PSP Endoplasmic Reticulum Stress and Unfolded Protein Response

PSP Endoplasmic Reticulum Stress and Unfolded Protein Response

> ER stress, UPR pathways (PERK, IRE1, ATF6), calcium dysregulation, protein misfolding, and therapeutic targeting in PSP

Overview

Endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) represent critical cellular stress pathways in Progressive Supranuclear Palsy (PSP). The ER is essential for protein folding, calcium homeostasis, and lipid biosynthesis. When these functions are disrupted, the UPR is activated to restore cellular homeostasis. In PSP, chronic ER stress contributes to neuronal dysfunction and tau pathology progression.

ER Stress in PSP

Pathological Evidence

ER stress is prominently activated in PSP brain tissue[@abdullah_er][@hiramoto_er_psp]:

| Region | ER Stress Markers | Severity |
|--------|-------------------|----------|
| Globus pallidus | CHOP, BiP upregulation | High |
| Substantia nigra | eIF2α phosphorylation | High |
| Frontal cortex | XBP1 splicing | Moderate |
| Brainstem nuclei | ATF4 expression | Moderate |

Molecular Triggers

Multiple mechanisms contribute to ER stress in PSP:

Tau accumulation — Hyperphosphorylated tau aggregates disrupt ER function

Calcium dysregulation — Altered calcium signaling impairs ER homeostasis

Oxidative stress — ROS damages ER membrane proteins

Lipid dysregulation — Altered membrane composition affects ER function

The Unfolded Protein Response (UPR)

Three UPR Pathways

The UPR is mediated by three ER transmembrane sensors[@schroder_er]:

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PSP Endoplasmic Reticulum Stress and Unfolded Protein Response

> ER stress, UPR pathways (PERK, IRE1, ATF6), calcium dysregulation, protein misfolding, and therapeutic targeting in PSP

Overview

Endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) represent critical cellular stress pathways in Progressive Supranuclear Palsy (PSP). The ER is essential for protein folding, calcium homeostasis, and lipid biosynthesis. When these functions are disrupted, the UPR is activated to restore cellular homeostasis. In PSP, chronic ER stress contributes to neuronal dysfunction and tau pathology progression.

ER Stress in PSP

Pathological Evidence

ER stress is prominently activated in PSP brain tissue[@abdullah_er][@hiramoto_er_psp]:

| Region | ER Stress Markers | Severity |
|--------|-------------------|----------|
| Globus pallidus | CHOP, BiP upregulation | High |
| Substantia nigra | eIF2α phosphorylation | High |
| Frontal cortex | XBP1 splicing | Moderate |
| Brainstem nuclei | ATF4 expression | Moderate |

Molecular Triggers

Multiple mechanisms contribute to ER stress in PSP:

Tau accumulation — Hyperphosphorylated tau aggregates disrupt ER function

Calcium dysregulation — Altered calcium signaling impairs ER homeostasis

Oxidative stress — ROS damages ER membrane proteins

Lipid dysregulation — Altered membrane composition affects ER function

The Unfolded Protein Response (UPR)

Three UPR Pathways

The UPR is mediated by three ER transmembrane sensors[@schroder_er]:

PERK Pathway

The PERK (PKR-like ER kinase) pathway is prominently activated in PSP[@chafekar_er]:

• eIF2α phosphorylation — Global translation attenuation
• ATF4 translation — Pro-apoptotic transcription factor
• CHOP expression — Key mediator of ER stress-induced apoptosis

| PERK Component | PSP Finding | Functional Impact |
|----------------|-------------|-------------------|
| p-eIF2α | 2-3 fold increase | Translation suppression |
| ATF4 | Elevated | Pro-apoptotic genes |
| CHOP | Strong upregulation | Cell death pathway |

IRE1 Pathway

The IRE1 (Inositol-requiring enzyme 1) pathway shows dysregulation in PSP:

XBP1 Splicing:

• XBP1 splicing increased in PSP neurons
• XBP1s drives chaperone expression (BiP, GRP94)
• May have dual protective and harmful effects

IRE1 RNase Activity:

• Regulated IED (IRE1-dependent decay)
• Can degrade specific mRNAs during stress
• May contribute to translational dysregulation

ATF6 Pathway

ATF6 (Activating transcription factor 6) activation in PSP:

• ATF6 cleavage increased in affected regions
• ATF6n migrates to nucleus
• Induces XBP1-like target genes
• Contributes to CHOP expression

ER-Calcium Dysregulation

Calcium Homeostasis in PSP

ER calcium handling is disrupted in PSP[@rao_er_calcium]:

| Calcium Channel | PSP Status | Effect |
|-----------------|------------|--------|
| SERCA pumps | Reduced | Impaired ER Ca²⁺ uptake |
| IP3 receptors | Dysregulated | Altered Ca²⁺ release |
| RyR channels | Altered | Calcium leak |
| Store-operated channels | Impaired | Reduced refill |

Consequences of Calcium Dysregulation

Chaperone function impairment — Calcium-dependent chaperones (BiP, calnexin) malfunction

Apoptosis induction — Calcium overload triggers mitochondrial apoptosis

Synaptic dysfunction — Altered neurotransmitter release

ER-mitochondria coupling disruption — Poor cellular energetics

ER-Mitochondria Crosstalk

Mitochondrial-ER Contact Sites

ER-mitochondria contact sites (MAMs) are altered in PSP[@brown_er_mito]:

• MAM integrity disrupted in PSP neurons
• Calcium transfer between organelles dysregulated
• Apoptosis signaling enhanced at contact sites
• Lipid transfer impaired

Implications for PSP

| MAM Function | PSP Status | Contribution |
|--------------|------------|--------------|
| Calcium signaling | Dysregulated | Apoptosis |
| Lipid synthesis | Altered | Membrane dysfunction |
| Autophagy regulation | Impaired | Protein clearance |
| Apoptosis initiation | Enhanced | Neuronal loss |

ER Stress and Tau Pathology

Tau-Induced ER Stress

Tau pathology directly induces ER stress[@sowers_er_tau][@cheung_er_tau]:

Tau oligomers disrupt ER membrane

Phosphorylated tau binds ER chaperones

Tau aggregation overwhelms protein quality control

ER stress accelerates tau phosphorylation (positive feedback)

UPR Effects on Tau

The UPR modulates tau pathology:

| UPR Pathway | Effect on Tau | Mechanism |
|-------------|--------------|-----------|
| PERK | Increased phosphorylation | eIF2α → GSK3β activation |
| IRE1 | Variable | JNK-mediated effects |
| ATF6 | May reduce aggregation | Chaperone upregulation |

Autophagy and ER Stress

ER Stress-Induced Autophagy

ER stress triggers autophagy as a protective response[@dupre_er_autophagy]:

Therapeutic Implications

• ER stress-induced autophagy may be protective in early PSP
• Later-stage autophagy failure contributes to tau accumulation
• Modulating autophagy could enhance protein clearance

Clinical Implications

Biomarker Potential

ER stress markers in PSP:

| Marker | Sample | Potential Use |
|--------|--------|---------------|
| BiP/GRP78 | CSF | Diagnostic |
| CHOP | Blood | Disease progression |
| XBP1 splicing | Blood | Therapeutic target |
| p-eIF2α | CSF | Prognosis |

Therapeutic Targets

ER stress modulators under investigation[@mazzo_er_therapy]:

| Target | Approach | Status |
|--------|----------|--------|
| PERK inhibitors | Small molecules | Preclinical |
| IRE1 RNase modulators | Natural compounds | Research |
| Chaperone inducers | Chemical chaperones | Early research |
| Calcium stabilizers | SERCA modulators | Research |

Specific for PSP

• Tau reduction may relieve ER stress burden
• Anti-tau immunotherapy could reduce ER stress
• Combination approaches targeting multiple pathways

Comparison with Other Tauopathies

ER Stress Across 4R-Tauopathies

| Feature | PSP | CBD | CBD Overlap |
|---------|-----|-----|------------|
| CHOP elevation | High | High | High |
| XBP1 splicing | Moderate | High | Moderate |
| ATF6 activation | Moderate | Moderate | Moderate |
| ER-calcium dysregulation | Prominent | Prominent | Present |

Summary

ER stress and UPR activation are prominent features of PSP pathogenesis. The chronic activation of all three UPR pathways (PERK, IRE1, ATF6) contributes to neuronal dysfunction through:

• Translational dysregulation
• Pro-apoptotic signaling (CHOP)
• Calcium homeostasis disruption
• ER-mitochondria coupling impairment

Key Takeaways:

• ER stress is elevated in PSP brain, particularly in globus pallidus and substantia nigra
• All three UPR pathways are activated, with PERK-CHOP pathway showing strongest pro-apoptotic signaling
• ER-calcium dysregulation contributes to both ER stress and mitochondrial dysfunction
• ER stress creates feedback loop that accelerates tau pathology
• ER stress modulators represent emerging therapeutic strategy

Related Mechanisms

• [PSP Mitochondrial Dysfunction](/mechanisms/psp-mitochondrial-dysfunction)
• [PSP Tau Oligomer Biology](/mechanisms/psp-tau-oligomer-biology)
• [PSP Excitotoxicity and Glutamatergic Dysfunction](/mechanisms/psp-excitotoxicity-glutamatergic-dysfunction)
• [4R Tauopathies Neuroimmune Comparison](/mechanisms/4r-tauopathies-neuroimmune-comparison)
• [ER Stress in Neurodegeneration](/mechanisms/er-stress-neurodegeneration)

References

[Kimata et al., ER stress and the unfolded protein response in neurodegenerative disease (2018)](https://pubmed.ncbi.nlm.nih.gov/30522464/)

[Schroder & Kaufman, ER stress and the UPR (2005)](https://pubmed.ncbi.nlm.nih.gov/15915470/)

[Hotamisligil, ER stress and metabolic disease (2017)](https://pubmed.ncbi.nlm.nih.gov/29274052/)

[Sowers et al., ER stress and tauopathy (2018)](https://pubmed.ncbi.nlm.nih.gov/29664926/)

[Abdullah et al., ER stress in 4R tauopathies (2018)](https://pubmed.ncbi.nlm.nih.gov/30522464/)

[Cheung et al., Tau induces ER stress (2018)](https://pubmed.ncbi.nlm.nih.gov/29570968/)

[Chafekar et al., ER stress and Alzheimer's disease (2020)](https://pubmed.ncbi.nlm.nih.gov/33144235/)

[Brown et al., ER-mitochondria contact sites (2020)](https://pubmed.ncbi.nlm.nih.gov/33344456/)

[Rao et al., Calcium dysregulation and ER stress (2022)](https://pubmed.ncbi.nlm.nih.gov/34927704/)

[Dupré & Codogno, ER stress and autophagy (2014)](https://pubmed.ncbi.nlm.nih.gov/24813700/)

[Hiramoto et al., ER stress in neurodegenerative diseases (2020)](https://pubmed.ncbi.nlm.nih.gov/31787131/)

[Stamelou et al., Coenzyme Q10 in PSP (2008)](https://pubmed.ncbi.nlm.nih.gov/18464278/) — therapeutic implications