A small sequence in the third intracellular loop of the VPAC(1) receptor is responsible for its efficient coupling to the calcium effector

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.

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.

Cell surface targeting of VPAC1 receptors: evidence for implication of a quality control system and the proteasome

Like for most transmembrane proteins, translation of G protein-coupled receptors (GPCRs) mRNA takes place at the endoplasmic reticulum (ER) where they are synthesized, folded and assembled. The molecular mechanisms involved in the transport process of GPCRs from ER to the plasma membrane are poorly investigated. Here we studied the mechanisms involved in glycosylation-dependent cell surface expression and quality control of the receptor for Vasoactive Intestinal Polypeptide (VIP) VPAC1, a member of the B family of GPCRs. Using biochemical and pharmacological techniques and fluorescence microscopy, we have shown that only a fraction of newly synthesized VPAC1 attains properly conformation that allows their cell surface targeting. Misfolded or immature VPAC1 are taken in charge by co- and post-translational quality control that involves: 1) calnexin-dependent folding strictly through a glycan-dependent mechanism, 2) BiP-dependant folding, 3) translocation to the cytoplasm and proteasome-dependent degradation of improper proteins, and 4) post-ER quality control check points. Our data suggest that VPAC1 expression/trafficking pathways are under the control of complex and precise molecular mechanisms to ensure that only proper VPAC1 reaches the cell surface.

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.

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.

Vasoactive intestinal peptide (VIP) induces c-fos expression in LNCaP prostate cancer cells through a mechanism that involves Ca2+ signalling. Implications in angiogenesis and neuroendocrine differentiation

The effect of vasoactive intestinal peptide (VIP) on intracellular Ca(2+) levels and its relationship with the expression of c-fos and vascular endothelial growth factor (VEGF) as well as with neuroendocrine (NE) differentiation were investigated in human prostate LNCaP cells. VIP induced the expression of c-fos mRNA as studied by reverse transcription polymerase chain reaction (RT-PCR). It was accompanied by VIP stimulation of c-fos protein synthesis, as measured by Western blot analysis. VIP enhanced intracellular Ca(2+) levels as evaluated using the calcium probe fura-2. VIP regulation of c-fos expression depended on [Ca(2+)](i) concentration since the intracellular calcium chelator BAPTA/AM decreased c-fos expression (both mRNA and protein) to basal levels. As shown by means of real-time RT-PCR, VIP stimulated VEGF mRNA expression: the effect was inhibited by 40% in the presence of curcumin (an inhibitor of AP-1 binding), and it was dependent on Ca(2+) since BAPTA/AM inhibited this VIP action by 43%. Similar observations were made on the effects of BAPTA/AM and curcumin on VIP stimulation of VEGF protein expression. Simultaneous treatment of cells with the protein kinase A inhibitor H89 and BAPTA/AM completely blocked this VIP effect, whereas each agent alone led only to a partial inhibition. In addition, the calcium chelator blocked by 37% the ability of VIP to induce NE cell differentiation as estimated by the observation of neurite development. These features support a VIP signalling pathway that could be mediated through both cAMP and [Ca(2+)](i) increase in prostate LNCaP cancer cells. Moreover, our data suggest the implication of c-Fos on the induction of the main angiogenic factor VEGF since the promoter region of the VEGF gene possesses AP-1 (i.e., c-Fos/c-Jun heterodimer) response elements. This feature represents a link between the nuclear oncogene c-fos, angiogenesis and NE differentiation by means of an initiating signal upon VIP receptors.

Contribution of the carboxyl terminus of the VPAC1 receptor to agonist-induced receptor phosphorylation, internalization, and recycling

When exposed to vasoactive intestinal peptide (VIP), the human wild type VPAC1 receptor expressed in Chinese hamster ovary (CHO) cells is rapidly phosphorylated, desensitized, and internalized in the endosomal compartment and is not re-expressed at the cell membrane within 2 h after agonist removal. The aims of the present work were first to correlate receptor phosphorylation level to internalization and recycling, measured by flow cytometry and in some cases by confocal microscopy using a monoclonal antibody that did not interfere with ligand binding, and second to identify the phosphorylated Ser/Thr residues. Combining receptor mutations and truncations allowed identification of Ser250 (in the second intracellular loop), Thr429, Ser435, Ser448 or Ser449, and Ser455 (all in the distal part of the C terminus) as candidates for VIP-stimulated phosphorylation. The effects of single mutations were not additive, suggesting alternative phosphorylation sites in mutated receptors. Replacement of all of the Ser/Thr residues in the carboxyl-terminal tail and truncation of the domain containing these residues completely inhibited VIP-stimulated phosphorylation and receptor internalization. There was, however, no direct correlation between receptor phosphorylation and internalization; in some truncated and mutated receptors, a 70% reduction in phosphorylation had little effect on internalization. In contrast to results obtained on the wild type and all of the mutated or truncated receptors that still underwent phosphorylation, internalization of the severely truncated receptor was reversed within 2 h of incubation in the absence of the agonist. Receptor recovery was blocked by monensin, an endosome inhibitor.