The protein quality control (PQC) network is the cell's comprehensive defense system against proteotoxic stress, comprising interconnected pathways that maintain proteostasis. This network encompasses molecular chaperones, the ubiquitin-proteasome system (UPS), autophagy-lysosomal pathways, the unfolded protein response (UPR), and ER-associated degradation (ERAD). Failure of these systems is a central mechanism in neurodegenerative diseases, where misfolded proteins accumulate as toxic aggregates.
Molecular chaperones are proteins that assist in proper protein folding and prevent aggregation. The major chaperone systems include:
| Chaperone | Type | Function | Disease Relevance |
|---|---|---|---|
| HSP70 | ATP-dependent | Folding, disaggregation | AD, PD, HD |
| HSP90 | ATP-dependent | Folding, stability | ALS, cancer |
| HSC70 | ATP-dependent | Co-chaperone, ERAD | PD, ALS |
| BiP/GRP78 | HSP70 family | ER chaperone, UPR regulation | AD, PD |
| DNAJ/HSP40 | Co-chaperone | HSP70 recruitment | HD |
The UPS is the primary pathway for targeted protein degradation: [4]
Autophagy degrades bulk protein aggregates and damaged organelles: [5]
The UPR is a transcriptional response to ER stress: [6]
| Sensor | Domain | Downstream Effect |
|---|---|---|
| PERK | Kinase | eIF2α phosphorylation → translation attenuation |
| IRE1 | Kinase/RNase | XBP1 splicing → chaperone upregulation |
| ATF6 | Transcription factor | ATF6f cleavage → ERAD component expression |
References: [1:1], [2:1], [7], [8], [9]
ERAD retrotranslocates misfolded proteins from the ER to the cytosol for proteasomal degradation:
| PQC Component | Defect | Evidence |
|---|---|---|
| UPS | Reduced proteasome activity | ↓20S proteasome in AD brain |
| Autophagy | Impaired lysosomal acidification | Cathepsin D deficiency in AD |
| UPR | Chronic activation → apoptosis | PERK, IRE1 hyperactivated in AD |
| Chaperones | HSP70 decreased | Reduced HSP70 in temporal cortex |
| PQC Component | Defect | Evidence |
|---|---|---|
| Autophagy | PINK1/Parkin mitophagy failure | Loss-of-function mutations in PD |
| Proteasome | GBA1 deficiency affects | Glucocerebrosidase mutations ↑ PD risk |
| Chaperones | HSP70 compromised | DNAJC proteins mutated in PD |
| ERAD | LRRK2 affects ER export | G2019S LRRK2 disrupts ERAD |
| PQC Component | Defect | Evidence |
|---|---|---|
| UPS | Ubiquitin inclusions | Bunina bodies, skein-like inclusions |
| Autophagy | p62, OPTN mutations | Autophagy receptor mutations cause ALS |
| ERAD | VCP mutations | Valosin-containing protein mutations |
| Proteostasis | C9orf72 affects | Hexanucleotide expansion disrupts |
| PQC Component | Defect | Evidence |
|---|---|---|
| UPR | Chronic ER stress | PERK, IRE1, ATF6 dysregulated |
| Autophagy | mTOR hyperactivation | Excessive autophagy |
| Chaperones | HTT sequesters chaperones | Mutant huntingtin traps HSP70/HSP90 |
| UPS | Impaired degradation | Ubiquitinated aggregates |
| PQC Component | Defect | Evidence |
|---|---|---|
| UPS | Overwhelmed by PrP^Sc | Proteasome inhibited |
| Autophagy | Dysregulated | Autophagosome accumulation |
| Chaperones | Failed clearance | HSP70 ineffective against prions |
| Strategy | Agent | Mechanism | Stage |
|---|---|---|---|
| HSP70 inducers | Geldanamycin derivatives | Hsp90 inhibition → Hsp70 upregulation | Preclinical |
| HSP90 inhibitors | 17-DMAG, PU-DZ8 | Release HSF1 → chaperone expression | Phase I/II |
| Chemical chaperones | TUDCA, glycerol | Protein stabilization | Phase II/III |
| Strategy | Agent | Mechanism | Stage |
|---|---|---|---|
| Proteasome activators | PA28γ | Enhanced proteasome activity | Preclinical |
| Ubiquitination modulators | E3 ligase modulators | Increase degradation of misfolded proteins | Discovery |
| Strategy | Agent | Mechanism | Stage |
|---|---|---|---|
| mTOR inhibitors | Rapamycin, Everolimus | Autophagy induction | Approved |
| mTOR-independent | Trehalose, lithium | TFEB activation | Phase II |
| TFEB activators | AAV-TFEB | Lysosomal biogenesis | Preclinical |
| Strategy | Agent | Mechanism | Stage |
|---|---|---|---|
| PERK inhibitors | GSK2656157 | Prevent eIF2α hyperphosphorylation | Preclinical |
| IRE1 inhibitors | MKC8866 | Reduce XBP1 splicing | Preclinical |
| BiP inducers | TUDCA | ER chaperone upregulation | Phase II |
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