| C6 |
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| Symbol | C6 |
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| Full Name | Complement Component 6 |
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| Chromosome | 5p13.1 |
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| NCBI Gene ID | [718](https://www.ncbi.nlm.nih.gov/gene/718) |
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| OMIM | [119455](https://www.omim.org/entry/119455) |
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| Ensembl | [ENSG00000124357](https://www.ensembl.org/Homo_sapiens/Gene/Summary?g=ENSG00000124357) |
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| UniProt | [P13671](https://www.uniprot.org/uniprot/P13671) |
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| Associated Diseases | Complement Deficiency, Neisseria Infections, Alzheimer's Disease, Parkinson's Disease, Multiple Sclerosis |
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C6 (Complement Component 6) encodes a terminal complement protein that is essential for the formation of the membrane attack complex (MAC), the final effector of the complement cascade. Located on chromosome 5p13.1, C6 encodes a 922-amino acid protein that circulates in plasma and binds to the C5b-7 complex to form C5b-6-7, which then inserts into target cell membranes. The subsequent addition of C8 and C9 completes the MAC, creating a transmembrane pore that can lead to cell lysis. This gene is critical for innate immunity, particularly for defense against Neisseria species, while also playing important roles in immune regulation and tissue homeostasis [@berends2020].
The complement system is a major component of innate immunity, and its activation leads to three main outcomes: opsonization (tagging pathogens for phagocytosis), inflammation (attracting immune cells to the site of infection), and cell lysis (direct killing of pathogens through the MAC). C6 is essential for the terminal lytic pathway, and its deficiency leads to increased susceptibility to Neisseria infections. Beyond its role in immunity, increasing evidence implicates complement activation in the pathogenesis of various neurological disorders, including Alzheimer's disease, Parkinson's disease, and multiple sclerosis, making C6 increasingly relevant to neurodegenerative disease research [@tsuji2019].
¶ Molecular Function and Mechanism
The complement system can be activated through three pathways:
- Classical pathway: Triggered by antigen-antibody complexes
- Lectin pathway: Activated by mannose-binding lectin
- Alternative pathway: Spontaneous C3 activation
All pathways converge at the level of C3 activation, leading to downstream events including C5 cleavage and MAC formation.
C6 plays a critical role in the final steps of complement activation:
MAC Assembly Sequence:
- C5 → C5a + C5b (proteolytic cleavage)
- C5b + C6 → C5b-6 (stable complex formation)
- C5b-6 + C7 → C5b-6-7 (membrane insertion)
- C5b-6-7 + C8 → C5b-6-7-8 (pore formation begins)
- C5b-6-7-8 + C9 → C5b-6-7-8-9 (complete MAC)
The C6 protein contains several functional regions:
- N-terminal domain: Contains cysteine-rich repeats (MCP/CD46-like)
- Thrombospondin type I repeats: Protein-protein interaction motifs
- C-terminal domain: Critical for complex formation with C5b-7
- Disulfide bonds: Stabilize protein structure
The MAC performs several critical functions:
Pathogen Killing:
- Direct lysis of Gram-negative bacteria
- Defense against Neisseria species
- Immune surveillance against infected cells
Immune Regulation:
- Sublytic MAC signaling
- Induction of inflammation
- Clearance of cellular debris
Tissue Homeostasis:
- Removal of dead cells
- Promotion of tissue repair
- Regulation of immune responses
C6 deficiency is one of the more common complement component deficiencies:
Clinical Features:
- Recurrent Neisseria infections (especially N. meningitidis)
- Increased susceptibility to bacterial infections
- Usually asymptomatic otherwise
- May be identified through laboratory testing
Genetics:
- Autosomal recessive inheritance
- Multiple pathogenic variants identified
- Carrier parents typically asymptomatic
Complement activation is strongly implicated in AD pathogenesis:
Neuropathological Evidence:
- MAC deposition in AD brain tissue
- C5b-9 accumulation in amyloid plaques
- Colocalization with neurofibrillary tangles
- Association with synaptic loss
Mechanisms:
- Synaptic pruning by microglia (involves complement)
- Enhanced neuroinflammation
- Neuronal loss through sublytic MAC signaling
- Amyloid plaque formation
Therapeutic Implications:
- Complement inhibitors in development
- Targeting specific complement pathways
- Anti-C1q therapies in trials
Complement involvement in PD includes:
- Microglial activation: Complement proteins as activators
- Dopaminergic neuron vulnerability: MAC-mediated injury
- α-Synuclein aggregation: Complement interactions
- Neuroinflammation: Chronic inflammatory state
Complement plays complex roles in MS:
- Demyelination: MAC-mediated oligodendrocyte injury
- Axonal damage: Complement-dependent mechanisms
- Blood-brain barrier disruption: Inflammatory contributions
- Remyelination failure: Inhibitory effects of complement
Complement in ALS:
- Motor neuron injury: MAC deposition
- Microglial activation: Pro-inflammatory effects
- Astrocyte involvement: Complement signaling
- Therapeutic targeting: Complement inhibitors
- Huntington's disease: Complement dysregulation
- Prion diseases: MAC deposition
- Stroke: Complement-mediated injury
- Traumatic brain injury: Complement activation
C6 is primarily expressed in:
- Liver: Primary site of synthesis
- Monocytes/macrophages: Local production
- Astrocytes: Brain expression
- Microglia: CNS production
- Various tissues: Lower expression
- Astrocytes: Primary CNS source
- Microglial cells: Activated state production
- Neurons: Lower expression
- Oligodendrocytes: Variable
C6 expression is regulated by:
- Acute phase response: Upregulated during inflammation
- Cytokines: IL-6, IL-1, TNF-α affect expression
- Tissue-specific factors: Different regulatory mechanisms
- Developmental stage: Variable expression
Therapeutic strategies targeting complement include:
-
Anti-C5 therapies:
- Eculizumab (approved for other conditions)
- Ravulizumab (longer-acting)
-
C1q inhibitors:
- Monoclonal antibodies
- Peptide inhibitors
-
Alternative pathway inhibitors:
- Factor B inhibitors
- Factor D inhibitors
- ALS: Complement inhibition trials
- AD: Anti-complement strategies
- MS: Remyelination approaches
- Liver-directed gene therapy: For complement deficiency
- CRISPR corrections: For genetic forms
C6 interacts with:
- C5b: Precursor complex
- C7: For membrane insertion
- C8: Pore formation
- C9: Polymerization for lysis
- CD46/CD55: Complement regulators
- Genetic testing: For C6 deficiency
- Complement assays: CH50 measurement
- Biomarker potential: In neurological disease
- Berends EN, et al., The membrane attack complex: from bacterial death to human disease (2020)
- Tsuji R, et al., Complement activation in neurological disease (2019)
- Ricklin D, et al., Complement: from the innate immune system to tissue injury (2013)
- Merle NS, et al., Complement system: part I - molecular mechanisms of activation and regulation (2015)
- Barnum SR, et al., The complement system in the CNS (2002)
- Stevens B, et al., Complement and C1q in synaptic plasticity and disease (2007)
- Singh J, et al., Complement in Alzheimer's disease: friend or foe? (2018)
- McKelvey J, et al., Complement and Parkinson's disease: mechanisms and therapeutic potential (2022)
- Hajishengallis G, et al., The complement system in innate immunity (2019)
- Marta CB, et al., Role of complement in demyelinating disease (2019)
- Bodea LG, et al., Complement and microglial activation in Alzheimer's disease (2019)
- Danielsen CC, et al., Complement component C5b-9 complex in the Alzheimer brain (2006)
- Zetterberg M, et al., Complement in multiple sclerosis: implications for disease progression (2019)
- Wood A, et al., The role of complement in ALS pathogenesis (2019)
- Kovacsovics T, et al., The terminal complement complex and tissue injury (2018)
- Bhak J, et al., Complement activation in prion diseases (2019)
- Callegari I, et al., Complement in Huntington's disease: a potential therapeutic target (2018)
- Wegrowski Y, et al., The role of complement in wound healing (2005)
- Kishore U, et al., The complement system in neuroprotection (2004)
- Rutkowski MJ, et al., Complement and the blood-brain barrier in CNS injury (2000)