Michel Goedert and Sjors Scheres of the MRC Laboratory of Molecular Biology, Cambridge, England, U.K., wondered if the cores of the tau fibrils differed, or if variations in their outer—the so-called “fuzzy coat” that does not integrate into the fibril core—might explain the findings. Goedert and Scheres have used cryo-EM to resolve the distinct folds tau takes in different tauopathies, including AD, and to date the fibril cores from different patients with the same disease have largely been the same (Jun 2017 news; Oct 2021 news). 

Over the years, studies in cell culture and in transgenic mouse models have suggested that different conformations, or “strains” of tau, can faithfully propagate their unique shape via templated misfolding, much as prions do (May 2014 news; Nov 2017 news). Cryo-EM studies of tau plucked from postmortem brain samples have revealed a limited number of distinct tau folds that each track remarkably well with the type of neurodegenerative tauopathy they cause (Shi et al., 2021). 

In their study, first author Chae Kim and colleagues asked whether tau contorts into many different shapes among people with the same tauopathy, and even within the same person. If so, could such differences relate to how fast tauopathy blazes through the brain? They extracted tau from the hippocampi of 40 people who had died with AD, including 23 who had suffered from a rapidly progressive form of the disease. Initially referred to the U.S. National Prion Disease Pathology Surveillance Center because they were suspected to have prion disease, these patients were later found to have had AD, based on neuropathological examination. On average, people with this form of AD died within 14 months of onset, while those with more typical AD lived for nearly seven years. At death, there were no significant differences in tau tangle burden or its distribution between those with slow versus fast forms of the disease.

What about tau conformation? The researchers used indirect approaches to address this. One, called conformation-dependent immunoassay (CDI), labels tau with antibodies to epitopes that are buried within the folds of the protein under native conditions and become exposed when the protein is denatured. By measuring the ratio of antibody binding under denaturing versus native conditions, scientists can surmise how unfolded the protein is, and estimate how many different forms exist. In a related method, called conformational stability assay (CSA), extracted tau is exposed to increasing concentrations of the denaturing agent guanidine-HCl. Every conformer of tau unfolds at a slightly different concentration of the agent, revealing the array of potential structures present in a sample. The authors applied these methods to each fraction of tau separated by size on a sucrose gradient. Together, this generated estimates of the size and structural diversity of tau forms in each brain sample, though no information on the structure of individual conformers, as can be had from cryo-EM.

In a nutshell, the scientists found a diverse portfolio of folded taus within each brain sample. Across all samples, there were roughly equal proportions of 3R and 4R isoforms among tau monomers, while 4R predominated within insoluble tau aggregates regardless of size. The 4R isoform—which contains four repeats of the microtubule binding domain—made up 80 percent of tau incorporated into insoluble, protease-resistant aggregates. Notably, different forms of this insoluble, 4R-predominant tau existed in people with rapidly progressing versus typical AD. Together, the data suggested that distinct structures—formed mostly from protease-resistant 4R tau—are associated with rapid progression of AD.

Might these differ in seeding potency? In in-vitro seeding assays, and in biosensor cell lines that measure tau aggregate formation, tau extracted from people with rapidly progressing AD more potently triggered aggregation of normally folded tau. The source of this greater seeding potential primarily lay in fractions of insoluble 4R tau. The findings hint that the way tau misfolds could set the pace of AD progression, but do not answer the question of which structural attributes might accelerate tauopathy.

Safar said that future studies will need to zero in on specific forms of tau associated with rapid progression. He plans to more closely map structural specifics, including post-translational modifications, of tau strains using hydroxyl radical footprinting, a technique his group has used to probe characteristics of prion strains (Siddiqi et al., 2021). If disease-accelerating strains of tau can be detected in biofluids, they could serve as prognostic biomarkers, Safar said, adding that strain-specific methods are already being employed to detect different types of prion disease (see Foutz et al., 2016). 

Scheres and Goedert used the study to offer a reminder of how the definition of the word conformer is changing. While the broad definition of the term includes any proteins with the same sequence but different structure, a narrower definition has evolved in prion research. “In this context, it represents a misfolded conformation of an aggregated protein that provides the template for other copies of that protein to misfold and become part of the aggregate, leading to spreading of the misfolded protein,” they wrote. “Although the molecular mechanisms of templated seeding are not fully understood, it is assumed that the ordered cores of amyloids provide the conformations for templated seeding of new proteins. Therefore, it is most likely that the structures of the ordered cores of amyloids define conformers,” they wrote (full comment below).

Whether the versions of tau that emerged in his study differ in their core structures or only their fuzzy coat is unclear. However, that tau strains from people with rapidly progressing AD had superior seeding capacity suggests that their structural differences could have a functional impact on propagation, Safar said. However, Scheres and Goedert point out that the tau constructs used in these seeding assays lacked the eight C-terminal amino acids that make up the core of the tau fibril, meaning they may not faithfully recapitulate seeding in the AD brain.—Jessica Shugart