Membrane Water Transport Proteins: Aquaporins

The mechanism of water movement across cell membranes remained unclear until 1992, when the first water specific channel was identified in the human erythrocyte membrane¹. It belongs to an ancient protein family (MIP) found in most living organisms, from prokaryotes to mammalians and plants². This protein, as well as the others recently discovered were named “aquaporin” (AQP’s). Today, ten water channels have been identified . They share the same 6 transmembrane domains structure but differ in their tissue distribution as well as in their regulatory mechanism.

AQP1 is found almost in all tissues. In kidney, it is located in the proximal tubule and in the descending thin limb. It is believed to be responsible for constitutive water reabsorption³. AQP1 deficient mice do not express any lethal pathology but are unable to produce concentrated urine⁴. Surprisingly, it has also been found that they had troubles in lipid metabolism when fed a lipid rich diet⁵.

Both AQP2 and AQP3 are found in the plasma membrane of the renal collecting duct epithelial cells. They are respectively located in the apical and basolateral side of the cells.

The abundance and the specific location of the different aquaporins in the kidney make them excellent markers of specific regions in this tissue.

AQP4 is the predominant channel water in brain and its distribution suggests it is involved in the regulation of extravascular water⁶. Most of the mechanisms of aquaporins functions are still unknown. However, the importance of water movement in living organism and the large distribution of these proteins suggest the importance of research in this field.

References

1. Denker, B.M. et al. (1988) J. Biol. Chem. 263, 15634.
2. Chepelinsky, A. B. (1994) In “Handbook of Membrane Channels,” pp.413-432. Academic Press, San Diego.
3. Nielsen, S. et al. (1995) Am. J. Physiol. 268, F1023.
4. Ma, T. et al. (1998) J. Biol. Chem. 273, 4296.
5. Ma, T. et al. (2001) Am. J. Physiol. Cell Physiol. 280 (1):C126.
6. Nielsen, S. et al. (1997) J. Neurosci. 17, 171.