Aquaporin-4

Protein coding gene in the species Homo sapiens

Aquaporin 4, also known as AQP 4, is a water channel protein encoded by the AQP4 gene in humans. AQP 4 belongs to the aquaporin family of integral membrane proteins that conduct water through the cell membrane. A limited number of aquaporins are found within the central nervous system (CNS): AQP1, 3, 4, 5, 8, 9, and 11, but more exclusive representation of AQP1, 4, and 9 are found in the brain and spinal cord. AQP4 shows the largest presence in the cerebellum and spinal cord grey matter. In the CNS, AQP4 is the most prevalent aquaporin channel, specifically located at the perimicrovessel astrocyte foot processes, glia limitans, and ependyma. In addition, this channel is commonly found facilitating water movement near cerebrospinal fluid and vasculature. Aquaporin 4 was first identified in 1986. It was the first evidence of the existence of water transport channels. The method that was used to discover the existence of the transport channels was through knockout experiments. With this technique they were able to show the significant role of AQP4 in CNS injuries and brain water imbalances.

The structure of AQP4 consists of six transmembrane domains and five connecting loops to form the channel. Through x ray crystallography, it was found that “each AQP4 monomer consists of six helical, membrane spanning domains and two short helical segments surrounding a narrow aqueous pore.” At the narrowest point the aqueous pore measures 2.8 angstroms, just large enough for the single file passage of water molecules. While each monomer is individually capable of water transport, the quaternary structure of the channel is a tetramer. In addition, AQP4 has two distinct structural isoforms located in the CNS: M1 and M23. AQP4 is also expressed in astrocytes and is upregulated by direct insult to the central nervous system. Specifically within the central nervous system (CNS), AQP4 can be found along the spinal cord and serves as the main water channel. In the kidneys, AQP4 is primarily found in the inner medulla, and shows little to no presence in the outer medulla and cortex. It is constitutively expressed in the basolateral cell membrane of principal collecting duct cells and provide a pathway for water to exit these cells.

Aquaporin 4's overall function is to provide fast water transportation as well as maintain homeostatic balance within the central nervous system. This channel can transport water up to speeds of 3E9 molecules per second. Other systems are also regulated by AQP4. Within the inner ear, the main role is to provide osmotic balance in supporting epithelium cells within the organ of Corti by recycling K+. Aquaporin 4 is the predominant autoimmune target in 2/3 neuromyelitis optica and higher AQP4 autoantibody levels are associated with the occurrence of optic neuritis (ON), however serum AQP4 IgG titer only moderately reflects disease activity, severity, or neurological prognosis. Specific AQP4 IgG autoantibody, or NMO IgG, binds to the extracellular surface of AQP4. This binding provides an opening for the development of targeted therapeutics in NMO. Therapy options are immunosuppression, such as corticosteroids and azathioprine immunosuppressive drugs, immunomodulation, and plasma exchange. A recent serum antibody (anti AQP4) has been detected for patients with NMO, which is currently used to diagnose this condition. Other clinical significant implications of AQP4 in the human body is the role in the regulation of cerebrospinal fluid (CSF) in the ventricles. Within the ventricles of the brain, AQP4 can be utilized in the removal of excess CSF in conditions such as hydrocephaly. The primary treatment for individuals with hydrocephaly is through the implementation of mechanical shunts into the ventricles to drain the excess fluid. With further research into the role of AQP4, it may be possible to modify the human body's system of upregulation of these channels to help in the reabsorption of CSF without the need to use physically invasive treatments.

Based on work in animal models, aquaporin 4 may have a role in several other diseases including Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, multiple sclerosis, and epilepsy, and appears to have a role in pathological response to traumatic brain injury and stroke. In rodent models, AQP4 appears plays a role in both the development and resolution of the cerebral edema that occurs following an injury like TBI or stroke and around brain tumors. In comparison with wild type mice, double knockout mice exhibited different diseases course post brain injury. It indicated reduced intracranial pressure, cell death, water accumulation, astrogliosis, and lesion volume. The expression of aquaporin 4 is reliant on the disease stage of TBI. In an acute stage of TBI, the lack of aquaporin 4 causes an decrease of excess water removal while for later stage TBI results in prevention of severe damage and swelling. In people who suffer from Alzheimer's disease, amyloid plaques sometimes develop in brain arteries—a condition is referred to as cerebral amyloid angiopathy, or CAA. Animal studies have found that the severity of CAA increases or decreases depending on aquaporin 4 expression. When there is an decrease in AQP4, CAA severity increases and vice versa; it is not known what causes changes in AQP4 expression levels, nor whether this is part of the disease process or an effort of the brain to adapt. In animal models of amyotrophic lateral sclerosis, AQP4 is overexpressed in the brainstem, cortex, and gray matter of the spinal cord which results in swollen astrocytes; the reason for this is not understood. Knockout mice display cognition problems; there is disruption in memory consolidation as well as disruption between memory acquisition, spatial recognition, and memory of where an object was after it has been moved.