[The pathway of vasopressin as a pharmacological target in nephrology: a narrative review]

ADH is a hormone secreted by neurohypophysis that plays different roles based on the target organ. At the renal level, this peptide is capable of causing electrolyte-free water absorption, thus playing a key role in the hydro-electrolytic balance. There are pathologies and disorders that jeopardize this balance and, in this field, ADH receptor inhibitors such as Vaptans could play a key role. By inhibiting the activation pathway of vasopressin, they are potentially useful in euvolemic and hypervolemic hypotonic hyponatremia. However, clinical trials in heart failure have not given favourable results on clinical outcomes. Even in SIADH, despite their wide use, there is no agreement by experts on their use. Since vaptans inhibit the cAMP pathway in tubular cells, their use has been proposed to inhibit cystogenesis. A clinical trial has shown favourable effects on ADPKD progression. Because vaptans have been shown to be effective in models of renal cysts disorders other than ADPKD, their use has been proposed in diseases such as nephronophthisis and recessive autosomal polycystic disease. Other possible uses of vaptans could be in kidney transplantation and cardiorenal syndrome. Due to the activity of ADH in coagulation and haemostasis, ADH's activation pathway by Desmopressin Acetate could be a useful strategy to reduce the risk of bleeding in biopsies in patients with haemorrhagic risk.

Quantitative evaluation of Oryeongsan and its action on water regulation in renal inner medullary collecting duct cells

ETHNOPHARMACOLOGICAL RELEVANCE

Oryeongsan (ORS, Wulingsan) has been reported to possess renal protective effects from renal diseases such as diabetes-induced renal damage, and nephrocalcinosis.

AIM OF THE STUDY

This study was conducted to evaluate the quantitative analysis and the inhibitory effect of ORS on hypertonic stress-induced water channel and apoptosis in murine inner medullary collecting duct cell line (mIMCD-3).

MATERIALS AND METHODS

Chromatographic and NMR spectroscopic analysis were performed and water balance regulation was determined by Western blot, RT-PCR, and immunofluorescnece.

RESULTS

Seven active principles (5-hydroxymethylfurfural, alismoxide, methyl(-)trans-cinnamate, adenine, guanosine, adenosine, and ferulic acid) in ORS were isolated and the structures were identified mainly by NMR spectroscopic analysis. In addition, contents of these metabolites in ORS were evaluated by HPLC analysis. Pretreatment with ORS significantly attenuated the hypertonic stress (175mM NaCl)-induced increase in protein levels of AQP2 and apical membrane insertion. ORS also attenuated osmolyte sodium-myo-inositol transporter (SMIT) expression and tonicity-responsive enhancer binding protein (TonEBP) mRNA under hypertonic stress. Those actions of ORS presented the similar effect of PKA inhibitor which AQP2 expression throughout the inhibition of vasopressin-mediated cAMP/PKA signal pathway. On the other hand, pretreatment with ORS attenuated hypertonic stress-induced cell death. Hypertonic stress-induced Bax or caspase-3 expression was decreased by ORS, resulting in anti-apoptotic effect.

CONCLUSIONS

The present data suggest that the beneficial effect of ORS in water balance and apoptosis against hypertonic stress of renal collecting ducts.

Mathematical Model of Ammonia Handling in the Rat Renal Medulla

The kidney is one of the main organs that produces ammonia and release it into the circulation. Under normal conditions, between 30 and 50% of the ammonia produced in the kidney is excreted in the urine, the rest being absorbed into the systemic circulation via the renal vein. In acidosis and in some pathological conditions, the proportion of urinary excretion can increase to 70% of the ammonia produced in the kidney. Mechanisms regulating the balance between urinary excretion and renal vein release are not fully understood. We developed a mathematical model that reflects current thinking about renal ammonia handling in order to investigate the role of each tubular segment and identify some of the components which might control this balance. The model treats the movements of water, sodium chloride, urea, NH₃ and NH4+, and non-reabsorbable solute in an idealized renal medulla of the rat at steady state. A parameter study was performed to identify the transport parameters and microenvironmental conditions that most affect the rate of urinary ammonia excretion. Our results suggest that urinary ammonia excretion is mainly determined by those parameters that affect ammonia recycling in the loops of Henle. In particular, our results suggest a critical role for interstitial pH in the outer medulla and for luminal pH along the inner medullary collecting ducts.

ENaC activity is increased in isolated, split-open cortical collecting ducts from protein kinase Cα knockout mice

The epithelial Na channel (ENaC) is negatively regulated by protein kinase C (PKC) as shown using PKC activators in a cell culture model. To determine whether PKCα influences ENaC activity in vivo, we examined the regulation of ENaC in renal tubules from PKCα⁻/⁻ mice. Cortical collecting ducts were dissected and split open, and the exposed principal cells were subjected to cell-attached patch clamp. In the absence of PKCα, the open probability (P₀) of ENaC was increased three-fold vs. wild-type SV129 mice (0.52 ± 0.04 vs. 0.17 ± 0.02). The number of channels per patch was also increased. Using confocal microscopy, we observed an increase in membrane localization of α-, β-, and γ-subunits of ENaC in principal cells in the cortical collecting ducts of PKCα⁻/⁻ mice compared with wild-type mice. To confirm this increase, one kidney from each animal was perfused with biotin, and membrane protein was pulled down with streptavidin. The nonbiotinylated kidney was used to assess total protein. While total ENaC protein did not change in PKCα⁻/⁻ mice, membrane localization of all the ENaC subunits was increased. The increase in membrane ENaC could be explained by the observation that ERK1/2 phosphorylation was decreased in the knockout mice. These results imply a reduction in ENaC membrane accumulation and P₀ by PKCα in vivo. The PKC-mediated increase in ENaC activity was associated with an increase in blood pressure in knockout mice fed a high-salt diet.

Urine concentration in the diabetic mouse requires both urea and water transporters

The regulation of the inner medullary collecting duct (IMCD) urea transporters (UT-A1, UT-A3) and aquaporin-2 (AQP2) and their interactions in diabetic animals is unknown. We investigated whether the urine concentrating defect in diabetic animals was a function of AQP2, the UT-As, or both transporters. UT-A1/UT-A3 knockout (UT-A1/A3 KO) mice produce dilute urine. We gave wild-type (WT) and UT-A1/A3 KO mice vasopressin via minipump for 7 days. In WT mice, vasopressin increased urine osmolality from 3,000 to 4,550 mosmol/kgH(2)O. In contrast, urine osmolality was low (800 mosmol/kgH(2)O) in the UT-A1/A3 KOs and remained low following vasopressin. Surprisingly, AQP2 protein abundance increased in UT-A1/A3 KO (114%) and WT (92%) mice. To define the role of UT-A1 and UT-A3 in the diabetic responses, WT and UT-A1/A3 KO mice were injected with streptozotocin (STZ). UT-A1/A3 KO mice showed only 40% survival at 7 days post-STZ injection compared with 70% in WT. AQP2 did not increase in the diabetic UT-A1/A3 KO mice compared with a 133% increase in WT diabetic mice. Biotinylation studies in rat IMCDs showed that membrane accumulation of UT-A1 increased by 68% in response to vasopressin in control rats but was unchanged by vasopressin in diabetic rat IMCDs. We conclude that, even with increased AQP2, UT-A1/UT-A3 is essential to optimal urine concentration. Furthermore, UT-A1 may be maximally membrane associated in diabetic rat inner medulla, making additional stimulation by vasopressin ineffective.

Urine concentrating and diluting ability during aging

Urine concentrating ability is reduced during normal aging in people and rats. The abundance of many of the key transport proteins that contribute to urine concentrating ability is reduced in the kidney medulla of aged rats. The reductions in water, sodium, and urea transport protein abundances, and their reduced response to water restriction, contribute to the reduced ability of aged rats to concentrate their urine and conserve water. If similar mechanisms occur in human kidneys, it would provide a molecular explanation for the reduced urine concentrating ability in aging and may provide opportunities for novel therapeutic approaches to improve urine concentrating ability and/or nocturnal polyuria.

Renal involvement in leptospirosis: the effect of glycolipoprotein on renal water absorption

Background

Leptospirotic renal lesions frequently produce a polyuric form of acute kidney injury with a urinary concentration defect. Our study investigated a possible effect of the glycolipoprotein, (GLPc) extracted from L. interrogans, on vasopressin (Vp) action in the guinea pig inner medullary collecting duct (IMCD).

Methods

The osmotic water permeability (Pf µm/s) was measured by the microperfusion in vitro technique. AQP2 protein abundance was determined by Western Blot. Three groups were established for study as follows: Group I, IMCD from normal (ngp, n = 5) and from leptospirotic guinea-pigs (lgp-infected with L. interrogans serovar Copenhageni, GLPc, n = 5); Group II, IMCD from normal guinea-pigs in the presence of GLPc (GLPc group, n = 54); Group III, IMCD from injected animals with GLPc ip (n = 8).

Results

In Group I, Pfs were: ngp- 61.8±22.1 and lgp- 8.8±12.4, p<0.01 and the urinary osmolalities were: lgp-735±64 mOsm/Kg and ngp- 1,632±120 mOsm/Kg. The lgp BUN was higher (176±36 mg%) than the ngp (56±9 mg%). In Group II, the Pf was measured under GLPc (250 µg/ml) applied directly to the bath solution of the microperfused normal guinea-pig IMCDs. GLPc blocked Vp (200 pg/ml,n = 5) action, did not block cAMP (10⁻⁴ M,) and Forskolin (Fors- 10⁻⁹ M) action, but partially blocked Cholera Toxin (ChT- 10⁻⁹ M) action. GLP from L.biflexa serovar patoc (GLPp, non pathogenic, 250 µg) did not alter Vp action. In Group III, GLPc (250 µg) injected intraperitoneally produced a decrease of about 20% in IMCD Aquaporin 2 expression.

Conclusion

The IMCD Pf decrease caused by GLP is evidence, at least in part, towards explaining the urinary concentrating incapacity observed in infected guinea-pigs.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

[Proteinuria: pathophysiology and clinical implications]

Pathological proteinuria is a sign of renal disease, either tubular or glomerular. Proteinuria is considered as a major renal and cardiovascular risk factor Screening, and quantification of proteinuria is part of the care of chronic kidney disease (CKD) patients, but also of high renal risk patients and high cardiovascular risk patients. CKD is now classified according to estimated GFR and proteinuria to improve prediction of adverse events. in this article, we summarize the pathophysiology of proteinuria, its clinical qualification and implications.

Trafficking mechanism of water channel aquaporin-2

Targeted positioning of the water channel AQP2 (aquaporin-2) strictly regulates body water homoeostasis. Trafficking of AQP2 to the apical membrane is critical for the reabsorption of water in renal collecting ducts. In addition to the cAMP-mediated effect of vasopressin on AQP2 trafficking to the apical membrane, other signalling cascades can also induce this sorting. Recently, AQP2-binding proteins which could regulate this trafficking have been discovered; SPA-1 (signal-induced proliferation-associated gene-1), a GAP (GTPase-activating protein) for Rap1, and the cytoskeletal protein actin. This review summarizes recent advances related to the trafficking mechanisms of AQP2.

Proliferation of acid-secretory cells in the kidney during adaptive remodelling of the collecting duct

The renal collecting duct adapts to changes in acid-base metabolism by remodelling and altering the relative number of acid or alkali secreting cells, a phenomenon termed plasticity. Acid secretory A intercalated cells (A-IC) express apical H(+)-ATPases and basolateral bicarbonate exchanger AE1 whereas bicarbonate secretory B intercalated cells (B-IC) express basolateral (and apical) H(+)-ATPases and the apical bicarbonate exchanger pendrin. Intercalated cells were thought to be terminally differentiated and unable to proliferate. However, a recent report in mouse kidney suggested that intercalated cells may proliferate and that this process is in part dependent on GDF-15. Here we extend these observations to rat kidney and provide a detailed analysis of regional differences and demonstrate that differentiated A-IC proliferate massively during adaptation to systemic acidosis. We used markers of proliferation (PCNA, Ki67, BrdU incorporation) and cell-specific markers for A-IC (AE1) and B-IC (pendrin). Induction of remodelling in rats with metabolic acidosis (with NH(4)Cl for 12 hrs, 4 and 7 days) or treatment with acetazolamide for 10 days resulted in a larger fraction of AE1 positive cells in the cortical collecting duct. A large number of AE1 expressing A-IC was labelled with proliferative markers in the cortical and outer medullary collecting duct whereas no labeling was found in B-IC. In addition, chronic acidosis also increased the rate of proliferation of principal collecting duct cells. The fact that both NH(4)Cl as well as acetazolamide stimulated proliferation suggests that systemic but not urinary pH triggers this response. Thus, during chronic acidosis proliferation of AE1 containing acid-secretory cells occurs and may contribute to the remodelling of the collecting duct or replace A-IC due to a shortened life span under these conditions.

[Vasopressin: molecular mechanisms of antidiuretic effect]

Body water balance is regulated by vasopressin, and multiple studies now have revealed essential role of aquaporins in this process. This review summarizes recent advances related to molecular events underlying the short-term control of water permeability which occurs via vesicular trafficking of AQP2 to the cell apical membrane, and long-term control through changes in the gene expression and the abundance of AQP-2, -3 and -4. The action of vasopressin on water permeability is counterbalanced by several factors such as prostaglandin E2, bradykinin, dopamin, endotelin-1 and others. Changes in AQP-2 expression / trafficking are of particular importance in pathological conditions characterized by both diluting and concentration defect of renal function.

[Modeling of kidney collecting ducts function in the cell culture conditions]

Continuous cell lines originating from the kidney collecting duct represent a powerful tool for the modeling of water and ions reabsorbtion processes. Present review considers the basic methodical approaches being utilized to study vasopressin-induced water transport mechanisms in the cell culture conditions--microscopical methods, electrophysiological measurements, various ways of evaluation of water flow across the cell monolayer, transfections of native and mutant proteins, GFP-technology. The results of the highest significance for the understanding of collecting ducts function which were obtained with usage of these methods are analyzed in the review.

Apical serine protease activity is necessary for assembly of a high-resistance renal collecting duct epithelium

AIM

We hypothesized that the serine protease prostasin is necessary for differentiation of a high-resistance renal collecting duct epithelium governed by glucocorticoid.

METHODS

Postnatal rat kidney and adult human kidney was used to study the expression and localization of prostasin. The murine collecting duct cell line (M-1) was cultured in Snapwell inserts to investigate the significance of prostasin for the development of transepithelial electrical resistance (TER).

RESULTS

In the cortex and medulla of rat kidney, prostasin mRNA and protein increased significantly between birth and weaning (day 21) and was detected in collecting ducts. Immunoreactive prostasin was associated with collecting ducts and loops of Henle in human kidney. In rat, adrenalectomy at day 10 had no effect on prostasin mRNA level in kidney at day 20. Cultured M-1 cells exhibited parallel increases in prostasin mRNA, protein and TER 5 days after seeding. Apical addition of the serine protease inhibitor aprotinin to M-1 cell cultures inhibited development of TER and led to aberrant localization of E-cadherin. This effect was mimicked by the protease inhibitor nafamostat. Apical addition of phospholipase C to cleave glycosylphosphatidylinositol (GPI) anchors released prostasin to the medium and attenuated development of TER with time of culture. Disruption of lipid rafts by methyl-β-cyclodextrin attenuated development of TER in M-1 cells. Omission of dexamethasone impaired development of TER in M-1 cells, while prostasin protein abundance and E-cadherin distribution did not change.

CONCLUSION

Apical, GPI-anchored, lipid raft-associated serine protease activity, compatible with prostasin, is necessary for the development of a high-resistance collecting duct epithelium.

The effect of aspartate-lysine-isoleucine and aspartate-arginine-tyrosine mutations on the expression and activity of vasopressin V2 receptor gene

BACKGROUND

Vasopressin type 2 receptor (V2R) plays an important role in the water reabsorption in the kidney collecting ducts. V2R is a G protein coupled receptor (GPCR) and the triplet of amino acids aspartate-arginine-histidine (DRH) in this receptor might significantly influence its activity similar to other GPCR. However, the role of this motif has not been fully confirmed. Therefore, the present study attempted to shed some more light on the role of DRH motif in G protein coupling and V2R function with the use of site-directed mutagenesis.

METHODS

Nested PCR using specific primers was used to produce DNA fragments containing aspartate-lysine-isoleucine and aspartate-arginine-tyrosine mutations with replacements of the arginine to lysine and histidine to tyrosine, respectively. After digestion, these inserts were ligated into the pcDNA3 vector and transformation into E. coli HB101 was performed using heat shock method. The obtained colonies were analyzed for the presence and orientation of the inserts using proper restriction enzymes. After transient transfection of COS-7 cells using diethylaminoethyl-dextran method, the adenylyl cyclase activity assay was performed for functional study. The cell surface expression was analyzed by indirect ELISA method.

RESULTS

The functional assay indicated that none of these mutations significantly altered cAMP production and cell surface expression of V2R in these cells.

CONCLUSION

Since some substitutions in arginine residue have shown to lead to the inactive V2 receptor, further studies are required to define the role of this residue more precisely. However, it seems that the role of the histidine residue is not critical in the V2 receptor function.

Use of optical tweezers to probe epithelial mechanosensation

Cellular mechanosensation mechanisms have been implicated in a variety of disease states. Specifically in renal tubules, the primary cilium and associated mechanosensitive ion channels are hypothesized to play a role in water and salt homeostasis, with relevant disease states including polycystic kidney disease and hypertension. Previous experiments investigating ciliary-mediated cellular mechanosensation have used either fluid flow chambers or micropipetting to elicit a biological response. The interpretation of these experiments in terms of the "ciliary hypothesis" has been difficult due the spatially distributed nature of the mechanical disturbance-several competing hypotheses regarding possible roles of primary cilium, glycocalyx, microvilli, cell junctions, and actin cytoskeleton exist. I report initial data using optical tweezers to manipulate individual primary cilia in an attempt to elicit a mechanotransduction response-specifically, the release of intracellular calcium. The advantage of using laser tweezers over previous work is that the applied disturbance is highly localized. I find that stimulation of a primary cilium elicits a response, while stimulation of the apical surface membrane does not. These results lend support to the hypothesis that the primary cilium mediates transduction of mechanical strain into a biochemical response in renal epithelia.

[Fast nongenomic aldosterone effect on the kinetics of intracellular sodium and cell volume in the cortical part of collecting ducts of the rat kidney]

The fast nongenomic aldosterone effect on intracellular sodium ([Na+]i) and cell volume was studied by the fluorescent microscopy in isolated cortical part of collecting duct of the rat kidney (CCD). It was shown that aldosterone (10 nM) raised [Na+]i in hyposodium outer medium (14 mM). The rate of [Na+]i changes in response to external sodium shift (137-14 mM) twice as low in the presence of aldosterone (p < 0.05). Corticosterone (100 nM) was unable to simulate aldosterone effect. Similarly to sodium channel blocker amiloride (10(-5) M), protein kinase C (PKC) inhibitor RO-31-8220 (10(-7) M) abolished aldosterone effect. Aldosterone (10 nM) significantly decreased the amplitude and increased the characteristic time of the cell volume restoration in hypotonic medium of the rat principle cells (p < 0.001). Corticosterone (50 nM) was also unable to reproduce aldosterone effect. Amiloride (10(-5) M) did not significantly influence either the amplitude or the characteristic time of cell volume restoration during hypoosmotic challenge (p > 0.05). For the first time the specificity and important role of Ca(2+)-dependent kinase in the nongenomic aldosterone effects on ENaC activity and cell volume regulation in rat CCD were demonstrated.

Phosphorylation of UT-A1 urea transporter at serines 486 and 499 is important for vasopressin-regulated activity and membrane accumulation

The UT-A1 urea transporter plays an important role in the urine concentrating mechanism. Vasopressin (or cAMP) increases urea permeability in perfused terminal inner medullary collecting ducts and increases the abundance of phosphorylated UT-A1, suggesting regulation by phosphorylation. We performed a phosphopeptide analysis that strongly suggested that a PKA consensus site(s) in the central loop region of UT-A1 was/were phosphorylated. Serine 486 was most strongly identified, with other potential sites at serine 499 and threonine 524. Phosphomutation constructs of each residue were made and transiently transfected into LLC-PK1 cells to assay for UT-A1 phosphorylation. The basal level of UT-A1 phosphorylation was unaltered by mutation of these sites. We injected oocytes, assayed [14C]urea flux, and determined that mutation of these sites did not alter basal urea transport activity. Next, we tested the effect of stimulating cAMP production with forskolin. Forskolin increased wild-type UT-A1 and T524A phosphorylation in LLC-PK1 cells and increased urea flux in oocytes. In contrast, the S486A and S499A mutants demonstrated loss of forskolin-stimulated UT-A1 phosphorylation and reduced urea flux. In LLC-PK1 cells, we assessed biotinylated UT-A1. Wild-type UT-A1, S486A, and S499A accumulated in the membrane in response to forskolin. However, in the S486A/S499A double mutant, forskolin-stimulated UT-A1 membrane accumulation and urea flux were totally blocked. We conclude that the phosphorylation of UT-A1 on both serines 486 and 499 is important for activity and that this phosphorylation may be involved in UT-A1 membrane accumulation.

Kir4.1/Kir5.1 channel forms the major K+ channel in the basolateral membrane of mouse renal collecting duct principal cells

K(+) channels in the basolateral membrane of mouse cortical collecting duct (CCD) principal cells were identified with patch-clamp technique, real-time PCR, and immunohistochemistry. In cell-attached membrane patches, three K(+) channels with conductances of approximately 75, 40, and 20 pS were observed, but the K(+) channel with the intermediate conductance (40 pS) predominated. In inside-out membrane patches exposed to an Mg(2+)-free medium, the current-voltage relationship of the intermediate-conductance channel was linear with a conductance of 38 pS. Addition of 1.3 mM internal Mg(2+) had no influence on the inward conductance (G(in) = 35 pS) but reduced outward conductance (G(out)) to 13 pS, yielding a G(in)/G(out) of 3.2. The polycation spermine (6 x 10(-7) M) reduced its activity on inside-out membrane patches by 50% at a clamp potential of 60 mV. Channel activity was also dependent on intracellular pH (pH(i)): a sigmoid relationship between pH(i) and channel normalized current (NP(o)) was observed with a pK of 7.24 and a Hill coefficient of 1.7. By real-time PCR on CCD extracts, inwardly rectifying K(+) (Kir)4.1 and Kir5.1, but not Kir4.2, mRNAs were detected. Kir4.1 and Kir5.1 proteins cellularly colocalized with aquaporin 2 (AQP2), a specific marker of CCD principal cells, while AQP2-negative cells (i.e., intercalated cells) showed no staining. Dietary K(+) had no influence on the properties of the intermediate-conductance channel, but a Na(+)-depleted diet increased its open probability by approximately 25%. We conclude that the Kir4.1/Kir5.1 channel is a major component of the K(+) conductance in the basolateral membrane of mouse CCD principal cells.

Newer insights into renal regulation of water homeostasis

The regulation of high osmolality is an important driving force for water reabsorption and urinary concentration--the key functions of the kidney for maintaining optimum body fluid volume. New evidence shows that transcription factor tonicity responsive enhancer binding protein (TonEBP) and calcineurin-nuclear factor of activated T cells through cross-talk enhance Aquaporin 2 (AQP2) expression. AQP2 is the predominant vasopressin regulated water channel of the kidney collecting duct and is essential for urinary concentration. The serine/threonine phosphatase calcineurin is an important signaling molecule involved in kidney development and function. One potential target of calcineurin action is the water channel AQP2. The nuclear factor of activated T cells (NFAT) family has recently been expanded by the discovery of a new member, NFAT 5, or Ton EBP. Ton EBP is the only known mammalian transcription factor that regulates gene expression in response to hypertonicity. This review examines the importance of AQP2, calcineurin, NFATc and TonEBP in the renal regulation of water homeostasis.

Role of SNAREs and H+-ATPase in the targeting of proton pump-coated vesicles to collecting duct cell apical membrane

Recycling of H(+)-ATPase to the apical plasma membrane, mediated by vesicular exocytosis and endocytosis, is an important mechanism for controlling H(+) secretion by the collecting duct. We hypothesized that SNAREs (soluble N-ethylmaleimide-sensitive factor attachment proteins) may be involved in the targeting of H(+)-ATPase-coated vesicles. Using a tissue culture model of collecting duct H(+) secretory cells (inner medullary collecting duct (IMCD) cells), we demonstrated that they express the proteins required for SNARE-mediated exocytosis and form SNARE-fusion complexes upon stimulation of H(+)-ATPase exocytosis. Furthermore, exocytic amplification of apical H(+)-ATPase is sensitive to clostridial toxins that cleave SNAREs and thereby inhibit secretion. Thus, SNAREs are critical for H(+)-ATPase cycling to the plasma membrane. The process in IMCD cells has a feature distinct from that of neuronal cells: the SNARE complex includes and requires the vesicular cargo (H(+)-ATPase) for targeting. Using chimeras and truncations of syntaxin 1, we demonstrated that there is a specific cassette within the syntaxin 1 H3 domain that mediates binding of the SNAREs and a second distinct H3 region that binds H(+)-ATPase. Utilizing point mutations of the B1 subunit of the H(+)-ATPase, we document that this subunit contains specific targeting information for the H(+)-ATPase itself. In addition, we found that Munc-18-2, a regulator of exocytosis, plays a multifunctional role in this system: it regulates SNARE complex formation and the affinity of syntaxin 1 for H(+)-ATPase.

Role of prostaglandins in collecting duct-derived endothelin-1 regulation of blood pressure and water excretion

Collecting duct (CD)-derived endothelin-1 (ET-1) exerts natriuretic, diuretic, and hypotensive effects. In vitro studies have implicated cyclooxygenase (COX) metabolites, and particularly PGE(2), as important mediators of CD ET-1 effects. However, it is unknown whether PGE(2) mediates CD-derived ET-1 actions in vivo. To test this, CD ET-1 knockout (KO) and control mice were studied. During normal salt and water intake, urinary PGE(2) excretion was unexpectedly increased in CD ET-1 KO mice compared with controls. Salt loading markedly increased urinary PGE(2) excretion in both groups of mice; however, the levels remained relatively higher in KO animals. Acutely isolated inner medullary collecting duct (IMCD) from KO mice also had increased PGE(2) production. The increased IMCD PGE(2) was COX-2 dependent, since NS-398 blocked all PGE(2) production. However, increased CD ET-1 KO COX-2 protein or mRNA could not be detected in inner medulla or IMCD, respectively. Inner medullary COX-1 mRNA and protein levels and IMCD COX-1 mRNA levels were unaffected by Na intake or CD ET-1 KO. KO mice on a normal or high-Na diet had elevated blood pressure compared with controls; this difference was not altered by indomethacin or NS-398 treatment. However, indomethacin or NS-398 did increase urine osmolality and reduce urine volume in KO, but not control, animals. In summary, IMCD COX-2-dependent PGE(2) production is increased in CD ET-1 KO mice, indicating that CD-derived ET-1 is not a primary regulator of IMCD PGE(2). Furthermore, the increased PGE(2) in CD ET-1 KO mice partly compensates for loss of ET-1 with respect to maintaining urinary water excretion, but not in blood pressure control.

Regulation of the basolateral chloride/base exchangers AE1 and SLC26A7 in the kidney collecting duct in potassium depletion

In the present study, the effect of potassium depletion on the expression of acid-base transporters in the collecting duct was examined. Toward this end rats were fed a potassium-free diet for 3 weeks. Thereafter, the expression of the basolateral chloride/bicarbonate exchangers AE1 and SLC26A7 and the apical H(+)-ATPase was examined by northern hybridization, immunoblot analysis and immunofluorescence labelling. The mRNA expression of AE1 increased by a robust approximately 500% in the cortex and approximately 70% in the outer medulla, which translated into a huge increase in AE1 protein abundance in the cortex and a moderate increase in the outer medulla in K-depletion. The mRNA expression of SLC26A7 did not change significantly but its protein abundance showed a robust increase in the outer medulla. The expression of SLC26A7 remained undetected in the cortex in K-depleted rats. The post translational increase in SLC26A7 membrane abundance in potassium depletion was recapitulated in vitro using epitope-tagged SLC26A7. H(+)-ATPase displayed enhanced apical plasma membrane immunoreactivity in the OMCD in K-depletion. We suggest that the up-regulation of SLC26A7 and AE1 on the basolateral membrane of A-intercalated cells in the OMCD and CCD, respectively, along with H(+)-ATPase on the apical membrane, contributes to enhanced bicarbonate absorption in the collecting duct in K-depletion.

Hydrogen peroxide stimulates chloride secretion in primary inner medullary collecting duct cells via mPGES-1-derived PGE2

We investigated the role and mechanism of H2O2 in regulation of NaCl transport in primary inner medullary collecting duct (IMCD) cells. IMCD cells were isolated from wild-type mice and grown onto semipermeable membranes, and short-circuit current (Isc) was determined by Ussing chamber. Exposure of IMCD cells to H2O2 at a range of 100-300 microM caused a rapid increase in Isc in a time- and dose-dependent manner. This increase was almost abolished by the cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel inhibitors diphenylamine-2-carboxylic acid (DPC) and CFTR inhibitor-172. In contrast, the magnitude of stimulation was unaffected by the epithelial Na+ channel (ENaC) inhibitor amiloride. The H2O2-induced Cl(-) secretion was significantly inhibited by indomethacin, as well as by microsomal PGE synthase-1 (mPGES-1) deficiency. Like H2O2, PGE2 treatment induced a twofold increase in Isc that was reduced by the protein kinase A (PKA) inhibitors H-89 and KT5720. These data suggest that H2O2 stimulates CFTR Cl(-) channel-mediated Cl(-) secretion through cyclooxygenase- and mPGES-1-dependent release of PGE2 and subsequent activation of PKA.

Hyponatremia: clinical diagnosis and management

Hyponatremia is a common clinical problem in hospitalized patients and nursing home residents. It also may occur in healthy athletes after endurance exercise. The majority of patients with hyponatremia are asymptomatic and do not require immediate correction of hyponatremia. Symptomatic hyponatremia is a medical emergency requiring rapid correction to prevent the worsening of brain edema. How fast we should increase the serum sodium levels depends on the onset of hyponatremia and still remains controversial. If the serum sodium levels are corrected too rapidly, patients may develop central pontine myelinolysis, but if they are corrected too slowly, patients may die of brain herniation. We review the epidemiology and mechanisms of hyponatremia, the sensitivity of women to hyponatremic injury, the adaptation and maladaptation of brain cells to hyponatremia and its correction, and the practical ways of managing hyponatremia. Because the majority of hyponatremia is caused by the non-osmotic release of vasopressin, the recent approval of the vasopressin receptor antagonist conivaptan for euvolemic hyponatremia may simplify hyponatremia management. However, physicians should be aware of the risk of rapid correction of hyponatremia, hypotension, and excessive fluid intake.

Pax transactivation-domain interacting protein is required for urine concentration and osmotolerance in collecting duct epithelia

Pax transactivation-domain interacting protein (PTIP) is a widely expressed nuclear protein that is essential for early embryonic development. PTIP was first identified on the basis of its interactions with the developmental regulator Pax2 but can also bind to other nuclear transcription factors. The Pax2 protein is essential for development of the renal epithelia and for regulating the response of mature collecting ducts to hyperosmotic stress. For determination of whether PTIP also functions in more differentiated cell types, the Cre-LoxP system was used to delete the ptip gene in the renal collecting ducts using Ksp-Cre driver mice. Collecting duct-specific ptip knockout mice were viable with little discernible phenotype under normal physiologic conditions. However, collecting duct-specific ptip mutants were unable to concentrate urine after the treatment of desamino-cis, D-arginine vasopressin, an antidiuretic hormone. Furthermore, aquaporin-2 (AQP2) expression in the inner medulla of the ptip knockout mice was decreased approximately 10-fold compared with that of wild-type littermates. Expression level of tonicity responsive enhancer binding protein, a transcription factor of AQP2, is not altered in the mutant mice, but its nuclear localization in the inner medulla is unresponsive after treatment with vasopressin agonists. This was due, at least in part, to decreased expression of the arginine vasopressin receptor 2 in ptip mutants. Furthermore, ptip null inner medullary collecting duct cells were sensitive to hyperosmolality in vitro. Thus, ptip is required for the urine concentration mechanism by modulating arginine vasopressin receptor 2 and AQP2 expression in the inner medulla. The data suggest an essential role for ptip in regulating urine concentration and in controlling survival of collecting duct epithelial cells in high osmolality.

TRPV4 as a flow sensor in flow-dependent K+ secretion from the cortical collecting duct

The transient receptor vanilloid-4 (TRPV4) is a mechanosensitive, swell-activated cation channel that is abundant in the renal distal tubules. Immunolocalization studies, however, present conflicting data as to whether TRPV4 is expressed along the apical and/or basolateral membranes. To disclose the role of TRPV4 in flow-dependent K+ secretion in distal tubules in vivo, urinary K+ excretion and net transports of K+ and Na+ in the cortical collecting duct (CCD) were measured with an in vitro microperfusion technique in TRPV4 +/+ and TRPV4 −/− mice. Both net K+ secretion and Na+ reabsorption were flow dependently increased in the CCDs isolated from TRPV4 +/+mice, which were significantly enhanced by a luminal application of 50 μM 4α-phorbol-12,13-didecanoate (4αPDD), an agonist of TRPV4. No flow dependence of net K+ and Na+ transports or effects of 4αPDD on CCDs were observed in TRPV4 −/− mice. A basolateral application of 4αPDD had little effect on these ion transports in the TRPV4 +/+ CCDs, while the luminal application did. Urinary K+ excretion was significantly smaller in TRPV4 −/− than in TRPV4 +/+ mice when urine production was stimulated by a venous application of furosemide. These observations suggested an essential role of the TRPV4 channels in the luminal or basolateral membrane as flow sensors in the mechanism underlying the flow-dependent K+ secretion in mouse CCDs.

Computer analysis of the significance of the effective osmolality for urea across the inner medullary collecting duct in the operation of a single effect for the counter-current multiplication system

BACKGROUND

Although urea and water are transported across separate pathways in the apical membrane of the inner medullary collecting duct (IMCD), the existence of a cellular diffusion barrier as an unstirred layer makes it possible to use coefficients of effective osmotic force (sigma*) as equivalent to reflection coefficients. The difference in effective osmolality between urea and NaCl across the IMCD becomes a driving force for water if the compositions of solutes are different between tubular lumen and interstitium. Since reported values for sigma*(urea) are discrepant, we compared the efficiency of a single effect in the counter-current system between an ascending thin limb (ATL) and the IMCD, with the interposition of capillary networks (CNW), between two models with sigma(urea)* = 0.7 (model 1) and sigma(urea)* = 1.0 (model 2).

METHODS

The time courses (within 3 s) of solute and the water transport profiles among ATL, CNW, and IMCD were simulated with a computer in the absence of flow in each compartment.

RESULTS

In spite of small differences in the profiles of urea and NaCl concentrations between the two models, model 1 displayed a larger volume flux in the IMCD than model 2, resulting in an increase of osmolality in the IMCD and a decrease of osmolality in the ATL. These findings are vital for the operation of the counter-current multiplication system.

CONCLUSIONS

The concept of coefficients for effective osmotic force can be applied to the counter-current model between the IMCD and the ATL with the interposition of CNW. The model of sigma(urea)* = 0.7 is more efficient than that of sigma(urea)* = 1.0.

Low Na intake suppresses expression of CYP2C23 and arachidonic acid-induced inhibition of ENaC

We previously demonstrated that arachidonic acid (AA) inhibits epithelial Na channels (ENaC) through the cytochrome P-450 (CYP) epoxygenase-dependent pathway ( 34 ). In the present study, we tested the hypothesis that low Na intake suppresses the expression of CYP2C23, which is mainly responsible for converting AA to epoxyeicosatrienoic acid (EET) in the kidney ( 11 ) and attenuates the AA-induced inhibition of ENaC. Immunostaining showed that CYP2C23 is expressed in the Tamm-Horsfall protein (THP)-positive and aquaporin 2 (AQP2)-positive tubules. This suggests that CYP2C23 is expressed in the thick ascending limb (TAL) and collecting duct (CD). Na restriction significantly suppressed the expression of CYP2C23 in the TAL and CD. Western blot also demonstrated that the expression of CYP2C23 in renal cortex and outer medulla diminished in rats on Na-deficient diet (Na-D) but increased in those on high-Na diet (4%). Moreover, the content of 11,12-epoxyeicosatrienoic acid (EET) decreased in the isolated cortical CD from rats on Na-D compared with those on a normal-Na diet (0.5%). Patch-clamp study showed that application of 15 μM AA inhibited the activity of ENaC by 77% in the CCD of rats on a Na-D for 3 days. However, the inhibitory effect of AA on ENaC was significantly attenuated in rats on Na-D for 14 days. Furthermore, inhibition of CYP epoxygenase with MS-PPOH increased the ENaC activity in the CCD of rats on a control Na diet. We also used microperfusion technique to examine the effect of MS-PPOH on Na transport in the distal nephron. Application of MS-PPOH significantly increased Na absorption in the distal nephron of control rats but had no significant effect on Na absorption in rats on Na-D for 14 days. We conclude that low Na intake downregulates the activity and expression of CYP2C23 and attenuates the inhibitory effect of AA on Na transport.

Proteomics in renal research

Proteomic technologies are used with increasing frequency in the renal community. In this review, we highlight the use in renal research of a number of available techniques including two-dimensional gel electrophoresis, liquid chromatography/mass spectrometry, surface-enhanced laser desorption/ionization, capillary electrophoresis/mass spectrometry, and antibody and tissue arrays. These techniques have been used to identify proteins or changes in proteins specific to regions of the kidney or associated with renal diseases or toxicity. They have also been used to examine protein expression changes and posttranslational modifications of proteins during signaling. A number of studies have used proteomic methodologies to look for diagnostic biomarkers in body fluids. The rapid rate of development of the technologies along with the combination of classic physiological and biochemical techniques with proteomics will enable new discoveries.

A Kir2.3-like K+ Conductance in Mouse Cortical Collecting Duct Principal Cells

K(+) channels play an important role in renal collecting duct cell function. The current study examined barium (Ba(2+))-sensitive whole-cell K(+) currents (IKBa) in mouse isolated collecting duct principal cells. IKBa demonstrated strong inward rectification and was inhibited by Ba(2+) in a dose- and voltage-dependent fashion, with the K (d) decreasing with hyperpolarization. The electrical distance of block by Ba(2+) was around 8.5%. As expected for voltage-dependent inhibition, the association constant increased with hyperpolarization, suggesting that the rate of Ba(2+) entry was increased at negative potentials. The dissociation constant also increased with hyperpolarization, consistent with the movement of Ba(2+) ions into the intracellular compartment at negative potentials. These properties are not consistent with ROMK but are consistent with the properties of Kir2.3. Kir2.3 is thought to be the dominant basolateral K(+) channel in principal cells. This study provides functional evidence for the expression of Kir2.3 in mouse cortical collecting ducts and confirms the expression of Kir2.3 in this segment of the renal tubule using reverse-transcriptase polymerase chain reaction. The conductance described here is the first report of a macroscopic K(+) conductance in mouse principal cells that shares the biophysical profile of Kir2.3. The properties and dominant nature of the conductance suggest that it plays an important role in K(+) handling in the principal cells of the cortical collecting duct.

High-conductance K channels in intercalated cells of the rat distal nephron

High-conductance (BK or maxi) K(+) channels were observed in cell-attached patches of the apical membrane of the isolated split-open rat connecting tubule (CNT). These channels were quite rare in cells identified visually as principal cells (PCs; 5/162 patches) but common in intercalated cells (ICs; 24/26 patches). The BK-expressing intercalated cells in the CNT and cortical collecting duct (CCD) were characterized by a low membrane potential (-36 mV) under short-circuit conditions, measured from the reversal potential of the channel currents with similar K(+) concentrations on both sides of the membrane. Under whole-cell clamp conditions with low intracellular Ca(2+), ICs had a very low K(+) conductance. When cell Ca(2+) was increased to 200 nM, a voltage-dependent, tetraethylammonium (TEA)-sensitive outward conductance was activated with a limiting value of 90 and 140 nS/cell in the CNT and CCD, respectively. Feeding animals a high-K diet for 1 wk did not increase these currents. TEA-sensitive currents were much smaller in PCs and usually below detection limits. To examine the possibility that the ICs participate in transepithelial K(+) secretion, we measured Na/K pump activity as a ouabain-sensitive current. Although these currents were easily observed in PCs, averaging 79 +/- 14 and 250 +/- 50 pA/cell in the CCD and CNT, respectively, they were below the level of detection in the ICs. We conclude that ICs have BK channel densities that are sufficient to support renal secretion of K(+) if cell Ca(2+) is elevated. However. a pathway for K(+) entry into these cells has not been identified.

Regulation of NaCl transport in the renal collecting duct: lessons from cultured cells

The fine control of NaCl absorption regulated by hormones takes place in the distal nephron of the kidney. In collecting duct principal cells, the epithelial sodium channel (ENaC) mediates the apical entry of Na(+), which is extruded by the basolateral Na(+),K(+)-ATPase. Simian virus 40-transformed and "transimmortalized" collecting duct cell lines, derived from transgenic mice carrying a constitutive, conditionally, or tissue-specific promoter-regulated large T antigen, have been proven to be valuable tools for studying the mechanisms controlling the cell surface expression and trafficking of ENaC and Na(+),K(+)-ATPase. These cell lines have made it possible to identify sets of aldosterone- and vasopressin-stimulated proteins, and have provided new insights into the concerted mechanism of action of serum- and glucocorticoid-inducible kinase 1 (Sgk1), ubiquitin ligase Nedd4-2 (neural precursor cell-expressed, developmentally down-regulated protein 4-2), and 14-3-3 regulatory proteins in modulating ENaC-mediated Na(+) currents. Epidermal growth factor and induced leucine zipper protein have also been shown to repress and stimulate ENaC-dependent Na(+) absorption, respectively, by activating or repressing the mitogen-activated protein kinase externally regulated kinase(1/2). Overall, these findings have provided evidence suggesting that multiple pathways are involved in regulating NaCl absorption in the distal nephron.

[Vasopressin receptors: structure/function relationships and signal transduction in target cells]

Vasopressin, a hypothalamic hormone, acts on its target tissues via three different G protein coupled receptors. The vasopressin V1a and V1b receptors, associated to Gq protein and phospholipase C, are responsible for vasoconstriction and regulation of the corticotroph axis respectively. The V2 vasopressin receptor is coupled to Gs protein and adenylyl cyclase and is responsible for water reabsorption in the renal collecting duct. Mutations of the V2 receptor are involved in diabetes insipidus and most of these mutations result in an endoplasmic reticulum (ER) retention of the mutated receptor. With the V1b receptor model, we have identified a proximal sequence of the C-terminal segment, which is crucial for ER export. Mutations in this short domain result in ER accumulation and degradation of the receptor. SSR 149415, a nonpeptide antagonist of V1bR, which is permeable to cell membrane, is able to rescue the mutant phenotype and acts as a pharmacological chaperone.

Collecting duct-specific knockout of the endothelin B receptor causes hypertension and sodium retention

Collecting duct (CD)-derived endothelin-1 (ET-1) inhibits renal Na reabsorption and its deficiency increases blood pressure (BP). The role of CD endothelin B (ETB) receptors in mediating these effects is unknown. CD-specific knockout of the ETB receptor was achieved using an aquaporin-2 promoter-Cre recombinase transgene and the loxP-flanked ETB receptor gene (CD ETB KO). Systolic BP in mice with CD-specific knockout of the ETB receptor, ETA receptor (CD ETA KO) and ET-1 (CD ET-1 KO), and their respective controls were compared during normal- and high-salt diet. On a normal-sodium diet, CD ETB KO mice had elevated BP, which increased further during high salt feeding. However, the degree of hypertension in CD ETB KO mice and the further increase in BP during salt feeding were lower than that of CD ET-1 KO mice, whereas CD ETA KO mice were normotensive. CD ETB KO mice had impaired sodium excretion following acute sodium loading. Aldosterone and plasma renin activity were decreased in CD ETB KO mice on normal- and high-sodium diets, while plasma and urinary ET-1 levels did not differ from controls. In conclusion, the CD ETB receptor partially mediates the antihypertensive and natriuretic effects of ET-1. CD ETA and ETB receptors do not fully account for the antihypertensive and natriuretic effects of CD-derived ET-1, suggesting paracrine effects of this peptide.

Regulation of cation transport in the distal nephron by mechanical forces

Thiazide and loop diuretics induce renal K(+) secretion, often leading to renal K(+) wasting and hypokalemia. This phenomenon has been proposed to reflect an increase in delivery to and reabsorption of Na(+) by the distal nephron, with a resultant increase in the driving force for passive K(+) efflux across the apical membrane. Recent studies suggest that cellular mechanisms that lead to enhanced rates of Na(+) reabsorption as well as K(+) secretion in response to increases tubular flow rates are more complex. Increases in tubular flow rates directly enhance the activity of apical membrane Na(+) channels and indirectly activate a class of K(+) channels, referred to as maxi-K, that are functionally inactive under low flow states. This review addresses the role of biomechanical forces, generated by variations in urinary flow rate and tubular fluid volume, in the regulation of transepithelial Na(+) and K(+) transport in the distal nephron. The question of why the distal nephron has evolved to include a component of flow-dependent K(+) secretion is also addressed.

Development and differentiation of the ureteric bud into the ureter in the absence of a kidney collecting system

Six1-/- mice were found to have apparently normal ureters in the absence of a kidney, suggesting that the growth and development of the unbranched ureter is largely independent of the more proximal portions of the UB which differentiates into the highly branched renal collecting system. Culture of isolated urinary tracts (from normal and mutant mice) on Transwell filters was employed to study the morphogenesis of this portion of the urogenital system. Examination of the ureters revealed the presence of a multi-cell layered tubule with a lumen lined by cells expressing uroplakin (a protein exclusively expressed in the epithelium of the lower urinary tract). Cultured ureters of both the wild-type and Six1 mutant become contractile and undergo peristalsis, an activity preceded by the expression of alpha-smooth muscle actin (alphaSMA). Treatment with a number of inhibitors of signaling molecules revealed that inhibition of PI3 kinase dissociates the developmental expression of alphaSMA from ureter growth and elongation. Epidermal growth factor also perturbed smooth muscle differentiation in culture. Moreover, the peristalsis of the ureter in the absence of the kidney in the Six1-/- mouse indicates that the development of this clinically important function of ureter (peristaltic movement of urine) is not dependent on fluid flow through the ureter. In keeping with this, isolated ureters cultured in the absence of surrounding tissues elongate, differentiate and undergo peristalsis when cultured on a filter and undergo branching morphogenesis when cultured in 3-dimensional extracellular matrix gels in the presence of a conditioned medium derived from a metanephric mesenchyme (MM) cell line. In addition, ureters of Six1-/- urinary tracts (i.e., lacking a kidney) displayed budding structures from their proximal ends when cultured in the presence of GDNF and FGFs reminiscent of UB budding from the wolffian duct. Taken together with the above data, this indicates that, although the distal ureter (at least early in its development) retains some of the characteristics of the more proximal UB, the growth and differentiation (i.e., development of smooth muscle actin, peristalsis and uroplakin expression) of the distal non-branching ureter are inherent properties of this portion of the UB, occurring independently of detectable influences of either the undifferentiated MM (unlike the upper portion of the ureteric bud) or more differentiated metanephric kidney. Thus, the developing distal ureter appears to be a unique anatomical structure which should no longer be considered as simply the non-branching portion of the ureteric bud. In future studies, the ability to independently analyze and study the portion of the UB that becomes the renal collecting system and that which becomes the ureter should facilitate distinguishing the developmental nephrome (renal ontogenome) from the ureterome.

Automated method for the isolation of collecting ducts

The structural and functional heterogeneity of the collecting duct present a tremendous experimental challenge requiring manual microdissection, which is time-consuming, labor intensive, and not amenable to high throughput. To overcome these limitations, we developed a novel approach combining the use of transgenic mice expressing green fluorescent protein (GFP) in the collecting duct with large-particle-based flow cytometry to isolate pure populations of tubular fragments from the whole collecting duct (CD), or inner medullary (IMCD), outer medullary (OMCD), or connecting segment/cortical collecting duct (CNT/CCD). Kidneys were enzymatically dispersed into tubular fragments and sorted based on tubular length and GFP intensity using large-particle-based flow cytometry or a complex object parametric analyzer and sorter (COPAS). A LIVE/DEAD assay demonstrates that the tubules were >90% viable. Tubules were collected as a function of fluorescent intensity and analyzed by epifluorescence and phase microscopy for count accuracy, GFP positivity, average tubule length, and time required to collect 100 tubules. Similarly, mRNA and protein from sorted tubules were analyzed for expression of tubule segment-specific genes using quantitative real-time RT-PCR and immunoblotting. The purity and yield of sorted tubules were related to sort stringency. Four to six replicates of 100 collecting ducts (9.68 ± 0.44–14.5 ± 0.66 cm or 9.2 ± 0.7 mg tubular protein) were routinely obtained from a single mouse in under 1 h. In conclusion, large-particle-based flow cytometry is fast, reproducible, and generates sufficient amounts of highly pure and viable collecting ducts from single or replicate animals for gene expression and proteomic analysis.

Effects of water restriction on gene expression in mouse renal medulla: identification of 3βHSD4 as a collecting duct protein

To identify novel gene targets of vasopressin regulation in the renal medulla, we performed a cDNA microarray study on the inner medullary tissue of mice following a 48-h water restriction protocol. In this study, 4,625 genes of the possible ∼12,000 genes on the array were included in the analysis, and of these 157 transcripts were increased and 63 transcripts were decreased by 1.5-fold or more. Quantitative, real-time PCR measurements confirmed the increases seen for 12 selected transcripts, and the decreases were confirmed for 7 transcripts. In addition, we measured transcript abundance for many renal collecting duct proteins that were not represented on the array; aquaporin-2 (AQP2), AQP3, Pax-8, and α- and β-Na-K-ATPase subunits were all significantly increased in abundance; the β- and γ-subunits of ENaC and the vasopressin type 1A receptor were significantly decreased. To correlate changes in mRNA expression with changes in protein expression, we carried out quantitative immunoblotting. For most of the genes examined, changes in mRNA abundances were not associated with concomitant protein abundance changes; however, AQP2 transcript abundance and protein abundance did correlate. Surprisingly, aldolase B transcript abundance was increased but protein abundance was decreased following 48 h of water restriction. Several transcripts identified by microarray were novel with respect to their expression in mouse renal medullary tissues. The steroid hormone enzyme 3β-hydroxysteroid dehydrogenase 4 (3βHSD4) was identified as a novel target of vasopressin regulation, and via dual labeling immunofluorescence we colocalized the expression of this protein to AQP2-expressing collecting ducts of the kidney. These studies have identified several transcripts whose abundances are regulated in mouse inner medulla in response to an increase in endogenous vasopressin levels and could play roles in the regulation of salt and water excretion.

Genetic restoration of aldose reductase to the collecting tubules restores maturation of the urine concentrating mechanism

To investigate the underlying causes for aldose reductase deficiency-induced diabetes insipidus, we carried out studies with three genotypic groups of mice. These included wild-type mice, knockout mice, and a newly created bitransgenic line that was homozygous for both the aldose reductase null mutation and an aldose reductase knockin transgene driven by the kidney-specific cadherin promoter to direct transgene expression in the collecting tubule epithelial cells. We found that from early renal developmental stages onward, urine osmolality did not exceed 1,000 mosmol/kgH2O in aldose reductase-deficient mice. The functional defects were correlated with significant renal cellular and structural abnormalities that included cell shrinkage, apoptosis, disorganized tubular and vascular structures, and segmental atrophy. In contrast, the transgenic aldose reductase expression in the bitransgenic mice largely but incompletely rescued urine concentrating capacity and significantly improved renal cell survival, cellular morphology, and renal structures. Together, these results suggest that aldose reductase not only plays important roles in osmoregulation and medullary cell survival but may also be essential for the full maturation of the urine concentrating mechanism.

Gamble's "economy of water" revisited: studies in urea transporter knockout mice

The Gamble phenomenon (initially described over 70 years ago as "an economy of water in renal function referable to urea") suggested that urea plays a special role in the urinary concentrating mechanism and that the concentrating mechanism depends in some complex way on an interaction between NaCl and urea. In this study, the role of collecting duct urea transporters in the Gamble phenomenon was investigated in wild-type mice and mice in which the inner medulla collecting duct (IMCD) facilitative urea transporters, UT-A1 and UT-A3, had been deleted (UT-A1/3-/- mice). The general features of the Gamble phenomenon were confirmed in wild-type mice, namely 1) the water requirement for the excretion of urea is less than for the excretion of an osmotically equivalent amount of NaCl; and 2) when fed various mixtures of urea and salt in the diet, less water is required for the excretion of the two substances together than the amount of water needed for the excretion of the two substances separately. In UT-A1/3-/- mice both of these elements of the phenomenon were absent, indicating that IMCD urea transporters play a central role in the Gamble phenomenon. A titration study in which wild-type mice were given progressively increasing amounts of urea showed that the ability of the kidney to reabsorb urea was saturable, resulting in osmotic diuresis above excretion rates of approximately 6,000 microosmol/day. In the same titration experiments, when increasing amounts of NaCl were added to the diet, mice were unable to increase urinary NaCl concentrations to >420 mM, resulting in osmotic diuresis at NaCl excretion rates of approximately 3,500 microosmol/day. Thus both urea and NaCl can induce osmotic diuresis when large amounts are given, supporting the conclusion that the decrease in water excretion with mixtures of urea and NaCl added to the diet (compared with pure NaCl or urea) is due to the separate abilities of urea and NaCl to induce osmotic diuresis, rather than to any specific interaction of urea transport and NaCl transport at an epithelial level.

Silencing and overexpression of the gamma-subunit of Na-K-ATPase directly affect survival of IMCD3 cells in response to hypertonic stress

The gamma-subunit of Na-K-ATPase is robustly expressed in inner medullary collecting duct (IMCD)3 cells either acutely challenged or adapted to hypertonicity but not under isotonic conditions. Circumstantial evidence suggests that this protein may be important for the survival of renal cells in a hypertonic environment. However, no direct proof for such a contention has been forthcoming. The complete mRNA sequences of either gamma-subunit isoforms were spliced into an expression vector and transfected into IMCD3 cells. Multiple clones stably expressed gamma-subunit protein under isotonic conditions. Clones expressing the gamma(b) isoform showed enhanced survival at lethal acute hypertonicity compared with either gamma(a) isoform or empty vector (control) expressing clones. We also evaluated the loss of gamma-subunit expression on the survival of IMCD3 cells exposed to hypertonicity employing silencing RNA techniques. Multiple stable gamma-subunit-specific siRNA clones were obtained and exposed to sublethal hypertonicity. Under these conditions, both the level of gamma mRNA and protein was essentially undetectable. The impact of silencing gamma-subunit expression resulted in a 70% reduction at 48 h (P < 0.01) in cell survival compared with empty vector (control) clones. gamma siRNA clones showed a 45% decrease in myo-inositol uptake compared with controls after an 18-h exposure to sublethal hypertonicity. Taken together, these data demonstrate a direct and critical role of the gamma-subunit on IMCD3 cell survival and/or adaptation in response to ionic hypertonic stress.

Claudins 6, 9, and 13 are developmentally expressed renal tight junction proteins

The adult proximal tubule is a low-resistance epithelium where there are high rates of both active transcellular and passive paracellular NaCl transport. We have previously demonstrated that the neonatal rabbit and rat proximal tubule have substantively different passive paracellular transport properties than the adult proximal tubule, which results in a maturational change in the paracellular passive flux of ions. Neonatal proximal tubules have a higher P(Na)/P(Cl) ratio and lower chloride and bicarbonate permeabilities than adult proximal tubules. Claudins are a large family of proteins which are the gate keepers of the paracellular pathway, and claudin isoform expression determines the permeability characteristics of the paracellular pathway. Previous studies have shown that claudins 1, 2, 3, 4, 5, 7, 8, 10, 11, 12, 15, and 16 are expressed in the adult mouse kidney. To determine whether there are developmental claudin isoforms, we compared the claudin isoforms present in the neonatal and adult kidney using RT-PCR to detect mRNA of claudin isoforms. Claudin 6, claudin 9, and claudin 13 were either not expressed or barely detectable in the adult mouse kidney using traditional PCR, but were expressed in the neonatal mouse kidney. Using real-time RT-PCR, we were able to detect a low level of claudin 6 mRNA expression in the adult kidney compared with the neonate, but claudin 9 and claudin 13 were only detected in the neonatal kidney. There was the same maturational decrease in these claudin proteins with Western blot analysis. Immunohistochemistry showed high levels of expression of claudin 6 in neonatal proximal tubules, thick ascending limb, distal convoluted tubules, and collecting ducts in a paracellular distribution but there was no expression of claudin 6 in the adult kidney. Using real-time RT-PCR claudin 6 and 9 mRNA were present in 1-day-old proximal convoluted tubules and were virtually undetectable in proximal convoluted tubules from adults. Claudin 13 was not detectable in neonatal or adult proximal convoluted tubules. In summary, we have identified developmentally expressed claudin isoforms, claudin 6, claudin 9, and claudin 13. These paracellular proteins may play a role in the maturational changes in paracellular permeability.

Nitric oxide stimulates COX-2 expression in cultured collecting duct cells through MAP kinases and superoxide but not cGMP

Collecting ducts are a major site of renal production and action of both prostaglandins and nitric oxide. Experiments were undertaken to examine whether nitric oxide regulates cyclooxygenase (COX)-2 expression and PGE(2) release in cultured collecting duct cells. In mIMCD-K2 cells, sodium nitroprusside (SNP) in the 50- to 800-microM range induced a marked dose- and time-dependent increase in COX-2 protein levels, determined by immunoblotting, and the induction was detectable at 4 h. This was preceded by induction of COX-2 mRNA as determined by real-time-RT-PCR. The COX-2 induction was accompanied by a significant rise in PGE(2) release as determined by enzyme immunoassay. S-nitroso-N-acetylpenicillamine (SNAP) had a similar stimulatory effect on COX-2 expression and PGE(2) release. 8-bromo-cGMP (200 microM) had no effect on COX-2 expression. The SNP-stimulated COX-2 expression was not affected by the guanylyl cyclase inhibitor methylene blue or the protein kinase G inhibitor KT-5823 (2.0 microM). In contrast, the SNP-stimulated COX-2 expression was significantly reduced by either the Erk1/2 inhibitor PD-98059 or the P38 inhibitor SB-203580 and was abolished by combination of the two kinase inhibitors. The stimulation was also significantly blocked by the SOD mimetic tempol. Thus we conclude that NO stimulates COX-2 expression in collecting duct cells through mechanisms involving MAP kinase and superoxide, but not cGMP.