AQP4 Protein — Aquaporin 4
Pathway Diagram
flowchart TD
AQP4["AQUAPORIN-4<br/>Water Channel Protein"]
GS["Glymphatic System<br/>Brain Clearance"]
NMOSD["Neuromyelitis Optica<br/>Spectrum Disorder"]
CNS["Central Nervous<br/>System Disorders"]
MS["Multiple Sclerosis"]
ALS["Amyotrophic Lateral<br/>Sclerosis"]
AD["Alzheimer's Disease"]
PD["Parkinson's Disease"]
NI["Neuroinflammation"]
AI["Autoimmune<br/>Response"]
INF["Inflammation"]
ND["Neurodegeneration"]
AGING["Aging Process"]
AQP4 --"facilitates"--> GS
AQP4 --"target antigen"--> NMOSD
AQP4 --"dysfunction in"--> CNS
AQP4 --"activates/regulates"--> MS
AQP4 --"dual role in"--> ALS
AQP4 --"associated with"--> AD
AQP4 --"inhibits"--> PD
AQP4 --"regulates"--> NI
AQP4 --"dual role in"--> AI
AQP4 --"modulates"--> INF
AQP4 --"involved in"--> ND
AQP4 --"regulates"--> AGING
GS --"impaired clearance"--> ND
NI --"promotes"--> ND
AI --"drives"--> NMOSD
style AQP4 fill:#006494,color:#e0e0e0
style GS fill:#1b5e20,color:#e0e0e0
style NMOSD fill:#ef5350,color:#0d0d1a
style CNS fill:#ef5350,color:#0d0d1a
style MS fill:#ef5350,color:#0d0d1a
style ALS fill:#ef5350,color:#0d0d1a
style AD fill:#ef5350,color:#0d0d1a
style PD fill:#1b5e20,color:#e0e0e0
style NI fill:#ef5350,color:#0d0d1a
style AI fill:#ef5350,color:#0d0d1a
style INF fill:#ef5350,color:#0d0d1a
style ND fill:#5d4400,color:#e0e0e0
style AGING fill:#4a1a6b,color:#e0e0e0
...
AQP4 Protein — Aquaporin 4
Pathway Diagram
Mermaid diagram (expand to render)
<div class="infobox infobox-protein">
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-feetM23 isoform (323 aa) : Truncated at the N-terminus, organizes into larger OAPs and is primarily end-foot localizedThe protein forms tetramers that assemble into larger orthogonal arrays:
Monomer : Six transmembrane alpha-helices with cytoplasmic N- and C-terminiTetramer : 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 microscopyThe 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 interstitiumAstrocyte volume regulation : AQP4 helps maintain optimal astrocyte hydrationCSF-ISF exchange : Facilitates bidirectional water flow between fluid compartmentsPotassium 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 patientsGlymphatic impairment : Sleep-dependent Aβ clearance is AQP4-dependent[Tau](/proteins/tau) pathology : AQP4 dysfunction may exacerbate tau spread via glymphatic pathwaysParkinson's Disease AQP4 influences PD through multiple mechanisms:
[Alpha-synuclein](/proteins/alpha-synuclein) clearance : [Glymphatic system](/entities/glymphatic-system) removes extracellular α-synImpaired clearance in PD : Reduced AQP4 expression observed in PD modelsBlood-brain barrier leakage : AQP4 dysregulation contributes to BBB breakdownNeuroinflammation : 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 resolutionTherapeutic window : Modulating AQP4 can reduce injury if timed correctlyMultiple 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/PDSleep modulation : Optimizing sleep architecture enhances AQP4-mediated glymphatic clearanceAnti-AQP4 antibodies : For NMO treatment, approaches to block pathogenic antibodiesEdema management : AQP4 modulators for stroke and TBIGene therapy : AAV-mediated AQP4 expression for neurodegenerative diseases[@bakker2016]Interactions AQP4 interacts with:
Kir4.1 (KCNJ10) : Potassium channel coordinate for potassium/water bufferingDystroglycan complex : Anchor at astrocyte end-feetMitochondrial proteins : AQP4 localizes to some mitochondrial membranesCytoskeletal proteins : Interaction with actin for membrane anchoringSee 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
[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) Show full description