An aquaporin is a specialized protein located in the cell membrane of body cells. It forms the mechanism responsible for pumping water into and out of the cell as needed. Aquaporins are part of the large family of major intrinsic proteins, proteins that form pores or channels in the cell membrane, and work to regulate the composition of the inside of the cell.
The aquaporin was discovered by Peter Agre of Johns Hopkins University in 1992. Agre won the 2003 Nobel Prize in Chemistry for his discovery. He discovered aquaporins serendipitously during a study on the Rh blood group antigen, confirming long-held suspicions by the scientific community that a mechanism for transporting water across the cell membrane existed.
Aquaporins conduct water into and out of the cell, but prevent the movement of ions and other solutes across the cell wall. A specialized form of aquaporin, called an aquaglyceroporin, does allow the movement of some solutes into and out of the cell, but like regular aquaporins, it does not allow charged particles, or ions, to pass through. Some solutes that aquaglyceroporins allow to cross the cell membrane are ammonia, carbon dioxide, and urea. The types of solutes allowed through by aquaporins depend on the size of the protein channel.
There are currently 13 known aquaporins in animals, six of which are located in the kidney. Biologists suspect that there are many more yet to be discovered. Plants also have aquaporins, which are integral to the transport of water from the soil, and through the roots, to various plant structures.
Since their discovery, aquaporins have been found to be implicated in a number of human diseases. If they could be manipulated, they might even be the key to curing some medical problems, such as fluid retention resulting from heart attack or stroke. Aquaporin mutations and deficiencies can lead to disease as well. Hereditary diabetes insipidus, a disorder characterized by excessive thirst and urination, is due to an aquaporin mutation, for example. Devic's disease, also called neuromyelitis optica, an autoimmune disorder characterized by inflammation of the optic nerve and spinal cord, is due to autoimmune reactions against an aquaporin.