Cell volume regulated transporters of compatible osmolytes

Oocyte-Specific Deletion of Slc6a9 Encoding the GLYT1 Glycine Transporter Eliminates Glycine Transport in Mouse Preimplantation Embryos and Their Ability to Counter Hypertonic Stress

Early preimplantation mouse embryos are sensitive to increased osmolarity, which can block their development. To overcome this, they accumulate organic osmolytes to maintain cell volume. The main organic osmolyte used by early mouse embryos is glycine. Glycine is transported during the mature egg and 1-cell to 4-cell embryo stages by a transporter identified as GLYT1, encoded by the Slc6a9 gene. Here, we have produced an oocyte-specific knockout of Slc6a9 by crossing mice that have a segment of the gene flanked by LoxP elements with transgenic mice expressing iCre driven by the oocyte-specific Gdf9 promoter. Slc6a9 null oocytes failed to develop glycine transport activity during meiotic maturation. However, females with these oocytes were fertile. When enclosed in their cumulus-oocyte complex, Slc6a9 null oocytes could accumulate glycine via GLYT1 transport in their coupled cumulus cells, which may support female fertility in vivo. In vitro, embryos derived from Slc6a9 null oocytes displayed a clear phenotype. While glycine rescued complete preimplantation development of wild type embryos from increased osmolarity, embryos derived from null oocytes failed to develop past the 2-cell stage even with glycine. Thus, Slc6a9 is required for glycine transport and protection against increased osmolarity in mouse eggs and early embryos.

Serine protease 35 regulates the fibroblast matrisome in response to hyperosmotic stress

Hyperosmotic stress occurs in several diseases, but its long-term effects are largely unknown. We used sorbitol-treated human fibroblasts in 3D culture to study the consequences of hyperosmotic stress in the skin. Sorbitol regulated many genes, which help cells cope with the stress condition. The most robustly regulated gene encodes serine protease 35 (PRSS35). Its regulation by hyperosmotic stress was dependent on the kinases p38 and JNK and the transcription factors NFAT5 and ATF2. We identified different collagens and collagen-associated proteins as putative PRSS35 binding partners. This is functionally important because PRSS35 affected the extracellular matrix proteome, which limited cell proliferation. The in vivo relevance of these findings is reflected by the coexpression of PRSS35 and its binding partners in human skin wounds, where hyperosmotic stress occurs as a consequence of excessive water loss. These results identify PRSS35 as a key regulator of the matrisome under hyperosmotic stress conditions.

Time-course changes in the regulation of ions and amino acids in the hard clam Meretrix lusoria upon lower salinity challenge

In this study, we examined ion and amino acid regulation in the gill and mantle of the hard clam Meretrix lusoria. We found that the osmolality and Na⁺ and Cl^- concentrations of hard clam hemolymph were significantly reduced after transferring clams from the salinity of their natural habitat [20‰ saltwater (SW)] to a lower salinity environment (10‰ SW). Specific activities of Na⁺ , K⁺ -ATPase (NKA), which provides the driving force for the secondary ion transport associated with cell osmoregulation in gills and mantles, were unaffected during the acclimation to lower salinity. In contrast, there was a significant decline in the contents of free amino acids (FAAs) in the gills and mantles of hard clams during lower salinity acclimation. Taurine was established to be the dominant FAA, the content of which is considerably higher than that of other FAAs in the hard clam. Following acclimation to the lower salinity environment, mRNA expression of the taurine transporter (TAUT), which plays a pivotal role in regulating intracellular taurine contents, was significantly upregulated in the gill and downregulated in the mantle of hard clams at different time points. However, the relative abundance of TAUT protein in the gill and mantle was significantly increased after transfer from 20‰ SW to 10‰ SW, which may reflect feedback regulation in response to reduced taurine contents in the gill and mantle of hard clams. Collectively, the findings of this study provide important insights on the dynamic processes of ion and amino acid regulation in the peripheral tissues of bivalves.

The C. elegans Hypertonic Stress Response: Big Insights from Shrinking Worms

Cell volume is one of the most aggressively defended physiological set points in biology. Changes in intracellular ion and water concentrations, which are induced by changes in metabolism or environmental exposures, disrupt protein folding, enzymatic activity, and macromolecular assemblies. To counter these challenges, cells and organisms have evolved multifaceted, evolutionarily conserved molecular mechanisms to restore cell volume and repair stress induced damage. However, many unanswered questions remain regarding the nature of cell volume 'sensing' as well as the molecular signaling pathways involved in activating physiological response mechanisms. Unbiased genetic screening in the model organism C. elegans is providing new and unexpected insights into these questions, particularly questions relating to the hypertonic stress response (HTSR) pathway. One surprising characteristic of the HTSR pathway in C. elegans is that it is under strong negative regulation by proteins involved in protein homeostasis and the extracellular matrix (ECM). The role of the ECM in particular highlights the importance of studying the HTSR in the context of a live organism where native ECM-tissue associations are preserved. A second novel and recently discovered characteristic is that the HTSR is regulated at the post-transcriptional level. The goal of this review is to describe these discoveries, to provide context for their implications, and to raise outstanding questions to guide future research.

Factors of the human embryo culture system that may affect media evaporation and osmolality

STUDY QUESTION

Which lab-related factors impact the culture system's capacity to maintain a stable osmolality during human embryo culture?

SUMMARY ANSWER

Incubator humidity, the volume of mineral oil, the type of culture media and the design of time-lapse dishes have been identified as important parameters that can cause an impact on media evaporation and consequently osmolality during culture.

WHAT IS KNOWN ALREADY

Culture medium is a critical component in human embryo culture. Minimizing its evaporation during culture is an adequate strategy to stabilize osmolality and, as a result, improving culture conditions and clinical outcomes.

STUDY DESIGN, SIZE, DURATION

The studied variables included media composition and supplementation; volume of mineral oil; incubator humidification; and the type of dish and incubator used. Additionally, six time-lapse dish models were compared in their ability to prevent evaporation.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Dishes were incubated in parallel to analyze osmolality during culture between groups: synthetic oviductal medium enriched with potassium versus human tubal fluid medium; protein versus no protein supplementation; dry versus humid atmosphere; high versus low volume of mineral oil. Additionally, media evaporation was compared between six models of time-lapse dishes with distinct designs, cultured in a joint incubator. Two of them were retested in their corresponding incubator to analyze the dish-incubator fit. Daily osmolality measurements were compared between groups. Linear regression was performed to analyze evaporation rates.

MAIN RESULTS AND THE ROLE OF CHANCE

Protein supplementation did not significantly affect evaporation. Contrarily, humidity levels inside the incubators, the volume of mineral oil and the type of culture media, played an important role in osmolality stabilization. The design of time-lapse dishes and their recommended preparation protocol heavily influenced their evaporation rates, which were further altered by each incubator's characteristics. Media with initially high osmolalities had a bigger risk of reaching hypertonic levels during culture.

LIMITATIONS, REASONS FOR CAUTION

While numerous, the studied variables are limited and therefore other factors could play a role in osmolality dynamics, as well. Incontrollable atmospheric factors could also result in some variation in the observed results between different centers and laboratories.

WIDER IMPLICATIONS OF THE FINDINGS

Published literature has extensively described how hypertonic media may impair embryo development and negatively affect clinical outcomes; therefore, maintaining a stable osmolality during culture should be considered essential. This work is of interest both for embryologists when analyzing their culture system and methodologies, as well as manufacturers in charge of designing IVF consumables.

STUDY FUNDING/COMPETING INTEREST(S)

This study was privately funded.

TRIAL REGISTRATION NUMBER

N/A.

The Potential Effects of Taurine in Mitigation of Radiation Nephropathy

Taurine (2-aminoethanesulfonic acid) is a sulfur-containing organic acid possessing several important effects, including antioxidant and anti-inflammatory ones. Exposure to ionizing radiation generates free radicals and reactive oxygen species (ROS) in irradiated cells, and free radical generation leads to oxidative stress. It is known that radiation nephropathy includes an inflammation-based process in which ROS and cytokines are responsible. Different doses of explored radiation can cause apoptosis, inflammation and a profound oxidative stress in kidneys. Oxidative stress is involved in renal injury after exposure to both ionizing radiation and inflammation. In this review, we describe the protective effect of taurine against several kidney diseases and the potential effects of taurine in the mitigation of radiation nephropathy. We also report that X-irradiation decreased the expression of taurine and TauT in the kidney. Taurine administration suppressed the decrease in the expression of taurine and TauT in the kidney after radiation exposure. Taurine might contribute to the mitigation of kidney injury induced by radiation.

Effect of Radiation on the Expression of Taurine Transporter in the Intestine of Mouse

There has been a growing interest on the effects of radiation since the Fukushima nuclear power plant accident of 2011. Taurine has been reported to have a radioprotective effect in irradiated mice. However, the detailed mechanism of this radioprotective effect is still awaiting clarification. The aim of this study was to investigation how radiation affects the expression of taurine and to shed light on the mechanism accounting for radioprotective and radiation mitigating effect. Six-week-old male mice were randomly divided into two groups: IR group (7 Gy irradiation) and IR + Tau group (7 Gy irradiation + taurine 3000 mg/kg/day). We examined the survival rate, the expression of taurine and taurine transporter in the small intestine and the urinary taurine concentration. In this study, no statistically significant difference was found in the survival rate between IR Group and IR + Tau Group. Three days and 7 days after irradiation, the urinary taurine concentration of IR + Tau group increased more than that of IR group. Three days and 10 days after irradiation, the expression of taurine and taurine transporter in the small intestine of IR group and IR + Tau group decreased more than that of normal small intestine. It is reported that radiation exposure increases the urinary taurine concentration. We found that the radiation exposure decreases the expression of the taurine transporter in the small intestine of mouse. This finding suggests that a decrease in the expression of the taurine transporter promotes the release of taurine from the tissue into the urine.

Differential localizations of the myo-inositol transporters HMIT and SMIT1 in the cochlear stria vascularis

The cochlear stria vascularis produces endolymph and thereby plays an active role in inner ear homeostasis. We recently reported that the H⁺/myo-inositol cotransporter (HMIT) gene is expressed in the stria vascularis. Here, we examined the protein localization of HMIT and Na⁺/myo-inositol cotransporter 1 (SMIT1) in the stria vascularis by immunohistochemistry. HMIT and SMIT1 were detected in the lateral wall of the cochlear duct. HMIT was widely detected throughout the stria vascularis, while SMIT1 was enriched in the strial basal cells. To examine the localization of HMIT in the stria vascularis in more detail, dissociated strial cells were immunostained, which resulted in the detection of HMIT immunoreactivity in marginal cells. These results indicate that HMIT is expressed in marginal cells and basal cells of the stria vascularis, while SMIT1 expression is enriched in basal cells. We speculate that HMIT and SMIT1 may play important roles in the homeostasis of cochlear fluids, for example by participating in pH regulation and osmoregulation.

Ionic and Amino Acid Regulation in Hard Clam (Meretrix lusoria) in Response to Salinity Challenges

Most marine mollusks are osmoconformers, in that, their body fluid osmolality changes in the direction of the change in environmental salinity. Marine mollusks exhibit a number of osmoregulatory mechanisms to cope with either hypo- or hyperosmotic stress. The effects of changes in salinity on the osmoregulatory mechanisms of the hard clam (Meretrix lusoria, an economically important species of marine bivalve for Taiwan) have not been determined. In this study, we examined the effect of exposure to hypo (10‰)- and hyper (35‰)-osmotic salinity on hard clams raised at their natural salinity (20‰). The osmolality, [Na(+)], and [Cl(-)] of the hard clam hemolymph were changed in the same direction as the surrounding salinity. Further, the contents of total free amino acids including taurine in the gills and mantles were significantly upregulated in hard clam with increasing salinity. The gill Na(+), K(+)-ATPase (NKA) activity, the important enzyme regulating cellular inorganic ions, was not affected by the changed salinity. Mantle NKA activity, however, was stimulated in the 35‰ SW treatment. The taurine transporter (TAUT) is related to the regulation of intracellular contents of taurine, the dominant osmolyte. Herein, a TAUT gene of hard clam was cloned and a TAUT antibody was derived for the immunoblotting. The TAUT mRNA expression of the mantle in hard clam was significantly stimulated in 35‰ SW, but protein expression was not modulated by the changed salinity. In gills of the hard clam with 10‰ SW, both TAUT mRNA and protein expressions were significantly stimulated, and it may reflect a feedback regulation from the decreased gills taurine content under long-term hypoosmotic acclimation. These findings suggest that TAUT expression is regulated differently in gills and mantles following exposure to alterations in environmental salinity. Taken together, this study used the physiological, biochemical and molecular approaches to simultaneously explore the osmoregulation in tissues of hard clam and may further help to understand the osmoregulation in bivalves.

Osmoregulatory inositol transporter SMIT1 modulates electrical activity by adjusting PI(4,5)P2 levels

Myo-inositol is an important cellular osmolyte in autoregulation of cell volume and fluid balance, particularly for mammalian brain and kidney cells. We find it also regulates excitability. Myo-inositol is the precursor of phosphoinositides, key signaling lipids including phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2]. However, whether myo-inositol accumulation during osmoregulation affects signaling and excitability has not been fully explored. We found that overexpression of the Na(+)/myo-inositol cotransporter (SMIT1) and myo-inositol supplementation enlarged intracellular PI(4,5)P2 pools, modulated several PI(4,5)P2-dependent ion channels including KCNQ2/3 channels, and attenuated the action potential firing of superior cervical ganglion neurons. Further experiments using the rapamycin-recruitable phosphatase Sac1 to hydrolyze PI(4)P and the P4M probe to visualize PI(4)P suggested that PI(4)P levels increased after myo-inositol supplementation with SMIT1 expression. Elevated relative levels of PIP and PIP2 were directly confirmed using mass spectrometry. Inositol trisphosphate production and release of calcium from intracellular stores also were augmented after myo-inositol supplementation. Finally, we found that treatment with a hypertonic solution mimicked the effect we observed with SMIT1 overexpression, whereas silencing tonicity-responsive enhancer binding protein prevented these effects. These results show that ion channel function and cellular excitability are under regulation by several "physiological" manipulations that alter the PI(4,5)P2 setpoint. We demonstrate a previously unrecognized linkage between extracellular osmotic changes and the electrical properties of excitable cells.

Role of N-glycosylation in renal betaine transport

The osmolyte and folding chaperone betaine is transported by the renal Na(+)-coupled GABA (γ-aminobutyric acid) symporter BGT-1 (betaine/GABA transporter 1), a member of the SLC6 (solute carrier 6) family. Under hypertonic conditions, the transcription, translation and plasma membrane (PM) insertion of BGT-1 in kidney cells are significantly increased, resulting in elevated betaine and GABA transport. Re-establishing isotonicity involves PM depletion of BGT-1. The molecular mechanism of the regulated PM insertion of BGT-1 during changes in osmotic stress is unknown. In the present study, we reveal a link between regulated PM insertion and N-glycosylation. Based on homology modelling, we identified two sites (Asn(171) and Asn(183)) in the extracellular loop 2 (EL2) of BGT-1, which were investigated with respect to trafficking, insertion and transport by immunogold-labelling, electron microscopy (EM), mutagenesis and two-electrode voltage clamp measurements in Xenopus laevis oocytes and uptake of radiolabelled substrate into MDCK (Madin-Darby canine kidney) and HEK293 (human embryonic kidney) cells. Trafficking and PM insertion of BGT-1 was clearly promoted by N-glycosylation in both oocytes and MDCK cells. Moreover, association with N-glycans at Asn(171) and Asn(183) contributed equally to protein activity and substrate affinity. Substitution of Asn(171) and Asn(183) by aspartate individually caused no loss of BGT-1 activity, whereas the double mutant was inactive, suggesting that N-glycosylation of at least one of the sites is required for function. Substitution by alanine or valine at either site caused a dramatic loss in transport activity. Furthermore, in MDCK cells PM insertion of N183D was no longer regulated by osmotic stress, highlighting the impact of N-glycosylation in regulation of this SLC6 transporter.

Hypotonic activation of the myo-inositol transporter SLC5A3 in HEK293 cells probed by cell volumetry, confocal and super-resolution microscopy

Swelling-activated pathways for myo-inositol, one of the most abundant organic osmolytes in mammalian cells, have not yet been identified. The present study explores the SLC5A3 protein as a possible transporter of myo-inositol in hyponically swollen HEK293 cells. To address this issue, we examined the relationship between the hypotonicity-induced changes in plasma membrane permeability to myo-inositol P_ino [m/s] and expression/localization of SLC5A3. P_ino values were determined by cell volumetry over a wide tonicity range (100–275 mOsm) in myo-inositol-substituted solutions. While being negligible under mild hypotonicity (200–275 mOsm), P_ino grew rapidly at osmolalities below 200 mOsm to reach a maximum of ∼3 nm/s at 100–125 mOsm, as indicated by fast cell swelling due to myo-inositol influx. The increase in P_ino resulted most likely from the hypotonicity-mediated incorporation of cytosolic SLC5A3 into the plasma membrane, as revealed by confocal fluorescence microscopy of cells expressing EGFP-tagged SLC5A3 and super-resolution imaging of immunostained SLC5A3 by direct stochastic optical reconstruction microscopy (dSTORM). dSTORM in hypotonic cells revealed a surface density of membrane-associated SLC5A3 proteins of 200–2000 localizations/μm². Assuming SLC5A3 to be the major path for myo-inositol, a turnover rate of 80–800 myo-inositol molecules per second for a single transporter protein was estimated from combined volumetric and dSTORM data. Hypotonic stress also caused a significant upregulation of SLC5A3 gene expression as detected by semiquantitative RT-PCR and Western blot analysis. In summary, our data provide first evidence for swelling-mediated activation of SLC5A3 thus suggesting a functional role of this transporter in hypotonic volume regulation of mammalian cells.

Connections between preimplantation embryo physiology and culture

PURPOSE

To review the history of experimental embryo culture and how culture media that permitted complete preimplantation development in vitro were first discovered, and the physiological insights gained.

METHODS

This article reviews the history of in vitro mammalian embryo culture, in particular the efforts that led to the current generation of successful culture media and how these reflect embryo physiology, highlighting the contributions of Dr. John D. Biggers and his colleagues and students.

RESULTS

The culture of mammalian embryos began about a century ago. However, defined media without biological fluids were only developed in the late 1950s, and the first live young born from cultured embryos, using these media, were produced by McLaren and Biggers in 1958. It wasn’t until the late 1980s, however, that preimplantation mammalian embryos could generally be cultured in vitro from fertilized eggs to blastocysts. These new media led to insights into embryo physiology, including the importance of cell volume homeostasis to early embryo viability.

CONCLUSIONS

The development of successful preimplantation embryo culture media has had a profound effect on assisted reproduction technologies and on research into early embryo physiology.

Role of ion transport in control of apoptotic cell death

Cell shrinkage is a hallmark and contributes to signaling of apoptosis. Apoptotic cell shrinkage requires ion transport across the cell membrane involving K(+) channels, Cl(-) or anion channels, Na(+)/H(+) exchange, Na(+),K(+),Cl(-) cotransport, and Na(+)/K(+)ATPase. Activation of K(+) channels fosters K(+) exit with decrease of cytosolic K(+) concentration, activation of anion channels triggers exit of Cl(-), organic osmolytes, and HCO3(-). Cellular loss of K(+) and organic osmolytes as well as cytosolic acidification favor apoptosis. Ca(2+) entry through Ca(2+)-permeable cation channels may result in apoptosis by affecting mitochondrial integrity, stimulating proteinases, inducing cell shrinkage due to activation of Ca(2+)-sensitive K(+) channels, and triggering cell-membrane scrambling. Signaling involved in the modification of cell-volume regulatory ion transport during apoptosis include mitogen-activated kinases p38, JNK, ERK1/2, MEKK1, MKK4, the small G proteins Cdc42, and/or Rac and the transcription factor p53. Osmosensing involves integrin receptors, focal adhesion kinases, and tyrosine kinase receptors. Hyperosmotic shock leads to vesicular acidification followed by activation of acid sphingomyelinase, ceramide formation, release of reactive oxygen species, activation of the tyrosine kinase Yes with subsequent stimulation of CD95 trafficking to the cell membrane. Apoptosis is counteracted by mechanisms involved in regulatory volume increase (RVI), by organic osmolytes, by focal adhesion kinase, and by heat-shock proteins. Clearly, our knowledge on the interplay between cell-volume regulatory mechanisms and suicidal cell death is still far from complete and substantial additional experimental effort is needed to elucidate the role of cell-volume regulatory mechanisms in suicidal cell death.

Media composition: salts and osmolality

The main components of embryo culture media are salts, which dissociate into their component inorganic ions in aqueous solution. All embryo culture media contain the same six inorganic ions: Na(+), K(+), Cl(-), Ca(2+), Mg(2+), and SO(4)(2-), while most also contain PO(4)(2-). The salts that are used to formulate embryo culture media can be traced back to classic saline solutions, particularly Krebs-Ringer Bicarbonate (KRB), that were developed for somatic cells in the first half of the twentieth century. The salt and inorganic ion concentrations in the first successful defined mouse embryo culture medium, Whittens medium, were identical to those in KRB. These remained largely unchanged in embryo culture media for decades, with similar levels found in the standard mouse embryo culture medium, M16, formulated in the 1970s. Human embryos were initially cultured in undefined somatic cell media such as Earles and Hams F-10 with serum added. This changed in the mid-1980s, however, with the development of Quinns HTF, a defined medium specifically formulated for human embryo culture, in which the inorganic ion concentrations are similar to those in M16 and Whittens. While these media were useful both for experimental work and clinically, embryos suffered developmental blocks in all of them, with mouse embryos blocking at the 2-cell stage and human embryos at the 4- to 8-cell stage. Starting in the late 1980s, however, mouse embryo culture media were first developed that alleviated these developmental blocks. These media, CZB and KSOM, had much lower osmolalities than previous media, mainly due to lower inorganic ion concentrations. Indeed, lowering total inorganic ion concentration and osmolality proved key to understanding how media that supported complete preimplantation development in vitro can be formulated. A subsequent improvement was the addition of amino acids to culture media for both mouse and human embryos. At least in part, their beneficial effect during the cleavage stages of development is due to the presence in early preimplantation embryos of mechanisms for cell volume regulation that depend on the accumulation of amino acids as organic osmolytes to provide intracellular osmotic support. These amino acids, principally glycine, replace a portion of the intracellular inorganic ions that would otherwise be needed to maintain cell size, preventing the intracellular ionic strength from rising to deleterious levels and blocking development. Thus, the optimum salts levels, osmolality, and amino acid contents of culture media are not independent, but interact strongly because of their roles in cell volume regulation. In the absence of compounds that preimplantation embryos can use as organic osmolytes, embryos will develop only at lower osmolalities and salt concentrations in the medium. However, when organic osmolytes such as some amino acids are present, embryos will develop in culture at higher osmolarities that are similar to those they experience in tubal fluid in vivo.

Effective T-cell recall responses require the taurine transporter Taut

T-cell activation and the subsequent transformation of activated T cells into T-cell blasts require profound changes in cell volume. However, the impact of cell volume regulation for T-cell immunology has not been characterized. Here we studied the role of the cell-volume regulating osmolyte transporter Taut for T-cell activation in Taut-deficient mice. T-cell mediated recall responses were severely impaired in taut(-/-) mice as shown with B16 melanoma rejection and hapten-induced contact hypersensitivity. CD4(+) and CD8(+) T cells were unequivocally located within peripheral lymph nodes of unprimed taut(-/-) mice but significantly decreased in taut(-/-) compared with taut(+/+) mice following in vivo activation. Further analysis revealed that Taut is critical for rescuing T cells from activation-induced cell death in vitro and in vivo as shown with TCR, superantigen, and antigen-specific activation. Consequently, reduction of CD4(+) and CD8(+) T cells in taut(-/-) mice upon antigen challenge resulted in impaired in vivo generation of T-cell memory. These findings disclose for the first time that volume regulation in T cells is an element in the regulation of adaptive immune responses and that the osmolyte transporter Taut is crucial for T-cell survival and T-cell mediated immune reactions.

Taurine: energy drink for T cells

The activation of T cells causes many cellular changes, including alterations in cell morphology, motility, and size. While all immunologists know that T cells increase their size and become "blasted" upon activation, little attention has been paid to the question of how cell size is regulated and how this process influences T-cell responses. In this issue of the European Journal of Immunology, Kaesler et al. [Eur. J. Immunol. 2012. 42: 831-841] demonstrate that the organic osmolyte taurine and its transporter Taut are instrumental in driving cell-volume regulation and therefore the T-cell response. In the absence of Taut, effector and memory T-cell responses in mice are severely impaired, mainly due to increased apoptosis of effector cells. Hence, this paper provides an important link between the regulation of cell size and effector T-cell responses.

Placental TonEBP/NFAT5 osmolyte regulation in an ovine model of intrauterine growth restriction

TonEBP/NFAT5 (the tonicity-responsive enhancer binding protein/nuclear factor of activated T cells) modulates cellular response to osmotic changes by accumulating inositol and sorbitol inside the cells. Our objective was to assess placental osmolytes, TonEBP/NFAT5 RNA and protein expression, and signaling molecules across gestation between control and intrauterine growth restriction (IUGR) ovine pregnancies. Pregnant sheep were placed in hyperthermic conditions to induce IUGR. Placental tissues were collected at 55, 95, and 130 days gestational age (dGA) to measure inositol, sorbitol, TonEBP/NFAT5 (NFAT5), sodium-dependent myo-inositol transporter (SMIT; official symbol SLC5A3), aldose reductase (AR), and NADPH (official symbol DE-CR1). Placental weight was reduced in IUGR compared to controls at 95 and 130 dGA. Osmolyte concentrations were similar between control and IUGR placentas, but both groups demonstrated a significant decrease in inositol concentration and an increase in sorbitol concentration with advancing gestation. Cytosolic NFAT5 protein decreased significantly from 55 to 95 dGA in both groups, and nuclear NFAT5 protein increased only at 130 dGA in the IUGR group, but no differences were seen between groups for either cytosolic or nuclear NFAT5 protein concentrations. DE-CR1 concentrations were similar between groups and increased significantly with advancing gestational age. AR was lowest at 55dGA, and SLC5A3 increased with advancing gestational age. We conclude that both placental osmolytes inositol and sorbitol (and their corresponding proteins SLC5A3 and AR) change with gestational age and are regulated, at least in part, by NFAT5 and DE-CR1 (NADPH). The inverse relationship between each osmolyte across gestation (e.g., inositol higher in early gestation and sorbitol higher in late gestation) may reflect nutritional needs that change across gestation.

Gap junctions favor normal rat kidney epithelial cell adaptation to chronic hypertonicity

Upon hypertonic stress most often resulting from high salinity, cells need to balance their osmotic pressure by accumulating neutral osmolytes called compatible osmolytes like betaine, myo-inositol, and taurine. However, the massive uptake of compatible osmolytes is a slow process compared with other defense mechanisms related to oxidative or heat stress. This is especially critical for cycling cells as they have to double their volume while keeping a hospitable intracellular environment for the molecular machineries. Here we propose that clustered cells can accelerate the supply of compatible osmolytes to cycling cells via the transit, mediated by gap junctions, of compatible osmolytes from arrested to cycling cells. Both experimental results in epithelial normal rat kidney cells and theoretical estimations show that gap junctions indeed play a key role in cell adaptation to chronic hypertonicity. These results can provide basis for a better understanding of the functions of gap junctions in osmoregulation not only for the kidney but also for many other epithelia. In addition to this, we suggest that cancer cells that do not communicate via gap junctions poorly cope with hypertonic environments thus explaining the rare occurrence of cancer coming from the kidney medulla.

NFAT5 regulates T lymphocyte homeostasis and CD24-dependent T cell expansion under pathologic hypernatremia

Immune cells rely on the transcription factor NFAT5 to adapt to hypertonic stress. The hypertonicity-dependent role of NFAT5 in T cells in vivo remains unclear because mouse models of NFAT5 deficiency have produced substantially different T cell phenotypes. In this study, we analyzed the T cell compartment in NFAT5-null and T cell-specific NFAT5 knockout mice. We found that NFAT5-null mice had constitutive, pronounced hypernatremia and suffered a severe immunodeficiency, with T cell lymphopenia, altered CD8 naive/memory homeostasis, and inability to reject allogeneic tumors. By contrast, T cell-specific NFAT5 knockout mice had normal plasma tonicity, rejected allogeneic tumors, and exhibited only a mild, low-penetrance memory bias in CD8 cells. Notably, when T cells from these mice were cultured ex vivo in hypernatremic media, they exhibited features found in NFAT5-null mice, with pronounced naive/memory imbalance and impaired homeostatic survival in response to IL-7, as well as a severe inhibition of their mitogen-induced proliferation. By analyzing surface receptors whose expression might be affected in NFAT5-deficient cells, we identified CD24 as a novel NFAT5 target induced by hypertonicity both in vitro and in vivo, and required to sustain T cell expansion under osmostress. NFAT5 bound to the Cd24 promoter in response to hypertonicity facilitated the local derepression of chromatin and enhanced the expression of CD24 mRNA and protein. Altogether, our results indicate that the systemic hypernatremia of NFAT5-null mice is a major contributor to their immunodeficiency, and highlight the role of NFAT5 and CD24 in the homeostasis of T cells under osmostress in vivo.

Cell volume regulation in oocytes and early embryos: connecting physiology to successful culture media

BACKGROUND

Preimplantation embryos are particularly susceptible to in vitro developmental blocks. These could be alleviated by lowering culture medium osmolarity. Because mammalian cells regulate their volumes by adjusting intracellular osmotic pressure, cell volume regulation could be critical to early embryos.

METHODS

We reviewed the literature on cell volume regulation in preimplantation embryos and the effects of increased osmolarity on embryo development, focusing also on the relation with improvements in embryo culture media.

RESULTS

Embryos failed to develop from fertilized oocytes when osmolarity is increased. This could be alleviated by decreasing osmolarity or including certain compounds such as certain amino acids. Early preimplantation mouse embryos require intracellular accumulation of glycine to provide osmotic support and thus control cell volume. The glycine-specific transporter, GLYT1, mediates osmoregulated glycine accumulation in mouse embryos and likely in human embryos. GLYT1 is activated during meiotic maturation starting at ovulation. Prior to this, oocyte size is not independently controlled but instead is determined by strong adhesion between the oocyte plasma membrane and the inner surface of the zona pellucida.

CONCLUSIONS

Early preimplantation embryos are particularly sensitive to increased osmolarity, and require the importation of glycine to regulate their cell volumes using a mechanism unique to early embryos. Cell volume regulation first appears when ovulation is triggered, oocyte zona pellucida adhesion is released, and glycine transport is activated. The requirement for supporting these physiological functions in oocytes and embryos should be taken into account when developing and improving systems for in vitro oocyte maturation and embryo culture.

Cell volume regulation is initiated in mouse oocytes after ovulation

Fertilized mouse eggs regulate their size principally by accumulating glycine as an intracellular osmolyte using the GLYT1 (SLC6A9) transporter, a mechanism of cell volume homeostasis apparently unique to early embryos before the morula stage. However, nothing was known of cell volume regulation in oocytes before fertilization. We show here that GLYT1 is quiescent in mouse germinal-vesicle-stage oocytes but becomes fully activated within hours after ovulation is triggered. This initiates accumulation of substantial amounts of intracellular glycine in oocytes during meiotic progression, reaching a maximal level in mature eggs. Measurements of endogenous free glycine showed that there were nearly undetectable levels in ovarian germinal-vesicle-stage oocytes, but high levels were present in mature ovulated eggs and in preimplantation embryos through the two-cell stage, but not in morulae. Furthermore, intracellular glycine was regulated in response to changes in external tonicity in eggs and embryos through the two-cell stage, but not in oocytes or embryos after the two-cell stage. Before activation of GLYT1,oocytes were unable to independently regulate their volume. As GLYT1 became active, however, oocyte volume decreased substantially and oocytes gained the ability to regulate their size, which required GLYT1 activity. Before ovulation, oocyte size was instead determined by a strong adhesion to the rigid extracellular matrix of the oocyte, the zona pellucida, which was released coincident with GLYT1 activation. The ability to acutely regulate cell size is thus acquired by the oocyte only after ovulation, when it first develops glycine-dependent cell volume regulation.

SIT1 is a betaine/proline transporter that is activated in mouse eggs after fertilization and functions until the 2-cell stage

Betaine (N,N,N-trimethylglycine) added to culture media is known to substantially improve the development of preimplantation mouse embryos in vitro, and to be imported into 1-cell embryos by a transporter that also accepts proline. Here, we found that the betaine/proline transporter is active in preimplantation mouse embryos only for a short period of development, between the 1- and 2-cell stages. Betaine/proline transport was activated after fertilization, beginning approximately 4 hours post-egg activation and reaching a maximum by approximately 10 hours. One- and 2-cell embryos contained endogenous betaine, indicating that a likely function for the transporter in vivo is the accumulation or retention of intracellular betaine. The appearance of transport activity after egg activation was independent of protein synthesis, but was reversibly blocked by disruption of the Golgi with brefeldin A. We assessed two candidates for the betaine/proline transporter: SIT1 (IMINO; encoded by Slc6a20a) and PROT (Slc6a7). mRNA from both genes was present in eggs and 1-cell embryos. However, when exogenously expressed in Xenopus oocytes, mouse PROT did not transport betaine and had an inhibition profile different from that of the embryonic transporter. By contrast, exogenously expressed mouse SIT1 transported both betaine and proline and closely resembled the embryonic transporter. A morpholino oligonucleotide designed to block translation of SIT1, when present from the germinal vesicle stage, blocked the appearance of betaine transport activity in parthenogenotes. Thus, SIT1 is likely to be a developmentally restricted betaine transporter in mouse preimplantation embryos that is activated by fertilization.

Activation of Ca2+/calmodulin-dependent protein kinase II is involved in hyperosmotic induction of the human taurine transporter

We investigated the signaling pathways participating in hyperosmotic regulation of the human taurine transporter (TAUT) by using specific inhibitors of various intracellular signaling molecules. Among them, the specific inhibitor of calcium/calmodulin-dependent protein kinase II (Ca(2+)/CaM kinase II) completely repressed the hyperosmotic regulation of TAUT. The osmosensitive upregulation of TAUT was also significantly inhibited by calmodulin antagonists and calcium-chelators. The increased expression level of TAUT mRNA by hypertonicity was repressed by the specific Ca(2+)/CaM kinase II inhibitor. The activated form of Ca(2+)/CaM kinase II protein could only be detected in Caco-2 cells under hypertonic conditions.

Fusion-pore expansion during syncytium formation is restricted by an actin network

Cell-cell fusion in animal development and in pathophysiology involves expansion of nascent fusion pores formed by protein fusogens to yield an open lumen of cell-size diameter. Here we explored the enlargement of micron-scale pores in syncytium formation, which was initiated by a well-characterized fusogen baculovirus gp64. Radial expansion of a single or, more often, of multiple fusion pores proceeds without loss of membrane material in the tight contact zone. Pore growth requires cell metabolism and is accompanied by a local disassembly of the actin cortex under the pores. Effects of actin-modifying agents indicate that the actin cortex slows down pore expansion. We propose that the growth of the strongly bent fusion-pore rim is restricted by a dynamic resistance of the actin network and driven by membrane-bending proteins that are involved in the generation of highly curved intracellular membrane compartments.

Ultraviolet A induces transport of compatible organic osmolytes in human dermal fibroblasts

Compatible organic osmolytes, such as betaine, myo-inositol and taurine, are involved in cell protection. Human dermal fibroblasts accumulate these osmolytes and express mRNA specific for their transporting systems betaine-/gamma-amino-n-butyric acid (GABA) transporter (BGT-1), sodium-dependent myo-inositol transporter (SMIT) and taurine transporter (TAUT). Taurine uptake was about sixfold higher than that of betaine and myo-inositol. Compared with normoosmotic (305 mOsm/l) control, hyperosmotic exposure (405 mOsm/l) led to a twofold induction of osmolyte uptake. Ultraviolet A (UVA) upregulated osmolyte transporter mRNA levels and increased osmolyte uptake. Taurine inhibited UVA-induced interleukin-6 (Il-6) mRNA expression by 40%. Furthermore, Il-6 accumulation in the supernatants of UVA-irradiated dermal fibroblasts was much slower when cells were preincubated with taurine. These data indicate that taurine accumulation seems to be part of the fibroblast response to UVA radiation and may protect against UVA-induced Il-6 overexpression.

Mechanisms and significance of cell volume regulation

Survival of human and animal cells requires avoidance of excessive alterations of cell volume. The osmolarity amassed by cellular accumulation of organic substances must be compensated by lowering cytosolic ion concentrations. The Na+/K+ ATPase extrudes Na+ in exchange for K+, which can permeate the cell membrane through K+ channels. K+ exit generates a cell-negative potential difference across the cell membrane, driving the exit of anions such as Cl-. The low cytosolic Cl- concentrations counterbalance the excess cellular osmolarity by organic substances. Cell volume regulation following cell swelling involves releasing ions through activation of K+ channels and/or anion channels, KCl-cotransport, or parallel activation of K+/H+ exchange and Cl-/HCO3- exchange. Cell volume regulation following cell shrinkage involves accumulation of ions through activation of Na+,K+,2Cl- cotransport, Na+/H+ exchange in parallel to Cl-/HCO3- exchange, or Na+ channels. The Na+ taken up is extruded by the Na+/K+ ATPase in exchange for K+. Shrunken cells further accumulate organic osmolytes such as sorbitol and glycerophosphorylcholine, and monomeric amino acids by altered metabolism and myoinositol (inositol), betaine, taurine, and amino acids by Na+ coupled transport. They release osmolytes during cell swelling. Challenges of cell volume homeostasis include transport, hormones, transmitters, and drugs. Moreover, alterations of cell volume participate in the machinery regulating cell proliferation and apoptotic cell death. Deranged cell volume regulation significantly contributes to the pathophysiology of several disorders such as liver insufficiency, diabetic ketoacidosis, hypercatabolism, fibrosing disease, sickle cell anemia, and infection.

Cell volume regulation in the perfused liver of a freshwater air-breathing cat fish Clarias batrachus under aniso-osmotic conditions: roles of inorganic ions and taurine

The roles of various inorganic ions and taurine, an organic osmolyte, in cell volume regulation were investigated in the perfused liver of a freshwater air-breathing catfish Clarias batrachus under aniso-osmotic conditions. There was a transient increase and decrease of liver cell volume following hypotonic (-80 mOsmol/l) and hypertonic (+80 mOsmol/l) exposures,respectively, which gradually decreased/increased near to the control level due to release/uptake of water within a period of 25-30 min. Liver volume decrease was accompanied by enhanced efflux of K+ (9.45 +/- 0.54 micromol/g liver) due to activation of Ba(2+)- and quinidine-sensitive K(+) channel, and to a lesser extent due to enhanced efflux of Cl(-) (4.35+/- 0.25 micromol/g liver) and Na+ (3.68+/- 0.37 micromol/g liver). Conversely, upon hypertonic exposure, there was amiloride-and ouabain-sensitive uptake of K+ (9.78+/- 0.65 micromol/g liver), and also Cl(-) (3.72 +/- 0.25 micromol/g liver).The alkalization/acidification of the liver effluents under hypo-/hypertonicity was mainly due to movement of various ions during volume regulatory processes. Taurine,an important organic osmolyte, appears also to play a very important role in hepatocyte cell volume regulation in the walking catfish as evidenced by the fact that hypo- and hyper-osmolarity caused transient efflux (5.68 +/- 0.38 micromol/g liver) and uptake (6.38 +/- 0.45 micromol/g liver) of taurine, respectively. The taurine efflux was sensitive to 4,4' -di-isothiocyanatostilbene-2,2'-disulphonic acid (DIDS, an anion channel blocker), but the uptake was insensitive to DIDS, thus indicating that the release and uptake of taurine during volume regulatory processes are unidirectional. Although the liver of walking catfish possesses the RVD and RVI mechanisms, it is to be noted that liver cells remain partly swollen and shrunken during anisotonic exposures,thereby possibly causing various volume-sensitive metabolic changes in the liver as reported earlier.

STATc is a key regulator of the transcriptional response to hyperosmotic shock

Background

Dictyostelium discoideum is frequently subjected to environmental changes in its natural habitat, the forest soil. In order to survive, the organism had to develop effective mechanisms to sense and respond to such changes. When cells are faced with a hypertonic environment a complex response is triggered. It starts with signal sensing and transduction and leads to changes in cell shape, the cytoskeleton, transport processes, metabolism and gene expression. Certain aspects of the Dictyostelium osmotic stress response have been elucidated, however, no comprehensive picture was available up to now.

Results

To better understand the D. discoideum response to hyperosmotic conditions, we performed gene expression profiling using DNA microarrays. The transcriptional profile of cells treated with 200 mM sorbitol during a 2-hour time course revealed a time-dependent induction or repression of 809 genes, more than 15% of the genes on the array, which peaked 45 to 60 minutes after the hyperosmotic shock. The differentially regulated genes were applied to cluster analysis and functional annotation using gene GO terms. Two main responses appear to be the down-regulation of the metabolic machinery and the up-regulation of the stress response system, including STATc. Further analysis of STATc revealed that it is a key regulator of the transcriptional response to hyperosmotic shock. Approximately 20% of the differentially regulated genes were dependent on the presence of STATc.

Conclusion

At least two signalling pathways are activated in Dictyostelium cells subjected to hypertonicity. STATc is responsible for the transcriptional changes of one of them.

Taurine deficiency and apoptosis: findings from the taurine transporter knockout mouse

Apoptosis is characterized by cell shrinkage, nuclear condensation, DNA-fragmentation and apoptotic body formation. Compatible organic osmolytes, e.g. taurine, modulate the cellular response to anisotonicity and may protect from apoptosis. Taurine transporter knockout mice (taut-/- mice) show strongly decreased taurine levels in a variety of tissues. They develop clinically important age-dependent diseases and some of them are characterized by apoptosis. Increased photoreceptor apoptosis leads to blindness of taut-/- mice at an early age. The taurine transporter may not be essential for the differentiation of photoreceptor cells, but many mature cells do not survive without an intact taurine transporter. The olfactory epithelium of taut-/- mice also exhibits structural and functional abnormalities. When compared with wild-types, taut-/- mice have a significantly higher proliferative activity of immature olfactory receptor neurons and an increased number of apoptotic cells. This is accompanied by electrophysiological findings indicating a reduced olfactory sensitivity. Furthermore, taut-/- and taut+/- mice develop moderate unspecific hepatitis and liver fibrosis beyond 1 year of age where hepatocyte apoptosis and activation of the CD95 system are pronounced.

A general model for the dynamics of the cell volume

The conservation of the cell volume within values compatible with the overall cell functions represents an ubiquitous property, shared by cells comprising the whole biological world. Water transport across membranes constitutes the main process associated to the dynamics of the cell volume, its chronic and acute regulations therefore represent crucial aspects of cell homeostasis. In spite of the biological diversity, the dynamics of the cell volume exhibits common basic features in the diverse types of cells. The purpose of this study is to show that there is a general model capable to describe the basic aspects of the dynamics of the cell volume. It is demonstrated here that the steady states of this model represent asymptotically stable configurations. As illustrations, several cases of non-polarized (i.e., symmetrical) and polarized (e.g., epithelial) cells performing water transport are shown here to represent particular cases of the general model. From a biological perspective, the existence of a general model for the dynamics of the cell volume reveals that, in spite of physiological and morphological peculiarities, there is a basic common design of the membrane transport processes. In view of its stability properties, this basic design may represent an ancestral property that has proven to be successful regarding the overall homeostatic properties of cells.

Effects of environmental salinity and temperature on osmoregulatory ability, organic osmolytes, and plasma hormone profiles in the Mozambique tilapia (Oreochromis mossambicus)

The Mozambique tilapia, Oreochromis mossambicus, is capable of surviving a wide range of salinities and temperatures. The present study was undertaken to investigate the influence of environmental salinity and temperature on osmoregulatory ability, organic osmolytes and plasma hormone profiles in the tilapia. Fish were acclimated to fresh water (FW), seawater (SW) or double-strength seawater (200% SW) at 20, 28 or 35 degrees C for 7 days. Plasma osmolality increased significantly as environmental salinity and temperature increased. Marked increases in gill Na(+), K(+)-ATPase activity were observed at all temperatures in the fish acclimated to 200% SW. By contrast, Na(+), K(+)-ATPase activity was not affected by temperature at any salinity. Plasma glucose levels increased significantly with the increase in salinity and temperature. Significant correlations were observed between plasma glucose and osmolality. In brain and kidney, content of myo-inositol increased in parallel with plasma osmolality. In muscle and liver, there were similar increases in glycine and taurine, respectively. Glucose content in liver decreased significantly in the fish in 200% SW. Plasma prolactin levels decreased significantly after acclimation to SW or 200% SW. Plasma levels of cortisol and growth hormone were highly variable, and no consistent effect of salinity or temperature was observed. Although there was no significant difference among fish acclimated to different salinity at 20 degrees C, plasma IGF-I levels at 28 degrees C increased significantly with the increase in salinity. Highest levels of IGF-I were observed in SW fish at 35 degrees C. These results indicate that alterations in gill Na(+), K(+)-ATPase activity and glucose metabolism, the accumulation of organic osmolytes in some organs as well as plasma profiles of osmoregulatory hormones are sensitive to salinity and temperature acclimation in tilapia.

Phenotype of the Taurine Transporter Knockout Mouse

This chapter reports present knowledge on the properties of mice with disrupted gene coding for the taurine transporter (taut-/- mice). Study of those mice unraveled some of the roles of taurine and its membrane transport for the development and maintenance of normal organ functions and morphology. When compared with wild-type controls, taut-/- mice have decreased taurine levels in skeletal and heart muscle by about 98%, in brain, kidney, plasma, and retina by 80 to 90%, and in liver by about 70%. taut-/- mice exhibit a lower body mass as well as a strongly reduced exercise capacity compared with taut+/- and wild-type mice. Furthermore, taut-/- mice show a variety of pathological features, for example, subtle derangement of renal osmoregulation, changes in neuroreceptor expression, and loss of long-term potentiation in the striatum, and they develop clinically relevant age-dependent disorders, for example, visual, auditory, and olfactory dysfunctions, unspecific hepatitis, and liver fibrosis. Taurine-deficient animal models such as acutely dietary-manipulated foxes and cats, pharmacologically induced taurine-deficient rats, and taurine transporter knockout mouse are powerful tools allowing identification of the mechanisms and complexities of diseases mediated by impaired taurine transport and taurine depletion (Chapman et al., 1993; Heller-Stilb et al., 2002; Huxtable, 1992; Lake, 1993; Moise et al., 1991; Novotny et al., 1991; Pion et al., 1987; Timbrell et al., 1995; Warskulat et al., 2004, 2006b). Taurine, which is the most abundant amino acid in many tissues, is normally found in intracellular concentrations of 10 to 70 mmol/kg in mammalian heart, brain, skeletal muscle, liver, and retina (Chapman et al., 1993; Green et al., 1991; Huxable, 1992; Timbrell et al., 1995). These high taurine levels are maintained by an ubiquitous expression of Na(+)-dependent taurine transporter (TAUT) in the plasma membrane (Burg, 1995; Kwon and Handler, 1995; Lang et al., 1998; Liu et al., 1992; Ramamoorthy et al., 1994; Schloss et al., 1994; Smith et al., 1992; Uchida et al., 1992; Vinnakota et al., 1997; Yancey et al., 1975). Taurine is not incorporated into proteins. It is involved in cell volume regulation, neuromodulation, antioxidant defense, protein stabilization, stress responses, and via formation of taurine-chloramine in immunomodulation (Chapman et al., 1993; Green et al., 1991; Huxtable, 1992; Timbrell et al., 1995). On the basis of its functions, taurine may protect cells against various types of injury (Chapman et al., 1993; Green et al., 1991; Huxtable, 1992; Kurz et al., 1998; Park et al., 1995; Stapleton et al., 1998; Timbrell et al., 1995; Welch and Brown, 1996; Wettstein and Häussinger, 1997). In order to examine the multiple taurine functions, murine models have several intrinsic advantages for in vivo research compared to other animal models, including lower cost, maintenance, and rapid reproduction rate. Further, experimental reagents for cellular and molecular studies are widely available for the mouse. In particular, mice can be easily genetically manipulated by making transgene and knockout mice. This chapter focuses on the phenotype of the TAUT-deficient murine model (taut-/-; Heller-Stilb et al., 2002), which may help researchers elucidate the diverse roles of taurine in development and maintenance of normal organ functions and morphology.

Chronic hyperglycemia modulates osteoblast gene expression through osmotic and non-osmotic pathways

Insulin dependent diabetes mellitus (IDDM; type I) is a chronic disease stemming from little or no insulin production and elevated blood glucose levels. IDDM is associated with osteoporosis and increased fracture rates. The mechanisms underlying IDDM associated bone loss are not known. Previously we demonstrated that osteoblasts exhibit a response to acute (1 and 24 h) hyperglycemia and hyperosmolality. Here we examined the influence of chronic hyperglycemia (30 mM) and its associated hyperosmolality on osteoblast phenotype. Our findings demonstrate that osteoblasts respond to chronic hyperglycemia through modulated gene expression. Specifically, chronic hyperglycemia increases alkaline phosphatase activity and expression and decreases osteocalcin, MMP-13, VEGF and GAPDH expression. Of these genes, only MMP-13 mRNA levels exhibit a similar suppression in response to hyperosmotic conditions (mannitol treatment). Acute hyperglycemia for a 48-h period was also capable of inducing alkaline phosphatase and suppressing osteocalcin, MMP-13, VEGF, and GAPDH expression in differentiated osteoblasts. This suggests that acute responses in differentiated cells are maintained chronically. In addition, hyperglycemic and hyperosmotic conditions increased PPARgamma2 expression, although this increase reached significance only in 21 days chronic glucose treated cultures. Given that osteocalcin is suppressed and PPARgamma2 expression is increased in type I diabetic mouse model bones, these findings suggest that diabetes-associated hyperglycemia may modulate osteoblast gene expression, function and bone formation and thereby contribute to type I diabetic bone loss.

The organic osmolytes betaine and proline are transported by a shared system in early preimplantation mouse embryos

Betaine and proline protect preimplantation mouse embryos against increased osmolarity and decreased cell volume, implying that they may function as organic osmolytes. However, the transport system(s) that mediates their accumulation in fertilized eggs and early embryos was unknown, and previously identified mammalian organic osmolyte transporters could not account for their transport. Here, we report that there is a single saturable transport component shared by betaine and proline in 1-cell mouse embryos. A series of inhibitors had nearly identical effects on both betaine and proline transport by this system. In addition, K(i) values for reciprocal inhibition of betaine and proline transport were approximately 100-300 microM, similar to K(m) values ( approximately 200-300 microM) for their transport, and both had similar maximal transport rates (V(max)). The K(i) values for inhibition of betaine and proline transport by dimethylglycine were similar ( approximately 2 mM), further supporting transport of both substrates by a single transport system. Finally, betaine and proline transport each required Na(+)- and Cl(-). These data were consistent with a single, Na(+)- and Cl(-)-requiring, betaine/proline transport system in 1-cell mouse embryos. While betaine was only transported by a single saturable system, we found an additional, less conspicuous proline transport route that was betaine-insensitive, Na(+)-sensitive, and inhibited by alanine, leucine, cysteine, and methionine. Furthermore, we showed that betaine, like proline, is present in the mouse oviduct and thus could serve as a physiological substrate. Finally, accumulation of both betaine and proline increased with increasing osmolarity, consistent with a possible role as organic osmolytes in early embryos.

Impaired ability to increase water excretion in mice lacking the taurine transporter gene TAUT

Cellular taurine uptake or release counteracts alterations of cell volume. Na+-coupled taurine transporter TAUT mediates concentrative cellular uptake of taurine. Inhibition of vasopressin secretion by hypotonicity may involve taurine release from glial cells of supraoptic nucleus. We compared renal function of mice lacking TAUT (taut-/-) and wild-type littermates (taut+/+). We observed renal taurine loss and subsequent hypotaurinemia in taut-/- mice. With free access to water, plasma and urine osmolality, urinary flow rate as well as urinary excretion and plasma concentrations of Na+ and K+ were similar in taut-/- and taut+/+ mice, whereas plasma concentrations of urea were enhanced in taut-/- mice. An oral water load (1 ml/16 g body weight) induced a similar diuresis in both genotypes. Repeating the oral water load immediately after normalization of urine flow rate, however, resulted in delayed diuresis and higher urinary vasopressin/creatinine ratios in taut-/- mice. In comparison, the repeated diuretic response to vasopressin V2 receptor blockade was not different between genotypes. Water deprivation for 36 h led to similar antidiuresis and increases of urinary osmolality in both genotypes. Upon free access to water after deprivation, taut-/- mice continued to concentrate urine up to 6 days, while taut+/+ mice rapidly returned to normal urinary osmolality. Urinary vasopressin/creatinine ratios and plasma aldosterone concentrations were not different under basal conditions but were significantly higher in taut-/- mice than in taut+/+ mice at 6 days after water deprivation. In conclusion, taut-/- mice suffer from renal taurine loss and impaired ability to lower urine osmolality and to increase urinary water excretion. The latter defect could reside extrarenally and result from a role of taurine in the suppression of vasopressin release which may be attenuated in taut-/- mice.

Regulation of intracellular glycine as an organic osmolyte in early preimplantation mouse embryos

GLYT1, a glycine transporter belonging to the neurotransmitter transporter family, has recently been identified as a novel cell volume-regulatory mechanism in the earliest stages of the mouse preimplantation embryo. It apparently acts by regulating the steady-state intracellular concentration of glycine, which functions as an organic osmolyte in embryos, to balance external osmolarity and thus maintain cell volume. GLYT1 in embryos was the first mammalian organic osmolyte transporter identified that appears to function in cell volume control under conditions of normal osmolarity, rather than being a response to the stress of chronic hypertonicity. Its maximal rate of transport was shown to be regulated by osmolarity. However, it was not known whether this osmotic regulation of the rate of glycine transport is sufficient to account for the observed control of steady-state intracellular glycine levels as a function of osmolarity in embryos. Here, we show that the intracellular accumulation of glycine in embryos is a direct function of the rate of glycine uptake via GLYT1. In addition, we have shown that the rate of efflux, likely via the volume-regulated anion and organic osmolyte channel in embryos, is also under osmotic regulation and contributes substantially to the control of steady-state glycine concentrations. Together, control of both the rate of uptake and rate of efflux of glycine underlies the mechanism of osmotic regulation of the steady-state concentration of glycine and hence cell volume in early embryos.

Effect of ammonium on the expression of osmosensitive genes in Madin-Darby canine kidney cells

The cells of the kidney medulla are exposed routinely to high extracellular concentrations of various solutes including NaCl, urea and ammonium (NH4+). Although it is well established that the expression of a variety of osmosensitive genes and proteins, which confer cytoprotection on renal medullary cells, is induced by high NaCl concentrations, the role of NH4+ in these cellular responses is unclear. This study thus addressed the effect of NH4+ on the expression of the betaine/GABA transporter (BGT-1), the sodium/myo-inositol cotransporter (SMIT), aldose reductase (AR), and heat shock protein 70 (HSP70) in Madin-Darby canine kidney (MDCK) cells, using Northern and Western blot analyses and enzyme-linked immunosorbent assay (ELISA). The incidence of apoptosis was monitored by determining caspase-3 activity and annexin V binding. Addition of NH4Cl (50 mM; total osmolality 400 mosmol (kg H2O)(-1) to the medium was more effective than equiosmolar NaCl in increasing BGT-1 and HSP70 mRNA abundance, but less effective in enhancing BGT-1 and HSP70 expression at the protein level. Qualitatively similar results were obtained for SMIT and AR mRNAs. Exposure to both isotonic and hypertonic, NH4Cl-containing medium enhanced apoptosis compared with equiosmolar, NaCl-containing media. These results suggest that, in addition to NaCl, NH4Cl may play a role in regulating the intracellular accumulation of organic osmolytes, the abundance of HSP70 and cell turnover in the renal medulla in vivo.

Osmotic regulation of cell function and possible clinical applications

Inflammation and immunosuppression can cause acute respiratory distress syndrome, multiple organ failure, and sepsis, all of which are lethal posttraumatic complications in trauma patients. Prevention of the inflammation and immunosuppression has been a main focus of trauma researcher for many years. Recently, hypertonic resuscitation has attracted attention as a possible therapeutic approach to counteract such deleterious immune responses in trauma patients. We have begun to understand how hypertonic fluids affect immune cell signaling, and a number of experimental and clinical studies have started to reveal valuable information on the clinical efficacy and the limitations of hypertonic resuscitation fluids. Knowledge of how osmotic cues regulate immune cell function will enable us to fully exploit the clinical potential of hypertonic resuscitation to reduce inflammatory and anergic complications in trauma patients.

Ammonium affects tight junctions and the cytoskeleton in MDCK cells

In the kidney medulla, tubule cells are exposed not only to elevated NaCl but also to high NH(4)Cl concentrations. Although it is well known that long-term exposure to high NaCl concentrations leads to reorganization of the actin-based cytoskeleton and to altered transport properties of renal epithelial cells, there have been no comparable studies on the effects of elevated extracellular NH(4)Cl concentrations. We therefore examined the effect of prolonged (up to 72 h) exposure of Madin-Darby canine kidney (MDCK) cells to increased NH(4)Cl concentrations on the actin-based cytoskeleton, the transepithelial electrical resistance (TER) and the expression and intracellular distribution of the tight junction protein occludin. NH(4)Cl exposure resulted in rarefaction of cytoplasmic stress fibres, formation of intense peripheral actin bands and reduced abundance of both F- and G-actin. While under control conditions occludin staining was restricted to the tight junction region, ample dot-like intracellular staining was apparent after NH(4)Cl exposure. These changes in cell structure were associated with an increase in TER and the enhanced expression of an additional putative, 40-kDa occludin isoform. Exposure to elevated extracellular NH(4)Cl concentrations thus leads to distinct alterations in the architecture and transepithelial transport properties of MDCK cells that may also be relevant for the tubule cells of the renal inner medulla.

The Osmolyte Strategy of Normal Human Keratinocytes in Maintaining Cell Homeostasis

Compatible organic osmolytes, such as betaine, myoinositol, and taurine, are involved in cell volume homeostasis as well as in cell protection, for example, against oxidative stress. This so-called osmolyte strategy requires the expression of specific osmolyte transporting systems such as the betaine/gamma-amino-n-butyric acid (GABA) transporter, the sodium-dependent myoinositol transporter and the taurine transporter (TAUT). In contrast to liver, kidney, and neural cells, nothing is known about osmolytes in the skin. Here we report that primary normal human keratinocytes (NHK) express mRNA specific for the betaine/GABA transporter, for the sodium-dependent myoinositol transporter and for the TAUT. In comparison to normoosmotic (305 mosmol per L) controls, a 3-5-fold induction of mRNA expression for the betaine/GABA-, the sodium-dependent myoinositol- and the TAUT was observed within 6-24 h after hyperosmotic exposure (405 mosmol per L). Expression of osmolyte transporters was associated with an increased uptake of radiolabeled osmolytes. Conversely, hypoosmotic (205 mosmol per L) stimulation induced significant efflux of these osmolytes. Exposure to ultraviolet B (290-315 nm) or ultraviolet A (340-400 nm) radiation, which are major sources of oxidative stress in skin, significantly stimulated osmolyte uptake. Increased osmolyte uptake was associated with upregulation of mRNA steady-state levels for osmolyte transporters in irradiated cells. These studies demonstrate that NHK possess an osmolyte strategy, which is important for their capacity to maintain cell volume homeostasis and seems to be part of their response to UV radiation.