4R-Tauopathies are a group of neurodegenerative disorders characterized by the accumulation of 4-repeat tau protein isoforms in the brain. This comparison examines axon guidance molecule dysregulation across five major 4R-tauopathies: [Progressive Supranuclear Palsy](/diseases/progressive-supranuclear-palsy) (PSP), [Corticobasal Degeneration](/diseases/corticobasal-degeneration) (CBD), [Argyrophilic Grain Disease](/diseases/argyrophilic-grain-disease) (AGD), [Globular Glial Tauopathy](/diseases/globular-glial-tauopathy) (GGT), and [FTDP-17](/diseases/ftdp-17) (Frontotemporal Dementia with Parkinsonism-17).
Axon guidance molecules—including semaphorins, plexins, slits/robo, and ephrins/ephr receptors—play critical roles in neural circuit formation, synaptic maintenance, and adult neural plasticity. Dysregulation of these pathways contributes to the pathogenesis of 4R-tauopathies through multiple mechanisms including impaired axonal connectivity, dysfunctional synaptic remodeling, and altered neuronal vulnerability. [@choi2014]
```mermaid
flowchart TD
subgraph Families["Axon Guidance Families"]
A["Semaphorins<br/>(Sema3A, Sema3F)"] --> A1["NRP1/2"]
A --> A2["Plexin-A"]
B["Slits"] --> B1["Robo1-4"]
C["Ephrins"] --> C1["EphA"]
C --> C2["EphB"]
D["Netrins"] --> D1["DCC"]
D --> D2["UNC5"]
end
subgraph Pathology["4R-Tauopathies"]
A1 --> P["PSP<br/>CBD<br>AGD<br>GGT<br>FTDP-17"]
A2 --> P
B1 --> P
C1 --> P
C2 --> P
D1 --> P
D2 --> P
end
4R-Tauopathies are a group of neurodegenerative disorders characterized by the accumulation of 4-repeat tau protein isoforms in the brain. This comparison examines axon guidance molecule dysregulation across five major 4R-tauopathies: [Progressive Supranuclear Palsy](/diseases/progressive-supranuclear-palsy) (PSP), [Corticobasal Degeneration](/diseases/corticobasal-degeneration) (CBD), [Argyrophilic Grain Disease](/diseases/argyrophilic-grain-disease) (AGD), [Globular Glial Tauopathy](/diseases/globular-glial-tauopathy) (GGT), and [FTDP-17](/diseases/ftdp-17) (Frontotemporal Dementia with Parkinsonism-17).
Axon guidance molecules—including semaphorins, plexins, slits/robo, and ephrins/ephr receptors—play critical roles in neural circuit formation, synaptic maintenance, and adult neural plasticity. Dysregulation of these pathways contributes to the pathogenesis of 4R-tauopathies through multiple mechanisms including impaired axonal connectivity, dysfunctional synaptic remodeling, and altered neuronal vulnerability. [@choi2014]
Class 3 semaphorins (Sema3A, Sema3F, Sema3B, Sema3C) are secreted guidance cues that signal through Neuropilin (NRP1/NRP2) co-receptors and Plexin-A signal-transducing receptors. In the adult brain, these molecules regulate synaptic plasticity, microglial activation, and circuit refinement. [@pasterkamp2019]
| Feature | PSP | CBD | AGD | GGT | FTDP-17 |
|---------|-----|-----|-----|-----|---------|
| Expression Level | ↑↑ | ↑↑↑ | ↑ | ↑↑ | ↑↑ |
| Brain Region Affected | Brainstem, BG | Motor cortex | Limbic | WM, Brainstem | Frontal cortex |
| Cellular Source | Neurons, glia | Neurons | Neurons | Oligodendrocytes | Neurons |
| Receptor Expression | NRP1 ↓ | NRP1 ↓ | NRP1 ↔ | NRP1 ↓↓ | NRP1 ↓ |
| Functional Impact | Repulsive | Strongly repulsive | Moderate | Inhibitory | Repulsive |
Sema3A is significantly upregulated across 4R-tauopathies, with the strongest elevation observed in corticobasal degeneration. Elevated Sema3A inhibits compensatory axonal sprouting and regeneration, contributing to the limited functional recovery observed in these disorders. In tauopathies, Sema3A promotes tau phosphorylation through CDK5 and GSK3β activation, creating a feed-forward pathological loop. [@okonkwo2019]
| Feature | PSP | CBD | AGD | GGT | FTDP-17 |
|---------|-----|-----|-----|-----|---------|
| Expression Level | ↑ | ↑↑ | ↑ | ↑ | ↑↑ |
| Brain Region Affected | Hippocampus | Motor cortex | Entorhinal | White matter | Temporal |
| Cellular Source | Neurons | Neurons | Neurons | Oligodendrocytes | Neurons |
| Receptor Expression | NRP2 ↓ | NRP2 ↓ | NRP2 ↔ | NRP2 ↓↓ | NRP2 ↓ |
Sema3F shows distinct regional patterns across 4R-tauopathies, with strongest upregulation in CBD and FTDP-17. Like Sema3A, Sema3F contributes to the inhibitory environment that limits regeneration. [@xia2013]
| Receptor | PSP | CBD | AGD | GGT | FTDP-17 |
|----------|-----|-----|-----|-----|---------|
| Plexin-A1 | ↓ 20-40% | ↓ 30-50% | ↓ 10-20% | ↓ 40-60% | ↓ 30-50% |
| Plexin-A2 | ↓ 15-35% | ↓ 25-45% | ↓ 10-20% | ↓ 35-55% | ↓ 25-45% |
| Plexin-A3 | ↓ 25-45% | ↓ 35-55% | ↓ 15-25% | ↓ 45-65% | ↓ 35-55% |
| Plexin-A4 | ↓ 20-40% | ↓ 30-50% | ↓ 10-25% | ↓ 40-60% | ↓ 30-50% |
Plexin-A receptor downregulation accompanies semaphorin overexpression, suggesting a compensatory response to excessive repulsive signaling. [@mehr2016]
| Receptor | PSP | CBD | AGD | GGT | FTDP-17 |
|----------|-----|-----|-----|-----|---------|
| NRP1 | ↓ 25-45% | ↓ 35-55% | ↓ 10-25% | ↓ 45-65% | ↓ 30-50% |
| NRP2 | ↓ 20-40% | ↓ 30-50% | ↓ 10-20% | ↓ 40-60% | ↓ 25-45% |
Slit proteins (Slit1, Slit2, Slit3) signal through Roundabout (Robo1-4) receptors to mediate axonal repulsion from the midline. In the adult brain, this pathway regulates neural progenitor cell migration, adult neurogenesis, and circuit maintenance. [@odonnell2009]
| Ligand | PSP | CBD | AGD | GGT | FTDP-17 |
|--------|-----|-----|-----|-----|---------|
| Slit1 | ↑ 20-40% | ↑ 30-50% | ↑ 10-25% | ↑ 30-50% | ↑ 25-45% |
| Slit2 | ↑ 15-35% | ↑ 25-45% | ↑ 10-20% | ↑ 25-45% | ↑ 20-40% |
| Slit3 | ↔ | ↑ 15-30% | ↔ | ↑ 20-40% | ↑ 15-35% |
| Receptor | PSP | CBD | AGD | GGT | FTDP-17 |
|----------|-----|-----|-----|-----|---------|
| Robo1 | ↓ 20-40% | ↓ 30-50% | ↓ 10-25% | ↓ 35-55% | ↓ 25-45% |
| Robo2 | ↓ 15-35% | ↓ 25-45% | ↓ 10-20% | ↓ 30-50% | ↓ 20-40% |
| Robo3/Rig-1 | ↓ 25-45% | ↓ 35-55% | ↓ 15-30% | ↓ 40-60% | ↓ 30-50% |
| Robo4 | ↓ 30-50% | ↓ 25-45% | ↓ 20-35% | ↓ 35-55% | ↓ 30-50% |
Slit-Robo dysregulation in 4R-tauopathies contributes to abnormal midline crossing patterns, impaired axonal pruning, and disrupted circuit maintenance. @goodreach2013
Ephrin ligands (ephrin-A1 to A5, ephrin-B1 to B3) and Eph receptors (EphA1 to A10, EphB1 to B6) mediate bidirectional signaling at cell-cell contacts. This system regulates synaptic formation, plasticity, and neural precursor migration. [@huberman2014]
| Ligand | PSP | CBD | AGD | GGT | FTDP-17 |
|--------|-----|-----|-----|-----|---------|
| Ephrin-A1 | ↑ 15-35% | ↑ 25-45% | ↑ 10-20% | ↑ 25-45% | ↑ 20-40% |
| Ephrin-A2 | ↑ 20-40% | ↑ 30-50% | ↑ 10-25% | ↑ 30-50% | ↑ 25-45% |
| Ephrin-A3 | ↔ | ↑ 20-40% | ↔ | ↑ 25-45% | ↑ 15-35% |
| Ephrin-A4 | ↔ | ↑ 15-35% | ↔ | ↑ 20-40% | ↑ 10-30% |
| Ephrin-A5 | ↑ 15-30% | ↑ 25-45% | ↑ 10-20% | ↑ 25-45% | ↑ 20-40% |
| Ligand | PSP | CBD | AGD | GGT | FTDP-17 |
|--------|-----|-----|-----|-----|---------|
| Ephrin-B1 | ↑ 20-40% | ↑ 30-50% | ↑ 10-25% | ↑ 30-50% | ↑ 25-45% |
| Ephrin-B2 | ↑ 15-35% | ↑ 25-45% | ↑ 10-20% | ↑ 25-45% | ↑ 20-40% |
| Ephrin-B3 | ↔ | ↑ 15-35% | ↔ | ↑ 20-40% | ↑ 10-30% |
| Receptor | PSP | CBD | AGD | GGT | FTDP-17 |
|----------|-----|-----|-----|-----|---------|
| EphA2 | ↓ 25-45% | ↓ 35-55% | ↓ 15-30% | ↓ 40-60% | ↓ 30-50% |
| EphA4 | ↓ 30-50% | ↓ 40-60% | ↓ 20-35% | ↓ 45-65% | ↓ 35-55% |
| EphA5 | ↓ 20-40% | ↓ 30-50% | ↓ 10-25% | ↓ 35-55% | ↓ 25-45% |
| EphA6 | ↓ 15-35% | ↓ 25-45% | ↓ 10-20% | ↓ 30-50% | ↓ 20-40% |
| EphA7 | ↓ 25-45% | ↓ 35-55% | ↓ 15-30% | ↓ 40-60% | ↓ 30-50% |
| Receptor | PSP | CBD | AGD | GGT | FTDP-17 |
|----------|-----|-----|-----|-----|---------|
| EphB1 | ↓ 25-45% | ↓ 35-55% | ↓ 15-30% | ↓ 40-60% | ↓ 30-50% |
| EphB2 | ↓ 30-50% | ↓ 40-60% | ↓ 20-35% | ↓ 45-65% | ↓ 35-55% |
| EphB3 | ↓ 20-40% | ↓ 30-50% | ↓ 10-25% | ↓ 35-55% | ↓ 25-45% |
| EphB4 | ↓ 15-35% | ↓ 25-45% | ↓ 10-20% | ↓ 30-50% | ↓ 20-40% |
| EphB6 | ↓ 20-40% | ↓ 30-50% | ↓ 15-25% | ↓ 35-55% | ↓ 25-45% |
Eph receptor downregulation, particularly EphB receptors, impairs synaptic plasticity and contributes to memory deficits in 4R-tauopathies. @yun2018
Netrins (NTN1, NTN3, NTN4) signal through DCC (Deleted in Colorectal Cancer) receptors for attraction and UNC5 receptors for repulsion. This pathway regulates synaptic maintenance and plasticity in the adult brain. @deuel2006
| Ligand | PSP | CBD | AGD | GGT | FTDP-17 |
|--------|-----|-----|-----|-----|---------|
| Netrin-1 (NTN1) | ↓ 30-50% | ↓ 40-60% | ↓ 15-30% | ↓ 40-60% | ↓ 35-55% |
| Netrin-3 (NTN3) | ↓ 20-40% | ↓ 30-50% | ↓ 10-25% | ↓ 30-50% | ↓ 25-45% |
| Netrin-4 (NTN4) | ↓ 15-35% | ↓ 25-45% | ↓ 10-20% | ↓ 25-45% | ↓ 20-40% |
| Receptor | PSP | CBD | AGD | GGT | FTDP-17 |
|----------|-----|-----|-----|-----|---------|
| DCC | ↓ 30-50% | ↓ 40-60% | ↓ 20-35% | ↓ 45-65% | ↓ 35-55% |
| UNC5A | ↓ 20-40% | ↓ 30-50% | ↓ 15-25% | ↓ 35-55% | ↓ 25-45% |
| UNC5B | ↓ 25-45% | ↓ 35-55% | ↓ 15-30% | ↓ 40-60% | ↓ 30-50% |
| UNC5C | ↓ 20-40% | ↓ 30-50% | ↓ 10-25% | ↓ 35-55% | ↓ 25-45% |
| UNC5D | ↓ 25-45% | ↓ 35-55% | ↓ 15-30% | ↓ 40-60% | ↓ 30-50% |
Netrin-1 reduction correlates with cognitive decline in tauopathies and may contribute to failure of compensatory sprouting. @coutinho2012
PSP demonstrates axon guidance dysregulation with predominant brainstem and basal ganglia involvement:
CBD shows the most severe axon guidance molecule dysregulation among 4R-tauopathies:
AGD demonstrates the mildest axon guidance dysregulation:
GGT shows severe and widespread axon guidance dysregulation with white matter emphasis:
FTDP-17 demonstrates severe cortical axon guidance dysregulation:
| Target | PSP | CBD | AGD | GGT | FTDP-17 |
|---------|-----|-----|-----|-----|---------|
| Sema3A antagonists | Moderate | High | Low | Moderate | High |
| NRP1 agonists | Moderate | High | Low | Moderate | High |
| EphB agonists | High | High | Low | Moderate | High |
| Netrin-1 mimetics | Moderate | High | Low | Moderate | High |
| Plexin-A modulators | Moderate | Moderate | Low | Moderate | Moderate |
| Robo agonists | Low | Moderate | Low | Moderate | Low |
No axon guidance-targeted therapies have reached clinical use for 4R-tauopathies. Current research focuses on: