Alexander A. Mongin

Volume-regulated anion channels (VRACs), formed by leucine-rich repeat-containing 8 (LRRC8) proteins, are ubiquitously expressed chloride channels essential for cell volume regulation and implicated in diverse physiological and pathological processes. Small-molecule VRAC inhibitors have been reported to modulate paracrine signaling, proliferation, differentiation, migration, and apoptosis, and have been patented for potential therapeutic applications in stroke, cardiovascular and metabolic diseases, and cancer. However, growing evidence indicates that many commonly used VRAC blockers exert substantial off-target effects and frequently fail to reproduce phenotypes observed after deletion of the essential VRAC subunit LRRC8A. Here, we systematically compared effects of several widely used pharmacological VRAC inhibitors with outcomes of molecular downregulation of LRRC8A in limiting proliferation of malignant glioblastoma cells derived from surgical specimens. NIH/3T3 fibroblasts served as a non-malignant control. In serum-containing media, structurally diverse VRAC blockers (DCPIB, DIDS, carbenoxolone, phloretin, and bromadiolone) reduced proliferation in a non-uniform manner, with potencies that did not correlate with reported VRAC affinities and varied markedly among cell lines. Radiotracer-based measurements of VRAC activity indicated that these discrepancies were largely attributable to binding of inhibitors to serum albumin. When experiments were repeated under serum-free conditions, all inhibitors except DIDS and phloretin induced extensive death of both malignant and non-malignant cells, confirmed by microscopy and LDH release assays. This cytotoxicity was accompanied by a significant reduction in intracellular ATP levels, consistent with previously reported mitochondrial uncoupling effects. In contrast, LRRC8A knockdown reduced proliferation without substantial cell death. Together, these findings demonstrate that most commercially available VRAC blockers limit proliferation and viability predominantly through VRAC-independent mechanisms. Under standard culture conditions, serum albumin masks much of their intrinsic cytotoxicity. These results underscore the need for rigorous molecular controls in pharmacological studies and provide basis for developing more selective and less toxic VRAC-targeting agents.

Volume-regulated anion channels (VRACs), formed by leucine-rich repeat-containing 8 (LRRC8) proteins, are ubiquitously expressed chloride channels essential for cell volume regulation and implicated in diverse physiological and pathological processes. Small-molecule VRAC inhibitors have been reported to modulate paracrine signaling, proliferation, differentiation, migration, and apoptosis, and have been patented for potential therapeutic applications in stroke, cardiovascular and metabolic diseases, and cancer. However, growing evidence indicates that many commonly used VRAC blockers exert substantial off-target effects and frequently fail to reproduce phenotypes observed after deletion of the essential VRAC subunit LRRC8A. Here, we systematically compared effects of several widely used pharmacological VRAC inhibitors with outcomes of molecular downregulation of LRRC8A in limiting proliferation of malignant glioblastoma cells derived from surgical specimens. NIH/3T3 fibroblasts served as a non-malignant control. In serum-containing media, structurally diverse VRAC blockers (DCPIB, DIDS, carbenoxolone, phloretin, and bromadiolone) reduced proliferation in a non-uniform manner, with potencies that did not correlate with reported VRAC affinities and varied markedly among cell lines. Radiotracer-based measurements of VRAC activity indicated that these discrepancies were largely attributable to binding of inhibitors to serum albumin. When experiments were repeated under serum-free conditions, all inhibitors except DIDS and phloretin induced extensive death of both malignant and non-malignant cells, confirmed by microscopy and LDH release assays. This cytotoxicity was accompanied by a significant reduction in intracellular ATP levels, consistent with previously reported mitochondrial uncoupling effects. In contrast, LRRC8A knockdown reduced proliferation without substantial cell death. Together, these findings demonstrate that most commercially available VRAC blockers limit proliferation and viability predominantly through VRAC-independent mechanisms. Under standard culture conditions, serum albumin masks much of their intrinsic cytotoxicity. These results underscore the need for rigorous molecular controls in pharmacological studies and provide basis for developing more selective and less toxic VRAC-targeting agents.

Volume-regulated anion channels (VRACs) are swelling-activated Cl ⁻ channels formed by heteromers of LRRC8 proteins (LRRC8A-E). VRACs mediate regulatory volume decrease by releasing Cl ⁻ and organic osmolytes, including taurine. In the brain, taurine functions as both an osmolyte and an inhibitory neuromodulator, acting as an agonist at glycine and GABA _A receptors. Here, we used RNAi and genetically modified mice to (i) determine the contribution of LRRC8 subunits to swelling-activated taurine release, and (ii) test whether brain VRACs contribute to systemic osmoregulation. In astrocyte cultures lacking the essential VRAC subunit LRRC8A, swelling-activated [ ³ H]taurine release was reduced by ∼90%. Similarly, in vivo brain microdialysis revealed a ≥90% reduction in swelling-activated taurine release in brain-wide LRRC8A knockout mice (bLRRC8A KO). RNAi experiments further confirmed LRRC8D as a key determinant of VRAC-mediated taurine release, with Lrrc8d knockdown reducing swelling activated [ ³ H]taurine efflux by >50%. RNA-seq revealed enrichment of Lrrc8d in the hypothalamus, where astrocytic taurine release is proposed to inhibit magnocellular neurons and suppress arginine vasopressin secretion during hypoosmotic conditions. We therefore tested whether disrupting brain VRACs impairs recovery from systemic hypoosmotic stress. bLRRC8A KO mice and mice with gene trap-mediated LRRC8D depletion were challenged with an intraperitoneal injection of sterile water (7.5% body weight, under anesthesia), and serum osmolality was monitored over time. Unexpectedly, neither model showed impaired recovery from hypoosmotic challenge. Overall, this study identifies LRRC8A/D-containing VRACs as the primary conduit for swelling-activated taurine release in the brain, yet indicates that this pathway is not essential for maintaining systemic fluid-electrolyte homeostasis.

Volume-regulated anion channels (VRACs), formed by leucine-rich repeat-containing 8 (LRRC8) proteins, are ubiquitously expressed chloride channels essential for cell volume regulation and implicated in diverse physiological and pathological processes. Small-molecule VRAC inhibitors have been reported to modulate paracrine signaling, proliferation, differentiation, migration, and apoptosis, and have been patented for potential therapeutic applications in stroke, cardiovascular and metabolic diseases, and cancer. However, growing evidence indicates that many commonly used VRAC blockers exert substantial off-target effects and frequently fail to reproduce phenotypes observed after deletion of the essential VRAC subunit LRRC8A. Here, we systematically compared effects of several widely used pharmacological VRAC inhibitors with outcomes of molecular downregulation of LRRC8A in limiting proliferation of malignant glioblastoma cells derived from surgical specimens. NIH/3T3 fibroblasts served as a non-malignant control. In serum-containing media, structurally diverse VRAC blockers (DCPIB, DIDS, carbenoxolone, phloretin, and bromadiolone) reduced proliferation in a non-uniform manner, with potencies that did not correlate with reported VRAC affinities and varied markedly among cell lines. Radiotracer-based measurements of VRAC activity indicated that these discrepancies were largely attributable to binding of inhibitors to serum albumin. When experiments were repeated under serum-free conditions, all inhibitors except DIDS and phloretin induced extensive death of both malignant and non-malignant cells, confirmed by microscopy and LDH release assays. This cytotoxicity was accompanied by a marked reduction in intracellular ATP levels, consistent with previously reported mitochondrial uncoupling effects. In contrast, LRRC8A knockdown reduced proliferation without substantial cell death. Together, these findings demonstrate that most commercially available VRAC blockers limit proliferation and viability predominantly through VRAC-independent mechanisms. Under standard culture conditions, serum albumin masks much of their intrinsic cytotoxicity. These results underscore the need for rigorous molecular controls in pharmacological studies and provide basis for developing more selective and less toxic VRAC-targeting agents.

Volume-regulated anion channels (VRACs) are ubiquitously expressed chloride channels composed of LRRC8A-E proteins. In human disorders, gain or loss of VRAC function leads to severe neurological phenotypes, potentially due to altered release of amino acid neurotransmitters. Here, we show that glutamate-permeable VRACs in brain astrocytes are primarily composed of LRRC8A and LRRC8C proteins. These findings provide insight into subunit organization of native VRACs in the CNS, with implications for normal brain function and neurological disease.

Leucine‐rich repeat‐containing protein 8A (LRRC8A) is an essential subunit of the ubiquitously expressed volume‐regulated anion channels (VRACs). Previous work has shown that LRRC8A is overexpressed in several cancers and is associated with poor survival outcomes. However, the underlying mechanisms remain obscure. In the present study, we investigated the role of LRRC8A and VRACs in the progression of glioblastoma (GBM), the most common and aggressive primary brain tumour. We found that, compared with healthy brain tissue, LRRC8A mRNA is significantly upregulated in surgical GBM specimens, patient‐derived GBM cell lines and GBM datasets from The Cancer Genome Atlas. GBM patients in the lowest quartile of LRRC8A expression exhibited a trend toward longer survival. In patient‐derived GBM cultures, RNA interference‐mediated knockdown of LRRC8A or pharmacological blockade of VRAC with 4,4′‐diisothiocyano‐2,2′‐stilbenedisulfonic acid (DIDS) reduced cell proliferation, lowered intracellular chloride levels and inhibited activity of mammalian target of rapamycin (mTOR) complex 2 (mTORC2). The anti‐proliferative effects of LRRC8A knockdown and DIDS were non‐additive, suggesting a shared mechanism. Biochemical and molecular analyses revealed that LRRC8A‐containing VRACs promote GBM cell proliferation through a new non‐enzymatic function of the chloride‐sensitive protein kinase WNK1. Specifically, VRAC activity facilitates WNK1‐dependent activation of mTORC2 and its downstream kinases AKT and SGK. In support of this model, either downregulation of WNK1 or pharmacological inhibition of mTOR or SGK/AKT suppressed GBM cell proliferation and mimicked the effect of LRRC8A knockdown. Together, these findings establish a new mTORC2‐centric signalling axis for VRAC‐dependent control of cellular functions and highlight several potential molecular targets for limiting GBM proliferation. image Key points: Volume‐regulated anion channels (VRACs) are considered to contribute to the progression of several human cancers. The essential VRAC subunit LRRC8A is significantly overexpressed in clinical specimens of glioblastoma, the most common and aggressive primary brain malignancy. RNA interference‐mediated downregulation of LRRC8A reduces proliferation in patient‐derived GBM cell cultures, suggesting that VRACs promote cancer cell growth. LRRC8A/VRAC‐mediated effects on cell proliferation are driven by a mechanism involving the chloride‐sensitive protein kinase WNK1, mTOR complex 2 and activation of downstream kinases AKT and SGK.

Stroke is a highly prevalent neurological condition, representing the second leading cause of mortality and the primary cause of disability worldwide. It occurs due to disruptions in cerebral blood flow or brain bleeding, resulting in rapid and extensive neuronal cell loss. As a result, most preclinical and translational stroke studies have focused on understanding the molecular mechanisms underlying neuronal death, neuroprotection, and neuroregeneration. Unfortunately, previous approaches targeting neuronal cells have consistently failed to reduce stroke-induced damage or improve functional recovery in clinical settings. This chapter highlights the emerging shift in the field toward exploring the role of non-neuronal cells, particularly astrocytes. Astrocytes are the most functionally diverse cells in the central nervous system, playing critical roles in maintaining brain health. We explore both the beneficial and detrimental effects of astrocytes in the context of stroke, summarizing well-established findings as well as emerging insights into how astrocytic pathology may be targeted in preclinical and translational research.

Volume-regulated anion channels (VRACs) are ubiquitous chloride channels that play important roles in cell volume regulation and numerous other physiological processes. VRACs are heteromeric complexes composed of leucine-rich repeat-containing proteins LRRC8A-E. LRRC8 subunit composition determines biophysical properties of VRACs, including permeability to small signaling molecules. Here, we used primary astrocyte cultures from wild-type and genetically modified C57BL/6 mice to investigate (i) subunit composition of native VRACs in the brain and (ii) subunit determinants of VRAC permeability to the excitatory neurotransmitter glutamate. qRT-PCR and RNA-seq analyses revealed high expression of Lrrc8a-d in mouse forebrain and astrocytes. Genetic deletion of the essential LRRC8A protein abolished swelling-activated glutamate release, measured as efflux of the non-metabolizable D-[ ³ H]aspartate, confirming the crucial role of VRACs in this process. RNAi-mediated knockdown of individual subunits identified LRRC8A and LRRC8C as key components of glutamate-permeable astrocytic VRACs. qRT-PCR and Western blot analyses further showed that knockdown of LRRC8A or LRRC8C reciprocally altered the protein stability of the partner subunit without affecting their mRNA levels. A similar pattern of mutual regulation was observed between LRRC8A and LRRC8D. In contrast to LRRC8C, downregulation of LRRC8D had a more limited impact on glutamate release. Additional double-knockdown experiments demonstrated that LRRC8C- and LRRC8D-containing channels form distinct VRAC populations. This model was further supported by Western blot results showing no reciprocal regulation of LRRC8C and LRRC8D stability. Together, these findings refine our understanding of how the subunit organization of native brain VRACs governs gliotransmitter release, with implications for normal brain function and neurological disease. NEW & NOTEWORTHY: Volume-regulated anion channels (VRACs) are ubiquitously expressed chloride channels composed of LRRC8A-E proteins. In human disorders, gain or loss of VRAC function leads to severe neurological phenotypes, potentially due to altered release of amino acid neurotrans-mitters. Here, we show that glutamate-permeable VRACs in brain astrocytes are primarily composed of LRRC8A and LRRC8C proteins. These findings provide insight into subunit organization of native VRACs in the CNS, with implications for normal brain function and neurological disease.

SUMMARY: Leucine-rich repeat-containing protein 8A (LRRC8A) is the essential subunit of ubiquitous volume-regulated anion channels (VRACs). LRCC8A is overexpressed in several cancers and promotes negative survival outcomes via a poorly defined mechanism. Here, we explored the role of LRRC8A and VRACs in the progression of glioblastoma (GBM), the most common and deadly primary brain tumor. We found that, as compared to healthy controls, LRRC8A mRNA was strongly upregulated in surgical GBM specimens, patient-derived GBM cell lines, and GBM datasets from The Cancer Genome Atlas (TCGA). Our in-silico analysis indicated that patients belonging to the lowest LRRC8A expression quartile demonstrated a trend for extended life expectancy. In patient-derived GBM cultures, siRNA-driven LRRC8A knockdown reduced cell proliferation and additionally decreased intracellular chloride levels and inhibited activity of mTOR complex 2. The antiproliferative effect of LRRC8A downregulation was recapitulated with a pharmacological inhibitor of VRAC. Our ensuing biochemical and molecular biology analyses established that the LRRC8A-containing VRACs facilitate GBM proliferation via a new mechanism involving non-enzymatic actions of the chloride-sensitive protein kinase WNK1. Accordingly, the chloride-bound WNK1 stimulates mTORC2 and the mTORC2-dependent protein kinases AKT and SGK, which promote proliferation. These findings establish the new mTORC2-centric axis for VRAC dependent regulation of cellular functions and uncover potential targets for GBM intervention. SUBJECT AREAS: Cell biology Molecular biology Cancer