Mechanisms of ribbon synapse function and plasticity

初级听觉神经元（螺旋神经节神经元）支配耳蜗（内耳听觉部分）的受体细胞（毛细胞），形成一种特殊类型的突触，称为带状突触，其特征是突触前带状结构。将讨论三个问题。(1)已知该带促进快速神经递质释放，因此对编码快速变化的信号至关重要，即时间处理能力。然而，带状突触是如何完成这项工作的尚不清楚。具体来说，每个内毛细胞（IHC）周围有15-30个带状突触，并且在IHC周围不同位置的突触形态不同。然而，目前尚不清楚在不同位置突触的神经元是否在其时间处理能力上有所不同。(2)视网膜感光细胞的突触带是动态的，在光照下解体，在黑暗中重组。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，我们发现带状突触在噪声损伤后的修复过程中具有有趣的突触可塑性。这包括分解、再生以及条带和突触后终端之间的相互作用，以重新分配突触。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如何影响时间加工仍有待探索。对这些问题的研究将有助于深入了解条带形态与功能之间的关系；以及脑带如何影响耳蜗的强度和时间编码。对突触可塑性的机制和结果的研究，将把对突触可塑性的操纵或控制引向有益的方向。目的和研究计划本研究的第一个目标是了解带状突触在ihc时间加工中的作用机制。以前，形态学差异是在免疫组织学上发现的。我们将使用透射电镜（TEM）进一步鉴定ihc周围条带和突触后末端的形态学差异。我将探讨形态差异如何与时间处理能力相关，这将在支配ihc不同位置的神经元的单个活动中得到验证。第二个目标是探索ihc中的带状结构是否在正常通信水平（70-90 dB SPL）的声刺激下动态分解，并在安静状态下重新组装。我将用透射电镜比较不同时间点声刺激的条带形态。分子生物学方法也将用于鉴定条带结构配置的变化（条带蛋白组织的变化）。功能观察还将与形态学和分子生物学相结合。第三个目标是探索修复过程中突触可塑性的机制。我将首先确定缎带的再生是否需要合成新的蛋白质，还是简单地将断裂的缎带重新组装起来。我将研究从生理分解和损伤转变的分子性质，以及修复的潜在基因控制。我将在TEM水平上确定修复后的带状是否在突触重建过程中吸引突触后末端，以及在修复过程中它们的重新定位，这是我最近的研究表明的。以往对粗诱发反应的观察表明，修复后的带状突触存在时间加工缺陷。我将进一步在单个单位水平上确定这一点，并探索这种缺陷的分子基础。

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.