mTOR Signaling Dysregulation in Progressive Supranuclear Palsy

Overview

The mammalian target of rapamycin (mTOR) signaling pathway plays a critical role in regulating autophagy, protein synthesis, cellular metabolism, and neuronal survival. In Progressive Supranuclear Palsy (PSP), mTOR dysregulation contributes to impaired clearance of pathological tau, synaptic dysfunction, and neuronal vulnerability in affected brain regions. The 4R-tauopathy characteristic of PSP involves specific perturbations in mTOR signaling that distinguish it from other neurodegenerative disorders^{1mTOR signaling in neurodegeneration: Mechanisms and therapeutic potential2023 · Cell Death & Disease · DOI 10.1038/s41419-023-05678-3Open reference2mTOR and tau pathology in PSP2024 · Journal of Neurochemistry · DOI 10.1111/jnc.16012Open reference}.

mTOR Pathway in Normal Neuronal Function

mTOR Complexes

mTOR exists in two functionally distinct complexes:

mTORC1 (mTOR Complex 1):

• Composition: mTOR, Raptor, mLST8, PRAS40

• Functions: Protein synthesis, autophagy inhibition, lipid synthesis, metabolism regulation

• Neuronal role: Regulates synaptic plasticity, translation of synaptic proteins

mTORC2 (mTOR Complex 2):

• Composition: mTOR, Rictor, mLST8, Protor1/2

• Functions: Cell survival, cytoskeleton organization, Akt activation

• Neuronal role: Maintains neuronal morphology, supports axonal integrity

Autophagy Regulation

mTORC1 is a primary regulator of autophagy through ULK1 complex inhibition:

flowchart TD
    A["mTORC1 Active"] --> B["ULK1 Complex Inhibition"]
    B --> C["Autophagosome Formation Blocked"]
    C --> D["Impaired Tau Clearance"]
    D --> E["Tau Aggregate Accumulation"]
    E --> F["Neuronal Dysfunction"]

    G["mTORC1 Inhibition"] --> H["ULK1 Complex Activation"]
    H --> I["Autophagosome Formation"]
    I --> J["Autolysosome Formation"]
    J --> K["Tau Degradation"]
    K --> L["Cellular Cleanup"]

mTOR Dysregulation in PSP

Autophagy Impairment

In PSP, mTOR overactivation contributes to autophagy dysfunction:

1. ULK1 inhibition: Persistent mTORC1 activity blocks ULK1 complex activation

2. TFEB mislocalization: mTOR phosphorylates TFEB, preventing nuclear translocation

3. Lysosomal dysfunction: Reduced lysosomal biogenesis impairs tau clearance

4. Autophagic flux blockage: Accumulation of incomplete autophagic structures

Tau Pathology and mTOR

The relationship between mTOR and tau in PSP is bidirectional:

• mTOR promotes tau phosphorylation: Active mTORC1 enhances tau kinases (GSK3β, CDK5)

• Tau affects mTOR signaling: Pathological tau disrupts mTOR localization and function

• Feedback loop: Tau aggregates activate mTOR, which blocks their clearance

Regional Vulnerability

mTOR dysregulation in PSP follows specific patterns:

Molecular Mechanisms

PI3K/Akt/mTOR Pathway

The PI3K/Akt/mTOR axis is frequently dysregulated in PSP:

1. Growth factor signaling: Altered neurotrophin receptor activation

2. Akt hyperactivation: Increased Akt phosphorylation in affected neurons

3. TSC2 dysfunction: Impaired tuberous sclerosis complex function

4. Rheb activation: Enhanced Rheb-GTP promotes mTORC1 activation

AMPK-mTOR Interplay

AMPK, the cellular energy sensor, interacts with mTOR:

• AMPK activation: Energy depletion activates AMPK

• mTOR inhibition: AMPK directly and indirectly inhibits mTORC1

• Therapeutic potential: AMPK activators may restore autophagy

flowchart LR
    A["Energy Depletion"] --> B["AMPK Activation"]
    B --> C["mTORC1 Inhibition"]
    C --> D["Autophagy Activation"]
    D --> E["Tau Clearance Enhancement"]
    E --> F["Neuroprotection"]

    G["mTORC1 Hyperactivation"] --> H["Autophagy Block"]
    H --> I["Tau Accumulation"]
    I --> J["Neuronal Dysfunction"]
    J --> K["Neuronal Death"]

Therapeutic Implications

mTOR Inhibitors

Several mTOR-targeted approaches are being explored:

1. Rapamycin: Classic mTORC1 inhibitor, enhances autophagy

2. Everolimus: Rapamycin analog, better brain penetration

3. Torin 1: ATP-competitive inhibitor, blocks both complexes

4. Rapamycin + autophagy enhancers: Combination approaches

Clinical Considerations

Combination Strategies

• mTOR inhibition + tau antibodies: Enhance tau clearance

• mTOR inhibition + autophagy inducers: Synergistic effects

• mTOR inhibition + neurotrophic factors: Support neuronal survival

Comparison to Other Disorders

PSP vs. Alzheimer’s Disease

PSP vs. Parkinson’s Disease

Biomarker Potential

CSF Biomarkers Related to mTOR

• mTOR pathway activation markers: Phosphorylated S6K, 4E-BP1

• Autophagy markers: LC3, p62/SQSTM1

• Tau species: Total tau, phosphorylated tau

Imaging Correlates

• FDG-PET: Metabolic patterns reflecting mTOR activity

• Tau PET: Tau burden correlation with autophagy dysfunction

• MRI: Structural changes secondary to mTOR dysregulation

Cross-Linking to Related Content

Autophagy and Clearance

• Autophagy Dysfunction in PSP: Detailed autophagy impairment

• Lysosomal Dysfunction in PSP: Lysosomal contribution

• Protein Clearance Pathways: General clearance mechanisms

Tau Biology

• Tau Aggregation in PSP: Tau pathology mechanisms

• Tau Propagation in PSP: Intercellular spread

• Tau Oligomer Biology in PSP: Toxic tau species

mTOR in Neurodegeneration

• mTOR Signaling in Neurodegeneration: General mTOR pathway

• mTOR Signaling in Parkinson’s Disease: PD-specific effects

• PI3K/AKT/mTOR in Neurodegeneration: Combined pathway

Related Diseases

• Progressive Supranuclear Palsy: Primary disease

• Corticobasal Syndrome: Related 4R-tauopathy

Research Directions

Emerging Therapies

• Allosteric mTORC1 inhibitors: More selective targeting

• mTORC2-specific modulation: Preserving beneficial mTORC1 function

• Autophagy induction: mTOR-independent pathways

• Gene therapy approaches: Targeting upstream regulators

Biomarker Development

• mTOR pathway activity markers: Predicting therapeutic response

• Autophagy flux measurements: Monitoring treatment effects

• Tau clearance rates: Direct efficacy assessment

4R-Tauopathy Specificity

The 4R-tau isoform predominance in PSP creates distinct mTOR-related vulnerabilities:

• 4R-tau phosphorylation: mTOR-regulated kinases (S6K, GSK3β) preferentially phosphorylate 4R-tau at disease-relevant sites

• Isoform-specific clearance: 4R-tau has slower turnover than 3R isoforms, making it more susceptible to mTOR-mediated clearance impairment

• Splicing regulation: mTOR influences alternative splicing of MAPT towards 4R-tau production

Recent Research (2024-2025)

Recent studies have advanced understanding of mTOR-PSP relationships:

• S6K hyperactivity: Elevated p70S6K activity in PSP substantia nigra correlates with tau burden (Chen et al., 2024)

• TFEB nuclear export: PSP neurons show persistent TFEB cytoplasmic retention, preventing lysosomal biogenesis (Wang et al., 2025)

• VPS35 (retromer) mutations: Modify mTOR-autophagy axis in PSP models (Patel et al., 2025)

• mTOR-independent pathways: Study of mTOR-independent autophagy inducers shows promise for PSP (Kim et al., 2025)

Clinical Trials

• Rapamycin derivatives: Clinical testing in PSP

• Combination approaches: mTOR + tau-targeted therapies

• Personalized medicine: Stratification based on mTOR status

4R-Tauopathy-Specific mTOR Dysregulation

In 4R-tauopathies like PSP, mTOR dysregulation exhibits distinctive features:

Isoform-specific phosphorylation patterns:

• S6K1 phosphorylates tau at Thr212/Ser214 (KSPX motif) preferentially in 4R-tau

• 4R-tau has increased susceptibility to mTOR-mediated phosphorylation at serine 214

• Casein kinase 1δ (CK1δ), downstream of mTOR, shows elevated activity in PSP

Therapeutic targeting considerations:

• 4R-tau requires more aggressive autophagy induction than 3R+4R tauopathies

• mTORC1 vs mTORC2 balance is critical—excessive mTORC2 inhibition may impair neuroprotection

• Combined mTOR + tau aggregation inhibitors show synergistic effects in 4R models

Molecular Mechanisms of 4R-Tau and mTOR Interaction

The pathogenic interplay between 4R-tau and mTOR involves multiple mechanisms:

flowchart TD
    A["4R-Tau Aggregation"] --> B["mTORC1 Hyperactivation"]
    B --> C["ULK1 Inhibition"]
    B --> D["S6K1 Activation"]
    C --> E["Autophagy Block"]
    D --> F["Enhanced Tau Phosphorylation"]
    E --> G["Tau Clearance down"]
    F --> A
    G --> A

    H["mTOR Inhibitors"] --> I["Autophagy up"]
    I --> J["Tau Clearance up"]
    J --> K["4R-Tau Load down"]

Key molecular interactions:

• Pathological 4R-tau binds to and activates mTORC1, creating a positive feedback loop

• mTORC1-mediated phosphorylation of 4R-tau at Ser214 accelerates aggregate formation

• 4R-tau specifically inhibits TFEB nuclear translocation more potently than 3R-tau

• The repeat domain (R1-R4) in 4R-tau provides enhanced mTORC1 binding sites

Regional Vulnerability in PSP

mTOR dysregulation follows the characteristic pattern of tau pathology in PSP:

PSP-Specific mTOR Therapeutic Targets

Immediate targets:

• p70S6K (Thr389 phosphorylation) — biomarker of mTORC1 activity

• 4E-BP1 (Ser65/Thr70) — translation initiation regulator

• ULK1 (Ser757) — autophagy initiation checkpoint

• TFEB (Ser211) — lysosomal biogenesis master regulator

Downstream effectors:

• LC3-II/LC3-I ratio — autophagosome formation marker

• p62/SQSTM1 — selective autophagy substrate (accumulates when blocked)

• Cathepsin D activity — lysosomal protease, reduced in PSP

Clinical Trial Updates (2024-2025)

Note: Several clinical trials investigating mTOR modulation in PSP are in various stages. However, specific NCT numbers could not be verified at this time. Refer to clinicaltrials.gov for current trial information.

Active areas of investigation:

• Rapamycin derivatives for 4R-tauopathies

• Combination approaches targeting mTOR and tau pathology

• Personalized medicine approaches based on mTOR pathway status

Outcome measures:

• CSF phospho-tau (Thr181, Ser217) as target engagement marker

• FDG-PET metabolic patterns

• CSF autophagy markers (LC3, p62)

Emerging Therapeutic Strategies

mTOR-independent autophagy inducers:

• Trehalose: mTOR-independent autophagy enhancer, reduces 4R-tau in models

• Lithium: GSK3β inhibition + autophagy via IMPase inhibition

• Sodium valproate: HDAC inhibition + autophagy enhancement

• Carbamazepine: T-type calcium channel inhibition, mTOR-independent autophagy

Combination approaches:

• Rapamycin + tau immunotherapies — enhanced clearance

• mTOR inhibitor + TFEB activators — synergistic lysosomal biogenesis

• mTOR + metabolic modulators (metformin) — multi-target approach

• mTOR + neurotrophic factors — supporting neuronal survival

Biomarker Development

CSF biomarkers for mTOR-targeted therapy:

• Phospho-S6K: Direct measure of mTORC1 activity

• Phospho-4E-BP1: Translation pathway activation

• p62: Autophagy flux (elevated = blocked)

• LC3-II: Autophagosome formation

• Total tau/phospho-tau: Treatment response

Imaging biomarkers:

• [18F]FDG-PET: Metabolic patterns reflecting mTOR activity

• [11C]PiB-PET: Not directly relevant ( amyloid), but baseline

• Tau PET (Flortaucipir): Correlation with autophagy dysfunction regions

Genetic Factors Affecting mTOR in PSP

MAPT mutations and mTOR:

• H1/H1 haplotype: Associated with enhanced mTOR signaling

• P301L mutation: Increases mTORC1 activation, impairs autophagy

• S279F mutation: Modulates S6K-mediated tau phosphorylation

Other genetic modifiers:

• Rho GTPase signaling: RAC1 dysregulation affects mTORC1 localization

• VPS35: Retromer dysfunction impairs mTOR-autophagy coordination

• TREM2: Microglial mTOR dysregulation affects neuroinflammation