Activation of microglia with zymosan promotes excitatory amino acid release via volume-regulated anion channels: the role of NADPH oxidases

NADPH Oxidases in Neurodegenerative Disorders: Mechanisms and Therapeutic Opportunities

Significance: The nicotinamide adenine dinucleotide phosphate oxidase (NOX) enzyme family, located in the central nervous system, is recognized as a source of reactive oxygen species (ROS) in the brain. Despite its importance in cellular processes, excessive ROS generation leads to cell death and is involved in the pathogenesis of neurodegenerative disorders. Recent advances: NOX enzymes contribute to the development of neurodegenerative diseases, such as Parkinson's disease (PD), Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), and stroke, highlighting their potential as targets for future therapeutic development. This review will discuss NOX's contribution and therapeutic targeting potential in neurodegenerative diseases, focusing on PD, AD, ALS, and stroke. Critical issues: Homeostatic and physiological levels of ROS are crucial for regulating several processes, such as development, memory, neuronal signaling, and vascular homeostasis. However, NOX-mediated excessive ROS generation is deeply involved in the damage of DNA, proteins, and lipids, leading to cell death in the pathogenesis of a wide range of diseases, namely neurodegenerative diseases. Future directions: It is essential to understand the role of NOX homologs in neurodegenerative disorders and the pathological mechanisms undergoing neurodegeneration mediated by increased levels of ROS. This further knowledge will allow the development of new specific NOX inhibitors and their application for neurodegenerative disease therapeutics.

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

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

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.

Crebanine ameliorates ischemia-reperfusion brain damage by inhibiting oxidative stress and neuroinflammation mediated by NADPH oxidase 2 in microglia

BACKGROUND

The urgent challenge for ischemic stroke treatment is the lack of effective neuroprotectants that target multiple pathological processes. Crebanine, an isoquinoline-like alkaloid with superior pharmacological activities, presents itself as a promising candidate for neuroprotection. However, its effects and mechanisms on ischemic stroke remain unknown.

METHODS

The effects of crebanine on brain damage following ischemic stroke were evaluated using the middle cerebral artery occlusion and reperfusion (MCAO/R) model. Mechanism of action was investigated using both MCAO/R rats and lipopolysaccharide (LPS)-activated BV-2 cells.

RESULTS

We initially demonstrated that crebanine effectively ameliorated the neurological deficits in MCAO/R rats, while also reducing brain edema and infarction. Treatment with crebanine resulted in the up-regulation of NeuN+ fluorescence density and down-regulation of FJB+ cell count, and mitigated synaptic damage. Crebanine attenuated the hyperactivation of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2) by downregulating NADP+ and NADPH levels, suppressing gp91phox and p47phox expressions, and reducing p47phox membrane translocation in Iba-1+ cells. Additionally, crebanine reduced the quantity of Iba-1+ cells and protein expression. Correlation analysis has demonstrated that the inhibition of NOX2 activation in microglia is beneficial for mitigating I/R brain injuries. Moreover, crebanine exhibited significant antioxidant properties by down-regulating the expression of superoxide anion and intracellular reactive oxygen species in vivo and in vitro, and reducing lipid and DNA peroxidation. Crebanine exerted anti-inflammatory effect, as evidenced by the reduction in the expressions of nitric oxide, interleukin 1β, tumor necrosis factor α, interleukin 6, and inducible nitric oxide synthase. The effect of crebanine was achieved through the suppression of nuclear factor-kappa B (NF-κB) and mitogen-activated protein kinases (MAPK) signaling pathway. This is supported by evidence showing reduced NF-κB p65 promoter activity and nucleus translocation, as well as suppressed IκBα phosphorylation and degradation. Additionally, it inhibited the phosphorylation of ERK, JNK, and p38 MAPKs. Importantly, the anti-oxidative stress and neuroinflammation effects of crebanine were further enhanced after silencing gp91phox and p47phox.

CONCLUSION

Crebanine alleviated the brain damages of MCAO/R rats by inhibiting oxidative stress and neuroinflammation mediated by NOX2 in microglia, implying crebanine might be a potential natural drug for the treatment of cerebral ischemia.

Krill Oil's Protective Benefits against Ultraviolet B-Induced Skin Photoaging in Hairless Mice and In Vitro Experiments

Krill oil (KO) shows promise as a natural marine-derived ingredient for improving skin health. This study investigated its antioxidant, anti-inflammatory, anti-wrinkle, and moisturizing effects on skin cells and UVB-induced skin photoaging in hairless mice. In vitro assays on HDF, HaCaT, and B16/F10 cells, as well as in vivo experiments on 60 hairless mice were conducted. A cell viability assay, diphenyl-1-picryhydrazyl (DPPH) radical scavenging activity test, elastase inhibition assay, procollagen content test, MMP-1 inhibition test, and hyaluronan production assay were used to experiment on in vitro cell models. Mice received oral KO administration (100, 200, or 400 mg/kg) once a day for 15 weeks and UVB radiation three times a week. L-Ascorbic acid (L-AA) was orally administered at 100 mg/kg once daily for 15 weeks, starting from the initial ultraviolet B (UVB) exposures. L-AA administration followed each UVB session (0.18 J/cm2) after one hour. In vitro, KO significantly countered UVB-induced oxidative stress, reduced wrinkles, and prevented skin water loss by enhancing collagen and hyaluronic synthesis. In vivo, all KO dosages showed dose-dependent inhibition of oxidative stress-induced inflammatory photoaging-related skin changes. Skin mRNA expressions for hyaluronan synthesis and collagen synthesis genes also increased dose-dependently after KO treatment. Histopathological analysis confirmed that krill oil (KO) ameliorated the damage caused by UVB-irradiated skin tissues. The results imply that KO could potentially act as a positive measure in diminishing UVB-triggered skin photoaging and address various skin issues like wrinkles and moisturization when taken as a dietary supplement.

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.

A SWELL time to develop the molecular pharmacology of the volume-regulated anion channel (VRAC)

Newly emerging roles of LRRC8 volume-regulated anion channels (VRAC) raise important questions about the therapeutic potential of VRAC in the treatment of epilepsy, type 2 diabetes, and other human diseases. A critical barrier to evaluating whether VRAC represents a viable drug target is the lack of potent and specific small-molecule inhibitors and activators of the channel. Here we review recent progress in developing the molecular pharmacology of VRAC made by screening a library of FDA-approved drugs for novel channel modulators. We discuss the discovery and characterization of cysteinyl leukotriene receptor antagonists Pranlukast and Zafirlukast as novel VRAC inhibitors, and zinc pyrithione (ZPT), which apparently activates VRAC through a reactive oxygen species (ROS)-dependent mechanism. These ongoing efforts set the stage for developing a pharmacological toolkit for probing the integrative physiology, molecular pharmacology, and therapeutic potential of VRAC.

Microglia subtypes show substrate- and time-dependent phagocytosis preferences and phenotype plasticity

Microglia are phagocytosis-competent CNS cells comprising a spectrum of subtypes with beneficial and/or detrimental functions in acute and chronic neurodegenerative disorders. The heterogeneity of microglia suggests differences in phagocytic activity and phenotype plasticity between microglia subtypes. To study these issues, primary murine glial cultures were cultivated in the presence of serum, different growth factors and cytokines to obtain M0-like, M1-like, and M2-like microglia as confirmed by morphology, M1/M2 gene marker expression, and nitric oxide assay. Single-cell analysis after 3 hours of phagocytosis of E.coli particles or IgG-opsonized beads showed equal internalization by M0-like microglia, whereas M1-like microglia preferably internalized E.coli particles and M2-like microglia preferably internalized IgG beads, suggesting subtype-specific preferences for different phagocytosis substrates. Time-lapse live-cells imaging over 16 hours revealed further differences between microglia subtypes in phagocytosis preference and internalization dynamics. M0- and, more efficiently, M1-like microglia continuously internalized E.coli particles for 16 hours, whereas M2-like microglia discontinued internalization after approximately 8 hours. IgG beads were continuously internalized by M0- and M1-like microglia but strikingly less by M2-like microglia. M2-like microglia initially showed continuous internalization similar to M0-like microglia but again discontinuation of internalization after 8 hours suggesting that the time of substrate exposure differently affect microglia subtypes. After prolonged exposure to E.coli particles or IgG beads for 5 days all microglia subtypes showed increased internalization of E.coli particles compared to IgG beads, increased nitric oxide release and up-regulation of M1 gene markers, irrespectively of the phagocytosis substrate, suggesting phenotype plasticity. In summary, microglia subtypes show substrate- and time-dependent phagocytosis preferences and phenotype plasticity. The results suggest that prolonged phagocytosis substrate exposure enhances M1-like profiles and M2-M1 repolarization of microglia. Similar processes may also take place in conditions of acute and chronic brain insults when microglia encounter different types of phagocytic substrates.

Microglia subtypes show substrate- and time-dependent phagocytosis preferences and phenotype plasticity

Microglia are phagocytosis-competent CNS cells comprising a spectrum of subtypes with beneficial and/or detrimental functions in acute and chronic neurodegenerative disorders. The heterogeneity of microglia suggests differences in phagocytic activity and phenotype plasticity between microglia subtypes. To study these issues, primary murine glial cultures were cultivated in the presence of serum, different growth factors and cytokines to obtain M0-like, M1-like, and M2-like microglia as confirmed by morphology, M1/M2 gene marker expression, and nitric oxide assay. Single-cell analysis after 3 hours of phagocytosis of E.coli particles or IgG-opsonized beads showed equal internalization by M0-like microglia, whereas M1-like microglia preferably internalized E.coli particles and M2-like microglia preferably internalized IgG beads, suggesting subtype-specific preferences for different phagocytosis substrates. Time-lapse live-cells imaging over 16 hours revealed further differences between microglia subtypes in phagocytosis preference and internalization dynamics. M0- and, more efficiently, M1-like microglia continuously internalized E.coli particles for 16 hours, whereas M2-like microglia discontinued internalization after approximately 8 hours. IgG beads were continuously internalized by M0- and M1-like microglia but strikingly less by M2-like microglia. M2-like microglia initially showed continuous internalization similar to M0-like microglia but again discontinuation of internalization after 8 hours suggesting that the time of substrate exposure differently affect microglia subtypes. After prolonged exposure to E.coli particles or IgG beads for 5 days all microglia subtypes showed increased internalization of E.coli particles compared to IgG beads, increased nitric oxide release and up-regulation of M1 gene markers, irrespectively of the phagocytosis substrate, suggesting phenotype plasticity. In summary, microglia subtypes show substrate- and time-dependent phagocytosis preferences and phenotype plasticity. The results suggest that prolonged phagocytosis substrate exposure enhances M1-like profiles and M2-M1 repolarization of microglia. Similar processes may also take place in conditions of acute and chronic brain insults when microglia encounter different types of phagocytic substrates.

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

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

Cell Death Induction and Protection by Activation of Ubiquitously Expressed Anion/Cation Channels. Part 1: Roles of VSOR/VRAC in Cell Volume Regulation, Release of Double-Edged Signals and Apoptotic/Necrotic Cell Death

Cell volume regulation (CVR) is essential for survival and functions of animal cells. Actually, normotonic cell shrinkage and swelling are coupled to apoptotic and necrotic cell death and thus called the apoptotic volume decrease (AVD) and the necrotic volume increase (NVI), respectively. A number of ubiquitously expressed anion and cation channels are involved not only in CVD but also in cell death induction. This series of review articles address the question how cell death is induced or protected with using ubiquitously expressed ion channels such as swelling-activated anion channels, acid-activated anion channels and several types of TRP cation channels including TRPM2 and TRPM7. The Part 1 focuses on the roles of the volume-sensitive outwardly rectifying anion channels (VSOR), also called the volume-regulated anion channel (VRAC), which is activated by cell swelling or reactive oxygen species (ROS) in a manner dependent on intracellular ATP. First we describe phenotypical properties, the molecular identity, and physical pore dimensions of VSOR/VRAC. Second, we highlight the roles of VSOR/VRAC in the release of organic signaling molecules, such as glutamate, glutathione, ATP and cGAMP, that play roles as double-edged swords in cell survival. Third, we discuss how VSOR/VRAC is involved in CVR and cell volume dysregulation as well as in the induction of or protection from apoptosis, necrosis and regulated necrosis under pathophysiological conditions.

Ischemia-Triggered Glutamate Excitotoxicity From the Perspective of Glial Cells

A plethora of neurological disorders shares a final common deadly pathway known as excitotoxicity. Among these disorders, ischemic injury is a prominent cause of death and disability worldwide. Brain ischemia stems from cardiac arrest or stroke, both responsible for insufficient blood supply to the brain parenchyma. Glucose and oxygen deficiency disrupts oxidative phosphorylation, which results in energy depletion and ionic imbalance, followed by cell membrane depolarization, calcium (Ca²⁺) overload, and extracellular accumulation of excitatory amino acid glutamate. If tight physiological regulation fails to clear the surplus of this neurotransmitter, subsequent prolonged activation of glutamate receptors forms a vicious circle between elevated concentrations of intracellular Ca²⁺ ions and aberrant glutamate release, aggravating the effect of this ischemic pathway. The activation of downstream Ca²⁺-dependent enzymes has a catastrophic impact on nervous tissue leading to cell death, accompanied by the formation of free radicals, edema, and inflammation. After decades of “neuron-centric” approaches, recent research has also finally shed some light on the role of glial cells in neurological diseases. It is becoming more and more evident that neurons and glia depend on each other. Neuronal cells, astrocytes, microglia, NG2 glia, and oligodendrocytes all have their roles in what is known as glutamate excitotoxicity. However, who is the main contributor to the ischemic pathway, and who is the unsuspecting victim? In this review article, we summarize the so-far-revealed roles of cells in the central nervous system, with particular attention to glial cells in ischemia-induced glutamate excitotoxicity, its origins, and consequences.

Functional activities of beta-glucans in the prevention or treatment of cervical cancer

Cervical cancer is the fourth-ranked cancer in the world and is associated with a large number of deaths annually. Chemotherapy and radiotherapy are known as the common therapeutic approaches in the treatment of cervical cancer, but because of their side effects and toxicity, researchers are trying to discovery alternative therapies. Beta-glucans, a group of glucose polymers that are derived from the cell wall of fungi, bacteria, and etc. it has been showed that beta-glucans have some anti-cancer properties which due to their impacts on adaptive and innate immunity. Along to these impacts, these molecules could be used as drug carriers. In this regard, the application of beta-glucans is a promising therapeutic option for the cancer prevention and treatment especially for cervical cancer. Herein, we have summarized the therapeutic potential of beta-glucans alone or as adjuvant therapy in the treatment of cervical cancer. Moreover, we highlighted beta-glucans as drug carriers for preventive and therapeutic purposes.

Functional activities of beta-glucans in the prevention or treatment of cervical cancer

Cervical cancer is the fourth-ranked cancer in the world and is associated with a large number of deaths annually. Chemotherapy and radiotherapy are known as the common therapeutic approaches in the treatment of cervical cancer, but because of their side effects and toxicity, researchers are trying to discovery alternative therapies. Beta-glucans, a group of glucose polymers that are derived from the cell wall of fungi, bacteria, and etc. it has been showed that beta-glucans have some anti-cancer properties which due to their impacts on adaptive and innate immunity. Along to these impacts, these molecules could be used as drug carriers. In this regard, the application of beta-glucans is a promising therapeutic option for the cancer prevention and treatment especially for cervical cancer. Herein, we have summarized the therapeutic potential of beta-glucans alone or as adjuvant therapy in the treatment of cervical cancer. Moreover, we highlighted beta-glucans as drug carriers for preventive and therapeutic purposes.

Effect of Glycine on BV-2 Microglial Cells Treated with Interferon-γ and Lipopolysaccharide

Microglia are first-line defense antigen-presenting phagocytes in the central nervous system. Activated microglial cells release pro-inflammatory cytokines and can trigger an oxidative burst. The amino acid glycine exerts anti-inflammatory, immunomodulatory and cytoprotective effects and influences cell volume regulation. This study aimed to investigate the role of glycine in the modulation of inflammatory processes in mouse BV-2 microglial cells. Inflammatory stress was induced by lipopolysaccharide/interferon-γ (LPS/IFN-γ) treatment for 24 h in the absence or presence of 1 or 5 mM glycine. Cells were analyzed by flow cytometry for cell volume, side scatter, apoptosis/necrosis and expression of activation-specific surface markers. Apoptosis progression was monitored by life cell imaging. Reduced glutathione/oxidized glutathione (GSH/GSSG) ratios and release of the pro-inflammatory cytokines IL-6 and TNF-α were measured using luminescence-based assays and ELISA, respectively. We found that LPS/IFN-γ-induced apoptosis was decreased and the fraction of living cells was increased by glycine. Expression of the surface markers CD11b, CD54 and CD80 was dose-dependently increased, while IL-6 and TNF-α release was not altered compared to LPS/IFN-γ-treated cells. We showed that in BV-2 microglial cells glycine improves viability and counteracts deleterious responses to LPS/IFN-γ, which might be relevant in neurodegenerative processes associated with inflammation, like Alzheimer’s or Parkinson’s disease.

Microglia in Alzheimer Disease: Well-Known Targets and New Opportunities

Microglia are the resident macrophages of the central nervous system. They play key roles in brain development, and physiology during life and aging. Equipped with a variety of molecular sensors and through the various functions they can fulfill, they are critically involved in maintaining the brain’s homeostasis. In Alzheimer disease (AD), microglia reaction was initially thought to be incidental and triggered by amyloid deposits and dystrophic neurites. However, recent genome-wide association studies have established that the majority of AD risk loci are found in or near genes that are highly and sometimes uniquely expressed in microglia. This leads to the concept of microglia being critically involved in the early steps of the disease and identified them as important potential therapeutic targets. Whether microglia reaction is beneficial, detrimental or both to AD progression is still unclear and the subject of intense debate. In this review, we are presenting a state-of-knowledge report intended to highlight the variety of microglial functions and pathways shown to be critically involved in AD progression. We first address both the acquisition of new functions and the alteration of their homeostatic roles by reactive microglia. Second, we propose a summary of new important parameters currently emerging in the field that need to be considered to identify relevant microglial targets. Finally, we discuss the many obstacles in designing efficient therapeutic strategies for AD and present innovative technologies that may foster our understanding of microglia roles in the pathology. Ultimately, this work aims to fly over various microglial functions to make a general and reliable report of the current knowledge regarding microglia’s involvement in AD and of the new research opportunities in the field.

More than just a pressure relief valve: physiological roles of volume-regulated LRRC8 anion channels

The volume-regulated anion channel (VRAC) is a key player in the volume regulation of vertebrate cells. This ubiquitously expressed channel opens upon osmotic cell swelling and potentially other cues and releases chloride and organic osmolytes, which contributes to regulatory volume decrease (RVD). A plethora of studies have proposed a wide range of physiological roles for VRAC beyond volume regulation including cell proliferation, differentiation and migration, apoptosis, intercellular communication by direct release of signaling molecules and by supporting the exocytosis of insulin. VRAC was additionally implicated in pathological states such as cancer therapy resistance and excitotoxicity under ischemic conditions. Following extensive investigations, 5 years ago leucine-rich repeat-containing family 8 (LRRC8) heteromers containing LRRC8A were identified as the pore-forming components of VRAC. Since then, molecular biological approaches have allowed further insight into the biophysical properties and structure of VRAC. Heterologous expression, siRNA-mediated downregulation and genome editing in cells, as well as the use of animal models have enabled the assessment of the proposed physiological roles, together with the identification of new functions including spermatogenesis and the uptake of antibiotics and platinum-based cancer drugs. This review discusses the recent molecular biological insights into the physiology of VRAC in relation to its previously proposed roles.

Cell Volume-Activated and Volume-Correlated Anion Channels in Mammalian Cells: Their Biophysical, Molecular, and Pharmacological Properties

There are a number of mammalian anion channel types associated with cell volume changes. These channel types are classified into two groups: volume-activated anion channels (VAACs) and volume-correlated anion channels (VCACs). VAACs can be directly activated by cell swelling and include the volume-sensitive outwardly rectifying anion channel (VSOR), which is also called the volume-regulated anion channel; the maxi-anion channel (MAC or Maxi-Cl); and the voltage-gated anion channel, chloride channel (ClC)-2. VCACs can be facultatively implicated in, although not directly activated by, cell volume changes and include the cAMP-activated cystic fibrosis transmembrane conductance regulator (CFTR) anion channel, the Ca²⁺-activated Cl^- channel (CaCC), and the acid-sensitive (or acid-stimulated) outwardly rectifying anion channel. This article describes the phenotypical properties and activation mechanisms of both groups of anion channels, including accumulating pieces of information on the basis of recent molecular understanding. To that end, this review also highlights the molecular identities of both anion channel groups; in addition to the molecular identities of ClC-2 and CFTR, those of CaCC, VSOR, and Maxi-Cl were recently identified by applying genome-wide approaches. In the last section of this review, the most up-to-date information on the pharmacological properties of both anion channel groups, especially their half-maximal inhibitory concentrations (IC₅₀ values) and voltage-dependent blocking, is summarized particularly from the standpoint of pharmacological distinctions among them. Future physiologic and pharmacological studies are definitely warranted for therapeutic targeting of dysfunction of VAACs and VCACs.

Crosstalk between Na+,K+-ATPase and a volume-regulated anion channel in membrane microdomains of human cancer cells

Low concentrations of cardiac glycosides including ouabain, digoxin, and digitoxin block cancer cell growth without affecting Na⁺,K⁺-ATPase activity, but the mechanism underlying this anti-cancer effect is not fully understood. Volume-regulated anion channel (VRAC) plays an important role in cell death signaling pathway in addition to its fundamental role in the cell volume maintenance. Here, we report cardiac glycosides-induced signaling pathway mediated by the crosstalk between Na⁺,K⁺-ATPase and VRAC in human cancer cells. Submicromolar concentrations of ouabain enhanced VRAC currents concomitantly with a deceleration of cancer cell proliferation. The effects of ouabain were abrogated by a specific inhibitor of VRAC (DCPIB) and knockdown of an essential component of VRAC (LRRC8A), and they were also attenuated by the disruption of membrane microdomains or the inhibition of NADPH oxidase. Digoxin and digitoxin also showed anti-proliferative effects in cancer cells at their therapeutic concentration ranges, and these effects were blocked by DCPIB. In membrane microdomains of cancer cells, LRRC8A was found to be co-immunoprecipitated with Na⁺,K⁺-ATPase α1-isoform. These ouabain-induced effects were not observed in non-cancer cells. Therefore, cardiac glycosides were considered to interact with Na⁺,K⁺-ATPase to stimulate the production of reactive oxygen species, and they also apparently activated VRAC within membrane microdomains, thus producing anti-proliferative effects.

The anti-parkinsonian drug zonisamide reduces neuroinflammation: Role of microglial Nav 1.6

Parkinson's disease (PD), the second most common age-related progressive neurodegenerative disorder, is characterized by dopamine depletion and the loss of dopaminergic (DA) neurons with accompanying neuroinflammation. Zonisamide is an-anti-convulsant drug that has recently been shown to improve clinical symptoms of PD through its inhibition of monoamine oxidase B (MAO-B). However, zonisamide has additional targets, including voltage-gated sodium channels (Na_v), which may contribute to its reported neuroprotective role in preclinical models of PD. Here, we report that Na_v1.6 is highly expressed in microglia of post-mortem PD brain and of mice treated with the parkinsonism-inducing neurotoxin MPTP. Administration of zonisamide (20 mg/kg, i.p. every 4 h × 3) following a single injection of MPTP (12.5 mg/kg, s.c.) reduced microglial Na_v 1.6 and microglial activation in the striatum, as indicated by Iba-1 staining and mRNA expression of F4/80. MPTP increased the levels of the pro-inflammatory cytokine TNF-α and gp91^phox, and this was significantly reduced by zonisamide. Together, these findings suggest that zonisamide may reduce neuroinflammation through the down-regulation of microglial Na_v 1.6. Thus, in addition to its effects on parkinsonian symptoms through inhibition of MAO-B, zonisamide may have disease modifying potential through the inhibition of Na_v 1.6 and neuroinflammation.

Blockade of microglial adenosine A2A receptor impacts inflammatory mechanisms, reduces ARPE-19 cell dysfunction and prevents photoreceptor loss in vitro

Age-related macular degeneration (AMD) is characterized by pathological changes in the retinal pigment epithelium (RPE) and loss of photoreceptors. Growing evidence has demonstrated that reactive microglial cells trigger RPE dysfunction and loss of photoreceptors, and inflammasome pathways and complement activation contribute to AMD pathogenesis. We and others have previously shown that adenosine A_2A receptor (A_2AR) blockade prevents microglia-mediated neuroinflammatory processes and mediates protection to the retina. However, it is still unknown whether blocking A_2AR in microglia protects against the pathological features of AMD. Herein, we show that an A_2AR antagonist, SCH58261, prevents the upregulation of the expression of pro-inflammatory mediators and the alterations in the complement system triggered by an inflammatory challenge in human microglial cells. Furthermore, blockade of A_2AR in microglia decreases the inflammatory response, as well as complement and inflammasome activation, in ARPE-19 cells exposed to conditioned medium of activated microglia. Finally, we also show that blocking A_2AR in human microglia increases the clearance of apoptotic photoreceptors. This study opens the possibility of using selective A_2AR antagonists in therapy for AMD, by modulating the interplay between microglia, RPE and photoreceptors.

The signaling role for chloride in the bidirectional communication between neurons and astrocytes

It is well known that the electrical signaling in neuronal networks is modulated by chloride (Cl^-) fluxes via the inhibitory GABA_A and glycine receptors. Here, we discuss the putative contribution of Cl^- fluxes and intracellular Cl^- to other forms of information transfer in the CNS, namely the bidirectional communication between neurons and astrocytes. The manuscript (i) summarizes the generic functions of Cl^- in cellular physiology, (ii) recaps molecular identities and properties of Cl^- transporters and channels in neurons and astrocytes, and (iii) analyzes emerging studies implicating Cl^- in the modulation of neuroglial communication. The existing literature suggests that neurons can alter astrocytic Cl^- levels in a number of ways; via (a) the release of neurotransmitters and activation of glial transporters that have intrinsic Cl^- conductance, (b) the metabotropic receptor-driven changes in activity of the electroneutral cation-Cl^- cotransporter NKCC1, and (c) the transient, activity-dependent changes in glial cell volume which open the volume-regulated Cl^-/anion channel VRAC. Reciprocally, astrocytes are thought to alter neuronal [Cl^-]_i through either (a) VRAC-mediated release of the inhibitory gliotransmitters, GABA and taurine, which open neuronal GABA_A and glycine receptor/Cl^- channels, or (b) the gliotransmitter-driven stimulation of NKCC1. The most important recent developments in this area are the identification of the molecular composition and functional heterogeneity of brain VRAC channels, and the discovery of a new cytosolic [Cl^-] sensor - the Wnk family protein kinases. With new work in the field, our understanding of the role of Cl^- in information processing within the CNS is expected to be significantly updated.

Markers and Biomarkers of Endothelium: When Something Is Rotten in the State

Endothelium is a community of endothelial cells (ECs), which line the blood and lymphatic vessels, thus forming an interface between the tissues and the blood or lympha. This strategic position of endothelium infers its indispensable functional role in controlling vasoregulation, haemostasis, and inflammation. The state of endothelium is simultaneously the cause and effect of many diseases, and this is coupled with modifications of endothelial phenotype represented by markers and with biochemical profile of blood represented by biomarkers. In this paper, we briefly review data on the functional role of endothelium, give definitions of endothelial markers and biomarkers, touch on the methodological approaches for revealing biomarkers, present an implicit role of endothelium in some toxicological mechanistic studies, and survey the role of reactive oxygen species (ROS) in modulation of endothelial status.

NADPH oxidases in Parkinson's disease: a systematic review

Parkinson's disease (PD) is a progressive movement neurodegenerative disease associated with a loss of dopaminergic neurons in the substantia nigra of the brain. Oxidative stress, a condition that occurs due to imbalance in oxidant and antioxidant status, is thought to play an important role in dopaminergic neurotoxicity. Nicotinamide adenine dinucleotide phosphate (NADPH) oxidases are multi-subunit enzymatic complexes that generate reactive oxygen species as their primary function. Increased immunoreactivities for the NADPH oxidases catalytic subunits Nox1, Nox2 and Nox4 have been reported in the brain of PD patients. Furthermore, knockout or genetic inactivation of NADPH oxidases exert a neuroprotective effect and reduce detrimental aspects of pathology in experimental models of the disease. However, the connections between NADPH oxidases and the biological processes believed to contribute to neuronal death are not well known. This review provides a comprehensive summary of our current understanding about expression and physiological function of NADPH oxidases in neurons, microglia and astrocytes and their pathophysiological roles in PD. It summarizes the findings supporting the role of both microglial and neuronal NADPH oxidases in cellular disturbances associated with PD such as neuroinflammation, alpha-synuclein accumulation, mitochondrial and synaptic dysfunction or disruption of the autophagy-lysosome system. Furthermore, this review highlights different steps that are essential for NADPH oxidases enzymatic activity and pinpoints major obstacles to overcome for the development of effective NADPH oxidases inhibitors for PD.

Microglia under psychosocial stressors along the aging trajectory: Consequences on neuronal circuits, behavior, and brain diseases

Mounting evidence indicates the importance of microglia for proper brain development and function, as well as in complex stress-related neuropsychiatric disorders and cognitive decline along the aging trajectory. Considering that microglia are resident immune cells of the brain, a homeostatic maintenance of their effector functions that impact neuronal circuitry, such as phagocytosis and secretion of inflammatory factors, is critical to prevent the onset and progression of these pathological conditions. However, the molecular mechanisms by which microglial functions can be properly regulated under healthy and pathological conditions are still largely unknown. We aim to summarize recent progress regarding the effects of psychosocial stress and oxidative stress on microglial phenotypes, leading to neuroinflammation and impaired microglia-synapse interactions, notably through our own studies of inbred mouse strains, and most importantly, to discuss about promising therapeutic strategies that take advantage of microglial functions to tackle such brain disorders in the context of adult psychosocial stress or aging-induced oxidative stress.

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

KEY POINTS

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

ABSTRACT

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

NOX Inhibitors - A Promising Avenue for Ischemic Stroke

NADPH-oxidase (NOX) mediated superoxide originally found on leukocytes, but now recognized in several types of cells in the brain. It has been shown to play an important role in the progression of stroke and related cerebrovascular disease. NOX is a multisubunit complex consisting of 2 membrane-associated and 4 cytosolic subunits. NOX activation occurs when cytosolic subunits translocate to the membrane, leading to transport electrons to oxygen, thus producing superoxide. Superoxide produced by NOX is thought to function in long-term potentiation and intercellular signaling, but excessive production is damaging and has been implicated to play an important role in the progression of ischemic brain. Thus, inhibition of NOX activity may prove to be a promising treatment for ischemic brain as well as an adjunctive agent to prevent its secondary complications. There is mounting evidence that NOX inhibition in the ischemic brain is neuroprotective, and targeting NOX in circulating immune cells will also improve outcome. This review will focus on therapeutic effects of NOX assembly inhibitors in brain ischemia and stroke. However, the lack of specificity and toxicities of existing inhibitors are clear hurdles that will need to be overcome before this class of compounds could be translated clinically.

ATP release during cell swelling activates a Ca2+-dependent Cl- current by autocrine mechanism in mouse hippocampal microglia

Microglia cells, resident immune cells of the brain, survey brain parenchyma by dynamically extending and retracting their processes. Cl⁻ channels, activated in the cellular response to stretch/swelling, take part in several functions deeply connected with microglia physiology, including cell shape changes, proliferation, differentiation and migration. However, the molecular identity and functional properties of these Cl⁻ channels are largely unknown. We investigated the properties of swelling-activated currents in microglial from acute hippocampal slices of Cx3cr1^+/GFP mice by whole-cell patch-clamp and imaging techniques. The exposure of cells to a mild hypotonic medium, caused an outward rectifying current, developing in 5–10 minutes and reverting upon stimulus washout. This current, required for microglia ability to extend processes towards a damage signal, was carried mainly by Cl⁻ ions and dependent on intracellular Ca²⁺. Moreover, it involved swelling-induced ATP release. We identified a purine-dependent mechanism, likely constituting an amplification pathway of current activation: under hypotonic conditions, ATP release triggered the Ca²⁺-dependent activation of anionic channels by autocrine purine receptors stimulation. Our study on native microglia describes for the first time the functional properties of stretch/swelling-activated currents, representing a key element in microglia ability to monitor the brain parenchyma.

Beneficial effects of dried pomegranate juice concentrated powder on ultraviolet B-induced skin photoaging in hairless mice

The present study investigated the anti-aging effects of pomegranate juice concentrated powder (PCP) in hairless mice following 15 weeks of UVB irradiation (three times a week; 0.18 J/cm²). Skin moisturizing effects were evaluated through skin water, collagen type I and hyaluronan contents, as well as collagen type I and hyaluronan synthesis-related transcript levels. Wrinkle formation and edema scores (skin weights) were also assessed, along with skin matrix metalloproteinase (MMP)-1, MMP-9 and MMP-13 transcript levels. To determine the anti-inflammatory effects of PCP, myeloperoxidase (MPO) activity, interleukin (IL)-1β and IL-10 contents were observed. Caspase-3 and cleaved poly(ADP-ribose) polymerase (PARP) were used as an apoptotic index in epidermal keratinocytes. To determine the anti-oxidative effects of PCP, nitrotyrosine and 4-hydroxynonenal immunoreactive cells were detected and glutathione (GSH) content, malondialdehyde levels, superoxide anion production, Nox2, and GSH reductase mRNA expression were all measured. The results indicated that skin wrinkles induced by photoaging were significantly reduced by PCP, whereas skin water contents, collagen type I and hyaluronan contents all increased. Furthermore, IL-1β levels in the PCP-treated groups were lower than those in the UVB-exposed control group. UVB-induced GSH depletion was also inhibited by PCP. Taken together, the results of the current study suggest that PCP has favorable protective effects against UVB-induced photoaging through anti-apoptotic effects, MMP activity inhibition and ECM (COL1 and hyaluronan) synthesis-related moisturizing, anti-inflammatory and anti-oxidative effects.

Microglia antioxidant systems and redox signalling

For many years, microglia, the resident CNS macrophages, have been considered only in the context of pathology, but microglia are also glial cells with important physiological functions. Microglia-derived oxidant production by NADPH oxidase (NOX2) is implicated in many CNS disorders. Oxidants do not stand alone, however, and are not always pernicious. We discuss in general terms, and where available in microglia, GSH synthesis and relation to cystine import and glutamate export, and the thioredoxin system as the most important antioxidative defence mechanism, and further, we discuss in the context of protein thiolation of target redox proteins the necessity for tightly localized, timed and confined oxidant production to work in concert with antioxidant proteins to promote redox signalling. NOX2-mediated redox signalling modulates the acquisition of the classical or alternative microglia activation phenotypes by regulating major transcriptional programs mediated through NF-κB and Nrf2, major regulators of the inflammatory and antioxidant response respectively. As both antioxidants and NOX-derived oxidants are co-secreted, in some instances redox signalling may extend to neighboring cells through modification of surface or cytosolic target proteins. We consider a role for microglia NOX-derived oxidants in paracrine modification of synaptic function through long term depression and in the communication with the adaptive immune system. There is little doubt that a continued foray into the functions of the antioxidant response in microglia will reveal antioxidant proteins as dynamic players in redox signalling, which in concert with NOX-derived oxidants fulfil important roles in the autocrine or paracrine regulation of essential enzymes or transcriptional programs.

LINKED ARTICLES

This article is part of a themed section on Redox Biology and Oxidative Stress in Health and Disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.12/issuetoc.

Bathing effects of east saline groundwater concentrates on allergic (atopic) dermatitis-like skin lesions induced by 2,4-dinitrochlorobenzene in hairless mice

In the present study, it was evaluated whether east saline groundwater concentration solution (ESGWc) exerted a favorable inhibitory effect on 2,4-dinitrochlorobenzene (DNCB)-induced allergic/atopic-like dermatitis (AD). AD was induced and boosted by sensitization with DNCB via topical application on the dorsal back skins. Mice with DNCB-induced AD were bathed in 100-, 200- and 400-fold diluted ESGWc. After 6 weeks bathing, changes to body weight, clinical skin severity scores, scratching behavior, serum total immunoglobulin (Ig)E levels, submandibular lymph node and spleen weights, splenic cytokine levels, skin cytokine mRNA expressions, antioxidant defense systems and superoxide anion productions were recorded to determine the effects of bathing on the histopathology of dorsal back skin tissues. All DNCB-induced mice demonstrated that the induction of AD through IgE-mediated hypersensitivities, oxidative stresses, activation of MMP and apoptosis of keratinocytes resulted in no significant differences in body weight between the different groups at each time point following initial sensitization. However, markers of DNCB-induced AD were significantly inhibited (P<0.05) in a concentration-dependent manner following bathing in all concentrations of ESGWc. The results obtained in the present study suggest that bathing in ESGWc may have favorable protective effects against DNCB-induced AD due to favorable systemic and local immunomodulatory effects, active cytoprotective anti-apoptotic effects, inhibitory effects of matrix metalloproteinase activity, and anti-inflammatory and antioxidative effects.

Signalling mechanisms mediating Zn2+-induced TRPM2 channel activation and cell death in microglial cells

Excessive Zn²⁺ causes brain damage via promoting ROS generation. Here we investigated the role of ROS-sensitive TRPM2 channel in H₂O₂/Zn²⁺-induced Ca²⁺ signalling and cell death in microglial cells. H₂O₂/Zn²⁺ induced concentration-dependent increases in cytosolic Ca²⁺ concentration ([Ca²⁺]_c), which was inhibited by PJ34, a PARP inhibitor, and abolished by TRPM2 knockout (TRPM2-KO). Pathological concentrations of H₂O₂/Zn²⁺ induced substantial cell death that was inhibited by PJ34 and DPQ, PARP inhibitors, 2-APB, a TRPM2 channel inhibitor, and prevented by TRPM2-KO. Further analysis indicate that Zn²⁺ induced ROS production, PARP-1 stimulation, increase in the [Ca²⁺]_c and cell death, all of which were suppressed by chelerythrine, a protein kinase C inhibitor, DPI, a NADPH-dependent oxidase (NOX) inhibitor, GKT137831, a NOX1/4 inhibitor, and Phox-I2, a NOX2 inhibitor. Furthermore, Zn²⁺-induced PARP-1 stimulation, increase in the [Ca²⁺]_c and cell death were inhibited by PF431396, a Ca²⁺-sensitive PYK2 inhibitor, and U0126, a MEK/ERK inhibitor. Taken together, our study shows PKC/NOX-mediated ROS generation and PARP-1 activation as an important mechanism in Zn²⁺-induced TRPM2 channel activation and, TRPM2-mediated increase in the [Ca²⁺]_c to trigger the PYK2/MEK/ERK signalling pathway as a positive feedback mechanism that amplifies the TRPM2 channel activation. Activation of these TRPM2-depenent signalling mechanisms ultimately drives Zn²⁺-induced Ca²⁺ overloading and cell death.

Long-Term Effects of Maternal Deprivation on Redox Regulation in Rat Brain: Involvement of NADPH Oxidase

Maternal deprivation (MD) causes perinatal stress, with subsequent behavioral changes which resemble the symptoms of schizophrenia. The NADPH oxidase is one of the major generators of reactive oxygen species, known to play a role in stress response in different tissues. The aim of this study was to elucidate the long-term effects of MD on the expression of NADPH oxidase subunits (gp91^phox, p22^phox, p67^phox, p47^phox, and p40^phox). Activities of cytochrome C oxidase and respiratory chain Complex I, as well as the oxidative stress parameters using appropriate spectrophotometric techniques were analyzed. Nine-day-old Wistar rats were exposed to a 24 h maternal deprivation and sacrificed at young adult age. The structures affected by perinatal stress, cortex, hippocampus, thalamus, and caudate nuclei were investigated. The most prominent findings were increased expressions of gp91^phox in the cortex and hippocampus, increased expression of p22^phox and p40^phox, and decreased expression of gp91^phox, p22^phox, and p47^phox in the caudate nuclei. Complex I activity was increased in all structures except cortex. Content of reduced glutathione was decreased in all sections while region-specific changes of other oxidative stress parameters were found. Our results indicate the presence of long-term redox alterations in MD rats.

Neuroprotective erythropoietin attenuates microglial activation, including morphological changes, phagocytosis, and cytokine production

Erythropoietin (EPO), a hematopoietic hormonal cytokine induced in response to hypoxia, has neuroprotective effects. EPO receptor (EPOR) is expressed in microglia, resident immune cells in the brain. However, the effect of EPO on microglial activation is not clear. In the present study, we demonstrated that the EPOR is highly expressed in microglia, rather than in neurons or astrocytes, in in vitro experiments. Therefore, we investigated whether EPO could attenuate lipopolysaccharide (LPS)-mediated activation of microglia in vitro. The BV-2 microglial cell line was treated with LPS in the absence or presence of EPO. In the presence of EPO, microglial expression of LPS-induced inflammatory cytokine genes was significantly decreased. In addition, EPO suppressed the LPS-induced phagocytic activity of BV-2 cells towards fluorescent beads, as well as induction of inducible nitric oxide synthase. In in vivo experiments, EPO significantly decreased the LPS-induced expression of inflammatory cytokine genes in mouse brains. Furthermore, morphological analysis of cortical microglia in the brains of mice stimulated with LPS revealed that combined treatment with EPO alleviated LPS-induced morphological changes in the microglia. These data indicate that EPO attenuates microglial activation, including morphological changes in vivo, phagocytosis in vitro, and the production of inflammatory cytokines in vivo and in vitro. Further investigation of EPO modulation of LPS-induced microglial activation may contribute to the development of novel neuroprotective therapies.

Protective effect of 3-(naphthalen-2-yl(propoxy)methyl)azetidine hydrochloride on hypoxia-induced toxicity by suppressing microglial activation in BV-2 cells

We recently reported the anti-inflammatory effects of 3-(naphthalen-2-yl(propoxy)methyl)azetidine hydrochloride (KHG26792) on the ATP-induced activation of the NFAT and MAPK pathways through the P2X7 receptor in microglia. To further investigate the underlying mechanism of KHG26792, we studied its protective effects on hypoxia-induced toxicity in microglia. The administration of KHG26792 significantly reduced the hypoxia-induced expression and activity of caspase-3 in BV-2 microglial cells. KHG26792 also reduced hypoxia-induced inducible nitric oxide synthase protein expression, which correlated with reduced nitric oxide accumulation. In addition, KHG26792 attenuated hypoxia-induced protein nitration, reactive oxygen species production, and NADPH oxidase activity. These effects were accompanied by the suppression of hypoxia-induced protein expression of hypoxia-inducible factor 1-alpha and NADPH oxidase-2. Although the clinical relevance of our findings remains to be determined, these data results suggest that KHG26792 prevents hypoxia-induced toxicity by suppressing microglial activation.

NADPH oxidase isoform expression is temporally regulated and may contribute to microglial/macrophage polarization after spinal cord injury

Spinal cord injury (SCI) results in both acute and chronic inflammation, as a result of activation of microglia, invasion of macrophages and activation of the NADPH oxidase (NOX) enzyme. The NOX enzyme is a primary source of reactive oxygen species (ROS) and is expressed by microglia and macrophages after SCI. These cells can assume either a pro- (M1) or anti-inflammatory (M2) polarization phenotype and contribute to tissue response to SCI. However, the contribution of NOX expression and ROS production to this polarization and vice versa is currently undefined. We therefore investigated the impact of SCI on NOX expression and microglial/macrophage polarization over time in a mouse model of contusion injury. Adult C57Bl/6 mice were exposed to a moderate T9 contusion SCI and tissue was assessed at acute, sub-acute and chronic time points for NOX isoform expression and co-expression with M1 and M2 microglia/macrophage polarization markers. Two NOX isoforms were increased after injury and were associated with both M1 and M2 markers, with an M1 preference for NOX2 acutely and NOX4 chronically. M2 cells were primarily found at acute time points only; the peak of NOX2 expression was associated with the decline in M2 polarization. In vitro, NOX2 inhibition shifted microglial polarization toward the M2 phenotype. These results now show that microglial/macrophage expression of NOX isoforms is independent of polarization state, but that NOX activity can influence subsequent polarization. These data can contribute to the therapeutic targeting of NOX as a therapy for SCI.

Complex molecular and functional outcomes of single versus sequential cytokine stimulation of rat microglia

BACKGROUND

Microglia are the "professional" phagocytes of the CNS. Phagocytosis is crucial for normal CNS development and maintenance, but it can be either beneficial or detrimental after injury or disease. For instance, white matter damage releases myelin debris that must be cleared by microglia in order for re-myelination to occur. However, phagocytosis can also produce damaging reactive oxygen species (ROS). Furthermore, microglia can acquire pro-inflammatory (M1) or anti-inflammatory (M2) activation states that affect cell functions. Although microglia are exposed to a changing cytokine environment after injury or disease, little is known about the molecular and functional consequences. Therefore, we applied several microglial activation paradigms, with or without myelin debris. We assessed (i) gene expression changes reflecting microglial activation and inflammatory states, and receptors and enzymes related to phagocytosis and ROS production, (ii) myelin phagocytosis and production of ROS, and (iii) expression and contributions of several ion channels that are considered potential targets for regulating microglial behavior.

METHODS

Primary rat microglia were exposed to cytokines, individually or sequentially. First, responses to individual M1 or M2 stimuli were compared: IFN-γ plus TNF-α ("I + T"; M1 activation), interleukin-4 (M2a/alternative activation), and interleukin-10 (M2c/acquired deactivation). Second, sequential cytokine addition was used to assess microglia repolarization and cell functions. The paradigms were M2a→M1, M2c→M1, M1→M2a, and M1→M2c.

RESULTS

M1 stimulation increased pro-inflammatory genes, phagocytosis, and ROS, as well as expression of Kv1.3, KCa3.1, and Kir2.1 channels. M2a stimulation increased anti-inflammatory genes, ROS production, and Kv1.3 and KCa3.1 expression. Myelin phagocytosis enhanced the M1 profile and dampened the M2a profile, and both phagocytosis and ROS production were dependent on NOX enzymes and Kir2.1 and CRAC channels. Importantly, microglia showed some capacity for re-polarization between M1 and M2a states, based on gene expression changes, myelin phagocytosis, and ROS production.

CONCLUSIONS

In response to polarizing and re-polarizing cytokine treatments, microglia display complex changes in gene transcription profiles, phagocytic capacity, NOX-mediated ROS production, and in ion channels involved in microglial activation. Because these changes might affect microglia-mediated CNS inflammation, they should be considered in future experimental, pre-clinical studies.

Repurposing psychiatric medicines to target activated microglia in anxious mild cognitive impairment and early Parkinson's disease

Anxiety is common in the Mild Cognitive Impairment (MCI) stage of Alzheimer's disease (AD) and the pre-motor stages of Parkinson's disease (PD). A concomitant and possible cause of this anxiety is microglial activation, also considered a key promoter of neurodegeneration in MCI and early PD via inflammatory mechanisms and the generation of degenerative proinflammatory cytokines. Psychiatric disorders, prevalent in AD and PD, are often treated with psychiatric drugs (psychotropics), raising the question of whether psychotropics might therapeutically affect microglial activation, MCI, and PD. The literature of common psychotropics used in treating psychiatric disorders was reviewed for preclinical and clinical findings regarding microglial activation. Findings potentially compatible with reduced microglial activation or reduced microglial inflammogen release were evident for: antipsychotics including neuroleptics (chlorpromazine, thioridazine, loxapine) and atypicals (aripiprazole, olanzapine, quetiapine, risperidone, ziprasidone); mood stabilizers (carbamazepine, valproate, lithium); antidepressants including tricyclics (amitriptyline, clomipramine, imipramine, nortriptyline), SSRIs (citalopram, escitalopram, fluoxetine, fluvoxamine, paroxetine, sertraline), venlafaxine, and bupropion; benzodiazepine anxiolytics (clonazepam, diazepam); cognitive enhancers (donepezil, galantamine, memantine); and other drugs (dextromethorphan, quinidine, amantadine). In contrast, pramipexole and methylphenidate might promote microglial activation. The most promising replicated findings of reduced microglial activation are for quetiapine, valproate, lithium, fluoxetine, donepezil, and memantine but further study is needed and translation of their microglial effects to human disease still requires investigation. In AD-relevant models, risperidone, valproate, lithium, fluoxetine, bupropion, donepezil, and memantine have therapeutic microglial effects in need of replication. Limited clinical data suggest some support for lithium and donepezil in reducing MCI progression, but other drugs have not been studied. In PD-relevant models, lamotrigine, valproate, fluoxetine, dextromethorphan, and amantadine have therapeutic microglial effects whereas methylphenidate induced microglial activation and pramipexole promoted NO release. Clinical data limited to pramipexole do not as of yet indicate faster progression of early PD while the other drugs remain to be investigated. These tantalizing psychotropic neuroprotective findings now invite replication and evidence in AD-and PD-specific models under chronic administration, followed by consideration for clinical trials in MCI and early stage PD. Psychiatric features in early disease may provide opportunities for clinical studies that also employ microglial PET biomarkers.

NADPH oxidases in oxidant production by microglia: activating receptors, pharmacology and association with disease

Microglia are the resident immune cells of the CNS and constitute a self-sustaining population of CNS-adapted tissue macrophages. As mononuclear phagocytic cells, they express high levels of superoxide-producing NADPH oxidases (NOX). The sole function of the members of the NOX family is to generate reactive oxygen species (ROS) that are believed to be important in CNS host defence and in the redox signalling circuits that shape the different activation phenotypes of microglia. NOX are also important in pathological conditions, where over-generation of ROS contributes to neuronal loss via direct oxidative tissue damage or disruption of redox signalling circuits. In this review, we assess the evidence for involvement of NOX in CNS physiopathology, with particular emphasis on the most important surface receptors that lead to generation of NOX-derived ROS. We evaluate the potential significance of the subcellular distribution of NOX isoforms for redox signalling or release of ROS to the extracellular medium. Inhibitory mechanisms that have been reported to restrain NOX activity in microglia and macrophages in vivo are also discussed. We provide a critical appraisal of frequently used and recently developed NOX inhibitors. Finally, we review the recent literature on NOX and other sources of ROS that are involved in activation of the inflammasome and discuss the potential influence of microglia-derived oxidants on neurogenesis, neural differentiation and culling of surplus progenitor cells. The degree to which excessive, badly timed or misplaced NOX activation in microglia may affect neuronal homeostasis in physiological or pathological conditions certainly merits further investigation.

LINKED ARTICLES

This article is part of a themed section on Redox Biology and Oxidative Stress in Health and Disease. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.12/issuetoc.

NADPH Oxidase: A Potential Target for Treatment of Stroke

Stroke is the third leading cause of death in industrialized nations. Oxidative stress is involved in the pathogenesis of stroke, and excessive generation of reactive oxygen species (ROS) by mitochondria is thought to be the main cause of oxidative stress. NADPH oxidase (NOX) enzymes have recently been identified and studied as important producers of ROS in brain tissues after stroke. Several reports have shown that knockout or deletion of NOX exerts a neuroprotective effect in three major experimental stroke models. Recent studies also confirmed that NOX inhibitors ameliorate brain injury and improve neurological outcome after stroke. However, the physiological and pathophysiological roles of NOX enzymes in the central nervous system (CNS) are not known well. In this review, we provide a comprehensive summary of our current understanding about expression and physiological function of NOX enzymes in the CNS and its pathophysiological roles in the three major types of stroke: ischemic stroke, intracerebral hemorrhage, and subarachnoid hemorrhage.

Volume-regulated anion channel--a frenemy within the brain

The volume-regulated anion channel (VRAC) is a ubiquitously expressed yet highly enigmatic member of the superfamily of chloride/anion channels. It is activated by cellular swelling and mediates regulatory cell volume decrease in a majority of vertebrate cells, including those in the central nervous system (CNS). In the brain, besides its crucial role in cellular volume regulation, VRAC is thought to play a part in cell proliferation, apoptosis, migration, and release of physiologically active molecules. Although these roles are not exclusive to the CNS, the relative significance of VRAC in the brain is amplified by several unique aspects of its physiology. One important example is the contribution of VRAC to the release of the excitatory amino acid neurotransmitters glutamate and aspartate. This latter process is thought to have impact on both normal brain functioning (such as astrocyte-neuron signaling) and neuropathology (via promoting the excitotoxic death of neuronal cells in stroke and traumatic brain injury). In spite of much work in the field, the molecular nature of VRAC remained unknown until less than 2 years ago. Two pioneer publications identified VRAC as the heterohexamer formed by the leucine-rich repeat-containing 8 (LRRC8) proteins. These findings galvanized the field and are likely to result in dramatic revisions to our understanding of the place and role of VRAC in the brain, as well as other organs and tissues. The present review briefly recapitulates critical findings in the CNS and focuses on anticipated impact on the LRRC8 discovery on further progress in neuroscience research.

Role of volume-regulated and calcium-activated anion channels in cell volume homeostasis, cancer and drug resistance

Volume-regulated channels for anions (VRAC) / organic osmolytes (VSOAC) play essential roles in cell volume regulation and other cellular functions, e.g. proliferation, cell migration and apoptosis. LRRC8A, which belongs to the leucine rich-repeat containing protein family, was recently shown to be an essential component of both VRAC and VSOAC. Reduced VRAC and VSOAC activities are seen in drug resistant cancer cells. ANO1 is a calcium-activated chloride channel expressed on the plasma membrane of e.g., secretory epithelia. ANO1 is amplified and highly expressed in a large number of carcinomas. The gene, encoding for ANO1, maps to a region on chromosome 11 (11q13) that is frequently amplified in cancer cells. Knockdown of ANO1 impairs cell proliferation and cell migration in several cancer cells. Below we summarize the basic biophysical properties of VRAC, VSOAC and ANO1 and their most important cellular functions as well as their role in cancer and drug resistance.

Bathing Effects of Various Seawaters on Allergic (Atopic) Dermatitis-Like Skin Lesions Induced by 2,4-Dinitrochlorobenzene in Hairless Mice

We evaluated the preventive effects of four types of seawater collected in Republic of Korea on hairless mice with 2,4-dinitrochlorobenzene- (DNCB-) induced allergic/atopic dermatitis (AD). The anti-inflammatory effects were evaluated by measuring tumor necrosis factor- (TNF-) α and interleukins (ILs). Glutathione (GSH), malondialdehyde (MDA), superoxide anion, and inducible nitric oxide synthase (iNOS) were measured to evaluate the antioxidant effects. Caspase-3 and poly (ADP-ribose) polymerase (PARP) were observed to measure the antiapoptotic effects; matrix metalloproteinase- (MMP-) 9 levels were also evaluated. Mice with AD had markedly higher clinical skin severity scores and scratching behaviors; higher TNF-α and ILs (1β, 10, 4, 5, and 13) levels; higher MDA, superoxide anion, caspase-3, PARP, and MMP-9 levels; and greater iNOS activity. However, the severity of AD was significantly decreased by bathing in seawaters, but it did not influence the dermal collagen depositions and skin tissue antioxidant defense systems. These results suggest that bathing in all four seawaters has protective effects against DNCB-induced AD through their favorable systemic and local immunomodulatory effects, active cytoprotective antiapoptotic effects, inhibitory effects of MMP activity and anti-inflammatory and antioxidative effects.

Oxidative stress, redox signalling and endothelial dysfunction in ageing-related neurodegenerative diseases: a role of NADPH oxidase 2

Chronic oxidative stress and oxidative damage of the cerebral microvasculature and brain cells has become one of the most convincing theories in neurodegenerative pathology. Controlled oxidative metabolism and redox signalling in the central nervous system are crucial for maintaining brain function; however, excessive production of reactive oxygen species and enhanced redox signalling damage neurons. While several enzymes and metabolic processes can generate intracellular reactive oxygen species in the brain, recently an O2−-generating enzyme, NADPH oxidase 2 (Nox2), has emerged as a major source of oxidative stress in ageing-related vascular endothelial dysfunction and neurodegenerative diseases. The currently available inhibitors of Nox2 are not specific, and general antioxidant therapy is not effective in the clinic; therefore, insights into the mechanism of Nox2 activation and its signalling pathways are needed for the discovery of novel drug targets to prevent or treat these neurodegenerative diseases. This review summarizes the recent developments in understanding the mechanisms of Nox2 activation and redox-sensitive signalling pathways and biomarkers involved in the pathophysiology of the most common neurodegenerative diseases, such as ageing-related mild cognitive impairment, Alzheimer's disease and Parkinson's disease.

The identification of a volume-regulated anion channel: an amazing Odyssey

The volume-regulated anion channel (VRAC) plays a pivotal role in cell volume regulation in essentially all cell types studied. Additionally, VRAC appears to contribute importantly to a wide range of other cellular functions and pathological events, including cell motility, cell proliferation, apoptosis and excitotoxic glutamate release in stroke. Although biophysically, pharmacologically and functionally thoroughly described, VRAC has until very recently remained a genetic orphan. The search for the molecular identity of VRAC has been long and has yielded multiple potential candidates, all of which eventually turned out to have properties not fully compatible with those of VRAC. Recently, two groups have independently identified the protein leucine-rich repeats containing 8A (LRRC8A), belonging to family of proteins (LRRC8A-E) distantly related to pannexins, as the likely pore-forming subunit of VRAC. In this brief review, we summarize the history of the discovery of VRAC, outline its basic biophysical and pharmacological properties, link these to several cellular functions in which VRAC appears to play important roles, and sketch the amazing search for the molecular identity of this channel. Finally, we describe properties of the LRRC8 proteins, highlight some features of the LRRC8A knockout mouse and discuss the impact of the discovery of LRRC8 as VRAC on future research.

c-Src deactivation by the polyphenol 3-O-caffeoylquinic acid abrogates reactive oxygen species-mediated glutamate release from microglia and neuronal excitotoxicity

3-O-caffeoylquinic acid (3-CQA) is an isomer of chlorogenic acid, which has been shown to regulate lipopolysaccharide-induced tumor necrosis factor production in microglia. Whereas overactivation of microglia is associated with neuronal loss in brain diseases via reactive oxygen species (ROS) production and glutamate excitotoxicity, naïve (nonactivated) microglia are believed to generate little ROS under basal conditions, contributing to the modulation of synaptic activity and nerve tissue repair. However, the signaling pathways controlling basal ROS homeostasis in microglial cells are still poorly understood. Here we used time-lapse microscopy coupled with highly sensitive FRET biosensors (for detecting c-Src activation, ROS generation, and glutamate release) and lentivirus-mediated shRNA delivery to study the pathways involved in antioxidant-regulated ROS generation and how this associates with microglia-induced neuronal cell death. We report that 3-CQA abrogates the acquisition of an amoeboid morphology in microglia triggered by Aβ oligomers or the HIV Tat peptide. Moreover, 3-CQA deactivates c-Src tyrosine kinase and abrogates c-Src activation during proinflammatory microglia stimulation, which shuts off ROS production in these cells. Moreover, forced increment of c-Src catalytic activity by overexpressing an inducible c-Src heteromerization construct in microglia increases ROS production, abrogating the 3-CQA effects. Whereas oxidant (hydrogen peroxide) stimulation dramatically enhances glutamate release from microglia, such release is diminished by the 3-CQA inhibition of c-Src/ROS generation, significantly alleviating cell death in cultures from embryonic neurons. Overall, we provide further mechanistic insight into the modulation of ROS production in cortical microglia, indicating antioxidant-regulated c-Src function as a pathway for controlling microglia-triggered oxidative damage.

The neuroprotective properties of the superoxide dismutase mimetic tempol correlate with its ability to reduce pathological glutamate release in a rodent model of stroke

The contribution of oxidative stress to ischemic brain damage is well established. Nevertheless, for unknown reasons, several clinically tested antioxidant therapies have failed to show benefits in human stroke. Based on our previous in vitro work, we hypothesized that the neuroprotective potency of antioxidants is related to their ability to limit the release of the excitotoxic amino acids glutamate and aspartate. We explored the effects of two antioxidants, tempol and edaravone, on amino acid release in the brain cortex, in a rat model of transient occlusion of the middle cerebral artery (MCAo). Amino acid levels were quantified using a microdialysis approach, with the probe positioned in the ischemic penumbra as verified by a laser Doppler technique. Two-hour MCAo triggered a dramatic increase in the levels of glutamate, aspartate, taurine, and alanine. Microdialysate delivery of 10mM tempol reduced the amino acid release by 60-80%, whereas matching levels of edaravone had no effect. In line with these data, an intracerebroventricular injection of tempol but not edaravone (500 nmol each, 15 min before MCAo) reduced infarction volumes by ~50% and improved neurobehavioral outcomes. In vitro assays showed that tempol was superior at removing superoxide anion, whereas edaravone was more potent at scavenging hydrogen peroxide, hydroxyl radical, and peroxynitrite. Overall, our data suggest that the neuroprotective properties of tempol are probably related to its ability to reduce tissue levels of the superoxide anion and pathological glutamate release and, in such a way, limit progression of brain infarction within ischemic penumbra. These new findings may be instrumental in developing new antioxidant therapies for treatment of stroke.