Aquaporins: their role in cholestatic liver disease

Abstract

This review focuses on current knowledge on hepatocyte aquaporins (AQPs) and their significance in bile formation and cholestasis. Canalicular bile secretion results from a combined interaction of several solute transporters and AQP water channels that facilitate water flow in response to the osmotic gradients created. During choleresis, hepatocytes rapidly increase their canalicular membrane water permeability by modulating the abundance of AQP8. The question was raised as to whether the opposite process, i.e. a decreased canalicular AQP8 expression would contribute to the development of cholestasis. Studies in several experimental models of cholestasis, such as extrahepatic obstructive cholestasis, estrogen-induced cholestasis, and sepsis-induced cholestasis demonstrated that the protein expression of hepatocyte AQP8 was impaired. In addition, biophysical studies in canalicular plasma membranes revealed decreased water permeability associated with AQP8 protein downregulation. The combined alteration in hepatocyte solute transporters and AQP8 would hamper the efficient coupling of osmotic gradients and canalicular water flow. Thus cholestasis may result from a mutual occurrence of impaired solute transport and decreased water permeability.

Figure 1

Topology, organization and functioning of the aquaporin water channel molecule. A: Each AQP monomer consist of six transmembrane domains (I-VI) connected by five loops (a-e) with two NPA boxes shaping the water pore, and the amino and carboxy termini oriented toward the cytoplasm; B: Aquaporins are arranged in tetramers. The water pore does not reside in the center of the molecule, but is formed by connecting loops b and e in each subunit that functions as a unique water pore allowing bidirectional water movement; C: The hourglass model for aquaporin structure. The channel consist of an extracellular and intracellular vestibule containing water in bulk solution joined in the center by a central constriction 20 Å in length where water molecules pass in single file. The a/R constriction delimited by arginine in the position 195 (R195) and histidine in the position 180 (H180) provides fixed positive charges which prevent proton passage. The second constriction is bounded by two asparagine residues from the highly conserved NPA motif. The single water molecule passes through the constriction with no resistance as it forms transient hydrogen bonds with the nearby asparagines.

Figure 2

Functional expression of AQP8 in normal and cholestatic liver. Superior panel: Immunogold electron microscopy for AQP8 in liver from control (left) and 7-d BDL rats (right). Arrowheads indicate AQP8 in bile canalicular (bc) membranes. Arrows indicate AQP8 in the pericanalicular cytoplasm. The upper inset shows an AQP8-containing vesicle, and the lower insert shows AQP8 in the tip of a microvillus and in the intermicrovillar plasma membrane region. (Original magnification X 60 000) Modified and reproduced with permission[57]. Inferior panel: On the left, anti-AQP8 immunoblot of the canalicular plasma membranes from normal and LPS-induced cholestatic liver. On the right, water permeability assessment of canalicular membranes from LPS-induced cholestatic liver. Typical tracings of a time course of scattered light intensity (osmotic water transport), along with single exponential fits in canalicular plasma membrane vesicles in response to a 250 mosM hypertonic sucrose gradient. Calculated P_f values (μm/s) are shown under each curve. Data are mean ± SE from 3 independent vesicle preparations. ^aP < 0.05 vs conrtol. Modified and reproduced with permission[64].

Figure 3

Proposed contribution of hepatocyte AQP8 to the development of cholestasis. On the left a normal hepatocyte is illustrated with AQP8 expressed at the canalicular membrane domain and in intracellular vesicles. Bile is formed by the active secretion of solute transporters (ST) such as Bsep and Mrp2, which generate the osmotic driving forces for water transport through canalicular AQP8. On the right, a cholestatic hepatocyte is illustrated with decreased expression and functioning of ST. AQP8 is downregulated at the canalicular domain, which impairs the water osmotic permeability thus contributing to decreased bile formation.