Sequence of neurofibrillary changes in aging and Alzheimer's disease: A confocal study with phospho-tau antibody, AD2

105

Filamentous inclusions composed of the microtubule-associated protein tau are found in Alzheimer disease and other tauopathic neurodegenerative diseases, but the mechanisms underlying their formation from full-length protein monomer under physiological conditions are unclear. To address this issue, the fibrillization of recombinant full-length four-repeat human tau was examined in vitro as a function of time and submicromolar tau concentrations using electron microscopy assay methods and a small-molecule inducer of aggregation, thiazine red. Data were then fit to a simple homogeneous nucleation model with rate constant constraints established from filament dissociation rate, critical concentration, and mass-per-unit length measurements. The model was then tested by comparing the predicted time-dependent evolution of length distributions to experimental data. Results indicated that once assembly-competent conformations were attained, the rate-limiting step in the fibrillization pathway was tau dimer formation. Filament elongation then proceeded by addition of tau monomers to nascent filament ends. Filaments isolated at reaction plateau contained ϳ2 tau protomers/␤-strand spacing on the basis of mass-per-unit length measurements. The model suggests four key steps in the aggregation pathway that must be surmounted for tau filaments to form in disease.

Section: Discussionmentioning

confidence: 99%

Nucleation-dependent Tau Filament Formation

Congdon¹,

Kim²,

Bonchak³

et al. 2008

105

“…When the critical concentration of tau molecules is achieved, unfolded monomer tau molecules, which have no substantial secondary structures, oligomerize leading to a conformational change to a ␤-sheet enriched structure (57, 58). The earliest secondary structure detectable with fluorescent dyes corresponds to tau aggregates associated with membranous structures (59), suggesting that the folding of tau protein into ␤-sheet-containing species may be facilitated by interaction with intracellular membranes and organelles. The final step involves the nucleation of tau filaments and formation of mature NFTs.…”

Section: Discussionmentioning

confidence: 99%

Activation of Glycogen Synthase Kinase 3β Promotes the Intermolecular Association of Tau

Wang

Johnson

2007

Tau is hyperphosphorylated and undergoes proteolysis in Alzheimer disease brain. Caspase-cleaved tau efficiently forms fibrillary structures in vitro and in situ. Glycogen synthase kinase 3␤ (GSK3␤) phosphorylates tau and induces the aggregation of caspase-cleaved tau in situ. Given the hypothesis that increased association of tau precedes the formation of fibrillar structures, we generated a cell model to quantitate the extent of tau association in situ using fluorescence resonance energy transfer (FRET) microscopy. The cyan and yellow fluorescent proteins were attached to full-length (T4) and caspase-cleaved (T4C3) tau at either the N or C termini, and a pair of cyan and yellow fluorescent protein-tagged tau were co-transfected into human embryonic kidney cells. The FRET efficiency was examined in the presence of a constitutively active or a kinase-dead GSK3␤. Active GSK3␤ significantly increased FRET efficiency with both T4 and T4C3, indicating that GSK3␤ activation resulted in an increase in the self-association of both T4 and T4C3, but interestingly only T4 is efficiently phosphorylated by GSK3␤. There was no significant difference in FRET efficiency between T4 and T4C3, although only T4C3 in the presence of active GSK3␤ leads to the formation of Sarkosyl-insoluble inclusions. These FRET studies demonstrate that GSK3␤ facilitates the association of T4 and T4C3, and the presence of caspase-cleaved tau is necessary for the evolution of tau oligomers into Sarkosyl-insoluble inclusions even though it is not extensively phosphorylated. These data imply that increased association of tau should not be regarded as a direct indicator of the formation of insoluble tau aggregates.The accumulation of polymeric filaments of the microtubule-associated protein tau as the intracellular neurofibrillary tangles (NFTs) 2 is one of the major neuropathological features of several diseases known as "tauopathies," which include Alzheimer disease (AD) and frontotemporal dementia with parkinsonism linked to chromosome 17, a group of autosomal dominant neurodegenerative diseases caused by mutations in the tau gene (1). NFTs are mainly composed of paired helical filaments (PHFs), which are formed from abnormally hyperphosphorylated tau (2, 3). Although in AD, the abundance of NFTs correlates positively with the severity of cognitive impairment (4), the role of NFTs as toxic mediators of neuronal dysfunction and death is still not clear. Several animal models show cognitive deficits and impaired axonal transport in the absence of NFTs (5-7). Furthermore, suppression of tau expression in a transgenic mouse model restored memory function and stabilized neuronal cell populations, whereas NFTs continued to accumulate (8). These studies suggest that NFTs are not sufficient to cause cognitive decline or neuronal death, and small soluble oligomers may be the toxic species (9). Therefore, elucidating the early steps in the process of tau oligomerization is of fundamental importance.Post-translational modifications of tau such as aberrant ph...

“…Brains of AD patients reveal extracellular amyloid-␤ (A␤) plaques and intracellular neurofibrillary tangles mainly composed of hyperphosphorylated tau protein (2)(3)(4)(5). Tau protein is classically known as a microtubule-interacting protein involved in regulating microtubule dynamics and stability (6 -8).…”

mentioning

confidence: 99%

Interaction of Endogenous Tau Protein with Synaptic Proteins Is Regulated by N-Methyl-d-aspartate Receptor-dependent Tau Phosphorylation

Mondragón‐Rodríguez

Trillaud-Doppia

Dudilot

et al. 2012

203

226

Background: Tau phosphorylation affects synaptic transmission, but the underlying mechanism remains elusive. Results: NMDA receptor activation leads to phosphorylation of endogenous tau, thereby regulating the interaction of tau with Fyn and postsynaptic scaffolding protein PSD95. Conclusion: Phosphorylation of tau controls the interaction of tau with the postsynaptic PSD95-Fyn-NMDA receptor complex leading to changes in synaptic activity. Significance: The here described physiological mechanism could go awry during the development of Alzheimer disease.