Blood-brain barrier (BBB) dysfunction is an emerging area of research in progressive supranuclear palsy (PSP), contributing to disease pathogenesis through impaired clearance of toxic proteins, neuroinflammation, and compromised neuronal support. While historically considered a primary tauopathy with minimal vascular involvement compared to Alzheimer's disease, recent studies reveal significant BBB alterations in PSP that impact disease progression and therapeutic delivery.
¶ BBB Structure and Function
The BBB is a specialized interface comprising:
- Endothelial cells: Tight junctions (claudin-5, occludin, ZO-1) creating a near-impermeable barrier
- Pericytes: Covering 80-90% of capillary surface area, regulating blood flow and BBB integrity
- Astrocyte end-feet: Form the neurovascular unit, releasing factors maintaining BBB integrity
- Basement membrane: Extracellular matrix supporting cellular components
- Restricts peripheral molecules from entering the brain
- Regulates ion homeostasis
- Facilitates transport of essential nutrients
- Enables waste clearance via glymphatic and perivascular pathways
- Protects from pathogens and immune cells
Studies using dynamic contrast-enhanced (DCE)-MRI have demonstrated:
- Increased BBB permeability in PSP compared to healthy controls
- Regional specificity: Enhanced leakage in basal ganglia, brainstem, and white matter
- Correlation with disease severity: Higher permeability correlates with worse clinical scores
- Progression marker: BBB leakiness increases with disease duration
- TSPO-PET: Shows microglial activation coinciding with regions of BBB disruption
- FDG-PET: Hypometabolism in regions with compromised BBB
- Tau PET: Accumulation patterns overlap with areas of enhanced permeability
| Biomarker |
Change in PSP |
Interpretation |
| Albumin ratio (CSF/serum) |
Elevated |
Reduced BBB integrity |
| IgG index |
Increased |
Intrathecal IgG synthesis |
| Matrix metalloproteinase-9 (MMP-9) |
Elevated |
Proteolytic degradation of tight junctions |
| Soluble platelet-derived growth factor receptor-β (sPDGFR-β) |
Elevated |
Pericyte injury marker |
| Claudin-5 |
Decreased in CSF |
Tight junction degradation |
Neuropathological studies reveal:
- Tight junction alterations: Reduced expression of claudin-5 and occludin
- Pericyte loss: Decreased pericyte coverage in PSP brain tissue
- Microhemorrhages: Evidence of previous BBB rupture
- Plasma protein extravasation: IgG and fibrinogen in brain parenchyma
- Astrocytic alterations: Reactive gliosis accompanying BBB breakdown
¶ Tau Pathology and BBB
- Tau oligomers can damage endothelial cells
- Tau-laden neurons release inflammatory mediators affecting BBB
- Hyperphosphorylated tau in pericytes and endothelial cells
- Impaired glymphatic function reduces interstitial tau clearance
- Reduced perivascular drainage of tau
- Decreased transcytosis across BBB for waste removal
¶ Neuroinflammation and BBB
- Activated microglia release pro-inflammatory cytokines:
- Interleukin-1β (IL-1β)
- Interleukin-6 (IL-6)
- Tumor necrosis factor-α (TNF-α)
- Cytokines disrupt tight junction proteins
- Matrix metalloproteinases (MMPs) degrade basement membrane
- Monocyte infiltration across compromised BBB
- T-cell presence in PSP brain tissue
- Potential for antigen-driven immune responses
- Cerebral amyloid angiopathy (CAA) can coexist with PSP
- Arteriosclerosis in PSP brains
- Reduced cerebral blood flow
- Impaired autoregulation of cerebral blood flow
- Reduced ability to respond to metabolic demands
- Orthostatic hypotension affecting cerebral perfusion
Vascular Endothelial Growth Factor (VEGF) signaling plays a critical role in maintaining cerebral vascular health, and its dysregulation contributes to the vascular aspects of PSP pathophysiology. The VEGF family includes VEGF-A (with isoforms VEGF121, VEGF165, VEGF189), VEGF-B, VEGF-C, and placental growth factor (PLGF), each with distinct roles in vascular maintenance and angiogenesis.
In PSP, VEGF signaling is compromised through multiple mechanisms:
Reduced VEGF production:
- Decreased VEGF-A expression in affected brain regions (basal ganglia, brainstem)
- Reduced cerebrospinal fluid VEGF levels correlating with disease severity
- Impaired hypoxic response due to defective VEGF upregulation
Receptor dysfunction:
- VEGFR-2 (KDR/Flk-1) signaling impaired in cerebral endothelial cells
- Reduced downstream PI3K/Akt and MAPK/ERK pathway activation
- Decreased endothelial cell survival signaling
Contributing factors:
- Tau pathology directly interferes with VEGF signaling
- Oxidative stress reduces VEGF production
- Neuroinflammation alters VEGF expression patterns
¶ VEGF and Angiogenesis Impairment
Angiogenesis in PSP is compromised through:
Endothelial progenitor cell dysfunction:
- Reduced circulating endothelial progenitor cells in PSP patients
- Impaired mobilization and vascular repair capacity
- Reduced capacity for neovascularization
Vascular rarefaction:
- 15-30% reduction in capillary density in basal ganglia
- Reduced vascular density in substantia nigra
- Progressive loss correlating with disease progression
flowchart TD
subgraph PSP_VEGF["VEGF Dysregulation in PSP"]
A["Tau Pathology"] --> B["Endothelial Dysfunction"]
A --> C["Oxidative Stress"]
B --> D["Reduced VEGF Production"]
C --> D
D --> E["VEGFR-2 Signaling Deficit"]
E --> F["Endothelial Cell Death"]
E --> G["Impaired Angiogenesis"]
F --> H["Capillary Rarefaction"]
G --> H
H --> I["Neuronal Hypoxia"]
I --> J["Neurodegeneration"]
end
style A fill:#fff3e0,stroke:#333
style J fill:#fff3e0,stroke:#333
¶ VEGF-B and Vascular Maintenance
VEGF-B primarily regulates vascular maintenance through VEGFR-1 (Flt-1):
- Reduced VEGF-B signaling contributes to cerebral vascular rarefaction
- Impaired pericyte recruitment and coverage
- Reduced endothelial cell survival
- Compromised neurovascular coupling
The VEGF-B/VEGFR-1 axis represents a therapeutic target for maintaining vascular health in PSP.
Neuropilin-1 (NRP1) and neuropilin-2 (NRP2) serve as VEGF co-receptors:
- NRP1 expression reduced on neurons and endothelial cells in PSP
- Impaired VEGF-mediated neuroprotection
- Altered semaphorin signaling affecting axonal guidance
- Combined deficits in vascular maintenance and neuronal connectivity
VEGF pathway modulation offers therapeutic opportunities:
VEGF replacement strategies:
- VEGF-A protein administration to restore angiogenic signaling
- Gene therapy approaches for sustained VEGF expression
- Small molecule VEGFR-2 agonists
Combination approaches:
- VEGF therapy with tau-directed treatments
- VEGF enhancement with anti-oxidants
- Cell-based delivery using engineered stem cells
| Approach |
Target |
Development Status |
| VEGFR-2 agonists |
Receptor signaling |
Preclinical |
| VEGF gene therapy |
Long-term expression |
Phase I/II |
| VEGF-B therapy |
Vascular maintenance |
Preclinical |
| Cell-based delivery |
Localized secretion |
Preclinical |
VEGF pathway markers may serve as PSP biomarkers:
- CSF VEGF-A: Reduced levels correlate with disease severity
- Plasma VEGF-B: Decreased in PSP vs. healthy controls
- sVEGFR-1: Soluble receptor as potential marker
These biomarkers may help identify patients for VEGF-targeted therapies and monitor treatment responses.
- Globus pallidus: Highest permeability changes
- Putamen: Significant leakage
- Caudate nucleus: Moderate involvement
- Substantia nigra: Vulnerable due to high iron content
- Pons: Pericyte loss documented
- Midbrain: Early involvement
- Periventricular white matter: Hyperintense lesions on MRI
- Internal capsule: Diffusion abnormalities
- Corpus callosum: Reduced integrity
- Tight junction alterations limit paracellular transport
- Active efflux transporters (P-glycoprotein, BCRP) may be upregulated
- Reduced transcytosis limits macromolecule delivery
| Approach |
Current Status |
Example |
| Liposomal formulations |
Research |
Nanoparticle delivery |
| Focused ultrasound |
Clinical trials |
Temporary BBB opening |
| Intranasal delivery |
Investigational |
Bypassing BBB |
| Trojan horse approaches |
Preclinical |
Antibody-tau fusion |
- Active immunization: Requires BBB penetration
- Passive immunization: Antibody engineering for brain delivery
- Small molecule inhibitors: Better BBB penetration potential
- Antisense oligonucleotides: Emerging delivery methods
- Anti-inflammatory agents: Targeting neuroinflammation-BBB axis
- Pericyte-protective strategies: PDGF-BB therapy
- Tight junction stabilizers: Claudin-5 targeting
- Antioxidants: Protecting endothelial function
| Feature |
PSP |
AD |
| Primary BBB mechanism |
Pericyte injury |
Amyloid-induced dysfunction |
| Tight junction loss |
Moderate |
Severe |
| Regional pattern |
Basal ganglia, brainstem |
Cortex, hippocampus |
| Aβ contribution |
Minimal |
Central |
- Similar pericyte involvement
- CBS shows more cortical BBB disruption
- Regional patterns differ
- PD: More focused on nigrostriatal BBB
- PSP: Broader basal ganglia and brainstem involvement
- Both show elevated CSF albumin ratio
- Pericyte-specific markers: sPDGFR-β in blood/CSF
- Endothelial dysfunction markers: Endothelin-1
- Neurofilament light chain: Correlates with BBB disruption
- Imaging biomarkers: DCE-MRI quantitative metrics
- Patient selection based on BBB integrity
- Pharmacodynamic markers of drug delivery
- Combination therapies targeting multiple pathways
- Timing of intervention
- Gene therapy: Crossing BBB efficiently
- Cell-based therapies: Stem cell delivery of trophic factors
- Biomaterial approaches: Engineered protein carriers
- Targeted nanoparticles: Tau-specific delivery vehicles