Understanding microglia-neuron cross talk: relevance of the microglia-neuron cocultures

Microglia in Ischemic Stroke: Pathogenesis Insights and Therapeutic Challenges

Ischemic stroke is the most common type of stroke, which is the main cause of death and disability on a global scale. As the primary immune cells in the brain that are crucial for preserving homeostasis of the central nervous system microenvironment, microglia have been found to exhibit dual or even multiple effects at different stages of ischemic stroke. The anti-inflammatory polarization of microglia and release of neurotrophic factors may provide benefits by promoting neurological recovery at the lesion in the early phase after ischemic stroke. However, the pro-inflammatory polarization of microglia and secretion of inflammatory factors in the later phase of injury may exacerbate the ischemic lesion, suggesting the therapeutic potential of modulating the balance of microglial polarization to predispose them to anti-inflammatory transformation in ischemic stroke. Microglia-mediated signaling crosstalk with other cells may also be key to improving functional outcomes following ischemic stroke. Thus, this review provides an overview of microglial functions and responses under physiological and ischemic stroke conditions, including microglial activation, polarization, and interactions with other cells. We focus on approaches that promote anti-inflammatory polarization of microglia, inhibit microglial activation, and enhance beneficial cell-to-cell interactions. These targets may hold promise for the creation of innovative therapeutic strategies.

TUNEL-n-DIFL Method for Detection and Estimation of Apoptosis Specifically in Neurons and Glial Cells in Mixed Culture and Animal Models of Central Nervous System Diseases and Injuries

Detection of merely apoptosis does not reveal the type of central nervous system (CNS) cells that are dying in the CNS diseases and injuries. In situ detection and estimation of amount of apoptosis specifically in neurons or glial cells (astrocytes, oligodendrocytes, and microglia) can unveil valuable information for designing therapeutics for protection of the CNS cells and functional recovery. A method was first developed and reported from our laboratory for in situ detection and estimation of amount of apoptosis precisely in neurons and glial cells using in vitro and in vivo models of CNS diseases and injuries. This is a combination of terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and double immunofluorescent labeling (DIFL) or simply TUNEL-n-DIFL method for in situ detection and estimation of amount of apoptosis in a specific CNS cell type. An anti-digoxigenin (DIG) IgG antibody conjugated with 7-amino-4-methylcoumarin-3-acetic acid (AMCA) for blue fluorescence, fluorescein isothiocyanate (FITC) for green fluorescence, or Texas Red (TR) for red fluorescence can be used for in situ detection of apoptotic cell DNA, which is earlier labeled with TUNEL using alkali-stable DIG-11-dUTP. A primary anti-NeuN (neurons), anti-GFAP (astrocytes), anti-MBP (oligodendrocytes), or anti-OX-42 (microglia) IgG antibody and a secondary IgG antibody conjugated with one of the above fluorophores (other than that of ani-DIG antibody) are used for in situ detection of apoptosis in a specific CNS cell type in the mixed culture and animal models of the CNS diseases and injuries.

A Neuron, Microglia, and Astrocyte Triple Co-culture Model to Study Alzheimer’s Disease

Glial cells are essential to understand Alzheimer’s disease (AD) progression, given their role in neuroinflammation and neurodegeneration. There is a need for reliable and easy to manipulate models that allow studying the mechanisms behind neuron and glia communication. Currently available models such as co-cultures require complex methodologies and/or might not be affordable for all laboratories. With this in mind, we aimed to establish a straightforward in vitro setting with neurons and glial cells to study AD. We generated and optimized a 2D triple co-culture model with murine astrocytes, neurons and microglia, based on sequential seeding of each cell type. Immunofluorescence, western blot and ELISA techniques were used to characterize the effects of oligomeric Aβ (oAβ) in this model. We found that, in the triple co-culture, microglia increased the expression of anti-inflammatory marker Arginase I, and reduced pro-inflammatory iNOS and IL-1β, compared with microglia alone. Astrocytes reduced expression of pro-inflammatory A1 markers AMIGO2 and C3, and displayed a ramified morphology resembling physiological conditions. Anti-inflammatory marker TGF-β1 was also increased in the triple co-culture. Lastly, neurons increased post-synaptic markers, and developed more and longer branches than in individual primary cultures. Addition of oAβ in the triple co-culture reduced synaptic markers and increased CD11b in microglia, which are hallmarks of AD. Consequently, we developed a straightforward and reproducible triple co-cultured model, where cells resemble physiological conditions better than in individual primary cultures: microglia are less inflammatory, astrocytes are less reactive and neurons display a more mature morphology. Moreover, we are able to recapitulate Aβ-induced synaptic loss and CD11b increase. This model emerges as a powerful tool to study neurodegeneration and neuroinflammation in the context of AD and other neurodegenerative diseases.

Salvianolic Acids for Injection alleviates cerebral ischemia/reperfusion injury by switching M1/M2 phenotypes and inhibiting NLRP3 inflammasome/pyroptosis axis in microglia in vivo and in vitro

ETHNOPHARMACOLOGICAL RELEVANCE

After cerebral ischemia/reperfusion injury, pro-inflammatory M1 and anti-inflammatory M2 phenotypes of microglia are involved in neuroinflammation, in which activation of NLRP3 inflammasome and subsequent pyroptosis play essential roles. Salvianolic Acids for Injection (SAFI) is Chinese medicine injection which composed of multiple phenolic acids extracted from Radix Salviae Miltiorrhizae, and has been reported to generate neuroprotective effects after cerebral ischemic insult in clinical and animal studies.

AIM OF THE STUDY

The present study was designed to investigate whether SAFI exerts neuroprotective effects by switching microglial phenotype and inhibiting NLRP3 inflammasome/pyroptosis axis in microglia.

MATERIALS AND METHODS

The middle cerebral artery occlusion/reperfusion (MCAO/R) model in rats and oxygen-glucose deprivation/reoxygenation (OGD/R) model in co-cultured primary neurons and primary microglia were utilized. The neuroprotective effect of SAFI was evaluated through measuring neurological deficit scores, neuropathological changes, inflammatory factors, cell phenotype markers, and related proteins of NLRP3 inflammasome/pyroptosis axis.

RESULTS

The results showed that SAFI treatment was able to: (1) produce a significant increase in neurological deficit scores and decrease in infarct volumes, and alleviate histological injury and neuronal apoptosis in cerebral cortex in MCAO/R model; (2) increase neuronal viability and reduce neuronal apoptosis in the OGD model; (3) reshape microglial polarization patterns from M1-like phenotype to M2-like phenotype; (4) inhibit the activation of the NLRP3 inflammasome and the expression of proteins related to NLRP3 inflammasome/pyroptosis axis in vivo and in vitro.

CONCLUSION

These findings indicate that SAFI exert neuroprotective effect, probably via reducing neuronal apoptosis, switching microglial phenotype from M1 towards M2, and inhibiting NLRP3 inflammasome/pyroptosis axis in microglia.

Manganese Exposure Aggravates β-Amyloid Pathology by Microglial Activation

Human epidemiological evidence and animal experimental data suggest that chronic manganese (Mn) exposure increases the risk of Alzheimer’s disease (AD) and amyloid plaques, a hallmark of AD brain pathology, but the underlying mechanisms were not fully understood. Using the transgenic APP/PS1/Tau triple transgenic AD (3×Tg-AD) mouse model and mouse-derived microglia and neuroblastoma cell lines, we found that chronic 5-month Mn treatment increased beta amyloid peptide (Aβ) expression and Aβ plaques in the cerebral cortex and hippocampus in these 3×Tg-AD mice. Furthermore, we found that the β- and γ-secretase cleavage activities were markedly increased, while α-secretase cleavage activity was reduced in the brain of Mn-treated AD mice; these effects increase Aβ production and thus are amyloidogenic. Equally important, Mn treatment alone did not alter β-secretase 1 (BACE1) gene expression or Aβ production in amyloidogenic mutant amyloid precursor protein (APP) gene hAPPsw-transfected N2a cells (APPsw-N2a), but in APPsw-N2a cells either co-cultured with microglia or cultured with microglia-conditioned media, Mn exposure increased BACE1 expression and amyloidogenesis. We further determined that Mn exposure promoted the activation of microglia both in 3×Tg-AD mouse brains and in cultured microglia cells, and increased the secretion of the inflammatory cytokines interleukin-1β (IL-1β) and tumor necrosis factor-α (TNF-α). Taken together, these results suggest that Mn may increase the release of IL-1β and TNF-α from microglia that in turn stimulates the expression of BACE1 gene and protein and consequently Aβ production; this novel molecular mechanism not only advances our understanding about the amyloidogenic effect of chronic Mn exposure reported for special human populations but also indicates Mn dyshomeostasis as a potential contributor to AD pathogenesis.

Repetitive Head Trauma Induces Chronic Traumatic Encephalopathy by Multiple Mechanisms

Exposure to repetitive neurotrauma increases lifetime risk for developing progressive cognitive deficits, neurobehavioral abnormalities, and chronic traumatic encephalopathy (CTE). CTE is a tau protein neurodegenerative disease first identified in boxers and recently described in athletes participating in other contact sports (notably American football, ice hockey, rugby, and wrestling) and in military veterans with blast exposure. Currently, CTE can only be diagnosed by neuropathological examination of the brain after death. The defining diagnostic lesion of CTE consists of patchy perivascular accumulations of hyperphosphorylated tau protein that localize in the sulcal depths of the cerebral cortex. Neuronal abnormalities, axonopathy, neurovascular dysfunction, and neuroinflammation are triggered by repetitive head impacts (RHIs) and likely act as catalysts for CTE pathogenesis and progression. However, the specific mechanisms that link RHI to CTE are unknown. This review will explore two important areas of CTE pathobiology. First, we will review what is known about the biomechanical properties of RHI that initiate CTE-related pathologies. Second, we will provide an overview of key features of CTE neuropathology and how these contribute to abnormal tau hyperphosphorylation, accumulation, and spread.

Kv1.3 channel blockade alleviates cerebral ischemia/reperfusion injury by reshaping M1/M2 phenotypes and compromising the activation of NLRP3 inflammasome in microglia

After cerebral ischemia/reperfusion injury, pro-inflammatory M1-like and anti-inflammatory M2-like phenotypes of microglia are involved in neuroinflammation, in which NLRP3 inflammasome plays an essential role. Kv1.3 channel has been recognized as neuro-immunomodulatory target, but it is not clear as to its role in the neuroinflammation after cerebral ischemic injury. The current study aimed to investigate the issue. Middle cerebral artery occlusion/reperfusion (MCAO/R) model in rats and oxygen-glucose deprivation/ reoxygenation (OGD/R) in primary microglia were utilized to mimic disease state of ischemic stroke. Treatment with PAP-1, a Kv1.3 channel blocker, produced a significant improvement in neurological deficit scores and a decrease in infarct volume in MCAO/R model. An increased number of M2-like phenotypic microglia and a reduced number of M1-like phenotypic microglia were observed by immunofluorescent staining in the in vivo model, which was further validated by flow cytometry in vitro. Western blot showed that PAP-1 treatment profoundly reduced cleavage of caspase-1 and IL-1β in vivo and in vitro. Furthermore, PAP-1 administration reduced the number of NLRP3⁺/Iba1⁺ cells and NLRP3 protein levels in vivo, while reduced mRNA and protein expression levels of NLRP3 in vitro. Reduced mRNA expression levels of IL-1β in vitro and protein level of IL-1β in vivo were also observed. Taken together, our findings suggested that Kv1.3 channel blockade effectively alleviated cerebral ischemic injury, possibly by reshaping microglial phenotypic response from M1 towards M2, compromising the activation of NLRP3 inflammasome in microglia, and inhibiting release of IL-1β.

Regional microglia are transcriptionally distinct but similarly exacerbate neurodegeneration in a culture model of Parkinson's disease

Background

Parkinson’s disease (PD) is characterized by selective degeneration of dopaminergic (DA) neurons of the substantia nigra pars compacta (SN) while neighboring ventral tegmental area (VTA) DA neurons are relatively spared. Mechanisms underlying the selective protection of the VTA and susceptibility of the SN are still mostly unknown. Here, we demonstrate the importance of balance between astrocytes and microglia in the susceptibility of SN DA neurons to the PD mimetic toxin 1-methyl-4-phenylpyridinium (MPP⁺).

Methods

Previously established methods were used to isolate astrocytes and microglia from the cortex (CTX), SN, and VTA, as well as embryonic midbrain DA neurons from the SN and VTA. The transcriptional profile of isolated microglia was examined for 21 canonical pro- and anti-inflammatory cytokines by qRT-PCR with and without MPP⁺ exposure. Homo- and heterotypic co-cultures of neurons and astrocytes were established, and the effect of altering the ratio of astrocytes and microglia in vitro on the susceptibility of midbrain DA neurons to the PD mimetic toxin MPP⁺ was investigated.

Results

We found that regionally isolated microglia (SN, VTA, CTX) exhibit basal differences in their cytokine profiles and that activation of these microglia with MPP⁺ results in differential cytokine upregulation. The addition of microglia to cultures of SN neurons and astrocytes was not sufficient to cause neurodegeneration; however, when challenged with MPP⁺, all regionally isolated microglia resulted in exacerbation of MPP⁺ toxicity which was alleviated by inhibition of microglial activation. Furthermore, we demonstrated that isolated VTA, but not SN, astrocytes were able to mediate protection of both SN and VTA DA neurons even in the presence of exacerbatory microglia; however, this protection could be reversed by increasing the numbers of microglia present.

Conclusion

These results suggest that the balance of astrocytes and microglia within the midbrain is a key factor underlying the selective vulnerability of SN DA neurons seen in PD pathogenesis and that VTA astrocytes mediate protection of DA neurons which can be countered by greater numbers of deleterious microglia.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1181-x) contains supplementary material, which is available to authorized users.

Regulatory role of NADPH oxidase 2 in the polarization dynamics and neurotoxicity of microglia/macrophages after traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of death and disability. Secondary injuries that develop after the initial trauma contribute to long-lasting neurophysiological deficits. Polarization of microglia/macrophages toward a pro-inflammatory (M1) phenotype may increase the progression of secondary injury following TBI; however, the regulatory and functional mechanisms underlying these changes remain poorly defined. In the present study, we showed elevated expression of NADPH oxidase 2 (NOX2) and activation of nuclear factor-kappa B (NF-κB) predominantly in microglia/macrophages at 4- and 7-days after controlled cortical impact in mice. Delayed inhibition of NOX2, beginning one day after TBI, reduced reactive oxygen species production of myeloid cells and protected neurons from oxidative damage. Moreover, delayed NOX inhibition or global genetic NOX2 knockout suppressed the M1 "pro-inflammatory" profile of microglia/macrophages and simultaneously increased the M2 "anti-inflammatory" profile in the injured brain. These changes were associated with marked down-regulation of the classical NF-κB pathway in microglia/macrophages and reduced production of pro-inflammatory cytokines, tumor necrosis factor-α and interleukin-1β, after TBI. Finally, we demonstrated that wild-type microglia/macrophages isolated from the ipsilateral cortex at 7 days post-TBI were neurotoxic to co-cultured primary neurons, whereas this neurotoxicity was largely attenuated in microglia/macrophages from NOX2-KO mice. Taken together, our study shows a direct link between NOX2 and the NF-κB pathway in microglia/macrophages after TBI, and it provides a novel mechanism by which NOX2 activation leads to the enhanced inflammatory response and neuronal damage after brain injury. Our data also supports the therapeutic potential of targeting NOX2, which may provide efficacy with an extended therapeutic window after TBI.

Phospholipase A2 of Peroxiredoxin 6 Plays a Critical Role in Cerebral Ischemia/Reperfusion Inflammatory Injury

Microglia-mediated inflammation is an important step in the progression of cerebral ischemia/reperfusion injury and the associated production of receptors of immunomoudulation, including Toll-like receptors (TLRs). Peroxiredoxin 6 (Prdx6) has been demonstrated as the endogenous antioxidant protein for its peroxidase properties. However, the role of the independent phospholipase A2 (iPLA2) activity of Prdx6 in stroke has not been well studied. In this study, we evaluated whether blocking the calcium-iPLA2 activity of Prdx6 using siRNA and inhibitors (1-hexadecyl-3-(trifluoroethgl)-sn-glycerol-2 phosphomethanol, MJ33) would have a critical effect on inflammatory brain damage. We conducted oxygen-glucose deprivation (OGD)/recovery (R) in vitro and middle cerebral artery occlusion (MCAO) in vivo in a microglia/neuron co-culture system and in rats. In vitro, we found that Prdx6-iPLA2 activity was associated with the secretion of neurotoxic inflammatory mediators interleukin1β (IL-1β), interleukin-17 (IL-17) and interleukin-23 (IL-23) and elevated expression of Toll-like receptor 2/4 (TLR2/4), leading to the formation of nuclear factor-kappa B (NF-κB), inducible nitric oxide synthase (iNOS) and cyclooxygenase-2 (COX-2) in microglial cells. In vivo, combined treatment with Prdx6-iPLA2 activity inhibitor MJ33 showed a greater diminution in neurologic deficits, cerebral infarction, brain water content and inflammatory molecules than Prdx6-siRNA treatment alone. Our findings provide new insight into Prdx6-iPLA2 function in the brain. Inhibition of Prdx6-iPLA2 activity by gene therapy and/or pharmacology may constitute a promising new therapeutic approach to the treatment of stroke.

The biphasic function of microglia in ischemic stroke

Microglia are brain resident macrophages originated from primitive progenitor cells in the yolk sac. Microglia can be activated within hours and recruited to the lesion site. Traditionally, microglia activation is considered to play a deleterious role in ischemic stroke, as inhibition of microglia activation attenuates ischemia induced brain injury. However, increasing evidence show that microglia activation is critical for attenuating neuronal apoptosis, enhancing neurogenesis, and promoting functional recovery after cerebral ischemia. Differential polarization of microglia could likely explain the biphasic role of microglia in ischemia. We comprehensively reviewed the mechanisms involved in regulating microglia activation and polarization. The latest discoveries of microRNAs in modulating microglia function are discussed. In addition, the interaction between microglia and other cells including neurons, astrocytes, oligodendrocytes, and stem cells were also reviewed. Future therapies targeting microglia may not exclusively aim at suppressing microglia activation, but also at modulating microglia polarization at different stages of ischemic stroke. More work is needed to elucidate the cellular and molecular mechanisms of microglia polarization under ischemic environment. The roles of microRNAs and transplanted stem cells in mediating microglia activation and polarization during brain ischemia also need to be further studied.