Role of extracellular cysteine residues in dimerization/oligomerization of the human prostacyclin receptor

Transfer mechanism of cell-free synthesized membrane proteins into mammalian cells

Nanodiscs are emerging to serve as transfer vectors for the insertion of recombinant membrane proteins into membranes of living cells. In combination with cell-free expression technologies, this novel process opens new perspectives to analyze the effects of even problematic targets such as toxic, hard-to-express, or artificially modified membrane proteins in complex cellular environments of different cell lines. Furthermore, transferred cells must not be genetically engineered and primary cell lines or cancer cells could be implemented as well. We have systematically analyzed the basic parameters of the nanotransfer approach and compared the transfer efficiencies from nanodiscs with that from Salipro particles. The transfer of five membrane proteins was analyzed: the prokaryotic proton pump proteorhodopsin, the human class A family G-protein coupled receptors for endothelin type B, prostacyclin, free fatty acids type 2, and the orphan GPRC5B receptor as a class C family member. The membrane proteins were cell-free synthesized with a detergent-free strategy by their cotranslational insertion into preformed nanoparticles containing defined lipid environments. The purified membrane protein/nanoparticles were then incubated with mammalian cells. We demonstrate that nanodiscs disassemble and only lipids and membrane proteins, not the scaffold protein, are transferred into cell membranes. The process is detectable within minutes, independent of the nanoparticle lipid composition, and the transfer efficiency directly correlates with the membrane protein concentration in the transfer mixture and with the incubation time. Transferred membrane proteins insert in both orientations, N-terminus in and N-terminus out, in the cell membrane, and the ratio can be modulated by engineering. The viability of cells is not notably affected by the transfer procedure, and transferred membrane proteins stay detectable in the cell membrane for up to 3 days. Transferred G-protein coupled receptors retained their functionality in the cell environment as shown by ligand binding, induction of internalization, and specific protein interactions. In comparison to transfection, the cellular membrane protein concentration is better controllable and more uniformly distributed within the analyzed cell population. A further notable difference to transfection is the accumulation of transferred membrane proteins in clusters, presumably determined by microdomain structures in the cell membranes.

A Review of Prostanoid Receptors: Expression, Characterization, Regulation, and Mechanism of Action

Prostaglandin signaling controls a wide range of biological processes from blood pressure homeostasis to inflammation and resolution thereof to the perception of pain to cell survival. Disruption of normal prostanoid signaling is implicated in numerous disease states. Prostaglandin signaling is facilitated by G-protein-coupled, prostanoid-specific receptors and the array of associated G-proteins. This review focuses on the expression, characterization, regulation, and mechanism of action of prostanoid receptors with particular emphasis on human isoforms.

International Union of Basic and Clinical Pharmacology. CIX. Differences and Similarities between Human and Rodent Prostaglandin E2 Receptors (EP1-4) and Prostacyclin Receptor (IP): Specific Roles in Pathophysiologic Conditions

Prostaglandins are derived from arachidonic acid metabolism through cyclooxygenase activities. Among prostaglandins (PGs), prostacyclin (PGI2) and PGE2 are strongly involved in the regulation of homeostasis and main physiologic functions. In addition, the synthesis of these two prostaglandins is significantly increased during inflammation. PGI2 and PGE2 exert their biologic actions by binding to their respective receptors, namely prostacyclin receptor (IP) and prostaglandin E2 receptor (EP) 1-4, which belong to the family of G-protein-coupled receptors. IP and EP1-4 receptors are widely distributed in the body and thus play various physiologic and pathophysiologic roles. In this review, we discuss the recent advances in studies using pharmacological approaches, genetically modified animals, and genome-wide association studies regarding the roles of IP and EP1-4 receptors in the immune, cardiovascular, nervous, gastrointestinal, respiratory, genitourinary, and musculoskeletal systems. In particular, we highlight similarities and differences between human and rodents in terms of the specific roles of IP and EP1-4 receptors and their downstream signaling pathways, functions, and activities for each biologic system. We also highlight the potential novel therapeutic benefit of targeting IP and EP1-4 receptors in several diseases based on the scientific advances, animal models, and human studies. SIGNIFICANCE STATEMENT: In this review, we present an update of the pathophysiologic role of the prostacyclin receptor, prostaglandin E2 receptor (EP) 1, EP2, EP3, and EP4 receptors when activated by the two main prostaglandins, namely prostacyclin and prostaglandin E2, produced during inflammatory conditions in human and rodents. In addition, this comparison of the published results in each tissue and/or pathology should facilitate the choice of the most appropriate model for the future studies.

Orphan G Protein–Coupled Receptor GPRC5B Controls Smooth Muscle Contractility and Differentiation by Inhibiting Prostacyclin Receptor Signaling

Background:

G protein–coupled receptors are important regulators of contractility and differentiation in vascular smooth muscle cells (SMCs), but the specific function of SMC-expressed orphan G protein–coupled receptor class C group 5 member B (GPRC5B) is unclear.

Methods:

We studied the role of GPRC5B in the regulation of contractility and dedifferentiation in human and murine SMCs in vitro and in iSM- Gprc5b -KO (tamoxifen-inducible, SMC-specific knockout) mice under conditions of arterial hypertension and atherosclerosis in vivo.

Results:

Mesenteric arteries from SMC-specific Gprc5b -KOs showed ex vivo significantly enhanced prostacyclin receptor (IP)–dependent relaxation, whereas responses to other relaxant or contractile factors were normal. In vitro, knockdown of GPRC5B in human aortic SMCs resulted in increased IP-dependent cAMP production and consecutive facilitation of SMC relaxation. In line with this facilitation of IP-mediated relaxation, iSM- Gprc5b -KO mice were protected from arterial hypertension, and this protective effect was abrogated by IP antagonists. Mechanistically, we show that knockdown of GPRC5B increased the membrane localization of IP both in vitro and in vivo and that GPRC5B, but not other G protein–coupled receptors, physically interacts with IP. Last, we show that enhanced IP signaling in GPRC5B-deficient SMCs not only facilitates relaxation but also prevents dedifferentiation during atherosclerosis development, resulting in reduced plaque load and increased differentiation of SMCs in the fibrous cap.

Conclusions:

Taken together, our data show that GPRC5B regulates vascular SMC tone and differentiation by negatively regulating IP signaling.

Role of prostacyclin in pulmonary hypertension

Prostacyclin is a powerful cardioprotective hormone released by the endothelium of all blood vessels. Prostacyclin exists in equilibrium with other vasoactive hormones and a disturbance in the balance of these factors leads to cardiovascular disease including pulmonary arterial hypertension. Since it's discovery in the 1970s concerted efforts have been made to make the best therapeutic utility of prostacyclin, particularly in the treatment of pulmonary arterial hypertension. This has centred on working out the detailed pharmacology of prostacyclin and then synthesising new molecules based on its structure that are more stable or more easily tolerated. In addition, newer molecules have been developed that are not analogues of prostacyclin but that target the receptors that prostacyclin activates. Prostacyclin and related drugs have without doubt revolutionised the treatment and management of pulmonary arterial hypertension but are seriously limited by side effects within the systemic circulation. With the dawn of nanomedicine and targeted drug or stem cell delivery systems it will, in the very near future, be possible to make new formulations of prostacyclin that can evade the systemic circulation allowing for safe delivery to the pulmonary vessels. In this way, the full therapeutic potential of prostacyclin can be realised opening the possibility that pulmonary arterial hypertension will become, if not curable, a chronic manageable disease that is no longer fatal. This review discusses these and other issues relating to prostacyclin and its use in pulmonary arterial hypertension.

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.

Molecular mechanisms regulating the vascular prostacyclin pathways and their adaptation during pregnancy and in the newborn

Prostacyclin (PGI(2)) is a member of the prostanoid group of eicosanoids that regulate homeostasis, hemostasis, smooth muscle function and inflammation. Prostanoids are derived from arachidonic acid by the sequential actions of phospholipase A(2), cyclooxygenase (COX), and specific prostaglandin (PG) synthases. There are two major COX enzymes, COX1 and COX2, that differ in structure, tissue distribution, subcellular localization, and function. COX1 is largely constitutively expressed, whereas COX2 is induced at sites of inflammation and vascular injury. PGI(2) is produced by endothelial cells and influences many cardiovascular processes. PGI(2) acts mainly on the prostacyclin (IP) receptor, but because of receptor homology, PGI(2) analogs such as iloprost may act on other prostanoid receptors with variable affinities. PGI(2)/IP interaction stimulates G protein-coupled increase in cAMP and protein kinase A, resulting in decreased [Ca(2+)](i), and could also cause inhibition of Rho kinase, leading to vascular smooth muscle relaxation. In addition, PGI(2) intracrine signaling may target nuclear peroxisome proliferator-activated receptors and regulate gene transcription. PGI(2) counteracts the vasoconstrictor and platelet aggregation effects of thromboxane A(2) (TXA(2)), and both prostanoids create an important balance in cardiovascular homeostasis. The PGI(2)/TXA(2) balance is particularly critical in the regulation of maternal and fetal vascular function during pregnancy and in the newborn. A decrease in PGI(2)/TXA(2) ratio in the maternal, fetal, and neonatal circulation may contribute to preeclampsia, intrauterine growth restriction, and persistent pulmonary hypertension of the newborn (PPHN), respectively. On the other hand, increased PGI(2) activity may contribute to patent ductus arteriosus (PDA) and intraventricular hemorrhage in premature newborns. These observations have raised interest in the use of COX inhibitors and PGI(2) analogs in the management of pregnancy-associated and neonatal vascular disorders. The use of aspirin to decrease TXA(2) synthesis has shown little benefit in preeclampsia, whereas indomethacin and ibuprofen are used effectively to close PDA in the premature newborn. PGI(2) analogs have been used effectively in primary pulmonary hypertension in adults and have shown promise in PPHN. Careful examination of PGI(2) metabolism and the complex interplay with other prostanoids will help design specific modulators of the PGI(2)-dependent pathways for the management of pregnancy-related and neonatal vascular disorders.

Extracellular disulfide bonds support scavenger receptor class B type I-mediated cholesterol transport

Scavenger receptor class B type I (SR-BI) binds high-density lipoprotein (HDL) and mediates the selective uptake of cholesteryl esters (CE). Although the extracellular domain of SR-BI is critical for function, the structural characteristics of this region remain elusive. Using sulfhydryl labeling strategies, we report the novel finding that all six cysteine (Cys) residues in the extracellular domain of SR-BI are involved in disulfide bond formation that is intramolecular by nature. We hypothesized that an SR-BI conformation stabilized by extracellular disulfide bonds is a prerequisite for SR-BI-mediated cholesterol transport. Thus, single-Cys mutant SR-BI receptors (C251S-, C280S-, C321S-, C323S-, C334S-, and C384S-SR-BI), as well as Cys-less SR-BI, a mutant SR-BI receptor void of all Cys residues, were created, and plasma membrane localization was confirmed. Functional assays revealed that C280S-, C321S-, C323S-, and C334S-SR-BI and Cys-less SR-BI mutant receptors displayed weakened HDL binding and subsequent selective uptake of HDL-CE. However, only C323S-SR-BI and Cys-less SR-BI were unable to mediate wild-type levels of efflux of free cholesterol (FC) to HDL. None of the Cys mutations disrupted SR-BI's ability to redistribute plasma membrane FC. Taken together, the intramolecular disulfide bonds in the extracellular domain of SR-BI appear to maintain the receptor in a conformation integral to its cholesterol transport functions.

Isolation and characterization of galactose-specific carbohydrate-binding protein from Guerin tumor cells

Carbohydrate-binding protein with specificity towards galactose was isolated from Guerin tumor cells. This protein had molecular weight of 51 kDa in dissociating and reducing conditions. It was phosphorylated, but not glycosylated, having two isoforms with pIs corresponding to 7.3 and 7.9. We found predominantly cytoplasmic and nuclear, but not plasma membrane, localization of the isolated protein. Oxidative conditions and presence of the ligand are required for the protein to oligomerize. Probing of the carbohydrate-binding domain with sugar derivatives showed that hydroxyl groups at C3, C4 and C6 positions of galactose, as well as at C3 and C6 positions of the glucose part of NAcLactosamine are involved in ligand binding. Tyrosine, tryptophan and histidine amino acids were found to participate in binding of the galactose ligand. N-linked multivalent macromolecular ligands, containing up to four antennae, bound to the isolated protein with positive cooperativity. Affinity for NAcLactosamine, as measured by its I(50) value, was 7918-times higher than that for galactose. Binding of galactose to the combining site was enthalpically driven, dH=-32.16 (kJ mol(-1)), with K(d) in the micromolar range, 32.25 x 10(4) mol(-1).

Dominant negative actions of human prostacyclin receptor variant through dimerization: implications for cardiovascular disease

OBJECTIVE

Prostacyclin and thromboxane mediate opposing cardiovascular effects through their receptors, the prostacyclin receptor (IP) and thromboxane receptor (TP). Individuals heterozygous for an IP variant, IP(R212C), displayed exaggerated loss of platelet IP responsiveness and accelerated cardiovascular disease. We examined association of IP(R212C) into homo- and heterodimeric receptor complexes and the impact on prostacyclin and thromboxane biology.

METHODS AND RESULTS

Dimerization of the IP, IP(R212C), and TPalpha was examined by bioluminesence resonance energy transfer in transfected HEK293 cells. We observed an equal propensity for formation of IPIP homodimers and IPTPalpha heterodimers. Compared with the IP alone, IP(R212C) displayed reduced cAMP generation and increased endoplasmic reticulum localization but underwent normal homo- and heterodimerization. When the IP(R212C) and IP were coexpressed, a dominant negative action of the variant was evident with enhanced wild-type IP localization to the endoplasmic reticulum and reduced agonist-dependent signaling. Further, the TPalpha activation response, which was shifted from inositol phosphate to cAMP generation following IPTPalpha heterodimerization, was normalized when the TPalpha instead dimerized with IP(R212C).

CONCLUSIONS

IP(R212C) exerts a dominant action on the wild-type IP and TPalpha through dimerization. This likely contributes to accelerated cardiovascular disease in individuals carrying 1 copy of the variant allele.

Rescue of defective G protein-coupled receptor function in vivo by intermolecular cooperation

G protein-coupled receptors (GPCRs) are ubiquitous mediators of signaling of hormones, neurotransmitters, and sensing. The old dogma is that a one ligand/one receptor complex constitutes the functional unit of GPCR signaling. However, there is mounting evidence that some GPCRs form dimers or oligomers during their biosynthesis, activation, inactivation, and/or internalization. This evidence has been obtained exclusively from cell culture experiments, and proof for the physiological significance of GPCR di/oligomerization in vivo is still missing. Using the mouse luteinizing hormone receptor (LHR) as a model GPCR, we demonstrate that transgenic mice coexpressing binding-deficient and signaling-deficient forms of LHR can reestablish normal LH actions through intermolecular functional complementation of the mutant receptors in the absence of functional wild-type receptors. These results provide compelling in vivo evidence for the physiological relevance of intermolecular cooperation in GPCR signaling.

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.

Rationale for prostaglandin I2 in bone marrow oedema--from theory to application

Introduction

Bone marrow oedema (BME) and avascular osteonecrosis (AVN) are disorders of unclear origin. Although there are numerous operative and non-operative treatments for AVN, pain management in patients with AVN remains challenging. Prostaglandins play an important role in inflammatory responses and cell differentiation. It is thought that prostaglandin I₂ ([PGI₂] or synonoma prostacyclin) and its analogues promote bone regeneration on a cellular or systemic level. The purpose of this study was to assess the curative and symptomatic efficacy of the prostacyclin analogue iloprost in BME and AVN patients.

Method

We are reporting on 50 patients (117 bones) affected by BME/AVN who were treated with iloprost. Pain levels before, during and 3 and 6 months after iloprost application were evaluated by a visual analogue scale (VAS). The short form(SF)-36 health survey served to judge general health status before and after treatment. Harris Hip Score (HHS) and Knee Society Score (KSS) were performed as functional scores and MRI and X-rays before and 3 and 6 months after iloprost application served as objective parameters for morphological changes of the affected bones.

Results

We found a significant improvement in pain, functional and radiological outcome in BME and early AVN stages after iloprost application, whereas patients with advanced AVN stages did not benefit from iloprost infusions. Mean pain level decreased from 5.26 (day 0) to 1.63 (6 months) and both HHS and KSS increased during follow-up. Moreover, the SF-36 increased from 353.2 (day 0) to 560.5 points (6 months). We found a significant decrease in BME on MRI scans after iloprost application.

Conclusions

In addition to other drugs, iloprost may be an alternative substance which should be considered in the treatment of BME/AVN-associated pain.

BRI2 homodimerizes with the involvement of intermolecular disulfide bonds

Familial British and Familial Danish Dementia (FBD and FDD) are two dominantly inherited neurodegenerative diseases that present striking similarities with Alzheimer's disease. The genetic defects underlying those dementias are mutations in the gene that encodes for BRI2 protein. Cleavage of mutated BRI2 by furin releases the peptides ABri or ADan, which accumulate in the brains of patients. BRI2 normal function is yet unknown. To unwind aspects of its cellular role, we investigated the possibility that BRI2 forms dimers, based on structural elements of the protein, the GXXXG motif within its transmembrane domain and the odd number of cysteine residues. We found that BRI2 dimerizes in cells and that dimers are held via non-covalent interactions and via disulfide bridges between the cysteines at position 89. Additionally, we showed that BRI2 dimers are formed in the ER and appear at the cell surface. Finally, BRI2 dimers were found to exist in mouse brain. Revealing the physiological properties of BRI2 is critical in the elucidation of the deviations that lead to neurodegeneration.

Prostaglandins in the kidney: developments since Y2K

There are five major PGs (prostaglandins/prostanoids) produced from arachidonic acid via the COX (cyclo-oxygenase) pathway: PGE(2), PGI(2) (prostacyclin), PGD(2), PGF(2alpha) and TXA(2) (thromboxane A(2)). They exert many biological effects through specific G-protein-coupled membrane receptors, namely EP (PGE(2) receptor), IP (PGI(2) receptor), DP (PGD(2) receptor), FP (PGF(2alpha) receptor) and TP (TXA(2) receptor) respectively. PGs are implicated in physiological and pathological processes in all major organ systems, including cardiovascular function, gastrointestinal responses, reproductive processes, renal effects etc. This review highlights recent insights into the role of each prostanoid in regulating various aspects of renal function, including haemodynamics, renin secretion, growth responses, tubular transport processes and cell fate. A thorough review of the literature since Y2K (year 2000) is provided, with a general overview of PGs and their synthesis enzymes, and then specific considerations of each PG/prostanoid receptor system in the kidney.

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.

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).

RO1138452 and RO3244794: characterization of structurally distinct, potent and selective IP (prostacyclin) receptor antagonists

Prostacyclin (PGI2) possesses various physiological functions, including modulation of nociception, inflammation and cardiovascular activity. Elucidation of these functions has been hampered by the absence of selective IP receptor antagonists. Two structurally distinct series of IP receptor antagonists have been developed: 4,5-dihydro-1H-imidazol-2-yl)-[4-(4-isopropoxy-benzyl)-phenyl]-amine (RO1138452) and R-3-(4-fluoro-phenyl)-2-[5-(4-fluoro-phenyl)-benzofuran-2-ylmethoxycarbonylamino]-propionic acid (RO3244794).RO1138452 and RO3244794 display high affinity for IP receptors. In human platelets, the receptor affinities (pKi) were 9.3 +/- 0.1 and 7.7 +/- 0.03, respectively; in a recombinant IP receptor system, pKi values were 8.7 +/- 0.06 and 6.9 +/- 0.1, respectively. Functional antagonism of RO1138452 and RO3244794 was studied by measuring inhibition of carbaprostacyclin-induced cAMP accumulation in CHO-K1 cells stably expressing the human IP receptor. The antagonist affinities (pKi) of RO1138452 and RO3244794 were 9.0 +/- 0.06 and 8.5 +/- 0.11, respectively. Selectivity profiles for RO1138452 and RO3244794 were determined via a panel of receptor binding and enzyme assays. RO1138452 displayed affinity at I2 (8.3) and PAF (7.9) receptors, while RO3244794 was highly selective for the IP receptor: pKi values for EP1 (< 5), EP3 (5.38), EP4 (5.74) and TP (5.09). RO1138452 (1-10 mg kg(-1), i.v.) and RO3244794 (1-30 mg kg(-1), i.v.) significantly reduced acetic acid-induced abdominal constrictions. RO1138452 (3-100 mg kg(-1), p.o.) and RO3244794 (0.3-30 mg kg(-1), p.o.) significantly reduced carrageenan-induced mechanical hyperalgesia and edema formation. RO3244794 (1 and 10 mg kg(-1), p.o.) also significantly reduced chronic joint discomfort induced by monoiodoacetate. These data suggest that RO1138452 and RO3244794 are potent and selective antagonists for both human and rat IP receptors and that they possess analgesic and anti-inflammatory potential.

MC4R oligomerizes independently of extracellular cysteine residues

The melanocortin 4 receptor (MC4R) plays an essential role in weight regulation. Recently we could show that the MC4R is able to form receptor dimers. In the present study we investigated the role of extracellular cysteine residues and the structure of the third extracellular loop for receptor dimerization. None of the four extracellular cysteine residues nor the structure of the third extracellular loop play a role for MC4R-MC4R interaction as all investigated mutants display the same dimerization pattern as the wild-type receptor. Therefore for MC4R dimerization structures of the transmembrane-spanning helices are more likely to be involved.

Oligomerization of the alpha and beta isoforms of the thromboxane A2 receptor: relevance to receptor signaling and endocytosis

Thromboxane A(2) (TXA(2)) is a potent mediator of inflammation, vasoconstriction and oxidative stress. The TXA(2) receptor (TP) is a G protein-coupled receptor (GPCR) that is expressed as two alternatively spliced isoforms, alpha (343 residues) and beta (407 residues) that share the first 328 residues. For many years GPCRs were assumed to exist and function as monomeric species, but increasing evidence suggests that a dimer is the minimal functional unit of GPCRs. In the present report, using co-immunoprecipitation of differentially tagged TP expressed in HEK293 cells, we demonstrate that TPalpha and TPbeta form homo- and hetero-oligomers. Immunoblotting of lysates from human platelets with an anti-TP specific antibody revealed the presence of endogenously expressed TP oligomers. We show that TP oligomerization is an agonist-independent process highly affected by the reducing agent dithiothreitol suggesting the involvement of disulfide bonds in TP oligomerization. Over-expression of G protein-coupled receptor kinases and arrestins did not modulate the extent of receptor dimerization/oligomerization. Co-expression of two TP signaling-deficient mutants, R60L and E2402R, resulted in rescuing of receptor signal transduction suggesting that dimers/oligomers constitute the functional units of this receptor. Interestingly, TPalpha which does not undergo constitutive or agonist-induced endocytosis on its own was subjected to both types of endocytosis when co-expressed with TPbeta, indicating that TPalpha can display intracellular trafficking when complexed through hetero-oligomerization with TPbeta.

Prostacyclin signaling in the kidney: implications for health and disease

The balance between vasodilator and vasoconstrictor pathways is key to the maintenance of homeostasis and the outcome of disease. In the kidney, prostaglandins (PGs) uphold this balance and regulate renal function: hemodynamics, renin secretion, growth responses, tubular transport processes, and cell fate. With the advent of cyclooxygenase (COX)-2-selective inhibitors, targeted deletions in mice (COX knockouts, PG receptor knockouts), and the discovery of intracrine signaling options for PGs (peroxisome proliferator-activated receptors and perinuclear PGE2receptors: EP1,3,4), many advances have been made in the study of arachidonic acid metabolites. Although prostacyclin (PGI2) is a major product of the COX pathway, there is very little emphasis on its importance to the kidney. This review will discuss PGI2biology and its relevance to different aspects of renal disease (growth, fibrosis, apoptosis), highlighting the most significant research from the past decade of PGI2literature, what we have learned from other organ systems, while stressing the significance of cross talk between various PGI2signaling pathways and its implications for renal health and disease.

Oligomerization of the fifth transmembrane domain from the adenosine A2A receptor

The human adenosine A2A receptor (A(2A)R) belongs to one of the largest family of membrane proteins, the G-protein coupled receptors (GPCRs), characterized by seven transmembrane (TM) helices. Little is known about the determinants of their structures, folding, assembly, activation mechanisms, and oligomeric states. Previous studies in our group showed that peptides corresponding to all seven TM domains form stable helical structures in detergent micelles and lipid vesicles. However, the peptides behave differently; TM5 is the only peptide to have a ratio [theta]222/[theta]208 obtained by circular dichroism (CD) spectroscopy>1. This finding suggested to us that TM5 might self-associate. In the present study, we investigate the unique properties of the TM5 domain. We performed detailed analyses of TM5 peptide behavior in membrane-mimetic environments using CD spectroscopy, fluorescence spectroscopy and Förster resonance energy transfer, and gel electrophoresis. We find that TM5 peptide has the ability to self-associate to form oligomeric structures in various hydrophobic milieus and that these oligomers are highly resistant to temperature and chemical denaturation. We also find that mutation of the full-length A(2A)R at position M193, which is located in the fifth TM domain, noticeably alters A(2A)R monomer: dimer ratio as observed on SDS-PAGE. Our results suggest that parallel association of TM5 dimers may play a role in the known adenosine A2A receptor dimerization. This study represents the first evidence of an individual GPCR transmembrane domain self-association.

Conserved extracellular cysteine residues and cytoplasmic loop-loop interplay are required for functionality of the heptahelical MLO protein

We performed a structure-function analysis of the plasma membrane-localized plant-specific barley (Hordeum vulgare) MLO (powdery-mildew-resistance gene o) protein. Invariant cysteine and proline residues, located either in extracellular loops or transmembrane domains that have been conserved in MLO proteins for more than 400 million years, were found to be essential for MLO functionality and/or stability. Similarly to many metazoan G-protein-coupled receptors known to function as homo- and hetero-oligomers, FRET (fluorescence resonance energy transfer) analysis revealed evidence for in planta MLO dimerization/oligomerization. Domain-swap experiments with closely related wheat and rice as well as diverged Arabidopsis MLO isoforms demonstrated that the identity of the C-terminal cytoplasmic tail contributes to MLO activity. Likewise, analysis of a progressive deletion series revealed that integrity of the C-terminus determines both MLO accumulation and functionality. A series of domain swaps of cytoplasmic loops with the wheat (Triticum aestivum) orthologue, TaMLO-B1, provided strong evidence for co-operative loop-loop interplay either within the protein or between MLO molecules. Our data indicate extensive intramolecular co-evolution of cytoplasmic domains in the evolutionary history of the MLO protein family.

A photoaffinity probe designed for host-specific signal flavonoid receptors in phytopathogenic Peronosporomycete zoospores of Aphanomyces cochlioides

Aphanomyces cochlioides zoospores show chemotaxis to cochliophilin A (5-hydroxy-6,7-methylenedioxyflavone, 1), a host derived attractant, and also respond to 5,7-dihydroxyflavone (2) known as an equivalent chemoattractant. To investigate the chemotactic receptors in the zoospores, we designed photoaffinity probes 4'-azido-5,7-dihydroxyflavone (3) and 4'-azido-7-O-biotinyl-5-hydroxyflavone (4) considering chemical structure of 2. Both 3 and 4 had zoospore attractant activity which was competitive with that of 1. When zoospores were treated with the biotinylated photoaffinity probe followed by UV irradiation and streptavidin-gold or peroxidase-conjugated streptavidin, probe-labeled proteins were detected on the cell membrane. This result indicated that the 1-specific-binding proteins, a candidate for hypothetical cochliophilin A receptor, were localized on the cell membrane of the zoospores. This is the first experimental evidence of flavonoid-binding proteins being present in zoospores, using chemically synthesized azidoflavone as photoaffinity-labeling reagent.

Dimerization of the human receptors for prostacyclin and thromboxane facilitates thromboxane receptor-mediated cAMP generation

Prostacyclin (PGI(2)) and thromboxane (TxA(2)) are biological opposites; PGI(2), a vasodilator and inhibitor of platelet aggregation, limits the deleterious actions of TxA(2), a vasoconstrictor and platelet activator. The molecular mechanisms involved in the counterregulation of PGI(2)/TxA(2) signaling are unclear. We examined the interaction of the receptors for PGI(2) (IP) and TxA(2) (TPalpha). IP-induced cAMP and TP-induced inositol phosphate generation were unaltered when the receptors were co-expressed in HEK 293 cells (IP/TPalpha-HEK). TP-cAMP generation, in response to TP agonists or a TP-dependent isoprostane, iPE(2)III, was evident in IP/TPalpha-HEK and in aortic smooth muscle cells, but not in cells expressing either receptor alone, or in IP-deficient aortic smooth muscle cells. Augmentation of TP-induced cAMP generation, with the IP agonist cicaprost, was ablated in IP-deficient cells and was independent of direct IP signaling. IP/TPalpha heterodimers were formed constitutively when the receptors were co-expressed, with no overt changes in ligand binding to the individual receptor sites. However, despite inefficient binding of iPE(2)III to either the IP or TPalpha, expressed alone or in combination, robust cAMP generation was evident in IP/TPalpha-HEK, suggesting the formation of an alternative receptor site. Thus, IP/TPalpha dimerization was coincident with TP-cAMP generation, promoting a "PGI(2)-like" cellular response to TP activation. This represents a previously unknown mechanism by which IP may limit the cellular effects of TP.