Distribution of aquaporin-4 water channel expression within rat kidney

TLDR

Aquaporins are transmembrane water channels, and the mercurial‑insensitive AQP4 has been cloned and shown to be strongly expressed in the kidney inner medulla and other tissues. The study aimed to map the regional distribution and cellular localization of AQP4 within the kidney. Using affinity‑purified anti‑AQP4 antibodies, the authors performed immunoblotting of membrane fractions, immunocytochemistry, immunoelectron microscopy, and differential centrifugation to characterize AQP4 localization. AQP4 was most abundant at the base of the inner medulla, localized to the basolateral membrane of principal cells in the proximal two‑thirds of the inner medullary collecting duct, absent from outer medulla and cortex, not upregulated by water restriction, suggesting a role in basolateral water exit.

Abstract

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.