SWELL1, a plasma membrane protein, is an essential component of volume-regulated anion channel

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.

Effect of plant flavonoids on the volume regulation of rat thymocytes under hypoosmotic stress

BACKGROUND

Cell volume regulation and volume-regulated anion channels are critical for cell survival in non-isosmotic conditions, and dysregulation of this system is detrimental. Although genes and proteins underlying this basic cellular machinery were recently identified, the pharmacology remains poorly explored.

METHODS

We examined effects of 16 flavonoids on the regulatory volume decrease (RVD) of thymocytes under hypoosmotic stress assessed by light transmittance and on the activity of volume-sensitive chloride channel by patch-clamp technique.

RESULTS

Comparison of effects of flavonoids on RVD revealed a group of four active substances with lehmannin being the strongest inhibitor (IC50 = 8.8 μM). Structure-functional comparison suggested that hydrophobicity brought about by methoxy, prenyl or lavandulyl groups as well as by the absence of glucosyl fragment together with localization of the phenyl ring B at the position C2 (which is at C3 in totally inactive isoflavones) are important structural determinants for the flavonoids activity as volume regulation inhibitors. All active flavonoids suppressed RVD under Gramicidin D-NMDG hypotonic stress conditions when cationic permeability was increased by an ionophore, gramicidin D, with all extracellular monovalent cations replaced with bulky NMDG+ suggesting that they target volume-sensitive anionic permeability. While effects of hispidulin and pulicarin were only partial, lehmannin and pinocembrin completely abolished RVD under Gramicidin D-NMDG conditions. In direct patch-clamp experiments, lehmannin and pinocembrin produced a strong inhibiting effect on the swelling-induced whole-cell chloride conductance in a voltage-independent manner.

CONCLUSION

Lehmannin, pinocembrin, and possibly hispidulin and pulicarin may serve as leads for developing effective low-toxic immunomodulators.

Clcn3 deficiency ameliorates high-fat diet-induced obesity and adipose tissue macrophage inflammation in mice

Obesity induces accumulation of adipose tissue macrophages (ATMs) and ATM-driven inflammatory responses that promote the development of glucose and lipid metabolism disorders. ClC-3 chloride channel/antiporter, encoded by the Clcn3, is critical for some basic cellular functions. Our previous work has shown significant alleviation of type 2 diabetes in Clcn3 knockout (Clcn3^−/−) mice. In the present study we investigated the role of Clcn3 in high-fat diet (HFD)-induced obesity and ATM inflammation. To establish the mouse obesity model, both Clcn3^−/− mice and wild-type mice were fed a HFD for 4 or 16 weeks. The metabolic parameters were assessed and the abdominal total adipose tissue was scanned using computed tomography. Their epididymal fat pad tissue and adipose tissue stromal vascular fraction (SVF) cells were isolated for analyses. We found that the HFD-fed Clcn3^−/− mice displayed a significant decrease in obesity-induced body weight gain and abdominal visceral fat accumulation as well as an improvement of glucose and lipid metabolism as compared with HFD-fed wild-type mice. Furthermore, the Clcn3 deficiency significantly attenuated HFD-induced ATM accumulation, HFD-increased F4/80⁺ CD11c⁺ CD206⁻ SVF cells as well as HFD-activated TLR-4/NF-κB signaling in epididymal fat tissue. In cultured human THP-1 macrophages, adenovirus-mediated transfer of Clcn3 specific shRNA inhibited, whereas adenovirus-mediated cDNA overexpression of Clcn3 enhanced lipopolysaccharide-induced activation of NF-κB and TLR-4. These results demonstrate a novel role for Clcn3 in HFD-induced obesity and ATM inflammation.

Absolute Protein Amounts and Relative Abundance of Volume-regulated Anion Channel (VRAC) LRRC8 Subunits in Cells and Tissues Revealed by Quantitative Immunoblotting

The volume-regulated anion channel (VRAC) plays an important role in osmotic cell volume regulation. In addition, it is involved in various physiological processes such as insulin secretion, glia-neuron communication and purinergic signaling. VRAC is formed by hetero-hexamers of members of the LRRC8 protein family, which consists of five members, LRRC8A-E. LRRC8A is an essential subunit for physiological functionality of VRAC. Its obligate heteromerization with at least one of its paralogues, LRRC8B-E, determines the biophysical properties of VRAC. Moreover, the subunit composition is of physiological relevance as it largely influences the activation mechanism and especially the substrate selectivity. However, the endogenous tissue-specific subunit composition of VRAC is unknown. We have now developed and applied a quantitative immunoblot study of the five VRAC LRRC8 subunits in various mouse cell lines and tissues, using recombinant protein for signal calibration. We found tissue-specific expression patterns of the subunits, and generally relative low expression of the essential LRRC8A subunit. Immunoprecipitation of LRRC8A also co-precipitates an excess of the other subunits, suggesting that non-LRRC8A subunits present the majority in hetero-hexamers. With this, we can estimate that in the tested cell lines, the number of VRAC channels per cell is in the order of 10,000, which is in agreement with earlier calculations from the comparison of single-channel and whole-cell currents.

Tannins, novel inhibitors of the volume regulation and the volume-sensitive anion channel

The volume-sensitive outwardly rectifying anion channel (VSOR) is a key component of volume regulation system critical for cell survival in non-isosmotic conditions. The aim of the present study was to test the effects of four tannin extracts with defined compositions on cell volume regulation and VSOR. Preparation I (98% of hydrolysable tannins isolated from leaves of sumac Rhus typhina L.) and Preparation II (100% of hydrolysable tannins isolated from leaves of broadleaf plantain Plantago major L) completely and irreversibly abolished swelling-activated VSOR currents in HCT116 cells. Both preparations profoundly suppressed the volume regulation in thymocytes with half-maximal effects of 40.9 μg/ml and 12.3 μg/ml, respectively. The inhibition was more efficient at lower concentrations but reverted at higher doses due to possible non-specific membrane-permeabilizing activity. Preparations III and IV (54,7% and 54.3% of hydrolysable tannins isolated, respectively, from roots and aboveground parts of Fergana spurge Euphorbia ferganensis B.Fedtch) inhibited VSOR activity in a partially reversible manner and suppressed the volume regulation with substantially higher half-maximal doses of 270 and 278 μg/ml, respectively, with no secondary reversion at higher doses. Hydrolysable tannins represent a novel class of VSOR channel inhibitors with the capacity to suppress the cell volume regulation machinery.

Mechanism and effects of pulsatile GABA secretion from cytosolic pools in the human beta cell

Pancreatic beta cells synthesize and secrete the neurotransmitter γ-aminobutyric acid (GABA) as a paracrine and autocrine signal to help regulate hormone secretion and islet homeostasis. Islet GABA release has classically been described as a secretory vesicle-mediated event. Yet, a limitation of the hypothesized vesicular GABA release from islets is the lack of expression of a vesicular GABA transporter in beta cells. Consequentially, GABA accumulates in the cytosol. Here we provide evidence that the human beta cell effluxes GABA from a cytosolic pool in a pulsatile manner, imposing a synchronizing rhythm on pulsatile insulin secretion. The volume regulatory anion channel (VRAC), functionally encoded by LRRC8A or Swell1, is critical for pulsatile GABA secretion. GABA content in beta cells is depleted and secretion is disrupted in islets from type 1 and type 2 diabetic patients, suggesting that loss of GABA as a synchronizing signal for hormone output may correlate with diabetes pathogenesis.

Hypotonic stress response of human keratinocytes involves LRRC8A as component of volume-regulated anion channels

The barrier function of the human epidermis is constantly challenged by environmental osmotic fluctuations. Hypotonic stress triggers cell swelling, which is counteracted by a compensatory mechanism called regulatory volume decrease (RVD) involving volume-regulated anion channels (VRACs). Recently, it was discovered that VRACs are composed of LRRC8 heteromers and that LRRC8A functions as the essential VRAC subunit in various mammalian cell types; however, the molecular identity of VRACs in the human epidermis remains to be determined. Here, we investigated the expression of LRRC8A and its role in hypotonic stress response of human keratinocytes. Immunohistological staining showed that LRRC8A is preferentially localized in basal and suprabasal epidermal layers. RNA sequencing revealed that LRRC8A is the most abundant subunit within the LRRC8 gene family in HaCaT cells as well as in primary normal human epidermal keratinocytes (NHEKs). To determine the contribution of LRRC8A to hypotonic stress response, we generated HaCaT- and NHEK-LRRC8A knockout cells by using CRISPR-Cas9. I^- influx assays using halide-sensitive YFP showed that LRRC8A is crucially important for mediating VRAC activity in HaCaTs and NHEKs. Moreover, cell volume measurements using calcein-AM dye further revealed that LRRC8A also substantially contributes to RVD. In summary, our study provides new insights into hypotonic stress response and suggests an important role of LRRC8A as VRAC component in human keratinocytes.

Caveolae-Mediated Activation of Mechanosensitive Chloride Channels in Pulmonary Veins Triggers Atrial Arrhythmogenesis

Background

Atrial fibrillation often occurs in the setting of hypertension and associated atrial dilation with pathologically increased cardiomyocyte stretch. In the setting of atrial dilation, mechanoelectric feedback has been linked to the development of ectopic beats that trigger paroxysmal atrial fibrillation mainly originating from pulmonary veins (PVs). However, the precise mechanisms remain poorly understood.

Methods and Results

We identify mechanosensitive, swelling‐activated chloride ion channels (I_Cl,swell) as a crucial component of the caveolar mechanosensitive complex in rat and human cardiomyocytes. In vitro optical mapping of rat PV, single rat PV, and human cardiomyocyte patch clamp studies showed that stretch‐induced activation of I_Cl,swell leads to membrane depolarization and decreased action potential amplitude, which trigger conduction discontinuities and both ectopic and reentrant activities within the PV. Reverse transcription quantitative polymerase chain reaction, immunofluorescence, and coimmunoprecipitation studies showed that I_Cl,swell likely consists of at least 2 components produced by mechanosensitive ClC‐3 (chloride channel‐3) and SWELL1 (also known as LRRC8A [leucine rich repeat containing protein 8A]) chloride channels, which form a macromolecular complex with caveolar scaffolding protein Cav3 (caveolin 3). Downregulation of Cav3 protein expression and disruption of caveolae structures during chronic hypertension in spontaneously hypertensive rats facilitates activation of I_Cl,swell and increases PV sensitivity to stretch 10‐ to 50‐fold, promoting the development of atrial fibrillation.

Conclusions

Our findings identify caveolae‐mediated activation of mechanosensitive I_Cl,swell as a critical cause of PV ectopic beats that can initiate atrial arrhythmias including atrial fibrillation. This mechanism is exacerbated in the setting of chronically elevated blood pressures.

SWELL1 promotes cell growth and metastasis of hepatocellular carcinoma in vitro and in vivo

Background

SWELL1 was recently demonstrated to be an indispensable part of the volume-regulated anion channel (VRAC). VRAC is reported to participate in cell proliferation, survival, and migration. However, the correlation between SWELL1 and hepatocellular carcinoma (HCC) remains poorly-understood. In this study, we tried to explore the role of SWELL1 in HCC.

Methods

Immunohistochemistry and quantitative real-time-PCR (qRT-PCR) was used to measure SWELL1 expression in HCC samples obtained from patients with HCC. The effects of SWELL1 on HCC cell proliferation, apoptosis, and metastasis were analysed by corresponding cytological experiments including Cell Counting Kit-8 (CCK8), colony-forming, 5-ethynyl-2′-deoxyuridine (EdU), cell cycle analysis, TUNEL, Annexin V and PI staining, wound healing, transwell, and so on. BALB/c nude mice were used for the in vivo assays. qRT-PCR and western blotting was performed for molecular mechanisms.

Findings

SWELL1 was highly expressed in HCC tissues, and related to the poor prognosis. In vitro, the over-expression of SWELL1 significantly induced cell proliferation and migration, and inhibited apoptosis, whereas suppressing SWELL1 had the opposite effects. Moreover, knockdown of SWELL1 suppressed the growth and metastasis of HCC in vivo. Further experiments revealed that SWELL1 induced cell growth by activating the cyclinD1/CDK2 pathway via the connection with PKCa at the signalling level, and regulated cell migration through the JNK pathway in HCC.

Interpretation

SWELL1 acts as a promoter in the growth and metastasis of HCC cells and may be a potential intervention target for HCC.

Fund

This work is supported by the National Natural Science Foundation of China (No. 81572422, 81700515).

Norcepharadione B attenuates H2O2-induced neuronal injury by upregulating cellular antioxidants and inhibiting volume-sensitive Cl- channel

Oxidative stress acts as an essential culprit factor in the development of stroke and Alzheimer’s disease. Norcepharadione B possesses various pharmacologic features as an extract obtained from Houttuynia cordata. Nevertheless, the anti-apoptotic and neuroprotective characteristics of norcepharadione B remain unclear. In this study, the neuronal protection effect provided by norcepharadione B against injury caused by hydrogen peroxide (H2O2) in HT22 cell as well as the fundamental mechanism was systematically explored. The neurotoxicity assays of hippocampal cells, which were co-cultured with H2O2, showed that norcepharadione B had the ability to insulate the toxicity induced by H2O2 with significant reduced cell apoptosis. Besides, norcepharadione B potentiated the activity of superoxide dismutase (SOD), increased the level of glutathione (GSH), and decreased malondialdehyde content. The H2O2-induced apoptotic protein Bax was suppressed, and the anti-apoptotic protein Bcl-2 was boosted by norcepharadione B. Norcepharadione B promoted Akt phosphorylation and further upregulated heme oxygenase (HO-1) in cells exposed to oxidative stress. However, the inductive effect of HO-1 by norcepharadione B was shut off via the PI3K/Akt inhibitor LY294002. Furthermore, 2-h incubation with H2O2 substantially increased cell volume in HT22 cells, while norcepharadione B effectively alleviated such effect by interrupting the activation of VSOR Cl− channel. Collectively, our data revealed protective properties of norcepharadione B in resisting oxidative stress induced by H2O2 through elevation of HO-1 in the dependence of PI3K/Akt and in inhibiting H2O2-induced cell swelling by VSOR Cl− channel obstruction in HT22 cells.

Impact statement

Norcepharadione B is an aporphine alkaloid compound extracted from Chinese herb Houttuynia cordata. It was well known for its anti-inflammatory, anti-cancer, and anti-platelet aggregation outcomes. Our study demonstrated that Norcepharadione B protected hippocampal neurons against oxidative stress and the resultant cell apoptosis upon H2O2 exposure. Meanwhile, Norcepharadione B also substantially reduced cell swelling induced by H2O2 via inhibiting VSOR Cl− channel in neurons. These findings uncovered the potential mechanisms of Norcepharadione B in protecting neuron apoptosis under oxidative stress and propose that Norcepharadione B may serve as a favorable herb medicine for restoring neuronal injury in the pathogenesis of stroke together with other neurodegenerative diseases.

CysLT1 receptor antagonists pranlukast and zafirlukast inhibit LRRC8-mediated volume regulated anion channels independently of the receptor

Volume-regulated anion channels (VRACs) encoded by the leucine-rich repeat containing 8 (LRRC8) gene family play critical roles in myriad cellular processes and might represent druggable targets. The dearth of pharmacological compounds available for studying VRAC physiology led us to perform a high-throughput screen of 1,184 of US Food and Drug Administration-approved drugs for novel VRAC modulators. We discovered the cysteinyl leukotriene receptor 1 (CysLT1R) antagonist, pranlukast, as a novel inhibitor of endogenous VRAC expressed in human embryonic kidney 293 (HEK293) cells. Pranlukast inhibits VRAC voltage-independently, reversibly, and dose-dependently with a maximal efficacy of only ~50%. The CysLT1R pathway has been implicated in activation of VRAC in other cell types, prompting us to test whether pranlukast requires the CysLT1R for inhibition of VRAC. Quantitative PCR analysis demonstrated that CYSLTR1 mRNA is virtually undetectable in HEK293 cells. Furthermore, the CysLT1R agonist leukotriene D4 had no effect on VRAC activity and failed to stimulate G_q-coupled receptor signaling. Heterologous expression of the CysLT1R reconstituted LTD4-CysLT1R- G_q-calcium signaling in HEK293 cells but had no effect on VRAC inhibition by pranlukast. Finally, we show the CysLT1R antagonist zafirlukast inhibits VRAC with an IC₅₀ of ~17 µM and does so with full efficacy. Our data suggest that both pranlukast and zafirlukast are likely direct channel inhibitors that work independently of the CysLT1R. This study provides clarifying insights into the putative role of leukotriene signaling in modulation of VRAC and identifies two new chemical scaffolds that can be used for development of more potent and specific VRAC inhibitors.

[Cancer cell-specific functional relation between Na+,K+-ATPase and volume-regulated anion channel]

Digitoxin and digoxin are plant-derived cardiac glycosides. They are Na⁺,K⁺-ATPase (sodium pump) inhibitors, and have been used clinically for treatment and prevention of heart failure and various tachycardia. On the other hand, some epidemiological studies showed that digoxin users have a lower cancer risk compared to the non-users, and that cancer patients who had been treated with digoxin face on improvement of their survival. In various in vitro studies, cardiac glycosides at sub-μM concentrations, which have no significant effect on enzymatic and ion-transporting activities of Na⁺,K⁺-ATPase, show anti-cancer effects. Na⁺,K⁺-ATPase is ubiquitously expressed, so it remains unclear why low concentrations of cardiac glycosides have cancer-specific effects. Recently, we found that the receptor-type Na⁺,K⁺-ATPase, which has no pumping activity, is associated with leucine-rich repeat-containing 8 family, member A(LRRC8A), one of the components of volume-regulated anion channel (VRAC), in the membrane microdomains of plasma membrane of cancer cells, and that this crosstalk contributes to the inhibition of the cancer cell growth by sub-μM cardiac glycosides. In this mechanism, cardiac glycosides bind to the receptor-type Na⁺,K⁺-ATPase, and then stimulate the production of reactive oxygen species (ROS) via NADPH oxidase. The ROS activate VRAC within the membrane microdomains, thus eliciting anti-proliferative effects. VRAC is ubiquitously expressed, and it is normally activated by cell swelling. However, VRAC is activated by cardiac glycoside without cell swelling. On the other hand, the cardiac glycosides-induced effects were not observed in non-cancer cells. Our findings can partly explain why cardiac glycosides elicit selective effects in cancer cells.

Ion Channels of the Islets in Type 2 Diabetes

Ca²⁺ is an essential signal for pancreatic β-cell function. Ca²⁺ plays critical roles in numerous β-cell pathways such as insulin secretion, transcription, metabolism, endoplasmic reticulum function, and the stress response. Therefore, β-cell Ca²⁺ handling is tightly controlled. At the plasma membrane, Ca²⁺ entry primarily occurs through voltage-dependent Ca²⁺ channels. Voltage-dependent Ca²⁺ channel activity is dependent on orchestrated fluctuations in the plasma membrane potential or voltage, which are mediated via the activity of many ion channels. During the pathogenesis of type 2 diabetes the β-cell is exposed to stressful conditions, which result in alterations of Ca²⁺ handling. Some of the changes in β-cell Ca²⁺ handling that occur under stress result from perturbations in ion channel activity, expression or localization. Defective Ca²⁺ signaling in the diabetic β-cell alters function, limits insulin secretion and exacerbates hyperglycemia. In this review, we focus on the β-cell ion channels that control Ca²⁺ handling and how they impact β-cell dysfunction in type 2 diabetes.

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.

Extracellular Nucleotides and P2 Receptors in Renal Function

The understanding of the nucleotide/P2 receptor system in the regulation of renal hemodynamics and transport function has grown exponentially over the last 20 yr. This review attempts to integrate the available data while also identifying areas of missing information. First, the determinants of nucleotide concentrations in the interstitial and tubular fluids of the kidney are described, including mechanisms of cellular release of nucleotides and their extracellular breakdown. Then the renal cell membrane expression of P2X and P2Y receptors is discussed in the context of their effects on renal vascular and tubular functions. Attention is paid to effects on the cortical vasculature and intraglomerular structures, autoregulation of renal blood flow, tubuloglomerular feedback, and the control of medullary blood flow. The role of the nucleotide/P2 receptor system in the autocrine/paracrine regulation of sodium and fluid transport in the tubular and collecting duct system is outlined together with its role in integrative sodium and fluid homeostasis and blood pressure control. The final section summarizes the rapidly growing evidence indicating a prominent role of the extracellular nucleotide/P2 receptor system in the pathophysiology of the kidney and aims to identify potential therapeutic opportunities, including hypertension, lithium-induced nephropathy, polycystic kidney disease, and kidney inflammation. We are only beginning to unravel the distinct physiological and pathophysiological influences of the extracellular nucleotide/P2 receptor system and the associated therapeutic perspectives.

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.

Novel insights into the role of LRRC8A in ameliorating alveolar fluid clearance in LPS induced acute lung injury

Leucine-rich repeat-containing 8A (LRRC8A) protein was recently identified as an essential component of volume-regulated anion channel which plays a central role in maintaining cell volume. The aim of this study was to elucidate the role of LRRC8A in alveolar fluid clearance (AFC) and the effect of inflammatory cytokines on LRRC8A and the underlying mechanism. Lipopolysaccharide (LPS) was used to generate a rat acute lung injury model. The results showed that the concentrations of IL-1β, TNF-α and IL-6 in bronchoalveolar lavage fluid increased significantly, but the expression of LRRC8A in the lung tissue decreased dramatically in the acute lung injury group followed by a decline in the AFC rate. Additionally, LRRC8A knockdown reduced AFC in normal rats. However, specific overexpression of LRRC8A in the lung could increase AFC. Furthermore, we observed the effects of LPS, IL-1β, TNF-α and IL-6 on the LRRC8A current in alveolar type II (ATII) cells, and IL-1β showed the greatest inhibition among them, which was involved in phospho-p38 activation. Overall, LRRC8A plays an essential role in the progression of AFC in LPS-induced acute lung injury, and chronic treatment with IL-1β or TNF-α could inhibit the function of LRRC8A in ATII cells by targeting phospho-p38. All of the findings suggested that LRRC8A could be a new partner in AFC and a potential target for the treatment of acute lung injury.

The LRRC8/VRAC anion channel facilitates myogenic differentiation of murine myoblasts by promoting membrane hyperpolarization

Skeletal muscle myoblast differentiation involves elaborate signaling networks, including the activity of various ion channels and transporters. Several K⁺ and Ca²⁺ channels have been shown to affect myogenesis, but little is known about roles of Cl^- channels in the associated processes. Here, we report that the leucine-rich repeat containing family 8 (LRRC8)/volume-regulated anion channel (VRAC) promotes mouse myoblast differentiation. All LRRC8 subunits of heteromeric VRAC were expressed during myotube formation of murine C2C12 myoblasts. Pharmacological VRAC inhibitors, siRNA-mediated knockdown of the essential VRAC subunit LRRC8A, or VRAC activity-suppressing overexpression of LRRC8A effectively reduced the expression of the myogenic transcription factor myogenin and suppressed myoblast fusion while not affecting myoblast proliferation. We found that inhibiting VRAC impairs plasma membrane hyperpolarization early during differentiation. At later times (more than 6 h after inducing differentiation), VRAC inhibition no longer suppressed myoblast differentiation, suggesting that VRAC acts upstream of K⁺ channel activation. Consequently, VRAC inhibition prevented the increase of intracellular steady-state Ca²⁺ levels that normally occurs during myogenesis. Our results may explain the mechanism for the thinning of skeletal muscle bundles observed in LRRC8A-deficient mice and highlight the importance of the LRRC8/VRAC anion channel in cell differentiation.

Effect of low temperature on the regulation of cell volume after hypotonic shock in gastric cancer cells

Although peritoneal lavage with distilled water performed after surgery prevents peritoneal seeding, cancer cells may avoid rupture under mild hypotonicity through regulatory volume decrease (RVD), which is the homeostatic regulation of ion and water transport. The aim of the present study was to investigate the effect of low temperature on cell volume and cell death under hypoosmolal conditions and determine the underlying molecular mechanisms in gastric cancer (GC). Three human GC cell lines (NUGC4, KATO‑III and MKN45) were exposed to hypotonic solutions, and the effects of low temperature on cell volume and viability were examined. Low temperature‑induced changes in membrane transporters were evaluated, and knockdown and overexpression experiments were conducted to determine their effects on cell volume during hypotonic stimulation. Low temperature (24˚C) during hypotonic stimulation inhibited RVD and enhanced the cytocidal effects on GC cells. The expression of leucine‑rich repeat containing protein A (LRRC8A), a component of a Cl‑ channel, was decreased, and aquaporin 5 (AQP5) expression was increased at low temperatures. LRRC8A knockdown markedly slowed the decrease in cell volume following cell swelling by hypotonic shock. AQP5 overexpression enhanced initial cell swelling after hypotonic shock and increased the final cell volume. These results suggest that a hypotonic solution at low temperature increased initial water influx via activation of AQP5 and decreased Cl‑ efflux via inhibition of LRRC8A. Therefore, low temperature enhanced the hypotonicity‑induced cytocidal effects on GC cells, and these results may contribute to the development of a novel lavage method effective in reducing peritoneal recurrence in GC.

Red Blood Cells: Chasing Interactions

Human red blood cells (RBC) are highly differentiated cells that have lost all organelles and most intracellular machineries during their maturation process. RBC are fundamental for the nearly all basic physiologic dynamics and they are key cells in the body's respiratory system by being responsible for the oxygen transport to all cells and tissues, and delivery of carbon dioxide to the lungs. With their flexible structure RBC are capable to deform in order to travel through all blood vessels including very small capillaries. Throughout their in average 120 days lifespan, human RBC travel in the bloodstream and come in contact with a broad range of different cell types. In fact, RBC are able to interact and communicate with endothelial cells (ECs), platelets, macrophages, and bacteria. Additionally, they are involved in the maintenance of thrombosis and hemostasis and play an important role in the immune response against pathogens. To clarify the mechanisms of interaction of RBC and these other cells both in health and disease as well as to highlight the role of important key players, we focused our interest on RBC membrane components such as ion channels, proteins, and phospholipids.

Identification of TMEM206 proteins as pore of PAORAC/ASOR acid-sensitive chloride channels

Acid-sensing ion channels have important functions in physiology and pathology, but the molecular composition of acid-activated chloride channels had remained unclear. We now used a genome-wide siRNA screen to molecularly identify the widely expressed acid-sensitive outwardly-rectifying anion channel PAORAC/ASOR. ASOR is formed by TMEM206 proteins which display two transmembrane domains (TMs) and are expressed at the plasma membrane. Ion permeation-changing mutations along the length of TM2 and at the end of TM1 suggest that these segments line ASOR’s pore. While not belonging to a gene family, TMEM206 has orthologs in probably all vertebrates. Currents from evolutionarily distant orthologs share activation by protons, a feature essential for ASOR’s role in acid-induced cell death. TMEM206 defines a novel class of ion channels. Its identification will help to understand its physiological roles and the diverse ways by which anion-selective pores can be formed.

LRRC8A Expression Influences Growth of Esophageal Squamous Cell Carcinoma

The volume-regulated anion channel is composed of leucine-rich repeat-containing protein A (LRRC8A) and is activated by hypotonic conditions to implement the process of regulatory volume decrease. The role of LRRC8A in regulating genes related to progression of esophageal squamous cell carcinoma (ESCC) was investigated, as well as the prognostic significance of LRRC8A expression in this tumor. Knockdown experiments were conducted using ESCC cell lines and LRRC8A siRNA to assess the influence of this protein on tumor function. In addition, the gene expression profile of ESCC was determined by microarray analysis. Immunohistochemistry was performed on 64 primary tumor samples from ESCC patients receiving radical esophagectomy. It was found that depletion of LRRC8A decreased cell proliferation and migration and also promoted apoptosis. Microarray data demonstrated G₁/S checkpoint regulation and up-regulation or down-regulation of phosphatidylinositol 3-kinase/AKT signaling, matrix metalloproteinase, and integrin signaling-related genes (including p21, p27, MMP1, and ITGAV) in LRRC8A-depleted cells. Immunohistochemistry showed that LRRC8A expression was related to the pathologic N and T stage categories, and strong LRRC8A expression was correlated with a worse prognosis of ESCC. These findings indicate that LRRC8A modulates tumor progression by influencing cell cycle, apoptosis, and migration, providing new insights into its function as an effector or biomarker of ESCC.

Acid- and Volume-Sensitive Chloride Currents in Microglial Cells

Many cell types express an acid-sensitive outwardly rectifying (ASOR) anion current of an unknown function. We characterized such a current in BV-2 microglial cells and then studied its interrelation with the volume-sensitive outwardly rectifying (VSOR) Cl⁻ current and the effect of acidosis on cell volume regulation. We used patch clamp, the Coulter method, and the pH-sensitive dye BCECF to measure Cl⁻ currents and cell membrane potentials, mean cell volume, and intracellular pH, respectively. The ASOR current activated at pH ≤ 5.0 and displayed an I⁻ > Cl⁻ > gluconate⁻ permeability sequence. When compared to the VSOR current, it was similarly sensitive to DIDS, but less sensitive to DCPIB, and insensitive to tamoxifen. Under acidic conditions, the ASOR current was the dominating Cl⁻ conductance, while the VSOR current was apparently inactivated. Acidification caused cell swelling under isotonic conditions and prevented the regulatory volume decrease under hypotonicity. We conclude that acidification, associated with activation of the ASOR- and inactivation of the VSOR current, massively impairs cell volume homeostasis. ASOR current activation could affect microglial function under acidotoxic conditions, since acidosis is a hallmark of pathophysiological events like inflammation, stroke or ischemia and migration and phagocytosis in microglial cells are closely related to cell volume regulation.

Regulation of Cell Behavior by Hydrostatic Pressure

Hydrostatic pressure (HP) regulates diverse cell behaviors including differentiation, migration, apoptosis, and proliferation. Abnormal HP is associated with pathologies including glaucoma and hypertensive fibrotic remodeling. In this review, recent advances in quantifying and predicting how cells respond to HP across several tissue systems are presented, including tissues of the brain, eye, vasculature and bladder, as well as articular cartilage. Finally, some promising directions on the study of cell behaviors regulated by HP are proposed.

Cell volume regulation in cancer cell migration driven by osmotic water flow

Cancer metastasis is the most frequent cause of death for patients with cancer. The main current treatment for cancer metastasis is chemotherapy targeting cancer cells’ ability to proliferate. However, some types of cancer cells show resistance to chemotherapy. Recently, cancer cell migration has become the subject of interest as a novel target of cancer therapy. Cell migration requires many factors, such as the cytoskeleton, cell‐matrix adhesion and cell volume regulation. Here, we focus on cell volume regulation and the role of ion/water transport systems in cell migration. Transport proteins, such as ion channels, ion carriers, and aquaporins, are indispensable for cell volume regulation under steady‐state conditions and during exposure to osmotic stress. Studies from the last ~25 years have revealed that cell volume regulation also plays an important role in the process of cell migration. Water flow in accordance with localized osmotic gradients generated by ion transport contributes to the driving force for cell migration. Moreover, it has been reported that metastatic cancer cells have higher expression of these transport proteins than nonmetastatic cancer cells. Thus, ion/water transport proteins involved in cell volume regulation and cell migration could be novel therapeutic targets for cancer metastasis. In this review, after presenting the importance of ion/water transport systems in cell volume regulation, we discuss the roles of transport proteins in a pathophysiological context, especially in the context of cancer cell migration.

Volume-regulated Cl- current: contributions of distinct Cl- channels and localized Ca2+ signals

The swelling-activated chloride current (I_Cl,swell) is induced when a cell swells and plays a central role in maintaining cell volume in response to osmotic stress. The major contributor of I_Cl,swell is the volume-regulated anion channel (VRAC). Leucine-rich repeat containing 8A (LRRC8A; SWELL1) was recently identified as an essential component of VRAC, but the mechanisms of VRAC activation are still largely unknown; moreover, other Cl^- channels, such as anoctamin 1 (ANO1), were also suggested to contribute to I_Cl,swell. In this present study, we investigated the roles of LRRC8A and ANO1 in activation of I_Cl,swell; we also explored the role of intracellular Ca²⁺ in I_Cl,swell activation. We used a CRISPR/Cas9 gene editing approach, electrophysiology, live fluorescent imaging, selective pharmacology, and other approaches to show that both LRRC8A and ANO1 can be activated by cell swelling in HEK293 cells. Yet, both channels contribute biophysically and pharmacologically distinct components to I_Cl,swell, with LRRC8A being the major component. Cell swelling induced oscillatory Ca²⁺ transients, and these Ca²⁺ signals were required to activate both the LRRC8A- and ANO1-dependent components of I_Cl,swell. Both I_Cl,swell components required localized rather than global Ca²⁺ for activation. Interestingly, while intracellular Ca²⁺ was necessary and sufficient to activate ANO1, it was necessary but not sufficient to activate LRRC8A-mediated currents. Finally, Ca²⁺ transients linked to the I_Cl,swell activation were mediated by the G protein-coupled receptor-independent PLC isoforms.

DCPIB, an Inhibitor of Volume-Regulated Anion Channels, Distinctly Modulates K2P Channels

K2P potassium channels stabilize the resting membrane potential in nearly all cells and have been implicated in several neuronal, cardiovascular, and immune diseases. DCPIB, a known specific and potent inhibitor of volume-regulated anion channels (VRAC), has been reported to activate TREK1 and TREK2 in astrocytes and in vitro recently. In the present study, we demonstrated DCPIB also voltage dependently activated TRAAK besides TREK1/TREK2, showing DCPIB activated all TREK subfamily members. In contrast, the compound potently inhibited several other K2P channels with no voltage dependence, including TRESK, TASK1, and TASK3. DCPIB displayed superior selectivity toward TRESK with an IC₅₀ of 0.14 μM, demonstrating at least 100-fold higher affinity over TREK1/TRAAK channels. Furthermore, the impaired ion selectivity filter region greatly impaired the activating effect of DCPIB on TREK1 but not the inhibitory effect of DCPIB on TRESK. This indicates distinct molecular determinants underlying the effect of DCPIB on TREK1 or TRESK channels. Our results showed that DCPIB played diverse effects on K2P channels and could be a useful tool for further investigating structure-function studies of K2P channels.

A FRET sensor of C-terminal movement reveals VRAC activation by plasma membrane DAG signaling rather than ionic strength

Volume-regulated anion channels (VRACs) are central to cell volume regulation. Recently identified as hetero-hexamers formed by LRRC8 proteins, their activation mechanism remains elusive. Here, we measured Förster resonance energy transfer (FRET) between fluorescent proteins fused to the C-termini of LRRC8 subunits. Inter-subunit FRET from LRRC8 complexes tracked VRAC activation. With patch-clamp fluorometry, we confirmed that the cytoplasmic domains rearrange during VRAC opening. With these FRET reporters, we determined VRAC activation, non-invasively, in live cells and their subcompartments. Reduced intracellular ionic strength did not directly activate VRACs, and VRACs were not activated on endomembranes. Instead, pharmacological manipulation of diacylglycerol (DAG), and protein kinase D (PKD) activity, activated or inhibited plasma membrane-localized VRACs. Finally, we resolved previous contradictory reports concerning VRAC activation, using FRET to detect robust activation by PMA that was absent during whole-cell patch clamp. Overall, non-invasive VRAC measurement by FRET is an essential tool for unraveling its activation mechanism.

Metabolic constraints of swelling-activated glutamate release in astrocytes and their implication for ischemic tissue damage

Volume-regulated anion channel (VRAC) is a glutamate-permeable channel that is activated by physiological and pathological cell swelling and promotes ischemic brain damage. However, because VRAC opening requires cytosolic ATP, it is not clear if and how its activity is sustained in the metabolically compromised CNS. In the present study, we used cultured astrocytes - the cell type which shows prominent swelling in stroke - to model how metabolic stress and changes in gene expression may impact VRAC function in the ischemic and post-ischemic brain. The metabolic state of primary rat astrocytes was modified with chemical inhibitors and examined using luciferin-luciferase ATP assays and a Seahorse analyzer. Swelling-activated glutamate release was quantified with the radiotracer D-[³ H]aspartate. The specific contribution of VRAC to swelling-activated glutamate efflux was validated by RNAi knockdown of the essential subunit, leucine-rich repeat-containing 8A (LRRC8A); expression levels of VRAC components were measured with qRT-PCR. Using this methodology, we found that complete metabolic inhibition with the glycolysis blocker 2-deoxy-D-glucose and the mitochondrial poison sodium cyanide reduced astrocytic ATP levels by > 90% and abolished glutamate release from swollen cells (via VRAC). When only mitochondrial respiration was inhibited by cyanide or rotenone, the intracellular ATP levels and VRAC activity were largely preserved. Bypassing glycolysis by providing the mitochondrial substrates pyruvate and/or glutamine led to partial recovery of ATP levels and VRAC activity. Unexpectedly, the metabolic block of VRAC was overridden when ATP-depleted cells were exposed to extreme cell swelling (≥ 50% reduction in medium osmolarity). Twenty-four hour anoxic adaptation caused a moderate reduction in the expression levels of the VRAC component LRRC8A, but no significant changes in VRAC activity. Overall, our findings suggest that (i) astrocytic VRAC activity and metabolism can be sustained by low levels of glucose and (ii) the inhibitory influence of diminishing ATP levels and the stimulatory effect of cellular swelling are the two major factors that govern VRAC activity in the ischemic brain.

TTYH1 and TTYH2 Serve as LRRC8A-Independent Volume-Regulated Anion Channels in Cancer Cells

Volume-regulated anion channels (VRACs) are involved in cellular functions such as regulation of cell volume, proliferation, migration, and cell death. Although leucine-rich repeat–containing 8A (LRRC8A) has been characterized as a molecular component of VRACs, here we show that Drosophila melanogaster tweety homologue 1 and 2 (TTYH1 and TTYH2) are critical for VRAC currents in cancer cells. LRRC8A-independent VRAC currents were present in the gastric cancer cell line SNU-601, but almost completely absent in its cisplatin-resistant derivative SNU-601-R10 (R10). The VRAC current in R10 was partially restored by treatment with trichostatin A (TSA), a histone deacetylase inhibitor. Based on microarray expression profiling of these cells, we selected two chloride channels, TTYH1 and TTYH2, as VRAC candidates. VRAC currents were completely absent from TTYH1- and TTYH2-deficient SNU-601 cells, and were clearly restored by expression of TTYH1 or TTYH2. In addition, we examined the expression of TTYH1 or TTYH2 in several cancer cell lines and found that VRAC currents of these cells were abolished by gene silencing of TTYH1 or TTYH2. Taken together, our data clearly show that TTYH1 and TTYH2 can act as LRRC8A-independent VRACs, suggesting novel therapeutic approaches for VRACs in cancer cells.

The Volume-Regulated Anion Channel LRRC8/VRAC Is Dispensable for Cell Proliferation and Migration

Cells possess the capability to adjust their volume for various physiological processes, presumably including cell proliferation and migration. The volume-regulated anion channel (VRAC), formed by LRRC8 heteromers, is critically involved in regulatory volume decrease of vertebrate cells. The VRAC has also been proposed to play a role in cell cycle progression and cellular motility. Indeed, recent reports corroborated this notion, with potentially important implications for the VRAC in cancer progression. In the present study, we examined the role of VRAC during cell proliferation and migration in several cell types, including C2C12 myoblasts, human colon cancer HCT116 cells, and U251 and U87 glioblastoma cells. Surprisingly, neither pharmacological inhibition of VRAC with 4-[(2-Butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl)oxy]butanoic acid (DCPIB), carbenoxolone or 5-nitro-2-(3-phenylpropyl-amino)benzoic acid (NPPB), nor siRNA-mediated knockdown or gene knockout of the essential VRAC subunit LRRC8A affected cell growth and motility in any of the investigated cell lines. Additionally, we found no effect of the VRAC inhibition using siRNA treatment or DCPIB on PI3K/Akt signaling in glioblastoma cells. In summary, our work suggests that VRAC is dispensable for cell proliferation or migration.

Inhibition of angiotensin II-induced cerebrovascular smooth muscle cell proliferation by LRRC8A downregulation through suppressing PI3K/AKT activation

Cerebrovascular smooth muscle cell proliferation is the major contributor to cerebrovascular remodeling and stroke. Chloride channels have been suggested to play an important role in the regulation of smooth muscle cell proliferation. This study aims to investigate the effect of leucine-rich repeat-containing 8A (LRRC8A), an essential component of volume-sensitive chloride channels, on cerebrovascular smooth muscle cell proliferation. The data showed that LRRC8A expression was increased in mouse brain artery during angiotensin II (AngII)-induced cerebrovascular remodeling. Similarly, AngII also increased the expression of LRRC8A in human brain vascular smooth muscle cells (HBVSMCs). Knockdown of LRRC8A by siRNA significantly inhibited AngII-induced the proliferation, migration, and invasion in HBVSMCs. The inhibition of HBVSMCs proliferation by LRRC8A downregulation appeared to be involved in suppression of cell-cycle transition. AngII-induced the decrease in p21 and p27 expression and the increase in CDK4 and cyclin D1 expression were attenuated by LRRC8A downregulation. Moreover, inhibition of LRRC8A suppressed AngII-induced PI3K/AKT activation and reactive oxygen species generation, but had no effect on JNK, ERK, and p38 phosphorylation. In addition, activation of PI3K/AKT-signaling pathways with specific agonists significantly abolished the effect of LRRC8A deficiency on HBVSMC proliferation. This present study demonstrates that knockdown of LRRC8A ameliorates AngII-induced cerebrovascular smooth muscle cell proliferation via inhibiting PI3K/AKT pathway, suggesting that LRRC8A may be a novel molecular target in the treatment of vascular remodeling and stroke.

Hormesis effects of phosphorus on the viability of Chlorella regularis cells under nitrogen limitation

BACKGROUND

Phosphorus (P) is an essential element of microalgae, which is either required for anabolism or for energy metabolism. When employing a nitrogen limitation strategy to trigger microalgal intracellular lipid accumulation, P supplementation was always simultaneously applied to compensate for the accompanied growth inhibition.

RESULTS

This study identified that P exerts hormesis effects on microalgae. Slight excess of P (≤ 45 mg L^-1) under nitrogen limitation condition stimulated the cell growth of Chlorella regularis and achieved a 10.2% biomass production increase. This also improved mitochondrial activity by 25.0% compared to control (P = 5.4 mg L^-1). The lipid productivity reached 354.38 mg (L d)^-1, which increased by 39.3% compared to control. Such an improvement was caused by the intracellularly stored polyphosphate energy pool. However, large excess of P (250 mg L^-1) inhibited the cell growth by 38.8% and mitochondrial activity decreased by 71.3%. C. regularis cells showed obvious poisoning status, such as enlarged size, plasmolysis, deformation of cell walls, and disorganization of organelles. This is probably because the over-accumulated P protonated the amide-N and disrupted membrane permeability.

CONCLUSIONS

These results provide new insight into the roles of P in microalgae lipid production: P does not always play a positive role under nitrogen limitation conditions.

Molecular Structure of the Hair Cell Mechanoelectrical Transduction Complex

Cochlear hair cells employ mechanically gated ion channels located in stereocilia that open in response to sound wave-induced motion of the basilar membrane, converting mechanical stimulation to graded changes in hair cell membrane potential. Membrane potential changes in hair cells cause neurotransmitter release from hair cells that initiate electrical signals in the nerve terminals of afferent fibers from spiral ganglion neurons. These signals are then propagated within the central nervous system (CNS) to mediate the sensation of hearing. Recent studies show that the mechanoelectrical transduction (MET) machinery of hair cells is formed by an ensemble of proteins. Candidate components forming the MET channel have been identified, but none alone fulfills all criteria necessary to define them as pore-forming subunits of the MET channel. We will review here recent findings on the identification and function of proteins that are components of the MET machinery in hair cells and consider remaining open questions.

Tweety-homolog (Ttyh) Family Encodes the Pore-forming Subunits of the Swelling-dependent Volume-regulated Anion Channel (VRACswell) in the Brain

In the brain, a reduction in extracellular osmolality causes water-influx and swelling, which subsequently triggers Cl^-- and osmolytes-efflux via volume-regulated anion channel (VRAC). Although LRRC8 family has been recently proposed as the pore-forming VRAC which is activated by low cytoplasmic ionic strength but not by swelling, the molecular identity of the pore-forming swelling-dependent VRAC (VRAC_swell) remains unclear. Here we identify and characterize Tweety-homologs (TTYH1, TTYH2, TTYH3) as the major VRAC_swell in astrocytes. Gene-silencing of all Ttyh1/2/3 eliminated hypo-osmotic-solution-induced Cl^- conductance (I_Cl,swell) in cultured and hippocampal astrocytes. When heterologously expressed in HEK293T or CHO-K1 cells, each TTYH isoform showed a significant I_Cl,swell with similar aquaporin-4 dependency, pharmacological properties and glutamate permeability as I_Cl,swell observed in native astrocytes. Mutagenesis-based structure-activity analysis revealed that positively charged arginine residue at 165 in TTYH1 and 164 in TTYH2 is critical for the formation of the channel-pore. Our results demonstrate that TTYH family confers the bona fide VRAC_swell in the brain.

PAC, an evolutionarily conserved membrane protein, is a proton-activated chloride channel

Severe local acidosis causes tissue damage and pain, and is one of the hallmarks of many diseases including ischemia, cancer, and inflammation. However, the molecular mechanisms of the cellular response to acid are not fully understood. We performed an unbiased RNA interference screen and identified PAC (TMEM206) as being essential for the widely observed proton-activated Cl^- (PAC) currents (I _Cl,H). Overexpression of human PAC in PAC knockout cells generated I _Cl,H with the same characteristics as the endogenous ones. Zebrafish PAC encodes a PAC channel with distinct properties. Knockout of mouse Pac abolished I _Cl,H in neurons and attenuated brain damage after ischemic stroke. The wide expression of PAC suggests a broad role for this conserved Cl^- channel family in physiological and pathological processes associated with acidic pH.

Vector Analysis of Cytoskeletal Structural Tension and the Mechanisms that Underpin Spectrin-Related Forces in Pyroptosis

Aims: Pyroptotic cells are characterized by plasma swelling, membrane blebbing, and disintegration of the cell membrane mediated by spectrin-based membrane skeleton and intercellular competitive tension activities. The spectrin-based membrane skeleton is involved in membrane organization through the regulation of intercellular tension. Using genetically encoded tension sensors to attain noninvasive force measurements in structural proteins, we investigated how cytoskeletal structural tension influences changes in plasma morphology during pyroptosis and the regulatory mechanism of cytoskeletal structural tension that underpins pyroptosis. Results: The results indicate that increasing spectrin tension is caused by osmotic swelling. Hightened tension of spectrin was closely associated with the shrink tension transmitted synergistically by microfilaments (MFs) and microtubules (MTs). However, the increment of spectrin tension in pyroptotic cells was controlled antagonistically by MF and MT forces. Different from MF tension, outward MT forces participated in the formation of membrane blebs. Spectrin tension caused by inward MF forces resisted pyroptosis swelling. Stabilization of MF and MT structure had little influence on intracellular tension and pyroptosis deformation. Pyroptosis-induced cytoskeletal structural tension was highly dependent on calcium signaling and reactive oxygen species generation. Blocking of membrane pores, nonselective ion flux, or elimination of caspase-1 cleavage resulted in the remission of structural forces associated with pyroptosis failure. Innovation and Conclusions: The data suggest that subcellular tension, in terms of magnitude and vector, is integral to pyroptosis through the mediation of swelling and blebbing and the elimination of structural tension, especially MT forces, may result in pyroptosis inhibition.

Glutamate-Releasing SWELL1 Channel in Astrocytes Modulates Synaptic Transmission and Promotes Brain Damage in Stroke

By releasing glutamate, astrocytes actively regulate synaptic transmission and contribute to excitotoxicity in neurological diseases. However, the mechanisms of astrocytic glutamate release have been debated. Here, we report non-vesicular release of glutamate through the glutamate-permeable volume-regulated anion channel (VRAC). Both cell swelling and receptor stimulation activated astrocytic VRAC, which requires its only obligatory subunit, Swell1. Astrocyte-specific Swell1 knockout mice exhibited impaired glutamatergic transmission due to the decreases in presynaptic release probability and ambient glutamate level. Consistently, the mutant mice displayed hippocampal-dependent learning and memory deficits. During pathological cell swelling, deletion of astrocytic Swell1 attenuated glutamate-dependent neuronal excitability and protected mice from brain damage after ischemic stroke. Our identification of a new molecular mechanism for channel-mediated glutamate release establishes a role for astrocyte-neuron interactions in both synaptic transmission and brain ischemia. It provides a rationale for targeting VRAC for the treatment of stroke and other neurological diseases associated with excitotoxicity.

The LRRC8-mediated volume-regulated anion channel is altered in glaucoma

Regulation of cellular volume is an essential process to balance volume changes during cell proliferation and migration or when intracellular osmolality increases due to transepithelial transport. We previously characterized the key role of volume-regulated anion channels (VRAC) in the modulation of the volume of trabecular meshwork (TM) cells and, in turn, the aqueous humour (AH) outflow from the eye. The balance between the secretion and the drainage of AH determines the intraocular pressure (IOP) that is the major casual risk factor for glaucoma. Glaucoma is an ocular disease that causes irreversible blindness due to the degeneration of retinal ganglion cells. The recent identification of Leucine-Rich Repeat-Containing 8 (LRRC8A-E) proteins as the molecular components of VRAC opens the field to elucidate their function in the physiology of TM and glaucoma. Human TM cells derived from non-glaucomatous donors and from open-angle glaucoma patients were used to determine the expression and the functional activity of LRRC8-mediated channels. Expression levels of LRRC8A-E subunits were decreased in HTM glaucomatous cells compared to normotensive HTM cells. Consequently, the activity of VRAC currents and volume regulation of TM cells were significantly affected. Impaired cell volume regulation will likely contribute to altered aqueous outflow and intraocular pressure.

Dexamethasone in Glioblastoma Multiforme Therapy: Mechanisms and Controversies

Glioblastoma multiforme (GBM) is the most common and malignant of the glial tumors. The world-wide estimates of new cases and deaths annually are remarkable, making GBM a crucial public health issue. Despite the combination of radical surgery, radio and chemotherapy prognosis is extremely poor (median survival is approximately 1 year). Thus, current therapeutic interventions are highly unsatisfactory. For many years, GBM-induced brain oedema and inflammation have been widely treated with dexamethasone (DEX), a synthetic glucocorticoid (GC). A number of studies have reported that DEX also inhibits GBM cell proliferation and migration. Nevertheless, recent controversial results provided by different laboratories have challenged the widely accepted dogma concerning DEX therapy for GBM. Here, we have reviewed the main clinical features and genetic and epigenetic abnormalities underlying GBM. Finally, we analyzed current notions and concerns related to DEX effects on cerebral oedema, cancer cell proliferation and migration and clinical outcome.

Mechanosensitive ATP release in the lungs: New insights from real-time luminescence imaging studies

Extracellular ATP and other nucleotides are important autocrine/paracrine mediators that stimulate purinergic receptors and regulate diverse processes in the normal lungs. They are also associated with pathogenesis of a number of respiratory diseases and clinical complications including acute respiratory distress syndrome and ventilator induced lung injury. Mechanical forces are major stimuli for cellular ATP release but precise mechanisms responsible for this release are still debated. The present review intends to provide the current state of knowledge of the mechanisms of ATP release in the lung. Putative pathways of the release, including the contribution of cell membrane injury and cell lysis are discussed addressing their strength, weaknesses and missing evidence that requires future study. We also provide an overview of the recent technical advances in studying cellular ATP release in vitro and ex vivo. Special attention is given to new insights into lung ATP release obtained with the real-time luminescence ATP imaging. This includes recent data on stretch-induced mechanosensitive ATP release in a model and primary cells of lung alveoli in vitro as well as inflation-induced ATP release in airspaces and pulmonary blood vessels of lungs, ex vivo.

Contribution of Anoctamins to Cell Survival and Cell Death

Before anoctamins (TMEM16 proteins) were identified as a family of Ca2+-activated chloride channels and phospholipid scramblases, the founding member anoctamin 1 (ANO1, TMEM16A) was known as DOG1, a marker protein for gastrointestinal stromal tumors (GIST). Meanwhile, ANO1 has been examined in more detail, and the role of ANO1 in cell proliferation and the development of different types of malignomas is now well established. While ANO5, ANO7, and ANO9 may also be relevant for growth of cancers, evidence has been provided for a role of ANO6 (TMEM16F) in regulated cell death. The cellular mechanisms by which anoctamins control cell proliferation and cell death, respectively, are just emerging; however, the pronounced effects of anoctamins on intracellular Ca2+ levels are likely to play a significant role. Recent results suggest that some anoctamins control membrane exocytosis by setting Ca2+i levels near the plasma membrane, and/or by controlling the intracellular Cl- concentration. Exocytosis and increased membrane trafficking induced by ANO1 and ANO6 may enhance membrane expression of other chloride channels, such as CFTR and volume activated chloride channels (VRAC). Notably, ANO6-induced phospholipid scrambling with exposure of phosphatidylserine is pivotal for the sheddase function of disintegrin and metalloproteinase (ADAM). This may support cell death and tumorigenic activity of IL-6 by inducing IL-6 trans-signaling. The reported anticancer effects of the anthelminthic drug niclosamide are probably related to the potent inhibitory effect on ANO1, apart from inducing cell cycle arrest through the Let-7d/CDC34 axis. On the contrary, pronounced activation of ANO6 due to a large increase in intracellular calcium, activation of phospholipase A2 or lipid peroxidation, can lead to ferroptotic death of cancer cells. It therefore appears reasonable to search for both inhibitors and potent activators of TMEM16 in order to interfere with cancer growth and metastasis.

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.

Long-Term Effects of Myoinositol on Behavioural Seizures and Biochemical Changes Evoked by Kainic Acid Induced Epileptogenesis

Epilepsy is one of the most devastating neurological diseases and despite significant efforts there is no cure available. Occurrence of spontaneous seizures in epilepsy is preceded by numerous functional and structural pathophysiological reorganizations in the brain—a process called epileptogenesis. Treatment strategies targeting this process may be efficient for preventing spontaneous recurrent seizures (SRS) in epilepsy, or for modification of disease progression. We have previously shown that (i) myoinositol (MI) pretreatment significantly decreases severity of acute seizures (status epilepticus: SE) induced by kainic acid (KA) in experimental animals and (ii) that daily post-SE administration of MI for 4 weeks prevents certain biochemical changes triggered by SE. However it was not established whether such MI treatment also exerts long-term effects on the frequency of SRS. In the present study we have shown that, in KA-induced post-SE epilepsy model in rats, MI treatment for 28 days reduces frequency and duration of behavioural SRS not only during the treatment, but also after its termination for the following 4 weeks. Moreover, MI has significant effects on molecular changes in the hippocampus, including mi-RNA expression spectrum, as well as mRNA levels of sodium-MI transporter and LRRC8A subunit of the volume regulated anionic channel. Taken together, these data suggest that molecular changes induced by MI treatment may counteract epileptogenesis. Thus, here we provide data indicating antiepileptogenic properties of MI, which further supports the idea of developing new antiepileptogenic and disease modifying drug that targets MI system.

Chloride Channels in Astrocytes: Structure, Roles in Brain Homeostasis and Implications in Disease

Astrocytes are the most abundant cell type in the CNS (central nervous system). They exert multiple functions during development and in the adult CNS that are essential for brain homeostasis. Both cation and anion channel activities have been identified in astrocytes and it is believed that they play key roles in astrocyte function. Whereas the proteins and the physiological roles assigned to cation channels are becoming very clear, the study of astrocytic chloride channels is in its early stages. In recent years, we have moved from the identification of chloride channel activities present in astrocyte primary culture to the identification of the proteins involved in these activities, the determination of their 3D structure and attempts to gain insights about their physiological role. Here, we review the recent findings related to the main chloride channels identified in astrocytes: the voltage-dependent ClC-2, the calcium-activated bestrophin, the volume-activated VRAC (volume-regulated anion channel) and the stress-activated Maxi-Cl⁻. We discuss key aspects of channel biophysics and structure with a focus on their role in glial physiology and human disease.

Biophysics and Structure-Function Relationships of LRRC8-Formed Volume-Regulated Anion Channels

Volume-regulated anion channels (VRACs) are key players in regulatory volume decrease of vertebrate cells by mediating the extrusion of chloride and organic osmolytes. They play additional roles in various physiological processes beyond their role in osmotic volume regulation. VRACs are formed by heteromers of LRRC8 proteins; LRRC8A (also called SWELL1) is an essential subunit that combines with any of its paralogs, LRRC8B–E, to form hexameric VRAC complexes. The subunit composition of VRACs determines electrophysiological characteristics of their anion transport such as single-channel conductance, outward rectification, and depolarization-dependent inactivation kinetics. In addition, differently composed VRACs conduct diverse substrates, such as LRRC8D enhancing VRAC permeability to organic substances like taurine or cisplatin. Here, after a recapitulation of the biophysical properties of VRAC-mediated ion and osmolyte transport, we summarize the insights gathered since the molecular identification of VRACs. We describe the recently solved structures of LRRC8 complexes and discuss them in terms of their structure-function relationships. These studies open up many potential avenues for future research.

Cryo-EM structures of the DCPIB-inhibited volume-regulated anion channel LRRC8A in lipid nanodiscs

Hypoosmotic conditions activate volume-regulated anion channels in vertebrate cells. These channels are formed by leucine-rich repeat-containing protein 8 (LRRC8) family members and contain LRRC8A in homo- or hetero-hexameric assemblies. Here, we present single-particle cryo-electron microscopy structures of Mus musculus LRRC8A in complex with the inhibitor DCPIB reconstituted in lipid nanodiscs. DCPIB plugs the channel like a cork in a bottle - binding in the extracellular selectivity filter and sterically occluding ion conduction. Constricted and expanded structures reveal coupled dilation of cytoplasmic LRRs and the channel pore, suggesting a mechanism for channel gating by internal stimuli. Conformational and symmetry differences between LRRC8A structures determined in detergent micelles and lipid bilayers related to reorganization of intersubunit lipid binding sites demonstrate a critical role for the membrane in determining channel structure. These results provide insight into LRRC8 gating and inhibition and the role of lipids in the structure of an ionic-strength sensing ion channel.

Mannitol decreases neocortical epileptiform activity during early brain development via cotransport of chloride and water

Seizures and brain injury lead to water and Cl^- accumulation in neurons. The increase in intraneuronal Cl^- concentration ([Cl^-]_i) depolarizes the GABA_A reversal potential (E_GABA) and worsens seizure activity. Neocortical neuronal membranes have a low water permeability due to the lack of aquaporins necessary to move free water. Instead, neurons use cotransport of ions including Cl^- to move water. Thus, increasing the extracellular osmolarity during seizures should result in an outward movement of water and salt, reducing [Cl^-]_i and improving GABA_A receptor-mediated inhibition. We tested the effects of hyperosmotic therapy with a clinically relevant dose of mannitol (20 mM) on epileptiform activity, spontaneous multiunit activity, spontaneous inhibitory post-synaptic currents (sIPSCs), [Cl^-]_i, and neuronal volume in layer IV/V of the developing neocortex of C57BL/6 and Clomeleon mice. Using electrophysiological techniques and multiphoton imaging in acute brain slices (post-natal day 7-12) and organotypic neocortical slice cultures (post-natal day 14), we observed that mannitol: 1) decreased epileptiform activity, 2) decreased neuronal volume and [Cl^-]_i through CCCs, 3) decreased spontaneous multi-unit activity frequency but not amplitude, and 4) restored the anticonvulsant efficacy of the GABA_A receptor modulator diazepam. Increasing extracellular osmolarity by 20 mOsm with hypertonic saline did not decrease epileptiform activity. We conclude that an increase in extracellular osmolarity by mannitol mediates the efflux of [Cl^-]_i and water through CCCs, which results in a decrease in epileptiform activity and enhances benzodiazepine actions in the developing neocortex in vitro. Novel treatments aimed to decrease neuronal volume may concomitantly decrease [Cl^-]_i and improve seizure control.

In vitro evaluation of the enantiomeric R- and S-1,1'-binaphthyl-2,2'-diaminodichlorido-Pt(ii) complexes in human Burkitt lymphoma cells: emphasis on cellular accumulation, cytotoxicity, DNA binding, and ability to induce apoptosis

The aim of this project is to gain insights into the uptake and cellular actions of the enantiomeric R- and S-1,1'-binaphthyl-2,2'-diaminodichlorido-Pt(ii) complexes (R- and S-[Pt(DABN)Cl₂]) in the cisplatin-sensitive human Burkitt lymphoma cell line (Gumbus, IC₅₀: 1.3 ± 0.2 μM) and its cisplatin-resistant sub-line (CDDPrGB, IC₅₀: 6.6 ± 1.2 μM). The cellular uptakes of R- and S-[Pt(DABN)Cl₂] are ca. 4-fold higher than cisplatin, and involve a transport mechanism independent of the volume-sensitive, organic anion-channel complex, which facilitates cisplatin accumulation. The cisplatin-resistant CDDPrGB cells are not cross-resistant to either S- or R-[Pt(DABN)Cl₂]. We also find that even though R-[Pt(DABN)Cl₂] has a higher maximal cellular uptake and binds at higher levels to calf-thymus DNA than S-[Pt(DABN)Cl₂], it appears that S-[Pt(DABN)Cl₂] is more cytotoxic for Gumbus (IC₅₀: 0.4 ± 0.1 μM) compared to R-[Pt(DABN)Cl₂] (IC₅₀: 0.7 ± 0.3 μM). The cellular action of R- and S-[Pt(DABN)Cl₂] involves G0/G1 cell cycle arrest and cell death involving the extrinsic and intrinsic apoptotic pathways.