Tau trimers are the minimal propagation unit spontaneously internalized to seed intracellular aggregation [Protein Structure and Folding]

April 17th, 2015 by Mirbaha, H., Holmes, B. B., Sanders, D. W., Bieschke, J., Diamond, M. I.

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 could be important for design of effective therapies. We studied recombinant tau repeat domain (RD), and tau assemblies purified from Alzheimer disease (AD) brain comprised 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 equivalently bound heparan sulfate proteoglycans on the cell surface, which are required for tau uptake and seeding, as they are 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 brain. AD brain contained aggregated species, whereas normal brain 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, while many larger sizes of soluble aggregates trigger uptake and seeding.