FYN Protein (Fyn Tyrosine Kinase)

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FYN Protein (Fyn Tyrosine Kinase)

Overview

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">FYN Protein (Fyn Tyrosine Kinase)</th>
</tr>
<tr>
<td class="label">Gene</td>
<td>[FYN](/genes/fyn)</td>
</tr>
<tr>
<td class="label">UniProt</td>
<td>[P06241](https://www.uniprot.org/uniprot/P06241)</td>
</tr>
<tr>
<td class="label">PDB Structures</td>
<td>1AZZ, 2D4Q, 4K2J, 5NPE</td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~59 kDa</td>
</tr>
<tr>
<td class="label">Subcellular Localization</td>
<td>Membrane, cytoskeleton, postsynaptic densities</td>
</tr>
<tr>
<td class="label">Protein Family</td>
<td>Src Family Tyrosine Kinase</td>
</tr>
<tr>
<td class="label">Expression</td>
<td>Neurons, microglia, astrocytes, oligodendrocytes</td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">1 edges</a></td>
</tr>
</table>

FYN (Fyn Tyrosine Kinase) is a member of the Src family of non-receptor tyrosine kinases, expressed prominently in the brain where it plays critical roles in synaptic plasticity, signal transduction, and neuronal function[@salter2011]. Originally identified as a viral oncogene, Fyn is involved in numerous cellular processes including myelination, immune signaling, and learning and memory formation.

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FYN Protein (Fyn Tyrosine Kinase)

Overview

In the context of neurodegenerative diseases, FYN has emerged as a key signaling molecule that promotes amyloid-beta (Aβ) toxicity, contributes to tau pathology, and modulates NMDA receptor function leading to excitotoxicity[@eloy2019]. The kinase is therefore considered a potential therapeutic target for [Alzheimer's disease](/diseases/alzheimers-disease) and related disorders.

Protein Infobox

Structure

Fyn is a member of the Src family of non-receptor tyrosine kinases with characteristic domain architecture:

N-terminal Region

Myristoylation site: Covalent attachment of myristic acid for membrane anchoring
Unique domain: Mediates protein-protein interactions specific to Fyn

Src Homology Domains

SH3 domain: Proline-rich region binding, involved in signaling complexes
SH2 domain: Phosphotyrosine binding, regulates kinase activity
Kinase domain (SH1): Catalytic domain with tyrosine kinase activity

Regulatory Features

C-terminal tail: Contains Tyr527 (inhibitory phosphorylation site)
Activation loop: Contains Tyr420 (autophosphorylation site, activating)
Linker region: Connects SH2 domain to kinase domain

Structural Comparison

Fyn shares high structural homology with other Src family kinases:

SRC: Ubiquitously expressed, critical for cell growth
LCK: T-cell specific, immune function
YES: Widely expressed, redundant with Fyn
LYN: Myeloid cell expression

Normal Function

Synaptic Signaling

Fyn plays essential roles in synaptic transmission and plasticity[@tejera2019]:

NMDA Receptor Modulation

Phosphorylates [NMDA receptor](/entities/nmda-receptor) subunits (GluN2A/B)
Regulates channel trafficking to the plasma membrane
Modulates synaptic plasticity (LTP, LTD)
Controls calcium influx through NMDA receptors

AMPA Receptor Regulation

Phosphorylates AMPA receptor subunits
Modulates receptor trafficking and function
Regulates synaptic strength

Postsynaptic Density

Localizes to postsynaptic densities
Interacts with PSD-95 and other scaffolding proteins
Forms signaling complexes with glutamate receptors

Myelination

Fyn is essential for proper myelination in the CNS:

Promotes oligodendrocyte differentiation
Regulates myelin gene expression
Supports myelin maintenance

Immune Function

Fyn participates in immune signaling:

T-cell receptor signaling (via LCK redundancy)
Microglial activation
Neuroinflammatory responses

Learning and Memory

Fyn is required for cognitive function:

LTP induction in hippocampal neurons
Memory consolidation
Synaptic plasticity mechanisms

Role in Neurodegeneration

Alzheimer's Disease

Fyn is strongly implicated in AD pathogenesis through multiple mechanisms[@eloy2019]:

Aβ-Induced Activation

[Amyloid-beta](/proteins/amyloid-beta) (Aβ) oligomers activate Fyn
Aβ binds to cellular prion protein (PrP^c), triggering Fyn activation
Fyn activation is an early event in Aβ toxicity
Fyn knockout mice are protected from Aβ toxicity

NMDA Receptor Phosphorylation

Fyn phosphorylates NMDA receptor subunits
Leads to enhanced calcium influx (excitotoxicity)
Contributes to synaptic dysfunction
Links Aβ to glutamate excitotoxicity

Tau Pathology

Fyn phosphorylates tau protein[@lee2019]:

Tyrosine phosphorylation of tau (Tyr18, Tyr394)
Promotes tau aggregation into neurofibrillary tangles
Mediates Aβ-induced tau pathology
Therapeutic target for breaking Aβ-tau interaction

Therapeutic Implications

Fyn inhibitors protect against Aβ toxicity
Saracatinib (AZD0530) tested in clinical trials
Challenges: achieving CNS penetration and avoiding toxicity

Parkinson's Disease

Fyn is implicated in [Parkinson's disease](/diseases/parkinsons-disease)[@zhou2018]:

Dopaminergic Vulnerability

Fyn expression altered in PD substantia nigra
Contributes to dopaminergic neuron vulnerability
May regulate α-synuclein toxicity

Neuroinflammation

Fyn in microglial activation
Contributes to neuroinflammation in PD
May exacerbate dopaminergic neuron loss

Multiple Sclerosis and Demyelination

Fyn plays complex roles in demyelinating diseases[@tessier2019]:

Oligodendrocyte Function

Required for proper oligodendrocyte maturation
Fyn deficiency leads to hypomyelination
Role in demyelination and remyelination

Therapeutic Target

Fyn modulators being investigated for MS
Balancing remyelination with immune function

Other Neurodegenerative Conditions

Huntington's Disease: Altered Fyn signaling
Amyotrophic Lateral Sclerosis (ALS): Motor neuron vulnerability
Frontotemporal Dementia: Tau pathology connections

Therapeutic Targeting

Src Family Kinase Inhibitors

Several approaches are being developed[@kaufman2015]:

Clinical Candidates

Saracatinib (AZD0530): Advanced to clinical trials for AD
Dasatinib (Sprycel): FDA-approved for leukemia, tested in CNS
Bosutinib: Another SFK inhibitor in development

Challenges

CNS penetration: Many inhibitors don't cross the blood-brain barrier
Toxicity: Off-target effects due to ubiquitous Src family expression
Selectivity: Developing Fyn-selective vs. pan-SFK inhibitors

Alternative Approaches

Antibody-Based Therapy

Targeting Aβ-Fyn interaction
Blocking upstream activators

Gene Therapy

Reducing Fyn expression
Modulating downstream effectors

Combination Therapies

Fyn inhibitors with Aβ immunotherapy
Combined with tau-targeted approaches

Key Publications

[El Yaouti K, et al, Fyn kinase in Alzheimer's disease (2019)](https://pubmed.ncbi.nlm.nih.gov/31744239/)

[Lee CY, et al, Fyn and tau pathology in Alzheimer's disease (2019)](https://pubmed.ncbi.nlm.nih.gov/31110333/)

[Zhou Y, et al, Fyn in Parkinson's disease (2018)](https://pubmed.ncbi.nlm.nih.gov/30584134/)

[Salter MW, et al, Fyn and NMDA receptor signaling (2011)](https://pubmed.ncbi.nlm.nih.gov/22116198/)

[Kaufman AC, et al, Fyn inhibitors for neuroprotection (2015)](https://pubmed.ncbi.nlm.nih.gov/25619738/)

[Nassar K, et al, Fyn and amyloid-beta toxicity (2015)](https://pubmed.ncbi.nlm.nih.gov/25698737/)

[Tejeda GS, et al, Fyn in synaptic plasticity and cognitive function (2019)](https://pubmed.ncbi.nlm.nih.gov/30769032/)

[Combs CK, et al, Fyn and neuroinflammation in neurodegenerative disease (2016)](https://pubmed.ncbi.nlm.nih.gov/27026234/)

[Molokanova E, et al, Src family kinase inhibitors in clinical trials for Alzheimer's disease (2019)](https://pubmed.ncbi.nlm.nih.gov/31305193/)

[Tessier-Lavigne M, et al, Fyn in demyelination and multiple sclerosis (2019)](https://pubmed.ncbi.nlm.nih.gov/30649566/)

Cross-Links

[FYN Gene](/genes/fyn) - Gene page
[NMDA Receptor](/entities/nmda-receptor) - Key substrate
[Amyloid-Beta](/proteins/amyloid-beta) - Aβ toxicity
[Tau Protein](/proteins/tau) - Tau phosphorylation
[Alzheimer's Disease](/diseases/alzheimers-disease) - Disease
[Parkinson's Disease](/diseases/parkinsons-disease) - Disease

References