Roles of Cdk5 in neurodevelopment and neurodegeneration

我们之前通过灭活蛋白激酶CDK5/p35建立了一种潜在的哺乳动物神经退行性疾病的果蝇模型。这是两种主要蛋白质之一的果蝇同源物，这两种蛋白质负责将tau磷酸化成在神经原纤维缠结中发现的形式，这是许多形式的人类神经退化的特征。这种蛋白激酶在苍蝇体内的失活会产生各种表型后果，这让人想起人类神经退行性疾病中看到的现象。在已经发表的实验中，我们发现CDK5也调节发育程序化形式的轴突和树突的分解，目前正在进行重新提交之前的最后修订。果蝇中央大脑蘑菇体的程序化修剪和重塑在机制上与疾病中看到的轴突丢失有许多相似之处。我们发现，CDK5控制轴突和树突分解的时间和速度，而且它部分是通过控制微管细胞骨架的稳定性来做到这一点的。然而，我们也发现，CDK5必须至少使用另一种机制，与其对微管的作用平行，后者更直接地针对轴突和树突在轴突分解级联反应后期的最终碎裂。这些数据开始为我们提供一种方法来区分不同的CDK5依赖机制，这些机制有助于在发育和疾病中全面分解轴突。 在这些针对CDK5/p35的有针对性的发育和表型研究的同时，我们也一直在对该激酶进行更广泛的系统分析。我们发现，缺乏激酶活性的果蝇的全基因组基因表达谱与具有高活性激酶的果蝇的基因表达谱显示出极强的相关性。这与哺乳动物表达不足/过度表达的生理学分析相一致，在哺乳动物中，CDK5/p35的获得和功能丧失都会导致细胞培养中的神经元死亡和体内的神经退化。然而，值得注意的是，CDK5/p35活性改变的果蝇的基因表达谱也显示出与衰老过程中观察到的变化谱密切相关。这或许开始暗示，长期以来公认的神经退行性疾病与衰老之间存在联系，这可能是一种机制基础。

We previously developed a potential Drosophila model of mammalian neurodegenerative disease by inactivating the protein kinase Cdk5/p35. This is the fly homolog of one of the two main proteins responsible for phosphorylating tau into the form found in the neurofibrillary tangles that are characteristic of many forms of human neurodegeneration. Inactivation of this protein kinase in vivo in the fly has a variety of phenotypic consequences that are reminiscent of phenomena seen in human neurodegenerative disease. In experiments that have been favorably reviewed for publication and are currently undergoing final revision prior to resubmission, we have now found that Cdk5 also regulates a developmentally-programmed form of axonal and dendritic disassembly in the fly. Programmed pruning and remodeling of the mushroom bodies in the Drosophila central brain bears many mechanistic similarities to the neurite loss seen in disease. We find that Cdk5 controls the timing and rate of axonal and dendritic disassembly, and that it does so in part by controlling the stability of the microtubule cytoskeleton. We also find, however, that Cdk5 must use at least one other mechanism, acting in parallel to its effects on microtubules, which more directly targets the final fragmentation of axons and dendrites late in the neurite disassembly cascade. These data begin to give us a way to discriminate the different Cdk5-dependent mechanisms that contribute to overall neurite disassembly in development and disease. In parallel with these targeted developmental and phenotypic studies of Cdk5/p35, we have also been performing a more broadly focused systems analysis of the kinase. We find that the genome-wide gene expression profile of flies lacking kinase activity shows extremely strong correlation to that of flies with hyper-activated kinase. This agrees with the physiological analysis of under/over expression in mammals, where gain and loss of function of Cdk5/p35 both result in neuronal death in cell culture and neurodegeneration in vivo. Remarkably, however, the gene expression profile of flies with altered Cdk5/p35 activity also shows close correlation with the profile of changes observed during aging. This begins to hint, perhaps, at a possible mechanistic basis for the long-recognized association between neurodegenerative disease and aging.