# Structural exposure of different microtubule binding domains determines the propagation and toxicity of pathogenic tau conformers in Alzheimer’s disease

## TL;DR

This study shows that the structural exposure of specific domains in misfolded tau proteins influences how quickly Alzheimer’s disease progresses and how toxic it is to neurons.

## Contribution

The study identifies the structural exposure of the fourth and first microtubule binding domains as key drivers of tau propagation and toxicity in Alzheimer’s disease.

## Key findings

- Structural exposure of the fourth microtubule binding domain (R4) drives tau replication and propagation.
- Exposure of the first microtubule binding domain (R1) is linked to neuronal toxicity and calcium influx.
- There is significant inter-individual structural variability in tau conformers from Alzheimer’s patients.

## Abstract

Deposits of misfolded tau proteins are leading indicators of cognitive decline in Alzheimer’s disease (AD), and our recent data implicate distinctly misfolded conformers of the tau protein with high seeding potency in rapid progression. We considered prion-like templated propagation of misfolding in neurons as an underlying mechanism and derived sensitive conformational assays to test this concept and identify critical structural drivers. Using novel photochemical hydroxylation monitored with a panel of Europium-labeled monoclonal antibodies, we investigated the structural organization of different microtubule binding domains (MTBDs) in brain-derived tau conformers in AD with different progression rates. We analyzed the impact of structural organization of different MTBDs on seeding potency in vitro and in primary neurons, and on the propagation rate of tau misfolding, compartmentalization, cytotoxicity, and calcium homeostasis in neuronally differentiated SH-SY5Y cells. Within the extensive inter-individual structural variability in all MTBDs and C-terminal tails, the most significant driver of seeding potency and propagation of tau protein misfolding in both in vitro seeding assays and in neuronal cultures was the structural exposure of the fourth MTBD (R4). In contrast, the major driver of calcium influx induced in neurons by the accumulation of misfolded tau was the structural exposure of the R1 domain. The data provide compelling evidence for a major diversity in the structural organization of MTBDs of misfolded AD brain-derived tau protein and implicate the structural exposure of distinct domains in different pathogenetic steps of AD — R4 tau domain in progression rate, and R1 domain in variable synaptic toxicity of misfolded tau, and thus in cognitive decline.

Within the spectrum of Alzheimer’s disease (AD) phenotypes, the disease stage, brain areas of atrophy, and ultimate cognitive decline tend to correlate with the progressive spread of misfolded tau protein aggregates, and our recent findings implicated the distinct, highly potent tau seed conformers in the rapid progression of AD. Our working hypothesis was that the specific structural organization of microtubule-binding domains (MTBDs) in pathogenic tau conformers is a critical driver of their replication by controlling their affinity to monomers of normal tau, thus leading to different rates of replication and propagation in neurons. The data presented here support the hypothesis and demonstrate (i) major inter-individual structural diversity of MTBDs in tau conformers isolated from AD with different progression rates, particularly in the fourth repeat (R4) tau domain, (ii) a significant role of the structural organization and exposure of the R4 domain within the MTBDs in the replication and propagation of tau conformers, and (iii) a striking correlation between structural exposure of the first repeat (R1) domain and toxicity in neuronal cultures. Although specifically targeting these domains in prion strain-like tau conformers with inhibitors will be challenging, it is a potentially explorable strategy in future studies.