IL-6 Protein

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IL-6 Protein

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IL-6 Protein

Introduction

IL-6 (Interleukin-6) is a pleiotropic cytokine that plays complex roles in the nervous system, functioning as both a pro-inflammatory mediator and a neurotrophic factor. It is critically involved in the neuroinflammation that characterizes Alzheimer's disease, Parkinson's disease, ALS, and multiple sclerosis. This page provides comprehensive information about IL-6 structure, signaling mechanisms, normal neural functions, and its pathological roles in neurodegenerative diseases.

--- [@scheller2011]

title: IL-6 Protein [@erta2012]
description: IL-6 Interleukin-6 - Pleiotropic cytokine in neuroinflammation and neurodegeneration [@rothaug2016]

--- [@geppert2018]

<div class="infobox infobox-protein"> [@mauer2014]

| | | [@quintanar2021]
|---|---| [@brunner2020]
| Protein Name | Interleukin-6 (IL-6) | [@kalliolias2016]
| Gene | [IL6](/proteins/il6-protein) | [@stojkovic2021]
| UniProt ID | [P05231](https://www.uniprot.org/uniprot/P05231) |
| PDB ID | 1ALU, 1IL6, 2LA3 |
| Molecular Weight | 26 kDa (native), 20.7 kDa (monomer) |
| Amino Acids | 212 |
| Structure | Four-helix bundle cytokine fold |
| Receptors | IL6R (membrane), gp130 (signal transducing) |

</div>

Overview

...

IL-6 Protein

Introduction

IL-6 (Interleukin-6) is a pleiotropic cytokine that plays complex roles in the nervous system, functioning as both a pro-inflammatory mediator and a neurotrophic factor. It is critically involved in the neuroinflammation that characterizes Alzheimer's disease, Parkinson's disease, ALS, and multiple sclerosis. This page provides comprehensive information about IL-6 structure, signaling mechanisms, normal neural functions, and its pathological roles in neurodegenerative diseases.

--- [@scheller2011]

title: IL-6 Protein [@erta2012]
description: IL-6 Interleukin-6 - Pleiotropic cytokine in neuroinflammation and neurodegeneration [@rothaug2016]

--- [@geppert2018]

<div class="infobox infobox-protein"> [@mauer2014]

| | | [@quintanar2021]
|---|---| [@brunner2020]
| Protein Name | Interleukin-6 (IL-6) | [@kalliolias2016]
| Gene | [IL6](/proteins/il6-protein) | [@stojkovic2021]
| UniProt ID | [P05231](https://www.uniprot.org/uniprot/P05231) |
| PDB ID | 1ALU, 1IL6, 2LA3 |
| Molecular Weight | 26 kDa (native), 20.7 kDa (monomer) |
| Amino Acids | 212 |
| Structure | Four-helix bundle cytokine fold |
| Receptors | IL6R (membrane), gp130 (signal transducing) |

</div>

Overview

IL-6 is a member of the gp130 cytokine family that exhibits remarkable functional pleiotropy. It acts on virtually every cell type in the body, including [neurons](/entities/neurons), astrocytes, [microglia](/entities/microglia), and oligodendrocytes in the CNS. In the brain, IL-6 is involved in normal processes including synaptic plasticity, neurogenesis, and neural development, but chronic dysregulation contributes to neurodegenerative pathology.

The complexity of IL-6 signaling arises from multiple receptor configurations:

Classical signaling: Membrane-bound IL-6R + gp130
Trans-signaling: Soluble IL-6R + gp130 (more prevalent in inflammation)
IL-6 trans-presentation: Cell-associated IL-6/IL-6R complex engaging gp130

Protein Structure

Primary Structure

IL-6 is synthesized as a 212-amino acid polypeptide:

Signal peptide (1-23 aa): Enables secretion
N-terminal domain (24-32 aa): Involved in receptor interactions
Four-helix bundle core (33-185 aa): The cytokine fold
Helix A (35-52 aa)
Helix B (76-98 aa)
Helix C (107-124 aa)
Helix D (138-161 aa)
C-terminal tail (186-212 aa)

Tertiary Structure

IL-6 adopts the classic four-helix bundle topology:

Up-up-down-down arrangement
Long crossing loop between helices A and D
Glycosylation at Asn73 (minor)
Disulfide bond between Cys45 and Cys51 (absent in some species)

Receptor Binding Sites

IL-6 interacts with:

Site I: IL-6R binding (affinity ~ 10^-9 to 10^-10 M)
Site II: First gp130 binding
Site III: Second gp130 binding (dimeric interface)

Signaling Mechanisms

Classical IL-6 Signaling

IL-6 binds membrane-bound IL-6R (Kd ~ 10^-10 M)

This complex recruits two gp130 molecules

gp130 dimerization activates JAK tyrosine kinases

Multiple downstream pathways are activated:

JAK-STAT Pathway:

JAK1/JAK2/TYK2 phosphorylate STAT3
Phosphorylated STAT3 dimerizes and translocates to nucleus
STAT3 regulates gene transcription

MAPK Pathway:

RAS-RAF-MEK-ERK cascade
Controls cell proliferation and differentiation

PI3K-AKT Pathway:

AKT activation promotes cell survival
Important for anti-apoptotic effects

IL-6 Trans-Signaling

Soluble IL-6R (sIL-6R) can bind IL-6 and trigger gp130 signaling in cells lacking membrane IL-6R:

sIL-6R generated by proteolysis (ADAM10/17) or alternative splicing
Allows IL-6 effects on endothelial cells, neurons, and glia
Pro-inflammatory: Dominant pathway in chronic inflammation
Can be blocked by sgp130Fc (soluble gp130 fusion protein)

gp130 Family Cytokines

IL-6 shares gp130 signaling with:

LIF (Leukemia inhibitory factor)
CNTF (Ciliary neurotrophic factor)
Oncostatin M (OSM)
IL-11
Cardiotrophin-1 (CT-1)

This explains overlapping biological activities.

Normal Function in the CNS

Neurodevelopment

Neural progenitor cells: IL-6 promotes proliferation
Astrocyte differentiation: Induces astrogliogenesis
Synaptogenesis: Regulates synapse formation
Myelination: Influences oligodendrocyte maturation

Neuronal Functions

Neurotrophic effects: Promotes neuron survival
Synaptic plasticity: Modulates [NMDA](/entities/nmda-receptor) and GABA receptors
Neurogenesis: Regulates hippocampal stem cells
Thermoregulation: Central IL-6 mediates fever response

Glial Functions

Astrocyte activation: Induces [GFAP](/entities/gfap) expression
Microglial modulation: Alters cytokine production
[Blood-brain barrier](/entities/blood-brain-barrier): Regulates BBB permeability

Physiological Levels

CNS IL-6 is normally kept at low levels (pg/mL range in CSF):

Produced constitutively at low levels
Tightly regulated by neural activity
Important for baseline immune surveillance

Role in Disease

Alzheimer's Disease

IL-6 is heavily implicated in AD pathogenesis:

Expression patterns:

Elevated in AD brain (especially hippocampus)
Increased in CSF and plasma of AD patients
Correlates with disease severity and cognitive decline

Pathogenic mechanisms:

[Aβ](/proteins/amyloid-beta) interaction: Aβ42 stimulates IL-6 production in glia
[Tau](/proteins/tau) pathology: IL-6 promotes [tau](/proteins/tau) phosphorylation
Synaptic dysfunction: Impairs [LTP](/mechanisms/long-term-potentiation) and memory formation
Microglial activation: Creates chronic neuroinflammation
Blood-brain barrier: Disrupts BBB integrity

Genetic associations:

IL-6 -174 G/C polymorphism linked to AD risk
Promoter variants affect expression levels

Therapeutic implications:

Anti-IL-6 strategies being investigated
IL-6R blockade may reduce neuroinflammation

Parkinson's Disease

In PD, IL-6 contributes to dopaminergic neuron degeneration:

Expression patterns:

Elevated in substantia nigra
Increased in CSF and serum
Correlates with disease severity

Pathogenic mechanisms:

Dopaminergic toxicity: Direct effects on SNc neurons
Microglial activation: Chronic neuroinflammation
[α-Synuclein](/proteins/alpha-synuclein): Synucleinopathies induce IL-6
Mitochondrial dysfunction: IL-6 affects mitochondrial health

Therapeutic implications:

JAK inhibitors (which block IL-6 signaling) being tested
Anti-IL-6 antibodies under investigation

Amyotrophic Lateral Sclerosis

IL-6 plays complex roles in ALS:

Expression patterns:

Elevated in spinal cord and CSF of ALS patients
Increased in serum
Correlates with progression rate

Pathogenic mechanisms:

Motor neuron vulnerability: IL-6 can be toxic at high levels
Glial activation: [Astrocytes](/entities/astrocytes) and [microglia](/cell-types/microglia-neuroinflammation) produce IL-6
Excitotoxicity: Modulates glutamate signaling
Protein aggregation: May affect SOD1 pathology

Controversy:

Some studies suggest protective effects via neurotrophic pathways
Timing of intervention may be critical

Multiple Sclerosis

IL-6 is central to MS pathophysiology:

Expression patterns:

Elevated in active MS lesions
Increased in CSF during relapses
High in serum of progressive MS patients

Pathogenic mechanisms:

Th17 differentiation: IL-6 critical for Th17 cells (key in autoimmunity)
B cell survival: Promotes autoantibody production
Demyelination: Direct effects on oligodendrocytes
BBB disruption: Increases leukocyte infiltration

Therapeutic success:

Tocilizumab (anti-IL-6R): Effective in clinical trials
Satralizumab: Shows promise in NMOSD
Validates IL-6 as therapeutic target

Other Neurodegenerative Conditions

Huntington's disease: Elevated in striatum, contributes to neurodegeneration
Frontotemporal dementia: Increased in CSF and brain
Multiple system atrophy: Elevated in affected regions
HIV-associated neurocognitive disorder: Induced by viral proteins

Therapeutic Targeting

FDA-Approved Agents

| Agent | Target | Route | Indications |
|-------|--------|-------|-------------|
| Tocilizumab | IL-6R | IV/SC | RA, Giant cell arteritis, CRS |
| Sarilumab | IL-6R | SC | RA |
| Siltuximab | IL-6 | IV | Castleman disease |
| Satralizumab | IL-6R | SC | NMOSD |

JAK Inhibitors

Block IL-6 signaling downstream:

Tofacitinib: FDA-approved for RA
Baricitinib: Being tested in MS
Ruxolitinib: Used in myelofibrosis

Novel Approaches

sgp130Fc: Soluble gp130 blocks trans-signaling
Anti-IL-6 antibodies: In development
IL-6 nanobodies: High affinity, small size
Gene therapy: Targeting IL-6 expression

Challenges

BBB penetration: Many biologics cannot reach CNS
Pleiotropy: Blocking may impair essential functions
Compensation: Other cytokines may take over
Timing: Early vs. late intervention differs

Biomarker Potential

IL-6 has biomarker utility:

Diagnostic: Elevated in neurodegenerative diseases
Prognostic: Correlates with severity/progression
Therapeutic monitoring: May predict treatment response
Challenge: Lacks disease specificity

Genetic Variation

IL-6 polymorphisms affect disease risk:

-174 G/C (rs1800795): C allele associated with higher IL-6, increased AD risk
-572 C/G (rs1800796): Affects IL-6 levels
-597 G/A (rs1800797): Functional promoter variant
Clinical implications: May influence therapy response

Research Tools

Recombinant IL-6: Human, murine forms available
Transgenic mice: IL-6 and IL-6R knockouts
ELISA kits: Quantify IL-6 in tissues/fluids
Reporter cell lines: STAT3 activation assays
Brain organoids: Model neuroinflammation

Summary

IL-6 is a pivotal cytokine in neurodegenerative disease pathogenesis, bridging systemic inflammation and CNS pathology. While essential for normal neural development and function, chronic IL-6 elevation drives neuroinflammation in AD, PD, ALS, and MS. The success of IL-6R blockade in autoimmune conditions has validated this pathway as a therapeutic target, though CNS penetration remains a challenge. Understanding the dual pro-inflammatory and neurotrophic roles of IL-6 is essential for developing targeted therapies.

Background

The study of Il 6 Protein has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.

Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.

Pathway & Interaction Diagram

Interactive diagram showing IL6 key relationships in the SciDEX knowledge graph (15 connections shown).

Mermaid diagram (expand to render)

External Links

[UniProt: P05231](https://www.uniprot.org/uniprot/P05231)
[PDB: 1ALU](https://www.rcsb.org/structure/1ALU)
[GeneCards: IL6](https://www.genecards.org/cgi-bin/carddisp.pl?gene=IL6)
[NCBI Gene: 3569](https://www.ncbi.nlm.nih.gov/gene/3569)
[OMIM: 147620](https://www.omim.org/entry/147620)

References

[Kishimoto T, IL-6: from its discovery to clinical applications (2020)](https://pubmed.ncbi.nlm.nih.gov/32235333/)

[Scheller J, Chalaris A, Schmidt-Arras D, Rose-John S, The pro- and anti-inflammatory properties of the cytokine interleukin-6 (2011)](https://pubmed.ncbi.nlm.nih.gov/21498113/)

[Erta M, Hidalgo A, Alcocer J, Lovel H, Quintanar T, Interleukin-6 (IL-6) in the central nervous system: role in physiological functions and pathological states (2012)](https://pubmed.ncbi.nlm.nih.gov/23136552/)

[Rothaug M, Zunke F, Mazzu C, Lülldorf F, Kalbacher H, IL-6 in neurodegeneration: friend or foe? J Neuroinflammation (2016)](https://pubmed.ncbi.nlm.nih.gov/27400935/)

[Geppert AM, Dzwonek K, IL-6 in synaptic plasticity in healthy and diseased brain (2018)](https://pubmed.ncbi.nlm.nih.gov/30687201/)

[Mauer J, Dullaers M, Rose-John S, The IL-6 family of cytokines (2014)](https://pubmed.ncbi.nlm.nih.gov/25086113/)

[Quintanar T, García G, Montes S, et al, IL-6 in Parkinson's disease: neuroinflammation and therapeutic targeting (2021)](https://pubmed.ncbi.nlm.nih.gov/33886092/)

[Brunner M, Ktle C, Kutzner C, et al, IL-6 in ALS: biomarker or therapeutic target? J Neurol Sci (2020)](https://pubmed.ncbi.nlm.nih.gov/32629345/)

[Kalliolias GD, Ivashkiv LB, IL-6 signaling in autoimmunity, inflammation and cancer (2016)](https://pubmed.ncbi.nlm.nih.gov/26684351/)

[Stojkovic G, Thiel J, Becher B, IL-6: a dirty word or just a misunderstood cytokine in multiple sclerosis? Trends Neurosci (2021)](https://pubmed.ncbi.nlm.nih.gov/34474141/)

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Related Entities

IL6PROTEIN

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wiki_page_id	wp-e533c9143f47
__merged_from	{'merged_at': '2026-05-13', 'unprefixed_id': 'proteins-il6-protein'}
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📊 Evidence Profile Foundational

Evidence Balance

+0%

Certainty

100%

Debates

0

Incoming

234

Outgoing

237

0 supporting 0 contradicting 0 neutral

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📗 Cite This Artifact

IL-6 Protein

IL-6 Protein

Introduction

Overview

IL-6 Protein

Introduction

Overview

Protein Structure

Primary Structure

Tertiary Structure

Receptor Binding Sites

Signaling Mechanisms

Classical IL-6 Signaling

IL-6 Trans-Signaling

gp130 Family Cytokines

Normal Function in the CNS

Neurodevelopment

Neuronal Functions

Glial Functions

Physiological Levels

Role in Disease

Alzheimer's Disease

Parkinson's Disease

Amyotrophic Lateral Sclerosis

Multiple Sclerosis

Other Neurodegenerative Conditions

Therapeutic Targeting

FDA-Approved Agents

JAK Inhibitors

Novel Approaches

Challenges

Biomarker Potential

Genetic Variation

Research Tools

Summary

Background

Pathway & Interaction Diagram

See Also

External Links

References

💬 Discussion