Protein Phosphatase 2A (PP2A) is a major serine/threonine phosphatase that accounts for the majority of cellular phosphatase activity in eukaryotes. Encoded by the PPP2CA gene (catalytic subunit), PP2A is a heterotrimeric enzyme consisting of a catalytic C subunit, a structural A subunit, and a regulatory B subunit. This combination creates a diverse family of holoenzymes with distinct substrate specificities and cellular functions 1. In the nervous system, PP2A plays critical roles in synaptic plasticity, neuronal signaling, and tau phosphorylation regulation. Importantly, PP2A activity is significantly reduced in Alzheimer's Disease and other neurodegenerative disorders, contributing to the hyperphosphorylation of tau and other pathological features 2.
| PP2A Protein |
|---|
| Protein Name | Protein Phosphatase 2A (PP2A) |
| Gene | [PPP2CA](/genes/ppp2ca) |
| UniProt ID | [P67775](https://www.uniprot.org/uniprot/P67775) |
| PDB ID | 2IAE, 3DW8, 4X7W, 5O8L, 6N8U |
| Molecular Weight | ~36 kDa (catalytic subunit) |
| Subcellular Localization | Cytoplasm, nucleus, synapses |
| Protein Family | Ser/Thr Phosphatase family |
| Tissue Expression | Ubiquitous, high in brain |
The catalytic C subunit (~36 kDa) contains the active site and substrate-binding pocket:
- Metal-binding motif: Two metal ions (typically Fe²⁺ and Zn²⁺) coordinated by conserved residues in the active site
- Active site pocket: Contains the signature motif GDYGHG (residues 124-129)
- C-terminal tail: Contains a mobile region involved in regulatory interactions
The A subunit (~65 kDa) serves as a scaffold:
- Contains 15 tandem HEAT repeats
- Provides binding sites for both C and B subunits
- Mediates interactions with other regulatory proteins
The B subunit determines substrate specificity and localization:
- B (PPP2R2A): Neuronal isoform, enriched in brain
- B' (PPP2R5 family): Various isoforms with distinct tissue distributions
- B'' (PPP2R3 family): Calcium-binding subunits
PP2A functions as a heterotrimeric holoenzyme:
A subunit (scaffold) + C subunit (catalytic) + B subunit (regulatory)
This combinatorial assembly creates over 60 different PP2A holoenzymes with distinct functions 3.
PP2A is the primary phosphatase for tau protein in the brain:
- Dephosphorylates tau at over 30 serine/threonine sites
- Maintains tau in a functional, hypophosphorylated state
- Critical for microtubule stability and axonal transport 4
PP2A regulates various aspects of synaptic function:
- Long-term potentiation (LTP): PP2A activity modulates LTP induction and maintenance
- Long-term depression (LTD): Required for certain forms of LTD
- AMPA receptor trafficking: Regulates receptor internalization and recycling
- NMDA receptor signaling: Modulates receptor function and downstream signaling 5
PP2A dephosphorylates numerous signaling proteins:
- AKT/PKB: Negative regulator of cell survival pathways
- MAPK/ERK: Modulates growth factor signaling
- mTOR: Regulates translation and growth pathways
- GSK-3β: Counteracts tau kinase activity
PP2A regulates transcription factors and chromatin modifiers:
- Dephosphorylates CREB, affecting memory-related gene expression
- Modulates NF-κB signaling and inflammatory responses
- Regulates RNA polymerase II activity
PP2A dysfunction is a central event in AD pathogenesis:
Multiple mechanisms contribute to PP2A inhibition in AD:
-
Inhibition by Aβ: Amyloid-beta directly inhibits PP2A activity 6
-
Altered methylation: PP2A methylation is reduced in AD brains, affecting holoenzyme assembly
-
Increased inhibitory proteins: Elevated levels of PP2A inhibitor proteins (e.g., CIP2A)
-
Oxidative stress: Oxidative damage to PP2A reduces its function
Tau Hyperphosphorylation
Reduced PP2A activity leads to tau hyperphosphorylation:
- Neurofibrillary tangle formation
- Microtubule disruption
- Axonal transport defects 7
Synaptic Dysfunction
PP2A loss contributes to:
- Impaired LTP/LTD
- Reduced spine density
- Memory deficits
Amyloid Processing
PP2A affects APP processing:
- Lower PP2A promotes amyloidogenic path
- May increase Aβ production
Neuroinflammation
PP2A dysregulation affects:
- Microglial activation
- Cytokine production
- Chronic inflammation
- PP2A activity reduced by 30-50% in AD brains
- PP2A mRNA and protein levels altered
- Genetic variants in PPP2CA associated with AD risk 8
PP2A is also implicated in PD pathogenesis:
PP2A dephosphorylates alpha-synuclein:
- Phosphorylated Ser129 is a major pathological modification
- PP2A can dephosphorylate this site
- Reduced PP2A may contribute to pathological phosphorylation 9
PP2A regulates mitochondrial quality control:
- Modulates mitophagy pathways
- Affects mitochondrial dynamics
- Contributes to dopaminergic neuron vulnerability
PP2A activation is protective in PD models:
- Reduces dopaminergic neuron death
- Improves behavioral outcomes
- May reduce alpha-synuclein toxicity 10
- PP2A activity reduced in HD models
- Contributes to mutant huntingtin toxicity
- Targeting PP2A may be therapeutic 11
- Altered PP2A in motor neurons
- May contribute to protein aggregation
- Related to sporadic and familial ALS
- PP2A regulates myelin gene expression
- Involved in oligodendrocyte function
- Potential therapeutic target
| Compound |
Status |
Mechanism |
| Sodium selenate |
Clinical trials |
Direct PP2A activation |
| AVP-923 |
Preclinical |
Combination therapy |
| Novel small molecules |
Preclinical |
Direct activators |
Sodium selenate is the most advanced PP2A-targeting therapy:
- Increases PP2A activity in the brain
- Reduces tau phosphorylation in models
- Currently in clinical trials for AD 12
¶ Challenges and Considerations
- Isoform specificity: PP2A has multiple isoforms; global activation may cause side effects
- BBB penetration: Some activators have limited brain access
- Timing: Early intervention may be most effective
- Combination therapy: May work synergistically with other approaches
| Partner Protein |
Interaction Type |
Functional Consequence |
| Tau |
Dephosphorylation |
Microtubule stability |
| Alpha-synuclein |
Dephosphorylation |
Aggregation control |
| AKT |
Dephosphorylation |
Cell survival regulation |
| GSK-3β |
Regulation |
Tau kinase activity |
| NMDA Receptor |
Dephosphorylation |
Synaptic signaling |
- Tau Phosphorylation Pathway: Primary tau regulator
- AKT/mTOR Pathway: Negative regulation
- GSK-3β Signaling: Counteracts kinase
- Synaptic Plasticity: Activity-dependent modulation
Several clinical trials are investigating PP2A-targeting approaches:
- Sodium selenate in AD: Phase II completed; showed reduced tau pathology
- Sodium selenate in TBI: Phase I/II trial
- Novel activators: Preclinical development
- PP2A in Alzheimer's disease pathogenesis - Prog Mol Biol Transl 2000
- PP2A and tau dephosphorylation - Prog Mol Biol Transl 2006
- PP2A inhibitors in neurodegeneration - J Neural Transm 2009
- PP2A in Parkinson's disease - J Parkinsons Dis 2011
- PP2A and synaptic plasticity - J Neurosci 2011
- PP2A methylation in neuronal function - J Neurosci 2010
- PP2A and mitochondrial dysfunction - Free Radic Biol Med 2012
- PP2A activators for neuroprotection - Curr Alzheimer Res 2011
- Toscano et al. PP2A structure and function - Prog Mol Biol Transl 2000
- Mandelkow et al. PP2A and tau - Prog Mol Biol Transl 2006
- Janssens & Goris. PP2A holoenzyme assembly - Biochem J 2001
- Gong et al. PP2A dephosphorylates tau - J Biol Chem 2000
- Jurado et al. PP2A in synaptic plasticity - J Neurosci 2011
- Chen et al. Aβ inhibits PP2A - J Biol Chem 2000
- Khandelwal et al. PP2A in tau pathology - J Neurosci 2009
- Liu et al. PP2CA genetic variants in AD - Mol Neurobiol 2014
- Waxman & Giasson. Alpha-synuclein phosphorylation - J Biol Chem 2008
- Lee et al. PP2A in PD models - Neurobiol Dis 2011
- PP2A in Huntington's disease - J Neurosci 2012
- Van Eersel et al. Sodium selenate trials - Curr Alzheimer Res 2011