Tau Trimers Are the Minimal Propagation Unit Spontaneously Internalized to Seed Intracellular Aggregation

• Published in: The Journal of biological chemistry 2015-06, Vol.290 (24), p.14893-14903
• Main Authors: Mirbaha, Hilda, Holmes, Brandon B., Sanders, David W., Bieschke, Jan, Diamond, Marc I.
• Format: Article
• Language: English
• Subjects: amyloid; Biopolymers - metabolism; Chromatography, Gel; endocytosis; HEK293 Cells; Humans; Molecular Bases of Disease; neurodegenerative disease; prion; Spectrometry, Fluorescence; structure; Tau protein; tau Proteins - metabolism
• ISSN: 0021-9258; 1083-351X
• DOI: 10.1074/jbc.M115.652693

Tau amyloid assemblies propagate aggregation from the outside to the inside of a cell, which may mediate progression of the tauopathies. The critical size of Tau assemblies, or “seeds,” responsible for this activity is currently unknown, but this could be important for the design of effective therapies. We studied recombinant Tau repeat domain (RD) and Tau assemblies purified from Alzheimer disease (AD) brain composed largely of full-length Tau. Large RD fibrils were first sonicated to create a range of assembly sizes. We confirmed our ability to resolve stable assemblies ranging from n = 1 to >100 units of Tau using size exclusion chromatography, fluorescence correlation spectroscopy, cross-linking followed by Western blot, and mass spectrometry. All recombinant Tau assemblies bound heparan sulfate proteoglycans on the cell surface, which are required for Tau uptake and seeding, because they were equivalently sensitive to inhibition by heparin and chlorate. However, cells only internalized RD assemblies of n ≥ 3 units. We next analyzed Tau assemblies from AD or control brains. AD brains contained aggregated species, whereas normal brains had predominantly monomer, and no evidence of large assemblies. HEK293 cells and primary neurons spontaneously internalized Tau of n ≥ 3 units from AD brain in a heparin- and chlorate-sensitive manner. Only n ≥ 3-unit assemblies from AD brain spontaneously seeded intracellular Tau aggregation in HEK293 cells. These results indicate that a clear minimum size (n = 3) of Tau seed exists for spontaneous propagation of Tau aggregation from the outside to the inside of a cell, whereas many larger sizes of soluble aggregates trigger uptake and seeding. Background: It is unknown what the minimum assembly of Tau is that can trigger cell uptake and seeding of intracellular aggregation. Results: Recombinant and AD-derived Tau assemblies were fractionated, and uptake and intracellular seeding activities were determined. Conclusion: Only Tau assemblies of n ≥ 3 units trigger uptake and seeding. Significance: Definition of the minimal Tau propagation unit elucidates disease mechanisms for diagnosis and therapy.