Mechanisms and significance of cell volume regulation

The profile of hippocampal metabolites differs between Alzheimer's disease and subcortical ischemic vascular dementia, as measured by proton magnetic resonance spectroscopy

Alzheimer's disease (AD) and subcortical ischemic vascular dementia (SIVD) have overlapping pathologies and risk factors, but their underlying neurodegenerative mechanisms are basically different. We performed magnetic resonance spectroscopy (MRS) to study metabolite differences between the two diseases in vivo. The subjects were 31 patients with SIVD and 99 with AD. Additionally, 45 elderly subjects were recruited as controls. We measured N-acetylaspartate (NAA), glutamine and glutamate (Glx), and myoinositol (mIns) concentration quantitatively using a 1.5-T MR scanner. N-acetylaspartate and Glx concentrations decreased in the hippocampus and cingulate/precuneal cortices (PCC) in both AD and SIVD patients, and the NAA decrease in the hippocampus was more prominent in AD than in SIVD. Interestingly, the pattern of mIns concentration changes differed between the two disorders; mIns was increased in AD but not increased in SIVD. If one differentiates between AD and SIVD by the mIns concentration in the hippocampus, the area under the receiver operating characteristic curve was 0.95, suggesting a high potential for discrimination. Our results suggest that proton MRS can provide useful information to differentiate between AD and SIVD. The difference of mIns concentrations in the hippocampus and PCC seems to reflect the different neurodegenerative mechanisms of the two disorders.

Subcellular carrier-based optical ion-selective nanosensors

In this review, two carrier systems based on nanotechnology for real-time sensing of biologically relevant analytes (ions or other biological molecules) inside cells in a non-invasive way are discussed. One system is based on inorganic nanoparticles with an organic coating, whereas the second system is based on organic microcapsules. The sensor molecules presented within this work use an optical read-out. Due to the different physicochemical properties, both sensors show distinctive geometries that directly affect their internalization patterns. The nanoparticles carry the sensor molecule attached to their surfaces whereas the microcapsules encapsulate the sensor within their cavities. Their different size (nano and micro) enable each sensors to locate in different cellular regions. For example, the nanoparticles are mostly found in endolysosomal compartments but the microcapsules are rather found in phagolysosomal vesicles. Thus, allowing creating a tool of sensors that sense differently. Both sensor systems enable to measure ratiometrically however, only the microcapsules have the unique ability of multiplexing. At the end, an outlook on how more sophisticated sensors can be created by confining the nano-scaled sensors within the microcapsules will be given.

pH-sensitive capsules as intracellular optical reporters for monitoring lysosomal pH changes upon stimulation

The concept of a long-term sensor for ion changes in the lysosome is presented. The sensor is made by layer-by-layer assembly of oppositely charged polyelectrolytes around ion-sensitive fluorophores, in this case for protons. The sensor is spontaneously incorporated by cells and resides over days in the lysosome. Intracellular changes of the concentration of protons upon cellular stimulation with pH-active agents are monitored by read-out of the sensor fluorescence at real time. With help of this sensor concept it is demonstrated that the different agents used (Monensin, Chloroquine, Bafilomycin A1, Amiloride) possessed different kinetics and mechanisms of action in affecting the intracellular pH values.

Basolateral membrane K+ channels in renal epithelial cells

The major function of epithelial tissues is to maintain proper ion, solute, and water homeostasis. The tubule of the renal nephron has an amazingly simple structure, lined by epithelial cells, yet the segments (i.e., proximal tubule vs. collecting duct) of the nephron have unique transport functions. The functional differences are because epithelial cells are polarized and thus possess different patterns (distributions) of membrane transport proteins in the apical and basolateral membranes of the cell. K(+) channels play critical roles in normal physiology. Over 90 different genes for K(+) channels have been identified in the human genome. Epithelial K(+) channels can be located within either or both the apical and basolateral membranes of the cell. One of the primary functions of basolateral K(+) channels is to recycle K(+) across the basolateral membrane for proper function of the Na(+)-K(+)-ATPase, among other functions. Mutations of these channels can cause significant disease. The focus of this review is to provide an overview of the basolateral K(+) channels of the nephron, providing potential physiological functions and pathophysiology of these channels, where appropriate. We have taken a "K(+) channel gene family" approach in presenting the representative basolateral K(+) channels of the nephron. The basolateral K(+) channels of the renal epithelia are represented by members of the KCNK, KCNJ, KCNQ, KCNE, and SLO gene families.

The plasma membrane potential and the organization of the actin cytoskeleton of epithelial cells

The establishment and maintenance of the polarized epithelial phenotype require a characteristic organization of the cytoskeletal components. There are many cellular effectors involved in the regulation of the cytoskeleton of epithelial cells. Recently, modifications in the plasma membrane potential (PMP) have been suggested to participate in the modulation of the cytoskeletal organization of epithelia. Here, we review evidence showing that changes in the PMP of diverse epithelial cells promote characteristic modifications in the cytoskeletal organization, with a focus on the actin cytoskeleton. The molecular paths mediating these effects may include voltage-sensitive integral membrane proteins and/or peripheral proteins sensitive to surface potentials. The voltage dependence of the cytoskeletal organization seems to have implications in several physiological processes, including epithelial wound healing and apoptosis.

Phosphorylation and activation of the plasma membrane Na+/H+ exchanger (NHE1) during osmotic cell shrinkage

The Na(+)/H(+)Exchanger isoform 1 (NHE1) is a highly versatile, broadly distributed and precisely controlled transport protein that mediates volume and pH regulation in most cell types. NHE1 phosphorylation contributes to Na(+)/H(+) exchange activity in response to phorbol esters, growth factors or protein phosphatase inhibitors, but has not been observed during activation by osmotic cell shrinkage (OCS). We examined the role of NHE1 phosphorylation during activation by OCS, using an ideal model system, the Amphiuma tridactylum red blood cell (atRBC). Na(+)/H(+) exchange in atRBCs is mediated by an NHE1 homolog (atNHE1) that is 79% identical to human NHE1 at the amino acid level. NHE1 activity in atRBCs is exceptionally robust in that transport activity can increase more than 2 orders of magnitude from rest to full activation. Michaelis-Menten transport kinetics indicates that either OCS or treatment with the phosphatase inhibitor calyculin-A (CLA) increase Na(+) transport capacity without affecting transport affinity (K(m)=44 mM) in atRBCs. CLA and OCS act non-additively to activate atNHE1, indicating convergent, phosphorylation-dependent signaling in atNHE1 activation. In situ(32)P labeling and immunoprecipitation demonstrates that the net phosphorylation of atNHE1 is increased 4-fold during OCS coinciding with a more than 2-order increase in Na(+) transport activity. This is the first reported evidence of increased NHE1 phosphorylation during OCS in any vertebrate cell type. Finally, liquid chromatography and mass spectrometry (LC-MS/MS) analysis of atNHE1 immunoprecipitated from atRBC membranes reveals 9 phosphorylated serine/threonine residues, suggesting that activation of atNHE1 involves multiple phosphorylation and/or dephosphorylation events.

EGF stimulates IClswell by a redistribution of proteins involved in cell volume regulation

BACKGROUND

ICln is a multifunctional protein involved in the generation of chloride currents activated during regulatory volume decrease (RVD) after cell swelling (ICl(swell)). Growth factor receptors play a key role in different cellular processes and epidermal growth factor (EGF) regulates swelling-activated chloride permeability.

AIM

We set out to investigate if the EGF-induced upregulation of ICl(swell) could be explained by a rearrangement of ICln subcellular distribution and interaction with its molecular partners.

METHODS

NIH-3T3 fibroblasts were serum-deprived for 24 hours and stimulated with EGF (40 ng/ml) for 30 minutes. ICl(swell) activation, ICln distribution and interaction with its molecular partner HSPC038 were assessed by whole cell patch clamp and fluorescence resonance energy transfer (FRET).

RESULTS

EGF treatment significantly enhanced the direct molecular interaction between ICln and HSPC038 and also resulted in an increase of ICln and HSPC038 association with the plasma membrane. Importantly, these events are associated with a significant increase of ICl(swell).

CONCLUSIONS

The present data indicate that EGF might exert its role in the modulation of volume-sensitive chloride currents in part through activation and translocation of ICln and HSPC038 to the plasma membrane.

Hypertonicity sensing in organum vasculosum lamina terminalis neurons: a mechanical process involving TRPV1 but not TRPV4

Primary osmosensory neurons in the mouse organum vasculosum lamina terminalis (OVLT) transduce hypertonicity via the activation of nonselective cation channels that cause membrane depolarization and increased action potential discharge, and this effect is absent in mice lacking expression of the transient receptor potential vanilloid 1 (Trpv1) gene (Ciura and Bourque, 2006). However other experiments have indicated that channels encoded by Trpv4 also contribute to central osmosensation in mice (Liedtke and Friedman, 2003; Mizuno et al., 2003). At present, the mechanism by which hypertonicity modulates cation channels in OVLT neurons is unknown, and it remains unclear whether Trpv1 and Trpv4 both contribute to this process. Here, we show that physical shrinking is necessary and sufficient to mediate hypertonicity sensing in OVLT neurons isolated from adult mice. Steps coupling progressive decreases in cell volume to increased neuronal activity were quantitatively equivalent whether shrinking was evoked by osmotic pressure or mechanical aspiration. Finally, modulation of OVLT neurons by tonicity or mechanical stimulation was unaffected by deletion of trpv4 but was abolished in cells lacking Trpv1 or wild-type neurons treated with the TRPV1 antagonist SB366791. Thus, hypertonicity sensing is a mechanical process requiring Trpv1, but not Trpv4.

Regulatory volume response following hypotonic stress in Atlantic salmon erythrocytes

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

The role of acidocalcisomes in the stress response of Trypanosoma cruzi

Acidocalcisomes of Trypanosoma cruzi are acidic calcium-containing organelles rich in phosphorus in the form of pyrophosphate (PP(i)) and polyphosphate (poly P). Acidification of the organelles is driven by vacuolar proton pumps, one of which, the vacuolar-type proton pyrophosphatase, is absent in mammalian cells. A calcium ATPase is involved in calcium uptake, and an aquaporin is important for water transport. Enzymes involved in the synthesis and degradation of PPi and poly P are present within the organelle. Acidocalcisomes function as storage sites for cations and phosphorus, participate in PP(i) and poly P metabolism and volume regulation and are essential for virulence. A signalling pathway involving cyclic AMP generation is important for fusion of acidocalcisomes to the contractile vacuole complex, transference of aquaporin and volume regulation. This pathway is an excellent target for chemotherapy as shown by the effects of phosphodiesterase C inhibitors on parasite survival.

Propeptide of aminopeptidase 1 protein mediates aggregation and vesicle formation in cytoplasm-to-vacuole targeting pathway

Misfolded protein aggregation causes disease and aging; autophagy counteracts this by eliminating damaged components, enabling cells to survive starvation. The cytoplasm-to-vacuole targeting pathway in yeast encompasses the aggregation of the premature form of aminopeptidase 1 (prApe1) in cytosol and its sequestration by autophagic proteins into a vesicle for vacuolar transport. We show that the propeptide of Ape1 is important for aggregation and vesicle formation and that it is sufficient for binding to prApe1 and Atg19. Defective aggregation disrupts vacuolar transport, suggesting that aggregate shape is important in vesicle formation, whereas Atg19 binding is not sufficient for vacuolar transport. Aggregation involves hydrophobicity, whereas Atg19 binding requires additional electrostatic interactions. Ape1 dodecamerization may cluster propeptides into trimeric structures, with sufficient affinity to form propeptide hexamers by binding to other dodecamers, causing aggregation. We show that Ape1 aggregates bind Atg19 and Atg8 in vitro; this could be used as a scaffold for an in vitro assay of autophagosome formation to elucidate the mechanisms of autophagy.

Adaptor protein-3 (AP-3) complex mediates the biogenesis of acidocalcisomes and is essential for growth and virulence of Trypanosoma brucei

Acidocalcisomes are acidic calcium and polyphosphate storage organelles found in a diverse range of organisms. Here we present evidence that the biogenesis of acidocalcisomes in Trypanosoma brucei is linked to the expression of adaptor protein-3 (AP-3) complex. Localization studies in cell lines expressing β3 and δ subunits of AP-3 fused to epitope tags revealed their partial co-localization with the vacuolar proton pyrophosphatase, a marker of acidocalcisomes, with the Golgi marker Golgi reassembly and stacking protein, and with antibodies against the small GTPase Rab11. Ablation of the β3 subunit by RNA interference (RNAi) resulted in disappearance of acidocalcisomes from both procyclic and bloodstream form trypanosomes, as revealed by immmunofluorescence and electron microscopy assays, with no alterations in trafficking of different markers to lysosomes. Knockdown of the β3 subunit resulted in lower acidic calcium, pyrophosphate, and polyphosphate content as well as defects in growth in culture, resistance to osmotic stress, and virulence in mice. Similar results were obtained by knocking down the expression of the δ subunit of AP-3. These results indicate that AP-3 is essential for the biogenesis of acidocalcisomes and for growth and virulence of T. brucei.

Permissive effect of EGFR-activated pathways on RVI and their anti-apoptotic effect in hypertonicity-exposed mIMCD3 cells

Hypertonicity is a stressful stimulus leading to cell shrinkage and apoptotic cell death. Apoptosis can be prevented if cells are able to activate the mechanism of RVI (regulatory volume increase). This study in mIMCD3 cells presents evidence of a permissive role of the EGFR (epidermal growth factor receptor) on RVI, achieved for the most part through the two main EGFR-triggered signalling chains, the MAPK (mitogen-activated protein kinase)/ERK (extracellular-signal-regulated kinase) and the PI3K (phosphoinositide 3-kinase)/Akt (also known as protein kinase B) pathways. Hyperosmotic solutions (450 mosM) made by addition of NaCl, increased EGFR phosphorylation, which is prevented by GM6001 and AG1478, blockers respectively, of MMPs (matrix metalloproteinases) and EGFR. Inhibition of EGFR, ERK (PD98059) or PI3K/Akt (wortmannin) phosphorylation reduced RVI by 60, 48 and 58% respectively. The NHE (Na+/H+ exchanger) seems to be the essential mediator of this effect since (i) NHE is the main contributor to RVI, (ii) EGFR, ERK and PI3K/Akt blockers added together with the NHE blocker zoniporide reduce RVI by non-additive effects and (iii) All the blockers significantly lowered the NHE rate in cells challenged by an NH4Cl pulse. Besides reducing RVI, the inhibition of MMP, EGFR and PI3K/Akt had a strong pro-apoptotic effect increasing cell death by 2–3.7-fold. This effect was significantly lower when RVI inhibition did not involve the EGFR-PI3K/Akt pathway. These results provide evidence that Akt and its permissive effect on RVI have a predominant influence on cell survival under hypertonic conditions in IMCD3 cells. This role of Akt operates under the influence of EGFR activation, promoted by MMP.

Acidic environment leads to ROS-induced MAPK signaling in cancer cells

Tumor micromilieu often shows pronounced acidosis forcing cells to adapt their phenotype towards enhanced tumorigenesis induced by altered cellular signalling and transcriptional regulation. In the presents study mechanisms and potential consequences of the crosstalk between extra- and intracellular pH (pH_e, pH_i) and mitogen-activated-protein-kinases (ERK1/2, p38) was analyzed. Data were obtained mainly in AT1 R-3327 prostate carcinoma cells, but the principle importance was confirmed in 5 other cell types. Extracellular acidosis leads to a rapid and sustained decrease of pH_i in parallel to p38 phosphorylation in all cell types and to ERK1/2 phosphorylation in 3 of 6 cell types. Furthermore, p38 phosphorylation was elicited by sole intracellular lactacidosis at normal pH_e. Inhibition of ERK1/2 phosphorylation during acidosis led to necrotic cell death. No evidence for the involvement of the kinases c-SRC, PKC, PKA, PI3K or EGFR nor changes in cell volume in acidosis-induced MAPK activation was obtained. However, our data reveal that acidosis enhances the formation of reactive oxygen species (ROS), probably originating from mitochondria, which subsequently trigger MAPK phosphorylation. Scavenging of ROS prevented acidosis-induced MAPK phosphorylation whereas addition of H₂O₂ enhanced it. Finally, acidosis increased phosphorylation of the transcription factor CREB via p38, leading to increased transcriptional activity of a CRE-reporter even 24 h after switching the cells back to a normal environmental milieu. Thus, an acidic tumor microenvironment can induce a longer lasting p38-CREB-medited change in the transcriptional program, which may maintain the altered phenotype even when the cells leave the tumor environment.

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

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

The ClC-3 chloride channels in cardiovascular disease

ClC-3 is a member of the ClC voltage-gated chloride (Cl(-)) channel superfamily. Recent studies have demonstrated the abundant expression and pleiotropy of ClC-3 in cardiac atrial and ventricular myocytes, vascular smooth muscle cells, and endothelial cells. ClC-3 Cl(-) channels can be activated by increase in cell volume, direct stretch of β1-integrin through focal adhesion kinase and many active molecules or growth factors including angiotensin II and endothelin-1-mediated signaling pathways, Ca(2+)/calmodulin-dependent protein kinase II and reactive oxygen species. ClC-3 may function as a key component of the volume-regulated Cl(-) channels, a superoxide anion transport and/or NADPH oxidase interaction partner, and a regulator of many other transporters. ClC-3 has been implicated in the regulation of electrical activity, cell volume, proliferation, differentiation, migration, apoptosis and intracellular pH. This review will highlight the major findings and recent advances in the study of ClC-3 Cl(-) channels in the cardiovascular system and discuss their important roles in cardiac and vascular remodeling during hypertension, myocardial hypertrophy, ischemia/reperfusion, and heart failure.

Identification of contractile vacuole proteins in Trypanosoma cruzi

Contractile vacuole complexes are critical components of cell volume regulation and have been shown to have other functional roles in several free-living protists. However, very little is known about the functions of the contractile vacuole complex of the parasite Trypanosoma cruzi, the etiologic agent of Chagas disease, other than a role in osmoregulation. Identification of the protein composition of these organelles is important for understanding their physiological roles. We applied a combined proteomic and bioinfomatic approach to identify proteins localized to the contractile vacuole. Proteomic analysis of a T. cruzi fraction enriched for contractile vacuoles and analyzed by one-dimensional gel electrophoresis and LC-MS/MS resulted in the addition of 109 newly detected proteins to the group of expressed proteins of epimastigotes. We also identified different peptides that map to at least 39 members of the dispersed gene family 1 (DGF-1) providing evidence that many members of this family are simultaneously expressed in epimastigotes. Of the proteins present in the fraction we selected several homologues with known localizations in contractile vacuoles of other organisms and others that we expected to be present in these vacuoles on the basis of their potential roles. We determined the localization of each by expression as GFP-fusion proteins or with specific antibodies. Six of these putative proteins (Rab11, Rab32, AP180, ATPase subunit B, VAMP1, and phosphate transporter) predominantly localized to the vacuole bladder. TcSNARE2.1, TcSNARE2.2, and calmodulin localized to the spongiome. Calmodulin was also cytosolic. Our results demonstrate the utility of combining subcellular fractionation, proteomic analysis, and bioinformatic approaches for localization of organellar proteins that are difficult to detect with whole cell methodologies. The CV localization of the proteins investigated revealed potential novel roles of these organelles in phosphate metabolism and provided information on the potential participation of adaptor protein complexes in their biogenesis.

A protective mechanism against antibiotic-induced ototoxicity: role of prestin

Hearing loss or ototoxicity is one of the major side effects associated with the use of the antibiotics, particularly aminoglycosides (AGs), which are the most commonly used antibiotics worldwide. However, the molecular and cellular events involved in the antibiotic-induced ototoxicity remains unclear. In the present study, we test the possibility that prestin, the motor protein specifically expressed in the basolateral membrane of outer hair cells (OHCs) in the cochlea with electromotility responsible for sound amplification, may be involved in the process of AG-induced apoptosis in OHCs. Our results from both mice model and cultured cell line indicate a previously unexpected role of prestin, in mediating antibiotic-induced apoptosis, the effect of which is associated with its anion-transporting capacity. The observed downregulation of prestin mRNA prior to detectable apoptosis in OHCs and hearing loss in the antibiotic-treated mice is interesting, which may serve as a protective mechanism against hearing loss induced by AGs in the early stage.

Choline and betaine in health and disease

Choline is an essential nutrient, but is also formed by de novo synthesis. Choline and its derivatives serve as components of structural lipoproteins, blood and membrane lipids, and as a precursor of the neurotransmitter acetylcholine. Pre-and postnatal choline availability is important for neurodevelopment in rodents. Choline is oxidized to betaine that serves as an osmoregulator and is a substrate in the betaine-homocysteine methyltransferase reaction, which links choline and betaine to the folate-dependent one-carbon metabolism. Choline and betaine are important sources of one-carbon units, in particular, during folate deficiency. Choline or betaine supplementation in humans reduces concentration of total homocysteine (tHcy), and plasma betaine is a strong predictor of plasma tHcy in individuals with low plasma concentration of folate and other B vitamins (B₂, B₆, and B₁₂) in combination TT genotype of the methylenetetrahydrofolate reductase 677 C->T polymorphism. The link to one-carbon metabolism and the recent availability of food composition data have motivated studies on choline and betaine as risk factors of chronic diseases previously studied in relation to folate and homocysteine status. High intake and plasma level of choline in the mother seems to afford reduced risk of neural tube defects. Intake of choline and betaine shows no consistent relation to cancer or cardiovascular risk or risk factors, whereas an unfavorable cardiovascular risk factor profile was associated with high choline and low betaine concentrations in plasma. Thus, choline and betaine showed opposite relations with key components of metabolic syndrome, suggesting a disruption of mitochondrial choline oxidation to betaine as part of the mitochondrial dysfunction in metabolic syndrome.

Nitric oxide/reactive oxygen species generation and nitroso/redox imbalance in heart failure: from molecular mechanisms to therapeutic implications

Adaptation of the heart to intrinsic and external stress involves complex modifications at the molecular and cellular levels that lead to tissue remodeling, functional and metabolic alterations, and finally to failure depending upon the nature, intensity, and chronicity of the stress. Reactive oxygen species (ROS) have long been considered as merely harmful entities, but their role as second messengers has gradually emerged. At the same time, our comprehension of the multifaceted role of nitric oxide (NO) and the related reactive nitrogen species (RNS) has been upgraded. The tight interlay between ROS and RNS suggests that their imbalance may implicate the impairment in physiological NO/redox-based signaling that contributes to the failing of the cardiovascular system. This review initially provides basic concepts on the role of nitroso/oxidative stress in the pathophysiology of heart failure with a particular focus on sources of ROS/RNS, their downstream targets, and endogenous modulators. Then, the role of NO/redox regulation of cardiomyocyte function, including calcium homeostasis, electrogenesis, and insulin signaling pathways, is described. Finally, an overview of old and emerging therapeutic opportunities in heart failure is presented, focusing on modulation of NO/redox mechanisms and discussing benefits and limitations.

Basolateral ion transporters involved in colonic epithelial electrolyte absorption, anion secretion and cellular homeostasis

Electrolyte transporters located in the basolateral membrane of the colonic epithelium are increasingly appreciated as elaborately regulated components of specific transport functions and cellular homeostasis: During electrolyte absorption, Na(+) /K(+) ATPase, Cl⁻ conductance, Cl⁻/HCO₃⁻ exchange, K(+) /Cl⁻ cotransport and K(+) channels are candidates for basolateral Na(+) , Cl⁻ and K(+) extrusion. The process of colonic anion secretion involves basolateral Na(+) /K(+) /2Cl⁻ , and probably also Na(+) /HCO₃⁻ cotransport, as well as Na(+) /K(+) ATPase and K(+) channels to supply substrate, stabilize the membrane potential and generate driving force respectively. Together with a multitude of additional transport systems, Na(+) /H(+) exchange and Na(+) /HCO₃⁻ cotransport have been implicated in colonocyte pH(i) and volume homeostasis. The purpose of this article is to summarize recently gathered information on the molecular identity, function and regulation of the involved basolateral transport systems in native tissue. Furthermore, we discuss how these findings can help to integrate these systems into the transport function and the cellular homoeostasis of colonic epithelial cells. Finally, disturbances of basolateral electrolyte transport during disease states such as mucosal inflammation will be reviewed.

Recent advances in the molecular and functional characterization of acid/base and electrolyte transporters in the basolateral membranes of gastric and duodenal epithelial cells

All segments of the gastrointestinal tract are comprised of an elaborately folded epithelium that expresses a variety of cell types and performs multiple secretory and absorptive functions. While the apical membrane expresses the electrolyte transporters that secrete or absorb electrolytes and water, basolateral transporters regulate the secretory or absorptive rates. During gastric acid formation, Cl⁻/HCO₃⁻ and Na(+) /H(+) exchange and other transporters secure Cl⁻ re-supply as well as pH and volume regulation. Gastric surface cells utilize ion transporters to secrete HCO₃⁻, maintain pH(i) during a luminal acid load and repair damaged surface areas during the process of epithelial restitution. Na(+)/H(+) exchange and Na(+)/HCO₃⁻ cotransport serve basolateral acid/base import for gastroduodenal HCO₃⁻ secretion. The gastric and duodenal epithelium also absorbs salt and water. Recent molecular information on novel ion transporters expressed in the gastric and duodenal epithelium has exploded; however, a function has not been found yet for all transporters. The purpose of this review is to summarize current knowledge on the molecular identity and cellular function of basolateral ion transporters in the gastric and duodenal epithelium.

Eag and HERG potassium channels as novel therapeutic targets in cancer

Voltage gated potassium channels have been extensively studied in relation to cancer. In this review, we will focus on the role of two potassium channels, Ether à-go-go (Eag), Human ether à-go-go related gene (HERG), in cancer and their potential therapeutic utility in the treatment of cancer. Eag and HERG are expressed in cancers of various organs and have been implicated in cell cycle progression and proliferation of cancer cells. Inhibition of these channels has been shown to reduce proliferation both in vitro and vivo studies identifying potassium channel modulators as putative inhibitors of tumour progression. Eag channels in view of their restricted expression in normal tissue may emerge as novel tumour biomarkers.

Angiotensin inhibition and longevity: a question of hydration

With the advancement of medical and investigative science, it is somewhat surprising that although it is possible to stabilise medical patients with hypertension and the associated kidney dysfunction, obesity, diabetes and even cancer, there is still no clear method of significantly reducing these chronic disease pathologies, and thus, extending life expectancy. There is one hormone common to these pathologies, the antagonism of which goes some way to clinical improvements, and this is angiotensin, which is released during hypovolaemia. Angiotensin antagonists are used to treat many of these pathologies, and it has been shown in the obesity literature that angiotensin antagonists decrease weight, but also increase the drinking of water. Increased cellular hydration, and hence, improved mitochondrial metabolism could be one of the mechanisms for the reduction in weight seen in these studies, as well as for reducing the other pathologies, all showing metabolic dysfunction. It appears that the application of straightforward physiological regulation might be an appropriate medical approach to the prevention of hypertension, kidney disease, obesity, diabetes and cancer, and thus, to an increased life expectancy.

Swelling-induced Ca²+ influx and K+ efflux in American alligator erythrocytes

The American alligator can hibernate during winter, which may lead to osmotic imbalance because of reduced kidney function and lack of food consumption during this period. Accordingly, we hypothesized that their red blood cells would have a well-developed regulatory volume decrease (RVD) to cope with the homeostatic challenges associated with torpor. Osmotic fragility was determined optically, mean cell volume was measured by electronic sizing, and changes in intracellular Ca²+ concentration were visualized using fluorescence microscopy and fluo-4-AM. Osmotic fragility increased and the ability to regulate volume was inhibited when extracellular Na+ was replaced with K+, or when cells were exposed to the K+ channel inhibitor quinine, indicating a requirement of K+ efflux for RVD. Addition of the ionophore gramicidin to the extracellular medium decreased osmotic fragility and also potentiated volume recovery, even in the presence of quinine. In addition, hypotonic shock (0.5 x Ringer) caused an increase in cytosolic Ca²+, which resulted from Ca²+ influx because it was not observed when extracellular Ca²+ was chelated with EGTA (ethylene glycol-bis(2-aminoethylether)-N,N,N',N'-tetraacetic acid). Furthermore, cells loaded with BAPTA-AM (1,2-bis(2-aminophenoxymethyl)ethane-N,N,N',N'-tetraacetic acid tetrakis(acetoxymethyl) ester) or exposed to a low Ca²+-EGTA hypotonic Ringer had a greater osmotic fragility and also failed to recover from cell swelling, indicating that extracellular Ca²+ was needed for RVD. Gramicidin reversed the inhibitory effect of low extracellular Ca²+. Finally, and surprisingly, the Ca²+ ionophore A23187 increased osmotic fragility and inhibited volume recovery. Taken together, our results show that cell swelling activated a K+ permeable pathway via a Ca²+-dependent mechanism, and this process mediated K+ loss during RVD.

Ion channels in volume regulation of clonal kidney cells

OBJECTIVES

Clonal kidney cells (Vero cells) are extensively utilized in the manufacture of biological preparations for disease diagnostics and therapeutics and also in preparation of vaccines. In all cells, regulation of volume is an essential function coupled to a variety of physiological processes and is a topic of interest. The objective here was to investigate involvement of ion channels in the process of volume regulation of Vero cells.

METHODS

Involvement of ion channels in cell volume regulation was studied using video-microscopy and flow cytometry. Pharmacologically unaltered cells of different sizes, which are presumably at different phases of the cell cycle, were used.

RESULTS

Ion transport inhibitors altered all phases of regulatory volume decrease (RVD) of Vero cells, rate of initial cell swelling, V(max) and volume recovery. Effects were dependent on type of inhibitor and on cell size (cell cycle phase). Participation of aquaporins in RVD was suggested. Inhibitors decelerated growth, arresting Vero cells at the G(0) /G(1) phase boundary. Electrophysiological study confirmed presence of volume-activated Cl(-) channels and K(+) channels in plasmatic membranes of the cells.

CONCLUSION

Vero cells of all sizes maintained the ability to recover from osmotic swelling. Activity of ion channels was one of the key factors that controlled volume regulation and proliferation of the cells.

Brain hypoxanthine concentration correlates to lactate/pyruvate ratio but not intracranial pressure in patients with acute liver failure

BACKGROUND & AIMS

The pathogenesis of cerebral edema in acute liver failure is suggested, in in vitro and animal studies, to involve a compromised oxidative metabolism with a decrease in cerebral ATP levels and an increase in purine concentrations. In this study we hypothesize that the cerebral concentrations of hypoxanthine, inosine, and lactate/pyruvate (LP) ratio are increased and correlated in patients with acute liver failure. Furthermore, we expect the purines and L/P ratio to correlate with intracranial pressure (ICP) (positively), and cerebral perfusion pressure (CPP) (negatively).

METHODS

In 17 patients (aged 18-60 years) with acute liver failure and severe hyperammonemia (182 ± 36 μM (mean ± SD)), cerebral microdialysis was performed, and ICP and CPP were monitored. Microdialysate concentrations of hypoxanthine, inosine, lactate, and pyruvate were measured.

RESULTS

The hypoxanthine concentration was 23.0 ± 12 μM in early samples and 11.7 ± 6.8 μM in late samples (normal level ~2.0 μM). The inosine concentration was 7.2 ± 7.1 μM and 2.8 ± 1.6 μM, and the LP ratio was 55.8 ± 21.6 and 45.6 ± 20.8, respectively (normal level ~18). Hypoxanthine correlated significantly to LP ratio (r(2)=0.40, p<0.01) while inosine did not. The purine levels and L/P ratio did not correlate to ICP or CPP, nor did they differ between patients with high ICP (>20 mmHg, n=9) and patients without (n=8).

CONCLUSIONS

This study shows that the high cerebral LP ratio correlates to the hypoxanthine level in patients with acute liver failure. However, these metabolic alterations were not related to the development of intracranial hypertension.

The beta1 subunit of the Na,K-ATPase pump interacts with megalencephalic leucoencephalopathy with subcortical cysts protein 1 (MLC1) in brain astrocytes: new insights into MLC pathogenesis

Megalencephalic leucoencephalopathy with subcortical cysts (MLC) is a rare congenital leucodystrophy caused by mutations in MLC1, a membrane protein of unknown function. MLC1 expression in astrocyte end-feet contacting blood vessels and meninges, along with brain swelling, fluid cysts and myelin vacuolation observed in MLC patients, suggests a possible role for MLC1 in the regulation of fluid and ion homeostasis and cellular volume changes. To identify MLC1 direct interactors and dissect the molecular pathways in which MLC1 is involved, we used NH2-MLC1 domain as a bait to screen a human brain library in a yeast two-hybrid assay. We identified the β1 subunit of the Na,K-ATPase pump as one of the interacting clones and confirmed it by pull-downs, co-fractionation assays and immunofluorescence stainings in human and rat astrocytes in vitro and in brain tissue. By performing ouabain-affinity chromatography on astrocyte and brain extracts, we isolated MLC1 and the whole Na,K-ATPase enzyme in a multiprotein complex that included Kir4.1, syntrophin and dystrobrevin. Because Na,K-ATPase is involved in intracellular osmotic control and volume regulation, we investigated the effect of hypo-osmotic stress on MLC1/Na,K-ATPase relationship in astrocytes. We found that hypo-osmotic conditions increased MLC1 membrane expression and favoured MLC1/Na,K-ATPase-β1 association. Moreover, hypo-osmosis induced astrocyte swelling and the reversible formation of endosome-derived vacuoles, where the two proteins co-localized. These data suggest that through its interaction with Na,K-ATPase, MLC1 is involved in the control of intracellular osmotic conditions and volume regulation in astrocytes, opening new perspectives for understanding the pathological mechanisms of MLC disease.

Metabolomic profiling reveals biochemical pathways and biomarkers associated with pathogenesis in cystic fibrosis cells

Cystic fibrosis (CF) is a life-shortening disease caused by a mutation in the cystic fibrosis transmembrane conductance regulator (CFTR) gene. To gain an understanding of the epithelial dysfunction associated with CF mutations and discover biomarkers for therapeutics development, untargeted metabolomic analysis was performed on primary human airway epithelial cell cultures from three separate cohorts of CF patients and non-CF subjects. Statistical analysis revealed a set of reproducible and significant metabolic differences between the CF and non-CF cells. Aside from changes that were consistent with known CF effects, such as diminished cellular regulation against oxidative stress and osmotic stress, new observations on the cellular metabolism in the disease were generated. In the CF cells, the levels of various purine nucleotides, which may function to regulate cellular responses via purinergic signaling, were significantly decreased. Furthermore, CF cells exhibited reduced glucose metabolism in glycolysis, pentose phosphate pathway, and sorbitol pathway, which may further exacerbate oxidative stress and limit the epithelial cell response to environmental pressure. Taken together, these findings reveal novel metabolic abnormalities associated with the CF pathological process and identify a panel of potential biomarkers for therapeutic development using this model system.

Manganese exposure inhibits the clearance of extracellular GABA and influences taurine homeostasis in the striatum of developing rats

Manganese (Mn) accumulation in the brain has been shown to alter the neurochemistry of the basal ganglia. Mn-induced alterations in dopamine biology are fairly well understood, but recently more evidence has emerged characterizing the role of γ-aminobutyric acid (GABA) in this dysfunction. The purpose of this study was to determine if the previously observed Mn-induced increase in extracellular GABA (GABA(EC)) was due to altered GABA transporter (GAT) function, and whether Mn perturbs other amino acid neurotransmitters, namely taurine and glycine (known modulators of GABA). Extracellular GABA, taurine, and glycine concentrations were collected from the striatum of control (CN) or Mn-exposed Sprague-Dawley rats using in vivo microdialysis, and the GAT inhibitor nipecotic acid (NA) was used to probe GAT function. Tissue and extracellular Mn levels were significantly increased, and the Fe:Mn ratio was decreased 36-fold in the extracellular space due to Mn-exposure. NA led to a 2-fold increase in GABA(EC) of CNs, a response that was attenuated by Mn. Taurine responded inversely to GABA, and a novel 10-fold increase in taurine was observed after the removal of NA in CNs. Mn blunted this response and nearly abolished extracellular taurine throughout collection. Striatal taurine transporter (Slc6a6) mRNA levels were significantly increased with Mn-exposure, and Mn significantly increased (3)H-Taurine uptake after 3-min exposure in primary rat astrocytes. These data suggest that Mn increases GABA(EC) by inhibiting the function of GAT, and that perturbed taurine homeostasis potentially impacts neural function by jeopardizing the osmoregulatory and neuromodulatory functions of taurine in the brain.

Perinatal taurine exposure alters renal potassium excretion mechanisms in adult conscious rats

Perinatal taurine exposure has long-term effects on the arterial pressure and renal function. This study tests its influence on renal potassium excretion in young adult, conscious rats. Female Sprague-Dawley rats were fed normal rat chow and given water alone (C), 3% beta-alanine in water (taurine depletion, TD) or 3% taurine in water (taurine supplementation, TS), either from conception until delivery (fetal period; TDF or TSF) or from delivery until weaning (lactation period; TDL or TSL). In Experiment 1, male offspring were fed normal rat chow and tap water, while in Experiment 2, beta-alanine and taurine were treated from conception until weaning and then female pups were fed normal rat chow and 5% glucose in drinking water (CG, TDG or TSG) or water alone (CW, TDW or TSW). At 7-8 weeks of age, renal potassium excretion was measured at rest and after an acute saline load (5% of body weight) in conscious, restrained rats. Although all male groups displayed similar renal potassium excretion, TSF rats slightly increased fractional potassium excretion at rest but not in response to saline load, whereas TDF did the opposite. Plasma potassium concentration was only slightly altered by the diet manipulations. In female offspring, none of the perinatal treatments significantly altered renal potassium excretion at rest or after saline load. High sugar intake slightly decreased potassium excretion at rest in TDG and TSG, but only the TDG group displayed a decreased response to saline load. The present data indicates that perinatal taurine exposure only mildly influences renal potassium excretion in adult male and female rats.

Aquaporins in spermatozoa and testicular germ cells: identification and potential role

Mammalian spermatozoa have relatively high water permeability and swell readily, as in the hypo-osmotic swelling test used in the andrology clinic. Physiologically, spermatozoa experience changes in the osmolality of the surrounding fluids in both the male and the female tracts on their journey from the testis to the ovum. Sperm volume regulation in response to such osmotic challenges is important to maintain a stable cell size for the normal shape and function of the sperm tail. Alongside ion channels for the fluxes of osmolytes, water channels would be crucial for sperm volume regulation. In contrast to the deep knowledge and numerous studies on somatic cell aquaporins (AQPs), the understanding of sperm AQPs is limited. Among the 13 AQPs, convincing evidence for their presence in spermatozoa has been confined to AQP7, AQP8 and AQP11. Overall, current findings indicate a major role of AQP8 in water influx and efflux for sperm volume regulation, which is required for natural fertilization. The preliminary data suggestive of a role for AQP7 in sperm glycerol metabolism needs further substantiation. The association of AQP11 with the residual cytoplasm of elongated spermatids and the distal tail of spermatozoa supports the hypothesis of more than just a role in conferring water permeability and also in the turnover and recycling of surplus cellular components made redundant during spermiogenesis and spermiation. This would be crucial for the maintenance of a germinal epithelium functioning efficiently in the production of spermatozoa.

Effects of supplementation with free glutamine and the dipeptide alanyl-glutamine on parameters of muscle damage and inflammation in rats submitted to prolonged exercise

In this study, we investigated the effect of the supplementation with the dipeptide L-alanyl-L-glutamine (DIP) and a solution containing L-glutamine and L-alanine on plasma levels markers of muscle damage and levels of pro-inflammatory cytokines and glutamine metabolism in rats submitted to prolonged exercise. Rats were submitted to sessions of swim training for 6 weeks. Twenty-one days prior to euthanasia, the animals were supplemented with DIP (n = 8) (1.5 g.kg(-1)), a solution of free L-glutamine (1 g.kg(-1)) and free L-alanine (0.61 g.kg(-1)) (G&A, n = 8) or water (control (CON), n = 8). Animals were killed at rest before (R), after prolonged exercise (PE-2 h of exercise). Plasma concentrations of glutamine, glutamate, tumour necrosis factor-alpha (TNF-alpha), prostaglandin E2 (PGE2) and activity of creatine kinase (CK), lactate dehydrogenase (LDH) and muscle concentrations of glutamine and glutamate were measured. The concentrations of plasma TNF-alpha, PGE2 and the activity of CK were lower in the G&A-R and DIP-R groups, compared to the CON-R. Glutamine in plasma (p < 0.04) and soleus muscle (p < 0.001) was higher in the DIP-R and G&A-R groups relative to the CON-R group. G&A-PE and DIP-PE groups exhibited lower concentrations of plasma PGE2 (p < 0.05) and TNF-alpha (p < 0.05), and higher concentrations of glutamine and glutamate in soleus (p < 0.001) and gastrocnemius muscles (p < 0.05) relative to the CON-PE group. We concluded that supplementation with free L-glutamine and the dipeptide LL-alanyl-LL-glutamine represents an effective source of glutamine, which may attenuate inflammation biomarkers after periods of training and plasma levels of CK and the inflammatory response induced by prolonged exercise.

Na, K-ATPase: Ubiquitous Multifunctional Transmembrane Protein and its Relevance to Various Pathophysiological Conditions

The Na(+), K(+)-ATPase (NKA) is an ubiquitous enzyme consisting of α, β and γ subunits, and is responsible for the creation and maintenance of the Na(+) and K(+) gradients across the cell membrane by transporting 3 Na(+) out and 2 K(+) into the cell. Sodium pump regulation is tissue as well as isoform specific. Intracellular messengers differentially regulate the activity of the individual NKA isozymes. Regulation of specific NKA isozymes gives cells the ability to precisely coordinate NKA activity to their physiological requirements. It is the only known receptor for the cardiac glycosides used to treat congestive heart failure and cardiac arrhythmias. Endogenous ligands structurally similar to cardiac glycosides may act as natural regulators of the sodium pump in heart and other tissues. Identification of naturally occurring regulators of  NKA could initiate the discovery of new hormone-like control systems involved in the etiology of selected disease processes, hence the importance of understanding the relation of the sodium pump and its ligands to disease. Diabetes has a marked effect on the metabolism of a variety of tissues and because the NKA is critical for the membrane potential and many transports, a change in its activity in diabetes would have profound consequence in these tissues. NKA is also involved in hypertension, salt balance, cardiovascular and renal disorders, sperm capacitation, cell volume regulation, apoptosis, rheumatoid arthritis, sepsis, neurological disorders, lung edema clearance and preeclampsia. NKA activity and expression in the collecting duct of kidney are modulated physiologically by hormones like aldosterone, vasopressin, and insulin. NKA enzyme activity and subunit levels are reduced in carcinoma, NKA-β levels were highly reduced in an invasive form of human renal clear cell carcinoma, androgen-dependent prostate cancer, in early stages of urothelial cancer, as well as in poorly differentiated, highly motile carcinoma cell lines obtained from various tissues suggesting a functional link between reduced NKA-β expression and cancer progression. It could be a target for the development of anticancer drugs as it serves as a signal transducer, it is a player in cell adhesion and its aberrant expression and activity are implicated in the development and progression of different cancers.

KEYWORDS

Na(+), K(+)-ATPase (NKA); Cardiotonic steroids (CTS); Diabetes; Hypertension; Cardiovascular and renal disorders; Signal transducer; Anticancer drugs.

Protective effect of betaine on changes in the levels of membrane-bound ATPase activity and mineral status in experimentally induced myocardial infarction in Wistar rats

Cardiovascular diseases are emerging as a major public health problem in most parts of the world even in developing countries still afflicted by infectious diseases, undernutrition, and other illnesses related to poverty. In the present study, we investigated the protective effect of betaine, a potent lipotropic molecule, on changes in the levels of membrane-bound ATPase activities, lipid peroxidation, sulfhydryl activities, and mineral status in isoprenaline-induced myocardial infarction in Wistar rats, an animal model of myocardial infarction in man. Oral administration of betaine (250 mg/kg body weight/day for a period of 30 days) significantly (p < 0.05) reduced the isoprenaline-induced abnormalities noted in the levels of sodium, potassium, and calcium in plasma and heart tissue. Pretreatment with betaine significantly attenuated isoprenaline-induced membrane-bound ATPase depletion in the heart tissue and preserved the myocardial membrane-bound ATPase activities at levels comparable to that of control rats. Oral administration of betaine significantly attenuated the isoprenaline-altered sulfhydryl groups in the heart tissue and preserved the myocardial sulfhydryl activities at levels comparable to that of control rats. It also significantly counteracted the isoprenaline-mediated lipid peroxidation and maintained the level at near normal. In the results of the present study, betaine administration significantly prevented the isoprenaline-induced alterations in the activities of membrane-bound ATPases, lipid peroxides, myocardial sulfhydryl levels, and maintained the mineral status at near normal.

Bestrophin 2: an anion channel associated with neurogenesis in chemosensory systems

The chemosensory neuroepithelia of the vertebrate olfactory system share a life-long ability to regenerate. Novel neurons proliferate from basal stem cells that continuously replace old or damaged sensory neurons. The sensory neurons of the mouse and rat olfactory system specifically express bestrophin 2, a member of the bestrophin family of calcium-activated chloride channels. This channel was recently proposed to operate as a transduction channel in olfactory sensory cilia. We raised a polyclonal antibody against bestrophin 2 and characterized the expression pattern of this protein in the mouse main olfactory epithelium, septal organ of Masera, and vomeronasal organ. Comparison with the maturation markers growth-associated protein 43 and olfactory marker protein revealed that bestrophin 2 was expressed in developing sensory neurons of all chemosensory neuroepithelia, but was restricted to proximal cilia in mature sensory neurons. Our results suggest that bestrophin 2 plays a critical role during differentiation and growth of axons and cilia. In mature olfactory receptor neurons, it appears to support growth and function of sensory cilia.

Cardiolipin and the osmotic stress responses of bacteria

Cells control their own hydration by accumulating solutes when they are exposed to high osmolality media and releasing solutes in response to osmotic down-shocks. Osmosensory transporters mediate solute accumulation and mechanosensitive channels mediate solute release. Escherichia coli serves as a paradigm for studies of cellular osmoregulation. Growth in media of high salinity alters the phospholipid headgroup and fatty acid compositions of bacterial cytoplasmic membranes, in many cases increasing the ratio of anionic to zwitterionic lipid. In E. coli, the proportion of cardiolipin (CL) increases as the proportion of phosphatidylethanolamine (PE) decreases when osmotic stress is imposed with an electrolyte or a non-electrolyte. Osmotic induction of the gene encoding CL synthase (cls) contributes to these changes. The proportion of phosphatidylglycerol (PG) increases at the expense of PE in cls(-) bacteria and, in Bacillus subtilis, the genes encoding CL and PG synthases (clsA and pgsA) are both osmotically regulated. CL is concentrated at the poles of diverse bacterial cells. A FlAsH-tagged variant of osmosensory transporter ProP is also concentrated at E. coli cell poles. Polar concentration of ProP is CL-dependent whereas polar concentration of its paralogue LacY, a H(+)-lactose symporter, is not. The proportion of anionic lipids (CL and PG) modulates the function of ProP in vivo and in vitro. These effects suggest that the osmotic induction of CL synthesis and co-localization of ProP with CL at the cell poles adjust the osmolality range over which ProP activity is controlled by placing it in a CL-rich membrane environment. In contrast, a GFP-tagged variant of mechanosensitive channel MscL is not concentrated at the cell poles but anionic lipids bind to a specific site on each subunit of MscL and influence its function in vitro. The sub-cellular locations and lipid dependencies of other osmosensory systems are not known. Varying CL content is a key element of osmotic adaptation by bacteria but much remains to be learned about its roles in the localization and function of osmoregulatory proteins.