Ligand-independent homomeric and heteromeric complexes between interleukin-2 or -9 receptor subunits and the gamma chain

Signaling via interleukin-2 (IL-2) and interleukin-9 receptors (IL-2R and IL-9R) involves heteromeric interactions between specific interleukin receptor subunits, which bind Janus kinase 1 (JAK1) and the JAK3 binding common gamma chain (gamma c). The potential existence and roles of homomeric and heteromeric complexes before ligand binding and their modulation by ligand and JAK3 are unclear. Using computerized antibody-mediated immunofluorescence co-patching of epitope-tagged receptors at the surface of live cells, we demonstrate that IL-2Rbeta, IL-9Ralpha, and gamma c each display a significant fraction of ligand-independent homomeric complexes (24-28% co-patching), whereas control co-patching levels with unrelated receptors are very low (7%). Heteromeric complex formation of IL2-Rbeta or IL-9Ralpha with gamma c is also observed in the absence of ligand (15-30%). Ligand binding increases this hetero-oligomerization 2-fold but does not affect homo-oligomerization. Co-expression of IL-2Ralpha does not affect the hetero-oligomerization of IL-2Rbeta and gamma c. Recruitment of gamma c into heterocomplexes is partly at the expense of its homo-oligomerization, suggesting that a functional role of the latter may be to keep the receptors inactive in the absence of ligand. At the same time, the preformed complexes between gamma c and IL-2Rbeta or IL-9Ralpha promote signaling by the JAK3 A572V mutant without ligand, supporting a pathophysiological role for the constitutive oligomerization in triggering ligand-independent activation of JAK3 (and perhaps other JAK mutants) mutants identified in several human cancers.     

Distinct impaired regulation of SOCS3 and long and short isoforms of the leptin receptor in visceral and subcutaneous fat of lean and obese women

Animal studies have illustrated the importance of the expression in adipose tissue of the leptin receptor (OB-R), and of SOCS3 an inhibitor of the leptin signaling pathway, in body weight regulation. The aim of the present study was to investigate in human adipose tissues of the same patients the OB-R isoforms and SOCS3 expression. Subcutaneous and omental adipose tissues were obtained from 6 lean and 18 morbidly obese women. The long isoform OB-Rb mRNA mediating leptin signaling, and SOCS3 mRNA are abundantly present in the subcutaneous fat of lean women, but are 90% and 70% decreased (P<0.0001) in obese women. In visceral fat from lean and obese women, both OB-Rb and SOCS3 mRNA are detected at very low levels. Subcutaneous/visceral ratios for OB-Ra the short OB-R isoform, OB-Rb, and SOCS3 mRNA abundance strongly correlate with the insulin sensitivity index, HOMA-% S, (r=0.49, P<0.0001, r=0.42, P=0.0002 and r=0.38, P=0.0002, respectively) in both lean and obese patients without type 2 diabetes. The near absence of OB-Rb mRNA and the similarly decreased SOCS3 expression in obese adipose tissue may reflect a defective leptin signaling pathway that could play a role in the impairment of insulin sensitivity associated with excess adiposity.     

Expression of OB receptor splice variants during prenatal development of the mouse

In adult animals, signaling through the leptin receptor (OB-R) has been shown to play a critical role in fat metabolism. However, it is not known when these receptors are first expressed and what their role may be during embryonic development. To date, at least 6 splice variants of the OB-R have been identified. Although the function of each of these individual splice variants are unknown, only one of them, ob-rL ,encodes a receptor with a long intracellular domain that is implicated in OB-R signaling. In this study we have used in situ hybridization to examine the localization of OB-R splice variants during embryonic development of C57B1/6J mice. Using a probe, ob-r, that recognizes all of the splice variants, ob-r mRNA was found to be distributed in developing bone, mesenchyme, notochord and liver. In addition, epithelial structures including leptomeninges, choroid plexi and hair follicles also expressed ob-r. No ob-r mRNA was detected in the CNS. ob-rL, expression was only detected in notochord, bone and mesenchyme. The differential expression of these two mRNA isoforms suggests that the extracellular and intracellular domains of the OB receptor perform different biological functions.     

Splice variants of the OB receptor gene are differentially expressed in brain and peripheral tissues of mice

A high affinity receptor for OB protein was recently cloned from the choroid plexus of mice. At least six alternatively spliced forms of the OB receptor (OB-R) gene have been described, all of which encode proteins containing the OB-R extracellular domain. One splice variant encodes a receptor with a long intracellular domain, OB-RL, that has been implicated in OB-R signaling. Here, we have used in situ hybridization to examine the localization of OB-R splice variants in brain and peripheral tissues of adult and newborn mice. Using a probe hybridizing with all known splice variants, we confirmed that OB-R mRNA was widely distributed in the adult tissues. In the CNS, choroid plexus was the major site of expression. We now demonstrate that OB-R mRNA is expressed in peripheral tissues; primarily associated with connective tissues. In addition, OB-R mRNA was detected at higher levels in peripheral tissues of newborn mice than in adult mice. With a probe specific for OB-RL, we confirmed that high mRNA expression was detected in hypothalamic nuclei, while low levels were observed in choroid plexus. We now report that in peripheral tissues of adult mice, OB-RL mRNA expression was either very low or undetectable. In newborn mice, the pattern of OB-RL message expression in the CNS was similar to that of adult mice, while bone was the site of highest OB-RL message expression in the peripheral tissue. These data suggest different biological roles for OB-R splice variants encoding the short and long forms of OB-R. The localization of OB-RL to hypothalamic nuclei supports the idea that OB-RL is the brain receptor that mediates OB protein signaling and actions. In addition, the expression of OB-R message in newborn mice also suggests a biological role of OB-R during development in mice.     

Novel level of signalling control in the JAK/STAT pathway revealed by in situ visualisation of protein-protein interaction during Drosophila development

It is commonly accepted that activation of most signalling pathways is induced by ligand receptor dimerisation. This belief has been challenged for some vertebrate cytokine receptors of the JAK/STAT pathway. Here we study whether DOME, the Drosophila receptor of the JAK/STAT pathway, can dimerise and if the dimerisation is ligand-dependent. To analyse DOME homo-dimerisation, we have applied a beta-gal complementation technique that allows the detection of protein interactions in situ. This technique has been used previously in cell culture but this is the first time that it has been applied to whole embryos. We show that this technique, which we rename betalue-betalau technique, can be used to detect DOME homo-dimerisation in Drosophila developing embryos. Despite DOME being ubiquitously expressed, dimerisation is developmentally regulated. We investigate the state of DOME dimerisation in the presence or absence of ligand and show that DOME dimerisation is not ligand-induced, indicating that ligand independent cytokine receptor dimerisation is a conserved feature across phyla. We have further analysed the functional significance of ligand-independent receptor dimerisation by comparing the effects of ectopic ligand expression in cells in which the receptor is, or is not, dimerised. We show that ligand expression can only activate STAT downstream targets or affect embryo development in cells in which the receptor is dimerised. These results suggest a model in which ligand-independent dimerisation of the JAK/STAT receptor confers cells with competence to activate the pathway prior to ligand reception. Thus, competence to induce the JAK/STAT signalling pathway in Drosophila can be regulated by controlling receptor dimerisation prior to ligand binding. These results reveal a novel level of JAK/STAT signalling regulation that could also apply to vertebrates.     

Constitutively active homo-oligomeric angiotensin II type 2 receptor induces cell signaling independent of receptor conformation and ligand stimulation

Members of the G-protein-coupled receptor superfamily (GPCRs) undergo homo- and/or hetero-oligomerization to induce cell signaling. Although some of these show constitutive activation, it is not clear how such GPCRs undergo homo-oligomerization with transmembrane helix movement. We previously reported that angiotensin II (Ang II) type 2 (AT(2)) receptor, a GPCR, showed constitutive activation and induced apoptosis independent of its ligand, Ang II. In the present study, we analyzed the translocation and oligomerization of the AT(2) receptor with transmembrane movement when the receptor induces cell signaling. Constitutively active homo-oligomerization, which was due to disulfide bonding between Cys(35) in one AT(2) receptor and Cys(290) in another AT(2) receptor, was localized in the cell membrane without Ang II stimulation and induced apoptosis without changes in receptor conformation. These results provide the direct evidence that the constitutively active homo-oligomeric GPCRs by intermolecular interaction in two extracellular loops is translocated to the cell membrane and induces cell signaling independent of receptor conformation and ligand stimulation.     

Expression of Ob Receptor Splice Variants During Prenatal Development of the Mouse

Abstract

In adult animals, signaling through the leptin receptor (OB-R) has been shown to play a critical role in fat metabolism. However, it is not known when these receptors are first expressed and what their role may be during embryonic development. to date, at least 6 splice variants of the OB-R have been identified. Although the function of each of these individual splice variants are unknown, only one of them, ob-r_L, encodes a receptor with a long intracellular domain that is implicated in OB-R signaling. in this study we have used in situ hybridization to examine the localization of OB-R splice variants during embryonic development of C57B1/6J mice. Using a probe, ob-r, that recognizes all of the splice variants, ob-r mRNA was found to be distributed in developing bone, mesenchyme, notochord and liver. in addition, epithelial structures including leptomeninges, choroid plexi and hair follicles also expressed ob-r No ob-r mRNA was detected in the CNS. ob-r_L, expression was only detected in notochord, bone and mesenchyme. the differential expression of these two mRNA isoforms suggests that the extracellular and intracellular domains of the OB receptor perform different biological functions.     

Dopamine D2 receptor isoforms expressed in AtT20 cells differentially couple to G proteins to acutely inhibit high voltage-activated calcium channels

The dopamine D2 receptor belongs to the serpentine superfamily of receptors, which have seven transmembrane segments and activate G proteins. D2 receptors are known to be linked, through Galpha(o)- and Galpha(i)-containing G proteins, to several signaling pathways in neuronal and secretory cells, including inhibition of adenylyl cyclase and high voltage-activated Ca2+ channels (HVA-CCs). The dopamine D2 receptor exists in two alternatively spliced isoforms, "long" and "short" (D2L, and D2S, respectively), which have identical ligand binding sites but differ by 29 amino acids in the third intracellular loop, the proposed site for G protein interaction. This has led to the speculation that the two isoforms may interact with different G proteins. We have transfected the AtT20 cell line with either D2L (KCL line) or D2S (KCS line) to facilitate experimentation on the individual isoforms. Both lines show dopamine agonist-dependent inhibition of Q-type HVA-CCs. We combined G protein antisense knock-down studies with multiwavelength fluorescence video microscopy to measure changes in HVA-CC inhibition to investigate the possibility of differential G protein coupling to this inhibition. The initial, rapid, K+ depolarization-induced increase in intracellular Ca2+ concentration is due to influx through HVA-CCs. Our studies reveal that both D2 isoforms couple to Galpha(o) to partially inhibit this influx. However, D2L also couples to Galpha(i)3, whereas D2S couples to Galpha(i)2. These data support the hypothesis of differential coupling of D2 receptor isoforms to G proteins.     

The N-terminal domain of Janus kinase 2 is required for Golgi processing and cell surface expression of erythropoietin receptor.

We show that Janus kinase 2 (JAK2), and more specifically just its intact N-terminal domain, binds to the erythropoietin receptor (EpoR) in the endoplasmic reticulum and promotes its cell surface expression. This interaction is specific as JAK1 has no effect. Residues 32 to 58 of the JAK2 JH7 domain are required for EpoR surface expression. Alanine scanning mutagenesis of the EpoR membrane proximal region reveals two modes of EpoR-JAK2 interaction. A continuous block of EpoR residues is required for functional, ligand-independent binding to JAK2 and cell surface receptor expression, whereas four specific residues are essential in switching on prebound JAK2 after ligand binding. Thus, in addition to its kinase activity required for cytokine receptor signaling, JAK is also an essential subunit required for surface expression of cytokine receptors.     

Characterization of a novel and functional human prolactin receptor isoform (deltaS1PRLr) containing only one extracellular fibronectin-like domain

Prolactin (PRL)-dependent signaling occurs as the result of ligand-induced homodimerization of the PRL receptor (PRLr). To date, short, intermediate, and long human PRLr isoforms have been characterized. To investigate the expression of other possible human PRLr isoforms, RT-PCR was performed on mRNA isolated from the breast carcinoma cell line T47D. A 1.5-kb PCR fragment was isolated, subcloned, and sequenced. The PCR product exhibited a nucleotide sequence 100% homologous to the human long isoform except bp 71-373 were deleted, which code for the S1 motif of the extracellular domain. Therefore, this isoform was designated the deltaS1 PRLr. Northern analysis revealed variable deltaS1 PRLr mRNA expression in a variety of tissues. Transfection of Chinese hamster ovary cells with deltaS1 cDNA showed the isoform is expressed at the protein level on the cell surface with a molecular mass of approximately 70 kDa. Kinetic studies indicated the deltaS1 isoform bound ligand at a lower affinity than wild-type receptor. The deltaS1 PRLr was also shown to activate the proximal signaling molecule Jak2 upon addition of ligand to transfected cells, and, unlike the long PRLr, high concentrations of ligand did not function as a self-antagonist to signaling during intervals of PRL serum elevation, i.e. stress and pregnancy. Given its apparent widespread expression, this PRLr isoform may contribute to PRL action. Furthermore, the functionality of this receptor raises interesting questions regarding the minimal extracellular domain necessary for ligand-induced receptor signaling.     

Expression of leptin receptor isoforms and effects of leptin on the proliferation and hormonal secretion in human pituitary adenomas

OBJECTIVE: To pursue whether leptin regulates anterior pituitary cells, we studied the ex vivo expression of several isoforms of the leptin receptor (OB-R) as well as the in vitro effects of leptin administration in human pituitary adenomas. METHODS: OB-R mRNA expression and in vitro response to leptin were studied in 39 pituitary macroadenomas. RESULTS: All 4 OB-R subtypes were expressed in most adenomas. The expression was significantly more pronounced in GH-secreting adenomas as compared to non-functioning tumor cells (p < 0.05). Leptin administration in vitro did not significantly influence cell proliferation or the secretion of GH, FSH, LH or alpha-subunit. CONCLUSIONS: (1) Several isoforms of the OB-R, including the signal transducing full-length receptor, are expressed in most human pituitary adenomas. (2) This expression ex vivo is not associated with significant effects of leptin in vitro.     

Constitutive oligomerization of human D2 dopamine receptors expressed in Spodoptera frugiperda 9 (Sf9) and in HEK293 cells. Analysis using co-immunoprecipitation and time-resolved fluorescence resonance energy transfer.

Human D2Long (D2L) and D2Short (D2S) dopamine receptor isoforms were modified at their N-terminus by the addition of a human immunodeficiency virus (HIV) or a FLAG epitope tag. The receptors were then expressed in Spodoptera frugiperda 9 (Sf9) cells using the baculovirus system, and their oligomerization was investigated by means of co-immunoprecipitation and time-resolved fluorescence resonance energy transfer (FRET). [3H]Spiperone labelled D2 receptors in membranes prepared from Sf9 cells expressing epitope-tagged D2L or D2S receptors, with a pKd value of approximately 10. Co-immunoprecipitation using antibodies specific for the tags showed constitutive homo-oligomerization of D2L and D2S receptors in Sf9 cells. When the FLAG-tagged D2S and HIV-tagged D2L receptors were co-expressed, co-immunoprecipitation showed that the two isoforms can also form hetero-oligomers in Sf9 cells. Time-resolved FRET with europium and XL665-labelled antibodies was applied to whole Sf9 cells and to membranes from Sf9 cells expressing epitope-tagged D2 receptors. In both cases, constitutive homo-oligomers were revealed for D2L and D2S isoforms. Time-resolved FRET also revealed constitutive homo-oligomers in HEK293 cells expressing FLAG-tagged D2S receptors. The D2 receptor ligands dopamine, R-(-)propylnorapomorphine, and raclopride did not affect oligomerization of D2L and D2S in Sf9 and HEK293 cells. Human D2 dopamine receptors can therefore form constitutive oligomers in Sf9 cells and in HEK293 cells that can be detected by different approaches, and D2 oligomerization in these cells is not regulated by ligands.     

Cooperation of JAK/STAT and Notch signaling in the Drosophila foregut

Temporal and spatial regulation of morphogenesis is pivotal to the formation of organs from simple epithelial tubes. In a genetic screen for novel genes controlling cell movement during posterior foregut development, we have identified and molecularly characterized two alleles of the domeless gene which encodes the Drosophila Janus kinase (JAK)/STAT receptor. We demonstrate that mutants for domeless or any other known component of the canonical JAK/STAT signaling pathway display a failure of coordinated cell movement during the development of the proventriculus, a multiply folded organ which is formed by stereotyped cell rearrangements in the posterior foregut. Whereas the JAK/STAT receptor is expressed in all proventricular precursor cells, expression of upd encoding its ligand and of STAT92E, the signal transducer of the pathway, is locally restricted to cells that invaginate during proventriculus development. We demonstrate by analyzing gene expression mediated by a model Notch response element and by studying the expression of the Notch target gene short stop, which encodes a cytoskeletal crosslinker protein, that JAK/STAT signaling is required for the activation of Notch-dependent gene expression in the foregut. Our results provide strong evidence that JAK/STAT and Notch signaling cooperate in the regulation of target genes that control epithelial morphogenesis in the foregut.     

Differential signaling of dopamine-D2S and -D2L receptors to inhibit ERK1/2 phosphorylation

Although they have distinct functions, the signaling of dopamine-D(2) receptor short and long isoforms (D(2)S and D(2)L) is virtually identical. We compared inhibitory regulation of extracellular signal-regulated kinases (ERK1/2) in GH4 pituitary cells separately transfected with these isoforms. Activation of rat or human dopamine-D(2)S, muscarinic or somatostatin receptors inhibited thyrotropin-releasing hormone-induced ERK1/2 phosphorylation, while the D(2)L receptor failed to inhibit this response. In order to address the structural basis for the differential signaling of D(2)S and D(2)L receptors, we examined the D(2)L-SS mutant, in which a protein kinase C (PKC) pseudosubstrate site that is present in the D(2)L but not D(2)S receptor was converted to a consensus PKC site. In transfected GH4 cells, the D(2)L-SS mutant inhibited thyrotropin-releasing hormone-induced ERK1/2 phosphorylation almost as strongly as the D(2)S receptor. A D(2)S-triple mutant that eliminates PKC sites involved in D(2)S receptor desensitization also inhibited ERK1/2 activation. Similarly, in striatal cultures, the D(2)-selective agonist quinpirole inhibited potassium-stimulated ERK1/2 phosphorylation, indicating the presence of this pathway in neurons. In conclusion, the D(2)S and D(2)L receptors differ in inhibitory signaling to ERK1/2 due to specific residues in the D(2)L receptor alternatively spliced domain, which may account for differences in their function in vivo.     

Ligand-independent dimerization of the human prolactin receptor isoforms: functional implications

Prolactin (PRL) contributes to the growth of normal and malignant breast tissues. PRL initiates signaling by engaging the PRL receptor (PRLr), a transmembrane (TM) receptor belonging to the cytokine receptor family. The accepted view has been that PRL activates the PRLr by inducing dimerization of the receptor, but recent reports show ligand-independent dimerization of other cytokine receptors. Using coimmunoprecipitation assays, we have confirmed ligand-independent dimerization of the PRLr in T47D breast cancer and HepG2 liver carcinoma cells. In addition, mammalian cells transfected with differentially epitope-tagged isoforms of the PRLr indicated that long, intermediate, and DeltaS1 PRLrs dimerized in a ligand-independent manner. To determine the domain(s) involved in PRLr ligand-independent dimerization, we generated PRLr constructs as follows: (1) the TM-ICD, which consisted of the TM domain and the intracellular domain (ICD) but lacked the extracellular domain (ECD), and (2) the ECD-TM, which consisted of the TM domain and the ECD but lacked the ICD. These constructs dimerized in a ligand-independent manner in mammalian cells, implicating a significant role for the TM domain in this process. These truncated PRLrs were functionally inert alone or in combination in cells lacking the PRLr. However, when introduced into cells containing endogenous PRLr, the ECD-TM inhibited human PRLr signaling, whereas the TM-ICD potentiated human PRLr signaling. These studies indicate that the ECD-TM and the TM-ICD are capable of modulating PRLr function. We also demonstrated an endogenous TM-ICD in T47D cells, suggesting that these findings are relevant to PRL-signaling pathways in breast cancer.     

Characterisation and application of antibodies specific for the long platelet-derived growth factor A and B chains

The platelet-derived growth factor (PDGF) family comprises important mitogens for mesenchymal cells. The active dimeric form of PDGF consists of four structurally related A, B, C, and D chains. All PDGF-variants bind to PDGF-receptors. The A and B chains occur with and without basic C-terminal amino acid extensions as long (A(L) and B(L)) and short (A(S) and B(S)) isoforms. PDGF-A and -B form homo- or heterodimers. The biological relevance of short and long isoforms is unknown, although it may relate to different affinities for glycosaminoglycans of the cell glycocalix and intercellular matrix. Commercially available anti-PDGF-A and anti-PDGF-B antibodies cannot discriminate between the short and the long isoforms. Thus, to investigate the function of the long and short isoforms, we raised antibodies specific for the long A and B chain isoforms. The antibodies were affinity-purified and their properties analysed by surface plasmon resonance. Inhibition studies with different PDGF homodimers and dot-blot studies proved their high specificity for the respective isoforms. Both antibodies recognised the target PDGF homodimers complexed to the glycocalix of human arterial smooth muscle cells and human monocyte-derived macrophages. By using these specific antibodies, we were able to confirm at the protein level the synthesis of PDGF-A and -B during differentiation of human monocyte-derived macrophages and to demonstrate the presence of the PDGF-A(L) and PDGF-B(L) isoforms in human arterial tissue.     

Unmodified prolactin (PRL) and S179D PRL-initiated bioluminescence resonance energy transfer between homo- and hetero-pairs of long and short human PRL receptors in living human cells

We have used bioluminescence resonance energy transfer (BRET) to examine the interaction between human prolactins (PRLs) and the long (LF) and two short isoforms (SF1a and SF1b) of the human PRL receptor in living cells. cDNA sequences encoding the LF, SF1a, and SF1b were subcloned into codon-humanized vectors containing cDNAs for either Renilla reniformis luciferase (Rluc) or a green fluorescent protein (GFP(2)) with a 12- or 13-amino acid linker connecting the parts of the fusion proteins. Transfection into human embryonic kidney 293 cells demonstrated maintained function of Rluc and GFP(2) when linked to the receptors, and confocal microscopy demonstrated the localization of tagged receptors in the plasma membrane by 48 h after transfection. All three tagged receptors transduced a signal, with the LF and SF1a stimulating, and SF1b inhibiting, promoter activity of an approximately 2.4-kb beta-casein-luc construct. Both unmodified PRL (U-PRL) and the molecular mimic of phosphorylated PRL, S179D PRL, induced BRET with all combinations of long and short receptor isoforms except SF1a plus SF1b. No BRET was observed with the site two-inactive mutant, G129R PRL. This is the first demonstration, 1) that species homologous PRL promotes both homo- and hetero-interaction of most long and short PRLR pairs in living cells, 2) that both U-PRL and S179D PRL are active in this regard, and 3) that there is some aspect of SF1a-SF1b structure that prevents this particular hetero-receptor pairing. In addition, we conclude that preferential pairing of different receptor isoforms is not the explanation for the different signaling initiated by U-PRL and S179D PRL.     

Agonist-independent and -dependent oligomerization of dopamine D(2) receptors by fusion to fluorescent proteins.

Oligomerization of the short (D(2S)) and long (D(2L)) isoforms of the dopamine D(2) receptor was explored in transfected Cos-7 cells by their C-terminal fusion to either an enhanced cyan or enhanced yellow fluorescent protein (ECFP or EYFP) and the fluorescent fusion protein interaction was monitored by a fluorescence resonance energy transfer (FRET) assay. The pharmacological properties of the fluorescent fusion proteins, as measured by both displacement of [(3)H]nemonapride binding and agonist-mediated stimulation of [(35)S]GTPgammaS binding upon co-expression with a G(alphao)Cys(351)Ile protein, were not different from the respective wild-type D(2S) and D(2L) receptors. Co-expression of D2S:ECFP+D2S:EYFP in a 1:1 ratio and D2L:ECFP+D2L:EYFP in a 27:1 ratio resulted, respectively, in an increase of 26% and 16% in the EYFP-specific fluorescent signal. These data are consistent with a close proximity of both D(2S) and D(2L) receptor pairs of fluorescent fusion proteins in the absence of ligand. The agonist-independent D(2S) receptor oligomerization could be attenuated by co-expression with either a wild-type, non-fluorescent D(2S) or D(2L) receptor subtype, but not with a distinct beta(2)-adrenoceptor. Incubation with the agonist (-)-norpropylapomorphine dose-dependently (EC(50): 0.23+/-0.06 nM) increased the FRET signal for the co-expression of D2S:ECFP and D2S:EYFP, in support of agonist-dependent D(2S) receptor oligomerization. In conclusion, our data strongly suggest the occurrence of dopamine D(2) receptor oligomers in intact Cos-7 cells.