Elemental And Structural Organization Of Neurons And Glia

谷氨酸受体(NMDARs)的NMDARs亚型在正常的中枢神经系统功能中发挥着重要而多样的作用。然而，这些受体的过度激活会导致钙过量内流、线粒体钙超载和功能障碍，是兴奋性损伤的关键早期事件。这建议将NMDAR作为抗兴奋性毒性治疗的靶点，但这种方法通常令人失望的患者结果强烈表明，NMDAR过度激活之外的因素正在发挥作用。这一信息促使人们寻找其他重要的钙依赖损伤途径。下文概述了最近的进展和正在进行的工作的具体目标。 目的#1：明确电压门控钙通道在钙依赖性神经退行性变中的作用。 尽管NMDAR明显在毒性钙负荷中起主导作用，但来自我们实验室和其他实验室的证据表明，钙进入的替代途径，例如通过电压门控钙通道(VGCC)，可以显著促进发育成熟神经元的毒性。因此，我们发现，虽然在一般情况下，在海马区和大脑皮层培养的神经元中，VGCC的激活并不会促进显著的细胞死亡，但在一小部分重要的神经元中，VGCC的激活确实会引起更强的钙升高。这些神经元的特点是VGCCs表达升高，导致细胞死亡的机制与经典的兴奋毒性途径相似，即过度的钙负荷和神经元变性前的线粒体功能障碍。结果表明，谷氨酸毒性的一个辅助途径，其重要性可能会在大脑老化或老年性痴呆中增加。 目的#2：探讨锌在谷氨酸兴奋性毒性和缺血性损伤中的作用。 短暂性脑缺血后细胞内锌升高有助于神经元损伤，但锌毒性的机制(S)尚不清楚。与钙类似，锌被认为通过线粒体功能障碍和/或ROS的产生来诱导毒性。最近在培养的海马神经元上的实验表明，锌可以通过VGCC和钙通透性AMPA受体进入神经元，但只有当细胞外培养液中含有异常高的外源性锌浓度(200微米)时，积累的锌才是有毒的，就像缺血后可能发生的那样。在有利于锌摄取的条件下，钙和锌都在线粒体内积累和共沉淀，但只有钙能够诱导线粒体肿胀、去极化和自由基产生，这是线粒体损伤的主要特征。这些数据支持这一工作假说，即兴奋性毒性线粒体功能障碍的经典机制必然是钙依赖的，因此锌毒性的机制(S)虽然仍不清楚，但肯定不同于钙的机制。

The NMDA subtype of the glutamate receptor (NMDARs) plays essential and diverse roles in normal CNS function. However over-activation of these receptors leads to excessive Ca2+ entry, mitochondrial calcium (Ca2+) overload and dysfunction, and is a crucial early event in excitotoxic injury. This recommends NMDARs as targets for anti-excitotoxic therapies, but the generally disappointing patient outcomes for such approaches strongly suggest that factors beyond the over-activation of NMDARs are at play. This information prompts searches for other important Ca2+-dependent injury pathways. Recent progress and the Specific Aims of ongoing work are summarized next. Aim #1: To define the role of voltage-gated calcium channels in calcium-dependent neurodegeneration. Although NMDARs clearly play the dominant role in toxic Ca2+ loading, evidence from our lab and others indicates that alternative routes of Ca2+ entry, for example, through voltage-gated calcium channels (VGCCs), can contribute significantly to toxicity in developmentally mature neurons. Thus, we find that while in general in hippocampal and cortical cultured neurons VGCC activation does not promote significant cell death, VGCC activation does evoke much stronger calcium elevations in a small but important subset of neurons. These neurons are characterized by elevated expression of VGCCs, which leads to cell death by mechanisms that are reminiscent of the classical excitotoxicity pathway, namely, excessive Ca2+ loading and mitochondrial dysfunction that precedes neuronal degeneration. The results demonstrate one ancillary pathway of glutamate toxicity, one whose significance is likely to increase during brain aging or in age-related dementia. Aim #2: To determine the role of zinc in glutamate excitotoxicity and ischemic injury. Elevation of intracellular zinc (Zn2+) following transient ischemia contributes to neuronal injury, but the mechanism(s) of Zn2+ toxicity remain unclear. In analogy to Ca2+, Zn2+ has been proposed to induce toxicity via mitochondrial dysfunction and/or ROS generation. Recent experiments in cultured hippocampal neurons reveal that Zn2+ can enter neurons through VGCCs and Ca2+-permeable AMPA receptors, but accumulated Zn2+ is only toxic when the extracellular medium contains unusually high (200 uM) exogenous Zn2+ concentrations, as might occur after ischemia. Under conditions that favor Zn2+ uptake, both Ca2+ and Zn2+ accumulate and co-precipitate within mitochondria, but only Ca2+ is capable of inducing mitochondrial swelling, depolarization, and free radical generation, which are the main hallmarks of mitochondrial damage. These data support the working hypothesis that classical mechanisms of excitotoxic mitochondrial dysfunction are necessarily Ca2+ dependent, so that mechanism(s) of Zn2+ toxicity, although still unknown, must be different from that of Ca2+.