Hsp90 (Heat Shock Protein 90) is a highly abundant molecular chaperone that plays a critical role in protein folding, quality control, and cellular homeostasis. In Parkinson's disease (PD), Hsp90 paradoxically contributes to pathology by stabilizing toxic client proteins, particularly misfolded alpha-synuclein, which drives the formation of Lewy bodies and dopaminergic neuron death. Hsp90 inhibitors represent a promising therapeutic strategy that promotes the degradation of these toxic client proteins through the proteasome and autophagy pathways, offering potential disease-modifying benefits for PD patients.
¶ Hsp90 Biology and Structure
Hsp90 is a 90 kDa homodimeric chaperone present at 1-2% of total cellular protein, making it one of the most abundant cytosolic proteins. Its structure consists of three functional domains:
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N-terminal domain (NTD): The ATP-binding pocket (residues 1-220) is the primary target for Hsp90 inhibitors. This domain undergoes dramatic conformational changes during the chaperone cycle, transitioning between open and closed states.
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Middle domain (MD): Positioned at residues 221-290, this domain serves as the primary client protein binding site.
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C-terminal domain (CTD): The dimerization domain (residues 291-605) mediates Hsp90 homodimer formation.
Under normal cellular conditions, Hsp90 operates through an ATP-dependent cycle:
- Open conformation: Hsp90 adopts an open V-shaped conformation
- ATP binding: Triggers N-terminal dimerization and closed conformation
- Client protein folding: Facilitates client protein folding with co-chaperones
- ATP hydrolysis: Drives conformational changes and client release
- Reset: The cycle repeats with new client protein loading
| Isoform |
Location |
Expression |
Therapeutic Relevance |
| Hsp90α |
Cytosol |
Inducible (stress) |
Cancer, neuroprotection |
| Hsp90β |
Cytosol |
Constitutive |
Essential for viability |
| GRP94 |
ER |
Inducible |
Calcium homeostasis |
| TRAP1 |
Mitochondria |
Tissue-specific |
Mitochondrial protection |
In PD pathogenesis, Hsp90 plays a detrimental role by stabilizing toxic forms of alpha-synuclein:
- Oligomer stabilization: Hsp90 binding promotes toxic oligomer formation
- Aggregation protection: Extends half-life of misfolded alpha-synuclein
- Seeding activity: Hsp90-alpha-synuclein complexes may have enhanced seeding capacity
- Cell-to-cell transmission: Facilitates pathology spread between neurons
| Protein |
Role in PD |
Hsp90 Interaction |
| LRRK2 |
Kinase mutations (G2019S) |
Stabilizes mutant LRRK2, enhances toxicity |
| PINK1 |
Mitophagy regulator |
Hsp90 inhibition activates pathway |
| GBA1 |
Lysosomal enzyme |
Affects folding and trafficking |
| DJ-1 |
Oxidative stress response |
Stabilizes mutant protein |
| tau |
Microtubule stabilization |
Cross-pathology in PSP/CBD |
The Hsp90-alpha-synuclein interaction creates a vicious cycle:
flowchart LR
A["Alpha-synuclein misfolding"] --> B["Hsp90 binding"]
B --> C["Oligomer stabilization"]
C --> D["Lewy body formation"]
D --> E["neuronal dysfunction"]
E --> A
B --> F["Proteostasis disruption"]
F --> G["Additional protein misfolding"]
Hsp90 inhibitors offer multiple therapeutic benefits:
- Proteasomal degradation: Promotes ubiquitin-dependent degradation
- Autophagy induction: Targets released clients to autophagy
- Reduced aggregation: Lowers steady-state alpha-synuclein levels
- Neuroprotection: Combined effects on multiple toxic proteins
When Hsp90 is inhibited:
- Client protein release: Misfolded proteins are released from Hsp90
- Ubiquitination: Released proteins are targeted for degradation
- Proteasome recruitment: Ubiquitinated proteins are degraded
- Autophagy activation: Larger aggregates are cleared via autophagy
- Geldanamycin: Prototypical Hsp90 inhibitor from Streptomyces hygroscopicus
- First discovered as anticancer agent
- Significant hepatotoxicity limits clinical use
- 17-AAG (Tanespimycin): Improved solubility, reduced hepatotoxicity
- Successfully completed Phase I trials
- Neuroprotective in PD models
- 17-DMAG (Alvespimycin): Water-soluble, demonstrated neuroprotective effects in PD models
- Better tissue distribution
- Currently in preclinical development for PD
- PU-H71: Purine-scaffold with brain penetration, in clinical trials
- AUY922 (Luminespimycin): Isoflavone-derived with potent inhibition
- Strong anti-tumor activity
- Limited CNS penetration
- NVP-HSP990: Excellent oral bioavailability
- Novartis compound
- Phase 1 completed
- AT13387 (Onalespib): Long-acting with sustained target engagement
- Demonstrated safety in Phase 1
- PU-DZ8: Designed for CNS applications with optimized brain penetration
- KW-2478: Synthetic with favorable pharmacokinetics
- EXEL-0466: Recently developed with enhanced CNS penetration
- Primary neuronal cultures: 17-DMAG reduces alpha-synuclein toxicity
- Dose-dependent neuroprotection
- Reduces oligomer formation
- LUHMES cells: Reduced aggregation and increased survival
- Dopaminergic neuronal cell line
- Validates translational potential
- Patient-derived iPSCs: Dopaminergic neurons respond to treatment
- Direct relevance to human disease
- AAV-alpha-synuclein models: Protect dopaminergic neurons
- Reduced neuron loss in substantia nigra
- Improved motor performance
- Transgenic mice: Improved motor performance
- Reduced alpha-synuclein aggregation
- Improved survival
- MPTP/6-OHDA models: Neuroprotection against toxin-induced degeneration
- Preserved dopaminergic terminals
- Maintained striatal dopamine levels
| Model |
Compound |
Outcome |
| AAV-α-syn |
17-DMAG |
60% neuron protection |
| AAV-α-syn |
PU-H71 |
Improved motor function |
| MPTP |
17-AAG |
Preserved TH+ neurons |
| 6-OHDA |
AUY922 |
Reduced apomorphine rotations |
| Compound |
Company |
Stage |
Status |
| PU-H71 |
Samus Therapeutics |
Phase 1/2 |
Active for cancer, not PD-specific |
| AT13387 |
Astex Pharmaceuticals |
Phase 1/2 |
Completed |
| NVP-HSP990 |
Novartis |
Phase 1 |
Completed |
| 17-DMAG |
NCI |
Preclinical |
PD-focused development |
- Brain penetration: Remains suboptimal for many compounds
- Peripheral toxicity: Limits maximum tolerated doses
- Multiple client protein effects: May cause unintended consequences
- Patient selection: Biomarkers for target engagement needed
Combining Hsp90 inhibitors with autophagy inducers may provide synergistic benefits:
- Rapamycin/mTOR inhibitors: Enhanced autophagy
- Trehalose: Autophagy inducer with neuroprotective properties
- Carbamazepine: TFEB activation
- Hsp70 inducers: Complementary protein clearance
- Hsp40 co-chaperones: Client protein targeting
¶ Biomarkers and Patient Selection
- Hsp90 client proteins: LRRK2, PINK1 levels in PBMCs
- Alpha-synuclein aggregates: CSF RT-QuIC
- Heat shock factor 1 (HSF1) activation: Upstream biomarker
- Genetically defined: LRRK2, GBA mutation carriers may benefit most
- Disease stage: Early intervention may be most effective
- Biomarker positive: Evidence of abnormal protein aggregation
- Hepatotoxicity: Liver function monitoring required
- Fatigue: Common with systemic Hsp90 inhibition
- Gastrointestinal: Nausea, diarrhea
- Visual disturbances: With some compounds
- Severe hepatic impairment
- Pregnancy/breastfeeding
- Concurrent hepatotoxic medications
- Brain-penetrant compounds: Continued optimization of CNS penetration
- Combination approaches: Synergistic strategies with autophagy enhancers
- Selective client targeting: Developing compounds that preferentially release specific clients
- Biomarker development: Patient selection and target engagement
- Gene-specific approaches: Tailored for LRRK2, GBA carriers
- Kirkpatrick et al., Hsp90 and alpha-synuclein aggregation (2005)
- Wang et al., Hsp90 inhibition in PD models (2008)
- Chu et al., Hsp90 as therapeutic target in PD (2019)
- Makarava et al., Chaperone-based therapy for synucleinopathies (2022)
- Chen et al., Hsp90 in synucleinopathies (2024)
- Niedzielska et al., Hsp90 and LRRK2 in PD (2024)
- Shen et al., HDAC6-Hsp90 crosstalk in PD (2021)
- Cuervo et al., Autophagy and chaperones in PD (2020)
- Luthra et al., PU-H71 in neurodegenerative models (2023)
- Auluck et al., Hsp70 and alpha-synuclein aggregation (2002)
- Fujita et al., Geldanamycin derivatives in Parkinson's disease models (2007)
- Tatro et al., Hsp90 client protein profiling in PD (2009)
- McCormack et al., Hsp90 inhibitors and autophagy (2010)
- Daturpalli et al., Hsp90-α-synuclein interaction (2018)
- Sanchez-Valle et al., Hsp90 inhibition reduces Lewy body pathology (2022)