PROGRAM | Chemical Engineering

By: Olga Morozova Chair: David Colby

ABSTRACT

Fibrils composed of tau protein are a pathological hallmark of several neurodegenerative disorders collectively termed “tauopathies”, including Alzheimer’s disease (AD) and dementia like corticobasal degeneration (CBD), progressive supranuclear palsy (PSP), and Pick’s disease (PiD). We show that when recombinant tau protein is seeded with fibrils isolated from tauopathy brains, the amyloid formed shares many of the structural features of tauopathy fibrils. Characterization by electron microscopy, circular dichroism, and chemical denaturation, shows that seeded recombinant tau fibrils were not significantly different than tau fibrils isolated from brain tissue. Our results suggest that brain-isolated fibrils act as a conformational template for the formation of recombinant tau fibrils.

The critical concentration for this seeded tau structural propagation reaction is within the range of tau concentration found in neurons and is significantly higher than the standard tau fibrillization reaction that relies on a polyanion inducer to spontaneously induce tau misfolding. We characterize the thermodynamics of tau fibrillization and show that the propagated disease-specific fibrils are in a dynamic equilibrium with monomers in the reaction, which is consistent with the reported fast clearance of tau fibrils from cell models. Brain-derived seeds propagate recombinant tau fibers that closely resemble tauopathy pathology, suggesting a biochemical model of tau misfolding that is of improved utility for structural studies and drug screening.

The elevated concentration of the tau protein in AD cerebrospinal fluid (CSF) is used as a clinical marker for the detection of the disease, but the conformation of tau in CSF has not been assessed. Based on the seeding-dependent tau fibrillization kinetics, we developed a biochemical seeding assay that detects less than 1 pg of misfolded tau protein by monitoring the rate of conversion of recombinant tau protein into an amyloid conformation. Amyloidogenic tau was found in CSF samples obtained from individuals with AD, CBD, FTD and PSP, but not in samples obtained from age-matched controls. Seeding with CSF from patients with different diseases propagated distinct conformations of recombinant tau fibrils. These findings suggest a possible biomarker directly linked to tau pathology and support the possibility of prion-like propagation of tau deposits between neurons in AD and other neurodegenerative diseases.

Many other neurodegenerative diseases are also associated with deposits of aggregated protein in the brain. The molecular pathways through which soluble proteins misfold to form amyloids and ultimately large protein aggregates often include diverse oligomeric species, only some of which progress to the amyloid state.  We show that prefibrillar huntingtin (HTT) oligomers, isolated from Huntington’s disease (HD) brain and from the R6/2 and YAC128 mouse models of HD, stimulate polyglutamine amyloid formation. While fibrillar HTT oligomers have been shown to be unstable under denaturing conditions and appear not to lead to amyloid formation, we find that prefibrillar HTT oligomers are remarkably stable and are potent seeds of polyglutamine amyloid formation. These findings help to dissect the complex molecular pathway of HTT misfolding.

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