Recent advances in cryo-electron microscopy (cryo-EM) have enabled the classification of tauopathies at near-atomic resolution, revealing disease-specific tau filament conformations that distinguish PSP from other 4R tauopathies and from Alzheimer's disease [@dong2026]. These microstructural differences influence intracellular localization, intercellular propagation, spatial distribution, and critically, the microscopic binding profiles and macroscopic imaging signatures of tau positron emission tomography (PET) tracers.
This page bridges neuropathological insights with in vivo PET findings, focusing on how tau filament structure determines what tau PET tracers "see" in PSP brains and what those signals reveal about disease biology.
Cryo-EM studies have resolved the atomic structure of tau filaments from PSP brains, revealing a unique fold pattern:
The tau filament fold determines PET tracer access and binding:
| Structural Feature | PET Tracer Implication |
|---|---|
| Fold geometry | Determines tracer access to binding sites |
| Cavity structure | Specific pockets accommodate tracer molecules |
| Surface charge | Influences electrostatic interactions |
| Conformational flexibility | Affects tracer on/off kinetics |
| Polymorphism | Multiple fold variants may explain PET signal heterogeneity |
Emerging evidence suggests PSP may contain multiple tau filament variants:
The most extensively studied tracer in PSP:
Binding characteristics:
Regional binding patterns [@malpetti2024]:
Clinical correlations [@nichol2022]:
Newer tracers specifically designed for 4R tauopathies:
PI-2620:
PMBBD3 (18F-PMBB3):
APN-1607 (18F-PMBB3 variant):
| Tracer | 4R Selectivity | Basal Ganglia Signal | Brainstem Signal | Cortical Signal |
|---|---|---|---|---|
| Flortaucipir (AV-1451) | Moderate | High | High | Low-Moderate |
| PI-2620 | High | Very high | High | Low |
| PMBBD3 | High | High | High | Low |
| MK-6240 | Moderate | High | High | Moderate |
The 2026 review by Dong et al. [@dong2026] proposes an integrative framework:
Tau Filament Conformation (Cryo-EM)
|
v
Intracellular Localization
- Neuronal perikarya
- Neuropil threads
- Glial inclusions (tufted astrocytes, oligodendrocytes)
|
v
Intercellular Propagation Patterns
- Trans-synaptic spread
- Glial cell uptake
- Extracellular release
|
v
Spatial Distribution (Neuroanatomy)
- Basal ganglia predilection
- Brainstem vulnerability
- Cortical sparing (relative)
|
v
Microscopic Binding Profiles
- Tracer access to filament binding sites
- Kinetic properties (on/off rates)
- Density of binding sites
|
v
Macroscopic PET Imaging Signatures
- Regional SUVR patterns
- Signal-to-noise ratios
- Longitudinal change rates
The cryo-EM-resolved tau filament structures explain several PET imaging observations:
Why PSP has lower cortical PET signal than AD: The PSP tau fold creates fewer accessible binding sites for AV-1451 in cortical regions compared to the AD tau fold
Why brainstem signal is high in PSP: The PSP tau fold is highly accessible to tracers in brainstem neurons, reflecting the high burden of brainstem tau pathology
Why different tracers show different patterns: Tracer molecules of different sizes and chemistries interact differently with the PSP tau fold geometry
Why PSP-P vs Richardson syndrome may differ on PET: Different filament variants associated with different clinical phenotypes may show different PET signal intensities
Tau PET provides in vivo evidence of tau pathology burden in PSP:
Differential diagnosis [@chen2024]:
Diagnostic accuracy:
Longitudinal tau PET studies demonstrate:
Tau PET could serve as a biomarker for disease-modifying therapies:
The structure-imaging correlation in PSP still has open questions:
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| Feature | PSP | AD |
|---|---|---|
| Primary tau isoform | 4R only | 3R + 4R |
| Filament type | Straight filaments | Paired helical + straight |
| Fold structure | C-shaped, unique | Bi-lobed PHF fold |
| Cortical PET signal | Low-Moderate | High |
| Basal ganglia PET | Very high | Low |
| Brainstem PET | High | Low |
| Hippocampal PET | Low | Very high |
| Feature | PSP | CBD |
|---|---|---|
| PET distribution | Symmetric, brainstem | Asymmetric, cortical |
| Basal ganglia signal | High, symmetric | High, asymmetric |
| Cortical signal | Low | Moderate-High |
| Tracer binding affinity | High for both | High for both |
| Phenotype correlation | Moderate | Moderate |
| Feature | PSP | AGD |
|---|---|---|
| Filament fold | C-shaped | Arrowhead |
| Primary location | Brainstem, basal ganglia | Limbic, cortical |
| PET signal pattern | Subthalamic nucleus | Amygdala, cortex |
| Clinical presentation | Subcortical syndrome | Dementia predominant |