Aquaporins - Water Channels
 Water crosses cell membranes by 
                diffusion through the lipid bilayer, 
                through water channel proteins called Aquaporins.

Functional characterization of the first aquaporin membrane protein was reported in 1992, but most membrane physiologists felt that there must be openings (pores or channels) in cell membranes to permit a flow of water,  because the osmotic permeability of some epithelial cells was much too large to be accounted for by simple diffusion through the plasma membrane. It is predicted that a single human aquaporin-1 channel protein facilitates water transport at a rate of roughly 3 billion water molecules per second. Such transport appears to be bidirectional, in accordance with the prevailing osmotic gradient.

In 1992 a "water channel" was identified and what its molecular machinery might look like was suggested; that is, proteins were identified  that formed an actual channel in membranes that facilitated water movement.

In the mid-1980s Peter Agre, M.D. (professor of Biological Chemistry and Medicine - John Hopkins Med School - 2003 Nobel Laureate in Chemistry) studied various membrane proteins isolated from the red blood cells. He also found one of these in the kidney cells. Having determined both its peptide sequence and the corresponding DNA sequence, he speculated that this might be the protein of the so-called cellular water channel. He termed this channel protein - aquaporin.

Agre tested his hypothesis that aquaporin might be a water channel protein in a simple experiment (fig. 1 - below). He compared cells which contained the protein in question with cells which did not have it. When the cells were placed in a water solution, those that had the protein in their membranes absorbed water by osmosis and swelled up while those that lacked the protein were not affected at all. Agre also ran trials with artificial cell membranes, termed liposomes, which are a simple lipid bound droplets of water. He found that the liposomes became permeable to water only if the aquaporin protein was implanted in their artificial membranes.

Fig 1. Agre’s experiment with cells containing or lacking aquaporin. Aquaporin is necessary for making the 'cell' absorb water and swell.

Agre also knew that mercury ions often prevent cells from taking up and releasing water, and he showed that water transport through his artificial membrane sacs with the aquaporin protein was prevented in the same way by mercury. This was further evidence that aquaporin might actually be a water channel.

How might a water channel work?

In 2000, together with other research teams, Agre reported the first high-resolution images of the three-dimensional structure of the aquaporin. With these data, it was possible to map in detail how a water channel might function. How is it that aquaporin only admits water molecules and not other molecules or ions? The membrane is, for instance, not allowed to leak protons. This is crucial because the difference in proton concentration between the inside and the outside of the cell is the basis of the cellular energy-storage system.

Aquaporins form tetramers in the cell membrane, and facilitate the transport of water and, in some cases, other small solutes, as glycerol, across the membrane. However, the water pores are completely impermeable to charged species, such as protons, a remarkable property that is critical for the conservation of membrane's electrochemical potential. Based on hydrophobicity plots of their amino acid sequences , the aquaporins are predicted to have six membrane-spanning segments, as depicted in the model of aquaporin-1 below.  Aquaporins exist in the plasma membrane as homotetramers. Each aquaporin monomer contains two hemi-pores, which fold together to form a water channel (fig 3.). 

The probable mechanism of action of aquaporin channels is studied using supercomputer simulations.  In the April 2002 issue of Science, the simulations run by researchers at the University of Illinois (Morten Jensen, Sanghyun Park, Emad Tajkhorshid, and Klaus Schulten) and the University of California at San Francisco (D. Fu, A. Libson, L.J.W. Miercke, C. Weitzman, P. Nollert, J. Krucinski, and R.M. Stroud) suggested the orientation of water molecules moving through aquaporins assures that only water, not ions such as protons, pass between cells.   The molecular dynamics (MD) computer simulations of the channels comprised a system of more than 100,000 atoms and revealed the formation of a single file inside the channel indicating that the water molecules pass through the channel single-file.   Upon entering, the water molecules face with their oxygen atom down the channel. Midstream, they reverse orientation, facing with the oxygen atom up. While passing through the channel, the ballet of water molecules streams through, always entering face down and leaving face up.

Selectivity is a central property of the channel. Water molecules worm their way through the narrow channel by orienting themselves in the local electrical field formed by the atoms of the channel wall. The strictly opposite orientations of the water molecules keep them from conducting protons, while still permitting a fast flux of water molecules.  Protons (or rather hydronium ions, H3O+) are stopped on the way and rejected because of their positive charges.

Jensen, Park, Tajkhorshid, & Schulten -
    
http://www.pnas.org/cgi/content/full/99/10/6731
Animations   courtesy of Tajkhorshid & Schulten or
                    de Groot and H. Grubmüller
Fig 2. Monomeric channel of
         aquaglyceroporin GlpF


    
Fig 3. Passage of water molecules through the aquaporin AQP1. Because of the positive charge at the center of the channel, positively charged ions such as H3O+, are deflected. This prevents proton leakage through the channel.

The physiological and medical significance of possible water channels.

The aquaporin proteins have been found to be a large protein family. More than 10 different mammalian aquaporins have been identified to date. Closely related water channel proteins have been isolated from plants, insects and bacteria. Aquaporin-1 from human red blood cells was the first to be discovered and is probably the best studied.  In the human body alone, at least eleven different aquaporin protein variants have been found.

The kidney removes waste substances the body wishes to dispose of. In the kidney water, ions and other small molecules leave the blood as ‘primary’ urine. Over 24 hours, about 170 liters of primary urine can be produced. Most of the water from this is reabsorbed so that finally about one liter of urine a day leaves the body.

From the glomerulus of the kidney, primary urine is passed on through a winding tube where about 70% of the water is reabsorbed to the blood by the aquaporin AQP1 protein. At the end of the glomerulus tube, another 10% of water is reabsorbed with a similar aquaporin, AQP2. Apart from this, sodium, potassium and chloride ions are also reabsorbed into the blood. Antidiuretic hormone (vasopressin) stimulates the transport of AQP2 to cell membranes in the tube walls and hence increases the water resorption from the urine. People with a deficiency of this hormone might be affected by the disease diabetes insipidus with a daily urine output of 10-15 liters.

References:

1. R. Bowen, Colorado State University, January 2, 2002
                                http://www.vivo.colostate.edu/hbooks/molecules/aquaporins.html
2. Nobel Museum - http://www.nobel.se/chemistry/laureates/2003/press.html
3. Emad Tajkhorshid, University of Illinois, July 23, 2003
                                 http://www.ks.uiuc.edu/Research/aquaporins/
4. R.M. Stroud, University of California, San Francisco (Science 290, 481-486, 2000)
                                 http://www-als.lbl.gov/als/science/sci_archive/glycerol.html.

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