Russell Isabella C, Lee Dongju, Wootten Denise, Sexton Patrick M, Bumbak Fabian
ARC Centre for Cryo-electron Microscopy of Membrane Proteins and Drug Discovery Biology Theme, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, Victoria, Australia.
ARC Centre for Cryo-electron Microscopy of Membrane Proteins and Drug Discovery Biology Theme, Monash Institute of Pharmaceutical Sciences, Monash University, Parkville, Victoria, Australia.
Pharmacol Rev. 2025 May;77(3):100056. doi: 10.1016/j.pharmr.2025.100056. Epub 2025 Apr 2.
G protein-coupled receptors (GPCRs) are critically important medicinal targets, and the cryogenic electron microscopy (cryo-EM) revolution is providing novel high-resolution GPCR structures at a rapid pace. Orphan G protein-coupled receptors (oGPCRs) are a group of approximately 100 nonolfactory GPCRs for which endogenous ligands are unknown or not validated. The absence of modulating ligands adds difficulties to understanding the physiologic significance of oGPCRs and in the determination of high-resolution structures of isolated receptors that could facilitate drug discovery. Despite the challenges, cryo-EM structures of oGPCR-G protein complexes are emerging. This is being facilitated by numerous developments to stabilize GPCR-G protein complexes such as the use of dominant-negative G proteins, mini-G proteins, complex-stabilizing nanobodies or antibody fragments, and protein tethering methods. Moreover, many oGPCRs are constitutively active, which can facilitate complex formation in the absence of a known activating ligand. Consequently, in addition to providing templates for drug discovery, active oGPCR structures shed light on constitutive GPCR activation mechanisms. These comprise self-activation, whereby mobile extracellular portions of the receptor act as tethered agonists by occupying a canonical orthosteric-binding site in the transmembrane core, constitutive activity due to alterations to conserved molecular switches that stabilize inactive states of GPCRs, as well as receptors activated by cryptic ligands that are copurified with the receptor. Cryo-EM structures of oGPCRs are now being determined at a rapid pace and are expected to be invaluable tools for oGPCR drug discovery. SIGNIFICANCE STATEMENT: Orphan G protein-coupled receptors (GPCRs) provide large untapped potential for development of new medicines. Many of these receptors display constitutive activity, enabling structure determination and insights into observed GPCR constitutive activity including (1) self-activation by mobile receptor extracellular portions that function as tethered agonists, (2) modification of conserved motifs canonically involved in receptor quiescence and/or activation, and (3) activation by cryptic lipid ligands. Collectively, these studies advance fundamental understanding of GPCR function and provide opportunities for novel drug discovery.
G蛋白偶联受体（GPCRs）是极其重要的药物靶点，而低温电子显微镜（cryo-EM）技术的革新正在迅速提供新型高分辨率的GPCR结构。孤儿G蛋白偶联受体（oGPCRs）是一组约100种非嗅觉GPCR，其内源配体未知或未经验证。缺乏调节性配体增加了理解oGPCR生理意义以及确定可促进药物发现的分离受体高分辨率结构的难度。尽管存在挑战，但oGPCR-G蛋白复合物的低温电子显微镜结构正在不断涌现。众多稳定GPCR-G蛋白复合物的进展推动了这一进程，例如使用显性负性G蛋白、微型G蛋白、复合物稳定纳米抗体或抗体片段以及蛋白质拴系方法。此外，许多oGPCR具有组成性活性，这可在没有已知激活配体的情况下促进复合物形成。因此，除了为药物发现提供模板外，活性oGPCR结构还揭示了组成性GPCR激活机制。这些机制包括自激活，即受体的可移动细胞外部分通过占据跨膜核心中的典型正构结合位点充当拴系激动剂；由于稳定GPCR非活性状态的保守分子开关发生改变而导致的组成性活性；以及由与受体共纯化的隐蔽配体激活的受体。oGPCR的低温电子显微镜结构目前正在快速确定，有望成为oGPCR药物发现的宝贵工具。意义声明：孤儿G蛋白偶联受体（GPCRs）为新药开发提供了巨大的未开发潜力。这些受体中的许多都表现出组成性活性，能够确定结构并深入了解观察到的GPCR组成性活性，包括（1）可移动的受体细胞外部分作为拴系激动剂的自激活；（2）对通常参与受体静止和/或激活的保守基序的修饰；以及（3）由隐蔽脂质配体激活。总体而言，这些研究推进了对GPCR功能的基本理解，并为新型药物发现提供了机会。
