Roles of Cdk5 in neurodevelopment and neurodegeneration

我们之前开发了一个潜在的哺乳动物神经退行性疾病的果蝇模型，方法是使蛋白激酶CDK5失活。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。我们发现，在果蝇的这个内源性过程中，神经退行性综合征与在人类和小鼠中观察到的神经退行性综合征在表型上有许多相似之处，从而验证了我们的果蝇突变体是哺乳动物疾病过程的模型。最近，我们发现CDK5还控制着轴突和树突发育分解的各个方面，特别是微管细胞骨架的稳定性。因此，我们发现，在变态过程中，CDK5活性的丧失推迟了苍蝇中央脑轴突和树突的发育性重塑的开始，而CDK5的过度激活则加速了重塑。遗传相互作用和药理学实验表明，这是部分地，但不是完全地，通过依赖于CDK的这些神经元中微管的失稳来介导的。鉴于神经元发育重塑与疾病病理性退变之间在解剖学和分子水平上的许多相似之处，CDK5的这种功能很可能与p35突变体的神经退变机制直接相关。 我们还启动了对CDK5失活后果的系统的、全基因组的研究。我们已经使用微阵列对具有或缺乏CDK5活性的成年果蝇进行了基因表达谱分析。我们通过分离幼蝇的RNA，在突变体出现明显的细胞或组织缺陷之前，将我们的努力集中在退化的早期事件上。初步分析发现，在缺乏CDK5活性的突变体中，存在两类基因表达变化。我们观察到对神经功能直接重要的基因的变化，包括感觉转导、突触传递和离子稳态。根据我们的表型分析，我们还观察到了与我们先前假设与CDK5功能相关的各种生物生理过程相关的基因表达的变化。这些包括蛋白质平衡、氧化还原状态、线粒体结构和功能以及应激敏感性。因此，微阵列分析作为表型分析的强大独立验证。此外，由于这种无偏见的全基因组阵列分析未能识别早期研究没有预测到的主要基因类别，这些数据表明我们已经确定了CDK5下游负责其病理后果的主要功能通路。值得注意的是，我们发现CDK5下游的许多过程也是自然衰老的生理特征，我们目前正在研究这种联系。

We previously developed a potential Drosophila model of mammalian neurodegenerative disease by inactivating the protein kinase Cdk5. 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. We showed that there are many phenotypic similarities between the neurodegenerative syndrome in this endogenous process of flies and that observed in humans and mice, thus validating our Drosophila mutant as a model of the mammalian disease process. More recently, we have found that Cdk5 also controls various aspects of developmental disassembly of axons and dendrites, in particular the stability of the microtubule cytoskeleton. Thus, we find that loss of Cdk5 activity delays the onset of developmental remodeling of axons and dendrites of the fly central brain during metamorphosis, while hyperactivation of Cdk5 accelerates remodeling. Genetic interaction and pharmacological experiments reveal that this is mediated partly, but not completely, through Cdk-dependent destabilization of microtubules in these neurons. Given the many anatomic and molecular similarities between developmental remodeling of neurons and their pathological degeneration in disease, it is likely that this function of Cdk5 is directly relevant to the mechanism of neurodegeneration in p35 mutants. We also initiated systematic, genome-wide investigations into the consequences of Cdk5 inactivation. We have used microarrays to perform gene expression profiling of adult flies having, or lacking, Cdk5 activity. We have focused our efforts on early events in degeneration by isolating RNA from young flies, before the onset of overt cellular or organismal defects in the mutants. Preliminary analysis reveals two classes of gene expression changes in mutants lacking Cdk5 activity. We observe alterations in genes directly important for neural function, including sensory transduction, synaptic transmission and ion homeostasis. We also observe changes in the expression of genes associated with a variety of organismal physiological processes we had previously hypothesized to be associated with Cdk5 function, based on our phenotypic analysis. These include proteostasis, redox state, mitochondrial structure and function and stress-sensitivity. The microarray analysis, therefore, serves as a powerful independent validation of the phenotypic analysis. Moreover, as this unbiased, genome-wide array analysis failed to identify major classes of genes not predicted by the earlier studies, these data suggest that we have identified the major functional pathways downstream of Cdk5 that are responsible for its pathological consequences. Remarkably, we find that many of the processes downstream of Cdk5 are also physiological hallmarks of natural aging, and we are currently investigating this connection.