Response of amoeboid microglia/brain macrophages in fetal rat brain exposed to a teratogen

Biology of Microglia in the Developing Brain

Microglia exist in different morphological forms in the developing brain. They show a small cell body with scanty cytoplasm with many branching processes in the grey matter of the developing brain. However, in the white matter such as the corpus callosum where the unmyelinated axons are loosely organized, they appear in an amoeboid form having a round cell body endowed with copious cytoplasm rich in organelles. The amoeboid cells eventually transform into ramified microglia in the second postnatal week when the tissue becomes more compact with the onset of myelination. Microglia serve as immunocompetent macrophages that act as neuropathology sensors to detect and respond swiftly to subtle changes in the brain tissues in pathological conditions. Microglial functions are broadly considered as protective in the normal brain development as they phagocytose dead cells and sculpt neuronal connections by pruning excess axons and synapses. They also secrete a number of trophic factors such as insulin-like growth factor-1 and transforming growth factor-β among many others that are involved in neuronal and oligodendrocyte survival. On the other hand, microglial cells when activated produce a plethora of molecules such as proinflammatory cytokines, chemokines, reactive oxygen species, and nitric oxide that are implicated in the pathogenesis of many pathological conditions such as epilepsy, cerebral palsy, autism, and perinatal hypoxic-ischemic brain injury. Although many studies have investigated the origin and functions of the microglia in the developing brain, in-depth in vivo studies along with analysis of their transcriptome and epigenetic changes need to be undertaken to elucidate their full potential be it protective or neurotoxic. This would lead to a better understanding of their roles in the healthy and diseased developing brain and advancement of therapeutic strategies to target microglia-mediated neurotoxicity.

Immunosuppression, peripheral inflammation and invasive infection from endogenous gut microbiota activate retinal microglia in mouse models

Although its actual role in the progression of degenerative processes is not fully known, the persistent activated state of retinal microglia and the concurrent secretion of inflammatory mediators may contribute to neuronal death and permanent vision loss. Our objective was to determine whether non-ocular conditions (immunosuppression and peripheral inflammation) could lead to activation of retinal microglia. Mouse models of immunosuppression induced by cyclophosphamide and/or peripheral inflammation by chemically induced sublethal colitis in C57BL/6J mice were used. Retinal microglia morphology, spatial distribution and complexity, as well as MHCII and CD11b expression levels were determined by flow cytometry and confocal immunofluorescence analysis with anti-CD11b, anti-IBA1 and anti-MHCIIRT1B antibodies. Retinas of mice with double treatment showed changes in microglial morphology, spatial distribution and expression levels of CD11b and MHCII. These effects were higher than those observed with any treatment separately. In addition, we also observed in these mice: (i) translocation of endogenous bacteria from gut to liver, and (ii) upregulation of TLR2 expression in retinal microglia. Using a mouse model of immunosuppression and gut colonization by Candida albicans, translocation of fungal cells was confirmed to occur in wild type and, to a higher extent, in TLR2 KO mice, which are more susceptible to fungal invasion; interestingly microglial changes were also higher in TLR2 KO mice. Hence, non-ocular injuries (immunosuppression, peripheral inflammation and invasive infection from endogenous gut microbiota) can activate retinal microglia and therefore could affect the progression of neurodegenerative disorders and should be taken into account to improve therapeutic options.

Macrophage-Colony Stimulating Factor Derived from Injured Primary Afferent Induces Proliferation of Spinal Microglia and Neuropathic Pain in Rats

Peripheral nerve injury induces proliferation of microglia in the spinal cord, which can contribute to neuropathic pain conditions. However, candidate molecules for proliferation of spinal microglia after injury in rats remain unclear. We focused on the colony-stimulating factors (CSFs) and interleukin-34 (IL-34) that are involved in the proliferation of the mononuclear phagocyte lineage. We examined the expression of mRNAs for macrophage-CSF (M-CSF), granulocyte macrophage-CSF (GM-CSF), granulocyte-CSF (G-CSF) and IL-34 in the dorsal root ganglion (DRG) and spinal cord after spared nerve injury (SNI) in rats. RT-PCR and in situ hybridization revealed that M-CSF and IL-34, but not GM- or G-CSF, mRNAs were constitutively expressed in the DRG, and M-CSF robustly increased in injured-DRG neurons. M-CSF receptor mRNA was expressed in naive rats and increased in spinal microglia following SNI. Intrathecal injection of M-CSF receptor inhibitor partially but significantly reversed the proliferation of spinal microglia and in early phase of neuropathic pain induced by SNI. Furthermore, intrathecal injection of recombinant M-CSF induced microglial proliferation and mechanical allodynia. Here, we demonstrate that M-CSF is a candidate molecule derived from primary afferents that induces proliferation of microglia in the spinal cord and leads to induction of neuropathic pain after peripheral nerve injury in rats.

Expression of inducible nitric oxide synthase (iNOS) in microglia of the developing quail retina

Inducible nitric oxide synthase (iNOS), which produce large amounts of nitric oxide (NO), is induced in macrophages and microglia in response to inflammatory mediators such as LPS and cytokines. Although iNOS is mainly expressed by microglia that become activated in different pathological and experimental situations, it was recently reported that undifferentiated amoeboid microglia can also express iNOS during normal development. The aim of this study was to investigate the pattern of iNOS expression in microglial cells during normal development and after their activation with LPS by using the quail retina as model. iNOS expression was analyzed by iNOS immunolabeling, western-blot, and RT-PCR. NO production was determined by using DAR-4M AM, a reliable fluorescent indicator of subcellular NO production by iNOS. Embryonic, postnatal, and adult in situ quail retinas were used to analyze the pattern of iNOS expression in microglial cells during normal development. iNOS expression and NO production in LPS-treated microglial cells were investigated by an in vitro approach based on organotypic cultures of E8 retinas, in which microglial cell behavior is similar to that of the in situ retina, as previously demonstrated in our laboratory. We show here that amoeboid microglia in the quail retina express iNOS during normal development. This expression is stronger in microglial cells migrating tangentially in the vitreal part of the retina and is downregulated, albeit maintained, when microglia differentiate and become ramified. LPS treatment of retina explants also induces changes in the morphology of amoeboid microglia compatible with their activation, increasing their lysosomal compartment and upregulating iNOS expression with a concomitant production of NO. Taken together, our findings demonstrate that immature microglial cells express iNOS during normal development, suggesting a certain degree of activation. Furthermore, LPS treatment induces overactivation of amoeboid microglia, resulting in a significant iNOS upregulation.

Quantitating the subtleties of microglial morphology with fractal analysis

It is well established that microglial form and function are inextricably linked. In recent years, the traditional view that microglial form ranges between “ramified resting” and “activated amoeboid” has been emphasized through advancing imaging techniques that point to microglial form being highly dynamic even within the currently accepted morphological categories. Moreover, microglia adopt meaningful intermediate forms between categories, with considerable crossover in function and varying morphologies as they cycle, migrate, wave, phagocytose, and extend and retract fine and gross processes. From a quantitative perspective, it is problematic to measure such variability using traditional methods, but one way of quantitating such detail is through fractal analysis. The techniques of fractal analysis have been used for quantitating microglial morphology, to categorize gross differences but also to differentiate subtle differences (e.g., amongst ramified cells). Multifractal analysis in particular is one technique of fractal analysis that may be useful for identifying intermediate forms. Here we review current trends and methods of fractal analysis, focusing on box counting analysis, including lacunarity and multifractal analysis, as applied to microglial morphology.

Mechanisms of neurotoxicity induced in the developing brain of mice and rats by DNA-damaging chemicals

It is not widely known how the developing brain responds to extrinsic damage, although the developing brain is considered to be sensitive to diverse environmental factors including DNA-damaging agents. This paper reviews the mechanisms of neurotoxicity induced in the developing brain of mice and rats by six chemicals (ethylnitrosourea, hydroxyurea, 5-azacytidine, cytosine arabinoside, 6-mercaptopurine and etoposide), which cause DNA damage in different ways, especially from the viewpoints of apoptosis and cell cycle arrest in neural progenitor cells. In addition, this paper also reviews the repair process following damage in the developing brain.

Microglia and neuronal cell death

Microglia, the brain's innate immune cell type, are cells of mesodermal origin that populate the central nervous system (CNS) during development. Undifferentiated microglia, also called ameboid microglia, have the ability to proliferate, phagocytose apoptotic cells and migrate long distances toward their final destinations throughout all CNS regions, where they acquire a mature ramified morphological phenotype. Recent studies indicate that ameboid microglial cells not only have a scavenger role during development but can also promote the death of some neuronal populations. In the mature CNS, adult microglia have highly motile processes to scan their territorial domains, and they display a panoply of effects on neurons that range from sustaining their survival and differentiation contributing to their elimination. Hence, the fine tuning of these effects results in protection of the nervous tissue, whereas perturbations in the microglial response, such as the exacerbation of microglial activation or lack of microglial response, generate adverse situations for the organization and function of the CNS. This review discusses some aspects of the relationship between microglial cells and neuronal death/survival both during normal development and during the response to injury in adulthood.

Repair process of fetal brain after 5-azacytidine-induced damage

The fetal brain is susceptible to many extrinsic stresses. Some of these stresses induce excessive cell death in the prenatal stage, leading to anomalies in the neonatal brain. However, it is unclear how the developing brain responds to and repairs the prenatal tissue damage. We treated pregnant rats on day 13 of gestation with 5-azacytidine, one of the compounds that induces excessive cell death and inhibits proliferation in neural progenitor cells, to damage the fetal brain, and investigated the repair process up to 60 h after treatment. Histological analysis showed that 5-azacytidine induced strong apoptosis of neural cells. By 60 h, apoptotic cells disappeared and the tissue was repaired, although the telencephalic wall remained thinner than in controls. Flow cytometry analysis showed that the cell cycle distribution also returned to control levels at 60 h, suggesting that the repair process was completed around 60 h. During the repair period, amoeboid microglia infiltrated the brain and ingested the apoptotic cells. These microglial cells were positive for the multiple microglial markers, and mRNAs for the microglia-related cytokines tumor necrosis factor alpha, interleukin 1beta and macrophage colony stimulating factor (M-CSF) were up-regulated. DNA microarray analysis showed the up-regulation of genes relevant to glial cells, inflammation, the extracellular matrix, glycolysis, proliferation and neural development. We show here that the developing brain has the capacity to respond to the damage induced by extrinsic chemical stresses, including changing the expression of numerous genes and the induction of microglia to aid the repair process.

Perinatal immunotoxicity: why adult exposure assessment fails to predict risk

Recent research has pointed to the developing immune system as a remarkably sensitive toxicologic target for environmental chemicals and drugs. In fact, the perinatal period before and just after birth is replete with dynamic immune changes, many of which do not occur in adults. These include not only the basic maturation and distribution of immune cell types and selection against autoreactive lymphocytes but also changes designed specifically to protect the pregnancy against immune-mediated miscarriage. The newborn is then faced with critical immune maturational adjustments to achieve an immune balance necessary to combat myriad childhood and later-life diseases. All these processes set the fetus and neonate completely apart from the adult regarding immunotoxicologic risk. Yet for decades, safety evaluation has relied almost exclusively upon exposure of the adult immune system to predict perinatal immune risk. Recent workshops and forums have suggested a benefit in employing alternative exposures that include exposure throughout early life stages. However, issues remain concerning when and where such applications might be required. In this review we discuss the reasons why immunotoxic assessment is important for current childhood diseases and why adult exposure assessment cannot predict the effect of xenobiotics on the developing immune system. It also provides examples of developmental immunotoxicants where age-based risk appears to differ. Finally, it stresses the need to replace adult exposure assessment for immune evaluation with protocols that can protect the developing immune system.

Microglia overexpressing the macrophage colony-stimulating factor receptor are neuroprotective in a microglial-hippocampal organotypic coculture system

Microglia with increased expression of the macrophage colony-stimulating factor receptor (M-CSFR; c-fms) are found surrounding plaques in Alzheimer's disease (AD) and in mouse models for AD and after ischemic or traumatic brain injury. Increased expression of M-CSFR causes microglia to adopt an activated state that results in proliferation, release of cytokines, and enhanced phagocytosis. To determine whether M-CSFR-induced microglial activation affects neuronal survival, we assembled a coculture system consisting of BV-2 microglia transfected to overexpress the M-CSFR and hippocampal organotypic slices treated with NMDA. Twenty-four hours after assembly of the coculture, microglia overexpressing M-CSFR proliferated at a higher rate than nontransfected control cells and exhibited enhanced migration toward NMDA-injured hippocampal cultures. Surprisingly, coculture with c-fms-transfected microglia resulted in a dramatic reduction in NMDA-induced neurotoxicity. Similar results were observed when cocultures were treated with the teratogen cyclophosphamide. Biolistic overexpression of M-CSFR on microglia endogenous to the organotypic culture also rescued neurons from excitotoxicity. Furthermore, c-fms-transfected microglia increased neuronal expression of macrophage colony-stimulating factor (M-CSF), the M-CSFR, and neurotrophin receptors in the NMDA-treated slices, as determined with laser capture microdissection. In the coculture system, direct contact between the exogenous microglia and the slice was necessary for neuroprotection. Finally, blocking expression of the M-CSF ligand by exogenous c-fms-transfected microglia with a hammerhead ribozyme compromised their neuroprotective properties. These results demonstrate a protective role for microglia overexpressing M-CSFR in our coculture system and suggest under certain circumstances, activated microglia can help rather than harm neurons subjected to excitotoxic and teratogen-induced injury.

Activation of immature microglia in response to stab wound in embryonic quail retina

Activation of mature (ramified) microglia in response to injury in the adult central nervous system (CNS) is well documented. However, the response of immature (ameboid) microglia to injury in the developing CNS has received little attention. In this study, a stab wound was made in embryonic quail retinas at incubation days 7 and 9, and the response of retinal microglial cells was analyzed at different times between days 1 and 37 postinjury. The appearance of microglial cells within the wound occurred at the same time as the arrival of the first migrating ameboid microglial cells at an equivalent area in control retinas. Therefore, no specific attraction of microglia toward the wound was observed. Microglial cells in the wound had phenotypic features similar to those of activated microglia in the adult CNS. Thus, their shape was more compact compared with microglial cells outside the wound, expression of the molecule recognized by the QH1 antibody was up-regulated, and their lysosomal compartment was markedly increased. Transitional forms between normal ameboid and activated-like microglial cells were seen at the wound edge, supporting the view that ameboid microglia become activated when they contact the wound during the normal course of their migration in the retina. The microglial reaction was maintained within the wound at 37 days postinjury. In addition to the stab wound, secondary damage areas were found in experimental retinas. Activated cells could still be observed in these areas at 37 days postinjury.

Retinoic acid inhibits expression of TNF-alpha and iNOS in activated rat microglia

The release of proinflammatory mediators such as tumor necrosis factor-alpha (TNF-alpha) and nitric oxide by microglia has been implicated in neurotoxicity in chronic neurodegenerative diseases such as Alzheimer's disease. As all-trans-retinoic acid (RA) has been reported to exert anti-inflammatory actions in various cell types, we have examined its effects on the expression of TNF-alpha and inducible nitric oxide synthase (iNOS) in microglia activated by beta-amyloid peptide (Abeta) and lipopolysaccharide (LPS). Exposure of primary cultures of rat microglial cells to Abeta or LPS stimulated the mRNA expression level of TNF-alpha (6-116-fold) and iNOS (8-500-fold) significantly. RA acted in a dose-dependent manner (0.1-10 microM) by attenuating both TNF-alpha (29-97%) and iNOS (61-96%) mRNA expression in microglia exposed to Abeta or LPS. RA-induced inhibition of TNF-alpha and iNOS mRNA expression in activated microglia was accompanied by the concomitant reduction in release of iNOS and TNF-alpha proteins as revealed by nitrite assay and ELISA, respectively. The anti-inflammatory effects of RA were correlated with the enhanced expression of retinoic acid receptor-beta, and transforming growth factor-beta1 as well as the inhibition of NF-kappaB translocation. These results suggest that RA may inhibit the neurotoxic effect of activated microglia by suppressing the production of inflammatory cytokines and cytotoxic molecules.

Enhanced inflammatory response in neural tubes of embryos derived from diabetic mice exposed to a teratogen

Exposure of embryos to the teratogen cyclophosphamide (CP) and maternal diabetes is linked to pathogenesis of neural tube defects during development. Maternal diabetes aggravates the teratogen-induced inflammatory reaction leading to increased risk of neural tube defects in mouse embryos. The inflammatory reaction in the developing neural tube has been characterized by the presence of activated amoeboid microglia/brain macrophages and altered expression levels of cytokines. Although there were no obvious anomalies observed in the neural tubes of embryos from CP-treated non-diabetic mice, the frequency of neural tube defects was increased significantly in embryos of CP-treated diabetic mice. Moreover, there were more activated amoeboid microglia in the forebrain of CP-treated diabetic embryos compared to that in CP-treated non-diabetic mice. The expression of cytokines such as tumor necrosis factor-alpha (TNF-alpha) and transforming growth factor-beta1 (TGF-beta1) in the fetal brain of normal and diabetic embryos was induced in the neural tubes after CP treatment. Furthermore, the mRNA expression levels of both genes were increased markedly in the neural tube of CP-treated diabetic embryos compared to that of CP-treated non-diabetic embryos as measured by quantitative real-time PCR. Immunohistochemically, more TNF-alpha- and TGF-beta1-positive cells, which included neurons and amoeboid microglia, were detected in CP-treated diabetic embryos than in CP-treated normal embryos. Maternal diabetes aggravates teratogen-induced inflammation, which is characterized in the developing neural tube by increased amoeboid microglia and enhanced expression of inflammatory cytokines. Although a definite link has yet to be elucidated, it is suggested that the increased rate of neural tube defects observed in CP-treated diabetic embryos may be due to upregulation of proinflammatory cytokines caused by maternal diabetes.

Intracellular signaling in M-CSF-induced microglia activation: role of Iba1

Microglia are reactively activated by various environmental stimulations caused by brain injury or disease. Activated microglia exhibit morphological transformation, proliferation, migration, phagocytosis, and the production of bioactive molecules. Various molecules are reported and suggested to activate microglia. Among them, macrophage-colony-stimulating factor (M-CSF) is considered one of the most convincing candidates responsible for maintaining activation properties of microglia. Therefore, the focus of the present study is on intracellular molecular events that arise downstream of M-CSF stimulation. M-CSF activates its receptor, Fms tyrosine kinase, and Fms sequentially activates a number of signaling molecules, including PI3K or phospholipase Cgamma (PLCgamma). Stimulation of continuing signaling cascades results in the activation of a small GTPase, Rac, the key molecule in microglia activation. Rac is known to be activated downstream of receptor tyrosine kinases and to regulate reorganization of the actin cytoskeleton, which profoundly underlies the above-mentioned properties of activated microglia. Iba1, a macrophage/microglia-specific calcium-binding protein, was identified by our group and was shown to be involved in the Rac signaling pathway. Further, we introduce a novel signaling pathway in which Rac is activated, dependent on PLCgamma and Iba1. However, to understand the molecular details of microglia activation, future work is required.

Expression of macrophage colony-stimulating factor and its receptor in microglia activation is linked to teratogen-induced neuronal damage

Prenatal exposure to teratogen agents is linked to the pathogenesis of neurodevelopment disorders, but the mechanisms leading to the neurodevelopmental disturbance are poorly understood. To elucidate this, an in vitro model of microglial activation induced by neuronal injury has been characterized. In this connection, exposure of primary microglial cells to the conditioned medium from the neuronal damage induced by teratogen, cyclophosphamide, is accompanied by a reactive microgliosis as assessed by reverse transcription-polymerase chain reaction, enzyme-linked immunosorbent assay, lectin histochemistry, double labeling immunohistochemistry and in situ hybridization. Our results showed that reactive microglia were capable of releasing various cytokines such as tumor necrosis factor-alpha, interleukin-1, interleukin-6, transforming growth factor-beta and nitric oxide. Also, we have shown that macrophage colony-stimulating factor (M-CSF) was in fact produced by the reactive microglia. Concomitant to this was the increased expression of M-CSF receptor in these cells following the teratogen-induced neuronal injury. The up-regulation of M-CSF receptor suggests that the cells are capable of responding to self-derived M-CSF in an autocrine fashion. Results with antibody neutralization further suggest that microglial proinflammatory response, as manifested by cytokine expression in culture, is mediated by M-CSF, which acts as a molecular signal that initiates a microglial reaction. We therefore suggest that microglial activation following cyclophosphamide treatment is not only a response to the neuronal damage, but is also a cause of the damage during pathogenesis of neurodevelopment disorders. To this end, the increased expression of M-CSF and its receptor on microglia would be directly linked to the active cell proliferation and proinflammatory response in the teratogen-induced injury.

Macrophage colony stimulating factor prevents NMDA-induced neuronal death in hippocampal organotypic cultures

Macrophage colony stimulating factor (M-CSF) and its receptor are up-regulated in the brain in Alzheimer's disease (AD), in transgenic mouse models for AD, and experimental models for traumatic and ischemic brain injury. M-CSF induces activation and proliferation of microglial cells and expression of proinflammatory cytokines. We examined the role of M-CSF in excitotoxic neuronal cell death in organotypic hippocampal cultures. NMDA treatment induced neuronal apoptosis and caspase-3 activation in organotypic hippocampal cultures, whereas treatment with M-CSF protected hippocampal neurons from NMDA-induced apoptosis. Caspase-3 activation was inhibited by M-CSF treatment to the same degree as with the caspase inhibitor Z-VAD-FMK. These results suggest that M-CSF has neuroprotective properties through inhibition of caspase-3 that could promote neuronal survival after excitotoxic insult. The role of M-CSF in neurological disease should be reevaluated as a microglial activator with potentially neuroprotective effects.

Regulatory Factors and Functions of Microglia during Development

This article reviews current knowledge on the origin and development of microglia as well as on regulatory factors that influence microglial development within the neural tube during embryogenesis. Ameboid microglia found in the developing neural tube originate from mesodermal precursors derived from the yolk sac. These microglial cells, which exhibit characteristic features of reactive microglia, undergo mitosis in situ in the nervous parenchyma and function as the full-blown phagocytes involved in the removal of cellular debris resulting from neural tube defect or normal cellular turnover. During embryogenesis, the microglia express various cytokines, growth factors and chemokines. Some of those factors together with other local factors may influence the proliferation and activation of microglia in the developing neural tube.

Origin of microglia

This paper reviews the various proposed hypotheses on the origin of microglia. The seminal study of del Rio-Hortega first stated that the cells were derived from the mesodermal pial cells that invaded the brain during embryonic development. Along with this was the description of precursor cells in the yolk sac in early development. Our results in the embryonic mouse brain have shown the occurrence of lectin-labelled precursor cells at the yolk sac that later appeared in the mesenchymal tissue associated with the neuroepithelium where they penetrated the nervous tissue to become the microglia. A second major view has held that microglia are of neuroectodermal origin; the cells either originate from glioblasts or the germinal matrix. Another school of thought is that microglia are derived from blood monocytes. In this connection, circulating monocytes enter the developing brain to assume the form as amoeboid microglia that subsequently evolve to become the ramified microglia. In traumatic brain lesions following an intravenous injection of colloidal carbon as a cytoplasmic marker for monocytes, it was found that carbon-labelled monocytes were the main source of brain macrophages, some of which transformed into microglia during the healing process. In conclusion, our results derived from the normal and altered brain development as well as from experimental lesions tend to favour the view of the monocytic nature of microglia. Recent studies by us also point to the possibility that some microglial cells may arise from the pial mesenchymal macrophages that appear to originate from the yolk sac precursors.