ABSTRACT
Title
Allosteric modulators for adenosine receptors: an alternative to the orthosteric ligands.
Authors
M.L. Trincavelli, S. Daniele, C. Martini
Dept. of Psychiatry, Neurobiology, Pharmacology and Biotechnology. University of Pisa, Pisa. Italy.
Dept. of Psychiatry, Neurobiology, Pharmacology and Biotechnology. University of Pisa, Pisa. Italy.
Abstract
Allosterism is the process by which the interaction of a chemical/protein at one location on a protein (the allosteric site) influences the binding or function of another ligand at a topographically distinct site [1]. The allosteric regulation of a receptor protein may occur through different mechanisms involving the homomeric and/or heteromeric proteins. For many years GPCRs were thought to exist as monomeric units. In this situation, an allosteric modulator must bind the same polypeptide as the orthosteric agonist but at a non-overlapping site. Binding of the allosteric modulator can result in an alteration (either positive or negative) in potency, affinity or apparent efficacy for the orthosteric agonist and viceversa. However, evidence both from native cells and heterologous expression systems has demonstrated that GPCRs can signal within dimers or higher-order complexes. In the allosteric regulation of GPCR heteromers, the ligand binding at the orthosteric site of one protomer results in allosteric modulation of ligand affinity at the orthosteric site of the second protomer. Allosterism across homo- or heteromers represents an additional topographic landscape that must now be considered pharmacologically for novel therapeutic approaches.
Adenosine receptors, classified as A1, A2A, A2B and A3, were among the first GPCRs discovered to be allosterically regulated at level of homo and heteromers. Heteromers of A2A and dopamine D2 receptors were one of the first GPCRs heteromers to be described. A close physical interaction between both receptors was shown using co-immunoprecipitation studies and by FRET and BRET analysis of living cells [2]. The A2A-D2 heteromers were found to be constitutive, since they were detected also in the absence of receptor agonists. However, the stechiometry of the A2A-D2 receptor heteromers is unknown: they could be A2A-D2 receptor heterodimers or high order heteromers. At the biochemical level, a functional antagonistic interaction between A2A-D2 receptor have been discovered: by means of an allosteric interaction in the receptor heteromer, stimulation of A2A receptor decreases the affinity of D2 receptor for their agonists. We have recently investigated the cross-regulation of A2A receptor induced by D2 receptor ligands: D2DR blockade by typical neuroleptic drugs causes an increase in A2A receptor binding affinity both in human platelets and in transfected cell lines [3, 4], suggesting D2 receptor antagonists are able to modulate A2A receptor-mediated response as well as it happens viceversa. We also provided evidences that the activation of D2 receptor by agonists, allosterically modulated A2A receptors at membrane level, causing a negative regulation of A2A receptor agonist binding to the orthosteric site, as well as a drop in A2A receptor functioning. These data suggest this interaction occurs not only at cytosolic level, but also involves a conformational change of A2A receptors at level of receptor binding protein transmitted through the heteromeric receptor association. In the view of the well known role of dopamine in Parkinson's disease, schizophrenia and drug addiction, the allosteric regulation of A2A-D2 heteromers may provide a new therapeutical approach for basal ganglia diseases associated with dopamine system disfunctions.
References
[1] Smith et al. (2010) Allostery at G protein-coupled receptor homo-and heteromers: uncharted pharmacological landscape. Pharmacol. Rev. 62, 701-725.
[2] Fuxe et al. (2005) Adenosine A2A and dopamine D2 heteromeric receptor complexes and their function. J. Mol. Neurosci. 26, 209-218
[3] Martini et al. (2006) Upregulation of A2A adenosine receptors in platelets from patients affected by bipolar disorders under treatment with typical antipsychotics. J. Psychiatric Res. 40, 81–88.
[4] Trincavelli et al. (2010) Receptor cross-talk: haloperidol treatment enhances A2A adenosine receptor functioning in a transfected cell model.Purinergic Signalling6, 373-381.
Adenosine receptors, classified as A1, A2A, A2B and A3, were among the first GPCRs discovered to be allosterically regulated at level of homo and heteromers. Heteromers of A2A and dopamine D2 receptors were one of the first GPCRs heteromers to be described. A close physical interaction between both receptors was shown using co-immunoprecipitation studies and by FRET and BRET analysis of living cells [2]. The A2A-D2 heteromers were found to be constitutive, since they were detected also in the absence of receptor agonists. However, the stechiometry of the A2A-D2 receptor heteromers is unknown: they could be A2A-D2 receptor heterodimers or high order heteromers. At the biochemical level, a functional antagonistic interaction between A2A-D2 receptor have been discovered: by means of an allosteric interaction in the receptor heteromer, stimulation of A2A receptor decreases the affinity of D2 receptor for their agonists. We have recently investigated the cross-regulation of A2A receptor induced by D2 receptor ligands: D2DR blockade by typical neuroleptic drugs causes an increase in A2A receptor binding affinity both in human platelets and in transfected cell lines [3, 4], suggesting D2 receptor antagonists are able to modulate A2A receptor-mediated response as well as it happens viceversa. We also provided evidences that the activation of D2 receptor by agonists, allosterically modulated A2A receptors at membrane level, causing a negative regulation of A2A receptor agonist binding to the orthosteric site, as well as a drop in A2A receptor functioning. These data suggest this interaction occurs not only at cytosolic level, but also involves a conformational change of A2A receptors at level of receptor binding protein transmitted through the heteromeric receptor association. In the view of the well known role of dopamine in Parkinson's disease, schizophrenia and drug addiction, the allosteric regulation of A2A-D2 heteromers may provide a new therapeutical approach for basal ganglia diseases associated with dopamine system disfunctions.
References
[1] Smith et al. (2010) Allostery at G protein-coupled receptor homo-and heteromers: uncharted pharmacological landscape. Pharmacol. Rev. 62, 701-725.
[2] Fuxe et al. (2005) Adenosine A2A and dopamine D2 heteromeric receptor complexes and their function. J. Mol. Neurosci. 26, 209-218
[3] Martini et al. (2006) Upregulation of A2A adenosine receptors in platelets from patients affected by bipolar disorders under treatment with typical antipsychotics. J. Psychiatric Res. 40, 81–88.
[4] Trincavelli et al. (2010) Receptor cross-talk: haloperidol treatment enhances A2A adenosine receptor functioning in a transfected cell model.Purinergic Signalling6, 373-381.