Tau has long been implicated in Alzheimer’s and other neurodegenerative diseases but it is not yet clear how this protein converts from its normal, functional form into a misfolded, harmful one. Now, researchers at Columbia University’s Zuckerman Institute and Mayo Clinic in Florida have used cryo-electron microscopy (cryo-EM) along with mass spectrometry to provide highly detailed snapshots of this important protein.  They found that post translational modifications (PTMs) can influence how tau misfolds in brain cells.

Anthony Fitzpatrick, Ph.D., a principal investigator at Columbia’s Mortimer B. Zuckerman Mind Brain Behavior Institute led the study, which appears in Cell.

Nearly 50 million people worldwide have Alzheimer’s Disease (AD) or a related dementia, and such diseases are the top cause of disability in later life. Tau is a microtubule-associated protein and the main component of the filamentous tangles that are hallmarks of tauopathies, a set of diseases that include AD, frontotemporal dementia with parkinsonism-17 (FTDP-17), Pick disease (PiD), progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD).

However, tau filaments are very hard to study as they are 10,000 times thinner than the width of a human hair.  To address this challenge, Fitzpatrick recently pioneered the use of cryo-EM to visualize individual tau filaments from diseased human brain tissue. Cryo-EM is a Nobel Prize-winning technology developed, in part, by researchers at Columbia University. Cryo-EM images samples using a beam of electrons and can be used on even extremely small biological structures. Using cryo-EM, paired with mass spectrometry, Fitzpatrick’s team has reconstructed the structures of tau filaments, providing new insights into how they form, grow, and spread throughout the brain.

“Cryo-EM does not provide a complete picture because it cannot fully recognize the microscopic PTMs on tau’s surface,” said Christina Lee, an undergraduate student and research assistant in the Fitzpatrick lab and the paper’s co-first author. “But mass spectrometry can pinpoint the chemical composition of PTMs on the surface of tau.”

Working with co-corresponding author Leonard Petrucelli, PhD, Ralph B. and Ruth K. Abrams Professor of Neuroscience at Mayo Clinic in Florida, and Nicholas Seyfried, PhD, professor of biochemistry at Emory University School of Medicine, the researchers used cryo-EM and mass spectrometry to analyze the brain tissue from patients diagnosed with two tauopathies: AD and CBD. CBD is a rare but extremely aggressive tauopathy. Unlike Alzheimer’s, which is thought to arise due to a number of factors, including tau, CBD is primarily associated with misfolded tau proteins.

This analysis of brain tissue samples revealed several key insights. Most notably, the researchers found that cross-talk between PTMs on the surface of tau influences the structure of the tau filaments, contributing to differences in the filaments across the various tauopathies — and even variations from patient to patient.

“Collectively, these results suggest that PTMs may not only be serving as markers on the proteins’ surface, but are actually influencing the behavior of tau,” said Fitzpatrick, who is also an assistant professor of biochemistry and molecular biophysics at Columbia’s Vagelos College of Physicians and Surgeons.

Today’s findings on Alzheimer’s and CBD hold promise for the field, particularly in the development of new disease models — such as lab-grown organoids, or mini-brains — that may serve to accurately recapitulate what is actually happening in the brains of patients. Moving forward, Fitzpatrick and his team plan to expand this work to other tauopathies.

“Our findings will inspire new approaches for developing diagnostic tools and designing drugs, such as targeting PTM vulnerabilities to slow disease progression,” said Fitzpatrick. “Neurodegenerative diseases are among the most complex and distressing class of illnesses, but through our work and that of our colleagues and collaborators, we are building a roadmap toward successful diagnostics and therapeutics.”