Aquaporin-4 Protein

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AQP4 Protein — Aquaporin 4

<div class="infobox infobox-protein">
<table>
<tr><th colspan="2" style="background:#e8f4f8; text-align:center; font-size:1.1em;">AQP4 — Aquaporin 4</th></tr>
<tr><td>Protein Name</td><td>Aquaporin 4</td></tr>
<tr><td>Gene</td><td><a href="/genes/aqp4">AQP4</a></td></tr>
<tr><td>UniProt ID</td><td><a href="https://www.uniprot.org/uniprot/P55064">P55064</a></td></tr>
<tr><td>PDB Structures</td><td>4QK0, 3DL8</td></tr>
<tr><td>Molecular Weight</td><td>34 kDa (monomer)</td></tr>
<tr><td>Subcellular Localization</td><td>Astrocyte end-feet, perivascular, ependymal</td></tr>
<tr><td>Protein Family</td><td>Aquaporin (AQP) family</td></tr>
<tr><td>Aliases</td><td>AQP4, M1-AQP4, M23-AQP4</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" style="color:#ef9a9a">ALZHEIMER'S</a></td>
</tr>
<tr>
<td class="label">SciDEX Hypotheses</td>
<td><a href="/hypothesis/h-807d7a82" style="color:#ce93d8" title="Score: 0.60">SASP-Driven Aquaporin-4 Dysregulation...</a> <a href="/hypothesis/h-62e56eb9" style="color:#ce93d8" title="Score: 0.53">Glymphatic System-Enhanced Antibody Clea...</a> <a href="/hypothesis/h-c8ccbee8" style="color:#ce93d8" title="Score: 0.46">Aquapor

...

AQP4 Protein — Aquaporin 4

Overview

Aquaporin-4 (AQP4) is the principal water channel in the brain, highly expressed in astrocyte end-feet that surround cerebral blood vessels and in ependymal cells lining the ventricles[@nielsen2003]. AQP4 plays a critical role in glymphatic clearance—a brain-wide waste clearance system that removes metabolic byproducts, toxic proteins, and interstitial solutes during sleep[@iliff2012]. Dysregulation of AQP4 has been implicated in [Alzheimer's disease](/diseases/alzheimers-disease) (AD), [Parkinson's disease](/diseases/parkinsons-disease) (PD), stroke, traumatic brain injury (TBI), and multiple sclerosis (MS)[@papadopoulos2013][@zhao2018]. The protein forms orthogonal arrays of particles (OAPs) in the plasma membrane that mediate rapid water flux between the cerebrospinal fluid (CSF) and interstitial fluid (ISF).

Structure

AQP4 exists as two major isoforms:

M1 isoform (323 aa): Contains the full sequence, localizes to both astrocyte cell bodies and end-feet
M23 isoform (323 aa): Truncated at the N-terminus, organizes into larger OAPs and is primarily end-foot localized

The protein forms tetramers that assemble into larger orthogonal arrays:

Monomer: Six transmembrane alpha-helices with cytoplasmic N- and C-termini
Tetramer: Four monomers form a functional water channel (~4 water molecules per nanosecond)
OAP (Orthogonal Array of Particles): Multiple tetramers cluster to form large assemblies visible by freeze-fracture electron microscopy

The M23 isoform has a shorter N-terminus that lacks the palmitoylation sites present in M1, allowing more efficient OAP formation[@jung1994].

Normal Function

Water Homeostasis

AQP4 mediates rapid water transport across the [blood-brain barrier](/entities/blood-brain-barrier):

Glymphatic clearance: CSF flows into the brain along perivascular spaces, then water exits through AQP4 into the interstitium
Astrocyte volume regulation: AQP4 helps maintain optimal astrocyte hydration
CSF-ISF exchange: Facilitates bidirectional water flow between fluid compartments

Potassium Buffering

AQP4 works with Kir4.1 potassium channels to buffer extracellular potassium:

During neuronal activity, potassium is released into the extracellular space
AQP4-mediated water flux helps disperse potassium
Coordinated with Kir4.1 for efficient potassium clearance[@kofuji2009]

Neuroinflammation

AQP4 plays complex roles in neuroinflammation:

Upregulated in reactive [astrocytes](/entities/astrocytes) surrounding lesions
Involved in edema formation during inflammation
Regulates immune cell trafficking across the BBB

Water Homeostasis in the Brain

AQP4 is the principal water channel in the central nervous system, critical for:

Brain water balance: Regulating extracellular and intracellular water
Cerebral edema resolution: Facilitating fluid clearance after injury
K+ buffering: Supporting neuronal excitability regulation
Metabolite transport: Enabling diffusion of small molecules
Sleep regulation: Associated with [glymphatic system](/entities/glymphatic-system) function

The perivascular localization of AQP4 (at astrocyte end-feet) positions it ideally for:

Rapid water flux between blood and brain
Cerebrospinal fluid circulation
Waste clearance via the glymphatic system

Role in Cerebral Edema

AQP4 plays a dual role in cerebral edema:

Cytotoxic edema (cell swelling):

AQP4 facilitates water entry into astrocytes
Loss of AQP4 reduces swelling in early phases
Knockout mice show improved outcomes in some edema models

Vasogenic edema (vessel leakage):

AQP4 helps clear interstitial fluid
Enhanced AQP4 expression may worsen edema resolution
Therapeutic modulation must consider edema type

Neuroinflammation and Disease

AQP4 dysfunction contributes to multiple neurological conditions:

Multiple sclerosis: Altered expression in demyelinated lesions
Epilepsy: Dysregulated AQP4 affects neuronal excitability
Traumatic brain injury: Edema formation and resolution
Stroke: Ischemic damage and reperfusion injury
Alzheimer's disease: Implicated in amyloid clearance
Parkinson's disease: May affect dopaminergic neuron survival

Therapeutic Modulation

Targeting AQP4 therapeutically includes:

AQP4 inhibitors: Reducing water flux in cytotoxic edema

AQP4 modulators: Altering channel gating

Gene therapy: Expressing modified AQP4 constructs

Combination approaches: AQP4 targeting with other therapies

Biomarkers: AQP4 autoantibodies in neuromyelitis optica

Role in Disease

Alzheimer's Disease

AQP4 dysfunction significantly impacts AD pathogenesis:

Impaired amyloid clearance: AQP4 deficiency reduces [Aβ](/proteins/amyloid-beta) clearance by ~40%[@iliff2012]
Perivascular AQP4 loss: Reduced AQP4 expression around cerebral vessels in AD patients
Glymphatic impairment: Sleep-dependent Aβ clearance is AQP4-dependent
[Tau](/proteins/tau) pathology: AQP4 dysfunction may exacerbate [tau](/proteins/tau) spread via glymphatic pathways

Parkinson's Disease

AQP4 influences PD through multiple mechanisms:

[Alpha-synuclein](/proteins/alpha-synuclein) clearance: Glymphatic system removes extracellular α-syn
Impaired clearance in PD: Reduced AQP4 expression observed in PD models
Blood-brain barrier leakage: AQP4 dysregulation contributes to BBB breakdown
Neuroinflammation: AQP4 affects microglial activation states[@zhao2018]

Stroke and Traumatic Brain Injury

AQP4 has dual roles in acute brain injury:

Early phase: Contributes to cytotoxic edema (cell swelling)
Late phase: Facilitates vasogenic edema resolution
Therapeutic window: Modulating AQP4 can reduce injury if timed correctly

Multiple Sclerosis

AQP4 is the autoantigen in neuromyelitis optica (NMO):

Autoantibodies against AQP4 cause demyelination
Loss of AQP4 in lesion centers
Astrocyte dysfunction precedes demyelination[@lennon2005]

Therapeutic Implications

AQP4 represents a therapeutic target for several conditions:

Glymphatic enhancement: Compounds that increase AQP4 expression or activity could improve waste clearance in AD/PD
Sleep modulation: Optimizing sleep architecture enhances AQP4-mediated glymphatic clearance
Anti-AQP4 antibodies: For NMO treatment, approaches to block pathogenic antibodies
Edema management: AQP4 modulators for stroke and TBI
Gene therapy: AAV-mediated AQP4 expression for neurodegenerative diseases[@bakker2016]

Interactions

AQP4 interacts with:

Kir4.1 (KCNJ10): Potassium channel coordinate for potassium/water buffering
Dystroglycan complex: Anchor at astrocyte end-feet
Mitochondrial proteins: AQP4 localizes to some mitochondrial membranes
Cytoskeletal proteins: Interaction with actin for membrane anchoring

Brain Atlas Resources

[Allen Human Brain Atlas - AQP4 Expression](https://human.brain-map.org/microarray/search/show?search_term=AQP4)
[Allen Cell Type Atlas - AQP4](https://celltypes.brain-map.org/)
[BrainSpan - AQP4 Developmental Expression](https://brainspan.org/)
[Allen Mouse Brain Atlas - AQP4](https://mouse.brain-map.org/)

Pathway & Interaction Diagram

Interactive diagram showing AQP4's key relationships in the SciDEX knowledge graph (15 connections shown).

Mermaid diagram (expand to render)

External Links

[UniProt: P55064](https://www.uniprot.org/uniprot/P55064)
[GeneCards: AQP4](https://www.genecards.org/cgi-bin/carddisp.pl?gene=AQP4)
[OMIM: 600030](https://www.omim.org/entry/600030)
[Allen Brain Atlas: AQP4 expression](https://human.brain-map.org/microarray/search/show?search_term=AQP4)

References

[Nielsen et al., AQP4 in brain (2003) (2003)](https://pubmed.ncbi.nlm.nih.gov/12665610/)

[Iliff et al., Glymphatic clearance of Aβ (2012) (2012)](https://doi.org/10.1126/science.1220363)

[Unknown, Papadopoulos & Verkman, AQP4 in brain disorders (2013) (2013)](https://doi.org/10.1038/nrm3520)

[Zhao et al., AQP4 in Parkinson's disease (2018) (2018)](https://pubmed.ncbi.nlm.nih.gov/29526542/)

[Jung et al., AQP4 structure and assembly (1994) (1994)](https://doi.org/10.1126/science.7512454)

[Unknown, Kofuji & Newman, AQP4 and Kir4.1 buffering (2009) (2009)](https://pubmed.ncbi.nlm.nih.gov/19447094/)

[Lennon et al., AQP4 autoantibody in NMO (2005) (2005)](https://doi.org/10.1084/jem.20051903)

[Bakker et al., AQP4 as therapeutic target (2016) (2016)](https://doi.org/10.1016/j.tips.2016.04.013)

From the [SciDEX Exchange](/exchange) — scored by multi-agent debate

[SASP-Driven Aquaporin-4 Dysregulation](/hypothesis/h-807d7a82) — 0.60 · Target: AQP4
[Glymphatic System-Enhanced Antibody Clearance Reversal](/hypothesis/h-62e56eb9) — 0.53 · Target: AQP4
[Aquaporin-4 Polarization Rescue](/hypothesis/h-c8ccbee8) — 0.46 · Target: AQP4

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Aquaporin-4 Protein

AQP4 Protein — Aquaporin 4

AQP4 Protein — Aquaporin 4

Overview

Structure

Normal Function

Water Homeostasis

Potassium Buffering

Neuroinflammation

Water Homeostasis in the Brain

Role in Cerebral Edema

Neuroinflammation and Disease

Therapeutic Modulation

Role in Disease

Alzheimer's Disease

Parkinson's Disease

Stroke and Traumatic Brain Injury

Multiple Sclerosis

Therapeutic Implications

Interactions

Brain Atlas Resources

Pathway & Interaction Diagram

See Also

External Links

References

Related Hypotheses