“Whether existing silk ends with 3R or 4R tau proteins, the silk can recruit any version of tau in the environment to add to the growing silk,” said Mei Hong, professor of chemistry at MIT. “For Alzheimer’s Having this property of randomly incorporating either version of the protein is very beneficial for the tau structure of the disease.”
Hong is the senior author of the study, which was recently published inNature Communications” magazine. MIT graduate student Aurelio Dregni and postdoc Pu Duan are lead authors of the paper.
In a healthy brain, tau functions as a stabilizer for microtubules in neurons. Each tau protein consists of three or four “repeats”, each consisting of 31 amino acid residues. Abnormal versions of 3R or 4R tau proteins can cause various diseases.
Chronic traumatic encephalopathy caused by repetitive head trauma is associated with abnormal accumulation of 3R and 4R tau proteins, similar to Alzheimer’s disease. However, most other neurodegenerative diseases involving tau are characterized by abnormal versions of either the 3R or 4R proteins, but not both.
In Alzheimer’s disease, the tau protein begins to form tangles in response to chemical modifications to the protein that interfere with its normal function. Each tangle consists of filaments of 3R and 4R tau proteins, but it is not known how these proteins combine at the molecular level to create these filaments.
One possibility Hong and her colleagues considered is that the filaments may be composed of alternating blocks of many 3R tau proteins or many 4R tau proteins. Alternatively, they hypothesized, individual molecules of 3R and 4R tau might alternate.
The researchers set out to explore these possibilities using nuclear magnetic resonance spectroscopy. By labeling the 3R and 4R tau proteins with carbon and nitrogen isotopes that can be detected by nuclear magnetic resonance, the researchers were able to calculate the probability that each 3R tau is followed by a 4R tau, as well as the probability that each 4R tau is followed by a 3R tau.
To produce their filaments, the researchers first extracted abnormal tau proteins from postmortem brain samples of Alzheimer’s patients. These “seeds” were added to a solution containing equal concentrations of normal 3R and 4R tau proteins, which were “recruited” by the seeds to form filaments.
To the researchers’ surprise, their NMR analysis revealed that the assembly of these 3R and 4R tau proteins in these seed filaments was almost random. A 4R tau protein has about a 40% chance of being followed by a 3R tau protein, and a 3R tau protein has a little over 50% chance of being followed by a 4R tau protein. Overall, 4R proteins account for 60% of Alzheimer’s disease tau filaments, although the pool of available tau proteins is evenly divided between 3R and 4R. The human brain also has roughly equal amounts of 3R and 4R tau proteins.
This type of assembly, which the researchers call a “fluid mixing of molecules,” may contribute to the Alzheimer’s epidemic compared to diseases that involve only 4R or 3R tau proteins, Hong said.
“Our interpretation is that this would favor the spread and growth of the toxic Alzheimer’s tau conformation,” she said.
Working with collaborators at the University of Pennsylvania School of Medicine (led by Professor Virginia Lee), the researchers showed that the structures of the tau filaments they generated in the lab were very similar to those seen in human Alzheimer’s patients, but they Not like filaments grown entirely from normal tau.
The tau filaments they generated also replicated the toxic effects of Alzheimer’s disease tangles, forming aggregates in the dendrites and axons of mouse neurons grown in lab dishes.
The current paper focuses on the structure of the rigid inner core of these filaments, but the researchers now hope to further study the structure of the softer protein segments that extend from this core. “We wanted to figure out how this protein went from a healthy and intrinsically disordered state to this toxic, misfolded and beta-sheet-rich state in the Alzheimer’s brain,” Hong said.
