Hetero-oligomerization and Specificity Changes of G Protein-Coupled Purinergic Receptors

[Hetero-oligomerization and Functional Interaction between Purinergic Receptors Expressed in Platelets to Regulate Platelet Shape Change]

Adenosine and its precursors, ATP and ADP, exert various physiological effects via binding to purinergic receptors. We previously used co-immunoprecipitation, bioluminescence resonance energy transfer (BRET) and immunoelectron microscopy to demonstrate the hetero-oligomerization of purinergic receptor subtypes. Furthermore, pharmacological studies found significant changes in receptor-mediated signaling in human embryonic kidney (HEK) 293T cells co-transfected with these receptors. These findings suggest that heterodimers of purinergic receptors may have distinct pharmacological profiles, possibly due to dimerization-induced conformational changes, further suggesting that hetero-dimerization may be employed to "fine-tune" purinergic receptor signaling. Adenosine A(2A) receptor (A(2A)R), P2Y1 receptor (P2Y1R) and P2Y12 receptor (P2Y12R) are predominantly expressed on human platelets. ADP activates human platelets by stimulating both P2Y1R and P2Y12R, which act sequentially and in concert to achieve complete platelet aggregation. In contrast, adenosine stimulates Gs-coupled A(2A)R, followed by activativation of adenylate cyclase, leading to an increase in intracellular cAMP levels, which potently inhibits platelet activation. We examined the hetero-oligomerization and functional interactions of A(2A)R, P2Y1R, and P2Y12R. In HEK293T cells triply expressing all three receptors, hetero-oligomerization was observed among the three receptors. Additionally, P2Y1R agonist-evoked Ca(2+) signaling was significantly inhibited by co-treatment with an A(2A)R antagonist in HEK293T cells. In human platelets, we identified endogenous A(2A)R/P2Y1R and A(2A)R/P2Y12R heterodimers. We also observed functional Ca(2+)-signaling-related cross-talk similar to those found in HEK293T cells, and found that they appeared to affect platelet shape. These results collectively suggest that intermolecular signal transduction and specific conformational changes occur among components of the hetero-oligomers formed by these three receptors.

A critical look at the function of the P2Y11 receptor

The P2Y11 receptor is a member of the purinergic receptor family. It has been overlooked, somewhat due to the lack of a P2ry11 gene orthologue in the murine genome, which prevents the generation of knockout mice, which have been so helpful for defining the roles of other P2Y receptors. Furthermore, some of the studies reported to date have methodological shortcomings, making it difficult to determine the function of P2Y11 with certainty. In this review, we discuss the lack of a murine "P2Y11-like receptor" and highlight the limitations of the currently available methods used to investigate the P2Y11 receptor. These methods include protein recognition with antibodies that show very little specificity, gene expression studies that completely overlook the existence of a fusion transcript between the adjacent PPAN gene and P2RY11, and agonists/antagonists reported to be specific for the P2Y11 receptor but which have not been tested for activity on numerous other adenosine 5'-triphosphate (ATP)-binding receptors. We suggest a set of criteria for evaluating whether a dataset describes effects mediated by the P2Y11 receptor. Following these criteria, we conclude that the current evidence suggests a role for P2Y11 in immune activation with cell type-specific effects.

Biochemical assay of G protein-coupled receptor oligomerization: adenosine A1 and thromboxane A2 receptors form the novel functional hetero-oligomer

G protein-coupled receptors (GPCRs) are classified into a family of seven transmembrane receptors. Receptor oligomerization may be the key to the expression and function of these receptors, for example, ligand binding, desensitization, membrane trafficking, and signaling. The accumulation of evidence that GPCRs form an oligomerization with a functional alternation may change the strategy for the discovery of novel drugs targeting GPCRs. Identification of the oligomer is essential to elucidate GPCR oligomerization. GPCR oligomerizations have been demonstrated using various biochemical approaches, which include the coimmunoprecipitation method, fluorescence resonance energy transfer assay, and bioluminescence RET assay. Thus, various assays are useful for the study of GPCR oligomerization, and we should choose the best method to match the purpose. We previously targeted adenosine A1 and thromboxane A2 (TP) receptors to form a functionally novel hetero-oligomer, since both receptors function in the same cells. This chapter describes the methods used to detect GPCR oligomerization and alterations in the signaling pathways, principally according to our findings on oligomerization between adenosine A1 and TPα receptors.

The G protein-coupled receptor heterodimer network (GPCR-HetNet) and its hub components

G protein-coupled receptors (GPCRs) oligomerization has emerged as a vital characteristic of receptor structure. Substantial experimental evidence supports the existence of GPCR-GPCR interactions in a coordinated and cooperative manner. However, despite the current development of experimental techniques for large-scale detection of GPCR heteromers, in order to understand their connectivity it is necessary to develop novel tools to study the global heteroreceptor networks. To provide insight into the overall topology of the GPCR heteromers and identify key players, a collective interaction network was constructed. Experimental interaction data for each of the individual human GPCR protomers was obtained manually from the STRING and SCOPUS databases. The interaction data were used to build and analyze the network using Cytoscape software. The network was treated as undirected throughout the study. It is comprised of 156 nodes, 260 edges and has a scale-free topology. Connectivity analysis reveals a significant dominance of intrafamily versus interfamily connections. Most of the receptors within the network are linked to each other by a small number of edges. DRD2, OPRM, ADRB2, AA2AR, AA1R, OPRK, OPRD and GHSR are identified as hubs. In a network representation 10 modules/clusters also appear as a highly interconnected group of nodes. Information on this GPCR network can improve our understanding of molecular integration. GPCR-HetNet has been implemented in Java and is freely available at http://www.iiia.csic.es/~ismel/GPCR-Nets/index.html.