Novel Approach Measures Tau Changes That Correlate With Dementia Stage
By Deborah Borfitz
February 18, 2021 | The chemistry of tau protein in the brains of Alzheimer’s patients looks different at early and later stages of the disease, suggesting it could serve as a biomarker of disease progression. Although dementia researchers have historically focused almost entirely on abnormal accumulations of beta-amyloid, tau correlates better with cognitive decline and thus understanding its chemistry is key, according to Judith Steen, Ph.D., associate professor of neurology at Harvard Medical School.
The problem to date is that there has been no methodology to properly characterize the chemical changes of either protein quantitatively, says Steen. Her lab has been using mass spectrometry, in conjunction with protein-specific assays, to measure molecules more precisely.
The technology platform, known as FLEXIQuant (full-length expressed stable isotope-labeled proteins for quantification), quantifies and identifies the chemical changes in proteins based on their mass and composition. The technique can sequence and quantify the makeup of tau with unprecedented accuracy, an application called FLEXITau, says Steen.
The platform was developed more than a decade ago before its clinical value was fully appreciated, Steen says, so the basic methodology was never patented. It is now being expanded to a variety of proteins beyond tau—e.g., FLEXISyn, for measuring alpha synuclein in Parkinson’s disease—and each of those applications will have intellectual property protections. Reagents for FLEXITau are developed by the Steen laboratory and shared with the research community.
By mining data from human tissues and associated clinical and pathological information, researchers have used the novel approach to identify changes in tau molecules linked to mild, moderate, and severe stages of dementia. The study recently published in Cell (DOI: 10.1016/j.cell.2020.10.029) and Steen was the lead investigator.
Mass spectrometry was used to analyze thousands of proteins in brain tissues collected by brain banks funded by the National Institutes of Health (NIH) from patients with neurodegenerative diseases.
Researchers looked at tau aggregates in tissues of two areas of the human brain—the frontal gyrus and the angular gyrus—from 49 patients with Alzheimer’s disease and 42 age-matched individuals without known Alzheimer's or dementia. They found that the chemistry of the tau protein changed in Alzheimer's patients, and that specific chemically modified forms of tau correlated with dementia stage.
Findings are informing development of PET reagents that bind better to tau aggregates in the brain, Steen says. Many existing PET reagents are only effective when imaging late-stage Alzheimer’s disease patients.
The Azrieli Centre for Neuro-Radiochemistry, affiliated with the University of Toronto, is using information provided by FLEXITau to develop better PET reagents, Steen continues. The research center is directed by Neil Vasdev, Ph.D., former director of radiochemistry at Massachusetts General Hospital (MGH).
Steen’s research has been supported for more than a decade by the Tau Consortium, a collaborative research program managed and funded by the Rainwater Charitable Foundation. She is one of approximately 40 contributing investigators collectively endeavoring to come up with diagnostics and treatments for primary tauopathies (abnormalities of the tau protein) like progressive supranuclear palsy, as well as secondary ones such as Alzheimer’s disease.
On the diagnostics front, lab members Patrick W. van Zalm and Hanno Steen have a study underway around the workflow for measuring changes in tau using cerebral spinal fluid or blood plasma in concert with FLEXITau. A study in larger cohorts from multiple locations, including Europe and the U.S., is planned to generate the statistically relevant data needed to bring the application into clinical use.
A Closer Look
If specific pathways are activated at each stage of Alzheimer’s disease, as it appears, the implication is that multiple drugs will be needed to effectively target the pathological tau protein changes that form over time, says Steen. Currently, no combination treatment approaches are being used to treat the disease based on these changing parameters.
Many of the companies developing Alzheimer’s drugs are using antibodies specific to beta-amyloid, and with limited success. Recent anti-amyloid trials have had mixed results and the central issue, she believes, is that certain changes in either tau or beta-amyloid were not sufficiently characterized before the therapies were developed.
“We have to take a closer look at the chemistry,” Steen says. As attention shifts to tau, she is hopeful that new understanding about the protein will guide drug discovery efforts on tau and beta-amyloid in a more promising direction.
The Steen lab is additionally trying to understand the progression of Alzheimer’s disease in collaboration with the lab of Bradley Hyman, M.D., Ph.D., at MGH. Some patients must be institutionalized within three years while others slowly deteriorate over a decade or more, Steen says. In the next year, pending grant funding from the NIH, the research team will be building on a small study published last year finding that individuals with “typical” Alzheimer’s disease may have distinct biochemical features of tau potentially requiring personalized therapeutic approaches.
The latest study published in Cell revealed 95 post-translational chemical modifications of tau and about one-third of them haven’t been previously described, says Steen. The chemical processing is achieved by a spectrum of different enzymes in a stepwise fashion, a process that appears to start with the addition of phosphate. Another project studying the action of these enzymes and the extent of the changes in specific areas of the tau protein is also in progress.
The analysis work is all enabled by mass spectrometry, to which Steen was introduced by Sir Peter Roepstorff at the University of Southern Denmark in Odense. The city was the place to learn proteomics technology, she says, noting that a renowned British researcher proclaimed, "Odense is to proteomics what Nashville is to country music.”
The pathology of tauopathies has been known for some time, says Steen, and “in Alzheimer’s, specific regions of the brain are affected early in disease.” Tau first appears in the locus coeruleus of the brainstem and in the entorhinal cortex, and spreads from there to other regions of the brain.
Why those regions are so vulnerable to tau aggregation is “one of the biggest questions in the field,” she says. The entorhinal cortex, which is directly connected to the olfactory bulb, can be a conduit for stressors entering the brain.
It is possible that a neurotropic virus can move from the nose to the brain, Steen says. The list of viruses that are neurotropic presumably includes the SARS-CoV-2 virus that causes COVID-19 and can result in the loss of smell.