Mouse model for ablation of proliferating microgliain acute CNS injuries

Chronically activated microglia in ALS gradually lose their immune functions and develop unconventional proteome

Neuroinflammation and chronic activation of microglial cells are the prominent features of amyotrophic lateral sclerosis (ALS) pathology. While alterations in the mRNA profile of diseased microglia have been well documented, the actual microglia proteome remains poorly characterized. Here we performed a functional characterization together with proteome analyses of microglial cells at different stages of disease in the SOD1-G93A model of ALS. Functional analyses of microglia derived from the lumbar spinal cord of symptomatic mice revealed: (i) remarkably high mitotic index (close to 100% cells are Ki67+) (ii) significant decrease in phagocytic capacity when compared to age-matched control microglia, and (iii) diminished response to innate immune challenges in vitro and in vivo. Proteome analysis revealed a development of two distinct molecular signatures at early and advanced stages of disease. While at early stages of disease, we identified several proteins implicated in microglia immune functions such as GPNMB, HMBOX1, at advanced stages of disease microglia signature at protein level was characterized with a robust upregulation of several unconventional proteins including rootletin, major vaults proteins and STK38. Upregulation of GPNMB and rootletin has been also found in the spinal cord samples of sporadic ALS. Remarkably, the top biological functions of microglia, in particular in the advanced disease, were not related to immunity/immune response, but were highly enriched in terms linked to RNA metabolism. Together, our results suggest that, over the course of disease, chronically activated microglia develop unconventional protein signatures and gradually lose their immune identity ultimately turning into functionally inefficient immune cells.

Shared inflammatory glial cell signature after stab wound injury, revealed by spatial, temporal, and cell-type-specific profiling of the murine cerebral cortex

Traumatic brain injury leads to a highly orchestrated immune- and glial cell response partially responsible for long-lasting disability and the development of secondary neurodegenerative diseases. A holistic understanding of the mechanisms controlling the responses of specific cell types and their crosstalk is required to develop an efficient strategy for better regeneration. Here, we combine spatial and single-cell transcriptomics to chart the transcriptomic signature of the injured male murine cerebral cortex, and identify specific states of different glial cells contributing to this signature. Interestingly, distinct glial cells share a large fraction of injury-regulated genes, including inflammatory programs downstream of the innate immune-associated pathways Cxcr3 and Tlr1/2. Systemic manipulation of these pathways decreases the reactivity state of glial cells associated with poor regeneration. The functional relevance of the discovered shared signature of glial cells highlights the importance of our resource enabling comprehensive analysis of early events after brain injury.

Neutrophil biology in injuries and diseases of the central and peripheral nervous systems

The role of inflammation in nervous system injury and disease is attracting increased attention. Much of that research has focused on microglia in the central nervous system (CNS) and macrophages in the peripheral nervous system (PNS). Much less attention has been paid to the roles played by neutrophils. Neutrophils are part of the granulocyte subtype of myeloid cells. These cells, like macrophages, originate and differentiate in the bone marrow from which they enter the circulation. After tissue damage or infection, neutrophils are the first immune cells to infiltrate into tissues and are directed there by specific chemokines, which act on chemokine receptors on neutrophils. We have reviewed here the basic biology of these cells, including their differentiation, the types of granules they contain, the chemokines that act on them, the subpopulations of neutrophils that exist, and their functions. We also discuss tools available for identification and further study of neutrophils. We then turn to a review of what is known about the role of neutrophils in CNS and PNS diseases and injury, including stroke, Alzheimer's disease, multiple sclerosis, amyotrophic lateral sclerosis, spinal cord and traumatic brain injuries, CNS and PNS axon regeneration, and neuropathic pain. While in the past studies have focused on neutrophils deleterious effects, we will highlight new findings about their benefits. Studies on their actions should lead to identification of ways to modify neutrophil effects to improve health.

Strategies for Manipulating Microglia to Determine Their Role in the Healthy and Diseased Brain

Microglia are the specialized macrophages of the central nervous system and play an important role in neural circuit development, modulating neurotransmission, and maintaining brain homeostasis. Microglia in normal brain is quiescent and show ramified morphology with numerous branching processes. They constantly survey their surrounding microenvironment through the extension and retraction of their processes and interact with neurons, astrocytes, and blood vessels using these processes. Microglia respond quickly to any pathological event in the brain by assuming ameboid morphology devoid of branching processes and restore homeostasis. However, when there is chronic inflammation, microglia may lose their homeostatic functions and secrete various proinflammatory cytokines and mediators that initiate neural dysfunction and neurodegeneration. In this article, we review the role of microglia in the normal brain and in various pathological brain conditions, such as Alzheimer's disease and multiple sclerosis. We describe strategies to manipulate microglia, focusing on depletion, repopulation, and replacement, and we discuss their therapeutic potential.

Restorative therapy using microglial depletion and repopulation for central nervous system injuries and diseases

Microglia are important resident immune cells in the central nervous system (CNS) and play an important role in its development, homeostasis, and disease treatments. Activated microglia perform diverse functions in mouse models of CNS neurodegenerative diseases or deficits. In humans, microglia have been linked to various neurodegenerative diseases. Following brain or spinal cord injury, microglia express pro- and anti-inflammatory phenotypes at different stages of recovery. With the development of pharmacological and genetic tools for microglial depletion, studies have demonstrated that microglial depletion exerts both positive and negative effects in the treatment of CNS diseases. Notably, microglial depletion provides an empty niche that stimulates production of new microglia. Microglial depletion and repopulation can not only treat diseases by eliminating dysfunctional microglia but can also provide an indication of the molecular mechanisms of diseases. Although this approach has shown impressive results, its use is still in its infancy. In this review, we summarize the current pharmacological and genetic tools for microglial depletion and highlight recent advances in microglial repopulation therapy for the treatment and functional recovery of neurological diseases and deficits. Finally, we briefly discuss the therapeutic challenges and prospective uses of microglial repopulation therapy.

Nogo receptor impairs the clearance of fibril amyloid-β by microglia and accelerates Alzheimer's-like disease progression

Alzheimer's disease (AD) is characterized by the progressive accumulation of β‐amyloid (Aβ)‐containing amyloid plaques, and microglia play a critical role in mediating Aβ clearance. Mounting evidence has confirmed that the ability of microglia in clearing Aβ decreased with aging and AD progress, but the underlying mechanisms are unclear. Previously, we have demonstrated that Nogo receptor (NgR), a receptor for three axon growth inhibitors associated with myelin, can decrease adhesion and migration of microglia to fibrils Aβ with aging. However, whether NgR expressed on microglia affect microglia phagocytosis of fibrils Aβ with aging remains unclear. Here, we found that aged but not young microglia showed increased NgR expression and decreased Aβ phagocytosis in APP/PS1 transgenic mice. NgR knockdown APP/PS1 mice showed simultaneous reduced amyloid burden and improved spatial learning and memory, which were associated with increased Aβ clearance. Importantly, Nogo‐P4, an agonist of NgR, enhanced the protein level of p‐Smad2/3, leading to a significant transcriptional inhibition of CD36 gene expression, which in turn decreased the microglial phagocytosis of Aβ. Moreover, ROCK accounted for Nogo‐P4‐induced activation of Smad2/3 signaling. Finally, the decreasing effect of NgR on microglial Aβ uptake was confirmed in a mouse model of intra‐hippocampal fAβ injection. Our findings suggest that NgR may play an important role in the regulation of Aβ homeostasis, and has potential as a therapeutic target for AD.

Should We Open Fire on Microglia? Depletion Models as Tools to Elucidate Microglial Role in Health and Alzheimer's Disease

Microglia play a critical role in both homeostasis and disease, displaying a wide variety in terms of density, functional markers and transcriptomic profiles along the different brain regions as well as under injury or pathological conditions, such as Alzheimer's disease (AD). The generation of reliable models to study into a dysfunctional microglia context could provide new knowledge towards the contribution of these cells in AD. In this work, we included an overview of different microglial depletion approaches. We also reported unpublished data from our genetic microglial depletion model, Cx3cr1CreER/Csf1rflx/flx, in which we temporally controlled microglia depletion by either intraperitoneal (acute model) or oral (chronic model) tamoxifen administration. Our results reported a clear microglial repopulation, then pointing out that our model would mimic a context of microglial replacement instead of microglial dysfunction. Next, we evaluated the origin and pattern of microglial repopulation. Additionally, we also reviewed previous works assessing the effects of microglial depletion in the progression of Aβ and Tau pathologies, where controversial data are found, probably due to the heterogeneous and time-varying microglial phenotypes observed in AD. Despite that, microglial depletion represents a promising tool to assess microglial role in AD and design therapeutic strategies.

Methamphetamine and sleep impairments: neurobehavioral correlates and molecular mechanisms

Methamphetamine is a potent and highly addictive psychostimulant, and one of the most widely used illicit drugs. Over recent years, its global usage and seizure have been on a rapid rise, with growing detrimental effects on mental and physical health, and devastating psychosocial impact pressing for intervention. Among the unwanted effects of methamphetamine, acute and long-term sleep impairments are of major concern, posing a significant therapeutic challenge, and a cause of addiction relapse. Unraveling mechanisms and functional correlates of methamphetamine-related sleep and circadian disruption are, therefore, of key relevance to translational and clinical psychiatry. In this article, we review the mounting evidence for the acute and long-term impairements of sleep-wake behavior and circadian activity caused by single or recurring methamphetamine usage and withdrawal. Factors contributing to the severity of sleep loss and related cognitive deficit, with risks of relapse are discussed. Key molecular players mediating methamphetamine-induced dopamine release and neuromodulation are considered, with wake-promoting effects in mesolimbic circuits. The effects on various sleep phases and related changes in dopamine levels in selected subcortical structures are reviewed and compared to other psychostimulants with similar action mechanisms. A critical appraisal is presented of the therapeutic use of modafinil, countering sleep, and circadian rhythm impairments. Finally, emerging knowledge gaps and methodical limitations are highlighted along with the areas for future research and therapeutic translation.

An improved TK-NOG mouse as a novel platform for humanized liver that overcomes limitations in both male and female animals

We developed a novel immunodeficient NOG mouse expressing HSVtk mutant clone 30 cDNA under the control of mouse transthyretin gene enhancer/promoter (NOG-TKm30) to acquire fertility in males and high inducibility of liver injury in females. Maximum human albumin levels (approx. 15 mg/mL plasma) in both male and female NOG-TKm30 mice engrafted with human hepatocytes (humanized liver mice) were observed 8-12 weeks after transplantation. Immunohistochemical analyses revealed abundant expression of major human cytochrome P450 (CYP) enzymes (CYP1A2, CYP2C9, CYP2D6, CYP2E1, and CYP3A4) in reconstituted liver with original zonal distribution. In vivo drug-drug interactions were observed in humanized liver mice as decreased area under the curve of midazolam (CYP3A4/5 substrate) and omeprazole (CYP3A4/5 and CYP2C19 substrate) after oral administration of rifampicin. Furthermore, we developed a pregnant model for evaluating prenatal exposure to drugs. The detection of thalidomide metabolites in the fetuses of pregnant humanized liver mice indicates that the novel TK model can be used for developmental toxicity studies requiring the assessment of human drug metabolism. These results suggest that the limitations of traditional TK-NOG mice can be addressed using NOG-TKm30 mice, which constitute a novel platform for humanized liver for both in vivo and in vitro studies.

Immunotherapies for Neurodegenerative Diseases

The current treatments for neurodegenerative diseases are mostly symptomatic without affecting the underlying cause of disease. Emerging evidence supports a potential role for immunotherapy in the management of disease progression. Numerous reports raise the exciting prospect that either the immune system or its derivative components could be harnessed to fight the misfolded and aggregated proteins that accumulate in several neurodegenerative diseases. Passive and active vaccinations using monoclonal antibodies and specific antigens that induce adaptive immune responses are currently under evaluation for their potential use in the development of immunotherapies. In this review, we aim to shed light on prominent immunotherapeutic strategies being developed to fight neuroinflammation-induced neurodegeneration, with a focus on innovative immunotherapies such as vaccination therapy.

Intrinsic DNA damage repair deficiency results in progressive microglia loss and replacement

The DNA excision repair protein Ercc1 is important for nucleotide excision, double strand DNA break, and interstrand DNA crosslink repair. In constitutive Ercc1‐knockout mice, microglia display increased phagocytosis, proliferation and an enhanced responsiveness to lipopolysaccharide (LPS)‐induced peripheral inflammation. However, the intrinsic effects of Ercc1‐deficiency on microglia are unclear. In this study, Ercc1 was specifically deleted from Cx3cr1‐expressing cells and changes in microglia morphology and immune responses at different times after deletion were determined. Microglia numbers were reduced with approximately 50% at 2–12 months after Ercc1 deletion. Larger and more ramified microglia were observed following Ercc1 deletion both in vivo and in organotypic hippocampal slice cultures. Ercc1‐deficient microglia were progressively lost, and during this period, microglia proliferation was transiently increased. Ercc1‐deficient microglia were gradually replaced by nondeficient microglia carrying a functional Ercc1 allele. In contrast to constitutive Ercc1‐deficient mice, microglia‐specific deletion of Ercc1 did not induce microglia activation or increase their responsiveness to a systemic LPS challenge. Gene expression analysis suggested that Ercc1 deletion in microglia induced a transient aging signature, which was different from a priming or disease‐associated microglia gene expression profile.

To Kill a Microglia: A Case for CSF1R Inhibitors

Microglia, the brain's immune sentinels, have garnered much attention in recent years. Researchers have begun to identify the manifold roles that these cells play in the central nervous system (CNS), and this work has been greatly facilitated by microglial depletion paradigms. The varying degrees of spatiotemporal manipulation afforded by such techniques allow microglial ablation before, during, and/or following insult, injury, or disease. We review the major methods of microglial depletion, including toxin-based, genetic, and pharmacological approaches, which differ in key factors including depletion onset, duration, and off-target effects. We conclude that pharmacological CSF1R inhibitors afford the most extensive versatility in manipulating microglia, making them ideal candidates for future studies investigating microglial function in health and disease.

Microglia alterations in neurodegenerative diseases and their modeling with human induced pluripotent stem cell and other platforms

Microglia are the main innate immune cells of the central nervous system (CNS). Unlike neurons and glial cells, which derive from ectoderm, microglia migrate early during embryo development from the yolk-sac, a mesodermal-derived structure. Microglia regulate synaptic pruning during development and induce or modulate inflammation during aging and chronic diseases. Microglia are sensitive to brain injuries and threats, altering their phenotype and function to adopt a so-called immune-activated state in response to any perceived threat to the CNS integrity. Here, we present a short overview on the role of microglia in human neurodegenerative diseases and provide an update on the current model systems to study microglia, including cell lines, iPSC-derived microglia with an emphasis in their transcriptomic profile and integration into 3D brain organoids. We present various strategies to model and study their role in neurodegeneration providing a relevant platform for the development of novel and more effective therapies.

Human iPSC-derived mature microglia retain their identity and functionally integrate in the chimeric mouse brain

Microglia, the brain-resident macrophages, exhibit highly dynamic functions in neurodevelopment and neurodegeneration. Human microglia possess unique features as compared to mouse microglia, but our understanding of human microglial functions is largely limited by an inability to obtain human microglia under homeostatic states. Here, we develop a human pluripotent stem cell (hPSC)-based microglial chimeric mouse brain model by transplanting hPSC-derived primitive macrophage progenitors into neonatal mouse brains. Single-cell RNA-sequencing of the microglial chimeric mouse brains reveals that xenografted hPSC-derived microglia largely retain human microglial identity, as they exhibit signature gene expression patterns consistent with physiological human microglia and recapitulate heterogeneity of adult human microglia. Importantly, the engrafted hPSC-derived microglia exhibit dynamic response to cuprizone-induced demyelination and species-specific transcriptomic differences in the expression of neurological disease-risk genes in microglia. This model will serve as a tool to study the role of human microglia in brain development and degeneration.

Diverging mRNA and Protein Networks in Activated Microglia Reveal SRSF3 Suppresses Translation of Highly Upregulated Innate Immune Transcripts

Uncontrolled microglial activation may lead to the development of inflammation-induced brain damage. Here, we uncover a ribosome-based mechanism/checkpoint involved in control of the innate immune response and microglial activation. Using an in vivo model system for analysis of the dynamic translational state of microglial ribosomes, with mRNAs as input and newly synthesized peptides as an output, we find a marked dissociation of microglia mRNA and protein networks following innate immune challenge. Highly upregulated and ribosome-associated mRNAs were not translated, resulting in two distinct microglial molecular signatures, a highly specialized pro-inflammatory mRNA signature and an immunomodulatory/homeostatic protein signature. We find that this is due to specific translational suppression of highly expressed mRNAs through a 3' UTR-mediated mechanism involving the RNA-binding protein SRSF3. This discovery suggests avenues for therapeutic modulation of innate immune response in resident microglia.

A story of birth and death: Insights into the formation and dynamics of the microglial population

Microglia are the main resident immunocompetent cells of the brain with key roles in brain development, homeostasis and function. Here we briefly review our current knowledge of the homeostatic mechanisms regulating the composition and turnover of the microglial population under physiological conditions from development to ageing. A greater understanding of these mechanisms may inform understanding of how dysregulation of microglial dynamics could contribute to the pathogenesis and/or progression of neurological disorders.

Proliferating NG2-Cell-Dependent Angiogenesis and Scar Formation Alter Axon Growth and Functional Recovery After Spinal Cord Injury in Mice

Spinal cord injury (SCI) induces a centralized fibrotic scar surrounded by a reactive glial scar at the lesion site. The origin of these scars is thought to be perivascular cells entering lesions on ingrowing blood vessels and reactive astrocytes, respectively. However, two NG2-expressing cell populations, pericytes and glia, may also influence scar formation. In the periphery, new blood vessel growth requires proliferating NG2⁺ pericytes; if this were also true in the CNS, then the fibrotic scar would depend on dividing NG2⁺ pericytes. NG2⁺ glial cells (also called oligodendrocyte progenitors or polydendrocytes) also proliferate after SCI and accumulate in large numbers among astrocytes in the glial scar. Their effect there, if any, is unknown. We show that proliferating NG2⁺ pericytes and glia largely segregate into the fibrotic and glial scars, respectively; therefore, we used a thymidine kinase/ganciclovir paradigm to ablate both dividing NG2⁺ cell populations to determine whether either scar was altered. Results reveal that loss of proliferating NG2⁺ pericytes in the lesion prevented intralesion angiogenesis and completely abolished the fibrotic scar. The glial scar was also altered in the absence of acutely dividing NG2⁺ cells, displaying discontinuous borders and significantly reduced GFAP density. Collectively, these changes enhanced edema, prolonged hemorrhage, and impaired forelimb functional recovery. Interestingly, after halting GCV at 14 d postinjury, scar elements and vessels entered the lesions over the next 7 d, as did large numbers of axons that were not present in controls. Collectively, these data reveal that acutely dividing NG2⁺ pericytes and glia play fundamental roles in post-SCI tissue remodeling.SIGNIFICANCE STATEMENT Spinal cord injury (SCI) is characterized by formation of astrocytic and fibrotic scars, both of which are necessary for lesion repair. NG2⁺ cells may influence both scar-forming processes. This study used a novel transgenic mouse paradigm to ablate proliferating NG2⁺ cells after SCI to better understand their role in repair. For the first time, our data show that dividing NG2⁺ pericytes are required for post-SCI angiogenesis, which in turn is needed for fibrotic scar formation. Moreover, loss of cycling NG2⁺ glia and pericytes caused significant multicellular tissue changes, including altered astrocyte responses and impaired functional recovery. This work reveals previously unknown ways in which proliferating NG2⁺ cells contribute to endogenous repair after SCI.

Microglial Activation in Traumatic Brain Injury

Microglia have a variety of functions in the brain, including synaptic pruning, CNS repair and mediating the immune response against peripheral infection. Microglia rapidly become activated in response to CNS damage. Depending on the nature of the stimulus, microglia can take a number of activation states, which correspond to altered microglia morphology, gene expression and function. It has been reported that early microglia activation following traumatic brain injury (TBI) may contribute to the restoration of homeostasis in the brain. On the other hand, if they remain chronically activated, such cells display a classically activated phenotype, releasing pro-inflammatory molecules, resulting in further tissue damage and contributing potentially to neurodegeneration. However, new evidence suggests that this classification is over-simplistic and the balance of activation states can vary at different points. In this article, we review the role of microglia in TBI, analyzing their distribution, morphology and functional phenotype over time in animal models and in humans. Animal studies have allowed genetic and pharmacological manipulations of microglia activation, in order to define their role. In addition, we describe investigations on the in vivo imaging of microglia using translocator protein (TSPO) PET and autoradiography, showing that microglial activation can occur in regions far remote from sites of focal injuries, in humans and animal models of TBI. Finally, we outline some novel potential therapeutic approaches that prime microglia/macrophages toward the beneficial restorative microglial phenotype after TBI.

An updated assessment of microglia depletion: current concepts and future directions

Microglia are the principal resident immune cells in the central nervous system and are believed to be versatile players in both inflammatory and physiological contexts. On the one hand, in order to safeguard the microenvironment microglia can be rapidly activated by contact with microbial products or cell debris, thereby exerting the functions of innate immunity via phagocytosis and secretion of cytokines and chemokines. Conversely, microglia can also assist in brain development, synaptic plasticity and neural repair through the production of neurotrophic factors and clearance of myelin debris. It is now well accepted that the dysfunction of microglia and microglia-induced neuroinflammation are implicated in the occurrence and progression of many neurological diseases. Although the past decade has witnessed major progress in understanding of multi-tasking microglia, what remains largely enigmatic is the relative importance of microglia at different disease stages and how microglia should be targeted for optimal therapeutic efficacy. Notably, microglia depletion through genetic targeting or pharmacological therapies can be viewed as effective tools to stimulate new microglia to repopulate the central nervous system. Microglia depletion and subsequent repopulation at defined stages in various experimental animal model disorders allow us to extend our knowledge of molecular mechanisms, thus holding promise for designing strategies to resolve neuroinflammation and promote recovery. Herein we highlight the highly plastic and diverse phenotypes of microglia and outline the lessons learned from microglia depletion approaches.

Glial Cells and Their Function in the Adult Brain: A Journey through the History of Their Ablation

Glial cells, consisting of microglia, astrocytes, and oligodendrocyte lineage cells as their major components, constitute a large fraction of the mammalian brain. Originally considered as purely non-functional glue for neurons, decades of research have highlighted the importance as well as further functions of glial cells. Although many aspects of these cells are well characterized nowadays, the functions of the different glial populations in the brain under both physiological and pathological conditions remain, at least to a certain extent, unresolved. To tackle these important questions, a broad range of depletion approaches have been developed in which microglia, astrocytes, or oligodendrocyte lineage cells (i.e., NG2-glia and oligodendrocytes) are specifically ablated from the adult brain network with a subsequent analysis of the consequences. As the different glial populations are very heterogeneous, it is imperative to specifically ablate single cell populations instead of inducing cell death in all glial cells in general. Thanks to modern genetic manipulation methods, the approaches can now directly be targeted to the cell type of interest making the ablation more specific compared to general cell ablation approaches that have been used earlier on. In this review, we will give a detailed summary on different glial ablation studies, focusing on the adult mouse central nervous system and the functional readouts. We will also provide an outlook on how these approaches could be further exploited in the future.

CD11b+Ly6G- myeloid cells mediate mechanical inflammatory pain hypersensitivity

Pain hypersensitivity at the site of inflammation as a result of chronic immune diseases, pathogenic infection, and tissue injury is a common medical condition. However, the specific contributions of the innate and adaptive immune system to the generation of pain during inflammation have not been systematically elucidated. We therefore set out to characterize the cellular and molecular immune response in two widely used preclinical models of inflammatory pain: (i) intraplantar injection of complete Freund's adjuvant (CFA) as a model of adjuvant- and pathogen-based inflammation and (ii) a plantar incisional wound as a model of tissue injury-based inflammation. Our findings reveal differences in temporal patterns of immune cell recruitment and activation states, cytokine production, and pain in these two models, with CFA causing a nonresolving granulomatous inflammatory response whereas tissue incision induced resolving immune and pain responses. These findings highlight the significant differences and potential clinical relevance of the incisional wound model compared with the CFA model. By using various cell-depletion strategies, we find that, whereas lymphocyte antigen 6 complex locus G (Ly)6G(+)CD11b(+) neutrophils and T-cell receptor (TCR) β(+) T cells do not contribute to the development of thermal or mechanical pain hypersensitivity in either model, proliferating CD11b(+)Ly6G(-) myeloid cells were necessary for mechanical hypersensitivity during incisional pain, and, to a lesser extent, CFA-induced inflammation. However, inflammatory (CCR2(+)Ly6C(hi)) monocytes were not responsible for these effects. The finding that a population of proliferating CD11b(+)Ly6G(-) myeloid cells contribute to mechanical inflammatory pain provides a potential cellular target for its treatment in wound inflammation.

Genetic targeting of microglia

Genetic targeting of microglia and other myeloid cells in the central nervous system (CNS) is highly desirable as they are critical effectors and regulators of changes in CNS homeostasis during development as well as in health and disease. Therefore, genetic reprogramming of microglia could constitute a central approach for potentially reducing disease burden. Previous attempts to target only microglia in vivo failed because of the similarities to their hematopoietic relatives in the circulation. However, this concept has been challenged by recent results of developmental and gene expression profiling studies which used novel molecular biological tools to unravel the origin of microglia and to define their role as specialized tissue macrophages clearly distinct from monocytes or monocyte-derived macrophages. The aim of this review is to recapitulate the history of microglia targeting approaches and finally highlight recent achievements in the field. We will discuss the pros and cons of the newly available genetic tools, their potential for future microglia research and genetic strategies.

Acute reduction of microglia does not alter axonal injury in a mouse model of repetitive concussive traumatic brain injury

The pathological processes that lead to long-term consequences of multiple concussions are unclear. Primary mechanical damage to axons during concussion is likely to contribute to dysfunction. Secondary damage has been hypothesized to be induced or exacerbated by inflammation. The main inflammatory cells in the brain are microglia, a type of macrophage. This research sought to determine the contribution of microglia to axon degeneration after repetitive closed-skull traumatic brain injury (rcTBI) using CD11b-TK (thymidine kinase) mice, a valganciclovir-inducible model of macrophage depletion. Low-dose (1 mg/mL) valganciclovir was found to reduce the microglial population in the corpus callosum and external capsule by 35% after rcTBI in CD11b-TK mice. At both acute (7 days) and subacute (21 days) time points after rcTBI, reduction of the microglial population did not alter the extent of axon injury as visualized by silver staining. Further reduction of the microglial population by 56%, using an intermediate dose (10 mg/mL), also did not alter the extent of silver staining, amyloid precursor protein accumulation, neurofilament labeling, or axon injury evident by electron microscopy at 7 days postinjury. Longer treatment of CD11b-TK mice with intermediate dose and treatment for 14 days with high-dose (50 mg/mL) valganciclovir were both found to be toxic in this injury model. Altogether, these data are most consistent with the idea that microglia do not contribute to acute axon degeneration after multiple concussive injuries. The possibility of longer-term effects on axon structure or function cannot be ruled out. Nonetheless, alternative strategies directly targeting injury to axons may be a more beneficial approach to concussion treatment than targeting secondary processes of microglial-driven inflammation.

The inflammasome pyrin contributes to pertussis toxin-induced IL-1β synthesis, neutrophil intravascular crawling and autoimmune encephalomyelitis

Microbial agents can aggravate inflammatory diseases, such as multiple sclerosis (MS) and its animal model, experimental autoimmune encephalomyelitis (EAE). An example is pertussis toxin (PTX), a bacterial virulence factor commonly used as an adjuvant to promote EAE, but whose mechanism of action is unclear. We have reported that PTX triggers an IL-6-mediated signaling cascade that increases the number of leukocytes that patrol the vasculature by crawling on its luminal surface. In the present study, we examined this response in mice lacking either TLR4 or inflammasome components and using enzymatically active and inactive forms of PTX. Our results indicate that PTX, through its ADP-ribosyltransferase activity, induces two series of events upstream of IL-6: 1) the activation of TLR4 signaling in myeloid cells, leading to pro-IL-1β synthesis; and 2) the formation of a pyrin-dependent inflammasome that cleaves pro-IL-1β into its active form. In turn, IL-1β stimulates nearby stromal cells to secrete IL-6, which is known to induce vascular changes required for leukocyte adhesion. Without pyrin, PTX does not induce neutrophil adhesion to cerebral capillaries and is less effective at inducing EAE in transgenic mice with encephalitogenic T lymphocytes. This study identifies the first microbial molecule that activates pyrin, a mechanism by which infections may influence MS and a potential therapeutic target for immune disorders.

What is microglia neurotoxicity (Not)?

Microglia most likely appeared early in evolution as they are not only present in vertebrates, but are also found in nervous systems of various nonvertebrate organisms. Mammalian microglia are derived from a specific embryonic, self-renewable myeloid cell population that is throughout lifetime not replaced by peripheral myeloid cells. These phylogenic and ontogenic features suggest that microglia serve vital functions. Yet, microglia often are described as neurotoxic cells, that actively kill (healthy) neurons. Since it is from an evolutionary point of view difficult to understand why an important and vulnerable organ like the brain should host numerous potential killers, we here review the concept of microglia neurotoxicity. On one hand it is discussed that most of our understanding about how microglia kill neurons is based on in vitro experiments or correlative staining studies that suffer from the difficulty to discriminate microglia and peripheral myeloid cells in the diseased brain. On the other hand it is described that a more functional approach by mutating, inactivating or deleting microglia is seldom associated with a beneficial outcome in an acute injury situation, suggesting that microglia are normally important protective elements in the brain. This might change in chronic disease or the aged brain, where; however, it remains to be established whether microglia simply lose their protective capacities or whether microglia become truly neurotoxic cells.

The brain's best friend: microglial neurotoxicity revisited

One long standing aspect of microglia biology was never questioned; their involvement in brain disease. Based on morphological changes (retracted processes and amoeboid shape) that inevitably occur in these cells in case of damage in the central nervous system, microglia in the diseased brain were called “activated.” Because “activated” microglia were always found in direct neighborhood to dead or dying neuron, and since it is known now for more than 20 years that cultured microglia release numerous factors that are able to kill neurons, microglia “activation” was often seen as a neurotoxic process. From an evolutionary point of view, however, it is difficult to understand why an important, mostly post-mitotic and highly vulnerable organ like the brain would host numerous potential killers. This review is aimed to critically reconsider the term microglia neurotoxicity and to discuss experimental problems around microglia biology, that often have led to the conclusion that microglia are neurotoxic cells.

GPR18 in microglia: implications for the CNS and endocannabinoid system signalling

A review of what is presently known about the G protein coupled receptor GPR18 in terms of its expression and distribution, pharmacology and potential implications for central nervous system and endocannabinoid system signalling. LINKED ARTICLES This article is part of a themed section on Cannabinoids. To view the other articles in this section visit http://dx.doi.org/10.1111/bph.2012.167.issue-8.

Insulin-like growth factor 1 and 2 (IGF1, IGF2) expression in human microglia: differential regulation by inflammatory mediators

BACKGROUND

Recent studies in experimental animals show that insulin-like growth factor 1 (IGF1) plays a trophic role during development and tissue injury and that microglia are important sources of IGF1. However, little information is available regarding the expression, regulation, and function of IGF1 and related proteins in human brain cells. In the current study, we examined the expression of IGF1 and IGF2 in human microglia in vivo and in vitro.

METHODS

Expression of IGF1 and IGF2 was examined by immunohistochemistry in post-mortem human brain sections derived from HIV+ and HIV- brains. In primary cultures of human fetal microglia, IGF1 and IGF2 mRNA and protein expression was examined by Q-PCR, ELISA, and Western blot analysis. Additionally, the role of IGF1 and IGF2 in neuroprotection was examined in primary human neuronal glial cultures.

RESULTS

Immunohistochemistry of human brain tissues showed that nonparenchymal cells (vessels and meninges), as well as parenchymal microglia and macrophages were positive for IGF1, in both HIV encephalitis and control brains, while IGF2 was undetectable. Cultured microglia expressed IGF1 mRNA and produced pg/ml levels of IGF1 protein; this was significantly suppressed by proinflammatory mediators, such as lipopolysaccharide (LPS), poly(I:C), and IFNγ. The Th2 cytokines IL-4 and IL-13 had no significant effect, but the cAMP analog (dibutyryl cAMP) significantly increased IGF1 production. In contrast, microglial IGF2 mRNA and protein (determined by Western blot) were upregulated by LPS. IGF1 receptor (IGF1R) immunoreactivity was predominantly expressed by neurons, and both IGF1 and IGF2 significantly protected neurons from cytokine (IL-1/IFNγ) induced death.

CONCLUSIONS

Our study in human brain tissues and cells indicates that microglia are important sources of neurotrophic growth factors IGF1 and IGF2, and that microglial activation phenotypes can influence the growth factor expression. Importantly, our results suggest that chronic neuroinflammation and upregulation of proinflammatory cytokines could lead to neurodegeneration by suppressing the production of microglia-derived neuronal growth factors, such as IGF1.

Galectin-3 is required for resident microglia activation and proliferation in response to ischemic injury

Growing evidence suggests that galectin-3 is involved in fine tuning of the inflammatory responses at the periphery, however, its role in injured brain is far less clear. Our previous work demonstrated upregulation and coexpression of galectin-3 and IGF-1 in a subset of activated/proliferating microglial cells after stroke. Here, we tested the hypothesis that galectin-3 plays a pivotal role in mediating injury-induced microglial activation and proliferation. By using a galectin-3 knock-out mouse (Gal-3KO), we demonstrated that targeted disruption of the galectin-3 gene significantly alters microglia activation and induces ∼4-fold decrease in microglia proliferation. Defective microglia activation/proliferation was further associated with significant increase in the size of ischemic lesion, ∼2-fold increase in the number of apoptotic neurons, and a marked deregulation of the IGF-1 levels. Next, our results revealed that contrary to WT cells, the Gal3-KO microglia failed to proliferate in response to IGF-1. Moreover, the IGF-1-mediated mitogenic microglia response was reduced by N-glycosylation inhibitor tunicamycine while coimmunoprecipitation experiments revealed galectin-3 binding to IGF-receptor 1 (R1), thus suggesting that interaction of galectin-3 with the N-linked glycans of receptors for growth factors is involved in IGF-R1 signaling. While the canonical IGF-1 signaling pathways were not affected, we observed an overexpression of IL-6 and SOCS3, suggesting an overactivation of JAK/STAT3, a shared signaling pathway for IGF-1/IL-6. Together, our findings suggest that galectin-3 is required for resident microglia activation and proliferation in response to ischemic injury.

IRF8 is a critical transcription factor for transforming microglia into a reactive phenotype

Microglia become activated by multiple types of damage in the nervous system and play essential roles in neuronal pathologies. However, how micro-glia transform into reactive phenotypes is poorly understood. Here, we identify the transcription factor interferon regulatory factor 8 (IRF8) as a critical regulator of reactive microglia. Within the spinal cord, IRF8 expression was normally low; however, the expression was markedly upregulated in microglia, but not in neurons or astrocytes, after peripheral nerve injury (PNI). IRF8 overexpression in cultured microglia promoted the transcription of genes associated with reactive states; conversely, IRF8 deficiency prevented these gene expressions in the spinal cord following PNI. Furthermore, IRF8-deficient mice were resistant to neuropathic pain, a common sequela of PNI, and transferring IRF8-over-expressing microglia spinally to normal mice produced pain. Therefore, IRF8 may activate a program of gene expression that transforms microglia into a reactive phenotype. Our findings provide a newly observed mechanism for microglial activation.

Clearance of amyloid-β peptides by microglia and macrophages: the issue of what, when and where

Accumulation of senile plaques consisting of amyloid-β peptide (Aβ) aggregates is a prominent pathological feature in Alzheimer's disease. Effective clearance of Aβ from the brain parenchyma is thought to regulate the development and progression of the disease. Macrophages in the brain play an important role in Aβ clearance by a variety of phagocytic and digestive mechanisms. Subpopulations of macrophages are heterogeneous such that resident microglia in the parenchyma, blood macrophages infiltrating from the periphery, and perivascular macrophages residing along cerebral vessels make functionally distinct contributions to Aβ clearance. Despite phenotypic similarities between the different macrophage subsets, a series of in vivo models have been derived to differentiate their relative impacts on Aβ dynamics as well as the molecular mechanisms underlying their activities. This review discusses the key findings from these models and recent research efforts to selectively enhance macrophage clearance of Aβ.

Using comparative anatomy in the axotomy model to identify distinct roles for microglia and astrocytes in synaptic stripping

The synaptic terminals' withdrawal from the somata and proximal dendrites of injured motoneuron by the processes of glial cells following facial nerve axotomy has been the subject of research for many years. This phenomenon is referred to as synaptic stripping, which is assumed to help survival and regeneration of neurons via reduction of synaptic inputs. Because there is no disruption of the blood-brain barrier or infiltration of macrophages, the axotomy paradigm has the advantage of being able to selectively investigate the roles of resident glial cells in the brain. Although there have been numerous studies of synaptic stripping, the detailed mechanisms are still under debate. Here we suggest that the species and strain differences that are often present in previous work might be related to the current controversies of axotomy studies. For instance, the survival ratios of axotomized neurons were generally found to be higher in rats than in mice. However, some studies have used the axotomy paradigm to follow the glial reactions and did not assess variations in neuronal viability. In the first part of this article, we summarize and discuss the current knowledge on species and strain differences in neuronal survival, glial augmentation and synaptic stripping. In the second part, we focus on our recent findings, which show the differential involvement of microglia and astrocytes in synaptic stripping and neuronal survival. This article suggests that the comparative study of the axotomy paradigm across various species and strains may provide many important and unexpected discoveries on the multifaceted roles of microglia and astrocytes in injury and repair.

Genetic approaches to study glial cells in the rodent brain

The development, function, and pathology of the brain depend on interactions of neurons and different types of glial cells, namely astrocytes, oligodendrocytes, microglia, and ependymal cells. Understanding neuron-glia interactions in vivo requires dedicated experimental approaches to manipulate each cell type independently. In this review, we first summarize techniques that allow for cell-specific gene modification including targeted mutagenesis and viral transduction. In the second part, we describe the genetic models that allow to target the main glial cell types in the central nervous system. The existing arsenal of approaches to study glial cells in vivo and its expansion in the future are key to understand neuron-glia interactions under normal and pathologic conditions.

Microglial depletion using intrahippocampal injection of liposome-encapsulated clodronate in prolonged hypothermic cardiac arrest in rats

Trauma patients who suffer cardiac arrest (CA) from exsanguination rarely survive. Emergency preservation and resuscitation using hypothermia was developed to buy time for resuscitative surgery and delayed resuscitation with cardiopulmonary bypass (CPB), but intact survival is limited by neuronal death associated with microglial proliferation and activation. Pharmacological modulation of microglia may improve outcome following CA. Systemic injection of liposome-encapsulated clodronate (LEC) depletes macrophages. To test the hypothesis that intrahippocampal injection of LEC would attenuate local microglial proliferation after CA in rats, we administered LEC or PBS into the right or left hippocampus, respectively. After rapid exsanguination and 6min no-flow, hypothermia was induced by ice-cold (IC) or room-temperature (RT) flush. Total duration of CA was 20min. Pre-treatment (IC, RTpre) and post-treatment (RTpost) groups were studied, along with shams (cannulation only) and CPB controls. On day 7, shams and CPB groups showed neither neuronal death nor microglial activation. In contrast, the number of microglia in hippocampus in each individual group (IC, RTpre, RTpost) was decreased with LEC vs. PBS by ∼34-46% (P<0.05). Microglial proliferation was attenuated in the IC vs. RT groups (P<0.05). Neuronal death did not differ between hemispheres or IC vs. RT groups. Thus, intrahippocampal injection of LEC attenuated microglial proliferation by ∼40%, but did not alter neuronal death. This suggests that microglia may not play a pivotal role in mediating neuronal death in prolonged hypothermic CA. This novel strategy provides us with a tool to study the specific effects of microglia in hypothermic CA.

Synergistic effects of galectin-1 and reactive astrocytes on functional recovery after contusive spinal cord injury

INTRODUCTION

Galectin-1 (Gal-1), a carbohydrate-binding protein, is differentially expressed by various normal and pathological tissues and appears to be functionally polyvalent. Recent evidence indicates that Gal-1 is involved in the proliferation of adult neural progenitor cells in neurogenic regions during adulthood. However, localization and functional roles of Gal-1 in the adult spinal cord have not been clarified.

METHOD

Here, we investigated the spatio-temporal profile of endogenous Gal-1 expression by in situ hybridization before and after experimental adult spinal cord injury and examined the correlation of Gal-1 with the fate of dividing cells in vivo, using double-labeling methods. Gal-1 mRNA was detectable at a relatively low level in uninjured spinal cord, but was markedly increased in the gray matter and/or white matter and in the ependyma rostral and caudal to the lesion site after injury.

RESULTS

Co-localization results revealed that Gal-1 was expressed predominantly by GFAP-positive reactive astrocytes. In addition, intrathecal infusion of recombinant Gal-1 enhanced cell division and reactive astrocytosis in the adult spinal cord. To explore further whether Gal-1 and reactive astrocytes provide a synergistic effect on neurological recovery following SCI, we investigated the differences in behavioral analysis between wild-type (WT) and reactive astrocyte-deficient transgenic mice after injury and found neuroprotective effects of Gal-1 appeared to be specifically mediated through reactive astrocytes.

CONCLUSION

These results indicate that Gal-1 exhibits great potential as a novel neuroprotective agent for the treatment of SCI.

Cerebral microglia mediate sleep/wake and neuroinflammatory effects of methamphetamine

Methamphetamine and modafinil exert their wake-promoting effects by elevating monoaminergic tone. The severity of hypersomnolence that occurs subsequent to induced wakefulness differs between these two agents. Microglia detects and modulates CNS reactions to agents such as D-methamphetamine that induce cellular stress. We therefore hypothesized that changes in the sleep/wake cycle that occur subsequent to administration of D-methamphetamine are modulated by cerebral microglia. In CD11b-herpes thymidine kinase transgenic mice (CD11b-TK(mt-30)), activation of the inducible transgene by intracerebroventricular (icv) ganciclovir results in toxicity to CD11b-positive cells (i.e. microglia), thereby reducing cerebral microglial cell counts. CD11b-TK(mt-30)and wild type mice were subjected to chronic icv ganciclovir or vehicle administration with subcutaneous mini-osmotic pumps. D-methamphetamine (1 and 2 mg/kg), modafinil (30 and 100 mg/kg) and vehicle were administered intraperitoneally to these animals. In CD11b-TK(mt-30) mice, but not wild type, icv infusion of ganciclovir reduced the duration of wake produced by D-methamphetamine at 2 mg/kg by nearly 1h. Nitric oxide synthase (NOS) activity, studied ex vivo, and NOS expression were elevated in CD11b-positive cerebral microglia from wild type mice acutely exposed to d-methamphetamine. Additionally, CD11b-positive microglia, but not other cerebral cell populations, exhibited changes in sleep-regulatory cytokine expression in response to d-METH. Finally, CD11b-positive microglia exposed to d-methamphetamine in vitro exhibited increased NOS activity relative to pharmacologically-naïve cells. CD11b-positive microglia from the brains of neuronal NOS (nNOS)-knockout mice failed to exhibit this effect. We propose that the effects of D-METH on sleep/wake cycles are mediated in part by actions on microglia, including possibly nNOS activity and cytokine synthesis.

Neurofibromatosis-1 heterozygosity increases microglia in a spatially and temporally restricted pattern relevant to mouse optic glioma formation and growth

Whereas carcinogenesis requires the acquisition of driver mutations in progenitor cells, tumor growth and progression are heavily influenced by the local microenvironment. Previous studies from our laboratory have used Neurofibromatosis-1 (NF1) genetically engineered mice to characterize the role of stromal cells and signals to optic glioma formation and growth. Previously, we have shown that Nf1+/- microglia in the tumor microenvironment are critical cellular determinants of optic glioma proliferation. To define the role of microglia in tumor formation and maintenance further, we used CD11b-TK mice, in which resident brain microglia (CD11b+, CD68+, Iba1+, CD45low cells) can be ablated at specific times after ganciclovir administration. Ganciclovir-mediated microglia reduction reduced Nf1 optic glioma proliferation during both tumor maintenance and tumor development. We identified the developmental window during which microglia are increased in the Nf1+/- optic nerve and demonstrated that this accumulation reflected delayed microglia dispersion. The increase in microglia in the Nf1+/- optic nerve was associated with reduced expression of the chemokine receptor, CX3CR1, such that reduced Cx3cr1 expression in Cx3cr1-GFP heterozygous knockout mice led to a similar increase in optic nerve microglia. These results establish a critical role for microglia in the development and maintenance of Nf1 optic glioma.

Regulation of innate immune responses in the brain

Microglial cells are the main innate immune cells of the complex cellular structure of the brain. These cells respond quickly to pathogens and injury, accumulate in regions of degeneration and produce a wide variety of pro-inflammatory molecules. These observations have resulted in active debate regarding the exact role of microglial cells in the brain and whether they have beneficial or detrimental functions. Careful targeting of these cells could have therapeutic benefits for several types of trauma and disease specific to the central nervous system. This Review discusses the molecular details underlying the innate immune response in the brain during infection, injury and disease.

Ablation of proliferating microglia does not affect motor neuron degeneration in amyotrophic lateral sclerosis caused by mutant superoxide dismutase

Microglial activation is a hallmark of all neurodegenerative diseases including amyotrophic lateral sclerosis (ALS). Here, a detailed characterization of the microglial cell population within the spinal cord of a mouse model of familial ALS was performed. Using flow cytometry, we detected three distinct microglial populations within the spinal cord of mice overexpressing mutant superoxide dismutase (SOD1): mature microglial cells (CD11b(+), CD45(low)), myeloid precursor cells (CD11b(+), CD45(int)), and macrophages (CD11b(+), CD45(high)). Characterization of cell proliferation within the CNS of SOD1(G93A) mice revealed that the expansion in microglial cell population is mainly attributable to the proliferation of myeloid precursor cells. To assess the contribution of proliferating microglia in motor neuron degeneration, we generated CD11b-TK(mut-30); SOD1(G93A) doubly transgenic mice that allow the elimination of proliferating microglia on administration of ganciclovir. Surprisingly, a 50% reduction in reactive microglia specifically in the lumbar spinal cord of CD11b-TK(mut-30); SOD1(G93A) doubly transgenic mice had no effect on motor neuron degeneration. This suggests that proliferating microglia-expressing mutant SOD1 are not central contributors of the neurodegenerative process in ALS caused by mutant SOD1.

Requirement of myeloid cells for axon regeneration

The role of CD11b+ myeloid cells in axonal regeneration was assessed using axonal injury models and CD11b-TK(mt-30) mice expressing a mutated HSV-1 thymidine kinase (TK) gene regulated by the myeloid-specific CD11b promoter. Continuous delivery of ganciclovir at a sciatic nerve lesion site greatly decreased the number of granulocytes/inflammatory monocytes and macrophages in the distal stump of CD11b-TK(mt-30) mice. Axonal regeneration and locomotor function recovery were severely compromised in ganciclovir-treated CD11b-TK(mt-30) mice. This was caused by an unsuitable growth environment rather than an altered regeneration capacity of neurons. In absence of CD11b+ cells, the clearance of inhibitory myelin debris was prevented, neurotrophin synthesis was abolished, and blood vessel formation/maintenance was severely compromised in the sciatic nerve distal stump. Spinal cord-injured axons also failed to regenerate through peripheral nerve grafts in the absence of CD11b+ cells. Therefore, myeloid cells support axonal regeneration and functional recovery by creating a growth-permissive milieu for injured axons.

Possible relation of hemin-induced HO-1 expression to the upregulation of VEGF and BDNF mRNA levels in rat C6 glioma cells

Glial cells are generally considered to contribute to retaining the integrity of neural function through the protection of neuronal cells against neurodegenerative insults and also expected to play a potential role in the protection of cerebrovascular systems from various toxic insults of hemorrhaged blood, thus proposing a possible implication of glial cells in the recovery of brain function from the damage caused by cerebral hemorrhage. Based on this hypothetical idea, the direct effect of hemin on the expression of genes encoding heme oxygenase-1 (HO-1), vascular endothelial growth factor (VEGF), and brain-derived neurotrophic factor (BDNF) in glial cells was examined using rat C6 glioma cells as an in vitro model system. Hemin elevated both HO-1 and VEGF mRNA levels in the glioma cells at the concentration causing no critical damage to the cells, and the elevation of BDNF mRNA levels was also observed by exposing the cells to hemin under the same conditions. Furthermore, the elevation of VEGF and BDNF mRNA levels induced by hemin was blocked by pretreatment of the cells with the agents inhibiting not only HO-1 gene expression but also its enzymatic activity. These pharmacological studies indicate that hemin can induce the enhancement of VEGF and BDNF gene expression probably through the mechanism mediated by HO-1 activity in the glioma cells, proposing the possibility that glial cells are capable of contributing to the recovery of brain function from the damage caused by cerebral hemorrhage through the production of neurogenic and angiogenic factors.

Modeling CNS microglia: the quest to identify predictive models

The mammalian central nervous system (CNS) is populated very early in development by tissue macrophages referred to as microglia. By adulthood, this CNS-resident population is found in all regions of the brain and spinal cord. Despite nearly a century of study, the in vivo function of microglia and the extent that they contribute to the onset, progression and recovery from neuroinflammatory disorders is still a subject of debate. Partly, the debate of whether activated microglia promote neuroprotection or neurodegeneration is fueled by the contrasting results derived from the different models used to assay microglial function. Here we discuss the strengths, weaknesses and utility of some of the most commonly used in vivo and in vitro models.

Neuroprotective effects of resident microglia following acute brain injury

Microglia quickly react to various neurodegenerative processes by producing cytokines and eliminating cellular debris via phagocytosis. These events are also associated with an increased proliferation of microglia, which derive from resident progenitors and those present in the bone marrow. However, it is not clear whether the innate immune response by resident or newly differentiated microglia is beneficial or detrimental to the central nervous system. The aim of this study was to determine the impact of an altered immune response following acute excitotoxicity. Sodium nitroprusside (SNP) or kainic acid (KA) was administered in the brain of various groups of mice, and the extent of neurodegeneration, myelin damage, and inflammation was evaluated within a period of 2 weeks. We used synthetic glucocorticoid (GC), myeloid differentiation factor 88 (MyD88)-deficient mice to suppress nuclear factor kappaB (NF-kappaB) signaling and transgenic mice that express the thymidine kinase (TK) protein under the control of the CD11b promoter to determine the role of proliferating and infiltrating microglia in acute models of brain injury. Neurodegeneration was more extensive in GC-treated and MyD88-deficient mice, suggesting that NF-kappaB signaling and microglia activation are potent neuroprotective mechanisms in the presence of SNP. KA was also highly toxic to neurons of the amygdala in MyD88 knockout mice but not in their WT littermates. Although bone marrow-derived cells are clearly attracted to neurodegenerative areas, preventing their infiltration and differentiation did not affect the extent of SNP-related damage. These data indicate that MyD88/NF-kappaB signaling in resident non-proliferating microglia plays a critical role by restricting damage during acute excitotoxicity.

Proliferating resident microglia after focal cerebral ischaemia in mice

Cerebral ischaemia usually results in the rapid death of neurons within the immediate territory of the affected artery. Neuronal loss is accompanied by a sequence of events, including brain oedema, blood-brain barrier (BBB) breakdown, and neuroinflammation, all of which contribute to further neuronal death. Although the role of macrophages and mononuclear phagocytes in the expansion of ischaemic injury has been widely studied, the relative contribution of these cells, either of exogenous or intrinsic central nervous system (CNS) origin is still not entirely clear. The purpose of this study, therefore, was to use different durations of transient middle cerebral artery occlusion (tMCAo) in the mouse to investigate fully post-occlusion BBB permeability and cellular changes in the brain during the 72 h post-MCAo period. This was achieved using in vivo magnetic resonance imaging (MRI) and cell labelling techniques. Our results show that BBB breakdown and formation of the primary ischaemic damage after tMCAo is not associated with significant infiltration of neutrophils, although more are observed with longer periods of MCAo. In addition, we observe very few infiltrating exogenous macrophages over a 72 h period after 30 or 60 mins of occlusion, instead a profound increase in proliferating resident microglia cells was observed. Interestingly, the more severe injury associated with 60 mins of MCAo leads to a markedly reduced proliferation of resident microglial cells, suggesting that these cells may play a protective function, possibly through phagocytosis of infiltrating neutrophils. These data further support possible beneficial actions of microglial cells in the injured brain.