Synaptic Defects in the Ca Channel Mutant Mouse

描述：（改编自申请人摘要） 编码A类（或P/Q型）基因的天然突变 钙通道与多种异常有关， 从偏头痛到运动性共济失调再到失神性癫痫发作。这些 异质性神经学表型强调了 P/Q型钙通道--中枢神经系统的主要胞吐通道 系统然而，P/Q不是唯一的钙通道控制类型。 CNS中的突触传递。N型（或B类）钙通道通常 与P/Q共存，它们共同管理许多项目的发布， 不是所有的发射机。P/Q和N通道是否发挥独特的功能作用 突触的位置还不清楚用一只P/Q通道突变小鼠进行的实验， 然而，这表明这两个渠道在功能上并不相同。 多余的，摇摇欲坠提供了一个机会，探索他们的不同 在exocytosis中的作用 纯合子蹒跚动物表现出戏剧性的神经学表型， 以共济失调和频繁的失神发作为特征。我们的初步 对摇摆的实验表明，P/Q的一个主要结果是， 通道突变是在某些情况下P/Q：N通道比例的变化（但不是 所有）神经末梢。例如，兴奋性递质的释放 谷氨酸和谷氨酸能突触传递 小脑的纤维-浦肯野细胞突触主要受N型 突变体中的钙通道，而不是野生型中的P/Q型 动物由于突触前钙通道的这些变化， 补体，兴奋性传递减少和G蛋白依赖性 在突变突触处抑制增强。相反，GABA从 抑制性神经末梢在蹒跚动物中似乎不受影响。 根据这些观察，我们假设， 在兴奋性传递上的tottering等位基因导致整体降低 浦肯野细胞的兴奋。有三个相互作用的因素：1） 兴奋性输入的谷氨酸盐释放受损， 参与P/Q通道; 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神经元的敏感性， 抑制（因为N通道更有效地由G蛋白调节 P/Q通道）;和3）未受损的抑制性，GABA能输入是 在面对减少的激励时相对更有效。浦肯野 细胞通过GABA能抑制性传递控制小脑输出， 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强小脑的净输出。最终，这样的 这些变化会刺激丘脑和运动皮层， 在这些动物中观察到的共济失调和癫痫发作。拟议的实验 这里将通过深入细胞和 钙通道突触探索和钙依赖性胞吐作用 摇晃的小脑结果将提供重要信息， 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.