The TonE/TonEBP pathway mediates tonicity-responsive regulation of UT-A urea transporter expression

Urea transport in the kidney

Urea transport proteins were initially proposed to exist in the kidney in the late 1980s when studies of urea permeability revealed values in excess of those predicted by simple lipid-phase diffusion and paracellular transport. Less than a decade later, the first urea transporter was cloned. Currently, the SLC14A family of urea transporters contains two major subgroups: SLC14A1, the UT-B urea transporter originally isolated from erythrocytes; and SLC14A2, the UT-A group with six distinct isoforms described to date. In the kidney, UT-A1 and UT-A3 are found in the inner medullary collecting duct; UT-A2 is located in the thin descending limb, and UT-B is located primarily in the descending vasa recta; all are glycoproteins. These transporters are crucial to the kidney's ability to concentrate urine. UT-A1 and UT-A3 are acutely regulated by vasopressin. UT-A1 has also been shown to be regulated by hypertonicity, angiotensin II, and oxytocin. Acute regulation of these transporters is through phosphorylation. Both UT-A1 and UT-A3 rapidly accumulate in the plasma membrane in response to stimulation by vasopressin or hypertonicity. Long-term regulation involves altering protein abundance in response to changes in hydration status, low protein diets, adrenal steroids, sustained diuresis, or antidiuresis. Urea transporters have been studied using animal models of disease including diabetes mellitus, lithium intoxication, hypertension, and nephrotoxic drug responses. Exciting new animal models are being developed to study these transporters and search for active urea transporters. Here we introduce urea and describe the current knowledge of the urea transporter proteins, their regulation, and their role in the kidney.

Vasopressin increases expression of UT-A1, UT-A3, and ER chaperone GRP78 in the renal medulla of mice with a urinary concentrating defect

Activation of V2 receptors (V2R) during antidiuresis increases the permeability of the inner medullary collecting duct to urea and water. Extracellular osmolality is elevated as the concentrating capacity of the kidney increases. Osmolality is known to contribute to the regulation of collecting duct water (aquaporin-2; AQP2) and urea transporter (UT-A1, UT-A3) regulation. AQP1KO mice are a concentrating mechanism knockout, a defect attributed to the loss of high interstitial osmolality. A V2R-specific agonist, deamino-8-D-arginine vasopressin (dDAVP), was infused into wild-type and AQP1KO mice for 7 days. UT-A1 mRNA and protein abundance were significantly increased in the medullas of wild-type and AQP1KO mice following dDAVP infusion. The mRNA and protein abundance of UT-A3, the basolateral urea transporter, was significantly increased by dDAVP in both wild-type and AQP1KO mice. Semiquantitative immunoblots revealed that dDAVP infusion induced a significant increase in the medullary expression of the endoplasmic reticulum (ER) chaperone GRP78. Immunofluorescence studies demonstrated that GRP78 expression colocalized with AQP2 in principal cells of the papillary tip of the renal medulla. Using immunohistochemistry and immunogold electron microscopy, we demonstrate that vasopressin induced a marked apical targeting of GRP78 in medullary principal cells. Urea-sensitive genes, GADD153 and ATF4 (components of the ER stress pathway), were significantly increased in AQP1KO mice by dDAVP infusion. These findings strongly support an important role of vasopressin in the activation of an ER stress response in renal collecting duct cells, in addition to its role in activating an increase in UT-A1 and UT-A3 abundance.

The expression of aquaporin-1 in the medulla of the kidney is dependent on the transcription factor associated with hypertonicity, TonEBP

Expression of aquaporin-1 (AQP1) and -2 (AQP2) channels in the kidney are critical for the maintenance of water homeostasis and the operation of the urinary concentrating mechanism. Hypertonic stress induced in inner medullary (IMCD3) cells by addition of NaCl to the medium substantially up-regulated the mRNA and protein expression of AQP1, suggesting that its activation occurs at a transcriptional and a translational levels. In contrast, no up-regulation of AQP1 was observed when these cells were exposed to the same tonicity by addition of urea. To explore the transcriptional activation of aqp1 under hypertonic stress, we examined the role of the transcription factor associated with hypertonicity, TonEBP. Treatment of IMCD3 cells with the TonEBP inhibitor rottlerin or silencing its expression with specific shRNA technology led to a substantial reduction in AQP1 expression under hypertonic conditions. Moreover, we defined a conserved TonEBP binding site located 811 bp upstream of the aqp1 exon that is essential for its expression. Single site-directed mutation of this TonE site led to a 54 ± 5% (p < 0.01) decrease in AQP1 luciferase-driven activity under hypertonic stress. TonEBP mutant mice display marked decrement in the expression of AQP1 in the inner medulla. In conclusion, these data demonstrate that TonEBP is necessary for the regulation of AQP1 expression in the inner medulla of the kidney under hypertonic conditions.

Subcellular distribution of urea transporter in the collecting tubule of shark kidney is dependent on environmental salinity

In the kidney of marine elasmobranchs, urea reabsorption from filtered urine is essential for maintaining high levels of urea in the body. In the kidney of the houndshark, Triakis scyllium, we previously found that a facilitative urea transporter (UT) is localized to a specific nephron segment, the collecting tubule, suggesting that the collecting tubule has an important role in the urea reabsorption process. To elucidate the roles of UT, we transferred T. scyllium to high (130%) and low (30%) salinity, and examined UT mRNA levels and UT distribution patterns in the kidney using real-time PCR and semi-quantitative fluorescence immunohistochemistry, respectively. Following transfer to low and high salinity, houndshark decreased and increased plasma urea concentrations, respectively, in order to control plasma osmolality. The abundance of UT mRNA did not differ among the experimental groups, whereas that of UT protein in the collecting tubule was significantly decreased in 30% seawater (SW). Furthermore, the subcellular UT distribution was dramatically changed. UT in the apical plasma membrane of collecting tubule almost disappeared in 30% SW, whereas it slightly increased in 130% SW compared with 100% SW. Conversely, reverse transfer of fish from 30 to100% SW restored UT in the apical membrane. These results indicate that the accumulation of UT to the apical plasma membrane of the collecting tubule of Triakis is an important factor for regulating urea reabsorption in the kidney.

Osmotic stress regulates mineralocorticoid receptor expression in a novel aldosterone-sensitive cortical collecting duct cell line

Aldosterone effects are mediated by the mineralocorticoid receptor (MR), a transcription factor highly expressed in the distal nephron. Given that MR expression level constitutes a key element controlling hormone responsiveness, there is much interest in elucidating the molecular mechanisms governing MR expression. To investigate whether hyper- or hypotonicity could affect MR abundance, we established by targeted oncogenesis a novel immortalized cortical collecting duct (CCD) cell line and examined the impact of osmotic stress on MR expression. KC3AC1 cells form domes, exhibit a high transepithelial resistance, express 11beta-hydroxysteroid dehydrogenase 2 and functional endogenous MR, which mediates aldosterone-stimulated Na(+) reabsorption through the epithelial sodium channel activation. MR expression is tightly regulated by osmotic stress. Hypertonic conditions induce expression of tonicity-responsive enhancer binding protein, an osmoregulatory transcription factor capable of binding tonicity-responsive enhancer response elements located in MR regulatory sequences. Surprisingly, hypertonicity leads to a severe reduction in MR transcript and protein levels. This is accompanied by a concomitant tonicity-induced expression of Tis11b, a mRNA-destabilizing protein that, by binding to the AU-rich sequences of the 3'-untranslated region of MR mRNA, may favor hypertonicity-dependent degradation of labile MR transcripts. In sharp contrast, hypotonicity causes a strong increase in MR transcript and protein levels. Collectively, we demonstrate for the first time that optimal adaptation of CCD cells to changes in extracellular fluid composition is accompanied by drastic modification in MR abundance via transcriptional and posttranscriptional mechanisms. Osmotic stress-regulated MR expression may represent an important molecular determinant for cell-specific MR action, most notably in renal failure, hypertension, or mineralocorticoid resistance.

The physiology of urinary concentration: an update

The renal medulla produces concentrated urine through the generation of an osmotic gradient extending from the cortico-medullary boundary to the inner medullary tip. This gradient is generated in the outer medulla by the countercurrent multiplication of a comparatively small transepithelial difference in osmotic pressure. This small difference, called a single effect, arises from active NaCl reabsorption from thick ascending limbs, which dilutes ascending limb flow relative to flow in vessels and other tubules. In the inner medulla, the gradient may also be generated by the countercurrent multiplication of a single effect, but the single effect has not been definitively identified. There have been important recent advances in our understanding of key components of the urine concentrating mechanism. In particular, the identification and localization of key transport proteins for water, urea, and sodium, the elucidation of the role and regulation of osmoprotective osmolytes, better resolution of the anatomical relationships in the medulla, and improvements in mathematic modeling of the urine concentrating mechanism. Continued experimental investigation of transepithelial transport and its regulation, both in normal animals and in knock-out mice, and incorporation of the resulting information into mathematic simulations, may help to more fully elucidate the inner medullary urine concentrating mechanism.

Urea promotes TonEBP expression and cellular adaptation in extreme hypertonicity

The transcriptional activator TonEBP is a central regulator of osmolality in the renal medulla and whole body water homeostasis. In order to understand the regulation of TonEBP in the renal medulla, we examined MDCK cells, a kidney-derived epithelial cell line, under conditions mimicking the renal medulla. Moderate changes in ambient tonicity, which was tolerated without prior adaptation, displayed lasting effects on TonEBP in bidirectional manner-stimulated by hypertonicity and inhibited by hypotonicity. TonEBP expression was further enhanced by extreme hypertonicity observed in the inner medullae of antidiuretic animals. Urea stimulated TonEBP expression and promoted cellular proliferation under the conditions of extreme hypertonicity. On the other hand, the TonEBP activity was negatively modulated under these conditions presumably to temper the highly abundant TonEBP. We conclude that urea is critical to the cellular adaptation to extreme hypertonicity and the high level of TonEBP expression in the inner medulla.

Hypertonic Stress in the Kidney: A Necessary Evil

The interstitium of the renal medulla is hypertonic, imposing deleterious effects on local cells. At the same time, the hypertonicity provides osmotic gradient for water reabsorption and is a local signal for tissue-specific gene expression and differentiation of the renal medulla, which is a critical organ for water homeostasis.

Tonicity-dependent induction of Sgk1 expression has a potential role in dehydration-induced natriuresis in rodents

In various mammalian species, including humans, water restriction leads to an acute increase in urinary sodium excretion. This process, known as dehydration natriuresis, helps prevent further accentuation of hypernatremia and the accompanying rise in extracellular tonicity. Serum- and glucocorticoid-inducible kinase (Sgk1), which is expressed in the renal medulla, is regulated by extracellular tonicity. However, the mechanism of its regulation and the physiological role of hypertonicity-induced SGK1 gene expression remain unclear. Here, we identified a tonicity-responsive enhancer (TonE) upstream of the rat Sgk1 transcriptional start site. The transcription factor NFAT5 associated with TonE in a tonicity-dependent fashion in cultured rat renal medullary cells, and selective blockade of NFAT5 activity resulted in suppression of the osmotic induction of the Sgk1 promoter. In vivo, water restriction of rats or mice led to increased urine osmolality, increased Sgk1 expression, increased expression of the type A natriuretic peptide receptor (NPR-A), and dehydration natriuresis. In cultured rat renal medullary cells, siRNA-mediated Sgk1 knockdown blocked the osmotic induction of natriuretic peptide receptor 1 (Npr1) gene expression. Furthermore, Npr1-/- mice were resistant to dehydration natriuresis, which suggests that Sgk1-dependent activation of the NPR-A pathway may contribute to this response. Collectively, these findings define a specific mechanistic pathway for the osmotic regulation of Sgk1 gene expression and suggest that Sgk1 may play an important role in promoting the physiological response of the kidney to elevations in extracellular tonicity.

The transcription factor NFAT5 is required for cyclin expression and cell cycle progression in cells exposed to hypertonic stress

Background

Hypertonicity can perturb cellular functions, induce DNA damage-like responses and inhibit proliferation. The transcription factor NFAT5 induces osmoprotective gene products that allow cells to adapt to sustained hypertonic conditions. Although it is known that NFAT5-deficient lymphocytes and renal medullary cells have reduced proliferative capacity and viability under hypertonic stress, less is understood about the contribution of this factor to DNA damage responses and cell cycle regulation.

Methodology/Principal Findings

We have generated conditional knockout mice to obtain NFAT5^−/− T lymphocytes, which we used as a model of proliferating cells to study NFAT5-dependent responses. We show that hypertonicity triggered an early, NFAT5-independent, genotoxic stress-like response with induction of p53, p21 and GADD45, downregulation of cyclins, and cell cycle arrest. This was followed by an NFAT5-dependent adaptive phase in wild-type cells, which induced an osmoprotective gene expression program, downregulated stress markers, resumed cyclin expression and proliferation, and displayed enhanced NFAT5 transcriptional activity in S and G2/M. In contrast, NFAT5^−/− cells failed to induce osmoprotective genes and exhibited poorer viability. Although surviving NFAT5^−/− cells downregulated genotoxic stress markers, they underwent cell cycle arrest in G1/S and G2/M, which was associated with reduced expression of cyclins E1, A2 and B1. We also show that pathologic hypertonicity levels, as occurring in plasma of patients and animal models of osmoregulatory disorders, inhibited the induction of cyclins and aurora B kinase in response to T cell receptor stimulation in fresh NFAT5^−/− lymphocytes.

Conclusions/Significance

We conclude that NFAT5 facilitates cell proliferation under hypertonic conditions by inducing an osmoadaptive response that enables cells to express fundamental regulators needed for cell cycle progression.

Controlled aquaporin-2 expression in the hypertonic environment

The corticomedullary osmolality gradient is the driving force for water reabsorption occurring in the kidney. In the collecting duct, this gradient allows luminal water to move across aquaporin (AQP) water channels, thereby increasing urine concentration. However, this same gradient exposes renal cells to great osmotic challenges. These cells must constantly adapt to fluctuations of environmental osmolality that challenge cell volume and incite functional change. This implies profound alterations of cell phenotype regarding water permeability. AQP2 is an essential component of the urine concentration mechanism whose controlled expression dictates apical water permeability of collecting duct principal cells. This review focuses on changes of AQP2 abundance and trafficking in hypertonicity-challenged cells. Intracellular mechanisms governing these events are discussed and the biological relevance of altered AQP2 expression by hypertonicity is outlined.

Urea and NaCl regulate UT-A1 urea transporter in opposing directions via TonEBP pathway during osmotic diuresis

In our previous studies of varying osmotic diuresis, UT-A1 urea transporter increased when urine and inner medullary (IM) interstitial urea concentration decreased. The purposes of this study were to examine 1) whether IM interstitial tonicity changes with different urine urea concentrations during osmotic dieresis and 2) whether the same result occurs even if the total urinary solute is decreased. Rats were fed a 4% high-salt diet (HSD) or a 5% high-urea diet (HUD) for 2 wk and compared with the control rats fed a regular diet containing 1% NaCl. The urine urea concentration decreased in HSD but increased in HUD. In the IM, UT-A1 and UT-A3 urea transporters, CLC-K1 chloride channel, and tonicity-enhanced binding protein (TonEBP) transcription factor were all increased in HSD and decreased in HUD. Next, rats were fed an 8% low-protein diet (LPD) or a 0.4% low-salt diet (LSD) to decrease the total urinary solute. Urine urea concentration significantly decreased in LPD but significantly increased in LSD. Rats fed the LPD had increased UT-A1 and UT-A3 in the IM base but decreased in the IM tip, resulting in impaired urine concentrating ability. The LSD rats had decreased UT-A1 and UT-A3 in both portions of the IM. CLC-K1 and TonEBP were unchanged by LPD or LSD. We conclude that changes in CLC-K1, UT-A1, UT-A3, and TonEBP play important roles in the renal response to osmotic diuresis in an attempt to minimize changes in plasma osmolality and maintain water homeostasis.

Interstitial tonicity controls TonEBP expression in the renal medulla

Cells in the hyperosmotic kidney medulla, express a transcriptional activator termed tonicity responsive enhancer binding protein (TonEBP). Genes targeted by TonEBP protect kidney cells from the deleterious effects of hyperosmolality by inducing the expression of organic osmolytes and molecular chaperones, and other genes that mediate urine concentration such as aquaporin-2 and urea transporters. We tested here the effect of hypertonicity and hyperosmotic salt in the renal medullary interstitium on the expression TonEBP. When massive water diuresis was induced in rats the medullary sodium concentrations did not change, neither did TonEBP expression. In these animals the medullary tonicity was unchanged despite the production of dilute urine. On the other hand, treatment with the loop diurectic furosemide resulted in a dose-dependent decrease in the medullary sodium concentration causing a reduction in interstitial tonicity. Here, TonEBP expression was blunted in the outer and inner medulla which was due, in part, to decreased mRNA abundance. As expected, the expression of TonEBP target genes in the renal medulla also decreased in response to furosemide. Hence TonEBP expression in the renal medulla is stimulated by interstitial hypertonicity.

Regulation of V2R transcription by hypertonicity and V1aR-V2R signal interaction

Arginine vasopressin (AVP) and hypertonicity in the renal medulla play a major role in the urine concentration mechanism. Previously, we showed that rat vasopressin V2 receptor (rV2R) promoter activity was increased by vasopressin V2R stimulation and decreased by vasopressin V1a receptor (V1aR) stimulation in a LLC-PK1 cell line stably expressing rat V1aR (LLC-PK1/rV1aR). In the present study, we investigated the effects of hypertonicity on the rV2R promoter activity and on the suppression of rV2R promoter activity by V1aR stimulation in LLC-PK1/rV1aR cells. rV2R promoter activity was increased in NaCl- or mannitol-induced hypertonicity. The hypertonicity-responsive site in the rV2R promoter region was limited to 10 bp, including the Sp1 motif. The increase of V2R promoter activity by hypertonicity was significantly inhibited by a JNK inhibitor (SP600125) and PKA inhibitor (H89). In contrast, rV2R promoter activity was remarkably suppressed by V1aR stimulation in the hypertonic condition rather than in the isotonic condition. The AVP-stimulated intracellular Ca2+ concentration was increased in the hypertonic condition, suggesting the functional activation of V1aR by hypertonicity. In conclusion, 1) V2R promoter activity is increased by hypertonicity via the JNK and PKA pathways, 2) suppression of V2R expression by the V1aR-Ca2+ pathway is enhanced by hypertonicity, and 3) hypertonicity enhances the V1aR-Ca2+ pathway. The counteractivity of V2R and V1aR could be required to maintain minimum urine volume in the dehydrated state.

Nucleoporin 88 (Nup88) is regulated by hypertonic stress in kidney cells to retain the transcription factor tonicity enhancer-binding protein (TonEBP) in the nucleus

Antibody microarray technology identified Nup88 (nucleoporin 88) as a highly up-regulated protein in response to osmotic stress in inner medullary collecting duct (IMCD3) cells. Changes in expression were verified by Western blot and quantitative PCR for protein and message expression. In mouse and human kidney, Nup88 expression was substantial in the papilla, whereas it was nearly absent in the cortex. Furthermore, the expression of Nup88 increased 410.4 +/- 22% in the papilla of mice after 36 h of thirsting. Nup88 protein expression in IMCD3 cells was significantly up-regulated in the first 8 h following exposure to acute osmotic stress, indicating that Nup88 is an early response protein. To define the function of Nup88 in the osmotic stress response, the transcription factor associated with hypertonicity, tonicity enhancer-binding protein (TonEBP), was cloned upstream of the green fluorescent protein. Employing this construct, we demonstrate that silencing Nup88 in IMCD3 cells acutely stressed to hypertonic conditions reduces nuclear retention of TonEBP, resulting in a substantial blunting in transcription of important osmotic stress response target genes and reduced cell viability. Finally, we show that in IMCD3 cells, nuclear export of TonEBP under isotonic conditions involves CRM-1 but under hypertonic stress is CRM1-independent. Our data, therefore, suggest that Nup88 is up-regulated in response to hypertonic stress and acts to retain TonEBP in the nucleus, activating transcription of critical osmoprotective genes.

Novel nuclear localization signal regulated by ambient tonicity in vertebrates

TonEBP is a Rel domain-containing transcription factor implicated in adaptive immunity, viral replication, and cancer. In the mammalian kidney, TonEBP is a central regulator of water homeostasis. Animals deficient in TonEBP suffer from life-threatening dehydration due to renal water loss. Ambient tonicity (effective osmolality) is the prominent signal for TonEBP in a bidirectional manner; TonEBP activity decreases in hypotonicity, whereas it increases in hypertonicity. Here we found that TonEBP displayed nuclear export in response to hypotonicity and nuclear import in response to hypertonicity. The nuclear export of TonEBP was not mediated by the nuclear export receptor CRM1 or discrete nuclear export signal. In contrast, a dominant nuclear localization signal (NLS) was found in a small region of 16 amino acid residues. When short peptides containing the NLS were fused to constitutively cytoplasmic proteins, the fusion proteins displayed tonicity-dependent nucleocytoplasmic trafficking like TonEBP. Thus, tonicity-dependent activation of the NLS is crucial in the nucleocytoplasmic trafficking of TonEBP. The novel NLS is present only in the vertebrates, indicating that it developed late in evolution.

Nitric oxide decreases expression of osmoprotective genes via direct inhibition of TonEBP transcriptional activity

During antidiuresis, renal medullary cells adapt to the hyperosmotic interstitial environment by increased expression of osmoprotective genes, which is driven by a common transcriptional activator, tonicity-responsive enhancer binding protein (TonEBP). Because nitric oxide (NO) is abundantly produced in the renal medulla, the present studies addressed the effect of NO on expression of osmoprotective genes and TonEBP activation in MDCK cells. Several structurally unrelated NO donors blunted tonicity-induced up-regulation of TonEBP target genes involved in intracellular accumulation of organic osmolytes. These effects were mediated by reduced transcriptional activity of TonEBP, as assessed by tonicity-responsive elements- and aldose reductase promoter-driven reporter constructs. Neither total TonEBP abundance nor nuclear translocation of TonEBP was affected by NO. Furthermore, 8-bromo-cGMP and peroxynitrite failed to reproduce the inhibitory effect of NO, indicating that NO acts directly on TonEBP rather than through classical NO signaling pathways. In support of this notion, electrophoretic mobility shift assays showed reduced binding of TonEBP to its target sequence in nuclear extracts prepared from MDCK cells treated with NO in vivo and in nuclear extracts exposed to NO in vitro. Furthermore, immunoprecipitation of S-nitrosylated proteins and the biotin-switch method identified TonEBP as a target for S-nitrosylation, which correlates with reduced DNA binding and transcriptional activity. These observations disclose a novel direct inhibitory effect of NO on TonEBP, a phenomenon that may be relevant for regulation of osmoprotective genes in the renal medulla.

Tonicity-regulated gene expression

Hypertonicity activates several different transcription factors, including TonEBP/OREBP, that in turn increase transcription of numerous genes. Hypertonicity elevates TonEBP/OREBP transcriptional activity by moving it into the nucleus, where it binds to its cognate DNA element (ORE), and by increasing its transactivational activity. This chapter presents protocols for measuring the transcriptional activity of TonEBP/OREBP and determining its subcellular localization, its binding to OREs, and activity of its transactivation domain.

Cellular Response to Hyperosmotic Stresses

Cells in the renal inner medulla are normally exposed to extraordinarily high levels of NaCl and urea. The osmotic stress causes numerous perturbations because of the hypertonic effect of high NaCl and the direct denaturation of cellular macromolecules by high urea. High NaCl and urea elevate reactive oxygen species, cause cytoskeletal rearrangement, inhibit DNA replication and transcription, inhibit translation, depolarize mitochondria, and damage DNA and proteins. Nevertheless, cells can accommodate by changes that include accumulation of organic osmolytes and increased expression of heat shock proteins. Failure to accommodate results in cell death by apoptosis. Although the adapted cells survive and function, many of the original perturbations persist, and even contribute to signaling the adaptive responses. This review addresses both the perturbing effects of high NaCl and urea and the adaptive responses. We speculate on the sensors of osmolality and document the multiple pathways that signal activation of the transcription factor TonEBP/OREBP, which directs many aspects of adaptation. The facts that numerous cellular functions are altered by hyperosmolality and remain so, even after adaptation, indicate that both the effects of hyperosmolality and adaptation to it involve profound alterations of the state of the cells.

Downregulation of renal TonEBP in hypokalemic rats

Hypokalemia causes a significant decrease in the tonicity of the renal medullary interstitium in association with reduced expression of sodium transporters in the distal tubule. We asked whether hypokalemia caused downregulation of the tonicity-responsive enhancer binding protein (TonEBP) transcriptional activator in the renal medulla due to the reduced tonicity. We found that the abundance of TonEBP decreased significantly in the outer and inner medullas of hypokalemic rats. Underlying mechanisms appeared different in the two regions because the abundance of TonEBP mRNA was lower in the outer medulla but unchanged in the inner medulla. Immunohistochemical examination of TonEBP revealed cell type-specific differences. TonEBP expression decreased dramatically in the outer and inner medullary collecting ducts, thick ascending limbs, and interstitial cells. In the descending and ascending thin limbs, TonEBP abundance decreased modestly. In the outer medulla, TonEBP shifted to the cytoplasm in the descending thin limbs. As expected, transcription of aldose reductase, a target of TonEBP, was decreased since the abundance of mRNA and protein was reduced. Downregulation of TonEBP appeared to have also contributed to reduced expression of aquaporin-2 and UT-A urea transporters in the renal medulla. In cultured cells, expression and activity of TonEBP were not affected by reduced potassium concentrations in the medium. These data support the view that medullary tonicity regulates expression and nuclear distribution of TonEBP in the renal medulla in cell type-specific manners.

Regulation of Urea Transporters by Tonicity-responsive Enhancer Binding Protein

Urea accumulation in the renal inner medulla plays a key role in the maintenance of maximal urinary concentrating ability. Urea transport in the kidney is mediated by transporter proteins that include renal urea transporter (UT-A) and erythrocyte urea transporter (UT-B). UT-A1 and UT-A2 are produced from the same gene. There is an active tonicity-responsive enhancer (TonE) in the promoter of UT-A1, and the UT-A1 promoter is stimulated by hypertonicity via tonicity-responsive enhancer binding protein (TonEBP). The downregulation of UT-A2 raises the possibility that TonEBP also regulates its promoter. There is some evidence that TonEBP regulates expression of UT-A in vivo; (1) during the renal development of the urinary concentrating ability, expression of TonEBP precedes that of UT-A1; (2) in transgenic mice expressing a dominant negative form of TonEBP, expression of UT-A1 and UT-A2 is severely impaired; (3) in treatment with cyclosporine A, TonEBP was significantly downregulated after 28 days. This downregulation involves mRNA levels of UT-A2; (4) in hypokalemic animals, downregulation of TonEBP contributed to the down regulation of UT-A in the inner medulla. These data support that TonEBP directly contributes to the urinary concentration and renal urea recycling by the regulation of urea transporters.

Mixed osmolytes: the degree to which one osmolyte affects the protein stabilizing ability of another

Mixtures of organic osmolytes occur in cells of many organisms, raising the question of whether their actions on protein stability are independent or synergistic. To investigate this question it is desirable to develop a system that permits evaluation of the effect of one osmolyte on the efficacy of another to either force-fold or denature a protein. A means of evaluating the efficacy of an osmolyte is provided by its m-value, an experimental quantity that measures the ability of the osmolyte to force a protein to unfold or fold. An experimental system is presented that enables evaluations of the m-values of osmolytes in the presence and absence of a second osmolyte. The experimental system involves use of a marginally stable protein in 10 mM buffer (pH 7, 200 mM salt, and 34 degrees C) that is at the midpoint of its native to denatured transition. These conditions enable determination of m-values for protecting and denaturing osmolytes in the presence and absence of a second osmolyte, permitting assessment of the extent to which the two osmolytes affect each other's efficacy. The two osmolytes investigated in this work are the denaturing osmolyte, urea, and the protecting osmolyte, sarcosine. Results show unequivocally that neither osmolyte alters the efficacy of the other in forcing the protein to fold or unfold-the osmolytes act independently on the protein despite their combined concentrations being in the multi-molar range. These osmolytes avoid altering one another's efficacy at these high concentrations because the number of osmolyte interaction sites on the protein is large and the binding constants are quite small. Consequently, the site occupancies are low enough in number that the two osmolytes neither compete nor cooperate in interacting with the protein.

The erythrocyte urea transporter UT-B

During the past decade significant progress has been made in our understanding of the role played by urea transporters in the production of concentrated urine by the kidney. Urea transporters have been cloned and characterized in a wide range of species. The genomic organization of the two major families of mammalian urea transporters, UT-A and UT-B, has been defined, providing new insight into the mechanisms that regulate their expression and function in physiological and pathological conditions. Beside the kidney, the presence of urea transporters has been documented in a variety of tissues, where their role is not fully known. Recently, mice with targeted deletion of the major urea transporters have been generated, which have shown variable impairment of urine concentrating ability, and have helped to clarify the physiological contribution of individual transporters to this process. This review focuses on the erythrocyte urea transporter UT-B.

Genomic organization of the mammalian SLC14a2 urea transporter genes

Urea transporters encoded by the UT-A gene play fundamental roles in the kidney and possibly other tissues. Knowledge of the genomic organization of the mouse, rat and human UT-A genes has enabled the engineering of transgenic and knockout animals and these have helped refine our understanding of the role of UT-A proteins. This review summarizes the published work that has accrued on the structure and regulation of these genes. It also documents a novel cDNA, human UT-A3, which has enabled a major refinement of the human UT-A gene structure. This and other information contained in this review should prove useful for future comparative genomic analysis, studies addressing gene regulation and for the engineering of transgenic and knockout animal strains.

Downregulation of renal sodium transporters and tonicity-responsive enhancer binding protein by long-term treatment with cyclosporin A

Tonicity-responsive enhancer binding protein (TonEBP) is a transcriptional activator that is regulated by ambient tonicity. TonEBP protects the renal medulla from the deleterious effects of hyperosmolality and regulates the urinary concentration by stimulating aquaporin-2 and urea transporters. The therapeutic use of cyclosporin A (CsA) is limited by nephrotoxicity that is manifested by reduced GFR, fibrosis, and tubular defects, including reduced urinary concentration. It was reported recently that long-term CsA treatment was associated with decreased renal expression of TonEBP target genes, including aquaporin-2, urea transporter, and aldose reductase. This study tested the hypothesis that long-term CsA treatment reduces the salinity/tonicity of the renal medullary interstitium as a result of inhibition of active sodium transporters, leading to downregulation of TonEBP. CsA treatment for 7 d did not affect TonEBP or renal function. Whereas expression of sodium transporters was altered, the medullary tonicity seemed unchanged. Conversely, 28 d of CsA treatment led to downregulation of TonEBP and overt nephrotoxicity. The downregulation of TonEBP involved reduced expression, cytoplasmic shift, and reduced transcription of its target genes. This was associated with reduced expression of active sodium transporters-sodium/potassium/chloride transporter type 2 (NKCC2), sodium/chloride transporter, and Na(+),K(+)-ATPase-along with increased sodium excretion and reduced urinary concentration. Infusion of vasopressin restored the expression of NKCC2 in the outer medulla as well as the expression and the activity of TonEBP. It is concluded that the downregulation of TonEBP in the setting of long-term CsA administration is secondary to the reduced tonicity of the renal medullary interstitium.

Specific TSC22 domain transcripts are hypertonically induced and alternatively spliced to protect mouse kidney cells during osmotic stress

We recently cloned a novel osmotic stress transcription factor 1 (OSTF1) from gills of euryhaline tilapia (Oreochromis mossambicus) and demonstrated that acute hyperosmotic stress transiently increases OSTF1 mRNA and protein abundance [Fiol DF, Kültz D (2005) Proc Natl Acad Sci USA102, 927-932]. In this study, a genome-wide search was conducted to identify nine distinct mouse transforming growth factor (TGF)-beta-stimulated clone 22 domain (TSC22D) transcripts, including glucocorticoid-induced leucine zipper (GILZ), that are orthologs of OSTF1. These nine TSC22D transcripts are encoded at four loci on chromosomes 14 (TSC22D1, two splice variants), 3 (TSC22D2, four splice variants), X (TSC22D3, two splice variants), and 5 (TSC22D4). All nine mouse TSC22D transcripts are expressed in renal cortex, medulla and papilla, and in the mIMCD3 cell line. The two TSC22D3 transcripts (including GILZ) are upregulated by aldosterone but not by hyperosmolality in mIMCD3 cells. In contrast, TSC22D4 is stably upregulated by hyperosmolality in mIMCD3 cells and increased in renal papilla compared with cortex. Moreover, all four TSC22D2 transcripts are transiently upregulated by hyperosmolality and resemble tilapia OSTF1 in this regard. All TSC22D2 transcripts depend on hypertonicity as the signal for their upregulation and are unresponsive to increases in cell-permeable osmolytes. mRNA stabilization is the mechanism for TSC22D2 upregulation by hyperosmolality. Overexpression of TSC22D2-4 in mIMCD3 cells confers protection towards osmotic stress, as evidenced by a 2.7-fold increase in cell survival after 3 days at 600 mOsmol x kg(-1). Based on variable responsiveness to aldosterone and hyperosmolality in kidney cells we conclude that mouse TSC22D genes have diverse physiological functions. TSC22D2 and TSC22D4 are involved in adaptation of renal cells to hypertonicity suggesting that they represent important elements of osmosensory signal transduction in mouse kidney cells.

Sequential expression of NKCC2, TonEBP, aldose reductase, and urea transporter-A in developing mouse kidney

This study was conducted to test the hypothesis that, during renal development, the Na-K-2Cl cotransporter type 2 (NKCC2) activates the tonicity-responsive enhancer binding protein (TonEBP) transcription factor by creating medullary hypertonicity. TonEBP, in turn, drives the expression of aldose reductase (AR) and urea transporter-A (UT-A). Kidneys from 13- to19-day-old fetuses (F13-F19), 1- to 21-day-old pups (P1-P21), and adult mice were examined by immunohistochemistry. NKCC2 was first detected on F14 in differentiating macula densa and thick ascending limb (TAL). TonEBP was first detected on F15 in the medullary collecting duct (MCD) and surrounding endothelial cells. AR was detected in the MCD cells of the renal medulla from F15. UT-A first appeared in the descending thin limb (DTL) on F16 and in the MCD on F18. After birth, NKCC2-positive TALs disappeared gradually from the tip of the renal papilla, becoming completely undetectable in the inner medulla on P21. TonEBP shifted from the cytoplasm to the nucleus in both vascular endothelial cells and MCD cells on P1, and its abundance increased gradually afterward. Immunoreactivity for AR and UT-A in the renal medulla increased markedly after birth. Treatment of neonatal animals with furosemide dramatically reduced expression of TonEBP, AR, and UT-A1. Furosemide also prevented the disappearance of NKCC2-expressing TALs in the papilla. The sequential expression of NKCC2, TonEBP, and its targets AR and UT-A and the reduced expression TonEBP and its targets in response to furosemide treatment support the hypothesis that local hypertonicity produced by the activity of NKCC2 activates TonEBP during development.

Transcriptional activator TonE-binding protein in cellular protection and differentiation

The TonE-binding protein (TonEBP) is a transcriptional activator in the Rel family that includes NFkappaB and NFAT. TonEBP is critical for the development and function of the renal medulla, which is a major regulator of water homeostasis. TonEBP is also implicated in diabetic nephropathy and inflammation. Established methods for biochemical and histochemical detection and functional analysis of TonEBP, including identification of novel TonEBP target genes, are described for those who are interested in investigating function and regulation of TonEBP.

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.

Postnatal expression of KLF12 in the inner medullary collecting ducts of kidney and its trans-activation of UT-A1 urea transporter promoter

Maturation of the inner medulla of the kidney occurs after birth and is vital for mammals to acquire maximal urinary concentrating ability. During this process, expression of several kidney transporters and channels involved in urine concentrating mechanisms is known to be regulated. We previously isolated KLF15 as a transcription factor that regulates the expression of the ClC-K1 chloride channel. We have now found that another KLF transcription factor, KLF12, is expressed in the kidney from around 15 days after birth. To gain insight into its involvement in the maturation process of the inner medulla, we first determined the expression site of KLF12 within the kidney by in situ hybridization. By comparing the AQP2 immunolocalization in sequential sections, KLF12 was found to be expressed in the collecting ducts. Because expression of the urea transporter UT-A1 and amiloride-sensitive epithelial sodium channels ENaC is known to be tightly regulated in the collecting ducts after birth, we tested whether KLF12 has a regulatory role in the promoter activities of these genes. KLF12 is able to increase UT-A1 but not ENaC promoter activity through the binding to CACCC motif. These results suggest that KLF12 is involved in the maturation processes of collecting ducts after birth, and that UT-A1 is a target gene of KLF12.

How tonicity regulates genes: story of TonEBP transcriptional activator

TonEBP stimulates genes whose products drive cellular accumulation of organic osmolytes and HSP70, which protect cells from the deleterious effects of hypertonicity and urea, respectively. Mice deficient in the TonEBP gene display severe atrophy of the renal medulla because cells failed to adapt to the hyperosmolality. Emerging data suggest that TonEBP plays a key role in the urinary concentrating mechanism by stimulating the UT-A urea transporters and possibly AQP2 water channel. Thus, TonEBP is an essential regulator in the urinary concentrating mechanism. Studies on structural basis of TonEBP function have revealed the structure of the DNA binding domain, and defined the transactivation domains. Molecular mechanisms underlying the nucleocytoplasmic trafficking, transactivation, and phosphorylation in response to changes in tonicity need to be understood in molecular detail. Such knowledge is needed for the identification of the sensor that detects changes in ambient tonicity and signals to TonEBP.

Tonicity-responsive enhancer binding protein is an essential regulator of aquaporin-2 expression in renal collecting duct principal cells

Tonicity-responsive enhancer binding protein (TonEBP) plays a key role in protecting renal cells from hypertonic stress by stimulating transcription of specific genes. Under hypertonic conditions, TonEBP activity is enhanced via increased nuclear translocation, transactivation, and abundance. It was reported previously that hypertonicity exerted a dual, time-dependent effect on vasopressin-inducible aquaporin-2 (AQP2) expression in immortalized mouse collecting duct principal cells (mpkCCDcl4). Whereas AQP2 abundance decreased after 3 h of hyperosmotic challenge, it increased after 24 h of hypertonic challenge. This study investigated the role that TonEBP may play in these events by subjecting mpkCCDcl4 cells to 3 or 24 h of hypertonic challenge. Hypertonic challenge increased TonEBP mRNA and protein content and enhanced TonEBP activity as illustrated by both increased TonEBP-dependent luciferase activity and mRNA expression of several genes that are targeted by TonEBP. Irrespective of the absence or presence of vasopressin, decreased TonEBP activity in cells that were transfected with either TonEBP small interfering RNA or an inhibitory form of TonEBP strongly reduced AQP2 mRNA and protein content under iso-osmotic conditions and blunted the increase of AQP2 abundance that was induced after 24 h of hypertonic challenge. Conversely, decreased TonEBP activity did not significantly alter reduced expression of AQP2 mRNA that was induced by 3 h of hypertonic challenge. Mutation of a TonE enhancer element located 489 bp upstream of the AQP2 transcriptional start site abolished the hypertonicity-induced increase of luciferase activity in cells that expressed AQP2 promoter-luciferase plasmid constructs, indicating that TonEBP influences AQP2 transcriptional activity at least partially by acting directly on the AQP2 promoter. These findings demonstrate that in collecting duct principal cells, TonEBP plays a central role in regulating AQP2 expression by enhancing AQP2 gene transcription.

Ultrastructural localization of UT-A and UT-B in rat kidneys with different hydration status

Urea transport in the kidney is mediated by a family of transporter proteins, including renal urea transporters (UT-A) and erythrocyte urea transporters (UT-B). We aimed to determine whether hydration status affects the subcellular distribution of urea transporters. Male Sprague-Dawley rats were divided into three groups: dehydrated rats (WD) given minimum water, hydrated rats (WL) given 3% sucrose in water for 3 days before death, and control rats given free access to water. We labeled kidney sections with antibodies against UT-A1 and UT-A2 (L194), UT-A3 (Q2), and UT-B using preembedding immunoperoxidase and immunogold methods. In control animals, UT-A1 and UT-A3 immunoreactivities were observed throughout the cytoplasm in inner medullary collecting duct (IMCD) cells, and weak labeling was observed on the basolateral plasma membrane. UT-A2 immunoreactivity in the descending thin limbs (DTL) was observed mainly on the apical and basolateral membranes of type I epithelium, and very faint labeling was observed in the long-loop DTL at the border between the outer and inner medulla. UT-A1 immunoreactivity intensity was markedly lower, and UT-A3 immunoreactivity was higher in IMCD of WD vs. controls. UT-A2 immunoreactivity intensities in the plasma membrane and cytoplasm of type I, II, and III epithelia of DTL were greater in WD vs. controls. In contrast, UT-A1 expression was greater and UT-A2 and UT-A3 expressions were lower in WL vs. controls. The subcellular distribution of UT-A in DTL or IMCD did not differ between control and experimental animals. UT-B was expressed in the plasma membrane of the descending vasa recta of both control and experimental animals. UT-B intensity was higher in WD and lower in WL vs. controls. These data indicate that changes in hydration status over 3 days affected urea transporter protein expression without changing its subcellular distribution.

Mitochondrial reactive oxygen species contribute to high NaCl-induced activation of the transcription factor TonEBP/OREBP

Hypertonicity activates the transcription factor tonicity-responsive enhancer/osmotic response element binding protein (TonEBP/OREBP), resulting in increased expression of genes involved in osmoprotective accumulation of organic osmolytes, including glycine betaine, and in increased expression of osmoprotective heat shock proteins. Our previous studies showed that high NaCl increases reactive oxygen species (ROS), which contribute to activation of TonEBP/OREBP. Mitochondria are a major source of ROS. The purpose of the present study was to examine whether mitochondria produce the ROS that contribute to activation of TonEBP/OREBP. We inhibited mitochondrial ROS production in HEK293 cells with rotenone and myxothiazol, which inhibit mitochondrial complexes I and III, respectively. Rotenone (250 nM) and myxothiazol (12 nM) reduce high NaCl-induced ROS over 40%, whereas apocynin (100 microM), an inhibitor of NADPH oxidase, and allopurinol (100 microM), an inhibitor of xanthine oxidase, have no significant effect. Rotenone and myxothiazol reduce high NaCl-induced increases in TonEBP/OREBP transcriptional activity (ORE/TonE reporter assay) and BGT1 (betaine transporter) mRNA abundance ranging from 53 to 69%. They inhibit high NaCl-induced TonEBP/OREBP transactivating activity, but not its nuclear translocation. Release of ATP into the medium on hypertonic stress has been proposed to be a signal that triggers cellular osmotic responses. However, we do not detect release of ATP into the medium or inhibition of high NaCl-induced ORE/TonE reporter activity by an ATPase, apyrase (20 U/ml), indicating that high NaCl-induced activation of TonEBP/OREBP is not mediated by release of ATP. We conclude that high NaCl increases mitochondrial ROS production, which contributes to the activation of TonEBP/OREBP by increasing its transactivating activity.

Regulated expression of renal and intestinal UT-B urea transporter in response to varying urea load

Production, recycling, and elimination of urea are important to maintain nitrogen balance. Adaptation to varying loads of urea due to different protein intake or in renal failure may involve changes in urea transport and may possibly affect urea transporters. In this study, we examined the expression of the UT-B urea transporter in rats fed a low-protein diet (LPD), a high-protein diet (HPD), and a 20% urea-supplemented diet. In the kidney, UT-B protein abundance increased in the outer medulla of both LPD-fed rats and 20% urea-fed rats, without changes in the inner medulla of either group compared with controls. In HPD-fed rats, UT-B protein decreased significantly in both the outer and inner medulla. We identified expression of UT-B in the rat colon, as a 2-kb mRNA transcript and as an approximately 45-kDa protein, with apical localization in superficial colon epithelial cells. UT-B also is expressed in rat small intestine. In rat colon, UT-B protein abundance was mildly, but significantly, decreased in LPD-fed and 20% urea-fed rats. UT-B abundance also was examined in the colon of 7/8 nephrectomized, uremic rats and in HPD-fed rats and was not significantly different from that in control rats. These findings indicate that UT-B expression is regulated in response to different loads of urea, with a pattern that suggests involvement of tissue-specific regulatory mechanism in kidney and colon.

DNA damage and osmotic regulation in the kidney

Renal medullary cells normally are exposed to extraordinarily high interstitial NaCl concentration as part of the urinary concentrating mechanism, yet they survive and function. Acute elevation of NaCl to a moderate level causes transient cell cycle arrest in culture. Higher levels of NaCl, within the range found in the inner medulla, cause apoptosis. Recently, it was surprising to discover that even moderately high levels of NaCl cause DNA double-strand breaks. The DNA breaks persist in cultured cells that are proliferating rapidly after adaptation to high NaCl, and DNA breaks normally are present in the renal inner medulla in vivo. High NaCl inhibits repair of broken DNA both in culture and in vivo, but the DNA is rapidly repaired if the level of NaCl is reduced. The inhibition of DNA repair is associated with suppressed activity of some DNA damage-response proteins like Mre11, Chk1, and H2AX but not that of others, like GADD45, p53, ataxia telangiectasia-mutated kinase (ATM), and Ku86. In this review, we consider possible mechanisms by which the renal cells escape the known dangerous consequences of persistent DNA damage. Furthermore, we consider that the persistent DNA damage may be a sensor of hypertonicity that activates ATM kinase to provide a signal that contributes to protective osmotic regulation.

Role and regulation of urea transporters

In the past few years, significant knowledge has been gained about the physiological role and regulation of urea transporters, which have been now cloned in many species. The two major mammalian urea transporters, UT-A and UT-B, have been best studied in the kidney, where they mediate the facilitated diffusion of urea across tubular, interstitial, and vascular compartments, necessary to maintain an osmolar gradient along the renal corticomedullary axis. The genes encoding these transporters, Slc14A2 for UT-A and Slc14A1 for UT-B, have been characterized in rodents and humans, allowing identification of transcriptional mechanisms involved in the regulation of UT-A expression. The crucial role that urea transporters play in renal physiology is underscored by the phenotypic characteristics of UT-A and UT-B knockout mice, in which lack of specific urea transporters impairs the ability to concentrate urine. Expression of the UT-A and UT-B transporters has also been identified in extra-renal sites, where their physiological significance is only beginning to be elucidated. More information on the mechanisms modulating urea transporter expression is becoming available, and the possible involvement of aberrant regulation of these transporters in pathological conditions, or as a result of certain pharmacological treatments, has emerged from recent studies.

Intracellular water homeostasis and the mammalian cellular osmotic stress response

The cellular response to osmotic stress ensures that the concentration of water inside the cell is maintained within a range that is compatible with biologic function. Single cell organisms are particularly dependent on mechanisms that permit adaptation to osmotic stress because each individual cell is directly exposed to the external environment. Mammals, however, limit osmotic stress by establishing an internal aqueous environment in which intravascular water and electrolytes are subject to sensitive and dynamic, organism-based homeostatic regulation. Recent studies of NFAT5/TonEBP, an essential mammalian osmoregulatory transcription factor, demonstrate the unexpected yet critical significance of cell-based osmotic regulation in vivo. These results highlight the fundamental importance of maintaining intracellular water homeostasis in the face of varying cellular metabolic activity and distinct tissue microenvironments.

Altered expression of selected genes in kidney of rats with lithium-induced NDI

Lithium treatment is associated with development of nephrogenic diabetes insipidus, caused in part by downregulation of collecting duct aquaporin-2 (AQP2) and AQP3 expression. In the present study, we carried out cDNA microarray screening of gene expression in the inner medulla (IM) of lithium-treated and control rats, and selected genes were then investigated at the protein level by immunoblotting and/or immunohistochemistry. The following genes exhibited significantly altered transcription and mRNA expression levels, and these were compatible with the changes in protein expression. 11β-Hydroxysteroid dehydrogenase type 2 protein expression in the IM was markedly increased (198 ± 25% of controls, n = 6), and immunocytochemistry demonstrated an increased labeling of IM collecting duct (IMCD) principal cells. This indicated altered renal mineralocorticoid/glucocorticoid responses in lithium-treated rats. The inhibitor of cyclin-dependent kinases p27 (KIP) protein expression was significantly decreased or undetectable in the IMCD cells, pointing to increased cellular proliferation and remodeling. Heat shock protein 27 protein expression was decreased in the IM (64 ± 6% of controls, n = 6), likely to be associated with the decreased medullary osmolality in lithium-treated rats. Consistent with this, lens aldose reductase protein expression was markedly decreased in the IM (16 ± 2% of controls, n = 6), and immunocytochemistry revealed decreased expression in the thin limb cells in the middle and terminal parts of the IM. Ezrin protein expression was upregulated in the IM (158 ± 16% of controls, n = 6), where it was predominantly expressed in the apical and cytoplasmic domain of the IMCD cells. Increased ezrin expression indicated remodeling of the actin cytoskeleton and/or altered regulation of IMCD transporters. In conclusion, the present study demonstrates changes in gene expression not only in the collecting duct but also in the thin limb of the loop of Henle in the IM, and several of these genes are linked to altered sodium and water reabsorption, cell cycling, and changes in interstitial osmolality.

Dual effects of hypertonicity on aquaporin-2 expression in cultured renal collecting duct principal cells

The driving force for renal water reabsorption is provided by the osmolarity gradient between the interstitium and the tubular lumen, which is subject to rapid physiologic variations as a consequence of water intake fluctuations. The effect of increased extracellular tonicity/osmolarity on vasopressin-inducible aquaporin-2 (AQP2) expression in immortalized mouse collecting duct principal cells (mpkCCD(cl4)) is investigated in this report. Increasing the osmolarity of the medium either by the addition of NaCl, sucrose, or urea first decreased AQP2 expression after 3 h. AQP2 expression then increased in cells exposed to NaCl- or sucrose-supplemented hypertonic medium after longer periods of time (24 h), while urea-supplemented hyperosmotic medium had no effect. Altered AQP2 expression induced by both short-term (3 h) and long-term (24 h) exposure of cells to hypertonicity arose from changes in AQP2 gene transcription because hypertonicity did not modify AQP2 mRNA stability nor AQP2 protein turnover. On the long-term, vasopressin (AVP) and hypertonicity increased AQP2 expression in a synergistic manner. Hypertonicity altered neither the dose-responsiveness of AVP-induced AQP2 expression nor cAMP-protein kinase (PKA) activity, while PKA inhibition did not reduce the extent of the hypertonicity-induced increase of AQP2 expression. These results indicate that in collecting duct principal cells: (1) a short-term increase of extracellular osmolarity decreases AQP2 expression through inhibition of AQP2 gene transcription; (2) a long-term increase of extracellular tonicity, but not osmolarity, enhances AQP2 expression via stimulation of AQP2 gene transcription; and (3) long-term hypertonicity and PKA increases AQP2 expression through synergistic but independent mechanisms.

Altered expression profile of transporters in the inner medullary collecting duct of aquaporin-1 knockout mice

Aquaporin-1 is the major protein responsible for transport of water across the epithelia of the proximal tubule and thin descending limbs. Rapid water efflux across the thin descending limb is required for the normal function of the countercurrent multiplier mechanism. Therefore, urinary concentrating capacity is severely impaired in aquaporin-1 knockout (AQP1 -/-) mice. Here, we have investigated the long-term consequences of deletion of the AQP1 gene product by profiling abundance changes in transporters expressed in the inner medullas of AQP1 (-/-) mice vs. heterozygotes [AQP1 (+/-)], which have a normal concentrating capacity. Semiquantitative immunoblotting demonstrated marked suppression of two proteins strongly expressed in the inner medullary collecting duct (IMCD): UT-A1 (a urea transporter) and AQP4 (a basolateral water channel). Furthermore, the urea permeability of the IMCD was significantly reduced in AQP1 (-/-) mice. In contrast, there was increased expression of three proteins normally expressed at higher levels in the cortical collecting duct (CCD) than in IMCD: AQP3 (another basolateral water channel) and the epithelial sodium channel subunits beta-ENaC and gamma-ENaC. Changes in expression of these proteins were confirmed by immunocytochemistry. Messenger RNA profiling (real-time RT-PCR) revealed changes in UT-A1, beta-ENaC, gamma-ENaC, and AQP3 transcript abundance that paralleled the changes in protein abundance. Thus, from the perspective of transport proteins, the IMCDs of AQP1 (-/-) mice have a significantly altered phenotype. To address whether these changes are specific to AQP1 (-/-) mice, we profiled IMCD transporter expression in a second knockout model manifesting a concentrating defect, that of ClC-nK1, a chloride channel in the ascending thin limb important for urinary concentration. As in the AQP1 knockout mice, ClC-nK1 (-/-) mice showed decreased expression of UT-A1 and increased expression of beta-ENaC and gamma-ENaC vs. WT controls. In conclusion, the expression profile of IMCD transporters is markedly altered in AQP1 -/- mice and this manifestation is related to the associated concentrating defect.

Osmotic response element-binding protein (OREBP) is an essential regulator of the urine concentrating mechanism

OREBP (osmotic response element-binding protein), also called TonEBP or NFAT5, is thought to induce the expression of genes that increase the accumulation of organic osmolytes to protect cells against a hypertonic environment. To investigate the consequences of lacking OREBP activity, transgenic (Tg) mice that overexpress OREBPdn (dominant negative form of OREBP) specifically in the epithelial cells of the renal collecting tubules were generated. These mice showed impairment in their urine concentrating mechanism, most likely due to reduced expression of the aquaporin AQP2 and the urea transporter UT-A1 and UT-A2 mRNAs. When deprived of water or after the administration of a vasopressin analogue, urine osmolality of the Tg mice was significantly increased but not to the same extent as that of the wild type mice. The expression of AQP2 and UT-A1, but not UT-A2 mRNAs, was increased to the same level as that of the wild type mice in the water deprivation state, indicating that the vasopressin regulatory mechanism was not affected by OREBPdn. These data indicate that in addition to vasopressin, OREBP is another essential regulator of the urine concentrating mechanism. Furthermore, the OREBPdn Tg mice developed progressive hydronephrosis soon after weaning, confirming the osmoprotective function of OREBP implicated by the in vitro experiments.

Renal urea transporters

PURPOSE OF REVIEW

Urea is transported across the kidney inner medullary collecting duct by urea-transporter proteins. Two urea-transporter genes have been cloned from humans and rodents: the UT-A (Slc14A2) gene encodes five protein and eight cDNA isoforms; the UT-B (Slc14A1) gene encodes a single isoform. In the past year, significant progress has been made in understanding the regulation of urea-transporter protein abundance in kidney, studies of genetically engineered mice that lack a urea transporter, identification of urea transporters outside of the kidney, cloning of urea transporters in nonmammalian species, and active urea transport in microorganisms.

RECENT FINDINGS

UT-A1 protein abundance is increased by 12 days of vasopressin, but not by 5 days. Analysis of the UT-A1 promoter suggests that vasopressin increases UT-A1 indirectly following a direct effect to increase the transcription of other genes, such as the Na(+)-K(+)-2Cl- cotransporter NKCC2/BSC1 and the aquaporin (AQP) 2 water channel, that begin to increase inner medullary osmolality. UT-A1 protein abundance is also increased by adrenalectomy, and is decreased by glucocorticoids or mineralocorticoids. However, each hormone works through its own receptor. Knockout mice that lack UT-A1 and UT-A3, or lack UT-B, have a urine-concentrating defect and a decrease in inner medullary interstitial urea content.

SUMMARY

Urea transporters play a critical role in the urine-concentrating mechanism. Their abundance is regulated by vasopressin, glucocorticoids, and mineralocorticoids. These regulatory mechanisms may be important in disease states such as diabetes because changes in urea-transporter abundance in diabetic rats require glucocorticoids and vasopressin.

Identification and characterization of a Kidd antigen/UT-B urea transporter expressed in human colon

We have identified a urea transporter from the mucosa of the human colon that has characteristics consistent with a Kidd antigen/UT-B urea transporter. This intestinal urea transporter encodes a 389-amino acid peptide with a sequence identical to that previously reported for the UT-B urea transporter in erythrocytes. Expression of a UT-B 2-kb mRNA transcript and of approximately 50- and approximately 98-kDa UT-B proteins is detected in human colonic mucosa by Northern and Western blot analysis. The UT-B protein is localized in the cell membrane and cytoplasm of the superficial intestinal epithelium and in the epithelial cells in the crypts. A 2-kb UT-B mRNA transcript and the UT-B protein were also identified in the intestinal cell line Caco-2. The transepithelial flux of (14)C urea was examined in Caco-2 cells growing on porous membrane support and was significantly inhibited by phloretin, 1,3-dimethylurea, and thiourea, suggesting that the transfer of urea across the Caco-2 monolayer could be mediated, at least in part, by the UT-B urea transporter. We conclude that the Kidd antigen/UT-B urea transporter is physiologically expressed in the human colon epithelium, where it could participate in the transport of urea across the colon mucosa.

Maturation of TonEBP expression in developing rat kidney

Tonicity-responsive enhancer binding protein (TonEBP) is a transcriptional activator of the Rel family. In the renal medulla, TonEBP stimulates genes encoding proteins involved in cellular accumulation of organic osmolytes, the vasopressin-regulated urea transporters (UT-A), and heat shock protein 70. To understand the role of TonEBP in the development of urinary concentrating ability, TonEBP expression during rat kidney development was investigated. In embryonic kidneys, TonEBP immunoreactivity was detected 16 days postcoitus in the cytoplasm of the endothelial cells surrounding the medullary collecting ducts (MCD). By 20 days, TonEBP was detected in most tubular profiles in the medulla, including the loop of Henle and MCD, and interstitial cells. The intensity of TonEBP immunoreactivity was much higher in the vasa recta than the tubules. In addition, immunoreactivity was localized predominantly to the cytoplasm. On postnatal day 1, two major changes were observed. TonEBP immunoreactivity shifted to the nucleus, and the intensity of TonEBP immunoreactivity of the tubules increased dramatically. These changes were associated with an increase in TonEBP and sodium-myo-inositol cotransporter mRNA abundance. Thereafter, TonEBP expression in tubular profiles increased moderately. The adult pattern of TonEBP expression was established at postnatal day 21 coincident with full maturation of the renal medulla. Thus expression of TonEBP in developing kidneys occurred predominantly in the medulla and preceded expression of its target genes, including UT-A. These data suggest that TonEBP contributes to the development of urine-concentrating ability.

Role of vasopressin in diabetes mellitus-induced changes in medullary transport proteins involved in urine concentration in Brattleboro rats

In rats with streptozotocin-induced diabetes mellitus for 10-20 days, we showed that the abundance of the major medullary transport proteins involved in the urinary concentrating mechanism, urea transporter (UT-A1), aquaporin-2 (AQP2), and the Na+-K+-2Cl- cotransporter (NKCC2/BSC1), is increased, despite the ongoing osmotic diuresis. To test whether vasopressin is necessary for these diabetes mellitus-induced changes in UT-A1, AQP2, or NKCC2/BSC1, we studied Brattleboro rats because they lack vasopressin. Brattleboro rats were given vasopressin (2.4 microg/day via osmotic minipump) for 5 or 12 days. At 5 days, vasopressin increased AQP2 protein abundance but decreased UT-A1 abundance compared with untreated Brattleboro rats. At 12 days, vasopressin increased the abundance of both UT-A1 and AQP2 proteins but did not alter NKCC2/BSC1. Next, untreated Brattleboro rats were made diabetic for 10 days by injecting them with streptozotocin (40 mg/kg). Diabetes mellitus increased the abundance of AQP2 and NKCC2/BSC1 proteins, but UT-A1 protein abundance did not increase. Third, vasopressin-treated Brattleboro rats were made diabetic with streptozotocin for 10 days. In vasopressin-treated Brattleboro rats, diabetes mellitus increased UT-A1, AQP2, and NKCC2/BSC1 protein abundances. Vasopressin significantly increased UT-A1 phosphorylation in vasopressin-treated diabetic Brattleboro rats but not in the other groups of Brattleboro rats. We conclude that 1) administering vasopressin to Brattleboro rats for 12 days, but not for 5 days, increases UT-A1 protein abundance and 2) vasopressin is necessary for the increase in UT-A1 protein in diabetic rats but is not necessary for the increase in AQP2 or NKCC2 proteins.

Gene regulation of renal-osmotic stress-induced Na-CL organic solute cotransporter

BACKGROUND

Na- and Cl-dependent organic solute cotransporters participate in transporting neurotransmitters in the brain and organic osmolytes in the kidney.

METHODS

We examined the intranephron localization and regulation of the renal osmotic-stress-induced Na-Cl organic solute cotransporter (ROSIT) mRNA expression using microdissected nephron segments from control and dehydrated rats and RT-PCR. To further know the mechanisms of gene regulation of ROSIT, microdissected proximal straight tubules (PST) were incubated in isotonic (290 mosm/kg H2O) or hyperosmotic (490- 1,090 mosm/kg H2O) solution.

RESULTS

ROSIT mRNA was expressed predominantly in PST and to a lesser extent in cortical thick ascending limbs and cortical collecting ducts in control rats, and dehydration caused an increase in the expression in whole nephron segments. ROSIT mRNA in PST was decreased with time by incubation in isotonic solution. Incubation of PST in hypertonic solution by adding NaCl increased mRNA expression as early as 15 min (1.5- and 3-fold at 15 and 30 min, respectively). This stimulating effect of NaCl was largest at 890 mosm/kg H2O. Hypertonicity by mannitol or myoinositol also increased ROSIT mRNA expression. In contrast, hyperosmolality by urea reduced ROSIT mRNA expression. GAPDH mRNA expression, an internal standard, did not change by incubation in NaCl or mannitol solution.

CONCLUSION

In summary, ROSIT mRNA expression was most abundant in PST in control and it was stimulated in whole nephron segments by dehydration. ROSIT mRNA expression in PST was stimulated by hypertonicity but not by urea. These data suggest that ROSIT may participate in the transport of amino acid under control conditions and organic osmolytes in dehydration.