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

Recent insights on the impact of SWELL1 on metabolic syndromes

SWELL1 is a key component of the volume-regulated anion channel (VRAC) and participates in cell volume regulation as an ion channel plasma membrane protein. While early studies focused on its role in immune cell development and tumor progression, recent studies have revealed that SWELL1 plays an important role in metabolic diseases. Studies have shown that SWELL1 is extensively involved in physiological processes in peripheral metabolic tissues, including adipocyte hypertrophy, skeletal muscle volume regulation, insulin secretion, and hepatic lipid metabolism through interactions with the insulin signaling pathway. These functions play key roles in the pathogenesis of obesity, type 2 diabetes mellitus (T2DM), and non-alcoholic fatty liver disease (NAFLD), suggesting that SWELL1 may be a new target for the treatment of metabolic diseases. In this review, we focus on the structural and functional characteristics of SWELL1 to provide an in-depth explanation of its role in the development of metabolic syndrome, especially the regulation of the insulin signaling pathway, and provide a basis for the development of therapeutic strategies for metabolic diseases targeting SWELL1.

Lysosomal LRRC8 complex regulates lysosomal pH, morphology and systemic glucose metabolism

The lysosome integrates anabolic signalling and nutrient-sensing to regulate intracellular growth pathways. The leucine-rich repeat containing 8 (LRRC8) channel complex forms a lysosomal anion channel and regulates PI3K-AKT-mTOR signalling, skeletal muscle differentiation, growth, and systemic glucose metabolism. Here, we define the endogenous LRRC8 subunits localized to a subset of lysosomes in differentiated myotubes. We show LRRC8A regulates leucine-stimulated mTOR, lysosome size, number, pH, and expression of lysosomal proteins LAMP2, P62, LC3B, suggesting impaired autophagic flux. Mutating a LRRC8A lysosomal targeting dileucine motif sequence (LRRC8A-L706A;L707A) in myotubes recapitulates the abnormal AKT signalling and altered lysosomal morphology and pH observed in LRRC8A KO cells. In vivo , LRRC8A-L706A;L707A KI mice exhibit increased adiposity, impaired glucose tolerance and insulin resistance characterized by reduced skeletal muscle glucose-uptake, and impaired incorporation of glucose into glycogen. These data reveal a lysosomal LRRC8 mediated metabolic signalling function that regulates lysosomal activity, systemic glucose homeostasis and insulin-sensitivity.

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

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

Physiology of the volume-sensitive/regulatory anion channel VSOR/VRAC. Part 1: from its discovery and phenotype characterization to the molecular entity identification

The volume-sensitive outwardly rectifying or volume-regulated anion channel, VSOR/VRAC, which was discovered in 1988, is expressed in most vertebrate cell types and is essentially involved in cell volume regulation after swelling and in the induction of cell death. This series of review articles describes what is already known and what remains to be uncovered about the functional and molecular properties as well as the physiological and pathophysiological roles of VSOR/VRAC. This Part 1 review article describes, from the physiological standpoint, first its discovery and significance in cell volume regulation, second its phenotypical properties, and third its molecular identification. Although the pore-forming core molecules and the volume-sensing subcomponent of VSOR/VRAC were identified as LRRC8 members and TRPM7 in 2014 and 2021, respectively, it is stressed that the identification of the molecular entity of VSOR/VRAC is still not complete enough to explain the full set of phenotypical properties.

Physiological Functions of the Volume-Regulated Anion Channel VRAC/LRRC8 and the Proton-Activated Chloride Channel ASOR/TMEM206

Volume-regulated anion channels (VRACs) and the acid-sensitive outwardly rectifying anion channel (ASOR) mediate flux of chloride and small organic anions. Although known for a long time, they were only recently identified at the molecular level. VRACs are heteromers consisting of LRRC8 proteins A to E. Combining the essential LRRC8A with different LRRC8 paralogues changes key properties of VRAC such as conductance or substrate selectivity, which is how VRACs are involved in multiple physiological functions including regulatory volume decrease, cell proliferation and migration, cell death, purinergic signalling, fat and glucose metabolism, insulin signalling, and spermiogenesis. VRACs are also involved in pathological conditions, such as the neurotoxic release of glutamate and aspartate. Certain VRACs are also permeable to larger, organic anions, including antibiotics and anti-cancer drugs, making them an interesting therapeutic target. ASOR, also named proton-activated chloride channel (PAC), is formed by TMEM206 homotrimers on the plasma membrane and on endosomal compartments where it mediates chloride flux in response to extracytosolic acidification and plays a role in the shrinking and maturation of macropinosomes. ASOR has been shown to underlie neuronal swelling which causes cell death after stroke as well as promoting the metastasis of certain cancers, making them intriguing therapeutic targets as well.

Structure-function relationships of the LRRC8 subunits and subdomains of the volume-regulated anion channel (VRAC)

Volume Regulated Anion Channels (VRAC) are critical contributors to cell volume homeostasis and are expressed ubiquitously in all vertebrate cells. VRAC sense increases in cell volume, and act to return cells to baseline volume in a process known as regulatory volume decrease (RVD) through the efflux of anions and organic osmolytes. This review will highlight seminal studies that elucidated the role of VRAC in RVD, their characteristics as a function of subunit specificity, and their clinical relevance in physiology and pathology. VRAC are also known as volume-sensitive outward rectifiers (VSOR) and volume-sensitive organic osmolyte/anion channels (VSOAC). In this review, the term VRAC will be used to refer to this family of channels.

Leucine-rich repeat containing 8A contributes to the expansion of The potential role of leucine-rich repeat-containing protein 8A in central nervous system: current situation and prospect

Cell swelling usually initiates the regulatory volume decrease (RVD) process mediated mainly by volume-regulated anion channels (VRACs), which are formed by multiple different leucine-rich repeat-containing protein 8 (LRRC8) family members. VRAC currents have been widely recorded in astrocytes, neurons and microglia in the brain, and VRACs have been suggested to be involved in the important pathogenesis of cell swelling-related central nervous system (CNS) diseases, such as ischemic stroke, epilepsy and epileptogenesis, glioblastoma (GBM), and so on. Recently, the increasing studies started to focus on LRRC8A (SWELL1), an obligatory subunit of VRAC indentified in 2014, which may be the key target to regulate the VRAC functions. After cerebral ischemia, the swollen astrocytes, neurons and microglia can activate LRRC8A-dependent VRACs, which may respectively promote the release of excitatory amino acids (EAA), interaction with ionotropic glutamate receptors, and regulating inflammation, suggesting the pleiotropic roles of LRRC8A in swollen brain cells. For the treatment of cell swelling-related CNS diseases, specific targeting LRRC8A may be a superior strategy to inhibit swollen-induced VRAC hyperactivity without blocking the normal VRAC function.

Macrophage activation in stellate ganglia contributes to lung injury-induced arrhythmogenesis in male rats

AIM

Patients suffering from acute lung injury (ALI) are at high risk of developing cardiac arrhythmias. We hypothesized that stellate ganglia (SG) neural inflammation contributes to ALI-induced arrhythmia.

METHODS

We created an ALI rat model using a single tracheal instillation of bleomycin (2.5 mg/kg), with saline as a sham control. We recorded ECGs by implanted radiotelemetry in male bleomycin and sham rats treated with and without oral minocycline (20 mg/kg/d), an anti-inflammatory drug that inhibits microglia/macrophage activation. The SG neuronal excitability was assessed by electrophysiology experiments.

RESULTS

ECG data showed that bleomycin-exposed rats exhibited significantly more spontaneous premature ventricular contractions (PVCs) from 1- to 3-week post-induction compared with sham rats, which was mitigated by chronic oral administration of minocycline. The bleomycin-exposed rats displayed a robust increase in both the number of Iba1-positive macrophages and protein expression of interferon regulatory factor 8 in the SG starting as early at 1-week post-exposure and lasted for at least 4 weeks, which was largely attenuated by minocycline. Heart rate variability analysis indicated autonomic imbalance during the first 2-week post-bleomycin, which was significantly attenuated by minocycline. Electrical stimulation of the decentralized SG triggered more PVCs in bleomycin-exposed rats than sham and bleomycin + minocycline rats. Patch-clamp data demonstrated enhanced SG neuronal excitability in the bleomycin-exposed rats, which was attenuated by minocycline. Co-culture of lipopolysaccharide (LPS)-pretreated macrophages with normal SG neurons enhanced SG neuronal excitability.

CONCLUSION

Macrophage activation in the SG contributes to arrhythmogenesis in bleomycin-induced ALI in male rats.

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

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

Cell death in the gut epithelium and implications for chronic inflammation

The intestinal epithelium has one of the highest rates of cellular turnover in a process that is tightly regulated. As the transit-amplifying progenitors of the intestinal epithelium generate ~300 cells per crypt every day, regulated cell death and sloughing at the apical surface keeps the overall cell number in check. An aberrant increase in the rate of intestinal epithelial cell (IEC) death underlies instances of extensive epithelial erosion, which is characteristic of several intestinal diseases such as inflammatory bowel disease and infectious colitis. Emerging evidence points to a crucial role of necroptosis, autophagy and pyroptosis as important modes of programmed cell death in the intestine in addition to apoptosis. The mode of cell death affects tissue restitution responses and ultimately the long-term risks of intestinal fibrosis and colorectal cancer. A vicious cycle of intestinal barrier breach, misregulated cell death and subsequent inflammation is at the heart of chronic inflammatory and infectious gastrointestinal diseases. This Review discusses the underlying molecular and cellular underpinnings that control programmed cell death in IECs, which emerge during intestinal diseases. Translational aspects of cell death modulation for the development of novel therapeutic alternatives for inflammatory bowel diseases and colorectal cancer are also discussed.

SGK1 mediates the hypotonic protective effect against H2O2-induced apoptosis of rat basilar artery smooth muscle cells by inhibiting the FOXO3a/Bim signaling pathway

Serum- and glucocorticoid-inducible kinease-1 (SGK1) is a serine/threonine kinase regulated by hypotonic stimuli, which is involved in regulation of cell cycle and apoptosis. Our previous study shows that activation of volume-regulated Cl^- channels (VRCCs) protects rat basilar artery smooth muscle cells (BASMCs) against hydrogen peroxide (H₂O₂)-induced apoptosis. In the present study, we investigated whether SGK1 was involved in the protective effect of VRCCs in BASMCs. We showed that hypotonic challenge significantly reduced H₂O₂-induced apoptosis, and increased SGK1 phosphorylation, but did not affect SGK1 protein expression. The protective effect of hypotonic challenge against H₂O₂-induced apoptosis was mediated through inhibiting mitochondria-dependent apoptotic pathway, evidenced by increased Bcl-2/Bax ratio, stabilizing mitochondrial membrane potential (MMP), decreased cytochrome c release from the mitochondria to the cytoplasm, and inhibition of the activation of caspase-9 and caspase-3. These protective effects of hypotonic challenge against H₂O₂-induced apoptosis was diminished and enhanced, respectively, by SGK1 knockdown and overexpression. We further revealed that SGK1 activation significantly increased forkhead box O3a (FOXO3a) phosphorylation, and then inhibited the translocation of FOXO3a into nucleus and the subsequent expression of Bcl-2 interacting mediator of cell death (Bim). In conclusion, SGK1 mediates the protective effect of VRCCs against H₂O₂-induced apoptosis in BASMCs via inhibiting FOXO3a/Bim signaling pathway. Our results provide compelling evidences that SGK1 is a critical link between VRCCs and apoptosis, and shed a new light on the treatment of vascular apoptosis-associated diseases, such as vascular remodeling, angiogenesis, and atherosclerosis.

Novel Analgesics with Peripheral Targets

A limited number of peripheral targets generate pain. Inflammatory mediators can sensitize these. The review addresses targets acting exclusively or predominantly on sensory neurons, mediators involved in inflammation targeting sensory neurons, and mediators involved in a more general inflammatory process, of which an analgesic effect secondary to an anti-inflammatory effect can be expected. Different approaches to address these systems are discussed, including scavenging proinflammatory mediators, applying anti-inflammatory mediators, and inhibiting proinflammatory or facilitating anti-inflammatory receptors. New approaches are contrasted to established ones; the current stage of progress is mentioned, in particular considering whether there is data from a molecular and cellular level, from animals, or from human trials, including an early stage after a market release. An overview of publication activity is presented, considering a IuPhar/BPS-curated list of targets with restriction to pain-related publications, which was also used to identify topics.

LRRC8A-dependent volume-regulated anion channels contribute to ischemia-induced brain injury and glutamatergic input to hippocampal neurons

Volume-regulated anion channels (VRACs) are critically involved in regulating cell volume, and leucine-rich repeat-containing protein 8A (LRRC8A, SWELL1) is an obligatory subunit of VRACs. Cell swelling occurs early after brain ischemia, but it is unclear whether neuronal LRRC8a contributes to ischemia-induced glutamate release and brain injury. We found that Lrrc8a conditional knockout (Lrrc8a-cKO) mice produced by crossing Nestin^Cre+/- with Lrrc8a^flox+/+ mice died 7-8 weeks of age, indicating an essential role of neuronal LRRC8A for survival. Middle cerebral artery occlusion (MCAO) caused an early increase in LRRC8A protein levels in the hippocampus in wild-type (WT) mice. Whole-cell patch-clamp recording in brain slices revealed that oxygen-glucose deprivation significantly increased the amplitude of VRAC currents in hippocampal CA1 neurons in WT but not in Lrrc8a-cKO mice. Hypotonicity increased the frequency of spontaneous excitatory postsynaptic currents (sEPSCs) in hippocampal CA1 neurons in WT mice, and this was abolished by DCPIB, a VRAC blocker. But in Lrrc8a-cKO mice, hypotonic solution had no effect on the frequency of sEPSCs in these neurons. Furthermore, the brain infarct volume and neurological severity score induced by MCAO were significantly lower in Lrrc8a-cKO mice than in WT mice. In addition, MCAO-induced increases in cleaved caspase-3 and calpain activity, two biochemical markers of neuronal apoptosis and death, in brain tissues were significantly attenuated in Lrrc8a-cKO mice compared with WT mice. These new findings indicate that cerebral ischemia increases neuronal LRRC8A-dependent VRAC activity and that VRACs contribute to increased glutamatergic input to hippocampal neurons and brain injury caused by ischemic stroke.

Dissecting the Role of Subtypes of Gastrointestinal Vagal Afferents

Gastrointestinal (GI) vagal afferents convey sensory signals from the GI tract to the brain. Numerous subtypes of GI vagal afferent have been identified but their individual roles in gut function and feeding regulation are unclear. In the past decade, technical approaches to selectively target vagal afferent subtypes and to assess their function has significantly progressed. This review examines the classification of GI vagal afferent subtypes and discusses the current available techniques to study vagal afferents. Investigating the distribution of GI vagal afferent subtypes and understanding how to access and modulate individual populations are essential to dissect their fundamental roles in the gut-brain axis.

Chloride - The Underrated Ion in Nociceptors

In contrast to pain processing neurons in the spinal cord, where the importance of chloride conductances is already well established, chloride homeostasis in primary afferent neurons has received less attention. Sensory neurons maintain high intracellular chloride concentrations through balanced activity of Na+-K+-2Cl- cotransporter 1 (NKCC1) and K+-Cl- cotransporter 2 (KCC2). Whereas in other cell types activation of chloride conductances causes hyperpolarization, activation of the same conductances in primary afferent neurons may lead to inhibitory or excitatory depolarization depending on the actual chloride reversal potential and the total amount of chloride efflux during channel or transporter activation. Dorsal root ganglion (DRG) neurons express a multitude of chloride channel types belonging to different channel families, such as ligand-gated, ionotropic γ-aminobutyric acid (GABA) or glycine receptors, Ca2+-activated chloride channels of the anoctamin/TMEM16, bestrophin or tweety-homolog family, CLC chloride channels and transporters, cystic fibrosis transmembrane conductance regulator (CFTR) as well as volume-regulated anion channels (VRACs). Specific chloride conductances are involved in signal transduction and amplification at the peripheral nerve terminal, contribute to excitability and action potential generation of sensory neurons, or crucially shape synaptic transmission in the spinal dorsal horn. In addition, chloride channels can be modified by a plethora of inflammatory mediators affecting them directly, via protein-protein interaction, or through signaling cascades. Since chloride channels as well as mediators that modulate chloride fluxes are regulated in pain disorders and contribute to nociceptor excitation and sensitization it is timely and important to emphasize their critical role in nociceptive primary afferents in this review.

A chloride channel in rat pancreatic acinar AR42J cells is sensitive to extracellular acidification and dependent on ROS

Extracellular acidification, playing a promoting role in the process of acute pancreatitis, has been reported to activate Cl^- channels in several types of cells. However, whether extracellular acidification aggravates acute pancreatitis via activating Cl^- channels remains unclear. Here, we investigated the effects of extracellular acidification on Cl^- channels in rat pancreatic acinar AR42J cells using whole-cell patch-clamp recordings. We found that extracellular acidification induced a moderately outward-rectified Cl^- current, with a selectivity sequence of I^- > Br^- ≥ Cl^- > gluconate^-, while intracellular acidification failed to induce the currents. The acid-sensitive currents were inhibited by Cl^- channel blockers, 4,4'-Diisothiocyanatostilbene-2,2'-disulfonic acid disodium salt hydrate and 5-Nitro-2-(3-phenylpropylamino) benzoic acid. After ClC-3 was silenced by ClC-3 shRNA, the acid-sensitive Cl^- currents were attenuated significantly, indicating that ClC-3 plays a vital role in the induction of acid-sensitive Cl^- currents. Extracellular acid elevated the intracellular level of reactive oxygen species (ROS) significantly, prior to inducing Cl^- currents. When ROS production was scavenged, the acid-sensitive Cl^- currents were abolished. Whereas, the level of acid-induced ROS was unaffected with silence of ClC-3. Our findings above demonstrate that extracellular acidification induces a Cl^- current in pancreatic acinar cells via promoting ROS generation, implying an underlying mechanism that extracellular acidification might aggravate acute pancreatitis through Cl^- channels.

LRRC8/VRAC channels exhibit a noncanonical permeability to glutathione, which modulates epithelial-to-mesenchymal transition (EMT)

Volume-regulated anion channels (VRAC) are chloride channels activated in response to osmotic stress to regulate cellular volume and also participate in other cellular processes, including cell division and cell death. Recently, members of the LRRC8 family have been identified as the main contributors of VRAC conductance. LRRC8/VRAC is permeable to chloride ions but also exhibits significant permeability to various substrates that vary strongly in charge and size. In this study, we explored the intriguing ability of LRRC8/VRAC to transport glutathione (GSH), the major cellular reactive oxygen species (ROS) scavenger, and its involvement in epithelial-to-mesenchymal transition (EMT), a cellular process in which cellular oxidative status is a crucial step. First, in HEK293-WT cells, we showed that a hypotonic condition induced LRRC8/VRAC-dependent GSH conductance (P_GSH/P_Cl of ~0.1) and a marked decrease in intracellular GSH content. GSH currents and GSH intracellular decrease were both inhibited by DCPIB, an inhibitor of LRRC8/VRAC, and were not observed in HEK293-LRRC8A KO cells. Then, we induced EMT by exposing renal proximal tubule epithelial cells to the pleiotropic growth factor TGFβ1, and we measured the contribution of LRRC8/VRAC in this process by measuring (i) EMT marker expression (assessed both at the gene and protein levels), (ii) cell morphology and (iii) the increase in migration ability. Interestingly, pharmacologic targeting of LRRC8/VRAC (DCPIB) or RNA interference-mediated inhibition (LRRC8A siRNA) attenuated the TGFβ1-induced EMT response by controlling GSH and ROS levels. Interestingly, TGFβ1 exposure triggered DCPIB-sensitive chloride conductance. These results suggest that LRRC8/VRAC, due to its native permeability to GSH and thus its ability to modulate ROS levels, plays a critical role in EMT and might contribute to other physiological and pathophysiological processes associated with oxidative stress.

The LRRC8 volume-regulated anion channel inhibitor, DCPIB, inhibits mitochondrial respiration independently of the channel

There has been a resurgence of interest in the volume-regulated anion channel (VRAC) since the recent cloning of the LRRC8A-E gene family that encodes VRAC. The channel is a heteromer comprised of LRRC8A and at least one other family member; disruption of LRRC8A expression abolishes VRAC activity. The best-in-class VRAC inhibitor, DCPIB, suffers from off-target activity toward several different channels and transporters. Considering that some anion channel inhibitors also suppress mitochondrial respiration, we systematically explored whether DCPIB inhibits respiration in wild type (WT) and LRRC8A-knockout HAP-1 and HEK-293 cells. Knockout of LRRC8A had no apparent effects on cell morphology, proliferation rate, mitochondrial content, or expression of several mitochondrial genes in HAP-1 cells. Addition of 10 µM DCPIB, a concentration typically used to inhibit VRAC, suppressed basal and ATP-linked respiration in part through uncoupling the inner mitochondrial membrane (IMM) proton gradient and membrane potential. Additionally, DCPIB inhibits the activity of complex I, II, and III of the electron transport chain (ETC). Surprisingly, the effects of DCPIB on mitochondrial function are also observed in HAP-1 and HEK-293 cells which lack LRRC8A expression. Finally, we demonstrate that DCPIB activates ATP-inhibitable potassium channels comprised of heterologously expressed Kir6.2 and SUR1 subunits. These data indicate that DCPIB suppresses mitochondrial respiration and ATP production by dissipating the mitochondrial membrane potential and inhibiting complexes I-III of the ETC. They further justify the need for the development of sharper pharmacological tools for evaluating the integrative physiology and therapeutic potential of VRAC in human diseases.

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.

High potassium exposure reveals the altered ability of astrocytes to regulate their volume in the aged hippocampus of GFAP/EGFP mice

In this study, we focused on age-related changes in astrocyte functioning, predominantly on the ability of astrocytes to regulate their volume in response to a pathological stimulus, namely extracellular 50 mM K⁺ concentration. The aim of our project was to identify changes in the expression and function of transport proteins in the astrocytic membrane and properties of the extracellular space, triggered by aging. We used three-dimensional confocal morphometry, gene expression profiling, immunohistochemical analysis, and diffusion measurement in the hippocampal slices from 3-, 9-, 12-, and 18-month-old mice, in which astrocytes are visualized by enhanced green fluorescent protein under the control of the promoter for human glial fibrillary acidic protein. Combining a pharmacological approach and the quantification of astrocyte volume changes evoked by hyperkalemia, we found that marked diversity in the extent of astrocyte swelling in the hippocampus during aging is due to the gradually declining participation of Na⁺-K⁺-Cl^- transporters, glutamate transporters (glutamate aspartate transporter and glutamate transporter 1), and volume-regulated anion channels. Interestingly, there was a redistribution of Na⁺-K⁺-Cl^- cotransporter and glutamate transporters from astrocytic soma to processes. In addition, immunohistochemical analysis confirmed an age-dependent decrease in the content of Na⁺-K⁺-Cl^- cotransporter in astrocytes. The overall extracellular volume changes revealed a similar age-dependent diversity during hyperkalemia as observed in astrocytes. In addition, the recovery of the extracellular space was markedly impaired in aged animals.

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.

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.

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.

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.

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.

TMEM16B determines cholecystokinin sensitivity of intestinal vagal afferents of nodose neurons

The satiety effects and metabolic actions of cholecystokinin (CCK) have been recognized as potential therapeutic targets in obesity for decades. We identified a potentially novel Ca2+-activated chloride (Cl-) current (CaCC) that is induced by CCK in intestinal vagal afferents of nodose neurons. The CaCC subunit Anoctamin 2 (Ano2/TMEM16B) is the dominant contributor to this current. Its expression is reduced, as is CCK current activity in obese mice on a high-fat diet (HFD). Reduced expression of TMEM16B in the heterozygote KO of the channel in sensory neurons results in an obese phenotype with a loss of CCK sensitivity in intestinal nodose neurons, a loss of CCK-induced satiety, and metabolic changes, including decreased energy expenditure. The effect on energy expenditure is further supported by evidence in rats showing that CCK enhances sympathetic nerve activity and thermogenesis in brown adipose tissue, and these effects are abrogated by a HFD and vagotomy. Our findings reveal that Ano2/TMEM16B is a Ca2+-activated chloride channel in vagal afferents of nodose neurons and a major determinant of CCK-induced satiety, body weight control, and energy expenditure, making it a potential therapeutic target in obesity.

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.

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.

Fluorescence-Based High Throughput Screening Technologies for Natural Chloride Ion Channel Blockers

Chloride channels represent a group of potential drug targets; their blockers showed significant protecting effect on impaired cells by modulating apoptosis, autophagy, and other cell signals. However, clinical drugs with chloride channel inhibitory properties have not yet been developed. Natural product extract becomes an underlying candidate satisfied the clinical requirements for its low toxicity, low cost, and abundant sources. Here, a fluorescence-based EYFP-H148Q/I153L-HeLa cell line model was constructed by molecular cloning and verified by real-time polymerase chain reaction and Western blotting assay. By using this chloride channel blocker screening model, seven hit compounds chosen from 6988 natural compounds showed the channel blocking activity. Then the hit compounds were further validated by electrophysiological patch-clamp analysis. Our study preliminarily identified PC-4 as a new chloride channel inhibitor and demonstrated the reliability and sensitivity of fluorescence-based high throughput screening technologies for discovery of biologically active compounds from natural herbal compounds.

Cell Volume Control in Healthy Brain and Neuropathologies

Regulation of cellular volume is a critical homeostatic process that is intimately linked to ionic and osmotic balance in the brain tissue. Because the brain is encased in the rigid skull and has a very complex cellular architecture, even minute changes in the volume of extracellular and intracellular compartments have a very strong impact on tissue excitability and function. The failure of cell volume control is a major feature of several neuropathologies, such as hyponatremia, stroke, epilepsy, hyperammonemia, and others. There is strong evidence that such dysregulation, especially uncontrolled cell swelling, plays a major role in adverse pathological outcomes. To protect themselves, brain cells utilize a variety of mechanisms to maintain their optimal volume, primarily by releasing or taking in ions and small organic molecules through diverse volume-sensitive ion channels and transporters. In principle, the mechanisms of cell volume regulation are not unique to the brain and share many commonalities with other tissues. However, because ions and some organic osmolytes (e.g., major amino acid neurotransmitters) have a strong impact on neuronal excitability, cell volume regulation in the brain is a surprisingly treacherous process, which may cause more harm than good. This topical review covers the established and emerging information in this rapidly developing area of physiology.

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.

LRRC8A is essential for swelling-activated chloride current and for regulatory volume decrease in astrocytes

Consolidated evidence indicates that astroglial cells are critical in the homeostatic regulation of cellular volume by means of ion channels and aquaporin-4. Volume-regulated anion channel (VRAC) is the chloride channel that is activated upon cell swelling and critically contributes to cell volume regulation in astrocytes. The molecular identity of VRAC has been recently defined, revealing that it belongs to the leucine-rich repeat-containing 8 (LRRC8) protein family. However, there is a lack of evidence demonstrating that LRRC8A underpins VRAC currents in astrocyte. Nonetheless, direct evidence of the role of LRRC8A in astrocytic regulatory volume decrease remains to be proved. Here, we aim to bridge this gap in knowledge by combining RNA interference specific for LRRC8A with patch-clamp analyses and a water-permeability assay. We demonstrated that LRRC8A molecular expression is essential for swelling-activated chloride current via VRAC in primary-cultured cortical astrocytes. The knockdown of LRRC8A with a specific short interference RNA abolished the recovery of the cell volume after swelling induced by hypotonic challenge. In addition, immunoblotting, immunofluorescence, confocal imaging, and immunogold electron microscopy demonstrated that LRRC8A is expressed in the plasma membrane of primary cortical astrocytes and in situ in astrocytes at the perivascular interface with endothelial cells. Collectively, our results suggest that LRRC8A is an essential subunit of VRAC and a key factor for astroglial volume homeostasis.-Formaggio, F., Saracino, E., Mola, M. G., Rao, S. B., Amiry-Moghaddam, M., Muccini, M., Zamboni, R., Nicchia, G. P., Caprini, M., Benfenati, V. LRRC8A is essential for swelling-activated chloride current and for regulatory volume decrease in astrocytes.

SWELL1 is a glucose sensor regulating β-cell excitability and systemic glycaemia

Insulin secretion is initiated by activation of voltage-gated Ca²⁺ channels (VGCC) to trigger Ca²⁺-mediated insulin vesicle fusion with the β-cell plasma membrane. The firing of VGCC requires β-cell membrane depolarization, which is regulated by a balance of depolarizing and hyperpolarizing ionic currents. Here, we show that SWELL1 mediates a swell-activated, depolarizing chloride current (I_Cl,SWELL) in both murine and human β-cells. Hypotonic and glucose-stimulated β-cell swelling activates SWELL1-mediated I_Cl,SWELL and this contributes to membrane depolarization and activation of VGCC-dependent intracellular calcium signaling. SWELL1 depletion in MIN6 cells and islets significantly impairs glucose-stimulated insulin secretion. Tamoxifen-inducible β-cell-targeted Swell1 KO mice have normal fasting serum glucose and insulin levels but impaired glucose-stimulated insulin secretion and glucose tolerance; and this is further exacerbated in mild obesity. Our results reveal that β-cell SWELL1 modulates insulin secretion and systemic glycaemia by linking glucose-mediated β-cell swelling to membrane depolarization and activation of VGCC-triggered calcium signaling.

SWELL1 is a glucose sensor regulating β-cell excitability and systemic glycaemia

Insulin secretion is initiated by activation of voltage-gated Ca2+ channels (VGCC) to trigger Ca2+-mediated insulin vesicle fusion with the β-cell plasma membrane. The firing of VGCC requires β-cell membrane depolarization, which is regulated by a balance of depolarizing and hyperpolarizing ionic currents. Here, we show that SWELL1 mediates a swell-activated, depolarizing chloride current (ICl,SWELL) in both murine and human β-cells. Hypotonic and glucose-stimulated β-cell swelling activates SWELL1-mediated ICl,SWELL and this contributes to membrane depolarization and activation of VGCC-dependent intracellular calcium signaling. SWELL1 depletion in MIN6 cells and islets significantly impairs glucose-stimulated insulin secretion. Tamoxifen-inducible β-cell-targeted Swell1 KO mice have normal fasting serum glucose and insulin levels but impaired glucose-stimulated insulin secretion and glucose tolerance; and this is further exacerbated in mild obesity. Our results reveal that β-cell SWELL1 modulates insulin secretion and systemic glycaemia by linking glucose-mediated β-cell swelling to membrane depolarization and activation of VGCC-triggered calcium signaling.

Subunit-dependent oxidative stress sensitivity of LRRC8 volume-regulated anion channels

KEY POINTS

Swelling-activated anion currents are modulated by oxidative conditions, but it is unknown if oxidation acts directly on the LRRC8 channel-forming proteins or on regulatory factors. We found that LRRC8A-LRRC8E heteromeric channels are dramatically activated by oxidation of intracellular cysteines, whereas LRRC8A-LRRC8C and LRRC8A-LRRC8D heteromers are inhibited by oxidation. Volume-regulated anion currents in Jurkat T lymphocytes were inhibited by oxidation, in agreement with a low expression of the LRRC8E subunit in these cells. Our results show that LRRC8 channel proteins are directly modulated by oxidation in a subunit-specific manner.

ABSTRACT

The volume-regulated anion channel (VRAC) is formed by heteromers of LRRC8 proteins containing the essential LRRC8A subunit and at least one among the LRRC8B-E subunits. Reactive oxygen species (ROS) play physiological and pathophysiological roles and VRAC channels are highly ROS sensitive. However, it is unclear if ROS act directly on the channels or on molecules involved in the activation pathway. We used fluorescently tagged LRRC8 proteins that yield large constitutive currents to test direct effects of oxidation. We found that 8A/8E heteromers are dramatically potentiated (more than 10-fold) by oxidation of intracellular cysteine residues by chloramine-T or tert-butyl hydroperoxide. Oxidation was, however, not necessary for hypotonicity-induced activation. In contrast, 8A/8C and 8A/8D heteromers were strongly inhibited by oxidation. Endogenous VRAC currents in Jurkat T lymphocytes were similarly inhibited by oxidation, in agreement with the finding that LRRC8C and LRRC8D subunits were more abundantly expressed than LRRC8E in Jurkat cells. Our results show that LRRC8 channels are directly modulated by oxidation in a subunit-dependent manner.