The PI3K/AKT signaling pathway is a critical pro-survival cascade that regulates neuronal survival, metabolism, synaptic plasticity, and protein homeostasis[@brazil2024]. Dysregulation of this pathway significantly contributes to neuronal death in Alzheimer's disease (AD), Parkinson's disease (PD), and other neurodegenerative disorders[@gong2023]. The pathway represents a crucial intersection between neurotrophic factor signaling and cellular survival mechanisms, making it a central focus for understanding neurodegeneration and developing therapeutic interventions[@kimelberg2022].
AKT (also known as PKB) is a serine/threonine protein kinase that promotes cell survival through multiple downstream effectors[@manning2007]. The PI3K/AKT signaling cascade is one of the most important cell survival pathways in neurons, linking extracellular growth factor signals to intracellular survival programs[@hetman2004]. This pathway is particularly important in the central nervous system, where post-mitotic neurons require robust survival signaling to maintain function throughout the lifespan[@zhang2024].
The pathway is activated by various extracellular signals that bind to receptor tyrosine kinases (RTKs) or cytokine receptors[@cheng2023]:
Growth Factors:
- BDNF (Brain-Derived Neurotrophic Factor): Activates TrkB receptor, providing critical survival signals for cortical and hippocampal neurons[@liu2023]
- IGF-1 (Insulin-like Growth Factor 1): Regulates neuronal metabolism and survival through IGF-1 receptor signaling[@wang2024]
- NGF (Nerve Growth Factor): Essential for sympathetic and sensory neuron survival, activates TrkA signaling[@cohen2023]
- GDNF (Glial Cell Line-Derived Neurotrophic Factor): Critical for dopaminergic neuron survival in the substantia nigra pars compacta[@ma2023]
Cytokines:
- IL-6 family cytokines activate the pathway through GP130 receptor signaling[@huang2024]
- TNF can activate PI3K/AKT in certain cellular contexts, with complex pro-survival and pro-inflammatory effects[@sanchez2023]
Insulin Signaling:
- Insulin receptor activation provides metabolic regulation and survival signaling[@xu2024]
- Cross-talk between neuronal insulin signaling and neurotrophic pathways is important for cognitive function[@li2023]
¶ PI3K Activation and Lipid Signaling
Class I PI3K Isoforms:
The class I PI3K isoforms are heterodimers consisting of a p85 regulatory subunit and a p110 catalytic subunit[@chen2024]:
- PI3Kα (PIK3CA): Contains p110α catalytic subunit, broadly expressed and important for growth factor signaling[@park2023]
- PI3Kβ (PIK3CB): Contains p110β, primarily expressed in blood cells and some neuronal populations[@zhang2024a]
- PI3Kγ (PIK3CG): Predominantly in immune cells, involved in inflammatory responses[@watanabe2023]
- PI3Kδ (PIK3CD): Leukocyte-specific isoform[@sun2024]
Molecular Mechanism:
The activation sequence proceeds as follows[@liu2023a]:
- Ligand binding induces RTK autophosphorylation on tyrosine residues
- The p85 regulatory subunit of PI3K binds to phosphotyrosine motifs via its SH2 domains
- This recruitment positions the p110 catalytic subunit at the plasma membrane
- PI3K phosphorylates phosphatidylinositol 4,5-bisphosphate (PIP2) to generate phosphatidylinositol 3,4,5-trisphosphate (PIP3)
- PIP3 recruits AKT and PDK1 to the plasma membrane through their PH domains
The lipid phosphatase PTEN (Phosphatase and Tensin Homolog) opposes PI3K activity by dephosphorylating PIP3 back to PIP2, providing crucial negative regulation of the pathway[@thompson2024].
¶ AKT Activation and Kinase Cascade
PDK1 (3-Phosphoinositide-Dependent Protein Kinase-1):
PDK1 is essential for AKT activation through phosphorylation at Thr308 in the activation loop[@koh2023]:
- PDK1 phosphorylates AKT at Thr308, providing partial activation
- Membrane recruitment is necessary for PDK1-mediated phosphorylation
- PDK1 activity is constitutive, but membrane localization ensures proper timing
mTORC2 (mTOR Complex 2):
mTORC2 phosphorylates AKT at Ser473 in the hydrophobic motif[@vanhaesebroeck2020]:
- This phosphorylation is required for full AKT activation
- mTORC2 regulates AKT substrate specificity
- Growth factor signaling enhances mTORC2 activity
Three AKT Isoforms:
AKT exists in three isoforms with distinct tissue distributions[@stambolic2001]:
- AKT1 (PKBα): Widely expressed, important for embryonic development
- AKT2 (PKBβ): Important for metabolic functions
- AKT3 (PKBγ): Highly expressed in brain, crucial for neuronal function
GSK-3β (Glycogen Synthase Kinase-3 Beta):
GSK-3β is a critical downstream target of AKT[@sarbassov2006]:
- AKT phosphorylates GSK-3β at Ser9, inhibiting its kinase activity
- This provides a key link between PI3K/AKT signaling and tau phosphorylation
- GSK-3β dysregulation contributes to both amyloid and tau pathology in AD[@atkins2022]
BAD (BCL2-Associated Agonist of Cell Death):
BAD is a pro-apoptotic BH3-only protein[@jope2022]:
- AKT phosphorylates BAD at Ser136, promoting its sequestration by 14-3-3 proteins
- This prevents BAD from inhibiting anti-apoptotic BCL-2 proteins
- Neuronal survival requires BAD inactivation through phosphorylation[@cheng2022]
FOXO Transcription Factors:
FOXOs are transcription factors that promote pro-apoptotic gene expression[@calnan2008]:
- AKT phosphorylates FOXO1 and FOXO3a, promoting their cytoplasmic retention
- Phosphorylated FOXOs are sequestered in the cytoplasm by 14-3-3 proteins
- This prevents transcription of genes like BIM, PUMA, and FasL[@kim2009]
mTOR (mammalian Target of Rapamycin):
mTOR is a central regulator of cell growth and metabolism[@saxton2017]:
- AKT activates mTORC1 through multiple mechanisms (TSC2 inhibition, PRAS40 phosphorylation)
- mTORC1 regulates protein synthesis through S6K1 and 4E-BP1
- mTORC1 also inhibits autophagy, linking growth factor signaling to protein homeostasis[@kuma2022]
CREB (cAMP Response Element-Binding Protein):
AKT can phosphorylate and activate CREB[@carlezon2000]:
- CREB activation promotes expression of survival genes
- CREB-mediated transcription is important for neuronal plasticity and memory
- BDNF expression is partly regulated by CREB[@lonze2002]
Multiple alterations in the PI3K/AKT pathway characterize Alzheimer's disease brain[@talbot2012]:
Reduced AKT Signaling:
- Decreased AKT phosphorylation at both Thr308 and Ser473 in AD hippocampus[@liu2023b]
- Impaired PI3K activity in cortical and hippocampal regions
- Reduced growth factor signaling through TrkB and IGF-1 receptors
PTEN Upregulation:
- Increased PTEN expression in AD brain correlates with reduced PIP3 levels
- PTEN mutations or inhibitors protect against amyloid-β toxicity in models[@knafo2022]
Growth Factor Decline:
- Reduced BDNF levels in AD hippocampus and cortex[@peng2023]
- Impaired IGF-1 signaling contributes to neuronal vulnerability
- Decreased neurotrophic support exacerbates neurodegeneration
Amyloid-β Effects:
Amyloid-β (Aβ) impairs PI3K/AKT signaling through multiple mechanisms[@shankar2008]:
- Aβ oligomers inhibit PI3K activity at synapses
- Synaptic PI3K/AKT dysfunction contributes to memory deficits
- Aβ-induced oxidative stress inactivates AKT signaling
Tau Pathology:
The relationship between PI3K/AKT and tau is complex[@avila2006]:
- AKT regulates GSK-3β activity, which directly phosphorylates tau
- Tau pathology disrupts postsynaptic signaling including PI3K/AKT
- Hyperphosphorylated tau may sequester AKT, impairing its function[@li2022]
Synaptic Dysfunction:
PI3K/AKT critically regulates synaptic plasticity[@horwood2023]:
- AKT regulates AMPA receptor trafficking during LTP
- Synaptic PI3K/AKT signaling is required for memory consolidation
- Synaptic deficits in AD correlate with PI3K/AKT dysregulation
AKT Activators:
Direct and indirect strategies to activate AKT are being explored[@lassen2022]:
- Phosphatase inhibitors that preserve AKT phosphorylation
- Growth factor mimetics that enhance upstream signaling
- Allosteric AKT activators in development
GSK-3 Inhibitors:
Targeting downstream GSK-3β offers therapeutic potential[@martinez2021]:
- Reduces tau phosphorylation and aggregation
- Improves cognitive function in AD models
- Multiple inhibitors in clinical trials for AD and bipolar disorder[@avila2022]
mTOR Modulators:
mTOR inhibitors like rapamycin show neuroprotective effects[@bove2011]:
- Induction of autophagy to clear protein aggregates
- Enhanced clearance of Aβ through autophagy
- Potential for combination with other therapeutic approaches[@liu2024]
The PI3K/AKT pathway is particularly important for dopaminergic neuron survival[@surmeier2017]:
- High basal PI3K/AKT activity in substantia nigra pars compacta (SNc)
- Dopaminergic neurons are vulnerable when pathway is compromised
- Growth factor dependence makes these neurons susceptible to PI3K/AKT dysfunction
GDNF provides critical survival signaling for dopaminergic neurons[@airavaara2019]:
- GDNF activates RET receptor tyrosine kinase
- PI3K/AKT signaling is the primary survival pathway downstream of RET
- GDNF and related factors have been tested clinically in PD patients[@bartus2013]
α-Synuclein pathology affects PI3K/AKT signaling[@chen2022]:
- α-Synuclein oligomers impair PI3K/AKT signaling
- Reduced neuronal survival signaling in PD models
- PI3K/AKT dysregulation may contribute to α-synuclein propagation
PI3K/AKT regulates mitochondrial function and dynamics[@rosario2022]:
- AKT promotes glucose uptake and mitochondrial biogenesis
- Mitochondrial dynamics are regulated through AKT signaling
- Anti-apoptotic effects include regulation of BCL-2 family proteins
PI3K/AKT signaling alterations in ALS include[@van2023]:
- Motor neuron vulnerability related to growth factor dependence
- Mutations in PI3K pathway genes identified in familial ALS
- Growth factor therapy approaches showing promise in models
Mutant huntingtin affects PI3K/AKT signaling[@twomey2023]:
- Impaired PI3K/AKT signaling contributes to neuronal dysfunction
- Therapeutic targeting of the pathway shows benefits in models
- Cross-talk with mutant huntingtin pathology
The pathway affects oligodendrocyte survival and myelin repair[@flores2022]:
- PI3K/AKT promotes oligodendrocyte progenitor cell survival
- Myelin repair mechanisms require AKT signaling
- Immune modulation through PI3K/AKT affects disease course
¶ Autophagy and Protein Homeostasis
AKT activates mTORC1, which regulates autophagy[@nixon2003]:
- mTORC1 inhibits autophagy initiation through ULK1 phosphorylation
- Autophagy inhibition by mTORC1 contributes to protein aggregate accumulation
- Dysregulated autophagy is a hallmark of neurodegenerative diseases
Modulating autophagy through PI3K/AKT has therapeutic potential[@rubinsztein2022]:
- mTOR inhibitors (rapamycin, everolimus) induce autophagy
- PI3K inhibitors have complex effects, depending on isoform selectivity
- Autophagy enhancers targeting downstream nodes show promise
| Compound |
Target |
Status |
Notes |
| GSK-3 inhibitors |
GSK-3β |
Clinical trials |
AD, bipolar disorder |
| Rapamycin |
mTORC1 |
Approved |
Immunosuppression, being repurposed |
| AKT inhibitors |
AKT |
Clinical trials |
Cancer applications |
| PI3K modulators |
PI3K |
Preclinical |
Pathway modulation |
Multiple growth factor approaches target PI3K/AKT signaling[@haggerty2023]:
- BDNF delivery through various routes
- IGF-1 therapy in clinical trials
- GDNF for PD has reached clinical testing[@nutt2013]
Viral vector-mediated gene delivery shows promise[@kells2024]:
- AAV-mediated AKT1 overexpression protects neurons
- Growth factor expression via viral vectors
- Combination approaches targeting multiple nodes
AMPK and PI3K/AKT share complex regulatory interactions[@hardie2012]:
- AMPK activation can inhibit mTORC1, complementing PI3K/AKT effects
- Energy sensing integrates with growth factor signaling
- Therapeutic targeting must consider cross-talk
PI3K/AKT and MAPK pathways intersect at multiple points[@huang2001]:
- Both pathways are activated by similar growth factors
- Cross-talk can be synergistic or antagonistic
- Combined targeting may provide benefits
The PI3K/AKT signaling pathway represents a central hub connecting neurotrophic factor signaling to neuronal survival, metabolic regulation, and protein homeostasis. Dysregulation of this pathway contributes to the pathogenesis of Alzheimer's disease, Parkinson's disease, and other neurodegenerative disorders. The pathway's importance is underscored by its multiple connections to key pathological features including amyloid-β toxicity, tau phosphorylation, α-synuclein aggregation, and mitochondrial dysfunction. Therapeutic strategies targeting this pathway, including growth factor therapies, GSK-3 inhibitors, and autophagy modulators, hold promise for disease-modifying treatments in neurodegeneration.
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PMID:36578234
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