17 research outputs found
GSK-3β phosphorylation of functionally distinct tau isoforms has differential, but mild effects
<p>Abstract</p> <p>Background</p> <p>Tau protein exists as six different isoforms that differ by the inclusion or exclusion of exons 2, 3 and 10. Exon 10 encodes a microtubule binding repeat, thereby resulting in three isoforms with three microtubule binding repeats (3R) and three isoforms that have four microtubule binding repeats (4R). In normal adult brain, the relative amounts of 3R tau and 4R tau are approximately equal. These relative protein levels are preserved in Alzheimer's disease, although in other neurodegenerative tauopathies such as progressive supranuclear palsy, corticobasal degeneration and Pick's disease, the ratio of 3R:4R is frequently altered. Because tau isoforms are not equally involved in these diseases, it is possible that they either have inherently unique characteristics owing to their primary structures or that post-translational modification, such as phosphorylation, differentially affects their properties.</p> <p>Results</p> <p>We have determined the effects of phosphorylation by a kinase widely believed to be involved in neurodegenerative processes, glycogen synthase kinase-3β (GSK-3β), on the microtubule binding and inducer-initiated polymerization of these isoforms in vitro. We have found that each isoform has a unique microtubule binding and polymerization profile that is altered by GSK-3β. GSK-3β phosphorylation had differential effects on the isoforms although there were similarities between isoforms and the effects were generally mild.</p> <p>Conclusion</p> <p>These results indicate that tau phosphorylation by a single kinase can have isoform specific outcomes. The mild nature of these changes, however, makes it unlikely that differential effects of GSK-3β phosphorylation on the isoforms are causative in neurodegenerative disease. Instead, the inherent differences in the isoform interactions themselves and local conditions in the diseased cells are likely the major determinant of isoform involvement in various neurodegenerative disorders.</p
Heat Shock Protein 70 Prevents both Tau Aggregation and the Inhibitory Effects of Preexisting Tau Aggregates on Fast Axonal Transport
Aggregation and accumulation of the microtubule-associated protein tau are associated with
cognitive decline and neuronal degeneration in Alzheimer's disease and other tauopathies. Thus,
preventing the transition of tau from a soluble state to insoluble aggregates and/or reversing the
toxicity of existing aggregates would represent a reasonable therapeutic strategy for treating these
neurodegenerative diseases. Here we demonstrate that molecular chaperones of the heat shock
protein 70 (Hsp70) family are potent inhibitors of tau aggregation in vitro, preventing the
formation of both mature fibrils and oligomeric intermediates. Remarkably, addition of Hsp70 to a
mixture of oligomeric and fibrillar tau aggregates prevents the toxic effect of these tau species on
fast axonal transport, a critical process for neuronal function. When incubated with preformed tau
aggregates, Hsp70 preferentially associated with oligomeric over fibrillar tau, suggesting that
prefibrillar oligomeric tau aggregates play a prominent role in tau toxicity. Taken together, our
data provide a novel molecular basis for the protective effect of Hsp70 in tauopathies
Pseudohyperphosphorylation has differential effects on polymerization and function of tau isoforms
The microtubule-associated protein tau exists as six isoforms created through the splicing of the second, third, and tenth exons. The isoforms are classified by their number of N-terminal exons (0N, 1N or 2N) and by their number of microtubule-binding repeat regions (3R or 4R). Hyperphosphorylated isoforms accumulate in insoluble aggregates in Alzheimer’s disease and other tauopathies. These neurodegenerative diseases can be categorized based on the isoform content of the aggregates they contain. Hyperphosphorylated tau has the general characteristics of an upward electrophoretic shift, decreased microtubule binding, and an association with aggregation. Previously we have shown that a combination of seven pseudophosphorylation mutations at sites phosphorylated by GSK-3β, referred to as 7-Phos, induced several of these characteristics in full-length 2N4R tau and led to the formation of fewer but longer filaments. We sought to determine whether the same phosphorylation pattern could cause differential effects in the other tau isoforms, possibly through varied conformational effects. Using in vitro techniques, we examined the electrophoretic mobility, aggregation properties and microtubule stabilization of all isoforms and their pseudophosphorylated counterparts. We found that pseudophosphorylation affected each isoform, but in several cases certain isoforms were affected more than others. These results suggest that hyperphosphorylation of tau isoforms could play a major role in determining the isoform composition of tau aggregates in disease
Hsp70 Alters Tau Function and Aggregation in an Isoform Specific Manner
Tauopathies are characterized by abnormal aggregation
of the microtubule associated protein tau. This aggregation is thought
to occur when tau undergoes shifts from its native conformation to
one that exposes hydrophobic areas on separate monomers, allowing
contact and subsequent association into oligomers and filaments. Molecular
chaperones normally function by binding to exposed hydrophobic stretches
on proteins and assisting in their refolding. Chaperones of the heat
shock protein 70 (Hsp70) family have been implicated in the prevention
of abnormal tau aggregation in adult neurons. Tau exists as six alternatively
spliced isoforms, and all six isoforms appear capable of forming the
pathological aggregates seen in Alzheimer’s disease. Because
tau isoforms differ in primary sequence, we sought to determine whether
Hsp70 would differentially affect the aggregation and microtubule
assembly characteristics of the various tau isoforms. We found that
Hsp70 inhibits tau aggregation directly and not through inducer-mediated
effects. We also determined that Hsp70 inhibits the aggregation of
each individual tau isoform and was more effective at inhibiting the
three repeat isoforms. Finally, all tau isoforms robustly induced
microtubule formation while in the presence of Hsp70. The results
presented herein indicate that Hsp70 affects tau isoform dysfunction
while having very little impact on the normal function of tau to mediate
microtubule assembly. This indicates that targeting Hsp70 to tau may
provide a therapeutic approach for the treatment of tauopathies that
avoids disruption of normal tau function