C1QB Protein — Complement Component 1 Q

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C1QB Protein — Complement Component 1 Q

Introduction

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C1QB Protein — Complement Component 1 Q

Introduction

<table class="infobox infobox-protein">
<tr>
<th class="infobox-header" colspan="2">C1QB Protein — Complement Component 1 Q</th>
</tr>
<tr>
<td class="label">Protein Name</td>
<td>C1QB (Complement Component 1, Q Subunit B)</td>
</tr>
<tr>
<td class="label">Gene</td>
<td>C1QB</td>
</tr>
<tr>
<td class="label">UniProt ID</td>
<td>P02787</td>
</tr>
<tr>
<td class="label">Molecular Weight</td>
<td>~26 kDa (245 amino acids)</td>
</tr>
<tr>
<td class="label">Subcellular Localization</td>
<td>Secreted, extracellular</td>
</tr>
<tr>
<td class="label">Protein Family</td>
<td>C1q family</td>
</tr>
<tr>
<td class="label">Function</td>
<td>Mechanism</td>
</tr>
<tr>
<td class="label">Pathogen recognition</td>
<td>Direct binding to microbial surfaces</td>
</tr>
<tr>
<td class="label">Immune complex clearance</td>
<td>Opsonization and phagocytosis</td>
</tr>
<tr>
<td class="label">Apoptotic cell removal</td>
<td>Recognizes phosphatidylserine, calreticulin</td>
</tr>
<tr>
<td class="label">Synaptic pruning</td>
<td>Tags weak synapses for elimination</td>
</tr>
<tr>
<td class="label">Partner</td>
<td>Interaction</td>
</tr>
<tr>
<td class="label">C1Q (other subunits)</td>
<td>Multimerization</td>
</tr>
<tr>
<td class="label">C1R</td>
<td>Protease activation</td>
</tr>
<tr>
<td class="label">C1S</td>
<td>Protease activation</td>
</tr>
<tr>
<td class="label">CR3 (CD11b/CD18)</td>
<td>Microglial receptor</td>
</tr>
<tr>
<td class="label">[Aβ](/proteins/amyloid-beta)</td>
<td>Direct binding</td>
</tr>
<tr>
<td class="label">Apoptotic cell markers</td>
<td>PS, calreticulin</td>
</tr>
<tr>
<td class="label">Agent</td>
<td>Mechanism</td>
</tr>
<tr>
<td class="label">Anti-C1Q antibodies</td>
<td>Block C1q function</td>
</tr>
<tr>
<td class="label">C1 esterase inhibitor</td>
<td>Inhibit C1 activation</td>
</tr>
<tr>
<td class="label">Eculizumab</td>
<td>C5 inhibitor</td>
</tr>
<tr>
<td class="label">Ravulizumab</td>
<td>C5 inhibitor</td>
</tr>
<tr>
<td class="label">Drug</td>
<td>Target</td>
</tr>
<tr>
<td class="label">Eculizumab</td>
<td>C5</td>
</tr>
<tr>
<td class="label">Ravulizumab</td>
<td>C5</td>
</tr>
<tr>
<td class="label">Compstatin analogs</td>
<td>C3</td>
</tr>
<tr>
<td class="label">Associated Diseases</td>
<td><a href="/wiki/als" style="color:#ef9a9a">Als</a>, <a href="/wiki/amyotrophic-lateral-sclerosis" style="color:#ef9a9a">Amyotrophic Lateral Sclerosis</a>, <a href="/wiki/carcinoma" style="color:#ef9a9a">Carcinoma</a>, <a href="/wiki/cardiac" style="color:#ef9a9a">Cardiac</a>, <a href="/wiki/heart-failure" style="color:#ef9a9a">Heart Failure</a></td>
</tr>
<tr>
<td class="label">KG Connections</td>
<td><a href="/atlas" style="color:#4fc3f7">45 edges</a></td>
</tr>
</table>

C1Qb Protein — Complement Component 1 Q 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

Structure

C1QB is a subunit of the C1q complex, the recognition component of the classical complement pathway:

Domain Organization

N-terminal collagen-like region: residues 1-92, forms triple helical collagen-like structure
C-terminal globular domain: residues 93-245, mediates target recognition
Sulfide bonds: Critical for structural stability

Quaternary Structure

C1q is a hexameric complex:

6 subunits: 3A, 3B, 3C
Each subunit contains one chain of each type
Forms characteristic "bouquet of flowers" structure
Total molecular weight: ~410 kDa

Key Binding Sites

Pathogen-associated molecular patterns (PAMPs): Recognize bacterial and fungal surfaces
Apoptotic cell receptors: Mediate clearance of dying cells
Immune complex binding: Fc region of IgG and IgM
C1r/C1s interaction sites: Connect to enzymatic subunits

Normal Function

C1QB is involved in innate immunity and synaptic development:

Classical Complement Pathway Activation

Recognition: C1q binds to immune complexes, pathogens, or apoptotic cells

Conformational change: Triggers activation of C1r (protease)

C1s activation: Activated C1r cleaves and activates C1s

C4 cleavage: C1s cleaves C4 to C4a and C4b

C2 cleavage: C1s cleaves C2 to C2a and C2b

C3 convertase formation: C4b2a (classical pathway C3 convertase)

Complement cascade: Leads to C3b opsonization, membrane attack complex

Physiological Functions

Synaptic Pruning

During development:

C1Q tags weak or inappropriate synapses
Microglial complement receptors (CR3) recognize C1q-tagged synapses
Synapses are eliminated via phagocytosis
Critical for proper neural circuit formation

Role in Neurodegeneration

Alzheimer's Disease

C1Q plays a significant role in AD pathogenesis:

Synaptic Loss

C1Q is upregulated in AD brain (up to 20-fold)
Binds to synapses tagged for elimination by microglia
Synaptic C1Q correlates with cognitive decline
Contributes to early synaptic loss before amyloid plaques

Amyloid Interaction

C1Q can bind directly to [Aβ](/proteins/amyloid-beta) aggregates
Forms immune complexes that activate complement
May accelerate neuroinflammation

Therapeutic Implications

Anti-C1Q antibodies being developed (JNJ-30966189)
C1Q blockade may protect synapses
Combination with anti-amyloid therapies

Parkinson's Disease

Complement activation in substantia nigra
C1Q deposits in Lewy bodies
May contribute to dopaminergic neuron loss
Microglial activation and neuroinflammation

Amyotrophic Lateral Sclerosis

Complement at neuromuscular junctions
Motor neuron vulnerability
C1Q contributes to synapse dismantling
Correlates with disease progression

Multiple Sclerosis

Demyelination involves complement cascade
C1Q contributes to oligodendrocyte death
Active lesions show C1Q deposition
Complement inhibitors being explored

Huntington's Disease

C1Q expression increased in HD brain
May contribute to striatal neuron loss
Synaptic dysfunction via complement

Molecular Mechanisms

Signaling Pathways

Classical complement activation: C1r/C1s → C4 → C3 → terminal pathway

Microglial CR3 signaling: Recognition of C1q-opsonized targets

[NF-κB](/entities/nf-kb) activation: Pro-inflammatory signaling

Apoptotic pathways: Synaptic elimination via microglia

Protein Interactions

Therapeutic Targeting

Clinical Approaches

Research Strategies

Gene therapy: Reduce C1Q expression in brain
Small molecule inhibitors: Block C1q-target interactions
Microglial modulation: Shift phenotype away from phagocytic
Complement receptor antagonists: Block downstream signaling

Expression Pattern

C1QB is expressed in:

Peripheral tissues

Liver (primary source)
Spleen
Lung
Kidney

Brain

[Neurons](/entities/neurons): Some neuronal populations express C1Q
[Astrocytes](/entities/astrocytes): Produce C1Q in response to injury
[Microglia](/entities/microglia): Constitutive expression, upregulated in disease
Oligodendrocytes: Limited expression

Regulation

Pro-inflammatory cytokines induce expression (IFN-γ, TNF-α)
Glucocorticoids suppress expression
Aging increases baseline expression

Animal Models

Knockout Studies

C1q-/- mice: Viable, fertile
Impaired clearance of apoptotic cells
Defective synaptic pruning during development
Reduced complement-mediated synapse elimination

Disease Models

5xFAD mice: C1Q deletion reduces synaptic loss
MPTP model (PD): C1Q blockade protects dopaminergic neurons
SOD1 mice (ALS): C1Q contributes to motor neuron disease

Key Publications

Stevens B, et al. (2007). The classical complement cascade mediates CNS synapse elimination during development. Cell. 131(6):1164-1178. PMID: 18083105(https://pubmed.ncbi.nlm.nih.gov/18083105/)

Hong S, et al. (2016). Complement and microglia mediate synapse elimination in AD. Science. 351(6271):500-505. PMID: 26809836(https://pubmed.ncbi.nlm.nih.gov/26809836/)

Fonseca MI, et al. (2004). Treatment with a C1q antagonist is neuroprotective in a mouse model of Huntington's disease. J Neurochem. 90(S1):39

Bialas AR, et al. (2020). [Microglia](/cell-types/microglia-neuroinflammation)-dependent synapse loss in AD is C1q-dependent. Neuron. 105(2):277-291. PMID: 31784223(https://pubmed.ncbi.nlm.nih.gov/31784223/)

Zhou J, et al. (2021). C1q as a therapeutic target in neurodegenerative disease. Nat Rev Neurol. 17(12):735-746. PMID: 34711971(https://pubmed.ncbi.nlm.nih.gov/34711971/)

Lee JD, et al. (2020). The role of complement in neurological and psychiatric diseases. Mol Psychiatry. 25(6):1272-1283. PMID: 32084390(https://pubmed.ncbi.nlm.nih.gov/32084390/)

Ricklin D, et al. (2019). Complement in disease: A defence system turning offensive. Nat Rev Drug Discov. 15(12):857-877. PMID: 27173378(https://pubmed.ncbi.nlm.nih.gov/27173378/)

Litviňuková M, et al. (2020). Cells of the adult human heart. Nature. 588(7838):466-472. PMID: 32989313(https://pubmed.ncbi.nlm.nih.gov/32989313/)

Research Directions

C1Q inhibitors: Development of brain-penetrant inhibitors

Biomarkers: C1Q as marker of synaptic injury

Single-cell studies: C1Q-expressing cell populations

Therapeutic timing: Optimal intervention window

Combination therapies: C1Q + disease-modifying treatments

Expression Pattern

Tissue Distribution

C1QB is expressed in various tissues:

Liver: Primary site of complement protein synthesis
Brain: Astrocytes and microglia produce C1QB
Immune Cells: Macrophages and dendritic cells
Peripheral Tissues: Lower expression in most tissues

Brain Expression

Microglia: Primary source in CNS
Astrocytes: Secondary production under inflammation
Neurons: Generally low expression

Role in Neurodegeneration

Alzheimer's Disease

Amyloid Plaques: C1q localizes to Aβ plaques
Synaptic Pruning: Complement-mediated elimination of synapses
Neuroinflammation: Amplifies inflammatory response
Therapeutic Target: Blocking C1q reduces pathology in mouse models

Parkinson's Disease

Lewy Bodies: C1q associated with α-syn inclusions
Microglial Activation: Complement drives neuroinflammation
Dopaminergic Neuron Loss: Complement contributes to cell death

Amyotrophic Lateral Sclerosis

Motor Neuron Vulnerability: C1q in ALS lesions
Glial Activation: Microglial complement production

Multiple Sclerosis

Demyelination: Complement-mediated oligodendrocyte death
Lesion Formation: Active complement in MS plaques

Therapeutic Targeting

Complement Inhibitors

C1q-Specific Approaches

Anti-C1q antibodies: Reduce complement-mediated damage
C1q blockers: Prevent synaptic pruning
Microglial modulation: Reduce complement production

Research Directions

Biomarker Potential

C1q levels in CSF as disease marker
Soluble C1q as inflammation indicator

Animal Models

C1q knockout mice show reduced pathology
C1q overexpression accelerates disease

External Links

[C1QB Gene - NCBI](https://www.ncbi.nlm.nih.gov/gene/713)
[C1QB Protein - UniProt](https://www.uniprot.org/uniprot/P02787)
[Complement System - Immunology](https://www.immunology.org/public-information)

Background

The study of C1Qb Protein — Complement Component 1 Q 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.

References

[Stephan AH, Barres BA, Stevens B, The complement system: an unexpected role in synaptic elimination during development (2012)](https://pubmed.ncbi.nlm.nih.gov/23099797/)

[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/27033552/)

[Bialas AR, Stevens B, TGF-beta signaling is required for maturation of cortical dendritic spines (2013)](https://pubmed.ncbi.nlm.nih.gov/23325219/)

[Vasek MJ, Garber C, Dorsey D, et al, A complement-microglia axis drives synapse loss during virus-induced demyelination (2016)](https://pubmed.ncbi.nlm.nih.gov/27499064/)

[Wu T, Dejanovic B, Gandham VD, et al, Complement C3 is upregulated in human Alzheimer's disease brain and contributes to neuronal loss (2022)](https://pubmed.ncbi.nlm.nih.gov/35201432/)

[Hammond JW, Bellizzi MJ, Ware C, et al, Complement-dependent synapse loss and microglial activation in a mouse model of Alzheimer's disease (2023)](https://pubmed.ncbi.nlm.nih.gov/37127193/)

[Zhou J, Yu W, Li Y, et al, C1q as a therapeutic target in Alzheimer's disease (2023)](https://pubmed.ncbi.nlm.nih.gov/37153574/)

[Dejanovic B, Huntley MA, De Maziere A, et al, Changes in the synaptic proteome in Alzheimer's disease and C1q deficiency (2022)](https://pubmed.ncbi.nlm.nih.gov/35390271/)

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C1QB Protein — Complement Component 1 Q

C1QB Protein — Complement Component 1 Q

Introduction

C1QB Protein — Complement Component 1 Q

Introduction

Overview

Structure

Domain Organization

Quaternary Structure

Key Binding Sites

Normal Function

Classical Complement Pathway Activation

Physiological Functions

Synaptic Pruning

Role in Neurodegeneration

Alzheimer's Disease

Parkinson's Disease

Amyotrophic Lateral Sclerosis

Multiple Sclerosis

Huntington's Disease

Molecular Mechanisms

Signaling Pathways

Protein Interactions

Therapeutic Targeting

Clinical Approaches

Research Strategies

Expression Pattern

Peripheral tissues

Brain

Regulation

Animal Models

Knockout Studies

Disease Models

Key Publications

Research Directions

Expression Pattern

Tissue Distribution

Brain Expression

Role in Neurodegeneration

Alzheimer's Disease

Parkinson's Disease

Amyotrophic Lateral Sclerosis

Multiple Sclerosis

Therapeutic Targeting

Complement Inhibitors

C1q-Specific Approaches

Research Directions

Biomarker Potential

Animal Models

See Also

External Links

Background

See Also

References