C. elegans UPR and Stress Integration Pathway Summary

Pathway Architecture

Priority 1: UPR-ER Sensors (6 genes)

IRE-1/XBP-1 Branch

Mechanism: IRE-1 senses unfolded proteins via its luminal domain, oligomerizes, and performs unconventional cytoplasmic splicing of xbp-1 mRNA. The spliced XBP-1(s) activates transcription of ER chaperones and ERAD components.

PEK-1/ATF-4 Branch

Mechanism: PEK-1 phosphorylates eIF2α, attenuating global translation while selectively enhancing ATF-4 translation. ATF-4 regulates amino acid metabolism, redox homeostasis, and H2S-dependent longevity pathways.

ATF-6 Branch

Key Discovery: ATF-6 deletion extends lifespan 43-57% by reducing ER-to-mitochondria calcium transfer, protecting mitochondria from calcium overload. This reveals an unexpected negative role for this UPR branch in longevity.

ER Chaperone

Usage: hsp-4::GFP is the standard reporter for UPR-ER activation in C. elegans. HSP-4 is transcriptionally upregulated by all three UPR-ER branches.

Priority 2: UPR-mt Pathway (5 genes)

Core UPR-mt Regulators

Critical Correction: UBL-5/Hub1 was incorrectly annotated with "protein tag activity" (GO:0031386). UBL-5 lacks the C-terminal di-glycine motif required for covalent conjugation. Instead, it functions through non-covalent protein-protein interactions as a splicing factor cofactor and DVE-1 transcriptional coactivator.

Mitochondrial Chaperones

Error Corrected: HSP-60 annotation GO:0061629 (RNA polymerase II TF binding) was removed. The cited paper (PMID:30057120) described DVE-1 binding the hsp-60 promoter, not HSP-60 protein binding to transcription factors.

Usage: hsp-6::GFP and hsp-60p::GFP are standard reporters for UPR-mt activation.

Priority 3: Regulators and Integration (6 genes)

Integrated Stress Response

Function: GCN-2 senses amino acid starvation (via uncharged tRNAs) and mitochondrial stress, phosphorylating eIF2α to activate the integrated stress response (ISR). Converges with PEK-1 on the same substrate.

Alternative UPR

Discovery: ABU proteins were discovered in C. elegans when researchers found that blocking the canonical UPR activated an alternative protective pathway. ABU-1 shows sequence similarity to scavenger receptors.

Epigenetic Regulators

Correction: JMJD-3.1 annotation GO:0000978 (sequence-specific DNA binding) was removed - JmjC domain proteins lack intrinsic DNA-binding activity and are recruited to chromatin via protein-protein interactions.

Key Mechanism: MET-2/LIN-65 establishes H3K9me2 marks at stress response genes, while JMJD-3.1 removes H3K27me3 to allow gene activation. This epigenetic regulation enables transgenerational inheritance of UPR-mt.

Non-Cell Autonomous Signaling

Discovery: OCR-2 in sensory neurons enables non-cell autonomous UPR signaling. Neurons sense stress and communicate to peripheral tissues via serotonin signaling, coordinating organismal stress responses.

Key Mechanistic Insights

1. Three-Branch Redundancy with Synthetic Lethality

• Individual UPR-ER branches are dispensable
• ire-1/xbp-1 deletion is synthetic lethal with atf-6 or pek-1
• Reveals parallel pathway requirements for survival

2. ATF-6 as Longevity Antagonist

• ATF-6 deletion extends lifespan 43-57%
• Mechanism: Reduced ER-mitochondrial calcium transfer
• Challenges assumption that UPR activation is purely protective

3. UBL-5: Non-Covalent Ubiquitin-Like Protein

• Lacks C-terminal di-Gly motif for covalent conjugation
• Functions via non-covalent interactions
• Dual roles: spliceosome and DVE-1 coactivator
• Distinct from UBL-3 which does have di-Gly motif

4. Epigenetic Transgenerational Inheritance

• MET-2/LIN-65 establishes H3K9me2 silencing marks
• JMJD-3.1 removes H3K27me3 for activation
• Enables transgenerational memory of mitochondrial stress
• Affects offspring longevity

5. Non-Cell Autonomous UPR Coordination

• OCR-2/TRPV channels in sensory neurons
• Serotonin signaling to peripheral tissues
• Enables organismal coordination of stress responses
• Neurons as stress sensors

6. ISR Integration

• Multiple kinases (PEK-1, GCN-2) converge on eIF2α
• ATF-4 as common downstream effector
• Integrates ER stress, amino acid starvation, mitochondrial dysfunction

7. HSP-4/HSP-6 as Pathway Markers

• hsp-4::GFP: UPR-ER reporter
• hsp-6::GFP/hsp-60p::GFP: UPR-mt reporters
• Standard tools for pathway dissection in C. elegans

Cross-Reference Table

Disease Relevance

Key References

1. Calfon M et al. (2002) Nature - XBP-1 unconventional splicing mechanism
2. Nargund AM et al. (2012) Science - ATFS-1 dual localization mechanism
3. Taylor RC & Bharat T (2014) Cell - Non-cell autonomous UPR signaling
4. Xu D et al. (2024) G3 - UPR-ER protects against DNA damage
5. Shpilka T & Bharat T (2024) Cell Reports - Germline-soma UPR-mt regulation
6. Merkwirth C et al. (2016) Cell - UPR-mt transgenerational inheritance
7. Tian Y et al. (2016) Cell - MET-2/LIN-65 in UPR-mt epigenetic memory

Generated: 2025-12-28
Project: CAEEL_UPR_STRESS
Status: COMPLETE (17 genes reviewed)