C4A Protein (Complement Component 4A)
Introduction
<table class="infobox infobox-protein">
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<th class="infobox-header" colspan="2">C4A Protein (Complement Component 4A)</th>
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<td class="label">Symbol</td>
<td><strong>C4A</strong></td>
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<td class="label">Full Name</td>
<td>C4A (Complement Component 4A)</td>
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<td class="label">Type</td>
<td>Protein</td>
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<td class="label">UniProt</td>
<td><a href="https://www.uniprot.org/uniprot/?query=C4A" target="_blank">Search UniProt</a></td>
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<td class="label">Associated Diseases</td>
<td><a href="/wiki/als" style="color:#ef9a9a">ALS</a>, <a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/ms" style="color:#ef9a9a">Ms</a>, <a href="/wiki/schizophrenia" style="color:#ef9a9a">Schizophrenia</a></td>
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<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">18 edges</a></td>
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C4A Protein (Complement Component 4A) is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.
Overview
Complement component 4A (C4A) is a core effector of the classical complement cascade. After activation, C4A is cleaved to generate covalently binding fragments that label surfaces for immune clearance and downstream complement amplification.[@law1997][@merle2015] In the CNS, complement signaling contributes to normal synapse refinement, but persistent activation can promote pathological synapse loss and inflammatory injury in neurodegenerative disorders.[@stevens2007][@hong2016]
Molecular Biology and Processing
C4A is synthesized as a precursor glycoprotein and processed into alpha, beta, and gamma chains linked by disulfide bonds.[@law1997] Upon classical pathway activation (typically via C1 complex recognition), C4 is cleaved to C4b, which exposes a reactive thioester and deposits on nearby membranes or protein aggregates.[@law1997][@merle2015]
Deposited C4b supports assembly of C3 convertase, coupling C4A activity to [C3 Protein (Complement Component 3)](/proteins/c3-protein) cleavage and broad complement amplification.[@merle2015][@hong2016] This places C4A upstream of many neuroimmune outcomes, including microglial phagocytic signaling.
CNS Complement Circuitry
Physiologic role: synapse pruning
Developmental studies established that classical complement components participate in activity-dependent synaptic pruning. C1q and C3 are best characterized, but C4 availability modulates pathway gain and therefore influences tagging pressure on vulnerable synapses.[@stevens2007][@sekar2016]
Pathologic role: maladaptive pruning and inflammation
In disease contexts, chronic complement tone can shift from selective remodeling to excessive synapse elimination and persistent microglial activation.[@hong2016][@dejanovic2018] This mechanism is especially relevant in networks with high protein-aggregate burden and local cytokine signaling, where complement deposition becomes spatially expanded rather than tightly restricted.
Disease Relevance
Alzheimer’s disease
In [Alzheimer's disease](/diseases/alzheimers-disease), complement factors accumulate around plaques and dystrophic neurites, and complement-dependent microglial phagocytic programs correlate with early synaptic loss.[@hong2016][@dejanovic2018] Although many studies focus on C1q/C3, C4A is mechanistically upstream and contributes to the same amplification architecture.
Parkinson’s disease and synucleinopathies
In [Parkinson's disease](/diseases/parkinsons-disease) and related synucleinopathies, innate immune activation and complement engagement are observed in vulnerable regions, suggesting that C4A-linked classical pathway activity may participate in neuron-glia injury loops.[@depboylu2011][@liddelow2017]
Genetic evidence for C4 dosage sensitivity
Human genetic studies demonstrate that C4 structural variation and expression level can alter disease risk in brain disorders, supporting the concept that complement dosage is biologically meaningful in neural circuits.[@sekar2016] This has strengthened interest in upstream complement control points for precision therapeutics.
Therapeutic Implications
Current complement-targeted strategies in neurodegeneration primarily inhibit upstream activation (C1 complex), amplification nodes (C3/C5), or receptor signaling on myeloid cells.[@dejanovic2018][@schartz2020] Because C4A sits early in the classical pathway, it represents a biologically coherent intervention point, but balancing host defense with CNS protection remains a central challenge.
Potential translational uses of C4A include:
- pathway stratification biomarker in neuroinflammatory endophenotypes
- pharmacodynamic readout for complement-modulating therapies
- integration with imaging and CSF markers of synapse injury
See Also
- [C1QA Gene](/genes/c1q)
- [C3 Protein (Complement Component 3)](/proteins/c3-protein)
- [Microglia in Neuroinflammation](/cell-types/microglia-in-neuroinflammation)
- [Neuroinflammation](/mechanisms/neuroinflammation)
- [Alzheimer's Disease](/diseases/alzheimers-disease)
Background
The study of C4A Protein (Complement Component 4A) 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.
External Links
- [PubMed](https://pubmed.ncbi.nlm.nih.gov/) - Biomedical literature
- [Alzheimer's Disease Neuroimaging Initiative](https://adni.loni.usc.edu/) - Research data
- [Allen Brain Atlas](https://brain-map.org/) - Brain gene expression data
References
[Law SKA, Dodds AW, The internal thioester and complement (1997)](https://pubmed.ncbi.nlm.nih.gov/2448463/)
[Merle NS, Church SE, Fremeaux-Bacchi V, Roumenina LT, Complement system part I: molecular mechanisms of activation and regulation (2015)](https://pubmed.ncbi.nlm.nih.gov/30393731/)
[Stevens B, Allen NJ, Vazquez LE, et al, The classical complement cascade mediates CNS synapse elimination (2007)](https://pubmed.ncbi.nlm.nih.gov/17972862/)
[Hong S, Beja-Glasser VF, Nfonoyim BM, et al, Complement and microglia mediate early synapse loss in Alzheimer mouse models (2016)](https://pubmed.ncbi.nlm.nih.gov/27476828/)
[Sekar A, Bialas AR, de Rivera H, et al, Schizophrenia risk from complex variation of complement component 4 (2016)](https://pubmed.ncbi.nlm.nih.gov/26814963/)
[Dejanovic B, Huntley MA, De Maziere A, et al, Changes in the synaptic proteome in tauopathy and rescue by C1q antibodies (2018)](https://pubmed.ncbi.nlm.nih.gov/30270045/)
[Depboylu C, Schäfer MKH, Arias-Carrión O, et al, Possible involvement of complement factor C1q in Parkinson's disease (2011)](https://pubmed.ncbi.nlm.nih.gov/16293288/)
[Liddelow SA, Guttenplan KA, Clarke LE, et al, Neurotoxic reactive astrocytes are induced by activated microglia (2017)](https://pubmed.ncbi.nlm.nih.gov/28099414/)
[Schartz ND, Tenner AJ, The good, the bad, and the opportunities of the complement system in neurodegenerative disease (2020)](https://pubmed.ncbi.nlm.nih.gov/28778796/)