Aquaporin water channels--from atomic structure to clinical medicine

Abstract

The water permeability of biological membranes has been a longstanding problem in physiology, but the proteins responsible for this remained unknown until discovery of the aquaporin 1 (AQP1) water channel protein. AQP1 is selectively permeated by water driven by osmotic gradients. The atomic structure of human AQP1 has recently been defined. Each subunit of the tetramer contains an individual aqueous pore that permits single-file passage of water molecules but interrupts the hydrogen bonding needed for passage of protons. At least 10 mammalian aquaporins have been identified, and these are selectively permeated by water (aquaporins) or water plus glycerol (aquaglyceroporins). The sites of expression coincide closely with the clinical phenotypes--ranging from congenital cataracts to nephrogenic diabetes insipidus. More than 200 members of the aquaporin family have been found in plants, microbials, invertebrates and vertebrates, and their importance to the physiology of these organisms is being uncovered.

Figure 1. Functional expression of AQP1 water channel in Xenopus laevis oocytes

Incubation in hypotonic buffer fails to cause swelling of a control oocyte (left). In contrast, an oocyte injected with AQP1 cRNA (right) exhibits high water permeability and has exploded. Reprinted from Science with permission (Preston et al. 1992).

Figure 2. Hourglass model for AQP1 topology

Arrangement of loops B and E with highly conserved NPA motifs forms a single aqueous pathway through the AQP1 subunit. Reprinted from Journal of Biological Chemistry with permission (Jung et al. 1994b).

Figure 3. AQP1 structure determined by electron crystallography

Space-filling model of AQP1 reveals three dimensional structure of aqueous pore. Top, horizontal section midway through AQP1 subunit shows 3 Å pore surrounded by hydrophobic residues (yellow) and tandem hydrogen-binding sites (Asn-192 and Asn-76, red). Bottom, vertical section shows aqueous channel lined by functionally important residues (Cys-189, His-180, Asn-76 and Asn-192; Arg-195 is not shown). Reprinted from Nature with permission (Murata et al. 2000).

Figure 4. Immunolocalization of AQP1 in rat kidney

Top, anti-AQP1 immunohistochemical evaluation of proximal convoluted tubule in outer medulla shows AQP1 in apical and basolateral membranes. Basal membrane (arrows); lumen (P); collecting duct (C). Bottom, anti-AQP1 immunogold electron microscopic examination of apical brush border (BB). Reprinted from Journal of Cell Biology with permission (Nielsen et al. 1993c).

Figure 5. Human aquaporin gene family

Water permeable (aquaporins) and glycerol permeable (aquaglyceroporins) family members are shown. AQP10 is included, but no confirmation of protein expression is yet published. Shown also are E. coli homologues (AqpZ and GlpF). The scale bar represents genetic distance between homologues.

Figure 6. Immunolocalization of AQP2 in isolated rat collecting duct

Top, anti-AQP2 immunogold electron microscopic examination of unstimulated collecting duct shows predominant labelling of intracellular vesicles. Bottom, after incubation with 100 pm vasopressin, collecting duct shows predominant labelling at apical surface (arrows). Reprinted from Proceedings of the National Academy of Sciences of the USA with permission (Nielsen et al. 1995).

Figure 7. Polarized expression of AQP4 in rat brain

A and B, anti-AQP4-immunogold electron microscopy demonstrates AQP4 in glial membranes facing blood vessels but not in membranes facing neuropil. C, anti-AQP4-immunogold electron microscopy demonstrates AQP4 in sub-pial astrocyte membranes. Scale bars: A and B, 0.5 μm; C, 1 μm. Reprinted from Journal of Neuroscience with permission (Nielsen et al. 1997b).