Aquaporins

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Aquaporins are transmembrane water transport channels that aid reabsorption and regulate the permeability of a cell to water. They are found most abundantly in regions involved in transporting particularly high rates of water, for example in the epithelial cells of the collecting duct at the end of a nephron in the kidney [1]. They are an extensive family of proteins, including Aquaporins 1-9, some being ubiquitous around the body for example Aquaporin 1; others are only present in specific regions such as Aquaporin 5 in the salivary gland. They are found both apically and basolaterally on membranes and also in a vesicular capacity (e.g. aquaporin 6). Aquaporins 3, 5 and 7, aquaglyceroporins, are capable of conducting glycerol. However, most aquaporins are very selctive and only allow the passage of water molecules, due to the protein's hour glass structure and the presence of two hydrophobic amino acids that line the inside of the pore [2].

The fusion of more aquaporins to the cell surface membrane is stimulated by the reception of ADH (anti-diuretic hormone) or vasopressin, by receptors found in the basolateral membrane of cells. For example, reabsorption of water through the principal cells in the collecting duct of the nephron by insertion of Aquaporin 2, is stimulated by the reception of vasopressin on the basolateral membrane of the cells. A G- protein coupled reaction takes place which stimulates adenylyl cyclase to produce cyclic AMP, a second messenger, which in turn stimulates protein kinase A to phosphorylate molecules of aquaporin 2. These are then trafficked, within vesicles, by actin and microtubular motors towards the apical membrane of the cell where the aquaporin is then inserted, increasing the reabsorption of water across the principal cell, via the the aquaporin 2 molecules on the apical membrane and the aquaporin 3/4 molecules already on the basolateral membrane to the blood. Therefore, ADH is secreted in times of water shortage (or low blood pressure) by the posterior pituitary gland, decreasing water loss through urine. In the absence of stimulation by ADH there are very few aquaporins present on the apical membrane, as most of the aquaporins will be stored in vesicles within the cell.  

References

  1. Alberts et al.(2008)Molecular Biology of the Cell,5th Edition: p673
  2. Alberts et al.(2008)Molecular Biology of the Cell,5th Edition: p674

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