The human vasoactive intestinal Peptide/Pituitary adenylate cyclase activating peptide receptor 1 (VPAC1): constitutive activation by mutations at threonine 343

Mechanisms involved in VPAC receptors activation and regulation: lessons from pharmacological and mutagenesis studies

Vasoactive intestinal peptide (VIP) plays diverse and important role in human physiology and physiopathology and their receptors constitute potential targets for the treatment of several diseases such as neurodegenerative disorder, asthma, diabetes, and inflammatory diseases. This article reviews the current knowledge regarding the two VIP receptors, VPAC(1) and VPAC(2), with respect to mechanisms involved in receptor activation, G protein coupling, signaling, regulation, and oligomerization.

Conformational switches in the VPAC(1) receptor

The vasoactive intestinal peptide receptor 1 (VPAC(1) ) belongs to family B of GPCRs and is activated upon binding of vasoactive intestinal peptide (VIP) and pituitary AC-activating polypeptide neuropeptides. Widely distributed throughout body, VPAC(1) plays important regulatory roles in human physiology and physiopathology. Like most members of the GPCR-B family, VPAC(1) receptor is predicted to follow the actual paradigm of a common 'two-domain' model of natural ligand action. However the precise structural basis for ligand binding, receptor activation and signal transduction are still incompletely understood due in part to the absence of X-ray crystal structure of the whole receptor and to significant structural differences with the most extensively studied family of receptor, the GPCR-A/rhodopsin family. Here, we try to summarize the current knowledge of the molecular mechanisms involved in VPAC(1) receptor activation and signal transduction. This includes search for amino acids involved in the two-step process of VIP binding, in the stabilization of VPAC(1) inactive and active conformations, and in binding and activation of G proteins.

Evidence that interaction between conserved residues in transmembrane helices 2, 3, and 7 are crucial for human VPAC1 receptor activation

The VPAC(1) receptor belongs to family B of G protein-coupled receptors (GPCR-B) and is activated upon binding of the vasoactive intestinal peptide (VIP). Despite the recent determination of the structure of the N terminus of several members of this receptor family, little is known about the structure of the transmembrane (TM) region and about the molecular mechanisms leading to activation. In the present study, we designed a new structural model of the TM domain and combined it with experimental mutagenesis experiments to investigate the interaction network that governs ligand binding and receptor activation. Our results suggest that this network involves the cluster of residues Arg(188) in TM2, Gln(380) in TM7, and Asn(229) in TM3. This cluster is expected to be altered upon VIP binding, because Arg(188) has been shown previously to interact with Asp(3) of VIP. Several point mutations at positions 188, 229, and 380 were experimentally characterized and were shown to severely affect VIP binding and/or VIP-mediated cAMP production. Double mutants built from reciprocal residue exchanges exhibit strong cooperative or anticooperative effects, thereby indicating the spatial proximity of residues Arg(188), Gln(380), and Asn(229). Because these residues are highly conserved in the GPCR-B family, they can moreover be expected to have a general role in mediating function.

Molecular mechanisms involved in vasoactive intestinal peptide receptor activation and regulation: current knowledge, similarities to and differences from the A family of G-protein-coupled receptors

An actual paradigm for activation and regulation of the GPCR (G-protein-coupled receptors)/seven-transmembrane helix family of receptors essentially emerges from extensive studies of the largest family of receptors, the GPCR-A/rhodopsin family. The mechanisms regulating the GPCR-B family signal transduction are less precisely understood due in part to the lack of the conserved signatures of the GPCR-A family (E/DRY, NPXXY) and in part to the absence of a reliable receptor modelling, although some studies suggest that both families share similar features. Here, we try to highlight the current knowledge of the activation and the regulation of the VIP (vasoactive intestinal peptide) receptors, namely VPAC (VIP/pituitary adenylate cyclase-activating peptide receptor) 1 and 2. This includes search for amino acids involved in the stabilization of the receptor active conformation and in coupling to G-proteins, signalling pathways activated in response to VIP, agonist-dependent receptor down-regulation, phosphorylation and internalization as well as pharmacological consequences of receptor hetero-dimerization.

Identification and characterization of five-transmembrane isoforms of human vasoactive intestinal peptide and pituitary adenylate cyclase-activating polypeptide receptors

The seven-transmembrane (7TM) G-protein-coupled neuroendocrine receptors VPAC1 (HGNC approved gene symbol VIPR1) and VPAC2 (HGNC approved gene symbol VIPR2) are expressed in different tissues and involved in the regulation of important biological functions. We now report the identification and characterization of novel five-transmembrane(5TM) forms of both human VPAC1 and human VPAC2. These alternatively spliced variant mRNAs result from the skipping of exons 10/11, spanning the third intracellular loop, the fourth extracellular loop, and the transmembrane regions 6 and 7, producing in-frame 5TM receptors predicted to lack a G-protein-binding motif. RT-PCR showed that these 5TM receptors are differentially expressed in transformed and normal cells. Translation of the 5TM protein was demonstrated by transfection and expression in CHO cells. Following agonist stimulation, differential signaling of the 7TM versus 5TM forms was shown both for the activation of adenylate cyclase and for tyrosine phosphorylation. The identification of these splice variants in various cells and their expression and differential signal transduction compared to the 7TM form suggest that these novel receptors have biological relevance.

Asn229 in the third helix of VPAC1 receptor is essential for receptor activation but not for receptor phosphorylation and internalization: comparison with Asn216 in VPAC2 receptor

After stimulation with agonist, G protein coupled receptors (GPCR) undergo conformational changes that allow activation of G proteins to transduce the signal, followed by phosphorylation by kinases and arrestin binding to promote receptor internalization. Actual paradigm, based on a study of GPCR-A/rhodopsin family, suggests that a network of interactions between conserved residues located in transmembrane (TM) domains (mainly TM3, TM6 and TM7) is involved in the molecular switch leading to GPCR activation. We evaluated in CHO cells expressing the VPAC(1) receptor the role of the third transmembrane helix in agonist signalling by point mutation into Ala of the residues highly conserved in the secretin-family of receptors: Y(224), N(229), F(230), W(232), E(236), G(237), Y(239), L(240). N(229)A VPAC(1) mutant was characterized by a decrease in both potency and efficacy of VIP stimulated adenylate cyclase activity, by the absence of agonist stimulated [Ca(2+)](i) increase, by a preserved receptor recognition of agonists and antagonist and by a preserved sensitivity to GTP suggesting the importance of that residue for efficient G protein activation. N(229)D mutant was not expressed at the membrane, and the N(229)Q with a conserved mutation was less affected than the A mutant. Agonist stimulated phosphorylation and internalization of N(229)A and N(229)Q VPAC(1) were unaffected. However, the re-expression of internalized mutant receptors, but not that of the wild type receptor, was rapidly reversed after VIP washing. Receptor phosphorylation, internalization and re-expression may be thus dissociated from G protein activation and linked to another active conformation that may influence its trafficking. Mutation of that conserved amino acid in VPAC(2) could be investigated only by a conservative mutation (N(216)Q) and led to a receptor with a low VIP stimulation of adenylate cyclase, receptor phosphorylation and internalization. This indicated the importance of the conserved N residue in the TM3 of that family of receptors.

beta-arrestin-dependent, G protein-independent ERK1/2 activation by the beta2 adrenergic receptor

Physiological effects of beta adrenergic receptor (beta2AR) stimulation have been classically shown to result from G(s)-dependent adenylyl cyclase activation. Here we demonstrate a novel signaling mechanism wherein beta-arrestins mediate beta2AR signaling to extracellular-signal regulated kinases 1/2 (ERK 1/2) independent of G protein activation. Activation of ERK1/2 by the beta2AR expressed in HEK-293 cells was resolved into two components dependent, respectively, on G(s)-G(i)/protein kinase A (PKA) or beta-arrestins. G protein-dependent activity was rapid, peaking within 2-5 min, was quite transient, was blocked by pertussis toxin (G(i) inhibitor) and H-89 (PKA inhibitor), and was insensitive to depletion of endogenous beta-arrestins by siRNA. beta-Arrestin-dependent activation was slower in onset (peak 5-10 min), less robust, but more sustained and showed little decrement over 30 min. It was insensitive to pertussis toxin and H-89 and sensitive to depletion of either beta-arrestin1 or -2 by small interfering RNA. In G(s) knock-out mouse embryonic fibroblasts, wild-type beta2AR recruited beta-arrestin2-green fluorescent protein and activated pertussis toxin-insensitive ERK1/2. Furthermore, a novel beta2AR mutant (beta2AR(T68F,Y132G,Y219A) or beta2AR(TYY)), rationally designed based on Evolutionary Trace analysis, was incapable of G protein activation but could recruit beta-arrestins, undergo beta-arrestin-dependent internalization, and activate beta-arrestin-dependent ERK. Interestingly, overexpression of GRK5 or -6 increased mutant receptor phosphorylation and beta-arrestin recruitment, led to the formation of stable receptor-beta-arrestin complexes on endosomes, and increased agonist-stimulated phospho-ERK1/2. In contrast, GRK2, membrane translocation of which requires Gbetagamma release upon G protein activation, was ineffective unless it was constitutively targeted to the plasma membrane by a prenylation signal (CAAX). These findings demonstrate that the beta2AR can signal to ERK via a GRK5/6-beta-arrestin-dependent pathway, which is independent of G protein coupling.

Identification of a VIP-specific receptor in guinea pig tenia coli

Vasoactive intestinal polypeptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP) interact with VPAC(2) receptors in rabbit and guinea pig (GP) gastric muscle but with functionally distinct VIP and PACAP receptors in GP tenia coli. This study examined whether selectivity for VIP was determined by two residues (40, 41) in the extracellular domain that differ in the VIP receptors of GP gastric and tenial muscle. A mutant rat VPAC(2) receptor (L40F, L41F), and two chimeric receptors in which the NH(2)-terminal domain of rat VPAC(2) receptor was replaced with that of GP gastric (chimeric-G) or tenia coli (chimeric-T) VIP receptors, were constructed and expressed in COS-1 cells. VIP and PACAP bound with equal affinity to wild-type and mutant rat VPAC(2) receptors and to chimeric-G receptor (IC(50): VIP 0.3 +/- 0.1 to 1.5 +/- 0.4 nM, PACAP 0.4 +/- 0.1 to 2.5 +/- 0.1 nM) and stimulated cAMP with equal potency (EC(50): VIP 13 +/- 5 to 48 +/- 8 nM, PACAP 8 +/- 3 to 31 +/- 14 nM). VIP bound with high affinity also to chimeric-T receptor (IC(50): 0.5 +/- 0.1 nM) and stimulated cAMP with high potency (EC(50): 3 +/- 1 nM). In contrast, PACAP exhibited >1,000-fold less affinity for binding or potency for stimulating cAMP. We conclude that GP tenia coli express a VIP-specific receptor and that selectivity is determined by a pair of extracellular phenylalanine residues.

Proline residue 280 in the second extracellular loop (EC2) of the VPAC2 receptor is essential for the receptor structure

Inspection of the amino acid sequence of the human VPAC1 and the VPAC2 receptors after alignment of the conserved residues indicates that the second extracellular loop (EC2) is one amino acid shorter in the VPAC1 receptor due to the lack of a proline residue in position 294. We hypothesized that this could be of importance for receptor structure and/or for ligand recognition. Insertion by directed mutagenesis of a proline in that position (<Pro>294 VPAC1) had little consequence on the binding of several agonists but reduced the affinity for the VPAC1 antagonist. Coupling of the <Pro>294 VPAC1 receptor to adenylate cyclase was improved, as demonstrated by an increased affinity for VIP and other agonists, and by a shift of the VPAC1 antagonist to partial agonist behavior. Deletion of the proline 280 (DeltaPro280 VPAC2) in the VPAC2 receptor markedly reduced the apparent affinity for all the agonists tested. Replacement of the proline by a glycine residue had a smaller effect on the ligands affinities. The proline residue in the VPAC2 receptor EC2 is thus essential for the receptor structure, and the EC2 domain is involved in ligand recognition and receptor functionality.

The human VPAC1 receptor: three-dimensional model and mutagenesis of the N-terminal domain

The human VPAC(1) receptor for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase activating peptide belongs to the class II family of G-protein-coupled receptors with seven transmembrane segments. Like for all class II receptors, the extracellular N-terminal domain of the human VPAC(1) receptor plays a predominant role in peptide ligand recognition. To determine the three-dimensional structure of this N-terminal domain (residues 1-144), the Protein Data Bank (PDB) was screened for a homologous protein. A subdomain of yeast lipase B was found to have 27% sequence identity and 50% sequence homology with the N-terminal domain (8) of the VPAC(1) receptor together with a good alignment of the hydrophobic clusters. A model of the N-terminal domain of VPAC(1) receptor was thus constructed by homology. It indicated the presence of a putative signal sequence in the N-terminal extremity. Moreover, residues (Glu(36), Trp(67), Asp(68), Trp(73), and Gly(109)) which were shown to be crucial for VIP binding are gathered around a groove that is essentially negatively charged. New putatively important residues for VIP binding were suggested from the model analysis. Site-directed mutagenesis and stable transfection of mutants in CHO cells indicated that Pro(74), Pro(87), Phe(90), and Trp(110) are indeed important for VIP binding and activation of adenylyl cyclase activation. Combination of molecular modeling and directed mutagenesis provided the first partial three-dimensional structure of a VIP-binding domain, constituted of an electronegative groove with an outspanning tryptophan shell at one end, in the N-terminal extracellular region of the human VPAC(1) receptor.

Two basic residues of the h-VPAC1 receptor second transmembrane helix are essential for ligand binding and signal transduction

We mutated the vasoactive intestinal peptide (VIP) Asp(3) residue and two VPAC(1) receptor second transmembrane helix basic residues (Arg(188) and Lys(195)). VIP had a lower affinity for R188Q, R188L, K195Q, and K195I VPAC(1) receptors than for VPAC(1) receptors. [Asn(3)] VIP and [Gln(3)] VIP had lower affinities than VIP for VPAC(1) receptors but higher affinities for the mutant receptors; the two basic amino acids facilitated the introduction of the negatively charged aspartate inside the transmembrane domain. The resulting interaction was necessary for receptor activation. 1/[Asn(3)] VIP and [Gln(3)] VIP were partial agonists at VPAC(1) receptors; 2/VIP did not fully activate the K195Q, K195I, R188Q, and R188L VPAC(1) receptors; a VIP analogue ([Arg(16)] VIP) was more efficient than VIP at the four mutated receptors; and [Asn(3)] VIP and [Gln(3)] VIP were more efficient than VIP at the R188Q and R188L VPAC(1) receptors; 3/the [Asp(3)] negative charge did not contribute to the recognition of the VIP(1) antagonist, [AcHis(1),D-Phe(2),Lys(15),Arg(16),Leu(27)] VIP ()/growth hormone releasing factor (8-27). This is the first demonstration that, to activate the VPAC(1) receptor, the Asp(3) side chain of VIP must penetrate within the transmembrane domain, in close proximity to two highly conserved basic amino acids from transmembrane 2.

The human vasoactive intestinal peptide/pituitary adenylate cyclase-activating peptide receptor 1 (VPAC1) promoter: characterization and role in receptor expression during enterocytic differentiation of the colon cancer cell line Caco-2Cl.20

The basic organization of the human vasoactive intestinal peptide/pituitary adenylate cyclase-activating peptide receptor (VPAC) 1 promoter was investigated after cloning the 5'-flanking region (1.4 kb) of the VPAC1 gene from a human genomic library. Subsequent functional analysis of various deletions of the 5'-flanking sequence, subcloned upstream of a luciferase reporter gene, was carried out in HT-29 cells. The minimal promoter region identified encompasses the -205/+76 sequence and contains a crucial CCAAT box (-182/-178) and a GC-rich sequence. Moreover a region (-1348/-933) containing a silencer element was identified. We previously showed that the expression of the VPAC1 receptor binding site is strictly dependent upon the enterocytic differentiation of human colon cancer Caco-2 cells [Laburthe, Rousset, Rouyer-Fessard, Couvineau, Chantret, Chevalier and Zweibaum (1987) J. Biol. Chem. 262, 10180-10184]. In the present study we show that VPAC1 mRNA increases dramatically when Caco-2Cl.20 cells differentiate, as measured by RNase protection assays and reverse transcriptase-PCR. A single transcript species of 3 kb is detected in differentiated cells by Northern-blot analysis. Accumulation of VPAC1 receptor mRNA is due to a 5-fold increase of transcription rate (run-on assay) without a change in mRNA half-life (9 h). Stable transfections of various constructs in Caco-2Cl.20 cells and subsequent analysis of reporter gene expression, during the enterocytic differentiation process over 25 days of culture, further indicated that the -254/+76 5'-flanking sequence is endowed with the regulatory element(s) necessary for transcriptional regulation of VPAC1 during differentiation. Altogether, these observations provide the first characterization of the basic organization of the human VPAC1 gene promoter and unravel the crucial role of a short promoter sequence in the strict transcriptional control of VPAC1 expression during differentiation of human colon cancer Caco-2 cells.