Microglia in cerebral ischemia: molecular actions and interactions

Potential of CBD Acting on Cannabinoid Receptors CB1 and CB2 in Ischemic Stroke

Stroke is one of the leading causes of death. It not only affects adult people but also many children. It is estimated that, every year, 15 million people suffer a stroke worldwide. Among them, 5 million people die, while 5 million people are left permanently disabled. In this sense, the research to find new treatments should be accompanied with new therapies to combat neuronal death and to avoid developing cognitive impairment and dementia. Phytocannabinoids are among the compounds that have been used by mankind for the longest period of history. Their beneficial effects such as pain regulation or neuroprotection are widely known and make them possible therapeutic agents with high potential. These compounds bind cannabinoid receptors CB1 and CB2. Unfortunately, the psychoactive side effect has displaced them in the vast majority of areas. Thus, progress in the research and development of new compounds that show efficiency as neuroprotectors without this psychoactive effect is essential. On the one hand, these compounds could selectively bind the CB2 receptor that does not show psychoactive effects and, in glia, has opened new avenues in this field of research, shedding new light on the use of cannabinoid receptors as therapeutic targets to combat neurodegenerative diseases such as Alzheimer's, Parkinson's disease, or stroke. On the other hand, a new possibility lies in the formation of heteromers containing cannabinoid receptors. Heteromers are new functional units that show new properties compared to the individual protomers. Thus, they represent a new possibility that may offer the beneficial effects of cannabinoids devoid of the unwanted psychoactive effect. Nowadays, the approval of a mixture of CBD (cannabidiol) and Δ9-THC (tetrahydrocannabinol) to treat the neuropathic pain and spasticity in multiple sclerosis or purified cannabidiol to combat pediatric epilepsy have opened new therapeutic possibilities in the field of cannabinoids and returned these compounds to the front line of research to treat pathologies as relevant as stroke.

Therapeutic management of ischemic stroke

Stroke is the third leading cause of years lost due to disability and the second-largest cause of mortality worldwide. Most occurrences of stroke are brought on by the sudden occlusion of an artery (ischemic stroke), but sometimes they are brought on by bleeding into brain tissue after a blood vessel has ruptured (hemorrhagic stroke). Alteplase is the only therapy the American Food and Drug Administration has approved for ischemic stroke under the thrombolysis category. Current views as well as relevant clinical research on the diagnosis, assessment, and management of stroke are reviewed to suggest appropriate treatment strategies. We searched PubMed and Google Scholar for the available therapeutic regimes in the past, present, and future. With the advent of endovascular therapy in 2015 and intravenous thrombolysis in 1995, the therapeutic options for ischemic stroke have expanded significantly. A novel approach such as vagus nerve stimulation could be life-changing for many stroke patients. Therapeutic hypothermia, the process of cooling the body or brain to preserve organ integrity, is one of the most potent neuroprotectants in both clinical and preclinical contexts. The rapid intervention has been linked to more favorable clinical results. This study focuses on the pathogenesis of stroke, as well as its recent advancements, future prospects, and potential therapeutic targets in stroke therapy.

Recent advances in tissue repair of the blood-brain barrier after stroke

The selective permeability of the blood-brain barrier (BBB) enables the necessary exchange of substances between the brain parenchyma and circulating blood and is important for the normal functioning of the central nervous system. Ischemic stroke inflicts damage upon the BBB, triggering adverse stroke outcomes such as cerebral edema, hemorrhagic transformation, and aggravated neuroinflammation. Therefore, effective repair of the damaged BBB after stroke and neovascularization that allows for the unique selective transfer of substances from the BBB after stroke is necessary and important for the recovery of brain function. This review focuses on four important therapies that have effects of BBB tissue repair after stroke in the last seven years. Most of these new therapies show increased expression of BBB tight-junction proteins, and some show beneficial results in terms of enhanced pericyte coverage at the injured vessels. This review also briefly outlines three effective classes of approaches and their mechanisms for promoting neoangiogenesis following a stroke.

Microbiota Metabolites and Immune Regulation Affect Ischemic Stroke Occurrence, Development, and Prognosis

The gut microbiota are not only related to the development and occurrence of digestive system disease, but also have a bidirectional relationship with nervous system diseases via the microbiota-gut-brain axis. At present, correlations between the gut microbiota and neurological diseases, including stroke, are one of the focuses of investigation and attention in the medical community. Ischemic stroke (IS) is a cerebrovascular disease accompanied by focal neurological deficit or central nervous system injury or death. In this review, we summarize the contemporary latest research on correlations between the gut microbiota and IS. Additionally, we discuss the mechanisms of gut microbiota implicated in IS and related to metabolite production and immune regulation. Moreover, the factors of gut microbiota that affecting IS occurrence, and research implicating the gut microbiota as potential therapeutic targets for IS, are highlighted. Our review highlights the evidential relationships and connections between the gut microbiota and IS pathogenesis and prognosis.

TMAO Promotes NLRP3 Inflammasome Activation of Microglia Aggravating Neurological Injury in Ischemic Stroke Through FTO/IGF2BP2

Objective

Stroke is a kind of cerebrovascular disease with high mortality. TMAO has been shown to aggravate stroke outcomes, but its mechanism remains unclear.

Materials and Methods

Mice were fed with 0.12% TMAO for 16 weeks. Then, mice were made into MCAO/R models. Neurological score, infarct volume, neuronal damage and markers associated with inflammation were assessed. Since microglia played a crucial role in ischemic stroke, microglia of MCAO/R mice were isolated for high-throughput sequencing to identify the most differentially expressed gene following TMAO treatment. Afterward, the downstream pathways of TMAO were investigated using primary microglia.

Results

TMAO promoted the release of inflammatory cytokines in the brain of MCAO/R mice and promoted the activation of OGD/R microglial inflammasome, thereby exacerbating ischemic stroke outcomes. FTO/IGF2BP2 inhibited NLRP3 inflammasome activation in OGD/R microglia by downregulating the m6A level of NLRP3. TMAO can inhibit the expression of FTO and IGF2BP2, thus promoting the activation of NLRP3 inflammasome in OGD/R microglia. In conclusion, these results demonstrated that TMAO promotes NLRP3 inflammasome activation of microglia aggravating neurological injury in ischemic stroke through FTO/IGF2BP2.

Conclusion

Our results demonstrated that TMAO promotes NLRP3 inflammasome activation of microglia aggravating neurological injury in ischemic stroke through FTO/IGF2BP2. These findings explained the molecular mechanism of TMAO aggravating ischemic stroke in detail and provided molecular mechanism for clinical treatment.

Mouse maternal odontogenic infection with Porphyromonas gingivalis induces cognitive decline in offspring

Introduction

Porphyromonas gingivalis (P. gingivalis), a major periodontal pathogen, causes intrauterine infection/inflammation. Offspring exposed to intrauterine infection/inflammation have an increased risk of neurological disorders, regardless of gestational age. However, the relationship between maternal periodontitis and offspring functional/histological changes in the brain has not yet been elucidated.

Methods

In this study, we used a gestational mouse model to investigate the effects of maternal odontogenic infection of P. gingivalis on offspring behavior and brain tissue.

Results

The step-through passive avoidance test showed that the latency of the acquisition trial was significantly shorter in the P. gingivalis group (p < 0.05), but no difference in spontaneous motor/exploratory parameters by open-field test. P. gingivalis was diffusely distributed throughout the brain, especially in the hippocampus. In the hippocampus and amygdala, the numbers of neuron cells and cyclic adenosine monophosphate response element binding protein-positive cells were significantly reduced (p < 0.05), whereas the number of ionized calcium binding adapter protein 1-positive microglia was significantly increased (p < 0.05). In the hippocampus, the number of glial fibrillary acidic protein-positive astrocytes was also significantly increased (p < 0.05).

Discussion

The offspring of P. gingivalis-infected mothers have reduced cognitive function. Neurodegeneration/neuroinflammation in the hippocampus and amygdala may be caused by P. gingivalis infection, which is maternally transmitted. The importance of eliminating maternal P. gingivalis-odontogenic infection before or during gestation in maintenance healthy brain function in offspring should be addressed in near future.

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.

Fucoidan (A sulfated polysaccharide) and Cerebroprotein in combination alleviate the neuroinflammation-mediated neural damage and functional deficits in the focal cerebral ischemia model of rat

Cerebral ischemic reperfusion injury could emanate a cascade of events ensuing in neural death and severe neurobehavioural deficits. The currently available interventions have failed to target the multimodal, interlinked mechanisms that operate cerebral ischemia-induced damage and functional loss. So an integrative intervention has become a mandate to overcome the deleterious mechanisms involved in cerebral ischemic pathophysiology. In this study, adult male Sprague dawley rats were exposed to 2 hours of right middle cerebral artery occlusion (rMCAo) followed by reperfusion, and the intervention group received Fucoidan alone at a dose of 50mg/kg, i.p (intraperitoneal), Cerebrolysin alone at a dose of 2.5mg/kg body weight and the combination of both. The sham rats were exposed to surgical procedures, except for the rMCAo. The assessments of the groups were made 24 hours after the rMCAo. The stand-alone treatment with Fucoidan, Cerebrolysin has shown a better outcome in the neurobehavioral and, histopathological assessments and the combination has made a significant reduction in the neurological deficits and the infarct volume when compared to the standalone groups. The BBB integrity was well preserved in the combination group when compared with the lesion and standalone groups. Moreover, the combined intervention reduced the level of pro-inflammatory cytokines TNFα, NFkB, IL1α, IL1-β, IL-6, CD68, COX-2, and mRNA expression of inflammatory genes IL1α, IL1-β, IL-6, IBA-1, and COX-2 effectively. In conclusion, the present study suggests that rMCAo induced neuroinflammation and neurobehavioural alterations were attenuated by intervention with a combination of Fucoidan and cerebrolysin; Further, Fucoidan and Cerebrolysin combination improved the ischemic tolerance level by promoting the proteins and genes that regulate the inflammatory cytokines and in aiding better recovery after ischemic reperfusion injury.

MicroRNA-210 Downregulates TET2 (Ten-Eleven Translocation Methylcytosine Dioxygenase 2) and Contributes to Neuroinflammation in Ischemic Stroke of Adult Mice

BACKGROUND

Stroke is a leading cause of morbidity and mortality worldwide. Neuroinflammation plays a key role in acute brain injury of ischemic stroke. MicroRNA-210 (miR210) is the master hypoxamir and regulates microglial activation and inflammation in a variety of diseases. In this study, we uncovered the mechanism of miR210 in orchestrating ischemic stroke-induced neuroinflammation through repression of TET2 (ten-eleven translocation methylcytosine dioxygenase 2) in the adult mouse brain.

METHODS

Ischemic stroke was induced in adult WT (wild type) or miR210 KO (miR210 deficient) mice by transient intraluminal middle cerebral artery occlusion. Injection of TET2 silencing RNA or miR210 complementary locked nucleic acid oligonucleotides, or miR210 KO mice were used to validate miR210-TET2 axis and its role in ischemic brain injury. Furthermore, the effect of TET2 overexpression on miR210-stimulated proinflammatory cytokines was examined in BV2 microglia. Post assays included magnetic resonance imaging scan for brain infarct size; neurobehavioral tests, reverse transcription-quantitative polymerase chain reaction, and Western blot for miR210; and TET2 levels, flow cytometry, and ELISA for neuroinflammation in the brain after stroke or microglia in vitro.

RESULTS

miR210 injection significantly reduced TET2 protein abundance in the brain, while miR210 complementary locked nucleic acid oligonucleotides or miR210 KO preserved TET2 regardless of ischemic brain injury. TET2 knockdown reversed the protective effects of miR210 inhibition or miR210 KO on ischemic stroke-induced brain infarct size and neurobehavioral deficits. Moreover, flow cytometry and ELISA assays showed that TET2 knockdown also significantly dampened the anti-inflammatory effect of miR210 inhibition on microglial activation and IL (interleukin)-6 release after stroke. In addition, overexpression of TET2 in BV2 microglia counteracted miR210-induced increase in cytokines.

CONCLUSIONS

miR210 inhibition reduced ischemic stroke-induced neuroinflammatory response via repression of TET2 in the adult mouse brain, suggesting that miR210 is a potential treatment target for acute brain injury after ischemic stroke.

Combined Treatment of Dichloroacetic Acid and Pyruvate Increased Neuronal Survival after Seizure

During seizure activity, glucose and Adenosine triphosphate (ATP) levels are significantly decreased in the brain, which is a contributing factor to seizure-induced neuronal death. Dichloroacetic acid (DCA) has been shown to prevent cell death. DCA is also known to be involved in adenosine triphosphate (ATP) production by activating pyruvate dehydrogenase (PDH), a gatekeeper of glucose oxidation, as a pyruvate dehydrogenase kinase (PDK) inhibitor. To confirm these findings, in this study, rats were given a per oral (P.O.) injection of DCA (100 mg/kg) with pyruvate (50 mg/kg) once per day for 1 week starting 2 h after the onset of seizures induced by pilocarpine administration. Neuronal death and oxidative stress were assessed 1 week after seizure to determine if the combined treatment of pyruvate and DCA increased neuronal survival and reduced oxidative damage in the hippocampus. We found that the combined treatment of pyruvate and DCA showed protective effects against seizure-associated hippocampal neuronal cell death compared to the vehicle-treated group. Treatment with combined pyruvate and DCA after seizure may have a therapeutic effect by increasing the proportion of pyruvate converted to ATP. Thus, the current research demonstrates that the combined treatment of pyruvate and DCA may have therapeutic potential in seizure-induced neuronal death.

Adenosine as a Key Mediator of Neuronal Survival in Cerebral Ischemic Injury

Several experimental studies have linked adenosine's neuroprotective role in cerebral ischemia. During ischemia, adenosine is formed due to intracellular ATP breakdown into ADP, further when phosphate is released from ADP, the adenosine monophosphate is formed. It acts via A1, A2, and A3 receptors found on neurons, blood vessels, glial cells, platelets, and leukocytes. It is related to various effector systems such as adenyl cyclase and membrane ion channels via G-proteins. Pharmacological manipulation of adenosine receptors by agonists (CCPA, ADAC, IB-MECA) increases ischemic brain damage in various in vivo and in vitro models of cerebral ischemia whereas, agonist can also be neuroprotective. Mainly, receptor antagonists (CGS15943, MRS1706) indicated neuroprotection. Later, various studies also revealed that the downregulation or upregulation of specific adenosine receptors is necessary during the recovery of cerebral ischemia by activating several downstream signaling pathways. In the current review, we elaborate on the dual roles of adenosine and its receptor subtypes A1, A2, and A3 and their involvement in the pathobiology of cerebral ischemic injury. Adenosine-based therapies have the potential to improve the outcomes of cerebral injury patients, thereby providing them with a more optimistic future.

Effects of Ceftriaxone on Oxidative Stress and Inflammation in a Rat Model of Chronic Cerebral Hypoperfusion

Ceftriaxone (CTX) exerts a neuroprotective effect by decreasing glutamate excitotoxicity. We further studied the underlying mechanisms and effects of CTX early post-treatment on behavior in a cerebral hypoperfusion rats. The rats’ common carotid arteries (2VO) were permanently ligated. CTX was treated after ischemia. Biochemical studies were performed to assess antioxidative stress and inflammation. Behavioral and histological studies were then tested on the ninth week after vessel ligation. The 2VO rats showed learning and memory deficits as well as working memory impairments without any motor weakness. The treatment with CTX was found to attenuate white matter damage, MDA production, and interleukin 1 beta and tumor necrosis factor alpha production, mainly in the hippocampal area. Moreover, CTX treatment could increase the expression of glia and the glial glutamate transporters, and the neuronal glutamate transporter. Taken together, our data indicate the neuroprotective mechanisms of CTX involving the upregulation of glutamate transporters’ expression. This increased expression contributes to a reduction in glutamate excitotoxicity and oxidative stress as well as pro-inflammatory cytokine production, thus resulting in the protection of neurons and tissue from further damage. The present study highlights the mechanism of the effect of CTX treatment and of the underlying ischemia-induced neuronal damage.

Effects of Ischemia on the Migratory Capacity of Microglia Along Collagen Microcontact Prints on Organotypic Mouse Cortex Brain Slices

Ischemic stroke is a severe insult in the brain causing cell death, inflammation, and activation of microglia. Microglia are the immune cells of the brain and play a role in any inflammatory process during neurodegeneration. Microglia are round ameboid and migrate to the lesion site, where they differentiate into ramified forms and activated phagocytic microglia. On the other hand, microglia can also release growth factors to repair degeneration. The aim of the present study is to explore the migratory capacity of microglia after ischemic insults. Organotypic brain slices of the mouse cortex (300 μm) were prepared. In order to study migration, the slices were connected to collagen-loaded microcontact prints (with or without monocyte chemoattractant protein-1, MCP-1) on the membranes. Slices were stimulated with lipopolysaccharide (LPS) for maximal microglial activation. Ischemic insults were simulated with oxygen-glucose deprivation (OGD) and acidosis (pH 6.5) for 3 days. After 3 weeks in culture, slices were fixed and immunohistochemically stained for the microglial markers Iba1, CD11b and macrophage-like antigen. Our data show that Iba1+ microglia migrated along the microcontact prints, differentiate and phagocyte 1.0 μm fluorescent microbeads. LPS significantly enhanced the number of round ameboid migrating microglia, while OGD and acidosis enhanced the number of ramified activated microglia. The effect was not visible on slices without any μCP and was most potent in μCP with MCP-1. We conclude that OGD and acidosis activate ramification and exhibit a similar mechanism, while LPS only activates round ameboid microglia. Collagen-loaded microcontact prints connected to mouse brain slices are a potent method to study activation and migration of microglia ex vivo.

Non-mitogenic fibroblast growth factor 1 protects against ischemic stroke by regulating microglia/macrophage polarization through Nrf2 and NF-κB pathways

Microglia are immune cells in the central nervous system (CNS) that participate in response to pathological process after ischemic injury. Non-mitogenic fibroblast growth factor 1 (nmFGF1) is an effective neuroprotective factor that is also known as a metabolic regulator. The present study aimed to investigate the effects and mechanism of the neuroprotective ability of nmFGF1 on microglia in mice after photothrombosis (PT) stroke model, to determine whether it could ameliorate ischemic injury in stroke experiment. We discovered that the intranasal administration of nmFGF1 reduced infarct size and ameliorated neurological deficits in behavioral assessment by regulating the secretion of proinflammatory and anti-inflammatory cytokines. Furthermore, in the in vitro experiments, we found that nmFGF1 regulated the expression levels of proinflammatory and anti-inflammatory cytokines in oxygen-glucose deprivation (OGD) and lipopolysaccharide (LPS) stimulation. Evidence have shown that when nuclear factor erythroid 2-related factor 2 (Nfr2) is activated, it inhibits nuclear factor-kappa B (NF-κB) activation to alleviate inflammation. Interestingly, nmFGF1 treatment in vivo remarkably inhibited NF-κB pathway activation and activated Nrf2 pathway. In addition, nmFGF1 and NF-κB inhibitor (BAY11-7082) inhibited NF-κB pathway in LPS-stimulated BV2 microglia. Moreover, in LPS-stimulated BV2 microglia, the anti-inflammatory effect produced by nmFGF1 was knocked down by Nrf2 siRNA. These results indicate that nmFGF1 promoted functional recovery in experimental stroke by modulating microglia/macrophage-mediated neuroinflammation via Nrf2 and NF-κB signaling pathways, making nmFGF1 a potential agent against ischemic stroke.

Neuroinflammation in Cerebral Ischemia and Ischemia/Reperfusion Injuries: From Pathophysiology to Therapeutic Strategies

Its increasing incidence has led stroke to be the second leading cause of death worldwide. Despite significant advances in recanalization strategies, patients are still at risk for ischemia/reperfusion injuries in this pathophysiology, in which neuroinflammation is significantly involved. Research has shown that in the acute phase, neuroinflammatory cascades lead to apoptosis, disruption of the blood-brain barrier, cerebral edema, and hemorrhagic transformation, while in later stages, these pathways support tissue repair and functional recovery. The present review discusses the various cell types and the mechanisms through which neuroinflammation contributes to parenchymal injury and tissue repair, as well as therapeutic attempts made in vitro, in animal experiments, and in clinical trials which target neuroinflammation, highlighting future therapeutic perspectives.

Electroacupuncture promotes microglial M2 polarization in ischemic stroke via annexin A1

BACKGROUND

Neuroinflammation is the leading cause of neurological sequelae in ischemic stroke. Recently, we reported that the anti-inflammatory mediator annexin A1 (ANXA1) favored microglial M2 polarization in brain injury. The purpose of this study was to investigate electroacupuncture (EA) treatment and its potentially ANXA1-mediated anti-inflammatory effects in the middle cerebral artery occlusion/reperfusion (MCAO/R) mouse model of stroke.

METHODS

Treatment with EA consisted of dense-sparse frequencies (alternating 4 Hz sparse waves for 1.5 s and 16 Hz dense waves for 1.5 s) at CV24 and GV26. Intracerebroventricular (ICV) injection of Boc-2 (5 µM) or short hairpin RNA (sh)ANXA1 (2 µL) 3 days before EA was performed to block the effects of ANXA1.

RESULTS

EA pretreatment enhanced expression of ANXA1 and its receptor, formyl peptide receptor (FPR), when compared to MCAO/R alone. EA treatment also rescued MCAO/R-induced deficits in neurological performance, and learning and memory, and reduced infarct volume. Double immunofluorescent labeling showed that EA prevented MCAO/R-induced changes in microglial activation and morphology. EA also reduced the release of pro-inflammatory cytokines, such as interleukin (IL)-1β, inducible nitric oxide synthase (iNOS) and tumor necrosis factor (TNF)-α, while increasing the release of anti-inflammatory cytokines, such as arginase-1 (Arg-1) and brain-derived neurotrophic factor (BDNF). All EA-induced effects were either partially or completely prevented by prior administration of FPR antagonist Boc-2 or shANXA1.

CONCLUSION

The current study provides strong evidence that EA treatment has protective effects against ischemic stroke in the MCAO/R mouse model and that the mechanism likely involves the promotion of M2 polarization in microglia via ANXA1.

Artesunate attenuates inflammatory injury and inhibits the NF-κB pathway in a mouse model of cerebral ischemia

OBJECTIVE

Inflammation is an important factor in the pathological process of cerebral ischemia. Artesunate exhibits a broad range of anti-inflammatory properties in many diseases. We investigated the potential protective effect of artesunate against cerebral ischemia and the related mechanisms.

METHODS

Mice were divided into distal middle cerebral artery occlusion (dMCAO), sham, low dose, and high dose groups and subjected to dMCAO, except for the sham group. The low and high dose groups were administered artesunate (15 and 30 mg/kg), and the neuroprotective effects were analyzed by evaluating infarct volumes and neurological deficits. Microglial activation and neutrophil infiltration were evaluated by immunofluorescence, immunohistochemical staining, and western blotting. Inflammatory mediators were measured by enzyme-linked immunosorbent assays. Nuclear factor (NF)-κB nuclear translocation was detected by immunofluorescence and western blotting.

RESULTS

Compared with the dMCAO group, artesunate significantly improved neurological deficit scores and infarct volumes and ameliorated inflammation by reducing neutrophil infiltration, suppressing microglial activation, and downregulating tumor necrosis factor-α and interleukin-1β expression. Furthermore, artesunate inhibited nuclear translocation of NF-κB and inhibitor protein α proteolysis.

CONCLUSIONS

Artesunate protected against inflammatory injury by reducing neutrophil infiltration and microglial activation, suppressing inflammatory cytokines, and inhibiting the NF-κB pathway. Therefore, artesunate is a potential ischemic stroke treatment.

Microglia at the Centre of Brain Research: Accomplishments and Challenges for the Future

Microglia are the immune guardians of the central nervous system (CNS), with critical functions in development, maintenance of homeostatic tissue balance, injury and repair. For a long time considered a forgotten 'third element' with basic phagocytic functions, a recent surge in interest, accompanied by technological progress, has demonstrated that these distinct myeloid cells have a wide-ranging importance for brain function. This review reports microglial origins, development, and function in the healthy brain. Moreover, it also targets microglia dysfunction and how it contributes to the progression of several neurological disorders, focusing on particular molecular mechanisms and whether these may present themselves as opportunities for novel, microglia-targeted therapeutic approaches, an ever-enticing prospect. Finally, as it has been recently celebrated 100 years of microglia research, the review highlights key landmarks from the past century and looked into the future. Many challenging problems have arisen, thus it points out some of the most pressing questions and experimental challenges for the ensuing century.

Differences in TNF‑α and TNF‑R1 expression in damaged neurons and activated astrocytes of the hippocampal CA1 region between young and adult gerbils following transient forebrain ischemia

Tumor necrosis factor (TNF)-α and TNF receptor 1 (TNF-R1) play diverse roles in modulating the neuronal damage induced by cerebral ischemia. The present study compared the time-dependent changes of TNF-α and TNF-R1 protein expression levels in the hippocampal subfield cornu ammonis 1 (CA1) between adult and young gerbils following transient forebrain ischemia (tFI), via western blot and immunohistochemistry analyses. In adult gerbils, delayed neuronal death of pyramidal neurons, the principal neurons in CA1, was recorded 4 days after tFI; however, in young gerbils, delayed neuronal death was recorded 7 days after tFI. TNF-α protein expression levels gradually increased in both groups following tFI; however, TNF-α expression was higher in young gerbils compared with adult gerbils. TNF-R1 protein expression levels markedly increased in both groups 1 day after tFI. Subsequently, TNF-R1 expression gradually decreased in young gerbils, whereas TNF-R1 expression levels were irregularly altered in adult gerbils following tFI. Notably, TNF-α immunoreactivity significantly increased in pyramidal neurons in both groups 1 day after tFI; however, the patterns altered between both groups. In adult gerbils, TNF-α immunoreactivity was rarely exhibited in pyramidal neurons 4 days after tFI due to neuronal death, suggesting that TNF-α immunoreactivity was newly expressed in astrocytes. In young gerbils, TNF-α immunoreactivity increased in pyramidal neurons 4 days after tFI, and TNF-α immunoreactivity was newly expressed in astrocytes. In addition, TNF-R1 immunoreactivity was exhibited in pyramidal cells of both sham groups, and significantly increased 1 day after tFI; however, the patterns altered between both groups. In adult gerbils, TNF-R1 immunoreactivity was rarely exhibited 4 days after tFI, and astrocytes newly expressed TNF-R1 immunoreactivity. In young gerbils, TNF-R1 immunoreactivity increased in pyramidal neurons 4 days after tFI; however, TNF-R1 immunoreactivity was not reported in pyramidal neurons and astrocytes thereafter. Taken together, the results of the present study suggest that different expression levels of TNF-α and TNF-R1 in ischemic CA1 between adult and young gerbils may be due to age-dependent differences of tFI-induced neuronal death.

[Central Nervous System Developmental Regulation of Microglia via Cytokines and Chemokines]

Microglia are immune cells resident in the central nervous system (CNS). It has been gradually clarified that microglia play various roles at the developmental stage of the CNS. From embryonic to early postnatal age, microglia remove apoptotic cells by phagocytosis and refine the neural circuits by synaptic pruning. In addition, microglia promote the proliferation and differentiation of neural stem cells by releasing physiologically active substances. Our group has focused on the physiological actions of microglia via cytokines and chemokines at the early postnatal developmental stage. We found that a large number of activated microglia accumulate in the early postnatal subventricular zone (SVZ). We demonstrated that the these SVZ microglia facilitate neurogenesis and oligodendrogenesis via inflammatory cytokines including IL-1β, TNFα, IL-6, IFNγ. We have also found that microglia regulate the functional maturation of the blood brain barrier (BBB) and identified the cytokines and chemokines involved in the effects of microglia. These findings indicate that microglia are physiologically more important than ever thought to reveal robust brain functions. Furthermore, the new mode of microglial action may lead to the discovery of drug targets of the incurable CNS diseases.

Trelagliptin Mitigates Macrophage Infiltration by Preventing the Breakdown of the Blood-Brain Barrier in the Brain of Middle Cerebral Artery Occlusion Mice

Stroke is a significant cardiovascular disease that influences the health of human beings all over the world, especially the elderly population. It is reported that the blood-brain barrier (BBB) can be easily destroyed by stroke, which is one of the main factors responsible for macrophage infiltration and central nervous inflammation. Here, we report the protective effects of Trelagliptin against BBB injury and macrophage infiltration. Our results indicate that the infraction volume, the neurological score, and macrophage infiltration staining with CD68 were increased in middle cerebral artery occlusion (MCAO) mice but significantly reversed by treatment with Trelagliptin. Additionally, Trelagliptin reduced the permeability of the BBB by increasing the expression of the tight junction zonula occludens protein-1 (ZO-1) in the cerebral cortex. In an in vitro hypoxia model of endothelial cells, the increased migration of macrophages, enlarged permeability of endothelial monolayer, downregulation of ZO-1, and elevated expression level of CXCL1 by hypoxic conditions were all reversed by treatment with Trelagliptin in a dose-dependent manner. Our results demonstrate that Trelagliptin might mitigate macrophage infiltration by preventing the breakdown of the blood-brain barrier in the brains of MCAO mice.

Therapeutic Hypothermia Inhibits the Classical Complement Pathway in a Rat Model of Neonatal Hypoxic-Ischemic Encephalopathy

OBJECTIVE

Complement activation is instrumental in the pathogenesis of Hypoxic-ischemic encephalopathy (HIE), a significant cause of neonatal mortality and disability worldwide. Therapeutic hypothermia (HT), the only available treatment for HIE, only modestly improves outcomes. Complement modulation as a therapeutic adjunct to HT has been considered, but is challenging due to the wide-ranging role of the complement system in neuroinflammation, homeostasis and neurogenesis in the developing brain. We sought to identify potential therapeutic targets by measuring the impact of treatment with HT on complement effector expression in neurons and glia in neonatal HIE, with particular emphasis on the interactions between microglia and C1q.

METHODS

The Vannucci model was used to induce HIE in term-equivalent rat pups. At P10-12, pups were randomly assigned to three different treatment groups: Sham (control), normothermia (NT), and hypothermia (HT) treatment. Local and systemic complement expression and neuronal apoptosis were measured by ELISA, TUNEL and immunofluorescence labeling, and differences compared between groups.

RESULTS

Treatment with HT is associated with decreased systemic and microglial expression of C1q, decreased systemic C5a levels, and decreased microglial and neuronal deposition of C3 and C9. The effect of HT on cytokines was variable with decreased expression of pro and anti-inflammatory effectors. HT treatment was associated with decreased C1q binding on cells undergoing apoptosis.

CONCLUSION

Our data demonstrate the extreme complexity of the immune response in neonatal HIE. We propose modulation of downstream effectors C3a and C5a as a therapeutic adjunct to HT to enhance neuroprotection in the developing brain.

Inflammatory mechanism of cerebral ischemia-reperfusion injury with treatment of stepharine in rats

BACKGROUND

Increasing evidence has shown that microglia-induced neuroinflammation is involved in the pathogenesis of ischemic stroke. Stepharine, one of the alkaloids extracted from Stephania japonica (Thunb.) Miers, exhibited strong inhibitory effect on microglial overactivation. However, it is not known whether it has the potential to prevent ischemic stroke.

METHODS

The neuroprotective and anti-neuroinflammatory effects of stepharine were investigated in vivo and in vitro, using a rat model of middle cerebral artery occlusion (MCAO) and lipopolysaccharide (LPS)-stimulated BV-2 cells, respectively.

RESULTS

In vivo, stepharine (500 μg/kg) suppressed neurological deficits scores, brain water content and cerebral infarct volume induced by MCAO. Moreover, stepharine (500 μg/kg) inhibited NeuN⁺ cells loss and Iba-1⁺ cells increase in the MCAO ischemic cortex. In vitro, stepharine (10, 30 μM) substantially inhibited nitric oxide release as well as the mRNA and protein expression of pro-inflammatory mediators [inducible nitric oxide synthase, interleukin (IL)-6, tumor necrosis factor (TNF)-α, IL-1β] in LPS-activated BV-2 cells. LPS-induced increase of TLR4 expression, IκBα phosphorylation, and NF-κB p65 nuclear translocation was inhibited by stepharine (10, 30 μM). Molecular docking analysis showed that stepharine directly interacted with TLR4. SPR assay further confirmed that stepharine could bind to the TLR4/MD2 complex. Meanwhile, stepharine exhibited neuroprotective effects on SH-SY5Y cells cultured with LPS-treated conditioned medium.

CONCLUSION

Our study demonstrated for the first time that stepharine improved the outcomes in MCAO rats, reduced neuronal loss, and suppressed microglial overactivation via the inhibition of TLR4/NF-κB pathway. These results suggest that stepharine might be a potential therapeutic agent for the treatment of ischemic stroke.

Synergic Therapeutic Potential of PEA-Um Treatment and NAAA Enzyme Silencing In the Management of Neuroinflammation

Inflammation is a key element in the pathobiology of neurodegenerative diseases and sees the involvement of different neuronal and non-neuronal cells as players able to respond to inflammatory signals of immune origin. Palmitoylethanolamide (PEA) is an endogenous potent anti-inflammatory agent, in which activity is regulated by N-acylethanolamine acid amidase (NAAA), that hydrolyzes saturated or monounsaturated fatty acid ethanolamides, such as PEA. In this research, an in vitro study was performed on different neuronal (SH-SY5Y) and non-neuronal cell lines (C6, BV-2, and Mo3.13) subjected to NAAA enzyme silencing and treated with PEA ultra-micronized (PEA-um) (1, 3, and 10 μM) to increase the amount of endogenous PEA available for counteract neuroinflammation provoked by stimulation with lipopolysaccharide (LPS) (1 μg/mL) and interferon gamma (INF-γ )(100 U/mL). Cell viability was performed by MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-Diphenyltetrazolium Bromide) staining, suggesting a protective effect of PEA-um (3 and 10 μM) on all cell lines studied. Western Blot analysis for inflammatory markers (Inducible nitric oxide synthase (iNOS) and cyclooxygenase 2 (COX-2)) was carried out in control and NAAA-silenced cells, highlighting how the concomitant treatment of the neuronal and non-neuronal cells with PEA-um after NAAA genic downregulation is satisfactory to counteract neuroinflammation. These in vitro findings support the protective role of endogenous PEA availability in the neuronal field, bringing interesting information for a translational point of view.

6-Gingerol attenuates microglia-mediated neuroinflammation and ischemic brain injuries through Akt-mTOR-STAT3 signaling pathway

Neuroinflammation is critical for the pathogenesis of ischemia brain damage. Over-activated microglia-mediated inflammation plays a very important role in ischemia cerebral injuries. 6-Gingerol, obtained from edible ginger (Zingiber Officinale) exhibits protective effects against inflammation. In this study, we found that 6-Gingerol could reduce the size of infarction (P = 0.0184) and improve neurological functions (P = 0.04) at the third day after ischemic brain injury in vivo. Since 6-Gingerol has the anti-inflammatory effects, we further investigated its impacts on neuroinflammation mediated by microglia both in vivo and in vitro. We found that the levels of pro-inflammatory cytokines Interleukin-1 beta (IL-1β, P = 0.0213), Interleukin-6 (IL-6, P = 0.0316), and inducible NO synthase (iNOS, P = 0.0229) in the infarct penumbra were lower in 6-Gingerol treated groups. Furthermore, microglia induced pro-inflammatory cytokines, such as IL-6, IL-1β, incremental intercellular nitric oxide (NO), as well as iNOS were blocked by the treatment of 6-Gingerol in lipopolysaccharide (LPS) stimulated microglia. In terms of mechanism, 6-Gingerol potently suppressed phosphorylation of serine-threonine protein kinase (Akt) - mammalian target of rapamycin (mTOR) - signal transducer and activator of transcription 3 (STAT3) in LPS-treated microglia. Taken together, the present study suggested that 6-Gingerol improved cerebral ischemia injury by suppressing microglia-mediated neuroinflammation by down-regulating Akt-mTOR-STAT3 pathway.

Periodontal Disease and Periodontal Disease-Related Bacteria Involved in the Pathogenesis of Alzheimer's Disease

Alzheimer’s disease (AD) is the most common cause of dementia, and it exhibits pathological properties such as deposition of extracellular amyloid β (Aβ) and abnormally phosphorylated Tau in nerve cells and a decrease of synapses. Conventionally, drugs targeting Aβ and its related molecules have been developed on the basis of the amyloid cascade hypothesis, but sufficient effects on the disease have not been obtained in past clinical trials. On the other hand, it has been pointed out that chronic inflammation and microbial infection in the brain may be involved in the pathogenesis of AD. Recently, attention has been focused on the relationship between the periodontopathic bacterium Porphylomonas gingivalis and AD. P. gingivalis and its toxins have been detected in autopsy brain tissues from patients with AD. In addition, pathological conditions of AD are formed or exacerbated in mice infected with P. gingivalis. Compounds that target the toxins of P. gingivalis ameliorate the pathogenesis of AD triggered by P. gingivalis infection. These findings indicate that the pathological condition of AD may be regulated by controlling the bacteria in the oral cavity and the body. In the current aging society, the importance of oral and periodontal care for preventing the onset of AD will increase.

TNF-α Pretreatment Improves the Survival and Function of Transplanted Human Neural Progenitor Cells Following Hypoxic-Ischemic Brain Injury

Neural progenitor cells (NPCs) therapy offers great promise in hypoxic-ischemic (HI) brain injury. However, the poor survival of implanted NPCs in the HI host environment limits their therapeutic effects. Tumor necrosis factor-alpha (TNF-α) is a pleiotropic cytokine that is induced in response to a variety of pathological processes including inflammation and immunity. On the other hand, TNF-α has protective effects on cell apoptosis and death and affects the differentiation, proliferation, and survival of neural stem/progenitor cells in the brain. The present study investigated whether TNF-α pretreatment on human NPCs (hNPCs) enhances the effectiveness of cell transplantation therapy under ischemic brain. Fetal brain tissue-derived hNPCs were pretreated with TNF-α before being used in vitro experiments or transplantation. TNF-α significantly increased expression of cIAP2, and the use of short hairpin RNA-mediated knockdown of cIAP2 demonstrated that cIAP2 protected hNPCs against HI-induced cytotoxicity. In addition, pretreatment of hNPCs with TNF-α mediated neuroprotection by altering microglia polarization via increased expression of CX3CL1 and by enhancing expression of neurotrophic factors. Furthermore, transplantation of TNF-α-treated hNPCs reduced infarct volume and improved neurological functions in comparison with non-pretreated hNPCs or vehicle. These findings show that TNF-α pretreatment, which protects hNPCs from HI-injured brain-induced apoptosis and increases neuroprotection, is a simple and safe approach to improve the survival of transplanted hNPCs and the therapeutic efficacy of hNPCs in HI brain injury.

Different Approaches to Modulation of Microglia Phenotypes After Spinal Cord Injury

Microglial cells, which are highly plastic, immediately respond to any change in the microenvironment by becoming activated and shifting the phenotype toward neurotoxicity or neuroprotection. The polarization of microglia/macrophages after spinal cord injury (SCI) seems to be a dynamic process and can change depending on the microenvironment, stage, course, and severity of the posttraumatic process. Effective methods to modulate microglia toward a neuroprotective phenotype in order to stimulate neuroregeneration are actively sought for. In this context, available approaches that can selectively impact the polarization of microglia/macrophages regulate synthesis of trophic factors and cytokines/chemokines in them, and their phagocytic function and effects on the course and outcome of SCI are discussed in this review.

Neuroinflammation: friend and foe for ischemic stroke

Stroke, the third leading cause of death and disability worldwide, is undergoing a change in perspective with the emergence of new ideas on neurodegeneration. The concept that stroke is a disorder solely of blood vessels has been expanded to include the effects of a detrimental interaction between glia, neurons, vascular cells, and matrix components, which is collectively referred to as the neurovascular unit. Following the acute stroke, the majority of which are ischemic, there is secondary neuroinflammation that both promotes further injury, resulting in cell death, but conversely plays a beneficial role, by promoting recovery. The proinflammatory signals from immune mediators rapidly activate resident cells and influence infiltration of a wide range of inflammatory cells (neutrophils, monocytes/macrophages, different subtypes of T cells, and other inflammatory cells) into the ischemic region exacerbating brain damage. In this review, we discuss how neuroinflammation has both beneficial as well as detrimental roles and recent therapeutic strategies to combat pathological responses. Here, we also focus on time-dependent entry of immune cells to the ischemic area and the impact of other pathological mediators, including oxidative stress, excitotoxicity, matrix metalloproteinases (MMPs), high-mobility group box 1 (HMGB1), arachidonic acid metabolites, mitogen-activated protein kinase (MAPK), and post-translational modifications that could potentially perpetuate ischemic brain damage after the acute injury. Understanding the time-dependent role of inflammatory factors could help in developing new diagnostic, prognostic, and therapeutic neuroprotective strategies for post-stroke inflammation.

Characterization of Inflammation in Delayed Cortical Transplantation

We previously reported that embryonic motor cortical neurons transplanted 1-week after lesion in the adult mouse motor cortex significantly enhances graft vascularization, survival, and proliferation of grafted cells, the density of projections developed by grafted neurons and improves functional repair and recovery. The purpose of the present study is to understand the extent to which post-traumatic inflammation following cortical lesion could influence the survival of grafted neurons and the development of their projections to target brain regions and conversely how transplanted cells can modulate host inflammation. For this, embryonic motor cortical tissue was grafted either immediately or with a 1-week delay into the lesioned motor cortex of adult mice. Immunohistochemistry (IHC) analysis was performed to determine the density and cell morphology of resident and peripheral infiltrating immune cells. Then, in situ hybridization (ISH) was performed to analyze the distribution and temporal mRNA expression pattern of pro-inflammatory or anti-inflammatory cytokines following cortical lesion. In parallel, we analyzed the protein expression of both M1- and M2-associated markers to study the M1/M2 balance switch. We have shown that 1-week after the lesion, the number of astrocytes, microglia, oligodendrocytes, and CD45+ cells were significantly increased along with characteristics of M2 microglia phenotype. Interestingly, the majority of microglia co-expressed transforming growth factor-β1 (TGF-β1), an anti-inflammatory cytokine, supporting the hypothesis that microglial activation is also neuroprotective. Our results suggest that the modulation of post-traumatic inflammation 1-week after cortical lesion might be implicated in the improvement of graft vascularization, survival, and density of projections developed by grafted neurons.

Methylmercury intoxication and cortical ischemia: Pre-clinical study of their comorbidity

Stroke is one of the main causes of human disability worldwide. Ischemic stroke is mostly characterized by metabolic collapse and fast tissue damage, followed by secondary damage in adjacent regions not previously affected. Heavy metals intoxication can be associated with stroke incidence, because of their damaging action in the vascular system. Mercury, in particular, possesses a high tropism by metabolically active regions, such as the brain. In the present study we sought to evaluate whether methylmercury (MeHg) intoxication can aggravate the tissue damage caused by an ischemic stroke induced by microinjections of endothelin-1 (ET-1) into the motor cortex of adult rats. Following MeHg intoxication by gavage (0.04 mg/kg/day) during 60 days, the animals were injected with ET-1 (1 μl, 40 pmol/μl) or vehicle (1 μl). After 7 days, all animals were submitted to behavioral tests and then their brains were processed to biochemical and immunohistochemical analyses. We observed that long-term MeHg intoxication promoted a significant Hg deposits in the motor cortex, with concomitant increase of microglial response, followed by reduction of the neuronal population following ischemia and MeHg intoxication, as well as disturbance in the antioxidant defense mechanisms by misbalance of oxidative biochemistry with increase of both lipid peroxidation and nitrite levels, associated to behavioral deficits. MeHg exposure and cortical ischemia demonstrated that both injuries are able of causing significant neurobehavioural impairments in motor coordination and learning accompanied of an exacerbated microglial activation, oxidative stress and neuronal loss in the motor cortex, indicating that MeHg as a source of metabolic disturbance can act as an important increasing factor of ischemic events in the brain.

Microglial Cells Depletion Increases Inflammation and Modifies Microglial Phenotypes in an Animal Model of Severe Sepsis

Sepsis-associated encephalopathy is highly prevalent and has impact both in early and late morbidity and mortality. The mechanisms by which sepsis induces brain dysfunction include neuroinflammation, disrupted blood-brain barrier, oxidative stress, and microglial activation, but the cellular and molecular mechanisms involved in these events are not completely understood. Our objective was to determine the effects of microglial depletion in the early systemic and brain inflammatory response and its impact in phenotypes expression in an animal model of sepsis. Animals were subjected to CLP, and depletion of microglial cells was accomplished by administration of (Lipo)-encapsulated clodronate and microglial repopulation by doxycycline. Clod-lip treatment was effective in decreasing microglia density in the hippocampus of animals. Pro-inflammatory cytokines were increased in the CLP+PBS, and liposomes administration increased even further these cytokines mainly 7 days, suggesting that microglial depletion exacerbates both local and systemic inflammation. In contrast, repopulation with doxycycline was able to revert the cytokine levels in both serum and cerebral structures on day 7 and 14 after repopulation. There were no differences in the correlation between M1 and M2 markers by real-time PCR, but immunohistochemistry showed significant increase in CD11b expression in CLP+PBS with greater expression in CLP + liposomes in the hippocampus. These results suggest that the depletion of microglia during severe sepsis development could be associated with early exacerbation of brain and systemic inflammation and repopulation is able to revert this condition, once a rapid neurological recovery is noticed until 7 days after sepsis.

Lanthanum chloride induces neuron damage by activating the nuclear factor-kappa B signaling pathway in activated microglia

Lanthanum is a rare earth element which can have adverse effects on the central nervous system (CNS).

Soluble epoxide hydrolase inhibition enhances anti-inflammatory and antioxidative processes, modulates microglia polarization, and promotes recovery after ischemic stroke

BACKGROUND

Ischemic stroke triggers inflammatory responses and oxidative stress in the brain, and microglia polarization affects the degree of neuroinflammation. It has been reported that the inhibition of soluble epoxide hydrolase (sEH) activity protects brain tissue. However, the anti-inflammatory and antioxidative effects of sEH inhibition in the ischemic brain are not fully understood. This study aimed to investigate the effects of a selective sEH inhibitor, 12-(3-adamantan-1-yl-ureido)-dodecanoic acid (AUDA), after ischemic stroke.

METHODS

Adult male rats with middle cerebral artery occlusion (MCAO) were administered with AUDA or a vehicle. Behavioral outcome, infarct volume, microglia polarization, and gene expression were assessed.

RESULTS

Rats treated with AUDA showed better behavioral outcomes and smaller infarct volumes after MCAO. After AUDA treatment, a reduction of M1 microglia and an increase of M2 microglia occurred at the ischemic cortex of rats. Additionally, there was an increase in the mRNA expressions of antioxidant enzymes and anti-inflammatory interleukin-10, and pro-inflammatory mediators were decreased after AUDA administration. Heme oxygenase-1 was mainly expressed by neurons, and AUDA was found to improve the survival of neurons.

CONCLUSION

The results of this study provided novel and significant insights into how AUDA can improve outcomes and modulate inflammation and oxidative stress after ischemic stroke.

Mesenchymal stem cells and the neuronal microenvironment in the area of spinal cord injury

Cell-based technologies are used as a therapeutic strategy in spinal cord injury (SCI). Mesenchymal stem cells (MSCs), which secrete various neurotrophic factors and cytokines, have immunomodulatory, anti-apoptotic and anti-inflammatory effects, modulate reactivity/phenotype of astrocytes and the microglia, thereby promoting neuroregeneration seem to be the most promising. The therapeutic effect of MSCs is due to a paracrine mechanism of their action, therefore the survival of MSCs and their secretory phenotype is of particular importance. Nevertheless, these data are not always reported in efficacy studies of MSC therapy in SCI. Here, we provide a review with summaries of preclinical trials data evaluating the efficacy of MSCs in animal models of SCI. Based on the data collected, we have tried (1) to establish the behavior of MSCs after transplantation in SCI with an evaluation of cell survival, migration potential, distribution in the area of injured and intact tissue and possible differentiation; (2) to determine the effects MSCs on neuronal microenvironment and correlate them with the efficacy of functional recovery in SCI; (3) to ascertain the conditions under which MSCs demonstrate their best survival and greatest efficacy.

Targeting vascular inflammation in ischemic stroke: Recent developments on novel immunomodulatory approaches

Ischemic stroke is a devastating and debilitating medical condition with limited therapeutic options. However, accumulating evidence indicates a central role of inflammation in all aspects of stroke including its initiation, the progression of injury, and recovery or wound healing. A central target of inflammation is disruption of the blood brain barrier or neurovascular unit. Here we discuss recent developments in identifying potential molecular targets and immunomodulatory approaches to preserve or protect barrier function and limit infarct damage and functional impairment. These include blocking harmful inflammatory signaling in endothelial cells, microglia/macrophages, or Th17/γδ T cells with biologics, third generation epoxyeicosatrienoic acid (EET) analogs with extended half-life, and miRNA antagomirs. Complementary beneficial pathways may be enhanced by miRNA mimetics or hyperbaric oxygenation. These immunomodulatory approaches could be used to greatly expand the therapeutic window for thrombolytic treatment with tissue plasminogen activator (t-PA). Moreover, nanoparticle technology allows for the selective targeting of endothelial cells for delivery of DNA/RNA oligonucleotides and neuroprotective drugs. In addition, although likely detrimental to the progression of ischemic stroke by inducing inflammation, oxidative stress, and neuronal cell death, 20-HETE may also reduce susceptibility of onset of ischemic stroke by maintaining autoregulation of cerebral blood flow. Although the interaction between inflammation and stroke is multifaceted, a better understanding of the mechanisms behind the pro-inflammatory state at all stages will hopefully help in developing novel immunomodulatory approaches to improve mortality and functional outcome of those inflicted with ischemic stroke.

Lentivirus-mediated overexpression of OTULIN ameliorates microglia activation and neuroinflammation by depressing the activation of the NF-κB signaling pathway in cerebral ischemia/reperfusion rats

Background

Ischemic stroke-induced neuroinflammation is mainly mediated by microglial cells. The nuclear factor kappa B (NF-κB) pathway is the key transcriptional pathway that initiates inflammatory responses following cerebral ischemia. OTULIN, a critical negative regulator of the NF-κΒ signaling pathway, exerts robust effects on peripheral immune cell-mediated inflammation and is regarded as an essential mediator for repressing inflammation in vivo. The effect of OTULIN on inflammatory responses in the central nervous system (CNS) was previously unstudied. This current study investigated the anti-inflammatory effect of OTULIN both in vitro and in vivo in ischemic stroke models.

Methods

Sprague-Dawley (SD) rats were subjected to transient middle cerebral artery occlusion (tMCAO) or an intraperitoneal injection of lipopolysaccharide (LPS). Overexpression of the OTULIN gene was utilized to observe the effect of OTULIN on ischemic stroke outcomes. The effect of OTULIN overexpression on microglia-mediated neuroinflammation was examined in rat primary microglia (PM) and in the microglial cell line N9 after induction by oxygen-glucose deprivation (OGD)-treated neuronal medium. The activation and inflammatory responses of microglia were detected using immunofluorescence, ELISA, and qRT-PCR. The details of molecular mechanism were assessed using Western blotting.

Results

In the tMCAO rats, the focal cerebral ischemia/reperfusion injury induced a continuous increase in OTULIN expression within 72 h, and OTULIN expression was increased in activated microglial cells. OTULIN overexpression obviously decreased the cerebral infarct volume, improved the neurological function deficits, and reduced neuronal loss at 72 h after reperfusion, and it also inhibited the activation of microglia and attenuated the release of TNF-α, IL-1β, and IL-6 by suppressing the NF-κB pathway at 24 h after tMCAO. In vitro, OTULIN overexpression inhibited the microglia-mediated neuroinflammation by reducing the production of TNF-α, IL-1β, and IL-6 via depressing the NF-κB pathway in both PM and N9 cells.

Conclusions

OTULIN provides a potential therapeutic target for ischemic brain injury by ameliorating the excessive activation of microglial cells and neuroinflammation through repressing the NF-κB signaling pathway.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1117-5) contains supplementary material, which is available to authorized users.

Combined Treatment With Dichloroacetic Acid and Pyruvate Reduces Hippocampal Neuronal Death After Transient Cerebral Ischemia

Transient cerebral ischemia (TCI) occurs when blood flow to the brain is ceased or dramatically reduced. TCI causes energy depletion and oxidative stress, which leads to neuronal death and cognitive impairment. Dichloroacetic acid (DCA) acts as an inhibitor of pyruvate dehydrogenase kinase (PDK). Additionally, DCA is known to increase mitochondrial pyruvate uptake and promotes glucose oxidation during glycolysis, thus enhancing pyruvate dehydrogenase (PDH) activity. In this study, we investigated whether the inhibition of PDK activity by DCA, which increases the rate of pyruvate conversion to adenosine triphosphate (ATP), prevents ischemia-induced neuronal death. We used a rat model of TCI, which was induced by common carotid artery occlusion and hypovolemia for 7 min while monitoring the electroencephalography for sustained isoelectric potential. Male Sprague-Dawley rats were given an intraperitoneal injection of DCA (100 mg/kg) with pyruvate (50 mg/kg) once per day for 2 days after insult. The vehicle, DCA only or pyruvate on rats was injected on the same schedule. Our study demonstrated that the combined administration of DCA with pyruvate significantly decreased neuronal death, oxidative stress, microglia activation when compared with DCA, or pyruvate injection alone. These findings suggest that the administration of DCA with pyruvate may enhance essential metabolic processes, which in turn promotes the regenerative capacity of the post-ischemic brain.

Neuroprotective effects of pretreatment with minocycline on memory impairment following cerebral ischemia in rats

Cerebral ischemia leads to memory impairment that is associated with loss of hippocampal CA1 pyramidal neurons. Neuroinflammation and oxidative stress may be implicated in the pathogenesis of ischemia/reperfusion damage. Minocycline has anti-inflammatory and antioxidant properties. We investigated the neuroprotective effects of minocycline in rats subjected to cerebral ischemia/reperfusion injury. Thirty male rats were divided into three groups: control, sham, and minocycline-pretreated group. Minocycline (40 mg/kg) was injected intraperitoneally immediately before surgery, and then ischemia was induced by occlusion of common carotid arteries for 20 min. Seven days after reperfusion, the Morris water-maze task was used to evaluate memory. Nissl staining was also performed to analyze pyramidal cell damage. We measured the contents of malondialdehyde and proinflammatory cytokines in the hippocampus by the thiobarbituric acid method and enzyme-linked immunosorbent assay, respectively. Microglial activation was also investigated by Iba1 immunostaining. The results showed that pretreatment with minocycline prevented memory impairment induced by cerebral ischemia/reperfusion. Minocycline pretreatment also significantly attenuated ischemia-induced pyramidal cell death and microglial activation in the CA1 region and reduced the levels of malondialdehyde and proinflammatory cytokines (interleukin-1β and tumor necrosis factor-α) in the hippocampus of ischemic rats. Minocycline showed neuroprotective effects on cerebral ischemia-induced memory deficit probably through its anti-inflammatory and antioxidant activities.

Deletion of the P2X4 receptor is neuroprotective acutely, but induces a depressive phenotype during recovery from ischemic stroke

INTRODUCTION

Acute ischemic injury leads to severe neuronal loss. One of the key mechanisms responsible for this effect is inflammation, which is characterized by the activation of myeloid cells, including resident microglia and infiltrating monocytes/macrophages. P2X4 receptors (P2X4Rs) present on these immune cells modulate the inflammatory response. For example, excessive release of adenosine triphosphate during acute ischemic stroke triggers stimulation of P2X4Rs, leading to myeloid cell activation and proliferation and further exacerbating post-ischemic inflammation. In contrast, during recovery P2X4Rs activation on microglia leads to the release of brain-derived neurotrophic factor (BDNF), which alleviate depression, maintain synaptic plasticity and hasten post-stroke behavioral recovery. Therefore, we hypothesized that deletion of the P2X4R specifically from myeloid cells would have differential effects on acute versus chronic recovery following stroke.

METHODS

We subjected global or myeloid-specific (MS) P2X4R knock-out (KO) mice and wild-type littermates of both sexes to right middle cerebral artery occlusion (60min). We performed histological, behavioral (sensorimotor and depressive), and biochemical (quantitative PCR and flow cytometry) analyses to determine the acute (three days after occlusion) and chronic (30days after occlusion) effects of receptor deletion.

RESULTS

Global P2X4R deletion led to reduced infarct size in both sexes. In MS P2X4R KO mice, only females showed reduced infarct size, an effect that did not change with ovariectomy. MS P2X4R KO mice of both sexes showed swift recovery from sensorimotor deficits during acute recovery but exhibited a more pronounced post-stroke depressive behavior phenotype that was independent of infarct size. Quantitative PCR analysis of whole cell lysate as well as flow-sorted myeloid cells from the perilesional cortex showed increased cellular interleukin 1 beta (IL-1β), interleukin 6 (IL-6), and tumor necrosis factor alpha (TNF-α) mRNA levels but reduced plasma levels of these cytokines in MS P2X4R KO mice after stroke. The expression levels of BDNF and other depression-associated genes were reduced in MS P2X4R KO mice after stroke.

CONCLUSIONS

P2X4R deletion protects against stroke acutely but predisposes to depression-like behavior chronically after stroke. Thus, a time-sensitive approach should be considered when targeting P2X4Rs after stroke.

Anti-Inflammatory Targets for the Treatment of Reperfusion Injury in Stroke

While the mainstay of acute stroke treatment includes revascularization via recombinant tissue plasminogen activator or mechanical thrombectomy, only a minority of stroke patients are eligible for treatment, as delayed treatment can lead to worsened outcome. This worsened outcome at the experimental level has been attributed to an entity known as reperfusion injury (R/I). R/I is occurred when revascularization is delayed after critical brain and vascular injury has occurred, so that when oxygenated blood is restored, ischemic damage is increased, rather than decreased. R/I can increase lesion size and also worsen blood barrier breakdown and lead to brain edema and hemorrhage. A major mechanism underlying R/I is that of poststroke inflammation. The poststroke immune response consists of the aberrant activation of glial cell, infiltration of peripheral leukocytes, and the release of damage-associated molecular pattern (DAMP) molecules elaborated by ischemic cells of the brain. Inflammatory mediators involved in this response include cytokines, chemokines, adhesion molecules, and several immune molecule effectors such as matrix metalloproteinases-9, inducible nitric oxide synthase, nitric oxide, and reactive oxygen species. Several experimental studies over the years have characterized these molecules and have shown that their inhibition improves neurological outcome. Yet, numerous clinical studies failed to demonstrate any positive outcomes in stroke patients. However, many of these clinical trials were carried out before the routine use of revascularization therapies. In this review, we cover mechanisms of inflammation involved in R/I, therapeutic targets, and relevant experimental and clinical studies, which might stimulate renewed interest in designing clinical trials to specifically target R/I. We propose that by targeting anti-inflammatory targets in R/I as a combined therapy, it may be possible to further improve outcomes from pharmacological thrombolysis or mechanical thrombectomy.

Blocking the CD38/cADPR pathway plays a double-edged role in LPS stimulated microglia

Whether the CD38/cyclic ADP-ribose (cADPR) pathway plays a protective or detrimental role in neuroinflammation remains controversial. This study aimed to determine the role of CD38 in neuroinflammation using lipopolysaccharide (LPS)-stimulated BV2 microglial cells and co-cultured Neuro-2a (N2a) cells. In monoculture experiments, BV2 cells were divided into control, CD38 interference (CD38Ri), negative control (NC), LPS, CD38Ri+LPS, NC+LPS and 8-Br-cADPR+LPS groups. In co-culture experiments, N2a cells were co-cultured with BV2 cells for 48h. Nicotinamide adenine dinucleotide (NAD⁺), cADPR and intracellular Ca²⁺ levels and CD38 expression increased significantly in LPS-stimulated BV2 cells. CD38 knockdown or 8-Br-cADPR treatment significantly reduced NAD⁺, cADPR and intracellular Ca²⁺ levels. CD38 knockdown increased iNOS and NO levels in BV2 cells without LPS treatment; however, CD38 knockdown or 8-Br-cADPR treatment reduced iNOS and NO levels in BV2 cells with LPS treatment. CD38 knockdown increased the ratio of TUNEL-positive cells and cleaved Caspase 3/Caspase 3 ratio, and decreased the Bcl-2/Bax ratio in BV2 cells without LPS treatment; however, CD38 knockdown reduced the TUNEL positivity in BV2 cells with LPS treatment. CD38 knockdown or 8-Br-cADPR inhibited TNF-α, IL-6 (interleukin-6) and IL-1β levels in LPS-stimulated BV2 cells. Co-culture with CD38 knockdown or 8-Br-cADPR-treated BV2 cells did not influence apoptosis or iNOS expression in N2a cells. In conclusion, our results indicate that blocking the CD38/cADPR pathway reduces intracellular Ca²⁺, NO and the secretion of proinflammatory cytokines. CD38 knockdown exerted a detrimental effect in apoptosis and NO production in normal microglia, but played a protective role in apoptosis and NO production in LPS-stimulated microglia.

The Sigma-1 Receptor Antagonist, S1RA, Reduces Stroke Damage, Ameliorates Post-Stroke Neurological Deficits and Suppresses the Overexpression of MMP-9

The glutamate N-methyl-D-aspartate receptor (NMDAR) plays an essential role in the excitotoxic neural damage that follows ischaemic stroke. Because the sigma-1 receptor (σ1R) can regulate NMDAR transmission, exogenous and putative endogenous regulators of σ1R have been investigated using animal models of ischaemic stroke. As both agonists and antagonists provide some neural protection, the selective involvement of σ1Rs in these effects has been questioned. The availability of S1RA (E-52862/MR309), a highly selective σ1R antagonist, prompted us to explore its therapeutic potential in an animal model of focal cerebral ischaemia. Mice were subjected to right middle cerebral artery occlusion (MCAO), and post-ischaemic infarct volume and neurological deficits were determined across a range of intervals after the stroke-inducing surgery. Intracerebroventricular or intravenous treatment with S1RA significantly reduced the cerebral infarct size and neurological deficits caused by permanent MCAO (pMCAO). Compared with the control/sham-operated mice, the neuroprotective effects of S1RA were observed when delivered up to 5 h prior to surgery and 3 h after ischaemic onset. Interestingly, neither mice with the genetic deletion of σ1R nor wild-type mice that were pre-treated with the σ1R agonist PRE084 showed beneficial effects after S1RA administration with regard to stroke infarction. S1RA-treated mice showed faster behavioural recovery from stroke; this finding complements the significant decreases in matrix metalloproteinase-9 (MMP-9) expression and reactive astrogliosis surrounding the infarcted cortex. Our data indicate that S1RA, via σ1R, holds promising potential for clinical application as a therapeutic agent for ischaemic stroke.

Neuroprotective effect of minocycline on cognitive impairments induced by transient cerebral ischemia/reperfusion through its anti-inflammatory and anti-oxidant properties in male rat

Memory deficit is the most visible symptom of cerebral ischemia that is associated with loss of pyramidal cells in CA1 region of the hippocampus. Oxidative stress and inflammation may be involved in the pathogenesis of ischemia/reperfusion (I/R) damage. Minocycline, a semi-synthetic tetracycline derived antibiotic, has anti-inflammatory and antioxidant properties. We evaluated the neuroprotective effect of minocycline on memory deficit induced by cerebral I/R in rat. I/R was induced by occlusion of common carotid arteries for 20min. Minocycline (40mg/kg, i.p.) was administered once daily for 7days after I/R. Learning and memory were assessed using the Morris water maze test. Nissl staining was used to evaluate the viability of CA1 pyramidal cells. The effects of minocycline on the microglial activation was also investigated by Iba1 (Ionized calcium binding adapter molecule 1) immunostaining. The content of malondialdehyde (MDA) and pro-inflammatory cytokines (IL-1β and TNF-α) in the hippocampus were measured by thiobarbituric acid reaction substances method and ELISA, respectively. Minocycline reduced the increase in escape latency time and in swimming path length induced by cerebral I/R. Furthermore, the ischemia-induced reduction in time spent in the target quadrant during the probe trial was increased by treatment with minocycline. Histopathological results indicated that minocycline prevented pyramidal cells death and microglial activation induced by I/R. Minocycline also reduced the levels of MDA and pro-inflammatory cytokines in the hippocampus in rats subjected to I/R. Minocycline has neuroprotective effects on memory deficit induced by cerebral I/R in rat, probably via its anti-inflammatory and antioxidant properties.

Effects of long‑term post‑ischemic treadmill exercise on gliosis in the aged gerbil hippocampus induced by transient cerebral ischemia

Therapeutic exercise is an integral component of the rehabilitation of patients who have suffered a stroke. The objective of the present study was to use immunohistochemistry to investigate the effects of post-ischemic exercise on neuronal damage or death and gliosis in the aged gerbil hippocampus following transient cerebral ischemia. Aged gerbils (male; age, 22–24 months) underwent ischemia and were subjected to treadmill exercise for 1 or 4 weeks. Neuronal death was detected in the stratum pyramidale of the hippocampal CA1 region and in the polymorphic layer of the dentate gyrus using cresyl violet and Fluoro-Jade B histofluorescence staining. No significant difference in neuronal death was identified following 1 or 4 weeks of post-ischemic treadmill exercise. However, post-ischemic treadmill exercise affected gliosis (the activation of astrocytes and microglia). Glial fibrillary acidic protein-immunoreactive astrocytes and ionized calcium binding adaptor molecule 1-immunoreactive microglia were activated in the CA1 and polymorphic layer of the dentate gyrus of the group without treadmill exercise. Conversely, 4 weeks of treadmill exercise significantly alleviated ischemia-induced astrocyte and microglial activation; however, 1 week of treadmill exercise did not alleviate gliosis. These findings suggest that long-term post-ischemic treadmill exercise following transient cerebral ischemia does not influence neuronal protection; however, it may effectively alleviate transient cerebral ischemia-induced astrocyte and microglial activation in the aged hippocampus.

Complex Roles of Microglial Cells in Ischemic Stroke Pathobiology: New Insights and Future Directions

Ischemic stroke constitutes the major cause of death and disability in the industrialized world. The interest in microglia arose from the evidence outlining the role of neuroinflammation in ischemic stroke pathobiology. Microglia constitute the powerhouse of innate immunity in the brain. Microglial cells are highly ramified, and use these ramifications as sentinels to detect changes in brain homeostasis. Once a danger signal is recognized, cells become activated and mount specialized responses that range from eliminating cell debris to secreting inflammatory signals and trophic factors. Originally, it was suggested that microglia play essentially a detrimental role in ischemic stroke. However, recent reports are providing evidence that the role of these cells is more complex than what was originally thought. Although these cells play detrimental role in the acute phase, they are required for tissue regeneration in the post-acute phases. This complex role of microglia in ischemic stroke pathobiology constitutes a major challenge for the development of efficient immunomodulatory therapies. This review aims at providing an overview regarding the role of resident microglia and peripherally recruited macrophages in ischemic pathobiology. Furthermore, the review will highlight future directions towards the development of novel fine-tuning immunomodulatory therapeutic interventions.

M2 Phenotype Microglia-derived Cytokine Stimulates Proliferation and Neuronal Differentiation of Endogenous Stem Cells in Ischemic Brain

Microglia play a key role in the immune response and inflammatory reaction that occurs in response to ischemic stroke. Activated microglia promote neuronal damage or protection in injured brain tissue. Extracellular signals polarize the microglia towards the M1/M2 phenotype. The M1/M2 phenotype microglia released pro- and anti-inflammatory cytokines which induce the activation of neural stem/progenitor cells (NSPCs). In this study, we investigated how the cytokines released by microglia affect the activation of NSPCs. First, we treated BV2 cells with a lipopolysaccharide (LPS; 20 ng/ml) for M1 phenotype microglia and interleukin-4 (IL-4; 20 ng/ml) for M2 phenotype microglia in BV2 cells. Mice were subjected to transient middle cerebral artery occlusion (tMCAO) for 1 h. In ex vivo, brain sections containing the subventricular zone (SVZ) were cultured in conditioned media of M1 and M2 phenotype-conditioned media for 3 d. We measured the expression of cytokines in the conditioned media by RT-PCR and ELISA. The M2 phenotype microglia-conditioned media led to the proliferation and neural differentiation of NSPCs in the ipsilateral SVZ after ischemic stroke. The RT-PCR and ELISA results showed that the expression of TGF-α mRNA was significantly higher in the M2 phenotype microglia-conditioned media. These data support that M2 phenotype microglia-derived TGF-α is one of the key factors to enhance proliferation and neural differntiation of NSPCs after ischemic stroke.