Dual roles of plasmalemmal chloride channels in induction of cell death

De novo variants in LRRC8C resulting in constitutive channel activation cause a human multisystem disorder

Volume-regulated anion channels (VRACs) are multimeric proteins composed of different paralogs of the LRRC8 family. They are activated in response to hypotonic swelling, but little is known about their specific functions. We studied two human individuals with the same congenital syndrome affecting blood vessels, brain, eyes, and bones. The LRRC8C gene harbored de novo variants in both patients, located in a region of the gene encoding the boundary between the pore and a cytoplasmic domain, which is depleted of sequence variations in control subjects. When studied by cryo-EM, both LRRC8C mutant proteins assembled as their wild-type counterparts, but showed increased flexibility, suggesting a destabilization of subunit interactions. When co-expressed with the obligatory LRRC8A subunit, the mutants exhibited enhanced activation, resulting in channel activity even at isotonic conditions in which wild-type channels are closed. We conclude that structural perturbations of LRRC8C impair channel gating and constitute the mechanistic basis of the dominant gain-of-function effect of these pathogenic variants. The pleiotropic phenotype of this novel clinical entity associated with monoallelic LRRC8C variants indicates the fundamental roles of VRACs in different tissues and organs.

Cell death induction and protection by activation of ubiquitously expressed anion/cation channels. Part 3: the roles and properties of TRPM2 and TRPM7

Cell volume regulation (CVR) is a prerequisite for animal cells to survive and fulfill their functions. CVR dysfunction is essentially involved in the induction of cell death. In fact, sustained normotonic cell swelling and shrinkage are associated with necrosis and apoptosis, and thus called the necrotic volume increase (NVI) and the apoptotic volume decrease (AVD), respectively. Since a number of ubiquitously expressed ion channels are involved in the CVR processes, these volume-regulatory ion channels are also implicated in the NVI and AVD events. In Part 1 and Part 2 of this series of review articles, we described the roles of swelling-activated anion channels called VSOR or VRAC and acid-activated anion channels called ASOR or PAC in CVR and cell death processes. Here, Part 3 focuses on therein roles of Ca2+-permeable non-selective TRPM2 and TRPM7 cation channels activated by stress. First, we summarize their phenotypic properties and molecular structure. Second, we describe their roles in CVR. Since cell death induction is tightly coupled to dysfunction of CVR, third, we focus on their participation in the induction of or protection against cell death under oxidative, acidotoxic, excitotoxic, and ischemic conditions. In this regard, we pay attention to the sensitivity of TRPM2 and TRPM7 to a variety of stress as well as to their capability to physicall and functionally interact with other volume-related channels and membrane enzymes. Also, we summarize a large number of reports hitherto published in which TRPM2 and TRPM7 channels are shown to be involved in cell death associated with a variety of diseases or disorders, in some cases as double-edged swords. Lastly, we attempt to describe how TRPM2 and TRPM7 are organized in the ionic mechanisms leading to cell death induction and protection.

Cancer as a Channelopathy—Appreciation of Complimentary Pathways Provides a Different Perspective for Developing Treatments

Simple Summary

While improvements in technology have improved our ability to treat many forms of cancer when diagnosed at an early stage of the disease, the ability to improve survival and quality of life for patients with late stage disease has been limited, largely due to the ability of cancer cells to evade destruction when treatments block preferred paths for survival. Here, we review the role that ions and ion channels play in normal cell function, the development of disease and their role in the life and death of a cell. It is hoped that viewing cancer from the perspective of altered ion channel expression and ion balance may provide a novel approach for developing more effective treatments for this devastating disease.

Abstract

Life depends upon the ability of cells to evaluate and adapt to a constantly changing environment and to maintain internal stability to allow essential biochemical reactions to occur. Ions and ion channels play a crucial role in this process and are essential for survival. Alterations in the expression of the transmembrane proteins responsible for maintaining ion balance that occur as a result of mutations in the genetic code or in response to iatrogenically induced changes in the extracellular environment is a characteristic feature of oncogenesis and identifies cancer as one of a constellation of diseases known as channelopathies. The classification of cancer as a channelopathy provides a different perspective for viewing the disease. Potentially, it may expand opportunities for developing novel ways to affect or reverse the deleterious changes that underlie establishing and sustaining disease and developing tolerance to therapeutic attempts at treatment. The role of ions and ion channels and their interactions in the cell’s ability to maintain ionic balance, homeostasis, and survival are reviewed and possible approaches that mitigate gain or loss of ion channel function to contribute to new or enhance existing cancer therapies are discussed.

Role of ANO1 in tumors and tumor immunity

Dysregulation of gene amplification, cell-signaling-pathway transduction, epigenetic and transcriptional regulation, and protein interactions drives tumor-cell proliferation and invasion, while ion channels also play an important role in the generation and development of tumor cells. Overexpression of Ca²⁺-activated Cl- channel anoctamin 1 (ANO1) is shown in numerous cancer types and correlates with poor prognosis. However, the mechanisms involved in ANO1-mediated malignant cellular transformation and the role of ANO1 in tumor immunity remain unknown. In this review, we discuss recent studies to determine the role of ANO1 in tumorigenesis and provide novel insights into the role of ANO1 in the context of tumor immunity. Furthermore, we analyze the roles and potential mechanisms of ANO1 in different types of cancers, and provide novel notions for the role of ANO1 in the tumor microenvironment and for potential use of ANO1 in clinical applications. Our review shows that ANO1 is involved in tumor immunity and microenvironment, and may, therefore, be an effective biomarker and therapeutic drug target.

Mechanisms Underlying Influence of Bioelectricity in Development

To execute the intricate process of development, cells coordinate across tissues and organs to determine where each cell divides and differentiates. This coordination requires complex communication between cells. Growing evidence suggests that bioelectrical signals controlled via ion channels contribute to cell communication during development. Ion channels collectively regulate the transmembrane potential of cells, and their function plays a conserved role in the development of organisms from flies to humans. Spontaneous calcium oscillations can be found in nearly every cell type and tissue, and disruption of these oscillations leads to defects in development. However, the mechanism by which bioelectricity regulates development is still unclear. Ion channels play essential roles in the processes of cell death, proliferation, migration, and in each of the major canonical developmental signaling pathways. Previous reviews focus on evidence for one potential mechanism by which bioelectricity affects morphogenesis, but there is evidence that supports multiple different mechanisms which are not mutually exclusive. Evidence supports bioelectricity contributing to development through multiple different mechanisms. Here, we review evidence for the importance of bioelectricity in morphogenesis and provide a comprehensive review of the evidence for several potential mechanisms by which ion channels may act in developmental processes.

Effect of Swell1 on regulating chondrocyte hypertrophy during the condylar osteochondral development process in mice

Chondrocyte hypertrophy is a significant factor in cartilage development, yet the molecular mechanism for cell volume expand during the process is remains unclear. In the present study, the relationship between Swell1, a cell volume regulated anion channel, and chondrocyte hypertrophy was explored. The results reveal that the spatiotemporal expression of Swell1 was similar with the development process of hypertrophic chondrocytes in condyles. Through Col10a1 mediated knock out of Swell1 in hypertrophy chondrocytes, we found that there are less obvious boundary between different condylar cartilage layers in which increased hypertrophic chondrocytes were scattered in all three cartilage layers. The cortical bone mass and bone mineral density in the subchondral bone significantly increased. Additionally, knock out of Swell1 could increase the expression of OCN in the femur condyle. Based on the aforementioned findings, a conclusion could be drawn that Swell1 is a significant factor in chondrocyte hypertrophy during the condylar osteochondral development process, and there was some difference between the mandibular and femur condyles, which will provide some new clues for understanding the development of cartilage and related diseases.

Volume-activated chloride channels contribute to lipopolysaccharide plus nigericin-induced pyroptosis in bone marrow-derived macrophages

The representative morphological features of pyroptosis are excessive cell swelling and subsequent membrane rupture. However, the mechanism underlying the cell's inherent inability to regulate volume during the progression of pyroptosis is poorly understood. In the current study, we found that both volume-activated chloride currents (I_{cl, vol}) and the regulatory volume decrease (RVD) were markedly decreased in bone marrow-derived macrophages (BMDMs) undergoing pyroptosis induced by lipopolysaccharides (LPS) and nigericin. The inhibition of I_{Cl, vol} and RVD by the chloride channel blockers, tamoxifen or DCPIB, led to the emergence of pyroptosis-like phenotypes such as activated-caspase-1, pyroptotic-body-like bubbles, and a fried-egg-like appearance. Moreover, the expression of the volume-activated chloride channel (VRAC) constituent protein Leucine-Rich Repeat-Containing 8A (LRRC8A) was significantly down-regulated in pyroptotic BMDMs treated with LPS and nigericin. The silencing of LRRC8A expression by small interfering RNA (si)-LRRC8A transfection not only reduced I_{Cl, vol} and RVD, but also caused BMDMs to show pyroptosis-like manifestations such as activated-caspase-1, membrane bubbles, and have a fried-egg-like appearance. These results reveal a new mechanism for the loss of volume regulation in the process of pyroptotic cell swelling and strongly suggest that a functional deficiency of VRAC/LRRC8A plays a key role in this disorder.

Cell Death Induction and Protection by Activation of Ubiquitously Expressed Anion/Cation Channels. Part 1: Roles of VSOR/VRAC in Cell Volume Regulation, Release of Double-Edged Signals and Apoptotic/Necrotic Cell Death

Cell volume regulation (CVR) is essential for survival and functions of animal cells. Actually, normotonic cell shrinkage and swelling are coupled to apoptotic and necrotic cell death and thus called the apoptotic volume decrease (AVD) and the necrotic volume increase (NVI), respectively. A number of ubiquitously expressed anion and cation channels are involved not only in CVD but also in cell death induction. This series of review articles address the question how cell death is induced or protected with using ubiquitously expressed ion channels such as swelling-activated anion channels, acid-activated anion channels and several types of TRP cation channels including TRPM2 and TRPM7. The Part 1 focuses on the roles of the volume-sensitive outwardly rectifying anion channels (VSOR), also called the volume-regulated anion channel (VRAC), which is activated by cell swelling or reactive oxygen species (ROS) in a manner dependent on intracellular ATP. First we describe phenotypical properties, the molecular identity, and physical pore dimensions of VSOR/VRAC. Second, we highlight the roles of VSOR/VRAC in the release of organic signaling molecules, such as glutamate, glutathione, ATP and cGAMP, that play roles as double-edged swords in cell survival. Third, we discuss how VSOR/VRAC is involved in CVR and cell volume dysregulation as well as in the induction of or protection from apoptosis, necrosis and regulated necrosis under pathophysiological conditions.

Ions, the Movement of Water and the Apoptotic Volume Decrease

The movement of water across the cell membrane is a natural biological process that occurs during growth, cell division, and cell death. Many cells are known to regulate changes in their cell volume through inherent compensatory regulatory mechanisms. Cells can sense an increase or decrease in their cell volume, and compensate through mechanisms known as a regulatory volume increase (RVI) or decrease (RVD) response, respectively. The transport of sodium, potassium along with other ions and osmolytes allows the movement of water in and out of the cell. These compensatory volume regulatory mechanisms maintain a cell at near constant volume. A hallmark of the physiological cell death process known as apoptosis is the loss of cell volume or cell shrinkage. This loss of cell volume is in stark contrast to what occurs during the accidental cell death process known as necrosis. During necrosis, cells swell or gain water, eventually resulting in cell lysis. Thus, whether a cell gains or loses water after injury is a defining feature of the specific mode of cell death. Cell shrinkage or the loss of cell volume during apoptosis has been termed apoptotic volume decrease or AVD. Over the years, this distinguishing feature of apoptosis has been largely ignored and thought to be a passive occurrence or simply a consequence of the cell death process. However, studies on AVD have defined an underlying movement of ions that result in not only the loss of cell volume, but also the activation and execution of the apoptotic process. This review explores the role ions play in controlling not only the movement of water, but the regulation of apoptosis. We will focus on what is known about specific ion channels and transporters identified to be involved in AVD, and how the movement of ions and water change the intracellular environment leading to stages of cell shrinkage and associated apoptotic characteristics. Finally, we will discuss these concepts as they apply to different cell types such as neurons, cardiomyocytes, and corneal epithelial cells.

Ano6 disruption impairs acinar cell regulatory volume decrease and protein secretion in murine submandibular salivary glands

The widely expressed Anoctamin 6 (Ano6) supports different Ca²⁺ -dependent functions, but little is known about its role in salivary glands. Mouse submandibular gland (SMG) acinar cells exhibited a robust regulatory volume decrease (RVD) following cell swelling that was reduced approximately 70% in Ano6^-/- mice. Ca²⁺ -free conditions nearly eliminated the RVD response suggesting that Ano6 is an obligatory component of the cell volume-activated, Ca²⁺ -dependent RVD pathway in salivary gland acinar cells. Ex vivo agonist-stimulated secretion of water and ions was unaffected by Ano6 disruption under both isotonic and hypotonic conditions suggesting that Ano6 does not play a major role in fluid and electrolyte secretion. In contrast, the total amount of β-adrenergic-dependent protein secretion by the SMG was significantly reduced in Ano6^-/- mice. Closer inspection of these latter results revealed that protein secretion was affected only in the female SMG by Ano6 disruption. These results indicate that Ano6 modulates the RVD response and protein secretion by salivary gland acinar cells.

MicroRNA-Mediated Metabolic Reprograming in Renal Cancer

Metabolic reprogramming is one of the hallmarks of renal cell cancer (RCC). We hypothesized that altered metabolism of RCC cells results from dysregulation of microRNAs targeting metabolically relevant genes. Combined large-scale transcriptomic and metabolic analysis of RCC patients tissue samples revealed a group of microRNAs that contribute to metabolic reprogramming in RCC. miRNAs expressions correlated with their predicted target genes and with gas chromatography-mass spectrometry (GC-MS) metabolome profiles of RCC tumors. Assays performed in RCC-derived cell lines showed that miR-146a-5p and miR-155-5p targeted genes of PPP (the pentose phosphate pathway) (G6PD and TKT), the TCA (tricarboxylic acid cycle) cycle (SUCLG2), and arginine metabolism (GATM), respectively. miR-106b-5p and miR-122-5p regulated the NFAT5 osmoregulatory transcription factor. Altered expressions of G6PD, TKT, SUCLG2, GATM, miR-106b-5p, miR-155-5p, and miR-342-3p correlated with poor survival of RCC patients. miR-106b-5p, miR-146a-5p, and miR-342-3p stimulated proliferation of RCC cells. The analysis involving >6000 patients revealed that miR-34a-5p, miR-106b-5p, miR-146a-5p, and miR-155-5p are PanCancer metabomiRs possibly involved in global regulation of cancer metabolism. In conclusion, we found that microRNAs upregulated in renal cancer contribute to disturbed expression of key genes involved in the regulation of RCC metabolome. miR-146a-5p and miR-155-5p emerge as a key “metabomiRs” that target genes of crucial metabolic pathways (PPP (the pentose phosphate pathway), TCA cycle, and arginine metabolism).

The Volume-Regulated Anion Channel in Glioblastoma

Malignancy of glioblastoma multiforme (GBM), the most common and aggressive form of human brain tumor, strongly depends on its enhanced cell invasion and death evasion which make surgery and accompanying therapies highly ineffective. Several ion channels that regulate membrane potential, cytosolic Ca²⁺ concentration and cell volume in GBM cells play significant roles in sustaining these processes. Among them, the volume-regulated anion channel (VRAC), which mediates the swelling-activated chloride current (IClswell) and is highly expressed in GBM cells, arguably plays a major role. VRAC is primarily involved in reestablishing the original cell volume that may be lost under several physiopathological conditions, but also in sustaining the shape and cell volume changes needed for cell migration and proliferation. While experimentally VRAC is activated by exposing cells to hypotonic solutions that cause the increase of cell volume, in vivo it is thought to be controlled by several different stimuli and modulators. In this review we focus on our recent work showing that two conditions normally occurring in pathological GBM tissues, namely high serum levels and severe hypoxia, were both able to activate VRAC, and their activation was found to promote cell migration and resistance to cell death, both features enhancing GBM malignancy. Also, the fact that the signal transduction pathway leading to VRAC activation appears to involve GBM specific intracellular components, such as diacylglicerol kinase and phosphatidic acid, reportedly not involved in the activation of VRAC in healthy tissues, is a relevant finding. Based on these observations and the impact of VRAC in the physiopathology of GBM, targeting this channel or its intracellular regulators may represent an effective strategy to contrast this lethal tumor.

Detection of apoptotic and live pre-osteoblast cell line using impedance-based biosensors with variable electrode design

Electrical impedance-based sensing of cell activity has become a powerful analytical tool that allows the monitoring of several relevant biological processes associated with cell evolution and morphology. In these types of biosensors, the electrode design has a direct impact on the sensitivity because it defines the capability of the biosensor to measure small changes in the impedance resulting from cell activities. Herein, impedance-based biosensors arrays with several configurations were successfully developed and used to study the impact of the electrode layout on the dynamics of cultured pre-osteoblast cells. The biosensor design was initially validated by measuring the effect of electrode design on the capacitance of a dielectric polymer (parylene) that mimics the dielectric characteristics of cell populations, results are shown in the Supplementary information section. Results from in vitro cell growth indicate that the optimized design of single electrodes with a diameter of 50 µm, are the most sensitive to cell motion whereas increasing the number of electrodes allows clear differentiation between living and dead cells after 3 h of inducing apoptosis. Apoptosis death was induced with Staurosporine, a chemical mediator of apoptosis in osteoblasts. These impedance results have been validated with optical imaging and flow cytometry analysis that were performed on parallel cultures. Frequency and electrolyte concentration effects are also discussed.

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 Identities and ATP Release Activities of Two Types of Volume-Regulatory Anion Channels, VSOR and Maxi-Cl

An elaborate volume regulation system based on interplay of ion channels and transporters was evolved to cope with constant osmotic challenges caused by intensive metabolism, transport and other physiological/pathophysiological events. In animal cells, two types of anion channels are directly activated by cell swelling and involved in the regulatory volume decrease (RVD): volume-sensitive outwardly rectifying anion channel (VSOR), also called volume-regulated anion channel (VRAC), and Maxi-Cl which is the most major type of maxi-anion channel (MAC). These two channels have very different biophysical profiles and exhibit opposite dependence on intracellular ATP. After several decades of verifying many false-positive candidates for VSOR and Maxi-Cl, LRRC8 family proteins emerged as major VSOR components, and SLCO2A1 protein as a core of Maxi-Cl. Still, neither of these proteins alone can fully reproduce the native channel phenotypes suggesting existence of missing components. Although both VSOR and Maxi-Cl have pores wide enough to accommodate bulky ATP^4- and MgATP^2- anions, evidence accumulated hitherto, based on pharmacological and gene silencing experiments, suggests that Maxi-Cl, but not VSOR, serves as one of the major pathways for the release of ATP from swollen and ischemic/hypoxic cells. Relations of VSOR and Maxi-Cl with diseases and their selective pharmacology are the topics promoted by recent advance in molecular identification of the two volume-activated, volume-regulatory anion channels.

Involvement of AMP-activated Protein Kinase (AMPK) in Regulation of Cell Membrane Potential in a Gastric Cancer Cell Line

Membrane potential (V_mem) is a key bioelectric property of non-excitable cells that plays important roles in regulating cell proliferation. However, the regulation of V_mem itself remains largely unexplored. We found that, under nutrient starvation, during which cell division is inhibited, MKN45 gastric cancer cells were in a hyperpolarized state associated with a high intracellular chloride concentration. AMP-activated protein kinase (AMPK) activity increased, and expression of cystic fibrosis transmembrane conductance regulator (CFTR) decreased, in nutrient-starved cells. Furthermore, the increase in intracellular chloride concentration level and V_mem hyperpolarization in nutrient-starved cells was suppressed by inhibition of AMPK activity. Intracellular chloride concentrations and hyperpolarization increased after over-activation of AMPK using the specific activator AICAR or suppression of CFTR activity using specific inhibitor GlyH-101. Under these conditions, proliferation of MKN45 cells was inhibited. These results reveal that AMPK controls the dynamic change in V_mem by regulating CFTR and influencing the intracellular chloride concentration, which in turn influences cell-cycle progression. These findings offer new insights into the mechanisms underlying cell-cycle arrest regulated by AMPK and CFTR.

Tripodal cyanurates as selective transmembrane Cl− transporters

Tris-carboxyamide and tris-sulfonamide-based anion receptors with cyanuric acid core are developed for transmembrane chloride transport.

TMEM16A/ANO1 Inhibits Apoptosis Via Downregulation of Bim Expression

Purpose: TMEM16A is a calcium-activated chloride channel that is amplified in a variety of cancers, including 30% of head and neck squamous cell carcinomas (HNSCCs), raising the possibility of an anti-apoptotic role in malignant cells. This study investigated this using a multimodal, translational investigation.Experimental Design: Combination of (i) in vitro HNSCC cell culture experiments assessing cell viability, apoptotic activation, and protein expression (ii) in vivo studies assessing similar outcomes, and (iii) molecular and staining analysis of human HNSCC samples.Results: TMEM16A expression was found to correlate with greater tumor size, increased Erk 1/2 activity, less Bim expression, and less apoptotic activity overall in human HNSCC. These findings were corroborated in subsequent in vitro and in vivo studies and expanded to include a cisplatin-resistant phenotype with TMEM16A overexpression. A cohort of 41 patients with laryngeal cancer demonstrated that cases that recurred after chemoradiation failure were associated with a greater TMEM16A overexpression rate than HNSCC that did not recur.Conclusions: Ultimately, this study implicates TMEM16A as a contributor to tumor progression by limiting apoptosis and as a potential biomarker of more aggressive disease. Clin Cancer Res; 23(23); 7324-32. ©2017 AACR.

Ion channels in regulated cell death

Activation of ion channels and pores are essential steps during regulated cell death. Channels and pores participate in execution of apoptosis, necroptosis and other forms of caspase-independent cell death. Within the program of regulated cell death, these channels are strategically located. Ion channels can shrink cells and drive them towards apoptosis, resulting in silent, i.e. immunologically unrecognized cell death. Alternatively, activation of channels can induce cell swelling, disintegration of the cell membrane, and highly immunogenic necrotic cell death. The underlying cell death pathways are not strictly separated as identical stimuli may induce cell shrinkage and apoptosis when applied at low strength, but may also cause cell swelling at pronounced stimulation, resulting in regulated necrosis. Nevertheless, the precise role of ion channels during regulated cell death is far from being understood, as identical channels may support regulated death in some cell types, but may cause cell proliferation, cancer development, and metastasis in others. Along this line, the phospholipid scramblase and Cl(-)/nonselective channel anoctamin 6 (ANO6) shows interesting features, as it participates in apoptotic cell death during lower levels of activation, thereby inducing cell shrinkage. At strong activation, e.g. by stimulation of purinergic P2Y7 receptors, it participates in pore formation, causes massive membrane blebbing, cell swelling, and membrane disintegration. The LRRC8 proteins deserve much attention as they were found to have a major role in volume regulation, apoptotic cell shrinkage and resistance towards anticancer drugs.

Mechanosensitivity of wild-type and G551D cystic fibrosis transmembrane conductance regulator (CFTR) controls regulatory volume decrease in simple epithelia

Mutations of cystic fibrosis transmembrane conductance regulator (CFTR), an epithelial ligand-gated anion channel, are associated with the lethal genetic disease cystic fibrosis. The CFTR G551D mutation impairs ATP hydrolysis and thereby makes CFTR refractory to cAMP stimulation. Both wild-type (WT) and G551D CFTR have been implicated in regulatory volume decrease (RVD), but the underlying mechanism remains incompletely understood. Here, we show that the channel activity of both WT and G551D CFTR is directly stimulated by mechanical perturbation induced by cell swelling at the single-channel, cellular, and tissue levels. Hypotonicity activated CFTR single channels in cell-attached membrane patches and WT-CFTR-mediated short-circuit current (Isc) in Calu-3 cells, and this was independent of Ca(2+)and cAMP/PKA signaling. Genetic suppression and ablation but not G551D mutation of CFTR suppressed the hypotonicity- and stretch-inducedIscin Calu-3 cells and mouse duodena. Moreover, ablation but not G551D mutation of the CFTR gene inhibited the RVD of crypts isolated from mouse intestine; more importantly, CFTR-specific blockers markedly suppressed RVD in both WT- and G551D CFTR mice, demonstrating for the first time that the channel activity of both WT and G551D CFTR is required for epithelial RVD. Our findings uncover a previously unrecognized mechanism underlying CFTR involvement in epithelial RVD and suggest that the mechanosensitivity of G551D CFTR might underlie the mild phenotypes resulting from this mutation.-Xie, C., Cao, X., Chen, X, Wang, D., Zhang, W. K., Sun, Y., Hu, W., Zhou, Z., Wang, Y., Huang, P. Mechanosensitivity of wild-type and G551D cystic fibrosis transmembrane conductance regulator (CFTR) controls regulatory volume decrease in simple epithelia.

Temperature sensitivity of acid-sensitive outwardly rectifying (ASOR) anion channels in cortical neurons is involved in hypothermic neuroprotection against acidotoxic necrosis

The acid-sensitive outwardly rectifying (ASOR) anion channel has been found in non-neuronal cell types and was shown to be involved in acidotoxic death of epithelial cells. We have recently shown that the ASOR channel is sensitive to temperature. Here, we extend those results to show that temperature-sensitive ASOR anion channels are expressed in cortical neurons and involved in acidotoxic neuronal cell death. In cultured mouse cortical neurons, reduction of extracellular pH activated anionic currents exhibiting phenotypic properties of the ASOR anion channel. The neuronal ASOR currents recorded at pH 5.25 were augmented by warm temperature, with a threshold temperature of 26 °C and the Q(10) value of 5.6. After 1 h exposure to acidic solution at 37 °C, a large population of neurons suffered from necrotic cell death which was largely protected not only by ASOR channel blockers but also by reduction of temperature to 25 °C. Thus, it is suggested that high temperature sensitivity of the neuronal ASOR anion channel provides, at least in part, a basis for hypothermic neuroprotection under acidotoxic situations associated with a number of pathological brain states.

Temporal Heterogeneity Metrics in Apoptosis Induced by Anticancer Drugs

The apoptotic process is highly heterogeneous and asynchronous. A long-standing question is how many parameters define the time and reversibility of the apoptotic response at a single-cell level. We characterized at the single-cell and population levels the time sequence of apoptotic events in response to anti-cancer drugs using extrinsic and intrinsic apoptotic stimuli. We show that the temporal sequence of major apoptotic events is the same in response to all anti-cancer drugs studied: the apoptotic volume decrease and Na+ influx occur rapidly and are tightly coordinated with mitochondrial outer membrane depolarization (MOMP), mitochondrial inner membrane depolarization and a decrease in the production of reactive oxygen species (ROS). Phosphatidylserine externalization usually starts after MOMP and precedes caspase 3/7 activation. Activation of caspases 3/7 is a slow process that always starts after MOMP, with significant delay. Cell-to-cell variability of the MOMP onset is described by Gaussian distribution, whereas the γ-distribution model describes cellular variability in the duration of MOMP-to-caspase activation stages. Cells from the pre-MOMP stage to the after-caspase 3/7 activation stage coexist for many hours. We demonstrated by FACS that cells in the pre-MOMP stage can recover after apoptotic stimuli, rarely recover after MOMP but before caspase 3/7 activation, and are unable to recover after caspase 3/7 activation. We propose a double-stroke model for apoptosis execution.

ClC-3 chloride channel/antiporter defect contributes to inflammatory bowel disease in humans and mice

BACKGROUND

ClC-3 channel/antiporter plays a critical role in a variety of cellular activities. ClC-3 has been detected in the ileum and colon.

OBJECTIVE

To determine the functions of ClC-3 in the gastrointestinal tract.

DESIGN

After administration of dextran sulfate sodium (DSS) or 2,4,6-trinitrobenzenesulfonic acid (TNBS), intestines from ClC-3-/- and wild-type mice were examined by histological, cellular, molecular and biochemical approaches. ClC-3 expression was determined by western blot and immunostaining.

RESULTS

ClC-3 expression was reduced in intestinal tissues from patients with UC or Crohn's disease and from mice treated with DSS. Genetic deletion of ClC-3 increased the susceptibility of mice to DSS- or TNBS-induced experimental colitis and prevented intestinal recovery. ClC-3 deficiency promoted DSS-induced apoptosis of intestinal epithelial cells through the mitochondria pathway. ClC-3 interacts with voltage-dependent anion channel 1, a key player in regulation of mitochondria cytochrome c release, but DSS treatment decreased this interaction. In addition, lack of ClC-3 reduced the numbers of Paneth cells and impaired the expression of antimicrobial peptides. These alterations led to dysfunction of the epithelial barrier and invasion of commensal bacteria into the mucosa.

CONCLUSIONS

A defect in ClC-3 may contribute to the pathogenesis of IBD by promoting intestinal epithelial cell apoptosis and Paneth cell loss, suggesting that modulation of ClC-3 expression might be a new strategy for the treatment of IBD.

Increase in hypotonic stress-induced endocytic activity in macrophages via ClC-3

Extracellular hypotonic stress can affect cellular function. Whether and how hypotonicity affects immune cell function remains to be elucidated. Macrophages are immune cells that play key roles in adaptive and innate in immune reactions. The purpose of this study was to investigate the role and underlying mechanism of hypotonic stress in the function of bone marrow-derived macrophages (BMDMs). Hypotonic stress increased endocytic activity in BMDMs, but there was no significant change in the expression of CD80, CD86, and MHC class II molecules, nor in the secretion of TNF-α or IL-10 by BMDMs. Furthermore, the enhanced endocytic activity of BMDMs triggered by hypotonic stress was significantly inhibited by chloride channel-3 (ClC-3) siRNA. Our findings suggest that hypotonic stress can induce endocytosis in BMDMs and that ClC-3 plays a central role in the endocytic process.

Downregulation of chloride channel ClC-2 by Janus kinase 3

Janus kinase-3 (JAK3) fosters proliferation and counteracts apoptosis of lymphocytes and tumor cells. The gain of function mutation (A572V)JAK3 has been discovered in acute megakaryoplastic leukemia. JAK3 is inactivated by replacement of lysine by alanine in the catalytic subunit ((K855A)JAK3). Regulation of cell proliferation and apoptosis involves altered activity of Cl(-) channels. The present study, thus, explored whether JAK3 modifies the function of the small conductance Cl(-) channel ClC-2. To this end, ClC-2 was expressed in Xenopus oocytes with or without wild-type JAK3, (A568V)JAK3 or (K851A)JAK3, and the Cl(-) channel activity determined by dual-electrode voltage clamp. Channel protein abundance in the cell membrane was determined utilizing chemiluminescence. As a result, expression of ClC-2 was followed by a marked increase of cell membrane conductance. The conductance was significantly decreased following coexpression of JAK3 or (A568V)JAK3, but not by coexpression of (K851A)JAK3. Exposure of the oocytes expressing ClC-2 together with (A568V)JAK3 to the JAK3 inhibitor WHI-P154 (4-[(3'-bromo-4'-hydroxyphenyl)amino]-6,7-dimethoxyquinazoline, 22 μM) increased the conductance. Coexpression of (A568V)JAK3 decreased the ClC-2 protein abundance in the cell membrane of ClC-2 expressing oocytes. The decline of conductance in ClC-2 and (A568V)JAK3 coexpressing oocytes following inhibition of channel protein insertion by brefeldin A (5 μM) was similar in oocytes expressing ClC-2 with (A568V)JAK3 and oocytes expressing ClC-2 alone, indicating that (A568V)JAK3 might slow channel protein insertion into rather than accelerating channel protein retrieval from the cell membrane. In conclusion, JAK3 downregulates ClC-2 activity and thus counteracts Cl(-) exit-an effect possibly influencing cell proliferation and apoptosis.

Ion channels and apoptosis in cancer

Humans maintain a constant cell number throughout their lifespan. This equilibrium of cell number is accomplished when cell proliferation and cell death are kept balanced, achieving a steady-state cell number. Abnormalities in cell growth or cell death can lead to an overabundance of cells known as neoplasm or tumours. While the perception of cancer is often that of an uncontrollable rate of cell growth or increased proliferation, a decrease in cell death can also lead to tumour formation. Most cells when detached from their normal tissue die. However, cancer cells evade cell death, tipping the balance to an overabundance of cell number. Therefore, overcoming this resistance to cell death is a decisive factor in the treatment of cancer. Ion channels play a critical role in cancer in regards to cell proliferation, malignant angiogenesis, migration and metastasis. Additionally, ion channels are also known to be critical components of apoptosis. In this review, we discuss the modes of cell death focusing on the ability of cancer cells to evade apoptosis. Specifically, we focus on the role ion channels play in controlling and regulating life/death decisions and how they can be used to overcome resistance to apoptosis in the treatment of cancer.

Membrane damaging activity of a maslinic acid analog

Close inspection of human ovarian cancer cells A2780 in the course of an antitumor screening using maslinic acid analogs revealed for one of the compounds, 4-oxa-4-phenyl-butyl 2,3-dihydroxy-olean-12-en-28-oate (1), an unusual behavior. During the incubation of the cells with 1, at the perimeter of the cells or close by crystals were formed consisting of cholesterol and excess 1. Compound 1 was incorporated into the cell's membrane followed by an extrusion of cholesterol from the lipid rafts. As a consequence of the alterations of the cell membrane, a volume decrease was initiated that triggered apoptosis; this extends previous models on apoptosis initiating mechanisms.

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.

Classification of potassium and chlorine ionic currents in retinal ganglion cell line (RGC-5) by whole-cell patch clamp

Retinal ganglion cell line (RGC-5) has been widely used as a valuable model for studying pathophysiology and physiology of retinal ganglion cells in vitro. However, the electrophysiological characteristics, especially a thorough classification of ionic currents in the cell line, remain to be elucidated in details. In the present study, we determined the resting membrane potential (RMP) in RGC-5 cell line and then identified different types of ionic currents by using the whole-cell patch-clamp technique. The RMP recorded in the cell line was between −30 and −6 mV (−17.6 ± 2.6 mV, n = 10). We observed the following voltage-gated ion channel currents: (1) inwardly rectifying Cl− current (ICl,ir), which could be blocked by Zn2+; (2) Ca2+-activated Cl− current (ICl,Ca), which was sensitive to extracellular Ca2+ and could be inhibited by disodium 4,4’-diisothiocyanatostilbene-2,2’-disulfonate; (3) inwardly rectifying K+ currents (IK1), which could be blocked by Ba2+; (4) a small amount of delayed rectifier K+ current (IK). On the other hand, the voltage-gated sodium channels current (INa) and transient outward potassium channels current (IA) were not observed in this cell line. These results further characterize the ionic currents in the RGC-5 cell line and are beneficial for future studies especially on ion channel (patho)physiology and pharmacology in the RGC-5 cell line.

The apoptotic volume decrease is an upstream event of MAP kinase activation during Staurosporine-induced apoptosis in HeLa cells

Persistent cell shrinkage, called apoptotic volume decrease (AVD), is a pivotal event of apoptosis. Activation of the volume-sensitive outwardly rectifying Cl⁻ channel (VSOR) is involved in the AVD induction. On the other hand, activation of the MAP kinase (MAPK) cascade is also known to play a critical role in apoptosis. In the present study, we investigated the relationship between the AVD induction and the stress-responsive MAPK cascade activation during the apoptosis process induced by staurosporine (STS) in HeLa cells. STS was found to induce AVD within 2–5 min and phosphorylation of c-Jun N-terminal kinase (JNK) and p38 MAPK after over 20–30 min. VSOR blockers suppressed not only STS-induced AVD but also phosphorylation of JNK and p38 as well as activation of caspase-3/7. Moreover, a p38 inhibitor, SB203580, and a JNK inhibitor, SP600125, failed to affect STS-induced AVD, whereas these compounds reduced STS-induced activation of caspase-3/7. Also, treatment with ASK1-specific siRNA suppressed STS-induced caspase-3/7 activation without affecting the AVD induction. Furthermore, sustained osmotic cell shrinkage per se was found to trigger phosphorylation of JNK and p38, caspase activation, and cell death. Thus, it is suggested that activation of p38 and JNK is a downstream event of AVD for the STS-induced apoptosis of HeLa cells.

Real-time characterization of cytotoxicity using single-cell impedance monitoring

Cellular impedance sensors have attracted great attention as a powerful characterization tool for real-time, label-free detection of cytotoxic agents. However, impedance measurements with conventional cell-based sensors that host multiple cells on a single electrode neither provide optimal cell signal sensitivity nor are capable of recording individual cell responses. Here we use a single-cell based platform to monitor cellular impedance on planar microelectrodes to characterize cellular death. In this study, individual cells were selectively patterned on microelectrodes with each hosting one live cell through ligand-mediated natural cell adhesion. Changes in cellular morphology and cell-electrode adherence were monitored after the patterned cells were treated with varying concentrations of hydrogen peroxide, sodium arsenite, and disodium hydrogen arsenate, three potent toxicants related to neurotoxicity and oxidative stress. At low toxicant concentrations, impedance waveforms acquired from individual cells showed variable responses. A time- and concentration-dependent response was seen in the averaged single-cell impedance waveform for all three toxicants. The apoptosis and necrosis characterizations were performed to validate cell impedance results. Furthermore, time constants of apoptosis and necrosis in response to toxicant exposure were analytically established using an equivalent circuit model that characterized the mechanisms of cell death.

The ΔC splice-variant of TRPM2 is the hypertonicity-induced cation channel in HeLa cells, and the ecto-enzyme CD38 mediates its activation

Hypertonicity-induced cation channels (HICCs) are key-players in proliferation and apoptosis but their molecular correlate remains obscure. Furthermore, the activation profile of HICCs is not well defined yet. We report here that, in HeLa cells, intracellular adenosine diphosphate ribose (ADPr) and cyclic ADPr (cADPr), as supposed activators of TRPM2, elicited cation currents that were virtually identical to the osmotic activation of HICCs. Silencing of the expression of TRPM2 and of the ecto-enzyme CD38 (as a likely source of ADPr and cADPr) inhibited HICC as well as nucleotide-induced currents and, in parallel, the hypertonic volume response of cells (the regulatory volume increase, RVI) was attenuated. Quantification of intracellular cADPr levels and the systematic application of extra- vs. intracellular nucleotides indicate that the outwardly directed gradient rather than the cellular activity of ADPr and cADPr triggers TRPM2 activation, probably along with a simultaneous biotransformation of nucleotides.Cloning of TRPM2 identified the ΔC-splice variant as the molecular correlate of the HICC, which could be strongly supported by a direct comparison of the respective Ca²⁺ selectivity. Finally, immunoprecipitation and high-resolution FRET/FLIM imaging revealed the interaction of TRPM2 and CD38 in the native as well as in a heterologous (HEK293T) expression system. We propose transport-related nucleotide export via CD38 as a novel mechanism of TRPM2/HICC activation. With the biotransformation of nucleotides running in parallel, continuous zero trans-conditions are achieved which will render the system infinitely sensitive.

Swelling-activated anion channels are essential for volume regulation of mouse thymocytes

Channel-mediated trans-membrane chloride movement is a key process in the active cell volume regulation under osmotic stress in most cells. However, thymocytes were hypothesized to regulate their volume by activating a coupled K-Cl cotransport mechanism. Under the patch-clamp, we found that osmotic swelling activates two types of macroscopic anion conductance with different voltage-dependence and pharmacology. At the single-channel level, we identified two types of events: one corresponded to the maxi-anion channel, and the other one had characteristics of the volume-sensitive outwardly rectifying (VSOR) chloride channel of intermediate conductance. A VSOR inhibitor, phloretin, significantly suppressed both macroscopic VSOR-type conductance and single-channel activity of intermediate amplitude. The maxi-anion channel activity was largely suppressed by Gd(3+) ions but not by phloretin. Surprisingly, [(dihydroindenyl)oxy] alkanoic acid (DIOA), a known antagonist of K-Cl cotransporter, was found to significantly suppress the activity of the VSOR-type single-channel events with no effect on the maxi-anion channels at 10 μM. The regulatory volume decrease (RVD) phase of cellular response to hypotonicity was mildly suppressed by Gd(3+) ions and was completely abolished by phloretin suggesting a major impact of the VSOR chloride channel and modulatory role of the maxi-anion channel. The inhibitory effect of DIOA was also strong, and, most likely, it occurred via blocking the VSOR Cl(-) channels.

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.

Volume regulated anion channel currents of rat hippocampal neurons and their contribution to oxygen-and-glucose deprivation induced neuronal death

Volume-regulated anion channels (VRAC) are widely expressed chloride channels that are critical for the cell volume regulation. In the mammalian central nervous system, the physiological expression of neuronal VRAC and its role in cerebral ischemia are issues largely unknown. We show that hypoosmotic medium induce an outwardly rectifying chloride conductance in CA1 pyramidal neurons in rat hippocampal slices. The induced chloride conductance was sensitive to some of the VRAC inhibitors, namely, IAA-94 (300 µM) and NPPB (100 µM), but not to tamoxifen (10 µM). Using oxygen-and-glucose deprivation (OGD) to simulate ischemic conditions in slices, VRAC activation appeared after OGD induced anoxic depolarization (AD) that showed a progressive increase in current amplitude over the period of post-OGD reperfusion. The OGD induced VRAC currents were significantly inhibited by inhibitors for glutamate AMPA (30 µM NBQX) and NMDA (40 µM AP-5) receptors in the OGD solution, supporting the view that induction of AD requires an excessive Na⁺-loading via these receptors that in turn to activate neuronal VRAC. In the presence of NPPB and DCPIB in the post-OGD reperfusion solution, the OGD induced CA1 pyramidal neuron death, as measured by TO-PRO-3-I staining, was significantly reduced, although DCPIB did not appear to be an effective neuronal VRAC blocker. Altogether, we show that rat hippocampal pyramidal neurons express functional VRAC, and ischemic conditions can initial neuronal VRAC activation that may contribute to ischemic neuronal damage.

Apoptotic volume decrease as a geometric determinant for cell dismantling into apoptotic bodies

Apoptosis is a mode of cell death through which cells are dismantled and cell remains are packed into small, membrane-bound, sealed vesicles called apoptotic bodies, which are easy to erase by phagocytosis by neighbouring and immune system cells. The end point of the process is to cleanly eliminate damaged or unnecessary cells without disrupting the surrounding tissue or eliciting an inflammatory response. The apoptotic process involves a series of specific events including deoxyribonucleic acid and nuclear fragmentation, protease-driven cleavage of specific substrates, which inhibits key survival functions and reorganizes the cell's structure, externalization of molecules involved in phagocytosis, membrane blebbing and cell shrinkage. Apoptotic volume decrease (AVD) leading to cell shrinkage is a core event in the course of apoptosis, the biological meaning of which has not been clearly ascertained. In this article we argue that volume loss is a geometrical requisite for cell dismantling into apoptotic bodies. This is derived from the cell's volume-to-surface ratio. Indeed, package of the original cell volume into smaller membrane-sealed vesicles requires that either cell membrane surface increase or cell volume decrease. In this sense, AVD provides a reservoir of membrane surface for apoptotic body formation. The strategic situation of AVD in the time course of apoptosis is also discussed in the context of apoptotic body formation.

Volume-sensitive outwardly rectifying chloride channel in white adipocytes from normal and diabetic mice

The volume-sensitive outwardly rectifying (VSOR) chloride channel is ubiquitously expressed and involved in cell volume regulation after osmotic swelling, called regulatory volume decrease (RVD), in various cell types. In adipocytes, the expression of the VSOR channel has not been explored to date. Here, by employing the whole-cell patch-clamp technique, we examined whether or not the VSOR channel is expressed in white adipocytes freshly isolated from epididymal fat pads of normal (C57BL/6 or KK) and diabetic (KKA(y)) mice. Whole cell voltage-clamp recordings revealed that Cl(-) currents were gradually activated upon cell swelling induced by application of a hypotonic solution, both in normal and diabetic adipocytes. Although both the mean cell size (or cell capacitance) and the current magnitude in KKA(y) adipocytes were larger than those in C57BL/6 cells, the current density was significantly lower in KKA(y) adipocytes (23.32 +/- 1.94 pA in C57BL/6 adipocytes vs. 13.04 +/- 2.41 pA in KKA(y) adipocytes at +100 mV). Similarly, the current density in diabetic KKA(y) adipocytes was lower than that in adipocytes from KK mice (a parental strain of KKA(y) mice), which do not present diabetes until an older age. The current was inhibited by Cl(-) channel blockers, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and glibenclamide, or hypertonic solution, and showed outward rectification and inactivation kinetics at large positive potentials. These electrophysiological and pharmacological properties are consistent with those of the VSOR channel in other cell types. Moreover, adipocytes showed RVD, which was inhibited by NPPB. In KKA(y) adipocytes, RVD was significantly slower (tau; 8.42 min in C57BL/6 adipocytes vs. 11.97 min in KKA(y) adipocytes) and incomplete during the recording period (25 min). It is concluded that the VSOR channel is functionally expressed and involved in volume regulation in white adipocytes. RVD is largely impaired in adipocytes from diabetic mice, presumably as a consequence of the lower density of the functional VSOR channel in the plasma membrane.

Deregulation of apoptotic volume decrease and ionic movements in multidrug-resistant tumor cells: role of chloride channels

Changes in cell volume and ion gradients across the plasma membrane play a pivotal role in the initiation of apoptosis. Here we explore the kinetics of apoptotic volume decrease (AVD) and ion content dynamics in wild-type (WT) and multidrug-resistant (MDR) Ehrlich ascites tumor cells (EATC). In WT EATC, induction of apoptosis with cisplatin (5 μM) leads to three distinctive AVD stages: an early AVD1 (4–12 h), associated with a 30% cell water loss; a transition stage AVDT (∼12 to 32 h), where cell volume is partly recovered; and a secondary AVD2 (past 32 h), where cell volume was further reduced. AVD1 and AVD2 were coupled to net loss of Cl−, K+, Na+, and amino acids (ninhydrin-positive substances), whereas during AVDT, Na+ and Cl− were accumulated. MDR EATC was resistant to cisplatin, showing increased viability and less caspase 3 activation. Compared with WT EATC, MDR EATC underwent a less pronounced AVD1, an augmented AVDT, and a delay in induction of AVD2. Changes in AVD were associated with inhibition of Cl− loss during AVD1, augmented NaCl uptake during AVDT, and a delay of Cl− loss during AVD2. Application of the anion channel inhibitor NS3728 inhibited AVD and completely abolished the differences in AVD, ionic movements, and caspase 3 activation between WT and MDR EATC. Finally, the maximal capacity of volume-regulated anion channel was found to be strongly repressed in MDR EATC. Together, these data suggest that impairment of AVD, primarily via modulation of NaCl movements, contribute to protection against apoptosis in MDR EATC.

TMEM16 proteins produce volume-regulated chloride currents that are reduced in mice lacking TMEM16A

All vertebrate cells regulate their cell volume by activating chloride channels of unknown molecular identity, thereby activating regulatory volume decrease. We show that the Ca(2+)-activated Cl(-) channel TMEM16A together with other TMEM16 proteins are activated by cell swelling through an autocrine mechanism that involves ATP release and binding to purinergic P2Y(2) receptors. TMEM16A channels are activated by ATP through an increase in intracellular Ca(2+) and a Ca(2+)-independent mechanism engaging extracellular-regulated protein kinases (ERK1/2). The ability of epithelial cells to activate a Cl(-) conductance upon cell swelling, and to decrease their cell volume (regulatory volume decrease) was dependent on TMEM16 proteins. Activation of I(Cl,swell) was reduced in the colonic epithelium and in salivary acinar cells from mice lacking expression of TMEM16A. Thus TMEM16 proteins appear to be a crucial component of epithelial volume-regulated Cl(-) channels and may also have a function during proliferation and apoptotic cell death.

Acid-sensitive outwardly rectifying anion channels in human erythrocytes

Acid-sensitive outwardly rectifying anion channels (ASOR) have been described in several mammalian cell types. The present whole-cell patch-clamp study elucidated whether those channels are expressed in erythrocytes. To this end whole-cell recordings were made in human erythrocytes from healthy donors treated with low pH and high osmotic pressure. When the pipette solution had a reduced Cl(-) concentration, treatment of the cells with Cl(-)-containing normal and hyperosmotic (addition of sucrose and polyethelene glycol 1000 [PEG-1000] to the Ringer) media with low pH significantly increased the conductance of the cells at positive voltages. Channel activity was highest in the PEG-1000 media (95 and 300 mM PEG-1000, pH 4.5 and 4.3, respectively) where the current-voltage curves demonstrated strong outward rectification and reversed at -40 mV. Substitution of the Cl(-)-containing medium with Cl(-)-free medium resulted in a decrease of the conductance at hyperpolarizing voltages, a shift in reversal potential (to 0 mV) and loss of outward rectification. The chloride currents were inhibited by chloride channels blockers DIDS and NPPB (IC(50) for both was approximately 1 mM) but not with niflumic acid and amiloride. The observations reveal expression of ASOR in erythrocytes.

Pathophysiology and puzzles of the volume-sensitive outwardly rectifying anion channel

Cell swelling activates or upregulates a number of anion channels. Of the volume-activated or -regulated anion channels (VAACs or VRACs), the volume-sensitive outwardly rectifying anion channel (VSOR) is most prominently activated and ubiquitously expressed. This channel is known to be involved in a variety of physiological processes including cell volume regulation, cell proliferation, differentiation and cell migration as well as cell turnover involving apoptosis. Recent studies have shown that VSOR activity is also involved in a number of pathophysiological processes including the acquisition of cisplatin resistance by cancer cells, ischaemia-reperfusion-induced death of cardiomyocytes and hippocampal neurons, glial necrosis under lactacidosis as well as neuronal necrosis under excitotoxicity. Moreover, VSOR serves as the pathway for glutamate release from astrocytes under ischaemic conditions and when stimulated by bradykinin, an initial mediator of inflammation. So far, many signalling molecules including the EGF receptor, PI3K, Src, PLCgamma and Rho/Rho kinase have been implicated in the regulation of VSOR activity. However, our pharmacological studies suggest that these signals are not essential components of the swelling-induced VSOR activation mechanism even though some of these signals may play permissive or modulatory roles. Molecular identification of VSOR is required to address the question of how cells sense volume expansion and activate VSOR.

alpha-ENaC is a functional element of the hypertonicity-induced cation channel in HepG2 cells and it mediates proliferation

The molecular correlate of hypertonicity-induced cation channels (HICCs) and their role in proliferation vs. apoptosis is a matter of debate. We report in this paper that, in whole-cell patch-clamp recordings, hypertonic stress (340→450 mosM) reversibly increased the Na⁺ conductance of HepG2 cells from 0.8 to 5.8 nS. The effect was dose-dependently inhibited by flufenamate and amiloride, known blockers of HICCs, with some 50% efficiency at 300 μM. In parallel, both drugs decreased HepG2 cell proliferation [in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays and with automatic cell counting]. Small interfering RNA (siRNA) silencing of the α-subunit of the epithelial Na⁺ channel (ENaC) reduced hypertonicity-induced Na⁺ currents to 60%, whereas the rate of HepG2 cell proliferation was approximately half of that of the control. Moreover, α-ENaC siRNA inhibited the regulatory volume increase of HepG2 cells (measured with scanning acoustic microscopy) by 60%. In florescence-activated cell sorting measurements, silencing of α-ENaC led to a significant decrease in the G1 and an increase in the G2/M phase of the cell cycle, whereas the S phase was not changing. Finally (determined by a caspase 3/7 assay), HICC inhibition by flufenamate and silencing of α-ENaC increased the rate of apoptosis in HepG2 cells. It is concluded that α-ENaC is one functional element of the HICC in HepG2 cells and that the channel is an important mediator of cell proliferation; likewise, HICC blockage shifts the system from a proliferative into a rather apoptotic one. This is the first report of a role of α-ENaC in cell proliferation.

CFTR regulation of intracellular pH and ceramides is required for lung endothelial cell apoptosis

The functional significance of the expression of cystic fibrosis transmembrane regulator (CFTR) on endothelial cells has not yet been elucidated. Since CFTR has been implicated in the regulation of intracellular sphingolipid levels, which are important regulators of endothelial cell apoptosis in response to various insults, we investigated the role of CFTR in the apoptotic responses of lung endothelial cells. CFTR was detected as a functional chloride channel in primary lung endothelial cells isolated from both pulmonary arteries (human or mouse) and bronchial arteries (sheep). Both specific CFTR inhibition with 2-(phenylamino) benzoic acid diphenylamine-2-carboxylic acid, 5-[(4-carboxyphenyl)methylene]-2-thioxo-3-[(3-trifluoromethyl)phenyl-4-thiazolidinone (CFTR(inh)-172), or 5-nitro-2-(3-phenylpropylamino)benzoic acid and CFTR knockdown significantly attenuated endothelial cell apoptosis induced by staurosporine or H(2)O(2). CFTR(inh)-172 treatment prevented the increases in the ceramide:sphingosine-1 phosphate ratio induced by H(2)O(2) in lung endothelial cells. Replenishing endogenous ceramides via sphingomyelinase supplementation restored the susceptibility of CFTR-inhibited lung endothelial cells to H(2)O(2)-induced apoptosis. Similarly, the anti-apoptotic phenotype of CFTR-inhibited cells was reversed by lowering the intracellular pH, and was reproduced by alkalinization before H(2)O(2) challenge. TUNEL staining and active caspase-3 immunohistochemistry indicated that cellular apoptosis was decreased in lung explants from patients with cystic fibrosis compared with those with smoking-induced chronic obstructive lung disease, especially in the alveolar tissue and vascular endothelium. In conclusion, CFTR function is required for stress-induced apoptosis in lung endothelial cells by maintaining adequate intracellular acidification and ceramide activation. These results may have implications in the pathogenesis of cystic fibrosis, where aberrant endothelial cell death may dysregulate lung vascular homeostasis, contributing to abnormal angiogenesis and chronic inflammation.

Acidic pH-activated Cl Current and Intracellular Ca Response in Human Keratinocytes

The layers of keratinocytes form an acid mantle on the surface of the skin. Herein, we investigated the effects of acidic pH on the membrane current and [Ca(2+)](c) of human primary keratinocytes from foreskins and human keratinocyte cell line (HaCaT). Acidic extracellular pH (pH(e)</= 5.5) activated outwardly rectifying Cl(-) current (I(Cl,pH)) with slow kinetics of voltage-dependent activation. I(Cl,pH) was potently inhibited by an anion channel blocker 4,4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS, 73.5% inhibition at 1 microM). I(Cl,pH) became more sensitive to pH(e) by raising temperature from 24 to 37. HaCaT cells also expressed Ca(2+)-activated Cl(-) current (I(Cl,Ca)), and the amplitude of I(Cl,Ca) was increased by relatively weak acidic pH(e) (7.0 and 6.8). Interestingly, the acidic pH(e) (5.0) also induced a sharp increase in the intracellular [Ca(2+)] (Delta[Ca(2+)](acid)) of HaCaT cells. The Delta[Ca(2+)](acid) was independent of extracellular Ca(2+), and was abolished by the pretreatment with PLC inhibitor, U73122. In primary human keratinocytes, 5 out of 28 tested cells showed Delta[Ca(2+)](acid). In summary, we found I(Cl,pH) and Delta[Ca(2+)](acid) in human keratinocytes, and these ionic signals might have implication in pathophysiological responses and differentiation of epidermal keratinocytes.