Volume regulation following hyposmotic shock in isolated turbot (Scophthalmus maximus) hepatocytes

Comparative transcriptomic analysis reveals mechanisms of divergence in osmotic regulation of the turbot Scophthalmus maximus

The turbot Scophthalmus maximus has evolved extensive physiological ability to adapt to multiple environmental salinities. The morphological changes of the kidney indicated the adaptability difference and similarity of turbot to salinity stress. Identify transcriptome-wide differences between low-salinity seawater (LSW, salinity 5)- and high-salinity seawater (HSW, salinity 50)-acclimated kidneys of turbot to decipher the osmotic regulation mechanism. We identified 688 differentially expressed genes (DEGs) in the LSW-acclimated kidneys and 2441 DEGs in the HSW-acclimated kidneys of turbot compared with seawater-acclimated kidneys, respectively. We investigated three patterns of gene regulation to salinity stress that involved in ion channels and transporters, functions of calcium regulation, organic osmolytes, energy demand, cell cycle regulation, and cell protection. Additionally, protein-protein interaction (PPI) analysis of DEGs suggested the presence of a frequent functional interaction pattern and that crucial genes in the PPI network are involved in hyper-osmotic regulation. Based on the analysis of comparative transcriptome data and related literature reports, we conclude that the mechanisms responsible for osmotic regulation and its divergence in turbot are related to various genes that are involved in canonical physiological functions. These findings provide insight into the divergence in osmoregulation of turbot and valuable information about osmoregulation mechanisms that will benefit other studies in this field.

Combined effects of high hydrostatic pressure and dispersed oil on the metabolism and the mortality of turbot hepatocytes (Scophthalmus maximus)

Since the DeepWater Horizon oil spill and the use at 1450 m depth of dispersant as a technical response, the need of relevant ecotoxicological data on deep-sea ecosystems becomes crucial. In this context, this study focused on the effect of high hydrostatic pressure (10.1 MPa) on turbot hepatocytes isolated from fish exposed either to chemically dispersed oil, mechanically dispersed oil or dispersant alone. Potential combined effects of oil/dispersant and hydrostatic pressure, were assessed on cell mortality (total cell death, necrosis and apoptosis), cell viability and on hepatocyte oxygen consumption (MO₂). No change in cell mortality was observed in any of the experimental conditions, whereas, the results of cell viability showed a strong and significant increase in the two oil groups independently of the pressure exposure. Finally, oil exposure and hydrostatic pressure have additive effects on oxygen consumption at a cellular level. Presence of dispersant prevent any MO₂ increase in our experimental conditions. These mechanistic effects leading to this increased energetic demand and its eventual inhibition by dispersant must be investigated in further experiments.

Oyster's cells regulatory volume decrease: A new tool for evaluating the toxicity of low concentration hydrocarbons in marine waters

Human activities require fossil fuels for transport and energy, a substantial part of which can accidentally or voluntarily (oil spillage) flow to the marine environment and cause adverse effects in human and ecosystems' health. This experiment was designed to estimate the suitability of an original cellular biomarker to early quantify the biological risk associated to hydrocarbons pollutants in seawater. Oocytes and hepatopancreas cells, isolated from oyster (Crassostrea gigas), were tested for their capacity to regulate their volume following a hypo-osmotic challenge. Cell volumes were estimated from cell images recorded at regular time intervals during a 90min-period. When exposed to diluted seawater (osmolalities from 895 to 712mosmkg(-1)), both cell types first swell and then undergo a shrinkage known as Regulatory Volume Decrease (RVD). This process is inversely proportional to the magnitude of the osmotic shock and is best fitted using a first-order exponential decay model. The Recovered Volume Factor (RVF) calculated from this model appears to be an accurate tool to compare cells responses. As shown by an about 50% decrease in RVF, the RVD process was significantly inhibited in cells sampled from oysters previously exposed to a low concentration of diesel oil (8.4mgL(-1) during 24h). This toxic effect was interpreted as a decreased permeability of the cell membranes resulting from an alteration of their lipidic structure by diesel oil compounds. In contrast, the previous contact of oysters with diesel did not induce any rise in the gills glutathione S-transferase specific activity. Therefore, this work demonstrates that the study of the RVD process of cells selected from sentinel animal species could be an alternative bioassay for the monitoring of hydrocarbons and probably, of various chemicals in the environment liable to alter the cellular regulations. Especially, given the high sensitivity of this biomarker compared with a proven one, it could become a relevant and accurate tool to estimate the biological hazards of micropollutants in the water.

Desiccation resistance in embryos of the killifish, Fundulus heteroclitus: single embryo measurements

Northern killifish, Fundulus heteroclitus macrolepidotus, spawn in estuaries at high tides. Embryos may be stranded in air at stream margins as the water recedes. These aerially incubated embryos are exposed to desiccation stress and may survive and develop normally to hatching at ∼14 days post-fertilization (dpf). We developed a technique to quantitatively measure the kinetics of water loss at various developmental stages from single embryos in controlled relative humidities (RHs). Embryos were able to tolerate short periods (2 hr) of severe desiccation and survive to hatching. Mid-stage (7 dpf) embryos showed the highest degree of desiccation tolerance compared to early-stage (2 dpf) and late-stage (14 dpf) embryos. We classified the patterns of water loss into four phases, the perivitelline space (PVS) phase, the resistance phase, the desiccation phase, and the equilibration phase. In the PVS phase, water loss was rapid at all developmental stages and all RHs (∼25% of total embryo weight). The water loss rate was slower during the resistance phase. It decreased as RH increased and length of this phase was longer in mid-stage than in early- and late-stage embryos. The water loss rate and length of the desiccation phase also depended on RH. These data support the hypothesis that low permeability embryonic compartment surface membranes retard water loss significantly and promote prolonged survival of these embryos during desiccation. We also show this mechanism cannot completely account for the survival of severely desiccated embryos (especially in 23% RH) and that there must also be complementary cellular responses.

Regulatory volume response following hypotonic stress in Atlantic salmon erythrocytes

The purpose of this study was to examine regulatory volume decrease (RVD) in Atlantic salmon red blood cells (RBCs). Osmotic fragility was determined optically, mean cell volume was measured electronically, and changes in intracellular Ca(2+) concentration were visualized using fluorescence microscopy and fluo-4-AM. Cells displayed an increase in osmotic fragility and an inhibition of volume recovery following hypotonic shock when they were exposed to a high taurine Ringer or when placed in a high K(+) medium. Interestingly, RVD in cells from fish collected during the summer depended more on taurine efflux, whereas fall cells relied more on the loss of K(+). In addition, RVD in fall cells was prevented with the K(+) channel inhibitor quinine, whereas the ionophore gramicidin decreased osmotic fragility and potentiated volume recovery. Further, hypotonic shock (0.5X Ringer) for both summer and fall cells caused an increase in cytosolic Ca(2+), which resulted from influx of this ion because it was not observed when extracellular Ca(2+) was chelated with EGTA (10 nM free Ca(2+)). Cells exposed to a low Ca(2+) hypotonic Ringer also had a greater osmotic fragility and failed to recover from hypotonic swelling. Finally, inhibition of phospholipase A(2) with ONO-RS-082 blocked volume recovery. In conclusion, Atlantic salmon RBCs displayed volume decrease in response to hypotonic shock, which depended on a swelling-induced influx of Ca(2+) and an increase in the efflux of K(+) and taurine.

Potentiation of regulatory volume decrease by a p2-like receptor and arachidonic acid in american alligator erythrocytes

This study examined the role of a P2 receptor and arachidonic acid (AA) in regulatory volume decrease (RVD) by American alligator red blood cells (RBCs). Osmotic fragility was determined optically, mean cell volume was measured by electronic sizing, and changes in intracellular Ca(2+) concentration were visualized using fluorescence microscopy. Gadolinium (50 μM), hexokinase (2.5 U/ml), and suramin (100 μM) increased osmotic fragility, blocked volume recovery after hypotonic shock, and prevented a rise in intracellular Ca(2+) that normally occurs during cell swelling. The P2X antagonists PPADS (50 μM) and TNP-ATP (10 μM) also increased fragility and inhibited volume recovery. In contrast, ATPγS (10 μM), α,β-methylene-ATP (50 μM) and Bz-ATP (50 μM) had the opposite effect, whereas 2-methylthio-ATP (50 μM) and UTP (10 μM) had no effect. In addition, the phospholipase A(2) (PLA(2)) inhibitors ONO-RS-082 (10 μM), chlorpromazine (10 μM), and isotetrandrine (10 μM) increased osmotic fragility and blocked volume recovery, whereas AA (10 μM) and its nonhydrolyzable analog eicosatetraynoic acid (ETYA, 10 μM) had the reverse effect. Further, AA (10 μM), but not ATPγS (10 μM), prevented the inhibitory effect of a low Ca(2+)-EGTA Ringer on RVD, whereas both AA (10 μM) and ATPγS (10 μM) caused cell shrinkage under isosmotic conditions. In conclusion, our results are consistent with the presence of a P2-like receptor whose activation stimulated RVD. In addition, AA also was important for volume recovery.

Regulatory volume decrease and P receptor signaling in fish cells: mechanisms, physiology, and modeling approaches

For animal cell plasma membranes, the permeability of water is much higher than that of ions and other solutes, and exposure to hyposmotic conditions almost invariably causes rapid water influx and cell swelling. In this situation, cells deploy regulatory mechanisms to preserve membrane integrity and avoid lysis. The phenomenon of regulatory volume decrease, the partial or full restoration of cell volume following cell swelling, is well-studied in mammals, with uncountable investigations yielding details on the signaling network and the effector mechanisms involved in the process. In comparison, cells from other vertebrates and from invertebrates received little attention, despite of the fact that e.g. fish cells could present rewarding model systems given the diversity in ecology and lifestyle of this animal group that may be reflected by an equal diversity of physiological adaptive mechanisms, including those related to cell volume regulation. In this review, we therefore present an overview on the most relevant aspects known on hypotonic volume regulation presently known in fish, summarizing transporters and signaling pathways described so far, and then focus on an aspect we have particularly studied over the past years using fish cell models, i.e. the role of extracellular nucleotides in mediating cell volume recovery of swollen cells. We, furthermore, present diverse modeling approaches developed on the basis of data derived from studies with fish and other models and discuss their potential use for gaining insight into the theoretical framework of volume regulation.

Cell volume regulation following hypotonic shock in hepatocytes isolated from Sparus aurata

The response of isolated hepatocytes of Sparus aurata to hypotonic shock was studied by the aid of videometric and light scattering methods. The isolated cells exposed to a rapid change (from 370 to 260 mOsm/kg) of the osmolarity of the bathing solution swelled but thereafter underwent a decrease of cell volume tending to recovery the original size. This homeostatic response RVD (regulatory volume decrease) was inhibited in the absence of extracellular Ca²+ and in the presence of TMB8, an inhibitor of Ca²+ release from intracellular stores. It is likely that Ca²+ entry through verapamil sensitive Ca²+-channels, probably leading to a release of Ca²+ from intracellular stores, is responsible for RVD since the blocker impaired the ability of the cell to recover its volume after the hypotonic shock. RVD tests performed in the presence of various inhibitors of different transport mechanisms, such as BaCl₂, quinine, glybenclamide and bumetanide as well as in the presence of a KCl activator, NEM, led us to suggest that the recovery of cell volume in hypotonic solution is accomplished by an efflux of K+ and Cl⁻ through conductive pathways paralleled by the operation of the KCl cotransport, followed by an obliged water efflux from the cells.

Activation of the MAPKs ERK1/2 by cell swelling in turbot hepatocytes

BACKGROUND INFORMATION

Activation of MAPKs (mitogen-activated protein kinases), in particular ERK1/2 (extracellular-signal-regulated kinase 1/2), has been reported to take place in a large variety of cell types after hypo-osmotic cell swelling. Depending on cell type, ERK1/2 phosphorylation can then serve or not the RVD (regulatory volume decrease) process. The present study investigates ERK1/2 activation after aniso-osmotic stimulations in turbot hepatocytes and the potential link between phosphorylation of these proteins and RVD.

RESULTS

In turbot hepatocytes, Western-blot analysis shows that a hypo-osmotic shock from 320 to 240 mOsm kg(-1) induced a rapid increase in ERK1/2 phosphorylation, whereas a hyper-osmotic shock from 320 to 400 mOsm kg(-1) induced no significant change in the phosphorylation of these proteins. The hypo-osmotic-induced ERK1/2 phosphorylation was significantly prevented when hypo-osmotic shock was performed in the presence of the specific MEK (MAPK/ERK kinase) inhibitor PD98059 (100 microM). In these conditions, the RVD process was not altered, suggesting that ERK1/2 did not participate in this process in turbot hepatocytes. Moreover, the hypo-osmotic-induced activation of ERK1/2 was significantly prevented by breakdown of extracellular ATP by apyrase (10 units ml(-1)), by inhibition of purinergic P2 receptors by suramin (100 microM) or by calcium depletion using EGTA (1 mM) and thapsigargin (1 microM).

CONCLUSIONS

In turbot hepatocytes, hypo-osmotic swelling but not hyper-osmotic shrinkage induced the activation of ERK1/2. However, these proteins do not seem to be involved in the RVD process. Their hypo-osmotic-induced activation is partially due to cascades of signalling events triggered by the binding of released ATP on purinergic P2 receptors and requires the presence of calcium.

Involvement of respiratory chain in the regulatory volume decrease process in turbot hepatocytes

Regulatory volume decrease (RVD) constitutes a fundamental process that turbot (Scophthalmus maximus) hepatocytes are able to perform when exposed to hypo-osmotic stress. RVD is an integrative mechanism that involves various elements of the cellular machinery. Among others, ATP is an essential protagonist: released following hypo-osmotic shock, it acts as an auto/paracrine factor to trigger other signalling events. The origin of this ATP remains unclear and, to the best of our knowledge, no information exists about the role of mitochondrial respiration in RVD. Therefore, we propose to analyse the potential link between RVD and the respiratory chain, with a focus on ATP release and exocytosis. Using inhibitors of mitochondrial respiration, RVD was shown to be dependent on respiratory chain activity. However, we demonstrated an indirect role of mitochondrial respiration: ATP could be synthesized and then stored in intracellular vesicles until the moment cells release it to face hypo-osmotic swelling. However, the involvement of exocytosis in this process needs to be further investigated.

Amiodarone pretreatment of organ donors exerts anti-oxidative protection but induces excretory dysfunction in liver preservation and reperfusion

The continuous shortage of organs necessitates the use of marginal organs from donors with various diseases, including arrhythmia-associated cardiac failure. One of the most frequently used anti-arrhythmic drugs is amiodarone (AM), which is given in particular in emergency situations. Apart from its anti-arrhythmic actions, AM provides anti-oxidative properties in cardiomyocytes. Thus, we were interested in whether AM donor pretreatment affects the organ quality and function of livers procured for preservation and transplantation. Donor rats were pretreated with AM (5 mg/kg of body weight) 10 minutes before flush-out of the liver with a cold (4 degrees C) histidine-tryptophan-ketoglutarate solution (n = 8). Livers were then stored for 24 hours at 4 degrees C before ex situ reperfusion with a 37 degrees C Krebs-Henseleit solution for 60 minutes in a nonrecirculating system. At the end of reperfusion, tissue samples were taken for histology and Western blot analysis. Animals with vehicle only (0.9% NaCl) served as ischemia/reperfusion controls (n = 8). Additionally, livers of untreated animals (n = 8) not subjected to 24 hours of cold ischemia served as sham controls. AM pretreatment effectively attenuated lipid peroxidation, stress protein expression, and apoptotic cell death. This was indicated by an AM-mediated reduction of malondialdehyde, heme oxygenase-1, and caspase-3 activation. However, AM treatment also induced mitochondrial damage and hepatocellular excretory dysfunction, as indicated by a significantly increased glutamate dehydrogenase concentration in the effluate and decreased bile production. In conclusion, AM donor pretreatment exerts anti-oxidative actions in liver preservation and reperfusion. However, these protective AM actions are counteracted by an induction of mitochondrial damage and hepatocellular dysfunction. Accordingly, AM pretreatment of donors for anti-arrhythmic therapy should be performed with caution.

Osmotic stress sensing and signaling in fishes

In their aqueous habitats, fish are exposed to a wide range of osmotic conditions and differ in their abilities to respond adaptively to these variations in salinity. Fish species that inhabit environments characterized by significant salinity fluctuation (intertidal zone, estuaries, salt lakes, etc.) are euryhaline and able to adapt to osmotic stress. Adaptive and acclimatory responses of fish to salinity stress are based on efficient mechanisms of osmosensing and osmotic stress signaling. Multiple osmosensors, including calcium sensing receptor likely act in concert to convey information about osmolality changes to downstream signaling and effector mechanisms. The osmosensory signal transduction network in fishes is complex and includes calcium, mitogen-activated protein kinase, 14-3-3 and macromolecular damage activated signaling pathways. This network controls, among other targets, osmosensitive transcription factors such as tonicity response element binding protein and osmotic stress transcription factor 1, which, in turn, regulate the expression of genes involved in osmotic stress acclimation. In addition to intracellular signaling mechanisms, the systemic response to osmotic stress in euryhaline fish is coordinated via hormone- and paracrine factor-mediated extracellular signaling. Overall, current insight into osmosensing and osmotic stress-induced signal transduction in fishes is limited. However, euryhaline fish species represent excellent models for answering critical emerging questions in this field and for elucidating the underlying molecular mechanisms of osmosensory signal transduction.

Effects of hyposmotic stress on exocytosis in isolated turbot, Scophthalmus maximus, hepatocytes

The effect of hyposmotic shock on exocytosis was examined in isolated hepatocytes of turbot, a marine flatfish, using the molecular probe FM1-43. Sudden exposure to a reduced osmolality caused an increase in cell exocytic activity related to the osmotic gradient between intra- and extracellular fluids. Cytoskeletal microtubules could contribute to this hyposmotic-induced exocytosis since colchicine inhibited the process. Protein kinase C, phosphatidylinositol-3 kinase, phospholipases A2, C and D could constitute key enzymes in the mechanism since their inhibition by specific agents altered the hyposmotic-induced exocytic activity. Moreover, arachidonic acid and derivates from the 5-lipoxygenase pathway as well as calcium could participate in the process. As regulatory volume decrease (RVD) exhibited by turbot hepatocytes following hyposmotic stimulation involves similar features, a potential role of exocytosis in volume regulation is suggested. In particular, exocytosis could serve RVD by contributing to ATP release since this latter process similarly appeared to be phospholipase D-dependent and related to the osmotic gradient. This study provides the first evidence of a volume-sensitive exocytosis that could aim at volume constancy in a marine teleost fish cell type.