Allostery-driven G protein selectivity in the adenosine A1 receptor

G蛋白偶联受体是一个受体大家族，35%的处方药都是这种受体的靶标。GPCRs的日常经验包括与组胺受体结合的抗组胺药物的抗过敏作用，咖啡因阻断腺苷受体导致的心率加快，以及吗啡与阿片受体结合时的疼痛缓解。腺苷受体有多种形式，标记为A1、A2A、A2B和A3。我们对腺苷A1受体(A1R)感兴趣，原则上它是许多需要新药治疗的疾病的靶点，包括青光眼、2型糖尿病、疼痛、癫痫和脑缺血。然而，许多科学家拒绝将腺苷A1R受体作为药物靶点，因为严重的副作用与靶点有关，而不是任何潜在的药物分子。问题的出现是因为当药物与A1R相互作用时，细胞内的许多通路都会被激活，无论是在中枢神经系统(CNS)还是在心肺系统。中枢神经系统中的通路可能会导致疼痛缓解，但心肺系统中的通路会减缓心脏、降低血压和抑制呼吸；心肺系统中的这些通路会导致不可接受的副作用，从而失去对A1R作为药物靶点的兴趣。这些可取和不可取的途径之所以出现，是因为A1R是一种与多个G蛋白偶联的GPCR；一些G蛋白产生有利的结果，而同时其他G蛋白可能产生不利的结果。最近，我们偶然发现了一种名为BnOCPA(BNO代表氧苄基，CPA是环戊基腺苷)的激动剂分子，它彻底改变了A1R作为药物靶点的格局。(激动剂是激活受体的分子，而不是咖啡因等阻断受体的拮抗剂。)BnOCPA激动剂完全改变了这种模式，因为它只激活一种G蛋白(Gob)，通过它在体内缓解疼痛。它不会激活关系非常密切的G蛋白GoA，因此不会对心血管产生副作用。BnOCPA现在允许我们提出一种合理的方法来设计只激活一种G蛋白的A1R激动剂。在偶然发现BnOCPA之后，我们提出了一个旨在合理设计类似化合物的研究计划。我们建议对BnOCPA及其类似分子如何与A1R相互作用进行实验研究。我们建议对BnOCPA和相关分子如何与受体相互作用进行计算研究。我们还建议研究这些受体/分子组合如何与相关G蛋白相互作用或不相互作用。这些研究将由根据结果设计的新分子的化学合成来补充。BnOCPA是一个相当大的分子，它从主要的A1R结合部位延伸到所谓的变构结合部位，在那里变构调节剂可以结合来帮助天然的腺苷激动剂。因此，这些研究将以存在变构调节剂的激动剂的研究为指导，因为部分变构调节剂可能会影响BnOCPA的氧苄基结合的位置，因此激动剂/变构调节剂组合可能显示出与BnOCPA类似的性质。收集到的信息将用于设计只与特定G蛋白相互作用的BnOCPA类似物。从这些研究中学到的原理可能会为设计除A1R之外的其他GPCR的G蛋白选择性激动剂打开大门。

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.