The synthetic tau protein reveals important insights into protein misfolding and fibril formation

The synthetic tau protein reveals important insights into protein misfolding and fibril formation

Scientists at Northwestern University and the University of California, Santa Barbara have created the first synthetic fragment of the tau protein that acts like a prion. The “mini-prion” folds and stacks into strands (or fibrils) of misfolded tau proteins, which then transmit their abnormally folded form to other normal tau proteins.

Misfolded prion-like proteins drive the progression of tauopathies, a group of neurodegenerative diseases – including Alzheimer's disease – through the abnormal accumulation of misfolded tau protein in the brain. By studying a minimal, full-length synthetic version of human tau, scientists can better recreate the fibril structure that contains misfolded tau proteins. This could potentially lead to targeted diagnostic and therapeutic tools that are urgently needed for neurodegenerative diseases.

During the development of the synthetic protein, the scientists also revealed new insights into the role of water around the protein surface in guiding the misfolding process. A mutation commonly used to model TAU-related diseases subtly alters the dynamic structure of water in the environment immediately surrounding the tau protein, the researchers found. This altered water structure affects the protein's ability to assume its abnormal shape.

The study will be published in the week of April 28thProceedings of this National Academy of Sciences.

“The scope of neurodegenerative diseases involving the protein tau is particularly broad,” said Northwestern’s Songi Han, who led the study. "It includes chronic traumatic encephalopathy that occurs in football players after head trauma, corticobasal degeneration or progressive supernuclear palsy. Generate self-propagating tau fragments that can produce the fibril structure and malfunction of the unique anterior anterior anterior anterior tauopathy.

Han is the Mark and Nancy Ratner Professor of Chemistry in Northwestern's Neeinberg College of Arts and Sciences and a member of the Institute for the Chemistry of Life Processes, Applied Physics Graduate Program, International Institute of Nanotechnology, Paula M. Trienens Institute for Sustainability and Energy, and Institute for Quantum Information Research and Engineering. Michael Vigers, a former Ph.D. Student in Hans's laboratory led the study and is the first author. UC Santa Barbara co-authors include Kenneth S. Kosik, Joan-Emma Shea and M. Scott Shell. The work was also made possible by several students and postdoctoral fellows, including Saeed Najafi, Samuel Lobo, Karen Tsay, Austin Dubose, and Andrew P. Longhini.

A chain reaction of incorrect folds

In many neurodegenerative diseases, the proteins fold into harmful, highly ordered fibrils that ultimately damage brain health but are difficult to diagnose. When a normal protein encounters the pathological tau fibrils, the normal protein changes to conform to the misfolded shape. This process leads to a chain reaction in which more and more proteins transform into the misfolded aggregation state. Although this behavior is prion-like, it does not involve actual prions, which can spread infectious diseases from person to person.

Using cryogenic electron microscopy (cryo-EM), the researchers solved the structure of fibrils from samples of brain tissue. Although the design of the structure was a significant breakthrough, brain samples can only be obtained after a patient's death. Despite the dramatic progress and intense interest in this area, the definitive diagnosis of TAU-related neurodegenerative diseases is only possible after death.

Today, when people show signs of a neurodegenerative disease, it is not diagnosed with a biomarker. Doctors determine the diagnosis by administering a patient survey and examining a collection of symptoms such as sleep patterns and memory. The bottleneck is the reliable generation of tau fibrils that reconstitute the critical and unique diseases to serve as targets for developing diagnostic strategies. “

Songi Han, Northwestern University

A simplified model

To address the current challenge, Han and her team sought to develop a synthetic, prion-like tau protein. Instead of recreating the entire length of the protein, which is long and unwieldy, Han's team aimed to determine the shortest piece of tau that could still assume a misfolded shape and form disease-like fibrils.

Ultimately, Han and her team focused on a short segment of tau, called JR2R3, which is just 19 amino acid segments long. The segment contains a mutation called P301L, which is commonly found in many diseases. The researchers found that this short peptide could form the damaging fibrils that are the hallmark of these diseases, acting as a “seed” to compartmentalize the misfolding and aggregation of full-length tau proteins.

“We made a mini version that is easier to control,” Han said. "But it does all the same things as the full-length version. They seed and cause normal tau protein to misfold the fibrils and bond together."

The team used cryo-EM to examine the structure of the synthetic fibrils. They found that the P301L mutation facilitates a specific type of misfolding that is commonly observed in samples from patients with neurodegeneration. The finding suggests that the mutation plays a crucial role in directing the protein into misfolding.

The shape of water

Han next wanted to understand how the initially disordered tau proteins become highly ordered fibril structures. She likened the mysterious phenomenon to throwing strands of limp spaghetti together and expecting them to form a neat pile.

“It is impossible that an intrinsically disordered protein would naturally fall into a perfect fold and stack that can regenerate forever,” Han said. "It makes no sense."

After hypothesizing that something must be holding the misfolded proteins together, Han found the key: water. The environment surrounding a protein, particularly the water molecules, play a crucial role in protein folding and aggregation. The P301L mutation appears to directly alter the structure of the tau protein and alter the behavior of water molecules around it.

“Water is a liquid molecule, but it still has structure,” Han said. "The mutation in the peptide could result in a more structured arrangement of water molecules around the mutation site. This structured water influences how the peptide interacts with other molecules and groups them together."

In other words, organized water sticks the proteins together and allows individual strands to bind together into a neat pile. The fibrils then use their prion-like behavior to recruit other proteins to precipitate and join the stack.

What's next?

The research team is now focusing on further characterizing the properties of the synthetic prion-like proteins. Ultimately, they plan to investigate potential applications, including the development of new diagnostic and therapeutic approaches for TAU-related diseases.

"Once a tau fibril is formed, it doesn't disappear," Han said. "It will grab naive tau and fold it into the same shape. It can do this forever and ever. If we can figure out how to block this activity, we can uncover new therapeutic agents."

The study, “Water Design is Key to Tau Prion Formation,” was supported by the National Institutes of Health (grant numbers R01AG05605 and R35GM136411), Deutsche Forschungsgemaft, and the WM Keck Foundation.

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