Impairment of Fast Axonal Transport in Alzheimer's Disease.

摘要

Alzheimer’s disease (AD) is the most common adult-onset neurodegenerative disease, and it affects over 5 million Americans. AD is also the most common cause of dementia, the loss of cognitive functions, among the elderly. Pathological characteristics of AD include the presence of extracellular amyloid plaques and intracellular neurofibrillary tangles, and significant loss of cholinergic neurons in hippocampus and basal forebrain. The cause of AD is largely unknown except for the genetically inherited familial form of AD which mutations in proteins such as Presenilin 1 (PS1), PS2, and amyloid precursor protein (APP) have been identified. Like other neurodegenerative diseases, only neurons are affected in AD although pathological proteins are ubiquitously expressed in many different types of tissues. Why neurons degenerate while other cell types remain normal is not fully elucidated.;Neurons are different from other cell types in that they have highly polarized structure with an axon which can be several thousand times the length of the cell body. There is little to no protein synthesis in the axon which means all the materials required for proper functioning of axon and its terminals must be transported from the site of their synthesis, the cell body in anterograde transport machinery. Also, signaling molecules and materials to be degraded are transported retrogradely in the axon. Cytoskeletal and soluble proteins are transported in the slow component of axonal transport, and fast axonal transport (FAT) transports membrane bounded organelles (MBOs). MBOs are transported in the anterograde direction by kinesins and in the retrograde direction by dyneins. Neurons’ high dependence on axonal transport for their functions and survival makes them more vulnerable to any insult that disrupts transport than other types of cells.;Recently, the role of impaired FAT has been implicated in the pathogenesis of a number of neurodegenerative diseases, including AD. Moreover, a dominant negative mutation of anterograde motor Kif5A is linked to Hereditary Spastic Paraplegia (HSP) Type 10, a progressive neurodegenerative disease that affects lower motor neurons (Reid et al, 2006). However, mutations in motor proteins themselves are rare, and no such mutation is linked to AD. An alternative explanation is that pathological proteins affect FAT’s regulatory processes. FAT is regulated by phosphorylation of motor proteins. Aberrant kinase and phosphatase activities can lead to untimely release of cargoes or abnormal transport velocities which will result in dying back neurodegenration.;In AD, hyperphosphorylation of tau protein provides a clear evidence for dysregulation of phosphotransferase activities. One of the kinases shown to have altered activity in AD is CK2, a pleiotropic Serine/Threonine kinase (Iimoto et al., 1990). Perfusion of isolated squid axoplasm with active CK2 inhibits both directions of FAT (Morfini et al., 2001). However molecular mechanism of this inhibition and its role in AD has not been previously studied. In this thesis, a detailed analysis of molecular events caused by elevated CK2 activity leading to impairment of FAT and their implications in the pathogenesis of AD are described. First, FAT velocity experiments with perfusion of the oligomeric Aβ42 (oAβ42) which is the major component of amyloid plaques, a pathological hallmark of AD, revealed oAβ42 directly activates CK2 and impairs both directions of FAT. Second, molecular mechanisms of FAT disruption by CK2 were studied using biochemical approach. Microtubule (MT) binding assays with primary mouse cortical neurons with CK2 activator revealed kinesin’s binding to MT is reduced upon activation of mouse endogenous CK2. A novel mechanism of alteration in dynein-based retrograde transport by CK2 activation was also identified. Iodixanol vesicle flotation assays failed to detect changes in motor protein binding to MBOs upon CK2 activation, but further study is needed to determine subpopulation specific effect of CK2.;Studies using primary cortical neurons of a mouse model of AD (PS1-ki M146V) demonstrated that both kinesin and dynein binding to MT are reduced in this transgenic mouse line while binding to the MBOs were not affected. Previous studies suggest kinesin light chains are more phosphorylated at CK2 consensus site although alteration in CK2 activity was not detected in permeabilized cortical neuron culture from this animal in current work. Very aggressive mouse model of AD (5xFAD) exhibited brain region specific activation of CK2. These results lead us to present a novel mechanism of the pathogenesis of AD which increased production of Aβ42 leads to altered activation of CK2 which impairs FAT causing neurons in specific brain region degenerate in dying back fashion.;To further understand late-onset nature of AD, additional studies revealed that expression levels of kinesin are developmentally regulated. With less kinesin available at later age, neurons are especially vulnerable to dysregulation of regulatory processes of FAT such as abnormal activation of CK2.