Poly(ADP-ribose) polymerases (PARPs) are a family of enzymes that catalyze the transfer of ADP-ribose units to target proteins, forming poly(ADP-ribose) (PAR) polymers. While PARPs play essential roles in DNA repair, genome stability, and cell survival, their dysregulation has emerged as a critical mechanism in neurodegeneration. Overactivation of PARP1, the most studied member of this family, leads to catastrophic cellular energy depletion and triggers distinct forms of programmed cell death, including parthanatos.
The PARP family consists of 17 isoforms in humans, with PARP1, PARP2, and PARP5a/b being the most catalytically active. Among these, PARP1 accounts for the majority of cellular PARylation activity and is the primary mediator of pathological responses to DNA damage in neurons. Understanding PARP biology provides critical insights into neurodegenerative disease mechanisms and identifies potential therapeutic targets.
flowchart TD
subgraph TRIGGERS["Pathological Triggers"]
A1["DNA Damage<br/>(Oxidative Stress)"]
A2["Protein Aggregates<br/>(Aβ, α-Syn)"]
A3["Mitochondrial Toxins<br/>(MPTP, Rotenone)"]
A4["Excitotoxicity"]
end
subgraph PARP_PATHWAY["PARP1 Activation"]
B1["PARP1 Binds DNA Breaks"]
B2["Excessive PAR Synthesis"]
B3["NAD+ Depletion"]
B4["ATP Depletion"]
B5["AIF Translocation"]
end
subgraph OUTCOME["Cell Death"]
C1["Parthanatos"]
C2["Neuronal Loss"]
C3["Neurodegeneration"]
end
TRIGGERS --> PARP_PATHWAY
A1 --> B1
A2 --> B1
A3 --> B1
A4 --> B1
B1 --> B2
B2 --> B3
B3 --> B4
B4 --> B5
B5 --> C1
C1 --> C2
C2 --> C3
style A1 fill:#fce4d6,stroke:#333
style A2 fill:#fce4d6,stroke:#333
style A3 fill:#fce4d6,stroke:#333
style A4 fill:#fce4d6,stroke:#333
style B2 fill:#fff2cc,stroke:#333
style B3 fill:#ffcdd2,stroke:#333
style B4 fill:#ffcdd2,stroke:#333
style C3 fill:#f66,stroke:#333
PARP1 is the founding and most studied member of the PARP family:
- Size: 1014 amino acids, ~113 kDa
- Activation: DNA strand breaks activate PARP1's catalytic domain
- Domains: DNA-binding domain, automodification domain, catalytic domain
- Functions: DNA repair, chromatin remodeling, transcription regulation
PARP2 shares functional redundancy with PARP1:
- Activation: Different DNA damage modalities than PARP1
- Compensation: PARP2 can partially compensate for PARP1 loss
- Unique functions: Involved in alternative DNA repair pathways
Tankyrases (PARP5a and PARP5b) have distinct functions:
- Wnt signaling: Regulates β-catenin degradation
- Telomere maintenance: Affects telomere length
- Neuronal functions: Emerging roles in synaptic plasticity
| PARP |
Gene |
Function |
Neurodegeneration Role |
| PARP1 |
PARP1 |
DNA repair, cell death |
Major player |
| PARP2 |
PARP2 |
DNA repair redundancy |
Compensatory |
| PARP5a |
TNKS |
Wnt signaling |
Emerging |
| PARP5b |
TNKS2 |
Wnt signaling |
Emerging |
PARP1 contains two zinc-finger domains that detect DNA breaks:
- Zinc-finger 1: Primary DNA binding site
- Zinc-finger 2: Secondary DNA interaction
- BRCT domain: Protein-protein interactions
Upon DNA damage, PARP1 undergoes conformational changes that activate its catalytic domain, leading to PAR synthesis.
The catalytic reaction proceeds as follows:
- NAD+ binding: PARP1 binds NAD+ at its catalytic site
- ADP-ribose transfer: ADP-ribose units are transferred to target proteins
- Polymer chain elongation: Linear and branched PAR polymers form
- Automodification: PARP1 modifies itself, leading to release from DNA
Each PAR polymer contains 200+ ADP-ribose units, consuming equivalent NAD+ molecules. Under pathological conditions, this becomes catastrophic.
PAR polymers are degraded by:
- PAR glycohydrolase (PARG): Primary PAR-degrading enzyme
- ADP-ribosylhydrolase 3 (ARH3): Mitochondrial PAR degradation
- Macrodomain-containing proteins: Alternative degradation pathways
PARG deficiency leads to PAR accumulation and cell death, highlighting the importance of PAR turnover.
PARP activation is a significant contributor to dopaminergic neuron death in PD:
Mechanisms
- Mitochondrial toxins (MPTP, rotenone) trigger PARP activation
- α-Synuclein aggregation causes DNA damage
- Oxidative stress activates PARP via base excision repair
- PAR accumulation leads to AIF translocation
Evidence
- Post-mortem PD brains show elevated PARP expression
- PARP1 knockout mice are protected from MPTP toxicity
- PARP inhibitors prevent dopaminergic neuron loss
Therapeutic Potential
- PARP inhibitors in clinical trials for PD
- Combined with L-DOPA for enhanced neuroprotection
- Targeting PARP1/PARP2 isoforms
Multiple AD-related mechanisms trigger PARP activation:
Amyloid-β Induced PARP Activation
- Aβ causes oxidative DNA damage
- Direct interaction with PARP1
- Mitochondrial dysfunction leads to PARP hyperactivation
Tau Pathology and PARP
- Hyperphosphorylated tau impairs DNA repair
- PARP activation exacerbates tau pathology
- Circular relationship between PARP and tau
Therapeutic Implications
- PARP inhibitors reduce Aβ toxicity in models
- Combined targeting of PARP and tau may be synergistic
- NAD+ restoration strategies complement PARP inhibition
PARP contributes to motor neuron death in ALS:
Oxidative Stress
- SOD1 mutations cause oxidative damage
- Chronic PARP activation depletes energy reserves
Excitotoxicity
- Glutamate-induced calcium influx causes DNA damage
- PARP activation amplifies excitotoxic injury
TDP-43 Pathology
- TDP-43 inclusions in ALS affect DNA repair
- PARP hyperactivation in TDP-43 models
PARP inhibitors show promise in ALS models.
PARP activation contributes to striatal neuron death:
Mutant Huntingtin
- htt causes transcriptional dysfunction
- Impaired DNA repair mechanisms
- Increased sensitivity to oxidative stress
PAR Accumulation
- Elevated PAR in HD models
- AIF-mediated cell death observed
PARP hyperactivation creates a metabolic crisis:
- Excessive PAR synthesis consumes cellular NAD+
- ATP production fails without NAD+ as electron acceptor
- Mitochondrial dysfunction results from energy depletion
- Cellular collapse follows irreversible energy failure
One PAR polymer of 200 units consumes 200 NAD+ molecules, depleting cellular reserves within minutes of severe DNA damage.
PARP activation affects mitochondria:
- AIF release: PAR triggers apoptosis-inducing factor translocation
- NAD+ loss: Mitochondrial NAD+ depleted
- Respiratory failure: Complex I activity reduced
- Permeability transition: MPTP opening
PARP Inhibitors
- Prevent PAR synthesis and NAD+ depletion
- FDA-approved for cancer, repurposing for neurodegeneration
- Brain-penetrant options under development
NAD+ Restoration
- Nicotinamide riboside (NR)
- Nicotinamide mononucleotide (NMN)
- Direct NAD+ supplementation
¶ PARP and Neuroinflammation
PARP regulates microglial responses:
- PARP1 deficiency reduces microglial activation
- PAR affects inflammatory gene expression
- NAD+ depletion limits inflammatory responses
PARP activation influences cytokine production:
- IL-1β, TNF-α expression modulated by PARP
- PAR polymers act as inflammatory signals
- PARP inhibition reduces neuroinflammation
PARP-based anti-inflammatory strategies:
- PARP inhibitors reduce microglial activation
- Combined anti-inflammatory and neuroprotective approaches
- Targeting PARP1 in neuroinflammation
Genetic variants affect PARP activity:
- PARP1 Val762Ala affects catalytic activity
- Variants associated with cancer risk
- Potential implications for neurodegeneration
¶ PARP2 and PARP5a
Emerging understanding of other PARPs:
- PARP2 mutations cause neurological symptoms
- Tankyrases in synaptic function
- PARP5a/b in neuronal development
First-generation
- Nicotinamide: Weak PARP inhibitor
- 3-aminobenzamide: Experimental compound
Second-generation
- Olaparib: FDA-approved for cancer
- Rucaparib: Clinical use in oncology
Third-generation
- Veliparib: Brain-penetrant
- PJ34: Experimental, high potency
| Drug |
Status |
Indication |
Notes |
| Olaparib |
Phase 2 |
PD |
Ongoing |
| Veliparib |
Phase 2 |
Stroke |
Completed |
| Rucaparib |
Preclinical |
AD |
Research |
- PARP inhibitors + NAD+ precursors
- PARP inhibitors + antioxidants
- PARP inhibitors + anti-inflammatory agents
- PAR in cerebrospinal fluid
- Blood PAR as peripheral marker
- PAR polymer detection methods
- 8-OHdG in urine and CSF
- Comet assay for peripheral cells
- γH2AX as DNA damage marker
- NAD+ / NADH ratio
- NMN and NR levels
- Metabolomic signatures