The Role of Intracellular Metabotropic Glutamate Receptor 5 at the Synapse

描述(由申请人提供)： 细胞内嗜代谢性谷氨酸受体5在突触计划中的作用综述通过与多种细胞内级联反应，代谢性谷氨酸受体5(MGluR5)在调节神经元活动和突触传递方面在整个中枢神经系统中发挥重要作用。MGluR5功能障碍与多种神经系统问题有关，包括焦虑、癫痫、成瘾、学习和记忆障碍以及脆性X综合征(FXS)。在FXS中，一种以智力低下和一系列自闭症特征为特征的单基因疾病，扩大的三核苷酸重复导致脆性X智力低下蛋白(FMRP)缺乏。FMRP通常作为翻译抑制物来对抗mGluR5信号；因此，FXS中FMRP的缺乏导致mGluR5的过度活性。为了纠正过量的mGluR5活性，mGluR5拮抗剂目前正在进行临床试验，作为FXS的治疗方法。虽然传统上认为像mGluR5这样的G蛋白偶联受体是从细胞表面启动其信号级联反应的，但越来越多的证据表明，位于核膜和内质网的细胞内受体也具有重要的生理意义。值得注意的是，高达90%的mGluR5位于细胞内，在分离的纹状体培养中，它引起独特的钙反应和下游信号级联。MGluR5的天然配体谷氨酸可以通过各种转运体和交换器穿过细胞膜，因此，突触释放的谷氨酸既可以激活细胞表面的mGluR5，也可以激活细胞内的mGluR5。由mGluR5激活启动的下游通路导致突触和AMPA受体(一种兴奋性的离子型谷氨酸受体)的局部蛋白质合成以及突触可塑性基因的转录；然而，细胞表面mGluR5对细胞内mGluR5的贡献尚未得到证实。这项提议的目的是检验这样一种假设，即细胞内mGluR5的激活和细胞表面mGluR5的激活对蛋白质合成和海马区AMPA受体内化具有不同的影响。海马区是一个适当的关注区域，因为它在学习和记忆中的重要性，以及它对细胞表面mGluR5被不透性的mGluR5激动剂DHPG激活的反应。使用一组可渗透和不能渗透的mGluR5激动剂和拮抗剂，可以单独激活细胞内的mGluR5或细胞表面的mGluR5，以描述它们的下游效应；例如，只有细胞内受体可以通过应用可渗透的激动剂和不渗透的拮抗剂来激活。确定细胞内mGluR5激活的作用在概念上，在显示细胞内G蛋白偶联受体的生理相关性方面，在临床上，在针对细胞内受体和/或细胞表面受体的药物靶向方面，都是重要的。对于脆性X综合征和可能的自闭症，了解细胞内mGluR5的作用对于开发合适的mGluR5拮抗剂用于治疗至关重要。 公共卫生相关性： 该项目将通过提高我们对脆性X综合征的潜在机制的了解，在寻找治疗脆性X综合征的适当方法方面有益于公众健康。这项研究将有助于阐明一种已知参与脆性X综合征的受体的功能。从这些研究中获得的知识可能也适用于自闭症谱系障碍，鉴于最近自闭症发病率的增加，这将对公共健康产生重大影响。

DESCRIPTION (provided by applicant): The Role of Intracellular Metabotropic Glutamate Receptor 5 at the Synapse Project Summary By coupling to various intracellular cascades, metabotropic glutamate receptor 5 (mGluR5) plays an important role throughout the CNS in modulating neuronal activity and synaptic transmission. Dysfunction of mGluR5 is implicated in a variety of neurological problems including anxiety, seizures, addiction, learning and memory disorders, and Fragile X Syndrome (FXS). In FXS, a single-gene disorder characterized by mental retardation and a range of autistic features, an expanded trinucleotide repeat results in a deficiency of Fragile X Mental Retardation Protein (FMRP). FMRP normally acts as a translational repressor to oppose mGluR5 signaling; thus, the lack of FMRP in FXS leads to overactivity of mGluR5. To correct the excess mGluR5 activity, mGluR5 antagonists are currently undergoing clinical trials as a treatment for FXS. Although G-protein coupled receptors like mGluR5 are traditionally thought to initiate their signaling cascades from the cell surface, there is mounting evidence that intracellular receptors, located on the nuclear membrane and endoplasmic reticulum, are also physiologically significant. Notably, up to 90% of mGluR5 is intracellularly located, where it gives rise to unique calcium responses and downstream signaling cascades in dissociated striatal cultures. The native ligand of mGluR5, glutamate, can cross the cell membrane via various transporters and exchangers; therefore, glutamate released at a synapse can activate both cell surface mGluR5 as well as intracellular mGluR5. The downstream pathways initiated by mGluR5 activation lead to local protein synthesis at the synapse and AMPA receptor (an excitatory ionotropic glutamate receptor) endocytosis as well as transcription of synaptic plasticity genes; however, the contributions of cell surface mGluR5 versus intracellular mGluR5 are not yet well-established. The goal of this proposal is to test the hypothesis that activation of intracellular mGluR5 versus cell surface mGluR5 has differential effects on protein synthesis and AMPA receptor internalization in the hippocampus. The hippocampus is an appropriate area on which to focus due to its importance in learning and memory and its established responses to cell surface mGluR5 activation by the impermeable mGluR5 agonist, DHPG. Using sets of permeable and impermeable mGluR5 agonists and antagonists, activation of intracellular mGluR5 or cell surface mGluR5 can be achieved in isolation in order to delineate their downstream effects; for instance, intracellular receptors alone can be activated with application of a permeable agonist and an impermeable antagonist. Defining the role of intracellular mGluR5 activation is important both conceptually, in showing the physiological relevance of intracellular G-protein coupled receptors, and clinically, in targeting drugs to intracellular receptors, cell surface receptors, or both. For Fragile X Syndrome and possibly autism, understanding the role of intracellular mGluR5 will be crucial in developing suitable mGluR5 antagonists for therapeutic treatments. PUBLIC HEALTH RELEVANCE: This project will benefit public health in the search for appropriate treatments for Fragile X Syndrome by improving our understanding of the underlying mechanisms of this disorder. This research will help clarify the function of a receptor known to be involved in Fragile X Syndrome. The knowledge gained from these studies may be applicable to autism spectrum disorders as well, which would have a major impact on public health given the recent increase in incidence of autism.