upr

Introduction

Unfolded Protein Response (Upr) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

Overview

flowchart TD
    UPR["UPR"] -->|"activates"| PROTEIN_FOLDING["PROTEIN_FOLDING"]
    UPR["UPR"] -->|"regulates"| SYNAPTIC_FUNCTION["SYNAPTIC_FUNCTION"]
    UPR["UPR"] -->|"activates"| APOPTOSIS["APOPTOSIS"]
    ER_STRESS["ER_STRESS"] -->|"activates"| UPR["UPR"]
    ER_STRESS["ER STRESS"] -->|"activates"| UPR["UPR"]
    RAPAMYCIN["RAPAMYCIN"] -->|"associated with"| UPR["UPR"]
    style UPR fill:#4fc3f7,stroke:#333,color:#000

The unfolded protein response (UPR) is an evolutionarily conserved cellular stress response activated when misfolded or unfolded proteins accumulate in the endoplasmic reticulum (ER), a condition known as ER stress. The endoplasmic-reticulum-stress is orchestrated by three ER-resident transmembrane sensor proteins — PERK (protein kinase R-like ER kinase), IRE1alpha (inositol-requiring enzyme 1alpha), and ATF6 (activating transcription factor 6) — which collectively initiate signaling cascades to restore protein homeostasis ([proteostasis). When adaptive endoplasmic-reticulum-stress mechanisms fail to resolve ER stress, the response shifts to a terminal phase that triggers apoptosis and neuronal death (Hetz & Saxena, 2017). ^{1Citation2011 · DOI 10.1126/science.1209038Open reference}

Given that protein misfolding and aggregation are hallmarks of virtually all neurodegenerative diseases — including ^{2- alzheimers — Major disease with UPR involvement}(/diseases/alzheimers-disease), parkinsons, ^{3- als — Clinical trial target for UPR modulators## External Links}(/diseases/als), huntington-pathway, and prion-diseases — the endoplasmic-reticulum-stress has emerged as a central mechanistic link between protein pathology and neuronal death. Pharmacological modulation of endoplasmic-reticulum-stress signaling, particularly the PERK pathway, has shown potent neuroprotective effects in preclinical models. ^{4Citation2013 · DOI 10.1038/nn.3486Open reference}

The Three UPR Branches

PERK Pathway

The PERK branch provides the earliest endoplasmic-reticulum-stress response: ^{5Citation2017 · DOI 10.1093/brain/awx074Open reference}

1. Activation: Under ER stress, BiP/GRP78 (the ER chaperone that normally keeps PERK inactive) dissociates from PERK’s luminal domain, allowing PERK homodimerization and autophosphorylation

2. eIF2α phosphorylation: Activated PERK phosphorylates eIF2α (eukaryotic initiation factor 2α), causing global translational attenuation — reducing the protein load entering the ER

3. ATF4 induction: Paradoxically, p-eIF2α selectively enhances translation of ATF4, a transcription factor that upregulates stress-response genes involved in amino acid metabolism, redox homeostasis, and ^{6- autophagy — Complementary protein clearance pathway}(/mechanisms/autophagy-lysosome-neurodegeneration)

4. CHOP activation: Sustained PERK signaling induces CHOP (C/EBP homologous protein, also known as DDIT3/GADD153), a pro-apoptotic transcription factor that marks the transition from adaptive to maladaptive unfolded-protein-response. CHOP promotes apoptosis by suppressing anti-apoptotic Bcl-2, activating ER oxidase ERO1α (generating oxidative-stress, and inducing GADD34 (which dephosphorylates eIF2α, restoring translation of misfolded proteins in a toxic positive feedback loop)

IRE1α Pathway

IRE1α is the most evolutionarily ancient UPR sensor: ^{7Citation2013 · DOI 10.1126/scitranslmed.3006767Open reference}

1. Activation: Similar to PERK, BiP dissociation triggers IRE1α oligomerization and trans-autophosphorylation

2. XBP1 splicing: Activated IRE1α’s endoribonuclease domain catalyzes unconventional cytoplasmic splicing of XBP1 mRNA, producing the active transcription factor XBP1s. XBP1s upregulates genes encoding ER chaperones, ERAD components, and lipid biosynthesis enzymes

3. RIDD: Under chronic stress, IRE1α’s RNase activity shifts to Regulated IRE1-Dependent Decay (RIDD), degrading ER-localized mRNAs to reduce the protein folding burden — but also degrading essential mRNAs, contributing to cell death

4. ASK1/JNK activation: IRE1α recruits TRAF2 and activates ASK1-JNK signaling, promoting apoptosis and neuroinflammation via nf-kb

ATF6 Pathway

ATF6 provides a transcriptional boost to ER folding capacity: ^{8Citation2020 · DOI 10.7554/eLife.62048Open reference}

1. Activation: Under ER stress, ATF6 (a type II transmembrane protein) is transported from ER to Golgi

2. Proteolytic processing: In the Golgi, Site-1 and Site-2 proteases cleave ATF6, releasing its cytoplasmic N-terminal bZIP transcription factor domain

3. Target genes: ATF6 translocates to the nucleus and upregulates ER chaperones (BiP, GRP94, calnexin), ERAD components, and XBP1 (amplifying the IRE1α pathway)

UPR in Neurodegenerative Diseases

Alzheimer’s Disease

UPR activation markers (phosphorylated PERK, p-eIF2α, activated IRE1α) are elevated in AD brain tissue, particularly in the hippocampus and cortex. Key connections include: ^{9Citation2009 · DOI 10.2353/ajpath.2009.080814Open reference}

• amyloid-beta oligomers induce ER stress and PERK activation in neurons

• tau-protein hyperphosphorylation is promoted by PERK-mediated translational changes

• psen1 mutations (causing familial AD) disrupt ER calcium homeostasis, exacerbating ER stress

• eIF2α phosphorylation impairs synaptic plasticity and long-term-potentiation, contributing to memory deficits (Ma et al., 2013)

Parkinson’s Disease

• alpha-synuclein oligomers directly bind to and activate IRE1α and PERK

• ER-mitochondria calcium transfer disruption (via MAMs is a key mechanism

• pink1 and prkn interact with UPR components; PINK1 deficiency sensitizes neurons to ER stress

• lrrk2 mutations impair ER-Golgi trafficking, triggering UPR

ALS/FTD

• tdp-43 cytoplasmic aggregation activates PERK and IRE1α

• sod1-protein mutant protein aggregates overwhelm ER quality control

• c9orf72 dipeptide repeat proteins cause ER stress through nucleocytoplasmic transport defects

• fus mutations disrupt ER-associated protein processing

Prion Diseases

• Prion protein (PrPSc) accumulation triggers sustained PERK activation

• The PERK-eIF2α-ATF4-CHOP pathway is a major driver of prion-induced neurodegeneration

• Genetic or pharmacological inhibition of PERK signaling is profoundly neuroprotective in prion-infected mice (Moreno et al., 2012)

Huntington’s Disease

• Mutant huntingtin protein disrupts ERAD, causing ER stress

• XBP1s overexpression is neuroprotective in HD models

• IRE1α-mediated RIDD degrades essential neuronal mRNAs

Therapeutic Targeting

PERK Pathway Modulation

The PERK-eIF2α axis has emerged as the most promising therapeutic target within the UPR: ^{10Citation2014 · DOI 10.1038/nrn3689Open reference}

• GSK2606414: First selective PERK inhibitor; showed potent neuroprotection in prion-infected mice but caused pancreatic toxicity

• ISRIB (Integrated Stress Response Inhibitor): Acts downstream of eIF2α phosphorylation; restores translation without inhibiting PERK directly. Shows broad neuroprotective effects and cognitive enhancement in aged mice (Krukowski et al., 2020). Currently in clinical development.

• Trazodone hydrochloride: Existing antidepressant that acts downstream of p-eIF2α; neuroprotective in prion and FTD mouse models (Halliday et al., 2017)

• Salubrinal: Inhibits eIF2α dephosphorylation; mixed results (protective vs. toxic depending on context)

IRE1α Modulators

• 4μ8C and STF-083010: IRE1α RNase inhibitors; under preclinical investigation

• KIRA6: Kinase-inhibiting RNase attenuator; protects retinal neurons from ER stress

Chaperone-Based Therapies

• 4-PBA (sodium phenylbutyrate): Chemical chaperone that reduces ER stress; component of AMX0035 (Relyvrio) for ALS

• TUDCA (tauroursodeoxycholic acid): Bile acid derivative with chemical chaperone properties; also a component of AMX0035

• Arimoclomol: Co-inducer of heat shock proteins; amplifies the chaperone response

Animal Models

Key animal studies have established the therapeutic relevance of UPR modulation: ^{2- alzheimers — Major disease with UPR involvement}0

• Prion-infected mice: PERK inhibition with GSK2606414 completely prevented neurodegeneration but caused pancreatic toxicity; ISRIB partially restored translation without pancreatic effects5,6

• rTg4510 tau mice: Chronic PERK-eIF2α activation contributes to translational repression and synapse loss; ISRIB restores memory function

• SOD1-G93A ALS mice: ISRIB fine-tunes UPR, improving motor neuron survival by maintaining protective ATF4 signaling while reducing translational block^{2- alzheimers — Major disease with UPR involvement}1

• 5xFAD AD mice: Genetic reduction of PERK rescues synaptic plasticity and spatial memory deficits

• Aged wild-type mice: ISRIB restores cognitive function in aged mice, reversing age-related translational decline^{2- alzheimers — Major disease with UPR involvement}2

Clinical Development (2024-2025 Updates)

Two major clinical programs testing eIF2B activators (ISRIB mechanism) reported results in 2025: ^{2- alzheimers — Major disease with UPR involvement}3

• DNL343 (Denali Therapeutics): A CNS-penetrant eIF2B activator tested in the Healy ALS Platform Trial. In January 2025, DNL343 failed to meet primary and key secondary endpoints — it did not change neurofilament light chain levels after 6 months of treatment or in a 7-month open-label extension^{2- alzheimers — Major disease with UPR involvement}4

• Fosigotifator (Calico/AbbVie): Another eIF2B activator tested in ALS. Also failed to meet the primary endpoint of slowing ALS progression in January 2025, with no effect on respiratory function or quality of life endpoints

• Trazodone + dibenzoylmethane: Existing drugs acting downstream of p-eIF2α showed neuroprotection in prion and FTD mice; trazodone repurposing is under investigation for neurodegenerative applications^{2- alzheimers — Major disease with UPR involvement}5

• AMX0035 (Relyvrio): Combination of 4-PBA + TUDCA (chemical chaperones reducing ER stress) was FDA-approved for ALS in 2022 but withdrawn in 2024 after Phase 3 PHOENIX trial failed to confirm efficacy

These clinical setbacks highlight that while preclinical UPR modulation is profoundly neuroprotective, translating these findings to human neurodegenerative disease remains challenging, possibly due to differences in disease stage, pathway redundancy, or the difficulty of achieving appropriate pathway modulation in the human CNS. ^{2- alzheimers — Major disease with UPR involvement}6

Brain Atlas Resources

• Allen Human Brain Atlas: Unfolded Protein Response expression search

• Allen Mouse Brain Atlas: Unfolded Protein Response search

• Allen Cell Type Atlas: Transcriptomic cell type reference

• BrainSpan Developmental Transcriptome: Unfolded Protein Response developmental expression

• Researchers Index — All researchers

• Diseases Index — Disease overview pages

External Links

• Google Scholar — Publications

• PubMed — Biomedical literature

Background

The study of Unfolded Protein Response (Upr) has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development. ^{2- alzheimers — Major disease with UPR involvement}7

Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions. ^{2- alzheimers — Major disease with UPR involvement}8

Additional evidence sources: ^{2- alzheimers — Major disease with UPR involvement}9 ^{3- als — Clinical trial target for UPR modulators## External Links}0 ^{3- als — Clinical trial target for UPR modulators## External Links}1 ^{3- als — Clinical trial target for UPR modulators## External Links}2 ^{3- als — Clinical trial target for UPR modulators## External Links}3 ^{3- als — Clinical trial target for UPR modulators## External Links}4 ^{3- als — Clinical trial target for UPR modulators## External Links}5 ^{3- als — Clinical trial target for UPR modulators## External Links}6 ^{3- als — Clinical trial target for UPR modulators## External Links}7