The Eph family of receptor tyrosine kinases and their ephrin ligands are key regulators of cell positioning, synaptic plasticity, and developmental patterning. The Eph/ephrin system mediates bidirectional signaling that controls axonal guidance, synapse formation, and neural circuit assembly. In neurodegenerative diseases, Eph/ephrin signaling is dysregulated and contributes to synaptic dysfunction, impaired regeneration, and neuroinflammation. Understanding this pathway offers therapeutic opportunities for conditions like Alzheimer's disease, Parkinson's disease, and stroke.
| Receptor |
Expression |
Primary Ligands |
Key Functions |
| EphA1 |
Various |
Ephrin-A1, A2, A3, A4, A5 |
Development, cancer |
| EphA2 |
Neurons, epithelium |
Ephrin-A1, A2, A3, A4 |
Angiogenesis, adhesion |
| EphA3 |
Brain |
Ephrin-A2, A5 |
Synaptic plasticity |
| EphA4 |
Neurons, glia |
Ephrin-A2, A3, A5, A6 |
Synapse, regeneration |
| EphA5 |
Neurons |
Ephrin-A2, A3, A5 |
Development |
| EphA6 |
Brain |
Ephrin-A2, A3, A5 |
Unknown |
| EphA7 |
Brain |
Ephrin-A2, A3, A4, A5 |
Development |
| EphA8 |
Brain |
Ephrin-A2, A5 |
Development |
| EphB1 |
Various |
Ephrin-B1, B2, B3 |
Development |
| EphB2 |
Neurons |
Ephrin-B1, B2, B3 |
Synapse, memory |
| EphB3 |
Various |
Ephrin-B1, B2, B3 |
Development |
| EphB4 |
Endothelium |
Ephrin-B2 |
Angiogenesis |
| EphB6 |
Thymus |
Ephrin-B1, B2, B3 |
Immune function |
¶ Ephrin Ligands
- Ephrin-A ligands (EFNA1-5): GPI-anchored, bind EphA receptors
- Ephrin-B ligands (EFNB1-3): Transmembrane, bind EphB receptors
The Eph/ephrin system operates through two complementary signaling directions:
Forward Signaling (receptor on signal-receiving cell):
- Ephrin binding activates Eph receptor tyrosine kinase activity
- Autophosphorylation creates docking sites for SH2 and PTB domain proteins
- Activates PI3K/Akt, MAPK/ERK, and Rho GTPase pathways
- Results in cytoskeletal reorganization, cell adhesion, and migration
Reverse Signaling (ephrin on signal-sending cell):
- Ephrin transmembrane ligands recruit PDZ domain proteins and Src family kinases
- Bidirectional communication allows for sophisticated tissue patterning
flowchart TD
subgraph "Forward Signaling"
A["Ephrin-A/B Ligand"] --> B["Eph Receptor (RTK)"]
B --> C["Autophosphorylation"]
C --> D["Adaptor Protein Recruitment"]
D --> E["PI3K/Akt Pathway"]
D --> F["MAPK/ERK Pathway"]
D --> G["Rho GTPases"]
E --> H["Cell Survival<br/>Protein Synthesis"]
F --> I["Proliferation<br/>Differentiation"]
G --> J["Cytoskeleton<br/>Shape Change"]
end
subgraph "Reverse Signaling"
K["Eph Receptor"] --> L["Ephrin Ligand (transmembrane)"]
L --> M["PDZ Domain<br/>Proteins"]
M --> N["Src Family<br/>Kinases"]
N --> O["Signal Transduction"]
end
H --> P["Synaptic<br/>Plasticity"]
I --> P
J --> P
O --> P
| Pathway |
Effectors |
Functions |
| PI3K/Akt |
PDK1, mTOR |
Cell survival, protein synthesis |
| MAPK/ERK |
RAF, MEK, ERK |
Proliferation, differentiation |
| Rho GTPases |
Rac, Rho, Cdc42 |
Cytoskeleton dynamics |
| Src |
Fyn, Src |
Adhesion, migration |
EphB2 is critically involved in synaptic function:
- Synaptic localization: EphB2 is enriched at excitatory synapses
- NMDA receptor interaction: EphB2 regulates NMDA receptor function and trafficking
- A beta-induced dysfunction: A beta reduces EphB2 expression and disrupts signaling
- Memory impairment: Restoring EphB2 reverses memory deficits in AD mouse models
The downregulation of EphB2 by amyloid-beta oligomers contributes to synaptic failure and cognitive decline in AD. Therapeutic approaches aimed at enhancing EphB2 signaling are being explored as potential treatments for synaptic dysfunction in AD.
- Ephrin-A5 modulates amyloid precursor protein (APP) processing
- EphA4 activation contributes to synaptic loss and dendritic spine simplification
- EphA4 antagonists promote synaptic repair and functional recovery
- Therapeutic targeting of EphA4 being explored in preclinical models
¶ Regeneration and Plasticity
- Impaired Eph/ephrin signaling contributes to regenerative failure after injury
- Blocking EphA4 promotes axon regeneration after CNS injury
- Bidirectional signaling regulates neural circuit assembly during development and adulthood
- EphB receptors regulate development of dopaminergic neurons in the substantia nigra
- Axonal guidance molecules influence substantia nigra connectivity and circuit formation
- Eph/ephrin signaling during development may determine vulnerability of dopaminergic neurons
- Dysregulated EphB signaling affects striatal synapse function and plasticity
- Contributes to basal ganglia circuit dysfunction in PD
- Alpha-synuclein pathology may disrupt Eph/ephrin signaling pathways
¶ Role in Stroke and Brain Injury
- EphA/ephrin-A signaling is upregulated after stroke and brain injury
- Mediates post-ischemic inflammation and angiogenesis
- Bidirectional signaling affects both neurons and vasculature
- EphA4 and ephrin-A5 are particularly implicated in post-injury responses
- Manipulating Eph/ephrin signaling can promote or inhibit regeneration
- EphA4 antagonists improve functional recovery after stroke
- EphB2 agonists may enhance synaptic repair
- Peptide-conjugated extracellular vesicles targeting EphA4 rejuvenate aged myelin
- EphB signaling affects motor neuron development and synapse formation at the neuromuscular junction
- Dysregulation contributes to neuromuscular junction disruption in ALS models
- EphA4 expression may influence motor neuron vulnerability
| Approach |
Target |
Status |
Clinical Candidates |
| EphA4 antagonists |
EphA4 |
Preclinical |
sEphA4-Fc, peptide conjugates |
| EphB2 agonists/modulators |
EphB2 |
Preclinical |
EphB2-Fc, EphB2 ligands |
| Soluble Eph receptors |
Multiple |
Preclinical |
sEphA5, sEphB2 |
| Peptide agonists |
Multiple |
Preclinical |
EphA4-targeting peptides |
- Complexity: Multiple receptors and ligands with overlapping functions
- Bidirectional signaling: Both forward and reverse signals must be considered
- Cell-type specificity: Achieving cell-type selective targeting in the CNS
- Blood-brain barrier: Delivery of large protein therapeutics to the CNS
- PI3K/AKT/mTOR: Shared downstream signaling with Eph/ephrin pathways
- Synaptic plasticity: EphB2 interaction with NMDA receptors
- Axonal guidance: Core Eph/ephrin function in neural development
- Neuroinflammation: Eph/ephrin modulation of microglial responses