EGFR kinase regulates volume-sensitive chloride current elicited by integrin stretch via PI-3K and NADPH oxidase in ventricular myocytes

Cell size regulates human endoderm specification through actomyosin-dependent AMOT-YAP signaling

Cell size is a crucial physical property that significantly impacts cellular physiology and function. However, the influence of cell size on stem cell specification remains largely unknown. Here, we investigated the dynamic changes in cell size during the differentiation of human pluripotent stem cells into definitive endoderm (DE). Interestingly, cell size exhibited a gradual decrease as DE differentiation progressed with higher stiffness. Furthermore, the application of hypertonic pressure or chemical to accelerate the reduction in cell size significantly and specifically enhanced DE differentiation. By functionally intervening in mechanosensitive elements, we have identified actomyosin activity as a crucial mediator of both DE differentiation and cell size reduction. Mechanistically, the reduction in cell size induces actomyosin-dependent angiomotin (AMOT) nuclear translocation, which suppresses Yes-associated protein (YAP) activity and thus facilitates DE differentiation. Together, our study has established a novel connection between cell size diminution and DE differentiation, which is mediated by AMOT nuclear translocation. Additionally, our findings suggest that the application of osmotic pressure can effectively promote human endodermal lineage differentiation.

Physiology of the volume-sensitive/regulatory anion channel VSOR/VRAC: part 2: its activation mechanisms and essential roles in organic signal release

The volume-sensitive outwardly rectifying or volume-regulated anion channel, VSOR/VRAC, which was discovered in 1988, is expressed in most vertebrate cell types, and is essentially involved in cell volume regulation after swelling and in the induction of cell death. This series of review articles describes what is already known and what remains to be uncovered about the functional and molecular properties as well as the physiological and pathophysiological roles of VSOR/VRAC. This Part 2 review article describes, from the physiological and pathophysiological standpoints, first the pivotal roles of VSOR/VRAC in the release of autocrine/paracrine organic signal molecules, such as glutamate, ATP, glutathione, cGAMP, and itaconate, as well as second the swelling-independent and -dependent activation mechanisms of VSOR/VRAC. Since the pore size of VSOR/VRAC has now well been evaluated by electrophysiological and 3D-structural methods, the signal-releasing activity of VSOR/VRAC is here discussed by comparing the molecular sizes of these organic signals to the channel pore size. Swelling-independent activation mechanisms include a physicochemical one caused by the reduction of intracellular ionic strength and a biochemical one caused by oxidation due to stimulation by receptor agonists or apoptosis inducers. Because some organic substances released via VSOR/VRAC upon cell swelling can trigger or augment VSOR/VRAC activation in an autocrine fashion, swelling-dependent activation mechanisms are to be divided into two phases: the first phase induced by cell swelling per se and the second phase caused by receptor stimulation by released organic signals.

Clues and new evidences in arterial hypertension: unmasking the role of the chloride anion

The present review will focus on the role of chloride anion in cardiovascular disease, with special emphasis in the development of hypertensive disease and vascular inflammation. It is known that acute and chronic overload of sodium chloride increase blood pressure and have pro-inflammatory and pro-fibrotic effects on different target organs, but it is unknown how chloride may influence these processes. Chloride anion is the predominant anion in the extracellular fluid and its intracellular concentration is dynamically regulated. As the queen of the electrolytes, it is of crucial importance to understand the physiological mechanisms that regulate the cellular handling of this anion including the different transporters and cellular chloride channels, which exert a variety of functions, such as regulation of cellular proliferation, differentiation, migration, apoptosis, intracellular pH and cellular redox state. In this article, we will also review the relationship between dietary, serum and intracellular chloride and how these different sources of chloride in the organism are affected in hypertension and their impact on cardiovascular disease. Additionally, we will discuss the approach of potential strategies that affect chloride handling and its potential effect on cardiovascular system, including pharmacological blockade of chloride channels and non-pharmacological interventions by replacing chloride by another anion.

From Pinocytosis to Methuosis-Fluid Consumption as a Risk Factor for Cell Death

The volumes of a cell [cell volume (CV)] and its organelles are adjusted by osmoregulatory processes. During pinocytosis, extracellular fluid volume equivalent to its CV is incorporated within an hour and membrane area equivalent to the cell's surface within 30 min. Since neither fluid uptake nor membrane consumption leads to swelling or shrinkage, cells must be equipped with potent volume regulatory mechanisms. Normally, cells respond to outwardly or inwardly directed osmotic gradients by a volume decrease and increase, respectively, i.e., they shrink or swell but then try to recover their CV. However, when a cell death (CD) pathway is triggered, CV persistently decreases in isotonic conditions in apoptosis and it increases in necrosis. One type of CD associated with cell swelling is due to a dysfunctional pinocytosis. Methuosis, a non-apoptotic CD phenotype, occurs when cells accumulate too much fluid by macropinocytosis. In contrast to functional pinocytosis, in methuosis, macropinosomes neither recycle nor fuse with lysosomes but with each other to form giant vacuoles, which finally cause rupture of the plasma membrane (PM). Understanding methuosis longs for the understanding of the ionic mechanisms of cell volume regulation (CVR) and vesicular volume regulation (VVR). In nascent macropinosomes, ion channels and transporters are derived from the PM. Along trafficking from the PM to the perinuclear area, the equipment of channels and transporters of the vesicle membrane changes by retrieval, addition, and recycling from and back to the PM, causing profound changes in vesicular ion concentrations, acidification, and-most importantly-shrinkage of the macropinosome, which is indispensable for its proper targeting and cargo processing. In this review, we discuss ion and water transport mechanisms with respect to CVR and VVR and with special emphasis on pinocytosis and methuosis. We describe various aspects of the complex mutual interplay between extracellular and intracellular ions and ion gradients, the PM and vesicular membrane, phosphoinositides, monomeric G proteins and their targets, as well as the submembranous cytoskeleton. Our aim is to highlight important cellular mechanisms, components, and processes that may lead to methuotic CD upon their derangement.

The SWELL1-LRRC8 complex regulates endothelial AKT-eNOS signaling and vascular function

The endothelium responds to numerous chemical and mechanical factors in regulating vascular tone, blood pressure, and blood flow. The endothelial volume-regulated anion channel (VRAC) has been proposed to be mechanosensitive and thereby sense fluid flow and hydrostatic pressure to regulate vascular function. Here, we show that the leucine-rich repeat-containing protein 8a, LRRC8A (SWELL1), is required for VRAC in human umbilical vein endothelial cells (HUVECs). Endothelial LRRC8A regulates AKT-endothelial nitric oxide synthase (eNOS) signaling under basal, stretch, and shear-flow stimulation, forms a GRB2-Cav1-eNOS signaling complex, and is required for endothelial cell alignment to laminar shear flow. Endothelium-restricted Lrrc8a KO mice develop hypertension in response to chronic angiotensin-II infusion and exhibit impaired retinal blood flow with both diffuse and focal blood vessel narrowing in the setting of type 2 diabetes (T2D). These data demonstrate that LRRC8A regulates AKT-eNOS in endothelium and is required for maintaining vascular function, particularly in the setting of T2D.

SWELL1 regulates skeletal muscle cell size, intracellular signaling, adiposity and glucose metabolism

Maintenance of skeletal muscle is beneficial in obesity and Type 2 diabetes. Mechanical stimulation can regulate skeletal muscle differentiation, growth and metabolism; however, the molecular mechanosensor remains unknown. Here, we show that SWELL1 (Lrrc8a) functionally encodes a swell-activated anion channel that regulates PI3K-AKT, ERK1/2, mTOR signaling, muscle differentiation, myoblast fusion, cellular oxygen consumption, and glycolysis in skeletal muscle cells. LRRC8A over-expression in Lrrc8a KO myotubes boosts PI3K-AKT-mTOR signaling to supra-normal levels and fully rescues myotube formation. Skeletal muscle-targeted Lrrc8a KO mice have smaller myofibers, generate less force ex vivo, and exhibit reduced exercise endurance, associated with increased adiposity under basal conditions, and glucose intolerance and insulin resistance when raised on a high-fat diet, compared to wild-type (WT) mice. These results reveal that the LRRC8 complex regulates insulin-PI3K-AKT-mTOR signaling in skeletal muscle to influence skeletal muscle differentiation in vitro and skeletal myofiber size, muscle function, adiposity and systemic metabolism in vivo.

Targeting Cardiac Myocyte Na+-K+ Pump Function With β3 Adrenergic Agonist in Rabbit Model of Severe Congestive Heart Failure

BACKGROUND

Abnormally high cytosolic Na⁺ concentrations in advanced heart failure impair myocardial contractility. Stimulation of the membrane Na⁺-K⁺ pump should lower Na⁺ concentrations, and the β3 adrenoceptor (β3 AR) mediates pump stimulation in myocytes. We examined if β3 AR-selective agonists given in vivo increase myocyte Na⁺-K⁺ pump activity and reverse organ congestion in severe heart failure (HF).

METHODS

Indices for HF were lung-, heart-, and liver: body weight ratios and ascites after circumflex coronary artery ligation in rabbits. Na⁺-K⁺ pump current, I_p, was measured in voltage-clamped myocytes from noninfarct myocardium. Rabbits were treated with the β3 AR agonists CL316,243 or ASP9531, starting 2 weeks after coronary ligation.

RESULTS

Coronary ligation caused ascites in most rabbits, significantly increased lung-, heart-, and liver: body weight ratios, and decreased I_p relative to that for 10 sham-operated rabbits. Treatment with CL316,243 for 3 days significantly reduced lung-, heart-, and liver: body weight ratios and prevalence of ascites in 8 rabbits with HF relative to indices for 13 untreated rabbits with HF. It also increased I_p significantly to levels of myocytes from sham-operated rabbits. Treatment with ASP9531 for 14 days significantly reduced indices of organ congestion in 6 rabbits with HF relative to indices of 6 untreated rabbits, and it eliminated ascites. β3 AR agonists did not significantly change heart rates or blood pressures.

CONCLUSIONS

Parallel β3 AR agonists-induced reversal of Na⁺-K⁺ pump inhibition and indices of congestion suggest pump inhibition is a useful target for treatment with β3 AR agonists in congestive HF.

SWELL signalling in adipocytes: can fat 'feel' fat?

Obesity is becoming a global epidemic, predisposing to Type 2 diabetes, cardiovascular disease, fatty liver disease, pulmonary disease, osteoarthritis and cancer. Therefore, understanding the biology of adipocyte expansion in response to overnutrition is critical to devising strategies to treat obesity, and the associated burden of morbidity and mortality. Through exploratory patch-clamp experiments in freshly isolated primary murine and human adipocytes, we recently determined that SWELL1/LRRC8a, a leucine-rich repeat containing transmembrane protein, functionally encoded an ion channel signalling complex (the volume-regulated anion channel, or VRAC) on the adipocyte plasma membrane. The SWELL1-/LRRC8 channel complex activates in response to increases in adipocyte volume and in the context of obesity. SWELL1 is also required for insulin-PI3K-AKT2 signalling to regulate adipocyte growth and systemic glycaemia. This commentary delves further into our working models for the molecular mechanisms of adipocyte SWELL1-mediated VRAC activation, proposed signal transduction mechanisms, and putative impact on adipocyte hypertrophy during caloric excess.

More than just a pressure relief valve: physiological roles of volume-regulated LRRC8 anion channels

The volume-regulated anion channel (VRAC) is a key player in the volume regulation of vertebrate cells. This ubiquitously expressed channel opens upon osmotic cell swelling and potentially other cues and releases chloride and organic osmolytes, which contributes to regulatory volume decrease (RVD). A plethora of studies have proposed a wide range of physiological roles for VRAC beyond volume regulation including cell proliferation, differentiation and migration, apoptosis, intercellular communication by direct release of signaling molecules and by supporting the exocytosis of insulin. VRAC was additionally implicated in pathological states such as cancer therapy resistance and excitotoxicity under ischemic conditions. Following extensive investigations, 5 years ago leucine-rich repeat-containing family 8 (LRRC8) heteromers containing LRRC8A were identified as the pore-forming components of VRAC. Since then, molecular biological approaches have allowed further insight into the biophysical properties and structure of VRAC. Heterologous expression, siRNA-mediated downregulation and genome editing in cells, as well as the use of animal models have enabled the assessment of the proposed physiological roles, together with the identification of new functions including spermatogenesis and the uptake of antibiotics and platinum-based cancer drugs. This review discusses the recent molecular biological insights into the physiology of VRAC in relation to its previously proposed roles.

Binding of the protein ICln to α-integrin contributes to the activation of IClswell current

ICl_swell is the chloride current induced by cell swelling, and plays a fundamental role in several biological processes, including the regulatory volume decrease (RVD). ICln is a highly conserved, ubiquitously expressed and multifunctional protein involved in the activation of ICl_swell. In platelets, ICln binds to the intracellular domain of the integrin αIIb chain, however, whether the ICln/integrin interaction plays a role in RVD is not known. Here we show that a direct molecular interaction between ICln and the integrin α-chain is not restricted to platelets and involves highly conserved amino acid motifs. Integrin α recruits ICln to the plasma membrane, thereby facilitating the activation of ICl_swell during hypotonicity. Perturbation of the ICln/integrin interaction prevents the transposition of ICln towards the cell surface and, in parallel, impedes the activation of ICl_swell. We suggest that the ICln/integrin interaction interface may represent a new molecular target enabling specific ICl_swell suppression in pathological conditions when this current is deregulated or plays a detrimental role.

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl^- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

Molecular Biology and Physiology of Volume-Regulated Anion Channel (VRAC)

The Volume-Regulated Anion Channel (VRAC) is activated by cell swelling and plays a key role in cell volume regulation. VRAC is ubiquitously expressed in vertebrate cells and also implicated in many other physiological and cellular processes including fluid secretion, glutamate release, membrane potential regulation, cell proliferation, migration, and apoptosis. Although its biophysical properties have been well characterized, the molecular identity of VRAC remained a mystery for almost three decades. The field was transformed by recent discoveries showing that the leucine-rich repeat-containing protein 8A (LRRC8A, also named SWELL1) and its four other homologs form heteromeric VRAC channels. The composition of LRRC8 subunits determines channel properties and substrate selectivity of a large variety of different VRACs. Incorporating purified SWELL1-containing protein complexes into lipid bilayers is sufficient to reconstitute channel activities, a finding that supports the decrease in intracellular ionic strength as the mechanism of VRAC activation during cell swelling. Characterization of Swell1 knockout mice uncovers the important role of VRAC in T cell development, pancreatic β-cell glucose-stimulated insulin secretion, and adipocyte metabolic function. The ability to permeate organic osmolytes and metabolites is a major feature of VRAC. The list of VRAC substrates is expected to grow, now also including some cancer drugs and antibiotics even under non-cell swelling conditions. Therefore, a critical role of VRAC in drug resistance and cell-cell communication is emerging. This review summarizes the exciting recent progress on the structure-function relationship and physiology of VRAC and discusses key future questions to be solved.

Search for Upstream Cell Volume Sensors: The Role of Plasma Membrane and Cytoplasmic Hydrogel

The plasma membrane plays a prominent role in the regulation of cell volume by mediating selective transport of extra- and intracellular osmolytes. Recent studies show that upstream sensors of cell volume changes are mainly located within the cytoplasm that displays properties of a hydrogel and not in the plasma membrane. Cell volume changes occurring in anisosmotic medium as well as in isosmotic environment affect properties of cytoplasmic hydrogel that, in turn, trigger rapid regulatory volume increase and decrease (RVI and RVD). The downstream signaling pathways include reorganization of 2D cytoskeleton and altered composition of polyphosphoinositides located on the inner surface of the plasma membrane. In addition to its action on physico-chemical properties of cytoplasmic hydrogel, cell volume changes in anisosmotic conditions affect the ionic strength of the cytoplasm and the [Na⁺]_i/[K⁺]_i ratio. Elevated intracellular ionic strength evoked by long term exposure of cells to hypertonic environment resulted in the activation of TonEBP and augmented expression of genes controlling intracellular organic osmolyte levels. The role of Na⁺_i/K⁺_i -sensitive, Ca²⁺_i -mediated and Ca²⁺_i-independent mechanisms of excitation-transcription coupling in cell volume-adjustment remains unknown.

Noradrenaline up-regulates volume-regulated chloride current by PKA-independent cAMP/exchange protein activated by cAMP pathway in human atrial myocytes

BACKGROUND AND PURPOSE

Adrenergic regulation of cell volume-regulated chloride current (I_Cl.vol ) is species-dependent. The present study investigates the mechanism underlying adrenergic regulation of I_Cl.vol in human atrial myocytes.

EXPERIMENTAL APPROACH

Conventional whole-cell patch voltage-clamp techniques were used to record membrane current in human atrial myocytes. I_Cl.vol was evoked by hyposmotic bath solution (0.6 times isosmotic, 0.6 T).

KEY RESULTS

I_Cl.vol was augmented by noradrenaline (1 μM) during cell swelling in 0.6 T but not under isosmotic (1 T) conditions. Up-regulation of I_Cl.vol in 0.6 T was blocked by the β-adrenoceptor antagonist propranolol (2 μM), but not by the α₁ -adrenoceptor antagonist prazosin (2 μM). This β-adrenergic response involved cAMP but was independent of PKA; the protein kinase inhibitor H-89 (2 μM) or PKI (10 μM in pipette solution) failed to prevent I_Cl.vol up-regulation by noradrenaline. Moreover, the PI3K/PKB inhibitor LY294002 (50 μM) and the PKG inhibitor KT5823 (10 μM) did not affect noradrenaline-induced increases in I_Cl.vol . Interestingly, the exchange protein directly activated by cAMP (Epac) agonist 8-pCPT-2'-O-Me-cAMP (50 μM) also up-regulated I_Cl.vol , and the noradrenaline-induced increase of I_Cl.vol in 0.6 T was reversed or prevented by the Epac inhibitor ESI-09 (10 μM).

CONCLUSION AND IMPLICATIONS

These data show that I_Cl.vol evoked by cell swelling of human atrial myocytes is up-regulated by noradrenaline via a PKA-independent cAMP/Epac pathway in human atrial myocytes. cAMP/Epac-induced I_Cl.vol is expected to shorten action potential duration during human atrial myocytes swelling and may be involved in abnormal cardiac electrical activity during cardiac pathologies that evoke β-adrenoceptor signalling.

The apoptotic effect of Zoledronic acid on the nasopharyngeal carcinoma cells via ROS mediated chloride channel activation

Zoledronic acid (ZA), a third-generation bisphosphonate, has been applied for treatment of bone metastases caused by malignant tumors. Recent studies have found its anti-cancer effects on various tumor cells. One of the mechanisms of anti-cancer effects of ZA is induction of apoptosis. However, the mechanisms of ZA-induced apoptosis in tumor cells have not been clarified clearly. In this study, we investigated the roles of chloride channels in ZA-induced apoptosis in nasopharyngeal carcinoma CNE-2Z cells. Apoptosis and chloride current were induced by ZA and suppressed by chloride channel blockers. After the knockdown of ClC-3 expression by ClC-3 siRNA, ZA-induced chloride current and apoptosis were significantly suppressed, indicating that the chloride channel participated in ZA-induced apoptosis may be ClC-3. When reactive oxygen species (ROS) generation was inhibited by the antioxidant N-acetyl-L-cysteine (L-NAC), ZA-induced apoptosis and chloride current were blocked accordingly, suggesting that ZA induces apoptosis through promoting ROS production and subsequently activating chloride channel.

Mechanisms of I/R-Induced Endothelium-Dependent Vasodilator Dysfunction

Ischemia/reperfusion (I/R) induces leukocyte/endothelial cell adhesive interactions (LECA) in postcapillary venules and impaired endothelium-dependent, NO-mediated dilatory responses (EDD) in upstream arterioles. A large body of evidence has implicated reactive oxygen species, adherent leukocytes, and proteases in postischemic EDD dysfunction in conduit arteries. However, arterioles represent the major site for the regulation of vascular resistance but have received less attention with regard to the mechanisms underlying their reduced responsiveness to EDD stimuli in I/R. Even though leukocytes do not roll along, adhere to, or emigrate across arteriolar endothelium in postischemic intestine, recent work indicates that I/R-induced venular LECA is causally linked to EDD in arterioles. An emerging body of evidence suggests that I/R-induced EDD in arterioles occurs by a mechanism that is triggered by LECA in postcapillary venules and involves the formation of signals in the interstitium elicited by the proteolytic activity of emigrated leukocytes. This activity releases matricryptins from or exposes matricryptic sites in the extracellular matrix that interact with the integrin α_vβ₃ to induce mast cell chymase-dependent formation of angiotensin II (Ang II). Subsequent activation of NAD(P)H oxidase by Ang II leads to the formation of oxidants which inactivate NO and leads to eNOS uncoupling, resulting in arteriolar EDD dysfunction. This work establishes new links between LECA in postcapillary venules, signals generated in the interstitium by emigrated leukocytes, mast cell degranulation, and impaired EDD in upstream arterioles. These fundamentally important findings have enormous implications for our understanding of blood flow dysregulation in conditions characterized by I/R.

Volume-sensitive outwardly rectifying chloride channel blockers protect against high glucose-induced apoptosis of cardiomyocytes via autophagy activation

Hyperglycemia is a well-characterized contributing factor for cardiac dysfunction and heart failure among diabetic patients. Apoptosis of cardiomyocytes plays a major role during the onset and pathogenesis of diabetic cardiomyopathy (DCM). Nonetheless, the molecular machinery underlying hyperglycemia-induced cardiac damage and cell death remains elusive. In the present study, we found that chloride channel blockers, 4,4'-diisothiocya-natostilbene-2,2'- disulfonic acid (DIDS) and 4-(2-butyl-6,7-dichlor-2-cyclopentyl-indan-1-on-5-yl) oxybutyric acid (DCPIB), inhibited high glucose-activated volume-sensitive outwardly rectifying (VSOR) Cl^- channel and improved the viability of cardiomyocytes. High glucose induced cardiomyocyte apoptosis by suppressing the autophagic stress, which can be reversed via blockade of VSOR Cl^- channel. VSOR activation in high glucose-treated cardiomyocytes was attributed to increased intracellular levels of reactive oxygen species (ROS). Taken together, our study unraveled a role of VSOR chloride currents in impaired autophagy and increased apoptosis of high glucose-exposed cardiomyocyte, and has implications for a therapeutic potential of VSOR chloride channel blockers in DCM.

The volume-regulated anion channel (LRRC8) in nodose neurons is sensitive to acidic pH

The leucine rich repeat containing protein 8A (LRRC8A), or SWELL1, is an essential component of the volume-regulated anion channel (VRAC) that is activated by cell swelling and ionic strength. We report here for the first time to our knowledge its expression in a primary cell culture of nodose ganglia neurons and its localization in the soma, neurites, and neuronal membrane. We show that this neuronal VRAC/SWELL1 senses low external pH (pH_o) in addition to hypoosmolarity. A robust sustained chloride current is seen in 77% of isolated nodose neurons following brief exposures to extracellular acid pH. Its activation involves proton efflux, intracellular alkalinity, and an increase in NOX-derived H₂O_2. The molecular identity of both the hypoosmolarity-induced and acid pH_o-conditioned VRAC as LRRC8A (SWELL1) was confirmed by Cre-flox-mediated KO, shRNA-mediated knockdown, and CRISPR/Cas9-mediated LRRC8A deletion in HEK cells and in primary nodose neuronal cultures. Activation of VRAC by low pH_o reduces neuronal injury during simulated ischemia and N-methyl-D-aspartate-induced (NMDA-induced) apoptosis. These results identify the VRAC (LRRC8A) as a dual sensor of hypoosmolarity and low pH_o in vagal afferent neurons and define the mechanisms of its activation and its neuroprotective potential.

VRACs and other ion channels and transporters in the regulation of cell volume and beyond

Cells need to regulate their volume to counteract osmotic swelling or shrinkage, as well as during cell division, growth, migration and cell death. Mammalian cells adjust their volume by transporting potassium, sodium, chloride and small organic osmolytes using plasma membrane channels and transporters. This generates osmotic gradients, which drive water in and out of cells. Key players in this process are volume-regulated anion channels (VRACs), the composition of which has recently been identified and shown to encompass LRRC8 heteromers. VRACs also transport metabolites and drugs and function in extracellular signal transduction, apoptosis and anticancer drug resistance.

Threonine532 phosphorylation in ClC-3 channels is required for angiotensin II-induced Cl(-) current and migration in cultured vascular smooth muscle cells

BACKGROUND AND PURPOSE

Angiotensin II (AngII) induces migration and growth of vascular smooth muscle cell (VSMC), which is responsible for vascular remodelling in some cardiovascular diseases. Ang II also activates a Cl(-) current, but the underlying mechanism is not clear.

EXPERIMENTAL APPROACH

The A10 cell line and primary cultures of VSMC from control, ClC-3 channel null mice and WT mice made hypertensive with AngII infusions were used. Techniques employed included whole-cell patch clamp, co-immunoprecipitation, site-specific mutagenesis and Western blotting,

KEY RESULTS

In VSMC, AngII induced Cl(-) currents was carried by the chloride ion channel ClC-3. This current was absent in VSMC from ClC-3 channel null mice. The AngII-induced Cl(-) current involved interactions between ClC-3 channels and Rho-kinase 2 (ROCK2), shown by N- or C-terminal truncation of ClC-3 protein, ROCK2 siRNA and co-immunoprecipitation assays. Phosphorylation of ClC-3 channels at Thr(532) by ROCK2 was critical for AngII-induced Cl(-) current and VSMC migration. The ClC-3 T532D mutant (mutation of Thr(532) to aspartate), mimicking phosphorylated ClC-3 protein, significantly potentiated AngII-induced Cl(-) current and VSMC migration, while ClC-3 T532A (mutation of Thr(532) to alanine) had the opposite effects. AngII-induced cell migration was markedly decreased in VSMC from ClC-3 channel null mice that was insensitive to Y27632, an inhibitor of ROCK2. In addition, AngII-induced cerebrovascular remodelling was decreased in ClC-3 null mice, possibly by the ROCK2 pathway.

CONCLUSIONS AND IMPLICATIONS

ClC-3 protein phosphorylation at Thr(532) by ROCK2 is required for AngII-induced Cl(-) current and VSMC migration that are involved in AngII-induced vascular remodelling in hypertension.

Transcriptomic analysis reveals specific osmoregulatory adaptive responses in gill mitochondria-rich cells and pavement cells of the Japanese eel

BACKGROUND

Homeostasis of ions and water is important for the maintenance of cellular functions. The regulation of the homeostasis is particularly important in euryhaline fish that migrate between freshwater (FW) and seawater (SW) environments. The fish gill, the major tissue that forms an interface separating the extracellular fluids and external water environment, has an effective transport system to maintain and regulate a constant body osmolality. In fish gills, the two major epithelial cells, pavement cells (PVCs) and mitochondria-rich cells (MRCs), are known to play key and complementary roles in ion transport at the interface. Discovering the robust mechanisms underlying the two cell types' response to osmotic stress would benefit our understanding of the fundamental mechanism allowing PVCs and MRCs to handle osmotic stress. Owing to the limited genomic data available on estuarine species, existing knowledge in this area is slim. In this study, transcriptome analyses were conducted using PVCs and MRCs isolated from Japanese eels adapted to FW or SW environments to provide a genome-wide molecular study to unravel the fundamental processes at work.

RESULTS

The study identified more than 12,000 transcripts in the gill cells. Interestingly, remarkable differential expressed genes (DEGs) were identified in PVCs (970 transcripts) instead of MRCs (400 transcripts) in gills of fish adapted to FW or SW. Since PVCs cover more than 90 % of the gill epithelial surface, the greater change in gene expression patterns in PVCs in response to external osmolality is anticipated. In the integrity pathway analysis, 19 common biological functions were identified in PVCs and MRCs. In the enriched signaling pathways analysis, most pathways differed between PVCs and MRCs; 14 enriched pathways were identified in PVCs and 12 in MRCs. The results suggest that the osmoregulatory responses in PVCs and MRCs are cell-type specific, which supports the complementary functions of the cells in osmoregulation.

CONCLUSIONS

This is the first study to provide transcriptomic analysis of PVCs and MRCs in gills of eels adapted to FW or SW environments. It describes the cell-type specific transcriptomic network in different tonicity. The findings consolidate the known osmoregulatory pathways and provide molecular insight in osmoregulation. The presented data will be useful for researchers to select their targets for further studies.

Role of volume-regulated and calcium-activated anion channels in cell volume homeostasis, cancer and drug resistance

Volume-regulated channels for anions (VRAC) / organic osmolytes (VSOAC) play essential roles in cell volume regulation and other cellular functions, e.g. proliferation, cell migration and apoptosis. LRRC8A, which belongs to the leucine rich-repeat containing protein family, was recently shown to be an essential component of both VRAC and VSOAC. Reduced VRAC and VSOAC activities are seen in drug resistant cancer cells. ANO1 is a calcium-activated chloride channel expressed on the plasma membrane of e.g., secretory epithelia. ANO1 is amplified and highly expressed in a large number of carcinomas. The gene, encoding for ANO1, maps to a region on chromosome 11 (11q13) that is frequently amplified in cancer cells. Knockdown of ANO1 impairs cell proliferation and cell migration in several cancer cells. Below we summarize the basic biophysical properties of VRAC, VSOAC and ANO1 and their most important cellular functions as well as their role in cancer and drug resistance.

The identification of a volume-regulated anion channel: an amazing Odyssey

The volume-regulated anion channel (VRAC) plays a pivotal role in cell volume regulation in essentially all cell types studied. Additionally, VRAC appears to contribute importantly to a wide range of other cellular functions and pathological events, including cell motility, cell proliferation, apoptosis and excitotoxic glutamate release in stroke. Although biophysically, pharmacologically and functionally thoroughly described, VRAC has until very recently remained a genetic orphan. The search for the molecular identity of VRAC has been long and has yielded multiple potential candidates, all of which eventually turned out to have properties not fully compatible with those of VRAC. Recently, two groups have independently identified the protein leucine-rich repeats containing 8A (LRRC8A), belonging to family of proteins (LRRC8A-E) distantly related to pannexins, as the likely pore-forming subunit of VRAC. In this brief review, we summarize the history of the discovery of VRAC, outline its basic biophysical and pharmacological properties, link these to several cellular functions in which VRAC appears to play important roles, and sketch the amazing search for the molecular identity of this channel. Finally, we describe properties of the LRRC8 proteins, highlight some features of the LRRC8A knockout mouse and discuss the impact of the discovery of LRRC8 as VRAC on future research.

Integrin β3 mediates cerebrovascular remodelling through Src/ClC-3 volume-regulated Cl(-) channel signalling pathway

BACKGROUND AND PURPOSE

Cerebrovascular remodelling is one of the important risk factors of stroke. The underlying mechanisms are unclear. Integrin β3 and volume-regulated ClC-3 Cl(-) channels have recently been implicated as important contributors to vascular cell proliferation. Therefore, we investigated the role of integrin β3 in cerebrovascular remodelling and related Cl(-) signalling pathway.

EXPERIMENTAL APPROACH

Cl(-) currents were recorded using a patch clamp technique. The expression of integrin β3 in hypertensive animals was examined by Western blot and immunohistochemisty. Immunoprecipitation, cDNA and siRNA transfection were employed to investigate the integrin β3/Src/ClC-3 signalling.

KEY RESULTS

Integrin β3 expression was up-regulated in stroke-prone spontaneously hypertensive rats, 2-kidney 2-clip hypertensive rats and angiotensin II-infused hypertensive mice. Integrin β3 expression was positively correlated with medial cross-sectional area and ClC-3 expression in the basilar artery of 2-kidney 2-clip hypertensive rats. Knockdown of integrin β3 inhibited the proliferation of rat basilar vascular smooth muscle cells induced by angiotensin II. Co-immunoprecipitation and immunofluorescence experiments revealed a physical interaction between integrin β3, Src and ClC-3 protein. The integrin β3/Src/ClC-3 signalling pathway was shown to be involved in the activation of volume-regulated chloride channels induced by both hypo-osmotic stress and angiotensin II. Tyrosine 284 within a concensus Src phosphorylation site was the key point for ClC-3 channel activation. ClC-3 knockout significantly attenuated angiotensin II-induced cerebrovascular remodelling.

CONCLUSIONS AND IMPLICATIONS

Integrin β3 mediates cerebrovascular remodelling during hypertension via Src/ClC-3 signalling pathway.

Nox family NADPH oxidases in mechano-transduction: mechanisms and consequences

SIGNIFICANCE

The majority of cells in a multi-cellular organism are continuously exposed to ever-changing physical forces. Mechano-transduction links these events to appropriate reactions of the cells involving stimulation of signaling cascades, reorganization of the cytoskeleton and alteration of gene expression.

RECENT ADVANCES

Mechano-transduction alters the cellular redox balance and the formation of reactive oxygen species (ROS). Nicotine amide adenine dinucleotide reduced form (NADPH) oxidases of the Nox family are prominent ROS generators and thus, contribute to this stress-induced ROS formation.

CRITICAL ISSUES

Different types and patterns of mechano-stress lead to Nox-dependent ROS formation and Nox-mediated ROS formation contributes to cellular responses and adaptation to physical forces. Thereby, Nox enzymes can mediate vascular protection during physiological mechano-stress. Despite this, over-activation and induction of Nox enzymes and a subsequent substantial increase in ROS formation also promotes oxidative stress in pathological situations like disturbed blood flow or extensive stretch.

FUTURE DIRECTIONS

Individual protein targets of Nox-mediated redox-signaling will be identified to better understand the specificity of Nox-dependent ROS signaling in mechano-transduction. Nox-inhibitors will be tested to reduce cellular activation in response to mechano-stimuli.

Role of ion transport in control of apoptotic cell death

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

Identification of key signaling molecules involved in the activation of the swelling-activated chloride current in human glioblastoma cells

The swelling-activated chloride current (I Cl,Vol) is abundantly expressed in glioblastoma (GBM) cells, where it controls cell volume and invasive migration. The transduction pathway mediating I Cl,Vol activation in GBM cells is, however, poorly understood. By means of pharmacological and electrophysiological approaches, on GL-15 human GBM cells we found that I Cl,Vol activation by hypotonic swelling required the activity of a U73122-sensitive phospholipase C (PLC). I Cl,Vol activation could also be induced by the membrane-permeable diacylglycerol (DAG) analog OAG. In contrast, neither calcium (Ca(2+)) chelation by BAPTA-AM nor changes in PKC activity were able to affect I Cl,Vol activation by hypotonic swelling. We further found that R59022, an inhibitor of diacylglycerol kinase (DGK), reverted I Cl,Vol activation, suggesting the involvement of phosphatidic acid. In addition, I Cl,Vol activation required the activity of a EHT1864-sensitive Rac1 small GTPase and the resulting actin polymerization, as I Cl,Vol activation was prevented by cytochalasin B. We finally show that I Cl,Vol can be activated by the promigratory fetal calf serum in a PLC- and DGK-dependent manner. This observation is potentially relevant because blood serum can likely come in contact with glioblastoma cells in vivo as a result of the tumor-related partial breakdown of the blood-brain barrier. Given the relevance of I Cl,Vol in GBM cell volume regulation and invasiveness, the several key signaling molecules found in this study to be involved in the activation of the I Cl,Vol may represent potential therapeutic targets against this lethal cancer.

Tumor angiogenesis is caused by single melanoma cells in a manner dependent on reactive oxygen species and NF-κB

Melanomas have a high angiogenic potential, but respond poorly to medical treatment and metastasize very early. To understand the early events in tumor angiogenesis, animal models with high tumor resolution and blood vessel resolution are required, which provide the opportunity to test the ability of small molecule inhibitors to modulate the angiogenic tumor program. We have established a transgenic melanoma angiogenesis model in the small laboratory fish species Japanese medaka. Here, pigment cells are transformed by an oncogenic receptor tyrosine kinase in fish expressing GFP throughout their vasculature. We show that angiogenesis occurs in a reactive oxygen species (ROS)- and NF-κB-dependent, but hypoxia-independent manner. Intriguingly, we observed that blood vessel sprouting is induced even by single transformed pigment cells. The oncogenic receptor as well as human melanoma cells harboring other oncogenes caused the production of pro-angiogenic factors, most prominently angiogenin, through NF-κB signaling. Inhibiting NF-κB prevented tumor angiogenesis and led to the regression of existing tumor blood vessels. In conclusion, our high-resolution medaka melanoma model discloses that ROS and NF-κB signaling from single tumor cells causes hypoxia-independent angiogenesis, thus, demonstrating that the intrinsic malignant tumor cell features are sufficient to initiate and maintain a pro-angiogenic signaling threshold.

17β-Oestradiol inhibits doxorubicin-induced apoptosis via block of the volume-sensitive Cl(-) current in rabbit articular chondrocytes

BACKGROUND AND PURPOSE Chondrocyte apoptosis contributes to disruption of cartilage integrity in osteoarthritis. Recent evidence suggested that the volume-sensitive organic osmolyte/anion channel [volume-sensitive (outwardly rectifying) Cl(-) current (I(Cl,vol) )] plays a functional role in the development of cell shrinkage associated with apoptosis (apoptotic volume decrease) in several cell types. In this study, we investigated the cellular effects of 17β-oestradiol on doxorubicin-induced apoptotic responses in rabbit articular chondrocytes. EXPERIMENTAL APPROACH Whole-cell membrane currents and cross-sectional area were measured from chondrocytes using a patch-clamp method and microscopic cell imaging, respectively. Caspase-3/7 activity was assayed as an index of apoptosis. KEY RESULTS Addition of doxorubicin (1 µM) to isosmotic bath solution rapidly activated the Cl(-) current with properties similar to those of I(Cl,vol) in chondrocytes. Doxorubicin also gradually decreased the cross-sectional area of chondrocytes, followed by enhanced caspase-3/7 activity; both of these responses were totally abolished by the I(Cl,vol) blocker DCPIB (20 µM). Pretreatment of chondrocytes with 17β-oestradiol (1 nM) for short (approximately 10 min) and long (24 h) periods almost completely prevented the doxorubicin-induced activation of I(Cl,vol) and subsequent elevation of caspase-3/7 activity. These effects of 17β-oestradiol were significantly attenuated by the oestrogen receptor blocker ICI 182780 (10 µM), as well as the phosphatidyl inositol-3-kinase (PI3K) inhibitors wortmannin (100 nM) and LY294002 (20 µM). Testosterone (10 nM) had no effect on the doxorubicin-induced Cl(-) current. CONCLUSIONS AND IMPLICATIONS 17β-Oestradiol prevents the doxorubicin-induced cell shrinkage mediated through activation of I(Cl,vol) and subsequent induction of apoptosis signals, through a membrane receptor-dependent PI3K pathway in rabbit articular chondrocytes.

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.

Osmosensory mechanisms in cellular and systemic volume regulation

Perturbations of cellular and systemic osmolarity severely challenge the function of all organisms and are consequently regulated very tightly. Here we outline current evidence on how cells sense volume perturbations, with particular focus on mechanisms relevant to the kidneys and to extracellular osmolarity and whole body volume homeostasis. There are a variety of molecular signals that respond to perturbations in cell volume and osmosensors or volume sensors responding to these signals. The early signals of volume perturbation include integrins, the cytoskeleton, receptor tyrosine kinases, and transient receptor potential channels. We also present current evidence on the localization and function of central and peripheral systemic osmosensors and conclude with a brief look at the still limited evidence on pathophysiological conditions associated with deranged sensing of cell volume.

New molecular mechanisms for cardiovascular disease: cardiac hypertrophy and cell-volume regulation

Cardiac hypertrophy is an increase in the muscle volume of the ventricle due to the enlargement of cardiac cells. Physiological cardiac hypertrophy is the normal response to healthy exercise, and pathological hypertrophy is the response to increased stress such as hypertension. Intracellular and extracellular aniosmotic conditions also change cell volume. Since persistent cell swelling or cell shrinkage during aniosmotic conditions results in cell death, the ability to regulate cell volume is important for the maintenance of cellular homeostasis. Cell swelling activates a regulatory volume decrease (RVD) response in which solute leakage pathways are stimulated and solute with water exits cells, reducing the cell volume towards the original value. In cardiac cells, one of the essential factors for cell-volume regulation is the volume-regulated anion channel (VRAC). However, the relationship between cardiac hypertrophy and cell-volume regulation is not clear. In this review, we introduce our recent findings showing that the impairment of VRAC current is exhibited in ventricular cells from mice with cardiac hypertrophy induced by transverse aortic constriction. Similar results were shown in caveolin-3-deficient mice, which develop cardiac hypertrophy without pressure overload. These results suggest that VRAC will be a new target for protection from the development of cardiac hypertrophy.

Cell volume homeostatic mechanisms: effectors and signalling pathways

Cell volume homeostasis and its fine-tuning to the specific physiological context at any given moment are processes fundamental to normal cell function. The understanding of cell volume regulation owes much to August Krogh, yet has advanced greatly over the last decades. In this review, we outline the historical context of studies of cell volume regulation, focusing on the lineage started by Krogh, Bodil Schmidt-Nielsen, Hans-Henrik Ussing, and their students. The early work was focused on understanding the functional behaviour, kinetics and thermodynamics of the volume-regulatory ion transport mechanisms. Later work addressed the mechanisms through which cellular signalling pathways regulate the volume regulatory effectors or flux pathways. These studies were facilitated by the molecular identification of most of the relevant channels and transporters, and more recently also by the increased understanding of their structures. Finally, much current research in the field focuses on the most up- and downstream components of these paths: how cells sense changes in cell volume, and how cell volume changes in turn regulate cell function under physiological and pathophysiological conditions.

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.

alpha-Adrenoceptor-mediated depletion of phosphatidylinositol 4, 5-bisphosphate inhibits activation of volume-regulated anion channels in mouse ventricular myocytes

BACKGROUND AND PURPOSE

Volume-regulated anion channels (VRACs) play an important role in cell-volume regulation. alpha(1)-Adrenoceptor stimulation by phenylephrine (PE) suppressed the hypotonic activation of VRAC current in mouse ventricular cells and regulatory volume decrease (RVD) was also absent in PE-treated cells. We examined whether the effects of alpha(1)-adrenoceptor stimuli on VRAC current were modulated by phosphatidylinositol signalling.

EXPERIMENTAL APPROACH

Whole-cell patch-clamp method was used to record the hypotonicity-induced VRAC current in mouse ventricular cells. RVD was analyzed by videomicroscopic measurement of cell images.

KEY RESULTS

The attenuation of VRAC current by PE was suppressed by alpha(1A)-adrenoceptor antagonists (prazosin and WB-4101), anti-G(q) protein antibody and a specific phosphoinositide-specific phospholipase C (PLC) inhibitor (U-73122), but not by antagonists for alpha(1B)-, alpha(1D)- or beta-adrenoceptor, or protein kinase C inhibitors. The inhibition of VRAC by PE was antagonized by intracellular excess phosphatidylinositol 4,5-bisphosphate (PIP(2)), while intracellular anti-PIP(2) antibody (PIP(2) Ab) inhibited the activation of VRAC currents. When cells were loaded with phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) with or without PIP(2) Ab, PE little affected the VRAC current. Extracellular m-3M3FBS (an activator of PLC) suppressed VRAC in the absence of PE, and this effect was reversed by intracellular excess PIP(2).

CONCLUSIONS AND IMPLICATIONS

Our results indicate that the stimulation of alpha(1A)-adrenoceptors by PE inhibited the activation of cardiac VRAC current via PIP(3) depletion brought about by PLC-dependent reduction of membrane PIP(2) level.

HIV protease inhibitors elicit volume-sensitive Cl- current in cardiac myocytes via mitochondrial ROS

HIV protease inhibitors (HIV PI) reduce morbidity and mortality of HIV infection but cause multiple untoward effects. Because certain HIV PI evoke production of reactive oxygen species (ROS) and volume-sensitive Cl(-) current (I(Cl,swell)) is activated by ROS, we tested whether HIV PI stimulate I(Cl,swell) in ventricular myocytes. Ritonavir and lopinavir elicited outwardly rectifying Cl(-) currents under isosmotic conditions that were abolished by the selective I(Cl,swell)-blocker DCPIB. In contrast, amprenavir, nelfinavir, and raltegravir, an integrase inhibitor, did not modulate I(Cl,swell) acutely. Ritonavir also reduced action potential duration, but amprenavir did not. I(Cl,swell) activation was attributed to ROS because ebselen, an H(2)O(2) scavenger, suppressed ritonavir- and lopinavir-induced I(Cl,swell). Major ROS sources in cardiomyocytes are sarcolemmal NADPH oxidase and mitochondria. The specific NADPH oxidase inhibitor apocynin failed to block ritonavir- or lopinavir-induced currents, although it blocks I(Cl,swell) elicited by osmotic swelling or stretch. In contrast, rotenone, a mitochondrial e(-) transport inhibitor, suppressed both ritonavir- and lopinavir-induced I(Cl,swell). ROS production was measured in HL-1 cardiomyocytes with C-H(2)DCFDA-AM and mitochondrial membrane potential (ΔΨ(m)) with JC-1. Flow cytometry confirmed that ritonavir and lopinavir but not amprenavir, nelfinavir, or raltegravir augmented ROS production, and HIV PI-induced ROS production was suppressed by rotenone but not NADPH oxidase blockade. Moreover, ritonavir, but not amprenavir, depolarized ΔΨ(m). These data suggest ritonavir and lopinavir activated I(Cl,swell) via mitochondrial ROS production that was independent of NADPH oxidase. ROS-dependent modulation of I(Cl,swell) and other ion channels by HIV PI may contribute to some of their actions in heart and perhaps other tissues.

Activation of swelling-activated chloride current by tumor necrosis factor-alpha requires ClC-3-dependent endosomal reactive oxygen production

ClC-3 is a Cl(-)/H(+) antiporter required for cytokine-induced intraendosomal reactive oxygen species (ROS) generation by Nox1. ClC-3 current is distinct from the swelling-activated chloride current (ICl(swell)), but overexpression of ClC-3 can activate currents that resemble ICl(swell). Because H(2)O(2) activates ICl(swell) directly, we hypothesized that ClC-3-dependent, endosomal ROS production activates ICl(swell). Whole-cell perforated patch clamp methods were used to record Cl(-) currents in cultured aortic vascular smooth muscle cells from wild type (WT) and ClC-3 null mice. Under isotonic conditions, tumor necrosis factor-alpha (TNF-alpha) (10 ng/ml) activated outwardly rectifying Cl(-) currents with time-dependent inactivation in WT but not ClC-3 null cells. Inhibition by tamoxifen (10 microm) and by hypertonicity (340 mosm) identified them as ICl(swell). ICl(swell) was also activated by H(2)O(2) (500 microm), and the effect of TNF-alpha was completely inhibited by polyethylene glycol-catalase. ClC-3 expression induced ICl(swell) in ClC-3 null cells in the absence of swelling or TNF-alpha, and this effect was also blocked by catalase. ICl(swell) activation by hypotonicity (240 mosm) was only partially inhibited by catalase, and the size of these currents did not differ between WT and ClC-3 null cells. Disruption of endosome trafficking with either mutant Rab5 (S34N) or Rab11 (S25N) inhibited TNF-alpha-mediated activation of ICl(swell). Thrombin also activates ROS production by Nox1 but not in endosomes. Thrombin caused H(2)O(2)-dependent activation of ICl(swell), but this effect was not ClC-3- or Rab5-dependent. Thus, activation of ICl(swell) by TNF-alpha requires ClC-3-dependent endosomal H(2)O(2) production. This demonstrates a functional link between two distinct anion currents, ClC-3 and ICl(swell).

Endothelin signalling regulates volume-sensitive Cl- current via NADPH oxidase and mitochondrial reactive oxygen species

AIMS

We assessed regulation of volume-sensitive Cl(-) current (I(Cl,swell)) by endothelin-1 (ET-1) and characterized the signalling pathway responsible for its activation in rabbit atrial and ventricular myocytes.

METHODS AND RESULTS

ET-1 elicited I(Cl,swell) under isosmotic conditions. Outwardly rectified Cl(-) current was blocked by the I(Cl,swell)-selective inhibitor DCPIB or osmotic shrinkage and involved ET(A) but not ET(B) receptors. ET-1-induced current was abolished by inhibiting epidermal growth factor receptor (EGFR) kinase or phosphoinositide-3-kinase (PI-3K), indicating that these kinases were downstream. Regarding upstream events, activation of I(Cl,swell) by osmotic swelling or angiotensin II (AngII) was suppressed by ET(A) blockade, whereas AngII AT(1) receptor blockade failed to alter ET-1-induced current. Reactive oxygen species (ROS) produced by NADPH oxidase (NOX) stimulate I(Cl,swell). As expected, blockade of NOX suppressed ET-1-induced I(Cl,swell), but blockade of mitochondrial ROS production with rotenone also suppressed I(Cl,swell). I(Cl,swell) was activated by augmenting complex III ROS production with antimycin A or diazoxide; in this case, I(Cl,swell) was insensitive to NOX inhibitors, indicating that mitochondria were downstream from NOX. ROS generation in HL-1 cardiomyocytes measured by flow cytometry confirmed the electrophysiological findings. ET-1-induced ROS production was inhibited by blocking either NOX or mitochondrial complex I, whereas complex III-induced ROS production was insensitive to NOX blockade.

CONCLUSION

ET-1-ET(A) signalling activated I(Cl,swell) via EGFR kinase, PI-3K, and NOX ROS production, which triggered mitochondrial ROS production. ET(A) receptors were downstream effectors when I(Cl,swell) was elicited by osmotic swelling or AngII. These data suggest that ET-1-induced ROS-dependent I(Cl,swell) is likely to participate in multiple physiological and pathophysiological processes.

Epidermal growth factor protects the heart against low-flow ischemia-induced injury

The role of ErbB4 and ErbB2 in the heart of adult mammals is well established. The heart also expresses ErbB1 (the epidermal growth factor (EGF) receptor), but this receptor has received less attention. We studied the effect of EGF on the response of isolated mouse heart to low-flow ischemia and reperfusion. Reducing perfusate flow to 10% for 30 min resulted in an increase in anaerobic metabolism and the leakage of lactate dehydrogenase during reperfusion. In addition, left ventricle +dP/dt and developed pressure were depressed (20-25%) during reperfusion. The addition of EGF 5 min before and throughout the ischemic period prevented the increase in anaerobic metabolism and the leakage of intracellular lactate dehydrogenase during reperfusion. EGF improved both +dP/dt and developed pressure during ischemia and prevented the decrease in dP/dt during reperfusion. To determine whether the effect of EGF on cell integrity depends on its effect on contractility, we studied nonbeating isolated myocytes. In these cells, anoxia and reoxygenation reduced cell viability by nearly 25%. EGF prevented such a decrease. Our results indicate that, like ErbB4 and ErbB2, ErbB1 also has an important role in the heart of adult animals.

Influence of the extracellular matrix and integrins on volume-sensitive osmolyte anion channels in C2C12 myoblasts

The purpose of this study was to determine whether extracellular matrix (ECM) composition through integrin receptors modulated the volume-sensitive osmolyte anion channels (VSOACs) in skeletal muscle-derived C2C12 cells. Cl(-) currents were recorded in whole cell voltage-clamped cells grown on laminin (LM), fibronectin (FN), or in the absence of a defined ECM (NM). Basal membrane currents recorded in isotonic media (300 mosmol/kg) were larger in cells grown on FN (3.8-fold at +100 mV) or LM (8.8-fold at +100 mV) when compared with NM. VSOAC currents activated by cell exposure to hypotonic solution were larger in cells grown on LM (1.72-fold at +100 mV) or FN (1.75-fold at +100 mV) compared with NM. Additionally, the kinetics of VSOAC activation was approximately 27% quicker on FN and LM. These currents were tamoxifen sensitive, displayed outward rectification, reversed at the equilibrium potential of Cl(-) and inactivated at potentials >+60 mV. Specific knockdown of beta(1)-integrin by short hairpin RNA interference strongly inhibited the VSOAC Cl(-) currents in cells plated on FN. In conclusion, ECM composition and integrins profoundly influence the biophysical properties and mechanisms of onset of VSOACs.

Integrin structure and functional relation with ion channels

Physical and functional link between cell adhesion molecules and ion channels provide a rapid connection between extracellular environment and cell physiology. Growing evidence does shows that frequent cross talk occurs between these classes of membrane proteins. These interactions are being addressed in ever increasing molecular detail. Recent advances have given X-ray resolved structure of the extracellular domains of integrin receptors. Such a level of resolution is still not available for the transmembrane and intracellular domains. Nonetheless, current molecular biological work is unraveling an intricate network connecting the cytoplasmic integrin domains with the cytoskeleton, ion channels and variety of cellular messengers. Overall, these studies show that integrins and ion channels both present bidirectional signaling features. Extracellular signals are usually transduced by integrins to trigger cellular responses that may involve ion fluxes, which can offer further relay. Intracellular processes and ion channel engagement can in turn affect integrin activation and expression and thus cell adhesion to the extracellular matrix. Moreover, ion channels themselves can communicate extracellular messages to both the cytoplasmic environment and integrin themselves. These interactions appear to often depend on formation of multiprotein membrane complexes that can recruit other elements, such as growth factor receptors and cytoplasmic signaling proteins. This chapter provides a general introduction to the field by giving a brief historical introduction and summarizing the main features of integrin structure and link to the cytoplasmic proteins. In addition, it outlines the main cellular processes inwhich channel-integrin interplay is known to exert clear physiological and pathological roles.

Exogenous and endogenous ceramides elicit volume-sensitive chloride current in ventricular myocytes

AIMS

Because ceramide accumulates in several forms of cardiovascular disease and ceramide-induced apoptosis may involve the volume-sensitive Cl(-) current, I(Cl,swell), we assessed whether ceramide activates I(Cl,swell).

METHODS AND RESULTS

I(Cl,swell) was measured in rabbit ventricular myocytes by whole-cell patch clamp after isolating anion currents. Exogenous C(2)-ceramide (C(2)-Cer), a membrane-permeant short-chain ceramide, elicited an outwardly rectifying Cl(-) current in both physiological and symmetrical Cl(-) solutions that was fully inhibited by DCPIB, a specific I(Cl,swell) blocker. In contrast, the metabolically inactive C(2)-Cer analogue C(2)-dihydroceramide (C(2)-H(2)Cer) failed to activate Cl(-) current. Bacterial sphingomyelinase (SMase), which generates endogenous long-chain ceramides as was confirmed by tandem mass spectrometry, also elicited an outwardly rectifying Cl(-) current that was inhibited by DCPIB and tamoxifen, another I(Cl,swell) blocker. Bacterial SMase-induced current was partially reversed by osmotic shrinkage and fully suppressed by ebselen, a scavenger of reactive oxygen species. Outward rectification with physiological and symmetrical Cl(-) gradients, block by DCPIB and tamoxifen, and volume sensitivity are characteristics that identify I(Cl,swell). Insensitivity to C(2)-H(2)Cer and block by ebselen suggest involvement of ceramide signalling rather than direct lipid-channel interaction.

CONCLUSION

Exogenous and endogenous ceramide elicited I(Cl,swell) in ventricular myocytes. This may contribute to persistent activation of I(Cl,swell) and aspects of altered myocyte function in cardiovascular diseases associated with by ceramide accumulation.