Simultaneous measurement of water volume and pH in single cells using BCECF and fluorescence imaging microscopy

Protective Effect of Peptide Calcium Channel Blocker Omega-Hexatoxin-Hv1a on Epithelial Cell during Ischemia-Reperfusion Injury

Ischemia-reperfusion injury (IRI) is a common phenomenon that develops both from natural causes and during major operations. Many intracellular processes mediated by calcium ions are involved in the development of IRI. Currently, chemical calcium channel blockers are used but they have a number of limitations. In this article, we study the effect of the omega-hexatoxin-Hv1a peptide toxin, an alternative to chemical calcium channel blockers, on the mechanisms of IRI development in epithelial cell culture. The toxin was produced using solid phase peptide synthesis. IRI was caused by deprivation of glucose, serum and oxygen. The data obtained demonstrate that the omega-hexatoxin-Hv1a toxin in nanomolar concentrations is able to prevent the development of apoptosis and necrosis in epithelial cells by reducing the concentration of calcium, sodium and potassium ions, as well as by delaying rapid normalization of the pH level, affecting the mitochondrial potential and oxidative stress. This toxin can be used as an alternative to chemical calcium channel blockers for preventing tissue and organ IRI due to its low-dose requirement and high bioavailability.

Peptide Sodium Channels Modulator Mu-Agatoxin-Aa1a Prevents Ischemia-Reperfusion Injury of Cells

Ischemia-reperfusion injury (IRI) is an irreversible functional and structural injury. Restoration of normal oxygen concentration exacerbates the emergence and development of deadly cells. One of the possible moments of reperfusion damage to cells is an increase in the intracellular concentration of sodium ions. In this article, we study the mu-agatoxin-Aa1a, a modulator of sodium channels, on the processes of IRI cells damage. The toxin was synthesized using an automatic peptide synthesizer. Hypoxia was induced by reducing the content of serum and oxygen in the CHO-K1 culture. The influence of the toxin on the level of apoptosis; intracellular concentration of sodium, calcium, and potassium ions; intracellular pH; totality of reactive oxygen species (ROS), nitric oxide (NO), and ATP; and changes in the mitochondrial potential were studied. The experiments performed show that mu-agatoxin-Aa1a effectively prevents IRI of cells. Toxin reduces the level of apoptosis and prevents a decrease in the intracellular concentration of sodium and calcium ions during IRI. Mu-agatoxin-Aa1a contributes to the maintenance of elevated intracellular pH, reduces the intracellular concentration of ROS, and prevents the decrease in intracellular NO concentration and mitochondrial potential under conditions of reoxygenation/reperfusion. An analysis of experimental data shows that the mu-agatoxin-Aa1a peptide has adaptogenic properties. In the future, this peptide can be used to prevent ischemia/reperfusion tissue damage different genesis.

Inhibition of aquaporin-1 prevents myocardial remodeling by blocking the transmembrane transport of hydrogen peroxide

Pathological remodeling of the myocardium has long been known to involve oxidant signaling, but strategies using systemic antioxidants have generally failed to prevent it. We sought to identify key regulators of oxidant-mediated cardiac hypertrophy amenable to targeted pharmacological therapy. Specific isoforms of the aquaporin water channels have been implicated in oxidant sensing, but their role in heart muscle is unknown. RNA sequencing from human cardiac myocytes revealed that the archetypal AQP1 is a major isoform. AQP1 expression correlates with the severity of hypertrophic remodeling in patients with aortic stenosis. The AQP1 channel was detected at the plasma membrane of human and mouse cardiac myocytes from hypertrophic hearts, where it colocalized with NADPH oxidase-2 and caveolin-3. We show that hydrogen peroxide (H₂O₂), produced extracellularly, is necessary for the hypertrophic response of isolated cardiac myocytes and that AQP1 facilitates the transmembrane transport of H₂O₂ through its water pore, resulting in activation of oxidant-sensitive kinases in cardiac myocytes. Structural analysis of the amino acid residues lining the water pore of AQP1 supports its permeation by H₂O₂ Deletion of Aqp1 or selective blockade of the AQP1 intrasubunit pore inhibited H₂O₂ transport in mouse and human cells and rescued the myocyte hypertrophy in human induced pluripotent stem cell-derived engineered heart muscle. Treatment of mice with a clinically approved AQP1 inhibitor, Bacopaside, attenuated cardiac hypertrophy. We conclude that cardiac hypertrophy is mediated by the transmembrane transport of H₂O₂ by the water channel AQP1 and that inhibitors of AQP1 represent new possibilities for treating hypertrophic cardiomyopathies.

Osmotic Response of Dorsal Root Ganglion Neurons Expressing Wild-Type and Mutant KCC3 Transporters

BACKGROUND/AIMS

Loss-of-Function (LOF) of the potassium chloride cotransporter 3 (KCC3) results in hereditary sensorimotor neuropathy with Agenesis of the Corpus Callosum (HSMN/ACC). Our KCC3 knockout mouse recapitulated axonal swelling and tissue vacuolization observed in autopsies of individuals with HSMN/ACC. We previously documented the first human case of a KCC3 gain-of-function (GOF) in which the patient also exhibited severe peripheral neuropathy. Furthermore, the GOF mouse model exhibited shrunken axons implicating the cotransporter in cell volume homeostasis. It is unclear how both KCC3 LOF and GOF lead to peripheral neuropathy. Thus, we sought to study differences in cell volume regulation of dorsal root ganglion neurons isolated from different mouse lines.

METHODS

Using wide-field microscopy, we measured calcein fluorescence intensity through pinhole measurements at the center of cells and compared cell swelling and cell volume regulation/recovery of wild-type, LOF, and GOF dorsal root ganglia neurons, as well as wild-type neurons treated with a KCC-specific inhibitor.

RESULTS

In contrast to control neurons that swell and volume regulate under a hypotonic challenge, neurons lacking KCC3 swell but fail to volume regulate. Similar data were observed in wild-type neurons treated with the KCC inhibitor. We also show that sensory neurons expressing a constitutively active KCC3 exhibited a blunted swelling phase compared to wild-type neurons, questioning the purely osmotic nature of the swelling phase.

CONCLUSION

These findings demonstrate the integral role of KCC3 in cell volume homeostasis and support the idea that cell volume homeostasis is critical to the health of peripheral nerves.

Genetic and pharmacological inactivation of apical Na+-K+-2Cl- cotransporter 1 in choroid plexus epithelial cells reveals the physiological function of the cotransporter

Choroid plexus epithelial cells (CPECs) secrete cerebrospinal fluid (CSF). They express Na+-K+-ATPase and Na+-K+-2Cl- cotransporter 1 (NKCC1) on their apical membrane, deviating from typical basolateral membrane location in secretory epithelia. Given this peculiarity, the direction of basal net ion fluxes mediated by NKCC1 in CPECs is controversial, and cotransporter function is unclear. Determining the direction of basal NKCC1-mediated fluxes is critical to understanding the function of apical NKCC1. If NKCC1 works in the net efflux mode, it may be directly involved in CSF secretion. Conversely, if NKCC1 works in the net influx mode, it would have an absorptive function, contributing to intracellular Cl- concentration ([Cl-]i) and cell water volume (CWV) maintenance needed for CSF secretion. We resolve this long-standing debate by electron microscopy (EM), live-cell-imaging microscopy (LCIM), and intracellular Na+ and Cl- measurements in single CPECs of NKCC1+/+ and NKCC1-/- mouse. NKCC1-mediated ion and associated water fluxes are tightly linked, thus their direction is inferred by measuring CWV changes. Genetic or pharmacological NKCC1 inactivation produces CPEC shrinkage. EM of NKCC1-/- CPECs in situ shows they are shrunken, forming large dilations of their basolateral extracellular spaces, yet remaining attached by tight junctions. Normarski LCIM shows in vitro CPECs from NKCC1-/- are ~17% smaller than NKCC1+/+. CWV measurements in calcein-loaded CPECs show that bumetanide (10 μM) produces ~16% decrease in CWV in NKCC1+/+ but not in NKCC1-/- CPECs. Our findings suggest that under basal conditions apical NKCC1 is continuously active and works in the net inward flux mode maintaining [Cl-]i and CWV needed for CSF secretion.

A Reliable and Non-destructive Method for Monitoring the Stromal pH in Isolated Chloroplasts Using a Fluorescent pH Probe

The proton gradient established by the pH difference across a biological membrane is essential for many physiological processes, including ATP synthesis and ion and metabolite transport. Currently, ionophores are used to study proton gradients, and determine their importance to biological functions of interest. Because of the lack of an easy method for monitoring the proton gradient across the inner envelope membrane of chloroplasts (ΔpH_env), whether the concentration of ionophores used can effectively abolish the ΔpH_env is not proven for most experiments. To overcome this hindrance, we tried to setup an easy method for real-time monitoring of the stromal pH in buffered, isolated chloroplasts by using fluorescent pH probes; using this method the ΔpH_env can be calculated by subtracting the buffer pH from the measured stromal pH. When three fluorescent dyes, BCECF-AM [2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester], CFDA-SE [5(6)-Carboxyfluorescein diacetate succinimidyl ester] and SNARF-1 carboxylic acid acetate succinimidyl ester were incubated with isolated chloroplasts, BCECF-AM and CFDA-SE, but not the ester-formed SNARF-1 were taken up by chloroplasts and digested with esterase to release high levels of fluorescence. According to its relatively higher pKa value (6.98, near the physiological pH of the stroma), BCECF was chosen for further development. Due to shielding of the excitation and emission lights by chloroplast pigments, the ratiometric fluorescence of BCECF was highly dependent on the concentration of chloroplasts. By using a fixed concentration of chloroplasts, a highly correlated standard curve of pH to the BCECF ratiometric fluorescence with an r-square value of 0.98 was obtained, indicating the reliability of this method. Consistent with previous reports, the light-dependent formation of ΔpH_env can be detected ranging from 0.15 to 0.33 pH units upon illumination. The concentration of the ionophore nigericin required to collapse the ΔpH_env was then studied. The establishment of a non-destructive method of monitoring the stromal pH will be valuable for studying the roles of the ΔpH_env in chloroplast physiology.

TRPM8 is a neuronal osmosensor that regulates eye blinking in mice

Specific peripheral sensory neurons respond to increases in extracellular osmolality but the mechanism responsible for excitation is unknown. Here we show that small increases in osmolality excite isolated mouse dorsal root ganglion (DRG) and trigeminal ganglion (TG) neurons expressing the cold-sensitive TRPM8 channel (transient receptor potential channel, subfamily M, member 8). Hyperosmotic responses were abolished by TRPM8 antagonists, and were absent in DRG and TG neurons isolated from Trpm8(-/-) mice. Heterologously expressed TRPM8 was activated by increased osmolality around physiological levels and inhibited by reduced osmolality. Electrophysiological studies in a mouse corneal preparation demonstrated that osmolality regulated the electrical activity of TRPM8-expressing corneal afferent neurons. Finally, the frequency of eye blinks was reduced in Trpm8(-/-) compared with wild-type mice and topical administration of a TRPM8 antagonist reduced blinking in wild-type mice. Our findings identify TRPM8 as a peripheral osmosensor responsible for the regulation of normal eye-blinking in mice.

Pathophysiology of cerebral oedema in acute liver failure

Cerebral oedema is a devastating consequence of acute liver failure (ALF) and may be associated with the development of intracranial hypertension and death. In ALF, some patients may develop cerebral oedema and increased intracranial pressure but progression to life-threatening intracranial hypertension is less frequent than previously described, complicating less than one third of cases who have proceeded to coma since the advent of improved clinical care. The rapid onset of encephalopathy may be dramatic with the development of asterixis, delirium, seizures and coma. Cytotoxic and vasogenic oedema mechanisms have been implicated with a preponderance of experimental data favouring a cytotoxic mechanism. Astrocyte swelling is the most consistent neuropathological finding in humans with ALF and ammonia plays a definitive role in the development of cytotoxic brain oedema. The mechanism(s) by which ammonia induces astrocyte swelling remains unclear but glutamine accumulation within astrocytes has led to the osmolyte hypothesis. Current evidence also supports an alternate ‘Trojan horse’ hypothesis, with glutamine as a carrier of ammonia into mitochondria, where its accumulation results in oxidative stress, energy failure and ultimately astrocyte swelling. Although a complete breakdown of the blood-brain barrier is not evident in human ALF, increased permeation to water and other small molecules such as ammonia has been demonstrated resulting from subtle alterations in the protein composition of paracellular tight junctions. At present, there is no fully efficacious therapy for cerebral oedema other than liver transplantation and this reflects our incomplete knowledge of the precise mechanisms underlying this process which remain largely unknown.

Effect of acidic pH on flow cytometric detection of bacteria stained with SYBR Green I and their distinction from background

Unspecific background caused by biotic or abiotic particles, cellular debris, or autofluorescence is a well-known interfering parameter when applying flow cytometry to the detection of microorganisms in combination with fluorescent dyes. We present here an attempt to suppress the background signal intensity and thus to improve the detection of microorganisms using the nucleic acid stain SYBR^® Green I. It has been observed that the fluorescent signals from SYBR Green I are greatly reduced at acidic pH. When lowering the pH of pre-stained samples directly prior to flow cytometric analysis, we hypothesized that the signals from particles and cells with membrane damage might therefore be reduced. Signals from intact cells, temporarily maintaining a neutral cytosolic pH, should not be affected. We show here that this principle holds true for lowering background interference, whereas the signals of membrane-compromised dead cells are only affected weakly. Signals from intact live cells at low pH were mostly comparable to signals without acidification. Although this study was solely performed with SYBR^® Green I, the principle of low pH flow cytometry (low pH-FCM) might hold promise when analyzing complex matrices with an abundance of non-cellular matter, especially when expanded to non-DNA binding dyes with a stronger pH dependence of fluorescence than SYBR Green I and a higher pK_a value.

Precipitants of hepatic encephalopathy induce rapid astrocyte swelling in an oxidative stress dependent manner

Hepatic encephalopathy (HE) is seen as the clinical manifestation of a low grade cerebral edema with formation of reactive oxygen and nitrogen species (RNOS). Astrocyte swelling is a crucial event and in cultured astrocytes HE-relevant factors almost instantaneously induce the formation of RNOS. However, short term effects of ammonia, inflammatory cytokines and RNOS on the volume of astrocytes and other brain cells as well as the underlying mechanisms are largely unknown, although a pathogenic link between RNOS formation and swelling in HE has been proposed. This issue was addressed in the present study by means of live-cell volume microscopy of brain cells in vitro. Ammonia, diazepam and pro-inflammatory cytokines such as tumor-necrosis factor-α (TNF-α), interferon-γ, interleukin-1β induced within 20min astrocyte swelling by about 25% accompanied by nuclear swelling of similar magnitude. Astrocyte swelling in response to NH4Cl, TNF-α or diazepam was abolished by the antioxidant epigallocatechin-gallate pointing to an involvement of RNOS. NH4Cl-induced astrocyte swelling was sensitive to inhibition of glutamine synthetase, NADPH oxidase or nitric oxide synthases. In line with a NMDA receptor-, prostanoid- and Ca(2+)-dependence of NH4Cl-induced RNOS formation, Ca(2+) chelation and inhibition of NMDA receptors or cyclooxygenase suppressed NH4Cl-induced astrocyte swelling, whereas the Ca(2+)-ionophore ionomycin, NMDA, glutamate and prostanoids induced rapid astrocyte swelling. NH4Cl also induced swelling of cultured microglia in a glutamine-synthesis dependent way, but had no effect on cell volume of cultured neurons. It is concluded that the pathways which trigger RNOS formation in astrocytes also trigger astrocyte swelling, whereas conversely and as shown previously hypoosmotic astrocyte swelling can induce RNOS formation. This establishes a complex interplay with an auto-amplificatory loop between RNOS formation and astrocyte swelling as an important event in the pathogenesis of HE.

Osmotic and oxidative/nitrosative stress in ammonia toxicity and hepatic encephalopathy

Hepatic encephalopathy (HE) is a neuropsychiatric complication of acute or chronic liver failure. Currently, HE in cirrhotic patients is seen as a clinical manifestation of a low grade cerebral edema which exacerbates in response to a variety of precipitating factors after an ammonia-induced exhaustion of the volume-regulatory capacity of the astrocyte. Astrocyte swelling triggers a complex signaling cascade which relies on NMDA receptor activation, elevation of intracellular Ca(2+) concentration and prostanoid-driven glutamate exocytosis, which result in increased formation of reactive nitrogen and oxygen species (RNOS) through activation of NADPH oxidase and nitric oxide synthase. Since RNOS in turn promote astrocyte swelling, a self-amplifying signaling loop between osmotic- and oxidative stress ensues, which triggers a variety of downstream consequences. These include protein tyrosine nitration (PTN), oxidation of RNA, mobilization of zinc, alterations in intra- and intercellular signaling and multiple effects on gene transcription. Whereas PTN can affect the function of a variety of proteins, such as glutamine synthetase, oxidized RNA may affect local protein synthesis at synapses, thereby potentially interfering with protein synthesis-dependent memory formation. PTN and RNA oxidation are also found in post mortem human cerebral cortex of cirrhotic patients with HE but not in those without HE, thereby confirming a role for oxidative stress in the pathophysiology of HE. Evidence derived from animal experiments and human post mortem brain tissue also indicates an up-regulation of microglia activation markers in the absence of increased synthesis of pro-inflammatory cytokines. However, the role of activated microglia in the pathophysiology of HE needs to be worked out in more detail. Most recent observations made in whole genome micro-array analyses of post mortem human brain tissue point to a hitherto unrecognized activation of multiple anti-inflammatory signaling pathways.

Molecular and functional expression of cation-chloride cotransporters in dorsal root ganglion neurons during postnatal maturation

GABA depolarizes and excites central neurons during early development, becoming inhibitory and hyperpolarizing with maturation. This "developmental shift" occurs abruptly, reflecting a decrease in intracellular Cl(-) concentration ([Cl(-)](i)) and a hyperpolarizing shift in Cl(-) equilibrium potential due to upregulation of the K(+)-Cl(-) cotransporter KCC2b, a neuron-specific Cl(-) extruder. In contrast, primary afferent neurons (PANs) are depolarized by GABA throughout adulthood because of expression of NKCC1, a Na(+)-K(+)-2Cl(-) cotransporter that accumulates Cl(-) above equilibrium. The GABA(A)-mediated depolarization of PANs determines presynaptic inhibition in the spinal cord, a key mechanism gating somatosensory information. Little is known about developmental changes in Cl(-) transporter expression and Cl(-) homeostasis in PANs. Whether NKCC1 is expressed in PANs of all phenotypes or is restricted to subpopulations (e.g., nociceptors) is debatable. Likewise, whether PANs express KCC2s is controversial. We investigated NKCC1 and K(+)-Cl(-) cotransporter expression in rat and mouse dorsal root ganglion (DRG) neurons with molecular methods. Using fluorescence imaging microscopy, we measured [Cl(-)](i) in acutely dissociated rat DRG neurons (P0-P21) loaded with N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide and classified with phenotypic markers. DRG neurons of all sizes express two NKCC1 mRNAs, one full-length and a shorter splice variant lacking exon 21. Immunolabeling with validated antibodies revealed ubiquitous expression of NKCC1 in DRG neurons irrespective of postnatal age and phenotype. As maturation progresses [Cl(-)](i) decreases gradually, persisting above equilibrium in >95% mature neurons. DRG neurons express mRNAs for KCC1, KCC3s, and KCC4, but not for KCC2s. Mechanisms underlying PANs' developmental changes in Cl(-) homeostasis are discussed and compared with those of central neurons.

Glucose is an adequate energy substrate for the depolarizing action of GABA and glycine in the neonatal rat spinal cord in vitro

In vitro studies have repeatedly demonstrated that the neurotransmitters γ-aminobutyric acid (GABA) and glycine depolarize immature neurons in many areas of the CNS, including the spinal cord. This widely accepted phenomenon was recently challenged by experiments showing that the depolarizing action of GABA on neonatal hippocampus and neocortex in vitro was prevented by adding energy substrates (ES), such as the ketone body metabolite dl-β-hydroxybutyric acid (DL-BHB), lactate, or pyruvate to the artificial cerebrospinal fluid (ACSF). It was suggested that GABA-induced depolarizations in vitro might be an artifact due to inadequate energy supply when glucose is the sole energy source, consistent with the energy metabolism of neonatal rat brain being largely dependent on ESs other than glucose. Here we examined the effects of these ESs (DL-BHB, lactate, pyruvate) on inhibitory postsynaptic potentials (IPSPs) recorded from neonatal rat lumbar spinal cord motoneurons (MNs), in vitro. We report that supplementing the ACSF with physiologic concentrations of DL-BHB, lactate, or pyruvate does not alter the reversal potential of IPSPs (E(IPSP)). Only high concentrations of pyruvate hyperpolarized E(IPSP). In addition, the depolarizing action of GABA on primary afferent terminals was not affected by supplementing the ACSF with ES at physiologic concentrations. We conclude that depolarizing IPSPs in immature MNs and the primary afferent depolarizations are not caused by inadequate energy supply. Glucose at its standard concentration appears to be an adequate ES for the neonatal spinal cord in vitro.

Brain energy metabolism and mitochondrial dysfunction in acute and chronic hepatic encephalopathy

One proposed mechanism for acute and chronic hepatic encephalopathy (HE) is a disturbance in cerebral energy metabolism. It also reviews the current status of this mechanism in both acute and chronic HE, as well as in other hyperammonemic disorders. It also reviews abnormalities in glycolysis, lactate metabolism, citric acid cycle, and oxidative phosphorylation as well as associated energy impairment. Additionally, the role of mitochondrial permeability transition (mPT), a recently established factor in the pathogenesis of HE and hyperammonemia, is emphasized. Energy failure appears to be an important pathogenetic component of both acute and chronic HE and a potential target for therapy.

Biological physics in México: Review and new challenges

Biological and physical sciences possess a long-standing tradition of cooperativity as separate but related subfields of science. For some time, this cooperativity has been limited by their obvious differences in methods and views. Biological physics has recently experienced a kind of revival (or better a rebirth) due to the growth of molecular research on animate matter. New avenues for research have been opened for both theoretical and experimental physicists. Nevertheless, in order to better travel for such paths, the contemporary biological physicist should be armed with a set of specialized tools and methods but also with a new attitude toward multidisciplinarity. In this review article, we intend to somehow summarize what has been done in the past (in particular, as an example we will take a closer look at the Mexican case), to show some examples of fruitful investigations in the biological physics area and also to set a proposal of new curricula for physics students and professionals interested in applying their science to get a better understanding of the physical basis of biological function.

Application of fluorescent indicators to analyse intracellular calcium and morphology in filamentous fungi

A novel staining and quantification method to investigate changes in intracellular calcium levels [Ca(2+)](i) and morphology in filamentous fungus is presented. Using a simple protocol, two fluorescent dyes, Fluo-4-AM and Cell trace calcein red-orange-AM were loaded into the filamentous fungus Penicillium chrysogenum. The present study investigates the applicability of using Ca(2+)-sensitive dye to quantify and image [Ca(2+)](i) in P. chrysogenum cultures chosen for its potential as an experimental system to study Ca(2+) signalling in elicited cultures. The dye loading was optimised and investigated at different pH loading conditions. It was observed that the fluorophore was taken up throughout the hyphae, retaining cell membrane integrity and no dye compartmentalisation within organelles was observed. From the fluorescent plate-reader studies a significant rise (p<0.001) in the relative fluorescence levels corresponding to [Ca(2+)](i) levels in the hyphae was observed when challenged with an elicitor (mannan oligosaccharide, 150mgL(-1)) which was dependent upon extracellular calcium. Concurrently a novel application of dye-loaded hyphae for morphological analysis was also examined using the imaging software Filament Tracer (Bitplane). Essential quantitative mycelial information including the length and diameter of the segments and number of branch points was obtained using this application based on the three-dimensional data.

Oxidative stress markers in the brain of patients with cirrhosis and hepatic encephalopathy

Cell culture studies and animal models point to an important role of oxidative/nitrosative stress in the pathogenesis of cerebral ammonia toxicity. However, it is unknown whether oxidative/nitrosative stress in the brain is also characteristic of hepatic encephalopathy (HE) in humans. We therefore analyzed post mortem cortical brain tissue samples from patients with cirrhosis dying with or without HE in comparison with brains from patients without liver disease. Significantly elevated levels of protein tyrosine-nitrated proteins, heat shock protein-27, and 8-hydroxyguanosine as a marker for RNA oxidation were found in the cerebral cortex of HE patients, but not of patients with cirrhosis but without HE. Glutamine synthetase (GS) activity was significantly decreased, whereas GS protein expression was not significantly affected. Protein expression of the glutamate/aspartate cotransporter was up-regulated in HE, whereas protein expression of neuronal and inducible nitric oxide synthases, manganese-dependent and copper/zinc-dependent superoxide dismutase, and glial glutamate transporter-1 were not significantly increased.

CONCLUSION

These data indicate that HE in patients with cirrhosis is associated with oxidative/nitrosative stress, protein tyrosine nitration, and RNA oxidation, suggesting a role of oxidative stress in the pathogenesis of HE in patients with cirrhosis.

Pathogenesis and diagnosis of hepatic encephalopathy

Hepatic encephalopathy (HE) is a common and potentially devastating neuropsychiatric complication of acute liver failure and cirrhosis. Even in its mildest form, minimal HE (MHE), the syndrome significantly impacts daily living and heralds progression to overt HE. There is maturity in the scientific understanding of the cellular processes that lead to functional and structural abnormalities in astrocytes. Hyperammonemia and subsequent cell swelling is a key pathophysiological abnormality, but this aspect alone is insufficient to fully explain the complex neurotransmitter abnormalities that may be observable using sophisticated imaging techniques. Inflammatory cytokines, reactive oxygen species activation and the role of neurosteroids on neurotransmitter binding sites are emerging pathological lines of inquiry that have yielded important new information on the processes underlying HE and offer promise of future therapeutic targets. Overt HE remains a clinical diagnosis and the neurophysiological and imaging modalities used in research studies have not transferred successfully to the clinical situation. MHE is best characterized by psychometric evaluation, but these tests can be lengthy to perform and require specific expertise to interpret. Simpler computer-based tests are now available and perhaps offer an opportunity to screen, diagnose and monitor MHE in a clinical scenario, although large-scale studies comparing the different techniques have not been undertaken. There is a discrepancy between the depth of understanding of the pathophysiology of HE and the translation of this understanding to a simple, easily understood diagnostic and longitudinal marker of disease. This is a present area of focus for the management of HE.

Interaction of oxidative stress, astrocyte swelling and cerebral ammonia toxicity

PURPOSE OF REVIEW

Description of the role of oxidative stress in the pathogenesis of cerebral ammonia toxicity and hepatic encephalopathy.

RECENT FINDINGS

Ammonia plays a key role in the pathogenesis of hepatic encephalopathy, which manifests as a neuropsychiatric syndrome accompanying acute and chronic liver failure. One consequence of ammonia action on the brain is astrocyte swelling, which triggers the generation of oxidative/nitrosative stress at the level of NADPH oxidase, nitric oxide synthases and the mitochondria. A self-amplifying signaling loop between oxidative stress and astrocyte swelling has been proposed. Consequences of the ammonia-induced oxidative/nitrosative stress response are protein modifications through nitration of tyrosine residues and oxidation of astrocytic and neuronal RNA. Nitrosative stress also mobilizes zinc from intracellular stores with impact on gene expression. These alterations may at least in part mediate cerebral ammonia toxicity through disturbances of intracellular and intercellular signaling and of synaptic plasticity.

SUMMARY

Oxidative/nitrosative stress and a low-grade cerebral edema as key events in the pathogenesis of ammonia toxicity and hepatic encephalopathy may offer potential new strategies for treatment. Ammonia-induced oxidation of RNA and proteins may impair postsynaptic protein synthesis, which is critically involved in learning and memory consolidation. RNA oxidation offers a novel explanation for multiple disturbances of neurotransmitter systems and gene expression and the cognitive deficits observed in hepatic encephalopathy.

Ammonium-evoked alterations in intracellular sodium and pH reduce glial glutamate transport activity

The clearance of extracellular glutamate is mainly mediated by pH- and sodium-dependent transport into astrocytes. During hepatic encephalopathy (HE), however, elevated extracellular glutamate concentrations are observed. The primary candidate responsible for the toxic effects observed during HE is ammonium (NH(4) (+)/NH(3)). Here, we examined the effects of NH(4) (+)/NH(3) on steady-state intracellular pH (pH(i)) and sodium concentration ([Na(+)](i)) in cultured astrocytes in two different age groups. Moreover, we assessed the influence of NH(4) (+)/NH(3) on glutamate transporter activity by measuring D-aspartate-induced pH(i) and [Na(+)](i) transients. In 20-34 days in vitro (DIV) astrocytes, NH(4) (+)/NH(3) decreased steady-state pH(i) by 0.19 pH units and increased [Na(+)](i) by 21 mM. D-Aspartate-induced pH(i) and [Na(+)](i) transients were reduced by 80-90% in the presence of NH(4) (+)/NH(3), indicating a dramatic reduction of glutamate uptake activity. In 9-16 DIV astrocytes, in contrast, pH(i) and [Na(+)](i) were minimally affected by NH(4) (+)/NH(3), and D-aspartate-induced pH(i) and [Na(+)](i) transients were reduced by only 30-40%. Next we determined the contribution of Na(+), K(+), Cl(-)-cotransport (NKCC). Immunocytochemical stainings indicated an increased expression of NKCC1 in 20-34 DIV astrocytes. Moreover, inhibition of NKCC with bumetanide prevented NH(4) (+)/NH(3)-evoked changes in steady-state pH(i) and [Na(+)](i) and attenuated the reduction of D-aspartate-induced pH(i) and [Na(+)](i) transients by NH(4) (+)/NH(3) to 30% in 20-34 DIV astrocytes. Our results suggest that NH(4) (+)/NH(3) decreases steady-state pH(i) and increases steady-state [Na(+)](i) in astrocytes by an age-dependent activation of NKCC. These NH(4) (+)/NH(3)-evoked changes in the transmembrane pH and sodium gradients directly reduce glutamate transport activity, and may, thus, contribute to elevated extracellular glutamate levels observed during HE.

Chloride accumulation drives volume dynamics underlying cell proliferation and migration

During brain development, progenitor cells migrate over long distances through narrow and tortuous extracellular spaces posing significant demands on the cell's ability to alter cell volume. This phenotype is recapitulated in primary brain tumors. We demonstrate here that volume changes occurring spontaneously in these cells are mediated by the flux of Cl- along with obligated water across the cell membrane. To do so, glioma cells accumulate Cl- to approximately 100 mM, a concentration threefold greater than predicted by the Nernst equation. Shunting this gradient through the sustained opening of exogenously expressed GABA-gated Cl- channels caused a 33% decrease in cell volume and impaired the ability of cells to migrate in a spatially constrained environment. Further, dividing cells condense their cytoplasm prior to mitosis, a phenomenon which is associated with the release of intracellular Cl- as indicated by a 40-mM decrease in [Cl-]i. These findings provide a new framework for considering the role of intracellular Cl- in glioma cells. Here, Cl- serves as an important osmotically active regulator of cell volume being the energetic driving force for volume changes required by immature cells in cell migration and proliferation. This mechanism that was studied in CNS malignancies may be shared with other immature cells in the brain as well.

Na+,K+,2Cl- cotransport and intracellular chloride regulation in rat primary sensory neurons: thermodynamic and kinetic aspects

Adult primary afferent neurons are depolarized by GABA throughout their entire surface, including their somata located in dorsal root ganglia (DRG). Primary afferent depolarization (PAD) mediated by GABA released from spinal interneurons determines presynaptic inhibition, a key mechanism in somatosensory processing. The depolarization is due to Cl(-) efflux through GABA(A) channels; the outward Cl(-) gradient is generated by a Na+,K+,2Cl(-) cotransporter (NKCC) as first established in amphibians. Using fluorescence imaging microscopy we measured [Cl(-)]i and cell water volume (CWV) in dissociated rat DRG cells (P0-P21) loaded with N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide and calcein, respectively. Basal [Cl(-)]i was 44.2 +/- 1.2 mM (mean +/- SE), Cl(-) equilibrium potential (E Cl) was -27.0 +/- 0.7 mV (n = 75). This [Cl(-)]i is about four times higher than electrochemical equilibrium. On isosmotic removal of external Cl(-), cells lost Cl(-) and shrank. On returning to control solution, cells reaccumulated Cl(-) and recovered CWV. Cl(-) reaccumulation had Na+-dependent (SDC) and Na+-independent (SIC) components. The SIC stabilized at [Cl(-)]i = 13.2 +/- 1.2 mM, suggesting that it was passive (E(Cl) = -60.5 +/- 3 mV). Bumetanide blocked CWV recovery and most (65%) of the SDC (IC50 = 5.7 microM), indicating that both were mediated by NKCC. Active Cl(-) uptake fell with increasing [Cl(-)]i and became negligible when [Cl(-)]i reached basal levels. The kinetics of active Cl(-) uptake suggests a negative feedback system in which intracellular Cl(-)regulates its own influx thereby keeping [Cl(-)]i constant, above electrochemical equilibrium but below the value that would attain if NKCC reached thermodynamic equilibrium.

Dynamic effects of Hg2+-induced changes in cell volume

Using a microfluidic volume sensor, we studied the dynamic effects of Hg2+ on hypotonic stress-induced volume changes in CHO cells. A hypotonic challenge to control cells caused them to swell but did not evoke a significant regulatory volume decrease (RVD). Treatment with 100 muM HgCl2 caused a substantial increase in the steady-state volume following osmotic stress. Continuous hypotonic challenge following a single 10-min exposure to HgCl2 produced a biphasic volume increase with a steady-state volume 100% larger than control cells. Repeated hypotonic challenges to cells exposed once to Hg2+ resulted in a sequential approach to the same steady-state volume. Stimulation after reaching steady state caused a reduction in peak cell volume. Repeated stimulation was different than continuous stimulation resulting in a more rapid approach to steady state. Substituting extracellular Na+ with impermeant NMDG+ in the hypotonic solution produced a rapid RVD-like volume decrease and eliminated the Hg2+-induced excess swelling. The volume decrease in the presence of Hg2+ was inhibited by tetraethylammonium and 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid disodium, blockers of K+ and Cl(-) channels, respectively, suggesting that part of the Hg2+ effect was increasing NaCl influx over KCl efflux. The presence of multiple phases of steady-state volume and their sensitivity to the stimulation history suggests that factors beyond solute fluxes, such as modification of mechanical stress within the cytoskeleton also plays a role in the response to hypotonic stress.

Interrogation of living cells using alternating current scanning electrochemical microscopy (AC-SECM)

In this paper we present the application of alternating current scanning electrochemical microscopy (AC-SECM) to the study of living cells. Commercial AFM instrumentation was modified to allow for performing robust AC-SECM measurements. Constant height AC imaging of the Cos-7 cells, performed directly in cell culture medium without the addition of a redox mediator, provided topographical information of the cell. Stationary tip measurements on the AC current were carried out to investigate the cellular activity of a single cell. The dependence of AC current magnitude on tip-to-sample separation distance was used to monitor real time changes in cell height of individual Cos-7 cells. Furthermore, AC-SECM was employed to observe changes in metabolic cellular activity stimulated by ethanol and phorbol-1,2-myristate-acetate-3. The effect of changing cellular activity on constant height AC-SECM imaging was also studied.

Hypoosmotic swelling and ammonia increase oxidative stress by NADPH oxidase in cultured astrocytes and vital brain slices

The role of NADPH oxidase (NOX) and the regulatory subunit p47(phox) for hypoosmotic ROS generation was studied in cultured rat astrocytes and brain slices of wilde type and p47(phox) knock-out mice. Cultured rat astrocytes express mRNAs encoding for the regulatory subunit p47(phox), NOX1, 2, and 4, and the dual oxidases (DUOX)1 and 2, but not NOX3. Hypoosmotic (205 mosmol/L) swelling of cultured astrocytes induced a rapid generation of ROS that was accompanied by serine phosphorylation of p47(phox) and prevented by the NADPH oxidase inhibitor apocynin. Apocynin also impaired the hypoosmotic tyrosine phosphorylation of Src. Both, hypoosmotic ROS generation and p47(phox) serine phosphorylation were sensitive to the acidic sphingomyelinase inhibitors AY9944 and desipramine, the protein kinase C (PKC)zeta-inhibitory pseudosubstrate peptide, the NMDA receptor antagonist MK-801 and the intracellular Ca(2+) chelator BAPTA-AM. Also hypoosmotic exposure of wilde type mouse cortical brain slices increased ROS generation, which was allocated in part to the astrocytes and which was absent in presence of apocynin and in cortical brain slices from p47(phox) knock-out mice. Also ammonia induced a rapid ROS production in cultured astrocytes and brain slices, which was sensitive to apocynin. The data suggest that astrocyte swelling triggers a p47(phox)-dependent NADPH oxidase-catalyzed ROS production. The findings further support a close interrelation between osmotic and oxidative stress in astrocytes, which may be relevant to different brain pathologies including hepatic encephalopathy.

AMPA-mediated excitotoxicity in oligodendrocytes: role for Na(+)-K(+)-Cl(-) co-transport and reversal of Na(+)/Ca(2+) exchanger

We investigated the role of Na(+)-K(+)-Cl(-) co-transporter isoform 1 (NKCC1) and reversal of Na(+)/Ca(2+) exchanger (NCX(rev)) in glutamate-mediated excitotoxicity in oligodendrocytes obtained from rat spinal cords (postnatal day 6-8). An immunocytochemical characterization showed that these cultures express NKCC1 and Na(+)/Ca(2+) exchanger isoforms 1, 2, and 3 (NCX1, NCX2, NCX3). Exposing the cultures to alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) plus cyclothiazide (CTZ) led to a transient rise in intracellular (), which was followed by a sustained overload, NKCC1 phosphorylation, and a NKCC1-mediated Na(+) influx. In the presence of a specific AMPA receptor inhibitor 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX), the AMPA/CTZ failed to elicit any changes in . The AMPA/CTZ-induced sustained rise led to mitochondrial Ca(2+) accumulation, release of cytochrome c from mitochondria, and cell death. The AMPA/CTZ-elicited increase, mitochondrial damage, and cell death were significantly reduced by inhibiting NKCC1 or NCX(rev). These data suggest that in cultured oligodendrocytes, activation of AMPA receptors leads to NKCC1 phosphorylation that enhances NKCC1-mediated Na(+) influx. The latter triggers NCX(rev) and NCX(rev)-mediated overload and compromises mitochondrial function and cellular viability.

Cytoplasmic volume condensation is an integral part of mitosis

Cell growth and osmotic volume regulation are undoubtedly linked to the progression of the cell cycle as with each division, a newly generated cell must compensate for loss of half of its volume to its sister cell. The extent to which size influences cell cycle decisions, however, is controversial in mammalian cells. Further, a mechanism by which cells can monitor and therefore regulate their size has not been fully elucidated. Despite an ongoing debate, there have been few studies which directly address the question in single cell real-time experiments. In this study we used fluorescent time-lapse imaging to quantitatively assess volume in individual spontaneously dividing cells throughout the cell cycle. Together with biophysical studies, these establish that the efflux of salt and water brings about a condensation of cytoplasmic volume as glioma cells progress through mitosis. As cells undergo this pre-mitotic condensation (PMC) they approach a preferred cell volume preceding each division. This is functionally linked to chromatin condensation, suggesting that PMC plays an integral role in mitosis.