Cryo-electron microscopy (cryo-EM) has revolutionized our understanding of tau filament structures in neurodegenerative diseases, revealing that distinct tauopathies are characterized by unique filament morphologies and conformations. Progressive Supranuclear Palsy (PSP), a 4R-tauopathy, exhibits characteristic straight filament (SF) structures that differ markedly from the paired helical filaments (PHFs) and straight filaments seen in Alzheimer's disease (AD), as well as the distinct structures observed in corticobasal degeneration (CBD). [@baker2021]
The structural analysis of tau filaments has provided unprecedented insights into the molecular basis of tau propagation, strain variation, and the relationship between protein conformation and clinical phenotype. This page provides a comprehensive overview of the cryo-EM structures of tau filaments in PSP, with detailed comparisons to AD and CBD. [@janning2022]
The [tau protein](/proteins/tau) (encoded by the [MAPT](/genes/mapt) gene) is a microtubule-associated protein that stabilizes neuronal microtubules. In the human brain, tau exists as six isoforms generated by alternative splicing of exon 2, exon 3, and exon 10. These isoforms differ in the number of N-terminal inserts (0, 1, or 2) and the presence of three (3R) or four (4R) microtubule-binding repeat domains. [@lvestam2022]
Cryo-electron microscopy (cryo-EM) has revolutionized our understanding of tau filament structures in neurodegenerative diseases, revealing that distinct tauopathies are characterized by unique filament morphologies and conformations. Progressive Supranuclear Palsy (PSP), a 4R-tauopathy, exhibits characteristic straight filament (SF) structures that differ markedly from the paired helical filaments (PHFs) and straight filaments seen in Alzheimer's disease (AD), as well as the distinct structures observed in corticobasal degeneration (CBD). [@baker2021]
The structural analysis of tau filaments has provided unprecedented insights into the molecular basis of tau propagation, strain variation, and the relationship between protein conformation and clinical phenotype. This page provides a comprehensive overview of the cryo-EM structures of tau filaments in PSP, with detailed comparisons to AD and CBD. [@janning2022]
The [tau protein](/proteins/tau) (encoded by the [MAPT](/genes/mapt) gene) is a microtubule-associated protein that stabilizes neuronal microtubules. In the human brain, tau exists as six isoforms generated by alternative splicing of exon 2, exon 3, and exon 10. These isoforms differ in the number of N-terminal inserts (0, 1, or 2) and the presence of three (3R) or four (4R) microtubule-binding repeat domains. [@lvestam2022]
The microtubule-binding repeats (R1-R4) constitute the core of tau filaments, while the N-terminal projection domain extends outward as the "fuzzy coat." In PSP, selective inclusion of exon 10 leads to an imbalance of 4R tau isoforms, which has important structural consequences for filament formation. [@song2023]
The aggregation of tau into insoluble filaments is driven by hyperphosphorylation at multiple sites, including serine and threonine residues such as Ser202, Thr205, Ser396, and Ser404. This phosphorylation reduces tau's affinity for microtubules, promoting its aggregation into β-sheet-rich fibrillar structures. [@yang2024]
Post-translational modifications beyond phosphorylation—including acetylation, truncation, oxidation, and ubiquitination—modulate filament formation and stability. Cryo-EM studies have revealed specific structural consequences of these modifications on filament architecture. [@vu2023]
Cryo-EM analysis of tau filaments involves extraction from post-mortem brain tissue, typically from the substantia nigra, globus pallidus, or cortical regions. The tissue is homogenized and fractionated to isolate Sarkosyl-insoluble tau filaments. Samples are then applied to cryo-EM grids, frozen in liquid ethane, and imaged using transmission electron microscopy. [@hemberg2024]
Advanced image processing techniques, including iterative helical real-space reconstruction (IHRSR) and cryo-EM single-particle analysis, enable determination of filament structures at near-atomic resolution. The recent cryo-EM structures of tau filaments have achieved resolutions of 3-4 Å, allowing detailed atomic modeling of the filament core. [@ishiyama2023]
Tau filaments in PSP primarily consist of straight filaments (SFs), which distinguish them from the twisted PHFs characteristic of AD. Cryo-EM structures have revealed that PSP straight filaments are composed of two C-shaped protofilaments that pair face-to-face, forming a hollow core region. [@brthen2022]
The filament core spans residues 254-378 of the 2N4R tau isoform, comprising portions of the microtubule-binding repeats R1-R4. This region adopts a cross-β sheet conformation, with β-strands running perpendicular to the filament axis. [@mudher2023]
Each protofilament in PSP tau filaments exhibits a C-shaped cross-sectional profile, formed by the stacking of β-sheets along the filament axis. The interface between protofilaments involves hydrophobic interactions and hydrogen bonding between residues in the microtubule-binding repeats.
The "fuzzy coat"—the peripheral region of tau filaments that extends from the core—contains the N-terminal projection domain and the remaining portions of the microtubule-binding repeats not incorporated into the core. This region is relatively disordered in cryo-EM structures but contributes to inter-filament interactions and potential templating properties.
While both PSP and AD can contain straight filaments, cryo-EM has revealed subtle but significant structural differences. AD straight filaments show variations in protofilament pairing and core region conformation compared to PSP straight filaments, suggesting disease-specific filament conformations or "strains."
AD is characterized by the presence of both paired helical filaments (PHFs) and straight filaments (SFs). PHFs exhibit a characteristic helical periodicity of approximately 80 nm, with a crossover spacing of about 40 nm. Cryo-EM structures of PHFs have revealed a double helical arrangement of two protofilaments.
The core of AD PHFs spans residues 306-378 (tau isoform 2N4R), forming a paired helical filament core (PHF6) that differs in conformation from the PSP straight filament core. The PHF core exhibits a more compact structure with distinct β-sheet topology.
Both AD and PSP tau filaments share a common cross-β sheet architecture, where β-strands run perpendicular to the filament axis, creating a "stacking" of β-sheets along the length of the filament. However, the specific β-sheet topology and hydrogen-bonding patterns differ between diseases.
The PHF6* motif (residues 378-380) and PHF6 (residues 306-378) form the core of AD PHFs, while the PSP core extends to include more N-terminal residues in the microtubule-binding repeats. These structural differences have implications for understanding disease-specific templating and the development of conformation-specific therapeutics.
Corticobasal degeneration (CBD) is another 4R-tauopathy, but cryo-EM studies have revealed that CBD tau filaments have distinct structural features from PSP. CBD filaments often show a wider cross-sectional profile and different protofilament organization compared to PSP straight filaments.
The CBD filament core includes residues 274-380 of 4R tau, with a distinct conformation that distinguishes it from PSP. Studies have shown that CBD and PSP can coexist in some cases, but their filament structures remain distinguishable.
Both PSP and CBD are classified as 4R-tauopathies due to the predominance of 4R tau isoforms. This contrasts with AD, which shows equal representation of 3R and 4R tau. The 4R tau isoforms include an additional microtubule-binding repeat (R2) encoded by exon 10, which is selectively included in PSP and CBD.
The presence of the R2 repeat influences filament formation by providing additional residues that can participate in β-sheet formation and protofilament interactions. This structural feature contributes to the distinct filament morphologies observed in 4R versus 3R/4R tauopathies.
| Feature | PSP | AD | CBD |
|---------|-----|----|-----|
| Primary Filament Type | Straight Filament (SF) | Paired Helical Filament (PHF) | Straight Filament (SF) |
| Tau Isoform | 4R | 3R/4R | 4R |
| Core Residues | 254-378 | 306-378 | 274-380 |
| Protofilaments | 2 | 2 | 2 |
| Helical Periodicity | None (linear) | ~80 nm | Variable |
| Cross-β Sheet | Present | Present | Present |
| Fuzzy Coat | Present | Present | Present |
Cryo-EM studies have revealed unexpected polymorphism in tau filaments both within and between diseases. Even within a single disease, multiple filament morphologies can coexist, suggesting the existence of distinct "strains" of tau filaments with different templating properties and clinical correlations.
In PSP, while straight filaments predominate, subtle variations in filament width, protofilament pairing, and core conformation have been observed between different brain regions and individual patients. This polymorphism may contribute to the clinical heterogeneity of PSP.
The structural differences between tau filament strains have implications for templated propagation (seeding). In vitro studies have shown that tau filaments can seed the aggregation of monomeric tau in a conformation-dependent manner, with different filament strains showing distinct seeding efficiencies and payload characteristics.
This templating ability has relevance for understanding the spread of tau pathology in the brain and for developing therapeutic strategies that target specific filament conformations.
The distinct filament structures observed in different tauopathies may contribute to the characteristic clinical presentations of each disease. The specific conformation of tau filaments influences their interaction with cellular components, their propagation patterns, and their effects on neuronal function.
PSP's characteristic vertical supranuclear gaze palsy and early postural instability may relate to the specific targeting of brainstem nuclei by 4R tau filaments with their unique structural features. Similarly, the cortical involvement in CBD correlates with distinct filament structures affecting cortical neurons.
Understanding the atomic structure of tau filaments has opened new avenues for therapeutic development. Conformation-specific antibodies and small molecules can now be designed to target the specific filament structures present in different tauopathies.
Immunotherapies targeting tau filaments must consider the structural differences between diseases. Antibodies that effectively recognize AD PHFs may have reduced affinity for PSP straight filaments, and vice versa. This has important implications for patient stratification in clinical trials.
The field of tau filament cryo-EM has been advanced by several landmark studies:
Recent advances have focused on:
Despite significant progress, key questions remain: