MOLECULAR CHARACTERIZATION OF AN AQUAPORIN CDNA FROM BRAIN - CANDIDATE OSMORECEPTOR AND REGULATOR OF WATER-BALANCE

The aquaporins transport water through membranes of numerous tissues, but the molecular mechanisms for sensing changes in extracellular osmo lality and regulating water balance in brain are unknown. We have isol ated a brain aquaporin by homology cloning. Like aquaporin 1 (AQP1, al so known as CHIP, channel-forming integral membrane protein of 28 kDa) , the deduced polypeptide has six putative transmembrane domains but l acks cysteines at the known mercury-sensitive sites. Two initiation si tes were identified encoding polypeptides of 301 and 323 amino acids; expression of each in Xenopus oocytes conferred a 20-fold increase in osmotic water permeability not blocked by 1 mM HgCl2, even after subst itution of cysteine at the predicted mercury-sensitive site. Northern analysis and RNase protection demonstrated the mRNA to be abundant in mature rat brain but only weakly detectable in eye, kidney, intestine, and Lung. In situ hybridization of brain localized the mRNA to ependy mal cells lining the aqueduct, glial cells forming the edge of the cer ebral cortex and brainstem, vasopressin-secretory neurons in supraopti c and paraventricular nuclei of hypothalamus, and Purkinje cells of ce rebellum. Its distinctive expression pattern implicates this fourth ma mmalian member of the aquaporin water channel family (designated gene symbol, AQP4) as the osmoreceptor which regulates body water balance a nd mediates water flow within the central nervous system.