Microglia: a sensor to threats in the nervous system?

CX3CL1-CX3CR1 axis protects retinal ganglion cells by inhibiting microglia activation in a distal optic nerve trauma model

BACKGROUND

The chemokine CX3CL1 has been reported to play an important role in optic nerve protection, but the underlying mechanism is still unclear. CX3CR1, the only receptor of CX3CL1, is specifically expressed on retinal microglia, whose activation plays a role in the pathological process of optic nerve injury. This study aimed to evaluate whether CX3CL1 exerts optic neuroprotection by affecting the activation of microglia by combining with CX3CR1.

METHODS

A mouse model of distal optic nerve trauma (ONT) was used to evaluate the effects of the CX3CL1-CX3CR1 axis on the activation of microglia and survival or axonal regeneration of retinal ganglion cells (RGCs). The activation of microglia, loss of RGCs, and damage to visual function were detected weekly till 4 weeks after modeling. CX3CL1 was injected intravitreally immediately or delayed after injury and the status of microglia and RGCs were examined.

RESULTS

Increases in microglia activation and optic nerve damage were accompanied by a reduced production of the CX3CL1-CX3CR1 axis after the distal ONT modeling. Both immediate and delayed intravitreal injection of CX3CL1 inhibited microglia activation, promoted survival of RGCs, and improved axonal regenerative capacity. Injection with CX3CL1 was no longer effective after 48 h post ONT. The CX3CL1-CX3CR1 axis promotes survival and axonal regeneration, as indicated by GAP43 protein and gene expression, of RGCs by inhibiting the microglial activation after ONT.

CONCLUSIONS

The CX3CL1-CX3CR1 axis could promote survival and axonal regeneration of RGCs by inhibiting the microglial activation after optic nerve injury. The CX3CL1-CX3CR1 axis may become a potential target for the treatment of optic nerve injury. Forty-eight hours is the longest time window for effective treatment after injury. The study is expected to provide new ideas for the development of targeted drugs for the repair of optic nerve.

A comparison of machine learning approaches for the quantification of microglial cells in the brain of mice, rats and non-human primates

Microglial cells are brain-specific macrophages that swiftly react to disruptive events in the brain. Microglial activation leads to specific modifications, including proliferation, morphological changes, migration to the site of insult, and changes in gene expression profiles. A change in inflammatory status has been linked to many neurodegenerative diseases such as Parkinson's disease and Alzheimer's disease. For this reason, the investigation and quantification of microglial cells is essential for better understanding their role in disease progression as well as for evaluating the cytocompatibility of novel therapeutic approaches for such conditions. In the following study we implemented a machine learning-based approach for the fast and automatized quantification of microglial cells; this tool was compared with manual quantification (ground truth), and with alternative free-ware such as the threshold-based ImageJ and the machine learning-based Ilastik. We first trained the algorithms on brain tissue obtained from rats and non-human primate immunohistochemically labelled for microglia. Subsequently we validated the accuracy of the trained algorithms in a preclinical rodent model of Parkinson's disease and demonstrated the robustness of the algorithms on tissue obtained from mice, as well as from images provided by three collaborating laboratories. Our results indicate that machine learning algorithms can detect and quantify microglial cells in all the three mammalian species in a precise manner, equipotent to the one observed following manual counting. Using this tool, we were able to detect and quantify small changes between the hemispheres, suggesting the power and reliability of the algorithm. Such a tool will be very useful for investigation of microglial response in disease development, as well as in the investigation of compatible novel therapeutics targeting the brain. As all network weights and labelled training data are made available, together with our step-by-step user guide, we anticipate that many laboratories will implement machine learning-based quantification of microglial cells in their research.

Relationship between mild traumatic brain injury and the gut microbiome: A scoping review

There is increasing evidence for the important role of gut microbiota (GMB) in the development and progression of neurologic pathologies. Some studies have shown that modifying the microbiome profile can confer benefits to patients. Mild traumatic brain injury (mTBI) is a common occurrence in the general population. Although most patients recover, in a minority, disabling symptoms can persist for several months. We carried out a review of the literature to assess the effect of mTBI on GMB and to determine whether alleviating dysbiosis can improve clinical outcomes in mTBI patients. We performed searches in Medline/PubMed and Embase using the keywords "MTBI" AND "microbiome" OR "microbiota". Additional articles were identified by manual searches and using the Google search engine. In animal models, a clear perturbation of GMB was reported following TBI and probiotic supplementation (Lactobacillus acidophilus or Clostridium butyricum) improved neurologic function. There were no studies on changes in GMB after mTBI in humans; however, pre- or probiotic supplementation reduced the infection rate in patients with severe TBI and shortened the time spent in the intensive care unit without conferring any neurologic benefits. Thus, although the findings from animal models are promising, clinical studies are needed to determine whether therapeutic strategies that restore gut microbiome profile can improve long-term outcomes of patients with mTBI.

SARS-CoV-2 Infection in Pregnant Women: Neuroimmune-Endocrine Changes at the Maternal-Fetal Interface

Coronavirus disease 2019 (COVID-19) caused by severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) has devastating effects on the population worldwide. Given this scenario, the extent of the impact of the disease on more vulnerable individuals, such as pregnant women, is of great concern. Although pregnancy may be a risk factor in respiratory virus infections, there are no considerable differences regarding COVID-19 severity observed between pregnant and nonpregnant women. In these circumstances, an emergent concern is the possibility of neurodevelopmental and neuropsychiatric harm for the offspring of infected mothers. Currently, there is no stronger evidence indicating vertical transmission of SARS-CoV-2; however, the exacerbated inflammatory response observed in the disease could lead to several impairments in the offspring's brain. Furthermore, in the face of historical knowledge on possible long-term consequences for the progeny's brain after infection by viruses, we must consider that this might be another deleterious facet of COVID-19. In light of neuroimmune interactions at the maternal-fetal interface, we review here the possible harmful outcomes to the offspring brains of mothers infected by SARS-CoV-2.

LncGBP9/miR-34a axis drives macrophages toward a phenotype conducive for spinal cord injury repair via STAT1/STAT6 and SOCS3

Background

Acute spinal cord injury (SCI) could cause mainly two types of pathological sequelae, the primary mechanical injury, and the secondary injury. The macrophage in SCI are skewed toward the M1 phenotype that might cause the failure to post-SCI repair.

Methods

SCI model was established in Balb/c mice, and the changes in macrophage phenotypes after SCI were monitored. Bioinformatic analyses were performed to select factors that might regulate macrophage polarization after SCI. Mouse bone marrow-derived macrophages (BMDMs) were isolated, identified, and induced for M1 or M2 polarization; the effects of lncRNA guanylate binding protein-9 (lncGBP9) and suppressor of cytokine signaling 3 (SOCS3) on macrophages polarization were examined in vitro and in vivo. The predicted miR-34a binding to lncGBP9 and SOCS3 was validated; the dynamic effects of lncGBP9 and miR-34a on SOCS3, signal transducer and activator of transcription 1 (STAT1)/STAT6 signaling, and macrophage polarization were examined. Finally, we investigated whether STAT6 could bind the miR-34a promoter to activate its transcription.

Results

In SCI Balb/c mice, macrophage skewing toward M1 phenotypes was observed after SCI. In M1 macrophages, lncGBP9 silencing significantly decreased p-STAT1 and SOCS3 expression and protein levels, as well as the production of Interleukin (IL)-6 and IL-12; in M2 macrophages, lncGBP9 overexpression increased SOCS3 mRNA expression and protein levels while suppressed p-STAT6 levels and the production of IL-10 and transforming growth factor-beta 1 (TGF-β1), indicating that lncGBP9 overexpression promotes the M1 polarization of macrophages. In lncGBP9-silenced SCI mice, the M2 polarization was promoted on day 28 after the operation, further indicating that lncGBP9 silencing revised the predominance of M1 phenotype at the late stage of secondary injury after SCI, therefore improving the repair after SCI. IncGBP9 competed with SOCS3 for miR-34a binding to counteract miR-34a-mediated suppression on SOCS3 and then modulated STAT1/STAT6 signaling and the polarization of macrophages. STAT6 bound the promoter of miR-34a to activate its transcription.

Conclusions

In macrophages, lncGBP9 sponges miR-34a to rescue SOCS3 expression, therefore modulating macrophage polarization through STAT1/STAT6 signaling. STAT6 bound the promoter of miR-34a to activate its transcription, thus forming two different regulatory loops to modulate the phenotype of macrophages after SCI.

Neuroprotective effect of tempeh against lipopolysaccharide-induced damage in BV-2 microglial cells

Objectives: This study evaluated the bioactive composition of tempeh products and examined the effects of tempeh on BV-2 microglial cell cytotoxicity, neurotrophic effects, and expression of inflammatory genes.Methods: Tempeh products included soybean fermented by Rhizopus, soybean fermented through cocultivation with Rhizopus and Lactobacillus, and red bean fermented through cocultivation with Rhizopus and Lactobacillus (RT-C). We analyzed the bioactive contents of tempeh extracts and evaluated the effects of tempeh water extract on lipopolysaccharide (LPS)-treated BV-2 cells.Results: The results showed that RT-C water extract had the highest concentrations of γ-aminobutyric acid (GABA) and anthocyanin. The tempeh water extracts, especially RT-C, reduced the formation of LPS-induced reactive oxygen species, downregulated the levels of nitric oxide synthase and phospho-cyclic-AMP response element-binding protein, and upregulated the expression of brain-derived neurotrophic factor (BDNF).Discussion: Our data demonstrate that RT-C has the highest concentrations of GABA and anthocyanin, more effectively reduces oxidative stress and inflammation, and increases the expression of BDNF in LPS-induced BV-2 cells.

Direct contacts of microglia on myelin sheath and Ranvier's node in the corpus callosum in rats

Over the recent years, it has been found that microglia pseudopodia contact synapses, detect sick ones and prune them, even in adult animals. Myelinated nerves also carry out plasticity in which microglia remove myelin debris by phagocytosis. However, it remains unknown whether microglia explore structures on nerve fibers, such as Ranvier's node (RN) or myelin sheath, before they become debris. By double or triple staining RNs or myelin sheathes and microglia in healthy rat corpus callosum, this study unveiled direct contacts of microglia pseudopodia with RNs and with para- and inter-nodal myelin sheathes, which was then verified by electron microscopic observations. Our data indicated that microglia also explore unmyelinated nerve fibers. Furthermore, we used the animals with matured white matter; therefore, microglia may be actively involved in plasticity of matured white matter tracts as it does for synapse pruning, instead of only passively phagocytize myelin debris.

Potential immunotherapies for traumatic brain and spinal cord injury

Traumatic injury of the central nervous system (CNS) including brain and spinal cord remains a leading cause of morbidity and disability in the world. Delineating the mechanisms underlying the secondary and persistent injury versus the primary and transient injury has been drawing extensive attention for study during the past few decades. The sterile neuroinflammation during the secondary phase of injury has been frequently identified substrate underlying CNS injury, but as of now, no conclusive studies have determined whether this is a beneficial or detrimental role in the context of repair. Recent pioneering studies have demonstrated the key roles for the innate and adaptive immune responses in regulating sterile neuroinflammation and CNS repair. Some promising immunotherapeutic strategies have been recently developed for the treatment of CNS injury. This review updates the recent progress on elucidating the roles of the innate and adaptive immune responses in the context of CNS injury, the development and characterization of potential immunotherapeutics, as well as outstanding questions in this field.

Microglia: An Interface between the Loss of Neuroplasticity and Depression

Depression has been widely accepted as a major psychiatric disease affecting nearly 350 million people worldwide. Research focus is now shifting from studying the extrinsic and social factors of depression to the underlying molecular causes. Microglial activity is shown to be associated with pathological conditions, such as psychological stress, pathological aging, and chronic infections. These are primary immune effector cells in the CNS and regulate the extensive dialogue between the nervous and the immune systems in response to different immunological, physiological, and psychological stressors. Studies have suggested that during stress and pathologies, microglia play a significant role in the disruption of neuroplasticity and have detrimental effects on neuroprotection causing neuroinflammation and exacerbation of depression. After a systematic search of literature databases, relevant articles on the microglial regulation of bidirectional neuroimmune pathways affecting neuroplasticity and leading to depression were reviewed. Although, several hypotheses have been proposed for the microglial role in the onset of depression, it is clear that all molecular pathways to depression are linked through microglia-associated neuroinflammation and hippocampal degeneration. Molecular factors such as an excess of glucocorticoids and changes in gene expression of neurotrophic factors, as well as neuro active substances secreted by gut microbiota have also been shown to affect microglial morphology and phenotype resulting in depression. This review aims to critically analyze the various molecular pathways associated with the microglial role in depression.

The Effect of Osteopontin on Microglia

Osteopontin (OPN) is a proinflammatory cytokine that can be secreted from many cells, including activated macrophages and T-lymphocytes, and is widely distributed in many tissues and cells. OPN, a key factor in tissue repairing and extracellular matrix remodeling after injury, is a constituent of the extracellular matrix of the central nervous system (CNS). Recently, the role of OPN in neurodegenerative diseases has gradually caused widespread concern. Microglia are resident macrophage-like immune cells in CNS and play a vital role in both physiological and pathological conditions, including restoring the integrity of the CNS and promoting the progression of neurodegenerative disorders. Microglia's major function is to maintain homeostasis and the normal function of the CNS, both during development and in response to CNS injury. Although the functional mechanism of OPN in CNS neurodegenerative diseases has yet to be fully elucidated, most studies suggest that OPN play a role in pathogenesis of neurodegenerative diseases or in neuroprotection by regulating the activation and function of microglia. Here, we summarize the functions of OPN on microglia in response to various stimulations in vitro and in vivo.

Neurons and astroglia govern microglial endotoxin tolerance through macrophage colony-stimulating factor receptor-mediated ERK1/2 signals

Endotoxin tolerance (ET) is a reduced responsiveness of innate immune cells like macrophages/monocytes to an endotoxin challenge following a previous encounter with the endotoxin. Although ET in peripheral systems has been well studied, little is known about ET in the brain. The present study showed that brain immune cells, microglia, being different from peripheral macrophages, displayed non-cell autonomous mechanisms in ET formation. Specifically, neurons and astroglia were indispensable for microglial ET. Macrophage colony-stimulating factor (M-CSF) secreted from these non-immune cells was essential for governing microglial ET. Neutralization of M-CSF deprived the neuron-glia conditioned medium of its ability to enable microglia to form ET when microglia encountered two lipopolysaccharide (LPS) treatments. Recombinant M-CSF protein rendered enriched microglia refractory to the second LPS challenge leading to microglial ET. Activation of microglial M-CSF receptor (M-CSFR; also known as CSF1R) and the downstream ERK1/2 signals was responsible for M-CSF-mediated microglial ET. Endotoxin-tolerant microglia in neuron-glia cultures displayed M2-like polarized phenotypes, as shown by upregulation of M2 marker Arg-1, elevated production of anti-inflammatory cytokine interleukin 10, and decreased secretion of pro-inflammatory mediators (tumor necrosis factor α, nitric oxide, prostaglandin E2 and interleukin 1β). Endotoxin-tolerant microglia protected neurons against LPS-elicited inflammatory insults, as shown by reduced neuronal damages in LPS pre-treatment group compared with the group without LPS pre-treatment. Moreover, while neurons and astroglia became injured during chronic neuroinflammation, microglia failed to form ET. Thus, this study identified a distinct non-cell autonomous mechanism of microglial ET. Interactions of M-CSF secreted by neurons and astroglia with microglial M-CSFR programed microglial ET. Loss of microglial ET could be an important pathogenetic mechanism of inflammation-associated neuronal damages.

Nuclear Factor Erythroid 2-Related Factor 2 (Nrf2) Mediates Neuroprotection in Traumatic Brain Injury at Least in Part by Inactivating Microglia

Background

Microglial activation has been reported to be involved in traumatic brain injury (TBI). Nuclear factor erythroid 2-related factor 2 (Nrf2) plays a significant role in protecting against TBI-induced secondary brain injury. However, the exact mechanism is not clearly understood. The present study aimed to explore whether Nrf2 protects against TBI partly by regulating microglia function.

Material/Methods

Microglia cells were isolated from C57BL/6 mouse brains (postnatal day 1–3). The expression of Nrf2 was suppressed by transfection with Nrf2-specific small interfering RNA (siRNA), and overexpressed by transfections with pcDNA3.1-Nrf2. The expression of Nrf2 was confirmed by real-time PCR and Western blotting. After transfection, cell viability, phagocytic ability, and the expression of pro-inflammatory cytokines (tumor necrosis factor (TNF)-α and interleukin (IL)-6) were determined by 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyltetrazolium bromide (MTT) colorimetric assay, phagocytosis assay, and enzyme-linked immunosorbent assay (ELISA), respectively.

Results

mRNA and protein expression levels of Nrf2 were significantly reduced by transfection with Nrf2-specific siRNA (both P<0.05) but were elevated by transfection with pcDNA3.1-Nrf2 (both P<0.01). The cell viability, phagocytic ability, and the expression of TNF-α and IL-6 were all significantly reduced by overexpression of Nrf2 but were significantly increased by silencing of Nrf2 compared with the control group.

Conclusions

Our results suggest that Nrf2 protects against TBI, at least part by regulating microglia function.

TREM2 Protein Expression Changes Correlate with Alzheimer's Disease Neurodegenerative Pathologies in Post-Mortem Temporal Cortices

Triggering receptor expressed by myeloid cells 2 (TREM2), a member of the immunoglobulin superfamily, has anti-inflammatory phagocytic function in myeloid cells. Several studies have shown that TREM2 gene variant rs75932628-T increased the risks for Alzheimer's disease (AD), Parkinson's disease, frontotemporal dementia and amyotrophic lateral sclerosis. It has been suggested that the risks could be resulted from the loss of TREM2 function caused by the mutation. Indeed, new evidence showed that several mutations in the immunoglobulin-like V-region led to low cell surface expression of TREM2 and reduced phagocytic function. Because of the emerging importance in understanding TREM2 expression and functions in human neurodegenerative diseases, we conducted biochemical and morphological studies of TREM2 expression in human post-mortem temporal cortical samples from AD and normal cases. Increased expression of TREM2 protein was found to significantly correlate with increases of phosphorylated-tau and active caspase 3, a marker of apoptosis, and also loss of the presynaptic protein SNAP25. Strong intensities of TREM2 immunoreactivity were observed in the microglia associated with amyloid plaques and in neuritic pathology-enriched areas. Based on the findings that TREM2 expression correlated with neurodegenerative markers, further investigation on whether there is abnormality of TREM2 functions in AD brains with nonmutated TREM2 is needed.

Development of NMDAR antagonists with reduced neurotoxic side effects: a study on GK11

The NMDAR glutamate receptor subtype mediates various vital physiological neuronal functions. However, its excessive activation contributes to neuronal damage in a large variety of acute and chronic neurological disorders. NMDAR antagonists thus represent promising therapeutic tools that can counteract NMDARs' overactivation. Channel blockers are of special interest since they are use-dependent, thus being more potent at continuously activated NMDARs, as may be the case in pathological conditions. Nevertheless, it has been established that NMDAR antagonists, such as MK801, also have unacceptable neurotoxic effects. Presently only Memantine is considered a safe NMDAR antagonist and is used clinically. It has recently been speculated that antagonists that preferentially target extrasynaptic NMDARs would be less toxic. We previously demonstrated that the phencyclidine derivative GK11 preferentially inhibits extrasynaptic NMDARs. We thus anticipated that this compound would be safer than other known NMDAR antagonists. In this study we used whole-genome profiling of the rat cingulate cortex, a brain area that is particularly sensitive to NMDAR antagonists, to compare the potential adverse effects of GK11 and MK801. Our results showed that in contrast to GK11, the transcriptional profile of MK801 is characterized by a significant upregulation of inflammatory and stress-response genes, consistent with its high neurotoxicity. In addition, behavioural and immunohistochemical analyses confirmed marked inflammatory reactions (including astrogliosis and microglial activation) in MK801-treated, but not GK11-treated rats. Interestingly, we also showed that GK11 elicited less inflammation and neuronal damage, even when compared to Memantine, which like GK11, preferentially inhibits extrasynaptic NMDAR. As a whole, our study suggests that GK11 may be a more attractive therapeutic alternative in the treatment of CNS disorders characterized by the overactivation of glutamate receptors.

Development of the microglial phenotype in culture

Selected morphological, molecular and functional aspects of various microglial cell populations were characterized in cell cultures established from the forebrains of E18 rat embryos. The mixed primary cortical cultures were maintained for up to 28days using routine culturing techniques when the microglial cells in the culture were not stimulated or immunologically challenged. During culturing, expansion of the microglial cell populations was observed, as evidenced by quantitative assessment of selected monocyte/macrophage/microglial cell-specific markers (human leukocyte antigen (HLA) DP, DQ, DR, CD11b/c and Iba1) via immunocyto- and histochemistry and Western blot analysis. The Iba1 immunoreactivity in Western blots steadily increased about 750-fold, and the number of Iba1-immunoreactive cells rose at least 67-fold between one day in vitro (DIV1) and DIV28. Morphometric analysis on binary (digital) silhouettes of the microglia revealed their evolving morphology during culturing. Microglial cells were mainly ameboid in the early stages of in vitro differentiation, while mixed populations of ameboid and ramified cell morphologies were characteristic of older cultures as the average transformation index (TI) increased from 1.96 (DIV1) to 15.17 (DIV28). Multiple immunofluorescence labeling of selected biomarkers revealed different microglial phenotypes during culturing. For example, while HLA DP, DQ, DR immunoreactivity was present exclusively in ameboid microglia (TI<3) between DIV1 and DIV10, CD11b/c- and Iba1-positive microglial cells were moderately (TI<13) and progressively (TI<81) more ramified, respectively, and always present throughout culturing. Regardless of the age of the cultures, proliferating microglia were Ki67-positive and characterized by low TI values (TI<3). The microglial function was assessed by an in vitro phagocytosis assay. Unstimulated microglia with low TI values were significantly more active in phagocytosing fluorescent microspheres than the ramified forms. In vitro studies on microglial population dynamics combined with phenotypic characterization can be of importance when different in vivo pathophysiological situations are modeled in vitro.

Preparation of primary microglia cultures from postnatal mouse and rat brains

Microglia are the inflammatory cells of the brain and are activated in neuropathological conditions. To study the biology of microglia, these cells can be isolated from the brain and analyzed in terms of pro- and anti-inflammatory cytokine production, involvement of intracellular signaling pathways upon inflammatory stimuli, phagocytosis, and several more biological aspects to understand their role in the brain. In this book chapter, I will discuss microglial cells and describe how these cells can be cultured from a postnatal mouse and rat brain.

Acute neuroimmune modulation attenuates the development of anxiety-like freezing behavior in an animal model of traumatic brain injury

Chronic anxiety is a common and debilitating result of traumatic brain injury (TBI) in humans. While little is known about the neural mechanisms of this disorder, inflammation resulting from activation of the brain's immune response to insult has been implicated in both human post-traumatic anxiety and in recently developed animal models. In this study, we used a lateral fluid percussion injury (LFPI) model of TBI in the rat and examined freezing behavior as a measure of post-traumatic anxiety. We found that LFPI produced anxiety-like freezing behavior accompanied by increased reactive gliosis (reflecting neuroimmune inflammatory responses) in key brain structures associated with anxiety: the amygdala, insula, and hippocampus. Acute peri-injury administration of ibudilast (MN166), a glial cell activation inhibitor, suppressed both reactive gliosis and freezing behavior, and continued neuroprotective effects were apparent several months post-injury. These results support the conclusion that inflammation produced by neuroimmune responses to TBI play a role in post-traumatic anxiety, and that acute suppression of injury-induced glial cell activation may have promise for the prevention of post-traumatic anxiety in humans.

Traumatic brain injury, microglia, and Beta amyloid

Recently, there has been growing interest in the association between traumatic brain injury (TBI) and Alzheimer's Disease (AD). TBI and AD share many pathologic features including chronic inflammation and the accumulation of beta amyloid (Aβ). Data from both AD and TBI studies suggest that microglia play a central role in Aβ accumulation after TBI. This paper focuses on the current research on the role of microglia response to Aβ after TBI.

Interaction of HmC1q with leech microglial cells: involvement of C1qBP-related molecule in the induction of cell chemotaxis

Background

In invertebrates, the medicinal leech is considered to be an interesting and appropriate model to study neuroimmune mechanisms. Indeed, this non-vertebrate animal can restore normal function of its central nervous system (CNS) after injury. Microglia accumulation at the damage site has been shown to be required for axon sprouting and for efficient regeneration. We characterized HmC1q as a novel chemotactic factor for leech microglial cell recruitment. In mammals, a C1q-binding protein (C1qBP alias gC1qR), which interacts with the globular head of C1q, has been reported to participate in C1q-mediated chemotaxis of blood immune cells. In this study, we evaluated the chemotactic activities of a recombinant form of HmC1q and its interaction with a newly characterized leech C1qBP that acts as its potential ligand.

Methods

Recombinant HmC1q (rHmC1q) was produced in the yeast Pichia pastoris. Chemotaxis assays were performed to investigate rHmC1q-dependent microglia migration. The involvement of a C1qBP-related molecule in this chemotaxis mechanism was assessed by flow cytometry and with affinity purification experiments. The cellular localization of C1qBP mRNA and protein in leech was investigated using immunohistochemistry and in situ hybridization techniques.

Results

rHmC1q-stimulated microglia migrate in a dose-dependent manner. This rHmC1q-induced chemotaxis was reduced when cells were preincubated with either anti-HmC1q or anti-human C1qBP antibodies. A C1qBP-related molecule was characterized in leech microglia.

Conclusions

A previous study showed that recruitment of microglia is observed after HmC1q release at the cut end of axons. Here, we demonstrate that rHmC1q-dependent chemotaxis might be driven via a HmC1q-binding protein located on the microglial cell surface. Taken together, these results highlight the importance of the interaction between C1q and C1qBP in microglial activation leading to nerve repair in the medicinal leech.

The spider effect: morphological and orienting classification of microglia in response to stimuli in vivo

The different morphological stages of microglial activation have not yet been described in detail. We transected the olfactory bulb of rats and examined the activation of the microglial system histologically. Six stages of bidirectional microglial activation (A) and deactivation (R) were observed: from stage 1A to 6A, the cell body size increased, the cell process number decreased, and the cell processes retracted and thickened, orienting toward the direction of the injury site; until stage 6A, when all processes disappeared. In contrast, in deactivation stages 6R to 1R, the microglia returned to the original site exhibiting a stepwise retransformation to the original morphology. Thin highly branched processes re-formed in stage 1R, similar to those in stage 1A. This reverse transformation mirrored the forward transformation except in stages 6R to 1R: cells showed multiple nuclei which were slowly absorbed. Our findings support a morphologically defined stepwise activation and deactivation of microglia cells.

Cannabinoids inhibit migration of microglial-like cells to the HIV protein Tat

Microglia are a population of macrophage-like cells in the central nervous system (CNS) which, upon infection by the human immunodeficiency virus (HIV), secrete a plethora of inflammatory factors, including the virus-specified trans-activating protein Tat. Tat has been implicated in HIV neuropathogenesis since it elicits chemokines, cytokines, and a chemotactic response from microglia. It also harbors a β-chemokine receptor binding motif, articulating a mode by which it acts as a migration stimulus. Since select cannabinoids have anti-inflammatory properties, cross the blood-brain barrier, and target specific receptors, they have potential to serve as agents for dampening untoward neuroimmune responses. The aim of this study was to investigate the effect of select cannabinoids on the migration of microglial-like cells toward Tat. Using a mouse BV-2 microglial-like cell model, it was demonstrated that the exogenous cannabinoids Delta-9-tetrahydrocannabinol (THC) and CP55940 exerted a concentration-related reduction in the migration of BV-2 cells towards Tat. A similar inhibitory response was obtained when the endogenous cannabinoid 2-arachidonoylglycerol (2-AG) was used. The CB(2) receptor (CB2R) antagonist SR144528, but not the CB(1) receptor (CB1R) antagonist SR141716A, blocked this inhibition of migration. Similarly, CB2R knockdown with small interfering RNA reversed the cannabinoid-mediated inhibition. In addition, the level of the β-chemokine receptor CCR-3 was reduced and its intracellular compartmentation was altered. These results indicate that cannabinoid-mediated inhibition of BV-2 microglial-like cell migration to Tat is linked functionally to the CB2R. Furthermore, the results indicate that activation of the CB2R leads to altered expression and compartmentation of the β-chemokine receptor CCR-3.

Inflammation after trauma: microglial activation and traumatic brain injury

OBJECTIVE

Patient outcome after traumatic brain injury (TBI) is highly variable. The underlying pathophysiology of this is poorly understood, but inflammation is potentially an important factor. Microglia orchestrate many aspects of this response. Their activation can be studied in vivo using the positron emission tomography (PET) ligand [11C](R)PK11195 (PK). In this study, we investigate whether an inflammatory response to TBI persists, and whether this response relates to structural brain abnormalities and cognitive function.

METHODS

Ten patients, studied at least 11 months after moderate to severe TBI, underwent PK PET and structural magnetic resonance imaging (including diffusion tensor imaging). PK binding potentials were calculated in and around the site of focal brain damage, and in selected distant and subcortical brain regions. Standardized neuropsychological tests were administered.

RESULTS

PK binding was significantly raised in the thalami, putamen, occipital cortices, and posterior limb of the internal capsules after TBI. There was no increase in PK binding at the original site of focal brain injury. High PK binding in the thalamus was associated with more severe cognitive impairment, although binding was not correlated with either the time since the injury or the extent of structural brain damage.

INTERPRETATION

We demonstrate that increased microglial activation can be present up to 17 years after TBI. This suggests that TBI triggers a chronic inflammatory response particularly in subcortical regions. This highlights the importance of considering the response to TBI as evolving over time and suggests interventions may be beneficial for longer intervals after trauma than previously assumed.

Astrogliosis in CNS pathologies: is there a role for microglia?

Astrogliosis, a cellular reaction with specific structural and functional characteristics, represents a remarkably homotypic response of astrocytes to all kinds of central nervous system (CNS) pathologies. Astrocytes play diverse functions in the brain, both harmful and beneficial. Mounting evidence indicates that astrogliosis is an underlying component of a diverse range of diseases and associated neuropathologies. The mechanisms that lead to astrogliosis are not fully understood, nevertheless, damaged neurons have long been reported to induce astrogliosis and astrogliosis has been used as an index for underlying neuronal damage. As the predominant source of proinflammatory factors in the CNS, microglia are readily activated under certain pathological conditions. An increasing body of evidence suggests that release of cytokines and other soluble products by activated microglia can significantly influence the subsequent development of astrogliosis and scar formation in CNS. It is well known that damaged neurons activate microglia very quickly, therefore, it is possible that activated microglia contribute factors/mediators through which damaged neuron induce astrogliosis. The hypothesis that activated microglia initiate and maintain astrogliosis suggests that suppression of microglial overactivation might effectively attenuate reactive astrogliosis. Development of targeted anti-microglial activation therapies might slow or halt the progression of astrogliosis and, therefore, help achieve a more beneficial environment in various CNS pathologies.

Intravitreous delivery of the corticosteroid fluocinolone acetonide attenuates retinal degeneration in S334ter-4 rats

PURPOSE

To study the neuroprotective properties of low-dose, sustained-release intravitreous fluocinolone acetonide (FA) in transgenic S334ter-4 rats.

METHODS

S334ter-4 rats aged 4 weeks were divided into four groups: 0.5 microg/d FA-loaded intravitreous drug delivery implant (IDDI); 0.2 microg/d FA-loaded IDDI; inactive IDDI; and unoperated controls. Electroretinography (ERG) was performed before surgery and every 2 weeks after surgery for 8 weeks. When the rats were 12 weeks of age, outer nuclear layer (ONL) and inner nuclear layer (INL) thicknesses were measured. Microglial cell counts were obtained from retinal wholemounts labeled for Iba-1.

RESULTS

At the end of the study, unoperated and inactive IDDI-implanted rats demonstrated 50% to 60% reductions in ERG amplitudes compared with those recorded at 4 weeks (P < 0.001 for both groups). FA 0.2-microg/d animals demonstrated 15% amplitude attenuation, while FA 0.5-microg/d animals showed 30% reduction. ONL thickness in FA 0.2-microg/d-treated eyes was 25.8% +/- 2.3% higher than in control group eyes (P < 0.001) and 30.0% +/- 2.1% higher than in inactive IDDI-implanted eyes (P < 0.001). In FA 0.5-microg/d-treated eyes, ONL thickness was 22.4% +/- 2.8% higher than in control group eyes (P < 0.001) and 22.3% +/- 3.7% higher than in inactive IDDI-implanted eyes (P < 0.01). No statistically significant difference was observed between the two control groups. No statistically significant difference between the two FA-treated groups was found. FA-treated groups demonstrated significantly fewer activated microglial cells than control groups.

CONCLUSIONS

Chronic intravitreous infusion of FA preserves ONL cell morphology and ERG a- and b-wave amplitudes and reduces retinal neuroinflammation in S334ter rats. Based on these findings, the synthetic corticosteroid FA may promise a therapeutic role in patients with retinal degeneration.

Cannabinoids as therapeutic agents for ablating neuroinflammatory disease

Cannabinoids have been reported to alter the activities of immune cells in vitro and in vivo. These compounds may serve as ideal agents for adjunct treatment of pathological processes that have a neuroinflammatory component. As highly lipophilic molecules, they readily access the brain. Furthermore, they have relatively low toxicity and can be engineered to selectively target cannabinoid receptors. To date, two cannabinoid receptors have been identified, characterized and designated CB(1) and CB(2). CB(1) appears to be constitutively expressed within the CNS while CB(2) apparently is induced during inflammation. The inducible nature of expression of CB(2) extends to microglia, the resident macrophages of the brain that play a critical role during early stages of inflammation in that compartment. Thus, the cannabinoid-cannabinoid receptor system may prove therapeutically manageable in ablating neuropathogenic disorders such as Alzheimer's disease, multiple sclerosis, amyotrophic lateral sclerosis, HIV encephalitis, closed head injury, and granulomatous amebic encephalitis.

Why neurodegenerative diseases are progressive: uncontrolled inflammation drives disease progression

Neurodegenerative diseases are a group of chronic, progressive disorders characterized by the gradual loss of neurons in discrete areas of the central nervous system (CNS). The mechanism(s) underlying their progressive nature remains unknown but a timely and well-controlled inflammatory reaction is essential for the integrity and proper function of the CNS. Substantial evidence has documented a common inflammatory mechanism in various neurodegenerative diseases. We hypothesize that in the diseased CNS, interactions between damaged neurons and dysregulated, overactivated microglia create a vicious self-propagating cycle causing uncontrolled, prolonged inflammation that drives the chronic progression of neurodegenerative diseases. We further propose that dynamic modulation of this inflammatory reaction by interrupting the vicious cycle might become a disease-modifying therapeutic strategy for neurodegenerative diseases.

CB2 receptors in the brain: role in central immune function

Recently, it has been recognized that the cannabinoid receptor CB2 may play a functionally relevant role in the central nervous system (CNS). This role is mediated primarily through microglia, a resident population of cells in the CNS that is morphologically, phenotypically, and functionally related to macrophages. These cells also express the cannabinoid receptor CB1. The CB1 receptor (CB1R) is constitutively expressed at low levels while the CB2 receptor (CB2R) is expressed at higher levels and is modulated in relation to cell activation state. The relatively high levels of the CB2R correspond with microglia being in 'responsive' and 'primed' states, suggesting the existence of a 'window' of functional relevance during which activation of the CB2R modulates microglial activities. Signature activities of 'responsive' and 'primed' microglia are chemotaxis and antigen processing, respectively. The endocannabinoid 2-arachidonylglycerol has been reported to stimulate a chemotactic response from these cells through the CB2R. In contrast, we have shown in vivo and in vitro that the exogenous cannabinoids delta-9-tetrahydrocannabinol and CP55940 inhibit the chemotactic response of microglia to Acanthamoeba culbertsoni, an opportunistic pathogen that is the causative agent of Granulomatous Amoebic Encephalitis, through activation of the CB2R. It is postulated that these exogenous cannabinoids superimpose an inhibitory effect on pro-chemotactic endocannabinoids that are elicited in response to Acanthamoeba. Furthermore, the collective results suggest that the CB2R plays a critical immune functional role in the CNS.

Reduced axon sprouting after treatment that diminishes microglia accumulation at lesions in the leech CNS

The role of mammalian microglia in central nervous system (CNS) repair is controversial. Microglia accumulate at lesions where they act as immune cells and phagocytize debris, and they may secrete neurotrophins, but they also produce molecules that can be cytotoxic, like nitric oxide (NO). To determine the importance of microglial accumulation at lesions on growth of severed CNS axons in the leech (Hirudo medicinalis), in which axon and synapse regeneration are notably successful even when isolated in tissue culture medium, microglial migration to lesions was reduced. Pressure (P) sensory neurons were injected with biocytin to reveal the extent of their sprouting 24 hours after lesioning. To reduce microglia accumulation at lesions, cords were treated for 3.5 hours with 3 mM ATP or 2 mM N(omega)-nitro-L-arginine methyl ester (L-NAME) or 50 microM Reactive blue-2 (RB2) beginning 30 minutes before injury. Lesioned controls were either not treated with drug or treated 3 hours later with one of the drugs, after the migration and subsequent accumulation of most microglia had occurred, but before the onset of axon sprouting, for a total of seven separate conditions. There was a significant reduction in total sprout lengths compared with controls when microglial accumulation was reduced. The results suggest that microglial cells are necessary for the usual sprouting of injured axons.

Using gene delivery to protect HIV-susceptible CNS cells: inhibiting HIV replication in microglia

Antiretroviral chemotherapy penetrates the CNS poorly. CNS HIV, thus sheltered, may injure the brain and complicate control of systemic HIV infection. Microglial cells play a major role in HIV persistence in the CNS but are rarely targeted for gene delivery. Because recombinant SV40 vectors (rSV40s) transduce other phagocytic cells efficiently, we tested rSV40 delivery of anti-HIV genetic therapy to microglial cells. Microglia prepared as enriched cultures from human fetal brain, were transduced with marker vectors, SV(RFP) and SV(Nef/FLAG), respectively, carrying DsRed and HIV-1 Nef bearing a FLAG epitope. By immunostaining and FACS, 95% of unselected cells expressed the transgenes, without detectable toxicity. Microglia were transduced with SV(AT), carrying human alpha1-antitrypsin (alpha1AT), which blocks Env and Gag processing. SV(AT)-treated microglia strongly resisted challenge with HIV-1BaL, even when microglia were transduced with SV(AT) following HIV challenge. Thus, rSV40s effectively transduce microglia and protect them from HIV.

Development of a culture system that supports adult microglial cell proliferation and maintenance in the resting state

Microglial cells constitute what is considered to be a fixed macrophage population in the central nervous system (CNS), which are broadly implicated in the regulation of neuroinflammation. In the normal adult CNS, microglial cells exist in a resting state characterized by a minimal or negative expression of MHC class II and the co-stimulatory molecules CD80, CD86 and CD40 and exhibit a unique ramified morphology. Microglial cell activation is associated with many inflammatory and neurogenerative CNS pathologies and is characterized by the transformation of resting microglia into cells with a macrophage morphology and up-regulation of MHC class II and co-stimulatory molecules. The cellular and molecular mechanisms required for microglial cell activation and their immunological functions in the adult brain still remain enigmatic, primarily due to the lack of an appropriate culture system that both facilitates microglial survival and expansion in the resting state. Here, we describe a new M-CSF-dependent culture system that overcomes these barriers and allows the long-term proliferation and maintenance of resting adult microglial cells isolated from the CNS. These cultured microglial cells retain their plasticity as indicated by their ability to up-regulate MHC class II and differentiate into cells with a macrophage morphology following the addition of IFN-gamma and GM-CSF, or activated T cells, which produce both cytokines. By measuring the proliferation of the T cells, we were also able to demonstrate that the microglial cells differentiated into fully functional antigen presenting cells. In addition, the replacement of the M-CSF with GM-CSF resulted in the differentiation of microglial cells into cells morphologically and phenotypically similar to dendritic cells. Our microglial cell culture system is the first described that allows the expansion of adult cells in the resting state and will facilitate studies examining the specific mechanisms of microglial cell activation and functions involved in a variety of CNS pathologies.

Ibuprofen and apigenin induce apoptosis and cell cycle arrest in activated microglia

In case of injury or disease, microglia are recruited to the site of the pathology and become activated as evidenced by morphological changes and expression of pro-inflammatory cytokines. Evidence suggests that microglia proliferate by cell division to create gliosis at the site of pathological conditions such as the amyloid plaques in Alzheimer's disease and the substantia nigra of Parkinson's disease patients. The hyperactivation of microglia contributes to neurotoxicity. In the present study we tested the hypothesis that anti-inflammatory compounds modulate the progression of cell cycle and induce apoptosis of the activated cells. We investigated the effects of ibuprofen (non-steroidal anti-inflammatory drug) and apigenin (a flavonoid with anti-inflammatory and anti-proliferative properties) on the cell cycle of the murine microglial cell line BV-2. The findings indicate that apigenin-induced cell cycle arrest preferentially in the G2/M phase and ibuprofen caused S phase arrest. The binding of annexin V-FITC to the membranes of cells which indicates the apoptotic process were examined, whereas the DNA was stained with propidium iodide. Both apigenin and ibuprofen induced apoptosis significantly in early and late stages. The induction of apoptosis by ibuprofen and apigenin was confirmed using TUNEL assay, revealing that 25 microM apigenin and 250 microM ibuprofen significantly increased apoptosis in BV-2 cells. The results from the present study suggest that anti-inflammatory compounds might inhibit microglial proliferation by modulating the cell cycle progression and apoptosis.

The Nogo signaling pathway for regeneration block

A hostile environment and decreased regenerative capacity may contribute to the failure of axon regeneration in the adult central nervous system. Recent studies leading to the identification of several myelin-associated inhibitors and their signaling molecules provide opportunitities to assess the contribution of these inhibitory molecules in restricting axon regeneration. These findings may ultimately allow for the development of strategies to alleviate the inhibitory effects of such molecules in an effort to encourage axon regeneration after spinal cord and brain injury.

Neurotrophins regulate proliferation and survival of two microglial cell lines in vitro

Microglia are thought to play a key role in the development and regeneration of the central nervous system although the mechanisms regulating their presence and activity are not fully understood. Substantial evidence suggests that members of the neurotrophin family such as nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 and -4 (NT-3/4) have a dramatic effect on both neurons and perineuronal cells. This study employed two murine microglial lines, BV-2 and N9, to examine the action of these neurotrophins on the mitotic activity and survival of microglia in vitro. Neurotrophins were incorporated into the media at the time of plating and cell number and levels of mitochondrial dehydrogenase activity (MTT) were determined at various time points in vitro. NGF increased cell number and MTT levels of both cell lines in a dose-dependent manner. BV-2 was more sensitive to NGF than N9. Similar responses were elicited by BDNF, although the sensitivity of each cell line was different than that found for NGF. NT-3 and NT-4 had no effect on cell proliferation. However, NT-4 had an effect on the survival of BV-2 and N9 cells. The response of these cells to neurotrophins was blocked by K252a, a tyrosine kinase inhibitor, suggesting that actions of neurotrophins were mediated by high-affinity tyrosine kinase receptors (Trk). Immunolocalization studies revealed positive Trk (pan) reactivity in the above cell lines and in primary microglia, but an absence of the low-affinity p75 neurotrophin receptor. Western blot analysis supported the above observations. These studies suggest that in addition to their neurotrophic actions, NGF and BDNF may also regulate microglial dynamics, thereby influencing the surrounding milieu during neuronal regeneration.

The scavenger receptor MARCO mediates cytoskeleton rearrangements in dendritic cells and microglia

Macrophage receptor with collagenous structure (MARCO) is a scavenger receptor expressed in peritoneal macrophages and in a subpopulation of macrophages in the marginal zone of the spleen and in the medullary cord of lymph nodes. By global gene expression analysis, it has been found that the MARCO mRNA was one of the most up-regulated in splenic dendritic cells (DCs) following lipopolysaccharide or bacterial activation and in granulocyte-macrophage colony-stimulating factor (GM-CSF)-treated microglial cells. Here we show that MARCO is expressed on splenic DCs at late time points after activation and that its expression correlates with profound changes in actin cytoskeleton organization in DCs and microglia. During maturation, DCs undergo profound rearrangements of actin cytoskeleton. Immature DCs are adherent with visible actin cables, while fully mature, MARCO-expressing, splenic DCs are nonadherent, round in shape, and have an actin cytoskeleton with a punctate distribution. The simple expression of MARCO was sufficient to induce these cytoskeleton modifications in DCs. MARCO-transfected immature DCs acquired a typical morphology of mature DCs and did not rearrange the actin cytoskeleton following activation. Moreover, DCs in which MARCO was knocked down did not reach the mature phenotype and maintained the typical morphology of transitional DCs. MARCO expression in DCs and microglial cells was also associated with a decrease of antigen internalization capacity. Thus, the MARCO receptor is important for actin cytoskeleton rearrangements and the down-regulation of antigen uptake function during DC and microglial cell maturation.

Activation of microglia: a neuroinflammatory role for CAP37

Recent evidence suggests that inflammation and immune function in the central nervous system (CNS) may play a considerable role in the progression of many neurodegenerative diseases. It is known that microglia, the CNS equivalent of peripheral blood monocytes, may be instrumental in causing neurotoxicity. However, the mediator(s) that activates microglia to produce toxic substances that orchestrate cell death has yet to be elucidated. We have identified a novel inflammatory molecule, cationic antimicrobial protein of molecular weight 37 kDa (CAP37), to the brains of patients dying from Alzheimer's disease. CAP37 is known to be a potent activator and regulator of monocyte function in the systemic circulation. We hypothesize that CAP37, a mediator previously shown to recruit and activate monocytes in the systemic circulation, may also play a role in CNS inflammation by modulating microglial function. Here we demonstrate that CAP37 is a chemoattractant for microglia and that CAP37-treated microglia express class II major histocompatibility antigens and produce proinflammatory cytokines and chemokines. We conclude that CAP37 has the ability to activate microglial cells and suggest that it has the potential to serve as a neuroinflammatory molecule.

Granulocyte-macrophage colony-stimulating factor induces an expression program in neonatal microglia that primes them for antigen presentation

Neonatal microglial cells respond to GM-CSF and M-CSF by acquiring different morphologies and phenotypes. To investigate the extent and consequences of this process, a global gene expression analysis was performed, with significant changes in transcript levels confirmed by biochemical analyses. Primary murine microglial cells underwent substantial expression reprogramming after treatment with GM-CSF or M-CSF with many differentially expressed transcripts important in innate and adaptive immunity. In particular, many gene products involved in Ag presentation were induced by GM-CSF, but not M-CSF, thus potentially priming relatively quiescent microglia cells for Ag presentation. This function of GM-CSF is distinct from its primary function in cell proliferation and survival.

Reduced inflammatory reactions to the inoculation of helper-dependent adenoviral vectors in traumatically injured rat brain

Traumatic brain injury (TBI) causes delayed neuronal deficits that in principle could be prevented by timely intervention with therapeutic genes. However, appropriate vectors for gene transfer to the brain with TBI remain to be developed. First-generation adenoviruses (fgAd) are usually associated with inflammatory and toxic effects when inoculated into brains, despite their high efficiency of gene transfer to these tissues. In this study the authors attempted to determine whether a less immunogenic gene-transfer protocol can be established in the traumatically injured rat brain using helper-dependent adenoviruses (hdAd), a novel adenoviral construct with full deletion of viral coding sequences. Their results show that transgene expression from intrahippocampally inoculated hdAd is maintained for at least 2 months after TBI, in contrast to the much shorter duration of fgAd-mediated gene expression. There was only minimal secretion of proinflammatory IL-1beta and TNF-alpha after inoculation of hdAd. Furthermore, the hdAd-mediated gene expression was associated with less microglial proliferation, astrocytic activation, and macrophage infiltration than observed in fgAd-inoculated brains. There was no additional tissue loss after hdAd inoculation compared with PBS injection. Although both anti-adenoviral and neutralizing antibodies were found in serum after brain inoculation of hdAd, they did not appear to affect transgene expression. The results suggest that hdAd are less immunogenic vectors than conventional adenoviral vectors, and offer improved vehicles for long-term therapeutic transgene transfer to traumatically injured brains.

Infection of baboon microglia with SIV-HIV recombinant viruses: role of CD4 and chemokine receptors

Microglia constitute the primary cell type infected with HIV in the brain and play a major role in viral persistence in the CNS and in the development of AIDS dementia. Lack of a suitable animal model and limitations in the availability of human tissues hinder most HIV/AIDS studies investigating the neuropathogenesis of AIDS dementia. The aims of this study were to determine whether baboon microglia can be productively infected with SIV-HIV (SHIV) recombinant viruses in vitro and whether they express HIV-1 receptors and coreceptors. Our results show the presence of mRNA for CD4, CCR5, and CXCR4 chemokine receptors on baboon microglial cells. Microglia lacked mRNA for the CCR3 chemokine receptor. We also show productive infection of baboon microglial cells by two SHIV isolates, SHIV-KU and SHIV-89.6P, and blockade of the infection with soluble CD4 protein, CCR5, and CXCR4 monoclonal antibodies. This study demonstrating the feasibility of infecting baboon microglia with SHIV isolates is an important first step in using the baboon as an alternative nonhuman primate model to study HIV neuropathogenesis.

Systemic administration of kainic acid in adult rat stimulates expression of the chemokine receptor CCR5 in the forebrain

As chemokines and their receptors are primarily expressed by glial cells in brain parenchyma, a model of glial cell proliferation may be useful to study the regulation of their expression in the brain. The well-established kainic acid seizure model was used in this study, focusing on the expression of the CCR5 chemokine receptor. Adult Sprague-Dawley rats were injected intraperitoneally with kainic acid (12 mg/kg), and in situ hybridization of CCR5 mRNA was performed at 12 h, 1, 3, or 7 days, posttreatment. Autoradiographic films and wet photographic emulsions demonstrated the very low expression of CCR5 mRNA in normal brain parenchyma, as well as in the microvasculature and ventricular/choroid plexus systems. After kainic acid treatment, brain CCR5 mRNA expression increased progressively from 12 h to 7 days, especially in the olfactory system, amygdaloid complex, thalamus, hippocampal formation, septum, and neocortex. This increase paralleled that of activated microglial cells as shown, using the microglial marker, OX-42. Moreover, CCR5 mRNA ISH combined with neuron-specific enolase immunocytochemistry showed that, in addition to its glial expression, CCR5 mRNA is expressed in neurons in the normal brain and, to a lesser extent, after kainate treatment due to neuronal losses. Finally, CCR5 protein is detected by immunocytochemistry in neurodegenerative areas in numerous glial cells, as well as in neurons, as clearly shown in the hippocampal formation. In summary, the chemokine receptor CCR5 is expressed by neuronal and non-neuronal cell types in the normal brain and is upregulated in both cell types after an insult.

Developmental plasticity of CNS microglia

Microglia arise from CD45(+) bone marrow precursors that colonize the fetal brain and play a key role in central nervous system inflammatory conditions. We report that parenchymal microglia are uncommitted myeloid progenitors of immature dendritic cells and macrophages by several criteria, including surface expression of "empty" class II MHC protein and their cysteine protease (cathepsin) profile. Microglia express receptors for stem cell factor and can be skewed toward more dendritic cell or macrophage-like profiles in response to the lineage growth factors granulocyte/macrophage colony-stimulating factor or macrophage colony-stimulating factor. Thus, in contrast to other organs, where terminally differentiated populations of resident dendritic cells and/or macrophages outnumber colonizing precursors, the majority of microglia within the brain remain in an undifferentiated state.

Characterization of a novel brain-derived microglial cell line isolated from neonatal rat brain

We observed highly aggressively proliferating immortalized (HAPI) cells growing in cultures that had been enriched for microglia. The cells were initially obtained from mixed glial cultures prepared from 3-day-old rat brains. HAPI cells are typically round with few or no processes when cultured in 10% serum containing medium. As the percentage of serum in the medium is decreased, the HAPI cells have more processes. HAPI cells stain for the isolectin B4, OX-42, and GLUT5, which are markers for microglial cells, but the cells do not immunolabel with A2B5, a marker of cells in the oligodendroglial cell lineage, or with the astrocyte-specific marker, glial fibrillary aciidic protein (GFAP). In addition, HAPI cells are capable of phagocytosis. We conclude that HAPI cells are of microglia/macrophage lineage. Exposing HAPI cells to lipopolysaccharide (LPS) induces the mRNAs for tumor necrosis factor-alpha (TNF-alpha) and inducible nitric oxide synthase (iNOS). LPS exposure also induces secretion of TNF-alpha and production of nitric oxide (NO) in HAPI cells. Because activation of microglia is associated with an increase in iron accumulation and ferritin expression, we tested the hypothesis that iron status affects the production of TNF-alpha and NO. Our studies demonstrate that both iron chelation and iron loading diminished the LPS-induced effect of TNF-alpha and NO. The results of this study indicate that HAPI cells possess the characteristics of microglia/brain macrophages, providing an alternative cell culture model for the study of microglia. In addition, we demonstrate that the activation of microglial cells could be modified by iron.

Helper-dependent adenoviral vector-mediated gene transfer in aged rat brain

Transfer of the neurotrophin gene into brain can attenuate age-related deficits such as neuronal atrophy and memory loss, but a suitable vector for this procedure has been lacking. The toxicity and immunogenicity of first-generation adenoviral vectors with E1 deletion (fgAdv) prohibit the application of gene transfer in the majority of central nervous system disorders. Here, we report less toxic and persistent gene expression mediated by helper-dependent adenovirus (hdAdv) in aged rat brain. After intrahippocampal or intraventricular inoculation of the vector, transgene expression was monitored by X-Gal staining and compared with fgAdv-mediated expression. Host inflammatory and immune responses against these vectors were evaluated by immunohistochemical detection of microglia, astrocytes, and infiltrating macrophages, as well as by enzyme-linked immunosorbent assay of cytokines TNF-alpha and IL-1beta. Transgene expression mediated by hdAdv persisted for more than 183 days regardless of inoculation site, as compared with 33 and 66 days for fgAdv-mediated expression after intraventricular and intrahippocampal inoculation, respectively. Inoculation with hdAdv was also associated with reduced numbers of activated microglial cells, astrocytes, and infiltrating macrophages in brain tissue. Secretion of the proinflammatory cytokines TNF-alpha and IL-1beta was minimal after hdAdv but not after fgAdv inoculation. These findings indicate that hdAdv would provide a safe and effective means to transfer therapeutic genes into aged brain.

T lymphocytes promote the development of bone marrow-derived APC in the central nervous system

Certain cells within the CNS, microglial cells and perivascular macrophages, develop from hemopoietic myelomonocytic lineage progenitors in the bone marrow (BM). Such BM-derived cells function as CNS APC during the development of T cell-mediated paralytic inflammation in diseases such as experimental autoimmune encephalomyelitis and multiple sclerosis. We used a novel, interspecies, rat-into-mouse T cell and/or BM cell-transfer method to examine the development and function of BM-derived APC in the CNS. Activated rat T cells, specific for either myelin or nonmyelin Ag, entered the SCID mouse CNS within 3-5 days of cell transfer and caused an accelerated recruitment of BM-derived APC into the CNS. Rat APC in the mouse CNS developed from transferred rat BM within an 8-day period and were entirely sufficient for induction of CNS inflammation and paralysis mediated by myelin-specific rat T cells. The results demonstrate that T cells modulate the development of BM-derived CNS APC in an Ag-independent fashion. This previously unrecognized regulatory pathway, governing the presence of functional APC in the CNS, may be relevant to pathogenesis in experimental autoimmune encephalomyelitis, multiple sclerosis, and/or other CNS diseases involving myelomonocytic lineage cells.

Microglial activation in the developing rat olfactory bulb

The development of the olfactory bulb, the primary central relay of the olfactory system, is characterized by a striking susceptibility to alterations in the amount of afferent input. For example, blocking airflow through one half of the nasal cavity during early life results in a number of dramatic changes in the bulb, including increased cell death. Previous studies reveal high levels of microglia in the olfactory bulb. Microglia function as phagocytes, aid in synaptogenesis, and produce important trophic and cytotoxic factors. In response to a number of tissue perturbations, microglia undergo an activation process that includes, among other changes, the up-regulation of complement receptor 3. Interestingly, a previous study reported that naris closure had no effect on microglia in the bulb; however, the research did not distinguish the functional activation state of microglia. We further examined the role of microglia in the normally developing and olfactory-deprived rat bulb using immunohistochemical detection of complement receptor 3 as a measure of microglial activation. Expression of the receptor in the bulb is relatively high during postnatal development, in particular when compared to levels in cortical regions caudal to the olfactory bulb. In addition, naris closure performed on the day after birth (but not after the first postnatal month) increases levels of the receptor in an age and laminar-dependent fashion. The presence of an inducible pool of activated microglia in the olfactory bulb may be important for normal development and contribute to the plethora of changes seen after early olfactory deprivation.

Expression of alpha(2)-macroglobulin receptor/low density lipoprotein receptor-related protein (LRP) in rat microglial cells

Low density lipoprotein receptor-related protein (LRP) participates in the uptake and degradation of several ligands implicated in neuronal pathophysiology including apolipoprotein E (apoE), activated alpha(2) -macroglobulin (alpha(2)M*) and beta-amyloid precursor protein (APP). The receptor is expressed in a variety of tissues. In the brain LRP is present in pyramidal-type neurons in cortical and hippocampal regions and in astrocytes that are activated as a result of injury or neoplasmic transformation. As LRP is expressed in the monocyte/macrophage cell system, we were interested in examining whether LRP is expressed in microglia. We isolated glial cells from the brain of neonatal rats and LRP was immunodetected both in microglial cells and in astrocytes expressing glial fibrillar acidic protein (GFAP). Microglial cells were able to bind and internalize LRP-specific ligand, alpha(2)M*. The internalization was inhibitable by RAP, with a Kd of 1.7 nM. The expression of LRP was up-regulated by dexamethasone, and down-regulated by lipopolysaccharide (LPS), gamma interferon (IFN-gamma) or a combination of both. LRP was less sensitive to dexamethasone in activated astrocytes than in microglia. We provided the first analysis of LRP expression and regulation in microglia. Our results open the possibility that microglial cells could be related to the participation of LRP and its ligands in different pathophysiological states in brain.

Microglial activation and neurological symptoms in the SIV model of NeuroAIDS: association of MHC-II and MMP-9 expression with behavioral deficits and evoked potential changes

HIV-1 causes cognitive and motor deficits and HIV encephalitis (HIVE) in a significant proportion of AIDS patients. Neurological impairment and HIVE are thought to result from release of cytokines and other harmful substances from infected, activated microglia. In this study, the quantitative relationship between microglial activation and neurological impairment was examined in the simian immunodeficiency model of HIVE. Macaque monkeys were infected with a passaged, neurovirulent strain of simian immunodeficiency virus, SIV(mac)239(R71/17E). In concurrent studies, functional impairment was assessed by motor and auditory brainstem evoked potentials and by measurements of cognitive and motor behavioral deficits. Brain tissue was examined by immunohistochemistry using two markers of microglia activation, MHC-II and matrix metalloproteinase-9 (MMP-9). The inoculated animals formed two groups: rapid progressors, which survived 6-14 weeks postinoculation, and slow progressors, which survived 87-109 weeks. In the rapid progressors, two patterns of MHC-II expression were present: (1) a widely disseminated pattern of MHC-II expressing microglia and microglial nodules in cortical gray matter and subcortical white matter, and (2) a more focal pattern in which MHC-II expressing microglia were concentrated into white matter. Animals exhibiting both patterns of microglial activation showed mild to severe changes in cognitive and motor behavior and evoked potentials. All rapid progressors showed expression of MMP-9 in microglia located in subcortical white matter. In the slow progressors MHC-II and MMP-9 staining was similar to uninoculated control macaques, and there was little or no evidence of HIVE. These animals showed behavioral deficits at the end of the disease course, but little changes in evoked potentials. Thus, increases in MHC-II and MMP-9 expression are associated with development of cognitive and motor deficits, alterations in evoked potentials, and rapid disease progression.

Evidence for a microglial reaction within the vestibular and cochlear nuclei following inner ear lesion in the rat

Following unilateral inner ear lesion, astrocytes undergo hypertrophy in the deafferented vestibular and cochlear nuclei as shown by an increase in the level of glial fibrillary acid. The present study extends our understanding of vestibular and cochlear system plasticity by examining microglial changes in these deafferented nuclei. The microglial reaction was studied 1, 2, 4, 8, 14, 21, 28 and 42 days following the lesion with a monoclonal OX-42 antibody and lectins (Griffonia simplicifolia, B4 isolectin) labelled with horseradish peroxidase or fluorescein. The deafferented nuclei were also examined for apoptotic cells by terminal transferase-mediated nick end labelling of nuclear DNA fragments. In control and sham-operated rats, the distribution of the resting microglial cells was uniform in both the vestibular and cochlear nuclei. In the deafferented vestibular complex, the microglial cells increased in number, became hypertrophied and were distributed in the medial, lateral, superior and inferior vestibular nuclei. Reactive microglial cells were also detected in the ipsilateral cochlear nuclei. Some of the immunostained cells were hypertrophic whereas others presented an ameboid morphology with few short and stout processes. The microglial reaction was confined to the antero- and posteroventral cochlear nuclei. Finally, reactive microglia was also observed in the prepositus hypoglossi ipsilateral to the lesion. The microglial reactions within the prepositus hypoglossi, the vestibular and the cochlear nuclei were detectable as early as one day after the lesion and persisted several weeks in both the vestibular and cochlear nuclei. Apoptotic cells were not detected in the vestibular nuclei at any stage following the lesion. In contrast, terminal deoxynucleotidyl transferase-mediated digoxygenin-11-dUTP nick end labelling-positive cells were first detected in the deafferented cochlear nuclei on the 3rd day following the lesion. They reached an apparent maximum by day 8 and then declined until day 24. Double labelling experiments demonstrate that these cochlear terminal deoxynucleotidyl transferase-mediated digoxygenin-11-dUTP nick end labelling-positive cells were also lectin-positive suggesting that reactive cochlear lectin-positive microglia cells were eliminated by a programmed cell death. Our results establish the two experimental models as reliable tools to understand the role of microglia in adult brain plasticity. The cochlear microglial reaction was probably induced by the degeneration of the acoustic nerve which follows the acoustic ganglion destruction. Interestingly, the same reasoning cannot apply to the vestibular microglial reaction following unilateral labyrinthectomy: the vestibular ganglion was spared and the primary vestibular neurons did not degenerate, at least during the first week following the lesion.

Homologous plasminogen N-terminal and plasminogen-related gene A and B peptides. Characterization of cDNAs and recombinant fusion proteins

The cDNA corresponding to exons 2-4 of the processed human plasminogen (Pgn) gene, encoding the N-terminal peptide domain (NTP), has been cloned, expressed in Escherichia coli as a recombinant protein (r-NTP) containing a hexahistidine tag, and refolded to the native structure that contains two internal cystine bridges. RNA expression of the two Pgn-related genes, PRG A and PRG B, that potentially encode 9-kDa polypeptides having extensive similarity to the NTP has been investigated. Using RNA-based PCR with liver RNA as template, we demonstrate that PRG A encodes a detectable mRNA species. PRG A and PRG B have been found to be transcribed in the liver and yield virtually identical mRNAs. Neither of the PRGs are expressed in a variety of other normal tissues, as determined by Northern blot analysis. Factor-Xa digestion of the tagged r-NTP yields cleavage products which indicates that the expressed r-NTP domain of Pgn is endowed with a flexible conformation. Recombinant PRG B protein (r-PRG B) fused to a hexahistidine tag was purified and analyzed for structural integrity. Preliminary 1H-NMR spectroscopic data for r-NTP and r-PRG B indicate relatively fast amide 1H-2H exchange in 2H2O and close conformational characteristics for the two homologous polypeptides. Far ultraviolet-CD spectra for r-NTP and r-PRG B at pH 7.0 indicate similar defined secondary structure content for both domains, with 13-17% alpha-helix and 24-27% antiparallel beta-sheet. The fact that two transcriptionally active genes encode almost identical polypeptides supports the hypothesis that the Pgn NTP, together with the putative polypeptides encoded by the PRGs, may serve an important function, such as controlling the conformation of Pgn and thus its susceptibility to tissue activators.