Alzheimer’s Disease Pathology as a Clue to Pathogenesis
Full text release has been delayed at the author’s request until August 12, 2013.
Alzheimer’s disease is characterized pathologically by two intracellular lesions, granulovacuolar degeneration and neurofibrillary tangles. Because Alzheimer’s disease is primarily sporadic, the traditional starting point for studying disease is pathology. It is still debated whether these hallmark lesions are markers or mediators of disease progression; however, the correlation between these lesions and disease progression warrants their use as clues to underlying disease processes. The molecular mechanisms of their development, their role in the disease process, and their connection to one another are not fully understood.
Granulovacuolar degeneration involves the accumulation of large, double membrane-bound bodies within certain neurons during the course of disease. Because of their ultrastructure, it is hypothesized that the bodies are autophagic. To test this, colocalization of autophagic and endocytic markers with a marker for granulovacuolar degeneration was investigated in hippocampal sections prepared from post mortem late stage Alzheimer’s disease cases using double-label confocal fluorescence microscopy. The resultant immunohistochemical signature suggests that granulovacuolar degeneration bodies accumulate at the nexus of autophagic and endocytic pathways and that failure to complete autolysosome formation may correlate with their formation. Due to its far-reaching roles in cells, disruption of the endocytic pathways has the potential to act as a nidus central to disease onset and the development of granulovacuolar degeneration as well as neurofibrillary tangles and amyloid plaques.
In sporadic Alzheimer’s disease, neurofibrillary tangle formation is preceded by extensive post-translational modification of tau. To identify the modification signature associated with tau lesion formation at single amino acid resolution, paired helical filaments immunopurified from Alzheimer’s disease brain were subjected to liquid chromatography, tandem mass spectrometry analysis. The resulting spectra identified Lys monomethylation as a new tau modification distributed among seven residues located in the projection and microtubule biding domains of tau protein. One site, K254, was found to be a substrate for a competing Lys modification, ubiquitylation. Double label confocal fluorescence microscopy demonstrated that methylation is wide-spread among neurofibrillary tangles in hippocampal sections prepared from post mortem late-stage Alzheimer’s disease cases. Together these data provide the first evidence that tau in neurofibrillary lesions is modified by Lys methylation.
The tau methylation signature involves sites known to mediate disease-related modifications including ubiquitylation and phosphorylation, suggesting that methylation is a candidate modification for influencing tau aggregation, toxicity, and protein turnover. To extend the correlation between methylation occupancy and disease, the modification state of soluble tau protein isolated from cognitively normal human brain was investigated using proteomic methods. Results showed that normal soluble tau is hypermethylated relative to disease-derived tau in its microtubule binding domain. Furthermore, recombinant human tau subjected to reductive methylation in vitro retains the normal function of tau as a microtubule-binding and stabilizing protein but its aggregation propensity is greatly attenuated. These data establish Lys methylation as a normal tau post-translational modification in human brain that may protect against pathological tau aggregation during aging. Furthermore, data suggests that enzymes responsible for tau methylation, including methyltransferases and demethylases, may be tractable targets for disease-modifying therapies focused on halting neurofibrillary lesion formation in Alzheimer’s disease.
Document number: osu1343231184
Permalink: http://rave.ohiolink.edu/etdc/view?acc_num=osu1343231184
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