Structural Basis of Signal Instigation Through Family C GPCRs

摘要 γ-氨基丁酸B受体（GABABR）和代谢型谷氨酸受体（mGluRs）属于 G蛋白偶联受体（GPCRs）C家族和关键调节神经元兴奋性、突触 传输和可塑性。许多中枢神经系统疾病与神经元兴奋性的改变有关 通过谷氨酸能和γ-氨基丁酸能系统因此，mGluRs和GABABR已经成为一个研究的主题。 巨大的药物发现努力，因为它们代表了治疗许多生理性疾病的主要治疗靶点。 功能障碍和神经退行性疾病和神经精神疾病。除了典型的七个 跨膜螺旋（7 TM）结构域，家族C GPCR还包括一个大的细胞外“venus fly trap”（VFT） 结构域，其构成正构配体结合位点。配体与细胞外VFT结构域的结合 触发VFT结构域从开放构象到闭合构象的大的构象变化。这只蛤蜊- 细胞外结构域的壳状闭合导致受体接合和G蛋白在细胞外表面上的活化。 跨膜结构域的细胞内侧，其机制尚不清楚。受体激活G 然后蛋白质起作用以增强或抑制第二信使信号级联。我们最近展示了 近全长mGluR 5和GABABR在非活性和活性构象中的cryoEM结构，揭示了 配体与VFT结合后，7 TM二聚体的组织发生了广泛的转变。尽管如此 尽管研究进展缓慢，但关于细胞膜上的变构通讯仍存在几个关键问题， 家族C GPCR，特别是G蛋白偶联和激活的机制。解决这些 问题，我们建议获得mGluR 2，mGluR 5和GABABR与其同源物复合的结构 G蛋白和探针的结构的见解，使用分子动力学模拟和诱变耦合到 功能测定这些受体-G蛋白复合物之间的相似性和差异将使我们能够 对比和比较我们的研究结果，并检查G蛋白偶联和选择性方面。总的来说， 这些研究将使我们能够建立一个详细的机制框架，以了解家族C GPCR 信号传导，并将形成针对这些受体的新的治疗策略的设计的基础。

Abstract The γ-aminobutyric acid B receptor (GABABR) and the metabotropic glutamate receptors (mGluRs) belong to the Family C of G protein coupled receptors (GPCRs) and critically regulate neuronal excitability, synaptic transmission and plasticity. Many disorders of the CNS have been linked to alterations in neuronal excitability via the glutamatergic and GABAergic system. Accordingly, mGluRs and GABABR have been the subject of an enormous drug discovery effort as they represent major therapeutic targets for treating numerous physiological dysfunctions and for neurodegenerative and neuropsychiatric conditions. Apart from the prototypical seven transmembrane helix (7TM) domain, Family C GPCRs also include a large extracellular ‘venus fly trap’ (VFT) domain that constitutes the orthosteric ligand binding site. Binding of ligand to the extracellular VFT domain triggers a large conformational change in the VFT domains from an open to a closed conformation. This clam- shell like closure of the extracellular domain results in receptor engagement and activation of G proteins on the intracellular side of the transmembrane domain with a mechanism that remains unclear. Receptor activated G proteins then act to either enhance or repress secondary messenger signaling cascades. We recently showed cryoEM structures of near-full length mGluR5 and GABABR in inactive and active conformations, revealing extensive transitions in the organization of the 7TM dimer upon ligand binding to the VFT. Notwithstanding this progress, several key questions remain regarding the allosteric communication across the cell membrane by Family C GPCRs, and particularly the mechanism of G protein coupling and activation. To address these questions, we propose to obtain the structures of mGluR2, mGluR5 and GABABR in complex with their cognate G proteins and probe the structural insights using molecular dynamics simulations and mutagenesis coupled to functional assays. The similarities and differences amongst these receptor-G protein complexes will allow us to contrast and compare our findings and examine aspects of G protein coupling and selectivity. Collectively, these studies will enable us to create a detailed mechanistic framework to understand Family C GPCR signaling and will form the basis for the design of novel therapeutic strategies targeting these receptors.