Cellular and Molecular Biology of the Aquaporin Water Channels

Abstract

The high water permeability characteristic of mammalian red cell membranes is now known to be caused by the protein AQP1. This channel freely permits movement of water across the cell membrane, but it is not permeated by other small, uncharged molecules or charged solutes. AQP1 is a tetramer with each subunit containing an aqueous pore likened to an hourglass formed by obversely arranged tandem repeats. Cryoelectron microscopy of reconstituted AQP1 membrane crystals has revealed the three-dimensional structure at 3-6 A. AQP1 is distributed in apical and basolateral membranes of renal proximal tubules and descending thin limbs as well as capillary endothelia. Ten mammalian aquaporins have been identified in water-permeable tissues and fall into two groupings. Orthodox aquaporins are water-selective and include AQP2, a vasopressin-regulated water channel in renal collecting duct, in addition to AQP0, AQP4, and AQP5. Multifunctional aquaglyceroporins AQP3, AQP7, and AQP9 are permeated by water, glycerol, and some other solutes. Aquaporins are being defined in numerous other species including amphibia, insects, plants, and microbials. Members of the aquaporin family are implicated in numerous physiological processes as well as the pathophysiology of a wide range of clinical disorders.

Conserved elements within first intron of aquaporin-5 (Aqp5) function as transcriptional enhancers

“…Aquaporin-5 (AQP5) is a member of a family of water channel proteins [22,23] and is expressed at high levels in the lung, salivary gland and lacrimal tissues [1][2][3][4][5][6][7][8]. In the human, mouse and rat genome, the Aqp5 gene is located in a closely spaced tandem arrangement with Aqp2 and Aqp6 [24].…”

Structural basis of water-specific transport through the AQP1 water channel

“…Members of the water channel superfamily have been found in a range of cell types from bacteria to human. In mammals, there are currently 10 families of water channels, referred to as the aquaporins (AQP): AQP0 -AQP9, which can be divided into two major groups 3,4 ; AQP0, AQP1, AQP2, AQP4-AQP6 and AQP8, permeable to water but not to small organic and inorganic molecules 5 , and AQP3, AQP7 and AQP9, permeable to glycerol or urea as well as water 3,5 .…”

“…AQP1 water channels allow water, but not ions including protons, to freely and bidirectionally move across the cell membrane 5,7 . Sequence analysis shows high homology among members of the AQP1 family and that the two halves of the sequence exhibit a high degree of similarity 4 .…”

Immunoexpression of aquaporin-1 in the rat periodontal ligament during experimental tooth movement

“…Although numerous studies over the past 100 years have produced hyalinization on the locally pressured side of the periodontal ligament, the relationship between force level, timing onset, and extent of hyalinization remains unclear (for review, 36). Aquaporins (AQPs) are a family of transmembrane channel-forming glycoproteins that provide a major pathway for osmotically driven water transport through cell membranes (2,5,7,31,35). AQPs have at least 13 subtypes and display a widespread tissue distribution ranging from mammals to plants and microorganism (17,33).…”