of the treatment.

RESULTS

Twenty out of 57 subjects were strong responders and lost 7.38 ± 1.74 compared to 2.11 ± 2.17 kg lost in poor responders. Carriers of at least one polymorphic rs10305420 allele had poor treatment response compared to carriers of two wild type alleles (OR = 0.27, 95% CI = 0.09-0.85, P = 0.025). Carriers of at least one polymorphic rs6923761 allele tended to have stronger treatment response compared to carriers of two wild type alleles (OR = 3.06, 95% CI = 0.96-9.74, P = 0.058). Fasting glucose and glucose after oral glucose tolerance test (OGTT) comparably decreased in both groups when compared to baseline, whereas no within treatment differences were found in androgen profile. Gastrointestinal adverse events were transit and balanced between strong and poor responders.

CONCLUSIONS

GLP-1R rs10305420 polymorphism explained some of the inter-individual differences in response to liraglutide regarding weight loss in obese PCOS women.

Apolipoprotein A-I increases insulin secretion and production from pancreatic β-cells via a G-protein-cAMP-PKA-FoxO1-dependent mechanism

OBJECTIVE

Therapeutic interventions that increase plasma levels of high-density lipoproteins and apolipoprotein A-I (apoA-I) A-I, the major high-density lipoprotein apolipoprotein, improve glycemic control in people with type 2 diabetes mellitus. High-density lipoproteins and apoA-I also enhance insulin synthesis and secretion in isolated pancreatic islets and clonal β-cell lines. This study identifies the signaling pathways that mediate these effects.

APPROACH AND RESULTS

Incubation with apoA-I increased cAMP accumulation in Ins-1E cells in a concentration-dependent manner. The increase in cAMP levels was inhibited by preincubating the cells with the cell-permeable, transmembrane adenylate cyclase inhibitor, 2'5' dideoxyadenosine, but not with KH7, which inhibits soluble adenylyl cyclases. Incubation of Ins-1E cells with apoA-I resulted in colocalization of ATP-binding cassette transporter A1 with the Gαs subunit of a heterotrimeric G-protein and a Gαs subunit-dependent increase in insulin secretion. Incubation of Ins-1E cells with apoA-I also increased protein kinase A phosphorylation and reduced the nuclear localization of forkhead box protein O1 (FoxO1). Preincubation of Ins-1E cells with the protein kinase A-specific inhibitors, H89 and PKI amide, prevented apoA-I from increasing insulin secretion and mediating the nuclear exclusion of FoxO1. Transfection of Ins-1E cells with a mutated FoxO1 that is restricted to the nucleus confirmed the requirement for FoxO1 nuclear exclusion by blocking insulin secretion in apoA-I-treated Ins-1E cells. ApoA-I also increased Irs1, Irs2, Ins1, Ins2, and Pdx1 mRNA levels.

CONCLUSIONS

ApoA-I increases insulin synthesis and secretion via a heterotrimeric G-protein-cAMP-protein kinase A-FoxO1-dependent mechanism that involves transmembrane adenylyl cyclases and increased transcription of key insulin response and β-cell survival genes.

A potentiator of orthosteric ligand activity at GLP-1R acts via covalent modification

We report that 4-(3-(benzyloxy)phenyl)-2-ethylsulfinyl-6-(trifluoromethyl)pyrimidine (BETP), which behaves as a positive allosteric modulator at the glucagon-like peptide-1 receptor (GLP-1R), covalently modifies cysteines 347 and 438 in GLP-1R. C347, located in intracellular loop 3 of GLP-1R, is critical to the activity of BETP and a structurally distinct GLP-1R ago-allosteric modulator, N-(tert-butyl)-6,7-dichloro-3-(methylsulfonyl)quinoxalin-2-amine. We further show that substitution of cysteine for phenylalanine 345 in the glucagon receptor is sufficient to confer sensitivity to BETP.

Regulation of glucose homeostasis by GLP-1

Glucagon-like peptide-1(7-36)amide (GLP-1) is a secreted peptide that acts as a key determinant of blood glucose homeostasis by virtue of its abilities to slow gastric emptying, to enhance pancreatic insulin secretion, and to suppress pancreatic glucagon secretion. GLP-1 is secreted from L cells of the gastrointestinal mucosa in response to a meal, and the blood glucose-lowering action of GLP-1 is terminated due to its enzymatic degradation by dipeptidyl-peptidase-IV (DPP-IV). Released GLP-1 activates enteric and autonomic reflexes while also circulating as an incretin hormone to control endocrine pancreas function. The GLP-1 receptor (GLP-1R) is a G protein-coupled receptor that is activated directly or indirectly by blood glucose-lowering agents currently in use for the treatment of type 2 diabetes mellitus (T2DM). These therapeutic agents include GLP-1R agonists (exenatide, liraglutide, lixisenatide, albiglutide, dulaglutide, and langlenatide) and DPP-IV inhibitors (sitagliptin, vildagliptin, saxagliptin, linagliptin, and alogliptin). Investigational agents for use in the treatment of T2DM include GPR119 and GPR40 receptor agonists that stimulate the release of GLP-1 from L cells. Summarized here is the role of GLP-1 to control blood glucose homeostasis, with special emphasis on the advantages and limitations of GLP-1-based therapeutics.

Insights into the structure of class B GPCRs

The secretin-like (class B) family of G protein-coupled receptors (GPCRs) are key players in hormonal homeostasis and are interesting drug targets for the treatment of several metabolic disorders (such as type 2 diabetes, osteoporosis, and obesity) and nervous system diseases (such as migraine, anxiety, and depression). The recently solved crystal structures of the transmembrane domains of the human glucagon receptor and human corticotropin-releasing factor receptor 1 have opened up new opportunities to study the structure and function of class B GPCRs. The current review shows how these structures offer more detailed explanations to previous biochemical and pharmacological studies of class B GPCRs, and provides new insights into their interactions with ligands.

Development of a cysteine-deprived and C-terminally truncated GLP-1 receptor

The glucagon-like peptide-1 receptor (GLP-1R) belongs to family B of the G-protein coupled receptors (GPCRs), and has become a promising target for the treatment of type 2 diabetes. Here we describe the development and characterization of a fully functional cysteine-deprived and C-terminally truncated GLP-1R. Single cysteines were initially substituted with alanine, and functionally redundant cysteines were subsequently changed simultaneously. Our results indicate that Cys(174), Cys(226), Cys(296) and Cys(403) are important for the GLP-1-mediated response, whereas Cys(236), Cys(329), Cys(341), Cys(347), Cys(438), Cys(458) and Cys(462) are not. Extensive deletions were made in the C-terminal tail of GLP-1R in order to determine the limit for truncation. As for other family B GPCRs, we observed a direct correlation between the length of the C-terminal tail and specific binding of (125)I-GLP-1, indicating that the membrane proximal part of the C-terminal is involved in receptor expression at the cell surface. The results show that seven cysteines and more than half of the C-terminal tail can be removed from GLP-1R without compromising GLP-1 binding or function.

Structure of the human glucagon class B G-protein-coupled receptor

Binding of the glucagon peptide to the glucagon receptor (GCGR) triggers the release of glucose from the liver during fasting; thus GCGR plays an important role in glucose homeostasis. Here we report the crystal structure of the seven transmembrane helical domain of human GCGR at 3.4 Å resolution, complemented by extensive site-specific mutagenesis, and a hybrid model of glucagon bound to GCGR to understand the molecular recognition of the receptor for its native ligand. Beyond the shared seven transmembrane fold, the GCGR transmembrane domain deviates from class A G-protein-coupled receptors with a large ligand-binding pocket and the first transmembrane helix having a 'stalk' region that extends three alpha-helical turns above the plane of the membrane. The stalk positions the extracellular domain (~12 kilodaltons) relative to the membrane to form the glucagon-binding site that captures the peptide and facilitates the insertion of glucagon's amino terminus into the seven transmembrane domain.

The activation of the CGRP receptor

The CGRP (calcitonin gene-related peptide) receptor is a family B GPCR (G-protein-coupled receptor). It consists of a GPCR, CLR (calcitonin receptor-like receptor) and an accessory protein, RAMP1 (receptor activity modifying protein 1). RAMP1 is needed for CGRP binding and also cell-surface expression of CLR. CLR is an example of a family B GPCR. Unlike family A GPCRs, little is known about how these receptors are activated by their endogenous ligands. This review considers what is known about the activation of family B GPCRs and then considers how this might be applied to CLR, particularly in light of new knowledge of the crystal structures of family A GPCRs.

Similarity between class A and class B G-protein-coupled receptors exemplified through calcitonin gene-related peptide receptor modelling and mutagenesis studies

Modelling class B G-protein-coupled receptors (GPCRs) using class A GPCR structural templates is difficult due to lack of homology. The plant GPCR, GCR1, has homology to both class A and class B GPCRs. We have used this to generate a class A–class B alignment, and by incorporating maximum lagged correlation of entropy and hydrophobicity into a consensus score, we have been able to align receptor transmembrane regions. We have applied this analysis to generate active and inactive homology models of the class B calcitonin gene-related peptide (CGRP) receptor, and have supported it with site-directed mutagenesis data using 122 CGRP receptor residues and 144 published mutagenesis results on other class B GPCRs. The variation of sequence variability with structure, the analysis of polarity violations, the alignment of group-conserved residues and the mutagenesis results at 27 key positions were particularly informative in distinguishing between the proposed and plausible alternative alignments. Furthermore, we have been able to associate the key molecular features of the class B GPCR signalling machinery with their class A counterparts for the first time. These include the [K/R]KLH motif in intracellular loop 1, [I/L]xxxL and KxxK at the intracellular end of TM5 and TM6, the NPXXY/VAVLY motif on TM7 and small group-conserved residues in TM1, TM2, TM3 and TM7. The equivalent of the class A DRY motif is proposed to involve Arg^2.39, His^2.43 and Glu^3.46, which makes a polar lock with T^6.37. These alignments and models provide useful tools for understanding class B GPCR function.

Calcitonin and calcitonin receptor-like receptors: common themes with family B GPCRs?

The calcitonin receptor (CTR) and calcitonin receptor-like receptor (CLR) are two of the 15 human family B (or Secretin-like) GPCRs. CTR and CLR are of considerable biological interest as their pharmacology is moulded by interactions with receptor activity-modifying proteins. They also have therapeutic relevance for many conditions, such as osteoporosis, diabetes, obesity, lymphatic insufficiency, migraine and cardiovascular disease. In light of recent advances in understanding ligand docking and receptor activation in both the family as a whole and in CLR and CTR specifically, this review reflects how applicable general family B GPCR themes are to these two idiosyncratic receptors. We review the main functional domains of the receptors; the N-terminal extracellular domain, the juxtamembrane domain and ligand interface, the transmembrane domain and the intracellular C-terminal domain. Structural and functional findings from the CLR and CTR along with other family B GPCRs are critically appraised to gain insight into how these domains may function. The ability for CTR and CLR to interact with receptor activity-modifying proteins adds another level of sophistication to these receptor systems but means careful consideration is needed when trying to apply generic GPCR principles. This review encapsulates current thinking in the realm of family B GPCR research by highlighting both conflicting and recurring themes and how such findings relate to two unusual but important receptors, CTR and CLR.

The structure and function of the glucagon-like peptide-1 receptor and its ligands

Glucagon-like peptide-1(7-36)amide (GLP-1) is a 30-residue peptide hormone released from intestinal L cells following nutrient consumption. It potentiates the glucose-induced secretion of insulin from pancreatic beta cells, increases insulin expression, inhibits beta-cell apoptosis, promotes beta-cell neogenesis, reduces glucagon secretion, delays gastric emptying, promotes satiety and increases peripheral glucose disposal. These multiple effects have generated a great deal of interest in the discovery of long-lasting agonists of the GLP-1 receptor (GLP-1R) in order to treat type 2 diabetes. This review article summarizes the literature regarding the discovery of GLP-1 and its physiological functions. The structure, function and sequence-activity relationships of the hormone and its natural analogue exendin-4 (Ex4) are reviewed in detail. The current knowledge of the structure of GLP-1R, a Family B GPCR, is summarized and discussed, before its known interactions with the principle peptide ligands are described and summarized. Finally, progress in discovering non-peptide ligands of GLP-1R is reviewed. GLP-1 is clearly an important hormone linking nutrient consumption with blood sugar control, and therefore knowledge of its structure, function and mechanism of action is of great importance.

Differential determinants for coupling of distinct G proteins with the class B secretin receptor

The secretin receptor is a prototypic class B G protein-coupled receptor that is activated by binding of its natural peptide ligand. The signaling effects of this receptor are mediated by coupling with Gs, which activates cAMP production, and Gq, which activates intracellular calcium mobilization. We have explored the molecular basis for the coupling of each of these G proteins to this receptor using systematic site-directed mutagenesis of key residues within each of the intracellular loop regions, and studying ligand binding and secretin-stimulated cAMP and calcium responses. Mutation of a conserved histidine in the first intracellular loop (H157A and H157R) markedly reduced cell surface expression, resulting in marked reduction in cAMP and elimination of measurable calcium responses. Mutation of an arginine (R153A) in the first intracellular loop reduced calcium, but not cAMP responses. Mutation of a dibasic motif in the second intracellular loop (R231A/K232A) had no significant effects on any measured responses. Mutations in the third intracellular loop involving adjacent lysine and leucine residues (K302A/L303A) or two arginine residues separated by a leucine and an alanine (R318A/R321A) significantly reduced cAMP responses, while the latter also reduced calcium responses. Additive effects were elicited by combining the effective mutations, while combining all the effective mutations resulted in a construct that continued to bind secretin normally, but that elicited no significant cAMP or calcium responses. These data suggest that, while some receptor determinants are clearly shared, there are also distinct determinants for coupling with each of these G proteins.

Mapping structural determinants within third intracellular loop that direct signaling specificity of type 1 corticotropin-releasing hormone receptor

The type 1 corticotropin-releasing hormone receptor (CRH-R1) influences biological responses important for adaptation to stressful stimuli, through activation of multiple downstream effectors. The structural motifs within CRH-R1 that mediate G protein activation and signaling selectivity are unknown. The aim of this study was to gain insights about important structural determinants within the third intracellular loop (IC3) of the human CRH-R1α important for cAMP and ERK1/2 pathways activation and selectivity. We investigated the role of the juxtamembrane regions of IC3 by mutating amino acid cassettes or specific residues to alanine. Although simultaneous tandem alanine mutations of both juxtamembrane regions Arg²⁹²-Met²⁹⁵ and Lys³¹¹-Lys³¹⁴ reduced ligand binding and impaired signaling, all other mutant receptors retained high affinity binding, indistinguishable from wild-type receptor. Agonist-activated receptors with tandem mutations at the proximal or distal terminal segments enhanced activation of adenylyl cyclase by 50–75% and diminished activation of inositol trisphosphate and ERK1/2 by 60–80%. Single Ala mutations identified Arg²⁹², Lys²⁹⁷, Arg³¹⁰, Lys³¹¹, and Lys³¹⁴ as important residues for the enhanced activation of adenylyl cyclase, partly due to reduced inhibition of adenylyl cyclase activity by pertussis toxin-sensitive G proteins. In contrast, mutation of Arg²⁹⁹ reduced receptor signaling activity and cAMP response. Basic as well as aliphatic amino acids within both juxtamembrane regions were identified as important for ERK1/2 phosphorylation through activation of pertussis toxin-sensitive G proteins as well as G_q proteins. These data uncovered unexpected roles for key amino acids within the highly conserved hydrophobic N- and C-terminal microdomains of IC3 in the coordination of CRH-R1 signaling activity.

Systems analysis of GLP-1 receptor signaling in pancreatic β-cells

Glucagon-like peptide-1 (GLP-1) elevates intracellular concentration of cAMP ([cAMP]) and facilitates glucose-dependent insulin secretion in pancreatic β-cells. There has been much evidence to suggest that multiple key players such as the GLP-1 receptor, G(s) protein, adenylate cyclase (AC), phosphodiesterase (PDE), and intracellular Ca(2+) concentration ([Ca(2+)]) are involved in the regulation of [cAMP]. However, because of complex interactions among these signaling factors, the kinetics of the reaction cascade as well as the activities of ACs and PDEs have not been determined in pancreatic β-cells. We have constructed a minimal mathematical model of GLP-1 receptor signal transduction based on experimental findings obtained mostly in β-cells and insulinoma cell lines. By fitting this theoretical reaction scheme to key experimental records of the GLP-1 response, the parameters determining individual reaction steps were estimated. The model reconstructed satisfactorily the dynamic changes in [cAMP] and predicted the activities of cAMP effectors, protein kinase A (PKA), and cAMP-regulated guanine nucleotide exchange factor [cAMP-GEF or exchange protein directly activated by cAMP (Epac)] during GLP-1 stimulation. The simulations also predicted the presence of two sequential desensitization steps of the GLP1 receptor that occur with fast and very slow reaction rates. The cross talk between glucose- and GLP-1-dependent signal cascades for cAMP synthesis was well reconstructed by integrating the direct regulation of AC and PDE by [Ca(2+)]. To examine robustness of the signaling system in controlling [cAMP], magnitudes of AC and PDE activities were compared in the presence or absence of GLP-1 and/or the PDE inhibitor IBMX.(1).

Third intracellular loop of glucagon like-peptide-1 receptor is coupled with endogenous mono-ADP-ribosyltransferase - novel type of receptor regulation?

Our previous studies revealed the main role of the third intracellular loop (IC(3)) of glucagon-like peptide-1 receptor (GLP-1 receptor), in G-protein activation, where the presence or absence of agonist and the receptor phosphorylation seemed to be the only regulatory mechanisms. In order to further study the signaling mechanisms of GLP-1 receptor, we investigated the effect of the third intracellular loop-derived peptide on endogenous mono-ADP-ribosyltransferase mediated mono-ADP-ribosylation of G-proteins β subunit in CHO cells. Results showed an inhibitory effect of IC(3) peptide on mono-ADP-ribosylation of β subunit, obviously via the mechanism of competitive inhibition. Excluding the activity of this inhibitory mechanism via pertussis toxin-sensitive G proteins, the direct functional coupling of IC(3) of GLP-1 receptor and endogenous mono-ADP-ribosyltransferase was confirmed. We suggest that this arginine specific enzymatic posttranslational modification of third intracellular loop of GLP-1 receptor might represent a possible novel mechanism of receptor activity regulation and the pharmacological potential in treatment of diabetes mellitus type 2.

Functional characterization of N-terminally GFP-tagged GLP-1 receptor

The glucagon-like peptide-1 receptor (GLP-1 receptor) mediates important effects on peripheral tissues and the central nervous system. It seems one of the most promising therapeutic targets for treatment of diabetes mellitus type 2. Surprisingly, very little is known about the cellular mechanisms that regulate its function in vivo. One of the approaches to study receptor dynamics, expression, or signaling is using GFP-tagged fluorescent proteins. In this study, we synthesized and characterized N-terminally GFP-tagged GLP-1 (GFP-GLP-1) receptor in CHO cells. We demonstrated that GFP-GLP-1 receptor is weakly expressed in the plasma membranes and is functionally coupled to adenylyl cyclase via heterotrimeric G-proteins, similarly as its wild type.

Pharmacological characterization of human incretin receptor missense variants

Glucagon-like peptide 1 (GLP-1) and glucose-dependent insulinotropic polypeptide (GIP) are gut-derived incretin hormones that regulate blood glucose levels. In addition to their widely accepted insulinotropic role, there is evidence that GLP-1 modulates feeding behavior and GIP regulates lipid metabolism, thereby promoting postprandial fat deposition. In this study, we investigated whether naturally occurring polymorphisms in the GLP-1 receptor (GLP-1R) and the GIP receptor (GIP-R) affect the pharmacological properties of these proteins. After transient expression of the receptors in human embryonic kidney 293 cells, basal and ligand-induced cAMP production were assessed by use of luciferase reporter gene assays. Our data reveal that the wild-type GIP-R displays a considerable degree of ligand-independent activity. In comparison, the GIP-R variants C46S, G198C, R316L, and E354Q show a marked decrease in basal signaling that may, at least in part, be explained by reduced cell surface expression. When stimulated with GIP, the C46S and R316L mutants display significantly reduced potency (>1000 and 25- fold, respectively) compared with wild type. Complementary competition binding assays further demonstrate that the C46S variant fails to bind radio-iodinated GIP, whereas all other GIP-R mutants maintain normal ligand affinity. In contrast to the GIP-R, the wild-type GLP-1R lacks constitutive activity. Furthermore, none of the 10 GLP-1R missense mutations showed an alteration in pharmacological properties versus wild type. The extent to which abnormalities in GIP-R function may lead to physiological changes or affect drug sensitivity in selected populations (e.g., obese, diabetic individuals) remains to be further investigated.

The third intracellular loop stabilizes the inactive state of the neuropeptide Y1 receptor

Constitutively active G-protein-coupled receptors (GPCRs) can signal even in the absence of ligand binding. Most Class I GPCRs are stabilized in the resting conformation by intramolecular interactions involving transmembrane domain (TM) 3 and TM6, particularly at loci 6.30 and 6.34 of TM6. Signaling by Gi/Go-coupled receptors such as the Neuropeptide Y1 receptor decreases already low basal metabolite levels. Thus, we examined constitutive activity using a biochemical assay mediated by a Gi/Gq chimeric protein and a more direct electrophysiological assay. Wild-type (WT-Y1) receptors express no measurable, agonist-independent activation, while mu-opioid receptors (MOR) and P2Y12 purinoceptors showed clear evidence of constitutive activation, especially in the electrophysiological assay. Neither point mutations at TM6 (T6.30A or N6.34A) nor substitution of the entire TM3 and TM6 regions from the MOR into the Y1 receptor increased basal WT-Y1 activation. By contrast, chimeric substitution of the third intracellular loop (ICL3) generated a constitutively active, Y1-ICL3-MOR chimera. Furthermore, the loss of stabilizing interactions from the native ICL3 enhanced the role of surrounding residues to permit basal receptor activation; because constitutive activity of the Y1-ICL3-MOR chimera was further increased by point mutation at locus 6.34, which did not alter WT-Y1 receptor activity. Our results indicate that the ICL3 stabilizes the Y1 receptor in the inactive state and confers structural properties critical for regulating Y receptor activation and signal transduction. These studies reveal the active participation of the ICL3 in the stabilization and activation of Class I GPCRs.

First cytoplasmic loop of glucagon-like peptide-1 receptor can function at the third cytoplasmic loop position of rhodopsin

G protein-coupled receptors (GPCRs) are classified into several families based on their amino acid sequences. In family 1, GPCRs such as rhodopsin and adrenergic receptor, the structure-function relationship has been extensively investigated to demonstrate that exposure of the third cytoplasmic loop is essential for selective G protein activation. In contrast, much less is known about other families. Here we prepared chimeric mutants between Gt-coupled rhodopsin and Gi/Go- and Gs-coupled glucagon-like peptide-1 (GLP-1) receptor of family 2 and tried to identify the loop region that functions at the third cytoplasmic loop position of rhodopsin. We succeeded in expressing a mutant having the first cytoplasmic loop of GLP-1 receptor and found that this mutant activated Gi and Go efficiently but did not activate Gt. Moreover, the rhodopsin mutant having the first loop of Gs-coupled secretin receptor of family 2 decreased the Gi and Go activation efficiencies. Therefore, the first loop of GLP-1 receptor would share a similar role to the third loop of rhodopsin in G protein activation. This result strongly suggested that different families of GPCRs have maintained molecular architectures of their ancestral types to generate a common mechanism, namely exposure of the cytoplasmic loop, to activate peripheral G protein.

Cysteine at position 217 in the intracellular loop 1 plays a critical role in human PTH receptor type 1 membrane translocation and function

PTHR1 mutants lacking endogenous cysteines in transmembrane and intracellular domains were generated. Mutant receptors were tested for their biological activities and mRNA and cell surface expression levels. C217 in intracellular loop 1 was determined to play a critical role in cell surface translocation and function of the receptor.

INTRODUCTION

Elucidating the role of different domains of PTH receptor 1 (PTHR1) is essential for understanding the mechanism of ligand-receptor interactions. Here we present a study directed at determining the importance of cysteine residues present in the intracellular and transmembrane (TM) domains of the receptor.

MATERIALS AND METHODS

Mutant receptors were generated by site-directed mutagenesis. Biological activities were characterized by adenylyl cyclase and competition binding assays. RT-PCR, ELISA, and immunofluorescence microscopy were carried out to determine receptor mRNA and protein expression levels.

RESULTS

Mutations C460L and C462L in TM7, C568L in the C-terminal intracellular domain of the receptor, and removal of C397 in intracellular loop (ICL)3 by insertion of cleavage sites for Factor Xa did not affect binding affinity of PTH or agonist-induced adenylyl cyclase activity, although maximal responses (IC(max) and EC(max)) were decreased. However, mutations C217L in ICL1 or both C217L and C568L simultaneously resulted in a decrease in binding and loss of adenylyl cyclase activity. RT-PCR results showed that the observed changes in binding and activity were not caused by changes in mRNA expression. Next, we determined cell surface and total expression of the wildtype and mutant receptors by ELISA. We found that mutations of C460/C462 to L moderately decreased transfer of receptors to the cell surface. However, mutation of C217 to L in the ICL1 drastically reduced cell surface expression. Immunofluorescence and confocal microscopy studies confirmed reduced cell surface expression of receptors containing the C217L mutation. Similar results were obtained when replacing C217 and C460/C462 of the receptor with A instead of L.

CONCLUSIONS

Our studies indicate that the cysteine at position 217 in ICL1 plays a critical role in translocation to the cell surface and biological function of PTHR1.

Mechanisms of action of glucagon-like peptide 1 in the pancreas

Glucagon-like peptide 1 (GLP-1) is a hormone that is encoded in the proglucagon gene. It is mainly produced in enteroendocrine L cells of the gut and is secreted into the blood stream when food containing fat, protein hydrolysate, and/or glucose enters the duodenum. Its particular effects on insulin and glucagon secretion have generated a flurry of research activity over the past 20 years culminating in a naturally occurring GLP-1 receptor (GLP-1R) agonist, exendin 4 (Ex-4), now being used to treat type 2 diabetes mellitus (T2DM). GLP-1 engages a specific guanine nucleotide-binding protein (G-protein) coupled receptor (GPCR) that is present in tissues other than the pancreas (brain, kidney, lung, heart, and major blood vessels). The most widely studied cell activated by GLP-1 is the insulin-secreting beta cell where its defining action is augmentation of glucose-induced insulin secretion. Upon GLP-1R activation, adenylyl cyclase (AC) is activated and cAMP is generated, leading, in turn, to cAMP-dependent activation of second messenger pathways, such as the protein kinase A (PKA) and Epac pathways. As well as short-term effects of enhancing glucose-induced insulin secretion, continuous GLP-1R activation also increases insulin synthesis, beta cell proliferation, and neogenesis. Although these latter effects cannot be currently monitored in humans, there are substantial improvements in glucose tolerance and increases in both first phase and plateau phase insulin secretory responses in T2DM patients treated with Ex-4. This review will focus on the effects resulting from GLP-1R activation in the pancreas.

Opsins and clusters of sensory G-protein-coupled receptors in the sea urchin genome

Rhodopsin-type G-protein-coupled receptors (GPCRs) contribute the majority of sensory receptors in vertebrates. With 979 members, they form the largest GPCR family in the sequenced sea urchin genome, constituting more than 3% of all predicted genes. The sea urchin genome encodes at least six Opsin proteins. Of these, one rhabdomeric, one ciliary and two G(o)-type Opsins can be assigned to ancient bilaterian Opsin subfamilies. Moreover, we identified four greatly expanded subfamilies of rhodopsin-type GPCRs that we call sea urchin specific rapidly expanded lineages of GPCRs (surreal-GPCRs). Our analysis of two of these groups revealed genomic clustering and single-exon gene structures similar to the most expanded group of vertebrate rhodopsin-type GPCRs, the olfactory receptors. We hypothesize that these genes arose by rapid duplication in the echinoid lineage and act as chemosensory receptors of the animal. In support of this, group B surreal-GPCRs are most prominently expressed in distinct classes of pedicellariae and tube feet of the adult sea urchin, structures that have previously been shown to react to chemical stimuli and to harbor sensory neurons in echinoderms. Notably, these structures also express different opsins, indicating that sea urchins possess an intricate molecular set-up to sense their environment.

Role of cysteine 341 and arginine 348 of GLP-1 receptor in G-protein coupling

We have demonstrated the ability of peptides derived from the third intracellular loop of GLP-1 receptor to differently modulate activity of four different types of G-proteins overexpressed in sf9 cells. In this respect, the involvement of Cys(341) in inhibition of G(s) and Cys(341) in activation of G(s) and in inhibition of G(i1,) G(o), and G(11), respectively, indicates their potential role in discrimination between different types of G-proteins. Moreover, these two amino acids from the third intracellular loop might represent an important novel targets for covalent modification by downstream regulators in signaling through GLP-1 receptor.

Isolation and structure-function studies of a glucagon-like peptide 1 receptor from goldfish Carassius auratus: identification of three charged residues in extracellular domains critical for receptor function

A better understanding of the molecular mechanism of ligand-receptor interaction of glucagon-like peptide 1 (GLP-1) receptors (GLP-1Rs) is useful for the design of potent GLP-1 analogs that could potentially be used as a treatment for diabetic patients. Changes in the ligand and receptor sequences during evolution provide invaluable clues to evaluate the functional motifs of the receptor that are responsible for ligand interaction. For these reasons, in the present study, we have isolated and functionally characterized a GLP-1R from goldfish. Its amino acid sequence shows 50.8% and 52.3% identity with the human glucagon (hGLU) and GLP-1Rs, respectively, and 84.1% with the zebrafish GLP-1R (the only other GLP-1R isolated from teleost fish). Peptides that are structurally different from goldfish (gf)GLP-1, such as gfGLU and hGLU and human GLP-1 (7-36)amide, are also capable of stimulating this receptor, albeit with lower potencies than gfGLP-1. gfGLP-1 stimulates the formation of cAMP through the recombinant gfGLP-1R with EC(50) = 0.18 nM, whereas EC(50) values for gfGLU, human GLP-1 (7-36)amide, and hGLU are 0.53 nM, 0.9 nM, and 1.2 nM, respectively. These results indicate that the gfGLP-1R is structurally more flexible than its mammalian counterpart and that its binding pocket can accommodate a wider spectrum of peptide ligands. Previous studies demonstrated that the charged residues in the extracellular domains of mammalian GLP-1R, particularly those found in the N-terminal domain and the first exoloop, are important for ligand binding. We investigated the roles of the conserved charged residues in the function of the gfGLP-1R. Eleven mutant receptors were constructed, and the effects of mutations were determined by functional assays. Our results demonstrated that three charged residues (D(113), R(197), and D(205)) present in the extracellular domains are critical for receptor function.

Mutational and computational analysis of the alpha(1b)-adrenergic receptor. Involvement of basic and hydrophobic residues in receptor activation and G protein coupling

To investigate their role in receptor coupling to G(q), we mutated all basic amino acids and some conserved hydrophobic residues of the cytosolic surface of the alpha(1b)-adrenergic receptor (AR). The wild type and mutated receptors were expressed in COS-7 cells and characterized for their ligand binding properties and ability to increase inositol phosphate accumulation. The experimental results have been interpreted in the context of both an ab initio model of the alpha(1b)-AR and of a new homology model built on the recently solved crystal structure of rhodopsin. Among the twenty-three basic amino acids mutated only mutations of three, Arg(254) and Lys(258) in the third intracellular loop and Lys(291) at the cytosolic extension of helix 6, markedly impaired the receptor-mediated inositol phosphate production. Additionally, mutations of two conserved hydrophobic residues, Val(147) and Leu(151) in the second intracellular loop had significant effects on receptor function. The functional analysis of the receptor mutants in conjunction with the predictions of molecular modeling supports the hypothesis that Arg(254), Lys(258), as well as Leu(151) are directly involved in receptor-G protein interaction and/or receptor-mediated activation of the G protein. In contrast, the residues belonging to the cytosolic extensions of helices 3 and 6 play a predominant role in the activation process of the alpha(1b)-AR. These findings contribute to the delineation of the molecular determinants of the alpha(1b)-AR/G(q) interface.

Functional segregation of the highly conserved basic motifs within the third endoloop of the human secretin receptor

In this study, a mutagenesis-based strategy was employed to assess the roles of two highly conserved motifs (KLR and RLAR) within the third endoloop of the human secretin receptor. Block deletion of KLRT and mutation of Lys323 (K(323)I) significantly reduced cAMP accumulation, and these mutations did not affect ligand interaction and receptor number expressed on the cell surface. Thus, the KLRT region at the N terminus of the third endoloop, particularly Lys323, is important for G protein coupling. For the RLAR motif, receptors with substitutions at positions 339 and 342 from Arg to Ala (R(339, 342)A), Glu (R(339, 342)E), or Ile (R(339, 342)I) as well as block deletion of the RLAR motif were all found to be defective in both secretin-binding and cAMP production. Interestingly, a single mutation at the corresponding positions of Arg339 or Arg342 responded as the wild-type human secretin receptor in all functional assays, indicating that the presence of one Arg at either position within the RLAR motif is sufficient for a normal receptor function. Immunofluorescent staining of these mutant receptors showed that these Arg residues are responsible for surface presentation and/or receptor stability.

Latrophilin, neurexin, and their signaling-deficient mutants facilitate alpha -latrotoxin insertion into membranes but are not involved in pore formation

Pure alpha-latrotoxin is very inefficient at forming channels/pores in artificial lipid bilayers or in the plasma membrane of non-secretory cells. However, the toxin induces pores efficiently in COS-7 cells transfected with the heptahelical receptor latrophilin or the monotopic receptor neurexin. Signaling-deficient (truncated) mutants of latrophilin and latrophilin-neurexin hybrids also facilitate pore induction, which correlates with toxin binding irrespective of receptor structure. This rules out the involvement of signaling in pore formation. With any receptor, the alpha-latrotoxin pores are permeable to Ca(2+) and small molecules including fluorescein isothiocyanate and norepinephrine. Bound alpha-latrotoxin remains on the cell surface without penetrating completely into the cytosol. Higher temperatures facilitate insertion of the toxin into the plasma membrane, where it co-localizes with latrophilin (under all conditions) and with neurexin (in the presence of Ca(2+)). Interestingly, on subsequent removal of Ca(2+), alpha-latrotoxin dissociates from neurexin but remains in the membrane and continues to form pores. These receptor-independent pores are inhibited by anti-alpha-latrotoxin antibodies. Our results indicate that (i) alpha-latrotoxin is a pore-forming toxin, (ii) receptors that bind alpha-latrotoxin facilitate its insertion into the membrane, (iii) the receptors are not physically involved in the pore structure, (iv) alpha-latrotoxin pores may be independent of the receptors, and (v) pore formation does not require alpha-latrotoxin interaction with other neuronal proteins.

A mutation in the second intracellular loop of the pituitary adenylate cyclase activating polypeptide type I receptor confers constitutive receptor activation

The pituitary adenylate cyclase activating polypeptide (PACAP) type I receptor belongs to the glucagon/secretin/vasoactive intestinal polypeptide (VIP) receptor family. We mutated and deleted an amino acid residue (E261) which is located within the second intracellular loop of the rat PACAP type I receptor and which is highly conserved among the receptor family. The wild-type receptor and the mutant receptors were efficiently expressed at the surface of COS-7 cells at nearly the same level and revealed the same high affinity for the agonist PACAP-27. The cAMP contents of COS cells transfected with the E261A, E261Q, and the deletion mutant receptor were 4.6-, 5.7-, and 6.7-fold higher as compared with COS cells transfected with the wild-type receptor. Thus, all the mutant PACAP receptors were constitutively active. The data suggest that the glutamic acid in the second intracellular loop of the PACAP receptor may be a key residue to constrain the receptor in the inactive conformation with respect to its coupling to G(s) proteins.

Expression and function of the gonadotropin-releasing hormone receptor are dependent on a conserved apolar amino acid in the third intracellular loop

The coupling of agonist-activated heptahelical receptors to their cognate G proteins is often dependent on the amino-terminal region of the third intracellular loop. Like many G protein-coupled receptors, the gonadotropin-releasing hormone (GnRH) receptor contains an apolar amino acid in this region at a constant distance from conserved Pro and Tyr/Asn residues in the fifth transmembrane domain (TM V). An analysis of the role of this conserved residue (Leu(237)) in GnRH receptor function revealed that the binding affinities of the L237I and L237V mutant receptors were unchanged, but their abilities to mediate GnRH-induced inositol phosphate signaling, G protein coupling, and agonist-induced internalization were significantly impaired. Receptor expression at the cell surface was reduced by replacement of Leu(237) with Val, and abolished by replacement with Ala, Arg, or Asp residues. These results are consistent with molecular modeling of the TM V and VI regions of the GnRH receptor, which predicts that Leu(237) is caged by several apolar amino acids (Ile(233), Ile(234), and Val(240) in TM V, and Leu(262), Leu(265), and Val(269) in TM VI) to form a tight hydrophobic cluster. These findings indicate that the conserved apolar residue (Leu(237)) in the third intracellular loop is an important determinant of GnRH receptor expression and activation, and possibly that of other G protein-coupled receptors.

The rat growth hormone-releasing hormone receptor gene: structure, regulation, and generation of receptor isoforms with different signaling properties

The interaction of GHRH with membrane-bound receptors on somatotroph cells of the anterior pituitary is an important step in the regulation of GH synthesis and secretion. The identification of a G protein-coupled receptor for GHRH has made it possible to investigate the pathway by which GHRH regulates pituitary somatotroph cell function. To initiate an analysis of the mechanisms regulating expression and function of the GHRH receptor, the structure of the gene and its promoter region were analyzed. The coding sequence of the rat GHRH receptor gene is contained within 14 exons spanning approximately 15 kb of genomic DNA. Four transcription start sites are located within 286 bp upstream of the initiation codon. The 5' flanking region of the GHRH receptor gene acts as a functional promoter in rat pituitary tumor GH3 cells, and basal promoter activity is enhanced in GH3 and COS7 cells by cotransfection of an expression construct encoding the pituitary-specific transcription factor Pit-1. The rat GHRH receptor gene is subject to at least 1 alternative RNA processing event that generates 2 receptor isoforms differing by 41 amino acids within the third intracellular loop (IL) of the protein. The short isoform of the GHRH receptor is predominant in pituitary cells. The MtT/S pituitary tumor cell line was found to express the GHRH receptor, and different populations of these cells produce predominantly the long or short isoforms of the receptor messenger RNA, suggesting that the alternative splicing can be regulated. Functional analysis of the two GHRH receptor isoforms demonstrates that both bind GHRH, but only the short isoform signals through a cAMP-mediated pathway. Neither receptor isoform is able to stimulate calcium mobilization from internal stores after GHRH treatment. Our findings indicate that the pituitary-specific transcription factor Pit-1 is involved in the somatotroph-specific expression of the GHRH receptor gene and that functionally distinct receptor proteins are generated by an alternative RNA processing mechanism.

Characterization of the carboxyl-terminal domain of the rat glucose-dependent insulinotropic polypeptide (GIP) receptor. A role for serines 426 and 427 in regulating the rate of internalization

Glucose-dependent insulinotropic polypeptide (GIP) is a gastrointestinal hormone involved in the regulation of insulin secretion. In non-insulin-dependent diabetes mellitus insulin responses to GIP are blunted, possibly due to altered signal transduction or reduced receptor number. Site-directed mutagenesis was used to construct truncated GIP receptors to study the importance of the carboxyl-terminal tail (CT) in binding, signaling, and receptor internalization. Receptors truncated at amino acids 425, 418, and 405, expressed in COS-7 or CHO-K1 cells, exhibited similar binding to wild type receptors. GIP-dependent cAMP production with the 405 mutant was decreased in COS-7 cells. Maximal cAMP production in CHO-K1 cells was reduced with all truncated forms. Binding was undetectable with a receptor truncated at amino acid 400; increasing tail length by adding 5 alanines restored binding and signaling. Mutants produced by alanine scanning of residues 394-401, adjacent to transmembrane domain 7, were all functional. CT truncation by 30 or more amino acids, mutation of serines 426/427, singly or combined, or complete CT serine knockout all reduced receptor internalization rate. The majority of the GIP receptor CT is therefore not required for signaling, a minimum chain length of approximately 405 amino acids is needed for receptor expression, and serines 426 and 427 are important for regulating rate of receptor internalization.