SRC Protein (SRC Proto-Oncogene)

SRC Protein (SRC Proto-Oncogene)

Overview

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">SRC Protein (SRC Proto-Oncogene)</th>
</tr>
<tr>
<td class="label">Drug</td>
<td>Type</td>
</tr>
<tr>
<td class="label">Dasatinib</td>
<td>Multi-kinase</td>
</tr>
<tr>
<td class="label">Bosutinib</td>
<td>Multi-kinase</td>
</tr>
<tr>
<td class="label">Saracatinib</td>
<td>Multi-kinase</td>
</tr>
<tr>
<td class="label">PP1</td>
<td>SRC selective</td>
</tr>
<tr>
<td class="label">PP2</td>
<td>SRC selective</td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>

SRC (SRC Proto-Oncogene, Non-Receptor Tyrosine Kinase) is the prototypical member of the Src family of non-receptor tyrosine kinases (SFKs), representing one of the most studied and functionally important kinases in cell biology. Originally discovered as the cellular homolog of the v-src oncogene from Rous sarcoma virus, SRC has emerged as a critical signaling molecule in virtually every cell type, with particularly important roles in neuronal function, synaptic plasticity, and neurodegenerative disease pathogenesis [@engen2008].

SRC Protein (SRC Proto-Oncogene)

Overview

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">SRC Protein (SRC Proto-Oncogene)</th>
</tr>
<tr>
<td class="label">Drug</td>
<td>Type</td>
</tr>
<tr>
<td class="label">Dasatinib</td>
<td>Multi-kinase</td>
</tr>
<tr>
<td class="label">Bosutinib</td>
<td>Multi-kinase</td>
</tr>
<tr>
<td class="label">Saracatinib</td>
<td>Multi-kinase</td>
</tr>
<tr>
<td class="label">PP1</td>
<td>SRC selective</td>
</tr>
<tr>
<td class="label">PP2</td>
<td>SRC selective</td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>

SRC (SRC Proto-Oncogene, Non-Receptor Tyrosine Kinase) is the prototypical member of the Src family of non-receptor tyrosine kinases (SFKs), representing one of the most studied and functionally important kinases in cell biology. Originally discovered as the cellular homolog of the v-src oncogene from Rous sarcoma virus, SRC has emerged as a critical signaling molecule in virtually every cell type, with particularly important roles in neuronal function, synaptic plasticity, and neurodegenerative disease pathogenesis [@engen2008].

The SRC gene encodes a 60 kDa protein that is ubiquitously expressed in mammals, with highest levels in platelets, neurons, and epithelial cells. As a non-receptor tyrosine kinase, SRC catalyzes the transfer of phosphate groups to tyrosine residues on substrate proteins, thereby regulating diverse cellular processes including cell growth, differentiation, adhesion, migration, and survival. In the nervous system, SRC plays essential roles in regulating NMDA receptor function, synaptic plasticity, and neuronal signaling cascades that are fundamental to learning and memory.

The central position of SRC in multiple signaling pathways, combined with its well-characterized structure and regulation, has made it an important therapeutic target. While initially pursued for cancer therapy, recent attention has turned to exploring SRC inhibition for neurodegenerative diseases, where its roles in neuroinflammation, excitotoxicity, and synaptic dysfunction make it an attractive target.

Structure and Biochemistry

Domain Architecture

SRC possesses the characteristic modular architecture shared by all Src family kinases, consisting of distinct functional domains that work together to regulate enzymatic activity and substrate specificity:

N-Terminal Unique Domain (residues 1-80): This domain contains a critical glycine at position 2 that undergoes myristoylation, enabling permanent membrane association. The unique domain also contributes to substrate recognition and protein-protein interactions that are specific to SRC.

SH3 Domain (residues 81-144): The Src Homology 3 domain binds to proline-rich motifs (PXXP sequences) in target proteins, facilitating the formation of signaling complexes. In neurons, SH3-mediated interactions are critical for localizing SRC to postsynaptic densities [@martin2001].

SH2 Domain (residues 145-241): The Src Homology 2 domain recognizes and binds to phosphotyrosine-containing motifs, enabling SRC to interact with activated receptors and signaling complexes. This domain is essential for targeting SRC to activated NMDA receptors.

Kinase Domain (residues 242-525): The catalytic domain contains the active site with essential residues for phosphotransferase activity (including K295, E310, and the DFG motif). The kinase domain undergoes dramatic conformational changes between active and inactive states.

C-Terminal Regulatory Tail (residues 526-536): This short region contains a critical tyrosine residue (Y527) whose phosphorylation state controls SRC activity. When phosphorylated, Y527 binds intramolecularly to the SH2 domain, maintaining an inactive conformation.

Structural Activation Mechanism

SRC activity is controlled through a sophisticated allosteric mechanism:

Inactive Conformation: In the inactive state, the C-terminal tail (Y527) is phosphorylated and engages the SH2 domain. The kinase domain adopts a closed conformation with the activation loop blocking the active site.

Activation Process: SRC becomes activated through multiple mechanisms:

Active Conformation: Upon activation, the kinase domain undergoes a conformational change that opens the active site for substrate binding. The activation loop is displaced, and catalytic residues are properly positioned.

Normal Physiological Functions

Role in Immune System

In immune cells, SRC family kinases are essential signaling molecules:

Receptor Signaling: SRC kinases couple various cell surface receptors to downstream signaling cascades. They are activated following engagement of T-cell receptors, B-cell receptors, integrins, and cytokine receptors.

Cell Adhesion and Migration: SRC regulates integrin signaling and cytoskeletal dynamics, controlling immune cell adhesion and migration.

Cytokine Production: SRC activation leads to activation of transcription factors (NF-κB, AP-1) that drive cytokine expression.

Role in Nervous System

Within the nervous system, SRC plays several critical roles:

NMDA Receptor Regulation: SRC is a key regulator of NMDA receptor function, phosphorylating NR2A and NR2B subunits on tyrosine residues. This phosphorylation enhances channel open probability, modulates trafficking, and regulates synaptic localization. The SRC-NMDAR interaction is essential for synaptic plasticity [@salter2000].

Synaptic Plasticity: Through its regulation of NMDARs and interactions with postsynaptic scaffolding proteins (PSD-95, SAP90), SRC plays a fundamental role in activity-dependent synaptic modifications including long-term potentiation (LTP) and long-term depression (LTD) [@ingraham2006].

Tau Phosphorylation: SRC can phosphorylate tau protein on tyrosine residues (Y18, Y29, Y197), potentially influencing tau pathology in Alzheimer's disease. This phosphorylation may promote tau aggregation and neurotoxicity.

Neuronal Signaling Cascades: SRC participates in signaling downstream of various neuronal receptors including:

• Neurotrophin receptors (TrkA, TrkB, p75^NTR)
• Epidermal growth factor receptor
• Insulin receptor
• Integrins

Role in Neurodegenerative Diseases

Alzheimer's Disease (AD)

SRC is implicated in multiple aspects of AD pathogenesis:

Amyloid-Beta Signaling: Aβ oligomers activate SRC family kinases, leading to downstream signaling that contributes to synaptic dysfunction. SRC is activated in response to Aβ exposure, and this activation mediates some toxic effects of Aβ.

NMDA Receptor Dysregulation: Altered SRC activity contributes to NMDAR hyperphosphorylation and dysfunction in AD. This affects calcium homeostasis, promoting excitotoxicity and synaptic failure.

Tau Pathology: SRC-mediated phosphorylation of tau on tyrosine residues may accelerate tau aggregation and NFT formation. The Y18 phosphorylation creates a pathological epitope.

Synaptic Failure: SRC-PSD-95 interactions are disrupted in AD brains, contributing to synaptic protein mislocalization and synaptic loss.

Therapeutic Target: SRC inhibitors have shown neuroprotective effects in AD models. The FDA-approved drug dasatinib has been investigated for potential use in AD [@xu2009].

Parkinson's Disease (PD)

In Parkinson's disease, SRC contributes to pathogenesis through several mechanisms:

Dopaminergic Neuron Vulnerability: SRC is activated in the substantia nigra of PD patients. Changes in SRC activity may contribute to the selective vulnerability of dopaminergic neurons.

Alpha-Synuclein Toxicity: SRC may modulate cellular responses to alpha-synuclein aggregation. Studies suggest that SRC activity influences the toxicity of Lewy body pathology.

Neuroinflammation: Microglial SRC activation drives inflammatory cytokine production in PD. Peripheral immune cell infiltration involves SRC-mediated signaling.

Therapeutic Potential: SRC inhibitors may have neuroprotective effects in PD models through anti-inflammatory and neuroprotective mechanisms [@li2010].

Multiple Sclerosis (MS)

SRC plays important roles in MS pathogenesis:

T-cell Activation: SRC is required for T-cell activation and migration into the CNS.

Demyelination: SRC signaling in oligodendrocytes and immune cells contributes to demyelination.

Blood-Brain Barrier: SRC regulates endothelial tight junctions and BBB permeability.

Therapeutic Target: SRC inhibitors have been investigated for MS treatment [@zhang2021].

Amyotrophic Lateral Sclerosis (ALS)

In ALS, SRC contributes to disease through:

Neuroinflammation: Chronic neuroinflammation driven by microglia and astrocyte activation involves SRC signaling.

Excitotoxicity: SRC regulates NMDA receptor function, which may influence excitotoxic motor neuron injury.

Therapeutic Considerations: SRC inhibitors may have potential to reduce neuroinflammation in ALS [@lee2019].

Stroke and Brain Ischemia

Following stroke and cerebral ischemia, SRC participates in:

Post-Ischemic Inflammation: SRC mediates inflammatory responses including leukocyte infiltration and cytokine release.

Blood-Brain Barrier Dysfunction: SRC regulates endothelial cell tight junctions, contributing to post-stroke BBB disruption.

Excitotoxicity: NMDAR activation leads to SRC-dependent signaling cascades that mediate excitotoxic injury.

Therapeutic Target: SRC inhibitors have shown neuroprotective effects in stroke models [@smith2015].

Therapeutic Targeting

SRC Inhibitors

Several SRC inhibitors have been developed, primarily for oncology indications:

Clinical Trials

Saracatinib (AZD0530): This brain-penetrant SRC inhibitor has been evaluated in Phase 2 clinical trials for Alzheimer's disease. Results showed some evidence of target engagement but limited cognitive benefit.

Dasatinib: Being repurposed for neurodegenerative diseases. Early-phase studies are evaluating safety and pharmacokinetics.

Challenges in CNS Drug Development

Blood-Brain Barrier Penetration: Most SRC inhibitors have limited CNS penetration. Achieving therapeutic concentrations in the brain remains challenging.

Selectivity: Achieving selectivity for SRC over other Src family kinases (LCK, FYN, LYN) is difficult. Off-target effects may contribute to toxicity.

Therapeutic Window: The optimal level of SRC inhibition for neuroprotection versus side effects remains unclear.

Mechanism-Based Toxicity: Long-term SRC inhibition may cause adverse effects through disruption of normal cellular functions.

Emerging Strategies

Brain-Penetrant Inhibitors: New SRC inhibitors with improved CNS penetration are under development. Saracatinib demonstrated that brain penetration is achievable.

Allosteric Modulators: Targeting allosteric sites may provide more selective modulation of SRC function.

Protein Degradation: PROTAC-based approaches could achieve sustained SRC degradation.

Combination Therapy: SRC inhibitors combined with other disease-modifying approaches may have synergistic effects.

Expression Patterns

Immune System Expression

SRC is expressed in virtually all immune cell types:

• T-cells, B-cells, NK cells
• Macrophages and microglia
• Dendritic cells
• Platelets

Nervous System Expression

In the CNS, SRC expression is widespread:

Neurons: SRC is expressed in most neuronal populations, with particularly high levels in:

• Hippocampal pyramidal neurons
• Cortical pyramidal neurons
• Cerebellar Purkinje cells

• Microglia (constitutively low, increases with activation)
• [Astrocytes](/cell-types/astrocytes)
• [Oligodendrocytes](/cell-types/oligodendrocytes)

Signaling Networks

Protein-Protein Interactions

SRC interacts with numerous proteins:

In Immune Cells:

• TCR/CD3 complex
• BCR components
• Integrins (β1, β2, β3)
• Cytokine receptors
• Adaptor proteins (LAT, SLP-76)

• NMDA receptor subunits (GRIN1, GRIN2A, GRIN2B)
• AMPA receptor subunits
• PSD-95 and related scaffold proteins
• Synaptic vesicles proteins
• Integrins
• Neurotrophin receptors

Signaling Pathways

SRC participates in multiple signaling cascades:

NMDAR Signaling: SRC → NR2A/B phosphorylation → enhanced Ca²⁺ influx → CaMKII/calcineurin activation → synaptic plasticity

Integrin Signaling: SRC → FAK activation → PI3K/AKT pathway → cell survival and cytoskeletal organization

Growth Factor Signaling: SRC → RAS/MAPK pathway → cell proliferation and differentiation

Inflammatory Signaling: SRC → NF-κB activation → cytokine production

Research Directions and Future Perspectives

Current Challenges

Understanding Cell-Type Specific Effects: Determining which cellular effects of SRC inhibition drive therapeutic benefit versus toxicity.

Biomarker Development: Patient selection biomarkers for SRC-targeted therapies are needed.

Delivery Strategies: Improving CNS delivery of SRC inhibitors remains critical.

Emerging Research Areas

SRC Isoforms: Alternative splicing may produce distinct SRC isoforms with different functions.

Post-Translational Modifications: Beyond phosphorylation, SRC undergoes other modifications that may regulate function.

Single-Cell Resolution: Understanding cell-type specific SRC functions using single-cell approaches.

Combination Therapies: SRC inhibition combined with other disease-modifying approaches.

Summary

SRC is the prototypical non-receptor tyrosine kinase with essential roles in both immune signaling and neuronal biology. Through its regulation of NMDA receptors, synaptic plasticity, tau phosphorylation, and neuroinflammatory responses, SRC contributes to multiple aspects of neurodegenerative diseases including Alzheimer's disease, Parkinson's disease, stroke, and multiple sclerosis.

The therapeutic targeting of SRC faces significant challenges including limited CNS penetration, off-target effects, and mechanism-based toxicity. However, the development of brain-penetrant inhibitors like saracatinib has demonstrated feasibility. Ongoing research continues to explore optimized dosing strategies, combination therapies, and cell-type selective targeting approaches.

The central role of SRC in multiple disease-relevant pathways makes it an attractive therapeutic target. Future research focused on improving brain penetration, achieving selectivity, and developing biomarker-driven patient selection strategies may enable successful exploitation of this target for neurodegenerative disease treatment.

See Also

• [SRC Gene](/genes/src)
• [LCK Protein](/proteins/lck-protein)
• [FYN Protein](/proteins/fyn-protein)
• [NMDA Receptor](/entities/nmda-receptor)
• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)
• [Neuroinflammation](/mechanisms/neuroinflammation-pathway)
• [Tau Protein](/proteins/tau)
• [Synaptic Plasticity](/mechanisms/synaptic-plasticity)

External Links

• [UniProt: P12931](https://www.uniprot.org/uniprot/P12931)
• [NCBI Gene: SRC](https://www.ncbi.nlm.nih.gov/gene/6713)
• [PDB: SRC Structures](https://www.rcsb.org/structure/1H3R)
• [AlphaFold: SRC](https://alphafold.ebi.ac.uk/entry/P12931)
• [Human Protein Atlas: SRC](https://www.proteinatlas.org/ENSG00000197122-SRC)
• [ClinicalTrials.gov: SRC inhibitors](https://clinicaltrials.gov/search?cond=Alzheimer%27s+disease&intr=SRC+inhibitor)

References