Regulation of heptaspanning-membrane-receptor function by dimerization and clustering

Adenosine A2A receptor and dopamine D3 receptor interactions: evidence of functional A2A/D3 heteromeric complexes

Adenosine A(2A) and dopamine D(2) receptors have been shown previously to form heteromeric complexes and interact at the level of agonist binding, G protein coupling, and trafficking. Because dopamine D(2) and D(3) receptors show a high degree of sequence homology, A(2A) and D(3) receptors may also interact in a similar manner. The present studies with confocal microscopy showed that A(2A)-yellow fluorescent protein (YFP) and D(3)-green fluorescent protein 2 (GFP2) receptors colocalize in the plasma membrane. Furthermore, fluorescence resonance energy transfer (FRET) analysis demonstrated that A(2A)-YFP and D(3)-GFP2 receptors give a positive FRET efficiency and are thereby likely to exist as heteromeric A(2A)/D(3) receptor complexes. Saturation experiments with [(3)H]dopamine demonstrated that the A(2A) receptor agonist 4-[2-[[6-amino-9(N-ethyl-beta-d-ribofuranuronaminoamidosyl)-9H-purin-2-yl]amino]ethyl]benzenepropanoic acid (CGS-21680) reduced the affinity of the high-affinity agonist binding state of the D(3) receptor for [(3)H]dopamine. The A(2A) and D(2A) receptors seem to interact also at the level of G protein coupling, because the adenosine A(2A) receptor agonist CGS-21680 fully counteracted the D(3) receptor-mediated inhibition of a forskolin-mediated increase in cAMP levels. Taken together, when coexpressed in the same neuron, A(2A) and D(3) receptors seem to form A(2A)/D(3) heteromeric receptor complexes in which A(2A) receptors antagonistically modulate both the affinity and the signaling of the D(3) receptors. D(3) receptor is one of the therapeutic targets for treatment of schizophrenia, and therefore, the A(2A)/D(3) receptor interactions could provide an alternative antischizophrenic treatment.

Factor XIIIA Transglutaminase Crosslinks AT1 Receptor Dimers of Monocytes at the Onset of Atherosclerosis

Many G protein-coupled receptors form dimers in cells. However, underlying mechanisms are barely understood. We report here that intracellular factor XIIIA transglutaminase crosslinks agonist-induced AT1 receptor homodimers via glutamine315 in the carboxyl-terminal tail of the AT1 receptor. The crosslinked dimers displayed enhanced signaling and desensitization in vitro and in vivo. Inhibition of angiotensin II release or of factor XIIIA activity prevented formation of crosslinked AT1 receptor dimers. In agreement with this finding, factor XIIIA-deficient individuals lacked crosslinked AT1 dimers. Elevated levels of crosslinked AT1 dimers were present on monocytes of patients with the common atherogenic risk factor hypertension and correlated with an enhanced angiotensin II-dependent monocyte adhesion to endothelial cells. Elevated levels of crosslinked AT1 receptor dimers on monocytes could sustain the process of atherogenesis, because inhibition of angiotensin II generation or of intracellular factor XIIIA activity suppressed the appearance of crosslinked AT1 receptors and symptoms of atherosclerosis in ApoE-deficient mice.

Factor XIIIA (Cross)Links AT1 Receptors to Atherosclerosis

Angiotensin II, a small peptide hormone that plays key roles in the regulation of blood pressure, also contributes to inflammatory processes that promote the development of atherosclerosis. In this issue of Cell, AbdAlla et al. (2004) provide evidence that pathogenic actions of angiotensin II involve covalent crosslinking of angiotensin AT1 receptors by factor XIIIA transglutaminase, resulting in stable receptor dimers with enhanced signaling properties.

Combining mass spectrometry and pull-down techniques for the study of receptor heteromerization. Direct epitope-epitope electrostatic interactions between adenosine A2A and dopamine D2 receptors

Previous results from FRET and BRET experiments and computational analysis (docking simulations) have suggested that a portion of the third intracellular loop (I3) of the human dopamine D2 receptor (D2R) and the C-tail from the human adenosine A2A receptor (A2AR) are involved in A2AR-D2R heteromerization. The results of the present studies, using pull-down and mass spectrometry experiments, suggest that A2AR-D2R heteromerization depends on an electrostatic interaction between an Arg-rich epitope from the I3 of the D2R (217RRRRKR222) and two adjacent Asp residues (DD401-402) or a phosphorylated Ser (S374) residue in the C-tail of the A2AR. A GST-fusion protein containing the C-terminal domain of the A2AR (GST-A2ACT) was able to pull down the whole D2R solubilized from D2R-tranfected HEK-293 cells. Second, a peptide corresponding to the Arg-rich I3 region of the D2R (215VLRRRRKRVN224) and bound to Sepharose was able to pull down both GST-A2ACT and the whole A2AR solubilized from A2AR-tranfected HEK-293 cells. Finally, mass spectometry and pull-down data showed that the Arg-rich D2R epitope binds to two different epitopes from the C-terminal part of the A2AR, containing the two adjacent Asp residues or the phosphorylated Ser residue (388HELKGVCPEPPGLDDPLAQDGAVGS412 and 370SAQEpSQGNT378). The present results are the first example of epitope-epitope electrostatic interaction underlying receptor heteromerization, a new, expanding area of protein-protein interactions.

The hydrodynamic properties of dark- and light-activated states of n-dodecyl beta-D-maltoside-solubilized bovine rhodopsin support the dimeric structure of both conformations

Rhodopsin (Rho) has been extracted in n-dodecyl beta-D-maltoside (DM) from bovine retinal rod outer segments and purified to homogeneity by affinity chromatography on concanavalin A-Sepharose. Because chemical cross-linking of Rho and photoactivated Rho (Rho*) provided initial evidence for the oligomeric nature of the photoreceptor protein, we carried out a hydrodynamic characterization of the native and activated conformations of detergent-solubilized Rho. The molecular weights of the complexes between dark and photoexcited states of Rho and DM were determined by gel filtration chromatography on Sephacryl S-300, in the presence of 0.1% DM. Subtracting the size of the corresponding detergent micelles resulted in molecular masses of 78 kDa for native Rho and 76 kDa for Rho*. The measured content of 0.97 g of detergent/g of protein resulted in a calculated partial specific volume of 0.765 cm(3)/g for the protein-detergent complex and a molar mass of 64-65 kDa for the protein moiety. The sizes of Rho.DM and Rho*.DM complexes were also evaluated by sedimentation on 10-30% sucrose gradients, in the presence of 0.1% DM, and molecular masses of about 60 kDa were estimated for both the dark- and light-activated states of the photoreceptor protein. The size of Rho was determined to be 65,300 and 69,800 Da, respectively, when the purified Rho.DM complex was either chromatographed on Sephacryl S-300 or ultracentrifuged on sucrose gradients in the absence of DM. All these results were consistent with a dimeric quaternary structure for both conformations of Rho. Additionally, the functional integrity of the purified photoreceptor protein following gel filtration chromatography and ultracentrifugation was demonstrated by three criteria as follows: (i) its characteristic UV-visible absorption spectra, (ii) its capability to photoactivate transducin, and (iii) its ability to serve as a substrate for rhodopsin kinase.

Negative cooperativity of glutamate binding in the dimeric metabotropic glutamate receptor subtype 1

Metabotropic glutamate receptor (mGluR) subtype 1 is a Class III G-protein-coupled receptor that is mainly expressed on the post-synaptic membrane of neuronal cells. The receptor has a large N-terminal extracellular ligand binding domain that forms a homodimer, however, the intersubunit communication of ligand binding in the dimer remains unknown. Here, using the intrinsic tryptophan fluorescence change as a probe for ligand binding events, we examined whether allosteric properties exist in the dimeric ligand binding domain of the receptor. The indole ring of the tryptophan 110, which resides on the upper surface of the ligand binding pocket, sensed the ligand binding events. From saturation binding curves, we have determined the apparent dissociation constants (K(0.5)) of representative agonists and antagonists for this receptor (3.8, 0.46, 40, and 0.89 microm for glutamate, quisqualate, (S)-alpha-methyl-4-carboxyphenylglycine ((S)-MCPG), and (+)-2-methyl-4-carboxyphenylglycine (LY367385), respectively). Calcium ions functioned as a positive modulator for agonist but not for antagonist binding (K(0.5) values were 1.3, 0.21, 59, and 1.2 microm for glutamate, quisqualate, (S)-MCPG, and LY367385, respectively, in the presence of 2.0 mm calcium ion). Moreover, a Hill analysis of the saturation binding curves revealed the strong negative cooperativity of glutamate binding between each subunit in the dimeric ligand binding domain. As far as we know, this is the first direct evidence that the dimeric ligand binding domain of mGluR exhibits intersubunit cooperativity of ligand binding.

Adenosine A2A-dopamine D2 receptor-receptor heteromerization: qualitative and quantitative assessment by fluorescence and bioluminescence energy transfer

There is evidence for strong functional antagonistic interactions between adenosine A2A receptors (A2ARs) and dopamine D2 receptors (D2Rs). Although a close physical interaction between both receptors has recently been shown using co-immunoprecipitation and co-localization assays, the existence of a A2AR-D2R protein-protein interaction still had to be demonstrated in intact living cells. In the present work, fluorescence resonance energy transfer (FRET) and bioluminescence resonance energy transfer (BRET) techniques were used to confirm the occurrence of A2AR-D2R interactions in co-transfected cells. The degree of A2AR-D2R heteromerization, measured by BRET, did not vary after receptor activation with selective agonists, alone or in combination. BRET competition experiments were performed using a chimeric D2R-D1R in which helices 5 and 6, the third intracellular loop (I3), and the third extracellular loop (E3) of the D2R were replaced by those of the dopamine D1 receptor (D1R). Although the wild type D2R was able to decrease the BRET signal, the chimera failed to achieve any effect. This suggests that the helix 5-I3-helix 6-E3 portion of D2R holds the site(s) for interaction with A2AR. Modeling of A2AR and D2R using a modified rhodopsin template followed by molecular dynamics and docking simulations gave essentially two different possible modes of interaction between D2R and A2AR. In the most probable one, helix 5 and/or helix 6 and the N-terminal portion of I3 from D2R approached helix 4 and the C-terminal portion of the C-tail from the A2AR, respectively.

Sensitization of neuronal A2A adenosine receptors after persistent D2 dopamine receptor activation

Acute activation of Galpha(i/o)-coupled D2 dopamine receptors inhibits A2A adenosine receptor stimulation of adenylate cyclase. This antagonistic interaction between D2 dopamine and A2A adenosine receptors has been well documented; however, the effects of persistent activation of D2 dopamine receptors on subsequent A2A adenosine receptor signaling have not been explored. The present study investigated the effects of short-term (3-h) and long-term (18-h) activation of D2L dopamine receptors on subsequent A2A adenosine receptor stimulation of adenylate cyclase in CAD-D2L and NS20Y-D2L neuroblastoma cells. Short- and long-term activation of D2L dopamine receptors markedly increased 5'-N-methylcarboxamidoadenosine (MECA)-stimulated cyclic AMP accumulation 1.4-fold and 1.7-fold, respectively. D2L receptor-induced sensitization of A2A-stimulated cyclic AMP accumulation was blocked by the D2 antagonist spiperone and pertussis toxin pretreatment. In addition, persistent activation of A2A adenosine receptors resulted in 50% desensitization of subsequent MECA-stimulated cyclic AMP accumulation; however, MECA-induced desensitization of A2A adenosine receptors did not prevent completely quinpirole-induced sensitization of adenylate cyclase. These studies revealed a novel mode of regulation between D2L dopamine and A2A adenosine receptors and suggest a cooperative interaction in the regulation of cyclic AMP signaling.