In recent years, support has grown for the idea that the same aggregating protein can fold up into different toxic shapes, giving rise to distinct neurodegenerative diseases. Now, in the August 29 Nature, researchers led by Michel Goedert and Sjors Scheres at the MRC Laboratory of Molecular Biology in Cambridge, England, in collaboration with Bernardino Ghetti and colleagues at Indiana University School of Medicine in Indianapolis, provide the first molecular-level proof that this happens.
- Cryo-EM reveals the molecular structure of tau filaments from a person with Pick’s disease.
- The molecule folds in a completely different way than it does in AD.
- The results may enable design of more specific reagents.
Using cryo-electron microscopy, they detailed the structure of tau filaments taken from the brain of a person who died of Pick’s disease. Intriguingly, these fibrils contained a fold quite different from that seen in cryo-EM of tau tangles in Alzheimer’s disease. Pick’s disease tau folded up into a J shape, instead of the C shape seen in AD tangles. Domains aligned in a different configuration as well, exposing a distinct set of residues on the surface. The findings explain some known differences in the composition of deposits in Pick’s and AD brains, and may enable the design of more specific reagents for each disease, the authors said. In particular, most current tau PET tracers do not recognize Pick’s deposits. “This provides a framework for designing specific tracers for each filament,” Goedert told Alzforum.
Others greeted the findings with enthusiasm. “By showing that tau filaments in Pick’s disease differ from those in Alzheimer's disease, these intriguing findings provide direct evidence that tau adopts different three-dimensional architectures, or proteopathic strains, in different diseases,” Lary Walker and David Lynn at Emory University, Atlanta, wrote to Alzforum (full comment below).
Different Strokes. In Pick’s disease (left), tau molecules adopt a long J-shaped fold, while in AD (right) they form a C that contains fewer domains. [Courtesy of Falcon et al., Nature.]
The findings have far-reaching implications, noted Byron Caughey at the National Institutes of Health in Hamilton, Montana. “The fact that tau filaments of Pick’s and Alzheimer’s diseases have distinct amyloid core folds implies that they will likely have different chemical surfaces, tendencies to interact with other factors and tissue components, preferred sites of accumulation, cytotoxicities, and thereby, neuropathological and clinical consequences,” he wrote to Alzforum (full comment below).
Scheres and Goedert previously solved the three-dimensional structure of Alzheimer’s tau. Tau molecules come in different isoforms, some of which contain three-repeat domains (3R tau), and some four-repeat (4R tau). AD filaments were known to equally incorporate 3R and 4R varieties. Cryo-EM revealed the reason, finding that the C-shaped core of the aggregate comprised the third and fourth repeat, which are present in all isoforms—it’s the second repeat that gets spliced out in the 3R isoform. Meanwhile, the free ends of each tau molecule waved around randomly in solution, forming a fuzzy coat around the fibril. Individual monomers stacked up to form filaments. These filaments pair up via hydrogen bonding along their backs (Jul 2017 news).
In contrast to AD, deposits in Pick’s disease are known to incorporate only 3R tau. To see why this might be, first author Benjamin Falcon isolated aggregates postmortem from the frontotemporal cortex of a woman with Pick’s disease. Like AD fibrils, the filaments were surrounded by a disordered, fuzzy coat made up of the free ends of the tau molecules. The researchers removed this with pronase treatment, and then used cryo-EM to map the structure of the aggregated protein core to 3.2 ångstrom resolution.
They found that the core consisted of residues 254 to 378 of 3R tau, making it longer than the AD core. This section includes repeats 1, 3, and 4. The core contained nine distinct β-strands; two from repeat 1, three from R3, three from R4, and one just past R4. The molecule kinks at Cys322 in R3 just after the fourth β-strand, creating a hairpin turn that enables the Pick’s fold (see image above). This allows β4/β5, β3/β6, β2/β7, and β1/β8 to pair up alongside each other, with β9 folding back onto the backside of β-8 along the short arm of the J.
This structure solves the riddle of why Pick’s deposits exclude 4R tau, the authors noted. For 4R tau to assume this J configuration, the R2 domain would have to replace R1. However, the R2 domain contains several side chains that would not pack properly into this structure. For example, glutamine at position 269 of 3R tau would be replaced by a valine, but the latter residue sports a branched side chain that would not fit. The authors confirmed the isoform selectivity in vitro, finding that the filaments used in this study seeded aggregation of 3R tau, but not 4R.
The Pick’s fold also explains why tau deposits in this disorder are not phosphorylated at Ser262. This residue packs into the interior of the Pick’s core at the first tight turn. In AD tau, Ser262 is exposed. The researchers saw other notable differences as well. While Cys322 is exposed in the Pick’s fold, it snuggles into the interior of the molecule in the AD fold. Conversely, Asp348, located in the middle of an R4 β-strand, pokes into solution in the AD fold, allowing the molecule to bend there and form one end of the C. Asp348 stays interior in the Pick’s fold, and the β-strand travels straight.
As in AD, the folded tau molecules stack up to form filaments, and these come in two varieties. Narrow Pick’s filaments consist of a single strand of stacked tau molecules, while in wide filaments, two strands bind each other at the hairpin turn through weak electrostatic interactions. i.e., van der Waals forces. This differs from AD, where tau filaments are always paired, but can form two filament types depending on their exact configuration.
Do all cases of sporadic Pick’s disease harbor this same structure? Goedert acknowledged that this is not yet known, but preliminary evidence suggests they might. The researchers analyzed tau aggregates from eight more Pick’s patients by immunogold EM. This technique uses antibody binding to characterize molecules, but cannot see fine details of structure. Deposits from all eight brains resembled those from the brain used for cryo-EM in that antibodies to tau’s R1, R3, and R4 domains did not recognize the aggregates, supporting the idea that all these domains were packed into the fibril core and inaccessible for antibody binding. When deposits were denatured and run on a gel, all three antibodies did recognize Pick’s tau. The researchers have not yet examined any Pick’s cases with mutations in tau. It is possible these could form distinct structures, Goedert noted.
The Pick’s and AD structures shed no light on what makes tau fold into one shape or the other. In solution, tau usually remains soluble, but can be triggered to aggregate by adding the polysaccharide heparin. Perhaps there are co-factors in neurons that induce tau to fold in particular ways, Scheres suggested. Alternatively, post-translational modifications such as phosphorylation may guide folding. For their part, Walker and Lynn wondered if smaller assemblies of tau push aggregation in one direction versus another. “Is the molecular architecture of fibrils related in some way to the formation, structure and/or pathogenicity of oligomers?” they asked.
Some of these possibilities could be investigated in mice that express human tau. Researchers could alter individual amino acids and see the effect on folding, providing clues to the earliest events in disease. First, however, Goedert and Scheres are continuing to investigate tau deposits in people. They are using cryo-EM to analyze co-aggregates of tau and PET tracers such as AV1451 to find out where these ligands bind. They are also attempting to isolate enough tau filaments from people with progressive supranuclear palsy and corticobasal degeneration to do cryo-EM. These deposits contain only 4R tau, and might assume a third configuration, distinct from the Pick’s and Alzheimer’s tau.—Madolyn Bowman Rogers
- Falcon B, Zhang W, Murzin AG, Murshudov G, Garringer HJ, Vidal R, Crowther RA, Ghetti B, Scheres SH, Goedert M. Structures of filaments from Pick's disease reveal a novel tau protein fold. Nature. 2018 Sep;561(7721):137-140. Epub 2018 Aug 29 PubMed.