Cell biology and physiology of CLC chloride channels and transporters

Chloride/proton antiporters ClC3 and ClC5 support bone formation in mice

Acid transport is required for bone synthesis by osteoblasts. The osteoblast basolateral surface extrudes acid by Na+/H+ exchange, but apical proton uptake is undefined. We found high expression of the Cl-/H+ exchanger ClC3 at the bone apical surface. In mammals ClC3 functions in intracellular vesicular chloride transport, but when we found Cl- dependency of H+ transport in osteoblast membranes, we queried whether ClC3 Cl-/H+ exchange functions in bone formation. We used ClC3 knockout animals, and closely-related ClC5 knockout animals: In vitro studies suggested that both ClC3 and ClC5 might support bone formation. Genotypes were confirmed by total exon sequences. Expression of ClC3, and to a lesser extent of ClC5, at osteoblast apical membranes was demonstrated by fluorescent antibody labeling and electron microscopy with nanometer gold labeling. Animals with ClC3 or ClC5 knockouts were viable. In ClC3 or ClC5 knockouts, bone formation decreased ~40 % by calcein and xylenol orange labeling in vivo. In very sensitive micro-computed tomography, ClC5 knockout reduced bone relative to wild type, consistent with effects of ClC3 knockout, but varied with specific histological parameters. Regrettably, ClC5-ClC3 double knockouts are not viable, suggesting that ClC3 or ClC5 activity are essential to life. We conclude that ClC3 has a direct role in bone formation with overlapping but probably slightly smaller effects of ClC5. The mechanism in mineral formation might include ClC H+ uptake, in contrast to ClC3 and ClC5 function in cell vesicles or other organs.

Expanding the genetic and phenotypic relevance of CLCN4 variants in neurodevelopmental condition: 13 new patients

OBJECTIVES

CLCN4 variations have recently been identified as a genetic cause of X-linked neurodevelopmental disorders. This study aims to broaden the phenotypic spectrum of CLCN4-related condition and correlate it with functional consequences of CLCN4 variants.

METHODS

We described 13 individuals with CLCN4-related neurodevelopmental disorder. We analyzed the functional consequence of the unreported variants using heterologous expression, biochemistry, confocal fluorescent microscopy, patch-clamp electrophysiology, and minigene splicing assay.

RESULTS

We identified five novel (p.R41W, p.L348V, p.G480R, p.R603W, c.1576 + 5G > A) and three known (p.T203I, p.V275M, p.A555V) pathogenic CLCN4 variants in 13 Chinese patients. The p.V275M variant is found at high frequency and seen in four unrelated individuals. All had global developmental delay (GDD)/intellectual disability (ID). Seizures were present in eight individuals, and 62.5% of them developed refractory epilepsy. Five individuals without seizures showed moderate to severe GDD/ID. Developmental delay precedes seizure onset in most patients. The variants p.R41W, p.L348V, and p.R603W compromise the anion/exchange function of ClC-4. p.R41W partially impairs ClC-3/ClC-4 association. p.G480R reduces ClC-4 expression levels and impairs the heterodimerization with ClC-3. The c.1576 + 5G > A variant causes 22 bp deletion of exon 10.

CONCLUSIONS

We further define and broaden the clinical and mutational spectrum of CLCN4-related neurodevelopmental conditions. The p.V275M variant may be a potential hotspot CLCN4 variant in Chinese patients. The five novel variants cause loss of function of ClC-4. Transport dysfunction, protein instability, intracellular trafficking defect, or failure of ClC-4 to oligomerize may contribute to the pathophysiological events leading to CLCN4-related neurodevelopmental disorder.

Effect of Roflumilast, a Selective PDE4 Inhibitor, on Bone Phenotypes in ADO2 Mice

Autosomal Dominant Osteopetrosis type II (ADO2) is a rare bone disease of impaired osteoclastic bone resorption that usually results from heterozygous missense mutations in the chloride channel 7 (CLCN7) gene. We previously created mouse models of ADO2 (p.G213R) with one of the most common mutations (G215R) as found in humans and demonstrated that this mutation in mice phenocopies the human disease of ADO2. Previous studies have shown that roflumilast (RF), a selective phosphodiesterase 4 (PDE4) inhibitor that regulates the cAMP pathway, can increase osteoclast activity. We also observed that RF increased bone resorption in both wild-type and ADO2 heterozygous osteoclasts in vitro, suggesting it might rescue bone phenotypes in ADO2 mice. To test this hypothesis, we administered RF-treated diets (0, 20 and 100 mg/kg) to 8-week-old ADO2 mice for 6 months. We evaluated bone mineral density and bone micro-architecture using longitudinal in-vivo DXA and micro-CT at baseline, and 6-, 12-, 18-, and 24-week post-baseline time points. Additionally, we analyzed serum bone biomarkers (CTX, TRAP, and P1NP) at baseline, 12-, and 24-week post-baseline. Our findings revealed that RF treatment did not improve aBMD (whole body, femur, and spine) and trabecular BV/TV (distal femur) in ADO2 mice compared to the control group treated with a normal diet. Furthermore, we did not observe any significant changes in serum levels of bone biomarkers due to RF treatment in these mice. Overall, our results indicate that RF does not rescue the osteopetrotic bone phenotypes in ADO2 heterozygous mice.

The PDE4 Inhibitors Roflumilast and Rolipram Rescue ADO2 Osteoclast Resorption Dysfunction

Autosomal Dominant Osteopetrosis type II (ADO2) is a rare bone disease of impaired osteoclastic bone resorption caused by heterozygous missense mutations in the chloride channel 7 (CLCN7). Adenylate cyclase, which catalyzes the formation of cAMP, is critical for lysosomal acidification in osteoclasts. We found reduced cAMP levels in ADO2 osteoclasts compared to wild-type (WT) osteoclasts, leading us to examine whether regulating cAMP would improve ADO2 osteoclast activity. Although forskolin, a known activator of adenylate cyclase and cAMP levels, negatively affected osteoclast number, it led to an overall increase in ADO2 and WT osteoclast resorption activity in vitro. Next, we examined cAMP hydrolysis by the phosphodiesterase 4 (PDE4) proteins in ADO2 versus WT osteoclasts. QPCR analysis revealed higher expression of the three major PDE4 subtypes (4a, 4b, 4d) in ADO2 osteoclasts compared in WT, consistent with reduced cAMP levels in ADO2 osteoclasts. In addition, we found that the PDE4 antagonists, rolipram and roflumilast, stimulated ADO2 and WT osteoclast formation in a dose-dependent manner. Importantly, roflumilast and rolipram displayed a concentration-dependent increase in osteoclast resorption activity which was greater in ADO2 than WT osteoclasts. Moreover, treatment with roflumilast rescued cAMP levels in ADO2 OCLs. The key findings from our studies demonstrate that osteoclasts from ADO2 mice exhibit reduced cAMP levels and PDE4 inhibition rescues cAMP levels and ADO2 osteoclast activity dysfunction in vitro. The mechanism of action of PDE4 inhibitors and their ability to reduce the high bone mass of ADO2 mice in vivo are currently under investigation. Importantly, these studies advance the understanding of the mechanisms underlying the ADO2 osteoclast dysfunction which is critical for the development of therapeutic approaches to treat clinically affected ADO2 patients.

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.

Neurotoxin-Derived Optical Probes for Biological and Medical Imaging

The superb specificity and potency of biological toxins targeting various ion channels and receptors are of major interest for the delivery of therapeutics to distinct cell types and subcellular compartments. Fused with reporter proteins or labelled with fluorophores and nanocomposites, animal toxins and their detoxified variants also offer expanding opportunities for visualisation of a range of molecular processes and functions in preclinical models, as well as clinical studies. This article presents state-of-the-art optical probes derived from neurotoxins targeting ion channels, with discussions of their applications in basic and translational biomedical research. It describes the design and production of probes and reviews their applications with advantages and limitations, with prospects for future improvements. Given the advances in imaging tools and expanding research areas benefiting from the use of optical probes, described here resources should assist the discovery process and facilitate high-precision interrogation and therapeutic interventions.

Novel variants in the CLCN4 gene associated with syndromic X-linked intellectual disability

Objective

The dysfunction of the CLCN4 gene can lead to X-linked intellectual disability and Raynaud-Claes syndrome (MRXSRC), characterized by severe cognitive impairment and mental disorders. This study aimed to investigate the genetic defects and clinical features of Chinese children with CLCN4 variants and explore the effect of mutant ClC-4 on the protein expression level and subcellular localization through in vitro experiments.

Methods

A total of 401 children with intellectual disabilities were screened for genetic variability using whole-exome sequencing (WES). Clinical data, including age, sex, perinatal conditions, and environmental exposure, were collected. Cognitive, verbal, motor, and social behavioral abilities were evaluated. Candidate variants were verified using Sanger sequencing, and their pathogenicity and conservation were analyzed using in silico prediction tools. Protein expression and localization of mutant ClC-4 were measured using Western blotting (WB) and immunofluorescence microscopy. The impact of a splice site variant was assessed with a minigene assay.

Results

Exome analysis identified five rare CLCN4 variants in six unrelated patients with intellectual disabilities, including two recurrent heterozygous de novo missense variants (p.D89N and p.A555V) in three female patients, and two hemizygous missense variants (p.N141S and p.R694Q) and a splicing variant (c.1390-12T > G) that are maternally inherited in three male patients. The p.N141S variant and the splicing variant c.1390-12(T > G were novel, while p.R694Q was identified in two asymptomatic heterozygous female patients. The six children with CLCN4 variants exhibited a neurodevelopmental spectrum disease characterized by intellectual disability (ID), delayed speech, autism spectrum disorders (ASD), microcephaly, hypertonia, and abnormal imaging findings. The minigene splicing result indicated that the c.1390-12T > G did not affect the splicing of CLCN4 mRNA. In vitro experiments showed that the mutant protein level and localization of mutant protein are similar to the wild type.

Conclusion

The study identified six probands with CLCN4 gene variants associated with X-linked ID. It expanded the gene and phenotype spectrum of CLCN4 variants. The bioinformatic analysis supported the pathogenicity of CLCN4 variants. However, these CLCN4 gene variants did not affect the ClC-4 expression levels and protein location, consistent with previous studies. Further investigations are necessary to investigate the pathogenetic mechanism.

Comprehensive analysis and experimental validation reveal elevated CLCN4 is a promising biomarker in endometrial cancer

Several studies have reported the role of CLCN4 in tumor progression. However, its mechanism remains to be thoroughly studied. The objective of this study was to explore the potential pathogenic role of CLCN4 in endometrial carcinoma (UCEC) with a better understanding of the pathological mechanisms involved. The potential roles of CLCN4 in different tumors were explored based on The Cancer Genome Atlas (TCGA), the expression difference, mutation, survival, pathological stage, Immunity subtypes, Immune infiltration, tumor microenvironment (TME), tumor mutation burden (TMB), microsatellite instability (MSI), mismatch repair (MMR) related to CLCN4 were analyzed. Then, the expression, prognosis, mutation, and functional enrichment of CLCN4 in UCEC were analyzed. Immunohistochemical experiment was used to verify the expression of CLCN4 in endometrial cancer tissues and normal tissues. In vitro, we knocked down of CLCN4 in HEC-1-A cells and performed CCK8, WB, RT-PCR, wound-healing, transwell assays to further validation of the molecular function. Results revealed that high expression of CLCN4 was observed in 20 cancer types of TCGA. CLCN4 expression correlates with poor survival in MESO, BLCA, THCA, especially UCEC tumors. CLCN4 expression was significantly associated with CD4+ T-cell infiltration, especially CD4+ Th1-cell. Immunohistochemical experiment reveals that CLCN4 is high expressed in endometrial tumors, in vitro experiment reveals that knockdown of CLCN4 inhibits the cells proliferation, migration and invasion. Our study is the first to offer a comprehensive understanding of the oncogenic roles of CLCN4 on different tumors. CLCN4 may become a potential biomarker in UCEC.

Potassium homeostasis - Physiology and pharmacology in a clinical context

Membrane voltage controls the function of excitable cells and is mainly a consequence of the ratio between the extra- and intracellular potassium concentration. Potassium homeostasis is safeguarded by balancing the extra-/intracellular distribution and systemic elimination of potassium to the dietary potassium intake. These processes adjust the plasma potassium concentration between 3.5 and 4.5 mmol/L. Several genetic and acquired diseases but also pharmacological interventions cause dyskalemias that are associated with increased morbidity and mortality. The thresholds at which serum K+ not only associates but also causes increased mortality are hotly debated. We discuss physiologic, pathophysiologic, and pharmacologic aspects of potassium regulation and provide informative case vignettes. Our aim is to help clinicians, epidemiologists, and pharmacologists to understand the complexity of the potassium homeostasis in health and disease and to initiate appropriate treatment strategies in dyskalemic patients.

Theoretical framework and experimental solution for the air-water interface adsorption problem in cryoEM

As cryogenic electron microscopy (cryoEM) gains traction in the structural biology community as a method of choice for determining atomic structures of biological complexes, it has been increasingly recognized that many complexes that behave well under conventional negative-stain electron microscopy tend to have preferential orientation, aggregate or simply mysteriously "disappear" on cryoEM grids. However, the reasons for such misbehavior are not well understood, which limits systematic approaches to solving the problem. Here, we have developed a theoretical formulation that explains these observations. Our formulation predicts that all particles migrate to the air-water interface (AWI) to lower the total potential surface energy-rationalizing the use of surfactant, which is a direct solution to reduce the surface tension of the aqueous solution. By performing cryogenic electron tomography (cryoET) on the widely-tested sample, GroEL, we demonstrate that, in a standard buffer solution, nearly all particles migrate to the AWI. Gradually reducing the surface tension by introducing surfactants decreased the percentage of particles exposed to the surface. By conducting single-particle cryoEM, we confirm that suitable surfactants do not damage the biological complex, thus suggesting that they might provide a practical, simple, and general solution to the problem for high-resolution cryoEM. Applying this solution to a real-world AWI adsorption problem involving a more challenging membrane protein, namely, the ClC-1 channel, has resulted in its near-atomic structure determination using cryoEM.

Biophysical Aspects of Neurodegenerative and Neurodevelopmental Disorders Involving Endo-/Lysosomal CLC Cl-/H+ Antiporters

Endosomes and lysosomes are intracellular vesicular organelles with important roles in cell functions such as protein homeostasis, clearance of extracellular material, and autophagy. Endolysosomes are characterized by an acidic luminal pH that is critical for proper function. Five members of the gene family of voltage-gated ChLoride Channels (CLC proteins) are localized to endolysosomal membranes, carrying out anion/proton exchange activity and thereby regulating pH and chloride concentration. Mutations in these vesicular CLCs cause global developmental delay, intellectual disability, various psychiatric conditions, lysosomal storage diseases, and neurodegeneration, resulting in severe pathologies or even death. Currently, there is no cure for any of these diseases. Here, we review the various diseases in which these proteins are involved and discuss the peculiar biophysical properties of the WT transporter and how these properties are altered in specific neurodegenerative and neurodevelopmental disorders.

Quantitative elemental imaging in eukaryotic algae

All organisms, fundamentally, are made from the same raw material, namely the elements of the periodic table. Biochemical diversity is achieved by how these elements are utilized, for what purpose, and in which physical location. Determining elemental distributions, especially those of trace elements that facilitate metabolism as cofactors in the active centers of essential enzymes, can determine the state of metabolism, the nutritional status, or the developmental stage of an organism. Photosynthetic eukaryotes, especially algae, are excellent subjects for quantitative analysis of elemental distribution. These microbes utilize unique metabolic pathways that require various trace nutrients at their core to enable their operation. Photosynthetic microbes also have important environmental roles as primary producers in habitats with limited nutrient supplies or toxin contaminations. Accordingly, photosynthetic eukaryotes are of great interest for biotechnological exploitation, carbon sequestration, and bioremediation, with many of the applications involving various trace elements and consequently affecting their quota and intracellular distribution. A number of diverse applications were developed for elemental imaging, allowing subcellular resolution, with X-ray fluorescence microscopy (XFM, XRF) being at the forefront, enabling quantitative descriptions of intact cells in a non-destructive method. This Tutorial Review summarizes the workflow of a quantitative, single-cell elemental distribution analysis of a eukaryotic alga using XFM.

Ion channels as a therapeutic target for renal fibrosis

Renal ion channel transport and electrolyte disturbances play an important role in the process of functional impairment and fibrosis in the kidney. It is well known that there are limited effective drugs for the treatment of renal fibrosis, and since a large number of ion channels are involved in the renal fibrosis process, understanding the mechanisms of ion channel transport and the complex network of signaling cascades between them is essential to identify potential therapeutic approaches to slow down renal fibrosis. This review summarizes the current work of ion channels in renal fibrosis. We pay close attention to the effect of cystic fibrosis transmembrane conductance regulator (CFTR), transmembrane Member 16A (TMEM16A) and other Cl− channel mediated signaling pathways and ion concentrations on fibrosis, as well as the various complex mechanisms for the action of Ca2+ handling channels including Ca2+-release-activated Ca2+ channel (CRAC), purinergic receptor, and transient receptor potential (TRP) channels. Furthermore, we also focus on the contribution of Na+ transport such as epithelial sodium channel (ENaC), Na+, K+-ATPase, Na+-H+ exchangers, and K+ channels like Ca2+-activated K+ channels, voltage-dependent K+ channel, ATP-sensitive K+ channels on renal fibrosis. Proposed potential therapeutic approaches through further dissection of these mechanisms may provide new therapeutic opportunities to reduce the burden of chronic kidney disease.

Genetic spectrum and founder effect of non-dystrophic myotonia: a Japanese case series study

Non-dystrophic myotonias (NDM) are rare skeletal muscle channelopathies, mainly linked to two voltage-gated ion channel genes, CLCN1 and SCN4A. The aim of this study is to identify the clinical and genetic features of patients with NDM in Japan. We collected a Japanese nationwide case series of patients with clinical diagnosis of NDM (1999-2021). Among 71 out of 88 pedigrees, using Sanger and next-generation sequencing targeting both CLCN1 and SCN4A genes, variants classified as pathogenic/likely pathogenic/unknown significance were detected from CLCN1 (31 probands), SCN4A (36 probands), or both genes (4 probands), and 11 of them were novel. Pedigrees carrying mono-allelic CLCN1 variants were more commonly seen than that with bi-allelic/double variants (24:7). Compared to patients with CLCN1 variants, patients harboring SCN4A variants showed younger onset age (5.64 ± 4.70 years vs. 9.23 ± 5.21 years), fewer warm-up phenomenon, but more paramyotonia, hyperCKemia, transient muscle weakness, and cold-induced myotonia. Haplotype analysis verified founder effects of the hot spot variants in both CLCN1 (p.T539A) and SCN4A (p.T1313M). This study reveals variants in CLCN1 and SCN4A from 80.7% of our case series, extending genetic spectrum of NDM, and would further our understanding of clinical similarity/diversity between CLCN1- and SCN4A-related NDM, as well as the genetic racial differences.

Anoctamin 1 controls bone resorption by coupling Cl- channel activation with RANKL-RANK signaling transduction

Osteoclast over-activation leads to bone loss and chloride homeostasis is fundamental importance for osteoclast function. The calcium-activated chloride channel Anoctamin 1 (also known as TMEM16A) is an important chloride channel involved in many physiological processes. However, its role in osteoclast remains unresolved. Here, we identified the existence of Anoctamin 1 in osteoclast and show that its expression positively correlates with osteoclast activity. Osteoclast-specific Anoctamin 1 knockout mice exhibit increased bone mass and decreased bone resorption. Mechanistically, Anoctamin 1 deletion increases intracellular Cl^- concentration, decreases H⁺ secretion and reduces bone resorption. Notably, Anoctamin 1 physically interacts with RANK and this interaction is dependent upon Anoctamin 1 channel activity, jointly promoting RANKL-induced downstream signaling pathways. Anoctamin 1 protein levels are substantially increased in osteoporosis patients and this closely correlates with osteoclast activity. Finally, Anoctamin 1 deletion significantly alleviates ovariectomy induced osteoporosis. These results collectively establish Anoctamin 1 as an essential regulator in osteoclast function and suggest a potential therapeutic target for osteoporosis.

The Role of the Lysosomal Cl−/H+ Antiporter ClC-7 in Osteopetrosis and Neurodegeneration

CLC proteins comprise Cl⁻ channels and anion/H⁺ antiporters involved in several fundamental physiological processes. ClC-7 is a lysosomal Cl⁻/H⁺ antiporter that together with its beta subunit Ostm1 has a critical role in the ionic homeostasis of lysosomes and of the osteoclasts’ resorption lacuna, although the specific underlying mechanism has so far remained elusive. Mutations in ClC-7 cause osteopetrosis, but also a form of lysosomal storage disease and neurodegeneration. Interestingly, both loss-of- and gain-of-function mutations of ClC-7 can be pathogenic, but the mechanistic implications of this finding are still unclear. This review will focus on the recent advances in our understanding of the biophysical properties of ClC-7 and of its role in human diseases with a focus on osteopetrosis and neurodegeneration.

Identification and validation of a signature involving voltage-gated chloride ion channel genes for prediction of prostate cancer recurrence

Voltage-gated chloride ion channels (CLCs) are transmembrane proteins that maintain chloride ion homeostasis in various cells. Accumulating studies indicated CLCs were related to cell growth, proliferation, and cell cycle. Nevertheless, the role of CLCs in prostate cancer (PCa) has not been systematically profiled. The purpose of this study was to investigate the expression profiles and biofunctions of CLCs genes, and construct a novel risk signature to predict biochemical recurrence (BCR) of PCa patients. We identified five differentially expressed CLCs genes in our cohort and then constructed a signature composed of CLCN2 and CLCN6 through Lasso-Cox regression analysis in the training cohort from the Cancer Genome Atlas (TCGA). The testing and entire cohorts from TCGA and the GSE21034 from the Gene Expression Omnibus (GEO) were used as internal and independent external validation datasets. This signature could divide PCa patients into the high and low risk groups with different prognoses, was apparently correlated with clinical features, and was an independent excellent prognostic indicator. Enrichment analysis indicated our signature was primarily concentrated in cellular process and metabolic process. The expression patterns of CLCN2 and CLCN6 were detected in our own cohort based immunohistochemistry staining, and we found CLCN2 and CLCN6 were highly expressed in PCa tissues compared with benign tissues and positively associated with higher Gleason score and shorter BCR-free time. Functional experiments revealed that CLCN2 and CLCN6 downregulation inhibited cell proliferation, colony formation, invasion, and migration, but prolonged cell cycle and promoted apoptosis. Furthermore, Seahorse assay showed that silencing CLCN2 or CLCN6 exerted potential inhibitory effects on energy metabolism in PCa. Collectively, our signature could provide a novel and robust strategy for the prognostic evaluation and improve treatment decision making for PCa patients.

The Role of Chloride Channels in the Multidrug Resistance

Nowadays, one of medicine's main and most challenging aims is finding effective ways to treat cancer. Unfortunately, although there are numerous anti-cancerous drugs, such as cisplatin, more and more cancerous cells create drug resistance. Thus, it is equally important to find new medicines and research the drug resistance phenomenon and possibilities to avoid this mechanism. Ion channels, including chloride channels, play an important role in the drug resistance phenomenon. Our article focuses on the chloride channels, especially the volume-regulated channels (VRAC) and CLC chloride channels family. VRAC induces multidrug resistance (MDR) by causing apoptosis connected with apoptotic volume decrease (AVD) and VRAC are responsible for the transport of anti-cancerous drugs such as cisplatin. VRACs are a group of heterogenic complexes made from leucine-rich repetition with 8A (LRRC8A) and a subunit LRRC8B-E responsible for the properties. There are probably other subunits, which can create those channels, for example, TTYH1 and TTYH2. It is also known that the ClC family is involved in creating MDR in mainly two mechanisms-by changing the cell metabolism or acidification of the cell. The most researched chloride channel from this family is the CLC-3 channel. However, other channels are playing an important role in inducing MDR as well. In this paper, we review the role of chloride channels in MDR and establish the role of the channels in the MDR phenomenon.

CLN7 is an organellar chloride channel regulating lysosomal function

Neuronal ceroid lipofuscinoses (NCLs) are a group of autosomal recessive lysosomal storage diseases. One variant form of late-infantile NCL (vLINCL) is caused by mutations of a lysosomal membrane protein CLN7, the function of which has remained unknown. Here, we identified CLN7 as a novel endolysosomal chloride channel. Overexpression of CLN7 increases endolysosomal chloride currents and enlarges endolysosomes through a Ca²⁺/calmodulin-dependent way. Human CLN7 and its yeast homolog exhibit characteristics of chloride channels and are sensitive to chloride channel blockers. Moreover, CLN7 regulates lysosomal chloride conductance, luminal pH, and lysosomal membrane potential and promotes the release of lysosomal Ca²⁺ through transient receptor potential mucolipin 1 (TRPML1). Knocking out CLN7 causes pathological features that are similar to those of patients with vLINCL, including retinal degeneration and autofluorescent lipofuscin. The pathogenic mutations in CLN7 lead to a decrease in chloride permeability, suggesting that reconstitution of lysosomal Cl⁻ homeostasis may be an effective strategy for the treatment of vLINCL.

Chloride transport modulators as drug candidates

Chloride transport across cell membranes is broadly involved in epithelial fluid transport, cell volume and pH regulation, muscle contraction, membrane excitability, and organellar acidification. The human genome encodes at least 53 chloride-transporting proteins with expression in cell plasma or intracellular membranes, which include chloride channels, exchangers, and cotransporters, some having broad anion specificity. Loss-of-function mutations in chloride transporters cause a wide variety of human diseases, including cystic fibrosis, secretory diarrhea, kidney stones, salt-wasting nephropathy, myotonia, osteopetrosis, hearing loss, and goiter. Although impactful advances have been made in the past decade in drug treatment of cystic fibrosis using small molecule modulators of the defective cystic fibrosis transmembrane conductance regulator (CFTR) chloride channel, other chloride channels and solute carrier proteins (SLCs) represent relatively underexplored target classes for drug discovery. New opportunities have emerged for the development of chloride transport modulators as potential therapeutics for secretory diarrheas, constipation, dry eye disorders, kidney stones, polycystic kidney disease, hypertension, and osteoporosis. Approaches to chloride transport-targeted drug discovery are reviewed herein, with focus on chloride channel and exchanger classes in which recent preclinical advances have been made in the identification of small molecule modulators and in proof of concept testing in experimental animal models.

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

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

Exogenous transforming growth factor-β1 prevents the inflow of fluoride to ameleoblasts through regulation of voltage-gated chloride channels 5 and 7

Dental fluorosis is a global issue. Although there are multiple causes of dental fluorosis, the precise mechanism remains controversial. Previous studies have demonstrated that extracellular fluoride may promote an accumulation of fluoride ions in ameloblasts, which may induce oxidative and endoplasmic reticulum stresses, leading to dental fluorosis. However, the exact process by which fluoride ions enter cells has not been determined. In the present study, intracellular fluoride concentration was determined using a newly developed specific fluorescent probe called probe 1. Under high extracellular fluoride concentrations, the fluorescence intensity of the ameloblasts increased, however, exogenous transforming growth factor-β1 (TGF-β1) was able to inhibit the increase. Furthermore, changes in the expression of the voltage-gated chloride channels 5 and 7 (ClC5 and ClC-7), which are responsible for the transport of fluoride were investigated. The results indicated that fluoride reduced the expression of endogenous TGF-β1 and increased the expression of ClC-5 and ClC-7. Additionally, exogenous TGF-β1 reduced the expression of ClC-5 and ClC-7. The results of the present study indicate that exogenous TGF-β1 may prevent accumulation of fluoride in ameloblasts through the regulation of ClC-5 and ClC-7 under high extracellular fluoride concentrations.

Neurodegeneration Upon Dysfunction of Endosomal/Lysosomal CLC Chloride Transporters

The regulation of luminal ion concentrations is critical for the function of, and transport between intracellular organelles. The importance of the acidic pH in the compartments of the endosomal-lysosomal pathway has been well-known for decades. Besides the V-ATPase, which pumps protons into their lumen, a variety of ion transporters and channels is involved in the regulation of the organelles' complex ion homeostasis. Amongst these are the intracellular members of the CLC family, ClC-3 through ClC-7. They localize to distinct but overlapping compartments of the endosomal-lysosomal pathway, partially with tissue-specific expression. Functioning as 2Cl⁻/H⁺ exchangers, they can support the vesicular acidification and accumulate luminal Cl⁻. Mutations in the encoding genes in patients and mouse models underlie severe phenotypes including kidney stones with CLCN5 and osteopetrosis or hypopigmentation with CLCN7. Dysfunction of those intracellular CLCs that are expressed in neurons lead to neuronal defects. Loss of endosomal ClC-3, which heteromerizes with ClC-4, results in neurodegeneration. Mutations in ClC-4 are associated with epileptic encephalopathy and intellectual disability. Mice lacking the late endosomal ClC-6 develop a lysosomal storage disease with reduced pain sensitivity. Human gene variants have been associated with epilepsy, and a gain-of-function mutation causes early-onset neurodegeneration. Dysfunction of the lysosomal ClC-7 leads to a lysosomal storage disease and neurodegeneration in mice and humans. Reduced luminal chloride, as well as altered calcium regulation, has been associated with lysosomal storage diseases in general. This review discusses the properties of endosomal and lysosomal Cl⁻/H⁺ exchange by CLCs and how various alterations of ion transport by CLCs impact organellar ion homeostasis and function in neurodegenerative disorders.

Roles of the ClC chloride channel CLH-1 in food-associated salt chemotaxis behavior of C. elegans

The ability of animals to process dynamic sensory information facilitates foraging in an ever-changing environment. However, molecular and neural mechanisms underlying such ability remain elusive. The ClC anion channels/transporters play a pivotal role in cellular ion homeostasis across all phyla. Here, we find a ClC chloride channel is involved in salt concentration chemotaxis of Caenorhabditis elegans. Genetic screening identified two altered-function mutations of clh-1 that disrupt experience-dependent salt chemotaxis. Using genetically encoded fluorescent sensors, we demonstrate that CLH-1 contributes to regulation of intracellular anion and calcium dynamics of salt-sensing neuron, ASER. The mutant CLH-1 reduced responsiveness of ASER to salt stimuli in terms of both temporal resolution and intensity, which disrupted navigation strategies for approaching preferred salt concentrations. Furthermore, other ClC genes appeared to act redundantly in salt chemotaxis. These findings provide insights into the regulatory mechanism of neuronal responsivity by ClCs that contribute to modulation of navigation behavior.

Determinants of Phagosomal pH During Host-Pathogen Interactions

The ability of phagosomes to halt microbial growth is intimately linked to their ability to acidify their luminal pH. Establishment and maintenance of an acidic lumen requires precise co-ordination of H⁺ pumping and counter-ion permeation to offset the countervailing H⁺ leakage. Despite the best efforts of professional phagocytes, however, a number of specialized pathogens survive and even replicate inside phagosomes. In such instances, pathogens target the pH-regulatory machinery of the host cell in an effort to survive inside or escape from phagosomes. This review aims to describe how phagosomal pH is regulated during phagocytosis, why it varies in different types of professional phagocytes and the strategies developed by prototypical intracellular pathogens to manipulate phagosomal pH to survive, replicate, and eventually escape from the phagocyte.

Determinants of Phagosomal pH During Host-Pathogen Interactions

The ability of phagosomes to halt microbial growth is intimately linked to their ability to acidify their luminal pH. Establishment and maintenance of an acidic lumen requires precise co-ordination of H⁺ pumping and counter-ion permeation to offset the countervailing H⁺ leakage. Despite the best efforts of professional phagocytes, however, a number of specialized pathogens survive and even replicate inside phagosomes. In such instances, pathogens target the pH-regulatory machinery of the host cell in an effort to survive inside or escape from phagosomes. This review aims to describe how phagosomal pH is regulated during phagocytosis, why it varies in different types of professional phagocytes and the strategies developed by prototypical intracellular pathogens to manipulate phagosomal pH to survive, replicate, and eventually escape from the phagocyte.

Osteoclast Fusion: Physiological Regulation of Multinucleation through Heterogeneity-Potential Implications for Drug Sensitivity

Classically, osteoclast fusion consists of four basic steps: (1) attraction/migration, (2) recognition, (3) cell-cell adhesion, and (4) membrane fusion. In theory, this sounds like a straightforward simple linear process. However, it is not. Osteoclast fusion has to take place in a well-coordinated manner-something that is not simple. In vivo, the complex regulation of osteoclast formation takes place within the bone marrow-in time and space. The present review will focus on considering osteoclast fusion in the context of physiology and pathology. Special attention is given to: (1) regulation of osteoclast fusion in vivo, (2) heterogeneity of osteoclast fusion partners, (3) regulation of multi-nucleation, (4) implications for physiology and pathology, and (5) implications for drug sensitivity and side effects. The review will emphasize that more attention should be given to the human in vivo reality when interpreting the impact of in vitro and animal studies. This should be done in order to improve our understanding of human physiology and pathology, as well as to improve anti-resorptive treatment and reduce side effects.

Chloride channel 7 protects from redox status impairment-induced renal tubular epithelial cell apoptosis by activating autophagy

AIM

Chloride channel 7 (CLC-7), broadly expressed in kidney tissues, affects the lysosome degradation pathway. And redox status impairment contributes to cell apoptosis and activates autophagy flux. This study mainly investigates the role and molecular mechanism of CLC-7 in redox status impairment-induced autophagic flux and apoptosis.

MAIN METHODS

When NRK52E cells, rat renal tubular epithelial cells, were exposed to H₂O₂ treatment, apoptosis, autophagy flux, and CLC-7 expression were detected. Further investigation was done to observe the change of apoptosis and autophagy flux in renal cells under overexpression or knocking down of CLC-7. The lysosomes acidity, lysosome enzyme Cathepsin D activity and phosphorylation of Ampk/mTOR were also examined when CLC-7 was overexpressed or knocked down.

KEY FINDINGS

Redox status impairment induced apoptosis and autophagy flux in NRK52E cells and upregulated CLC-7. Overexpression of CLC-7 increased lysosome acidity and Cathepsin D activity. In cells with CLC-7 overexpression, we observed a significant increase of autophagy flux and decline of apoptosis, as well as an apparent increase of p-Ampk and decrease of p-mTOR. On the contrary, cells with knocking down CLC-7 led to opposite results.

SIGNIFICANCES

CLC-7 is essential to maintain and enhance acidity and enzyme activity in lysosome. Through activating autophagy flux, it exerts survival against renal tubular epithelial cell apoptosis induced by redox status impairment. Its function to modulate Ampk/mTOR pathway is the possible reason why CLC-7 can trigger autophagy flux.

Physiological and pathophysiological role of ion channels and transporters in the colorectum and colorectal cancer

The incidence of colorectal cancer has increased annually, and the pathogenesis of this disease requires further investigation. In normal colorectal tissues, ion channels and transporters maintain the water‐electrolyte balance and acid/base homeostasis. However, dysfunction of these ion channels and transporters leads to the development and progression of colorectal cancer. Therefore, this review focuses on the progress in understanding the roles of ion channels and transporters in the colorectum and in colorectal cancer, including aquaporins (AQPs), Cl⁻ channels, Cl⁻/HCO3‐ exchangers, Na⁺/HCO3‐ transporters and Na⁺/H⁺ exchangers. The goal of this review is to promote the identification of new targets for the treatment and prognosis of colorectal cancer.

CLIC2α Chloride Channel Orchestrates Immunomodulation of Hemocyte Phagocytosis and Bactericidal Activity in Crassostrea gigas

Chloride ion plays critical roles in modulating immunological interactions. Herein, we demonstrated that the anion channel CLIC2α mediates Cl⁻ flux to regulate hemocytes functions in the Pacific oyster (Crassostrea gigas). Specifically, during infection by Vibrio parahemolyticus, chloride influx was activated following onset of phagocytosis. Phosphorylation of Akt was stimulated by Cl⁻ ions entering host cells, further contributing to signal transduction regulating internalization of bacteria through the PI3K/Akt signaling pathway. Concomitantly, Cl⁻ entered phagosomes, promoted the acidification and maturation of phagosomes, and contributed to production of HOCl to eradicate engulfed bacteria. Finally, genomic screening reveals CLIC2α as a major Cl⁻ channel gene responsible for regulating Cl⁻ influx in oysters. Knockdown of CLIC2α predictably impeded phagosome acidification and restricted bacterial killing in oysters. In conclusion, our work has established CLIC2α as a prominent regulator of Cl⁻ influx and thus Cl⁻ function in C. gigas in bacterial infection contexts.

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Influx of chloride ions is switched on during phagocytosis in oyster hemocytes

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PI3K/Akt signaling pathway mediates chloride-dependent activation of phagocytosis

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Cl⁻ promotes phagosomal acidification and HOCl production

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CLIC2α is the principal chloride channel encoding gene within oyster genome

Divergent Cl- and H+ pathways underlie transport coupling and gating in CLC exchangers and channels

The CLC family comprises H⁺-coupled exchangers and Cl^- channels, and mutations causing their dysfunction lead to genetic disorders. The CLC exchangers, unlike canonical 'ping-pong' antiporters, simultaneously bind and translocate substrates through partially congruent pathways. How ions of opposite charge bypass each other while moving through a shared pathway remains unknown. Here, we use MD simulations, biochemical and electrophysiological measurements to identify two conserved phenylalanine residues that form an aromatic pathway whose dynamic rearrangements enable H⁺ movement outside the Cl^- pore. These residues are important for H⁺ transport and voltage-dependent gating in the CLC exchangers. The aromatic pathway residues are evolutionarily conserved in CLC channels where their electrostatic properties and conformational flexibility determine gating. We propose that Cl^- and H⁺ move through physically distinct and evolutionarily conserved routes through the CLC channels and transporters and suggest a unifying mechanism that describes the gating mechanism of both CLC subtypes.

Molecular Basis of CLC Antiporter Inhibition by Fluoride

CLC channels and transporters conduct or transport various kinds of anions, with the exception of fluoride, which acts as an effective inhibitor. Here, we performed sub-nanosecond DFT-based QM/MM simulations of the E. coli anion/proton exchanger ClC-ec1 and observed that fluoride binds incoming protons within the selectivity filter, with excess protons shared with the gating glutamate E148. Depending on E148 conformation, the competition for the proton can involve either a direct F^-/E148 interaction or the modulation of water molecules bridging the two anions. The direct interaction locks E148 in a conformation that does not allow for proton transport, and thus inhibits protein function.

Dynamical model of the CLC-2 ion channel reveals conformational changes associated with selectivity-filter gating

This work reports a dynamical Markov state model of CLC-2 "fast" (pore) gating, based on 600 microseconds of molecular dynamics (MD) simulation. In the starting conformation of our CLC-2 model, both outer and inner channel gates are closed. The first conformational change in our dataset involves rotation of the inner-gate backbone along residues S168-G169-I170. This change is strikingly similar to that observed in the cryo-EM structure of the bovine CLC-K channel, though the volume of the intracellular (inner) region of the ion conduction pathway is further expanded in our model. From this state (inner gate open and outer gate closed), two additional states are observed, each involving a unique rotameric flip of the outer-gate residue GLUex. Both additional states involve conformational changes that orient GLUex away from the extracellular (outer) region of the ion conduction pathway. In the first additional state, the rotameric flip of GLUex results in an open, or near-open, channel pore. The equilibrium population of this state is low (∼1%), consistent with the low open probability of CLC-2 observed experimentally in the absence of a membrane potential stimulus (0 mV). In the second additional state, GLUex rotates to occlude the channel pore. This state, which has a low equilibrium population (∼1%), is only accessible when GLUex is protonated. Together, these pathways model the opening of both an inner and outer gate within the CLC-2 selectivity filter, as a function of GLUex protonation. Collectively, our findings are consistent with published experimental analyses of CLC-2 gating and provide a high-resolution structural model to guide future investigations.

β2-adrenergic Agonists Rescue Lysosome Acidification and Function in PSEN1 Deficiency by Reversing Defective ER-to-lysosome Delivery of ClC-7

Lysosomal dysfunction is considered pathogenic in Alzheimer disease (AD). Loss of presenilin-1 (PSEN1) function causing AD impedes acidification via defective vacuolar ATPase (vATPase) V0a1 subunit delivery to lysosomes. We report that isoproterenol (ISO) and related β2-adrenergic agonists reacidify lysosomes in PSEN1 Knock out (KO) cells and fibroblasts from PSEN1 familial AD patients, which restores lysosomal proteolysis, calcium homeostasis, and normal autophagy flux. We identify a novel rescue mechanism involving Portein Kinase A (PKA)-mediated facilitation of chloride channel-7 (ClC-7) delivery to lysosomes which reverses markedly lowered chloride (Cl^-) content in PSEN1 KO lysosomes. Notably, PSEN1 loss of function impedes Endoplasmic Reticulum (ER)-to-lysosome delivery of ClC-7. Transcriptomics of PSEN1-deficient cells reveals strongly downregulated ER-to-lysosome transport pathways and reversibility by ISO, thus accounting for lysosomal Cl^- deficits that compound pH elevation due to deficient vATPase and its rescue by β2-adrenergic agonists. Our findings uncover a broadened PSEN1 role in lysosomal ion homeostasis and novel pH modulation of lysosomes through β2-adrenergic regulation of ClC-7, which can potentially be modulated therapeutically.

An Up-to-Date Overview of the Complexity of Genotype-Phenotype Relationships in Myotonic Channelopathies

Myotonic disorders are inherited neuromuscular diseases divided into dystrophic myotonias and non-dystrophic myotonias (NDM). The latter is a group of dominant or recessive diseases caused by mutations in genes encoding ion channels that participate in the generation and control of the skeletal muscle action potential. Their altered function causes hyperexcitability of the muscle membrane, thereby triggering myotonia, the main sign in NDM. Mutations in the genes encoding voltage-gated Cl⁻ and Na⁺ channels (respectively, CLCN1 and SCN4A) produce a wide spectrum of phenotypes, which differ in age of onset, affected muscles, severity of myotonia, degree of hypertrophy, and muscle weakness, disease progression, among others. More than 200 CLCN1 and 65 SCN4A mutations have been identified and described, but just about half of them have been functionally characterized, an approach that is likely extremely helpful to contribute to improving the so-far rather poor clinical correlations present in NDM. The observed poor correlations may be due to: (1) the wide spectrum of symptoms and overlapping phenotypes present in both groups (Cl⁻ and Na⁺ myotonic channelopathies) and (2) both genes present high genotypic variability. On the one hand, several mutations cause a unique and reproducible phenotype in most patients. On the other hand, some mutations can have different inheritance pattern and clinical phenotypes in different families. Conversely, different mutations can be translated into very similar phenotypes. For these reasons, the genotype-phenotype relationships in myotonic channelopathies are considered complex. Although the molecular bases for the clinical variability present in myotonic channelopathies remain obscure, several hypotheses have been put forward to explain the variability, which include: (a) differential allelic expression; (b) trans-acting genetic modifiers; (c) epigenetic, hormonal, or environmental factors; and (d) dominance with low penetrance. Improvements in clinical tests, the recognition of the different phenotypes that result from particular mutations and the understanding of how a mutation affects the structure and function of the ion channel, together with genetic screening, is expected to improve clinical correlation in NDMs.

A highly-selective chloride microelectrode based on a mercuracarborand anion carrier

The chloride gradient plays an important role in regulating cell volume, membrane potential, pH, secretion, and the reversal potential of inhibitory glycine and GABA_A receptors. Measurement of intracellular chloride activity, \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${{\boldsymbol{a}}}_{{\boldsymbol{Cl}}}^{{\boldsymbol{i}}}$$\end{document}aCli, using liquid membrane ion-selective microelectrodes (ISM), however, has been limited by the physiochemical properties of Cl⁻ ionophores which have caused poor stability, drift, sluggish response times, and interference from other biologically relevant anions. Most importantly, intracellular \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\bf{HC}}{{\bf{O}}}_{{\bf{3}}}^{-}$$\end{document}HCO3− may be up to 4 times more abundant than Cl⁻ (e.g. skeletal muscle) which places severe constraints on the required selectivity of a Cl⁻ – sensing ISM. Previously, a sensitive and highly-selective Cl⁻ sensor was developed in a polymeric membrane electrode using a trinuclear Hg(II) complex containing carborane-based ligands, [9]-mercuracarborand-3, or MC3 for short. Here, we have adapted the use of the MC3 anion carrier in a liquid membrane ion-selective microelectrode and show the MC3-ISM has a linear Nernstian response over a wide range of a_Cl (0.1 mM to 100 mM), is highly selective for Cl⁻ over other biological anions or inhibitors of Cl⁻ transport, and has a 10% to 90% settling  time of 3  sec. Importantly, over the physiological range of a_Cl (1 mM to 100 mM) the potentiometric response of the MC3-ISM is insensitive to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\bf{HC}}{{\bf{O}}}_{{\bf{3}}}^{-}$$\end{document}HCO3− or changes in pH. Finally, we demonstrate the biological application of an MC3-ISM by measuring intracellular a_Cl, and the response to an external Cl-free challenge, for an isolated skeletal muscle fiber.

A large intragenic deletion in the CLCN1 gene causes Hereditary Myotonia in pigs

Mutations in the CLCN1 gene are the primary cause of non-dystrophic Hereditary Myotonia in several animal species. However, there are no reports of Hereditary Myotonia in pigs to date. Therefore, the objective of the present study was to characterize the clinical and molecular findings of Hereditary Myotonia in an inbred pedigree. The clinical, electromyographic, histopathological, and molecular findings were evaluated. Clinically affected pigs presented non-dystrophic recessive Hereditary Myotonia. Nucleotide sequence analysis of the entire coding region of the CLCN1 gene revealed the absence of the exons 15 and 16 in myotonic animals. Analysis of the genomic region flanking the deletion unveiled a large intragenic deletion of 4,165 nucleotides. Interestingly, non-related, non-myotonic pigs expressed transcriptional levels of an alternate transcript (i.e., X2) that was identical to the deleted X1 transcript of myotonic pigs. All myotonic pigs and their progenitors were homozygous recessive and heterozygous, respectively, for the 4,165-nucleotide deletion. This is the first study reporting Hereditary Myotonia in pigs and characterizing its clinical and molecular findings. Moreover, to the best of our knowledge, Hereditary Myotonia has never been associated with a genomic deletion in the CLCN1 gene in any other species.

A Mathematical Model of Lysosomal Ion Homeostasis Points to Differential Effects of Cl- Transport in Ca2+ Dynamics

The establishment and maintenance of ion gradients between the interior of lysosomes and the cytosol are crucial for numerous cellular and organismal functions. Numerous ion transport proteins ensure the required variation in luminal concentrations of the different ions along the endocytic pathway to fit the needs of the organelles. Failures in keeping proper ion homeostasis have pathological consequences. Accordingly, several human diseases are caused by the dysfunction of ion transporters. These include osteopetrosis, caused by the dysfunction of Cl-/H+ exchange by the lysosomal transporter ClC-7. To better understand how chloride transport affects lysosomal ion homeostasis and how its disruption impinges on lysosomal function, we developed a mathematical model of lysosomal ion homeostasis including Ca2+ dynamics. The model recapitulates known biophysical properties of ClC-7 and enables the investigation of its differential activation kinetics on lysosomal ion homeostasis. We show that normal functioning of ClC-7 supports the acidification process, is associated with increased luminal concentrations of sodium, potassium, and chloride, and leads to a higher Ca2+ uptake and release. Our model highlights the role of ClC-7 in lysosomal acidification and shows the existence of differential Ca2+ dynamics upon perturbations of Cl-/H+ exchange and its activation kinetics, with possible pathological consequences.

Mutation of external glutamate residue reveals a new intermediate transport state and anion binding site in a CLC Cl-/H+ antiporter

The CLC family of proteins are involved in a variety of physiological processes to control cellular chloride concentration. Two distinct classes of CLC proteins, Cl^- channels and Cl^-/H⁺ antiporters, have been functionally and structurally investigated over the last several decades. Previous studies have suggested that the conformational heterogeneity of the critical glutamate residue, Glu_ex, could explain the transport cycle of CLC-type Cl^-/H⁺ antiporters. However, the presence of multiple conformations (Up, Middle, and Down) of the Glu_ex has been suggested from combined structural snapshots of 2 different CLC antiporters: CLC-ec1 from Escherichia coli and cmCLC from a thermophilic red alga, Cyanidioschyzon merolae Thus, we aimed to investigate further the heterogeneity of Glu_ex-conformations in CLC-ec1, the most deeply studied CLC antiporter, at both functional and structural levels. Here, we show that the crystal structures of the Glu_ex mutant E148D and wild-type CLC-ec1 with varying anion concentrations suggest a structural intermediate, the "Midlow" conformation. We also found that an extra anion can be located above the external Cl^--binding site in the E148D mutant when the anion concentration is high. Moreover, we observed that a carboxylate in solution can occupy either the external or central Cl^--binding site in the ungated E148A mutant using an anomalously detectable short carboxylic acid, bromoacetate. These results lend credibility to the idea that the Glu_ex can take at least 3 distinct conformational states during the transport cycle of a single CLC antiporter.

Physiology of Electrolyte Transport in the Gut: Implications for Disease

We now have an increased understanding of the genetics, cell biology, and physiology of electrolyte transport processes in the mammalian intestine, due to the availability of sophisticated methodologies ranging from genome wide association studies to CRISPR-CAS technology, stem cell-derived organoids, 3D microscopy, electron cryomicroscopy, single cell RNA sequencing, transgenic methodologies, and tools to manipulate cellular processes at a molecular level. This knowledge has simultaneously underscored the complexity of biological systems and the interdependence of multiple regulatory systems. In addition to the plethora of mammalian neurohumoral factors and their cross talk, advances in pyrosequencing and metagenomic analyses have highlighted the relevance of the microbiome to intestinal regulation. This article provides an overview of our current understanding of electrolyte transport processes in the small and large intestine, their regulation in health and how dysregulation at multiple levels can result in disease. Intestinal electrolyte transport is a balance of ion secretory and ion absorptive processes, all exquisitely dependent on the basolateral Na⁺ /K⁺ ATPase; when this balance goes awry, it can result in diarrhea or in constipation. The key transporters involved in secretion are the apical membrane Cl^- channels and the basolateral Na⁺ -K⁺ -2Cl^- cotransporter, NKCC1 and K⁺ channels. Absorption chiefly involves apical membrane Na⁺ /H⁺ exchangers and Cl^- /HCO₃ ^- exchangers in the small intestine and proximal colon and Na⁺ channels in the distal colon. Key examples of our current understanding of infectious, inflammatory, and genetic diarrheal diseases and of constipation are provided. © 2019 American Physiological Society. Compr Physiol 9:947-1023, 2019.

Characterizing chloride-dependent acidification in brain clathrin-coated vesicles 1

Endocytic organelles maintain their acidic pH using the V-type ATPase proton pump. However, proton accumulation across the membrane generates a voltage and requires the movement of an additional ion, known as a counterion, to dissipate charge buildup. The role of counterion movement in endosomes is not clear, but a subpopulation of early endosomes, clathrin-coated vesicles (CCVs), has previously been shown to use external chloride (Cl-) to allow V-ATPase-dependent acidification. We aimed to determine the identity and function of this presumed Cl- transporting protein. Our sample of highly enriched bovine brain CCVs exhibited V-type ATPase-facilitated acidification in the presence of external Cl-, independent of the monovalent cations present. While unsuccessful at identifying the mechanism of anion transport, we used glutamate-facilitated acidification, density gradients, and mass spectrometry to show that most brain CCVs are synaptic vesicles, complementing results from earlier studies that argued similarity only on the basis on protein content. The source of Cl--dependent acidification in brain CCVs may be vGLUT1, a synaptic vesicle glutamate transporter with known Cl- permeability, although CCVs in other tissues are likely to utilize different proteins to facilitate acidification.

Structure of the human ClC-1 chloride channel

ClC-1 protein channels facilitate rapid passage of chloride ions across cellular membranes, thereby orchestrating skeletal muscle excitability. Malfunction of ClC-1 is associated with myotonia congenita, a disease impairing muscle relaxation. Here, we present the cryo-electron microscopy (cryo-EM) structure of human ClC-1, uncovering an architecture reminiscent of that of bovine ClC-K and CLC transporters. The chloride conducting pathway exhibits distinct features, including a central glutamate residue ("fast gate") known to confer voltage-dependence (a mechanistic feature not present in ClC-K), linked to a somewhat rearranged central tyrosine and a narrower aperture of the pore toward the extracellular vestibule. These characteristics agree with the lower chloride flux of ClC-1 compared with ClC-K and enable us to propose a model for chloride passage in voltage-dependent CLC channels. Comparison of structures derived from protein studied in different experimental conditions supports the notion that pH and adenine nucleotides regulate ClC-1 through interactions between the so-called cystathionine-β-synthase (CBS) domains and the intracellular vestibule ("slow gating"). The structure also provides a framework for analysis of mutations causing myotonia congenita and reveals a striking correlation between mutated residues and the phenotypic effect on voltage gating, opening avenues for rational design of therapies against ClC-1-related diseases.

Knockout of ClC-2 reveals critical functions of adherens junctions in colonic homeostasis and tumorigenicity

Adherens junctions (AJs), together with tight junctions (TJs), form an apical junctional complex that regulates intestinal epithelial cell-to-cell adherence and barrier homeostasis. Within the AJ, membrane-bound E-cadherin binds β-catenin, which functions as an essential intracellular signaling molecule. We have previously identified a novel protein in the region of the apical junction complex, chloride channel protein-2 (ClC-2), that we have used to study TJ regulation. In this study, we investigated the possible effects of ClC-2 on the regulation of AJs in intestinal mucosal epithelial homeostasis and tumorigenicity. Mucosal homeostasis and junctional proteins were examined in wild-type (WT) and ClC-2 knockout (KO) mice as well as associated colonoids. Tumorigenicity and AJ-associated signaling were evaluated in a murine colitis-associated tumor model and in a colorectal cancer cell line (HT-29). Colonic tissues from ClC-2 KO mice had altered ultrastructural morphology of intercellular junctions with reduced colonocyte differentiation, whereas jejunal tissues had minimal changes. Colonic crypts from ClC-2 KO mice had significantly higher numbers of less-differentiated forms of colonoids compared with WT. Furthermore, the absence of ClC-2 resulted in redistribution of AJ proteins and increased β-catenin activity. Downregulation of ClC-2 in colorectal cells resulted in significant increases in proliferation associated with disruption of AJs. Colitis-associated tumors in ClC-2 KO mice were significantly increased, associated with β-catenin transcription factor activation. The absence of ClC-2 results in less differentiated colonic crypts and increased tumorigenicity associated with colitis via dysregulation of AJ proteins and activation of β-catenin-associated signaling. NEW & NOTEWORTHY Disruption of adherens junctions in the absence of chloride channel protein-2 revealed critical functions of these junctional structures, including maintenance of colonic homeostasis and differentiation as well as driving tumorigenicity by regulating β-catenin signaling.

Ion Channels in Cancer: Are Cancer Hallmarks Oncochannelopathies?

Genomic instability is a primary cause and fundamental feature of human cancer. However, all cancer cell genotypes generally translate into several common pathophysiological features, often referred to as cancer hallmarks. Although nowadays the catalog of cancer hallmarks is quite broad, the most common and obvious of them are 1) uncontrolled proliferation, 2) resistance to programmed cell death (apoptosis), 3) tissue invasion and metastasis, and 4) sustained angiogenesis. Among the genes affected by cancer, those encoding ion channels are present. Membrane proteins responsible for signaling within cell and among cells, for coupling of extracellular events with intracellular responses, and for maintaining intracellular ionic homeostasis ion channels contribute to various extents to pathophysiological features of each cancer hallmark. Moreover, tight association of these hallmarks with ion channel dysfunction gives a good reason to classify them as special type of channelopathies, namely oncochannelopathies. Although the relation of cancer hallmarks to ion channel dysfunction differs from classical definition of channelopathies, as disease states causally linked with inherited mutations of ion channel genes that alter channel's biophysical properties, in a broader context of the disease state, to which pathogenesis ion channels essentially contribute, such classification seems absolutely appropriate. In this review the authors provide arguments to substantiate such point of view.

Modulation of Cl- signaling and ion transport by recruitment of kinases and phosphatases mediated by the regulatory protein IRBIT

IRBIT is a multifunctional protein that controls the activity of various epithelial ion transporters including NBCe1-B. Interaction with IRBIT increases NBCe1-B activity and exposes two cryptic Cl^--sensing GXXXP sites that enable regulation of NBCe1-B by intracellular Cl^- (Cl^- _in). Here, phosphoproteomic analysis revealed that IRBIT controlled five phosphorylation sites in NBCe1-B that determined both the active conformation of the transporter and its regulation by Cl^- _in Mutational analysis suggested that the phosphorylation status of Ser²³², Ser²³³, and Ser²³⁵ was regulated by IRBIT and determined whether NBCe1 transporters are in active or inactive conformations. The absence of phosphorylation at Ser²³², Ser²³³, or Ser²³⁵ produced NBCe1-B in the conformations pSer²³³/pSer²³⁵, pSer²³²/pSer²³⁵, or pSer²³²/pSer²³³, respectively. The activity of the pSer²³³/pSer²³⁵ form was similar to that of IRBIT-activated NBCe1-B, but it was insensitive to inhibition by Cl^- _in The properties of the pSer²³²/pSer²³⁵ form were similar to those of wild-type NBCe1-B, whereas the pSer²³²/pSer²³³ form was partially active, further activated by IRBIT, but retained inhibition by Cl^- _in Furthermore, IRBIT recruited the phosphatase PP1 and the kinase SPAK to control phosphorylation of Ser⁶⁵, which affected Cl^- _in sensing by the ³²GXXXP³⁶ motif. IRBIT also recruited the phosphatase calcineurin and the kinase CaMKII to control phosphorylation of Ser¹², which affected Cl^- _in sensing by the ¹⁹⁴GXXXP¹⁹⁸ motif. Ser²³², Ser²³³, and Ser²³⁵ are conserved in all NBCe1 variants and affect their activity. These findings reveal how multiple kinase and phosphatase pathways use phosphorylation sites to fine-tune a transporter, which have important implications for epithelial fluid and HCO₃ ^- secretion.

Modulation of ClC-3 gating and proton/anion exchange by internal and external protons and the anion selectivity filter

KEY POINTS

The ClC-3 2Cl^- /1H⁺ exchanger modulates endosome pH and Cl^- concentration. We investigated the relationships between ClC-3-mediated ion transport (steady-state transport current, I_SS ), gating charge (Q) and cytoplasmic alkalization. ClC-3 transport is functionally unidirectional. ClC-5 and ClC-3 display indistinguishable exchange ratios, but ClC-3 cycling is less "efficient", as reflected by a large Q/I_SS . An M531A mutation predicted to increase water-wire stability and cytoplasmic proton supply improves efficiency. Protonation (pH 5.0) of the outer glutamate gate (Glu_ext ; E224) reduces Q, inhibits transport, and weakens coupling. Removal of the central tyrosine anion gate (Y572S) greatly increases uncoupled anion current. Tyrosine -OH removal (Y572F) alters anion selectivity and impairs coupling. E224 and Y572 act as anion barriers, and contribute to gating. The Y572 side chain and -OH regulate Q movement kinetics and voltage dependence. E224 and Y572 interact to create a "closed" inner gate conformation that maintains coupling during cycling.

ABSTRACT

We utilized plasma membrane-localized ClC-3 to investigate relationships between steady-state transport current (I_SS ), gating charge (Q) movement, and cytoplasmic alkalization rate. ClC-3 exhibited lower transport efficiency than ClC-5, as reflected by a larger Q/I_SS ratio, but an indistinguishable Cl^- /H⁺ coupling ratio. External SCN^- reduced H⁺ transport rate and uncoupled anion/H⁺ exchange by 80-90%. Removal of the external gating glutamate ("Glu_ext ") (E224A mutation) reduced Q and abolished H⁺ transport. We hypothesized that Methionine 531 (M531) impedes "water wire" H⁺ transfer from the cytoplasm to E224. Accordingly, an M531A mutation decreased the Q/I_SS ratio by 50% and enhanced H⁺ transport. External protons (pH 5.0) inhibited I_SS and markedly reduced Q while shifting the Q-voltage (V) relationship positively. The Cl^- /H⁺ coupling ratio at pH 5.0 was significantly increased, consistent with externally protonated Glu_ext adopting an outward/open position. Internal "anion gate" removal (Y572S) dramatically increased I_SS and impaired coupling, without slowing H⁺ transport rate. Loss of both gates (Y572S/E224A) resulted in a large "open pore" conductance. Y572F (removing only the phenolic hydroxide) and Y572S shortened Q duration similarly, resulting in faster Q kinetics at all voltages. These data reveal a complex relationship between Q and ion transport. Q/I_SS must be assessed together with coupling ratio to properly interpret efficiency. Coupling and transport rate are influenced by the anion, internal proton supply and external protons. Y572 regulates H⁺ coupling as well as anion selectivity, and interacts directly with E224. Disruption of this "closed gate" conformation by internal protons may represent a critical step in the ClC-3 transport cycle.