NATURE MEDICINE • VOLUME 6 • NUMBER 2 • FEBRUARY 2000 159 ARTICLES Brain edema plays a critical role in the pathophysiology and mor- bidity of a wide variety of nervous system disorders including head trauma, tumors, stroke, infections, and metabolic disor- ders 1,2 . However, little is known about the molecular mechanisms responsible for these alterations in cerebral water balance. Consequently, at present the therapeutic options are limited to neurosurgical decompression and intravenous administration of hyperosmolar agents, therapies that were introduced more than 70 years ago 3 . A family of molecular water channels called aqua- porins, which has been recently identified in mammals 4,5 , may provide a molecular mechanism for brain edema and thus offer therapeutic alternatives. Aquaporins are small integral membrane proteins (MW ∼30,000) that provide a major pathway for water transport in many cell types in the kidney, lung and other fluid-transporting tissues. Several aquaporins (AQP) have been identified in areas of the central nervous system that are known to participate in the production and reabsorption of brain fluid. AQP1 is selectively ex- pressed in the choroid plexus where it may play a role in cere- brospinal (CSF) production 6,7 . AQP4 is expressed widely throughout the brain, particularly at brain–blood and brain–CSF interfaces. AQP4 is abundantly expressed by glial cells lining the ependymal and pial surfaces that are in contact with CSF in the ventricular system and subarachnoid space 8 . Highly polarized AQP4 expression is also found in astrocytic foot processes near, or in direct contact with blood vessels 9 . AQP4 functions as an effi- cient water-selective transporting protein that is expressed to a much lesser extent in tissues outside of the nervous system, in- cluding kidney collecting duct, airways, stomach, colon, and skeletal muscle 10 . The specific localization of AQP4 to these anatomical and cellu- lar regions of the central nervous system suggests a role for AQP4 in cerebral water balance. The purpose of this study was to test the hypothesis that AQP4 is involved in cerebral edema. Well-charac- terized models of cellular brain edema (acute water intoxication 11 ) and combined cellular and vasogenic brain edema (ischemic stroke 12,13 ) were used to produce cerebral edema in mice. The ex- periments were done in a blinded manner on matched wild-type (AQP4 +/+ ) and AQP4 null (AQP4 –/– ) mice. The results indicate that AQP4 deletion in mice is associated with greatly reduced cerebral edema in response to water intoxication and stroke, with im- proved clinically relevant indices including survival and neurolog- ical status. These findings provide a specific molecular mechanism for brain water transport and suggest that AQP4 inhibition by pharmacological blockers might offer a new therapeutic option for some forms of cerebral edema. Brain edema after water intoxication AQP4 –/– mice were evaluated for neurological deficits and gross neuroanatomical abnormalities. They had no general behavioral, motor, sensory, or coordination deficits. Comparison of brains from AQP4 –/– and AQP4 +/+ mice showed no gross differences in size or anatomy. There were no gross differences in the posterior, ante- rior, or middle cerebral artery anatomy. The distribution of the middle cerebral artery territory seemed to be identical for AQP4 –/– and AQP4 +/+ mice. Microscopic examination of hematoxylin and eosin stained brain sections showed no histological differences. Blood–brain barrier permeability in AQP4 –/– and AQP4 +/+ control mice was evaluated with Evans blue. The blood–brain barrier was intact in both groups of mice except in choroid plexus where there Aquaporin-4 deletion in mice reduces brain edema after acute water intoxication and ischemic stroke GEOFFREY T. MANLEY 1 , MIKI FUJIMURA 2 , TONGHUI MA 3 , NOBUO NOSHITA 2 , FERDA FILIZ 3 , ANDREW W. BOLLEN 4 , PAK CHAN 2 & A.S. VERKMAN 3 1 Department of Neurosurgery, University of California, San Francisco, California 94143, USA 2 Department of Neurosurgery, Stanford University Medical School, Stanford, California 94305, USA 3 Departments of Medicine and Physiology, Cardiovascular Research Institute, University of California, San Francisco, California 94143, USA, and 4 Department of Pathology, University of California, San Francisco, California 94143, USA Correspondence should be addressed to G.T.M.; email: manley@itsa.ucsf.edu; http://www.ucsf.edu/verklab Cerebral edema contributes significantly to morbidity and death associated with many common neurological disorders. However, current treatment options are limited to hyperosmolar agents and surgical decompression, therapies introduced more than 70 years ago. Here we show that mice deficient in aquaporin-4 (AQP4), a glial membrane water channel, have much better sur- vival than wild-type mice in a model of brain edema caused by acute water intoxication. Brain tissue water content and swelling of pericapillary astrocytic foot processes in AQP4-deficient mice were significantly reduced. In another model of brain edema, focal ischemic stroke pro- duced by middle cerebral artery occlusion, AQP4-deficient mice had improved neurological out- come. Cerebral edema, as measured by percentage of hemispheric enlargement at 24 h, was decreased by 35% in AQP4-deficient mice. These results implicate a key role for AQP4 in modu- lating brain water transport, and suggest that AQP4 inhibition may provide a new therapeutic option for reducing brain edema in a wide variety of cerebral disorders. © 2000 Nature America Inc. • http://medicine.nature.com © 2000 Nature America Inc. • http://medicine.nature.com