The critical role of transmembrane prolines in human prostacyclin receptor activation

Insights into the molecular mechanism of positive cooperativity between partial agonist MK-8666 and full allosteric agonist AP8 of hGPR40 by Gaussian accelerated molecular dynamics (GaMD) simulations

Activation of human free fatty acid receptor 1 (FFAR1, also called hGPR40) enhances insulin secretion in a glucose-dependent manner. Hence, the development of selective agonist targeting hGPR40 has been proposed as a therapeutic strategy of type 2 diabetes mellitus. Some agonists targeting hGPR40 were reported. The radioligand-binding studies and the crystal structures reveal that there are multiple sites on GPR40, and there exists positive binding cooperativity between the partial agonist MK-8666 and full allosteric agonist (AgoPAM) AP8. In this work, we carried out long-time Gaussian accelerated molecular dynamics (GaMD) simulations on hGPR40 to shed light on the mechanism of the cooperativity between the two agonists at different sites. Our results reveal that the induced-fit conformational coupling is bidirectional between the two sites. The movements and rotations of TM3, TM4, TM5 and TM6 due to their inherent flexibility are crucial in coupling the conformational changes of the two agonists binding sites. These helices adopt similar conformational states upon alternative ligand or both ligands binding. The Leu138^4.57, Leu186^5.42 and Leu190^5.46 play roles in coordinating the rearrangements of residues in the two pockets, which makes the movements of residues in the two sites like gear movements. These results provide detailed information at the atomic level about the conformational coupling between different sites of GPR40, and also provide the structural information for further design of new agonists of GPR40. In addition, these results suggest that it is necessary by considering the effect of other site bound in structure-based ligands discovery.

Revealing the Positive Binding Cooperativity Mechanism between the Orthosteric and the Allosteric Antagonists of CCR2 by Metadynamics and Gaussian Accelerated Molecular Dynamics Simulations

CC chemokine receptor 2 (CCR2) and its endogenous CC chemokine ligands are associated with numerous inflammatory, neurodegenerative diseases, and cancer. CCR2 is becoming an attractive target in the treatment of autoimmune disease and neurodegenerative diseases. The orthosteric antagonist BMS-681 and allosteric antagonist CCR2-RA-[R] of CCR2 show positive binding cooperativity. We performed well-tempered metadynamics simulations and Gaussian accelerated MD simulations to reveal the influence of the orthosteric antagonist on the unbinding of allosteric antagonist of CCR2. We revealed different unbinding pathways of CCR2-RA-[R] in binary complex CCR2-VT5 and ternary complex CCR2-73R-VT5. The different unbinding pathways of CCR2-RA-[R] are due to the conformational dynamics of TM6. We obtained the significant conformational differences of the intracellular side of TM6 upon CCR2 binding to different ligands by GaMD simulation. The conformational dynamics of TM6 are consistent with the unbinding pathway analysis. GaMD simulations indicate that BMS-681 binding restricts the bend of intracellular side of TM6 by stabilizing the extracellular sides of TM6 and TM7. The charged residues Arg206^5.43 of TM5 and Glu291^7.39 of TM7 play key roles in stabling TM7 and TM6. TM6 and TM7 are crucial components in the orthosteric and allosteric binding sites. Our results illustrate the conformational details about the effect of the orthosteric antagonist on the allosteric antagonist of CCR2. The conformational dynamics of CCR2 upon binding to different ligands can provide a rational basis for development of allosteric ligands of CCR2.

Conformational Change in the Ligand-Binding Pocket via a KISS1R Mutation (P147L) Leads to Isolated Gonadotropin-Releasing Hormone Deficiency

Context:

Kisspeptin receptor (KISS1R) is expressed in hypothalamic gonadotropin-releasing hormone neurons and responsible for pubertal onset and reproductive functions. KISS1R mutations remain a rare cause of congenital hypogonadotropic hypogonadism (CHH).

Objective:

The aim of this study was to identify the genetic cause of CHH in a patient and to functionally characterize a KISS1R mutation.

Design:

The patient was a 47-year-old Japanese man whose parents were first cousins. He lacked secondary sexual characteristics owing to normosmic CHH. Exon segments for the KISS1R gene in this patient were screened for mutations. Functional analyses were performed using HEK293 cells expressing KISS1R mutants. Molecular dynamics simulations were performed to compare the ligand-KISS1R mutant complex with those of wild-type KISS1R variants.

Results:

A homozygous mutation (c.440C>T, p.P147L) in KISS1R was identified. The P147L mutation did not affect either receptor expression level or subcellular localization in the recombinant expression system. Intracellular calcium measurements and cellular dielectric spectroscopy demonstrated that the P147L mutation impaired receptor function to an extent more severe than that of a previously reported L148S mutation. A receptor-ligand binding assay showed the P147L mutation causes a substantial loss of ligand-binding affinity. Molecular dynamics simulations revealed the P147L mutation decreases the contact surface area of the ligand-receptor complex in an expanded ligand-binding pocket.

Conclusion:

We identified a loss-of-function mutation in KISS1R associated with CHH. Our results demonstrated that the P147L mutation causes a severe phenotype and functional impairment resulting from the loss of ligand-binding affinity due to an expanded ligand-binding pocket.

Free backbone carbonyls mediate rhodopsin activation

Conserved prolines in the transmembrane helices of G-protein-coupled receptors (GPCRs) are often considered to function as hinges that divide the helix into two segments capable of independent motion. Depending on their potential to hydrogen-bond, the free C=O groups associated with these prolines can facilitate conformational flexibility, conformational switching or stabilization of the receptor structure. To address the role of conserved prolines in family A GPCRs through solid-state NMR spectroscopy, we focus on bovine rhodopsin, a GPCR in the visual receptor subfamily. The free backbone C=O groups on helices H5 and H7 stabilize the inactive rhodopsin structure through hydrogen-bonds to residues on adjacent helices. In response to light-induced isomerization of the retinal chromophore, hydrogen-bonding interactions involving these C=O groups are released, thus facilitating repacking of H5 and H7 onto the transmembrane core of the receptor. These results provide insights into the multiple structural and functional roles of prolines in membrane proteins.

High throughput mutagenesis for identification of residues regulating human prostacyclin (hIP) receptor expression and function

The human prostacyclin receptor (hIP receptor) is a seven-transmembrane G protein-coupled receptor (GPCR) that plays a critical role in vascular smooth muscle relaxation and platelet aggregation. hIP receptor dysfunction has been implicated in numerous cardiovascular abnormalities, including myocardial infarction, hypertension, thrombosis and atherosclerosis. Genomic sequencing has discovered several genetic variations in the PTGIR gene coding for hIP receptor, however, its structure-function relationship has not been sufficiently explored. Here we set out to investigate the applicability of high throughput random mutagenesis to study the structure-function relationship of hIP receptor. While chemical mutagenesis was not suitable to generate a mutagenesis library with sufficient coverage, our data demonstrate error-prone PCR (epPCR) mediated mutagenesis as a valuable method for the unbiased screening of residues regulating hIP receptor function and expression. Here we describe the generation and functional characterization of an epPCR derived mutagenesis library compromising >4000 mutants of the hIP receptor. We introduce next generation sequencing as a useful tool to validate the quality of mutagenesis libraries by providing information about the coverage, mutation rate and mutational bias. We identified 18 mutants of the hIP receptor that were expressed at the cell surface, but demonstrated impaired receptor function. A total of 38 non-synonymous mutations were identified within the coding region of the hIP receptor, mapping to 36 distinct residues, including several mutations previously reported to affect the signaling of the hIP receptor. Thus, our data demonstrates epPCR mediated random mutagenesis as a valuable and practical method to study the structure-function relationship of GPCRs.

Acyl-coenzyme A:cholesterol acyltransferase 1 - significance of single-nucleotide polymorphism at residue 526 and the role of Pro347 near the fifth transmembrane domain

Acyl-coenzyme A:cholesterol acyltransferases (ACATs), which are members of the membrane-bound O-acyltransferase family, catalyze the conversion of cholesterol to cholesteryl esters. Mammals have two isoenzymes: ACAT1 and ACAT2. Both enzymes are drug targets for treating human diseases. ACAT1 is present in various cell types. It contains nine transmembrane domains (TMDs), with the active site His460 located within TMD7, and the active site Asn421 located within the fourth large cytoplasmic loop. In human ACAT1, a single-nucleotide polymorphism exists for residue 526: the codon is either CAG for Gln, or CGG for Arg. Gln526/Arg526 is present within the C-terminal loop. Its biochemical significance is unknown. In addition, within the C-terminal half of ACAT1, numerous residues conserved with those of ACAT2 are present; the functions of these conserved residues are largely unknown. Here, we performed single-substitution mutagenesis experiments to investigate the roles of individual residues present in the C-terminal loop, including Gln526/Arg526, and the eight conserved Pro residues located near/in various TMDs. The results show that the enzyme activity of ACAT1 with Gln526 is less active than that of ACAT1 with Arg526 by 40%. In addition, several residues in the C-terminal loop are important for maintaining proper ACAT1 protein stability. Other results show that Pro347 plays an important role in modulating enzyme catalysis. Overall, our results imply that the CAG/CGG polymorphism can be utilized to perform ACAT1 activity/human disease susceptibility studies, and that Pro347 located near TMD5 plays an important role in modulating enzyme catalysis.

Dissecting the functions of conserved prolines within transmembrane helices of the D2 dopamine receptor

G protein-coupled receptors (GPCRs) contain a number of conserved proline residues in their transmembrane helices, and it is generally assumed these play important functional and/or structural roles. Here we use unnatural amino acid mutagenesis, employing α-hydroxy acids and proline analogues, to examine the functional roles of five proline residues in the transmembrane helices of the D2 dopamine receptor. The well-known tendency of proline to disrupt helical structure is important at all sites, while we find no evidence for a functional role for backbone amide cis-trans isomerization, another feature associated with proline. At most proline sites, the loss of the backbone NH is sufficient to explain the role of the proline. However, at one site, P210(5.50), a substituent on the backbone N appears to be essential for proper function. Interestingly, the pattern in functional consequences that we see is mirrored in the pattern of structural distortions seen in recent GPCR crystal structures.

Comprehensive biochemical analysis of rare prostacyclin receptor variants: study of association of signaling with coronary artery obstruction

Currently, pharmacogenetic studies are at an impasse as the low prevalence (<2%) of most variants hinder their pharmacogenetic analysis with population sizes often inadequate for sufficiently powered studies. Grouping rare mutations by functional phenotype rather than mutation site can potentially increase sample size. Using human population-based studies (n = 1,761) to search for dysfunctional human prostacyclin receptor (hIP) variants, we recently discovered 18 non-synonymous mutations, all with frequencies less than 2% in our study cohort. Eight of the 18 had defects in binding, activation, and/or protein stability/folding. Mutations (M113T, L104R, and R279C) in three highly conserved positions demonstrated severe misfolding manifested by impaired binding and activation of cell surface receptors. To assess for association with coronary artery disease, we performed a case-control study comparing coronary angiographic results from patients with reduced cAMP production arising from the non-synonymous mutations (n = 23) with patients with non-synonymous mutations that had no reduction in cAMP (n = 17). Major coronary artery obstruction was significantly increased in the dysfunctional mutation group in comparison with the silent mutations. We then compared the 23 dysfunctional receptor patients with 69 age- and risk factor-matched controls (1:3). This verified the significantly increased coronary disease in the non-synonymous dysfunctional variant cohort. This study demonstrates the potential utility of in vitro functional characterization in predicting clinical phenotypes and represents the most comprehensive characterization of human prostacyclin receptor genetic variants to date.

Influence of proline upon the folding and geometry of the WALP19 transmembrane peptide

The orientations, geometries, and lipid interactions of designed transmembrane (TM) peptides have attracted significant experimental and theoretical interest. Because the amino acid proline will introduce a known discontinuity into an alpha helix, we have sought to measure the extent of helix kinking caused by a single proline within the isolated TM helical domain of WALP19. For this purpose, we synthesized acetyl-GWWLALALAP(10)ALALALWWA-ethanolamide and included pairs of deuterated alanines by using 60-100% Fmoc-l-Ala-d(4) at selected sequence positions. Solid-state deuterium ((2)H) magnetic resonance spectra from oriented, hydrated samples (1/40, peptide/lipid; using several lipids) reveal signals from many of the alanine backbone C(alpha) deuterons as well as the alanine side-chain C(beta) methyl groups, whereas signals from C(alpha) deuterons generally have not been observed for similar peptides without proline. It is conceivable that altered peptide dynamics may be responsible for the apparent "unmasking" of the backbone resonances in the presence of the proline. Data analysis using the geometric analysis of labeled alanines (GALA) method reveals that the peptide helix is distorted due to the presence of the proline. To provide additional data points for evaluating the segmental tilt angles of the two halves of the peptide, we substituted selected leucines with l-Ala-d(4). Using this approach, we were able to deduce that the apparent average tilt of the C-terminal increases from approximately 4 degrees to approximately 12 degrees when Pro(10) is introduced. The segment N-terminal to proline is more complex and possibly is more dynamically flexible; Leu to Ala mutations within the N-terminal segment alter the average orientations of alanines in both segments. Nevertheless, in DOPC, we could estimate an apparent kink angle of approximately 19 degrees . Together, the results suggest that the central proline influences not only the geometry but also the dynamics of the membrane-spanning peptide. The results make up an important basis for understanding the functional role of proline in several families of membrane proteins.

The human prostacyclin receptor from structure function to disease

Thirty years have passed since Vane and colleagues first described a substance, prostanoid X, from microsomal fractions (later called prostacyclin) that relaxed rather than contracted mesenteric arteries. The critical role of prostacyclin in many pathophysiological conditions, such as atherothrombosis, has only recently become appreciated (through receptor knockout mice studies, selective cyclooxygenase-2 inhibition clinical trials, and the discovery of dysfunctional prostacyclin receptor genetic variants). Additionally, important roles in such diverse areas as pain and inflammation, and parturition are being uncovered. Prostacyclin-based therapies, currently used for pulmonary hypertension, are accordingly emerging as possible treatments for such diseases, fueling interests in structure function studies for the receptor and signal transduction pathways in native cells. The coming decade is likely to yield many further exciting advances.

Novel signaling pathways promote a paracrine wave of prostacyclin-induced vascular smooth muscle differentiation

The important athero-protective role of prostacyclin is becoming increasingly evident as recent studies have revealed adverse cardiovascular effects in mice lacking the prostacyclin receptor, in patients taking selective COX-2 inhibitors, and in patients in the presence of a dysfunctional prostacyclin receptor genetic variant. We have recently reported that this protective mechanism includes the promotion of a quiescent differentiated phenotype in human vascular smooth muscle cells (VSMC). Herein, we address the intriguing question of how localized endothelial release of the very unstable eicosanoid, prostacyclin, exerts a profound effect on the vascular media, often 30 cell layers thick. We report a novel PKA-, Akt-1- and ERK1/2-dependent prostacyclin-induced prostacyclin release that appears to play an important role in propagation of the quiescent, differentiated phenotype through adjacent arterial smooth muscle cells in the vascular media. Treating VSMC with the prostacyclin analog iloprost induced differentiation (contractile protein expression and contractile morphology), and also up-regulated COX-2 expression, leading to prostacyclin release by VSMC. This paracrine prostacyclin release, in turn, promoted differentiation and COX-2 induction in neighboring VSMC that were not exposed to iloprost. Using siRNA and pharmacologic inhibitors, we report that this positive feedback mechanism, prostacyclin-induced prostacyclin release, is mediated by cAMP/PKA signaling, ERK1/2 activation, and a novel prostacyclin receptor signaling pathway, inhibition of Akt-1. Furthermore, these pathways appear to be regulated by the prostacyclin receptor independently of one another. We conclude that prevention of de-differentiation and proliferation through a paracrine positive feedback mechanism is a major cardioprotective function of prostacyclin.

The role of proline residues in the structure and function of human MT2 melatonin receptor

Melatonin functions as an essential regulator of various physiological processes in all vertebrate species. In mammals, two G protein-coupled melatonin receptors (GPCR) mediate some melatonin's actions: MT1 and MT2. Transmembrane domains (TM) of most GPCRs contain a set of highly conserved proline residues that presumably play important structural and functional roles. As TM segments of MT2 receptor display several interesting differences in expression of specific proline residues compared to other rhodopsin-like receptors (rGPCRs), we investigated the role of proline residues in the structure and function of this receptor. All prolines in TM segments of MT2 receptor were individually replaced with alanine and/or glycine. In addition, the unusual NAxxY motif located in TM7 was mutated to generate highly conserved NPxxY motif found in the majority of rGPCR proteins. Following transient expression in CHO-K1 cells, binding properties of the mutant receptors and their ability to transduce signals were analyzed using (125)I-mel- and [(35)S]GTPgammaS-binding assays, respectively. The impact of the performed mutations on the receptor structure was assessed by molecular dynamic simulations of MT2 receptors embedded in the fully hydrated phospholipid bilayer. Our results indicate that residues P174, P212 and P266 are important for the ligand binding and/or signaling of the human MT2 receptor. We also show that changes within the unusual NAxxY sequence in the TM7 (mutations A305P and A305V) produce defective MT2 receptors indicating an important role of this motif in the function of melatonin receptors.

Influence of proline on the thermostability of the active site and membrane arrangement of transmembrane proteins

Proline residues play a fundamental and subtle role in the dynamics, structure, and function in many membrane proteins. Temperature derivative spectroscopy and differential scanning calorimetry have been used to determine the effect of proline substitution in the structural stability of the active site and transmembrane arrangement of bacteriorhodopsin. We have analyzed the Pro-to-Ala mutation for the helix-embedded prolines Pro50, Pro91, and Pro186 in the native membrane environment. This information has been complemented with the analysis of the respective crystallographic structures by the FoldX force field. Differential scanning calorimetry allowed us to determine distorted membrane arrangement for P50A and P186A. The protein stability was severely affected for P186A and P91A. In the case of Pro91, a single point mutation is capable of strongly slowing down the conformational diffusion along the denaturation coordinate, becoming a barrier-free downhill process above 371 K. Temperature derivative spectroscopy, applied for first time to study thermal stability of proteins, has been used to monitor the stability of the active site of bacteriorhodopsin. The mutation of Pro91 and Pro186 showed the most striking effects on the retinal binding pocket. These residues are the Pro in closer contact to the active site (activation energies for retinal release of 60.1 and 76.8 kcal/mol, respectively, compared to 115.8 kcal/mol for WT). FoldX analysis of the protein crystal structures indicates that the Pro-to-Ala mutations have both local and long-range effects on the structural stability of residues involved in the architecture of the protein and the active site and in the proton pumping function. Thus, this study provides a complete overview of the substitution effect of helix-embedded prolines in the thermodynamic and dynamic stability of a membrane protein, also related to its structure and function.

Acceleration of cardiovascular disease by a dysfunctional prostacyclin receptor mutation: potential implications for cyclooxygenase-2 inhibition

Recent increased adverse cardiovascular events observed with selective cyclooxygenase-2 inhibition led to the withdrawal of rofecoxib (Vioxx) and valdecoxib (Bextra), but the mechanisms underlying these atherothrombotic events remain unclear. Prostacyclin is the major end product of cyclooxygenase-2 in vascular endothelium. Using a naturally occurring mutation in the prostacyclin receptor, we report for the first time that a deficiency in prostacyclin signaling through its G protein-coupled receptor contributes to atherothrombosis in human patients. We report that a prostacyclin receptor variant (R212C) is defective in adenylyl cyclase activation in both patient blood and in an in vitro COS-1 overexpression system. This promotes increased platelet aggregation, a hallmark of atherothrombosis. Our analysis of patients in 3 separate white cohorts reveals that this dysfunctional receptor is not likely an initiating factor in cardiovascular disease but that it accelerates the course of disease in those patients with the greatest risk factors. R212C was associated with cardiovascular disease only in the high cardiovascular risk cohort (n=980), with no association in the low-risk cohort (n=2293). In those at highest cardiovascular risk, both disease severity and adverse cardiovascular events were significantly increased with R212C when compared with age- and risk factor-matched normal allele patients. We conclude that for haploinsufficient mutants, such as the R212C, the enhanced atherothrombotic phenotype is likely dependent on the presence of existing atherosclerosis or injury (high risk factors), analogous to what has been observed in the cyclooxygenase-2 inhibition studies or prostacyclin receptor knockout mice studies. Combining both biochemical and clinical approaches, we conclude that diminished prostacyclin receptor signaling may contribute, in part, to the underlying adverse cardiovascular outcomes observed with cyclooxygenase-2 inhibition.

A profile of the residues in the second extracellular loop that are critical for ligand recognition of human prostacyclin receptor

The residues in the second extracellular loop (eLP2) of the prostanoid receptors, which are important for specific ligand recognition, were previously predicted in our earlier studies of the thromboxane A2 receptor (TP) using a combination of NMR spectroscopy and recombinant protein approaches. To further test this hypothesis, another prostanoid receptor, the prostacyclin receptor (IP), which has opposite biological characteristics to that of TP, was used as a model for these studies. A set of recombinant human IPs with site-directed mutations at the nonconserved eLP2 residues were constructed using an Ala-scanning approach, and then expressed in HEK293 and COS-7 cells. The expression levels of the recombinant receptors were six-fold higher in HEK293 cells than in COS-7 cells. The residues important for ligand recognition and binding within the N-terminal segment (G159, Q162, and C165) and the C-terminal segment (L172, R173, M174, and P179) of IP eLP2 were identified by mutagenesis analyses. The molecular mechanisms for the specific ligand recognition of IP were further demonstrated by specific site-directed mutagenesis using different amino acid residues with unique chemical properties for the key residues Q162, L172, R173, and M174. A comparison with the corresponding functional residues identified in TP eLP2 revealed that three (Q162, R173, and M174) of the four residues are nonconserved, and these are proposed to be involved in specific ligand recognition. We discuss the importance of G159 and P179 in ligand recognition through configuration of the loop conformation is discussed. These studies have further indicated that characterization of the residues in the eLP2 regions for all eight prostanoid receptors could be an effective approach for uncovering the molecular mechanisms of the ligand selectivities of the G-protein-coupled receptors.

Functional role of the conserved proline in helix 6 of the human bradykinin B2 receptor

Pro258 in transmembrane domain (TMD) 6 of the bradykinin (BK) B(2) receptor (B(2)R) is highly conserved among G-protein coupled receptors (GPCRs). Using mutagenesis, we show that Pro258 is required for normal trafficking of the receptor to the plasma membrane and that mutation of Pro258 to Ala or Leu but not Gly, enhances BK efficacy to induce receptor activation. Furthermore, P258A mutation suppresses the constitutive activity of a constitutively activated N113A-B(2)R mutant but preserves the antagonist to agonist efficacy shift previously observed on the N113A single mutant. Our data suggest that Pro258 in TMD6 is required for agonist-independent activation of the B(2)R and that straightening of TMD6 at the Pro-kink might favor G-protein coupling. It is also shown that Asn113 is a contact point of BK interaction and it is proposed that the release of a TMD3-TMD6 interaction involving Asn113 is crucial for the efficacy shift from antagonism toward agonism.

New insights into human prostacyclin receptor structure and function through natural and synthetic mutations of transmembrane charged residues

BACKGROUND AND PURPOSE

The human prostacyclin receptor (hIP), a G-protein coupled receptor (GPCR) expressed mainly on platelets and vascular smooth muscle cells, plays important protective roles in the cardiovascular system. We hypothesized that significant insights could be gained into the structure and function of the hIP through mutagenesis of its energetically unfavourably located transmembrane charged residues.

EXPERIMENTAL APPROACH

Within its putative transmembrane helices fourteen hydrophilic residues, both unique and conserved across GPCRs, were systematically mutated to assess for effects on receptor structure and function.

KEY RESULTS

Mutations of ten of the fourteen charged residues to alanine exhibited defective binding and/or activation. Key potential interactions were identified between 6 core residues; E116(3.49)-R117(3.50) (salt bridge TMIII), D274(7.35)-R279(7.40) (salt bridge TMVII), and D60(2.50)-D288(7.49) (H-bond network TMII-TMVII). Further detailed investigation of E116(3.49) (TMIII) with mutation to a glutamine showed a 2.6-fold increase in agonist-independent basal activity. This increase in activity accounts for a proportion ( approximately 13%) of full agonist induced activation. We further characterized two novel naturally occurring human mutations, R77(2.33)C and R279(7.40)C recently identified in a 1455 human genomic DNA sample screen. The R77(2.33)C variant appeared to exclusively affect expression, while the R279(7.40)C variant, exhibited considerable deficiencies in both agonist binding and activation.

CONCLUSIONS AND IMPLICATIONS

Transmembrane charged residues play important roles in maintaining the hIP binding pocket and ensuring normal activation. The critical nature of these charged residues and the presence of naturally occurring mutations have important implications in the rational design of prostacyclin agonists for treating cardiovascular disease.

Agonist-dependent consequences of proline to alanine substitution in the transmembrane helices of the calcitonin receptor

BACKGROUND AND PURPOSE

Transmembrane proline (P) residues in family A G protein-coupled receptors (GPCRs) form functionally important kinks in their helices. These residues are little studied in family B GPCRs but experiments with the VPAC1 receptor and calcitonin receptor-like receptor (CL) show parallels with family A receptors. We sought to determine the function of these residues in the insert negative form of the human calcitonin receptor, a close relative of CL.

EXPERIMENTAL APPROACH

Proline residues within the transmembrane domains of the calcitonin receptor (P246, P249, P280, P326, P336) were individually mutated to alanine (A) using site-directed mutagenesis. Receptors were transiently transfected into Cos-7 cells using polyethylenimine and salmon and human calcitonin-induced cAMP responses measured. Salmon and human calcitonin competition binding experiments were also performed and receptor cell-surface expression assessed by whole cell ELISA.

KEY RESULTS

P246A, P249A and P280A were wild-type in terms of human calcitonin-induced cAMP activation. P326A and P336A had reduced function (165 and 12-fold, respectively). In membranes, human calcitonin binding was not detectable for any mutant receptor but in whole cells, binding was detected for all mutants apart from P326A. Salmon calcitonin activated mutant and wild-type receptors equally, although B(max) values were reduced for all mutants apart from P326A.

CONCLUSIONS AND IMPLICATIONS

P326 and P336 are important for the function of human calcitonin receptors and are likely to be involved in generating receptor conformations appropriate for agonist binding and receptor activation. However, agonist-specific effects were observed , implying distinct conformations of the human calcitonin receptor.

Role of extracellular domains in PBAN/pyrokinin GPCRs from insects using chimera receptors

Pheromone biosynthesis-activating neuropeptide (PBAN) is a peptide used by a variety of moths to regulate pheromone production. Pyrokinins are peptides that activate muscle contraction in a variety of insects. These peptides have a common FXPRLamide C-terminal ending that is required for activity. Receptors have been identified from a moth and Drosophila as belonging to the rhodopsin family of G-protein coupled receptors (GPCRs) with sequence similarity to neuromedin U receptors from vertebrates. No insect GPCR has been characterized with regard to role of extracellular domains required for peptide binding and receptor activation. To begin characterizing these GPCRs we created chimera receptors using a PBAN-receptor from a moth and pyrokinin-receptors from Drosophila where extracellular domains were swapped. The N-terminal of the moth GPCR has two N-glycosylation sites that when replaced with glutamines, activity was reduced but not absent, indicating these sites contribute to receptor stability. Activity was greatly reduced by replacing the 2nd extracellular loop that has an N-glycosylation site and a cysteine that can form a disulfide bridge with a cysteine at the beginning of the 3rd transmembrane domain. Exchange of the 3rd extracellular loop between the moth and Drosophila receptor resulted in differential activation by PBAN or a diapause hormone peptide. This result indicates that the 3rd extracellular loop is directly involved in peptide ligand recognition. Results are discussed in context of the structural features of insect GPCRs that are required for receptor activation as compared to vertebrate receptors.

Human prostacyclin receptor structure and function from naturally-occurring and synthetic mutations

Prostacyclin (PGI2) is released by vascular endothelial cells and serves as a potent vasodilator, inhibitor of platelet aggregation (anti-thrombotic), and moderator of vascular smooth muscle cell proliferation-migration-differentiation (anti-atherosclerotic). These actions are mediated via a seven transmembrane-spanning G-protein coupled receptor (GPCR), known as the human prostacyclin receptor or hIP. Animal studies using prostacyclin receptor knock-out (IP-/-) mice have revealed increased propensities towards thrombosis, intimal hyperplasia, atherosclerosis, restenosis, as well as reperfusion injury. Of further importance has been the world-wide withdrawal of selective COX-2 inhibitors, due to their discriminating suppression of COX-2-derived PGI2 and its cardioprotective effects, leading to increased cardiovascular events, including myocardial infarction and thrombotic stroke. Over the last decade, mutagenesis studies of the IP receptor, in conjunction with in vitro functional assays and molecular modeling, have provided critical insights into the molecular mechanisms of both agonist binding and receptor activation. Most recently, the discovery of naturally-occurring and dysfunctional mutations within the hIP has provided additional insights into the proposed cardioprotective role of prostacyclin. The aim of this review is to summarize the most recent findings regarding hIP receptor structure-function that have developed through the study of both synthetic and naturally-occurring mutations.

G-protein coupled receptor structure

Because of their central role in regulation of cellular function, structure/function relationships for G-protein coupled receptors (GPCR) are of vital importance, yet only recently have sufficient data been obtained to begin mapping those relationships. GPCRs regulate a wide range of cellular processes, including the senses of taste, smell, and vision, and control a myriad of intracellular signaling systems in response to external stimuli. Many diseases are linked to GPCRs. A critical need exists for structural information to inform studies on mechanism of receptor action and regulation. X-ray crystal structures of only one GPCR, in an inactive state, have been obtained to date. However considerable structural information for a variety of GPCRs has been obtained using non-crystallographic approaches. This review begins with a review of the very earliest GPCR structural information, mostly derived from rhodopsin. Because of the difficulty in crystallizing GPCRs for X-ray crystallography, the extensive published work utilizing alternative approaches to GPCR structure is reviewed, including determination of three-dimensional structure from sparse constraints. The available X-ray crystallographic analyses on bovine rhodopsin are reviewed as the only available high-resolution structures for any GPCR. Structural information available on ligand binding to several receptors is included. The limited information on excited states of receptors is also reviewed. It is concluded that while considerable basic structural information has been obtained, more data are needed to describe the molecular mechanism of activation of a GPCR.

Versatility and differential roles of cysteine residues in human prostacyclin receptor structure and function

Prostacyclin plays important roles in vascular homeostasis, promoting vasodilatation and inhibiting platelet thrombus formation. Previous studies have shown that three of six cytoplasmic cysteines, particularly those within the C-terminal tail, serve as important lipidation sites and are differentially conjugated to palmitoyl and isoprenyl groups (Miggin, S. M., Lawler, O. A., and Kinsella, B. T. (2003) J. Biol. Chem. 278, 6947-6958). Here we report distinctive roles for extracellular- and transmembrane-located cysteine residues in human prostacyclin receptor structure-function. Within the extracellular domain, all cysteines (4 of 4) appear to be involved in disulfide bonding interactions (i.e. a highly conserved Cys-92-Cys-170 bond and a putative non-conserved Cys-5-Cys-165 bond), and within the transmembrane (TM) region there are several cysteines (3 of 8) that maintain critical hydrogen bonding interactions (Cys-118 (TMIII), Cys-251 (TMVI), and Cys-202 (TMV)). This study highlights the necessity of sulfhydryl (SH) groups in maintaining the structural integrity of the human prostacyclin receptor, as 7 of 12 extracellular and transmembrane cysteines studied were found to be differentially indispensable for receptor binding, activation, and/or trafficking. Moreover, these results also demonstrate the versatility and reactivity of these cysteine residues within different receptor environments, that is, extracellular (disulfide bonds), transmembrane (H-bonds), and cytoplasmic (lipid conjugation).

The role of proline residues in the dynamics of transmembrane helices: the case of bacteriorhodopsin

Proline residues in transmembrane helices have been found to have important roles in the functioning of membrane proteins. Moreover, Pro residues occur with high frequency in transmembrane alpha-helices, as compared to alpha-helices for soluble proteins. Here, we report several properties of the bacteriorhodopsin mutants P50A (helix B), P91A (helix C) and P186A (helix F). Compared to wild type, strongly perturbed behaviour has been found for these mutants. In the resting state, increased hydroxylamine accessibility and altered Asp-85 pKa and light-dark adaptation were observed. On light activation, hydroxylamine accessibility was increased and proton transport activity, M formation kinetics and FTIR difference spectra of M and N intermediates showed clear distortions. On the basis of these alterations and the near identity of the crystalline structures of mutants with that of wild type, we conclude that the transmembrane proline residues of bacteriorhodopsin fulfil a dynamic role in both the resting and the light-activated states. Our results are consistent with the notion that mutation of Pro to Ala allows the helix to increase its flexibility towards the direction originally hindered by the steric clash between the ring Cgamma and the carbonyl O of the i-4 residue, at the same time decreasing the mobility towards the opposite direction. Due to their properties, transmembrane Pro residues may serve as transmission elements of conformational changes during the transport process. We propose that these concepts can be extended to other transmembrane proteins.

Extracellular Loop 3 (EL3) and EL3-Proximal Transmembrane Helix 7 of the Mammalian Type I and Type II Gonadotropin-Releasing Hormone (GnRH) Receptors Determine Differential Ligand Selectivity to GnRH-I and GnRH-II

Mammalian type I and II gonadotropin-releasing hormone (GnRH) receptors (GnRHRs) show differential ligand preference for GnRH-I and GnRH-II, respectively. Using a variety of chimeric receptors based on green monkey GnRHR-2 (gmGnRHR-2), a representative type II GnRHR, and rat GnRHR, a representative type I GnRHR, this study elucidated specific domains responsible for this ligand selectivity. A chimeric gmGnRHR-2 with the extracellular loop 3 (EL3) and EL3-proximal transmembrane helix 7 (TMH7) of rat GnRHR showed a great increase in ligand sensitivity to GnRH-I but not to GnRH-II. Point-mutation studies indicate that four amino acids, Leu/Phe(7.38), Leu/Phe(7.43), Ala/Pro(7.46), and Pro/Cys(7.47) in TMH7 are critical for ligand selectivity as well as receptor conformation. Furthermore, a combinatory mutation (Pro(7.31)-Pro(7.32)-Ser(7.33) motif to Ser-Glu-Pro in EL3 and Leu(7.38), Leu(7.43), Ala(7.46), and Pro(7.47) to those of rat GnRHR) in gmGnRH-2 exhibited an approximately 500-fold increased sensitivity to GnRH-I, indicating that these residues are critical for discriminating GnRH-II from GnRH-I. [Trp(7)]GnRH-I and [Trp(8)]GnRH-I but not [His(5)]GnRH-I exhibit a higher potency in activating wild-type gmGnRHR-2 than native GnRH-I, indicating that amino acids at positions 7 and 8 of GnRHs are more important than position 5 for differential recognition by type I and type II GnRHRs. As a whole, these data suggest a molecular coevolution of ligands and their receptors and facilitate the understanding of the molecular interaction between GnRHs and their cognate receptors.

Heterologous expression of limulus rhodopsin

Invertebrates such as Drosophila or Limulus assemble their visual pigment into the specialized rhabdomeric membranes of photoreceptors where phototransduction occurs. We have investigated the biosynthesis of rhodopsin from the Limulus lateral eye with three cell culture expression systems: mammalian COS1 cells, insect Sf9 cells, and amphibian Xenopus oocytes. We extracted and affinity-purified epitope-tagged Limulus rhodopsin expressed from a cDNA or cRNA from these systems. We found that all three culture systems could efficiently synthesize the opsin polypeptide in quantities comparable with that found for bovine opsin. However, none of the systems expressed a protein that stably bound 11-cis-retinal. The protein expressed in COS1 and Sf9 cells appeared to be misfolded, improperly localized, and proteolytically degraded. Similarly, Xenopus oocytes injected with Limulus opsin cRNA did not evoke light-sensitive currents after incubation with 11-cis-retinal. However, injecting Xenopus oocytes with mRNA from Limulus lateral eyes yielded light-dependent conductance changes after incubation with 11-cis-retinal. Also, expressing Limulus opsin cDNA in the R1-R6 photoreceptors of transgenic Drosophila yielded a visual pigment that bound retinal, had normal spectral properties, and coupled to the endogenous phototransduction cascade. These results indicate that Limulus opsin may require one or more photoreceptor-specific proteins for correct folding and/or chromophore binding. This may be a general property of invertebrate opsins and may underlie some of the functional differences between invertebrate and vertebrate visual pigments.

The unique ligand-binding pocket for the human prostacyclin receptor. Site-directed mutagenesis and molecular modeling

The human prostacyclin receptor is a seven-transmembrane alpha-helical G-protein coupled receptor, which plays important roles in both vascular smooth muscle relaxation as well as prevention of blood coagulation. The position of the native ligand-binding pocket for prostacyclin as well as other derivatives of the 20-carbon eicosanoid, arachidonic acid, has yet to be determined. Through the use of prostanoid receptor sequence alignments, site-directed mutagenesis, and the 2.8-A x-ray crystallographic structure of bovine rhodopsin, we have developed a three-dimensional model of the agonist-binding pocket within the seven-transmembrane (TM) domains of the human prostacyclin receptor. Upon mutation to alanine, 11 of 29 candidate residues within TM domains II, III, IV, V, and VII exhibited a marked decrease in agonist binding. Of this group, four amino acids, Arg-279 (TMVII), Phe-278 (TMVII), Tyr-75 (TMII), and Phe-95 (TMIII), were identified (via receptor amino acid sequence alignment, ligand structural comparison, and computer-assisted homology modeling) as having direct molecular interactions with ligand side-chain constituents. This binding pocket is distinct from that of the biogenic amine receptors and rhodopsin where the native ligands (also composed of a carbon ring and a carbon chain) are accommodated in an opposing direction. These findings should assist in the development of novel and highly specific ligands including selective antagonists for further molecular pharmacogenetic studies of the human prostacyclin receptor.

The crystallographic model of rhodopsin and its use in studies of other G protein-coupled receptors

G protein-coupled receptors (GPCRs) are integral membrane proteins that respond to environmental signals and initiate signal transduction pathways activating cellular processes. Rhodopsin is a GPCR found in rod cells in retina where it functions as a photopigment. Its molecular structure is known from cryo-electron microscopic and X-ray crystallographic studies, and this has reshaped many structure/function questions important in vision science. In addition, this first GPCR structure has provided a structural template for studies of other GPCRs, including many known drug targets. After presenting an overview of the major structural elements of rhodopsin, recent literature covering the use of the rhodopsin structure in analyzing other GPCRs will be summarized. Use of the rhodopsin structural model to understand the structure and function of other GPCRs provides strong evidence validating the structural model.