NO mediates microglial response to acute spinal cord injury under ATP control in vivo

In vivo spectrally unmixed multi-photon imaging of longitudinal axon-glia changes in injured spinal white matter

Understanding the sequence of cellular responses and their contributions to pathomorphogical changes in spinal white matter injuries is a prerequisite for developing efficient therapeutic strategies for spinal cord injury (SCI) as well as neurodegenerative and inflammatory diseases of the spinal cord such as amyotrophic lateral sclerosis and multiple sclerosis. We have developed several types of surgical procedures suitable for acute one-time and chronic recurrent in vivo multiphoton microscopy of spinal white matter [1]. Sophisticated surgical procedures were combined with transgenic mouse technology to image spinal tissue labeled with up to four fluorescent proteins (FPs) in axons, astrocytes, microglia, and blood vessels. To clearly separate the simultaneously excited FPs, spectral unmixing including iterative procedures was performed after imaging the diversely labeled spinal white matter with a custom-made 4-channel two-photon laser-scanning microscope. In our longitudinal multicellular studies of injured spinal white matter, we imaged axonal dynamics and invasion of microglia and astrocytes for a time course of over 200 days after SCI. Our methods offer ideal platforms for investigating acute and chronic cellular dynamics, cell-cell interactions, and metabolite fluctuations in health and disease as well as pharmacological manipulations in vivo.

Exploring the Role of Apigenin in Neuroinflammation: Insights and Implications

Neuroinflammation, a hallmark of various central nervous system disorders, is often associated with oxidative stress and neuronal or oligodendrocyte cell death. It is therefore very interesting to target neuroinflammation pharmacologically. One therapeutic option is the use of nutraceuticals, particularly apigenin. Apigenin is present in plants: vegetables (parsley, celery, onions), fruits (oranges), herbs (chamomile, thyme, oregano, basil), and some beverages (tea, beer, and wine). This review explores the potential of apigenin as an anti-inflammatory agent across diverse neurological conditions (multiple sclerosis, Parkinson's disease, Alzheimer's disease), cancer, cardiovascular diseases, cognitive and memory disorders, and toxicity related to trace metals and other chemicals. Drawing upon major studies, we summarize apigenin's multifaceted effects and underlying mechanisms in neuroinflammation. Our review underscores apigenin's therapeutic promise and calls for further investigation into its clinical applications.

Histone deacetylase 3 regulates microglial function through histone deacetylation

As the primary innate immune cells of the brain, microglia respond to damage and disease through pro-inflammatory release of cytokines and neuroinflammatory molecules. Histone acetylation is an activating transcriptional mark that regulates inflammatory gene expression. Inhibition of histone deacetylase 3 (Hdac3) has been utilized in pre-clinical models of depression, stroke, and spinal cord injury to improve recovery following injury, but the molecular mechanisms underlying Hdac3's regulation of inflammatory gene expression in microglia is not well understood. To address this lack of knowledge, we examined how pharmacological inhibition of Hdac3 in an immortalized microglial cell line (BV2) impacted histone acetylation and gene expression of pro- and anti-inflammatory genes in response to immune challenge with lipopolysaccharide (LPS). Flow cytometry and cleavage under tags & release using nuclease (CUT & RUN) revealed that Hdac3 inhibition increases global and promoter-specific histone acetylation, resulting in the release of gene repression at baseline and enhanced responses to LPS. Hdac3 inhibition enhanced neuroprotective functions of microglia in response to LPS through reduced nitric oxide release and increased phagocytosis. The findings suggest Hdac3 serves as a regulator of microglial inflammation, and that inhibition of Hdac3 facilitates the microglial response to inflammation and its subsequent clearing of debris or damaged cells. Together, this work provides new mechanistic insights into therapeutic applications of Hdac3 inhibition which mediate reduced neuroinflammatory insults through microglial response.

HMGB1 in depression: An overview of microglial HMBG1 in the pathogenesis of depression

Depression is a prevalent psychiatric disorder with elusive pathogenesis. Studies have proposed that enhancement and persistence of aseptic inflammation in the central nervous system (CNS) may be closely associated with the development of depressive disorder. High mobility group box 1 (HMGB1) has obtained significant attention as an evoking and regulating factor in various inflammation-related diseases. It is a non-histone DNA-binding protein that can be released as a pro-inflammatory cytokine by glial cells and neurons in the CNS. Microglia, as the immune cell of the brain, interacts with HMGB1 and induces neuroinflammation and neurodegeneration in the CNS. Therefore, in the current review, we aim to investigate the role of microglial HMGB1 in the pathogenetic process of depression.

α-Tocopherol Protects Lipopolysaccharide-Activated BV2 Microglia

Microglia, the resident macrophage-like population in the central nervous system, play a crucial role in the pathogenesis of many neurodegenerative disorders by triggering an inflammatory response that leads to neuronal death. Neuroprotective compounds to treat or prevent neurodegenerative diseases are a new field of study in modern medicine. Microglia are activated in response to inflammatory stimuli. The pathogenesis of various neurodegenerative diseases is closely related to the constant activation of microglia due to their fundamental role as a mediator of inflammation in the brain environment. α-Tocopherol, also known as vitamin E, is reported to possess potent neuroprotective effects. The goal of this study was to investigate the biological effects of vitamin E on BV2 microglial cells, as a possible neuroprotective and anti-inflammatory agent, following stimulation with lipopolysaccharide (LPS). The results showed that the pre-incubation of microglia with α-tocopherol can guarantee neuroprotective effects during microglial activation induced by LPS. α-Tocopherol preserved the branched morphology typical of microglia in a physiological state. It also reduced the migratory capacity; the production of pro-inflammatory and anti-inflammatory cytokines such as TNF-α and IL-10; and the activation of receptors such as TRL4 and CD40, which modulate the PI3K-Akt signaling pathway. The results of this study require further insights and research, but they present new scenarios for the application of vitamin E as an antioxidant for the purpose of greater neuroprotection in vivo for the prevention of possible neurodegenerative diseases.

The Interplay between cGMP and Calcium Signaling in Alzheimer's Disease

Cyclic guanosine monophosphate (cGMP) is a ubiquitous second messenger and a key molecule in many important signaling cascades in the body and brain, including phototransduction, olfaction, vasodilation, and functional hyperemia. Additionally, cGMP is involved in long-term potentiation (LTP), a cellular correlate of learning and memory, and recent studies have identified the cGMP-increasing drug Sildenafil as a potential risk modifier in Alzheimer's disease (AD). AD development is accompanied by a net increase in the expression of nitric oxide (NO) synthases but a decreased activity of soluble guanylate cyclases, so the exact sign and extent of AD-mediated imbalance remain unclear. Moreover, human patients and mouse models of the disease present with entangled deregulation of both cGMP and Ca2+ signaling, e.g., causing changes in cGMP-mediated Ca2+ release from the intracellular stores as well as Ca2+-mediated cGMP production. Still, the mechanisms governing such interplay are poorly understood. Here, we review the recent data on mechanisms underlying the brain cGMP signaling and its interconnection with Ca2+ signaling. We also discuss the recent evidence stressing the importance of such interplay for normal brain function as well as in Alzheimer's disease.

The Putative Role of Astaxanthin in Neuroinflammation Modulation: Mechanisms and Therapeutic Potential

Neuroinflammation is a protective mechanism against insults from exogenous pathogens and endogenous cellular debris and is essential for reestablishing homeostasis in the brain. However, excessive prolonged neuroinflammation inevitably leads to lesions and disease. The use of natural compounds targeting pathways involved in neuroinflammation remains a promising strategy for treating different neurological and neurodegenerative diseases. Astaxanthin, a natural xanthophyll carotenoid, is a well known antioxidant. Mounting evidence has revealed that astaxanthin is neuroprotective and has therapeutic potential by inhibiting neuroinflammation, however, its functional roles and underlying mechanisms in modulating neuroinflammation have not been systematically summarized. Hence, this review summarizes recent progress in this field and provides an update on the medical value of astaxanthin. Astaxanthin modulates neuroinflammation by alleviating oxidative stress, reducing the production of neuroinflammatory factors, inhibiting peripheral inflammation and maintaining the integrity of the blood-brain barrier. Mechanistically, astaxanthin scavenges radicals, triggers the Nrf2-induced activation of the antioxidant system, and suppresses the activation of the NF-κB and mitogen-activated protein kinase pathways. With its good biosafety and high bioavailability, astaxanthin has strong potential for modulating neuroinflammation, although some outstanding issues still require further investigation.

Microglia in depression: an overview of microglia in the pathogenesis and treatment of depression

Major depressive disorder is a highly debilitating psychiatric disorder involving the dysfunction of different cell types in the brain. Microglia are the predominant resident immune cells in the brain and exhibit a critical role in depression. Recent studies have suggested that depression can be regarded as a microglial disease. Microglia regulate inflammation, synaptic plasticity, and the formation of neural networks, all of which affect depression. In this review, we highlighted the role of microglia in the pathology of depression. First, we described microglial activation in animal models and clinically depressed patients. Second, we emphasized the possible mechanisms by which microglia recognize depression-associated stress and regulate conditions. Third, we described how antidepressants (clinical medicines and natural products) affect microglial activation. Thus, this review aimed to objectively analyze the role of microglia in depression and focus on potential antidepressants. These data suggested that regulation of microglial actions might be a novel therapeutic strategy to counteract the adverse effects of devastating mental disorders.

Long-term in vivo imaging of mouse spinal cord through an optically cleared intervertebral window

The spinal cord accounts for the main communication pathway between the brain and the peripheral nervous system. Spinal cord injury is a devastating and largely irreversible neurological trauma, and can result in lifelong disability and paralysis with no available cure. In vivo spinal cord imaging in mouse models without introducing immunological artifacts is critical to understand spinal cord pathology and discover effective treatments. We developed a minimally invasive intervertebral window by retaining the ligamentum flavum to protect the underlying spinal cord. By introducing an optical clearing method, we achieve repeated two-photon fluorescence and stimulated Raman scattering imaging at subcellular resolution with up to 15 imaging sessions over 6-167 days and observe no inflammatory response. Using this optically cleared intervertebral window, we study neuron-glia dynamics following laser axotomy and observe strengthened contact of microglia with the nodes of Ranvier during axonal degeneration. By enabling long-term, repetitive, stable, high-resolution and inflammation-free imaging of mouse spinal cord, our method provides a reliable platform in the research aiming at interpretation of spinal cord physiology and pathology.

Astrocytes and Microglia Exhibit Cell-Specific Ca2+ Signaling Dynamics in the Murine Spinal Cord

The spinal cord is the main pathway connecting brain and peripheral nervous system. Its functionality relies on the orchestrated activity of both neurons and glial cells. To date, most advancement in understanding the spinal cord inner mechanisms has been made either by in vivo exposure of its dorsal surface through laminectomy or by acute ex vivo slice preparation, likely affecting spinal cord physiology in virtue of the necessary extensive manipulation of the spinal cord tissue. This is especially true of cells immediately responding to alterations of the surrounding environment, such as microglia and astrocytes, reacting within seconds or minutes and for up to several days after the original insult. Ca2+ signaling is considered one of the most immediate, versatile, and yet elusive cellular responses of glia. Here, we induced the cell-specific expression of the genetically encoded Ca2+ indicator GCaMP3 to evaluate spontaneous intracellular Ca2+ signaling in astrocytes and microglia. Ca2+ signals were then characterized in acute ex vivo (both gray and white matter) as well as in chronic in vivo (white matter) preparations using MSparkles, a MATLAB-based software for automatic detection and analysis of fluorescence events. As a result, we were able to segregate distinct astroglial and microglial Ca2+ signaling patterns along with method-specific Ca2+ signaling alterations, which must be taken into consideration in the reliable evaluation of any result obtained in physiological as well as pathological conditions. Our study revealed a high degree of Ca2+ signaling diversity in glial cells of the murine spinal cord, thus adding to the current knowledge of the astonishing glial heterogeneity and cell-specific Ca2+ dynamics in non-neuronal networks.

Microglia form satellites with different neuronal subtypes in the adult murine central nervous system

Microglia are the innate immune cells of the central nervous system (CNS). In the adult uncompromised CNS, they have a highly ramified morphology and continuously extend and retract their processes. A subpopulation of microglial cells forms close soma-to-soma contacts with neurons and have been termed satellite microglia, yet the role of such interaction is largely unknown. Here, we analyzed the distribution of satellite microglia in different areas of the CNS of adult male mice applying transgenic- and immunolabeling of neuronal subtypes and microglia followed by three-dimensional imaging analysis. We quantified satellite microglia associated with GABAergic and glutamatergic neurons in the somatosensory cortex, striatum, and thalamus; with dopaminergic and serotonergic neurons in the basal forebrain and raphe nucleus, respectively; and with cerebellar Purkinje cell neurons. Satellite microglia in the retina were assessed qualitatively. Microglia form satellites with all neuronal subtypes studied, whereas a preference for a specific neuron subtype was not found. The occurrence and frequency of satellite microglia is determined by the histo-architectural organization of the brain area and the densities of neuronal somata therein.

Microglial TREM2 in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, is an aggressive motor neuron degenerative disease characterized by selective loss of both upper and lower motor neurons. The mechanisms underlying disease initiation and progression are poorly understood. The involvement of nonmotor neuraxis emphasizes the contribution of glial cells in disease progress. Microglia comprise a unique subset of glial cells and are the principal immune cells in the central nervous system (CNS). Triggering receptor expressed on myeloid cell 2 (TREM2) is a surface receptor that, within the CNS, is exclusively expressed on microglia and plays crucial roles in microglial proliferation, migration, activation, metabolism, and phagocytosis. Genetic evidence has linked TREM2 to neurodegenerative diseases including ALS, but its function in ALS pathogenesis is largely unknown. In this review, we summarize how microglial activation, with a specific focus on TREM2 function, affects ALS progression clinically and experimentally. Understanding microglial TREM2 function will help pinpoint the molecular target for ALS treatment.

Biomaterial and Therapeutic Approaches for the Manipulation of Macrophage Phenotype in Peripheral and Central Nerve Repair

Injury to the peripheral or central nervous systems often results in extensive loss of motor and sensory function that can greatly diminish quality of life. In both cases, macrophage infiltration into the injury site plays an integral role in the host tissue inflammatory response. In particular, the temporally related transition of macrophage phenotype between the M1/M2 inflammatory/repair states is critical for successful tissue repair. In recent years, biomaterial implants have emerged as a novel approach to bridge lesion sites and provide a growth-inductive environment for regenerating axons. This has more recently seen these two areas of research increasingly intersecting in the creation of 'immune-modulatory' biomaterials. These synthetic or naturally derived materials are fabricated to drive macrophages towards a pro-repair phenotype. This review considers the macrophage-mediated inflammatory events that occur following nervous tissue injury and outlines the latest developments in biomaterial-based strategies to influence macrophage phenotype and enhance repair.

Isolation of microglia from retinas of chronic ocular hypertensive rats

Microglia are the principal glial cells involved in the processes of immune inflammation within both retina and optic nerve, especially under the context of glaucomatous neuropathy. Considering the distinguishing role of retinal microglia in glaucoma and the lack of established protocol for microglia isolation from animal glaucoma model, the present study aimed to develop and validate a method with characteristics of both simplicity and efficiency for retinal microglia isolation from chronic ocular hypertensive (COH) rats. A Percoll gradient of various concentrations was used to separate microglia from whole retinal cells of the COH rats and control group. The finally isolated microglia were identified by CD11b and Iba-1 immunofluorescence staining, and the cell viability was determined by trypan blue staining. Additionally, the proportion of microglia in the whole retina cells was identified by flow cytometry. Results showed that the survival rates of isolated retinal microglia with the Percoll gradient method were 67.2 ± 4% and 67.6 ± 3% in control and COH groups, respectively. The proportion of the microglia population in the whole retinal cells was about 0.4–0.93%. To conclude, the present study confirmed that the application of Percoll gradient could effectively separate microglia from retinas of COH rats, which will probably enrich the tool kit for basic researchers of glaucoma specialty and help with scientific investigations.

Plasticity of microglia

Microglial cells are the scions of foetal macrophages which invade the neural tube early during embryogenesis. The nervous tissue environment instigates the phenotypic metamorphosis of foetal macrophages into idiosyncratic surveilling microglia, which are generally characterised by a small cell body and highly ramified motile processes that constantly scan the nervous tissue for signs of changes in homeostasis and allow microglia to perform crucial homeostatic functions. The surveilling microglial phenotype is evolutionarily conserved from early invertebrates to humans. Despite this evolutionary conservation, microglia show substantial heterogeneity in their gene and protein expression, as well as morphological appearance. These differences are age, region and context specific and reflect a high degree of plasticity underlying the life-long adaptation of microglia, supporting the exceptional adaptive capacity of the central nervous system. Microgliocytes are essential elements of cellular network formation and refinement in the developing nervous tissue. Several distinct patrolling modes of microglial processes contribute to the formation, modification, and pruning of synapses; to the support and protection of neurones through microglial-somatic junctions; and to the control of neuronal and axonal excitability by specific microglia-axonal contacts. In pathology, microglia undergo proliferation and reactive remodelling known as microgliosis, which is context dependent, yet represents an evolutionarily conserved defence response. Microgliosis results in the emergence of multiple disease and context-specific reactive states; in addition, neuropathology is associated with the appearance of specific protective or recovery microglial forms. In summary, the plasticity of microglia supports the development and functional activity of healthy nervous tissue and provides highly sophisticated defences against disease.

Suppression of LPS-Induced Inflammation and Cell Migration by Azelastine through Inhibition of JNK/NF-κB Pathway in BV2 Microglial Cells

The c-Jun N-terminal kinases (JNKs) are implicated in many neuropathological conditions, including neurodegenerative diseases. To explore potential JNK3 inhibitors from the U.S. Food and Drug Administration-approved drug library, we performed structure-based virtual screening and identified azelastine (Aze) as one of the candidates. NMR spectroscopy indicated its direct binding to the ATP-binding site of JNK3, validating our observations. Although the antihistamine effect of Aze is well documented, the involvement of the JNK pathway in its action remains to be elucidated. This study investigated the effects of Aze on lipopolysaccharide (LPS)-induced JNK phosphorylation, pro-inflammatory mediators, and cell migration in BV2 microglial cells. Aze was found to inhibit the LPS-induced phosphorylation of JNK and c-Jun. It also inhibited the LPS-induced production of pro-inflammatory mediators, including interleukin-6, tumor necrosis factor-α, and nitric oxide. Wound healing and transwell migration assays indicated that Aze attenuated LPS-induced BV2 cell migration. Furthermore, Aze inhibited LPS-induced IκB phosphorylation, thereby suppressing nuclear translocation of NF-κB. Collectively, our data demonstrate that Aze exerts anti-inflammatory and anti-migratory effects through inhibition of the JNK/NF-κB pathway in BV2 cells. Based on our findings, Aze may be a potential candidate for drug repurposing to mitigate neuroinflammation in various neurodegenerative disorders, including Alzheimer’s and Parkinson’s diseases.

Bioactive Lipid Mediators in the Initiation and Resolution of Inflammation after Spinal Cord Injury

Neuroinflammation is a prominent feature of the response to CNS trauma. It is also an important hallmark of various neurodegenerative diseases in which inflammation contributes to the progression of pathology. Inflammation in the CNS can contribute to secondary damage and is therefore an excellent therapeutic target for a range of neurological conditions. Inflammation in the nervous system is complex and varies in its fine details in different conditions. It involves a wide variety of secreted factors such as chemokines and cytokines, cell adhesion molecules, and different cell types that include resident cell of the CNS, as well as immune cells recruited from the peripheral circulation. Added to this complexity is the fact that some aspects of inflammation are beneficial, while other aspects can induce secondary damage in the acute, subacute and chronic phases. Understanding these aspects of the inflammatory profile is essential for developing effective therapies. Bioactive lipids constitute a large group of molecules that modulate the initiation and the resolution of inflammation. Dysregulation of these bioactive lipid pathways can lead to excessive acute inflammation, and failure to resolve this by specialized pro-resolution lipid mediators can lead to the development of chronic inflammation. The focus of this review is to discuss the effects of bioactive lipids in spinal cord trauma and their potential for therapies.

Assessing Microglial Dynamics by Live Imaging

Microglia are highly dynamic in the brain in terms of their ability to migrate, proliferate, and phagocytose over the course of an individual's life. Real-time imaging is a useful tool to examine how microglial behavior is regulated and how it affects the surrounding environment. However, microglia are sensitive to environmental stimuli, so they possibly change their state during live imaging in vivo, mainly due to surgical damage, and in vitro due to various effects associated with culture conditions. Therefore, it is difficult to perform live imaging without compromising the properties of the microglia under physiological conditions. To overcome this barrier, various experimental conditions have been developed; recently, it has become possible to perform live imaging of so-called surveillant microglia in vivo, ex vivo, and in vitro, although there are various limitations. Now, we can choose in vivo, ex vivo, or in vitro live imaging systems according to the research objective. In this review, we discuss the advantages and disadvantages of each experimental system and outline the physiological significance and molecular mechanisms of microglial behavior that have been elucidated by live imaging.

A Novel 1,8-Naphthyridine-2-Carboxamide Derivative Attenuates Inflammatory Responses and Cell Migration in LPS-Treated BV2 Cells via the Suppression of ROS Generation and TLR4/Myd88/NF-κB Signaling Pathway

Novel 1,8-naphthyridine-2-carboxamide derivatives with various substituents (HSR2101-HSR2113) were synthesized and evaluated for their effects on the production of pro-inflammatory mediators and cell migration in lipopolysaccharide (LPS)-treated BV2 microglial cells. Among the tested compounds, HSR2104 exhibited the most potent inhibitory effects on the LPS-stimulated production of inflammatory mediators, including nitric oxide (NO), tumor necrosis factor-α, and interleukin-6. Therefore, this compound was chosen for further investigation. We found that HSR2104 attenuated levels of inducible NO synthase and cyclooxygenase 2 in LPS-treated BV2 cells. In addition, it markedly suppressed LPS-induced cell migration as well as the generation of intracellular reactive oxygen species (ROS). Moreover, HSR2104 abated the LPS-triggered nuclear translocation of nuclear factor-κB (NF-κB) through inhibition of inhibitor kappa Bα phosphorylation. Furthermore, it reduced the expressions of Toll-like receptor 4 (TLR4) and myeloid differentiation factor 88 (MyD88) in LPS-treated BV2 cells. Similar results were observed with TAK242, a specific inhibitor of TLR4, suggesting that TLR4 is an upstream regulator of NF-κB signaling in BV2 cells. Collectively, our findings demonstrate that HSR2104 exhibits anti-inflammatory and anti-migratory activities in LPS-treated BV2 cells via the suppression of ROS and TLR4/MyD88/NF-κB signaling pathway. Based on our observations, HSR2104 may have a beneficial impact on inflammatory responses and microglial cell migration involved in the pathogenesis of various neurodegenerative disorders.

Long-term environmental enrichment reduces microglia morphological diversity of the molecular layer of dentate gyrus

We investigated long-term environmental influences on morphology of microglia from the outer and middle thirds of molecular layer of the dentate gyrus (MolDG), and on microglia from dorsal and ventral dentate gyrus molecular layer. We also estimated the total number of MolDG microglia using stereology. For this purpose, microglia of the molecular layer of the dentate gyrus of 20-month-old female Swiss albino mice, housed from 21st postnatal day onwards, in the impoverished environment of the standard laboratory cages (SEA), or in a cage with an enriched environment (EEA), were reconstructed microscopically in three dimensions and compared with each other and with microglia of 6-month-old female Swiss albino mice, also housed from weaning onwards in an enriched cage (EEY). All mice had their brains sectioned and processed for immunolabeling for IBA-1, a selective microglia marker. Random and systematic microglia samples were reconstructed in three dimensions and classified morphologically using hierarchical cluster analysis, followed by discriminant function analysis. SEA and EEY showed two morphological phenotypes of microglia in both the outer and middle thirds of MolDG. EEA mice showed such a reduction in the morphological diversity of microglia that essentially a single morphotype was found. EEA mouse microglia showed an intermediate morphological complexity between types I and II SE microglia. We suggest that type I and type II microglia in SE mice may have different physiological roles and that long-term EE may be associated with adaptive responses of microglial phenotypes to somatomotor and cognitive stimuli.

Nanoscale Surveillance of the Brain by Microglia via cAMP-Regulated Filopodia

Microglia, the brain's immune cells, maintain homeostasis and sense pathological changes by continuously surveying the parenchyma with highly motile large processes. Here, we demonstrate that microglia also use thin actin-dependent filopodia that allow fast nanoscale sensing within discrete regions. Filopodia are distinct from large processes by their size, speed, and regulation mechanism. Increasing cyclic AMP (cAMP) by activating norepinephrine G_s-coupled receptors, applying nitric oxide, or inhibiting phosphodiesterases rapidly increases filopodia but collapses large processes. Alternatively, G_i-coupled P2Y12 receptor activation collapses filopodia but triggers large processes extension with bulbous tips. Similar control of cytoskeletal dynamics and microglial morphology by cAMP is observed in ramified primary microglia, suggesting that filopodia are intrinsically generated sensing structures. Therefore, nanoscale surveillance of brain parenchyma by microglia requires localized cAMP increases that drive filopodia formation. Shifting intracellular cAMP levels controls the polarity of microglial responses to changes in brain homeostasis and alters the scale of immunosurveillance.

Extracellular Vesicles Can Deliver Anti-inflammatory and Anti-scarring Activities of Mesenchymal Stromal Cells After Spinal Cord Injury

Spinal cord injury is characterized by initial neural tissue disruption that triggers secondary damage and extensive non-resolving inflammation, which aggravates loss of function and hinders recovery. The early onset of inflammation following traumatic spinal cord injury underscores the importance of acute intervention after the initial trauma. Injections of mesenchymal stromal cells (MSCs) can reduce inflammation following spinal cord injury. We asked if extracellular vesicles (EVs) can substitute the anti-inflammatory and anti-scarring activities of their parental MSCs in a rat model of contusion spinal cord injury. We report that MSC-EVs were as potent as the parental intact cells in reducing the level of neuroinflammation for up to 2 weeks post-injury. Acute application of EVs after spinal cord injury was shown to robustly decrease the expression of pro-inflammatory cytokines in the spinal cord parenchyma in the very early phase of secondary damage. Moreover, the anti-scarring impact of MSC-EVs was even more efficient than the parental cells. We therefore conclude that anti-inflammatory and anti-scarring activities of MSC application can be mediated by their secreted EVs. In light of their substantial safety and druggability advantages, EVs may have a high potential in early therapeutic treatment following traumatic spinal cord injury.

Intravital imaging of immune cells and their interactions with other cell types in the spinal cord: Experiments with multicolored moving cells

Intravital imaging of the immune system is a powerful technique for studying biology of the immune response in the spinal cord using a variety of disease models ranging from traumatic injury to autoimmune disorders. Here, we will discuss specific technical aspects as well as many intriguing biological phenomena that have been revealed with the use of intravital imaging for investigation of the immune system in the spinal cord. We will discuss surgical techniques for exposing and stabilizing the spine that are critical for obtaining images, visualizing immune and CNS cells with genetically expressed fluorescent proteins, fluorescent labeling techniques and briefly discuss some of the challenges of image analysis.

Phagocytosis of Apoptotic Cells Is Specifically Upregulated in ApoE4 Expressing Microglia in vitro

Alzheimer’s disease (AD) is characterized by intracellular tau aggregates and extracellular deposition of amyloid-β (Aβ). The major genetic risk factor to develop AD is the Apolipoprotein E isoform 4 (ApoE4). ApoE4 may directly affect Aβ pathology, yet the exact role of ApoE4 in the progression of AD remains unclear. Although astrocytes are the main source of ApoE in brain tissue, other cell types might contribute to ApoE isotype-dependent effects. While ApoE expression does not play a relevant role in homeostatic microglia, we and others could recently show that ApoE expression is significant upregulated in disease-associated microglia including AD-mouse models and human AD. ApoE has been supposed to have an anti-inflammatory effect, with ApoE4 being less effective than ApoE3. However, ApoE-isotype specific effects on microglia function in disease have not been thoroughly investigated to date. In contrast to this, the role of ApoE2, the third most common major ApoE isoform, in neurodegeneration has not been characterized in detail, but it has been shown to delay the onset of disease in familial AD. To elucidate the differential roles of the three-major human ApoE isoforms on microglia function we each expressed the human ApoE isoforms in murine N9 microglia cells. We could show that ApoE4 specifically influences actin cytoskeleton rearrangement and morphology. In migration assays, ApoE4 significantly promotes cell motility. To quantify phagocytosis by microglia we established an uptake assay based on imaging flow cytometry. Although expression of ApoE4 led to significantly reduced uptake of Aβ in contrast to the other isoforms, we could show that ApoE4 specifically increased phagocytosis of apoptotic neuronal cells. Our findings show that ApoE4 intrinsically affects microglia physiology by upregulating motility and phagocytic behavior in vitro and may therefore specifically contribute to microglia dysregulation in AD.

Transplantation of Microglia in the Area of Spinal Cord Injury in an Acute Period Increases Tissue Sparing, but Not Functional Recovery

Microglial cells are known as important mediators of inflammation and immune response in the central nervous system (CNS). However, a neuroprotective role of these cells in post-traumatic processes should not be overlooked. Microglial cells are the first to respond to CNS injury and are further involved in all critical events of pathogenesis. When activated microglia clear the cellular debris and release anti- and proinflammatory cytokines and chemokines, nitric oxide, neurotrophins, and antioxidants capable of producing both neurotoxic and neuroprotective effects. The aim of this study was to determine to what extent the phagocytic activity of microglia in an acute period of spinal cord injury (SCI) in rats can effect the post-traumatic processes. For this purpose we implanted genetically modified Ad5-EGFP or Ad5-GDNF microglial cells into the area of acute SCI. Our experiments demonstrate that the area of intact tissue was lower in the group transplanted with Ad5-GDNF-transduced microglial cells with reduced phagocytic activity than that in the group of animals transplanted with Ad5-EGFP-transduced microglia cells which did not affect the cell activity. At the same time, there was no significant difference in the functional recovery index between these groups. Thus, the increased number of microglia cells with good phagocytic activity in the area of acute SCI may contribute to the improved nervous tissue integrity without a significant effect on the functional recovery within 30 days after injury.

Bidirectional Microglia-Neuron Communication in Health and Disease

Microglia are ramified cells that exhibit highly motile processes, which continuously survey the brain parenchyma and react to any insult to the CNS homeostasis. Although microglia have long been recognized as a crucial player in generating and maintaining inflammatory responses in the CNS, now it has become clear, that their function are much more diverse, particularly in the healthy brain. The innate immune response and phagocytosis represent only a little segment of microglia functional repertoire that also includes maintenance of biochemical homeostasis, neuronal circuit maturation during development and experience-dependent remodeling of neuronal circuits in the adult brain. Being equipped by numerous receptors and cell surface molecules microglia can perform bidirectional interactions with other cell types in the CNS. There is accumulating evidence showing that neurons inform microglia about their status and thus are capable of controlling microglial activation and motility while microglia also modulate neuronal activities. This review addresses the topic: how microglia communicate with other cell types in the brain, including fractalkine signaling, secreted soluble factors and extracellular vesicles. We summarize the current state of knowledge of physiological role and function of microglia during brain development and in the mature brain and further highlight microglial contribution to brain pathologies such as Alzheimer’s and Parkinson’s disease, brain ischemia, traumatic brain injury, brain tumor as well as neuropsychiatric diseases (depression, bipolar disorder, and schizophrenia).

Microenvironment Imbalance of Spinal Cord Injury

Spinal cord injury (SCI), for which there currently is no cure, is a heavy burden on patient physiology and psychology. The microenvironment of the injured spinal cord is complicated. According to our previous work and the advancements in SCI research, ‘microenvironment imbalance’ is the main cause of the poor regeneration and recovery of SCI. Microenvironment imbalance is defined as an increase in inhibitory factors and decrease in promoting factors for tissues, cells and molecules at different times and spaces. There are imbalance of hemorrhage and ischemia, glial scar formation, demyelination and re-myelination at the tissue’s level. The cellular level imbalance involves an imbalance in the differentiation of endogenous stem cells and the transformation phenotypes of microglia and macrophages. The molecular level includes an imbalance of neurotrophic factors and their pro-peptides, cytokines, and chemokines. The imbalanced microenvironment of the spinal cord impairs regeneration and functional recovery. This review will aid in the understanding of the pathological processes involved in and the development of comprehensive treatments for SCI.

Axonal regeneration in zebrafish spinal cord

In the present review we discuss two interrelated events-axonal damage and repair-known to occur after spinal cord injury (SCI) in the zebrafish. Adult zebrafish are capable of regenerating axonal tracts and can restore full functionality after SCI. Unlike fish, axon regeneration in the adult mammalian central nervous system is extremely limited. As a consequence of an injury there is very little repair of disengaged axons and therefore functional deficit persists after SCI in adult mammals. In contrast, peripheral nervous system axons readily regenerate following injury and hence allow functional recovery both in mammals and fish. A better mechanistic understanding of these three scenarios could provide a more comprehensive insight into the success or failure of axonal regeneration after SCI. This review summarizes the present understanding of the cellular and molecular basis of axonal regeneration, in both the peripheral nervous system and the central nervous system, and large scale gene expression analysis is used to focus on different events during regeneration. The discovery and identification of genes involved in zebrafish spinal cord regeneration and subsequent functional experimentation will provide more insight into the endogenous mechanism of myelination and remyelination. Furthermore, precise knowledge of the mechanism underlying the extraordinary axonal regeneration process in zebrafish will also allow us to unravel the potential therapeutic strategies to be implemented for enhancing regrowth and remyelination of axons in mammals.

Effects of the ecto-ATPase apyrase on microglial ramification and surveillance reflect cell depolarization, not ATP depletion

Microglia, the brain's innate immune cells, have highly motile processes which constantly survey the brain to detect infection, remove dying cells, and prune synapses during brain development. ATP released by tissue damage is known to attract microglial processes, but it is controversial whether an ambient level of ATP is needed to promote constant microglial surveillance in the normal brain. Applying the ATPase apyrase, an enzyme which hydrolyzes ATP and ADP, reduces microglial process ramification and surveillance, suggesting that ambient ATP/ADP maintains microglial surveillance. However, attempting to raise the level of ATP/ADP by blocking the endogenous ecto-ATPase (termed NTPDase1/CD39), which also hydrolyzes ATP/ADP, does not affect the cells' ramification or surveillance, nor their membrane currents, which respond to even small rises of extracellular [ATP] or [ADP] with the activation of K+ channels. This indicates a lack of detectable ambient ATP/ADP and ecto-ATPase activity, contradicting the results with apyrase. We resolve this contradiction by demonstrating that contamination of commercially available apyrase by a high K+ concentration reduces ramification and surveillance by depolarizing microglia. Exposure to the same K+ concentration (without apyrase added) reduced ramification and surveillance as with apyrase. Dialysis of apyrase to remove K+ retained its ATP-hydrolyzing activity but abolished the microglial depolarization and decrease of ramification produced by the undialyzed enzyme. Thus, applying apyrase affects microglia by an action independent of ATP, and no ambient purinergic signaling is required to maintain microglial ramification and surveillance. These results also have implications for hundreds of prior studies that employed apyrase to hydrolyze ATP/ADP.

In vivo two-photon imaging of motoneurons and adjacent glia in the ventral spinal cord

BACKGROUND

Interactions between motoneurons and glial cells are pivotal to regulate and maintain functional states and synaptic connectivity in the spinal cord. In vivo two-photon imaging of the nervous system provided novel and unexpected knowledge about structural and physiological changes in the grey matter of the forebrain and in the dorsal white matter of the spinal cord.

NEW METHOD

Here, we describe a novel experimental strategy to investigate the spinal grey matter, i.e. the ventral horn motoneurons and their adjacent glial cells by employing in vivo two-photon laser-scanning microscopy (2P-LSM) in anesthetized transgenic mice.

RESULTS

After retrograde tracer labelling in transgenic mice with cell-specific expression of fluorescent proteins and surgical exposure of the lumbar intumescence groups of motoneurons could be visualized deeply localized in the ventral horn. In this region, morphological responses of microglial cells to ATP could be recorded for an hour. In addition, using in mice with expression of GCaMP3 in astrocytes, physiological Ca²⁺ signals could be recorded after local noradrenalin application.

COMPARISON WITH EXISTING METHODS

Previous in vivo imaging protocols were restricted to the superficial dorsal white matter or upper layers of the dorsal horn. Here, we modified a multi-step procedure originally established for a root-crush injury. We adapted it to simultaneously visualize motoneurons and adjacent glial cells in living animals.

CONCLUSION

A modified surgery approach is presented to visualize fluorescently labelled motoneurons and glial cells at a depth of more than 200 μm in the grey matter ventral horn of the mouse spinal cord.

The microglial activation profile and associated factors after experimental spinal cord injury in rats

BACKGROUND

Spinal cord injury (SCI) has imposed a great impact on the quality of life of patients due to its relatively young age of onset. The pathophysiology of SCI has been proven to be complicated. Microglia plays an important role in neuroinflammation and second injuries after SCI. Different environment and other factors may determine the microglial activation profile and what role they play. However, neither accurate time-course profiles of microglial activation nor influence factors have been demonstrated in varied SCI models.

METHODS

A rat compressive SCI model was used. Microglial activation profile and contents of inflammatory factors including IL-1β, IL-6 and TNF-α were detected. Myelination status as well as levels of iron and glutamate concentration, adenosine triphosphate (ATP) and potassium are also assessed.

RESULTS

Our results showed that the activated microglia participating in immune-mediated responses peaked at day 7 post SCI and gradually decreased during the following 3 weeks. Contrarily, myelination and oligodendroglia showed an opposite trend, indicating that microglia may be a key factor partly through inflammatory reaction. Iron and glutamate concentration were found to be the highest at day 7 after SCI while both ATP and potassium reached a low valley at the same time.

CONCLUSION

These findings showed a microglial activation profile and the alterations of associated factors after experiment SCI model. Moreover, our data suggest that high iron and glutamate concentration may be released by damaged oligodendroglia and contribute to the activation of microglial after SCI.

The in situ morphology of microglia is highly sensitive to the mode of tissue fixation

Microglia are known as the most motile cells in the central nervous system (CNS). It was shown in vivo that they permanently scan their direct microenvironment and react to pathological conditions within minutes. Many studies of brain pathologies use fixed brain tissue to investigate cellular changes. Unfortunately, due to technical reasons, the time span between the induction of the fixation procedure (start of the perfusion) and the finally-fixed tissue lasts several minutes, giving time to microglia to start reacting to the ischemic conditions due to perfusion start. Here, we investigated the microglial changes generated by the fixation itself in TgH(CX3CR1-EGFP) mice with fluorescent labelled microglia using confocal laser scanning microscopy (CLSM) of fixed brain tissue as well as two-photon laser scanning microscopy (2P-LSM) during the perfusion of a living animal. We revealed the impact of fixation and buffer parameters on cell morphology. The largest morphological differences compared to physiological in vivo branch arborization were observed when the directly dissected brain was immersed in paraformaldehyde fixation solution overnight, without prior fixative perfusion of the animal. But even perfusion with a fixative, followed by post-fixation leads to small changes in microglial process length and number and could not be prevented when compared to physiological in vivo microglia morphology acquired using in vivo 2P-LSM. Interestingly, perfusion with different buffers either oxygenated artificial cerebrospinal fluid or phosphate buffered saline prior to perfusion-fixation showed minor microglia changes in arborization and/or number of processes. Fixation methods influence microglia morphology. Therefore, to define microglia activation states immunohistochemical stainings or genetic labelling of the cells have to be included in addition to morphological analysis.

In vivo recording of nerve conduction velocity of spinal CNS fibers in the mouse

Anesthetic and surgical procedures and an electrophysiological method were developed for recording nerve conduction velocity (NCV) of CNS fibers in the murine spinal cord. Under intravenous anesthesia and artificial ventilation the lumbar spinal cord segments L1 to L4 and dorsal roots L3 to L5 on the left side were exposed by laminectomy. After stimulation of the dorsal root L4, a compound action potential (CAP) was recorded at the ipsilateral left fasciculus gracilis at the spinal cord level L1. The latency from stimulation to the CAP together with the measured distance between the electrodes was used for the determination of the NCV. NCV of the fastest fibers in the fasciculus gracilis was observed to be approximately 28 m/s. Reversible decrease of the NCV was measured, in vivo, under general hypothermia. The technique described serves for in vivo electrophysiological investigations of spinal central fibers in wildtype and mutant mice.

Two-photon laser-scanning microscopy for single and repetitive imaging of dorsal and lateral spinal white matter in vivo

We developed appropriate surgical procedures for single and repetitive multi-photon imaging of spinal cord in vivo. By intravenous anesthesia, artificial ventilation and laminectomy, acute experiments were performed in the dorsal and lateral white matter. By volatile anesthesia and minimal-invasive surgery, chronic repetitive imaging up to 8 months were performed in the dorsal column through the window between two adjacent spines. Transgenic mouse technology enabled simultaneous imaging of labeled axons, astrocytes and microglia. Repetitive imaging showed positional shifts of microglia over time. These techniques serve for investigations of cellular dynamics and cell-cell interactions in intact and pathologically changed spinal tissue.

Rapid neuroinflammatory response localized to injured neurons after diffuse traumatic brain injury in swine

Despite increasing appreciation of the critical role that neuroinflammatory pathways play in brain injury and neurodegeneration, little is known about acute microglial reactivity following diffuse traumatic brain injury (TBI) - the most common clinical presentation that includes all concussions. Therefore, we investigated acute microglial reactivity using a porcine model of closed-head rotational velocity/acceleration-induced TBI that closely mimics the biomechanical etiology of inertial TBI in humans. We observed rapid microglial reactivity within 15min of both mild and severe TBI. Strikingly, microglial activation was restrained to regions proximal to individual injured neurons - as denoted by trauma-induced plasma membrane disruption - which served as epicenters of acute reactivity. Single-cell quantitative analysis showed that in areas free of traumatically permeabilized neurons, microglial density and morphology were similar between sham or following mild or severe TBI. However, microglia density increased and morphology shifted to become more reactive in proximity to injured neurons. Microglial reactivity around injured neurons was exacerbated following repetitive TBI, suggesting further amplification of acute neuroinflammatory responses. These results indicate that neuronal trauma rapidly activates microglia in a highly localized manner, and suggest that activated microglia may rapidly influence neuronal stability and/or pathophysiology after diffuse TBI.

Microglial P2Y12 receptors regulate microglial activation and surveillance during neuropathic pain

Microglial cells are critical in the pathogenesis of neuropathic pain and several microglial receptors have been proposed to mediate this process. Of these receptors, the P2Y12 receptor is a unique purinergic receptor that is exclusively expressed by microglia in the central nervous system (CNS). In this study, we set forth to investigate the role of P2Y12 receptors in microglial electrophysiological and morphological (static and dynamic) activation during spinal nerve transection (SNT)-induced neuropathic pain in mice. First, we found that a genetic deficiency of the P2Y12 receptor (P2Y12(-/-) mice) ameliorated pain hypersensitivities during the initiation phase of neuropathic pain. Next, we characterised both the electrophysiological and morphological properties of microglia in the superficial spinal cord dorsal horn following SNT injury. We show dramatic alterations including a peak at 3days post injury in microglial electrophysiology while high resolution two-photon imaging revealed significant changes of both static and dynamic microglial morphological properties by 7days post injury. Finally, in P2Y12(-/-) mice, these electrophysiological and morphological changes were ameliorated suggesting roles for P2Y12 receptors in SNT-induced microglial activation. Our results therefore indicate that P2Y12 receptors regulate microglial electrophysiological as well as static and dynamic microglial properties after peripheral nerve injury, suggesting that the microglial P2Y12 receptor could be a potential therapeutic target for the treatment of neuropathic pain.

Microglia and monocytes synergistically promote the transition from acute to chronic pain after nerve injury

Microglia and peripheral monocytes contribute to hypersensitivity in rodent models of neuropathic pain. However, the precise respective function of microglia and peripheral monocytes has not been investigated in these models. To address this question, here we combined transgenic mice and pharmacological tools to specifically and temporally control the depletion of microglia and monocytes in a mouse model of spinal nerve transection (SNT). We found that although microglia and monocytes are required during the initiation of mechanical allodynia or thermal hyperalgesia, these cells may not be as important for the maintenance of hypersensitivity. Moreover, we demonstrated that either resident microglia or peripheral monocytes are sufficient in gating neuropathic pain after SNT. We propose that resident microglia and peripheral monocytes act synergistically to initiate hypersensitivity and promote the transition from acute to chronic pain after peripheral nerve injury.

Microglia and monocytes contribute to neuropathic pain states, but the precise role of the two cell types is not clear. Here Peng et al. use temporally controlled ablation of monocytes and microglia in mice to show that these cells work together to initiate neuropathic-pain like behaviour, but are less important in the maintenance phase.

Microglia-Neuron Communication in Epilepsy

Epilepsy has remained a significant social concern and financial burden globally. Current therapeutic strategies are based primarily on neurocentric mechanisms that have not proven successful in at least a third of patients, raising the need for novel alternative and complementary approaches. Recent evidence implicates glial cells and neuroinflammation in the pathogenesis of epilepsy with the promise of targeting these cells to complement existing strategies. Specifically, microglial involvement, as a major inflammatory cell in the epileptic brain, has been poorly studied. In this review, we highlight microglial reaction to experimental seizures, discuss microglial control of neuronal activities, and propose the functions of microglia during acute epileptic phenotypes, delayed neurodegeneration, and aberrant neurogenesis. Future research that would help fill in the current gaps in our knowledge includes epilepsy-induced alterations in basic microglial functions, neuro-microglial interactions during chronic epilepsy, and microglial contribution to developmental seizures. Studying the role of microglia in epilepsy could inform therapies to better alleviate the disease. GLIA 2016;65:5-18.

Positively Charged Oligo[Poly(Ethylene Glycol) Fumarate] Scaffold Implantation Results in a Permissive Lesion Environment after Spinal Cord Injury in Rat

Positively charged oligo[poly(ethylene glycol) fumarate] (OPF+) scaffolds loaded with Schwann cells bridge spinal cord injury (SCI) lesions and support axonal regeneration in rat. The regeneration achieved is not sufficient for inducing functional recovery. Attempts to increase regeneration would benefit from understanding the effects of the scaffold and transplanted cells on lesion environment. We conducted morphometric and stereological analysis of lesions in rats implanted with OPF+ scaffolds with or without loaded Schwann cells 1, 2, 3, 4, and 8 weeks after thoracic spinal cord transection. No differences were found in collagen scarring, cyst formation, astrocyte reactivity, myelin debris, or chondroitin sulfate proteoglycan (CSPG) accumulation. However, when scaffold-implanted animals were compared with animals with transection injuries only, these barriers to regeneration were significantly reduced, accompanied by increased activated macrophages/microglia. This distinctive and regeneration permissive tissue reaction to scaffold implantation was independent of Schwann cell transplantation. Although the tissue reaction was beneficial in the short term, we observed a chronic fibrotic host response, resulting in scaffolds surrounded by collagen at 8 weeks. This study demonstrates that an appropriate biomaterial scaffold improves the environment for regeneration. Future targeting of the host fibrotic response may allow increased axonal regeneration and functional recovery.

Microglial migration and interactions with dendrimer nanoparticles are altered in the presence of neuroinflammation

Background

Microglial cells have been implicated in neuroinflammation-mediated injury in the brain, including neurodevelopmental disorders such as cerebral palsy (CP) and autism. Pro-inflammatory activation of microglial cells results in the impairment of their neuroprotective functions, leading to an exaggerated, ongoing immune dysregulation that can persist long after the initial insult. We have previously shown that dendrimer-mediated delivery of an anti-inflammatory agent can attenuate inflammation in a rabbit model of maternal inflammation-induced CP and significantly improve the motor phenotype, due to the ability of the dendrimer to selectively localize in activated microglia.

Methods

To elucidate the interactions between dendrimers and microglia, we created an organotypic whole-hemisphere brain slice culture model from newborn rabbits with and without exposure to inflammation in utero. We then used this model to analyze the dynamics of microglial migration and their interactions with dendrimers in the presence of neuroinflammation.

Results

Microglial cells in animals with CP had an amoeboid morphology and impaired cell migration, demonstrated by decreased migration distance and velocity when compared to cells in healthy, age-matched controls. However, this decreased migration was associated with a greater, more rapid dendrimer uptake compared to microglial cells from healthy controls.

Conclusions

This study demonstrates that maternal intrauterine inflammation is associated with impaired microglial function and movement in the newborn brain. This microglial impairment may play a role in the development of ongoing brain injury and CP in the offspring. Increased uptake of dendrimers by the “impaired” microglia can be exploited to deliver drugs specifically to these cells and modulate their functions. Host tissue and target cell characteristics are important aspects to be considered in the design and evaluation of targeted dendrimer-based nanotherapeutics for improved and sustained efficacy. This ex vivo model also provides a rapid screening tool for evaluation of the effects of various therapies on microglial function.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0529-3) contains supplementary material, which is available to authorized users.

Evaluation of TgH(CX3CR1-EGFP) mice implanted with mCherry-GL261 cells as an in vivo model for morphometrical analysis of glioma-microglia interaction

Background

Glioblastoma multiforme is the most aggressive brain tumor. Microglia are prominent cells within glioma tissue and play important roles in tumor biology. This work presents an animal model designed for the study of microglial cell morphology in situ during gliomagenesis. It also allows a quantitative morphometrical analysis of microglial cells during their activation by glioma cells.

Methods

The animal model associates the following cell types: 1- mCherry red fluorescent GL261 glioma cells and; 2- EGFP fluorescent microglia, present in the TgH(CX3CR1-EGFP) mouse line. First, mCherry-GL261 glioma cells were implanted in the brain cortex of TgH(CX3CR1-EGFP) mice. Epifluorescence − and confocal laser-scanning microscopy were employed for analysis of fixed tissue sections, whereas two-photon laser-scanning microscopy (2P-LSM) was used to track tumor cells and microglia in the brain of living animals.

Results

Implanted mCherry-GL261 cells successfully developed brain tumors. They mimic the aggressive behavior found in human disease, with a rapid increase in size and the presence of secondary tumors apart from the injection site. As tumor grows, mCherry-GL261 cells progressively lost their original shape, adopting a heterogeneous and diffuse morphology at 14–18 d. Soma size increased from 10–52 μm. At this point, we focused on the kinetics of microglial access to glioma tissues. 2P-LSM revealed an intense microgliosis in brain areas already shortly after tumor implantation, i.e. at 30 min. By confocal microscopy, we found clusters of microglial cells around the tumor mass in the first 3 days. Then cells infiltrated the tumor area, where they remained during all the time points studied, from 6–18 days. Microglia in contact with glioma cells also present changes in cell morphology, from a ramified to an amoeboid shape. Cell bodies enlarged from 366 ± 0.0 μm², in quiescent microglia, to 1310 ± 146.0 μm², and the cell processes became shortened.

Conclusions

The GL261/CX3CR1 mouse model reported here is a valuable tool for imaging of microglial cells during glioma growth, either in fixed tissue sections or living animals. Remarkable advantages are the use of immunocompetent animals and the simplified imaging method without the need of immunohistochemical procedures.

Microglia in central nervous system repair after injury

Accumulating evidence suggests that immune cells perform crucial inflammation-related functions including clearing dead tissue and promoting wound healing. Thus, they provide a conducive environment for better neuronal regeneration and functional recovery after adult mammalian central nervous system (CNS) injury. However, activated immune cells can also induce secondary damage of intact tissue and inhibit post-injury CNS repair. The inflammation response is due to the microglial production of cytokines and chemokines for the recruitment of peripheral immune cell populations, such as monocytes, neutrophils, dendritic cells and T lymphocytes. Interestingly, microglia and T lymphocytes can be detected at the injured site in both the early and later stages after nerve injury, whereas other peripheral immune cells infiltrate the injured parenchyma of the brain and spinal cord only in the early post-injury phase, and subsequently disappear. This suggests that microglia and T cells may play crucial roles in the post-injury functional recovery of the CNS. In this review, we summarize the current studies on microglia that examined neuronal regeneration and the molecular signalling mechanisms in the injured CNS. Better understanding of the effects of microglia on neural regeneration will aid the development of therapy strategies to enhance CNS functional recovery after injury.

Deletion of Nuclear Factor kappa B p50 Subunit Decreases Inflammatory Response and Mildly Protects Neurons from Transient Forebrain Ischemia-induced Damage

Transient forebrain ischemia induces delayed death of the hippocampal pyramidal neurons, particularly in the CA2 and medial CA1 area. Early pharmacological inhibition of inflammatory response can ameliorate neuronal death, but it also inhibits processes leading to tissue regeneration. Therefore, research efforts are now directed to modulation of post-ischemic inflammation, with the aim to promote beneficial effects of inflammation and limit adverse effects. Transcription factor NF-κB plays a key role in the inflammation and cell survival/apoptosis pathways. In the brain, NF-κB is predominantly found in the form of a heterodimer of p65 (RelA) and p50 subunit, where p65 has a transactivation domain while p50 is chiefly involved in DNA binding. In this study, we subjected middle-aged Nfkb1 knockout mice (lacking p50 subunit) and wild-type controls of both sexs to 17 min of transient forebrain ischemia and assessed mouse performance in a panel of behavioral tests after two weeks of post-operative recovery. We found that ischemia failed to induce clear memory and motor deficits, but affected spontaneous locomotion in genotype- and sex-specific way. We also show that both the lack of the NF-κB p50 subunit and female sex independently protected CA2 hippocampal neurons from ischemia-induced cell death. Additionally, the NF-κB p50 subunit deficiency significantly reduced ischemia-induced microgliosis, astrogliosis, and neurogenesis. Lower levels of hippocampal microgliosis significantly correlated with faster spatial learning. We conclude that NF-κB regulates the outcome of transient forebrain ischemia in middle-aged subjects in a sex-specific way, having an impact not only on neuronal death but also specific inflammatory responses and neurogenesis.

Methylene blue exerts a neuroprotective effect against traumatic brain injury by promoting autophagy and inhibiting microglial activation

Traumatic brain injury (TBI) leads to permanent neurological impairment, and methylene blue (MB) exerts central nervous system neuroprotective effects. However, only one previous study has investigated the effectiveness of MB in a controlled cortical impact injury model of TBI. In addition, the specific mechanisms underlying the effect of MB against TBI remain to be elucidated. Therefore, the present study investigated the neuroprotective effect of MB on TBI and the possible mechanisms involved. In a mouse model of TBI, the animals were randomly divided into sham, vehicle (normal saline) or MB groups. The treatment time-points were 24 and 72 h (acute phase of TBI), and 14 days (chronic phase of TBI) post-TBI. The brain water content (BWC), and levels of neuronal death, and autophagy were determined during the acute phase, and neurological deficit, injury volume and microglial activation were assessed at all time-points. The injured hemisphere BWC was significantly increased 24 h post-TBI, and this was attenuated following treatment with MB. There was a significantly higher number of surviving neurons in the MB group, compared with the Vehicle group at 24 and 72 h post-TBI. In the acute phase, the MB-treated animals exhibited significantly upregulated expression of Beclin 1 and increased LC3-II to LC3-I ratios, compared with the vehicle group, indicating an increased rate of autophagy. Neurological functional deficits, measured using the modified neurological severity score, were significantly lower in the acute phase in the MB-treated animals and cerebral lesion volumes in the MB-treated animals were significantly lower, compared with the other groups at all time-points. Microglia were activated 24 h after TBI, peaked at 72 h and persisted until 14 days after TBI. Although the number of Iba-1-positive cells in the vehicle and MB groups 24 h post-TBI were not significantly different, marked microglial inhibition was observed in the MB group 72 h and 14 days after-TBI. These results indicated that MB exerts a neuroprotective effect by increasing autophagy, decreasing brain edema and inhibiting microglial activation.

Extracellular ATP and other nucleotides-ubiquitous triggers of intercellular messenger release

Extracellular nucleotides, and ATP in particular, are cellular signal substances involved in the control of numerous (patho)physiological mechanisms. They provoke nucleotide receptor-mediated mechanisms in select target cells. But nucleotides can considerably expand their range of action. They function as primary messengers in intercellular communication by stimulating the release of other extracellular messenger substances. These in turn activate additional cellular mechanisms through their own receptors. While this applies also to other extracellular messengers, its omnipresence in the vertebrate organism is an outstanding feature of nucleotide signaling. Intercellular messenger substances released by nucleotides include neurotransmitters, hormones, growth factors, a considerable variety of other proteins including enzymes, numerous cytokines, lipid mediators, nitric oxide, and reactive oxygen species. Moreover, nucleotides activate or co-activate growth factor receptors. In the case of hormone release, the initially paracrine or autocrine nucleotide-mediated signal spreads through to the entire organism. The examples highlighted in this commentary suggest that acting as ubiquitous triggers of intercellular messenger release is one of the major functional roles of extracellular nucleotides. While initiation of messenger release by nucleotides has been unraveled in many contexts, it may have been overlooked in others. It can be anticipated that additional nucleotide-driven messenger functions will be uncovered with relevance for both understanding physiology and development of therapy.