This installment of Neuroscience Today, Neurology Tomorrow explores amyloid-beta peptide biology and discovers a specific effect on axonal transport. Though still at much too early a stage to consider clinical trials, this represents the opening of a new door for the development of treatments targeting amyloid function rather than amyloid itself.

Oligomeric amyloid-beta appears to cause neurotoxicity via a mechanism that disrupts the transport of proteins and organelles in both directions along an axon, according to two studies that are the first to establish specific mechanisms responsible for the early synaptic dysfunction seen in Alzheimer’s disease.

Working mostly independent of each other, two research groups focused on finding the intraneuronal targets affected by small, soluble aggregates of amyloid-beta (Abeta), especially oligomers, by examining their effect on the processes of synaptic transmission and fast axonal transport (FAT) in squid giant axons. FAT is essential for the proper function and survival of neurons because axons–unlike neuronal dendrites or cell bodies–lack the machinery for protein synthesis. So axons must transport essential molecules and organelles from the cell body.

Both groups found that tiny amounts of oligomers of the 42-amino acid species of Abeta peptide (Abeta42) increased the activity of casein kinase 2 (CK2), an enzyme involved in the regulation of FAT.

One group, led by Gustavo Pigino, Ph.D., of the University of Illinois at Chicago, and the Marine Biological Laboratory at Woods Hole, Mass., found that the activation of CK2 by oligomeric Abeta42 inhibited FAT by causing a component of one of the molecular motors involved in the transport, kinesin-1, to release its vesicular cargo. This effect occurred independently of transcription or translation because the researchers were working with extruded axoplasmsthat were separated from cell bodies (Proc. Natl. Acad. Sci. U.S.A. 2009 [doi:10.1073/pnas.0901229106]).

The other group, led by Dr. Herman W. Moreno of the State University of New York Downstate Medical Center, Brooklyn, and the Marine Biological Laboratory, found additional evidence in support of FAT inhibition when they observed that high-frequency stimulation of presynaptic terminals in axons treated with oligomeric Abeta42 led to a significant increase in the number of nondocked vesicles (Proc. Natl. Acad. Sci. U.S.A. 2009 [doi:10.1073/pnas.0900944106]).

“Dysregulation of FAT results in reduced activity of critical synaptic elements required for integrity, maintenance, and function of synapses, leading to synaptic failure and a dying-back pattern of neurodegeneration,” Dr. Pigino and his coinvestigators wrote.

Dr. Pigino and his associates found that FAT was not inhibited by intracellular unaggregated or fibrillar forms of Abeta42, while Dr. Moreno’s group also found that acute extracellular administration of oligomeric Abeta42 did not inhibit synaptic transmission.

Both groups found no effect of Abeta40 on synaptic dysfunction. Under normal cellular conditions, Abeta40 is normally produced to a greater extent than Abeta42, which is produced more often under the pathogenic conditions of Alzheimer’s disease.

Dr. Caselli’s comment: The amyloid cascade hypothesis places the generation of Abeta amyloid at the center of the destructive cycle that results in Alzheimer’s disease, and for many years, attention was focused on actual plaques, conglomerations of aggregated fibrillar amyloid, and dystrophic neurites that are one of its neuropathologic hallmarks. Intracellular Abeta, which derives from intracellular production and reuptake from extracellular sources, was described in 1993 (Proc. Natl. Acad. Sci. U.S.A. 1993;90:9513-7), though its pathogenic potential was not fully understood. Over the past 5-10 years, there has been a growing understanding of the potential pathogenic role played by soluble Abeta monomers and oligomers distinct from plaques (Nat. Neurosci. 2005;8:79-84), and of intraneuronal Abeta oligomers (Neurobiol. Aging 2005;26:1235-44). The studies from Dr. Pigino and colleagues and Dr. Moreno and colleagues have shown that soluble oligomers of Abeta, and not fibrillar or aggregated Abeta, disrupt synaptic function through intraneuronal rather than extracellular mechanisms. The synaptic dysfunction is the result of impaired bidirectional axonal transport due to Abeta-mediated activation of a specific enzyme system (CK2) that Dr. Moreno and associates have shown to be pharmacologically reversible with the CK2 inhibitor, DMAT (2-dimethylamino-4,5,6,7 tetrabromo-1H-benzimidazole).

Therapeutic strategies targeting amyloid have been less successful that predicted, but none to date has blocked a specifically identified amyloid function. The discoveries made by these investigators raise the possibility of a therapeutic strategy targeting amyloid function rather than amyloid itself. Whether this will prove to be the critical function, however, remains to be determined. The current experiments were conducted on squid giant axons, and the next logical step would seem to be development of a pharmacologic agent that might be tested in transgenic animal models of Alzheimer’s disease. Drug discovery and animal model testing foretell a long and difficult road, but may represent the way to more effective therapy.

Clinical perspective by Dr. Caselli, chair of neurology at the Mayo Clinic, Scottsdale, Ariz., and professor of neurology at the Mayo Medical School, Rochester, Minn.

Research report by Jeff Evans, clinical news editor, Elsevier Global Medical News.

Copyright (c) 2009 Elsevier Global Medical News. All rights reserved. This material may not be published, broadcast, rewritten, or redistributed.

“今天的神经科学，未来的神经病学”专栏的一篇文章探讨了β淀粉样蛋白的生物学特征，并发现了它对轴突运输的一种特殊的作用。虽然距离进行临床试验阶段尚远，但是却为治疗淀粉样蛋白功能而不是淀粉样蛋白本身开辟了一条新思路。

根据两项阿尔茨海默病患者早期突触功能障碍机制的研究结果显示，β淀粉样蛋白通过干扰轴突对蛋白质和细胞器的双向运输导致其神经毒性。

两个独立工作的小组进行了两项研究，他们研究的对象都是小的、可溶性的β淀粉样蛋白聚合物，特别是低聚物对神经细胞内某些靶点的作用。检测方法是观察他们对乌贼巨轴突的突触传递和快速轴突运输(FAT)的作用。FAT是神经元功能和存活的基础。因为与树突和胞体不同，轴突缺乏蛋白质合成的结构，因此，轴突必须从胞体运输必需的分子和细胞器。

两组均发现少量的42氨基酸组成的低聚物——Abeta肽（Abeta42）可以增加酪蛋白激酶2的活性，而酪蛋白激酶2参与FAT的调节。

其中，由芝加哥伊利诺伊大学和马萨诸塞州Woods Hole海洋生物实验室的Gustavo Pigino教授领导的小组发现，Abeta42低聚物通过促进参与运输的驱动蛋白1（一种分子马达成分）释放其囊泡内的物质，激活酪蛋白激酶2而抑制FAT。因为研究者是将轴浆从细胞体内分离出来以后进行试验，所以这一作用与转录或者翻译过程无关。(Proc.Natl.Acad.Sci.U.S.A.2009[doi:10.1073/pnas.0901229106])

由位于布鲁克林的纽约州立大学医学中心和海洋生物实验室的Herman W.Moreno医生领导另外一组，发现了更多支持FAT被抑制的证据。他们发现高频刺激经低聚物Abeta42处理过的轴突突触前膜，可以显著增加未靠近前膜并释放囊内物质的囊泡数量。(Proc. Natl. Acad. Sci. U.S.A. 2009 [doi:10.1073/pnas.0900944106])

 “FAT的功能失调原因是保持突触完整性、稳定性以及功能决定性成分的活性降低，进而导致突触的破坏和不可逆性的神经变性，”Pigino医生和他的研究人员写到。

Pigino医生和他的同事发现细胞内未凝集的Abeta42，或者纤维状的Abeta42并不会抑制FAT，而Moreno医生的小组同样发现在细胞外快速给予低聚Abeta42注射也不会抑制突触传递。

两个小组均认为Abeta40对轴突功能障碍没有作用。在正常细胞环境下，Abeta40的合成量远远高于Abeta42，但是在阿尔茨海默病的病理条件下，后者的合成会明显增多。

Caselli医生评论：淀粉样蛋白级联假说把Abeta淀粉样蛋白的产生视为阿尔茨海默病的破坏性循环的核心步骤。多年以来，人们的注意力一直放在已经存在的斑块和聚集的纤维状淀粉样蛋白上，其主要神经病原学特征是神经突触的营养不良。人们在1993年就发现，细胞内的Abeta在细胞内产生，并在细胞外重吸收(Proc. Natl. Acad. Sci. U.S.A. 1993;90:9513-7)，但是它的潜在的致病机理尚不清楚。在过去的5到10年间，对于可溶性Abeta单体以及斑块外的低聚物(Nat. Neurosci. 2005;8:79-84)，以及细胞内Abeta低聚物(Neurobiol. Aging 2005;26:1235-44)的潜在致病性的认识不断加深。Pigino等人和Moreno等人的研究发现，可溶性的Abeta低聚物以及未纤维化或者聚集的Abeta通过细胞内而不是细胞外的机制破坏了突触的功能。突触功能障碍的原因是Abeta介导的特殊的酶系统（酪蛋白激酶2）的活化破坏了轴突的双向运输功能。Moreno医生和他的同事发现酪蛋白激酶2的抑制药物DMAT可以逆转这一过程。

针对淀粉样蛋白的治疗策略并没有获得预期的成功，迄今为止，尚没有阻断某种特殊的被确认的淀粉样蛋白的方法。以上这些研究的发现增加了开展针对淀粉样蛋白的功能，而不是淀粉样蛋白本身的治疗策略的可能性。这些功能是否非常重要尚有待于证实。目前的实验研究对象是乌贼的巨型轴突，下一步研究主要是针对转基因的阿尔茨海默病动物模型进行药理试验。药物的研发和动物模型试验都是漫长的道路，但是这条道路可能通向更加有效的治疗方法。

Casell博士是亚利桑那州斯科特斯德Mayo医院神经内科主任，明尼苏达州罗彻斯特Mayo医学院神经病学教授。

本新闻由EGMN临床医学主任Jeff Evans报道。

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