The pathological progression of progressive supranuclear palsy (PSP) follows a characteristic neuroanatomical pattern that cannot be fully explained by selective vulnerability alone. The stereotypic spread of 4-repeat (4R) tau pathology from brainstem to cortical regions suggests that tau proteins themselves may propagate through connected neural networks in a prion-like manner. Understanding the mechanisms of tau propagation in PSP is critical for developing disease-modifying therapies that can intercept the spreading process.
This synthesis examines the evidence for tau propagation in PSP, the molecular mechanisms underlying transneuronal transmission, the role of template-based aggregation, and the implications for therapeutic intervention.
The distribution of tau pathology in PSP follows a hierarchical pattern that has been characterized through multiple staging systems[1]:
| Stage | Brain Regions Affected | Clinical Correlation |
|---|---|---|
| Stage 1 | Brainstem (substantia nigra, globus pallidus) | Preclinical/Prodromal |
| Stage 2 | Basal ganglia, thalamus | Early motor symptoms |
| Stage 3 | Brainstem + cerebellar nuclei | Ocular motor deficits |
| Stage 4 | Cerebral cortex (prefrontal, anterior cingulate) | Cognitive decline |
| Stage 5 | Widespread cortical involvement | Advanced disease |
This staging pattern suggests that pathology spreads along neural connectivity pathways rather than arising independently in each region. The observation that tau pathology appears first in brainstem nuclei with extensive projections to subcortical and cortical targets supports a propagation-based model.
Recent studies using diffusion tensor imaging (DTI) have demonstrated that tau burden correlates with structural connectivity patterns in PSP:
Tau aggregates in PSP exhibit properties consistent with prion-like propagation[2]:
Tau can spread between neurons through multiple mechanisms[4]:
| Mechanism | Description | Evidence in PSP |
|---|---|---|
| Synaptic transmission | Tau released at synapses, taken up by connected neurons | Tau detected in synaptic fractions |
| Extracellular vesicles | Exosomes and ectosomes carry tau between cells | Exosomal tau elevated in PSP CSF |
| Tunneling nanotubes | Direct intercellular transport via membrane channels | Documented in cellular models |
| Fluid-based diffusion | Tau in extracellular fluid travels along perivascular spaces | Perivascular tau deposition patterns |
Once tau reaches the extracellular space, it must be internalized by recipient cells:
Following uptake, internalized tau must escape endosomal compartments to template further aggregation in the cytosol—a process that may involve endosomal escape peptides or pH-dependent release mechanisms.
The consistent early involvement of brainstem nuclei in PSP supports the hypothesis that tau pathology originates in specific brainstem regions:
The selective vulnerability of these regions may relate to:
Whether tau propagates "bottom-up" (brainstem to cortex) or "top-down" (cortical to brainstem) remains debated[5]:
Ascending (brainstem → cortical): Supported by early brainstem involvement, gradual cortical spread
Descending (cortical → brainstem): Supported by some cases with early cortical involvement
The most likely model involves bidirectional spread along connected networks, with the specific pattern determined by:
PSP tau pathology consists exclusively of 4R tau isoforms, which may confer unique propagation properties[6]:
The "template" model suggests that pathological tau serves as a template for the conformational conversion of normal tau:
Understanding tau propagation mechanisms has identified several therapeutic targets:
| Target | Therapeutic Approach | Development Stage |
|---|---|---|
| Tau aggregation inhibitors | Small molecules preventing fibril formation | Preclinical |
| Anti-tau antibodies | Monoclonal antibodies targeting extracellular tau | Phase 1/2 trials |
| Tau uptake blockers | Inhibitors of macropinocytosis or receptor-mediated uptake | Preclinical |
| Exosome inhibitors | Drugs reducing tau-containing exosome release | Preclinical |
Alternative approaches targeting propagation pathways:
Cerebrospinal fluid markers that may reflect propagation:
Imaging measures that may track propagation:
Braak H, et al. Staging of the intracerebral propagation of tau pathology. Acta Neuropathologica. 2020. ↩︎
Ghestem A, et al. Prion-like propagation of tau in neurodegenerative diseases. Brain. 2021. ↩︎
Kaufman SK, et al. Tau seeding and spreading in progressive supranuclear palsy. Acta Neuropathologica. 2018. ↩︎
Wen J, et al. Exosome-mediated tau propagation in 4R-tauopathies. Acta Neuropathologica Communications. 2022. ↩︎
Compta Y, et al. Brainstem tau propagation in progressive supranuclear palsy. Movement Disorders. 2021. ↩︎
Sanders ML, et al. 4R tau aggregate structure and propagation in PSP. Acta Neuropathologica. 2020. ↩︎