The expanding toolbox to study the LRRC8-formed volume-regulated anion channel VRAC

Sphingosine-1-phosphate activates LRRC8 volume-regulated anion channels through Gβγ signalling

Volume-regulated anion channels (VRACs) formed by leucin-rich repeat containing 8 (LRRC8) proteins play a pivotal role in regulatory volume decrease by mediating the release of chloride and organic osmolytes. Apart from the regulation of cell volume, LRRC8/VRAC function underlies numerous physiological processes in vertebrate cells including membrane potential regulation, glutamate release and apoptosis. LRRC8/VRACs are also permeable to antibiotics and anti-cancer drugs, representing therefore important therapeutic targets. The activation mechanisms for LRRC8/VRACs are still unclear. Besides through osmotic cell swelling, LRRC8/VRACs can be activated by various stimuli under isovolumetric conditions. Sphingosine-1-phosphate (S1P), an important signalling lipid, which signals through a family of G protein-coupled receptors (GPCRs), has been reported to activate LRRC8/VRACs in several cell lines. Here, we measured inter-subunit Förster resonance energy transfer (FRET) and used whole-cell patch clamp electrophysiology to investigate S1P-induced LRRC8/VRAC activation. We systematically assessed the involvement of GPCRs and G protein-mediated signal transduction in channel activation. We found that S1P-induced channel activation is mediated by S1PR1 in HeLa cells. Following the downstream signalling pathway of S1PR1 and using toxin-mediated inhibition of the associated G proteins, we showed that Gβγ dimers rather than Gαi or Gαq play a critical role in S1P-induced VRAC activation. We could also show that S1P causes protein kinase D (PKD) phosphorylation, suggesting that Gβγ recruits phospholipase Cβ (PLCβ) with the consequent PKD activation by diacylglycerol. Notably, S1P did not activate LRRC8/VRAC in HEK293 cells, but overexpression of Gβγ-responsive PLCβ isoform could facilitate S1P-induced LRRC8/VRAC currents. We thus identified S1PR1-mediated Gβγ-PLCβ signalling as a key mechanism underlying isosmotic LRRC8/VRAC activation. KEY POINTS: Leucin-rich repeat containing 8 (LRRC8) anion/osmolyte channels are involved in multiple physiological processes where they can be activated as volume-regulated anion channels (VRACs) by osmotic cell swelling or isovolumetric stimuli such as sphingosine-1-phosphate (S1P). In the present study, using pharmacological modulation and gene-depleted cells in patch clamp recording and optical monitoring of LRRC8 activity, we find that LRRC8/VRAC activation by S1P is mediated by the G protein-coupled receptor S1PR1 coupled to G proteins of the Gi family. The signal transduction to LRRC8/VRAC activation specifically involves phospholipase Cβ activation by βγ subunits of pertussis toxin-insensitive heteromeric Gi proteins. S1P-mediated and hypotonicity-induced LRRC8/VRAC activation pathways converge in protein kinase D activation.

Deep-Learning-Based Segmentation of Cells and Analysis (DL-SCAN)

With the recent surge in the development of highly selective probes, fluorescence microscopy has become one of the most widely used approaches to studying cellular properties and signaling in living cells and tissues. Traditionally, microscopy image analysis heavily relies on manufacturer-supplied software, which often demands extensive training and lacks automation capabilities for handling diverse datasets. A critical challenge arises if the fluorophores employed exhibit low brightness and a low signal-to-noise ratio (SNR). Consequently, manual intervention may become a necessity, introducing variability in the analysis outcomes even for identical samples when analyzed by different users. This leads to the incorporation of blinded analysis, which ensures that the outcome is free from user bias to a certain extent but is extremely time-consuming. To overcome these issues, we developed a tool called DL-SCAN that automatically segments and analyzes fluorophore-stained regions of interest such as cell bodies in fluorescence microscopy images using deep learning. We demonstrate the program's ability to automate cell identification and study cellular ion dynamics using synthetic image stacks with varying SNR. This is followed by its application to experimental Na+ and Ca2+ imaging data from neurons and astrocytes in mouse brain tissue slices exposed to transient chemical ischemia. The results from DL-SCAN are consistent, reproducible, and free from user bias, allowing efficient and rapid analysis of experimental data in an objective manner. The open-source nature of the tool also provides room for modification and extension to analyze other forms of microscopy images specific to the dynamics of different ions in other cell types.

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.

Trends in volume-regulated anion channel (VRAC) research: visualization and bibliometric analysis from 2014 to 2022

Objective: In this study, we utilized bibliometric methods to assess the worldwide scientific output and identify hotspots related to the research on the volume-regulated anion channel (VRAC) from 2014 to 2022. Methods: From Web of Science, we obtained studies related to VRAC published from 2014 to 2022. To analyzed the data, we utilized VOSviewer, a tool for visualizing network, to create networks based on the collaboration between countries, institutions, and authors. Additionally, we performed an analysis of journal co-citation, document citation, and co-occurrence of keywords. Furthermore, we employed CiteSpace (6.1. R6 Advanced) to analyzed keywords and co-cited references with the strongest burst. Results: The final analysis included a total of 278 related articles and reviews, covering the period from 2014 to 2022. The United States emerged as the leading country contributing to this field, while the University of Copenhagen stood out as the most prominent institution. The author with most publications and most citations was Thomas J. Jentsch. Among the cited references, the article by Voss et al. published in Science (2014) gained significant attention for its identification of LRRC8 heteromers as a crucial component of the volume-regulated anion channel VRAC. Pflügers Archiv European Journal of Physiology and Journal of Physiology-London were the leading journals in terms of the quantity of associated articles and citations. Through the analysis of keyword co-occurrence, it was discovered that VRAC is involved in various physiological processes including cell growth, migration, apoptosis, swelling, and myogenesis, as well as anion and organic osmolyte transport including chloride, taurine, glutamate and ATP. VRAC is also associated with related ion channels such as TMEM16A, TMEM16F, pannexin, and CFTR, and associated with various diseases including epilepsy, leukodystrophy, atherosclerosis, hypertension, cerebral edema, stroke, and different types of cancer including gastric cancer, glioblastoma and hepatocellular carcinoma. Furthermore, VRAC is involved in anti-tumor drug resistance by regulating the uptake of platinum-based drugs and temozolomide. Additionally, VRAC has been studied in the context of pharmacology involving DCPIB and flavonoids. Conclusion: The aim of this bibliometric analysis is to provide an overall perspective for research on VRAC. VRAC has become a topic of increasing interest, and our analysis shows that it continues to be a prominent area. This study offers insights into the investigation of VRAC channel and may guide researchers in identifying new directions for future research.

Ion Channels and Transporters in Muscle Cell Differentiation

Investigations on ion channels in muscle tissues have mainly focused on physiological muscle function and related disorders, but emerging evidence supports a critical role of ion channels and transporters in developmental processes, such as controlling the myogenic commitment of stem cells. In this review, we provide an overview of ion channels and transporters that influence skeletal muscle myoblast differentiation, cardiac differentiation from pluripotent stem cells, as well as vascular smooth muscle cell differentiation. We highlight examples of model organisms or patients with mutations in ion channels. Furthermore, a potential underlying molecular mechanism involving hyperpolarization of the resting membrane potential and a series of calcium signaling is discussed.