Constitutive interaction of the P2Y2 receptor with the hematopoietic cell-specific G protein G(alpha16) and evidence for receptor oligomers

Quantitative Super-Resolution Imaging for the Analysis of GPCR Oligomerization

G-protein coupled receptors (GPCRs) are known to form homo- and hetero- oligomers which are considered critical to modulate their function. However, studying the existence and functional implication of these complexes is not straightforward as controversial results are obtained depending on the method of analysis employed. Here, we use a quantitative single molecule super-resolution imaging technique named qPAINT to quantify complex formation within an example GPCR. qPAINT, based upon DNA-PAINT, takes advantage of the binding kinetics between fluorescently labelled DNA imager strands to complementary DNA docking strands coupled to protein targeting antibodies to quantify the protein copy number in nanoscale dimensions. We demonstrate qPAINT analysis via a novel pipeline to study the oligomerization of the purinergic receptor Y2 (P2Y₂), a rhodopsin-like GPCR, highly expressed in the pancreatic cancer cell line AsPC-1, under control, agonistic and antagonistic conditions. Results reveal that whilst the density of P2Y₂ receptors remained unchanged, antagonistic conditions displayed reduced percentage of oligomers, and smaller numbers of receptors in complexes. Yet, the oligomeric state of the receptors was not affected by agonist treatment, in line with previous reports. Understanding P2Y₂ oligomerization under agonistic and antagonistic conditions will contribute to unravelling P2Y₂ mechanistic action and therapeutic targeting.

G protein-coupled purinergic P2Y receptor oligomerization: Pharmacological changes and dynamic regulation

P2Y receptors (P2YRs) are a δ group of rhodopsin-like G protein-coupled receptors (GPCRs) with many essential functions in physiology and pathology, such as platelet aggregation, immune responses, neuroprotective effects, inflammation, and cellular proliferation. Thus, they are among the most researched therapeutic targets used for the clinical treatment of diseases (e.g., the antithrombotic drug clopidogrel and the dry eye treatment drug diquafosol). GPCRs transmit signals as dimers to increase the diversity of signalling pathways and pharmacological activities. Many studies have frequently confirmed dimerization between P2YRs and other GPCRs due to their functions in cardiovascular and cerebrovascular processes in vivo and in vitro. Recently, some P2YR dimers that dynamically balance physiological functions in the body were shown to be involved in effective signal transduction and exert pathological responses. In this review, we summarize the types, pharmacological changes, and active regulators of P2YR-related dimerization, and delineate new functions and pharmacological activities of P2YR-related dimers, which may be a novel direction to improve the effectiveness of medications.

Functional and molecular evidence for heteromeric association of P2Y1 receptor with P2Y2 and P2Y4 receptors in mouse granulocytes

BACKGROUND

All hematopoietic cells express P2 receptors, however pharmacological characteristics such as expression and affinity in granulocytes are unknown.

METHODS

Pharmacological characteristics of P2 receptors were evaluated by Ca(2+) measurements using Fura-2 fluorophore. P2 receptors expression were analyzed by flow cytometry and RT-PCR. P2 interaction were shown by coimmunoprecipitation, western blotting and FRET.

RESULTS

Granulocytes were responsive to P2Y agonists, whereas P2X agonists were ineffective. Ca(2+) increase, elicited by ADP and UTP was dependent on intracellular stocks and sensitive to G-coupled receptor inhibition. Moreover, MRS2179, a specific antagonist of the P2Y1 receptor, abolished ADP response. Interestingly, ADP and UTP exhibited full heterologous desensitization, suggesting that these agonists interact with the same receptor. The heteromeric association between P2Y1 receptor and the P2Y2 and P2Y4 receptors was shown by immunoprecipitation and FRET analysis.

CONCLUSION

Clear evidence of heteromeric association of P2Y receptors was found during the evaluation of P2 receptors present in mice granulocytes, which could impact in the classical pharmacology of P2Y receptors in granulocytes.

Interaction of purinergic receptors with GPCRs, ion channels, tyrosine kinase and steroid hormone receptors orchestrates cell function

Extracellular purines and pyrimidines have emerged as key regulators of a wide range of physiological and pathophysiological cellular processes acting through P1 and P2 cell surface receptors. Increasing evidence suggests that purinergic receptors can interact with and/or modulate the activity of other classes of receptors and ion channels. This review will focus on the interactions of purinergic receptors with other GPCRs, ion channels, receptor tyrosine kinases, and steroid hormone receptors. Also, the signal transduction pathways regulated by these complexes and their new functional properties are discussed.

P2 receptor signaling in neurons and glial cells of the central nervous system

Purine and pyrimidine nucleotides are extracellular signaling molecules in the central nervous system (CNS) leaving the intracellular space of various CNS cell types via nonexocytotic mechanisms. In addition, ATP is a neuro-and gliotransmitter released by exocytosis from neurons and neuroglia. These nucleotides activate P2 receptors of the P2X (ligand-gated cationic channels) and P2Y (G protein-coupled receptors) types. In mammalians, seven P2X and eight P2Y receptor subunits occur; three P2X subtypes form homomeric or heteromeric P2X receptors. P2Y subtypes may also hetero-oligomerize with each other as well as with other G protein-coupled receptors. P2X receptors are able to physically associate with various types of ligand-gated ion channels and thereby to interact with them. The P2 receptor homomers or heteromers exhibit specific sensitivities against pharmacological ligands and have preferential functional roles. They may be situated at both presynaptic (nerve terminals) and postsynaptic (somatodendritic) sites of neurons, where they modulate either transmitter release or the postsynaptic sensitivity to neurotransmitters. P2 receptors exist at neuroglia (e.g., astrocytes, oligodendrocytes) and microglia in the CNS. The neuroglial P2 receptors subserve the neuron-glia cross talk especially via their end-feets projecting to neighboring synapses. In addition, glial networks are able to communicate through coordinated oscillations of their intracellular Ca(2+) over considerable distances. P2 receptors are involved in the physiological regulation of CNS functions as well as in its pathophysiological dysregulation. Normal (motivation, reward, embryonic and postnatal development, neuroregeneration) and abnormal regulatory mechanisms (pain, neuroinflammation, neurodegeneration, epilepsy) are important examples for the significance of P2 receptor-mediated/modulated processes.

A membrane network of receptors and enzymes for adenine nucleotides and nucleosides

Most cells express more than one receptor plus degrading enzymes for adenine nucleotides or nucleosides, and cellular responses to purines are rarely compatible with the actions of single receptors. Therefore, these receptors are viewed as components of a combinatorial receptor web rather than self-dependent entities, but it remained unclear to what extent they can associate with each other to form signalling units. P2Y(1), P2Y(2), P2Y(12), P2Y(13), P2X(2), A(1), A(2A) receptors and NTPDase1 and -2 were expressed as fluorescent fusion proteins which were targeted to membranes and signalled like the unlabelled counterparts. When tested by FRET microscopy, all the G protein-coupled receptors proved able to form heterooligomers with each other, and P2Y(1), P2Y(12), P2Y(13), A(1), A(2A), and P2X(2) receptors also formed homooligomers. P2Y receptors did not associate with P2X, but G protein-coupled receptors formed heterooligomers with NTPDase1, but not NTPDase2. The specificity of prototypic interactions (P2Y(1)/P2Y(1), A(2A)/P2Y(1), A(2A)/P2Y(12)) was corroborated by FRET competition or co-immunoprecipitation. These results demonstrate that G protein-coupled purine receptors associate with each other and with NTPDase1 in a highly promiscuous manner. Thus, purinergic signalling is not only determined by the expression of receptors and enzymes but also by their direct interaction within a previously unrecognized multifarious membrane network.

Activation of P2Y receptors by ATP and by its analogue, ADPbetaS, triggers two calcium signal pathways in the longitudinal muscle of mouse distal colon

Our previous research showed that ATP and adenosine 5'-O-2-thiodiphosphate (ADPbetaS) induce contractile effects in the longitudinal muscle of mouse distal colon via activation of P2Y receptors which are not P2Y(1) or P2Y(12) subtypes. This study investigated the nature of the P2Y receptor subtype(s) and the mechanisms leading to the intracellular calcium concentration increase necessary to trigger muscular contraction. Motor responses of mouse colonic longitudinal muscle to P2Y receptor agonists were examined in vitro as changes in isometric tension. ATP or ADPbetaS induced muscular contraction, which was not affected by P2Y(11) or P2Y(13) selective antagonists. Calcium-free solution or the calcium channel blocker, nifedipine, failed to modify the contractile responses to ATP or ADPbetaS, which were virtually abolished by depletion of calcium intracellular stores after repetitive addition of carbachol in calcium-free medium with addition of cyclopiazonic acid. Neomycin or U-73122, phospholipase C inhibitors, or 2-aminoethoxy-diphenylborate (2-APB), membrane-permeant IP(3) receptor inhibitor reduced the response to ATP, whilst ryanodine or ruthenium red, inhibiting calcium release from ryanodine-sensitive stores, abolished the response to ADPbetaS. Responses to maximally effective concentrations of ATP and ADPbetaS were not fully additive. Desensitisation with ADPbetaS antagonized the contractile effects of ATP, as desensitisation with ATP antagonized the response to ADPbetaS. In the longitudinal muscle of mouse distal colon, ATP and ADPbetaS induce muscular contraction via a P2Y receptor, coupled to differential signal pathways leading to intracellular calcium increase.

Neutrophil apoptosis mediated by nicotinic acid receptors (GPR109A)

G protein-coupled receptor (GPR)109A (HM74A) is a G(i) protein-coupled receptor, which is activated by nicotinic acid (NA), a lipid-lowering drug. Here, we demonstrate that mature human neutrophils, but not eosinophils, express functional GPR109A receptors. The induction of the GPR109A gene appears to occur late in the terminal differentiation process of neutrophils, since a mixed population of immature bone marrow neutrophils did not demonstrate evidence for its expression. NA accelerated apoptosis in cultured neutrophils in a concentration-dependent manner, as assessed by phosphatidylserine redistribution, caspase-3 activation, and DNA fragmentation assays. The pro-apoptotic effect of NA was abolished by pertussis toxin, which was used to block G(i) proteins, suggesting a receptor-mediated mechanism. Activation of GPR109A by NA resulted in decreased levels of cyclic adenosine monophosphate (cAMP), most likely due to G(i)-mediated inhibition of adenylyl cyclase activity. NA-induced apoptosis was reversed by the addition of cell-permeable cAMP, pointing to the possibility that reduced cAMP levels promote apoptosis in neutrophils. Distal mechanism involved in this process may include the post-translational modification of members of the Bcl-2 family, such as dephosphorylation of pro-apoptotic Bad and antiapoptotic Mcl-1 proteins. Taken together, following maturation in the bone marrow, neutrophils express functional GPR109A receptors, which might be involved in the regulation of neutrophil numbers. Moreover, this study identified a new cellular target of NA and future drugs activating GPR109A receptors, the mature neutrophil.

Modeling P2Y receptor-Ca2+ response coupling in taste cells

Here we elaborated an analytical approach for the simulation of dose-response curves mediated by cellular receptors coupled to PLC and Ca(2+) mobilization. Based on a mathematical model of purinergic Ca(2+) signaling in taste cells, the analysis of taste cells responsiveness to nucleotides was carried out. Consistently with the expression of P2Y(2) and P2Y(4) receptors in taste cells, saturating ATP and UTP equipotently mobilized intracellular Ca(2+). Cellular responses versus concentration of BzATP, a P2Y(2) agonist and a P2Y(4) antagonist, implicated high and low affinity BzATP receptors. Suramin modified the BzATP dose-response curve in a manner that suggested the low affinity receptor to be weakly sensitive to this P2Y antagonist. Given that solely P2Y(2) and P2Y(11) are BzATP receptors, their high sensitivity to suramin is poorly consistent with the suramin effects on BzATP responses. We simulated a variety of dose-response curves for different P2Y receptor sets and found that the appropriate fit of the overall pharmacological data was achievable only with dimeric receptors modeled as P2Y(2)/P2Y(4) homo- and heterodimers. Our computations and analytical analysis of experimental dose-response curves raise the possibility that ATP responsiveness of mouse taste cells is mediated by P2Y(2) and P2Y(4) receptors operative mostly in the dimeric form.

Comparative analysis of P2Y4 and P2Y6 receptor architecture in native and transfected neuronal systems

Although extensive studies provided molecular and pharmacological characterization of metabotropic P2Y receptors for extracellular nucleotides, little is still known about their quaternary structure. By the use of transfected cellular systems and SDS-PAGE, in our previous work we established the propensity of P2Y(4) receptor to form dimeric interactions. Here we focused on endogenously expressed P2Y(4) and P2Y(6) subtypes, comparing their oligomeric complexes under Blue Native (BN) gel electrophoresis. We provided evidence that P2Y(4) and P2Y(6) receptors form high order complexes in native neuronal phenotypes and that the oligomers can be disaggregated down to the dimeric P2Y(4) or to the dimeric and monomeric P2Y(6) receptor. Moreover, dimeric P2Y(4) and monomeric P2Y(6) proteins display selective microdomain partitioning in lipid rafts from specialized subcellular compartments such as synaptosomes. Ligand activation by UTP shifted the oligomerization of P2Y(6) but not of P2Y(4) receptor, as analysed by BN electrophoresis. Finally, whereas transfected P2Y(4) and P2Y(6) proteins homo-interact and posses the appropriate domains to associate with all P2Y(1,2,4,6,11) subtypes, in naive PC12 cells the endogenous P2Y(4) forms hetero-oligomers only with the P2Y(6) subunit. In conclusion, our results indicate that quaternary structure distinguishing P2Y(4) from P2Y(6) receptors might be crucial for specific ligand activation, membrane partitioning and consequent functional regulation.

14-3-3γ affects dynamics and integrity of glial filaments by binding to phosphorylated GFAP

Recent findings indicated a protective role of GFAP in ischemic brain, injured spinal cord, and in neurodegenerative disease. We previously demonstrated that 14-3-3γ, once thought to be neuronal specific, was up-regulated by ischemia in astrocytes and may play a specific protective role in astrocytes. Here we report that 14-3-3γ associates with both soluble and filamentous GFAP in a phosphorylation- and cell-cycle-dependent manner in primary cultured astrocytes. The amount of association increases during G2/M phase due to more phosphorylated GFAP. Moreover, this interaction is independent of vimentin, another type III intermediate filament protein in astrocytes which forms glial filaments with GFAP. A series of domain deletion mutants and substitution mutations at phosphorylation sites (from serine to alanine) on GFAP demonstrated that serine 8 in the head domain is essential for the direct association of GFAP to 14-3-3γ. Overexpression of 14-3-3γ destroyed the integrity and affected the movement of GFAP intermediate filaments. This data demonstrates that 14-3-3γ contributes to the regulation of dynamics of GFAP filaments, which may contribute to the stability of the cytoskeleton and the mechanisms of central nervous system neurodegenerative disease.

P2 receptor web: Complexity and fine-tuning

The present review offers a new perspective on a family of receptors, termed P2 receptors, specific for nucleoside tri- and diphosphates of purines/pyrimidines. We emphasize here that while decoding the inputs of various related extracellular ligands, P2 receptors are a clear example of increasing biological complexity. They are represented by 7 ionotropic P2X and 8 metabotropic P2Y receptors; they have very heterogeneous ligands and binding characteristics, molecular properties, transduction mechanisms, cellular localization and protein-protein interactions. While the reason for this sophistication is unknown, a few compelling issues emerge while looking at such a rich variety. We ask, for instance, why so many different receptor subtypes are necessary for triggering biological properties and functions, and if these receptors are more than the sum of their single entities. A first possibility is that newly synthesized P2 proteins are casually located on the cell surface (stochastic hypothesis). Alternatively, distinct subunits are engaged on different cell phenotypes by genetic control (genetic determinism) and/or selective recruitment under physiopathological conditions and epigenetic stimuli (epigenetic determinism). Nevertheless, an appropriate way to both dissect the vast biological scenario and molecular complexity among P2 receptors and to integrate and upgrade their assortment is to regard them as a "combinatorial receptor web", that is, a dynamic architecture of P2 proteins demonstrating economic efficiency and involving a process of "fine-tuning", a mechanism which endorses the dynamic nature of all biological reactions. In the present analysis, we stimulate a scientific query about what contributes to such a vast P2 receptor sophistication.

International Union of Pharmacology LVIII: update on the P2Y G protein-coupled nucleotide receptors: from molecular mechanisms and pathophysiology to therapy

There have been many advances in our knowledge about different aspects of P2Y receptor signaling since the last review published by our International Union of Pharmacology subcommittee. More receptor subtypes have been cloned and characterized and most orphan receptors de-orphanized, so that it is now possible to provide a basis for a future subdivision of P2Y receptor subtypes. More is known about the functional elements of the P2Y receptor molecules and the signaling pathways involved, including interactions with ion channels. There have been substantial developments in the design of selective agonists and antagonists to some of the P2Y receptor subtypes. There are new findings about the mechanisms underlying nucleotide release and ectoenzymatic nucleotide breakdown. Interactions between P2Y receptors and receptors to other signaling molecules have been explored as well as P2Y-mediated control of gene transcription. The distribution and roles of P2Y receptor subtypes in many different cell types are better understood and P2Y receptor-related compounds are being explored for therapeutic purposes. These and other advances are discussed in the present review.

Muscarinic receptor-activated cationic channels in murine ileal myocytes

BACKGROUND AND PURPOSE

There is little information about the excitatory cholinergic mechanisms of mouse small intestine although this model is important for gene knock-out studies.

EXPERIMENTAL APPROACH

Using patch-clamp techniques, voltage-dependent and pharmacological properties of carbachol- or intracellular GTPgammaS-activated cationic channels in mouse ileal myocytes were investigated.

KEY RESULTS

Three types of cation channels were identified in outside-out patches (17, 70 and 140 pS). The voltage-dependent behaviour of the 70 pS channel, which was also the most abundantly expressed channel (approximately 0.35 micro(-2)) was most consistent with the properties of the whole-cell muscarinic current (half-maximal activation at -72.3+/-9.3 mV, slope of -9.1+/-7.4 mV and mean open probability of 0.16+/-0.01 at -40 mV; at near maximal activation by 50 microM carbachol). Both channel conductance and open probability depended on the permeant cation in the order: Cs+ (70 pS) >Rb+ (66pS) >Na+ (47 pS) >Li+ (30 pS). External application of divalent cations, quinine, SK&F 96365 or La3+ strongly inhibited the whole-cell current. At the single channel level the nature of the inhibitory effects appeared to be very different. Either reduction of the open probability (quinine and to some extent SK&F 96365 and La3+) or of unitary current amplitude (Ca2+, Mg2+, SK&F 96365, La3+) was observed implying significant differences in the dissociation rates of the blockers.

CONCLUSIONS AND IMPLICATIONS

The muscarinic cation current of murine small intestine is very similar to that in guinea-pig myocytes and murine genetic manipulation should yield important information about muscarinic receptor transduction mechanisms.

Severe acute respiratory syndrome-associated coronavirus 3a protein forms an ion channel and modulates virus release

Fourteen ORFs have been identified in the severe acute respiratory syndrome-associated coronavirus (SARS-CoV) genome. ORF 3a of SARS-CoV codes for a recently identified transmembrane protein, but its function remains unknown. In this study we confirmed the 3a protein expression and investigated its localization at the surface of SARS-CoV-infected or 3a-cDNA-transfected cells. Our experiments showed that recombinant 3a protein can form a homotetramer complex through interprotein disulfide bridges in 3a-cDNA-transfected cells, providing a clue to ion channel function. The putative ion channel activity of this protein was assessed in 3a-complement RNA-injected Xenopus oocytes by two-electrode voltage clamp. The results suggest that 3a protein forms a potassium sensitive channel, which can be efficiently inhibited by barium. After FRhK-4 cells were transfected with an siRNA, which is known to suppress 3a expression, followed by infection with SARS-CoV, the released virus was significantly decreased, whereas the replication of the virus in the infected cells was not changed. Our observation suggests that SARS-CoV ORF 3a functions as an ion channel that may promote virus release. This finding will help to explain the highly pathogenic nature of SARS-CoV and to develop new strategies for treatment of SARS infection.

Tyrosine kinase modulation of protein kinase C activity regulates G protein-linked Ca2+ signaling in leukemic hematopoietic cells

We have used a recombinant mouse pre-B cell line (TonB210.1, expressing Bcr/Abl under the control of an inducible promoter) and several human leukemia cell lines to study the effect of high tyrosine kinase activity on G protein-coupled receptor (GPCR) agonist-stimulated cellular Ca(2+) release and store-operated Ca(2+) entry (SOCE). After induction of Bcr/Abl expression, GPCR-linked SOCE increased. The effect was reverted in the presence of the specific Abl inhibitor imatinib (1microM) and the Src inhibitor PP2 (10microM). In leukemic cell lines constitutively expressing high tyrosine kinase activity, Ca(2+) transients were reduced by imatinib and/or PP2. Ca(2+) transients were enhanced by specific inhibitors of PKC subtypes and this effect was amplified by tyrosine kinase inhibition in Bcr/Abl expressing TonB210.1 and K562 cells. Under all conditions Ca(2+) transients were essentially blocked by the PKC activator PMA. In Bcr/Abl expressing (but not in native) TonB210.1 cells, tyrosine kinase inhibitors enhanced PKCalpha catalytic activity and PKCalpha co-immunoprecipitated with Bcr/Abl. Unlike native TonB210.1 cells, Bcr/Abl expressing cells showed a high rate of cell death if Ca(2+) influx was reduced by complexing extracellular Ca(2+) with BAPTA. Our data suggest that tonic inhibition of PKC represents a mechanism by which high tyrosine kinase activity can enhance cellular Ca(2+) transients and thus exert profound effects on the proliferation, apoptosis and chemotaxis of leukemic cells.

Nucleotide signaling in nervous system development

The development of the nervous system requires complex series of cellular programming and intercellular communication events that lead from the early neural induction to the formation of a highly structured central and peripheral nervous system. Neurogenesis continuously takes place also in select regions of the adult mammalian brain. During the past years, a multiplicity of cellular control mechanisms has been identified, ranging from differential transcriptional mediators to inducers or inhibitors of cell specification or neurite outgrowth. While the identification of transcription factors typical for the stage-specific progression has been a topic of key interest for many years, less is known concerning the potential multiplicity of relevant intercellular signaling pathways and the fine tuning of epigenetic gene regulation. Nucleotide receptors can induce a multiplicity of cellular signaling pathways and are involved in multiple molecular interactions, thus opening the possibility of cross talk between several signaling pathways, including growth factors, cytokines, and extracellular matrix components. An increasing number of studies provides evidence for a role of nucleotide signaling in nervous system development. This includes progenitor cell proliferation, cell migration, neuronal and glial cellular interaction and differentiation, and synaptic network formation.