Liposomal clodronate selectively eliminates microglia from primary astrocyte cultures

Glycogenolysis-Induced Astrocytic Serping1 Expression Regulates Neuroinflammatory Effects on Hippocampal neuron

The bacterial pathogen, lipopolysaccharide (LPS), elicits microglial response and induces cytokine secretion that subsequently activates astrocytes. Recent findings have indicated that LPS-induced activation of postnatal glial cells has led to alterations in synapse formation in hippocampal and cortical neurons, thereby resulting in a prolonged increased risk for seizure or depression. Nevertheless, its mechanisms remain to be fully elucidated. Cellular metabolism has recently gained recognition as a critical regulatory mechanism for the activation of peripheral immune cells, as it supplies the requisite energy and metabolite for their activation. In the present study, we report that LPS did not change the expression of reported astrocyte-derived synaptogenic genes in the postnatal hippocampus; however, it induced upregulation of astrocytic complement component regulator Serping1 within the postnatal hippocampus. As a regulatory mechanism, activation of glycogen degradation (glycogenolysis) governs the expression of a subset of inflammatory-responsive genes including Serping1 through reactive oxygen species (ROS)-NF-κB axis. Our study further demonstrated that glycogenolysis is implicated in neurotoxic phenotypes of astrocytes, such as impaired neuronal synaptogenesis or cellular toxicity. These findings suggested that activation of glycogenolysis in postnatal astrocytes is an essential metabolic pathway for inducing responses in inflammatory astrocytes.

Microglia Depletion Attenuates the Pro-Resolving Activity of the Formyl Peptide Receptor 2 Agonist AMS21 Related to Inhibition of Inflammasome NLRP3 Signalling Pathway: A Study of Organotypic Hippocampal Cultures

Microglial cells have been demonstrated to be significant resident immune cells that maintain homeostasis under physiological conditions. However, prolonged or excessive microglial activation leads to disturbances in the resolution of inflammation (RoI). Formyl peptide receptor 2 (FPR2) is a crucial player in the RoI, interacting with various ligands to induce distinct conformational changes and, consequently, diverse biological effects. Due to the poor pharmacokinetic properties of endogenous FPR2 ligands, the aim of our study was to evaluate the pro-resolving effects of a new ureidopropanamide agonist, compound AMS21, in hippocampal organotypic cultures (OHCs) stimulated with lipopolysaccharide (LPS). Moreover, to assess whether AMS21 exerts its action via FPR2 specifically located on microglial cells, we conducted a set of experiments in OHCs depleted of microglial cells using clodronate. We demonstrated that the protective and anti-inflammatory activity of AMS21 manifested as decreased levels of lactate dehydrogenase (LDH), nitric oxide (NO), and proinflammatory cytokines IL-1β and IL-6 release evoked by LPS in OHCs. Moreover, in LPS-stimulated OHCs, AMS21 treatment downregulated NLRP3 inflammasome-related factors (CASP1, NLRP3, PYCARD) and this effect was mediated through FPR2 because it was blocked by the FPR2 antagonist WRW4 pre-treatment. Importantly this beneficial effect of AMS21 was only observed in the presence of microglial FPR2, and absent in OHCs depleted with microglial cells using clodronate. Our results strongly suggest that the compound AMS21 exerts, at nanomolar doses, protective and anti-inflammatory properties and an FPR2 receptor located specifically on microglial cells mediates the anti-inflammatory response of AMS21. Therefore, microglial FPR2 represents a promising target for the enhancement of RoI.

Procyanidin improves experimental colitis by regulating macrophage polarization

BACKGROUND

Inflammatory bowel disease (IBD) is a chronic disease with an unclear pathogenesis for which successful treatments are still lacking. It has been reported that procyanidin, a natural antioxidant, relieves colitis, but the specific mechanism is elusive.

PURPOSE

Our present study was designed to investigate the effects of procyanidin on colitis and the regulation of the M1 macrophage phenotype and related signaling pathways.

METHODS

In vivo, we used two classic colitis models to observe the effect of procyanidin on macrophage polarization. In vitro, we further validated the therapeutic effect of procyanidin in the RAW264.7 cell line and peritoneal macrophages.

RESULTS

The current findings provide new evidence that procyanidin ameliorated dextran sulfate sodium (DSS)-induced colitis by preventing the polarization of macrophages to the M1 type and downregulating the levels of proinflammatory factors in cells. We also showed that procyanidin prevented lipopolysaccharide (LPS)-induced elevation of inflammatory cytokines and the activation of proinflammatory macrophages, which was achieved by activating the STAT3 and NF-κB pathways.

CONCLUSIONS

This is the first study to demonstrate that procyanidin alleviates experimental colitis by inhibiting the polarization of proinflammatory macrophages. These data reveal new ideas for the pathogenesis and treatment of inflammatory diseases.

Prefrontal microglia deficiency during adolescence disrupts adult cognitive functions and synaptic structures: A follow-up study in female mice

The prefrontal cortex (PFC) provides executive top-down control of a variety of cognitive processes. A distinctive feature of the PFC is its protracted structural and functional maturation throughout adolescence to early adulthood, which is necessary for acquiring mature cognitive abilities. Using a mouse model of cell-specific, transient and local depletion of microglia, which is based on intracerebral injection of clodronate disodium salt (CDS) into the PFC of adolescent male mice, we recently demonstrated that microglia contribute to the functional and structural maturation of the PFC in males. Because microglia biology and cortical maturation are partly sexually dimorphic, the main objective of the present study was to examine whether microglia similarly regulate this maturational process in female mice as well. Here, we show that a single, bilateral intra-PFC injection of CDS in adolescent (6-week-old) female mice induces a local and transient depletion (70 to 80% decrease from controls) of prefrontal microglia during a restricted window of adolescence without affecting neuronal or astrocytic cell populations. This transient microglia deficiency was sufficient to disrupt PFC-associated cognitive functions and synaptic structures at adult age. Inducing transient prefrontal microglia depletion in adult female mice did not cause these deficits, demonstrating that the adult PFC, unlike the adolescent PFC, is resilient to transient microglia deficiency in terms of lasting cognitive and synaptic maladaptations. Together with our previous findings in males, the present findings suggest that microglia contribute to the maturation of the female PFC in a similar way as to the prefrontal maturation occurring in males.

Annexin A1 upregulates hematoma resolution via the FPR2/p-ERK(1/2)/DUSP1/CD36 signaling pathway after germinal matrix hemorrhage

Germinal matrix hemorrhage (GMH) is one of the leading causes of morbidity and mortality in preterm infants in the United States, with little progress made in its clinical management. Blood clots disrupting normal cerebrospinal fluid circulation and absorption after germinal matrix hemorrhage are key contributors towards post-hemorrhagic hydrocephalus development. n-formyl peptide receptor 2 (FPR2), a G-protein-coupled receptor, has been associated with the activation of p-ERK1/2, which in turn promotes the transcription of the DUSP1 gene, which may play a role in CD36 signaling. CD36 scavenger, a transmembrane glycoprotein, plays an essential role in microglia phagocytic blood clot clearance after GMH. FPR2's role in blood clot clearance after hemorrhagic stroke is unknown. We hypothesize that FPR2 activation by FPR2 agonist Annexin A1 (AnxA1) will enhance hematoma resolution via the upregulation of the CD36 signaling pathway, thereby improving short- and long-term neurological outcomes. Bacterial collagenase (0.3 U) was infused intraparenchymally into the right hemispheric ganglionic eminence in P7 rat pups to induce GMH. AnxA1 and FPR2 Inhibitor (Boc2) were given at 1-h post-GMH via intranasal administration. FPR2 CRISPR was given 48-h prior to GMH induction. Short-term neurological deficits were assessed using negative geotaxis test. Hematoma volume was assessed using hemoglobin assay. Protein expression was assessed using western blots. Long-term neurocognitive deficits and motor coordination were assessed using Morris water maze, rotarod, and foot fault tests. We have demonstrated that AnxA1 treatment enhances hematoma resolution and improved short and long-term outcomes. Lastly, FPR2 agonist AnxA1 treatment resulted in the upregulation of the FPR2/p-ERK(1/2)/DUSP1/CD36 signaling pathway.

Replacement of Mouse Microglia With Human Induced Pluripotent Stem Cell (hiPSC)-Derived Microglia in Mouse Organotypic Slice Cultures

Microglia, the major immune cells in the brain, are reported to differ in gene expression patterns among species. Therefore, it would be preferable in some cases to use human microglia rather than mouse microglia in microglia-targeted disease research. In the past half a decade, researchers have developed in vivo transplantation methods in which human induced pluripotent stem cell-derived microglia (hiPSC-MG) are transplanted into a living mouse brain. However, in vivo transplantation methods are not necessarily accessible to all researchers due to the difficulty of obtaining the materials needed and the transplantation technique itself. In addition, for in vivo systems for microglia-targeted drug screening, it is difficult to control the pharmacokinetics, especially blood-brain barrier permeability. Therefore, in addition to existing in vivo transplantation systems, the development of an ex vivo transplantation system would help to further evaluate the properties of hiPSC-MG. In this study, we aimed to establish a method to efficiently transplant hiPSC-MG into cultured mouse hippocampal slices. We found that approximately 80% of the total microglia in a cultured slice were replaced by hiPSC-derived microglia when innate microglia were pharmacologically removed prior to transplantation. Furthermore, when neuronal death was induced by applying Kainic acid (KA) to slice cultures, transplanted hiPSC-MG changed their morphology and phagocytosed cell debris. Thus, this study provides a method to transplant hiPSC-MG into the mouse hippocampal slice cultures with a high replacement rate. Because the transplanted microglia survived and exerted phagocytic functions, this method will be useful for evaluating the properties of hiPSC-MG ex vivo.

Microglia Depletion from Primary Glial Cultures Enables to Accurately Address the Immune Response of Astrocytes

Astrocytes are the most abundant cells in the CNS parenchyma and play an essential role in several brain functions, such as the fine-tuning of synaptic transmission, glutamate uptake and the modulation of immune responses, among others. Much of the knowledge on the biology of astrocytes has come from the study of rodent primary astrocytic cultures. Usually, the culture is a mixed population of astrocytes and a small proportion of microglia. However, it is critical to have a pure culture of astrocytes if one wants to address their inflammatory response. If present, microglia sense the stimulus, rapidly proliferate and react to it, making it unfeasible to assess the individual responsiveness of astrocytes. Microglia have been efficiently eliminated in vivo through PLX-3397, a colony-stimulating factor-1 receptor (CSF-1R) inhibitor. In this work, the effectiveness of PLX-3397 in eradicating microglia from primary mixed glial cultures was evaluated. We tested three concentrations of PLX-3397—0.2 μM, 1 μM and 5 μM—and addressed its impact on the culture yield and viability of astrocytes. PLX-3397 is highly efficient in eliminating microglia without affecting the viability or response of cultured astrocytes. Thus, these highly enriched monolayers of astrocytes allow for the more accurate study of their immune response in disease conditions.

A method for the selective depletion of microglia in the dorsal hippocampus in the juvenile rat brain

BACKGROUND

To understand the role of microglia in brain function and development, methods have emerged to deplete microglia throughout the brain. Liposome-encapsulated clodronate (LEC) can be infused into the brain to deplete microglia in a brain-region and time-specific manner.

NEW METHOD

This study validates methodology to deplete microglia in the rat dorsal hippocampus (dHP) during a specific period of juvenile development. Stereotaxic surgery was performed to infuse LEC at postnatal day (P) 16 or 19 into dHP. Rat brains were harvested at various ages to determine specificity of infusion and duration of depletion.

RESULTS

P19 infusion of LEC into dHP with a 27G syringe depleted microglia in dHP subregions CA1, dentate gyrus (DG), and CA3 from P24-P30. There was also evidence of depletion in parietal cortex above the infusion site. P16 infusion of LEC with a 32 G syringe depleted microglia only in dHP subregions CA1 and DG from P21-P40.

COMPARISON WITH EXISTING METHOD(S)

Previous methods have infused LEC intra-hippocampally in adult rats or intra-cerebroventricularly in neonatal rats. This study is the first to publish methodology to deplete microglia in a brain-region specific manner during juvenile rat development.

CONCLUSIONS

The timing of LEC infusion during the juvenile period can be adjusted to achieve maximal microglia depletion by a specific postnatal day. A 27G needle results in LEC backflow during the infusion, but also allows LEC to reach all subregions of dHP. Infusion with a 32 G needle prevents backflow during infusion, but results in a more local spread of LEC within dHP.

Teriflunomide shifts the astrocytic bioenergetic profile from oxidative metabolism to glycolysis and attenuates TNFα-induced inflammatory responses

Astrocytes utilize both glycolytic and mitochondrial pathways to power cellular processes that are vital to maintaining normal CNS functions. These cells also mount inflammatory and acute phase reactive programs in response to diverse stimuli. While the metabolic functions of astrocytes under homeostatic conditions are well-studied, the role of cellular bioenergetics in astrocyte reactivity is poorly understood. Teriflunomide exerts immunomodulatory effects in diseases such as multiple sclerosis by metabolically reprogramming lymphocytes and myeloid cells. We hypothesized that teriflunomide would constrain astrocytic inflammatory responses. Purified murine astrocytes were grown under serum-free conditions to prevent acquisition of a spontaneous reactive state. Stimulation with TNFα activated NFκB and increased secretion of Lcn2. TNFα stimulation increased basal respiration, maximal respiration, and ATP production in astrocytes, as assessed by oxygen consumption rate. TNFα also increased glycolytic reserve and glycolytic capacity of astrocytes but did not change the basal glycolytic rate, as assessed by measuring the extracellular acidification rate. TNFα specifically increased mitochondrial ATP production and secretion of Lcn2 required ATP generated by oxidative phosphorylation. Inhibition of dihydroorotate dehydrogenase via teriflunomide transiently increased both oxidative phosphorylation and glycolysis in quiescent astrocytes, but only the increased glycolytic ATP production was sustained over time, resulting in a bias away from mitochondrial ATP production even at doses down to 1 μM. Preconditioning with teriflunomide prevented the TNFα-induced skew toward oxidative phosphorylation, reduced mitochondrial ATP production, and reduced astrocytic inflammatory responses, suggesting that this drug may limit neuroinflammation by acting as a metabolomodulator.

The effect of macrophage polarization on the expression of the oxytocin signalling system in enteric neurons

BACKGROUND

The aim of the current study was to investigate the effect of macrophage polarization on the expression of oxytocin (OT) and the oxytocin receptor (OTR) in enteric neurons.

METHODS

In this study, we used a classic colitis model and D-mannose model to observe the correlation between macrophage polarization and OT signalling system. In order to further demonstrate the effect of macrophages, we examined the expression of OT signalling system after depletion of macrophages.

RESULTS

The data showed that, in vitro, following polarization of macrophages to the M1 type by LPS, the macrophage supernatant contained proinflammatory cytokines (IL-1β, IL-6 and TNF-α) that inhibited the expression of OT and OTR in cultured enteric neurons; following macrophage polarization to the M2 type by IL4, the macrophage supernatant contained anti-inflammatory cytokines (TGF-β) that promoted the expression of OT and OTR in cultured enteric neurons. Furthermore, M1 macrophages decreased the expression of the OT signalling system mainly through STAT3/NF-κB pathways in cultured enteric neurons; M2 macrophages increased the expression of the OT signalling system mainly through activation of Smad2/3 and inhibition of the expression of Peg3 in cultured enteric neurons. In a colitis model, we demonstrated that macrophages were polarized to the M1 type during the inflammatory phase, with significant decreased in the expression of OT and OTR. When macrophages were polarized to the M2 type during the recovery phase, OT and OTR expression increased significantly. In addition, we found that D-mannose increased the expression of OT and OTR through polarization of macrophages to the M2 type.

CONCLUSIONS

This is the first study to demonstrate that macrophage polarization differentially regulates the expression of OT and OTR in enteric neurons.

Targeting Microglial Disturbances to Protect the Brain From Neurodevelopmental Disorders Associated With Prematurity

Microglial activation during critical phases of brain development can result in short- and long-term consequences for neurological and psychiatric health. Several studies in humans and rodents have shown that microglial activation, leading to a transition from the homeostatic state toward a proinflammatory phenotype, has adverse effects on the developing brain and neurodevelopmental disorders. Targeting proinflammatory microglia may be an effective strategy for protecting the brain and attenuating neurodevelopmental disorders induced by inflammation. In this review we focus on the role of inflammation and the activation of immature microglia (pre-microglia) soon after birth in prematurity-associated neurodevelopmental disorders, and the specific features of pre-microglia during development. We also highlight the relevance of immunomodulatory strategies for regulating activated microglia in a rodent model of perinatal brain injury. An original neuroprotective approach involving a nanoparticle-based therapy and targeting microglia, with the aim of improving myelination and protecting the developing brain, is also addressed.

Microglial inflammation after chronic spinal cord injury is enhanced by reactive astrocytes via the fibronectin/β1 integrin pathway

BACKGROUND

After spinal cord injury (SCI), glial scarring is mainly formed around the lesion and inhibits axon regeneration. Recently, we reported that anti-β1 integrin antibody (β1Ab) had a therapeutic effect on astrocytes by preventing the induction of glial scar formation. However, the cellular components within the glial scar are not only astrocytes but also microglia, and whether or not β1Ab treatment has any influence on microglia within the glial scar remains unclear.

METHODS

To evaluate the effects of β1Ab treatment on microglia within the glial scar after SCI, we applied thoracic contusion SCI to C57BL/6N mice, administered β1Ab in the sub-acute phase, and analyzed the injured spinal cords with immunohistochemistry in the chronic phase. To examine the gene expression in microglia and glial scars, we selectively collected microglia with fluorescence-activated cell sorting and isolated the glial scars using laser-captured microdissection (LMD). To examine the interaction between microglia and astrocytes within the glial scar, we stimulated BV-2 microglia with conditioned medium of reactive astrocytes (RACM) in vitro, and the gene expression of TNFα (pro-inflammatory M1 marker) was analyzed via quantitative polymerase chain reaction. We also isolated both naïve astrocytes (NAs) and reactive astrocytes (RAs) with LMD and examined their expression of the ligands for β1 integrin receptors. Statistical analyses were performed using Wilcoxon's rank-sum test.

RESULTS

After performing β1Ab treatment, the microglia were scattered within the glial scar and the expression of TNFα in both the microglia and the glial scar were significantly suppressed after SCI. This in vivo alteration was attributed to fibronectin, a ligand of β1 integrin receptors. Furthermore, the microglial expression of TNFα was shown to be regulated by RACM as well as fibronectin in vitro. We also confirmed that fibronectin was secreted by RAs both in vitro and in vivo. These results highlighted the interaction mediated by fibronectin between RAs and microglia within the glial scar.

CONCLUSION

Microglial inflammation was enhanced by RAs via the fibronectin/β1 integrin pathway within the glial scar after SCI. Our results suggested that β1Ab administration had therapeutic potential for ameliorating both glial scar formation and persistent neuroinflammation in the chronic phase after SCI.

Don't know what you got till it's gone: microglial depletion and neurodegeneration

In the central nervous system, immunologic surveillance and response are carried out, in large part, by microglia. These resident macrophages derive from myeloid precursors in the embryonic yolk sac, migrating to the brain and eventually populating local tissue prior to blood-brain barrier formation. Preserved for the duration of lifespan, microglia serve the host as more than just a central arm of innate immunity, also contributing significantly to the development and maintenance of neurons and neural networks, as well as neuroregeneration. The critical nature of these varied functions makes the characterization of key roles played by microglia in neurodegenerative disorders, especially Alzheimer's disease, of paramount importance. While genetic models and rudimentary pharmacologic approaches for microglial manipulation have greatly improved our understanding of central nervous system health and disease, significant advances in the selective and near complete in vitro and in vivo depletion of microglia for neuroscience application continue to push the boundaries of research. Here we discuss the research efficacy and utility of various microglial depletion strategies, including the highly effective CSF1R inhibitor models, noteworthy insights into the relationship between microglia and neurodegeneration, and the potential for therapeutic repurposing of microglial depletion and repopulation.

Amyloid-β impairs the phagocytosis of dystrophic synapses by astrocytes in Alzheimer's disease

Reactive astrocytes and dystrophic neurites, most aberrant presynaptic elements, are found surrounding amyloid-β plaques in Alzheimer's disease (AD). We have previously shown that reactive astrocytes enwrap, phagocytose, and degrade dystrophic synapses in the hippocampus of APP mice and AD patients, but affecting less than 7% of dystrophic neurites, suggesting reduced phagocytic capacity of astrocytes in AD. Here, we aimed to gain insight into the underlying mechanisms by analyzing the capacity of primary astrocyte cultures to phagocytose and degrade isolated synapses (synaptoneurosomes, SNs) from APP (containing dystrophic synapses and amyloid-β peptides), Tau (containing AT8- and AT100-positive phosphorylated Tau) and WT (controls) mice. We found highly reduced phagocytic and degradative capacity of SNs-APP, but not AT8/AT100-positive SNs-Tau, as compared with SNs-WT. The reduced astrocyte phagocytic capacity was verified in hippocampus from 12-month-old APP mice, since only 1.60 ± 3.81% of peri-plaque astrocytes presented phagocytic structures. This low phagocytic capacity did not depend on microglia-mediated astrocyte reactivity, because removal of microglia from the primary astrocyte cultures abrogated the expression of microglia-dependent genes in astrocytes, but did not affect the phagocytic impairment induced by oligomeric amyloid-β alone. Taken together, our data suggest that amyloid-β, but not hyperphosphorylated Tau, directly impairs the capacity of astrocytes to clear the pathological accumulation of oligomeric amyloid-β, as well as of peri-plaque dystrophic synapses containing amyloid-β, perhaps by reducing the expression of phagocytosis receptors such as Mertk and Megf10, thus increasing neuronal damage in AD. Therefore, the potentiation or recovery of astrocytic phagocytosis may be a novel therapeutic avenue in AD.

CCR2 downregulation attenuates spinal cord injury by suppressing inflammatory monocytes

Specific elimination of blood-derived macrophages/monocytes following spinal cord injury (SCI) may suppress neurotoxicity without affecting the neuroprotective microglia at the injury sites. We aimed to deplete hematogenous monocytes by downregulating CCR2 through siCCR2-loaded nanoparticles and investigated its outcome in the recovery of locomotor function of SCI mice. We induced SCI in mice and examined the influx of blood-derived monocytes into the injury site. We constructed nanoparticles loaded with siRNA targeting CCR2 and examined its efficiency in downregulating the CCR2 expression in cultured RAW264.7 cells and monocytes in vivo. Finally, we assessed the effects of CCR2 downregulation in pro-inflammatory cytokine production, axon regeneration, and locomotor recovery of the SCI mice. We found that SCI significantly increased the CCL2 expression and number of blood-derived macrophages/monocytes in the lesion area. Nanoparticles loaded with siCCR2 significantly suppressed the CCR2 expression in hematogenous macrophages/monocytes, reduced the number of hematogenous macrophages/monocytes, and reduced pro-inflammatory cytokine production at the injury site. Finally, CCR2 downregulation promoted axon regeneration and improved locomotor recovery in SCI mice. Our study suggests that siCCR2 loading nanoparticles are efficient and specific in downregulating hematogenous macrophages/monocytes without affecting the neuroprotective microglia and its efficacy in promoting locomotor recovery in SCI mice warrants further investigation for its clinical application in SCI.

Liposomes embedded within fibrin gels facilitate localized macrophage manipulations within nerve

BACKGROUND

Understanding the role of macrophages at discrete spatial locations during nerve regeneration after injury is important. But, methodologies that systemically manipulate macrophages can obscure their roles within discrete spatial locations within nerve.

NEW METHOD

Liposomes were embedded within fibrin gels to construct a delivery system that facilitated macrophage-specific manipulations at a sole spatial region, as macrophages accumulated within the fibrin. Clodronate liposomes were characterized for their toxicity to specific cells composing nerve in vitro, then tested for macrophage-specific depletion in vivo. This delivery system using clodronate liposomes was used to repair a mouse sciatic nerve gap to evaluate its efficacy and effects.

RESULT

Clodronate liposomes showed specific toxicity to macrophages without affecting dorsal root ganglia (DRG)-derived neurons, endothelial cells, or Schwann cells in culture. The delivery system demonstrated sustained release of liposomes for more than 7 days while still retaining liposomes within the fibrin. In vivo, the delivery system demonstrated macrophages were targeted by liposomes, and the use of clodronate liposomes minimized macrophage accumulation within fibrin, while not affecting macrophage accumulation within DRG. Nerve regeneration across the nerve gap repaired using this delivery system was associated with decreased angiogenesis, Schwann cell accumulation, axon growth, and reinnervation of affected muscle.

COMPARISON WITH EXISTING METHODS

This delivery system allowed specific perturbation of macrophages locally in nerve. This method could be applicable across species without the need for genetic manipulations or systemic pharmaceuticals.

CONCLUSION

Liposomes embedded within fibrin gels locally target macrophages at the site of nerve injury, which enables greater precision in conclusions regarding their roles in nerve.

Microglia prevent beta-amyloid plaque formation in the early stage of an Alzheimer's disease mouse model with suppression of glymphatic clearance

Background

Soluble beta-amyloid (Aβ) can be cleared from the brain through various mechanisms including enzymatic degradation, glial cell phagocytosis, transport across the blood-brain barrier, and glymphatic clearance. However, the relative contribution of each clearance system and their compensatory effects in delaying the pathological process of Alzheimer’s disease (AD) are currently unknown.

Methods

Fluorescent trace, immunofluorescence, and Western blot analyses were performed to compare glymphatic clearance ability and Aβ accumulation among 3-month-old APP695/PS1-dE9 transgenic (APP/PS1) mice, wild-type mice, aquaporin 4 knock out (AQP4^−/−) mice, and AQP4^−/−/APP/PS1 mice. The consequence of selectively eliminating microglial cells, or downregulating apolipoprotein E (apoE) expression, on Aβ burden, was also investigated in the frontal cortex of AQP4^−/−/APP/PS1 mice and APP/PS1 mice.

Results

AQP4 deletion in APP/PS1 mice significantly exaggerated glymphatic clearance dysfunction, and intraneuronal accumulation of Aβ and apoE, although it did not lead to Aβ plaque deposition. Notably, microglia, but not astrocytes, increased activation and phagocytosis of Aβ in the cerebral cortex of AQP4^−/−/APP/PS1 mice, compared with APP/PS1 mice. Selectively eliminating microglia in the frontal cortex via local injection of clodronate liposomes resulted in deposition of Aβ plaques in AQP4^−/−/APP/PS1 mice, but not APP/PS1 mice. Moreover, knockdown of apoE reduced intraneuronal Aβ levels in both APP/PS1 mice and AQP4^−/−/APP/PS1 mice, indicating an inhibitory effect of apoE on Aβ clearance.

Conclusion

The above results suggest that the glymphatic system mediated Aβ and apoE clearance and microglia mediated Aβ degradation synergistically prevent Aβ plague formation in the early stages of the AD mouse model. Protecting one or both of them might be beneficial to delaying the onset of AD.

Abnormal Cannabidiol Affects Production of Pro-Inflammatory Mediators and Astrocyte Wound Closure in Primary Astrocytic-Microglial Cocultures

Abnormal cannabidiol (abn-CBD) exerts neuroprotective effects in vivo and in vitro. In the present study, we investigated the impact of abn-CBD on the glial production of proinflammatory mediators and scar formation within in vitro models. Primary astrocytic-microglial cocultures and astrocytic cultures from neonatal C57BL/6 mice and CB2 receptor knockout mice were stimulated with lipopolysaccharide (LPS), and the concentrations of tumor necrosis factor α (TNFα), interleukin-6 (IL-6) and nitrite were determined. Furthermore, we performed a live cell microscopy-based scratch-wound assay. After LPS stimulation, TNFα, IL-6 and nitrite production was more strongly increased in cocultures than in isolated astrocytes. Abn-CBD treatment attenuated the LPS-induced production of TNFα and nitrite in cocultures, while IL-6 production remained unaltered. In isolated astrocytes, only LPS-induced TNFα production was reduced by abn-CBD. Similar effects were observed after abn-CBD application in cocultures of CB2 knockout mice. Interestingly, LPS-induced TNFα and nitrite levels were far lower in CB2 knockout cultures compared to wildtypes, while IL-6 levels did not differ. In the scratch-wound assay, treatment with abn-CBD decelerated wound closure when microglial cells were present. Our data shows a differential role of abn-CBD for modulation of glial inflammation and astrocytic scar formation. These findings provide new explanations for mechanisms behind the neuroprotective potential of abn-CBD.

The neuroprotective role of microglial cells against amyloid beta-mediated toxicity in organotypic hippocampal slice cultures

During Alzheimer's disease (AD) progression, microglial cells play complex roles and have potentially detrimental as well as beneficial effects. The use of appropriate model systems is essential for characterizing and understanding the roles of microglia in AD pathology. Here, we used organotypic hippocampal slice cultures (OHSCs) to investigate the impact of microglia on amyloid beta (Aβ)-mediated toxicity. Neurons in OHSCs containing microglia were not vulnerable to cell death after 7 days of repeated treatment with Aβ_1-42 oligomer-enriched preparations. However, when clodronate was used to remove microglia, treatment with Aβ_1-42 resulted in significant neuronal death. Further investigations indicated signs of endoplasmic reticulum stress and caspase activation after Aβ_1-42 challenge only when microglia were absent. Interestingly, microglia provided protection without displaying any classic signs of activation, such as an amoeboid morphology or the release of pro-inflammatory mediators (e.g., IL-6, TNF-α, NO). Furthermore, depleting microglia or inhibiting microglial uptake mechanisms resulted in significant more Aβ deposition compared to that observed in OHSCs containing functional microglia, suggesting that microglia efficiently cleared Aβ. Because inhibiting microglial uptake increased neuronal cell death, the ability of microglia to engulf Aβ is thought to contribute to its protective properties. Our study argues for a beneficial role of functional ramified microglia whereby they act against the accumulation of neurotoxic forms of Aβ and support neuronal resilience in an in situ model of AD pathology.

Central Nervous System Remyelination: Roles of Glia and Innate Immune Cells

In diseases such as multiple sclerosis (MS), inflammation can injure the myelin sheath that surrounds axons, a process known as demyelination. The spontaneous regeneration of myelin, called remyelination, is associated with restoration of function and prevention of axonal degeneration. Boosting remyelination with therapeutic intervention is a promising new approach that is currently being tested in several clinical trials. The endogenous regulation of remyelination is highly dependent on the immune response. In this review article, we highlight the cell biology of remyelination and its regulation by innate immune cells. For the purpose of this review, we discuss the roles of microglia, and also astrocytes and oligodendrocyte progenitor cells (OPCs) as they are being increasingly recognized to have immune cell functions.

Pharmacology of bisphosphonates in pain

The treatment of pain, in particular, chronic pain, remains a clinical challenge. This is particularly true for pain associated with severe or rare conditions, such as bone cancer pain, vulvodynia, or complex regional pain syndrome. Over the recent years, there is an increasing interest in the potential of bisphosphonates in the treatment of pain, although there are few papers describing antinociceptive and anti-hypersensitizing effects of bisphosphonates in various animal models of pain. There is also increasing evidence for clinical efficacy of bisphosphonates in chronic pain states, although the number of well-controlled studies is still limited. However, the mechanisms underlying the analgesic effects of bisphosphonates are still largely elusive. This review provides an overview of preclinical and clinical studies of bisphosphonates in pain and discusses various pharmacological mechanisms that have been postulated to explain their analgesic effects. LINKED ARTICLES: This article is part of a themed issue on The molecular pharmacology of bone and cancer-related bone diseases. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v178.9/issuetoc.

Tranexamic acid reduces heme cytotoxicity via the TLR4/TNF axis and ameliorates functional recovery after spinal cord injury

BACKGROUND

Spinal cord injury (SCI) is a catastrophic trauma accompanied by intralesional bleeding and neuroinflammation. Recently, there is increasing interest in tranexamic acid (TXA), an anti-fibrinolytic drug, which can reduce the bleeding volume after physical trauma. However, the efficacy of TXA on the pathology of SCI remains unknown.

METHODS

After producing a contusion SCI at the thoracic level of mice, TXA was intraperitoneally administered and the bleeding volume in the lesion area was quantified. Tissue damage was evaluated by immunohistochemical and gene expression analyses. Since heme is one of the degraded products of red blood cells (RBCs) and damage-associated molecular pattern molecules (DAMPs), we examined the influence of heme on the pathology of SCI. Functional recovery was assessed using the open field motor score, a foot print analysis, a grid walk test, and a novel kinematic analysis system. Statistical analyses were performed using Wilcoxon's rank-sum test, Dunnett's test, and an ANOVA with the Tukey-Kramer post-hoc test.

RESULTS

After SCI, the intralesional bleeding volume was correlated with the heme content and the demyelinated area at the lesion site, which were significantly reduced by the administration of TXA. In the injured spinal cord, toll-like receptor 4 (TLR4), which is a DAMP receptor, was predominantly expressed in microglial cells. Heme stimulation increased TLR4 and tumor necrosis factor (TNF) expression levels in primary microglial cells in a dose-dependent manner. Similarly to the in vitro experiments, the injection of non-lysed RBCs had little pathological influence on the spinal cord, whereas the injection of lysed RBCs or heme solution significantly upregulated the TLR4 and TNF expression in microglial cells. In TXA-treated SCI mice, the decreased expressions of TLR4 and TNF were observed at the lesion sites, accompanied by a significant reduction in the number of apoptotic cells and better functional recovery in comparison to saline-treated control mice.

CONCLUSION

The administration of TXA ameliorated the intralesional cytotoxicity both by reducing the intralesional bleeding volume and preventing heme induction of the TLR4/TNF axis in the SCI lesion. Our findings suggest that TXA treatment may be a therapeutic option for acute-phase SCI.

Lipopolysaccharide-induced neuroinflammation induces presynaptic disruption through a direct action on brain tissue involving microglia-derived interleukin 1 beta

BACKGROUND

Systemic inflammation has been linked to synapse loss and cognitive decline in human patients and animal models. A role for microglial release of pro-inflammatory cytokines has been proposed based on in vivo and primary culture studies. However, mechanisms are hard to study in vivo as specific microglial ablation is challenging and the extracellular fluid cannot be sampled without invasive methods. Primary cultures have different limitations as the intricate multicellular architecture in the brain is not fully reproduced. It is essential to confirm proposed brain-specific mechanisms of inflammatory synapse loss directly in brain tissue. Organotypic hippocampal slice cultures (OHSCs) retain much of the in vivo neuronal architecture, synaptic connections and diversity of cell types whilst providing convenient access to manipulate and sample the culture medium and observe cellular reactions.

METHODS

OHSCs were generated from P6-P9 C57BL/6 mice. Inflammation was induced via addition of lipopolysaccharide (LPS), and cultures were analysed for changes in synaptic proteins, gene expression and protein secretion. Microglia were selectively depleted using clodronate, and the effect of IL1β was assessed using a specific neutralising monoclonal antibody.

RESULTS

LPS treatment induced loss of the presynaptic protein synaptophysin without altering PSD95 or Aβ protein levels. Depletion of microglia prior to LPS application prevented the loss of synaptophysin, whilst microglia depletion after the inflammatory insult was partially effective, although less so than pre-emptive treatment, indicating a time-critical window in which microglia can induce synaptic damage. IL1β protein and mRNA were increased after LPS addition, with these effects also prevented by microglia depletion. Direct application of IL1β to OHSCs resulted in synaptophysin loss whilst pre-treatment with IL1β neutralising antibody prior to LPS addition prevented a significant loss of synaptophysin but may also impact basal synaptic levels.

CONCLUSIONS

The loss of synaptophysin in this system confirms LPS can act directly within brain tissue to disrupt synapses, and we show that microglia are the relevant cellular target when all major CNS cell types are present. By overcoming limitations of primary culture and in vivo work, our study strengthens the evidence for a key role of microglia-derived IL1β in synaptic dysfunction after inflammatory insult.

Depletion of microglia immediately following traumatic brain injury in the pediatric rat: Implications for cellular and behavioral pathology

The inflammatory response is a significant component of the pathophysiology of pediatric traumatic brain injury. High levels of inflammatory mediators have been found in the cerebrospinal fluid of brain-injured children which have been linked to poor prognosis. Targeting aspects of the inflammatory response in the hopes of finding a viable post-injury therapeutic option has gained attention. Microglia are largely responsible for perpetuating the injury-induced inflammatory response but in the developing brain they play beneficial roles in both normal and disease states. Following closed head injury in the neonate rat, depletion of microglia with intracerebral injections of liposomes containing clodronate was associated with an increase in neurodegeneration in the early post-injury period (3 days) relative to those injected with empty liposomes suggestive of a decrease in clearance of dying cells. In sham-injured animals, microglia repopulated the clodrosome-mediated depleted brain regions over a period of 2-4 weeks and exhibited morphology typical of a resting phenotype. In brain-injured animals, the repopulated microglia in clodrosome-injected animals exhibited rod-like and amoeboid morphologies. However, fluoro-Jade B reactivity in these brain regions was more extensive than in empty liposome-injected animals suggesting that the active microglia may be unable to clear dying neurons. This was accompanied by an induction of hyperexcitability in the local cortical circuitry. Depletion of microglia within the white matter tracts and the thalamus did not affect the extent of injury-induced traumatic axonal injury. Increased neurodegeneration in the dorsal subiculum was not accompanied by any changes to injury-induced deficits in spatial learning and memory. These data suggest that activation of microglia may be important for removal of dying neurons in the traumatically-injured immature brain.

Microglial Depletion with Clodronate Liposomes Increases Proinflammatory Cytokine Levels, Induces Astrocyte Activation, and Damages Blood Vessel Integrity

Investigators are increasingly interested in using microglial depletion to study the role of microglia under pathologic conditions. Liposome-encapsulated clodronate is commonly used to eliminate macrophage populations because it causes functionally irreversible inhibition and apoptosis once phagocytized by macrophages. Recent studies have shown that microglia can be depleted in disease models by injecting clodronate liposomes into the brain parenchyma. However, it is unclear whether intracerebral administration of clodronate liposomes is a practical method of eliminating microglia under physiologic conditions or whether microglial depletion induces damage to other brain cells. In this study, injecting 1 μL of clodronate liposomes (7 μg/μL) into the striatum of mice caused ablation of microglia at 1 day that persisted for 3 days. Microglia reappeared in the boundary regions of microglia elimination after 5 days. Importantly, we observed an increase in proinflammatory cytokine levels and an increase in neural/glial antigen 2 and glial fibrillary acidic protein expression in the perilesional region. In contrast, expression levels of myelin basic protein, microtubule-associated protein 2, and postsynaptic protein-95 decreased in the periphery of regions where microglia were depleted. Moreover, clodronate liposome administration decreased the density and integrity of blood vessels in the perilesional regions. In cultured primary neurons, clodronate liposome exposure also attenuated ATP synthesis. Together, these findings suggest that intracerebral administration of clodronate liposomes into brain parenchyma can deplete microglia, but can also damage other brain cells and blood vessel integrity.

Blockade of microglial adenosine A2A receptor suppresses elevated pressure-induced inflammation, oxidative stress, and cell death in retinal cells

Glaucoma is a retinal degenerative disease characterized by the loss of retinal ganglion cells and damage of the optic nerve. Recently, we demonstrated that antagonists of adenosine A_2A receptor (A_2A R) control retinal inflammation and afford protection to rat retinal cells in glaucoma models. However, the precise contribution of microglia to retinal injury was not addressed, as well as the effect of A_2A R blockade directly in microglia. Here we show that blocking microglial A_2A R prevents microglial cell response to elevated pressure and it is sufficient to protect retinal cells from elevated pressure-induced death. The A_2A R antagonist SCH 58261 or the knockdown of A_2A R expression with siRNA in microglial cells prevented the increase in microglia response to elevated hydrostatic pressure. Furthermore, in retinal neural cell cultures, the A_2A R antagonist decreased microglia proliferation, as well as the expression and release of pro-inflammatory mediators. Microglia ablation prevented neural cell death triggered by elevated pressure. The A_2A R blockade recapitulated the effects of microglia depletion, suggesting that blocking A_2A R in microglia is able to control neurodegeneration in glaucoma-like conditions. Importantly, in human organotypic retinal cultures, A_2A R blockade prevented the increase in reactive oxygen species and the morphological alterations in microglia triggered by elevated pressure. These findings place microglia as the main contributors for retinal cell death during elevated pressure and identify microglial A_2A R as a therapeutic target to control retinal neuroinflammation and prevent neural apoptosis elicited by elevated pressure.

Post-stroke DHA Treatment Protects Against Acute Ischemic Brain Injury by Skewing Macrophage Polarity Toward the M2 Phenotype

Systemic docosahexaenoic acid (DHA) has been explored as a clinically feasible protectant in stroke models. However, the mechanism for DHA-afforded neuroprotection remains elusive. Transient middle cerebral artery occlusion (tMCAO) was induced for 1 h. DHA (i.p., 10 mg/kg) was administered immediately after reperfusion and repeated daily for 3 days. Stroke outcomes, systemic inflammatory status, and microglia/macrophage phenotypic alterations were assessed 3 days after stroke. Macrophage depletion was induced by clodronate liposomes injection. Primary macrophage cultures were used to evaluate the direct effect of DHA on macrophages. We demonstrated that post-stroke DHA injection efficiently reduced brain infarct and ameliorated neurological deficits 3 days after tMCAO. Systemic DHA treatment significantly inhibited immune cell infiltration (macrophages, neutrophils, T lymphocytes, and B lymphocytes) and promoted macrophage polarization toward an anti-inflammatory M2 phenotype in the ischemic brain. Meanwhile, systemic DHA administration inhibited the otherwise elevated pro-inflammatory factors in blood and shifted circulating macrophage polarity toward M2 phenotype after ischemic stroke. The numbers of circulating immune cells in blood and spleen, however, were equivalent between DHA- and vehicle-treated groups. The protective effects of DHA were macrophage-dependent, as macrophage depletion abolished DHA-afforded neuroprotection. In vitro studies confirmed that DHA suppressed production of chemokines and pro-inflammatory cytokines from macrophages under inflammatory stimulation. These data indicate that post-stroke DHA treatment ameliorated acute ischemic brain injury in a macrophage-dependent manner and DHA enhanced macrophage phenotypic shift toward an anti-inflammatory phenotype to reduced central and peripheral inflammation after stroke.

Microglia extensively survey the developing cortex via the CXCL12/CXCR4 system to help neural progenitors to acquire differentiated properties

Neocortical development proceeds through the formation of new zones in which neural-lineage cells are organized based on their differentiation status. Although microglia initially distribute homogeneously throughout the growing cerebral wall, they accumulate in the inner cytogenic zone, the ventricular zone (VZ) and the subventricular zone (SVZ) in the mid-embryonic stage. However, the roles of these cells remain to be elucidated. In this study, we found that microglia, despite being only a minor population of the cells that constitute the cerebral wall, promote the differentiation of neural progenitor cells by frequently moving throughout the cortex; their migration is mediated by the CXCL12/CXCR4 system. Pulse-chase experiments confirmed that microglia help Pax6⁺ stem-like cells to differentiate into Tbr2⁺ intermediate progenitors. Further, monitoring of microglia by live imaging showed that administration of AMD3100, an antagonist of CXCR4, dampened microglial movement and decreased microglial surveillance throughout the cortex. In particular, arrest of microglial motion led to a prominent decrease in the abundance of Tbr2⁺ cells in the SVZ. Based on our findings, we propose that extensive surveillance by microglia contributes to the efficient functioning of these cells, thereby regulating the differentiation of neural stem-like cells.

Liposome-encapsulated clodronate specifically depletes spinal microglia and reduces initial neuropathic pain

Liposome-encapsulated clodronate (LEC) is a specific depletor of macrophages. Our study characterized the LEC depletory effects, given intrathecally, on spinal microglia and assessed its effects on initiation and maintenance of neuropathic pain. Measured by using the MTT assay, LEC treatment specifically inhibited cell viability of cultured primary microglia, but not astrocytes or neurons, from neonatal rats, with an IC₅₀ of 43 μg/mL. In spinal nerve ligation-induced neuropathic rats, pretreatment (1 day but not 5 days earlier) with intrathecal LEC specifically depleted microglia (but not astrocytes or neurons) in both contralateral and ipsilateral dorsal horns by the same degree (63% vs. 71%). Intrathecal injection of LEC reversibly blocked the antinociceptive effects of the GLP-1 receptor agonist exenatide and dynorphin A stimulator bulleyaconitine, which have been claimed to be mediated by spinal microglia, whereas it failed to alter morphine- or the glycine receptor agonist gelsemine-induced mechanical antiallodynia which was mediated via the neuronal mechanisms. Furthermore, intrathecal LEC injection significantly attenuated initial (one day after nerve injury) but not existing (2 weeks after nerve injury) mechanical allodynia. Our study demonstrated that LEC, given intrathecally, is a specific spinal microglial inhibitor and significantly reduces initiation but not maintenance of neuropathic pain, highlighting an opposite role of spinal microglia in different stages of neuropathic pain.

Intravenous immune-modifying nanoparticles as a therapy for spinal cord injury in mice

Intravenously infused synthetic 500nm nanoparticles composed of poly(lactide-co-glycolide) are taken up by blood-borne inflammatory monocytes via a macrophage scavenger receptor (macrophage receptor with collagenous structure), and the monocytes no longer traffic to sites of inflammation. Intravenous administration of the nanoparticles after experimental spinal cord injury in mice safely and selectively limited infiltration of hematogenous monocytes into the injury site. The nanoparticles did not bind to resident microglia, and did not change the number of microglia in the injured spinal cord. Nanoparticle administration reduced M1 macrophage polarization and microglia activation, reduced levels of inflammatory cytokines, and markedly reduced fibrotic scar formation without altering glial scarring. These findings thus implicate early-infiltrating hematogenous monocytes as highly selective contributors to fibrosis that do not play an indispensable role in gliosis after SCI. Further, the nanoparticle treatment reduced accumulation of chondroitin sulfate proteoglycans, increased axon density inside and caudal to the lesion site, and significantly improved functional recovery after both moderate and severe injuries to the spinal cord. These data provide further evidence that hematogenous monocytes contribute to inflammatory damage and fibrotic scar formation after spinal cord injury in mice. Further, since the nanoparticles are simple to administer intravenously, immunologically inert, stable at room temperature, composed of an FDA-approved material, and have no known toxicity, these findings suggest that the nanoparticles potentially offer a practical treatment for human spinal cord injury.

Identification of a vesicular ATP release inhibitor for the treatment of neuropathic and inflammatory pain

Despite the high incidence of neuropathic and inflammatory pain worldwide, effective drugs with few side effects are currently unavailable for the treatment of chronic pain. Recently, researchers have proposed that inhibitors of purinergic chemical transmission, which plays a key role in the pathological pain response, may allow for targeted treatment of pathological neuropathic and inflammatory pain. However, such therapeutic analgesic agents have yet to be developed. In the present study, we demonstrated that clodronate, a first-generation bisphosphonate with comparatively fewer side effects than traditional treatments, significantly attenuates neuropathic and inflammatory pain unrelated to bone abnormalities via inhibition of vesicular nucleotide transporter (VNUT), a key molecule for the initiation of purinergic chemical transmission. In vitro analyses indicated that clodronate inhibits VNUT at a half-maximal inhibitory concentration of 15.6 nM without affecting other vesicular neurotransmitter transporters, acting as an allosteric modulator through competition with Cl^- A low concentration of clodronate impaired vesicular ATP release from neurons, microglia, and immune cells. In vivo analyses revealed that clodronate is more effective than other therapeutic agents in attenuating neuropathic and inflammatory pain, as well as the accompanying inflammation, in wild-type but not VNUT ^-/- mice, without affecting basal nociception. These findings indicate that clodronate may represent a unique treatment strategy for chronic neuropathic and inflammatory pain via inhibition of vesicular ATP release.

Microglial Activation Promotes Cell Survival in Organotypic Cultures of Postnatal Mouse Retinal Explants

The role of microglia during neurodegeneration remains controversial. We investigated whether microglial cells have a neurotoxic or neuroprotective function in the retina. Retinal explants from 10-day-old mice were treated in vitro with minocycline to inhibit microglial activation, with LPS to increase microglial activation, or with liposomes loaded with clodronate (Lip-Clo) to deplete microglial cells. Flow cytometry was used to assess the viability of retinal cells in the explants and the TUNEL method to show the distribution of dead cells. The immunophenotypic and morphological features of microglia and their distribution were analyzed with flow cytometry and immunocytochemistry. Treatment of retinal explants with minocycline reduced microglial activation and simultaneously significantly decreased cell viability and increased the presence of TUNEL-labeled cell profiles. This treatment also prevented the migration of microglial cells towards the outer nuclear layer, where cell death was most abundant. The LPS treatment increased microglial activation but had no effect on cell viability or microglial distribution. Finally, partial microglial removal with Lip-Clo diminished the cell viability in the retinal explants, showing a similar effect to that of minocycline. Hence, cell viability is diminished in retinal explants cultured in vitro when microglial cells are removed or their activation is inhibited, indicating a neurotrophic role for microglia in this system.

Selective Depletion of Microglia from Cerebellar Granule Cell Cultures Using L-leucine Methyl Ester

Microglia, the resident immunocompetent cells of the CNS, play multifaceted roles in modulating and controlling neuronal function, as well as mediating innate immunity. Primary rodent cell culture models have greatly advanced our understanding of neuronal-glial interactions, but only recently have methods to specifically eliminate microglia from mixed cultures been utilized. One such technique - described here - is the use of L-leucine methyl ester, a lysomotropic agent that is internalized by macrophages and microglia, wherein it causes lysosomal disruption and subsequent apoptosis(13,14). Experiments using L-leucine methyl ester have the power to identify the contribution of microglia to the surrounding cellular environment under diverse culture conditions. Using a protocol optimized in our laboratory, we describe how to eliminate microglia from P5 rodent cerebellar granule cell culture. This approach allows one to assess the relative impact of microglia on experimental data, as well as determine whether microglia are playing a neuroprotective or neurotoxic role in culture models of neurological conditions, such as stroke, Alzheimer's or Parkinson's disease.

What Elements of the Inflammatory System Are Necessary for Epileptogenesis In Vitro?

The inflammatory and central nervous systems share many signaling molecules, compromising the utility of traditional pharmacological and knockout approaches in defining the role of inflammation in CNS disorders such as epilepsy. In an in vitro model of post-traumatic epileptogenesis, the development of epilepsy proceeded in the absence of the systemic inflammatory system, and was unaffected by removal of cellular mediators of inflammation, including macrophages and T-lymphocytes.

Epileptogenesis in vivo can be altered by manipulation of molecules such as cytokines and complement that subserve intercellular signaling in both the inflammatory and central nervous systems. Because of the dual roles of these signaling molecules, it has been difficult to precisely define the role of systemic inflammation in epileptogenesis. Organotypic hippocampal brain slices can be maintained in culture independently of the systemic inflammatory system, and the rapid course of epileptogenesis in these cultures supports the idea that inflammation is not necessary for epilepsy. However, this preparation still retains key cellular inflammatory mediators. Here, we found that rodent hippocampal organotypic slice cultures depleted of T lymphocytes and microglia developed epileptic activity at essentially the same rate and to similar degrees of severity as matched control slice cultures. These data support the idea that although the inflammatory system, neurons, and glia share key intercellular signaling molecules, neither systemic nor CNS-specific cellular elements of the immune and inflammatory systems are necessary components of epileptogenesis.

Microglia dictate the impact of saturated fat consumption on hypothalamic inflammation and neuronal function

Diets rich in saturated fat produce inflammation, gliosis, and neuronal stress in the mediobasal hypothalamus (MBH). Here, we show that microglia mediate this process and its functional impact. Although microglia and astrocytes accumulate in the MBH of mice fed a diet rich in saturated fatty acids (SFAs), only the microglia undergo inflammatory activation, along with a buildup of hypothalamic SFAs. Enteric gavage specifically with SFAs reproduces microglial activation and neuronal stress in the MBH, and SFA treatment activates murine microglia, but not astrocytes, in culture. Moreover, depleting microglia abrogates SFA-induced inflammation in hypothalamic slices. Remarkably, depleting microglia from the MBH of mice abolishes inflammation and neuronal stress induced by excess SFA consumption, and in this context, microglial depletion enhances leptin signaling and reduces food intake. We thus show that microglia sense SFAs and orchestrate an inflammatory process in the MBH that alters neuronal function when SFA consumption is high.

LPS and IL-1 differentially activate mouse and human astrocytes: role of CD14

Treatment of cultures with toll-like receptor (TLR) ligands or cytokines has become a popular approach to investigate astrocyte neuroinflammatory responses and to simulate the neural environment in various CNS disorders. However, despite much effort, the mechanism of astrocyte activation such as their responses to the TLR ligands and IL-1 remain highly debated. We compared highly pure primary mouse and human astrocyte cultures in their ability to produce proinflammatory mediators (termed "A1") and immunoregulatory mediators (termed "A2") in response to LPS, poly IC, and IL-1 stimulation. In human astrocytes, IL-1 induced both A1 and A2 responses, poly IC induced mostly A2, and LPS induced neither. In mouse astrocytes, LPS induced mostly an A1-predominant response, poly IC induced both A1 and A2, and IL-1 neither. In addition, mouse astrocytes produce abundant IL-1 protein, whereas human astrocytes did not, despite robust IL-1 mRNA expression. Of the TLR4 receptor complex proteins, human astrocytes expressed TLR4 and MD2 but not CD14, whereas mouse astrocytes expressed all three. Mouse astrocyte CD14 (cell-associated and soluble) was potently upregulated by LPS. Silencing TLR4 or CD14 by siRNA suppressed LPS responses in mouse astrocytes. In vivo, astrocytes in LPS-injected mouse brains also expressed CD14. Our results show striking differences between human and mouse astrocytes in the use of TLR/IL-1R and subsequent downstream signaling and immune activation. IL-1 translational block in human astrocytes may be a built-in mechanism to prevent autocrine and paracrine cell activation and neuroinflammation. These results have important implications for translational research of human CNS diseases.

Microglia regulate the number of neural precursor cells in the developing cerebral cortex

Neurogenesis must be properly regulated to ensure that cell production does not exceed the requirements of the growing cerebral cortex, yet our understanding of mechanisms that restrain neuron production remains incomplete. We investigated the function of microglial cells in the developing cerebral cortex of prenatal and postnatal macaques and rats and show that microglia limit the production of cortical neurons by phagocytosing neural precursor cells. We show that microglia selectively colonize the cortical proliferative zones and phagocytose neural precursor cells as neurogenesis nears completion. We found that deactivating microglia in utero with tetracyclines or eliminating microglia from the fetal cerebral cortex with liposomal clodronate significantly increased the number of neural precursor cells, while activating microglia in utero through maternal immune activation significantly decreased the number of neural precursor cells. These data demonstrate that microglia play a fundamental role in regulating the size of the precursor cell pool in the developing cerebral cortex, expanding our understanding of the mechanisms that regulate cortical development. Furthermore, our data suggest that any factor that alters the number or activation state of microglia in utero can profoundly affect neural development and affect behavioral outcomes.