Synaptic Defects in the Ca Channel Mutant Mouse

描述：(改编自申请者摘要) 编码A类(或P/Q型)基因的自然发生突变 钙通道与多种异常有关，范围从 偏头痛从运动性共济失调到失神癫痫发作。这些 不同的神经学表型强调了 P/Q型钙通道--中枢神经主要的胞吐通道 系统。然而，P/Q并不是钙通道控制的唯一类型 中枢神经系统的突触传递。N型(或B类)钙通道通常 与P/Q共存，并共同管理许多产品的发布，如果 不是全部，是传送器。P/Q和N通道是否发挥独特的功能作用 突触的位置尚不清楚。用一只P/Q通道突变小鼠进行实验， 然而，摇摇欲坠表明，这两个通道在功能上并不是 这是多余的，这种摇摇欲坠提供了一个机会来探索他们的不同 在胞吐作用中的作用。 纯合子摇摇欲坠的动物表现出戏剧性的神经表型， 以共济失调和频繁的失神发作为特征。我们的预赛 关于摇摇欲坠的实验表明，P/Q的一个主要后果 通道突变是某些(但不是)通道中P/Q：N通道比率的变化 全部)神经末梢。例如，释放兴奋性传送器 谷氨酸和谷氨酸能突触的平行传递 小脑纤维-浦肯野细胞突触主要受N型调控 突变体中的钙通道，而不是野生型的P/Q型 动物。由于突触前钙通道的这些变化 补体、兴奋性传递减少并依赖G蛋白 突触突触的抑制作用增强。相比之下，GABA从 在摇摇欲坠的动物身上，抑制性神经末梢似乎没有受到影响。 在这些观察的基础上，我们假设 兴奋性传递上的摇摇欲坠的等位基因导致总体下降 浦肯野细胞的兴奋。三个相互作用的因素起作用：1) 兴奋性输入的谷氨酸释放受到损害，因为 P/Q通道受累；2)N通道介导的相对增加 释放进一步增强了这些输入对突触前的敏感性 抑制(因为N通道更有效地由G蛋白调节 P/Q通道)；以及3)未受损的抑制性GABA能输入是 相对更有效的面对减少的激励。作为Purkinje 细胞通过GABA能抑制传递控制小脑输出 小脑深部核团的输出神经元，我们预测 浦肯野细胞的活动将增加小脑的净输出。最终，这样的 变化会刺激丘脑和运动皮质，提供一种合理的机制 在这些动物身上观察到的共济失调和癫痫。建议进行的实验 这里将通过深入的细胞和 钙通道与钙依赖性胞吐作用的突触探索 摇摇欲坠的小脑。结果将为以下方面提供重要信息 了解突变对小脑回路行为的影响 从长远来看，可能会为新的治疗干预措施提供建议 共济失调和其他运动障碍。

DESCRIPTION:(adapted from applicant's abstract) Naturally-occurring mutations in the gene encoding class A (or P/Q-type) calcium channels are associated with multiple abnormalities, ranging from migraine headache to motor ataxias to absence epileptic seizures. These heterogeneous neurological phenotypes underscore the central importance of P/Q-type calcium channels-the dominant exocytotic channels in central nervous system. P/Q is not, however, the only type of calcium channel controlling synaptic transmission in the CNS. N-type (or class B) calcium channels usually co-exist with P/Q and, together, they jointly govern the release of many, if not all, transmitters. Whether P/Q and N channels play unique functional roles at the synapse is unclear. Experiments with one P/Q channel mutant mouse, tottering, suggest, however, that the two channels are not functionally redundant and that tottering offers an opportunity to explore their different roles in exocytosis. Homozygous tottering animals display a dramatic neurological phenotype, characterized by ataxia and frequent absence seizures. Our preliminary experiments on tottering demonstrate that a primary consequence of the P/Q channel mutation is a shift in the ratio of P/Q:N channels in some (but not all) nerve terminals. For example, release of the excitatory transmitter glutamate and glutamatergic synaptic transmission at the parallel fiber-Purkinje cell synapse in cerebellum are controlled largely by N-type calcium channels in the mutant, rather than P/Q-type as they are in wild-type animals. As a consequence of these changes in the presynaptic calcium channel complement, excitatory transmission is reduced and G protein-dependent inhibition is enhanced at mutant synapses. In contrast, GABA release from inhibitory nerve terminals appears to be unaffected in tottering animals. On the basis of these observations, we hypothesize that the selective effect of the tottering allele on excitatory transmission leads to an overall decreased excitation of Purkinje cells. Three interacting factors contribute: 1) glutamate release from excitatory inputs is impaired due to the decreased involvement of P/Q channels; 2) the relative increase in N channel-mediated release further enhances susceptibility of these inputs to presynaptic inhibition (because N channels are more effectively modulated by G proteins than are P/Q channels); and 3) unimpaired inhibitory, GABAergic inputs are relatively more efficacious in the face of reduced excitation. As Purkinje cells control cerebellar output via GABAergic inhibitory transmission onto output neurons in deep cerebellar nuclei, we predict that a reduction in Purkinje cell activity will enhance net cerebellar output. Ultimately, such changes would excite thalamus and motor cortex, providing a plausible mechanism for the ataxia and seizures observed in these animals. Experiments proposed here will stringently test this hypothesis through in-depth cellular and synaptic exploration of calcium channels and calcium-dependent exocytosis in tottering cerebellum. Results will provide essential information for understanding the consequences of the mutation on cerebellar circuit behavior and may, in the long term, offer suggestions for new therapeutic interventions into ataxia and other motor disorders.