¶ Hypothesis
Primary Hypothesis: 4R-tau pathology in PSP initiates in specific brainstem nuclei (particularly the sublaterodorsal nucleus and cholinergic tegmental nuclei) before spreading to subcortical structures and ultimately the cortex in a predictable pattern that correlates with clinical phenotypes.
Secondary Hypotheses:
- The specific initiation zone determines whether patients develop PSP (brainstem predilection) vs CBS (cortical predilection)
- Functional connectivity patterns facilitate templated propagation along specific neural networks
- The presence of specific genetic variants (MAPT H1/H2, GWAS hits) modifies the initiation zone
¶ Open Question Source
This experiment addresses the critical understanding gap identified in the CBS/PSP Cure Roadmap Phase 2:
- "Where does tau initiate and how does it spread through networks?"
Also complements existing experiments:
- Tau Spreading Network Mapping via Spatial Transcriptomics (existing experiment 22, focuses on molecular signatures)
- This experiment focuses on anatomical initiation and propagation patterns
¶ Validation Protocol
¶ Study Design
- Type: Multi-center cross-sectional + longitudinal post-mortem and imaging study
- Cohort:
- Early-stage PSP (n=50): PSP-RS, disease duration
years - Early-stage CBS (n=30): Probable CBS, disease duration years
- Advanced PSP (n=30): Disease duration >5 years
- Controls (n=30): Age-matched, no neurodegenerative pathology
- Early-stage PSP (n=50): PSP-RS, disease duration
¶ Phase 1: Post-Mortem Mapping
¶ Tissue Collection
-
Brain regions (systematic sampling):
- Brainstem: Sublaterodorsal nucleus, pedunculopontine nucleus, dorsal raphe, locus coeruleus
- Basal ganglia: Striatum (caudate, putamen), globus pallidus internus/externus, subthalamic nucleus
- Thalamus: Anterior, mediodorsal, pulvinar
- Cortex: Motor (BA4), premotor (BA6), prefrontal (BA9/46), posterior cingulate (BA23)
- Cerebellum: Deep nuclei
-
Processing:
- Alternating sections for histology (H&E, silver stain) and biochemistry
- Fresh frozen for protein extraction and spatial proteomics
- RNA preservation for spatial transcriptomics
¶ Assays
-
AT8 immunohistochemistry (phospho-tau at Ser202/Thr205)
- Semi-quantitative scoring (0-3) in each region
- Automated image analysis for burden quantification
-
4R-tau specific assays
- RD4 antibody (detects 4R-tau specifically)
- Comparison with 3R+4R antibodies
-
Spatial proteomics (CODEX or imaging mass cytometry)
- 50+ protein panel including:
- Tau species (total, phosphorylated at multiple sites)
- Neuronal markers (NeuN, synaptophysin)
- Glial markers (Iba1, GFAP)
- Connectivity markers (synaptic proteins)
- 50+ protein panel including:
-
Functional connectivity modeling
- Human connectome data (HCP) for normative templates
- Model predicted spread patterns based on anatomical connectivity
- Compare with observed patterns
¶ Phase 2: In Vivo Validation
¶ Imaging Sub-Study
- Tau PET (flortaucipir, PI-2620 for 4R specificity when available)
- MRI (T1, DTI for connectivity, SWI for iron)
- Participants: Early-stage PSP (n=30), age-matched controls (n=15)
¶ Protocol
| Scan | Timepoint | Purpose |
|---|---|---|
| Tau PET (FTP) | Baseline, 12mo | Regional tau burden |
| Tau PET (PI-2620) | Baseline | 4R-tau specificity |
| MR T1 | Baseline, 12mo | Volumetric analysis |
| DTI | Baseline, 12mo | Structural connectivity |
| rs-fMRI | Baseline | Functional connectivity |
¶ Modeling Approach
- Spread rate estimation: Compare baseline to 12-month tau PET
- Network-based spread model: Fit models using human connectome
- Prediction of future spread: Validate against longitudinal data
¶ Phase 3: Genetic Modifiers
- Genotyping: MAPT H1/H2 haplotype, GWAS hits for PSP risk
- Analysis: Association between genetic variants and initiation zone location
¶ Expected Outcomes
¶ Primary Outcomes
- Initiation zone identification: Brainstem nuclei identified as primary initiation site in >75% of PSP cases
- Propagation pattern: Consistent brainstem → basal ganglia → cortex progression documented
- Early detection targets: Identify 2-3 brain regions where tau appears first that could serve as early biomarkers
¶ Secondary Outcomes
- CBS vs PSP differentiation: Identify distinct initiation zones for CBS vs PSP
- Connectivity-based spread: Confirm network connectivity facilitates templated spread
- Genetic modification: Identify genetic variants that influence initiation zone
¶ Exploratory Outcomes
- Initiation threshold: Determine burden level at which pathology becomes detectable
- Regional vulnerability factors: Identify molecular signatures that explain regional susceptibility
- Therapeutic implications: Identify intervention windows before cortical spread
¶ Feasibility Assessment
¶ Strengths
- Integration of post-mortem (gold standard for anatomical mapping) with in vivo imaging
- Multi-center collaboration provides adequate case numbers
- Spatial proteomics provides unprecedented resolution
- Longitudinal component enables spread rate estimation
¶ Challenges
- Early-stage post-mortem tissue is rare (short survival)
- Tau PET has limited sensitivity for brainstem tau
- Post-mortem tissue quality varies
- 4R-specific PET tracers still in development
¶ Timeline
| Milestone | Expected Date |
|---|---|
| Protocol finalization | Month 1-2 |
| Tissue collection (retrospective + prospective) | Month 3-18 |
| Histology and imaging analysis | Month 12-24 |
| In vivo imaging sub-study enrollment | Month 6-18 |
| Longitudinal imaging completion | Month 18-30 |
| Integrated analysis | Month 30-36 |
| Publication | Month 36-42 |
¶ Cost Estimate
| Category | Cost (USD) |
|---|---|
| Personnel (2 FTE, 1 PI, 1 biostatistician) | $1,100,000 |
| Post-mortem tissue procurement | $300,000 |
| Histology and immunohistochemistry | $250,000 |
| Spatial proteomics (CODEX) | $400,000 |
| In vivo imaging (PET/MRI) | $350,000 |
| Data analysis and modeling | $200,000 |
| Indirect costs (20%) | $520,000 |
| Total | $3,120,000 |
¶ Cross-Disease Relevance
This experiment provides insights relevant for:
- AD (similar propagation questions)
- Other tauopathies (FTD, CBD)
- Network spread models (applicable to alpha-synuclein, TDP-43)
- Therapeutic development (identify intervention windows)