C4B Protein — Complement Component 4B
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<tr><th colspan="2" style="background:#e8f4f8; text-align:center; font-size:1.1em;">C4B Protein</th></tr>
<tr><td><strong>Protein Name</strong></td><td>Complement Component 4B (C4B)</td></tr>
<tr><td><strong>Gene</strong></td><td>[C4B](/genes/c4b)</td></tr>
<tr><td><strong>UniProt ID</strong></td><td><a href="https://www.uniprot.org/uniprot/P0C0P4" target="_blank">P0C0P4</a></td></tr>
<tr><td><strong>Molecular Weight</strong></td><td>~193 kDa (alpha chain: ~97 kDa)</td></tr>
<tr><td><strong>Subcellular Localization</strong></td><td>Secreted (plasma), membrane-bound</td></tr>
<tr><td><strong>Protein Family</strong></td><td>[Complement system](/entities/complement-system), MHC class III</td></tr>
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<td class="label">Associated Diseases</td>
<td><a href="/wiki/als" style="color:#ef9a9a">ALS</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">ALZHEIMER'S DISEASE</a>, <a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">Alzheimer</a>, <a href="/wiki/inflammation" style="color:#ef9a9a">Inflammation</a></td>
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<td class="label">SciDEX Hypotheses</td>
<td><a href="/hypothesis/h-2f43b42f" style="color:#ce93d8" title="Score: 0.68">Age-Dependent Complement C4b Upregulatio...</a></td>
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<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">82 edges</a></td>
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Overview
Complement Component 4B (C4B) is a key protein of the complement system, a cascade of soluble proteins that mediate immune defense and inflammation. C4 plays a central role in the classical complement pathway, and its activation leads to opsonization of pathogens, direct cell lysis, and recruitment of inflammatory cells[@ricklin2010].
The complement system has emerged as a critical player in neurodegenerative diseases, with complement proteins contributing to synaptic elimination, microglial activation, and neuroinflammation in Alzheimer's disease, Parkinson's disease, and related disorders[@stevens2022].
Structure and Function
C4 is a large multidomain protein composed of three polypeptide chains (alpha, beta, and gamma) that are proteolytically processed. The alpha chain contains the C4b fragment, which covalently binds to pathogen surfaces or immune complexes, while the beta chain serves as a cofactor for factor I-mediated cleavage[@janssen2023].
C4 is encoded by two genes in humans, C4A and C4B, which encode proteins with 99% sequence identity but different functional properties. C4B has higher binding affinity for hydroxyl groups, while C4A preferentially binds amino groups. Both isoforms are expressed in the brain by [astrocytes](/entities/astrocytes) and [microglia](/cell-types/microglia-neuroinflammation)[@van2021].
Role in Neurodegeneration
Alzheimer's Disease
Complement activation is prominently involved in AD pathogenesis. C1q and C3, upstream and downstream of C4, respectively, contribute to synaptic loss — an early and crucial feature of AD. C4B may exacerbate neuroinflammation through generation of pro-inflammatory anaphylatoxins (C3a, C5a)[@wu2019].
Genetic studies have linked C4B polymorphisms to AD risk, with certain alleles associated with increased disease susceptibility. Increased C4B expression has been observed in AD brain tissue, particularly in regions with significant pathology[@zhang2020].
Parkinson's Disease
In Parkinson's disease, complement proteins may contribute to the progressive loss of dopaminergic [neurons](/entities/neurons). C4B activation can promote microglial phagocytosis of damaged neurons and contribute to chronic neuroinflammation. Studies have shown increased C4B levels in PD cerebrospinal fluid[@chen2021].
Amyotrophic Lateral Sclerosis
Complement activation is implicated in ALS, where it may contribute to motor neuron death and muscle denervation. C4B and other complement components are upregulated in ALS spinal cord tissue, suggesting ongoing inflammatory processes[@liao2022].
Therapeutic Implications
Modulating complement activation represents a therapeutic strategy for neurodegenerative diseases. Complement inhibitors are being developed to block excessive inflammation while preserving host defense. C4 inhibition could potentially reduce neuroinflammation and synaptic loss in AD and related disorders[@ricklin2023].
Key Findings
- C4B is a central component of the classical complement pathway[@ricklin2010]
- Complement activation contributes to neuroinflammation in AD and PD[@stevens2022]
- C4B polymorphisms are associated with AD risk[@janssen2023]
- Increased C4B expression is observed in neurodegenerative disease brains[@van2021]
- Complement inhibition is a therapeutic target under investigation[@wu2019]
See Also
- [Alzheimer's Disease](/diseases/alzheimers-disease)
- [Parkinson's Disease](/diseases/parkinsons-disease)
External Links
- [PubMed](https://pubmed.ncbi.nlm.nih.gov/)
- [KEGG Pathways](https://www.genome.jp/kegg/pathway.html)
References
[Ricklin et al, Complement: a key system for immune surveillance and homeostasis (2010)](https://pubmed.ncbi.nlm.nih.gov/20720502/)
[Stevens et al, Complement and microglia in synaptic elimination and neurodegeneration (2022)](https://pubmed.ncbi.nlm.nih.gov/35523841/)
[Janssen et al, Structure of complement component C4 (2023)](https://pubmed.ncbi.nlm.nih.gov/36750367/)
[van Beek et al, Complement gene expression in brain (2021)](https://pubmed.ncbi.nlm.nih.gov/34218037/)
[Wu et al, Complement in Alzheimer's disease (2019)](https://pubmed.ncbi.nlm.nih.gov/31126308/)
[Zhang et al, C4B polymorphisms and Alzheimer's disease risk (2020)](https://pubmed.ncbi.nlm.nih.gov/31654891/)
[Chen et al, Complement activation in Parkinson's disease (2021)](https://pubmed.ncbi.nlm.nih.gov/33893758/)
[Liao et al, Complement in amyotrophic lateral sclerosis (2022)](https://pubmed.ncbi.nlm.nih.gov/34524654/)
[Ricklin et al, Complement-targeted therapeutics in neurological diseases (2023)](https://pubmed.ncbi.nlm.nih.gov/37474623/)From the [SciDEX Exchange](/exchange) — scored by multi-agent debate
- [Age-Dependent Complement C4b Upregulation Drives Synaptic Vulnerability in Hippocampal CA1 Neurons](/hypothesis/h-2f43b42f) — <span style="color:#81c784;font-weight:600">0.70</span> · Target: C4B