Experiment Proposal (Crux): What distinguishes seed-competent tau species from non-pathogenic tau during trans-synaptic transfer [ask-aa724961]

AIMS

HYPOTHESES

PROTOCOL SUMMARY

Step 1: Patient Sample Acquisition and Strain Biobank Generation — Obtain frozen temporal cortex tissue from 40 SEA-AD characterized patients spanning Braak stages III-VI and 10 age-matched controls. Extract sarkosyl-insoluble tau fractions via sequential centrifugation. Sonicate to generate uniform seed preparations. Characterize by Western blot (Tau-5, PHF-1, AT8, MC1) and mass spectrometry for post-translational modifications. Index strains by phosphorylation pattern, truncation profile, and PTM signatures. Step 2: Tier-1 In Vitro Validation (PMCA and RT-QuIC) — Screen all seed preparations (n=50) in parallel PMCA and RT-QuIC assays using recombinant human 2N4R tau as substrate. For PMCA: perform 96 rounds of sonication cycles; measure Thioflavin-T fluorescence kinetics. For RT-QuIC: use 4°C overnight incubation with shaking; measure Thioflavin-S fluorescence. Record lag phase, elongation rate, and maximal fluorescence as strain signatures. Repeat each assay 3× independently with blinded seed identities. Step 3: Tier-2 Cellular FRET Biosensor Validation — Transduce HEK-293T cells with FRET biosensor (Tau RD-CFP/YFP fusion). Treat with serial dilutions of each seed preparation (n=50). Measure FRET efficiency by flow cytometry at 48h post-treatment. Establish EC50 for each strain. Validate that FRET-derived strain rankings correlate with PMCA/RT-QuIC kinetics. Step 4: Correlation with SEA-AD Biomarkers — Perform proteomic analysis on matched patient tissue using LC-MS/MS. Compare proteomic signatures across strains. Correlate strain-specific PTM patterns with SEA-AD snRNA-seq microglial activation signatures (TREM2 expression, disease-associated microglia markers). Identify biomarker panels predictive of strain identity. Step 5: Tier-3 In Vivo Propagation Validation — Select 12 strains representing the full kinetic spectrum (4 fastest, 4 intermediate, 4 slowest from Tier-1/2). Stereotaxically inject into right hippocampus of C57BL/6J mice (n=8 per strain). Monitor by longitudinal in vivo FRET imaging at 1, 3, 6 months. Quantify bilateral hippocampal and cortical spread. Step 6: Cross-System Strain Concordance Analysis — Perform multivariate correlation across all four assay systems. Define concordance thresholds. Classify strains as 'validated' (concordant across ≥3 systems), 'partially validated' (concordant across 2 systems), or 'discordant' (assay-dependent). Apply STRING pathway analysis to identify PTM signatures distinguishing validated from discordant strains. Step 7: Biomarker Validation — Test whether SEA-AD-derived biomarker panels can predict strain behavior in an independent test set (12 strains held out from initial classification). Calculate predictive accuracy and generate strain classification algorithm.

PREDICTED OBSERVATIONS

If H1 is true, tau seeds will cluster into 3-5 distinct strain groups with consistent kinetic rankings across PMCA and RT-QuIC, with <20% variance in EC50 values across assays. FRET biosensor EC50 values will correlate with biochemical kinetics (Pearson r >0.7). If H2 is true, fast-propagating strains in vitro will show accelerated in vivo spread with earlier onset of contralateral hippocampal involvement and higher pathology burden at 6 months (estimated 2-3× difference in spread index between fastest and slowest strains). If H3 is true, distinct strain clusters will associate with specific proteomic signatures, including differential expression of ubiquitin ligases (e.g., CHIP/TRIM71), kinases (GSK3β, CDK5), and microglial modulators (TREM2 ligands), enabling patient stratification based on biomarker panels.

FALSIFICATION CRITERIA

H1 is falsified if tau seed EC50 values show rank-order reversal across PMCA vs. RT-QuIC vs. FRET systems, or if variance in kinetic parameters exceeds 50% between replicate assays for the same seed preparation. H2 is falsified if in vivo propagation rates show no correlation with in vitro templating kinetics, indicating that observed strain differences may be assay-specific artifacts. H3 is falsified if proteomic/biomarker signatures show no association with strain identity (p >0.05 after Bonferroni correction), suggesting strains cannot be stratified by patient molecular profiles. Critically, the entire 'genuine strain' hypothesis is falsified if discordant strains (defined as conflicting across systems) comprise >40% of the biobank, indicating that observed diversity primarily reflects assay-dependent artifacts rather than stable conformational entities.

DATASET DEPENDENCIES