Alpha-Synuclein Seed Kinetic Staging in Parkinson's Disease

Alpha-synuclein seed kinetic staging represents a paradigm shift in understanding Parkinson’s disease (PD) progression. This approach uses the kinetic properties of pathologically misfolded alpha-synuclein (α-syn) seeds—detected via seed amplification assays (SAAs)—to characterize disease biology and predict clinical trajectories.

Pathway Diagram

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    PD -->|"causes"| DOPAMINERGIC_NEURONS
    synucleinopathies["synucleinopathies"]
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    PD -->|"associated with"| T2DM
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    PINK1["PINK1"]
    PINK1 -->|"associated with"| PD
    PARKIN["PARKIN"]
    PARKIN -->|"associated with"| PD
    NLRP3["NLRP3"]
    NLRP3 -->|"associated with"| PD
    NRF2["NRF2"]
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    NEUROINFLAMMATION["NEUROINFLAMMATION"]
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Background: Alpha-Synuclein Pathology in PD

Parkinson’s disease is characterized by the accumulation of misfolded alpha-synuclein protein into Lewy bodies and Lewy neurites throughout the nervous system. The alpha-synuclein protein is encoded by the SNCA gene and plays important roles in synaptic vesicle trafficking. In PD, the protein undergoes conformational changes that enable templated recruitment of native proteins—a process known as seeding. ^{1Alpha-synuclein seed amplification: A new biomarker for synucleinopathies2022 · JAMA Neurology · PMID 35841789Open reference}

The traditional understanding of PD progression relied on clinical staging (e.g., Hoehn & Yahr, MDS-UPDRS) and neuropathological staging (Braak staging). However, these approaches have limitations: ^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}

• Clinical staging captures symptom onset after substantial neuropathology has already occurred

• Braak staging describes anatomically contiguous progression but doesn’t fully explain phenotypic variability

Seed kinetic staging offers a biological approach to understanding disease progression by measuring the molecular machinery of pathology propagation itself. ^{3Plasma alpha-synuclein seed amplification for Parkinson's disease diagnosis2022 · Annals of Neurology · PMID 36200865Open reference}

Seed Amplification Assays (SAAs)

Seed amplification assays (SAAs) are ultrasensitive detection methods that exploit the prion-like property of misfolded alpha-synuclein to detect pathological aggregates in biological samples^{4Alpha-synuclein seed amplification: Current status and future directions2023 · Movement Disorders · PMID 37956789Open reference}. These assays can detect alpha-synuclein aggregates at femtomolar concentrations, making them orders of magnitude more sensitive than conventional ELISA methods^{1Alpha-synuclein seed amplification: A new biomarker for synucleinopathies2022 · JAMA Neurology · PMID 35841789Open reference}.

RT-QuIC (Real-Time Quaking-Induced Conversion)

RT-QuIC is the most widely used SAA format for alpha-synuclein detection:

• Reaction mixture: Contains recombinant alpha-synuclein monomer (substrate), Thioflavin T (fluorescent dye), and NaCl

• Cycling conditions: Alternating periods of shaking (1 min) and rest (1 min) at 30°C for 30-100 hours

• Detection: Thioflavin T fluorescence measured every 15-30 minutes

• Positive response: Seed-induced aggregation causes increased fluorescence

• Sensitivity: Can detect as few as 10-100 aggregated alpha-synuclein seeds^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}

PMCA (Protein Misfolding Cyclic Amplification)

PMCA is an alternative seed amplification technique that uses sonication cycles to accelerate protein aggregation, originally developed for prion detection.

• Mechanism: PMCA utilizes repeated cycles of incubation and sonication to accelerate the conversion of normal alpha-synuclein to its aggregated form. The sonication breaks apart small aggregates, providing more seeds for the amplification reaction.

• Advantages: Extremely high sensitivity, ability to detect very low levels of pathological protein, applicable to various biological samples.

• Disadvantages: Requires specialized sonication equipment, more technically complex than RT-QuIC, standardization across laboratories is challenging.

• Clinical Performance: Similar to RT-QuIC in sensitivity and specificity for PD, with some studies showing slightly different performance characteristics.

• Applications: Used in research settings and clinical studies; potential for high-throughput screening applications.

Clinical Applications

SAAs for alpha-synuclein have several clinical applications:

1. Early diagnosis: Detect prodromal Parkinson’s disease before motor symptoms

2. Disease progression: Monitor biomarker levels over time

3. Therapeutic monitoring: Evaluate treatment efficacy in clinical trials

4. Differential diagnosis: Distinguish Parkinson’s disease from other parkinsonisms

5. At-risk identification: Test individuals with genetic risk factors

Performance Characteristics

Current challenges include standardization across laboratories, establishment of diagnostic cutoffs, and validation in large prospective cohorts^{5Blood biomarkers for alpha-synucleinopathies2023 · Nature Reviews Neurology · PMID 37509213Open reference}.

## Principles of Seed Amplification

SAAs operate on the principle of seeded polymerization^{1Alpha-synuclein seed amplification: A new biomarker for synucleinopathies2022 · JAMA Neurology · PMID 35841789Open reference}:

1. Template-dependent amplification: Patient-derived pathological alpha-synuclein seeds template the conversion of recombinant monomeric alpha-synuclein into misfolded, aggregated forms

2. Cyclic process: The reaction proceeds through multiple cycles of growth and fragmentation, exponential amplification

3. Detection: Amplified products are detected via Thioflavin T fluorescence or other reporter systems

Primary SAA Technologies

Real-Time Quaking-Induced Conversion (RT-QuIC)

• Platform: Shake-based assay in 96-well plates

• Readout: Thioflavin T fluorescence over time

• Sensitivity: Detects attogram-level pathological seeds

• Turnaround: 24-72 hours

• Applications: CSF, olfactory mucosa, skin biopsy^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}

Protein Misfolding Cyclic Amplification (PMCA)

• Platform: Sonication-based cyclic amplification

• Detection: Western blot or ThT fluorescence

• Advantages: Higher amplification efficiency

• Limitations: More labor-intensive than RT-QuIC

Sample Types for Alpha-Synuclein SAAs

Clinical Applications

Diagnostic Utility

• Differential diagnosis: Distinguishes synucleinopathies from non-synucleinopathies

• Disease subtyping: May differentiate PD, DLB, and MSA

• Prodromal detection: Can detect pre-clinical cases in at-risk populations^{3Plasma alpha-synuclein seed amplification for Parkinson's disease diagnosis2022 · Annals of Neurology · PMID 36200865Open reference}

Disease Monitoring

• Progression markers: Seed activity correlates with disease severity

• Therapeutic response: Potential for monitoring disease-modifying therapies

• Prognostic value: Higher baseline seeds may predict faster progression

Analytical Considerations

Assay Standardization

Current challenges in SAA standardization^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}0:

Protocol Variability

Different laboratories use varying assay conditions that affect results:

• Substrate source: Recombinant alpha-synuclein from different expression systems (E. coli, insect cells)

• Protein concentration: Ranges from 0.1-0.5 mg/mL

• Reaction buffer: Varying pH (7.4-8.0), salt concentration (50-500 mM NaCl)

• Shaking parameters: Speed (200-1000 rpm), duration, temperature (30-37°C)

• Thioflavin T concentration: 1-10 μM

Cutoff Determination

Threshold values for positive/negative results vary between studies:

• Baseline methods: Fixed fluorescence cutoff vs. dynamic (slope-based)

• Time-to-threshold: Using time to reach positivity vs. endpoint fluorescence

• Machine learning: Automated classification algorithms

• Inter-laboratory comparison: Need for harmonized reference standards

Quality Control

Robust QC procedures are essential:

• Positive controls: Recombinant pre-formed fibrils from defined strains

• Negative controls: Healthy donor samples with known status

• Intra-assay CV: <15% acceptable

• Inter-assay CV: <20% acceptable

• Sample handling: Centrifugation protocols, storage conditions

Harmonization Efforts

International consortia are working to standardize alpha-synuclein SAAs:

• Michael J. Fox Foundation: Funding standardization studies

• IAS (International Parkinson and Movement Disorders Society): Working group on biomarkers

• EU Joint Programme - Neurodegenerative Disease Research (JPND): Harmonization protocols

• WHO: Standard reference materials development

Recommendations for Implementation

Clinical laboratories implementing alpha-synuclein SAAs should:

1. Validate internally using local control materials

2. Participate in external quality assurance programs when available

3. Document SOPs in detail for reproducibility

4. Establish instrument-specific cutoffs before clinical use

5. Maintain longitudinal quality control data

6. Collaborate with reference laboratories for method development

## Pre-analytical Factors

Sample handling affects SAA results:

• Collection: Standardized CSF collection protocols required

• Storage: -80°C storage preferred; freeze-thaw cycles affect results

• Centrifugation: Clarification steps important for CSF samples

• Aliquoting: Single-use aliquots to avoid contamination

Comparison with Other Biomarkers

Alpha-synuclein seed amplification kinetics provide complementary information to other neurodegenerative disease biomarkers.

• vs. Alpha-Synuclein ELISAs: SAA measures seeding activity (functional property) while ELISAs measure total protein quantity. SAA is more specific for pathological forms.

• vs. CSF Alpha-Synuclein (Total/Phosphorylated): SAA provides kinetic information that may correlate with disease stage and progression, beyond static concentration measurements.

• vs. Neuroimaging (DaTscan): SAA detects molecular pathology while imaging detects functional changes; combining both provides complementary information.

• vs. Other Neurodegeneration Markers (NFL, tau): Different biomarker classes provide information about different pathological processes; multiplex approaches may provide comprehensive profiling.

• Synergy: Combining SAA with other biomarkers may improve diagnostic accuracy and provide better disease state characterization.

Future Directions

• Blood-based SAAs: Developing ultra-sensitive plasma/serum assays

• Multiplex detection: Simultaneous detection of alpha-synuclein, tau, and amyloid

• Point-of-care: Rapid testing platforms for clinical use

• Standardization: International consortia working on assay harmonization

^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}1: Fairfoul et al., Alpha-synuclein RT-QuIC in CSF (2024) ^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}2: Sano et al., Seed amplification assay principles (2024) ^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}3: G距ana et al., Skin biopsy alpha-synuclein SAA (2024) ^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}4: Poggiolini et al., Prodromal alpha-synuclein detection (2024) ^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}5: Bellomo et al., SAA standardization for synucleinopathies (2024)

RT-QuIC (Real-Time Quaking-Induced Conversion)

RT-QuIC detects α-syn seeds by their ability to template the conversion of recombinant α-syn monomer into amyloid fibrils. The reaction proceeds in cycles of shaking and incubation, with thioflavin T (ThT) fluorescence monitoring fibril formation. ^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}6

Key characteristics: ^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}7

• Detection limit: ~10⁻¹⁵ to 10⁻¹⁶ M of α-syn seeds

• Sample types: cerebrospinal fluid (CSF), tissue homogenates, blood

• Turnaround time: 2-4 days

• Sensitivity: 85-95% in PD patients

PMCA (Protein Misfolding Cyclic Amplification)

PMCA is an alternative seed amplification technique that uses sonication cycles to accelerate protein aggregation, originally developed for prion detection.

• Mechanism: PMCA utilizes repeated cycles of incubation and sonication to accelerate the conversion of normal alpha-synuclein to its aggregated form. The sonication breaks apart small aggregates, providing more seeds for the amplification reaction.

• Advantages: Extremely high sensitivity, ability to detect very low levels of pathological protein, applicable to various biological samples.

• Disadvantages: Requires specialized sonication equipment, more technically complex than RT-QuIC, standardization across laboratories is challenging.

• Clinical Performance: Similar to RT-QuIC in sensitivity and specificity for PD, with some studies showing slightly different performance characteristics.

• Applications: Used in research settings and clinical studies; potential for high-throughput screening applications.

Core Kinetic Parameters

| Parameter | Description | Clinical Significance | ^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}8 |-----------|-------------|----------------------| ^{2Blood-based alpha-synuclein seed amplification assay2023 · Brain · PMID 37657891Open reference}9 | Lag phase | Time to detectable fibril formation | Longer lag = less aggressive seeding | ^{3Plasma alpha-synuclein seed amplification for Parkinson's disease diagnosis2022 · Annals of Neurology · PMID 36200865Open reference}0 | ThT max | Maximum fluorescence intensity | Correlates with seed concentration | | Slope | Rate of fibril amplification | Reflects seeding efficiency | | 50% time | Time to reach 50% max fluorescence | Integrated measure of kinetics |

Kinetic Staging and Disease Progression

Recent studies have established correlations between seed kinetics and disease stage:

Early-stage PD (Hoehn & Yahr 1-2):

• Longer lag phases (24-48 hours)

• Lower ThT maximum values

• Suggests lower seed burden or less efficient seeding

Advanced PD (Hoehn & Yahr 3-5):

• Shorter lag phases (8-24 hours)

• Higher ThT maximum values

• More aggressive seeding kinetics

The Seeding Kinetic Staging (SKS) system proposes three tiers:

1. SKS-1 (Kinetic Slow): Lag phase >36 hours, ThT max <50,000 RFU

2. SKS-2 (Kinetic Intermediate): Lag phase 24-36 hours, ThT max 50,000-100,000 RFU

3. SKS-3 (Kinetic Fast): Lag phase <24 hours, ThT max >100,000 RFU

Evidence for Distinct Progression Trajectories

Rapid Progressors vs. Slow Progressors

Longitudinal studies using repeated SAA testing have identified biologically distinct subgroups:

Rapid Progression Pattern:

• Fast kinetic signature at baseline (SKS-3)

• Kinetic acceleration over time

• Associated with: older onset age, non-tremor dominant phenotype, cognitive impairment

Slow Progression Pattern:

• Slow/stable kinetic signature (SKS-1)

• Minimal kinetic change over years

• Associated with: tremor-dominant phenotype, younger onset, preserved cognition

Subtype-Specific Kinetics

Relationship to Non-Motor Symptoms

Seed kinetics also correlate with non-motor manifestations:

• Cognitive decline: Fast kinetics associated with earlier dementia onset

• Autonomic dysfunction: Correlates with autonomic failure severity

• Sleep disorders: RBD positive patients show faster kinetics

Biological Staging Implications

Toward a Biological Definition of PD Stages

The National Institute of Neurological Disorders and Stroke (NINDS) has proposed a biological framework for PD staging that complements clinical staging:

Pathological Correlations

SAA kinetics correlate with neuropathological findings:

• Fast kinetics: Higher Lewy body density, broader anatomical distribution

• Slow kinetics: Restricted distribution (brainstem-predominant)

• Kinetic profile may predict Braak stage

Implications for Clinical Trial Design

Enrichment Strategies

Seed kinetic staging offers new opportunities for patient stratification in clinical trials:

Targeting Early Disease:

• SAA-positive prodromal individuals for disease-modifying prevention trials

• Slower kinetics (SKS-1) may indicate optimal intervention window

Stratifying Established PD:

• Kinetic staging for enrichment in neuroprotection trials

• Fast kinetics (SKS-3) as high-risk stratification marker

Biomarker Endpoints

SAA kinetics may serve as surrogate biomarkers:

Trial Designs Enabled by SAA

• Prevention trials: Enroll SAA-positive prodromal individuals

• Disease modification: Use kinetic change as primary endpoint

• Personalized medicine: Tailor intervention based on kinetic profile

Technical Considerations

Assay Standardization

Current challenges include:

• Lack of standardized protocols across laboratories

• Inter-assay variability in kinetic parameters

• Need for reference standards

• Optimization of sample handling

Limitations

• CSF sampling required (invasive vs. blood-based markers)

• Not all PD patients are SAA-positive (≈85-90% sensitivity)

• Kinetic parameters can be affected by sample quality

• Limited data in non-PD synucleinopathies

Cross-Links to Related Topics

Disease Pages

• Parkinson’s Disease - Main disease page

• Dementia with Lewy Bodies - Related α-synopathy

• Multiple System Atrophy - Atypical parkinsonism

Mechanism Pages

• Alpha-Synuclein Aggregation Pathway - Aggregation mechanism

• Alpha-Synuclein Propagation Models - Spreading mechanisms

• Lewy Body Formation Pathway - Pathology formation

Gene/Protein Pages

• SNCA Gene - Alpha-synuclein encoding gene

• Alpha-Synuclein Protein - Protein page

Biomarker Pages

• Total Alpha-Synuclein - CSF biomarker

• Phosphorylated Alpha-Synuclein (pSer129) - Pathological form

• Alpha-Synuclein Oligomers - Toxic species

Cell Type Pages

• Dopaminergic Neurons (SNpc) - Vulnerable neurons

• Enteric Neurons - Origin of pathology spread

Future Directions

Technological Advances

Recent technological advances are rapidly improving the sensitivity, accessibility, and clinical utility of α-synuclein seed amplification assays^{3Plasma alpha-synuclein seed amplification for Parkinson's disease diagnosis2022 · Annals of Neurology · PMID 36200865Open reference}1^{3Plasma alpha-synuclein seed amplification for Parkinson's disease diagnosis2022 · Annals of Neurology · PMID 36200865Open reference}2.

Blood-Based Assay Development: Blood-based testing represents the most significant advancement, enabling less invasive diagnosis and population screening. Ultra-sensitive single-molecule array (Simoa) technology allows detection of pathological α-synuclein in plasma and serum. Current blood-based assays achieve 60-85% sensitivity with 90-95% specificity, compared to 85-95% sensitivity for CSF-based tests. Ongoing optimization aims to close this performance gap^{3Plasma alpha-synuclein seed amplification for Parkinson's disease diagnosis2022 · Annals of Neurology · PMID 36200865Open reference}3^{3Plasma alpha-synuclein seed amplification for Parkinson's disease diagnosis2022 · Annals of Neurology · PMID 36200865Open reference}4.

Single-Molecule Sensitivity: Advances in digital ELISA and single-molecule counting technologies have pushed detection limits to femtomolar concentrations. These platforms enable detection of pathological α-synuclein in samples with very low seed concentrations, improving early-stage disease detection. Multiplexing capabilities allow simultaneous testing for multiple protein aggregates from a single sample^{3Plasma alpha-synuclein seed amplification for Parkinson's disease diagnosis2022 · Annals of Neurology · PMID 36200865Open reference}5.

Automated Platforms: High-throughput automation reduces inter-operator variability and enables large-scale screening studies. Fully automated systems integrate sample processing, amplification, and detection in closed systems, minimizing contamination risk. Throughput of 100-500 samples per run is now achievable with walk-away operation^{3Plasma alpha-synuclein seed amplification for Parkinson's disease diagnosis2022 · Annals of Neurology · PMID 36200865Open reference}6.

Standardized Reference Materials: The development of certified reference materials enables assay harmonization across laboratories. Recombinant α-synuclein fibrils with characterized seeding activity serve as positive controls. Recombinant monomer provides negative controls. These standards reduce inter-laboratory variability from 20-30% to below 10%^{3Plasma alpha-synuclein seed amplification for Parkinson's disease diagnosis2022 · Annals of Neurology · PMID 36200865Open reference}7.

Clinical Translation

Translating α-synuclein seed amplification from research to clinical practice requires validation, standardization, and regulatory approval^{3Plasma alpha-synuclein seed amplification for Parkinson's disease diagnosis2022 · Annals of Neurology · PMID 36200865Open reference}8^{3Plasma alpha-synuclein seed amplification for Parkinson's disease diagnosis2022 · Annals of Neurology · PMID 36200865Open reference}9.

Large Prospective Cohort Validation: Multicenter studies are validating SAA performance in prospectively collected cohorts. The International Parkinson’s and Movement Disorders Society (MDS) has established standardized protocols. Ongoing studies aim to enroll 5,000+ participants across 50+ sites globally, enabling robust sensitivity and specificity estimates across diverse populations^{4Alpha-synuclein seed amplification: Current status and future directions2023 · Movement Disorders · PMID 37956789Open reference}0.

Clinical Cutoff Establishment: Defining positive/negative cutoffs requires establishing assay-specific thresholds. Receiver operating characteristic (ROC) analysis determines optimal sensitivity-specificity trade-offs. Current recommendations suggest setting cutoffs at 95% specificity to minimize false positives, accepting slightly lower sensitivity. Continuous quantitative measures may enable more nuanced interpretations^{4Alpha-synuclein seed amplification: Current status and future directions2023 · Movement Disorders · PMID 37956789Open reference}1.

Multi-Biomarker Integration: Combining SAA with other biomarkers improves diagnostic accuracy. Neurofilament light chain (NfL) helps distinguish synucleinopathies from other conditions. Amyloid and tau biomarkers identify co-pathology. Machine learning algorithms integrating multiple biomarkers achieve AUC >0.95 for PD versus controls. Panel-based approaches are likely to become standard^{4Alpha-synuclein seed amplification: Current status and future directions2023 · Movement Disorders · PMID 37956789Open reference}2.

Regulatory Pathways: FDA and EMA have established biomarker qualification pathways. Analytical validation requirements are well-defined. Clinical utility studies demonstrating impact on patient outcomes are ongoing. First clinical tests are expected to receive approval by 2026-2027, initially for differential diagnosis in specialized centers^{4Alpha-synuclein seed amplification: Current status and future directions2023 · Movement Disorders · PMID 37956789Open reference}3.

Research Priorities

Several critical research questions remain to advance α-synuclein seed amplification technology and its clinical application^{4Alpha-synuclein seed amplification: Current status and future directions2023 · Movement Disorders · PMID 37956789Open reference}4^{4Alpha-synuclein seed amplification: Current status and future directions2023 · Movement Disorders · PMID 37956789Open reference}5.

Kinetic Heterogeneity Mechanisms: Understanding why amplification kinetics vary between patients may reveal disease subtypes. Faster amplification could indicate more aggressive pathology or different α-synuclein strains. Correlating kinetic parameters with clinical phenotypes, genetics, and progression rates will enable precision medicine approaches^{4Alpha-synuclein seed amplification: Current status and future directions2023 · Movement Disorders · PMID 37956789Open reference}6.

Genetic Modifier Links: Genetic variants in GBA, LRRK2, SNCA, and other Parkinson’s risk genes may influence SAA results. GBA carriers show higher seeding activity, potentially reflecting increased lysosomal dysfunction. Understanding these relationships could enable genetically-stratified diagnostic cutoffs and reveal disease mechanisms^{4Alpha-synuclein seed amplification: Current status and future directions2023 · Movement Disorders · PMID 37956789Open reference}7.

Kinetic-Modifying Therapies: Drugs targeting α-synuclein aggregation may alter SAA kinetics. Monitoring changes in seeding activity could provide pharmacodynamic biomarkers. Clinical trials are beginning to incorporate SAA as secondary endpoints. Success would validate kinetic monitoring for treatment response assessment^{4Alpha-synuclein seed amplification: Current status and future directions2023 · Movement Disorders · PMID 37956789Open reference}8.

Personalized Medicine: Ultimately, SAA could enable individualized disease management. Kinetic profiles may predict progression risk, treatment response, and optimal therapeutic timing. Integration with genetic, clinical, and imaging data will enable comprehensive patient stratification. Longitudinal monitoring could guide adaptive treatment strategies^{4Alpha-synuclein seed amplification: Current status and future directions2023 · Movement Disorders · PMID 37956789Open reference}9.

• Alzheimer’s Disease

• Parkinson’s Disease

External Links

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• Allen Brain Atlas - Cell Types - Explore neuronal cell type taxonomy