Molecular basis for osmoregulation of organic osmolytes in renal medullary cells

Betaine as a Functional Ingredient: Metabolism, Health-Promoting Attributes, Food Sources, Applications and Analysis Methods

Betaine is a non-essential amino acid with proven functional properties and underutilized potential. The most common dietary sources of betaine are beets, spinach, and whole grains. Whole grains-such as quinoa, wheat and oat brans, brown rice, barley, etc.-are generally considered rich sources of betaine. This valuable compound has gained popularity as an ingredient in novel and functional foods due to the demonstrated health benefits that it may provide. This review study will provide an overview of the various natural sources of betaine, including different types of food products, and explore the potential of betaine as an innovative functional ingredient. It will thoroughly discuss its metabolic pathways and physiology, disease-preventing and health-promoting properties, and further highlight the extraction procedures and detection methods in different matrices. In addition, gaps in the existing scientific literature will be emphasized.

Metabolomic Analysis in Neurocritical Care Patients

Metabolomics is the analytical study of metabolites in biological matrices using high-throughput profiling. Traditionally, the metabolome has been studied to identify various biomarkers for the diagnosis and pathophysiology of disease. Over the last decade, metabolomic research has grown to include the identification of prognostic markers, the development of novel treatment strategies, and the prediction of disease severity. In this review, we summarized the available evidence on the use of metabolome profiling in neurocritical care populations. Specifically, we focused on aneurysmal subarachnoid hemorrhage, traumatic brain injury, and intracranial hemorrhage to identify the gaps in the current literature and to provide direction for future studies. A primary literature search of the Medline and EMBASE databases was conducted. Upon removing duplicate studies, abstract screening and full-text screening were performed. We screened 648 studies and extracted data from 17 studies. Based on the current evidence, the utility of metabolomic profiling has been limited due to inconsistencies amongst studies and a lack of reproducible data. Studies identified various biomarkers for diagnosis, prognosis, and treatment modification. However, studies evaluated and identified different metabolites, resulting in an inability to compare the study results. Future research towards addressing the gaps in the current literature, including reproducing data on the use of specific metabolite panels, is needed.

Osmoregulatory Role of Betaine and Betaine/γ-Aminobutyric Acid Transporter 1 in Post-Traumatic Syringomyelia

Syringomyelia (SM) is primarily characterized by the formation of a fluid-filled cyst that forms in the parenchyma of the spinal cord following injury or other pathology. Recent omics studies in animal models have identified dysregulation of solute carriers, channels, transporters, and small molecules associated with osmolyte regulation during syrinx formation/expansion in the spinal cord. However, their connections to syringomyelia etiology are poorly understood. In this study, the biological functions of the potent osmolyte betaine and its associated solute carrier betaine/γ-aminobutyric acid (GABA) transporter 1 (BGT1) were studied in SM. First, a rat post-traumatic SM model was used to demonstrate that the BGT1 was primarily expressed in astrocytes in the vicinity of syrinxes. In an in vitro system, we found that astrocytes uptake betaine through BGT1 to regulate cell size under hypertonic conditions. Treatment with BGT1 inhibitors, especially NNC 05-2090, demonstrated midhigh micromolar range potency in vitro that reversed the osmoprotective effects of betaine. Finally, the specificity of these BGT1 inhibitors in the CNS was demonstrated in vivo, suggesting feasibility for targeting betaine transport in SM. In summary, these data provide an enhanced understanding of the role of betaine and its associated solute carrier BGT1 in cell osmoregulation and implicates the active role of betaine and BGT1 in syringomyelia progression.

Distribution and function of the muscarinic receptor subtypes in the cardiovascular system

Muscarinic acetylcholine receptors belong to the G protein-coupled receptor superfamily and are widely known to mediate numerous functions within the central and peripheral nervous system. Thus, they have become attractive therapeutic targets for various disorders. It has long been known that the parasympathetic system, governed by acetylcholine, plays an essential role in regulating cardiovascular function. Unfortunately, due to the lack of pharmacologic selectivity for any one muscarinic receptor, there was a minimal understanding of their distribution and function within this region. However, in recent years, advancements in research have led to the generation of knockout animal models, better antibodies, and more selective ligands enabling a more thorough understanding of the unique role muscarinic receptors play in the cardiovascular system. These advances have shown muscarinic receptor 2 is no longer the only functional subtype found within the heart and muscarinic receptors 1 and 3 mediate both dilation and constriction in the vasculature. Although muscarinic receptors 4 and 5 are still not well characterized in the cardiovascular system, the recent generation of knockout animal models will hopefully generate a better understanding of their function. This mini review aims to summarize recent findings and advances of muscarinic involvement in the cardiovascular system.

The Development of Sugar-Based Anti-Melanogenic Agents

The regulation of melanin production is important for managing skin darkness and hyperpigmentary disorders. Numerous anti-melanogenic agents that target tyrosinase activity/stability, melanosome maturation/transfer, or melanogenesis-related signaling pathways have been developed. As a rate-limiting enzyme in melanogenesis, tyrosinase has been the most attractive target, but tyrosinase-targeted treatments still pose serious potential risks, indicating the necessity of developing lower-risk anti-melanogenic agents. Sugars are ubiquitous natural compounds found in humans and other organisms. Here, we review the recent advances in research on the roles of sugars and sugar-related agents in melanogenesis and in the development of sugar-based anti-melanogenic agents. The proposed mechanisms of action of these agents include: (a) (natural sugars) disturbing proper melanosome maturation by inducing osmotic stress and inhibiting the PI3 kinase pathway and (b) (sugar derivatives) inhibiting tyrosinase maturation by blocking N-glycosylation. Finally, we propose an alternative strategy for developing anti-melanogenic sugars that theoretically reduce melanosomal pH by inhibiting a sucrose transporter and reduce tyrosinase activity by inhibiting copper incorporation into an active site. These studies provide evidence of the utility of sugar-based anti-melanogenic agents in managing skin darkness and curing pigmentary disorders and suggest a future direction for the development of physiologically favorable anti-melanogenic agents.

Responses of the Human Brain to Mild Dehydration and Rehydration Explored In Vivo by 1H-MR Imaging and Spectroscopy

BACKGROUND AND PURPOSE

As yet, there are no in vivo data on tissue water changes and associated morphometric changes involved in the osmo-adaptation of normal brains. Our aim was to evaluate osmoadaptive responses of the healthy human brain to osmotic challenges of de- and rehydration by serial measurements of brain volume, tissue fluid, and metabolites.

MATERIALS AND METHODS

Serial T1-weighted and (1)H-MR spectroscopy data were acquired in 15 healthy individuals at normohydration, on 12 hours of dehydration, and during 1 hour of oral rehydration. Osmotic challenges were monitored by serum measures, including osmolality and hematocrit. MR imaging data were analyzed by using FreeSurfer and LCModel.

RESULTS

On dehydration, serum osmolality increased by 0.67% and brain tissue fluid decreased by 1.63%, on average. MR imaging morphometry demonstrated corresponding decreases of cortical thickness and volumes of the whole brain, cortex, white matter, and hypothalamus/thalamus. These changes reversed during rehydration. Continuous fluid ingestion of 1 L of water for 1 hour within the scanner lowered serum osmolality by 0.96% and increased brain tissue fluid by 0.43%, on average. Concomitantly, cortical thickness and volumes of the whole brain, cortex, white matter, and hypothalamus/thalamus increased. Changes in brain tissue fluid were related to volume changes of the whole brain, the white matter, and hypothalamus/thalamus. Only volume changes of the hypothalamus/thalamus significantly correlated with serum osmolality.

CONCLUSIONS

This is the first study simultaneously evaluating changes in brain tissue fluid, metabolites, volume, and cortical thickness. Our results reflect cellular volume regulatory mechanisms at a macroscopic level and emphasize that it is essential to control for hydration levels in studies on brain morphometry and metabolism in order to avoid confounding the findings.

Hyperosmotic stress reduces melanin production by altering melanosome formation

Many tissues of the human body encounter hyperosmotic stress. The effect of extracellular osmotic changes on melanin production has not yet been elucidated. In this study, we determined that hyperosmotic stress induced by organic osmolytes results in reduced melanin production in human melanoma MNT-1 cells. Under hyperosmotic stress, few pigmented mature melanosomes were detected, but there was an increase in swollen vacuoles. These vacuoles were stained with an anti-M6PR antibody that recognizes late endosomal components and with anti-TA99 and anti-HMB45 antibodies, implying that melanosome formation was affected by hyperosmotic stress. Electron microscopic analysis revealed that the M6PR-positive swollen vacuoles were multi-layered and contained melanized granules, and they produced melanin when L-DOPA was applied, indicating that these vacuoles were still capable of producing melanin, but the inner conditions were not compatible with melanin production. The vacuolation phenomenon induced by hyperosmotic conditions disappeared with treatment with the PI3K activator 740 Y-P, indicating that the PI3K pathway is affected by hyperosmotic conditions and is responsible for the proper formation and maturation of melanosomes. The microarray analysis showed alterations of the vesicle organization and transport under hyperosmotic stress. Our findings suggest that melanogenesis could be regulated by physiological conditions, such as osmotic pressure.

The Caenorhabditis elegans mucin-like protein OSM-8 negatively regulates osmosensitive physiology via the transmembrane protein PTR-23

The molecular mechanisms of animal cell osmoregulation are poorly understood. Genetic studies of osmoregulation in yeast have identified mucin-like proteins as critical regulators of osmosensitive signaling and gene expression. Whether mucins play similar roles in higher organisms is not known. Here, we show that mutations in the Caenorhabditis elegans mucin-like gene osm-8 specifically disrupt osmoregulatory physiological processes. In osm-8 mutants, normal physiological responses to hypertonic stress, such as the accumulation of organic osmolytes and activation of osmoresponsive gene expression, are constitutively activated. As a result, osm-8 mutants exhibit resistance to normally lethal levels of hypertonic stress and have an osmotic stress resistance (Osr) phenotype. To identify genes required for Osm-8 phenotypes, we performed a genome-wide RNAi osm-8 suppressor screen. After screening ~18,000 gene knockdowns, we identified 27 suppressors that specifically affect the constitutive osmosensitive gene expression and Osr phenotypes of osm-8 mutants. We found that one suppressor, the transmembrane protein PTR-23, is co-expressed with osm-8 in the hypodermis and strongly suppresses several Osm-8 phenotypes, including the transcriptional activation of many osmosensitive mRNAs, constitutive glycerol accumulation, and osmotic stress resistance. Our studies are the first to show that an extracellular mucin-like protein plays an important role in animal osmoregulation in a manner that requires the activity of a novel transmembrane protein. Given that mucins and transmembrane proteins play similar roles in yeast osmoregulation, our findings suggest a possible evolutionarily conserved role for the mucin-plasma membrane interface in eukaryotic osmoregulation.

Metabolomics-based identification of apoptosis-inducing metabolites in recombinant fed-batch CHO culture media

A liquid chromatography-mass spectrometry (LC-MS) based metabolomics platform was previously established to identify and profile extracellular metabolites in culture media of mammalian cells. This presented an opportunity to isolate novel apoptosis-inducing metabolites accumulating in the media of antibody-producing Chinese hamster ovary (CHO mAb) fed-batch bioreactor cultures. Media from triplicate cultures were collected daily for the metabolomics analysis. Concurrently, cell pellets were obtained for determination of intracellular caspase activity. Metabolite profiles from the LC-MS data were subsequently examined for their degree of correlation with the caspase activity. A panel of extracellular metabolites, the majority of which were nucleotides/nucleosides and amino acid derivatives, exhibited good (R² > 0.8) and reproducible correlation. Some of these metabolites, such as oxidized glutathione, AMP and GMP, were later shown to induce apoptosis when introduced to fresh CHO mAb cultures. Finally, metabolic engineering targets were proposed to potentially counter the harmful effects of these metabolites.

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.

1H-Nuclear magnetic resonance pattern recognition studies withN-phenylanthranilic acid in the rat: time- and dose-related metabolic effects

N-Phenylanthranilic acid (NPAA) causes renal papillary necrosis (RPN) in the rat following repeated oral dosing. Non-invasive early detection of RPN is difficult, but a number of potential biomarkers have been investigated, including phospholipid and uronic acid excretion. This study used 1H-nuclear magnetic resonance (NMR) spectroscopic analysis of urine to investigate urinary metabolic perturbations occurring in the rat following exposure to NPAA. Male Alderley Park rats received NPAA (300, 500 or 700 mg kg(-1) day(-1) orally) for 7 days, and urine was collected on days 7-8, 14-15, 21-22 and 28-29. In a separate study, urine was collected on days 1-2, 3-4, 5-6 and 7-8 from rats receiving 500 mg kg(-1) day(-1). Samples were analysed by 1H NMR spectroscopy combined with multivariate data analysis and clinical chemistry. Histopathology and clinical chemistry analysis of terminal blood samples was carried out following termination on days 4, 6, 8 and 29 (4 week time course) and days 2, 4, 6 and 8 (8 day study). Urine analysis revealed a marked, though variable, excretion of beta-hydroxybutyrate, acetoacetate and acetone (ketone bodies) seen on days 3-4, 5-6 and 7-8 of the study. It is postulated that the ketonuria might be secondary to an alteration in fatty acid metabolism due to inhibition of prostaglandin synthesis. In addition, an elevation in urinary ascorbate was observed during the first 8 days of the study. Ascorbate is considered to be a biomarker of hepatic response, probably reflecting an increased hepatic activity due to glucuronidation of NPAA.

Metabonomic modeling of drug toxicity

Global metabolic profiling (metabonomics/metabolomics) has shown particular promise in the area of toxicology and drug development. In both preclinical screening and mechanistic exploration, metabolic profiling can offer rapid, noninvasive toxicological information that is robust and reproducible, with little or no added technical resources to existing studies in drug metabolism and toxicity. In this review, the study design and analytical technology required for metabonomics are discussed, along with key examples of how fundamental questions in drug development can be addressed. Strategies for metabonomic data analysis in toxicity assessment are detailed in both principle and practice, together with a description of toxicologically relevant metabolic biomarkers. Extended into the assessment of efficacy and toxicity in the clinic, metabonomics may prove crucial in making personalized therapy and pharmacogenomics a reality.

Structure of the tetrameric form of human L-Xylulose reductase: Probing the inhibitor-binding site with molecular modeling and site-directed mutagenesis

L-Xylulose reductase (XR) is a member of the short-chain dehydrogenase/reductase (SDR) superfamily. In this study we report the structure of the biological tetramer of human XR in complex with NADP(+) and a competitive inhibitor solved at 2.3 A resolution. A single subunit of human XR is formed by a centrally positioned, seven-stranded, parallel beta-sheet surrounded on either side by two arrays of three alpha-helices. Two helices located away from the main body of the protein form the variable substrate-binding cleft, while the dinucleotide coenzyme-binding motif is formed by a classical Rossmann fold. The tetrameric structure of XR, which is held together via salt bridges formed by the guanidino group of Arg203 from one monomer and the carboxylate group of the C-terminal residue Cys244 from the neighboring monomer, explains the ability of human XR to prevent the cold inactivation seen in the rodent forms of the enzyme. The orientations of Arg203 and Cys244 are maintained by a network of hydrogen bonds and main-chain interactions of Gln137, Glu238, Phe241, and Trp242. These interactions are similar to those defining the quaternary structure of the closely related carbonyl reductase from mouse lung. Molecular modeling and site-directed mutagenesis identified the active site residues His146 and Trp191 as forming essential contacts with inhibitors of XR. These results could provide a structural basis in the design of potent and specific inhibitors for human XR.

Betaine in human nutrition

Betaine is distributed widely in animals, plants, and microorganisms, and rich dietary sources include seafood, especially marine invertebrates ( approximately 1%); wheat germ or bran ( approximately 1%); and spinach ( approximately 0.7%). The principal physiologic role of betaine is as an osmolyte and methyl donor (transmethylation). As an osmolyte, betaine protects cells, proteins, and enzymes from environmental stress (eg, low water, high salinity, or extreme temperature). As a methyl donor, betaine participates in the methionine cycle-primarily in the human liver and kidneys. Inadequate dietary intake of methyl groups leads to hypomethylation in many important pathways, including 1) disturbed hepatic protein (methionine) metabolism as determined by elevated plasma homocysteine concentrations and decreased S-adenosylmethionine concentrations, and 2) inadequate hepatic fat metabolism, which leads to steatosis (fatty accumulation) and subsequent plasma dyslipidemia. This alteration in liver metabolism may contribute to various diseases, including coronary, cerebral, hepatic, and vascular diseases. Betaine has been shown to protect internal organs, improve vascular risk factors, and enhance performance. Databases of betaine content in food are being developed for correlation with population health studies. The growing body of evidence shows that betaine is an important nutrient for the prevention of chronic disease.

Activation of extracellular signal-regulated kinase ERK after hypo-osmotic stress in renal epithelial A6 cells

Activation of mitogen-activated protein (MAP) kinases has been reported to occur after a hypo-osmotic cell swelling in various types of cells. In renal epithelial A6 cells, the hypo-osmotic shock induced a rapid increase in the phosphorylation of an extracellular signal-regulated kinase (ERK)-like protein that was maximal 10 min after osmotic stress. Activation of ERK was significantly increased when hypo-osmotic stress was performed in the absence of extracellular Ca2+, a condition that inhibits regulatory volume decrease (RVD). Exposure of cells to PD98059, an inhibitor of the MAP kinase kinase MEK, at a concentration that fully cancelled ERK activation, did not inhibit RVD. On the contrary, RVD was abolished when osmotic shock was induced in the presence of SB203580, an inhibitor of stress-activated protein kinases (SAPKs). These results suggest that different MAP kinases are activated after hypo-osmotic stress in A6 cells. SAPKs would be involved in the control of RVD, while ERK would lead to later events, such as gene expression or energy metabolism.

Short and long-term variation of plasma glycine betaine concentrations in humans

BACKGROUND

Glycine betaine is important in cell volume regulation and in remethylating homocysteine, a vascular risk factor.

OBJECTIVE

We investigated changes in circulating glycine betaine concentrations in human volunteers both under acute osmotic stress and over longer time scales.

DESIGN

Plasma glycine betaine concentrations were measured in normal human volunteers in three studies: (1) during acute diuresis and antidiuresis; (2) during prolonged diuresis for 5 days, and antidiuresis for 5 days followed by further diuresis for the final 5 days; (3) repeated samples taken 3 years apart.

RESULTS

Circulating glycine betaine concentrations remained almost unchanged for several hours after acute diuretic or antidiuretic stresses. There was more (3-10-fold) interindividual variation than intraindividual variation. A similar pattern was found on day 15 of the study. In a 3-year follow-up, plasma glycine betaine concentrations on the two occasions were highly correlated with no systematic change, showing that individual set points remain stable for years. In contrast, there was no relationship among plasma proline betaine concentrations at these times. Urinary glycine betaine excretions measured 3 years apart were also found to correlate once the perturbing effect of dietary proline betaine excretion was allowed for.

CONCLUSIONS

Human circulating glycine betaine is homeostatically controlled with a distinct control value for each individual. In contrast, peripheral blood concentrations of proline betaine, which is present in the diet (and has no known metabolic or physiological role in mammals), is not controlled.

Ethanol-induced swelling in neonatal rat primary astrocyte cultures

We tested the hypothesis that astrocytes swell in response to ethanol (EtOH) exposure. The experimental approach consisted of an electrical impedance method designed to measure cell volume. In chronic experiments, EtOH (100 mM) was added to the culture media for 1, 3, or 7 days. The cells were subsequently exposed for 15 min to isotonic buffer (122 mM NaCl) also containing 100 mM EtOH. Subsequently, the cells were washed and exposed to hypotonic buffer (112 mM NaCl) containing 100 mM mannitol. Chronic exposure to EtOH led to a marked increase in cell volume compared with control cells. Specific anion cotransport blockers, such as SITS, DIDS, furosemide, or bumetanide, when simultaneously added with EtOH to hyponatremic buffer, failed to reverse the EtOH-induced effect on swelling. In acute experiments, confluent neonatal rat primary astrocyte cultures were exposed to isotonic media (122 mM NaCl) for 15 min, followed by 45-min exposure to hypotonic media (112 mM NaCl, mimicking in vivo hyponatremic conditions associated with EtOH withdrawal) in the presence of 0-100 mM EtOH. This exposure led to a concentration-dependent increase in cell volume. Combined, these studies suggest that astrocytes exposed to EtOH accumulate compensatory organic solutes to maintain cell volume, and that in response to hyponatremia and EtOH withdrawal their volume increases to a greater extent than in cells exposed to hyponatremia alone. Furthermore, the changes associated with EtOH are osmotic in nature, and they are not reversed by anion cotransport blockers.

Intestinal uptake of betaine in vitro and the distribution of methyl groups from betaine, choline, and methionine in the body of broiler chicks

The efficiency of betaine absorption into small intestinal slices of broiler chicks was studied in vitro with 14C-labeled betaine. The relative proportion of Na+-coupled betaine uptake, as well as the total uptake capacity was larger in the duodenum than in the jejunum. Dietary betaine increased the Na+-coupled uptake in the duodenum. In in vivo-experiments, methyl-14C-labeled betaine, methionine, or choline was fed to broiler chicks. Betaine appeared in the blood more rapidly, and reached a higher total concentration than choline or methionine. The data suggest that choline and methionine were associated with plasma lipoproteins whereas betaine remained free in the plasma. The label distribution in liver, kidney, and intestinal tissues was studied 24 h after label ingestion. Most of the label from betaine was found in the aquaeous phase in the muscle, while in the liver and jejunum the label from betaine was distributed more evenly between the aquaeous, lipid, and protein phases. Label from choline accumulated in the lipid fraction, particularly so in the liver, whereas label from methionine showed a more variable distribution pattern. The distribution results are interpreted in terms of specific roles of betaine, choline, and methionine in methyl group metabolism.

Wortmannin and LY294002 inhibit myo-inositol accumulation by cultured bovine aorta endothelial cells and murine 3T3-L1 adipocytes

We have previously reported that myo-inositol uptake and metabolism is reduced in human fibroblasts derived from patients with ataxia telangiectasia (AT). Treating normal fibroblasts with 10-100 microM wortmannin duplicates some of the phenotypic properties of AT fibroblasts including the decrease in myo-inositol accumulation. In the present study we examined whether treatment of other types of mammalian cells with wortmannin or LY294002 altered myo-inositol uptake. Cultured bovine aorta endothelial cells or 3T3-L1 adipocytes were incubated with either wortmannin or LY294002, and afterwards, myo-inositol uptake and SMIT mRNA levels were determined. Incubating cultured bovine aorta endothelial cells and 3T3-L1 adipocytes with either wortmannin or LY294002 caused a time- and concentration-dependent decrease in myo-inositol accumulation that was independent of changes in SMIT mRNA levels. The effect of wortmannin and LY294002 on myo-inositol accumulation was not due to an increase in myo-inositol secretion. The effect of LY294002 on myo-inositol accumulation was reversible. Furthermore, the LY294002-induced decrease in myo-inositol accumulation was specific since the uptake of serine or choline by cultured bovine aorta endothelial cells and 3T3-L1 adipocytes treated with LY294002 was not significantly decreased. Co-incubation of cultured bovine aorta endothelial cells and 3T3-L1 adipocytes with either wortmannin or LY294002 and hyperosmotic medium caused a significant decrease in the induction of myo-inositol accumulation by hyperosmolarity without significantly affecting the hyperosmotic-induced increase in SMIT mRNA levels. These data suggest that myo-inositol accumulation is regulated post-translationally by wortmannin and LY294002.

Isolation of differentially expressed aldose reductase in ovaries after estrogen withdrawal from hypophysectomized diethylstilbestrol treated rats: increased expression during apoptosis

More than 99% of the follicles are eliminated by apoptosis before reaching ovulation. Several growth factors and hormones inhibit apoptosis in the ovary, including estrogen. Using differential display of mRNA, aldose reductase was shown to increase in the ovary of diethylstilbestrol treated hypophysectomized rats after estrogen withdrawal, inducing apoptosis. The aldose reductase mRNA expression was confirmed to be 2.2 +/- 0.2-fold higher after estrogen withdrawal using northern blot analysis. In addition, untreated immature rats showed a 1.7 +/- 0.3-fold higher expression of ovarian aldose reductase mRNA compared to ovaries 24 h after pregnant mare's serum gonadotropin treatment, decreasing apoptosis in the ovary. In the prostate, the level of aldose reductase was increased 3.1 +/- 1.1-fold 2 days after castration induced apoptosis. Although the physiological role of aldose reductase in the ovary is not known, these data suggest that aldose reductase may be part of a hormonally regulated apoptotic pathway in the ovary and prostate.

Normalization of hyperosmotic-induced inositol uptake by renal and endothelial cells is regulated by NF-kappaB

Hyperosmolarity is a stress factor that has been shown to cause an increase in the transcription of the Na(+)-dependent myo-inositol cotransporter (SMIT). However, regulation of the reversion of SMIT mRNA levels and transporter activity following removal of hyperosmotic stress is less understood. Previously we have shown that postinduction normalization of SMIT mRNA levels and myo-inositol accumulation following removal of hyperosmotic stress is inhibited by actinomycin D and cycloheximide, suggesting that normalization requires RNA transcription and protein synthesis. We now demonstrate that removal of hyperosmotic stress causes an activation of the transcription factor NF-kappaB in renal and endothelial cells. Inhibiting NF-kappaB activation with pyrrolidine dithiocarbamate (PD) blocks the normalization of SMIT mRNA levels and myo-inositol accumulation on removal of the cells from hyperosmotic medium. These studies demonstrate that the downregulation of the myo-inositol transporter following reversal of hyperosmotic induction is regulated via the activation of NF-kappaB.

Evidence that taurine modulates osmoregulation by modification of osmolarity sensor protein ENVZ--expression

Although the involvement of taurine in osmoregulation is well-documented and widely accepted, no detailed mechanism for this function has been reported so far. We used subtractive hybridization to study mRNA steady state levels of genes up- or downregulated by taurine. Rats were fed taurine 100 mg/kg body weight per day for a period of three days and hearts (total ventricular tissue) of experimental animals and controls were pooled and used for mRNA extraction. mRNAs from two groups were used for subtractive hybridization. Clones of the subtractive library were sequenced and the obtained sequences were identified by gen bank assignment. Two clones were found to contain sequences which could be assigned to the osmolarity sensor protein envZ, showing homologies of 61 and 65%. EnvZ is an inner membrane protein in bacteria, important for osmosensing and required for porine gene regulation. It undergoes autophosphorylation and subsequently phosphorylates OmpR, which in turn binds to the porine (outer membrane protein) promoters to regulate the expression of OmpF and OmpC, major outer membrane porines. This is the first report of an osmosensing mechanism in the mammalian system, which was described in bacteria only. Furthermore, we are assigning a tentative role for taurine in the osmoregulatory process by modifying the expression of the osmoregulatory sensor protein ENVZ.

Functional characterization of the Betaine/gamma-aminobutyric acid transporter BGT-1 expressed in Xenopus oocytes

Betaine is an osmolyte accumulated in cells during osmotic cell shrinkage. The canine transporter mediating cellular accumulation of the osmolyte betaine and the neurotransmitter gamma-aminobutyric acid (BGT-1) was expressed in Xenopus oocytes and analyzed by two-electrode voltage clamp and tracer flux studies. Exposure of oocytes expressing BGT-1 to betaine or gamma-aminobutyric acid (GABA) depolarized the cell membrane in the current clamp mode and induced an inward current under voltage clamp conditions. At 1 mM substrate the induced currents decreased in the following order: betaine = GABA > diaminobutyric acid = beta-alanine > proline = quinidine > dimethylglycine > glycine > sarcosine. Both the Vmax and Km of GABA- and betaine-induced currents were voltage-dependent, and GABA- and betaine-induced currents and radioactive tracer uptake were strictly Na+-dependent but only partially dependent on the presence of Cl-. The apparent affinity of GABA decreased with decreasing Na+ concentrations. The Km of Na+ also depended on the GABA and Cl- concentration. A decrease of the Cl- concentration reduced the apparent affinity for Na+ and GABA, and a decrease of the Na+ concentration reduced the apparent affinity for Cl- and GABA. A comparison of 22Na+-, 36Cl--, and 14C-labeled GABA and 14C-labeled betaine fluxes and GABA- and betaine-induced currents yielded a coupling ratio of Na+/Cl-/organic substrate of 3:1:1 or 3:2:1. Based on the data, a transport model of ordered binding is proposed in which GABA binds first, Na+ second, and Cl- third. In conclusion, BGT-1 displays significant functional differences from the other members of the GABA transporter family.

Hypertonicity stimulates taurine uptake and transporter gene expression in Caco-2 cells

The osmoregulation of taurine transport in intestinal epithelial cells was investigated using human intestinal Caco-2 cells. The activity of taurine transport in the Caco-2 cells was increased by hypertonic conditions. This hypertonicity-induced up-regulation was dependent on both the culturing time and the osmotic pressure. Hypertonicity did not affect the activity of L-leucine, L-lysine, or L-glutamic acid transport, suggesting that osmoregulation was specific to taurine transport. The intracellular taurine content of Caco-2 cells was also increased by culturing in a hypertonic medium. These hypertonicity-induced changes in the intracellular taurine content and transport activity were reversible. A kinetic analysis of taurine transport in the control and hypertonic cells suggested that the up-regulation was associated with an increase in the amount of the taurine transporter. The mRNA level of the taurine transporter in hypertonic cells was markedly higher than that in the control cells, indicating that this osmotic regulation was due to the increased expression of the taurine transporter gene.

Signalling and gene regulation by urea and NaCl in the renal medulla

1. Cells of the mammalian renal medulla are routinely subjected to an enormously elevated and labile ambient osmolality as a consequence of the renal concentrating mechanism. The present review focuses on the most recent advances in hyperosmotic solute-mediated signal transduction and regulation of gene transcription in cells of the kidney medulla. 2. On the basis of osmolality alone, NaCl and urea are the principal renal medullary solutes. 3. Urea, which is membrane permeant, activates transcription of immediate-early genes via an extracellular signal-regulated kinase (ERK)/Elk-1-dependent pathway. Urea also activates multiple effectors characteristic of a receptor tyrosine kinase-like signalling cascade. 4. In contrast, the functionally impermeant solute NaCl activates transcription of tonicity responsive genes (principally genes encoding proteins essential for osmolyte uptake or synthesis) via a unique consensus element contained within their 5' flanking sequences. 5. An activity exhibiting tonicity inducible sequence-specific interaction with this DNA element has been identified. 6. Hypertonicity, like thermal stress, activates transcription of genes encoding heat shock proteins. The relationship between signalling events leading to tonicity mediated and heat shock-mediated gene transcription remains to be established.

Organic osmolyte transport in quiescent and activated rat hepatic stellate cells (Ito cells)

Activation of hepatic stellate cells (HSCs) results in multiple alterations of cell function, but nothing is known about organic osmolytes in these cells. Organic osmolyte transport and transporter messenger RNA (mRNA) expression was studied in quiescent rat HSCs and after their transformation into alpha1-smooth muscle actin-positive myofibroblastlike cells. Quiescent stellate cells expressed in an osmosensitive manner the mRNA levels of the transporters for taurine (TAUT) and myoinositol (SMIT), whereas that for betaine was not detectable. However, these cells showed osmosensitive uptake not only of taurine and myoinositol but also of betaine. Osmosensitive betaine uptake was mediated by amino acid transport system A. After transformation into myofibroblasts, taurine and myoinositol uptake increased 5.5-fold and 4.5-fold, respectively, together with the respective transporter mRNA levels. Betaine uptake increased twofold because of osmosensitive induction of BGT1 expression. In both quiescent and activated HSCs, hypoosmotic cell swelling induced a rapid and 4, 4'-diisothiocyanatostilbene-2,2'-disulphonic acid-sensitive osmolyte efflux. In quiescent HSCs, hyperosmotic exposure increased the messenger RNA (mRNA) level of cyclooxygenase-2, which was counteracted by taurine but not by betaine or myoinositol. The study identifies taurine, myoinositol, and betaine as osmolytes in HSCs. Transformation of HSCs is accompanied by enhanced osmolyte transport activity and induction of the BGT1 transporter, which may be another activation marker of HSCs.

Osmotic regulation of the Na+/myo-inositol cotransporter and postinduction normalization

BACKGROUND

In renal cells, hyperosmolarity has been shown to induce the accumulation of myo-inositol, via the Na+/myo-inositol cotransporter (SMIT). Previously we showed that SMIT mRNA in the kidney is localized in the medullary thick ascending limb of Henle (TALH). Here we used renal cells derived from the rabbit outer medullary TALH to examine the regulation of myo-inositol transport by hyperosmolarity. In addition, using both cultured renal and endothelial cells, we examined the normalization of SMIT activity and mRNA levels following induction by hyperosmolarity.

METHODS

TALH cells were exposed to isotonic or hyperosmotic medium, and then SMIT mRNA levels and myo-inositol accumulation were determined. To examine postinduction normalization, cultured endothelial and renal cells were first exposed to hyperosmotic medium and then to isotonic medium containing actinomycin D or cycloheximide. Afterwards, SMIT mRNA levels and myo-inositol accumulation were determined.

RESULTS

Hyperosmolarity increased SMIT mRNA levels and myo-inositol accumulation in TALH cells. The hyperosmolarity-induced increase in myo-inositol uptake by TALH cells was characterized by an increase in the Vmax for the high-affinity myo-inositol transport system, with no change in the Km. This increase was blocked by actinomycin D or cycloheximide. Examination of postinduction normalization showed that returning hyperosmotic-treated cells to isotonic medium caused a rapid reversion of SMIT mRNA levels, followed by a return of myo-inositol accumulation to basal values. However, the addition of cycloheximide or actinomycin D partially to totally prevented the reversal in SMIT mRNA levels and activity.

CONCLUSIONS

These results suggest that RNA and protein synthesis is required for the hyperosmotic induction of SMIT mRNA levels and myo-inositol accumulation by TALH cells. Furthermore, normalization of SMIT mRNA levels and myo-inositol accumulation following hyperosmotic induction requires RNA transcription and protein synthesis.

Exclusion and retention of compensatory kosmotropes by HPLC columns

With water as the elution solvent, zwitterionic solutes and polyols were retained on HPLC columns, more than was water, by totally hydrophobic packing materials. Relative retentions were systematically affected by oxygen functional groups in the packing material, explicable as specific retention of water. Reproducible elution sequences of 20 solutes at a variety of hydrophobic surfaces (aromatic and both long- and short-alkyl aliphatic surfaces) showed there is a general process, consistent with interactions with hydration water at the surface having solvent properties distinct from bulk water. Early eluting solutes included glycine, sarcosine and taurine. Glycine betaine followed both these and N,N-dimethylglycine. The natural betaines propionobetaine and dimethylsulfoniopropionate also preceded glycine betaine. Dimethylsulfoxide was strongly retained, as (to a lesser extent) was proline betaine. Polyols eluted in the sequence sorbitol, trehalose, glycerol. Changes in the chemical nature of the surface or base material affected relative retentions of water and solutes. The presence of hydrogen-bonding functions increased retention of polyols, as well as water, relative to zwitterionic solutes. Specific effects retention, constraining models based on the formation of low-density water.

Compatible organic osmolytes in rat liver sinusoidal endothelial cells

Compatible organic osmolytes, such as betaine and taurine are involved in the regulation of Kupffer cell (KC) function, but nothing is known about osmolytes in liver endothelial cells. This was investigated here by studying the effect of aniso-osmotic exposure of rat liver sinusoidal endothelial cells (SEC) on osmolyte transport and the messenger RNA (mRNA) levels for the transport systems for betaine (BGT1), taurine (TAUT), and myo-inositol (SMIT). Compared with normo-osmotic exposure (305 mosmol/L), hyperosmotic exposure (405 mosmol/L) of SEC led to an increase in the mRNA levels for these transport systems and simultaneously to a stimulation of betaine, taurine, and myo-inositol uptake, which led to an increase of cell volume. Conversely, hypo-osmotic exposure decreased osmolyte uptake. When hyperosmotically pre-exposed SEC were loaded with betaine, taurine, or myoinositol, hypo-osmotic stress stimulated the efflux of these osmolytes from the cells. Studies on osmolyte tissue levels revealed that taurine was an important compatible organic osmolyte under normo-osmotic conditions and predominantly released following hypo-osmotic stress. Conversely, following hyperosmotic exposure, the increase in cellular betaine and myo-inositol exceeded that of taurine. In lipopolysaccharide (LPS)-treated SEC, hyperosmotic exposure markedly raised the mRNA levels for cyclo-oxygenase-2 (COX-2), but not for inducible nitric oxide synthase (iNOS). The increase of COX-2 mRNA levels was counteracted by betaine and taurine and, to a lesser extent, by myo-inositol. The findings indicate that SEC use taurine, betaine, and myo-inositol as compatible organic osmolytes.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Glucose-specific regulation of aldose reductase in capan-1 human pancreatic duct cells In vitro

Impaired pancreatic duct secretion is frequently observed in insulin-dependent diabetes mellitus (IDDM), although the cellular mechanism(s) of dysfunction remains unknown. Studies in other tissues have suggested that a hyperglycemia-induced decrease in Na, K-ATPase activity could contribute to the metabolic complications of IDDM and that increased polyol metabolism is involved in this response. The present studies examined the effects of glucose on Na, K-ATPase activity and on expression and activity of aldose reductase (AR), a primary enzyme of polyol metabolism, in Capan-1 human pancreatic duct cells. Increasing medium glucose from 5.5 to 22 mM caused a 29% decrease in Na,K-ATPase activity. The decrease was corrected by 100 microM sorbinil, a specific AR inhibitor. Increasing glucose from 5.5 to 110 mM also resulted in concentration-dependent increases in AR mRNA and enzyme activity that could be resolved into two components, one that was glucose specific and observed at pathophysiological concentrations (< 55 mM) and a second that was osmotically induced at high concentrations (> 55 mM) and which was not glucose specific. The present study demonstrates that pathophysiological levels of glucose specifically activate polyol metabolism with a consequent decrease in Na,K-ATPase activity in pancreatic duct epithelial cells, and that this response to hyperglycemia could contribute to decreased pancreatic secretion observed in IDDM. This is the first report of AR regulation in the pancreatic duct epithelium.

"Compensatory" organic osmolytes in high osmolarity and dehydration stresses: history and perspectives

As stated in the conclusion, "life is a thing of macromolecular cohesion in salty water." This brief historical overview shows that "compensatory" organic osmolytes take an essential place in this cohesion. It reviews the major steps of the study of these compounds over more than 100 years, from the early beginnings of 1885 until now, showing some of its fascinating developments and ending on the idea that the most fascinating is still to come. This study can be taken as an example of the richness of the comparative approach.

Osmoregulated taurine transport in H4IIE hepatoma cells and perfused rat liver

The effects of aniso-osmotic exposure on taurine transport were studied in H4IIE rat hepatoma cells. Hyperosmotic (405 mosmol/l) exposure of H4IIE cells stimulated Na+-dependent taurine uptake and led to an increase in taurine transporter (TAUT) mRNA levels, whereas hypo-osmotic (205 mosmol/l) exposure diminished both taurine uptake and TAUT mRNA levels when compared with normo-osmotic (305 mosmol/l) control incubations. Taurine uptake increased 30-40-fold upon raising the ambient osmolarity from 205 to 405 mosmol/l. When H4IIE cells and perfused livers were preloaded with taurine, hypo-osmotic cell swelling led to a rapid release of taurine from the cells. The taurine efflux, but not taurine uptake, was sensitive to 4,4'-di-isothiocyanatostilbene-2,2'-disulphonic acid (DIDS), suggestive of an involvement of DIDS-sensitive channels in mediating volume-regulatory taurine efflux. Whereas in both H4IIE rat hepatoma cells and primary hepatocytes TAUT mRNA levels were strongly dependent upon ambient osmolarity, mRNAs for other osmolyte transporters, i.e. the betaine transporter BGT-1 and the Na+/myo-inositol transporter SMIT, were not detectable. In line with this, myo-inositol uptake by H4IIE hepatoma cells was low and was not stimulated by hyperosmolarity. However, despite the absence of BGT-1 mRNA, a slight osmosensitive uptake of betaine was observed, but the rate was less than 10% of that of taurine transport. This study identifies a constitutively expressed and osmosensitive TAUT in H4IIE cells and the use of taurine as a main osmolyte, whereas betaine and myo-inositol play little or no role in the osmolyte strategy in these cells. This is in contrast with rat liver macrophages, in which betaine has been shown to be a major osmolyte.

Mannitol, a novel bacterial compatible solute in Pseudomonas putida S12

The aim of this study was to identify the compatible solutes accumulated by Pseudomonas putida S12 subjected to osmotic stress. In response to reduced water activity, P. putida S12 accumulated Nalpha-acetylglutaminylglutamine amide (NAGGN) simultaneously with a novel compatible solute identified as mannitol (using 13C- and 1H-nuclear magnetic resonance, liquid chromatography-mass spectroscopy and high-performance liquid chromatography methods) to maximum concentrations of 74 and 258 micromol g (dry weight) of cells(-1), respectively. The intracellular amounts of each solute varied with both the type and amount of osmolyte applied to induce osmotic stress in the medium. Both solutes were synthesized de novo. Addition of betaine to the medium resulted in accumulation of this compound and depletion of both NAGGN and mannitol. Mannitol and NAGGN were accumulated when sucrose instead of salts was used to reduce the medium water activity. Furthermore, both compatible solutes were accumulated when glucose was substituted by other carbon sources. However, the intracellular quantities of mannitol decreased when fructose, succinate, or lactate were applied as a carbon source. Mannitol was also raised to high intracellular concentrations by other salt-stressed Pseudomonas putida strains. This is the first study demonstrating a principal role for the de novo-synthesized polyol mannitol in osmoadaptation of a heterotrophic eubacterium.

Localization and regulation of renal Na+/myo-inositol cotransporter in diabetic rats

We have examined the effect of diabetes on sodium/myo-inositol cotransporter (SMIT) mRNA levels and myo-inositol content in the kidney to test the hypothesis that diabetes-induced changes in renal myo-inositol levels are due to the regulation of SMIT mRNA levels. In streptozotocin-induced diabetic rats, after 3, 7 and 28 days of diabetes, SMIT mRNA levels in the whole kidney were increased three to fivefold, and remained increased by about twofold after six months of diabetes. Insulin treatment of diabetic rats normalized blood glucose levels and prevented the increase in SMIT mRNA levels. Treating diabetic rats with sorbinil, an aldose reductase inhibitor, corrected the abnormal accumulation of sorbitol but had no effect on the diabetes-induced increase in renal SMIT mRNA levels. The regional distribution of SMIT mRNA from normal rats showed a relative abundance in cortex, outer medulla, and inner medulla of 1.0:3.4:7.0. After seven days of diabetes, the levels of SMIT mRNA and myo-inositol content were significantly increased only in the outer medulla. In situ hybridization studies revealed that SMIT mRNA in the outer medulla was predominately localized to the medullary thick ascending limbs of Henle's loop and was not localized to any specific cell in the inner medulla. This distribution pattern was unchanged in diabetic rats. These studies show that diabetes causes an increase in renal SMIT mRNA, which is primarily localized to the outer medulla. Accumulation of myo-inositol by the thick ascending limb of Henle's loop may account for most of the increase caused by diabetes.

[Regulation of cell function by level of hydration]

The hydration state of mammalian cells is dynamic and changes within minutes under the influence of nutrients, hormones and oxidative stress. Such changes in cell hydration act as an independent signal which modulates cellular metabolism and gene expression by activating intracellular signalling systems. Although the structures which sense hydration changes are unknown, this creates a novel and elegant mechanism for adaptation of cell function and gene expression to environmental challenges and provides new aspects for clinical medicine.

Osmotic regulation of Na-myo-inositol cotransporter mRNA level and activity in endothelial and neural cells

Myo-inositol (MI) is an important factor in the synthesis of phosphoinositides, and as an osmolyte, MI contributes to the regulation of cell volume. In cells of renal origin, hypertonicity causes an increase in sodium-dependent MI transporter (SMIT) mRNA levels and MI transport. However, it is unknown whether changes in osmolarity regulate transport of MI in neural or endothelial cells. IN these studies, neural and endothelial cells were exposed to hyperosmotic medium for up to 48 h, and the effect on MI transport was determined. Transport of MI was maximally increased by exposing the cells to hyperosmotic medium for 24 h. Kinetic analysis of high-affinity MI transport demonstrated an increase in the apparent maximal velocity with no significant change in the apparent Km. The hyperosmotic induction of MI transport was blocked by the addition of cycloheximide, indicating a requirement for protein synthesis, and was associated with increased levels of SMIT mRNA. In contrast to the effect of hypertonicity, exposure of neural and endothelial cells to hypotonic conditions caused a decrease in SMIT mRNA levels and MI transport in endothelial cells. These studies demonstrate that, in extrarenal cell types, changes in osmolarity also regulate SMIT activity and mRNA levels.

AlphaB-crystallin protects glial cells from hypertonic stress

AlphaB-crystallin and the small stress protein, heat shock protein of 27 kDa (HSP27), share structural similarities and are coordinately induced by classical stress stimuli. We have recently observed that hypertonic stress produced by high NaCl concentrations selectively induces alphaB-crystallin in glial cells. To examine divergence of the functional properties of these two related proteins, we have constructed stable alphaB-crystallin-expressing glial cell lines from the U-251 MG glioma cells, which are normally deficient in alphaB-crystallin expression but constitutively express HSP27. These transfected cells lines are more resistant to acute hypertonic stress than the parental line from which they were derived. Moreover, the parental line acclimates to stepwise increases in hypertonicity and upregulates endogenous alphaB-crystallin in the process but not HSP27. The overexpression of HSP27 and alphaB-crystallin in NIH/3T3 fibroblasts, a cell line that normally expresses little alphaB-crystallin and no HSP27, does not result in increased survival. This suggests that alphaB-crystallin interacts with cell-type specific mechanisms to aid in protection from hypertonic stress.

Changes in organic solutes, volume, energy state, and metabolism associated with osmotic stress in a glial cell line: a multinuclear NMR study

Diffusion-weighted in vivo 1H-NMR spectroscopy of F98 glioma cells embedded in basement membrane gel threads showed that the initial cell swelling to about 180% of the original volume induced under hypotonic stress was followed by a regulatory volume decrease to nearly 100% of the control volume in Dulbecco's modified Eagle's medium (DMEM) but only to 130% in Krebs-Henseleit buffer (KHB, containing only glucose as a substrate) after 7 h. The initial cell shrinkage to approx. 70% induced by the hypertonic stress was compensated by a regulatory volume increase which after 7 h reached almost 100% of the control value in KHB and 75% in DMEM. 1H-, 13C- and 31P-NMR spectroscopy of perchloric acid extracts showed that these volume regulatory processes were accompanied by pronounced changes in the content of organic osmolytes. Adaptation of intra- to extracellular osmolarity was preferentially mediated by a decrease in the cytosolic taurine level under hypotonic stress and by an intracellular accumulation of amino acids under hypertonic stress. If these solutes were not available in sufficient quantities (as in KHB), the osmolarity of the cytosol was increasingly modified by biosynthesis of products and intermediates of essential metabolic pathways, such as alanine, glutamate and glycerophosphocholine in addition to ethanolamine. The cellular nucleoside triphosphate level measured by in vivo 31P-NMR spectroscopy indicated that the energy state of the cells was more easily sustained under hypotonic than hypertonic conditions.