Abstract
The self-assembly of the protein tau into neurofibrillary tangles is one of the hallmarks of Alzheimer’s disease and related tauopathies. Still, the molecular mechanism of tau aggregation is largely unknown. This problem may be addressed by systematically obtaining reproducible in vitro kinetics measurements under quiescent conditions in the absence of triggering substances. Here, we implement this strategy by developing protocols for obtaining an ultrapure tau fragment (residues 304–380 of tau441) and for performing spontaneous aggregation assays with reproducible kinetics under quiescent conditions. We are thus able to identify the mechanism of fibril formation of the tau 304–380 fragment at physiological pH using fluorescence spectroscopy and mass spectrometry. We find that primary nucleation is slow, and that secondary processes dominate the aggregation process once the initial aggregates are formed. Moreover, our results further show that secondary nucleation of monomers on fibril surfaces dominates over fragmentation of fibrils. Using separate isotopes in monomers and fibrils, through mass spectroscopy measurements, we verify the isotope composition of the intermediate oligomeric species, which reveals that these small aggregates are generated from monomer through secondary nucleation. Our results provide a framework for understanding the processes leading to tau aggregation in disease and for selecting possible tau forms as targets in the development of therapeutic interventions in Alzheimer’s disease.
Highlights
Alzheimer’s disease (AD) is a devastating neurodegenerative disease of increasing prevalence.[1−3] The pathology of AD is linked to the self-assembly of the amyloid β (Aβ) peptide into extracellular plaques and of the tubulin-associated unit protein into intracellular tangles in the brains of AD patients
The protein may misfold and aggregate into neurofibrillary tangles. Such misfolded forms of tau have been identified in several human neurodegenerative diseases, collectively known as tauopathies, including AD, Danish and British dementias, white matter tauopathy with globular glial inclusions, frontotemporal dementia, Pick’s disease, progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), argyrophilic grain disease (AGD), Guam Parkinsonism− dementia complex, tangle-only dementia, Parkinsonism linked to chromosome 17 (FTDP-17T), and Gerstmann−Straü ssler− Scheinker disease.[14−17] Several reports indicate that posttranslational modifications may differentiate between these tauopathies.[18−21] Tau can be secreted from neurons[22] and has been observed in blood and cerebrospinal fluid (CSF23), in which the levels of phosphorylated tau isoforms serve as biomarkers for AD.[24]
The results reveal that the aggregation mechanism of the tau fragment is governed by secondary processes and that secondary nucleation on fibril surfaces is the dominant source of new oligomers
Summary
Alzheimer’s disease (AD) is a devastating neurodegenerative disease of increasing prevalence.[1−3] The pathology of AD is linked to the self-assembly of the amyloid β (Aβ) peptide into extracellular plaques and of the tubulin-associated unit (tau) protein into intracellular tangles in the brains of AD patients. Tau was isolated and identified in 1975,11 and its concentration in neuronal cells is found to be around 2 μM.[12,13] The protein may misfold and aggregate into neurofibrillary tangles Such misfolded forms of tau have been identified in several human neurodegenerative diseases, collectively known as tauopathies, including AD, Danish and British dementias, white matter tauopathy with globular glial inclusions, frontotemporal dementia, Pick’s disease, progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), argyrophilic grain disease (AGD), Guam Parkinsonism− dementia complex, tangle-only dementia, Parkinsonism linked to chromosome 17 (FTDP-17T), and Gerstmann−Straü ssler− Scheinker disease.[14−17] Several reports indicate that posttranslational modifications may differentiate between these tauopathies.[18−21] Tau can be secreted from neurons[22] and has been observed in blood and cerebrospinal fluid (CSF23), in which the levels of phosphorylated tau isoforms serve as biomarkers for AD.[24] Since aggregated forms of tau have been identified in the extracellular space,[25] both intracellular and extracellular tau may play a role in the disease. Several reports indicate that misfolded and aggregated forms of tau may be Received: July 9, 2021 Accepted: October 19, 2021 Published: November 16, 2021
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