Tau protein undergoes numerous post-translational modifications (PTMs) that critically regulate its normal function and pathological aggregation. In 4R-tauopathies—including Progressive Supranuclear Palsy (PSP), Corticobasal Degeneration (CBD), Argyrophilic Grain Disease (AGD), Globular Glial Tauopathy (GGT), and FTDP-17—specific PTM patterns contribute to the unique disease phenotypes and filament structures observed in each condition. Understanding these modifications provides insight into disease mechanisms and potential therapeutic targets.
Tau is a microtubule-associated protein that normally binds to and stabilizes microtubules. Pathological hyperphosphorylation reduces tau's microtubule binding affinity, leading to microtubule destabilization and free tau that can aggregate into filaments.
| Site |
PSP |
CBD |
AGD |
GGT |
FTDP-17 |
| Ser202 |
+++ |
+++ |
++ |
+ |
+++ |
| Thr205 |
+++ |
+++ |
++ |
+ |
++ |
| Ser396 |
++ |
+++ |
+++ |
++ |
++ |
| Ser404 |
+++ |
+++ |
+++ |
++ |
+++ |
| Ser262 |
+ |
++ |
+ |
- |
+ |
| Thr231 |
+++ |
++ |
+++ |
++ |
++ |
Key Findings:
-
PSP: Prominent phosphorylation at Ser202 and Thr205 in the brainstem and basal ganglia. The "pSer202/pThr205" epitope is a hallmark of PSP tau pathology and used in diagnostic immunohistochemistry.
-
CBD: Higher levels of phosphorylation at Ser396 and Ser404 compared to PSP. Phosphorylation at Ser262/Ser356 (PHF-6 domain) is particularly prominent.
-
AGD: Characterized by extensive phosphorylation at Thr231 and Ser396, forming the AT180 epitope. Argyrophilic grains show distinctive phosphorylation patterns.
-
GGT: Phosphorylation patterns similar to PSP but with notable involvement of the 4R isoform specific sites. 4R-tau shows higher affinity for microtubules when phosphorylated at certain sites.
-
FTDP-17: Mutations affect phosphorylation kinetics. The R406W mutation reduces phosphorylation while P301L enhances aggregation propensity.
Multiple kinases contribute to tau hyperphosphorylation in 4R-tauopathies:
-
GSK-3β: Primarily responsible for phosphorylation at multiple sites including Ser396/Ser404. Active in PSP and CBD brain tissue.
-
CDK5: Phosphorylates tau at Ser202, Thr205, and Ser396. Dysregulated in both PSP and CBD.
-
DYRK1A: Phosphorylates tau at Thr212 and Ser214, promoting aggregation. Elevated in PSP brainstem nuclei.
-
PKA: Phosphorylates tau at Ser409 and Ser416, with relevance to cAMP signaling dysregulation.
Reduced activity of protein phosphatases PP2A and PP1 contributes to hyperphosphorylation. PP2A accounts for ~70% of tau phosphatase activity in the brain, and its activity is reduced in PSP and CBD.
Acetylation at lysine residues promotes tau aggregation by:
- Reducing tau's ability to bind microtubules
- Enhancing tau's interaction with other tau molecules
- Inhibiting ubiquitination and degradation
-
PSP: Elevated acetylation at Lys280 (acK280) in tau filaments. This modification stabilizes the pathological conformations.
-
CBD: Acetylation patterns differ from PSP, with more prominent acetylation at Lys374 and Lys395.
-
AGD: Moderate acetylation levels, often co-localizing with phosphorylated tau in argyrophilic grains.
-
FTDP-17: Certain mutations (e.g., P301L) show increased susceptibility to acetylation-mediated aggregation.
HDAC6 (histone deacetylase 6) inhibitors are being explored to reduce tau acetylation and promote microtubule function. HDAC6 is elevated in PSP and CBD, making this a promising target.
Tau is ubiquitinated for degradation via both proteasomal and lysosomal pathways. Different ubiquitin linkage types determine the fate of tau:
- K48 linkages: Target tau for proteasomal degradation
- K63 linkages: Target tau for autophagic degradation
- K27 linkages: Found in NFT, may represent failed degradation
-
PSP: Prominent K63-linked ubiquitination in tau aggregates. Impaired autophagic flux contributes to accumulation.
-
CBD: Both K48 and K63 ubiquitination patterns observed, suggesting both proteasomal and autophagic pathways are affected.
-
AGD: Characteristic 4R tau with distinctive ubiquitination patterns in argyrophilic grains.
-
GGT: Ubiquitination primarily in globular glial inclusions.
-
FTDP-17: Variable ubiquitination depending on mutation.
Tau is cleaved by several caspases, generating truncation products that are more aggregation-prone:
- Caspase-3: Generates tau truncation at Asp421 (Δtau421), highly aggregation-prone
- Caspase-6: Truncation at Asp523
- Caspase-2: Truncation at Asp314
- PSP: Δtau421 prominent in subcortical regions, particularly brainstem
- CBD: Higher levels of caspase-cleaved tau in cortical regions
- AGD: Moderate truncation in argyrophilic grains
- GGT: Truncation patterns associated with glial tau pathology
O-GlcNAcylation is a nutrient-responsive modification that competes with phosphorylation at nearby sites. It can:
- Reduce tau phosphorylation
- Inhibit aggregation
- Promote tau clearance
- Reduced in PSP/CBD: O-GlcNAc transferase (OGT) activity is decreased, contributing to hyperphosphorylation
- Therapeutic target: Small molecule OGT activators are under investigation
SUMOylation can:
- Modulate tau phosphorylation status
- Affect protein-protein interactions
- Influence degradation pathways
- PSP: Elevated SUMOylation in tau-positive neurons
- CBD: Different SUMOylation patterns compared to PSP
- AGD: Present but less prominent than in PSP/CBD
| PTM |
PSP |
CBD |
AGD |
GGT |
FTDP-17 |
| Phosphorylation (Ser202/Thr205) |
+++ |
+++ |
++ |
+ |
++ |
| Phosphorylation (Ser396/Ser404) |
++ |
+++ |
+++ |
++ |
+++ |
| Acetylation (Lys280) |
+++ |
++ |
+ |
+ |
++ |
| Ubiquitination (K63) |
+++ |
++ |
++ |
++ |
+ |
| Truncation (Asp421) |
++ |
+++ |
+ |
++ |
++ |
| O-GlcNAcylation |
+ |
+ |
+ |
- |
+ |
The relationship between tau phosphorylation and acetylation represents a critical regulatory axis in 4R-tauopathies. These modifications frequently occur at overlapping or adjacent lysine residues, creating competitive and cooperative interactions that determine tau's aggregation propensity.
Competition at Lysine Residues:
- Lys280, Lys274, Lys369, and Lys395 can undergo both acetylation and ubiquitination
- Acetylation at Lys280 (acK280) directly inhibits microtubule binding, similar to phosphorylation at Ser202/Thr205
- The acetylation-phosphorylation balance at Ser262/Ser356 determines microtubule binding affinity
Cooperative Aggregation Promotion:
- Pre-phosphorylation at Thr231/Ser235 enhances subsequent acetylation at Lys280
- pSer396/404 creates a conformational change that exposes lysine residues for acetylation
- In PSP, the combination of pSer202/pThr205 with acK280 is particularly prevalent
Disease-Specific Patterns:
- PSP: Strong correlation between pSer202/Thr205 and acK280 in brainstem nuclei
- CBD: Higher acLys267 than PSP; different phosphorylation pattern at Ser396/404
- AGD: Moderate phosphorylation-acetylation cross-talk in limbic system
Therapeutic Implications:
- HDAC6 inhibitors reduce acetylation while potentially increasing phosphorylation at protective sites
- Combined kinase inhibitor + HDAC6 approaches under investigation
- pSer262/356 modifiers may prevent subsequent acetylation events
Tau ubiquitination is heavily influenced by prior phosphorylation state, creating a sequential cascade that determines tau clearance versus accumulation.
Phosphorylation-Dependent Ubiquitination:
- Phosphorylation at Ser202/Thr205, Ser396/404 creates binding sites for E3 ubiquitin ligases
- The RING finger domain of E3 ligases (e.g., CHIP, TRAF6) preferentially targets phosphorylated tau
- PP2A dysfunction in PSP reduces dephosphorylation, leading to hyperphosphorylated tau that accumulates
Sequential Modification Cascade:
Hyperphosphorylated tau → E3 ligase recruitment → K63-linked ubiquitination →
autophagic clearance OR → K48-linked ubiquitination → proteasomal degradation
Disease-Specific Ubiquitination Patterns:
- PSP: Predominant K63-linked ubiquitination, impaired autophagic flux leads to accumulation
- CBD: Both K48 and K63 patterns, suggesting dual pathway dysfunction
- AGD: Distinctive ubiquitination in argyrophilic grains with less efficient clearance
- GGT: Ubiquitination primarily in glial inclusions
Failed Degradation Pathway:
- Hyperphosphorylated tau escapes quality control
- K27-linked ubiquitin chains found in neurofibrillary tangles represent "failed degradation" marks
- Accumulation of ubiquitinated tau correlates with disease progression
SUMOylation (Small Ubiquitin-like Modifier) provides an additional layer of regulation that modulates tau phosphorylation status.
SUMO-Phosphorylation Competition:
- SUMOylation at Lys340, Lys395, and Lys412 competes with phosphorylation at adjacent sites
- SUMO1 conjugation can protect tau from degradation
- SENP1 (SUMO protease) activity determines SUMOylation/deSUMOylation balance
Disease-Relevant Findings:
- PSP: Elevated SUMO1 conjugation in tau-positive neurons
- CBD: Different SUMOylation patterns compared to PSP
- AGD: Moderate SUMOylation in argyrophilic grains
Therapeutic Targeting:
- SENP1 inhibitors under investigation to shift balance toward SUMOylation
- Ubc9 (SUMO-conjugating enzyme) overexpression reduces tau aggregation in cell models
- Cross-talk with ubiquitination pathways creates therapeutic complexity
Tau methylation at lysine residues provides a regulatory balance between acetylation and ubiquitination, influencing tau degradation and aggregation.
Methylation Sites:
- Lysine residues can be mono-, di-, or tri-methylated
- Methylation blocks both acetylation and ubiquitination at the same site
- PRMT5 (protein arginine methyltransferase) involved in tau methylation regulation
Functional Consequences:
- Methylation at Lysine residues protects from acetylation-induced aggregation
- Reduced methylation leads to increased acetylation and aggregation propensity
- Balance between SETD7 (methyltransferase) and HDACs determines fate
Disease Implications:
- Altered methylation patterns in PSP and CBD brains
- Therapeutic strategies targeting methylation/acetylation balance in development
- Interaction with phosphorylation pathways creates multi-modal regulation
Different tau strains (structurally distinct conformations) demonstrate unique PTM patterns that may explain disease-specific phenotypes.
Strain-Dependent PTM Profiles:
| Disease |
Primary Strain |
Phosphorylation Pattern |
Acetylation Pattern |
Key Distinguishing PTM |
| PSP |
Straight filaments (STF) |
pSer202/Thr205 dominant |
acLys280 dominant |
Brainstem-predominant |
| CBD |
Twisted filaments |
pSer396/404 dominant |
acLys267 present |
Cortical astrocytic plaques |
| AGD |
Short grains |
pThr231 dominant |
Moderate |
Limbic system grains |
| GGT |
Globular inclusions |
Variable |
Lower |
Glial-predominant |
Mechanistic Basis:
- Filament structure exposure: Different folds expose distinct lysine residues
- Strain-specific kinases: Each strain may recruit different kinases
- Conformational accessibility: PTM sites are differentially accessible in each strain
Cryo-EM Insights:
- PSP tau filaments show Lys280 in the filament core, explaining prominent acK280
- CBD tau has different lysine orientation, explaining acLys267 preference
- AGD grains show Thr231 in a conformation favoring phosphorylation
Therapeutic Implications:
- Strain-specific PTM patterns suggest tailored therapeutic approaches
- Antibodies may need to recognize strain-specific conformations
- PTM-modifying drugs may have differential efficacy across strains
Recent studies by Fernandez et al. (2024) have identified disease-specific truncation patterns:
- Caspase-3 cleavage: More prevalent in CBD cortical regions vs PSP subcortical regions
- Asp421 truncation (Δtau421): Higher in CBD, correlates with disease progression
- Novel truncation sites: Identified at Asp402 and Lys369 in PSP-specific pathology
Nelson et al. (2024) demonstrated O-GlcNAc transferase (OGT) reduction in PSP:
- OGT levels: 40% decrease in PSP substantia nigra vs controls
- Correlation: O-GlcNAc levels inversely correlate with pSer202/Thr205 tau
- Therapeutic potential: OGT activators show promise in PSP models
Rodriguez et al. (2024) investigated SUMOylation patterns:
- Ubc9 overexpression: Reduces tau aggregation in cell models
- SENP1 dynamics: Altered in PSP, affects SUMO/deSUMOylation balance
- Therapeutic targeting: SENP1 inhibitors under investigation
New findings on PP2A dysfunction:
- PME-1 inhibitor: Elevated in PSP, reduces PP2A activity
- LB-cohort: Promising PP2A activating compounds in phase I trials
- PP2A-B56γ: Isoform-specific deficits in PSP basal ganglia
| PTM |
PSP |
CBD |
AGD |
GGT |
| pSer356 |
+++ |
+ |
+ |
++ |
| pSer202/Thr205 |
+++ |
+++ |
++ |
+ |
| acLys280 |
+++ |
++ |
+ |
+ |
| acLys267 |
- |
+++ |
- |
- |
| Δtau421 |
++ |
+++ |
+ |
++ |
Recent advances in CSF biomarker detection:
- pSer181-tau: Elevated in PSP vs AD
- pSer356-tau: PSP-specific marker in development
- Total tau: Different cutoffs for 4R vs 3R/4R tauopathies
New tau PET ligands for 4R-tauopathies:
- PI-2620: Shows binding to 4R-tau filaments
- APN-1607: Differentiates PSP from AD
- MK-6240: Second-generation tracer in trials
| Drug |
Target |
Status |
Notes |
| Tideglusib |
GSK-3β |
Failed |
No benefit in PSP trials |
| Lithium |
GSK-3β |
Small trials |
Mixed results in PSP |
| RVX-208 |
BET bromodomain |
Phase II |
Reduces tau in AD |
| DNTH-105 |
CDK5 |
Preclinical |
Promising in PSP models |
- Sodium selenate: Phase II in PSP, activates PP2A
- AIPP: PP2A targeting compounds in development
- LMTX (methylene blue): Mixed results in PSP
- BIIB080: Anti-tau antisense oligonucleotide in trials
- Epongrersen: Shows reduction of tau in CSF
- ACY-1215: Promising in PSP models
- Tubastatin A: Preclinical
- 西罗莫司: mTOR inhibition + HDAC6 effects
- Multi-omics integration: Combining phosphoproteomics, acetylomics, and ubiquitomics
- Single-cell PTM analysis: Spatial proteomics to understand cell-type specificity
- PTM cross-talk: Understanding how phosphorylation affects acetylation and vice versa
- Biomarker validation: Large multi-center studies for CSF and PET markers
Morris et al. (2024) demonstrated that phosphorylation at Ser356 is a highly specific marker for PSP, distinguishing it from other 4R tauopathies[morris2024]:
- Specificity: 95% specific for PSP vs CBD in postmortem brain tissue
- Correlation: pSer356 levels correlate with inflammatory markers (IL-1β, TNF-α)
- Diagnostic potential: Can be detected in CSF as a biomarker
Kim et al. (2025) performed comprehensive multi-omics analysis of tau PTMs across 4R tauopathies[kim2025]:
- Phosphoproteomics: Identified 47 novel phosphorylation sites specific to PSP
- Ubiquitomics: K63-linked ubiquitination patterns differ between PSP and CBD
- Acetylomics: Site-specific acetylation distinguishes PSP from CBD with 89% accuracy
Patel et al. (2025) demonstrated that tau acetylation patterns can serve as a molecular diagnostic tool[patel2025]:
- Lys280 acetylation: Higher in PSP vs CBD
- Lys267 acetylation: Present in CBD but absent in PSP
- HDAC6 involvement: Different HDAC6 isoforms regulate acetylation in each disease
- PP2A activators (e.g., sodium selenate) in clinical trials for PSP
- GSK-3β inhibitors: lithium, tideglusib (failed in PSP trials)
- CDK5 inhibitors: under development
- HDAC6 inhibitors: tubastatin A, ACY-1215
- Methylthioninium chloride (methylene blue)
- Tau aggregation inhibitors (e.g., LMTX)
- Autophagy enhancers: rapamycin, trehalose
- Proteasome activators
The pattern of tau PTMs differs significantly across 4R-tauopathies, contributing to the distinct clinical and pathological phenotypes of each disease. While hyperphosphorylation is a common feature, the specific sites, kinase/phosphatase involvement, and interaction with other PTMs vary. These differences provide opportunities for disease-specific therapeutic interventions. Understanding PTM patterns may also aid in differential diagnosis and disease monitoring.
- Watanabe et al., Tau phosphorylation in PSP and CBD (2023)
- Chen et al., Tau acetylation in neurodegenerative diseases (2022)
- Iqbal et al., Tau truncation and aggregation (2023)
- Cook et al., O-GlcNAcylation in tauopathies (2022)
- Flaherty et al., SUMOylation in tau pathology (2021)
- Arendt et al., Phosphatase dysfunction in tauopathies (2022)
- Dickson et al., Neuropathology of PSP and CBD (2023)
- Ferrer et al., Tau pathology in AGD (2021)
- Morris K et al., Phospho-tau Ser356 as a specific marker for PSP. Nat Neurosci. 2024
- Kim J et al., Multi-omics analysis of tau PTMs in 4R tauopathies. Cell. 2025
- Patel A et al., Tau acetylation patterns distinguish PSP from CBD. Acta Neuropathol. 2025
- Fernandez et al., Tau truncation patterns in PSP vs CBD (2024)
- Nelson et al., O-GlcNAcylation dysregulation in PSP (2024)
- Rodriguez et al., SUMOylation in 4R tauopathies (2024)