The role of a conserved region of the second intracellular loop in AT1 angiotensin receptor activation and signaling

An Emerging Strategy for Neuroinflammation Treatment: Combined Cannabidiol and Angiotensin Receptor Blockers Treatments Effectively Inhibit Glial Nitric Oxide Release

Cannabidiol (CBD), the major non-psychoactive phytocannabinoid found in cannabis, has anti-neuroinflammatory properties. Despite the increasing use of CBD, little is known about its effect in combination with other substances. Combination therapy has been gaining attention recently, aiming to produce more efficient effects. Angiotensin II activates the angiotensin 1 receptor and regulates neuroinflammation and cognition. Angiotensin receptor 1 blockers (ARBs) were shown to be neuroprotective and prevent cognitive decline. The present study aimed to elucidate the combined role of CBD and ARBs in the modulation of lipopolysaccharide (LPS)-induced glial inflammation. While LPS significantly enhanced nitric oxide synthesis vs. the control, telmisartan and CBD, when administered alone, attenuated this effect by 60% and 36%, respectively. Exposure of LPS-stimulated cells to both compounds resulted in the 95% inhibition of glial nitric oxide release (additive effect). A synergistic inhibitory effect on nitric oxide release was observed when cells were co-treated with losartan (5 μM) and CBD (5 μM) (by 80%) compared to exposure to each compound alone (by 22% and 26%, respectively). Telmisartan and CBD given alone increased TNFα levels by 60% and 40%, respectively. CBD and telmisartan, when given together, attenuated the LPS-induced increase in TNFα levels without statistical significance. LPS-induced IL-17 release was attenuated by CBD with or without telmisartan (by 75%) or telmisartan alone (by 60%). LPS-induced Interferon-γ release was attenuated by 80% when telmisartan was administered in the absence or presence of CBD. Anti-inflammatory effects were recorded when CBD was combined with the known anti-inflammatory agent dimethyl fumarate (DMF)/monomethyl fumarate (MMF). A synergistic inhibitory effect of CBD and MMF on glial release of nitric oxide (by 77%) was observed compared to cells exposed to MMF (by 35%) or CBD (by 12%) alone. Overall, this study highlights the potential of new combinations of CBD (5 μM) with losartan (5 μM) or MMF (1 μM) to synergistically attenuate glial NO synthesis. Additive effects on NO production were observed when telmisartan (5 μM) and CBD (5 μM) were administered together to glial cells.

Endocytosis and signaling of angiotensin II type 1 receptor

A vasoactive octapeptide angiotensin II (Ang II) hormone is the key regulator of the renin-angiotensin system (RAS). It binds with the two different plasma membrane receptors like angiotensin II type 1 (AT1) and type 2 (AT2) and consequence various biological responses occur. Further, AT1 has two subtypes such as AT1A and AT1B. These angiotensin receptors are classified to be G protein-coupled receptors (GPCRs). The main constituent of RAS is the AT1 receptor (AT1R), and its activation, signal transduction, and regulation have been extensively studied. After Ang II stimulation, the ligand-receptor complexes internalized and trafficked through the early endosome, recycling endosome, and some receptors skipped the recycling endosome and trafficked to the lysosome for metabolic degradation. Moreover, some short sequence motifs located in the carboxyl-terminus (CT) of the receptor play a vital role in the internalization, phosphorylation, subcellular trafficking, signaling, and desensitization. Furthermore, in endocytosis, the various proteins interact with the CT region of the receptor. This chapter highlights the basic mechanism of AT1 receptor internalization, trafficking and signaling in both physiological and pathophysiological conditions.

Serodolin, a β-arrestin-biased ligand of 5-HT7 receptor, attenuates pain-related behaviors

Transmembrane signaling through G protein–coupled receptors (GPCRs), originally described as requiring coupling to intracellular G proteins, also uses G protein–independent pathways through β-arrestin recruitment. Biased ligands, by favoring one of the multiple bioactive conformations of GPCRs, allow selective signaling through either of these pathways. Here, we identified Serodolin as the first β-arrestin–biased agonist of the serotonin 5-HT₇ receptor. This new ligand, while acting as an inverse agonist on G_s signaling, selectively induces ERK activation in a β-arrestin–dependent way. Importantly, we report that Serodolin decreases pain intensity caused by thermal, mechanical, or inflammatory stimuli. Our findings suggest that targeting the 5-HT₇R with β-arrestin–biased ligand could be a valid alternative strategy to the use of opioids for the relief of pain.

G protein–coupled receptors (GPCRs) are involved in regulation of manifold physiological processes through coupling to heterotrimeric G proteins upon ligand stimulation. Classical therapeutically active drugs simultaneously initiate several downstream signaling pathways, whereas biased ligands, which stabilize subsets of receptor conformations, elicit more selective signaling. This concept of functional selectivity of a ligand has emerged as an interesting property for the development of new therapeutic molecules. Biased ligands are expected to have superior efficacy and/or reduced side effects by regulating biological functions of GPCRs in a more precise way. In the last decade, 5-HT₇ receptor (5-HT₇R) has become a promising target for the treatment of neuropsychiatric disorders, sleep and circadian rhythm disorders, and pathological pain. In this study, we showed that Serodolin is unique among a number of agonists and antagonists tested: it behaves as an antagonist/inverse agonist on G_s signaling while inducing ERK activation through a β-arrestin–dependent signaling mechanism that requires c-SRC activation. Moreover, we showed that Serodolin clearly decreases hyperalgesia and pain sensation in response to inflammatory, thermal, and mechanical stimulation. This antinociceptive effect could not be observed in 5-HT₇R knockout (KO) mice and was fully blocked by administration of SB269-970, a specific 5-HT₇R antagonist, demonstrating the specificity of action of Serodolin. Physiological effects of 5-HT₇R stimulation have been classically shown to result from G_s-dependent adenylyl cyclase activation. In this study, using a β-arrestin–biased agonist, we provided insight into the molecular mechanism triggered by 5-HT₇R and revealed its therapeutic potential in the modulation of pain response.

Modulation of the rat angiotensin type 1a receptor by an upstream short open reading frame

The rat angiotensin type 1a receptor (AT_1aR) is a peptide hormone G protein-coupled receptor (GPCR) that plays a key role in electrolyte homeostasis and blood pressure control. There is a highly conserved short open reading frame (sORF) in exon 2 (E2) that is downstream from exon 1 (E1) and upstream of the AT_1aR coding region located in exon 3 (E3). To determine the role of this E2 sORF in AT_1aR signaling, human embryonic kidney-293 (HEK293) cells were transfected with plasmids containing AT_1aR cDNA with either an intact or disrupted E2 sORF. The intact sORF attenuated the efficacy of angiotensin (Ang) II (p < 0.001) and sarcosine¹,Ile⁴,Ile⁸-Ang II (SII), (p < 0.01) to activate AT_1aR signaling through extracellular signal-related kinases 1/2 (ERK1/2). A time-course showed agonist-induced AT_1aR-mediated ERK1/2 activation was slower in the presence of the intact compared to the disrupted sORF [Ang II: p < 0.01 and SII: p < 0.05]. Ang II-induced ERK1/2 activation was completely inhibited by the protein kinase C (PKC) inhibitor Ro 31-8220 regardless of whether the sORF was intact or disrupted. Flow cytometric analyses suggested the intact sORF improved cell survival; the percentage of live cells increased (p < 0.05) while the percentage of early apoptotic cells decreased (p < 0.01) in cells transfected with the AT_1aR plasmid containing the intact sORF. These findings have implications for the regulation of AT₁Rs in physiological and pathological conditions and warrant investigation of sORFs in the 5' leader sequence (5'LS) of other GPCRs.

Beta-arrestins operate an on/off control switch for focal adhesion kinase activity

Focal adhesion kinase (FAK) regulates key biological processes downstream of G protein-coupled receptors (GPCRs) in normal and cancer cells, but the modes of kinase activation by these receptors remain unclear. We report that after GPCR stimulation, FAK activation is controlled by a sequence of events depending on the scaffolding proteins β-arrestins and G proteins. Depletion of β-arrestins results in a marked increase in FAK autophosphorylation and focal adhesion number. We demonstrate that β-arrestins interact directly with FAK and inhibit its autophosphorylation in resting cells. Both FAK-β-arrestin interaction and FAK inhibition require the FERM domain of FAK. Following the stimulation of the angiotensin receptor AT1AR and subsequent translocation of the FAK-β-arrestin complex to the plasma membrane, β-arrestin interaction with the adaptor AP-2 releases inactive FAK from the inhibitory complex, allowing its activation by receptor-stimulated G proteins and activation of downstream FAK effectors. Release and activation of FAK in response to angiotensin are prevented by an AP-2-binding deficient β-arrestin and by a specific inhibitor of β-arrestin/AP-2 interaction; this inhibitor also prevents FAK activation in response to vasopressin. This previously unrecognized mechanism of FAK regulation involving a dual role of β-arrestins, which inhibit FAK in resting cells while driving its activation at the plasma membrane by GPCR-stimulated G proteins, opens new potential therapeutic perspectives in cancers with up-regulated FAK.

Structure, dynamics and lipid interactions of serotonin receptors: excitements and challenges

Serotonin (5-hydroxytryptamine, 5-HT) is an intrinsically fluorescent neurotransmitter found in organisms spanning a wide evolutionary range. Serotonin exerts its diverse actions by binding to distinct cell membrane receptors which are classified into many groups. Serotonin receptors are involved in regulating a diverse array of physiological signaling pathways and belong to the family of either G protein-coupled receptors (GPCRs) or ligand-gated ion channels. Serotonergic signaling appears to play a key role in the generation and modulation of various cognitive and behavioral functions such as sleep, mood, pain, anxiety, depression, aggression, and learning. Serotonin receptors act as drug targets for a number of diseases, particularly neuropsychiatric disorders. The signaling mechanism and efficiency of serotonin receptors depend on their amazing ability to rapidly access multiple conformational states. This conformational plasticity, necessary for the wide variety of functions displayed by serotonin receptors, is regulated by binding to various ligands. In this review, we provide a succinct overview of recent developments in generating and analyzing high-resolution structures of serotonin receptors obtained using crystallography and cryo-electron microscopy. Capturing structures of distinct conformational states is crucial for understanding the mechanism of action of these receptors, which could provide important insight for rational drug design targeting serotonin receptors. We further provide emerging information and insight from studies on interactions of membrane lipids (such as cholesterol) with serotonin receptors. We envision that a judicious combination of analysis of high-resolution structures and receptor-lipid interaction would allow a comprehensive understanding of GPCR structure, function and dynamics, thereby leading to efficient drug discovery.

The nonselective cation channel TRPV4 inhibits angiotensin II receptors

G-protein-coupled receptors (GPCRs) are a ubiquitously expressed family of receptor proteins that regulate many physiological functions and other proteins. They act through two dissociable signaling pathways: the exchange of GDP to GTP by linked G-proteins and the recruitment of β-arrestins. GPCRs modulate several members of the transient receptor potential (TRP) channel family of nonselective cation channels. How TRP channels reciprocally regulate GPCR signaling is less well-explored. Here, using an array of biochemical approaches, including immunoprecipitation and fluorescence, calcium imaging, phosphate radiolabeling, and a β-arrestin-dependent luciferase assay, we characterize a GPCR-TRP channel pair, angiotensin II receptor type 1 (AT1R), and transient receptor potential vanilloid 4 (TRPV4), in primary murine choroid plexus epithelial cells and immortalized cell lines. We found that AT1R and TRPV4 are binding partners and that activation of AT1R by angiotensin II (ANGII) elicits β-arrestin-dependent inhibition and internalization of TRPV4. Activating TRPV4 with endogenous and synthetic agonists inhibited angiotensin II-mediated G-protein-associated second messenger accumulation, AT1R receptor phosphorylation, and β-arrestin recruitment. We also noted that TRPV4 inhibits AT1R phosphorylation by activating the calcium-activated phosphatase calcineurin in a Ca2+/calmodulin-dependent manner, preventing β-arrestin recruitment and receptor internalization. These findings suggest that when TRP channels and GPCRs are co-expressed in the same tissues, many of these channels can inhibit GPCR desensitization.

Not So Dry After All: DRY Mutants of the AT1A Receptor and H1 Receptor Can Induce G-Protein-Dependent Signaling

G-protein-coupled receptors (GPCRs) are seven transmembrane spanning receptors that regulate a wide array of intracellular signaling cascades in response to various stimuli. To do so, they couple to different heterotrimeric G proteins and adaptor proteins, including arrestins. Importantly, arrestins were shown to regulate GPCR signaling through G proteins, as well as promote G protein-independent signaling events. Several research groups have reported successful isolation of exclusively G protein-dependent and arrestin-dependent signaling downstream of GPCR activation using biased agonists or receptor mutants incapable of coupling to either arrestins or G proteins. In the latter category, the DRY mutant of the angiotensin II type 1 receptor was extensively used to characterize the functional selectivity downstream of AT1_AR. In an attempt to understand histamine 1 receptor signaling, we characterized the signaling capacity of the H1R DRY mutant in a panel of dynamic, live cell biosensor assays, including arrestin recruitment, heterotrimeric G protein activation, Ca²⁺ signaling, protein kinase C activity, GTP binding of RhoA, and activation of ERK1/2. Here, we show that both H1R DRY mutant and the AT1_AR DRY mutant are capable of efficient activation of G protein-mediated signaling. Therefore, contrary to the common belief, they do not constitute suitable tools for the dissection of the arrestin-mediated, G protein-independent signaling downstream of these receptors.

Ligand-induced activation of ERK1/2 signaling by constitutively active Gs-coupled 5-HT receptors

5-HT₄R, 5-HT₆R, and 5-HT_7AR are three constitutively active G_s-coupled 5-HT receptors that have key roles in brain development, learning, memory, cognition, and other physiological processes in the central nervous system. In addition to G_s signaling cascade mediated by these three 5-HT receptors, the ERK1/2 signaling which is dependent on cyclic adenosine monophosphate (cAMP) production and protein kinase A (PKA) activation downstream of G_s signaling has also been widely studied. In this study, we investigated these two signaling pathways originating from the three G_s-coupled 5-HT receptors in AD293 cells. We found that the phosphorylation and activation of ERK1/2 are ligand-induced, in contrast to the constitutively active G_s signaling. This indicates that G_s signaling alone is not sufficient for ERK1/2 activation in these three 5-HT receptors. In addition to G_s, we found that β-arrestin and Fyn are essential for the activation of ERK1/2. Together, these results put forth a novel mechanism for ERK1/2 activation involving the cooperative action of G_s, β-arrestin, and Fyn.

The Role of β-Arrestin Proteins in Organization of Signaling and Regulation of the AT1 Angiotensin Receptor

AT1 angiotensin receptor plays important physiological and pathophysiological roles in the cardiovascular system. Renin-angiotensin system represents a target system for drugs acting at different levels. The main effects of ATR1 stimulation involve activation of Gq proteins and subsequent IP3, DAG, and calcium signaling. It has become evident in recent years that besides the well-known G protein pathways, AT1R also activates a parallel signaling pathway through β-arrestins. β-arrestins were originally described as proteins that desensitize G protein-coupled receptors, but they can also mediate receptor internalization and G protein-independent signaling. AT1R is one of the most studied receptors, which was used to unravel the newly recognized β-arrestin-mediated pathways. β-arrestin-mediated signaling has become one of the most studied topics in recent years in molecular pharmacology and the modulation of these pathways of the AT1R might offer new therapeutic opportunities in the near future. In this paper, we review the recent advances in the field of β-arrestin signaling of the AT1R, emphasizing its role in cardiovascular regulation and heart failure.

Novel mechanisms of G-protein-coupled receptors functions: AT1 angiotensin receptor acts as a signaling hub and focal point of receptor cross-talk

AT₁ angiotensin receptor (AT₁R), a prototypical G protein-coupled receptor (GPCR), is the main receptor, which mediates the effects of the renin-angiotensin system (RAS). AT₁R plays a crucial role in the regulation of blood pressure and salt-water homeostasis, and in the development of pathological conditions, such as hypertension, heart failure, cardiovascular remodeling, renal fibrosis, inflammation, and metabolic disorders. Stimulation of AT₁R leads to pleiotropic signal transduction pathways generating arrays of complex cellular responses. Growing amount of evidence shows that AT₁R is a versatile GPCR, which has multiple unique faces with distinct conformations and signaling properties providing new opportunities for functionally selective pharmacological targeting of the receptor. Biased ligands of AT₁R have been developed to selectively activate the β-arrestin pathway, which may have therapeutic benefits compared to the conventional angiotensin converting enzyme inhibitors and angiotensin receptor blockers. In this review, we provide a summary about the most recent findings and novel aspects of the AT₁R function, signaling, regulation, dimerization or oligomerization and its cross-talk with other receptors, including epidermal growth factor (EGF) receptor, adrenergic receptors and CB₁ cannabinoid receptor. Better understanding of the mechanisms and structural aspects of AT₁R activation and cross-talk can lead to the development of novel type of drugs for the treatment of cardiovascular and other diseases.

Central Hypothyroidism Due to a TRHR Mutation Causing Impaired Ligand Affinity and Transactivation of Gq

CONTEXT

Central congenital hypothyroidism (CCH) is an underdiagnosed disorder characterized by deficient production and bioactivity of thyroid-stimulating hormone (TSH) leading to low thyroid hormone synthesis. Thyrotropin-releasing hormone (TRH) receptor (TRHR) defects are rare recessive disorders usually associated with incidentally identified CCH and short stature in childhood.

OBJECTIVES

Clinical and genetic characterization of a consanguineous family of Roma origin with central hypothyroidism and identification of underlying molecular mechanisms.

DESIGN

All family members were phenotyped with thyroid hormone profiles, pituitary magnetic resonance imaging, TRH tests, and dynamic tests for other pituitary hormones. Candidate TRH, TRHR, TSHB, and IGSF1 genes were screened for mutations. A mutant TRHR was characterized in vitro and by molecular modeling.

RESULTS

A homozygous missense mutation in TRHR (c.392T > C; p.I131T) was identified in an 8-year-old boy with moderate hypothyroidism (TSH: 2.61 mIU/L, Normal: 0.27 to 4.2; free thyroxine: 9.52 pmol/L, Normal: 10.9 to 25.7) who was overweight (body mass index: 20.4 kg/m2, p91) but had normal stature (122 cm; -0.58 standard deviation). His mother, two brothers, and grandmother were heterozygous for the mutation with isolated hyperthyrotropinemia (TSH: 4.3 to 8 mIU/L). The I131T mutation, in TRHR intracellular loop 2, decreases TRH affinity and increases the half-maximal effective concentration for signaling. Modeling of TRHR-Gq complexes predicts that the mutation disrupts the interaction between receptor and a hydrophobic pocket formed by Gq.

CONCLUSIONS

A unique missense TRHR defect identified in a consanguineous family is associated with central hypothyroidism in homozygotes and hyperthyrotropinemia in heterozygotes, suggesting compensatory elevation of TSH with reduced biopotency. The I131T mutation decreases TRH binding and TRHR-Gq coupling and signaling.

Angiotensin type 1A receptor regulates β-arrestin binding of the β2-adrenergic receptor via heterodimerization

Heterodimerization between angiotensin type 1A receptor (AT₁R) and β₂-adrenergic receptor (β₂AR) has been shown to modulate G protein-mediated effects of these receptors. Activation of G protein-coupled receptors (GPCRs) leads to β-arrestin binding, desensitization, internalization and G protein-independent signaling of GPCRs. Our aim was to study the effect of heterodimerization on β-arrestin coupling. We found that β-arrestin binding of β₂AR is affected by activation of AT₁Rs. Costimulation with angiotensin II and isoproterenol markedly enhanced the interaction between β₂AR and β-arrestins, by prolonging the lifespan of β₂AR-induced β-arrestin2 clusters at the plasma membrane. While candesartan, a conventional AT₁R antagonist, had no effect on the β-arrestin2 binding to β₂AR, TRV120023, a β-arrestin biased agonist, enhanced the interaction. These findings reveal a new crosstalk mechanism between AT₁R and β₂AR, and suggest that enhanced β-arrestin2 binding to β₂AR can contribute to the pharmacological effects of biased AT₁R agonists.

Angiotensin II receptors and peritoneal dialysis-induced peritoneal fibrosis

The vasoactive hormone angiotensin II initiates its major hemodynamic effects through interaction with AT1 receptors, a member of the class of G protein-coupled receptors. Acting through its AT1R, angiotensin II regulates blood pressure and renal salt and water balance. Recent evidence points to additional pathological influences of activation of AT1R, in particular inflammation, fibrosis and atherosclerosis. The transcription factor nuclear factor κB, a key mediator in inflammation and atherosclerosis, can be activated by angiotensin II through a mechanism that may involve arrestin-dependent AT1 receptor internalization. Peritoneal dialysis is a therapeutic modality for treating patients with end-stage kidney disease. The effectiveness of peritoneal dialysis at removing waste from the circulation is compromised over time as a consequence of peritoneal dialysis-induced peritoneal fibrosis. The non-physiological dialysis solution used in peritoneal dialysis, i.e. highly concentrated, hyperosmotic glucose, acidic pH as well as large volumes infused into the peritoneal cavity, contributes to the development of fibrosis. Numerous trials have been conducted altering certain components of the peritoneal dialysis fluid in hopes of preventing or delaying the fibrotic response with limited success. We hypothesize that structural activation of AT1R by hyperosmotic peritoneal dialysis fluid activates the internalization process and subsequent signaling through the transcription factor nuclear factor κB, resulting in the generation of pro-fibrotic/pro-inflammatory mediators producing peritoneal fibrosis.

Biased ligand quantification in drug discovery: from theory to high throughput screening to identify new biased μ opioid receptor agonists

BACKGROUND AND PURPOSE

Biased GPCR ligands are able to engage with their target receptor in a manner that preferentially activates distinct downstream signalling and offers potential for next generation therapeutics. However, accurate quantification of ligand bias in vitro is complex, and current best practice is not amenable for testing large numbers of compound. We have therefore sought to apply ligand bias theory to an industrial scale screening campaign for the identification of new biased μ receptor agonists.

EXPERIMENTAL APPROACH

μ receptor assays with appropriate dynamic range were developed for both Gαi -dependent signalling and β-arrestin2 recruitment. Δlog(Emax /EC50 ) analysis was validated as an alternative for the operational model of agonism in calculating pathway bias towards Gαi -dependent signalling. The analysis was applied to a high throughput screen to characterize the prevalence and nature of pathway bias among a diverse set of compounds with μ receptor agonist activity.

KEY RESULTS

A high throughput screening campaign yielded 440 hits with greater than 10-fold bias relative to DAMGO. To validate these results, we quantified pathway bias of a subset of hits using the operational model of agonism. The high degree of correlation across these biased hits confirmed that Δlog(Emax /EC50 ) was a suitable method for identifying genuine biased ligands within a large collection of diverse compounds.

CONCLUSIONS AND IMPLICATIONS

This work demonstrates that using Δlog(Emax /EC50 ), drug discovery can apply the concept of biased ligand quantification on a large scale and accelerate the deliberate discovery of novel therapeutics acting via this complex pharmacology.

The N-terminus of the yeast G protein-coupled receptor Ste2p plays critical roles in surface expression, signaling, and negative regulation

G protein-coupled receptors (GPCRs) are found in all eukaryotic cells examined to date where they function as membrane-bound proteins that bind a multitude of extracellular ligands to initiate intracellular signal transduction systems controlling cellular physiology. GPCRs have seven heptahelical membrane spanning domains connected by extracellular and intracellular loops with an extracellular N-terminus and an intracellular C-terminus. The N-terminus has been the least studied domain of most GPCRs. The yeast Ste2p protein, the receptor for the thirteen amino acid peptide pheromone α-factor, has been used extensively as a model to study GPCR structure and function. In this study we constructed a number of deletions of the Ste2p N-terminus and uncovered an unexpected function as a negative regulatory domain. We examined the role of the N-terminus in expression, signaling function and ligand-binding properties and found that the residues 11-30 play a critical role in receptor expression on the cell surface. The studies also indicated that residues 2-10 of the N-terminus are involved in negative regulation of signaling as shown by the observation that deletion of these residues enhanced mating and gene induction. Furthermore, our results indicated that the residues 21-30 are essential for optimal signaling. Overall, we propose that the N-terminus of Ste2p plays multiple regulatory roles in controlling receptor function.

International Union of Basic and Clinical Pharmacology. XCIX. Angiotensin Receptors: Interpreters of Pathophysiological Angiotensinergic Stimuli [corrected]

The renin angiotensin system (RAS) produced hormone peptides regulate many vital body functions. Dysfunctional signaling by receptors for RAS peptides leads to pathologic states. Nearly half of humanity today would likely benefit from modern drugs targeting these receptors. The receptors for RAS peptides consist of three G-protein-coupled receptors—the angiotensin II type 1 receptor (AT1 receptor), the angiotensin II type 2 receptor (AT2 receptor), the MAS receptor—and a type II trans-membrane zinc protein—the candidate angiotensin IV receptor (AngIV binding site). The prorenin receptor is a relatively new contender for consideration, but is not included here because the role of prorenin receptor as an independent endocrine mediator is presently unclear. The full spectrum of biologic characteristics of these receptors is still evolving, but there is evidence establishing unique roles of each receptor in cardiovascular, hemodynamic, neurologic, renal, and endothelial functions, as well as in cell proliferation, survival, matrix-cell interaction, and inflammation. Therapeutic agents targeted to these receptors are either in active use in clinical intervention of major common diseases or under evaluation for repurposing in many other disorders. Broad-spectrum influence these receptors produce in complex pathophysiological context in our body highlights their role as precise interpreters of distinctive angiotensinergic peptide cues. This review article summarizes findings published in the last 15 years on the structure, pharmacology, signaling, physiology, and disease states related to angiotensin receptors. We also discuss the challenges the pharmacologist presently faces in formally accepting newer members as established angiotensin receptors and emphasize necessary future developments.

Identification of Distinct Conformations of the Angiotensin-II Type 1 Receptor Associated with the Gq/11 Protein Pathway and the β-Arrestin Pathway Using Molecular Dynamics Simulations

Biased signaling represents the ability of G protein-coupled receptors to engage distinct pathways with various efficacies depending on the ligand used or on mutations in the receptor. The angiotensin-II type 1 (AT1) receptor, a prototypical class A G protein-coupled receptor, can activate various effectors upon stimulation with the endogenous ligand angiotensin-II (AngII), including the Gq/11 protein and β-arrestins. It is believed that the activation of those two pathways can be associated with distinct conformations of the AT1 receptor. To verify this hypothesis, microseconds of molecular dynamics simulations were computed to explore the conformational landscape sampled by the WT-AT1 receptor, the N111G-AT1 receptor (constitutively active and biased for the Gq/11 pathway), and the D74N-AT1 receptor (biased for the β-arrestin1 and -2 pathways) in their apo-forms and in complex with AngII. The molecular dynamics simulations of the AngII-WT-AT1, N111G-AT1, and AngII-N111G-AT1 receptors revealed specific structural rearrangements compared with the initial and ground state of the receptor. Simulations of the D74N-AT1 receptor revealed that the mutation stabilizes the receptor in the initial ground state. The presence of AngII further stabilized the ground state of the D74N-AT1 receptor. The biased agonist [Sar(1),Ile(8)]AngII also showed a preference for the ground state of the WT-AT1 receptor compared with AngII. These results suggest that activation of the Gq/11 pathway is associated with a specific conformational transition stabilized by the agonist, whereas the activation of the β-arrestin pathway is linked to the stabilization of the ground state of the receptor.

Effect of a toggle switch mutation in TM6 of the human adenosine A₃ receptor on Gi protein-dependent signalling and Gi-independent receptor internalization

BACKGROUND AND PURPOSE

The highly conserved tryptophan (W6.48) in transmembrane domain 6 of GPCRs has been shown to play a central role in forming an active conformation in response to agonist binding. We set out to characterize the effect of this mutation on the efficacy of two agonists at multiple signalling pathways downstream of the adenosine A₃ receptor.

EXPERIMENTAL APPROACH

Residue W6.48 in the human adenosine A₃ receptor fused to yellow fluorescent protein was mutated to phenylalanine and expressed in CHO-K1 cells containing a cAMP response element reporter gene. The effects on agonist-mediated receptor internalization were monitored by automated confocal microscopy and image analysis. Further experiments were carried out to investigate agonist-mediated ERK1/2 phosphorylation, inhibition of [³H]-cAMP accumulation and β-arrestin2 binding.

KEY RESULTS

NECA was able to stimulate agonist-mediated internalization of the W6.48F mutant receptor, while the agonist HEMADO was inactive. Investigation of other downstream signalling pathways indicated that G-protein coupling was impaired for both agonists tested. Mutation of W6.48F therefore resulted in differential effects on agonist efficacy, and introduced signalling pathway bias for HEMADO at the adenosine A₃ receptor.

CONCLUSIONS AND IMPLICATIONS

Investigation of the pharmacology of the W6.48F mutant of the adenosine A₃ receptor confirms that this region is important in forming the active conformation of the receptor for stimulating a number of different signalling pathways and that mutations in this residue can lead to changes in agonist efficacy and signalling bias.

Investigation of the fate of type I angiotensin receptor after biased activation

Biased agonism on the type I angiotensin receptor (AT1-R) can achieve different outcomes via activation of G protein-dependent and -independent cellular responses. In this study, we investigated whether the biased activation of AT1-R can lead to different regulation and intracellular processing of the receptor. We analyzed β-arrestin binding, endocytosis, and subsequent trafficking steps, such as early and late phases of recycling of AT1-R in human embryonic kidney 293 cells expressing wild-type or biased mutant receptors in response to different ligands. We used Renilla luciferase-tagged receptors and yellow fluorescent protein-tagged β-arrestin2, Rab5, Rab7, and Rab11 proteins in bioluminescence resonance energy transfer measurements to follow the fate of the receptor after stimulation. We found that not only is the signaling of the receptor different upon using selective ligands, but the fate within the cells is also determined by the type of the stimulation. β-arrestin binding and the internalization kinetics of the angiotensin II-stimulated AT1-R differed from those stimulated by the biased agonists. Similarly, angiotensin II-stimulated wild-type AT1-R showed differences compared with a biased mutant AT1-R (DRY/AAY AT1-R) with regards to β-arrestin binding and endocytosis. We found that the differences in the internalization kinetics of the receptor in response to biased agonist stimulation are due to the differences in plasma membrane phosphatidylinositol 4,5-bisphosphate depletion. Moreover, the stability of the β-arrestin binding is a major determinant of the later fate of the internalized AT1-R receptor.

Mutations in the 'DRY' motif of the CB1 cannabinoid receptor result in biased receptor variants

The role of the highly conserved 'DRY' motif in the signaling of the CB1 cannabinoid receptor (CB1R) was investigated by inducing single-, double-, and triple-alanine mutations into this site of the receptor. We found that the CB1R-R3.50A mutant displays a partial decrease in its ability to activate heterotrimeric Go proteins (∼80% of WT CB1R (CB1R-WT)). Moreover, this mutant showed an enhanced basal β-arrestin2 (β-arr2) recruitment. More strikingly, the double-mutant CB1R-D3.49A/R3.50A was biased toward β-arrs, as it gained a robustly increased β-arr1 and β-arr2 recruitment ability compared with the WT receptor, while its G-protein activation was decreased. In contrast, the double-mutant CB1R-R3.50A/Y3.51A proved to be G-protein-biased, as it was practically unable to recruit β-arrs in response to agonist stimulus, while still activating G-proteins, although at a reduced level (∼70% of CB1R-WT). Agonist-induced ERK1/2 activation of the CB1R mutants showed a good correlation with their β-arr recruitment ability but not with their G-protein activation or inhibition of cAMP accumulation. Our results suggest that G-protein activation and β-arr binding of the CB1R are mediated by distinct receptor conformations, and the conserved 'DRY' motif plays different roles in the stabilization of these conformations, thus mediating both G-protein- and β-arr-mediated functions of CB1R.

Role for endocytosis of a constitutively active GPCR (GPR185) in releasing vertebrate oocyte meiotic arrest

Vertebrate oocytes are naturally arrested at prophase of meiosis I for sustained periods of time before resuming meiosis in a process called oocyte maturation that prepares the egg for fertilization. Members of the constitutively active GPR3/6/12 family of G-protein coupled receptors represent important mediators of meiotic arrest. In the frog oocyte the GPR3/12 homolog GPRx (renamed GPR185) has been shown to sustain meiotic arrest by increasing intracellular cAMP levels through GαSβγ. Here we show that GPRx is enriched at the cell membrane (~80%), recycles through an endosomal compartment at steady state, and loses its ability to signal once trapped intracellularly. Progesterone-mediated oocyte maturation is associated with significant internalization of both endogenous and overexpressed GPRx. Furthermore, a GPRx mutant that does not internalize in response to progesterone is significantly more efficient than wild-type GPRx at blocking oocyte maturation. Collectively our results argue that internalization of the constitutively active GPRx is important to release oocyte meiotic arrest.

Structural determinants for binding, activation, and functional selectivity of the angiotensin AT1 receptor

The renin-angiotensin system (RAS) plays an important role in the pathophysiology of cardiovascular disorders. Pharmacologic interventions targeting the RAS cascade have led to the discovery of renin inhibitors, angiotensin-converting enzyme inhibitors, and AT(1) receptor blockers (ARBs) to treat hypertension and some cardiovascular and renal disorders. Mutagenesis and modeling studies have revealed that differential functional outcomes are the results of multiple active states conformed by the AT(1) receptor upon interaction with angiotensin II (Ang II). The binding of agonist is dependent on both extracellular and intramembrane regions of the receptor molecule, and as a consequence occupies more extensive area of the receptor than a non-peptide antagonist. Both agonist and antagonist bind to the same intramembrane regions to interfere with each other's binding to exhibit competitive, surmountable interaction. The nature of interactions with the amino acids in the receptor is different for each of the ARBs given the small differences in the molecular structure between drugs. AT(1) receptors attain different conformation states after binding various Ang II analogues, resulting in variable responses through activation of multiple signaling pathways. These include both classical and non-classical pathways mediated through growth factor receptor transactivations, and provide cross-communication between downstream signaling molecules. The structural requirements for AT(1) receptors to activate extracellular signal-regulated kinases 1 and 2 through G proteins, or G protein-independently through β-arrestin, are different. We review the structural and functional characteristics of Ang II and its analogs and antagonists, and their interaction with amino acid residues in the AT(1) receptor.

Identification of serine 348 on the apelin receptor as a novel regulatory phosphorylation site in apelin-13-induced G protein-independent biased signaling

Phosphorylation plays vital roles in the regulation of G protein-coupled receptor (GPCR) functions. The apelin and apelin receptor (APJ) system is involved in the regulation of cardiovascular function and central control of body homeostasis. Here, using tandem mass spectrometry, we first identified phosphorylated serine residues in the C terminus of APJ. To determine the role of phosphorylation sites in APJ-mediated G protein-dependent and -independent signaling and function, we induced a mutation in the C-terminal serine residues and examined their effects on the interaction between APJ with G protein or GRK/β-arrestin and their downstream signaling. Mutation of serine 348 led to an elimination of both GRK and β-arrestin recruitment to APJ induced by apelin-13. Moreover, APJ internalization and G protein-independent ERK signaling were also abolished by point mutation at serine 348. In contrast, this mutant at serine residues had no demonstrable impact on apelin-13-induced G protein activation and its intracellular signaling. These findings suggest that mutation of serine 348 resulted in inactive GRK/β-arrestin. However, there was no change in the active G protein thus, APJ conformation was biased. These results provide important information on the molecular interplay and impact of the APJ function, which may be extrapolated to design novel drugs for cardiac hypertrophy based on this biased signal pathway.

Functionally biased signalling properties of 7TM receptors - opportunities for drug development for the ghrelin receptor

The ghrelin receptor is a 7 transmembrane (7TM) receptor involved in a variety of physiological functions including growth hormone secretion, increased food intake and fat accumulation as well as modulation of reward and cognitive functions. Because of its important role in metabolism and energy expenditure, the ghrelin receptor has become an important therapeutic target for drug design and the development of anti-obesity compounds. However, none of the compounds developed so far have been approved for commercial use. Interestingly, the ghrelin receptor is able to signal through several different signalling pathways including Gαq , Gαi/o , Gα12/13 and arrestin recruitment. These multiple signalling pathways allow for functionally biased signalling, where one signalling pathway may be favoured over another either by selective ligands or through mutations in the receptor. In the present review, we have described how ligands and mutations in the 7TM receptor may bias the receptors to favour either one G-protein over another or to promote G-protein independent signalling pathways rather than G-protein-dependent pathways. For the ghrelin receptor, both agonist and inverse agonists have been demonstrated to signal more strongly through the Gαq -coupled pathway than the Gα12/13 -coupled pathway. Similarly a ligand that promotes Gαq coupling over Gαi coupling has been described and it has been suggested that several different active conformations of the receptor may exist dependent on the properties of the agonist. Importantly, ligands with such biased signalling properties may allow the development of drugs that selectively modulate only the therapeutically relevant physiological functions, thereby decreasing the risk of side effects.

LINKED ARTICLES

This article is part of a themed section on Neuropeptides. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2013.170.issue-7.

Biased ligands: pathway validation for novel GPCR therapeutics

G protein-coupled receptors (GPCRs), in recent years, have been shown to signal via multiple distinct pathways. Furthermore, biased ligands for some receptors can differentially stimulate or inhibit these pathways versus unbiased endogenous ligands or drugs. Biased ligands can be used to gain a deeper understanding of the molecular targets and cellular responses associated with a GPCR, and may be developed into therapeutics with improved efficacy, safety and/or tolerability. Here we review examples and approaches to pathway validation that establish the relevance and therapeutic potential of distinct pathways that can be selectively activated or blocked by biased ligands.

A type 1 cholecystokinin receptor mutant that mimics the dysfunction observed for wild type receptor in a high cholesterol environment

Cholecystokinin (CCK) stimulates the type 1 CCK receptor (CCK1R) to elicit satiety after a meal. Agonists with this activity, although potentially useful for treatment of obesity, can also have side effects and toxicities of concern, making the development of an intrinsically inactive positive allosteric modulator quite attractive. Positive allosteric modulators also have the potential to correct the defective receptor-G protein coupling observed in the high membrane cholesterol environment described in metabolic syndrome. Current model systems to study CCK1R in such an environment are unstable and expensive to maintain. We now report that the Y140A mutation within a cholesterol-binding motif and the conserved, class A G protein-coupled receptor-specific (E/D)RY signature sequence results in ligand binding and activity characteristics similar to wild type CCK1R in a high cholesterol environment. This is true for natural CCK, as well as ligands with distinct chemistries and activity profiles. Additionally, the Y140A construct also behaved like CCK1R in high cholesterol in regard to its internalization, sensitivity to a nonhydrolyzable GTP analog, and anisotropy of a bound fluorescent CCK analog. Chimeric CCK1R/CCK2R constructs that systematically changed the residues in the allosteric ligand-binding pocket were studied in the presence of Y140A. This established increased importance of unique residues within TM3 and reduced the importance of TM2 for binding in the presence of this mutation, with the agonist trigger likely pulled away from its Leu(356) target on TM7. The distinct conformation of this intramembranous pocket within Y140A CCK1R provides an opportunity to normalize this by using a small molecule allosteric ligand, thereby providing safe and effective correction of the coupling defect in metabolic syndrome.

ERK5 activation by Gq-coupled muscarinic receptors is independent of receptor internalization and β-arrestin recruitment

G-protein-coupled receptors (GPCRs) are known to activate both G protein- and β-arrestin-dependent signalling cascades. The initiation of mitogen-activated protein kinase (MAPK) pathways is a key downstream event in the control of cellular functions including proliferation, differentiation, migration and apoptosis. Both G proteins and β-arrestins have been reported to mediate context-specific activation of ERK1/2, p38 and JNK MAPKs. Recently, the activation of ERK5 MAPK by Gq-coupled receptors has been described to involve a direct interaction between Gαq and two novel effectors, PKCζ and MEK5. However, the possible contribution of β-arrestin towards this pathway has not yet been addressed. In the present work we sought to investigate the role of receptor internalization processes and β-arrestin recruitment in the activation of ERK5 by Gq-coupled GPCRs. Our results show that ERK5 activation is independent of M1 or M3 muscarinic receptor internalization. Furthermore, we demonstrate that phosphorylation-deficient muscarinic M1 and M3 receptors are still able to fully activate the ERK5 pathway, despite their reported inability to recruit β-arrestins. Indeed, the overexpression of Gαq, but not that of β-arrestin1 or β-arrestin2, was found to potently enhance ERK5 activation by GPCRs, whereas silencing of β-arrestin2 expression did not affect the activation of this pathway. Finally, we show that a β-arrestin-biased mutant form of angiotensin II (SII; Sar1-Ile4-Ile8 AngII) failed to promote ERK5 phosphorylation in primary cardiac fibroblasts, as compared to the natural ligand. Overall, this study shows that the activation of ERK5 MAPK by model Gq-coupled GPCRs does not depend on receptor internalization, β-arrestin recruitment or receptor phosphorylation but rather is dependent on Gαq-signalling.

NF-κB and CREB are required for angiotensin II type 1 receptor upregulation in neurons

Nuclear factor kappa B (NF-κB) and the Ets like gene-1 (Elk-1) are two transcription factors that have been previously established to contribute to the Angiotensin II mediated upregulation of Angiotensin II type 1 receptor (AT1R) in neurons. The cAMP response element binding protein (CREB) is another transcription factor that has also been implicated in AT1R gene transcription. The goal of the current study was to determine if NF-κB and CREB association was required for AT1R upregulation. We hypothesized that the transcription of the AT1R gene occurs via an orchestration of transcription factor interactions including NF-κB, CREB, and Elk-1. The synergistic role of CREB and NFκB in promoting AT1R gene expression was determined using siRNA-mediated silencing of CREB. Electrophorectic Mobility Shift Assay studies employing CREB and NF-κB demonstrated increased protein - DNA binding as a result of Ang II stimulation which was blunted by siRNA silencing of CREB. Upstream inhibition of p38 mitogen activated protein kinase (p38 MAPK) with SB203580 or inhibition of the calmodulin kinase (CAMK) pathway using KN-62 blunted changes in CREB and NF-κB expression. These findings suggest that Ang II may activate multiple signaling pathways involving p38 MAPK leading to the activation of NF-κB and CREB, which feed back to upregulate the AT1R gene. This study provides insight into the molecular mechanisms involving multiple transcription factor activation in a coordinated fashion which may be partially responsible for sympathoexcitation in clinical conditions associated with increased activation of the renin angiotensin system.

Structural features of the G-protein/GPCR interactions

BACKGROUND

The details of the functional interaction between G proteins and the G protein coupled receptors (GPCRs) have long been subjected to extensive investigations with structural and functional assays and a large number of computational studies.

SCOPE OF REVIEW

The nature and sites of interaction in the G-protein/GPCR complexes, and the specificities of these interactions selecting coupling partners among the large number of families of GPCRs and G protein forms, are still poorly defined.

MAJOR CONCLUSIONS

Many of the contact sites between the two proteins in specific complexes have been identified, but the three dimensional molecular architecture of a receptor-Gα interface is only known for one pair. Consequently, many fundamental questions regarding this macromolecular assembly and its mechanism remain unanswered.

GENERAL SIGNIFICANCE

In the context of current structural data we review the structural details of the interfaces and recognition sites in complexes of sub-family A GPCRs with cognate G-proteins, with special emphasis on the consequences of activation on GPCR structure, the prevalence of preassembled GPCR/G-protein complexes, the key structural determinants for selective coupling and the possible involvement of GPCR oligomerization in this process.

The second intracellular loop of the human cannabinoid CB2 receptor governs G protein coupling in coordination with the carboxyl terminal domain

The major effects of cannabinoids and endocannabinoids are mediated via two G protein-coupled receptors, CB1 and CB2, elucidation of the mechanism and structural determinants of the CB2 receptor coupling with G proteins will have a significant impact on drug discovery. In the present study, we systematically investigated the role of the intracellular loops in the interaction of the CB2 receptor with G proteins using chimeric receptors alongside the characterization of cAMP accumulation and ERK1/2 phosphorylation. We provided evidence that ICL2 was significantly involved in G protein coupling in coordination with the C-terminal end. Moreover, a single alanine substitution of the Pro-139 in the CB2 receptor that corresponds to Leu-222 in the CB1 receptor resulted in a moderate impairment in the inhibition of cAMP accumulation, whereas mutants P139F, P139M and P139L were able to couple to the Gs protein in a CRE-driven luciferase assay. With the ERK activation experiments, we further found that P139L has the ability to activate ERK through both Gi- and Gs-mediated pathways. Our findings defined an essential role of the second intracellular loop of the CB2 receptor in coordination with the C-terminal tail in G protein coupling and receptor activation.

Shear stress-induced Ang II AT1 receptor activation: G-protein dependent and independent mechanisms

Mechanotransduction enables cells to sense and respond to stimuli, such as strain, pressure and shear stress (SS), critical for maintenance of cardiovascular homeostasis or pathological states. The angiotensin II type 1 receptor (AT1R) was the first G protein-coupled receptor described to display stretch-induced activation in cardiomyocytes independent of its ligand Ang II. Here, we assessed whether SS (15 dynes/cm(2), 10 min), an important mechanical force present in the cardiovascular system, activates AT1R independent of its ligand. SS induced extracellular signal-regulated kinase (ERK) activation, used as a surrogate of AT1R activation, in Chinese hamster ovary cells expressing the AT1R (CHO+AT1) but not in wild type cells (CHO). AT1R dependent SS-induced ERK activation involves Ca(2+) inflow and activation of Gαq since Ca(2+) chelator EGTA or Gαq-specific inhibitor YM-254890 decreased SS-induced ERK activation. On the other hand, the activation of JAK-2 and Src, two intracellular signaling molecules independent of G protein activation, were not differently modulated in the presence of AT1R. Also, ERK activation by SS was observed in CHO cells expressing the mutated AT1R DRY/AAY, which has impaired ability to activate Gαq dependent intracellular signaling. Altogether we provided evidence that SS activates AT1R in the absence of its ligand by both a G protein-dependent and -independent pathways. The biological relevance of these observations deserves to be further investigated since the novel mechanisms described extend the knowledge of the activation of GPCRs independent of its traditional ligand.

Molecular weight of different angiotensin II polymers directly determines: density of endothelial membrane AT1 receptors and coronary vasoconstriction

We have shown that angiotensin II (Ang II) does not diffuse across the vessel wall, remaining intravascularly confined and acting solely on the coronary endothelial luminal membrane (CELM) receptors. A sustained intracoronary infusion of Ang II causes transient coronary vasoconstriction (desensitization) due to membrane internalization of CELM Ang II type 1 receptors (CELM-AT1R). In contrast, sustained intracoronary infusion of a non-diffusible polymer of Ang II (Ang II-Pol, 15,000 kDa) causes a sustained vasoconstriction by preventing CELM-AT1R internalization. In addition, a sustained intracoronary infusion of Ang II leads to a depressed response following a secondary Ang II administration (tachyphylaxis) that is reversed by Ang II-Pol. These findings led us to hypothesize that the rate of desensitization, tachyphylaxis, and AT1R internalization were dependent on Ang II-Pol molecular weight. To test this hypothesis, we synthesized Ang II-Pols of the following molecular weights (in kDa): 1.3, 2.7, 11, 47, 527, 3270 and 15,000. Vasoconstriction was measured following intracoronary infusion of Ang II-Pols in Langendorff-perfused guinea pig hearts at constant flow. The CELM protein fraction was extracted using the silica pellicle technique at different time points in order to determine the rate of AT1R internalization following each Ang II-Pol infusion. CELM-AT1R density was quantified by Western blot. We found that the rate of desensitization and the tachyphylaxis effect varied inversely with the molecular weight of the Ang II-Pols. Inversely proportional to the molecular weight of Ang II-Pol the CELM-AT1R density decreases over time. These results indicate that the mechanism responsible for the decreased rate of desensitization and tachyphylaxis by higher molecular weight Ang II polymers is due to reduction in the rate of CELM-AT1R internalization. These Ang II polymers would be valuable tools for studying the relationship between AT1R internalization and physiological effects.

Advances in methods to characterize ligand-induced ionic lock and rotamer toggle molecular switch in G protein-coupled receptors

Structural biology of GPCRs has made significant progress upon recently developed technologies for GPCRs expression/purification and elucidation of GPCRs crystal structures. The crystal structures provide a snapshot of the receptor structural disposition of GPCRs itself or with cocrystallized ligands, and the results are congruent with biophysical and computer modeling studies reported about GPCRs conformational and dynamics flexibility, regulated activation, and the various stabilizing interactions, such as "molecular switches." The molecular switches generally constitute the most conserved domains within a particular GPCR superfamily. Often agonist-induced receptor activation proceeds by the disruption of majority of these interactions, while antagonist and inverse agonist act as blockers and structural stabilizers, respectively. Several elegant studies, particularly for the β2AR, have demonstrated the relationship between ligand structure, receptor conformational changes, and corresponding pharmacological outcomes. Thus, it is of great importance to understand GPCRs activation related to cell signaling pathways. Herein, we summarize the steps to produce functional GPCRs, generate suitably fluorescent labeled GPCRs and the procedure to use that to understand if ligand-induced activation can proceed by activation of the GPCRs via ionic lock switch and/or rotamer toggle switch mechanisms. Such understanding of ligand structure and mechanism of receptor activation will provide great insight toward uncovering newer pathways of GPCR activation and aid in structure-based drug design.

β-arrestin 2 regulates Aβ generation and γ-secretase activity in Alzheimer's disease

β-arrestins are associated with numerous aspects of G protein-coupled receptor (GPCR) signaling and regulation and accordingly influence diverse physiological and pathophysiological processes. Here we report that β-arrestin 2 expression is elevated in two independent cohorts of individuals with Alzheimer's disease. Overexpression of β-arrestin 2 leads to an increase in amyloid-β (Aβ) peptide generation, whereas genetic silencing of Arrb2 (encoding β-arrestin 2) reduces generation of Aβ in cell cultures and in Arrb2(-/-) mice. Moreover, in a transgenic mouse model of Alzheimer's disease, genetic deletion of Arrb2 leads to a reduction in the production of Aβ(40) and Aβ(42). Two GPCRs implicated previously in Alzheimer's disease (GPR3 and the β(2)-adrenergic receptor) mediate their effects on Aβ generation through interaction with β-arrestin 2. β-arrestin 2 physically associates with the Aph-1a subunit of the γ-secretase complex and redistributes the complex toward detergent-resistant membranes, increasing the catalytic activity of the complex. Collectively, these studies identify β-arrestin 2 as a new therapeutic target for reducing amyloid pathology and GPCR dysfunction in Alzheimer's disease.

Therapeutic potential of functional selectivity in the treatment of heart failure

Adrenergic and angiotensin receptors are prominent targets in pharmacological alleviation of cardiac remodeling and heart failure, but their use is associated with cardiodepressant side effects. Recent advances in our understanding of seven transmembrane receptor signaling show that it is possible to design ligands with "functional selectivity," acting as agonists on certain signaling pathways while antagonizing others. This represents a major pharmaceutical opportunity to separate desired from adverse effects governed by the same receptor. Accordingly, functionally selective ligands are currently pursued as next-generation drugs for superior treatment of heart failure.

Allosteric interactions within the AT₁ angiotensin receptor homodimer: role of the conserved DRY motif

G protein coupled receptor (GPCR) dimerization has a remarkable impact on the diversity of receptor signaling. Allosteric communication between the protomers of the dimer can alter ligand binding, receptor conformation and interactions with different effector proteins. In this study we investigated the allosteric interactions between wild type and mutant protomers of type 1 angiotensin receptor (AT₁R) dimers transiently expressed in CHO cells. In our experimental setup, one protomer of the dimer was selectively stimulated and the β-arrestin2 binding and conformation alteration of the other protomer was followed. The interaction between β-arrestin2 and the non-stimulated protomer was monitored through a bioluminescence resonance energy transfer (BRET) based method. To measure the conformational alterations in the non-stimulated protomer directly, we also used a BRET based intramolecular receptor biosensor, which was created by inserting yellow fluorescent protein (YFP) into the 3rd intracellular loop of AT₁R and fusing Renilla luciferase (RLuc) to its C terminal region. We have detected β-arrestin2 binding, and altered conformation of the non-stimulated protomer. The cooperative ligand binding of the receptor homodimer was also observed by radioligand dissociation experiments. Mutation of the conserved DRY sequence in the activated protomer, which is also required for G protein activation, abolished all the observed allosteric effects. These data suggest that allosteric interactions in the homodimers of AT₁R significantly affect the function of the non-stimulated protomer, and the conserved DRY motif has a crucial role in these interactions.

Acute depletion of plasma membrane phosphatidylinositol 4,5-bisphosphate impairs specific steps in endocytosis of the G-protein-coupled receptor

Receptor endocytosis plays an important role in regulating the responsiveness of cells to specific ligands. Phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P(2)] has been shown to be crucial for endocytosis of some cell surface receptors, such as EGF and transferrin receptors, but its role in G-protein-coupled receptor internalization has not been investigated. By using luciferase-labeled type 1 angiotensin II (AT1R), type 2C serotonin (5HT2CR) or β(2) adrenergic (β2AR) receptors and fluorescently tagged proteins (β-arrestin-2, plasma-membrane-targeted Venus, Rab5) we were able to follow the sequence of molecular interactions along the endocytic route of the receptors in HEK293 cells using the highly sensitive method of bioluminescence resonance energy transfer and confocal microscopy. To study the role of plasma membrane PtdIns(4,5)P(2) in receptor endocytosis, we used our previously developed rapamycin-inducible heterodimerization system, in which the recruitment of a 5-phosphatase domain to the plasma membrane degrades PtdIns(4,5)P(2). Here we show that ligand-induced interaction of AT1, 5HT2C and β(2)A receptors with β-arrestin-2 was unaffected by PtdIns(4,5)P(2) depletion. However, trafficking of the receptors to Rab5-positive early endosomes was completely abolished in the absence of PtdIns(4,5)P(2). Remarkably, removal of the receptors from the plasma membrane was reduced but not eliminated after PtdIns(4,5)P(2) depletion. Under these conditions, stimulated AT1 receptors clustered along the plasma membrane, but did not enter the cells. Our data suggest that in the absence of PtdIns(4,5)P(2), these receptors move into clathrin-coated membrane structures, but these are not cleaved efficiently and hence cannot reach the early endosomal compartment.

Mapping of the localization of type 1 angiotensin receptor in membrane microdomains using bioluminescence resonance energy transfer-based sensors

Initiation and termination of signaling of the type I angiotensin receptor (AT(1)-R) can lead to dynamic changes in its localization in plasma membrane microdomains. Several markers were recently developed to investigate membrane microdomains. Here, we used several YFP-labeled fusion constructs (i.e. raft or non-raft plasma membrane markers) to analyze the agonist-induced changes in compartmentalization of AT(1)-R, including internalization or lateral movement between plasma membrane compartments in response to stimulation using bioluminescence resonance energy transfer measurements. Our data demonstrate that angiotensin II (AngII) stimulus changes the microdomain localization of wild type or mutated (DRY → AAY or TSTS → AAAA) AT(1)-Rs co-expressed with the fluorescent probes in HEK293 cells. The comparison of the trafficking of AT(1)-R upon AngII stimulus with those of [Sar(1),Ile(8)]AngII or [Sar(1),Ile(4),Ile(8)]AngII stimulus revealed different types of changes, depending on the nature of the ligand. The observed changes in receptor compartmentalization of the AT(1)-R are strikingly different from those of 5HT-2C and EGF receptors, which demonstrate the usefulness of the bioluminescence resonance energy transfer-based measurements in the investigation of receptor trafficking in the plasma membrane in living cell experiments.

Sensitivity of cholecystokinin receptors to membrane cholesterol content

Cholesterol represents a structurally and functionally important component of the eukaryotic cell membrane, where it increases lipid order, affects permeability, and influences the lateral mobility and conformation of membrane proteins. Several G protein-coupled receptors have been shown to be affected by the cholesterol content of the membrane, with functional impact on their ligand binding and signal transduction characteristics. The effects of cholesterol can be mediated directly by specific molecular interactions with the receptor and/or indirectly by altering the physical properties of the membrane. This review focuses on the importance and differential effects of membrane cholesterol on the activity of cholecystokinin (CCK) receptors. The type 1 CCK receptor is quite sensitive to its cholesterol environment, while the type 2 CCK receptor is not. The possible structural basis for this differential impact is explored and the implications of pathological states, such as metabolic syndrome, in which membrane cholesterol may be increased and CCK1R function may be abnormal are discussed. This is believed to have substantial potential importance for the development of drugs targeting the CCK receptor.

Differential sensitivity of types 1 and 2 cholecystokinin receptors to membrane cholesterol

Recent studies indicate that membrane cholesterol can associate with G protein-coupled receptors (GPCRs) and affect their function. Previously, we reported that manipulation of membrane cholesterol affects ligand binding and signal transduction of the type 1 cholecystokinin receptor (CCK1R), a Class A GPCR. We now demonstrate that the closely related type 2 cholecystokinin receptor (CCK2R) does not share this cholesterol sensitivity. The sequences of both receptors reveal almost identical cholesterol interaction motifs in analogous locations in transmembrane segments two, three, four, and five. The disparity in cholesterol sensitivity between these receptors, despite their close structural relationship, provides a unique opportunity to define the possible structural basis of cholesterol sensitivity of CCK1R. To evaluate the relative contributions of different regions of CCK1R to cholesterol sensitivity, we performed ligand binding studies and biological activity assays of wild-type and CCK2R/CCK1R chimeric receptor-bearing Chinese hamster ovary cells after manipulation of membrane cholesterol. We also extended these studies to site-directed mutations within the cholesterol interaction motifs. The results contribute to a better understanding of the structural requirements for cholesterol sensitivity in CCK1R and provides insight into the function of other cholesterol-sensitive Class A GPCRs.

Unexpected diversity and photoperiod dependence of the zebrafish melanopsin system

Animals have evolved specialized photoreceptors in the retina and in extraocular tissues that allow them to measure light changes in their environment. In mammals, the retina is the only structure that detects light and relays this information to the brain. The classical photoreceptors, rods and cones, are responsible for vision through activation of rhodopsin and cone opsins. Melanopsin, another photopigment first discovered in Xenopus melanophores (Opn4x), is expressed in a small subset of retinal ganglion cells (RGCs) in the mammalian retina, where it mediates non-image forming functions such as circadian photoentrainment and sleep. While mammals have a single melanopsin gene (opn4), zebrafish show remarkable diversity with two opn4x-related and three opn4-related genes expressed in distinct patterns in multiple neuronal cell types of the developing retina, including bipolar interneurons. The intronless opn4.1 gene is transcribed in photoreceptors as well as in horizontal cells and produces functional photopigment. Four genes are also expressed in the zebrafish embryonic brain, but not in the photoreceptive pineal gland. We discovered that photoperiod length influences expression of two of the opn4-related genes in retinal layers involved in signaling light information to RGCs. Moreover, both genes are expressed in a robust diurnal rhythm but with different phases in relation to the light-dark cycle. The results suggest that melanopsin has an expanded role in modulating the retinal circuitry of fish.

Biased Ligands for Better Cardiovascular Drugs

Drug discovery efforts targeting G-protein-coupled receptors (GPCR) have been immensely successful in creating new cardiovascular medicines. Currently marketed GPCR drugs are broadly classified as either agonists that activate receptors or antagonists that prevent receptor activation by endogenous stimuli. However, GPCR couple to a multitude of intracellular signaling pathways beyond classical G-protein signals, and these signals can be independently activated by biased ligands to vastly expand the potential for new drugs at these classic targets. By selectively engaging only a subset of a receptor's potential intracellular partners, biased ligands may deliver more precise therapeutic benefit with fewer side effects than current GPCR-targeted drugs. In this review, we discuss the history of biased ligand research, the current understanding of how biased ligands exert their unique pharmacology, and how research into GPCR signaling has uncovered previously unappreciated capabilities of receptor pharmacology. We focus on several receptors to illustrate the approaches taken and discoveries made, and how these are steadily illuminating the intricacies of GPCR pharmacology. Discoveries of biased ligands targeting the angiotensin II type 1 receptor and of separable pharmacology suggesting the potential value of biased ligands targeting the β-adrenergic receptors and nicotinic acid receptor GPR109a highlight the powerful clinical promise of this new category of potential therapeutics.

Simplified modeling approach suggests structural mechanisms for constitutive activation of the C5a receptor

Molecular modeling of conformational changes occurring in the transmembrane region of the complement factor 5a receptor (C5aR) during receptor activation was performed by comparing two constitutively active mutants (CAMs) of C5aR, NQ (I124N/L127Q), and F251A, to those of the wild-type C5aR and NQ-N296A (I124N/L127Q/N296A), which have the wild-type phenotype. Modeling involved comprehensive sampling of various rotations of TM helices aligned to the crystal template of the dark-adapted rhodopsin along their long axes. By assuming that the relative energies of the spontaneously activated states of CAMs should be lower or at least comparable to energies characteristic for the ground states, we selected the plausible models for the conformational states associated with constitutive activation in C5aR. The modeling revealed that the hydrogen bonds between the side chains of D82-N119, S85-N119, and S131-C221 characteristic for the ground state were replaced by the hydrogen bonds D82-N296, N296-Y300, and S131-R134, respectively, in the activated states. Also, conformational transitions that occurred upon activation were hindered by contacts between the side chains of L127 and F251. The results rationalize the available data of mutagenesis in C5aR and offer the first specific molecular mechanism for the loss of constitutive activity in NQ-N296A. Our results also contributed to understanding the general structural mechanisms of activation in G-protein-coupled receptors lacking the "ionic lock", R(3.50) and E/D(6.30). Importantly, these results were obtained by modeling approaches that deliberately simplify many elements in order to explore potential conformations of GPCRs involving large-scale molecular movements.

Biased signaling of the angiotensin II type 1 receptor can be mediated through distinct mechanisms

BACKGROUND

Seven transmembrane receptors (7TMRs) can adopt different active conformations facilitating a selective activation of either G protein or β-arrestin-dependent signaling pathways. This represents an opportunity for development of novel therapeutics targeting selective biological effects of a given receptor. Several studies on pathway separation have been performed, many of these on the Angiotensin II type 1 receptor (AT1R). It has been shown that certain ligands or mutations facilitate internalization and/or recruitment of β-arrestins without activation of G proteins. However, the underlying molecular mechanisms remain largely unresolved. For instance, it is unclear whether such selective G protein-uncoupling is caused by a lack of ability to interact with G proteins or rather by an increased ability of the receptor to recruit β-arrestins. Since uncoupling of G proteins by increased ability to recruit β-arrestins could lead to different cellular or in vivo outcomes than lack of ability to interact with G proteins, it is essential to distinguish between these two mechanisms.

METHODOLOGY/PRINCIPAL FINDINGS

We studied five AT1R mutants previously published to display pathway separation: D74N, DRY/AAY, Y292F, N298A, and Y302F (Ballesteros-Weinstein numbering: 2.50, 3.49-3.51, 7.43, 7.49, and 7.53). We find that D74N, DRY/AAY, and N298A mutants are more prone to β-arrestin recruitment than WT. In contrast, receptor mutants Y292F and Y302F showed impaired ability to recruit β-arrestin in response to Sar1-Ile4-Ile8 (SII) Ang II, a ligand solely activating the β-arrestin pathway.

CONCLUSIONS/SIGNIFICANCE

Our analysis reveals that the underlying conformations induced by these AT1R mutants most likely represent principally different mechanisms of uncoupling the G protein, which for some mutants may be due to their increased ability to recruit β-arrestin2. Hereby, these findings have important implications for drug discovery and 7TMR biology and illustrate the necessity of uncovering the exact molecular determinants for G protein-coupling and β-arrestin recruitment, respectively.

Β-arrestin: a signaling molecule and potential therapeutic target for heart failure

Currently, some of the most effective treatments for heart failure target GPCRs such as the beta-adrenergic receptors (β1AR and β2AR) and angiotensin II type IA receptors (AT1aR). Ligands for these receptors not only function by blocking the deleterious G-protein mediated pathway leading to heart failure, but also signal via G-protein independent pathways that involve receptor phosphorylation by G-protein receptor kinases (GRKs) leading to recruitment of the multifunctional protein, β-arrestin. Originally thought to play a role in GPCR desensitization and internalization, β-arrestin has recently been shown to mediate signaling independent of classical second messengers in a way that is often protective to the heart. The multi-functionality of β-arrestin makes it an intriguing molecule in the development of the next generation of drugs for cardiac diseases with the potential to simultaneously inhibit deleterious G-protein dependent pathways while activating beneficial β-arrestin mediated signaling. In this review, we explore various facets of β-arrestin signaling and offer a perspective on its potential role as a key signaling molecule in the treatment of heart failure. This article is part of a special issue entitled "Key Signaling Molecules in Hypertrophy and Heart Failure."