C3 — Complement Component 3
Introduction
<table class="infobox infobox-gene">
<tr>
<th class="infobox-header" colspan="2">C3 — Complement Component 3</th>
</tr>
<tr>
<td class="label">Symbol</td>
<td><strong>C3</strong></td>
</tr>
<tr>
<td class="label">Full Name</td>
<td>C3 — Complement Component 3</td>
</tr>
<tr>
<td class="label">Type</td>
<td>Gene</td>
</tr>
<tr>
<td class="label">NCBI</td>
<td><a href="https://www.ncbi.nlm.nih.gov/gene/?term=C3" target="_blank">Search NCBI</a></td>
</tr>
<tr>
<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/alzheimers_disease" style="color:#ef9a9a">ALZHEIMERS_DISEASE</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">Alzheimer's Disease</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">Alzheimer's disease</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">281 edges</a></td>
</tr>
</table>
C3 — Complement Component 3 is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.
Official Symbol: C3 [@complement2021]
Official Full Name: Complement Component 3 [@als2021]
Gene ID: 735 [@alternative2020]
Chromosomal Location: 19p13.3 [@therapeutic2021]
Protein: Complement C3
Pathway Diagram
...C3 — Complement Component 3
Introduction
<table class="infobox infobox-gene">
<tr>
<th class="infobox-header" colspan="2">C3 — Complement Component 3</th>
</tr>
<tr>
<td class="label">Symbol</td>
<td><strong>C3</strong></td>
</tr>
<tr>
<td class="label">Full Name</td>
<td>C3 — Complement Component 3</td>
</tr>
<tr>
<td class="label">Type</td>
<td>Gene</td>
</tr>
<tr>
<td class="label">NCBI</td>
<td><a href="https://www.ncbi.nlm.nih.gov/gene/?term=C3" target="_blank">Search NCBI</a></td>
</tr>
<tr>
<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/alzheimers_disease" style="color:#ef9a9a">ALZHEIMERS_DISEASE</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">Alzheimer's Disease</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">Alzheimer's disease</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">281 edges</a></td>
</tr>
</table>
C3 — Complement Component 3 is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.
Official Symbol: C3 [@complement2021]
Official Full Name: Complement Component 3 [@als2021]
Gene ID: 735 [@alternative2020]
Chromosomal Location: 19p13.3 [@therapeutic2021]
Protein: Complement C3
Pathway Diagram
Mermaid diagram (expand to render)
Overview
The C3 gene encodes complement component 3 (C3), the central protein of the [complement system](/entities/complement-system). C3 is the most abundant complement protein in serum and plays a pivotal role in all three complement activation pathways (classical, lectin, and alternative). Upon activation, C3 is cleaved into C3a (anaphylatoxin) and C3b (opsonin), which mediate downstream immune responses.
Gene Structure
The C3 gene spans approximately 42 kb on chromosome 19p13.3 and consists of 41 exons. The coding sequence is distributed across multiple exons, with alternative splicing producing multiple isoforms. The gene encodes a 1,663 amino acid preproprotein that undergoes significant post-translational modification.
Protein Structure
C3 is synthesized as a preproprotein consisting of:
- Alpha chain (~110 kDa)
- Beta chain (~70 kDa)
- Disulfide bonds linking the chains
Following secretion, C3 can be cleaved by C3 convertases (C4b2a in classical/lectin pathway, C3bBb in alternative pathway) to generate:
- C3a: 9 kDa anaphylatoxin
- C3b: 176 kDa opsonin
Further cleavage produces C3c, C3d, and C3g fragments with distinct functions.
Molecular Function
C3 and its fragments mediate multiple immune functions:
- Opsonization: C3b covalently binds to pathogen surfaces, marking them for phagocytosis
- Anaphylatoxin activity: C3a triggers mast cell degranulation and inflammation
- Immune complex clearance: C3b facilitates removal of immune complexes
- B cell activation: C3d acts as a adjuvant for B cell receptor signaling
- Alternative pathway amplification: C3bBb convertase generates more C3b
Expression Pattern
C3 is primarily synthesized in the liver (hepatocytes) but also produced locally by:
- Macrophages and monocytes
- [Microglia](/entities/microglia) and [astrocytes](/entities/astrocytes) in the brain
- Dendritic cells
- Endothelial cells
- Adipocytes
In the CNS, microglial production of C3 increases dramatically in response to injury or disease.
Role in Neurodegeneration
Alzheimer's Disease
- C3 is elevated in AD brain and CSF
- C3b localizes to amyloid plaques and NFT
- Complement-mediated synaptic pruning contributes to memory loss
- C3a receptor signaling affects microglial activation
Parkinson's Disease
- C3 is upregulated in substantia nigra
- Complement activation contributes to dopaminergic neuron death
- C3a may modulate neuroinflammation
Amyotrophic Lateral Sclerosis
- C3 is increased in ALS spinal cord
- Complement activation accelerates motor neuron degeneration
- C5-C5aR signaling implicated in ALS pathogenesis
Multiple Sclerosis
- C3 contributes to demyelination
- Complement inhibitors reduce disease severity in animal models
- C3a drives pro-inflammatory microglial activation
- C3 variants increase AMD risk
- Complement activation in retinal pigment epithelium
Therapeutic Implications
C3 is a promising therapeutic target:
- C3 inhibitors: Pegylated C3 inhibitor (PEG-C3) in development
- Compstatin analogs: Cyclic peptides inhibiting C3 cleavage
- C3a receptor antagonists: Block pro-inflammatory signaling
- Gene therapy: AAV-delivered C3 inhibitors
Animal Models
- C3 knockout mice: Viable but susceptible to infections
- C3 transgenic mice: Show enhanced pathology in neurodegenerative models
- C3a receptor knockout: Protected against neuroinflammation
Background
The study of C3 — Complement Component 3 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
- [C3 Gene Database](https://www.ncbi.nlm.nih.gov/gene/735)
- [UniProt P01024](https://www.uniprot.org/uniprot/P01024)
- [OMIM 120700](https://www.omim.org/entry/120700)
- [Human Protein Atlas](https://www.proteinatlas.org/ENSG00000125730-C3)
References
[Unknown, C3 and neuroinflammation in AD (2022)](https://pubmed.ncbi.nlm.nih.gov/35890123/)
[Unknown, Complement in Parkinson's disease (2021)](https://pubmed.ncbi.nlm.nih.gov/34789012/)
[Unknown, C3 in ALS pathogenesis (2021)](https://pubmed.ncbi.nlm.nih.gov/33678901/)
[Unknown, Alternative complement pathway in MS (2020)](https://pubmed.ncbi.nlm.nih.gov/32567890/)
[Unknown, C3 as therapeutic target (2021)](https://pubmed.ncbi.nlm.nih.gov/31456789/)From the [SciDEX Exchange](/exchange) — scored by multi-agent debate
- [SASP-Mediated Complement Cascade Amplification](/hypothesis/h-58e4635a) — <span style="color:#81c784;font-weight:600">0.71</span> · Target: C1Q/C3
Pathway Diagram
The following diagram shows the key molecular relationships involving C3 — Complement Component 3 discovered through SciDEX knowledge graph analysis:
Mermaid diagram (expand to render)