Water is critical for life, and the threat of osmosis is a major factor in the development and maintenance of salt balance in all cells. The extreme variation across tissues in cell membrane resistance to water diffusion is due in part to the presence and regulation of integral membrane protein "aquaporin" channels.

Aquaporin channels are highly permeable to water while completely excluding other solutes including protons. A number of aquaporin channel proteins have now been cloned from a variety of tissue types starting with the 28kDa AQPl isolated from red cell membranes. The individual members of the aquaporin family (there are at least 10 in humans) have identical predicted secondary structures with up to 6 highly conserved hydrophobic membrane-spanning domains (about 18-25 AA each) and two conserved NPA motifs; however, significant variation exists in the N and C terminal regions where only about 20% of the sequence is conserved.

Types of Aquaporins
  • AQPO: major lens membrane protein
  • AQP1: abundant in renal proximal tubules and capillary endothelium.
  • AQP2: vasopressin-regulated water channel of the apical membrane of collecting duct cells
  • AQP3: wide tissue distribution.
  • AQP4: expressed significantly in the brain and other tissues
  • AQP5: epithelial tissues (salivary and lachrymal glands, etc)
  • AQP6: only found in kidney with low water permeability
  • AQP7: abundantly expressed in testis; also in adipose tissue, kidney and heart
  • AQP8: abundant in testis; also in liver, pancreas, placenta and salivary gland
  • AQP9: peripheral leukocytes
  • AQPadipose: adipose-specific, recently cloned