Corticobasal Syndrome (CBS) is a progressive neurodegenerative disorder characterized by asymmetric cortical atrophy, basal ganglia degeneration, and prominent movement abnormalities. A defining pathological feature of CBS is the abnormal accumulation of four-repeat (4R) tau protein in neurons and glia. This mechanism page focuses on the dysregulation of tau phosphorylation in CBS, examining the key kinases and phosphatases involved, how 4R tau isoform imbalance affects phosphorylation patterns, and how this differs from Alzheimer's disease tau pathology.
Tau is a microtubule-associated protein primarily expressed in neurons where it stabilizes microtubules in axons and modulates axonal transport. Under normal conditions, tau phosphorylation is tightly regulated, with approximately 2-3 moles of phosphate per mole of tau. This phosphorylation state allows dynamic regulation of microtubule binding affinity in response to cellular signaling needs.
Tau has over 85 potential phosphorylation sites, primarily on serine and threonine residues, with a smaller number on tyrosine residues. These sites are clustered in:
- Proline-rich domain (PRD): Ser199, Ser202, Thr205, Thr212, Ser214, Ser235
- Microtubule-binding repeats (R1-R4): Ser262, Ser356, Ser396, Ser404, Ser422
The phosphorylation state at these sites determines tau's functional properties, including microtubule binding affinity, aggregation propensity, and subcellular localization.
GSK3β is the primary kinase implicated in tau hyperphosphorylation in CBS and other tauopathies.
- Activity: Constitutively active serine/threonine kinase
- Key phosphorylation sites: Ser199, Ser202, Thr205, Ser396, Ser404
- Regulation: Inhibited by Akt-mediated phosphorylation at Ser9
- Role in CBS: GSK3β activity is elevated in CBS brain tissue, and its activation correlates with the density of tau-positive inclusions
GSK3β preferentially phosphorylates tau at sites flanking the microtubule-binding repeats, leading to reduced microtubule binding and increased aggregation propensity.
CDK5 is a neuron-specific kinase critical for neuronal development and function.
- Activation: Requires binding to regulatory subunits p35 or p39
- Key phosphorylation sites: Ser199, Ser202, Thr205, Ser235
- Dysregulation in CBS: Calpain-mediated cleavage of p35 to p25 leads to prolonged CDK5 activation
- Role in CBS: CDK5 hyperactivity contributes to pathological tau phosphorylation in CBS, particularly in neurons with 4R tau inclusions
The p25/CDK5 complex shows increased activity in CBS brains, making it a therapeutic target of interest.
The MARK family (MARK1-4) regulates microtubule dynamics through tau phosphorylation.
- Key phosphorylation sites: Ser262, Ser356 (within microtubule-binding repeats)
- Role in CBS: Phosphorylation at these sites dramatically reduces tau's microtubule binding affinity
- Specificity: MARK preferentially phosphorylates 4R tau isoforms due to structural differences in the repeat domain
MARK-mediated phosphorylation at Ser262 is considered an early event in tau pathology, as it displaces tau from microtubules, making it available for aggregation.
PKA is a cAMP-dependent kinase that modulates tau phosphorylation in response to neurotransmitter signaling.
- Key phosphorylation sites: Ser396, Ser404, Ser214
- Role in CBS: PKA activity is dysregulated in CBS, potentially due to altered cAMP signaling
- Interaction with other kinases: PKA can prime tau for subsequent phosphorylation by GSK3β
CaMKII is abundant in neurons and activated by calcium influx.
- Key phosphorylation sites: Ser416, Thr231
- Role in CBS: Excitotoxicity and calcium dysregulation in CBS may lead to CaMKII activation
- Pathological significance: CaMKII-mediated tau phosphorylation may contribute to synaptic dysfunction
flowchart TD
A["Normal 4R Tau"] --> B["Kinase Hyperactivation"]
B --> C["GSK3β"]
B --> D["CDK5/p25"]
B --> E["MARK"]
B --> F["CaMKII"]
C --> G["Ser199/Ser202/Thr205 Phosphorylation"]
D --> G
E --> H["Ser262/Ser356 Phosphorylation"]
F --> I["Ser416/Thr231 Phosphorylation"]
G --> J["Hyperphosphorylated 4R Tau"]
H --> J
I --> J
K["PP2A Deficit"] --> J
J --> L["Microtubule Detachment"]
J --> M["4R Tau Aggregation"]
M --> N["CBS Neurodegeneration"]
PP2A is the major phosphatase responsible for tau dephosphorylation.
- Activity: Accounts for approximately 70% of tau dephosphorylation activity in the brain
- Key dephosphorylation sites: All major phospho-sites including Ser199, Ser202, Thr205, Ser396, Ser404
- Regulation: PP2A activity is regulated by methylation, phosphorylation, and inhibitor proteins
- Role in CBS: PP2A activity is reduced in CBS brains, contributing to net tau hyperphosphorylation
The downregulation of PP2A in CBS may result from:
- Reduced expression of PP2A subunits
- Increased methylation of PP2A
- Accumulation of endogenous PP2A inhibitors
PP1 also contributes to tau dephosphorylation but has a more limited role.
- Key dephosphorylation sites: Primarily Ser396 and Ser404
- Role in CBS: PP1 activity is also reduced in CBS, though less dramatically than PP2A
- Therapeutic potential: PP1 activators have been proposed as therapeutic agents
flowchart LR
A["Hyperphosphorylated Tau"] --> B["Phosphatase Activity"]
subgraph P["hosphatases"]
B --> C["PP2A"]
B --> D["PP1"]
end
C --> C1 [Major<br/>dephosphorylation<br/>70%]
D --> D1 [Minor<br/>dephosphorylation]
C1 --> E["Normal Tau"]
D1 --> E
E --> F["Proper Microtubule Binding"]
Unlike Alzheimer's disease, where both 3R and 4R tau are present in neurofibrillary tangles, CBS is characterized by a predominance of 4R tau isoforms (approximately 75% 4R, 25% 3R).
The 4R tau isoform has distinct phosphorylation characteristics:
- Additional microtubule-binding repeat (R2): Contains additional phosphorylation sites that are absent in 3R tau
- Higher basal phosphorylation: 4R tau has inherently higher phosphorylation at some sites due to structural accessibility
- Distinct phosphorylation patterns: Cryo-EM studies show different phosphorylation patterns in 4R tau filaments compared to 3R/4R mixed filaments
The MAPT H1 haplotype, which is the major genetic risk factor for CBS, promotes exon 10 inclusion, leading to increased 4R tau expression.
| Site |
3R Tau |
4R Tau |
CBS Relevance |
| Ser202 |
✓ |
✓ |
Early marker |
| Thr205 |
✓ |
✓ |
Pathological |
| Ser262 |
— |
✓ |
4R-specific |
| Ser356 |
— |
✓ |
4R-specific |
| Ser396 |
✓ |
✓ |
Aggregation |
| Ser404 |
✓ |
✓ |
Late-stage |
| Feature |
Alzheimer's Disease |
Corticobasal Syndrome |
| Tau isoform |
3R + 4R (mixed) |
Predominantly 4R |
| Primary kinases |
GSK3β, CDK5 |
GSK3β, CDK5, MARK |
| Phosphatase deficit |
Severe PP2A reduction |
Moderate PP2A reduction |
| Phosphorylation pattern |
Broad, many sites |
Specific sites predominant |
| Filament type |
Paired helical filaments (PHFs) |
Straight filaments, 4R-specific |
| Cell types affected |
Pyramidal neurons |
Neurons + astrocytes |
| Regional pattern |
Braak staging |
Asymmetric cortical |
Cryo-EM studies have revealed distinct tau filament structures in CBS compared to AD:
- CBS filaments: Composed of 4R tau with a distinct C-shaped fold
- AD filaments: Mixed 3R/4R tau with paired helical filament structure
- CBD-specific structures: "Astrogliopathic" tau with distinct protofilament arrangements
These structural differences explain the distinct phosphorylation patterns and clinical presentations.
flowchart TD
subgraph "Tauopathies"
A["Tau Pathology"]
end
subgraph "Alzheimer's Disease"
B["3R + 4R Mixed"]
B --> C["Paired Helical Filaments"]
C --> D["Braak Staging Pattern"]
end
subgraph "Corticobasal Syndrome"
E["Predominantly 4R"]
E --> F["Straight Filaments"]
F --> G["Asymmetric Cortical Pattern"]
end
A --> B
A --> E
Clinical status: Preclinical to Phase 1
- Lithium: First-generation GSK3β inhibitor, shown to reduce tau phosphorylation in models
- Tideglusib: Small-molecule GSK3β inhibitor, completed Phase 2 trials for AD and PSP
- Challenges: Toxicity due to broad kinase inhibition, limited brain penetration
Clinical status: Preclinical
- Roscovitine: CDK5 inhibitor shown to reduce tau phosphorylation
- Challenges: CDK5 is essential for neuronal function, systemic inhibition may be toxic
Clinical status: Preclinical
- Target: Specifically inhibit MARK-mediated Ser262 phosphorylation
- Advantage: More selective targeting possible
- Challenge: Blood-brain barrier penetration
Given that multiple kinases contribute to tau hyperphosphorylation, combination approaches are being explored:
- Rationale: Simultaneous inhibition of GSK3β, CDK5, and other kinases
- Challenge: Balancing efficacy with toxicity
An alternative approach involves activating PP2A:
- Calyculin A: PP2A activator, shown to reduce tau phosphorylation
- Challenges: PP2A activation may have off-target effects