Aquaporin water channels and endothelial cell function

Abstract

The aquaporins (AQP) are a family of homologous water channels expressed in many epithelial and endothelial cell types involved in fluid transport. AQP1 protein is strongly expressed in most microvascular endothelia outside of the brain, as well as in endothelial cells in cornea, intestinal lacteals, and other tissues. AQP4 is expressed in astroglial foot processes adjacent to endothelial cells in the central nervous system. Transgenic mice lacking aquaporins have been useful in defining their role in mammalian physiology. Mice lacking AQP1 manifest defective urinary concentrating ability, in part because of decreased water permeability in renal vasa recta microvessels. These mice also show a defect in dietary fat processing that may involve chylomicron absorption by intestinal lacteals, as well as defective active fluid transport across the corneal endothelium. AQP1 might also play a role in tumour angiogenesis and in renal microvessel structural adaptation. However, AQP1 in most endothelial tissues does not appear to have a physiological function despite its role in osmotically driven water transport. For example, mice lacking AQP1 have low alveolar-capillary water permeability but unimpaired lung fluid absorption, as well as unimpaired saliva and tear secretion, aqueous fluid outflow, and pleural and peritoneal fluid transport. In the central nervous system mice lacking AQP4 are partially protected from brain oedema in water intoxication and ischaemic models of brain injury. Therefore, although the role of aquaporins in epithelial fluid transport is in most cases well-understood, there remain many questions about the role of aquaporins in endothelial cell function. It is unclear why many leaky microvessels strongly express AQP1 without apparent functional significance. Improved understanding of aquaporin-endothelial biology may lead to novel therapies for human disease, such as pharmacological modulation of corneal fluid transport, renal fluid clearance and intestinal absorption.

Fig. 1

AQP1 facilitates osmotic water transport across renal vasa recta endothelium. (A) Location of the four principal aquaporin water channels in kidney tubules and microvasculature. (B) Light micrographs of OMDVR microdissected from wildtype (+/+) and AQP1 null (−/−) mice. Unperfused OMDVR from wildtype mice have diameter −10 μm. (C) Osmotic water permeability (P_f) measured in isolated microperfused OMDVR. P_f was measured in response to 200 mOsm gradients of NaCl or raffinose. Data from Pallone et al. (2000).

Fig. 2

AQP1 facilitates osmotic water transport across alveolar microvascular endothelium. (A) Schematic of barriers in airways and lung showing sites of aquaporin expression. (B) Osmotic water permeability across the air space–capillary barrier measured in perfused mouse lung in which the air space was filled with isosmolar saline containing a membrane-impermeant fluorescent indicator. Fluorescence changes as water moves into or out of the air spaces in response to changes in pulmonary artery perfusate osmolality. (C) Data shown for wildtype and AQP1 null mice. (D) Measurement of lung microvascular endothelial water permeability. The air space was filled with an inert perfluorocarbon, and the pulmonary artery was perfused with solutions of indicated osmolalities containing FITC-dextran. In response to an increase in perfusate osmolality, water moves into capillaries, resulting in fluorophore dilution and a prompt decrease in pleural surface fluorescence signal. Fluorescence returns to its initial level as osmotic equilibrium is established between capillary fluid and interstitium. (E) Gravimetric measurement of lung water permeability. The pulmonary artery was perfused with solutions of specified osmolality and lung weight was measured continuously by a gravimetric transducer. Data from Bai et al. (1999), Ma et al. (2000a) and Song et al. (2000b).

Fig. 3

Involvement of AQP1 in water transport across the pleural barrier. (A) Immunofluorescence localization of AQP1 protein in pleural microvascular endothelia (arrows) from wildtype and AQP1 null mice. Arrowheads indicate pleural surface. Scale bar = 150 μm. (B) Reduced osmotically induced water transport across the pleural barrier in AQP1 null mice. After anaesthesia and mechanical ventilation, the pleural space was infused with 0.25 mL of a hyperosmolar (500 mOsm) solution and pleural fluid osmolality was measured at indicated times. (C) Isosmolar fluid absorption from the pleural space. The pleural space was infused with 0.25 mL of an isosmolar solution containing 1% albumin and pleural fluid volume was measured at indicated times. Adapted from Song et al. (2000c).

Fig. 4

AQP1 in central lacteal endothelium in small intestine facilitates fat absorption. (A) Immunofluorescence localization of AQP1 in duodenum showing strong AQP1 immunostaining in central lacteals of wildtype mice. Scale bar = 50 μm. (B) Weight loss and appearance of mice on a 50% fat diet. (left) Weight curves of wildtype, heterozygous, and AQP1 null mice on a high-fat diet. Initial mouse weight was 10–12 g. Mice were switched to normal diet on day 8. (right) Photograph of mice on day 4 of a high-fat diet showing oily appearing fur and smaller size of AQP1 null mice. (C) Lipase activity in faeces and small intestinal lumen. Samples were collected from weight matched mice (10–12 g) before and after 3 days on high-fat diet. Data taken from Ma et al. (2000b).

Fig. 5

Aquaporins and the blood–brain barrier. (A) Immunofluorescence showing AQP1 protein expression in the apical surface of choroid plexus cells. (B) Diagram of a brain microvessel in cross-section. Endothelial cells (red) are joined by tight junctions (black) and surrounded by astrocyte foot processes (blue). AQP4 water channels (yellow) are expressed at the astrocyte foot processes. (C) Human brain showing AQP4 immunoreactivity (brown) around microvessels. Scale bar = 10 μm. (D) Freeze-fracture electron micrograph (E-face) of rat brain showing AQP4 square arrays in an astrocyte foot process. Inset: arrays in P-face micrograph. (E) Survival of wildtype vs. AQP4 knockout mice after acute water intoxication produced by intraperitoneal water infusion. (F) (top) Brain sections of mice at 24 h after ischaemic stroke produced by permanent middle cerebral artery occlusion. Note midline shift and marked oedema in brain from wildtype mice. (bottom) Averaged hemispheric enlargement expressed as a percentage determined by image analysis of brain sections. Adapted from Manley et al. (2000) and Papadopoulos et al. (2002).