The expression of the gamma subunit of Na-K-ATPase is regulated by osmolality via C-terminal Jun kinase and phosphatidylinositol 3-kinase-dependent mechanisms

Renal Mg handling, FXYD2 and the central role of the Na,K-ATPase

This article examines the central role of Na,K-ATPase (α1β1FXYD2) in renal Mg handling, especially in distal convoluted tubule (DCT), the segment responsible for final regulation of Mg balance. By considering effects of Na,K-ATPase on intracellular Na and K concentrations, and driving forces for Mg transport, we propose a consistent rationale explaining basal Mg reabsorption in DCT and altered Mg reabsorption in some human diseases. FXYD2 (γ subunit) is a regulatory subunit that adapts functional properties of Na,K-ATPase to cellular requirements. Mutations in FXYD2 (G41R), and transcription factors (HNF-1B and PCBD1) that affect FXYD2 expression are associated with hypomagnesemia with hypermagnesuria. These mutations result in impaired interactions of FXYD2 with Na,K-ATPase. Renal Mg wasting implies that Na,K-ATPase is inhibited, but in vitro studies show that FXYD2 itself inhibits Na,K-ATPase activity, raising K0.5 Na. However, FXYD2 also stabilizes the protein by amplifying specific interactions with phosphatidylserine and cholesterol within the membrane. Renal Mg wasting associated with impaired Na,K-ATPase/FXYD2 interactions is explained simply by destabilization and inactivation of Na,K-ATPase. We consider also the role of the Na,K-ATPase in Mg (and Ca) handling in Gitelman syndrome and Familial hyperkalemia and hypertension (FHHt). Renal Mg handling serves as a convenient marker for Na,K-ATPase activity in DCT.

The Role of Phospholipase D and MAPK Signaling Cascades in the Adaption of Lichen Microalgae to Desiccation: Changes in Membrane Lipids and Phosphoproteome

Classically, lichen phycobionts are described as poikilohydric organisms able to undergo desiccation due to the constitutive presence of molecular protection mechanisms. However, little is known about the induction of cellular responses in lichen phycobionts during drying. The analysis of the lipid composition of the desiccated lichen microalga Asterochloris erici revealed the unusual accumulation of highly polar lipids (oligogalactolipids and phosphatidylinositol), which prevents the fusion of membranes during stress, but also the active degradation of cone-shaped lipids (monogalactosyldiacylglycerol and phosphatidylethanolamine) to stabilize membranes in desiccated cells. The level of phosphatidic acid increased 7-fold during desiccation, implicating a possible role for phospholipase D (PLD) in the response to osmotic stress. Inhibition of PLD with 1-butanol markedly impaired the recovery of photosynthesis activity in A. erici upon desiccation and salt stress (2 M NaCl). These two hyperosmotic stresses caused the phosphorylation of c-Jun N-terminal kinase (JNK) and p38-like mitogen-activated protein kinase (MAPK) and the dephosphorylation of extracellular signal-regulated kinase (ERK). The incubation with 1-butanol reduced the phosphorylation of JNK-like proteins and increased the dephosphorylation of ERK-like proteins, which indicates an upstream control of MAPK cascades by PLD. The phosphoproteome showed that desiccation caused the phosphorylation of several proteins in A. erici, most of them involved in protein turnover. The results demonstrate that lichen phycobionts possess both constitutive and inducible protective mechanisms to acquire desiccation tolerance. Among others, these responses are controlled by the PLD pathway through the activation of MAPK cascades.

Cell survival after UV radiation stress in the unicellular chlorophyte Dunaliella tertiolecta is mediated by DNA repair and MAPK phosphorylation

Ultraviolet radiation (UVR) induces damage in a variety of organisms, and cells may adapt by developing repair or tolerance mechanisms to counteract such damage; otherwise, the cellular fate is cell death. Here, the effect of UVR-induced cell damage and the associated signalling and repair mechanisms by which cells are able to survive was studied in Dunaliella tertiolecta. UVR did not cause cell death, as shown by the absence of SYTOX Green-positive labelling cells. Ultrastructure analysis by transmission electron microscopy demonstrated that the cells were alive but were subjected to morphological changes such as starch accumulation, chromatin disaggregation, and chloroplast degradation. This behaviour paralleled a decrease in F(v)/F(m) and the formation of cyclobutane-pyrimidine dimers, showing a 10-fold increase at the end of the time course. There was a high accumulation of the repressor of transcriptional gene silencing (ROS1), as well as the cell proliferation nuclear antigen (PCNA) in UVR-treated cells, revealing activation of DNA repair mechanisms. The degree of phosphorylation of c-Jun N-terminal kinase (JNK) and p38-like mitogen-activated protein kinases was higher in UVR-exposed cells; however, the opposite occurred with the phosphorylated extracellular signal-regulated kinase (ERK). This confirmed that both JNK and p38 need to be phosphorylated to trigger the stress response, as well as the fact that cell division is arrested when an ERK is dephosphorylated. In parallel, both DEVDase and WEHDase caspase-like enzymatic activities were active even though the cells were not dead, suggesting that these proteases must be considered within a wider frame of stress proteins, rather than specifically being involved in cell death in these organisms.

Patterns of gene and metabolite define the effects of extracellular osmolality on kidney collecting duct

To investigate the effects of changes in extracellular osmolality on the function of kidney collecting duct cells, particularly on water and sodium reabsorption in the conditions of diuresis and antidiuresis, we generated transcriptome and metabolome profiles of primary cultured inner medullary collecting duct (IMCD) cells. They were grown in hyperosmolar culture medium (640 mOsm) for 4 days and then exposed to either reduced (300 mOsm) or same osmolality for 1 or 2 days more. Integrated analysis of the transcriptome and metabolome revealed that decreased extracellular osmolality was associated with decreased levels of organic osmolytes, glucose, intermediates of citric acid cycle, and branched-chain amino acids (BCAA) in IMCD cells, along with significantly decreased gene expression and protein abundance of P-type transporters (ATP1B1), ABC transporters (ABCC5 and ABCG1), and insulin signaling pathways (IRS2). Quantitative real-time RT-PCR and semiquantitative immunoblotting confirmed the changes of transcript levels of differentially expressed genes and protein levels. Taken together, integrated analysis of omics data demonstrated that water and sodium reabsorption could be reduced by decreased extracellular osmolality per se, through decreased levels of ABC transporters and IRS2, which play a potential role in the transport of organic osmolytes, BCAA, glucose, and trafficking of epithelial sodium channel.

Post-transcriptional control of Na,K-ATPase activity and cell growth by a splice variant of FXYD2 protein with modified mRNA

In kidney, FXYD proteins regulate Na,K-ATPase in a nephron segment-specific way. FXYD2 is the most abundant renal FXYD but is not expressed in most renal cell lines unless induced by hypertonicity. Expression by transfection of FXYD2a or FXYD2b splice variants in NRK-52E cells reduces the apparent Na(+) affinity of the Na,K-ATPase and slows the cell proliferation rate. Based on RT-PCR, mRNAs for both splice variants were expressed in wild type NRK-52E cells as low abundance species. DNA sequencing of the PCR products revealed a base alteration from C to T in FXYD2b but not FXYD2a from both untreated and hypertonicity-treated NRK-52E cells. The 172C→T sequence change exposed a cryptic KKXX endoplasmic reticulum retrieval signal via a premature stop codon. The truncation affected trafficking of FXYD2b and its association with Na,K-ATPase and blocked its effect on enzyme kinetics and cell growth. The data may be explained by altered splicing or selective RNA editing of FXYD2b, a supplementary process that would ensure that it was inactive even if transcribed and translated, in these cells that normally express only FXYD2a. 172C→T mutation was also identified after mutagenesis of FXYD2b by error-prone PCR coupled with a selection for cell proliferation. Furthermore, the error-prone PCR alone introduced the mutation with high frequency, implying a structural peculiarity. The data confirm truncation of FXYD2b as a potential mechanism to regulate the amount of FXYD2 at the cell surface to control activity of Na,K-ATPase and cell growth.

FXYD proteins stabilize Na,K-ATPase: amplification of specific phosphatidylserine-protein interactions

FXYD proteins are a family of seven small regulatory proteins, expressed in a tissue-specific manner, that associate with Na,K-ATPase as subsidiary subunits and modulate kinetic properties. This study describes an additional property of FXYD proteins as stabilizers of Na,K-ATPase. FXYD1 (phospholemman), FXYD2 (γ subunit), and FXYD4 (CHIF) have been expressed in Escherichia coli and purified. These FXYD proteins associate spontaneously in vitro with detergent-soluble purified recombinant human Na,K-ATPase (α1β1) to form α1β1FXYD complexes. Compared with the control (α1β1), all three FXYD proteins strongly protect Na,K-ATPase activity against inactivation by heating or excess detergent (C(12)E(8)), with effectiveness FXYD1 > FXYD2 ≥ FXYD4. Heating also inactivates E(1) ↔ E(2) conformational changes and cation occlusion, and FXYD1 protects strongly. Incubation of α1β1 or α1β1FXYD complexes with guanidinium chloride (up to 6 m) causes protein unfolding, detected by changes in protein fluorescence, but FXYD proteins do not protect. Thus, general protein denaturation is not the cause of thermally mediated or detergent-mediated inactivation. By contrast, the experiments show that displacement of specifically bound phosphatidylserine is the primary cause of thermally mediated or detergent-mediated inactivation, and FXYD proteins stabilize phosphatidylserine-Na,K-ATPase interactions. Phosphatidylserine probably binds near trans-membrane segments M9 of the α subunit and the FXYD protein, which are in proximity. FXYD1, FXYD2, and FXYD4 co-expressed in HeLa cells with rat α1 protect strongly against thermal inactivation. Stabilization of Na,K-ATPase by three FXYD proteins in a mammalian cell membrane, as well the purified recombinant Na,K-ATPase, suggests that stabilization is a general property of FXYD proteins, consistent with a significant biological function.

Response of IMCD3 cells to hypertonic challenges as analyzed by electron microscopy

This work defines the ultrastructural responses of immortalized cells from the inner medullary collecting duct cells (IMCD3 cells) to hypertonic challenges. The cultured cells were either acutely exposed to hypertonic medium (550 mOsm/kgH₂O) for 24-72 h or gradually adapted to 600 or 900 mOsm/kgH₂O media with sodium chloride. After short (24 h) hypertonic challenges, there was an expansion of the Golgi apparatus with distinct expression of the γ subunit of Na,K-ATPase. The frequency of active caspase-3-positive cells was unchanged as was also the measured activity of caspase-3. Immunoelectron microscopy showed that active caspase-3 in the positive cells was localized in cytoplasmic bodies 0.5-1 μm in diameter but not in other structures. Apoptotic bodies with the nuclei were only rarely observed following acute hypertonicity for 24 to 72 h. Following prolonged hypertonic challenges, some cells showed condensation of the chromatin but still few apoptotic bodies. Gradual hypertonicity to 900 mOsm/kgH₂O led to a decrease of microvilli, dilated cisternae of the endoplasmic reticulum (ER), increased abundance of free ribosomes and longitudinal mitochondrial cristae. Virus particles were present inside and outside the cells in all experimental conditions and appeared unrelated to the apoptotic process. The results suggest that cultured IMCD3 cells are resistant to short hypertonic challenge or gradual adaptation to moderate hypertonicity and only rarely exhibit more ultrastructural apoptotic changes than control cells. The presence of caspase-3-containing bodies is a novel finding, and we suggest that they arise from the ER and are involved in the apoptotic signaling system.

FXYD2 and Na,K-ATPase expression in isolated human proximal tubular cells: disturbed upregulation on renal hypomagnesemia?

Autosomal dominant renal hypomagnesemia (OMIM 154020), associated with hypocalciuria, has been linked to a 121G to A mutation in the FXYD2 gene. To gain insight into the molecular mechanisms linking this mutation to the clinical phenotype, we studied isolated proximal tubular cells from urine of a patient and a healthy subject. Cells were immortalized and used to assess the effects of hypertonicity-induced overexpression of FXYD2 on amount, activity and apparent affinities for Na⁺, K⁺ and ATP of Na,K-ATPase. Both cell lines expressed mRNA for FXYD2a and FXYD2b, and patient cells contained both the wild-type and mutated codons. FXYD2 protein expression was lower in patient cells and could be increased in both cell lines upon culturing in hyperosmotic medium but to a lesser extent in patient cells. Similarly, hyperosmotic culturing increased Na,K-ATPase protein expression and ATP hydrolyzing activity but, again, to a lesser extent in patient cells. Apparent affinities of Na,K-ATPase for Na⁺, K⁺ and ATP did not differ between patient and control cells or after hyperosmotic induction. We conclude that human proximal tubular cells respond to a hyperosmotic challenge with an increase in FXYD2 and Na,K-ATPase protein expression, though to a smaller absolute extent in patient cells.

Different ways to die: cell death modes of the unicellular chlorophyte Dunaliella viridis exposed to various environmental stresses are mediated by the caspase-like activity DEVDase

Programmed cell death is necessary for homeostasis in multicellular organisms and it is also widely recognized to occur in unicellular organisms. However, the mechanisms through which it occurs in unicells, and the enzymes involved within the final response is still the subject of heated debate. It is shown here that exposure of the unicellular microalga Dunaliella viridis to several environmental stresses, induced different cell death morphotypes, depending on the stimulus received. Senescent cells demonstrated classical and unambiguous apoptotic-like characteristics such as chromatin condensation, DNA fragmentation, intact organelles, and blebbing of the cell membrane. Acute heat shock caused general swelling and altered plasma membrane, but the presence of chromatin clusters and DNA strand breaks suggested a necrotic-like event. UV irradiated cells presented changes typical for necrosis, together with apoptotic characteristics resembling an intermediate cell-death phenotype termed aponecrosis-like. Cells subjected to hyperosmotic shock revealed chromatin spotting without DNA fragmentation, and extensive cytoplasmic swelling and vacuolization, comparable to a paraptotic-like cell death phenotype. Nitrogen-starved cells showed pyknosis, blebbing, and cytoplasmic consumption, indicating a similarity to autophagic/vacuolar-like cell death. The caspase-like activity DEVDase was measured by using the fluorescent substrate Ac-DEVD-AMC and antibodies against the human caspase-3 active enzyme cross-reacted with bands, the intensity of which paralleled the activity. All the environmental stresses tested produced a substantial increase in both DEVDase activity and protein levels. The irreversible caspase-3 inhibitor Z-DEVD-FMK completely inhibited the enzymatic activity whereas serine and aspartyl proteases inhibitors did not. These results show that cell death in D. viridis does not conform to a single pattern and that environmental stimuli may produce different types of cell death depending on the type and intensity of the stimulus, all of which help to understand the cell death-dependent and cell death-independent functions of caspase-like proteins. Hence, these data support the theory that alternative, non-apoptotic programmed cell death (PCDs), exist either in parallel or in an independent manner with apoptosis and were already present in single-celled organisms that evolved some 1.2-1.6 billion years ago.

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.

How do kidney cells adapt to survive in hypertonic inner medulla?

The hypertonic inner medulla poses challenges to the cells that inhabit this area of the nephron. We employed discovery tools including proteomics and genomics to identify proteins that subserve the adaptive response. The gamma subunit of the Na/K-ATPase is critical to the survival of cells in hypertonic conditions, as silencing it increases osmosensitvity, and overexpression increases osmotolerance. The inner medullary collecting duct (IMCD) has high transepithelial resistance (TER). Proteins responsible for tight junction integrity are upregulated in hypertonic states. Multi PDZ protein 1 (MUPP1), a PDZ scaffolding protein, targets Claudin 4 to the tight junction. The silencing of either of these proteins decreases TER and renders the epithelium leaky. The accumulation of inert osmolytes is integral to the adaptive response. The genes involved are regulated by the transcription factor Tonicity Enhancer Binding Protein. An osmoregulated nuclear protein Nup88 is critical to the retention of this transcription factor in the nucleus and to the generation of the osmolytes. In summary, IMCD cells bring forth a coordinated response to hypertoncity that is necessary for cell survival and function of these cells in anisotonic conditions.

Expression and trafficking of the gamma subunit of Na,K-ATPase in hypertonically challenged IMCD3 cells

The gamma subunit (FXYD2) of Na,K-ATPase is an important regulator of the sodium pump. In this investigation we have analysed the trafficking of gamma to the plasma membrane in cultures of inner medullary collecting duct cells (IMCD3) following acute hypertonic challenge and brefeldin A (BFA) treatment. Following hypertonic challenging for 24 hr immunofluorescence labeling revealed initial co-localization of the gamma subunit and 58K Golgi protein in the cytoplasm, but no co-localization of alpha1 and Golgi protein. Exposure of the challenged cells to BFA prevented the subsequent incorporation of gamma into the basolateral plasma membrane. The gamma subunit instead remained in cytoplasmic vesicles while cell proliferation and cell viability decreased simultaneously. Following removal of BFA from the hypertonic medium the IMCD3 cells recovered with distinct expression of gamma in the basolateral membrane. The alpha1 subunit was only marginally influenced by BFA. The results demonstrate that the gamma subunit trafficks to the plasma membrane via the Golgi apparatus, despite the absence of a signal sequence. The results also suggest that the gamma and alpha subunits do not traffic together to the plasma membrane, and that the gamma and alpha subunit have different turnover rates during these experimental conditions.

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.

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.

Structural and functional interactions between FXYD5 and the Na+-K+-ATPase

FXYD5 is a member of a family of tissue-specific regulators of the Na(+)-K(+)-ATPase expressed in kidney tubules. Previously, we have shown that FXYD5 interacts with the alphabeta-subunits of the Na(+)-K(+)-ATPase and increases its V(max) (Lubarski I, Pihakaski-Maunsbach K, Karlish SJ, Maunsbach AB, Garty H. J Biol Chem 280: 37717-37724, 2005). The current study further characterizes structural interaction and structure-function relationships of FXYD5. FXYD5/FXYD4 chimeras expressed in Xenopus laevis oocytes have been used to demonstrate that both the high-affinity association with the pump and the increase in V(max) are mediated by the transmembrane domain of FXYD5. Several amino acids that participate in the high-affinity interaction between FXYD5 and the alpha-subunit of the Na(+)-K(+)-ATPase have been identified. The data suggest that different FXYD proteins interact similarly with the Na(+)-K(+)-ATPase and their transmembrane domains play a key role in both the structural interactions and functional effects. Other experiments have identified at least one splice variant of FXYD5 with 10 additional amino acids at the COOH terminus, suggesting the possibility of other functional effects not mediated by the transmembrane domain. FXYD5 could be specifically bound to wheat germ agglutinin beads, indicating that it is glycosylated. However, unlike previous findings in metastatic cells, such glycosylation does not evoke a large increase in the size of the protein expressed in native epithelia and X. laevis oocytes.

Expression of the calcium-binding protein S100A4 is markedly up-regulated by osmotic stress and is involved in the renal osmoadaptive response

Proteomic analysis of Inner Medullary Collecting Duct (IMCD3) cells adapted to increasing levels of tonicity (300, 600, and 900 mosmol/kg H(2)O) by two-dimensional difference gel electrophoresis and mass spectrometry revealed several proteins as yet unknown to be up-regulated in response to hypertonic stress. Of these proteins, one of the most robustly up-regulated (22-fold) was S100A4. The identity of the protein was verified by high pressure liquid chromatography-mass spectrometry. Western blot analysis confirmed increased expression with increased tonicity, both acute and chronic. S100A4 protein expression was further confirmed by immunocytochemical analysis. Cells grown in isotonic conditions showed complete absence of immunostaining, whereas chronically adapted IMCD3 cells had uniform cytoplasmic localization. The protein is also regulated in vivo as in mouse kidney tissues S100A4 expression was many -fold greater in the papilla as compared with the cortex and increased further in the papilla upon 36 h of thirsting. Increased expression of S100A4 was also observed in the medulla and papilla, but not the cortex of a human kidney. Data from Affymetrix gene chip analysis and quantitative PCR also revealed increased S100A4 message in IMCD3 cells adapted to hypertonicity. The initial expression of message increased at 8-10 h following exposure to acute sublethal hypertonic stress (550 mosmol/kg H(2)O). Protein and message half-life in IMCD3 cells were 85.5 and 6.8 h, respectively. Increasing medium tonicity with NaCl, sucrose, mannitol, and choline chloride stimulated S100A4 expression, whereas urea did not. Silencing of S100A4 expression using a stable siRNA vector (pSM2; Open Biosystems) resulted in a 48-h delay in adaptation of IMCD3 cells under sublethal osmotic stress, suggesting S100A4 is involved in the osmoadaptive response. In summary, we describe the heretofore unrecognized up-regulation of a small calcium-binding protein, both in vitro and in vivo, whose absence profoundly delays osmoadaptation and slows cellular growth under hypertonic conditions.

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.

Role of FXYD proteins in ion transport

The FXYD proteins are a family of seven homologous single transmembrane segment proteins (FXYD1-7), expressed in a tissue-specific fashion. The FXYD proteins modulate the function of Na,K-ATPase, thus adapting kinetic properties of active Na+ and K+ transport to the specific needs of different cells. Six FXYD proteins are known to interact with Na,K-ATPase and affect its kinetic properties in specific ways. Although effects of FXYD proteins on parameters such as K(1/2)Na+, K(1/2)K+, K(m)ATP, and V(max) are modest, usually twofold, these effects may have important long-term consequences for homeostasis of cation balance. In this review we summarize basic features of FXYD proteins and present recent evidence for functional effects, structure-function relations and structural interactions with Na,K-ATPase. We then discuss possible physiological roles, based on in vitro observations and newly available knockout mice models. Finally, we also consider evidence that FXYD proteins affect functioning of other ion transport systems.

Locations, abundances, and possible functions of FXYD ion transport regulators in rat renal medulla

The gamma-subunit of Na-K-ATPase (FXYD2) and corticosteroid hormone-induced factor (CHIF; FXYD4) are considered pump regulators in kidney tubules. The aim of this study was to expand the information about their locations in the kidney medulla and to evaluate their importance for electrolyte excretion in an animal model. The cellular and subcellular locations and abundances of gamma and CHIF in the medulla of control and sodium-depleted rats were analyzed by immunofluorescence and immunoelectron microscopy and semiquantitative Western blotting. The results showed that antibodies against the gamma-subunit COOH terminus and splice variant gamma(a), but not splice variant gamma(b), labeled intercalated cells, but not principal cells, in the initial part of the inner medullary collecting duct (IMCD1). In subsequent segments (IMCD2 and IMCD3), all principal cells exhibited distinct basolateral labeling for both the gamma-subunit COOH terminus, splice variant gamma(a), and CHIF. Splice variant gamma(b) was abundant in the inner stripe of the outer medulla but absent in the inner medulla (IM). Double labeling by high-resolution immunoelectron microscopy showed close structural association between CHIF and the Na-K-ATPase alpha(1)-subunit in basolateral membranes. The present observations provide new information about the cellular and subcellular locations of gamma and CHIF in the renal medulla and show a new gamma variant in the IM. Extensive NaCl depletion did not induce significant changes in the locations or abundances of the gamma-subunit COOH terminus and CHIF in different kidney zones. We conclude that the unchanged levels of these two FXYD proteins suggest that they are not primary determinants for urine electrolyte composition during NaCl depletion.

FXYD proteins: new regulators of Na-K-ATPase

FXYD proteins belong to a family of small-membrane proteins. Recent experimental evidence suggests that at least five of the seven members of this family, FXYD1 (phospholemman), FXYD2 (gamma-subunit of Na-K-ATPase), FXYD3 (Mat-8), FXYD4 (CHIF), and FXYD7, are auxiliary subunits of Na-K-ATPase and regulate Na-K-ATPase activity in a tissue- and isoform-specific way. These results highlight the complexity of the regulation of Na+ and K+ handling by Na-K-ATPase, which is necessary to ensure appropriate tissue functions such as renal Na+ reabsorption, muscle contractility, and neuronal excitability. Moreover, a mutation in FXYD2 has been linked to cases of human hypomagnesemia, indicating that perturbations in the regulation of Na-K-ATPase by FXYD proteins may be critically involved in pathophysiological states. A better understanding of this novel regulatory mechanism of Na-K-ATPase should help in learning more about its role in pathophysiological states. This review summarizes the present knowledge of the role of FXYD proteins in the modulation of Na-K-ATPase as well as of other proteins, their regulation, and their structure-function relationship.

FXYD Proteins: Tissue-Specific Regulators of the Na,K-ATPase

Work in several laboratories has led to the identification of a family of short single-span transmembrane proteins named after the invariant extracellular motif: FXYD. Four members of this group have been shown to interact with the Na,K-adenosine triphosphatase (ATPase) and alter the pump kinetics. Thus, it is assumed that FXYD proteins are tissue-specific regulatory subunits, which adjust the kinetic properties of the pump to the specific needs of the relevant tissue, cell type, or physiologic state, without affecting it elsewhere. A number of studies have provided evidence for additional and possibly unrelated functions of the FXYD proteins. This review summarizes current knowledge on the structure, function, and cellular distribution of FXYD proteins with special emphasis on their role in kidney electrolyte homeostasis.

The γ-subunit of Na-K-ATPase is incorporated into plasma membranes of mouse IMCD3 cells in response to hypertonicity

Hypertonicity mediated by chloride upregulates the expression of the γ-subunit of Na-K-ATPase in cultured cells derived from the murine inner medullary collecting duct (IMCD3; Capasso JM, Rivard CJ, Enomoto LM, and Berl T. Proc Natl Acad Sci USA 100: 6428–6433, 2003). The purpose of this study was to examine the cellular locations and the time course of γ-subunit expression after long-term adaptation and acute hypertonic challenges induced with different salts. Cells were analyzed by confocal immunofluorescence and immunoelectron microscopy with antibodies against the COOH terminus of the Na-K-ATPase γ-subunit or the γbsplice variant. Cells grown in 300 mosmol/kgH2O showed no immunoreactivity for the γ-subunit, whereas cells adapted to 600 or 900 mosmol/kgH2O demonstrated distinct reactivity located at the plasma membrane of all cells. IMCD3 cell cultures acutely challenged to 550 mosmol/kgH2O with sodium chloride or choline chloride showed incorporation of γ into plasma membrane 12 h after osmotic challenge and distinct membrane staining in ∼40% of the cells 48 h after osmotic shock. In contrast, challenging the IMCD3 cells to 550 mosmol/kgH2O by addition of sodium acetate did not result in expression of the γ-subunit in the membranes of surviving cells after 48 h. The present results demonstrate that the Na-K-ATPase γ-subunit becomes incorporated into the basolateral membrane of IMCD3 cells after both acute hyperosmotic challenge and hyperosmotic adaptation. We conclude that the γ-subunit has an important role in the function of Na-K-ATPase to sustain the cellular cation balance over the plasma membrane in a hypertonic environment.

Synthesis of the Na-K-ATPase γ-subunit is regulated at both the transcriptional and translational levels in IMCD3 cells

We previously reported that hypertonicity-mediated upregulation of the γ-subunit of Na-K-ATPase is dependent on both the JNK and the PI3 kinase pathways ( Proc Natl Acad Sci USA 98: 13414, 2001). The present experiments were undertaken to explore the mechanisms whereby these pathways regulate the expression of the γ-subunit in inner medullary collecting duct cells (IMCD3). Inhibition of JNK with SP-600125 (20 μM), a concentration that causes an ∼95% inhibition of hypertonicity-stimulated JNK activation, markedly decreased the amount of the γ-subunit in response to 550 mosmol/kgH2O for 48 h. This was accompanied by a parallel decrease in the γ-subunit mRNA. The rate at which the γ-subunit mRNA decreased was unaffected by actinomycin D. In contrast, inhibition of PI3 kinase with LY-294002 results in a marked decrease in the amount of γ-subunit protein but without alteration in γ-subunit message. The rate at which the γ-subunit protein decreased was unaffected by cyclohexamide. Transfection of IMCD3 cells with a γ-subunit construct results in the expression of both γ-subunit message and protein. However, in cortical collecting duct cells (M1 cells) such transfection resulted in expression of only the message and not the protein. We conclude that JNK regulates the γ-subunit at the transcriptional level while PI3 kinase regulates γ-subunit expression at the translational level. There is also posttranscriptional cell specificity in the expression of the γ -subunit of Na-K-ATPase.

MAP kinases and the adaptive response to hypertonicity: functional preservation from yeast to mammals

The adaptation to hypertonicity in mammalian cells is driven by multiple signaling pathways that include p38 kinase, Fyn, the catalytic subunit of PKA, ATM, and JNK2. In addition to the well-characterized tonicity enhancer (TonE)-TonE binding protein interaction, other transcription factors (and their respective cis elements) can potentially respond to hypertonicity. This review summarizes the current knowledge about the signaling pathways that regulate the adaptive response to osmotic stress and discusses new insights from yeast that could be relevant to the osmostress response in mammals.

EphA2: expression in the renal medulla and regulation by hypertonicity and urea stress in vitro and in vivo

EphA2, a member of the large family of Eph receptor tyrosine kinases, is highly expressed in epithelial tissue and has been implicated in cell-cell and cell-matrix interactions, as well as cell growth and survival. Expression of EphA2 mRNA and protein was markedly upregulated by both hypertonic stress and by elevated urea concentrations in cells derived from the murine inner medullary collecting duct. This upregulation likely required transactivation of the epidermal growth factor (EGF) receptor tyrosine kinase and metalloproteinase-dependent ectodomain cleavage of an EGF receptor ligand, based on pharmacological inhibitor studies. A human EphA2 promoter fragment spanning nucleotides -4030 to +21 relative to the putative EphA2 transcriptional start site was responsive to tonicity but insensitive to urea. A promoter fragment spanning -1890 to +128 recapitulated both tonicity- and urea-dependent upregulation of expression, consistent with transcriptional activation. Neither the bona fide p53 response element at approximately -1.5 kb nor a pair of putative TonE elements at approximately -3 kb conferred the tonicity responsiveness. EphA2 mRNA and protein were expressed at low levels in rat renal cortex but at high levels in the collecting ducts of the renal medulla and papilla. Water deprivation in rats increased EphA2 expression in renal papilla, whereas dietary supplementation with 20% urea increased EphA2 expression in outer medulla. These data indicate that transcription and expression of the EphA2 receptor tyrosine kinase are regulated by tonicity and urea in vitro and suggest that this phenomenon is also operative in vivo. Renal medullary EphA2 expression may represent an adaptive response to medullary hypertonicity or urea exposure.

Finding of TRE (TPA responsive element) in the sequence of human taurine transporter promoter

Activity of the taurine transporter (TAUT) is regulated by signal transduction in response to diverse stimuli including tumor promoters such as phobol ester. Regulation of the transcription rate of TAUT appears to play an important role in exerting biological roles of taurine in mammalian tissues in adverse environments. Although cDNA of human TAUT has been cloned and sequenced in placenta, thyroid cells, and retinal pigment epithelial cells, the promoter region of TAUT has never been reported. In order to clone the upstream region of the human TAUT promoter, we have compared TAUT cDNA sequences with the entire human genome sequence. Polymerase chain reaction (PCR) was performed from genomic DNA prepared from a SK-Hep-1 cell line for the amplification of the TAUT promoter region including the partial exon (150 bp) and the 5' untranslated region (UTR, 380 bp). The PCR product of the promoter region, which was 1800 bp long, was ligated into the pGEM-T vector, and sequenced. The 5' flanking region of the TAUT promoter was analysed for the identification of enhancer and regulation motifs. Surprisingly we found the consensus TPA responsive element (TGAGTCAG) which is responsible for gene regulation by the protein kinase C (PKC)-mediated signal transduction pathway. The well known fact that proto-oncogene AP1 (cFos/cJun heterodimer or cJun/cJun homodimer) binds to TRE implies that TAUT expression might be closely linked to tumor promotion. Since AP1 activity is also tightly regulated in nerve cells, AP1-regulated TAUT transcription might be an important step in nerve cell function. Furthermore, the TFIID binding site, cap signal for transcription initiation, PEA3 motif, heat shock factor binding motif, and many other motifs were found in the TAUT promoter region, and require characterization.

Phosphorylation of MAP kinase-like proteins mediate the response of the halotolerant alga Dunaliella viridis to hypertonic shock

The microalga Dunaliella viridis has the ability to adapt to a variety of environmental stresses including osmotic and thermal shocks, UV irradiation and nitrogen starvation. Lacking a rigid cell wall, Dunaliella provides an excellent model to study stress signaling in eukaryotic unicellular organisms. When exposed to hyperosmotic stress, UV irradiation or high temperature, a 57-kDa protein is recognized by antibodies specific to mammalian p38, to its yeast homologue Hog1, and to the phospho-p38 MAP kinase motif. This 57-kDa protein appears to be both up-regulated and phosphorylated. Three other proteins (50, 45, 43 kDa) were transiently phosphorylated under stress conditions as detected with an antibody specific to the mammalian phospho c-Jun N-terminal kinase (JNK) motif. Treatment with specific inhibitors of p38 MAP kinase (SB203580) and JNK (SP600125) activities markedly impaired the adaptation of Dunaliella to osmotic stress. From an evolutionary standpoint, these data strongly suggest that MAP kinase signaling pathways, other than ERK, were already operating in the common ancestor of plant and animal kingdoms, probably as early as 1400 million years ago.

Chloride, not sodium, stimulates expression of the gamma subunit of Na/K-ATPase and activates JNK in response to hypertonicity in mouse IMCD3 cells

Hypertonicity induced by NaCl, but not by urea or mannitol, up-regulates expression of the gamma subunit of Na/K-ATPase in cells of the murine inner medullary collecting duct line (IMCD3) by activation of the Jun kinase 2 (JNK2) pathways. We examined the ionic mediators of the osmosensitive response. An increase in osmolality to 550 milliosmoles per kg of water (mosmol/kgH2O) for 48 h by replacement of NaCl with choline chloride did not prevent the up-regulation of the gamma subunit. Neither Na+ ionophores nor inhibitors of cellular Na+ uptake altered the up-regulation of the gamma subunit or JNK activation. Changes in cell cation concentrations driven by incubation in low-K+ medium were effective in up-regulating the alpha1 subunit of Na/K-ATPase but did not have any effect on the gamma subunit. The replacement of NaCl with choline chloride did not down-regulate gamma-subunit expression in cells adapted to hypertonicity. In contrast, the replacement of NaCl with sodium acetate, or pretreatment of cells with the Cl- channel inhibitor 5-nitro-2-(3-phenylpropyl-amino)benzoic acid (NPPB) completely blocked gamma-subunit up-regulation, inhibited JNK activation, and caused a significant decrement in cell survival in hypertonic but not isotonic conditions. In adapted cells, replacement of 300 mosmol/kgH2O NaCl with sodium acetate resulted in down-regulation of the gamma subunit. In conclusion, we describe a Na+-independent, Cl--dependent mechanism for hypertonicity-mediated activation of the JNK and the subsequent synthesis of the gamma subunit of Na/K-ATPase, which are necessary for cellular survival in these anisotonic conditions.

A specific functional interaction between CHIF and Na,K-ATPase: role of FXYD proteins in the cellular regulation of the pump

CHIF (corticosteroid hormone-induced factor) is a member of the FXYD family that shares approximately 50% homology with the gamma subunit of Na,K-ATPase. It is expressed in renal collecting duct and distal colon, and is upregulated by Na(+) deprivation and high K(+) diet. Both CHIF and gamma are coimmunoprecipitated by an anti-alpha subunit antibody, and alpha is immunoprecipitated by anti-gamma and anti-CHIF antibodies. (86)Rb(+) flux experiments in CHIF-transfected HeLa cells demonstrate that CHIF increases the affinity for cytoplasmic Na(+), but does not affect the affinity for extracellular K(Rb). A physiological role of CHIF in kidney function is further elucidated by the phenotypic analysis of CHIF knockout mice. Taken together with data by others, it appears that FXYD proteins are tissue-specific subunits or regulators of the Na,K-ATPase whose function is to adjust the pump kinetics to particular physiological needs.

Immunocytochemical localization of Na,K-ATPase gamma subunit and CHIF in inner medulla of rat kidney

The gamma subunit of Na,K-ATPase and CHIF both belong to the FXYD single-membrane-spanning protein family and have been suggested to have regulatory functions in kidney tubules. CHIF is known to be present in the collecting duct, and gamma has been demonstrated in several segments of the rat kidney tubule, but never clearly in the inner medullary collecting duct (IMCD). Here, we demonstrate the cellular and subcellular localization of the gamma subunit and CHIF in the IMCD in inner medulla by using Western blotting, laser-scanning confocal immunofluorescence, and immunoelectron microscopy. In the initial quarter of the IMCD (next to the outer medulla), antibodies against the C-terminal of gamma as well as splice variant gammaa labeled the basolateral surface of intercalated cells (ICs), while principal cells (PCs) remained unlabeled. In the middle segment of the IMCD, all PCs exhibited distinct basolateral staining for the gammaC-terminal as well as gammaa and CHIF. Immunoelectron microscopy showed that the gammaC-terminal and CHIF were associated with the inner leaflet of the basolateral plasma membrane in the labeled cells. Immunoblotting demonstrated the presence of both the gammaC-terminal and gammaa in inner medullary tissue. However, splice variant gammab was not detected in inner medulla by immunocytochemistry or immunoblotting. The present observations demonstrate that the Na,K-ATPase gamma subunit and CHIF are strategically located in the inner medulla to participate in the fine-tuning of urine ion composition through the regulation of the Na,K-ATPase activity in the IMCD.

Adaptation of murine inner medullary collecting duct (IMCD3) cell cultures to hypertonicity

Recently, we have adapted IMCD3 cell cultures to survive under increasing hypertonic conditions (i.e., 600 and 900 mOsmol/kg H(2)O). In adapted cells, ATPase activity is increased by one order of magnitude, while the expression of the alpha and beta subunit is increased by a factor of 4 to 5 over controls (300 mOsmol/kg H(2)O). Corresponding increases in mRNAs were also detected. The gamma subunit has been described as being uniquely expressed in some areas of the kidney, but never in cell cultures (even those derived from kidney tissues). However, the gamma subunit was detected at the protein and mRNA levels in the adapted IMCD3 cells. In contrast to the alpha and beta subunits, the levels of gamma protein and mRNA expression continue to increase as a function of the media ion concentration. We have also demonstrated that signaling pathways that upregulate the alpha, beta, and gamma subunits are very different. Increasing concentrations of the PI3 kinase inhibitor, LY294002, resulted in a dose-dependent reduction in the expression of the gamma subunit, with total abolition at 10 micro M. However, LY294002 had no significant effect on the expression of the alpha subunit. Inhibition of the JNK2 (but not of the JNK1) pathways by dominant negative transfections abolished the upregulation of the gamma, but not the a subunit. Failure to upregulate the expression of the gamma subunit was associated with a marked decrease in cell viability upon stress.

FXYD proteins: new tissue-specific regulators of the ubiquitous Na,K-ATPase

Maintenance of the Na+ and K+ gradients between the intracellular and extracellular milieus of animal cells is a prerequisite for basic cellular homeostasis and for functions of specialized tissues. The Na,K-ATPase, an oligomeric P-type adenosine triphosphatase (ATPase), is composed of a catalytic alpha subunit and a regulatory beta subunit and is the main player that fulfils these tasks. A variety of regulatory mechanisms are necessary to guarantee appropriate Na,K-ATPase expression and activity adapted to changing physiological demands. Recently, a regulatory mechanism was defined that is mediated by interaction of Na,K-ATPase with small proteins of the FXYD family, which possess a single transmembrane domain and so far have been considered as channels or regulators of ion channels. The mammalian FXYD proteins FXYD1 through FXYD7 exhibit tissue-specific distribution. Phospholemman (FXYD1) in heart and skeletal muscle, the gamma subunit of Na,K-ATPase (FXYD2) and corticosteroid hormone-induced factor (FXYD4, also known as CHIF) in the kidney, and FXYD7 in the brain associate preferentially with the widely expressed Na,K-ATPase alpha1-beta1 isozyme and modulate its transport activity in a way that conforms to tissue-specific requirements. Thus, tissue- and isozyme-specific interaction of Na,K-ATPase with FXYD proteins contributes to proper handling of Na+ and K+ by the Na,K-ATPase, and ensures correct function in such processes as renal Na+-reabsorption, muscle contraction, and neuronal excitability.

A functional interaction between CHIF and Na-K-ATPase: implication for regulation by FXYD proteins

Like the gamma-subunit of Na-K-ATPase, the corticosteroid hormone-induced factor (CHIF) is a member of the FXYD family of one-transmembrane-segment proteins. Both CHIF and two splice variants of gamma, gamma(a) and gamma(b), are expressed in the kidney. Immunolocalization experiments demonstrate mutually exclusive expression of CHIF and gamma in different nephron segments. Specific coimmunoprecipitation experiments demonstrate the existence in kidney membranes of the complexes alpha/beta/gamma(a), alpha/beta/gamma(b), and alpha/beta/CHIF and exclude mixed complexes such as alpha/beta/gamma(a)/gamma(b) and alpha/beta/gamma/CHIF. CHIF has been expressed in HeLa cells harboring the rat alpha(1)-subunit of Na-K-ATPase. (86)Rb flux experiments demonstrate that CHIF induces a two- to threefold increase in apparent affinity for cytoplasmic Na (K'(Na)) but does not affect affinity for extracellular K (Rb) ions (K'(K)) or V(max). Measurements of Na-K-ATPase using isolated membranes show similar but smaller effects of CHIF on K'(Na), whereas K'(K) and K'(ATP) are unaffected. The functional effects of CHIF differ from those of gamma. An implication of these findings is that other FXYD proteins could act as tissue-specific modulators of Na-K-ATPase.