Why do some people with AD slide rapidly into severe dementia, while others decline gradually over more than a decade? Part of the answer could come down to which biochemical forms of tau inhabit a person’s brain, suggests a study published June 22 in Nature Medicine. Among postmortem brain samples from people with advanced AD, Bradley Hyman at Massachusetts General Hospital in Charlestown and colleagues found a striking variability in tau’s ability to seed aggregation. The scientists tied aggregation-prone forms of tau in the postmortem brain to a more rapid course of disease during life. They pegged large, soluble tau oligomers—phosphorylated on specific residues—as the most hazardous species. Antibodies trained against these types of tau stopped its aggregation. The findings cast tau’s behavior as a major prognostic determinant for AD, and support the concept of targeting these troublesome forms of the protein with therapeutics.
- Tau seeding activity varied by an order of magnitude among 32 people with AD.
- Seeding activity, hyperphosphorylation, and oligomerization of tau correlated with clinical aggressiveness.
- Phosphorylation of specific tau residues associated with rate of decline.
“This is a well-designed study underlining once more the importance of the soluble tau oligomeric assemblies over long tau filaments in Alzheimer’s disease, as well as heterogeneity in tau oligomers,” commented Rakez Kayed of the University of Texas Medical Branch in Galveston.
“The study clearly highlights the complexity and heterogeneity of tau proteins in people with AD,” noted Hilal Lashuel of École Polytechnique Fédérale de Lausanne in Switzerland. The complexity calls for cautious interpretation of the findings, Lashuel added, noting that enigmatic tau oligomers are in dynamic equilibrium that can be influenced by disease stage and all manner of other factors. “It would be very difficult to identify a specific oligomer species that consistently correlates with tau propagation or disease progression,” he said.
Throughout the course of AD, neurofibrillary tangles of tau overtake the brain in a stereotypical sequence (Braak and Braak, 1991). Fueled at least in part by a templated misfolding mechanism, the propagation of tau tangles throughout the brain is tied closely to clinical progression of the disease (Jan 2020 news; May 2019 news; Jun 2019 news).
But even among people with the typical, amnestic form of AD, how aggressively their clinical disease gets worse varies strikingly from one person to the next (Komarova et al., 2011). This not only creates uncertainty for patients and their families, but also poses a risk for clinical trials, whose success depends on being able to measure a treatment effect within a set time, typically six months for Phase 2. Too many slow progressors in a trial cohort can sink a study even if the drug did what it was intended to do. Might molecular variations in tau species—particularly those that influence its propagation—explain this clinical heterogeneity?
First author Simon Dujardin and colleagues addressed this question by probing myriad aspects of tau taken from the postmortem brains of 32 people who had died in the advanced stages of AD. At the time of death, each had extensive tau tangles in the brain, at Braak stage V/VI. However, their clinical trajectories had been remarkably variable. Their age at onset ranged from 45 to 81, and the time between symptom onset and when they died ranged from five to 19 years. Their rates of cognitive decline, as gauged by serial tests on the clinical dementia rating scale sum of boxes (CDR-SOB), also varied widely.
The researchers started by measuring the capacity of tau to seed aggregation in biosensor cell lines. Equipped with a tau fragment tagged with fluorescent donor and acceptor molecules, these cells light up when potent seeds spark aggregation (Oct 2014 news). Although the researchers normalized the amount of tau used from each brain, they found wide variation in seeding activity. It ranged by an order of magnitude across samples. Notably, the three samples with the highest seeding activity came from people who carried two copies of ApoE4, suggesting the risk factor influences tau propagation.
Seed Span. When added to biosensor cell lines (left), soluble tau proteins extracted from the brains of people with AD were strikingly heterogeneous in a seeding assay. [Courtesy of Dujardin et al., Nature Medicine, 2020.]
The physiological relevance of this cell-based biosensor assay has been challenged recently (May 2020 news). So as not to rely entirely on this assay as a proxy for tau’s propagation potential, the researchers also conducted a series of alternative seeding experiments with a subset of the samples deemed low, intermediate, or high seeders based on the biosensor assay. Whether treating primary neuron cultures with these extracts or injecting the extracts directly into the brains of mice expressing P301S tau, the researchers observed similar relative trends in seeding activity among the samples. This suggested that the cell-based biosensor assay provided a meaningful gauge of the relative potency of tau seeds in each sample.
To figure out what about tau determines its seeding potency, the researchers subjected tau in the brain extracts to a barrage of biochemical assays and cross-referenced the results with the seeding activity gleaned from biosensor assays. They report that seeding activity correlated not with a person’s amount of total tau, but with levels of oligomeric, hyperphosphorylated tau in each sample. Compared with intermediate or low "seeders," high seeders had an abundance of soluble, high-molecular-weight tau oligomers.
Using mass spectrometry, the researchers mapped the phosphorylation landscape of tau across samples, noting that tau doubly phosphorylated on Thr231 and Ser235, or singly phosphorylated on Ser262, correlated with seeding activity. Curiously, neither ptau-181 nor ptau-217—the species that rise in the cerebrospinal fluid in the preclinical stages of AD—were significantly tied to seeding activity (Mar 2020 news; Apr 2020 conference news).
Does tau seeding activity, or any of its biochemical correlates, relate to a patient’s clinical progression? Indeed, the researchers found that the higher the tau seeding activity, the steeper the person’s rate of decline on the CDR-SOB, and the younger his or her age at symptom onset. The abundance of oligomeric, hyperphosphorylated species of tau also correlated with disease progression, as did levels of the same phospho-tau species that associated with seeding activity.
Collectively, tau seeding activity accounted for about 25 percent of the clinical heterogeneity among people with typical AD, the researchers reported. This suggests that tau antibodies that block tau seeding might also stem clinical progression of the disease. To identify antibodies that might do the trick, the researchers used a panel of seven antibodies trained against different parts of the tau protein, or against specific phospho-residues, to deplete tau from the brain extracts, then tested the remaining seeding activity. They found that some antibodies quashed seeding more effectively than others, and that there was significant variability between samples. Overall, antibodies such as AT8 and PHF1, which bind to pathological forms of tau, inhibited seeding most consistently across samples.
At first glance, the heterogeneity in tau’s seeding capacity and biochemical forms across AD brains may seem at odds with cryo-electron microscopy studies, which identified two predominant conformations of tau fibrils in people with AD (Jul 2017 news). That’s not the case, Dujardin noted. Soluble tau oligomers—not fibrils—are the source of tau’s biochemical variability in this postmortem study. He said it would be fascinating to examine tau oligomers via cryoEM.
Lashuel noted that the researchers did not analyze insoluble fractions of tau in their assays, biasing them to zero in on soluble species. He suggested that attention be paid to understanding the near-total lack of seeding activity in the "low seeders," who still ultimately developed AD. Perhaps more answers would be found in insoluble fractions, he said.
Why is one person’s tau not like another’s? Dujardin suspects genetic differences that influence cellular processes such as degradation and autophagy, which may selectively degrade certain forms of tau. Though microglial function theoretically influences these pathways, Dujardin noted that inflammatory markers in the 32 brains did not correlate with seeding activity, at least at this end stage of disease. Individual differences in kinase activity could also influence which phosphorylations tau accumulates over a person’s lifetime.
How might researchers leverage the findings to inform a person’s prognosis? Dujardin noted several possibilities. A handful of studies have managed to detect seeding activity in tau derived from CSF, though Dujardin noted that these assays need to become more sensitive (Takeda et al., 2016). In lieu of directly measuring seeding activity, perhaps quantification of tau oligomers, and/or specific phospho-tau species associated with seeding activity, could serve the same purpose.—Jessica Shugart
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