Response of amoeboid and ramified microglial cells to lipopolysaccharide injections in postnatal rats--a lectin and ultrastructural study

Neurovascular Unit Alterations in the Growth-Restricted Newborn Are Improved Following Ibuprofen Treatment

The developing brain is particularly vulnerable to foetal growth restriction (FGR) and abnormal neurodevelopment is common in the FGR infant ranging from behavioural and learning disorders to cerebral palsy. No treatment exists to protect the FGR newborn brain. Recent evidence suggests inflammation may play a key role in the mechanism responsible for the progression of brain impairment in the FGR newborn, including disruption to the neurovascular unit (NVU). We explored whether ibuprofen, an anti-inflammatory drug, could reduce NVU disruption and brain impairment in the FGR newborn. Using a preclinical FGR piglet model, ibuprofen was orally administered for 3 days from birth. FGR brains demonstrated a proinflammatory state, with changes to glial morphology (astrocytes and microglia), and blood-brain barrier disruption, assessed by IgG and albumin leakage into the brain parenchyma and a decrease in blood vessel density. Loss of interaction between astrocytic end-feet and blood vessels was evident where plasma protein leakage was present, suggestive of structural deficits to the NVU. T-cell infiltration was also evident in the parenchyma of FGR piglet brains. Ibuprofen treatment reduced the pro-inflammatory response in FGR piglets, reducing the number of activated microglia and enhancing astrocyte interaction with blood vessels. Ibuprofen also attenuated plasma protein leakage, regained astrocytic end-feet interaction around vessels, and decreased T-cell infiltration into the FGR brain. These findings suggest postnatal administration of ibuprofen modulates the inflammatory state, allowing for stronger interaction between vasculature and astrocytic end-feet to restore NVU integrity. Modulation of the NVU improves the FGR brain microenvironment and may be key to neuroprotection.

Developing a mouse model of acute encephalopathy using low-dose lipopolysaccharide injection and hyperthermia treatment

IMPACT STATEMENT

Acute encephalopathy (AE), mainly reported in East Asia, is classified into four categories based on clinical and neuropathological findings. Among them, AE caused by cytokine storm is known as the severest clinical entity that causes cerebral edema with poor prognosis. Because suitable and convenient model animal of AE had not been developed, the treatment of patients with AE is not established. In the present study, we established a simple and convenient protocol to mimic AE due to cytokine storm. Our model animal should be useful to elucidate the pathogenesis of AE.

Comparative morphology and phagocytic capacity of primary human adult microglia with time-lapse imaging

Microglia provide immune surveillance within the brain and spinal cord. Various microglial morphologies include ramified, amoeboid, and pseudopodic. The link between form and function is not clear, especially for human adult microglia which are limited in availability for study. Here, we examined primary human microglia isolated from normal-appearing white matter. Pseudopodic and amoeboid microglia were effective phagocytes, taking up E. coli bioparticles using ruffled cell membrane sheets and retrograde transport. Pseudopodic and amoeboid microglia were more effective phagocytes as compared to ramified microglia or monocyte-derived dendritic cells. Thus, amoeboid and pseudopodic microglia may both be effective as brain scavengers.

Neuregulin upregulates microglial α7 nicotinic acetylcholine receptor expression in immortalized cell lines: implications for regulating neuroinflammation

Neuregulin, previously known as ARIA, is a signaling protein involved in cell survival, synaptic plasticity, cell communication and differentiation. Neuregulin has also been described as a potent inducer of acetylcholine receptor transcription in muscle and although both neuregulin and acetylcholine have been individually described to have neuroprotective roles, their relationship in the cholinergic anti-inflammatory pathway of the brain has not been examined. Using three cell lines, BV-2, EOC-20 and RAW 264.7, we investigated the role that neuregulin signaling through the Erb family of tyrosine kinases may play in the anti-inflammatory process mediated by the α7 nicotinic acetylcholine receptors. Here we show that ErbB4 is expressed in all of our cell lines and is phosphorylated upon treatment with neuregulin. Neuregulin treatment further increases the expression of α7 nicotinic acetylcholine receptors in the microglial lines tested. Given the central role of α7 nicotinic acetylcholine receptors in regulating system inflammation we analyzed the expression of several pro-inflammatory cytokines in our system. Using ELISAs for TNF-α and IL-6 we show that treatment with NRG can produce a nearly a 33% decrease in the levels of tumor necrosis factor-α secreted by activated microglia and a nearly 88% decrease in IL-6. Given these results we propose a neuroprotective role for neuregulin wherein it modulates the expression of TNF-α and thus inflammation in the CNS via the upregulation of α7 nicotinic acetylcholine receptor expression in microglia in vitro. We suggest that the disregulation of neuregulin expression may be pivotal in neurological disorders characterized by inflammation.

Inflammation and α-synuclein's prion-like behavior in Parkinson's disease--is there a link?

Parkinson's disease patients exhibit progressive spreading of aggregated α-synuclein in the nervous system. This slow process follows a specific pattern in an inflamed tissue environment. Recent research suggests that prion-like mechanisms contribute to the propagation of α-synuclein pathology. Little is known about factors that might affect the prion-like behavior of misfolded α-synuclein. In this review, we suggest that neuroinflammation plays an important role. We discuss causes of inflammation in the olfactory bulb and gastrointestinal tract and how this may promote the initial misfolding and aggregation of α-synuclein, which might set in motion events that lead to Parkinson's disease neuropathology. We propose that neuroinflammation promotes the prion-like behavior of α-synuclein and that novel anti-inflammatory therapies targeting this mechanism could slow disease progression.

Microglia/macrophages responses to kainate-induced injury in the rat retina

The present study was aimed to elucidate how retinal microglia/macrophages would respond to neuronal death after intravitreal kainate injection. An increased expression of the complement receptor type 3 (CR3) and an induction of the major histocompatibility complex (MHC) class II and ED-1 antigens were mainly observed in the inner retina after kainate injection. Prominent cell death revealed by Fluoro Jade B (FJB) staining and ultrastructural examination appeared at the inner border of the inner nuclear layer (INL) at 1 day post-injection. Interestingly, some immunoreactive cells appeared at the outer segment of photoreceptor layer (OSPRL) at different time intervals. Our quantitative analysis further showed that CR3 immunoreactivity was drastically increased peaking at 7 days but subsided thereafter. MHC class II and ED-1 immunoreactivities showed a moderate but steady increase peaking at 3 days and declined thereafter. Double labeling study further revealed that retinal microglia/macrophages expressed concurrently CR3 and ED-1 antigens (OX-42+/ED-1+) or MHC class II molecules (OX-42+/OX-6+) and remained branched in shape at early stage of kainate challenge. By electron microscopy, microglia/macrophages with CR3 immunoreactivity displayed abundant cytoplasm containing a few vesicles and phagosomes. Other cells ultrastructurally similar to Müller cells or astrocytes could also engulf exogenous substances. In conclusion, retinal microglia/macrophages responded vigorously to kainate-induced neuronal cell death that may also trigger the recruitment of macrophages from neighboring tissues and induce the phagocytotic activity of cells other than retinal microglia/macrophages.

Responses of microglia in vitro to the gram-positive bacterial component, lipoteichoic acid

An increase in incidence and severity of gram-positive infections has emerged in the past decade. In this regard, attention has been focused recently on immune responses of microglial cells in the central nervous system to gram-positive bacteria. The underlying immunological and cellular events in microglial activation induced by specific bacterial toxin of gram-positive bacteria, however, have not yet been clarified fully. This study reports that a simple cell wall product, lipoteichoic acid (LTA), derived from gram-positive bacteria (Staphylococcus aureus) could trigger microglial activation in vitro. Microglia challenged with LTA showed intense ruffling of plasma membrane in the form of lamellipodia or rounded up forming cell aggregates. MTT assay and Western blot analysis with anti-proliferating cell nuclear antigen antibody showed a significant microglial proliferation that may be induced at the later phases of LTA treatment with low doses but at the early period with a high dose. Concentrated LTA also caused apoptotic death of cultured microglia showing fragmented nuclei and increased expression of annexin V or caspase 3. In response to LTA, isolated microglia increased the expression of inducible nitric oxide synthase and major histocompatibility complex class II antigen. Microglial LTA receptors such as CD14 molecule, complement receptor type 3, and macrophage scavenger receptor were upregulated concurrently. In conclusion, staphylococcal LTA can exert an immunomodulatory effect on microglial morphology, cell cycle, and immunomolecules, including its receptors.

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.

Breakdown of the blood-brain barrier to proteins in white matter of the developing brain following systemic inflammation

Compromised blood-brain barrier permeability resulting from systemic inflammation has been implicated as a possible cause of brain damage in fetuses and newborns and may underlie white matter damage later in life. Rats at postnatal day (P) 0, P8 and P20 and opossums (Monodelphis domestica) at P15, P20, P35, P50 and P60 and adults of both species were injected intraperitoneally with 0.2-10 mg/kg body weight of 055:B5 lipopolysaccharide. An acute-phase response occurred in all animals. A change in the permeability of the blood-brain barrier to plasma proteins during a restricted period of postnatal development in both species was determined immunocytochemically by the presence of proteins surrounding cerebral blood vessels and in brain parenchyma. Blood vessels in white matter, but not grey matter, became transiently permeable to proteins between 10 and 24 h after lipopolysaccharide injection in P0 and P8 rats and P35-P60 opossums. Brains of Monodelphis younger than P35, rats older than P20 and adults of both species were not affected. Permeability of the blood-cerebrospinal fluid (CSF) barrier to proteins was not affected by systemic inflammation for at least 48 h after intraperitoneal injection of lipopolysaccharide. These results show that there is a restricted period in brain development when the blood-brain barrier, but not the blood-CSF barrier, to proteins is susceptible to systemic inflammation; this does not appear to be attributable to barrier "immaturity" but to its stage of development and only occurs in white matter.

Increased morphological diversity of microglia in the activated hypothalamic supraoptic nucleus

Microglia are the immune cells of the CNS. In the normal adult mammalian brain, the majority of these cells is quiescent and exhibits a ramified morphology. Microglia are perhaps best known for their swift transformation to an activated ameboid morphology in response to pathological insults. Here we have observed the responsiveness of these cells to events surrounding the normal activation of neurosecretory neurons in the hypothalamic supraoptic nucleus (SON), a well studied model of structural plasticity in the CNS. Neurons in the SON were activated by substituting 2% saline for drinking water. Brain sections were collected from four experimental groups [controls (C), 2 d-dehydrated (2D), 7 d-dehydrated (D7), and 7 d-dehydrated/21 d-rehydrated animals (R21)] and stained with Isolectin-B4-HRP to visualize microglial cells. Based on morphological criteria, we quantified ramified, hypertrophied, and ameboid microglia using unbiased stereological techniques. Statistical analyses showed significant increases in the number of hypertrophied microglia in the D2 and D7 groups. Moreover, there was a significant increase in the number of ameboid microglia in the D7 group. No changes were seen across conditions in the number of ramified cells, nor did we observe any significant phenotypic changes in a control area of the cingulate gyrus. Hence, increased morphological diversity of microglia was found specifically in the SON and was reversible with the cessation of stimulation. These results indicate that phenotypic plasticity of microglia may be a feature of the normal structural remodeling that accompanies neuronal activation in addition to the activation that accompanies brain pathology.

Prenatal infection obliterates glutamate-related protection against free hydroxyl radicals in neonatal rat brain

Prenatal infection constitutes an important risk factor for brain injury, in both premature and full-term infants. Unfortunately, as the mechanisms involved are far from understood, no therapeutic strategy emerges to prevent the damage. We tested the hypothesis that administration of lipopolysaccharide (LPS) to gravid female rats enhanced glutamate-induced oxidative stress in brain of pups. A microdialysis probe was implanted into the striatum of 14-day-old animals and the release of hydroxyl radicals (.OH) in the perfusion medium was evaluated. Glutamate promoted a delayed.OH release in the offspring of dams given LPS, contrasting with the.OH decreases observed in control animals. A similar response occurred after infusion of (R,S)-3,5-dihydroxyphenylglycine (DHPG), a Group I metabotropic glutamate receptor (mGluR) agonist. This response was not consecutive to a remote activation of N-methyl-D-aspartate (NMDA) receptors, as it was unaffected by an NMDA receptor antagonist. Furthermore, the response to NMDA itself decreased in the offspring of dams given LPS. Massive amounts of DHPG, however, likely internalizing the mGlu receptor, still blunted the response to NMDA, as in controls. No quantitative variation occurred in mGluR1, mGluR5, or the NR1 subunit of the NMDA receptor between controls and neonates born from LPS-treated dams. Direct LPS injection into age-matched pups, by contrast, affected the response to neither glutamate nor DHPG. These results confirm that normally during perinatal development, the brain is protected from any oxidative stress resulting from excess glutamate, and the results support the hypothesis that maternal infection before delivery may lead to critical brain damage via the release of toxic free radicals.

Microglial reaction induced by noncytotoxic methylmercury treatment leads to neuroprotection via interactions with astrocytes and IL-6 release

Microglial cells react early to a neurotoxic insult. However, the bioactive factors and the cell-cell interactions leading to microglial activation and finally to a neuroprotective or neurodegenerative outcome remain to be elucidated. Therefore, we analyzed the microglial reaction induced by methylmercury (MeHgCl) using cell cultures of different complexity. Isolated microglia were found to be directly activated by MeHgCl (10(-10) to 10(-6) M), as indicated by process retraction, enhanced lectin staining, and cluster formation. An association of MeHgCl-induced microglial clusters with astrocytes and neurons was observed in three-dimensional cultures. Close proximity was found between the clusters of lectin-stained microglia and astrocytes immunostained for glial fibrillary acidic protein (GFAP), which may facilitate interactions between astrocytes and reactive microglia. In contrast, immunoreactivity for microtubule-associated protein (MAP-2), a neuronal marker, was absent in the vicinity of the microglial clusters. Interactions between astrocytes and microglia were studied in cocultures treated for 10 days with MeHgCl. Interleukin-6 release was increased at 10(-7) M of MeHgCl, whereas it was decreased when each of these two cell types was cultured separately. Moreover, addition of IL-6 to three-dimensional brain cell cultures treated with 3 x 10(-7) M of MeHgCl prevented the decrease in immunostaining of the neuronal markers MAP-2 and neurofilament-M. IL-6 administered to three-dimensional cultures in the absence of MeHgCl caused astrogliosis, as indicated by increased GFAP immunoreactivity. Altogether, these results show that microglial cells are directly activated by MeHgCl and that the interaction between activated microglia and astrocytes can increase local IL-6 release, which may cause astrocyte reactivity and neuroprotection.

Response of amoeboid and differentiating ramified microglia to glucocorticoids in postnatal rats: a lectin histochemical and ultrastructural study

After glucocorticoid injection(s), the number of amoeboid microglial cells (AMC) in the corpus callosum labelled by lectin was markedly reduced when compared with the corresponding control rats. In rats killed at the age of 7 days, all the labeled cells differentiated to become ramified microglia. Ultrastructurally, the AMC in glucocorticoid-injected rats were extremely vacuolated and showed increased lipid droplets. Furthermore, the cells displayed varied lectin labelling patterns especially at both the trans saccules of the Golgi apparatus and lysosomes. In differentiating ramified microglia, massive cellular debris and lectin-stained vesicles or vacuoles were observed; some of the latter appeared to fuse with the plasma membrane. The most striking feature after glucocorticoid (GCC) treatment was the complete diminution of lectin labelling at the Golgi saccules in some differentiating ramified microglia. The present results have demonstrated different effects of glucocorticoids on AMC and differentiating ramified microglia. The differential response of AMC and differentiating ramified microglia to the immunosuppressive drugs may be attributed to the fact that these cells in the postnatal brains subserve different functions or that they are at different differentiation stages. In other words, the sensitivity of microglial cells to the immunosuppressive drugs is dependent upon the stage of cell maturation/differentiation.

Microglial response to the neurotoxicity of 6-hydroxydopamine in neonatal rat cerebellum

Depletion of noradrenaline in newborn rats by 6-hydroxydopamine (6-OHDA) affects the postnatal development and reduces the granular cell area in the neocerebellum (lobules V-VII). During the first postnatal month, Bergmann glial fibers guide the migration of immature granule cells to the internal granule cell layer. Microglia and Bergmann glia may play an important role in this process, but the exact mechanism behind this phenomenon is not known. We studied the effect of systemic administration of 6-OHDA on the expression and localization on microglia and Bergmann glia in the neonatal cerebellum by immunohistochemistry. In the neocerebellum, 6-OHDA treatment caused a significant increase in the number of activated microglia. The increase was observed mainly in the granule cell layer and the cerebellar medulla. Bergmann glial cells in treated brains were abnormally located, did not form intimate associations with Purkinje cells, and the glial fibers were structurally different. Our findings indicate that a noradrenergic influence may be necessary for the normal maturation and migration of granule cells, and abnormal migration may be the result of Bergmann glia destruction and the activation of microglia. Activated microglia in the granule cell layer may be used as a marker for an injured cerebellar area.

Response of macrophage/microglial cells to experimental neuronal degeneration in the avian isthmo-optic nucleus during development

Blockade of the retrograde axonal transport of isthmo-optic nucleus (ION) neurons in the avian embryo results in their massive degeneration. We used this system to investigate the response of macrophage/microglial cells to neuronal degeneration in the embryonic brain. Colchicine was injected into the right eye of quail or chick embryos at a time when the survival of ION neurons depends on retrograde trophic support from the retina, and the chronology of the subsequent macrophage/microglial response in the ION was analyzed. This response was restricted to the ION contralateral to the injected eye; no modifications of the normal state were observed in the surrounding parenchyma or in the opposite ION, used as control. The response was first detected 18 hours after the colchicine injection (18 hours pi), when an increase of the macrophage/microglial cell number was evident. The number of these cells in the affected ION increased, peaking at 40-48 hours pi. At later survival times, macrophage/microglial cells were progressively less abundant in the affected ION, which gradually diminished in size. At 120 hours pi the only remnant of the ION was a small cluster of macrophage/microglial cells, surrounded by a clear area with scarce nonmicroglial cells, in the region formerly occupied by the ION. This study reveals that a strong macrophage/microglial response occurs in the embryonic brain in response to neuronal degeneration but that these cells do not trigger the neuronal death, as they only appear after pyknotic fragments are already observable.