IL-34 Protein

IL-34 Protein

Introduction

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">IL-34 Protein</th>
</tr>
<tr>
<td class="label">Feature</td>
<td>IL-34</td>
</tr>
<tr>
<td class="label">Primary expression</td>
<td>Neurons</td>
</tr>
<tr>
<td class="label">Receptor specificity</td>
<td>CSF1R only</td>
</tr>
<tr>
<td class="label">Brain function</td>
<td>Microglial maintenance</td>
</tr>
<tr>
<td class="label">Disease relevance</td>
<td>Chronic neurodegeneration</td>
</tr>
<tr>
<td class="label">Strategy</td>
<td>Mechanism</td>
</tr>
<tr>
<td class="label">CSF1R Inhibitors</td>
<td>Reduce microglial activation</td>
</tr>
<tr>
<td class="label">IL-34 Neutralization</td>
<td>Modulate microglial phenotype</td>
</tr>
<tr>
<td class="label">CSF1R Modulation</td>
<td>Fine-tune microglial function</td>
</tr>
<tr>
<td class="label">Compound</td>
<td>Selectivity</td>
</tr>
<tr>
<td class="label">PLX3397</td>
<td>CSF1R/Flt3</td>
</tr>
<tr>
<td class="label">PLX5622</td>
<td>CSF1R</td>
</tr>
<tr>
<td class="label">BLZ945</td>
<td>CSF1R</td>
</tr>
<tr>
<td class="label">NCT ID</td>
<td>Intervention</td>
</tr>
<tr>
<td class="label">NCT03822606</td>
<td>PLX3397</td>
</tr>
<tr>
<td class="label">NCT04050327</td>
<td>CSF1R inhibitor</td>
</tr>
</table>

IL-34 Protein

Introduction

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">IL-34 Protein</th>
</tr>
<tr>
<td class="label">Feature</td>
<td>IL-34</td>
</tr>
<tr>
<td class="label">Primary expression</td>
<td>Neurons</td>
</tr>
<tr>
<td class="label">Receptor specificity</td>
<td>CSF1R only</td>
</tr>
<tr>
<td class="label">Brain function</td>
<td>Microglial maintenance</td>
</tr>
<tr>
<td class="label">Disease relevance</td>
<td>Chronic neurodegeneration</td>
</tr>
<tr>
<td class="label">Strategy</td>
<td>Mechanism</td>
</tr>
<tr>
<td class="label">CSF1R Inhibitors</td>
<td>Reduce microglial activation</td>
</tr>
<tr>
<td class="label">IL-34 Neutralization</td>
<td>Modulate microglial phenotype</td>
</tr>
<tr>
<td class="label">CSF1R Modulation</td>
<td>Fine-tune microglial function</td>
</tr>
<tr>
<td class="label">Compound</td>
<td>Selectivity</td>
</tr>
<tr>
<td class="label">PLX3397</td>
<td>CSF1R/Flt3</td>
</tr>
<tr>
<td class="label">PLX5622</td>
<td>CSF1R</td>
</tr>
<tr>
<td class="label">BLZ945</td>
<td>CSF1R</td>
</tr>
<tr>
<td class="label">NCT ID</td>
<td>Intervention</td>
</tr>
<tr>
<td class="label">NCT03822606</td>
<td>PLX3397</td>
</tr>
<tr>
<td class="label">NCT04050327</td>
<td>CSF1R inhibitor</td>
</tr>
</table>

Interleukin-34 (IL-34) is a cytokine that plays critical roles in the regulation of innate immune cells, particularly microglia in the central nervous system. Discovered in 2008 as a second ligand for the colony-stimulating factor 1 receptor (CSF1R), IL-34 has emerged as a key regulator of microglial survival, proliferation, differentiation, and functional activation in the context of neurodegenerative diseases.

IL-34 is distinct from the more well-known cytokine CSF1 (macrophage colony-stimulating factor, M-CSF) in its expression patterns, receptor binding characteristics, and biological functions. While both cytokines signal through CSF1R, IL-34 is predominantly expressed in neuronal populations in the brain, making it a critical neuron-microglia communication signal. This spatial specificity positions IL-34 as a central player in neuroinflammation and neurodegenerative disease pathogenesis.

This comprehensive review examines the structure-function relationships of IL-34, its normal physiological roles in the nervous system, its dysregulation in Alzheimer's disease (AD), Parkinson's disease (PD), and related neurodegenerative conditions, and current therapeutic strategies targeting the IL-34/CSF1R axis.

Structure

Primary Structure and Biochemistry

IL-34 is a secreted glycoprotein with a molecular weight of approximately 39 kDa as a homodimer. Each monomer consists of approximately 200 amino acids, with the protein forming a stable antiparallel homodimer through disulfide bond formation at C-terminal cysteine residues [@n'Diaye2019]. The protein lacks significant sequence homology to other cytokines, making it a unique member of the cytokine family.

Three-Dimensional Structure

Crystallographic studies have revealed the structural basis for IL-34's interaction with CSF1R. The protein adopts a long-chain helical bundle fold characteristic of cytokine growth factors, with four helices (A-D) arranged in an up-up-down-down topology. The receptor binding interface involves multiple contact points between IL-34 and the extracellular domains of CSF1R, with distinct binding kinetics compared to CSF1.

Post-Translational Modifications

IL-34 undergoes several post-translational modifications:

• N-linked glycosylation: Multiple N-glycosylation sites in the N-terminal region affect protein stability and secretion
• O-linked glycosylation: Present in the flexible loop regions
• Disulfide bonding: Two conserved cysteine residues (C122 and C141) form an inter-subunit disulfide bond stabilizing the homodimer

Receptor Binding Specificity

IL-34 signals exclusively through CSF1R, unlike CSF1 which can also bind the related receptor FLT3. The binding affinity of IL-34 for CSF1R (Kd ~10-20 nM) is comparable to CSF1, but the kinetics and thermodynamic parameters differ. Notably, IL-34 can also bind to the receptor-like protein PTP-ζ (PTPRZ1) in the CNS, expanding its signaling repertoire.

Normal Function

Microglial Regulation

IL-34 is the primary cytokine sustaining microglial populations in the healthy brain. Unlike peripheral macrophages that depend on both CSF1 and IL-34, brain microglia are critically dependent on IL-34 signaling for their survival and maintenance [@greter2012].

The essential functions of IL-34 in microglial biology include:

CNS Regional Specificity

IL-34 expression is not uniform across the brain:

• High expression: Cerebral cortex, hippocampus (CA1/CA3), substantia nigra
• Moderate expression: Cerebellum, thalamus
• Low expression: Brainstem

Comparison with CSF1

While both IL-34 and CSF1 signal through CSF1R, they have distinct biological roles:

Role in Neurodegenerative Diseases

Alzheimer's Disease

IL-34 Expression Changes in AD

Multiple studies have demonstrated that IL-34 expression is significantly altered in Alzheimer's disease brains. Elevated IL-34 levels have been reported in:

• Postmortem AD brain tissue (frontal cortex, hippocampus)
• Cerebrospinal fluid of AD patients
• Plasma of patients with mild cognitive impairment (MCI)

Mechanisms of IL-34 Dysregulation in AD

Several interconnected mechanisms contribute to IL-34 alterations in AD:

Amyloid-β Effects: Aβ1-42 oligomers directly stimulate neuronal IL-34 expression through NF-κB-dependent pathways. This creates a feedback loop where amyloid pathology triggers increased IL-34, which in turn modulates microglial responses to plaques.

Tau Pathology Impact: Hyperphosphorylated tau accumulation is associated with reduced IL-34 expression in specific brain regions, suggesting region-specific dysregulation.

Neuroinflammation: Pro-inflammatory cytokines (IL-1β, TNF-α) downregulate IL-34 expression in neurons through negative feedback mechanisms.

Microglial Interactions: IL-34 modulates microglial phenotype around amyloid plaques, promoting a disease-associated microglia (DAM) phenotype that may have both protective and pathogenic functions.

Therapeutic Implications for AD

Targeting the IL-34/CSF1R axis in AD includes several approaches:

Parkinson's Disease

IL-34 in PD Pathogenesis

In Parkinson's disease, IL-34 plays a role in microglial responses within the substantia nigra and other affected regions:

Dopaminergic Neuron Vulnerability: IL-34 is expressed by dopaminergic neurons and may serve as a trophic signal for microglia in the substantia nigra.

Microglial Activation: IL-34 promotes a pro-inflammatory microglial phenotype that may contribute to dopaminergic neuron death. Studies have shown elevated IL-34 in the substantia nigra of PD patients.

α-Synuclein Interactions: IL-34 expression is modulated by α-synuclein pathology, creating bidirectional feedback between protein aggregation and microglial activation.

Therapeutic Targeting in PD

Multiple Sclerosis and Related Conditions

IL-34 has been implicated in demyelinating diseases:

• Multiple Sclerosis: Elevated IL-34 in CSF and brain lesions; correlates with disease activity
• Experimental Autoimmune Encephalomyelitis (EAE): IL-34 blockade ameliorates disease progression
• Amyotrophic Lateral Sclerosis: Dysregulated IL-34 in motor cortex and CSF

Signaling Pathways and Mechanisms

CSF1R Signaling Cascade

IL-34 binding to CSF1R triggers the following canonical signaling pathways:

Microglial Phenotype Modulation

IL-34 differentially affects microglial phenotypes:

• Pro-inflammatory: IL-34 can promote expression of pro-inflammatory cytokines (IL-1β, TNF-α, IL-6)
• Anti-inflammatory: IL-34 also promotes expression of anti-inflammatory markers (Arg1, IL-10)
• Phagocytic Activity: IL-34 enhances microglial phagocytosis of Aβ and debris

Cross-talk with Other Pathways

IL-34 signaling intersects with multiple neuroimmune pathways:

• TREM2 Signaling: Both IL-34 and TREM2 regulate microglial responses to amyloid
• Complement System: IL-34 modulates expression of complement proteins
• Cytokine Networks: IL-34 interacts with IL-1β, TNF-α, and TGF-β signaling

Therapeutic Targeting

CSF1R Inhibitors

CSF1R inhibitors have shown promise in preclinical models of AD and PD, reducing microglial burden and improving behavioral outcomes.

IL-34-Targeting Strategies

Clinical Trials

Genetic Variations

The IL34 gene contains several polymorphisms associated with disease risk:

• rs1512714: Promoter variant associated with AD risk
• rs6738835: Intron variant linked to PD susceptibility
• rs1129563: Coding variant affecting protein function

Cross-Links to Related Topics

• [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)
• [Disease-Associated Microglia (DAM)](/mechanisms/disease-associated-microglia)
• [TREM2 Signaling](/mechanisms/trem2-signaling)

Key Publications

See Also

• [CSF1R Protein](/proteins/csf1r-protein)
• [IL34 Gene](/genes/il34)
• [Neuroinflammation](/mechanisms/neuroinflammation)
• [Microglia](/cell-types/microglia)
• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)

External Links

• [UniProt: Q86SZ3](https://www.uniprot.org/uniprot/Q86SZ3)
• [PDB: 4O8W](https://www.rcsb.org/structure/4O8W)
• [PDB: 5BVC](https://www.rcsb.org/structure/5BVC)
• [AlphaFold: IL-34](https://alphafold.ebi.ac.uk/entry/Q86SZ3)
• [GeneCards: IL34](https://www.genecards.org/cgi-bin/carddisp.pl?gene=IL34)
• [IUPHAR: CSF1R](https://www.guidetopharmacology.org/GTP/RCDb.html)

Brain Atlas Resources

• [Allen Human Brain Atlas](https://human.brain-map.org/) — protein expression data
• [Allen Cell Type Atlas](https://celltypes.brain-map.org/) — cell type specific expression
• [BrainSpan Atlas](https://brainspan.org/) — developmental transcriptome
• [Allen Mouse Brain Atlas](https://mouse.brain-map.org/) — mouse brain expression

References