Corriden, Ross and Kilpatrick, Laura E. and Kellam, Barrie and Briddon, Stephen J. and Hill, Stephen J. (2014) Kinetic analysis of antagonist-occupied adenosine-A3 receptors within membrane microdomains of individual cells provides evidence of receptor

In our previous work, using a fluorescent adenosine-A3 receptor (A3AR) agonist and fluorescence correlation spectroscopy (FCS), we demonstrated highaffinity labeling of the active receptor (R*) conformation. In the current study, we used a fluorescent A3AR antagonist (CA200645) to study the binding characteristics of antagonist-occupied inactive receptor (R) conformations in membrane microdomains of individual cells. FCS analysis of CA200645-occupied A3ARs revealed 2 species, D2 and D3, that diffused at 2.29 0.35 and 0.09 0.03 m/s, respectively. FCS analysis of a green fluorescent protein (GFP)-tagged A3AR exhibited a single diffusing species (0.105 m/s). The binding of CA200645 to D3 was antagonized by nanomolar concentrations of the A3 antagonist MRS 1220, but not by the agonist NECA (up to 300 nM), consistent with labeling of R. CA200645 normally dissociated slowly from the A3AR, but inclusion of xanthine amine congener (XAC) or VUF 5455 during washout markedly accelerated the reduction in the number of particles exhibiting D3 characteristics. It is notable that this effect was accompanied by a significant increase in the number of particles with D2 diffusion. These data show that FCS analysis of ligand-occupied receptors provides a unique means of monitoring ligand A3AR residence times that are significantly reduced as a consequence of allosteric interaction across the dimer interface.—Corriden, R., Kilpatrick, L. E., Kellam, B., Briddon, S. J., Hill, S. J. Kinetic analysis of antagonistoccupied adenosine-A3 receptors within membrane microdomains of individual cells provides evidence for receptor dimerization and allosterism. FASEB J. 28, 4211–4222 (2014). www.fasebj.org