| C4B Protein | |
|---|---|
| Protein Name | Complement Component 4B (C4B) |
| Gene | [C4B](/genes/c4b) |
| UniProt ID | P0C0P4 |
| Molecular Weight | ~193 kDa (alpha chain: ~97 kDa) |
| Subcellular Localization | Secreted (plasma), membrane-bound |
| Protein Family | [Complement system](/entities/complement-system), MHC class III |
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[1].
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[2].
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[3].
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 and microglia[4].
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)[5].
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[6].
In Parkinson's disease, complement proteins may contribute to the progressive loss of dopaminergic 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[7].
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[8].
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[9].
Ricklin et al. Complement: a key system for immune surveillance and homeostasis. Nature Immunology. 2010. ↩︎ ↩︎
Stevens et al. Complement and microglia in synaptic elimination and neurodegeneration. Nature Reviews Neuroscience. 2022. ↩︎ ↩︎
Janssen et al. Structure of complement component C4. Annual Review of Immunology. 2023. ↩︎ ↩︎
van Beek et al. Complement gene expression in brain. Journal of Neuroimmunology. 2021. ↩︎ ↩︎
Wu et al. Complement in Alzheimer's disease. Molecular Neurodegeneration. 2019. ↩︎ ↩︎
Zhang et al. C4B polymorphisms and Alzheimer's disease risk. Neurobiology of Aging. 2020. ↩︎
Chen et al. Complement activation in Parkinson's disease. Movement Disorders. 2021. ↩︎
Liao et al. Complement in amyotrophic lateral sclerosis. Brain Pathology. 2022. ↩︎
Ricklin et al. Complement-targeted therapeutics in neurological diseases. Nature Reviews Drug Discovery. 2023. ↩︎