Masataka Ifuku

Bradykinin-Induced Microglial Migration Mediated by B1-Bradykinin Receptors Depends on Ca2+ Influx via Reverse-Mode Activity of the Na+/Ca2+ Exchanger

Bradykinin (BK) is produced and acts at the site of injury and inflammation. In the CNS, migration of microglia toward the lesion site plays an important role pathologically. In the present study, we investigated the effect of BK on microglial migration. Increased motility of cultured microglia was mimicked by B1 receptor agonists and markedly inhibited by a B1 antagonist, but not by a B2 receptor antagonist. BK induced chemotaxis in microglia isolated from wild-type and B2-knock-out mice but not from B1-knock-out mice. BK-induced motility was not blocked by pertussis toxin but was blocked by chelating intracellular Ca2+ or by low extracellular Ca2+, implying that Ca2+ influx is prerequisite. Blocking the reverse mode of Na+/Ca2+ exchanger (NCX) completely inhibited BK-induced migration. The involvement of NCX was further confirmed by using NCX+/− mice; B1-agonist-induced motility and chemotaxis was decreased compared with that in NCX+/+ mice. Activation of NCX seemed to be dependent on protein kinase C and phosphoinositide 3-kinase, and resultant activation of intermediate-conductance (IK-type) Ca2+-dependent K+ currents (IK(Ca)) was activated. Despite these effects, BK did not activate microglia, as judged from OX6 staining. Using in vivo lesion models and pharmacological injection to the brain, it was shown that microglial accumulation around the lesion was also dependent on B1 receptors and IK(Ca). These observations support the view that BK functions as a chemoattractant by using the distinct signal pathways in the brain and, thus, attracts microglia to the lesion site in vivo.

Neuroprotective role of bradykinin because of the attenuation of pro‐inflammatory cytokine release from activated microglia

Bradykinin (BK) has been reported to be a mediator of brain damage in acute insults. Receptors for BK have been identified on microglia, the pathologic sensors of the brain. Here, we report that BK attenuated lipopolysaccharide (LPS)‐induced release of tumor necrosis factor‐alpha (TNF‐α) and interleukin‐1β from microglial cells, thus acting as an anti‐inflammatory mediator in the brain. This effect was mimicked by raising intracellular cAMP or stimulating the prostanoid receptors EP2 and EP4, while it was abolished by a cAMP antagonist, a prostanoid receptor antagonist, or by an inhibitor of the inducible cyclooxygenase (cyclooxygenase‐2). BK also enhanced formation of prostaglandin E2 and expression of microsomal prostaglandin E synthase. Expression of BK receptors and EP2/EP4 receptors were also enhanced. Using physiological techniques, we identified functional BK receptors not only in culture, but also in microglia from acute brain slices. BK reduced LPS‐induced neuronal death in neuron–microglia co‐cultures. This was probably mediated via microglia as it did not affect TNF‐α‐induced neuronal death in pure neuronal cultures. Our data imply that BK has anti‐inflammatory and neuroprotective effects in the central nervous system by modulating microglial function.

Translocator protein (18 kDa)/peripheral benzodiazepine receptor specific ligands induce microglia functions consistent with an activated state.

In the brain, translocator protein (18 kDa) (TSPO), previously called peripheral benzodiazepine receptor (PBR), is a glial protein that has been extensively used as a biomarker of brain injury and inflammation. However, the functional role of TSPO in glial cells is not well characterized. In this study, we show that the TSPO‐specific ligands R‐PK11195 (PK) and Ro5‐4864 (Ro) increased microglia proliferation and phagocytosis with no effect on migration. Both ligands increased reactive oxygen species (ROS) production, and this effect may be mediated by NADPH‐oxidase. PK and Ro also produced a small but detectable increase in IL‐1β release. We also examined the effect of PK and Ro on the expression of proinflammatory genes and cytokine release in lipopolysaccharide (LPS) and adenosine triphosphate (ATP) activated microglia. PK or Ro had no effect on LPS‐induced increase of pro‐inflammatory genes, but they both decreased the ATP‐induced increase of COX‐2 gene expression. Ro, but not PK, enhanced the LPS‐induced release of IL‐1β. However, Ro decreased the ATP‐induced release of IL‐1β and TNF‐α, and PK decreased the ATP‐induced release of TNF‐α. Exposure to Ro in the presence of LPS increased the number of apoptotic microglia, an effect that could be blocked by PK. These findings show that TSPO ligands modulate cellular functions consistent with microglia activation. Further, when microglia are activated, these ligands may have therapeutic potential by reducing the expression of pro‐inflammatory genes and cytokine release. Finally, Ro‐like ligands may be involved in the elimination of activated microglia via apoptosis. ©2010 Wiley‐Liss, Inc.

Anti-inflammatory/anti-amyloidogenic effects of plasmalogens in lipopolysaccharide-induced neuroinflammation in adult mice

BackgroundNeuroinflammation involves the activation of glial cells in neurodegenerative diseases such as Alzheimer’s disease (AD). Plasmalogens (Pls) are glycerophospholipids constituting cellular membranes and play significant roles in membrane fluidity and cellular processes such as vesicular fusion and signal transduction.MethodsIn this study the preventive effects of Pls on systemic lipopolysaccharide (LPS)-induced neuroinflammation were investigated using immunohistochemistry, real-time PCR methods and analysis of brain glycerophospholipid levels in adult mice.ResultsIntraperitoneal (i.p.) injections of LPS (250 μg/kg) for seven days resulted in increases in the number of Iba-1-positive microglia and glial fibrillary acidic protein (GFAP)-positive astrocytes in the prefrontal cortex (PFC) and hippocampus accompanied by the enhanced expression of IL-1β and TNF-α mRNAs. In addition, β-amyloid (Aβ3–16)-positive neurons appeared in the PFC and hippocampus of LPS-injected animals. The co-administration of Pls (i.p., 20 mg/kg) after daily LPS injections significantly attenuated both the activation of glial cells and the accumulation of Aβ proteins. Finally, the amount of Pls in the PFC and hippocampus decreased following the LPS injections and this reduction was suppressed by co-treatment with Pls.ConclusionsThese findings suggest that Pls have anti-neuroinflammatory and anti-amyloidogenic effects, thereby indicating the preventive or therapeutic application of Pls against AD.

Plasmalogens Rescue Neuronal Cell Death through an Activation of AKT and ERK Survival Signaling

Neuronal cells are susceptible to many stresses, which will cause the apoptosis and neurodegenerative diseases. The precise molecular mechanism behind the neuronal protection against these apoptotic stimuli is necessary for drug discovery. In the present study, we have found that plasmalogens (Pls), which are glycerophospholipids containing vinyl ether linkage at sn-1 position, can protect the neuronal cell death upon serum deprivation. Interestingly, caspse-9, but not caspase-8 and caspase-12, was cleaved upon the serum starvation in Neuro-2A cells. Pls treatments effectively reduced the activation of caspase-9. Furthermore, cellular signaling experiments showed that Pls enhanced phosphorylation of the phosphoinositide 3-kinase (PI3K)-dependent serine/threonine-specific protein kinase AKT and extracellular-signal-regulated kinases ERK1/2. PI3K/AKT inhibitor LY294002 and MAPK/ERK kinase (MEK) inhibitor U0126 treatments study clearly indicated that Pls-mediated cell survival was dependent on the activation of these kinases. In addition, Pls also inhibited primary mouse hippocampal neuronal cell death induced by nutrient deprivation, which was associated with the inhibition of caspase-9 and caspase-3 cleavages. It was reported that Pls content decreased in the brain of the Alzheimer’s patients, which indicated that the reduction of Pls content could endanger neurons. The present findings, taken together, suggest that Pls have an anti-apoptotic action in the brain. Further studies on precise mechanisms of Pls-mediated protection against cell death may lead us to establish a novel therapeutic approach to cure neurodegenerative disorders.

Multifunctional effects of bradykinin on glial cells in relation to potential anti-inflammatory effects

Kinins have been reported to be produced and act at the site of injury and inflammation. Despite many reports that they are likely to initiate a particular cascade of inflammatory events, bradykinin (BK) has anti-inflammatory effects in the brain mediated by glial cells. In the present review, we have attempted to describe the complex responses and immediate reaction of glial cells to BK. Glial cells express BK receptors and induce Ca(2+)-dependent signal cascades. Among them, production of prostaglandin E(2) (PGE(2)), via B(1) receptors in primary cultured microglia, has a negative feedback effect on lipopolysaccharide (LPS)-induced release of tumor necrosis factor-alpha (TNF-alpha) via increasing intracellular cyclic adenosine monophosphate (cAMP). In addition, BK up-regulates the production of neurotrophic factors such as nerve growth factor (NGF) via B(2) receptors in astrocytes. These results suggest that BK may have anti-inflammatory and neuroprotective effects in the brain through multiple functions on glial cells. These observations may help to understand the paradox on the role of kinins in the central nervous system and may be useful for therapeutic strategy.

Galectin-1 promotes basal and kainate-induced proliferation of neural progenitors in the dentate gyrus of adult mouse hippocampus

We examined the expression of galectin-1, an endogenous lectin with one carbohydrate-binding domain, in the adult mouse hippocampus after systemic kainate administration. We found that the expression of galectin-1 was remarkably increased in activated astrocytes of the CA3 subregion and dentate gyrus of the hippocampus, and in nestin-positive neural progenitors in the dentate gyrus. Quantitative reverse transcription PCR (RT-PCR) analysis revealed that the galectin-1 mRNA level in hippocampus began to increase 1 day after kainate administration and that a 13-fold increase was attained within 3 days. Western blotting analysis confirmed that the level of galectin-1 protein increased to more than three-fold a week after the exposure. We showed that isolated astrocytes express and secrete galectin-1. To clarify the significance of the increased expression of galectin-1 in hippocampus, we compared the levels of hippocampal cell proliferation in galectin-1 knockout and wild-type mice after saline or kainate administration. The number of 5-bromo-2′-deoxyuridine (BrdU)-positive cells detected in the subgranular zone (SGZ) of galectin-1 knockout mice decreased to 62% with saline, and to 52% with kainate, as compared with the number seen in the wild-type mice. Most of the BrdU-positive cells in SGZ expressed doublecortin and neuron-specific nuclear protein, indicating that they are immature neurons. We therefore concluded that galectin-1 promotes basal and kainate-induced proliferation of neural progenitors in the hippocampus.

Induction of interleukin‐1β by activated microglia is a prerequisite for immunologically induced fatigue

We previously reported that an intraperitoneal (i.p.) injection of synthetic double‐stranded RNA, polyriboinosinic:polyribocytidylic acid (poly‐I:C), produced prolonged fatigue in rats, which might serve as a model for chronic fatigue syndrome. The poly‐I:C‐induced fatigue was associated with serotonin transporter (5‐HTT) overexpression in the prefrontal cortex (PFC), a brain region that has been suggested to be critical for fatigue sensation. In the present study, we demonstrated that microglial activation in the PFC was important for poly‐I:C‐induced fatigue in rats, as pretreatment with minocycline, an inhibitor of microglial activation, prevented the decrease in running wheel activity. Poly‐I:C injection increased the microglial interleukin (IL)‐1β expression in the PFC. An intracerebroventricular (i.c.v.) injection of IL‐1β neutralising antibody limited the poly‐I:C‐induced decrease in activity, whereas IL‐1β (i.c.v.) reduced the activity in a dose‐dependent manner. 5‐HTT expression was enhanced by IL‐1β in primary cultured astrocytes but not in microglia. Poly‐I:C injection (i.p.) caused an increase in 5‐HTT expression in astrocytes in the PFC of the rat, which was inhibited by pretreatment with minocycline (i.p.) and rat recombinant IL‐1 receptor antagonist (i.c.v.). Poly‐I:C injection (i.p.) led to a breakdown of the blood–brain barrier and enhanced Toll‐like receptor 3 signaling in the brain. Furthermore, direct application of poly‐I:C enhanced IL‐1β expression in primary microglia. We therefore propose that poly‐I:C‐induced microglial activation, which may be at least partly caused by a direct action of poly‐I:C, enhances IL‐1β expression. Then, IL‐1β induces 5‐HTT expression in astrocytes, resulting in the immunologically induced fatigue.

Expression, subunit composition, and function of AMPA‐type glutamate receptors are changed in activated microglia; possible contribution of GluA2 (GluR‐B)‐deficiency under pathological conditions

Microglia express AMPA (α‐amino‐hydroxy‐5‐methyl‐isoxazole‐4‐propionate)‐type of glutamate (Glu) receptors (AMPAR), which are highly Ca2+ impermeable due to the expression of GluA2. However, the functional importance of AMPAR in microglia remains to be investigated, especially under pathological conditions. As low expression of GluA2 was reported in some neurodegenerative diseases, GluA2−/− mice were used to show the functional change of microglial AMPARs in response to Glu or kainate (KA). Here we found that Glu‐induced currents in the presence of 100 μM cyclothiazide, an inhibitor of AMPAR desensitization, showed time‐dependent decrease after activation of microglia with lipopolysaccharide (LPS) in GluA2+/+ microglia, but not in GluA2−/− microglia. Upon activation of microglia, expression level of GluA2 subunits significantly increased, while expression of GluA1, A3 and A4 subunits on membrane surface significantly decreased. These results suggest that nearly homomeric GluA2 subunits were the main reason for low conductance of AMPAR in activated microglia. Increased expression of GluA2 in microglia was also detected partially in brain slices from LPS‐injected mice. Cultured microglia from GluA2−/− mice showed higher Ca2+‐permeability, consequently inducing significant increase in the release of proinflammatory cytokine, such as TNF‐α. The conditioning medium from KA‐treated GluA2−/− microglia had more neurotoxic effect on wild type cultured neurons than that from KA‐treated GluA2+/+ microglia. These results suggest that membrane translocation of GluA2‐containing AMPARs in activated microglia has functional importance and thus, dysfunction or decreased expression of GluA2 may accelerate Glu neurotoxicity via excess release of proinflammatory cytokines from microglia.

Functional importance of inositol-1,4,5-triphosphate-induced intracellular Ca2+ mobilization in galanin-induced microglial migration

J. Neurochem. (2011) 117, 61–70.