Distribution of aquaporin-4 water channel expression within rat kidney.
The aquaporins are a family of transmembrane proteins that function as molecular water channels. Recently, a mercurial-insensitive water channel [MIWC or aquaporin-4 (AQP4)] has been cloned, and its mRNA was found to be expressed strongly in kidney inner medulla and several nonrenal tissues. We prepared affinity-purified polyclonal antipeptide antibodies to AQP4 to define the regional distribution and cellular location of this water channel within the kidney. Immunoblotting of membrane fractions from different regions of the kidney revealed strongest expression in the base of the renal inner medulla, with detectable levels also in the inner medullary tip, but little or no expression in the outer medulla or cortex. Immunocytochemistry (light microscopy) revealed renal AQP4 labeling exclusively in the collecting duct principal cells, chiefly in the proximal two-thirds of the inner medullary collecting duct (IMCD). Little or no expression was seen in the outer medullary and cortical collecting ducts. Immunoelectron microscopy demonstrated AQP4 labeling of the basolateral membrane of IMCD cells, with relatively little labeling of intracellular vesicles. Differential centrifugation of inner medullary homogenates also revealed a lack of distribution to the vesicle-enriched fraction, which contains the vasopressin-regulated water channel, aquaporin-2. In contrast to aquaporin-2 and aquaporin-3, water restriction of rats did not increase the level of AQP4 expression. These results suggest a possible role for AQP4 in the basolateral exit of water from the IMCD.
Terris
J
J
Laboratory of Kidney and Electrolyte Metabolism, National Heart, Lung and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892, USA.
Ecelbarger
C A
CA
Marples
D
D
Knepper
M A
MA
Nielsen
S
S
D000595:Amino Acid Sequence; D000818:Animals; D051400:Aquaporin 3; D051401:Aquaporin 4; D020346:Aquaporins; D002462:Cell Membrane / Q000378:metabolism; D007150:Immunohistochemistry; D007473:Ion Channels / Q000235:genetics / Q000378:metabolism*; D007668:Kidney / Q000378:metabolism*; D016253:Microscopy, Immunoelectron; D008969:Molecular Sequence Data; D051381:Rats; D017207:Rats, Sprague-Dawley; D013347:Subcellular Fractions / Q000378:metabolism; D014018:Tissue Distribution; D014868:Water Deprivation / Q000502:physiology; D014882:Water-Electrolyte Balance
C496029:Aqp3 protein, rat; C496032:Aqp4 protein, rat; D051401:Aquaporin 4; D020346:Aquaporins; D007473:Ion Channels; D051400:Aquaporin 3