Protein Coupling Specificity Research Articles

Signalling of Adrenoceptors: Canonical Pathways and New Paradigms.

The concept of G protein-coupled receptors initially arose from studies of the β-adrenoceptor, adenylyl cyclase, and cAMP signalling pathway. Since then both canonical G protein-coupled receptor signalling pathways and emerging paradigms in receptor signalling have been defined by experiments focused on adrenoceptors. Here, we discuss the evidence for G protein coupling specificity of the nine adrenoceptor subtypes. We summarise the ability of each of the adrenoceptors to activate proximal signalling mediators including cAMP, calcium, mitogen-activated protein kinases, and protein kinase C pathways. Finally, we highlight the importance of precise spatial and temporal control of adrenoceptor signalling that is controlled by the localisation of receptors at intracellular membranes and in larger protein complexes.

Functional profiling of the G protein-coupled receptor C3aR1 reveals ligand-mediated biased agonism

G protein-coupled receptors (GPCRs) are leading druggable targets for several medicines, but many GPCRs are still untapped for their therapeutic potential due to poor understanding of specific signaling properties. The complement C3a receptor 1 (C3aR1) has been extensively studied for its physiological role in C3a-mediated anaphylaxis/inflammation, and in TLQP-21-mediated lipolysis, but direct evidence for the functional relevance of the C3a and TLQP-21 ligands and signal transduction mechanisms are still limited. In addition, C3aR1 G protein coupling specificity is still unclear, and whether endogenous ligands, or drug-like compounds, show ligand-mediated biased agonism is unknown. Here, we demonstrate that C3aR1 couples preferentially to Gi/o/z proteins and can recruit β-arrestins to cause internalization. Furthermore, we showed that in comparison to C3a63-77, TLQP-21 exhibits a preference toward Gi/o-mediated signaling compared to β-arrestin recruitment and internalization. We also show that the purported antagonist SB290157 is a very potent C3aR1 agonist, where antagonism of ligand-stimulated C3aR1 calcium flux is caused by potent β-arrestin-mediated internalization. Finally, ligand-mediated signaling bias impacted cell function as demonstrated by the regulation of calcium influx, lipolysis in adipocytes, phagocytosis in microglia, and degranulation in mast cells. Overall, we characterize C3aR1 as a Gi/o/z-coupled receptor, and demonstrate functional relevance of ligand-mediated signaling bias in key cellular models. Due to C3aR1 and its endogenous ligands being implicated in inflammatory and metabolic diseases, these results are of relevance toward future C3aR1 drug discovery.

Structural genomics of the human dopamine receptor system.

The dopaminergic system, including five dopamine receptors (D1R to D5R), plays essential roles in the central nervous system (CNS); and ligands that activate dopamine receptors have been used to treat many neuropsychiatric disorders, including Parkinson's Disease (PD) and schizophrenia. Here, we report cryo-EM structures of all five subtypes of human dopamine receptors in complex with G protein and bound to the pan-agonist, rotigotine, which is used to treat PD and restless legs syndrome. The structures reveal the basis of rotigotine recognition in different dopamine receptors. Structural analysis together with functional assays illuminate determinants of ligand polypharmacology and selectivity. The structures also uncover the mechanisms of dopamine receptor activation, unique structural features among the five receptor subtypes, and the basis of G protein coupling specificity. Our work provides a comprehensive set of structural templates for the rational design of specific ligands to treat CNS diseases targeting the dopaminergic system.

Abstract 2868: A chemogenetic approach reveals a GPCR-Gαs-PKA signaling axis promoting T cell dysfunction and cancer immunotherapy failure

Abstract Recent advances in immune checkpoint blockade (ICB) inhibiting programmed death-1 (PD-1) and cytotoxic T-lymphocyte-associated protein (CTLA-4) have revolutionized the standard of care for cancer treatment. However, the limited response rates to ICB across multiple cancer types suggest that new approaches and targets are clearly needed to fully elucidate the underlying biology of dysfunctional and exhausted CD8 T cells in cancer in order to achieve durable responses (cure). G protein-coupled receptors (GPCRs) are the most intensively studied drug targets, primarily due to their druggability and relevance to most physiological processes and disease conditions. However, the role of GPCRs in cancer and particularly in cancer immunology has been largely underexplored. We have applied a new computational pipeline to cross-integrate hundreds of thousands of CD8 T cells from multiple single cell RNA-seq datasets from 13 distinct cancer types. Using this approach, we identified many GPCRs that are overrepresented in exhausted CD8 T cells. A new computational strategy enabled us to predict the G protein coupling specificity of most GPCRs. This led to the discovery that Gαs-coupled GPCRs are significantly enriched in exhausted CD8 T cells. These include EP2, EP4, A2AR, β1AR, and β2AR, and we found that all of these receptors promote CD8 T cell dysfunction by inhibiting cytotoxicity and cytokine secretion. However, these receptors are also expressed in other immune cells within the tumor microenvironment. To address the direct impact of these receptors on T cell function we used a novel synthetic biology approach, involving the development of a chemogenetic CD8-restricted Gαs-DREADD (Designer Receptor Exclusively Activated by A Designer Drug) transgenic mouse model in which activation of Gαs signaling is temporally and spatially controlled. Utilizing this Gαs-DREADD model, we discovered that the Gαs-signaling axis represents a previously uncharacterized signaling axis that dampens the anti-tumor CD8 T cell activity and leads to ICB immunotherapy failure. Our central hypothesis is that secretion of GPCR ligands in the tumor microenvironment and their actions on CD8 T cells lead to the activation of the Gαs signaling cascade leading to T cell dysfunction, by decreasing cytotoxic and migratory activity that abolished the effectiveness of ICB. Our findings reveal that Gαs-coupled GPCRs may represent new targetable immune checkpoints that can be combined with ICB as part of novel multimodal precision approaches to enhance the response to immunotherapies. Citation Format: Bryan S. Yung, Victoria H. Wu, Farhoud Faraji, Robert Saddawi-Konefka, Zhiyong Wang, Alexander T. Wenzel, Miranda Song, Meghana S. Pagadala, Lauren M. Clubb, Joshua Chiou, Sanju Sinha, Marin Matic, Francesco Raimondi, Thomas Hoang, Rebecca Berdeaux, Dario A. Vignali, Ramiro Iglesias-Bartolome, Hannah Carter, Eytan Ruppin, Jill P. Mesirov, J. Silvio Gutkind. A chemogenetic approach reveals a GPCR-Gαs-PKA signaling axis promoting T cell dysfunction and cancer immunotherapy failure [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 2868.

Analysis of β2AR-Gs and β2AR-Gi complex formation by NMR spectroscopy

The β2-adrenergic receptor (β2AR) is a prototypical G protein-coupled receptor (GPCR) that preferentially couples to the stimulatory G protein Gs and stimulates cAMP formation. Functional studies have shown that the β2AR also couples to inhibitory G protein Gi, activation of which inhibits cAMP formation [R. P. Xiao, Sci. STKE 2001, re15 (2001)]. A crystal structure of the β2AR-Gs complex revealed the interaction interface of β2AR-Gs and structural changes upon complex formation [S. G. Rasmussen et al., Nature 477, 549-555 (2011)], yet, the dynamic process of the β2AR signaling through Gs and its preferential coupling to Gs over Gi is still not fully understood. Here, we utilize solution nuclear magnetic resonance (NMR) spectroscopy and supporting molecular dynamics (MD) simulations to monitor the conformational changes in the G protein coupling interface of the β2AR in response to the full agonist BI-167107 and Gs and Gi1 These results show that BI-167107 stabilizes conformational changes in four transmembrane segments (TM4, TM5, TM6, and TM7) prior to coupling to a G protein, and that the agonist-bound receptor conformation is different from the G protein coupled state. While most of the conformational changes observed in the β2AR are qualitatively the same for Gs and Gi1, we detected distinct differences between the β2AR-Gs and the β2AR-Gi1 complex in intracellular loop 2 (ICL2). Interactions with ICL2 are essential for activation of Gs These differences between the β2AR-Gs and β2AR-Gi1 complexes in ICL2 may be key determinants for G protein coupling selectivity.

Structural Insights into the Process of GPCR-G Protein Complex Formation

The crystal structure of the β2-adrenergic receptor (β2AR) bound to the G protein adenylyl cyclase stimulatory G protein (Gs) captured the complex in a nucleotide-free state (β2AR-Gsempty). Unfortunately, the β2AR-Gsempty complex does not provide a clear explanation for G protein coupling specificity. Evidence from several sources suggests the existence of a transient complex between the β2AR and GDP-bound Gs protein (β2AR-GsGDP) that may represent an intermediate on the way to the formation of β2AR-Gsempty and may contribute to coupling specificity. Here we present a structure of the β2AR in complex with the carboxyl terminal 14 amino acids from Gαs along with the structure of the GDP-bound Gs heterotrimer. These structures provide evidence for an alternate interaction between the β2AR and Gs that may represent an intermediate that contributes to Gs coupling specificity.

Mechanism of Specific G Protein Coupling to Adenosine Receptors

Four subtypes of adenosine receptors (ARs), the A1AR, A2AAR, A2BAR and A3AR, mediate a broad range of physiological functions. They have emerged as important therapeutic targets for treating human diseases including cardiac ischemia, neuropathic pain and cancer. The odd-numbered subtypes prefer to couple with the Gi/o proteins, while the even-numbered subtypes predominantly couple with the Gs proteins. Despite extensive experimental and computational studies, the exact determinants of ARs-G protein coupling specificity remain unclear. Here, all-atom enhanced sampling simulations using a robust Gaussian accelerated molecular dynamics (GaMD) method have been applied to determine the mechanism of specific coupling between the A1AR and A2AAR and the Gi and Gs proteins, respectively. The GaMD simulations have revealed differentiated fluctuations and distinct low-energy conformational states of the ARs upon binding of decoy versus cognate G proteins. Important motifs that are responsible for the receptor-G protein coupling specificity have been also identified. In addition, our GaMD simulation findings are consistent with recent experimental data. The GaMD simulations have provided important insights into the mechanism of specific coupling between ARs and the G proteins at an atomistic level, which will facilitate future computer-aided design of selective drug molecules targeting the ARs.

Structure of the µ-opioid receptor-Gi protein complex.

The μ opioid receptor (μOR) is a G protein-coupled receptor (GPCR) that is the target of most clinically and recreationally used opioids. The induced positive effects of analgesia and euphoria are mediated by μOR signaling through the adenylyl cyclase-inhibiting heterotrimeric G protein Gi. We present the 3.5-Å resolution cryo-electron microscopy (cryo-EM) structure of the μOR bound to the agonist peptide DAMGO and nucleotide-free Gi. DAMGO occupies the morphinan ligand pocket, with its N-terminus interacting with conserved receptor residues while its C-terminus engages regions important for opioid ligand selectivity. Comparison of the μOR-Gi complex to previously determined structures of other GPCRs bound to the stimulatory G protein Gs reveals differences in the movement of transmembrane receptor helix 6 and in the interactions between the G protein α subunit and the receptor core. Together, these results shed light into the structural features that contribute to the Gi protein coupling specificity of the μOR.

Decoding the Signaling through Homomeric and Heteromeric Dopamine D2 and Cannabinioid CB1 Receptors

G protein-coupled receptors (GPCRs) play critical roles in signal transduction and represent around half of drug targets. Accumulating evidence supports that GPCRs form both homomers and heteromers. Utilizing electron microscopic immunocytochemistry, cannabinoid 1 (CB1) receptors were found to co-localize with dopamine 2 (D2) receptors in the somata and dendrites of striato-pallidal GABA neurons. Fluorescence resonance energy transfer and co-immunoprecipitation studies have provided evidence for heteromerization of D2 and CB1 receptors. In addition, the D2-CB1 receptor complex shows an interesting switching behavior of its G protein coupling specificity from Gi/o to Gs signaling. Both homomeric D2 and CB1 receptors are preferentially Gi/o coupled and decrease cAMP levels induced by forskolin stimulation. However, application of CB1 receptor agonist in a cell line co-expressing D2 and CB1 receptors increased cAMP levels, consistent with Gs signaling.Electrophysiological assays using GIRK channels as reporters of Gs activity corroborated the switching behavior of the CB1 receptor, although not reproducibly. We sought additional assays to study the switch in signaling specificity of CB1 receptors in CB1-D2 complexes. G alpha 16 belongs to the Gq superfamily and its activation leads to phospholipase C activation and Ca2+ release. We constructed G alpha 16 chimeras in which the last 11 or 30 amino acids were replaced with the corresponding sequence of Go and Gs. HEK 293 cells were transfected transiently with D2 or CB1 receptor and chimeric G alpha 16 cDNA and calcium responses were monitored by Fluo4-AM in a FlexStation. Stimulation of the D2 receptor showed increases in intracellular calcium levels in cells co-transfected with the D2 receptor and G16/o chimeras but not with G16/s or G16 wt. Signaling through the CB1 receptor and D2-CB1 receptor complex is currently being tested.

Constitutive Gαi Coupling Activity of Very Large G Protein-coupled Receptor 1 (VLGR1) and Its Regulation by PDZD7 Protein

The very large G protein-coupled receptor 1 (VLGR1) is a core component in inner ear hair cell development. Mutations in the vlgr1 gene cause Usher syndrome, the symptoms of which include congenital hearing loss and progressive retinitis pigmentosa. However, the mechanism of VLGR1-regulated intracellular signaling and its role in Usher syndrome remain elusive. Here, we show that VLGR1 is processed into two fragments after autocleavage at the G protein-coupled receptor proteolytic site. The cleaved VLGR1 β-subunit constitutively inhibited adenylate cyclase (AC) activity through Gαi coupling. Co-expression of the Gαiq chimera with the VLGR1 β-subunit changed its activity to the phospholipase C/nuclear factor of activated T cells signaling pathway, which demonstrates the Gαi protein coupling specificity of this subunit. An R6002A mutation in intracellular loop 2 of VLGR1 abolished Gαi coupling, but the pathogenic VLGR1 Y6236fsx1 mutant showed increased AC inhibition. Furthermore, overexpression of another Usher syndrome protein, PDZD7, decreased the AC inhibition of the VLGR1 β-subunit but showed no effect on the VLGR1 Y6236fsx1 mutant. Taken together, we identified an independent Gαi signaling pathway of the VLGR1 β-subunit and its regulatory mechanisms that may have a role in the development of Usher syndrome.

G-Protein-gekoppelte Designerrezeptoren

The signalling via G protein-coupled receptor (GPCR) stimulation is complex, as a particular GPCR may activate more than one G protein. A ligand can also activate several GPCRs with different G protein coupling specificity. To reduce the complexity of GPCR signalling, “G protein coupled designer receptors exclusively activated by designer drugs” (DREADDs) have been developed that can be activated by the compound clozapine-N-oxide. These designer receptors are powerful chemical-genetic tools to investigate the physiological functions of G protein-coupled receptors in vivo.

Regulation of G protein-coupling specificity via cis and trans activation of the LH/chorionic gonadotrophin receptor (LHCGR)

Regulation of G protein-coupling specificity via cis and trans activation of the LH/chorionic gonadotrophin receptor (LHCGR)

Molecular characterization of prostaglandin F receptor (FP) and E receptor subtype 1 (EP1) in zebrafish

Prostaglandins E (PGE) and F (PGF) mediate diverse physiological functions via their cell surface receptors – prostaglandin E receptor (EP) subtypes 1, 2, 3 and 4 (EP1; EP2; EP3; EP4) and F receptor (FP). In teleost fishes, PGE was implicated in gill epithelium ion transport, while both PGE and PGF were involved in oocyte maturation, follicular rupture and coordination of reproductive behaviors. However, little is known about the mechanisms behind their actions. In present study, we first identified the full-length ORF cDNA clones of three zebrafish prostaglandin E receptor subtype 1 (zEP1) isoforms – zEP1a, zEP1b and zEP1c – and FP (zFP) from adult ovary. RT-PCR showed that zEP1a, zEP1b and zFP are widely expressed in adult tissues, while zEP1c mRNA expression is mainly confined in brain and kidney. Using a pGL3-NFAT-RE luciferase reporter system, both zEP1a and zEP1b expressed in DF-1 cells were shown to be activated by PGE2 potently while zEP1c and zFP were activated by PGF2a effectively, suggesting that the four receptors are functionally coupled to intracellular Ca2+-signaling pathway. Furthermore, EP1a and EP1b, but not EP1c were suggested to couple to cAMP-PKA signaling pathway using a pGL3-CRE luciferase reporter assay. Although zEP1c might originate as a paralog to zEP1a and zEP1b, its functional coupling to PGF2α instead of PGE2 suggested that zEP1 isoforms might have sub-functionalized in their ligand binding and G protein coupling specificity, in addition to differential tissue distribution. Characterization of these receptors undoubtedly furthered our understanding on the diverse yet highly target-specific responses of prostaglandins in teleosts.

G Protein Coupling and Ligand Selectivity of the D2Land D3Dopamine Receptors

The human dopamine D(2L) receptor couples promiscuously to multiple members of the Galpha(i/o) subfamily. Despite the high homology of the D(2L) and D(3) receptors, the G protein coupling specificity of the human D(3) receptor is less clearly characterized. The primary aim of this study, then, was the parallel characterization of the G protein coupling specificity of the D(2L) and D(3) receptors. By using both receptor-G protein fusion proteins and stable cell lines in which pertussis toxin-resistant mutants of individual Galpha(i)-family G proteins were expressed in an inducible fashion, we demonstrated highly selective coupling of the D(3) receptor to Galpha(o1). Furthermore, by using the fusion proteins to ensure identical stoichiometry of receptor to G protein for each pairing, a range of ligands displayed higher potency and, for partial agonists, higher efficacy at the D(3) receptor when coupled to Galpha(o1) compared with the D(2L) receptor. The second aim of this study was to investigate the molecular basis of the above differential G protein coupling specificity. The importance of a 12-amino acid sequence from the C-terminal end of the third intracellular loop of the D(2L) receptor in providing promiscuity in G protein coupling was demonstrated using a chimeric D(3)/D(2) receptor in which the equivalent region of the D(3) receptor was exchanged for this sequence. This chimera displayed D(3)-like affinity for [(3)H]spiperone and potency for agonists but gained D(2)-like ability to couple to each of Galpha(i1-3) as well as Galpha(o1).

Multiple G‐protein‐coupling specificity of β‐adrenoceptor in macrophages

Adrenergic signalling of the immune system is one of the important modulator pathways of the inflammatory immune response realized via G protein-mediated pathways. The resulted signal depends on the type of the receptor-coupled G-protein (GPCR) that, according to the classical paradigm in the case of beta-adrenergic receptor (beta-AR), is Gs-type. Recently, alternate and/or multiple G protein coupling specificity of GPCRs have been demonstrated including a switch from Gs to Gi binding. The possibility of a Gs/Gi switch and its role in the immune response of macrophages has not been investigated yet. In this study, we demonstrate that beta-adrenergic stimulation itself is able to induce a transient mitogen-activated protein kinase phosphorylation in murine peritoneal macrophages in a pertussis toxin-sensitive manner, suggesting that the Gs/Gi switch also occurs in the immune system. Although this process is very rapid, it can influence different signalling pathways and can reprogramme effector functions suggesting that sympathetic modulation of the defence mechanism of the innate immune system has an additional, Gs/Gi switch-dependent component.

Prediction of GPCR-G Protein Coupling Specificity Using Features of Sequences and Biological Functions

Understanding the coupling specificity between G protein-coupled receptors (GPCRs) and specific classes of G proteins is important for further elucidation of receptor functions within a cell. Increasing information on GPCR sequences and the G protein family would facilitate prediction of the coupling properties of GPCRs. In this study, we describe a novel approach for predicting the coupling specificity between GPCRs and G proteins. This method uses not only GPCR sequences but also the functional knowledge generated by natural language processing, and can achieve 92.2% prediction accuracy by using the C4.5 algorithm. Furthermore, rules related to GPCR-G protein coupling are generated. The combination of sequence analysis and text mining improves the prediction accuracy for GPCR-G protein coupling specificity, and also provides clues for understanding GPCR signaling.

Molecular characterization of two G protein-coupled receptor splice variants as FLP2 receptors in Caenorhabditis elegans

Two alternatively spliced Caenorhabditis elegans G protein-coupled receptors, T19F4.1a and T19F4.1b, were cloned and functionally characterized. The T19F4.1b receptor protein is 30 amino acids longer than T19F4.1a, and the difference in amino acid constitution is exclusively conferred to the intracellular C-terminal region, suggesting a potential difference in G protein-coupling specificity. Following cloning of the receptor cDNAs into the pcDNA3 vector and stable or transient transfection into Chinese hamster ovary cells, the aequorin bioluminescence/Ca 2+ assay was used to investigate receptor activation. This is the first report of the construction of a cell line stably expressing a C. elegans neuropeptide receptor. Our experiments identified both receptors as being cognate receptors for two FMRFamide-related peptides encoded by the flp-2 precursor: SPREPIRFamide (FLP2-A) and LRGEPIRFamide (FLP2-B). Pharmacological profiling using truncated forms of FLP2-A and -B revealed that the active core of both peptides is EPIRFamide. Screening of peptides encoded by other flps did not result in a significant activation of the receptor. In contrast to other C. elegans receptors tested in heterologous expression systems, the functional activation of both T19F4.1a and T19F4.1b was not temperature-dependent. Screening in cells lacking the promiscuous G α16 suggests that T19F4.1a and b are both linked to the G q pathway.

The influence of G protein subtype on agonist action at D 2 dopamine receptors

In previous studies, we have shown that agonists influence the ability of D 2 dopamine receptors to couple to G proteins and here we extend this work. The human D 2Short dopamine receptor and a natural polymorphism of this D 2Short(Ser 311Cys), have been studied by co-expressing the receptors in insect cells with Gβ 1γ 2 and either Gα o, Gα i1, Gα i2 or Gα i3 G protein subunits. These preparations have been used to study the G protein coupling profiles of the two receptors and the influence of agonists. Receptor/G protein coupling was analysed in dopamine/[ 3H]spiperone competition binding experiments and through stimulation of [ 35S]GTPγS binding. Although the Ser 311Cys polymorphism itself had no appreciable effect on the G protein coupling specificity of the D 2 receptor, agonist stimulation of [ 35S]GTPγS binding, revealed that both dopamine and (+)-3PPP showed a clear preference for Gα o compared to the Gα i subtypes, but quinpirole did not. These results indicate that agonists are able to stabilise different receptor conformations with different abilities to couple to G proteins.

Identification of a novel site within G protein alpha subunits important for specificity of receptor-G protein interaction.

Several domains of G protein alpha subunits are implicated in the control of receptor-G protein coupling specificity. Among these are the extreme N-and C-termini, the alpha4/beta6-loops, and the loop linking the N-terminal alpha-helix to the beta1-strand of the ras-like domain. In this study, we illustrate that single-point mutations of a highly conserved glycine residue within the linker I region of the Galpha(q) subunit confers upon the mutant Galpha(q) the ability to be activated by Galpha(i)- and Galpha(s) -coupled receptors, as evidenced by guanosine 5'-O-(3-[(35)S]thio)triphosphate binding and inositol phosphate turnover assays. The mutations did not affect expression of Galpha(q) proteins nor their ability to stimulate phospholipase Cbeta. It is noteworthy that both mutant and wild-type Galpha(q) proteins are indistinguishable in their ability to reconstitute a functional Gq-PLCbeta-calcium signaling pathway when cotransfected with the Galpha(q)-coupled neurokinin 1 or muscarinic M3 receptor into mouse embryonic fibroblasts derived from Galpha(q/11) knockout mice. On a three-dimensional model of the receptor-G protein complex, the highly conserved linker I region connecting the helical and the GTPase domain of the Galpha protein is inaccessible to the intracellular surface of the receptors. Our data indicate that receptor-G protein coupling specificity is not exclusively governed by direct receptor-G protein interaction and that it even bypasses the requirement of the extreme C terminus of Galpha, a well accepted receptor recognition domain, suggesting a novel allosteric mechanism for G protein-coupled receptor-G protein selectivity.

ATP priming of macrophage-derived chemokine responses in CHO cells expressing the CCR4 receptor.

The mechanism by which ATP primes for subsequent macrophage-derived chemokine (MDC) mediated intracellular calcium (Ca2+(i)) responses at the human CCR4 receptor stably expressed in Chinese hamster ovary (CHO) cells was investigated. MDC alone was unable to elicit a Ca2+(i) response, but pre-stimulation of cells with ATP enabled a subsequent MDC-mediated Ca2+(i) response with a pEC50 of 8.66+/-0.16. The maximal response elicited by MDC was dependent upon the concentration of ATP used to prime, but the pEC50 was stable at all ATP concentrations tested. Pertussis toxin pre-treatment did not effect the ATP response, but abolished that to MDC, demonstrating that priming with ATP did not alter G protein-coupling specificity of the CCR4 receptor. Ionomycin and thapsigargin both increased Ca2+(i) concentrations (pEC50s of 7.59+/-0.57 and 6.81+/-0.31 respectively), but were unable to prime for MDC responses, suggesting the priming mechanism was not dependent upon increases in Ca2+(i) concentrations. Priming of the MDC response was still observed when experiments were performed with low Ca2+(e) (70 microM), indicating that Ca2+ influx was not required for ATP to prime the CCR4 receptor. Neither Ro31-8220 nor wortmannin affected priming, suggesting that protein kinase C and phosphoinositol 3-kinase were not involved. In conclusion, pre-stimulation of endogenous P2Y receptors with ATP facilitates Ca2+ signalling at the recombinant CCR4 receptor in CHO cells, although the mechanism by which this occurs remains to be defined.