Diseases such as Alzheimer's and frontotemporal dementia are considered neurodegenerative tauopathies, in which tau proteins in the brain misfold and lead to cognitive decline. Until now, it has been unclear how misfolded tau recruits other seemingly healthy tau and causes it to misfold and clump.
A new study led by Northwestern University researchers, in collaboration with researchers with the University of California, Santa Barbara, found that tau vulnerable to misfolding carries a single dominant hotspot. That site binds the matching site on a neighboring tau, or on a tau at the end of an existing fibril, and the incoming tau then misfolds to match. The hotspot is hydrated by water that is wrapping over and protecting a hydrophobic site, and releasing that water is what pulls the two together. This finding could help scientists diagnose tauopathies, block misfolded tau from converting other normal tau to misfolded shapes, and design therapeutic solutions in the future.
The study was published today (Oct. 12) in the Proceedings of the National Academy of Sciences.
"Once you know that aggregation-prone tau proteins always have a hotspot that is first attracted to the same site of tau that is stacked to fibrils, then you know there's always some part of the protein that you can target to neutralize," said Songi Han, senior author on the study. "If you know where to block its action of misfolding, you could block it to prevent it from propagating."
Songi Han is a Mark and Nancy Ratner Professor of Chemistry at Northwestern's Weinberg College of Arts and Sciences and the vice chair of the American Chemical Society's physical chemistry division.
When it comes to understanding why they misfold and propagate their misfolded shape, tau proteins have been a black box for researchers. Unlike other proteins, tau protein monomers are not structured. They stack together, however, in perfect register to form long fibrils, with every tau molecule adopting the same shape upon attaching to the growing fibril end. How do thousands of noodle-like, unstructured molecules know to find each other, recreate that same misfolded shape and stack in perfect alignment, like a tower of poker chips thousands high? This is where Han and her team stepped in.
"The proteins have to misfold and neatly stack to fibrils to elongate while propagating the same misfolded shape, and the only way that this process is forever is if the same site of the protein stacks to the same," Han said. "If you have a tiny mismatch in the alignment of the tau shapes that stack to the fibril end, the structure will eventually stop growing."
The chemistry of tau proteins dictates the architecture of the fibrils they build, with each misfolded tau stacking on top of the one beneath it to hold the fibril together. These tau fibrils build up inside neurons as neurofibrillary tangles, disrupt communication among neurons and, when the cell dies and the misfolded tau proteins are released, they can "infect" other tau proteins that are floating around and waiting to be used.
Like all proteins in a cell, tau is surrounded by water. But that water does not interact with every part of the protein in the same way. Some regions readily attract and interact with water. But other regions are hydrophobic, meaning they tend to repel water. Han and her team found unusual water structuring around hotspots within tau's hydrophobic regions. At these spots, water forms a cap reminiscent of ice, except that the water molecules stay dynamic. These caps mark where the two tau molecules will meet.
"You have a certain site, and water cannot quite coat it because it doesn't like water. But that part still needs to be protected, because if you don't, then the protein crashes out and is no longer available," Han said. "What water tends to do is build a stabilizing cap around it and basically these water caps attract each other."
When two hotspots meet, the capping water is released, and the energy gained by freeing it locks them together. This ensures the tau proteins will line up the same way, every time. When the team capped the hotspot with a chemical tag, the fibrils stopped growing, compared to when they capped two other sites away from the hotspot that did not stop fibril growth. Now that researchers understand how tau continues to misfold new tau and where this interaction begins, scientists can focus their efforts on blocking that hotspot before the fibrils recruit more tau and spreads to other neurons.
"We might have found a shortcut for the pharmacist or biochemist because instead of searching for everything, we can say, 'Look for that hotspot. That's the spot that matters,'" Han said. "We are trying to give it the principle of how it works so that others can find solutions better, faster or more rationally."
Understanding how tau works also could allow researchers to create synthetic tau fibrils that mimic the various tauopathy diseases, and to develop diagnostic and therapeutic agents. Researchers currently do not have good models for neurodegenerative disease research. "In order to cure a disease, one needs to be able to recreate it," Han said. "There's a real problem that disease-mimicking tau fibrils cannot be synthetically made easily. It's really difficult to try to do any development with human patient tissue."
To synthetically make a protein, researchers need to understand how the protein works. This study could be a critical step in creating synthetic proteins, and not necessarily just tau proteins, for future medical research.
This study was supported by the National Institutes of Health (award numbers R01AG056058 and R35GM136411), the Tau Consortium of the Rainwater Charitable Fund and the German Research Foundation. The W. M. Keck Foundation supported the ongoing experimental and computational method developments for the tau shape propagation study. Computational modeling was supported by the National Science Foundation (NSF-ANR MCB/PHY 2423885).
