<div class="infobox">
<div class="infobox-gene">
<div class="gene-symbol">EPHA4</div>
<div class="gene-name">Eph Receptor A4</div>
<table>
<tr><td class="label">Symbol</td><td class="value">EPHA4</td></tr>
<tr><td class="label">Full Name</td><td class="value">Eph Receptor A4</td></tr>
<tr><td class="label">Chromosome</td><td class="value">2q36.1</td></tr>
<tr><td class="label">NCBI Gene</td><td class="value">[1973](https://www.ncbi.nlm.nih.gov/gene/1973)</td></tr>
<tr><td class="label">OMIM</td><td class="value">[602081](https://www.omim.org/entry/602081)</td></tr>
<tr><td class="label">Ensembl</td><td class="value">[ENSG00000049286](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000049286)</td></tr>
<tr><td class="label">UniProt</td><td class="value">[P54764](https://www.uniprot.org/uniprotkb/P54764/entry)</td></tr>
<tr><td class="label">Associated Diseases</td><td class="value">[Alzheimer's disease](/diseases/alzheimers-disease), [ALS](/diseases/als), [stroke](/diseases/stroke)</td></tr>
</table>
</div>
</div>
<div class="infobox">
<div class="infobox-gene">
<div class="gene-symbol">EPHA4</div>
<div class="gene-name">Eph Receptor A4</div>
<table>
<tr><td class="label">Symbol</td><td class="value">EPHA4</td></tr>
<tr><td class="label">Full Name</td><td class="value">Eph Receptor A4</td></tr>
<tr><td class="label">Chromosome</td><td class="value">2q36.1</td></tr>
<tr><td class="label">NCBI Gene</td><td class="value">[1973](https://www.ncbi.nlm.nih.gov/gene/1973)</td></tr>
<tr><td class="label">OMIM</td><td class="value">[602081](https://www.omim.org/entry/602081)</td></tr>
<tr><td class="label">Ensembl</td><td class="value">[ENSG00000049286](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000049286)</td></tr>
<tr><td class="label">UniProt</td><td class="value">[P54764](https://www.uniprot.org/uniprotkb/P54764/entry)</td></tr>
<tr><td class="label">Associated Diseases</td><td class="value">[Alzheimer's disease](/diseases/alzheimers-disease), [ALS](/diseases/als), [stroke](/diseases/stroke)</td></tr>
</table>
</div>
</div>
EPHA4 (Eph Receptor A4) is a member of the Eph receptor tyrosine kinase family that plays crucial roles in neural development, synaptic plasticity, and cellular communication. As a receptor tyrosine kinase that binds ephrin-A ligands, EPHA4 regulates dendritic spine morphology, synaptic function, neuromuscular junction formation, and neural circuit formation. Dysregulated EPHA4 signaling has been implicated in neurodegenerative diseases, particularly [Alzheimer's disease](/diseases/alzheimers-disease), [amyotrophic lateral sclerosis (ALS)](/diseases/als), and stroke recovery. PMID: 28777749
The [Eph-ephrin signaling system](/mechanisms/eph-ephrin-signaling) is one of the most important receptor-ligand systems in the nervous system, governing development and maintaining function throughout life. EPHA4 stands out among the Eph receptors due to its particularly strong expression in the hippocampus and cortex, regions critical for learning and memory, and its involvement in motor neuron biology.[@liu2020] PMID: 30999031
The EPHA4 gene is located on chromosome 2q36.1 and encodes a 986-amino acid transmembrane receptor protein. The protein structure follows the canonical Eph receptor architecture, comprising an N-terminal ephrin-binding domain (ligand-binding domain), a cysteine-rich region, two fibronectin type III repeats, a transmembrane helix, and a cytoplasmic domain containing the tyrosine kinase domain.[@chen2020] PMID: 32169356
The ephrin-binding domain of EPHA4 shows unique binding characteristics compared to other EPHA receptors. EPHA4 exhibits high affinity for ephrin-A2, ephrin-A3, and ephrin-A5 ligands. This binding specificity is determined by specific residues in the ligand-binding pocket that form hydrogen bonds and van der Waals interactions with the ephrin ligands. The binding triggers receptor clustering and autophosphorylation, initiating downstream signaling cascades. PMID: 28777749
The cytoplasmic tyrosine kinase domain of EPHA4 contains multiple tyrosine residues that undergo autophosphorylation upon ligand binding. Key phosphorylation sites include Tyr-602, Tyr-618, Tyr-779, and Tyr-797, which serve as docking sites for downstream signaling molecules containing SH2 domains. The kinase activity is tightly regulated and can be modulated by protein tyrosine phosphatases. PMID: 30999031
EPHA4 exhibits high expression in the adult [hippocampus](/brain-regions/hippocampus), particularly in the CA1 and CA3 regions, as well as in the [cortex](/brain-regions/cortex), specifically layer V pyramidal neurons. In the developing brain, EPHA4 expression is highest during embryency and early postnatal development, correlating with its roles in neuronal migration, axon guidance, and synapse formation. PMID: 32169356
Within the hippocampus, EPHA4 is prominently expressed in pyramidal neurons of the CA1 and CA3 regions, as well as in dentate gyrus granule cells. The receptor is particularly abundant at synapses, where it regulates synaptic structure and function. This hippocampal expression correlates with the role of EPHA4 in learning and memory. PMID: 28777749
In the cortex, EPHA4 expression is enriched in pyramidal neurons of layers II/III and V, which are the primary output neurons of the cortex. These neurons receive dense glutamatergic innervation and are involved in corticocortical and corticospinal communication. EPHA4 on these neurons regulates synaptic inputs and outputs. PMID: 30999031
EPHA4 is also highly expressed in motor neurons of the spinal cord and brainstem. This expression pattern is clinically relevant because EPHA4 dysregulation has been implicated in ALS, a disease that primarily affects motor neurons. PMID: 32169356
The [Eph-ephrin system](/mechanisms/eph-ephrin-signaling) plays a fundamental role in regulating synaptic plasticity, the cellular basis of learning and memory. EPHA4 contributes to synaptic plasticity through multiple mechanisms, including regulation of dendritic spine morphology, modulation of glutamatergic synaptic transmission, and influence on long-term potentiation (LTP) and long-term depression (LTD).
Dendritic spines are small actin-rich protrusions from dendritic shafts that receive the majority of excitatory synaptic inputs in the brain. EPHA4 signaling regulates spine morphogenesis through modulation of the actin cytoskeleton. Upon ephrin-A binding, EPHA4 activates downstream effectors such as Vav2/3 (guanine nucleotide exchange factors for Rho GTPases) and α-chimaerin, which in turn regulate Rac1 and Cdc42 activity.[@zhao2020]
Studies have demonstrated that EPHA4 localizes to both presynaptic and postsynaptic compartments, enabling bidirectional synaptic signaling. At the postsynaptic terminal, EPHA4 regulates the formation, maintenance, and plasticity of dendritic spines. Loss of EPHA4 leads to altered spine morphology and reduced spine density.
EPHA4 modulates long-term potentiation (LTP), the persistent strengthening of synapses that underlies learning and memory. The receptor regulates NMDA receptor function and trafficking, influencing the calcium influx necessary for LTP induction. EPHA4 also modulates AMPA receptor trafficking, affecting synaptic strength.
Interestingly, EPHA4 also regulates long-term depression (LTD), the weakening of synaptic connections. This bidirectional modulation allows precise control of synaptic strength. The balance between LTP and LTD is critical for proper cognitive function, and dysregulated EPHA4 signaling disrupts this balance.
EPHA4 regulates excitatory synaptic transmission through modulation of presynaptic release machinery and postsynaptic receptor function. At the presynaptic terminal, EPHA4 influences vesicle release probability and the size of the readily releasable pool of vesicles.
At the postsynaptic density, EPHA4 interacts with NMDA receptor subunits and influences NMDA receptor-mediated calcium influx. The receptor also affects AMPA receptor trafficking and localization, influencing synaptic strength and plasticity.
EPHA4 plays a critical role in the formation and maintenance of the neuromuscular junction (NMJ), the synapse between motor neurons and skeletal muscle fibers. This function distinguishes EPHA4 from many other Eph receptors and explains its involvement in motor neuron diseases.
During NMJ development, EPHA4 on the motor neuron terminal binds to ephrin-A ligands on the muscle membrane. This interaction triggers bidirectional signaling that coordinates presynaptic differentiation (motor neuron terminal) and postsynaptic differentiation (muscle membrane).
The presynaptic signaling through EPHA4 regulates the clustering of synaptic vesicles and the formation of active zones, the specialized regions where neurotransmitter release occurs. The postsynaptic signaling through ephrin-A ligands regulates the formation of the motor endplate, the specialized region of the muscle membrane that receives innervation.
Multiple lines of evidence implicate EPHA4 in the pathogenesis of [Alzheimer's disease](/diseases/alzheimers-disease). EPHA4 dysregulation contributes to several hallmark features of AD, including tau pathology, synaptic dysfunction, and neuroinflammation.
EPHA4 has been shown to promote tau phosphorylation and aggregation, key features of AD neuropathology. The receptor activates kinases that phosphorylate tau, including GSK-3β and CDK5, while potentially reducing phosphatase activity. This creates a pro-tau-pathology signaling environment ([Gong et al., 2016](https://pubmed.ncbi.nlm.nih.gov/27185467/)).
The interaction between EPHA4 and tau is bidirectional. Tau pathology can also affect EPHA4 signaling, creating a vicious cycle that accelerates neurodegeneration. Hyperphosphorylated tau accumulates at synapses and interferes with normal Eph-ephrin signaling, disrupting synaptic function.
EPHA4 plays a crucial role in maintaining synaptic integrity, and its dysregulation contributes to synaptic loss, an early and correlate of cognitive decline in AD. The receptor regulates the function of both glutamatergic and GABAergic synapses through distinct mechanisms.
At glutamatergic synapses, EPHA4 modulates NMDA receptor function and trafficking, affecting calcium signaling essential for synaptic plasticity. EPHA4 also regulates AMPA receptor trafficking, influencing synaptic strength and connectivity.
EPHA4 has emerged as an important player in the pathogenesis of [amyotrophic lateral sclerosis (ALS)](/diseases/als), a progressive neurodegenerative disease that affects motor neurons. Studies have shown that EPHA4 is dysregulated in ALS and contributes to motor neuron dysfunction and death.
The role of EPHA4 in ALS reflects its importance in motor neuron biology. The receptor regulates motor neuron migration and positioning during development, as well as NMJ formation and function. Dysregulated EPHA4 signaling in ALS may affect multiple aspects of motor neuron health.
Genetic studies have identified EPHA4 variants associated with ALS susceptibility, suggesting a causal relationship. Targeting EPHA4 signaling has been proposed as a therapeutic strategy for ALS, although the complexity of Eph-ephrin signaling requires careful consideration.
EPHA4 plays a dual role in stroke pathophysiology and recovery. Immediately after stroke, EPHA4-mediated repulsive signaling may contribute to excitotoxic damage. During recovery, however, EPHA4 signaling promotes neural circuit reorganization and functional recovery.
The role of EPHA4 in stroke recovery involves its effects on axonal sprouting, synaptogenesis, and plasticity. EPHA4 blockade during the recovery phase enhances functional recovery in mouse models of stroke, suggesting that temporary EPHA4 inhibition may be beneficial.
During development, EPHA4 guides neuronal migration and axon pathfinding through contact-dependent repulsion. The receptor interacts with ephrin-A ligands expressed on neighboring cells, creating repulsive cues that shape neural circuit formation.
In the corticospinal system, EPHA4 regulates the guidance of corticospinal axons from the cortex to the spinal cord. This pathway is essential for voluntary movement, and disruption leads to motor deficits.
EPHA4 also plays a role in the establishment of topographic maps in sensory systems. The receptor contributes to the retinotectal map, where it helps establish the orderly representation of visual space in the superior colliculus.
Given its central role in AD, ALS, and stroke, EPHA2 represents a promising therapeutic target. Several strategies have been explored:
The timing of EPHA4 modulation is critical, particularly in stroke, where inhibition may be beneficial during recovery but could impair long-term function.
EPHA4 transduces signals through multiple downstream pathways upon ephrin ligand binding. The canonical forward signaling cascade begins with receptor dimerization and autophosphorylation of tyrosine residues in the cytoplasmic domain. Key phosphorylation sites include Tyr-602, Tyr-618, Tyr-779, and Tyr-797, which create docking sites for SH2 domain-containing signaling proteins.
The primary downstream pathways activated by EPHA4 include:
Like other Eph receptors, EPHA4 participates in bidirectional signaling. When EPHA4-expressing cells contact ephrin-A-expressing cells, reverse signaling can be transduced into the ephrin-bearing cell. This is particularly important in:
EPHA4 plays a significant role in neuroinflammation, a key feature of neurodegenerative diseases. The receptor is expressed on microglia and astrocytes, where it modulates their inflammatory responses.
EPHA4 regulates microglial activation states. Upon ephrin-A binding, EPHA4 can promote either pro-inflammatory or anti-inflammatory responses depending on context. Studies have shown that EPHA4 signaling influences:
EPHA4 is also expressed on astrocytes, where it regulates their support functions for neurons. Astrocytic EPHA4 influences:
EPHA4 contributes to neuronal excitability through its effects on ion channel function and synaptic transmission. The receptor modulates:
EPHA4 regulates NMDA receptor function, affecting calcium influx and downstream signaling. The receptor also influences AMPA receptor trafficking, impacting excitatory synaptic strength.
EPHA4 is involved in regulating GABAergic inhibitory synapses. The receptor modulates GABA release and GABA receptor function, influencing the balance between excitation and inhibition.
Through its effects on both excitatory and inhibitory synapses, EPHA4 influences neuronal network oscillations, particularly in the hippocampus. Disrupted network activity contributes to cognitive deficits in AD.
While most studied in AD and ALS, EPHA4 has also been implicated in [Parkinson's disease](/diseases/parkinsons-disease). The receptor is expressed in brain regions affected by PD, including the substantia nigra and striatum.
EPHA4 in PD may contribute to:
EPHA4 signaling influences mitochondrial function, which is relevant to neurodegeneration. The receptor modulates:
Small molecule inhibitors targeting EPHA4 kinase activity are being developed. These compounds may reduce pathological signaling in conditions where EPHA4 promotes neurodegeneration.
Anti-EPHA4 antibodies can block ephrin binding and receptor activation. This approach allows precise targeting of EPHA4 without affecting other Eph receptors.
For conditions where enhanced EPHA4 signaling would be beneficial, ephrin-A mimetics that selectively activate EPHA4 are being explored.
Viral vectors delivering functional EPHA3 or EPHA4 may restore normal signaling in developmental disorders or neurodegenerative conditions.
EPHA4 interacts with numerous proteins beyond its ephrin ligands:
Mouse models lacking EPHA4 show developmental abnormalities including defective axon guidance, altered neuronal migration, and impaired NMJ formation. EPHA4 knockout mice exhibit motor deficits and impaired learning and memory.
Transgenic mouse models overexpressing EPHA4 show enhanced synaptic plasticity but also exhibit certain pathological features. These models have been used to study EPHA4 function and test therapeutic interventions.