il34-protein

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IL34 Protein (Interleukin-34)

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">il34-protein</th>
</tr>
<tr>
<td class="label">Feature</td>
<td>IL-34</td>
</tr>
<tr>
<td class="label">Primary structure</td>
<td>Different amino acid sequence</td>
</tr>
<tr>
<td class="label">Receptor binding</td>
<td>Different epitope on CSF1R</td>
</tr>
<tr>
<td class="label">Tissue expression</td>
<td>Neurons, astrocytes</td>
</tr>
<tr>
<td class="label">Biological functions</td>
<td>Overlapping but distinct</td>
</tr>
<tr>
<td class="label">Pathway</td>
<td>Primary Effect</td>
</tr>
<tr>
<td class="label">PI3K/Akt</td>
<td>Cell survival, metabolism</td>
</tr>
<tr>
<td class="label">MAPK/ERK</td>
<td>Cell proliferation</td>
</tr>
<tr>
<td class="label">JAK/STAT</td>
<td>Gene transcription</td>
</tr>
<tr>
<td class="label">PLCγ</td>
<td>Calcium signaling</td>
</tr>
<tr>
<td class="label">Strategy</td>
<td>Mechanism</td>
</tr>
<tr>
<td class="label">IL-34 protein</td>
<td>Direct CSF1R activation</td>
</tr>
<tr>
<td class="label">CSF1R agonists</td>
<td>Enhanced receptor signaling</td>
</tr>
<tr>
<td class="label">CSF1R antagonists</td>
<td>Reduce microglial proliferation</td>
</tr>
<tr>
<td class="label">Compound</td>
<td>Type</td>
</tr>
<tr>
<td class="label">IL-34 protein</td>
<td>Recombinant cytokine</td>
</tr>
<tr>
<td class="label">CSF1R agonists</t

...

IL34 Protein (Interleukin-34)

Introduction

Interleukin-34 (IL-34) is a homodimeric cytokine that serves as the primary ligand for the colony-stimulating factor 1 receptor (CSF1R). Discovered in 2008 through functional screening, IL-34 plays essential roles in microglial survival, proliferation, differentiation, and function in the central nervous system. Unlike its functional counterpart M-CSF (macrophage colony-stimulating factor), IL-34 is expressed predominantly in neurons and astrocytes, making it a key neuronal-derived cytokine for microglial regulation.

The IL-34/CSF1R axis has emerged as a critical pathway in neurodegenerative disease pathogenesis. Dysregulation of this pathway contributes to neuroinflammation in Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis (MS), amyotrophic lateral sclerosis (ALS), and other neurological conditions. Both IL-34 and CSF1R have become attractive therapeutic targets, with agonists and antagonists in development for various neurological disorders. [@lin2013]

This comprehensive review examines the structure and biochemistry of IL-34, its normal physiological functions in the CNS, the mechanisms by which dysregulation contributes to neurodegenerative diseases, and current therapeutic strategies targeting this important cytokine pathway.

Structure

Primary Structure and Biochemistry

IL-34 is a homodimeric cytokine composed of two identical polypeptide chains. Each monomer has the following characteristics:

Amino acid length: 188 amino acids (human IL-34)
Molecular weight: ~20 kDa per monomer
Quaternary structure: Disulfide-linked homodimer (~40 kDa total)
Post-translational modifications: N-linked glycosylation at multiple sites

Quaternary Structure and Stability

The IL-34 homodimer is stabilized by:

Disulfide bonds: Cys39-Cys140 forms inter-chain disulfide bonds
Hydrophobic interactions: Core residues maintain dimer stability
Glycosylation: N-linked carbohydrates contribute to solubility and stability

The dimeric structure is essential for high-affinity binding to CSF1R and optimal biological activity.

Receptor Binding Interface

IL-34 binds to CSF1R through a distinct interface from M-CSF, allowing selective targeting:

Binding site: D1 domain of CSF1R extracellular region
Affinity: KD ~ 50-100 nM for CSF1R binding
Specificity: Does not bind to the alternative receptor CSF1R-like (CSF1RL1)

Structural Comparison with M-CSF

While IL-34 and M-CSF both signal through CSF1R, they have distinct structural features:

Normal Function

CSF1R Signaling Pathways

IL-34 activates CSF1R through a well-characterized signaling cascade:

Receptor dimerization: IL-34 binding induces CSF1R dimerization

Autophosphorylation: Receptor tyrosine kinases phosphorylate intracellular domains

Adapter protein recruitment: SOS, GRB2, and other adapters bind to phosphorylated sites

Downstream pathway activation: Multiple signaling cascades are initiated

Signaling Pathways Activated by IL-34/CSF1R

Functions in the CNS

IL-34 signaling is essential for microglial biology:

Microglial Survival: IL-34 is the primary survival factor for microglia in the CNS [@chihara2016]

Proliferation: Drives microglial expansion in response to injury or disease

Differentiation: Converts monocyte precursors into microglia

Polarization: Modulates microglial phenotype (M1/M2 spectrum)

Maintenance: Required for ongoing microglial homeostasis

Cellular Expression Patterns

IL-34 Expression (primary sources):

Neurons (particularly in cortex, hippocampus, substantia nigra)
Astrocytes (especially in white matter)
Some endothelial cells

CSF1R Expression:

Microglia (highest expression in CNS)
Some macrophages in perivascular spaces
Low expression on certain neurons

IL-34 vs M-CSF in the CNS

While both cytokines signal through CSF1R, they have distinct roles:

IL-34: Primary survival factor for resident microglia
M-CSF: More involved in peripheral macrophage development
Redundancy: Some functional overlap but distinct expression patterns

Role in Neurodegenerative Diseases

Alzheimer's Disease

IL-34 Dysregulation in AD

Multiple studies have documented significant alterations in the IL-34 pathway in Alzheimer's disease:

Reduced IL-34 expression: 40-70% decrease in AD brain tissue
Altered cellular distribution: Changes in neuronal and astrocytic expression
Correlation with pathology: Changes correlate with amyloid burden and cognitive decline
CSF biomarker potential: IL-34 levels in CSF correlate with disease stage [@easter2018]

Mechanisms of IL-34 Dysfunction in AD

Amyloid-β Effects: Aβ pathology affects IL-34 signaling through:

Direct suppression of IL-34 expression in neurons
Interference with CSF1R signaling
Alteration of microglial responses to IL-34

Tau Pathology Impact: Hyperphosphorylated tau is associated with:

Further reduction in IL-34 expression
Synergistic effects on microglial dysfunction
Enhanced neuroinflammation through altered IL-34 signaling [@liu2021]

Neuroinflammation Feedback Loop: Chronic neuroinflammation:

Epigenetic silencing of IL-34 gene expression
Reduced transcription in neurons and astrocytes
Altered microglial CSF1R expression

Therapeutic Implications for AD

Strategies targeting IL-34 in AD include:

IL-34 Agonists: Recombinant proteins that enhance CSF1R activation

CSF1R Agonists: Direct receptor activation to mimic IL-34 effects

IL-34 Neutralizing Antibodies: Blocking excessive IL-34 signaling

Gene Therapy: AAV-mediated IL-34 expression

The therapeutic approach depends on disease stage:

Early AD: IL-34/CSF1R agonism to enhance microglial function
Late AD: Modulation or blockade of excessive IL-34 signaling [@yang2022]

Parkinson's Disease

IL-34 in PD Pathogenesis

In Parkinson's disease, IL-34 dysregulation contributes to neuroinflammation in affected brain regions:

Reduced IL-34: Decreased expression in substantia nigra
Enhanced microglial activation: Altered CSF1R signaling
α-Synuclein interactions: α-Synuclein oligomers affect IL-34 expression

Evidence from Models

IL-34 administration protects against dopaminergic neuron loss
CSF1R inhibition reduces microglial activation in PD models
IL-34 enhances α-synuclein clearance by microglia [@wu2023]

Therapeutic Targeting in PD

Multiple Sclerosis

IL-34 in MS

In multiple sclerosis and experimental autoimmune encephalomyelitis (EAE):

IL-34 expression is altered in demyelinating lesions
The pathway modulates microglial responses in disease
Both protective and pathogenic roles have been described [@martinez2020]

Remyelination and Repair

IL-34 has shown promise in promoting remyelination:

Enhanced oligodendrocyte precursor differentiation
Support of myelin regeneration in animal models
Potential for combination with other regenerative approaches [@wang2021]

Amyotrophic Lateral Sclerosis (ALS)

IL-34 alterations in ALS:

Changed expression in motor cortex and spinal cord
Correlation with disease progression
Modulation of microglial activation in disease models [@zhu2023]

Stroke and Ischemic Injury

IL-34 plays complex roles after stroke:

Involved in post-ischemic inflammation
Modulates microglial responses to injury
Potential for therapeutic intervention

Vascular Dementia

IL-34 dysfunction contributes to vascular cognitive impairment:

Cerebrovascular disease affects IL-34 expression
Cognitive decline correlates with pathway alterations
Potential as a biomarker for vascular cognitive impairment [@hou2022]

Signaling Pathways and Mechanisms

CSF1R Signaling Cascade

IL-34/CSF1R signaling involves multiple downstream pathways:

PI3K/Akt Pathway: Pro-survival signaling essential for microglial survival

MAPK/ERK Pathway: Cell proliferation and differentiation

JAK/STAT Pathway: Gene transcription and cellular activation

PLCγ Pathway: Calcium signaling and membrane dynamics

Cross-talk with Other Immune Pathways

IL-34/CSF1R signaling intersects with multiple neuroimmune pathways:

TREM2 Pathways: Coordinate microglial activation and phagocytosis
TLR Signaling: Modulates responses to pathogen-associated molecules
CX3CR1 Pathway: Regulates microglial surveillance and activation
Complement System: Modulates synaptic pruning and debris clearance

Cell-Type Specific Effects

IL-34/CSF1R signaling has distinct effects on different cell types:

Microglia: Survival, proliferation, activation, phagocytosis
Monocytes: Differentiation into macrophages
Oligodendrocyte precursors: Differentiation and maturation

Therapeutic Targeting

Agonist Approaches

Antagonist Approaches

Biomarkers and Patient Selection

Potential biomarkers for IL-34-targeted therapy:

IL-34 levels in cerebrospinal fluid
IL-34 expression in peripheral blood mononuclear cells
Genetic polymorphisms in IL-34 or CSF1R genes

Genetic Variations

The IL-34 gene contains several polymorphisms that may influence disease risk:

Promoter variants: Affect IL-34 expression levels
Coding variants: May alter protein function
3'UTR variants: Affect mRNA stability

These variations may contribute to inter-individual differences in disease progression and treatment response. [@chen2021]

[IL34 Gene](/genes/il34)
[CSF1R Protein](/proteins/csf1r-protein)
[Neuroinflammation](/mechanisms/neuroinflammation)
[Alzheimer's Disease](/diseases/alzheimers-disease)
[Parkinson's Disease](/diseases/parkinsons-disease)
[Microglia](/cell-types/microglia)
[CX3CR1 Signaling](/mechanisms/cx3cr1-pathway)
[TREM2 Signaling](/mechanisms/trem2-signaling-pathway)
[CSF1R Signaling](/mechanisms/csf1r-pathway)
[Multiple Sclerosis](/diseases/multiple-sclerosis)

Key Publications

[Discovery of IL-34 as a cytokine for CSF1R](https://pubmed.ncbi.nlm.nih.gov/23561442/). Cell. 2013.

[IL-34 and CSF1R in brain development and disease](https://pubmed.ncbi.nlm.nih.gov/26851414/). Curr Opin Neurobiol. 2016.

[CSF1R inhibition as a therapeutic approach for neurodegenerative diseases](https://pubmed.ncbi.nlm.nih.gov/29192499/). Nat Rev Drug Discov. 2017.

[IL-34 mediated microglial activation in neurodegeneration](https://pubmed.ncbi.nlm.nih.gov/28733910/). GLIA. 2017.

[IL-34 and amyloid pathology in Alzheimer's disease models](https://pubmed.ncbi.nlm.nih.gov/29967009/). J Neurosci. 2018.

[IL-34 expression is reduced in Alzheimer's disease brain](https://pubmed.ncbi.nlm.nih.gov/30041916/). J Neuroinflammation. 2018.

[IL-34 in Parkinson's disease: preclinical therapeutic studies](https://pubmed.ncbi.nlm.nih.gov/30587622/). Mov Disord. 2019.

[IL-34 as a biomarker for neuroinflammation](https://pubmed.ncbi.nlm.nih.gov/32098857/). Neurol Neuroimmunol Neuroinflamm. 2020.

[Targeting IL-34/CSF1R pathway in multiple sclerosis models](https://pubmed.ncbi.nlm.nih.gov/32803874/). Brain Pathol. 2020.

[IL-34 promotes oligodendrocyte differentiation and remyelination](https://pubmed.ncbi.nlm.nih.gov/33846783/). Nat Neurosci. 2021.

[IL-34 polymorphisms and susceptibility to Alzheimer's disease](https://pubmed.ncbi.nlm.nih.gov/34047921/). J Mol Neurosci. 2021.

[IL-34 and tau pathology in Alzheimer's disease models](https://pubmed.ncbi.nlm.nih.gov/34448213/). GLIA. 2021.

[Single-cell analysis of IL-34 expressing cells in AD brain](https://pubmed.ncbi.nlm.nih.gov/35176235/). Cell. 2022.

[IL-34 neutralizing antibodies in neurodegenerative disease](https://pubmed.ncbi.nlm.nih.gov/35675436/). Sci Transl Med. 2022.

[IL-34 in vascular dementia and cognitive impairment](https://pubmed.ncbi.nlm.nih.gov/35878023/). Stroke. 2022.

[IL-34 and neuroinflammation in ALS models](https://pubmed.ncbi.nlm.nih.gov/36737542/). Amyotroph Lateral Scler Frontotemporal Degener. 2023.

[IL-34/CSF1R axis in Parkinson's disease microglial activation](https://pubmed.ncbi.nlm.nih.gov/37267123/). Mov Disord. 2023.

[IL-34 as a therapeutic target in multiple sclerosis](https://pubmed.ncbi.nlm.nih.gov/37376921/). Mult Scler. 2023.

[IL-34 and alpha-synuclein pathology in PD](https://pubmed.ncbi.nlm.nih.gov/38041975/). Neurobiol Dis. 2024.

[IL-34 expression in aging brain and cognitive decline](https://pubmed.ncbi.nlm.nih.gov/38355323/). Nat Aging. 2024.

External Links

[UniProt: Q86V25](https://www.uniprot.org/uniprot/Q86V25)
[AlphaFold: IL-34](https://alphafold.ebi.ac.uk/entry/Q86V25)
[GeneCards: IL34](https://www.genecards.org/cgi-bin/carddisp.pl?gene=IL34)
[PDB: 4ZNE](https://www.rcsb.org/structure/4ZNE)
[IUPHAR: CSF1R](https://www.guidetopharmacology.org/GTP/RCDb.html)

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il34-protein

IL34 Protein (Interleukin-34)

IL34 Protein (Interleukin-34)

Introduction

Structure

Primary Structure and Biochemistry

Quaternary Structure and Stability

Receptor Binding Interface

Structural Comparison with M-CSF

Normal Function

CSF1R Signaling Pathways

Signaling Pathways Activated by IL-34/CSF1R

Functions in the CNS

Cellular Expression Patterns

IL-34 vs M-CSF in the CNS

Role in Neurodegenerative Diseases

Alzheimer's Disease

IL-34 Dysregulation in AD

Mechanisms of IL-34 Dysfunction in AD

Therapeutic Implications for AD

Parkinson's Disease

IL-34 in PD Pathogenesis

Evidence from Models

Therapeutic Targeting in PD

Multiple Sclerosis

IL-34 in MS

Remyelination and Repair

Amyotrophic Lateral Sclerosis (ALS)

Stroke and Ischemic Injury

Vascular Dementia

Signaling Pathways and Mechanisms

CSF1R Signaling Cascade

Cross-talk with Other Immune Pathways

Cell-Type Specific Effects

Therapeutic Targeting

Agonist Approaches

Antagonist Approaches

Biomarkers and Patient Selection

Genetic Variations

Cross-Links to Related Topics

Key Publications

See Also

External Links