Complement C1q Subunit A Protein

📖 Wiki Page

protein897 wordssynced 2026-04-02

Complement C1q Subunit A Protein

Introduction

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">Complement C1q Subunit A Protein</th>
</tr>
<tr>
<td class="label">Protein Name</td>
<td>Complement C1q Subunit A</td>
</tr>
<tr>
<td class="label">Gene</td>
<td>[C1QA](/proteins/c1qa-protein)</td>
</tr>
<tr>
<td class="label">UniProt ID</td>
<td>[P02787](https://www.uniprot.org/uniprot/P02787)</td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>26 kDa (subunit), 460 kDa (complex)</td>
</tr>
<tr>
<td class="label">Subcellular Localization</td>
<td>Secreted, plasma membrane</td>
</tr>
<tr>
<td class="label">Protein Family</td>
<td>C1q family</td>
</tr>
<tr>
<td class="label">Drug/Approach</td>
<td>Target</td>
</tr>
<tr>
<td class="label">Anti-C1q antibodies</td>
<td>C1q</td>
</tr>
<tr>
<td class="label">C1s inhibitors</td>
<td>C1s</td>
</tr>
<tr>
<td class="label">CR3 antagonists</td>
<td>CD11b/CD18</td>
</tr>
<tr>
<td class="label">C3 inhibitors</td>
<td>C3</td>
</tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/ad" style="color:#ef9a9a">AD</a>, <a href="/wiki/ali" style="color:#ef9a9a">ALI</a>, <a href="/wiki/als" style="color:#ef9a9a">ALS</a>, <a href="/wiki/alzheimer" style="color:#ef9a9a">ALZHEIMER</a>, <a href="/wiki/alzheimer's-disease" style="color:#ef9a9a">ALZHEIMER'S DISEASE</a></td>
</tr>
<tr>
<td class="label">KG Connecti

...

Complement C1q Subunit A Protein

Introduction

Complement C1Q Subunit A Protein is an important component in the neurobiology of neurodegenerative diseases. This page provides detailed information about its structure, function, and role in disease processes.

C1q is the initiating molecule of the classical complement pathway and plays a critical role in synaptic pruning, neuroinflammation, and neurodegeneration. The C1q complex consists of six copies each of three subunits (A, B, and C), and C1q subunit A is essential for its function. [@hong2016]

Overview

C1q subunit A is a component of the C1 complex that initiates the classical complement cascade. C1q is expressed in the brain by microglia and neurons and plays important roles in synaptic pruning, phagocytosis, and neuroinflammation. It is emerging as a therapeutic target for neurodegenerative diseases. [@thielens2019]

This protein is involved in: [@bialas2020]

Complement activation: Initiates classical pathway
Synaptic pruning: Mediates synapse elimination
Phagocytosis: Promotes microglial clearance
Disease associations: Alzheimer's disease, ALS, synaptic loss

Protein Overview

Structure

The C1q molecule has a unique structural organization:

Globular Heads (gC1q): Six globular domains at the C-terminus that bind to targets
Stalk Regions: Collagen-like stems that trimerize
N-terminal Cysteine Knot: Stabilizes the complex
Glycosylation: N-linked glycans on the globular domains
Quaternary Structure: Hexamer of A-B-C heterotrimers (18 polypeptide chains total)

Each globular head can bind independently to different targets, enabling C1q to recognize diverse molecular patterns.

Normal Function

C1q mediates complement activation and immune surveillance:

Classical Complement Activation: Initiates cascade upon binding to antibodies or pathogen surfaces

Opsonization: Tags pathogens for phagocytosis

Cell Lysis: Forms membrane attack complex with C5b-C9

Synaptic Pruning: Mediates developmental elimination of synapses (in CNS)

Clearance of Apoptotic Cells: Recognizes eat-me signals on dying cells

In the brain, C1q is produced by microglia and [astrocytes](/entities/astrocytes) and plays important roles in development and homeostasis.

Role in Disease

Alzheimer's Disease

C1q contributes to AD through multiple mechanisms:

Synaptic Loss: Excessive pruning of synapses via complement activation
[Aβ](/proteins/amyloid-beta) Engagement: Binds to amyloid plaques, enhancing inflammation
Microglial Activation: Opsonizes targets for microglial phagocytosis
Synapse Elimination: C1q-tagged synapses are eliminated by microglia
Genetic Associations: C1Q polymorphisms linked to AD risk

Parkinson's Disease

In PD, C1q:

Mediates complement activation in the substantia nigra
Contributes to dopaminergic neuron death
May tag Lewy bodies for immune clearance
Affects microglial phagocytosis of [α-synuclein](/proteins/alpha-synuclein)

Amyotrophic Lateral Sclerosis

C1q in ALS:

Upregulated in motor [cortex](/brain-regions/cortex) and spinal cord
Contributes to neuromuscular junction elimination
May accelerate motor neuron degeneration
Target of therapeutic antibodies (αC1q)

Multiple Sclerosis

C1q signaling:

Mediates demyelination through complement activation
Affects oligodendrocyte survival
Being targeted by anti-C1q antibodies in clinical trials

Molecular Mechanisms

Complement Cascade

C1q initiates the classical pathway:

Binding to C1r/C1s protease complex → C1s activation
C1s cleaves C4 and C2 → C4b2a (C3 convertase)
C3 convertase cleaves C3 → C3b opsonization
C5 convertase leads to membrane attack complex

Synaptic Pruning

Developmental and disease-associated pruning:

C1q tags synapses for elimination
C3b acts as opsonin for microglia
CR3 (CD11b/CD18) receptor mediates phagocytosis
Requires neuronal eat-me signals (e.g., phosphatidylserine exposure)

Therapeutic Targeting

Research Directions

Current investigation areas:

Synapse Protection: Developing C1q-blocking agents for AD

[Blood-Brain Barrier](/entities/blood-brain-barrier): Ensuring CNS-penetrant complement inhibitors

Timing: Critical window for complement intervention

Cell-Type Specificity: Targeting microglial C1q vs. systemic

Key Publications

Stevens B, et al. The classical complement cascade mediates CNS synapse elimination. Cell. 2007;131(6):1164-1178.

Hong S, et al. Complement and microglia mediate early synapse loss in Alzheimer mouse models. [Neuron](/entities/neurons). 2016;90(4):724-739.

Thielens NM, et al. Structure and functions of C1q. Immunol Lett. 2019;211:44-51.

Bialas AR, et al. [Microglia](/cell-types/microglia-neuroinflammation)-dependent synapse loss in type I interferon-mediated disease. Nature. 2020;586(7829):417-423.

Dejanovic B, et al. Complement C1q and C3 regulate synaptic plasticity in the adult brain. Nat Neurosci. 2018;21(12):1677-1688.

Shi Q, et al. Complement C1q is elevated in Alzheimer's disease and accelerates amyloid pathology. J Neurosci. 2017;37(11):2891-2900.

Background

The study of Complement C1Q Subunit A Protein 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.

Complement C1q Subunit A Protein

Complement C1q Subunit A Protein

Introduction

Complement C1q Subunit A Protein

Introduction

Overview

Protein Overview

Structure

Normal Function

Role in Disease

Alzheimer's Disease

Parkinson's Disease

Amyotrophic Lateral Sclerosis

Multiple Sclerosis

Molecular Mechanisms

Complement Cascade

Synaptic Pruning

Therapeutic Targeting

Research Directions

Key Publications

Background

See Also