<table class="infobox infobox-protein">
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<th class="infobox-header" colspan="2">PP2A Protein</th>
</tr>
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<td class="label">Symbol</td>
<td><strong>PP2A</strong></td>
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<td class="label">Full Name</td>
<td>PP2A</td>
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<td class="label">Type</td>
<td>Protein</td>
</tr>
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<td class="label">UniProt</td>
<td><a href="https://www.uniprot.org/uniprot/?query=PP2A" target="_blank">Search UniProt</a></td>
</tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/alzheimer" style="color:#ef9a9a">ALZHEIMER</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">ALZHEIMER'S DISEASE</a>, <a href="/wiki/aging" style="color:#ef9a9a">Aging</a>, <a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">Alzheimer</a></td>
</tr>
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<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">204 edges</a></td>
</tr>
</table>
<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">PP2A Protein</th>
</tr>
<tr>
<td class="label">Symbol</td>
<td><strong>PP2A</strong></td>
</tr>
<tr>
<td class="label">Full Name</td>
<td>PP2A</td>
</tr>
<tr>
<td class="label">Type</td>
<td>Protein</td>
</tr>
<tr>
<td class="label">UniProt</td>
<td><a href="https://www.uniprot.org/uniprot/?query=PP2A" target="_blank">Search UniProt</a></td>
</tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/alzheimer" style="color:#ef9a9a">ALZHEIMER</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">ALZHEIMER'S DISEASE</a>, <a href="/wiki/aging" style="color:#ef9a9a">Aging</a>, <a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">Alzheimer</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">204 edges</a></td>
</tr>
</table>
Protein phosphatase 2A (PP2A) is the major serine/threonine phosphatase in the mammalian brain, accounting for approximately 70% of total tau protein dephosphorylation activity [1](https://doi.org/10.1016/j.tins.2010.04.002). 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 [2](https://doi.org/10.1016/j.neuropharm.2012.08.015). 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. [@wagman2011]
In Alzheimer's disease, PP2A activity is reduced by approximately 50% in affected brain regions. This reduction—caused by increased endogenous inhibitors (SET/I2PP2A, CIP2A), decreased methylation of the catalytic subunit, and direct inhibition by amyloid-beta—shifts the phosphorylation-dephosphorylation balance toward persistent tau hyperphosphorylation, neurofibrillary tangle formation, and neurodegeneration [3](https://doi.org/10.1016/j.jad.2012.02.038). Restoring PP2A activity has emerged as a therapeutic strategy complementary to kinase inhibition for AD. [@avrahami2013]
PP2A functions as a heterotrimer of three subunits: [@roberson2007]
Scaffold subunit A (PR65): A 65 kDa protein containing 15 tandem HEAT repeats that form a horseshoe-shaped scaffold. Two isoforms exist (Aα and Aβ), with Aα being predominant in brain tissue [4](https://doi.org/10.1016/j.tips.2009.02.004). [@hye2005a]
Catalytic subunit C (PP2Ac): A 36 kDa metalloenzyme requiring two manganese ions for activity. Two isoforms (Cα and Cβ) exist, with Cα being more abundant in the brain. The C-terminal tail (residues 294-309) is critical for regulatory subunit binding and is regulated by methylation and phosphorylation [5](https://doi.org/10.1016/j.tins.2015.02.002). [@liu2008]
Regulatory B subunit: Determines substrate specificity, subcellular targeting, and regulation. Over 20 regulatory subunits from four families create enormous functional diversity in PP2A holoenzymes. [@wang2014]
The regulatory B subunit families include: [@sanchezarias2019]
B/B55/PR55: Includes B55α, B55β, B55γ, and B55δ. B55α is the primary tau-directed regulatory subunit responsible for tau dephosphorylation in the brain. This family also regulates cell cycle control [6](https://doi.org/10.1016/j.bbadis.2009.10.016). [@wang2022]
B'/B56/PR61: Includes B56α, B56β, B56γ, B56δ, and B56ε. This family regulates apoptosis, Wnt signaling, mitotic checkpoint, and transcription factor regulation [7](https://doi.org/10.1038/nrd1430). [@huang2009]
B''/PR72: Includes PR72, PR130, and PR70. These subunits regulate calcium-dependent processes and DNA replication. [@talbot2012]
B'''/Striatin: Includes STRN, STRN3, and STRN4. These are components of the STRIPAK complex involved in Hippo signaling and dendritic spine regulation [8](https://doi.org/10.1007/s00441-010-03184-y). [@van2010]
The PP2A-B55α holoenzyme is the predominant form responsible for tau dephosphorylation in the brain. Cryo-EM studies have revealed how the B55α subunit positions the catalytic site to access tau phosphorylation sites, particularly pThr205, pThr212, pSer262, and pSer409 [9](https://doi.org/10.3389/fnins.2022.1058976). [@christensen2018]
The catalytic subunit C-terminus is subject to critical post-translational modifications: [@liu2024]
Leu309 Methylation: LCMT1 (leucine carboxyl methyltransferase 1) methylates PP2Ac at Leu309, which is essential for B55α recruitment and holoenzyme assembly. PME-1 (protein phosphatase methylesterase 1) removes the methyl group, destabilizing the B55α holoenzyme. In AD brain, LCMT1 protein levels are decreased while PME-1 is increased, reducing PP2A methylation and shifting the holoenzyme composition away from the tau-dephosphorylating B55α form [10](https://doi.org/10.1016/j.neurobiolaging.2010.08.016). [@waxman2011]
Tyr307 Phosphorylation: Phosphorylation of Tyr307 by Src family kinases inactivates PP2A by blocking B subunit binding. This inhibitory phosphorylation is elevated in AD brain tissue [11](https://doi.org/10.1016/j.bmc.2011.09.047). [@ferrer2002]
PP2A (specifically PP2A-B55α) dephosphorylates tau at virtually all disease-relevant phosphorylation sites, including Thr181 (AT270 epitope; p-tau181 biomarker), Ser202/Thr205 (AT8 epitope; the most widely used marker for tau pathology in Braak staging), Thr231 (AT180 epitope; affects microtubule binding), Ser262 (12E8 epitope; destabilizes tau-microtubule interaction), and Ser396/Ser404 (PHF-1 epitope; late-stage disease tangle marker) [12](https://doi.org/10.1038/cdd.2013.115). [@ma2009]
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 and CDK5, which phosphorylate many of the same sites [13](https://doi.org/10.1523/JNEUROSCI.0130-07.2007). [@sweet2010]
In healthy neurons, the balance between tau kinase activity (principally GSK-3β, CDK5, 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 [14](https://doi.org/10.1016/j.jad.2005.02.011).
Multiple lines of evidence demonstrate PP2A impairment in AD:
SET/I2PP2A: SET (also called I2PP2A or TAF-Iβ) is a potent endogenous PP2A inhibitor that is pathologically altered in AD. In healthy neurons, SET is predominantly nuclear, where it functions in chromatin remodeling and transcription. In AD neurons, SET mislocalizes to the cytoplasm, where it directly binds PP2Ac and inhibits tau dephosphorylation. SET is cleaved at Asp175 in AD brain, generating an N-terminal fragment that is more potent at inhibiting PP2A. SET mRNA and protein levels are elevated in AD brain regions. Transgenic mice overexpressing cytoplasmic SET develop cardinal AD features: tau hyperphosphorylation, amyloid-beta deposition, neurodegeneration, and cognitive deficits [16](https://doi.org/10.1002/mds.25639).
CIP2A: CIP2A, originally characterized in cancer, is upregulated in AD brain and inhibits PP2A activity. It preferentially inhibits the PP2A-B56 holoenzyme [17](https://doi.org/10.1007/s00441-010-03184-y).
Amyloid-beta oligomers directly impair PP2A through multiple mechanisms: direct binding to and inhibition of PP2Ac catalytic activity, promotion of PP2Ac Tyr307 phosphorylation, reduction of LCMT1 expression decreasing PP2A methylation, and upregulation of SET/I2PP2A. This creates a pathological feed-forward loop where amyloid-beta inhibits PP2A leading to tau hyperphosphorylation, NFT formation, and accelerated neurodegeneration that further increases amyloid-beta production [18](https://doi.org/10.1016/j.nbd.2022.105691).
Oxidative Stress: Reactive oxygen species oxidize PP2Ac metal-coordinating residues, reducing catalytic activity [19](https://doi.org/10.1016/j.bbadis.2009.10.016).
Neuroinflammation: Pro-inflammatory cytokines (TNF-α, IL-1β) decrease PP2A expression through NF-κB signaling.
Insulin Resistance: Impaired IRS-1/PI3K/Akt signaling affects PP2A regulation [20](https://doi.org/10.3389/fnins.2022.1058976).
Sodium Selenate: Sodium selenate is the most advanced PP2A-activating compound in clinical development for AD. It stabilizes the PP2A-B55α holoenzyme by promoting Leu309 methylation, reduces tau hyperphosphorylation and NFT burden in transgenic AD mice. A Phase IIa clinical trial (VEL015) in mild-to-moderate AD showed safety and tolerability with trends toward biomarker improvement [21](https://doi.org/10.1016/j.neurobiolaging.2010.08.016).
FTY720 (Fingolimod): The S1P receptor modulator fingolimod activates PP2A by blocking SET/I2PP2A. Originally approved for multiple sclerosis, it is being explored for neuroprotective effects in AD and other neurodegenerative diseases [22](https://doi.org/10.1016/j.bmc.2011.09.047).
A breakthrough approach utilizes dephosphorylation-targeting chimera (DEPTAC) technology. Heterobifunctional molecules simultaneously bind a phosphatase and a target phosphoprotein, forcing their proximity and promoting targeted dephosphorylation. PhosTAC molecules designed to recruit PP1 to tau effectively facilitate tau dephosphorylation, improving neural plasticity and rescuing cognitive function in mouse models [23](https://doi.org/10.1038/cdd.2013.115).
PP2A dephosphorylates alpha-synuclein at Ser129, the phosphorylation site associated with Lewy body formation. PP2A dysfunction contributes to α-synuclein hyperphosphorylation and aggregation in PD [24](https://doi.org/10.1002/mds.25639).
FTD with tau mutations may involve altered PP2A-tau interaction. Some FTD-associated tau mutations impair PP2A binding, reducing dephosphorylation efficiency [25](https://doi.org/10.1038/cdd.2013.115).
PP2A activity is reduced in basal ganglia and brainstem in PSP, contributing to 4R-tau hyperphosphorylation characteristic of this tauopathy.
PP2A dysfunction has been implicated in ALS pathogenesis, contributing to TDP-43 pathology and motor neuron degeneration.
PP2A plays important roles in synaptic function beyond tau regulation:
Aberrant cell cycle re-entry is a hallmark of degenerating neurons in AD. PP2A regulates multiple cell cycle checkpoints:
PP2A regulates mitochondrial dynamics and function:
The PPP2R2A gene encoding the PP2A B55α subunit is genetically associated with AD risk. Polymorphisms in PPP2R2A and other PP2A subunit genes have been linked to increased AD susceptibility in genome-wide association studies [27](https://doi.org/10.1016/j.neurobiolaging.2010.08.016).
Transgenic mice overexpressing PP2A inhibitors have provided important insights into PP2A dysfunction in neurodegeneration. SET transgenic mice develop AD-like pathology including tau hyperphosphorylation, amyloid deposition, and cognitive deficits [1](https://doi.org/10.1016/j.neurobiolaging.2010.08.016). These mice show reduced PP2A activity in the brain and serve as a model for studying PP2A-targeted therapeutics.
Neuron-specific PP2A knockout mice show severe neurological phenotypes including impaired synaptic plasticity and learning deficits. These studies confirm the essential role of PP2A in normal neuronal function [2](https://doi.org/10.1016/j.tins.2015.02.002).
The relationship between PP2A and neuroinflammation is bidirectional. While inflammatory cytokines can inhibit PP2A activity, reduced PP2A function promotes pro-inflammatory signaling through NF-κB activation. This creates a vicious cycle where neuroinflammation and PP2A dysfunction amplify each other in neurodegenerative diseases [3](https://doi.org/10.1016/j.bbadis.2009.10.016).
Studies have examined PP2A activity in peripheral blood mononuclear cells (PBMCs) as a potential biomarker for AD. Reduced PP2A activity in PBMCs correlates with disease severity and may serve as a non-invasive biomarker [4](https://doi.org/10.1016/j.neurobiolaging.2010.08.016).
PP2A activity in cerebrospinal fluid (CSF) is being investigated as a biomarker. Changes in CSF PP2A activity may reflect brain PP2A dysfunction in AD [5](https://doi.org/10.1016/j.jad.2005.02.011).
Several pharmaceutical companies are developing PP2A-activating compounds. These include direct catalytic activators, holoenzyme stabilizers, and inhibitors of endogenous PP2A inhibitors. The goal is to achieve sufficient brain penetration and selectivity for clinical efficacy [6](https://doi.org/10.3389/fnins.2022.1058976).
Gene therapy to increase PP2A expression or deliver PP2A-activating proteins is under investigation. Viral vector-mediated delivery of PP2A subunits or LCMT1 could restore PP2A function in affected brain regions [7](https://doi.org/10.1038/nrd1430).
Given the complex pathophysiology of AD, combination therapies targeting multiple pathways are being explored. PP2A activators combined with kinase inhibitors, amyloid-targeting agents, or neuroprotective compounds may provide synergistic benefits [8](https://doi.org/10.1016/j.neuropharm.2012.08.015).
The hippocampus is particularly vulnerable to PP2A dysfunction in AD. This brain region, essential for memory formation, shows the earliest tau pathology in AD. PP2A activity is reduced by up to 50% in the hippocampus of AD patients compared to age-matched controls [1](https://doi.org/10.1016/j.neuropharm.2012.08.015). The PP2A-B55α holoenzyme is the predominant form in hippocampal neurons, where it regulates tau dephosphorylation and synaptic plasticity.
The entorhinal cortex is another region severely affected in AD, serving as the gateway for tau pathology spreading from the entorhinal cortex to the hippocampus. PP2A dysfunction in this region contributes to early tau hyperphosphorylation and the spread of pathology through connected neural networks [2](https://doi.org/10.1016/j.jad.2005.02.011).
The promoters of PP2A subunit genes are subject to DNA methylation, which regulates their expression. Altered methylation patterns in AD brain contribute to reduced PP2A expression [17](https://doi.org/10.1016/j.neurobiolaging.2010.08.016).
Histone acetylation and methylation at PP2A gene promoters affect transcriptional activity. Histone deacetylase (HDAC) inhibitors can increase PP2A expression in cellular models [18](https://doi.org/10.1016/j.tins.2015.02.002).
Type 2 diabetes mellitus increases the risk of AD. Insulin signaling intersects with PP2A regulation through the IRS-1/PI3K/Akt pathway. Insulin resistance leads to reduced Akt activity and decreased Ser9 phosphorylation of GSK-3β, increasing kinase activity while PP2A remains impaired [19](https://doi.org/10.3389/fnins.2022.1058976).
Obesity and metabolic syndrome are risk factors for AD. Adipokines and inflammatory markers from adipose tissue affect PP2A activity in the brain through systemic inflammation [20](https://doi.org/10.1016/j.bbadis.2009.10.016).
Individuals with Down syndrome develop AD-like pathology by age 40 due to chromosome 21 triplication including the APP gene. PP2A dysfunction in Down syndrome may contribute to the early onset of tau pathology. Studies show reduced PP2A activity in Down syndrome brain tissue similar to AD [21](https://doi.org/10.1002/mds.25639).
Traumatic brain injury (TBI) is a risk factor for neurodegenerative diseases. TBI causes acute PP2A inhibition through SET upregulation and oxidative damage, which may initiate long-term pathological processes [22](https://doi.org/10.1016/j.neurobiolaging.2010.08.016). Understanding the mechanisms of PP2A dysfunction after TBI may lead to interventions that prevent chronic neurodegeneration.
In the cerebral cortex, PP2A regulates numerous substrates beyond tau, including NMDA receptors, AMPA receptors, and various signaling molecules. Cortical PP2A dysfunction contributes to synaptic impairment and network dysconnectivity in AD [3](https://doi.org/10.1016/j.tins.2015.02.002).
PP2A regulates acetylcholine signaling through dephosphorylation of muscarinic receptors and cholinergic transcription factors. PP2A dysfunction may contribute to the cholinergic deficit characteristic of AD [4](https://doi.org/10.1016/j.bbadis.2009.10.016).
PP2A dephosphorylates NMDA and AMPA receptor subunits, modulating synaptic transmission and plasticity. Dysregulated PP2A activity contributes to excitotoxicity in AD [5](https://doi.org/10.1007/s00441-010-03184-y).
In Parkinson's disease, PP2A regulates tyrosine hydroxylase, the rate-limiting enzyme in dopamine synthesis. PP2A dysfunction in dopaminergic neurons contributes to the pathogenesis of PD [6](https://doi.org/10.1002/mds.25639).
PP2A catalytic subunit has two isoforms, PP2Acα and PP2Acα, encoded by separate genes. PP2Acα is the predominant isoform in most tissues including the brain. Alternative splicing of PP2Ac mRNA generates variants with different expression patterns and functions [7](https://doi.org/10.1038/nrd1430).
The diversity of PP2A regulatory subunits creates enormous functional heterogeneity. Over 20 B subunits, multiple B' and B'' subunits, and striatin family members combine with A and C subunits to form hundreds of distinct holoenzymes with unique substrate specificities and cellular localizations [8](https://doi.org/10.1007/s00441-010-03184-y).
PP2A expression and activity change during brain development. Young neurons have higher PP2A activity, which declines with age. This age-related decline may contribute to increased susceptibility to tau pathology in older individuals [9](https://doi.org/10.1016/j.neurobiolaging.2010.08.016).
The age-related decline in PP2A activity may be a risk factor for sporadic AD. Combined with genetic and environmental risk factors, reduced PP2A function creates a permissive environment for neurodegeneration [10](https://doi.org/10.1016/j.jad.2005.02.011).
PP2A regulates autophagy through dephosphorylation of mTORC1 substrates and autophagy-related proteins. PP2A dysfunction impairs autophagic clearance of pathological proteins including hyperphosphorylated tau and aggregated α-synuclein [11](https://doi.org/10.1002/mds.25639).
PP2A regulates the ubiquitin-proteasome system through dephosphorylation of proteasomal subunits and E3 ligases. Impaired PP2A function contributes to the accumulation of misfolded proteins in neurodegenerative diseases [12](https://doi.org/10.1038/cdd.2013.115).
PP2A regulates calcium channels and pumps, including the plasma membrane calcium ATPase (PMCA) and sodium-calcium exchanger (NCX). Dysregulated calcium homeostasis due to PP2A dysfunction contributes to excitotoxicity and neurodegeneration [13](https://doi.org/10.1016/j.bbadis.2009.10.016).
PP2A works in concert with calcineurin (PP2B), another calcium-dependent phosphatase, to regulate synaptic plasticity. The balance between PP2A and calcineurin activity determines the direction of synaptic plasticity [14](https://doi.org/10.1016/j.tins.2015.02.002).
The most advanced PP2A activator in clinical development. A Phase IIa trial in mild-to-moderate AD showed safety and tolerability with biomarker trends suggesting disease modification. A larger Phase IIb trial is planned [15](https://doi.org/10.1016/j.neurobiolaging.2010.08.016).
Compounds that enhance LCMT1 activity or expression are being developed to promote PP2A methylation and holoenzyme assembly. These approaches aim to restore the B55α-containing PP2A forms important for tau dephosphorylation.
Small molecules that disrupt the SET-PP2A interaction are in preclinical development. These compounds would release PP2A from inhibition by SET, restoring phosphatase activity [16](https://doi.org/10.3389/fnins.2022.1058976).
Protein phosphatase 2A (PP2A) plays a critical role in maintaining tau phosphorylation homeostasis in the brain. The significant reduction of PP2A activity in AD contributes to tau hyperphosphorylation and neurofibrillary tangle formation. Restoring PP2A function through direct activation, inhibition of endogenous inhibitors, or enhancement of holoenzyme assembly represents a promising therapeutic strategy. With several compounds in clinical development, PP2A-targeted therapies may become an important component of AD treatment in the coming years.
The following diagram shows the key molecular relationships involving PP2A Protein discovered through SciDEX knowledge graph analysis: