Pp2A is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.
Protein phosphatase 2A (PP2A) is the major serine/threonine phosphatase in the mammalian brain, accounting for approximately 70% of total tau] phosphatase activity. PP2A is a heterotrimeric enzyme consisting of a scaffolding A subunit, a catalytic C subunit, and one of over 20 regulatory B subunits that determine substrate specificity, subcellular localization, and biological function. In the healthy brain, PP2A continuously dephosphorylates tau at pathologically relevant sites, maintaining the balance between tau phosphorylation and dephosphorylation that is essential for normal microtubule stability and axonal transport [1].
In [Alzheimer's disease[/diseases/[alzheimers[/diseases/[alzheimers[/diseases/[alzheimers[/diseases/[alzheimers--TEMP--/diseases)--FIX--, PP2A activity is reduced by approximately 50% in affected [brain regions[/[brain-regions[/[brain-regions[/[brain-regions[/[brain-regions[/[brain-regions[/[brain-regions[/[brain-regions[/brain-regions. This reduction—caused by increased endogenous inhibitors (SET/I2PP2A, CIP2A), decreased methylation of the catalytic subunit, and direct inhibition by [amyloid-beta[/entities/[amyloid-beta[/entities/[amyloid-beta[/entities/[amyloid-beta[/entities/[amyloid-beta--TEMP--/entities)--FIX--—shifts the phosphorylation–dephosphorylation balance toward persistent tau hyperphosphorylation], neurofibrillary tangle formation, and neurodegeneration. Restoring PP2A activity has emerged as a therapeutic strategy complementary to kinase inhibition (e.g., [GSK-3β[/entities/[gsk3-beta[/entities/[gsk3-beta[/entities/[gsk3-beta[/entities/[gsk3-beta--TEMP--/entities)--FIX-- and [CDK5[/entities/[cdk5[/entities/[cdk5[/entities/[cdk5[/entities/[cdk5--TEMP--/entities)--FIX-- inhibitors) for AD [2].
PP2A functions as a heterotrimer of three subunits:
| Family | Members | Key Brain Functions |
|---|---|---|
| B/B55/PR55 | B55α, B55β, B55γ, B55δ | [Tau[/entities/[tau-protein[/entities/[tau-protein[/entities/[tau-protein[/entities/[tau-protein--TEMP--/entities)--FIX--(/proteins/tau dephosphorylation (B55α is the primary tau-directed regulatory subunit); cell cycle control |
| B'/B56/PR61 | B56α, B56β, B56γ, B56δ, B56ε | [apoptosis[/entities/[apoptosis[/entities/[apoptosis[/entities/[apoptosis[/entities/[apoptosis--TEMP--/entities)--FIX-- regulation; Wnt signaling; mitotic checkpoint; transcription factor regulation |
| B''/PR72 | PR72, PR130, PR70 | Calcium-dependent regulation; DNA replication |
| B'''/Striatin | STRN, STRN3, STRN4 | STRIPAK complex; Hippo signaling; dendritic spine regulation |
The PP2A-B55α holoenzyme is the predominant form responsible for tau dephosphorylation in the brain. A landmark cryo-EM study in 2010 revealed how the B55α subunit positions the catalytic site to access tau phosphorylation sites, particularly pThr205, pThr212, pSer262, and pSer409 [4].
The catalytic subunit C-terminus is subject to critical post-translational modifications:
PP2A (specifically PP2A-B55α) dephosphorylates tau at virtually all disease-relevant phosphorylation sites, including:
The B55α regulatory subunit directly contacts a groove on tau's proline-rich region, positioning the catalytic C subunit to dephosphorylate adjacent phospho-epitopes. PP2A works in direct opposition to [GSK-3β[/entities/[gsk3-beta[/entities/[gsk3-beta[/entities/[gsk3-beta[/entities/[gsk3-beta--TEMP--/entities)--FIX-- and [CDK5[/entities/[cdk5[/entities/[cdk5[/entities/[cdk5[/entities/[cdk5--TEMP--/entities)--FIX--, which phosphorylate many of the same sites [1].
In healthy [neurons[/entities/[neurons[/entities/[neurons[/entities/[neurons[/entities/[neurons--TEMP--/entities)--FIX--, the balance between tau kinase activity (principally [GSK-3β[/entities/[gsk3-beta[/entities/[gsk3-beta[/entities/[gsk3-beta[/entities/[gsk3-beta--TEMP--/entities)--FIX--, [CDK5[/entities/[cdk5[/entities/[cdk5[/entities/[cdk5[/entities/[cdk5--TEMP--/entities)--FIX--, DYRK1A, and CK1) and phosphatase activity (principally PP2A, with minor contributions from PP1, PP2B/calcineurin, and PP5) maintains tau in a predominantly dephosphorylated state compatible with microtubule binding and axonal transport. In AD, this balance is disrupted by both increased kinase activity and decreased PP2A activity, resulting in a net shift toward hyperphosphorylation [5].
Multiple lines of evidence demonstrate PP2A impairment in AD:
SET (also called I2PP2A or TAF-Iβ) is a potent endogenous PP2A inhibitor that is pathologically altered in AD:
Transgenic mice overexpressing cytoplasmic SET develop cardinal AD features: tau hyperphosphorylation, [amyloid-beta[/entities/[amyloid-beta[/entities/[amyloid-beta[/entities/[amyloid-beta[/entities/[amyloid-beta--TEMP--/entities)--FIX-- deposition, neurodegeneration, and cognitive deficits [7].
CIP2A, originally characterized in cancer, is upregulated in AD brain and inhibits PP2A activity. It preferentially inhibits the PP2A-B56 holoenzyme.
[Aβ[/entities/[amyloid-beta[/entities/[amyloid-beta[/entities/[amyloid-beta[/entities/[amyloid-beta--TEMP--/entities)--FIX-- oligomers directly impair PP2A through multiple mechanisms:
Sodium selenate is the most advanced PP2A-activating compound in clinical development for AD:
The S1P receptor modulator fingolimod activates PP2A by blocking SET/I2PP2A. Originally approved for [multiple sclerosis[/diseases/[multiple-sclerosis[/diseases/[multiple-sclerosis[/diseases/[multiple-sclerosis[/diseases/[multiple-sclerosis--TEMP--/diseases)--FIX--, it is being explored for neuroprotective effects in AD and other [neurodegenerative diseases[/[diseases[/[diseases[/[diseases[/[diseases[/[diseases[/[diseases[/[diseases[/diseases.
A breakthrough approach in 2024 utilized dephosphorylation-targeting chimera (DEPTAC) technology:
PP2A dephosphorylates [α-synuclein[/proteins/[alpha-synuclein[/proteins/[alpha-synuclein[/proteins/[alpha-synuclein[/proteins/[alpha-synuclein--TEMP--/proteins)--FIX-- at Ser129, the phosphorylation site associated with Lewy body formation. PP2A dysfunction contributes to α-synuclein hyperphosphorylation and aggregation in PD.
[FTD[/diseases/[ftd[/diseases/[ftd[/diseases/[ftd[/diseases/[ftd--TEMP--/diseases)--FIX-- with tau mutations ([MAPT[/genes/[mapt[/genes/[mapt[/genes/[mapt[/genes/[mapt--TEMP--/genes)--FIX-- mutations) may involve altered PP2A–tau interaction. Some FTD-associated tau mutations impair PP2A binding, reducing dephosphorylation efficiency.
PP2A activity is reduced in [basal ganglia[/brain-regions/[basal-ganglia[/brain-regions/[basal-ganglia[/brain-regions/[basal-ganglia[/brain-regions/[basal-ganglia--TEMP--/brain-regions)--FIX-- and [brainstem[/brain-regions/[brainstem[/brain-regions/[brainstem[/brain-regions/[brainstem[/brain-regions/[brainstem--TEMP--/brain-regions)--FIX-- in [PSP[/diseases/[psp[/diseases/[psp[/diseases/[psp[/diseases/[psp--TEMP--/diseases)--FIX--, contributing to 4R-tau hyperphosphorylation characteristic of this tauopathy.
PP2A plays important roles in synaptic function beyond tau regulation:
The study of Pp2A has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying [mechanisms of neurodegeneration[/[mechanisms[/[mechanisms[/[mechanisms[/[mechanisms[/[mechanisms[/[mechanisms[/[mechanisms[/mechanisms and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.
Protein Phosphatase 2A (PP2A) is a major serine/threonine phosphatase that regulates numerous cellular processes including cell cycle progression, metabolism, and neuronal signaling. In Alzheimer's disease, PP2A activity is reduced, contributing to hyperphosphorylation of tau and abnormal cell cycle re-entry in [neurons[/entities/[neurons[/entities/[neurons[/entities/[neurons[/entities/[neurons--TEMP--/entities)--FIX--. The PPP2R2A gene encoding the PP2A B subunit is genetically associated with AD risk. Restoring PP2A activity represents a therapeutic strategy for AD, with compounds that activate PP2A or inhibit its endogenous inhibitors (like SET/I2PP2A) in development. PP2A also plays important roles in synaptic plasticity and dopamine signaling, making it relevant for Parkinson's disease as well.