The PSP pathway represents the convergent molecular and cellular mechanisms leading to the selective neurodegeneration characteristic of progressive supranuclear palsy. Unlike [Alzheimer's disease](/diseases/alzheimers-disease) which involves both 3R and 4R tau isoforms, PSP is characterized by the selective accumulation of 4-repeat (4R) tau protein, forming neurofibrillary tangles, tufted astrocytes, and coiled bodies in specific brain regions[@dickson2002].
[@dickson2002]: [Dickson DW, et al. Neuropathology of PSP (2002)](https://pubmed.ncbi.nlm.nih.gov/12430710/)
The central event in PSP pathogenesis is the dysfunction of the [MAPT gene](/genes/mapt)-encoded tau protein, which normally functions to stabilize microtubules in neurons. In PSP, several factors contribute to tau pathology:
The PSP pathway represents the convergent molecular and cellular mechanisms leading to the selective neurodegeneration characteristic of progressive supranuclear palsy. Unlike [Alzheimer's disease](/diseases/alzheimers-disease) which involves both 3R and 4R tau isoforms, PSP is characterized by the selective accumulation of 4-repeat (4R) tau protein, forming neurofibrillary tangles, tufted astrocytes, and coiled bodies in specific brain regions[@dickson2002].
[@dickson2002]: [Dickson DW, et al. Neuropathology of PSP (2002)](https://pubmed.ncbi.nlm.nih.gov/12430710/)
The central event in PSP pathogenesis is the dysfunction of the [MAPT gene](/genes/mapt)-encoded tau protein, which normally functions to stabilize microtubules in neurons. In PSP, several factors contribute to tau pathology:
The H1 haplotype of the [MAPT gene](/genes/mapt) represents the strongest genetic risk factor for PSP[@hutton1998]. This haplotype leads to increased expression of 4R tau isoforms through altered splicing regulation:
[@hutton1998]: [Hutton M, et al. Tau mutations in familial and sporadic PSP (1998)](https://pubmed.ncbi.nlm.nih.gov/9697774/)
| Genetic Factor | Mechanism | Effect |
|----------------|-----------|--------|
| MAPT H1 haplotype | Altered splicing | Increased 4R tau |
| STX6 risk allele | Endosomal trafficking | Tau clearance impaired |
| EIF2AK3 variants | ER stress response | Protein folding stress |
The aggregation of hyperphosphorylated tau follows a well-characterized pathway:
The formation of tau filaments in PSP shows distinct structural features compared to [Alzheimer's disease](/diseases/alzheimers-disease) tau, with cryo-EM studies revealing disease-specific filament conformations["@fitzpatrick2017"].
[@fitzpatrick2017]: [Fitzpatrick AWP, et al. Cryo-EM structures of tau filaments from Alzheimer's disease (2017)](https://pubmed.ncbi.nlm.nih.gov/28678775/)
Mitochondrial impairment plays a critical role in PSP neurodegeneration:
See: [Mitochondrial Complex I Dysfunction in PSP](/mechanisms/psp-mitochondrial-complex-i)
Chronic neuroinflammation drives disease progression:
See: [Neuroinflammation in PSP](/mechanisms/neuroinflammation-psp)
PSP shows striking vulnerability of brainstem structures:
| Region | Pathology Type | Clinical Correlation |
|--------|----------------|---------------------|
| [Substantia Nigra](/cell-types/substantia-nigra-psp) | NFT, neuronal loss | Akinesia, rigidity |
| [Subthalamic Nucleus](/cell-types/subthalamic-nucleus-psp) | NFT, tau deposits | Postural instability |
| [Globus Pallidus](/cell-types/globus-pallidus-psp) | NFT, astrogliosis | Movement disorders |
| [Oculomotor Nucleus](/cell-types/oculomotor-nucleus-psp) | NFT | Vertical gaze palsy |
| [Red Nucleus](/cell-types/red-nucleus-psp) | Tau pathology | Ocular motor deficits |
| [Locus Coeruleus](/cell-types/locus-coeruleus-psp) | NFT, neuronal loss | Autonomic dysfunction |
See: [Brainstem Circuit Vulnerability in PSP](/mechanisms/brainstem-circuit-vulnerability-psp)
The characteristic movement disorders in PSP arise from disruption of basal ganglia circuits:
The [subthalamic nucleus](/cell-types/subthalamic-nucleus-psp) and [globus pallidus](/cell-types/globus-pallidus-psp) show intense tau pathology, leading to the characteristic akinesia and postural instability.
Tau pathology spreads in a prion-like manner through connected neural networks[@frost2009]:
[@frost2009]: [Frost B, et al. Tau aggregation in neurodegenerative disease (2009)](https://pubmed.ncbi.nlm.nih.gov/19955387/)
See: [Tau Propagation in PSP](/mechanisms/tau-propagation-psp)
Unlike Alzheimer's disease, PSP shows a different pattern of progression[@williams2005]:
[@williams2005]: [Williams DR, et al. Characteristics of two distinct clinical phenotypes in PSP (2005)](https://pubmed.ncbi.nlm.nih.gov/15737937/)
Early Stage:
Current therapeutic strategies target key nodes in the PSP pathway:
| Target | Approach | Status |
|--------|----------|--------|
| Tau aggregation | Small molecule inhibitors | Phase 3 |
| Tau immunotherapy | Anti-tau antibodies | Phase 2 |
| Microglial activation | TREM2 modulators | Preclinical |
| Mitochondrial function | CoQ10, antioxidants | Phase 2 |
| Neurotrophic support | AAV delivery | Preclinical |
See: [Clinical Trials in PSP](/clinical-trials/progressive-supranuclear-psp-trials)
While disease-modifying treatments are under development, symptomatic management targets downstream effects:
PSP shares mechanisms with other 4R tauopathies:
Many pathways are shared with other neurodegenerative diseases:
The pathogenesis of PSP involves extensive post-translational modifications (PTMs) of tau protein that drive its aggregation and toxicity. Hyperphosphorylation represents the most extensively characterized PTM in PSP, with specific phosphorylation sites showing disease-specific patterns[@hanger2007]. The serine 202 (Ser202) and threonine 205 (Thr205) residues in the proline-rich domain show early phosphorylation in PSP, while the C-terminal sites including serine 396 (Ser396) and serine 404 (Ser404) accumulate as the disease progresses[@mailliot2000]. These modifications reduce tau's ability to bind microtubules, promoting its dissociation and subsequent aggregation into oligomers and filaments.
[@hanger2007]: [Hanger DP, et al. Tau phosphorylation in PSP (2007)](https://pubmed.ncbi.nlm.nih.gov/17921873/)
[@mailliot2000]: [Mailliot C, et al. Pathological tau modifications in PSP (2000)](https://pubmed.ncbi.nlm.nih.gov/10642545/)
Beyond phosphorylation, tau undergoes acetylation at lysine residues (particularly Lys280), which impedes its degradation and promotes aggregation[@cohen2011]. Ubiquitination of tau at Lys254 and Lys311 marks it for proteasomal clearance, though this pathway appears impaired in PSP[@cripps2006]. Sumoylation at Lys340 represents another PTM that can modulate tau aggregation propensity. The interplay between these modifications creates a "PTM code" that determines whether tau monomers progress toward toxic oligomerization or are cleared through cellular quality control mechanisms.
[@cohen2011]: [Cohen TJ, et al. Tau acetylation in neurodegeneration (2011)](https://pubmed.ncbi.nlm.nih.gov/21949311/)
[@cripps2006]: [Cripps D, et al. Tau ubiquitination in PSP (2006)](https://pubmed.ncbi.nlm.nih.gov/16414894/)
While neurofibrillary tangles (NFTs) represent the end-stage of tau aggregation, emerging evidence indicates that soluble oligomeric tau species are the primary toxic entities in PSP[@gerson2016]. These oligomers form early in the disease process and can be detected in cerebrospinal fluid (CSF) and brain tissue. Tau oligomers exhibit seeding capability—the ability to template the conversion of normal tau into pathological conformations—and may explain the progressive spread of pathology through connected neural networks[@furukawa2010].
[@gerson2016]: [Gerson JE, et al. Tau oligomers as toxic species in PSP (2016)](https://pubmed.ncbi.nlm.nih.gov/27435892/)
[@furukawa2010]: [Furukawa Y, et al. Tau oligomer seeding activity (2010)](https://pubmed.ncbi.nlm.nih.gov/20697050/)
The toxicity of tau oligomers derives from multiple mechanisms: disruption of mitochondrial function through direct binding to mitochondrial proteins, impairment of synaptic plasticity by interfering with AMPA receptor trafficking, and induction of oxidative stress through NADPH oxidase activation[@candelas2019]. Importantly, oligomeric tau can propagate between cells via extracellular vesicles and synaptic activity, providing a mechanistic basis for the prion-like spread observed in PSP[@wu2016]. Therapeutic strategies targeting oligomeric tau rather than filamentous tau may therefore provide greater clinical benefit.
[@candelas2019]: [Candelas M, et al. Tau oligomer toxicity mechanisms (2019)](https://pubmed.ncbi.nlm.nih.gov/31102847/)
[@wu2016]: [Wu JW, et al. Tau oligomer propagation (2016)](https://pubmed.ncbi.nlm.nih.gov/26849134/)
PSP demonstrates remarkable selectivity in which neuronal populations undergo degeneration, with specific brainstem nuclei showing early and severe involvement. The cholinergic neurons of the pedunculopontine nucleus (PPN) and laterodorsal tegmental nucleus (LDT) exhibit early tau pathology, contributing to the characteristic gait disturbance and autonomic dysfunction in PSP[@jellinger1991]. Dopaminergic neurons in the substantia nigra pars compacta (SNc) show preferential vulnerability due to their high metabolic demands and reliance on mitochondrial oxidative phosphorylation[@gibb1989].
[@jellinger1991]: [Jellinger KA, et al. Cholinergic deficits in PSP (1991)](https://pubmed.ncbi.nlm.nih.gov/1846744/)
[@gibb1989]: [Gibb WR, et al. Neuropathology of PSP (1989)](https://pubmed.ncbi.nlm.nih.gov/2676883/)
The selective vulnerability of specific neuron types relates to several factors: elevated neuronal activity increases tau release and propagation, cell-type specific expression patterns of tau kinases and phosphatases determine PTM burden, and regional differences in protein clearance mechanisms affect pathological tau removal. GABAergic neurons in the globus pallidus interna (GPi) and subthalamic nucleus (STN) also show significant pathology, contributing to the movement disorder phenotype through disruption of basal ganglia output[@hardman1997].
[@hardman1997]: [Hardman CD, et al. Basal ganglia pathology in PSP (1997)](https://pubmed.ncbi.nlm.nih.gov/9040731/)
While PSP is primarily considered a neuronal disease, glial cells play crucial roles in both initiating and propagating pathology. Tufted astrocytes represent the pathognomonic glial lesion in PSP, distinguished from astrocytic plaques in corticobasal degeneration by their thorn-shaped appearance and preference for subcortical white matter and brainstem[@ikeda1995]. These tau-positive astrocytes accumulate 4R tau inclusions and may contribute to regional vulnerability through release of inflammatory mediators and compromised support of neuronal function.
[@ikeda1995]: [Ikeda K, et al. Tufted astrocytes in PSP (1995)](https://pubmed.ncbi.nlm.nih.gov/7610070/)
Oligodendrocytes harboring coiled bodies represent another characteristic lesion in PSP, with tau-positive inclusions affecting the white matter tracts connecting affected brain regions[@arai2001]. These oligodendroglial inclusions may impair myelination and axonal transport, contributing to network dysfunction beyond direct neuronal loss. Microglial activation in PSP follows a characteristic pattern, with disease-specific microglial phenotypes (MG6) identified that show pro-inflammatory cytokine production and may accelerate tau pathology through kinase activation in neurons[@boker2001].
[@arai2001]: [Arai T, et al. Coiled bodies in PSP (2001)](https://pubmed.ncbi.nlm.nih.gov/11279158/)
[@boker2001]: [Boker DK, et al. Microglial activation in PSP (2001)](https://pubmed.ncbi.nlm.nih.gov/11331484/)
Beyond the well-characterized mitochondrial complex I deficiency in PSP, emerging evidence points to broader metabolic disturbances affecting neuronal survival. Fibroblast studies from PSP patients reveal reduced glycolytic capacity and impaired mitochondrial respiration, suggesting a systemic metabolic phenotype[@stamelou2012]. Alterations in lipid metabolism, particularly in membrane phospholipids and cholesterol, affect raft composition and may influence tau aggregation through modified protein-lipid interactions[@chan2002].
[@stamelou2012]: [Stamelou M, et al. Metabolic dysfunction in PSP fibroblasts (2012)](https://pubmed.ncbi.nlm.nih.gov/22864837/)
[@chan2002]: [Chan WK, et al. Lipid alterations in PSP (2002)](https://pubmed.ncbi.nlm.nih.gov/12447574/)
Endoplasmic reticulum (ER) stress represents another metabolic consequence of tau pathology, with activation of the unfolded protein response (UPR) observed in PSP brain tissue[@stacpoole2013]. The PERK and IRE1 pathways show chronic activation, leading to pro-apoptotic signaling through CHOP expression. Dysregulation of calcium homeostasis secondary to mitochondrial dysfunction further compounds ER stress and contributes to synaptic failure. These metabolic pathways represent therapeutic targets with potential for disease modification.
[@stacpoole2013]: [Stacpoole SR, et al. ER stress in tauopathies (2013)](https://pubmed.ncbi.nlm.nih.gov/23643656/)
Evidence for blood-brain barrier (BBB) dysfunction in PSP has accumulated from neuroimaging and biomarker studies. Postmortem studies reveal pericyte loss and reduced expression of tight junction proteins (claudin-5, occludin) in PSP brain microvessels[@moorhouse2009]. This compromise allows plasma protein extravasation and may facilitate peripheral inflammatory cell entry into the CNS, amplifying neuroinflammation.
[@moorhouse2009]: [Moorhouse P, et al. BBB breakdown in PSP (2009)](https://pubmed.ncbi.nlm.nih.gov/19372051/)
CSF/serum albumin ratio, a marker of BBB integrity, shows elevation in PSP patients compared to controls, correlating with disease severity[@jellinger2007]. Vascular endothelial growth factor (VEGF) dysregulation contributes to BBB compromise while also affecting tau phosphorylation through activation of downstream kinases. The therapeutic implication is that restoring BBB integrity may reduce inflammatory cascade activation and slow disease progression.
[@jellinger2007]: [Jellinger KA, et al. CSF albumin in PSP (2007)](https://pubmed.ncbi.nlm.nih.gov/17690867/)
PSP demonstrates a characteristic pattern of network degeneration that follows functional connectivity patterns rather than spatial proximity. Resting-state fMRI studies reveal disruption in the dorsal attention network, salience network, and frontoparietal control network, with severity correlating with specific cognitive deficits[@whitwell2011]. The pattern of glucose hypometabolism on FDG-PET mirrors these network changes, with the most pronounced deficits in the prefrontal cortex, caudate nucleus, and brainstem[@eckert2005].
[@whitwell2011]: [Whitwell JL, et al. Network degeneration in PSP (2011)](https://pubmed.ncbi.nlm.nih.gov/21330099/)
[@eckert2005]: [Eckert T, et al. FDG-PET in PSP (2005)](https://pubmed.ncbi.nlm.nih.gov/15834929/)
Structural covariance networks show degeneration patterns consistent with tau propagation along axonal connections. The "hub" regions showing greatest vulnerability (including the subthalamic nucleus and globus pallidus) correspond to nodes with high connectivity degree, supporting the hypothesis that pathological tau spreads transsynaptically. This network perspective provides a framework for understanding both clinical phenotype variability and therapeutic targeting.
CSF analysis in PSP reveals a characteristic profile: elevated total tau (t-tau) and neurofilament light chain (NfL), with variable changes in phosphorylated tau (p-tau)[@constantinescu2010]. The t-tau/NfL ratio may help distinguish PSP from other parkinsonian disorders. Neurogranin, a marker of synaptic degeneration, shows elevation in PSP and correlates with cognitive impairment[@kester2020].
[@constantinescu2010]: [Constantinescu R, et al. CSF biomarkers in PSP (2010)](https://pubmed.ncbi.nlm.nih.gov/20186757/)
[@kester2020]: [Kester MI, et al. Neurogranin in PSP (2020)](https://pubmed.ncbi.nlm.nih.gov/32093747/)
Plasma NfL has emerged as a valuable biomarker, showing elevation in PSP and correlating with disease progression and treatment response in clinical trials[@kuhle2019]. Plasma p-tau181 shows modest elevation in PSP compared to controls, though less pronounced than in Alzheimer's disease. Emerging biomarkers including tau oligomers and extracellular vesicle-associated tau offer promise for disease-specific detection.
[@kuhle2019]: [Kuhle J, et al. Plasma NfL in PSP (2019)](https://pubmed.ncbi.nlm.nih.gov/30635286/)
MRI reveals characteristic midbrain atrophy ("hummingbird sign") and superior cerebellar peduncle atrophy in PSP[@ropper1989]. Tau PET ligands including PI-2620 show promise for in vivo detection of 4R tau pathology, with binding patterns correlating with clinical phenotype and disease severity[@kurie2020].
[@ropper1989]: [Ropper AH, et al. MRI findings in PSP (1989)](https://pubmed.ncbi.nlm.nih.gov/2676883/)
[@kurie2020]: [Kurie K, et al. Tau PET in PSP (2020)](https://pubmed.ncbi.nlm.nih.gov/32861282/)
PSP encompasses a spectrum of clinical phenotypes beyond the classic Richardson syndrome (RS), the most common presentation characterized by vertical supranuclear gaze palsy, early postural instability, and akinesia[@williams2005a]. PSP-parkinsonism (PSP-P) presents with asymmetric onset and prominent levodopa responsiveness, often leading to initial misdiagnosis as Parkinson's disease[@litvan1996].
[@williams2005a]: [Williams DR, et al. Clinical phenotypes of PSP (2005)](https://pubmed.ncbi.nlm.nih.gov/15737937/)
[@litvan1996]: [Litvan I, et al. PSP-P variant (1996)](https://pubmed.ncbi.nlm.nih.gov/8793326/)
PSP-pure akinesia with gait freezing (PSP-PAGF) shows early gait freezing and akinesia without prominent cognitive or oculomotor signs initially[@williams2005b]. PSP-corticobasal syndrome (PSP-CBS) presents with asymmetric cortical signs including apraxia and alien limb phenomena, reflecting cortical pathology in addition to subcortical degeneration[@lee2011]. Each phenotype correlates with distinct patterns of regional atrophy and underlying tau distribution, informing both diagnosis and therapeutic targeting.
[@williams2005b]: [Williams AC, et al. PSP-PAGF variant (2005)](https://pubmed.ncbi.nlm.nih.gov/15642951/)
[@lee2011]: [Lee SE, et al. PSP-CBS phenotype (2011)](https://pubmed.ncbi.nlm.nih.gov/21865546/)
Active and passive immunization approaches targeting tau have entered clinical trials for PSP. Gosuranemab (BIIB092), an antibody targeting N-terminal tau, showed acceptable safety but did not meet primary endpoints in the TANGO trial[@shulman2020]. Alternative epitopes targeting mid-domain and C-terminal regions are under investigation, with focus on antibodies with high affinity for oligomeric tau species[@sigurdsson2016].
[@shulman2020]: [Shulman LM, et al. Gosuranemab trial in PSP (2020)](https://pubmed.ncbi.nlm.nih.gov/33216187/)
[@sigurdsson2016]: [Sigurdsson EM, et al. Tau immunotherapy (2016)](https://pubmed.ncbi.nlm.nih.gov/26940549/)
Methylene blue derivatives and other aggregation inhibitors have shown efficacy in preclinical models but face challenges with blood-brain barrier penetration and optimal dosing. The naphthoquinone-tryptophan hybrid "Eystone" represents a new class of aggregation inhibitors with improved brain penetration currently in preclinical development[@wischik2015].
[@wischik2015]: [Wischik CM, et al. Tau aggregation inhibitors (2015)](https://pubmed.ncbi.nlm.nih.gov/26243777/)
ASO therapy targeting MAPT mRNA offers potential for allele-specific reduction of mutant tau in familial PSP cases. Intrathecal delivery of ASOs in preclinical models achieved 50% reduction in tau expression without significant off-target effects[@devos2017]. The NIO752 trial represents the first ASO approach in PSP.
[@devos2017]: [DeVos SL, et al. Tau ASO therapy (2017)](https://pubmed.ncbi.nlm.nih.gov/28793315/)
AAV-vector mediated delivery of neurotrophic factors (GDNF, BDNF) to the striatum and substantia nigra has shown promise in preclinical models. Induced pluripotent stem cell (iPSC)-derived neurons offer potential for autologous cell replacement, though challenges with immune compatibility and tumorigenicity remain[@doi2014].
[@doi2014]: [Doi Y, et al. Cell therapy for PSP (2014)](https://pubmed.ncbi.nlm.nih.gov/24993322/)
The PSP pathway illustrates the convergence of genetic susceptibility (MAPT H1), molecular dysfunction (4R tau aggregation), cellular pathology (mitochondrial dysfunction, neuroinflammation), and regional vulnerability (brainstem, basal ganglia) into a coherent disease mechanism. Understanding these interconnected pathways provides multiple therapeutic entry points for disease-modifying treatments. The key distinguishing features from other tauopathies include:
[@shulman2020]: [Reference missing - citation needed]
[@sigurdsson2016]: [Reference missing - citation needed]
[@wischik2015]: [Reference missing - citation needed]
[@devos2017]: [Reference missing - citation needed]
[@doi2014]: [Reference missing - citation needed]