Aquaporin

The second water channel to be cloned, aquaporin 2 or AQP2, is 42% identical to AQP1 at the amino acid level (Fushimi et al., 1993). Human AQP2 is a 271 amino acid protein with a molecular mass of 29 kDa and a predicted transmembrane structure similar to that of AQP1 (Sasaki et al., 1994). The gene is located on chromosome 12q13 and contains four exons (Table 19.1).

AQP2 is expressed exclusively in the collecting duct of the kidney and plays a fundamental role in the production of a concentrated urine (Fig. 19.4). The concentration of the urine is regulated by varying the extent to which water is absorbed from the collecting ducts of the kidney tubule. The membranes of the collecting duct cells are relatively impermeable to water. Water uptake is achieved by the regulated insertion of AQP2 channels into the apical membranes of the principal cells of the collecting duct, thereby increasing their water permeability (Fig. 19.5). This process is under the control of arginine-vasopressin (AVP). Binding of AVP to its receptor on the basal membrane of the principal cells results in activation of adenylate cyclase (via the heterotrimeric G-protein Gs) and thus in elevation of cyclic AMP. This stimulates protein kinase A (PKA) and initiates a chain of (as yet, poorly understood) events that ultimately result in the translocation of vesicles containing water channels to the apical membrane. Here, they fuse, inserting AQP2 channels into the surface membrane and increasing its water permeability. The vesicles containing water channels are subsequently retrieved by endocytosis and are then continuously recycled for as long as AVP continues to be present. Removal of AVP allows the continued uptake of AQP2 channels by endocytosis, but there is no longer any concomitant insertion, so that water uptake ceases. AQP2 is expressed exclusively in the apical membranes of the collecting duct cells, and the efflux of water across their basal membranes is mediated by another type of aquaporin, AQP3. In addition to stimulating the insertion of AQP2 in the apical membrane, AVP also exerts a longer-term effect on water permeability by increasing the level of AQP2 expression.

The link between AQP2 and the production of a concentrated urine was firmly established when it was discovered that the AQP2 gene is mutated in

Kidney Tubule Structure

Cortex

Outer medulla

Inner medulla

AQP2 (apical), AQP3 (basolateral) AQP1

FIGURE 19.4 EXPRESSION PATTERN OF DIFFERENT AQUAPORINS IN THE KIDNEY

The structure of a single kidney tubule. The distribution of the different aquaporins along the tubule is indicated.

Cortex

Outer medulla

Inner medulla

AQP2 (apical), AQP3 (basolateral) AQP1

FIGURE 19.4 EXPRESSION PATTERN OF DIFFERENT AQUAPORINS IN THE KIDNEY

The structure of a single kidney tubule. The distribution of the different aquaporins along the tubule is indicated.

some patients with familial nephrogenic diabetes insipidus (NDI; Deen et al., 1994a). This is an inherited disease in which water uptake by the kidney tubules is impaired. The disorder manifests within the first few weeks of life. The most typical symptoms are the excretion of large amounts of hypotonic urine and excessive thirst. In early infancy these may not be noticed, and the disease is often recognised by signs of dehydration, such as poor feeding, poor weight gain, irritability and fever. Without treatment, the severe dehydration associated with NDI can lead to mental retardation or death.

In most cases, familial NDI is caused by a mutation in the vasopressin receptor. In some families, however, the disease results from a mutation in the AQP2 gene and is inherited in an autosomal recessive fashion. Six missense mutations (G64R, N68S, T126M, A147T, R187C, and S216P) and two nucleotide deletions (at codons 310 and 369) have been identified to date (Fig. 19.2A). Both deletions shift the reading frame, the latter producing premature termination of translation and a truncated protein while the former produces a longer protein. When the missense mutations were engineered in AQP2 and expressed in Xenopus oocytes, either no increase or only a very small increase, in water permeability was observed (Deen et al., 1995a; Mulders et al., 1997). This is because incorporation of the mutant protein into the plasma membrane is impaired. Coinjection of mRNAs encoding mutant and wildtype AQP2 did not affect water permeability, which is consistent with the

Aquaporine Collecting Duct

o AQP3

FIGURE 19.5 ROLE OF AQP2 IN URINE CONCENTRATION

Cellular events involved in water uptake in the principal cells of the kidney collecting duct. Agonists such as AVP stimulate the production of cyclic AMP, which promotes the fusion of membrane vesicles containing AQP2 channels with the apical membrane. Water entering the cell through apical AQP2 channels exits via AQP3 channels in the basolateral membrane. This gives rise to a transcellular uptake of water.

o AQP3

FIGURE 19.5 ROLE OF AQP2 IN URINE CONCENTRATION

Cellular events involved in water uptake in the principal cells of the kidney collecting duct. Agonists such as AVP stimulate the production of cyclic AMP, which promotes the fusion of membrane vesicles containing AQP2 channels with the apical membrane. Water entering the cell through apical AQP2 channels exits via AQP3 channels in the basolateral membrane. This gives rise to a transcellular uptake of water.

autosomal recessive inheritance of the disease. As it is believed that water channels exist as tetrameric complexes, this argues that the mutant AQP2 subunits either do not complex with wild-type subunits or that the wild-type subunit is able to offset the deleterious effect of the mutant subunit.

Both prolonged hypokalaemia or lithium treatment result in nephrogenic diabetes insipidus. Reduced AQP2 levels are also found in animal models of both these syndromes (Marples et al., 1995,1996), further emphasizing the clinical importance of this water channel. In contrast, AQP2 was found to be upregulated in the apical membranes of renal-collecting duct cells in an animal model of congestive heart failure with fluid retention (Nielsen et al, 1997b).

Interestingly, AQP2 can be detected in the urine, and in normal people its excretion is increased by dehydration or AVP (Kanno et al., 1995). By contrast, in patients with mutations in AQP2, the protein was not detected in the urine even in response to dehydration. This suggests that urinary excretion of AQP2 may provide a simple way to detect its presence in the collecting duct membranes. This could be useful diagnostically, for example, to distinguish between central and nephrogenic NDI or to determine whether secondary NDI (such as that caused by drugs or hypokalaemia) results from a decrease in the expression of AQP2 or its delivery to the plasma membrane.

Diabetes 2

Diabetes 2

Diabetes is a disease that affects the way your body uses food. Normally, your body converts sugars, starches and other foods into a form of sugar called glucose. Your body uses glucose for fuel. The cells receive the glucose through the bloodstream. They then use insulin a hormone made by the pancreas to absorb the glucose, convert it into energy, and either use it or store it for later use. Learn more...

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