PSPN Mouse

What are the most commonly used transgenic mouse models for PSP research?

Several transgenic mouse models have been developed for PSP research, with the P301L and A152T mutations in the MAPT gene being the most extensively utilized. The P301L mutation was initially associated with frontotemporal dementia and has been widely used to generate transgenic mice that develop rapid and reliable tau pathology. These models can be manipulated using different promoters and tau isoforms to study various aspects of tauopathy .

The A152T mutation is another important model as it is a risk variant for multiple neurodegenerative proteinopathies, including cases with symptoms consistent with PSP with pallido-nigro-luysial atrophy. A152T transgenic mice exhibit progressive aggregation of hyperphosphorylated tau in neurons of the hippocampus and cortex, along with synaptic loss, neurodegeneration, and disrupted proteostasis with accompanying cognitive behavioral impairment .

What are the limitations of current PSP mouse models?

Current PSP mouse models face several significant limitations:

• Lack of comprehensive pathology: No existing animal model fully replicates the key anatomical and cytopathologic hallmarks, spatiotemporal spread of pathology, progressive neurodegeneration, and symptoms that characterize PSP in humans .

• Translational challenges with P301L models: The P301L mutation produces "Pick-like bodies" that differ significantly from PSP pathology. Additionally, the P301L residue is located within the filament core of misfolded PSP tau where a leucine residue likely cannot be accommodated, making it unlikely that P301L tau can replicate the filament structure associated with PSP .

• Promoter-related expression issues: The spatial expression of tau in transgenic models is inextricably linked to the promoter used, creating artificial expression patterns .

• Overexpression artifacts: Many models rely on overexpression of tau, which may create artifacts not seen in the natural disease state .

• Difficulty modeling tufted astrocytes: Modeling the tufted astrocyte, a key hallmark of PSP, poses a particular challenge, with the numbers seen reportedly scarce in most models .

How do mouse tau expression patterns differ from humans, and why is this important?

This difference is crucial for PSP research:

• Isoform ratios: Human brains express both 3R and 4R tau isoforms in approximately equal ratio (1:1), while wildtype rodents only express 4R tau .

• PSP-specific alterations: In PSP, the normal 1:1 ratio shifts in favor of 4R isoforms .

• Modeling challenges: This fundamental difference makes it difficult for standard mouse models to naturally recapitulate the human disease process, particularly the shift from balanced isoform expression to 4R predominance .

• Innovative approaches: Researchers have attempted to address this by generating mice expressing all 6 isoforms of human tau. While early models did not maintain the 1:1 3R:4R ratio, more recent models by He et al. (2020) have preserved this ratio, offering improved opportunities to model tauopathies in mice .

What is the role of glial cells in PSP pathology and how are they modeled?

Glial cells play a critical and increasingly recognized role in PSP pathology:

• Astrocyte involvement: Tufted astrocytes are a key hallmark of PSP, but they have been difficult to model in animals. Recent research indicates astrocytes may not only sequester tau but may actively participate in tau aggregation and cellular dysfunction .

• Oligodendrocyte pathology: PSP features coiled bodies in oligodendrocytes. Tau deposits have been observed predominantly in oligodendrocytes of the inoculated corpus callosum in some models .

• Expression of tau in glial cells: Recent studies have found that MAPT mRNA is expressed not only in neurons but also in oligodendrocytes and astrocytes, and this expression is maintained in cells bearing aggregated tau in PSP .

• Locomotor phenotypes: In PSP-inoculated macaques, researchers noted a locomotor phenotype accompanied by strong astrocytic pathology in the peri-nigral region, suggesting glial involvement in PSP symptoms even in the absence of significant dopamine deficits .

What are the methodological approaches for creating PSP tau seeding models, and how do they compare?

PSP tau seeding models represent an important approach to modeling disease pathology:

• Human PSP brain-derived preparations: In pioneering studies, PBS-soluble tau preparations from human PSP putamen were inoculated into the hippocampus and overlying cortex of both non-transgenic and ALZ17 mice. These animals developed widespread silver-positive tau aggregates in interconnected brain regions, including neurofibrillary tangles, oligodendroglial coiled bodies, and astrocytic inclusions resembling the tufted astrocytes characteristic of PSP .

• Sarkosyl-insoluble tau inoculation: Subsequent studies utilized sarkosyl-insoluble tau from human PSP brain tissue. These studies found:

  • Neuronal inclusions, coiled bodies, and astrocytic tau that propagated via the neuronal connectome

  • Variation in distribution and spread of tau pathology depending on inoculation sites

  • Predominant tau deposits in oligodendrocytes when inoculated into corpus callosum

• Regional specificity: Different injection sites (hippocampus, thalamus, corpus callosum) produce different patterns of pathology spread, demonstrating the importance of anatomical targeting in model development .

• Non-human primate models: Recent studies have extended this approach to rhesus macaques, offering closer approximation to human disease due to their similar tau isoform expression and cerebral connectivity .

The table below summarizes key seeding approaches and their outcomes:

How can researchers overcome the limited availability of human PSP brain tissue for seeding studies?

The limited availability of human PSP brain tissue presents a significant challenge for seeding studies. Researchers have developed innovative approaches to address this constraint:

• In vitro amplification: Researchers have developed methodologies to amplify PSP tau seeds in vitro, using recombinant tau as a substrate. The resulting recombinant seeds share biophysical properties with the original human brain-derived seed. When inoculated into 6hTau mice, these amplified products can develop both neurofibrillary tangles and tufted astrocytes, demonstrating retention of disease-specific features .

• Cellular amplification: Another approach involves cellular amplification of PSP brain-derived seeds. Tau4RD LM-YFP biosensor cells can be seeded with PSP brain-derived tau to induce seeding of endogenous tau. Infected cells can then generate monoclonal cell lines that stably propagate misfolded tau. Cell lysates can subsequently be used to seed recombinant tau .

• Challenges and limitations: Despite their promise, these approaches face challenges:

  • Amplification efficiency is influenced by the yield and purity of PSP tau

  • Amplified tau may not perform similarly in successive rounds of in vitro amplification

  • Important cofactors might be diluted during amplification

  • Replication of these methodologies across laboratories remains to be demonstrated

These approaches represent cutting-edge solutions that could revolutionize PSP modeling by generating unlimited quantities of seeding-competent material capable of faithfully replicating disease-specific features.

What are the comparative advantages of different animal species in modeling PSP, and how should researchers select the appropriate model?

Different animal species offer distinct advantages for modeling PSP, with selection depending on research questions:

• Mouse models:

  • Advantages: Genetic manipulation capabilities, rapid breeding, cost-effectiveness, availability of transgenic lines

  • Limitations: Express only 4R tau (unlike human 3R/4R mix), difficulty modeling tufted astrocytes, different brain connectivity from humans

  • Best for: Genetic studies, high-throughput screening, initial pathophysiological investigations

• Rat models:

  • Advantages: More complex behaviors than mice, good for cognitive/behavioral testing

  • Examples: SHR72, SHR24, SHR318 lines with varying tau pathology; hTau-40/P301L rats

  • Best for: Behavioral studies, examining "pretangle" pathology, specific symptom modeling like abnormal startle reflex

• Non-human primates (Rhesus macaques):

  • Advantages: Expression of both 3R and 4R tau, human-like cerebral connectivity, naturally develop PSP-like pathology with aging

  • Features: Aged macaques (>25 years) develop 4R tau-positive deposits mimicking PSP ultrastructure, PSP-like tufted astrocytes

  • Best for: Translational studies, complex cognitive/motor assessments, long-term progression studies

  • Limitations: Ethical considerations, cost, specialized facilities required, longer timeframes

Selection criteria should include:

• Research question specificity (genetic factors, cellular mechanisms, symptom manifestation)

• Temporal considerations (acute vs. chronic studies)

• Required pathological features (neuronal, oligodendroglial, astrocytic pathology)

• Behavioral/functional endpoints

• Practical constraints (budget, facilities, expertise)

What are the current approaches to modeling the pedunculopontine tegmental nucleus (PPTg) pathology in PSP, and how do they relate to clinical symptoms?

The pedunculopontine tegmental nucleus (PPTg) is critically involved in PSP pathology and contributes to key clinical symptoms:

• AAV-based selective expression: A recent approach used CRE-dependent AAV system to selectively express 1N4R human tau in the cholinergic neurons of the PPTg. This model replicated several features of PSP including:

  • Loss of acoustic startle response (ASR)

  • Moderate locomotor deficit

  • Reduction in PPTg cholinergic neurons

  • Reduction in nigral dopaminergic neurons

  • Deposition of hyperphosphorylated tau

• PPTg lesion studies: Complementary approaches have used direct lesioning of the PPTg in rats to model specific symptoms:

  • Successfully modeled ASR, a very specific phenotype of PSP

  • Revealed that while spontaneous locomotion remained unaffected, when challenged, a locomotor deficit attributed to nigral dopaminergic loss can be observed

  • Suggests this simplified approach may offer additional opportunity for modeling PSP-related behaviors

• Limitations and future directions:

  • Current AAV models don't encompass the glial contribution to PSP

  • Long-term studies in hTau rats are needed to determine if a more aggressive PSP-like phenotype emerges over time

  • Combined approaches targeting multiple brain regions may better recapitulate the full spectrum of PSP pathology

This research highlights the importance of the PPTg in PSP and provides targeted approaches to model specific symptoms, offering a valuable platform to investigate early tau pathology and symptom development.

How can researchers distinguish between artifacts of overexpression versus disease-relevant pathology in transgenic PSP mouse models?

Distinguishing overexpression artifacts from disease-relevant pathology is critical for translational validity:

• Comparison with human pathology: Careful comparison of mouse model pathology with human PSP brain samples is essential. Key features to evaluate include:

  • Tau isoform ratios (4R vs 3R)

  • Regional distribution of pathology (should match PSP-affected regions)

  • Cellular morphology of inclusions (neurofibrillary tangles, tufted astrocytes, coiled bodies)

  • Ultrastructural characteristics of tau filaments

• Use of knock-in models: Newer knock-in approaches that maintain physiological expression levels can help avoid overexpression artifacts. The mouse expressing all 6 human tau isoforms with preserved 1:1 ratio of 3R:4R represents an example of this improved approach .

• Temporal progression analysis: True disease-relevant pathology typically shows:

  • Age-dependent progression

  • Stereotypical spread through connected brain regions

  • Correlation between pathology and functional deficits

• Cross-model validation: Findings should be validated across multiple model systems:

  • Compare observations in transgenic models with seeding models and AAV-based models

  • Test if phenotypes can be reproduced in wildtype animals using seeding approaches

  • Validate in higher species when possible

• Molecular signatures: Examine whether the molecular characteristics of tau aggregates match those seen in human PSP:

  • Hyperphosphorylation patterns

  • Presence of PSP-specific tau fragments (30-34kDa)

  • Similar insolubility and biochemical properties

By applying these analytical approaches, researchers can better differentiate model artifacts from disease-relevant findings, improving the translational value of their research.

What are the optimal protocols for assessing tau pathology in PSP mouse models?

Comprehensive assessment of tau pathology in PSP mouse models requires multiple complementary approaches:

• Histological techniques:

  • Silver staining methods (Gallyas, Campbell-Switzer) for argyrophilic inclusions

  • Thioflavin S staining for beta-sheet aggregates

  • Immunohistochemistry using antibodies against:

    • Total tau

    • Phospho-tau epitopes (AT8, AT100, PHF-1)

    • 4R-specific tau

    • Oligomeric tau species

• Biochemical approaches:

  • Sequential extraction (PBS-soluble, sarkosyl-soluble, sarkosyl-insoluble fractions)

  • Western blotting to identify:

    • Shift toward 4R tau isoforms

    • Hyperphosphorylated species

    • PSP-specific tau fragments (30-34kDa)

  • ELISA for quantitative measurement of different tau species

• Advanced imaging:

  • Electron microscopy to visualize filament structure

  • Two-photon imaging for longitudinal monitoring in living animals

  • PET imaging with tau-specific tracers to monitor progression

• Cell-specific assessments:

  • Double immunolabeling to identify cell types containing tau pathology:

    • NeuN for neurons

    • GFAP for astrocytes

    • Olig2 for oligodendrocytes

  • Quantification of different tau cytopathologies:

    • Neurofibrillary tangles

    • Tufted astrocytes

    • Coiled bodies

• Functional correlates:

  • Correlate pathology measurements with behavioral deficits

  • Track regional neurodegeneration alongside tau accumulation

These methodologies should be applied systematically across brain regions implicated in PSP, including midbrain, basal ganglia, brainstem, and frontal cortex, with particular attention to the pedunculopontine tegmental nucleus.

What behavioral and functional assessments best capture PSP-like symptoms in mouse models?

Effective assessment of PSP-like symptoms requires a comprehensive battery of tests targeting specific domains affected in human PSP:

• Motor function tests:

  • Acoustic startle response (ASR): A key test that specifically reflects PPTg dysfunction in PSP

  • Challenging locomotor tests: While spontaneous locomotion may appear normal, tests that challenge the motor system can reveal deficits attributed to nigral dopaminergic loss

  • Balance beam: Assesses coordination and balance

  • Rotarod: Measures motor coordination and learning

  • Gait analysis: Evaluates stride length, width, and rhythm disturbances

  • Vertical grid test: Assesses hindlimb strength and coordination

• Oculomotor assessments:

  • Eye-tracking methods to evaluate saccadic eye movements

  • Optokinetic response testing

  • Pupillary light reflex

• Cognitive function:

  • Working memory: Y-maze, T-maze alternation

  • Executive function: 5-choice serial reaction time task

  • Cognitive flexibility: Reversal learning paradigms

  • Attention: 5-choice serial reaction time task

• Speech/vocalization correlates:

  • Ultrasonic vocalization recording and analysis

  • Laryngeal function assessment

• Assessment timeline:

  • Establish baseline measurements before pathology onset

  • Conduct longitudinal testing to track symptom progression

  • Correlate behavioral changes with pathological development

• Practical considerations:

  • Use standardized testing protocols to ensure reproducibility

  • Control for confounding factors (time of day, handling stress)

  • Employ automated systems to reduce experimenter bias

  • Consider sex differences in symptom manifestation

By employing this multifaceted approach to behavioral assessment, researchers can better characterize the functional impact of PSP-like pathology and establish more robust translational endpoints for therapeutic testing.

How can researchers quantitatively compare different PSP mouse models to determine their translational value?

Objective quantitative comparison of PSP mouse models requires a standardized multi-dimensional assessment framework:

• Pathological fidelity scoring:

  • Develop a weighted scoring system that evaluates:

    • Presence and distribution of all three PSP cytopathologies (NFTs, tufted astrocytes, coiled bodies)

    • Regional pattern similarity to human PSP (midbrain, basal ganglia, brainstem predominance)

    • Tau isoform ratio (4R predominance)

    • Biochemical characteristics (sarkosyl-insolubility, hyperphosphorylation)

    • Presence of PSP-specific tau fragments (30-34kDa)

• Symptom replication index:

  • Quantify the degree to which models reproduce cardinal PSP symptoms:

    • Postural instability and falls

    • Oculomotor dysfunction

    • Acoustic startle response abnormalities

    • Cognitive/behavioral changes

    • Progressive nature of symptoms

• Neurodegeneration mapping:

  • Compare patterns of cell loss with human PSP:

    • Neuronal loss in specific nuclei (PPTg, substantia nigra, etc.)

    • Glial pathology in affected regions

    • White matter tract integrity

    • Correlation between cell loss and symptom severity

• Biomarker correlation:

  • Assess whether biomarkers in the model correlate with human PSP:

    • CSF tau levels and phosphorylation profile

    • Neuroinflammatory markers

    • Neuroimaging correlates

• Therapeutic predictivity:

  • Test known compounds with clinical data to validate predictive value:

    • Response to tau-directed therapies

    • Effects of symptomatic treatments

This framework allows for systematic comparison across models and helps researchers select the most appropriate model for specific research questions. A quantitative scoring system enables objective assessment of translational value and identifies specific strengths and limitations of each model.

What are the optimal methods for inducing and tracking the spread of tau pathology in PSP mouse models?

Inducing and tracking tau pathology spread in PSP models requires sophisticated methodologies:

• Optimized induction protocols:

  • Seeding approach selection:

    • Human PSP brain-derived preparations (PBS-soluble or sarkosyl-insoluble)

    • In vitro amplified PSP seeds

    • Cellular amplified PSP seeds

    • Synthetic tau preformed fibrils (PFFs) with PSP-like characteristics

  • Strategic injection targeting:

    • Hippocampus and overlying cortex: Produces widespread pathology involving neurons, oligodendrocytes and astrocytes

    • Thalamus: Generates different distribution pattern compared to hippocampal injection

    • Corpus callosum: Results in predominantly oligodendrocytic tau deposits

    • PPTg: Specifically targets region critically affected in PSP

• Tracking methodologies:

  • Time-course analysis:

    • Harvest tissues at multiple timepoints (1, 3, 6, 9, 12 months post-injection)

    • Map progression through anatomically connected regions

    • Quantify pathology intensity in each region at each timepoint

  • Advanced imaging approaches:

    • Two-photon microscopy with fluorescently labeled tau

    • Transparent tissue techniques (CLARITY, iDISCO) for whole-brain mapping

    • PET imaging with tau-specific tracers for longitudinal in vivo monitoring

  • Cell-type specific tracking:

    • Use of CRE-dependent reporter systems to track affected cell populations

    • Differentiate between neuronal and glial spread mechanisms

    • Retrograde and anterograde tracing combined with tau immunostaining

• Quantitative assessment:

  • Stereological quantification of affected cells by region and cell type

  • Digital image analysis for automated quantification

  • Heat-map generation showing pathology intensity across brain regions

• Mechanistic evaluation:

  • Manipulate specific cellular pathways to assess impact on spread

  • Compare spread patterns in mice with different genetic backgrounds

  • Evaluate the role of neuronal activity in pathology propagation

These approaches provide comprehensive tools for inducing and tracking tau pathology in a manner that recapitulates the progressive nature of PSP, enabling improved understanding of disease mechanisms and evaluation of potential therapeutics.

What considerations should guide the development of the next generation of PSP mouse models?

The development of improved PSP mouse models should address current limitations through several key considerations:

• Tau isoform expression:

  • Utilize models expressing all 6 isoforms of human tau with preserved 1:1 ratio of 3R:4R

  • Implement conditional systems to induce shift toward 4R predominance, mimicking PSP pathophysiology

  • Consider knock-in approaches rather than overexpression to maintain physiological expression levels

• Regional and cellular specificity:

  • Target expression to brain regions most affected in PSP

  • Develop methods to induce pathology in specific cell types:

    • Neurons in vulnerable regions

    • Oligodendrocytes for coiled body formation

    • Astrocytes for tufted astrocyte development

  • Consider multi-vector approaches targeting different cell populations simultaneously

• Genetic risk factor incorporation:

  • Integrate known PSP genetic risk factors beyond MAPT

  • Develop models combining multiple risk alleles to better recapitulate disease complexity

  • Consider gene-environment interaction models

• Progressive pathology development:

  • Design models that develop pathology gradually over time

  • Create systems allowing for controlled acceleration of pathological processes

  • Incorporate mechanisms for spatiotemporal spread of pathology

• Reproducible symptomatology:

  • Prioritize models that develop key PSP symptoms:

    • PPTg dysfunction and acoustic startle response abnormalities

    • Progressive motor impairment

    • Oculomotor dysfunction

    • Cognitive changes

• Translational enhancement:

  • Validation against human postmortem tissue

  • Development of parallel models in higher species

  • Standardized assessment protocols for cross-laboratory comparison

  • Correlation with human biomarkers

• Technological integration:

  • Incorporate reporter systems for live imaging of pathology progression

  • Develop models compatible with optogenetic and chemogenetic manipulations

  • Consider organ-on-chip and 3D culture systems as complementary approaches

By addressing these considerations, the next generation of PSP mouse models will more faithfully recapitulate human disease, enhancing their utility for mechanistic studies and therapeutic development.