The phosphatase inhibitor okadaic acid induces AQP2 translocation independently from AQP2 phosphorylation in renal collecting duct cells

The prostaglandin E2 analogue sulprostone antagonizes vasopressin-induced antidiuresis through activation of Rho

Arginine-vasopressin (AVP) facilitates water reabsorption in renal collecting duct principal cells by activation of vasopressin V2 receptors and the subsequent translocation of water channels (aquaporin-2, AQP2) from intracellular vesicles into the plasma membrane. Prostaglandin E2 (PGE2) antagonizes AVP-induced water reabsorption; the signaling pathway underlying the diuretic response is not known. Using primary rat inner medullary collecting duct (IMCD) cells, we show that stimulation of prostaglandin EP3 receptors induced Rho activation and actin polymerization in resting IMCD cells, but did not modify the intracellular localization of AQP2. However, AVP-, dibutyryl cAMP- and forskolin-induced AQP2 translocation was strongly inhibited. This inhibitory effect was independent of increases in cAMP and cytosolic Ca2+. In addition, stimulation of EP3 receptors inhibited the AVP-induced Rho inactivation and the AVP-induced F-actin depolymerization. The data suggest that the signaling pathway underlying the diuretic effects of PGE2 and probably those of other diuretic agents include cAMP- and Ca2+-independent Rho activation and F-actin formation.

The ins and outs of aquaporin-2 trafficking

This review outlines recent advances related to the molecular mechanisms and pathways of aquaporin-2 (AQP2) water channel trafficking. AQP2 is a fascinating protein, whose sorting signals can be interpreted by different cell types to achieve apical or basolateral membrane insertion, in both regulated and constitutive trafficking pathways. In addition to the well-known cAMP-mediated, stimulatory effect of vasopressin on AQP2 membrane insertion, other signaling and trafficking events can also lead to AQP2 membrane accumulation via cAMP-independent mechanisms. These include 1) elevation of cGMP, mediated by sodium nitroprusside (a nitric oxide donor), atrial natriuretic factor, and l-arginine (via nitric oxide synthase); 2) disruption of the actin cytoskeleton; and 3) inhibition of the clathrin-mediated endocytotic arm of the AQP2 recycling pathway by dominant-negative dynamin expression and by membrane cholesterol depletion. Recent data also indicate that AQP2 recycles constitutively in epithelial cells, it can be inserted into different membrane domains in different cell types both in vitro and in vivo, and these pathways can be modulated by factors including hypertonicity. The roles of accessory proteins, including small GTPases and soluble N-ethylmaleimide-sensitive factor attachment protein receptor proteins in AQP2 membrane insertion, are also being uncovered. Understanding cAMP-independent mechanisms for membrane insertion of AQP2 is especially relevant to the therapeutic bypassing of the mutated, dysfunctional vasopressin receptor in patients with X-linked nephrogenic diabetes insipidus.

cAMP-induced AQP2 translocation is associated with RhoA inhibition through RhoA phosphorylation and interaction with RhoGDI

We have recently demonstrated that inhibition of Rho GTPase with Clostridium difficile toxin B, or with Clostridium botulinum C3 toxin, causes actin depolymerization and translocation of aquaporin 2 (AQP2) in renal CD8 cells in the absence of hormonal stimulation. Here we demonstrate that Rho inhibition is part of the signal transduction cascade activated by vasopressin leading to AQP2 insertion into the apical membrane. Quantitation of active RhoA (GTP-bound) by selective pull down experiments demonstrated that the amount of active RhoA decreased upon stimulation of CD8 cells with the cAMP-elevating agent forskolin. Consistent with this observation, forskolin treatment resulted in a decreased expression of membrane-associated (active) Rho, as assessed by cell fractionation followed by western blotting analysis. In addition, the abundance of the endogenous Rho GDP dissociation inhibitor (Rho-GDI) was found to have decreased in the membrane fraction after forskolin stimulation. Co-immunoprecipitation experiments revealed that, after forskolin stimulation, the amount of Rho-GDI complexed with RhoA increased, suggesting that Rho GTPase inhibition occurs through association of RhoA with Rho-GDI. Finally, forskolin stimulation was associated with an increase in Rho phosphorylation on a serine residue, a protein modification known to stabilize the inactive form of RhoA and to increase its interaction with Rho-GDI. Taken together, these data demonstrate that RhoA inhibition through Rho phosphorylation and interaction with Rho-GDI is a key event for cytoskeletal dynamics controlling cAMP-induced AQP2 translocation.

A rat kidney tubule suspension for the study of vasopressin-induced shuttling of AQP2 water channels

AVP increases the osmotic water permeability of renal collecting ducts by inducing the translocation of specific aquaporin-2 (AQP2) water channels from cytoplasmic vesicles to the apical plasma membrane of the principal cells. Here, we report a novel inner medullary tubule suspension for the study of this phenomenon that overcomes some of the drawbacks faced by present techniques; both primary cultures of inner medullary collecting duct cells and cell lines expressing AQP2 show aberrant trafficking and/or signaling pathways. The tubule suspensions were prepared by proteolytic digestion of inner medullas dissected from freshly isolated rat kidneys. After drug treatment, cellular distribution of AQP2 was determined by membrane fractionation and Western blotting or by immunocytochemistry. Treatment of suspensions with 1 nM AVP caused redistribution of AQP2 to the apical plasma membrane of the principal cells, a process inhibited by microtubule disruption or PKA inhibition. We conclude that this method provides a valuable new approach to the study of the cellular mechanisms involved in the response of the collecting duct to AVP.

Functional involvement of VAMP/synaptobrevin-2 in cAMP-stimulated aquaporin 2 translocation in renal collecting duct cells

The involvement of soluble N-ethylmaleimide sensitive factor-attachment protein receptor (SNARE) proteins in the cAMP-induced exocytosis of aquaporin 2 (AQP2)-containing vesicles was investigated in AQP2-transfected renal CD8 cells. RT-PCR and western blot analysis confirmed the presence of the SNARE homologs VAMP/synaptobrevin-2, syntaxin-1, syntaxin-4 and SNAP-23 in CD8 cells. Tetanus neurotoxin (TeNT) was efficient in cleaving synaptobrevin-like protein both in vitro and in intact CD8 cells incubated with the toxin. TeNT treatment in intact CD8 cells completely abolished cAMP-stimulated AQP2 targeting to the plasma membrane, as assessed by quantification of cell-surface immunoreactivity to an anti-AQP2 antibody raised against a peptide reproducing the extracellular AQP2 C-loop. These results represent the first evidence for the functional involvement of VAMP-2 in cAMP-induced AQP2 exocytosis in renal cells.

Regulated transport of the glucose transporter GLUT4

In muscle and fat cells, insulin stimulates the delivery of the glucose transporter GLUT4 from an intracellular location to the cell surface, where it facilitates the reduction of plasma glucose levels. Understanding the molecular mechanisms that mediate this translocation event involves integrating our knowledge of two fundamental processes--the signal transduction pathways that are triggered when insulin binds to its receptor and the membrane transport events that need to be modified to divert GLUT4 from intracellular storage to an active plasma membrane shuttle service.

Aquaporin-2: COOH terminus is necessary but not sufficient for routing to the apical membrane

Renal regulation of mammalian water homeostasis is mediated by the aquaporin-1 (AQP1) water channel, which is expressed in the apical and basolateral membranes of proximal tubules and descending limbs of Henle, and aquaporin-2 (AQP2), which is redistributed from intracellular vesicles to the apical membrane (AM) of collecting duct cells with vasopressin. In transfected Madin-Darby canine kidney cells, AQP1 and AQP2 are regulated similarly, which indicates that routing elements reside in their primary sequences. We studied the role of the AQP2 COOH terminus in apical routing and AQP2 shuttling. An AQP1 chimera (AQP1 with an AQP2 tail: AQP1/2-N220) was located only in the AM independent of forskolin treatment. Forskolin increased the apical expression of AQP1 and AQP1/2-N220 less than twofold; that of AQP2 increased more than fourfold with concomitant changes in osmotic water permeabilities. The dimeric AQP2 tail coupled to placental alkaline phosphatase (AQP2-Plap) was retained in intracellular vesicles different from those of homotetrameric wild-type AQP2; the same protein without the AQP2 tail (TMR-Plap) was only expressed in the AM. The study shows that the AQP2 COOH tail is necessary but not sufficient for routing to the AM and suggests that other parts of AQP2 are needed for AQP2 accumulation in intracellular vesicles.

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.

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.

The subcellular localization of an aquaporin-2 tetramer depends on the stoichiometry of phosphorylated and nonphosphorylated monomers

In renal principal cells, vasopressin regulates the shuttling of the aquaporin (AQP)2 water channel between intracellular vesicles and the apical plasma membrane. Vasopressin-induced phosphorylation of AQP2 at serine 256 (S256) by protein kinase A (PKA) is essential for its localization in the membrane. However, phosphorylated AQP2 (p-AQP2) has also been detected in intracellular vesicles of noninduced principal cells. As AQP2 is expressed as homotetramers, we hypothesized that the number of p-AQP2 monomers in a tetramer might be critical for the its steady state distribution. Expressed in oocytes, AQP2-S256D and AQP2-S256A mimicked p-AQP2 and non-p-AQP2, respectively, as routing and function of AQP2-S256D and wild-type AQP2 (wt-AQP2) were identical, whereas AQP2-S256A was retained intracellularly. In coinjection experiments, AQP2-S256A and AQP2-S256D formed heterotetramers. Coinjection of different ratios of AQP2-S256A and AQP2-S256D cRNAs revealed that minimally three AQP2-S256D monomers in an AQP2 tetramer were essential for its plasma membrane localization. Therefore, our results suggest that in principal cells, minimally three monomers per AQP2 tetramer have to be phosphorylated for its steady state localization in the apical membrane. As other multisubunit channels are also regulated by phosphorylation, it is anticipated that the stoichiometry of their phosphorylated and nonphosphorylated subunits may fine-tune the activity or subcellular localization of these complexes.