The multiple roles of mitochondria in synaptic transmission

描述(申请人提供)：我们的总体目标是确定影响神经递质释放的线粒体机制，以及这些机制对不同类型突触的影响。神经末梢中的线粒体可以很好地影响神经递质的释放，但它们的影响方式一直拒绝澄清。线粒体功能的许多方面都直接与突触可塑性有关，但由于这些活动(ATP产生；钙和钠离子的处理；质子的排出；活性氧的释放)的交织性质，很难确定哪些方面对突触可塑性产生了最初的影响。第二个需要澄清的领域是线粒体在不同形式的短期突触可塑性中的作用。虽然线粒体在强直后突触强度的增强中起着既定的作用，但对于它们对其他形式的突触短期可塑性的影响却知之甚少。最后，虽然我们知道线粒体影响大神经末梢的神经递质释放和突触可塑性，但对它们在哺乳动物中枢神经系统典型的小神经末梢的影响知之甚少。这些都是我们知识中的明显差距，特别是突触的可塑性允许突触强度的变化，这是学习和记忆的一种现象。也许更令人不安的是，线粒体功能障碍在许多神经退行性疾病的中心被发现，其发病机制和进展情况尚不清楚。中心假说是线粒体通过多种机制影响神经递质的释放，而神经末梢的结构及其激发历史决定了哪种机制是有影响的。我们提出了一种电生理、成像和遗传学相结合的方法，在体内的果蝇神经末梢上解决了这一假说，并引入了一种新的具有单一释放位点的外周突触作为具有相同结构的中枢性突触的模型。我们将测试线粒体钙摄取的能力，以限制动作电位短串期间钙瞬变的幅度和神经递质的释放--这是中枢神经元常见的一种放电模式(目标1)。重点将放在单释放部位的神经末梢上，在那里我们观察到线粒体对钙离子有贪婪的胃口。我们将确定这些末端的线粒体是否能够更有效地吸收钙，因为它们能够直接从钙微域中吸收钙(目标2)。我们将确定线粒体ATP的产生，而不是钙的摄取，是否是在持续的神经放电过程中保持同步释放的主要机制(目标3)。最后，我们将测试强直后递质释放增强过程中对线粒体钙释放的要求，以及线粒体和内质网之间钙离子的转移(目标4)。了解线粒体功能在非病理条件下如何影响突触传递，将为理解线粒体在病理条件下的作用提供必要的基础。与公共健康相关：线粒体是人体所有细胞中的细胞器，产生我们大部分的能量。它们集中在神经末梢，在那里它们为神经之间的交流提供动力，这是大脑的一项基本活动。然而，人们对它们对神经系统功能的贡献知之甚少，这一点令人担忧，因为线粒体故障与许多神经系统疾病有关。我们目前正在研究线粒体如何影响健康神经之间的通讯，以便我们可能了解它们参与神经退行性疾病的方式。

DESCRIPTION (provided by applicant): Our overall goal is to determine the mitochondrial mechanisms that influence neurotransmitter release and the impact of these mechanisms across different synapse types. Mitochondria in nerve terminals are well placed to influence neurotransmitter release but their means of influence have resisted clarification. Many facets of mitochondrial function have been directly implicated in synaptic plasticity but due to the interwoven nature of these activities (ATP production; Ca2+ and Na+ handling; extrusion of protons; release of reactive oxygen species) it has been difficult to identify those that make the primary impact. A second area that requires clarification is the role of mitochondria in different forms of short-term synaptic plasticity. Although mitochondria have an established role in the post-tetanic potentiation of synaptic strength, little is known about their impact on other forms of short-term synaptic plasticity. Lastly, while we know that mitochondria influence neurotransmitter release and synaptic plasticity in large nerve terminals very little is known about their influence in small terminals, typical of the mammalian CNS. These are glaring gaps in our knowledge, particularly as synaptic plasticity allows for changes in synaptic strength, a phenomenon underlying learning and memory. More troubling perhaps, is that mitochondrial dysfunction is found at the epicenter of many neurodegenerative conditions for which the pathogenesis and progression are poorly understood. The central hypothesis is that mitochondria influence neurotransmitter release through multiple mechanisms, and the architecture of the nerve terminal and its firing history determines which mechanism is influential. We bring a combined electrophysiological, imaging and genetic approach to address this hypothesis at Drosophila nerve terminals in vivo, and we introduce a novel peripheral synapse with a single release-site as a model for central synapses with the same architecture. We will test the ability of mitochondrial Ca2+ uptake to limit the amplitude of Ca2+ transients and neurotransmitter release during short trains of action potentials - a firing pattern common in central neurons (Aim 1). Emphasis will be placed on single release-site nerve terminals where we observe mitochondria to have a voracious appetite for Ca2+. We will determine if mitochondria in these terminals are more effective at taking up Ca2+ because they are able to take up Ca2+ directly from Ca2+ microdomains (Aim 2). We will determine whether mitochondrial ATP production, rather than Ca2+ uptake, is the principle mechanism that maintains synchronous release during sustained nerve firing (Aim 3). Finally we will test the requirement for mitochondrial Ca2+ release in the post-tetanic potentiation of transmitter release, and examine the transfer of Ca2+ between mitochondria and the endoplasmic reticulum (Aim 4). An understanding of how mitochondrial function influences synaptic transmission under non-pathological conditions will provide the foundation required to understand the role of mitochondria in pathological conditions. PUBLIC HEALTH RELEVANCE: Mitochondria are organelles within all cells of the human body that generate most of our energy. They concentrate within nerve endings where they power communication between nerves, a fundamental activity of the brain. However, little is known about the way in which they contribute to the function of the nervous system and this is troubling, as mitochondrial malfunction is implicated in many diseases of the nervous system. We are currently examining how mitochondria influence the communication between healthy nerves so that we may understand the ways in which they may become involved in neurodegenerative conditions.