Allostery-driven G protein selectivity in the adenosine A1 receptor

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Lay SummaryG protein-coupled receptors, GPCRs, are a large family of receptors that are the target for 35% of prescription drugs. Everyday experiences of GPCRs include the anti-allergy effect of anti-histamines binding to the histamine receptor, the increase in the heart rate from caffeine blocking the adenosine receptor and pain relief from morphine binding to the opioid receptor. The adenosine receptor comes in multiple forms, labelled A1, A2A, A2B and A3. We are interested in the adenosine A1 receptor (A1R), which in principle is a target for a number of conditions where new drugs are needed, including glaucoma, type 2 diabetes mellitus, pain, epilepsy and cerebral ischemia. However, many scientists have rejected the adenosine A1R receptor as a drug target because of serious side effects intrinsically linked to the target rather than to any potential drug molecule. The problem arises because when a drug interacts with the A1R, a number of pathways are activated inside the cell, whether this be in the central nervous system (CNS) or in the cardiorespiratory system. The pathways in the CNS may lead to pain relief but the pathways in the cardiorespiratory system slow the heart, reduce blood pressure, and supress respiration; these pathways in the cardiorespiratory system lead to unacceptable side effects, and consequently to a loss in interest in the A1R as a drug target. These desirable and undesirable pathways arise because the A1R is a GPCR that couples to multiple G proteins; some G proteins give rise to favourable outcomes while simultaneously, other G proteins may give rise to unfavourable outcomes. Recently, we discovered, by chance, an agonist molecule called BnOCPA (BnO stands for oxybenzyl, CPA is cyclopentyladenosine), which has totally transformed the landscape with regards to the A1R as a drug target. (An agonist is a molecule that activates the receptor, as opposed to an antagonist, like caffeine, that blocks the receptor.) The BnOCPA agonist has totally shifted the paradigm as it only activates one G protein (Gob), through which it confers pain relief in vivo. It does not activate the very closely related G protein Goa and so there are no cardiovascular side effects. BnOCPA now allows us to propose a rational approach to designing A1R agonists that only activate one G protein. Having discovered BnOCPA by chance, we propose a programme of research aimed at rational design of similar compounds. We propose experimental studies of how BnOCPA and similar molecules interact with the A1R. We propose computational studies of how BnOCPA and related molecules interact with the receptor. We also propose studies of how these receptor/molecule combinations interact, or don't interact, with relevant G proteins. These studies will be supplemented by chemical synthesis of new molecules designed from the results. BnOCPA is a rather large molecule that extends beyond the main A1R binding site into the so called, allosteric binding site, where allosteric modulators can bind to help the natural adenosine agonist. These studies will therefore be guided by studies of agonists in the presence of allosteric modulators in the understanding that parts of the allosteric modulators may influence where the oxybenzyl group of BnOCPA binds and so the agonist/allosteric modulator combination may show similar properties to BnOCPA. The information gathered will be used to design BnOCPA analogues that can interact with only specified G proteins. The principles learned in these studies may open the door to the design of G protein selective agonists for other GPCRs besides the A1R.