Neuropathology of Progressive Supranuclear Palsy

Neuropathology of Progressive Supranuclear Palsy

Overview

Progressive supranuclear palsy (PSP), first described by John Steele, Jerome Richardson, and Jerzy Olszewski in 1964, is a neurodegenerative disorder classified as a 4-repeat (4R) tauopathy. The disease is characterized neuropathologically by neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau protein, tufted astrocytes, and widespread neuronal loss with prominent gliosis. Understanding the neuropathology of PSP is essential for distinguishing it from other neurodegenerative diseases, particularly other 4R tauopathies like corticobasal degeneration (CBD), and for developing targeted therapeutics that address the underlying tau pathology. [@dickson2010]

PSP affects approximately 5-7 per 100,000 individuals, making it the most common atypical parkinsonian syndrome after Parkinson's disease itself. The neuropathological hallmark is the accumulation of abnormal tau protein in neurons, astrocytes, and oligodendrocytes, leading to progressive neuronal dysfunction and death in specific brain regions responsible for motor control, balance, and cognitive function. [@litvan1996]

Pathway Diagram

Neuropathology of Progressive Supranuclear Palsy

Overview

Progressive supranuclear palsy (PSP), first described by John Steele, Jerome Richardson, and Jerzy Olszewski in 1964, is a neurodegenerative disorder classified as a 4-repeat (4R) tauopathy. The disease is characterized neuropathologically by neurofibrillary tangles (NFTs) composed of hyperphosphorylated tau protein, tufted astrocytes, and widespread neuronal loss with prominent gliosis. Understanding the neuropathology of PSP is essential for distinguishing it from other neurodegenerative diseases, particularly other 4R tauopathies like corticobasal degeneration (CBD), and for developing targeted therapeutics that address the underlying tau pathology. [@dickson2010]

PSP affects approximately 5-7 per 100,000 individuals, making it the most common atypical parkinsonian syndrome after Parkinson's disease itself. The neuropathological hallmark is the accumulation of abnormal tau protein in neurons, astrocytes, and oligodendrocytes, leading to progressive neuronal dysfunction and death in specific brain regions responsible for motor control, balance, and cognitive function. [@litvan1996]

Pathway Diagram

Historical Background and Discovery

The original description of PSP by Steele, Richardson, and Olszewski in 1964 established the clinical triad of vertical gaze palsy, pseudobulbar palsy, and parkinsonism with axial rigidity. This clinical syndrome, initially termed "progressive supranuclear ophthalmoplegia," was later renamed PSP to reflect the broader clinical spectrum beyond ocular motor involvement. The neuropathological correlates were subsequently characterized, establishing the diagnostic criteria that remain in use today. [@steele1964]

Tau Protein Biology and the 4R Tauopathy Classification

Tau Protein Structure and Function

The tau protein is encoded by the MAPT (Microtubule-Associated Protein Tau) gene located on chromosome 17q21.31. It plays a critical role in microtubule stabilization, axonal transport, and neuronal polarity. The tau gene undergoes alternative splicing to produce six isoforms ranging from 352 to 441 amino acids, distinguished by the presence of 3 or 4 repeats (3R or 4R) of the microtubule-binding domain in the C-terminal half of the protein. [@goedert2000]

Tau Isoforms:

• 3-repeat tau (3R): Lacks exon 10, contains 3 microtubule-binding repeats
• 4-repeat tau (4R): Includes exon 10, contains 4 microtubule-binding repeats

Pathological Tau Conformations

In PSP, tau protein undergoes abnormal post-translational modifications including hyperphosphorylation, truncation, and aggregation into paired helical filaments (PHFs) and straight filaments (SFs). These filaments form the insoluble inclusions that characterize the disease:

• Paired Helical Filaments (PHFs): The classic filament morphology in tauopathies, visible by electron microscopy
• Straight Filaments (SFs): Also present, particularly in PSP
• Tau Oligomers: Intermediate aggregates increasingly recognized as toxic species

Hyperphosphorylation Sites

The tau protein in PSP is abnormally phosphorylated at numerous sites, including:

• Ser202, Thr205 (PHF-1 epitope)
• Ser396, Ser404 (PHF-6 epitope)
• Thr181, Thr231 (AT270, AT100 epitopes)

Characteristic Tau Inclusions

Neurofibrillary Tangles (NFTs)

NFTs are the most well-characterized tau inclusion in PSP, composed of intracellular aggregates of hyperphosphorylated tau protein arranged as paired helical filaments. Unlike Alzheimer's disease where NFTs are predominantly cortical, in PSP they are especially abundant in subcortical structures:

NFT Distribution in PSP:

• Substantia nigra pars compacta: Severe involvement with NFT-laden neurons
• Globus pallidus: High density of NFTs
• Subthalamic nucleus: Among the most affected regions
• Red nucleus: Prominent involvement
• Superior colliculus: Associated with vertical gaze palsy
• Brainstem reticular formation: Widespread involvement
• Cerebral cortex: Variable, generally less affected than in AD

Tufted Astrocytes

Tufted astrocytes represent one of the most pathognomonic neuropathological features of PSP, distinguishing it from other tauopathies including CBD. These are astrocytic inclusions characterized by tau-positive fibrils arranged in a dense, tufted pattern within astrocyte processes. They are most commonly observed in the striatum, motor cortex, and brainstem.

Key Features of Tufted Astrocytes:

• Tau immunoreactivity concentrated in astrocyte processes
• Dense, fibrillar appearance (tufted configuration)
• Variable astrocyte cell body size
• Primarily found in gray matter regions
• Absent in Alzheimer's disease (differentiating feature)

Coiled Bodies

Coiled bodies are oligodendroglial inclusions containing aggregated tau protein. They appear as argyrophilic, filamentous inclusions in the cytoplasm of oligodendrocytes, particularly affecting white matter tracts. These inclusions are a consistent finding in PSP and contribute to the white matter pathology observed in the disease.

Coiled Body Characteristics:

• Located in oligodendrocyte cytoplasm
• Composed of 4R tau filaments
• Associated with myelin abnormalities
• Predominant in subcortical white matter
• May contribute to axonal dysfunction

Thread-Like Inclusions

Thread-like inclusions represent distorted neuronal processes containing tau filaments, creating a "thread-like" appearance in affected brain regions. These tau-positive neurites are a major component of the pathological burden in PSP and contribute to synaptic dysfunction and neuronal disconnection. [@duyckaerts2019]

Regional Distribution of Pathology

Brainstem Involvement

The brainstem is disproportionately affected in PSP, explaining many of the characteristic clinical features:

Substantia Nigra:

• Severe loss of dopaminergic neurons in the pars compacta
• NFT accumulation in surviving neurons
• Pigment incontinence
• Contributes to parkinsonian features

• Critical for vertical gaze control
• NFT burden correlates with vertical gaze palsy
• Neuronal loss and gliosis

• Involvement explains the characteristic supranuclear gaze palsy
• Early vertical gaze impairment
• Later horizontal gaze involvement

• Extensive NFT deposition
• Basis pontis degeneration
• Contributes to gait disturbance

• Autonomic dysfunction correlation
• Respiratory and cardiovascular regulation nuclei affected

Basal Ganglia Pathology

Globus Pallidus (interna and externa):

• Marked neuronal loss
• High NFT density
• Associated gliosis
• Contributes to akinesia and rigidity

• Severe involvement—one of the most affected regions
• NFT burden correlates with motor symptoms
• Key node in motor circuitry

• Prominent NFT deposition
• Rubral tremor correlation

Diencephalic Involvement

Thalamus:

• Variable involvement
• Input to cortical motor areas
• May contribute to cognitive symptoms

• Autonomic dysfunction correlates
• Sleep disturbance mechanisms

Cortical Involvement

Compared to Alzheimer's disease, the cerebral cortex is relatively spared in PSP, though not entirely unaffected:

Motor Cortex (Brodmann area 4):

• Moderate NFT burden
• Upper motor neuron signs
• Contributes to pseudobulbar palsy

• Some NFT deposition
• Cognitive impairment correlates
• Executive dysfunction

• Generally spared
• Helps differentiate from AD

Neuronal Loss Patterns

Selective Vulnerability

PSP demonstrates remarkable regional specificity in neuronal loss, affecting specific populations while sparing others:

Most Vulnerable Neurons:

• Dopaminergic neurons of substantia nigra pars compacta
• Large pyramidal neurons of globus pallidus interna
• Neurons of subthalamic nucleus
• Purkinje cells of the cerebellum (variable)

• Hippocampal neurons (unlike AD)
• Basal forebrain cholinergic neurons
• Serotonergic raphe neurons
• Noradrenergic locus coeruleus neurons

Mechanisms of Neuronal Death

Multiple mechanisms contribute to neuronal loss in PSP:

Glial Responses

Reactive Astrocytosis

Astrocytosis is prominent in affected regions of PSP brains:

Astrocyte Activation:

• GFAP upregulation
• Cellular hypertrophy
• Process extension
• Tufted astrocytes as pathognomonic feature

• Highest in basal ganglia and brainstem
• Correlates with NFT burden
• Variable cortical involvement

Microglial Activation

Microglial activation is consistently observed in PSP:

Activation Markers:

• CD68 upregulation
• HLA-DR expression
• Morphological transformation

• Pro-inflammatory cytokine production
• ROS generation
• Phagocytic activity
• May contribute to disease progression

Oligodendroglial Pathology

Oligodendrocyte involvement in PSP includes:

• Coiled body formation
• Myelin abnormalities
• White matter tract degeneration
• Axonal dysfunction

Comparison with Other Tauopathies

PSP vs. Corticobasal Degeneration (CBD)

Both PSP and CBD are classified as 4R tauopathies but have distinct neuropathological features:

| Feature | PSP | CBD |
|---------|-----|-----|
| Astrocytic inclusions | Tufted astrocytes | Astrocytic plaques (cortico-basal) |
| Neuronal inclusions | NFTs in subcortex | NFTs in cortex |
| Cortical involvement | Moderate | Severe |
| Ballooned neurons | Rare | Characteristic |
| Microtubule pathology | Less prominent | Prominent |
| Distribution | Brainstem predominant | Cortical/subcortical |

The distinction can be challenging, and some cases show overlapping features, leading to the concept of "atypical" or "overlap" tauopathies. [@dickson2008]

PSP vs. Alzheimer Disease

While both involve tau pathology, key differences include:

• Tau isoform: PSP = exclusively 4R tau; AD = both 3R and 4R tau (3R+4R PHFs)
• NFT distribution: AD favors hippocampus and entorhinal cortex; PSP favors brainstem and basal ganglia
• Amyloid: Present in ~70% of AD cases; typically absent in primary PSP
• Clinical correlation: Memory prominent in AD; postural instability/gaze palsy in PSP

PSP vs. Frontotemporal Lobar Degeneration (FTD)

• PSP shares features with FTD as both can be 4R tauopathies
• FTD has more variable tau isoform involvement
• Clinical overlap exists between PSP and FTD variants
• Some cases meet criteria for both entities

Neuropathological Diagnosis

NINDS-SPSP Criteria

The National Institute of Neurological Disorders and Stroke (NINDS) and the Society for PSP (SPSP) established neuropathological diagnostic criteria:

Required Findings:

• NFT density in pallidum ≥ certain threshold
• NFT density in subthalamic nucleus ≥ certain threshold
• NFT density in substantia nigra ≥ certain threshold

• Tufted astrocytes
• Coiled bodies
• Moderate cortical involvement

Recommended Staging Schemes

Various staging systems have been proposed:

Braak Staging for PSP:

• Stage I-II: Brainstem involvement
• Stage III-IV: Basal ganglia involvement
• Stage V-VI: Cortical involvement

Postmortem Findings

Common autopsy findings in PSP include:

• Atrophy patterns:
• Midbrain atrophy ("hummingbird" sign on MRI)
• Superior cerebellar peduncle atrophy
• Frontotemporal atrophy
• Gross examination:
• Dilated fourth ventricle
• Discoloration of the substantia nigra
• Small cavitary lesions in the basal ganglia
• Pallor of the subthalamic nucleus

Molecular Mechanisms of Tau Pathology

Tau Phosphorylation Dynamics

The balance between tau kinases and phosphatases is disrupted in PSP:

Kinases implicated:

• GSK-3β
• CDK5
• MARK
• PKA

• PP2A (principal tau phosphatase)
• PP1
• PP5

Aggregation Mechanisms

Tau aggregation in PSP involves:

Prion-Like Propagation

Evidence supports prion-like spreading of tau pathology:

• Template-based misfolding
• Neuronal connectivity-based spread
• Possible involvement of extracellular vesicles
• Experimental transmission in animal models

Associated Proteinopathies

Alpha-Synuclein Co-Pathology

• Present in ~30% of PSP cases
• Lewy bodies may coexist
• Influences clinical phenotype

TDP-43 Pathology

• Less common than in AD
• Occasionally observed
• May modify clinical presentation

Biomarker Development

Neuropathological findings guide biomarker development:

Neuroimaging Biomarkers

• PET ligands: [^18F]Flortaucipir (AV-1451) binds to tau filaments
• MRI: Atrophy patterns characteristic
• Diffusion imaging: White matter tract involvement

Cerebrospinal Fluid Biomarkers

• Total tau: Elevated
• Phosphorylated tau: May be elevated
• Neurofilament light chain: Elevated
• Novel 4R tau-specific assays in development

Blood-Based Biomarkers

• Neurofilament light chain
• Tau species
• Emerging platforms

Research Implications and Therapeutic Targets

Tau-Targeted Therapies

Understanding neuropathology informs therapeutic development:

Tau Aggregation Inhibitors:

• Methylene blue derivatives
• Small molecule inhibitors
• Natural products

• Passive immunization approaches
• Active immunization
• Antibody engineering

• MAPT gene silencing
• Tau expression modulation
• AAV delivery vectors

Clinical Trial Implications

• Neuropathological staging informs patient selection
• Biomarker development enables monitoring
• Understanding of mechanisms guides combination therapies

See Also

• [Progressive Supranuclear Palsy](/diseases/progressive-supranuclear-palsy)
• [Tauopathies Overview](/mechanisms/tauopathies-overview)
• [Tau Protein Biology](/proteins/tau)
• [4R Tauopathies](/mechanisms/4r-tauopathies)
• [Frontotemporal Lobar Degeneration](/diseases/frontotemporal-lobar-degeneration)
• [Corticobasal Degeneration](/diseases/corticobasal-degeneration)

Clinical-Pathological Correlations

Richardson Syndrome (Classic PSP)

The classic PSP phenotype (Richardson syndrome) correlates with maximum NFT burden in subthalamic nucleus and globus pallidus, severe brainstem involvement, and prominent tufted astrocytes. The characteristic "hummingbird" sign on MRI reflects midbrain atrophy. Patients present with the triad of vertical supranuclear gaze palsy, pseudobulbar palsy, and parkinsonism with axial rigidity. Neuropathologically, this variant shows the highest densities of NFTs in the brainstem and basal ganglia, with relative cortical sparing. The progression is typically more rapid than other PSP variants, with survival averaging 5-7 years from symptom onset. [@wider2010]

PSP-Parkinsonism (PSP-P)

The parkinsonian variant (PSP-P) presents with more asymmetric parkinsonian features that may initially resemble idiopathic Parkinson's disease. Neuropathologically, PSP-P shows less severe brainstem involvement and greater cortical NFT burden compared to Richardson syndrome. This variant may show more significant involvement of the motor cortex and has been estimated to represent 20-30% of PSP cases. The clinical differentiation is important as PSP-P patients may initially respond to levodopa, while Richardson syndrome patients typically do not. Differentiation becomes clearer over time as vertical gaze palsy develops. [@williams2006]

PSP with Pure Akinesia with Gait Freezing (PAGF)

PAGF represents a distinct clinical variant characterized by early gait freezing and akinesia without the classic supranuclear gaze palsy early in the disease course. Neuropathologically, PAGF shows minimal cortical involvement with predominant subcortical NFT burden. This variant typically has an earlier age of onset (mean ~58 years) and slower progression compared to Richardson syndrome. The neuropathological findings may be more focal, affecting primarily the globus pallidus and striatum. PAGF was initially described as a separate entity but is now recognized as part of the PSP spectrum. [@williams2006]

Corticobasal Syndrome (CBS)-PSP

Some patients present with features of both PSP and corticobasal degeneration, reflecting overlapping neuropathology. This variant shows features of both 4R tauopathies, with astrocytic plaques typically found in CBD alongside tufted astrocytes characteristic of PSP. Cortical involvement is severe, and the clinical presentation includes asymmetric rigidity, apraxia, and cortical sensory loss. The distinction between PSP and CBD can be challenging both clinically and neuropathologically, and some cases are classified as "atypical" or "overlap" tauopathies. The existence of a unified CBD-PSP spectrum is supported by genetic studies showing shared risk alleles. [@dickson2008]

Genetic Factors in PSP

MAPT Gene and H1 Haplotype

The MAPT (Microtubule-Associated Protein Tau) gene located on chromosome 17q21.31 encodes the tau protein. The H1 haplotype, spanning the entire MAPT gene region, represents the major genetic risk factor for sporadic PSP. The H1/H1 genotype is associated with increased expression of 4R tau isoforms and significantly increases PSP risk (odds ratio ~3-5). This haplotype is also a risk factor for CBD and certain forms of FTD, supporting the concept of a shared 4R tauopathy spectrum. [@hutton2000] [@baker1999]

The H2 haplotype appears protective, with reduced 4R tau expression. Interestingly, the H1 haplotype originated from a single founding event approximately 2,000 years ago and has since expanded in populations of European ancestry. The functional consequences of the H1 haplotype include:

• Increased 4R tau expression
• Altered exon 10 splicing
• Enhanced aggregation propensity
• Possible effects on neuroinflammation

Known Mutations

While no specific mutations have been identified that cause PSP, several MAPT mutations cause related tauopathies:

• P301L/S: Causes FTD with parkinsonism (FTDP-17)
• Exon 10 mutations: Alter 3R/4R tau ratio
• Splicing mutations: Lead to 4R tau predominance

Genome-Wide Association Studies

Large GWAS have identified additional risk loci:

• MOBP: Myelin-associated oligodendrocyte basic protein
• SLCO1A2: Solute carrier organic anion transporter
• DUSP10: Dual-specificity phosphatase 10

Experimental Models of PSP

Transgenic Mouse Models

Several mouse models have been developed to study PSP pathophysiology:

P301S Tau Transgenic Mice:

• Express human tau with the P301S mutation
• Develop NFTs and neuronal loss
• Motor and cognitive phenotypes
• Used for therapeutic testing

• Specific 4R tau isoform expression
• Replicate 4R tau predominance
• Show NFT formation
• Glial responses

• Targeted brain region injection
• Rapid pathology development
• Prion-like spreading models
• Used to study propagation

In Vitro Models

Cell Culture Systems:

• Tau aggregation assays
• Neuronal-glial co-cultures
• iPSC-derived neurons from PSP patients
• 3D brain organoids

• Ex vivo culture systems
• Tau pathology propagation
• Drug penetration studies
• Mechanism investigation

Therapeutic Development

Current Clinical Trials

Multiple therapeutic approaches are in various stages of clinical development:

Tau Aggregation Inhibitors:

• Methylene blue derivatives (Phase 3)
• Small molecule inhibitors (Phase 1/2)
• Natural product derivatives

• Passive antibodies targeting tau
• Active immunization approaches
• Antibody engineering for brain penetration
• Various epitopes (N-terminal, mid-domain, phospho-tau)

• ASO targeting MAPT mRNA
• Gene therapy vectors
• Kinase inhibitors
• Phosphatase activators

Challenges in Therapeutic Development

Several factors complicate PSP therapeutic development:

Biomarker Limitations:

• Lack of validated progression biomarkers
• Difficulty in monitoring target engagement
• Need for sensitive outcome measures

• Clinical heterogeneity within PSP spectrum
• Need for neuropathological confirmation in trials
• Variable progression rates

• Relatively rare disease
• Small patient populations
• Short trial durations
• Regulatory pathways

References