Researchers have discovered how the tau protein turns from a normal to a disease state in Alzheimer's disease (AD) and have shown how the discovery could potentially deliver a therapeutic target.

Published in the July 6, 2022, issue of Science Advances, the team's findings provide hope for preventing the tau transformation process from happening, thereby keeping tau in a healthy state and avoiding toxic effects on brain cells.

During AD development, tau accumulates in deposits inside brain cells. During this process, tau gets heavily modified, with various deposits made up of tau carrying multiple small changes at many different positions within the tau molecule.

While such changes to tau have been known to neuropathologists for decades, it remained unclear how tau arrives at this multi-modified stage.

The research was centered on how the modification of one spot on the tau protein is linked with the modification of many spots, called multi-site phosphorylation.

Tau hyperphosphorylation is associated with the hallmarks of AD, such as degradation and cognitive deficits.

"What we wanted to do was try to understand if we have a modification at one point, how will that influence modification at another point," lead study author Kristie Stefanoska, research fellow in dementia at Flinders University, told BioWorld.

To answer that question, the team started with cell cultures and made individual modifications at 17 sites on the tau protein, then transfected this with a kinase and then looked at the phosphorylation patterns.

That resulted in a large dataset of such patterns that the team then mined to look at changes at multiple sites.

Discovery of master sites

That was when they discovered what they termed "master sites," which were spots on the tau protein that change very early, and as a result are capable of causing multiple changes at other spots.

Phosphorylation and modification of the tau protein have been studied for a long time, senior author Arne Ittner, senior research fellow in Neuroscience in the Flinders Health and Medical Research Institute in Adelaide, Australia, told BioWorld.

But it was not known how tau arrives at such a state of modification, and that's where the idea that if these master sites could be identified, "we could identify how one change may affect these other changes subsequently," he said.

"We wanted to try and get ahead of that disease cascade, and there are certain positions on this tau molecule that have been identified with disease, but we wanted to backtrack to see how it goes from point A to point B, and that's where we found these master sites that are so important in triggering that event of multiple modification," Stefanoska added.

Experiments were conducted to find out how this happens, because the modifications in the tau protein are mediated by enzymes, or protein kinases, which are usually very specific, adding one modification to one particular spot.

"But we discovered there are some enzymes that when they meet tau and place a modification in one spot, they somehow change their abilities and become enzymes that can target multiple spots. We've characterized one tau and kinase that was particularly interesting – p38 alpha – that was a new behavior of protein kinases that was a sort of conversation between the kinase and the tau," Ittner said.

"We believe this enzyme could be very important in making these changes seen in tau in Alzheimer's."

The team used CRISPR-Cas9 genome-edited mice (T205 mice) such that a specific section of the tau molecule was removed.

"We wanted to see how this would affect, in a physiological setting, the phosphorylation, or changes, within tau. And from there we moved on to use a gene therapeutic approach to re-introduce a version of tau, and we did that in a disease model," Stefanoska said.

Two different models were used to answer this question of tau phosphorylation master sites.

The team discovered the role of these master sites in health and disease in the gene therapeutic approach with the AD mouse model (APP.23 tau knockout) that re-introduced a healthier version of tau than the normal tau that seems to undergo those changes.

"What we found is that by removing a single master site, we could abolish the multi-site changes, so that the tau molecule wouldn't undergo multi-site phosphorylation," she said.

"We could also improve memory in a spatiotemporal paradigm where we showed that when you have the normal tau that contains the master site that can cause that disease cascade, it seems to have poorer cognitive flexibility and learning.

"But once we re-introduced a version of the tau molecule that doesn't have that master site, we can actually see an improvement in their learning."

"What is most exciting for me was to find the mechanism that links site-specific changes with this hyper- or multi-site changes. I've been wondering about that for a long time, and I think the field is quite interested in that answer," Ittner said.

Striking discovery

Seeing that removing a master site improved memory in mice was a striking discovery, he said.

The team will now investigate how its findings could be translated into a treatment.

"Slowing down the changes at master sites of tau in these diseases may put the brakes on tau toxicity and dementia," Ittner said.

One approach could be a gene therapy whereby removing a master site could improve some cognitive deficits and alleviate some problems associated with AD.

The other approach is to use a vaccine in which master sites could be targeted to remove some of the phosphorylation before it cascades.

"We have shown that this new concept has therapeutic potential, but future work is needed to understand the role of these master sites in health and disease," Stefanoska said.

"Tau modification in Alzheimer's disease is a complicated process. Ours is the first study to link an initial change in tau with multi-site modification along the entire protein."

The authors say the new mechanism and the master sites at its center could apply to a range of neurological disorders in which tau is involved, including Parkinson's disease, concussion-induced chronic brain injury and stroke.