Cell culture models and animal models for studying the patho-physiological role of renal aquaporins.

Aquaporins (AQPs) are key players regulating urinary-concentrating ability. To date, eight aquaporins have been characterized and localized along the nephron, namely, AQP1 located in the proximal tubule, thin descending limb of Henle, and vasa recta; AQP2, AQP3 and AQP4 in collecting duct principal cells; AQP5 in intercalated cell type B; AQP6 in intercalated cells type A in the papilla; AQP7, AQP8 and AQP11 in the proximal tubule. AQP2, whose expression and cellular distribution is dependent on vasopressin stimulation, is involved in hereditary and acquired diseases affecting urine-concentrating mechanisms. Due to the lack of selective aquaporin inhibitors, the patho-physiological role of renal aquaporins has not yet been completely clarified, and despite extensive studies, several questions remain unanswered. Until the recent and large-scale development of genetic manipulation technology, which has led to the generation of transgenic mice models, our knowledge on renal aquaporin regulation was mainly based on in vitro studies with suitable renal cell models. Transgenic and knockout technology approaches are providing pivotal information on the role of aquaporins in health and disease. The main goal of this review is to update and summarize what we can learn from cell and animal models that will shed more light on our understanding of aquaporin-dependent renal water regulation.

Aquaporin 2 and apical calcium-sensing receptor: new players in polyuric disorders associated with hypercalciuria.

The kidney plays a critical role in regulating water homeostasis through specific proteins highly expressed in the kidney, called aquaporins, allowing water permeation at a high rate. This brief review focuses on some nephropathies associated with impaired urinary concentrating ability and in particular analyzes the role of aquaporin 2 in hypercalciuria, the most common metabolic abnormality in patients with nephrolithiasis. Specifically, this review discusses the relationship between hypercalciuria and impaired aquaporin 2-mediated water handling in both acquired and inherited disorders characterized by hypercalciuria, including those affecting the sensor of extracellular calcium concentration, the calcium-sensing receptor, which represents the principal target for extracellular calcium regulation of several tissues including parathyroid glands and kidney. In the kidney, the calcium-sensing receptor regulates renal calcium excretion and influences the transepithelial movement of water and other electrolytes. Understanding the molecular basis of alteration of kidney concentrating ability found in hypercalciuria will help for devising strategies for reducing the risk of nephrocalcinosis, nephrolithiasis, and renal insufficiency.

Aquaporin-2 excretion and renal function during the 1st week of life in preterm newborn infants.

In many preterm infants, a characteristic pattern of fluid and electrolyte homeostasis occurs during the 1st week of life, consisting of three phases: prediuretic, diuretic, and postdiuretic. In this study, we evaluated the possible role of aquaporin-2 (AQP2) in renal concentrating ability and correlated it with other markers of the renal function in healthy preterm infants. Daily urine and spot blood samples were collected from 9 healthy preterm (32 +/- 1 weeks) infants at postnatal ages 1, 3, and 7 days. Urine and serum osmolality, creatinine, electrolytes, and AQP2 excretion were measured. All infants showed a significant (about 7%) weight loss on day 3 associated with a more than threefold increase in urine output without a significant change in fluid intake (diuretic phase). The creatinine clearance increased on day 3, indicating an increase in glomerular filtration rate. Interestingly, on day 3, the level of total excreted AQP2 (pmol/h) was significantly higher when compared to day 1 and day 7, and the same tendency was observed for urine osmolality. To conclude, the observed increase in urine osmolality and creatinine clearance during the diuretic phase, paralleled by an increase in total AQP2 excretion, suggests that AQP2 can contribute to the urinary concentrating ability early in postnatal life.

Water immersion is associated with an increase in aquaporin-2 excretion in healthy volunteers.

Here, we report the alterations in renal water handling in healthy volunteers during a 6 h thermoneutral water immersion at 34 to 36 degrees C. We found that water immersion is associated with a reversible increase in total urinary AQP2 excretion.

Rho inhibits cAMP-induced translocation of aquaporin-2 into the apical membrane of renal cells.

First published August 8, 2001; 10.1152/ajprenal.00091.2001.-We have recently demonstrated that actin depolymerization is a prerequisite for cAMP-dependent translocation of the water channel aquaporin-2 (AQP2) into the apical membrane in AQP2-transfected renal CD8 cells (29). The Rho family of small GTPases, including Cdc42, Rac, and Rho, regulates the actin cytoskeleton. In AQP2-transfected CD8 cells, inhibition of Rho GTPases with Clostridium difficile toxin B or with C. limosum C3 fusion toxin, as well as incubation with the Rho kinase inhibitor, Y-27632, caused actin depolymerization and translocation of AQP2 in the absence of the cAMP-elevating agent forskolin. Both forskolin and C3 fusion toxin-induced AQP2 translocation were associated with a similar increase in the osmotic water permeability coefficient. Expression of constitutively active RhoA induced formation of stress fibers and abolished AQP2 translocation in response to forskolin. Cytochalasin D induced both depolymerization of F-actin and AQP2 translocation, suggesting that depolymerization of F-actin is sufficient to induce AQP2 translocation. Together, these data indicate that Rho inhibits cAMP-dependent translocation of AQP2 into the apical membrane of renal principal cells by controlling the organization of the actin cytoskeleton.

Ht31: the first protein kinase A anchoring protein to integrate protein kinase A and Rho signaling.

In an attempt to isolate protein kinase A anchoring proteins (AKAPs) involved in vasopressin-mediated water reabsorbtion, the complete sequence of the human AKAP Ht31 was determined and a partial cDNA of its rat orthologue (Rt31) was cloned. The Ht31 cDNA includes the estrogen receptor cofactor Brx and the RhoA GDP/GTP exchange factor proto-lymphoid blast crisis (Lbc) sequences. The Ht31 gene was assigned to chromosome 15 (region q24-q25). It encodes Ht31 and the smaller splice variants Brx and proto-Lbc. A protein of the predicted size of Ht31 (309 kDa) was detected in human mammary carcinoma and HeLa cells. Anti-Ht31/Rt31 antibodies immunoprecipitated RhoA from primary cultured rat renal inner medullary collecting duct cells, indicating an interaction between the AKAP and RhoA in vivo. These results suggest that Ht31/Rt31 represent a new type of AKAP, containing both an anchoring and a catalytic domain, which appears to be capable of modulating the activity of an interacting partner. Ht31/Rt31 have the potential to integrate Rho and protein kinase A signaling pathways, and thus, are prime candidates to regulate vasopressin-mediated water reabsorbtion.

An inhibitory role of Rho in the vasopressin-mediated translocation of aquaporin-2 into cell membranes of renal principal cells.

Vasopressin regulates water reabsorption in renal collecting duct principal cells by a cAMP-dependent translocation of the water channel aquaporin-2 (AQP2) from intracellular vesicles into the cell membrane. In the present work primary cultured inner medullary collecting duct cells were used to study the role of the proteins of the Rho family in the translocation of AQP2. Clostridium difficile toxin B, which inhibits all members of the Rho family, Clostridium limosum C3 toxin, which inactivates only Rho, and the Rho kinase inhibitor, Y-27632, induced both depolymerization of actin stress fibers and AQP2 translocation in the absence of vasopressin. The data suggest an inhibitory role of Rho in this process, whereby constitutive membrane localization is prevented in resting cells. Expression of constitutively active RhoA induced formation of actin stress fibers and abolished AQP2 translocation in response to elevation of intracellular cAMP, confirming the inhibitory role of Rho. Cytochalasin D induced both depolymerization of the F-actin cytoskeleton and AQP2 translocation, indicating that depolymerization of F-actin is sufficient to induce AQP2 translocation. Thus Rho is likely to control the intracellular localization of AQP2 via regulation of the F-actin cytoskeleton.