Mechanisms of axon guidance during development

发育过程中轴突引导的机制

• 批准号: 7735301
• 负责人: edward giniger
• 金额: $ 104.63万
• 依托单位: NATIONAL INSTITUTE OF NEUROLOGICAL DISORDERS AND STROKE
• 依托单位国家: 美国
• 资助国家: 美国
• 起止时间: 至
• 项目状态: 未结题
• 来源: https://reporter.nih.gov/project-details/7735301
• 关键词: Actins; Adult; Affect; Aging; Alzheimer' s Disease; Amyotrophic Lateral Sclerosis; Animals; Axon; Behavioral; Biochemical Genetics; Biological Models; Biology; Birth; Brain; Brain region; Cancer Etiology; Cell Adhesion; Cell Nucleus; Cell Shape; Cell Surface Receptors; Cell membrane; Cells; Cessation of life; Characteristics; Class; Complex; Coupling; Cytoskeletal Proteins; Cytoskeleton; Defect; Dendrites; Development; Disabled Persons; Disease; Drosophila genus; Drosophila notch protein; Elements; Embryo; Endopeptidases; Event; Genes; Genetic; Growth; Growth Cones; Head; Health; Histology; Homologous Gene; Human; Image; Invertebrates; Lead; Learning; Lesion; Life; Ligands; Link; Localized; Longevity; Malignant Neoplasms; Mammals; Methods; Modeling; Modification; Molecular; Molecular Genetics; Morphology; Motor; Mus; Muscle Rigidity; Mutate; Mutation; Nerve; Nerve Degeneration; Nervous system structure; Neurodegenerative Disorders; Neurofibrillary Tangles; Neurons; Neuropil; Notch Signaling Pathway; Orthologous Gene; Pathway interactions; Patients; Pattern; Peptide Hydrolases; Phosphotransferases; Play; Population; Process; Property; Protein Kinase; Protein Tyrosine Kinase; Proteins; Receptor Signaling; Regulation; Rhabdomyosarcoma; Role; Signal Pathway; Signal Transduction; Standards of Weights and Measures; Stroke; Structure; Synapses; Syndrome; Tauopathies; Tissues; Tyrosine; Tyrosine Phosphorylation; Week; abl Oncogene; axon guidance; base; cell motility; cell type; computerized data processing; embryonic stem cell; fly; genetic manipulation; human disease; interest; intracellular protein transport; leukemia; malignant breast neoplasm; medulloblastoma; mutant; neuronal cell body; neuronal survival; notch protein; null mutation; prevent; protein localization location; receptor; relating to nervous system; research study; response; tissue fixing; trafficking

How is the proper pattern of neural connections established during development? And how is that pattern maintained in the adult nervous system? These are the questions that the Axon Guidance and Neural Connectivity Unit seeks to answer. To understand the mechanisms underlying the establishment of neural connections, we focus on what might be termed the "elementary event" in the process of neural wiring, the mechanism by which a single cell-surface receptor tells a developing neuron where to grow in order to find its synaptic partners. We study a particular cell surface receptor called Notch. Notch is notable because, in addition to directing nerve growth, it also controls the branching of dendrites, the identities of neurons (and many other types of cells), how many neurons are born and whether cells live or die. As such, it controls many aspects of animal development and is responsible for a wide array of human diseases, including some kinds of cancer and stroke. What we learn about Notch in axons, therefore, has implications for biology and health far beyond the particular process we are examining. Previous studies of Notch have focused on a single signaling mechanism for this ubiquitous receptor. We have found, however, that this is only half of the story. About 5% of the Notch protein in the embryo is tyrosine phosphorylated, and this population of molecules associates specifically with an alternate group of downstream effectors, the Abl oncogene and its associated accessory factors, to directly control cell-cell contacts, cell shape and cell migration. We have shown, moreover, that this alternate Notch signaling pathway acts at the plasma membrane (as opposed to the standard signaling pathway, which targets events in the cell nucleus), and that it acts via a protein called Rac that is a direct regulator of the actin cytoskeleton and of cell-adhesion complexes. In growing nerves, the activity of the Notch/Abl/Rac machinery is revealed as regulation of the direction and extent of nerve growth. Current experiments are directed at continuing to elucidate the molecular mechanism of this alternate signaling pathway, and to determine where besides growing axons it may act in biology and disease. We are particularly interested by evidence that the alternate Notch signaling pathway we have discovered may be central to controlling the survival of neural and embryonic stem cells, and that it is key to the mechanism by which activation of Notch can cause cancers, including medulloblastoma, leukemia, rhabdomyosarcoma and breast cancer. There is a great deal of interest in developing models of neurodegenerative diseases in simple, invertebrate model systems that provide unequaled experimental power for characterizing the cellular events of a complex process and establishing its molecular genetic basis. Use of Drosophila for studies of neurodegeneration have been problematic, however, since in general they have either relied on highly artificial manipulations, such as high-level expression of mutated human genes in the fly, or have identified genes that clearly affect neuronal survival in the fly but are not related to any gene or pathway demonstrated to play a role in neurodegeneration in mammals. We have now identified a natural, adult-onset neurodegenerative syndrome of Drosophila in flies mutant for the ortholog of a gene directly implicated in human diseases including Alzheimer Disease and ALS. The protein kinase Cdk5, together with its regulatory subunit, p35, is one of the major kinases that phosphorylates cytoskeletal proteins to generate the neurofibrillary tangles that are characteristic of the "tauopathy" class of neurodegenerative diseases. Moreover, activated Cdk5 is found concentrated in degenerating tissue in the brains of Alzheimer patients, and experimental activation of Cdk5 induces degenerating lesions in the mouse brain. We have generated a null mutation of the gene encoding the fly homolog of the Cdk5 activating subunit, p35. We find the mutants are viable and fertile, and are behaviorally normal at birth. However, within a few weeks they show progressive loss of motor coordination, culminating in rigidity and then death, with a significantly shortened lifespan (30% shorter than matched controls). Sectioning the heads of aging p35 mutant flies reveals degenerative lesions in the brain, initially in the neuropil but also around the cell bodies. Remarkably, these lesions are highly localized, being present bilaterally in specific brain nuclei, but not generally distributed through the brain, even though p35 and Cdk5 are present and active throughout the brain. Therefore, these mutants may allow us not only to uncover the genetic pathway leading to Cdk5-associated neurodegeneration, but also to understand how and why disease processes that occur throughout the brain lead to very specific and characteristic structural and behavioral defects in particular brain regions. Moreover, in mutant animals we observe widespread defects in axon patterning, synaptic morphology and protein localization within axons long before we see overt degeneration, raising the possibility that late onset degeneration may actually reflect a delayed response to defective nervous system structure early in development.

在发育过程中，神经连接的适当模式是如何建立的？这种模式是如何在成人神经系统中维持的？这些都是Axon指导和神经连接单位试图回答的问题。 为了了解神经连接建立的潜在机制，我们将重点放在神经连接过程中可能被称为“初级事件”的过程中，即单个细胞表面受体告诉发育中的神经元在哪里生长以寻找其突触伙伴的机制。我们研究一种特殊的细胞表面受体，称为Notch。Notch之所以引人注目，是因为它除了指导神经生长外，还控制树突的分支、神经元(和许多其他类型的细胞)的身份、多少神经元的诞生以及细胞的存活或死亡。因此，它控制着动物发育的许多方面，并对人类的一系列疾病负责，包括一些种类的癌症和中风。因此，我们对轴突Notch的了解对生物学和健康的影响远远超出了我们正在研究的特定过程。以前对Notch的研究主要集中在这种普遍存在的受体的单一信号机制上。然而，我们发现这只是故事的一半。胚胎中约5%的Notch蛋白是酪氨酸磷酸化的，这组分子与另一组下游效应因子Abl癌基因及其相关辅助因子特异性地结合，直接控制细胞与细胞的接触、细胞形状和细胞迁移。此外，我们还表明，这种替代的Notch信号通路作用于质膜(与以细胞核中的事件为靶点的标准信号通路相反)，它通过一种名为Rac的蛋白质发挥作用，Rac是肌动蛋白细胞骨架和细胞黏附复合体的直接调节因子。在生长神经中，Notch/Abl/Rac机制的活动表现为对神经生长方向和程度的调节。目前的实验旨在继续阐明这种替代信号通路的分子机制，并确定除了生长轴突外，它还可能在生物学和疾病中发挥作用。我们特别感兴趣的证据是，我们发现的替代Notch信号通路可能是控制神经和胚胎干细胞存活的核心，并且它是Notch激活可导致癌症(包括髓母细胞瘤、白血病、横纹肌肉瘤和乳腺癌)的机制的关键。 在简单的无脊椎动物模型系统中建立神经退行性疾病的模型引起了人们的极大兴趣，这些模型系统为描述复杂过程的细胞事件并建立其分子遗传学基础提供了无与伦比的实验力量。然而，利用果蝇来研究神经退行性变是有问题的，因为通常它们要么依赖于高度人工的操作，例如在果蝇中高水平表达突变的人类基因，要么已经识别出明显影响果蝇神经元存活的基因，但与任何被证明在哺乳动物神经退行性变中发挥作用的基因或途径无关。我们现在已经在果蝇中发现了一种自然的、成人起病的神经退行性综合征，该基因的同源突变直接与包括阿尔茨海默病和ALS在内的人类疾病有关。蛋白激酶CDK5及其调节亚基p35是一种主要的激酶，它使细胞骨架蛋白磷酸化，产生神经原纤维缠结，这是一类神经退行性疾病的特征。此外，激活的CDK5被发现集中在阿尔茨海默病患者大脑中的退行性组织中，实验中激活的CDK5会导致小鼠大脑中的退行性病变。我们已经产生了编码CDK5激活亚基p35的苍蝇同源基因的零突变。我们发现这些突变体是可行的和可生育的，并且在出生时行为正常。然而，在几周内，他们表现出运动协调性的进行性丧失，最终导致僵硬，然后死亡，寿命显著缩短(比匹配的对照组短30%)。对老化的p35突变果蝇的头部进行切片，发现了大脑中的退行性损伤，最初是在神经纤维，但也在细胞体周围。值得注意的是，这些损伤是高度局部性的，存在于特定的脑核，但并不是普遍分布在大脑中，尽管p35和CDK5存在并活跃于整个大脑。因此，这些突变体可能不仅可以让我们揭示导致CDK5相关神经退化的遗传途径，还可以了解整个大脑中发生的疾病过程如何以及为什么会导致特定脑区非常特定和特征性的结构和行为缺陷。此外，在突变动物中，我们观察到轴突模式、突触形态和轴突内蛋白质定位的广泛缺陷，远在我们看到明显的变性之前，这增加了迟发性变性实际上可能反映了对发育早期有缺陷的神经系统结构的延迟反应的可能性。