Role of TASK2 potassium channels regarding volume regulation in primary cultures of mouse proximal tubules

Several papers reported the role of TASK2 channels in cell volume regulation and regulatory volume decrease (RVD). To check the possibility that the TASK2 channel modulates the RVD process in kidney, we performed primary cultures of proximal convoluted tubules (PCT) and distal convoluted tubules (DCT) from wild-type and TASK2 knockout (KO) mice. In KO mice, the TASK2 coding sequence was in part replaced by the lac-Z gene. This allows for the precise localization of TASK2 in kidney sections using beta-galactosidase staining. TASK2 was only localized in PCT cells. K+ currents were analyzed by the whole-cell clamp technique with 125 mM K-gluconate in the pipette and 140 mM Na-gluconate in the bath. In PCT cells from wild-type mice, hypotonicity induced swelling-activated K+ currents insensitive to 1 mM tetraethylammonium, 10 nM charybdotoxin, and 10 microM 293B, but blocked by 500 microM quinidine and 10 microM clofilium. These currents were increased in alkaline pH and decreased in acidic pH. In PCT cells from TASK2 KO, swelling-activated K+ currents were completely impaired. In conclusion, the TASK2 channel is expressed in kidney proximal cells and could be the swelling-activated K+ channel responsible for the cell volume regulation process during osmolyte absorptions in the proximal tubules.     

On the theory of apoptotic decrease of the cell volume

The dynamic model of membrane transport, which describes the changing of ion contents in the cell, cell volume and membrane potential, for the first time, is applied to analysis of the apoptotic processes. It is shown that increasing of permeability of K+, and Cl(-)-channels, decreasing of permeability of Na+ together with degradation of Na+/K+ pump, KCC and NC cotransporters lead to decreasing of cell U937 volume and plasma membrane depolarization at apoptosis induced by staurosporine in concentration 1 microM. The experimental data using at calculations was published in paper (Yurinskaya et al., 2010).     

Role of potassium channels in chlorogenic acid-induced apoptotic volume decrease and cell cycle arrest in Candida albicans

BackgroundChlorogenic acid (CRA) is an abundant phenolic compound in the human diet. CRA has a potent antifungal effect, inducing cell death in Candida albicans. However, there are no further studies to investigate the antifungal mechanism of CRA, associated with ion channels.MethodsTo evaluate the inhibitory effects on CRA-induced cell death, C. albicans cells were pretreated with potassium and chloride channel blockers, separately. Flow cytometry was carried out to detect several hallmarks of apoptosis, such as cell cycle arrest, caspase activation, and DNA fragmentation, after staining of the cells with SYTOX green, FITC-VAD-FMK, and TUNEL.ResultsCRA caused excessive potassium efflux, and an apoptotic volume decrease (AVD) was observed. This change, in turn, induced cytosolic calcium uptake and cell cycle arrest in C. albicans. Moreover, CRA induced caspase activation and DNA fragmentation, which are considered apoptotic markers. In contrast, the potassium efflux and proapoptotic changes were inhibited when potassium channels were blocked, whereas there was no inhibitory effect when chloride channels were blocked.ConclusionsCRA induces potassium efflux, leading to AVD and G2/M cell cycle arrest in C. albicans. Therefore, potassium efflux via potassium channels regulates the CRA-induced apoptosis, stimulating several apoptotic processes.General significanceThis study improves the understanding of the antifungal mechanism of CRA and its association with ion homeostasis, thereby pointing to a role of potassium channels in CRA-induced apoptosis.     

Regulatory volume increase (RVI) and apoptotic volume decrease (AVD) in U937 cells in hypertonic medium

Changes in intracellular water, K⁺ and Na⁺ of U937 cells incubated in hyperosmolar medium supplemented with 200 mM sucrose have been studied. Cells were stained with acrydine orange, ethydium bromide, APOPercentage dye, which marks the phosphatidyl serine distribution on the plasma membrane; and FLICA polycaspase fluorescent dye. It was found that cell shrinkage produced by direct osmotic effect induced both a regulatory volume increase and apoptotic volume decrease. The regulatory volume increase dominated at the early stage, whereas apoptotic volume decrease prevailed at the later stage. Therefore, U937 cells were capable of triggering apoptosis and apoptotic volume decrease, despite the unimpaired regulatory volume increase response, and the current opinion that the dysfunction of the regulatory volume increase is a prerequisite for apoptosis and apoptotic volume decrease (Subramanyam et al., 2010) should be revised. It is concluded that the apoptotic volume decrease plays a significant role in preventing osmotic lysis in apoptotic cells, rather than in initiating apoptosis.     

Regulatory volume increase (RVI) and apoptotic volume decrease (AVD) in U937 cells placed into hyperosmotic medium

Time course of changes in intracellular water, K+ and Na+ of U937 cells incubated in hyperosmolar medium with addition of 200 mM sucrose was studied. Ouabain-sensitive and ouabain-resistant Rb+ (K+) influxes were measured during regulatory cell volume increase (RVI) and apoptotic volume decrease (AVD). Microscopy of cells stained by Acrydine orange, Ethydium bromide, APOPercenrage Dye and polycaspase marker FLICA was performed. We found that initial osmotic cell shrinkage induced both RVI and AVD responses. RVI dominated at the early stage whereas AVD prevailed at the later stage. In view of the data obtained in U937 cells the current opinion that RVI "dysfunction" is a prerequisite for apoptosis and AVD (Subramanyam et al., 2010) should be revised. U937 cells are capable to trigger of apoptosis and AVD in spite of the unimpaired RVI response. It is concluded that AVD plays a significant role in preventing osmotic lysis of apoptotic cells rather than in the initiation of apoptosis.     

Staurosporine-induced cell death in salmonid cells: the role of apoptotic volume decrease, ion fluxes and MAP kinase signaling

Apoptotic cell death in mammalian models is frequently associated with cell shrinkage. Inhibition of apoptotic volume decrease (AVD) is cytoprotective, suggesting that cell shrinkage is an important early event in apoptosis. In salmonid hepatoma and gill cells staurosporine induced apoptosis, as assessed by activation of effector caspases, nuclear condensation, and a decrease of mitochondrial membrane potential (MMP), and these changes were accompanied by cell shrinkage. The Cl⁻ transport inhibitor DIDS and the K⁺ channel inhibitor quinidine prevented AVD, but only DIDS inhibited apoptosis. Other Cl⁻ flux inhibitors, as well as a pan-caspase inhibitor, did not prevent cell shrinkage, but still prevented caspase activation. Furthermore, regulatory volume decrease (RVD) under hypotonic conditions was not facilitated, but diminished in apoptotic cells. Since all transport inhibitors used blocked RVD, but only DIDS and quinidine inhibited AVD, the ion transporters involved in both processes are apparently not identical. In addition, our data indicate that inhibition of Cl⁻ fluxes rather than blocking cell shrinkage or K⁺ fluxes is important for preventing apoptosis. In line with this, inhibition of MAP kinases reduced RVD and not AVD, but still diminished caspase activation. Finally, we observed that MAP kinases were activated upon staurosporine treatment and that at least activation of ERK was prevented when AVD was inhibited.     

Regulatory volume decrease in cultured kidney cells (A6): role of amino acids

Volume regulation was studied in A6 epithelia grown on permeable supports by measuring cell thickness (Tc) while simultaneously recording short circuit current (ISC) and transepithelial conductance (Gt). Lowering the tonicity of the basolateral solution (pi b) from 250 or 215 to 140 mOsm/kg elicited a rapid rise in Tc followed by a regulation of the cell volume towards control. This decrease in Tc displays the characteristics of the regulatory volume decrease (RVD). Upon restoring the isoosmotic conditions, Tc decreased rapidly below its control value. A post RVD regulatory volume increase (RVI) as described for other cell types was not observed. The subsequent reduction of the basolateral osmolality increased Tc to the level recorded at the end of the first hypoosmotic pulse. Because cell content was not altered during the isoosmotic period the second hypoosmotic challenge was isotonic with the cell and did therefore not evoke an RVD. However, the cell did not lose its ability to volume regulate since an RVD could be elicited by further reduction of pi b from 140 to 100 mOsm/kg. The possibility of an involvement of amino acids in the RVD was tested. The amount of amino acids in the cell as well as excreted in the bath was determined by amino acid analysis. Millimolar concentrations of threonine, serine, alanine, glutamate, glycine and aspartate were found in the cell extract. The cellular amino acid concentration was 28.8 +/- 0.4 mM. The amounts of glycine, aspartate and glutamate excreted from the cell during the hypotonic treatment were significantly larger than in control conditions. The excretion of these amino acids during hypotonicity decreased the cellular amino acid concentration by 8.4 +/- 0.2 mM. This quantity cannot completely account for the RVD during the first hypotonic challenge. The addition of glycine, aspartate and glutamate to the bathing solutions, although used at concentrations higher than intracellularly, did not reduce RVD. On the contrary, this maneuver increased the amplitude of the RVD following both hypoosmotic pulses. This result suggests a stimulatory role of the amino acids on the processes responsible for the RVD.     

Induction of apoptosis by nitric oxide in macrophages is independent of apoptotic volume decrease

Apoptosis occurs through a sequence of specific biochemical and morphological alterations that define the progress of cell death. The changes of the mitochondrial inner membrane potential (DeltaPsi(m)), the release of cytochrome c to the cytosol, the apoptotic volume decrease (AVD) and the activation of caspases have been measured in RAW 264.7, HeLa and Jurkat T cells incubated with molecules that induce apoptosis through the mitochondrial pathway. Our data show that NO, staurosporine, etoposide and camptothecin increased DeltaPsi(m) in macrophages but not in HeLa and Jurkat cells, that exhibited a DeltaPsi(m) decrease. Moreover, the apoptosis induced by NO in macrophages, but not that promoted by staurosporine, might occur in the absence of AVD. Analysis of the sequence of apoptotic manifestations shows that DeltaPsi(m) precedes AVD and caspase activation in RAW 264.7 cells. Inhibition of AVD abrogates apoptosis in HeLa and Jurkat T cells regardless of the stimuli used. These data suggest that the changes of DeltaPsi(m) are cell-type dependent and that AVD is dispensable for apoptosis in macrophages.     

Regulatory volume decrease in human esophageal epithelial cells

In vivo human esophageal epithelial cells are regularly exposed to hyposmolal stress. This stress, however, only becomes destructive when the surface epithelial cell (barrier) layers are breached and there is contact of the hyposmolal solution with the basolateral cell membranes. The present investigation was designed to examine the effects of hyposmolal stress in the latter circumstance using as a model for human esophageal epithelial cells the noncancer-derived HET-1A cell line. Cell volume and the response to hyposmolal stress in suspensions of HET-1A cells were determined by cell passage through a Coulter Counter Multisizer II. HET-1A cells behaved as osmometers over the range of 280 to 118 mosmol/kg H(2)O with rapid increases in cell volume < or = 15-20% above baseline. Following swelling, the cells exhibited regulatory volume decrease (RVD), restoring baseline volume within 30 min, despite continued hyposmolal stress. With the use of pharmacologic agents and ion substitutions, RVD appeared to result from rapid activation of parallel K(+) and Cl(-) conductance pathways and this was subsequently joined by activation of a KCl cotransporter. Exposure to hyposmolal stress in an acidic environment, pH 6.6, inhibited, but did not abolish, RVD. These data indicate that human esophageal epithelial cells can protect against hyposmolal stress by RVD and that the redundancy in mechanisms may, to some extent, serve as added protection in patients with reflux disease when hyposmolal stress may occur in an acidic environment.     

Role of caspase 2 in apoptotic signaling in primate and murine germ cells

This study investigates the role of caspase 2 in apoptotic signaling of nonhuman primate male germ cells triggered by mild testicular hyperthermia, testosterone (T(e)) implants, or by combined interventions. Mean incidence of germ cell apoptosis increased significantly by Day 3 in the heat (H(e)) alone group and by Day 8 in the Te alone group but peaked at Day 3 in H(e) + T(e) group. We found activation of caspase 2 in both germ cells and Sertoli cells after induction of apoptosis. Most notably, active caspase 2 immunoreactivity was detected only in those germ cells susceptible to apoptosis compared with controls, where little or no such staining is detected. To further explore the role of caspase 2 in regulating male germ cell death, we next evaluated the efficacy of caspase 2 inhibition in preventing or attenuating heat-induced germ cell apoptosis in rats. Caspase 2 inhibition significantly (P < 0.05) prevented such heat-induced germ cell apoptosis. The protection offered by the caspase 2 inhibitor occurred upstream of mitochondria, involving suppression of mitogen-activated protein kinase (MAPK) 14 activation and inducible nitric oxide synthase (NOS2) induction and, in turn, suppression of cytochrome c-mediated death pathway. Together, our results show that caspase 2 is activated in male germ cells undergoing apoptosis in nonhuman primates after heat stress, hormonal deprivation, or after combined interventions. Blockade of caspase 2 activation prevents heat-induced germ cell apoptosis in rats by suppressing the MAPK14- and NO-mediated intrinsic pathway signaling.     

NMR measurement of intracellular water volume in rat kidney proximal tubules

Knowledge of cell water volume is essential for the measurement of concentrations of intracellular ions and metabolites in kidney proximal tubules. We have developed a method which utilizes 35Cl-NMR as a measure of extracellular volume and 2H-NMR, in combination with a membrane-impermeable shift-reagent [Dy-DTPA]2-, as a measure of the ratio of intra- and extracellular water volumes. Measurement of extracellular volume by 35Cl-NMR is possible, since the resonance of intracellular 35Cl is too broad to be detectable in kidney cells. The 2H-NMR measurement exploits the fact that only extracellular water is in direct contact with [Dy-DTPA]2-. However, rapid exchange of water across the cell membrane results in only a single 2H2O resonance at a chemical shift which is a weighted average of the shifted extra- and unshifted intracellular water resonances. Expression of the extracellular volume as a fraction of the total volume by fCl and as a fraction of the total water-volume by fD, permits the calculation of the fractional cell-water content fw = [(1/fD)-1]/[(1/fCl)-1]. This approach was applied to proximal tubular suspensions prepared from the rat kidney. The water content was found to be 76.9 +/- 1.8% (n = 6) at 37 degrees C. Increasing extracellular osmolality from 295 to 390 mOsm/kg H2O, by addition of mannitol, decreased the water content by 21%. Our results are in good agreement with those obtained by the gravimetric method.     

Live fluorescence and transmission-through-dye microscopic study of actinomycin D-induced apoptosis and apoptotic volume decrease

The effect of actinomycin D on HeLa cells was studied by live fluorescence and transmission-through-dye microscopy—a recently developed technique that permits volume measurements in live cells. In particular, it is well suited for the observation and quantification of the apoptotic volume decrease (AVD), which is widely viewed as an essential feature of apoptosis. The main results from our study are as follows. (1) Apoptosis caused in HeLa cells by actinomycin D proceeds in two morphologically distinct stages: the early stage is characterized by extensive blebbing, and the late stage by a more compact shape. The loss of mitochondrial membrane potential occurs at about the same time as blebbing, and chromatin condensation follows 30–90 min later. Caspase-3 and 7 become activated during the late stage. (2) Because blebbing occurs before activation of caspase-3, it has to be initiated by a different mechanism. Although blebbing is one of the earliest observable changes, it can be selectively inhibited without affecting other apoptotic reactions. (3) The majority of cells experience a temporary volume increase after the appearance of blebs. Eventually, AVD takes over and the cells shrink by approximately 40 % of their initial volume; the volume loss becomes noticeable at the end of the blebbing phase and continues through the late stage. Sometimes, at the end of long incubations, shrinkage gives way to swelling, possibly indicating secondary necrosis. (4) Both early and late apoptosis are accompanied by intracellular accumulation of Na⁺, while low-sodium medium prevents apoptosis. Except for a partial protective effect of quinine, all of the tested blockers of Na⁺, K⁺ and Cl^? channels failed to prevent apoptosis or AVD.     

Calcium-dependent control of volume regulation in renal proximal tubule cells: I. Swelling-Activated Ca2+ entry and release

Summary The mechanism of Ca²⁺-dependent control of hypotonic cell volume regulation was investigated in the isolated, nonperfused renal proximal straight tubule. When proximal tubules were exposed to hypotonic solution with 1 mM Ca²⁺, cells swelled rapidly and then underwent regulatory volume decrease (RVD). This treatment resulted in an increase in intracellular free calcium concentration ([Ca²⁺]_i) by a mechanism that had two phases: the first was a transient increase from baseline (136 nM) to a peak (413 nM) that occurred in the first 15–20 sec, but was followed by a rapid decay toward the pre-swelling levels. The second phase was characterized by a sustained elevation of [Ca²⁺]_i above the baseline (269 nM), which was maintained over several minutes. The dependence of these two phases on extracellular Ca²⁺ was determined. Reduction of bath [Ca²⁺] to 10 or 1 M partially diminished the transient phase, but abolished the sustained phase completely, such that [Ca²⁺]_i fell below the base-line levels during RVD. It was concluded that the transient increase resulted predominantly from swelling-activated release of intracellular Ca²⁺ stores and that the sustained phase was due to swelling-activated Ca²⁺ entry across the plasma membrane. Ca²⁺ entry probably also contributed to the transient increase in [Ca²⁺]_i. The time dependence of swelling-activated Ca²⁺ entry was also investigated, since it was previously shown that RVD was characterized by a calcium window period (<60 sec). during which extracellular Ca²⁺ was required. Outside of this time period, RVD would inactivate and could not be reactivated by subsequent addition of Ca²⁺. It was found that the Ca²⁺ permeability did not inactivate over several minutes, indicating that the temporal dependence of RVD on extracellular Ca²⁺ is not due to the transient activation of a Ca²⁺ entry pathway.     

Involvement of regulatory volume decrease in the migration of nasopharyngeal carcinoma cells

The transwell chamber migration assay and CCD digital camera imaging techniques were used to investigate the relationship between regulatory volume decrease (RVD) and cell migration in nasopharyngeal carcinoma cells (CNE-2Z cells). Both migrated and non-migrated CNE-2Z cells, when swollen by 47% hypotonic solution, exhibited RVD which was inhibited by extracellular application of chloride channel blockers adenosine 5‘-triphosphate (ATP), 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and tamoxifen. However, RVD rate in migrated CNE-2Z cells was bigger than that of non-migrated cells and the sensitivity of migrated cells to NPPB and tamoxifen was higher than that of nonmigrated cells. ATP, NPPB and tamoxifen also inhibited migration of CNE-2Z cells. The inhibition of migration was positively correlated to the blockage of RVD, with a correlation coefficient (r) = 0.99, suggesting a functional relationship between RVD and cell migration. We conclude that RVD is involved in cell migration and RVD may play an important role in migratory process in CNE-2Z cells.     

Regulation of gluconeogenesis in swine kidney proximal tubule cells

Summary Proximal tubule cells were isolated from swine kidney and cultured for periods of more than 30 days. The cells formed confluent monolayers after plating on a collagen surface and they were passaged more than 5 times on this matrix. The cells maintain several metabolic functions of proximal tubule cells, including gluconeogenesis and the ability to respond to epinephrine and parathyroid hormone. Gluconeogenesis, a principal metabolic pathway in proximal tubule cells, was examined as a function of days in culture. The isolated cells showed a nearly constant rate of gluconeogenesis from ¹⁴C-lactate, ¹⁴C-alkaine and 14C-glycerol with no significant loss of activity for at least 30 days in culture. Likewise, the activities of several cytosolic and membrane associated enzymes including, alkaline phosphatase, -glutamyltransferase, fructose-1,6-bisphosphatase and phosphofructokinase were nearly constant over the same time period.The cells responded to treatment with epinephrine and parathyroid hormone, and the rate of gluconeogenesis from ¹⁴C-lactate doubled in the presence of these hormones. The morphological and biochemical evidence obtained in these studies show that the proximal tubule cells isolated from swine kidney provide an excellent well defined system for studying the hormonal regulation of carbohydrate metabolism in this tissue.Abbreviations PTH Parathyroid Hormone - cAMP cyclic 3,5-adenosine Monophosphate     

Plasma membrane aquaporin activity can affect the rate of apoptosis but is inhibited after apoptotic volume decrease

Apoptosis is characterized by a conserved series of morphological events beginning with the apoptotic volume decrease (AVD). This study investigated a role for aquaporins (AQPs) during the AVD. Inhibition of AQPs blocked the AVD in ovarian granulosa cells undergoing growth factor withdrawal and blocked downstream apoptotic events such as cell shrinkage, changes in the mitochondrial membrane potential, DNA degradation, and caspase-3 activation. The effects of AQP inhibition on the AVD and DNA degradation were consistent in thymocytes and with two additional apoptotic signals, thapsigargin and C₆-ceramide. Overexpression of AQP-1 in Chinese hamster ovary (CHO-AQP-1) cells enhanced their rate of apoptosis. The AVD is driven by loss of K⁺ from the cell, and we hypothesize that after the AVD, AQPs become inactive, which halts further water loss and allows K⁺ concentrations to decrease to levels necessary for apoptotic enzyme activation. Swelling assays on granulosa cells, thymocytes, and CHO-AQP-1 cells revealed that indeed, the shrunken (apoptotic) subpopulation has very low water permeability compared with the normal-sized (nonapoptotic) subpopulation. In thymocytes, AQP-1 is present and was shown to colocalize with the plasma membrane receptor tumor necrosis factor receptor-1 (TNF-R1) both before and after the AVD, which suggests that this protein is not proteolytically cleaved and remains on the cell membrane. Overall, these data indicate that AQP-mediated water loss is important for the AVD and downstream apoptotic events, that the water permeability of the plasma membrane can control the rate of apoptosis, and that inactivation after the AVD may help create the low K⁺ concentration that is essential in apoptotic cells. Furthermore, inactivation of AQPs after the AVD does not appear to be through degradation or removal from the cell membrane. water movement; major intrinsic protein; channel; enzyme     

Extracellular ATP stimulates volume decrease in Necturus red blood cells

This study examined whether extracellular ATP stimulatesregulatory volume decrease (RVD) in Necturusmaculosus (mudpuppy) red blood cells (RBCs). Thehemolytic index (a measure of osmotic fragility) decreased withextracellular ATP (50 µM). In contrast, the ATP scavenger hexokinase(2.5 U/ml, 1 mM glucose) increased osmotic fragility. In addition, theATP-dependent K⁺ channelantagonist glibenclamide (100 µM) increased the hemolytic index, andthis inhibition was reversed with ATP (50 µM). We also measured cellvolume recovery in response to hypotonic shock electronically with aCoulter counter. Extracellular ATP (50 µM) enhanced cell volumedecrease in a hypotonic (0.5×) Ringer solution. In contrast, hexokinase (2.5 U/ml) and apyrase (an ATP diphosphohydrolase, 2.5 U/ml)inhibited cell volume recovery. The inhibitory effect of hexokinase wasreversed with the Ca²⁺ ionophoreA-23187 (1 µM); it also was reversed with the cationophore gramicidin(5 µM in a choline-Ringer solution), indicating that ATP was linkedto K⁺ efflux. In addition,glibenclamide (100 µM) and gadolinium (10 µM) inhibited cell volumedecrease, and the effect of these agents was reversed with ATP (50 µM) and A-23187 (1 µM). Using the whole cell patch-clamp technique,we found that ATP (50 µM) stimulated a whole cell current underisosmotic conditions. In addition, apyrase (2.5 U/ml), glibenclamide(100 µM), and gadolinium (10 µM) inhibited whole cell currents thatwere activated during hypotonic swelling. The inhibitory effect ofapyrase was reversed with the nonhydrolyzable analog adenosine5'-O-(3-thiotriphosphate) (50 µM), and the effect of glibenclamide or gadolinium was reversed withATP (50 µM). Finally, anionic whole cell currents were activated withhypotonic swelling when ATP was the only significant charge carrier,suggesting that increases in cell volume led to ATP efflux through aconductive pathway. Taken together, these results indicate thatextracellular ATP stimulated cell volume decrease via aCa²⁺-dependent step that led toK⁺ efflux.