PheVI:09 (Phe6.44) as a sliding microswitch in seven-transmembrane (7TM) G protein-coupled receptor activation

Thermodynamic architecture and conformational plasticity of GPCRs

G-protein-coupled receptors (GPCRs) are ubiquitous integral membrane proteins involved in diverse cellular signaling processes. Here, we carry out a large-scale ensemble thermodynamic study of 45 ligand-free GPCRs employing a structure-based statistical mechanical framework. We find that multiple partially structured states co-exist in the GPCR native ensemble, with the TM helices 1, 6 and 7 displaying varied folding status, and shaping the conformational landscape. Strongly coupled residues are anisotropically distributed, accounting for only 13% of the residues, illustrating that a large number of residues are inherently dynamic. Active-state GPCRs are characterized by reduced conformational heterogeneity with altered coupling-patterns distributed throughout the structural scaffold. In silico alanine-scanning mutagenesis reveals that extra- and intra-cellular faces of GPCRs are coupled thermodynamically, highlighting an exquisite structural specialization and the fluid nature of the intramolecular interaction network. The ensemble-based perturbation methodology presented here lays the foundation for understanding allosteric mechanisms and the effects of disease-causing mutations in GCPRs.

Concerted conformational dynamics and water movements in the ghrelin G protein-coupled receptor

There is increasing support for water molecules playing a role in signal propagation through G protein-coupled receptors (GPCRs). However, exploration of the hydration features of GPCRs is still in its infancy. Here, we combined site-specific labeling with unnatural amino acids to molecular dynamics to delineate how local hydration of the ghrelin receptor growth hormone secretagogue receptor (GHSR) is rearranged upon activation. We found that GHSR is characterized by a specific hydration pattern that is selectively remodeled by pharmacologically distinct ligands and by the lipid environment. This process is directly related to the concerted movements of the transmembrane domains of the receptor. These results demonstrate that the conformational dynamics of GHSR are tightly coupled to the movements of internal water molecules, further enhancing our understanding of the molecular bases of GPCR-mediated signaling.

The discovery of a new antibody for BRIL-fused GPCR structure determination

G-protein-coupled receptors (GPCRs)—the largest family of cell-surface membrane proteins—mediate the intracellular signal transduction of many external ligands. Thus, GPCRs have become important drug targets. X-ray crystal structures of GPCRs are very useful for structure-based drug design (SBDD). Herein, we produced a new antibody (SRP2070) targeting the thermostabilised apocytochrome b562 from Escherichia coli M7W/H102I/R106L (BRIL). We found that a fragment of this antibody (SRP2070Fab) facilitated the crystallisation of the BRIL-tagged, ligand bound GPCRs, 5HT_1B and AT₂R. Furthermore, the electron densities of the ligands were resolved, suggesting that SPR2070Fab is versatile and adaptable for GPCR SBDD. We anticipate that this new tool will significantly accelerate structure determination of other GPCRs and the design of small molecular drugs targeting them.

Come Fly with Me: An overview of dopamine receptors in Drosophila melanogaster

Dopamine (DA) receptors play critical roles in a wide range of behaviours, including sensory processing, motor function, reward and arousal. As such, aberrant DA signalling is associated with numerous neurological and psychiatric disorders. Therefore, understanding the mechanisms by which DA neurotransmission drives intracellular signalling pathways that modulate behaviour can provide critical insights to guide the development of targeted therapeutics. Drosophila melanogaster has emerged as a powerful model with unique advantages to study the mechanisms underlying DA neurotransmission and associated behaviours in a controlled and systematic manner. Many regions in the fly brain innervated by dopaminergic neurons have been mapped and linked to specific behaviours, including associative learning and arousal. Here, we provide an overview of the homology between human and Drosophila dopaminergic systems and review the current literature on the pharmacology, molecular signalling mechanisms and behavioural outcome of DA receptor activation in the Drosophila brain.

Structural Model of Ghrelin Bound to its G Protein-Coupled Receptor

The peptide ghrelin targets the growth hormone secretagogue receptor 1a (GHSR) to signal changes in cell metabolism and is a sought-after therapeutic target, although no structure is known to date. To investigate the structural basis of ghrelin binding to GHSR, we used solid-state nuclear magnetic resonance (NMR) spectroscopy, site-directed mutagenesis, and Rosetta modeling. The use of saturation transfer difference NMR identified key residues in the peptide for receptor binding beyond the known motif. This information combined with assignment of the secondary structure of ghrelin in its receptor-bound state was incorporated into Rosetta using an approach that accounts for flexible binding partners. The NMR data and models revealed an extended binding surface that was confirmed via mutagenesis. Our results agree with a growing evidence of peptides interacting via two sites at G protein-coupled receptors.

The Homeostatic Force of Ghrelin

Ghrelin, a gastric-derived acylated peptide, regulates energy homeostasis by transmitting information about peripheral nutritional status to the brain, and is essential for protecting organisms against famine. Ghrelin operates brain circuits to regulate homeostatic and hedonic feeding. Recent research advances have shed new light on ghrelin's multifaceted roles in cellular homeostasis, which could maintain the internal environment and overcome metaflammation in metabolic organs. Here, we highlight our current understanding of the regulatory mechanisms of the ghrelin system in energy metabolism and cellular homeostasis and its clinical trials. Future studies of ghrelin will further elucidate how the stomach regulates systemic homeostasis.

Structural insights into positive and negative allosteric regulation of a G protein-coupled receptor through protein-lipid interactions

Lipids are becoming known as essential allosteric modulators of G protein-coupled receptor (GPCRs). However, how they exert their effects on GPCR conformation at the atomic level is still unclear. In light of recent experimental data, we have performed several long-timescale molecular dynamics (MD) simulations, totalling 24 μs, to rigorously map allosteric modulation and conformational changes in the β₂ adrenergic receptor (β2AR) that occur as a result of interactions with three different phospholipids. In particular, we identify different sequential mechanisms behind receptor activation and deactivation, respectively, mediated by specific lipid interactions with key receptor regions. We show that net negatively charged lipids stabilize an active-like state of β2AR that is able to dock G_sα protein. Clustering of anionic lipids around the receptor with local distortion of membrane thickness is also apparent. On the other hand, net-neutral zwitterionic lipids inactivate the receptor, generating either fully inactive or intermediate states, with kinetics depending on lipid headgroup charge distribution and hydrophobicity. These chemical differences alter membrane thickness and density, which differentially destabilize the β2AR active state through lateral compression effects.

Structure of the Nanobody-Stabilized Active State of the Kappa Opioid Receptor

The κ-opioid receptor (KOP) mediates the actions of opioids with hallucinogenic, dysphoric, and analgesic activities. The design of KOP analgesics devoid of hallucinatory and dysphoric effects has been hindered by an incomplete structural and mechanistic understanding of KOP agonist actions. Here, we provide a crystal structure of human KOP in complex with the potent epoxymorphinan opioid agonist MP1104 and an active-state-stabilizing nanobody. Comparisons between inactive- and active-state opioid receptor structures reveal substantial conformational changes in the binding pocket and intracellular and extracellular regions. Extensive structural analysis and experimental validation illuminate key residues that propagate larger-scale structural rearrangements and transducer binding that, collectively, elucidate the structural determinants of KOP pharmacology, function, and biased signaling. These molecular insights promise to accelerate the structure-guided design of safer and more effective κ-opioid receptor therapeutics.

G Protein-Coupled Receptors Contain Two Conserved Packing Clusters

G protein-coupled receptors (GPCRs) have evolved a seven-transmembrane helix framework that is responsive to a wide range of extracellular signals. An analysis of the interior packing of family A GPCR crystal structures reveals two clusters of highly packed residues that facilitate tight transmembrane helix association. These clusters are centered on amino acid positions 2.47 and 4.53, which are highly conserved as alanine and serine, respectively. Ala2.47 mediates the interaction between helices H1 and H2, while Ser4.53 mediates the interaction between helices H3 and H4. The helical interfaces outside of these clusters are lined with residues that are more loosely packed, a structural feature that facilitates motion of helices H5, H6, and H7, which is required for receptor activation. Mutation of the conserved small side chain at position 4.53 within packing cluster 2 is shown to disrupt the structure of the visual receptor rhodopsin, whereas sites in packing cluster 1 (e.g., positions 1.46 and 2.47) are more tolerant to mutation but affect the overall stability of the protein. These findings reveal a common structural scaffold of GPCRs that is important for receptor folding and activation.

Structural insights into the extracellular recognition of the human serotonin 2B receptor by an antibody

Monoclonal antibodies provide an attractive alternative to small-molecule therapies for a wide range of diseases. Given the importance of G protein-coupled receptors (GPCRs) as pharmaceutical targets, there has been an immense interest in developing therapeutic monoclonal antibodies that act on GPCRs. Here we present the 3.0-Å resolution structure of a complex between the human 5-hydroxytryptamine 2B (5-HT_2B) receptor and an antibody Fab fragment bound to the extracellular side of the receptor, determined by serial femtosecond crystallography with an X-ray free-electron laser. The antibody binds to a 3D epitope of the receptor that includes all three extracellular loops. The 5-HT_2B receptor is captured in a well-defined active-like state, most likely stabilized by the crystal lattice. The structure of the complex sheds light on the mechanism of selectivity in extracellular recognition of GPCRs by monoclonal antibodies.

Structure and dynamics of a constitutively active neurotensin receptor

Many G protein-coupled receptors show constitutive activity, resulting in the production of a second messenger in the absence of an agonist; and naturally occurring constitutively active mutations in receptors have been implicated in diseases. To gain insight into mechanistic aspects of constitutive activity, we report here the 3.3 Å crystal structure of a constitutively active, agonist-bound neurotensin receptor (NTSR1) and molecular dynamics simulations of agonist-occupied and ligand-free receptor. Comparison with the structure of a NTSR1 variant that has little constitutive activity reveals uncoupling of the ligand-binding domain from conserved connector residues, that effect conformational changes during GPCR activation. Furthermore, molecular dynamics simulations show strong contacts between connector residue side chains and increased flexibility at the intracellular receptor face as features that coincide with robust signalling in cells. The loss of correlation between the binding pocket and conserved connector residues, combined with altered receptor dynamics, possibly explains the reduced neurotensin efficacy in the constitutively active NTSR1 and a facilitated initial engagement with G protein in the absence of agonist.

The Quantum Nature of Drug-Receptor Interactions: Deuteration Changes Binding Affinities for Histamine Receptor Ligands

In this article we report a combined experimental and computational study concerning the effects of deuteration on the binding of histamine and two other histaminergic agonists to 3H-tiotidine-labeled histamine H2 receptor in neonatal rat astrocytes. Binding affinities were measured by displacing radiolabeled tiotidine from H2 receptor binding sites present on cultured neonatal rat astrocytes. Quantum-chemical calculations were performed by employing the empirical quantization of nuclear motion within a cluster model of the receptor binding site extracted from the homology model of the entire H2 receptor. Structure of H2 receptor built by homology modelling is attached in the supporting information (S1 Table) Experiments clearly demonstrate that deuteration affects the binding by increasing the affinity for histamine and reducing it for 2-methylhistamine, while basically leaving it unchanged for 4-methylhistamine. Ab initio quantum-chemical calculations on the cluster system extracted from the homology H2 model along with the implicit quantization of the acidic N-H and O-H bonds demonstrate that these changes in the binding can be rationalized by the altered strength of the hydrogen bonding upon deuteration known as the Ubbelohde effect. Our computational analysis also reveals a new mechanism of histamine binding, which underlines an important role of Tyr250 residue. The present work is, to our best knowledge, the first study of nuclear quantum effects on ligand receptor binding. The ligand H/D substitution is relevant for therapy in the context of perdeuterated and thus more stable drugs that are expected to enter therapeutic practice in the near future. Moreover, presented approach may contribute towards understanding receptor activation, while a distant goal remains in silico discrimination between agonists and antagonists based on the receptor structure.

Helix 3 acts as a conformational hinge in Class A GPCR activation: An analysis of interhelical interaction energies in crystal structures

A collection of crystal structures of rhodopsin, β2-adrenergic and adenosine A2A receptors in active, intermediate and inactive states were selected for structural and energetic analyses to identify the changes involved in the activation/deactivation of Class A GPCRs. A set of helix interactions exclusive to either inactive or active/intermediate states were identified. The analysis of these interactions distinguished some local conformational changes involved in receptor activation, in particular, a packing between the intracellular domains of transmembrane helices H3 and H7 and a separation between those of H2 and H6. Also, differential movements of the extracellular and intracellular domains of these helices are apparent. Moreover, a segment of residues in helix H3, including residues L/I3.40 to L3.43, is identified as a key component of the activation mechanism, acting as a conformational hinge between extracellular and intracellular regions. Remarkably, the influence on the activation process of some glutamic and aspartic acidic residues and, as a consequence, the influence of variations on local pH is highlighted. Structural hypotheses that arose from the analysis of rhodopsin, β2-adrenergic and adenosine A2A receptors were tested on the active and inactive M2 muscarinic acetylcholine receptor structures and further discussed in the context of the new mechanistic insights provided by the recently determined active and inactive crystal structures of the μ-opioid receptor. Overall, the structural and energetic analyses of the interhelical interactions present in this collection of Class A GPCRs suggests the existence of a common general activation mechanism featuring a chemical space useful for drug discovery exploration.

Uncovering the triggers for GPCR activation using solid-state NMR spectroscopy

G protein-coupled receptors (GPCRs) span cell membranes with seven transmembrane helices and respond to a diverse array of extracellular signals. Crystal structures of GPCRs have provided key insights into the architecture of these receptors and the role of conserved residues. However, the question of how ligand binding induces the conformational changes that are essential for activation remains largely unanswered. Since the extracellular sequences and structures of GPCRs are not conserved between receptor subfamilies, it is likely that the initial molecular triggers for activation vary depending on the specific type of ligand and receptor. In this article, we describe NMR studies on the rhodopsin subfamily of GPCRs and propose a mechanism for how retinal isomerization switches the receptor to the active conformation. These results suggest a general approach for determining the triggers for activation in other GPCR subfamilies using NMR spectroscopy.

Functional roles of evolutionary conserved motifs and residues in vertebrate chemokine receptors

Chemokine receptors regulate cell migration and homing. They belong to the rhodopsin-like family of GPCRs. Their ancestor genes emerged in the early stages of vertebrate evolution. Since then, the family has been greatly expanded through whole and segmental genome duplication events. During evolution, many amino acid changes have been introduced in individual chemokine receptors, but certain motifs and residues are highly conserved. Previously, we proposed a nomenclature system of the vertebrate chemokine receptors based on their evolutionary history and phylogenetic analyses. With the use of this classification system, we are now able to confidently assign the species orthologs of vertebrate chemokine receptors. Here, we systematically analyze conserved motifs and residues of each group of orthologous chemokine receptors that may play important roles in their signaling and biologic functions. Our present analysis may provide useful information on how individual chemokine receptors are activated upon ligand binding.

Gating function of isoleucine-116 in TM-3 (position III:16/3.40) for the activity state of the CC-chemokine receptor 5 (CCR5)

BACKGROUND AND PURPOSE

A conserved amino acid within a protein family indicates a significance of the residue. In the centre of transmembrane helix (TM)-5, position V:13/5.47, an aromatic amino acid is conserved among class A 7TM receptors. However, in 37% of chemokine receptors - a subgroup of 7TM receptors - it is a leucine indicating an altered function. Here, we describe the significance of this position and its possible interaction with TM-3 for CCR5 activity.

EXPERIMENTAL APPROACH

The effects of [L203F]-CCR5 in TM-5 (position V:13/5.47), [I116A]-CCR5 in TM-3 (III:16/3.40) and [L203F;G286F]-CCR5 (V:13/5.47;VII:09/7.42) were determined in G-protein- and β-arrestin-coupled signalling. Computational modelling monitored changes in amino acid conformation.

KEY RESULTS

[L203F]-CCR5 increased the basal level of G-protein coupling (20-70% of Emax ) and β-arrestin recruitment (50% of Emax ) with a threefold increase in agonist potency. In silico, [I116A]-CCR5 switched χ1-angle in [L203F]-CCR5. Furthermore, [I116A]-CCR5 was constitutively active to a similar degree as [L203F]-CCR5. Tyr(244) in TM-6 (VI:09/6.44) moved towards TM-5 in silico, consistent with its previously shown function for CCR5 activation. On [L203F;G286F]-CCR5 the antagonist aplaviroc was converted to a superagonist.

CONCLUSIONS AND IMPLICATIONS

The results imply that an aromatic amino acid in the centre of TM-5 controls the level of receptor activity. Furthermore, Ile(116) acts as a gate for the movement of Tyr(244) towards TM-5 in the active state, a mechanism proposed previously for the β2 -adrenoceptor. The results provide an understanding of chemokine receptor function and thereby information for the development of biased and non-biased antagonists and inverse agonists.

Biased and g protein-independent signaling of chemokine receptors

Biased signaling or functional selectivity occurs when a 7TM-receptor preferentially activates one of several available pathways. It can be divided into three distinct forms: ligand bias, receptor bias, and tissue or cell bias, where it is mediated by different ligands (on the same receptor), different receptors (with the same ligand), or different tissues or cells (for the same ligand–receptor pair). Most often biased signaling is differentiated into G protein-dependent and β-arrestin-dependent signaling. Yet, it may also cover signaling differences within these groups. Moreover, it may not be absolute, i.e., full versus no activation. Here we discuss biased signaling in the chemokine system, including the structural basis for biased signaling in chemokine receptors, as well as in class A 7TM receptors in general. This includes overall helical movements and the contributions of micro-switches based on recently published 7TM crystals and molecular dynamics studies. All three forms of biased signaling are abundant in the chemokine system. This challenges our understanding of “classic” redundancy inevitably ascribed to this system, where multiple chemokines bind to the same receptor and where a single chemokine may bind to several receptors – in both cases with the same functional outcome. The ubiquitous biased signaling confers a hitherto unknown specificity to the chemokine system with a complex interaction pattern that is better described as promiscuous with context-defined roles and different functional outcomes in a ligand-, receptor-, or cell/tissue-defined manner. As the low number of successful drug development plans implies, there are great difficulties in targeting chemokine receptors; in particular with regard to receptor antagonists as anti-inflammatory drugs. Un-defined and putative non-selective targeting of the complete cellular signaling system could be the underlying cause of lack of success. Therefore, biased ligands could be the solution.

Computational analysis of negative and positive allosteric modulator binding and function in metabotropic glutamate receptor 5 (in)activation

Metabotropic glutamate receptors (mGluRs) are high-profile G-protein coupled receptors drug targets because of their involvement in several neurological disease states, and mGluR5 in particular is a subtype whose controlled allosteric modulation, both positive and negative, can potentially be useful for the treatment of schizophrenia and relief of chronic pain, respectively. Here we model mGluR5 with a collection of positive and negative allosteric modulators (PAMs and NAMs) in both active and inactive receptor states, in a manner that is consistent with experimental information, using a specialized protocol that includes homology to increase docking accuracy, and receptor relaxation to generate an individual induced fit with each allosteric modulator. Results implicate two residues in particular for NAM and PAM function: NAM interaction with W785 for receptor inactivation, and NAM/PAM H-bonding with S809 for receptor (in)activation. Models suggest the orientation of the H-bond between allosteric modulator and S809, controlled by PAM/NAM chemistry, influences the position of TM7, which in turn influences the shape of the allosteric site, and potentially the receptor state. NAM-bound and PAM-bound mGluR5 models also reveal that although PAMs and NAMs bind in the same pocket and share similar binding modes, they have distinct effects on the conformation of the receptor. Our models, together with the identification of a possible activation mechanism, may be useful in the rational design of new allosteric modulators for mGluR5.

Unraveling structural mechanisms of allosteric drug action

Orthosteric drugs block the active site to obstruct function; allosteric drugs modify the population of the active state, to modulate function. Available data lead us to propose that allosteric drugs can constitute anchors and drivers. The anchor docks into an allosteric pocket. The conformation with which it interacts is unchanged during the transition between the inactive and active states. The anchor provides the foundation that allows the driver to exert a 'pull' and/or 'push' action that shifts the receptor population from the inactive to the active state. The presence or absence of driver atom in an allosteric drug can exert opposite agonism. We map a strategy for driver identification and expect the allosteric trigger concept to transform agonist/antagonist drug discovery.

Unifying family A GPCR theories of activation

Several new pairs of active and inactive GPCR structures have recently been solved enabling detailed structural insight into the activation process, not only of rhodopsin but now also of the β2 adrenergic, M2 muscarinic and adenosine A2A receptors. Combined with structural analyses they have enabled us to examine the different recent theories proposed for GPCR activation and show that they are all indeed parts of the same process, and are intrinsically related through their effect on the central hydrophobic core of GPCRs. This new unifying general process of activation is consistent with the identification of known constitutively active mutants and an in-depth conservational analysis of significant residues implicated in the process.

Molecular determinants of allosteric modulation at the M1 muscarinic acetylcholine receptor

Benzylquinolone carboxylic acid (BQCA) is an unprecedented example of a selective positive allosteric modulator of acetylcholine at the M1 muscarinic acetylcholine receptor (mAChR). To probe the structural basis underlying its selectivity, we utilized site-directed mutagenesis, analytical modeling, and molecular dynamics to delineate regions of the M1 mAChR that govern modulator binding and transmission of cooperativity. We identified Tyr-85(2.64) in transmembrane domain 2 (TMII), Tyr-179 and Phe-182 in the second extracellular loop (ECL2), and Glu-397(7.32) and Trp-400(7.35) in TMVII as residues that contribute to the BQCA binding pocket at the M1 mAChR, as well as to the transmission of cooperativity with the orthosteric agonist carbachol. As such, the BQCA binding pocket partially overlaps with the previously described "common" allosteric site in the extracellular vestibule of the M1 mAChR, suggesting that its high subtype selectivity derives from either additional contacts outside this region or through a subtype-specific cooperativity mechanism. Mutation of amino acid residues that form the orthosteric binding pocket caused a loss of carbachol response that could be rescued by BQCA. Two of these residues (Leu-102(3.29) and Asp-105(3.32)) were also identified as indirect contributors to the binding affinity of the modulator. This new insight into the structural basis of binding and function of BQCA can guide the design of new allosteric ligands with tailored pharmacological properties.

Functional residues essential for the activation of the CB1 cannabinoid receptor

Recently developed X-ray crystal structures of active state G-protein-coupled receptors have opened the way for detailed examination of the movement of the transmembrane (TM) helices and the specific residues involved in the receptor activation upon ligand binding. Previous modeling studies have indicated that the brain cannabinoid (CB1) receptor binds with a ligand at least in part through a hydrophobic tail on the ligand. This interaction is believed to be similar to the rotameric toggle switch proposed for the β2 adrenergic receptor (β2AR). In the present study, an active state model for the CB1 receptor, guided by the X-ray structure of the active state for β2AR, was constructed with HU210 bound as a ligand. Molecular dynamics (MD) simulations were employed to provide a smooth progression between inactive and active states of the receptor. This model was compared with our previously published CB1 receptor model to identify the functional residues that play key roles in triggering receptor conformational changes upon agonist binding. Movements in TM helices and functional residues are discussed. W279(5.43), contributing to an inward movement of the fifth TM helix (TM5) to the helical core, could serve as another rotameric switch for receptor activation. V282(5.46), interacting with the ligand's hydrophobic C3 alkyl chain, appears to play a key role in TM5 inward movement centered at L286(5.50) and subsequent coupling to V204(3.40). V204(3.40), closely interacting with the TM5 and TM6 hydrophobic patch residues in the middle of the receptor, particularly I290(5.54) and L352(6.44), appears to facilitate helical rearrangements, leading to the breakage of the ionic lock and the rotameric change of Y397(7.53), which are key features of the active state.

Stimulation of proglucagon gene expression by human GPR119 in enteroendocrine L-cell line GLUTag

GPR119 is a G protein-coupled receptor expressed on enteroendocrine L-cells that synthesize and secrete the incretin hormone glucagon-like peptide-1 (GLP-1). Although GPR119 agonists stimulate L-cell GLP-1 secretion, there is uncertainty concerning whether GLP-1 biosynthesis is under the control of GPR119. Here we report that GPR119 is functionally coupled to increased proglucagon (PG) gene expression that constitutes an essential first step in GLP-1 biosynthesis. Using a mouse L-cell line (GLUTag) that expresses endogenous GPR119, we demonstrate that PG gene promoter activity is stimulated by GPR119 agonist AS1269574. Surprisingly, transfection of GLUTag cells with recombinant human GPR119 (hGPR119) results in a constitutive and apparently ligand-independent increase of PG gene promoter activity and PG mRNA content. These constitutive actions of hGPR119 are mediated by cAMP-dependent protein kinase (PKA) but not cAMP sensor Epac2. Thus, the constitutive action of hGPR119 to stimulate PG gene promoter activity is diminished by: 1) a dominant-negative Gαs protein, 2) a dominant-negative PKA regulatory subunit, and 3) a dominant-negative A-CREB. Interestingly, PG gene promoter activity is stimulated by 6-Bn-cAMP-AM, a cAMP analog that selectively activates α and β isoforms of type II, but not type I PKA regulatory subunits expressed in GLUTag cells. Finally, our analysis reveals that a specific inhibitor of Epac2 activation (ESI-05) fails to block the stimulatory action of 6-Bn-cAMP-AM at the PG gene promoter, nor is PG gene promoter activity stimulated by: 1) a constitutively active Epac2, or 2) cAMP analogs that selectively activate Epac proteins. Such findings are discussed within the context of ongoing controversies concerning the relative contributions of PKA and Epac2 to the control of PG gene expression.

GHS-R1a constitutive activity and its physiological relevance

Abundant evidences have shown that ghrelin, by its binding to GHS-R1a, plays an important role for fundamental physiological functions. Increasing attention is given to the GHS-R1a unusually high constitutive activity and its contribution to downstream signaling and physiological processes. Here, we review recent lines of evidences showing that the interaction between ligand-binding pocket TM domains and the ECL2 could be partially responsible for this high constitutive activity. Interestingly, GHSR-1a constitutive activity activates in turn the downstream PLC, PKC, and CRE signaling pathways and this activation is reversed by the inverse agonist [D-Arg¹, D-Phe⁵, D-Trp^7,9, Leu¹¹]-substance P (MSP). Noteworthy, GHSR-1a exhibits a C-terminal-dependent constitutive internalization. Non-sense GHS-R1a mutation (Ala204Glu), first discovered in Moroccan patients, supports the role of GHSR-1a constitutive activity in physiological impairments. Ala204Glu-point mutation, altering exclusively the GHSR-1a constitutive activity, was associated with familial short stature syndrome. Altogether, these findings suggest that GHS-R1a constitutive activity could contribute to GH secretion or body weight regulation. Consequently, future research on basic and clinical applications of GHS-R1a inverse agonists will be challenging and potentially rewarding.