SDF-1α and LPA modulate microglia potassium channels through rho gtpases to regulate cell morphology

Synthesis, Characterization, and in vivo Evaluation of a Novel Potent Autotaxin-Inhibitor

The autotaxin-lysophosphatidic acid (ATX-LPA) signaling pathway plays a role in a variety of autoimmune diseases, such as rheumatoid arthritis or neurodegeneration. A link to the pathogenesis of glaucoma is suggested by an overactive ATX-LPA axis in aqueous humor samples of glaucoma patients. Analysis of such samples suggests that the ATX-LPA axis contributes to the fibrogenic activity and resistance to aqueous humor outflow through the trabecular meshwork. In order to inhibit or modulate this pathway, we developed a new series of ATX-inhibitors containing novel bicyclic and spirocyclic structural motifs. A potent lead compound (IC50 against ATX: 6 nM) with good in vivo PK, favorable in vitro property, and safety profile was generated. This compound leads to lowered LPA levels in vivo after oral administration. Hence, it was suitable for chronic oral treatment in two rodent models of glaucoma, the experimental autoimmune glaucoma (EAG) and the ischemia/reperfusion models. In the EAG model, rats were immunized with an optic nerve antigen homogenate, while controls received sodium chloride. Retinal ischemia/reperfusion (I/R) was induced by elevating the intraocular pressure (IOP) in one eye to 140 mmHg for 60 min, followed by reperfusion, while the other untreated eye served as control. Retinae and optic nerves were evaluated 28 days after EAG or 7 and 14 days after I/R induction. Oral treatment with the optimized ATX-inhibitor lead to reduced retinal ganglion cell (RGC) loss in both glaucoma models. In the optic nerve, the protective effect of ATX inhibition was less effective compared to the retina and only a trend to a weakened neurofilament distortion was detectable. Taken together, these results provide evidence that the dysregulation of the ATX-LPA axis in the aqueous humor of glaucoma patients, in addition to the postulated outflow impairment, might also contribute to RGC loss. The observation that ATX-inhibitor treatment in both glaucoma models did not result in significant IOP increases or decreases after oral treatment indicates that protection from RGC loss due to inhibition of the ATX-LPA axis is independent of an IOP lowering effect.

ProBDNF induces sustained elevation of intracellular Ca2+ possibly mediated by TRPM7 channels in rodent microglial cells

Microglia are intrinsic immune cells that release factors including pro- and anti-inflammatory cytokines, nitric oxide (NO) and neurotrophins following activation in the brain. Elevation of intracellular Ca²⁺ concentration ([Ca²⁺ ]i) is important for microglial functions, such as the release of cytokines or NO from activated microglia. Brain-derived neurotrophic factor (BDNF) is a neurotrophin well known for its roles in the activation of microglia. Interestingly, proBDNF, the precursor form of mature BDNF, and mature BDNF elicit opposing neuronal responses in the brain. Mature BDNF induces sustained intracellular Ca²⁺ elevation through the upregulation of the surface expression of TRPC3 channels in rodent microglial cells. In addition, TRPC3 channels are important for the BDNF-induced suppression of NO production in activated microglia. In this study, we observed that proBDNF and mature BDNF have opposite effects on the relative expression of surface p75 neurotrophin receptor (p75^NTR ) in rodent microglial cells. ProBDNF induces a sustained elevation of [Ca²⁺ ]i through binding to the p75^NTR , which is possibly mediated by Rac 1 activation and TRPM7 channels in rodent microglial cells. Flow cytometry showed that proBDNF increased the relative surface expression of TRPM7. Although proBDNF did not affect either mRNA expression of pro- and anti-inflammatory cytokines or the phagocytic activity, proBDNF potentiates the generation of NO induced by IFN-γ and TRPM7 channels could be involved in the proBDNF-induced potentiation of IFN-γ-mediated production of NO. We show direct evidence that rodent microglial cells are able to respond to proBDNF, which might be important for the regulation of inflammatory responses in the brain.

Gintonin mitigates experimental autoimmune encephalomyelitis by stabilization of Nrf2 signaling via stimulation of lysophosphatidic acid receptors

Gintonin (GT), a glycolipoprotein fraction isolated from ginseng, exerts neuroprotective effects in models of neurodegenerative diseases such as Alzheimer's disease. However, the in vivo role of GT in multiple sclerosis (MS) has not been clearly resolved. We investigated the effect of GT in myelin oligodendrocyte glycoprotein (MOG_35-55)-induced experimental autoimmune encephalomyelitis (EAE), an animal model of MS. GT alleviated behavioral symptoms of EAE associated with reduced demyelination, diminished infiltration and activation of immune cells (microglia and macrophage), and decreased expression of inflammatory mediators in the spinal cord of the EAE group compared to that of the sham group. GT reduced the percentages of CD4⁺/IFN-γ⁺ (Th1) and CD4⁺/IL-17⁺ (Th17) cells but increased the population of CD4⁺/CD25⁺/Foxp3⁺ (Treg) cells in the spinal cord, in agreement with altered mRNA expression of IFN-γ, IL-17, and TGF-ß in the spinal cord in concordance with mitigated blood-brain barrier disruption. The underlying mechanism is related to inhibition of the ERK and p38 mitogen-activated protein kinases (MAPKs) and nuclear factor-kappa B (NF-κB) pathways and the stabilization of nuclear factor erythroid 2-related factor 2 (Nrf2) via increased expression of lysophosphatidic acid receptor (LPAR) 1-3. Impressively, these beneficial effects of GT were completely neutralized by inhibiting LPARs with Ki16425, a LPAR1/3 antagonist. Our results strongly suggest that GT may be able to alleviate EAE due to its anti-inflammatory and antioxidant activities through LPARs. Therefore, GT is a potential therapeutic option for treating autoimmune disorders including MS.

Escherichia coli Rho GTPase-activating toxin CNF1 mediates NLRP3 inflammasome activation via p21-activated kinases-1/2 during bacteraemia in mice

Inflammasomes are signalling platforms that are assembled in response to infection or sterile inflammation by cytosolic pattern recognition receptors. The consequent inflammasome-triggered caspase-1 activation is critical for the host defence against pathogens. During infection, NLRP3, which is a pattern recognition receptor that is also known as cryopyrin, triggers the assembly of the inflammasome-activating caspase-1 through the recruitment of ASC and Nek7. The activation of the NLRP3 inflammasome is tightly controlled both transcriptionally and post-translationally. Despite the importance of the NLRP3 inflammasome regulation in autoinflammatory and infectious diseases, little is known about the mechanism controlling the activation of NLRP3 and the upstream signalling that regulates the NLRP3 inflammasome assembly. We have previously shown that the Rho-GTPase-activating toxin from Escherichia coli cytotoxic necrotizing factor-1 (CNF1) activates caspase-1, but the upstream mechanism is unclear. Here, we provide evidence of the role of the NLRP3 inflammasome in sensing the activity of bacterial toxins and virulence factors that activate host Rho GTPases. We demonstrate that this activation relies on the monitoring of the toxin's activity on the Rho GTPase Rac2. We also show that the NLRP3 inflammasome is activated by a signalling cascade that involves the p21-activated kinases 1 and 2 (Pak1/2) and the Pak1-mediated phosphorylation of Thr 659 of NLRP3, which is necessary for the NLRP3-Nek7 interaction, inflammasome activation and IL-1β cytokine maturation. Furthermore, inhibition of the Pak-NLRP3 axis decreases the bacterial clearance of CNF1-expressing UTI89 E. coli during bacteraemia in mice. Taken together, our results establish that Pak1 and Pak2 are critical regulators of the NLRP3 inflammasome and reveal the role of the Pak-NLRP3 signalling axis in vivo during bacteraemia in mice.

Ion channels and transporters in microglial function in physiology and brain diseases

Microglial cells interact with all components of the central nervous system (CNS) and are increasingly recognized to play essential roles during brain development, homeostasis and disease pathologies. Functions of microglia include maintaining tissue integrity, clearing cellular debris and dead neurons through the process of phagocytosis, and providing tissue repair by releasing anti-inflammatory cytokines and neurotrophic factors. Changes of microglial ionic homeostasis (Na⁺, Ca²⁺, K⁺, H⁺, Cl^-) are important for microglial activation, including proliferation, migration, cytokine release and reactive oxygen species production, etc. These are mediated by ion channels and ion transporters in microglial cells. Here, we review the current knowledge about the role of major microglial ion channels and transporters, including several types of Ca²⁺ channels (store-operated Ca²⁺ entry (SOCE) channels, transient receptor potential (TRP) channels and voltage-gated Ca²⁺ channels (VGCCs)) and Na⁺ channels (voltage-gated Na⁺ channels (Nav) and acid-sensing ion channels (ASICs)), K⁺ channels (inward rectifier K⁺ channels (K_ir), voltage-gated K⁺ channels (K_V) and calcium-activated K⁺ channels (K_Ca)), proton channels (voltage-gated proton channel (Hv1)), and Cl^- channels (volume (or swelling)-regulated Cl^- channels (VRCCs) and chloride intracellular channels (CLICs)). In addition, ion transporter proteins such as Na⁺/Ca²⁺ exchanger (NCX), Na⁺-K⁺-Cl^- cotransporter (NKCC1), and Na⁺/H⁺ exchanger (NHE1) are also involved in microglial function in physiology and brain diseases. We discussed microglial activation and neuroinflammation in relation to the ion channel/transporter stimulation under brain disease conditions and therapeutic aspects of targeting microglial ion channels/transporters for neurodegenerative disease, ischemic stroke, traumatic brain injury and neuropathic pain.

Actin-mediated Microglial Chemotaxis via G-Protein Coupled Purinergic Receptor in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disease mainly associated with aging, oxidative stress and genetic mutations. There are two pathological proteins involved in AD; Amyloid-β peptide and microtubule-associated protein Tau (MAPT). The β- and γ-secretase enzyme cleaves the Amyloid precursor protein, which results in the formation of extracellular plaques in brain. While, Tau undergoes hyperphosphorylation and other post-translational modifications (PTMs), which eventually generates Tau oligomers, and intracellular neurofibrillary tangles (NFTs) in neurons. Moreover, the brain-resident glia and infiltrated macrophages elevate the level of CNS inflammation, which trigger the oxidative damage of neuronal circuits by reactive oxygen species (ROS) and Nitric oxide (NO). Microglia is the primary immune cell in the CNS, which is continuously surveilling the neuronal synapses and pathogen invasion. Microglia in the resting state is called 'Ramified', which possess long surveilling extensions with a small cell body. But, upon activation, microglia retracts the cellular extensions and transform into round migratory cells, called as 'Amoeboid' state. Activated microglia undergoes actin remodeling by forming lamellipodia and filopodia, which directs the migratory axis while podosomes formed are involved in extracellular matrix degradation for invasion. Protein-aggregates in malfunctioning synapses and in CNS milieu can be detected by microglia, which results in its activation and migration. Subsequently, the phagocytosis of synapses leads to the inflammatory burst and memory loss. The extracellular nucleotides released from damaged neurons and the cytokine-chemokine gradients allow the neighboring microglia and macrophages to migrate-infiltrate at the site of neuronal-damage. The ionotropic (P2XR) and metabotropic (P2YR) purinergic receptor recognize extracellular ATP/ADP, which propagates through the intracellular calcium signaling, chemotaxis, phagocytosis and inflammation. The P2Y receptors give 'find me' or 'eat me' signals to microglia to either migrate or phagocytose cellular debris. Further, the actin cytoskeleton helps microglia to mediate directed chemotaxis and neuronal repair during neurodegeneration. Hence, we aim to emphasize the connection between purinergic signaling and actin-driven mechanical movements of microglia for migration and inflammation in AD.

Lysophospholipids and Their G-Coupled Protein Signaling in Alzheimer's Disease: From Physiological Performance to Pathological Impairment

Lysophospholipids (LPLs) are bioactive signaling lipids that are generated from phospholipase-mediated hydrolyzation of membrane phospholipids (PLs) and sphingolipids (SLs). Lysophosphatidic acid (LPA) and sphingosine-1-phosphate (S1P) are two of the best-characterized LPLs which mediate a variety of cellular physiological responses via specific G-protein coupled receptor (GPCR) mediated signaling pathways. Considerable evidence now demonstrates the crucial role of LPA and S1P in neurodegenerative diseases, especially in Alzheimer’s disease (AD). Dysfunction of LPA and S1P metabolism can lead to aberrant accumulation of amyloid-β (Aβ) peptides, the formation of neurofibrillary tangles (NFTs), neuroinflammation and ultimately neuronal death. Summarizing LPA and S1P signaling profile may aid in profound health and pathological processes. In the current review, we will introduce the metabolism as well as the physiological roles of LPA and S1P in maintaining the normal functions of the nervous system. Given these pivotal functions, we will further discuss the role of dysregulation of LPA and S1P in promoting AD pathogenesis.

The inhibition of Kir2.1 potassium channels depolarizes spinal microglial cells, reduces their proliferation, and attenuates neuropathic pain

Spinal microglia change their phenotype and proliferate after nerve injury, contributing to neuropathic pain. For the first time, we have characterized the electrophysiological properties of microglia and the potential role of microglial potassium channels in the spared nerve injury (SNI) model of neuropathic pain. We observed a strong increase of inward currents restricted at 2 days after injury associated with hyperpolarization of the resting membrane potential (RMP) in microglial cells compared to later time-points and naive animals. We identified pharmacologically and genetically the current as being mediated by Kir2.1 ion channels whose expression at the cell membrane is increased 2 days after SNI. The inhibition of Kir2.1 with ML133 and siRNA reversed the RMP hyperpolarization and strongly reduced the currents of microglial cells 2 days after SNI. These electrophysiological changes occurred coincidentally to the peak of microglial proliferation following nerve injury. In vitro, ML133 drastically reduced the proliferation of BV2 microglial cell line after both 2 and 4 days in culture. In vivo, the intrathecal injection of ML133 significantly attenuated the proliferation of microglia and neuropathic pain behaviors after nerve injury. In summary, our data implicate Kir2.1-mediated microglial proliferation as an important therapeutic target in neuropathic pain.

Transcriptomic and computational analysis identified LPA metabolism, KLHL14 and KCNE3 as novel regulators of Epithelial-Mesenchymal Transition

Epithelial-mesenchymal transition (EMT) is a complex biological program between physiology and pathology. Here, amniotic epithelial cells (AEC) were used as in vitro model of transiently inducible EMT in order to evaluate the transcriptional insights underlying this process. Therefore, RNA-seq was used to identify the differentially expressed genes and enrichment analyses were carried out to assess the intracellular pathways involved. As a result, molecules exclusively expressed in AEC that experienced EMT (GSTA1-1 and GSTM3) or when this process is inhibited (KLHL14 and KCNE3) were identified. Lastly, the network theory was used to obtain a computational model able to recognize putative controller genes involved in the induction and in the prevention of EMT. The results suggested an opposite role of lysophosphatidic acid (LPA) synthesis and degradation enzymes in the regulation of EMT process. In conclusion, these molecules may represent novel EMT regulators and also targets for developing new therapeutic strategies.

Gintonin, a ginseng-derived ingredient, as a novel therapeutic strategy for Huntington's disease: Activation of the Nrf2 pathway through lysophosphatidic acid receptors

Gintonin (GT), a ginseng-derived lysophosphatidic acid receptor ligand, regulates various cellular effects and represses inflammation. However, little is known about the potential value of GT regarding inflammation in the neurodegenerative diseases, such as Huntington's disease (HD). In this study, we investigated whether GT could ameliorate the neurological impairment and striatal toxicity in cellular or animal model of HD. Pre-, co-, and onset-treatment with GT (25, 50, or 100 mg/kg/day, p.o.) alleviated the severity of neurological impairment and lethality following 3-nitropropionic acid (3-NPA). Pretreatment with GT also attenuated mitochondrial dysfunction i.e. succinate dehydrogenase and MitoSOX activities, apoptosis, microglial activation, and mRNA expression of inflammatory mediators i.e. IL-1β, IL-6, TNF-α, COX-2, and iNOS in the striatum after 3-NPA-intoxication. Its action mechanism was associated with lysophosphatidic acid receptors (LPARs) and nuclear factor erythroid 2-related factor 2 (Nrf2) signaling pathway activations and the inhibition of mitogen-activated protein kinases (MAPKs) and nuclear factor-κB (NF-κB) signaling pathways. These beneficial effects of GT were neutralized by pre-inhibiting LPARs with Ki16425 (a LPAR1/3 antagonist). Interestingly, GT reduced cell death and mutant huntingtin (HTT) aggregates in STHdh cells. It also mitigated neurological impairment in mice with adeno-associated viral (AAV) vector serotype DJ-mediated overexpression of N171-82Q-mutant HTT in the striatum. Taken together, our findings firstly suggested that GT has beneficial effects with a wide therapeutic time-window in 3-NPA-induced striatal toxicity by antioxidant and anti-inflammatory activities through LPA. In addition, GT exerts neuroprotective effects in STHdh cells and AAV vector-infected model of HD. Thus GT might be an innovative therapeutic candidate to treat HD-like syndromes.

Identifying lysophosphatidic acid receptor subtype 1 (LPA1) as a novel factor to modulate microglial activation and their TNF-α production by activating ERK1/2

Microglia regulate immune responses in the brain, and their activation is key to the pathogenesis of diverse neurological diseases. Receptor-mediated lysophosphatidic acid (LPA) signaling has been known to regulate microglial biology, but it is still unclear which receptor subtypes guide the biology, particularly, microglial activation. Here, we investigated the pathogenic aspects of LPA receptor subtype 1 (LPA₁) in microglial activation using a systemic lipopolysaccharide (LPS) administration-induced septic mouse model in vivo and LPS-stimulated rat primary microglia in vitro. LPA₁ knockdown in the brain with its specific shRNA lentivirus attenuated the sepsis-induced microglia activation, morphological transformation, and proliferation. LPA₁ knockdown also resulted in the downregulation of TNF-α, at both mRNA and protein levels in septic brains, but not IL-1β or IL-6. In rat primary microglia, genetic or pharmacological blockade of LPA₁ attenuated gene upregulation and secretion of TNF-α in LPS-stimulated cells. In particular, the latter was associated with the suppressed TNF-α converting enzyme (TACE) activity. We reaffirmed these biological aspects using a BV2 microglial cell line in which LPA₁ expression was negligible. LPA₁ overexpression in BV2 cells led to significant increments in TNF-α production upon stimulation with LPS, whereas inhibiting LPA₁ reversed the production. We further identified ERK1/2, but not p38 MAPK or Akt, as the underlying effector pathway after LPA₁ activation in both septic brains and stimulated microglia. The current findings of the novel role of LPA₁ in microglial activation along with its mechanistic aspects could be applied to understanding the pathogenesis of diverse neurological diseases that involve microglial activation.

Role of lysophosphatidic acid in ion channel function and disease

Lysophosphatidic acid (LPA) is a bioactive phospholipid that exhibits a wide array of functions that include regulation of protein synthesis and adequate development of organisms. LPA is present in the membranes of cells and in the serum of several mammals and has also been shown to participate importantly in pathophysiological conditions. For several decades it was known that LPA produces some of its effects in cells through its interaction with specific G protein-coupled receptors, which in turn are responsible for signaling pathways that regulate cellular function. Among the target proteins for LPA receptors are ion channels that modulate diverse aspects of the physiology of cells and organs where they are expressed. However, recent studies have begun to unveil direct effects of LPA on ion channels, highlighting this phospholipid as a direct agonist and adding to the knowledge of the field of lipid-protein interactions. Moreover, the roles of LPA in pathophysiological conditions associated with the function of some ion channels have also begun to be clarified, and molecular mechanisms have been identified. This review focuses on the effects of LPA on ion channel function under normal and pathological conditions and highlights our present knowledge of the mechanisms by which it regulates the function and expression of N- and T-type Ca⁺⁺ channels; M-type K⁺ channel and inward rectifier K⁺ channel subunit 2.1; transient receptor potential (TRP) melastatin 2, TRP vanilloid 1, and TRP ankyrin 1 channels; and TWIK-related K⁺ channel 1 (TREK-1), TREK-2, TWIK-related spinal cord K⁺ channel (TRESK), and TWIK-related arachidonic acid-stimulated K⁺ channel (TRAAK).

Intrathecal Administration of CXCL1 Enhances Potassium Currents in Microglial Cells

The functioning of microglial cells inside the central nervous system depends on their ion channels expression. Microglia are capable of synthesizing different cytokines and chemokines, including CXCL1, and responding to their action via specific receptors. In this study, we explore the effect of intrathecal injection of CXCL1 on potassium currents, expressed in CX3CR1-Green Fluorescent Protein labeled microglia in transgenic mice. The results showed that CXCL1 hyperpolarized the cells by enhancing inward rectifying potassium currents and increasing the membrane area, suggesting an activating effect on microglia.

Responses of rat and mouse primary microglia to pro- and anti-inflammatory stimuli: molecular profiles, K+ channels and migration

BACKGROUND

Acute CNS damage is commonly studied using rat and mouse models, but increasingly, molecular analysis is finding species differences that might affect the ability to translate findings to humans. Microglia can undergo complex molecular and functional changes, often studied by in vitro responses to discrete activating stimuli. There is considerable evidence that pro-inflammatory (M1) activation can exacerbate tissue damage, while anti-inflammatory (M2) states help resolve inflammation and promote tissue repair. However, in assessing potential therapeutic targets for controlling inflammation, it is crucial to determine whether rat and mouse microglia respond the same.

METHODS

Primary microglia from Sprague-Dawley rats and C57BL/6 mice were cultured, then stimulated with interferon-γ + tumor necrosis factor-α (I + T; M1 activation), interleukin (IL)-4 (M2a, alternative activation), or IL-10 (M2c, acquired deactivation). To profile their activation responses, NanoString was used to monitor messenger RNA (mRNA) expression of numerous pro- and anti-inflammatory mediators, microglial markers, immunomodulators, and other molecules. Western analysis was used to measure selected proteins. Two potential targets for controlling inflammation-inward- and outward-rectifier K⁺ channels (Kir2.1, Kv1.3)-were examined (mRNA, currents) and specific channel blockers were applied to determine their contributions to microglial migration in the different activation states.

RESULTS

Pro-inflammatory molecules increased after I + T treatment but there were several qualitative and quantitative differences between the species (e.g., iNOS and nitric oxide, COX-2). Several molecules commonly associated with an M2a state differed between species or they were induced in additional activation states (e.g., CD206, ARG1). Resting levels and/or responses of several microglial markers (Iba1, CD11b, CD68) differed with the activation state, species, or both. Transcripts for several Kir2 and Kv1 family members were detected in both species. However, the current amplitudes (mainly Kir2.1 and Kv1.3) depended on activation state and species. Treatment-induced changes in morphology and migratory capacity were similar between the species (migration reduced by I + T, increased by IL-4 or IL-10). In both species, Kir2.1 block reduced migration and Kv1.3 block increased it, regardless of activation state; thus, these channels might affect microglial migration to damage sites.

CONCLUSIONS

Caution is recommended in generalizing molecular and functional responses of microglia to activating stimuli between species.

Phospholipid localization implies microglial morphology and function via Cdc42 in vitro

Under a quiescent state, microglia exhibit a ramified shape, rather than the amoeboid-like morphology following injury or inflammation. The manipulation of microglial morphology in vitro has not been very successful, which has impeded the progress of microglial studies. We demonstrate that lysophosphatidylserine (LysoPS), a kind of lysophospholipids, rapidly and substantially alters the morphology of primary cultured microglia to an in vivo-like ramified shape in a receptor independent manner. This mechanism is mediated by Cdc42 activity. LysoPS is incorporated into the plasma membrane and converted to phosphatidylserine (PS) via the Lands' cycle. The accumulated PS on the membrane recruits Cdc42. Both Cdc42 and PS colocalize predominantly in primary and secondary processes, but not in peripheral branches or tips of microglia. Along with the morphological changes LysoPS suppresses inflammatory cytokine production and NF-kB activity. The present study provides a tool to manipulate a microglial phenotype from an amoeboid to a fully ramified in vitro, which certainly contributes to studies exploring microglial physiology and pathology.

1-Oleyl-lysophosphatidic acid (LPA) promotes polarization of BV-2 and primary murine microglia towards an M1-like phenotype

Background

Microglia, the immunocompetent cells of the CNS, rapidly respond to brain injury and disease by altering their morphology and phenotype to adopt an activated state. Microglia can exist broadly between two different states, namely the classical (M1) and the alternative (M2) phenotype. The first is characterized by the production of pro-inflammatory cytokines/chemokines and reactive oxygen and/or nitrogen species. In contrast, alternatively activated microglia are typified by an anti-inflammatory phenotype supporting wound healing and debris clearance. The objective of the present study was to determine the outcome of lysophosphatidic acid (LPA)-mediated signaling events on microglia polarization.

Methods

LPA receptor expression and cyto-/chemokine mRNA levels in BV-2 and primary murine microglia (PMM) were determined by qPCR. M1/M2 marker expression was analyzed by Western blotting, immunofluorescence microscopy, or flow cytometry. Cyto-/chemokine secretion was quantitated by ELISA.

Results

BV-2 cells express LPA receptor 2 (LPA2), 3, 5, and 6, whereas PMM express LPA1, 2, 4, 5, and 6. We show that LPA treatment of BV-2 and PMM leads to a shift towards a pro-inflammatory M1-like phenotype. LPA treatment increased CD40 and CD86 (M1 markers) and reduced CD206 (M2 marker) expression. LPA increased inducible nitric oxide synthase (iNOS) and COX-2 levels (both M1), while the M2 marker Arginase-1 was suppressed in BV-2 cells. Immunofluorescence studies (iNOS, COX-2, Arginase-1, and RELMα) extended these findings to PMM. Upregulation of M1 markers in BV-2 and PMM was accompanied by increased cyto-/chemokine transcription and secretion (IL-1β, TNFα, IL-6, CCL5, and CXCL2). The pharmacological LPA5 antagonist TCLPA5 blunted most of these pro-inflammatory responses.

Conclusions

LPA drives BV-2 and PMM towards a pro-inflammatory M1-like phenotype. Suppression by TCLPA5 indicates that the LPA/LPA5 signaling axis could represent a potential pharmacological target to interfere with microglia polarization in disease.

Activation of mitochondrial transient receptor potential vanilloid 1 channel contributes to microglial migration

Microglia, the resident immune cells in the brain, survey the environment of the healthy brain. Microglial migration is essential for many physiological and pathophysiological processes. Although microglia express some members of the transient receptor potential (TRP) channel family, there is little knowledge regarding the physiological roles of TRP channels in microglia. Here, we explored the role of TRP vanilloid 1 (TRPV1), a channel opened by capsaicin, heat, protons, and endovanilloids, in microglia. We found that application of capsaicin induced concentration-dependent migration in microglia derived from wild-type mice but not in those derived from TRPV1 knockout (TRPV1-KO) mice. Capsaicin-induced microglial migration was significantly inhibited by co-application of the TRPV1 blocker SB366791 and the Ca(2+) chelator BAPTA-AM. Using RT-PCR and immunocytochemistry, we validated that TRPV1 was expressed in microglia. Electrophysiological recording, intracellular Ca(2+) imaging, and immunocytochemistry indicated that TRPV1 was localized primarily in intracellular organelles. Treatment with capsaicin induced an increase in intramitochondrial Ca(2+) concentrations and mitochondrial depolarization. Furthermore, microglia derived from TRPV1-KO mice showed delayed Ca(2+) efflux compared with microglia derived from wild-type mice. Capsaicin-induced microglial migration was inhibited by membrane-permeable antioxidants and MAPK inhibitors, suggesting that mitochondrial TRPV1 activation induced Ca(2+) -dependent production of ROS followed by MAPK activation, which correlated with an augmented migration of microglia. Moreover, a mixture of three endovanilloids augmented microglial migration via TRPV1 activation. Together, these results indicate that mitochondrial TRPV1 plays an important role in inducing microglial migration. Activation of TRPV1 triggers an increase in intramitochondrial Ca(2+) concentration and following depolarization of mitochondria, which results in mtROS production, MAPK activation, and enhancement of chemotactic activity in microglia.

Expression and contributions of the Kir2.1 inward-rectifier K(+) channel to proliferation, migration and chemotaxis of microglia in unstimulated and anti-inflammatory states

When microglia respond to CNS damage, they can range from pro-inflammatory (classical, M1) to anti-inflammatory, alternative (M2) and acquired deactivation states. It is important to determine how microglial functions are affected by these activation states, and to identify molecules that regulate their behavior. Microglial proliferation and migration are crucial during development and following damage in the adult, and both functions are Ca²⁺-dependent. In many cell types, the membrane potential and driving force for Ca²⁺ influx are regulated by inward-rectifier K⁺ channels, including Kir2.1, which is prevalent in microglia. However, it is not known whether Kir2.1 expression and contributions are altered in anti-inflammatory states. We tested the hypothesis that Kir2.1 contributes to Ca²⁺ entry, proliferation and migration of rat microglia. Kir2.1 (KCNJ2) transcript expression, current amplitude, and proliferation were comparable in unstimulated microglia and following alternative activation (IL-4 stimulated) and acquired deactivation (IL-10 stimulated). To examine functional roles of Kir2.1 in microglia, we first determined that ML133 was more effective than the commonly used blocker, Ba²⁺; i.e., ML133 was potent (IC₅₀ = 3.5 μM) and voltage independent. Both blockers slightly increased proliferation in unstimulated or IL-4 (but not IL-10)-stimulated microglia. Stimulation with IL-4 or IL-10 increased migration and ATP-induced chemotaxis, and blocking Kir2.1 greatly reduced both but ML133 was more effective. In all three activation states, blocking Kir2.1 with ML133 dramatically reduced Ca²⁺ influx through Ca²⁺-release-activated Ca²⁺ (CRAC) channels. Thus, Kir2.1 channel activity is necessary for microglial Ca²⁺ signaling and migration under resting and anti-inflammatory states but the channel weakly inhibits proliferation.

Lysophosphatidic Acid signaling in the nervous system

The brain is composed of many lipids with varied forms that serve not only as structural components but also as essential signaling molecules. Lysophosphatidic acid (LPA) is an important bioactive lipid species that is part of the lysophospholipid (LP) family. LPA is primarily derived from membrane phospholipids and signals through six cognate G protein-coupled receptors (GPCRs), LPA1-6. These receptors are expressed on most cell types within central and peripheral nervous tissues and have been functionally linked to many neural processes and pathways. This Review covers a current understanding of LPA signaling in the nervous system, with particular focus on the relevance of LPA to both physiological and diseased states.