Mechanisms of ribbon synapse function and plasticity

研究问题和意义初级听觉神经元(螺旋神经节神经元)通过一种特殊类型的突触(称为带状突触)来支配耳蜗(内耳的听力部分)中的受体细胞(毛细胞)，这种突触的特征是突触前带状结构。将回答三个问题。(1)已知功能区有助于神经递质的快速释放，因此对编码信号的快速变化至关重要--称为时间处理能力。然而，功能区突触到底是如何完成这一任务的尚不清楚。具体地说，每个内毛细胞(IHC)周围有15-30个带状突触，在IHC周围不同位置的突触具有不同的形态。然而，尚不清楚在不同位置突触的神经元在时间处理能力上是否有所不同。(2)视网膜光感受器细胞中的突触带是动态的：它们在光中分解，在黑暗中重新组装。目前尚不清楚这种动态过程是否存在于IHC中，以服务于耳蜗的强度编码。(3)最近，我们展示了噪声损伤后带状突触修复过程中一种有趣的突触可塑性。这涉及到突触的分解、再生以及突触带和突触后终末之间的相互作用，以重新分配突触。在功能上，修复后的突触显示出时间处理缺陷。然而，修复的性质和机制以及它对时间加工的影响仍有待探索。对这些问题的研究将提供深入的认识，包括带状结构如何与功能相关，以及带状结构如何对耳蜗内的强度和时间编码做出贡献。对突触可塑性的机制和后果的研究将使突触可塑性的调控朝着有益的方向发展。目的和研究计划第一个目标是了解带状突触对间质细胞进行时间处理的机制。在此之前，形态差异是在免疫组织学中发现的。我们将使用透射电子显微镜进一步鉴定IHC周围带和突触后终末的形态差异。我将探索形态差异与时间处理能力的关系，这将在支配IHC不同位置的神经元的单个单位活动中得到验证。第二个目标是探索IHC中的条带是否在正常通讯水平(70-90dBSPL)下被声刺激动态分解并在安静的情况下重新组装。我将使用透射电子显微镜比较不同时间点Re：声音刺激的丝带的形态。还将使用分子生物学方法来鉴定条带结构构型的变化(条带蛋白组织的变化)。功能观察也将与形态学和分子生物学结合使用。第三个目标是探索突触可塑性在修复过程中的机制。我将首先确定带子的再生是需要合成新的蛋白质，还是需要简单地重新组装破碎的带子。我将研究从生理性破坏和损伤转变的分子本质，以及修复的潜在基因控制。正如我最近的研究所指出的，我将在透射电子显微镜水平上确定修复的条带是否会在突触重建期间吸引突触后终末，并在修复期间重新定位突触后终末。先前对总诱发反应的观察表明，修复的带状突触存在时间处理缺陷。我将在单个单位的水平上进一步确定这一点，并探索赤字的分子基础。

Research Questions and Significance Primary auditory neurons (spiral ganglion neurons) innervate the receptor cells (hair cells) in the cochlea (the hearing part of inner ear) with a special type of synapses called ribbon synapses that are characterized by the pre-synaptic ribbon structure. Three questions will be addressed. (1) The ribbon is known to facilitate fast neurotransmitter release and therefore critical for encoding fast change of signals—referred as temporal processing ability. However, exactly how the ribbon synapses fulfill this job is not clear. Specifically, there are 15-30 ribbon synapses around each inner hair cell (IHC), and morphology differences are seen for synapses at different locations around an IHC. However, it is not clear if neurons that synapse at the different locations are different in their temporal processing ability. (2) The synaptic ribbons in retina photo receptor cells are dynamic: they disassembled in light and reassembled in dark. It is not clear if this dynamic process exists in IHCs to serve intensity coding of the cochlea. (3) Recently, we demonstrated an interesting synaptic plasticity during the repair process of ribbon synapses after noise-induced damage. This involves breaking down, regeneration and the interaction between ribbons and post-synaptic terminals for the reallocation of the synapses. Functionally, the repaired synapses show temporal processing deficits. However the natures and the mechanisms for the repair and how it impacts the temporal processing remain to be explored. Research on these questions will provide insight how ribbon morphology is connected with the function; and how the ribbon contributes to intensity and temporal coding in the cochlea. The study on the mechanisms and the consequence of the synaptic plasticity will lead the manipulation or control of the plasticity to beneficial direction. Objectives and Research Plan The first goal is to understand the mechanisms of ribbon synapse for temporal processing of IHCs. Previously, the morphology differences were identified in immunohistology. We will use transmission electronic microscope (TEM) to further identify morphology differences in ribbons and post-synaptic terminals around IHCs. I will explore how the morphology differences are related with the temporal processing ability that will be verified in the single unit activities of neurons innervating different positions of IHCs. The second goal is to explore if the ribbons in IHCs are dynamically broken down by acoustic stimulation at the level for normal communication (70-90 dB SPL) and reassembled in quiet. I will compare the morphology of ribbons across different time points re: sound stimulation using TEM. Molecular biological methods will also be used to identify the changes in structure configuration of ribbons (the changes in ribbon protein organization). Functional observation will also used in association with morphology and molecular biology. The third goal is to explore the mechanisms of synaptic plasticity during the repair. I will first identify if the regeneration of ribbons require the synthesis of new proteins, or a simply reassembly of broken ribbons. I will investigate the molecular nature of the shifting from physiological breaking down and the damage, as well as the potential gene control of the repair. I will identify at TEM level if repaired ribbons attract post-synaptic terminals during the re-establishment of the synapses and the relocation of them during the repair as indicated by my recent study. Previous observation in gross evoked responses has shown the temporal processing deficit of repaired ribbon synapses. I will further identify this at single unit level and explore the molecular bases for the deficit.