Physiological mechanisms of lysophosphatidylcholine-induced de-ramification of murine microglia

Identification of Metabolomic Signatures for Ischemic Hypoxic Encephalopathy Using a Neonatal Rat Model

A study was performed to determine early metabolomic markers of ischemic hypoxic encephalopathy (HIE) using a Rice-Vannucci model for newborn rats. Dried blood spots from 7-day-old male and female rat pups, including 10 HIE-affected animals and 16 control animals, were analyzed by liquid chromatography coupled with mass spectrometry (HPLC-MS) in positive and negative ion recording modes. Multivariate statistical analysis revealed two distinct clusters of metabolites in both HPLC-MS modes. Subsequent univariate statistical analysis identified 120 positive and 54 negative molecular ions that exhibited statistically significant change in concentration, with more than a 1.5-fold difference after HIE. In the HIE group, the concentrations of steroid hormones, saturated mono- and triglycerides, and phosphatidylcholines (PCs) were significantly decreased in positive mode. On the contrary, the concentration of unsaturated PCs was increased in the HIE group. Among negatively charged molecular ions, the greatest variations were found in the categories of phosphatidylcholines, phosphatidylinositols, and triglycerides. The major metabolic pathways associated with changed metabolites were analyzed for both modes. Metabolic pathways such as steroid biosynthesis and metabolism fatty acids were most affected. These results underscored the central role of glycerophospholipid metabolism in triggering systemic responses in HIE. Therefore, lipid biomarkers' evaluation by targeted HPLC-MS research could be a promising approach for the early diagnosis of HIE.

Microglial morphometric analysis: so many options, so little consistency

Quantification of microglial activation through morphometric analysis has long been a staple of the neuroimmunologist's toolkit. Microglial morphological phenomics can be conducted through either manual classification or constructing a digital skeleton and extracting morphometric data from it. Multiple open-access and paid software packages are available to generate these skeletons via semi-automated and/or fully automated methods with varying degrees of accuracy. Despite advancements in methods to generate morphometrics (quantitative measures of cellular morphology), there has been limited development of tools to analyze the datasets they generate, in particular those containing parameters from tens of thousands of cells analyzed by fully automated pipelines. In this review, we compare and critique the approaches using cluster analysis and machine learning driven predictive algorithms that have been developed to tackle these large datasets, and propose improvements for these methods. In particular, we highlight the need for a commitment to open science from groups developing these classifiers. Furthermore, we call attention to a need for communication between those with a strong software engineering/computer science background and neuroimmunologists to produce effective analytical tools with simplified operability if we are to see their wide-spread adoption by the glia biology community.

Maternal antibiotic administration during gestation can affect the memory and brain structure in mouse offspring

Maternal antibiotics administration (MAA) is among the widely used therapeutic approaches in pregnancy. Although published evidence demonstrates that infants exposed to antibiotics immediately after birth have altered recognition memory responses at one month of age, very little is known about in utero effects of antibiotics on the neuronal function and behavior of children after birth. Therefore, this study aimed to evaluate the impact of MAA at different periods of pregnancy on memory decline and brain structural alterations in young mouse offspring after their first month of life. To study the effects of MAA on 4-week-old offspring, pregnant C57BL/6J mouse dams (2-3-month-old; n = 4/group) were exposed to a cocktail of amoxicillin (205 mg/kg/day) and azithromycin (51 mg/kg/day) in sterile drinking water (daily/1 week) during either the 2nd or 3rd week of pregnancy and stopped after delivery. A control group of pregnant dams was exposed to sterile drinking water alone during all three weeks of pregnancy. Then, the 4-week-old offspring mice were first evaluated for behavioral changes. Using the Morris water maze assay, we revealed that exposure of pregnant mice to antibiotics at the 2nd and 3rd weeks of pregnancy significantly altered spatial reference memory and learning skills in their offspring compared to those delivered from the control group of dams. In contrast, no significant difference in long-term associative memory was detected between offspring groups using the novel object recognition test. Then, we histologically evaluated brain samples from the same offspring individuals using conventional immunofluorescence and electron microscopy assays. To our knowledge, we observed a reduction in the density of the hippocampal CA1 pyramidal neurons and hypomyelination in the corpus callosum in groups of mice in utero exposed to antibiotics at the 2nd and 3rd weeks of gestation. In addition, offspring exposed to antibiotics at the 2nd or 3rd week of gestation demonstrated a decreased astrocyte cell surface area and astrocyte territories or depletion of neurogenesis in the dentate gyrus and hippocampal synaptic loss, respectively. Altogether, this study shows that MAA at different times of pregnancy can pathologically alter cognitive behavior and brain development in offspring at an early age after weaning.

Dynamic Responses of Microglia in Animal Models of Multiple Sclerosis

Microglia play an essential role in maintaining central nervous system (CNS) homeostasis, as well as responding to injury and disease. Most neurological disorders feature microglial activation, a process whereby microglia undergo profound morphological and transcriptional changes aimed at containing CNS damage and promoting repair, but often resulting in overt inflammation that sustains and propagates the neurodegenerative process. This is especially evident in multiple sclerosis (MS), were microglial activation and microglia-driven neuroinflammation are considered key events in the onset, progression, and resolution of the disease. Our understanding of microglial functions in MS has widened exponentially in the last decade by way of new tools and markers to discriminate microglia from other myeloid populations. Consequently, the complex functional and phenotypical diversity of microglia can now be appreciated. This, in combination with a variety of animal models that mimic specific features and processes of MS, has contributed to filling the gap of knowledge in the cascade of events underlying MS pathophysiology. The purpose of this review is to present the most up to date knowledge of the dynamic responses of microglia in the commonly used animal models of MS, specifically the immune-mediated experimental autoimmune encephalomyelitis (EAE) model, and the chemically-induced cuprizone and lysolecithin models. Elucidating the spectrum of microglial functions in these models, from detrimental to protective, is essential to identify emerging targets for therapy and guide drug discovery efforts.

Engineering biomaterial microenvironments to promote myelination in the central nervous system

Promoting remyelination and/or minimizing demyelination are key therapeutic strategies under investigation for diseases and injuries like multiple sclerosis (MS), spinal cord injury, stroke, and virus-induced encephalopathy. Myelination is essential for efficacious neuronal signaling. This myelination process is originated by oligodendrocyte progenitor cells (OPCs) in the central nervous system (CNS). Resident OPCs are capable of both proliferation and differentiation, and also migration to demyelinated injury sites. OPCs can then engage with these unmyelinated or demyelinated axons and differentiate into myelin-forming oligodendrocytes (OLs). However this process is frequently incomplete and often does not occur at all. Biomaterial strategies can now be used to guide OPC and OL development with the goal of regenerating healthy myelin sheaths in formerly damaged CNS tissue. Growth and neurotrophic factors delivered from such materials can promote proliferation of OPCs or differentiation into OLs. While cell transplantation techniques have been used to replace damaged cells in wound sites, they have also resulted in poor transplant cell viability, uncontrollable differentiation, and poor integration into the host. Biomaterial scaffolds made from extracellular matrix (ECM) mimics that are naturally or synthetically derived can improve transplanted cell survival, support both transplanted and endogenous cell populations, and direct their fate. In particular, stiffness and degradability of these scaffolds are two parameters that can influence the fate of OPCs and OLs. The future outlook for biomaterials research includes 3D in vitro models of myelination / remyelination / demyelination to better mimic and study these processes. These models should provide simple relationships of myelination to microenvironmental biophysical and biochemical properties to inform improved therapeutic approaches.

TRPM2 Channel in Microglia as a New Player in Neuroinflammation Associated With a Spectrum of Central Nervous System Pathologies

Microglial cells in the central nervous system (CNS) are crucial in maintaining a healthy environment for neurons to function properly. However, aberrant microglial cell activation can lead to excessive generation of neurotoxic proinflammatory mediators and neuroinflammation, which represents a contributing factor in a wide spectrum of CNS pathologies, including ischemic stroke, traumatic brain damage, Alzheimer’s disease, Parkinson’s disease, multiple sclerosis, psychiatric disorders, autism spectrum disorders, and chronic neuropathic pain. Oxidative stress is a salient and common feature of these conditions and has been strongly implicated in microglial cell activation and neuroinflammation. The transient receptor potential melastatin-related 2 (TRPM2) channel, an oxidative stress-sensitive calcium-permeable cationic channel, is highly expressed in microglial cells. In this review, we examine the recent studies that provide evidence to support an important role for the TRPM2 channel, particularly TRPM2-mediated Ca²⁺ signaling, in mediating microglial cell activation, generation of proinflammatory mediators and neuroinflammation, which are of relevance to CNS pathologies. These findings lead to a growing interest in the TRPM2 channel, a new player in neuroinflammation, as a novel therapeutic target for CNS diseases.

Toxin-Based Models to Investigate Demyelination and Remyelination

Clinical myelin diseases, and our best experimental approximations, are complex entities in which demyelination and remyelination proceed unpredictably and concurrently. These features can make it difficult to identify mechanistic details. Toxin-based models offer lesions with predictable spatiotemporal patterns and relatively discrete phases of damage and repair: a simpler system to study the relevant biology and how this can be manipulated. Here, we discuss the most widely used toxin-based models, with a focus on lysolecithin, ethidium bromide, and cuprizone. This includes an overview of their respective mechanisms, strengths, and limitations and step-by-step protocols for their use.

Multi-modal imaging of long-term recovery post-stroke by positron emission tomography and matrix-assisted laser desorption/ionisation mass spectrometry

RATIONALE

Stroke is a leading cause of disability worldwide. Understanding the recovery process post-stroke is essential; however, longer-term recovery studies are lacking. In vivo positron emission tomography (PET) can image biological recovery processes, but is limited by spatial resolution and its targeted nature. Untargeted mass spectrometry imaging offers high spatial resolution, providing an ideal ex vivo tool for brain recovery imaging.

METHODS

Magnetic resonance imaging (MRI) was used to image a rat brain 48 h after ischaemic stroke to locate the infarcted regions of the brain. PET was carried out 3 months post-stroke using the tracers [¹⁸ F]DPA-714 for TSPO and [¹⁸ F]IAM6067 for sigma-1 receptors to image neuroinflammation and neurodegeneration, respectively. The rat brain was flash-frozen immediately after PET scanning, and sectioned for matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS) imaging.

RESULTS

Three months post-stroke, PET imaging shows minimal detection of neurodegeneration and neuroinflammation, indicating that the brain has stabilised. However, MALDI-MS images reveal distinct differences in lipid distributions (e.g. phosphatidylcholine and sphingomyelin) between the scar and the healthy brain, suggesting that recovery processes are still in play. It is currently not known if the altered lipids in the scar will change on a longer time scale, or if they are stabilised products of the brain post-stroke.

CONCLUSIONS

The data demonstrates the ability to combine MALD-MS with in vivo PET to image different aspects of stroke recovery.

Reduced cerebral vascularization in experimental neuronopathic Gaucher disease

The glycosphingolipidosis, Gaucher disease, in which a range of neurological manifestations occur, results from a deficiency of acid β-glucocerebrosidase, with subsequent accumulation of β-glucocerebroside, its upstream substrates, and the non-acylated congener β-glucosylsphingosine. However, the mechanisms by which end-organ dysfunction arise are poorly understood. Here, we report strikingly diminished cerebral microvascular density in a murine model of disease, and provide a detailed analysis of the accompanying cerebral glycosphingolipidome in these animals, with marked elevations of β-glucosylsphingosine. Further in vitro studies confirmed a concentration-dependent impairment of endothelial cytokinesis upon exposure to quasi-pathological concentrations of β-glucosylsphingosine. These findings support a premise for pathogenic disruption of cerebral angiogenesis as an end-organ effect, with potential for therapeutic modulation in neuronopathic Gaucher disease. Copyright © 2017 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

NLR members NLRC4 and NLRP3 mediate sterile inflammasome activation in microglia and astrocytes

Lysophosphatidylcholine is associated with neurodegeneration and demyelination. Freeman et al. demonstrate that lysophosphatidylcholine triggers NLRP3- and NLRC4-dependent inflammasome activation, and in a synergistic fashion, NLRP3 and NLRC4 contribute to a cuprizone-induced demyelination model in vivo.

Inflammation in the brain accompanies several high-impact neurological diseases including multiple sclerosis (MS), stroke, and Alzheimer’s disease. Neuroinflammation is sterile, as damage-associated molecular patterns rather than microbial pathogens elicit the response. The inflammasome, which leads to caspase-1 activation, is implicated in neuroinflammation. In this study, we reveal that lysophosphatidylcholine (LPC), a molecule associated with neurodegeneration and demyelination, elicits NLRP3 and NLRC4 inflammasome activation in microglia and astrocytes, which are central players in neuroinflammation. LPC-activated inflammasome also requires ASC (apoptotic speck containing protein with a CARD), caspase-1, cathepsin-mediated degradation, calcium mobilization, and potassium efflux but not caspase-11. To study the physiological relevance, Nlrc4^−/− and Nlrp3^−/− mice are studied in the cuprizone model of neuroinflammation and demyelination. Mice lacking both genes show the most pronounced reduction in astrogliosis and microglial accumulation accompanied by decreased expression of the LPC receptor G2A, whereas MS patient samples show increased G2A. These results reveal that NLRC4 and NLRP3, which normally form distinct inflammasomes, activate an LPC-induced inflammasome and are important in astrogliosis and microgliosis.

TRPM2 contributes to LPC-induced intracellular Ca2+ influx and microglial activation

Microglia are the resident immune cells which become activated in some pathological conditions in central nervous system (CNS). Lysophosphatidylcholine (LPC), an endogenous inflammatory phospholipid, is implicated in immunomodulatory function of glial cells in the CNS. Although several studies uncovered that LPC induces intracellular Ca²⁺ influx and morphologic change in microglia, there is still no direct evidence showing change of phosphorylation of mitogen-activated protein kinase (MAPK) p38 (p-p38), a widely used microglia activation marker, by LPC. Furthermore, the cellular mechanism of LPC-induced microglia activation remains unknown. In this study, we found that LPC induced intracellular Ca²⁺ increase in primary cultured microglia, which was blocked in the presence of Gd³⁺, non-selective transient receptor potential (TRP) channel blocker. RT-PCR and whole cell patch clamp recordings revealed molecular and functional expression of TRP melastatin 2 (TRPM2) in microglia. Using western blotting, we also observed that LPC increased phosphorylation of p38 MAPK, and the increase of p-p38 expression is also reversed in TRPM2-knockout (KO) microglia. Moreover, LPC induced membrane trafficking of TRPM2 and intrathecal injection of LPC increased Iba-1 immunoreactivity in the spinal cord, which were significantly reduced in KO mice. In addition, LPC-induced intracellular Ca²⁺ increase and inward currents were abolished in TRPM2-KO microglia. Taken together, our results suggest that LPC induces intracellular Ca²⁺ influx and increases phosphorylation of p38 MAPK via TRPM2, which in turn activates microglia.

Phospholipase A₂-Generated Lipid Mediators in the Brain: The Good, the Bad, and the Ugly

Phospholipase A2 (PLA2) generates arachidonic acid, docosahexaenoic acid, and lysophospholipids from neural membrane phospholipids. These metabolites have a variety of physiological effects by themselves and also are substrates for the synthesis of more potent lipid mediators such as eicosanoids, platelet activating factor, and 4-hydroxynonenal (4-HNE). At low concentrations, these mediators act as second messengers. They affect and modulate several cell functions, including signal transduction, gene expression, and cell proliferation, but at high concentrations, these lipid mediators cause neurotoxicity. Among the metabolites generated by PLA2, 4-HNE is the most cytotoxic metabolite and is associated with the apoptotic type of neural cell death. Levels of 4-HNE are markedly increased in neurological disorders such as Alzheimer disease, Parkinson disease, ischemia, spinal cord trauma, and head injury. The purpose of this review is to summarize and integrate the vast literature on metabolites generated by PLA2 for a wider audience. The authors hope that this discussion will jump-start more studies not only on the involvement of PLA2 in neurological disorders but also on the importance of PLA2-generated lipid mediators in physiological and pathological processes.

An integrated metabolomics approach for the research of new cerebrospinal fluid biomarkers of multiple sclerosis

(1) Lipid profiling in MuS and OND patients. (2) Search of alterations associated with MuS. (3) Characterization of differences.

IL-4 type 1 receptor signaling up-regulates KCNN4 expression, and increases the KCa3.1 current and its contribution to migration of alternative-activated microglia

The Ca²⁺-activated K⁺ channel, KCa3.1 (KCNN4/IK1/SK4), contributes to “classical,” pro-inflammatory activation of microglia, and KCa3.1 blockers have improved the outcome in several rodent models of CNS damage. For instance, blocking KCa3.1 with TRAM-34 rescued retinal ganglion neurons after optic nerve damage in vivo and, reduced p38 MAP kinase activation, production of reactive oxygen and nitrogen species, and neurotoxicity by microglia in vitro. In pursuing the therapeutic potential of KCa3.1 blockers, it is crucial to assess KCa3.1 contributions to other microglial functions and activation states, especially the IL-4-induced “alternative” activation state that can counteract pro-inflammatory states. We recently found that IL-4 increases microglia migration – a crucial function in the healthy and damaged CNS – and that KCa3.1 contributes to P₂Y₂ receptor-stimulated migration. Here, we discovered that KCa3.1 is greatly increased in alternative-activated rat microglia and then contributes to an enhanced migratory capacity. IL-4 up-regulated KCNN4 mRNA (by 6 h) and greatly increased the KCa3.1 current by 1 day, and this required de novo protein synthesis. The increase in current was sustained for at least 6 days. IL-4 increased microglial migration and this was reversed by blocking KCa3.1 with TRAM-34. A panel of inhibitors of signal-transduction mediators was used to analyze contributions of IL-4-related signaling pathways. Induction of KCNN4 mRNA and KCa3.1 current was mediated specifically through IL-4 binding to the type I receptor and, surprisingly, it required JAK3, Ras/MEK/ERK signaling and the transcription factor, activator protein-1, rather than JAK2, STAT6, or phosphatidylinositol 3-kinase.The same receptor subtype and pathway were required for the enhanced KCa3.1-dependent migration. In providing the first direct signaling link between an IL-4 receptor, expression and roles of an ion channel, this study also highlights the potential importance of KCa3.1 in alternative-activated microglia.

Inhibition of microglia activation as a phenotypic assay in early drug discovery

Complex biological processes such as inflammation, cell death, migration, proliferation, and the release of biologically active molecules can be used as outcomes in phenotypic assays during early stages of drug discovery. Although target-based approaches have been widely used over the past decades, a disproportionate number of first-in-class drugs have been identified using phenotypic screening. This review details phenotypic assays based on inhibition of microglial activation and their utility in primary and secondary screening, target validation, and pathway elucidation. The role of microglia, both in normal as well as in pathological conditions such as chronic neurodegenerative diseases, is reviewed. Methodologies to assess microglia activation in vitro are discussed in detail, and classes of therapeutic drugs known to decrease the proinflammatory and cytotoxic responses of activated microglia are appraised, including inhibitors of glutaminase, cystine/glutamate antiporter, nuclear factor κB, and mitogen-activated protein kinases.

The road to remyelination in demyelinating diseases: current status and prospects for clinical treatment

Within CNS disorders, demyelinating diseases are among the most devastating and cost intensive due to long-term disabilities affecting relatively young patients. Multiple sclerosis, a chronic inflammatory demyelinating disease in which the persistent inhibitory microenvironment of the resident oligodendrocyte precursor cells abrogates regeneration of myelin sheaths, is the most prominent disease in the spectrum of demyelinating diseases. The essential goal is to stimulate creation of new myelin sheaths on the demyelinated axons, leading to restoration of saltatory conduction and resolving functional deficits. The past few decades witnessed significant efforts to understand the cellular interactions at the lesion site with studies suggesting efficient remyelination as a prerequisite for functional repair. Despite its proven efficacy in experimental models, immunosuppression has not had profound clinical consequences in multiple sclerosis, which argued for a paradigm shift in the design of therapeutics aiming to achieve remyelination. For example, targeting oligodendrocytes themselves may drive remyelination in the CNS. This group and others have demonstrated that natural autoreactive antibodies directed at oligodendrocyte progenitors participate in remyelination. Accordingly, the authors developed a recombinant autoreactive natural human IgM antibody with therapeutic potential for remyelination.

Differential response of macrophages to core-shell Fe3O4@Au nanoparticles and nanostars

Murine RAW 264.7 cells were exposed to spheroidal core-shell Fe(3)O(4)@Au nanoparticles (SCS-NPs, ca. 34 nm) or nanostars (NSTs, ca. 100 nm) in the presence of bovine serum albumin, with variable effects observed after macrophagocytosis. Uptake of SCS-NPs caused macrophages to adopt a rounded, amoeboid form, accompanied by an increase in surface detachment. In contrast, the uptake of multibranched NSTs did not induce gross changes in macrophage shape or adhesion, but correlated instead with cell enlargement and signatures of macrophage activation such as TNF-α and ROS. MTT assays indicate a low cytotoxic response to either SCS-NPs or NSTs despite differences in macrophage behavior. These observations show that differences in NP size and shape are sufficient to produce diverse responses in macrophages following uptake.

Significance of brain tissue oxygenation and the arachidonic acid cascade in stroke

The significance of the hypoxia component of stroke injury is highlighted by hypermetabolic brain tissue enriched with arachidonic acid (AA), a 22:6n-3 polyunsaturated fatty acid. In an ischemic stroke environment in which cerebral blood flow is arrested, oxygen-starved brain tissue initiates the rapid cleavage of AA from the membrane phospholipid bilayer. Once free, AA undergoes both enzyme-independent and enzyme-mediated oxidative metabolism, resulting in the formation of number of biologically active metabolites which themselves contribute to pathological stroke outcomes. This review is intended to examine two divergent roles of molecular dioxygen in brain tissue as (1) a substrate for life-sustaining homeostatic metabolism of glucose and (2) a substrate for pathogenic metabolism of AA under conditions of stroke. Recent developments in research concerning supplemental oxygen therapy as an intervention to correct the hypoxic component of stroke injury are discussed.

Administration of 2-arachidonoylglycerol ameliorates both acute and chronic experimental autoimmune encephalomyelitis

BACKGROUND AND PURPOSE

Experimental autoimmune encephalomyelitis (EAE) is a widely used model of multiple sclerosis (MS) and both conditions have been reported to exhibit reduced endocannabinoid activity. The purpose of this study was to address the effect of exogenously administered 2-arachidonoylglycerol (2AG), an endocannabinoid receptor ligand, on acute phase and chronic disability in EAE.

EXPERIMENTAL APPROACH

Acute and chronic EAE models were induced in susceptible mice and 2AG-treatment was applied for 14 days from day of disease induction.

KEY RESULTS

2AG-treatment ameliorated acute phase of disease with delay of disease onset in both EAE models and reduced disease mortality and long-term (70 days post-induction) clinical disability in chronic EAE. Reduced axonal pathology in the chronic EAE- (p<0.0001) and increased activation and ramification of microglia in the 2AG-treated acute EAE- (p<0.05) model were noticed. The latter was accompanied by a 2- to 4-fold increase of the M2-macrophages in the perivascular infiltrations (p<0.001) of the 2AG-treated animals in the acute (day 22), although not the chronic (day 70), EAE model. Expression of cannabinoid receptors 1 (CB1R) and 2 (CB2R) was increased in 2AG-treated animals of acute EAE vs. controls (p<0.05). In addition, ex vivo viability assays exhibited reduced proliferation of activated lymph node cells when extracted from 2AG-treated EAE animals, whereas a dose-dependent response of activated lymphocytes to 2AG-treatment in vitro was noticed.

CONCLUSION AND IMPLICATIONS

Our data indicate for the first time that 2AG treatment may provide direct (via CBRs) and immune (via M2 macrophages) mediated neuroprotection in EAE.

A phospholipase A₂ isolated from Lachesis muta snake venom increases the survival of retinal ganglion cells in vitro

We have previously showed that a phospholipase A₂ isolated from Lachesis muta snake venom and named LM-PLA₂-I displayed particular biological activities, as hemolysis, inhibition on platelet aggregation, edema induction and myotoxicity. In the present work, we evaluated the effect of LM-PLA₂-I on the survival of axotomized rat retinal ganglion cells kept in vitro, as well as its mechanism of action. Our results clearly showed that treatment with LM-PLA₂-I increased the survival of ganglion cells (100% when compared to control cultures) and the treatment of LM-PLA₂-I with p-bromophenacyl bromide abolished this effect. This result indicates that the effect of LM-PLA₂-I on ganglion cell survival is entirely dependent on its enzymatic activity and the generation of lysophosphatidylcholine (LPC) may be a prerequisite to the observed survival. In fact, commercial LPC mimicked the effect of LM-PLA₂-I upon ganglion cell survival. To investigate the mechanism of action of LM-PLA₂-I, cultures were treated with chelerythrine chloride, BAPTA-AM, rottlerin and also with an inhibitor of c-junc kinase (JNKi). Our results showed that rottlerin and JNK inhibitor abolished the LM-PLA₂-I on ganglion cell survival. Taken together, our results showed that LM-PLA₂-I and its enzymatic product, LPC promoted survival of retinal ganglion cells through the protein kinase C pathway and strongly suggest a possible role of the PLA₂ enzyme and LPC in controlling the survival of axotomized neuronal cells.

Importance of lipid rafts for lysophosphatidylcholine-induced caspase-1 activation and reactive oxygen species generation

Lipid rafts play an important role in regulating cellular processes and functions. Here, we demonstrate that in microglia stimulated with the pro-inflammatory lipid lysophosphatidylcholine (LPC), caspase-1 activation and NADPH oxidase activity depend on intact lipid rafts. Disruption of lipid rafts with methyl-β-cyclodextrin, fumonisin B1 or nystatin prevented LPC-stimulated caspase-1 activation and reactive oxygen species (ROS) production, whereas LPC-induced Na(+) influx remained unaffected. Since ROS regulate caspase-1 activity in LPC-stimulated microglia, the effects of lipid raft-disrupting agents on caspase-1 activation can be related to their inhibition of NADPH oxidase-mediated ROS production.

Sodium dependence of lysophosphatidylcholine-induced caspase-1 activity and reactive oxygen species generation

The proinflammatory cytokines interleukin (IL)-1β and IL-18 play pivotal roles in neuroinflammatory diseases. Caspase-1-mediated proteolytic cleavage is required to convert the premature, biologically inactive cytokines to their biologically active forms capable of promoting tissue inflammation. Although caspases have been recognized as potential therapeutic targets in inflammatory diseases, mechanisms regulating caspase-1 activation are not fully understood. Here we demonstrate that the proinflammatory lipid lysophosphatidylcholine (LPC) initiates microglial caspase-1 activation in a Na(+)-dependent manner. LPC-induced caspase-1 activity was almost completely inhibited upon omission of extracellular Na(+), but was unaffected by inhibition of Na(+)/K(+)-ATPase with ouabain or by inhibition of Na(+)/H(+) antiport with amiloride. Inhibition of caspase-1-mediated IL-1β processing by Na(+)-free medium led to reduced amounts of mature IL-1β released from LPC-stimulated microglia. Furthermore, LPC-induced production of reactive oxygen species (ROS) was abolished by Na(+)-free medium, indicating Na(+) dependence of NADPH oxidase activity in LPC-stimulated microglia. Since ROS production was found to be crucial to caspase-1 activation in LPC-stimulated microglia, the Na(+) dependence of caspase-1 can be related to the Na(+) dependence of NADPH oxidase. In summary, it is suggested that in LPC-activated microglia, Na(+) influx is required for the production of NADPH oxidase-mediated ROS, which subsequently stimulate caspase-1 activity.

Lysophosphatidylcholine- and MCP-1-induced chemotaxis of monocytes requires potassium channel activity

One of the earliest cellular responses in atherogenesis is the focal recruitment of circulating monocytes, while the most important atherogenic chemoattractants are monocyte chemoattractant protein-1 (MCP-1) and lysophosphatidylcholine (LPC). Invading monocytes transform into activated macrophages and foam cells, which stimulate inflammatory processes and promote atherosclerosis. In this study, we have searched for common mechanisms involved in MCP-1- and LPC-stimulated monocyte migration. We have found that migration of THP-1 monocytes stimulated with MCP-1 was reduced upon inhibition of G(i/o) proteins with pertussis toxin and upon inhibition of platelet activating factor receptors with BN52021, whereas LPC-stimulated monocyte chemotaxis remained unaffected by both inhibitors. Furthermore, Cl(-) channels were only required for MCP-1-induced chemotaxis. However, activity of voltage-gated K+ channels and of Ca2+-activated K+ channels was found to be involved in migration of monocytes stimulated with either MCP-1 or LPC. Inhibition of voltage-gated K+ channels with 4-aminopyridine or margatoxin partially inhibited MCP-1- and LPC-stimulated migration of monocytes. Blockade of Ca2+-activated K+ channels with TRAM-34 also partially reduced migration of MCP-1- and LPC-stimulated monocytes. Simultaneous inhibition of voltage-gated and Ca2+-activated K+ channels abolished MCP-1- and LPC-induced chemotaxis of monocytes. Thus, K+ channel inhibition may represent a novel powerful strategy to reduce monocyte infiltration and subsequent inflammation in atherosclerosis.

Lysophosphatidylcholine induces glial cell activation: role of rho kinase

Lysophosphatidylcholine (LPC), a major phospholipid component of atherogenic oxidized LDL, is implicated in atherosclerosis and, recently, in neurodegenerative diseases. We investigated the immunomodulatory functions of LPC in the central nervous system (CNS) using both an in vivo rat model, and in vitro culture systems of human primary astrocytes and a microglia cell line, HMO6. Compared with PBS injection, 20 nmol LPC-injection into the rat striatum increased astrocyte and microglial accumulation and elevated iNOS expression; concomitantly a time-dependent decrease in number of neurons was exhibited. In vitro studies on astrocytes and HMO6 cells showed that LPC increased the gene expression of proinflammatory factors IL-1beta, COX-2, and GM-CSF. LPC also induced chemotactic responses in HMO6 cells. Inhibition of rho kinase by fasudil, Y27632, or expressing a dominant negative form of rho kinase inhibited the LPC-induced IL-1beta mRNA expression in both astrocytes and HMO6. Moreover, intraperitoneal fasudil injection inhibited the LPC-induced microglial accumulation and iNOS expression and also was effective in protecting against neuronal loss. Silencing G2A, a specific receptor for LPC, inhibited proinflammatory gene expression and HMO6 migration. Overall, our results indicate that LPC induced considerable neuroinflammatory reactivity in glia mediated by rho kinase-dependent pathways with inhibition of these pathways conferring significant extents of neuroprotection.

Immunosuppression after traumatic or ischemic CNS damage: it is neuroprotective and illuminates the role of microglial cells

Acute traumatic and ischemic events in the central nervous system (CNS) invariably result in activation of microglial cells as local representatives of the immune system. It is still under debate whether activated microglia promote neuronal survival, or whether they exacerbate the original extent of neuronal damage. Protagonists of the view that microglial cells cause secondary damage have proposed that inhibition of microglial activation by immunosuppression is beneficial after acute CNS damage. It is the aim of this review to analyse the effects of immunosuppressants on isolated microglial cells and neurons, and to scrutinize the effects of immunosuppression in different in vivo models of acute CNS trauma or ischemia. It is found that the immunosuppressants cytosine-arabinoside, different steroids, cyclosporin A, FK506, rapamycin, mycophenolate mofetil, and minocycline all have direct inhibitory effects on microglial cells. These effects are mainly exerted by inhibiting microglial proliferation or microglial secretion of neurotoxic substances such as proinflammatory cytokines and nitric oxide. Furthermore, immunosuppression after acute CNS trauma or ischemia results in improved structure preservation and, mostly, in enhanced function. However, all investigated immunosuppressants also have direct effects on neurons, and some immunosuppressants affect other glial cells such as astrocytes. In summary, it is safe to conclude that immunosuppression after acute CNS trauma or ischemia is neuroprotective. Furthermore, circumferential evidence indicates that microglial activation after traumatic or ischemic CNS damage is not beneficial to adjacent neurons in the immediate aftermath of such acute lesions. Further experiments with more specific agents or genetic approaches that specifically inhibit microglial cells are needed in order to fully answer the question of whether microglial activation is "good or bad".

Lipopolysaccharide-induced down-regulation of Ca2+ release-activated Ca2+ currents (I CRAC) but not Ca2+-activated TRPM4-like currents (I CAN) in cultured mouse microglial cells

Microglia are the main immunocompetent cells of the mammalian central nervous system (CNS). Activation of cultured microglial cells and subsequent release of nitric oxide and cytokines critically depends on intracellular calcium levels. Since microglia undergo dramatic morphological, biochemical and electrophysiological changes in response to pathological events in the CNS, we investigated temporal changes in expression levels of ion channels involved in cellular calcium homeostasis in mouse cortical microglial cells in culture. Specifically, we assessed the inward and delayed outward rectifier potassium currents (I IRK and I DRK), calcium (Ca2+) release-activated Ca2+ currents (I CRAC) and Ca2+-activated TRPM4-like currents (I CAN) in non-activated microglia and cells that were activated by exposure to lipopolysaccharide (LPS) between 3 and 48 h. Unstimulated microglial cells, subcultured from an astrocyte coculture, typically exhibited a ramified, rod-shaped morphology. During the first 3 days of culture cell size and shape were maintained, but the percentage of cells showing prominent I IRK went up and those expressing I DRK went down. Cells retaining I DRK exhibited smaller amplitudes, whereas those of I IRK and I CRAC were not affected. However, after 24 h of exposure to 1 microg ml(-1) LPS, most cells showed an amoeboid ('fried egg'-shaped) morphology with a 62% increase in cell capacitance. At that point in time, only 14% of the cells revealed I IRK and 3% had I DRK exclusively, whereas the majority of cells expressed both currents. The amplitudes of I CRAC and I IRK progressively decreased after stimulation, whereas I DRK transiently reached a maximum after 6 h of LPS exposure and then returned to pre-stimulation expression levels. Cultured microglia also revealed TRPM4-like, Ca2+-activated non-selective currents (I CAN) with an EC50 of 1.2 microm [Ca2+]i. The expression levels of this current did not change significantly during and after 24 h of LPS exposure. We propose that LPS-induced down-regulation of I IRK and I CRAC will reduce the cell's capacity to produce significant calcium influx upon receptor activation and result in decreased sensitivity to exogenous stimulation. In this scenario, I CAN expression would remain constant, although its activity would automatically be reduced due to the diminished calcium influx capacity of the cell.

Small-conductance Cl- channels contribute to volume regulation and phagocytosis in microglia

The shape and volume of microglia (brain immune cells) change when they activate during brain inflammation and become migratory and phagocytic. Swollen rat microglia express a large Cl(-) current (I(Clswell)), whose biophysical properties and functional roles are poorly understood and whose molecular identity is unknown. We constructed a fingerprint of useful biophysical properties for comparison with I(Clswell) in other cell types and with cloned Cl(-) channels. The microglial I(Clswell) was rapidly activated by cell swelling but not by voltage, and showed no time-dependence during voltage-clamp steps. Like I(Clswell) in many cell types, the halide selectivity sequence was I(-) > Br(-) > Cl(-) > F(-). However, it differed in lacking inactivation, even at +100 mV with high extracellular Mg(2+), and in having a much lower single-channel conductance: 1-3 pS. Based on these fundamental differences, the microglia channel is apparently a different gene product than the more common intermediate-conductance I(Clswell). Microglia express several candidate genes, with relative mRNA expression levels of: CLIC1 > ClC3 > I(Cln) > or = ClC2 > Best2 > Best1 > or = Best3 > Best4. Using a pharmacological toolbox, we show that all drugs that reduced the microglia current (NPPB, IAA-94, flufenamic acid and DIOA) increased the resting cell volume in isotonic solution and inhibited the regulatory volume decrease that followed cell swelling in hypotonic solution. Both channel blockers tested (NPPB and flufenamic acid) dose-dependently inhibited microglia phagocytosis of E. coli bacteria. Because I(Clswell) is involved in microglia functions that involve shape and volume changes, it is potentially important for controlling their ability to migrate to damage sites and phagocytose dead cells and debris.

ATP-induced chemotaxis of microglial processes requires P2Y receptor-activated initiation of outward potassium currents

Microglial cells are the resident macrophages that are involved in brain injuries and infections. Recent studies using transcranial two-photon microscopy have shown that ATP and P2Y receptors mediated rapid chemotactic responses of miroglia to local injury. However, the molecular mechanism for microglial chemotaxis toward ATP is still unknown. To address this question, we employed a combination of simultaneous perforated whole-cell recordings and time-lapse confocal imaging in GFP-labeled microglia in acute brain slices from adult mice. We found that ATP-induced rapid chemotaxis is correlated with P2Y receptor associated-outward potassium current in microglia. Activation of both P2Y receptor and its associated potassium channels are required for ATP-induced chemotaxis and baseline motility of microglial cells. The chemotaxis required the activation of phosphoinositide 3-kinase but not mitogen-activated protein kinase pathway. Our results provide strong evidence that P2Y receptor-associated outward potassium channels and the phosphoinositide 3-kinase pathway are important for ATP-induced microglial motility in acute brain slices.

Mechanism of Lysophosphatidylcholine-Induced Lysosome Destabilization

Lysosomal destabilization is critical for the organelle and living cells. Phospholipase A(2 )(PLA(2)) was shown to be able to destabilize lysosomes under some conditions. By what mechanism the enzyme affects lysosomal stability is not fully studied. In this study, we investigated the effects of lysophosphatidylcholine (lysoPC), a PLA(2)-produced lipid metabolite, on lysosomal ion permeability, osmotic sensitivity and stability. By measuring lysosomal beta-hexosaminidase free activity, membrane potential, proton leakage and their enzyme latency loss in hypotonic sucrose medium, we established that lysoPC could increase the lysosomal permeability to both potassium ions and protons and enhance lysosomal osmotic sensitivity. These changes in lysosomal membrane properties promoted entry of potassium ions into lysosomes via K(+)/H(+) exchange. The resultant osmotic imbalance across the membranes led to losses of lysosomal integrity. The enhancement of lysosomal osmotic sensitivity caused the lysosomes to become more liable to destabilization in osmotic shock. These results suggest that lysoPC may play a key role in PLA(2)-induced lysosomal destabilization.

Lysophosphatidylcholine produced by the phospholipase A2 isolated from Lachesis muta snake venom modulates natural killer activity as a protein kinase C effector

We have showed that a phospholipase A(2) isolated from Lachesis muta snake venom, denoted LM-PLA(2)-I, had some biological effects. Here, we examined its effects on lymphocytes. Pre-incubation of human peripheral blood lymphocytes with LM-PLA(2)-I plus phosphatidylcholine (PC) stimulated the natural killer (NK) activity. This was accompanied by DNA binding of nuclear transcription factor kappaB and the increase in PKC activity with translocation of the enzyme from the cytoplasma into the plasma membrane. These effects were reproduced when lymphocytes were pre-incubated with commercial lysophosphatidylcholine (LPC) and abolished by stausrosporin or p-bromophenacyl bromide. Evaluation of phosphorylated PKC isoforms showed that pre-incubation with LPC activated the autophosphorylation of the PKCzeta isoform. Taken together, these results confirm that the enzymatic activity of the phospholipase A(2) present in L. muta venom is for the biological activity of the snake venom, and strongly suggest that the LPC produced may be acting as a modulator of PKC isoforms.

Lysophosphatidylcholine potentiates Ca2+ influx, pore formation and p44/42 MAP kinase phosphorylation mediated by P2X7 receptor activation in mouse microglial cells

The P2X7 receptor (P2X7R) is an ATP-gated ion channel highly expressed in microglia. P2X7R plays important roles in inflammatory responses in the brain. However, little is known about the mechanisms regulating its functions in microglia. Lysophosphatidylcholine (LPC), an inflammatory phospholipid that promotes microglial activation, may have some relevance to P2X7R signaling in terms of microglial function. In this study, we examined its effects on P2X7R signaling in a mouse microglial cell line (MG6) and primary microglia. LPC facilitated the sustained increase in the intracellular Ca(2+) concentration ([Ca(2+)](i)) through P2X7R channels activated by ATP or BzATP. The potentiated increase in [Ca(2+)](i) was actually inhibited by P2X7R antagonists, brilliant blue G and oxidized ATP. The potentiating effect of LPC was not observed with P2Y receptor systems, which are also expressed in MG6 cells. G2A, a receptor for LPC, was expressed in MG6 cells, but not involved in the facilitating effect of LPC on the P2X7R-mediated change in [Ca(2+)](i). Furthermore, LPC enhanced the P2X7R-associated formation of membrane pores and the activation of p44/42 mitogen-activated protein kinase. These results suggest that LPC may regulate microglial functions in the brain by enhancing the sensitivity of P2X7R.

TRAM-34 inhibits nonselective cation channels

TRAM-34 has been demonstrated to inhibit intermediate-conductance Ca(2+)-activated K(+) channels in a wide variety of cell types, including immune cells. In the present study, we investigated effects of TRAM-34 on microglial cells stimulated with lysophosphatidylcholine (LPC). LPC-induced increases in the intracellular Ca(2+) concentration of microglial cells were effectively reduced in the presence of TRAM-34. At a concentration of 1 microM, TRAM-34 inhibited LPC-induced Ca(2+) signals by 60%. The TRAM-34-induced reduction of LPC-induced Ca(2+) increases cannot be related to the inhibition of Ca(2+)-activated K(+) channels. In contrast to TRAM-34, the Ca(2+)-activated K(+) channel inhibitor charybdotoxin did not affect LPC-induced increases in the intracellular Ca(2+) concentration of microglial cells. Patch clamp experiments revealed a direct inhibitory effect of TRAM-34 on nonselective cation channels. Half-maximal inhibition of LPC-induced nonselective cation currents was determined at 38 nM TRAM-34. These data indicate that TRAM-34 may cause additional effects on immune cells that are unrelated to the well-described inhibition of Ca(2+)-activated K(+) channels.

Modulation of inflammation in brain: a matter of fat

Neuroinflammation is a host defense mechanism associated with neutralization of an insult and restoration of normal structure and function of brain. Neuroinflammation is a hallmark of all major CNS diseases. The main mediators of neuroinflammation are microglial cells. These cells are activated during a CNS injury. Microglial cells initiate a rapid response that involves cell migration, proliferation, release of cytokines/chemokines and trophic and/or toxic effects. Cytokines/chemokines stimulate phospholipases A2 and cyclooxygenases. This results in breakdown of membrane glycerophospholipids with the release of arachidonic acid (AA) and docosahexaenoic acid (DHA). Oxidation of AA produces pro-inflammatory prostaglandins, leukotrienes, and thromboxanes. One of the lyso-glycerophospholipids, the other products of reactions catalyzed by phospholipase A2, is used for the synthesis of pro-inflammatory platelet-activating factor. These pro-inflammatory mediators intensify neuroinflammation. Lipoxin, an oxidized product of AA through 5-lipoxygenase, is involved in the resolution of inflammation and is anti-inflammatory. Docosahexaenoic acid is metabolized to resolvins and neuroprotectins. These lipid mediators inhibit the generation of prostaglandins, leukotrienes, and thromboxanes. Levels of prostaglandins, leukotrienes, and thromboxanes are markedly increased in acute neural trauma and neurodegenerative diseases. Docosahexaenoic acid and its lipid mediators prevent neuroinflammation by inhibiting transcription factor NFkappaB, preventing cytokine secretion, blocking the synthesis of prostaglandins, leukotrienes, and thromboxanes, and modulating leukocyte trafficking. Depending on its timing and magnitude in brain tissue, inflammation serves multiple purposes. It is involved in the protection of uninjured neurons and removal of degenerating neuronal debris and also in assisting repair and recovery processes. The dietary ratio of AA to DHA may affect neurodegeneration associated with acute neural trauma and neurodegenerative diseases. The dietary intake of docosahexaenoic acid offers the possibility of counter-balancing the harmful effects of high levels of AA-derived pro-inflammatory lipid mediators.

Lysophosphatidylcholine stimulates IL-1beta release from microglia via a P2X7 receptor-independent mechanism

IL-1beta released from activated macrophages contributes significantly to tissue damage in inflammatory, degenerative, and autoimmune diseases. In the present study, we identified a novel mechanism of IL-1beta release from activated microglia (brain macrophages) that occurred independently of P2X(7) ATP receptor activation. Stimulation of LPS-preactivated microglia with lysophosphatidylcholine (LPC) caused rapid processing and secretion of mature 17-kDa IL-1beta. Neither LPC-induced IL-1beta release nor LPC-stimulated intracellular Ca(2+) increases were affected by inhibition of P2X(7) ATP receptors with oxidized ATP. Microglial LPC-induced IL-1beta release was suppressed in Ca(2+)-free medium or during inhibition of nonselective cation channels with Gd(3+) or La(3+). It was also attenuated when Ca(2+)-activated K(+) channels were blocked with charybdotoxin (CTX). The electroneutral K(+) ionophore nigericin did not reverse the suppressive effects of CTX on LPC-stimulated IL-1beta release, demonstrating the importance of membrane hyperpolarization. Furthermore, LPC-stimulated caspase activity was unaffected by Ca(2+)-free medium or CTX, suggesting that secretion but not processing of IL-1beta is Ca(2+)- and voltage-dependent. In summary, these data indicate that the activity of nonselective cation channels and Ca(2+)-activated K(+) channels is required for optimal IL-1beta release from LPC-stimulated microglia.

Regulation of microglial behavior by ion channel activity

Microglia play an important role in the central nervous system, where these cells, it is believed, have both neuroprotective and neurotoxic effects. In response to acute brain injury or during neurodegenerative and neuroinflammatory diseases, activated microglial cells undergo shape changes, migrate to the affected sites of neuronal damage, proliferate, and release a variety of substances, such as cytokines and reactive oxygen species (ROS). This review summarizes the physiological mechanisms underlying microglial activation and deactivation processes, with particular focus on the involvement of microglial ion channels. Microglial ion channels have been shown to be capable, by regulating membrane potential, cell volume, and intracellular ion concentrations, of modulating or facilitating proliferation, migration, cytokine secretion, shape changes, and the respiratory burst of microglial cells.

Blockade of chloride conductance antagonizes PMA-induced ramification in the murine microglial cell line, BV-2

In microglial cells, activation of ion channels and ion transporters is associated with the transformation from an amoeboid to a ramified phenotype and vice versa. In the present study, we evaluated the contributions of protein kinase C (PKC) activity and ion conductance to the phorbol 12-myristate 13-acetate (PMA)-dependent ramification in the murine microglial cell line, BV-2. In a first set of experiments, we showed that PMA, a commonly used activator of PKC, but not the bioinactive analog 4 alpha-phorbol 12,13-didecanoate (4 alpha-PDD), induces ramification in BV-2 cells. Surprisingly, the PKC inhibitors calphostin C, chelerythrine, or bisindolylmaleimide II did not antagonize PMA-induced ramification. In a further set of experiments, we found that 4,4'-diisocyanatostilbene-2,2' disulfonic acid (DIDS), 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid (SITS), which block chloride channels and K-Cl cotransporters, and SKF 96365, a non-selective ion channel blocker, consistently suppressed PMA-induced ramification in BV-2 cells. Additional ion channel blockers, including lanthanides, amiloride, Ba2+, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), and flufenamic acid did not affect PMA-induced ramification in BV-2 cells. Cs+ accentuated the PMA-dependent ramification in BV-2 cells. Thus, our results indicate (1) that a PMA-binding protein, excluding PKC isoforms, is critical in structural remodeling of microglial cells and (2) that chloride conductance plays a pivotal role in induction of ramification in microglial cells.

A novel physiological mechanism of glycine-induced immunomodulation: Na+-coupled amino acid transporter currents in cultured brain macrophages

Glycine is known to modulate immune cell responses. However, the physiological mechanisms underlying inhibitory effects of glycine on macrophages are not well understood. Here we show that glycine is capable of inducing inward currents in brain macrophages (microglia). In contrast to glycine, the glycine receptor agonist taurine failed to elicit currents. Glycine-evoked currents of brain macrophages were unaffected by strychnine, Cl(-)-free extracellular solution, N-[3-(4'-fluorophenyl)-3-(4'-phenylphenoxy)propyl])sarcosine (NFPS) and amoxapine, but were abolished upon omission of extracellular Na(+). Furthermore, glycine caused increases in the intracellular Na(+) concentration and pronounced membrane depolarization. Glycine-evoked depolarization was Na(+) dependent and occurred independently of the intracellular Cl(-) concentration. Similarly to glycine, glutamine and alpha-(methylamino)isobutyric acid (MeAIB) elicited inward currents in brain macrophages. In the presence of either glutamine or MeAIB, glycine-induced currents were inhibited. It is concluded that neither functional glycine receptors nor glycine transporters are expressed in brain macrophages. We suggest that glycine mediates its effects by activation of system A Na(+)-coupled neutral amino acid transporters.