Aquaporin - 4

Aquaporin‑4 is the main water channel in the central nervous system, concentrated in astrocyte endfeet at blood–brain and cerebrospinal fluid interfaces. It facilitates rapid bidirectional water movement to regulate brain water balance, extracellular volume, and cerebrospinal fluid flow, supports potassium buffering and glymphatic clearance of metabolites, and plays a key role in conditions like cerebral edema and autoimmune neuromyelitis optica.

Aquaporin‑4 is the target antigen in neuromyelitis optica spectrum disorder (NMOSD), an autoimmune inflammatory demyelinating disease of the central nervous system. Pathogenic AQP4‑IgG antibodies attack astrocyte water channels, causing optic neuritis, longitudinally extensive transverse myelitis, and other brainstem or cerebral symptoms. Detection of AQP4 antibodies is highly specific for NMOSD and guides diagnosis and immunotherapy.

Aquaporin-4 (AQP4) positive means a person has antibodies against the AQP4 water‑channel protein, usually detected in blood or cerebrospinal fluid. This autoantibody signals an autoimmune attack on astrocytes and strongly supports a diagnosis of neuromyelitis optica spectrum disorder (NMOSD). AQP4 positivity helps distinguish NMOSD from multiple sclerosis and guides immunosuppressive treatment and prognosis.

Aquaporin‑4 (AQP4) is a water channel highly concentrated in the central nervous system, especially in astrocyte endfeet at the blood–brain barrier, perivascular and subpial regions, and ependymal surfaces lining ventricles. It’s also present in the retina (Müller cells), optic nerve and spinal cord. Smaller amounts appear in some peripheral tissues, but its primary role is CNS water homeostasis.

Aquaporins are membrane channel proteins whose main purpose is to facilitate rapid, selective water transport across cell membranes, enabling fluid balance and osmotic homeostasis in tissues. They support kidney urine concentration, secretion and absorption in glands, brain water regulation, and cellular volume control. Some aquaglyceroporins also transport small solutes like glycerol, contributing to metabolism and cell signaling.

Another name for the aquaporin‑4 antibody is AQP4‑IgG, often called NMO‑IgG (neuromyelitis optica immunoglobulin G). These terms refer to autoantibodies that target the aquaporin‑4 water channel on astrocytes and are strongly associated with neuromyelitis optica spectrum disorder (NMOSD). Detection of AQP4‑IgG in blood or cerebrospinal fluid supports diagnosis and guides treatment decisions.