Galectin-8 modulates human osteoclast activity partly through isoform-specific interactions

In overactive human osteoclasts, we previously identified an alternative splicing event in LGALS8, encoding galectin-8, resulting in decreased expression of the long isoform. Galectin-8, which modulates cell-matrix interactions and functions intracellularly as a danger recognition receptor, has never been associated with osteoclast biology. In human osteoclasts, inhibition of galectin-8 expression revealed its roles in bone resorption, osteoclast nuclearity, and mTORC1 signaling regulation. Galectin-8 isoform-specific inhibition asserted a predominant role for the short isoform in bone resorption. Moreover, a liquid chromatography with tandem mass spectrometry (LC-MS/MS) proteomic analysis of galectin-8 isoforms performed in HEK293T cells identified 22 proteins shared by both isoforms. Meanwhile, nine interacting partners were specific for the short isoform, and none were unique to the long isoform. Interactors specific for the galectin-8 short isoform included cell adhesion proteins and lysosomal proteins. We confirmed the interactions of galectin-8 with CLCN3, CLCN7, LAMP1, and LAMP2, all known to localize to secretory vesicles, in human osteoclasts. Altogether, our study reveals direct roles of galectin-8 in osteoclast activity, mostly attributable to the short isoform.

Distribution of delta and mu opioid receptor mRNA in rodent dorsal root ganglia neurons

Primary afferents are responsible for transmitting signals produced by noxious stimuli from the periphery to the spinal cord. Mu and delta opioid receptors (MOP and DOP) have analgesic properties and are highly expressed in dorsal root ganglia (DRG) neurons. In humans, spinal DOP is almost exclusively located on central terminals of DRG neurons, whereas in rodents, it is expressed both on presynaptic terminals and spinal neurons. In this study, we aimed to assess the distribution of MOP and DOP in the DRGs of mice and rats. Using in situ hybridization and immunofluorescence, we visualized MOP and DOP mRNA together with various neuronal markers. In rats and mice, we show that both receptors are expressed, albeit to different extents, in all types of neurons, namely, large and medium myelinated neurons (NF200-positive), small nonpeptidergic (IB4- or P2X3R-positive) and peptidergic C fibres (Tac1-positive). Overall, DOP mRNA was found to be mainly expressed in large and medium myelinated neurons, whereas MOP mRNA was mainly found in C fibres. The distribution of MOP and DOP, however, slightly differs between rats and mice, with a higher proportion of small nonpeptidergic C fibres expressing DOP mRNA in mice than in rats. We further found that neither morphine nor inflammation affected the distribution of the receptor mRNA. Because of their location, our results confirm that MOP and DOP have the potential to alleviate similar types of pain and that this effect could slightly differ between species.

Knock-In Mouse Models to Investigate the Functions of Opioid Receptors in vivo

Due to their low expression levels, complex multi-pass transmembrane structure, and the current lack of highly specific antibodies, the assessment of endogenous G protein-coupled receptors (GPCRs) remains challenging. While most of the research regarding their functions was performed in heterologous systems overexpressing the receptor, recent advances in genetic engineering methods have allowed the generation of several unique mouse models. These animals proved to be useful to investigate numerous aspects underlying the physiological functions of GPCRs, including their endogenous expression, distribution, interactome, and trafficking processes. Given their significant pharmacological importance and central roles in the nervous system, opioid peptide receptors (OPr) are often referred to as prototypical receptors for the study of GPCR regulatory mechanisms. Although only a few GPCR knock-in mouse lines have thus far been generated, OPr are strikingly well represented with over 20 different knock-in models, more than half of which were developed within the last 5 years. In this review, we describe the arsenal of OPr (mu-, delta-, and kappa-opioid), as well as the opioid-related nociceptin/orphanin FQ (NOP) receptor knock-in mouse models that have been generated over the past years. We further highlight the invaluable contribution of such models to our understanding of the in vivo mechanisms underlying the regulation of OPr, which could be conceivably transposed to any other GPCR, as well as the limitations, future perspectives, and possibilities enabled by such tools.

Differential barcoding of opioid receptors trafficking

Over the past several years, studies have highlighted the δ-opioid receptor (DOPr) as a promising therapeutic target for chronic pain management. While exhibiting milder undesired effects than most currently prescribed opioids, its specific agonists elicit effective analgesic responses in numerous animal models of chronic pain, including inflammatory, neuropathic, diabetic, and cancer-related pain. However, as compared with the extensively studied μ-opioid receptor, the molecular mechanisms governing its trafficking remain elusive. Recent advances have denoted several significant particularities in the regulation of DOPr intracellular routing, setting it apart from the other members of the opioid receptor family. Although they share high homology, each opioid receptor subtype displays specific amino acid patterns potentially involved in the regulation of its trafficking. These precise motifs or "barcodes" are selectively recognized by regulatory proteins and therefore dictate several aspects of the itinerary of a receptor, including its anterograde transport, internalization, recycling, and degradation. With a specific focus on the regulation of DOPr trafficking, this review will discuss previously reported, as well as potential novel trafficking barcodes within the opioid and nociceptin/orphanin FQ opioid peptide receptors, and their impact in determining distinct interactomes and physiological responses.

Identification of the interactome of the DP1 receptor for Prostaglandin D2: Regulation of DP1 receptor signaling and trafficking by IQGAP1

BACKGROUND

Mechanisms governing localization, trafficking and signaling of G protein-coupled receptors (GPCRs) are critical in cell function. Protein-protein interactions are determinant in these processes. However, there are very little interacting proteins known to date for the DP1 receptor for prostaglandin D₂.

METHODS

We performed LC-MS/MS analyses of the DP1 receptor interactome in HEK293 cells. To functionally validate our LC-MS/MS data, we studied the implications of the interaction with the IQGAP1 scaffold protein in the trafficking and signaling of DP1.

RESULTS

In addition to expected interacting proteins such as heterotrimeric G protein subunits, we identified proteins involved in signaling, trafficking, and folding localized in various cell compartments. Endogenous DP1-IQGAP1 co-immunoprecipitation was observed in colon cancer HT-29 cells. The interaction was augmented by DP1 agonist activation in HEK293 cells and GST-pulldown assays showed that IQGAP1 binds to intracellular loops 2 and 3 of DP1. Co-localization of the two proteins was observed by confocal microscopy at the cell periphery and in intracellular vesicles in the basal state. PGD₂ treatment resulted in the redistribution of the DP1-IQGAP1 co-localization in the perinuclear vicinity. DP1 receptor internalization was promoted by overexpression of IQGAP1, while it was diminished by IQGAP1 knockdown with DsiRNAs. DP1-mediated ERK1/2 activation was augmented and sustained overtime by overexpression of IQGAP1 when compared to DP1 expressed alone. IQGAP1 knockdown decreased ERK1/2 activation by DP1 stimulation. Interestingly, ERK1/2 signaling by DP1 was increased when IQGAP2 was silenced, while it was impaired by IQGAP3 knockdown.

CONCLUSIONS

Our findings define the putative DP1 interactome, a patho-physiologically important receptor, and validated the interaction with IQGAP1 in DP1 function. Our data also reveal that IQGAP proteins may differentially regulate GPCR signaling.

GENERAL SIGNIFICANCE

The identified putative DP1-interacting proteins open multiple lines of research in DP1 and GPCR biology in various cell compartments.

In-depth NMR characterization of Rab4a structure, nucleotide exchange and hydrolysis kinetics reveals an atypical GTPase profile

Rab4a is a small GTPase associated with endocytic compartments and a key regulator of early endosomes recycling. Gathering evidence indicates that its expression and activation are required for the development of metastases. Rab4a-intrinsic GTPase properties that control its activity, i.e. nucleotide exchange and hydrolysis rates, have not yet been thoroughly studied. The determination of these properties is of the utmost importance to understand its functions and contributions to tumorigenesis. Here, we used the constitutively active (Rab4aQ67L) and dominant negative (Rab4aS22N) mutants to characterize the thermodynamical and structural determinants of the interaction between Rab4a and GTP (GTPγS) as well as GDP. We report the first ¹H, ¹³C, ¹⁵N backbone NMR assignments of a Rab GTPase family member with Rab4a in complex with GDP and GTPγS. We also provide a qualitative description of the extent of structural and dynamical changes caused by the Q67L and S22N mutations. Using a real-time NMR approach and the two aforementioned mutants as controls, we evaluated Rab4a intrinsic nucleotide exchange and hydrolysis rates. Compared to most small GTPases such as Ras, a rapid GTP exchange rate along with slow hydrolysis rate were observed. This suggests that, in a cellular context, Rab4a can self-activate and persist in an activated state in absence of regulatory mechanisms. This peculiar profile is uncommon among the Ras superfamily members, making Rab4a an atypical fast-cycling GTPase and may explain, at least in part, how it contributes to metastases.

L-type prostaglandin D synthase regulates the trafficking of the PGD2 DP1 receptor by interacting with the GTPase Rab4

Accumulating evidence indicates that G protein-coupled receptors (GPCRs) interact with Rab GTPases during their intracellular trafficking. How GPCRs recruit and activate the Rabs is unclear. Here, we report that depletion of endogenous L-type prostaglandin D synthase (L-PGDS) in HeLa cells inhibited recycling of the prostaglandin D₂ (PGD₂) DP1 receptor (DP1) to the cell surface after agonist-induced internalization and that L-PGDS overexpression had the opposite effect. Depletion of endogenous Rab4 prevented l-PGDS-mediated recycling of DP1, and l-PGDS depletion inhibited Rab4-dependent recycling of DP1, indicating that both proteins are mutually involved in this pathway. DP1 stimulation promoted its interaction through its intracellular C terminus with Rab4, which was increased by l-PGDS. Confocal microscopy revealed that DP1 activation induces l-PGDS/Rab4 co-localization. l-PGDS/Rab4 and DP1/Rab4 co-immunoprecipitation levels were increased by DP1 agonist treatment. Pulldown assays with purified GST-l-PGDS and His₆-Rab4 indicated that both proteins interact directly. l-PGDS interacted preferentially with the inactive, GDP-locked Rab4S22N variant rather than with WT Rab4 or with constitutively active Rab4Q67L proteins. Overexpression and depletion experiments disclosed that l-PGDS partakes in Rab4 activation following DP1 stimulation. Experiments with deletion mutants and synthetic peptides revealed that amino acids 85-92 in l-PGDS are involved in its interaction with Rab4 and in its effect on DP1 recycling. Of note, GTPγS loading and time-resolved FRET assays with purified proteins suggested that l-PGDS enhances GDP-GTP exchange on Rab4. Our results reveal how l-PGDS, which produces the agonist for DP1, regulates DP1 recycling by participating in Rab4 recruitment and activation.

MURC/CAVIN-4 facilitates store-operated calcium entry in neonatal cardiomyocytes

Intact store-operated calcium entry (SOCE) mechanisms ensure the maintenance of Ca²⁺ homeostasis in cardiomyocytes while their dysregulation promotes the development of cardiomyopathies. To better understand this calcium handling process in cardiomyocytes, we sought to identify unknown protein partners of stromal interaction molecule 1 (STIM1), a main regulatory protein of SOCE. We identified the muscle-related coiled-coil protein (MURC), also known as Cavin-4, as a candidate and showed that MURC interacts with STIM1 in cardiomyocytes. This interaction occurs via the HR1 and ERM domains of MURC and STIM1, respectively. Our results also demonstrated that the overexpression of MURC in neonatal rat ventricular myocytes (NRVM) is sufficient to potentiate SOCE and that its HR1 domain is required to mediate this effect. Interestingly, the R140W-MURC mutant, a missense variant of the HR1 domain associated with human dilated cardiomyopathy, exacerbates the SOCE increase in NRVM. Although the endogenous expression of STIM1 and Ca²⁺ channel Orai1 is not modulated under these conditions, we showed that MURC increases the interaction between these proteins under resting conditions. Our study provides novel evidence that MURC regulates SOCE by interacting with STIM1 in cardiomyocytes. In addition, we identified a first potential mechanism by which the R140W mutation of MURC may contribute to calcium mishandling and the development of cardiomyopathies.

Monitoring the Aggregation of GPCRs by Fluorescence Microscopy

G protein-coupled receptors (GPCRs) contain highly hydrophobic domains that are subject to aggregation when exposed to the crowded environment of the cytoplasm. Many events can lead to protein aggregation such as mutations, endoplasmic reticulum (ER) stress, and misfolding. These processes have been widely known to impact GPCR folding, maturation, and localization. Protein aggregates are transported toward the microtubule-organizing center via dynein to form a large juxta-nuclear structure called the aggresome, and in due course, are then targeted for degradation. Here, we describe a method to study aggregation of GPCRs by fluorescence microscopy.

Adeno-associated virus 2/9 delivery of Cre recombinase in mouse primary afferents

Genetically-modified animal models have significantly increased our understanding of the complex central nervous system circuits. Among these models, inducible transgenic mice whose specific gene expression can be modulated through a Cre recombinase/LoxP system are useful to study the role of specific peptides and proteins in a given population of cells. In the present study, we describe an efficient approach to selectively deliver a Cre-GFP to dorsal root ganglia (DRG) neurons. First, mice of different ages were injected in both hindpaws with a recombinant adeno-associated virus (rAAV2/9-CBA-Cre-GFP). Using this route of injection in mice at 5 days of age, we report that approximately 20% of all DRG neurons express GFP, 6 to 8 weeks after the infection. The level of infection was reduced by 50% when the virus was administered at 2 weeks of age. Additionally, the virus-mediated delivery of the Cre-GFP was also investigated via the intrathecal route. When injected intrathecally, the rAAV2/9-CBA-Cre-GFP virus infected a much higher proportion of DRG neurons than the intraplantar injection, with up to 51.6% of infected lumbar DRG neurons. Noteworthy, both routes of injection predominantly transduced DRG neurons over spinal and brain neurons.

Thromboxane A2 receptor (TBXA2R) is a potent survival factor for triple negative breast cancers (TNBCs)

Triple Negative Breast Cancer (TNBC) is defined by the lack of ERα, PR expression and HER2 overexpression and is the breast cancer subtype with the poorest clinical outcomes. Our aim was to identify genes driving TNBC proliferation and/or survival which could represent novel therapeutic targets.

We performed microarray profiling of primary TNBCs and generated differential genelists based on clinical outcomes following the chemotherapy regimen FEC (5-Fluorouracil/Epirubicin/Cyclophosphamide -‘good’ outcome no relapse > 3 years; ‘poor’ outcome relapse < 3 years). Elevated expression of thromboxane A2 receptor (TBXA2R) was observed in ‘good’ outcome TNBCs. TBXA2R expression was higher specifically in TNBC cell lines and TBXA2R knockdowns consistently showed dramatic cell killing in TNBC cells. TBXA2R mRNA and promoter activities were up-regulated following BRCA1 knockdown, with c-Myc being required for BRCA1-mediated transcriptional repression.

We demonstrated that TBXA2R enhanced TNBC cell migration, invasion and activated Rho signalling, phenotypes which could be reversed using Rho-associated Kinase (ROCK) inhibitors. TBXA2R also protected TNBC cells from DNA damage by negatively regulating reactive oxygen species levels. In summary, TBXA2R is a novel breast cancer-associated gene required for the survival and migratory behaviour of a subset of TNBCs and could provide opportunities to develop novel, more effective treatments.

Involvement of the coatomer protein complex I in the intracellular traffic of the delta opioid receptor

The delta opioid receptor (DOPr) is known to be mainly expressed in intracellular compartments. It remains unknown why DOPr is barely exported to the cell surface, but it seems that a substantial proportion of the immature receptor is trapped within the endoplasmic reticulum (ER) and the Golgi network. In the present study, we performed LC-MS/MS analysis to identify putative protein partners involved in the retention of DOPr. Analysis of the proteins co-immunoprecipitating with Flag-DOPr in transfected HEK293 cells revealed the presence of numerous subunits of the coatomer protein complex I (COPI), a vesicle-coating complex involved in recycling resident proteins from the Golgi back to the ER. Further analysis of the amino acid sequence of DOPr identified multiple consensus di-lysine and di-arginine motifs within the intracellular segments of DOPr. Using cell-surface ELISA and GST pulldown assays, we showed that DOPr interacts with COPI through its intracellular loops 2 and 3 (ICL2 and ICL3, respectively) and that the mutation of the K¹⁶⁴AK¹⁶⁶ (ICL2) or K²⁵⁰EK²⁵² (ICL3) putative COPI binding sites increased the cell-surface expression of DOPr in transfected cells. Altogether, our results indicate that COPI is a binding partner of DOPr and provide a putative mechanism to explain why DOPr is highly retained inside the cells.

Regulation of GPCR expression through an interaction with CCT7, a subunit of the CCT/TRiC complex

A direct and functional interaction between a subunit of the CCT/TCP-1 ring complex (TRiC) chaperonin complex and G protein–coupled receptor (GPCRs) is shown. Evidence is provided that distinct nascent GPCRs can undergo alternative folding pathways and that CCT/TRiC is critical in preventing aggregation of some GPCRs and in promoting their proper maturation and expression.

Mechanisms that prevent aggregation and promote folding of nascent G protein–coupled receptors (GPCRs) remain poorly understood. We identified chaperonin containing TCP-1 subunit eta (CCT7) as an interacting partner of the β-isoform of thromboxane A₂ receptor (TPβ) by yeast two-hybrid screening. CCT7 coimmunoprecipitated with overexpressed TPβ and β₂-adrenergic receptor (β₂AR) in HEK 293 cells, but also with endogenous β₂AR. CCT7 depletion by small interfering RNA reduced total and cell-surface expression of both receptors and caused redistribution of the receptors to juxtanuclear aggresomes, significantly more so for TPβ than β₂AR. Interestingly, Hsp90 coimmunoprecipitated with β₂AR but virtually not with TPβ, indicating that nascent GPCRs can adopt alternative folding pathways. In vitro pull-down assays showed that both receptors can interact directly with CCT7 through their third intracellular loops and C-termini. We demonstrate that Trp³³⁴ in the TPβ C-terminus is critical for the CCT7 interaction and plays an important role in TPβ maturation and cell-surface expression. Of note, introducing a tryptophan in the corresponding position of the TPα isoform confers the CCT7-binding and maturation properties of TPβ. We show that an interaction with a subunit of the CCT/TCP-1 ring complex (TRiC) chaperonin complex is involved in regulating aggregation of nascent GPCRs and in promoting their proper maturation and expression.

Characterization of the interaction between the prostaglandin D2 DP1 receptor and the intracellular L-prostaglandin D synthase

Identification of G protein-coupled receptor (GPCR)-interacting proteins is an intense subject of current research. However, confirmation and characterization of identified interactions can be difficult with GPCRs, especially at the endogenous level. Here, we describe how we characterized the interaction between the prostaglandin D2 DP1 receptor and the intracellular L-type prostaglandin D synthase by in vitro pull-down assays using purified recombinant GST- and His-tagged proteins, by co-immunoprecipitation of overexpressed Flag- and HA-tagged proteins, and by co-immunoprecipitation of endogenous proteins.

New insights in the regulation of Rab GTPases by G protein-coupled receptors

Cargo-mediated regulation of vesicular transport has received great attention lately. Rab GTPases, forming the largest branch of the Ras GTPase superfamily, regulate almost every step of vesicle-mediated trafficking. Growing evidence suggests that mutations, aberrant expression, and altered post-translational modifications of Rab GTPases are associated with human diseases. However, their regulatory mechanisms and how they are connected to cargo proteins are still poorly understood. Accumulating data indicate that G protein-coupled receptors (GPCRs) directly associate with Rab GTPases and that these interactions dictate receptor trafficking. Yet, it remained unclear whether the receptors could regulate the targeting and activity of Rab GTPases in various cell compartments. It is only in recent years that experimental studies showed that GPCR signaling and interaction with Rab-associated regulatory proteins modulate the localization and activity of Rab GTPases. This research is revealing novel regulatory mechanisms of these small GTPases and should contribute to the progress in effective drug development. Recently published in the Journal of Cell Science, Lachance et al. present a novel role for ubiquitylation of Rab11a by a β2AR/HACE1 complex in regulating Rab11a activity and β2AR trafficking.

A G protein-coupled receptor and the intracellular synthase of its agonist functionally cooperate

The GPCR DP1 promotes the activity of L-PGDS, the enzyme that produces the DP1 agonist PGD₂, while at the same time L-PGDS promotes the export and activity of DP1 in response to PGD₂.

Export of newly synthesized G protein–coupled receptors (GPCRs) remains poorly characterized. We show in this paper that lipocalin-type prostaglandin D₂ (PGD₂) synthase (L-PGDS) interacts intracellularly with the GPCR DP1 in an agonist-independent manner. L-PGDS promotes cell surface expression of DP1, but not of other GPCRs, in HEK293 and HeLa cells, independent of L-PGDS enzyme activity. In addition, formation of a DP1–Hsp90 complex necessary for DP1 export to the cell surface is dependent on the interaction between L-PGDS and the C-terminal MEEVD residues of Hsp90. Surprisingly, PGD₂ synthesis by L-PGDS is promoted by coexpression of DP1, suggesting a possible intracrine/autocrine signaling mechanism. In this regard, L-PGDS increases the formation of a DP1–ERK1/2 complex and increases DP1-mediated ERK1/2 signaling. Our findings define a novel cooperative mechanism in which a GPCR (DP1) promotes the activity of the enzyme (L-PGDS) that produces its agonist (PGD₂) and in which this enzyme in turn acts as a cofactor (of Hsp90) to promote export and agonist-dependent activity of the receptor.

Prostaglandin D(2) induces apoptosis of human osteoclasts through ERK1/2 and Akt signaling pathways

In a recent study we have shown that prostaglandin D2 (PGD2) induces human osteoclast (OC) apoptosis through the activation of the chemoattractant receptor homologous molecule expressed on T-helper type 2 cell (CRTH2) receptor and the intrinsic apoptotic pathway. However, the molecular mechanisms underlying this response remain elusive. The objective of this study is to investigate the intracellular signaling pathways mediating PGD2-induced OC apoptosis. OCs were generated by in vitro differentiation of human peripheral blood mononuclear cells (PBMCs), and then treated with or without the selective inhibitors of mitogen-activated protein kinase-extracellular signal-regulated kinase (ERK) kinase, (MEK)-1/2, phosphatidylinositol3-kinase (PI3K) and NF-κB/IκB kinase-2 (IKK2) prior to the treatments of PGD2 as well as its agonists and antagonists. Fluorogenic substrate assay and immunoblotting were performed to determine the caspase-3 activity and key proteins involved in Akt, ERK1/2 and NF-κB signaling pathways. Treatments with both PGD2 and a CRTH2 agonist decreased ERK1/2 (Thr202/Tyr204) and Akt (Ser473) phosphorylation, whereas both treatments increased β-arrestin-1 phosphorylation (Ser412) in the presence of naproxen, which was used to eliminate endogenous prostaglandin production. In the absence of naproxen, treatment with a CRTH2 antagonist increased both ERK1/2 and Akt phosphorylations, and reduced the phosphorylation of β-arrestin-1. Treatment of OCs with a selective MEK-1/2 inhibitor increased caspase-3 activity and OC apoptosis induced by both PGD2 and a CRTH2 agonist. Moreover, a CRTH2 antagonist diminished the selective MEK-1/2 inhibitor-induced increase in caspase-3 activity in the presence of endogenous prostaglandins. In addition, treatment of OCs with a selective PI3K inhibitor decreased ERK1/2 (Thr202/Tyr204) phosphorylation caused by PGD2, whereas increased ERK1/2 (Thr202/Tyr204) phosphorylation by a CRTH2 antagonist was attenuated with a PI3K inhibitor treatment. The DP receptor was not implicated in any of the parameters evaluated. Treatment of OCs with PGD2 as well as its receptor agonists and antagonists did not alter the phosphorylation of RelA/p65 (Ser536). Moreover, the caspase-3 activity was not altered in OCs treated with a selective IKK2/NF-κB inhibitor. In conclusion, endogenous or exogenous PGD2 induces CRTH2-dependent apoptosis in human differentiated OCs; β-arrestin-1, ERK1/2, and Akt, but not IKK2/NF-κB are probably implicated in the signaling pathways of this receptor in the model studied.

Novel, gel-free proteomics approach identifies RNF5 and JAMP as modulators of GPCR stability

The maturation and folding of G protein-coupled receptors are governed by mechanisms that remain poorly understood. In an effort to characterize these biological events, we optimized a novel, gel-free proteomic approach to identify partners of the β2-adrenergic receptor (β2AR). In addition to a number of known interacting proteins such as heterotrimeric G protein subunits, this allowed us to identify proteins involved in endoplasmic reticulum (ER) QC of the receptor. Among β2AR-associated proteins is Ring finger protein 5 (RNF5), an E3 ubiquitin ligase anchored to the outer membrane of the ER. Coimmunoprecipitation assays confirmed, in a cellular context, the interaction between RNF5 and the β2AR as well as the prostaglandin D2 receptor (DP). Confocal microscopy revealed that DP colocalized with RNF5 at the ER. Coexpression of RNF5 with either receptor increased levels of their expression, whereas small interfering RNA-mediated knockdown of endogenous RNF5 promoted the opposite. RNF5 did not modulate the ubiquitination state of β2AR or DP. Instead, RNF5 ubiquitinated JNK-associated membrane protein (JAMP), a protein that recruits the proteasome to the ER membrane and that is negatively regulated by RNF5-mediated ubiquitination. JAMP coimmunoprecipitated with both β2AR and DP and decreased total receptor protein levels through proteasomal degradation. Expression of DP, a receptor largely retained in the ER, promoted proteasome recruitment by JAMP. Degradation of both receptors via JAMP was increased when RNF5 was depleted. Our data suggest that RNF5 regulates the turnover of specific G protein-coupled receptors by ubiquitinating JAMP and preventing proteasome recruitment.

Inverse agonist and pharmacochaperone properties of MK-0524 on the prostanoid DP1 receptor

Prostaglandin D₂ (PGD₂) acts through two G protein-coupled receptors (GPCRs), the prostanoid DP receptor and CRTH2 also known as DP1 and DP2, respectively. Several previously characterized GPCR antagonists are now classified as inverse agonists and a number of GPCR ligands are known to display pharmacochaperone activity towards a given receptor. Here, we demonstrate that a DP1 specific antagonist, MK-0524 (also known as laropiprant), decreased basal levels of intracellular cAMP produced by DP1, a Gα(s)-coupled receptor, in HEK293 cells. This reduction in cAMP levels was not altered by pertussis toxin treatment, indicating that MK-0524 did not induce coupling of DP1 to Gα(i/o) proteins and that this ligand is a DP1 inverse agonist. Basal ERK1/2 activation by DP1 was not modulated by MK-0524. Interestingly, treatment of HEK293 cells expressing Flag-tagged DP1 with MK-0524 promoted DP1 cell surface expression time-dependently to reach a maximum increase of 50% compared to control after 24 h. In contrast, PGD₂ induced the internalization of 75% of cell surface DP1 after the same time of stimulation. The increase in DP1 cell surface targeting by MK-0524 was inhibited by Brefeldin A, an inhibitor of transport from the endoplasmic reticulum-Golgi to the plasma membrane. Confocal microscopy confirmed that a large population of DP1 remained trapped intracellularly and co-localized with calnexin, an endoplasmic reticulum marker. Redistribution of DP1 from intracellular compartments to the plasma membrane was observed following treatment with MK-0524 for 24 h. Furthermore, MK-0524 promoted the interaction between DP1 and the ANKRD13C protein, which we showed previously to display chaperone-like effects towards the receptor. We thus report that MK-0524 is an inverse agonist and a pharmacochaperone of DP1. Our findings may have important implications during therapeutic treatments with MK-0524 and for the development of new molecules targeting DP1.

Ubiquitination and activation of a Rab GTPase promoted by a β2-Adrenergic Receptor/HACE1 complex

We and others have shown that trafficking of G protein-coupled receptors is regulated by Rab GTPases. Cargo-mediated regulation of vesicular transport has received great attention lately. Rab GTPases, forming the largest branch of the Ras GTPase superfamily, regulate almost every step of vesicle-mediated trafficking. Rab GTPases are well-recognized targets of human diseases but their regulation and the mechanisms connecting them to cargo proteins are still poorly understood. Herein, we show by overexpression/depletion studies that HACE1, a HECT domain-containing ubiquitin ligase, promotes the recycling of the β2-adrenergic receptor (β2AR), a prototypical G protein-coupled receptor, through a Rab11a-dependent mechanism. Interestingly, the β2AR in conjunction with HACE1 triggered ubiquitination of Rab11a, as observed by Western blot analysis. LC-MS/MS experiments determined that Rab11a is ubiquitnatied on Lys145. A Rab11a-K145R mutant failed to undergo β2AR/HACE1-induced ubiquitination and inhibited the HACE1-mediated recycling of the β2AR. Rab11a, but not Rab11a-K145R, was activated by β2AR/HACE1 indicating that ubiquitination of Lys145 is involved in Rab11a activation. β2AR/HACE1 co-expression also potentiated ubiquitination of Rab6a and Rab8a, but not of other Rab GTPases that were tested. We report a novel regulatory mechanism of Rab GTPases by their ubiquitination with associated functional effects demonstrated on Rab11a. This partakes into a new pathway whereby a cargo protein, like a G protein-coupled receptor, can regulate its own trafficking by inducing the ubiquitination and activation of a Rab GTPase.

The C-terminal domains of melanocortin-2 receptor (MC2R) accessory proteins (MRAP1) influence their localization and ACTH-induced cAMP production

ACTH binding to the human melanocortin-2 receptor (MC2R) requires the presence of the MC2R accessory protein1 isoforms, MRAPα or MRAPβ. This study evaluated the role of the isoform-specific C-terminal domains of MRAP with regard to their cellular localization, topology, interaction with MRAP2 and cAMP production. When stably expressed in HEK293/FRT cells or in B16-G4F mouse melanoma cells (an MSH receptor-deficient cell clone), MRAPα and MRAPdCT (truncated MRAP1, N-terminal only) localized mainly around the nuclear envelope and within dense intracellular endosomes, while MRAPβ exhibited a strong localization at the plasma membrane, and partially with rapid recycling endosomes. MRAPβ and MRAPdCT both exhibited dual-topology (N(cyto)/C(exo) and N(exo)/C(cyto)) at the plasma membrane whereas MRAPα exhibited only N(cyto)/C(exo) topology at the plasma membrane while adopting dual-topology in intracellular compartments. Both MRAPα and MRAP2 colocalized in intracellular compartments, as opposed to weak colocalization between MRAPβ and MRAP2. MRAP2 and MC2R enhanced the expression of MRAP1 isoforms and vice versa. Moreover, in both HEK293/FRT and B16-G4F cells, ACTH failed to activate MC2R unless MRAP1 was present. MRAP1 expression enhanced MC2R cell-surface expression as well as concentration-dependent cAMP accumulation. In the presence of human or zebrafish MC2R, MRAPβ induced the highest cAMP accumulation while MRAPdCT induced the lowest. Together, the present findings indicate that the C-terminal domains of MRAP dictate their intracellular localization in addition to regulating ACTH-induced cAMP production. These preferential localizations suggest that MRAPα is involved in MC2R targeting to the plasma membrane, while MRAPβ may enhance ACTH-MC2R coupling to cAMP production.

Regulation of β2-adrenergic receptor maturation and anterograde trafficking by an interaction with Rab geranylgeranyltransferase: modulation of Rab geranylgeranylation by the receptor

Previous reports by us and others demonstrated that G protein-coupled receptors interact functionally with Rab GTPases. Here, we show that the β(2)-adrenergic receptor (β(2)AR) interacts with the Rab geranylgeranyltransferase α-subunit (RGGTA). Confocal microscopy showed that β(2)AR co-localizes with RGGTA in intracellular compartments and at the plasma membrane. Site-directed mutagenesis revealed that RGGTA binds to the L(339)L(340) motif in the β(2)AR C terminus known to be involved in the transport of the receptor from the endoplasmic reticulum to the cell surface. Modulation of the cellular levels of RGGTA protein by overexpression or siRNA-mediated knockdown of the endogenous protein demonstrated that RGGTA has a positive role in the maturation and anterograde trafficking of the β(2)AR, which requires the interaction of RGGTA with the β(2)AR L(339)L(340) motif. Furthermore, the β(2)AR modulates the geranylgeranylation of Rab6a, Rab8a, and Rab11a, but not of other Rab proteins tested in this study. Regulation of Rab geranylgeranylation by the β(2)AR was dependent on the RGGTA-interacting L(339)L(340) motif. Interestingly, a RGGTA-Y107F mutant was unable to regulate Rab geranylgeranylation but still promoted β(2)AR maturation, suggesting that RGGTA may have functions independent of Rab geranylgeranylation. We demonstrate for the first time an interaction between a transmembrane receptor and RGGTA which regulates the maturation and anterograde transport of the receptor, as well as geranylgeranylation of Rab GTPases.

WDR36 acts as a scaffold protein tethering a G-protein-coupled receptor, Gαq and phospholipase Cβ in a signalling complex

We identified the WD-repeat-containing protein, WDR36, as an interacting partner of the β isoform of thromboxane A2 receptor (TPβ) by yeast two-hybrid screening. We demonstrated that WDR36 directly interacts with the C-terminus and the first intracellular loop of TPβ by in vitro GST-pulldown assays. The interaction in a cellular context was observed by co-immunoprecipitation, which was positively affected by TPβ stimulation. TPβ–WDR36 colocalization was detected by confocal microscopy at the plasma membrane in non-stimulated HEK293 cells but the complex translocated to intracellular vesicles following receptor stimulation. Coexpression of WDR36 and its siRNA-mediated knockdown, respectively, increased and inhibited TPβ-induced Gαq signalling. Interestingly, WDR36 co-immunoprecipitated with Gαq, and promoted TPβ–Gαq interaction. WDR36 also associated with phospholipase Cβ (PLCβ) and increased the interaction between Gαq and PLCβ, but prevented sequestration of activated Gαq by GRK2. In addition, the presence of TPβ in PLCβ immunoprecipitates was augmented by expression of WDR36. Finally, disease-associated variants of WDR36 affected its ability to modulate Gαq-mediated signalling by TPβ. We report that WDR36 acts as a new scaffold protein tethering a G-protein-coupled receptor, Gαq and PLCβ in a signalling complex.

Mechanisms of melanocortin-2 receptor (MC2R) internalization and recycling in human embryonic kidney (hek) cells: identification of Key Ser/Thr (S/T) amino acids

ACTH is the most important stimulus of the adrenal cortex. The precise molecular mechanisms underlying the ACTH response are not yet clarified. The functional ACTH receptor includes melanocortin-2 receptor (MC2R) and MC2R accessory proteins (MRAP). In human embryonic kidney 293/Flp recombinase target cells expressing MC2R, MRAP1 isoforms, and MRAP2, we found that ACTH induced a concentration-dependent and arrestin-, clathrin-, and dynamin-dependent MC2R/MRAP1 internalization, followed by intracellular colocalization with Rab (Ras-like small guanosine triphosphate enzyme)4-, Rab5-, and Rab11-positive recycling endosomes. Preincubation of cells with monensin and brefeldin A revealed that 28% of the internalized receptors were recycled back to the plasma membrane and participated in total accumulation of cAMP. Moreover, certain intracellular Ser and Thr (S/T) residues of MC2R were found to play important roles not only in plasma membrane targeting and function but also in promoting receptor internalization. The S/T residues T131, S140, T204, and S280 were involved in MRAP1-independent cell-surface MC2R expression. Other mutants (S140A, S208A, and S202D) had lower cell-surface expressions in absence of MRAPβ. In addition, T143A and T147D drastically impaired cell-surface expression and function, whereas T131A, T131D, and S280D abrogated MC2R internalization. Thus, the modification of MC2R intracellular S/T residues may positively or negatively regulate its plasma membrane expression and the capacity of ACTH to induce cAMP accumulation. Mutations of T131, T143, T147, and S280 into either A or D had major repercussions on cell-surface expression, cAMP accumulation, and/or internalization parameters, pointing mostly to the second intracellular loop as being crucial for MC2R expression and functional regulation.

An interaction between L-prostaglandin D synthase and arrestin increases PGD2 production

L-type prostaglandin synthase (L-PGDS) produces PGD(2), a lipid mediator involved in neuromodulation and inflammation. Here, we show that L-PGDS and arrestin-3 (Arr3) interact directly and can be co-immunoprecipitated endogenously from MG-63 osteoblasts. Perinuclear L-PGDS/Arr3 co-localization is observed in PGD(2)-producing MG-63 cells and is induced by the addition of the L-PGDS substrate or co-expression of COX-2 in HEK293 cells. Inhibition of L-PGDS activity in MG-63 cells triggers redistribution of Arr3 and L-PGDS to the cytoplasm. Perinuclear localization of L-PGDS is detected in wild-type mouse embryonic fibroblasts (MEFs) but is more diffused in MEFs-arr-2(-/-)-arr-3(-/-). Arrestin-3 promotes PGD(2) production by L-PGDS in vitro. IL-1β-induced PGD(2) production is significantly lower in MEFs-arr-2(-/-)-arr-3(-/-) than in wild-type MEFs but can be rescued by expressing Arr2 or Arr3. A peptide corresponding to amino acids 86-100 of arrestin-3 derived from its L-PGDS binding domain stimulates L-PGDS-mediated PGD(2) production in vitro and in MG-63 cells. We report the first characterization of an interactor/modulator of a PGD(2) synthase and the identification of a new function for arrestin, which may open new opportunities for improving therapies for the treatment of inflammatory diseases.

ANKRD13C acts as a molecular chaperone for G protein-coupled receptors

Although the mechanisms that regulate folding and maturation of newly synthesized G protein-coupled receptors are crucial for their function, they remain poorly characterized. By yeast two-hybrid screening, we have isolated ANKRD13C, a protein of unknown function, as an interacting partner for the DP receptor for prostaglandin D(2). In the present study we report the characterization of this novel protein as a regulator of DP biogenesis and trafficking in the biosynthetic pathway. Co-localization by confocal microscopy with an endoplasmic reticulum (ER) marker, subcellular fractionation experiments, and demonstration of the interaction between ANKRD13C and the cytoplasmic C terminus of DP suggest that ANKRD13C is a protein associated with the cytosolic side of ER membranes. Co-expression of ANKRD13C with DP initially increased receptor protein levels, whereas siRNA-mediated knockdown of endogenous ANKRD13C decreased them. Pulse-chase experiments indicated that ANKRD13C can promote the biogenesis of DP by inhibiting the degradation of newly synthesized receptors. However, a prolonged interaction between ANKRD13C and DP resulted in ER retention of misfolded/unassembled forms of the receptor and to their proteasome-mediated degradation. ANKRD13C also regulated the expression of other GPCRs tested (CRTH2, thromboxane A(2) (TPα), and β2-adrenergic receptor), whereas it did not affect the expression of green fluorescent protein, GRK2 (G protein-coupled receptor kinase 2), and VSVG (vesicular stomatitis virus glycoprotein), showing specificity toward G protein-coupled receptors. Altogether, these results suggest that ANKRD13C acts as a molecular chaperone for G protein-coupled receptors, regulating their biogenesis and exit from the ER.

Characterization of C-terminal tail determinants involved in CRTH2 receptor trafficking: identification of a recycling motif

The molecular mechanisms regulating the trafficking of the CRTH2 receptor are poorly understood. In the present study, we characterize C-terminal tail determinants involved in the agonist-induced trafficking of the CRTH2 receptor for prostaglandin D(2). Our results showed that progressive deletion of C-terminal tail residues from amino acid 395 up to 337 gradually impaired CRTH2 internalization by approximately 50% as measured by ELISA in HEK293 cells. Surprisingly, further deletion of the C-tail to amino acid 328 or 317 resulted in receptor mutants displaying internalization similar to the wild-type receptor. Individual mutations of Asp(330), Ser(331), Glu(332), and Leu(333) to Ala in the C-tail of the full length receptor resulted in a 45% increase in internalization of the receptor mutants relative to the wild-type receptor. Pretreatment with the recycling inhibitor monensin increased internalization of the wild-type receptor but did not affect that of the D330A, S331A, E332A and L333A mutants, indicating that these residues are part of a recycling motif. Further experiments revealed that Asp(330), Ser(331) and Glu(332) are not only involved in receptor recycling, but are also required for promotion of CRTH2 internalization by GRK2 and GRK5. Site-directed mutagenesis identified Thr(347) as a major site for PKC-induced internalization of the receptor. Confocal microscopy revealed that arrestin-3 dissociated from the receptor after agonist stimulation and internalization, suggesting that CRTH2 is a class A G protein-coupled receptor. Our study identified specific amino acids in the CRTH2 receptor C-tail implicated in the agonist-induced internalization and the recycling of the receptor.

Metastases suppressor NM23-H2 interaction with G-quadruplex DNA within c-MYC promoter nuclease hypersensitive element induces c-MYC expression

Regulatory influence of the G-quadruplex or G4 motif present within the nuclease hypersensitive element (NHE) in the promoter of c-MYC has been noted. On the other hand, association of NM23-H2 to the NHE leads to c-MYC activation. Therefore, NM23-H2 interaction with the G4 motif within the c-MYC NHE presents an interesting mechanistic possibility. Herein, using luciferase reporter assay and chromatin immunoprecipitation we show NM23-H2 mediated c-MYC activation involves NM23-H2-G4 motif binding within the c-MYC NHE. G4 motif complex formation with recombinant NM23-H2 was independently confirmed using fluorescence energy transfer, which also indicated that the G4 motif was resolved to an unfolded state within the protein-bound complex. Taken together, this supports transcriptional role of NM23-H2 via a G4 motif.

The N-terminal coiled-coil domain of the cytohesin/ARNO family of guanine nucleotide exchange factors interacts with Galphaq

Cytohesins are guanine-nucleotide exchange factors (GEF) for the Arf family of GTPases. One member of the Arf family, ARF6, plays an active role in the intracellular trafficking of G protein-coupled receptors. We have previously reported that Galphaq signaling leads to the activation of ARF6, possibly through a direct interaction with cytohesin-2/ARNO. Here, we report that Galphaq can directly interact with cytohesin-1, another Arf-GEF of the ARNO/cytohesin family. Cytohesin-1 preferentially associated with a constitutively active mutant of Galphaq (Galphaq-Q209L) compared to wild-type Galphaq in HEK293 cells. Stimulation of TPbeta, a Galphaq-coupled receptor, to activate Galphaq resulted in the promotion of a protein complex between Galphaq and cytohesin-1. Confocal immunofluorescence microscopy revealed that wild-type Galphaq and cytohesin-1 co-localized in intracellular compartments and at or near the plasma membrane. In contrast, expression of Galphaq-Q209L induced a drastic increase in the localization of cytohesin-1 at the plasma membrane. Expression of a dominant-negative mutant of cytohesin-1 reduced by 40% the agonist-induced internalization of TPbeta, a process that we previously demonstrated to be dependent on Galphaq-mediated signaling and Arf6 activation. Using deletion mutants, we show that cytohesin-1 interacts with Galphaq through its N-terminal coiled-coil domain. Cytohesin-1 and cytohesin-2/ARNO mutants lacking the coiled-coil domain were unable to relay Galphaq-mediated activation of Arf6. This is the first report of an interaction between the coiled-coil domain of the cytohesin/ARNO family of Arf-GEFs and a member of the heterotrimeric G proteins.

Peroxiredoxin-4 interacts with and regulates the thromboxane A(2) receptor

We identified peroxiredoxin-4 (Prx-4) as a protein interacting with the beta isoform of the thromboxane A(2) receptor (TPbeta) by yeast two-hybrid analysis. Prx-4 co-immunoprecipitated constitutively with TPbeta in HEK293 cells. The second and third intracellular loops as well as the C-terminus of TPbeta interacted directly with Prx-4. Co-expression of Prx-4 caused a 60% decrease in cell surface expression of TPbeta. Prx-4 and TPbeta predominantly co-localized in the endoplasmic reticulum. Co-expression of Prx-4 in cells treated with H(2)O(2) targeted TPbeta for degradation. We show for the first time an interaction between a receptor involved in oxidative stress and Prx-4, an anti-oxidative enzyme.

Differential regulation of the signaling and trafficking of the two prostaglandin D2 receptors, prostanoid DP receptor and CRTH2

Prostaglandin D2 (PGD2) exerts its actions on two G protein-coupled receptors, the prostanoid DP receptor and CRTH2 (chemoattractant homologous receptor expressed on TH2 cells). Here, we characterize the regulation of the signaling and trafficking of the prostanoid DP receptor and CRTH2. Time-course and dose-response curves showed that both receptors expressed in HEK293 cells internalized maximally after 2 h of stimulation with 1 microM PGD2. Co-expression of the G protein-coupled receptor kinases GRK2, GRK5 or GRK6 increased agonist-induced internalization of CRTH2, while only GRK2 had an effect on the internalization of the prostanoid DP receptor. Protein kinase C (PKC) activation stimulated the internalization of both receptors. Interestingly, only PGD2-induced internalization of CRTH2, and not of prostanoid DP receptor, was decreased by inhibition of PKC or protein kinase A (PKA). Our data also indicate that CRTH2 is subjected to basal phosphorylation by PKA, which appears to be involved in CRTH2 internalization. Prostanoid DP receptor internalization was promoted by co-expression of arrestin-2 and -3, while the internalization of CRTH2 was increased by co-expression of arrestin-3 only. The detection of prostanoid DP receptor and CRTH2 internalization was reduced by the co-expression of Rab4 and Rab11, respectively, suggesting differential regulation of receptor recycling. Moreover, immunofluorescence microscopy experiments showed that the prostanoid DP receptor specifically co-localized with Rab4, and CRTH2 with Rab11. The signaling of the prostanoid DP receptor was regulated by GRK2 overexpression, while that of CRTH2 was modulated by overexpression of GRK2, -5 and -6. Our results show a differential regulation of the prostanoid DP receptor and CRTH2, two receptors for PGD2.

Prostaglandin production by human osteoclasts in culture

OBJECTIVE

Prostaglandins (PG) are important mediators of bone metabolism with direct and indirect effects on bone cells. They may have important effects on osteoclasts, but it is not known if these cells can synthesize PG. We used 2 experimental models in order (1) to determine the presence and functionality of cyclooxygenase (COX) and phospholipase A2 (PLA2) enzymes in human osteoclasts and (2) to study their role in cell metabolism.

METHODS

Experiments were undertaken on authentic human osteoclasts extracted from human fetuses (fhOC) and on human osteoclast-like (hOCL) cells differentiated from peripheral blood mononuclear cells. The presence of COX proteins was determined by immunohistochemistry. COX and PLA2 enzymatic activity was evaluated at the single-cell level by fluorescence microscopy. An enriched population of hOCL cells was used to evaluate total PG production and the influence of COX activity on bone resorption.

RESULTS

COX-1 was expressed in the cytoplasm and COX-2 was distributed mainly near the nuclear membrane of osteoclasts. These cells showed a high basal level of COX activity that could be inhibited by pretreatment with COX inhibitors. Cytosolic PLA2 was present in both models. Human osteoclasts actively produced PG, and the COX-1 pathway was implicated in the control of bone resorption.

CONCLUSION

These results indicate that PG may be important autacoids for the control of osteoclast biology and that the COX-1 pathway is implicated in the inhibition of bone resorption.

Implication of prostaglandin receptors in the accumulation of osteoprotegerin in human osteoblast cultures

OBJECTIVE

Prostaglandins are important mediators in bone metabolism and in pathologies such as rheumatoid arthritis and osteoarthritis. We investigated the roles of cyclooxygenases (COX) and prostaglandin receptors in the accumulation of osteoprotegerin (OPG) in the supernatants of human osteoblasts in culture.

METHODS

Three different cellular models were used, the human osteosarcoma cell lines MG-63 and Saos-2, and primary cultures of human osteoblasts. OPG concentrations were determined by ELISA.

RESULTS

RT-PCR analysis showed that, like primary human osteoblasts, MG-63 cells express DP, EP4, FP, IP, and TP receptors, whereas the Saos-2 cells lack IP. Concentration of OPG was highest in MG-63 cell supernatants (36 +/- 12.5 ng/ml), followed by human osteoblasts (12.77 +/- 2.2 ng/ml) and Saos-2 (3.6 +/- 0.76 ng/ml). COX inhibitors did not alter these values. Prostaglandin E2 and BW 245C (a synthetic DP receptor agonist) decreased OPG in the supernatants of human osteoblasts but not in immortalized cell lines. These effects were concentration-dependent and were inhibited by EP4 and DP receptor antagonists. Fluprostenol, an FP receptor agonist, increased the accumulation of OPG in MG-63 but not in primary human osteoblasts or Saos-2.

CONCLUSION

Our results show that activation of EP4 or DP receptors decreased the accumulation of OPG in supernatants of osteoblasts in culture, and suggest that these receptors could be interesting pharmacological targets in bone diseases. They also demonstrate important differences between primary osteoblasts and immortalized cell lines, both in the distribution and in the effects mediated by prostaglandin receptors.

ARF6 activation by Galpha q signaling: Galpha q forms molecular complexes with ARNO and ARF6

G protein-coupled receptors (GPCRs) are widely expressed hepta-helical receptors with tightly regulated pleiotropic effects. ADP-Ribosylation Factor 6 (ARF6) plays an important role in GPCR trafficking and is the subject of intense research. However, the mechanisms underlying activation and regulation of ARF6 by GPCRs are poorly characterized. Here we report that Galpha(q) signaling leads to the activation of ARF6. Stimulation of the TPbeta receptor triggered ARF6 activation which was completely inhibited by the RGS domain of GRK2 known to specifically bind and sequester Galpha(q). Co-immunoprecipitation studies revealed that ARNO (a guanine nucleotide exchange factor for ARF6) and ARF6 formed complexes preferentially with activated Galpha(q) compared to non-activated Galpha(q). Formation of the Galpha(q) complexes with ARNO and ARF6 was detected early and was optimal after 30 min of receptor stimulation corresponding with the profile of ARF6 activation. Interestingly, binding experiments using purified proteins showed that Galpha(q) interacted directly with ARNO. Galpha(q)-dependent TPbeta receptor-mediated activation of ARF6 resulted in phosphoinositol-4,5-bisphosphate production which was potently inhibited by dominant negative mutants of ARNO and ARF6. Furthermore, our data show that the expression of ARNO and ARF6 promoted, whereas dominant negative mutants of these proteins inhibited the internalization of the TPbeta receptor. This further elucidates our previous data on the PLCbeta- and PKC-independent mechanism involved in Galpha(q)-mediated internalization of the TPbeta receptor. Taken altogether, our results support a novel model where activated Galpha(q) forms molecular complexes with ARNO and ARF6, possibly through a direct interaction with ARNO, leading to ARF6 activation.

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.

The Intracellular Trafficking of the G Protein-coupled Receptor TPβ Depends on a Direct Interaction with Rab11

Intracellular trafficking pathways of cell surface receptors following their internalization are the subject of intense research efforts. However, the mechanisms by which they recycle back to the cell surface are still poorly defined. We have recently demonstrated that the small Rab11 GTPase protein is a determinant factor in controlling the recycling to the cell surface of the beta-isoform of the thromboxane A2 receptor (TPbeta) following its internalization. Here, we demonstrate with co-immunoprecipitation studies in HEK293 cells that there is a Rab11-TPbeta association occurring in the absence of agonist, which is not modulated by stimulation of TPbeta. We show with purified TPbeta intracellular domains fused to GST and HIS-Rab11 proteins that Rab11 interacts directly with the first intracellular loop and the C-tail of TPbeta. Amino acids 335-344 of the TPbeta C-tail were determined to be essential for the interaction of Rab11 with this receptor domain. This identified sequence appears to be important in directing the intracellular trafficking of the receptor from the Rab5-positive intracellular compartment to the perinuclear recycling endosome. Interestingly, our data indicate that TPbeta interacts with the GDP-bound form, and not the GTP-bound form, of Rab11 which is necessary for recycling of the receptor back to the cell surface. To our knowledge, this is the first demonstration of a direct interaction between Rab11 and a transmembrane receptor.

Death receptors, Fas and TRAIL receptors, are involved in human osteoclast apoptosis

Survival and apoptosis are crucial aspects of the osteoclast life cycle. Although osteoclast survival has been extensively studied, little is known about the mechanisms involved in human osteoclast apoptosis. In the present study, cord blood monocytes (CBMs) were used as the source of human osteoclast precursors. When cultured in the presence of M-CSF and RANKL, CBMs formed multinucleated cells that expressed RANK and calcitonin receptor, and were able to resorb bone. These cells expressed TRAIL receptors (R1-R4). Surprisingly, although TRAIL-receptor expression was not detectable in osteoclasts from normal bone, osteoclasts from myeloma specimens did express TRAIL receptors to a variable extent. Significantly, we have shown for the first time that this pathway is indeed functional in human osteoclasts, and that apoptosis occurred and was significantly greater in the presence of TRAIL. In addition, we have shown that a Fas-activating antibody is also able to induce osteoclast apoptosis, as did TGFbeta, whereas the survival factor M-CSF decreased apoptosis. Overall, these findings suggest that death receptors, TRAIL receptors and Fas, could be involved in osteoclast apoptosis in humans.

Involvement of Actin in Agonist-induced Endocytosis of the G Protein-coupled Receptor for Thromboxane A2

The role of actin in endocytosis of G protein-coupled receptors is poorly defined. In the present study, we demonstrate that agents that depolymerize (latrunculin B and cytochalasin D) or stabilize (jasplakinolide) the actin cytoskeleton blocked agonist-induced endocytosis of the beta isoform of the thromboxane A(2) receptor (TPbeta) in HEK293 cells. This suggests that endocytosis of TPbeta requires active remodeling of the actin cytoskeleton. On the other hand, disruption of microtubules with colchicine did not affect endocytosis of the receptor. Expression of wild-type and mutant forms of the small GTPases RhoA and Cdc42 potently inhibited endocytosis of TPbeta, further indicating a role for the dynamic regulation of the actin cytoskeleton in this pathway. Agonist treatment of TPbeta in HEK293 cells resulted in the formation of actin stress fibers through Galpha(q/11) signaling. Because we previously showed that endocytosis of TPbeta is dependent on arrestins, we decided to explore the relation between arrestin-2 and -3 and actin in endocytosis of this receptor. Interestingly, we show that the inhibition of TPbeta endocytosis by the actin toxins in HEK293 cells was overcome by the overexpression of arrestin-3, but not of arrestin-2. These results indicate that the actin cytoskeleton is not essential in arrestin-3-mediated endocytosis of TPbeta. However, arrestin-3 could not promote endocytosis of the TPbetaY339A and TPbetaI343A carboxyl-terminal mutants when the actin cytoskeleton was disrupted. Our data provide new evidence that the actin cytoskeleton plays an essential role in TPbeta endocytosis. Furthermore, our work suggests the existence of actin-dependent and -independent arrestin-mediated pathways of endocytosis.

Production of prostaglandin D(2) by human osteoblasts and modulation of osteoprotegerin, RANKL, and cellular migration by DP and CRTH2 receptors

Human osteoblasts produce PGD(2), which acts on the DP receptor to decrease osteoprotegerin production and on the CRTH2 receptor to decrease RANKL expression and to induce osteoblast chemotaxis. These results indicate that activation of CRTH2 may lead to an anabolic response in bone.

INTRODUCTION

Whereas the actions of prostaglandin (PG)E(2) as a modulator of bone and osteoblast function are relatively well characterized, little is known about PGD(2) and bone metabolism. The objectives of this study were to determine if human osteoblasts can produce PGD(2), which prostaglandin D(2) synthases are implicated in this synthesis, to identify the PGD(2) receptors (DP and CRTH2) on these cells and to characterize the biological effects resulting from their activation.

MATERIALS AND METHODS

RT-PCR analysis and immunohistochemistry were used to detect PGD(2) receptor and synthases in cultured human osteoblasts. Immunohistochemistry was used to identify the synthases and receptors in human bone tissue. Intracellular cAMP and calcium levels were determined to verify receptor activation. The cells were stimulated with PGD(2) or the specific agonists BW 245C (DP) and DK-PGD(2) (CRTH2), and the resulting effects on osteoprotegerin (OPG) secretion, RANKL expression, and chemotaxis were determined. Osteoblast production of PGD(2) was evaluated by measuring PGD(2) in the culture supernatants after stimulation with interleukin (IL)-1, TNF-alpha, PTH, vascular endothelial growth factor (VEGF), and insulin-like growth factor I (IGF-I).

RESULTS

Human osteoblasts in culture generated PGD(2) when stimulated. Both osteoblasts in culture and in situ present the lipocalin-type PGD(2) synthase only. Both DP and CRTH2 receptors were present in human osteoblasts in culture and in situ. Stimulation of DP resulted in an increase in cAMP, whereas CRTH2 increased the intracellular calcium level. OPG production was reduced by 60% after DP receptor stimulation, whereas CRTH2 receptor stimulation decreased RANKL expression on human osteoblasts. As reported for other cell types, CRTH2 was a potent inducer of chemotaxis for human osteoblasts in culture.

CONCLUSIONS

Human osteoblasts in culture produce PGD(2) under biologically relevant stimuli through the lipocalin-type PGD(2) synthase (L-PGDS) pathway. As an autacoid, PGD(2) can act on DP and CRTH2 receptors, both present on these cells. Specific activation of CRTH2 could lead directly and indirectly to an anabolic response in bone.

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

Prostacyclin activation of prostanoid IP receptors may result in pain sensation, inflammatory responses, inhibition of platelet aggregation, and vasodilation in vascular tissue. The prostanoid IP receptor is a G-protein-coupled receptor. In the present study, we investigated the determinants responsible, at least in part, for the prostacyclin receptor (IP) dimerization/oligomerization. Using co-immunoprecipitation of differentially tagged IP expressed in COS-7 cells, we demonstrate that IP can form dimers and oligomers. Treatment of IP-expressing cells with the stable agonist carbaprostacyclin failed to alter the ratios of oligomeric/dimeric/monomeric forms of the receptor, suggesting that IP dimerization/oligomerization is an agonist-independent process. The reducing agents dithiothreitol and 2-mercaptoethanol were highly efficient in converting the receptor from its oligomeric form to the monomeric state, indicating the involvement of disulfide bonds in IP oligomerization. Immunoblotting of the osteoblastic MG-63 cell line lysates with an anti-IP specific antibody revealed the presence of endogenous IP oligomers which were converted to dimers and monomers upon treatment with dithiothreitol. Individual substitutions of the four extracellular IP Cys residues (Cys(5), Cys(92), Cys(165) and Cys(170)) for Ser resulted in greatly decreased receptor protein expression in COS-7 cells. The C92-170S double mutant showed receptor protein expression level similar to the individual mutants. However, expression of the C92-165S and C165-170S mutants was drastically reduced, suggesting that there was formation of disulfide bonds between Cys(5) and Cys(165), and between Cys(92) and Cys(170). The Cys receptor mutants showed altered oligomer/dimer/monomer ratios. Dimerization/oligomerization likely occurs intracellularly since these Cys receptor mutants could still form dimers/oligomers despite their lack of expression at the cell surface.

Role of the Rab11-Associated Intracellular Pool of Receptors Formed by Constitutive Endocytosis of the β Isoform of the Thromboxane A2 Receptor (TPβ)

Intracellular trafficking pathways of G protein-coupled receptors (GPCRs), following their agonist-induced endocytosis and their consequences on receptor function, are the subject of intense research efforts. However, less is known regarding their constitutive endocytosis. We previously demonstrated that the beta isoform of the thromboxane A(2) receptor (TPbeta) undergoes constitutive and agonist-induced endocytosis. Constitutive endocytosis of GPCRs can lead to the formation of an intracellular pool of receptors from which they can recycle back to the cell surface. In the present report, we show with the help of two TPbeta mutants (TPbeta-Y339A and TPbeta-I343A) specifically deficient in constitutive endocytosis that this intracellular pool of receptors serves to maintain agonist sensitivity over prolonged receptor stimulation in HEK293 cells. Second messenger generation by the TPbeta-Y339A and TPbeta-I343A mutants was drastically reduced compared to the wild-type receptor as suggested by dose-response and time-course experiments of inositol phosphates production following agonist treatment, despite normal coupling between the receptors and the Galpha(q) protein. Moreover, second messenger production after receptor activation was dramatically reduced when cells were pretreated with monensin, a recycling inhibitor. Receptor cell surface expression and endocytosis experiments further revealed that the small GTPase Rab11 protein is a determinant factor in controlling TPbeta recycling back to the cell surface. Co-localization experiments performed by immunofluorescence microscopy indicated that both constitutive and agonist-triggered endocytosis resulted in targeting of TPbeta to the Rab11-positive recycling endosome. Thus, we provide evidence that constitutive endocytosis of TPbeta forms a pool of receptors in the perinuclear recycling endosome from which they recycle to the cell surface, a process involved in preserving receptor sensitivity to agonist stimulation.

Nm23-H2 Interacts with a G Protein-coupled Receptor to Regulate Its Endocytosis through an Rac1-dependent Mechanism

G protein-coupled receptors (GPCRs) represent a vast family of transmembrane proteins involved in the regulation of several physiological responses. The thromboxane A2 receptor (present as two isoforms: TP alpha and TP beta) is a GPCR displaying diverse pharmacological effects. As seen for many other GPCRs, TP beta is regulated by agonist-induced internalization. In the present study, we report the identification by yeast two-hybrid screening of Nm23-H2, a nucleoside diphosphate kinase, as a new interacting molecular partner with the C-terminal tail of TP beta. This interaction was confirmed in a cellular context when Nm23-H2 was co-immunoprecipitated with TP beta in HEK293 cells, a process dependent on agonist stimulation of the receptor. We observed that agonist-induced internalization of TP beta was regulated by Nm23-H2 through modulation of Rac1 signaling. Immunofluorescence microscopy in HEK293 cells revealed that Nm23-H2 had a cytoplasmic and nuclear localization but was induced to translocate to the plasma membrane upon stimulation of TP beta to show extensive co-localization with the receptor. Our findings represent the first demonstration of an interaction of an Nm23 protein with a membrane receptor and constitute a novel molecular regulatory mechanism of GPCR endocytosis.

Activation-independent parathyroid hormone receptor internalization is regulated by NHERF1 (EBP50)

Parathyroid hormone (PTH) regulates extracellular calcium homeostasis through the type 1 PTH receptor (PTH1R) expressed in kidney and bone. The PTH1R undergoes beta-arrestin/dynamin-mediated endocytosis in response to the biologically active forms of PTH, PTH-(1-34), and PTH-(1-84). We now show that amino-truncated forms of PTH that do not activate the PTH1R nonetheless induce PTH1R internalization in a cell-specific pattern. Activation-independent PTH1R endocytosis proceeds through a distinct arrestin-independent mechanism that is operative in cells lacking the adaptor protein Na/H exchange regulatory factor 1 (NHERF1) (ezrin-binding protein 50). Using a combination of radioligand binding experiments and quantitative, live cell confocal microscopy of fluorescently tagged PTH1Rs, we show that in kidney distal tubule cells and rat osteosarcoma cells, which lack NHERF1, the synthetic antagonist PTH-(7-34) and naturally circulating PTH-(7-84) induce internalization of PTH1R in a beta-arrestin-independent but dynamin-dependent manner. Expression of NHERF1 in these cells inhibited antagonist-induced endocytosis. Conversely, expression of dominant-negative forms of NHERF1 conferred internalization sensitivity to PTH-(7-34) in cells expressing NHERF1. Mutation of the PTH1R PDZ-binding motif abrogated interaction of the receptor with NHERF1. These mutated receptors were fully functional but were now internalized in response to PTH-(7-34) even in NHERF1-expressing cells. Removing the NHERF1 ERM domain or inhibiting actin polymerization allowed otherwise inactive ligands to internalize the PTH1R. These results demonstrate that NHERF1 acts as a molecular switch that legislates the conditional efficacy of PTH fragments. Distinct endocytic pathways are determined by NHERF1 that are operative for the PTH1R in kidney and bone cells.

Trafficking, ubiquitination, and down-regulation of the human platelet-activating factor receptor

Platelet-activating factor (PAF) is a potent phospholipid mediator involved in various disease states such as allergic asthma, atherosclerosis and psoriasis. The human PAF receptor (PAFR) is a member of the G protein-coupled receptor family. Following PAF stimulation, cells become rapidly desensitized; this refractory state can be maintained for hours and is dependent on PAFR phosphorylation, internalization, and down-regulation. In this report, we characterized ligand-induced, long term PAFR desensitization, and pathways leading to its degradation. Some GPCRs are known to be targeted to proteasomes for degradation while others traffic via the early/late endosomes toward lysosomes. Specific inhibitors of lysosomal proteases and inhibitors of the proteasome were effective in reducing the ligand-induced PAFR down-regulation by 40 and 25%, respectively, indicating the importance of receptor targeting to both lysosomes and proteasomes in long term cell desensitization to PAF. The effects of the proteasome and lysosomal protease inhibitors were additive and, together, completely blocked ligand-induced degradation of PAFR. Using dominant-negative Rab5 and 7 and colocalization of the PAFR with the early endosome autoantigen I (EEAI) or transferrin, we confirmed that ligand-induced PAFR down-regulation was Rab5/7-dependent and involved lysosomal degradation. In addition, we also demonstrated that PAFR was ubiquitinated in an agonist-independent manner. However, a dominant negative ubiquitin ligase (NCbl) reduced PAFR ubiquitination and inhibited ligand-induced but not basal receptor degradation. Our results indicate that PAFR degradation can occur via both the proteasome and lysosomal pathways and ligand-stimulated degradation is ubiquitin-dependent.

Galphaq-coupled receptor internalization specifically induced by Galphaq signaling. Regulation by EBP50

In the present report, we investigated the effect of ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50) expression on the agonist-induced internalization of the thromboxane A(2) beta receptor (TPbeta receptor). Interestingly, we found that EBP50 almost completely blocked TPbeta receptor internalization, which could not be reversed by overexpression of G protein-coupled receptor (GPCR) kinases and arrestins. Because we recently demonstrated that EBP50 can bind to and inhibit Galpha(q), we next studied whether Galpha(q) signaling could induce TPbeta receptor internalization, addressing the long standing question about the relationship between GPCR signaling and their internalization. Expression of a constitutively active Galpha(q) mutant (Galpha(q)-R183C) resulted in a robust internalization of the TPbeta receptor, which was unaffected by expression of dominant negative mutants of arrestin-2 and -3, but inhibited by expression of EBP50 or dynamin-K44A, a dominant negative mutant of dynamin. Phospholipase Cbeta and protein kinase C did not appear to significantly contribute to internalization of the TPbeta receptor, suggesting that Galpha(q) induces receptor internalization through a phospholipase Cbeta- and protein kinase C-independent pathway. Surprisingly, there appears to be specificity in Galpha protein-mediated GPCR internalization. Galpha(q)-R183C also induced the internalization of CXCR4 (Galpha(q)-coupled), whereas it failed to do so for the beta(2)-adrenergic receptor (Galpha(s)-coupled). Moreover, Galpha(s)-R201C, a constitutively active form of Galpha(s), had no effect on internalization of the TPbeta, CXCR4, and beta(2)-adrenergic receptors. Thus, we showed that Galpha protein signaling can lead to internalization of GPCRs, with specificity in both the Galpha proteins and GPCRs that are involved. Furthermore, a new function has been described for EBP50 in its capacity to inhibit receptor endocytosis.

Residues 293 and 294 are ligand contact points of the human angiotensin type 1 receptor

The human angiotensin II type 1 receptor (hAT(1)) was photolabeled with a high-affinity radiolabeled photoreactive analogue of AngII, (125)I-[Sar(1), Val(5), p-Benzoyl-L-phenylalanine(8)]AngII ((125)I-[Sar(1),Bpa(8)]AngII). Chemical cleavage with CNBr produced a 7 kDa fragment (285-334) of the C-terminal portion of the hAT(1). Manual Edman radiosequencing of photolabeled, per-acetylated, and CNBr-fragmented receptor showed that ligand incorporation occurred through Phe(293) and Asn(294) within the seventh transmembrane domain of the hAT(1). Receptor mutants with Met introduced at the presumed contact residues, F293M and N294M, were photolabeled and then digested with CNBr. SDS-PAGE analysis of those digested mutant receptors confirmed the contact positions 293 and 294 through ligand release induced by CNBr digestion. Additional receptor mutants with Met residues introduced into the N- and C-terminal proximity of those residues 293 and 294 of the hAT(1) produced, upon photolabeling and CNBr digestion, fragmentation patterns compatible only with the above contact residues. These data indicate that the C-terminal residue of AngII interacts with residues 293 and 294 of the seventh transmembrane domain of the human AT(1) receptor. Taking into account a second receptor-ligand contact at the second extracellular loop and residue 3 of AngII (Boucard, A. A., Wilkes, B. C., Laporte, S. A., Escher, E., Guillemette, G., and Leduc, R. (2000) Biochemistry 39, 9662-70) the Ang II molecule must adopt an extended structure in the AngII binding pocket.

Regulation of GTP-binding protein alpha q (Galpha q) signaling by the ezrin-radixin-moesin-binding phosphoprotein-50 (EBP50)

Although ezrin-radixin-moesin-binding phosphoprotein 50 (EBP50) is a PDZ domain-containing protein known to bind to various channels, receptors, cytoskeletal elements, and cytoplasmic proteins, there is still very little evidence for a role of EBP50 in the regulation of receptor signal transduction. In this report, we show that EBP50 inhibits the phospholipase C (PLC)-beta-mediated inositol phosphate production of a Galpha(q)-coupled receptor as well as PLC-beta activation by the constitutively active Galpha(q)-R183C mutant. Coimmunoprecipitation experiments revealed that EBP50 interacts with Galpha(q) and to a greater extent with Galpha(q)-R183C. Agonist stimulation of the thromboxane A(2) receptor (TP receptor) resulted in an increased interaction between EBP50 and Galpha(q), suggesting that EBP50 preferentially interacts with activated Galpha(q). We also demonstrate that EBP50 inhibits Galpha(q) signaling by preventing the interaction between Galpha(q) and the TP receptor and between activated Galpha(q) and PLC-beta1. Investigation of the EBP50 regions involved in Galpha(q) binding indicated that its two PDZ domains are responsible for this interaction. This study constitutes the first demonstration of an interaction between a G protein alpha subunit and another protein through a PDZ domain, with broad implications in the regulation of diverse physiological systems.