AQP4 Protein — Aquaporin 4

AQP4 Protein — Aquaporin 4

Pathway Diagram

AQP4 Protein — Aquaporin 4

Pathway Diagram

<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><strong>Protein Name</strong></td><td>Aquaporin 4</td></tr>
<tr><td><strong>Gene</strong></td><td><a href="/genes/aqp4">AQP4</a></td></tr>
<tr><td><strong>UniProt ID</strong></td><td><a href="https://www.uniprot.org/uniprot/P55064">P55064</a></td></tr>
<tr><td><strong>PDB Structures</td><td>4QK0, 3DL8</td></tr>
<tr><td><strong>Molecular Weight</strong></td><td>34 kDa (monomer)</td></tr>
<tr><td><strong>Subcellular Localization</strong></td><td>Astrocyte end-feet, perivascular, ependymal</td></tr>
<tr><td><strong>Protein Family</strong></td><td>Aquaporin (AQP) family</td></tr>
<tr><td><strong>Aliases</strong></td><td>AQP4, M1-AQP4, M23-AQP4</td></tr>
</table>
</div>

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

• 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

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

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 spread via glymphatic pathways

Parkinson's Disease

AQP4 influences PD through multiple mechanisms:

• [Alpha-synuclein](/proteins/alpha-synuclein) clearance: [Glymphatic system](/entities/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

See Also

• AQP4 Gene
• [Glymphatic System](/mechanisms/glymphatic-system)
• [Astrocytes in Neurodegeneration](/cell-types/astrocytes)
• [Alzheimer's Disease](/diseases/alzheimers-disease)
• [Parkinson's Disease](/diseases/parkinsons-disease)

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